Acetylcarnitine and cellular stress response: roles in nutritional redox homeostasis and regulation of longevity genes

Aging is associated with a reduced ability to cope with physiological challenges. Although the mechanisms underlying age-related alterations in stress tolerance are not well defined, many studies support the validity of the oxidative stress hypothesis, which suggests that lowered functional capacity in aged organisms is the result of an increased generation of reactive oxygen and nitrogen species. Increased production of oxidants in vivo can cause damage to intracellular macromolecules, which can translate into oxidative injury, impaired function and cell death in vulnerable tissues such as the brain. To survive different types of injuries, brain cells have evolved networks of responses, which detect and control diverse forms of stress. This is accomplished by a complex network of the so-called longevity assurance processes, which are composed of several genes termed vitagenes. Among these, heat shock proteins form a highly conserved system responsible for the preservation and repair of the correct protein conformation. The heat shock response contributes to establishing a cytoprotective state in a wide variety of human diseases, including inflammation, cancer, aging and neurodegenerative disorders. Given the broad cytoprotective properties of the heat shock response, there is now a strong interest in discovering and developing pharmacological agents capable of inducing the heat shock response. Acetylcarnitine is proposed as a therapeutic agent for several neurodegenerative disorders, and there is now evidence that it may play a critical role as modulator of cellular stress response in health and disease states. In the present review, we first discuss the role of nutrition in carnitine metabolism, followed by a discussion of carnitine and acetyl-l-carnitine in mitochondrial dysfunction, in aging, and in age-related disorders. We then review the evidence for the role of acetylcarnitine in modulating redox-dependent mechanisms leading to up-regulation of vitagenes in brain, and we also discuss new approaches for investigating the mechanisms of lifetime survival and longevity.     

Age related mitochondrial degenerative disorders in humans

Disorders caused by mitochondrial respiratory chain deficiency due to mutations in mitochondrial DNA have varied phenotypes but many involve neurological features often associated with cell loss within specific brain regions. These disorders, along with the increasing evidence of decline in mitochondrial function with ageing, have raised speculation that primary changes in mitochondria could have an important role in age-related neurodegenerative diseases such as Parkinson's disease (PD) and Alzheimer's disease (AD). Evidence supporting a role for mitochondria in common neurodegenerative diseases comes from studies with the toxin MPP+ and familial PD, which has been shown to involve proteins such as DJ-1 and Pink1 (both of which are predicted to have a role in mitochondrial function and oxidative stress). Mutations within the mitochondrial genome have been shown to accumulate with age and in common neurodegenerative diseases. Mitochondrial DNA haplogroups have also been shown to be associated with certain neurodegenerative conditions. This review covers the primary mitochondrial diseases but also discuss the potential role of mitochondria and mitochondrial DNA mutations in mitochondrial and neurodegenerative diseases, in particular in PD and in AD.     

Peripheral Mitochondrial Dysfunction in Alzheimer’s Disease: Focus on Lymphocytes

Alzheimer's disease (AD) is the most common progressive neurodegenerative disease. Today, AD affects millions of people worldwide and the number of AD cases will increase with increased life expectancy. The AD brain is marked by severe neurodegeneration like the loss of synapses and neurons, atrophy and depletion of neurotransmitter systems in the hippocampus and cerebral cortex. Recent findings suggest that these pathological changes are causally induced by mitochondrial dysfunction and increased oxidative stress. These changes are not only observed in the brain of AD patients but also in the periphery. In this review, we discuss the potential role of elevated apoptosis, increased oxidative stress and especially mitochondrial dysfunction as peripheral markers for the detection of AD in blood cells especially in lymphocytes. We discuss recent not otherwise published findings on the level of complex activities of the respiratory chain comprising mitochondrial respiration and the mitochondrial membrane potential (MMP). We obtained decreased basal MMP levels in lymphocytes from AD patients as well as enhanced sensitivity to different complex inhibitors of the respiratory chain. These changes are in line with mitochondrial defects obtained in AD cell and animal models, and in post-mortem AD tissue. Importantly, these mitochondrial alterations where not only found in AD patients but also in patients with mild cognitive impairment (MCI). These new findings point to a relevance of mitochondrial function as an early peripheral marker for the detection of AD and MCI.     

