New insights into the protein aggregation pathology in myotilinopathy by combined proteomic and immunolocalization analyses

A brief history of the congenital myopathies - the myopathological perspective

Congenital myopathies are defined by early clinical onset, slow progression, hereditary nature and disease-specific myopathological lesions - however, with exceptions - demanding special techniques in regard to morphological diagnostic and research work-up. To identify an index disease in a family requires a muscle biopsy - and no congenital myopathy has ever been first described at autopsy. The nosographic history commenced when - in addition to special histopathological techniques in the earliest classical triad of central core disease, 1956, nemaline myopathy, 1963, and centronuclear myopathy, 1966/67, within a decade - electron microscopy and enzyme histochemistry were applied to unfixed frozen muscle tissue and, thus, revolutionized diagnostic and research myopathology. During the following years, the list of structure-defined congenital myopathies grew to some 40 conditions. Then, the introduction of immunohistochemistry allowed myopathological documentation of proteins and their abnormalities in individual congenital myopathies. Together with the diagnostic evolution of molecular genetics, many more congenital myopathies were described, without new disease-specific lesions or only already known ones. These were nosographically defined by individual mutations in hitherto congenital myopathies-unrelated genes. This latter development may also affect the nomenclature of congenital myopathies in that the mutant gene needs to be attached to the individually identified congenital myopathies with or without the disease-specific lesion, such as CCD-RYR1 or CM-RYR1. This principle is similar to that of the nomenclature of Congenital Disorders of Glycosylation. Retroactive molecular characterization of originally and first described congenital myopathies has only rarely been achieved.

Mass Spectrometry-Based Proteomic Technology and Its Application to Study Skeletal Muscle Cell Biology

Voluntary striated muscles are characterized by a highly complex and dynamic proteome that efficiently adapts to changed physiological demands or alters considerably during pathophysiological dysfunction. The skeletal muscle proteome has been extensively studied in relation to myogenesis, fiber type specification, muscle transitions, the effects of physical exercise, disuse atrophy, neuromuscular disorders, muscle co-morbidities and sarcopenia of old age. Since muscle tissue accounts for approximately 40% of body mass in humans, alterations in the skeletal muscle proteome have considerable influence on whole-body physiology. This review outlines the main bioanalytical avenues taken in the proteomic characterization of skeletal muscle tissues, including top-down proteomics focusing on the characterization of intact proteoforms and their post-translational modifications, bottom-up proteomics, which is a peptide-centric method concerned with the large-scale detection of proteins in complex mixtures, and subproteomics that examines the protein composition of distinct subcellular fractions. Mass spectrometric studies over the last two decades have decisively improved our general cell biological understanding of protein diversity and the heterogeneous composition of individual myofibers in skeletal muscles. This detailed proteomic knowledge can now be integrated with findings from other omics-type methodologies to establish a systems biological view of skeletal muscle function.

Bi-allelic variants of FILIP1 cause congenital myopathy, dysmorphism and neurological defects

Filamin-A-interacting protein 1 (FILIP1) is a structural protein that is involved in neuronal and muscle function and integrity and interacts with FLNa and FLNc. Pathogenic variants in filamin-encoding genes have been linked to neurological disorders (FLNA) and muscle diseases characterized by myofibrillar perturbations (FLNC), but human diseases associated with FILIP1 variants have not yet been described. Here, we report on five patients from four unrelated consanguineous families with homozygous FILIP1 variants (two nonsense and two missense). Functional studies indicated altered stability of the FILIP1 protein carrying the p.[Pro1133Leu] variant. Patients exhibit a broad spectrum of neurological symptoms including brain malformations, neurodevelopmental delay, muscle weakness and pathology and dysmorphic features. Electron and immunofluorescence microscopy on the muscle biopsy derived from the patient harbouring the homozygous p.[Pro1133Leu] missense variant revealed core-like zones of myofibrillar disintegration, autophagic vacuoles and accumulation of FLNc. Proteomic studies on the fibroblasts derived from the same patient showed dysregulation of a variety of proteins including FLNc and alpha-B-crystallin, a finding (confirmed by immunofluorescence) which is in line with the manifestation of symptoms associated with the syndromic phenotype of FILIP1opathy. The combined findings of this study show that the loss of functional FILIP1 leads to a recessive disorder characterized by neurological and muscular manifestations as well as dysmorphic features accompanied by perturbed proteostasis and myopathology.

Desmin mutations impact the autophagy flux in C2C12 cell in mutation-specific manner

Desmin is the main intermediate filament of striated and smooth muscle cells and plays a crucial role in maintaining the stability of muscle fiber during contraction and relaxation cycles. Being a component of Z-disk area, desmin integrates autophagic pathways, and the disturbance of Z-disk proteins' structure negatively affects chaperone-assisted selective autophagy (CASA). In the present study, we focused on alteration of autophagy flux in myoblasts expressing various Des mutations. We applied Western blotting, immunocytochemistry, RNA sequencing, and shRNA approach to demonstrate that DesS12F, DesA357P, DesL345P, DesL370P, and DesD399Y mutations. Mutation-specific effect on autophagy flux being most severe in aggregate-prone Des mutations such as DesL345P, DesL370P, and DesD399Y. RNA sequencing data confirmed the most prominent effect of these mutations on expression profile and, in particular, on autophagy-related genes. To verify CASA contribution to desmin aggregate formation, we suppressed CASA by knocking down Bag3 and demonstrated that it promoted aggregate formation and lead to downregulation of Vdac2 and Vps4a and upregulation of Lamp, Pink1, and Prkn. In conclusion, Des mutations showed a mutation-specific effect on autophagy flux in C2C12 cells with either a predominant impact on autophagosome maturation or on degradation and recycling processes. Aggregate-prone desmin mutations lead to the activation of basal autophagy level while suppressing the CASA pathway by knocking down Bag3 can promote desmin aggregate formation.

Human Mutated MYOT and CRYAB Genes Cause a Myopathic Phenotype in Zebrafish

Myofibrillar myopathies (MFMs) are a group of hereditary neuromuscular disorders sharing common histological features, such as myofibrillar derangement, Z-disk disintegration, and the accumulation of degradation products into protein aggregates. They are caused by mutations in several genes that encode either structural proteins or molecular chaperones. Nevertheless, the mechanisms by which mutated genes result in protein aggregation are still unknown. To unveil the role of myotilin and αB-crystallin in the pathogenesis of MFM, we injected zebrafish fertilized eggs at the one-cell stage with expression plasmids harboring cDNA sequences of human wildtype or mutated MYOT (p.Ser95Ile) and human wildtype or mutated CRYAB (p.Gly154Ser). We evaluated the effects on fish survival, motor behavior, muscle structure and development. We found that transgenic zebrafish showed morphological defects that were more severe in those overexpressing mutant genes. which developed a myopathic phenotype consistent with that of human myofibrillar myopathy, including the formation of protein aggregates. Results indicate that pathogenic mutations in myotilin and αB-crystallin genes associated with MFM cause a structural and functional impairment of the skeletal muscle in zebrafish, thereby making this non-mammalian organism a powerful model to dissect disease pathogenesis and find possible druggable targets.

