Amyotrophic lateral sclerosis mutant vesicle-associated membrane protein-associated protein-B transgenic mice develop TAR-DNA-binding protein-43 pathology

Spinal cord neurone loss and foot placement changes in a rat knock-in model of amyotrophic lateral sclerosis Type 8

Amyotrophic lateral sclerosis is an age-dependent cell type-selective degenerative disease. Genetic studies indicate that amyotrophic lateral sclerosis is part of a spectrum of disorders, ranging from spinal muscular atrophy to frontotemporal dementia that share common pathological mechanisms. Amyotrophic lateral sclerosis Type 8 is a familial disease caused by mis-sense mutations in VAPB. VAPB is localized to the cytoplasmic surface of the endoplasmic reticulum, where it serves as a docking point for cytoplasmic proteins and mediates inter-organelle interactions with the endoplasmic reticulum membrane. A gene knock-in model of amyotrophic lateral sclerosis Type 8 based on the VapBP56S mutation and VapB gene deletion has been generated in rats. These animals display a range of age-dependent phenotypes distinct from those previously reported in mouse models of amyotrophic lateral sclerosis Type 8. A loss of motor neurones in VapBP56S/+ and VapBP56S/P56S animals is indicated by a reduction in the number of large choline acetyl transferase-staining cells in the spinal cord. VapB-/- animals exhibit a relative increase in cytoplasmic TDP-43 levels compared with the nucleus, but no large protein aggregates. Concomitant with these spinal cord pathologies VapBP56S/+ , VapBP56S/P56S and VapB-/- animals exhibit age-dependent changes in paw placement and exerted pressures when traversing a CatWalk apparatus, consistent with a somatosensory dysfunction. Extramotor dysfunction is reported in half the cases of motor neurone disease, and this is the first indication of an associated sensory dysfunction in a rodent model of amyotrophic lateral sclerosis. Different rodent models may offer complementary experimental platforms with which to understand the human disease.

Disrupted endoplasmic reticulum-mediated autophagosomal biogenesis in a Drosophila model of C9-ALS-FTD

ABBREVIATIONS

3R: UAS construct expressing 3 G4C2 repeats (used as control); 3WJ: three-way junction; 12R: UAS construct expressing leader sequence and 12 G4C2 repeats; 30R: UAS construct expressing 30 G4C2 repeats; 36R: UAS construct expressing 36 G4C2 repeats; 44R: UAS construct expressing leader sequence and 44 G4C2 repeats; ALS: amyotrophic lateral sclerosis; Atg: autophagy related; atl: atlastin; C9-ALS-FTD: ALS or FTD caused by hexanuleotide repeat expansion in C9orf72; ER: endoplasmic reticulum; FTD: frontotemporal dementia; HRE: GGGGCC hexanucleotide repeat expansion; HSP: hereditary spastic paraplegia; Lamp1: lysosomal associated membrane protein 1; MT: microtubule; NMJ: neuromuscular junction; Rab: Ras-associated binding GTPase; RAN: repeat associated non-AUG (RAN) translation; RO-36: UAS construct expression "RNA-only" version of 36 G4C2 repeats in which stop codons in all six reading frames are inserted.; Rtnl1: Reticulon-like 1; SN: segmental nerve; TFEB/Mitf: transcription factor EB/microphthalmia associated transcription factor (Drosophila ortholog of TFEB); TrpA1: transient receptor potential cation channel A1; VAPB: VAMP associated protein B and C; VNC: ventral nerve cord (spinal cord in Drosophila larvae).

Yeast Models of Amyotrophic Lateral Sclerosis Type 8 Mimic Phenotypes Seen in Mammalian Cells Expressing Mutant VAPBP56S

Amyotrophic lateral sclerosis (ALS) is a complex neurodegenerative disease that results in the loss of motor neurons and can occur sporadically or due to genetic mutations. Among the 30 genes linked to familial ALS, a P56S mutation in VAPB, an ER-resident protein that functions at membrane contact sites, causes ALS type 8. Mammalian cells expressing VAPBP56S have distinctive phenotypes, including ER collapse, protein and/or membrane-containing inclusions, and sensitivity to ER stress. VAPB is conserved through evolution and has two homologs in budding yeast, SCS2 and SCS22. Previously, a humanized version of SCS2 bearing disease-linked mutations was described, and it caused Scs2-containing inclusions when overexpressed in yeast. Here, we describe a yeast model for ALS8 in which the two SCS genes are deleted and replaced with a single chromosomal copy of either wild-type or mutant yeast SCS2 or human VAPB expressed from the SCS2 promoter. These cells display ER collapse, the formation of inclusion-like structures, and sensitivity to tunicamycin, an ER stress-inducing drug. Based on the phenotypic similarity to mammalian cells expressing VAPBP56S, we propose that these models can be used to study the molecular basis of cell death or dysfunction in ALS8. Moreover, other conserved ALS-linked genes may create opportunities for the generation of yeast models of disease.

Autophagy Dysfunction in ALS: from Transport to Protein Degradation

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting upper and lower motor neurons (MNs). Since the identification of the first ALS mutation in 1993, more than 40 genes have been associated with the disorder. The most frequent genetic causes of ALS are represented by mutated genes whose products challenge proteostasis, becoming unable to properly fold and consequently aggregating into inclusions that impose proteotoxic stress on affected cells. In this context, increasing evidence supports the central role played by autophagy dysfunctions in the pathogenesis of ALS. Indeed, in early stages of disease, high levels of proteins involved in autophagy are present in ALS MNs; but at the same time, with neurodegeneration progression, autophagy-mediated degradation decreases, often as a result of the accumulation of toxic protein aggregates in affected cells. Autophagy is a complex multistep pathway that has a central role in maintaining cellular homeostasis. Several proteins are involved in its tight regulation, and importantly a relevant fraction of ALS-related genes encodes products that directly take part in autophagy, further underlining the relevance of this key protein degradation system in disease onset and progression. In this review, we report the most relevant findings concerning ALS genes whose products are involved in the several steps of the autophagic pathway, from phagophore formation to autophagosome maturation and transport and finally to substrate degradation.

Progress in biophysics and molecular biology proteostasis impairment and ALS

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive and fatal neurodegenerative disease that results from the loss of both upper and lower motor neurons. It is the most common motor neuron disease and currently has no effective treatment. There is mounting evidence to suggest that disturbances in proteostasis play a significant role in ALS pathogenesis. Proteostasis is the maintenance of the proteome at the right level, conformation and location to allow a cell to perform its intended function. In this review, we present a thorough synthesis of the literature that provides evidence that genetic mutations associated with ALS cause imbalance to a proteome that is vulnerable to such pressure due to its metastable nature. We propose that the mechanism underlying motor neuron death caused by defects in mRNA metabolism and protein degradation pathways converges on proteostasis dysfunction. We propose that the proteostasis network may provide an effective target for therapeutic development in ALS.

Modelling amyotrophic lateral sclerosis in rodents

The efficient study of human disease requires the proper tools, one of the most crucial of which is an accurate animal model that faithfully recapitulates the human condition. The study of amyotrophic lateral sclerosis (ALS) is no exception. Although the majority of ALS cases are considered sporadic, most animal models of this disease rely on genetic mutations identified in familial cases. Over the past decade, the number of genes associated with ALS has risen dramatically and, with each new genetic variant, there is a drive to develop associated animal models. Rodent models are of particular importance as they allow for the study of ALS in the context of a living mammal with a comparable CNS. Such models not only help to verify the pathogenicity of novel mutations but also provide critical insight into disease mechanisms and are crucial for the testing of new therapeutics. In this Review, we aim to summarize the full spectrum of ALS rodent models developed to date.