Mitochondrial involvement in Parkinson's disease, Huntington's disease, hereditary spastic paraplegia and Friedreich's ataxia

Respiratory chain dysfunction has been identified in several neurodegenerative disorders. In Friedreich's ataxia (FA) and Huntington's disease (HD), where the respective mutations are in nuclear genes encoding non-respiratory chain mitochondrial proteins, the defects in oxidative phosphorylation are clearly secondary. In Parkinson's disease (PD) the situation is less clear, with some evidence for a primary role of mitochondrial DNA in at least a proportion of patients. The pattern of the respiratory chain defect may provide some clue to its cause; in PD there appears to be a selective complex I deficiency; in HD and FA the deficiencies are most severe in complex II/III with a less severe defect in complex IV. Aconitase activity in HD and FA is severely decreased in brain and muscle, respectively, but appears to be normal in PD brain. Free radical generation is thought to be of importance in both HD and FA, via excitotoxicity in HD and abnormal iron handling in FA. The oxidative damage observed in PD may be secondary to the mitochondrial defect. Whatever the cause(s) and sequence of events, respiratory chain deficiencies appear to play an important role in the pathogenesis of neurodegeneration. The mitochondrial abnormalities induced may converge on the function of the mitochondrion in apoptosis. This mode of cell death is thought to play an important role in neurodegenerative diseases and it is tempting to speculate that the observed mitochondrial defects in PD, HD and FA result directly in apoptotic cell death, or in the lowering of a cell's threshold to undergo apoptosis. Clarifying the role of mitochondria in pathogenesis may provide opportunities for the development of treatments designed to reverse or prevent neurodegeneration.     

Mitochondrial quality control: a matter of life and death for neurons

Neuronal survival critically depends on the integrity and functionality of mitochondria. A hierarchical system of cellular surveillance mechanisms protects mitochondria against stress, monitors mitochondrial damage and ensures the selective removal of dysfunctional mitochondrial proteins or organelles. Mitochondrial proteases emerge as central regulators that coordinate different quality control (QC) pathways within an interconnected network of mechanisms. A failure of this system causes neuronal loss in a steadily increasing number of neurodegenerative disorders, which include Parkinson's disease, spinocerebellar ataxia, spastic paraplegia and peripheral neuropathies. Here, we will discuss the role of the mitochondrial QC network for neuronal survival and neurodegeneration.     

Mitochondria and Neurodegeneration

Many lines of evidence suggest that mitochondria have a central role in ageing-related neurodegenerative diseases. However, despite the evidence of morphological, biochemical and molecular abnormalities in mitochondria in various tissues of patients with neurodegenerative disorders, the question “is mitochondrial dysfunction a necessary step in neurodegeneration?” is still unanswered. In this review, we highlight some of the major neurodegenerative disorders (Alzheimer’s disease, Parkinson’s disease, Amyotrophic lateral sclerosis and Huntington’s disease) and discuss the role of the mitochondria in the pathogenetic cascade leading to neurodegeneration.     

Mitophagy, mitochondrial dynamics and the general stress response in yeast

Autophagy is a fundamental cellular process promoting survival under various environmental stress conditions. Selective types of autophagy have gained much interest recently as they are involved in specific quality control mechanisms removing, for example, aggregated proteins or dysfunctional mitochondria. This is considered to counteract the development of a number of neurodegenerative disorders and aging. Here we review the role of mitophagy and mitochondrial dynamics in ensuring quality control of mitochondria. In particular, we provide possible explanations why mitophagy in yeast, in contrast with the situation in mammals, was found to be independent of mitochondrial fission. We further discuss recent findings linking these processes to nutrient sensing pathways and the general stress response in yeast. In particular, we propose a model for how the stress response protein Whi2 and the Ras/PKA (protein kinase A) signalling pathway are possibly linked and thereby regulate mitophagy.     

