Microbiota-Derived Tryptophan Metabolite Indole-3-Propionic Acid-Emerging Role in Neuroprotection

Aryl hydrocarbon receptor (AhR) is an 848-amino-acid transcription factor encoded by the AHR gene on chromosome 7p21.1 in humans [61,62,63,64]. Inactive cytoplasmic AhR forms a complex with cochaperone p23, a heat-shock protein, an AhR-interacting protein, and an SRC protein kinase. Upon binding an AhR ligand, the complex undergoes conformational changes, translocates to the nucleus, and there controls the transcription of a variety of genes [62]. The major role of AhR was initially ascribed to xenobiotic detoxification and protection against environmental toxins. Currently, it is broadly accepted that the role of AhR in human physiology is quite complex. The net outcome of its activation depends on the cell type, ontogenetic period of life, and the presence or absence of other signaling molecules/pathogenetic factors [65,66].

Apart from regulating transcription, AhR may target the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), estrogen, or retinoic acid receptors [63,67,68]. AhR may also act as a ubiquitin ligase, which impacts the processes of ubiquitination and proteasomal degradation, and contributes to the regulation of cell division [69]. AhR changes histone acetylation and methylation and influences the long non-coding RNA and microRNAs [63,68]. Furthermore, activation of AhR may impact the metabolism of Trp along the kynurenine pathway. The protein kinase SCR, released upon ligand binding to AhR, phosphorylates the indoleamine 2,3-dioxygenase 1 (IDO1), and, thus, affects the fate of Trp metabolism [70].

In peripheral organs, AhR is expressed in vascular cells and impacts the endothelial barriers, including the gut–brain axis [67,68]. It is considered essential for the balance between the development of antigen tolerance, on one side, and the immune defense against pathogenic microbial species on the other [68]. In the brain, apart from vascular cells, AhR is extensively expressed in neurons, astrocytes, and microglia [55,58]. The expression of AhR depends on the stage of development and is region- and cell-dependent [71].

The gut–brain axis, which involves interactions of intestinal ligands with peripheral and central AhR receptors, is considered an important element of proper brain function [63,68]. Microbiota-produced indoles, as well as many kynurenines, are recognized as potent AhR ligands [72]. However, despite several publications referring to IPA as an AhR ligand, the available data are ambiguous.

Studies that support IPA as an AhR agonist employed various experimental approaches. First, the effects of IPA were reversed in vivo by an AhR antagonist, as demonstrated, e.g., in a murine sepsis model [73]. Secondly, the level of AhR protein following exposure to certain indoles was increased, as shown in beta-amyloid (Aβ) precursor protein (APP)/PS1 mice in which a mixture of indoles, including IPA, upregulated the brain expression of AhR protein [43]. However, this effect was not specific to IPA, and the authors did not assess the affinity of the studied indoles to AhR. More precise conclusions were drawn in the study on an osteoarthritis model, where AhR knockdown abolished the IPA-evoked anti-inflammatory effects [74]. Other authors were unable to confirm the activation of AhR by IPA, as shown in 293T cells [75].

The net effect of IPA may depend on the presence of other ligands in the milieu. IPA blocked the effect of another AhR agonist, indoxyl sulfate, which is considered a uremic toxin. In cultured proximal tubular cells, IPA diminished the augmented expression of AHR, cytochrome P450, family 1, subfamily A, polypeptide 1 (CYP1A1), transforming growth factor β1 (TGF-β1), and monocyte chemoattractant protein-1 (MCP-1), possibly through the signal transducer and activator of transcription 3 (STAT3) suppression [76]. In at least two other studies, in an environment with different indoles present, the action of IPA on AhR was suggested to be unpredictable and varying [77,78]. Thus, considering the data gathered so far, the AhR receptor mediates the action of IPA. However, the degree of the effect is not fully predictable, since AHR signaling depends on other transcription factors, the proportion of various ligands in the extracellular milieu, and the cell type expressing the receptor.

