Kynurenic acid and kynurenine aminotransferases in retinal aging and neurodegeneration

Customized flexible platform - starting point for the development of wearable sensor for the direct electrochemical detection of kynurenic acid in biological samples

Kynurenic acid (KA) is an active metabolite of tryptophan with notable biological effects, such as antioxidant, neuroprotective, and anti-inflammatory properties. It often undergoes changes of the concentration in biological fluids in chronic diseases. Thus, detecting KA is of great importance for diagnosing inflammatory and neurodegenerative conditions, monitoring disease progression, and assessing responses to pharmacological treatment. This study aimed to design a tailored, flexible platform for sensitive and direct electrochemical detection of KA in biological fluids. Carbon-based electrodes were custom-printed in the lab using specialized inks and flexible substrates. The working electrodes were further functionalized with graphene oxide and subsequently electrochemically reduced to increase the sensitivity toward the analyte. An optimized differential pulse voltammetry protocol was developed for KA detection. The elaborated platform was firstly characterized and then evaluated regarding the analytical performances. It showed a good limit of detection (3 nM and demonstrated the capability to detect KA across a broad concentration range (0.01-500 μM). Finally, the elaborated flexible platform, was succesfully applied for KA determination in serum and saliva samples, in comparison with an optimized HPLC-UV method. The developed platform is the first example of in-lab printed flexible platform reported in literature so far for KA detection. It is also the first study reported in the literature of detection of KA in raw saliva collected from 10 subjects. The sensitivity towards the target analyte, coupled with the adaptability and portability, showcases the potential of this platform for thus illustrating great potential for further development of wearable sensors and biomedical applications.

The Complex World of Kynurenic Acid: Reflections on Biological Issues and Therapeutic Strategy

It has been unequivocally established that kynurenic acid has a number of actions in a variety of cells and tissues, raising, in principle, the possibility of targeting its generation, metabolism or sites of action to manipulate those effects to a beneficial therapeutic end. However, many basic aspects of the biology of kynurenic acid remain unclear, potentially leading to some confusion and misinterpretations of data. They include questions of the source, generation, targets, enzyme expression, endogenous concentrations and sites of action. This essay is intended to raise and discuss many of these aspects as a source of reference for more balanced discussion. Those issues are followed by examples of situations in which modulating and correcting kynurenic acid production or activity could bring significant therapeutic benefit, including neurological and psychiatric conditions, inflammatory diseases and cell protection. More information is required to obtain a clear overall view of the pharmacological environment relevant to kynurenic acid, especially with respect to the active concentrations of kynurenine metabolites in vivo and changed levels in disease. The data and ideas presented here should permit a greater confidence in appreciating the sites of action and interaction of kynurenic acid under different local conditions and pathologies, enhancing our understanding of kynurenic acid itself and the many clinical conditions in which manipulating its pharmacology could be of clinical value.

New insights into healthy ageing, inflammageing and frailty using metabolomics

Human ageing is a normal process and does not necessarily result in the development of frailty. A mix of genetic, environmental, dietary, and lifestyle factors can have an impact on ageing, and whether an individual develops frailty. Frailty is defined as the loss of physiological reserve both at the physical and cellular levels, where systemic processes such as oxidative stress and inflammation contribute to physical decline. The newest "omics" technology and systems biology discipline, metabolomics, enables thorough characterisation of small-molecule metabolites in biological systems at a particular time and condition. In a biological system, metabolites-cellular intermediate products of metabolic reactions-reflect the system's final response to genomic, transcriptomic, proteomic, epigenetic, or environmental alterations. As a relatively newer technique to characterise metabolites and biomarkers in ageing and illness, metabolomics has gained popularity and has a wide range of applications. We will give a comprehensive summary of what is currently known about metabolomics in studies of ageing, with a focus on biomarkers for frailty. Metabolites related to amino acids, lipids, carbohydrates, and redox metabolism may function as biomarkers of ageing and/or frailty development, based on data obtained from human studies. However, there is a complexity that underpins biological ageing, due to both genetic and environmental factors that play a role in orchestrating the ageing process. Therefore, there is a critical need to identify pathways that contribute to functional decline in people with frailty.

