Characterization of myosin heavy chains in cultured aorta smooth muscle cells. A comparative study

Cell-cell contacts via N-cadherin induce a regulatory renin secretory phenotype in As4.1 cells

The lack of a clonal renin-secreting cell line has greatly hindered the investigation of the regulatory mechanisms of renin secretion at the cellular, biochemical, and molecular levels. In the present study, we investigated whether it was possible to induce phenotypic switching of the renin-expressing clonal cell line As4.1 from constitutive inactive renin secretion to regulated active renin secretion. When grown to postconfluence for at least two days in media containing fetal bovine serum or insulin-like growth factor-1, the formation of cell-cell contacts via N-cadherin triggered downstream cellular signaling cascades and activated smooth muscle-specific genes, culminating in phenotypic switching to a regulated active renin secretion phenotype, including responding to the key stimuli of active renin secretion. With the use of phenotype-switched As4.1 cells, we provide the first evidence that active renin secretion via exocytosis is regulated by phosphorylation/dephosphorylation of the 20 kDa myosin light chain. The molecular mechanism of phenotypic switching in As4.1 cells described here could serve as a working model for full phenotypic modulation of other secretory cell lines with incomplete phenotypes.

Filament evanescence of myosin II and smooth muscle function

Smooth muscle is an integral part of hollow organs. Many of them are constantly subjected to mechanical forces that alter organ shape and modify the properties of smooth muscle. To understand the molecular mechanisms underlying smooth muscle function in its dynamic mechanical environment, a new paradigm has emerged that depicts evanescence of myosin filaments as a key mechanism for the muscle’s adaptation to external forces in order to maintain optimal contractility. Unlike the bipolar myosin filaments of striated muscle, the side-polar filaments of smooth muscle appear to be less stable, capable of changing their lengths through polymerization and depolymerization (i.e., evanescence). In this review, we summarize accumulated knowledge on the structure and mechanism of filament formation of myosin II and on the influence of ionic strength, pH, ATP, myosin regulatory light chain phosphorylation, and mechanical perturbation on myosin filament stability. We discuss the scenario of intracellular pools of monomeric and filamentous myosin, length distribution of myosin filaments, and the regulatory mechanisms of filament lability in contraction and relaxation of smooth muscle. Based on recent findings, we suggest that filament evanescence is one of the fundamental mechanisms underlying smooth muscle’s ability to adapt to the external environment and maintain optimal function. Finally, we briefly discuss how increased ROCK protein expression in asthma may lead to altered myosin filament stability, which may explain the lack of deep-inspiration–induced bronchodilation and bronchoprotection in asthma.

Development of an electrospun biomimetic polyurea scaffold suitable for vascular grafting

The optimization of biomechanical and biochemical properties of a vascular graft to render properties relevant to physiological environments is a major challenge today. These critical properties of a vascular graft not only regulate its stability and integrity, but also control invasion of cells for scaffold remodeling permitting its integration with native tissue. In this work, we have synthesized a biomimetic scaffold by electrospinning a blend of a polyurea, poly(serinol hexamethylene urea) (PSHU), and, a polyester, poly-ε-caprolactone (PCL). Mechanical properties of the scaffold were varied by varying polymer blending ratio and electrospinning flow rate. Mechanical characterization revealed that scaffolds with lower PSHU content relative to PCL content resulted in elasticity close to native mammalian arteries. We also found that increasing electrospinning flow rates also increased the elasticity of the matrix. Optimization of elasticity generated scaffolds that enabled vascular smooth muscle cells (SMCs) to adhere, grow and maintain a SMC phenotype. The 30/70 scaffold also underwent slower degradation than scaffolds with higher PSHU content, thereby, providing the best option for in vivo remodeling. Further, Gly-Arg-Gly-Asp-Ser (RGD) covalently conjugated to the polyurea backbone in 30/70 scaffold resulted in significantly increased clotting times. Reducing surface thrombogenicity by the conjugation of RGD is critical to avoiding intimal hyperplasia. Hence, biomechanical and biochemical properties of a vascular graft can be balanced by optimizing synthesis parameters and constituent components. For these reasons, the optimized RGD-conjugated 30/70 scaffold electrospun at 2.5 or 5 mL/h has great potential as a suitable material for vascular grafting applications. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 278-290, 2018.

Vascular Mural Cells in Healing Canine Myocardial Infarcts

Angiogenesis is a critical process in healing of myocardial infarcts, leading to the formation of highly vascular granulation tissue. However, effective cardiac repair depends on mechanisms that inhibit the angiogenic process after a mature scar is formed, preventing inappropriate expansion of the fibrotic process. Using a canine model of reperfused myocardial infarction, we demonstrated that maturation of the infarct leads to the formation of neovessels, with a thick muscular coat, that demonstrate distinct morphological characteristics. Many of these “neoarterioles” lack a defined internal elastic lamina and demonstrate irregular deposits of extracellular matrix in the media. Vascular mural cells in healing infarcts undergo phenotypic changes, showing minimal expression of desmin during the proliferative phase (1 hr occlusion/7 days reperfusion) but in the mature scar (8 weeks reperfusion) acquire a phenotype similar to that of vascular smooth muscle cells in control areas. Non-muscle myosin heavy chains A and B are induced in infarct endothelial cells and myofibroblasts, respectively, but are not expressed in neovascular mural cells. Recruitment of a muscular coat and formation of neoarterioles in mature scars may inhibit endothelial cell proliferation and vascular sprouting, stabilizing the infarct vasculature.

Blebbistatin inhibits the chemotaxis of vascular smooth muscle cells by disrupting the myosin II-actin interaction

Blebbistatin is a myosin II-specific inhibitor. However, the mechanism and tissue specificity of the drug are not well understood. Blebbistatin blocked the chemotaxis of vascular smooth muscle cells (VSMCs) toward sphingosylphosphorylcholine (IC(50) = 26.1 +/- 0.2 and 27.5 +/- 0.5 microM for GbaSM-4 and A7r5 cells, respectively) and platelet-derived growth factor BB (IC(50) = 32.3 +/- 0.9 and 31.6 +/- 1.3 muM for GbaSM-4 and A7r5 cells, respectively) at similar concentrations. Immunofluorescence and fluorescent resonance energy transfer analysis indicated a blebbistatin-induced disruption of the actin-myosin interaction in VSMCs. Subsequent experiments indicated that blebbistatin inhibited the Mg(2+)-ATPase activity of the unphosphorylated (IC(50) = 12.6 +/- 1.6 and 4.3 +/- 0.5 microM for gizzard and bovine stomach, respectively) and phosphorylated (IC(50) = 15.0 +/- 0.6 microM for gizzard) forms of purified smooth muscle myosin II, suggesting a direct effect on myosin II motor activity. It was further observed that the Mg(2+)-ATPase activities of gizzard myosin II fragments, heavy meromyosin (IC(50) = 14.4 +/- 1.6 microM) and subfragment 1 (IC(50) = 5.5 +/- 0.4 microM), were also inhibited by blebbistatin. Assay by in vitro motility indicated that the inhibitory effect of blebbistatin was reversible. Electron-microscopic evaluation showed that blebbistatin induced a distinct conformational change (i.e., swelling) of the myosin II head. The results suggest that the site of blebbistatin action is within the S1 portion of smooth muscle myosin II.

