The structure of the sarcomeric M band: localization of defined domains of myomesin, M-protein, and the 250-kD carboxy-terminal region of titin by immunoelectron microscopy.

The M band of vertebrate cross-striated myofibrils has remained an enigmatic structure. In addition to myosin thick filaments, two major structural proteins, myomesin and M-protein, have been localized to the M band. Also, titin is expected to be anchored in this structure. To begin to understand the molecular layout of these three proteins, a panel of 16 polyclonal and monoclonal antibodies directed against unique epitopes of defined sequence was assembled, and immunoelectron microscopy was used to locate the position of the epitopes at the sarcomere level. The results allow the localization and orientation of defined domains of titin, myomesin, and M-protein at high resolution. The 250-kD carboxy-terminal region of titin clearly enters the M band with the kinase domain situated approximately 52 nm from the central M1-line. The positions of three additional epitopes are compatible with the view that the titin molecule reaches approximately 60 nm into the opposite sarcomere half. Myomesin also seems to bridge the central M1-line and is oriented parallel to the long axis of the myofibril. The neighboring molecules are oriented in an antiparallel and staggered fashion. The amino-terminal portion of the protein, known to contain a myosin binding site, seems to adopt a specific three-dimensional arrangement. While myomesin is present in both slow and fast fibers, M-protein is restricted to fast fibers. It appears to be organized in a fundamentally different manner: the central portion of the polypeptide is around the M1-line, while the terminal epitopes seem to be arranged along thick filaments. This orientation fits the conspicuously stronger M1-lines in fast twitch fibers. Obvious implications of this model are discussed.

Indications for a novel muscular dystrophy pathway. gamma-filamin, the muscle-specific filamin isoform, interacts with myotilin.

gamma-Filamin, also called ABP-L, is a filamin isoform that is specifically expressed in striated muscles, where it is predominantly localized in myofibrillar Z-discs. A minor fraction of the protein shows subsarcolemmal localization. Although gamma-filamin has the same overall structure as the two other known isoforms, it is the only isoform that carries a unique insertion in its immunoglobulin (Ig)-like domain 20. Sequencing of the genomic region encoding this part of the molecule shows that this insert is encoded by an extra exon. Transient transfections of the insert-bearing domain in skeletal muscle cells and cardiomyocytes show that this single domain is sufficient for targeting to developing and mature Z-discs. The yeast two-hybrid method was used to identify possible binding partners for the insert-bearing Ig-like domain 20 of gamma-filamin. The two Ig-like domains of the recently described alpha-actinin-binding Z-disc protein myotilin were found to interact directly with this filamin domain, indicating that the amino-terminal end of gamma-filamin may be indirectly anchored to alpha-actinin in the Z-disc via myotilin. Since defects in the myotilin gene were recently reported to cause a form of autosomal dominant limb-girdle muscular dystrophy, our findings provide a further contribution to the molecular understanding of this disease.

Mapping of a myosin-binding domain and a regulatory phosphorylation site in M-protein, a structural protein of the sarcomeric M band.

The myofibrils of cross-striated muscle fibers contain in their M bands cytoskeletal proteins whose main function seems to be the stabilization of the three-dimensional arrangement of thick filaments. We identified two immunoglobin domains (Mp2-Mp3) of M-protein as a site binding to the central region of light meromyosin. This binding is regulated in vitro by phosphorylation of a single serine residue (Ser76) in the immediately adjacent amino-terminal domain Mp1. M-protein phosphorylation by cAMP-dependent kinase A inhibits binding to myosin LMM. Transient transfection studies of cultured cells revealed that the myosin-binding site seems involved in the targeting of M-protein to its location in the myofibril. Using the same method, a second myofibril-binding site was uncovered in domains Mp9-Mp13. These results support the view that specific phosphorylation events could be also important for the control of sarcomeric M band formation and remodeling.