The impact of IPA on the brain seems to be mediated by at least two pathways. First, intestinal IPA may alleviate local inflammation, and reduce the release of proinflammatory molecules, thus breaking the vicious cycle culminating in brain disorders. A rapidly growing amount of data supports the concept of the gut–brain axis viewed as the interaction of intestinal microbiota- and host cells-produced metabolites with the central nervous system. Special emphasis concerns the detrimental role of even subtle, yet chronic, inflammation. In such a scenario, a deficit of anti-inflammatory compounds may initiate processes culminating in morphological alteration and dysfunction of the brain. Secondly, when circulating IPA reaches the central compartment, it can directly modulate cellular processes through genomic, epigenetic, and receptor-linked switches.

The pregnane X receptor (PXR) is encoded by the nuclear receptor subfamily 1 group I member 2 (NR1I2) gene on chromosome 3q13.33 [79]. PXR is expressed within the cells of the enterohepatic system, immunocompetent cells such as lymphocytes, monocytes, or macrophages [80]. However, other cells and organs, including neurons and blood–brain-barrier endothelium, express PXR [81]. The primary action of PXR, similarly to AhR, relates to modulation of xenobiotics and drug metabolism and transport, e.g., through changes in the hepatic cytochrome P450, family 3, subfamily A (CYP3A) enzyme or the organic anion-transporting polypeptide 1a4 (Oatp1a4). Subsequently, drugs and xenobiotics are cleared from the body [82,83,84]. Again, similarly to AhR, PXR may exert non-genomic effects, including, e.g., interactions with NFkB, Toll-like receptors (TLRs), or the inflammasome. It also impacts cellular energetics and apoptotic processes [85,86].

An important aspect of brain PXR activation is associated with the metabolism of neurosteroids, which, in turn, may affect behavior and neuronal survival [87]. It is assumed that the neurotoxic effect of various xenobiotics may be in part mediated by PXR, which changes the expression of mitochondrial CYP450 and alters mitochondrial function [81]. Interestingly, there are significant inter-species differences in ligand affinity or selectivity in the case of PXR, similar to those in the case of AhR [86]. Moreover, the effects of PXR activation vary between species and cell types, and depend on the used ligand [81].

IPA seems to be a rather weak agonist for human PXR, with a half maximal effective concentration (EC₅₀) of 120 μM. However, when co-administered with indole, IPA activates PXR in a much stronger manner. Mouse PXR is stimulated by IPA with much higher specificity (EC₅₀ of 0.55 μM) [75]. In mouse models of PD, administration of IPA decreased the permeability of the intestinal barrier and blood–brain barrier (BBB), ameliorated inflammation, and improved dopaminergic neuronal function in a PXR-dependent manner, with a possible involvement of the Janus kinase 1 (JAK1)/STAT6 pathway [37].

IPA is a hydroxyl radical scavenger acting over two-fold more potent than melatonin, a structurally and biochemically similar Trp derivative. The compound acts as an endogenous electron donor, scavenging free radicals effectively and preventing oxidative damage without undergoing autoxidation during the redox recycling of transition metals. It detoxifies highly reactive free radicals by donating electrons to hydroxyl anions [88,89,90].

IPA penetrates mitochondria, binds to complex I of the respiratory chain, and stabilizes energy metabolism, thereby lowering reactive oxygen species production [2]. Additionally, IPA has been observed to work in synergy with glutathione, another antioxidant, to inhibit the formation of cationic free radicals, further enhancing its protective effects [28]. IPA and its derivative, indolepropionamide (IPAM), reduced hydroxyl radical generation and inhibited the age-dependent impairment of mitochondrial function [91]. Furthermore, IPA protects DNA from oxidative injury, as shown in various models of DNA damage [92,93]. The action of IPA results from the prevention of lipid peroxidation and inhibition of the synthesis of proinflammatory cytokines [35,94,95]. Antioxidant effects of IPA are also evident in vivo. In Mongolian gerbils subjected to transient forebrain ischemia, orally administered IPA decreased lipid peroxidation in the hippocampus and reduced DNA damage in pyramidal neurons of the CA1 area [30].

Compelling data indicate that IPA acts as a potent anti-inflammatory molecule in various experimental paradigms [9,96]. The beneficial effects of IPA were demonstrated in liver steatosis, metabolic disorders, insulin resistance, cardiovascular diseases, and bone diseases [97,98,99].

