Genes and the Environment in Neurodegeneration

Neurodegenerative diseases are a heterogeneous group of pathologies which includes complex multifactorial diseases, monogenic disorders and disorders for which inherited, sporadic and transmissible forms are known. Factors associated with predisposition and vulnerability to neurodegenerative disorders may be described usefully within the context of gene–environment interplay. There are many identified genetic determinants for neurodegeneration, and it is possible to duplicate many elements of recognized human neurodegenerative disorders in animal models of the disease. However, there are similarly several identifiable environmental influences on outcomes of the genetic defects; and the course of a progressive neurodegenerative disorder can be greatly modified by environmental elements. In this review we highlight some of the major neurodegenerative disorders (Alzheimer’s disease, Parkinson’s disease, Amyotrophic lateral sclerosis, Huntington’s disease, and prion diseases.) and discuss possible links of gene–environment interplay including, where implicated, mitochondrial genes.     

Mitochondrial matters of the brain: amyloid formation and Alzheimer’s disease introduction

In recent years mitochondria, as the most abundant organelles in animal and human cells, have come to the forefront of biomedical research as they are now recognized not only as the major producers of ATP needed to drive cellular functions critical for life, but they are also the instruments of cell death. Not surprisingly, therefore, mitochondria are now known to be involved in many different diseases ranging from those that affect millions worldwide to those that affect only a few, i.e., rare diseases. These diseases include in addition to cardio-myopathies and cancer also diseases that affect many other organs/tissues including the brain/nervous system, the latter diseases now commonly referred to as “neurodegenerative diseases”. Specifically, the subject of this mini-review series focuses on the role of mitochondria in Alzheimer’s disease, a major age related neurodegenerative disease that results in loss or decline of memory and other cognitive abilities. This devastating disease affects millions of Americans, and globally multi-millions with very grim predictions for the future. Although the molecular and gene-related details that underlie Alzheimer’s disease remain to be clearly elucidated, mitochondria appear to be very intimately involved. The purpose of this mini-review series is to summarize how various investigators working on this subject envision the role(s) of mitochondria in Alzheimer’s disease. The development of future therapies for this disease is likely to rely heavily on the new knowledge gained.     

Redox regulation of mitochondrial fission,protein misfolding,synaptic damage,and neuronal cell death: potential implications for Alzheimer’s and Parkinson’s diseases

Normal mitochondrial dynamics consist of fission and fusion events giving rise to new mitochondria, a process termed mitochondrial biogenesis. However, several neurodegenerative disorders manifest aberrant mitochondrial dynamics, resulting in morphological abnormalities often associated with deficits in mitochondrial mobility and cell bioenergetics. Rarely, dysfunctional mitochondrial occur in a familial pattern due to genetic mutations, but much more commonly patients manifest sporadic forms of mitochondrial disability presumably related to a complex set of interactions of multiple genes (or their products) with environmental factors (G × E). Recent studies have shown that generation of excessive nitric oxide (NO), in part due to generation of oligomers of amyloid-β (Aβ) protein or overactivity of the NMDA-subtype of glutamate receptor, can augment mitochondrial fission, leading to frank fragmentation of the mitochondria. S-Nitrosylation, a covalent redox reaction of NO with specific protein thiol groups, represents one mechanism contributing to NO-induced mitochondrial fragmentation, bioenergetic failure, synaptic damage, and eventually neuronal apoptosis. Here, we summarize our evidence in Alzheimer’s disease (AD) patients and animal models showing that NO contributes to mitochondrial fragmentation via S-nitrosylation of dynamin-related protein 1 (Drp1), a protein involved in mitochondrial fission. These findings may provide a new target for drug development in AD. Additionally, we review emerging evidence that redox reactions triggered by excessive levels of NO can contribute to protein misfolding, the hallmark of a number of neurodegenerative disorders, including AD and Parkinson’s disease. For example, S-nitrosylation of parkin disrupts its E3 ubiquitin ligase activity, and thereby affects Lewy body formation and neuronal cell death.     

