Linking Protein Misfolding and Heat Stress: A Common Role for the Kynurenine Pathway

1. Introduction

The kynurenine pathway (KP), initiated through the oxidative degradation of tryptophan, has been the subject of investigation since the early twentieth century. Metabolites of tryptophan (TRP) exert diverse biological effects, including the modulation of excitotoxic neurotransmission, oxidative stress, neurotransmitter transport, neuroinflammatory responses, protein aggregation, microtubule dynamics, and gut microbiota homeostasis. Beyond the central nervous system (CNS), the KP also contributes to allergic reactions, immune regulation, mitochondrial function, and cellular bioenergetics through its role in maintaining cellular redox balance. More recently, the KP has emerged as a critical regulator of both central and peripheral immune and inflammatory processes. Its metabolites can suppress the proliferation of intracellular pathogens, promote maternal immune tolerance, and regulate apoptosis, thereby influencing cell cycle progression [1]. Dysregulation of the KP is strongly associated with the development of neurological and neurodegenerative disorders. These shared mechanisms highlight the relevance of KP-related processes not only in chronic neurodegenerative diseases but also in acute syndromes, where rapid neuroinflammation and excitotoxic stress contribute to neuronal damage.

Proteinopathies, characterized by the abnormal aggregation of specific proteins in the brain, underlie the majority of neurodegenerative diseases. This group includes conditions such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS). These disorders are typically associated with chronic neuroinflammation, involving activation of resident brain immune cells and infiltration of peripheral immune cells into the CNS [2]. Prolonged or chronic stimuli drive a sustained inflammatory state, leading to progressive neuronal damage [3]. Neuroinflammation is characterized by the release of a broad spectrum of cellular and molecular mediators, including pro-inflammatory cytokines, chemokines and complement components which contribute to immune activation. In the CNS, microglia and astrocytes serve as the primary source of pro-inflammatory molecules [4] such as interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) [5].

Neuroinflammation is the immune-mediated inflammatory response that occurs following injury or exposure to pathological stimuli. CNS insults rapidly activate immune mechanisms, particularly through the recruitment and activation of resident glial cells. Acute neurological conditions, such as heatstroke, are accompanied by inflammatory responses and disruption of the blood–brain barrier (BBB) [6]. Heatstroke is characterized by a systemic inflammatory response involving elevated pro-inflammatory cytokines, which correlate with disease severity and contribute to multiorgan dysfunction [7]. These processes converge on shared pathological pathways, including mitochondrial dysfunction, inflammatory activation, and impairment of protein degradation systems. Notably, activation of the kynurenine pathway during heat stress further links metabolic and inflammatory responses to excitotoxicity and protein aggregation. Together, these overlapping mechanisms suggest that heatstroke may not only mimic key features of proteinopathies but also contribute to their initiation or progression under certain conditions.

Overall, the study of tryptophan metabolism and the mechanisms by which its metabolites regulate inflammation and neurodegeneration remains a highly promising field. It offers significant potential for the development of novel therapeutic strategies targeting neurodegenerative diseases, cognitive disorders, and acute neurological conditions such as heatstroke.

2. Tryptophan Metabolism

The essential amino acid L-tryptophan (TRP) is a precursor of many important biologically active molecules. The human body cannot synthesize TRP, so it must be obtained from the diet. However, some types of gut microorganisms, such as Enterococcus, Escherichia, Candida, or Streptococcus, are capable of expressing the enzyme tryptophan synthase, which serves as a catalyst in the biosynthesis of TRP from serine and indole-3-glycerol phosphate [8].

From a chemical perspective, the structure of TRP is based on an indole (a bicyclic ring composed of pyrrole and benzene), connected to the α-carbon by a single -CH₂ group. The presence of the indole ring is responsible for the hydrophobic properties of TRP. In addition, it contains 11 carbon atoms, the highest among all amino acids.

After absorption, TRP is transformed into a series of small bioactive molecules, all of which influence numerous cellular metabolic pathways and physiological responses (regulation of immune, gut, endocrine, cardiovascular, and neurological functions). Changes in their concentrations are therefore often associated with various diseases and syndromes, including neurodegenerative disorders such as AD and PD [9].

In humans, up to 90% of TRP circulates in the blood in a free (unbound) form, while the remaining 10% is bound to albumin [10]. Most TRP undergoes various reactions (such as oxidation, hydroxylation, deamination, decarboxylation, or transamination), and it is catabolized/metabolized via three pathways: the kynurenine pathway (occurring mainly in the liver: 90%, and in the brain and gut: 10%), the serotonin pathway (occurring mainly in the gastrointestinal tract: 90%, and in the nervous system: 10%), and the indole pathway (mediated by gut microbes and occurring in the intestine). An illustration of these three TRP metabolic pathways is shown in Figure 1. The remaining absorbed TRP is used for protein synthesis [8].

