硫酸吲哚酚在脑内的作用机制

Aim: This study investigates the correlation between indoxyl sulfate (IS) levels and cognitive impairment in end-stage renal disease (ESRD) patients from human study, in vivo and in vitro study. Materials and Methods: Comparison of demographic and biochemical data, including IS concentrations, was conducted between a control group(n=16) and the ESRD with cognitive impairment group (n=14) and without cognitive impairment (n=17). A CKD animal model induced renal impairment in adenine-fed C57BL/6 mice, assessing memory loss and behavioral changes. Immunohistochemistry evaluated choline acetyltransferase activity and GFAP expression. Differentiating SH-SY5Y cells were treated with IS, assessing cell viability and apoptosis via annexin V and propidium iodide staining and western blotting. Reactive oxidized species generation was measured using DCFCA fluorescence and NAC pretreatment. Results: In ESRD patients with cognitive impairment, IS levels were significantly higher compared to healthy controls, along with older age. CKD mice exhibited renal impairment and memory loss, accompanied by altered choline acetyltransferase activity and GFAP expression. IS treatment induced early apoptosis in SH-SY5Y cells, associated with increased cleaved caspase 3 levels and Fas/Fas-ligand activity, altered Bax/Bcl2 ratio, and reactive oxidized species generation. Conclusion: Elevated IS levels are associated with cognitive impairment and neuronal apoptosis, potentially mediated by oxidative stress. IS could be a therapeutic target for cognitive dysfunction in CKD, necessitating further research into its mechanisms and therapeutic interventions.

It is unknown whether indoles, metabolites of tryptophan that are derived entirely from bacterial metabolism in the gut, are associated with symptoms of depression and anxiety. Serum samples (baseline, 12 weeks) were drawn from participants (n = 196) randomized to treatment with cognitive behavioral therapy (CBT), escitalopram, or duloxetine for major depressive disorder. Baseline indoxyl sulfate abundance was positively correlated with severity of psychic anxiety and total anxiety and with resting state functional connectivity to a network that processes aversive stimuli (which includes the subcallosal cingulate cortex (SCC-FC), bilateral anterior insula, right anterior midcingulate cortex, and the right premotor areas). The relation between indoxyl sulfate and psychic anxiety was mediated only through the metabolite’s effect on the SCC-FC with the premotor area. Baseline indole abundances were unrelated to post-treatment outcome measures, and changes in symptoms were not correlated with changes in indole concentrations. These results suggest that CBT and antidepressant medications relieve anxiety via mechanisms unrelated to modulation of indoles derived from gut microbiota; it remains possible that treatment-related improvement stems from their impact on other aspects of the gut microbiome. A peripheral gut microbiome-derived metabolite was associated with altered neural processing and with psychiatric symptom (anxiety) in humans, which provides further evidence that gut microbiome disruption can contribute to neuropsychiatric disorders that may require different therapeutic approaches. Given the exploratory nature of this study, findings should be replicated in confirmatory studies. Clinical trial NCT00360399 “Predictors of Antidepressant Treatment Response: The Emory CIDAR” https://clinicaltrials.gov/ct2/show/NCT00360399.

Background Neurologic complications, such as cognitive and emotional dysfunction, have frequently been observed in chronic kidney disease (CKD) patients. Previous research shows that uremic toxins play a role in the pathogenesis of CKD-associated cognitive impairment. Since astrocytes contribute to the protection and survival of neurons, astrocyte function and brain metabolism may contribute to the pathogenesis of neurodegeneration. Indoxyl sulfate (IS) is the most popular uremic toxin. However, how IS-induced astrocyte injury brings about neurologic complications in CKD patients has not been elucidated. Methods The rate of extracellular acidification was measured in astrocytes when IS (0.5–3 mM, 4 or 7 days) treatment was applied. The hexokinase 1 (HK1), pyruvate kinase isozyme M2 (PKM2), pyruvate dehydrogenase (PDH), and phosphofructokinase (PFKP) protein levels were also measured. The activation of the apoptotic pathway was investigated using a confocal microscope, fluorescence-activated cell sorting, and cell three-dimensional imaging was used. Results In astrocytes, IS affected glycolysis in not only dose-dependently but also time-dependently. Additionally, HK1, PKM2, PDH, and PFKP levels were decreased in IS-treated group when compared to the control. The results were prominent in cases with higher doses and longer exposure duration. The apoptotic features after IS treatment were also observed. Conclusion Our results showed that the inhibition of glycolysis by IS in astrocytes leads to cell death via apoptosis. Specifically, long-term and higher-dose exposures had more serious effects on astrocytes. Our results suggest that the glycolysis pathway and related targets could provide a novel approach to cognitive dysfunction in CKD patients.

Chronic Kidney Disease (CKD) and neurodegenerative diseases are aging-related diseases. CKD with declined renal function is associated with an elevation of circulating indoxyl sulfate, a metabolite synthesized by gut microbes. We explored the roles of gut microbial metabolites in linking with Central Nervous System (CNS) diseases by administrating indoxyl sulfate intraperitoneally to male C57BL/6 mice with unilateral nephrectomy. Upon exposure, the accumulation of indoxyl sulfate was noted in the blood, prefrontal cortical tissues, and cerebrospinal fluid. Mice showed behavioral signs of mood disorders and neurodegeneration such as anxiety, depression, and cognitive impairment. Those behavioral changes were accompanied by disturbed neuronal survival, neural stem cell activity, expression of Brain-Derived Neurotrophic Factor, serotonin, corticosterone, and Repressor Element-1 Silencing Transcription Factor, and post-receptor intracellular signaling, as well as upregulated oxidative stress and neuroinflammation. Uremic toxin adsorbent AST-120 improved the above mentioned changes. Intriguingly, intracerebroventricular indoxyl sulfate administration only caused limited alterations in the normal mice and the alterations were reversed by aryl hydrocarbon receptor antagonism. The findings suggest pathogenic roles of indoxyl sulfate in the development of CNS diseases, and highlight gut microbiota as alternative targets for intervention with the aim of slowing down the progression of CKD and decreasing CNS complications.

Indoxyl sulfate—a bacterially derived metabolite—has been identified as a toxin that is elevated in children with autism spectrum disorder (ASD). As a neurotoxin, uremic toxin, nephrotoxin, cardiotoxin, osteotoxin, and myotoxin, indoxyl sulfate has been associated with several other conditions, including chronic kidney disease, acute kidney injury, Parkinson’s disease, cognitive disorders, and mood disorders such as anxiety and depression. Indoxyl sulfate is derived from bacterial modification of host tryptophan, and elevated levels of indoxyl sulfate are associated with decreased levels of important neurotransmitters including serotonin, dopamine, and norepinephrine. This article will review what is currently known about indoxyl sulfate in relation to ASD and its comorbidities. A systematic review identified six studies of levels of indoxyl sulfate in children with ASD. All six studies found that indoxyl sulfate was significantly elevated in the urine of children with ASD compared to typically developing children. Through this review, indoxyl sulfate was identified as a toxic microbially derived metabolite that is significantly increased in a subset of children with ASD and may contribute to both core and co-morbid ASD symptoms.

Patients with end-stage renal disease often have neurological disorders, with a higher incidence of memory impairment or epilepsy than in the general population. Patients undergoing hemodialysis are particularly exposed to the biological effects of uremic toxins. Indoxyl sulfate (IS) is one of the most potent uremic toxins; however, its possible effects on seizure susceptibility or memory functions have yet to be elucidated. In the current study, we focused on investigating the possible convulsant and amnesic effects of IS in recognized animal models. The study was performed on adult male Swiss mice. IS and scopolamine (SCO) were administered intraperitoneally (i.p.), and pentylenetetrazole (PTZ) was injected subcutaneously (s.c.). All substances were given as single injections. Acute IS administration (400 mg/kg) led to its accumulation in the brain. IS at doses of 200 and 400 mg/kg decreased the PTZ convulsive threshold, and at the same doses, it did not significantly affect the threshold for electroconvulsions. IS (200 and 400 mg/kg) did not impair learning in the passive avoidance test and did not increase the SCO-induced memory impairment in this test. IS increased lipid peroxidation, decreased the level of reduced glutathione, and reduced the activity of superoxide dismutase and catalase in mouse brains. Exposure to IS did not significantly change the activity of acetylcholinesterase in the brain tissue. This study shows that acute exposure to IS induces oxidative stress in the brain and potentiates PTZ-induced seizures in mice. Further studies are needed to find out whether IS-induced oxidative stress may affect epileptic seizures and/or epileptogenesis.

Rates of pregnancy-related acute kidney injury (PR-AKI) have increased in the U.S over the past two decades, but how PR-AKI affects the blood–brain barrier (BBB) is understudied. AKI is associated with increased amounts of uremic toxins, like indoxyl sulfate (I.S), whose chronic administration leads to BBB and cognitive changes. This study’s objective was to determine if (1) PR-AKI increases I.S and (2) if administration of I.S during pregnancy elicits renal injury and/or increases BBB permeability. From gestational day (GD) 11 to GD19, Sprague Dawley rats were given either 100 or 200 mg/kg body-weight dose of I.S. PR-AKI was induced on GD18 via 45 min bilateral renal ischemic reperfusion surgery. On GD18, metabolic cage metrics and metabolic waste was collected and on GD19 blood pressure, and BBB permeability (by Evan’s Blue infusion) were measured. I.S and creatinine were measured in both urine and circulation, respectively. One-way ANOVA or student t-tests were performed using GraphPad Prism with a p < 0.05 significance. I.S and PR-AKI led to oliguria. I.S administration led to increased BBB permeability compared to normal pregnant and PR-AKI animals. These results suggest that I.S administration during pregnancy leads to increased BBB permeability and evidence of renal injury comparable to PR-AKI animals.

