αB-crystallin is a sensor for assembly intermediates and for the subunit topology of desmin intermediate filaments

Mutations in the small heat shock protein chaperone CRYAB (αB-crystallin/HSPB5) and the intermediate filament protein desmin, phenocopy each other causing cardiomyopathies. Whilst the binding sites for desmin on CRYAB have been determined, desmin epitopes responsible for CRYAB binding and also the parameters that determine CRYAB binding to desmin filaments are unknown. Using a combination of co-sedimentation centrifugation, viscometric assays and electron microscopy of negatively stained filaments to analyse the in vitro assembly of desmin filaments, we show that the binding of CRYAB to desmin is subject to its assembly status, to the subunit organization within filaments formed and to the integrity of the C-terminal tail domain of desmin. Our in vitro studies using a rapid assembly protocol, C-terminally truncated desmin and two disease-causing mutants (I451M and R454W) suggest that CRYAB is a sensor for the surface topology of the desmin filament. Our data also suggest that CRYAB performs an assembly chaperone role because the assembling filaments have different CRYAB-binding properties during the maturation process. We suggest that the capability of CRYAB to distinguish between filaments with different surface topologies due either to mutation (R454W) or assembly protocol is important to understanding the pathomechanism(s) of desmin-CRYAB myopathies.

Metallothionein as a clonable high-density marker for cryo-electron microscopy

Cryo-electron microscopy is expanding its scope from macromolecules towards much larger and more complex cellular specimens such as organelles, cells and entire tissues. While isolated macromolecular specimens are typically composed of only very few different components that may be recognized by their shape, size or state of polymerization, cellular specimens combine large numbers of proteinaceous structures as well as nucleic acids and lipid arrays. Consequently, an unambiguous identification of these structures within the context of a whole cell may create a very difficult challenge. On plastic-embedded specimens, or Tokuyasu sections (Tokuyasu, 1980), epitopes that are exposed at the surface can be tagged by antibodies. However, vitrified sections have to be kept at strict cryo-conditions (below -140 °C) and therefore do not allow any post-sectioning treatment of the specimens other than data acquisition in the microscope. Hence, the labels have to be placed into the specimen before freezing. Here we report on the application of a small metal-clustering protein, metallothionein (MTH), as a clonable label capable of clustering metal atoms into a high-density particle with high spatial resolution. We tested MTH as a label for kinesin-decorated microtubules (MTs) as well as the building blocks of desmin intermediate filaments (IFs).

Multiple sites in αB-crystallin modulate its interactions with desmin filaments assembled in vitro

The β3- and β8-strands and C-terminal residues 155–165 of αB-crystallin were identified by pin arrays as interaction sites for various client proteins including the intermediate filament protein desmin. Here we present data using 5 well-characterised αB-crystallin protein constructs with substituted β3- and β8-strands and with the C-terminal residues 155–165 deleted to demonstrate the importance of these sequences to the interaction of αB-crystallin with desmin filaments. We used electron microscopy of negatively stained samples to visualize increased interactions followed by sedimentation assays to quantify our observations. A low-speed sedimentation assay measured the ability of αB-crystallin to prevent the self-association of desmin filaments. A high-speed sedimentation assay measured αB-crystallin cosedimentation with desmin filaments. Swapping the β8-strand of αB-crystallin or deleting residues 155–165 increased the cosedimentation of αB-crystallin with desmin filaments, but this coincided with increased filament-filament interactions. In contrast, substitution of the β3-strand with the equivalent αA-crystallin sequences improved the ability of αB-crystallin to prevent desmin filament-filament interactions with no significant change in its cosedimentation properties. These data suggest that all three sequences (β3-strand, β8-strand and C-terminal residues 155–165) contribute to the interaction of αB-crystallin with desmin filaments. The data also suggest that the cosedimentation of αB-crystallin with desmin filaments does not necessarily correlate with preventing desmin filament-filament interactions. This important observation is relevant not only to the formation of the protein aggregates that contain both desmin and αB-crystallin and typify desmin related myopathies, but also to the interaction of αB-crystallin with other filamentous protein polymers.

Primary desminopathies

Mutations of the human desmin gene on chromosome 2q35 cause a familial or sporadic form of skeletal myopathy frequently associated with cardiac abnormalities. Skeletal and cardiac muscle from patients with primary desminopathies characteristically display cytoplasmic accumulation of desmin-immunoreactive material and myofibrillar changes. However, desmin-positive protein aggregates in conjunction with myofibrillar abnormalities are also the morphological hallmark of the large group of secondary desminopathies (synonyms: myofibrillar myopathies, desmin-related myopathies), which comprise sporadic and familial neuromuscular conditions of considerable clinical and genetic heterogeneity. Here, we will give an overview on the functional role of desmin in striated muscle as well as the main clinical, myopathological, genetic and patho-physiological aspects of primary desminopathies. Furthermore, we will discuss recent genetic and biochemical advances in distinguishing primary from secondary desminopathies.

Impact of disease mutations on the desmin filament assembly process

It has been documented that mutations in the human desmin gene lead to a severe type of myofibrillar myopathy, termed more specifically desminopathy, which affects cardiac and skeletal as well as smooth muscle. We showed recently that 14 recombinant versions of these disease-causing desmin variants, all involving single amino acid substitutions in the alpha-helical rod domain, interfere with in vitro filament formation at distinct stages of the assembly process. We now provide mechanistic details of how these mutations affect the filament assembly process by employing analytical ultracentrifugation, time-lapse electron microscopy of negatively stained and glycerol-sprayed/low-angle rotary metal-shadowed samples, quantitative scanning transmission electron microscopy, and viscometric studies. In particular, the soluble assembly intermediates of two of the mutated proteins exhibit unusually high s-values, compatible with octamers and other higher-order complexes. Moreover, several of the six filament-forming mutant variants deviated considerably from wild-type desmin with respect to their filament diameters and mass-per-length values. In the heteropolymeric situation with wild-type desmin, four of the mutant variants caused a pronounced "hyper-assembly", when assayed by viscometry. This indicates that the various mutations may cause abortion of filament formation by the mutant protein at distinct stages, and that some of them interfere severely with the assembly of wild-type desmin. Taken together, our findings provide novel insights into the basic intermediate filament assembly mechanisms and offer clues as to how amino acid changes within the desmin rod domain may interfere with the normal structural organization of the muscle cytoskeleton, eventually leading to desminopathy.

The cytoskeleton and related proteins in the human failing heart

In addition to functional alterations, heart failure has a structural basis as well. This concerns all components of the cardiac myocytes as well as the extracellular space. Proteins of the cardiomyocyte can be subdivided in 5 different categories: 1) Contractile proteins including myosin, actin, tropomyosin and the troponins. 2) Sarcomeric skeleton: titin, myosin binding protein C, alpha-actinin, myomesin, and M-protein. 3) True 'cytoskeletal' proteins: tubulin, desmin and actin. 4) Membrane-associated proteins: dystrophin, spectrin, talin, vinculin, ankyrin and others. 5) Proteins of the intercalated disc: desmosomes consisting of desmoplakin, desmocollin, desmoglein and desmin; adherens junctions with N-cadherin, the catenins and vinculin, and gap junctions with connexin. Failing myocardium obtained from patients undergoing cardiac transplantation exhibits ultrastuctural degeneration and an altered nucleus/cytoplasm relationship. The contractile proteins and those of the sarcomeric skeleton, especially titin, are downregulated, the cytoskeletal proteins desmin and tubulin and membrane-associated proteins such as vinculin and dystrophin are upregulated and those of the intercalated disc are irregularly arranged. Elevation of cytoskeletal proteins correlates well with diastolic and contractile dysfunction in these patients. The enlarged interstitial space contains fibrosis, i.e. accumulations of fibroblasts and extracellular matrix components, in addition to macrophages and microvascular elements. Loss of the contractile machinery and related proteins such as titin and alpha-actinin may be the first and decisive event initiating an adaptive increase in cytoskeleton and membrane associated components. Fibrosis may be stimulated by subcellular degeneration. The hypothesis is put forward that all proteins of the different myocardial compartments contribute to the deterioration of cardiac function in heart failure.

