Protein aggregate myopathies

An Update on Reported Variants in the Skeletal Muscle α-Actin (ACTA1) Gene

The ACTA1 gene encodes skeletal muscle alpha-actin, which forms the core of the sarcomeric thin filament in adult skeletal muscle. ACTA1 represents one of six highly conserved actin proteins that have all been associated with human disease. The first 15 pathogenic variants in ACTA1 were reported in 1999, which expanded to 177 in 2009. Here, we update on the now 607 total variants reported in LOVD, HGMD, and ClinVar, which includes 343 reported pathogenic/likely pathogenic (P/LP) variants. We also provide suggested ACTA1-specific modifications to ACMG variant interpretation guidelines based on our analysis of known variants, gnomAD reports, and pathogenicity in other actin isoforms. Using these criteria, we report a total of 447 P/LP ACTA1 variants. From a clinical perspective, the number of reported ACTA1 disease phenotypes has grown from five to 20, albeit with some overlap. The vast majority (74%) of ACTA1 variants cause nemaline myopathy (NEM), but there are increasing numbers that cause cardiomyopathy and novel phenotypes such as distal myopathy. We highlight challenges associated with identifying genotype-phenotype correlations for ACTA1. Finally, we summarize key animal models and review the current state of preclinical treatments for ACTA1 disease. This update provides important resources and recommendations for the study and interpretation of ACTA1 variants.

Protein Aggregates and Aggrephagy in Myopathies

A number of muscular disorders are hallmarked by the aggregation of misfolded proteins within muscle fibers. A specialized form of macroautophagy, termed aggrephagy, is designated to remove and degrade protein aggregates. This review aims to summarize what has been studied so far about the direct involvement of aggrephagy and the activation of the key players, among others, p62, NBR1, Alfy, Tollip, Optineurin, TAX1BP1 and CCT2 in muscular diseases. In the first part of the review, we describe the aggrephagy pathway with the involved proteins; then, we illustrate the muscular disorder histologically characterized by protein aggregates, highlighting the role of aggrephagy pathway abnormalities in these muscular disorders.

Myocardial protein aggregates in pet guinea pigs

A retrospective study of guinea pigs submitted for necropsy revealed intracytoplasmic inclusions in the cardiomyocytes of 26 of 30 animals. The inclusions were found with approximately the same frequency in male and female guinea pigs and were slightly more common in older animals. In most cases, the animals did not have clinical signs or necropsy findings suggestive of heart failure, and the cause of death or reason for euthanasia was attributed to concurrent disease processes. However, the 4 guinea pigs with the highest inclusion body burden all had pulmonary edema, sometimes with intra-alveolar hemosiderin-laden macrophages, suggestive of heart failure. The inclusions were found in both the left and right ventricular myocardium, mainly in the papillary muscles, but were most common in the right ventricular free wall. No inclusions were detected in the atrial myocardium or in skeletal muscle. The inclusions did not stain with Congo red or periodic acid-Schiff. Electron microscopy revealed dense aggregates of disorganized myofilaments and microtubules that displaced and compressed the adjacent organelles. By immunohistochemistry, there was some scattered immunoreactivity for desmin and actin at the periphery of the inclusions and punctate actin reactivity within the aggregates. The inclusions did not react with antibodies to ubiquitin or cardiac myosin, but were variably reactive for alpha B crystallin, a small heat shock chaperone protein. The inclusions were interpreted as evidence of impaired proteostasis.

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.

Human eukaryotic elongation factor 1A forms oligomers through specific cysteine residues

Eukaryotic elongation factor 1A (eEF1A) is a multifunctional protein involved in bundling actin, severing microtubule, activating the phosphoinositol-4 kinase, and recruiting aminoacyl-tRNAs to ribosomes during protein biosynthesis. Although evidence has shown the presence of the isoform eEF1A1 oligomers, the substantial mechanism of the self-association remains unclear. Herein, we found that human eEF1A1 could spontaneously form oligomers. Specifically, mutagenesis screen on cysteine residues demonstrated that Cys(234) was essential for eEF1A1 oligomerization. In addition, we also found that hydrogen peroxide treatment could induce the formation of eEF1A oligomers in cells. By cysteine replacement, eEF1A2 isoform displayed the ability to oligomerize in cells under the oxidative environment. In summary, in this study we characterized eEF1A1 oligomerization and demonstrated that specific cysteine residues are required for this oligomerization activity.

