Endothelin-1 alters the contractile phenotype of cultured embryonic smooth muscle cells

Development of Adaptive Immunity and Its Role in Lung Remodeling

Asthma is characterized by airflow limitations resulting from bronchial closure, which can be either reversible or fixed due to changes in airway tissue composition and structure, also known as remodeling. Airway remodeling is defined as increased presence of mucins-producing epithelial cells, increased thickness of airway smooth muscle cells, angiogenesis, increased number and activation state of fibroblasts, and extracellular matrix (ECM) deposition. Airway inflammation is believed to be the main cause of the development of airway remodeling in asthma. In this chapter, we will review the development of the adaptive immune response and the impact of its mediators and cells on the elements defining airway remodeling in asthma.

Chronic hypoxia alters fetal cerebrovascular responses to endothelin-1

In utero hypoxia influences the structure and function of most fetal arteries, including those of the developing cerebral circulation. Whereas the signals that initiate this hypoxic remodeling remain uncertain, these appear to be distinct from the mechanisms that maintain the remodeled vascular state. The present study explores the hypothesis that chronic hypoxia elicits sustained changes in fetal cerebrovascular reactivity to endothelin-1 (ET-1), a potent vascular contractant and mitogen. In fetal lambs, chronic hypoxia (3,820-m altitude for the last 110 days of gestation) had no significant effect on plasma ET-1 levels or ETA receptor density in cerebral arteries but enhanced contractile responses to ET-1 in an ETA-dependent manner. In organ culture (24 h), 10 nM ET-1 increased medial thicknesses less in hypoxic than in normoxic arteries, and these increases were ablated by inhibition of PKC (chelerythrine) in both normoxic and hypoxic arteries but were attenuated by inhibition of CaMKII (KN93) and p38 (SB203580) in normoxic but not hypoxic arteries. As indicated by Ki-67 immunostaining, ET-1 increased medial thicknesses via hypertrophy. Measurements of colocalization between MLCK and SMαA revealed that organ culture with ET-1 also promoted contractile dedifferentiation in normoxic, but not hypoxic, arteries through mechanisms attenuated by inhibitors of PKC, CaMKII, and p38. These results support the hypothesis that chronic hypoxia elicits sustained changes in fetal cerebrovascular reactivity to ET-1 through pathways dependent upon PKC, CaMKII, and p38 that cause increased ET-1-mediated contractility, decreased ET-1-mediated smooth muscle hypertrophy, and a depressed ability of ET-1 to promote contractile dedifferentiation.

Myosin phosphatase isoforms as determinants of smooth muscle contractile function and calcium sensitivity of force production

The dephosphorylation of myosin by the MP causes smooth muscle relaxation. MP is also a key target of signals that regulate vascular tone and thus blood flow and pressure. Here, we review studies from the past two decades that support the hypothesis that the regulated expression of MP subunits is a critical determinant of smooth muscle responses to constrictor and dilator signals. In particular, the highly regulated splicing of the regulatory subunit Mypt1 Exon 24 is proposed to tune sensitivity to NO/cGMP-mediated relaxation. The regulated transcription of the MP inhibitory subunit CPI-17 is proposed to determine sensitivity to agonist-mediated constriction. The expression of these subunits is specific in the microcirculation and varies in developmental and disease contexts. To date, the relationship between MP subunit expression and vascular function in these different contexts is correlative; confirmation of the hypothesis will require the generation of genetically engineered mice to test the role of MP subunits and their isoforms in the specificity of vascular smooth muscle responses to constrictor and dilator signals.

Smooth muscle contractile diversity in the control of regional circulations

Each regional circulation has unique requirements for blood flow and thus unique mechanisms by which it is regulated. In this review we consider the role of smooth muscle contractile diversity in determining the unique properties of selected regional circulations and its potential influence on drug targeting in disease. Functionally smooth muscle diversity can be dichotomized into fast versus slow contractile gene programs, giving rise to phasic versus tonic smooth muscle phenotypes, respectively. Large conduit vessel smooth muscle is of the tonic phenotype; in contrast, there is great smooth muscle contractile diversity in the other parts of the vascular system. In the renal circulation, afferent and efferent arterioles are arranged in series and determine glomerular filtration rate. The afferent arteriole has features of phasic smooth muscle, whereas the efferent arteriole has features of tonic smooth muscle. In the splanchnic circulation, the portal vein and hepatic artery are arranged in parallel and supply blood for detoxification and metabolism to the liver. Unique features of this circulation include the hepatic-arterial buffer response to regulate blood flow and the phasic contractile properties of the portal vein. Unique features of the pulmonary circulation include the low vascular resistance and hypoxic pulmonary vasoconstriction, the latter attribute inherent to the smooth muscle cells but the mechanism uncertain. We consider how these unique properties may allow for selective drug targeting of regional circulations for therapeutic benefit and point out gaps in our knowledge and areas in need of further investigation.

