Kynurenic Acid Protects Against Ischemia/Reperfusion-Induced Retinal Ganglion Cell Death in Mice

Neuroactive Kynurenines as Pharmacological Targets: New Experimental Tools and Exciting Therapeutic Opportunities

Both preclinical and clinical studies implicate functional impairments of several neuroactive metabolites of the kynurenine pathway (KP), the major degradative cascade of the essential amino acid tryptophan in mammals, in the pathophysiology of neurologic and psychiatric diseases. A number of KP enzymes, such as tryptophan 2,3-dioxygenase (TDO2), indoleamine 2,3-dioxygenases (IDO1 and IDO2), kynurenine aminotransferases (KATs), kynurenine 3-monooxygenase (KMO), 3-hydroxyanthranilic acid oxygenase (3-HAO), and quinolinic acid phosphoribosyltransferase (QPRT), control brain KP metabolism in health and disease and are therefore increasingly considered to be promising targets for the treatment of disorders of the nervous system. Understanding the distribution, cellular expression, and regulation of KP enzymes and KP metabolites in the brain is therefore critical for the conceptualization and implementation of successful therapeutic strategies. SIGNIFICANCE STATEMENT: Studies have implicated the kynurenine pathway of tryptophan in the pathophysiology of neurologic and psychiatric diseases. Key enzymes of the kynurenine pathway regulate brain metabolism in both health and disease, making them promising targets for treating these disorders. Therefore, understanding the distribution, cellular expression, and regulation of these enzymes and metabolites in the brain is critical for developing effective therapeutic strategies. This review endeavors to describe these processes in detail.

The Biology and Biochemistry of Kynurenic Acid, a Potential Nutraceutical with Multiple Biological Effects

Kynurenic acid (KYNA) is an antioxidant degradation product of tryptophan that has been shown to have a variety of cytoprotective, neuroprotective and neuronal signalling properties. However, mammalian transporters and receptors display micromolar binding constants; these are consistent with its typically micromolar tissue concentrations but far above its serum/plasma concentration (normally tens of nanomolar), suggesting large gaps in our knowledge of its transport and mechanisms of action, in that the main influx transporters characterized to date are equilibrative, not concentrative. In addition, it is a substrate of a known anion efflux pump (ABCC4), whose in vivo activity is largely unknown. Exogeneous addition of L-tryptophan or L-kynurenine leads to the production of KYNA but also to that of many other co-metabolites (including some such as 3-hydroxy-L-kynurenine and quinolinic acid that may be toxic). With the exception of chestnut honey, KYNA exists at relatively low levels in natural foodstuffs. However, its bioavailability is reasonable, and as the terminal element of an irreversible reaction of most tryptophan degradation pathways, it might be added exogenously without disturbing upstream metabolism significantly. Many examples, which we review, show that it has valuable bioactivity. Given the above, we review its potential utility as a nutraceutical, finding it significantly worthy of further study and development.

The role of the kynurenine pathway in cardiovascular disease

The kynurenine pathway (KP) serves as the primary route for tryptophan metabolism in most mammalian organisms, with its downstream metabolites actively involved in various physiological and pathological processes. Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) serve as the initial and pivotal enzymes of the KP, with IDO playing important and intricate roles in cardiovascular diseases. Multiple metabolites of KP have been observed to exhibit elevated concentrations in plasma across various cardiovascular diseases, such as atherosclerosis, hypertension, and acute myocardial infarction. Multiple studies have indicated that kynurenine (KYN) may serve as a potential biomarker for several adverse cardiovascular events. Furthermore, Kynurenine and its downstream metabolites have complex roles in inflammation, exhibiting both inhibitory and stimulatory effects on inflammatory responses under different conditions. In atherosclerosis, upregulation of IDO stimulates KYN production, mediating aromatic hydrocarbon receptor (AhR)-induced exacerbation of vascular inflammation and promotion of foam cell formation. Conversely, in arterial calcification, this mediation alleviates osteogenic differentiation of vascular smooth muscle cells. Additionally, in cardiac remodeling, KYN-mediated AhR activation exacerbates pathological left ventricular hypertrophy and fibrosis. Interventions targeting components of the KP, such as IDO inhibitors, 3-hydroxyanthranilic acid, and anthranilic acid, demonstrate cardiovascular protective effects. This review outlines the mechanistic roles of KP in coronary atherosclerosis, arterial calcification, and myocardial diseases, highlighting the potential diagnostic, prognostic, and therapeutic value of KP in cardiovascular diseases, thus providing novel insights for the development and application of related drugs in future research.

