The serum kynurenine pathway metabolic profile is associated with overweight and obesity in multiple sclerosis

Relationship Between Obesity and Depression Considering the Inflammatory Theory

Numerous scientific findings indicate that excess adipose tissue, particularly visceral fat, is associated with a chronic inflammatory state manifested by elevated levels of proinflammatory cytokines and an imbalance in the T helper type 1/type 2 (Th1/Th2) response, which carries numerous metabolic consequences. Obesity induces, among other effects, the activation of the kynurenine pathway and a reduction in serotonin synthesis, alterations in adipokine profiles, modifications of the hypothalamic-pituitary-adrenal (HPA) axis, disturbances in fatty acid ratios, oxidative stress, and dysfunction of the gamma-aminobutyric acid (GABA)ergic system. These neuroimmunological and metabolic disturbances, along with obesity-induced neurotransmission abnormalities that may represent a common underlying model of depression, could provide valuable insights into the pathomechanisms of depression, allowing for prediction of disease progression and individualized therapeutic strategies in overweight patients. Furthermore, the analysis of inflammation-associated biomarkers opens up new therapeutic perspectives, suggesting that interventions aimed at reducing inflammation might lead to potential advances in the treatment of depression.

Vitamin B6 status is related to disease severity and modulated by endurance exercise in individuals with multiple sclerosis: a secondary analysis of a randomized controlled trial

BACKGROUND

Low circulating concentrations of B vitamins are linked to various chronic and neurodegenerative diseases. Notably, pyridoxal 5'-phosphate (vitamin B6) deficiency is linked to altered inflammatory responses and cellular immune function, both critical in multiple sclerosis (MS). Nevertheless, most MS research has focused on folate (vitamin B9) and vitamin B12, leaving other B vitamins understudied.

OBJECTIVES

This secondary analysis investigated B-vitamin serum concentrations and related metabolites across MS phenotypes (primary progressive MS, relapsing-remitting MS, and secondary progressive MS) and disease severity levels. Additionally, the impact of endurance exercise on B-vitamin concentrations was investigated.

METHODS

In total, 106 individuals with MS participated in a randomized controlled trial, including different endurance exercise conditions. Serum B-vitamin concentrations were analyzed in 99 participants before and after 3 wk of intervention. Before analysis, participants were dichotomized to 1 of the 2 disability groups based on their expanded disability status scale (EDSS) score: EDSS≥4.5 (n = 47, EDSS: 5.86 ± 0.56) and EDSS<4 (n = 52, EDSS: 3.59 ± 0.83).

RESULTS

Higher EDSS scores were associated with lower pyridoxal 5'-phosphate (vitamin B-6) concentrations (rs: -0.32; 95% CI: -0.49, -0.12; P = 0.011), with the EDSS≥4.5 group also showing lower baseline pyridoxal 5'-phosphate (vitamin B6) concentrations (β: -0.18; 95% CI: -0.30, -0.07; P = 0.007) than the EDSS<4 group. Significant time × EDSS group interactions were evident for pyridoxal 5'-phosphate (vitamin B6; β: 0.05; 95% CI: 0.02, 0.08; P = 0.011), pyridoxal (vitamin B6; β: 0.05; 95% CI: 0.02, 0.09; P = 0.005), and riboflavin (vitamin B2; β: 0.06; 95% CI: 0.02, 0.09; P = 0.008), showing increases in these vitamers in the EDSS≥4.5 group postexercise. N1-Methylnicotinamide (vitamin B3; β: -0.11; 95% CI: -0.15, -0.06; P < 0.001) decreased in both groups over time.

CONCLUSIONS

Disease severity is associated with distinct B-vitamin profiles in individuals with MS, although endurance exercise appears to modify specific B-vitamin concentrations. This trial was registered at clinicaltrials.gov as NCT04356248.