Immunohistochemical demonstration of blood vessels alpha-actin down-regulation in LPS-treated pregnant mice

Lipopolysaccharide (LPS), produced by gram-negative bacteria, mediates vasodilatation, changing the action of contractile smooth muscle by increasing expression of nitric oxide synthase and prostaglandin. For the first time we demonstrate, by immunohistochemical methods, that administration of LPS to pregnant mice causes alpha-actin-mediated down-regulation of contractile filaments in uterine blood vessels, thereby potentially increasing vessels permeability, blood supply, and immune cells homing to this environment, culminating in the reestablishment of uterine homeostasis.

Vascular smooth muscle phenotypic diversity and function

The control of force production in vascular smooth muscle is critical to the normal regulation of blood flow and pressure, and altered regulation is common to diseases such as hypertension, heart failure, and ischemia. A great deal has been learned about imbalances in vasoconstrictor and vasodilator signals, e.g., angiotensin, endothelin, norepinephrine, and nitric oxide, that regulate vascular tone in normal and disease contexts. In contrast there has been limited study of how the phenotypic state of the vascular smooth muscle cell may influence the contractile response to these signaling pathways dependent upon the developmental, tissue-specific (vascular bed) or disease context. Smooth, skeletal, and cardiac muscle lineages are traditionally classified into fast or slow sublineages based on rates of contraction and relaxation, recognizing that this simple dichotomy vastly underrepresents muscle phenotypic diversity. A great deal has been learned about developmental specification of the striated muscle sublineages and their phenotypic interconversions in the mature animal under the control of mechanical load, neural input, and hormones. In contrast there has been relatively limited study of smooth muscle contractile phenotypic diversity. This is surprising given the number of diseases in which smooth muscle contractile dysfunction plays a key role. This review focuses on smooth muscle contractile phenotypic diversity in the vascular system, how it is generated, and how it may determine vascular function in developmental and disease contexts.

(+)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.

Competition of PTB with TIA proteins for binding to a U-rich cis-element determines tissue-specific splicing of the myosin phosphatase targeting subunit 1

A considerable amount of smooth muscle phenotypic diversity is generated by tissue-specific and developmentally regulated splicing of alternative exons. The control mechanisms are unknown. We are using a myosin phosphatase targeting subunit-1 (MYPT1) alternative exon as a model to investigate this question. In the present study, we show that the RNA binding proteins TIA and PTB function as antagonistic enhancers and suppressors of splicing of the alternative exon, respectively. Each functions through a single U-rich element, containing two UCUU motifs, just downstream of the alternative exon 5' splice site. Tissue-specific down-regulation of TIA protein in the perinatal period allows PTB to bind to the U-rich element and suppress splicing of the alternative exon as the visceral smooth muscle acquires the fast-phasic smooth muscle contractile phenotype. This provides a novel role for PTB in the tissue-specific regulation of splicing of alternative exons during the generation of smooth muscle phenotypic diversity.

Decreased phosphatase activity, increased Ca2+ sensitivity, and myosin light chain phosphorylation in urinary bladder smooth muscle of newborn mice

