Differential involvement of sarcomeric proteins in myofibrillar myopathies: a morphological and immunohistochemical study

Desmin and its molecular chaperone, the αB-crystallin: How post-translational modifications modulate their functions in heart and skeletal muscles?

Maintenance of the highly organized striated muscle tissue requires a cell-wide dynamic network through protein-protein interactions providing an effective mechanochemical integrator of morphology and function. Through a continuous and complex trans-cytoplasmic network, desmin intermediate filaments ensure this essential role in heart and in skeletal muscle. Besides their role in the maintenance of cell shape and architecture (permitting contractile activity efficiency and conferring resistance towards mechanical stress), desmin intermediate filaments are also key actors of cell and tissue homeostasis. Desmin participates to several cellular processes such as differentiation, apoptosis, intracellular signalisation, mechanotransduction, vesicle trafficking, organelle biogenesis and/or positioning, calcium homeostasis, protein homeostasis, cell adhesion, metabolism and gene expression. Desmin intermediate filaments assembly requires αB-crystallin, a small heat shock protein. Over its chaperone activity, αB-crystallin is involved in several cellular functions such as cell integrity, cytoskeleton stabilization, apoptosis, autophagy, differentiation, mitochondria function or aggresome formation. Importantly, both proteins are known to be strongly associated to the aetiology of several cardiac and skeletal muscles pathologies related to desmin filaments disorganization and a strong disturbance of desmin interactome. Note that these key proteins of cytoskeleton architecture are extensively modified by post-translational modifications that could affect their functional properties. Therefore, we reviewed in the herein paper the impact of post-translational modifications on the modulation of cellular functions of desmin and its molecular chaperone, the αB-crystallin.

Synaptopodin-2 Isoforms Have Specific Binding Partners and Display Distinct, Muscle Cell Type-Specific Expression Patterns

Synaptopodin-2 (SYNPO2) is a protein associated with the Z-disc in striated muscle cells. It interacts with α-actinin and filamin C, playing a role in Z-disc maintenance under stress by chaperone-assisted selective autophagy (CASA). In smooth muscle cells, SYNPO2 is a component of dense bodies. Furthermore, it has been proposed to play a role in tumor cell proliferation and metastasis in many different kinds of cancers. Alternative transcription start sites and alternative splicing predict the expression of six putative SYNPO2 isoforms differing by extended amino- and/or carboxy-termini. Our analyses at mRNA and protein levels revealed differential expression of SYNPO2 isoforms in cardiac, skeletal and smooth muscle cells. We identified synemin, an intermediate filament protein, as a novel binding partner of the PDZ-domain in the amino-terminal extension of the isoforms mainly expressed in cardiac and smooth muscle cells, and demonstrated colocalization of SYNPO2 and synemin in both cell types. A carboxy-terminal extension, mainly expressed in smooth muscle cells, is sufficient for association with dense bodies and interacts with α-actinin. SYNPO2 therefore represents an additional and novel link between intermediate filaments and the Z-discs in cardiomyocytes and dense bodies in smooth muscle cells, respectively. In pathological skeletal muscle samples, we identified SYNPO2 in the central and intermediate zones of target fibers of patients with neurogenic muscular atrophy, and in nemaline bodies. Our findings help to understand distinct functions of individual SYNPO2 isoforms in different muscle tissues, but also in tumor pathology.

Deep Characterization of a Greek Patient with Desmin-Related Myofibrillar Myopathy and Cardiomyopathy

Desmin is a class III intermediate filament protein highly expressed in cardiac, smooth and striated muscle. Autosomal dominant or recessive mutations in the desmin gene (DES) result in a variety of diseases, including cardiomyopathies and myofibrillar myopathy, collectively called desminopathies. Here we describe the clinical, histological and radiological features of a Greek patient with a myofibrillar myopathy and cardiomyopathy linked to the c.734A>G,p.(Glu245Gly) heterozygous variant in the DES gene. Moreover, through ribonucleic acid sequencing analysis in skeletal muscle we show that this variant provokes a defect in exon 3 splicing and thus should be considered clearly pathogenic.

Cardiac Overexpression of XIN Prevents Dilated Cardiomyopathy Caused by TNNT2 ΔK210 Mutation

TNNT2 mutation is associated with a range of cardiac diseases, including dilated cardiomyopathy (DCM). However, the mechanisms underlying the development of DCM and heart failure remain incompletely understood. In the present study, we found the expression of cardiac XIN protein was reduced in TNNT2-ΔK210 hESCs-derived cardiomyocytes and mouse heart tissues. We further investigated whether XIN protects against TNNT2 mutation-induced DCM. Overexpression of the repeat-containing isoform XINB decreased the percentage of myofilaments disorganization and increased cell contractility of TNNT2-ΔK210 cardiomyocytes. Moreover, overexpression of XINB by heart-specific delivery via AAV9 ameliorates DCM remodeling caused by TNNT2-ΔK210 mutation in mice, revealed by partially reversed cardiac dilation, systolic dysfunction and heart fibrosis. These results suggest that deficiency of XIN may play a critical role in the development of DCM. Consequently, our findings may provide a new mechanistic insight and represent a therapeutic target for the treatment of idiopathic DCM.

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.

Skeletal and Cardiac Muscle Disorders Caused by Mutations in Genes Encoding Intermediate Filament Proteins

Intermediate filaments are major components of the cytoskeleton. Desmin and synemin, cytoplasmic intermediate filament proteins and A-type lamins, nuclear intermediate filament proteins, play key roles in skeletal and cardiac muscle. Desmin, encoded by the DES gene (OMIM *125660) and A-type lamins by the LMNA gene (OMIM *150330), have been involved in striated muscle disorders. Diseases include desmin-related myopathy and cardiomyopathy (desminopathy), which can be manifested with dilated, restrictive, hypertrophic, arrhythmogenic, or even left ventricular non-compaction cardiomyopathy, Emery–Dreifuss Muscular Dystrophy (EDMD2 and EDMD3, due to LMNA mutations), LMNA-related congenital Muscular Dystrophy (L-CMD) and LMNA-linked dilated cardiomyopathy with conduction system defects (CMD1A). Recently, mutations in synemin (SYNM gene, OMIM *606087) have been linked to cardiomyopathy. This review will summarize clinical and molecular aspects of desmin-, lamin- and synemin-related striated muscle disorders with focus on LMNA and DES-associated clinical entities and will suggest pathogenetic hypotheses based on the interplay of desmin and lamin A/C. In healthy muscle, such interplay is responsible for the involvement of this network in mechanosignaling, nuclear positioning and mitochondrial homeostasis, while in disease it is disturbed, leading to myocyte death and activation of inflammation and the associated secretome alterations.

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.

The p.Ala2430Val mutation in filamin C causes a "hypertrophic myofibrillar cardiomyopathy"

Hypertrophic cardiomyopathy (HCM) often leads to heart failure. Mutations in sarcomeric proteins are most frequently the cause of HCM but in many patients the gene defect is not known. Here we report on a young man who was diagnosed with HCM shortly after birth. Whole exome sequencing revealed a mutation in the FLNC gene (c.7289C > T; p.Ala2430Val) that was previously shown to cause aggregation of the mutant protein in transfected cells. Myocardial tissue from patients with this mutation has not been analyzed before and thus, the underlying etiology is not well understood. Myocardial tissue of our patient obtained during myectomy at the age of 23 years was analyzed in detail by histochemistry, immunofluorescence staining, electron microscopy and western blot analysis. Cardiac histology showed a pathology typical for myofibrillar myopathy with myofibril disarray and abnormal protein aggregates containing BAG3, desmin, HSPB5 and filamin C. Analysis of sarcomeric and intercalated disc proteins showed focally reduced expression of the gap junction protein connexin43 and Xin-positive sarcomeric lesions in the cardiomyocytes of our patient. In addition, autophagy pathways were altered with upregulation of LC3-II, WIPI1 and HSPB5, 6, 7 and 8. We conclude that the p.Ala2430Val mutation in FLNC most probably is associated with HCM characterized by abnormal intercalated discs, disarray of myofibrils and aggregates containing Z-disc proteins similar to myofibrillar myopathy, which supports the pathological effect of the mutation.