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

INTRODUCTION: Myofibrillar myopathies are characterized by progressive muscle weakness and impressive abnormal protein aggregation in muscle fibers. In about 10 % of patients, the disease is caused by mutations in the MYOT gene encoding myotilin. The aim of our study was to decipher the composition of protein deposits in myotilinopathy to get new information about aggregate pathology. RESULTS: Skeletal muscle samples from 15 myotilinopathy patients were included in the study. Aggregate and control samples were collected from muscle sections by laser microdissection and subsequently analyzed by a highly sensitive proteomic approach that enables a relative protein quantification. In total 1002 different proteins were detected. Seventy-six proteins showed a significant over-representation in aggregate samples including 66 newly identified aggregate proteins. Z-disc-associated proteins were the most abundant aggregate components, followed by sarcolemmal and extracellular matrix proteins, proteins involved in protein quality control and degradation, and proteins with a function in actin dynamics or cytoskeletal transport. Forty over-represented proteins were evaluated by immunolocalization studies. These analyses validated our mass spectrometric data and revealed different regions of protein accumulation in abnormal muscle fibers. Comparison of data from our proteomic analysis in myotilinopathy with findings in other myofibrillar myopathy subtypes indicates a characteristic basic pattern of aggregate composition and resulted in identification of a highly sensitive and specific diagnostic marker for myotilinopathy. CONCLUSIONS: Our findings i) indicate that main protein components of aggregates belong to a network of interacting proteins, ii) provide new insights into the complex regulation of protein degradation in myotilinopathy that may be relevant for new treatment strategies, iii) imply a combination of a toxic gain-of-function leading to myotilin-positive protein aggregates and a loss-of-function caused by a shift in subcellular distribution with a deficiency of myotilin at Z-discs that impairs the integrity of myofibrils, and iv) demonstrate that proteomic analysis can be helpful in differential diagnosis of protein aggregate myopathies.

Xin-deficient mice display myopathy, impaired contractility, attenuated muscle repair and altered satellite cell functionality.

AIM: Xin is an F-actin-binding protein expressed during development of cardiac and skeletal muscle. We used Xin-/- mice to determine the impact of Xin deficiency on different aspects of skeletal muscle health, including functionality and regeneration. METHODS: Xin-/- skeletal muscles and their satellite cell (SC) population were investigated for the presence of myopathic changes by a series of histological and immunofluorescent stains on resting uninjured muscles. To further understand the effect of Xin loss on muscle health and its SCs, we studied SCs responses following cardiotoxin-induced muscle injury. Functional data were determined using in situ muscle stimulation protocol. RESULTS: Compared to age-matched wild-type (WT), Xin-/- muscles exhibited generalized myopathy and increased fatigability with a significantly decreased force recovery post-fatiguing contractions. Muscle regeneration was attenuated in Xin-/- mice. This impaired regeneration prompted an investigation into SC content and functionality. Although SC content was not different, significantly more activated SCs were present in Xin-/- vs. WT muscles. Primary Xin-/- myoblasts displayed significant reductions (approx. 50%) in proliferative capacity vs. WT; a finding corroborated by significantly decreased MyoD-positive nuclei in 3 days post-injury Xin-/- muscle vs. WT. As more activated SCs did not translate to more proliferating myoblasts, we investigated whether Xin-/- SCs displayed an exaggerated loss by apoptosis. More apoptotic SCs (TUNEL+/Pax7+) were present in Xin-/- muscle vs. WT. Furthermore, more Xin-/- myoblasts were expressing nuclear caspase-3 compared to WT at 3 days post-injury. CONCLUSION: Xin deficiency leads to a myopathic condition characterized by increased muscle fatigability, impaired regeneration and SC dysfunction.

Characterization of cardiotin, a structural component in the myocard.