Fibin regulates cardiomyocyte hypertrophy and causes protein-aggregate-associated cardiomyopathy in vivo

Despite the identification of numerous molecular pathways modulating cardiac hypertrophy its pathogenesis is not completely understood. In this study we define an unexpected role for Fibin ("fin bud initiation factor homolog") in cardiomyocyte hypertrophy. Via gene expression profiling in hypertrophic murine hearts after transverse aortic constriction we found a significant induction of Fibin. Moreover, Fibin was upregulated in another mouse model of cardiac hypertrophy (calcineurin-transgenics) as well as in patients with dilated cardiomyopathy. Immunoflourescence microscopy revealed subcellular localization of Fibin at the sarcomeric z-disc. Overexpression of Fibin in neonatal rat ventricular cardiomyocytes revealed a strong anti-hypertrophic effect through inhibiting both, NFAT- and SRF-dependent signalling. In contrast, transgenic mice with cardiac-restricted overexpression of Fibin developed dilated cardiomyopathy, accompanied by induction of hypertrophy-associated genes. Moreover, Fibin overexpression accelerated the progression to heart failure in the presence of prohypertrophic stimuli such as pressure overload and calcineurin overexpression. Histological and ultrastructural analyses surprisingly showed large protein aggregates containing Fibin. On the molecular level, aggregate formation was accompanied by an induction of the unfolded protein response subsequent UPR-mediated apoptosis and autophagy. Taken together, we identified Fibin as a novel potent negative regulator of cardiomyocyte hypertrophy in vitro. Yet, heart-specific Fibin overexpression in vivo causes development of a protein-aggregate-associated cardiomyopathy. Because of close similarities to myofibrillar myopathies, Fibin represents a candidate gene for cardiomyopathy and Fibin transgenic mice may provide additional mechanistic insight into aggregate formation in these diseases.

Silencing MYOT Expression May Inhibit Autophagy in Human Skeletal Muscle Cells

Muscle diseases are closely related to autophagy disorders. Studies of autophagy inhibition indicated the importance of autophagy in muscle regeneration, while activation of autophagy can restore muscle function in some myopathies. Previous studies have revealed that mutations in the MYOT gene may lead to several kinds of hereditary myopathies. However, whether the autophagy played a crucial role in hereditary myopathy caused by MYOT mutations was still not clear. In this study, we established the MYOT knockdown human skeletal muscle cell models (HSkMCs) by small interfering RNA. Real-time PCR and Western blot studies found that the expression of p62 and LC3B-II was decreased dramatically, which suggested that silencing MYOT expression may regulate the autophagy in HSkMCs. Further immunofluorescence study on Ad-mCherry-GFP-LC3B adenovirus transfection and monodansylcadaverine (MDC) staining revealed that knocking down the expression of MYOT may inhibit the autophagy. Next, we used the autophagy inducer Earle's balanced salt solution (EBSS) and late-autophagy inhibitor bafilomycin A1 (BAF A1) to treat the HSkMCs, respectively, and found that silencing MYOT expression can inhibit the activation of autophagy by EBSS and aggravate the inhibition of autophagy by BAF A1. Finally, we also found that silencing MYOT expression can downregulate the expression of ATG7 and ATG5, two important autophagy regulatory molecules. Hence, our study may first reveal that knocking down the expression of MYOT may inhibit the autophagy. Hereditary myopathies caused by MYOT mutations may partly result from the inhibition of autophagy in HSkMCs.

Molecular signatures of inherited and acquired sporadic late onset nemaline myopathies

Acquired sporadic late onset nemaline myopathy (SLONM) and inherited nemaline myopathy (iNM) both feature accumulation of nemaline rods in muscle fibers. Unlike iNM, SLONM is amenable to therapy. The distinction between these disorders is therefore crucial when the diagnosis remains ambiguous after initial investigations. We sought to identify biomarkers facilitating this distinction and to investigate the pathophysiology of nemaline rod formation in these different disorders. Twenty-two muscle samples from patients affected by SLONM or iNM underwent quantitative histological analysis, laser capture microdissection for proteomic analysis of nemaline rod areas and rod-free areas, and transcriptomic analysis. In all iNM samples, nemaline rods were found in subsarcolemmal or central aggregates, whereas they were diffusely distributed within muscle fibers in most SLONM samples. In SLONM, muscle fibers harboring nemaline rods were smaller than those without rods. Necrotic fibers, increased endomysial connective tissue, and atrophic fibers filled with nemaline rods were more common in SLONM. Proteomic analysis detected differentially expressed proteins between nemaline rod areas and rod-free areas, as well as between SLONM and iNM. These differentially expressed proteins implicated immune, structural, metabolic, and cellular processes in disease pathophysiology. Notably, immunoglobulin overexpression with accumulation in nemaline rod areas was detected in SLONM. Transcriptomic analysis corroborated proteomic findings and further revealed substantial gene expression differences between SLONM and iNM. Overall, we identified unique pathological and molecular signatures associated with SLONM and iNM, suggesting distinct underlying pathophysiological mechanisms. These findings represent a step towards enhanced diagnostic tools and towards development of treatments for SLONM.

Genetic-Based Treatment Strategies for Muscular Dystrophy and Congenital Myopathies

PURPOSE OF REVIEW

This article discusses the foundational concepts of genetic treatment strategies employed in neuromuscular medicine, as well as the importance of genetic testing as a requirement for applying gene-based therapy.

RECENT FINDINGS

Gene therapies have become a reality for several neuromuscular disorders. Exon-skipping and (in Europe) ribosomal read-through approaches are currently available to a subset of patients with Duchenne muscular dystrophy. Microdystrophin gene replacement has shown promise and is nearing the final stages of clinical trials. Numerous gene-based therapies for other muscular dystrophies and congenital myopathies are progressing toward approval as well.

SUMMARY

Muscular dystrophies and congenital myopathies are a heterogenous group of hereditary muscle disorders. Confirming a diagnosis with genetic testing is not only critical for guiding management, but also an actual prerequisite for current and future gene therapies. Recessive loss-of-function or dominant haploinsufficiency disorders may be treated with gene replacement strategies, whereas dominant negative and toxic gain-of-function disorders are best addressed with a variety of knockdown approaches. It is important to recognize that many therapeutics are mutation specific and will only benefit a subset of individuals with a specific disease.

Proteomic profiling of sporadic late-onset nemaline myopathy

Objective

To define the proteomic profile of sporadic late‐onset nemaline myopathy (SLONM) and explore its pathogenesis.