TRP and its metabolites have emerged as potential biomarkers for the diagnosis and monitoring of both chronic neurodegenerative diseases, including AD and PD, and acute neurological syndromes such as heatstroke. In the context of heatstroke, alterations in TRP metabolism may reflect acute neuroinflammation, excitotoxic stress, and blood–brain barrier (BBB) dysfunction, providing insight into the severity of neuronal injury. Alterations in TRP metabolism reflect ongoing neuroinflammatory processes and excitotoxic stress, providing insight into disease severity and progression. Measurement of TRP and its metabolites in blood or CSF may help in early diagnosis and ongoing monitoring of neurodegenerative and neurological diseases.

2.1. Kynurenine Pathway

The kynurenine pathway (KP) is the dominant catabolic pathway of TRP [11]. Generally around 90–95% of absorbed TRP enters the KP and is transformed into the unstable N-formylkynurenine via indoleamine 2,3-dioxygenase (IDO1) or tryptophan 2,3-dioxygenase, which is subsequently converted into the aryl hydrocarbon receptor (AhR) ligand—KYN—in a reaction catalyzed by N-formylkynurenine formamidase [12]. KYN can then be metabolized either into KA (in astrocytes) via kynurenine aminotransferases or into 3-OH KYN (in microglia) via kynurenine monooxygenase (KMO). 3-OH KYN is further converted either into XA or into 3-OH AA, which can also be formed directly from KYN through AA. Through a series of enzymatic reactions, additional intermediates such as PA and QA are generated, with the final product of this pathway being NAD⁺, a molecule crucial for energy metabolism. This pathway is also linked to the production of NA, a precursor of NAD⁺ [13].

TRP is actively transported across the blood–brain barrier (BBB) via large neutral amino acid (LNAA) transporters, primarily LAT1 (L-type amino acid transporter 1). During inflammatory conditions, enhanced degradation of TRP through the KP increases the availability of peripheral kynurenine, which can also cross the BBB and enter the CNS. It has been estimated that up to 60% of brain KYN is derived from peripheral sources [14]. Under physiological conditions, LAT-1 transporter is nearly saturated; however, in AD, alterations in the TRP/LAT-1 ratio may influence TRP availability in the brain [15].

Because energy demands are significantly increased during neuroimmune responses, the KP represents a key regulatory mechanism of the immune system in the brain [16]. In addition to KYN, other TRP metabolites, such as KA, AA, and 3-OH AA, are endogenous ligands for AhR, which in turn regulates immune cell functions [17]. Intermediates of this pathway represent important molecules often associated not only with neurological disorders [10] and neurodegenerative diseases, such as Huntington’s disease [18], Parkinson’s disease [19] or AD [20], but also with cardiovascular diseases [21] and cancer [22].

Some metabolites generated in the KP are neuroactive, in part through their effects on N-methyl-D-aspartate (NMDA) receptor signaling. For example, KA (an NMDA antagonist) and PA are neuroprotective molecules, whereas QA (an NMDA agonist), 3-OH AA, and 3-OH KYN are neurotoxic [23]. Neuropathy is accompanied by elevated QA levels, suggesting that QA plays a significant role in neurological diseases. In addition to QA, increased peripheral plasma levels of KYN, 3-OH AA, XA, and TRP have been observed in AD compared to healthy controls. Moreover, plasma levels of PA and KYN indirectly correlate with phosphorylated tau levels in CSF [24]. Changes in TRP metabolism have been shown to have various effects on the potential neurotoxicity of Aβ and α-synuclein or tau protein aggregates [25]. Metabolic disturbances in the KP can be reflected by an increased KYN/TRP ratio, which may also indicate a higher risk of CNS disorders, suggesting that this ratio could serve as a useful biomarker [20].

2.2. Serotonin Pathway

A small portion (approximately 5% of TRP) enters the serotonin pathway (SP), where it is first converted into 5-OH TRP, which is then transformed into the neurotransmitter 5-HT (serotonin) [13]. These reactions are catalyzed by tryptophan hydroxylase and 5-hydroxytryptophan decarboxylase. Acetylation of 5-HT catalyzed by arylalkylamine N-acetyltransferase produces N-acetylserotonin (NAS), which is a precursor of melatonin (MEL, the main endogenous regulator of circadian rhythms). The final metabolite in this part of the SP is AFMK, which can be formed either enzymatically or via MEL oxidation [26].

5-HT can also be catabolized into FHKA by IDO1, into MES by indolethylamine N-methyltransferase, or by monoamine oxidase into HIAa, which is subsequently converted by aldehyde dehydrogenase into the major metabolite 5-OH IAA [17]. 5-OH IAA is associated with dementia with Lewy bodies (DLB) and AD [27]. It has been shown that 5-OH IAA, together with homovanillic acid (a dopamine metabolite), may serve as potential biomarkers to distinguish DLB from AD [27].

Alongside other neurotransmitter systems (cholinergic, dopaminergic), the serotonergic system is also involved in AD progression. In AD, not only is a decrease in 5-HT receptors observed in various brain regions (cortex and hippocampus), but also a reduction in 5-HT and 5-OH IAA levels. Therefore, therapies based on selective serotonin reuptake inhibitors have proven effective in this pathway. These drugs increase 5-HT concentrations in the CSF and improve cognitive function and memory in patients with AD [28].