Exploring the mechanical behavior of single intermediate filaments

Intermediate filaments (IFs) are structural elements of eukaryotic cells with distinct mechanical properties. Tissue integrity is severely impaired, in particular in skin and muscle, when IFs are either absent or malfunctioning due to mutations. Our knowledge on the mechanical properties of IFs is mainly based on tensile testing of macroscopic fibers and on the rheology of IF networks. At the single filament level, the only piece of data available is a measure of the persistence length of vimentin IFs. Here, we have employed an atomic force microscopy (AFM) based protocol to directly probe the mechanical properties of single cytoplasmic IFs when adsorbed to a solid support in physiological buffer environment. Three IF types were studied in vitro: recombinant murine desmin, recombinant human keratin K5/K14 and neurofilaments isolated from rat brains, which are composed of the neurofilament triplet proteins NF-L, NF-M and NF-H. Depending on the experimental conditions, the AFM tip was used to laterally displace or to stretch single IFs on the support they had been adsorbed to. Upon applying force, IFs were stretched on average 2.6-fold. The maximum stretching that we encountered was 3.6-fold. A large reduction of the apparent filament diameter was observed concomitantly. The observed mechanical properties therefore suggest that IFs may indeed function as mechanical shock absorbers in vivo.

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.

Desmin filaments are stably associated with the outer nuclear surface in chick myoblasts

Eukaryotic cells have highly organized, interconnected intracellular compartments. The nuclear surface and cytoplasmic cytoskeletal filaments represent compartments involved in such an association. Intermediate filaments are the major cytoskeletal elements in this association. Desmin is a muscle-specific structural protein and one of the earliest known muscle-specific genes to be expressed during cardiac and skeletal muscle development. Desmin filaments have been shown to be associated with the nuclear surface in the myogenic cell line C2C12. Previous studies have revealed that mice lacking desmin develop imperfect muscle, exhibiting the loss of nuclear shape and positioning. In the present work, we have analyzed the association between desmin filaments and the outer nuclear surface in nuclei isolated from pectoral skeletal muscle of chick embryos and in primary chick myogenic cell cultures by using immunofluorescence microscopy, negative staining, immunogold, and transmission electron microscopy. We show that desmin filaments remain firmly attached to the outer nuclear surface after the isolation of nuclei. Furthermore, positive localization of desmin persists after gentle washing of the nuclei with high ionic strength solutions. These data suggest that desmin intermediate filaments are stably and firmly connected to the outer nuclear surface in skeletal muscles cells in vivo and in vitro.

Unusual aspects of desmoplastic small round cell tumor

Desmoplastic small round cell tumor (DSRCT) is a neoplasia that occurs mainly in childhood and involves abdominal or peritoneal sites, coexpressing ectodermal and mesenchimal immunophenotypic markers, and is endowed with an impressive stromal desmoplasia that tends to decrease on tumor relapse. To date, over 150 cases have been collected in the literature. Its presumed neuroectodermal histogenesis has been challenged by cytogenetic findings different from those usually associated with neoplasms of neuroectodermal origin. The authors report a case bearing clinical and histologic aspects of typical desmoplastic retroperitoneal small cell tumor, with intense and diffuse nuclear immunopositivity for WT1, but lacking divergent immunophenotype. Ultrastructural investigation revealed that desmoplasia could result from fibrillary synthesis by neoplastic cells.

The expression of cytokeratin in hepatic stellate cells of the cod

In the current study, we examined the cytoskeletal architecture of cod hepatic stellate cells. We found that the cod hepatic stellate cells contain abundant cytoplasmic filaments. Deep-etch electron microscopy showed that the major component of the cytoplasmic filaments was intermediate filaments, although microtubules and microfilaments were also found in the cytoplasmic filament bundles. Immunoelectron microscopy revealed the presence of beta-tubulin, alpha-smooth muscle actin, smooth muscle type myosin, desmin and cytokeratin but not vimentin or glial fibrillar acidic protein. These results demonstrate that the cytoplasmic filaments of cod hepatic stellate cells are composed of desmin and cytokeratin intermediate filaments, acto-myosin complexes and microtubules, suggesting that the cod hepatic stellate cells have both contractile and structural functions. The expression of cytokeratin in cod hepatic stellate cells indicates that they serve for mechanical support in the extremely soft liver tissues of cods with their abundant lipids.

Desmin filaments influence myofilament spacing and lateral compliance of slow skeletal muscle fibers

Intermediate filaments composed of desmin interlink Z-disks and sarcolemma in skeletal muscle. Depletion of desmin results in lower active stress of smooth, cardiac, and skeletal muscles. Structural functions of intermediate filaments in fast (psoas) and slow (soleus) skeletal muscle were examined using x-ray diffraction on permeabilized muscle from desmin-deficient mice (Des-/-) and controls (Des+/+). To examine lateral compliance of sarcomeres and cells, filament distances and fiber width were measured during osmotic compression with dextran. Equatorial spacing (x-ray diffraction) of contractile filaments was wider in soleus Des-/- muscle compared to Des+/+, showing that desmin is important for maintaining lattice structure. Osmotic lattice compression was similar in Des-/- and Des+/+. In width measurements of single fibers and bundles, Des-/- soleus were more compressed by dextran compared to Des+/+, showing that intermediate filaments contribute to whole-cell compliance. For psoas fibers, both filament distance and cell compliance were similar in Des-/- and Des+/+. We conclude that desmin is important for stabilizing sarcomeres and maintaining cell compliance in slow skeletal muscle. Wider filament spacing in Des-/- soleus cannot, however, explain the lower active stress, but might influence resistance to stretch, possibly minimizing stretch-induced cell injury.

Desmin aggregate formation by R120G alphaB-crystallin is caused by altered filament interactions and is dependent upon network status in cells

The R120G mutation in alphaB-crystallin causes desmin-related myopathy. There have been a number of mechanisms proposed to explain the disease process, from altered protein processing to loss of chaperone function. Here, we show that the mutation alters the in vitro binding characteristics of alphaB-crystallin for desmin filaments. The apparent dissociation constant of R120G alphaB-crystallin was decreased while the binding capacity was increased significantly and as a result, desmin filaments aggregated. These data suggest that the characteristic desmin aggregates seen as part of the disease histopathology can be caused by a direct, but altered interaction of R120G alphaB-crystallin with desmin filaments. Transfection studies show that desmin networks in different cell backgrounds are not equally affected. Desmin networks are most vulnerable when they are being made de novo and not when they are already established. Our data also clearly demonstrate the beneficial role of wild-type alphaB-crystallin in the formation of desmin filament networks. Collectively, our data suggest that R120G alphaB-crystallin directly promotes desmin filament aggregation, although this gain of a function can be repressed by some cell situations. Such circumstances in muscle could explain the late onset characteristic of the myopathies caused by mutations in alphaB-crystallin.

