Synemin may function to directly link muscle cell intermediate filaments to both myofibrillar Z-lines and costameres

Roles for the stem cell associated intermediate filament Nestin in prostate cancer migration and metastasis

The intermediate filament protein Nestin identifies stem/progenitor cells in adult tissues, but the function of Nestin is poorly understood. We investigated Nestin expression and function in common lethal cancers. Nestin mRNA was detected in cell lines from small cell lung, and breast cancers, and particularly elevated in cell lines derived from prostate cancer metastases. Whereas the androgen-independent lines PC3, 22RV1, and DU145 all expressed Nestin transcripts under standard culture conditions, the androgen-dependent line LnCaP expressed Nestin only on androgen withdrawal. We confirmed associations of Nestin expression, androgen withdrawal, and metastatic potential by immunohistochemical analysis of samples from 254 prostate cancer patients. Cytoplasmic Nestin protein was readily identifiable in prostate cancer cells from 75% of patients with lethal androgen-independent disease, even in cancer sampled from the prostate itself. However, Nestin expression was undetectable in localized androgen-deprived tumors and in metastases without prior androgen deprivation. To address its function, we reduced Nestin levels with short hairpin RNAs, markedly inhibiting in vitro migration and invasion in prostate cancer cells but leaving cell growth intact. Nestin knockdown also diminished metastases 5-fold compared with controls despite uncompromised tumorigenicity at the site of inoculation. These results specify a function for Nestin in cell motility and identify a novel pathway for prostate cancer metastasis. Activity of this pathway may be selected by the extraprostatic environment or, as supported by our data, may originate within the prostate after androgen deprivation. Further dissection of this novel Nestin migration pathway may lead to strategies to prevent and neutralize metastatic spread.

Molecular evolution of type VI intermediate filament proteins

BACKGROUND

Tanabin, transitin and nestin are type VI intermediate filament (IF) proteins that are developmentally regulated in frogs, birds and mammals, respectively. Tanabin is expressed in the growth cones of embryonic vertebrate neurons, whereas transitin and nestin are found in myogenic and neurogenic cells. Another type VI IF protein, synemin, is expressed in undifferentiated and mature muscle cells of birds and mammals. In addition to an IF-typical alpha-helical core domain, type VI IF proteins are characterized by a long C-terminal tail often containing distinct repeated motifs. The molecular evolution of type VI IF proteins remains poorly studied.

RESULTS

To examine the evolutionary history of type VI IF proteins, sequence comparisons, BLAST searches, synteny studies and phylogenic analyses were performed. This study provides new evidence that tanabin, transitin and nestin are indeed orthologous type VI IF proteins. It demonstrates that tanabin, transitin and nestin genes share intron positions and sequence identities, have a similar chromosomal context and display closely related positions in phylogenic analyses. Despite this homology, fast evolution rates of their C-terminal extremity have caused the appearance of repeated motifs with distinct biological activities. In particular, our in silico and in vitro analyses of their tail domain have shown that (avian) transitin, but not (mammalian) nestin, contains a repeat domain displaying nucleotide hydrolysis activity.

CONCLUSION

These analyses of the evolutionary history of the IF proteins fit with a model in which type VI IFs form a branch distinct from NF proteins and are composed of two major proteins: synemin and nestin orthologs. Rapid evolution of the C-terminal extremity of nestin orthologs could be responsible for their divergent functions.

Intermediate filament scaffolds fulfill mechanical, organizational, and signaling functions in the cytoplasm

Intermediate filaments (IFs) are cytoskeletal polymers whose protein constituents are encoded by a large family of differentially expressed genes. Owing in part to their properties and intracellular organization, IFs provide crucial structural support in the cytoplasm and nucleus, the perturbation of which causes cell and tissue fragility and accounts for a large number of genetic diseases in humans. A number of additional roles, nonmechanical in nature, have been recently uncovered for IF proteins. These include the regulation of key signaling pathways that control cell survival, cell growth, and vectorial processes including protein targeting in polarized cellular settings. As this discovery process continues to unfold, a rationale for the large size of this family and the context-dependent regulation of its members is finally emerging.

Cytoplasmic gamma-actin expression in diverse animal models of muscular dystrophy

We recently showed that cytoplasmic gamma-actin (gamma(cyto)-actin) is dramatically elevated in striated muscle of dystrophin-deficient mdx mice. Here, we demonstrate that gamma(cyto)-actin is markedly increased in golden retriever muscular dystrophy (GRMD), which better recapitulates the dystrophinopathy phenotype in humans. Gamma(cyto)-Actin was also elevated in muscle from alpha-sarcoglycan null mice, but not in several other dystrophic animal models, including mice deficient in beta-sarcoglycan, alpha-dystrobrevin, laminin-2, or alpha7 integrin. Muscle from mice lacking dystrophin and utrophin also expressed elevated gamma(cyto)-actin, which was not restored to normal by transgenic overexpression of alpha7 integrin. However, gamma(cyto)-actin was further elevated in skeletal muscle from GRMD animals treated with the glucocorticoid prednisone at doses shown to improve the dystrophic phenotype and muscle function. These data suggest that elevated gamma(cyto)-actin is part of a compensatory cytoskeletal remodeling program that may partially stabilize dystrophic muscle in some cases where the dystrophin-glycoprotein complex is compromised.