Developmental changes in the regulation of smooth muscle contraction were examined in urinary bladder smooth muscle from mice. Maximal active stress was lower in newborn tissue compared with adult, and it was correlated with a lower content of actin and myosin. Sensitivity to extracellular Ca²⁺ during high-K⁺ contraction, was higher in newborn compared with 3-wk-old and adult bladder strips. Concentrations at half maximal tension (EC₅₀) were 0.57 ± 0.01, 1.14 ± 0.12, and 1.31 ± 0.08 mM. Force of the newborn tissue was inhibited by ∼45% by the nonmuscle myosin inhibitor Blebbistatin, whereas adult tissue was not affected. The calcium sensitivity in newborn tissue was not affected by Blebbistatin, suggesting that nonmuscle myosin is not a primary cause for increased calcium sensitivity. The relation between intracellular [Ca²⁺] and force was shifted toward lower [Ca²⁺] in the newborn bladders. This increased Ca²⁺ sensitivity was also found in permeabilized muscles (EC₅₀: 6.10 ± 0.07, 5.77 ± 0.08, and 5.55 ± 0.02 pCa units, in newborn, 3-wk-old, and adult tissues). It was associated with an increased myosin light chain phosphorylation and a decreased rate of dephosphorylation. No difference was observed in the myosin light chain phosphorylation rate, whereas the rate of myosin light chain phosphatase–induced relaxation was about twofold slower in the newborn tissue. The decreased rate was associated with a lower expression of the phosphatase regulatory subunit MYPT-1 in newborn tissue. The results show that myosin light chain phosphatase activity can be developmentally regulated in mammalian urinary bladders. The resultant alterations in Ca²⁺ sensitivity may be of importance for the nervous and myogenic control of the newborn bladders.

The kinetic properties of smooth muscle: how a little extra weight makes myosin faster

The contractile properties of smooth muscle (SM) are often described as fast and slow, but the molecular basis for the diversity in contractile properties has yet to be fully elucidated. Studies have shown that the differences in the contractile parameters are seen at the level of the contractile proteins. Experiments have implicated both the splicing of the SM myosin heavy chain (MHC) and the SM myosin essential myosin light chain as possible molecular determinants of the contractile properties of SM. This communication will focus on the role of the 7 aa insert in the smooth muscle MHC in determining the contractile properties of SM and the possible mechanism by which this insert could alter the kinetics of the SM actomyosin ATPase.

TIA proteins are necessary but not sufficient for the tissue-specific splicing of the myosin phosphatase targeting subunit 1

We are using the tissue-specific splicing of myosin phosphatase targeting subunit (MYPT1) as a model to investigate smooth muscle phenotypic diversity. We previously identified a U-rich intronic enhancer flanking the 5' splice site (IE1), and a bipartite exonic enhancer/suppressor, that regulate splicing of the MYPT1 central alternative exon. Here we show that T-cell inhibitor of apoptosis (TIA-1) and T-cell inhibitor of apoptosis-related (TIAR) proteins bind to the IE1. Co-transfection of TIA expression vectors with a MYPT1 mini-gene construct increase splicing of the central alternative exon. TIA proteins do not enhance splicing when the palindromic exonic splicing enhancer (ESE) is mutated, indicating that TIAs are necessary but not sufficient for splicing. The ESE specifically binds SRp55 and SRp20 proteins, supporting a model in which both SR and TIA proteins binding to their cis-elements are required for the recruitment of the splicing complex to a weak 5' splice site. Inactivation of TIA proteins in the DT40 cell line (TIA-1(-/-)TIAR(+/-)) reduced the splicing of the central alternative exon of the endogenous MYPT1 as well as stably transfected MYPT1 minigene constructs. Splicing of the MYPT1 3' alternative exon and the MLC(17) alternative exon were unaffected, suggesting that TIA proteins regulate a subset of smooth muscle/nonmuscle alternative splicing reactions. Finally, reduced RNA binding and reduced expression of the TIA and SR proteins in phasic (gizzard) smooth muscle around hatching coincided with the switch from exon inclusion to exon skipping, suggesting that loss of TIA and SR enhancer activity may play a role in the developmental switch in MYPT1 splicing.

Dynamic changes in expression of myosin phosphatase in a model of portal hypertension

Myosin phosphatase is a target for signaling pathways that modulate calcium sensitivity of force production in smooth muscle. Myosin phosphatase targeting subunit 1 (MYPT1) isoforms are generated by cassette-type alternative splicing of exons in the central and 3' portion of the transcript. Exclusion of the 3' alternative exon, coding for the leucine zipper (LZ)-positive MYPT1 isoform, is associated with the ability to desensitize to calcium (relax) in response to NO/cGMP-dependent signaling. We examined expression of MYPT1 isoforms and smooth muscle phenotype in normal rat vessels and in a prehepatic model of portal hypertension characterized by arteriolar dilation. The large capacitance vessels, aorta, pulmonary artery, and inferior vena cava expressed predominantly the 3' exon-out/LZ-positive MYPT1 isoform. The first-order mesenteric resistance artery (MA1) and portal vein (PV) expressed severalfold higher levels of MYPT1 with predominance of the 3' exon-included/LZ-negative isoform. There was minor variation in the presence of the MYPT1 central alternative exons. Myosin heavy and light chain splice variants in part cosegregated with MYPT1 isoforms. In response to portal hypertension induced by PV ligature, abundance of MYPT1 in PV and MA1 was significantly reduced and switched to the LZ-positive isoform. These changes were evident within 1 day of PV ligature and were maintained for up to 10 days before reverting to control values at day 14. Alteration of MYPT1 expression was part of a complex change in protein expression that can be generalized as a modulation from a phasic (fast) to a tonic (slow) contractile phenotype. Implications of vascular smooth muscle phenotypic diversity and reversible phenotypic modulation in portal hypertension with regards to regulation of blood flow are discussed.

