Central nervous system-specific alterations in the tryptophan metabolism in the 3-nitropropionic acid model of Huntington's disease

Kynurenine pathway and its role in neurologic, psychiatric, and inflammatory bowel diseases

Tryptophan metabolism along the kynurenine pathway is of central importance for the immune function. It prevents hyperinflammation and induces long-term immune tolerance. Accumulating evidence also demonstrates cytoprotective and immunomodulatory properties of kynurenine pathway in conditions affecting either central or peripheral nervous system as well as other conditions such as inflammatory bowel disease (IBD). Although multilevel association exists between the inflammatory bowel disease (IBD) and various neurologic (e.g., neurodegenerative) disorders, it is believed that the kynurenine pathway plays a pivotal role in the development of both IBD and neurodegenerative disorders. In this setting, there is strong evidence linking the gut-brain axis with intestinal dysfunctions including IBD which is consistent with the fact that the risk of neurodegenerative diseases is higher in IBD patients. This review aims to highlight the role of kynurenine metabolic pathway in various neurologic and psychiatric diseases as well as relationship between IBD and neurodegenerative disorders in the light of the kynurenine metabolic pathway.

Brain Functional Network and Amino Acid Metabolism Association in Females with Subclinical Depression

This study aimed to investigate the association between complex brain functional networks and the metabolites in urine in subclinical depression. Electroencephalography (EEG) signals were recorded from 78 female college students, including 40 with subclinical depression (ScD) and 38 healthy controls (HC). The phase delay index was utilized to construct functional connectivity networks and quantify the topological properties of brain networks using graph theory. Meanwhile, the urine of all participants was collected for non-targeted LC-MS metabolic analysis to screen differential metabolites. The global efficiency was significantly increased in the α-2, β-1, and β-2 bands, while the characteristic path length of β-1 and β-2 and the clustering coefficient of β-2 were decreased in the ScD group. The severity of depression was negatively correlated with the level of cortisone (p = 0.016, r = −0.40). The metabolic pathways, including phenylalanine metabolism, phenylalanine tyrosine tryptophan biosynthesis, and nitrogen metabolism, were disturbed in the ScD group. The three metabolic pathways were negatively correlated (p = 0.014, r = −0.493) with the global efficiency of the brain network of the β-2 band, whereas they were positively correlated (p = 0.014, r = 0.493) with the characteristic path length of the β-2 band. They were mainly associated with low levels of L-phenylalanine, and the highest correlation sparsity was 0.11. The disturbance of phenylalanine metabolism and the phenylalanine, tryptophan, tyrosine biosynthesis pathways cause depressive symptoms and changes in functional brain networks. The decrease in the L-phenylalanine level may be related to the randomization trend of the β-1 frequency brain functional network.

Tryptophan Metabolism in Central Nervous System Diseases: Pathophysiology and Potential Therapeutic Strategies

The metabolism of L-tryptophan (TRP) regulates homeostasis, immunity, and neuronal function. Altered TRP metabolism has been implicated in the pathophysiology of various diseases of the central nervous system. TRP is metabolized through two main pathways, the kynurenine pathway and the methoxyindole pathway. First, TRP is metabolized to kynurenine, then kynurenic acid, quinolinic acid, anthranilic acid, 3-hydroxykynurenine, and finally 3-hydroxyanthranilic acid along the kynurenine pathway. Second, TRP is metabolized to serotonin and melatonin along the methoxyindole pathway. In this review, we summarize the biological properties of key metabolites and their pathogenic functions in 12 disorders of the central nervous system: schizophrenia, bipolar disorder, major depressive disorder, spinal cord injury, traumatic brain injury, ischemic stroke, intracerebral hemorrhage, multiple sclerosis, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease. Furthermore, we summarize preclinical and clinical studies, mainly since 2015, that investigated the metabolic pathway of TRP, focusing on changes in biomarkers of these neurologic disorders, their pathogenic implications, and potential therapeutic strategies targeting this metabolic pathway. This critical, comprehensive, and up-to-date review helps identify promising directions for future preclinical, clinical, and translational research on neuropsychiatric disorders.