VAP Proteins - From Organelle Tethers to Pathogenic Host Interactors and Their Role in Neuronal Disease

Vesicle-associated membrane protein (VAMP)-associated proteins (VAPs) are ubiquitous ER-resident tail-anchored membrane proteins in eukaryotic cells. Their N-terminal major sperm protein (MSP) domain faces the cytosol and allows them to interact with a wide variety of cellular proteins. Therefore, VAP proteins are vital to many cellular processes, including organelle membrane tethering, lipid transfer, autophagy, ion homeostasis and viral defence. Here, we provide a timely overview of the increasing number of VAPA/B binding partners and discuss the role of VAPA/B in maintaining organelle-ER interactions and cooperation. Furthermore, we address how viruses and intracellular bacteria hijack VAPs and their binding partners to induce interactions between the host ER and pathogen-containing compartments and support pathogen replication. Finally, we focus on the role of VAP in human disease and discuss how mutated VAPB leads to the disruption of cellular homeostasis and causes amyotrophic lateral sclerosis.

The Link between VAPB Loss of Function and Amyotrophic Lateral Sclerosis

The VAP proteins are integral adaptor proteins of the endoplasmic reticulum (ER) membrane that recruit a myriad of interacting partners to the ER surface. Through these interactions, the VAPs mediate a large number of processes, notably the generation of membrane contact sites between the ER and essentially all other cellular membranes. In 2004, it was discovered that a mutation (p.P56S) in the VAPB paralogue causes a rare form of dominantly inherited familial amyotrophic lateral sclerosis (ALS8). The mutant protein is aggregation-prone, non-functional and unstable, and its expression from a single allele appears to be insufficient to support toxic gain-of-function effects within motor neurons. Instead, loss-of-function of the single wild-type allele is required for pathological effects, and VAPB haploinsufficiency may be the main driver of the disease. In this article, we review the studies on the effects of VAPB deficit in cellular and animal models. Several basic cell physiological processes are affected by downregulation or complete depletion of VAPB, impinging on phosphoinositide homeostasis, Ca2+ signalling, ion transport, neurite extension, and ER stress. In the future, the distinction between the roles of the two VAP paralogues (A and B), as well as studies on motor neurons generated from induced pluripotent stem cells (iPSC) of ALS8 patients will further elucidate the pathogenic basis of p.P56S familial ALS, as well as of other more common forms of the disease.

Patient-Specific Cells for Modeling and Decoding Amyotrophic Lateral Sclerosis: Advances and Challenges

Motor neuron loss or degeneration is the typical characteristic of amyotrophic lateral sclerosis (ALS), which often leads to weakness, paralysis, or even death. The underlying mechanisms of motor neuron degeneration and ALS progression remain elusive, and there is no effective treatment for ALS. The advances of stem cells and reprogramming techniques has made it possible to generate patient-specific motor neurons as cell models for studying disease mechanisms and drug discovery. This review comprehensively discusses recent approaches to generate motor neurons from stem cells and somatic cells and highlights the application of induced motor neurons to modeling ALS diseases, dissecting the pathogenesis, and screening new drugs. New perspectives are also discussed on generating patient-specific motor neuron subtypes that are affected by ALS or creating 3D spinal cord organoid models for better recapitulating and understanding ALS.

Pathomechanisms of ALS8: altered autophagy and defective RNA binding protein (RBP) homeostasis due to the VAPB P56S mutation

Mutations in RNA binding proteins (RBPs) and in genes regulating autophagy are frequent causes of familial amyotrophic lateral sclerosis (fALS). The P56S mutation in vesicle-associated membrane protein-associated protein B (VAPB) leads to fALS (ALS8) and spinal muscular atrophy (SMA). While VAPB is primarily involved in the unfolded protein response (UPR), vesicular trafficking and in initial steps of the autophagy pathway, the effect of mutant P56S-VAPB on autophagy regulation in connection with RBP homeostasis has not been explored yet. Examining the muscle biopsy of our index ALS8 patient of European origin revealed globular accumulations of VAPB aggregates co-localised with autophagy markers LC3 and p62 in partially atrophic and atrophic muscle fibres. In line with this skin fibroblasts obtained from the same patient showed accumulation of P56S-VAPB aggregates together with LC3 and p62. Detailed investigations of autophagic flux in cell culture models revealed that P56S-VAPB alters both initial and late steps of the autophagy pathway. Accordingly, electron microscopy complemented with live cell imaging highlighted the impaired fusion of accumulated autophagosomes with lysosomes in cells expressing P56S-VAPB. Consistent with these observations, neuropathological studies of brain and spinal cord of P56S-VAPB transgenic mice revealed signs of neurodegeneration associated with altered protein quality control and defective autophagy. Autophagy and RBP homeostasis are interdependent, as demonstrated by the cytoplasmic mis-localisation of several RBPs including pTDP-43, FUS, Matrin 3 which often sequestered with P56S-VAPB aggregates both in cell culture and in the muscle biopsy of the ALS8 patient. Further confirming the notion that aggregation of the RBPs proceeds through the stress granule (SG) pathway, we found persistent G3BP- and TIAR1-positive SGs in P56S-VAPB expressing cells as well as in the ALS8 patient muscle biopsy. We conclude that P56S-VAPB-ALS8 involves a cohesive pathomechanism of aberrant RBP homeostasis together with dysfunctional autophagy.

The Mitochondrial-associated ER membrane (MAM) compartment and its dysregulation in Amyotrophic Lateral Sclerosis (ALS)

The endoplasmic reticulum (ER) and mitochondria connect at multiple contact sites to form a unique cellular compartment, termed the 'mitochondria-associated ER membranes' (MAMs). MAMs are hubs for signalling pathways that regulate cellular homeostasis and survival, metabolism, and sensitivity to apoptosis. MAMs are therefore involved in vital cellular functions, but they are dysregulated in several human diseases. Whilst MAM dysfunction is increasingly implicated in the pathogenesis of neurodegenerative diseases, its role in amyotrophic lateral sclerosis (ALS) is poorly understood. However, in ALS both ER and mitochondrial dysfunction are well documented pathophysiological events. Moreover, alterations to lipid metabolism in neurons regulate processes linked to neurodegenerative diseases, and a link between dysfunction of lipid metabolism and ALS has also been proposed. In this review we discuss the structural and functional relevance of MAMs in ALS and how targeting MAM could be therapeutically beneficial in this disorder.

TAR DNA-Binding Protein 43 is Cleaved by the Protease 3C of Enterovirus A71

Enterovirus A71 (EV-A71) is one of the etiological pathogens leading to hand, foot, and mouth disease (HFMD), which can cause severe neurological complications. The neuropathogenesis of EV-A71 infection is not well understood. The mislocalization and aggregation of TAR DNA-binding protein 43 (TDP-43) is the pathological hallmark of amyotrophic lateral sclerosis (ALS). However, whether TDP-43 was impacted by EV-A71 infection is unknown. This study demonstrated that TDP-43 was cleaved during EV-A71 infection. The cleavage of TDP-43 requires EV-A71 replication rather than the activated caspases due to viral infection. TDP-43 is cleaved by viral protease 3C between the residues 331Q and 332S, while mutated TDP-43 (Q331A) was not cleaved. In addition, mutated 3C which lacks the protease activity failed to induce TDP-43 cleavage. We also found that TDP-43 was translocated from the nucleus to the cytoplasm, and the mislocalization of TDP-43 was induced by viral protease 2A rather than 3C. Taken together, we demonstrated that TDP-43 was cleaved by viral protease and translocated to the cytoplasm during EV-A71 infection, implicating the possible involvement of TDP-43 in the pathogenesis of EV-A71infection.

