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.

Filamin-A-interacting protein 1 (FILIP1) is a dual compartment protein linking myofibrils and microtubules during myogenic differentiation and upon mechanical stress

Variations in the gene encoding filamin-A-interacting protein 1 (FILIP1) were identified to be associated with a combination of neurological and muscular symptoms. While FILIP1 was shown to regulate motility of brain ventricular zone cells, a process important for corticogenesis, the function of the protein in muscle cells has been less well characterized. The expression of FILIP1 in regenerating muscle fibres predicted a role in early muscle differentiation. Here we analysed expression and localization of FILIP1 and its binding partners filamin-C (FLNc) and microtubule plus-end-binding protein EB3 in differentiating cultured myotubes and adult skeletal muscle. Prior to the development of cross-striated myofibrils, FILIP1 is associated with microtubules and colocalizes with EB3. During further myofibril maturation its localization changes, and FILIP1 localizes to myofibrillar Z-discs together with the actin-binding protein FLNc. Forced contractions of myotubes by electrical pulse stimulation (EPS) induce focal disruptions in myofibrils and translocation of both proteins from Z-discs to these lesions, suggesting a role in induction and/or repair of these structures. The immediate proximity of tyrosylated, dynamic microtubules and EB3 to lesions implies that also these play a role in these processes. This implication is supported by the fact that in nocodazole-treated myotubes that lack functional microtubules, the number of lesions induced by EPS is significantly reduced. In summary, we here show that FILIP1 is a cytolinker protein that is associated with both microtubules and actin filaments, and might play a role in the assembly of myofibrils and their stabilization upon mechanical stress to protect them from damage.

Novel Filamin C Myofibrillar Myopathy Variants Cause Different Pathomechanisms and Alterations in Protein Quality Systems

Myofibrillar myopathies (MFM) are a group of chronic muscle diseases pathophysiologically characterized by accumulation of protein aggregates and structural failure of muscle fibers. A subtype of MFM is caused by heterozygous mutations in the filamin C (FLNC) gene, exhibiting progressive muscle weakness, muscle structural alterations and intracellular protein accumulations. Here, we characterize in depth the pathogenicity of two novel truncating FLNc variants (p.Q1662X and p.Y2704X) and assess their distinct effect on FLNc stability and distribution as well as their impact on protein quality system (PQS) pathways. Both variants cause a slowly progressive myopathy with disease onset in adulthood, chronic myopathic alterations in muscle biopsy including the presence of intracellular protein aggregates. Our analyses revealed that p.Q1662X results in FLNc haploinsufficiency and p.Y2704X in a dominant-negative FLNc accumulation. Moreover, both protein-truncating variants cause different PQS alterations: p.Q1662X leads to an increase in expression of several genes involved in the ubiquitin-proteasome system (UPS) and the chaperone-assisted selective autophagy (CASA) system, whereas p.Y2704X results in increased abundance of proteins involved in UPS activation and autophagic buildup. We conclude that truncating FLNC variants might have different pathogenetic consequences and impair PQS function by diverse mechanisms and to varying extents. Further studies on a larger number of patients are necessary to confirm our observations.

Target formation in muscle fibres indicates reinnervation - A proteomic study in muscle samples from peripheral neuropathies

AIMS

Target skeletal muscle fibres - defined by different concentric areas in oxidative enzyme staining - can occur in patients with neurogenic muscular atrophy. Here, we used our established hypothesis-free proteomic approach with the aim of deciphering the protein composition of targets. We also searched for potential novel interactions between target proteins.

METHODS

Targets and control areas were laser microdissected from skeletal muscle sections of 20 patients with neurogenic muscular atrophy. Samples were analysed by a highly sensitive mass spectrometry approach, enabling relative protein quantification. The results were validated by immunofluorescence studies. Protein interactions were investigated by yeast two-hybrid assays, coimmunoprecipitation experiments and bimolecular fluorescence complementation.

RESULTS

More than 1000 proteins were identified. Among these, 55 proteins were significantly over-represented and 40 proteins were significantly under-represented in targets compared to intraindividual control samples. The majority of over-represented proteins were associated with the myofibrillar Z-disc and actin dynamics, followed by myosin and myosin-associated proteins, proteins involved in protein biosynthesis and chaperones. Under-represented proteins were mainly mitochondrial proteins. Functional studies revealed that the LIM domain of the over-represented protein LIMCH1 interacts with isoform A of Xin actin-binding repeat-containing protein 1 (XinA).

CONCLUSIONS

In particular, proteins involved in myofibrillogenesis are over-represented in target structures, which indicate an ongoing process of sarcomere assembly and/or remodelling within this specific area of the muscle fibres. We speculate that target structures are the result of reinnervation processes in which filamin C-associated myofibrillogenesis is tightly regulated by the BAG3-associated protein quality system.

Proteomic and morphological insights and clinical presentation of two young patients with novel mutations of BVES (POPDC1)

Popeye domain containing protein 1 (POPDC1) is a highly conserved transmembrane protein essential for striated muscle function and homeostasis. Pathogenic variants in the gene encoding POPDC1 (BVES, Blood vessel epicardial substance) are causative for limb-girdle muscular dystrophy (LGMDR25), associated with cardiac arrhythmia. We report on four affected children (age 7-19 years) from two consanguineous families with two novel pathogenic variants in BVES c.457C>T(p.Q153X) and c.578T>G (p.I193S). Detailed analyses were performed on muscle biopsies from an affected patient of each family including immunofluorescence, electron microscopy and proteomic profiling. Cardiac abnormalities were present in all patients and serum creatine kinase (CK) values were variably elevated despite lack of overt muscle weakness. Detailed histological analysis of skeletal muscle, however indicated a myopathy with reduced sarcolemmal expression of POPDC1 accompanied by altered sarcolemmal and sarcoplasmatic dysferlin and Xin/XIRP1 abundance. At the electron microscopic level, the muscle fiber membrane was focally disrupted. The proteomic signature showed statistically significant dysregulation of 191 proteins of which 173 were increased and 18 were decreased. Gene ontology-term analysis of affected biological processes revealed - among others - perturbation of muscle fibril assembly, myofilament sliding, and contraction as well as transition between fast and slow fibers. In conclusion, these findings demonstrate that the phenotype of LGMDR25 is highly variable and also includes younger children with conduction abnormalities, no apparent muscular problems, and only mildly elevated CK values. Biochemical studies suggest that BVES mutations causing loss of functional POPDC1 can impede striated muscle function by several mechanisms.