Protein signature of human skin fibroblasts allows the study of the molecular etiology of rare neurological diseases

BACKGROUND

The elucidation of pathomechanisms leading to the manifestation of rare (genetically caused) neurological diseases including neuromuscular diseases (NMD) represents an important step toward the understanding of the genesis of the respective disease and might help to define starting points for (new) therapeutic intervention concepts. However, these "discovery studies" are often limited by the availability of human biomaterial. Moreover, given that results of next-generation-sequencing approaches frequently result in the identification of ambiguous variants, testing of their pathogenicity is crucial but also depending on patient-derived material.

METHODS

Human skin fibroblasts were used to generate a spectral library using pH8-fractionation of followed by nano LC-MS/MS. Afterwards, Allgrove-patient derived fibroblasts were subjected to a data independent acquisition approach. In addition, proteomic signature of an enriched nuclear protein fraction was studied. Proteomic findings were confirmed by immunofluorescence in a muscle biopsy derived from the same patient and cellular lipid homeostasis in the cause of Allgrove syndrome was analysed by fluorescence (BODIPY-staining) and coherent anti-Stokes Raman scattering (CARS) microscopy.

RESULTS

To systematically address the question if human skin fibroblasts might serve as valuable biomaterial for (molecular) studies of NMD, we generated a protein library cataloguing 8280 proteins including a variety of such linked to genetic forms of motoneuron diseases, congenital myasthenic syndromes, neuropathies and muscle disorders. In silico-based pathway analyses revealed expression of a diversity of proteins involved in muscle contraction and such decisive for neuronal function and maintenance suggesting the suitability of human skin fibroblasts to study the etiology of NMD. Based on these findings, next we aimed to further demonstrate the suitability of this in vitro model to study NMD by a use case: the proteomic signature of fibroblasts derived from an Allgrove-patient was studied. Dysregulation of paradigmatic proteins could be confirmed in muscle biopsy of the patient and protein-functions could be linked to neurological symptoms known for this disease. Moreover, proteomic investigation of nuclear protein composition allowed the identification of protein-dysregulations according with structural perturbations observed in the muscle biopsy. BODIPY-staining on fibroblasts and CARS microscopy on muscle biopsy suggest altered lipid storage as part of the underlying disease etiology.

CONCLUSIONS

Our combined data reveal that human fibroblasts may serve as an in vitro system to study the molecular etiology of rare neurological diseases exemplified on Allgrove syndrome in an unbiased fashion.

Commercial genetic testing for type 2 polysaccharide storage myopathy and myofibrillar myopathy does not correspond to a histopathological diagnosis

Background

Commercial genetic tests for type 2 polysaccharide storage myopathy (PSSM2) and myofibrillar myopathy (MFM) have not been validated by peer‐review, and formal regulation of veterinary genetic testing is lacking.

Objectives

To compare genotype and allele frequencies of commercial test variants (P variants) in MYOT (P2; rs1138656462), FLNC (P3a; rs1139799323), FLNC (P3b; rs1142918816) and MYOZ3 (P4; rs1142544043) between Warmblood (WB) and Arabian (AR) horses diagnosed with PSSM2/MFM by muscle histopathology, and phenotyped breed‐matched controls. To quantify variant frequency in public repositories of ancient and modern horse breeds.

Study design

Cross sectional using archived clinical material and publicly available data.

Methods

We studied 54 control‐WB, 68 PSSM2/MFM‐WB, 30 control‐AR, 30 PSSM2/MFM‐AR and 205 public genotypes. Variants were genotyped by pyrosequencing archived DNA. Genotype and allele frequency, and number of variant alleles or loci were compared within breed between controls, PSSM2/MFM combined and MFM or PSSM2 horses considered separately using additive/genotypic and dominant models (Fisher's exact tests). Variant frequencies in modern, early domestic and Przewalski horses were determined from a public data repository.

Results

There was no significant association between any P locus and a histopathological diagnosis of PSSM2/MFM, and no difference between control and myopathic horses in total loci with alternative alleles, or total alternate alleles when PSSM2/MFM was considered combined or separately as PSSM2 or MFM. For all tests, sensitivity was <0.33. Allele frequencies in WB (controls/cases) were: 8%/15% (P2), 5%/6% (P3a/b) and 9%/13% (P4); in AR, frequencies were: 12%/17% (P2), 2%/2% (P3a/b) and 7%/12% (P4). All P variants were present in early domestic (400‐ to 5500‐year‐old) horses and P2 present in the Przewalski.

Conclusions

Because of the lack of significant association between a histopathological diagnosis of PSSM2 or MFM and the commercial genetic test variants P2, P3 and P4 in WB and AR, we cannot recommend the use of these variant genotypes for selection and breeding, prepurchase examination or diagnosis of a myopathy.

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.

First clinical and myopathological description of a myofibrillar myopathy with congenital onset and homozygous mutation in FLNC

Filamin C (encoded by the FLNC gene) is a large actin-cross-linking protein involved in shaping the actin cytoskeleton in response to signaling events both at the sarcolemma and at myofibrillar Z-discs of cross-striated muscle cells. Multiple mutations in FLNC are associated with myofibrillar myopathies of autosomal-dominant inheritance. Here, we describe for the first time a boy with congenital onset of generalized muscular hypotonia and muscular weakness, delayed motor development but no cardiac involvement associated with a homozygous FLNC mutation c.1325C>G (p.Pro442Arg). We performed ultramorphological, proteomic, and functional investigations as well as immunological studies of known marker proteins for dominant filaminopathies. We show that the mutant protein is expressed in similar quantities as the wild-type variant in control skeletal muscle fibers. The proteomic signature of quadriceps muscle is altered and ultrastructural perturbations are evident. Moreover, filaminopathy marker proteins are comparable both in our homozygous and a dominant control case (c.5161delG). Biochemical investigations demonstrate that the recombinant mutant protein is less stable and more prone to degradation by proteolytic enzymes than the wild-type variant. The unusual congenital presentation of the disease clearly demonstrates that homozygosity for mutations in FLNC severely aggravates the phenotype.

Phosphoproteomics identifies dual-site phosphorylation in an extended basophilic motif regulating FILIP1-mediated degradation of filamin-C

The PI3K/Akt pathway promotes skeletal muscle growth and myogenic differentiation. Although its importance in skeletal muscle biology is well documented, many of its substrates remain to be identified. We here studied PI3K/Akt signaling in contracting skeletal muscle cells by quantitative phosphoproteomics. We identified the extended basophilic phosphosite motif RxRxxp[S/T]xxp[S/T] in various proteins including filamin-C (FLNc). Importantly, this extended motif, located in a unique insert in Ig-like domain 20 of FLNc, is doubly phosphorylated. The protein kinases responsible for this dual-site phosphorylation are Akt and PKCα. Proximity proteomics and interaction analysis identified filamin A-interacting protein 1 (FILIP1) as direct FLNc binding partner. FILIP1 binding induces filamin degradation, thereby negatively regulating its function. Here, dual-site phosphorylation of FLNc not only reduces FILIP1 binding, providing a mechanism to shield FLNc from FILIP1-mediated degradation, but also enables fast dynamics of FLNc necessary for its function as signaling adaptor in cross-striated muscle cells.

