Density-related expression of caldesmon and vinculin in cultured rabbit aortic smooth muscle cells

Bladder smooth muscle organ culture preparation maintains the contractile phenotype

Smooth muscle cells, when subjected to culture, modulate from a contractile to a secretory phenotype. This has hampered the use of cell culture for molecular techniques to study the regulation of smooth muscle biology. The goal of this study was to develop a new organ culture model of bladder smooth muscle (BSM) that would maintain the contractile phenotype and aid in the study of BSM biology. Our results showed that strips of BSM subjected to up to 9 days of organ culture maintained their contractile phenotype, including the ability to achieve near-control levels of force with a temporal profile similar to that of noncultured tissues. The technical aspects of our organ culture preparation that were responsible, in part, for the maintenance of the contractile phenotype were a slight longitudinal stretch during culture and subjection of the strips to daily contraction-relaxation. The tissues contained viable cells throughout the cross section of the strips. There was an increase in extracellular collagenous matrix, resulting in a leftward shift in the passive length-tension relationship. There were no significant changes in the content of smooth muscle-specific α-actin, calponin, h-caldesmon, total myosin heavy chain, protein kinase G, Rho kinase-I, or the ratio of SM1 to SM2 myosin isoforms. Moreover the organ cultured tissues maintained functional voltage-gated calcium channels and large-conductance calcium-activated potassium channels. Therefore, we propose that this novel BSM organ culture model maintains the contractile phenotype and will be a valuable tool for the use in cellular/molecular biology studies of bladder myocytes.

Characterization of statically loaded tissue-engineered mitral valve chordae tendineae

Chordae tendineae are essential to the proper function of the mitral valve. Native chordae contain a dense collagenous core and an outer elastin sheath. We have been using the principle of directed collagen gel shrinkage to fabricate tissue-engineered mitral valve chordae. Because the microstructure of biologic tissues determines their mechanical behavior, the morphology of collagen and elastin in tissue-engineered chordae should mimic that of native chordae. The objective of this study, therefore, was to examine the morphology of our tissue-engineered constructs in comparison to native chordae. A collagen-cell suspension was cast into silicon rubber wells with microporous anchors at the ends and cultured in an incubator. The anchors allowed shrinkage to occur only transverse to the long axis of the wells, thus creating highly aligned collagen fibril constructs. The collagen constructs were cultured for 8 weeks and characterized mechanically, histologically, and biochemically at different culture time points. Histologic sections showed that in all mature constructs collagen fibers were oriented parallel to the long axis of the constructs. At the edge of the tissue collagen fibers were in general straight, whereas in the middle of the tissue they were wavy. Transmission electron microscopy showed a progressive increase in the density and longitudinal orientation of collagen fibrils with culture time. Light and scanning electron microscopy showed the presence of an elastin sheath around the collagen core. Immunostaining demonstrated that smooth muscle cells differentiate during tissue development and TUNEL assay showed that cells in the interior of the constructs undergo apoptosis. This study has demonstrated that collagen-cell constructs, with material properties and microstructure similar to native mitral valve chordae, can be developed using static culture.

A role for rho in smooth muscle phenotypic regulation

The role of the small GTP-binding protein Rho in the process of smooth muscle cell (SMC) phenotypic modulation was investigated using cultured rabbit aortic SMCs. Both Rho transcription and Rho protein expression were high for the first 3 days of culture ("contractile" state cells), with expression decreasing after change to the "synthetic" state and peaking upon return to the contractile phenotype. Activation of Rho (indicated by translocation to the membrane) also peaked upon return to the contractile state and was low in synthetic state SMCs. Transient transfection of synthetic state rabbit SMCs with constitutively active Rho (vall4rho) caused a dramatic decrease in cell size and reorganization of cytoskeletal proteins to resemble those of the contractile phenotype; alpha-actin and myosin adopted a tightly packed, highly organized arrangement, whereas vimentin localized to the immediate perinuclear region and focal adhesions were enlarged. Conversely, specific inhibition of endogenous Rho, by expression of C3 transferase, resulted in the complete loss of actin and myosin filaments without affecting the distribution of vimentin. Focal adhesions were reduced in number. Thus, Rho plays a key role in regulating SMC phenotypic expression.

