Immunocytochemical studies of desmin and vimentin in pericapillary cells of chicken

The composition of intermediate filaments in pericytes was examined by immunofluorescent and immunoelectron microscopic labeling of frozen sections of various chicken microvascular beds in situ. Pericytes in capillaries of cardiac muscle, exocrine pancreas, and kidney (peritubular capillary) were found to contain both desmin and vimentin. In some capillaries where pericytes do not exist, cells apposed to endothelial cells--the Ito cell in the hepatic sinusoid and the reticular cell in the splenic sinusoid--were shown to contain both of the intermediate filament proteins. In contrast, podocytes and mesangial cells around renal glomerular capillaries contained only vimentin. The presence of desmin supports the hypothesis that pericytes may have a contractile apparatus similar to that of vascular smooth muscle cells. Our results also revealed that even in microvascular beds where pericytes are not found, cells having both desmin and vimentin exist next to endothelial cells and may assume similar functions to pericytes.     

Expression of intermediate filament-associated proteins paranemin and synemin in chicken development

The expression of two intermediate filament-associated proteins, paranemin (280,000 mol wt) and synemin (230,000 mol wt), was investigated with respect to the expression of two core intermediate filament proteins, desmin and vimentin, in various embryonic and adult chicken muscle and nonmuscle cells. All developing muscle cells, regardless of their type, simultaneously express desmin, vimentin, paranemin, and synemin. However, a difference is observed in the expression of paranemin in adult muscle. This protein is removed during differentiation of both fast and slow skeletal muscle, visceral smooth muscle, and the smooth muscle of muscular arteries, but remains in mature myocardial cells, cardiac conducting fibers, and the smooth muscle cells of elastic arteries. Some of these cells express vimentin, others desmin, and still others a mixture of the two. On the other hand, synemin is expressed in all the above types of adult muscle cells except myocardial cells. Adult myocardial cells also lack vimentin, and its presence is gradually reduced after hatching. Since in adult striated muscle all expressed intermediate filament proteins are found predominantly in association with the peripheries of myofibrillar Z discs, these results suggest that a change in the composition of skeletal and cardiac muscle Z discs occurs during chicken development and maturation. Erythrocytes that express synemin and vimentin do not express paranemin, while both embryonic and adult Schwann cells co- express paranemin and vimentin, but not synemin. Endothelial cells of muscular vessels express paranemin, while those of elastic vessels do not, and neither contains synemin. Paranemin and synemin are not expressed in neurons, epithelial, and most glial cells, suggesting that these two polypeptides are expressed only in conjunction with desmin or vimentin. These results suggest that the composition of intermediate filaments changes during chicken development, not only with respect to their core subunit proteins but also with respect to two associated polypeptides, particularly in muscle cells.     

Localization of vimentin and desmin in BHK21/C13 cells and in baby hamster kidney

Specific antibodies against the intermediate filament protein subunits, desmin and vimentin, were used to characterize the fibroblastic tissue culture cell line BHK21/C13 and the cells comprising baby hamster kidney (BHK). The BHK21/C13 cells have previously been shown to contain desmin and vimentin by biochemical techniques. The results from double immunofluorescence analysis show that both immunologically distinct intermediate filament subunit proteins are expressed simultaneously within the same BHK21/C13 cell, and that the filamentous patterns are very similar, if not superimposable even in cells treated with colchicine. There are some cells that may contain vimentin only. Double immunofluorescence on cryostat sections of BHKs and preparations of dissociated kidney cells demonstrate that the cells, most likely smooth muscle, comprising the blood vessel walls contain vimentin and desmin simultaneously. The simultaneous expression of vimentin and desmin is not a phenomenon which is restricted to tissue culture cells. Thus, the simultaneous presence of these two intermediate filament proteins within the BHK21/C13 cell may not be the result of growth in tissue culture.     

