The neurobiology of amyotrophic lateral sclerosis

Progress in therapy development for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease that cannot be slowed substantially using any currently-available clinical tools. Through decades of studying sporadic and familial ALS (SALS and FALS), researchers are coming to understand ALS as a complex syndrome with diverse genetic and environmental etiologies. It is know appreciated that motor neuron degeneration in ALS requires active (gain of function) and passive (loss of function) events to occur in non-neuronal cells, especially astrocytes and microglia. These neuroinflammatory processes produce paracrine factors that detrimentally affect motor neurons, precipitating protein aggregation and compromising cytoskeletal integrity. The result is a loss of neuronal homeostasis and progressive die-back of motor axons culminating in death of the afflicted motor neurons. This review will discuss experimental therapeutics that have been tested in murine ALS models, with an emphasis on those that have progressed to human clinical trials. Reasons will be considered for the frequent failure of preclinical successes to translate into positive clinical outcomes. Finally, this review will explore current trends in experimental therapeutics for ALS with emphasis on the emerging interest in axon guidance signaling pathways as novel targets for pharmacological support of neural cytoskeletal structure and function in order to slow ALS.

Gas7-deficient mouse reveals roles in motor function and muscle fiber composition during aging

Background

Growth arrest-specific gene 7 (Gas7) has previously been shown to be involved in neurite outgrowth in vitro; however, its actual role has yet to be determined. To investigate the physiological function of Gas7 in vivo, here we generated a Gas7-deficient mouse strain with a labile Gas7 mutant protein whose functions are similar to wild-type Gas7.

Methodology/Principal Findings

Our data show that aged Gas7-deficient mice have motor activity defects due to decreases in the number of spinal motor neurons and in muscle strength, of which the latter may be caused by changes in muscle fiber composition as shown in the soleus. In cross sections of the soleus of Gas7-deficient mice, gross morphological features and levels of myosin heavy chain I (MHC I) and MHC II markers revealed significantly fewer fast fibers. In addition, we found that nerve terminal sprouting, which may be associated with slow and fast muscle fiber composition, was considerably reduced at neuromuscular junctions (NMJ) during aging.

Conclusions/Significance

These findings indicate that Gas7 is involved in motor neuron function associated with muscle strength maintenance.

Mechanisms of loss of functions of human angiogenin variants implicated in amyotrophic lateral sclerosis

Background

Mutations in the coding region of angiogenin (ANG) gene have been found in patients suffering from Amyotrophic Lateral Sclerosis (ALS). Neurodegeneration results from the loss of angiogenic ability of ANG (protein coded by ANG). In this work, we performed extensive molecular dynamics (MD) simulations of wild-type ANG and disease associated ANG variants to elucidate the mechanism behind the loss of ribonucleolytic activity and nuclear translocation activity, functions needed for angiogenesis.

Methodology/Principal Findings

MD simulations were carried out to study the structural and dynamic differences in the catalytic site and nuclear localization signal residues between WT-ANG (Wild-type ANG) and six mutants. Variants K17I, S28N, P112L and V113I have confirmed association with ALS, while T195C and A238G single nucleotide polymorphisms (SNPs) encoding L35P and K60E mutants respectively, have not been associated with ALS. Our results show that loss of ribonucleolytic activity in K17I is caused by conformational switching of the catalytic residue His114 by 99°. The loss of nuclear translocation activity of S28N and P112L is caused by changes in the folding of the residues ³¹RRR³³ that result in the reduction in solvent accessible surface area (SASA). Consequently, we predict that V113I will exhibit loss of angiogenic properties by loss of nuclear translocation activity and L35P by loss of both ribonucleolytic activity and nuclear translocation activity. No functional loss was inferred for K60E. The MD simulation results were supported by hydrogen bond interaction analyses and molecular docking studies.

Conclusions/Significance

Conformational switching of catalytic residue His114 seems to be the mechanism causing loss of ribonucleolytic activity and reduction in SASA of nuclear localization signal residues ³¹RRR³³ results in loss of nuclear translocation activity in ANG mutants. Therefore, we predict that L35P mutant, would exhibit loss of angiogenic functions, and hence would correlate with ALS while K60E would not show any loss.

Axonal transport deficits and degeneration can evolve independently in mouse models of amyotrophic lateral sclerosis

Axonal transport deficits have been reported in many neurodegenerative conditions and are widely assumed to be an immediate causative step of axon and synapse loss. By imaging changes in axonal morphology and organelle transport over time in several animal models of amyotrophic lateral sclerosis (ALS), we now find that deficits in axonal transport of organelles (mitochondria, endosomes) and axon degeneration can evolve independently. This conclusion rests on the following results: (i) Axons can survive despite long-lasting transport deficits: In the SOD(G93A) model of ALS, transport deficits are detected soon after birth, months before the onset of axon degeneration. (ii) Transport deficits are not necessary for axon degeneration: In the SOD(G85R) model of ALS, motor axons degenerate, but transport is unaffected. (iii) Axon transport deficits are not sufficient to cause immediate degeneration: In mice that overexpress wild-type superoxide dismutase-1 (SOD(WT)), axons show chronic transport deficits, but survive. These data suggest that disturbances of organelle transport are not a necessary step in the emergence of motor neuron degeneration.

The preclinical discovery of amyotrophic lateral sclerosis drugs

INTRODUCTION

Amyotrophic lateral sclerosis (ALS), also referred to as Lou Gehrig's disease, is characterized by the progressive loss of cells in the brain and spinal cord that leads to debilitation and death in 3 - 5 years. Only one therapeutic drug, riluzole, has been approved for ALS and this drug improves survival by 2 - 3 months. The need for new therapeutics that can postpone or slow the progression of the motor deficits and prolong survival is still a strong unmet medical need.

