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Neuromuscular disorders comprise a diverse group of human inborn diseases that arise from defects in the structure and/or function of the muscle tissue — encompassing the muscle cells (myofibres) themselves and their extracellular matrix — or muscle fibre innervation. Since the identification in 1987 of the first genetic lesion associated with a neuromuscular disorder — mutations in dystrophin as an underlying cause of Duchenne muscular dystrophy — the field has made tremendous progress in understanding the genetic basis of these diseases, with pathogenic variants in more than 500 genes now identified as underlying causes of neuromuscular disorders. The subset of neuromuscular disorders that affect skeletal muscle are referred to as myopathies or muscular dystrophies, and are due to variants in genes encoding muscle proteins. Many of these proteins provide structural stability to the myofibres or function in regulating sarcolemmal integrity, whereas others are involved in protein turnover, intracellular trafficking, calcium handling and electrical excitability — processes that ensure myofibre resistance to stress and their primary activity in muscle contraction. In this Review, we discuss how defects in muscle proteins give rise to muscle dysfunction, and ultimately to disease, with a focus on pathologies that are most common, best understood and that provide the most insight into muscle biology.Myopathies are genetically inherited diseases that affect the structure and/or function of skeletal muscles and often result in muscle degeneration (muscular dystrophy). This Review discusses our current understanding of the cellular and molecular mechanisms that underlie the most common of these pathologies, which provide key insights into muscle biology.

Macrophages specialize in removing lipids and debris present in the atherosclerotic plaque. However, plaque progression renders macrophages unable to degrade exogenous atherogenic material and endogenous cargo including dysfunctional proteins and organelles. Here we show that a decline in the autophagy–lysosome system contributes to this as evidenced by a derangement in key autophagy markers in both mouse and human atherosclerotic plaques. By augmenting macrophage TFEB, the master transcriptional regulator of autophagy–lysosomal biogenesis, we can reverse the autophagy dysfunction of plaques, enhance aggrephagy of p62-enriched protein aggregates and blunt macrophage apoptosis and pro-inflammatory IL-1β levels, leading to reduced atherosclerosis. In order to harness this degradative response therapeutically, we also describe a natural sugar called trehalose as an inducer of macrophage autophagy–lysosomal biogenesis and show trehalose’s ability to recapitulate the atheroprotective properties of macrophage TFEB overexpression. Our data support this practical method of enhancing the degradative capacity of macrophages as a therapy for atherosclerotic vascular disease. Dysfunction of autophagy in plaque macrophages aggravates atherosclerosis. Here the authors show that induction of macrophage autophagy–lysosomal biogenesis either genetically by overexpression of the master transcriptional regulator of this process, TFEB, or pharmacologically with trehalose is atheroprotective.

Algorithms designed to identify canonical yeast prions predict that around 250 human proteins, including several RNA-binding proteins associated with neurodegenerative disease, harbour a distinctive prion-like domain (PrLD) enriched in uncharged polar amino acids and glycine. PrLDs in RNA-binding proteins are essential for the assembly of ribonucleoprotein granules. However, the interplay between human PrLD function and disease is not understood. Here we define pathogenic mutations in PrLDs of heterogeneous nuclear ribonucleoproteins (hnRNPs) A2B1 and A1 in families with inherited degeneration affecting muscle, brain, motor neuron and bone, and in one case of familial amyotrophic lateral sclerosis. Wild-type hnRNPA2 (the most abundant isoform of hnRNPA2B1) and hnRNPA1 show an intrinsic tendency to assemble into self-seeding fibrils, which is exacerbated by the disease mutations. Indeed, the pathogenic mutations strengthen a ‘steric zipper’ motif in the PrLD, which accelerates the formation of self-seeding fibrils that cross-seed polymerization of wild-type hnRNP. Notably, the disease mutations promote excess incorporation of hnRNPA2 and hnRNPA1 into stress granules and drive the formation of cytoplasmic inclusions in animal models that recapitulate the human pathology. Thus, dysregulated polymerization caused by a potent mutant steric zipper motif in a PrLD can initiate degenerative disease. Related proteins with PrLDs should therefore be considered candidates for initiating and perhaps propagating proteinopathies of muscle, brain, motor neuron and bone. The identification of pathogenic mutations within prion-like domains (PrLDs) of the RNA-binding proteins hnRNPA2B1 and hnRNPA1 add to our understanding of how mutations in these proteins lead to degenerative disease, and highlight the potential importance of PrLDs in degenerative diseases of the nervous system, muscle and bone. Disease link to prion-like RNA-binding protein How do mutations in RNA-binding proteins cause human disease, and neurodegeneration in particular? Hong Joo Kim et al . have identified mutations in two RNA-binding proteins, hnRNPA2B1 and hnRNPA1, in two families with inclusion body myopathy with frontotemporal dementia. Both of the mutations lie within a highly conserved part of a protein domain that has similarities to prion proteins, and a tendency to aggregate. This aggregation is enhanced by the mutations. The mutated prion-like domain of hnRNPA2 can functionally replace that of a yeast prion protein and reproduce its prion-like behaviour. These findings have relevance to the pathogenesis of degenerative diseases and proteinopathies such as amyotrophic lateral sclerosis.