Hypoxic preconditioning protects against ischemic kidney injury through the IDO1/kynurenine pathway

Prolonged cellular hypoxia leads to energetic failure and death. However, sublethal hypoxia can trigger an adaptive response called hypoxic preconditioning. While prolyl-hydroxylase (PHD) enzymes and hypoxia-inducible factors (HIFs) have been identified as key elements of oxygen-sensing machinery, the mechanisms by which hypoxic preconditioning protects against insults remain unclear. Here, we perform serum metabolomic profiling to assess alterations induced by two potent cytoprotective approaches, hypoxic preconditioning and pharmacologic PHD inhibition. We discover that both approaches increase serum kynurenine levels and enhance kynurenine biotransformation, leading to preservation of NAD+ in the post-ischemic kidney. Furthermore, we show that indoleamine 2,3-dioxygenase 1 (Ido1) deficiency abolishes the systemic increase of kynurenine and the subsequent renoprotection generated by hypoxic preconditioning and PHD inhibition. Importantly, exogenous administration of kynurenine restores the hypoxic preconditioning in the context of Ido1 deficiency. Collectively, our findings demonstrate a critical role of the IDO1-kynurenine axis in mediating hypoxic preconditioning.

Indoleamine-2,3-Dioxygenase 1 Deficiency Suppresses Seizures in Epilepsy

Background: Indoleamine-2,3-dioxygenase 1 (IDO1) is the initial and rate-limiting enzyme in the metabolism of tryptophan (TRP) to kynurenine (KYN). IDO1-dependent neurotoxic KYN metabolism plays a crucial role in the pathogenesis of many neurodegenerative disorders. However, the function of IDO1 in epilepsy is still unclear. Objective: In this study, we investigated whether IDO1 deficiency could affect epilepsy in a lithium-pilocarpine-induced model. Methods: Patients with epilepsy and controls were enrolled. Male C57BL/6 mice and IDO1 knockout (KO, IDO1^-/-) mice were subjected to intraperitoneal injection of lithium and pilocarpine to induce epilepsy. The levels of IDO1 and concentrations of TRP and KYN in patients with epilepsy and epileptic mice were evaluated by enzyme-linked immunosorbent assay (ELISA) and liquid chromatography-mass spectrometry (LC-MS), respectively. Then, behavioral phenotypes related to epileptic seizures and neuronal damage were compared between KO and wild-type (WT) mice with lithium-pilocarpine-induced epilepsy. To explore the underlying pathways involved in the effects of IDO1 deficiency, the concentrations of kynurenic acid (KYNA) and quinolinic acid (QUIN), glial cell activation, the levels of major pro-inflammatory cytokines, and antioxidant enzyme activity were measured by LC-MS, immunohistochemistry, and ELISA. Results: In this study, IDO1 levels and the KYN/TRP ratio in the sera and cerebrospinal fluid (CSF) were increased in patients with epilepsy. Also, IDO1 levels, the KYN/TRP ratio, and the levels of pro-inflammatory cytokines in the sera and hippocampi were increased in mice during the acute phase and chronic phase after status epilepticus (SE). Furthermore, IDO1 was localized in microglial cells in epileptic mice. IDO1 deficiency delayed SE onset and attenuated the frequency, duration, and severity of spontaneous recurrent seizures (SRSs). Moreover, IDO1 deficiency improved neuronal survival. Additionally, IDO1^-/- epileptic mice showed progressive declines in QUIN production, glial cell activation and pro-inflammatory cytokines levels, and enhanced antioxidant enzyme activity. Conclusions: IDO1 deletion suppressed seizures and alleviated neuronal damage by reducing the IDO1-dependent production of neurotoxic metabolites, which finally inhibited glial cell activation and pro-inflammatory cytokine production and improved antioxidant enzyme activity. Our study demonstrates that IDO1 may be involved in the pathogenesis of epilepsy and has the potential to be a therapeutic target for epilepsy treatment.

Cuprizone markedly decreases kynurenic acid levels in the rodent brain tissue and plasma

Background

The kynurenine (KYN) pathway (KP) of the tryptophan (TRP) metabolism seems to play a role in the pathomechanism of multiple sclerosis (MS). Cuprizone (CPZ) treated animals develop both demyelination (DEM) and remyelination (REM) in lack of peripheral immune response, such as the lesion pattern type III and IV in MS, representing primary oligodendrogliopathy.

Objective

To measure the metabolites of the KP in the CPZ treated animals, including TRP, KYN and kynurenic acid (KYNA). We proposed that KYNA levels might be decreased in the CPZ-induced demyelinating phase of the animal model of MS, which model represents the progressive phase of the disease.

Methods

A total of 64 C57Bl/6J animals were used for the study. Immunohistochemical (IHC) measurements were performed to prove the effect of CPZ, whereas high-performance liquid chromatography (HPLC) was used to quantify the metabolites of the KP (n = 10/4 groups; DEM, CO1, REM, CO2).

Results

IHC measurements proved the detrimental effects of CPZ. HPLC measurements demonstrated a decrease of KYNA in the hippocampus (p < 0.05), somatosensory cortex (p < 0.01) and in plasma (p < 0.001).

Conclusion

This is the first evidence of marked reduction in KYNA levels in a non-immune mediated model of MS. Our results suggest an involvement of the KP in the pathomechanism of MS, which needs to be further elucidated.