Methods

We performed mass spectrometry on laser‐dissected frozen muscle samples from five patients with SLONM, three of whom with an associated monoclonal protein (MP), and four controls, to determine the proteomic profile of SLONM. Furthermore, we assessed the role of the MP by evaluating the expression of the immunoglobulin light chain variable regions (IGVL).

Results

There were 294 differentially expressed proteins: 272 upregulated and 22 downregulated. Among the top 100 upregulated proteins, the most common categories were: nuclear or nucleic acid metabolism (24%), extracellular matrix and basal lamina (17%), immune response (13%), and actin dynamics (8%). Downregulated proteins consisted mostly of contractile proteins. Among upregulated proteins, there were 65 with a role related to the immune system, including eight proteins involved in major histocompatibility complex 1 (MHC1) and antigen processing, 15 in MHCII complex and phagocytosis, and 23 in B and/or T‐cell function. Among nine upregulated immunoglobulin proteins, there were two IGVL genes. However, these were also detected in SLONM cases without an MP, with no evidence of clonally dominant immunoglobulin deposition. In muscle sections from SLONM patients, nemaline rods tended to accumulate in atrophic fibers with marked rarefaction of the myofibrils. Increased MHC1 reactivity was present in fibers containing nemaline rods as well as adjacent nonatrophic fibers.

Conclusion

Our findings suggest that aberrant immune activation is present in SLONM, but do not support a direct causal relationship between the MP and SLONM.

Nanoenviroments of the β-Subunit of L-Type Voltage-Gated Calcium Channels in Adult Cardiomyocytes

In cardiomyocytes, Ca²⁺ influx through L-type voltage-gated calcium channels (LTCCs) following membrane depolarization regulates crucial Ca²⁺-dependent processes including duration and amplitude of the action potentials and excitation-contraction coupling. LTCCs are heteromultimeric proteins composed of the Ca_vα₁, Ca_vβ, Ca_vα₂δ and Ca_vγ subunits. Here, using ascorbate peroxidase (APEX2)-mediated proximity labeling and quantitative proteomics, we identified 61 proteins in the nanoenvironments of Ca_vβ₂ in cardiomyocytes. These proteins are involved in diverse cellular functions such as cellular trafficking, cardiac contraction, sarcomere organization and excitation-contraction coupling. Moreover, pull-down assays and co-immunoprecipitation analyses revealed that Ca_vβ₂ interacts with the ryanodine receptor 2 (RyR2) in adult cardiomyocytes, probably coupling LTCCs and the RyR2 into a supramolecular complex at the dyads. This interaction is mediated by the Src-homology 3 domain of Ca_vβ₂ and is necessary for an effective pacing frequency-dependent increase of the Ca²⁺-induced Ca²⁺ release mechanism in cardiomyocytes.

Proteomic characterisation of polyglucosan bodies in skeletal muscle in RBCK1 deficiency

AIMS

Several neurodegenerative and neuromuscular disorders are characterised by storage of polyglucosan, consisting of proteins and amylopectin-like polysaccharides, which are less branched than in normal glycogen. Such diseases include Lafora disease, branching enzyme deficiency, glycogenin-1 deficiency, polyglucosan body myopathy type 1 (PGBM1) due to RBCK1 deficiency and others. The protein composition of polyglucosan bodies is largely unknown.

METHODS

We combined quantitative mass spectrometry, immunohistochemical and western blot analyses to identify the principal protein components of polyglucosan bodies in PGBM1. Histologically stained tissue sections of skeletal muscle from four patients were used to isolate polyglucosan deposits and control regions by laser microdissection. Prior to mass spectrometry, samples were labelled with tandem mass tags that enable quantitative comparison and multiplexed analysis of dissected samples. To study the distribution and expression of the accumulated proteins, immunohistochemical and western blot analyses were performed.

RESULTS

Accumulated proteins were mainly components of glycogen metabolism and protein quality control pathways. The majority of fibres showed depletion of glycogen and redistribution of key enzymes of glycogen metabolism to the polyglucosan bodies. The polyglucosan bodies also showed accumulation of proteins involved in the ubiquitin-proteasome and autophagocytosis systems and protein chaperones.

CONCLUSIONS

The sequestration of key enzymes of glycogen metabolism to the polyglucosan bodies may explain the glycogen depletion in the fibres and muscle function impairment. The accumulation of components of the protein quality control systems and other proteins frequently found in protein aggregate disorders indicates that protein aggregation may be an essential part of the pathobiology of polyglucosan storage.

The β2-Subunit of Voltage-Gated Calcium Channels Regulates Cardiomyocyte Hypertrophy

L-type voltage-gated calcium channels (LTCCs) regulate crucial physiological processes in the heart. They are composed of the Ca_vα₁ pore-forming subunit and the accessory subunits Ca_vβ, Ca_vα₂δ, and Ca_vγ. Ca_vβ is a cytosolic protein that regulates channel trafficking and activity, but it also exerts other LTCC-independent functions. Cardiac hypertrophy, a relevant risk factor for the development of congestive heart failure, depends on the activation of calcium-dependent pro-hypertrophic signaling cascades. Here, by using shRNA-mediated Ca_vβ silencing, we demonstrate that Ca_vβ₂ downregulation enhances α1-adrenergic receptor agonist-induced cardiomyocyte hypertrophy. We report that a pool of Ca_vβ₂ is targeted to the nucleus in cardiomyocytes and that the expression of this nuclear fraction decreases during in vitro and in vivo induction of cardiac hypertrophy. Moreover, the overexpression of nucleus-targeted Ca_vβ₂ in cardiomyocytes inhibits in vitro-induced hypertrophy. Quantitative proteomic analyses showed that Ca_vβ₂ knockdown leads to changes in the expression of diverse myocyte proteins, including reduction of calpastatin, an endogenous inhibitor of the calcium-dependent protease calpain. Accordingly, Ca_vβ₂-downregulated cardiomyocytes had a 2-fold increase in calpain activity as compared to control cells. Furthermore, inhibition of calpain activity in Ca_vβ₂-downregulated cells abolished the enhanced α1-adrenergic receptor agonist-induced hypertrophy observed in these cells. Our findings indicate that in cardiomyocytes, a nuclear pool of Ca_vβ₂ participates in cellular functions that are independent of LTCC activity. They also indicate that a downregulation of nuclear Ca_vβ₂ during cardiomyocyte hypertrophy promotes the activation of calpain-dependent hypertrophic pathways.

