Oligomerization of mutant SOD1 in mitochondria of motoneuronal cells drives mitochondrial damage and cell toxicity

Intracellular spatially-targeted chemical chaperones increase native state stability of mutant SOD1 barrel

Amyotrophic lateral sclerosis (ALS) is a progressive neurological disorder with currently no cure. Central to the cellular dysfunction associated with this fatal proteinopathy is the accumulation of unfolded/misfolded superoxide dismutase 1 (SOD1) in various subcellular locations. The molecular mechanism driving the formation of SOD1 aggregates is not fully understood but numerous studies suggest that aberrant aggregation escalates with folding instability of mutant apoSOD1. Recent advances on combining organelle-targeting therapies with the anti-aggregation capacity of chemical chaperones have successfully reduce the subcellular load of misfolded/aggregated SOD1 as well as their downstream anomalous cellular processes at low concentrations (micromolar range). Nevertheless, if such local aggregate reduction directly correlates with increased folding stability remains to be explored. To fill this gap, we synthesized and tested here the effect of 9 ER-, mitochondria- and lysosome-targeted chemical chaperones on the folding stability of truncated monomeric SOD1 (SOD1bar) mutants directed to those organelles. We found that compound ER-15 specifically increased the native state stability of ER-SOD1bar-A4V, while scaffold compound FDA-approved 4-phenylbutyric acid (PBA) decreased it. Furthermore, our results suggested that ER15 mechanism of action is distinct from that of PBA, opening new therapeutic perspectives of this novel chemical chaperone on ALS treatment.

Acute Toxicity and Stress Behaviour of Heterobranchus bidorsalis Exposed to the Detergent Nittol® NTL

The acute toxicity of the detergent Nittol® 0.8, 1.0, 1.2, 1.4, 1.6, and 0.0 mg NTL/L of clean water on Heterobrunchus bidorsalis, 5.5 ± 0.3 g, 6.4 ± 0.5 cm were investigated, using semi-static bioassay, for 96 h in 50 L capacity plastic test bowls. The fingerlings of the same brood stock and age were collected from Onose Farms Limited, Ughelli, Delta State to the University Research Laboratory, Enugu Lat. 7.4 N; 8°7′5 and long 6°8′ E. 7°6′ W. The test fish were acclimatized for 14 days, and fed at 3% body weight once daily, on a 40% CP commercial diet. Feeding was suspended 24 h before and during the range finding and acute tests. The whole set-up was replicated three times, and no death was recorded during the acclimatization period and in the control. A total of 180 fingerlings were used, and 10 fingerlings were assigned to each replicate. The test set-up was monitored daily for water quality parameters, opercular ventilation, tail fin beat frequency, and mortality. Dose and time-dependent behavioural patterns exhibited by the test fish, during the exposure periods include rapid swimming, air gulping, loss of balance, and a period of convulsion before death. Significant elevation in pH and temperature, reduction of DO compared to the control (p < 0.05) in the water quality, and dose-dependent early elevation of the tail and fin movements declined towards the end of the experiment. The 96 h LC50 was determined to be 1.41 mg/L, indicating that the detergent NTL is toxic to the test fish. The haematological parameters were significantly (p < 0.05) reduced in the treated ranges of RBC 5.20 ± 0.07–8.00 ± 0.02 × 106 mm3, HB 7.53 ± 0.50–10.72 ± 0.14 g/dl, PCV 13.20 ± 0.8.50–18.00 ± 0.43 % below their elevated respective controls of 10.50 ± 0.01 × 10 6 mm3, 11.00 ± 0.01 g/dl, and 23.48 ± 0.2.6 %. The white blood cells (WBC) recorded a significant (p < 0.05) increase in ranges of 23.72 ± 0.14–51.80 ± 1.9 × 103 mm3 above the control value of 11.00 ± 0.01 × 103 mm3 Therefore, values greater than the safe amount of 0.014 mg/L should not be allowed in the receiving culture waters for Heterobrunchus bidorsalis fingerlings.

S-Glutathionylation and S-Nitrosylation in Mitochondria: Focus on Homeostasis and Neurodegenerative Diseases

Redox post-translational modifications are derived from fluctuations in the redox potential and modulate protein function, localization, activity and structure. Amongst the oxidative reversible modifications, the S-glutathionylation of proteins was the first to be characterized as a post-translational modification, which primarily protects proteins from irreversible oxidation. However, a growing body of evidence suggests that S-glutathionylation plays a key role in core cell processes, particularly in mitochondria, which are the main source of reactive oxygen species. S-nitrosylation, another post-translational modification, was identified >150 years ago, but it was re-introduced as a prototype cell-signaling mechanism only recently, one that tightly regulates core processes within the cell’s sub-compartments, especially in mitochondria. S-glutathionylation and S-nitrosylation are modulated by fluctuations in reactive oxygen and nitrogen species and, in turn, orchestrate mitochondrial bioenergetics machinery, morphology, nutrients metabolism and apoptosis. In many neurodegenerative disorders, mitochondria dysfunction and oxidative/nitrosative stresses trigger or exacerbate their pathologies. Despite the substantial amount of research for most of these disorders, there are no successful treatments, while antioxidant supplementation failed in the majority of clinical trials. Herein, we discuss how S-glutathionylation and S-nitrosylation interfere in mitochondrial homeostasis and how the deregulation of these modifications is associated with Alzheimer’s, Parkinson’s, amyotrophic lateral sclerosis and Friedreich’s ataxia.

Fly for ALS: Drosophila modeling on the route to amyotrophic lateral sclerosis modifiers

Amyotrophic lateral sclerosis (ALS) is a rare, devastating disease, causing movement impairment, respiratory failure and ultimate death. A plethora of genetic, cellular and molecular mechanisms are involved in ALS signature, although the initiating causes and progressive pathological events are far from being understood. Drosophila research has produced seminal discoveries for more than a century and has been successfully used in the past 25 years to untangle the process of ALS pathogenesis, and recognize potential markers and novel strategies for therapeutic solutions. This review will provide an updated view of several ALS modifiers validated in C9ORF72, SOD1, FUS, TDP-43 and Ataxin-2 Drosophila models. We will discuss basic and preclinical findings, illustrating recent developments and novel breakthroughs, also depicting unsettled challenges and limitations in the Drosophila-ALS field. We intend to stimulate a renewed debate on Drosophila as a screening route to identify more successful disease modifiers and neuroprotective agents.

Therapeutic Potential of Polyphenols in Amyotrophic Lateral Sclerosis and Frontotemporal Dementia

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are severe neurodegenerative disorders that belong to a common disease spectrum. The molecular and cellular aetiology of the spectrum is a highly complex encompassing dysfunction in many processes, including mitochondrial dysfunction and oxidative stress. There is a paucity of treatment options aside from therapies with subtle effects on the post diagnostic lifespan and symptom management. This presents great interest and necessity for the discovery and development of new compounds and therapies with beneficial effects on the disease. Polyphenols are secondary metabolites found in plant-based foods and are well known for their antioxidant activity. Recent research suggests that they also have a diverse array of neuroprotective functions that could lead to better treatments for neurodegenerative diseases. We present an overview of the effects of various polyphenols in cell line and animal models of ALS/FTD. Furthermore, possible mechanisms behind actions of the most researched compounds (resveratrol, curcumin and green tea catechins) are discussed.

