Patterns of abnormal protein expression in target formations and unstructured cores

Characterisation of phenotypic patterns in equine exercise-associated myopathies

BACKGROUND

Equine exercise-associated myopathies are prevalent, clinically heterogeneous, generally idiopathic disorders characterised by episodes of myofibre damage that occur in association with exercise. Episodes are intermittent and vary within and between affected horses and across breeds. The aetiopathogenesis is often unclear; there might be multiple causes. Poor phenotypic characterisation hinders genetic and other disease analyses.

OBJECTIVES

The aim of this study was to characterise phenotypic patterns across exercise-associated myopathies in horses.

STUDY DESIGN

Historical cross-sectional study, with subsequent masked case-control validation study.

METHODS

Historical clinical and histological features from muscle samples (n = 109) were used for k-means clustering and validated using principal components analysis and hierarchical clustering. For further validation, a blinded histological study (69 horses) was conducted comparing two phenotypic groups with selected controls and horses with histopathological features characterised by myofibrillar disruption.

RESULTS

We identified two distinct broad phenotypes: a non-classic exercise-associated myopathy syndrome (EAMS) subtype was associated with practitioner-described signs of apparent muscle pain (p < 0.001), reluctance to move (10.85, p = 0.001), abnormal gait (p < 0.001), ataxia (p = 0.001) and paresis (p = 0.001); while a non-specific classic RER subtype was not uniquely associated with any particular variables. No histological differences were identified between subtypes in the validation study, and no identifying histopathological features for other equine myopathies identified in either subtype.

MAIN LIMITATIONS

Lack of an independent validation population; small sample size of smaller identified subtypes; lack of positive control myofibrillar myopathy cases; case descriptions derived from multiple independent and unblinded practitioners.

CONCLUSIONS

This is the first study using computational clustering methods to identify phenotypic patterns in equine exercise-associated myopathies, and suggests that differences in patterns of presenting clinical signs support multiple disease subtypes, with EAMS a novel subtype not previously described. Routine muscle histopathology was not helpful in sub-categorising the phenotypes in our population.

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.

The ultrastructure of muscle fibers and satellite cells in experimental autoimmune encephalomyelitis after treatment with transcranial magnetic stimulation

In this study, we investigated the effect of transcranial magnetic stimulation (TMS) on the ultrastructure of muscle fibers and satellite cells in rats with experimental autoimmune encephalomyelitis (EAE). EAE-induced animals were treated with TMS (60 Hz at 0.7 mT) for 2 hours in the morning, once a day, 5 days a week, for 3 weeks, starting on day 15 post-immunization. The rats were sacrificed on day 36 post-immunization, and the soleus muscles were evaluated by light microscopy and transmission electron microscopy. Findings were compared with a non-treated EAE group. Electron microscopy analysis showed the presence of degenerated mitochondria, autophagic vacuoles, and altered myofibrils in non-treated EAE group. This correlates with the presence of acid phosphatase activity in muscle fibers and core-targetoid lesions with desmin immunohistochemistry. Most myonuclei in the EAE group showed apoptotic features. In contrast, EAE induced-TMS treated animals had less ultrastructural changes in the mitochondria and the myofibrils, together with less frequent apoptotic nuclear features. Peripheral desmin+ protrusions, as a marker of active satellite cells, were significantly increased in TMS-treated group. This correlates ultrastructurally with the presence of active features in satellite cells in the TMS group. In conclusion, the attenuation of ultrastructural alterations in muscle fibers and activation response of satellite cells caused by EAE indicated that skeletal muscle had a regenerative response to TMS.

Myopathic changes in murine skeletal muscle lacking synemin

Diseases of striated muscle linked to intermediate filament (IF) proteins are associated with defects in the organization of the contractile apparatus and its links to costameres, which connect the sarcomeres to the cell membrane. Here we study the role in skeletal muscle of synemin, a type IV IF protein, by examining mice null for synemin (synm-null). Synm-null mice have a mild skeletal muscle phenotype. Tibialis anterior (TA) muscles show a significant decrease in mean fiber diameter, a decrease in twitch and tetanic force, and an increase in susceptibility to injury caused by lengthening contractions. Organization of proteins associated with the contractile apparatus and costameres is not significantly altered in the synm-null. Elastimetry of the sarcolemma and associated contractile apparatus in extensor digitorum longus myofibers reveals a reduction in tension consistent with an increase in sarcolemmal deformability. Although fatigue after repeated isometric contractions is more marked in TA muscles of synm-null mice, the ability of the mice to run uphill on a treadmill is similar to controls. Our results suggest that synemin contributes to linkage between costameres and the contractile apparatus and that the absence of synemin results in decreased fiber size and increased sarcolemmal deformability and susceptibility to injury. Thus synemin plays a moderate but distinct role in fast twitch skeletal muscle.