Integrated proteomic and transcriptomic profiling identifies aberrant gene and protein expression in the sarcomere, mitochondrial complex I, and the extracellular matrix in Warmblood horses with myofibrillar myopathy

Background

Myofibrillar myopathy in humans causes protein aggregation, degeneration, and weakness of skeletal muscle. In horses, myofibrillar myopathy is a late-onset disease of unknown origin characterized by poor performance, atrophy, myofibrillar disarray, and desmin aggregation in skeletal muscle. This study evaluated molecular and ultrastructural signatures of myofibrillar myopathy in Warmblood horses through gluteal muscle tandem-mass-tag quantitative proteomics (5 affected, 4 control), mRNA-sequencing (8 affected, 8 control), amalgamated gene ontology analyses, and immunofluorescent and electron microscopy.

Results

We identified 93/1533 proteins and 47/27,690 genes that were significantly differentially expressed. The top significantly differentially expressed protein CSRP3 and three other differentially expressed proteins, including, PDLIM3, SYNPO2, and SYNPOL2, are integrally involved in Z-disc signaling, gene transcription and subsequently sarcomere integrity. Through immunofluorescent staining, both desmin aggregates and CSRP3 were localized to type 2A fibers. The highest differentially expressed gene CHAC1, whose protein product degrades glutathione, is associated with oxidative stress and apoptosis. Amalgamated transcriptomic and proteomic gene ontology analyses identified 3 enriched cellular locations; the sarcomere (Z-disc & I-band), mitochondrial complex I and the extracellular matrix which corresponded to ultrastructural Z-disc disruption and mitochondrial cristae alterations found with electron microscopy.

Conclusions

A combined proteomic and transcriptomic analysis highlighted three enriched cellular locations that correspond with MFM ultrastructural pathology in Warmblood horses. Aberrant Z-disc mechano-signaling, impaired Z-disc stability, decreased mitochondrial complex I expression, and a pro-oxidative cellular environment are hypothesized to contribute to the development of myofibrillar myopathy in Warmblood horses. These molecular signatures may provide further insight into diagnostic biomarkers, treatments, and the underlying pathophysiology of MFM.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07758-0.

Advanced Fiber Type-Specific Protein Profiles Derived from Adult Murine Skeletal Muscle

Skeletal muscle is a heterogeneous tissue consisting of blood vessels, connective tissue, and muscle fibers. The last are highly adaptive and can change their molecular composition depending on external and internal factors, such as exercise, age, and disease. Thus, examination of the skeletal muscles at the fiber type level is essential to detect potential alterations. Therefore, we established a protocol in which myosin heavy chain isoform immunolabeled muscle fibers were laser microdissected and separately investigated by mass spectrometry to develop advanced proteomic profiles of all murine skeletal muscle fiber types. All data are available via ProteomeXchange with the identifier PXD025359. Our in-depth mass spectrometric analysis revealed unique fiber type protein profiles, confirming fiber type-specific metabolic properties and revealing a more versatile function of type IIx fibers. Furthermore, we found that multiple myopathy-associated proteins were enriched in type I and IIa fibers. To further optimize the assignment of fiber types based on the protein profile, we developed a hypothesis-free machine-learning approach, identified a discriminative peptide panel, and confirmed our panel using a public data set.

FLNC-Associated Myofibrillar Myopathy: New Clinical, Functional, and Proteomic Data

Objective

To determine whether a new indel mutation in the dimerization domain of filamin C (FLNc) causes a hereditary myopathy with protein aggregation in muscle fibers, we clinically and molecularly studied a German family with autosomal dominant myofibrillar myopathy (MFM).

Methods

We performed mutational analysis in 3 generations, muscle histopathology, and proteomic studies of IM protein aggregates. Functional consequences of the FLNC mutation were investigated with interaction and transfection studies and biophysics molecular analysis.

Results

Eight patients revealed clinical features of slowly progressive proximal weakness associated with a heterozygous c.8025_8030delCAAGACinsA (p.K2676Pfs*3) mutation in FLNC. Two patients exhibited a mild cardiomyopathy. MRI of skeletal muscle revealed lipomatous changes typical for MFM with FLNC mutations. Muscle biopsies showed characteristic MFM findings with protein aggregation and lesion formation. The proteomic profile of aggregates was specific for MFM-filaminopathy and indicated activation of the ubiquitin-proteasome system (UPS) and autophagic pathways. Functional studies revealed that mutant FLNc is misfolded, unstable, and incapable of forming homodimers and heterodimers with wild-type FLNc.

Conclusions

This new MFM-filaminopathy family confirms that expression of mutant FLNC leads to an adult-onset muscle phenotype with intracellular protein accumulation. Mutant FLNc protein is biochemically compromised and leads to dysregulation of protein quality control mechanisms. Proteomic analysis of MFM protein aggregates is a potent method to identify disease-relevant proteins, differentiate MFM subtypes, evaluate the relevance of gene variants, and identify novel MFM candidate genes.

Quantitative Muscle-MRI Correlates with Histopathology in Skeletal Muscle Biopsies

BACKGROUND

Skeletal muscle biopsy is one of the gold standards in the diagnostic workup of muscle disorders. By histopathologic analysis, characteristic features like inflammatory cellular infiltrations, fat and collagen replacement of muscle tissue or structural defects of the myofibers can be detected. In the past years, novel quantitative MRI (qMRI) techniques have been developed to quantify tissue parameters, thus providing a non-invasive diagnostic tool in several myopathies.

OBJECTIVE

This proof-of-principle study was performed to validate the qMRI-techniques to skeletal muscle biopsy results.

METHODS

Ten patients who underwent skeletal muscle biopsy for diagnostic purposes were examined by qMRI. Fat fraction, water T2-time and diffusion parameters were measured in the muscle from which the biopsy was taken. The proportion of fat tissue, the severity of degenerative and inflammatory parameters and the amount of type 1- and type 2- muscle fibers were determined in all biopsy samples. The qMRI-data were then correlated to the histopathological findings.

RESULTS

The amount of fat tissue in skeletal muscle biopsy correlated significantly with the fat fraction derived from the Dixon sequence. The water T2-time, a parameter for tissue edema, correlated with the amount of vacuolar changes of myofibers and endomysial macrophages in the histopathologic analysis. No significant correlations were found for diffusion parameters.

CONCLUSION

In this proof-of-principle study, qMRI techniques were related to characteristic histopathologic features in neuromuscular disorders. The study provides the basis for further development of qMRI methods in the follow-up of patients with neuromuscular disorders, especially in the context of emerging treatment strategies.

The Role of Z-disc Proteins in Myopathy and Cardiomyopathy

The Z-disc acts as a protein-rich structure to tether thin filament in the contractile units, the sarcomeres, of striated muscle cells. Proteins found in the Z-disc are integral for maintaining the architecture of the sarcomere. They also enable it to function as a (bio-mechanical) signalling hub. Numerous proteins interact in the Z-disc to facilitate force transduction and intracellular signalling in both cardiac and skeletal muscle. This review will focus on six key Z-disc proteins: α-actinin 2, filamin C, myopalladin, myotilin, telethonin and Z-disc alternatively spliced PDZ-motif (ZASP), which have all been linked to myopathies and cardiomyopathies. We will summarise pathogenic variants identified in the six genes coding for these proteins and look at their involvement in myopathy and cardiomyopathy. Listing the Minor Allele Frequency (MAF) of these variants in the Genome Aggregation Database (GnomAD) version 3.1 will help to critically re-evaluate pathogenicity based on variant frequency in normal population cohorts.

