The role of glial cells in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) has traditionally been considered a neuron-centric disease. This view is now outdated, with increasing recognition of cell autonomous and non-cell autonomous contributions of central and peripheral nervous system glia to ALS pathomechanisms. With glial research rapidly accelerating, we comprehensively interrogate the roles of astrocytes, microglia, oligodendrocytes, ependymal cells, Schwann cells and satellite glia in nervous system physiology and ALS-associated pathology. Moreover, we highlight the inter-glial, glial-neuronal and inter-system polylogue which constitutes the healthy nervous system and destabilises in disease. We also propose classification based on function for complex glial reactive phenotypes and discuss the pre-requisite for integrative modelling to advance translation. Given the paucity of life-enhancing therapies currently available for ALS patients, we discuss the promising potential of harnessing glia in driving ALS therapeutic discovery.

Therapeutic Effect of an Ursolic Acid-Based Nutraceutical on Neuronal Regeneration after Sciatic Nerve Injury

Peripheral nerve injuries lead to severe functional impairments and long recovery times, with limited effectiveness and accessibility of current treatments. This has increased interest in natural bioactive compounds, such as ursolic acid (UA). Our study evaluated the effect of an oleolyte rich in UA from white grape pomace (WGPO) on neuronal regeneration in mice with induced sciatic nerve resection, administered concurrently with the induced damage (the WGPO group) and 10 days prior (the PRE-WGPO group). The experiment was monitored at two-time points (4 and 10 days) after injury. After 10 days, the WGPO group demonstrated a reduction in muscle atrophy, evidenced by an increased number and diameter of muscle fibers and a decreased Atrogin-1 and Murf-1 expression relative to the denervated control. It was also observed that 85.7% of neuromuscular junctions (NMJs) were fully innervated, as indicated by the colocalization of α-bungarotoxin and synaptophysin, along with the significant modulation of Oct-6 and S-100. The PRE-WGPO group showed a more beneficial effect on nerve fiber reformation, with a significant increase in myelin protein zero and 95.2% fully innervated NMJs, and a pro-hypertrophic effect in resting non-denervated muscles. Our findings suggest WGPO as a potential treatment for various conditions that require the repair of nerve and muscle injuries.

Amyotrophic lateral sclerosis

The scientific landscape surrounding amyotrophic lateral sclerosis has shifted immensely with a number of well-defined ALS disease-causing genes, each with related phenotypical and cellular motor neuron processes that have come to light. Yet in spite of decades of research and clinical investigation, there is still no etiology for sporadic amyotrophic lateral sclerosis, and treatment options even for those with well-defined familial syndromes are still limited. This chapter provides a comprehensive review of the genetic basis of amyotrophic lateral sclerosis, highlighting factors that contribute to its heritability and phenotypic manifestations, and an overview of past, present, and upcoming therapeutic strategies.

The Cell Autonomous and Non-Cell Autonomous Aspects of Neuronal Vulnerability and Resilience in Amyotrophic Lateral Sclerosis

Simple Summary

Amyotrophic lateral sclerosis (ALS) is a fatal disease characterized by a progressive paralysis due to the loss of particular neurons in our nervous system called motor neurons, that exert voluntary control of all our skeletal muscles. It is not entirely understood why motor neurons are particularly vulnerable in ALS, neither is it completely clear why certain groups of motor neurons, including those that regulate eye movement, are rather resilient to this disease. However, both vulnerability and resilience to ALS likely reflect cell intrinsic properties of different motor neuron subpopulations as well as non-cell autonomous events regulated by surrounding cell types. In this review we dissect the particular properties of different motor neuron types and their responses to disease that may underlie their respective vulnerabilities and resilience. Disease progression in ALS involves multiple cell types that are closely connected to motor neurons and we here also discuss their contributions to the differential vulnerability of motor neurons.

Abstract

Amyotrophic lateral sclerosis (ALS) is defined by the loss of upper motor neurons (MNs) that project from the cerebral cortex to the brain stem and spinal cord and of lower MNs in the brain stem and spinal cord which innervate skeletal muscles, leading to spasticity, muscle atrophy, and paralysis. ALS involves several disease stages, and multiple cell types show dysfunction and play important roles during distinct phases of disease initiation and progression, subsequently leading to selective MN loss. Why MNs are particularly vulnerable in this lethal disease is still not entirely clear. Neither is it fully understood why certain MNs are more resilient to degeneration in ALS than others. Brain stem MNs of cranial nerves III, IV, and VI, which innervate our eye muscles, are highly resistant and persist until the end-stage of the disease, enabling paralyzed patients to communicate through ocular tracking devices. MNs of the Onuf’s nucleus in the sacral spinal cord, that innervate sphincter muscles and control urogenital functions, are also spared throughout the disease. There is also a differential vulnerability among MNs that are intermingled throughout the spinal cord, that directly relate to their physiological properties. Here, fast-twitch fatigable (FF) MNs, which innervate type IIb muscle fibers, are affected early, before onset of clinical symptoms, while slow-twitch (S) MNs, that innervate type I muscle fibers, remain longer throughout the disease progression. The resilience of particular MN subpopulations has been attributed to intrinsic determinants and multiple studies have demonstrated their unique gene regulation and protein content in health and in response to disease. Identified factors within resilient MNs have been utilized to protect more vulnerable cells. Selective vulnerability may also, in part, be driven by non-cell autonomous processes and the unique surroundings and constantly changing environment close to particular MN groups. In this article, we review in detail the cell intrinsic properties of resilient and vulnerable MN groups, as well as multiple additional cell types involved in disease initiation and progression and explain how these may contribute to the selective MN resilience and vulnerability in ALS.

Pathogenic Extracellular Vesicle (EV) Signaling in Amyotrophic Lateral Sclerosis (ALS)

Extracellular vesicles (EVs), once considered a pathway for cells to remove waste, have now emerged as an important mechanism for intercellular communication. EVs are particularly appealing in understanding the central nervous system (CNS) communication, given that there are very diverse cell types in the CNS and constant communications among various cells to respond to the frequently changing environment. While they are heterogeneous and new vesicles are continuously to be discovered, EVs are primarily classified as plasma membrane-derived microvesicles (MVs) and endosome-derived exosomes. Secretion of EVs has been shown from all CNS cell types in vitro and intercellular EV signaling has been implicated in neural development, axon integrity, neuron to glia communication, and propagation of protein aggregates formed by disease pathogenic proteins. However, significant hurdles remain to be tackled in understanding their physiological and pathological roles as well as how they can be developed as biomarkers or new therapeutics. Here we provide our summary on the known cell biology of EVs and discuss opportunities and challenges in understanding EV biology in the CNS and particularly their involvement in ALS pathogenesis.