… indoxyl sulfate across BBB, indicating that alterations in function of brain OAT3 are also involved in process of some brain … Our findings may pave the way for fresh mechanism in central …

Abstract Gut microbiota in interaction with intestinal host tissues influences many brain functions and microbial dysbiosis has been linked with brain disorders, such as neuropsychiatric conditions and Alzheimer’s disease (AD). l -tryptophan metabolites and short-chained fatty acids (SCFA) are major messengers in the microbiota-brain axis. Aryl hydrocarbon receptors (AhR) are main targets of tryptophan metabolites in brain microvessels which possess an enriched expression of AhR protein. The Ah receptor is an evolutionarily conserved, ligand-activated transcription factor which is not only a sensor of xenobiotic toxins but also a pleiotropic regulator of both developmental processes and age-related tissue degeneration. Major microbiota-produced tryptophan metabolites involve indole derivatives, e.g., indole 3-pyruvic acid, indole 3-acetaldehyde, and indoxyl sulfate, whereas indoleamine and tryptophan 2,3-dioxygenases (IDO/TDO) of intestine host cells activate the kynurenine (KYN) pathway generating KYN metabolites, many of which are activators of AhR signaling. Chronic kidney disease (CKD) increases the serum level of indoxyl sulfate which promotes AD pathogenesis, e.g., it disrupts integrity of blood–brain barrier (BBB) and impairs cognitive functions. Activation of AhR signaling disturbs vascular homeostasis in brain; (i) it controls blood flow via the renin-angiotensin system, (ii) it inactivates endothelial nitric oxide synthase (eNOS), thus impairing NO production and vasodilatation, and (iii) it induces oxidative stress, stimulates inflammation, promotes cellular senescence, and enhances calcification of vascular walls. All these alterations are evident in cerebral amyloid angiopathy (CAA) in AD pathology. Moreover, AhR signaling can disturb circadian regulation and probably affect glymphatic flow. It seems plausible that dysbiosis of gut microbiota impairs the integrity of BBB via the activation of AhR signaling and thus aggravates AD pathology. Key messages Dysbiosis of gut microbiota is associated with dementia and Alzheimer’s disease. Tryptophan metabolites are major messengers from the gut host-microbiota to brain. Tryptophan metabolites activate aryl hydrocarbon receptor (AhR) signaling in brain. The expression of AhR protein is enriched in brain microvessels and blood-brain barrier. Tryptophan metabolites disturb brain vascular integrity via AhR signaling. Dysbiosis of gut microbiota promotes inflammation and AD pathology via AhR signaling.

… -oxide, indoxyl sulfate, and p-cresyl sulfate impair hepatocyte energy … within the brain, and affect behavior and brain functions. … , through different mechanisms including aryl hydrocarbon …

Emerging evidence suggests that parkinson's disease (PD) extends beyond the brain and involves early disturbances along the gut-brain axis. Among the metabolites shaping this communication, bacterial products derived from tryptophan, particularly indole compounds, are gaining attention as key biochemical links between intestinal dysbiosis and neurodegeneration. Multi-omics studies consistently show a reduction in commensal bacteria capable of producing beneficial indoles and an enrichment of Enterobacteriaceae that redirect tryptophan catabolism toward toxic intermediates. This shift disrupts epithelial and blood-brain barrier function and amplifies inflammatory and oxidative stress pathways within the central nervous system. Protective metabolites such as indole-3-propionic acid (IPA), indole-3-acetic acid (IAA), indole-3-lactic acid (ILA), and indole-3-carbinol (I3C) exert antioxidant, barrier-stabilizing, and anti-inflammatory effects through receptors including the aryl hydrocarbon and pregnane X receptors. Conversely, uremic indoles such as indoxyl sulfate (IS) and p-cresyl sulfate (pCS) activate microglia and astrocytes, promote α-synuclein aggregation, and accelerate dopaminergic neuron loss. Together, these findings support a view of PD as a metabolic imbalance between neuroprotective and neurotoxic indoles. Understanding how microbial and host pathways regulate this balance may open opportunities for early diagnosis and targeted interventions that integrate metabolism, immunity, and neuroprotection.

The abnormal indole metabolism is associated with the progression of Autism Spectrum Disorder (ASD). Indoxyl sulfate (IS), one of the active products of indole metabolism, still has an unknown role in ASD progression. This study investigates the role of IS/Aryl hydrocarbon receptor (AhR)/iNOS pathway in microglial activation in the prefrontal cortex (PFC) of ASD-like rats. Pregnant LPS-exposed induced autism-like behaviors offspring rats, concomitant with increased IS levels in the PFC. The levels of nuclear-AhR, IBA1, CD16 and iNOS proteins expression were increased in the PFC of LPS-exposed rats, whereas ARG1 protein expression level decreased, indicates microglia hyperactivation coupled with altered microglia morphology. ELISA analysis and further measure of synapses changes showed significantly increased inflammatory factors (TNF-α and IL-1β) and synaptic alterations. In vitro experiments demonstrated that IS treatment significantly upregulated the expression level of nuclear-AhR, enhanced microglia marker (IBA1, CD16 and iNOS) proteins and pro-inflammation factors levels (TNF-α and IL-1β), while concurrently reducing ARG1 protein expression and IL-10 levels in BV2 microglial cells. Moreover, the IS treatment significantly enhanced AhR enrichment in iNOS promoter region by chromatin immunoprecipitation and dual luciferase gene reporter assays, thereby significantly elevating the iNOS expression. However, the AhR-specific antagonist CH-223191 could block this activation and reverse the above proteins and inflammation factors changes. In a word, increased IS levels in the PFC of ASD-like offspring rats activate the AhR/iNOS pathway, driving microglial hyperresponsiveness and contributing to the development of ASD disease.

Alzheimer’s disease (AD) is an age-dependent neurodegenerative disease characterized by extracellular Amyloid Aβ peptide (Aβ) deposition and intracellular Tau protein aggregation. Glia, especially microglia and astrocytes are core participants during the progression of AD and these cells are the mediators of Aβ clearance and degradation. The microbiota-gut-brain axis (MGBA) is a complex interactive network between the gut and brain involved in neurodegeneration. MGBA affects the function of glia in the central nervous system (CNS), and microbial metabolites regulate the communication between astrocytes and microglia; however, whether such communication is part of AD pathophysiology remains unknown. One of the potential links in bilateral gut-brain communication is tryptophan (Trp) metabolism. The microbiota-originated Trp and its metabolites enter the CNS to control microglial activation, and the activated microglia subsequently affect astrocyte functions. The present review highlights the role of MGBA in AD pathology, especially the roles of Trp per se and its metabolism as a part of the gut microbiota and brain communications. We (i) discuss the roles of Trp derivatives in microglia-astrocyte crosstalk from a bioinformatics perspective, (ii) describe the role of glia polarization in the microglia-astrocyte crosstalk and AD pathology, and (iii) summarize the potential of Trp metabolism as a therapeutic target. Finally, we review the role of Trp in AD from the perspective of the gut-brain axis and microglia, as well as astrocyte crosstalk, to inspire the discovery of novel AD therapeutics.

… in the expanding comprehension of the gut–brain axis, which serves as a bidirectional communication network between the gastrointestinal system and the central nervous system. The …

In recent years, a substantial amount of data have supported an active role of gut microbiota in mediating mammalian brain function and health. Mining gut microbiota and their metabolites for neuroprotection is enticing but requires that the fundamental biochemical details underlying such microbiota-brain crosstalk be deciphered. While a neuronal gut-brain axis (through the vagus nerve) is not disputable, accumulating studies also point to a humoral route (via blood/lymphatic circulation) by which innumerable microbial molecular cues translocate from local gut epithelia to circulation with potentials to further cross the blood-brain barrier and reach the brain. In this Perspective, we review a realm of gut microbial molecules to evaluate their fate, function, and neuroactivities in vivo as mediated by microbiota. We turn to seminal studies of neurophysiology and neurologic disease models for the elucidation of biochemical pathways that link microbiota to gut-brain signaling. In addition, we discuss opportunities and challenges for advancing the microbiota-brain axis field while calling for high-throughput discovery of microbial molecules and studies for resolving the interspecies, interorgan, and interclass interaction among these neuroactive microbial molecules.

The aryl hydrocarbon receptor (AhR) functions as a ligand‐dependent transcription factor, serving as a pivotal environmental sensor that significantly influences both physiological and pathological processes. The inactivated state of AhR is present in the cell cytoplasm and transfer into the nucleus upon activation by a variety of ligands. It subsequently regulates a variety of processes including cellular metabolism, organ and tissue development, and maintenance of immune homeostasis. Despite substantial advancements over the past decade, the mechanisms by which AhR specifically regulates immune cell function in response to environmental factors and influences disease progression remain not fully elucidated. This review systematically analyzes the basic structure and major signaling pathways of AhR, its physiological functions in maintaining organismal homeostasis and its mechanism of action on various types of immune cells, and their therapeutic potential in autoimmune diseases, inflammatory disorders, tumor microenvironment, and neurodegenerative diseases. Translating immune‐metabolic reprogramming mechanisms into clinical applications represents a pivotal challenge in AhR research. And this review integrates and analyzes the great potential of AhR as a pleiotropic therapeutic target for regulating immunity and treating a series of diseases, offering actionable frameworks for future exploration.