Heterogeneity of Intermediate Filament Expression in Vascular Smooth Muscle: A Gradient in Desmin Positive Cells from the Rat Aortic Arch to the Level of the Arteria Iliaca Communis

The display of the two distinct intermediate filament proteins, desmin and vimentin, in rat vascular smooth muscle tissue was studied by immunofluorescence microscopy on frozen sections of aorta and other blood vessels. Vascular smooth muscle cells present in these vessels always appeared rich in vimentin. However, staining of sections covering six distinct but contiguous parts of the aorta showed that the number of desmin containing cells was low distal to the truncus brachiocephalicus, but increases until in distal parts of the aorta and in the arteria iliaca communis almost all cells appear positive for desmin. Thus blood vessels show heterogeneity of intermediate filament expression not only in cross-section but can also display heterogeneity along their length. Muscular arteries such as the renal artery and the arteria femoralis, as well as arterioles and veins including the vena jugularis and the vena cava also contain desmin. Thus it may be that low numbers of desmin-positive cells are typical of elastic arteries, while muscular arteries and other blood vessels are characterized by large numbers of desmin-positive cells. We discuss whether desmin-positive and desmin-negative vascular smooth muscle cells may perform different functions and raise the possibility that desmin expression may coincide with the turn on of a specially regulated contractility program.     

In vitro translation of the intermediate filament proteins desmin and vimentin.

Polyadenylated ribonucleic acid (RNA) was isolated from chicken skeletal and smooth muscle and translated in a cell-free rabbit reticulocyte system. Both types of muscle tissue contain messenger RNAs that code for the intermediate filament proteins desmin and vimentin, and the relative concentrations of the two translation products reflect the prevalence of the two proteins in vivo. Desmin synthesis represents a greater proportion of the total protein synthesis from smooth muscle RNA than from skeletal muscle RNA, whereas the converse is true of vimentin synthesis. Fractionation of the RNA on formamide-containing sucrose gradients before translation indicates that the desmin messenger RNA is larger than the vimentin messenger RNA and contains an extensive noncoding segment. The desmin and vimentin messages code predominantly for the non-phosphorylated forms of desmin and vimentin. However, the ratio of phosphorylated to unphosphorylated forms of the proteins could be increased by adding cyclic adenosine monophosphate-dependent kinase activity to the translation mixtures. These results suggest that desmin and vimentin are each synthesized from a single messenger RNA species and that posttranslational phosphorylation generates the additional isoelectric variants of each which are observed in vivo.     

Intermediate filaments in smooth muscle from pregnant and non-pregnant human uterus.

The intermediate filament proteins desmin and vimentin from pregnant and non-pregnant uterine muscle and smooth-muscle cells in culture were analysed using SDS/PAGE. The desmin content in uterine muscle increases dramatically during pregnancy, whereas vimentin remains unchanged or changes very little. When muscle cells are kept in culture, a considerable increase in vimentin content is observed as compared with vimentin in freshly isolated non-pregnant uterine tissue. Our results strengthen the view that vimentin and desmin filaments have independent function and turnover, and point to a predominantly structural role for desmin filaments.     

Visualization of intermediate filaments in living cells using fluorescently labeled desmin

Fluorescently labeled desmin was incorporated into intermediate filaments when microinjected into living tissue culture cells. The desmin, purified from chicken gizzard smooth muscle and labeled with the fluorescent dye iodoacetamido rhodamine, was capable of forming a network of 10-nm filaments in solution. The labeled protein associated specifically with the native vimentin filaments in permeabilized, unfixed interphase and mitotic PtK2 cells. The labeled desmin was microinjected into living, cultured embryonic skeletal myotubes, where it became incorporated in straight fibers aligned along the long axis of the myotubes. Upon exposure to nocodazole, microinjected myotubes exhibited wavy, fluorescent filament bundles around the muscle nuclei. In PtK2 cells, an epithelial cell line, injected desmin formed a filamentous network, which colocalized with the native vimentin intermediate filaments but not with the cytokeratin networks and microtubular arrays. Exposure of the injected cells to nocadazole or acrylamide caused the desmin network to collapse and form a perinuclear cap that was indistinguishable from vimentin caps in the same cells. During mitosis, labeled desmin filaments were excluded from the spindle area, forming a cage around it. The filaments were partitioned into two groups either during anaphase or at the completion of cytokinesis. In the former case, the perispindle desmin filaments appeared to be stretched into two parts by the elongating spindle. In the latter case, a continuous bundle of filaments extended along the length of the spindle and appeared to be pinched in two by the contracting cleavage furrow. In these cells, desmin filaments were present in the midbody where they gradually were removed as the desmin filament network became redistributed throughout the cytoplasm of the spreading daughter cells.     