Light microscopic methods to visualize mitochondria on tissue sections

Mitochondria are cytoplasmic, double-membrane organelles, a main role of which is to synthesize ATP, the universal energy ‘supply’ of cells. In the last three decades, molecular genetic, biochemical, immunological and cell biological techniques have been applied in a coordinated fashion to unveil the pathogenesis of known mitochondrial disorders, as well as to explore the role of mitochondria in aging and neurodegenerative diseases. Once to be thought to be rare, it is now clear that mitochondrial dysfunction is an important cause of neurological and cardiac diseases, and age-related disorders such as cancer. Here, we review, illustrate, and provide updated protocols of two histochemical, and three immunohistochemical methods that in our opinion are the most reliable tools to visualize mitochondria on tissue sections from normal and disease specimens.     

Oxidative stress in mitochondria

In mitochondria, oxidative phosphorylation and enzymatic oxidation of biogenic amines by monoamine oxidase produce reactive oxygen and nitrogen species, which are proposed to cause neuronal cell death in neurodegenerative disorders, including Parkinson’s and Alzheimer’s disease. In these disorders, mitochondrial dysfunction, increased oxidative stress, and accumulation of oxidation-modified proteins are involved in cell death in definite neurons. The interactions among these factors were studied by use of a peroxynitrite-generating agent, N-morpholino sydnonimine (SIN-1) and an inhibitor of complex I, rotenone, in human dopaminergic SH-SY5Y cells. In control cells, peroxynitrite nitrated proteins, especially the subunits of mitochondrial complex I, as 3-nitrotyrosine, suggesting that neurons are exposed to constant oxidative stress even under physiological conditions. SIN-1 and an inhibitor of proteasome, carbobenzoxy-l-isoleucyl-γ-t-butyl-l-analyl-l-leucinal (PSI), increased markedly the levels of nitrated proteins with concomitant induction of apoptosis in the cells. Rotenone induced mitochondrial dysfunction and accumulation and aggregation of proteins modified with acrolein, an aldehyde product of lipid peroxidation in the cells. At the same time, the activity of the 20S β-subunit of proteasome was reduced significantly, which degrades oxidative-modified protein. The mechanism was proved to be the result of the modification of the 20S β-subunit with acrolein and to the binding of other acrolein-modified proteins to the 20S β-subunit. Increased oxidative stress caused by SIN-1 treatment induced a decline in the mitochondrial membrane potential, ΔΨm, and activated mitochondrial apoptotic signaling and induced cell death in SH-SY5Y cells. As another pathway, p38 mitogen-activated protein (MAP) kinase and exracellular signal-regulated kinase (ERK) mediated apoptosis induced by SIN-1. On the other hand, a series of neuroprotective propargylamine derivatives, including rasagiline [N-propargyl-1(R)aminoindan]and (−)deprenyl, intervened in the activation of apoptotic cascade by reactive oxygen species-reactive nitrogen species in mitochondria through stabilization of the membrane potential, ΔΨm. In addition, rasagiline induced antiapoptotic Bcl-2 and glial cell line-derived neurotrophic factor (GDNF) in SH-SY5Y cells, which was mediated by the ERK-nuclear factor (NF)-κB pathway. These results are discussed in relation to the interaction of oxidative stress and mitochondria in the regulation of neuronal death and survival in neurodegenerative diseases.     

Mitochondrial Optic Neuropathies: How Two Genomes may Kill the Same Cell Type?

Ocular involvement is a prevalent feature in mitochondrial diseases. Leber’s hereditary optic neuropathy (LHON) and dominant optic atrophy (DOA) are both non-syndromic optic neuropathies with a mitochondrial etiology. LHON is associated with point mutations in the mitochondrial DNA (mtDNA), which affect subunit genes of complex I. The majority of DOA patients harbor mutations in the nuclear-encoded protein OPA1, which is targeted to mitochondria and participates to cristae organization and mitochondrial network dynamics. In both disorders the retinal ganglion cells (RGCs) are specific cellular targets of the degenerative process. We here review the clinical features and the genetic bases, and delineate the possible common pathomechanism for both these disorders.     

The cerebral proteopathies

The abnormal assembly and deposition of specific proteins in the brain is the probable cause of most neurodegenerative disease afflicting the elderly. These “cerebral proteopathies” include Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), prion diseases, and a variety of other disorders. Evidence is accumulating that the anomalous aggregation of the proteins, and not a loss of protein function, is central to the pathogenesis of these diseases. Thus, therapeutic strategies that reduce the production, accumulation, or polymerization of pathogenic proteins might be applicable to a wide range of some of the most devastating diseases of old age.     