Mutant VAPB: Culprit or Innocent Bystander of Amyotrophic Lateral Sclerosis?

Nearly twenty years ago a mutation in the VAPB gene, resulting in a proline to serine substitution (p.P56S), was identified as the cause of a rare, slowly progressing, familial form of the motor neuron degenerative disease Amyotrophic Lateral Sclerosis (ALS). Since then, progress in unravelling the mechanistic basis of this mutation has proceeded in parallel with research on the VAP proteins and on their role in establishing membrane contact sites between the ER and other organelles. Analysis of the literature on cellular and animal models reviewed here supports the conclusion that P56S-VAPB, which is aggregation-prone, non-functional and unstable, is expressed at levels that are insufficient to support toxic gain-of-function or dominant negative effects within motor neurons. Instead, insufficient levels of the product of the single wild-type allele appear to be required for pathological effects, and may be the main driver of the disease. In light of the multiple interactions of the VAP proteins, we address the consequences of specific VAPB depletion and highlight various affected processes that could contribute to motor neuron degeneration. In the future, distinction of specific roles of each of the two VAP paralogues should help to further elucidate the basis of p.P56S familial ALS, as well as of other more common forms of the disease.

VAPB ER-Aggregates, A Possible New Biomarker in ALS Pathology

A point mutation (P56S) in the gene-encoding vesicle-associated membrane-protein-associated protein B (VAPB) leads to an autosomal-dominant form of amyotrophic lateral sclerosis (ALS), classified as ALS-8. The mutant VAPB is characterized by ER-associated aggregates that lead to a complete reorganization of ER structures. Growing evidences suggest VAPB involvement in ALS pathomechanisms. In fact, numerous studies demonstrated VAPB alteration also in sporadic ALS (sALS) and showed the presence of its aggregates when others ALS-related gene are mutant. Recently, the identification of new biomarkers in peripheral blood mononuclear cells (PBMCs) has been proposed as a good noninvasive option for studying ALS. Here, we evaluated VAPB as a possible ALS pathologic marker analyzing PBMCs of sALS patients. Immunofluorescence analysis (IFA) showed a peculiar pattern of VAPB aggregates in sALS, not evident in healthy control (HC) subjects and in Parkinson’s disease (PD) PBMCs. This specific pattern led us to suppose that VAPB could be misfolded in sALS. The data indirectly confirmed by flow cytometry assay (FCA) showed a reduction of VAPB fluorescent signals in sALS. However, our observations were not associated with the presence of a genetic mutation or altered gene expression of VAPB. Our study brings further evidences of the VAPB role in ALS as a diagnostic biomarker.

VAPB depletion alters neuritogenesis and phosphoinositide balance in motoneuron-like cells: relevance to VAPB-linked amyotrophic lateral sclerosis

VAPB and VAPA are ubiquitously expressed endoplasmic reticulum membrane proteins that play key roles in lipid exchange at membrane contact sites. A mutant, aggregation-prone, form of VAPB (P56S) is linked to a dominantly inherited form of amyotrophic lateral sclerosis; however, it has been unclear whether its pathogenicity is due to toxic gain of function, to negative dominance, or simply to insufficient levels of the wild-type protein produced from a single allele (haploinsufficiency). To investigate whether reduced levels of functional VAPB, independently from the presence of the mutant form, affect the physiology of mammalian motoneuron-like cells, we generated NSC34 clones, from which VAPB was partially or nearly completely depleted. VAPA levels, determined to be over fourfold higher than those of VAPB in untransfected cells, were unaffected. Nonetheless, cells with even partially depleted VAPB showed an increase in Golgi- and acidic vesicle-localized phosphatidylinositol-4-phosphate (PI4P) and reduced neurite extension when induced to differentiate. Conversely, the PI4 kinase inhibitors PIK93 and IN-10 increased neurite elongation. Thus, for long-term survival, motoneurons might require the full dose of functional VAPB, which may have unique function(s) that VAPA cannot perform.

SOD1 activity threshold and TOR signalling modulate VAP(P58S) aggregation via reactive oxygen species-induced proteasomal degradation in a Drosophila model of amyotrophic lateral sclerosis

Familial amyotrophic lateral sclerosis (ALS) is an incurable, late-onset motor neuron disease, linked strongly to various causative genetic loci. ALS8 codes for a missense mutation, P56S, in VAMP-associated protein B (VAPB) that causes the protein to misfold and form cellular aggregates. Uncovering genes and mechanisms that affect aggregation dynamics would greatly help increase our understanding of the disease and lead to potential therapeutics. We developed a quantitative high-throughput Drosophila S2R+ cell-based kinetic assay coupled with fluorescent microscopy to score for genes involved in the modulation of aggregates of the fly orthologue, VAP(P58S), fused with GFP. A targeted RNA interference screen against 900 genes identified 150 hits that modify aggregation, including the ALS loci Sod1 and TDP43 (also known as TBPH), as well as genes belonging to the mTOR pathway. Further, a system to measure the extent of VAP(P58S) aggregation in the Drosophila larval brain was developed in order to validate the hits from the cell-based screen. In the larval brain, we find that reduction of SOD1 levels or decreased mTOR signalling reduces aggregation, presumably by increasing the levels of cellular reactive oxygen species (ROS). The mechanism of aggregate clearance is, primarily, proteasomal degradation, which appears to be triggered by an increase in ROS. We have thus uncovered an interesting interplay between SOD1, ROS and mTOR signalling that regulates the dynamics of VAP aggregation. Mechanistic processes underlying such cellular regulatory networks will lead to better understanding of the initiation and progression of ALS.This article has an associated First Person interview with the first author of the paper.

Loss of VAPB Regulates Autophagy in a Beclin 1-Dependent Manner

Autophagy is an evolutionarily-conserved self-degradative process that maintains cellular homeostasis by eliminating protein aggregates and damaged organelles. Recently, vesicle-associated membrane protein-associated protein B (VAPB), which is associated with the familial form of amyotrophic lateral sclerosis, has been shown to regulate autophagy. In the present study, we demonstrated that knockdown of VAPB induced the up-regulation of beclin 1 expression, which promoted LC3 (microtubule-associated protein light chain 3) conversion and the formation of LC3 puncta, whereas overexpression of VAPB inhibited these processes. The regulation of beclin 1 by VAPB was at the transcriptional level. Moreover, knockdown of VAPB increased autophagic flux, which promoted the degradation of the autophagy substrate p62 and neurodegenerative disease proteins. Our study provides evidence that the regulation of autophagy by VAPB is associated with the autophagy-initiating factor beclin 1.

ER Dynamics and Derangement in Neurological Diseases

The endoplasmic reticulum (ER) is a morphologically dynamic organelle containing different membrane subdomains with distinct cellular functions. Numerous observations have revealed that ER stress response induced by disturbed ER homeostasis is linked to various neurological/neurodegenerative disorders. In contrast, recent findings unveil that ER structural derangements are linked to the progression of several neurological diseases. The derangements involve two distinct, and likely opposing pathways. One is dysfunction of ER dynamics machinery, leading to disruption of ER network organization. Another one is facilitation of pre-existing machinery, leading to generation of markedly-ordered de novo membranous structure. Restoring the ER network can be the effective way toward the cure of ER-deranged neurological disorders.