Characterization of mammalian synemin, an intermediate filament protein present in all four classes of muscle cells and some neuroglial cells: co-localization and interaction with type III intermediate filament proteins and keratins

Using a monoclonal antibody, we have detected a high molecular weight muscle protein, co-localized and co-isolating with desmin. Searching a human cDNA database with partial amino acid sequences of the protein, we found a cDNA clone encoding a 1565-amino-acid polypeptide, identified as a mammalian (human) synemin, a member of the intermediate filament (IF) protein family. Immunoblotting showed the presence of a 180-kDa polypeptide in skeletal muscle and 180- and 200-kDa polypeptides in cardiac and smooth muscles. Interestingly, synemin was also found in myoepithelial cells, which have keratin filaments instead of desmin. Moreover, synemin was also found in astrocytes of optic nerves and non-myelin-forming Schwann cells, together with glial fibrillary acidic protein (GFAP) and vimentin. Blot overlays pointed to molecular interactions of synemin with desmin, vimentin, GFAP and keratin 5 and 6, but not with keratin 14. The experimental data also suggested a possible link with nebulin, a skeletal muscle protein. Purified synemin was coassembled with desmin in different molar ratios, and at 1:25, as typically found in vivo, IFs were formed which were comparable in length to desmin filaments. However, at molar ratios of 3:25 and 6:25, much shorter and irregular shaped filamentous polymers were generated. The fact that synemin is present in all four classes of muscle cells and a specific type of glial cells is indicative of important functions. Its incorporation may give structural and functional versatility to the IF cytoskeleton.

Myofibrillar (desmin-related) myopathy: clinico-pathological spectrum in 3 cases and review of the literature

Myofibrillar or desmin-related myopathies encompass neuromuscular disorders with abnormal deposits of desmin and myofibrillar alterations. We report 3 unrelated patients presenting with proximal and distal myopathy, and, as a unique congenital syndrome, diffusely distributed myopathy, osteoporosis and myopia. Muscle biopsies shared cytoplasmic inclusions, rimmed vacuoles, and ragged-red-like fibers. Sarcoplasmic inclusions, either plaque-like or amorphous, strongly immunoreacted on dystrophin and variably for desmin, alphaB crystallin and ubiquitin. Cyclin-dependent kinases CDK1, CDK2 and CDK5 were overexpressed in affected fibers. Ultrastructurally, focal myofibrillar disruption was accompanied by tubulo-filamentous inclusions in one case and abundant glycogen and enlarged mitochondria displaying respiratory chain dysfunction at biochemistry in another case. Molecular analysis of the alphaB crystallin gene coding sequence and exons 4, 5 and 6 of the desmin gene did not reveal any mutation. The morphologic denominator of hyaline structures and areas of myofibrillar destruction occurs in heterogeneous conditions and may overlap with features of inclusion body myopathy and mitochondrial myopathy.

p21-activated kinase PAK phosphorylates desmin at sites different from those for Rho-associated kinase

p21-activated kinase (PAK) and Rho-associated kinase (Rho-kinase) have been shown to induce Ca(2+)-independent contraction of smooth muscle. PAK-induced contraction of Triton-skinned smooth muscle correlates with increased phosphorylation of caldesmon and desmin, although the role of desmin phosphorylation has remained obscure. Here we report that desmin serves as an excellent substrate for PAK in vitro. PAK phosphorylated desmin in a GTP. Cdc42/Rac-dependent manner. Phosphorylation of desmin by PAK dramatically inhibited its filament-forming ability. PAK phosphorylated mainly serine residues of the head domain of desmin, and the major phosphorylation sites differed from those for Rho-kinase. These results suggest that different site-specific phosphorylation of desmin via two divergent protein kinases downstream of Rho family GTPases would seem to increase the regulatory potential for organization of desmin filaments.

The intermediate filament protein consensus motif of helix 2B: its atomic structure and contribution to assembly

Nearly all intermediate filament proteins exhibit a highly conserved amino acid motif (YRKLLEGEE) at the C-terminal end of their central alpha-helical rod domain. We have analyzed its contribution to the various stages of assembly by using truncated forms of Xenopus vimentin and mouse desmin, VimIAT and DesIAT, which terminate exactly before this motif, by comparing them with the wild-type and tailless proteins. It is surprising that in buffers of low ionic strength and high pH where the full-length proteins form tetramers, both VimIAT and DesIAT associated into various high molecular weight complexes. After initiation of assembly, both VimIAT and DesIAT aggregated into unit-length-type filaments, which rapidly longitudinally annealed to yield filaments of around 20 nm in diameter. Mass measurements by scanning transmission electron microscopy revealed that both VimIAT and DesIAT filaments contained considerably more subunits per cross-section than standard intermediate filaments. This indicated that the YRKLLEGEE-motif is crucial for the formation of authentic tetrameric complexes and also for the control of filament width, rather than elongation, during assembly. To determine the structure of the YRKLLEGEE domain, we grew crystals of peptides containing the last 28 amino acid residues of coil 2B, chimerically fused at its amino-terminal end to the 31 amino acid-long leucine zipper domain of the yeast transcription factor GCN4 to facilitate appropriate coiled-coil formation. The atomic structure shows that starting from Tyr400 the two helices gradually separate and that the coiled coil terminates with residue Glu405 while the downstream residues fold away from the coiled-coil axis.

Association of plectin with Z-discs is a prerequisite for the formation of the intermyofibrillar desmin cytoskeleton

Plectin is a high-molecular mass protein (approximately 500 kd) that binds actin, intermediate filaments, and microtubules. Mutations of the plectin gene cause a generalized blistering skin disorder and muscular dystrophy. In adult muscle, plectin is colocalized with desmin at structures forming the intermyofibrillar scaffold and beneath the plasma membrane. To study the involvement of plectin in myofibrillogenesis, we analyzed the spatial and temporal expression patterns of plectin in cultured differentiating human skeletal muscle cells and its relationship to desmin intermediate filaments during this process. Northern and Western blot analyses demonstrated that at least two different plectin isoforms are expressed at all developmental stages from proliferating myoblasts to mature myotubes. Using immunocytochemistry, we show that the localization of plectin dramatically changes from a network-like distribution into a cross-striated distribution during maturation of myocytes. Double immunofluorescence experiments revealed that desmin and plectin are colocalized in premyofibrillar stages and in mature myotubes. Interestingly, plectin was often found to localize to the periphery of Z-discs during the actual alignment of neighboring myofibrils, and an obvious cross-striated plectin staining pattern was observed before desmin was localized in the Z-disc region. We conclude that the association of plectin with Z-discs is an early event in the lateral alignment of myofibrils that precedes the formation of the intermyofibrillar desmin cytoskeleton.