The characterization and subcellular distribution of cardiotin, a structural component of striated muscle, is described using a monoclonal antibody. This high molecular mass component (> 300 kDa) is expressed in the myocard of several species and to a lesser extent also in skeletal muscle. Cardiotin is not found in smooth muscle tissues, other mesenchymal or epithelial tissues. The cardiotin distribution pattern is independent of other sarcomeric components, such as desmin, myosin, actin, titin, nebulin, and desmoplakin, and shows a longitudinal filamentous localization between myofibrils. The average distance between parallel running cardiotin filaments is approximately 2.3 microns, as concluded from confocal scanning laser microscopic analysis of double-immunolabeled muscle preparations. The cardiotin filamentous staining reaction is oriented perpendicularly to the typical cross-striations observed with antibodies to desmin, spanning several sarcomeres and showing a length between 12 to 80 microns in frozen sections. Its localization pattern suggests a possible link with the sarcoplasmic reticulum. We have never observed cardiotin filaments to cross the intercalated disks, stained by antibodies to desmoplakins or desmin. Cardiotin cannot be solubilized from cardiac muscle by nonionic detergents of high concentrations of KCl or KI, suggesting a structural role in the myocard. The protein could so far not be detected in developing embryonic heart, but expression seems to be initiated after birth, depending on the species examined.

Integration of titin into the sarcomeres of cultured differentiating human skeletal muscle cells.

Titin is amongst the first sarcomeric proteins to be detected in the process of myofibrillogenesis of striated muscle. During embryogenesis this high molecular weight protein is initially observed in a punctate staining pattern in immunohistochemical studies, while during maturation titin organizes into a cross-striated pattern. The dynamic process of titin assembly up to its integration into the sarcomeres of cultured human skeletal muscle cells has been studied in subsequent stages of differentiation with antibodies to four well-defined titin epitopes. Since in maturated muscle cells these epitopes are clearly distinguishable on the extended titin molecule we wondered how these epitopes reorganize during myofibrillogenesis, and whether such a reorganization would reveal important clues about its supramolecular organization during development. Immunofluorescence staining of postmitotic mononuclear myoblasts indicate that the investigated epitopes of the titin molecule are displayed in a punctate pattern with neighboring, but clearly separate spots in the cytoplasm of the cells. During elongation and fusion of the cells, these titin spots associate with stress fiber-like structures to finally reach their position at either the Z-line, the A-I junction or the A-band. We propose that during this transition the large titin molecule is unfolded, with the amino terminus of the molecule migrating in the direction of the Z-line and the carboxy terminus moving towards the M-line. In maturated, fused myotubes the final cross-striated patterns of all investigated titin epitopes are observed. While this process of unfolding of the titin molecule progresses, other compounds of the Z-line and the A-band migrate to their specific positions in the nascent sarcomere. A-band components such as sarcomeric myosin and C-protein, are also observed as dot-like aggregates during initial stages of muscle cell differentiation and organize into a cross-striated pattern in the sarcomere virtually simultaneously with titin. The Z-line associated component desmin organizes into a cross-striated pattern at a later stage.

Immunogold EM reveals a close association of plectin and the desmin cytoskeleton in human skeletal muscle.

Plectin is a multifunctional cytoskeletal linker protein with an intermediate filament-binding site and sequence elements with high homology to actin-binding domains. Mutations of the human plectin gene as well as the targeted inactivation of its murine analog cause a generalized blistering skin disorder and muscular dystrophy, thus implying its essential role in cells that are exposed to mechanical stress. In the present study we report the characterization of two new domain-specific plectin antibodies as well as ultrastructural localization of plectin in normal human skeletal muscle. Using immunogold electron microscopy, we localized plectin at three prominent sites: 1) Plectin is found at regularly spaced intervals along the cytoplasmic face of the plasma membrane. 2) It is distinctly localized at filamentous bridges between Z-lines of peripheral myofibrils and the sarcolemma and 3) at structures forming the intermyofibrillar scaffold. At the latter two locations, plectin and desmin were found to colocalize. Our ultrastructural analysis suggests that plectin may have a central role in the structural and functional organization of the intermediate filament cytoskeleton in mature human skeletal muscle.

Two immunoglobulin-like domains of the Z-disc portion of titin interact in a conformation-dependent way with telethonin.