Modeling of Retina and Optic Nerve Ischemia-Reperfusion Injury through Hypoxia-Reoxygenation in Human Induced Pluripotent Stem Cell-Derived Retinal Ganglion Cells

Retinal ganglion cells (RGCs) are specialized projection neurons that constitute part of the retina, and the death of RGCs causes various eye diseases, but the mechanism of RGC death is still unclear. Here, we induced cell death in human induced pluripotent stem cell (hiPSC)-derived RGC-rich retinal tissues using hypoxia-reoxygenation in vitro. Flow cytometry, immunochemistry, and Western blotting showed the apoptosis and necrosis of RGCs under hypoxia-reoxygenation, and they were rescued by an apoptosis inhibitor but not by a necrosis inhibitor. This revealed that the cell death induced in our model was mainly due to apoptosis. To our knowledge, this is the first model to reproduce ischemia-reperfusion in hiPSC-derived RGCs. Thus, the efficacy of apoptosis inhibitors and neuroprotective agents can be evaluated using this model, bringing us closer to clinical applications.

Tryptophan metabolism in health and disease

Tryptophan (Trp) metabolism primarily involves the kynurenine, 5-hydroxytryptamine, and indole pathways. A variety of bioactive compounds produced via Trp metabolism can regulate various physiological functions, including inflammation, metabolism, immune responses, and neurological function. Emerging evidence supports an intimate relationship between Trp metabolism disorder and diseases. The levels or ratios of Trp metabolites are significantly associated with many clinical features. Additionally, studies have shown that disease progression can be controlled by modulating Trp metabolism. Indoleamine-2,3-dioxygenase, Trp-2,3-dioxygenase, kynurenine-3-monooxygenase, and Trp hydroxylase are the rate-limiting enzymes that are critical for Trp metabolism. These key regulatory enzymes can be targeted for treating several diseases, including tumors. These findings provide novel insights into the treatment of diseases. In this review, we have summarized the recent research progress on the role of Trp metabolites in health and disease along with their clinical applications.

Kynurenic acid promotes osteogenesis via the Wnt/β-catenin signaling

The role of kynurenic acid (KynA) in neurological and mental diseases has been widely studied. Emerging studies disclosed that KynA has a protective effect on tissues including heart, kidney, and retina. However, the role of KynA in osteoporosis has not been reported so far. To elucidate the role of KynA in age-related osteoporosis, both control and osteoporosis mice were administrated KynA for three consecutive months, and micro-computed tomography (μCT) analysis was then performed. In addition, primary bone marrow mesenchymal stem cells (BMSCs) were isolated for osteogenic differentiation induction and treated with KynA in vitro. Our data suggested that KynA administration rescued age-related bone loss in vivo, and KynA treatment promotes BMSC osteogenic differentiation in vitro. Moreover, KynA activated the Wnt/β-catenin signaling during BMSC osteogenic differentiation. Wnt inhibitor MSAB inhibited KynA-induced osteogenic differentiation. Further data demonstrated that KynA exerted its effect on BMSC osteogenic differentiation and Wnt/β-catenin signaling activation via G protein-coupled receptor 35 (GPR35). In conclusion, the protective effect of KynA on age-related osteoporosis was disclosed. Additionally, the promoting effect of KynA on osteoblastic differentiation via Wnt/β-catenin signaling was verified and the effect dependent on GPR35. These data suggest that KynA administration potentially contributes to the treatment of age-related osteoporosis.