A novel intronic single nucleotide polymorphism in the myosin heavy polypeptide 4 gene is responsible for the mini-muscle phenotype characterized by major reduction in hind-limb muscle mass in mice

Replicated artificial selection for high levels of voluntary wheel running in an outbred strain of mice favored an autosomal recessive allele whose primary phenotypic effect is a 50% reduction in hind-limb muscle mass. Within the High Runner (HR) lines of mice, the numerous pleiotropic effects (e.g., larger hearts, reduced total body mass and fat mass, longer hind-limb bones) of this hypothesized adaptive allele include functional characteristics that facilitate high levels of voluntary wheel running (e.g., doubling of mass-specific muscle aerobic capacity, increased fatigue resistance of isolated muscles, longer hind-limb bones). Previously, we created a backcross population suitable for mapping the responsible locus. We phenotypically characterized the population and mapped the Minimsc locus to a 2.6-Mb interval on MMU11, a region containing ∼100 known or predicted genes. Here, we present a novel strategy to identify the genetic variant causing the mini-muscle phenotype. Using high-density genotyping and whole-genome sequencing of key backcross individuals and HR mice with and without the mini-muscle mutation, from both recent and historical generations of the HR lines, we show that a SNP representing a C-to-T transition located in a 709-bp intron between exons 11 and 12 of the Myosin heavy polypeptide 4 (Myh4) skeletal muscle gene (position 67,244,850 on MMU11; assembly, December 2011, GRCm38/mm10; ENSMUSG00000057003) is responsible for the mini-muscle phenotype, Myh4(Minimsc). Using next-generation sequencing, our approach can be extended to identify causative mutations arising in mouse inbred lines and thus offers a great avenue to overcome one of the most challenging steps in quantitative genetics.

Myofibrillar myopathy caused by a mutation in the motor domain of mouse MyHC IIb

Ariel is a mouse mutant that suffers from skeletal muscle myofibrillar degeneration due to the rapid accumulation of large intracellular protein aggregates. This fulminant disease is caused by an ENU-induced recessive mutation resulting in an L342Q change within the motor domain of the skeletal muscle myosin protein MYH4 (MyHC IIb). Although normal at birth, homozygous mice develop hindlimb paralysis from Day 13, consistent with the timing of the switch from developmental to adult myosin isoforms in mice. The mutated myosin (MYH4(L342Q)) is an aggregate-prone protein. Notwithstanding the speed of the process, biochemical analysis of purified aggregates showed the presence of proteins typically found in human myofibrillar myopathies, suggesting that the genesis of ariel aggregates follows a pathogenic pathway shared with other conformational protein diseases of skeletal muscle. In contrast, heterozygous mice are overtly and histologically indistinguishable from control mice. MYH4(L342Q) is present in muscles from heterozygous mice at only 7% of the levels of the wild-type protein, resulting in a small but significant increase in force production in isolated single fibres and indicating that elimination of the mutant protein in heterozygotes prevents the pathological changes observed in homozygotes. Recapitulation of the L342Q change in the functional equivalent of mouse MYH4 in human muscles, MYH1, results in a more aggregate-prone protein.

Protein aggregation in congenital myopathies

Protein aggregation in congenital myopathies may be encountered among different myofibrillar myopathies such as granulofilamentous myopathy, cytoplasmic body myopathy, or spheroid body myopathy, which are designated as αB crystallinopathy, desminopathy, and myotilinopathy, respectively, according to the respective mutant proteins. Caps in cap disease and reducing bodies in reducing body myopathy were disclosed to contain numerous proteins. The multitude of diverse proteins aggregating within muscle fibers suggests impaired extralysosomal degradation of proteins, a disturbance of catabolism. The lack of different proteins accruing, but the mutant ones at an early age of affected patients in actin filament aggregating myopathy (AFAM) and hyaline body myopathy (HBM), suggests defects in maturation of sarcomeres and failure to integrate the possible mutant proteins, sarcomeric actin and heavy chain myosin in AFAM and HBM, a disturbance of anabolic metabolism.

Myopathy causing camptocormia in idiopathic Parkinson's disease: a multidisciplinary approach

Extreme forward flexion of the spine, named camptocormia (CC), and head drop syndrome (HD) may be among the most disabling symptoms in Parkinson's disease (PD). This study aims to eludicate the etiology of PD-associated CC and HD via a multidisciplinary approach (clinical examination, electromyography, MRI, genetic analysis, muscle morphology) centering on the histology of the paraspinal muscles. We studied 17 patients with the clinical diagnosis of PD and CC or head drop syndrome and six controls. We performed muscle biopsies of paraspinal muscles and deep neck extensor muscles. Mean age at onset of postural abnormality was 66 years and mean latency between onset of parkinsonian symptoms to first signs of CC or head drop was 7 years. The electromyogram of paraspinal muscles was abnormal in 13-14 patients. Histopathology revealed chronic myopathic changes in 14 of 17 biopsies, consisting of abnormal variation in fiber size, increase in internal nuclei, and increase in connective tissue, myofibrillar disarray and similarities to protein surplus myopathies. Interestingly, heterozygous variants in the Parkin gene were found in 2 of 9 investigated patients. We conclude that CC and HD in PD are predominantly myopathic. Aberrant protein aggregation may link PD and CC.