Muscle intermediate filaments and their links to membranes and membranous organelles

Intermediate filaments (IFs) play a key role in the integration of structure and function of striated muscle, primarily by mediating mechanochemical links between the contractile apparatus and mitochondria, myonuclei, the sarcolemma and potentially the vesicle trafficking apparatus. Linkage of all these membranous structures to the contractile apparatus, mainly through the Z-disks, supports the integration and coordination of growth and energy demands of the working myocyte, not only with force transmission, but also with de novo gene expression, energy production and efficient protein and lipid trafficking and targeting. Desmin, the most abundant and intensively studied muscle intermediate filament protein, is linked to proper costamere organization, myoblast and stem cell fusion and differentiation, nuclear shape and positioning, as well as mitochondrial shape, structure, positioning and function. Similar links have been established for lysosomes and lysosome-related organelles, consistent with the presence of widespread links between IFs and membranous structures and the regulation of their fusion, morphology and stabilization necessary for cell survival.

Synemin is expressed in reactive astrocytes in neurotrauma and interacts differentially with vimentin and GFAP intermediate filament networks

Immature astrocytes and astrocytoma cells contain synemin and three other intermediate filament (IF) proteins: glial fibrillary acidic protein (GFAP), vimentin and nestin. Here, we show that, after neurotrauma, reactive astrocytes produce synemin and thus propose synemin as a new marker of reactive astrocytes. Comparison of synemin mRNA and protein levels in brain tissues and astrocyte cultures from wild-type, Vim-/- and Gfap-/-Vim-/- mice showed that in the absence of vimentin, synemin protein was undetectable although synemin mRNA was present at wild-type levels. By contrast, in Gfap-/- astrocytes, synemin protein and mRNA levels, as well as synemin incorporation into vimentin IFs, were unaltered. Biochemical assays with purified proteins suggested that synemin interacts with GFAP IFs like an IF-associated protein rather than like a polymerization partner, whereas the opposite was true for synemin interaction with vimentin. In transfection experiments, synemin did not incorporate into normal, filamentous GFAP networks, but integrated into vimentin and GFAP heteropolymeric networks. Thus, alongside GFAP, vimentin and nestin, reactive astrocytes contain synemin, whose accumulation is suppressed post-transcriptionally in the absence of a polymerization partner. In astrocytes, this partner is vimentin and not GFAP, which implies a functional difference between these two type III IF proteins.

Plectin 1f scaffolding at the sarcolemma of dystrophic (mdx) muscle fibers through multiple interactions with beta-dystroglycan

In skeletal muscle, the cytolinker plectin is prominently expressed at Z-disks and the sarcolemma. Alternative splicing of plectin transcripts gives rise to more than eight protein isoforms differing only in small N-terminal sequences (5-180 residues), four of which (plectins 1, 1b, 1d, and 1f) are found at substantial levels in muscle tissue. Using plectin isoform-specific antibodies and isoform expression constructs, we show the differential regulation of plectin isoforms during myotube differentiation and their localization to different compartments of muscle fibers, identifying plectins 1 and 1f as sarcolemma-associated isoforms, whereas plectin 1d localizes exclusively to Z-disks. Coimmunoprecipitation and in vitro binding assays using recombinant protein fragments revealed the direct binding of plectin to dystrophin (utrophin) and beta-dystroglycan, the key components of the dystrophin-glycoprotein complex. We propose a model in which plectin acts as a universal mediator of desmin intermediate filament anchorage at the sarcolemma and Z-disks. It also explains the plectin phenotype observed in dystrophic skeletal muscle of mdx mice and Duchenne muscular dystrophy patients.

Different expression of synemin isoforms in glia and neurons during nervous system development

The synemin gene encodes proteins belonging to the intermediate filament family. These proteins confer resistance to mechanical stress and modulate cell shape. Three synemin isoforms, of 180 (H), 150 (M) and 41 (L) kDa, are produced by alternative splicing of the pre-mRNA and are regulated differently during development. The three isoforms differ in their C-terminal tail domains, while their IF rod domains are identical. Synemins H/M occurred together with nestin and vimentin in glial progenitors during the early differentiation of the developing mouse central nervous system. They are later found in GFAP-labeled cells. In contrast, the L isoform appeared only in neurons, together with neurofilaments and betaIII-tubulin in the brain after birth. However, synemin L appeared from E13 in the peripheral nervous system, where it was confined to the neurons of spinal ganglia. In the meantime, the synemin H/M isoforms were found in both the neurons and Schwann cells of the sensorial ganglia from E11. Tissue fractionation and purification of IFs from adult mouse spinal cord revealed that the synemin L isoform binds to neurofilaments associated with the membrane compartment. This report describes the synthesis of the three synemin isoforms by selective cell types, and their temporal and spatial distributions. Mechanisms specific to neurons and glia probably control the splicing of the common synemin mRNA and the synthesis of each synemin isoform.

Interactions of intermediate filament protein synemin with dystrophin and utrophin

Synemin is a unique, very large intermediate filament (IF) protein present in all types of muscle cells, which forms heteropolymeric intermediate filaments (IFs) with the major IF proteins desmin and/or vimentin. We show herein that tissue-purified avian synemin directly interacts with both dystrophin and utrophin, and that specific expressed regions of both of the mammalian (human) synemin isoforms (alpha-synemin and beta-synemin) directly interact with specific expressed domains/regions of the dystrophin and utrophin molecules. Mammalian synemin is also shown to colocalize with dystrophin within muscle cell cultures. These results indicate that synemin is an important IF protein in muscle cells that helps fortify the linkage between the peripheral layer of cellular myofibrils and the costameric regions located along the sarcolemma and the sarcolemma region located within the neuromuscular and myotendinous junctions (NMJs and MTJs).