… central nervous system (CNS) by affecting the maturation and homeostasis of microglia [14, 15]. In addition, the microbial metabolite of dietary tryptophan, indole-3-sulfate (… neurotoxicity […

Alzheimer's disease (AD) is a neurodegenerative disorder predominantly affecting the elderly population, imposing significant burdens on both society and the families of affected individuals. As investigations into AD advance, the involvement of the aromatic hydrocarbon receptor (AHR) signaling pathway in the AD progression has garnered increasing attention. AHR functions as a receptor that detects and responds to a various of environmental stimuli, playing a crucial role in numerous biological processes, including cellular differentiation, metabolic regulation, and immune responses. During the progression of AD, the AHR signaling pathway may be intricately linked to several pathological factors, including β-amyloid (Aβ) accumulation, neuroinflammation, oxidative stress, mitochondrial dysfunction, changes in the blood‒brain barrier (BBB) permeability, and alterations in neuronal metabolic processes. Inhibitors of AHR such as indoleamine 2,3-dioxygenase 1 (IDO1) present potential therapeutic approaches for AD treatment. A comprehensive understanding of the AHR signaling pathway’s mechanisms in the context of AD is essential for elucidating the disease’s pathogenesis and for the development of novel therapeutic strategies. This review aims to provide a detailed overview of recent research advancements related to the AHR signaling pathway in AD, thereby offering valuable references and insights for future studies.

Chronic kidney disease (CKD)‐associated mental disorders have been attributed to the excessive accumulation of hemodialysis‐resistant indoxyl‐3‐sulfate (I3S) in the brain. I3S not only induces oxidative stress but is also a potent endogenous agonist of the aryl hydrocarbon receptor (AhR). Here, we investigated the role of AhR in CKD‐induced brain disorders using a 5/6 nephrectomy‐induced CKD mouse model, which showed increased I3S concentration in both blood and brain, anxiety and impaired novelty recognition, and AhR activation in the anterior cortex. GFAP+ reactive astrocytes were increased accompanied with the reduction of glutamate transporter 1 (GLT1) on perineuronal astrocytic processes (PAPs) in the anterior cingulate cortex (ACC) in CKD mice, and these alterations were attenuated in both neural lineage‐specific and astrocyte‐specific Ahr conditional knockout mice (nAhrCKO and aAhrCKO). By using chronic I3S treatment in primary astrocytes and glia‐neuron (GN) mix cultures to mimic the CKD brain microenvironment, we also found significant reduction of GLT1 expression and activity in an AhR‐dependent manner. Chronic I3S treatment induced AhR‐dependent pro‐oxidant Nox1 and AhR‐independent anti‐oxidant HO‐1 expressions. Notably, AhR mediates chronic I3S‐induced neuronal activity enhancement and synaptotoxicity in GN mix, not neuron‐enriched cortical culture. In CKD mice, neuronal activity enhancement was observed in ACC and hippocampal CA1, and these responses were abrogated by both nAhrCKO and aAhrCKO. Finally, intranasal AhR antagonist CH‐223191 administration significantly ameliorated the GLT1/PAPs reduction, increase in c‐Fos+ neurons, and memory impairment in the CKD mice. Thus, astrocytic AhR plays a crucial role in the CKD‐induced disturbance of neuron‐astrocyte interaction and mental disorders.

Gut microbiota alterations might affect the development of Alzheimer's disease (AD) through microbiota-derived metabolites. For example, microbiota-derived Indoles via tryptophan metabolism prevented Aβ accumulation and Tau hyperphosphorylation, restored synaptic plasticity, and then promoted the cognitive and behavioral ability of APP/PS1 mice. The imbalanced compositions of Indoles-producing bacteria with tryptophan deficiency were found in male APP/PS1 mice, but the molecular mechanisms remained unclear. Our current study revealed that Indoles (including indole, indole-3-acetic acid and indole-3-propionic acid) upregulated the production of aryl hydrocarbon receptor (AhR), inhibited the activation of the NF-κB signal pathway as well as the formation of the NLRP3 inflammasome, reduced the release of inflammatory cytokines, including TNF-α, IL-6, IL-1β and IL-18, alleviating the inflammatory response of APP/PS1 mice. These findings demonstrated the roles of Indoles-producing bacteria in activating the AhR pathway to regulate neuroinflammation of AD through gut microbiota-derived Indoles, which implied a novel way for AD treatment.

The Aryl hydrocarbon receptor (AHR) is a cytosolic receptor and ligand-activated transcription factor widely expressed across various cell types in the body. Its signaling is vital for host responses at barrier sites, regulating epithelial renewal, barrier integrity, and the activities of several types of immune cells. This makes AHR essential for various cellular responses during aging, especially those governing inflammation and immunity. In this review, we provided an overview of the mechanisms by which the AHR mediates inflammatory response at gut and brain level through signals from intestinal microbes. The age-related reduction of gut microbiota functions is perceived as a trigger of aberrant immune responses linking gut and brain inflammation to neurodegeneration. Thus, we explored gut microbiome impact on the nature and availability of AHR ligands and outcomes for several signaling pathways involved in neurodegenerative diseases and age-associated decline of brain functions, with an insight on Parkinson’s and Alzheimer’s diseases, the most common neurodegenerative diseases in the elderly. Specifically, we focused on microbial tryptophan catabolism responsible for the production of several AHR ligands. Perspectives for the development of microbiota-based interventions targeting AHR activity are presented for a healthy aging.

AhR regulates remyelination following cuprizone-induced demyelination. Deletion of AhR in adult microglia restrains efficient remyelination. Microglial AhR deficiency causes myelin debris accumulation and impairs phagocytic function. Microglial AhR mediates phagocytosis of microglia via SYK. Multiple sclerosis (MS) is an inflammatory-mediated demyelinating disease of the central nervous system (CNS). Although studies have demonstrated that microglia facilitate remyelination in demyelinating diseases, the underlying mechanisms are still not fully characterized. We found that aryl hydrocarbon receptor (AhR), an environment sensor, was upregulated within the corpus callosum in the cuprizone model of CNS demyelination, and upregulated AhR was mainly confined to microglia. Deletion of AhR in adult microglia inhibited efficient remyelination. Transcriptome analysis using RNA-seq revealed that AhR-deficient microglia displayed impaired gene expression signatures associated with lysosome and phagocytotic pathways. Furthermore, AhR-deficient microglia showed impaired clearance of myelin debris and defected phagocytic capacity. Further investigation of target genes of AhR revealed that spleen tyrosine kinase (SYK) is the downstream effector of AhR and mediated the phagocytic capacity of microglia. Additionally, AhR deficiency in microglia aggravated CNS inflammation during demyelination. Altogether, our study highlights an essential role for AhR in microglial phagocytic function and suggests the therapeutic potential of AhR in demyelinating diseases. Graphical Abstract

Recent studies have highlighted the crucial role of microglia (MG) and their interactions with the gut microbiome in post-stroke neuroinflammation. The activation of immunoregulatory pathways, including the aryl hydrocarbon receptor (AHR) pathway, is influenced by a dynamic balance of ligands derived from both the host and microbiota. This study aimed to investigate the association between stroke-induced dysbiosis and the resultant imbalance in AHR ligand sources (loss of microbiota-derived [indole-based] and increase of host-derived [kynurenine-based]) after stroke. Microbiota-derived AHR ligands decreased in human plasma and remained low for days following an ischemic stroke highlighting the translational significance. Transient-middle-cerebral-artery-occlusion was performed in aged wild-type and germ-free male mice. MG-AHR expression and activity increased in both in vivo and ex vivo stroke models. Germ-free mice showed altered neuroinflammation and antigen presentation while aged mice showed reduced infarct volume and neurological deficits following treatment with microbiota-derived AHR ligands after stroke. Restoring a balanced pool of host- and microbiota-derived AHR ligands may be beneficial after stroke and may represent a therapeutic target. Peesh et al. show that ischemic stroke reduces microbiota-derived and increases host-derived aryl (AHR) hydrocarbon ligands. Post-stroke treatment with indole-based AHR ligands improved microglia-mediated antigen processing and co-stimulatory immune functions.

Several metabolites of the essential amino acid tryptophan have emerged as key players in gut homeostasis through different cellular pathways, particularly through metabolites which can activate the aryl hydrocarbon receptor (AHR). This study aimed to map the metabolism of tryptophan in early life and investigate the effects of specific metabolites on epithelial cells and barrier integrity. Twenty-one tryptophan metabolites were measured in the feces of full-term and preterm neonates as well as in human milk and formula. The ability of specific AHR metabolites to regulate cytokine-induced IL8 expression and maintain barrier integrity was assessed in Caco2 cells and human fetal organoids (HFOs). Overall, higher concentrations of tryptophan metabolites were measured in the feces of full-term neonates compared to those of preterm ones. Within AHR metabolites, indole-3-lactic acid (ILA) was significantly higher in the feces of full-term neonates. Human milk contained different levels of several tryptophan metabolites compared to formula. Particularly, within the AHR metabolites, indole-3-sulfate (I3S) and indole-3-acetic acid (IAA) were significantly higher compared to formula. Fecal-derived ILA and milk-derived IAA were capable of reducing TNFα-induced IL8 expression in Caco2 cells and HFOs in an AHR-dependent manner. Furthermore, fecal-derived ILA and milk-derived IAA significantly reduced TNFα-induced barrier disruption in HFOs.