In vitro analysis of rod composition and actin dynamics in inherited myopathies

Rods are the pathological hallmark of nemaline myopathy, but they can also occur as a secondary phenomenon in other disorders, including mitochondrial myopathies such as complex I deficiency. The mechanisms of rod formation are not well understood, particularly when rods occur in diverse disorders with very different structural and metabolic defects. We compared the characteristics of rods associated with abnormalities in structural components of skeletal muscle thin filament (3 mutations in the skeletal actin gene ACTA1) with those of rods induced by the metabolic cell stress of adenosine triphosphate depletion. C2C12 and NIH/3T3 cell culture models and immunocytochemistry were used to study rod composition and conformation. Fluorescent recovery after photobleaching was used to measure actin dynamics inside the rods. We demonstrate that not all rods are the same. Rods formed under different conditions contain a unique fingerprint of actin-binding proteins (cofilin and alpha-actinin) and display differences in actin dynamics that are specific to the mutation, to the cellular location of the rods (intranuclear vs cytoplasmic), and/or to the underlying pathological process (i.e. mutant actin or adenosine triphosphate depletion). Thus, rods likely represent a common morphological end point of a variety of different pathological processes, either structural or metabolic.

Extralysosomal protein degradation in myofibrillar myopathies

Myofibrillar myopathies (MFMs) are a group of heterogeneous muscle disorders morphologically defined by the presence of foci of dissolution of the myofibrils, accumulation of the products of myofibrillar degradation and ectopic expression of multiple proteins. MFMs represent the paradigm of conformational protein diseases of skeletal and cardiac muscles. Protein aggregation in MFMs is now considered to be the result of a failure of the extralysosomal proteolytic degradation system. Several factors including mutant proteins, aggresome formation and oxidative stress may compromise the ubiquitin-proteasome system, promoting the accumulation of potentially toxic protein aggregates within muscle cells.

Hyperphosphorylation and aggregation of Tau in laforin-deficient mice, an animal model for Lafora disease

Lafora progressive myoclonous epilepsy (Lafora disease; LD) is caused by mutations in the EPM2A gene encoding a dual specificity protein phosphatase named laforin. Our analyses on the Epm2a gene knock-out mice, which developed most of the symptoms of LD, reveal the presence of hyperphosphorylated Tau protein (Ser(396) and Ser(202)) as neurofibrillary tangles (NFTs) in the brain. Intriguingly, NFTs were also observed in the skeletal muscle tissues of the knock-out mice. The hyperphosphorylation of Tau was associated with increased levels of the active form of GSK3 beta. The observations on Tau protein were replicated in cell lines using laforin overexpression and knockdown approaches. We also show here that laforin and Tau proteins physically interact and that the interaction was limited to the phosphatase domain of laforin. Finally, our in vitro and in vivo assays demonstrate that laforin dephosphorylates Tau, and therefore laforin is a novel Tau phosphatase. Taken together, our study suggests that laforin is one of the critical regulators of Tau protein, that the NFTs could underlie some of the symptoms seen in LD, and that laforin can contribute to the NFT formation in Alzheimer disease and other tauopathies.

Molecular pathology of myofibrillar myopathies

Myofibrillar myopathies (MFMs) are clinically and genetically heterogeneous muscle disorders that are defined morphologically by the presence of foci of myofibril dissolution, accumulation of myofibrillar degradation products, and ectopic expression of multiple proteins. MFMs are the paradigm of conformational protein diseases of the skeletal (and cardiac) muscles characterised by intracellular protein accumulation in muscle cells. Understanding of this group of disorders has advanced in recent years through the identification of causative mutations in various genes, most of which encode proteins of the sarcomeric Z-disc, including desmin, alphaB-crystallin, myotilin, ZASP and filamin C. This review focuses on the MFMs arising from defects in these proteins, summarising genetic and clinical features of the disorders and then discussing emerging understanding of the molecular pathogenic mechanisms leading to muscle fibre degeneration. Defective extralysosomal degradation of proteins is now recognised as an important element in this process. Several factors--including mutant proteins, a defective ubiquitin-proteasome system, aggresome formation, mutant ubiquitin, p62, oxidative stress and abnormal regulation of some transcription factors--are thought to participate in the cascade of events occurring in muscle fibres in MFMs.