Changes in three-dimensional architecture of microfilaments in cultured vascular smooth muscle cells during phenotypic modulation

To investigate changes in the three-dimensional microfilament architecture of vascular smooth muscle cells (SMC) during the process of phenotypic modulation, rabbit aortic SMCs cultured under different conditions and at different time points were either labelled with fluorescein-conjugated probes to cytoskeletal and contractile proteins for observation by confocal laser scanning microscopy, or extracted with Triton X-100 for scanning electron microscopy. Densely seeded SMCs in primary culture, which maintain a contractile phenotype, display prominent linear myofilament bundles (stress fibres) that are present throughout the cytoplasm with alpha-actin filaments predominant in the central part and beta-actin filaments in the periphery of the cell. Intermediate filaments form a meshed network interconnecting the stress fibres and linking directly to the nucleus. Moderately and sparsely seeded SMCs, which modulate toward the synthetic phenotype during the first 5 days of culture, undergo a gradual redistribution of intermediate filaments from the perinuclear region toward the peripheral cytoplasm and a partial disassembly of stress fibres in the central part of the upper cortex of the cytoplasm, with an obvious decrease in alpha-actin and myosin staining. These changes are reversed in moderately seeded SMCs by day 8 of culture when they have reached confluence. The results reveal two changes in microfilament architecture in SMCs as they undergo a change in phenotype: the redistribution of intermediate filaments probably due to an increase in synthetic organelles in the perinuclear area, and the partial disassembly of stress fibres which may reflect a degradation of contractile components.

Intraluminal pressure is essential for the maintenance of smooth muscle caldesmon and filamin content in aortic organ culture

Different forms of mechanical stimulation are among the physiological factors constantly acting on the vessel wall. We previously demonstrated that subjecting vascular smooth muscle cells (VSMCs) in culture to cyclic stretch increased the expression of high-molecular-weight caldesmon, a marker protein of a differentiated, contractile, VSMC phenotype. In the present work the effects of mechanical factors, in the form of circumferential stress and shear stress, on the characteristics of SM contractile phenotype were studied in an organ culture of rabbit aorta. Application of an intralumininal pressure of 80 mm Hg to aortic segments cultured in Dulbecco's modified Eagle's medium containing 20% fetal calf serum for 3 days prevented the decrease in high-molecular-weight caldesmon content (70+/-4% of initial level in nonpressurized vessel, 116+/-17% at 80 mm Hg) and filamin content (80+/-5% in nonpressurized vessel, 100+/-2% at 80 mm Hg). SM myosin and low-molecular-weight caldesmon contents showed no dependence on vessel pressurization. Neither endothelial denudation nor alteration of intraluminal flow rates affected marker protein content in 3-day vessel culture, thus excluding the possibility of any shear or endothelial effects. Maintenance of high high-molecular-weight caldesmon and filamin levels in the organ cultures of pressurized and stretched vessels demonstrates the positive role of mechanical factors in the control of the VSMC differentiated phenotype.

Smooth muscle cell expression of type I cyclic GMP-dependent protein kinase is suppressed by continuous exposure to nitrovasodilators, theophylline, cyclic GMP, and cyclic AMP