Abstract Background : Stroke is a major cause of morbidity and mortality, and its incidence increases with age. While acute therapies for stroke are currently limited to intravenous thrombolytics and endovascular thrombectomy, recent studies have implicated an important role for the gut microbiome in post-stroke neuroinflammation. After stroke, several immuno-regulatory pathways, including the aryl hydrocarbon receptor (AHR) pathway, become activated. AHR is a master regulatory pathway that mediates neuroinflammation. Among various cell types, microglia (MG), as the resident immune cells of the brain, play a vital role in regulating post-stroke neuroinflammation and antigen presentation. Activation of AHR is dependent on a dynamic balance between host-derived and microbiota-derived ligands. While previous studies have shown that activation of MG AHR by host-derived ligands, such as kynurenine, is detrimental after stroke, the effects of post-stroke changes in microbiota-derived ligands of AHR, such as indoles, is unknown. Our study builds on the concept that differential activation of MG AHR by host-derived versus microbiome-derived metabolites affects outcomes after ischemic stroke. We examined the link between stroke-induced dysbiosis and loss of essential microbiota-derived AHR ligands. We hypothesize that restoring the balance between host-derived (kynurenine) and microbiota-derived (indoles) ligands of AHR is beneficial after stroke, offering a new potential avenue for therapeutic intervention in post-stroke neuroinflammation. Method : We performed immunohistochemical analysis of brain samples from stroke patients to assess MG AHR expression after stroke. We used metabolomics analysis of plasma samples from stroke and non-stroke control patients with matched comorbidities to determine the levels of indole-based AHR ligands after stroke. We performed transient middle cerebral artery occlusion (MCAO) in aged (18 months) wild-type (WT) and germ-free (GF) mice to investigate the effects of post-stroke treatment with microbiota-derived indoles on outcome. To generate our results, we employed a range of methodologies, including flow cytometry, metabolomics, and 16S microbiome sequencing. Results : We found that MG AHR expression is increased in human brain after stroke and after ex vivo oxygen-glucose deprivation and reperfusion (OGD/R). Microbiota-derived ligands of AHR are decreased in the human plasma at 24 hours after ischemic stroke. Kynurenine and indoles exhibited differential effects on aged WT MG survival after ex vivo OGD/R. We found that specific indole-based ligands of AHR (indole-3-propionic acid and indole-3-aldehyde) were absent in GF mice, thus their production depends on the presence of a functional gut microbiota. Additionally, a time-dependent decrease in the concentration of these indole-based AHR ligands occurred in the brain within the first 24 hours after stroke in aged WT mice. Post-stroke treatment of GF mice with a cocktail of microbiota-derived indole-based ligands of AHR regulated MG-mediated neuroinflammation and molecules involved in antigen presentation (increased CD80, MHC-II, and CD11b). Post-stroke treatment of aged WT mice with microbiota-derived indole-based ligands of AHR reduced both infarct volume and neurological deficits at 24 hours. Conclusion : Our novel findings provide compelling evidence that the restoration of a well-balanced pool of host-derived kynurenine-based and microbiota-derived indole-based ligands of AHR holds considerable therapeutic potential for the treatment of ischemic stroke.

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor initially identified as the receptor for dioxin. Almost half a century after its discovery, AHR is now recognized as a receptor for multiple physiological ligands, with important roles in health and disease. In this review, we discuss the role of AHR in the gut–brain axis and its potential value as a therapeutic target for immune-mediated diseases.

Neurological disorders (NDs) represent a significant global health challenges, with neurodegeneration being a common pathological feature. Recent investigations indicate the involvement of gut microbiota-derived metabolites in these disorders, such as neuroinflammation, oxidative stress, and cognitive decline. The gut-brain axis, a communication network between the gut and the central nervous system (CNS), is influenced by microbial metabolites, which can cross the blood-brain barrier and impact brain function. Key metabolites such as trimethylamine N-oxide (TMAO), para-cresol sulfate (pCS), 4-ethylphenyl sulfate (4-EPS), and indoxyl sulfate (IS) have been linked with the progression of neurological disorders. TMAO disrupts blood-brain barrier integrity, promotes oxidative stress, and activates microglial cells, which lead to the apoptosis of neurons, resulting in neuroinflammation. This could also result in psychiatric changes and behavioral disorders. pCS produced from gut bacteria metabolizing dietary proteins is correlated with amplified oxidative stress, neuroinflammation, and cognitive impairments in disorders like Parkinson's disease and Alzheimer's disease. Similarly, elevated 4-EPS levels are linked to autism spectrum disorder, contributing to anxiety-like behavior and blood-brain barrier disruption. Understanding the mechanisms by which gut-derived metabolites affect neurological health could lead to novel therapeutic strategies that can target gut microbiota for the medication and treatment of neurological disorders. Dietary precursors and gut microbiota metabolites, modulated by probiotics, prebiotics, postbiotics, and synbiotics, play a critical role in maintaining microbiota homeostasis and influencing neurological health, needing sophisticated biosensors to enable real-time monitoring and early intervention in disorders linked to gut metabolite imbalances.

… chemical to enter the systemic circulation, cross the blood brain barrier (BBB) or blood-cerebrospinal fluid (CSF) barrier and attain bioactive brain concentrations. Identifying such …

ABSTRACT Alterations in the gut microbiota composition have been associated with a range of neurodevelopmental, neurodegenerative, and neuropsychiatric disorders. The gut microbes transform and metabolize dietary- and host-derived molecules generating a diverse group of metabolites with local and systemic effects. The bi-directional communication between brain and the microbes residing in the gut, the so-called gut–brain axis, consists of a network of immunological, neuronal, and endocrine signaling pathways. Although the full variety of mechanisms of the gut–brain crosstalk is yet to be established, the existing data demonstrates that a single metabolite or its derivatives are likely among the key inductors within the gut–brain axis communication. However, more research is needed to understand the molecular mechanisms underlying how gut microbiota associated metabolites alter brain functions, and to examine if different interventional approaches targeting the gut microbiota could be used in prevention and treatment of neurological disorders, as reviewed herein. Abbreviations:4-EPS 4-ethylphenylsulfate; 5-AVA(B) 5-aminovaleric acid (betaine); Aβ Amyloid beta protein; AhR Aryl hydrocarbon receptor; ASD Autism spectrum disorder; BBB Blood–brain barrier; BDNF Brain-derived neurotrophic factor; CNS Central nervous system; GABA ɣ-aminobutyric acid; GF Germ-free; MIA Maternal immune activation; SCFA Short-chain fatty acid; 3M-4-TMAB 3-methyl-4-(trimethylammonio)butanoate; 4-TMAP 4-(trimethylammonio)pentanoate; TMA(O) Trimethylamine(-N-oxide); TUDCA Tauroursodeoxycholic acid; ZO Zonula occludens proteins

… direct impact on critical barriers of the microbiota-gut-brain axis, including the intestinal epithelial barrier and the blood-brain barrier (BBB) 3 . These barriers act as dynamic gatekeepers…

… toxins may disrupt the blood brain barrier and damage the brain … of indoxyl sulfate leads to the accumulation of indoxyl sulfate in … Interestingly, in this study, indoxyl sulfate, methylglyoxal, …

The gut-brain-kidney axis represents a dynamic interplay among the gut microbiota, renal function, and neurological processes, emerging as a critical factor in chronic kidney disease (CKD) pathophysiology. This paper reviews recent data on the mechanisms and pathways that integrate gut-brain-kidney signaling and communication, advances in our understanding of this axis, and potential diagnostic and prognostic biomarkers and interventions for CKD. Literature search was conducted on PubMed, Scopus, Web of Science, and Embase using a combination of the keywords gut microbiota, gut microbiome, gut-brain axis, gut-kidney axis, gut-brain-kidney axis, chronic kidney disease, dysbiosis, therapy, metabolites, and neuroinflammation.” Relevant studies were selected and synthesized in this narrative review. Gut dysbiosis, characterized by microbial composition and function alterations, contributes to systemic inflammation and metabolic imbalances, exacerbating CKD progression. Uremic toxins such as indoxyl sulfate and p-cresyl sulfate, derived from microbial metabolism, impair kidney function and disrupt neurocognitive health via oxidative stress and neuroinflammation, highlighting the interconnectedness of these systems. Recent advances in high-throughput sequencing and metabolomics have elucidated mechanisms linking gut microbiota and associated metabolites to kidney and brain health, revealing the role of microbial diversity and metabolite profiles in disease outcomes. Studies demonstrate that probiotics, prebiotics, and dietary interventions targeting the gut microbiota can modulate systemic inflammation and reduce uremic toxin levels, offering therapeutic potential. Understanding the bidirectional signaling within the gut-brain-kidney axis opens avenues for novel biomarkers and interventions in CKD management.