Epicardial cells of human adults can undergo an epithelial-to-mesenchymal transition and obtain characteristics of smooth muscle cells in vitro

Myocardial and coronary development are both critically dependent on epicardial cells. During cardiomorphogenesis, a subset of epicardial cells undergoes an epithelial-to-mesenchymal transition (EMT) and invades the myocardium to differentiate into various cell types, including coronary smooth muscle cells and perivascular and cardiac interstitial fibroblasts. Our current knowledge of epicardial EMT and the ensuing epicardium-derived cells (EPDCs) comes primarily from studies of chick and mouse embryonic development. Due to the absence of an in vitro culture system, very little is known about human EPDCs. Here, we report for the first time the establishment of cultures of primary epicardial cells from human adults and describe their immunophenotype, transcriptome, transducibility, and differentiation potential in vitro. Changes in morphology and beta-catenin staining pattern indicated that human epicardial cells spontaneously undergo EMT early during ex vivo culture. The surface antigen profile of the cells after EMT closely resembles that of subepithelial fibroblasts; however, only EPDCs express the cardiac marker genes GATA4 and cardiac troponin T. After infection with an adenovirus vector encoding the transcription factor myocardin or after treatment with transforming growth factor-beta1 or bone morphogenetic protein-2, EPDCs obtain characteristics of smooth muscle cells. Moreover, EPDCs can undergo osteogenesis but fail to form adipocytes or endothelial cells in vitro. Cultured epicardial cells from human adults recapitulate at least part of the differentiation potential of their embryonic counterparts and represent an excellent model system to explore the biological properties and therapeutic potential of these cells.

Chronic Hypoxia Diminishes the Proliferative Response of Guinea Pig Uterine Artery Vascular Smooth Muscle Cells in Vitro

The pregnancy-related size enlargement of the guinea pig uterine artery is partially accomplished by hyperplasia in all layers of the vessel wall. We sought to determine the separate and combined effects of chronic hypoxia and pregnancy on the proliferative capacity of uterine artery vascular smooth muscle cells (UA VSMCs). We established primary UA VSMC cultures from a total of 13 guinea pigs using an enzymatic digestion technique. Animals were bred and kept in normoxia or hypoxia (P(B) = 463 mmHg, simulated elevation = 3962 m) for 45 days, a duration equivalent to midpregnancy in the guinea pig 63-day gestation. Nonpregnant matched controls were included. The proliferative response of UA VSMCs to 1, 3, 5 or 7 days of serum stimulation in vitro was compared. Exposure to hypoxia reduced UA VSMC proliferative response to serum stimulation relative to that seen in cells harvested from normoxic females. The inhibitory effect was present both in cells harvested from nonpregnant and pregnant animals and resulted in a lower UA VSMC proliferative response in the cells harvested from hypoxic compared with normoxic pregnant animals. Our data were consistent with our hypothesis that chronic maternal hypoxia compromises the capacity for growth and remodeling of the uterine artery during pregnancy, perhaps by interfering with the ability of vascular smooth muscle cells to de-differentiate to a proliferative phenotype. Noteworthy was that such effects of chronic hypoxia were retained in cultured cells.

(+)Insert smooth muscle myosin heavy chain (SM-B): From single molecule to human

In smooth muscle, alternative mRNA splicing of a single gene produces four myosin heavy chain (SMMHC) isoforms. Two of these isoforms differ by the presence [(+)insert] or absence [(-)insert] of a seven amino acid insert in the motor domain. This insert enhances the kinetic properties of myosin at the molecular level but its exact role at the cell and tissue levels still has to be elucidated. This review focuses on the expression and biological functions of the (+)insert isoform. Current knowledge is summarized regarding its tissue distribution in animals and humans. Studies at the molecular, cellular and tissue levels that aimed at understanding the contribution of this isoform to smooth muscle mechanical function are presented with a particular focus on velocity of shortening. In addition, the altered expression of the (+)insert isoform in diseases and models of diseases and the compensatory mechanisms that occur when the (+)insert is knocked out are discussed. The need for additional studies on the relationship of this isoform to contractile performance and how expression of this isoform is regulated are also considered.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Hypoxia-induced pulmonary artery adventitial remodeling and neovascularization: contribution of progenitor cells

Information is rapidly emerging regarding the important role of the arterial vasa vasorum in a variety of systemic vascular diseases. In addition, increasing evidence suggests that progenitor cells of bone marrow (BM) origin may contribute to postnatal neovascularization and/or vascular wall thickening that is characteristic in some forms of systemic vascular disease. Little is known regarding postnatal vasa formation and the role of BM-derived progenitor cells in the setting of pulmonary hypertension (PH). We sought to determine the effects of chronic hypoxia on the density of vasa vasorum in the pulmonary artery and to evaluate if BM-derived progenitor cells contribute to the increased vessel wall mass in a bovine model of hypoxia-induced PH. Quantitative morphometric analyses of lung tissue from normoxic and hypoxic calves revealed that hypoxia results in a dramatic expansion of the pulmonary artery adventitial vasa vasorum. Flow cytometric analysis demonstrated that cells expressing the transmembrane tyrosine kinase receptor for stem cell factor, c-kit, are mobilized from the BM in the circulation in response to hypoxia. Immunohistochemistry revealed an increase in the expression of c-kit+ cells together with vascular endothelial growth factor, fibronectin, and thrombin in the hypoxia-induced remodeled pulmonary artery vessel wall. Circulating mononuclear cells isolated from neonatal calves exposed to hypoxia were found to differentiate into endothelial and smooth muscle cell phenotypes depending on culture conditions. From these observations, we suggest that the vasa vasorum and circulating progenitor cells could be involved in vessel wall thickening in the setting of hypoxia-induced PH.

Airway structure, function and development in health and disease

Until they are fully mature, the airways are highly susceptible to damage. Factors that may contribute to vulnerability of immature airways and the occurrence of bronchopulmonary dysplasia (BPD) in preterm neonates include decreased contractility of smooth muscles of the airway, which leads to generation of lower forces, and immaturity of airway cartilage, leading to increased compressibility of developing airways. Mechanical ventilation has little effect on adult airways, but affects the dimensions and mechanical properties of preterm and newborn airways. Techniques for clinical evaluation of airway function include: (i). measurements of airway function during tidal breathing (airway resistance and reactivity are significantly elevated in infants with BPD); (ii). forced expiratory flow measurements [small-airway obstruction in infants with BPD is indicated by markedly reduced maximal volume measurements (Vmax)]; (iii). radiography procedures (plain radiographs, fluoroscopy, computed tomography and virtual bronchoscopy); and (iv). endoscopy procedures (rigid or flexible bronchoscopy, with or without measurement of oesophageal pressure). Imaging has demonstrated an excessively decreased airway cross-sectional area during exhalation in infants with BPD and acquired tracheomegaly in very preterm infants who had received mechanical ventilatory support. To further advance our understanding of how the airways develop, and to design less damaging protocols for mechanical ventilation in preterm neonates, basic laboratory studies of airway ultrastructure need to be performed and the results correlated with clinical pulmonary function studies.

Bovine distal pulmonary arterial media is composed of a uniform population of well-differentiated smooth muscle cells with low proliferative capabilities

The media of the normal bovine main pulmonary artery (MPA) is composed of phenotypically heterogeneous smooth muscle cells (SMC) with markedly different proliferative capabilities in response to serum, mitogens, and hypoxia. Little, however, is known of the SMC phenotype in distal pulmonary arteries (PA), particularly in arterioles, which regulate the pulmonary circulation. With a panel of muscle-specific antibodies against alpha-smooth muscle (SM)-actin, SM-myosin heavy chains (SM-MHC), SM-MHC-B isoform, desmin, and meta-vinculin, we demonstrate a progressive increase in phenotypic uniformity and level of differentiation of SMC along the proximal-to-distal axis of normal adult bovine pulmonary circulation so that the media of distal PA (1,500- to 100-microm diameter) is composed of a phenotypically uniform population of "well-differentiated" SMC. Similarly, when isolated and assessed in vitro, distal PA-SMC is composed of a single, uniform population of differentiated SMC that exhibited minimal growth responses to a variety of mitogens while their cell size increased substantially in response to serum. Their growth was inhibited by hypoxic exposure under all conditions tested. Distal PA-SMC also differed from MPA-SMC by exhibiting a distinct pattern of DNA synthesis in response to serum and mitogens. Thus, in contrast to the MPA, distal PA media is composed of an apparently uniform population of well-differentiated SMC that are proliferation resistant and have a substantial capacity to hypertrophy in response to growth-promoting stimuli. We thus speculate that distinct SMC phenotypes present in distal vs. proximal PA may confer different response mechanisms during remodeling in conditions such as hypertension.