Pathophysiology of inflammatory and autoimmune myopathies

The main subtypes of inflammatory myopathies include dermatomyositis (DM), polymyositis (PM), necrotizing autoimmune myositis (NAM) and sporadic inclusion-body myositis (sIBM). The review provides an update on the main clinical characteristics unique to each subset, including fundamental aspects on muscle pathology helpful to assure accurate diagnosis, underlying immunopathomechanisms and therapeutic strategies. DM is a complement-mediated microangiopathy leading to destruction of capillaries, distal hypoperfusion and inflammatory cell stress on the perifascicular regions. NAM is an increasingly recognized subacute myopathy triggered by statins, viral infections, cancer or autoimmunity with macrophages as the final effector cells mediating fiber injury. PM and IBM are characterized by cytotoxic CD8-positive T cells which clonally expand in situ and invade MHC-I-expressing muscle fibers. In IBM, in addition to autoimmunity, there is vacuolization and intrafiber accumulation of degenerative and stressor molecules. Pro-inflammatory mediators, such as gamma interferon and interleukin IL1-β, seem to enhance the accumulation of stressor and amyloid-related misfolded proteins. Current therapies using various immunosuppressive and immunomodulating drugs are discussed for PM, DM and NAM, and the principles for effective treatment strategies in IBM are outlined.

Abnormal Distribution of Calcium-Handling Proteins

Central core disease (CCD) and multi-minicore disease (MmD) are muscle disorders characterized by foci of mitochondria depletion and sarcomere disorganization ("cores") in muscle fibers. Although core myopathies are the most frequent congenital myopathies, their pathogenesis remains elusive and specific diagnostic markers are lacking. Core myopathies are mostly caused by mutations in 2 sarcoplasmic reticulum proteins: the massive Ca-release channel RyR1 or the selenoprotein N (SelN) of unknown function. To search for distinctive markers and to obtain further pathophysiological insight, we identified the molecular defects in 12 core myopathy patients and analyzed the immunolocalization of 6 proteins of the Ca-release complex in their muscle biopsies. In 7 cases with RYR1 mutations (6 CCD, one MmD), RyR1 was depleted from the cores; in contrast, the other proteins of the sarcoplasmic reticulum (calsequestrin, SERCA1/2, and triadin) and the T-tubule (dihydropyridine receptor-alpha1subunit) accumulated within or around the lesions, suggesting an original modification of the Ca-release complex protein arrangement. Conversely, all Ca-related proteins were distributed normally in 5 MmD cases with SelN mutations. Our results provide an appropriate tool to orientate the differential and molecular diagnosis of core myopathies and suggest that different pathophysiological mechanisms lead to core formation in SelN- and in RyR1-related core myopathies.

Protein aggregate myopathies

Protein aggregate myopathies (PAMs) based on the morphologic phenomenon of aggregation of proteins within muscle fibers may occur in children (selenoproteinopathies, actinopathies, and myosinopathies) or adults (certain myofibrillar myopathies and myosinopathies). They may be mutation related, which includes virtually all childhood forms but certain other forms as well, or sporadic, which are largely seen in adults. Their classification as myofibrillar or desmin-related myopathies, actinopathies, or myosinopathies is based on the identification of respective mutant proteins, most of them components of the sarcomeres. Recognition of PAM requires muscle biopsy and an extensive immunohistochemical and electron microscopic workup of the biopsied muscle tissue after which molecular analysis of morphologically ascertained proteins should ensue to permit recognition of individual entities and genetic counseling of patients and families. Because pathogenetic principles in PAMs are still incompletely known, causative therapy, at this time, is not available.

Congenital myopathies in the new millennium

Few medical disciplines have benefited so enormously from the molecular revolution as myology. Whereas the congenital myopathies have flourished from enzyme histochemistry and electron microscopy, defining individual congenital myopathies by structural abnormalities, genetic research has only recently focused on congenital myopathies. However, a number of congenital myopathies have been molecularly elucidated: central and multiminicore diseases, nemaline myopathy, myotubular myopathy, and congenital myopathy marked by aggregation of proteins, giving rise to the concept of protein aggregate myopathies, to which now desminopathies, alpha-B crystallinopathies, selenoproteinopathy, myotilinopathy, actinopathies, and myosinopathies belong. Based on recent identification of mutations in respective genes, the principle "from morphology, that is, immunohistochemistry, to molecular analysis" through recognition of certain accrued proteins within muscle fibers and subsequent analysis of their respective genes has resulted in a wealth of genetic data and in reconsidering classification and nosologic interpretation of certain congenital myopathies. This heuristic principle needs to be further applied to other genetically still obscure congenital myopathies.