2.3. Indole Pathway

This route of TRP degradation is primarily mediated by gut microorganisms, which produce indole-based metabolites through the indole pathway (IP) [29]. Examples of such indole metabolites include IPA, I3AA, and I3LA. These metabolites can cross the BBB into the brain. They act as AhR ligands with neuroprotective properties and participate in the modulation and regulation of various processes (e.g., modulation of the inflammatory response, regulation of CNS inflammation) [30].

In addition, these indoles may provide protection against oxidative stress, which is associated with AD. For example, IPA has antioxidant effects and can reduce Aβ aggregation. Not only IPA but also other metabolites such as TRM, I3AA, and I3LA may potentially inhibit Aβ aggregation and slow AD progression. Neuroprotective effects have also been demonstrated for I3AA, which activates AhR in microglia and astrocytes, leading to reduced neuroinflammation in AD. Serum and CSF levels of I3AA were lower in AD patients compared to non-AD individuals. Despite these findings, further research is needed on the impact of IP metabolites on AD progression [31,32].

TRP and its metabolites have emerged as potential biomarkers for the diagnosis and monitoring of neurodegenerative disease progression, including AD and PD. Alterations in TRP metabolism, particularly along the KP, reflect ongoing neuroinflammatory processes and excitotoxic stress, providing insight into disease severity and progression. Measurement of TRP and its metabolites in blood or CSF may help in early diagnosis and ongoing monitoring of neurodegenerative diseases.

3. The Neuromodulatory Roles of Kynurenines

Acute or chronic stress, infections, inflammatory responses, and the accumulation of toxins disrupt homeostasis in the KP, leading to dysregulation of the pathway and altered production of neuroactive metabolites [33,34]. Nevertheless, the factors that determine whether brain kynurenine metabolism shifts toward neuroprotective or excitotoxic metabolites remain poorly understood.

Pro-inflammatory cytokines play a key role in regulating the activity of indoleamine 2,3-dioxygenase (IDO), leading to its activation and a subsequent shift in TRP metabolism toward the KP. IDO expression is induced by pro-inflammatory cytokines, including IL-1β, IL-6, and TNF-α; chemokine ligands such as C-C motif chemokine ligand 1 and 5 (CCL1 and CCL5); and small signaling molecules, including prostaglandins, nitric oxide (NO), and reactive oxygen species (ROS) [35,36]. The production of ROS, increased TNF-α, and cytokine secretion further promote interferon-gamma (IFN-γ) production, which in turn stimulates IDO activity in astrocytes, microglia, and other inflammatory cells [37]. Activation of IDO results in decreased TRP availability and increased levels of tryptophan catabolites [38]. The KP generates several neuroactive and potentially neurotoxic metabolites, including QA, XA, 3-HAA, 3-HK, and PA. These metabolites exhibit pro-oxidant properties, contributing to the generation of reactive oxygen and nitrogen species, such as superoxide radicals and peroxides, and may also promote copper-dependent protein cross-linking, ultimately leading to cellular damage [39,40].

The neuroactive effects of kynurenine metabolites are mediated primarily by two mechanisms: modulation of cellular receptor activity and regulation of redox processes within neurons and glial cells. Kynurenine 3-monooxygenase (KMO) is a key enzyme in the KP and a major determinant of tryptophan metabolism. Pharmacological inhibition of KMO does not significantly alter brain QA levels, and even complete genetic deletion reduces QA in the brain by only ~20% [41]. Nonetheless, KMO inhibition using JM6 reversed spatial memory deficits and anxiety-like behaviors in mice [42], and in a mouse model of AD, KMO inhibition prevented synapse loss and improved spatial memory. Similarly, downregulation of KMO expression alleviated pathological phenotypes in a Drosophila model, including motor deficits and other neurodegenerative changes [43].

These findings highlight how alterations in KMO activity shift the balance of downstream kynurenine metabolites, such as KYNA, 3-HK, and QA, toward either neuroprotective or excitotoxic route. Understanding how the KP flux is regulated under physiological and pathological conditions is therefore critical for identifying therapeutic strategies to mitigate excitotoxicity while enhancing neuroprotection.

3.1. Quinolinic Acid

QA exerts neurotoxic effects and contributes to neuronal and glial dysfunction through multiple, interconnected mechanisms. Notably, QA acts as an agonist of NMDA receptors containing NR2A and NR2B subunits promoting Ca²⁺ influx, activation of proteases, and the generation of ROS and nitrogen species, including superoxide (O₂⁻) and NO, which contribute to neurotoxicity [44]. In addition, QA can chelate Fe²⁺ ions, leading to lipid peroxidation [45,46]. Furthermore, QA disrupts actin cytoskeleton dynamics in neurons and astrocytes, impairing intracellular protein transport necessary for maintaining synaptic homeostasis [47].

