Myosin heavy-chain isoforms in adult and developing rabbit vascular smooth muscle

Two monoclonal antibodies specific for smooth muscle myosin (designated SM-E7 and SM-A9) and one monoclonal anti-(human platelet myosin) antibody (designated NM-G2) have been used to study myosin heavy chain composition of smooth muscle cells in adult and in developing rabbit aorta. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis and Western blotting experiments revealed that adult aortic muscle consisted of two myosin heavy chains (MCH) of smooth muscle type, named MHC-1 (205 kDa), and MHC-2 (200 kDa). In the fetal/neonatal stage of development, vascular smooth muscle was found to contain only MHC-1 but not MHC-2. Non-muscle myosin heavy chain, which showed the same electrophoretic mobility as the slower migrating MHC, was expressed in an inverse manner with respect to MHC-2, i.e. it was detectable only in the early stages of development. The distinct pattern of smooth and non-muscle myosin isoform expression during development may be related to the different functional properties of smooth muscle cells during vascular myogenesis.     

Myosin heavy-chain isoforms in human smooth muscle

The myosin heavy-chain composition of human smooth muscle has been investigated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, enzyme immunoassay, and enzyme-immunoblotting procedures. A polyclonal and a monoclonal antibody specific for smooth muscle myosin heavy chains were used in this study. The two antibodies were unreactive with sarcomeric myosin heavy chains and with platelet myosin heavy chain on enzyme immunoassay and immunoblots, and stained smooth muscle cells but not non-muscle cells in cryosections and cultures processed for indirect immunofluorescence. Two myosin heavy-chain isoforms, designated MHC-1 and MHC-2 (205 kDa and 200 kDa, respectively) were reactive with both antibodies on immunoblots of pyrophosphate extracts from different smooth muscles (arteries, veins, intestinal wall, myometrium) electrophoresed in 4% polyacrylamide gels. In the pulmonary artery, a third myosin heavy-chain isoform (MHC-3, 190 kDa) electrophoretically and antigenically distinguishable from human platelet myosin heavy chain, was specifically recognized by the monoclonal antibody. Analysis of muscle samples, directly solubilized in a sodium dodecyl sulfate solution, and degradation experiments performed on pyrophosphate extracts ruled out the possibility that MHC-3 is a proteolytic artefact. Polypeptides of identical electrophoretic mobility were also present in the other smooth muscle preparations, but were unreactive with this antibody. The presence of three myosin heavy-chain isoforms in the pulmonary artery may be related to the unique physiological properties displayed by the smooth muscle of this artery.     

The heavy-chain stoichiometry of smooth muscle myosin is a characteristic of smooth muscle tissues

The stoichiometry of the two heavy chains of myosin in smooth muscle was determined by electrophoresing extracts of native myosin and of dissociated myosin on sodium dodecyl sulfate (SDS) 4%-polyacrylamide gels. The slower migrating heavy chain was 3.6 times more abundant in toad stomach, 2.3 in rabbit myometrium, 2.0 in rat femoral artery, 1.3 in guinea pig ileum, 0.93 in pig trachea and 0.69 in human bronchus, than the more rapidly migrating chain. Both heavy chains were identified as smooth muscle myosin by immunoblotting using antibodies to smooth muscle and non-muscle myosin. The unequal proportion of heavy chains suggested the possibility of native isoforms of myosin comprised of heavy-chain homodimers. To test this, native myosin extracts wer electrophoresed on non-dissociating (pyrophosphate) gels. When each band was individually analysed on SDS-polyacrylamide gel the slowest was found to be filamin and the other bands were myosin in which the relative proportion of the heavy chains was unchanged from that found in the original tissue extracts. Since this is incompatible with either a heterodimeric or a homodimeric arrangement it suggests that pyrophosphate gel electrophoresis is incapable of separating putative isoforms of native myosin.     