A key component of the nitric oxide-cyclic guanosine monophosphate (cGMP) pathway in smooth muscle cells (SMC) is the type I GMP-dependent protein kinase (PK-G I). Activation of PK-G I mediates the reduction of cytoplasmic calcium concentrations and vasorelaxation. In this manuscript, we demonstrate that continuous exposure of SMC in culture to the nitrovasodilators S-nitroso-N-acetylpenicillamine (SNAP) or sodium nitroprusside (SNP) results in approximately 75% suppression of PK-G I mRNA by 48 h. PK-G I mRNA and protein were also suppressed by continuous exposure to cGMP analogues 8-bromo- and 8-(4-chlorophenylthio) guanosine-3,5-monophosphate or the cAMP analogue dibutyryl cAMP. These results suggest that activation of one or both of the cyclic nucleotide-dependent protein kinases mediates PK-G I mRNA suppression. Using isoform-specific cDNA probes, only the PK-G I alpha was detected in SMC, either at baseline or after suppression, while PK-G I beta was not detected, indicating that isoform switch was not contributing to the gene regulation. Using the transcription inhibitor actinomycin D, the PK-G I mRNA half-life in bovine SMC was observed to be 5 h. The half-life was not affected by the addition of SNAP to actinomycin D, indicating no effect on PK-G I mRNA stability. Nuclear runoff studies indicated a suppression of PK-G I gene transcription by SNAP. PK-G I suppression was also observed in vivo in rats given isosorbide dinitrate in the drinking water, with a dose-dependent suppression of PK-G I protein in the aorta. PK-G I antigen in whole rat lung extract was also suppressed by administration of isosorbide or theophylline in the drinking water. These data may contribute to our understanding of nitrovasodilator resistance, a phenomenon resulting from continuous exposure to nitroglycerin or other nitrovasodilators.

Expression of myogenic marker proteins in human leiomyosarcoma

A series of formalin-fixed, paraffin-embedded leiomyosarcomas (n = 11) was studied immunohistochemically for their expression of various myogenic marker proteins. According to their predominant histological appearance, the tumors were classified as well (n = 4), moderately (n = 5), or poorly (n = 2) differentiated. Using monoclonal anti-muscle specific actin antibodies from clone HHF35 all examined tumors were positively stained. Desmin was not always found in leiomyosarcomas, since positive staining could be demonstrated only in eight cases. As revealed by staining with anti-vinculin antibodies from clone hVIN-1 using the APAAP technique, all leiomyosarcomas with the exception of one expressed vinculin. Typically, the vinculin immunoreactivity was detected diffusely throughout the majority of neoplastic cells as well as in vascular smooth muscle cells of blood vessels. Nine leiomyosarcomas displayed a positive staining for calponin, an actin-binding protein expressed in smooth muscle cells and their precursors. The distribution of calponin resembled that of vinculin in decorating myofibrils of nearly all tumor cells. Actinin immunoreactivity was present in tumor cells of all cases, but was expressed also in nontumor cells such as epithelia. These results suggest that the monoclonal antibodies against vinculin and calponin may serve as additional diagnostic markers for myogenic differentiation in leiomyosarcomas and related tumors.

Identification of two types of smooth muscle cells from rabbit urinary bladder

Cells from the muscular layer of neonatal (3-day-old) rabbit urinary bladders were dissociated with collagenase, and cultured in M199 supplemented with 10% fetal bovine serum and antibiotic-antimycotic. Cells in culture were of two types: long and short. The short cells were thick and spindle-shaped, and the long cells were flat and elongated. The long cells can be about 15 times longer than the short cells. The short cells do not divide, but the long cells divide readily. Expressions of smooth muscle and non-muscle myosins, alpha-smooth muscle actin, vimentin, and h-caldesmon were determined by immuno-fluorescence microscopy using specific antibodies. Both types of cells react strongly with antibodies against smooth and non-muscle myosins. Unlike the short cells, the long cells also contain alpha-actin and vimentin. The expression of h-caldesmon was very weak in both cell types. Also, cells dissociated from the smooth muscle layers of adult (6-month-old) rabbit bladder were cultured under the same conditions as the cells from the neonatal bladders to see if the heterogeneity of smooth muscle cells, exhibited by cells from neonatal rabbits, is also shown by cells from adult bladder. Two types of cells were also identified. The cells were then fixed and examined with the same panel of antibodies that we used for the neonatal cells. The long cells from adult bladder muscle express similar proteins to those in the neonatal long cells, and the short cells were stained positively with smooth muscle myosin, non-muscle myosin, alpha-smooth muscle actin, and lightly with caldesmon. Although the absence of vimentin in the short cells from adults is similar to that from neonatal, the strong expression of alpha-actin in the adult short cells is unlike the short cells from neonatal rabbits, in which their expression is barely detectable.

