RNA quality control and protein aggregates in amyotrophic lateral sclerosis: A review

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron disease in adults. The biologic basis of ALS remains unknown. However, ALS research has taken a dramatic turn over the past 4 years. Ground breaking discoveries of mutations of genes that encode RNA processing proteins, and demonstration that abnormal aggregates of these and other proteins precede motor neuron loss in familial and sporadic ALS, have initiated a paradigm shift in understanding the pathogenic mechanisms of ALS. Curiously, some of these RNA binding proteins have prion‐like domains, with a propensity to self‐aggregation. The emerging hypothesis that a focal cascade of toxic protein aggregates, and their consequent non–cell‐autonomous spread to neighborhood groups of neurons, fits the classical temporo‐spatial progression of ALS. This article reviews the current research efforts toward understanding the role of RNA‐processing regulation and protein aggregates in ALS. Muscle Nerve 47:330‐338, 2013     

Amyotrophic lateral sclerosis and frontotemporal lobar degeneration: A spectrum of TDP‐43 proteinopathies

It is now established that pathological transactive response DNA‐binding protein with a Mr of 43 kD (TDP‐43) on sodium dodecyl sulfate‐polyacrylamide gel electrophoresis is the major disease protein in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) with ubiquitin‐positive inclusions (now known as FTLD‐TDP). In fact, the discovery of pathological TDP‐43 solidified the idea that these disorders are multi‐system diseases and this led to the concept of a TDP‐43 proteinopathy as a spectrum of disorders comprised of different clinical and pathological entities extending from ALS to ALS with cognitive impairment/dementia and FTLD‐TDP without or with motor neuron disease (FTLD‐MND). These align along a broad disease continuum sharing similar pathogenetic mechanisms linked to pathological TDP‐43. We here review salient findings in the development of a concept of TDP‐43 proteinopathy as a novel group of neurodegenerative diseases similar in concept to α‐synucleinopathies and tauopathies.     

<Emphasis Type="Italic">Drosophila</Emphasis> models of neurodegenerative disease

Over the last two decades, a number of mutations have been identified that give rise to neurodegenerative disorders, including familial forms of Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis. Although in most cases sporadic cases vastly outnumber familial forms of such diseases, study of such inherited forms has the potential to provide powerful clues regarding the pathophysiological basis of neurodegeneration. One powerful approach to analyzing disease mechanisms is the development of transgenic animal models, most notably in the mouse. However, development and analysis of such models can be costly and time consuming. Development of improved transgenic technologies have contributed to the development of Drosophila models of a number of neurodegenerative disorders that have shown striking similarities to the human diseases. Moreover, genetic screens using such models have begun to unravel aspects of the pathophysiological basis of neurodegenerative disorders. Here, we provide a general overview of fly models pertinent to trinucleotide repeat expansion disorders, Alzheimer’s, and Parkinson’s diseases, and highlight key genetic modifiers that have been identified to date using such models.     

TDP-43-based animal models of neurodegeneration: new insights into ALS pathology and pathophysiology

The clinical and pathological overlap between amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) suggests these diseases share common underlying mechanisms, a suggestion underscored by the discovery that TDP-43 inclusions are a key pathologic feature in both ALS and FTLD. This finding, combined with the identification of TDP-43 mutations in ALS, directly implicates this DNA/RNA binding protein in disease pathogenesis in ALS and FTLD. However, many key questions remain, including what is the normal function of TDP-43, and whether disease-associated mutations produce toxicity in the nucleus, cytoplasm or both. Furthermore, although pathologic TDP-43 inclusions are clearly associated with many forms of neurodegeneration, whether TDP-43 aggregation is a key step in the pathogenesis in ALS, FTLD and other disorders remains to be proven. This review will compare the features of numerous recently developed animal models of TDP-43-related neurodegeneration, and discuss how they contribute to our understanding of the pathogenesis of human ALS and FTLD.     