The role of mitochondria in inherited neurodegenerative diseases

In the past decade, the genetic causes underlying familial forms of many neurodegenerative disorders, such as Huntington's disease, Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, Friedreich ataxia, hereditary spastic paraplegia, dominant optic atrophy, Charcot-Marie-Tooth type 2A, neuropathy ataxia and retinitis pigmentosa, and Leber's hereditary optic atrophy have been elucidated. However, the common pathogenic mechanisms of neuronal death are still largely unknown. Recently, mitochondrial dysfunction has emerged as a potential 'lowest common denominator' linking these disorders. In this review, we discuss the body of evidence supporting the role of mitochondria in the pathogenesis of hereditary neurodegenerative diseases. We summarize the principal features of genetic diseases caused by abnormalities of mitochondrial proteins encoded by the mitochondrial or the nuclear genomes. We then address genetic diseases where mutant proteins are localized in multiple cell compartments, including mitochondria and where mitochondrial defects are likely to be directly caused by the mutant proteins. Finally, we describe examples of neurodegenerative disorders where mitochondrial dysfunction may be 'secondary' and probably concomitant with degenerative events in other cell organelles, but may still play an important role in the neuronal decay. Understanding the contribution of mitochondrial dysfunction to neurodegeneration and its pathophysiological basis will significantly impact our ability to develop more effective therapies for neurodegenerative diseases.     

Endogenous mitochondrial oxidative stress: neurodegeneration, proteomic analysis, specific respiratory chain defects, and efficacious antioxidant therapy in superoxide dismutase 2 null mice

Oxidative stress and mitochondrial dysfunction have been linked to neurodegenerative disorders such as Parkinson's and Alzheimer's disease. However, it is not yet understood how endogenous mitochondrial oxidative stress may result in mitochondrial dysfunction. Most prior studies have tested oxidative stress paradigms in mitochondria through either chemical inhibition of specific components of the respiratory chain, or adding an exogenous insult such as hydrogen peroxide or paraquat to directly damage mitochondria. In contrast, mice that lack mitochondrial superoxide dismutase (SOD2 null mice) represent a model of endogenous oxidative stress. SOD2 null mice develop a severe neurological phenotype that includes behavioral defects, a severe spongiform encephalopathy, and a decrease in mitochondrial aconitase activity. We tested the hypothesis that specific components of the respiratory chain in the brain were differentially sensitive to mitochondrial oxidative stress, and whether such sensitivity would lead to neuronal cell death. We carried out proteomic differential display and examined the activities of respiratory chain complexes I, II, III, IV, V, and the tricarboxylic acid cycle enzymes alpha-ketoglutarate dehydrogenase and citrate synthase in SOD2 null mice in conjunction with efficacious antioxidant treatment and observed differential sensitivities of mitochondrial proteins to oxidative stress. In addition, we observed a striking pattern of neuronal cell death as a result of mitochondrial oxidative stress, and were able to significantly reduce the loss of neurons via antioxidant treatment.     

Neuropathological Aspects of Mitochondrial DNA Disease

Mitochondrial DNA disorders are an important cause of neurological disease, yet despite our awareness of the importance of these conditions, relatively little is known about the neuropathology of these disorders and even less about the mechanisms involved in neuronal dysfunction and death. In this review we detail important features from neuropathological studies available and highlight deficiencies that are currently limiting our understanding of mitochondrial DNA disease. We also discuss possible future approaches that might resolve some of these outstanding issues. Further study of these disorders is critical because mitochondria play a central role in neuronal survival and it is likely that an understanding of the mechanisms involved in neuronal dysfunction and cell death in mitochondrial DNA disease may have implications for other neurodegenerative diseases.     