Differentiated properties and proliferation of arterial smooth muscle cells in culture

The smooth muscle cell is the sole cell type normally found in the media of mammalian arteries. In the adult, it is a terminally differentiated cell that expresses cytoskeletal marker proteins like smooth muscle alpha-actin and smooth muscle myosin heavy chains, and contracts in response to chemical and mechanical stimuli. However, it is able to revert to a proliferative and secretory active state equivalent to that seen during vasculogenesis in the fetus, and this is a prerequisite for the involvement of the smooth muscle cell in the formation of atherosclerotic and restenotic lesions. A similar transition from a contractile to a synthetic phenotype occurs when smooth muscle cells are established in culture. Accordingly, an in vitro system has been used extensively to study the regulation of differentiated properties and proliferation of these cells. During the first few days after seeding, the cells are reorganized structurally with a loss of myofilaments and formation of a widespread endoplasmic reticulum and a prominent Golgi complex. In parallel, they lose their contractility and instead become competent to divide in response to a large variety of mitogens, including platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF). After entering the cell cycle, they start to produce these and other mitogens on their own, and continue to replicate in the absence of exogenous stimuli for a restricted number of generations. Furthermore, they start to secrete extracellular matrix components such as collagen, elastin, and proteoglycans. The mechanisms that control this change in morphology and function of the smooth muscle cells are still poorly understood. Adhesive proteins such as fibronectin and laminin apparently have an important role in determining the basic phenotypic state of the cells and exert their effects via integrin receptors. The proliferative and secretory activities of the cells are influenced by a multitude of growth factors, cytokines, and other molecules. Although much work remains before an integrated view of this regulatory machinery can be achieved, there is no doubt that the cell culture technique has contributed substantially to our knowledge of smooth muscle differentiation and growth. At the same time, it has been crucial in exploring the role of these cells in vascular disease and developing new therapeutic strategies to cope with major causes of human death and disability.

Vascular smooth muscle cells of H-2Kb-tsA58 transgenic mice. Characterization of cell lines with distinct properties

BACKGROUND

The vascular wall is composed of at least two different populations of smooth muscle cells that are distinct in their structure and protein composition. According to the developmental stage of tissue taken for culture, the ratio between cells of epithelioid phenotype and spindle-shaped cells is variable. In particular, the epithelioid cells display characteristic features associated with immaturity. Because their increased appearance can be observed in endothelial denudation, the represent a dedifferentiated, proliferative smooth muscle cell type with a repair function in vascular injury.

METHODS AND RESULTS

To investigate this cellular heterogeneity, we established vascular smooth muscle cell lines from H-2Kb-tsA58 transgenic mice. Due to temperature-sensitive expression of the SV 40 large T-antigen in cells derived from this mouse strain, our smooth muscle lines were conditionally immortalized from the onset of their life in culture. Thus, we were able to clone cell lines representing the two different phenotypes described so far. Epithelioid cells derived from newborn animals are characterized by their expression of cytokeratins and the development of tight junctional complexes. Spindle-shaped cells, which could be isolated from newborn or adult animals, corresponded in phenotype and protein expression to smooth muscle cell lines established previously.

CONCLUSIONS

The special properties of vascular smooth muscle cells of the epithelioid phenotype suggest an endothelial replacement function in the course of injury to the vascular wall.