Superoxide Dismutase 1 in Health and Disease: How a Frontline Antioxidant Becomes Neurotoxic

Cu/Zn superoxide dismutase (SOD1) is a frontline antioxidant enzyme catalysing superoxide breakdown and is important for most forms of eukaryotic life. The evolution of aerobic respiration by mitochondria increased cellular production of superoxide, resulting in an increased reliance upon SOD1. Consistent with the importance of SOD1 for cellular health, many human diseases of the central nervous system involve perturbations in SOD1 biology. But far from providing a simple demonstration of how disease arises from SOD1 loss-of-function, attempts to elucidate pathways by which atypical SOD1 biology leads to neurodegeneration have revealed unexpectedly complex molecular characteristics delineating healthy, functional SOD1 protein from that which likely contributes to central nervous system disease. This review summarises current understanding of SOD1 biology from SOD1 genetics through to protein function and stability.

Multiple ways to a dead end: diverse mechanisms by which ALS mutant genes induce cell death

Amyotrophic Lateral Sclerosis (ALS) is a deadly neuromuscular disorder caused by progressive motor neuron loss in the brain and spinal cord. Over the past decades, a number of genetic mutations have been identified that cause or are associated with ALS disease progression. Numerous genes harbor ALS mutations, and they encode proteins displaying a wide range of physiological functions, with limited overlap. Despite the divergent functions, mutations in these genes typically trigger protein aggregation, which can confer gain- and/or loss-of-function to a number of essential cellular processes. Nuclear processes such as mRNA splicing and the response to DNA damage are significantly affected in ALS patients. Cytoplasmic organelles such as mitochondria are damaged by ALS mutant proteins. Processes that maintain cellular homeostasis such as autophagy, nonsense-mediated mRNA decay and nucleocytoplasmic transport, are also impaired by ALS mutations. Here, we review the multiple mechanisms by which mutations in major ALS-associated genes, such as TARDBP, C9ORF72 and FUS, lead to impairment of essential cellular processes.

Exposure of β6/β7-Loop in Zn/Cu Superoxide Dismutase (SOD1) Is Coupled to Metal Loss and Is Transiently Reversible During Misfolding

Upon losing its structural integrity (misfolding), SOD1 acquires neurotoxic properties to become a pathogenic protein in ALS, a neurodegenerative disease targeting motor neurons; understanding the mechanism of misfolding may enable new treatment strategies for ALS. Here, we reported a monoclonal antibody, SE21, targeting the β6/β7-loop region of SOD1. The exposure of this region is coupled to metal loss and is entirely reversible during the early stages of misfolding. By using SE21 mAb, we demonstrated that, in apo-SOD1 incubated under the misfolding-promoting conditions, the reversible phase, during which SOD1 is capable of restoring its nativelike conformation in the presence of metals, is followed by an irreversible structural transition, autocatalytic in nature, which takes place prior to the onset of SOD1 aggregation and results in the formation of atypical apo-SOD1 that is unable to bind metals. The reversible phase defines a window of opportunity for pharmacological intervention using metal mimetics that stabilize SOD1 structure in its nativelike conformation to attenuate the spreading of the misfolding signal and disease progression by preventing the exposure of pathogenic SOD1 epitopes. Phenotypically similar apo-SOD1 species with impaired metal binding properties may also be produced via oxidation of Cys¹¹¹, underscoring the diversity of SOD1 misfolding pathways.

The proteasome: friend and foe of mitochondrial biogenesis

Most mitochondrial proteins are synthesized in the cytosol and subsequently translocated as unfolded polypeptides into mitochondria. Cytosolic chaperones maintain precursor proteins in an import-competent state. This post-translational import reaction is under surveillance of the cytosolic ubiquitin-proteasome system, which carries out several distinguishable activities. On the one hand, the proteasome degrades nonproductive protein precursors from the cytosol and nucleus, import intermediates that are stuck in mitochondrial translocases, and misfolded or damaged proteins from the outer membrane and the intermembrane space. These surveillance activities of the proteasome are essential for mitochondrial functionality, as well as cellular fitness and survival. On the other hand, the proteasome competes with mitochondria for nonimported cytosolic precursor proteins, which can compromise mitochondrial biogenesis. In order to balance the positive and negative effects of the cytosolic protein quality control system on mitochondria, mitochondrial import efficiency directly regulates the capacity of the proteasome via transcription factor Rpn4 in yeast and nuclear respiratory factor (Nrf) 1 and 2 in animal cells. In this review, we provide a thorough overview of how the proteasome regulates mitochondrial biogenesis.

The Ubiquitin Proteasome System in Neuromuscular Disorders: Moving Beyond Movement

Neuromuscular disorders (NMDs) affect 1 in 3000 people worldwide. There are more than 150 different types of NMDs, where the common feature is the loss of muscle strength. These disorders are classified according to their neuroanatomical location, as motor neuron diseases, peripheral nerve diseases, neuromuscular junction diseases, and muscle diseases. Over the years, numerous studies have pointed to protein homeostasis as a crucial factor in the development of these fatal diseases. The ubiquitin-proteasome system (UPS) plays a fundamental role in maintaining protein homeostasis, being involved in protein degradation, among other cellular functions. Through a cascade of enzymatic reactions, proteins are ubiquitinated, tagged, and translocated to the proteasome to be degraded. Within the ubiquitin system, we can find three main groups of enzymes: E1 (ubiquitin-activating enzymes), E2 (ubiquitin-conjugating enzymes), and E3 (ubiquitin-protein ligases). Only the ubiquitinated proteins with specific chain linkages (such as K48) will be degraded by the UPS. In this review, we describe the relevance of this system in NMDs, summarizing the UPS proteins that have been involved in pathological conditions and neuromuscular disorders, such as Spinal Muscular Atrophy (SMA), Charcot-Marie-Tooth disease (CMT), or Duchenne Muscular Dystrophy (DMD), among others. A better knowledge of the processes involved in the maintenance of proteostasis may pave the way for future progress in neuromuscular disorder studies and treatments.

Primate differential redoxome (PDR) - A paradigm for understanding neurodegenerative diseases

Despite different phenotypic manifestations, mounting evidence points to similarities in the molecular basis of major neurodegenerative diseases (ND). CNS has evolved to be robust against hazard of ROS, a common perturbation aerobic organisms are confronted with. The trade-off of robustness is system's fragility against rare and unexpected perturbations. Identifying the points of CNS fragility is key for understanding etiology of ND. We postulated that the 'primate differential redoxome' (PDR), an assembly of proteins that contain cysteine residues present only in the primate orthologues of mammals, is likely to associate with an added level of regulatory functionalities that enhanced CNS robustness against ROS and facilitated evolution. The PDR contains multiple deterministic and susceptibility factors of major ND, which cluster to form coordinated redox networks regulating various cellular processes. The PDR analysis revealed a potential CNS fragility point, which appears to associates with a non-redundant PINK1-PRKN-SQSTM1(p62) axis coordinating protein homeostasis and mitophagy.