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.

Kynurenine pathway, NAD+ synthesis, and mitochondrial function: Targeting tryptophan metabolism to promote longevity and healthspan

Aging is characterized by a progressive decline in the normal physiological functions of an organism, ultimately leading to mortality. Nicotinamide adenine dinucleotide (NAD⁺) is an essential cofactor that plays a critical role in mitochondrial energy production as well as many enzymatic redox reactions. Age-associated decline in NAD⁺ is implicated as a driving factor in several categories of age-associated disease, including metabolic and neurodegenerative disease, as well as deficiency in the mechanisms of cellular defense against oxidative stress. The kynurenine metabolic pathway is the sole de novo NAD⁺ biosynthetic pathway, generating NAD⁺ from ingested tryptophan. Altered kynurenine pathway activity is associated with both aging and a variety of age-associated diseases. Kynurenine pathway interventions can extend lifespan in both fruit flies and nematodes, and altered NAD⁺ metabolism represents one potential mediating mechanism. Recent studies demonstrate that supplementation with NAD⁺ or NAD⁺-precursors increase longevity and promote healthy aging in fruit flies, nematodes, and mice. NAD⁺ levels and the intrinsic relationship to mitochondrial function have been widely studied in the context of aging. Mitochondrial function and dynamics have both been implicated in longevity determination in a range of organisms from yeast to humans, at least in part due to their intimate link to regulating an organism's cellular energy economy and capacity to resist oxidative stress. Recent findings support the idea that complex communication between the mitochondria and the nucleus orchestrates a series of events and stress responses involving mitophagy, mitochondrial number, mitochondrial unfolded protein response (UPR^mt), and mitochondria fission and fusion events. In this review, we discuss how mitochondrial morphological changes and dynamics operate during aging, and how altered metabolism of tryptophan to NAD⁺ through the kynurenine pathway interacts with these processes.

Tryptophan Metabolism: A Versatile Area Providing Multiple Targets for Pharmacological Intervention

The essential amino acid L-tryptophan (Trp) undergoes extensive metabolism along several pathways, resulting in production of many biologically active metabolites which exert profound effects on physiological processes. The disturbance in Trp metabolism and disposition in many disease states provides a basis for exploring multiple targets for pharmaco-therapeutic interventions. In particular, the kynurenine pathway of Trp degradation is currently at the forefront of immunological research and immunotherapy. In this review, I shall consider mammalian Trp metabolism in health and disease and outline the intervention targets. It is hoped that this account will provide a stimulus for pharmacologists and others to conduct further studies in this rich area of biomedical research and therapeutics.

Increased IL-6 trans-signaling in depression: focus on the tryptophan catabolite pathway, melatonin and neuroprogression

Depression has been conceptualized as a disorder driven by immuno-inflammatory pathways and oxidative and nitrosative stress. These factors couple to the induction of neuroregulatory tryptophan catabolites via the activation of indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO). Oxidative damage to neoepitopes increases autoimmune responses, changing the nature of the neural substrate of recurrent depression, which leads to neuroprogression and drives treatment resistance. A number of pro-inflammatory cytokines are linked to these processes. Here, we focus on the role of interleukin (IL)-6 in depression and its associated disorders; we highlight the progress made since the first paper showing increased IL-6 levels was published 20 years ago by Maes and colleagues. When coupled with increased levels of the soluble IL-6 receptor in depression, higher levels of IL-6 may indicate increased IL-6 trans-signaling, whereby IL-6 receptor signaling occurs in cells not normally expressing the IL-6 receptor. It has been suggested that IL-6 is intimately associated with two crucial aspects of depression, as well as central inflammation more broadly. First, the regulation of the local inflammatory response via its interactions with macrophage and glia melatonin production is coupled to local epigenetic modulation via methyl CpG-binding protein 2 (MeCP2). Second, the more systemic regulation of tryptophan availability occurs via the IL-6 induction of IDO. Coupled to its role in the regulation of autoimmune associated T-helper 17 cells and IL-17 production, IL-6 has wide and differential impacts on processes driving depression and a wider range of psychiatric and neurodegenerative conditions.