Stretch affects phenotype and proliferation of vascular smooth muscle cells

The exertion of periodic dynamic strain on the arterial wall is hypothesized to be relevant to smooth muscle cell morphology and function. This study has investigated the effect of cyclic mechanical stretching on rabbit aortic smooth muscle cell proliferation and expression of contractile phenotype protein markers. Cells were cultured on flexible-bottomed dishes and cyclic stretch was applied (frequency 30 cycles/min, 15% elongation) using a Flexercell Strain unit. Cyclic stretch potentiated smooth muscle cell proliferation in serum-activated cultures but not in cultures maintained in 0.5% fetal calf serum. Stretching induced a serum-independent increase of h-caldesmon expression and this effect was reversible following termination of mechanical stimulation. Strain was without effect on smooth muscle myosin or calponin expression. In cells grown on laminin stretch-induced h-caldesmon expression was more prominent than in cells cultured on collagen types I and IV, poly-L-lysine and gelatin. These data suggest that cyclic mechanical stimulation possesses dual effect on vascular smooth muscle cell phenotype characteristics since it: 1) potentiates proliferation, an attribute of a dedifferentiated phenotype; and 2) increases expression of h-caldesmon considered a marker of a differentiated smooth muscle cell state.

Heterogeneity of smooth muscle-associated proteins in mammalian brain microvasculature

In the brain, the microvascular system is composed of endothelial cells surrounded by a layer of pericytes. The lack of smooth muscle cells in this tissue suggests that any contractile function must be performed by one or both of these cell types. The present study was undertaken in order to identify cells in terminal blood vessels that contain smooth muscle-like contractile machinery. Endothelial cells were reactive with antibodies against smooth muscle myosin but showed no other smooth muscle-related features. In contrast, pericytes of intact microvessels showed a pattern of protein expression similar to that of smooth muscle cells. Pericytes also behaved in tissue culture like cultured smooth muscle cells, with regard to the changes in expression of smooth muscle-related proteins. These data confirm the close relationship between smooth muscle cells and pericytes, and point to their contractile function in the brain microvessels.

Changes in the caldesmon isoform content and intimal thickening in the rabbit carotid artery induced by a silicone elastomer collar

The presence of a silicone elastomer collar around one carotid artery of a rabbit induces thickening of the tunica intima. We used immunoblotting to study quantitatively changes in the isoforms of caldesmon, a protein implicated in the regulation of contractility in smooth muscle, while also monitoring the histological changes during 28 days after collaring. Control rabbit carotid arteries (n = 28) contained 245 +/- 6.4 nmol/g protein of the larger isoform of caldesmon (CDh) and 68.3 +/- 3.6 nmol/g protein of the smaller isoform (CD1). Four days after collaring, intimal thickening was slight, but 44% of arterial CDh had been lost; this loss of CDh was therefore from the tunica media. At 10 days, CDh fell to 37% of the control level. Immunofluorescence using CDh-specific antibodies showed that the CDh level was diminished but remained uniform across the wall of collared arteries. At 14 days, when intimal thickening was maximal, there was 30% more CD1 than in controls. At 28 days, the neointima had thinned, and CD1 had fallen to below control levels. Thus, CD1 levels reflected the development and regression of neointima. Changes in caldesmon isoforms showed that smooth muscle cell phenotypic changes occurred throughout the arterial wall.

Expression of meta-vinculin in human coronary arteriosclerosis is related to the histological grade of plaque formation

In the present study we demonstrate that the quantitative reduction of meta-vinculin expression parallels histological changes during the course of coronary arteriosclerosis. Immunofluorescence stainings of coronary arteries revealed that vinculin distribution resembled that of other smooth muscle-specific cytoskeletal proteins like alpha-actin, caldesmon or myosin light chain kinase in labeling smooth muscle cells brightly. Although close to arteriosclerotic plaques, the cellularity as measured by the density of nuclei was often not significantly altered. Cells of this location expressed markedly reduced amounts of vinculin, suggesting that they are smooth muscle cells of a synthetic phenotype. To determine the fractional meta-vinculin content in arteriosclerotic lesions, we performed densitometric scanning of immunoblots incubated with anti-vinculin monoclonal antibodies reacting with both meta-vinculin (150 kDa) and vinculin (130 kDa). In parallel, each tissue sample was evaluated histologically for the degree of arteriosclerotic alterations according to the morphometric atheroma score of Stratford et al. (n = 13). In type 1 lesions covering slight intimal thickening, meta-vinculin represented 36% (mean, range 35%-39%) of the total vinculin immunoreactivity. In type 2 lesions consisting of fibrous plaques of up to twice the original artery wall thickness, meta-vinculin accounted for 28% (mean, range 22%-35%) of the total vinculin content. Meta-vinculin was substantially reduced in type 3 lesions (mean 13%, range 8%-18%) which are characterized by extensive atheromatous plaques. Thus, the meta-vinculin/vinculin ratio differed significantly between early, intermediate and advanced phases of coronary arteriosclerotic plaque formation.