Mitochondrial Dysfunction, Neurogenesis, and Epigenetics: Putative Implications for Amyotrophic Lateral Sclerosis Neurodegeneration and Treatment

Amyotrophic lateral sclerosis (ALS) is a progressive and devastating multifactorial neurodegenerative disorder. Although the pathogenesis of ALS is still not completely understood, numerous studies suggest that mitochondrial deregulation may be implicated in its onset and progression. Interestingly, mitochondrial deregulation has also been associated with changes in neural stem cells (NSC) proliferation, differentiation, and migration. In this review, we highlight the importance of mitochondrial function for neurogenesis, and how both processes are correlated and may contribute to the pathogenesis of ALS; we have focused primarily on preclinical data from animal models of ALS, since to date no studies have evaluated this link using human samples. As there is currently no cure and no effective therapy to counteract ALS, we have also discussed how improving neurogenic function by epigenetic modulation could benefit ALS. In support of this hypothesis, changes in histone deacetylation can alter mitochondrial function, which in turn might ameliorate cellular proliferation as well as neuronal differentiation and migration. We propose that modulation of epigenetics, mitochondrial function, and neurogenesis might provide new hope for ALS patients, and studies exploring these new territories are warranted in the near future.

Silence superoxide dismutase 1 (SOD1): a promising therapeutic target for amyotrophic lateral sclerosis (ALS)

Introduction: Amyotrophic lateral sclerosis (ALS) is a progressive and incurable neurodegenerative disorder that targets upper and lower motor neurons and leads to fatal muscle paralysis. Mutations in the superoxide dismutase 1 (SOD1) gene are responsible for 15% of familial ALS cases, but several studies have indicated that SOD1 dysfunction may also play a pathogenic role in sporadic ALS. SOD1 induces numerous toxic effects through the pathological misfolding and aggregation of mutant SOD1 species, hence a reduction of the levels of toxic variants appears to be a promising therapeutic strategy for SOD1-related ALS. Several methods are used to modulate gene expression in vivo; these include RNA interference, antisense oligonucleotides (ASOs) and CRISPR/Cas9 technology.Areas covered: This paper examines the current approaches for gene silencing and the progress made in silencing SOD1 in vivo. It progresses to shed light on the key results and pitfalls of these studies and highlights the future challenges and new perspectives for this exciting research field.Expert opinion: Gene silencing strategies targeting SOD1 may represent effective approaches for familial and sporadic ALS-related neurodegeneration; however, the risk of off-target effects must be minimized, and effective and minimally invasive delivery strategies should be fine-tuned.

ALS Genetics, Mechanisms, and Therapeutics: Where Are We Now?

The scientific landscape surrounding amyotrophic lateral sclerosis (ALS) continues to shift as the number of genes associated with the disease risk and pathogenesis, and the cellular processes involved, continues to grow. Despite decades of intense research and over 50 potentially causative or disease-modifying genes identified, etiology remains unexplained and treatment options remain limited for the majority of ALS patients. Various factors have contributed to the slow progress in understanding and developing therapeutics for this disease. Here, we review the genetic basis of ALS, highlighting factors that have contributed to the elusiveness of genetic heritability. The most commonly mutated ALS-linked genes are reviewed with an emphasis on disease-causing mechanisms. The cellular processes involved in ALS pathogenesis are discussed, with evidence implicating their involvement in ALS summarized. Past and present therapeutic strategies and the benefits and limitations of the model systems available to ALS researchers are discussed with future directions for research that may lead to effective treatment strategies outlined.

The biophysics of superoxide dismutase-1 and amyotrophic lateral sclerosis

Few proteins have come under such intense scrutiny as superoxide dismutase-1 (SOD1). For almost a century, scientists have dissected its form, function and then later its malfunction in the neurodegenerative disease amyotrophic lateral sclerosis (ALS). We now know SOD1 is a zinc and copper metalloenzyme that clears superoxide as part of our antioxidant defence and respiratory regulation systems. The possibility of reduced structural integrity was suggested by the first crystal structures of human SOD1 even before deleterious mutations in the sod1 gene were linked to the ALS. This concept evolved in the intervening years as an impressive array of biophysical studies examined the characteristics of mutant SOD1 in great detail. We now recognise how ALS-related mutations perturb the SOD1 maturation processes, reduce its ability to fold and reduce its thermal stability and half-life. Mutant SOD1 is therefore predisposed to monomerisation, non-canonical self-interactions, the formation of small misfolded oligomers and ultimately accumulation in the tell-tale insoluble inclusions found within the neurons of ALS patients. We have also seen that several post-translational modifications could push wild-type SOD1 down this toxic pathway. Recently we have come to view ALS as a prion-like disease where both the symptoms, and indeed SOD1 misfolding itself, are transmitted to neighbouring cells. This raises the possibility of intervention after the initial disease presentation. Several small-molecule and biologic-based strategies have been devised which directly target the SOD1 molecule to change the behaviour thought to be responsible for ALS. Here we provide a comprehensive review of the many biophysical advances that sculpted our view of SOD1 biology and the recent work that aims to apply this knowledge for therapeutic outcomes in ALS.

Altered glucose catabolism in the presynaptic and perisynaptic compartments of SOD1G93A mouse spinal cord and motor cortex indicates that mitochondria are the site of bioenergetic imbalance in ALS

Amyotrophic lateral sclerosis is an adult-onset neurodegenerative disease that develops because of motor neuron death. Several mechanisms occur supporting neurodegeneration, including mitochondrial dysfunction. Recently, we demonstrated that the synaptosomes from the spinal cord of SOD1^G93A mice, an in vitro model of presynapses, displayed impaired mitochondrial metabolism at early pre-symptomatic stages of the disease, whereas perisynaptic astrocyte particles, or gliosomes, were characterized by mild energy impairment only at symptomatic stages. This work aimed to understand whether mitochondrial impairment is a consequence of upstream metabolic damage. We analyzed the critical pathways involved in glucose catabolism at presynaptic and perisynaptic compartments. Spinal cord and motor cortex synaptosomes from SOD1^G93A mice displayed high activity of hexokinase and phosphofructokinase, key glycolysis enzymes, and of citrate synthase and malate dehydrogenase, key Krebs cycle enzymes, but did not display high lactate dehydrogenase activity, the key enzyme in lactate fermentation. This enhancement was evident in the spinal cord from the early stages of the disease and in the motor cortex at only symptomatic stages. Conversely, an increase in glycolysis and lactate fermentation activity, but not Krebs cycle activity, was observed in gliosomes from the spinal cord and motor cortex of SOD1^G93A mice although only at the symptomatic stages of the disease. The cited enzymatic activities were enhanced in spinal cord and motor cortex homogenates, paralleling the time-course of the effect observed in synaptosomes and gliosomes. The observed metabolic modifications might be considered an attempt to restore altered energetic balance and indicate that mitochondria represent the ultimate site of bioenergetic impairment.