The major mechanisms underlying the anti-inflammatory IPA effect comprise upregulated expression of AhR, decreased synthesis of the proinflammatory NF-κB, tumor necrosis factor-α (TNF-α), and other cytokines, reduced generation of the NLR family pyrin domain containing 3 (NLRP3) inflammasome, or enhanced synthesis of neurotrophic factors [41,96]. IPA may also inhibit the AKT protein pathway, with subsequent higher expression of the anti-inflammatory interleukin 10 (IL-10) and a decline in inflammation [100]. Recently, the ability of IPA to interfere with matrix metalloproteinase 9 (MMP9) was suggested [101].

IPA suppressed IL-1β-induced inflammation, extracellular matrix (ECM) degradation, and NF-κB signaling activation in chondrocytes in an AhR-mediated way [74]. AhR-dependent reduction in bacterial burden, protection against organ damage, and decreased sepsis-related mortality following IPA administration were shown [73]. IPA regulated PPT1 expression to modulate the PI3K-AKT-mTOR pathway, thereby restoring autophagic activity in senescent macrophages and suppressing both inflammation and aging-related myocardial fibrosis. Additionally, IPA influenced the cGAS-STING signaling pathway to regulate PPT1 expression [102].

Anti-inflammatory and neuroprotective effects of IPA are evident in the central nervous system. In primary human astrocytes, IPA attenuated the lipopolysaccharide (LPS)-evoked release of inflammatory cytokines, such as monocyte chemoattractant protein-1 (MCP-1), IL-12, IL-13, and TNF-α, yet it did not change the level of KYN or the KYN/Trp ratio [41]. A mixture of indole, indole-3-acetic acid, and IPA upregulated the expression of AhR, decreased the NF-κB and generation of the NLRP3 inflammasome, and lowered the level of proinflammatory cytokines in APP/PS1 mice, a mouse model of Alzheimer's disease (AD). Indoles also improved the integrity of the intestinal barrier, enhanced cognition, and ameliorated brain neuropathology in APP/PS1 mice [43]. However, in the above study, the effect of IPA alone was not assessed.

Incubation of LPS-activated murine microglial BV2 cells in the presence of 1–5 μM IPA for 6 h diminished the release of proinflammatory TNF-α into incubation media. Furthermore, incubation with conditioned media from IPA-treated microglia restored BDNF and nerve growth factor (NGF) expression in neuroblastoma cells inhibited by LPS [33].

In mice, microdeletion of chromosomal region 16p11.2, known to contribute in humans to neurodevelopmental disorders, cognitive impairment, and dysbiosis, resulted in decreased IPA levels. In this model, orally administered IPA reversed hippocampal alterations and improved cognition, possibly as a result of an increased phosphorylation of ERK1 protein [53].

A study in 32 healthy elderly individuals receiving probiotics for 12 weeks revealed a 1.91-fold increase in serum IPA. IPA level was linked with bacterial diversity and with serum BDNF level. In cultured murine microglial cells treated with 5 μM IPA, the production of TNF-α decreased. Moreover, in human neuroblastoma SH-SY5Y cells, application of media from IPA-treated microglia enhanced BDNF and nerve growth factor synthesis [33].

More than 95% of dietary Trp is degraded along the kynurenine pathway (KP) into derivatives collectively called kynurenines [103]. The metabolism of Trp starts with the conversion of N-formylkynurenine by two step-limiting enzymes, indoleamine 2,3-dioxygenase (IDO), and Trp 2,3-dioxygenase (TDO) [104]. The expression of IDO is induced by inflammatory mediators, e.g., interferon gamma (IFN-γ), IL-6, or TNF-α [105]. A novel way of the Trp metabolism through an enzyme, IL-4-induced 1 (IL4I1), acting as an L-amino acid oxidase, was recently described [106].

N-formylkynurenine is rapidly converted by formidase to the stable metabolite kynurenine, constituting the core of the pathway. Kynurenine is further metabolized along three major pathways. Firstly, it is transformed into KYNA by kynurenine aminotransferases (KATs) I-IV. Kynurenine can also be cleaved by kynureninase into anthranilic acid. Finally, kynurenine 3-monooxygenase (KMO) converts kynurenine to 3-hydroxykynurenine (3-HK). 3-HK is converted to xanthurenic acid or, through enzymatic and non-enzymatic conversion, with 3-hydroxyanthranilic acid as an intermediate, yields quinolinic acid (QUIN). The latter one is metabolized to nicotinamide adenine dinucleotide (NAD). Alternatively, 3-hydroxyanthranilic acid is converted to picolinic acid [103,107,108].