Pig brain homogenates contain smooth muscle myosin and cytoplasmic myosin isoforms

Summary Three myosin isoforms, two of smooth muscle and one of cytoplasmic origin, were found in porcine brain by Western blotting analysis with antibodies specific for smooth and cytoplasmic myosins. The smooth muscle isoforms comprise at least 30% of the total myosin present. Brain tissue is therefore not a suitable source for the isolation of pure cytoplasmic contractile proteins.Abbreviations DTE dithioerythritol - EDTA ethylenediaminetetraacetic acid - FITC fluorescein isothiocyanate - PBS phosphate buffered saline - SDS PAGE polyacrylamide electrophoresis in the presence of sodium dodecylsulphate - TRIS tris(hydroxymethyl)aminomethane     

Nonmuscle and smooth muscle myosin isoforms in bovine endothelial cells

A panel of monoclonal antibodies, specific for human platelet (NM-A9, NM-F6, and NM-G2) and for bovine smooth muscle (SM-E7) myosin heavy chains (MHC), were used to study the composition and the distribution of myosin isoforms in bovine endothelial cells (EC), in vivo and in vitro. Using indirect and double immunofluorescence techniques, we have found that in the intact aortic endothelium there is expression of nonmuscle MHC (NM-MHC), exclusively. By contrast, hepatic sinusoidal endothelium as well as cultured bovine aortic EC (BAEC) in the subconfluent phase of growth show coexistence of NM- and smooth muscle MHC (SM-MHC) isoforms. SM myosin immunoreactivity disappears when cultured BAEC become confluent. In this phase of cell growth, NM-MHC isoforms are localized differently within the cells, i.e., in the cytoplasm around the nucleus or in the cortical, submembranous region of EC cytoplasm. A third type of intracellular distribution of NM-MHC immunoreactivity was evident in the cell periphery of binucleated, confluent BAEC. These data indicate that (1) several myosin isoforms are differently distributed in bovine endothelia; and (2) SM myosin expression and the specific subcellular localization of NM myosin isoforms within EC might be regulated by cell-cell interactions.     

The expression and functional activities of smooth muscle myosin and non‐muscle myosin isoforms in rat prostate

Benign prostatic hyperplasia (BPH) is mainly caused by increased prostatic smooth muscle (SM) tone and volume. SM myosin (SMM) and non‐muscle myosin (NMM) play important roles in mediating SM tone and cell proliferation, but these molecules have been less studied in the prostate. Rat prostate and cultured primary human prostate SM and epithelial cells were utilized. In vitro organ bath studies were performed to explore contractility of rat prostate. SMM isoforms, including SM myosin heavy chain (MHC) isoforms (SM1/2 and SM‐A/B) and myosin light chain 17 isoforms (LC_17a/b), and isoform ratios were determined via competitive RT‐PCR. SM MHC and NM MHC isoforms (NMMHC‐A, NMMHC‐B and NMMHC‐C) were further analysed via Western blotting and immunofluorescence microscopy. Prostatic SM generated significant force induced by phenylephrine with an intermediate tonicity between phasic bladder and tonic aorta type contractility. Correlating with this kind of intermediate tonicity, rat prostate mainly expressed LC_17a and SM1 but with relatively equal expression of SM‐A/SM‐B at the mRNA level. Meanwhile, isoforms of NMMHC‐A, B, C were also abundantly present in rat prostate with SMM present only in the stroma, while NMMHC‐A, B, C were present both in the stroma and endothelial. Additionally, the SMM selective inhibitor blebbistatin could potently relax phenylephrine pre‐contracted prostate SM. In conclusion, our novel data demonstrated the expression and functional activities of SMM and NMM isoforms in the rat prostate. It is suggested that the isoforms of SMM and NMM could play important roles in BPH development and bladder outlet obstruction.     

Regulation of expression of avian slow myosin heavy-chain isoforms.

The slow tonic anterior latissimus dorsi (ALD) muscle of the chicken contains two isomyosins, namely SM-1 and SM-2. The proportions of the two isoforms change with age, SM-2 expression increasing at the expense of SM-1. Applying a load on the wing increases the rate and extent of SM-1 replacement. Here we have demonstrated that decreasing the load by removal of the distal portion of the wing in 1-week-old chickens had an effect opposite to that of overloading in that it slowed muscle growth and the rate of SM-1 elimination. Experimental unloading of muscles previously weighted for 1 or 3 weeks slowed the growth rate of muscles, with consequent regression of relative hypertrophy; however, it did not lead to the reexpression of SM-1 myosin. This indicates that the overload-induced changes in myosin expression are not readily reversible. Nerve section produced unexpected results, in that it advanced the normal developmental shift in myosin expression toward predominance of the SM-2 isoform, similar to the effect of muscle overload.     