Isoform-specific functions of synaptopodin-2 variants in cytoskeleton stabilization and autophagy regulation in muscle under mechanical stress

Protein homeostasis (proteostasis) in multicellular organisms depends on the maintenance of force-bearing and force-generating cellular structures. Within myofibrillar Z-discs of striated muscle, isoforms of synaptopodin-2 (SYNPO2/myopodin) act as adapter proteins that are engaged in proteostasis of the actin-crosslinking protein filamin C (FLNc) under mechanical stress. SYNPO2 directly binds F-actin, FLNc and α-actinin and thus contributes to the architectural features of the actin cytoskeleton. By its association with autophagy mediating proteins, i.e. BAG3 and VPS18, SYNPO2 is also engaged in protein quality control and helps to target mechanical unfolded and damaged FLNc for degradation. Here we show that deficiency of all SYNPO2-isoforms in myotubes leads to decreased myofibrillar stability and deregulated autophagy under mechanical stress. In addition, isoform-specific proteostasis functions were revealed. The PDZ-domain containing variant SYNPO2b and the shorter, PDZ-less isoform SYNPO2e both localize to Z-discs. Yet, SYNPO2e is less stably associated with the Z-disc than SYNPO2b, and is dynamically transferred into FLNc-containing myofibrillar lesions under mechanical stress. SYNPO2e also recruits BAG3 into these lesions via interaction with the WW domain of BAG3. Our data provide evidence for a role of myofibrillar lesions as a transient quality control compartment essential to prevent and repair contraction-induced myofibril damage in muscle and indicate an important coordinating activity for SYNPO2 therein.

Maintaining proteostasis under mechanical stress

Cell survival, tissue integrity and organismal health depend on the ability to maintain functional protein networks even under conditions that threaten protein integrity. Protection against such stress conditions involves the adaptation of folding and degradation machineries, which help to preserve the protein network by facilitating the refolding or disposal of damaged proteins. In multicellular organisms, cells are permanently exposed to stress resulting from mechanical forces. Yet, for long time mechanical stress was not recognized as a primary stressor that perturbs protein structure and threatens proteome integrity. The identification and characterization of protein folding and degradation systems, which handle force-unfolded proteins, marks a turning point in this regard. It has become apparent that mechanical stress protection operates during cell differentiation, adhesion and migration and is essential for maintaining tissues such as skeletal muscle, heart and kidney as well as the immune system. Here, we provide an overview of recent advances in our understanding of mechanical stress protection.

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

Objective

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

Methods

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

Results

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

Conclusions

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

Molecular basis of F-actin regulation and sarcomere assembly via myotilin

Sarcomeres, the basic contractile units of striated muscle cells, contain arrays of thin (actin) and thick (myosin) filaments that slide past each other during contraction. The Ig-like domain-containing protein myotilin provides structural integrity to Z-discs-the boundaries between adjacent sarcomeres. Myotilin binds to Z-disc components, including F-actin and α-actinin-2, but the molecular mechanism of binding and implications of these interactions on Z-disc integrity are still elusive. To illuminate them, we used a combination of small-angle X-ray scattering, cross-linking mass spectrometry, and biochemical and molecular biophysics approaches. We discovered that myotilin displays conformational ensembles in solution. We generated a structural model of the F-actin:myotilin complex that revealed how myotilin interacts with and stabilizes F-actin via its Ig-like domains and flanking regions. Mutant myotilin designed with impaired F-actin binding showed increased dynamics in cells. Structural analyses and competition assays uncovered that myotilin displaces tropomyosin from F-actin. Our findings suggest a novel role of myotilin as a co-organizer of Z-disc assembly and advance our mechanistic understanding of myotilin's structural role in Z-discs.

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.

Recessive Mutations in SYNPO2 as a Candidate of Monogenic Nephrotic Syndrome

Introduction

Most of the approximately 60 genes that if mutated cause steroid-resistant nephrotic syndrome (SRNS) are highly expressed in the glomerular podocyte, rendering SRNS a “podocytopathy.”

Methods

We performed whole-exome sequencing (WES) in 1200 nephrotic syndrome (NS) patients.

Results

We discovered homozygous truncating and homozygous missense mutation in SYNPO2 (synaptopodin-2) (p.Lys1124∗ and p.Ala1134Thr) in 2 patients with childhood-onset NS. We found SYNPO2 expression in both podocytes and mesangial cells; however, notably, immunofluorescence staining of adult human and rat kidney cryosections indicated that SYNPO2 is localized mainly in mesangial cells. Subcellular localization studies reveal that in these cells SYNPO2 partially co-localizes with α-actinin and filamin A−containing F-actin filaments. Upon transfection in mesangial cells or podocytes, EGFP-SYNPO2 co-localized with α-actinin-4, which gene is mutated in autosomal dominant SRNS in humans. SYNPO2 overexpression increases mesangial cell migration rate (MMR), whereas shRNA knockdown reduces MMR. Decreased MMR was rescued by transfection of wild-type mouse Synpo2 cDNA but only partially by cDNA representing mutations from the NS patients. The increased mesangial cell migration rate (MMR) by SYNPO2 overexpression was inhibited by ARP complex inhibitor CK666. SYNPO2 shRNA knockdown in podocytes decreased active Rac1, which was rescued by transfection of wild-type SYNPO2 cDNA but not by cDNA representing any of the 2 mutant variants.