TDP‐43 pathology in polyglutamine diseases: With reference to amyotrphic lateral sclerosis

A nuclear protein, transactivation response (TAR) DNA binding protein 43 kDa (TDP‐43), is the major component of neuronal cytoplasmic inclusions (NCIs) in frontotemporal lobar degeneration with ubiquitin inclusions (FTLD‐U) and sporadic amyotrophic lateral sclerosis (SALS). While initially thought to be relatively specific to FTLD‐U and ALS, TDP‐43 pathology has now been detected in a number of other neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. In such tauopathies and α‐synucleinopathies, occurrence of TDP‐43‐positive neuronal cytoplasmic inclusions may be associated with other distinct molecular pathologic processes primarily involving their own pathological proteins, tau and α–synuclein, respectively (secondary TDP‐43 proteinopathies). On the other hand, in several polyglutamine (polyQ) diseases, TDP‐43 appears to play an important pathomechanistic role. Interestingly, intermediate‐length polyQ expansions (27–33 Qs) in ataxin 2, the causative gene of spinocerebellar ataxia type 2, have recently been reported to be a genetic risk factor for SALS. Here, with a review of the literature, we discuss the relationship between ALS and polyQ diseases from the viewpoint of TDP‐43 neuropathology.     

ALS and FTLD: two faces of TDP-43 proteinopathy

Major discoveries have been made in the recent past in the genetics, biochemistry and neuropathology of frontotemporal lobar degeneration (FTLD). TAR DNA-binding protein 43 (TDP-43), encoded by the TARDBP gene, has been identified as the major pathological protein of FTLD with ubiquitin-immunoreactive (ub-ir) inclusions (FTLD-U) with or without amyotrophic lateral sclerosis (ALS) and sporadic ALS. Recently, mutations in the TARDBP gene in familial and sporadic ALS have been reported which demonstrate that abnormal TDP-43 alone is sufficient to cause neurodegeneration. Several familial cases of FTLD-U, however, are now known to have mutations in the progranulin ( GRN ) gene, but granulin is not a component of the TDP-43- and ub-ir inclusions. Further, TDP-43 is found to be a component of the inclusions of an increasing number of neurodegenerative diseases. Other FTLD-U entities with TDP-43 proteinopathy include: FTLD-U with valosin-containing protein ( VCP ) gene mutation and FTLD with ALS linked to chromosome 9p. In contrast, chromosome 3-linked dementia, FTLD-U with chromatin modifying protein 2B ( CHMP2B ) mutation, has ub-ir, TDP-43-negative inclusions. In summary, recent discoveries have generated new insights into the pathogenesis of a spectrum of disorders called TDP-43 proteinopathies including: FTLD-U, FTLD-U with ALS, ALS, and a broadening spectrum of other disorders. It is anticipated that these discoveries and a revised nosology of FTLD will contribute toward an accurate diagnosis, and facilitate the development of new diagnostic tests and therapeutics.     

Axonal transport defects: a common theme in neurodegenerative diseases

A core pathology central to most neurodegenerative diseases is the misfolding, fibrillization and aggregation of disease proteins to form the hallmark lesions of specific disorders. The mechanisms underlying these brain-specific neurodegenerative amyloidoses are the focus of intense investigation and defective axonal transport has been hypothesized to play a mechanistic role in several neurodegenerative disorders; however, this hypothesis has not been extensively examined. Discoveries of mutations in human genes encoding motor proteins responsible for axonal transport do provide direct evidence for the involvement of axonal transport in neurodegenerative diseases, and this evidence is supported by studies of animal models of neurodegeneration. In this review, we summarize recent findings related to axonal transport and neurodegeneration. Focusing on specific neurodegenerative diseases from a neuropathologic perspective, we highlight discoveries of human motor protein mutations in some of these diseases, as well as illustrate new insights from animal models of neurodegenerative disorders. We also review the current understanding of the biology of axonal transport including major recent findings related to slow axonal transport.     