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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 α-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 a-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.     

Glomus tumor cells: evaluation of smooth muscle and endothelial cell properties

Six cases of glomus tumor in superficial soft tissues were investigated immunohistochemically for the presence of different types of intermediate filaments, myosin, laminin, a basal lamina glycoprotein, and the endothelial cell markers, factor VIII-related antigen (FVIIIR:Ag) and Ulex europaeus I lectin (UEA I) binding sites. The tumor cells appeared to contain only vimentin, the fibroblast-type of intermediate filament protein. They were also positive for myosin, and were invested by laminin-positive basal lamina-like material, but did not express endothelial cell markers. Ultrastructural studies revealed prominent arrays of both intermediate filaments and microfilaments, the latter resembling the myofilament bundles seen in smooth muscle cells. The results show that glomus tumor cells resemble smooth muscle cells in their content of myosin and in some ultrastructural features. In their lack of desmin, however, they differ from most types of smooth muscle cell, although they are similar in this respect to some vascular smooth muscle cells.     

Association of glycosphingolipids with intermediate filaments of mesenchymal, epithelial, glial, and muscle cells.

We reported recently that two glycosphingolipids (GSLs), globoside (Gb4) and ganglioside GM3, colocalized with vimentin intermediate filaments of human umbilical vein endothelial cells. To determine whether this association is unique to endothelial cells or to vimentin, we analyzed a variety of cell types. Double-label immunofluorescent staining of fixed, permeabilized cells, with and without colcemid treatment, was performed with antibodies against glycolipids and intermediate filaments. Globoside colocalized with vimentin in human and mouse fibroblasts, with desmin in smooth muscle cells, with keratin in keratinocytes and hepatoma cells, and with glial fibrillary acidic protein (GFAP) in glial cells. Globoside colocalization was detected only with vimentin in MDCK and HeLa cells, which contain separate vimentin and keratin networks. GM3 ganglioside also colocalized with vimentin in human fibroblasts. Association of other GSLs with intermediate filaments was not detected by immunofluorescence, but all cell GSLs were detected in cytoskeletal fractions of metabolically labelled endothelial cells. These observations indicate that globoside colocalizes with vimentin, desmin, kertain and GFAP, with a preference for vimentin in cells that contain both vimentin and keratin networks. The nature of the association is not yet known. Globoside and GM3 may be present in vesicles associated with intermediate filaments (IF), or bound directly to IF or IF associated proteins. The prevalence of this association suggests that colocalization of globoside with the intermediate filament network has functional significance. We are investigating the possibility that intermediate filaments participate in the intracellular transport and sorting of glycosphingolipids.     

Intermediate filaments in the testis of the teleost mosquito fish Gambusia affinis holbrooki: a light and electron microscope immunocytochemical study and Western blotting analysis

Summary A light and electron microscope immunocytochemical study and Western blotting analysis has been performed on intermediate filaments (vimentin, desmin and cytokeratins) in the testis of the teleost fish Gambusia affinis holbrooki. An immunoreaction to vimentin was observed in the epithelium of the efferent ducts, testicular canal and their surrounding peritubular cells. Positive vimentin immunostaining was also observed in the cells located around seminiferous tubules (boundary cells), Leydig cells, interstitial fibroblasts, chromatophores, and blood vessel endothelial cells. In contrast to mammals, no vimentin immunoreactivity was found in the Sertoli cells. Immunoreactivity to desmin was weak in the epithelial cells of the efferent ducts and testicular canal and intense in the peritubular cells that surrounded these ducts. Desmin immunoreactivity was also observed in the seminiferous tubule boundary cells. The immunoreactivity was weak in the boundary cells that surrounded germ cell cysts containing spermatogonia or spermatocytes and intense in the boundary cells around cysts with elongated or mature spermatids. Immunoreactivity towards cytokeratins was observed only in testicular blood vessels. Cytokeratin immunolabelling was intense in the endothelium and weak in the vascular smooth muscle cells. No cytokeratin immunoreactivity was found in the Sertoli cells, germ cells, interstitial cells or in the efferent duct epithelium. The absence of intermediate filaments in the Sertoli cells, the absence of cytokeratins in the epithelium of the sperm excretory ducts, and the presence of desmin filaments in these epithelial cells are the most important differences with regards to the intermediate filament phenotype in mammalian testes.     