Conclusion

We show that SYNPO2 variants may lead to Rac1-ARP3 dysregulation, and may play a role in the pathogenesis of nephrotic syndrome.

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.

HspB1 phosphorylation regulates its intramolecular dynamics and mechanosensitive molecular chaperone interaction with filamin C

Mechanical force-induced conformational changes in proteins underpin a variety of physiological functions, typified in muscle contractile machinery. Mutations in the actin-binding protein filamin C (FLNC) are linked to musculoskeletal pathologies characterized by altered biomechanical properties and sometimes aggregates. HspB1, an abundant molecular chaperone, is prevalent in striated muscle where it is phosphorylated in response to cues including mechanical stress. We report the interaction and up-regulation of both proteins in three mouse models of biomechanical stress, with HspB1 being phosphorylated and FLNC being localized to load-bearing sites. We show how phosphorylation leads to increased exposure of the residues surrounding the HspB1 phosphosite, facilitating their binding to a compact multidomain region of FLNC proposed to have mechanosensing functions. Steered unfolding of FLNC reveals that its extension trajectory is modulated by the phosphorylated region of HspB1. This may represent a posttranslationally regulated chaperone-client protection mechanism targeting over-extension during mechanical stress.

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

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

The novel cardiac z-disc protein CEFIP regulates cardiomyocyte hypertrophy by modulating calcineurin signaling

The z-disc is a structural component at the lateral borders of the sarcomere and is important for mechanical stability and contractility of both cardiac and skeletal muscles. Of note, the sarcomeric z-disc also represents a nodal point in cardiomyocyte function and signaling. Mutations of numerous z-disc proteins are associated with cardiomyopathies and muscle diseases. To identify additional z-disc proteins that might contribute to cardiac disease, we employed an in silico screen for cardiac-enriched cDNAs. This screen yielded a previously uncharacterized protein named cardiac-enriched FHL2-interacting protein (CEFIP), which exhibited a heart- and skeletal muscle-specific expression profile. Importantly, CEFIP was located at the z-disc and was up-regulated in several models of cardiomyopathy. We also found that CEFIP overexpression induced the fetal gene program and cardiomyocyte hypertrophy. Yeast two-hybrid screens revealed that CEFIP interacts with the calcineurin-binding protein four and a half LIM domains 2 (FHL2). Because FHL2 binds calcineurin, a phosphatase controlling hypertrophic signaling, we examined the effects of CEFIP on the calcineurin/nuclear factor of activated T-cell (NFAT) pathway. These experiments revealed that CEFIP overexpression further enhances calcineurin-dependent hypertrophic signal transduction, and its knockdown repressed hypertrophy and calcineurin/NFAT activity. In summary, we report on a previously uncharacterized protein CEFIP that modulates calcineurin/NFAT signaling in cardiomyocytes, a finding with possible implications for the pathogenesis of cardiomyopathy.

Myofibrillar Z-discs Are a Protein Phosphorylation Hot Spot with Protein Kinase C (PKCα) Modulating Protein Dynamics

The Z-disc is a protein-rich structure critically important for the development and integrity of myofibrils, which are the contractile organelles of cross-striated muscle cells. We here used mouse C2C12 myoblast, which were differentiated into myotubes, followed by electrical pulse stimulation (EPS) to generate contracting myotubes comprising mature Z-discs. Using a quantitative proteomics approach, we found significant changes in the relative abundance of 387 proteins in myoblasts versus differentiated myotubes, reflecting the drastic phenotypic conversion of these cells during myogenesis. Interestingly, EPS of differentiated myotubes to induce Z-disc assembly and maturation resulted in increased levels of proteins involved in ATP synthesis, presumably to fulfill the higher energy demand of contracting myotubes. Because an important role of the Z-disc for signal integration and transduction was recently suggested, its precise phosphorylation landscape further warranted in-depth analysis. We therefore established, by global phosphoproteomics of EPS-treated contracting myotubes, a comprehensive site-resolved protein phosphorylation map of the Z-disc and found that it is a phosphorylation hotspot in skeletal myocytes, underscoring its functions in signaling and disease-related processes. In an illustrative fashion, we analyzed the actin-binding multiadaptor protein filamin C (FLNc), which is essential for Z-disc assembly and maintenance, and found that PKCα phosphorylation at distinct serine residues in its hinge 2 region prevents its cleavage at an adjacent tyrosine residue by calpain 1. Fluorescence recovery after photobleaching experiments indicated that this phosphorylation modulates FLNc dynamics. Moreover, FLNc lacking the cleaved Ig-like domain 24 exhibited remarkably fast kinetics and exceedingly high mobility. Our data set provides research community resource for further identification of kinase-mediated changes in myofibrillar protein interactions, kinetics, and mobility that will greatly advance our understanding of Z-disc dynamics and signaling.

The cochaperone BAG3 coordinates protein synthesis and autophagy under mechanical strain through spatial regulation of mTORC1

The cochaperone BAG3 is a central protein homeostasis factor in mechanically strained mammalian cells. It mediates the degradation of unfolded and damaged forms of the actin-crosslinker filamin through chaperone-assisted selective autophagy (CASA). In addition, BAG3 stimulates filamin transcription in order to compensate autophagic disposal and to maintain the actin cytoskeleton under strain. Here we demonstrate that BAG3 coordinates protein synthesis and autophagy through spatial regulation of the mammalian target of rapamycin complex 1 (mTORC1). The cochaperone utilizes its WW domain to contact a proline-rich motif in the tuberous sclerosis protein TSC1 that functions as an mTORC1 inhibitor in association with TSC2. Interaction with BAG3 results in a recruitment of TSC complexes to actin stress fibers, where the complexes act on a subpopulation of mTOR-positive vesicles associated with the cytoskeleton. Local inhibition of mTORC1 is essential to initiate autophagy at sites of filamin unfolding and damage. At the same time, BAG3-mediated sequestration of TSC1/TSC2 relieves mTORC1 inhibition in the remaining cytoplasm, which stimulates protein translation. In human muscle, an exercise-induced association of TSC1 with the cytoskeleton coincides with mTORC1 activation in the cytoplasm. The spatial regulation of mTORC1 exerted by BAG3 apparently provides the basis for a simultaneous induction of autophagy and protein synthesis to maintain the proteome under mechanical strain.