Mitochondrial Transport and Turnover in the Pathogenesis of Amyotrophic Lateral Sclerosis

Neurons are high-energy consuming cells, heavily dependent on mitochondria for ATP generation and calcium buffering. These mitochondrial functions are particularly critical at specific cellular sites, where ionic currents impose a large energetic burden, such as at synapses. The highly polarized nature of neurons, with extremely large axoplasm relative to the cell body, requires mitochondria to be efficiently transported along microtubules to reach distant sites. Furthermore, neurons are post-mitotic cells that need to maintain pools of healthy mitochondria throughout their lifespan. Hence, mitochondrial transport and turnover are essential processes for neuronal survival and function. In neurodegenerative diseases, the maintenance of a healthy mitochondrial network is often compromised. Numerous lines of evidence indicate that mitochondrial impairment contributes to neuronal demise in a variety of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), where degeneration of motor neurons causes a fatal muscle paralysis. Dysfunctional mitochondria accumulate in motor neurons affected by genetic or sporadic forms of ALS, strongly suggesting that the inability to maintain a healthy pool of mitochondria plays a pathophysiological role in the disease. This article critically reviews current hypotheses on mitochondrial involvement in the pathogenesis of ALS, focusing on the alterations of mitochondrial axonal transport and turnover in motor neurons.

Cross talk between SOD1 and the mitochondrial UPR in cancer and neurodegeneration

The mitochondrial unfolded protein response (UPR^mt) is rapidly gaining attention. While the CHOP (ATF4/5) axis of the UPR^mt was the first to be described, other axes have subsequently been reported. Validation of this complex pathway in C. elegans has been extensively studied. However, validation of the UPR^mt in mouse models of disease known to implicate mitochondrial reprogramming or dysfunction, such as cancer and neurodegeneration, respectively, is only beginning to emerge. This review summarizes recent findings and highlights the major role of the superoxide dismutase SOD1 in the communication between the mitochondria and the nucleus in these settings. While SOD1 has mostly been studied in the context of familial amyotrophic lateral sclerosis (fALS), recent studies suggest that SOD1 may be a potentially important mediator of the UPR^mt and converge to emphasize an increasingly vital role of SOD1 as a therapeutic target in cancer.

Mitochondria and Reactive Oxygen Species in Aging and Age-Related Diseases

Aging has been linked to several degenerative processes that, through the accumulation of molecular and cellular damage, can progressively lead to cell dysfunction and organ failure. Human aging is linked with a higher risk for individuals to develop cancer, neurodegenerative, cardiovascular, and metabolic disorders. The understanding of the molecular basis of aging and associated diseases has been one major challenge of scientific research over the last decades. Mitochondria, the center of oxidative metabolism and principal site of reactive oxygen species (ROS) production, are crucial both in health and in pathogenesis of many diseases. Redox signaling is important for the modulation of cell functions and several studies indicate a dual role for ROS in cell physiology. In fact, high concentrations of ROS are pathogenic and can cause severe damage to cell and organelle membranes, DNA, and proteins. On the other hand, moderate amounts of ROS are essential for the maintenance of several biological processes, including gene expression. In this review, we provide an update regarding the key roles of ROS-mitochondria cross talk in different fundamental physiological or pathological situations accompanying aging and highlighting that mitochondrial ROS may be a decisive target in clinical practice.

Characterization of the Mitochondrial Aerobic Metabolism in the Pre- and Perisynaptic Districts of the SOD1G93A Mouse Model of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult-onset fatal neurodegenerative disease characterized by muscle wasting, weakness, and spasticity due to a progressive degeneration of cortical, brainstem, and spinal motor neurons. The etiopathological causes are still largely obscure, although astrocytes definitely play a role in neuronal damage. Several mechanisms have been proposed to concur to neurodegeneration in ALS, including mitochondrial dysfunction. We have previously shown profound modifications of glutamate release and presynaptic plasticity in the spinal cord of the SOD1^G93A mouse model of ALS. In this work, we characterized, for the first time, the aerobic metabolism in two specific compartments actively involved in neurotransmission (i.e. the presynaptic district, using purified synaptosomes, and the perisynaptic astrocyte processes, using purified gliosomes) in SOD1^G93A mice at different stages of the disease. ATP/AMP ratio was lower in synaptosomes isolated from the spinal cord, but not from other brain areas, of SOD1^G93A vs. control mice. The energy impairment was linked to altered oxidative phosphorylation (OxPhos) and increment of lipid peroxidation. These metabolic dysfunctions were present during disease progression, starting at the very pre-symptomatic stages, and did not depend on a different number of mitochondria or a different expression of OxPhos proteins. Conversely, gliosomes showed a reduction of the ATP/AMP ratio only at the late stages of the disease and an increment of oxidative stress also in the absence of a significant decrement in OxPhos activity. Data suggest that the presynaptic neuronal moiety plays a pivotal role for synaptic energy metabolism dysfunctions in ALS. Changes in the perisynaptic compartment seem subordinated to neuronal damage.

A molecular chaperone activity of CCS restores the maturation of SOD1 fALS mutants

Superoxide dismutase 1 (SOD1) is an important metalloprotein for cellular oxidative stress defence, that is mutated in familiar variants of Amyotrophic Lateral Sclerosis (fALS). Some mutations destabilize the apo protein, leading to the formation of misfolded, toxic species. The Copper Chaperone for SOD1 (CCS) transiently interacts with SOD1 and promotes its correct maturation by transferring copper and catalyzing disulfide bond formation. By in vitro and in-cell NMR, we investigated the role of the SOD-like domain of CCS (CCS-D2). We showed that CCS-D2 forms a stable complex with zinc-bound SOD1 in human cells, that has a twofold stabilizing effect: it both prevents the accumulation of unstructured mutant SOD1 and promotes zinc binding. We further showed that CCS-D2 interacts with apo-SOD1 in vitro, suggesting that in cells CCS stabilizes mutant apo-SOD1 prior to zinc binding. Such molecular chaperone function of CCS-D2 is novel and its implications in SOD-linked fALS deserve further investigation.