Beyond its direct excitotoxic action as an NMDA receptor agonist on neurons, QA activates glial cells and increases the production of chemokines such as monocyte chemoattractant protein-1 (MCP-1), as well as the expression of corresponding chemokine receptors, in a manner similar to pro-inflammatory cytokines including TNF-α, IL-1, and IFN-γ [48]. Excessive QA also contribute indirectly to pathological post-translational modifications of tau protein by altering gene expression and reducing the activity of serine/threonine phosphatases in primary cultures of human neurons [49]. In human neurons and astrocytes, QA enhances the activity of inducible nitric oxide synthase (iNOS) and neuronal nitric oxide synthase (nNOS), resulting in cytotoxic and excitotoxic effects and triggering an energy crisis through NAD⁺ depletion. The NAD⁺ depletion activates the NAD⁺-dependent nuclear DNA repair enzyme poly(ADP-ribose) polymerase-1 (PARP-1), which, while attempting to restore energy homeostasis, can exacerbate cytotoxic damage when overactivated due to impaired energy levels and oxidative stress. Importantly, inhibition of iNOS and nNOS prevents these effects, highlighting the central role of NO in QA-mediated excitotoxicity [50].

In addition to activating NMDA receptors, QA stimulates the expression of several immunologically active and pro-inflammatory cytokines, including interleukin-8 (IL-8), CCL5, and macrophage inflammatory protein-1 (MIP-1), as well as multiple C-C and C-X-C chemokine receptors, such as CCR3, CCR5, CXCR4, and CXCR6 [51]. These chemokines promote leukocyte recruitment across the BBB. In parallel, QA enhances glial cell proliferation while continuing to stimulate NMDA receptors, further contributing to neuroinflammatory and excitotoxic processes [52].

3.2. Kynurenic Acid

KYNA acts as an NMDA receptor antagonist and can protect neurons against QA-induced excitotoxicity. In the brain, KYNA is synthesized de novo in astrocytes by KAT II following uptake of kynurenine via the LAT-1 [53]. In addition to KAT-mediated synthesis, KYNA can be produced via alternative mechanisms, including conversion of D-kynurenine by D-amino acid oxidase and non-enzymatic reactions involving ROS [54].

KYNA exerts diverse neuromodulatory effects, influencing glutamatergic, cholinergic, GABAergic, and dopaminergic neurotransmission [55].

At low concentrations, it prevents excitotoxic neuronal damage, although the exact CNS levels required for neuroprotection remain unclear. Dysregulation of kynurenine 3-monooxygenase activity can lead to significantly elevated KYNA levels, as observed in patients with AD compared to controls [56].

KYNA also modulates oxidative stress by attenuating activation of nuclear factor erythroid 2-related factor 2 (Nrf2), a key regulator of cellular antioxidant defense, thereby reducing the impact of ROS, lipid peroxidation, protein damage, and mitochondrial respiratory dysfunction induced by QA [57].

Although generally considered neuroprotective, excessive accumulation of KYNA above physiological levels can impair glutamatergic signaling and contribute to cognitive decline [58]. This is partly due to its antagonistic action on α7-nicotinic acetylcholine receptors (α7nACh-R) and its regulatory effects on other acetylcholine receptor subtypes. Moreover, KYNA functions as an agonist of the AhR and the G-protein-coupled receptor GPR35, mobilizing intracellular calcium (Ca²⁺) and stimulating inositol phosphate production [59].

3.3. 3-Hydroxykynurenine

3-hydroxykynurenine, exhibits both antioxidant and pro-oxidant properties depending on its concentration. Lower concentrations are associated with strong pro-oxidant activity and neurotoxicity [60], whereas higher concentrations confer resistance to oxidative stress [61]. 3-HK can stimulate glutathione transferase, superoxide dismutase, and Nrf2, which are critical for antioxidant regulation. Therefore, changes in 3-HK concentration can modulate antioxidant systems and activate cellular redox sensors during inflammation. The generation of free radicals also enhances excitotoxic effects during the combined action of 3-HK and QA.

3.4. Xanthurenic and Picolinic Acid

The roles of XA and PA are less well characterized, though they exhibit antioxidant and immunomodulatory effects via agonistic interactions with metabotropic glutamate receptors (mGluR2 and mGluR3). XA is produced through the transamination of 3-HK catalyzed by KAT II and has recently been shown to possess neurotransmitter activity in the nervous system [62].

PA promotes macrophage activation by upregulating IFN-dependent nitric oxide synthase and inducing MIP-1 expression. Intracranial, but not subcutaneous, administration of PA lowers seizure thresholds in mice, although the precise mechanism remains unclear [63]. PA can modulate glutamatergic neurotransmission depending on its concentration and the presence of other agonists such as kainates [64], and may exert anti-inflammatory and neuroprotective effects relevant to neuroinflammatory and neurodegenerative conditions.

4. Proteinopathies

A relatively large group of neurodegenerative diseases consists of proteinopathies including tauopathies, synucleinopathies, and amyloidoses [65].

Proteinopathies represent a group of disorders associated with conformational changes in proteins. As a result of these conformational changes (misfolding of proteins), their function is altered, often leading to the formation of oligomers that can subsequently aggregate into pathological structures. A very important fact is that aggregated forms of misfolded proteins have the ability to induce the conversion of correctly folded proteins [66]. The individual stages of proper and improper protein folding are illustrated in Figure 2.

Proteins most commonly fold in the cytoplasm; however, this process can also occur in mitochondria or the endoplasmic reticulum, which contains molecular chaperones. When intracellular proteins fail to fold efficiently, the cell activates the so-called “protein quality control system,” which can induce either refolding of the proteins or their degradation via molecular chaperones and proteasomes. If an imbalance occurs between correctly and misfolded proteins, misfolded proteins can aggregate, forming structures that are implicated in various human diseases [67].