The intimate association between the endoplasmic reticulum (ER) and mitochondrial membranes at ER-Mitochondria contact sites (ERMCS) is a platform for critical cellular processes, particularly lipid synthesis. How contacts are remodeled and the impact of altered contacts on lipid metabolism remains poorly understood. We show that the p97 AAA-ATPase and its adaptor ubiquitin-X domain adaptor 8 (UBXD8) regulate ERMCS. The p97-UBXD8 complex localizes to contacts and its loss increases contacts in a manner that is dependent on p97 catalytic activity. Quantitative proteomics and lipidomics of ERMCS demonstrates alterations in proteins regulating lipid metabolism and a significant change in membrane lipid saturation upon UBXD8 deletion. Loss of p97-UBXD8 increased membrane lipid saturation via SREBP1 and the lipid desaturase SCD1. Aberrant contacts can be rescued by unsaturated fatty acids or overexpression of SCD1. We find that the SREBP1-SCD1 pathway is negatively impacted in the brains of mice with p97 mutations that cause neurodegeneration. We propose that contacts are exquisitely sensitive to alterations to membrane lipid composition and saturation. Inter-organellar contacts are hubs for several critical cellular processes, such as lipid synthesis. Here Ganji et al. show that the p97-UBXD8 complex modulates contacts by regulating membrane lipid composition and saturation.

Genetic testing is essential for patients with a suspected hereditary myopathy. More than 50% of patients clinically diagnosed with a myopathy carry a variant of unknown significance in a myopathy gene, often leaving them without a genetic diagnosis. Limb-girdle muscular dystrophy (LGMD) type R4/2E is caused by mutations in β-sarcoglycan (SGCB). Together, β-, α-, γ-, and δ-sarcoglycan form a 4-protein transmembrane complex (SGC) that localizes to the sarcolemma. Biallelic loss-of-function mutations in any subunit can lead to LGMD. To provide functional evidence for the pathogenicity of missense variants, we performed deep mutational scanning of SGCB and assessed SGC cell surface localization for all 6,340 possible amino acid changes. Variant functional scores were bimodally distributed and perfectly predicted pathogenicity of known variants. Variants with less severe functional scores more often appeared in patients with slower disease progression, implying a relationship between variant function and disease severity. Amino acid positions intolerant to variation mapped to points of predicted SGC interactions, validated in silico structural models, and enabled accurate prediction of pathogenic variants in other SGC genes. These results will be useful for clinical interpretation of SGCB variants and improving diagnosis of LGMD; we hope they enable wider use of potentially life-saving gene therapy.

The misfolding and aggregation of proteins is often implicated in the development and progression of degenerative diseases. Heat shock proteins (HSPs), such as the ubiquitously expressed Type II Hsp40 molecular chaperone, DNAJB6, assist in protein folding and disaggregation. Historically, mutations within the DNAJB6 G/F domain have been associated with Limb-Girdle Muscular Dystrophy type 1D, now referred to as LGMDD1, a dominantly inherited degenerative disease. Recently, novel mutations within the J domain of DNAJB6 have been reported in patients with LGMDD1. Since novel myopathy-causing mutations in the Hsp40 J domain have yet to be characterized and both the function of DNAJB6 in skeletal muscle and the clients of this chaperone are unknown, we set out to assess the effect of these mutations on chaperone function using the genetically tractable yeast system. The essential yeast Type II Hsp40, Sis1, is homologous to DNAJB6 and is involved in the propagation of yeast prions. Using phenotypic, biochemical, and functional assays we found that homologous mutations in the Sis1 J domain differentially alter the processing of specific yeast prion strains, as well as a non-prion substrate. These data suggest that the newly-identified mutations in the J domain of DNAJB6 cause aberrant chaperone function that leads to the pathogenesis in LGMDD1.

