Preliminary biochemical investigations of the intermediate filaments

Immunofluorescent localization of desmin and vimentin in developing cardiac muscle of Syrian hamster

The distributions of desmin and vimentin were examined in frozen sections of cardiac muscle from embryonic, newborn, and adult Syrian hamster by using immunofluorescent methods. Frozen sections of newborn and adult skeletal muscle were used for comparison. Cardiac myocytes from day 9 in utero embryos already show a clear association of desmin with the sarcomeric myofibrils. In newborn hearts, desmin is localized in the myofibrillar Z-line areas as well as in the peripheral cytoplasm of the cell. Three days after birth, desmin is associated with the intercalated discs. Thus, in adult cardiac muscle, desmin is present in both Z-bands and intercalated discs. Skeletal muscle of newborn and adult hamster also contains desmin associated with the Z-lines of myofibrils. Vimentin is associated with the myofibrils of day 9 in utero cardiac muscle cells. The protein remains associated with the myofibrillar Z-lines in the newborns and adults. No detectable staining for vimentin was observed in newborn or adult hamster skeletal muscle. The existence of vimentin as well as desmin in differentiated cardiac muscle may be a consequence of the somewhat more epithelial-like nature of cardiac cells as compared to skeletal muscle syncitia.

The synthesis and distribution of desmin and vimentin during myogenesis in vitro

Electrophoretic and autoradiographic analyses of the incorporation of 35S--methionine into newly synthesized proteins during myogenesis reveal that presumptive chicken myoblasts synthesize primarily one intermediate filament protein: vimentin. Desmin synthesis is initiated at the onset of fusion. Synthesis rates of both filament subunits increase during the first three days in culture, relative to the total protein synthesis rate. The observed increase in the rate of desmin synthesis (at least 10 fold) is significantly greater than that observed for vimentin, and is responsible for a net increase in the cellular desmin content relative to vimentin. Both filament subunits continue to be synthesized through at least 20 days in culture. Immunofluorescent staining using desmin- and vimentin-specific antisera supports the conclusion that desmin is synthesized only in fusing or multinucleate cells. These results indicate that the synthesis of the two filament subunits is not coordinately regulated during myogenesis. The distributions of desmin and vimentin in multinucleate chicken myotubes are indistinguishable, as determined by double immunofluorescence techniques. In early myotubes, both proteins are found in an intricate network of free cytoplasmic filaments. Later in myogenesis, several days after the appearance of alpha--actinin-containing Z line striations, both filament proteins become associated with the Z lines of newly assembled myofibrils, with a corresponding decrease in the number of cytoplasmic filaments. This transition corresponds to the time when the alpha--actinin-containing Z lines become aligned laterally. These data suggest that the two intermediate filament systems, desmin and vimentin, have an important role in the lateral organization and registration of myofibrils and that the synthesis of desmin and assembly of desmin-containing intermediate filaments during myogenesis is directly related to these functions. These results also indicate that the Z disc is assembled in at least two distinct steps during myogenesis.

Desmin and vimentin coexist at the periphery of the myofibril Z disc

Two-dimensional gel electrophoresis has revealed that vimentin, the predominant subunit of intermediate filaments in cells of mesenchymal origin, is a component of isolated skeletal myofibrils. It thus coexists in mature muscle fibers with desmin, the major subunit of muscle intermediate filaments. Antisera to desmin and vimentin, shown to be specific for their respective antigens by two-dimensional immunoautoradiography, have been used in immunofluorescence to demonstrate that vimentin has the same distribution as desmin in skeletal muscle. Both desmin and vimentin surround each myofibril Z disc and form honeycomb-like networks within each Z plane of the muscle fiber. This distribution is complementary to that of alpha-actinin within a given Z plane. Desmin and vimentin may thus be involved in maintaining the lateral registration of sarcomeres by transversely linking adjacent myofibrils at their Z discs. This linkage would support and integrate the fiber as a whole, and provide a molecular basis for the cross-striated appearance of skeletal muscle.