Splicing of a myosin phosphatase targeting subunit 1 alternative exon is regulated by intronic cis-elements and a novel bipartite exonic enhancer/silencer element

Isoforms of the smooth muscle myosin phosphatase targeting subunit 1 (MYPT1) are generated by cassette-type alternative splicing of exons. Tissue-specific expression of these isoforms is thought to determine smooth muscle-relaxant properties and unique responses to signaling pathways. We used mini-gene deletion/mutation constructs to identify cis regulators of splicing of the chicken MYPT1 central alternative exon. Comparisons of alternative exon splicing were made between smooth muscle cells of the fast-phasic contractile phenotype (gizzard), in which the central alternative exon is skipped, and slow tonic contractile phenotype (aorta), in which the alternative exon is included. We demonstrate that splicing of the alternative exon requires a cis-enhancer complex in the vicinity of the alternative exon 5'-splice site. This complex consists of two UCUU motifs in an intronic U-rich sequence (putative PTB (polypyrimidine tract binding) or T cell inhibitor of apoptosis-1 binding sites), an intronic 67-nucleotide enhancer that has similarities with the cardiac Troponin T MSE3 enhancer, and a potentially novel exonic splicing enhancer. The exonic enhancer contains the palindromic sequence UCCUACAUCCU present in many other transcripts where alternative splicing of exons occurs, suggesting that it may be more broadly active. The exonic enhancer is adjacent to a potentially novel exonic silencer element that contains a 13-nucleotide imperfect palindromic sequence. This silencer, in conjunction with a distal intronic silencer, is proposed to mediate the silencing of splicing of the MYPT1 central alternative exon in the fast phasic smooth muscle phenotype.

The smooth muscle myosin seven amino acid heavy chain insert's kinetic role in the crossbridge cycle for mouse bladder

The seven amino acid insert in the smooth muscle myosin heavy chain is thought to regulate the kinetics of contraction, contributing to the differences between fast and slow smooth muscle. The effects of this insert on force and stiffness were determined in bladder tissue of a transgenic mouse line expressing the insert SMB at one of three levels: an SMB wild type (+/+), an SMA homozygous type (-/-) and a heterozygous type (+/-). For skinned muscle, an increase in MgADP or inorganic phosphate (Pi) should shift the distribution of crossbridges in the actomyosin ATPase (AMATPase) to increase the relative population of the crossbridge state prior to ADP release and Pi release, respectively. Exogenous ADP increased force and stiffness in a manner consistent with increasing the Ca2+ concentration in both the +/+ and +/- mouse types. However, the -/- type showed a significantly greater increase in force than in stiffness suggesting that immediately prior to ADP release, the AMATPase either has an additional force producing isomerization state or a slower ADP dissociation rate for the -/- type compared to the +/+ or +/- types. Exogenous Pi led to a significantly greater decrease in stiffness than in force for all three mouse types suggesting that there is a force producing state prior to Pi release. In addition, the increase in Pi showed similar changes in the +/+ and -/- types whereas in the +/- type the decreases in both force and stiffness were greater than the other two mouse types indicating that the insert can affect the cooperativity between myosin heads. In conclusion, the seven amino acid insert modulates the kinetics and/or states of the AMATPase, which could lead to differences in the kinetics of contraction between fast and slow smooth muscle.