AREAS COVERED

Although there are a number of drugs currently in clinical trials for ALS, this review provides an overview of the most promising biological targets and preclinical strategies that are currently being developed and deployed. The list of targets for ALS was compiled from a variety of websites including individual companies that have ALS programs and include those from the author's experience.

EXPERT OPINION

Progress is being made in the identification of possible new therapeutics for ALS with recent efforts in understanding the genetic causes of the disease, susceptibility factors and the development of additional preclinical animal models. However, many challenges remain in the identification of new ALS therapeutics including: the use of relevant biomarkers, the need for an earlier diagnosis of the disease and additional animal models. Multiple strategies need to be tested in the clinic in order to determine what will be effective in patients.

Protein misdirection inside and outside motor neurons in Amyotrophic Lateral Sclerosis (ALS): a possible clue for therapeutic strategies

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease characterized by progressive muscle wasting and weakness with no effective cure. Emerging evidence supports the notion that the abnormal conformations of ALS-linked proteins play a central role in triggering the motor neuron degeneration. In particular, mutant types of superoxide dismutase 1 (SOD1) and TAR DNA binding protein 43kDa (TDP-43) are key molecules involved in the pathogenesis of familial and sporadic ALS, respectively. The commonalities of the two proteins include a propensity to aggregate and acquire detrimental conformations through oligomerization, fragmentation, or post-translational modification that may drive abnormal subcellular localizations. Although SOD1 is a major cytosolic protein, mutated SOD1 has been localized to mitochondria, endoplasmic reticulum, and even the extracellular space. The nuclear exclusion of TDP-43 is a pathological hallmark for ALS, although the pathogenic priority remains elusive. Nevertheless, these abnormal behaviors based on the protein misfolding are believed to induce diverse intracellular and extracellular events that may be tightly linked to non-cell-autonomous motor neuron death. The generation of mutant- or misfolded protein-specific antibodies would help to uncover the distribution and propagation of the ALS-linked proteins, and to design a therapeutic strategy to clear such species. Herein we review the literature regarding the mislocalization of ALS-linked proteins, especially mutant SOD1 and TDP-43 species, and discuss the rationale of molecular targeting strategies including immunotherapy.

The role of environmental exposures in neurodegeneration and neurodegenerative diseases

Neurodegeneration describes the loss of neuronal structure and function. Numerous neurodegenerative diseases are associated with neurodegeneration. Many are rare and stem from purely genetic causes. However, the prevalence of major neurodegenerative diseases is increasing with improvements in treating major diseases such as cancers and cardiovascular diseases, resulting in an aging population. The neurological consequences of neurodegeneration in patients can have devastating effects on mental and physical functioning. The causes of most cases of prevalent neurodegenerative diseases are unknown. The role of neurotoxicant exposures in neurodegenerative disease has long been suspected, with much effort devoted to identifying causative agents. However, causative factors for a significant number of cases have yet to be identified. In this review, the role of environmental neurotoxicant exposures on neurodegeneration in selected major neurodegenerative diseases is discussed. Alzheimer's disease, Parkinson's disease, multiple sclerosis, and amyotrophic lateral sclerosis were chosen because of available data on environmental influences. The special sensitivity the nervous system exhibits to toxicant exposure and unifying mechanisms of neurodegeneration are explored.

Mesenchymal stromal cells prolong the lifespan in a rat model of amyotrophic lateral sclerosis

BACKGROUND AIMS

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder characterized by the loss of brain and spinal cord motor neurons (MN). The intraspinal and systemic grafting of mesenchymal stromal cells (MSC) was used to treat symptomatic transgenic rats overexpressing human superoxide dismutase 1 (SOD1) in order to alleviate the disease course and prolong the animals' lifespan.

METHODS

At the age of 16 weeks (disease onset) the rats received two grafts of MSC expressing green fluorescent protein (GFP(+) MSC) on the same day, intraspinally (10(5) cells) and intravenously (2 × 10(6) cells). Sham-treated animals were injected with phosphate-buffered saline (PBS). Motor activity, grip strength and body weight were tested, followed by immunohistochemical analysis.

RESULTS

The combined grafting of MSC into symptomatic rats had a significant effect on motor activity and grip strength starting 4 weeks after transplantation. The lifespan of animals in the treated group was 190 ± 3.33 days compared with 179 ± 3.6 days in the control group of animals. Treated rats had a larger number of MN at the thoracic and lumbar levels; these MN were of larger size, and the intensity of terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick-end labeling (TUNEL) staining in the somas of apoptotic MN at the thoracic level was much lower than in sham-treated animals. Transplanted GFP(+) MSC survived in the spinal cord until the end stage of the disease and migrated both rostrally and caudally from the injection site.

CONCLUSIONS

Intraspinal and intravenous transplantation of MSC has a beneficial and possibly synergistic effect on the lifespan of ALS animals.

The benefits and limitations of animal models for translational research in neurodegenerative diseases

Age-related neurodegenerative diseases are largely limited to humans and rarely occur spontaneously in animals. Genetically engineered mouse models recapitulate aspects of the corresponding human diseases and are instrumental in studying disease mechanisms and testing therapeutic strategies. If considered within the range of their validity, mouse models have been predictive of clinical outcome. Translational failure is less the result of the incomplete nature of the models than of inadequate preclinical studies and misinterpretation of the models. This commentary summarizes current models and highlights key questions we should be asking about animal models, as well as questions that cannot be answered with the current models.