Cu/Zn Superoxide Dismutase Forms Amyloid Fibrils under Near-Physiological Quiescent Conditions: The Roles of Disulfide Bonds and Effects of Denaturant

Cu/Zn superoxide dismutase (SOD1) forms intracellular aggregates that are pathological indicators of amyotrophic lateral sclerosis. A large body of research indicates that the entry point to aggregate formation is a monomeric, metal-ion free (apo), and disulfide-reduced species. Fibril formation by SOD1 in vitro has typically been reported only for harsh solvent conditions or mechanical agitation. Here we show that monomeric apo-SOD1 in the disulfide-reduced state forms fibrillar aggregates under near-physiological quiescent conditions. Monomeric apo-SOD1 with an intact intramolecular disulfide bond is highly resistant to aggregation under the same conditions. A cysteine-free variant of SOD1 exhibits fibrillization behavior and fibril morphology identical to those of disulfide-reduced SOD1, firmly establishing that intermolecular disulfide bonds or intramolecular disulfide shuffling are not required for aggregation and fibril formation. The decreased lag time for fibril formation resulting from reduction of the intramolecular disulfide bond thus primarily reflects the decreased stability of the folded state relative to partially unfolded states, rather than an active role of free sulfhydryl groups in mediating aggregation. Addition of urea to increase the amount of fully unfolded SOD1 increases the lag time for fibril formation, indicating that the population of this species does not dominate over other factors in determining the onset of aggregation. Our results contrast with previous results obtained for agitated samples, in which case amyloid formation was accelerated by denaturant. We reconcile these observations by suggesting that denaturants destabilize monomeric and aggregated species to different extents and thus affect nucleation and growth.

Neurobiology of axonal transport defects in motor neuron diseases: Opportunities for translational research?

Intracellular trafficking of cargoes is an essential process to maintain the structure and function of all mammalian cell types, but especially of neurons because of their extreme axon/dendrite polarisation. Axonal transport mediates the movement of cargoes such as proteins, mRNA, lipids, membrane-bound vesicles and organelles that are mostly synthesised in the cell body and in doing so is responsible for their correct spatiotemporal distribution in the axon, for example at specialised sites such as nodes of Ranvier and synaptic terminals. In addition, axonal transport maintains the essential long-distance communication between the cell body and synaptic terminals that allows neurons to react to their surroundings via trafficking of for example signalling endosomes. Axonal transport defects are a common observation in a variety of neurodegenerative diseases, and mutations in components of the axonal transport machinery have unequivocally shown that impaired axonal transport can cause neurodegeneration (reviewed in El-Kadi et al., 2007, De Vos et al., 2008; Millecamps and Julien, 2013). Here we review our current understanding of axonal transport defects and the role they play in motor neuron diseases (MNDs) with a specific focus on the most common form of MND, amyotrophic lateral sclerosis (ALS).

Sex specific activation of the ERα axis of the mitochondrial UPR (UPRmt) in the G93A-SOD1 mouse model of familial ALS

The mitochondrial unfolded protein response (UPRmt) is a transcriptional program aimed at restoring proteostasis in mitochondria. Upregulation of mitochondrial matrix proteases and heat shock proteins was initially described. Soon thereafter, a distinct UPRmt induced by misfolded proteins in the mitochondrial intermembrane space (IMS) and mediated by the estrogen receptor alpha (ERα), was found to upregulate the proteasome and the IMS protease OMI. However, the IMS-UPRmt was never studied in a neurodegenerative disease in vivo. Thus, we investigated the IMS-UPRmt in the G93A-SOD1 mouse model of familial ALS, since mutant SOD1 is known to accumulate in the IMS of neural tissue and cause mitochondrial dysfunction. As the ERα is most active in females, we postulated that a differential involvement of the IMS-UPRmt could be linked to the longer lifespan of females in the G93A-SOD1 mouse. We found a significant sex difference in the IMS-UPRmt, because the spinal cords of female, but not male, G93A-SOD1 mice showed elevation of OMI and proteasome activity. Then, using a mouse in which G93A-SOD1 was selectively targeted to the IMS, we demonstrated that the IMS-UPRmt could be specifically initiated by mutant SOD1 localized in the IMS. Furthermore, we showed that, in the absence of ERα, G93A-SOD1 failed to activate OMI and the proteasome, confirming the ERα dependence of the response. Taken together, these results demonstrate the IMS-UPRmt activation in SOD1 familial ALS, and suggest that sex differences in the disease phenotype could be linked to differential activation of the ERα axis of the IMS-UPRmt.

Enhancing Mitofusin/Marf ameliorates neuromuscular dysfunction in Drosophila models of TDP-43 proteinopathies

Transactive response DNA-binding protein 43 kDa (TDP-43) is considered a major pathological protein in amyotrophic lateral sclerosis and frontotemporal lobar degeneration. The precise mechanisms by which TDP-43 dysregulation leads to toxicity in neurons are not fully understood. Using TDP-43-expressing Drosophila, we examined whether mitochondrial dysfunction is a central determinant in TDP-43 pathogenesis. Expression of human wild-type TDP-43 in Drosophila neurons results in abnormally small mitochondria. The mitochondrial fragmentation is correlated with a specific decrease in the mRNA and protein levels of the Drosophila profusion gene mitofusin/marf. Importantly, overexpression of Marf ameliorates defects in spontaneous walking activity and startle-induced climbing response of TDP-43-expressing flies. Partial inactivation of the mitochondrial profission factor, dynamin-related protein 1, also mitigates TDP-43-induced locomotor deficits. Expression of TDP-43 impairs neuromuscular junction transmission upon repetitive stimulation of the giant fiber circuit that controls flight muscles, which is also ameliorated by Marf overexpression. We show here for the first time that enhancing the profusion gene mitofusin/marf is beneficial in an in vivo model of TDP-43 proteinopathies, serving as a potential therapeutic target.

Cysteine Modifications in the Pathogenesis of ALS

Several proteins are found misfolded and aggregated in sporadic and genetic forms of amyotrophic lateral sclerosis (ALS). These include superoxide dismutase (SOD1), transactive response DNA-binding protein (TDP-43), fused in sarcoma/translocated in liposarcoma protein (FUS/TLS), p62, vasolin-containing protein (VCP), Ubiquilin-2 and dipeptide repeats produced by unconventional RAN-translation of the GGGGCC expansion in C9ORF72. Up to date, functional studies have not yet revealed a common mechanism for the formation of such diverse protein inclusions. Consolidated studies have demonstrated a fundamental role of cysteine residues in the aggregation process of SOD1 and TDP43, but disturbance of protein thiols homeostatic factors such as protein disulfide isomerases (PDI), glutathione, cysteine oxidation or palmitoylation might contribute to a general aberration of cysteine residues proteostasis in ALS. In this article we review the evidence that cysteine modifications may have a central role in many, if not all, forms of this disease.

Oxygen-dependent activation of Cu,Zn-superoxide dismutase-1

Copper zinc superoxide dismutase (Sod1) is a critical enzyme in limiting reactive oxygen species in both the cytosol and the mitochondrial intermembrane space.