M20, the small subunit of PP1M, binds to microtubules

Myosin light chain phosphatase (PP1M) is composed of three subunits, i.e., M20, MBS, and a catalytic subunit. Whereas MBS is assigned as a myosin binding subunit, the function of M20 is unknown. In the present study, we found that M20 binds to microtubules. The binding activity was revealed by cosedimentation of M20 with microtubules and binding of tubulin to M20 affinity resin. Green fluorescent protein (GFP)-tagged M20 (M20-GFP) was expressed in chicken primary smooth muscle cells and COS-7 cells and was used as a probe for studying the association between M20 and microtubules in living cells. M20-GFP was localized on filamentous structures in both cell types. Colocalization analysis revealed that M20-GFP colocalized with tubulin. Treatment with nocodazole, but not cytochalasin B, abolished the filamentous structure of M20-GFP. These results indicate that M20-GFP associates with microtubules in cells. Microinjection of rhodamine-tubulin into the M20-expressing cells revealed that incorporation of rhodamine-tubulin into microtubules was significantly facilitated by microtubule-associated M20. Consistent with this result, M20 enhanced the rate of tubulin polymerization in vitro and produced elongated microtubules. These results suggest that M20 has a microtubule binding activity and plays a role in regulating microtubule dynamics.

Decreased shortening velocity and altered myosin isoforms in guinea-pig hypertrophic intestinal smooth muscle

The aims of this study were to investigate whether hypertrophy of the small intestinal smooth muscle leads to alterations of myosin isoform composition and shortening velocity and whether possible changes correlate with a change in the sensitivity to ADP of shortening velocity in this tissue. A partial occlusion was introduced in the distal part of the ileum of guinea-pigs. After 2 weeks, the part of the small intestine just proximal of the stenosis was hypertrophied (indicated by a significantly increased cross-sectional area). The most proximal part of the small intestine was used as control, thus enabling comparisons between hypertrophic and normal tissue from the same animal. The outer longitudinal layer of the intestinal wall was gently peeled off and used for biochemistry, RT-PCR and mechanical experiments. The desmin/actin ratio was significantly increased following hypertrophy, although myosin and actin expression were similar in control and hypertrophic tissue. In hypertrophic tissue, the myosin heavy chain mRNA with a 21 base pair insert decreased significantly. The composition of the mRNA encoding the myosin essential light chains changed towards more of the basic type (LC17b). No change in the expression of non-muscle myosin heavy chains A and B was detected. The maximal shortening velocity (V(max)) of maximally activated skinned preparations was significantly lower in the hypertrophic tissue (~50 % of control). The sensitivity of V(max) to ADP was increased in the hypertrophic smooth muscle tissue. We conclude that myosin expression is altered following intestinal hypertrophy and that these alterations affect reactions in the cross-bridge interaction, leading to a slower and more economical contractile function.

Maternal dexamethasone treatment alters myosin isoform expression and contractile dynamics in fetal arteries

We tested the hypothesis that maternal glucocorticoid treatment modulates 17-kDa myosin light chain (myosin LC17) isoform expression and contractile dynamics in fetal ovine carotid arteries. In the single course group, ewes received 6 mg dexamethasone or placebo over 48 h. In the repeated course group, ewes received 6 mg dexamethasone or placebo weekly for 5 wk. In response to 1 microM phenylephrine, arteries from fetuses of dexamethasone-treated ewes exhibited biphasic contractions, characterized by an intermediate relaxation phase. The relaxation rate constant was significantly higher in arteries from the fetuses of dexamethasone than placebo-treated ewes. The observed biphasic contractions suggest the appearance of functional sarcoplasmic reticulum in the arteries from the fetuses of dexamethasone-treated ewes. The myosin LC17(a) isoform expression was lower in the arteries from the fetuses of the placebo-treated ewes than in those from the ewes. Repeated maternal administration of dexamethasone induced an almost twofold increase in myosin LC17(a) isoform expression in the fetal arteries. In contrast, maternal myosin LC17a isoform expression was not affected by dexamethasone treatment. We speculate that dexamethasone-induced increases in fetal myosin LC17(a) isoform expression represent accelerated differentiation of a subpopulation of vascular smooth muscle cells from the fetal to adult phenotype.