Reimann, Schwäble et al. perform quantitative proteomics to study PI3K/Akt signaling in contracting myotubes. They identify a dual-site phosphorylation motif in the actin cross-linker and signaling adaptor filamin C, which regulates its degradation and mobility, suggesting the importance of dual phosphorylation for filamin C function in striated muscle cells.

The regulatory role of Myomaker and Myomixer-Myomerger-Minion in muscle development and regeneration

Skeletal muscle plays essential roles in motor function, energy, and glucose metabolism. Skeletal muscle formation occurs through a process called myogenesis, in which a crucial step is the fusion of mononucleated myoblasts to form multinucleated myofibers. The myoblast/myocyte fusion is triggered and coordinated in a muscle-specific way that is essential for muscle development and post-natal muscle regeneration. Many molecules and proteins have been found and demonstrated to have the capacity to regulate the fusion of myoblast/myocytes. Interestingly, two newly discovered muscle-specific membrane proteins, Myomaker and Myomixer (also called Myomerger and Minion), have been identified as fusogenic regulators in vertebrates. Both Myomaker and Myomixer-Myomerger-Minion have the capacity to directly control the myogenic fusion process. Here, we review and discuss the latest studies related to these two proteins, including the discovery, structure, expression pattern, functions, and regulation of Myomaker and Myomixer-Myomerger-Minion. We also emphasize and discuss the interaction between Myomaker and Myomixer-Myomerger-Minion, as well as their cooperative regulatory roles in cell-cell fusion. Moreover, we highlight the areas for exploration of Myomaker and Myomixer-Myomerger-Minion in future studies and consider their potential application to control cell fusion for cell-therapy purposes.

Neuromuscular Diseases Due to Chaperone Mutations: A Review and Some New Results

Skeletal muscle and the nervous system depend on efficient protein quality control, and they express chaperones and cochaperones at high levels to maintain protein homeostasis. Mutations in many of these proteins cause neuromuscular diseases, myopathies, and hereditary motor and sensorimotor neuropathies. In this review, we cover mutations in DNAJB6, DNAJB2, αB-crystallin (CRYAB, HSPB5), HSPB1, HSPB3, HSPB8, and BAG3, and discuss the molecular mechanisms by which they cause neuromuscular disease. In addition, previously unpublished results are presented, showing downstream effects of BAG3 p.P209L on DNAJB6 turnover and localization.

Genetic Mutations and Demographic, Clinical, and Morphological Aspects of Myofibrillar Myopathy in a French Cohort

BACKGROUND

Protein aggregate myopathies (PAM) represent a group of familial or sporadic neuromuscular conditions with marked clinical and genetic heterogeneity that occur in children and adults. Familial PAM includes myofibrillar myopathies defined by the presence of desmin-positive protein aggregates and degenerative intermyofibrillar network changes. PAM is often caused by dysfunctional genes, such as DES, PLEC 1, CRYAB, FLNC, MYOT, ZASP, BAG3, FHL1, and DNAJB6.

OBJECTIVE

To retrospectively analyze genetic mutations and demographic, clinical, and morphological aspects of PAM in a French population.

METHODS

Forty-eight PAM patients (29 men, 19 women) were divided into two groups, those with genetically (GIM) and nongenetically identified (NGIM) mutations associated with myofibrillar myopathy.

RESULTS

Age of myopathy onset ranged from 13 to 68 years. GIM group mutations (81.25%) included DES (14), ZASP (8), FLNC (4), MYOT (4), BAG3 (1), CRYAB (2), and DNAJB6 (6). The MYOT subgroup displayed a significantly older onset age (p = 0.029). Autosomal dominant inheritance was found in 74.3% of GIM and 44.4% of NGIM subjects. Overall, 22.9% had Maghrebian heritage, 72.9% Caucasian, and 4.2% Asian. The most common clinical sign was distal muscle weakness (66%) followed by simultaneous distal and proximal weakness in 49%. Eleven patients had contractures, one had a rigid spine, and five had respiratory dysfunction. GIM subjects had greater cardiac involvement (51.7%) versus the NGIM group (33.3%). The average serum creatine kinase level was 393 U/L in GIM and 382 U/L in NGIM subjects. Morphological analysis showed significant differences among GIM subgroups, including the number of vacuoles and regenerated fibers (ZASP), group atrophy (ZASP), and rubbed out fibers (MYOT). Ultrastructural findings showed significant differences in intranuclear rods, Z-disc thickness, and intranuclear inclusions between gene mutation subgroups. Paracrystalline inclusions were present in three patients (DNAJB6). The most frequent mutation in was in DES, followed by ZASP.

CONCLUSIONS

GIM and NGIM PAM subjects showed similar results, suggesting that any unknown genes, which cause this disease have characteristics similar to those already described. Considering the complexity of clinical, morphological, and genetic data related to PAM, particularly myofibrillar myopathies, physicians should be careful when diagnosing patients with sporadic PAM.

Dysregulated autophagy in restrictive cardiomyopathy due to Pro209Leu mutation in BAG3

Myofibrillary myopathies (MFM) are hereditary myopathies histologically characterized by degeneration of myofibrils and aggregation of proteins in striated muscle. Cardiomyopathy is common in MFM but the pathophysiological mechanisms are not well understood. The BAG3-Pro209Leu mutation is associated with early onset MFM and severe restrictive cardiomyopathy (RCM), often necessitating heart transplantation during childhood. We report on a young male patient with a BAG3-Pro209Leu mutation who underwent heart transplantation at eight years of age. Detailed morphological analyses of the explanted heart tissue showed intracytoplasmic inclusions, aggregation of BAG3 and desmin, disintegration of myofibers and Z-disk alterations. The presence of undegraded autophagosomes, seen by electron microscopy, as well as increased levels of p62, LC3-I and WIPI1, detected by immunohistochemistry and western blot analyses, indicated a dysregulation of autophagy. Parkin and PINK1, proteins involved in mitophagy, were slightly increased whereas mitochondrial OXPHOS activities were not altered. These findings indicate that altered autophagy plays a role in the pathogenesis and rapid progression of RCM in MFM caused by the BAG3-Pro209Leu mutation, which could have implications for future therapeutic strategies.

Clinical and histopathological features of myofibrillar myopathy in Warmblood horses

BACKGROUND

To report a novel exertional myopathy, myofibrillar myopathy (MFM) in Warmblood (WB) horses.

OBJECTIVES

To 1) describe the distinctive clinical and myopathic features of MFM in Warmblood horses and 2) investigate the potential inheritance of MFM in a Warmblood family.

STUDY DESIGN

Retrospective selection of MFM cases and prospective evaluation of a Warmblood family.

METHODS

Retrospectively, muscle biopsies were selected from Warmblood horses diagnosed with MFM and clinical histories obtained (n = 10). Prospectively, muscle biopsies were obtained from controls (n = 8) and a three generation WB family (n = 11). Samples were assessed for histopathology [scored 0-3], fibre types, cytoskeletal and Z disc protein aggregates, electron microscopic alterations (EM) and muscle glycogen concentrations.