Expression of myosin heavy chain isoforms by smooth muscle cells in cerebral arteriovenous malformations

We have characterised the blood vessels found in normal cerebral vasculature and in arteriovenous malformations (AVMs), based on the expression of smooth muscle cell (SMC)-specific proteins. The marker proteins used were smooth muscle alpha-actin and four myosin heavy chain isoforms (SM1, SM2, SMemb and NMHC-A). Specimens of AVM obtained during surgery, and normal cerebral vessels from autopsy cases were studied immunohistochemically and compared. The arterial components of AVM contained an abundance of SMCs of the contractile phenotype, which were positive for alpha-actin, SM1 and SM2, but not for SMemb and NMHC-A. These components showed the same staining pattern as mature normal arteries. Two different types of abnormal veins were found in the AVM specimens: large veins with a thick and fibrous wall (so-called 'arterialised' veins) and intraparenchymal thin-walled sinusoidal veins. The former expressed alpha-actin, SM1, SM2, and SMemb, the latter expressed alpha-actin, SM1, and SM2. These marker expression patterns resembled those of normal cerebral arteries, and the results were compatible with arterialisation of the cerebral veins caused by arteriovenous shunting. However, the expression of SMemb was found only in the arterialised type of veins, not in the sinusoidal type or the arteries that had sustained abnormal blood flow in the AVMs. The thick-walled veins in the AVMs showed the same staining pattern as normal veins of dural plexus origin (large subarachnoid veins and dural sinuses). It is therefore possible to assume that they originated from the dural plexus, and extended into the brain during the formation of AVMs.

Nonmuscle Myosin motor of smooth muscle

Nonmuscle myosin can generate force and shortening in smooth muscle, as revealed by studies of the urinary bladder from mice lacking smooth muscle myosin heavy chain (SM-MHC) but expressing the nonmuscle myosin heavy chains A and B (NM-MHC A and B; Morano, I., G.X. Chai, L.G. Baltas, V. Lamounier-Zepter, G. Lutsch, M. Kott, H. Haase, and M. Bader. 2000. Nat. Cell Biol. 2:371-375). Intracellular calcium was measured in urinary bladders from SM-MHC-deficient and SM-MHC-expressing mice in relaxed and contracted states. Similar intracellular [Ca2+] transients were observed in the two types of preparations, although the contraction of SM-MHC-deficient bladders was slow and lacked an initial peak in force. The difference in contraction kinetics thus do not reflect differences in calcium handling. Thick filaments were identified with electron microscopy in smooth muscle cells of SM-MHC-deficient bladders, showing that NM-MHC can form filaments in smooth muscle cells. Maximal shortening velocity of maximally activated, skinned smooth muscle preparations from SM-MHC-deficient mice was significantly lower and more sensitive to increased MgADP compared with velocity of SM-MHC-expressing preparations. Active force was significantly lower and less inhibited by increased inorganic phosphate. In conclusion, large differences in nucleotide and phosphate binding exist between smooth and nonmuscle myosins. High ADP binding and low phosphate dependence of nonmuscle myosin would influence both velocity of actin translocation and force generation to promote slow motility and economical force maintenance of the cell.

Isolation of murine aortic endothelial cells in culture and the effects of sex steroids on their growth

The lack of commercially available primary murine endothelial cells prompted us to isolate and cultivate this cell type. We report here the effect of sex steroids on the in vitro growth of murine aortic endothelial cells. Murine aortic endothelial cells were isolated by a combination of explant outgrowth from aortic rings and enzymatic digestion. The endothelial nature of the cells was verified by uptake of acylated low-density lipoprotein and positive staining for CD-31. Murine aortic endothelial cell growth is stimulated by physiological concentrations of estrogen. Progesterone, when given simultaneously with estrogen, inhibited the stimulatory growth effect of estrogen. Murine aortic endothelial cells grown in vitro continue to express messenger ribonucleic acid for proteins related to endothelial growth. These include vascular endothelial growth factor, its receptors Flt-1 and Flk-1, and the angiogenesis-associated transcription factor, Ets-1.

Immunocytochemistry of myoepithelial cells in the salivary glands

MECs are distributed on the basal aspect of the intercalated duct and acinus of human and rat salivary glands. However, they do not occur in the acinus of rat parotid glands, and sometimes occur in the striated duct of human salivary glands. MECs, as the name implies, have structural features of both epithelial and smooth muscle cells. They contract by autonomic nervous stimulation, and are thought to assist the secretion by compressing and/or reinforcing the underlying parenchyma. MECs can be best observed by immunocytochemistry. There are three types of immunocytochemical markers of MECs in salivary glands. The first type includes smooth muscle protein markers such as alpha-SMA, SMMHC, h-caldesmon and basic calponin, and these are expressed by MECs and the mesenchymal vasculature. The second type is expressed by MECs and the duct cells and includes keratins 14, 5 and 17, alpha 1 beta 1 integrin, and metallothionein. Vimentin is the third type and, in addition to MECs, is expressed by the mesenchymal cells and some duct cells. The same three types of markers are used for studying the developing gland. Development of MECs starts after the establishment of an extensively branched system of cellular cords each of which terminates as a spherical cell mass, a terminal bud. The pluripotent stem cell generates the acinar progenitor in the terminal bud and the ductal progenitor in the cellular cord. The acinar progenitor differentiates into MECs, acinar cells and intercalated duct cells, whereas the ductal progenitor differentiates into the striated and excretory duct cells. Both in the terminal bud and in the cellular cord, the immediate precursors of all types of the epithelial cells appear to express vimentin. The first identifiable MECs are seen at the periphery of the terminal bud or the immature acinus (the direct progeny of the terminal bud) as somewhat flattened cells with a single cilium projecting toward them. They express vimentin and later alpha-SMA and basic calponin. At the next developmental stage, MECs acquire cytoplasmic microfilaments and plasmalemmal caveolae but not as much as in the mature cell. They express SMMHC and, inconsistently, K14. This protein is consistently expressed in the mature cell. K14 is expressed by duct cells, and vimentin is expressed by both mesenchymal and epithelial cells. After development, the acinar progenitor and the ductal progenitor appear to reside in the acinus/intercalated duct and the larger ducts, respectively, and to contribute to the tissue homeostasis. Under unusual conditions such as massive parenchymal destruction, the acinar progenitor contributes to the maintenance of the larger ducts that result in the occurrence of striated ducts with MECs. The acinar progenitor is the origin of salivary gland tumors containing MECs. MECs in salivary gland tumors are best identified by immunocytochemistry for alpha-SMA. There are significant numbers of cells related to luminal tumor cells in the non-luminal tumor cells that have been believed to be neoplastic MECs.

Cutting edge: association of the motor protein nonmuscle myosin heavy chain-IIA with the C terminus of the chemokine receptor CXCR4 in T lymphocytes

The binding of chemokines to their receptors guides lymphocyte migration. However, the precise mechanism that links the chemotactic signals with the energy and traction force generated by the actomyosin complex of the cell has not been elucidated. Using biochemical approaches and mass spectrometry analysis, we found an association between the C-termini of CXCR4 and CCR5 and the motor protein nonmuscle myosin H chain-IIA. Immunoprecipitation experiments revealed that this association also occurs between the endogenous molecules in T lymphocytes. As expected, myosin L chain was also associated with CXCR4. Confocal microscopy analysis showed that CXCR4 and motor protein nonmuscle myosin H chain-IIA colocalize at the leading edge of migrating T lymphocytes, together with filamentous actin and myosin L chain. These results provide the first evidence of a biochemical association between chemokine receptors and motor proteins, a mechanosignaling mechanism that may have a key role in lymphocyte migration.