Autophagy activation by rapamycin eliminates mouse Mallory-Denk bodies and blocks their proteasome inhibitor-mediated formation

The proteasomal and lysosomal/autophagy pathways in the liver and other tissues are involved in several biological processes including the degradation of misfolded proteins. Exposure of hepatocyte cell lines to proteasome inhibitors (PIs) results in the formation of inclusions that resemble Mallory-Denk bodies (MDBs). Keratins are essential for MDB formation and keratin 8 (K8)-overexpressing transgenic mice are predisposed to MDB formation. We tested the hypothesis that PIs induce MDBs in vivo and that autophagy participates in MDB turnover. The effect of the PI bortezomib (which is used to treat some malignancies) on MDB formation was tested in K8-overexpressing mice and in cultured cells. Inclusion formation was examined using immune and conventional electron microscopy (EM). Bortezomib induced MDB-like inclusions composed of keratins, ubiquitin, and p62 in cultured cells. Short-term exposure to bortezomib induced similar inclusions in K8-overexpressing but not in nontransgenic mice, without causing liver injury. In bortezomib-treated mice, autophagy was activated in hepatocytes as determined by EM and biochemical analysis. Further activation of autophagy by rapamycin (Rap) decreased the number of inclusions in bortezomib-treated K8 transgenic mice significantly. Rap also led to resorption of spontaneously formed MDBs in aging K8-overexpressing mice. Immune EM demonstrated K8-positive and ubiquitin-positive structures in autophagic vacuoles in the mouse liver.

CONCLUSION

PIs alone are sufficient to induce MDBs in susceptible animals, while Rap-mediated activation of autophagy prevents MDB formation and causes MDB resorption. These findings suggest that some patients treated with PIs may become predisposed to MDB formation. Autophagy provides a potential cellular mechanism for the resorption of cytoplasmic inclusions.

Autophagy modulates keratin-containing inclusion formation and apoptosis in cell culture in a context-dependent fashion

The major pathways for protein degradation are the proteasomal and lysosomal systems. Derangement of protein degradation causes the formation of intracellular inclusions, and apoptosis and is associated with several diseases. We utilized hepatocyte-derived cell lines to examine the consequences of the cytoplasmic hepatocyte Mallory-Denk body-like inclusions on organelle organization, autophagy and apoptosis, and tested the hypothesis that autophagy affects inclusion turnover. Proteasome inhibitors (PIs) generate keratin-containing Mallory-Denk body-like inclusions in cultured cells and cause reorganization of mitochondria and other organelles, autophagy and apoptosis. In cultured hepatoma cells, caspase inhibition blocks PI-induced apoptosis but not inclusion formation or autophagy activation. Autophagy induction by rapamycin decreases the extent of PI-induced inclusions and apoptosis in Huh7 and OUMS29 cells. Surprisingly, blocking of autophagy sequestration by 3 methyl adenine or beclin 1 siRNA, but not bafilomycin A1 inhibition of autophagic degradation, also inhibits inclusion formation in the tested cells. Therefore, autophagy can be upstream of apoptosis and may promote or alleviate inclusion formation in cell culture in a context-dependent manner via putative autophagy-associated molecular triggers. Manipulation of autophagy may offer a strategy to address the importance of inclusion formation and its significance in inclusion-associated diseases.

Non-pathogenic protein aggregates in skeletal muscle in MLF1 transgenic mice

Protein aggregate formation in muscle is thought to be pathogenic and associated with clinical weakness. Over-expression of either wild type or a mutant form of myeloid leukemia factor 1 (MLF1) in transgenic mouse skeletal muscle and in cultured cells resulted in aggregate formation. Aggregates were detected in MLF1 transgenic mice at 6 weeks of age, and increased in size with age. However, histological examination of skeletal muscles of MLF1 transgenic mice revealed no pathological changes other than the aggregates, and RotaRod testing did not detect functional deficits. MLF1 has recently been identified as a protein that could neutralize the toxicity of intracellular protein aggregates in a Drosophila model of Huntington's disease (HD). We also demonstrate that MLF1 interacts with MRJ, a heat shock protein, which can independently neutralize the toxicity of intracellular protein aggregates in the Drosophila HD model. Our data suggest that over-expression of MLF1 has no significant impact on skeletal muscle function in mice; that progressive formation of protein aggregates in muscle are not necessarily pathogenic; and that MLF1 and MRJ may function together to ameliorate the toxic effects of polyglutamine or mutant proteins in myodegenerative diseases such as inclusion body myositis and oculopharyngeal muscular dystrophy, as well as neurodegenerative disease.