RESULTS

Myofibrillar myopathy-affected cases experienced exercise intolerance, reluctance to go forward, stiffness and poorly localised lameness. Abnormal aggregates of the cytoskeletal protein desmin were found in up to 120 type 2a and a few type 2x myofibres of MFM cases. Desmin positive fibres did not stain for developmental myosin, α actinin or dystrophin. Scores for internalised myonuclei (score MFM 0.83 ± 0.67, controls 0.22 ± 0.45), anguloid atrophy (MFM 0.95 ± 0.55, controls 0.31 ± 0.37) and total myopathic scores (MFM 5.85 ± 2.10, controls 1.41 ± 2.17) were significantly higher in MFM cases vs.

CONTROLS

Focal Z disc degeneration, myofibrillar disruption and accumulation of irregular granular material was evident in MFM cases. Muscle glycogen concentrations were similar between MFM cases and controls. In the Warmblood family, desmin positive aggregates were found in myofibres of the founding dam and in horses from two subsequent generations.

MAIN LIMITATIONS

Restricted sample size due to limited availability of well phenotyped cases.

CONCLUSIONS

A distinctive and potentially heritable form of MFM exists in Warmblood horses that present with exercise intolerance and abnormal hindlimb gait. Muscle tissue is characterised by ectopic accumulation of desmin and Z disc and myofibrillar degeneration.

A novel mutation in the PDZ-like motif of ZASP causes distal ZASP-related myofibrillar myopathy

Mutations in the LDB3 gene have been identified in patients with Z-disc-associated, alternatively spliced, PDZ motif-containing protein (ZASP)-related myofibrillar myopathy (ZASP-MFM) characterized by late-onset distal myopathy with signs of cardiomyopathy and neuropathy. We describe an autosomal dominant inherited pedigree with ZASP-MFM that is in line with the typical phenotype of distal myopathy without cardiomyopathy and neuropathy, while mild asymmetrical muscle atrophy can be observed in some affected members. Muscle MRI revealed considerable fatty degeneration involved in the posterior compartment of thigh and lower leg, but relatively preserved in rectus femoris, sartorius, gracilis, adductor longus and biceps femoris breve muscles in the later stage. In addition, fatty infiltration of medial gastrocnemius muscle can be initiated as early as in the third decade in asymptomatic individuals. Myopathological features showed sarcoplasmic accumulation of multiple protein deposits and electron dense filamentous bundle aggregates. A novel heterozygous missense mutation (p.N155H) in a highly conserved PDZ-like motif of ZASP was identified. The results indicate that typical ZASP-MFM presenting with late-onset distal myopathy is commonly associated with mutations in PDZ-like motif of ZASP. The development of fatty degeneration is consistent with the typical pattern of ZASP-MFM, and the initial fatty infiltration might be started from medial gastrocnemius muscle. Our study expands the clinical and mutational spectrum of ZASP-MFM.

Filamin C is a highly dynamic protein associated with fast repair of myofibrillar microdamage

Filamin c (FLNc) is a large dimeric actin-binding protein located at premyofibrils, myofibrillar Z-discs and myofibrillar attachment sites of striated muscle cells, where it is involved in mechanical stabilization, mechanosensation and intracellular signaling. Mutations in the gene encoding FLNc give rise to skeletal muscle diseases and cardiomyopathies. Here, we demonstrate by fluorescence recovery after photobleaching that a large fraction of FLNc is highly mobile in cultured neonatal mouse cardiomyocytes and in cardiac and skeletal muscles of live transgenic zebrafish embryos. Analysis of cardiomyocytes from Xirp1 and Xirp2 deficient animals indicates that both Xin actin-binding repeat-containing proteins stabilize FLNc selectively in premyofibrils. Using a novel assay to analyze myofibrillar microdamage and subsequent repair in cultured contracting cardiomyocytes by live cell imaging, we demonstrate that repair of damaged myofibrils is achieved within only 4 h, even in the absence of de novo protein synthesis. FLNc is immediately recruited to these sarcomeric lesions together with its binding partner aciculin and precedes detectable assembly of filamentous actin and recruitment of other myofibrillar proteins. These data disclose an unprecedented degree of flexibility of the almost crystalline contractile machinery and imply FLNc as a dynamic signaling hub, rather than a primarily structural protein. Our myofibrillar damage/repair model illustrates how (cardio)myocytes are kept functional in their mechanically and metabolically strained environment. Our results help to better understand the pathomechanisms and pathophysiology of early stages of FLNc-related myofibrillar myopathy and skeletal and cardiac diseases preceding pathological protein aggregation.

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

Introduction

Myofibrillar myopathies are characterized by progressive muscle weakness and impressive abnormal protein aggregation in muscle fibers. In about 10 % of patients, the disease is caused by mutations in the MYOT gene encoding myotilin. The aim of our study was to decipher the composition of protein deposits in myotilinopathy to get new information about aggregate pathology.

Results

Skeletal muscle samples from 15 myotilinopathy patients were included in the study. Aggregate and control samples were collected from muscle sections by laser microdissection and subsequently analyzed by a highly sensitive proteomic approach that enables a relative protein quantification. In total 1002 different proteins were detected. Seventy-six proteins showed a significant over-representation in aggregate samples including 66 newly identified aggregate proteins. Z-disc-associated proteins were the most abundant aggregate components, followed by sarcolemmal and extracellular matrix proteins, proteins involved in protein quality control and degradation, and proteins with a function in actin dynamics or cytoskeletal transport. Forty over-represented proteins were evaluated by immunolocalization studies. These analyses validated our mass spectrometric data and revealed different regions of protein accumulation in abnormal muscle fibers. Comparison of data from our proteomic analysis in myotilinopathy with findings in other myofibrillar myopathy subtypes indicates a characteristic basic pattern of aggregate composition and resulted in identification of a highly sensitive and specific diagnostic marker for myotilinopathy.

Conclusions

Our findings i) indicate that main protein components of aggregates belong to a network of interacting proteins, ii) provide new insights into the complex regulation of protein degradation in myotilinopathy that may be relevant for new treatment strategies, iii) imply a combination of a toxic gain-of-function leading to myotilin-positive protein aggregates and a loss-of-function caused by a shift in subcellular distribution with a deficiency of myotilin at Z-discs that impairs the integrity of myofibrils, and iv) demonstrate that proteomic analysis can be helpful in differential diagnosis of protein aggregate myopathies.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-016-0280-0) contains supplementary material, which is available to authorized users.

Myofibrillar instability exacerbated by acute exercise in filaminopathy

Filamin C (FLNC) mutations in humans cause myofibrillar myopathy (MFM) and cardiomyopathy, characterized by protein aggregation and myofibrillar degeneration. We generated the first patient-mimicking knock-in mouse harbouring the most common disease-causing filamin C mutation (p.W2710X). These heterozygous mice developed muscle weakness and myofibrillar instability, with formation of filamin C- and Xin-positive lesions streaming between Z-discs. These lesions, which are distinct from the classical MFM protein aggregates by their morphology and filamentous appearance, were greatly increased in number upon acute physical exercise in the mice. This pathology suggests that mutant filamin influences the mechanical stability of myofibrillar Z-discs, explaining the muscle weakness in mice and humans. Re-evaluation of biopsies from MFM-filaminopathy patients with different FLNC mutations revealed a similar, previously unreported lesion pathology, in addition to the classical protein aggregates, and suggested that structures previously interpreted as aggregates may be in part sarcomeric lesions. We postulate that these lesions define preclinical disease stages, preceding the formation of protein aggregates.

Suspected myofibrillar myopathy in Arabian horses with a history of exertional rhabdomyolysis

REASONS FOR PERFORMING STUDY

Although exertional rhabdomyolysis (ER) is common in Arabian horses, there are no dedicated studies describing histopathological characteristics of muscle from Arabian horses with ER.