Peptains block retinal ganglion cell death in animal models of ocular hypertension: implications for neuroprotection in glaucoma

Ocular hypertension is a significant risk factor for vision loss in glaucoma due to the death of retinal ganglion cells (RGCs). This study investigated the effects of the antiapoptotic peptides peptain-1 and peptain-3a on RGC death in vitro in rat primary RGCs and in mouse models of ocular hypertension. Apoptosis was induced in primary rat RGCs by trophic factor deprivation for 48 h in the presence or absence of peptains. The effects of intravitreally injected peptains on RGC death were investigated in mice subjected to retinal ischemic/reperfusion (I/R) injury and elevated intraocular pressure (IOP). I/R injury was induced in mice by elevating the IOP to 120 mm Hg for 1 h, followed by rapid reperfusion. Ocular hypertension was induced in mice by injecting microbeads (MB) or silicone oil (SO) into the anterior chamber of the eye. Retinal flatmounts were immunostained with RGC and activated glial markers. Effects on anterograde axonal transport were determined by intravitreal injection of cholera toxin-B. Peptain-1 and peptain-3a inhibited neurotrophic factor deprivation-mediated RGC apoptosis by 29% and 35%, respectively. I/R injury caused 52% RGC loss, but peptain-1 and peptain-3a restricted RGC loss to 13% and 16%, respectively. MB and SO injections resulted in 31% and 36% loss in RGCs following 6 weeks and 4 weeks of IOP elevation, respectively. Peptain-1 and peptain-3a inhibited RGC death; the loss was only 4% and 12% in MB-injected eyes and 16% and 15% in SO-injected eyes, respectively. Anterograde transport was defective in eyes with ocular hypertension, but this defect was substantially ameliorated in peptain-injected eyes. Peptains suppressed ocular hypertension-mediated retinal glial activation. In summary, our results showed that peptains block RGC somal and axonal damage and neuroinflammation in animal models of glaucoma. We propose that peptains have the potential to be developed as therapeutics against neurodegeneration in glaucoma.

Mitochondrial remodeling and ischemic protection by G protein-coupled receptor 35 agonists

Kynurenic acid (KynA) is tissue protective in cardiac, cerebral, renal, and retinal ischemia models, but the mechanism is unknown. KynA can bind to multiple receptors, including the aryl hydrocarbon receptor, the a7 nicotinic acetylcholine receptor (a7nAChR), multiple ionotropic glutamate receptors, and the orphan G protein-coupled receptor GPR35. Here, we show that GPR35 activation was necessary and sufficient for ischemic protection by KynA. When bound by KynA, GPR35 activated G_i- and G_12/13-coupled signaling and trafficked to the outer mitochondria membrane, where it bound, apparantly indirectly, to ATP synthase inhibitory factor subunit 1 (ATPIF1). Activated GPR35, in an ATPIF1-dependent and pertussis toxin-sensitive manner, induced ATP synthase dimerization, which prevented ATP loss upon ischemia. These findings provide a rationale for the development of specific GPR35 agonists for the treatment of ischemic diseases.

Kynurenic Acid and Its Analogue SZR-72 Ameliorate the Severity of Experimental Acute Necrotizing Pancreatitis

The pathophysiology of acute pancreatitis (AP) is not well understood, and the disease does not have specific therapy. Tryptophan metabolite L-kynurenic acid (KYNA) and its synthetic analogue SZR-72 are antagonists of the N-methyl-D-aspartate receptor (NMDAR) and have immune modulatory roles in several inflammatory diseases. Our aims were to investigate the effects of KYNA and SZR-72 on experimental AP and to reveal their possible mode of action. AP was induced by intraperitoneal (i.p.) injection of L-ornithine-HCl (LO) in SPRD rats. Animals were pretreated with 75-300 mg/kg KYNA or SZR-72. Control animals were injected with physiological saline instead of LO, KYNA and/or SZR-72. Laboratory and histological parameters, as well as pancreatic and systemic circulation were measured to evaluate AP severity. Pancreatic heat shock protein-72 and IL-1β were measured by western blot and ELISA, respectively. Pancreatic expression of NMDAR1 was investigated by RT-PCR and immunohistochemistry. Viability of isolated pancreatic acinar cells in response to LO, KYNA, SZR-72 and/or NMDA administration was assessed by propidium-iodide assay. The effects of LO and/or SZR-72 on neutrophil granulocyte function was also studied. Almost all investigated laboratory and histological parameters of AP were significantly reduced by administration of 300 mg/kg KYNA or SZR-72, whereas the 150 mg/kg or 75 mg/kg doses were less or not effective, respectively. The decreased pancreatic microcirculation was also improved in the AP groups treated with 300 mg/kg KYNA or SZR-72. Interestingly, pancreatic heat shock protein-72 expression was significantly increased by administration of SZR-72, KYNA and/or LO. mRNA and protein expression of NMDAR1 was detected in pancreatic tissue. LO treatment caused acinar cell toxicity which was reversed by 250 µM KYNA or SZR-72. Treatment of acini with NMDA (25, 250, 2000 µM) did not influence the effects of KYNA or SZR-72. Moreover, SZR-72 reduced LO-induced H₂O₂ production of neutrophil granulocytes. KYNA and SZR-72 have dose-dependent protective effects on LO-induced AP or acinar toxicity which seem to be independent of pancreatic NMDA receptors. Furthermore, SZR-72 treatment suppressed AP-induced activation of neutrophil granulocytes. This study suggests that administration of KYNA and its derivative could be beneficial in AP.