Synemin expression is widespread in liver fibrosis and is induced in proliferating and malignant biliary epithelial cells

The expression profile of intermediate filament proteins provides valuable information on the differentiation of specific cell populations and their contributions to disease. Synemin is one of the few intermediate filament proteins whose expression pattern during pathological situations is poorly characterized. We conducted a systematic immunohistochemical investigation of synemin expression in human liver diseases. In normal liver and in the early prefibrotic phase of chronic viral hepatitis or steatohepatitis, synemin was localized in hepatic stellate cells (HSCs) and vascular cells. Fibrotic or cirrhotic liver disease promoted intense synemin staining of HSCs in parenchymal and fibrous zones. In portal tract fibroblasts, synemin expression was rare under normal conditions but was widespread in severe inflammatory diseases associated with portal expansion, consistent with the notion that some fibrotic reactions involve HSCs, whereas others involve both HSCs and portal fibroblasts. Most sinusoidal endothelial cells were synemin negative in normal liver but were positive in hepatocellular carcinomas. Synemin was also expressed in the epithelial component of the ductular reaction in various liver diseases and in cholangiocarcinoma cells but not in hepatocellular carcinoma cells. Myofibroblasts in stromal reaction to carcinomas were synemin positive. Thus, synemin helps delineate different types of liver fibrotic reactions and provides a marker for sinusoidal capillarization and for proliferating biliary epithelial and cholangiocarcinoma cells.

The intermediate filament protein, synemin, is an AKAP in the heart

Targeting of protein kinase A (PKA) by A-kinase anchoring proteins (AKAPs) contributes to high specificity of PKA signaling pathways. PKA phosphorylation of myofilament and cytoskeletal proteins may regulate myofibrillogenesis and myocyte remodeling during heart disease; however, known cardiac AKAPs do not localize to these regions. To identify novel AKAPs which target PKA to the cytoskeleton or myofilaments, a human heart cDNA library was screened and the intermediate filament (IF) protein, synemin, was identified as a putative RII (PKA regulatory subunit type II) binding protein. A predicted RII binding region was mutated and resulted in loss of RII binding. Furthermore, synemin co-localized with RII in SW13/cl.1-vim+ cells and co-immunoprecipitated with RII from adult rat cardiomyocytes. Synemin was localized at the level of Z-lines with RII and desmin in adult hearts, however, neonatal cardiomyocytes showed differential synemin and desmin localization. Quantitative Western blots also showed significantly more synemin was present in failing human hearts. We propose that synemin provides temporal and spatial targeting of PKA in adult and neonatal cardiac myocytes.

Dystrophin, its interactions with other proteins, and implications for muscular dystrophy

Duchenne muscular dystrophy is the most prevalent and severe form of human muscular dystrophy. Investigations into the molecular basis for Duchenne muscular dystrophy were greatly facilitated by seminal studies in the 1980s that identified the defective gene and its major protein product, dystrophin. Biochemical studies revealed its tight association with a multi-subunit complex, the so-named dystrophin-glycoprotein complex. Since its description, the dystrophin-glycoprotein complex has emerged as an important structural unit of muscle and also as a critical nexus for understanding a diverse array of muscular dystrophies arising from defects in several distinct genes. The dystrophin homologue utrophin can compensate at the cell/tissue level for dystrophin deficiency, but functions through distinct molecular mechanisms of protein-protein interaction.

Photoradiation could influence the cytoskeleton organization and inhibit the survival of human hepatoma cells in vitro

Low-power laser therapy has become popular in clinical applications including promoting wound healing and pain relief. However, effects of this photoradiation on human hepatoma cells are rarely studied. Previously, we found 808 nm gallium aluminum arsenide (GaAlAs) continuous wave laser had an inhibitory effect on the proliferation of human hepatoma cell lines HepG2 and J-5 at the energy density of 5.85 and 11.7 J/cm(2), respectively. The aim of the present study was to evaluate the possible mechanism of action of this photoradiation on HepG2 and J-5 cells. HepG2 and J-5 cells were cultured in 24-well plates for 24 h. After photoradiation by 130 mW 808 nm GaAlAs continuous wave laser for different time intervals (0, 30, 60, 90, 120, 150, and 180 s), Western blot and immunofluorescent staining were used to examine the expression and distribution of histone and cytoskeletal proteins. The cell counts as well as histone and synemin expression of HepG2 and J-5 cells were reduced by photoradiation at the energy density of 5.85 and 11.7 J/cm(2), respectively. Furthermore, the architecture of cytoskeletons and the distribution of intermediate filament-associated proteins (plectin and synemin) were disorganized by photoradiation. Photoradiation by 808 nm GaAlAs continuous wave laser at the energy density of 5.85 and 7.8 J/cm(2) inhibited the survival of human hepatoma cell lines. The mechanism might reduce synthesis of histone and synemin. Reduced histone synthesis might further reduce the proliferation rate of these cells. Reduced synemin synthesis might result in the destruction of the cytoskeleton. Therefore, the net effects by this photoradiation were reduced cell survival.