Therapeutic targets for amyotrophic lateral sclerosis: current treatments and prospects for more effective therapies

Although amyotrophic lateral sclerosis (ALS) was described more than 130 years ago, the cause(s) of most cases of this adult motor neuron disease remains a mystery. With the discovery of mutations in one gene (Cu/Zn superoxide dismutase) as a primary cause of some forms of ALS, model systems have been developed that have helped us begin to understand mechanisms involved in motor neuron death and enabled testing of potential new therapies. Several other genes have been implicated as risk factors in motor neuron diseases, including neurofilaments, cytoplasmic dynein and dynactin, vascular endothelial growth factor, and angiogenin. With advances in the basic research of the disease, many hypotheses accounting for motor neuron death are being explored, including loss of trophic support, protein mishandling, mitochondrial dysfunction, excitotoxicity, axonal abnormalities and inflammation. Many of these mechanisms are the focus of research in other neurodegenerative disorders, such as Parkinson's, Alzheimer's and Huntington's disease.     

RNA oxidation in Alzheimer disease and related neurodegenerative disorders

RNA oxidation and its biological effects are less well studied compared to DNA oxidation. However, RNA may be more susceptible to oxidative insults than DNA, for RNA is largely single-stranded and its bases are not protected by hydrogen bonding and less protected by specific proteins. Also, cellular RNA locates in the vicinity of mitochondria, the primary source of reactive oxygen species. Oxidative modification can occur not only in protein-coding RNAs, but also in non-coding RNAs that have been recently revealed to contribute towards the complexity of the mammalian brain. Damage to coding and non-coding RNAs will cause errors in proteins and disturbances in the regulation of gene expression. While less lethal than mutations in the genome and not inheritable, such sublethal damage to cells might be associated with underlying mechanisms of degeneration, especially age-associated neurodegeneration that is commonly found in the elderly population. Indeed, oxidative RNA damage has been described recently in most of the common neurodegenerative disorders including Alzheimer disease, Parkinson disease, dementia with Lewy bodies and amyotrophic lateral sclerosis. Of particular interest, the accumulating evidence obtained from studies on either human samples or experimental models coincidentally suggests that oxidative RNA damage is a feature in vulnerable neurons at early-stage of these neurodegenerative disorders, indicating that RNA oxidation actively contributes to the onset or the development of the disorders. Further investigations aimed at understanding of the processing mechanisms related to oxidative RNA damage and its consequences may provide significant insights into the pathogenesis of neurodegenerative disorders and lead to better therapeutic strategies.     

Huntington's disease: from molecular pathogenesis to clinical treatment

Huntington's disease is a progressive, fatal, neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin gene, which encodes an abnormally long polyglutamine repeat in the huntingtin protein. Huntington's disease has served as a model for the study of other more common neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease. These disorders all share features including: delayed onset; selective neuronal vulnerability, despite widespread expression of disease-related proteins during the whole lifetime; abnormal protein processing and aggregation; and cellular toxic effects involving both cell autonomous and cell-cell interaction mechanisms. Pathogenic pathways of Huntington's disease are beginning to be unravelled, offering targets for treatments. Additionally, predictive genetic testing and findings of neuroimaging studies show that, as in some other neurodegenerative disorders, neurodegeneration in affected individuals begins many years before onset of diagnosable signs and symptoms of Huntington's disease, and it is accompanied by subtle cognitive, motor, and psychiatric changes (so-called prodromal disease). Thus, Huntington's disease is also emerging as a model for strategies to develop therapeutic interventions, not only to slow progression of manifest disease but also to delay, or ideally prevent, its onset.     