Cyclosporine A affects the organization of cytoskeletal fibrillar proteins in rat thymus

We have evaluated whether cyclosporine A affects cell structure and cytoskeletal proteins of the thymus of Wistar rats. Immunohistochemical analysis showed that expression of the cytoskeletal proteins vimentin and desmin was much higher in epithelial cells, dendritic cells and lymphocytes in the thymus of treated rats than in untreated controls. Protein expression was observed as a positive condensation in a distinct area near the nucleus with a capping-like configuration. An ultrastructural study showed that the amount of cytoskeletal fibrillar structures was increased in the treated rats. The structures were assembled in a limited area of the cell with a nuclear capping-like configuration which was in agreement with the light microscopical observations. Immunoblotting analysis demonstrated that vimentin and desmin had a lower molecular weight in treated rats than in controls (57 and 53 kDa versus 55 and 51 kDa, respectively). The results clearly indicate that cyclosporine A affects the structure of the cytoskeleton suggesting that this could be the first step in its immunosuppressive effects by altering nucleus/cytoplasm signaling.

Association of mitochondria with plectin and desmin intermediate filaments in striated muscle

Plectin (M(r) > 500,000) is a versatile and widely expressed cytolinker protein. In striated muscle it is predominantly found at the Z-disc level where it colocalizes with the intermediate filament protein desmin. Both proteins show altered labeling patterns in tissues of muscular dystrophy patients. Moreover, mutations in the plectin gene lead to the autosomal recessive human disorder epidermolysis bullosa simplex with muscular dystrophy, and defects in the desmin gene have been shown to cause familiar cardiac and skeletal myopathy. Since intermediate filaments (IFs) in striated muscle tissue have been found to be intimately associated with mitochondria, we investigated whether plectin is involved in this association. Using postembedding immunogold labeling of Lowicryl sections and immunogold labeling of ultrathin cryosections, we show that plectin is associated with desmin IFs linking myofibrils to mitochondria at the level of the Z-disc and along the entire length of the sarcomere. The localization of plectin label at the mitochondrial membrane itself was consistent with a putative linker function of plectin between desmin IFs and the mitochondrial surface. In mitochondrion-rich muscle fibers, both plectin and desmin were part of an ordered arrangement of mitochondrial side branches, which wound around myofibrils adjacent to the Z-discs and were anchored into a filamentous network transversing from one fibril to the other. The association of mitochondria with plectin and IFs was seen also in tissues without regular distribution patterns of mitochondria, such as heart muscle and neonatal skeletal muscle tissues. These data were supplemented with in vitro binding assays showing direct interaction of plectin with desmin via its carboxy-terminal IF-binding domain. As a cytolinker protein associated with mitochondria and desmin IFs, plectin could play an important role in the positioning and shape formation, in particular branching, of mitochondrial organelles in striated muscle tissues.

Desmin filaments studied by quasi-elastic light scattering

We studied polymers of desmin, a muscle-specific type III intermediate filament protein, using quasi-elastic light scattering. Desmin was purified from chicken gizzard. Polymerization was induced either by 2 mM MgCl(2) or 150 mM NaCl. The polymer solutions were in the semidilute regime. We concluded that the persistence length of the filaments is between 0.1 and 1 microm. In all cases, we found a hydrodynamic diameter of desmin filaments of 16-18 nm. The filament dynamics exhibits a characteristic frequency in the sense that correlation functions measured on one sample but at different scattering vectors collapse onto a single master curve when time is normalized by the experimentally determined initial decay rate.

Intermediate filament assembly: temperature sensitivity and polymorphism

Intermediate filament (IF) proteins are encoded by a large multigene family and form polymers with a uniform diameter of approximately 10 nm. However, although the cytoplasmic representatives all confirm to a unit-type structural principle leading to the formation of extended coiled coils, it is becoming increasingly clear that subunit arrangements and physical properties vary among the different filaments. Thus, the intricate tissue-specific expression pattern of individual IF proteins (especially, their co-expression with other members of the IF protein family or with IF-associated proteins to form obligatory heteropolymers) points to distinct functions acquired during evolution relevant to cellular homeostasis in various tissues.

The effects of mono-ADP-ribosylation on desmin assembly-disassembly

Previous studies have shown that desmin, the muscle-specific intermediate filament protein, is a substrate for the endogenous muscle arginine-specific mono-ADP-ribosyltransferase and that ADP-ribosylation inhibits assembly of desmin into intermediate filaments (Huang et al., Exp. Cell Res. 226, 147-153, 1996). In this paper, the effects of mono-ADP-ribosylation on assembly and disassembly of desmin intermediate filaments were further characterized. First, it was found that ADP-ribosylated desmin does not coassemble with unmodified desmin and has no effect on assembly of unmodified desmin. Second, incubation of assembled desmin filaments with mono-ADP-ribosyltransferase and NAD+ results in disassembly of the filaments. Finally, the structural components of the attached ADP-ribose moiety responsible for altering the assembly of desmin into filaments were investigated by a stepwise cleavage of ADP-ribose with snake venom phosphodiesterase and alkaline phophatase, followed by analysis of assembly. The reactions catalyzed by these two enzymes were established using a desmin peptide as a substrate. Our results show that ribosylated desmin, but not phosphoribosylated desmin, was able to self-assemble into intermediate filaments, suggesting that the presence of a phosphate group is needed to alter desmin's assembly ability.

Characterization of distinct early assembly units of different intermediate filament proteins

We have determined the mass-per-length (MPL) composition of distinct early assembly products of recombinant intermediate filament (IF) proteins from the four cytoplasmic sequence homology classes, and compared these values with those of the corresponding mature filaments. After two seconds under standard assembly conditions (i.e. 25 mM Tris-HCl (pH 7.5), 50 mM NaCl, 37 degrees C), vimentin, desmin and the neurofilament triplet protein NF-L aggregated into similar types of "unit-length filaments" (ULFs), whereas cytokeratins (CKs) 8/18 already yielded long IFs at this time point, so the ionic strength had to be reduced. The number of molecules per filament cross-section, as deduced from the MPL values, was lowest for CK8/18, i.e. 16 and 25 at two seconds compared to 16 and 21 at one hour. NF-L exhibited corresponding values of 26 and 30. Vimentin ULFs yielded a pronounced heterogeneity, with major peak values of 32 and 45 at two seconds and 30, 37 and 44 after one hour. Desmin formed filaments of distinctly higher mass with 47 molecules per cross-section, at two seconds and after one hour of assembly. This indicates that individual types of IF proteins generate filaments with distinctly different numbers of molecules per cross-section. Also, the observed significant reduction of apparent filament diameter of ULFs compared to the corresponding mature IFs is the result of a "conservative" radial compaction-type reorganization within the filament, as concluded from the fact that both the immature and mature filaments contain very similar numbers of subunits per cross-section. Moreover, the MPL composition of filaments is strikingly dependent on the assembly conditions employed. For example, vimentin fibers formed in 0.7 mM phosphate (pH 7.5), 2.5 mM MgCl2, yield a significantly increased number of molecules per cross-section (56 and 84) compared to assembly under standard conditions. Temperature also strongly influences assembly: above a certain threshold temperature "pathological" ULFs form that are arrested in this state, indicating that the system is forced into strong but unproductive interactions between subunits. Similar "dead-end" structures were obtained with vimentins mutated to introduce principal alterations in subdomains presumed to be of general structural importance, indicating that these sequence changes led to new modes of intermolecular interactions.

Rho-associated kinase phosphorylates desmin, the myogenic intermediate filament protein, at unique amino-terminal sites

We obtained evidence that Rho-associated kinase (Rho-kinase) phosphorylates desmin, the myogenic intermediate filament protein, with approximately 2 mol phosphate per mole of desmin in vitro. Desmin phosphorylated by Rho-kinase lost the potential to form 10-nm filaments. Thr-16, Thr-75, and Thr-76 on desmin proved to be the major phosphorylation sites for Rho-kinase. All these sites are located within the head domain and are different from the reported phosphorylation sites of protein kinase. A, protein kinase C, and cdc2 kinase. We are entertaining the notion that Rho-kinase may regulate filament structures of desmin by site-specific phosphorylation.