VCP (also called p97) recognizes and interacts with ubiquitylated cargo molecules that are destined for proteasomal degradation. Meyer and colleagues show that VCP, together with its cofactor UBXD1, sorts ubiquitylated caveolin-1 to the endolysosome system. Mutations in VCP that are associated with human degenerative diseases lack this ability. The AAA-ATPase VCP (also known as p97) cooperates with distinct cofactors to process ubiquitylated proteins in different cellular pathways 1 , 2 , 3 . VCP missense mutations cause a systemic degenerative disease in humans, but the molecular pathogenesis is unclear 4 , 5 . We used an unbiased mass spectrometry approach and identified a VCP complex with the UBXD1 cofactor, which binds to the plasma membrane protein caveolin-1 (CAV1) and whose formation is specifically disrupted by disease-associated mutations. We show that VCP–UBXD1 targets mono-ubiquitylated CAV1 in SDS-resistant high-molecular-weight complexes on endosomes, which are en route to degradation in endolysosomes 6 . Expression of VCP mutant proteins, chemical inhibition of VCP, or siRNA-mediated depletion of UBXD1 leads to a block of CAV1 transport at the limiting membrane of enlarged endosomes in cultured cells. In patient muscle, muscle-specific caveolin-3 accumulates in sarcoplasmic pools and specifically delocalizes from the sarcolemma. These results extend the cellular functions of VCP to mediating sorting of ubiquitylated cargo in the endocytic pathway and indicate that impaired trafficking of caveolin may contribute to pathogenesis in individuals with VCP mutations.

BAG3 is a multi-domain hub that connects two classes of chaperones, small heat shock proteins (sHSPs) via two isoleucine-proline-valine (IPV) motifs and Hsp70 via a BAG domain. Mutations in either the IPV or BAG domain of BAG3 cause a dominant form of myopathy, characterized by protein aggregation in both skeletal and cardiac muscle tissues. Surprisingly, for both disease mutants, impaired chaperone binding is not sufficient to explain disease phenotypes. Recombinant mutants are correctly folded, show unaffected Hsp70 binding but are impaired in stimulating Hsp70-dependent client processing. As a consequence, the mutant BAG3 proteins become the node for a dominant gain of function causing aggregation of itself, Hsp70, Hsp70 clients and tiered interactors within the BAG3 interactome. Importantly, genetic and pharmaceutical interference with Hsp70 binding completely reverses stress-induced protein aggregation for both BAG3 mutations. Thus, the gain of function effects of BAG3 mutants act as Achilles heel of the HSP70 machinery. BAG3 is a Hsp70 co-chaperone that is highly expressed in muscles. Here the authors show that several myofibrillar myopathy causing BAG3 mutations are not impaired in Hsp70 binding, but rather impair the ADP-ATP exchange step of the Hsp70 cycle, causing the aggregation of BAG3, Hsp70 and Hsp70 clients and leading to a collapse of protein homeostasis.

Dominant mutations in p97/VCP (valosin-containing protein) cause a rare multisystem degenerative disease with varied phenotypes that include inclusion body myopathy, Paget’s disease of bone, frontotemporal dementia, and amyotrophic lateral sclerosis. p97 disease mutants have altered N-domain conformations, elevated ATPase activity, and altered cofactor association. We have now discovered a previously unidentified disease-relevant functional property of p97 by identifying how the cofactors p37 and p47 regulate p97 ATPase activity. We define p37 as, to our knowledge, the first known p97-activating cofactor, which enhances the catalytic efficiency ( k cₐₜ/ K ₘ) of p97 by 11-fold. Whereas both p37 and p47 decrease the K ₘ of ATP in p97, p37 increases the k cₐₜ of p97. In contrast, regulation by p47 is biphasic, with decreased k cₐₜ at low levels but increased k cₐₜ at higher levels. By deleting a region of p47 that lacks homology to p37 (amino acids 69–92), we changed p47 from an inhibitory cofactor to an activating cofactor, similar to p37. Our data suggest that cofactors regulate p97 ATPase activity by binding to the N domain. Induced conformation changes affect ADP/ATP binding at the D1 domain, which in turn controls ATPase cycling. Most importantly, we found that the D2 domain of disease mutants failed to be activated by p37 or p47. Our results show that cofactors play a critical role in controlling p97 ATPase activity, and suggest that lack of cofactor-regulated communication may contribute to p97-associated disease pathogenesis. Significance Age-associated degenerative diseases have similar pathogenic mechanisms related to defects in protein homeostasis. p97/VCP (valosin-containing protein) is essential for coordinating protein degradation and is mutated in a multisystem degenerative disease that affects the central nervous system, muscle, and bone. p97/VCP is an enzyme in the AAA ATPases (ATPases associated with diverse cellular activities) family, which takes apart ATP and uses this energy to perform pivotal functions. We found that p97/VCP cofactors control its enzymatic activity. p97/VCP disease mutants behave abnormally due to lack of appropriate control by these cofactors. Correcting the function of the disease-associated proteins may be a desirable approach to developing safe treatment for fatal degenerative diseases. The next steps are to screen and characterize large panels of compounds to identify potential drugs that may correct the malfunction.