ABSTRACT In addition to the high neurotoxicity, depression, and anxiety are the most prominent characteristics of methamphetamine (Meth) withdrawal. Studies to date on the issue of Meth-associated depression and anxiety are focused on the brain, however, whether peripheral homeostasis, especially the “microbiota-gut” axis participates in these adverse outcomes, remains poorly understood. In the current study, with the fecal microbiota transplantation (FMT) assay, the mice received microbiota from Meth withdrawal mice displayed marked depression and anxiety behaviors. The 16S rRNA sequencing results showed that Meth withdrawal contributed to a striking reduction of Akkermansia, Bacteroides, Faecalibaculum, Desulfovibrio, and Anaerostipes, which are known to be associated with tryptophan (TRP) metabolism. Noteworthily, the substantial decreases of the indole derivatives from the TRP metabolic pathway, including IAA, IPA, ILA, IET, IArA, IAld, and TRM were observed in the serum of both Meth abusing humans and mice during Meth withdrawal with the UHPLC-MS/MS analysis. Combining the high and low TRP diet mouse model, the mice with high TRP diet obviously impeded Meth-associated depression and anxiety behaviors, and these results were further strengthened by the evidence that administration of IPA, IAA, and indole dramatically ameliorated the Meth induced aberrant behaviors. Importantly, these protective effects were remarkably counteracted in aryl hydrocarbon receptor knockout (AhR KO) mice, underlining the key roles of microbiota-indoles-AhR signaling in Meth-associated depression and anxiety. Collectively, the important contribution of the present work is that we provide the first evidence that peripheral gut homeostasis disturbance but not limited to the brain, plays a key role in driving the Meth-induced depression and anxiety in the periods of withdrawal, especially the microbiota and the indole metabolic disturbance. Therefore, targeting AhR may provide novel insight into the therapeutic strategies for Meth-associated psychological disorders.

Stroke, as a serious cerebral vascular disease with high incidence and high rates of disability and mortality, has limited therapeutic options due to the narrow time window. Compelling evidence has highlighted the significance of the gut microbiota and gut–brain axis as critical regulatory factors affecting stroke. Along the microbiota–gut–brain axis, tryptophan metabolism further acquires increasing attention for its intimate association with central nervous system diseases. For the purpose of exploring the potential role of tryptophan metabolism in stroke and providing systematic insights into the intricate connection of the microbiota–gut–brain axis with the pathological procedure of stroke, this review first summarized the practical relationship between microbiota and stroke by compiling the latest case-control research. Then, the microbiota–gut–brain axis, as well as its interaction with stroke, were comprehensively elucidated on the basis of the basic anatomical structure and physiological function. Based on the crosstalk of microbiota–gut–brain, we further focused on the tryptophan metabolism from the three major metabolic pathways, namely, the kynurenine pathway, serotonin pathway, and microbial pathway, within the axis. Moreover, the effects of tryptophan metabolism on stroke were appreciated and elaborated here, which is scarcely found in other reviews. Hopefully, the systematic illustration of the mechanisms and pathways along the microbiota–gut–brain axis will inspire more translational research from metabolic perspectives, along with more attention paid to tryptophan metabolism as a promising pharmaceutical target in order to reduce the risk of stroke, mitigate the stroke progression, and ameliorate the stroke prognosis.

Neurodegenerative disorders such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS) share key molecular features, including neuroinflammation, oxidative stress, mitochondrial dysfunction, and progressive neuronal loss. Increasing evidence indicates that gut dysbiosis and alterations in microbiota-derived metabolites are involved in these processes through multiple pathways along the gut–brain axis. However, while broad compositional changes are well-documented, a critical knowledge gap remains regarding the specific biochemical signal transduction pathways translating dysbiosis into pathology. This narrative review addresses this gap by synthesizing current human and experimental studies addressing gut microbiota alterations in AD, PD, and ALS, with particular emphasis on the biochemical and molecular mechanisms mediated by gut-derived metabolites. Dysbiosis in neurodegenerative diseases is frequently associated with reduced abundance of short-chain fatty acid (SCFA)-producing bacteria and altered metabolism of SCFAs, bile acids, tryptophan-derived indoles, trimethylamine-N-oxide (TMAO), and lipopolysaccharides (LPS). These microbial metabolites have been shown to modulate intestinal and blood–brain barrier integrity, influence Toll-like receptor- and G protein-coupled receptor-dependent signaling, regulate microglial activation, and affect molecular pathways related to protein aggregation in experimental models. In addition, emerging evidence highlights the involvement of oxidative and nitrosative stress, immune–metabolic crosstalk, and altered xenobiotic metabolism in microbiota–host interactions during neurodegeneration. By integrating microbiological, metabolic, and molecular perspectives, this review underscores the important and emerging role of microbiota-derived molecules in neurodegenerative disorders and outlines key chemical and metabolic pathways that may represent targets for future mechanistic studies and therapeutic strategies.

As an essential amino acid, tryptophan (Trp) serves as a pivotal mediator in gut-brain axis (GBA) communication through three primary metabolic pathways: kynurenine (Kyn), indole, and serotonin (5-HT), which together regulate neuroimmune and neuroendocrine homeostasis via the vagus and spinal afferent nerves, circulatory system, and hypothalamic-pituitary-adrenal (HPA) axis. This review systematically examines Trp metabolism’s critical roles in GBA, emphasizing molecular pathways, rate-limiting enzymes, and receptor-mediated signaling. We discuss the bidirectional interplay between gut microbiota and host Trp metabolism, encompassing microbial modulation of host enzyme activities such as indoleamine 2,3-dioxygenase and direct production of bioactive indole derivatives like indole-3-propionic acid. Characteristic disruptions in Trp metabolism patterns are identified across GBA-associated disorders including irritable bowel syndrome, inflammatory bowel disease, depression, Alzheimer’s disease, schizophrenia and Parkinson’s disease, marked by aberrant neurotoxic to neuroprotective metabolite ratios and enzymatic dysregulation. The aryl hydrocarbon receptor (AhR) emerges as a molecular hub connecting Trp metabolites to GBA functions, with distinct metabolites eliciting opposing effects through AhR activation. Therapeutic strategies targeting Trp metabolism are critically evaluated, including fecal microbiota transplantation, probiotic supplementation, metabolite administration, and enzyme inhibitors. Future research directions address mechanistic gaps and translational challenges in restoring GBA homeostasis via Trp pathway modulation.

The biochemical processes of childhood-onset epilepsy remain unclear, with no reliable biomarkers for prognosis or management. Untargeted plasma metabolomics offers a valuable approach to uncover underlying pathomechanisms and identify actionable biomarkers. In this study, plasma samples from 18 pediatric patients with epilepsy and 11 age-matched healthy controls were analyzed using liquid chromatography-mass spectrometry. Data were analyzed using univariate and multivariate statistical methods and pathway enrichment analysis. Multivariate analyses demonstrated separation between the patient and control groups. A total of 19 endogenous metabolites (VIP > 1, adjusted p < 0.05) emerged as key differentiators. Compared with controls, patients exhibited significant reductions in tryptophan (Trp), 5-Hydroxyindoleacetic acid (5-HIAA), several gut microbiota-derived metabolites, including indole, indoxyl sulfate, and p-cresyl sulfate, as well as in niacin metabolism end-products - N1-Methyl-2-pyridone-5-carboxamide (Met2PY) and N1-Methyl-4-pyridone-3-carboxamide (Met4PY). In addition, patients showed decreased levels of tricarboxylic acid (TCA) cycle intermediates, concomitant with an increase in fatty acid derivatives and N-acetylneuraminic acid (Neu5Ac). The most substantially altered metabolic pathways in epilepsy patients involved the TCA cycle, vitamin A and C metabolism, prostaglandin synthesis, and D4/E4-neuroprostane formation. Observed alterations in tryptophan and microbiota-derived metabolites suggest gut dysbiosis may contribute to epilepsy development through the gut-brain axis. Moreover, the circulatory metabolic markers indicating an energy deficit and oxidative stress underscore the systemic impact of seizure activity.

Endogenous indole and its derivatives (indoles), considered as promising N-substituted heterocyclic compounds, are tryptophan metabolites derived from intestinal microbiota and exhibit a range of biological activities. Recent studies indicate that indoles contribute to maintaining the biological barrier of the human intestine, which exert the anti-inflammatory activities mainly through activating AhR and PXR receptors to affect the immune system’s function, significantly improving intestinal health (inflammatory bowel disease, hemorrhagic colitis, colorectal cancer) and further promote human health (diabetes mellitus, central system inflammation, and vascular regulation). However, the revealed toxic influences cannot be ignored. Indoxyl sulfate, an indole derivative, performs nephrotoxicity and cardiovascular toxicity. We addressed the interaction between indoles and intestinal microbiota and the indoles’ effects on human health as double-edged swords. This review provides scientific bases for the correlation of indoles with diseases moreover highlights several directions for subsequent indoles-related studies.

Objective Our study aimed to explore the influence of gut microbiota and their metabolites on intracranial aneurysms (IA) progression and pinpoint-related metabolic biomarkers derived from the gut microbiome. Design We recruited 358 patients with unruptured IA (UIA) and 161 with ruptured IA (RIA) from two distinct geographical regions for conducting an integrated analysis of plasma metabolomics and faecal metagenomics. Machine learning algorithms were employed to develop a classifier model, subsequently validated in an independent cohort. Mouse models of IA were established to verify the potential role of the specific metabolite identified. Results Distinct shifts in taxonomic and functional profiles of gut microbiota and their related metabolites were observed in different IA stages. Notably, tryptophan metabolites, particularly indoxyl sulfate (IS), were significantly higher in plasma of RIA. Meanwhile, upregulated tryptophanase expression and indole-producing microbiota were observed in gut microbiome of RIA. A model harnessing gut-microbiome-derived tryptophan metabolites demonstrated remarkable efficacy in distinguishing RIA from UIA patients in the validation cohort (AUC=0.97). Gut microbiota depletion by antibiotics decreased plasma IS concentration, reduced IA formation and rupture in mice, and downregulated matrix metalloproteinase-9 expression in aneurysmal walls with elastin degradation reduction. Supplement of IS reversed the effect of gut microbiota depletion. Conclusion Our investigation highlights the potential of gut-microbiome-derived tryptophan metabolites as biomarkers for distinguishing RIA from UIA patients. The findings suggest a novel pathogenic role for gut-microbiome-derived IS in elastin degradation in the IA wall leading to the rupture of IA.