Intermediate (skeletin) filaments in heart Purkinje fibers. A correlative morphological and biochemical identification with evidence of a cytoskeletal function

Cow Purkinje fibers contain a population of free cytoplasmic filaments which consistently differ in ultrastructural appearance from actin and myosin filaments, irrespective of preparation technique. The fixation and staining techniques, however, influenced the filament diameter, which was found to be 7.4--9.5 nm for filaments in plastic-embedded material, and 7.0 nm in cryo-sectioned material, thus intermediate as compared to actin and myosin filaments. Cross-sectional profiles suggested that the intermediate-sized filaments are composed of four subfilaments. To provide a basis for further biochemical investigations on the filaments, extraction procedures were carried out to remove other cell organelles. Electron microscopy showed that undulating bundles of intermediate filaments converging towards desmosomes still remained, after the extractions, together with Z-disk material. In spite of the extensive extraction, the shape of the individual cells and the assemblies of cell bundles remained intact. This confirms that the intermediate filaments of cow Purkinje fibers together with desmosomes do in fact have a cytoskeletal function. On account of (a) the cytoskeletal function of the filaments, (b) the similarities to the smooth muscle "100-A filament" protein subunit skeletin, and (c) the inadequate and confusing existing terminology, we suggest that the filaments be named "skeletin filaments."

Effect of anabolic steroids on rat heart muscle cells. I. Intermediate filaments

Long-term treatment of female rats with the anabolic steroid hormone Methandrostenolone results in a conspicuous increase of intermediate sized, nonmyofibrillar filaments in muscle cells of the left cardiac ventricle, as revealed by electron microscopy. These filaments, measuring 70 - 110 A in diameter, form a characteristic network at the Z-level of the sarcomere, either encircling or penetrating the Z-bands, and appear to insert into the nuclear membrane. The T-system is accompanied by the filaments adjacent to the site of the couplings. Here they are attached to subsarcolemmal electron-dense patches, which may be Z-line precursor material. The filaments may function as a cytoskeleton, to provide passive support in the mechanism of contraction and to mediate nucleo-sarcolemmal and nucleo-myofibrillar exchange.

Mixed Müllerian tumors of the uterus. Ultrastructural studies on the differentiation of rhabdomyoblasts

In the present study the differentiation of rhabdomyoblasts of two carcinosarcomas of the uterus is analysed electronmicroscopically. During the development of rhabdomyoblasts three cell types can be distinguished: 1. the undifferentiated mesenchymal cell with abundant ribosomes but few other cell organelles. Usually these cells are already associated to each other in strands of 2-4 cells. Occasionally cytoplasmic areas with numerous nonspecific filaments can be observed. 2. the light rhabdomyoblast which is characterized by its conspicuous nonspecific cytoplasmic filaments and its reduction in ribosomes. Specific myofilaments can be visualized only occasionally in this cell type. 3. more differentiated rhabdomyoblasts. These cells can be identified clearly by the presence of large numbers of myofibrils and myofibrillar fragments. Higly organized myofibrils as seen in skeletal muscle are seldom found. Transitional forms between these cell types are also observed. The functions of Z-bodies and of the sarcotubular system in the process of myofibrillar differentiation are discussed.

Myocardial ultrastructure in idiopathic hypertrophic subaortic stenosis. A study of operatively excised left ventricular outflow tract muscle in 14 patients

Electron microscopic studies revealed distinctive abnormalities in operatively resected myocardium from the left ventricular outflow tract in 14 patients with idiopathic hypertrophic subaortic stenosis. Bundles of muscle cells were severely disorganized, with cells running in different directions instead of in parallel. Muscle cells were wider and shorter than in hypertrophy due to other causes and showed increased cellular branching, extensive side-to-side intercellular junctions, widened Z bands, and evidence of formation of new sarcomeres. Some myofibrils were oriented obliquely or perpendicular to the longitudinal axes of the cells and some myofilaments that originated from a single Z band inserted into Z bands of other myofibrils. Examination of left ventricular apical myocardium in two patients revealed hypertrophied but normally arranged muscle cells. It is concluded that abnormal architecture of muscle cells is the basic morphologic feature of idiopathic hypertrophic subaortic stenosis.