Focus on the Role of D-serine and D-amino Acid Oxidase in Amyotrophic Lateral Sclerosis/Motor Neuron Disease (ALS)

We have investigated a pathogenic mutation in D-amino acid oxidase (DAO), DAO^R199W, associated with familial Amyotrophic Lateral Sclerosis (ALS) that impairs D-serine metabolism and causes protein aggregation, autophagy and cell death in motor neuron cell lines. These features are consistent with the pathogenic processes occurring in ALS but most importantly, we have demonstrated that activation of the formation of ubiquitinated protein inclusions, increased autophagosome production and apoptotic cell death caused by the mutation in cell lines are attenuated by 5,7-dichlorokynurenic acid (DCKA), a selective inhibitor of the glycine/D-serine binding site of the NMDA receptor. D-serine is an essential co-agonist at this glutamate receptor. This data provides insight into potential upstream mechanisms that involve the action of D-serine at the NMDA receptor and might contribute to neurodegeneration. This is highly relevant to sporadic ALS (SALS), familial ALS, as well as ALS models, where elevated levels of D-serine have been reported and hence has broader clinical therapeutic implications. In order to investigate this further, we have generated a transgenic line expressing the pathogenic mutation, in order to determine whether mice expressing DAO^R199W develop a motor phenotype and whether crossing the SOD1^G93A model of ALS with mice expressing DAO^R199W affects disease progression. We found that heterozygous expression of DAO^R199W led to a significant loss of spinal cord motor neurons at 14 months, which is similar to that found in homozygous mice expressing DAO^G181R. We hypothesize that DAO has potential for development as a therapeutic agent in ALS.

Neuronal overexpression of human VAPB slows motor impairment and neuromuscular denervation in a mouse model of ALS

Four mutations in the VAMP/synaptobrevin-associated protein B (VAPB) gene have been linked to amyotrophic lateral sclerosis (ALS) type 8. The mechanism by which VAPB mutations cause motor neuron disease is unclear, but studies of the most common P56S variant suggest both loss of function and dominant-negative sequestration of wild-type protein. Diminished levels of VAPB and its proteolytic cleavage fragment have also been reported in sporadic ALS cases, suggesting that VAPB loss of function may be a common mechanism of disease. Here, we tested whether neuronal overexpression of wild-type human VAPB would attenuate disease in a mouse model of familial ALS1. We used neonatal intraventricular viral injections to express VAPB or YFP throughout the brain and spinal cord of superoxide dismutase (SOD1) G93A transgenic mice. Lifelong elevation of neuronal VAPB slowed the decline of neurological impairment, delayed denervation of hindlimb muscles, and prolonged survival of spinal motor neurons. Collectively, these changes produced a slight but significant extension in lifespan, even in this highly aggressive model of disease. Our findings lend support for a protective role of VAPB in neuromuscular health.

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.

Characterisation of the pathogenic effects of the in vivo expression of an ALS-linked mutation in D-amino acid oxidase: Phenotype and loss of spinal cord motor neurons

Amyotrophic lateral sclerosis (ALS) is the most common adult-onset neuromuscular disorder characterised by selective loss of motor neurons leading to fatal paralysis. Current therapeutic approaches are limited in their effectiveness. Substantial advances in understanding ALS disease mechanisms has come from the identification of pathogenic mutations in dominantly inherited familial ALS (FALS). We previously reported a coding mutation in D-amino acid oxidase (DAO^R199W) associated with FALS. DAO metabolises D-serine, an essential co-agonist at the N-Methyl-D-aspartic acid glutamate receptor subtype (NMDAR). Using primary motor neuron cultures or motor neuron cell lines we demonstrated that expression of DAO^R199W, promoted the formation of ubiquitinated protein aggregates, activated autophagy and increased apoptosis. The aim of this study was to characterise the effects of DAO^R199Win vivo, using transgenic mice overexpressing DAO^R199W. Marked abnormal motor features, e.g. kyphosis, were evident in mice expressing DAO^R199W, which were associated with a significant loss (19%) of lumbar spinal cord motor neurons, analysed at 14 months. When separated by gender, this effect was greater in females (26%; p< 0.0132). In addition, we crossed the DAO^R199W transgenic mouse line with the SOD1^G93A mouse model of ALS to determine whether the effects of SOD1^G93A were potentiated in the double transgenic line (DAO^R199W/SOD1^G93A). Although overall survival was not affected, onset of neurological signs was significantly earlier in female double transgenic animals than their female SOD1^G93A littermates (125 days vs 131 days, P = 0.0239). In summary, some significant in vivo effects of DAO^R199W on motor neuron function (i.e. kyphosis and loss of motor neurons) were detected which were most marked in females and could contribute to the earlier onset of neurological signs in double transgenic females compared to SOD1^G93A littermates, highlighting the importance of recognizing gender effects present in animal models of ALS.

Protein Homeostasis in Amyotrophic Lateral Sclerosis: Therapeutic Opportunities?

Protein homeostasis (proteostasis), the correct balance between production and degradation of proteins, is essential for the health and survival of cells. Proteostasis requires an intricate network of protein quality control pathways (the proteostasis network) that work to prevent protein aggregation and maintain proteome health throughout the lifespan of the cell. Collapse of proteostasis has been implicated in the etiology of a number of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), the most common adult onset motor neuron disorder. Here, we review the evidence linking dysfunctional proteostasis to the etiology of ALS and discuss how ALS-associated insults affect the proteostasis network. Finally, we discuss the potential therapeutic benefit of proteostasis network modulation in ALS.

Insights into ALS pathomechanisms: from flies to humans

Amyotrophic Lateral Sclerosis (ALS) is a devastating neurodegenerative disease causing the death of motor neurons with consequent muscle atrophy and paralysis. Several neurodegenerative diseases have been modeled in Drosophila and genetic studies on this model organism led to the elucidation of crucial aspects of disease mechanisms. ALS, however, has lagged somewhat behind possibly because of the lack of a suitable genetic model. We were the first to develop a fly model for ALS and over the last few years, we have implemented and used this model for a large scale, unbiased modifier screen. We also report an extensive bioinformatic analysis of the genetic modifiers and we show that most of them are associated in a network of interacting genes controlling known as well as novel cellular processes involved in ALS pathogenesis. A similar analysis for the human homologues of the Drosophila modifiers and the validation of a subset of them in human tissues confirm and expand the significance of the data for the human disease. Finally, we analyze a possible application of the model in the process of therapeutic discovery in ALS and we discuss the importance of novel “non-obvious” models for the disease.

TDP-43 or FUS-induced misfolded human wild-type SOD1 can propagate intercellularly in a prion-like fashion

Amyotrophic lateral sclerosis (ALS), which appears to spread through the neuroaxis in a spatiotemporally restricted manner, is linked to heritable mutations in genes encoding SOD1, TDP-43, FUS, C9ORF72, or can occur sporadically without recognized genetic mutations. Misfolded human wild-type (HuWt) SOD1 has been detected in both familial and sporadic ALS patients, despite mutations in SOD1 accounting for only 2% of total cases. We previously showed that accumulation of pathological TDP-43 or FUS coexist with misfolded HuWtSOD1 in patient motor neurons, and can trigger its misfolding in cultured cells. Here, we used immunocytochemistry and immunoprecipitation to demonstrate that TDP-43 or FUS-induced misfolded HuWtSOD1 can propagate from cell-to-cell via conditioned media, and seed cytotoxic misfolding of endogenous HuWtSOD1 in the recipient cells in a prion-like fashion. Knockdown of SOD1 using siRNA in recipient cells, or incubation of conditioned media with misfolded SOD1-specific antibodies, inhibits intercellular transmission, indicating that HuWtSOD1 is an obligate seed and substrate of propagated misfolding. In this system, intercellular spread of SOD1 misfolding is not accompanied by transmission of TDP-43 or FUS pathology. Our findings argue that pathological TDP-43 and FUS may exert motor neuron pathology in ALS through the initiation of propagated misfolding of SOD1.