Alzheimer’s disease (AD) is the leading cause of dementia, mainly affecting elderly individuals. AD is characterized by β-amyloid plaques, abnormal tau tangles, neuronal loss, and metabolic disruptions. Recent studies have revealed the involvement of the kynurenine (KP) pathway and the aryl hydrocarbon receptor (AhR) in AD development. The KP pathway metabolizes tryptophan to produce neuroactive substances like kynurenine, kynurenic acid, and quinolinic acid. In AD, high levels of kynurenine and the neurotoxic quinolinic acid are associated with increased neuroinflammation and excitotoxicity; conversely, reduced levels of kynurenic acid, which acts as a glutamate receptor antagonist, compromise neuroprotection. Research has indicated elevated KP metabolites and enzymes in the hippocampus of AD patients and other tissues such as blood, cerebrospinal fluid, and urine. However, the finding that KP metabolites are AD biomarkers in blood, cerebrospinal fluid, and urine has been controversial. This controversy, stemming from the lack of consideration of the specific stage of AD, details of the patient’s treatment, cognitive deficits, and psychiatric comorbidities, underscores the need for more comprehensive research. AhR, a ligand-activated transcription factor, regulates immune response, oxidative stress, and xenobiotic metabolism. Various ligands, including tryptophan metabolites, can activate it. Some studies suggest that AhR activation contributes to AD, while others propose that it provides neuroprotection. This discrepancy may be explained by the specific ligands that activate AhR, highlighting the complex relationship between the KP pathway, AhR activation, and AD, where the same pathway can produce both neuroprotective and harmful effects.

Significance While the effects of gut microbes on brain development and function have been described, the mechanisms remain largely unknown. Here, we report that tryptophan-metabolizing gut microbes secrete indoles that regulate neurogenesis in the adult hippocampus. This stimulatory effect on adult neurogenesis is mediated by the metabolic- and immune-linked aryl hydrocarbon receptor (AhR). Another AhR ligand, the tryptophan metabolite kynurenine, failed to induce neurogenesis, suggesting ligand specificity of AhR-mediated regulation of adult neurogenesis. The indole-AhR signaling pathway elevates transcription factors and signaling proteins that promote adult neurogenesis, as well as key markers of synaptic maturation. Our data demonstrate a symbiotic gut–brain coregulatory axis that connects the metabolic status of gut microbes to the control of neurogenesis in the adult hippocampus. While modulatory effects of gut microbes on neurological phenotypes have been reported, the mechanisms remain largely unknown. Here, we demonstrate that indole, a tryptophan metabolite produced by tryptophanase-expressing gut microbes, elicits neurogenic effects in the adult mouse hippocampus. Neurogenesis is reduced in germ-free (GF) mice and in GF mice monocolonized with a single-gene tnaA knockout (KO) mutant Escherichia coli unable to produce indole. External administration of systemic indole increases adult neurogenesis in the dentate gyrus in these mouse models and in specific pathogen-free (SPF) control mice. Indole-treated mice display elevated synaptic markers postsynaptic density protein 95 and synaptophysin, suggesting synaptic maturation effects in vivo. By contrast, neurogenesis is not induced by indole in aryl hydrocarbon receptor KO (AhR−/−) mice or in ex vivo neurospheres derived from them. Neural progenitor cells exposed to indole exit the cell cycle, terminally differentiate, and mature into neurons that display longer and more branched neurites. These effects are not observed with kynurenine, another AhR ligand. The indole-AhR–mediated signaling pathway elevated the expression of β-catenin, Neurog2, and VEGF-α genes, thus identifying a molecular pathway connecting gut microbiota composition and their metabolic function to neurogenesis in the adult hippocampus. Our data have implications for the understanding of mechanisms of brain aging and for potential next-generation therapeutic opportunities.

In recent years, aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, has been considered to be involved in aging phenotypes across several species. This receptor is a highly conserved biosensor that is activated by numerous exogenous and endogenous molecules, including microbiota metabolites, to mediate several physiological and toxicological functions. Brain aging hallmarks, which include glial cell activation and inflammation, increased oxidative stress, mitochondrial dysfunction, and cellular senescence, increase the vulnerability of humans to various neurodegenerative diseases. Interestingly, many studies have implicated AhR signaling pathways in the aging process and longevity across several species. This review provides an overview of the impact of AhR pathways on various aging hallmarks in the brain and the implications for AhR signaling as a mechanism in regulating aging-related diseases of the brain. We also explore how the nature of AhR ligands determines the outcomes of several signaling pathways in brain aging processes.

Circadian clocks maintain diurnal rhythms of sleep–wake cycle of 24 h that regulate not only the metabolism of an organism but also many other periodical processes. There is substantial evidence that circadian regulation is impaired in Alzheimer's disease. Circadian clocks regulate many properties known to be disturbed in Alzheimer's patients, such as the integrity of the blood–brain barrier (BBB) as well as the diurnal glymphatic flow that controls waste clearance from the brain. Interestingly, an evolutionarily conserved transcription factor, that is, aryl hydrocarbon receptor (AhR), impairs the function of the core clock proteins and thus could disturb diurnal rhythmicity in the BBB. There is abundant evidence that the activation of AhR signalling inhibits the expression of the major core clock proteins, such as the brain and muscle arnt‐like 1 (BMAL1), clock circadian regulator (CLOCK) and period circadian regulator 1 (PER1) in different experimental models. The expression of AhR is robustly increased in the brains of Alzheimer's patients, and protein level is enriched in astrocytes of the BBB. It seems that AhR signalling inhibits glymphatic flow since it is known that (i) activation of AhR impairs the function of the BBB, which is cooperatively interconnected with the glymphatic system in the brain, and (ii) neuroinflammation and dysbiosis of gut microbiota generate potent activators of AhR, which are able to impair glymphatic flow. I will examine current evidence indicating that activation of AhR signalling could disturb circadian functions of the BBB and impair glymphatic flow and thus be involved in the development of Alzheimer's pathology.

Arsenic, a pervasive environmental contaminant in groundwater, poses a severe global threat to public health. Chronic arsenic exposure has been linked to neurological impairment, however, its specific pathogenic mechanism and whether the gut-brain axis plays a key role remain unclear. This study investigated the role of gut microbiota and its metabolite indoxyl sulfate (IS) in mediating chronic exposure to arsenic-induced cognitive impairment and Alzheimer's disease (AD)-like pathology, with a specific focus on microglial pyroptosis. We found that chronic arsenic exposure induced cognitive dysfunction and intestinal barrier injury, disrupted gut microbiota composition, promoted IS accumulation in serum and brain, and activated the AhR/NF-κB/NLRP3 signaling pathway, triggering microglial pyroptosis and elevating AD-like pathological markers in mice. Meanwhile, fecal microbiota transplantation (FMT) from arsenic-exposed mice recapitulated cognitive impairment, elevated IS levels, and neuroinflammation in recipient mice. Furthermore, arsenic upregulated hepatic IS-synthesis genes (CYP2E1, Sult1d1) and downregulated renal IS-excretion gene (ABCG2). In vitro, arsenic and IS co-exposure promoted M1 polarization and enhanced pyroptosis by activating the AhR/NF-κB/NLRP3 signaling pathway, while suppressing phagocytosis-related proteins (TREM2, SYK and CD36). Furthermore, SiAhR treatment could alleviated microglial inflammatory injury and enhancing the microglia's phagocytic capacity induced by arsenic and IS co-exposure in BV2 cells through inhibiting the AhR/NF-κB/NLRP3-mediated pyroptosis signaling pathway. In conclusion, chronic arsenic exposure induced cognitive impairment and AD-like pathological via the gut microbiota-AhR-pyroptosis cascade, where in IS accumulation served a key mediator. These findings provide new insights into preventing arsenic-related cognitive damage.

The microbiota-gut-brain axis (MGBA) has been recognized as an important communication network between the gut and the brain. This network operates through immune, neural, and endocrine pathways, wherein microbiota-derived metabolites act as essential messengers regulating MGBA. Among gut metabolites, indole and its derivatives derived from tryptophan by gut microbiota are emerging as critical factors along the MGBA. By activating the aryl hydrocarbon receptor (AhR), these metabolites help modulate neuroimmune responses by regulating microglial activation, astrocyte reactivity, and the integrity of the blood-brain barrier (BBB), thereby exerting impacts on neuroinflammation, nerve regeneration, and BBB function. Although animal studies are promising, turning these findings into clinical translation is still difficult due to the conditional effects of AhR signaling, the reliable biomarkers, and the challenges in gut metabolite delivery. This review aims to summarize recent advances in understanding the indole-brain connection, critically evaluate current therapeutic strategies, and highlight the need for more targeted therapies.