Endomorphins and morphine limit anoxia-reoxygenation-induced brain mitochondrial dysfunction in the mouse

The protection of brain mitochondria from oxidative stress is an important therapeutic strategy against ischemia-reperfusion injury and neurodegenerative disorders. Isolated brain mitochondria subjected to a 5 min period of anoxia followed by 5 min reoxygenation mirrored the effect of oxidative stress in the brain. The present study attempts to evaluate the protective effects of endomorphin 1 (EM1), endomorphin 2 (EM2), and morphine (Mor) in an in vitro mouse brain mitochondria anoxia-reoxygenation model. Endomorphins (EM1/2) and Mor were added to mitochondria prior to anoxia or reoxygenation. EM1/2 and Mor markedly improved mitochondrial respiratory activity with a decrease in state 4 and increases in state 3, respiratory control ratio (RCR) and the oxidative phosphorylation efficiency (ADP/O ratio), suggesting that they may play a protective role in mitochondria. These drugs inhibited alterations in mitochondrial membrane fluidity, lipoperoxidation, and cardiolipin (CL) release, which indicates protection of the mitochondrial membranes from oxidative damage. The protective effects of these drugs were concentration-dependent. Furthermore, these drugs blocked the enhanced release of cytochrome c (Cyt c), and consequently inhibited the cell apoptosis induced by the release of Cyt c. Our results suggest that EM1/2 and Mor effectively protect brain mitochondria against oxidative stresses induced by in vitro anoxia-reoxygenation and may play an important role in the prevention of deleterious effects during brain ischemia-reperfusion and neurodegenerative diseases.     

Autophagy of Mitochondria: A Promising Therapeutic Target for Neurodegenerative Disease

The autophagic process is the only known mechanism for mitochondrial turnover and it has been speculated that dysfunction of autophagy may result in mitochondrial error and cellular stress. Emerging investigations have provided new understanding of how autophagy of mitochondria (also known as mitophagy) is associated with cellular oxidative stress and its impact on neurodegeneration. This impaired autophagic function may be considered as a possible mechanism in the pathogenesis of several neurodegenerative disorders including Parkinson’s disease, Alzheimer’s disease, multiple sclerosis, amyotrophic lateral sclerosis, and Huntington disease. It can be suggested that autophagy dysfunction along with oxidative stress is considered main events in neurodegenerative disorders. New therapeutic approaches have now begun to target mitochondria as a potential drug target. This review discusses evidence supporting the notion that oxidative stress and autophagy are intimately associated with neurodegenerative disease pathogenesis. This review also explores new approaches that can prevent mitochondrial dysfunction, improve neurodegenerative etiology, and also offer possible cures to the aforementioned neurodegenerative diseases.     

Sex-dependent differences in aged rat brain mitochondrial function and oxidative stress

Females show lower incidences of several neurodegenerative diseases related to oxidative stress and mitochondrial dysfunction than males. In addition, female rats show more differentiated mitochondria than males in several tissues. The aim of this work was to investigate the existence of sex-dependent differences in brain mitochondrial bioenergetics and oxidative balance in aged rats. Results showed that aged female rat brain had a lower mitochondria content than aged male brain but with a greater differentiation degree given the higher mitochondrial protein content and mitochondrial complex activities in females. Female rat brain also showed a better oxidative balance than that of males, reflected by the fact that higher mitochondrial respiratory chain function is accompanied by a similar ROS production and greater antioxidant enzyme activities, which could be responsible for the lesser oxidative damage observed in proteins and lipids in this sex. Interestingly, levels of UCP4 and UCP5--proteins related to a decrease in ROS production--were also higher in females. In conclusion, aged female rat brain had more differentiated mitochondria than male brain and showed a better control of oxidative stress balance, which could be due, in part, to the neuroprotective effect of UCPs.     