Effects of thyroxine on myosin isoform expression and mechanical properties in guinea-pig smooth muscle

Information on the effects of thyroid hormone on smooth muscle contractile protein expression and mechanical properties is sparse. We have addressed the following questions. (1) Can thyroxine hormone alter myosin isoform composition in smooth muscle? (2) Can a change in myosin isoform composition lead to altered mechanical properties in smooth muscle? (3) Are alterations, if occurring, equal in fast and slow smooth muscle types? Guinea-pigs were treated with thyroxine (T(4)) for 12 days. Control animals were given physiological saline solution. Maximal unloaded shortening velocity (V(max)) was measured in chemically skinned, maximally activated muscle preparations from the aorta and the taenia coli. V(max) increased following thyroxine treatment, by approximately 20 % in the taenia coli. In the aorta, no significant increase in V(max) could be detected. The sensitivity of isometric force to inorganic phosphate (P(i)) was increased in the taenia coli following thyroxine treatment. The expression of mRNA (determined with RT-PCR) for the myosin heavy chain with the seven amino acid insert increased by approximately 70 % in the aorta and about 25 % in the taenia coli following thyroxine treatment. Western blot analysis showed an increase in the inserted myosin heavy chain form in the taenia coli. Expression of mRNA for the myosin essential light chains and the corresponding proteins did not change significantly in either muscle type. No alterations in non-muscle myosin heavy chain isoforms could be detected after thyroxine treatment. In conclusion, thyroxine treatment alters the isoform composition of myosin in fast and slow smooth muscles in vivo. This change is sufficient to increase shortening velocity and sensitivity of isometric force to P(i) in the fast, but not in the slow, smooth muscle type.

Agonist-induced force enhancement: the role of isoforms and phosphorylation of the myosin-targeting subunit of myosin light chain phosphatase

The magnitude of agonist-induced Ca(2+) sensitization of force is tissue-dependent, but an explanation for this diversity is unknown. Ca(2+) sensitization is thought to involve a G-protein-mediated inhibition of myosin light chain phosphatase activity by phosphorylation of the myosin-targeting subunit (MYPT). The MYPT has two isoforms that differ by a central insert, which lies near this phosphorylation site. Expression of MYPT isoforms is both developmentally regulated and tissue-specific. We hypothesized that the presence or absence of the central insert determines the magnitude of agonist-induced Ca(2+) sensitization. Throughout development, the chicken aorta exclusively expresses the splice-in MYPT isoform, and guanosine 5'-O-(thiotriphosphate) (GTPgammaS) produces a significant force enhancement. Early during development, the chicken gizzard expresses the splice-in MYPT isoform, and GTPgammaS produced a Ca(2+) sensitization. In the gizzard coincident with the shift in expression from the splice-in to splice-out MYPT isoform, GTPgammaS no longer produced force enhancement. In addition, adenosine 5'-O-(thiotriphosphate) (ATPgammaS) phosphorylated only adult gizzard tissue, the only tissue that did not demonstrate a Ca(2+) sensitization. These results suggest that the relative expression of splice-in/splice-out MYPT isoforms determines the magnitude of agonist-induced force enhancement and that MYPT phosphorylation is not required for Ca(2+) sensitization.

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.

Determinants of the contractile properties in the embryonic chicken gizzard and aorta

Smooth muscle is generally grouped into two classes of differing contractile properties. Tonic smooth muscles show slow rates of force activation and relaxation and slow speeds of shortening (V(max)) but force maintenance, whereas phasic smooth muscles show poor force maintenance but have fast V(max) and rapid rates of force activation and relaxation. We characterized the development of gizzard and aortic smooth muscle in embryonic chicks to identify the cellular determinants that define phasic (gizzard) and tonic (aortic) contractile properties. Early during development, tonic contractile properties are the default for both tissues. The gizzard develops phasic contractile properties between embryonic days (ED) 12 and 20, characterized primarily by rapid rates of force activation and relaxation compared with the aorta. The rapid rate of force activation correlates with expression of the acidic isoform of the 17-kDa essential myosin light chain (MLC(17a)). Previous data from in vitro motility assays (Rover AS, Frezon Y, and Trybus KM. J Muscle Res Cell Motil 18: 103-110, 1997) have postulated that myosin heavy chain (MHC) isoform expression is a determinant for V(max) in intact tissues. In the current study, differences in V(max) did not correlate with previously published differences in MHC or MLC(17a) isoforms. Rather, V(max) was increased with thiophosphorylation of the 20-kDa regulatory myosin light chain (MLC(20)) in the gizzard, suggesting that a significant internal load exists. Furthermore, V(max) in the gizzard increased during postnatal development without changes in MHC or MLC(17) isoforms. Although the rate of MLC(20) phosphorylation was similar at ED 20, the rate of MLC(20) dephosphorylation was significantly higher in the gizzard versus the aorta, correlating with expression of the M130 isoform of the myosin binding subunit in the myosin light chain phosphatase (MLCP) holoenzyme. These results indicate that unique MLCP and MLC(17) isoform expression marks the phasic contractile phenotype.