Nanoscale intracellular organization and functional architecture mediating cellular behavior

Cells function based on a complex set of interactions that control pathways resulting in ultimate cell fates including proliferation, differentiation, and apoptosis. The inter-workings of this immensely dense network of intracellular molecules are influenced by more than random protein and nucleic acid distribution where their interactions culminate in distinct cellular function. By probing the design of these biological systems from an engineering perspective, researchers can gain great insight that will aid in building and utilizing systems that are on this size scale where traditional large-scale rules may fail to apply. The organized interaction and gradient distribution in intracellular space imply a structural architecture that modulates cellular processes by influencing biochemical interactions including transport and binding-reactions. One significant structure that plays a role in this modulation is the cell cytoskeleton. Here, we discuss the cytoskeleton as a central and integrating functional structure in influencing cell processes and we describe technology useful for probing this structure. We explain the nanometer scale science of cytoskeletal structure with respect to intracellular organization, mechanotransduction, cytoskeletal-associated proteins, and motor molecules, as well as nano- and microtechnologies that are applicable for experimental studies of the cytoskeleton. This biological architecture of the cytoskeleton influences molecular, cellular, and physiological processes through structured multimodular and hierarchical principles centered on these functional filaments. Through investigating these organic systems that have evolved over billions of years, understanding in biology, engineering, and nanometer-scaled science will be advanced.

Severe muscle disease-causing desmin mutations interfere with in vitro filament assembly at distinct stages

Desmin is the major intermediate filament (IF) protein of muscle. Recently, mutations of the desmin gene have been reported to cause familial or sporadic forms of human skeletal, as well as cardiac, myopathy, termed desmin-related myopathy (DRM). The impact of any of these mutations on filament assembly and integration into the cytoskeletal network of myocytes is currently not understood, despite the fact that all cause the same histopathological defect, i.e., desmin aggregation. To gain more insight into the molecular basis of this process, we investigated how mutations within the alpha-helical rod domain of desmin affect both the assembly of the recombinant protein in vitro as well as the filament-forming capacity in cDNA-transfected cells. Whereas 6 of 14 mutants assemble into seemingly normal IFs in the test tube, the other mutants interfere with the assembly process at distinct stages, i.e., tetramer formation, unit-length filament (ULF) formation, filament elongation, and IF maturation. Correspondingly, the mutants with in vitro assembly defects yield dot-like aggregates in transfected cells, whereas the mutants that form IFs constitute a seemingly normal IF cytoskeleton in the cellular context. At present, it is entirely unclear why the latter mutant proteins also lead to aggregate formation in myocytes. Hence, these findings may be a starting point to dissect the contribution of the individual subdomains for desmin pathology and, eventually, the development of therapeutic interventions.

Specific interaction of the actin-binding domain of dystrophin with intermediate filaments containing keratin 19

Cytokeratins 8 and 19 concentrate at costameres of striated muscle and copurify with the dystrophin-glycoprotein complex, perhaps through the interaction of the cytokeratins with the actin-binding domain of dystrophin. We overexpressed dystrophin's actin-binding domain (Dys-ABD), K8 and K19, as well as closely related proteins, in COS-7 cells to assess the basis and specificity of their interaction. Dys-ABD alone associated with actin microfilaments. Expressed with K8 and K19, which form filaments, Dys-ABD associated preferentially with the cytokeratins. This interaction was specific, as the homologous ABD of betaI-spectrin failed to interact with K8/K19 filaments, and Dys-ABD did not associate with desmin or K8/K18 filaments. Studies in COS-7 cells and in vitro showed that Dys-ABD binds directly and specifically to K19. Expressed in muscle fibers in vivo, K19 accumulated in the myoplasm in structures that contained dystrophin and spectrin and disrupted the organization of the sarcolemma. K8 incorporated into sarcomeres, with no effect on the sarcolemma. Our results show that dystrophin interacts through its ABD with K19 specifically and are consistent with the idea that cytokeratins associate with dystrophin at the sarcolemma of striated muscle.

Intermediate filament protein synemin is present in human reactive and malignant astrocytes and associates with ruffled membranes in astrocytoma cells

Synemin, a very unique type VI intermediate filament (IF) protein, exhibits alternative splice variants termed alpha and beta. Unlike other IF proteins, synemin binds to actin-associated proteins, including alpha-actinin, vinculin, and alpha-dystrobrevin. Our previous work has demonstrated the presence of synemin in differentiating astrocytes. In this study, we have examined the presence of synemin in human astrocytes under pathological conditions, using rabbit antibodies raised against the C-terminal domain of human synemin produced in bacteria. Western blotting shows that astrocytic tumors contain greater amounts of alpha-synemin than do normal brain tissues. These tumors also contain beta-synemin, which is not detectable in normal brain. Immunohistochemistry demonstrates that, while synemin is present in normal adult brain only in vascular smooth muscle cells, it is newly synthesized by reactive and neoplastic astrocytes. Alpha- and beta-Synemins have also been detected by Western blotting and polymerase chain reaction in several human glioblastoma cell lines. In these cell lines, surprisingly, synemin is associated with ruffled membranes in addition to being distributed along the IF network. In ruffled membranes, synemin was found to co-localize with alpha-actinin. This unusual cellular localization for an IF protein is maintained after nocodazole-induced perinuclear coiling of the vimentin IF network. In addition, immunoprecipitation experiments demonstrate that synemin forms a complex with alpha-actinin in glioblastoma cells. Taken together with synemin localization within ruffled membranes, this finding suggests that synemin plays a role in motility of glioblastoma cells.

The biology of desmin filaments: how do mutations affect their structure, assembly, and organisation?

Desmin, the major intermediate filament (IF) protein of muscle, is evolutionarily highly conserved from shark to man. Recently, an increasing number of mutations of the desmin gene has been described to be associated with human diseases such as certain skeletal and cardiac myopathies. These diseases are histologically characterised by intracellular aggregates containing desmin and various associated proteins. Although there is progress regarding our knowledge on the cellular function of desmin within the cytoskeleton, the impact of each distinct mutation is currently not understood at all. In order to get insight into how such mutations affect filament assembly and their integration into the cytoskeleton we need to establish IF structure at atomic detail. Recent progress in determining the dimer structure of the desmin-related IF-protein vimentin allows us to assess how such mutations may affect desmin filament architecture.