Kynurenines manifest a plethora of biological effects in peripheral and central compartments, including modulation of immune response, cellular survival, and metabolism (for a detailed review see: [109,110,111]). A broad array of data from various experimental paradigms revealed compromised function of the KP in cardiovascular, gastrointestinal, kidney tract, endocrine, or neuropsychiatric disorders, to name a few [64,112,113]. These discoveries resulted in various pharmacological attempts aimed at manipulating the KP, which is considered a promising therapeutic target in human pathology. However, the interplay between various kynurenines, and their net impact on a given cell, and the organ dynamically changes. It depends primarily on the efficacy of biosynthetic and catabolic processes and the proportion between metabolites known to exert the opposing action [114]. Interestingly, the oxidative status of the milieu seems increasingly important as it may alter the properties of metabolites from antioxidant to promoting oxidative stress [109].

The classical neurotoxic metabolites of the KP include QUIN, an endogenous agonist of glutamatergic receptors of N-methyl-D-aspartate type (NMDA), and 3-HK, which generates free radicals. However, anthranilic acid, 3-hydroxyanthranilic acid, xanthurenic acid, picolinic acid, or kynurenine itself may all generate free radicals [109,115].

On the other hand, KYNA is a pleiotropic molecule, for years considered an endogenous antagonist of glutamate receptors with neuroprotective properties [116]. With the years, the complex biological effects of KYNA have become unveiled [105,117]. Apart from the antagonism of iono- and metabotropic glutamate receptors, KYNA was reported to stimulate G-protein-coupled receptor 35 (GPR35), to act as a ligand of adrenoceptor alpha 2B (ADRA2B) and hydroxycarboxylic acid receptor 3 (HCAR3), and to stimulate the α-7 nicotinic acetylcholine receptor (α7nAChR), although the latter effect is a matter of debate [64,105,110].

Furthermore, KYNA, similarly to other metabolites of the KP, such as kynurenine or xanthurenic acid, is an agonist of AhR, yet with higher potency than other kynurenines, showing an ED₅₀ of 100 nM-1 μM [105,118,119].

KYNA occurs in the serum, CSF, and various organs, including the brain. The concentration is within the 40–100 nM range. However, KYNA may attain much higher levels locally, for example, during inflammation [105]. The classical KYNA synthesis pathway in the brain involves the conversion of L-kynurenine by KATs, and was initially ascribed to astroglial cells, but it is now believed that microglial and neuronal cells may also synthesize KYNA [120]. It is noteworthy that peripheral KYNA practically does not reach the central compartment [121].

Therefore, attempts to alter central KYNA level were aimed mostly to increase the brain concentration of KYNA substrate, kynurenine, or to use compounds that, upon penetration through the blood–brain barrier, may change the activity of KATs, or impede the conversion of kynurenine along the other two arms of the KP [107,122].

Recently, it was demonstrated that orally administered IPA may boost serum and brain KYNA, but not kynurenine levels in rodents [34]. Moreover, sub-chronic treatment with IPA provided in the chow caused several-fold increases in both IPA and KYNA levels in the plasma and prefrontal cortex of the animals. In contrast, KYNA levels in medial prefrontal cortex and striatum were not affected by in situ IPA administration via microdialysis probe. Similarly, the addition of IPA to homogenates of rat prefrontal cortex tissue and recombinant human KATII did not influence KATII activity in concentrations up to 1 mM [34]. Thus, IPA is a potent stimulator of KYNA production, acting most probably within the intestinal compartment. However, the precise mechanisms underlying this effect remain to be elucidated.

AD, the major cause of dementia in the elderly, is predicted to affect 150 million people worldwide by the year 2050 [134]. It is characterized by the progressive accumulation of Aβ in amyloid plaques in the brain and vasculature, and by deposits of hyperphosphorylated tau protein within neurons, neurofibrillary tangles, and increasing loss of neurons and cognitive abilities [135,136]. The social and economic costs of disease are tremendous, and, despite our increasing understanding of the underlying mechanism, currently available therapeutic options are merely symptomatic. Novel drugs introduced to therapy and targeting amyloid plaques, such as lecanemab and donanemab, offer moderate improvement in the brain pathology and cognition at the early stage of the disease; furthermore, they are linked with several serious adverse effects [137].