Effects of Kynurenic Acid on the Rat Aorta Ischemia-Reperfusion Model: Pharmacological Characterization and Proteomic Profiling

Kynurenic acid (KYNA) is derived from tryptophan, formed by the kynurenic pathway. KYNA is being widely studied as a biomarker for neurological and cardiovascular diseases, as it is found in ischemic conditions as a protective agent; however, little is known about its effect after ischemia-reperfusion in the vascular system. We induced ischemia for 30 min followed by 5 min reperfusion (I/R) in the rat aorta for KYNA evaluation using functional assays combined with proteomics. KYNA recovered the exacerbated contraction induced by phenylephrine and relaxation induced by acetylcholine or sodium nitroprussiate in the I/R aorta, with vessel responses returning to values observed without I/R. The functional recovery can be related to the antioxidant activity of KYNA, which may be acting on the endothelium-injury prevention, especially during reperfusion, and to proteins that regulate neurotransmission and cell repair/growth, expressed after the KYNA treatment. These proteins interacted in a network, confirming a protein profile expression for endothelium and neuron repair after I/R. Thus, the KYNA treatment had the ability to recover the functionality of injured ischemic-reperfusion aorta, by tissue repairing and control of neurotransmitter release, which reinforces its role in the post-ischemic condition, and can be useful in the treatment of such disease.

Tryptophan Metabolism via Kynurenine Pathway: Role in Solid Organ Transplantation

Solid organ transplantation is a gold standard treatment for patients suffering from an end-stage organ disease. Patient and graft survival have vastly improved during the last couple of decades; however, the field of transplantation still encounters several unique challenges, such as a shortage of transplantable organs and increasing pool of extended criteria donor (ECD) organs, which are extremely prone to ischemia-reperfusion injury (IRI), risk of graft rejection and challenges in immune regulation. Moreover, accurate and specific biomarkers, which can timely predict allograft dysfunction and/or rejection, are lacking. The essential amino acid tryptophan and, especially, its metabolites via the kynurenine pathway has been widely studied as a contributor and a therapeutic target in various diseases, such as neuropsychiatric, autoimmune disorders, allergies, infections and malignancies. The tryptophan-kynurenine pathway has also gained interest in solid organ transplantation and a variety of experimental studies investigating its role both in IRI and immune regulation after allograft implantation was first published. In this review, the current evidence regarding the role of tryptophan and its metabolites in solid organ transplantation is presented, giving insights into molecular mechanisms and into therapeutic and diagnostic/prognostic possibilities.

Effective Activation by Kynurenic Acid and Its Aminoalkylated Derivatives on M-Type K+ Current