The accumulation of misfolded proteins occurs either intracellularly (e.g., tau protein, α-synuclein) or extracellularly (e.g., amyloid-β) and can take on various histopathological forms. They may result from increased translation of specific mediator ribonucleic acids (mRNAs), transcriptional activation, or reduced protein degradation rates due to impairment of the proteasomal or lysosomal pathways [68].

4.1. Tauopathies

Tauopathies represent a biochemically and morphologically heterogeneous group of neurodegenerative diseases. A characteristic feature of this group is the deposition—the formation of insoluble aggregates of abnormal (hyperphosphorylated) tau protein in the brain, primarily in neurons, glial cells, and the extracellular space. Tau protein, associated with microtubules (microtubule-associated protein tau; MAPT), binds to tubulin and promotes its polymerization and stabilization into microtubules [69].

Since tau protein was originally identified as a microtubule-associated protein, it is assumed that neurodegeneration results from the loss of tau’s ability to bind microtubules, which may occur due to hyperphosphorylation. Hyperphosphorylation is mediated by activated kinases capable of transferring phosphate groups to tau protein. Phosphorylated tau detaches from microtubules, aggregates, and forms neurofibrillary tangles (NFTs). Destabilized microtubules lose their signaling functions, which in some cases may lead to apoptosis [70].

Currently, more than 26 tauopathies are known [71], and several classification schemes exist. In diseases where tau protein accumulation is the primary feature, these are referred to as primary tauopathies. Conversely, in conditions such as trauma or autoimmune disorders, where tau accumulation plays a secondary role, these are called secondary tauopathies [72].

Another classification is based on the predominant tau isoforms present in aggregates: 3R tauopathies, 4R tauopathies, and 3R/4R tauopathies (with roughly equal 3R/4R ratios). A distinct group of tauopathies is known as “geographically isolated tauopathies,” which include, for example, nodding syndrome of northern Uganda, Guadeloupean parkinsonism, amyotrophic lateral sclerosis in the western Pacific, and the Parkinsonism-dementia complex. The etiopathogenesis of these disorders is not yet fully understood, although multiple studies have highlighted the influence of environmental factors [73].

4.2. Amyloidoses

The origin of this group of diseases is the amyloid precursor protein (APP), which is encoded by a gene located on chromosome 21 [74]. APP is a transmembrane protein with three domains: intracellular, membrane-bound, and extracellular. It is synthesized in the endoplasmic reticulum and subsequently transported to the Golgi apparatus, where it matures before being delivered to the plasma membrane [75].

Under normal conditions, APP can be cleaved via two pathways: non-amyloidogenic and amyloidogenic. In the first pathway, APP is processed by α-secretase to generate a soluble fragment (sAPPα), which remains in the extracellular space, and an 83-amino-acid C-terminal fragment, which remains in the membrane. Subsequent cleavage by γ-secretase produces the non-amyloidogenic extracellular fragment (p3) and the APP intracellular domain (AICD).

In the second pathway, APP is cleaved by β- and γ-secretases. In the first step, β-secretase generates a C-terminal fragment, which is then cleaved by γ-secretase in the second step to produce an insoluble peptide known as amyloid-β (Aβ). Aβ aggregates and forms Aβ plaques [76]. The major final forms of Aβ are 38 amino acids (Aβ-38), 40 amino acids (Aβ-40), and 42 amino acids (Aβ-42) [77], with Aβ42 considered the most neurotoxic [78].

Neuronal damage occurs when an immune response is triggered, leading to inflammation. Additionally, deposition of Aβ plaques on blood vessels can result in angiopathy. The inflammatory response induced by Aβ and disruption of the BBB leads to the release of pro-inflammatory cytokines and chemokines. The cytokines secreted by brain immune cells can attract blood-derived immune cells that can infiltrate the brain due to increased permeability of the BBB [79].

All three of these processes occur extracellularly, in contrast to tau pathology, which occurs intracellularly. The combination of these two pathological processes leads to the most well-known and prevalent amyloidosis: Alzheimer’s disease.

4.3. Synucleinopathies

Synucleinopathies are a group of progressive neurodegenerative disorders characterized by the abnormal accumulation of misfolded α-synuclein protein, leading to neuronal death. Major types include Parkinson’s disease, Dementia with Lewy bodies (DLB), and Multiple system atrophy (MSA), which cause motor, cognitive, and autonomic dysfunction. Clinically, they are characterized by a chronic and progressive decline in motor, cognitive, behavioural, and autonomic functions, depending on the distribution of the lesions. In PD and DLB, the protein is primarily found in Lewy bodies (LBs) within neurons, whereas in MSA, it is predominantly observed in glial cytoplasmic inclusions (GCIs) [80]. Neuron-derived α-synuclein activates microglia. It was shown that α-synuclein inclusions trigger microglial activation with increased expression of inflammation-related genes. Different α-synuclein conformations induce distinct inflammatory responses, including cytokine and chemokine production via TLR signaling [81].

