Mitochondria, metabolic disturbances, oxidative stress and the kynurenine system, with focus on neurodegenerative disorders

Abstract

The mitochondria have several important functions in the cell. A mitochondrial dysfunction causes an abatement in ATP production, oxidative damage and the induction of apoptosis, all of which are involved in the pathogenesis of numerous disorders. This review focuses on mitochondrial dysfunctions and discusses their consequences and potential roles in the pathomechanism of neurodegenerative disorders. Other pathogenetic factors are also briefly surveyed. The second part of the review deals with the kynurenine metabolic pathway, its alterations and their potential association with cellular energy impairment in certain neurodegenerative diseases.

During energy production, most of the O₂ consumed by the mitochondria is reduced fully to water, but 1–2% of the O₂ is reduced incompletely to give the superoxide anion (O₂⁻). If the function of one or more respiratory chain complexes is impaired for any reason, the enhanced production of free radicals further worsens the mitochondrial function by causing oxidative damage to macromolecules, and by opening the mitochondrial permeability transition pores thereby inducing apoptosis. These high-conductance pores offer a pathway which can open in response to certain stimuli, leading to the induction of the cells' own suicide program. This program plays an essential role in regulating growth and development, in the differentiation of immune cells, and in the elimination of abnormal cells from the organism. Both failure and exaggeration of apoptosis in a human body can lead to disease. The increasing amount of superoxide anions can react with nitric oxide to yield the highly toxic peroxynitrite anion, which can destroy cellular macromolecules. The roles of oxidative, nitrative and nitrosative damage are discussed. Senescence is accompanied by a higher degree of reactive oxygen species production, and by diminished functions of the endoplasmic reticulum and the proteasome system, which are responsible for maintenance of the normal protein homeostasis of the cell. In the event of a dysfunction of the endoplasmic reticulum, unfolded proteins aggregate in it, forming potentially toxic deposits which tend to be resistant to degradation. Cells possess adaptive mechanisms with which to avoid the accumulation of incorrectly folded proteins. These involve molecular chaperones that fold proteins correctly, and the ubiquitin proteasome system which degrades misfolded, unwanted proteins. Both the endoplasmic reticulum and the ubiquitin proteasome system fulfill cellular protein quality control functions.

The kynurenine system: Tryptophan is metabolized via several pathways, the main one being the kynurenine pathway. A central compound of the pathway is kynurenine (KYN), which can be metabolized in two separate ways: one branch furnishing kynurenic acid, and the other 3-hydroxykynurenine and quinolinic acid, the precursors of NAD. An important feature of kynurenic acid is the fact that it is one of the few known endogenous excitatory amino acid receptor blockers with a broad spectrum of antagonistic properties in supraphysiological concentrations. One of its recently confirmed sites of action is the α7-nicotinic acetylcholine receptor and interestingly, a more recently identified one is a higher affinity positive modulatory binding site at the AMPA receptor. Kynurenic acid has proven to be neuroprotective in several experimental settings. On the other hand, quinolinic acid is a specific agonist at the N-methyl-d-aspartate receptors, and a potent neurotoxin with an additional and marked free radical-producing property. There are a number of neurodegenerative disorders whose pathogenesis has been demonstrated to involve multiple imbalances of the kynurenine pathway metabolism. These changes may disturb normal brain function and can add to the pathomechanisms of the diseases. In certain disorders, there is a quinolinic acid overproduction, while in others the alterations in brain kynurenic acid levels are more pronounced. A more precise knowledge of these alterations yields a basis for getting better therapeutic possibilities. The last part of the review discusses metabolic disturbances and changes in the kynurenine metabolic pathway in Parkinson's, Alzheimer's and Huntington's diseases.

Introduction

The mitochondria are responsible for the energy supply of cells. The brain is the organ that uses the most energy in the human body, accounting for 20% of the total oxygen consumption despite accounting for only 2% of the total body mass [1]. The mitochondria play crucial roles in other cell processes too, the most important of which being their roles in signaling processes, calcium homeostasis, cell cycle regulation, apoptosis, free radical production and thermogenesis. They are key components in the processes of development, ageing and cell death (both apoptotic and necrotic).

A mitochondrial dysfunction and oxidative damage play roles in the pathogenesis of numerous disorders, e.g. Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), Wilson's disease, Friedreich's ataxia, multiple sclerosis and a number of inherited disorders of the mitochondrial genome, the mitochondrial encephalomyopathies (e.g. Leber's disease with optic atrophy and dystonia, MELAS, MERRF, Leigh's disease, Kearns–Sayre syndrome) [2], [3], [4], [5]. The list of mitochondria-related diseases is growing rapidly: cancer, heart failure, diabetes, obesity, ischemia-reperfusion injury, atherosclerosis, certain liver diseases and asbestosis. They all share the common features of disturbances of the mitochondrial Ca²⁺, ATP or reactive oxygen species (ROS) metabolism [6].