The prion protein in human neuromuscular diseases

The basis of human prion diseases affecting the nervous system is accumulation of a disease-associated conformer (PrPSc) of the normal cellular prion protein (PrPC). Earlier studies demonstrated increased expression of PrPC in inclusion body myositis (IBM), dermato-, and polymyositis, as well as neurogenic muscle atrophy. To define the spectrum and reliability of PrPC immunoreactivity, its expression was examined systematically in a series of pathologically characterized muscular disorders by means of immunohistochemistry, confocal laser microscopy, and immunogold electron microscopy. Anti-PrPC immunolabelling of rimmed vacuoles was observed in IBM, inclusions of myofibrillary myopathy, targets, regenerating, and atrophic fibres, mononuclear cells, in addition to ragged red fibres in mitochondrial myopathies, and focal sarcolemmal immunostaining in non-diseased controls. Quantitative analysis demonstrated that, in neurogenic muscle lesions, anti-PrPC staining detects a significantly broader spectrum of fibres than anti-vimentin or anti-NCAM. In dystrophic muscle, PrPC expression was mainly restricted to regenerating fibres. In IBM, PrPC expression was not confined to rimmed vacuoles or vacuolated fibres and only a small percentage (7.1%) of rimmed vacuoles were PrPC positive. Ultrastructurally, PrPC was observed in the cytoplasm of lymphocytes, in the myofibrillar network of targets, and in rimmed vacuoles. Knowledge of disease circumstances with altered expression of PrPC is important in the setting of a potentially increased chance for extraneural PrPC-PrPSc conversion. In addition, our observations suggest that PrPC may have a general stress-response effect in various neuromuscular disorders.

Disturbed trafficking of dystrophin and associated proteins in targetoid phenomena after chronic muscle denervation

Dystrophin and associated proteins form a complex with an important role at the sarcolemma. Expression of this protein complex is highly regulated during development and regeneration. In order to better understand assembling patterns of these proteins, we have studied their expression in targetoid-like phenomena found in human muscle after chronic denervation, a situation known to give rise to abnormal protein trafficking. In eight biopsies of patients with chronic denervation, mainly resulting from amyotrophic lateral sclerosis, we found a number of targetoid phenomena. Selective accumulation of a number of sarcolemmal and sarcoplasmatic proteins occurred in targetoid phenomena. The larger majority of them contained gamma-sarcoglycan (gammaSG), but none contained the developmental heavy chain myosin isoform. In a series of 166 targetoid phenomena which could be studied with 17 different antibodies recognizing sarcolemmal and cytoplasmatic proteins, a high level of colocalization of gammaSG with desmin and alpha-actinin was found. Colocalization rate was much lower with other proteins, including other members of the dystrophin-associated protein complex. These data show that selective changes in expression of otherwise closely related proteins occur during disturbed trafficking leading to target formation. Because members of the dystrophin-associated protein complex do not accumulate in a similar fashion within targets, we suggest that a complex molecular control of gene expression and trafficking of this complex is involved after chronic muscle denervation.

Beyond LGMD1A: myotilin is a component of central core lesions and nemaline rods

Myotilin is a Z-disc protein that binds alpha-actinin, gamma-filamin and F-actin. The essential role of myotilin in skeletal muscle is highlighted by the recent observation that autosomal dominant limb girdle muscular dystrophy type 1A is caused by mutations in the human myotilin gene. We studied the expression and subcellular distribution of myotilin in nemaline myopathy, central core disease, centronuclear myopathy, and myopathies with tubular aggregates. A prominent myotilin immunostaining of nemaline rods and core lesions was detected in all ten cases of nemaline myopathy and five cases of central core disease. This renders myotilin a sensitive, though non-specific marker for these structural lesions. Western blot analysis did not indicate an increased myotilin expression in nemaline myopathy muscle. However, the analysis indicated upregulation of a 75 kDa immunoreactive band, very weak in control muscle but previously detected in limb girdle muscular dystrophy 1A samples. Our findings indicate that myotilin is a core structural molecule in nemaline rods and central core lesions and suggest modification of myotilin in nemaline myopathy, and further support the notion that myotilin may have a key role in the dynamic molecular events mediating myofibril assembly in normal and diseased human skeletal muscle.

Expression of plectin in muscle fibers with cytoarchitectural abnormalities

Plectin is a protein belonging to the cytoskeletal anchoring system, concentrated at sites of mechanical stress in different cell types. In normal skeletal muscle, plectin is located at level of Z-discs, sarcolemma, post-synaptic membrane, and intermyofibrillar network. We investigated plectin immunocytochemistry in lobulated fibers, fibers with tubular aggregates, target fibers, central core disease and centronuclear myopathy. Thirty to forty percent of lobulated fibers had patchy increase of plectin immunoreactivity at sarcolemmal level with focal subsarcolemmal increases. Tubular aggregates revealed a low binding for plectin. Ten percent of central cores exhibited faint focal increase of plectin immunoreactivity. Target formations had a normal plectin pattern. In centronuclear myopathy, plectin immunoreactivity was increased around the centrally located nuclei in 8-12% of the fibers, at the sarcolemma of 50% of type 2 fibers, and at the membrane of small vacuoles located peripherally around the central nuclei. We postulate that plectin may play a role in the subsarcolemmal aggregation of mitochondria in the lobulated fibers, and in the central position of nuclei as well as in shape formation, positioning and moving of the vacuoles in centronuclear myopathy.