Differential location of different types of intermediate-sized filaments in various tissues of the chicken embryo.

The location of constitutive proteins of different types of intermediate-sized (about 10 mm) filaments (cytokeratin, vimentin, desmin, brain filament protein) was examined in various tissues of 11--20 day chick embryos, using specific antibodies against the isolated proteins and immunofluorescence microscopy on frozen sections and on isolated serous membrane. The tissues studied which contained epithelia were small intestine, gizzard, esophagus, crop, liver, kidney, thymus, mesenteries, and epidermis. The results show that the different intermediate filament proteins, as seen in the same organ, are characteristic of specific lines of differentiation: Cytokeratin filaments are restricted to--and specific for--epithelial cells; vimentin filaments are seen--at this stage of embryogenesis--only in mesenchymal cells, including connective tissue, endothelial and blood cells, and chondrocytes; filaments containing protein(s) related to the subunit protein prepared from gizzard 10 nm filaments (i.e., desmin) are significant only in muscle cells; and intermediate filament protein of brain, most probably neurofilament protein, is present only in nerve cells. We conclude that for most tissues the expression of filaments of cytokeratin, vimentin, desmin, and neurofilament protein is mutually exclusive, and that these protein structurees provide useful markers for histochemical and cytochemical differentiation of cells of epithelial, mesenchymal, myogenic, and neurogenic differentiation.     

Differential Location of Different Types of Intermediate-Sized Filaments in Various Tissues of the Chicken Embryo

The location of constitutive proteins of different types of intermediate-sized (about 10 mm) filaments (cytokeratin, vimentin, desmin, brain filament protein) was examined in various tissues of 11–20 day chick embryos, using specific antibodies against the isolated proteins and immunofluorescence microscopy on frozen sections and on isolated serous membrane. The tissues studied which contained epithelia were small intestine, gizzard, esophagus, crop, liver, kidney, thymus, mesenteries, and epidermis. The results show that the different intermediate filament proteins, as seen in the same organ, are characteristic of specific lines of differentiation: Cytokeratin filaments are restricted to – and specific for – epithelial cells; vimentin filaments are seen – at this stage of embryogenesis – only in mesenchymal cells, including connective tissue, endothelial and blood cells, and chondrocytes; filaments containing protein(s) related to the subunit protein prepared from gizzard 10 nm filaments (i.e., desmin) are significant only in muscle cells; and intermediate filament protein of brain, most probably neurofilament protein, is present only in nerve cells. We conclude that for most tissues the expression of filaments of cytokeratin, vimentin, desmin, and neurofilament protein is mutually exclusive, and that these protein structures provide useful markers for histochemical and cytochemical differentiation of cells of epithelial, mesenchymal, myogenic, and neurogenic differentiation.     

Identification and classification of interstitial cells in the canine proximal colon by ultrastructure and immunocytochemistry

The ultrastructure and immunocytochemistry of interstitial cells (ICs) in the canine proximal colon were investigated. Three types of ICs were found within the tunica muscularis. (1) ICs were located along the submucosal surface of the circular muscle (IC-SM). These cells shared many features of smooth muscle cells, including myosin thick filaments and immunoreactivity to smooth muscle gamma actin, myosin light chain, and calponin antibodies. IC-SM were clearly different from smooth muscle cells in that contractile filaments were less abundant and intermediate filaments consisted of vimentin instead of desmin. (2) ICs in the region of the myenteric plexus (IC-MY) were similar to IC-SM, but these cells had no thick filaments or immunoreactivity to smooth muscle gamma actin or calponin antibodies. (3) The fine structures and immunoreactivity of ICs within the muscle layers (IC-BU) were similar to IC-MY, but IC-BU lacked a definite basal lamina and membrane caveolae. IC-BU and IC-MY were both immunopositive for vimentin. Since all ICs were immunopositive for vimentin, vimentin antibodies may be a useful tool for distinguishing between ICs and smooth muscle cells. Each class of ICs was closely associated with nerve fibers, made specialized contacts with smooth muscle cells, and formed multicellular networks. A combination of ultrastructural and immunocytochemical techniques helps the identification and classification of ICs by revealing the fine structures and determining the chemical coding of each class of ICs.     