Maintenance of type 2 glycolytic myofibers with age by Mib1-Actn3 axis

Age-associated muscle atrophy is a debilitating condition associated with loss of muscle mass and function with age that contributes to limitation of mobility and locomotion. However, the underlying mechanisms of how intrinsic muscle changes with age are largely unknown. Here we report that, with age, Mind bomb-1 (Mib1) plays important role in skeletal muscle maintenance via proteasomal degradation-dependent regulation of α-actinin 3 (Actn3). The disruption of Mib1 in myofibers (Mib1^ΔMF) results in alteration of type 2 glycolytic myofibers, muscle atrophy, impaired muscle function, and Actn3 accumulation. After chronic exercise, Mib1^ΔMF mice show muscle atrophy even at young age. However, when Actn3 level is downregulated, chronic exercise-induced muscle atrophy is ameliorated. Importantly, the Mib1 and Actn3 levels show clinical relevance in human skeletal muscles accompanied by decrease in skeletal muscle function with age. Together, these findings reveal the significance of the Mib1-Actn3 axis in skeletal muscle maintenance with age and suggest the therapeutic potential for the treatment or amelioration of age-related muscle atrophy.

Loss of supervillin causes myopathy with myofibrillar disorganization and autophagic vacuoles

The muscle specific isoform of the supervillin protein (SV2), encoded by the SVIL gene, is a large sarcolemmal myosin II- and F-actin-binding protein. Supervillin (SV2) binds and co-localizes with costameric dystrophin and binds nebulin, potentially attaching the sarcolemma to myofibrillar Z-lines. Despite its important role in muscle cell physiology suggested by various in vitro studies, there are so far no reports of any human disease caused by SVIL mutations. We here report four patients from two unrelated, consanguineous families with a childhood/adolescence onset of a myopathy associated with homozygous loss-of-function mutations in SVIL. Wide neck, anteverted shoulders and prominent trapezius muscles together with variable contractures were characteristic features. All patients showed increased levels of serum creatine kinase but no or minor muscle weakness. Mild cardiac manifestations were observed. Muscle biopsies showed complete loss of large supervillin isoforms in muscle fibres by western blot and immunohistochemical analyses. Light and electron microscopic investigations revealed a structural myopathy with numerous lobulated muscle fibres and considerable myofibrillar alterations with a coarse and irregular intermyofibrillar network. Autophagic vacuoles, as well as frequent and extensive deposits of lipoproteins, including immature lipofuscin, were observed. Several sarcolemma-associated proteins, including dystrophin and sarcoglycans, were partially mis-localized. The results demonstrate the importance of the supervillin (SV2) protein for the structural integrity of muscle fibres in humans and show that recessive loss-of-function mutations in SVIL cause a distinctive and novel myopathy.

Panorama of the distal myopathies

Distal myopathies are genetic primary muscle disorders with a prominent weakness at onset in hands and/or feet. The age of onset (from early childhood to adulthood), the distribution of muscle weakness (upper versus lower limbs) and the histological findings (ranging from nonspecific myopathic changes to myofibrillar disarrays and rimmed vacuoles) are extremely variable. However, despite being characterized by a wide clinical and genetic heterogeneity, the distal myopathies are a category of muscular dystrophies: genetic diseases with progressive loss of muscle fibers. Myopathic congenital arthrogryposis is also a form of distal myopathy usually caused by focal amyoplasia. Massive parallel sequencing has further expanded the long list of genes associated with a distal myopathy, and contributed identifying as distal myopathy-causative rare variants in genes more often related with other skeletal or cardiac muscle diseases. Currently, almost 20 genes (ACTN2, CAV3, CRYAB, DNAJB6, DNM2, FLNC, HNRNPA1, HSPB8, KHLH9, LDB3, MATR3, MB, MYOT, PLIN4, TIA1, VCP, NOTCH2NLC, LRP12, GIPS1) have been associated with an autosomal dominant form of distal myopathy. Pathogenic changes in four genes (ADSSL, ANO5, DYSF, GNE) cause an autosomal recessive form; and disease-causing variants in five genes (DES, MYH7, NEB, RYR1 and TTN) result either in a dominant or in a recessive distal myopathy. Finally, a digenic mechanism, underlying a Welander-like form of distal myopathy, has been recently elucidated. Rare pathogenic mutations in SQSTM1, previously identified with a bone disease (Paget disease), unexpectedly cause a distal myopathy when combined with a common polymorphism in TIA1. The present review aims at describing the genetic basis of distal myopathy and at summarizing the clinical features of the different forms described so far.

The first versatile human iPSC-based model of ectopic virus induction allows new insights in RNA-virus disease

A detailed description of pathophysiological effects that viruses exert on their host is still challenging. For the first time, we report a highly controllable viral expression model based on an iPS-cell line from a healthy human donor. The established viral model system enables a dose-dependent and highly localized RNA-virus expression in a fully controllable environment, giving rise for new applications for the scientific community.

Homozygous expression of the myofibrillar myopathy-associated p.W2710X filamin C variant reveals major pathomechanisms of sarcomeric lesion formation

Filamin C (FLNc) is mainly expressed in striated muscle cells where it localizes to Z-discs, myotendinous junctions and intercalated discs. Recent studies have revealed numerous mutations in the FLNC gene causing familial and sporadic myopathies and cardiomyopathies with marked clinical variability. The most frequent myopathic mutation, p.W2710X, which is associated with myofibrillar myopathy, deletes the carboxy-terminal 16 amino acids from FLNc and abolishes the dimerization property of Ig-like domain 24. We previously characterized "knock-in" mice heterozygous for this mutation (p.W2711X), and have now investigated homozygous mice using protein and mRNA expression analyses, mass spectrometry, and extensive immunolocalization and ultrastructural studies. Although the latter mice display a relatively mild myopathy under normal conditions, our analyses identified major mechanisms causing the pathophysiology of this disease: in comparison to wildtype animals (i) the expression level of FLNc protein is drastically reduced; (ii) mutant FLNc is relocalized from Z-discs to particularly mechanically strained parts of muscle cells, i.e. myotendinous junctions and myofibrillar lesions; (iii) the number of lesions is greatly increased and these lesions lack Bcl2-associated athanogene 3 (BAG3) protein; (iv) the expression of heat shock protein beta-7 (HSPB7) is almost completely abolished. These findings indicate grave disturbances of BAG3-dependent and -independent autophagy pathways that are required for efficient lesion repair. In addition, our studies reveal general mechanisms of lesion formation and demonstrate that defective FLNc dimerization via its carboxy-terminal domain does not disturb assembly and basic function of myofibrils. An alternative, more amino-terminally located dimerization site might compensate for that loss. Since filamins function as stress sensors, our data further substantiate that FLNc is important for mechanosensing in the context of Z-disc stabilization and maintenance.