Old versus New Mechanisms in the Pathogenesis of ALS

Amyotrophic Lateral Sclerosis (ALS) is recognized as a very complex disease. As we have learned in the past 20 years from studies in patients and in models based on the expression of mutant SOD1, ALS is not a purely motor neuron disease as previously thought. While undoubtedly motor neurons are lost in patients, a number of alterations in those cell-types that interact functionally with motor neurons (astrocytes, microglia, muscle fibers, oligodendrocytes) take place even long before onset of symptoms. At the same time, disturbance of several, only partly inter-related physiological functions play some role in the onset and progression of the disease. Traditionally, mitochondrial damage and oxidative stress, excitotoxicity, neuroinflammation, altered axonal transport, ER stress, protein aggregation and defective removal of toxic proteins have been considered as key factors in the pathogenesis of ALS, with the relatively recent addition of disturbances in RNA metabolism. This complexity makes the search for an effective treatment extremely difficult and prompts further studies to reveal other possible, previously unappreciated aspects of the pathogenesis of ALS. In this review, we focus on previous knowledge on ALS mechanisms as well as new facets emerging from studies on genetic ALS patients and models that may both provide precious information for a novel therapeutic approach.

Pathways to mitochondrial dysfunction in ALS pathogenesis

Alterations in the structure and functions of mitochondria are a typical trait of Amyotrophic Lateral Sclerosis, a neurodegenerative disease characterized by a prominent degeneration of upper and lower motor neurons. The known gene mutations that are responsible for a small fraction of ALS cases point to a complex interplay between different mechanisms in the disease pathogenesis. Here we will briefly overview the genetic and mechanistic evidence that make dysfunction of mitochondria a candidate major player in this process.

Role of mitochondrial raft-like microdomains in the regulation of cell apoptosis

Lipid rafts are envisaged as lateral assemblies of specific lipids and proteins that dissociate and associate rapidly and form functional clusters in cell membranes. These structural platforms are not confined to the plasma membrane; indeed lipid microdomains are similarly formed at subcellular organelles, which include endoplasmic reticulum, Golgi and mitochondria, named raft-like microdomains. In addition, some components of raft-like microdomains are present within ER-mitochondria associated membranes. This review is focused on the role of mitochondrial raft-like microdomains in the regulation of cell apoptosis, since these microdomains may represent preferential sites where key reactions take place, regulating mitochondria hyperpolarization, fission-associated changes, megapore formation and release of apoptogenic factors. These structural platforms appear to modulate cytoplasmic pathways switching cell fate towards cell survival or death. Main insights on this issue derive from some pathological conditions in which alterations of microdomains structure or function can lead to severe alterations of cell activity and life span. In the light of the role played by raft-like microdomains to integrate apoptotic signals and in regulating mitochondrial dynamics, it is conceivable that these membrane structures may play a role in the mitochondrial alterations observed in some of the most common human neurodegenerative diseases, such as Amyotrophic lateral sclerosis, Huntington's chorea and prion-related diseases. These findings introduce an additional task for identifying new molecular target(s) of pharmacological agents in these pathologies.

SOD1 misplacing and mitochondrial dysfunction in amyotrophic lateral sclerosis pathogenesis

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease presenting as sporadic (sALS) or familial (fALS) forms. Even if the list of the genes underlining ALS greatly expanded, defects in superoxide dismutase 1 (SOD1), encoding the copper/zinc SOD1, still remain a major cause of fALS and are likely involved also in apparently sporadic presentations. The pathogenesis of ALS is still unknown, but several lines of evidence indicate that the mitochondrial accumulation of mutant SOD1 is an important mechanism of mitochondrial dysfunction, leading to motor neuron pathology and death. The intramitochondrial localization of mutant SOD1 is debated. Mutant SOD1 might accumulate inside the intermembrane space (IMS), overriding the physiological retention regulated by the copper chaperone for superoxide dismutase (CCS). On the other hand, misfolded SOD1 might deposit onto the outer mitochondrial membrane (OMM), clumping the transport across mitochondrial membranes and engaging mitochondrial-dependent cell apoptosis. The elucidation of the mechanisms ruling SOD1 localization and misplacing might shed light on peculiar ALS features such as cell selectivity and late onset. More importantly, these studies might disclose novel targets for therapeutic intervention in familial ALS as well as non-genetic forms. Finally, pharmacological or genetic manipulation aimed to prevent or counteract the intracellular shifting of mutant SOD1 could be effective for other neurodegenerative disorders featuring the toxic accumulation of misfolded proteins.

Mitochondrial metals as a potential therapeutic target in neurodegeneration

Transition metals are critical for enzyme function and protein folding, but in excess can mediate neurotoxic oxidative processes. As mitochondria are particularly vulnerable to oxidative damage due to radicals generated during ATP production, mitochondrial biometal homeostasis must therefore be tightly controlled to safely harness the redox potential of metal enzyme cofactors. Dysregulation of metal functions is evident in numerous neurological disorders including Alzheimer's disease, stroke, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and Friedrich's ataxia. This review describes the mitochondrial metal defects in these disorders and highlights novel metal-based therapeutic approaches that target mitochondrial metal homeostasis in neurological disorders.

A novel p.E121G SOD1 mutation in slowly progressive form of amyotrophic lateral sclerosis induces cytoplasmic aggregates in cultured motor neurons and reduces cell viability

Mutations in the SOD1 gene encoding the Cu/Zn superoxide dismutase-1 protein are responsible for amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease. To date a large number of mutations have been reported in SOD1, but only few of them have been studied and validated by functional studies. We present a novel mutation in SOD1 in a female suffering from slowly progressive ALS. This dominant mutation (c.365A > G) in exon 5 resulted in a substitution of a highly conserved amino acid (p.E121G) of the protein. Functional studies in the motor neuronal cell line NSC34 and in primary culture of mouse motor neurons revealed that this mutation p.E121G induced aggregates positive for SOD1 and ubiquitin, as well as reduced cell viability. These findings identified a novel causal mutation in ALS in close proximity with one of the three histidine residues (H120) of SOD1 interacting with copper.

SOD1, an unexpected novel target for cancer therapy

Cancer cells have elevated levels of reactive oxygen species (ROS), which are generated in majority by the mitochondria. In the mitochondrial matrix, the manganese dismutase SOD2 acts as a major anti-oxidant enzyme. The deacetylase SIRT3 regulates the activity of SOD2. Recently, SIRT3 was reported to be decreased in 87% of breast cancers, resulting therefore in a decrease in the activity of SOD2 and an elevation in ROS. In addition to SIRT3, we recently reported that SOD2 itself is down-regulated in breast cancer cell lines upon activation of oncogenes, such as Ras. Since in absence of SOD2, superoxide levels are elevated and may cause irreversible damage, mechanisms must exist to retain superoxide below a critical threshold and maintain viability of cancer cells. The copper/zinc dismutase SOD1 localizes in the cytoplasm, the inter-membrane space of the mitochondria and the nucleus. Emerging evidences from several groups now indicate that SOD1 is overexpressed in cancers and that the activity of SOD1 may be essential to maintain cellular ROS under this critical threshold. This review summarizes the studies reporting important roles of SOD1 in cancer and addresses the potential cross-talk between the overexpression of SOD1 and the regulation of the mitochondrial unfolded protein response (UPR(mt)). While mutations in SOD1 is the cause of 20% of cases of familial amyotrophic lateral sclerosis (fALS), a devastating neurodegenerative disease, these new studies expand the role of SOD1 to cancer.