Tissue-engineered in vitro modeling of the impact of Schwann cells in amyotrophic lateral sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a devastating neurodegenerative disease affecting upper and lower motor neurons (MN). To investigate whether Schwann cells could be involved in the disease pathogenesis, we developed a tissue-engineered 3D in vitro model that combined MNs cocultured with astrocytes and microglia seeded on top of a collagen sponge populated with epineurium fibroblasts to enable 3D axonal migration. C2C12 myoblasts were seeded underneath the sponge in presence or absence of Schwann cells. To reproduce an ALS cellular microenvironment, MNs, astrocytes and microglia were extracted from SOD1^G93A mice recapitulating many aspects of the human disease. This 3D ALS in vitro model was compared with a 3D control made of cells isolated from SOD1^WT mice. We showed that normal Schwann cells strongly enhanced MN axonal migration in the 3D control model but had no effect in the ALS model. However, ALS-derived Schwann cells isolated from SOD1^G93A mice failed to significantly improve axonal migration in both models. These results suggest that a cell therapy using healthy Schwann cells may not be effective in promoting axonal regeneration in ALS. In addition, this 3D ALS model could be used to study the impact of other cell types on ALS by various combinations of normal and diseased cells. This article is protected by copyright. All rights reserved.

Mini-Review: Induced pluripotent stem cells and the search for new cell-specific ALS therapeutic targets

Amongst the most important discoveries in ALS pathobiology are the works demonstrating that multiple cell types contribute to disease onset and progression. However, a significant limitation in ALS research is the inability to obtain tissues from ALS patient brain and spinal cord during the course of the disease. In vivo modeling has provided insights into the role of these cell subtypes in disease onset and progression. However, in vivo models also have shortcomings, including the reliance on a limited number of models based upon hereditary forms of the disease. Therefore, using human induced pluripotent stem cells (iPSC) reprogrammed from somatic cells of ALS patients, with both hereditary and sporadic forms of the disease, and differentiated into cell subtypes of both the central nervous system (CNS) and peripheral nervous system (PNS), have become powerful complementary tools for investigating basic mechanisms of disease as well as a platform for drug discovery. Motor neuron and other neuron subtypes, as well as non-neuronal cells have been differentiated from human iPSC and studied for their potential contributions to ALS pathobiology. As iPSC technologies have advanced, 3D modeling with multicellular systems organised in microfluidic chambers or organoids are the next step in validating the pathways and therapeutic targets already identified. Precision medicine approaches with iPSC using either traditional strategies of screening drugs that target a known pathogenic mechanism as well as "blind-to-target" drug screenings that allow for patient stratification based on drug response rather than clinical characteristics are now being employed.

AAV9-mediated gene delivery of MCT1 to oligodendrocytes does not provide a therapeutic benefit in a mouse model of ALS

Oligodendrocyte dysfunction has been implicated in the pathophysiology of amyotrophic lateral sclerosis (ALS), a neurodegenerative disorder characterized by progressive motor neuron loss. The failure of trophic support provided by oligodendrocytes is associated with a concomitant reduction in oligodendroglial monocarboxylate transporter 1 (MCT1) expression and is detrimental for the long-term survival of motor neuron axons. Therefore, we established an adeno-associated virus 9 (AAV9)-based platform by which MCT1 was targeted mostly to white matter oligodendrocytes to investigate whether this approach could provide a therapeutic benefit in the SOD1^G93A mouse model of ALS. Despite good oligodendrocyte transduction and AAV-mediated MCT1 transgene expression, the disease outcome of SOD1^G93A mice was not altered. Our study further increases our current understanding about the complex nature of oligodendrocyte pathology in ALS and provides valuable insights into the future development of therapeutic strategies to efficiently modulate these cells.

Delayed onset of inherited ALS by deletion of the BDNF receptor TrkB.T1 is non-cell autonomous

The pathophysiology of Amyotrophic Lateral Sclerosis (ALS), a disease caused by the gradual degeneration of motoneurons, is still largely unknown. Insufficient neurotrophic support has been cited as one of the causes of motoneuron cell death. Neurotrophic factors such as BDNF have been evaluated in ALS human clinical trials, but yielded disappointing results attributed to the poor pharmacokinetics and pharmacodynamics of BDNF. In the inherited ALS G93A SOD1 animal model, deletion of the BDNF receptor TrkB.T1 delays spinal cord motoneuron cell death and muscle weakness through an unknown cellular mechanism. Here we show that TrkB.T1 is expressed ubiquitously in the spinal cord and its deletion does not change the SOD1 mutant spinal cord inflammatory state suggesting that TrkB.T1 does not influence microglia or astrocyte activation. Although TrkB.T1 knockout in astrocytes preserves muscle strength and co-ordination at early stages of disease, its specific conditional deletion in motoneurons or astrocytes does not delay motoneuron cell death during the early stage of the disease. These data suggest that TrkB.T1 may limit the neuroprotective BDNF signaling to motoneurons via a non-cell autonomous mechanism providing new understanding into the reasons for past clinical failures and insights into the design of future clinical trials employing TrkB agonists in ALS.

Properties of Glial Cell at the Neuromuscular Junction Are Incompatible with Synaptic Repair in the SOD1G37R ALS Mouse Model

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting motoneurons (MNs) in a motor-unit (MU)-dependent manner. Glial dysfunction contributes to numerous aspects of the disease. At the neuromuscular junction (NMJ), early alterations in perisynaptic Schwann cell (PSC), glial cells at this synapse, may impact their ability to regulate NMJ stability and repair. Indeed, muscarinic receptors (mAChRs) regulate the repair phenotype of PSCs and are overactivated at disease-resistant NMJs [soleus muscle (SOL)] in SOD1^G37R mice. However, it remains unknown whether this is the case at disease-vulnerable NMJs and whether it translates into an impairment of PSC-dependent repair mechanisms. We used SOL and sternomastoid (STM) muscles from SOD1^G37R mice and performed Ca²⁺-imaging to monitor PSC activity and used immunohistochemistry to analyze their repair and phagocytic properties. We show that PSC mAChR-dependent activity was transiently increased at disease-vulnerable NMJs (STM muscle). Furthermore, PSCs from both muscles extended disorganized processes from denervated NMJs and failed to initiate or guide nerve terminal sprouts at disease-vulnerable NMJs, a phenomenon essential for compensatory reinnervation. This was accompanied by a failure of numerous PSCs to upregulate galectin-3 (MAC-2), a marker of glial axonal debris phagocytosis, on NMJ denervation in SOD1 mice. Finally, differences in these PSC-dependent NMJ repair mechanisms were MU type dependent, thus reflecting MU vulnerability in ALS. Together, these results reveal that neuron-glia communication is ubiquitously altered at the NMJ in ALS. This appears to prevent PSCs from adopting a repair phenotype, resulting in a maladapted response to denervation at the NMJ in ALS.SIGNIFICANCE STATEMENT Understanding how the complex interplay between neurons and glial cells ultimately lead to the degeneration of motor neurons and loss of motor function is a fundamental question to comprehend amyotrophic lateral sclerosis (ALS). An early and persistent alteration of glial cell activity takes place at the neuromuscular junction (NMJ), the output of motor neurons, but its impact on NMJ repair remains unknown. Here, we reveal that glial cells at disease-vulnerable NMJs often fail to guide compensatory nerve terminal sprouts and to adopt a phagocytic phenotype on denervated NMJs in SOD1^G37R mice. These results show that glial cells at the NMJ elaborate an inappropriate response to NMJ degeneration in a manner that reflects motor-unit (MU) vulnerability and potentially impairs compensatory reinnervation.