The rescue of microtubule-dependent traffic recovers mitochondrial function in Parkinson's disease

In Parkinson's disease mitochondrial dysfunction can lead to a deficient ATP supply to microtubule protein motors leading to mitochondrial axonal transport disruption. Compromised axonal transport will then lead to a disorganized distribution of mitochondria and other organelles in the cell, as well as, the accumulation of aggregated proteins like alpha-synuclein. Moreover, axonal transport disruption can trigger synaptic accumulation of autophagosomes packed with damaged mitochondria and protein aggregates promoting synaptic failure.We previously observed that neuronal-like cells with an inherent mitochondrial impairment derived from PD patients contain a disorganized microtubule network, as well as, alpha-synuclein oligomer accumulation. In this work we provide new evidence that an agent that promotes microtubule network assembly, NAP (davunetide), improves microtubule-dependent traffic, restores the autophagic flux and potentiates autophagosome–lysosome fusion leading to autophagic vacuole clearance in Parkinson's disease cells. Moreover, NAP is capable of efficiently reducing alpha-synuclein oligomer content and its sequestration by the mitochondria. Most interestingly, NAP decreases mitochondrial ubiquitination levels, as well as, increases mitochondrial membrane potential indicating a rescue in mitochondrial function.Overall, we demonstrate that by improving microtubule-mediated traffic, we can avoid mitochondrial-induced damage and thus recover cell homeostasis. These results prove that NAP may be a promising therapeutic lead candidate for neurodegenerative diseases that involve axonal transport failure and mitochondrial impairment as hallmarks, like Parkinson's disease and related disorders.     

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.     

The mitochondrial kinase PINK1, stress response and Parkinson’s disease

Mitochondrial dysfunction is well documented in presymptomatic brain tissue with Parkinson’s disease (PD). Identification of the autosomal recessive variant PARK6 caused by loss-of-function mutations in the mitochondrial kinase PINK1 provides an opportunity to dissect pathogenesis. Although PARK6 shows clinical differences to PD, the induction of alpha-synuclein “Lewy” pathology by PINK1-deficiency proves that mitochondrial pathomechanisms are relevant for old-age PD. Mitochondrial dysfunction is induced by PINK1 deficiency even in peripheral tissues unaffected by disease, consistent with the ubiquitous expression of PINK1. It remains unclear whether this dysfunction is due to PINK1-mediated phosphorylation of proteins inside or outside mitochondria. Although PINK1 deficiency affects the mitochondrial fission/fusion balance, cell stress is required in mammals to alter mitochondrial dynamics and provoke apoptosis. Clearance of damaged mitochondria depends on pathways including PINK1 and Parkin and is critical for postmitotic neurons with high energy demand and cumulative stress, providing a mechanistic concept for the tissue specificity of disease.     

Huntington’s disease and mitochondrial alterations: emphasis on experimental models

Huntington’s disease (HD) is an inheritable neurological disorder coursing with degeneration of basal ganglia and producing chorea and dementia. One common factor accounting for neurodegeneration in this disorder is mitochondrial deterioration at both morphologic and functional levels. The development of experimental models in animals or cell preparations to resemble pathologic and pathogenic conditions of this disorder has served for more than four decades to describe part of the mechanistic alterations that could be occurring in mitochondria of HD patients, and the subsequent design of therapeutic alternatives where mitochondrial alterations are the primary target. In this miniriview we describe some of the most relevant studies at the experimental level, giving support to the hypothesis that mitochondria play a central role in HD pathogenesis.     

MicroRNAs: Possible role in pathogenesis of Parkinson’s disease

Parkinson’s disease is one of the most common human neurodegenerative disorders caused by the loss of dopaminergic neurons from the substantia nigra pars compacta of human brain. However, causes and mechanisms of the progression of the disease are not yet fully clarified. To date, investigation of the role of miRNAs in norm and pathology is one of the most intriguing and actively developing areas in molecular biology. MiRNAs regulate expression of a variety of genes and can be implicated in pathogenesis of various diseases. Possible role of miRNAs in pathogenesis of Parkinson’s disease is discussed in this review.     