Mitochondrial Ca(2+) and neurodegeneration

Mitochondria are essential for ensuring numerous fundamental physiological processes such as cellular energy, redox balance, modulation of Ca(2+) signaling and important biosynthetic pathways. They also govern the cell fate by participating in the apoptosis pathway. The mitochondrial shape, volume, number and distribution within the cells are strictly controlled. The regulation of these parameters has an impact on mitochondrial function, especially in the central nervous system, where trafficking of mitochondria is critical to their strategic intracellular distribution, presumably according to local energy demands. Thus, the maintenance of a healthy mitochondrial population is essential to avoid the impairment of the processes they regulate: for this purpose, cells have developed mechanisms involving a complex system of quality control to remove damaged mitochondria, or to renew them. Defects of these processes impair mitochondrial function and lead to disordered cell function, i.e., to a disease condition. Given the standard role of mitochondria in all cells, it might be expected that their dysfunction would give rise to similar defects in all tissues. However, damaged mitochondrial function has pleiotropic effects in multicellular organisms, resulting in diverse pathological conditions, ranging from cardiac and brain ischemia, to skeletal muscle myopathies to neurodegenerative diseases. In this review, we will focus on the relationship between mitochondrial (and cellular) derangements and Ca(2+) dysregulation in neurodegenerative diseases, emphasizing the evidence obtained in genetic models. Common patterns, that recognize the derangement of Ca(2+) and energy control as a causative factor, have been identified: advances in the understanding of the molecular regulation of Ca(2+) homeostasis, and on the ways in which it could become perturbed in neurological disorders, may lead to the development of therapeutic strategies that modulate neuronal Ca(2+) signaling.     

Mitochondrial control of neuron death and its role in neurodegenerative disorders

Genetic or functional mitochondrial alterations can result in the initiation of cell death programs that are believed to contribute to cell death in diabetes, ageing and neurodegenerative disorders. Mitochondria are being considered the main link between cellular stress signals activated during acute and chronic nerve cell injury, and the execution of nerve cell death. This second function of mitochondria is regulated by several families of proteins that can trigger an increase in permeability of the outer and/or inner mitochondrial membrane. One example of this is the formation of the mitochondrial permeability transition pore (MPTP). This process can trigger the release of cell death-inducing factors from mitochondria, as well as a dissipation of the mitochondrial transmembrane potential, depletion of ATP, and increased free radical formation. Among the factors released from mitochondria are cytochrome c, the apoptosis inductor factor (AIF), and caspases. We review the role of the MPTP in diverse physiological and pathological processes, including neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis (ALS). The design of drugs that could interfere with the functions of the MPTP could allow novel therapeutic approaches for the treatment of acute and chronic nerve cell injury.     

Mitochondria as targets for chemotherapy

Mitochondrial malfunctioning is implicated in the pathogenesis of a variety of disorders, including cancer and multiple neurodegenerative diseases, such as Parkinson’s disease, Alzheimer’s disease, amyotrophic lateral sclerosis, and Huntington’s disease. Disturbance of mitochondrial vital functions, e.g., production of ATP, calcium buffering capacity, and generation of reactive oxygen species, can be potentially involved in disease pathogenesis. Neurological disorders caused by mitochondrial deterioration are often associated with cell loss within specific brain regions. In contrast, mitochondrial alterations in tumor cells and the “Warburg effect” might lead to cell survival and resistance of tumor cells to chemotherapy. This review is devoted to the role of mitochondria in neurodegeneration and tumor formation, and describes how targeting of mitochondria can be beneficial in the therapy of these diseases, which affect a large human population.     

Understanding Miro GTPases: Implications in the Treatment of Neurodegenerative Disorders

The Miro GTPases represent an unusual subgroup of the Ras superfamily and have recently emerged as important mediators of mitochondrial dynamics and for maintaining neuronal health. It is now well-established that these enzymes act as essential components of a Ca²⁺-sensitive motor complex, facilitating the transport of mitochondria along microtubules in several cell types, including dopaminergic neurons. The Miros appear to be critical for both anterograde and retrograde mitochondrial transport in axons and dendrites, both of which are considered essential for neuronal health. Furthermore, the Miros may be significantly involved in the development of several serious pathological processes, including the development of neurodegenerative and psychiatric disorders. In this review, we discuss the molecular structure and known mitochondrial functions of the Miro GTPases in humans and other organisms, in the context of neurodegenerative disease. Finally, we consider the potential human Miros hold as novel therapeutic targets for the treatment of such disease.