Advances in the diagnosis of inherited neuromuscular diseases and implications for therapy development

Advances in DNA sequencing technologies have resulted in a near doubling, in under 10 years, of the number of causal genes identified for inherited neuromuscular disorders. However, around half of patients, whether children or adults, do not receive a molecular diagnosis after initial diagnostic workup. Massively parallel technologies targeting RNA, proteins, and metabolites are being increasingly used to diagnose these unsolved cases. The use of these technologies to delineate pathways, biomarkers, and therapeutic targets has led to new approaches entering the drug development pipeline. However, these technologies might give rise to misleading conclusions if used in isolation, and traditional techniques including comprehensive neurological evaluation, histopathology, and biochemistry continue to have a crucial role in diagnostics. For optimal diagnosis, prognosis, and precision medicine, no single ruling technology exists. Instead, an interdisciplinary approach combining novel and traditional neurological techniques with computer-aided analysis and international data sharing is needed to advance the diagnosis and treatment of neuromuscular disorders.

Drosophila NUAK functions with Starvin/BAG3 in autophagic protein turnover

The inability to remove protein aggregates in post-mitotic cells such as muscles or neurons is a cellular hallmark of aging cells and is a key factor in the initiation and progression of protein misfolding diseases. While protein aggregate disorders share common features, the molecular level events that culminate in abnormal protein accumulation cannot be explained by a single mechanism. Here we show that loss of the serine/threonine kinase NUAK causes cellular degeneration resulting from the incomplete clearance of protein aggregates in Drosophila larval muscles. In NUAK mutant muscles, regions that lack the myofibrillar proteins F-actin and Myosin heavy chain (MHC) instead contain damaged organelles and the accumulation of select proteins, including Filamin (Fil) and CryAB. NUAK biochemically and genetically interacts with Drosophila Starvin (Stv), the ortholog of mammalian Bcl-2-associated athanogene 3 (BAG3). Consistent with a known role for the co-chaperone BAG3 and the Heat shock cognate 71 kDa (HSC70)/HSPA8 ATPase in the autophagic clearance of proteins, RNA interference (RNAi) of Drosophila Stv, Hsc70-4, or autophagy-related 8a (Atg8a) all exhibit muscle degeneration and muscle contraction defects that phenocopy NUAK mutants. We further demonstrate that Fil is a target of NUAK kinase activity and abnormally accumulates upon loss of the BAG3-Hsc70-4 complex. In addition, Ubiquitin (Ub), ref(2)p/p62, and Atg8a are increased in regions of protein aggregation, consistent with a block in autophagy upon loss of NUAK. Collectively, our results establish a novel role for NUAK with the Stv-Hsc70-4 complex in the autophagic clearance of proteins that may eventually lead to treatment options for protein aggregate diseases.

A metastable subproteome underlies inclusion formation in muscle proteinopathies

Protein aggregation is a pathological feature of neurodegenerative disorders. We previously demonstrated that protein inclusions in the brain are composed of supersaturated proteins, which are abundant and aggregation-prone, and form a metastable subproteome. It is not yet clear, however, whether this phenomenon is also associated with non-neuronal protein conformational disorders. To respond to this question, we analyzed proteomic datasets from biopsies of patients with genetic and acquired protein aggregate myopathy (PAM) by quantifying the changes in composition, concentration and aggregation propensity of proteins in the fibers containing inclusions and those surrounding them. We found that a metastable subproteome is present in skeletal muscle from healthy patients. The expression of this subproteome escalate as proteomic samples are taken more proximal to the pathologic inclusion, eventually exceeding its solubility limits and aggregating. While most supersaturated proteins decrease or maintain steady abundance across healthy fibers and inclusion-containing fibers, proteins within the metastable subproteome rise in abundance, suggesting that they escape regulation. Taken together, our results show in the context of a human conformational disorder that the supersaturation of a metastable subproteome underlies widespread aggregation and correlates with the histopathological state of the tissue.

Myofibril diameter is set by a finely tuned mechanism of protein oligomerization in Drosophila

Myofibrils are huge cytoskeletal assemblies embedded in the cytosol of muscle cells. They consist of arrays of sarcomeres, the smallest contractile unit of muscles. Within a muscle type, myofibril diameter is highly invariant and contributes to its physiological properties, yet little is known about the underlying mechanisms setting myofibril diameter. Here we show that the PDZ and LIM domain protein Zasp, a structural component of Z-discs, mediates Z-disc and thereby myofibril growth through protein oligomerization. Oligomerization is induced by an interaction of its ZM domain with LIM domains. Oligomerization is terminated upon upregulation of shorter Zasp isoforms which lack LIM domains at later developmental stages. The balance between these two isoforms, which we call growing and blocking isoforms sets the stereotyped diameter of myofibrils. If blocking isoforms dominate, myofibrils become smaller. If growing isoforms dominate, myofibrils and Z-discs enlarge, eventually resulting in large pathological aggregates that disrupt muscle function.

Chaperones in sporadic inclusion body myositis-Validation of proteomic data

INTRODUCTION

Sporadic inclusion body myositis (sIBM) is characterized by myopathological features including rimmed vacuoles (RVs) and proteins associated with protein aggregation, autophagy, and inflammation. Previous proteomic studies of RV areas revealed an overrepresentation of several chaperones and subunits of the T-complex protein 1 (TCP-1), which is involved in prevention of protein aggregation.

METHODS

To validate our proteomic findings, immunofluorescence analyses of selected chaperones and quantitative Western blot analysis of TCP-1 proteins were performed in five sIBM patients and five healthy controls.

RESULTS

Immunofluorescence studies confirmed increased immunoreactivity for VCP, UNC45B, GRP-75, αB-crystallin, LAMP-2, Rab-7a, and TCP-1α and TCP-θ in RVs. Quantitative Western blot analysis revealed a significantly higher level of TCP-1 in sIBM muscle tissue when compared with healthy controls.

DISCUSSION

Our study findings validate new insights in protein quality control and degradation processes that seem to be relevant in sIBM. These data provide an important basis for future functional and therapeutic studies.