A missense mutation in the alphaB-crystallin chaperone gene causes a desmin-related myopathy

Desmin-related myopathies (DRM) are inherited neuromuscular disorders characterized by adult onset and delayed accumulation of aggregates of desmin, a protein belonging to the type III intermediate filament family, in the sarcoplasma of skeletal and cardiac muscles. In this paper, we have mapped the locus for DRM in a large French pedigree to a 26-cM interval in chromosome 11q21-23. This region contains the alphaB-crystallin gene (CRYAB), a candidate gene encoding a 20-kD protein that is abundant in lens and is also present in a number of non-ocular tissues, including cardiac and skeletal muscle. AlphaB-crystallin is a member of the small heat shock protein (shsp) family and possesses molecular chaperone activity. We identified an R120G missense mutation in CRYAB that co-segregates with the disease phenotype in this family. Muscle cell lines transfected with the mutant CRYAB cDNA showed intracellular aggregates that contain both desmin and alphaB-crystallin as observed in muscle fibers from DRM patients. These results are the first to identify a defect in a molecular chaperone as a cause for an inherited human muscle disorder.

Desmin myopathy involving cardiac, skeletal, and vascular smooth muscle: report of a case with immunoelectron microscopy

Desmin myopathy is a rare idiopathic disorder characterized by abnormal aggregates of desmin-type intermediate filaments, which affects cardiac and skeletal muscle, and rarely the intestinal smooth muscle. We report a 42-year-old woman with atrial fibrillation and progressive restrictive cardiomyopathy. Left ventricular biopsy, cardiac explant, and subsequent autopsy study of skeletal muscle revealed cytoplasmic granulo-filamentous inclusions that were continuous with Z-lines and were immunoreactive for desmin filaments both at the light immunohistochemical and electron microscopic level. In addition, we report the presence of characteristic inclusions within the smooth muscle of intramural coronary blood vessels. This is the first description of desmin inclusions within vascular smooth muscle, and underscores the systemic nature of this rare myopathy.

Formation of binucleated cardiac myocytes in rat heart: II. Cytoskeletal organisation

Neonatal cardiac myocytes continue to undergo nuclear division, but lose their ability to complete cell division between 3 and 4 days of age. To examine cytoskeletal organisation of cardiac myocytes during mitosis, freshly isolated cardiac myocytes from 2-, 4-, 6- and 8-day-old rats were fixed and labeled with anti-tubulin, vinculin, desmin and sarcomeric alpha-actinin antibodies. The central, nuclear region of cardiac myocytes is expanded to form a balloon-like structure when they entered prophase. The organisation of microtubules, vinculin and desmin in mitotic myocytes from 4-, 6- and 8-day-old rats was identical to that in dividing myocytes from 2-day-old animals. Microtubules emanating from the nuclear membrane mainly ran along the longitudinal axis of cardiac myocytes in interphase. Microtubules were disassembled and reorganised into the mitotic spindle during mitosis. Desmin was disassembled, either diffusely distributed in the cytoplasm or formed spotty cytoplasmic aggregates during mitosis. Vinculin was disassembled in prometaphase, diffusely distributed in the cytoplasm and associated with cell membranes. During telophase it concentrated in the equator of mitotic spindles. Sarcomeric alpha-actinin became dispersed in the cytoplasm of mitotic myocytes from 2-day-old rats in prometaphase. It remained diffusely distributed in the cytoplasm and associated with cell membranes until the completion of cytokinesis. However, sarcomeric alpha-actinin was only partially disassembled in 4-, 6- and 8-day-old myocytes. Striations of alpha-actinin with full sarcomere length were observed in the cytoplasm as well as in the region of furrow formation. Thus, incomplete disassembly and presence of myofibrils in the equator region where cleavage furrows from may physically impede the furrowing of sarcolemma driven by the contractile ring, resulting in the formation of binucleated cardiac myocytes.

Prosome cytodistribution relative to desmin and actin filaments in dividing C2.7 myoblasts and during myotube formation in vitro

Prosomes constitute the multicatalytic proteinase (MCP) core of the 26S proteasomes, but were first observed as subcomplexes of untranslated mRNP; this suggests that they play a putative role in the control of protein biosynthesis in addition to their catabolic enzymatic function. In previous investigations it was shown that some prosomes colocalize with the intermediate filaments (IF) of the cytoskeleton, of the cytokeratin type in epithelial cells, and of the vimentin type in fibroblasts. Studies on adult rat muscle carried out with prosome-specific monoclonal antibodies (p-mAbs) have shown, surprisingly, that specific types of prosomes predominantly occupy a particular zone in between the M and the Z lines of the sarcomeric structure. The data presented here show that the subunit composition of prosomes changes when the dividing C2.7 myoblasts fuse into myotubes. We show furthermore that, in dividing C2.7 myoblasts, prosomes colocalize with the desmin network as well as with that of actin, in a distribution that changes with the subunit pattern of the prosomes investigated by individual p-mAbs. Surprisingly, when myogenic fusion is induced, specific types of prosomes move first to the nuclei; later on, they reappear in the cytoplasm. There, superimposing initially onto the reorganizing desmin filaments that run from one pole of the prefusion myoblast to the other, prosomes gradually colocalize with the actin fibers in the fusing myotubes, finally forming a "pearl on a string" pattern. These results are discussed in relation to parallel observations of prosome distribution between the actin and IF networks not only in epithelial cells but also in fusing muscle satellite cells, which made it possible to monitor the complete buildup of the sarcomeric structure.

Characterization of ADP-ribosylation sites on desmin and restoration of desmin intermediate filament assembly by de-ADP-ribosylation

Desmin is an intermediate filament protein that can be ADP-ribosylated by arginine-specific mono(ADP-ribosyl) transferase. Stoichiometric modification of desmin by the transferase causes inhibition of assembly of desmin into 10-nm intermediate filaments (Huang et al., 1993, Biochem. Biophys. Res. Commun. 197, 570-577). In this work, the sites of modification that can affect disassembly have been identified. ADP-ribosylated desmin (1.2 mol ADP-ribose/mol desmin) was digested with lysyl endopeptidase followed by trypsin. Two ADP-ribosylated peptides were obtained, sequenced by Edman degradation, and analyzed by the use of matrix-assisted laser desorption/ionization mass spectrometry. Arginines 48 and 68 of desmin's head domain were shown to be sites of modification, with arginine 48 the major ADP-ribosylation site. ADP-ribosylated desmin (4 mol ADP-ribose/mol desmin) was treated with ADP-ribosylarginine hydrolase. Removal of more than three ADP-ribose groups results in partial restoration of desmin's ability to form intermediate filaments. It is necessary to remove all ADP-ribose groups from desmin to restore its complete ability to form intermediate filaments. The fact that the effect of ADP-ribosylation on the filament-forming properties of desmin is fully reversible suggests that ADP-ribosylation alone is responsible for the changes noted in desmin.