Congenital myopathies at their molecular dawning

The introduction and application of molecular techniques have commenced to influence and alter the nosology of congenital myopathies. Long-known entities such as nemaline myopathies, core diseases, and desmin-related myopathies have now been found to be caused by unequivocal mutations. Several of these mutations and their genes have been identified by analyzing aggregates of proteins within muscle fibers as a morphological hallmark as in desminopathy and actinopathy, the latter a subtype among the nemaline myopathies. Immunohistochemistry has played a crucial role in recognizing this new group of protein aggregate myopathies within the spectrum of congenital myopathies. It is to be expected that other congenital myopathies marked by inclusion bodies may turn out to be such protein aggregate myopathies, depending on analysis of individual proteins within these protein aggregates and their association with putative gene mutations.

Filamin C accumulation is a strong but nonspecific immunohistochemical marker of core formation in muscle

Filamin C is the muscle isoform of a group of large actin-crosslinking proteins. On the one hand, filamin C is associated with the Z-disk of the myofibrillar apparatus and binds to myotilin; on the other hand, it interacts with the sarcoglycan complex at the sarcolemma. Filamin C may be involved in reorganizing the cytoskeleton in response to signalling events and in muscle it may, in addition, fulfill structural functions at the Z-disk. An examination of biopsies from patients with multi-minicore myopathy, central core myopathy and neurogenic target fibers with core-like target formations (TF) revealed strong reactivity of all the cores and target formations with two different anti-filamin C antibodies. In all three conditions, the immunoreactivity in the cores for filamin C was considerably stronger than that for desmin. Only for alphaB-crystallin were comparable levels of immunoreactivity detected. There was no difference in intensity for filamin C between the three pathological conditions. Thus, filamin C along with alphaB-crystallin is a strong and robust, but nonspecific marker of core formation. The reason why filamin C accumulates in cores is unclear at present, but we postulate that it may be critically involved in the chain of events eventually leading to myofibrillar degeneration.

Familial cardioneuromyopathy with hyaline masses and nemaline rods: a novel phenotype

Two siblings (patients 1 and 2) had adult-onset muscle weakness that was greater distally than proximally, as well as respiratory insufficiency, cardiomyopathy, and cervical spine anomalies. Electromyography studies indicated myopathy and findings consistent with neuropathy in both. In the deltoid muscle of patient 1 and the anterior tibial muscle of patient 2, myriad type 1 fibers harbored large, irregularly polygonal, and mostly central hyaline masses, small vacuoles, and nemaline rods flanking the hyaline masses or congregated under the sarcolemma. The hyaline masses are intensely congophilic; react strongly for desmin, alphaB-crystallin, alpha1-antichymotrypsin, and ubiquitin and variably for gelsolin and dystrophin; and are devoid of alpha-actinin, nebulin, titin, and slow myosin. The presence of ubiquitin, gelsolin, and fragmented filaments, and the absence of nebulin, titin, alpha-actinin, and slow myosin in the hyaline masses, signal nonlysosomal protein degradation. Ultrastructurally, the hyaline masses are composed of intermediate-density amorphous material intermingled with fragmented filaments and irregularly branching, pleomorphic, highly electron-dense material, resembling the hyaline structures of myofibrillar myopathy. We conclude that the pathological process in this syndrome is one that induces destruction of myofibrillar components, resulting in aggregation of the degraded residues in hyaline masses, and causes replication of Z disks, resulting in formation of nemaline rods.

A missense mutation in the desmin rod domain is associated with autosomal dominant distal myopathy, and exerts a dominant negative effect on filament formation

In some myopathies of distal onset, the intermediate filament desmin is abnormally accumulated in skeletal and cardiac muscle. We report the first point mutation in desmin cosegregating with an autosomal dominant form of desmin-related myopathy. The L345P desmin missense mutation occurs in a large, six generation Ashkenazi Jewish family. The mutation is located in an evolutionarily highly conserved position of the desmin coiled-coil rod domain important for dimer formation. L345P desmin is incapable of forming filamentous networks in transfected HeLa and SW13 cells. We conclude that the L345P desmin missense mutation causes myopathy by interfering in a dominant-negative manner with the dimerization-polymerization process of intermediate filament assembly.