Generation of a cell line with smooth muscle phenotype from hypertrophied urinary bladder

We have established a cell line from hypertrophied rabbit urinary bladder smooth muscle (SM) that stably expresses SM myosin (SMM). These cells, termed BSM, are spindle shaped and form swirls, similar to the "hills and valleys" described for cultured aortic SM cells. Western blotting revealed that BSM expresses the amino-terminal SMM heavy chain isoform SM-B, the carboxy-terminal SM1 and SM2 isoforms, and SM alpha-actin. In addition, they express cGMP-dependent protein kinase G, made by contractile SM cells in vitro but not by noncontractile cells synthesizing extracellular matrix. Immunofluorescence studies indicate a homogeneous population of cells expressing alpha-actin and SMM, including the SM-B isoform, and karyotyping demonstrates a stable 4N chromosomal pattern. These cells also express calcium-dependent myosin light chain kinase and phosphatase activity and contract in response to the muscarinic agonist bethanechol. To our knowledge, BSM is the first visceral SM cell line that expresses the SM-B isoform and might serve as a useful model to study the transcriptional regulation of tissue-specific SMM isoforms in differentiation and pathological SM.

Selective expansion of fibroblast subpopulations from pulmonary artery adventitia in response to hypoxia

Proliferation of fibroblasts contributes to the adventitial thickening observed during the development of hypoxia-induced pulmonary hypertension. However, whether all or only specific subpopulations of fibroblasts proliferate during this process is unknown. Because lung, skin, and gingiva contain multiple fibroblast subpopulations, we hypothesized that the pulmonary artery (PA) adventitia of neonatal calves is composed of multiple fibroblast subpopulations and that only selective subpopulations expand under chronic hypoxic conditions. Fibroblast subpopulations were isolated from PA adventitia of control calves using limited dilution cloning techniques. These subpopulations exhibited marked differences in morphology, actin expression, and serum-stimulated growth. Only select fibroblast subpopulations demonstrated the ability to proliferate in response to hypoxia. Fibroblast subpopulations were similarly isolated from calves exposed to hypoxia (14 days). With regard to morphology, actin expression, and serum-stimulated growth of subpopulations, there were no obvious differences in fibroblast subpopulations between the hypoxic and the control calves. However, the number of fibroblast subpopulations with about a twofold increase in hypoxia-induced DNA synthesis was significantly greater in the hypoxic calves (26%) compared with control calves (10%). We conclude that the bovine PA adventitia comprises numerous phenotypically and biochemically distinct fibroblast subpopulations and that select subpopulations expand in response to chronic hypoxia.

Heterogeneity of bladder myocytes in vitro: modulation of myosin isoform expression

We studied the expression of myosin heavy chain isoforms differing at the N-terminal (SM-A, SM-B) and the C-terminal (SM1, SM2) regions and non-muscle myosin heavy chain II-A and II-B (NMMHC II-A and B) in newborn and adult rabbit bladder smooth muscle cells (SMCs) and in cultures of enzymatically dissociated neonatal detrusor. RT-PCR analyses revealed that 94.5+/-3.27% of MHC transcripts of the adult bladder SMCs contained the 21-nucleotide insert (SM-B) compared with 83.8+/-3.2% in the newborn bladder, with the remainder of the mRNA being non-inserted (SM-A). In 3, 7, and 10 days of primary culture (proliferating, confluent, and post-confluent, respectively) and up to 4 subculture passages, bladder myocytes expressed predominantly SM-A. Immunofluorescence microscopy revealed heterogeneity in cultured myocytes, i.e. SM-B positive cells coexisting with negatively stained cells. In adult bladder, the C-terminal isoforms SM1 and SM2 represented, 43.1+/-4.3% and 56.89 + 4.3% of the mRNA, respectively, while newborn bladders expressed 72.5+/-7% SM1 and 27.5+/-7% SM2. Upon culturing, cells predominantly expressed SM1 at both the mRNA and protein levels. NMMHC II-A was expressed by both adult and newborn bladders and in culture, whereas NMMHC II-B was expressed at low levels only in newborn bladders, but upregulated in culture. These data indicate that bladder myocytes in vitro undergo modulation with relative overexpression of SM-A and SM1 and upregulation of NMMHC II-B. Information on the mechanisms responsible for this modulation in vitro might provide an understanding of the nature of altered myosin isoform expression associated with smooth muscle dysfunction in certain bladder diseases.

Changes in Ca2+ signaling and contractile protein isoforms in smooth muscle cells from guinea pig ileum during culture

Single smooth muscle cells (SMCs) isolated from guinea pig ileum using collagenase and papain were cultured on laminin-coated dishes in MEM containing fetal calf serum. Temporal changes in intracellular calcium ion concentration in response to carbachol and to ATP were investigated using fluo-3/AM and fluorescence microscopy. It was observed that carbachol caused an increased intracellular calcium ion in freshly isolated single SMCs but a reduced or negative response of cultured SMCs before confluence. On the other hand, ATP was observed to cause an increase in the calcium ion content of SMCs throughout the culture. SDS-PAGE and Western blot analyses revealed changes in the expression of contractile proteins as follows. l-Caldesmon and non-muscle type myosin heavy chain (NMHC) (considered to be marker molecules for dedifferentiation in smooth muscle cells) and non-muscle type tropomyosin were not observed in freshly isolated single SMCs. l-Caldesmon and NMHC appeared in the cultured SMCs within 2 days and the tropomyosin isoform was observed 6 days following seeding. Simultaneously, smooth muscle type myosin heavy chain (SMHC) decreased strikingly and the 41 kDa tropomyosin monomer was lost. The content of alpha-actin decreased gradually to a minimum on day 6 when non-muscle type tropomyosin appeared, and the cells began to proliferate rapidly. These results suggest that the loss of contractility in cultured smooth muscle cells is more closely related to changes in contractile protein profiles than to receptor-mediated signal transduction and that in addition to NMHC and l-caldesmon, non-muscle type tropomyosin may be useful as a marker molecule for de-differentiation of smooth muscle cells.

Nitric oxide regulates smooth-muscle-specific myosin heavy chain gene expression at the transcriptional level-possible role of SRF and YY1 through CArG element

Nitric oxide (NO) plays an important role in vascular regulation through its vasodilatory, antiatherogenic, and antithrombotic properties. NO inhibits platelet adhesion and aggregation and modulates smooth muscle cell (SMC) proliferation and migration. In animals with experimentally induced vascular injury, ec-NOS gene transfection not only restored NO production to normal levels but also increased vascular reactivity of the injured vessels. However, it is unclear whether NO regulates smooth-muscle-specific gene expression. We report here that addition of PDGF-BB to vascular smooth muscle cells suppressed SM-MHC expression but treatment with the NO donors FK409 and SNAP restored SM-MHC mRNA/protein expression. In vitro transfection and subsequent CAT assays demonstrated that exogenous NO can restore PDGF-BB-induced suppression of SM-MHC promoter activity. Promoter deletion analysis revealed that a CArG-3 box located at -1276 bp in the SM-MHC promoter was important for NO-dependent promoter regulation and as well as high level promoter activity. Gel mobility shift assays showed that CArG-3 contained the SRF binding site and a binding site for YY1, a nuclear factor which acts as a negative regulator on muscle-specific promoters. Interestingly, NO donor FK409 reduced YY1 binding to the CArG-3 element but increased SRF binding, suggesting that these two factors bind competitively to the overlapping sites. We also found that mutation to the YY1 binding site in the CArG-3 element resulted in a loss of PDGF-BB-induced suppression of the SM-MHC promoter activity. These findings indicate that NO regulates SM-MHC gene expression at the transcriptional level at least partially through the regulation of transcription factor binding activities on the CArG element. Thus we propose that NO plays a positive role in maintaining the differentiated state of VSMCs.