The role of gut dysbiosis in the development of AD was the subject of numerous studies, presented in recent reviews [2,10,138]. Experimental data support the view of IPA as beneficial in AD. The initial discovery came from studies in neuroblastoma cells and in neuronal primary cultures, in which IPA prevented oxidative stress and inhibited Aβ-induced cell death [35]. In neuroblastoma cells expressing the human APP gene, IPA increased mitochondrial membrane potential and respiration rate, and reduced free radical production [138]. Recently, in silico and in vitro research demonstrated that esters of IPA inhibit acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activities, which are pharmacological targets in AD [139,140].

IPA reduced Aβ fibril formation by 50% and reduced the stimulatory effect of apolipoprotein E4 (ApoE4), measured by fluorescence intensity (unpublished observation of Pappolla et al. cited in a review by [141]). Melatonin, a compound structurally similar to IPA, exerts corresponding effects, as it reverses the generation of neurofibrillary tangles evoked by ApoE4 on Aβ [142]. Both IPA and melatonin were demonstrated to improve mitochondrial respiratory rate in APP-expressing neuroblastoma cells [138]. Melatonin has antioxidant properties, augments Aβ clearance by the lymphatic system, and decreases oligomeric Aβ40 in the brain [3,143,144]. The effect of both IPA and melatonin is cumulative when it comes to preventing lipid peroxidation [145].

In vivo, a strong positive correlation was observed between the levels of IPA generated by gut microbiota and behavioral changes in the 5xFAD transgenic AD mouse model [44]. A 30-day treatment with Clostridium sporogenes improved cognitive performance and spatial memory, restored the synaptic ultrastructure and attenuated neuroinflammatory responses, and reduced cortical and hippocampal Aβ accumulation. Concomitant increase in IPA levels, the relative prevalence of IPA-synthesizing bacteria in the gut, and a decrease in bacteria characteristic for AD, e.g., Aquabacterium, Corynebacterium, and Romboutsia, were shown [44]. In transgenic APP/PS1 mice fed a Trp-free diet and treated orally with indole, IAA, and IPA for 4 weeks, a prominent reduction in inflammatory markers was observed. Upregulated AhR, inhibition of the NF-κB pathway, and decreased formation of the NLR family pyrin domain containing 3 (NLRP3) inflammasome, reduced the levels of TNF-α, IL-6, IL-1β, and IL-18, and alleviated the inflammatory response, were all observed [43]. IPA also ameliorated cognitive deficits and neuroinflammation in LPS-treated mice, possibly by inhibition of the RAGE/JAK2/STAT3 signaling pathway [131]. In the murine APP/PS1 AD model, IPA was shown to interact with peroxisome proliferator-activated receptor α (PPARα) and to activate neuroprotective genes, i.e., Bdnf, Pparα, acyl-CoA synthetase, bubblegum family, member 1 (Acsbg1), stearoyl-CoA desaturase 2 (Scd2), and Scd3 [45]. In male mice, increased abundance of indole-producing bacteria Lactobacillus reuteri, elevated levels of IPA, and enhanced PPARα signaling were linked an improvement of working memory and reduction of neuronal damage [45]. Tetradecyl 2,3-dihydroxybenzoate (ABG-001), a molecule with NGF-mimicking activity and anti-aging properties, enhanced the production of IPA by gut microbiota in a murine model of AD. In this study, IPA was shown to target the heat shock cognate 70 kDa protein (Hsc70) and regulate the signaling pathways Hsc70/pyruvate kinase muscle isozyme/hexokinase 2/light chain 3 (PKM2/HK2/LC3) and forkhead box O3/sirtuin 1 (FOXO3a/SIRT1). ABG-001 diminished the memory dysfunctions of the experimental animals by modulating inflammation through IPA and autophagy via Hsc70 [46]. Intermittent fasting, increasingly recognized as cardioprotective, neuroprotective, and promoting longer life span, was shown to improve mitochondrial bioenergetics, and to reverse cognitive deficits in diabetic mice. Alterations in microbial metabolites included an increase in IPA levels [47]. Furthermore, the effects of intermittent fasting were mimicked by intraperitoneal administration of IPA [47].