Cerebellar Predominant Increase in mRNA Expression Levels of Sirt1 and Sirt3 Isoforms in a Transgenic Mouse Model of Huntington's Disease

The potential role of Sirt1 and Sirt2 subtypes of Sirtuins (class III NAD⁺-dependent deacetylases) in the pathogenesis of Huntington’s disease (HD) has been extensively studied yielding some controversial results. However, data regarding the involvement of Sirt3 and their variants in HD are considerably limited. The aim of this study was to assess the expression pattern of Sirt1 and three Sirt3 mRNA isoforms (Sirt3-M1/2/3) in the striatum, cortex and cerebellum in respect of the effect of gender, age and the presence of the transgene using the N171-82Q transgenic mouse model of HD. Striatal, cortical and cerebellar Sirt1-Fl and Sirt3-M1/2/3 mRNA levels were measured in 8, 12 and 16 weeks old N171-82Q transgenic mice and in their wild-type littermates. Regarding the striatum and cortex, the presence of the transgene resulted in a significant increase in Sirt3-M3 and Sirt1 mRNA levels, respectively, whereas in case of the cerebellum the transgene resulted in increased expression of all the assessed subtypes and isoforms. Aging exerted minor influence on Sirt mRNA expression levels, both in transgene carriers and in their wild-type littermates, and there was no interaction between the presence of the transgene and aging. Furthermore, there was no difference between genders. The unequivocal cerebellar Sirtuin activation with presumed compensatory role suggests that the cerebellum might be another key player in HD in addition to the most severely affected striatum. The mitochondrially acting Sirt3 may serve as an interesting novel therapeutic target in this deleterious condition.

Indoleamine 2,3-Dioxygenase Activity in Chlamydia muridarum and Chlamydia pneumoniae Infected Mouse Lung Tissues

Chlamydia trachomatis infections are the most prevalent sexually transmitted infections with potentially debilitating sequelae, such as infertility. Mouse models are generally used for vaccine development, to study the immune response and histopathology associated with Chlamydia infection. An important question regarding murine models is the in vivo identification of murine host genes responsible for the elimination of the murine and human Chlamydia strains. RNA sequencing of the Chlamydia muridarum infected BALB/c lung transcriptome revealed that several genes with direct antichlamydial functions were induced at the tissue level, including the already described and novel members of the murine interferon-inducible GTPase family, the CXCL chemokines CXCL9, CXCL11, immunoresponsive gene 1, nitric oxide synthase-2 (iNOS), and lipocalin-2. Indoleamine 2,3-dioxygenase 1-2 (IDO1-2) previously described potent antichlamydial host enzymes were also highly expressed in the infected murine lungs. This finding was novel, since IDO was considered as a unique human antichlamydial defense gene. Besides a lower level of epithelial cell positivity, immunohistochemistry showed that IDO1-2 proteins were expressed prominently in macrophages. Detection of the tryptophan degradation product kynurenine and the impact of IDO inhibition on Chlamydia muridarum growth proved that the IDO1-2 proteins were functionally active. IDO1-2 activity also increased in Chlamydia muridarum infected C57BL/6 lung tissues, indicating that this phenomenon is not mouse strain specific. Our study shows that the murine antichlamydial response includes a variety of highly up-regulated defense genes in vivo. Among these genes the antichlamydial effectors IDO1-2 were identified. The potential impact of murine IDO1-2 expression on Chlamydia propagation needs further investigation.

HPLC method for the assessment of tryptophan metabolism utilizing separate internal standard for each detector

The development of a validated method, applicable for the measurement of tryptophan (TRP) and serotonin (5-HT), and that of the neuroprotective branch of the kynurenine pathway from several different biological matrices, including mouse brain, is described. Following the spectral analysis of the metabolites, they were quantified with reversed-phase high-performance liquid chromatography (HPLC), using separate internal standards (ISs) for UV (3-nitro-L-tyrosine) and fluorescent (the newly utilized 4-hydroxyquinazoline-2-carboxylic acid) detectors. With regard to validation parameters, selectivity, linearity, limit of detection, limit of quantification, precision and recovery were determined. Although the linearity ranges were different for the assessed matrices, the correlation coefficient was >0.999 in each case. Furthermore, good intra- and inter-day precision values were obtained with coefficient of variation <5%, and bias <6.5% (except the 5-HT level in brain samples), respectively. The recoveries varied between 82.5% and 116%. The currently developed methods yield opportunities for the assessment of concentration changes in the TRP metabolism from a wide range of biological matrices, therefore they may well be utilized in future clinical and preclinical studies, especially in view that so many metabolites with the application of ISs have not been detected from mouse brain with such a simple HPLC method before.