5. Changes of the Kynurenine Pathway in Selected Neurological Conditions

5.1. Parkinson’s Disease

Parkinson’s disease is a neurodegenerative disorder characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and the accumulation of misfolded α-synuclein in Lewy bodies throughout the brainstem and other regions. Striatal dopamine loss drives both the classic motor symptoms and non-motor features of PD, with non-motor signs often appearing before motor symptoms [82]. The motor features include resting tremor, limb rigidity, bradykinesia, and postural and gait instability.

The pathogenesis of PD is associated with disruptions in the ubiquitin-proteasome pathway, mitochondrial dysfunction, oxidative stress, and apoptosis [83].

The first evidence associating the KP in PD pathogenesis came from studies using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). This lipophilic compound, initially identified in 1983 as a contaminant in street heroin, rapidly induced PD in young users. [84]. It selectively induces nigral degeneration and PD-like behavior in rodents and primates, making it a valuable research tool [85]. MPTP toxicity arises from glial conversion to pyridinium metabolite (MPP+), which inhibits neuronal mitochondrial respiratory chain and generates free radicals [86]. In vivo studies demonstrated that MPTP reduces KYNA synthesis and decreases the density of KAT I–positive neurons in the substantia nigra of mice, representing a chronic change associated with persistent dopaminergic degeneration [87] However, KYNA pretreatment protects neurons and preserving mitochondrial function in cells exposed to MPP+ [88].

Moreover, disruptions in glutamate signaling, as shown in various animal models, can reduce the neuroprotective metabolite KYNA and contribute to PD development. Supporting this, in rodent 6-OHDA models of PD, treatment with L-KYN and probenecid elevates KYNA and dopamine levels, decreases glial activation and neuronal degeneration, and improves motor function, likely by preventing glutamate-induced excitotoxicity through NMDA receptor inhibition [89,90].

Excitotoxicity, oxidative stress, and inflammatory changes disrupt the KP, promoting the activation of its excitotoxic branches. This results in a significant decrease in the neuroprotective metabolite KYNA and an increase in the neurotoxic metabolite QA, which further exacerbates excitotoxicity and inflammatory response, contributing to the onset and progression of PD [91]. Low endogenous KYNA levels directly impair the brain’s capacity to protect against QA- and glutamate-induced excitotoxicity through NMDA receptors [92]. The pathway also enhances the production of 3-HK, a strong inducer of neurotoxic free radicals, which play a important role in PD pathogenesis. It was shown that high concentration of 3-HK can cause cell death by producing free radicals in PD [93].

Alterations in KP metabolites have been detected in the brain tissue, plasma, and CSF of patients with PD. Compared to healthy controls, these patients exhibit a reduced KYNA/KYN ratio, elevated levels of QA and an increased QA/KYNA ratio. Overall, metabolomic studies indicate a shift in the kynurenine pathway toward enhanced production of the neurotoxic metabolite QA and reduced synthesis of the neuroprotective metabolite KYNA [94].

Postmortem studies reported significanty decreased KYNA and KYN levels in frontal cortex, putamen, cerebellum and substantia nigra, while the concentration of the neurotoxin 3-HK in the striatum and SNpc was significantly increased and without change in TRP/KYN and KYN/KYNA ratios, in the brains of PD patients [95].

The disturbances of KYN pathway metabolites were measured as potential biomarkers in plasma as well as CSF [96,97]. An increase in KYN/TRP ratio, depletion of plasma TRP level, and increase in KYN and KYNA were reported in serum samples of PD patients [98]. Increase in serum KYNA was also observed among patients without dyskinesia, but not in dyskinetic PD patients [96]. In contrast, the deficiency of KYNA was revealed in a metabolomic study performed on a larger cohort of PD patients. Findings included lower plasma KYNA/KYN ratio, higher QA level, and increased QA/KYNA ratio. PD subjects had increased levels of 3-HK and decreased amounts of 3-HAA acid in plasma [99].

In PD patients was found increased neuroexcitatory QA/KYNA ratio in both plasma and CSF of PD participants associated with peripheral and inflammation and vitamin B₆ deficiency [94]. Similarly, lower KYNA, higher QA, and an elevated QA/PA ratio in CSF, as well as high 3-HK in plasma, were detected. Furthermore, elevated QA levels were associated with CSF tau, soluble TREM2 (sTREM2), and the severity of both motor and non-motor symptoms in PD. In addition, patient subgroups characterized by distinct KP profiles exhibited differences in clinical manifestations of the disease.

Also the treament may affect tryptophan metabolism via the KP. Altered levels of kynurenine metabolites can affect glutamatergic transmission and may play a role in the development of L-DOPA-induced dyskinesia [97]. Compared to patients with healthy controls, individuals with PD exhibit a lower KYNA/KYN ratio, along with higher QA levels and an increased QA/KYNA ratio. Moreover, patients at an advanced stage of PD show further reductions in KYNA and the KYNA/KYN ratio, as well as elevations in QA levels and the QA/KYNA ratio, relative to those at an early stage and to healthy controls. In contrast, within advanced PD, KYNA and QA levels, as well as the KYNA/KYN and QA/KYNA ratios, do not differ significantly between patients with and without psychiatric symptoms, dementia, or levodopa-induced dyskinesia [99].