Glial Cells-The Strategic Targets in Amyotrophic Lateral Sclerosis Treatment

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disease, which is characterized by the degeneration of motor neurons in the motor cortex and the spinal cord and subsequently by muscle atrophy. To date, numerous gene mutations have been linked to both sporadic and familial ALS, but the effort of many experimental groups to develop a suitable therapy has not, as of yet, proven successful. The original focus was on the degenerating motor neurons, when researchers tried to understand the pathological mechanisms that cause their slow death. However, it was soon discovered that ALS is a complicated and diverse pathology, where not only neurons, but also other cell types, play a crucial role via the so-called non-cell autonomous effect, which strongly deteriorates neuronal conditions. Subsequently, variable glia-based in vitro and in vivo models of ALS were established and used for brand-new experimental and clinical approaches. Such a shift towards glia soon bore its fruit in the form of several clinical studies, which more or less successfully tried to ward the unfavourable prognosis of ALS progression off. In this review, we aimed to summarize current knowledge regarding the involvement of each glial cell type in the progression of ALS, currently available treatments, and to provide an overview of diverse clinical trials covering pharmacological approaches, gene, and cell therapies.

The Regulation of the Small Heat Shock Protein B8 in Misfolding Protein Diseases Causing Motoneuronal and Muscle Cell Death

Misfolding protein diseases are a wide class of disorders in which the aberrantly folded protein aggregates accumulate in affected cells. In the brain and in the skeletal muscle, misfolded protein accumulation induces a variety of cell dysfunctions that frequently lead to cell death. In motoneuron diseases (MNDs), misfolded proteins accumulate primarily in motoneurons, glial cells and/or skeletal muscle cells, altering motor function. The deleterious effects of misfolded proteins can be counteracted by the activity of the protein quality control (PQC) system, composed of chaperone proteins and degradative systems. Here, we focus on a PQC system component: heat shock protein family B (small) member 8 (HSPB8), a chaperone induced by harmful stressful events, including proteotoxicity. In motoneuron and muscle cells, misfolded proteins activate HSPB8 transcription and enhance HSPB8 levels, which contributes to prevent aggregate formation and their harmful effects. HSPB8 acts not only as a chaperone, but also facilitates the autophagy process, to enable the efficient clearance of the misfolded proteins. HSPB8 acts as a dimer bound to the HSP70 co-chaperone BAG3, a scaffold protein that is also capable of binding to HSP70 (associated with the E3-ligase CHIP) and dynein. When this complex is formed, it is transported by dynein to the microtubule organization center (MTOC), where aggresomes are formed. Here, misfolded proteins are engulfed into nascent autophagosomes to be degraded via the chaperone-assisted selective autophagy (CASA). When CASA is insufficient or impaired, HSP70 and CHIP associate with an alternative co-chaperone, BAG1, which routes misfolded proteins to the proteasome for degradation. The finely tuned equilibrium between proteasome and CASA activity is thought to be crucial for maintaining the functional cell homeostasis during proteotoxic stresses, which in turn is essential for cell survival. This fine equilibrium seems to be altered in MNDs, like Amyotrophic lateral sclerosis (ALS) and spinal and bulbar muscular atrophy (SBMA), contributing to the onset and the progression of disease. Here, we will review how misfolded proteins may affect the PQC system and how the proper activity of this system can be restored by boosting or regulating HSPB8 activity, with the aim to ameliorate disease progression in these two fatal MNDs.

Dynamic neuromuscular remodeling precedes motor-unit loss in a mouse model of ALS

Despite being an early event in ALS, it remains unclear whether the denervation of neuromuscular junctions (NMJ) is simply the first manifestation of a globally degenerating motor neuron. Using in vivo imaging of single axons and their NMJs over a three-month period, we identify that single motor-units are dismantled asynchronously in SOD1^G37R mice. We reveal that weeks prior to complete axonal degeneration, the dismantling of axonal branches is accompanied by contemporaneous new axonal sprouting resulting in synapse formation onto nearby NMJs. Denervation events tend to propagate from the first lost NMJ, consistent with a contribution of neuromuscular factors extrinsic to motor neurons, with distal branches being more susceptible. These results show that NMJ denervation in ALS is a complex and dynamic process of continuous denervation and new innervation rather than a manifestation of sudden global motor neuron degeneration.

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease, is a fatal neurodegenerative disorder. It occurs when the neurons that control muscles – the motoneurons – disconnect from their target muscles and die. This causes the muscles to weaken and waste away. More and more muscles become affected over time until eventually the muscles that control breathing also become paralyzed. Most patients die within two to five years of diagnosis.

Motoneurons consist of a cell body plus a cable-like structure called the axon. The cell body of each motoneuron sits within the spinal cord, and the axon extends out of the spinal cord to the motoneuron’s target muscle. Within the muscle the axon divides into branches, each of which connects with multiple muscle fibers. The breakdown of these connections, known as neuromuscular junctions, is one of the first signs of ALS.

Does a motoneuron lose all of its connections with muscle fibers at once, or do the connections break down a few at a time? This distinction is important as it will help to identify the events that lead to muscle paralysis in ALS. To find out, Martineau et al. studied mice that had two genetic mutations: one that causes ALS and another that produces fluorescent molecules in some motoneurons. This allowed the branches of the motoneurons to be tracked over time with a fluorescence microscope.

Martineau et al. found that individual neurons lose their connections to muscle fibers gradually. Moreover, motoneurons grow new branches and form new connections even while losing their old ones. This dual process of pruning and budding lasts for several weeks, until eventually the motoneuron dies.

Developing drugs to stabilize neuromuscular junctions during the period when motoneurons gradually disconnect from muscles could be a promising avenue to explore to improve the quality of life of ALS patients. One advantage of this treatment strategy is that neuromuscular junctions in muscles are easier to access than motoneurons inside the spinal cord. To identify potential drugs, future studies will need to focus on the proteins and signals that cause the neuromuscular junctions to break down.