Immunocytochemical colocalization of desmin and vimentin in human fetal skeletal muscle cells

BACKGROUND

Desmin and vimentin are the major intermediate filaments in muscle. They have been extensively studied in animal experiments. This study is the first to identify the distribution and to analyse the correlation of desmin and vimentin in human fetal skeletal muscle. Vimentin might be replaced by or transformed into desmin during myogenesis in chick embryo, although the precise process remains to be elucidated. The aim of this report is to evaluate the ratio of desmin to vimentin in human fetal muscle.

METHODS

By double-labeling immunoelectron microscopy, desmin and vimentin intermediate filaments were localized in developing skeletal muscles of 20-29-week-old human fetuses. The ratio of desmin and vimentin was analyzed statistically.

RESULTS

Two sizes of colloidal gold particles, 5 nm (vimentin) and 10 nm (desmin), were distributed along the intermediate filaments. The commonest distance between gold particles was approximately 40-50 nm. Desmin and vimentin labeled with gold particles were arranged very close together in the same intermediate filament. The ratio of vimentin to desmin varied but the amount of vimentin decreased progressively from the undifferentiated myoblast to the differentiated myocytes. As the fetuses developed, desmin increased and vimentin decreased. Desmin and vimentin intermediate filaments were identified in the intermyofibrils of differentiated myocytes, in subsarcolemmal space, and in myoblast.

CONCLUSIONS

Desmin and vimentin were colocalized in the same intermediate filaments. More vimentin existed in the less differentiated myocytes, although a small number of desmin filaments were already found in undifferentiated myoblasts. These intermediate filaments may not only connect myofibril bundles, cell organelles, and cell membrane but also provide a basis for myofibrillogenesis that is similar to relation between connective fibers and parenchymal cells.

Ischemia affects cardiac proteins in healthy animals less severely than in human patients

BACKGROUND

Recently, our group showed that in human hearts proteins are extremely sensitive to ischemic injury. The purpose of this investigation was to evaluate the effects of ischemia on contractile and cytoskeletal proteins in rabbit and pig hearts and to compare these findings with those obtained in humans.

METHODS

Rabbit hearts were arrested by perfusion with Euro-Collins solution at different temperatures. Hearts perfused with buffer served as controls. Tissue samples were incubated for varying time intervals and processed for immunohistochemistry and electron microscopy. Porcine hearts were treated in the same manner. Changes in the localization of myosin, desmin, and tropomyosin antibodies were evaluated and the degree of ischemic injury was determined by electron microscopy.

RESULTS

Healthy animal hearts tolerate ischemia better than human hearts. Cardiac proteins are more sensitive to ischemia than the ultrastructural cellular organelles. Temperatures as low as 0 degree C produce more cell damage than 4 degrees C and should therefore be avoided. The Euro-Collins solution protects the myocardium better than buffer.

CONCLUSIONS

We conclude that healthy animal hearts are more resistant to ischemia than diseased human hearts and that results from experimental studies should be interpreted with caution with regard to the human situation.

The pattern of desmin filaments in myocardial disarray

"Myofiber disarray" defines a nonparallel arrangement of cardiac myocytes. The presence of a sufficient quantity of myocardial fibers showing this change is considered to be a specific histological feature of hypertrophic cardiomyopathy (HCM). However, small zones of myofiber disarray are found in both cardiac hypertrophy and other pathological conditions. Recently, we demonstrated an altered pattern of desmin intermediate filaments in disarrayed myofibers from specimens of HCM. To test the hypothesis that desmin alterations might be specific for cardiomyopathy, we performed an immunohistochemical study on myocardial surgical samples from 11 patients with HCM and from 12 patients with tetralogy of Fallot (toF) on 14 endomyocardial biopsy specimens (EMBs) from transplant recipients with myofiber disarray surrounding areas of scarring (previous biopsy site) and on specimens of four autoptic hearts with severe acquired left ventricular hypertrophy. Disarrayed myofibers from all specimens of HCM showed the following abnormalities in the pattern of desmin intermediate filament distribution: (1) decrease or loss of labeling of intercalated discs and Z bands, (2) longitudinal arrangement of desmin intermediate filaments, and (3) intense, granular staining of several myocytes. This spectrum of desmin alterations was never observed in disarrayed myofibers in specimens of toF or acquired myocardial hypertrophy or in EMBs. Altered distribution of desmin intermediate filaments seems to be specific to myofiber disarray in HCM and it may play a role in the altered myocyte arrangement in HCM.

Differential organization of desmin and vimentin in muscle is due to differences in their head domains

In most myogenic systems, synthesis of the intermediate filament (IF) protein vimentin precedes the synthesis of the muscle-specific IF protein desmin. In the dorsal myotome of the Xenopus embryo, however, there is no preexisting vimentin filament system and desmin's initial organization is quite different from that seen in vimentin-containing myocytes (Cary and Klymkowsky, 1994. Differentiation. In press.). To determine whether the organization of IFs in the Xenopus myotome reflects features unique to Xenopus or is due to specific properties of desmin, we used the injection of plasmid DNA to drive the synthesis of vimentin or desmin in myotomal cells. At low levels of accumulation, exogenous vimentin and desmin both enter into the endogenous desmin system of the myotomal cell. At higher levels exogenous vimentin forms longitudinal IF systems similar to those seen in vimentin-expressing myogenic systems and massive IF bundles. Exogenous desmin, on the other hand, formed a reticular IF meshwork and non-filamentous aggregates. In embryonic epithelial cells, both vimentin and desmin formed extended IF networks. Vimentin and desmin differ most dramatically in their NH2-terminal "head" regions. To determine whether the head region was responsible for the differences in the behavior of these two proteins, we constructed plasmids encoding chimeric proteins in which the head of one was attached to the body of the other. In muscle, the vimentin head-desmin body (VDD) polypeptide formed longitudinal IFs and massive IF bundles like vimentin. The desmin head-vimentin body (DVV) polypeptide, on the other hand, formed IF meshworks and non-filamentous structures like desmin. In embryonic epithelial cells DVV formed a discrete filament network while VDD did not. Based on the behavior of these chimeric proteins, we conclude that the head domains of vimentin and desmin are structurally distinct and not interchangeable, and that the head domain of desmin is largely responsible for desmin's muscle-specific behaviors.

Rearrangement of mRNAs for costamere proteins during costamere development in cultured skeletal muscle from chicken

Mature skeletal myofibrils are surrounded by costameres, ribs of metavinculin, vinculin, intermediate filaments, and other proteins that connect the myofibril to the extracellular matrix. Costameres have recently been shown to be the sites at which the forces generated by the myofibril are transduced laterally into the extracellular matrix. We observed costameres developing in cultured skeletal muscles, grown in micromass culture from cells taken from embryonic chicken leg. We detected proteins by immunofluorescence and mRNA by in situ hybridization. Antibody and probe signals were imaged by laser scanning confocal microscopy. Antibody to vimentin protein is first detected in stripes in register with the Z line of the myofibril, at approximately day 12 after fusion; soon thereafter probe to vimentin mRNA is also detected in the same stripes. Optical sections indicate that vimentin mRNA and protein are very close, no more than 0.1 mm apart and possibly in immediate contact. Antibody to vimentin is detected in stripes only in cells that twitch spontaneously. Antibodies and probes to desmin and vinculin protein and mRNA are next detected in stripes of the same periodicity, at approximately day 17 after fusion. Vinculin protein (but not mRNA) is detected at focal contacts much earlier in development. Controls for bleed through of fluorescence, RNase H sensitivity, hybridization without probe, and binding to the myofibril all gave appropriate results. Probes to glyceraldehyde-3-phosphate dehydrogenase, a glycolytic enzyme, stained diffusely and did not associate with the myofibril. These results show that components of the costamere arrive at the structure in a defined sequence, and that mRNA organization is a conspicuous, precise and temporally controlled aspect of costamere development. These results may have wider implications. In these cells, some mRNAs are positioned with submicrometer precision in space and differentially over time. Particular mRNAs differ in the time and place of such positioning. This implies both that cellular structures provide physical cues for such positioning and that mRNA contains information that interacts with such cues in a message-specific manner. If such precision in mRNA location is found in other somatic cells, it could have significant implications for the ways in which cells generate and maintain cellular structures.