OBJECTIVES

To prospectively identify distinctive histopathological features of muscle from Arabian endurance horses with a history of ER (pro-ER) and to retrospectively determine their prevalence in archived samples from Arabian horses with exertional myopathies (retro-ER).

STUDY DESIGN

Prospective and retrospective histopathological description.

METHODS

Middle gluteal muscle biopsies obtained from Arabian controls (n = 14), pro-ER (n = 13) as well as archived retro-ER (n = 25) muscle samples previously classified with type 2 polysaccharide storage myopathy (15/25), recurrent exertional rhabdomyolysis (7/25) and no pathology (3/25) were scored for histopathology and immunohistochemical staining of cytoskeletal proteins. Glutaraldehyde-fixed samples (2 pro-ER, one control) were processed for electron microscopy. Pro-ER and retro-ER groups were compared with controls using Mann-Whitney U and Fisher's exact tests.

RESULTS

Centrally located myonuclei in mature myofibres were found in significantly more (P<0.05) pro-ER (12/13) and retro-ER (21/25) horses than controls (4/14). Degenerating myofibres were not evident in any biopsies. Retro-ER horses had amylase-resistant polysaccharide (6/25, P<0.05) and higher scores for cytoplasmic glycogen, rimmed vacuoles and rod-like bodies. A few control horses (3/14) and significantly (P<0.05) more pro-ER (12/13) and retro-ER (18/25) horses had disrupted myofibrillar alignment and large desmin and αβ-crystallin positive cytoplasmic aggregates. Prominent Z-disc degeneration and focal myofibrillar disruption with regional accumulation of β-glycogen particles were identified on electron microscopy of the 2 pro-ER samples.

CONCLUSIONS

In a subset of Arabian horses with intermittent episodes of exertional rhabdomyolysis, ectopic accumulation of cytoskeletal proteins and Z-disc degeneration bear a strong resemblance to a myofibrillar myopathy. While many of these horses were previously diagnosed with type 2 polysaccharide storage myopathy, pools of glycogen forming within disrupted myofibrils appeared to give the false appearance of a glycogen storage disorder.

New cardiac and skeletal protein aggregate myopathy associated with combined MuRF1 and MuRF3 mutations

Protein aggregate myopathies (PAMs) define muscle disorders characterized by protein accumulation in muscle fibres. We describe a new PAM in a patient with proximal muscle weakness and hypertrophic cardiomyopathy, whose muscle fibres contained inclusions containing myosin and myosin-associated proteins, and aberrant distribution of microtubules. These lesions appear as intact A- and M-bands lacking thin filaments and Z-discs. These features differ from inclusions in myosin storage myopathy (MSM), but are highly similar to those in mice deficient for the muscle-specific RING finger proteins MuRF1 and MuRF3. Sanger sequencing excluded mutations in the MSM-associated gene MYH7 but identified mutations in TRIM63 and TRIM54, encoding MuRF1 and MuRF3, respectively. No mutations in other potentially disease-causing genes were identified by Sanger and whole exome sequencing. Analysis of seven family members revealed that both mutations segregated in the family but only the homozygous TRIM63 null mutation in combination with the heterozygous TRIM54 mutation found in the proband caused the disease phenotype. Both MuRFs are microtubule-associated proteins localizing to sarcomeric M-bands and Z-discs. They are E3 ubiquitin ligases that play a role in degradation of sarcomeric proteins, stabilization of microtubules and myogenesis. Lack of ubiquitin and the 20S proteasome subunit in the inclusions found in the patient suggested impaired turnover of thick filament proteins. Disruption of microtubules in cultured myotubes was rescued by transient expression of wild-type MuRF1. The unique features of this novel myopathy point to defects in homeostasis of A-band proteins in combination with instability of microtubules as cause of the disease.

Zebrafish models of BAG3 myofibrillar myopathy suggest a toxic gain of function leading to BAG3 insufficiency

Mutations in the co-chaperone Bcl2-associated athanogene 3 (BAG3) can cause myofibrillar myopathy (MFM), a childhood-onset progressive muscle disease, characterized by the formation of protein aggregates and myofibrillar disintegration. In contrast to other MFM-causing proteins, BAG3 has no direct structural role, but regulates autophagy and the degradation of misfolded proteins. To investigate the mechanism of disease in BAG3-related MFM, we expressed wild-type BAG3 or the dominant MFM-causing BAG3 (BAG3(P209L)) in zebrafish. Expression of the mutant protein results in the formation of aggregates that contain wild-type BAG3. Through the stimulation and inhibition of autophagy, we tested the prevailing hypothesis that impaired autophagic function is responsible for the formation of protein aggregates. Contrary to the existing theory, our studies reveal that inhibition of autophagy is not sufficient to induce protein aggregation. Expression of the mutant protein, however, did not induce myofibrillar disintegration and we therefore examined the effect of knocking down Bag3 function. Loss of Bag3 resulted in myofibrillar disintegration, but not in the formation of protein aggregates. Remarkably, BAG3(P209L) is able to rescue the myofibrillar disintegration phenotype, further demonstrating that its function is not impaired. Together, our knockdown and overexpression experiments identify a mechanism whereby BAG3(P209L) aggregates form, gradually reducing the pool of available BAG3, which eventually results in BAG3 insufficiency and myofibrillar disintegration. This mechanism is consistent with the childhood onset and progressive nature of MFM and suggests that reducing aggregation through enhanced degradation or inhibition of nucleation would be an effective therapy for this disease.

Unusual multisystemic involvement and a novel BAG3 mutation revealed by NGS screening in a large cohort of myofibrillar myopathies

Background

Myofibrillar myopathies (MFM) are a group of phenotypically and genetically heterogeneous neuromuscular disorders, which are characterized by protein aggregations in muscle fibres and can be associated with multisystemic involvement.

Methods

We screened a large cohort of 38 index patients with MFM for mutations in the nine thus far known causative genes using Sanger and next generation sequencing (NGS). We studied the clinical and histopathological characteristics in 38 index patients and five additional relatives (n = 43) and particularly focused on the associated multisystemic symptoms.

Results

We identified 14 heterozygous mutations (diagnostic yield of 37%), among them the novel p.Pro209Gln mutation in the BAG3 gene, which was associated with onset in adulthood, a mild phenotype and an axonal sensorimotor polyneuropathy, in the absence of giant axons at the nerve biopsy. We revealed several novel clinical phenotypes and unusual multisystemic presentations with previously described mutations: hearing impairment with a FLNC mutation, dysphonia with a mutation in DES and the first patient with a FLNC mutation presenting respiratory insufficiency as the initial symptom. Moreover, we described for the first time respiratory insufficiency occurring in a patient with the p.Gly154Ser mutation in CRYAB. Interestingly, we detected a polyneuropathy in 28% of the MFM patients, including a BAG3 and a MYOT case, and hearing impairment in 13%, including one patient with a FLNC mutation and two with mutations in the DES gene. In four index patients with a mutation in one of the MFM genes, typical histological findings were only identified at the ultrastructural level (29%).

Conclusions

We conclude that extraskeletal symptoms frequently occur in MFM, particularly cardiac and respiratory involvement, polyneuropathy and/or deafness. BAG3 mutations should be considered even in cases with a mild phenotype or an adult onset. We identified a genetic defect in one of the known genes in less than half of the MFM patients, indicating that more causative genes are still to be found. Next generation sequencing techniques should be helpful in achieving this aim.