The giant muscle protein titin/connectin plays a crucial role in myofibrillogenesis as a molecular ruler for sarcomeric protein sorting. We describe here that the N-terminal titin immunoglobulin domains Z1 and Z2 interact specifically with telethonin in yeast two-hybrid analysis and protein binding assays. Immunofluorescence with antibodies against the N-terminal region of titin and telethonin detects both proteins at the Z-disc of human myotubes. Longer titin fragments, comprising a serine-proline-rich phosphorylation site and the next domain, do not interact. The interaction of telethonin with titin is therefore conformation-dependent, reflecting a possible phosphorylation regulation during myofibrillogenesis.

Abnormal expression of intermediate filament proteins in X-linked myotubular myopathy is not reproduced in vitro.

Expression patterns of the intermediate filament proteins (IFPs) desmin and vimentin, in biopsy material taken from a 1 day old boy with fatal neonatal X-linked myotubular myopathy (XLMTM) were compared with the expression of these proteins in cultured myotubes, from the same patient. Immunohistochemical studies revealed the persistence of high levels of desmin in virtually all, and vimentin in most, of the myofibres within the patient's biopsy. Analysis of intermediate filament expression in differentiating, cultured muscle cells did not reveal overt differences between XLMTM cultures and cultures of control muscle. Titin distribution patterns indicated a normal process of myofibrillogenesis in XLMTM myotubes. We conclude that the failure to properly regulate IFP-expression is not intrinsic to XLMTM muscle fibres. The possibility that this failure is due to a defective external, possibly neural factor, is discussed.

Postnatal centralization of muscle fibre nuclei in centronuclear myopathy.

Postnatal centralization of muscle fibre nuclei, which were previously located subsarcolemmally, is described in a case of centronuclear myopathy (CNM) in a male patient with generalized muscle weakness since birth. A muscle biopsy was taken at the age of 11 months; no particular abnormalities were observed at this stage apart from an unusual variation in fibre size. A distinctly below average muscle fibre diameter, increased endomysial connective tissue, and features typical for CNM were found in a biopsy taken 9 yr later. Immunohistochemical studies using antibodies to desmin, vimentin, laminin and type IV collagen revealed altered staining patterns compared with normal fibres. The abnormalities in the patterns of cytoskeletal proteins point to a defective regulation of the composition and organization of the cytoskeletal network during development, paralleled by abnormalities in the extracellular matrix.

Induction of corticospinal target finding by release of a diffusible, chemotropic factor in cervical spinal grey matter.

The outgrowth of corticospinal tract axons in rat spinal cord primarily occurs during the first postnatal week. Axons originating from a group of layer V pyramidal cell bodies situated in the anterior part of the cerebral sensorimotor cortex project mainly to the cervical gray matter (Joosten et al., Dev. Brain Res., 36 (1987) 121-130). By co-culturing explants of the anterior part of the sensorimotor cortex and of cervical spinal gray matter in 3-D collagen gels, a target-specific directional growth of cortical axons towards the cervical spinal gray explant could be demonstrated. After retrograde filling with the fluorescent tracer 1,1-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate (DiI), in vivo as well as in vitro, most of the DiI-labelled cortical neurons were located in layer V of the cortical explant, and were characterized by a pyramidal shape. These data suggest that the cervical spinal gray matter target area becomes innervated by corticospinal axons through the release of a diffusible chemotropic factor.

Chlorpromazine inhibition of muscarinic-cholinergic responses in the rat parotid gland.

The ability of chlorpromazine (CPZ) to inhibit muscarinic-cholinergic secretory events was studied in vitro in rat parotid acinar cells. CPZ inhibited carbachol-induced amylase release in a dose-dependent fashion but had no effect on that elicited by isoproterenol. The inhibition of parotid protein synthesis induced by carbachol, but not that induced by A23187, was blocked by CPZ. CPZ exhibited a dose-dependent inhibition of [3H] quinuclidinyl benzilate (QNB) binding to muscarinic receptors, and altered the KD of the receptor for the ligand. These results are consistent with an ability of CPZ to inhibit muscarinic-cholinergic-induced salivary secretion by complex interference with receptor binding. In addition, CPZ may block parotid-muscarinic responses by impeding a post-receptor signaling step which is proximal to Ca2+ mobilization.