Identification of multiple genes in bovine retinal pericytes altered by exposure to elevated levels of glucose by using mRNA differential display

Loss of capillary pericytes, a characteristic finding in diabetic retinopathy, is strongly associated with hyperglycemia. The pathologic aberrations associated with diabetic retinopathy are localized primarily in the retinal capillaries and are only poorly reversed by subsequent euglycemic control. Since hyperglycemia significantly inhibits pericyte growth in culture, we investigated the regulation of gene expression in retinal pericytes exposed to physiologic (5.5mM) and pathologic (20 mM) glucose concentrations. By utilizing modifications of the mRNA differential display technique, over 14,000 mRNA species were screened, and 35 candidate clones were obtained. Partial DNA sequence demonstrated that 25 of these were distinct genes, including 7 known, 16 previously unreported, and 2 sequences with known homologues. Northern blot analysis demonstrated altered gene expression in 10 (40%), undetectable signals in 12 (48%), and nonregulation in 3 (12%). Genes with glucose-regulated expression included those encoding fibronectin (51% +/- 15%, P = 0.003; mean percentage of control +/- SD), caldesmon (68% +/- 18%; P = 0.026), two ribosomal proteins (201% +/- 72%, P = 0.011; 136% +/- 16%, P = 0.036), Rieske FeS reductase (66% +/- 17%; P = 0.029), three previously unreported sequences (57%, 167%, 271%), and molecules homologous to autoantigens (213%) and tyrosine kinases (down 16- to 33-fold). Caldesmon protein concentrations in pericytes and smooth muscle cells demonstrated decreases by Western blot analysis concordant with mRNA levels. These studies identify genes whose expression is significantly altered after 7 days of exposure to elevated glucose levels and provide new targets for understanding the adverse effects of hyperglycemia on vascular cells. In addition, this study provides strong support for the use of differential mRNA display as a method to rapidly isolate differentially expressed genes in metabolic systems.

A novel phosphoglucomutase-related protein is concentrated in adherens junctions of muscle and nonmuscle cells

Using five monoclonal antibodies raised against a human uterine smooth muscle extract, we have identified a novel antigen which runs as a closely spaced doublet in SDS-gels. The proteins (60/63 kDa) co-purify, are present in a 1:1 ratio as judged by Coomassie Blue staining, and are immunologically closely related, if not identical. No N-terminal sequence could be obtained from a mixture of the 60/63 kDa proteins, but the sequence of four polypeptides liberated by V8 protease or cyanogen bromide cleavage showed that the proteins are closely related to the glycolytic enzyme phosphoglucomutase type 1. Affinity-purified polyclonal antibodies and three different monoclonal antibodies to the 60/63 kDa proteins cross-reacted with rabbit skeletal muscle phosphoglucomutase type 1, whilst two additional monoclonal antibodies were specific for the 60/63 kDa proteins. Peptide maps of the 60/63 kDa proteins and phosphoglucomutase 1 are markedly different, and the purified proteins have no detectable phosphoglucomutase activity. Staining of cultured smooth muscle cells and fibroblasts with antibodies to 60/63 kDa proteins showed that the antigen is concentrated in focal contacts at the ends of actin bundles and is also associated with actin filaments. About 60% of the cellular 60/63 kDa proteins were found in the detergent-insoluble fraction, suggesting a physical association with the cytoskeleton. The highest levels of protein immunoreactivity were found in muscles. The antigen is concentrated in muscle adherens junctions, including smooth muscle dense plaques, cardiomyocyte intercalated disks, and striated muscle myotendinous junctions. Among epithelial cells, the 63 kDa isoform of the protein was found only in cultured keratinocytes where immunofluorescent staining was localized in cell-to-cell adherens junctions. Expression of the 60/63 kDa proteins in vascular smooth muscle cells is developmentally regulated and correlates with the differentiated contractile phenotype of these cells.