Alterations in expression of myosin and myosin light chain kinases in response to vascular injury

Histochemical analysis of balloon-injured rat carotid arteries revealed a coordinated expression of nonmuscle myosin heavy chain-A and -B (NM-A and NM-B) in response to injury. Expression of these nonmuscle myosin forms shifts from the media to the adventitia and intima. In contrast, expression of smooth muscle myosin heavy chain-1 (SM-1) within the media is not altered, whereas smooth muscle myosin heavy chain-2 (SM-2) expression declines. Western blotting shows a statistically significant increase in expression of NM-A that occurs within 6 h in response to carotid injury, suggesting this myosin form may be an appropriate experimental marker for proliferating, migrating cells in injured vessels. No overall change in the relative expression level of NM-B was detected, suggesting that compensatory declines in media expression are balanced by increases in the intima and adventitia. Expression of SM-1 did not change in response to injury, whereas the expression of SM-2 significantly declined between 24 h and 7 days. Expression of myosin light chain kinase is also negatively regulated, and the decline in its expression parallels downregulation of SM-2.

Cell-specific differences in ET-1 system in adjacent layers of main pulmonary artery. A new source of ET-1

Endothelin-1 (ET-1) is a potent vasoconstrictor that causes sustained constriction of the pulmonary artery and modulates normal vascular tone. Endothelial cells were thought to be the major source of ET-1, but recent studies show that vascular smooth muscle cells (SMCs) are also capable of its synthesis. We examined the ET-1 and endothelin-converting enzyme-1 (ECE-1) system in cells cultured from two adjacent layers, subendothelial (L1) and inner medial (L2), of normal sheep main pulmonary artery and the response of this system to exogenous ET-1 and transforming growth factor-beta1 (TGF-beta1). End points include assessment of preproET-1 (ppET-1) and ECE-1 gene coexpression, measurement of intracellular and released ET-1, and ECE-1 activity. RT-PCR analysis revealed that ppET-1 and ECE-1 transcripts were greater in L1 than in L2 cells. The L1 cells also synthesized (L1, 3.2 +/- 0.1; L2, 1.2 +/- 0.1 fmol/10(6) cells) and released (L1, 9.2 +/- 0.5; L2, 2.3 +/-0.1 fmol/ml) greater amounts of ET-1 than L2 cells. The L2 cells internalized exogenous ET-1 in a dose-dependent manner (EC(50) 8 nmol/l) and were more responsive to exogenous ET-1 than L1 cells, showing upregulation of both the ppET-1 and ECE genes. TGF-beta1 downregulated ET-1-stimulated ppET-1 and ECE-1 transcripts but only in L2 cells. In addition, L1 cells showed greater ECE-1 activity than L2 cells, and in both, the activity was sensitive to the metalloprotease inhibitor phosphoramidon. We conclude that the ET-1 system in L1 and L2 cells is distinct. The data suggest that the two cell types have diverse functions in the arterial wall; the L1 cells, like endothelial cells, provide a local source of ET-1; and since the L2 cells are more responsive to exogenous ET-1, they are likely to affect normal pulmonary vascular tone.

Characterization of an E-box-dependent cis element in the smooth muscle alpha-actin promoter

Identification of the regulators of smooth muscle specific gene expression is critical for understanding smooth muscle cell (SMC) differentiation and the alterations in SMC phenotype seen in vascular diseases. Previous studies have identified that a 2-bp mutation in a conserved cis-acting element (TGTTTATC) in the promoter of the chicken smooth muscle (SM) alpha-actin gene abolished nuclear factor binding and decreased transcriptional activity of a 271-bp SM alpha-actin promoter fragment when transfected into rat aortic SMC. However, the promoter region containing this conserved sequence has negative cis regulatory activity when studied in homologous systems. The goal of the present studies was to further characterize the transcriptional activity of the rat SM alpha-actin promoter region between -224 and -236 that is conserved across mammals. DNAse I analysis and electrophoretic mobility shift assays demonstrated that SMC nuclear proteins bound an extended sequence (TGTTTATCCCCATAA). Transient transfection experiments of wild-type and mutant rat SM alpha-actin promoter-luciferase constructs into rat aortic SMC revealed that promoter activity was enhanced by mutations of specific nucleotides in the TGTTTATCCCCA region. Interestingly, the TGTTTATCCCCA element in the rat SM alpha-actin promoter is centered between 2 canonical E-boxes. Mutations of the flanking E-boxes abolished the enhancement in promoter activity seen with mutation of the TGTTTATCCCCA element alone. Thus studies provide evidence for a regulatory cassette in the rat SM alpha-actin promoter that regulates gene expression via combinatorial interactions between 2 E-boxes and a newly described TGTTTATCCCCA element.

Long-term effects of Ca(2+) on structure and contractility of vascular smooth muscle

Culture of dispersed smooth muscle cells is known to cause rapid modulation from the contractile to the synthetic cellular phenotype. However, organ culture of smooth muscle tissue, with maintained extracellular matrix and cell-cell contacts, may facilitate maintenance of the contractile phenotype. To test the influence of culture conditions, structural, functional, and biochemical properties of rat tail arterial rings were investigated after culture. Rings were cultured for 4 days in the absence and presence of 10% FCS and then mounted for physiological experiments. Intracellular Ca(2+) concentration ([Ca(2+)](i)) after stimulation with norepinephrine was similar in rings cultured with and without FCS, whereas force development after FCS was decreased by >50%. The difference persisted after permeabilization with beta-escin. These effects were associated with the presence of vasoconstrictors in FCS and were dissociated from its growth-stimulatory action. FCS treatment increased lactate production but did not affect ATP, ADP, or AMP contents. The contents of actin and myosin were decreased by culture but similar for all culture conditions. There was no effect of FCS on calponin contents or myosin SM1/SM2 isoform composition, nor was there any appearance of nonmuscle myosin. FCS-stimulated rings showed evidence of cell degeneration not found after culture without FCS or with FCS + verapamil (1 microM) to lower [Ca(2+)](i). The decreased force-generating ability after culture with FCS is thus associated with increased [Ca(2+)](i) during culture and not primarily caused by growth-associated modulation of cells from the contractile to the synthetic phenotype.

Phosphorylation and regulation of choline kinase from Saccharomyces cerevisiae by protein kinase A

The CKI1-encoded choline kinase (ATP:choline phosphotransferase, EC 2.7.1.32) from Saccharomyces cerevisiae was phosphorylated in vivo on multiple serine residues. Activation of protein kinase A activity in vivo resulted in a transient increase in the phosphorylation of choline kinase. This phosphorylation was accompanied by a stimulation in choline kinase activity. In vitro, protein kinase A phosphorylated choline kinase on a serine residue with a stoichiometry (0.44 mol of phosphate/mol of choline kinase) consistent with one phosphorylation site/choline kinase subunit. The major phosphopeptide derived from the enzyme phosphorylated in vitro by protein kinase A was common to one of the major phosphopeptides derived from the enzyme phosphorylated in vivo. Protein kinase A activity was dose- and time-dependent and dependent on the concentrations of ATP (Km 2.1 microM) and choline kinase (Km 0.12 microM). Phosphorylation of choline kinase with protein kinase A resulted in a stimulation (1.9-fold) in choline kinase activity whereas alkaline phosphatase treatment of choline kinase resulted in a 60% decrease in choline kinase activity. The mechanism of the protein kinase A-mediated stimulation in choline kinase activity involved an increase in the apparent Vmax values with respect to ATP (2.6-fold) and choline (2.7-fold). Overall, the results reported here were consistent with the conclusion that choline kinase was regulated by protein kinase A phosphorylation.

Angiotensin II type 1 receptor antagonist downregulates nonmuscle myosin heavy chains in spontaneously hypertensive rat aorta

The aim of this study was to clarify the differences between the angiotensin II type 1 (AT1) receptor antagonist and the angiotensin-converting enzyme (ACE) inhibitor on smooth muscle and nonmuscle myosin heavy chain isoforms in aortic smooth muscle cells of Wistar-Kyoto rats and spontaneously hypertensive rats. All 4 myosin heavy chain isoforms are heterogeneously expressed in the smooth muscle cells of the aortic tunica media in 20-week-old rats, and the contractile-type myosin heavy chains are highly expressed in smooth muscle cells of the aortic tunica media compared with the synthetic-type myosin heavy chains. Both the AT1 receptor antagonist and the ACE inhibitor had the same effects on hemodynamics, smooth muscle cell hypertrophy and proliferation, fibrosis, and vascular remodeling in spontaneously hypertensive rats. However, the AT1 receptor antagonist had a more potent effect on the downregulation of the synthetic-type myosin heavy chains than the ACE inhibitor in spontaneously hypertensive rat aortic tunica media. In contrast, these effects of the AT1 receptor antagonist and the ACE inhibitor on hemodynamics, morphology, fibrosis, and expression of myosin heavy chain isoforms in smooth muscle cells of the aortic tunica media were not observed in Wistar-Kyoto rats. Thus, within 6 weeks, the AT1 receptor antagonist might modulate the cellular composition of myosin heavy chain isoforms in smooth muscle cells more efficiently than the ACE inhibitor, without morphological changes in the spontaneously hypertensive rat aorta.