In recent years, gut–brain axis signaling has been recognized as an essential factor modifying behavior, mood, cognition, and cellular viability under physiological and pathological conditions. Consequently, the intestinal microbiome has become a potential therapeutic target in neurological and psychiatric disorders. The microbiota-derived metabolite of tryptophan (Trp), indole-3-propionic acid (IPA), was discovered to target a number of molecular processes and to impact brain function. In this review, we outline the key mechanisms by which IPA may affect neuronal activity and survival and provide an update on the evidence supporting the neuroprotective action of the compound in various experimental paradigms. Accumulating data indicates that IPA is a free radical scavenger, a ligand of aryl hydrocarbon receptors (AhR) and pregnane X receptors (PXR), and an anti-inflammatory molecule. IPA decreases the synthesis of the proinflammatory nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), tumor necrosis factor-α (TNF-α), and other cytokines, reduces the generation of the NLR family pyrin domain containing 3 (NLRP3) inflammasome, and enhances the synthesis of neurotrophic factors. Furthermore, produced in the gut, or administered orally, IPA boosts the central levels of kynurenic acid (KYNA), a neuroprotective metabolite of Trp. IPA reduces the release of proinflammatory molecules in the gut, breaking the gut–inflammation–brain vicious cycle, which otherwise leads to neuronal loss. Moreover, as a molecule that easily enters central compartment, IPA may directly impact brain function and cellular survival. Overall, the gathered data confirms neuroprotective features of IPA, and supports its potential use in high-risk populations, in order to delay the onset and ameliorate the course of neurodegenerative disorders and cognitive impairment. Clinical trials evaluating IPA as a promising therapeutic add-on, able to slow down the progress of neurodegenerative disorders such as Alzheimer’s or Parkinson’s disease and to limit the morphological and behavioral consequences of ischemic stroke, are urgently needed.

Blood-brain barrier (BBB) damage significantly affects the prognosis of ischemic stroke patients. This project employed multi-omics analysis to identify key factors regulating BBB disruption during cerebral ischemia-reperfusion. An integrated analysis of three transcriptome sequencing datasets from mouse middle cerebral artery occlusion/reperfusion (MCAO/R) models identified eight downregulated genes in endothelial cells. Additionally, transcriptome analysis of BBB (cortex) and non-BBB (lung) endothelium of E13.5 mice revealed 2,102 upregulated genes potentially associated with BBB integrity. The eight downregulated genes were intersected with the 2,102 BBB-related genes and mapped using single-cell RNA sequencing data, revealing that solute carrier family 22 member 8 (Slc22a8) is specifically expressed in endothelial cells and pericytes and significantly decreases after MCAO/R. This finding was validated in the mouse MCAO/R model at both protein and mRNA levels in this study. External overexpression of Slc22a8 using a lentivirus carrying Tie2 improved Slc22a8 and tight junction protein levels and reduced BBB leakage after MCAO/R, accompanied by Wnt/β-catenin signaling activation. In conclusion, this study suggested that MCAO/R-induced downregulation of Slc22a8 expression may be a crucial mechanism underlying BBB disruption. Interventions that promote Slc22a8 expression or enhance its function hold promise for improving the prognosis of patients with cerebral ischemia.

The blood–brain barrier (BBB) is a highly selective and restrictive semipermeable network of cells and blood vessel constituents. All components of the neurovascular unit give to the BBB its crucial and protective function, i.e., to regulate homeostasis in the central nervous system (CNS) by removing substances from the endothelial compartment and supplying the brain with nutrients and other endogenous compounds. Many transporters have been identified that play a role in maintaining BBB integrity and homeostasis. As such, the restrictive nature of the BBB provides an obstacle for drug delivery to the CNS. Nevertheless, according to their physicochemical or pharmacological properties, drugs may reach the CNS by passive diffusion or be subjected to putative influx and/or efflux through BBB membrane transporters, allowing or limiting their distribution to the CNS. Drug transporters functionally expressed on various compartments of the BBB involve numerous proteins from either the ATP-binding cassette (ABC) or the solute carrier (SLC) superfamilies. Pathophysiological stressors, age, and age-associated disorders may alter the expression level and functionality of transporter protein elements that modulate drug distribution and accumulation into the brain, namely, drug efficacy and toxicity. This review focuses and sheds light on the influence of inflammatory conditions and diseases such as Alzheimer’s disease, epilepsy, and stroke on the expression and functionality of the BBB drug transporters, the consequential modulation of drug distribution to the brain, and their impact on drug efficacy and toxicity.

Introduction: Thyroid hormone transporters are essential for thyroid hormones to enter target cells. Monocarboxylate transporter (MCT) 8 is a key transporter and is expressed at the blood–brain barrier (BBB), in neural cells and many other tissues. Patients with MCT8 deficiency have severe neurodevelopmental delays because of cerebral hypothyroidism and chronic sequelae of peripheral thyrotoxicosis. The T3 analog 3,3′,5-triiodothyroacetic acid (TRIAC) rescued neurodevelopmental features in animal models mimicking MCT8 deficiency and improved key metabolic features in patients with MCT8 deficiency. However, the identity of the transporter(s) that facilitate TRIAC transport are unknown. Here, we screened candidate transporters that are expressed at the human BBB and/or brain–cerebrospinal fluid barrier and known thyroid hormone transporters for TRIAC transport. Materials and Methods: Plasma membrane expression was determined by cell surface biotinylation assays. Intracellular accumulation of 1 nM TRIAC was assessed in COS-1 cells expressing candidate transporters in Dulbecco’s phosphate-buffered saline (DPBS)/0.1% glucose or Dulbecco’s modified Eagle’s medium (DMEM) with or without 0.1% bovine serum albumin (BSA). Expression of Slc22a8 was determined by fluorescent in situ hybridization in brain sections from wild-type and Mct8/Oatp1c1 knockout mice at postnatal days 12, 21, and 120. Results: In total, 59 plasma membrane transporters were selected for screening of TRIAC accumulation (n = 40 based on expression at the human BBB and/or brain–cerebrospinal fluid barrier and having small organic molecules as substrates; n = 19 known thyroid hormone transporters). Screening of the selected transporter panel showed that 18 transporters facilitated significant intracellular accumulation of TRIAC in DPBS/0.1% glucose or DMEM in the absence of BSA. In the presence of BSA, substantial transport was noted for SLCO1B1 and SLC22A8 (in DPBS/0.1% glucose and DMEM) and SLC10A1, SLC22A6, and SLC22A24 (in DMEM). The zebrafish and mouse orthologs of these transporters similarly facilitated intracellular accumulation of TRIAC. Highest Slc22a8 mRNA expression was detected in mouse brain capillary endothelial cells and choroid plexus epithelial cells at early postnatal time points, but was reduced at P120. Conclusions: Human SLC10A1, SLCO1B1, SLC22A6, SLC22A8, and SLC22A24 as well as their mouse and zebrafish orthologs are efficient TRIAC transporters. These findings contribute to the understanding of TRIAC treatment in patients with MCT8 deficiency and animal models thereof.

Metabolomic analysis of cerebrospinal fluid (CSF) is used to improve diagnostics and pathophysiological understanding of neurological diseases. Alterations in CSF metabolite levels can partly be attributed to changes in brain metabolism, but relevant transport processes influencing CSF metabolite concentrations should be considered. The entry of molecules including metabolites into the central nervous system (CNS), is tightly controlled by the blood-brain, blood-CSF, and blood-spinal cord barriers, where aquaporins and membrane-bound carrier proteins regulate influx and efflux via passive and active transport processes. This report therefore provides reference for future CSF metabolomic work, by providing a detailed summary of the current knowledge on the location and function of the involved transporters and routing of metabolites from blood to CSF and from CSF to blood.

The topic of uremic toxicity has received broad attention from the nephrological community over the past few decades. An aspect that is much less often considered is the possibility that the metabolic pathways that generate uremic toxins also may produce molecules that benefit body functions. Here, we discuss this dualism based on the example of tryptophan-derived metabolites, which comprise elements that are mainly toxic, such as indoxyl sulfate, kynurenine and kynurenic acid, but also beneficial compounds, such as indole, melatonin and indole-3-propionic acid, and ambivalent (beneficial for some aspects and harmful for others) compounds such as serotonin. This dualism can also be perceived at the level of the main receptor of the tryptophan-derived metabolites, the aryl hydrocarbon receptor (AHR), which has also been linked to both harm and benefit. We hypothesize that these beneficial effects are the reason why uremic toxin generation remained preserved throughout evolution. This duality is also not unique for the tryptophan-derived metabolites, and in this broader context we discuss the remote sensing and signaling theory (RSST). The RSST proposes that transporters (e.g., organic anion transporter 1—OAT1; ATP-binding cassette transporter G—ABCG2) and drug metabolizing enzymes form a large network of proteins interacting to promote small molecule remote communication at the inter-organ (e.g., gut–liver–heart–brain–kidney) and inter-organismal (e.g., gut microbe–host) levels. These small molecules include gut microbe-derived uremic toxins as well as beneficial molecules such as those discussed here. We emphasize that this positive side of uremic metabolite production needs more attention, and that this dualism especially needs to be considered when assessing and conceiving of therapeutic interventions. These homeostatic considerations are central to the RSST and suggest that interventions be aimed at preserving or restoring the balance between positive and negative components rather than eliminating them all without distinction.