Selected contribution: mechanical strain increases force production and calcium sensitivity in cultured airway smooth muscle cells

Cultured airway smooth muscle cells subjected to cyclic deformational strain have increased cell content of myosin light chain kinase (MLCK) and myosin and increased formation of actin filaments. To determine how these changes may increase cell contractility, we measured isometric force production with changes in cytosolic calcium in individual permeabilized cells. The pCa for 50% maximal force production was 6.6+/-0.4 in the strain cells compared with 5.9+/-0.3 in control cells, signifying increased calcium sensitivity in strain cells. Maximal force production was also greater in strain cells (8.6+/-2.9 vs. 5.7+/-3.1 microN). The increased maximal force production in strain cells persisted after irreversible thiophosphorylation of myosin light chain, signifying that increased force could not be explained by differences in myosin light chain phosphorylation. Cells strained for brief periods sufficient to increase cytoskeletal organization but insufficient to increase contractile protein content also produced more force, suggesting that strain-induced cytoskeletal reorganization also increases force production.

Mitogen-induced up-regulation of non-smooth muscle isoform of alpha-tropomyosin in rat aortic smooth muscle cells

Correlation between the expression of the alpha-tropomyosin isoforms and cell growth was investigated in rat aortic smooth muscle cells. The levels of exon 1a, exons 1a+2a (smooth muscle type) and exons 1a+2b (non-smooth muscle type) were determined by reverse transcription-polymerase chain reaction (RT-PCR). When the cells were cultured, the level of exons 1a+2b transiently increased while reaching a maximum at 3-5 days. When the serum-deprived confluent cells were stimulated with 3-20% serum for 1.5 h, the level of exons 1a+2b increased by about twofold. The 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7) but not 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimi de (GF 109203X) inhibited this up-regulation. Phorbol-12, 13-dibutyrate (PDB) mimicked the effect of serum. The DNA synthesis as determined by the incorporation of 5-bromo-2'-deoxy-uridine (BrdU) was not enhanced by the 1.5 h stimulation with serum or phorbol ester. The up-regulation of non-smooth muscle isoform of alpha-tropomyosin occurred during G(0)/G(1) transition before entering S phase. Protein phosphorylation is suggested to be involved in the up-regulation. However, the responsible kinase(s) remain to be elucidated.

The maximal velocity of vascular smooth muscle shortening is independent of the expression of calponin

In smooth muscle, the phosphorylation/dephosphorylation of the 20-kDa regulatory light chain of myosin (MLC20) is known to regulate actomyosin interaction and force. However, a thin filament based regulatory system for actomyosin interaction has been suggested to exist in parallel to MLC20 phosphorylation. Calponin is a thin filament associated protein that in vitro inhibits actomyosin interaction, and has been suggested to reduce maximal shortening velocity (vmax). Using antibodies to h1- and h2-calponin, we demonstrated that calponin was present in smooth muscle from Sprague Dawley (SD) rats, while calponin was not detectable in the smooth muscle from Wistar Kyoto (WKY) rats. vmax determined from the force vs. velocity relationship at maximal Ca2+ activation was not different for either the aorta or the portal vein of SD vs. WKY rats. These results suggest that physiological levels of calponin do not contribute to a thin filament-based secondary regulation to inhibit smooth muscle contraction.

Vascular smooth muscle cell effect on endothelial cell endothelin-1 production

Endothelin-1 (ET-1) is a potent mitogen secreted by endothelial cells (ECs) in culture and is a putative factor in vascular lesion development. The purpose of this study was to examine whether smooth muscle cells (SMCs) inhibit EC secretion of ET-1. The effect of SMCs on EC ET-1 and constitutively expressed nitric oxide (NO) synthase activity was examined by using a bilayer co-culture model. SMCs inhibited both EC ET-1 protein and RNA levels, compared with ECs cultured alone. SMCs increased EC NO production when compared with ECs cultured alone. In addition, SMC inhibition of EC ET-1 production could be blocked by the NO synthase inhibitor N(G)-nitro-L-arginine-methyl ester. ECs stimulated SMC proliferation, and the ET-1 AB and B receptor blockers inhibited EC stimulation of SMC proliferation. The ET-1 A blocker had no effect on SMC proliferation. We conclude that SMCs regulate EC ET-1 and ecNOS synthase transcript levels and protein levels. SMC inhibition of ET-1 production by ECs may be mediated through SMC-modulated changes in EC NO activity. Finally, EC stimulation of SMC proliferation in bilayer co-culture is mediated by ET-1 through the ET-1 B receptor.