Mitochondria and Alzheimer’s disease: amyloid-β peptide uptake and degradation by the presequence protease,hPreP

Several lines of evidence suggest mitochondrial dysfunction as a possible underlying mechanism of Alzheimer’s disease (AD). Accumulation of the amyloid-β peptide (Aβ), a neurotoxic peptide implicated in the pathogenesis of AD, has been detected in brain mitochondria of AD patients and AD transgenic mouse models. In vitro evidence suggests that the Aβ causes mitochondrial dysfunction e.g. oxidative stress, mitochondrial fragmentation and decreased activity of cytochrome c oxidase and TCA cycle enzymes. Here we review the link between mitochondrial dysfunctions and AD. In particular we focus on the mechanism for Aβ uptake by mitochondria and on the recently identified Aβ degrading protease in human brain mitochondria.     

Alteration of mitochondrial function and cell sensitization to death

Stimulation of cell death is a powerful instrument in the organism’s struggle with cancer. Apoptosis represents one mode of cell death. However, in a variety of tumor cells proapoptotic mechanisms are downregulated, or not properly activated, whereas antiapoptotic mechanisms are upregulated. Mitochondria are known as key players in the regulation of apoptotic pathways. Specifically, permeabilization of the mitochondrial outer membrane and subsequent release of proapoptotic proteins from the intermembrane space are viewed as decisive events in the initiation and/or execution of apoptosis. Disruption of mitochondrial functions by anticancer drugs, which induce oxidative stress, inhibit mitochondrial respiration, or uncouple oxidative phosphorylation, can sensitize mitochondria in these cells and facilitate outer membrane permeabilization.     

Mitochondrial dynamics,cell death and the pathogenesis of Parkinson’s disease

The structure and function of the mitochondrial network is regulated by mitochondrial biogenesis, fission, fusion, transport and degradation. A well-maintained balance of these processes (mitochondrial dynamics) is essential for neuronal signaling, plasticity and transmitter release. Core proteins of the mitochondrial dynamics machinery play important roles in the regulation of apoptosis, and mutations or abnormal expression of these factors are associated with inherited and age-dependent neurodegenerative disorders. In Parkinson’s disease (PD), oxidative stress and mitochondrial dysfunction underlie the development of neuropathology. The recessive Parkinsonism-linked genes PTEN-induced kinase 1 (PINK1) and Parkin maintain mitochondrial integrity by regulating diverse aspects of mitochondrial function, including membrane potential, calcium homeostasis, cristae structure, respiratory activity, and mtDNA integrity. In addition, Parkin is crucial for autophagy-dependent clearance of dysfunctional mitochondria. In the absence of PINK1 or Parkin, cells often develop fragmented mitochondria. Whereas excessive fission may cause apoptosis, coordinated induction of fission and autophagy is believed to facilitate the removal of damaged mitochondria through mitophagy, and has been observed in some types of cells. Compensatory mechanisms may also occur in mice lacking PINK1 that, in contrast to cells and Drosophila, have only mild mitochondrial dysfunction and lack dopaminergic neuron loss. A better understanding of the relationship between the specific changes in mitochondrial dynamics/turnover and cell death will be instrumental to identify potentially neuroprotective pathways steering PINK1-deficient cells towards survival. Such pathways may be manipulated in the future by specific drugs to treat PD and perhaps other neurodegenerative disorders characterized by abnormal mitochondrial function and dynamics.     

Role of Ubiquitin Protein Ligases in the Pathogenesis of Polyglutamine Diseases

The accumulation of intracellular protein deposits as inclusion bodies is the common pathological hallmark of most age related neurodegenerative disorders including polyglutamine diseases. Appearances of aggregates of the misfolded mutant disease proteins suggest that the cells are unable to efficiently degrade them, and failure of clearance leads to the severe disturbances of the cellular quality control system. The quality control ubiquitin ligases are now increasingly implicated in the biology of polyglutamine diseases, Parkinson’s diseases, Amyotrophic lateral sclerosis and Alzheimer’s disease. Here we review the recent studies that have revealed a critical role of E3 ubiquitin ligases in understanding the pathogenesis of polyglutamine diseases.     

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.