Temporospatial Analysis and New Players in the Immunology of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by progressive loss of lower and upper motor neurons (MN) leading to muscle weakness, paralysis and eventually death. Although a highly varied etiology results in ALS, it broadly manifests itself as sporadic and familial forms that have evident similarities in clinical symptoms and disease progression. There is a tremendous amount of knowledge on molecular mechanisms leading to loss of MNs and neuromuscular junctions (NMJ) as major determinants of disease onset, severity and progression in ALS. Specifically, two main opposing hypotheses, the dying forward and dying back phenomena, exist to account for NMJ denervation. The former hypothesis proposes that the earliest degeneration occurs at the central MNs and proceeds to the NMJ, whereas in the latter, the peripheral NMJ is the site of precipitating degeneration progressing backwards to the MN cell body. A large body of literature strongly indicates a role for the immune system in disease onset and progression via regulatory involvement at the level of both the central and peripheral nervous systems (CNS and PNS). In this review, we discuss the earliest reported immune responses with an emphasis on newly identified immune players in mutant superoxide dismutase 1 (mSOD1) transgenic mice, the gold standard mouse model for ALS.

Emerging understanding of the genotype-phenotype relationship in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive, noncurable neurodegenerative disorder of the upper and lower motor neurons causing weakness and death within a few years of symptom onset. About 10% of patients with ALS have a family history of the disease; however, ALS-associated genetic mutations are also found in sporadic cases. There are over 100 ALS-associated mutations, and importantly, several genetic mutations, including C9ORF72, SOD1, and TARDBP, have led to mechanistic insight into this complex disease. In the clinical realm, knowledge of ALS genetics can also help explain phenotypic heterogeneity, aid in genetic counseling, and in the future may help direct treatment efforts.

Neuromuscular Junction Dismantling in Amyotrophic Lateral Sclerosis

Neuromuscular junction assembly and plasticity during embryonic, postnatal, and adult life are tightly regulated by the continuous cross-talk among motor nerve endings, muscle fibers, and glial cells. Altered communications among these components is thought to be responsible for the physiological age-related changes at this synapse and possibly for its destruction in pathological states. Neuromuscular junction dismantling plays a crucial role in the onset of Amyotrophic Lateral Sclerosis (ALS). ALS is characterized by the degeneration and death of motor neurons leading to skeletal muscle denervation, atrophy and, most often, death of the patient within five years from diagnosis. ALS is a non-cell autonomous disease as, besides motor neuron degeneration, glial cells, and possibly muscle fibers, play a role in its onset and progression. Here, we will review the recent literature regarding the mechanisms leading to neuromuscular junction disassembly and muscle denervation focusing on the role of the three players of this peripheral tripartite synapse.

The Role of the Heat Shock Protein B8 (HSPB8) in Motoneuron Diseases

Amyotrophic lateral sclerosis (ALS) and spinal and bulbar muscular atrophy (SBMA) are two motoneuron diseases (MNDs) characterized by aberrant protein behavior in affected cells. In familial ALS (fALS) and in SBMA specific gene mutations lead to the production of neurotoxic proteins or peptides prone to misfold, which then accumulate in form of aggregates. Notably, some of these proteins accumulate into aggregates also in sporadic ALS (sALS) even if not mutated. To prevent proteotoxic stresses detrimental to cells, misfolded and/or aggregated proteins must be rapidly removed by the protein quality control (PQC) system. The small heat shock protein B8 (HSPB8) is a chaperone induced by harmful events, like proteasome inhibition. HSPB8 is expressed both in motoneuron and muscle cells, which are both targets of misfolded protein toxicity in MNDs. In ALS mice models, in presence of the mutant proteins, HSPB8 is upregulated both in spinal cord and muscle. HSPB8 interacts with the HSP70 co-chaperone BAG3 and enhances the degradation of misfolded proteins linked to sALS, or causative of fALS and of SBMA. HSPB8 acts by facilitating autophagy, thereby preventing misfolded protein accumulation in affected cells. BAG3 and BAG1 compete for HSP70-bound clients and target them for disposal to the autophagy or proteasome, respectively. Enhancing the selective targeting of misfolded proteins by HSPB8-BAG3-HSP70 to autophagy may also decrease their delivery to the proteasome by the BAG1-HSP70 complex, thereby limiting possible proteasome overwhelming. Thus, approaches aimed at potentiating HSPB8-BAG3 may contribute to the maintenance of proteostasis and may delay MNDs progression.

Motor neuron intrinsic and extrinsic mechanisms contribute to the pathogenesis of FUS-associated amyotrophic lateral sclerosis

Motor neuron-extrinsic mechanisms have been shown to participate in the pathogenesis of ALS-SOD1, one familial form of amyotrophic lateral sclerosis (ALS). It remains unclear whether such mechanisms contribute to other familial forms, such as TDP-43 and FUS-associated ALS. Here, we characterize a single-copy mouse model of ALS-FUS that conditionally expresses a disease-relevant truncating FUS mutant from the endogenous murine Fus gene. We show that these mice, but not mice heterozygous for a Fus null allele, develop similar pathology as ALS-FUS patients and a mild motor neuron phenotype. Most importantly, CRE-mediated rescue of the Fus mutation within motor neurons prevented degeneration of motor neuron cell bodies, but only delayed appearance of motor symptoms. Indeed, we observed downregulation of multiple myelin-related genes, and increased numbers of oligodendrocytes in the spinal cord supporting their contribution to behavioral deficits. In all, we show that mutant FUS triggers toxic events in both motor neurons and neighboring cells to elicit motor neuron disease.

Unraveling gene expression profiles in peripheral motor nerve from amyotrophic lateral sclerosis patients: insights into pathogenesis

The aim of the present study is to investigate the molecular pathways underlying amyotrophic lateral sclerosis (ALS) pathogenesis within the peripheral nervous system. We analyzed gene expression changes in human motor nerve diagnostic biopsies obtained from eight ALS patients and seven patients affected by motor neuropathy as controls. An integrated transcriptomics and system biology approach was employed. We identified alterations in the expression of 815 genes, with 529 up-regulated and 286 down-regulated in ALS patients. Up-regulated genes clustered around biological process involving RNA processing and protein metabolisms. We observed a significant enrichment of up-regulated small nucleolar RNA transcripts (p = 2.68*10-11) and genes related to endoplasmic reticulum unfolded protein response and chaperone activity. We found a significant down-regulation in ALS of genes related to the glutamate metabolism. Interestingly, a network analysis highlighted HDAC2, belonging to the histone deacetylase family, as the most interacting node. While so far gene expression studies in human ALS have been performed in postmortem tissues, here specimens were obtained from biopsy at an early phase of the disease, making these results new in the field of ALS research and therefore appealing for gene discovery studies.