Pathogenic effects of a novel heterozygous R350P desmin mutation on the assembly of desmin intermediate filaments in vivo and in vitro

Mutations of the human desmin gene on chromosome 2q35 cause a familial or sporadic form of skeletal myopathy frequently associated with cardiac abnormalities. Here, we report the pathogenic effects of a novel heterozygous R350P desmin missense mutation, which resides in the evolutionary highly conserved coil 2B domain of the alpha-helical coiled-coil desmin rod domain, on the assembly of desmin intermediate filaments (IF) in cultured cells and in vitro. By transfection experiments, we show that R350P desmin is incapable of de novo formation of a desmin IF network in vimentin-free BMGE+H, MCF7 and SW13 cells and that it disrupts the endogenous vimentin cytoskeleton in 3T3 fibroblast cells. Hence, transfected cells displayed abnormal cytoplasmic protein aggregates reminiscent of desmin-positive protein deposits seen in the immunohistochemical and ultrastructural analysis of skeletal muscle derived from the index patient of the affected family. To study the functional effects of the R350P desmin mutation at the protein level, we performed in vitro assembly studies with wild-type (WT) and mutant desmin protein. Our analysis revealed that the in vitro assembly process of R350P desmin is already disturbed at the unit length filament level and that further association reactions generate huge, tightly packed protein aggregates. On assessing the pathogenic effects of R350P desmin in various mixtures with WT desmin, we show that a ratio of 1 : 3 (R350P desmin/WT desmin) is sufficient to effectively block the normal polymerization process of desmin IFs. Our findings indicate that the heterozygous R350P desmin mutation exerts a dominant negative effect on the ordered lateral arrangement of desmin subunits. This disturbance of the lateral packing taking place in the first phase of assembly is ultimately leading to abnormal protein aggregation.

Intermediate filament associated proteins

Intermediate filament associated proteins (IFAPs) coordinate interactions between intermediate filaments (IFs) and other cytoskeletal elements and organelles, including membrane-associated junctions such as desmosomes and hemidesmosomes in epithelial cells, costameres in striated muscle, and intercalated discs in cardiac muscle. IFAPs thus serve as critical connecting links in the IF scaffolding that organizes the cytoplasm and confers mechanical stability to cells and tissues. However, in recent years it has become apparent that IFAPs are not limited to structural crosslinkers and bundlers but also include chaperones, enzymes, adapters, and receptors. IF networks can therefore be considered scaffolding upon which associated proteins are organized and regulated to control metabolic activities and maintain cell homeostasis.

Desminopathies in muscle disease

A recently identified class of myopathies is produced by abnormal desmin, and is characterized by a disorganization of the desmin filament network, the accumulation of insoluble desmin-containing aggregates, and destructive changes in the sarcomeric organization of striated muscles. The desmin filaments interact with various other cytoskeletal proteins. The distinct clinical phenotypes are heterogeneous, with progressive skeletal myopathy, cardiomyopathy, and respiratory insufficiency as the most prominent features. Most of the desmin mutations are autosomal dominant. Identification of the causal genetic mutations shows that the desmin gene is not the only gene implicated in desminopathies; other genes encoding desmin-associated proteins, such as alpha-B-crystallin, and synemin may also be involved. Patients with mutations in their alpha-B-crystallin gene, which produce similar skeletal and cardiac myopathies, also have opaque lenses. Knockout mice have helped to reveal the fundamental role of desmin filaments in cell architecture, sarcomere alignment, myofibril organization, and the distribution of mitochondria. Transgenic mice, which accumulate aggregates of desmin and associated proteins in their muscles, show that the loss of desmin intermediate function as a result of mutations in desmin itself, or in the desmin-associated constituents, is important for disease progression.

Desmin: molecular interactions and putative functions of the muscle intermediate filament protein

Desmin is the intermediate filament (IF) protein occurring exclusively in muscle and endothelial cells. There are other IF proteins in muscle such as nestin, peripherin, and vimentin, besides the ubiquitous lamins, but they are not unique to muscle. Desmin was purified in 1977, the desmin gene was characterized in 1989, and knock-out animals were generated in 1996. Several isoforms have been described. Desmin IFs are present throughout smooth, cardiac and skeletal muscle cells, but can be more concentrated in some particular structures, such as dense bodies, around the nuclei, around the Z-line or in costameres. Desmin is up-regulated in muscle-derived cellular adaptations, including conductive fibers in the heart, electric organs, some myopathies, and experimental treatments with drugs that induce muscle degeneration, like phorbol esters. Many molecules have been reported to associate with desmin, such as other IF proteins (including members of the membrane dystroglycan complex), nebulin, the actin and tubulin binding protein plectin, the molecular motor dynein, the gene regulatory protein MyoD, DNA, the chaperone alphaB-crystallin, and proteases such as calpain and caspase. Desmin has an important medical role, since it is used as a marker of tumors' origin. More recently, several myopathies have been described, with accumulation of desmin deposits. Yet, after almost 30 years since its identification, the function of desmin is still unclear. Suggested functions include myofibrillogenesis, mechanical support for the muscle, mitochondrial localization, gene expression regulation, and intracellular signaling. This review focuses on the biochemical interactions of desmin, with a discussion of its putative functions.