Cross-bridge cycling in smooth muscle: a short review

This review is focused on the cross-bridge interaction of the organized contractile system of smooth muscle fibres. By using chemically skinned preparations the different enzymatic reactions of actin-myosin interaction have been associated with mechanical events. A rigor state has been identified in smooth muscle and the binding of ATP causes dissociation of rigor cross-bridges at rates slightly slower than those in skeletal muscle, but fast enough not to be rate-limiting for cross-bridge turn over in the muscle fibre. The release of inorganic phosphate (Pi) is associated with force generation, and this process is not rate-limiting for maximal shortening velocity (Vmax) in the fully activated muscle. The binding of ADP to myosin is strong in the smooth muscle contractile system, a property that might be associated with the generally slow cross-bridge turn over. Both force and Vmax are modulated by the extent of myosin light chain phosphorylation. Low levels of activation are considered to be associated with the recruitment of slowly cycling dephosphorylated cross-bridges which reduces shortening velocity. The attachment of these cross-bridge states in skinned smooth muscles can be regulated by cooperative mechanisms and thin filament associated systems. Smooth muscles exhibit a large diversity in their Vmax and the individual smooth muscle tissue can alter its Vmax under physiological conditions. The diversity and the long-term modulation of phenotype are associated with changes in myosin heavy and light chain isoform expression.

Nitric oxide--cyclic GMP pathway regulates vascular smooth muscle cell phenotypic modulation: implications in vascular diseases

The role of cGMP-dependent protein kinase (PKG) in the regulation of rat aortic vascular smooth muscle cells (VSMC) phenotype was examined using a transfected cell culture system. Repetitively passaged VSMC do not express PKG and exist in the synthetic phenotype. Transfection of PKG-l alpha cDNA, or the active catalytic domain of PKG-l alpha, resulted in the appearance of VSMC having a morphology consistent with the contractile phenotype. PKG-expressing cells also contained markers for the contractile phenotype (for example, smooth muscle specific myosin heavy chain, calponin, alpha-actin) and reduced levels of synthetic phenotype markers (osteopontin, thrombospondin). PKG-transfected VSMC have also reduced the levels of fibroblast growth factor receptors 1 and 2, consistent with the establishment of a more contractile phenotype. The regulation of PKG expression in VSMC is largely undefined; however, continuous exposure of cultured bovine aortic smooth muscle cells with nitric oxide (NO)-donor drugs or cyclic nucleotide analogues reduced the expression of PKG. These results suggest that PKG occupies a critical role in VSMC phenotype and that suppression of PKG expression during inflammation or injury promotes a more synthetic state of the VSMC.

Molecular control of vascular smooth muscle cell differentiation

Changes in the differentiated state of the vascular smooth muscle cell (SMC) including enhanced growth responsiveness, altered lipid metabolism, and increased matrix production are known to play a key role in development of atherosclerotic disease. As such, there has been extensive interest in understanding the molecular mechanisms and factors that regulate differentiation of vascular SMC, and how this regulation might be disrupted in vascular disease. Key questions include determination of mechanisms that control the coordinate expression of genes required for the differentiated function of the smooth muscle cell, and determination as to how these regulatory processes are influenced by local environmental cues known to be important to control of smooth muscle differentiation. Of particular interest, a number of common cis regulatory elements including highly conserved CArG [CC(A/T)6GG] motifs or CArG-like motifs and a TGF beta control element have been identified in the promoters of virtually all smooth muscle differentiation marker genes characterized to date including smooth muscle alpha-actin, smooth muscle myosin heavy chain, telokin, and SM22 alpha and shown to be required for expression of these genes both in vivo and in vitro. In addition, studies have identified a number of trans factors that interact with these cis elements, and shown how the expression or activity of these factors is modified by local environmental cues such as contractile agonists that are known to influence differentiation of smooth muscle.

Regulation and tuning of smooth muscle myosin

Smooth muscle myosin is regulated by phosphorylation of one of the two myosin light chains. This phosphorylation causes an unfolding of the myosin that allows it to interact with actin to produce force. The inactive state involves trapping the myosin in a conformation wherein the myosin heads interact with a segment of the myosin rod. Phosphorylation of the regulatory light chain weakens these interactions and allows the myosin to be activated. Smooth muscle myosin has a large movement of its light chain binding domain that is coupled to ADP release. This structural change may be necessary for the generation of "latch." Smooth muscle myosin has three different regions that vary to generate different isoforms: (1) an alternative insertion within the myosin head; (2) two possible essential light chains; and (3) an alternative tail at the end of the myosin rod. There is substantial evidence that the insertion in the myosin head increases the enzymatic activity of the myosin and leads to greater shortening velocity. The function of the other two variants is as yet unclear.

Differential expression of non-muscle myosin heavy chain genes during Xenopus embryogenesis

Class II non-muscle myosins are implicated in diverse biological processes such as cytokinesis, cellularization, cell shape changes and gastrulation. Two distinct non-muscle myosin heavy chain genes have been reported in all vertebrates: non-muscle myosin heavy chain-A (NMHC-A) and -B (NMHC-B). We report here the isolation of the Xenopus homolog of NMHC-A and present a comparative analysis of the developmental and spatial expression patterns of NMHC-A and the previously isolated NMHC-B to address the role of NMHCs in Xenopus development. A 7.5 kb NMHC-A mRNA is present, maternally in unfertilized eggs and throughout embryogenesis, as well as in all adult tissues examined. An additional 8.3 kb zygotic transcript for NMHC-A is also detected, but only during embryonic stages. Whole mount in situ hybridization with tailbud stage embryos shows that NMHC-A mRNA is predominantly expressed in the epidermis, whereas NMHC-B mRNA is expressed in the somites, brain, eyes and branchial arches. Interestingly, the expression of NMHC-B in developing somites is gradually restricted to the center of each somite as differentiation proceeds. DAPI nuclear staining demonstrated that NMHC-B mRNA is colocalized with the nuclei or perinuclear area. In animal cap experiments, treatment with activin A or ectopic expression of Xbra and an activated form of Xlim1 markedly up-regulates NMHC-B as well as muscle actin mRNAs and slightly down-regulates NMHC-A mRNA, consistent with NMHC-B expression in the somitic muscle and NMHC-A expression in the epidermis.

Differences in contractile protein content and isoforms in phasic and tonic smooth muscles

The basis of tonic vs. phasic contractile phenotypes of visceral smooth muscles is poorly understood. We used gel electrophoresis and quantitative scanning densitometry to measure the content and isoform composition of contractile proteins in opossum lower esophageal sphincter (LES), to represent tonic muscle, and circular muscle of the esophageal body (EB), to represent phasic smooth muscle. The amount of protein in these two types of muscles is similar: approximately 27 mg/g of frozen tissue. There is no difference in the relative proportion of myosin, actin, calponin, and tropomyosin in the two muscle types. However, the EB contains approximately 2.4-times more caldesmon than the LES. The relative ratios of alpha- to gamma-contractile isoforms of actin are 0.9 in the LES and 0.3 in EB. The ratio between acidic (LC17a) and basic (LC17b) isoforms of the 17-kDa essential light chain of myosin is 0.7:1 in the LES, compared with 2.7:1 in the EB. There is no significant difference in the ratios of smooth muscle myosin SM1 and SM2 isoforms in the two muscle types. The level of the myosin heavy chain isoform, which contains the seven-amino acid insert in the myosin head, is about threefold higher in the EB compared with LES. In conclusion, the esophageal phasic muscle in contrast to the tonic LES contains proportionally more caldesmon, LC17a, and seven-amino acid-inserted myosin and proportionally less alpha-actin. These differences may provide a basis for functional differences between tonic and phasic smooth muscles.