Sarcomeric lesions and remodeling proximal to intercalated disks in overload-induced cardiac hypertrophy

Pressure overload induces cardiac remodeling involving both the contractile machinery and intercalated disks (IDs). Filamin C (FlnC) and Xin actin-binding repeat-containing proteins (XIRPs) are multi-adapters localizing in IDs of higher vertebrates. Knockout of the gene encoding Xin (Xirp1) in mice leads to a mild cardiac phenotype with ID mislocalization. In order to amplify this phenotype, we performed transverse aortic constriction (TAC) on control and Xirp1-deficient mice. TAC induced similar left ventricular hypertrophy in both genotypes, suggesting that the lack of Xin does not lead to higher susceptibility to cardiac overload. However, in both genotypes, FlnC appeared in "streaming" localizations across multiple sarcomeres proximal to the IDs, suggesting a remodeling response. Furthermore, FlnC-positive areas of remodeling, reminiscent of sarcomeric lesions previously described for skeletal muscles (but so far unreported in the heart), were also observed. These adaptations reflect a similarly strong effect of the pressure induced by TAC in both genotypes. However, 2 weeks post-operation TAC-treated knockout hearts had reduced levels of connexin43 and slightly increased incidents of ventricular tachycardia compared to their wild-type (WT) counterparts. Our findings highlight the FlnC-positive sarcomeric lesions and ID-proximal streaming as general remodeling responses in cardiac overload-induced hypertrophy.

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.

Breaking sarcomeres by in vitro exercise

Eccentric exercise leads to focal disruptions in the myofibrils, referred to as “lesions”. These structures are thought to contribute to the post-exercise muscle weakness, and to represent areas of mechanical damage and/or remodelling. Lesions have been investigated in human biopsies and animal samples after exercise. However, this approach does not examine the mechanisms behind lesion formation, or their behaviour during contraction. To circumvent this, we used electrical pulse stimulation (EPS) to simulate exercise in C2C12 myotubes, combined with live microscopy. EPS application led to the formation of sarcomeric lesions in the myotubes, resembling those seen in exercised mice, increasing in number with the time of application or stimulation intensity. Furthermore, transfection with an EGFP-tagged version of the lesion and Z-disc marker filamin-C allowed us to observe the formation of lesions using live cell imaging. Finally, using the same technique we studied the behaviour of these structures during contraction, and observed them to be passively stretching. This passive behaviour supports the hypothesis that lesions contribute to the post-exercise muscle weakness, protecting against further damage. We conclude that EPS can be reliably used as a model for the induction and study of sarcomeric lesions in myotubes in vitro.

Interplay of Matrix Stiffness and c-SRC in Hepatic Fibrosis

Introduction: In liver fibrosis activation of hepatic stellate cells (HSC) comprises phenotypical change into profibrotic and myofibroplastic cells with increased contraction and secretion of extracellular matrix (ECM) proteins. The small GTPase RhoA orchestrates cytoskeleton formation, migration, and mobility via non-receptor tyrosine-protein kinase c-SRC (cellular sarcoma) in different cells. Furthermore, RhoA and its downstream effector Rho-kinase also play a crucial role in hepatic stellate cells and hepatic fibrogenesis. Matrix stiffness promotes HSC activation via cytoskeleton modulation. This study investigated the interaction of c-SRC and RhoA under different matrix stiffness conditions.

Methods: Liver fibrosis was induced in rats using bile duct ligation (BDL), thioacetamide (TAA) or carbon tetrachloride (CCl₄) models. mRNA levels of albumin, PDGF-R, RHOA, COL1A1, and αSMA were analyzed via qRT-PCR. Western Blots using phospho-specific antibodies against p-c-SRC418 and p-c-SRC530 analyzed the levels of activating and inactivating c-SRC, respectively. LX2 cells and hepatocytes were cultured on acrylamide gels of 1 and 12 kPa or on plastic to mimic non-fibrotic, fibrotic, or cirrhotic environments then exposed to SRC-inhibitor PP2. Overexpression of RhoA was performed by transfection using RhoA-plasmids. Additionally, samples from cirrhotic patients and controls were collected at liver transplantations and tumor resections were analyzed for RhoA and c-SRC protein expression by Western Blot.

Results: Transcription of albumin and RhoA was decreased, whereas transcription and activation of c-SRC was increased in hepatocytes cultured on 12 kPa compared to 1 kPa gels. LX2 cells cultured on 12 kPa gels showed upregulation of RHOA, COL1A1, and αSMA mRNA levels. Inhibition of c-SRC by PP2 in LX2 cells led to an increase in COL1A1 and αSMA most prominently in 12 kPa gels. In LX2 cells with RhoA overexpression, c-SRC inhibition by PP2 failed to improve fibrosis. RhoA expression was significantly elevated in human and experimental liver fibrosis, while c-SRC was inactivated.

Conclusions: This study shows that c-SRC is inactive in activated myofibroblast-like HSC in liver cirrhosis. Inactivation of c-SRC is mediated by a crosstalk with RhoA upon hepatic stellate cell activation and fibrosis progression.

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.

In vivo characterization of human myofibrillar myopathy genes in zebrafish

Myofibrillar myopathies (MFM) are progressive diseases of human heart and skeletal muscle with a severe impact on life quality and expectancy of affected patients. Although recently several disease genes for myofibrillar myopathies could be identified, today most genetic causes and particularly the associated mechanisms and signaling events that lead from the mutation to the disease phenotype are still mostly unknown. To assess whether the zebrafish is a suitable model system to validate MFM candidate genes using targeted antisense-mediated knock-down strategies, we here specifically inactivated known human MFM disease genes and evaluated the resulting muscular and cardiac phenotypes functionally and structurally. Consistently, targeted ablation of MFM genes in zebrafish led to compromised skeletal muscle function mostly due to myofibrillar degeneration as well as severe heart failure. Similar to what was shown in MFM patients, MFM gene-deficient zebrafish showed pronounced gene-specific phenotypic and structural differences. In summary, our results indicate that the zebrafish is a suitable model to functionally and structurally evaluate novel MFM disease genes in vivo.