Role of myosin phosphatase isoforms in cGMP-mediated smooth muscle relaxation

In vitro experiments showing the activation of the myosin phosphatase via heterophilic leucine zipper interactions between its targeting subunit (MYPT1) and cGMP-dependent protein kinase I suggested a pathway for smooth muscle relaxation (Surks, H. K., Mochizuki, N., Kasai, Y., Georgescu, S. P., Tang, K. M., Ito, M., Lincoln, T. M., and Mendelsohn, M. E. (1999) Science 286, 1583-1587). The relationship between MYPT1 isoform expression and smooth muscle responses to cGMP signaling in vivo has not been explored. MYPT1 isoforms that contain or lack a C-terminal leucine zipper are generated in birds and mammals by cassette-type alternative splicing of a 31-nucleotide exon. The avian and mammalian C-terminal isoforms are highly conserved and expressed in a tissue-specific fashion. In the mature chicken the tonic contracting aorta and phasic contracting gizzard exclusively express the leucine zipper positive and negative MYPT1 isoforms, respectively. Expression of the MYPT1 isoforms is also developmentally regulated in the gizzard, which switches from leucine zipper positive to negative isoforms around the time of hatching. This switch coincides with the development in the gizzard of a cGMP-resistant phenotype, i.e. inability to dephosphorylate myosin and relax in response to 8-bromo-cGMP after calcium activation. Furthermore, association of cGMP-dependent protein kinase I with MYPT1 is detected by immunoprecipitation only in the tissue that expresses the leucine zipper positive isoform of MYPT1. These results suggest that the regulated splicing of MYPT1 is an important determinant of smooth muscle phenotypic diversity and the variability in the response of smooth muscles to the calcium desensitizing effect of cGMP signaling.     

Embryonic chicken gizzard: smooth muscle and non-muscle myosin isoforms

Antibodies to smooth muscle and non-muscle myosin allow the development of smooth muscle and its capillary system in the embryonic chicken gizzard to be followed by immunofluorescent techniques. Although smooth muscle development proceeds in a serosal to luminal direction, angiogenetic cell clusters develop independently at the luminal side close to the epithelial layer, and the presumptive capillaries invade the developing muscle in a luminal to serosal direction. The smooth muscle and non-muscle myosin heavy chains in this avian system cannot be separated by SDS polyacrylamide gel electrophoresis and do not show isoform specificity in immunoblotting, unlike the system found in mammals. Only two myosin heavy chains with M_r of 200 and 196 kDa were separable and considerable immunological cross-reactivity was found between the denatured myosin isoform heavy chains.     

The carboxyl-terminal isoforms of smooth muscle myosin heavy chain determine thick filament assembly properties.

The alternatively spliced SM1 and SM2 smooth muscle myosin heavy chains differ at their respective carboxyl termini by 43 versus 9 unique amino acids. To determine whether these tailpieces affect filament assembly, SM1 and SM2 myosins, the rod region of these myosin isoforms, and a rod with no tailpiece (tailless), were expressed in Sf 9 cells. Paracrystals formed from SM1 and SM2 rod fragments showed different modes of molecular packing, indicating that the tailpieces can influence filament structure. The SM2 rod was less able to assemble into stable filaments than either SM1 or the tailless rods. Expressed full-length SM1 and SM2 myosins showed solubility differences comparable to the rods, establishing the validity of the latter as a model for filament assembly. Formation of homodimers of SM1 and SM2 rods was favored over the heterodimer in cells coinfected with both viruses, compared with mixtures of the two heavy chains renatured in vitro. These results demonstrate for the first time that the smooth muscle myosin tailpieces differentially affect filament assembly, and suggest that homogeneous thick filaments containing SM1 or SM2 myosin could serve distinct functions within smooth muscle cells.     