Desmin forms toxic, seeding-competent amyloid aggregates that persist in muscle fibers

Protein aggregation and the deposition of amyloid is a common feature in neurodegeneration, but can also be seen in degenerative muscle diseases known as myofibrillar myopathies (MFMs). Hallmark pathology in MFM patient muscle is myofibrillar disarray, aggregation of the muscle-specific intermediate filament, desmin, and amyloid. In some cases, a missense mutation in desmin leads to its destabilization and aggregation. The present study demonstrates that similar to neurodegenerative proteins, desmin can form amyloid and template the amyloidogenic conversion of unaggregated desmin protein. This desmin-derived amyloid is toxic to myocytes and persists when introduced into skeletal muscle, in contrast to unaggregated desmin. These data demonstrate that desmin itself can form amyloid and expand the mechanism of proteinopathies to skeletal muscle.

Desmin-associated myofibrillar myopathy (MFM) has pathologic similarities to neurodegeneration-associated protein aggregate diseases. Desmin is an abundant muscle-specific intermediate filament, and disease mutations lead to its aggregation in cells, animals, and patients. We reasoned that similar to neurodegeneration-associated proteins, desmin itself may form amyloid. Desmin peptides corresponding to putative amyloidogenic regions formed seeding-competent amyloid fibrils. Amyloid formation was increased when disease-associated mutations were made within the peptide, and this conversion was inhibited by the anti-amyloid compound epigallocatechin-gallate. Moreover, a purified desmin fragment (aa 117 to 348) containing both amyloidogenic regions formed amyloid fibrils under physiologic conditions. Desmin fragment-derived amyloid coaggregated with full-length desmin and was able to template its conversion into fibrils in vitro. Desmin amyloids were cytotoxic to myotubes and disrupted their myofibril organization compared with desmin monomer or other nondesmin amyloids. Finally, desmin fragment amyloid persisted when introduced into mouse skeletal muscle. These data suggest that desmin forms seeding-competent amyloid that is toxic to myofibers. Moreover, small molecules known to interfere with amyloid formation and propagation may have therapeutic potential in MFM.

Molecular insights into cardiomyopathies associated with desmin (DES) mutations

Increasing usage of next-generation sequencing techniques pushed during the last decade cardiogenetic diagnostics leading to the identification of a huge number of genetic variants in about 170 genes associated with cardiomyopathies, channelopathies, or syndromes with cardiac involvement. Because of the biochemical and cellular complexity, it is challenging to understand the clinical meaning or even the relevant pathomechanisms of the majority of genetic sequence variants. However, detailed knowledge about the associated molecular pathomechanism is essential for the development of efficient therapeutic strategies in future and genetic counseling. Mutations in DES, encoding the muscle-specific intermediate filament protein desmin, have been identified in different kinds of cardiac and skeletal myopathies. Here, we review the functions of desmin in health and disease with a focus on cardiomyopathies. In addition, we will summarize the genetic and clinical literature about DES mutations and will explain relevant cell and animal models. Moreover, we discuss upcoming perspectives and consequences of novel experimental approaches like genome editing technology, which might open a novel research field contributing to the development of efficient and mutation-specific treatment options.

Autophagy inhibition promotes SNCA/alpha-synuclein release and transfer via extracellular vesicles with a hybrid autophagosome-exosome-like phenotype

The autophagy-lysosome pathway (ALP) regulates intracellular homeostasis of the cytosolic protein SNCA/alpha-synuclein and is impaired in synucleinopathies, including Parkinson disease and dementia with Lewy bodies (DLB). Emerging evidence suggests that ALP influences SNCA release, but the underlying cellular mechanisms are not well understood. Several studies identified SNCA in exosome/extracellular vesicle (EV) fractions. EVs are generated in the multivesicular body compartment and either released upon its fusion with the plasma membrane, or cleared via the ALP. We therefore hypothesized that inhibiting ALP clearance 1) enhances SNCA release via EVs by increasing extracellular shuttling of multivesicular body contents, 2) alters EV biochemical profile, and 3) promotes SNCA cell-to-cell transfer. Indeed, ALP inhibition increased the ratio of extra- to intracellular SNCA and upregulated SNCA association with EVs in neuronal cells. Ultrastructural analysis revealed a widespread, fused multivesicular body-autophagosome compartment. Biochemical characterization revealed the presence of autophagosome-related proteins, such as LC3-II and SQSTM1. This distinct "autophagosome-exosome-like" profile was also identified in human cerebrospinal fluid (CSF) EVs. After a single intracortical injection of SNCA-containing EVs derived from CSF into mice, human SNCA colocalized with endosome and neuronal markers. Prominent SNCA immunoreactivity and a higher number of neuronal SNCA inclusions were observed after DLB patient CSF EV injections. In summary, this study provides compelling evidence that a) ALP inhibition increases SNCA in neuronal EVs, b) distinct ALP components are present in EVs, and c) CSF EVs transfer SNCA from cell to cell in vivo. Thus, macroautophagy/autophagy may regulate EV protein composition and consequently progression in synucleinopathies.

A novel desmin (DES) indel mutation causes severe atypical cardiomyopathy in combination with atrioventricular block and skeletal myopathy

Background

DES mutations cause different cardiac and skeletal myopathies. Most of them are missense mutations.

Methods

Using a next‐generation sequencing cardiac 174 gene panel, we identified a novel heterozygous in‐frame indel mutation (DES‐c.493_520del28insGCGT, p.Q165_A174delinsAS) in a Caucasian patient with cardiomyopathy in combination with atrioventricular block and skeletal myopathy. This indel mutation is located in the coding region of the first exon. Family anamnesis revealed a history of sudden cardiac death. We performed cell transfection experiments and in vitro assembly experiments to prove the pathogenicity of this novel DES indel mutation.

Results

These experiments revealed a severe filament formation defect of mutant desmin supporting the pathogenicity. In addition, we labeled a skeletal muscle biopsy from the mutation carrier revealing cytoplasmic desmin positive protein aggregates. In summary, we identified and functionally characterized a pathogenic DES indel mutation causing cardiac and skeletal myopathy.

Conclusion

Our study has relevance for the clinical and genetic interpretation of further DES indel mutations causing cardiac or skeletal myopathies and might be helpful for risk stratification.

Intersection of Proteomics and Genomics to "Solve the Unsolved" in Rare Disorders such as Neurodegenerative and Neuromuscular Diseases

Despite recent rapid advances in sequencing technologies, a significant proportion of patients with rare genetic disorders do not receive a genetic diagnosis after exhaustive testing, and even fewer have a potential therapeutic target identified. Taking rare neuromuscular and neurodegenerative disorders as a paradigm that can be extended to other rare Mendelian disorders, this viewpoint explores the opportunities that are brought about by the integration of genomics and proteomics, as well as the limitations and remaining challenges of this newly emerging field of proteogenomics. The relevance of combining proteomic findings with genetic results for diagnosis and gene discovery is illustrated, highlighting the insights the combined analysis provides into the underlying biology and aetiology as well as the limitations of the experimental techniques. A final discussion focuses on the importance of mechanisms to enable the sharing, reuse, and analysis of source experimental data and describes some of the international initiatives that are making progress in this area.