Mechanisms of mutant SOD1 induced mitochondrial toxicity in amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), mitochondrial dysfunction is recognized as one of the key elements contributing to the pathology. Mitochondria are the major source of intracellular reactive oxygen species (ROS). Increased production of ROS as well as oxidative damage of proteins and lipids have been demonstrated in many models of ALS. Moreover, these changes were also observed in tissues of ALS patients indicative of important role for oxidative stress in the disease pathology. However, the origin of oxidative stress in ALS has remained unclear. ALS linked mutant Cu/Zn-superoxide dismutase 1 (SOD1) has been shown to significantly associate with mitochondria, especially in the spinal cord. In animal models, increased recruitment of mutant SOD1 (mutSOD1) to mitochondria appears already before the disease onset, suggestive of causative role for the manifestation of pathology. Recently, substantial in vitro and in vivo evidence has accumulated demonstrating that localization of mutSOD1 to the mitochondrial intermembrane space (IMS) inevitably leads to impairment of mitochondrial functions. However, the exact mechanisms of the selectivity and toxicity have remained obscure. Here we discuss the current knowledge on the role of mutSOD1 in mitochondrial dysfunction in ALS from the novel perspective emphasizing the misregulation of dismutase activity in IMS as a major mechanism for the toxicity.

Solid-state NMR studies of metal-free SOD1 fibrillar structures

Copper-zinc superoxide dismutase 1 (SOD1) is present in the protein aggregates deposited in motor neurons of amyotrophic lateral sclerosis (ALS) patients. ALS is a neurodegenerative disease that can be either sporadic (ca. 90%) or familial (fALS). The most widely studied forms of fALS are caused by mutations in the sequence of SOD1. Ex mortuo SOD1 aggregates are usually found to be amorphous. In vitro SOD1, in its immature reduced and apo state, forms fibrillar aggregates. Previous literature data have suggested that a monomeric SOD1 construct, lacking loops IV and VII, (apoSODΔIV-VII), shares the same fibrillization properties of apoSOD1, both proteins having the common structural feature of the central β-barrel. In this work, we show that structural information can be obtained at a site-specific level from solid-state NMR. The residues that are sequentially assignable are found to be located at the putative nucleation site for fibrillar species formation in apoSOD, as detected by other experimental techniques.

Regulation of CuZnSOD and its redox signaling potential: implications for amyotrophic lateral sclerosis

SIGNIFICANCE

Molecular oxygen is a Janus-faced electron acceptor for biological systems, serving as a reductant for respiration, or as the genesis for oxygen-derived free radicals that damage macromolecules. Superoxide is well known to perturb nonheme iron proteins, including Fe/S proteins such as aconitase and succinate dehydrogenase, as well as other enzymes containing labile iron such as the prolyl hydroxylase domain-containing family of enzymes; whereas hydrogen peroxide is more specific for two-electron reactions with thiols on glutathione, glutaredoxin, thioredoxin, and the peroxiredoxins.

RECENT ADVANCES

Over the past two decades, familial cases of amyotrophic lateral sclerosis (ALS) have been shown to have an association with commonly altered superoxide dismutase 1 (SOD1) activity, expression, and protein structure. This has led to speculation that an altered redox balance may have a role in creating the ALS phenotype.

CRITICAL ISSUES

While SOD1 alterations in familial ALS are manifold, they generally create perturbations in the flux of electrons. The nexus of SOD1 between one- and two-electron signaling processes places it at a key signaling regulatory checkpoint for governing cellular responses to physiological and environmental cues.

FUTURE DIRECTIONS

The manner in which ALS-associated mutations adjust SOD1's role in controlling the flow of electrons between one- and two-electron signaling processes remains obscure. Here, we discuss the ways in which SOD1 mutations influence the form and function of copper zinc SOD, the consequences of these alterations on free radical biology, and how these alterations might influence cell signaling during the onset of ALS.

Defective mitochondrial dynamics is an early event in skeletal muscle of an amyotrophic lateral sclerosis mouse model

Mitochondria are dynamic organelles that constantly undergo fusion and fission to maintain their normal functionality. Impairment of mitochondrial dynamics is implicated in various neurodegenerative disorders. Amyotrophic lateral sclerosis (ALS) is an adult-onset neuromuscular degenerative disorder characterized by motor neuron death and muscle atrophy. ALS onset and progression clearly involve motor neuron degeneration but accumulating evidence suggests primary muscle pathology may also be involved. Here, we examined mitochondrial dynamics in live skeletal muscle of an ALS mouse model (G93A) harboring a superoxide dismutase mutation (SOD1^G93A). Using confocal microscopy combined with overexpression of mitochondria-targeted photoactivatable fluorescent proteins, we discovered abnormal mitochondrial dynamics in skeletal muscle of young G93A mice before disease onset. We further demonstrated that similar abnormalities in mitochondrial dynamics were induced by overexpression of mutant SOD1^G93A in skeletal muscle of normal mice, indicating the SOD1 mutation drives ALS-like muscle pathology in the absence of motor neuron degeneration. Mutant SOD1^G93A forms aggregates inside muscle mitochondria and leads to fragmentation of the mitochondrial network as well as mitochondrial depolarization. Partial depolarization of mitochondrial membrane potential in normal muscle by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) caused abnormalities in mitochondrial dynamics similar to that in the SOD1^G93A model muscle. A specific mitochondrial fission inhibitor (Mdivi-1) reversed the SOD1^G93A action on mitochondrial dynamics, indicating SOD1^G93A likely promotes mitochondrial fission process. Our results suggest that accumulation of mutant SOD1^G93A inside mitochondria, depolarization of mitochondrial membrane potential and abnormal mitochondrial dynamics are causally linked and cause intrinsic muscle pathology, which occurs early in the course of ALS and may actively promote ALS progression.

The effect of SOD1 mutation on cellular bioenergetic profile and viability in response to oxidative stress and influence of mutation-type

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disorder characterized by the progressive degeneration of motor neurons. Substantial evidence implicates oxidative stress and mitochondrial dysfunction as early events in disease progression. Our aim was to ascertain whether mutation of the SOD1 protein increases metabolic functional susceptibility to oxidative stress. Here we used a motor neuron-like cell line (NSC34) stably transfected with various human mutant SOD1 transgenes (G93A, G37R, H48Q) to investigate the impact of oxidative stress on cell viability and metabolic function within intact cells. NSC34 cells expressing mutant SOD1 showed a dose dependent reduction in cell viability when exposed to oxidative stress induced by hydrogen peroxide, with variation between mutations. The G93A transfectants showed greater cell death and LDH release compared to cells transfected with the other SOD1 mutations, and H48Q showed an accelerated decline at later time points. Differences in mitochondrial bioenergetics, including mitochondrial respiration, coupling efficiency and proton leak, were identified between the mutations, consistent with the differences observed in viability. NSC34 cells expressing G93A SOD1 displayed reduced coupled respiration and mitochondrial membrane potential compared to controls. Furthermore, the G93A mutation had significantly increased metabolic susceptibility to oxidative stress, with hydrogen peroxide increasing ROS production, reducing both cellular oxygen consumption and glycolytic flux in the cell. This study highlights bioenergetic defects within a cellular model of ALS and suggests that oxidative stress is not only detrimental to oxygen consumption but also glycolytic flux, which could lead to an energy deficit in the cell.