Golgi Fragmentation in ALS Motor Neurons. New Mechanisms Targeting Microtubules, Tethers, and Transport Vesicles

Pathological alterations of the Golgi apparatus, such as its fragmentation represent an early pre-clinical feature of many neurodegenerative diseases and have been widely studied in the motor neuron disease amyotrophic lateral sclerosis (ALS). Yet, the underlying molecular mechanisms have remained cryptic. In principle, Golgi fragmentation may result from defects in three major classes of proteins: structural Golgi proteins, cytoskeletal proteins and molecular motors, as well as proteins mediating transport to and through the Golgi. Here, we present the different mechanisms that may underlie Golgi fragmentation in animal and cellular models of ALS linked to mutations in SOD1, TARDBP (TDP-43), VAPB, and C9Orf72 and we propose a novel one based on findings in progressive motor neuronopathy (pmn) mice. These mice are mutated in the TBCE gene encoding the cis-Golgi localized tubulin-binding cofactor E, one of five chaperones that assist in tubulin folding and microtubule polymerization. Loss of TBCE leads to alterations in Golgi microtubules, which in turn impedes on the maintenance of the Golgi architecture. This is due to down-regulation of COPI coat components, dispersion of Golgi tethers and strong accumulation of ER-Golgi SNAREs. These effects are partially rescued by the GTPase ARF1 through recruitment of TBCE to the Golgi. We hypothesize that defects in COPI vesicles, microtubules and their interaction may also underlie Golgi fragmentation in human ALS linked to other mutations, spinal muscular atrophy (SMA), and related motor neuron diseases. We also discuss the functional relevance of pathological Golgi alterations, in particular their potential causative, contributory, or compensatory role in the degeneration of motor neuron cell bodies, axons and synapses.

Vapb/Amyotrophic lateral sclerosis 8 knock-in mice display slowly progressive motor behavior defects accompanying ER stress and autophagic response

Missense mutations (P56S) in Vapb are associated with autosomal dominant motor neuron diseases: amyotrophic lateral sclerosis and lower motor neuron disease. Although transgenic mice overexpressing the mutant vesicle-associated membrane protein-associated protein B (VAPB) protein with neuron-specific promoters have provided some insight into the toxic properties of the mutant proteins, their role in pathogenesis remains unclear. To identify pathological defects in animals expressing the P56S mutant VAPB protein at physiological levels in the appropriate tissues, we have generated Vapb knock-in mice replacing wild-type Vapb gene with P56S mutant Vapb gene and analyzed the resulting pathological phenotypes. Heterozygous P56S Vapb knock-in mice show mild age-dependent defects in motor behaviors as characteristic features of the disease. The homozygous P56S Vapb knock-in mice show more severe defects compared with heterozygous mice reflecting the dominant and dose-dependent effects of P56S mutation. Significantly, the knock-in mice demonstrate accumulation of P56S VAPB protein and ubiquitinated proteins in cytoplasmic inclusions, selectively in motor neurons. The mutant mice demonstrate induction of ER stress and autophagic response in motor neurons before obvious onset of behavioral defects, suggesting that these cellular biological defects might contribute to the initiation of the disease. The P56S Vapb knock-in mice could be a valuable tool to gain a better understanding of the mechanisms by which the disease arises.

Autophagy and Neurodegeneration: Insights from a Cultured Cell Model of ALS

Autophagy plays a major role in the elimination of cellular waste components, the renewal of intracellular proteins and the prevention of the build-up of redundant or defective material. It is fundamental for the maintenance of homeostasis and especially important in post-mitotic neuronal cells, which, without competent autophagy, accumulate protein aggregates and degenerate. Many neurodegenerative diseases are associated with defective autophagy; however, whether altered protein turnover or accumulation of misfolded, aggregate-prone proteins is the primary insult in neurodegeneration has long been a matter of debate. Amyotrophic lateral sclerosis (ALS) is a fatal disease characterized by selective degeneration of motor neurons. Most of the ALS cases occur in sporadic forms (SALS), while 10%–15% of the cases have a positive familial history (FALS). The accumulation in the cell of misfolded/abnormal proteins is a hallmark of both SALS and FALS, and altered protein degradation due to autophagy dysregulation has been proposed to contribute to ALS pathogenesis. In this review, we focus on the main molecular features of autophagy to provide a framework for discussion of our recent findings about the role in disease pathogenesis of the ALS-linked form of the VAPB gene product, a mutant protein that drives the generation of unusual cytoplasmic inclusions.

Calcium dysregulation links ALS defective proteins and motor neuron selective vulnerability

More than 20 distinct gene loci have so far been implicated in amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disorder characterized by progressive neurodegeneration of motor neurons (MN) and death. Most of this distinct set of ALS-related proteins undergoes toxic deposition specifically in MN for reasons which remain unclear. Here we overview a recent body of evidence indicative that mutations in ALS-related proteins can disrupt fundamental Ca²⁺ signalling pathways in MN, and that Ca²⁺ itself impacts both directly or indirectly in many ALS critical proteins and cellular processes that result in MN neurodegeneration. We argue that the inherent vulnerability of MN to dysregulation of intracellular Ca²⁺ is deeply associated with discriminating pathogenicity and aberrant crosstalk of most of the critical proteins involved in ALS. Overall, Ca²⁺ deregulation in MN is at the cornerstone of different ALS processes and is likely one of the factors contributing to the selective susceptibility of these cells to this particular neurodegenerative disease.

Network analyses reveal novel aspects of ALS pathogenesis

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease characterized by selective loss of motor neurons, muscle atrophy and paralysis. Mutations in the human VAMP-associated protein B (hVAPB) cause a heterogeneous group of motor neuron diseases including ALS8. Despite extensive research, the molecular mechanisms underlying ALS pathogenesis remain largely unknown. Genetic screens for key interactors of hVAPB activity in the intact nervous system, however, represent a fundamental approach towards understanding the in vivo function of hVAPB and its role in ALS pathogenesis. Targeted expression of the disease-causing allele leads to neurodegeneration and progressive decline in motor performance when expressed in the adult Drosophila, eye or in its entire nervous system, respectively. By using these two phenotypic readouts, we carried out a systematic survey of the Drosophila genome to identify modifiers of hVAPB-induced neurotoxicity. Modifiers cluster in a diverse array of biological functions including processes and genes that have been previously linked to hVAPB function, such as proteolysis and vesicular trafficking. In addition to established mechanisms, the screen identified endocytic trafficking and genes controlling proliferation and apoptosis as potent modifiers of ALS8-mediated defects. Surprisingly, the list of modifiers was mostly enriched for proteins linked to lipid droplet biogenesis and dynamics. Computational analysis reveals that most modifiers can be linked into a complex network of interacting genes, and that the human genes homologous to the Drosophila modifiers can be assembled into an interacting network largely overlapping with that in flies. Identity markers of the endocytic process were also found to abnormally accumulate in ALS patients, further supporting the relevance of the fly data for human biology. Collectively, these results not only lead to a better understanding of hVAPB function but also point to potentially relevant targets for therapeutic intervention.

Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease causing loss of motor neurons and consequently a progressive deterioration of motor functions. ALS is uniformly fatal with death occurring 5 years after onset of symptoms. There is currently no effective treatment for ALS. Several mutations in a gene called hVAPB have shown that this gene is causative of a type of ALS known as ALS8. In this study we sought to identify genes and cellular processes that are involved in the toxicity conferred by the defective ALS8 allele. By using the power of Drosophila genetics, we performed a large scale genomic screen and identified a number of genes that can affect hVAPB-mediated toxicity. These modifiers cluster into functional pathways known to be involved in ALS as well as novel ones. The relevance of these modifiers and mechanisms for the human disease was confirmed by showing that the human homologues of the fly modifiers can be organized into a network that closely resembles that of the Drosophila genes. Identifying cellular processes and proteins that modulate hVAPB pathological activity can facilitate the discovery of an effective treatment for ALS.

Amyotrophic lateral sclerosis-linked mutant VAPB inclusions do not interfere with protein degradation pathways or intracellular transport in a cultured cell model

VAPB is a ubiquitously expressed, ER-resident adaptor protein involved in interorganellar lipid exchange, membrane contact site formation, and membrane trafficking. Its mutant form, P56S-VAPB, which has been linked to a dominantly inherited form of Amyotrophic Lateral Sclerosis (ALS8), generates intracellular inclusions consisting in restructured ER domains whose role in ALS pathogenesis has not been elucidated. P56S-VAPB is less stable than the wild-type protein and, at variance with most pathological aggregates, its inclusions are cleared by the proteasome. Based on studies with cultured cells overexpressing the mutant protein, it has been suggested that VAPB inclusions may exert a pathogenic effect either by sequestering the wild-type protein and other interactors (loss-of-function by a dominant negative effect) or by a more general proteotoxic action (gain-of-function). To investigate P56S-VAPB degradation and the effect of the inclusions on proteostasis and on ER-to-plasma membrane protein transport in a more physiological setting, we used stable HeLa and NSC34 Tet-Off cell lines inducibly expressing moderate levels of P56S-VAPB. Under basal conditions, P56S-VAPB degradation was mediated exclusively by the proteasome in both cell lines, however, it could be targeted also by starvation-stimulated autophagy. To assess possible proteasome impairment, the HeLa cell line was transiently transfected with the ERAD (ER Associated Degradation) substrate CD3δ, while autophagic flow was investigated in cells either starved or treated with an autophagy-stimulating drug. Secretory pathway functionality was evaluated by analyzing the transport of transfected Vesicular Stomatitis Virus Glycoprotein (VSVG). P56S-VAPB expression had no effect either on the degradation of CD3δ or on the levels of autophagic markers, or on the rate of transport of VSVG to the cell surface. We conclude that P56S-VAPB inclusions expressed at moderate levels do not interfere with protein degradation pathways or protein transport, suggesting that the dominant inheritance of the mutant gene may be due mainly to haploinsufficiency.

A genetic screen identifies Tor as an interactor of VAPB in a Drosophila model of amyotrophic lateral sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a progressive neurodegenerative disorder characterized by selective death of motor neurons. In 5–10% of the familial cases, the disease is inherited because of mutations. One such mutation, P56S, was identified in human VAPB that behaves in a dominant negative manner, sequestering wild type protein into cytoplasmic inclusions.

We have conducted a reverse genetic screen to identify interactors of Drosophila VAPB. We screened 2635 genes and identified 103 interactors, of which 45 were enhancers and 58 were suppressors of VAPB function. Interestingly, the screen identified known ALS loci – TBPH, alsin2 and SOD1. Also identified were genes involved in cellular energetics and homeostasis which were used to build a gene regulatory network of VAPB modifiers. One key modifier identified was Tor, whose knockdown reversed the large bouton phenotype associated with VAP(P58S) expression in neurons. A similar reversal was seen by over-expressing Tuberous Sclerosis Complex (Tsc1,2) that negatively regulates TOR signaling as also by reduction of S6K activity. In comparison, the small bouton phenotype associated with VAP(wt) expression was reversed with Tsc1 knock down as well as S6K-CA expression. Tor therefore interacts with both VAP(wt) and VAP(P58S), but in a contrasting manner. Reversal of VAP(P58S) bouton phenotypes in larvae fed with the TOR inhibitor Rapamycin suggests upregulation of TOR signaling in response to VAP(P58S) expression.

The VAPB network and further mechanistic understanding of interactions with key pathways, such as the TOR cassette, will pave the way for a better understanding of the mechanisms of onset and progression of motor neuron disease.

Gain-of-function mutations in the ALS8 causative gene VAPB have detrimental effects on neurons and muscles

Amyotrophic Lateral Sclerosis (ALS) is a motor neuron degenerative disease characterized by a progressive, and ultimately fatal, muscle paralysis. The human VAMP-Associated Protein B (hVAPB) is the causative gene of ALS type 8. Previous studies have shown that a loss-of-function mechanism is responsible for VAPB-induced ALS. Recently, a novel mutation in hVAPB (V234I) has been identified but its pathogenic potential has not been assessed. We found that neuronal expression of the V234I mutant allele in Drosophila (DVAP-V260I) induces defects in synaptic structure and microtubule architecture that are opposite to those associated with DVAP mutants and transgenic expression of other ALS-linked alleles. Expression of DVAP-V260I also induces aggregate formation, reduced viability, wing postural defects, abnormal locomotion behavior, nuclear abnormalities, neurodegeneration and upregulation of the heat-shock-mediated stress response. Similar, albeit milder, phenotypes are associated with the overexpression of the wild-type protein. These data show that overexpressing the wild-type DVAP is sufficient to induce the disease and that DVAP-V260I is a pathogenic allele with increased wild-type activity. We propose that a combination of gain- and loss-of-function mechanisms is responsible for VAPB-induced ALS.

Converging mechanisms in ALS and FTD: disrupted RNA and protein homeostasis

Breakthrough discoveries identifying common genetic causes for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) have transformed our view of these disorders. They share unexpectedly similar signatures, including dysregulation in common molecular players including TDP-43, FUS/TLS, ubiquilin-2, VCP, and expanded hexanucleotide repeats within the C9ORF72 gene. Dysfunction in RNA processing and protein homeostasis is an emerging theme. We present the case here that these two processes are intimately linked, with disease-initiated perturbation of either leading to further deviation of both protein and RNA homeostasis through a feedforward loop including cell-to-cell prion-like spread that may represent the mechanism for relentless disease progression.

FE65 interacts with ADP-ribosylation factor 6 to promote neurite outgrowth

FE65 is an adaptor protein that binds to the amyloid precursor protein (APP). As such, FE65 has been implicated in the pathogenesis of Alzheimer's disease. In addition, evidence suggests that FE65 is involved in brain development. It is generally believed that FE65 participates in these processes by recruiting various interacting partners to form functional complexes. Here, we show that via its first phosphotyrosine binding (PTB) domain, FE65 binds to the small GTPase ADP-ribosylation factor 6 (ARF6). FE65 preferentially binds to ARF6-GDP, and they colocalize in neuronal growth cones. Interestingly, FE65 stimulates the activation of both ARF6 and its downstream GTPase Rac1, a regulator of actin dynamics, and functions in growth cones to stimulate neurite outgrowth. We show that transfection of FE65 and/or ARF6 promotes whereas small interfering RNA knockdown of FE65 or ARF6 inhibits neurite outgrowth in cultured neurons as compared to the mock-transfected control cells. Moreover, knockdown of ARF6 attenuates FE65 stimulation of neurite outgrowth and defective neurite outgrowth seen in FE65-deficient neurons is partially corrected by ARF6 overexpression. Notably, the stimulatory effect of FE65 and ARF6 on neurite outgrowth is abrogated either by dominant-negative Rac1 or knockdown of Rac1. Thus, we identify FE65 as a novel regulator of neurite outgrowth via controlling ARF6-Rac1 signaling.