Myosin heavy chain isoform expression in rat smooth muscle development

Smooth muscle myosin heavy chains (MHCs), the motor proteins that power smooth muscle contraction, are produced by alternative splicing from a single gene. The smooth muscle MHC gene is capable of producing four isoforms by utilizing alternative splice sites located at the regions encoding the carboxy terminus and the junction of the 25- and 50-kDa tryptic peptides. These four isoforms, SM1A, SM1B, SM2A, and SM2B, are a combination of one of two heavy chains containing different carboxy-terminal tails (1 or 2) without (A) or with (B) an additional motif in the myosin head. In the present study, using RNA analysis and isoform-specific antibodies, we demonstrate the expression patterns of MHC isoforms during development in rat smooth muscle tissues. RNase protection analysis indicates that the mRNAs for SMA and SMB isoforms, which differ by a 21-nucleotide insertion in the region encoding the S1 head region of the myosin molecule, are differentially expressed during development in a highly tissue-specific manner. Smooth muscle MHC transcripts are first detectable in developing rat smooth muscle tissues at 17 days of fetal development. The SMB mRNA is shown to be expressed in smooth muscle from fetal bladder, intestine, and stomach and from neonatal aorta; however, it is not expressed in cultured smooth muscle cells from rat aorta. The SMA mRNA is also present at all stages of development in the smooth muscles examined; however, it is much less abundant than SMB mRNA in most fetal smooth muscles. We show here that the SMB isoform, which contains a unique seven-amino acid insertion at the junction of the 25- and 50-kDa tryptic peptides, is present in conjunction with SM1 and SM2 tails on immunoblots of smooth muscle from stomach, intestine, bladder, and uterus and is expressed during development in a pattern distinct from that of the SM1 and SM2 tail isoforms.

Differential development of umbilical and systemic arteries. II. Contractile proteins

In fetal sheep, umbilical responsiveness to ANG II exceeds systemic vascular responsiveness. Fetal systemic vascular smooth muscle (VSM) exhibits an immature phenotype with decreased contractile protein contents, low 200-kDa myosin heavy chain (MHC) SM2, and significant nonmuscle MHC-B expression, whereas umbilical VSM phenotype is incompletely described. We tested the hypothesis that differences in vascular responsiveness could reflect dissimilarities in VSM phenotype. Actin, MHC, MHC isoforms, and active stresses were compared in strips of femoral arteries and aorta from near-term fetal (n = 12) and adult (n = 12) sheep to those in external and intra-abdominal umbilical arteries. Actin contents in fetal femoral artery and aorta were less (P </= 0.006) than in external umbilical artery (7.37 +/- 1.4 and 7.53 +/- 0.7 vs. 21.6 +/- 2.2 microg/mg wet wt, respectively) as were MHC contents (3.17 +/- 0.4 and 2.84 +/- 0. 3 vs. 7.16 +/- 0.7, respectively). Whereas 204- and 200-kDa MHC were expressed equally in fetal systemic arteries, umbilical and adult arteries predominantly expressed the 204-kDa isoform (SM1); only fetal systemic VSM expressed MHC-B. Fetal systemic artery stresses and myosin light chain phosphorylation were less than those in umbilical and adult arteries (P < 0.001). Compared with umbilical and adult arteries, fetal systemic VSM is biochemically and functionally immature and thus umbilical VSM demonstrates precocious maturation resembling adult VSM in protein expression and function.

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-specific expression of the smooth muscle myosin heavy chain gene in transgenic mice requires 5'-flanking and first intronic DNA sequence

The smooth muscle myosin heavy chain (SM-MHC) gene encodes a major contractile protein whose expression exclusively marks the smooth muscle cell (SMC) lineage. To better understand smooth muscle differentiation at the transcriptional level, we have initiated studies to identify those DNA sequences critical for expression of the SM-MHC gene. Here we report the identification of an SM-MHC promoter-intronic DNA fragment that directs smooth muscle-specific expression in transgenic mice. Transgenic mice harboring an SM-MHC-lacZ reporter construct containing approximately 16 kb of the SM-MHC genomic region from -4.2 to + 11.6 kb (within the first intron) expressed the lacZ transgene in all smooth muscle tissue types. The inclusion of the intronic sequence was required for transgene expression, since 4.2 kb of the 5'-flanking region alone was not sufficient for expression. In the adult mouse, transgene expression was observed in both arterial and venous smooth muscle, in airway smooth muscle of the trachea and bronchi, and in the smooth muscle layers of all abdominal organs, including the stomach, intestine, ureters, and bladder. During development, transgene expression was first detected in airway SMCs at embryonic day 12.5 and in vascular and visceral SMC tissues by embryonic day 14.5. Of interest, expression of the SM-MHC transgene was markedly reduced or absent in some SMC tissues, including the pulmonary circulation. Moreover, the transgene exhibited a heterogeneous pattern between individual SMCs within a given tissue, suggesting the possibility of the existence of different SM-MHC gene regulatory programs between SMC subpopulations and/or of episodic rather than continuous expression of the SM-MHC gene. To our knowledge, results of these studies are the first to identify a promoter region that confers complete SMC specificity in vivo, thus providing a system with which to define SMC-specific transcriptional regulatory mechanisms and to design vectors for SMC-specific gene targeting.

Myosin isoform shifts and decreased reactivity in hypoxia-induced hypertensive pulmonary arterial muscle

The principal stimulus that evokes pulmonary hypertension is chronic alveolar hypoxia. Pulmonary hypertension is associated with remodeling of the vessel walls, involving hypertrophy and hyperplasia of pulmonary arterial smooth muscle (PASM) and a concomitant increase in the deposition of connective tissue, resulting in increased wall thickness. The purpose of the present study was to determine the effect of hypoxia-induced hypertension on the structure and function of PASM. Experiments were designed to determine whether hypoxia-induced pulmonary hypertension is associated with alterations in PASM: 1) reactivity to a variety of agonists, 2) contractile protein proportions and isoforms, and 3) structural properties. Young adult male rats were made hypoxic by lowering the fraction of inspired O2 (10%) for 14 days. Pulmonary arterial segments were isolated and dose-response curves to various agonists (high K+, norepinephrine, serotonin, angiotensin II, and adenosine) were generated. Gel electrophoresis was used to measure changes in the relative amounts of actin or myosin and of myosin heavy chain (MHC) isoforms. Structural changes were correlated with the pharmacological and biochemical data. Hypoxia-induced pulmonary hypertension caused a general decreased reactivity, an increase in the proportion of nonmuscle to muscle MHC isoforms in PASM, and an increase in arterial wall thickness with PASM hypertrophy or hyperplasia.

Beta3 integrins are upregulated after vascular injury and modulate thrombospondin- and thrombin-induced proliferation of cultured smooth muscle cells

BACKGROUND

Treatment with an antibody that binds beta3 integrins (abciximab; c7E3 Fab) at the time of coronary angioplasty decreases the need for repeat revascularization. Two potential mechanisms have been proposed to explain this effect: (1) inhibition of platelet aggregation or (2) interruption of ligand binding to beta3 integrins on the smooth muscle cell (SMC) surface. We examined the latter hypothesis by determining (1) if beta3 integrin expression is upregulated after vascular injury in the baboon, (2) if 7E3 binds beta3 integrins on cultured SMC, and (3) if beta3 integrin activation plays a role in proliferation of cultured SMC.