Kynurenic acid (KYNA, 4-oxoquinoline-2-carboxylic acid), an intermediate of the tryptophan metabolism, has been recognized to exert different neuroactive actions; however, the need of how it or its aminoalkylated amide derivative N-(2-(dimethylamino)ethyl)-3-(morpholinomethyl)-4-oxo-1,4-dihydroquinoline-2-carboxamide (KYNA-A4) exerts any effects on ion currents in excitable cells remains largely unmet. In this study, the investigations of how KYNA and other structurally similar KYNA derivatives have any adjustments on different ionic currents in pituitary GH₃ cells and hippocampal mHippoE-14 neurons were performed by patch-clamp technique. KYNA or KYNA-A4 increased the amplitude of M-type K⁺ current (I_K(M)) and concomitantly enhanced the activation time course of the current. The EC₅₀ value required for KYNA- or KYNA-A4 -stimulated I_K(M) was yielded to be 18.1 or 6.4 μM, respectively. The presence of KYNA or KYNA-A4 shifted the relationship of normalized I_K(M)-conductance versus membrane potential to more depolarized potential with no change in the gating charge of the current. The voltage-dependent hysteretic area of I_K(M) elicited by long-lasting triangular ramp pulse was observed in GH₃ cells and that was increased during exposure to KYNA or KYNA-A4. In cell-attached current recordings, addition of KYNA raised the open probability of M-type K⁺ channels, along with increased mean open time of the channel. Cell exposure to KYNA or KYNA-A4 mildly inhibited delayed-rectifying K⁺ current; however, neither erg-mediated K⁺ current, hyperpolarization-activated cation current, nor voltage-gated Na⁺ current in GH₃ cells was changed by KYNA or KYNA-A4. Under whole-cell, current-clamp recordings, exposure to KYNA or KYNA-A4 diminished the frequency of spontaneous action potentials; moreover, their reduction in firing frequency was attenuated by linopirdine, yet not by iberiotoxin or apamin. In hippocampal mHippoE-14 neurons, the addition of KYNA also increased the I_K(M) amplitude effectively. Taken together, the actions presented herein would be one of the noticeable mechanisms through which they modulate functional activities of excitable cells occurring in vivo.

Tryptophan Pathway Abnormalities in a Murine Model of Hereditary Glaucoma

BACKGROUND

It has been shown that a possible pathogenetic mechanism of neurodegeneration in the mouse model of glaucoma (DBA/2J) may be an alteration of kynurenic acid (KYNA) in the retina. This study aimed to verify the hypothesis that alterations of tryptophan (TRP) metabolism in DBA/2J mice is not limited to the retina.

METHODS

Samples of the retinal tissue and serum were collected from DBA/2J mice (6 and 10 months old) and control C57Bl/6 mice of the same age. The concentration of TRP, KYNA, kynurenine (KYN), and 3-hydroxykynurenine (3OH-K) was measured by HPLC. The activity of indoleamine 2,3-dioxygenase (IDO) was also determined as a KYN/TRP ratio.

RESULTS

TRP, KYNA, L-KYN, and 3OH-K concentration were significantly lower in the retinas of DBA/2J mice than in C57Bl/6 mice. 3OH-K concentration was higher in older mice in both strains. Serum TRP, L-KYN, and KYNA concentrations were lower in DBA/2J than in age-matched controls. However, serum IDO activity did not differ significantly between compared groups and strains.

CONCLUSIONS

Alterations of the TRP pathway seem not to be limited to the retina in the murine model of hereditary glaucoma.

Tryptophan Derivatives by Saccharomyces cerevisiae EC1118: Evaluation, Optimization, and Production in a Soybean-Based Medium

Given the pharmacological properti es and the potential role of kynurenic acid (KYNA) in human physiology and the pleiotropic activity of the neurohormone melatonin (MEL) involved in physiological and immunological functions and as regulator of antioxidant enzymes, this study aimed at evaluating the capability of Saccharomyces cerevisiae EC1118 to release tryptophan derivatives (dTRPs) from the kynurenine (KYN) and melatonin pathways. The setting up of the spectroscopic and chromatographic conditions for the quantification of the dTRPs in LC-MS/MS system, the optimization of dTRPs' production in fermentative and whole-cell biotransformation approaches and the production of dTRPs in a soybean-based cultural medium naturally enriched in tryptophan, as a case of study, were included in the experimental plan. Variable amounts of dTRPs, with a prevalence of metabolites of the KYN pathway, were detected. The LC-MS/MS analysis showed that the compound synthesized at highest concentration is KYNA that reached 9.146 ± 0.585 mg/L in fermentation trials in a chemically defined medium at 400 mg/L TRP. Further experiments in a soybean-based medium confirm KYNA as the main dTRPs, whereas the other dTRPs reached very lower concentrations. While detectable quantities of melatonin were never observed, two MEL isomers were successfully measured in laboratory media.