Neurodegenerative diseases are among the most prevalent age-related diseases in humans with increasing incidence and no cure. In the past years, research on (poly)phenols focused on flavonoids has been tackling the missing links of (poly)phenol potential for the prevention and treatment of neurodegenerative diseases. (Poly)phenols from dietary sources have been emerging as potential targets to modulate the progression and development of such multifactorial diseases through diet. The most abundant circulating metabolites of dietary (poly)phenols, the low molecular weight (poly)phenol metabolites resulting from phase I and II metabolism and microbiota transformations have been listed. These metabolites are known to reach human circulation. However, their brain permeability and role in neuroinflammation are two topics that still have research gaps, therefore hampering a comprehensive view of their impact to mitigate brain inflammation through the blood-brain barrier and the underlying mechanisms.

The Solute Carrier Protein Family (SLC) is responsible for the uptake and transport of a variety of substances across the cell membrane. It plays a central role in maintaining the stability of the intracellular environment through participation in metabolic processes and the transport of drugs and toxins. The highly tissue-specific expression of SLC proteins endows them with potential applications in disease treatment and drug development. Transplant immune reactions are a major challenge in the field of organ transplantation, as graft rejection is a key factor determining the success of transplantation and long-term organ survival. SLC proteins are increasingly drawing attention for their roles in modulating immune responses, influencing transplant immune tolerance, and controlling graft rejection. By regulating the metabolism and function of immune cells, SLC proteins affect the formation and tolerance of transplant immune responses. Among them, 7 SLC proteins are “validated targets” with approved or phase III drugs, 9 are “candidate targets” in active clinical trials, and 14 remain “potential targets” supported by genetic and pre-clinical evidence. This article elucidates the functions of SLC proteins in transplant immunology, inflammation and autoimmune diseases, tumor immunology, metabolic diseases, and neurological diseases, as well as the new targets and strategies for treating these diseases that SLC proteins provide.

Drug transporters play an important role in the maintenance of chemical balance and homeostasis in different tissues. In addition to their physiological functions, they are crucial for the absorption, distribution, and elimination of many clinically important drugs, thereby impacting therapeutic efficacy and toxicity. Increasing evidence has demonstrated that infectious, metabolic, inflammatory, and neurodegenerative diseases alter the expression and function of drug transporters. However, the current knowledge on transporter regulation in critical protective barriers, such as the brain and placenta, is still limited and requires more research. For instance, while many studies have examined P-glycoprotein, it is evident that research on the regulation of highly expressed transporters in the blood–brain barrier and blood–placental barrier are lacking. The aim of this review is to summarize the currently available literature in order to better understand transporter regulation in these critical barriers.

Indoxyl sulfate is a microbially derived uremic toxin that accumulates in late-stage chronic kidney disease and contributes to both renal and cardiovascular toxicity. Indoxyl sulfate is generated by the metabolism of indole, a compound created solely by gut microbial tryptophanases. Here, we characterize the landscape of tryptophanase enzymes in the human gut microbiome and find remarkable structural and functional similarities across diverse taxa. We leverage this homology through a medicinal chemistry campaign to create a potent pan-inhibitor, (3S) ALG-05, and validate its action as a transition-state analog. (3S) ALG-05 successfully reduces indole production in microbial culture and displays minimal toxicity against microbial and mammalian cells. Mice treated with (3S) ALG-05 show reduced cecal indole and serum indoxyl sulfate levels with minimal changes in other tryptophan-metabolizing pathways. These studies present a non-bactericidal pan-inhibitor of gut microbial tryptophanases with potential promise for reducing indoxyl sulfate in chronic kidney disease.

Pieces of evidence support the view that the accumulation of uremic toxins enhances oxidative stress and downstream regulation of signaling pathways, contributing to both endothelial microangiography and cell dysfunction. This study is to address the impact of protein-binding uremic toxins on the severity of peripheral nerve function in patients with chronic kidney disease (CKD). Fifty-four patients with CKD were included in the Toronto Clinical Neuropathy Score (TCNS), nerve conduction study (NCS), and laboratory studies including protein-binding uremic toxin (indoxyl sulfate [IS] and p-cresyl sulfate [PCS]), oxidative stress (Thiol and thiobarbituric acid reacting substances [TBARS]), and endothelial dysfunction (serum intercellular adhesion molecule 1 [sICAM-1] and serum vascular adhesion molecule 1 [sVCAM-1]) at enrollment. We used composite amplitude scores (CAS) to analyze the severity of nerve conductions on peripheral nerve function. TCNS and CAS were higher in the diabetic CKD group (p = 0.02 and 0.01, respectively). The NCS revealed the compound muscle action potential of ulnar and peroneal nerves and the sensory nerve action potential of ulnar and sural nerves (p = 0.004, p = 0.004, p = 0.004, and p = 0.001, respectively), which was found to be significantly low in the diabetic group. CAS was significantly correlated with age (r = 0.27, p = 0.04), urine albumin-creatinine ratio (UACR) (r = 0.29, p = 0.046), free-form IS (r = 0.39, p = 0.009), sICAM-1 (r = 0.31, p = 0.02), sVCAM-1 (r = 0.44, p < 0.0001), TBARS (r = 0.35, p = 0.002), and thiols (r = −0.28, p = 0.045). Linear regression revealed that only TBARS and free-form IS were strongly associated with CAS. The mediation analysis shows that the sVCAM-1 level serves as the mediator between higher IS and higher CAS. IS and oxidative stress contribute to the severity of peripheral nerve dysfunction in patients with CKD, and chronic glycemic impairment can worsen the conditions.

The intestinal microbiota metabolic activity towards the available substrates generates myriad bacterial metabolites that may accumulate in the luminal fluid. Among them, indole and indole-related compounds are produced by specific bacterial species from tryptophan. Although indole-related compounds are, first, involved in intestinal microbial community communication, these molecules are also active on the intestinal mucosa, exerting generally beneficial effects in different experimental situations. After absorption, indole is partly metabolized in the liver into the co-metabolite indoxyl sulfate. Although some anti-inflammatory actions of indole on liver cells have been shown, indoxyl sulfate is a well-known uremic toxin that aggravates chronic kidney disease, through deleterious effects on kidney cells. Indoxyl sulfate is also known to provoke endothelial dysfunction. Regarding the central nervous system, emerging research indicates that indole at excessive concentrations displays a negative impact on emotional behavior. The indole-derived co-metabolite isatin appears, in pre-clinical studies, to accumulate in the brain, modulating brain function either positively or negatively, depending on the doses used. Oxindole, a bacterial metabolite that enters the brain, has shown deleterious effects on the central nervous system in experimental studies. Lastly, recent studies performed with indoxyl sulfate report either beneficial or deleterious effects depending once again on the dose used, with missing information on the physiological concentrations that are reaching the central nervous system. Any intervention aiming at modulating indole and indole-related compound concentrations in the biological fluids should crucially take into account the dual effects of these compounds according to the host tissues considered.

There is a high prevalence of ventricular arrhythmias related to sudden cardiac death in patients with chronic kidney disease (CKD). To explored the possible mechanism of CKD-related ventricular arrhythmias, a CKD rat model was created, and indoxyl sulfate (IS) was further used in vivo and in vitro. This project used the following methods: patch clamp, electrocardiogram, and some molecular biology experimental techniques. IS was found to be significantly elevated in the serum of CKD rats. Interestingly, the expression levels of the fast transient outward potassium current–related (Ito,f-related) proteins (Kv4.2, Kv4.3, and KChIP2) in the heart of CKD rats and rats treated with IS decreased. IS dose-dependently reduced Ito,f density, accompanied by the decreases in Kv4.2, Kv4.3, and KChIP2 proteins in vitro. IS also prolonged the action potential duration and QT interval, and paroxysmal ventricular tachycardia could be induced by IS. In-depth studies have shown that ROS/p38MAPK, ROS–p44/42 MAPK, and NF-κB signaling pathways play key roles in the reduction of Ito,f density and Ito,f-related proteins caused by IS. These data suggest that IS reduces Ito,f-related proteins and Ito,f density by activating ROS/MAPK and NF-κB signaling pathways, and the action potential duration and QT interval are subsequently prolonged, which contributes to increasing the susceptibility to arrhythmia in CKD.

Impaired hepatic and renal function influence Alzheimer's disease (AD) progression; however, whether AD progression affects these important organ functions remains unclear. Here, we investigated the impact of AD progression, characterized by brain amyloid-beta (Aβ) accumulation, on liver and kidney function of AppNL-G-F/NL-G-F (APP-KI) mice using quantitative proteomics. SWATH-based quantitative proteomics revealed changes in mitochondrial, drug metabolism, and pharmacokinetic-related proteins in mouse liver and kidneys during the early (2-month-old) and intermediate (5-month-old) stages of Aβ accumulation. Notably, in 5-month-old APP-KI mouse liver, 25 phase I/II metabolizing enzymes (8 CYPs, 7 UGTs, 7 CESs, and 3 SLCs) and five transporters (2 ABCs and 3 SLCs) were significantly altered; specifically, Ugt1a9 and Slc33a1 protein abundances increased, whereas Ugt1a1 and Abcc3 protein abundances decreased. In the kidneys, 13 phase I/II metabolizing enzymes and 10 ABC-SLC transporters were altered, including Ugt1a6, Ugt1a7, Slc22a7, and Abcb1a. Additionally, plasma proteins, such as albumin and alpha-1-acid glycoprotein, which are critical for drug binding and distribution, were also altered. These results underscore the significant role of progressive brain Aβ accumulation in modifying hepatic and renal protein abundances, potentially influencing drug metabolism and disposition in AD. Our findings provide novel insights into the complex relationship between AD progression and organ dysfunction.