ADP-ribosylation of the intermediate filament protein desmin and inhibition of desmin assembly in vitro by muscle ADP-ribosyltransferase

Arginine-specific mono(ADP-ribosyl)transferase purified from rabbit skeletal muscle catalyzes stoichiometric ADP-ribosylation of the intermediate filament protein, desmin. In contrast, cholera toxin catalyzes a much lower level of ADP-ribosylation of desmin. Modification results in potent inhibition of desmin's ability to assemble into filaments. Phosphorylation of desmin by the catalytic subunit of cAMP dependent protein kinase is also inhibited by ADP-ribosylation. ADP-ribosylation site(s) are located within the N-terminal head domain of desmin.

Cytoskeletal and cytocontractile protein composition of smooth muscle cells in developing and obstructed rabbit bladder

The differentiation patterns of smooth muscle cells (SMC) in rabbit bladder during development and in the hypertrophic response to partial outflow obstruction induced in adult animals were evaluated by biochemical and immunochemical techniques and by using a panel of monoclonal antibodies specific for desmin, vimentin, alpha-actin of smooth muscle (SM) type, SM myosin, and nonmuscle (NM) myosin isoforms. Desmin and SM alpha-actin were homogeneously distributed in SMC of developing, adult, and obstructed bladders. Conversely, marked changes in the ratio and antigenicity of SM myosin isoforms were observed by SDS electrophoresis and Western blotting, respectively. In particular, the 205 K (SM1) isoform was down-regulated with development whereas the 200 K (SM2) isoform was up-regulated around 7 days after birth and down-regulated in the obstructed bladder. Vimentin was expressed in SMC of the fetal bladder and declined markedly during postnatal, physiological hypertrophy of SMC, which occurs concomitantly with diminution of DNA synthesis. This polypeptide became detectable, however, in SMC of obstructed bladders. The 196 K (NM) myosin isoform recognized by NM-A9 antibody, present only in endothelium of blood vessels and in mucosa of normal fetal and adult bladders, became expressed in detrusor muscle, when SMC underwent a process of pathological hypertrophy. The reexpression of vimentin and the de novo appearance of NM myosin isoform in hypertrophic bladders can be reversed when the tissue mass is reduced, such as in bladders after 1-month recovery from partial obstruction. Thus, a specific NM myosin isoform can be used as a marker of SMC hypertrophy in obstructed bladder. In addition, the combined use of anti-vimentin and NM-A9 antibodies can distinguish between SMC which are in the physiological or in the pathological condition of adaptive bladder hypertrophy.

Familial desminopathy: myopathy with accumulation of desmin-type intermediate filaments

Two siblings developed cardiomyopathy several years before slowly progressive muscle weakness. Skeletal muscle biopsy specimens showed subsarcolemmal crescents of dark eosinophilic material in both type I and type II fibres. Immunohistochemically the subsarcolemmal material stained positively for the intermediate filament protein desmin and for the heat shock protein ubiquitin but for no other cytoskeletal proteins. Ultrastructurally the subsarcolemmal deposits consisted of aggregates of granular and filamentous material arising from Z-bands. Follow up muscle biopsies six years later showed an increased number of the muscle fibres that contained subsarcolemmal aggregates that stained positively for desmin and ubiquitin. These clinical and pathological features characterise a rare familial myopathy associated with an unusual distribution of desmin intermediate filament proteins in skeletal and probably also cardiac muscle.

Peptides from the conserved ends of the rod domain of desmin disassemble intermediate filaments and reveal unexpected structural features: a circular dichroism, Fourier transform infrared, and electron microscopic study

Synthetic peptides representing the conserved ends of the rod domain of desmin are shown to disassemble preformed desmin filaments when added in moderate molar excess. This argues for a similar importance of both ends of the rod for filament stability. Recent structural models of intermediate filaments suggest close proximity of the ends and perhaps even an interaction (N. Geisler, J. Schünemann, and K. Weber, 1992, Eur. J. Biochem. 206, 841-852; P. M. Steinert, L. N. Marekov, R. D. B. Fraser, and D. A. D. Parry, 1993, J. Mol. Biol. 230, 436-452). Since the disassembling activity of the peptides, in addition to their sequences, should be related in some way to their secondary structure, we have investigated the structures of a number of related peptides which all arise from the ends of the rod using electron microscopic and spectroscopic methods. All peptides showed the expected alpha-helical structure at low concentrations in the presence of trifluoroethanol, as revealed by circular dichroism. At higher concentrations the peptides showed extensive self-aggregation into various types of filaments. The filaments contain the peptides in beta-sheet conformation as shown by Fourier transform infrared spectroscopy.

Cleavage of human and mouse cytoskeletal and sarcomeric proteins by human immunodeficiency virus type 1 protease. Actin, desmin, myosin, and tropomyosin

HeLa cell actin was cleaved by human immunodeficiency virus type 1 protease when in its soluble, globular form (G-actin). No cleavage of the polymerized, filamentous form of actin (F-actin) was observed when examined by denaturing gel electrophoresis; however, electron microscopy revealed a low level of cleavage of F-actin. Immunoblotting of mouse skeletal and human pectoral muscle myofibrils treated in vitro with human immunodeficiency virus type 1 protease showed that myosin heavy chain, desmin, tropomyosin, and a fraction of the actin were all cleaved. Electron microscopy of these myofibrils demonstrated changes consistent with cleavage of these proteins: Z-lines were rapidly lost, the length of the A bands was shortened, and the thick filaments (myosin filaments) were often laterally frayed such that the structures disintegrated. Nonmuscle myosin heavy chains were also cleaved by this enzyme in vitro. These data demonstrate that this protease can cause alterations in muscle cell ultrastructure in vitro that may be of clinical relevance in infected individuals.

p13suc1 suppresses the catalytic function of p34cdc2 kinase for intermediate filament proteins, in vitro

The regulation of p34cdc2 kinase activity controls the entry into and exit from mitosis. Although genetic and biochemical evidence suggested close interactions between cyclins, p13suc1 and p34cdc2 kinase, the roles of p13suc1 on p34cdc2 kinase functions remain unclear. To examine the effects of p13suc1 on p34cdc2 kinase function we developed a simple purification procedure for p34cdc2 kinase, unassociated with p13suc1. The key to the purification procedures we used was buffer containing 0.5 M NaCl and 50% ethylene glycol, as a specific elutant of p34cdc2 kinase from p13suc1-Sepharose. This purified p34cdc2 kinase stoichiometrically phosphorylated vimentin and desmin. Exogenous p13suc1 suppressed the phosphorylation of these filament proteins by the kinase and prevented disassembly, although histone H1 phosphorylation was not affected. Peptide mapping analysis showed a similar extent of inhibition by p13suc1 for all five phosphorylation sites by p34cdc2 kinase of vimentin and desmin, hence these p13suc1-induced inhibitions are probably not site-specific. It thus appears that p13suc1 has a selective effect on the catalytic activity of p34cdc2 kinase for these filament proteins.