The most pronounced statistically significant differences between the PD and control groups were identified for KYN levels in CSF, as well as for 3-HK and 5-HT in blood serum. These findings enabled the identification of distinct correlation patterns between CSF and serum metabolites, which may serve as a basis for the differential diagnosis of synucleinopathies [25].

KYNA analogues have been shown to suppress glutamatergic activity while also modulating NMDA receptor function, highlighting their therapeutic potential in PD. Improvement in motor symptoms has been observed in PD patients treated with zonisamide, a sulfonamide antiepileptic drug, although its precise mechanism of action remains unclear. Notably, zonisamide has been reported to enhance astroglial release of KP metabolites, including KYNA, providing further evidence for the involvement of KYNA in PD therapy [100]. The neuroimmunophilin ligand FK506 is used as an immunosuppressive agent in PD treatment. In addition to increasing KYNA production in the cortex, FK506 attenuates KYNA inhibition induced by 3-nitropropionic acid and MPP⁺ [101]. Studies indicate that treatment with this ligand can promote the survival of dopaminergic neurons in a dose-dependent manner. Furthermore, FK506 significantly reduces the infiltration and activation of cytotoxic T cells, as well as various subtypes of macrophages and microglia [102]. These findings suggest that FK506 possesses anti-inflammatory properties and contributes to the reduction of neurodegeneration [103].

5.2. Alzheimer’s Disease

AD is currently the most prevalent cause of dementia worldwide. AD pathology is characterized by the presence of amyloid-β plaques and neurofibrillary tangles in the brains of affected individuals. Despite extensive research, the underlying mechanisms of AD pathology remain incompletely understood.

Alterations in the KP in AD arise from a complex interplay of inflammatory, metabolic, and neurodegenerative processes. A major driver is chronic neuroinflammation, where activated microglia and astrocytes release pro-inflammatory cytokines such as IL-1β, IL-6, TNF-α, and IFN-γ, which induce IDO1 and shift TRP metabolism toward KYN production [40,104]. This is further exacerbated by amyloid-β accumulation and tau pathology, which amplify inflammatory signaling and create a feed-forward loop promoting KP activation [105,106]. Dysregulation of key enzymes, including increased IDO1, tryptophan 2,3-dioxygenase (TDO), and KMO, drives metabolism toward neurotoxic metabolites such as 3-HK and QA, while potentially reducing the formation of the neuroprotective KYNA. Oxidative stress and mitochondrial dysfunction further modulate KP activity and contribute to redox imbalance and impaired energy metabolism. Aging also plays a significant role, as it is associated with increased baseline inflammation and enhanced KP activation [107]. In addition, peripheral immune activation and systemic conditions such as obesity or insulin resistance elevate circulating KYN levels, which can cross the BBB and influence central metabolism, particularly in the context of BBB disruption commonly observed in AD [108]. Altered TRP availability and competition with other large neutral amino acids, as well as gut microbiota dysbiosis, further modulate KP dynamics [109,110]. Together, these factors lead to an imbalance between neuroprotective and neurotoxic kynurenine metabolites, contributing to excitotoxicity, neuroinflammation, synaptic dysfunction, and ultimately cognitive decline in AD.

Alterations in the KP in Alzheimer’s disease remain complex, heterogeneous, and not entirely consistent across studies. Nevertheless, both peripheral and central levels of KP metabolites have been associated with disease severity and cognitive decline. Specifically, KYN (plasma, serum, and CSF), KYNA (serum, urine, CSF), TRP (plasma, serum, urine), the KYN/TRP ratio (plasma, serum, urine, CSF), and QA (plasma, CSF) have all been reported to correlate with cognitive impairment. Notably, positive correlations have been observed between cognitive performance and KYNA levels, whereas QA shows inverse associations, supporting the concept of a neuroprotective–neurotoxic balance within the KP. In contrast, some metabolites such as KYN, 3-HK, and AA exhibit only modest or non-significant changes [111].

A recent meta-analysis further highlighted compartment-specific alterations, showing decreased TRP levels exclusively in peripheral blood and an increased KYN/TRP ratio only in the periphery of AD patients. In the CSF, 3-HK levels were reduced and displayed greater variability in cognitively normal individuals compared to AD patients, KYNA was also decreased peripherally. Interestingly, no consistent differences were observed for KYN or QA between AD and control groups [112]. These findings suggest that KP dysregulation in AD may differ between central and peripheral compartments, reflecting complex regulatory mechanisms.

Consistent with this, analyses of peripheral blood from AD patients revealed dysregulated levels of multiple KP metabolites, including KYN, 3-HK, KYNA, and QA, further implicating KP imbalance in AD pathophysiology. Moreover, studies measuring TRP, KYN, QA, and the KYN/TRP ratio in both plasma and CSF demonstrated significant correlations with core AD biomarkers such as amyloid-β and tau, linking KP metabolism directly to neurodegenerative processes [113]. In addition, clinical studies across cognitively normal, mild cognitive impairment, and AD populations have shown that an increased KYN/TRP ratio correlates with pro-inflammatory cytokines (IL-1ra, IL-12p40, IL-18) and memory performance, whereas an elevated KYN/5-HT ratio is associated with broader neuropsychiatric disturbances, including negative affect, executive dysfunction, and global cognitive decline [114].