Involvement of α-synuclein in Parkinson's disease and other neurodegenerative disorders

Summary. A major step in the elucidation of the pathogenesis of neurodegenerative disorders was the identification of a mutation in the α-synuclein gene in autosomal dominant Parkinson's disease (PD). α-Synuclein is the main component of Lewy bodies (LB), the neuropathological hallmark of PD. Moreover, a fragment of α-synuclein (NAC) is the second major component of amyloid plaques in Alzheimer's disease (AD). Recent studies of other neurodegenerative disorders such as dementia with LB (DLB), multiple system atrophy (MSA) and amyotrophic lateral sclerosis (ALS) also revealed intracellular accumulations of α-synuclein in affected brain regions. This may indicate that these disorders partially share common pathogenic mechanisms. Recent data provide first insights into the physiological function of α-synuclein and support the concept of an essential role of α-synuclein in neurodegeneration. Increasing knowledge on the pathogenic molecular mechanisms of neurodegeneration and of the pathophysiological function of α-synuclein in particular may influence future development of therapeutic strategies in neurodegenerative disorders. Received April 9, 1999; accepted June 16, 1999     

Review: Iron metabolism and the role of iron in neurodegenerative disorders

Iron plays a role for the biogenesis of two important redox‐reactive prosthetic groups of enzymes, iron sulphur clusters (ISC) and heme. A part of these biosynthetic pathways takes plays in the mitochondria. While several important proteins of cellular iron uptake and storage and of mitochondrial iron metabolism are well‐characterized, limited knowledge exists regarding the mitochondrial iron importers (mitoferrins). A disturbed distribution of iron, hampered Fe‐dependent biosynthetic pathways and eventually oxidative stress resulting from an increased labile iron pool are suggested to play a role in several neurodegenerative diseases. Friedreich's ataxia is associated with mitochondrial iron accumulation and hampered ISC/heme biogenesis due to reduced frataxin expression, thus representing a monogenic mitochondrial disorder, which is clearly elicited solely by a disturbed iron metabolism. Less clear are the controversially discussed impacts of iron dysregulation and iron‐dependent oxidative stress in the most common neurodegenerative disorders, i.e. Alzheimer's disease (AD) and Parkinson's disease (PD). Amyotrophic lateral sclerosis (ALS) may be viewed as a disease offering a better support for a direct link between iron, oxidative stress and regional neurodegeneration. Altogether, despite significant progress in molecular knowledge, the true impact of iron on the sporadic forms of AD, PD and ALS is still uncertain. Here we summarize the current knowledge of iron metabolism disturbances in neurodegenerative disorders.     

Biochemistry and molecular biology of tauopathies

Filamentous tau deposits in neurons or glial cells are the hallmark lesions of neurodegenerative tauopathies, such as Alzheimer’s disease, Pick’s disease, corticobasal degeneration and progressive supranuclear palsy. Biochemical analyses of Sarkosyl‐insoluble tau from brains with tauopathies have revealed that tau deposits in different diseases consisted of different tau isoforms (i.e., all six tau isoforms occur in Alzheimer’s disease, four repeat tau isoforms occur in corticobasal degeneration or progressive supranuclear palsy, and three repeat tau isoforms occur in Pick’s disease). The discovery of mutations in the tau gene in FTDP‐17 has established that abnormalities in tau function or expression are sufficient to cause filamentous aggregation of hyperphosphorylated tau and neurodegeneration similar to that seen in sporadic tauopathies. Because the number of tau inclusions and their regional distribution correlate with clinical symptoms, inhibition of tau aggregation or filament formation in neurons or glial cells may prevent neurodegeneration. We have investigated the effects of 42 compounds belonging to nine different chemical classes on tau filament formation, and found that several phenothiazine and polyphenol compounds, and one porphyrin compound inhibit tau filament formation.     

Neurodegeneration and oxidative stress: prion disease results from loss of antioxidant defence