Evolution of homologous domains of cytoplasmic intermediate filament proteins and lamins

The earliest gene duplications in the evolution of the intermediate filament proteins created the ancestors of acidic keratins, basic keratins, nonepithelial intermediate filament proteins, and lamins. Biochemistry and function of cytoplasmic intermediate filaments differ greatly from those of lamins. Cytoplasmic intermediate filament proteins have a different cellular location than lamins, form different types of supramolecular structures, and are missing a protein segment found in lamins; but the data presented here indicate that the cytoplasmic intermediate filaments do not have a common ancestor separate from the ancestor of lamins. In the non-epithelial intermediate filament branch, the ancestor of neurofilament proteins and the common ancestor of desmin, vimentin, and glial fibrillary acidic protein (GFAP) diverged first. By evolutionary criteria, the intermediate filament protein recently discovered in neuronal cells does not belong to the neurofilament family but is more closely related to desmin, vimentin, and GFAP. Sequences of different sub-domains yield different evolutionary trees, possibly indicating existence of sub- domain-specific functions.      

Immunocytochemical analysis of intermediate filaments in embryonic heart cells with monoclonal antibodies to desmin

Monoclonal antibodies ( McAbs ) have been generated against a preparation of intermediate filament proteins (IFP) from adult chicken gizzard. Two antibodies, D3 and D76 , have been characterized in detail. They bind specifically to desmin but recognize different epitopes. In the adult chicken, both McAbs produced equivalent immunofluorescent staining patterns, reacting in frozen sections with all forms of muscle tissue, including vascular smooth muscle, but with no other tissue types. In isolated skeletal myofibrils and in longitudinal frozen sections of cardiac and skeletal muscle, desmin was detected with both McAbs at the Z-band and in longitudinally-oriented filament bundles between myofibrils. In contrast to these results in the adult, the intermediate filaments (IF) of embryonic cardiac myocytes in primary cultures were decorated only with McAb D3, whereas McAb D76 was completely unreactive with these cells. Similarly, frozen sections through the heart at early stages of embryonic chick development (Hamburger-Hamilton stages 17-18) revealed regions of myocytes, identified by double immunofluorescence with myosin-specific McAbs , that were unstained with McAb D76 even though similar regions were stained by McAb D3. That McAb D76 reacted with desmin in all adult cardiac myocytes but not with all embryonic heart cells indicates that embryonic and adult cardiac IF are immunologically distinct and implies a conversion in IF immunoreactivity during cardiac development.     

Immunological relationship between different types of bovine intermediate filaments

In order to examine the relationship between the intermediate filaments from Purkinje fibres of the cow heart conduction system and five proposed subclasses of mammalian intermediate filaments, the gel electrophoresis-derived enzyme-linked immunosorbent assay (GEDELISA) has been used to examine the specificity and crossreactivity of our antibodies against the Purkinje fibre intermediate filament protein, skeletin. Bovine tissues known to contain intermediate filaments of the five main subclasses were examined with antiskeletin and with preimmune serum and the specific antiserum absorbed with pure skeletin as controls. The antibodies raised against Purkinje fibre skeletin reacted with all three polypeptides of the "neurofilament triplet", with glial fibrillary acidic protein (GFAP), with smooth muscle desmin and also slightly with some prekeratin subunits and with endothelial vimentin. From studies with monoclonal antibodies and amino acid sequencing, certain regions of all intermediate filaments are suggested to be structurally related. Here we show that Purkinje fibre skeletin seems to share antigenic determinants with the proposed five main classes of intermediate filaments. Our antibody is the first carefully controlled experimentally induced antibody having such properties. This might be due to the special attributes of the intermediate filament system in Purkinje fibres, which themselves have unique properties.     