Two isoforms of regulatory light chain of abalone smooth muscle myosin

Abalone myosin contains two kinds of light chain, regulatory light chain (LC2) and essential light chain (LC1) according to SDS-PAGE. Three distinct light chain bands were observed on polyacrylamide gel electrophoresis of purified abalone myosin in the presence of urea (urea-PAGE). The slower two components showed had mobility on SDS-PAGE and they also showed regulatory activity as the regulatory light chain. They were termed LC2-a and LC2-b in order of increasing mobility on urea-PAGE and isolated by DE-32 ion exchange column chromatography in the presence 8 M urea. The ratio of LC2-a and LC2-b in the central portion of adductor muscle of abalone (LC2-a: LC2-b = 7:3) was different from that (1:1) in the peripheral portion. These results suggest that the two light chains are isoforms of the regulatory light chain. The amino acid compositions of LC2-a and LC2-b were very similar to each other except for the Cys content. The UV absorption spectra were also quite similar, as were the UV difference absorption spectra induced by Ca2+. Phosphorylation was not detectable with the myosin light chain kinase of chicken gizzard. The Ca2+ concentration dependencies of Mg-ATPase activity of LC2-a or LC2-b hybridized abalone myosin (a-myosin, b-myosin) were similar to each other in the absence of rabbit F-actin, but differed in the presence of actin. The b-myosin had a higher maximum value of actomyosin ATPase activity and a lower apparent binding constant of actin and myosin than a-myosin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Detection and distribution of myosin isozymes in vertebrate smooth muscle

Crude extracts of taenia coli (guinea-pig), gizzard (chicken), stomach, colon, ureter, bladder, mesenteric vein, mesenteric artery, uterus and vas deferens (dog) were electrophoresed under conditions which do not denature myosin (pyrophosphate gels). Two isozymes (G1 and G2) were observed in all cases. Their mobilities are the same in all organs, but there are some variations in their relative proportions. They have an ATPase activity. Based on electrophoretic mobility the light chains (L20 and L17) seem to be the same for both isozymes whilst the heavy chains are different. Isozyme G2 contains one type of heavy chain of an apparent molecular mass of 230 kDa, whilst isozyme G1 contains two types of heavy chains: one of apparent molecular mass of 230 kDa, and the other of apparent molecular mass of 200 kDa.     

Developmentally regulated expression of vascular smooth muscle myosin heavy chain isoforms

Two types of smooth muscle myosin heavy chain (MHC) isoforms, SM1 and SM2, were recently identified to have different carboxyl termini (Nagai, R., Kuro-o, M., Babij, P., and Periasamy, M. (1989) J. Biol. Chem. 264, 9734-9737). SM1 and SM2 are considered to be generated from a single gene through alternative RNA splicing. In this study we investigated expression of vascular MHC isoforms during development in rabbits at the mRNA, protein, and histological levels. In adults, all smooth muscle cells reacted with both anti-SM1 and anti-SM2 antibodies on immunofluorescence, suggesting the coexpression of SM1 and SM2 in a single cell. In fetal and perinatal rabbits, however, only anti-SM1 antibody consistently reacted with smooth muscles. Reactivity with anti-SM2 antibody was negative in the fetal and neonatal blood vessels and gradually increased during 30 days after birth. These developmental changes in SM1 and SM2 expression at the histological level coincided with mRNA expression of each MHC isoform as determined by S1 nuclease mapping, indicating that expression of SM1 and SM2 is controlled at the level of RNA splicing. However, sodium dodecyl sulfate-polyacrylamide gel electrophoresis of myosin from fetal and perinatal aortas revealed the presence of large amount of SM2. Interestingly, fetal SM2 did not cross-react with our anti-SM2 antibody on immunoblotting. We conclude that expression of SM1 and SM2 are differentially regulated during development and that a third type of MHC isoform may exist in embryonic and perinatal vascular smooth muscles.     

Expression and function of COOH-terminal myosin heavy chain isoforms in mouse smooth muscle