Xin is a marker of skeletal muscle damage severity in myopathies

Xin is a striated muscle-specific protein that is localized to the myotendinous junction in skeletal muscle. However, in injured mouse muscle, Xin expression is up-regulated and observed throughout skeletal muscle fibers and within satellite cells. In this study, Xin was analyzed by immunofluorescent staining in skeletal muscle samples from 47 subjects with various forms of myopathy, including muscular dystrophies, inflammatory myopathies, mitochondrial/metabolic myopathy, and endocrine myopathy. Results indicate that Xin immunoreactivity is positively and significantly correlated (rs = 0.6175, P = <0.0001) with the severity of muscle damage, regardless of myopathy type. Other muscle damage measures also showed a correlation with severity [Xin actin-binding repeat-containing 2 (rs = -0.7108, P = 0.0006) and collagen (rs = 0.4683, P = 0.0783)]. However, because only Xin lacked immunoreactivity within the healthy muscle belly, any detectable immunoreactivity for Xin was indicative of muscle damage. We also investigated the expression of Xin within the skeletal muscle of healthy individuals subjected to damaging eccentric exercise. Consistent with our previously mentioned results, Xin immunoreactivity was increased 24 hours after exercise in damaged muscle fibers and within the activated muscle satellite cells. Taken together, these data demonstrate Xin as a useful biomarker of muscle damage in healthy individuals and in patients with myopathy. The strong correlation between the degree of muscle damage and Xin immunoreactivity suggests that Xin may be a suitable outcome measure to evaluate disease progression and treatment effects in clinical trials.

Prostate cancer cell migration induced by myopodin isoforms is associated with formation of morphologically and biochemically distinct actin networks

Myopodin is an actin-binding protein that promotes cancer cell migration in response to serum stimulation and is associated with invasive tumor development. To determine whether enhanced migration reflects changes in actin cytoskeleton remodeling, fluorescence confocal microscopy was used to examine the composition and morphology of filamentous actin structures in mock-transduced cells vs. stably transduced PC3 cells expressing human myopodin isoforms, and the chemokinetic response of cells was quantified using transwell assays. The same approaches were used to analyze the effects of external migration stimuli, actin polymerization inhibitors or deletion of the isoform-specific amino- and/or carboxy termini on cell migration and actin bundle formation. Results indicate that the termini of the myopodin isoforms differentially alter the formation of morphologically distinct F-actin networks that also differ in their myosin and myopodin staining patterns. Furthermore, enhanced cell migration was reduced by >50% when actin bundle formation was impaired by myopodin-truncation, low concentrations of an actin polymerization inhibitor, or in the absence of an external migration stimulus. Human myopodin isoforms are therefore potent regulators of stress fiber formation, inducing the formation of biochemically and morphologically distinct F-actin networks in the cell body whose presence directly correlates with increased cell migration.

Identification of Xin-repeat proteins as novel ligands of the SH3 domains of nebulin and nebulette and analysis of their interaction during myofibril formation and remodeling

The striated muscle–specific actin-binding proteins Xin and Xirp2 are identified as novel ligands of the SH3 domains of the thin filament ruler nebulin and nebulette. The interaction is spatially restricted to structures associated with myofibril development or remodeling, indicating a role for these proteins in myofibril assembly and repair.

The Xin actin-binding repeat–containing proteins Xin and XIRP2 are exclusively expressed in striated muscle cells, where they are believed to play an important role in development. In adult muscle, both proteins are concentrated at attachment sites of myofibrils to the membrane. In contrast, during development they are localized to immature myofibrils together with their binding partner, filamin C, indicating an involvement of both proteins in myofibril assembly. We identify the SH3 domains of nebulin and nebulette as novel ligands of proline-rich regions of Xin and XIRP2. Precise binding motifs are mapped and shown to bind both SH3 domains with micromolar affinity. Cocrystallization of the nebulette SH3 domain with the interacting XIRP2 peptide PPPTLPKPKLPKH reveals selective interactions that conform to class II SH3 domain–binding peptides. Bimolecular fluorescence complementation experiments in cultured muscle cells indicate a temporally restricted interaction of Xin-repeat proteins with nebulin/nebulette during early stages of myofibril development that is lost upon further maturation. In mature myofibrils, this interaction is limited to longitudinally oriented structures associated with myofibril development and remodeling. These data provide new insights into the role of Xin actin-binding repeat–containing proteins (together with their interaction partners) in myofibril assembly and after muscle damage.

Differential proteomic analysis of abnormal intramyoplasmic aggregates in desminopathy

Desminopathy is a subtype of myofibrillar myopathy caused by desmin mutations and characterized by protein aggregates accumulating in muscle fibers. The aim of this study was to assess the protein composition of these aggregates. Aggregates and intact myofiber sections were obtained from skeletal muscle biopsies of five desminopathy patients by laser microdissection and analyzed by a label-free spectral count-based proteomic approach. We identified 397 proteins with 22 showing significantly higher spectral indices in aggregates (ratio >1.8, p<0.05). Fifteen of these proteins not previously reported as specific aggregate components provide new insights regarding pathomechanisms of desminopathy. Results of proteomic analysis were supported by immunolocalization studies and parallel reaction monitoring. Three mutant desmin variants were detected directly on the protein level as components of the aggregates, suggesting their direct involvement in aggregate-formation and demonstrating for the first time that proteomic analysis can be used for direct identification of a disease-causing mutation in myofibrillar myopathy. Comparison of the proteomic results in desminopathy with our previous analysis of aggregate composition in filaminopathy, another myofibrillar myopathy subtype, allows to determine subtype-specific proteomic profile that facilitates identification of the specific disorder.

BIOLOGICAL SIGNIFICANCE

Our proteomic analysis provides essential new insights in the composition of pathological protein aggregates in skeletal muscle fibers of desminopathy patients. The results contribute to a better understanding of pathomechanisms in myofibrillar myopathies and provide the basis for hypothesis-driven studies. The detection of specific proteomic profiles in different myofibrillar myopathy subtypes indicates that proteomic analysis may become a useful tool in differential diagnosis of protein aggregate myopathies.

Nebulin binding impedes mutant desmin filament assembly

Although a clear picture of the in vitro assembly process is established for vimentin, the role of associated partner proteins and their effect on intermediate filament assembly has not been fully examined. This study finds delayed dynamics of desminopathy-linked mutant desmin in myocytes and hindered assembly when associated to nebulin.

Desmin intermediate filaments (DIFs) form an intricate meshwork that organizes myofibers within striated muscle cells. The mechanisms that regulate the association of desmin to sarcomeres and their role in desminopathy are incompletely understood. Here we compare the effect nebulin binding has on the assembly kinetics of desmin and three desminopathy-causing mutant desmin variants carrying mutations in the head, rod, or tail domains of desmin (S46F, E245D, and T453I). These mutants were chosen because the mutated residues are located within the nebulin-binding regions of desmin. We discovered that, although nebulin M160–164 bound to both desmin tetrameric complexes and mature filaments, all three mutants exhibited significantly delayed filament assembly kinetics when bound to nebulin. Correspondingly, all three mutants displayed enhanced binding affinities and capacities for nebulin relative to wild-type desmin. Electron micrographs showed that nebulin associates with elongated normal and mutant DIFs assembled in vitro. Moreover, we measured significantly delayed dynamics for the mutant desmin E245D relative to wild-type desmin in fluorescence recovery after photobleaching in live-cell imaging experiments. We propose a mechanism by which mutant desmin slows desmin remodeling in myocytes by retaining nebulin near the Z-discs. On the basis of these data, we suggest that for some filament-forming desmin mutants, the molecular etiology of desminopathy results from subtle deficiencies in their association with nebulin, a major actin-binding filament protein of striated muscle.