Assembly of amino-terminally deleted desmin in vimentin-free cells

To study the role of the amino-terminal domain of the desmin subunit in intermediate filament (IF) formation, several deletions in the sequence encoding this domain were made. The deleted hamster desmin genes were fused to the RSV promoter. Expression of such constructs in vimentin- free MCF-7 cells as well as in vimentin-containing HeLa cells, resulted in the synthesis of mutant proteins of the expected size. Single- and double-label immunofluorescence assays of transfected cells showed that in the absence of vimentin, desmin subunits missing amino acids 4-13 are still capable of filament formation, although in addition to filaments large numbers of desmin dots are present. Mutant desmin subunits missing larger portions of their amino terminus cannot form filaments on their own. It may be concluded that the amino-terminal region comprising amino acids 7-17 contains residues indispensable for desmin filament formation in vivo. Furthermore it was shown that the endogenous vimentin IF network in HeLa cells masks the effects of mutant desmin on IF assembly. Intact and mutant desmin colocalized completely with endogenous vimentin in HeLa cells. Surprisingly, in these cells endogenous keratin also seemed to colocalize with endogenous vimentin, even if the endogenous vimentin filaments were disturbed after expression of some of the mutant desmin proteins. In MCF-7 cells some overlap between endogenous keratin and intact exogenous desmin filaments was also observed, but mutant desmin proteins did not affect the keratin IF structures. In the absence of vimentin networks (MCF-7 cells), the initiation of desmin filament formation seems to start on the preexisting keratin filaments. However, in the presence of vimentin (HeLa cells) a gradual integration of desmin in the preexisting vimentin filaments apparently takes place.     

Immunohistochemistry of chaetognath body wall muscles

Abstract. A light and electron immunohistochemical study was carried out on the body wall muscles of the chaetognath Sagitta friderici for the presence of a variety of contractile proteins (myosin, paramyosin, actin), regulatory proteins (tropomyosin, troponin), and structural proteins (α‐actinin, desmin, vimentin). The primary muscle (～80% of body wall volume) showed the characteristic structure of transversely striated muscles, and was comparable to that of insect asynchronous flight muscles. In addition, the body wall had a secondary muscle with a peculiar structure, displaying two sarcomere types (S1 and S2), which alternated along the myofibrils. S1 sarcomeres were similar to those in the slow striated fibers of many invertebrates. In contrast, S2 sarcomeres did not show a regular sarcomeric pattern, but instead exhibited parallel arrays of 2 filament types. The thickest filaments (～10–15 nm) were arranged to form lamellar structures, surrounded by the thinnest filaments (～6 nm). Immunoreactions to desmin and vimentin were negative in both muscle types. The primary muscle exhibited the classical distribution of muscle proteins: actin, tropomyosin, and troponin were detected along the thin filaments, whereas myosin and paramyosin were localized along the thick filaments; immunolabeling of α‐actinin was found at Z‐bands. Immunoreactions in the S1 sarcomeres of the secondary muscle were very similar to those found in the primary muscle. Interestingly, the S2 sarcomeres of this muscle were labeled with actin and tropomyosin antibodies, and presented no immunore‐actions to both myosin and paramyosin. α‐Actinin in the secondary muscle was only detected at the Z‐lines that separate S1 from S2. These findings suggest that S2 are not true sarcomeres. Although they contain actin and tropomyosin in their thinnest filaments, their thickest filaments do not show myosin or paramyosin, as the striated muscle thick myofilaments do. These peculiar S2 thick filaments might be an uncommon type of intermediate filament, which were labeled neither with desmin or vimentin antibodies.     

Intermediate filaments in smooth muscle

The intermediate filament (IF) network is one of the three cytoskeletal systems in smooth muscle. The type III IF proteins vimentin and desmin are major constituents of the network in smooth muscle cells and tissues. Lack of vimentin or desmin impairs contractile ability of various smooth muscle preparations, implying their important role for smooth muscle force development. The IF framework has long been viewed as a fixed cytostructure that solely provides mechanical integrity for the cell. However, recent studies suggest that the IF cytoskeleton is dynamic in mammalian cells in response to various external stimulation. In this review, the structure and biological properties of IF proteins in smooth muscle are summarized. The role of IF proteins in the modulation of smooth muscle force development and redistribution/translocation of signaling partners (such as p130 Crk-associated substrate, CAS) is depicted. This review also summarizes our latest understanding on how the IF network may be regulated in smooth muscle. cytoskeleton; force development; vimentin; desmin