Isoforms of the smooth muscle myosin motor, SM1 and SM2, differ in length at the carboxy terminal tail region. Their proportion changes with development, hormonal status and disease, but their function is unknown. We developed mice carrying the myosin heavy chain (MyHC) transgenes SM1, cMyc-tagged SM1, SM2, and V5-tagged SM2, and all transgenes corresponded to the SMa NH₂-terminal isoform. Transgene expression was targeted to smooth muscle by the smooth muscle -actin promoter. Immunoblot analysis showed substantial expression of the cMyc-tagged SM1 and V5-tagged SM2 MyHC protein in aorta and bladder and transgene mRNA was expressed in mice carrying unlabeled SM1 or SM2 transgenes. Despite significant protein expression of tagged MyHCs we found only small changes in the SM1:SM2 protein ratio. Significant changes in functional phenotype were observed in mice carrying unlabeled SM1 or SM2 transgenes. Force in aorta and bladder was increased (72 ± 14%, 92 ± 11%) in SM1 and decreased to 57 ± 1% and 80 ± 3% in SM2 transgenic mice. SM1 transgenic bladders had faster (1.8 ± 0.3 s) and SM2 slower (7.1 ± 0.5 s) rates of force redevelopment following a rapid step shortening. We hypothesize that small changes in the SM1:SM2 ratio could be amplified if they are associated with changes in thick filament assembly and underlie the altered contractility. These data provide evidence indicating an in vivo function for the COOH-terminal isoforms of smooth muscle myosin and suggest that the SM1:SM2 ratio is tightly regulated in smooth muscle tissues. myosin heavy chain; transgenic mice     

Role of 17-kDa essential light chain isoforms of aorta smooth muscle myosin.

Aorta smooth muscle myosin contains two kinds of 17-kDa essential light chain, LC17nm (nonmuscle-type) and LC17gi (gizzard-type) [Hasegawa, Y., Ueda, Y., Watanabe, M., & Morita, F. (1992) J. Biochem. 111, 798-803]. The LC17 isoforms were released from porcine aorta myosin by incubation at 46 degrees C. The rate of release was 1.5 to 2 times higher with LC17gi than with LC17nm. Aorta myosins containing the two LC17 isoforms in various ratios could be reconstituted. The actin-activated ATPase activity was measured as a function of LC17nm content. The Vm value was lower with myosin which contained more LC17nm. The apparent dissociation constant for F-actin, Km, was 20-fold less with myosin which contained 81% LC17nm than myosin which contained 23% LC17nm. A similar difference in the dissociation constants of myosin for F-actin was observed in the presence of adenylyl imidodiphosphate. The role of LC17nm appears to be to make aorta myosin suitable for maintaining the muscle tension with a low expenditure of energy. The isoform-dependent difference in the F-actin-binding affinities of myosin seems partly due to the difference in the affinities of LC17 isoforms themselves for F-actin. We found that the isolated LC17nm itself could bind with F-actin with a dissociation constant of 64 microM, but LC17gi could not.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification of a 94-bp GC-rich element in the smooth muscle myosin heavy-chain promoter controlling vascular smooth muscle cell-specific gene expression

The previously described rabbit 2.3-kilobase smooth muscle myosin haevy-chain (SMHC_wt) promoter targets gene expression in transgenic animals to vascular smooth muscle cells (SMCs), including coronary arteries. Therefore, SMHC_wt is thought to provide a promising tool for human gene therapy. In the present study, we examined tissue specificity and expression levels of wild-type and mutated SMHC promoters within the system of high-capacity adenoviral (hcAd) vectors. SMHC_wt and a series of SMHC promoter deletion mutants, a triple promoter as well as a cytomegalovirus-SMHC hybrid promoter driving the enhanced green fluorescence protein (EGFP) reporter gene were transiently transfected into aortic SMCs. Fluorescence intensity was measured by flow cytometric analysis. Consecutively, hcAd vectors were constructed with the SMHC_wt and the mutant promoter with the highest fluorescence activity. Levels of EGFP expression were determined after transduction of SMCs derived from human coronary arteries. For analysis of tissue specificity, embryonic stem (ES) cell-derived SMCs (ESdSMHCs) and cardiomyocytes, (ESdCMs) were used. In comparison with SMHC_wt, only the SMHC_del94 mutant lacking a 94-bp GC-rich element revealed a 1.5-fold increased fluorescence activity. Transduction of primary SMCs of human coronary arteries with hcAd vectors confirmed an increased EGFP expression driven by the SMHC_del94 promoter. In ES-cell-derived embryoid bodies, SMHC_wt was exclusively active in transduced ESdSMCs. In contrast, expression of SMHC_del94 was also found in ESdCMs and other nontarget cells of the embryoid body. The tissue-specific rabbit SMHC_wt promoter seems to be suitable for adenoviral gene transfer in SMCs of human coronary arteries and deletion of a 94-bp negative cis-acting GC-rich element results in loss of specificity. These authors contributed equally to the study.