Kynurenine pathway dysregulation as a mechanistic link between cognitive impairment and brain damage: Implications for multiple sclerosis

Cognitive impairment is a core symptom of multiple sclerosis (MS), resulting from inflammation-related brain damage and brain network dysfunction. Inflammation also causes dysregulation of the kynurenine pathway, which is the primary route of tryptophan metabolism. Kynurenine pathway dysregulation is characterised by a shift in concentrations of tryptophan catabolites, also referred to as kynurenines. Some kynurenines have neurotoxic effects that partly resemble the molecular mechanisms of MS pathophysiology underpinning brain damage and brain network dysfunction. The kynurenine pathway may therefore qualify as a mechanistic link between systemic inflammation, brain damage, and cognitive impairment in MS. This perspective article (1) provides an overview of inflammation-related kynurenine pathway dysregulation and MS-relevant neuroimmune properties of kynurenines and (2) summarises the current evidence on associations between systemic kynurenines, imaging metrics of brain structure or related markers, and cognitive performance in populations that present with kynurenine pathway dysregulation and are prone to cognitive impairment. These findings are used to (3) set a research agenda for future studies aimed at clarifying the role of the kynurenine pathway in brain damage and cognitive impairment in MS.

Constitutive loss of kynurenine-3-monooxygenase changes circulating kynurenine metabolites without affecting systemic energy metabolism

Kynurenic acid (KYNA) and quinolinic acid (QUIN) are metabolites of the kynurenine pathway of tryptophan degradation with opposing biological activities in the central nervous system. In the periphery, KYNA is known to positively affect metabolic health, whereas the effects of QUIN remain less explored. Interestingly, metabolic stressors, including exercise and obesity, differentially change the balance between circulating KYNA and QUIN. Here, we hypothesized that chronically elevated levels of circulating KYNA and reduced levels of QUIN would manifest as differences in whole body energy metabolism. To test this, we used a mouse model lacking the enzyme kynurenine 3-monooxygenase (KMO), thus shunting kynurenine away from QUIN synthesis and toward KYNA production. KMO-deficient and wild-type littermate male and female mice were evaluated under chow and high-fat diets. Comprehensive kynurenine pathway metabolite profiling in plasma showed that the loss of KMO elicits robust changes in circulating levels of kynurenine metabolites. This included a 45-fold increase in kynurenine, a 26-fold increase in KYNA, and a 99% decrease in QUIN levels, depending on the diet. However, despite these changes, loss of KMO did not significantly impact whole body energy metabolism or change the transcriptomic profile of subcutaneous adipose tissue on either diet. With KMO inhibitors being considered therapeutic candidates for various disorders, this work shows that chronic systemic KMO inhibition does not have widespread metabolic effects. Our data also indicate that the beneficial effects of KYNA on metabolism may depend on its acute, intermittent elevation in circulation, akin to transient exercise-induced signals that mediate improved metabolic health.NEW & NOTEWORTHY The kynurenine pathway of tryptophan degradation is influenced by metabolic stressors: exercise raises circulating KYNA levels, while obesity is linked to increased QUIN. We investigated whether a mouse model lacking KMO-leading to increased circulating KYNA and decreased QUIN-would exhibit changes in energy metabolism. We found that energy metabolism was largely unaffected despite robust changes in circulating kynurenine metabolites, suggesting that systemic KMO inhibition may not have widespread metabolic effects.

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.

Alteration in kynurenine pathway metabolites in young women with autoimmune thyroiditis

The kynurenine pathway (KP) of tryptophan degradation includes several compounds that reveal immunomodulatory properties. The present study aimed to investigate the alteration in KP metabolites in young women with autoimmune thyroiditis (AIT) and their associations with thyroid function. The thyroid function tests, antithyroid antibodies measurement and ultrasonography of the thyroid gland have been performed in 57 young women with AIT and 38 age-matched healthy controls. The serum levels of tryptophan, kynurenine (KYN) and its metabolites were determined, and the activity of KP enzymes was calculated indirectly as product-to-substrate ratios. KP was activated and dysregulated in AIT, along with significantly elevated levels of KYN and anthranilic acid (AA), at the expense of the reduction of kynurenic acid (KYNA), which was reflected by the increase in the AA/KYNA ratio (p < 0.001). In univariate and multiple regression analyses, peripheral deiodinase (SPINA-GD) activity in AIT was positively associated with KYNA, AA, and quinolinic acid (QA). The merger of AA, AA/KYNA ratio, QA and SPINA-GD exhibited the highest sensitivity and specificity to predict AIT (p < 0.001) in receiver operating characteristic (ROC) analysis. In conclusion, the serum KYN metabolite profile is dysregulated in young women with AIT and could serve as a new predictor of AIT risk.