Myofibrillar myopathies

Myofibrillar myopathies (MFMs) are rare, inherited or sporadic, progressive neuromuscular disorders with considerable clinical and genetic heterogeneity. MFMs are defined morphologically by foci of myofibril dissolution that begins at the Z-disk, accumulation of myofibrillar degradation products, and ectopic expression of a large number of proteins including desmin. To date, mutations in six genes are known to cause MFMs, accounting for approximately half of the MFM patients identified. The causative genes encode mainly sarcomeric Z-disk(-related) proteins: desmin, αB-crystallin, myotilin, Z-band alternatively spliced PDZ motif containing protein (ZASP), filamin C and the antiapoptotic BCL2-associated athanogene 3 (Bag3). Although in most MFM patients the disease presents in adulthood and evolves slowly, some patients with desminopathy, αB-crystallinopathy or Bag3opathies have an infantile or juvenile disease onset. Cardiac involvement is very common in desminopathies and can sometimes be the initial or only symptom of the disease. Respiratory symptoms are noted during childhood in αB-crystallinopathies. Early severe cardiac and respiratory involvement is seen in Bag3opathies. Optical microscopic and immunohistochemical features are similar in MFMs; however, ultrastructural findings can be useful to differentiate between the distinct MFM subtypes. No curative treatment for MFMs is currently available. Careful follow-up, especially of cardiac and respiratory function, is important.

Myopodin is an F-actin bundling protein with multiple independent actin-binding regions

The assembly of striated muscle myofibrils is a multistep process in which a variety of proteins is involved. One of the first and most important steps in myofibrillogenesis is the arrangement of thin myofilaments into ordered I-Z-I brushes, requiring the coordinated activity of numerous actin binding proteins. The early expression of myopodin prior to sarcomeric α-actinin, as well as its binding to actin, α-actinin and filamin indicate an important role for this protein in actin cytoskeleton remodelling with the precise function of myopodin in this process yet remaining to be resolved. While myopodin was previously described as a protein capable of cross-linking actin filaments into thick bundles upon transient transfections, it has remained unclear whether myopodin alone is capable of bundling actin, or if additional proteins are involved. We have therefore investigated the in vitro actin binding properties of myopodin. High speed cosedimentation assays with skeletal muscle actin confirmed direct binding of myopodin to F-actin and showed that this interaction is mediated by at least two independent actin binding sites, found in all myopodin isoforms identified to date. Furthermore, low-speed cosedimentation assays revealed that not only full length myopodin, but also the fragment containing only the second binding site, bundles microfilaments in the absence of accessory proteins. Ultrastructural analysis demonstrated that this bundling activity resembled that of α-actinin. Biochemical experiments revealed that bundling was not achieved by myopodin's ability to dimerize, indicating the presence of two individual F-actin binding sites within the second binding segment. Thus full length myopodin contains at least three F-actin binding sites. These data provide further understanding of the mechanisms by which myopodin contributes to actin reorganization during myofibril assembly.

A combined laser microdissection and mass spectrometry approach reveals new disease relevant proteins accumulating in aggregates of filaminopathy patients

Filaminopathy is a subtype of myofibrillar myopathy caused by mutations in FLNC, the gene encoding filamin C, and histologically characterized by pathologic accumulation of several proteins within skeletal muscle fibers. With the aim to get new insights in aggregate composition, we collected aggregates and control tissue from skeletal muscle biopsies of six myofibrillar myopathy patients harboring three different FLNC mutations by laser microdissection and analyzed the samples by a label-free mass spectrometry approach. A total of 390 proteins were identified, and 31 of those showed significantly higher spectral indices in aggregates compared with patient controls with a ratio >1.8. These proteins included filamin C, other known myofibrillar myopathy associated proteins, and a striking number of filamin C binding partners. Across the patients the patterns were extremely homogeneous. Xin actin-binding repeat containing protein 2, heat shock protein 27, nebulin-related-anchoring protein, and Rab35 could be verified as new filaminopathy biomarker candidates. In addition, further experiments identified heat shock protein 27 and Xin actin-binding repeat containing protein 2 as novel filamin C interaction partners and we could show that Xin actin-binding repeat containing protein 2 and the known interaction partner Xin actin-binding repeat containing protein 1 simultaneously associate with filamin C. Ten proteins showed significant lower spectral indices in aggregate samples compared with patient controls (ratio <0.56) including M-band proteins myomesin-1 and myomesin-2. Proteomic findings were consistent with previous and novel immunolocalization data. Our findings suggest that aggregates in filaminopathy have a largely organized structure of proteins also interacting under physiological conditions. Different filamin C mutations seem to lead to almost identical aggregate compositions. The finding that filamin C was detected as highly abundant protein in aggregates in filaminopathy indicates that our proteomic approach may be suitable to identify new candidate genes among the many MFM patients with so far unknown mutation.

Proteomic and protein interaction network analysis of human T lymphocytes during cell-cycle entry

Proteomic analysis of T cells emerging from quiescence identifies dynamic network-level changes in key cellular processes. Disruption of two such processes, ribosome biogenesis and RNA splicing, reveals that the programs controlling cell growth and cell-cycle entry are separable.

The authors conduct a proteomic and protein interaction network analysis of human T lymphocytes during entry into the first cell cycle.

Inhibiting the induction of eIF6 (60S ribosome biogenesis) causes T cells to enter the cell cycle without growing in size.

Inhibiting the induction of SF3B2/SF3B4 (U2/U12-dependent RNA splicing) allows an increase in cell size without entering the cell cycle.

These results provide proof of principle that blastogenesis and proliferation programs are separable in primary human T cells.

Regulating the transition of cells such as T lymphocytes from quiescence (G₀) into an activated, proliferating state involves initiation of cellular programs resulting in entry into the cell cycle (proliferation), the growth cycle (blastogenesis, cell size) and effector (functional) activation. We show the first proteomic analysis of protein interaction networks activated during entry into the first cell cycle from G₀. We also provide proof of principle that blastogenesis and proliferation programs are separable in primary human T cells. We employed a proteomic profiling method to identify large-scale changes in chromatin/nuclear matrix-bound and unbound proteins in human T lymphocytes during the transition from G₀ into the first cell cycle and mapped them to form functionally annotated, dynamic protein interaction networks. Inhibiting the induction of two proteins involved in two of the most significantly upregulated cellular processes, ribosome biogenesis (eIF6) and hnRNA splicing (SF3B2/SF3B4), showed, respectively, that human T cells can enter the cell cycle without growing in size, or increase in size without entering the cell cycle.

In vivo characterization of mutant myotilins

Myofibrillar myopathy (MFM) is a group of disorders that are pathologically defined by the disorganization of the myofibrillar alignment associated with the intracellular accumulation of Z-disk-associated proteins. MFM is caused by mutations in genes encoding Z-disk-associated proteins, including myotilin. Although a number of MFM mutations have been identified, it has been difficult to elucidate the precise roles of the mutant proteins. Here, we present a useful method for the characterization of mutant proteins associated with MFM. Expression of mutant myotilins in mouse tibialis anterior muscle by in vivo electroporation recapitulated both the pathological changes and the biochemical characteristics observed in patients with myotilinopathy. In mutant myotilin-expressing muscle fibers, myotilin aggregates and is costained with polyubiquitin, and Z-disk-associated proteins and myofibrillar disorganization were commonly seen. In addition, the expressed S60C mutant myotilin protein displayed marked detergent insolubility in electroporated mouse muscle, similar to that observed in human MFM muscle with the same mutation. Thus, in vivo electroporation can be a useful method for evaluating the pathogenicity of mutations identified in MFM.