Desmin is oxidized and nitrated in affected muscles in myotilinopathies and desminopathies

Degenerative diseases with abnormal protein aggregates are characterized by the accumulation of proteins with variable posttranslational modifications including phosphorylation, glycoxidation, oxidation, and nitration. Myofibrillar myopathies, including myotilinopathies and desminopathies, are characterized by the intracytoplasmic focal accumulation of proteins in insoluble aggregates in muscle cells. By using single immunohistochemistry, monodimensional gel electrophoresis and Western blotting, and bidimensional gel electrophoresis, in-gel digestion, and mass spectometry, desmin was demonstrated to be a major target of oxidation and nitration in both desminopathies and myotilinopathies. Because oxidized and nitrated proteins may have toxic effects and may impair ubiquitin-proteasomal function, modified desmin can be considered to be an additional element in the pathogenesis of myofibrillar myopathies. In addition to desmin, pyruvate kinase muscle splice form M1 is oxidized, thus supporting complemental mitochondrial damage, at least in some cases of myotilinopathy.

Expression of mutant ubiquitin (UBB+1) and p62 in myotilinopathies and desminopathies

Protein aggregates in muscle cells are the morphological hallmark of myofibrillar myopathies, including myotilinopathies and desminopathies. The aim of the present study is to analyse the expression of mutant ubiquitin (UBB+1), an aberrant form of ubiquitin which accumulates in certain disorders characterized by intracellular aggregates of proteins, and p62, a multimeric signal protein which plays an active role in aggregate formation, in muscle biopsies from patients suffering from myotilinopathy and desminopathy in order to gain understanding of the mechanisms leading to protein aggregation in these disorders. Single immunohistochemistry, and single- and double-labelling immunofluorescence and confocal microscopy for UBB+1 and p62, has been performed in muscle biopsies from patients suffering from myotilinopathy and desminopathy. Strong UBB+1 immunoreactivity, colocalizing with myotilin aggregates, was found in muscle fibres in myotilinopathies. UBB+1 accumulation, colocalizing with desmin aggregates, also occurs in desminopathies. In addition, strong p62 immunoreactivity colocalizing with myotilin aggregates was observed in myotilinopathies. Similarly, p62 immunoreactivity colocalizing with desmin aggregates was found in desminopathies. The present findings suggest that accumulation of protein aggregates in myotilinopathies and in desminopathies may be related with UBB+1/abnormal protein complexes which are resistant to proteasome degradation. Furthermore, these observations suggest a relationship between the presence of p62 and the formation of inclusions in different subtypes of myofibrillar myopathies.

Oxidative stress in desminopathies and myotilinopathies: a link between oxidative damage and abnormal protein aggregation

Myotilinopathies and desminopathies are subgroups of myofibrillar myopathies (MFM) caused by mutations in myotilin and desmin genes, respectively. They are characterized by the presence of protein aggregates in muscle cells. As oxidation of proteins facilitates their aggregation and makes them more resistant to proteolysis, the present study was geared to analyze oxidative stress in MFM. For this purpose, markers of glycoxidation, lipoxidation and nitration were examined with gel electrophoresis and Western blotting, single immunohistochemistry, and double- and triple-labeling immunofluorescence and confocal microscopy in muscle biopsies from patients suffering from myotilinopathy and desminopathy. Increased levels of glycation-end products (AGEs), N-carboxymethyl-lysine (CML) and N-carboxyethyl-lysine (CEL), malondialdehyde-lysine (MDAL), 4-hydroxynonenal (HNE) and nitrotyrosine (N-tyr) were found in MFM. Furthermore, aberrant expression of AGE, CML, CEL, MDAL and HNE, as well as of neuronal, inducible and endothelial nitric oxide synthases (nNOS, iNOS, eNOS), and superoxide dismutase 2 (SOD2), was found in muscle fibers containing protein aggregates in myotilinopathies and desminopathies. AGE, ubiquitin and p62 co-localized in several muscle fibers in MFM. As oxidized proteins are vulnerable to misfolding and are resistant to degradation by the UPS, the present observations support a link between oxidative stress, protein aggregation and abnormal protein deposition in MFMs.