Cloning and characterization of cytokeratins 8 and 19 in adult rat striated muscle. Interaction with the dystrophin glycoprotein complex

We used degenerate primers for the amino- and carboxyl-terminal ends of the rod domains of intermediate filament proteins in reverse transcriptase-PCR experiments to identify and clone cytokeratins 8 and 19 (K8 and K19) from cardiac muscle of the adult rat. Northern blots showed that K8 has a 2.2-kb transcript and K19 has a 1.9-kb transcript in both adult cardiac and skeletal muscles. Immunolocalization of the cytokeratins in adult cardiac muscle with isoform-specific antibodies for K8 and K19 showed labeling at Z-lines within the muscle fibers and at Z-line and M-line domains at costameres at the sarcolemmal membrane. Dystrophin and K19 could be co-immunoprecipitated and co-purified from extracts of cardiac muscle, suggesting a link between the cytokeratins and the dystrophin-based cytoskeleton at the sarcolemma. Furthermore, transfection experiments indicate that K8 and K19 may associate with dystrophin through a specific interaction with its actin-binding domain. Consistent with this observation, the cytokeratins are disrupted at the sarcolemmal membrane of skeletal muscle of the mdx mouse that lacks dystrophin. Together these results indicate that at least two cytokeratins are expressed in adult striated muscle, where they may contribute to the organization of both the myoplasm and sarcolemma.

Desmin-related myopathy: clinical, electrophysiological, radiological, neuropathological and genetic studies

Ten Spanish patients from six unrelated families diagnosed with desmin-related myopathy (DRM) were studied. The pattern of DRM inheritance was autosomal dominant in three families, autosomal recessive in one, and there was no family history in two cases. The disease onset was in early adulthood. Cardiac myopathy was the initial presentation in two patients, respiratory insufficiency in one, and lower limb weakness in all others. Cardiac involvement was observed in four patients. Lens opacities were found in four. CK level was normal or slightly elevated, and electrophysiological examination was consistent with myopathy. Muscle biopsies identified intracytoplasmic desmin-immunoreactive inclusions. In addition to desmin, synemin, actin, gelsolin, ubiquitin, alphaB-crystallin and amyloid betaA4 were also present in the deposits. Ultrastructural examination revealed areas of myofibrillary disruption, abnormal electron-dense structures and accumulations of granulofilamentous material. A missense R406W mutation and a novel single amino acid deletion in the desmin gene were identified in two patients; the other patients did not show mutations in desmin, synemin, syncoilin or alphaB-crystallin genes. Analysis of 10 Spanish DRM cases illustrates a wide clinical, myopathological and genetic spectrum of DRM, reinforcing the need for further exploration of genetic causes for this group of disorders.

?-synemin localizes to regions of high stress in human skeletal myofibers

Synemin is an intermediate filament protein shown previously to interact with alpha-dystrobrevin and desmin. Immunoblot analysis detects a beta-synemin protein of 170 kDa in human skeletal muscle and an alpha-synemin protein of 225 kDa in monkey brain. Low-resolution immunohistochemical analysis localizes beta-synemin within muscle along the sarcolemma, whereas confocal microscopic analysis further refines localization to the costamere and muscle Z-lines. In addition to these locations, beta-synemin is also enriched at the neuromuscular and myotendinous junctions, other regions that undergo high stress during myofiber contraction. Based on its localization and its expression pattern, it is proposed that beta-synemin functions as a structural protein involved in maintaining muscle integrity through its interactions with alpha-dystrobrevin, desmin, and other structural proteins.

Expression of the intermediate filament protein synemin in myofibrillar myopathies and other muscle diseases

Synemin is a member of the intermediate protein superfamily. Previous studies in avian and rodent skeletal and cardiac muscles have demonstrated that synemin localises at the Z-band, where it associates with desmin and alpha-actinin. In the present study, the distribution of synemin was examined using immunohistochemistry in muscle biopsy specimens from patients suffering from myofibrillar myopathy (MM, n=6), dermatomyositis (DM, n=3), inclusion body myositis (IBM, n=5), oculopharyngeal muscular dystrophy (OPD, n=3) and denervation atrophy (DA, n=3), to investigate the possible participation of this protein in the pathogenesis of various muscular diseases. Of patients affected by MM, two showed the presence of mutations in the desmin gene; none had mutations in the alphaB-crystallin gene; and no mutations were identified in synemin or syncoilin genes of three patients. Synemin immunohistochemistry disclosed a faint staining corresponding to the Z-bands in the cytoplasm of control muscle fibres; in contrast, focal aggregates of synemin were seen in patients with MM. Increased synemin immunoreactivity was identified diffusely or in the subsarcolemmal space of scattered fibres in patients with DM, and in vacuolated fibres of patients with IBM and OPD. Strong synemin immunoreactivity was observed in target formations and atrophic fibres of patients with denervating disorders, as well as in atrophic fibres, regardless of their origin, in all patients studied. Synemin co-localised with desmin, as seen on consecutive serial sections immunostained with anti-synemin or anti-desmin antibodies. These observations demonstrate abnormal accumulations containing both synemin and desmin in muscle fibres in patients with MM, IBM, DM, OPD and DA. Considering the important role of synemin as one of intermediate filaments of skeletal and cardiac muscle, its destruction and accumulation in the intracellular debris suggest that synemin may participate in the pathogenesis of these disorders.