Chemical cross-linking indicates a staggered and antiparallel protofilament of desmin intermediate filaments and characterizes one higher-level complex between protofilaments

Tetrameric rods, protofilaments and assembled filaments of desmin, the intermediate filament protein of muscle, have been chemically cross-linked with the lysine specific cross-linkers EGS [ethylene glycol bis(succinimidylsuccinate), 1.61 nm span] and bis(sulfosuccinimidyl) suberate (1.14 nm span). One bis(sulfosuccinimidyl)suberate and two EGS cross-links were isolated from the rod and characterized. They show that the two coiled coils in the rod tetramer are staggered by approximately 15-20 nm and strongly indicate an antiparallel arrangement in which the inner overlapping part of the rod is formed by the amino-terminal helices 1A, 1B and 2A. Both EGS cross-links identified in the rod were also isolated from cross-linked filaments. The isolated rod, therefore, represents a complex also present in identical, or very similar form in protofilaments and in assembled filaments. Cross-linked filaments yielded a third EGS cross-link that must have been formed between neighboring protofilaments. It connects the highly conserved carboxy-terminus of helix 2B of the first protofilament to the overlap region formed by helices 1A and 2A of the second protofilament. The restrictions posed by these cross-links on current filament models are discussed.

Peripheral neuropathy with giant axons and cardiomyopathy associated with desmin type intermediate filaments in skeletal muscle

A sporadic case (female, aged 14 years) is reported who was affected by myopathy, restrictive cardiomyopathy and sensory motor polyneuropathy. A muscle biopsy showed accumulation of osmiophilic granular and filamentous material on electron microscopy, which stained positively in immunofluorescence for desmin. Increased desmin phosphorylated isoforms have been demonstrated by one- and two-dimensional electrophoresis. Sural nerve biopsy showed a peripheral neuropathy with giant axons, filled with closely packed neurofilaments. Clinical and morphological aspects of this new disease entity are discussed with regards to the classical form of giant axonal neuropathy and to other conditions of peripheral neuropathy with giant axons.

A synthetic peptide representing the consensus sequence motif at the carboxy-terminal end of the rod domain inhibits intermediate filament assembly and disassembles preformed filaments

All intermediate filament (IF) proteins share a highly conserved sequence motif at the COOH-terminal end of their rod domains. We have studied the influence of a 20-residue peptide, representing the consensus motif on filament formation and stability. Addition of the peptide at a 10-20-fold molar excess over keratins K8 plus K18 had a severe effect on subsequent IF assembly. Filaments displayed a rough surface and variable diameters with a substantial amount present in unravelled form. At higher peptide concentration (50-100-fold molar excess), IF formation was completely inhibited and instead only loose aggregates of "globular" particles were formed. The peptide also influenced performed keratin IF in a dose-dependent manner. While a three-fold molar excess was sufficient to cause partial fragmentation of IF, a 50-fold molar excess caused complete disassembly within 5 min. Loosely associated protofibrils, short needlelike IF fragments, and aggregates of globular particles were detected. The motif peptide also caused the disassembly of filaments formed by desmin, a type III IF protein. Peptide concentrations and incubation times required for complete disassembly were somewhat higher than for the filaments containing K8 plus K18. A 50-fold molar excess was sufficient to cause complete disassembly within 1 h. Peptides unrelated in sequence to the motif did not interfere with filament formation or stability even when present for more than 12 h at a 100-fold molar excess. The results suggest that the motif sequence normally binds to a specific acceptor site for which the motif peptide can successfully compete. Taken together with current models of IF structure the results indicate that normal binding of the motif sequence to its acceptor must play an essential role in IF formation, possibly by directing the proper alignment of neighboring tetramers or protofilaments. Finally we show that in vitro formed IF are much more sensitive and dynamic strutures than previously thought.

Determination of the critical concentration required for desmin assembly

The critical concentration required for filament assembly in vitro from highly purified desmin was determined by both turbidity and centrifugation assays. Assembly was done in the presence of 2 mM-Ca2+, 2 mM-Mg2+ or 150 mM-Na+ at 2, 22 and 37 degrees C. Similar values for critical concentration were obtained by both assays. As temperature increased, critical concentration decreased for each cation. The critical concentration was lowest in the presence of Ca2+ at 2, 22 and 37 degrees C, but was highest in the presence of 150 mM-Na+ at 2 degrees C. Negative staining showed that supernatants from the centrifugation assays contained protofilaments, protofibrils and short particles (less than 300 nm), but pellets contained long filaments (greater than 1 micron) with an average diameter of 10 nm. As the temperature increased, both the average diameter and average length of particles in the supernatant increased. Thermodynamic analysis indicated that hydrophobic interactions were dominant during desmin assembly, but that ionic interactions might also be involved. Our results demonstrated that the specific cation and temperature and temperature-cation interactions all are important in assembly of desmin intermediate filaments.

Cardiomyopathy and multicore myopathy with accumulation of intermediate filaments

A girl affected by a restrictive cardiomyopathy with neuromuscular involvement is described. Morphological examination showed a pattern of multicore myopathy and with electron microscopy a sarcoplasmic accumulation of electron dense granular and filamentous material was demonstrated both in skeletal muscle and heart. This peculiar electron dense material corresponded to increased desmin in muscle and cardiac fibres and was demonstrated immunohistochemically.

The two coiled coils in the isolated rod domain of the intermediate filament protein desmin are staggered. A hydrodynamic analysis of tetramers and dimers

Desmin protofilaments and the proteolytically derived alpha-helical rod domain have been characterized by high-resolution gel permeation chromatography (GPC) using columns calibrated for the determination of viscosity radii. Additional characterization by chemical cross-linking and the determination of sedimentation values allowed the calculation of the molecular dimensions of the molecular species isolated. In dilute buffers GPC separated desmin rod preparations into two complexes: a dimer species (single coiled coil) with a length of 50 +/- 5 nm and a tetramer species (two coiled coils) with a length of 65 +/- 5 nm. Thus the two coiled coils in the tetramer are staggered by approximately 15 nm. The hydrodynamically derived lengths of the rod dimer and tetramer are supported by electron microscopy after metal shadowing. The hydrodynamic properties of desmin protofilaments follow that of the rod tetramer. The data on the hydrodynamic analysis of the rod tetramer of desmin in solution are in full agreement with the structural information recently deduced from paracrystals of the rod of glial fibrillary acid protein [Stewart, M., Quinlan, R.A. & Moir, R.D. (1989) J. Cell Biol. 109, 225-234]. Our results explain the inhomogeneity of molecules encountered in previous electron microscopical analyses.

Desmin-containing intermediate filaments: structural analysis using monoclonal antibodies

1. Monoclonal antibodies have been raised against N- and C-terminal desmin sequence regions. 2. The antibodies decorated desmin intermediate filaments in a helical fashion with four antibody molecules per helix turn. 3. The filaments could be decorated with both types of antibody consecutively. 4. These results support a model of intermediate filament assembly in which the tetrameric protofilaments are aligned in a staggered fashion with partial overlapping of the central rod domain regions of the desmin sequence.