METHODS AND RESULTS

Results demonstrated that immunostaining for beta3 integrins was present in the neointima 1 week after balloon withdrawal injury of baboon brachial arteries and that beta3 integrin expression colocalized with alpha-actin-positive cells. In contrast, staining for beta3 integrins was undetectable in contralateral uninjured brachial arteries. 7E3 bound to cultured human aortic SMC with an affinity (KD=3.3 nmol/L) similar to 7E3 binding to endothelial cells or platelets. Cotreatment with 7E3 partially inhibited thrombospondin-induced or alpha-thrombin-induced proliferation but not PDGF-induced or serum-induced proliferation.

CONCLUSIONS

In summary, these studies demonstrate that vascular cell beta3 integrin expression is increased after injury, that 7E3 binds to cultured SMC with high affinity, and that beta3 activation is important for thrombospondin-induced or alpha-thrombin-induced proliferation. These results support the hypothesis that beta3 integrins play a role in SMC growth responses after balloon injury.

Airway smooth muscle contractile, regulatory and cytoskeletal protein expression in health and disease

The major part of research dealing with the biophysical and biochemical properties of airway smooth muscle is based on the assumption that the cells constituting the tissue are homogenous. For striated muscle this has been shown untenable. In recent years almost every property of vascular smooth muscle has been also demonstrated to be heterogeneous. This realization has been late in arriving on the airway smooth muscle research scene. Our own studies have shown that mechanical properties are, in quantitative terms, heterogeneously distributed down the airways and that contractility, for example, in extrapulmonary and intrapulmonary airways differs markedly. Another indication of heterogeneity is derived from studies of the biochemical properties of airway smooth muscle cells (ASMCs) in culture. Dramatic changes in phenotype expression were found with days in culture. Just after isolation from the tissue, the cells were of contractile type and contained mature isoforms of contractile, regulatory and cytoskeletal proteins. After the fourth day in culture the cellular phenotype changed such that contractile filaments diminished rapidly with smooth muscle isoforms being replaced by non-muscle isoforms. The cell assumed secretory or synthetic properties and commenced proliferating rapidly. It is possible that similar changes in phenotype could occur in vivo in cells undergoing hypertrophy or hyperplasia. Thus, a thickened medial layer of the type seen in the walls of airways from asthmatic airways is not necessarily one endowed with increased contractility and, in fact, the latter may be subnormal. Finally, using the so-called motility assay, we studied the velocity of translation of actin filaments by myosin molecules obtained from antigen-sensitized and control airway smooth muscle. We found no change in maximum velocity of actin translation. This was under conditions where the myosin light chain (MLC) was fully phosphorylated. However, in these tissues we found heterogeneity in myosin light chain kinase (MLCK) content which, we inferred, accounted for the difference in shortening velocity between control and sensitized muscle strips in vitro.

Airway smooth muscle cell proliferation: characterization of subpopulations by sensitivity to heparin inhibition

Growth and maturation state of airway smooth muscle cells (SMCs) are determinants of asthma pathophysiology. Heparin reduces airway SMC proliferation and arterial SMC replication and phenotypic modulation. Distinct arterial SMC subtypes, differing in heparin sensitivity, have been characterized. We assessed the cellular mechanisms underlying the growth and phenotype of heparin-treated canine tracheal myocytes in primary culture. Heparin reduced replication by 40%. Immunoblot assay of myosin, actin, and myosin light chain kinase revealed heparin had no effect on rapid spontaneous phenotypic modulation after the cells were plated. Heparin increased cellular protein and vimentin contents in confluent cultures, suggesting that it may induce hypertrophic growth. Cell cycle analysis revealed that heparin decreased serum-stimulated replicating myocyte number by 40%. Also, G2-M transit was 20% slower for the set of SMCs that proceeded past G1 in the presence of heparin. These data indicate that heparin does not inhibit airway SMC replication by blocking modulation from the contractile state. Moreover, airway smooth muscle is composed of distinct SMC populations differing in mitogen and antiproliferative mediator responsiveness. Identification of functionally divergent subgroups suggests that distinct sets of SMCs may contribute differentially to airway physiology and pathophysiology.

Smooth muscle cells isolated from discrete compartments of the mature vascular media exhibit unique phenotypes and distinct growth capabilities

Heterogeneity of smooth muscle cell (SMC) phenotype and function is rapidly emerging as an important concept. We have recently described that phenotypically distinct SMC subpopulations in bovine pulmonary arteries exhibit unique proliferative and matrix-producing responses to hypoxic pulmonary hypertension. To provide better understanding of the molecular mechanisms contributing to this phenomenon, experimental studies will require a reliable in vitro model. The purpose of the present study was first to determine if distinct SMC subpopulations, similar to those observed in vivo, could be selectively isolated from the mature arterial media, and then to evaluate whether select SMC subpopulations would exhibit heightened responses to growth-promoting stimuli and hypoxia. We were able to reproducibly isolate at least four phenotypically unique cell subpopulations from the inner, middle, and outer compartments of the arterial media. Differences in cell phenotype were demonstrated by morphological appearance and differential expression of muscle-specific proteins. The isolated cell subpopulations exhibited markedly different growth capabilities. Two SMC subpopulations grew slowly in 10% serum and were quiescent in plasma-based medium. The other two cell subpopulations, exhibiting nonmuscle characteristics, grew rapidly in 10% serum and proliferated in plasma-based medium and in response to hypoxia. Certain colonies of the nonmuscle-like cell subpopulations were found to grow autonomously under serum-deprived conditions and to secrete mitogenic factors. Our data, demonstrating that phenotypically distinct cells with enhanced growth potential exist within the normal arterial media, support the idea that these unique cells could contribute selectively to the pathogenesis of vascular disease.

Characterisation of the promoter which regulates expression of a phosphoglucomutase-related protein, a component of the dystrophin/utrophin cytoskeleton predominantly expressed in smooth muscle

We have recently characterised a 60-kDa muscle-specific phosphoglucomutase-related protein (PGM-RP) which is expressed predominantly in adult visceral and vascular smooth muscle. Here we show that the adult vascular smooth muscle cell line PAC1, which retains the capacity to synthesise metavinculin (a marker of the contractile phenotype) also expressed PGM-RP. However, an embryonic smooth muscle cell line A10, which lacks metavinculin, expressed low levels of PGM-RP. Levels of PGM-RP increased in quiescent PAC1 and A10 cells, and were elevated in response to angiotensin II. PGM-RP is therefore a good marker of the contractile/differentiated smooth muscle phenotype. We have sequenced 1.8 kb of the human PGM-RP promoter and shown that it lacks a conventional TATA box. There are multiple transcription start sites, the most predominant of which are inside an initiator sequence (Inr), which is close to two CT boxes and a GATA element. A minimal promoter-CAT construct (p57-CAT) containing the Inr, a CT box and GATA element directed high-level chloramphenicol acetyltransferase (CAT) expression in the differentiated smooth muscle cell line PAC1, and low-level expression in the embryonic smooth muscle cell line A10. This fits well with the pattern of expression of the endogenous gene. A construct (p146-CAT) containing all of the mRNA initiation sites directed a reduced level of CAT expression, and constructs containing 1.8 kb and 3.3 kb upstream of the major transcription start site displayed even lower activity. Sequence comparisons suggest that the PGM-RP promoter evolved from the main phosphoglucomutase promoter which is active in wide range of cell types. The PGM-RP promoter may have acquired negative regulatory elements as expression of the gene became muscle-specific.

A novel aortic smooth muscle cell line obtained from p53 knock out mice expresses several differentiation characteristics

Here we report that we could obtain a highly differentiated smooth muscle cell line by screening the expression of a-smooth muscle actin from p53 knook out mice aorta. This cell revealed extended bipolar shape and expressed h-caldesmon and calponin as well as a-smooth muscle actin as protein markers of differentiated smooth muscle. Further intracellular calcium increase was induced by application of noradrenaline in a dose dependent manner and calcium oscillation was also observed in a higher dose (100 microM). Appropriate application of 5-azacytidine enhanced these tendencies and induced slow contraction by endothelin-1 and phenylephrine.