Striated muscle cytoarchitecture: an intricate web of form and function

Striated muscle is an intricate, efficient, and precise machine that contains complex interconnected cytoskeletal networks critical for its contractile activity. The individual units of the sarcomere, the basic contractile unit of myofibrils, include the thin, thick, titin, and nebulin filaments. These filament systems have been investigated intensely for some time, but the details of their functions, as well as how they are connected to other cytoskeletal elements, are just beginning to be elucidated. These investigations have advanced significantly in recent years through the identification of novel sarcomeric and sarcomeric-associated proteins and their subsequent functional analyses in model systems. Mutations in these cytoskeletal components account for a large percentage of human myopathies, and thus insight into the normal functions of these proteins has provided a much needed mechanistic understanding of these disorders. In this review, we highlight the components of striated muscle cytoarchitecture with respect to their interactions, dynamics, links to signaling pathways, and functions. The exciting conclusion is that the striated muscle cytoskeleton, an exquisitely tuned, dynamic molecular machine, is capable of responding to subtle changes in cellular physiology.

Lateral force transmission across costameres in skeletal muscle

In skeletal muscle, contractile force can be transmitted laterally between the z-disks and M-lines of neighboring myofibrils, across the sarcolemma, and through the extracellular matrix to the tendon. Here we examine the data that support this model and the sarcolemmal properties and structures, termed "costameres," that are consistent with it.

On noxious desmin: functional effects of a novel heterozygous desmin insertion mutation on the extrasarcomeric desmin cytoskeleton and mitochondria

Recent studies in desmin (-/-) mice have shown that the targeted ablation of desmin leads to pathological changes of the extrasarcomeric intermediate filament cytoskeleton, as well as structural and functional abnormalities of mitochondria in striated muscle. Here, we report on a novel heterozygous single adenine insertion mutation (c.5141_5143insA) in a 40-year-old patient with a distal myopathy. The insertion mutation leads to a frameshift and a truncated desmin (K239fs242). Using transfection studies in SW13 and BHK21 cells, we show that the K239fsX242 desmin mutant is incapable of forming a desmin intermediate filament network. Furthermore, it induces the collapse of a pre-existing desmin cytoskeleton, alters the subcellular distribution of mitochondria and leads to abnormal cytoplasmic protein aggregates reminiscent of desmin-immunoreactive granulofilamentous material seen in the ultrastructural analysis of the patient's muscle. Analysis of mitochondrial function in isolated saponin-permeablized skeletal muscle fibres from our patient showed decreased maximal rates of respiration with the NAD-dependent substrate combination glutamate and malate, as well as a higher amytal sensitivity of respiration, indicating an in vivo inhibition of complex I activity. Our findings suggest that the heterozygous K239fsX242 desmin insertion mutation has a dominant negative effect on the polymerization process of desmin intermediate filaments and affects not only the subcellular distribution, but also biochemical properties of mitochondria in diseased human skeletal muscle. As a consequence, the intermediate filament pathology-induced mitochondrial dysfunction may contribute to the degeneration/regeneration process leading to progressive muscle dysfunction in human desminopathies.

Acute effects of desmin mutations on cytoskeletal and cellular integrity in cardiac myocytes

Mutations in desmin have been associated with a subset of human myopathies. Symptoms typically appear in the second to third decades of life, but in the most severe cases can manifest themselves earlier. How desmin mutations lead to aberrant muscle function, however, remains poorly defined. We created a series of four mutations in rat desmin and tested their in vitro filament assembly properties. RDM-G, a chimera between desmin and green fluorescent protein, formed protofilament-like structures in vitro. RDM-1 and RDM-2 blocked in vitro assembly at the unit-length filament stage, while RDM-3 had more subtle effects on assembly. When expressed in cultured rat neonatal cardiac myocytes via adenovirus infection, these mutant proteins disrupted the endogenous desmin filament to an extent that correlated with their defects in in vitro assembly properties. Disruption of the desmin network by RDM-1 was also associated with disruption of plectin, myosin, and alpha-actinin organization in a significant percentage of infected cells. In contrast, expression of RDM-2, which is similar to previously characterized human mutant desmins, took longer to disrupt desmin and plectin organization and had no significant effect on myosin or alpha-actinin organization over the 5-day time course of our studies. RDM-3 had the mildest effect on in vitro assembly and no discernable effect on either desmin, plectin, myosin, or alpha-actinin organization in vivo. These results indicate that mutations in desmin have both direct and indirect effects on the cytoarchitecture of cardiac myocytes.

Plectin tethers desmin intermediate filaments onto subsarcolemmal dense plaques containing dystrophin and vinculin

Plectin is a versatile cytoskeletal linker protein that preferentially localizes at interfaces between intermediate filaments and the plasma membrane in muscle, epithelial cells, and other tissues. Its deficiency causes muscular dystrophy with epidermolysis bullosa simplex. To better understand the functional roles of plectin beneath the sarcolemma of skeletal muscles and to gain some insights into the underlying mechanism of plectin-deficient muscular dystrophy, we studied in vivo structural and molecular relationships of plectin to subsarcolemmal cytoskeletal components, such as desmin, dystrophin, and vinculin, in rat skeletal muscles. Immunogold electron microscopy revealed that plectin fine threads tethered desmin intermediate filaments onto subsarcolemmal dense plaques overlying Z-lines and I-bands. These dense plaques were found to contain dystrophin and vinculin, and thus may be the structural basis of costameres. The in vivo association of plectin with desmin, (meta-)vinculin, dystrophin, and actin was demonstrated by immunoprecipitation experiments. Treatment of plectin immunoprecipitates with gelsolin reduced actin, dystrophin, and (meta-)vinculin but not desmin, implicating that subsarcolemmal actin could partly mediate the interaction between plectin and dystrophin or (meta-)vinculin. Altogether, our data suggest that plectin, along with desmin intermediate filaments, might serve a vital structural role in the stabilization of the subsarcolemmal cytoskeleton.