Purinergic contribution to amyotrophic lateral sclerosis

By signalling through purinergic receptors classified as ionotropic P2X (for ATP) and metabotropic P1 (for adenosine) and P2Y (mainly for ADP, UDP, UTP, ATP), the extracellular nucleotides and their metabolic derivatives originated by extracellular activity of several different ectonucleotidases, are involved in the functioning of the nervous system. Here they exert a central role during physiological processes, but also in the precarious balance between beneficial and noxious events. Indeed, in recent years, the dysregulation of extracellular purinergic homeostasis has been correlated to well-characterized acute and chronic neurodegenerative and neuroinflammatory diseases. Among these, we focus our attention on purinergic signalling occurring in amyotrophic lateral sclerosis (ALS), the most common late onset motoneuron disease, characterized by specific loss of motoneurons in brain stem and ventral horns of spinal cord. ALS is a progressive non-cell-autonomous and multifactorial neuroinflammatory disease, whose aetiology and pathological mechanisms are unidentified for most patients and initiate long before any sign or symptom becomes apparent. By combining purinergic with ALS knowledge, in this work we thus present and sustain a novel line of investigation on the purinergic contribution to ALS. In particular, here we recapitulate very early results about P2X4, P2X7 and P2Y6 receptor expression in tissues from ALS animal and cell models and patients, and more recent achievements about purinergic signalling mainly performed in vitro in microglia and lately in astrocytes and motoneurons. We finally highlight how purinergic signalling has progressively evolved up to preclinical trials, to the point of deserving now full consideration with reference to ALS. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Altered terminal Schwann cell morphology precedes denervation in SOD1 mice

In mice that express SOD1 mutations found in human motor neuron disease, degeneration begins in the periphery for reasons that remain unknown. At the neuromuscular junction (NMJ), terminal Schwann cells (TSCs) have an intimate relationship with motor terminals and are believed to help maintain the integrity of the motor terminal. Recent evidence indicates that TSCs in some SOD1 mice exhibit abnormal functional properties, but other aspects of possible TSC involvement remain unknown. In this study, an analysis of TSC morphology and number was performed in relation to NMJ innervation status in mice which express the G93A SOD1 mutation. At P30, all NMJs of the fast medial gastrocnemius (MG) muscle were fully innervated by a single motor axon but 50% of NMJs lacked TSC cell bodies and were instead covered by the processes of Schwann cells with cell bodies located on the preterminal axons. NMJs in P30 slow soleus muscles were also fully innervated by single motor axons and only 5% of NMJs lacked a TSC cell body. At P60, about 25% of MG NMJs were denervated and lacked labeling for TSCs while about 60% of innervated NMJs lacked TSC cell bodies. In contrast, 96% of P60 soleus NMJs were innervated while 9% of innervated NMJs lacked TSC cell bodies. The pattern of TSC abnormalities found at P30 thus correlates with the pattern of denervation found at P60. Evidence from mice that express the G85R SOD1 mutation indicate that TSC abnormalities are not unique for mice that express G93A SOD1 mutations. These results add to an emerging understanding that TSCs may play a role in motor terminal degeneration and denervation in animal models of motor neuron disease.

Amyotrophic lateral sclerosis: Current perspectives from basic research to the clinic

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive degeneration of upper and lower motoneurons, leading to muscle weakness and paralysis, and finally death. Considerable recent advances have been made in basic research and preclinical therapeutic attempts using experimental models, leading to increasing clinical and translational research in the context of this disease. In this review we aim to summarize the most relevant findings from a variety of aspects about ALS, including evaluation methods, animal models, pathophysiology, and clinical findings, with particular emphasis in understanding the role of every contributing mechanism to the disease for elucidating the causes underlying degeneration of motoneurons and the development of new therapeutic strategies.

Characterization of the Contribution of Genetic Background and Gender to Disease Progression in the SOD1 G93A Mouse Model of Amyotrophic Lateral Sclerosis: A Meta-Analysis

Background: The SOD1 G93A mouse model of amyotrophic lateral sclerosis (ALS) is the most frequently used model to examine ALS pathophysiology. There is a lack of homogeneity in usage of the SOD1 G93A mouse, including differences in genetic background and gender, which could confound the field’s results.

Objective: In an analysis of 97 studies, we characterized the ALS progression for the high transgene copy control SOD1 G93A mouse on the basis of disease onset, overall lifespan, and disease duration for male and female mice on the B6SJL and C57BL/6J genetic backgrounds and quantified magnitudes of differences between groups.

Methods: Mean age at onset, onset assessment measure, disease duration, and overall lifespan data from each study were extracted and statistically modeled as the response of linear regression with the sex and genetic background factored as predictors. Additional examination was performed on differing experimental onset and endpoint assessment measures.

Results: C57BL/6 background mice show delayed onset of symptoms, increased lifespan, and an extended disease duration compared to their sex-matched B6SJL counterparts. Female B6SJL generally experience extended lifespan and delayed onset compared to their male counterparts, while female mice on the C57BL/6 background show delayed onset but no difference in survival compared to their male counterparts. Finally, different experimental protocols (tremor, rotarod, etc.) for onset determination result in notably different onset means.

Conclusions: Overall, the observed effect of sex on disease endpoints was smaller than that which can be attributed to the genetic background. The often-reported increase in lifespan for female mice was observed only for mice on the B6SJL background, implicating a strain-dependent effect of sex on disease progression that manifests despite identical mutant SOD1 expression.

Rodent Models of Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a motor neuron disease affecting upper and lower motor neurons in the central nervous system. Patients with ALS develop extensive muscle wasting and atrophy leading to paralysis and death 3 to 5 years after disease onset. The condition may be familial (fALS 10%) or sporadic ALS (sALS, 90%). The large majority of fALS cases are due to genetic mutations in the Superoxide dismutase 1 gene (SOD1, 15% of fALS) and repeat nucleotide expansions in the gene encoding C9ORF72 (∼ 40% to 50% of fALS and ∼ 10% of sALS). Studies suggest that ALS is mediated through aberrant protein homeostasis (i.e., ER stress and autophagy) and/or changes in RNA processing (as in all non-SOD1-mediated ALS). In all of these cases, animal models suggest that the disorder is mediated non-cell autonomously, i.e., not only motor neurons are involved, but glial cells including microglia, astrocytes, and oligodendrocytes, and other neuronal subpopulations are also implicated in the pathogenesis. Provided in this unit is a review of ALS rodent models, including discussion of their relative advantages and disadvantages. Emphasis is placed on correlating the model phenotype with the human condition and the utility of the model for defining the disease process. Information is also presented on RNA processing studies in ALS research, with particular emphasis on the newest ALS rodent models.