Translocation of molecular chaperones to the titin springs is common in skeletal myopathy patients and affects sarcomere function

Myopathies encompass a wide variety of acquired and hereditary disorders. The pathomechanisms include structural and functional changes affecting, e.g., myofiber metabolism and contractile properties. In this study, we observed increased passive tension (PT) of skinned myofibers from patients with myofibrillar myopathy (MFM) caused by FLNC mutations (MFM-filaminopathy) and limb-girdle muscular dystrophy type-2A due to CAPN3 mutations (LGMD2A), compared to healthy control myofibers. Because the giant protein titin determines myofiber PT, we measured its molecular size and the titin-to-myosin ratio, but found no differences between myopathies and controls. All-titin phosphorylation and site-specific phosphorylation in the PEVK region were reduced in myopathy, which would be predicted to lower PT. Electron microscopy revealed extensive ultrastructural changes in myofibers of various hereditary myopathies and also suggested massive binding of proteins to the sarcomeric I-band region, presumably heat shock proteins (HSPs), which can translocate to elastic titin under stress conditions. Correlative immunofluorescence and immunoelectron microscopy showed that two small HSPs (HSP27 and αB-crystallin) and the ATP-dependent chaperone HSP90 translocated to the titin springs in myopathy. The small HSPs, but not HSP90, were upregulated in myopathic versus control muscles. The titin-binding pattern of chaperones was regularly observed in Duchenne muscular dystrophy (DMD), LGMD2A, MFM-filaminopathy, MFM-myotilinopathy, titinopathy, and inclusion body myopathy due to mutations in valosin-containing protein, but not in acquired sporadic inclusion body myositis. The three HSPs also associated with elastic titin in mouse models of DMD and MFM-filaminopathy. Mechanical measurements on skinned human myofibers incubated with exogenous small HSPs suggested that the elevated PT seen in myopathy is caused, in part, by chaperone-binding to the titin springs. Whereas this interaction may be protective in that it prevents sarcomeric protein aggregation, it also has detrimental effects on sarcomere function. Thus, we identified a novel pathological phenomenon common to many hereditary muscle disorders, which involves sarcomeric alterations.

Proteomics of rimmed vacuoles define new risk allele in inclusion body myositis

OBJECTIVE

Sporadic inclusion body myositis (sIBM) pathogenesis is unknown; however, rimmed vacuoles (RVs) are a constant feature. We propose to identify proteins that accumulate within RVs.

METHODS

RVs and intact myofibers were laser microdissected from skeletal muscle of 18 sIBM patients and analyzed by a sensitive mass spectrometry approach using label-free spectral count-based relative protein quantification. Whole exome sequencing was performed on 62 sIBM patients. Immunofluorescence was performed on patient and mouse skeletal muscle.

RESULTS

A total of 213 proteins were enriched by >1.5 -fold in RVs compared to controls and included proteins previously reported to accumulate in sIBM tissue or when mutated cause myopathies with RVs. Proteins associated with protein folding and autophagy were the largest group represented. One autophagic adaptor protein not previously identified in sIBM was FYCO1. Rare missense coding FYCO1 variants were present in 11.3% of sIBM patients compared with 2.6% of controls (p = 0.003). FYCO1 colocalized at RVs with autophagic proteins such as MAP1LC3 and SQSTM1 in sIBM and other RV myopathies. One FYCO1 variant protein had reduced colocalization with MAP1LC3 when expressed in mouse muscle.

INTERPRETATION

This study used an unbiased proteomic approach to identify RV proteins in sIBM that included a novel protein involved in sIBM pathogenesis. FYCO1 accumulates at RVs, and rare missense variants in FYCO1 are overrepresented in sIBM patients. These FYCO1 variants may impair autophagic function, leading to RV formation in sIBM patient muscle. FYCO1 functionally connects autophagic and endocytic pathways, supporting the hypothesis that impaired endolysosomal degradation underlies the pathogenesis of sIBM. Ann Neurol 2017;81:227-239.

Proteins that accumulate with age in human skeletal-muscle aggregates contribute to declines in muscle mass and function in Caenorhabditis elegans

Protein aggregation increases with age in normal tissues, and with pathology and age in Alzheimer's hippocampus and mouse cardiac muscle. We now ask whether human skeletal muscle accumulates aggregates with age. Detergent-insoluble protein aggregates were isolated from vastus lateralis biopsies from 5 young (23–27 years of age) and 5 older (64-80 years) adults. Aggregates, quantified after gel electrophoresis, contain 2.1-fold more protein (P<0.0001) when isolated from older subjects relative to young. Of 515 proteins identified by liquid chromatography coupled to tandem mass spectrometry, 56 (11%) were significantly more abundant in older muscle, while 21 (4%) were depleted with age (each P<0.05). Orthologs to seven of these proteins were then targeted in C. elegans by RNA interference. Six of the seven knockdown treatments decreased protein aggregation (range 6-45%, P<0.01 to <0.0001) and increased muscle mass (range 1.5- to 1.85-fold, P<0.01 to <0.0001) in aged nematodes, and rescued mobility (range 1.4 to 1.65-fold, P≤0.0005 each) in a nematode amyloidopathy model. We conclude that specific aggregate proteins, discovered as differentially abundant in aging human muscle, have orthologs that contribute functionally to aggregation and age-associated muscle loss in nematodes, and thus can be considered potential drug targets for sarcopenia in humans.

Mutant desmin substantially perturbs mitochondrial morphology, function and maintenance in skeletal muscle tissue

Secondary mitochondrial dysfunction is a feature in a wide variety of human protein aggregate diseases caused by mutations in different proteins, both in the central nervous system and in striated muscle. The functional relationship between the expression of a mutated protein and mitochondrial dysfunction is largely unknown. In particular, the mechanism how this dysfunction drives the disease process is still elusive. To address this issue for protein aggregate myopathies, we performed a comprehensive, multi-level analysis of mitochondrial pathology in skeletal muscles of human patients with mutations in the intermediate filament protein desmin and in muscles of hetero- and homozygous knock-in mice carrying the R349P desmin mutation. We demonstrate that the expression of mutant desmin causes disruption of the extrasarcomeric desmin cytoskeleton and extensive mitochondrial abnormalities regarding subcellular distribution, number and shape. At the molecular level, we uncovered changes in the abundancy and assembly of the respiratory chain complexes and supercomplexes. In addition, we revealed a marked reduction of mtDNA- and nuclear DNA-encoded mitochondrial proteins in parallel with large-scale deletions in mtDNA and reduced mtDNA copy numbers. Hence, our data demonstrate that the expression of mutant desmin causes multi-level damage of mitochondria already in early stages of desminopathies.