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.

Angiotensin-II type 1 receptor-mediated Janus kinase 2 activation induces liver fibrosis

Activation of the renin angiotensin system resulting in stimulation of angiotensin-II (AngII) type I receptor (AT1R) is an important factor in the development of liver fibrosis. Here, we investigated the role of Janus kinase 2 (JAK2) as a newly described intracellular effector of AT1R in mediating liver fibrosis. Fibrotic liver samples from rodents and humans were compared to respective controls. Transcription, protein expression, activation, and localization of JAK2 and downstream effectors were analyzed by real-time polymerase chain reaction, western blotting, immunohistochemistry, and confocal microscopy. Experimental fibrosis was induced by bile duct ligation (BDL), CCl4 intoxication, thioacetamide intoxication or continuous AngII infusion. JAK2 was inhibited by AG490. In vitro experiments were performed with primary rodent hepatic stellate cells (HSCs), Kupffer cells (KCs), and hepatocytes as well as primary human and human-derived LX2 cells. JAK2 expression and activity were increased in experimental rodent and human liver fibrosis, specifically in myofibroblastic HSCs. AT1R stimulation in wild-type animals led to activation of HSCs and fibrosis in vivo through phosphorylation of JAK2 and subsequent RhoA/Rho-kinase activation. These effects were prevented in AT1R(-/-) mice. Pharmacological inhibition of JAK2 attenuated liver fibrosis in rodent fibrosis models. In vitro, JAK2 and downstream effectors showed increased expression and activation in activated HSCs, when compared to quiescent HSCs, KCs, and hepatocytes isolated from rodents. In primary human and LX2 cells, AG490 blocked AngII-induced profibrotic gene expression. Overexpression of JAK2 led to increased profibrotic gene expression in LX2 cells, which was blocked by AG490.

CONCLUSION

Our study substantiates the important cell-intrinsic role of JAK2 in HSCs for development of liver fibrosis. Inhibition of JAK2 might therefore offer a promising therapy for liver fibrosis.

Aciculin interacts with filamin C and Xin and is essential for myofibril assembly, remodeling and maintenance

Filamin C (FLNc) and Xin actin-binding repeat-containing proteins (XIRPs) are multi-adaptor proteins that are mainly expressed in cardiac and skeletal muscles and which play important roles in the assembly and repair of myofibrils and their attachment to the membrane. We identified the dystrophin-binding protein aciculin (also known as phosphoglucomutase-like protein 5, PGM5) as a new interaction partner of FLNc and Xin. All three proteins colocalized at intercalated discs of cardiac muscle and myotendinous junctions of skeletal muscle, whereas FLNc and aciculin also colocalized in mature Z-discs. Bimolecular fluorescence complementation experiments in developing cultured mammalian skeletal muscle cells demonstrated that Xin and aciculin also interact in FLNc-containing immature myofibrils and areas of myofibrillar remodeling and repair induced by electrical pulse stimulation (EPS). Fluorescence recovery after photobleaching (FRAP) experiments showed that aciculin is a highly dynamic and mobile protein. Aciculin knockdown in myotubes led to failure in myofibril assembly, alignment and membrane attachment, and a massive reduction in myofibril number. A highly similar phenotype was found upon depletion of aciculin in zebrafish embryos. Our results point to a thus far unappreciated, but essential, function of aciculin in myofibril formation, maintenance and remodeling.

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.

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.

Cellular mechanotransduction relies on tension-induced and chaperone-assisted autophagy

Mechanical tension is an ever-present physiological stimulus essential for the development and homeostasis of locomotory, cardiovascular, respiratory, and urogenital systems. Tension sensing contributes to stem cell differentiation, immune cell recruitment, and tumorigenesis. Yet, how mechanical signals are transduced inside cells remains poorly understood. Here, we identify chaperone-assisted selective autophagy (CASA) as a tension-induced autophagy pathway essential for mechanotransduction in muscle and immune cells. The CASA complex, comprised of the molecular chaperones Hsc70 and HspB8 and the cochaperone BAG3, senses the mechanical unfolding of the actin-crosslinking protein filamin. Together with the chaperone-associated ubiquitin ligase CHIP, the complex initiates the ubiquitin-dependent autophagic sorting of damaged filamin to lysosomes for degradation. Autophagosome formation during CASA depends on an interaction of BAG3 with synaptopodin-2 (SYNPO2). This interaction is mediated by the BAG3 WW domain and facilitates cooperation with an autophagosome membrane fusion complex. BAG3 also utilizes its WW domain to engage in YAP/TAZ signaling. Via this pathway, BAG3 stimulates filamin transcription to maintain actin anchoring and crosslinking under mechanical tension. By integrating tension sensing, autophagosome formation, and transcription regulation during mechanotransduction, the CASA machinery ensures tissue homeostasis and regulates fundamental cellular processes such as adhesion, migration, and proliferation.