Smooth muscle differentiation in human myometrium and uterine leiomyoma

Smooth muscle differentiation has been analysed in human myometrium and leiomyoma by Western blotting with antibodies to smooth muscle specific proteins. No differences in the expression of h-caldesmon, metavinculin, desmin, alpha-smooth muscle actin and calponin were observed. The technique of two-dimensional gel electrophoresis was used, therefore, to further analyse differences between normal smooth muscle cells and their neoplastic counterparts. By comparing the protein patterns of normal myometrium and leiomyoma, it was possible to identify a protein with a molecular weight of approximately 27 kD that is selectively expressed in normal uterine smooth muscle cells. This protein proved to be a low molecular weight variant of calponin, a smooth muscle specific protein of as yet unknown function. Its immediate downregulation in tissue culture of normal myometrium points to a possible role in the process of dedifferentiation.

Expression of chicken vinculin complements the adhesion-defective phenotype of a mutant mouse F9 embryonal carcinoma cell

A mutant cell line, derived from the mouse embryonal carcinoma cell line F9, is defective in cell-cell adhesion (compaction) and in cell-substrate adhesion. We have previously shown that neither uvomorulin (E-cadherin) nor integrins are responsible for the mutant phenotype (Calogero, A., M. Samuels, T. Darland, S. A. Edwards, R. Kemler, and E. D. Adamson. 1991. Dev. Biol. 146:499-508). Several cytoskeleton proteins were assayed and only vinculin was found to be absent in mutant (5.51) cells. A chicken vinculin expression vector was transfected into the 5.51 cells together with a neomycin-resistance vector. Clones that were adherent to the substrate were selected in medium containing G418. Two clones, 5.51Vin3 and Vin4, were analyzed by Nomarski differential interference contrast and laser confocal microscopy as well as by biochemical and molecular biological techniques. Both clones adhered well to substrates and both exhibited F-actin stress fibers with vinculin localized at stress fiber tips in focal contacts. This was in marked contrast to 5.51 parental cells, which had no stress fibers and no vinculin. The mutant and complemented F9 cell lines will be useful models for examining the complex interactions between cytoskeletal and cell adhesion proteins.

Expression of calponin in rabbit and human aortic smooth muscle cells

Polyclonal antibodies to chicken gizzard calponin were used to localize calponin and determine calponin expression in rabbit and human aortic smooth muscle cells in culture. Calponin was localized on the microfilament bundles of cultured smooth muscle cells. Early in primary culture, calponin staining was accumulated preferentially in the central part of the cell body. With time in culture, the number of calponin-negative smooth muscle cells increased while the distribution of calponin in calponin-positive cells became more even along the stress fibers. Calponin content and the calponin/actin ratio decreased about 5-fold in rabbit aortic smooth muscle cells during the first week in primary culture and remained low in proliferating cells. The same tendency in calponin expression was observed when human vascular smooth muscle was studied. On cryostat sections of human umbilical cord, calponin antibodies mainly stained vessel walls of both the arteries and veins, although less intensive labelling was also observed in non-vascular tissue. When primary isolates of human aortic intimal and medial smooth muscle cells were compared with corresponding passaged cultures, it was found that calponin content was reduced about 9-fold in these cells in culture and was similar to the amount of calponin in endothelial cells and fibroblasts. Thus, high calponin expression may be used as an additional marker of vascular smooth muscle cell contractile phenotype.