To our knowledge, just one study reported contrasting results. In patients with mild cognitive impairment, metabolomics and in silico studies suggested that the IPA level is a predictor of progression to AD. However, the trend towards increased plasma IPA was statistically insignificant, and the conclusion was based on the model constructed with the use of eight metabolites [58].

In 2002, IPA was suggested as a promising therapeutic agent in AD [141]. Plans for pre-clinical and clinical studies were also mentioned [141]; however, no data about such clinical trials are available. In a recent review, the authors proposed that the health of the aging brain can be promoted, and dementia development may be prevented by inducing a higher production of indoles, such as IPA, by the microbiota [2].

PD is the second most prevalent neurodegenerative disorder [146]. The accumulation and toxic aggregates of α-synuclein and the degeneration of dopaminergic neurons in the substantia nigra are hallmarks of the disease [147,148,149]. Genetic and environmental factors, which result in oxidative stress, decreased antioxidant capacity, neuroinflammation, lipid peroxidation, mitochondrial dysfunction, and pathological aggregation of α-synuclein, have been implicated in PD pathogenesis [149,150,151,152].

Therapeutic options are symptomatic and involve enhancement of central dopaminergic transmission and acetylcholinesterase inhibitors (AChE-I) [153]. Increasing evidence implies gut microbiota as a potential therapeutic target in PD [64,154].

IPA has been demonstrated to act as a chemical chaperone, reducing endoplasmic reticulum (ER) stress in cells that overexpress Parkin-associated endothelin-like receptor (GPR37) and α-synuclein, thus modeling PD [42]. IPA derivatives demonstrated neuroprotective and antioxidant activities in neuroblastoma cells and inhibited MAO-B, an enzyme that oxidizes dopamine, thus depleting its level and generating free radicals [36]. Furthermore, IPA esters were shown to inhibit the AChE and BChE activities [139]. In rotenone-stimulated enteric glial cells, IPA, in a PXR-mediated manner, reduced mRNA expression of IL-6, IL-1β, and TNF-α, impaired IL13 signaling with subsequent decline of JAK1-STAT6 pathway [37]. The protective effects of IPA were also evident in vivo, in a rotenone model of PD in mice, where IPA alleviated enteric gliosis and restored the integrity of the intestinal and brain barriers [37].

A study on 56 PD patients revealed surprisingly higher IPA levels in the plasma compared to controls (1.26 vs. 0.83 μM); however, IPA did not correlate with cognitive and motor status scores of the patients [27]. Furthermore, the analyses were not performed separately for patients with early (N = 19) and advanced (N = 37) disease, and the impact of medications and diet was not taken into consideration [27]. The elevation of plasma IPA in PD patients could be a compensatory protective mechanism.

IPA may also protect the brain after hypoxia and ischemia, as has been shown in several studies. IPA had a protective effect on the disruption of the blood–brain barrier following hypoxic-ischemic brain injury in neonatal rats, possibly by modulating the PXR signaling pathway [39]. Indeed, its protective effects were reversed by PXR antagonists [39]. In this study, IPA alleviated inflammation by inhibiting the NF-κB pathway. This, in turn, reduced the brain injury-evoked expression of inducible nitric oxide synthase (iNOS), TNF-α, IL-6, and IL-1β, and decreased intracellular ROS levels. PXR overexpression in the rat brain microvascular endothelial cells ameliorated the decline in junctional proteins expression. Furthermore, it reduced the activity of the NF-κB pathway in the model of oxygen-glucose deprivation injury. Additionally, IPA downregulated the activation of MMPs in rat brain microvascular endothelial cells [39].

In a study on Mongolian gerbils, 2-week oral pretreatment with IPA reduced hippocampal CA1 damage by 50%, decreased expression of proinflammatory markers, and ameliorated DNA damage in pyramidal cells [30]. In a murine model of middle cerebral artery occlusion (MCAO), post-stroke intragastric IPA ameliorated neuroinflammation and neuronal damage [48]. It also improved the composition of gut microbiome with subsequent recovery of intestinal permeability and modulated the activity of regulatory T (Treg) and T helper 17 (Th17) cells in intestinal lymphoid tissue [48]. Boosting microbial IPA production by acupuncture exerted beneficial effects in a glucose/oxygen deprivation model [155].