Amyotrophic lateral sclerosis: mechanisms and therapeutics in the epigenomic era

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease of the motor neurons, which results in weakness and atrophy of voluntary skeletal muscles. Treatments do not modify the disease trajectory effectively, and only modestly improve survival. A complex interaction between genes, environmental exposure and impaired molecular pathways contributes to pathology in patients with ALS. Epigenetic mechanisms control the hereditary and reversible regulation of gene expression without altering the basic genetic code. Aberrant epigenetic patterns-including abnormal microRNA (miRNA) biogenesis and function, DNA modifications, histone remodeling, and RNA editing-are acquired throughout life and are influenced by environmental factors. Thus, understanding the molecular processes that lead to epigenetic dysregulation in patients with ALS might facilitate the discovery of novel therapeutic targets and biomarkers that could reduce diagnostic delay. These achievements could prove crucial for successful disease modification in patients with ALS. We review the latest findings regarding the role of miRNA modifications and other epigenetic mechanisms in ALS, and discuss their potential as therapeutic targets.

Specific marker expression and cell state of Schwann cells during culture in vitro

Schwann cells (SCs) in animals exist in different developmental stages or wound repair phases, distinguished mainly by the expression of SC-specific markers. No study has yet determined SC state under in vitro culture conditions, and the specific markers expressed in SC are obscure as well. In this study, we harvested sciatic nerves from newborn mice and isolated SCs by an enzyme-digestion method, then we examined the expression profiles of ten markers (S100, p75^NTR, Sox10, Sox2, GAP43, NCAM, Krox20, Oct6, MBP, and MPZ) at both the RNA and protein levels in in vitro mouse SCs and speculated their relation with in vivo SC stages. We assayed RNA and protein levels of SC specific markers by immunofluorescence, Western Blot, and real-time quantitative RT-PCR. The results show that the expression of most markers (S100, p75^NTR, GAP43, NCAM, Krox20, Oct6, MBP and MPZ) was not detectable in all of early stage cultured SCs. The expression of transcription factors Sox10 and Sox2 was, however, detectable in all SCs. After 8 days, the positive expression rate of all markers except GAP43 and Oct6 was almost 100%.These results indicates Sox10 is a necessary marker for SC identification, while S100 is not reliable. SCs cultured in vitro express Sox2, P75^NTR, NCAM, GAP43, Oct6, and MPZ, suggesting that they are similar to in vivo undifferentiated iSCs or dedifferentiated iSCs after nerve injury.

Early and persistent abnormal decoding by glial cells at the neuromuscular junction in an ALS model

Amyotrophic lateral sclerosis (ALS) is a late-onset neuromuscular disease characterized by progressive loss of motor neurons (MNs) preceded by neuromuscular junction (NMJ) denervation. Despite the importance of NMJ denervation in ALS, the mechanisms involved remain unexplored and ill defined. The contribution of glial cells in the disease has been highlighted, including axonal Schwann cell activation that precedes the decline of motor function and the onset of hindlimb paralysis. Because NMJ denervation occurs early in the process and that perisynaptic Schwann cells (PSCs), glial cells at the NMJ, regulate morphological stability, integrity, and repair of the NMJ, one could predict that PSC functions would be altered even before denervation, contributing to NMJ malfunctions. We tested this possibility using a slowly progressive model of ALS (SOD1(G37R) mice). We observed a normal NMJ organization at a presymptomatic stage of ALS (120 d), but PSC detection of endogenous synaptic activity revealed by intracellular Ca(2+) changes was enhanced compared with their wild-type littermates. This inappropriate PSC decoding ability was associated with an increased level of neurotransmitter release and dependent on intrinsic glial properties related to enhanced muscarinic receptor activation. The alteration of PSC muscarinic receptor functions also persists during the preonset stage of the disease and became dependent on MN vulnerability with age. Together, these results suggest that PSC properties are altered in the disease process in a manner that would be detrimental for NMJ repair. The impairments of PSC functions may contribute to NMJ dysfunction and ALS pathogenesis.

ALS as a distal axonopathy: molecular mechanisms affecting neuromuscular junction stability in the presymptomatic stages of the disease

Amyotrophic Lateral Sclerosis (ALS) is being redefined as a distal axonopathy, in that many molecular changes influencing motor neuron degeneration occur at the neuromuscular junction (NMJ) at very early stages of the disease prior to symptom onset. A huge variety of genetic and environmental causes have been associated with ALS, and interestingly, although the cause of the disease can differ, both sporadic and familial forms of ALS show a remarkable similarity in terms of disease progression and clinical manifestation. The NMJ is a highly specialized synapse, allowing for controlled signaling between muscle and nerve necessary for skeletal muscle function. In this review we will evaluate the clinical, animal experimental and cellular/molecular evidence that supports the idea of ALS as a distal axonopathy. We will discuss the early molecular mechanisms that occur at the NMJ, which alter the functional abilities of the NMJ. Specifically, we focus on the role of axon guidance molecules on the stability of the cytoskeleton and how these molecules may directly influence the cells of the NMJ in a way that may initiate or facilitate the dismantling of the neuromuscular synapse in the presymptomatic stages of ALS.

Glial cells in amyotrophic lateral sclerosis

For more than twenty years glial cells have been implicated in the pathogenetic cascades for genetic and sporadic forms of ALS. The biological role of glia, including the principal CNS glia, astroglia and oligodendroglia, as well as the myeloid derived microglia, has uniformly led to converging data sets that implicate these diverse cells in the degeneration of neurons in ALS. Originating as studies in postmortem human brain implicating astroglia, the research progressed to strongly implicate microglia and contributors to CNS injury in all forms of ALS. Most recently and unexpectedly, oligodendroglia have also been shown in animal model systems and human brain to play an early role in the dysfunction and death of ALS neurons. These studies have identified a number of diverse cellular cascades that could be, or have already been, the target of therapeutic interventions. Understanding the temporal and regional role of these cells and the magnitude of their contribution will be important for future interventions. Employing markers of these cell types may also allow for future important patient subgrouping and pharmacodynamic drug development tools.

The role of oxidative stress in degeneration of the neuromuscular junction in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive loss of motoneurons and degradation of the neuromuscular junctions (NMJ). Consistent with the dying-back hypothesis of motoneuron degeneration the decline in synaptic function initiates from the presynaptic terminals in ALS. Oxidative stress is a major contributory factor to ALS pathology and affects the presynaptic transmitter releasing machinery. Indeed, in ALS mouse models nerve terminals are sensitive to reactive oxygen species (ROS) suggesting that oxidative stress, along with compromised mitochondria and increased intracellular Ca(2+) amplifies the presynaptic decline in NMJ. This initial dysfunction is followed by a neurodegeneration induced by inflammatory agents and loss of trophic support. To develop effective therapeutic approaches against ALS, it is important to identify the mechanisms underlying the initial pathological events. Given the role of oxidative stress in ALS, targeted antioxidant treatments could be a promising therapeutic approach. However, the complex nature of ALS and failure of monotherapies suggest that an antioxidant therapy should be accompanied by anti-inflammatory interventions to enhance the restoration of the redox balance.