Filamin C-related myopathies: pathology and mechanisms

The term filaminopathy was introduced after a truncating mutation in the dimerization domain of filamin C (FLNc) was shown to be responsible for a devastating muscle disease. Subsequently, the same mutation was found in patients from diverse ethnical origins, indicating that this specific alteration is a mutational hot spot. Patients initially present with proximal muscle weakness, while distal and respiratory muscles become affected with disease progression. Muscle biopsies of these patients show typical signs of myofibrillar myopathy, including disintegration of myofibrils and aggregation of several proteins into distinct intracellular deposits. Highly similar phenotypes were observed in patients with other mutations in Ig-like domains of FLNc that result in expression of a noxious protein. Biochemical and biophysical studies showed that the mutated domains acquire an abnormal structure causing decreased stability and eventually becoming a seed for abnormal aggregation with other proteins. The disease usually presents only after the fourth decade of life possibly as a result of ageing-related impairments in the machinery that is responsible for disposal of damaged proteins. This is confirmed by mutations in components of this machinery that cause a highly similar phenotype. Transfection studies of cultured muscle cells reflect the events observed in patient muscles and, therefore, may provide a helpful model for testing future dedicated therapeutic strategies. More recently, FLNC mutations were also found in families with a distal myopathy phenotype, caused either by mutations in the actin-binding domain of FLNc that result in increased actin-binding and non-specific myopathic abnormalities without myofibrillar myopathy pathology, or a nonsense mutation in the rod domain that leads to RNA instability, haploinsufficiency with decreased expression levels of FLNc in the muscle fibers and myofibrillar abnormalities, but not to the formation of desmin-positive protein aggregates required for the diagnosis of myofibrillar myopathy.

Impairment of protein degradation in myofibrillar myopathy caused by FLNC/filamin C mutations

Myofibrillar myopathy caused by FLNC/filamin C mutations is characterized by disintegration of myofibrils and a massive formation of protein aggregates within skeletal muscle fibers. We performed immunofluorescence studies in skeletal muscle sections from filaminopathy patients to detect disturbances of protein quality control mechanisms. Our analyses revealed altered expression of chaperone proteins and components of proteasomal and autophagic degradation pathways in abnormal muscle fibers that harbor protein deposits but not in neighboring muscle fibers without pathological protein aggregation. These findings suggest a dysfunction of protein stabilizing and degrading mechanisms that leads to a pathological accumulation of protein aggregates in abnormal fibers. Accordingly, a pharmacological modulation of chaperone activity may be a promising therapeutic strategy to prevent protein aggregation and to reduce disease progression. Newly established filaminopathy cell culture models provide a suitable basis for testing such pharmacological approaches.

Pathophysiology of protein aggregation and extended phenotyping in filaminopathy

Mutations in FLNC cause two distinct types of myopathy. Disease associated with mutations in filamin C rod domain leading to expression of a toxic protein presents with progressive proximal muscle weakness and shows focal destructive lesions of polymorphous aggregates containing desmin, myotilin and other proteins in the affected myofibres; these features correspond to the profile of myofibrillar myopathy. The second variant associated with mutations in the actin-binding domain of filamin C is characterized by weakness of distal muscles and morphologically by non-specific myopathic features. A frameshift mutation in the filamin C rod domain causing haploinsufficiency was also found responsible for distal myopathy with some myofibrillar changes but no protein aggregation typical of myofibrillar myopathies. Controversial data accumulating in the literature require re-evaluation and comparative analysis of phenotypes associated with the position of the FLNC mutation and investigation of the underlying disease mechanisms. This is relevant and necessary for the refinement of diagnostic criteria and developing therapeutic approaches. We identified a p.W2710X mutation in families originating from ethnically diverse populations and re-evaluated a family with a p.V930_T933del mutation. Analysis of the expanded database allows us to refine clinical and myopathological characteristics of myofibrillar myopathy caused by mutations in the rod domain of filamin C. Biophysical and biochemical studies indicate that certain pathogenic mutations in FLNC cause protein misfolding, which triggers aggregation of the mutant filamin C protein and subsequently involves several other proteins. Immunofluorescence analyses using markers for the ubiquitin-proteasome system and autophagy reveal that the affected muscle fibres react to protein aggregate formation with a highly increased expression of chaperones and proteins involved in proteasomal protein degradation and autophagy. However, there is a noticeably diminished efficiency of both the ubiquitin-proteasome system and autophagy that impairs the muscle capacity to prevent the formation or mediate the degradation of aggregates. Transfection studies of cultured muscle cells imitate events observed in the patient's affected muscle and therefore provide a helpful model for testing future therapeutic strategies.

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.

Atorvastatin inhibits proliferation and apoptosis, but induces senescence in hepatic myofibroblasts and thereby attenuates hepatic fibrosis in rats

Hepatic myofibroblasts (MFB) show increased proliferation, migration and collagen production, which are crucial for hepatic fibrogenesis. Atorvastatin treatment inhibits proliferation, apoptosis and cytokine production of MFB in bile duct-ligated (BDL) rats in vivo. Here, we have further investigated the underlying mechanisms. Primary rat hepatic stellate cells (HSC) were isolated and culture-activated to hepatic MFB. Following 3 days of incubation with atorvastatin (10(-4), 10(-5) and 10(-6) M), transcription levels of profibrotic cytokines (transforming growth factor-β1, connective tissue growth factor and TIMP1) and procollagen Ia were analyzed by real time PCR. Proliferation was investigated by 5'-bromo-2'-deoxyuridine assays. α-Smooth muscle actin protein expression was examined by western blotting. Fluorescence-activated cell sorting analysis of Annexin V and propidium iodide were used to measure apoptosis. Furthermore, p21 western blotting and β-galactosidase staining were investigated in MFB as senescence markers. Subsequently, hepatic expression of desmin and senescence markers were analyzed in the livers of rats receiving atorvastatin (15 mg/kg*d) for 1 week starting 3 and 5 weeks after BDL. Atorvastatin inhibited the activation of HSC to MFB and decreased cytokine and collagen production in MFB in vitro. In addition, proliferation, cytokine and collagen production of MFB were reduced by atorvastatin. Atorvastatin initiated apoptosis at 10(-4) M and attenuated it at 10(-5) M. Atorvastatin induced p21 protein expression and β-galactosidase staining of MFB in vitro and in vivo. Atorvastatin elicits similiar effects on MFB as previously seen in vivo: it decreases MFB turnover and fibrogenesis. We suggest that a further mechanism explaining these effects is senescence of cells.