Genetics of amyotrophic lateral sclerosis: an update

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder involving both upper motor neurons (UMN) and lower motor neurons (LMN). Enormous research has been done in the past few decades in unveiling the genetics of ALS, successfully identifying at least fifteen candidate genes associated with familial and sporadic ALS. Numerous studies attempting to define the pathogenesis of ALS have identified several plausible determinants and molecular pathways leading to motor neuron degeneration, which include oxidative stress, glutamate excitotoxicity, apoptosis, abnormal neurofilament function, protein misfolding and subsequent aggregation, impairment of RNA processing, defects in axonal transport, changes in endosomal trafficking, increased inflammation, and mitochondrial dysfunction. This review is to update the recent discoveries in genetics of ALS, which may provide insight information to help us better understanding of the disease neuropathogenesis.

Amyotrophic lateral sclerosis-related VAPB P56S mutation differentially affects the function and survival of corticospinal and spinal motor neurons

The substitution of Proline with Serine at residue 56 (P56S) of vesicle-associated membrane protein-associated protein B (VAPB) has been linked to an atypical autosomal dominant form of familial amyotrophic lateral sclerosis 8 (ALS8). To investigate the pathogenic mechanism of P56S VAPB in ALS, we generated transgenic (Tg) mice that heterologously express human wild-type (WT) and P56S VAPB under the control of a pan-neuronal promoter Thy1.2. While WT VAPB Tg mice did not exhibit any overt motor behavioral phenotypes, P56S VAPB Tg mice developed progressive hyperactivities and other motor abnormalities. VAPB protein was accumulated as large punctate in the soma and proximal dendrites of both corticospinal motor neurons (CSMNs) and spinal motor neurons (SMNs) in P56S VAPB Tg mice. Concomitantly, a significant increase of endoplasmic reticulum stress and unfolded protein response and the resulting up-regulation of pro-apoptotic factor CCAAT/enhancer-binding protein homologous protein expression were observed in the CSMNs and SMNs of P56S VAPB Tg mice. However, only a progressive loss of CSMNs but not SMNs was found in P56S VAPB Tg mice. In SMNs, P56S VAPB promoted a rather selective translocation of VAPB protein onto the postsynaptic site of C-boutons that altered the morphology of C-boutons and impaired the spontaneous rhythmic discharges of SMNs. Therefore, these findings provide new pathophysiological mechanisms of P56S VAPB that differentially affect the function and survival of CSMNs and SMNs in ALS8.

Amyotrophic lateral sclerosis (ALS)-associated VAPB-P56S inclusions represent an ER quality control compartment

BACKGROUND

Protein aggregation and the formation of intracellular inclusions are a central feature of many neurodegenerative disorders, but precise knowledge about their pathogenic role is lacking in most instances. Here we have characterized inclusions formed in transgenic mice carrying the P56S mutant form of VAPB that causes various motor neuron syndromes including ALS8.

RESULTS

Inclusions in motor neurons of VAPB-P56S transgenic mice are characterized by the presence of smooth ER-like tubular profiles, and are immunoreactive for factors that operate in the ER associated degradation (ERAD) pathway, including p97/VCP, Derlin-1, and the ER membrane chaperone BAP31. The presence of these inclusions does not correlate with signs of axonal and neuronal degeneration, and axotomy leads to their gradual disappearance, indicating that they represent reversible structures. Inhibition of the proteasome and knockdown of the ER membrane chaperone BAP31 increased the size of mutant VAPB inclusions in primary neuron cultures, while knockdown of TEB4, an ERAD ubiquitin-protein ligase, reduced their size. Mutant VAPB did not codistribute with mutant forms of seipin that are associated with an autosomal dominant motor neuron disease, and accumulate in a protective ER derived compartment termed ERPO (ER protective organelle) in neurons.

CONCLUSIONS

The data indicate that the VAPB-P56S inclusions represent a novel reversible ER quality control compartment that is formed when the amount of mutant VAPB exceeds the capacity of the ERAD pathway and that isolates misfolded and aggregated VAPB from the rest of the ER. The presence of this quality control compartment reveals an additional level of flexibility of neurons to cope with misfolded protein stress in the ER.

Widespread aggregation of mutant VAPB associated with ALS does not cause motor neuron degeneration or modulate mutant SOD1 aggregation and toxicity in mice

BACKGROUND

A proline-to-serine substitution at position-56 (P56S) of vesicle-associated membrane protein-associated protein B (VAPB) causes a form of dominantly inherited motor neuron disease (MND), including typical and atypical amyotrophic lateral sclerosis (ALS) and a mild late-onset spinal muscular atrophy (SMA). VAPB is an integral endoplasmic reticulum (ER) protein and has been implicated in various cellular processes, including ER stress, the unfolded protein response (UPR) and Ca2+ homeostasis. However, it is unclear how the P56S mutation leads to neurodegeneration and muscle atrophy in patients. The formation of abnormal VAPB-positive inclusions by mutant VAPB suggests a possible toxic gain of function as an underlying mechanism. Furthermore, the amount of VAPB protein is reported to be reduced in sporadic ALS patients and mutant SOD1G93A mice, leading to the hypothesis that wild type VAPB plays a role in the pathogenesis of ALS without VAPB mutations.

RESULTS

To investigate the pathogenic mechanism in vivo, we generated human wild type (wtVAPB) and mutant VAPB (muVAPB) transgenic mice that expressed the transgenes broadly in the CNS. We observed robust VAPB-positive aggregates in the spinal cord of muVAPB transgenic mice. However, we failed to find an impairment of motor function and motor neuron degeneration. We also did not detect any change in the endogenous VAPB level or evidence for induction of the unfolded protein response (UPR) and coaggregation of VAPA with muVAPB. Furthermore, we crossed these VAPB transgenic mice with mice that express mutant SOD1G93A and develop motor neuron degeneration. Overexpression of neither wtVAPB nor muVAPB modulated the protein aggregation and disease progression in the SOD1G93A mice.

CONCLUSION

Overexpression of VAPBP56S mutant to approximately two-fold of the endogenous VAPB in mouse spinal cord produced abundant VAPB aggregates but was not sufficient to cause motor dysfunction or motor neuron degeneration. Furthermore, overexpression of either muVAPB or wtVAPB does not modulate the course of ALS in SOD1G93A mice. These results suggest that changes in wild type VAPB do not play a significant role in ALS cases that are not caused by VAPB mutations. Furthermore, these results suggest that muVAPB aggregates are innocuous and do not cause motor neuron degeneration by a gain-of-toxicity, and therefore, a loss of function may be the underlying mechanism.

XBP1 depletion precedes ubiquitin aggregation and Golgi fragmentation in TDP-43 transgenic rats

Protein inclusion is a prominent feature of neurodegenerative diseases including frontotemporal lobar degeneration (FTLD) that is characterized by the presence of ubiquitinated TDP-43 inclusion. Presence of protein inclusions indicates an interruption to protein degradation machinery or the overload of misfolded proteins. In response to the increase in misfolded proteins, cells usually initiate a mechanism called unfolded protein response (UPR) to reduce misfolded proteins in the lumen of endoplasmic reticules. Here, we examined the effects of mutant TDP-43 on the UPR in transgenic rats that express mutant human TDP-43 restrictedly in the neurons of the forebrain. Over-expression of mutant TDP-43 in rats caused prominent aggregation of ubiquitin and remarkable fragmentation of Golgi complexes prior to neuronal loss. While ubiquitin aggregates and Golgi fragments were accumulating, neurons expressing mutant TDP-43 failed to up-regulate chaperones residing in the endoplasmic reticules and failed to initiate the UPR. Prior to ubiquitin aggregation and Golgi fragmentation, neurons were depleted of X-box-binding protein 1 (XBP1), a key player of UPR machinery. Although it remains to determine how mutation of TDP-43 leads to the failure of the UPR, our data demonstrate that failure of the UPR is implicated in TDP-43 pathogenesis.