[Desmin filaments and their disorganization associated with myofibrillar myopathies]

Desmin, the muscle-specific intermediate filament protein, is one of the earliest markers expressed in all muscle tissues during development. It forms a three-dimensional scaffold around the myofibril Z-disc and connects the entire contractile apparatus to the subsarcolemmal cytoskeleton, the nuclei and other cytoplasmic organelles. Desmin is essential for tensile strength and muscle integrity. In humans, disorganization of the desmin network is associated with cardiac and/or skeletal myopathies characterized by accumulation of desmin-containing aggregates in the cells. Currently, 49 mutations have been identified in desmin gene. The majority of these mutations alter desmin filament assembly process through different molecular mechanisms and also its interaction with its protein partners. Here, we will give an overview of desmin network organization as well as the impact of desmin mutations on this process. Furthermore, we will discuss the different molecular mechanisms implicated in perturbation of the desmin filament assembly process.

A novel CRYAB mutation resulting in multisystemic disease

Mutations in the CRYAB gene, encoding alpha-B crystallin, cause distinct clinical phenotypes including isolated posterior polar cataract, myofibrillar myopathy, cardiomyopathy, or a multisystemic disorder combining all these features. Genotype/phenotype correlations are still unclear. To date, multisystemic involvement has been reported only in kindred harboring the R120G substitution. We report a novel CRYAB mutation, D109H, associated with posterior polar cataract, myofibrillar myopathy and cardiomyopathy in a two-generation family with five affected individuals. Age of onset, clinical presentation, and muscle abnormalities were very similar to those described in the R120G family. Alpha-B crystallin may form dimers and acts as a chaperone for a number of proteins. It has been suggested that the phenotypic diversity could be related to the various interactions between target proteins of individual mutant residues. Molecular modeling indicates that residues D109 and R120 interact with each other during dimerization of alpha-B crystallin; interestingly, the two substitutions affecting these residues (D109H and R120G) are associated with the same clinical phenotype, thus suggesting a similar pathogenic mechanism. We propose that impairment of alpha-B crystallin dimerization may also be relevant to the pathogenesis of these disorders.

BAG3-related myofibrillar myopathy in a Chinese family

In contrast to the usual slow disease progression in myofibrillar myopathies, patients with Bag3opathy often have a rapidly progressive and more severe phenotype with a worse prognosis. We describe a Chinese patient, born to non-consanguineous parents, who first presented at age 6 with clumsy walking and difficult climbing staircase. With a history of restrictive lung disease previously diagnosed as asthma, she progressed rapidly with proximal myopathy, rigid spine and bilateral tightening of the Achilles tendons requiring surgical elongation. Hypertrophic cardiomyopathy with restrictive physiology was shown by echocardiogram. Moreover, prolonged QT interval was also noted in the patient. Family history was unremarkable yet her father was incidentally found to have prolonged QT interval. Mutation analysis with genomic DNA of the proband showed heterozygous de novo known mutation c.626C>T (p.Pro209Leu) and a germline variation c.772C>T (p.Arg258Trp) in BAG3. Her father was found to be a carrier of c.772C>T. Muscle biopsy findings were suggestive of myofibrillar myopathy on light microscopy and ultrastructural studies. To our knowledge, this is the first Chinese case of Bag3opathy so far reported.

Synaptopodin family of natively unfolded, actin binding proteins: physical properties and potential biological functions

The synaptopodin family of proteins consists of at least 3 members: synaptopodin, the synaptopodin 2 proteins, and the synaptopodin 2-like proteins. Each family member has at least 3 isoforms that are produced by alternative splicing. Synaptopodin family members are basic proteins that are rich in proline and have little regular 2° or 3° structure at physiological temperature, pH and ionic strength. Like other natively unfolded proteins, synaptopodin family members have multiple binding partners including actin and other actin-binding proteins. Several members of the synaptopodin family have been shown to stimulate actin polymerization and to bundle actin filaments either on their own or in collaboration with other proteins. Synaptopodin 2 has been shown to accelerate nucleation of actin filament formation and to induce actin bundling. The actin polymerization activity is inhibited by Ca²⁺-calmodulin. Synaptopodin 2 proteins are localized in Z-bands of striated and heart muscle and dense bodies of smooth muscle cells. Depending on the developmental status and stress, at least one member of the synaptopodin family can occupy nuclei of some cells. Members of the synaptopodin 2 subfamily have been implicated in cancers.

The sarcomeric Z-disc component myopodin is a multiadapter protein that interacts with filamin and alpha-actinin

Here we introduce myopodin as a novel filamin C binding partner. Corroborative yeast two-hybrid and biochemical analyses indicate that the central part of myopodin that shows high homology to the closely related protein synaptopodin and that is common to all its currently known or predicted variants interacts with filamin C immunoglobulin-like domains 20-21. A detailed characterization of the previously described interaction between myopodin and alpha-actinin demonstrates for the first time that myopodin contains three independent alpha-actinin-binding sites. Newly developed myopodin-specific antibodies reveal expression at the earliest stages of in vitro differentiation of human skeletal muscle cells preceding the expression of sarcomeric alpha-actinin. Myopodin colocalizes with filamin and alpha-actinin during all stages of muscle development. By contrast, colocalization with its previously identified binding partner zyxin is restricted to early developmental stages. Genetic and cellular analyses of skeletal muscle provided direct evidence for an alternative transcriptional start site in exon three, corroborating the expression of a myopodin variant lacking the PDZ domain encoded by exons 1 and 2 in skeletal muscle. We conclude that myopodin is a multiadapter protein of the sarcomeric Z-disc that links nascent myofibrils to the sarcolemma via zyxin, and might play a role in early assembly and stabilization of the Z-disc. Mutations in FLNC, ACTN2 and several other genes encoding Z-disc-related proteins cause myopathy and cardiomyopathy. Its localization and its association with the myopathy-associated proteins filamin C and alpha-actinin make myopodin an interesting candidate for a muscle disease gene.

Sarcomeric α-actinins and their role in human muscle disease

In skeletal muscle, the sarcomeric α-actinins (α-actinin-2 and -3) are a major component of the Z-line and crosslink actin thin filaments to maintain the structure of the sarcomere. Based on their known protein binding partners, the sarcomeric α-actinins are likely to have a number of structural, signaling and metabolic roles in skeletal muscle. In addition, the α-actinins interact with many proteins responsible for inherited muscle disorders. In this paper, we explore the role of the sarcomeric α-actinins in normal skeletal muscle and in the pathogenesis of a range of neuromuscular disorders.

Myofibrillar myopathies: a clinical and myopathological guide

Myofibrillar myopathies (MFMs) are histopathologically characterized by desmin-positive protein aggregates and myofibrillar degeneration. Because of the marked phenotypic and pathomorphological variability, establishing the diagnosis of MFM can be a challenging task. While MFMs are partly caused by mutations in genes encoding for extramyofibrillar proteins (desmin, alphaB-crystallin, plectin) or myofibrillar proteins (myotilin, Z-band alternatively spliced PDZ-containing protein, filamin C, Bcl-2-associated athanogene-3, four-and-a-half LIM domain 1), a large number of these diseases are caused by still unresolved gene defects. Although recent years have brought new insight into the pathogenesis of MFMs, the precise molecular pathways and sequential steps that lead from an individual gene defect to progressive muscle damage are still unclear. This review focuses on the clinical and myopathological aspects of genetically defined MFMs, and shall provide a diagnostic guide for this numerically significant group of protein aggregate myopathies.