Non-motor Behavioral Alterations of PGC-1α-Deficient Mice - A Peculiar Phenotype With Slight Male Preponderance and No Apparent Progression

Dysfunction of peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α) has been linked to various neurodegenerative and neuropsychiatric disorders; however, reports on psychic behavioral alterations on PGC-1α-deficient animals are sparse. The present study revisited prior observations of anxiety-related, depression-related, and hippocampal memory-related observations having been made on different PGC-1α-deficient murine strains, in a large-scale analysis on whole-body full-length (FL-)PGC-1α-deficient mice. The examinations were performed on animals covering a wide age range enrolled from both sexes, and included paradigms such as the open-field, elevated plus maze, light-dark box, tail suspension test, and spatial recognition two-trial Y-maze. The findings revealed no signs of previously reported anxiety-like behavior, but revealed an unexpected phenotype with decreased anxiety behavior consistent throughout different paradigms, with slight male preponderance. This was associated with despair-like anhedonic behavior, consistent with that reported previously, but did not associate with either peripheral or brain alterations in kynurenic acid synthesis, which was previously proposed. Though male FL-PGC-1α-deficient mice tended to perform poorer in the hippocampus-based spatial learning paradigm, the genotype overall was not associated with impairment in spatial memory, contradicting with prior observations. None of the observed alterations deteriorated with age, similarly to motor alterations as reported previously. The most likely contributors of this peculiar phenotype are discussed, with clinicopathological correlations drawn. Being the first to address these behavioral domains within the same PGC-1α-deficient strain, our findings extend the knowledge about the complex in vivo effect of PGC-1α dysfunction and add important notes to research in the field of PGC-1α in connection with neuropsychiatric disorders.

Tolfenamic Acid Attenuates 3-Nitropropionic Acid-Induced Biochemical Alteration in Mice

Tolfenamic acid (TA), a nonsteroidal anti-inflammatory drug, shows neuroprotective effects and alleviates cognitive deficits in transgenic mouse models of Alzheimer's disease. However, whether TA can prevent the biochemical alterations induced by intraperitoneal injection of 3-nitropropionic acid (3-NP) in mice is still unknown. In this study, the striatal lesion area was measured by 2,3,5-triphenyltetrazolium chloride staining. Glutamate, SDH and ATP levels were tested using colorimetric assay kits. The neuroinflammatory cytokine levels were tested by ELISA kits. The expression of synaptic proteins and the subtypes of the NMDA receptor were tested by western blotting. TA was orally administered 10 days before 3-NP injection (pretreatment) or on the same day as 3-NP injection (co-treatment). TA pretreatment showed the strongest neuroprotective effects: pretreatment significantly attenuated the 3-NP-induced muscular weakness in the forelimb and alterations in glutamate level, mitochondrial function, and pro-inflammatory cytokine release in the brains of mice. These results suggest that TA has preventive and protective effects on 3-NP-induced neurotoxicity.

Lack of age-related clinical progression in PGC-1α-deficient mice - implications for mitochondrial encephalopathies

Impaired peroxisome proliferator-activated receptor-gamma coactivator 1-alpha (PGC-1α) function has been demonstrated in several neurodegenerative diseases, and murine whole-body knockouts of PGC-1α have been considered as models for Huntington's disease. Recent neuropathological studies, however, rather propose these animals to be morphological models of mitochondrial encephalopathies, with special reminiscence of Kearns-Sayre syndrome. PGC-1α-deficient animals have already been subjected to behavioral assessments; however, the contradictory findings and the paucity of data assessing long-term progression necessitated further examinations. This study provides a comprehensive neurological phenotypic profiling of full-length-(FL-)PGC-1α-deficient mice in a broad age spectrum, with special focus on previously controversial findings, the issue of long-term phenotypic progression, the histopathological assessment of previously non-characterized tissues of potential clinicopathological relevance, and the gene expression profile of novel brain-specific isoforms of PGC-1α. Our findings demonstrate moderate hypomotility with signs of gait and trunk ataxia in addition to severe impairments in coordination and muscle strength in FL-PGC-1α-deficient mice, phenotypic features consistent of a mitochondrial disease. Intriguingly, however, these early alterations did not progress with age, the understanding of which may unveil mechanisms of potential therapeutic relevance, as discussed. The observed phenotype did not associate with retinal or spinal cord alterations, and was accompanied by mild myopathic changes. Based on these, FL-PGC-1α-deficient mice can be regarded not only as morphological but behavioral models of mitochondrial encephalopathies, with an important temporal limitation that has now been clarified. The mechanisms capable of halting a potentially lethal phenotype are to be unveiled, as they may hold therapeutic value for mitochondrial diseases.