The TRK-fused gene is mutated in hereditary motor and sensory neuropathy with proximal dominant involvement

Hereditary motor and sensory neuropathy with proximal dominant involvement (HMSN-P) is an autosomal-dominant neurodegenerative disorder characterized by widespread fasciculations, proximal-predominant muscle weakness, and atrophy followed by distal sensory involvement. To date, large families affected by HMSN-P have been reported from two different regions in Japan. Linkage and haplotype analyses of two previously reported families and two new families with the use of high-density SNP arrays further defined the minimum candidate region of 3.3 Mb in chromosomal region 3q12. Exome sequencing showed an identical c.854C>T (p.Pro285Leu) mutation in the TRK-fused gene (TFG) in the four families. Detailed haplotype analysis suggested two independent origins of the mutation. Pathological studies of an autopsied patient revealed TFG- and ubiquitin-immunopositive cytoplasmic inclusions in the spinal and cortical motor neurons. Fragmentation of the Golgi apparatus, a frequent finding in amyotrophic lateral sclerosis, was also observed in the motor neurons with inclusion bodies. Moreover, TAR DNA-binding protein 43 kDa (TDP-43)-positive cytoplasmic inclusions were also demonstrated. In cultured cells expressing mutant TFG, cytoplasmic aggregation of TDP-43 was demonstrated. These findings indicate that formation of TFG-containing cytoplasmic inclusions and concomitant mislocalization of TDP-43 underlie motor neuron degeneration in HMSN-P. Pathological overlap of proteinopathies involving TFG and TDP-43 highlights a new pathway leading to motor neuron degeneration.

VAPB interacts with the mitochondrial protein PTPIP51 to regulate calcium homeostasis

A proline to serine substitution at position 56 in the gene encoding vesicle-associated membrane protein-associated protein B (VAPB) causes some dominantly inherited familial forms of motor neuron disease including amyotrophic lateral sclerosis (ALS) type-8. VAPB is an integral endoplasmic reticulum (ER) protein whose amino-terminus projects into the cytosol. Overexpression of ALS mutant VAPBP56S disrupts ER structure but the mechanisms by which it induces disease are not properly understood. Here we show that VAPB interacts with the outer mitochondrial membrane protein, protein tyrosine phosphatase-interacting protein 51 (PTPIP51). ER and mitochondria are both stores for intracellular calcium (Ca(2+)) and Ca(2+) exchange between these organelles occurs at regions of ER that are closely apposed to mitochondria. These are termed mitochondria-associated membranes (MAM). We demonstrate that VAPB is a MAM protein and that loss of either VAPB or PTPIP51 perturbs uptake of Ca(2+) by mitochondria following release from ER stores. Finally, we demonstrate that VAPBP56S has altered binding to PTPIP51 and increases Ca(2+) uptake by mitochondria following release from ER stores. Damage to ER, mitochondria and Ca(2+) homeostasis are all seen in ALS and we discuss the implications of our findings in this context.

Amyotrophic lateral sclerosis: update and new developments

Amyotrophic lateral sclerosis (ALS) is the most common form of motor neuron disease. It is typically characterized by adult-onset degeneration of the upper and lower motor neurons, and is usually fatal within a few years of onset. A subset of ALS patients has an inherited form of the disease, and a few of the known mutant genes identified in familial cases have also been found in sporadic forms of ALS. Precisely how the diverse ALS-linked gene products dictate the course of the disease, resulting in compromised voluntary muscular ability, is not entirely known. This review addresses the major advances that are being made in our understanding of the molecular mechanisms giving rise to the disease, which may eventually translate into new treatment options.

Amyotrophic lateral sclerosis-associated mutant VAPBP56S perturbs calcium homeostasis to disrupt axonal transport of mitochondria

A proline-to-serine substitution at position 56 in the gene encoding vesicle-associated membrane protein-associated protein B (VAPB; VAPBP56S) causes some dominantly inherited familial forms of motor neuron disease, including amyotrophic lateral sclerosis (ALS) type-8. Here, we show that expression of ALS mutant VAPBP56S but not wild-type VAPB in neurons selectively disrupts anterograde axonal transport of mitochondria. VAPBP56S-induced disruption of mitochondrial transport involved reductions in the frequency, velocity and persistence of anterograde mitochondrial movement. Anterograde axonal transport of mitochondria is mediated by the microtubule-based molecular motor kinesin-1. Attachment of kinesin-1 to mitochondria involves the outer mitochondrial membrane protein mitochondrial Rho GTPase-1 (Miro1) which acts as a sensor for cytosolic calcium levels ([Ca²⁺]c); elevated [Ca²⁺]c disrupts mitochondrial transport via an effect on Miro1. To gain insight into the mechanisms underlying the VAPBP56S effect on mitochondrial transport, we monitored [Ca²⁺]c levels in VAPBP56S-expressing neurons. Expression of VAPBP56S but not VAPB increased resting [Ca²⁺]c and this was associated with a reduction in the amounts of tubulin but not kinesin-1 that were associated with Miro1. Moreover, expression of a Ca²⁺ insensitive mutant of Miro1 rescued defective mitochondrial axonal transport and restored the amounts of tubulin associated with the Miro1/kinesin-1 complex to normal in VAPBP56S-expressing cells. Our results suggest that ALS mutant VAPBP56S perturbs anterograde mitochondrial axonal transport by disrupting Ca²⁺ homeostasis and effecting the Miro1/kinesin-1 interaction with tubulin.

Restructured Endoplasmic Reticulum, Generated by Mutant, Amyotrophic Lateral Sclerosis-Linked VAPB, is Cleared by the Proteasome

VAPB (Vesicle-Associated Membrane Protein-Associated Protein B) is a ubiquitously expressed, ER-resident tail-anchored protein that functions as adaptor for lipid-exchange proteins. Its mutant form, P56S-VAPB, is linked to a dominantly inherited form of amyotrophic lateral sclerosis (ALS8). P56S-VAPB forms intracellular inclusions, whose role in ALS pathogenesis has not yet been elucidated. We recently demonstrated that these inclusions are formed by profoundly remodelled stacked ER cisternae (Fasana E. et al., FASEB J. 24:1419, 2010). Here, we used stable HeLa-TetOff cell lines inducibly expressing wild type and P56S-VAPB, as well as microinjection protocols in non-transfected cells, to investigate the dynamics of inclusion generation and degradation. Shortly after synthesis, the mutant protein forms small, polyubiquitinated clusters, which then congregate in the juxtanuclear region independently from the integrity of the microtubule cytoskeleton. The rate of degradation of the aggregated mutant is higher than that of the wild type protein, so that the inclusions are cleared only a few hours after cessation of P56S-VAPB synthesis. At variance with other inclusion bodies linked to neurodegenerative diseases, clearance of P56S-VAPB inclusions involves the proteasome, with no apparent participation of macro-autophagy. Transfection of a dominant negative form of the AAA ATPase, p97/VCP, stabilizes mutant VAPB, suggesting a role for this ATPase in extracting the aggregated protein from the inclusions. Our results demonstrate that the structures induced by P56S-VAPB stand apart from other inclusion bodies, both in the mechanism of their genesis and of their clearance from the cell, with possible implications for the pathogenic mechanism of the mutant protein.