Increased axonal ribosome numbers is an early event in the pathogenesis of amyotrophic lateral sclerosis

Myelinating glia cells support axon survival and functions through mechanisms independent of myelination, and their dysfunction leads to axonal degeneration in several diseases. In amyotrophic lateral sclerosis (ALS), spinal motor neurons undergo retrograde degeneration, and slowing of axonal transport is an early event that in ALS mutant mice occurs well before motor neuron degeneration. Interestingly, in familial forms of ALS, Schwann cells have been proposed to slow disease progression. We demonstrated previously that Schwann cells transfer polyribosomes to diseased and regenerating axons, a possible rescue mechanism for disease-induced reductions in axonal proteins. Here, we investigated whether elevated levels of axonal ribosomes are also found in ALS, by analysis of a superoxide dismutase 1 (SOD1)^G93A mouse model for human familial ALS and a patient suffering from sporadic ALS. In both cases, we found that the disorder was associated with an increase in the population of axonal ribosomes in myelinated axons. Importantly, in SOD1^G93A mice, the appearance of axonal ribosomes preceded the manifestation of behavioral symptoms, indicating that upregulation of axonal ribosomes occurs early in the pathogenesis of ALS. In line with our previous studies, electron microscopy analysis showed that Schwann cells might serve as a source of axonal ribosomes in the disease-compromised axons. The early appearance of axonal ribosomes indicates an involvement of Schwann cells early in ALS neuropathology, and may serve as an early marker for disease-affected axons, not only in ALS, but also for other central and peripheral neurodegenerative disorders.

The multifaceted role of glial cells in amyotrophic lateral sclerosis

Despite indisputable progress in the molecular and genetic aspects of amyotrophic lateral sclerosis (ALS), a mechanistic comprehension of the neurodegenerative processes typical of this disorder is still missing and no effective cures to halt the progression of this pathology have yet been developed. Therefore, it seems that a substantial improvement of the outcome of ALS treatments may depend on a better understanding of the molecular mechanisms underlying neuronal pathology and survival as well as on the establishment of novel etiological therapeutic strategies. Noteworthy, a convergence of recent data from multiple studies suggests that, in cellular and animal models of ALS, a complex pathological interplay subsists between motor neurons and their non-neuronal neighbours, particularly glial cells. These observations not only have drawn attention to the physiopathological changes glial cells undergo during ALS progression, but they have moved the focus of the investigations from intrinsic defects and weakening of motor neurons to glia-neuron interactions. In this review, we summarize the growing body of evidence supporting the concept that different glial populations are critically involved in the dreadful chain of events leading to motor neuron sufferance and death in various forms of ALS. The outlined observations strongly suggest that glial cells can be the targets for novel therapeutic interventions in ALS.

Death receptor 6 (DR6) antagonist antibody is neuroprotective in the mouse SOD1G93A model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the death of motor neurons, axon degeneration, and denervation of neuromuscular junctions (NMJ). Here we show that death receptor 6 (DR6) levels are elevated in spinal cords from post-mortem samples of human ALS and from SOD1^G93A transgenic mice, and DR6 promotes motor neuron death through activation of the caspase 3 signaling pathway. Blocking DR6 with antagonist antibody 5D10 promotes motor neuron survival in vitro via activation of Akt phosphorylation and inhibition of the caspase 3 signaling pathway, after growth factor withdrawal, sodium arsenite treatment or co-culture with SOD1^G93A astrocytes. Treatment of SOD1^G93A mice at an asymptomatic stage starting on the age of 42 days with 5D10 protects NMJ from denervation, decreases gliosis, increases survival of motor neurons and CC1⁺ oligodendrocytes in spinal cord, decreases phosphorylated neurofilament heavy chain (pNfH) levels in serum, and promotes motor functional improvement assessed by increased grip strength. The combined data provide clear evidence for neuroprotective effects of 5D10. Blocking DR6 function represents a new approach for the treatment of neurodegenerative disorders involving motor neuron death and axon degeneration, such as ALS.

Is SOD1 loss of function involved in amyotrophic lateral sclerosis?

Mutations in the gene superoxide dismutase 1 (SOD1) are causative for familial forms of the neurodegenerative disease amyotrophic lateral sclerosis. When the first SOD1 mutations were identified they were postulated to give rise to amyotrophic lateral sclerosis through a loss of function mechanism, but experimental data soon showed that the disease arises from a—still unknown—toxic gain of function, and the possibility that loss of function plays a role in amyotrophic lateral sclerosis pathogenesis was abandoned. Although loss of function is not causative for amyotrophic lateral sclerosis, here we re-examine two decades of evidence regarding whether loss of function may play a modifying role in SOD1–amyotrophic lateral sclerosis. From analysing published data from patients with SOD1–amyotrophic lateral sclerosis, we find a marked loss of SOD1 enzyme activity arising from almost all mutations. We continue to examine functional data from all Sod1 knockout mice and we find obvious detrimental effects within the nervous system with, interestingly, some specificity for the motor system. Here, we bring together historical and recent experimental findings to conclude that there is a possibility that SOD1 loss of function may play a modifying role in amyotrophic lateral sclerosis. This likelihood has implications for some current therapies aimed at knocking down the level of mutant protein in patients with SOD1–amyotrophic lateral sclerosis. Finally, the wide-ranging phenotypes that result from loss of function indicate that SOD1 gene sequences should be screened in diseases other than amyotrophic lateral sclerosis.

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.

The complex molecular biology of amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disorder that causes selective death of motor neurons followed by paralysis and death. A subset of ALS cases is caused by mutations in the gene for Cu, Zn superoxide dismutase (SOD1), which impart a toxic gain of function to this antioxidant enzyme. This neurotoxic property is widely believed to stem from an increased propensity to misfold and aggregate caused by decreased stability of the native homodimer or a tendency to lose stabilizing posttranslational modifications. Study of the molecular mechanisms of SOD1-related ALS has revealed a complex array of interconnected pathological processes, including glutamate excitotoxicity, dysregulation of neurotrophic factors and axon guidance proteins, axonal transport defects, mitochondrial dysfunction, deficient protein quality control, and aberrant RNA processing. Many of these pathologies are directly exacerbated by misfolded and aggregated SOD1 and/or cytosolic calcium overload, suggesting the primacy of these events in disease etiology and their potential as targets for therapeutic intervention.