Prion diseases or transmissible spongiform encephalopathies (TSEs) are rare neurodegenerative disorders that can be acquired either by direct transmission, inherited through dominant mutations in the prion protein gene or via an unknown sporadic cause. This latter group constitutes the vast majority of cases. Like many neurodegenerative diseases the hallmarks of oxidative damage can be readily detected throughout the brain of the affected individual. However, unlike most other neurodegenerative diseases, prion diseases are connected with a dramatic loss of antioxidant defence. As abnormal protein accumulates in the diseased brain there is both an increase of oxidative substances and a loss of the defences that keep them in check. In particular the normal cellular prion protein has been shown to be an antioxidant. Conversion of this protein to the protease resistant isoform is accompanied by a loss of this antioxidant activity. This change creates a paradox as the loss of activity is not accompanied by a loss of protein expression. It is likely that this prevents other cellular defences from responding sufficiently to protect neurons from the heightened oxidative burden. Recent experiments with transgenic mice have shown that when prion protein expression is switched off during the course of prion disease, cell death is dramatically halted and the mouse recovers from the disease. This result clearly illustrates that the continued expression of non-function prion protein is essential for disease progression. This implies that the presence of this abnormal protein during prion disease causes a failure of cellular antioxidant defence. This failed defence is the fundamental cause of the massive neurodegeneration that results in the fatal nature of TSEs. The role of oxidative stress in TSEs and other neurodegenerative disorders are discussed in this review.     

Are pathological lesions in neurodegenerative disorders the cause or the effect of the degeneration?

Pathological lesions in the form of extracellular protein deposits, intracellular inclusions and changes in cell morphology occur in the brain in the majority of neurodegenerative disorders. Studies of the presence, distribution, and molecular determinants of these lesions are often used to define individual disorders and to establish the mechanisms of lesion pathogenesis. In most disorders, however, the relationship between the appearance of a lesion and the underlying disease process is unclear. Two hypotheses are proposed which could explain this relationship: (i) lesions are the direct cause of the observed neurodegeneration (‘causal’ hypothesis); and (ii) lesions are a reaction to neurodegeneration (‘reaction’ hypothesis). These hypotheses are considered in relation to studies of the morphology and molecular determinants of lesions, the effects of gene mutations, degeneration induced by head injury, the effects of experimentally induced brain lesions, transgenic studies and the degeneration of anatomical pathways. The balance of evidence suggests that in many disorders, the appearance of the pathological lesions is a reaction to degenerative processes rather than being their cause. Such a conclusion has implications both for the classification of neurodegenerative disorders and for studies of disease pathogenesis.     

Optineurin is potentially associated with TDP‐43 and involved in the pathogenesis of inclusion body myositis

S. Yamashita, E. Kimura, N. Tawara, H. Sakaguchi, T. Nakama, Y. Maeda, T. Hirano, M. Uchino and Y. Ando (2013) Neuropathology and Applied Neurobiology 39, 406–416 Optineurin is potentially associated with TDP‐43 and involved in the pathogenesis of inclusion body myositis Aims: Increasing evidences suggest a similarity in the pathophysiological mechanisms of neuronal cell death in amyotrophic lateral sclerosis (ALS) and myofibre degeneration in sporadic inclusion body myositis (sIBM). The aim of this study is to elucidate the involvement of ALS‐causing proteins in the pathophysiological mechanisms in sIBM. Methods: Skeletal muscle biopsy specimens of five patients with sIBM, two with oculopharyngeal muscular dystrophy (OPMD), three with polymyositis (PM), three with dermatomyositis (DM), three with neurogenic muscular atrophy, and three healthy control subjects were examined. We analysed the expression and localization of familial ALS‐causing proteins, including transactive response DNA binding protein‐43 (TDP‐43), fused in sarcoma/translocated in liposarcoma (FUS/TLS), Cu/Zn superoxide dismutase (SOD1) and optineurin (OPTN) by immunohistochemistry. Results: TDP‐43, OPTN and, to a lesser extent, FUS/TLS were more frequently accumulated in the cytoplasm in patients with sIBM and OPMD than in patients with PM, DM, neurogenic muscular atrophy, or healthy control subjects. SOD1 was accumulated in a small percentage of myofibres in patients with sIBM and OPMD, and to a very small extent in patients with PM and DM. Confocal microscopy imaging showed that TDP‐43 proteins more often colocalized with OPTN than with FUS/TLS, p62 and phosphorylated Tau. Conclusions: These findings suggest that OPTN in cooperation with TDP‐43 might be involved in the pathophysiological mechanisms of skeletal muscular degeneration in myopathy with rimmed vacuoles. Further investigation into these mechanisms is therefore warranted.