Xirp proteins mark injured skeletal muscle in zebrafish

Myocellular regeneration in vertebrates involves the proliferation of activated progenitor or dedifferentiated myogenic cells that have the potential to replenish lost tissue. In comparison little is known about cellular repair mechanisms within myocellular tissue in response to small injuries caused by biomechanical or cellular stress. Using a microarray analysis for genes upregulated upon myocellular injury, we identified zebrafish Xin-actin-binding repeat-containing protein1 (Xirp1) as a marker for wounded skeletal muscle cells. By combining laser-induced micro-injury with proliferation analyses, we found that Xirp1 and Xirp2a localize to nascent myofibrils within wounded skeletal muscle cells and that the repair of injuries does not involve cell proliferation or Pax7(+) cells. Through the use of Xirp1 and Xirp2a as markers, myocellular injury can now be detected, even though functional studies indicate that these proteins are not essential in this process. Previous work in chicken has implicated Xirps in cardiac looping morphogenesis. However, we found that zebrafish cardiac morphogenesis is normal in the absence of Xirp expression, and animals deficient for cardiac Xirp expression are adult viable. Although the functional involvement of Xirps in developmental and repair processes currently remains enigmatic, our findings demonstrate that skeletal muscle harbours a rapid, cell-proliferation-independent response to injury which has now become accessible to detailed molecular and cellular characterizations.

Skeletal muscle regeneration is delayed by reduction in Xin expression: consequence of impaired satellite cell activation?

Xin is a striated muscle-specific actin-binding protein whose mRNA expression has been observed in damaged skeletal muscle. Here we demonstrate increased Xin protein expression early postinjury (≤12 h) and localization primarily to the periphery of damaged myofibers. At 1 day postinjury, Xin is colocalized with MyoD, confirming expression in activated satellite cells (SCs). By 5 days postinjury, Xin is evident in newly regenerated myofibers, with a return to preinjury levels by 14 days of regeneration. To determine whether the increased Xin expression is functionally relevant, tibialis anterior muscles of wild-type mice were infected with Xin-short hairpin RNA (shRNA) adenovirus, whereas the contralateral tibialis anterior received control adenovirus (Control). Four days postinfection, muscles were harvested or injured with cardiotoxin and collected at 3, 5, or 14 days thereafter. When compared with Control, Xin-shRNA infection attenuated muscle regeneration as demonstrated by Myh3 expression and fiber areas. Given the colocalization of Xin and MyoD, we isolated single myofibers from infected muscles to investigate the effect of silencing Xin on SC function. Relative to Control, SC activation, but not proliferation, was significantly impaired in Xin-shRNA-infected muscles. To determine whether Xin affects the G0-G1 transition, cell cycle reentry was assessed on infected C2C12 myoblasts using a methylcellulose assay. No difference in reentry was noted between groups, suggesting that Xin contributes to SC activation by means other than affecting G0-G1 transition. Together these data demonstrate a critical role for Xin in SC activation and reduction in Xin expression results in attenuated skeletal muscle repair.

Distal myopathy with upper limb predominance caused by filamin C haploinsufficiency

OBJECTIVE

In this study, we investigated the detailed clinical findings and underlying genetic defect in 3 presumably related Bulgarian families displaying dominantly transmitted adult onset distal myopathy with upper limb predominance.

METHODS

We performed neurologic, electrophysiologic, radiologic, and histopathologic analyses of 13 patients and 13 at-risk but asymptomatic individuals from 3 generations. Genome-wide parametric linkage analysis was followed by bidirectional sequencing of the filamin C (FLNC) gene. We characterized the identified nonsense mutation at cDNA and protein level.

RESULTS

Based on clinical findings, no known myopathy subtype was implicated in our distal myopathy patients. Light microscopic analysis of affected muscle tissue showed no specific hallmarks; however, the electron microscopy revealed changes compatible with myofibrillar myopathy. Linkage studies delineated a 9.76 Mb region on chromosome 7q22.1-q35 containing filamin C (FLNC), a gene previously associated with myofibrillar myopathy. Mutation analysis revealed a novel c.5160delC frameshift deletion in all patients of the 3 families. The mutation results in a premature stop codon (p.Phe1720LeufsX63) that triggers nonsense-mediated mRNA decay. FLNC transcript levels were reduced in muscle and lymphoblast cells from affected subjects and partial loss of FLNC in muscle tissue was confirmed by protein analysis.

CONCLUSIONS

The FLNC mutation that we identified is distinct in terms of the associated phenotype, muscle morphology, and underlying molecular mechanism, thus extending the currently recognized clinical and genetic spectrum of filaminopathies. We conclude that filamin C is a dosage-sensitive gene and that FLNC haploinsufficiency can cause a specific type of myopathy in humans.

DNA sequencing errors in molecular diagnostics of filamin myopathy

BACKGROUND

Filamin myopathy is a neuromuscular disorder manifesting with predominantly limb-girdle muscle weakness and in many patients with diaphragm paralysis and cardiomyopathy, caused by mutations in the filamin C (FLNC) gene. Molecular diagnosis of filamin myopathy based on direct DNA sequencing of coding exons is compromised by the presence of a high homology pseudogene (pseFLNC) located approximately 53.6 kb downstream of the functional FLNC gene on chromosome 7q.

METHODS

Molecular cloning, RT-PCR and real-time PCR methods were used to detect sequence differences between the FLNC and pseFLNC that are implicated in known or potential molecular diagnostic errors. Overall, 50 patients with a phenotype resembling filamin myopathy have been screened for mutations in FLNC.

RESULTS

FLNC sequence inconsistencies caused by the interference from pseFLNC were identified and diagnostic errors involving, in particular, the detection of the most frequent disease-causing FLNC p.W2710X mutation resolved. Mismatches between the FLNC and pseFLNC sequences were tabulated for future use.

CONCLUSIONS

We devise a strategy that allows one to discern mutations occurring in the functional FLNC from those harbored in pseFLNC, thus preventing possible complications in future research and patient genetic testing.

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.