Selective knockdown of mutant SOD1 in Schwann cells ameliorates disease in G85R mutant SOD1 transgenic mice

Mutants of superoxide dismutase type 1 (mtSOD1) that have full dismutase activity (e.g., G37R) as well as none (e.g., G85R) cause familial amyotrophic lateral sclerosis (FALS), indicating that mtSOD1-induced FALS results from a toxicity rather than loss in SOD1 enzymatic activity. Still, it has remained unclear whether mtSOD1 dismutase activity can influence disease. A previous study demonstrated that Cre-mediated knockdown of G37R expression in Schwann cells (SCs) of G37R transgenic mice shortened the late phase of disease and survival. These results suggested that the neuroprotective effect of G37R expressed in SCs was greater than its toxicity, presumably because its dismutase activity counteracted reactive oxygen species (ROS). In order to further investigate this, we knocked down G85R in SCs by crossing G85R(flox) mice with myelin-protein-zero (P(0)):Cre mice, which express Cre recombinase in SCs. Knockdown of G85R in SCs of G85R mice delayed disease onset and extended survival indicating that G85R expression in SCs is neurotoxic. These results demonstrate differences in the effect on disease of dismutase active vs. inactive mtSOD1 suggesting that both a loss as well as gain in function of mtSOD1 influence FALS pathogenesis. The results suggest that mtSOD1-induced FALS treatment may have to be adjusted depending on the cell type targeted and particular mtSOD1 involved.

Molecular pathways of motor neuron injury in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a genetically diverse disease. At least 15 ALS-associated gene loci have so far been identified, and the causative gene is known in approximately 30% of familial ALS cases. Less is known about the factors underlying the sporadic form of the disease. The molecular mechanisms of motor neuron degeneration are best understood in the subtype of disease caused by mutations in superoxide dismutase 1, with a current consensus that motor neuron injury is caused by a complex interplay between multiple pathogenic processes. A key recent finding is that mutated TAR DNA-binding protein 43 is a major constituent of the ubiquitinated protein inclusions in ALS, providing a possible link between the genetic mutation and the cellular pathology. New insights have also indicated the importance of dysregulated glial cell-motor neuron crosstalk, and have highlighted the vulnerability of the distal axonal compartment early in the disease course. In addition, recent studies have suggested that disordered RNA processing is likely to represent a major contributing factor to motor neuron disease. Ongoing research on the cellular pathways highlighted in this Review is predicted to open the door to new therapeutic interventions to slow disease progression in ALS.

SOD1 and TDP-43 animal models of amyotrophic lateral sclerosis: recent advances in understanding disease toward the development of clinical treatments

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease with no cure. Breakthroughs in understanding ALS pathogenesis came with the discovery of dominant mutations in the superoxide dismutase 1 gene (SOD1) and other genes, including the gene encoding transactivating response element DNA binding protein-43 (TDP-43). This has led to the creation of animal models to further our understanding of the disease and identify a number of ALS-causing mechanisms, including mitochondrial dysfunction, protein misfolding and aggregation, oxidative damage, neuronal excitotoxicity, non-cell autonomous effects and neuroinflammation, axonal transport defects, neurotrophin depletion, effects from extracellular mutant SOD1, and aberrant RNA processing. Here we summarise the SOD1 and TDP-43 animal models created to date, report on recent findings supporting the potential mechanisms of ALS pathogenesis, and correlate this understanding with current developments in the clinic.

Glial cells in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an adult motor neuron disease characterized by premature death of upper and lower motor neurons. Two percent of ALS cases are caused by the dominant mutations in the gene for superoxide dismutase 1 (SOD1) through a gain of toxic property of mutant protein. Genetic and chimeric mice studies using SOD1 models indicate that non-neuronal cells play important roles in neurodegeneration through non-cell autonomous mechanism. We review the contribution of each glial cell type in ALS pathology from studies of the rodent models and ALS patients. Astrogliosis and microgliosis are not only considerable hallmarks of the disease, but the intensity of microglial activation is correlated with severity of motor neuron damage in human ALS. The impaired astrocytic functions such as clearance of extracellular glutamate and release of neurotrophic factors are implicated in disease. Further, the damage within astrocytes and microglia is involved in accelerated disease progression. Finally, other glial cells such as NG2 cells, oligodendrocytes and Schwann cells are under the investigation to determine their contribution in ALS. Accumulating knowledge of active role of glial cells in the disease should be carefully applied to understanding of the sporadic ALS and development of therapy targeted for glial cells.

Astrocyte loss of mutant SOD1 delays ALS disease onset and progression in G85R transgenic mice

Approximately 10% of patients with amyotrophic lateral sclerosis (ALS) have familial ALS (FALS), and 20% of FALS are caused by mutations of superoxide dismutase type 1 (MTSOD1). The fact that some MTSOD1s that cause FALS have full dismutase activity (e.g. G37R) and others no dismutase activity (e.g. G85R) suggests that MTSOD1 causes FALS due to toxicity of the protein rather than a loss in enzymatic function. Compelling data have demonstrated that motor neuron (MN) degeneration can result from a non-cell autonomous effect of the MTSOD1. In order to clarify the role of astrocytes in FALS, we deleted MTSOD1 in astrocytes of G85R transgenic mice. In contrast to a similar study using G37R mice in which astrocyte MTSOD1 loss affected only the late phase of ALS disease, we found that astrocyte MTSOD1 loss in G85R mice delayed disease onset and prolonged the early phase of disease progression, without affecting the late phase. In addition, astrocyte G85R knockdown resulted in decreased microgliosis, decreased SOD1-immunoreactive inclusions and preservation of GLT-1 transporter expression. The differential effects of astrocyte G85R versus G37R knockdown on MN death demonstrate SOD1 mutation-specific effects on ALS pathogenesis; these differences may be a result of the different dismutase activities of the two mutants. The effect of the knockdown of G85R expression in astrocytes on onset as well as disease duration highlights the importance of this cell type in FALS.

Motor neuron-immune interactions: the vicious circle of ALS

Because microglial cells, the resident macrophages of the CNS, react to any lesion of the nervous system, they have for long been regarded as potential players in the pathogenesis of several neurodegenerative disorders including amyotrophic lateral sclerosis, the most common motor neuron disease in the adult. In recent years, this microglial reaction to motor neuron injury, in particular, and the innate immune response, in general, has been implicated in the progression of the disease, in mouse models of ALS. The mechanisms by which microglial cells influence motor neuron death in ALS are still largely unknown. Microglial activation increases over the course of the disease and is associated with an alteration in the production of toxic factors and also neurotrophic factors. Adding to the microglial/macrophage response to motor neuron degeneration, the adaptive immune system can likewise influence the disease process. Exploring these motor neuron-immune interactions could lead to a better understanding in the physiopathology of ALS to find new pathways to slow down motor neuron degeneration.