Binding of an ankyrin-1 isoform to obscurin suggests a molecular link between the sarcoplasmic reticulum and myofibrils in striated muscles

Assembly of specialized membrane domains, both of the plasma membrane and of the ER, is necessary for the physiological activity of striated muscle cells. The mechanisms that mediate the structural organization of the sarcoplasmic reticulum with respect to the myofibrils are, however, not known. We report here that ank1.5, a small splice variant of the ank1 gene localized on the sarcoplasmic reticulum membrane, is capable of interacting with a sequence of 25 aa located at the COOH terminus of obscurin. Obscurin is a giant sarcomeric protein of approximately 800 kD that binds to titin and has been proposed to mediate interactions between myofibrils and other cellular structures. The binding sites and the critical aa required in the interaction between ank1.5 and obscurin were characterized using the yeast two-hybrid system, in in vitro pull-down assays and in experiments in heterologous cells. In differentiated skeletal muscle cells, a transfected myc-tagged ank1.5 was found to be selectively restricted near the M line region where it colocalized with endogenous obscurin. The M line localization of ank1.5 required a functional obscurin-binding site, because mutations of this domain resulted in a diffused distribution of the mutant ank1.5 protein in skeletal muscle cells. The interaction between ank1.5 and obscurin represents the first direct evidence of two proteins that may provide a direct link between the sarcoplasmic reticulum and myofibrils. In keeping with the proposed role of obscurin in mediating an interaction with ankyrins and sarcoplasmic reticulum, we have also found that a sequence with homology to the obscurin-binding site of ank1.5 is present in the ank2.2 isoform, which in striated muscles has been also shown to associate with the sarcoplasmic reticulum. Accordingly, a peptide containing the COOH terminus of ank2.2 fused with GST was found to bind to obscurin. Based on reported evidence showing that the COOH terminus of ank2.2 is necessary for the localization of ryanodine receptors and InsP3 receptors in the sarcoplasmic reticulum, we propose that obscurin, through multiple interactions with ank1.5 and ank2.2 isoforms, may assemble a large protein complex that, in addition to a structural function, may play a role in the organization of specific subdomains in the sarcoplasmic reticulum.

Muscular dystrophies, the cytoskeleton and cell adhesion

Muscular dystrophies are associated with mutations in genes encoding several classes of proteins. These range from extracellular matrix and integral membrane proteins to cytoskeletal proteins, but also include a heterogeneous group of proteins including proteases, nuclear proteins, and signalling molecules. Muscular dystrophy phenotypes have also become evident in studies on various knockout mice defective in proteins not previously considered or known to be mutated in muscular dystrophies. Some unifying themes are beginning to emerge from all of these data. This review will consider recent advances in our understanding of the molecules involved and bring together data that suggest a role for the cytoskeleton and cell adhesion in muscular dystrophies.

Association of syncoilin and desmin: linking intermediate filament proteins to the dystrophin-associated protein complex

We recently identified a novel protein called syncoilin, a putative intermediate filament protein that interacts with alpha-dystrobrevin, a member of the dystrophin-associated protein complex. Syncoilin is found at the neuromuscular junction, sarcolemma, and Z-lines and is thought to be important for muscle fiber integrity. Based on the similar protein structure and cellular localization of syncoilin and desmin, we proposed that these proteins interact in vivo. The data presented confirm an interaction between syncoilin and desmin and demonstrate their co-localization in skeletal muscle. Intriguingly, whereas these proteins interact, COS-7 cell expression studies show that desmin and syncoilin do not assemble into heterofilaments. Furthermore, fractionation assay and immunofluorescence study of H2K myoblasts and myotubes suggest that, unlike typical intermediate filament proteins, syncoilin does not participate in filament formation with any protein. However, it is possible that syncoilin is involved in the anchoring of the desmin intermediate filament network at the sarcolemma and the neuromuscular junction. This interaction is likely to be important for maintaining muscle fiber integrity and may also link the dystrophin-associated protein complex to the cytoskeleton. The dysfunction or absence of syncoilin may result in the disruption of the intermediate filament network leading to muscle necrosis. Syncoilin is therefore an ideal candidate gene for muscular dystrophies and desmin-related myopathies.

Human synemin gene generates splice variants encoding two distinct intermediate filament proteins

Intermediate filament (IF) proteins are constituents of the cytoskeleton, conferring resistance to mechanical stress, and are encoded by a dispersed multigene family. In man we have identified two isoforms (180 and 150 kDa) of the IF protein synemin. Synemin alpha and beta have a very short N-terminal domain of 10 amino acids and a long C-terminal domain consisting of 1243 amino acids for the alpha isoform and 931 amino acids for the beta isoform. An intronic sequence of the synemin beta isoform is used as a coding sequence for synemin alpha. Both mRNA isoforms (6.5 and 7.5 kb) result from alternative splicing of the same gene, which has been assigned to human chromosome 15q26.3. Analyses by Northern and Western blot revealed that isoform beta is the predominant isoform in striated muscles, whereas both isoforms (alpha and beta) are present in almost equal quantities in smooth muscles. Co-transfection and immunolabeling experiments indicate that both synemin isoforms are incorporated with desmin to form heteropolymeric IFs. Furthermore synemin and desmin are found aggregated together in certain pathological situations.