Immunophenotyping of congenital myopathies: disorganization of sarcomeric, cytoskeletal and extracellular matrix proteins.

We have studied the expression and distribution patterns of the intermediate filament proteins desmin and vimentin, the sarcomere components titin, nebulin and myosin, the basement membrane constituents collagen type IV and laminin, and the reticular layer component collagen type VI in skeletal muscle of patients with "classic" congenital myopathies (CM), using indirect immunofluorescence assays. In all biopsy specimens obtained from patients with central core disease (CCD), nemaline myopathy (NM), X-linked myotubular myopathy (XLMTM) and centronuclear myopathy (CNM), disease-specific desmin disturbances were observed. Vimentin was present in immature fibres in severe neonatal NM, and as sarcoplasmic aggregates in one case of CNM, while the amounts of vimentin and embryonic myosin, observed in XLMTM, decreased with age of the patients. Abnormal expression of myosin isoforms was found in several CM biopsies, although the organization of myosin and other sarcomere components was rarely disturbed. Basement membrane and reticular layer proteins were often prominently increased in severe cases of CM. We conclude that (i) desmin is a marker for individual types of CM and might be used for diagnostic purposes; (ii) the expression patterns of the differentiation markers desmin, vimentin and embryonic myosin in XLMTM, point either to a postnatal muscle fibre maturation or to a variable time-point of maturational arrest in individual patients; (iii) the correlation between the distribution patterns of extracellular matrix proteins and clinical presentation points to a role of these proteins in pathophysiology of CM.

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.

Increased expression of a 68-kDa protein in the corpus cavernosum of some men with erectile dysfunction.

Erectile dysfunction (ED) may be caused by abnormalities of intracavernous penile structures. In order to investigate whether specific proteins could be identified that might be related to ED, the composition of structural proteins in cavernous tissues of patients with ED was compared to that of normal cavernous tissues by gel electrophoresis. Increased expression of a 68-kDa nonionic detergent extraction-resistant protein was demonstrated in tissues of more than half of the patients with vasculogenic ED, whereas only one out of nine normal cavernous tissues showed the same phenomenon. Increased expression was not related to a specific type of vascular insufficiency, aging, or diabetic constituency. Histochemical and immunochemical studies revealed that the increased amount of the 68-kDa protein is not merely the result of a surplus of nervous, smooth muscle, or elastic tissues. Furthermore, antibodies specific for 68-kDa neurofilament and 62- to 67.5-kDa tropoelastin did not recognize the 68-kDa protein on Western blots. The possibility that the 68-kDa protein may help us understand the etiology of certain cases of erectile dysfunction is discussed.

Xerostomia: evaluation of a symptom with increasing significance.

Xerostomia is the subjective sensation of oral dryness. Although it is most commonly associated with salivary gland dysfunction, it may also occur with normal gland activity. Xerostomia may be an early symptom of several morbid systemic conditions with important implications for the medical and dental management of patients. Oral dryness also has negative effects on an individual's emotional well-being and quality of life. The complaint of xerostomia necessitates a complete evaluation of a patient's general health, salivary gland function, and oral motor and sensory abilities. The salivary gland assessment includes symptom review, analysis of glandular secretions, scintiscanning, and minor labial gland biopsy. No single component is sufficient to adequately diagnose the presence, extent, or cause of salivary dysfunction. Treatment of a dry mouth, to date, is mainly palliative in nature, with the intent of preserving oral structures and functions. Better therapies are essential in the management of xerostomia, whatever the cause. The importance of xerostomia as a symptom is increasingly recognized in medicine and dentistry. The dentist is commonly the first health professional to hear this complaint and may be critical in directing a full and appropriate evaluation.

Case challenge: painful swelling of the temporomandibular joint.

A 59-year old woman was referred by her dentist to an Oral Medicine practice for evaluation of acute pain and swelling in her right temporomandibular joint (TMJ).

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.