Gene expression profile of SOD1-G93A mouse spinal cord, blood and muscle

The exact pathway leading to neuron death and muscle atrophy in amyotrophic lateral sclerosis (ALS) has not yet been elucidated. Gene expression profile of spinal cord, blood and muscle could provide signalling pathways and systemic alterations useful for future biomarker development. In our study we compared whole genome expression profiles of lumbar spinal cord with peripheral blood and tibialis anterior muscle in 16 mutant SOD1-G93A mice and 15 wild-type littermates. In SOD1-G93A mice, 11 genes were significantly differentially expressed in spinal cord, and 16 genes in blood, while much larger transcriptional changes were noted in muscle (1745 genes significant; six overlapping with spinal cord (0.3%)) probably due to muscle atrophy. Overlap with spinal cord was enriched for significant genes in blood (six of 16 overlapping with spinal cord (37.5%)). Three genes were significantly down-regulated in all three tissues, and were closely related to mitochondrial function. Furthermore, clustering the significant genes in spinal cord and in blood, but not in muscle, could identify the SOD1-G93A mice. We conclude that blood gene expression profile overlapped with profile of spinal cord, allowing differentiation of SOD1-G93A mice from wild-type littermates. Blood gene expression profiling may be a promising biomarker for ALS patients.

Amyotrophic lateral sclerosis: new insights into underlying molecular mechanisms and opportunities for therapeutic intervention

Recent years have witnessed a renewed interest in the pathogenic mechanisms of amyotrophic lateral sclerosis (ALS), a late-onset progressive degeneration of motor neurons. The discovery of new genes associated with the familial form of the disease, along with a deeper insight into pathways already described for this disease, has led scientists to reconsider previous postulates. While protein misfolding, mitochondrial dysfunction, oxidative damage, defective axonal transport, and excitotoxicity have not been dismissed, they need to be re-examined as contributors to the onset or progression of ALS in the light of the current knowledge that the mutations of proteins involved in RNA processing, apparently unrelated to the previous "old partners," are causative of the same phenotype. Thus, newly envisaged models and tools may offer unforeseen clues on the etiology of this disease and hopefully provide the key to treatment.

S-nitrosylated protein disulfide isomerase contributes to mutant SOD1 aggregates in amyotrophic lateral sclerosis

A major hallmark of mutant superoxide dismutase (SOD1)-linked familial amyotrophic lateral sclerosis is SOD1-immunopositive inclusions found within motor neurons. The mechanism by which SOD1 becomes aggregated, however, remains unclear. In this study, we aimed to investigate the role of nitrosative stress and S-nitrosylation of protein disulfide isomerase (PDI) in the formation of SOD1 aggregates. Our data show that with disease progression inducible nitric oxide synthase (iNOS) was up-regulated, which generated high levels of nitric oxide (NO) and subsequently induced S-nitrosylation of PDI in the spinal cord of mutant SOD1 transgenic mice. This was further confirmed by in vitro observation that treating SH-SY5Y cells with NO donor S-nitrosocysteine triggered a dose-dependent formation of S-nitrosylated PDI. When mutant SOD1 was over-expressed in SH-SY5Y cells, the iNOS expression was up-regulated, and NO generation was consequently increased. Furthermore, both S-nitrosylation of PDI and the formation of mutant SOD1 aggregates were detected in the cells expressing mutant SOD1(G93A). Blocking NO generation with the NOS inhibitor N-nitro-L-arginine attenuated the S-nitrosylation of PDI and inhibited the formation of mutant SOD1 aggregates. We conclude that NO-mediated S-nitrosylation of PDI is a contributing factor to the accumulation of mutant SOD1 aggregates in amyotrophic lateral sclerosis.

Mitochondrial dysfunction in familial amyotrophic lateral sclerosis

A growing body of evidence suggests that mitochondrial dysfunctions play a crucial role in the pathogenesis of various neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS), a neurodegenerative disease affecting both upper and lower motor neurons. Although ALS is predominantly a sporadic disease, approximately 10% of cases are familial. The most frequent familial form is caused by mutations in the gene encoding Cu/Zn superoxide dismutase 1 (SOD1). A dominant toxic gain of function of mutant SOD1 has been considered as the cause of the disease and mitochondria are thought to be key players in the pathogenesis. However, the exact nature of the link between mutant SOD1 and mitochondrial dysfunctions remains to be established. Here, we briefly review the evidence for mitochondrial dysfunctions in familial ALS and discuss a possible link between mutant SOD1 and mitochondrial dysfunction.

In vivo pathogenic role of mutant SOD1 localized in the mitochondrial intermembrane space

Mutations in Cu,Zn superoxide dismutase (SOD1) are associated with familial amyotrophic lateral sclerosis (ALS). Mutant SOD1 causes a complex array of pathological events, through toxic gain of function mechanisms, leading to selective motor neuron degeneration. Mitochondrial dysfunction is among the well established toxic effects of mutant SOD1, but its mechanisms are just starting to be elucidated. A portion of mutant SOD1 is localized in mitochondria, where it accumulates mostly on the outer membrane and inside the intermembrane space (IMS). Evidence in cultured cells suggests that mutant SOD1 in the IMS causes mitochondrial dysfunction and compromises cell viability. Therefore, to test its pathogenic role in vivo we generated transgenic mice expressing G93A mutant or wild-type (WT) human SOD1 targeted selectively to the mitochondrial IMS (mito-SOD1). We show that mito-SOD1 is correctly localized in the IMS, where it oligomerizes and acquires enzymatic activity. Mito-G93ASOD1 mice, but not mito-WTSOD1 mice, develop a progressive disease characterized by body weight loss, muscle weakness, brain atrophy, and motor impairment, which is more severe in females. These symptoms are associated with reduced spinal motor neuron counts and impaired mitochondrial bioenergetics, characterized by decreased cytochrome oxidase activity and defective calcium handling. However, there is no evidence of muscle denervation, a cardinal pathological feature of ALS. Together, our findings indicate that mutant SOD1 in the mitochondrial IMS causes mitochondrial dysfunction and neurodegeneration, but per se it is not sufficient to cause a full-fledged ALS phenotype, which requires the participation of mutant SOD1 localized in other cellular compartments.