Ponsin interacts with Nck adapter proteins: implications for a role in cytoskeletal remodelling during differentiation of skeletal muscle cells

Skeletal muscle differentiation is a complex process: It is characterised by changes in gene expression and protein composition. Simultaneously, a dramatic remodelling of the cytoskeleton and associated cell-matrix contacts, the costameres, occurs. The expression and localisation of the protein ponsin at cell-matrix contacts marks the establishment of costameres. In this report we show that skeletal muscle cells are characterised by a novel ponsin isoform, which contains a large insertion in its carboxy-terminus. This skeletal muscle-specific module binds the adapter proteins Nck1 and Nck2, and increased co-localisation of ponsin with Nck2 is observed at remodelling cell-matrix contacts of differentiating skeletal muscle cells. Since this ponsin insertion can be phosphorylated, it may adjust the interaction affinity with Nck adapter proteins. The novel ponsin isoform and its interaction with Nck1/2 provide exciting insight into the convergence of signalling pathways at the costameres, and its crucial role for skeletal muscle differentiation and re-generation.

Chaperone-assisted selective autophagy is essential for muscle maintenance

How are biological structures maintained in a cellular environment that constantly threatens protein integrity? Here we elucidate proteostasis mechanisms affecting the Z disk, a protein assembly essential for actin anchoring in striated muscles, which is subjected to mechanical, thermal, and oxidative stress during contraction [1]. Based on the characterization of the Drosophila melanogaster cochaperone Starvin (Stv), we define a conserved chaperone machinery required for Z disk maintenance. Instead of keeping Z disk proteins in a folded conformation, this machinery facilitates the degradation of damaged components, such as filamin, through chaperone-assisted selective autophagy (CASA). Stv and its mammalian ortholog BAG-3 coordinate the activity of Hsc70 and the small heat shock protein HspB8 during disposal that is initiated by the chaperone-associated ubiquitin ligase CHIP and the autophagic ubiquitin adaptor p62. CASA is thus distinct from chaperone-mediated autophagy, previously shown to facilitate the ubiquitin-independent, direct translocation of a client across the lysosomal membrane [2]. Impaired CASA results in Z disk disintegration and progressive muscle weakness in flies, mice, and men. Our findings reveal the importance of chaperone-assisted degradation for the preservation of cellular structures and identify muscle as a tissue that highly relies on an intact proteostasis network, thereby shedding light on diverse myopathies and aging.

Complete loss of murine Xin results in a mild cardiac phenotype with altered distribution of intercalated discs

AIMS

Xin is a striated muscle-specific F-actin binding protein that has been implicated in cardiomyopathies. In cardiomyocytes, Xin is localized at intercalated discs (IDs). Mice lacking only two of the three Xin isoforms (XinAB(-/-) mice) develop severe cardiac hypertrophy. To further investigate the function of Xin variants in the mammalian heart, we generated XinABC(-/-) mice deficient in all Xin isoforms.

METHODS AND RESULTS

XinABC(-/-) mice showed a very mild phenotype: heart weight, heart weight to tibia length ratios, and cardiac dimensions were not altered. Increased perivascular fibrosis was only observed in hearts of young XinABC(-/-) mice. Striking differences were revealed in isolated cardiomyocytes: XinABC(-/-) cells demonstrated a significantly increased number of non-terminally localized ID-like structures. Furthermore, resting sarcomere length was increased, sarcomere shortening, peak shortening at 0.5-1 Hz, and the duration of shortening were decreased, and shortening and relengthening velocities were accelerated at frequencies above 4 Hz in XinABC(-/-) cardiomyocytes. ECG showed a significantly shorter HV interval and a trend towards shorter QRS interval in XinABC(-/-) mice, suggesting a faster conduction velocity of the ventricular-specific conduction system. In human cardiac tissue, expression of XinC protein was detected solely in samples from patients with cardiac hypertrophy.

CONCLUSION

Total Xin deficiency leads to topographical ID alterations, premature fibrosis and subtle changes in contractile behaviour; this is a milder cardiac phenotype than that observed in XinAB(-/-) mice, which still can express XinC. Together with the finding that XinC is detected solely in cardiomyopathic human tissues, this suggests that its expression is responsible for the stronger dominant phenotype in XinAB(-/-) mice. Furthermore, it indicates that XinC may be involved in the development of human cardiac hypertrophy.

Multipurpose modular lentiviral vectors for RNA interference and transgene expression

We have created a multipurpose modular lentiviral vector system for expressing both transgenes and miRNA 30-based short hairpins (shRNAmirs) for RNAi. The core of the resulting vector system, pLVmir, allows a simple two step cloning procedure for expressing shRNAmirs under the control of a Pol II promoter in both a constitutive and conditional manner. The adapted cloning method includes a PCR-free method for transferring shRNAmir based RNAi clones from a publicly available library (Open Biosystems). The addition of a Pol II promoter-driven shRNAmir cassette and broadening the choice of Pol III promoters and silencing triggers offers great flexibility to this system. The combination of several preexisting and additional modules created here caters to common needs of researchers. Our modular vector system was validated regarding functionality of promoters, inducibility and reversibility. We successfully applied the system to knockdown Xirp2 mRNA expression in H2kb-tsA58 muscle cells and determined that this had no spurious effect on the expression of a closely related protein. Finally, our set of lentiviral vectors may be used to achieve synergistic effects, for simultaneous knockdown of two genes, as a rescue plasmid and for studying mutant proteins in a physiological context.

Enrichment and terminal differentiation of striated muscle progenitors in vitro

Enrichment and terminal differentiation of mammalian striated muscle cells is severely hampered by fibroblast overgrowth, de-differentiation and/or lack of functional differentiation. Herein we report a new, reproducible and simple method to enrich and terminally differentiate muscle stem cells and progenitors from mice and humans. We show that a single gamma irradiation of muscle cells induces their massive differentiation into structurally and functionally intact myotubes and cardiomyocytes and that these cells can be kept in culture for many weeks. Similar results are also obtained when treating skeletal muscle-derived stem cells and progenitors with Mitomycin C.