A clinical study on 197 patients with acute cerebral infarction revealed that low serum IPA is an independent predictor of acute stroke and poor prognosis [59]. In line with this report, it was recently discovered that mice bacterial phyla, known to generate AHR ligands, are significantly reduced by aging and experimental stroke. Furthermore, in the middle cerebral artery occlusion stroke model, a decreased serum IPA in mice was observed [49]. Conversely, IPA treatment reduced the expression of IL-1β, IL−4, and IL-10 in microglial cells exposed to oxygen-glucose deprivation. In the same study it was shown that, in patients suffering from stroke, plasma IPA was decreased up to 7 days after the event [49].

To our knowledge, there is only a single report on IPA status in patients with HD. The study included patients with premanifest (N = 52) and early symptomatic (N = 102) HD. Plasma IPA levels were decreased in both groups, reaching 138.5 ng/mL and 107.7 ng/mL, respectively, vs. control (191.1 ng/mL; N = 140) [25].

The data on the role of IPA in multiple sclerosis are ambiguous. An increased IPA synthesis was observed in the relapsing–remitting experimental autoimmune encephalomyelitis in mice, considered a multiple sclerosis model [156]. Similarly, higher IPA was observed by others in patients with the relapsing–remitting course of multiple sclerosis [60]. Furthermore, Expanded Disability Status Scale (EDSS) scores, demonstrating the disability level in patients with multiple sclerosis, were significantly correlated with the urine concentration of IPA (r = 0.5, p < 0.001) [60]. On the other hand, activation of astrocytic AHR by IPA was shown to reduce the central nervous system inflammation and to decrease immune-mediated damage in experimental autoimmune encephalomyelitis [54].

IPA's protective properties were observed in models of autism spectrum disorder. IPA deficiency disrupts the IPA/AHR/NF-κB signaling pathway, resulting in the overactivation of hippocampal microglia and excessive pruning of neuronal synapses. This dysregulation is associated with neural patterns and behaviors similar to those observed in autism spectrum disorder [52].

IPA was also found to promote axon regeneration and functional recovery after sciatic nerve crush in mice [56]. Administration of IPA for 10 days before injury and up to the third day after injury enhanced axonal regeneration and the recovery of sensory function. IPA increased the expression of the PXR gene and genes involved in neutrophil chemotaxis, thereby regulating the regenerative capacity of dorsal root ganglia. Moreover, the inhibition of neutrophil chemotaxis prevented IPA-mediated axonal regeneration [56].

In a rat traumatic brain injury model, the serum levels of IPA exhibited an initial downward trend, which became reversed after three days [57]. These results suggested that IPA may serve to monitor the time that elapsed from the injury [57].

Research on the potential beneficial effects of IPA in aging revealed that elevated IPA levels were associated with an increase in serum BDNF in healthy elderly subjects [33].

There are still a number of unanswered questions concerning IPA action. The majority of data implicate the beneficial effects of IPA, yet some findings warrant further research. It is not clear whether IPA will offer clinical improvement in multiple sclerosis. Furthermore, the contribution of AhR activity to neuronal survival should be studied in more detail, including potentially confounding factors such as diet composition, microbiome variability, and pharmacokinetics of the compound. Similarly, the impact of IPA on KYNA synthesis must be clarified. Currently, it is not understood what intestinal factors are necessary to stimulate the generation of KYNA, and whether they are of microbial or intestinal origin.

In view of the accumulated data and considering a huge demand for novel neuroprotective compounds, clinical trials evaluating the potential use of IPA are essential. We suggest that IPA should be evaluated as (1) a preventive measure aimed to delay the onset of AD- and PD-related neurodegeneration and cognitive impairment in high-risk populations, (2) a therapeutic add-on intervention aimed to slow down the progress of AD and PD, and (3) a therapeutic approach aimed to limit the morphological and behavioral consequences of ischemic stroke. Further experimental and clinical research should also bring more details on the prospective use of IPA in other clinical conditions associated with neuronal loss.