Metabolomic analyses provide further support, demonstrating significantly reduced serum TRP, KYN, and XA levels in AD patients, with XA positively correlating with cognitive scores such as the Mini-Mental State Examination (MMSE), reinforcing the link between KP metabolites and cognitive status [115]. However, findings regarding KYNA remain inconsistent. While decreased brain KYNA levels have been reported in transgenic AD mouse models, human studies show conflicting results, with some reporting increased, unchanged, or even decreased KYNA levels depending on the brain region and biological matrix examined. For example, postmortem analyses revealed region-specific alterations, with elevated KYNA in the putamen and caudate nucleus but not in cortical areas [116], whereas other studies reported reduced KYNA levels in AD brain tissue.

Age-related changes in KP metabolism may further contribute to these discrepancies. Aging itself has been associated with increased levels of KYN, KYNA, and QA in both serum and CSF, independent of disease status. Interestingly, although KYNA concentrations were found to be reduced in the CSF of AD patients, estimates of KYN transport into the brain did not differ between patients and controls. Additionally, serum KYN levels negatively correlated with AD severity, while strong correlations were observed between peripheral and central concentrations of KYN and QA, suggesting systemic contributions to central KP alterations [117].

Overall, these findings support a significant, yet complex involvement of the KP in AD pathogenesis, highlighting both neurotoxic and neuroprotective mechanisms. This dual role makes the KP an attractive target for therapeutic intervention. Accordingly, various strategies have been explored to either enhance endogenous KYNA levels or reduce the production of neurotoxic metabolites such as QA. In animal models of AD, such approaches have shown promise in slowing disease progression [118]. To overcome the limited stability and bioavailability of endogenous metabolites, several KYNA and L-KYN analogues have been developed, including 7-chlorokynurenic acid (7-Cl-KYNA), L-4-chlorokynurenine (4-Cl-L-KYN), and other derivatives designed to improve pharmacokinetic properties [119]. Notably, 4-Cl-L-KYN can cross the blood–brain barrier and inhibit QA-mediated toxicity at the glycine site of NMDA receptors [120]. Furthermore, recent experimental work demonstrated that chronic administration of the KYNA analogue KYNA-1 modulates tau pathology and reduces neuroinflammation in a rat model of tauopathy [118].

Taken together, the growing body of evidence suggests that KP dysregulation is intricately linked to neuroinflammation, metabolic imbalance, and neurodegeneration in AD, and may represent a promising avenue for the development of novel diagnostic biomarkers and therapeutic strategies.

5.3. Heatstroke

Heatstroke is a life threatening condition characterised by the elevation of core body temperature above 40° celsius with the manifestation of central nervous system (CNS) disorder [121]. It represents the most severe form of heat-related illnesses (HRI) and is associated with systemic inflammation, oxidative stress, and metabolic dysregulation. Emerging evidence suggests that the kynurenine pathway (KP), the primary route of tryptophan degradation plays a significant role in mediating these pathophysiological processes [122].

Physical exertion or exposure to elevated temperatures leads to an increase in metabolic rate and elevation in core body temperature, failure of the body’s thermoregulatory mechanisms leads to systemic inflammatory response syndrome and multiple organ failure [123]. This response is characterized by the elevation of pro-inflammatory cytokines, including interferon-gamma IFN-γ, interleukin-1β, interleukin-6, and tumor necrosis factor -α [124]. These cytokines act as potent inducers of IDO, resulting in the degradation of tryptophan in KP and the subsequent release of KYN and active downstream metabolites, including KYNA, PIC, and QUIN. Consequently, circulating tryptophan levels decline with a subsequent increase in kynurenine, promoting regulatory T cell expansion, and the kynurenine-to-tryptophan ratio can serve as a functional indicator of pathway activation and systemic immune engagement [125] (Figure 3).

The KP and the accumulation of kynurenine metabolites play a central role in heatstroke pathology. An increase in body temperature raises peripheral levels of several KP metabolites. These levels return to baseline after cessation of the stimulus, indicating that KP enzymes are adaptable to physiological stress [122]. Heatstroke further promotes the disruption of the blood-brain barrier facilitating the permeability of kynurenine which are metabolised into the neuroactive compounds linking peripheral metabolic changes to central nervous system dysfunction [121].

6. Conclusions

In conclusion, the overlap of mechanisms underlying heat stroke and proteinopathies highlights a shared vulnerability of the central nervous system to proteotoxic, inflammatory, and metabolic stress. Acute hyperthermia induces processes—such as protein misfolding, oxidative damage, and neuroinflammation—that closely mirror those observed in chronic neurodegenerative disorders, including Alzheimer’s disease and Parkinson’s disease. The involvement of the KP further supports a link between systemic stress responses and neurodegenerative cascades. These insights suggest that heatstroke may act not only as an acute neurological insult but also as a potential modifier or accelerator of proteinopathic processes, warranting further investigation into its long-term neurological consequences and therapeutic targeting of shared pathways.