A myosin phosphatase targeting subunit isoform transition defines a smooth muscle developmental phenotypic switch

Smooth muscle myosin phosphatase dephosphorylates the regulatory myosin light chain and thus mediates smooth muscle relaxation. The activity of this myosin phosphatase is dependent upon its myosin-targeting subunit (MYPT1). Isoforms of MYPT1 have been identified, but how they are generated and their relationship to smooth muscle phenotypes is not clear. Cloning of the middle section of chicken and rat MYPT1 genes revealed that each gene gave rise to isoforms by cassette-type alternative splicing of exons. In chicken, a 123-nucleotide exon was included or excluded from the mature mRNA, whereas in rat two exons immediately downstream were alternative. MYPT1 isoforms lacking the alternative exon were only detected in mature chicken smooth muscle tissues that display phasic contractile properties, but the isoform ratios were variable. The patterns of expression of rat MYPT1 mRNA isoforms were more complex, with three major and two minor isoforms present in all smooth muscle tissues at varying stoichiometries. Isoform switching was identified in the developing chicken gizzard, in which the exon-skipped isoform replaced the exon-included isoform around the time of hatching. This isoform switch occurred after transitions in myosin heavy chain and myosin light chain (MLC(17)) isoforms and correlated with a severalfold increase in the rate of relaxation. The developmental switch of MYPT1 isoforms is a good model for determining the mechanisms and significance of alternative splicing in smooth muscle.

Forced expression of essential myosin light chain isoforms demonstrates their role in smooth muscle force production

The molecular determinants of the contractile properties of smooth muscle are poorly understood, and have been suggested to be controlled by splice variant expression of the myosin heavy chain near the 25/50-kDa junction (Kelley, C. A., Takahashi, M., Yu, J. H., and Adelstein, R. S. (1993) J. Biol. Chem. 268, 12848-12854) as well as by differences in the expression of an acidic (MLC(17a)) and a basic (MLC(17b)) isoform of the 17-kDa essential myosin light chain (Nabeshima, Y., Nonomura, Y., and Fujii-Kuriyama, Y. (1987) J. Biol. Chem. 262, 106508-10612). To investigate the molecular mechanism that regulates the mechanical properties of smooth muscle, we determined the effect of forced expression of MLC(17a) and MLC(17b) on the rate of force activation during agonist-stimulated contractions of single cultured chicken embryonic aortic and gizzard smooth muscle cells. Forced expression of MLC(17a) in aortic smooth muscle cells increased (p < 0.05) the rate of force activation, forced expression of MLC(17b) in gizzard smooth muscle cells decreased (p < 0.05) the rate of force activation, while forced expression of the endogenous MLC(17) isoform had no effect on the rate of force activation. These results demonstrate that MLC(17) is a molecular determinant of the contractile properties of smooth muscle. MLC(17) could affect the contractile properties of smooth muscle by either changing the stiffness of the myosin lever arm or modulating the rate of a load-dependent step and/or transition in the actomyosin ATPase cycle.

Vascular biology of endothelin signal transduction

1. Endothelins regulate cell function by interacting with two classes of cell surface receptors, ETA and ETB receptors. Both receptor types are members of the heptahelical transmembrane-spanning receptor superfamily and couple via G-proteins to multiple intracellular effectors. 2. Many of the cellular responses induced by endothelins are mediated by changes in cytoplasmic Ca2+ concentration. Stimulation of inositol 1,4,5-trisphosphate (IP3) formation promotes release of Ca2+ from intracellular stores via IP3-sensitive Ca2+ channels. This mechanism accounts for the initial transient peak of the Ca2+ elevation. The entry of Ca2+ across the plasma membrane through multiple types of Ca2+ channels is responsible for the sustained phase of Ca2+ elevation and, together, both mechanisms regulate cell function. 3. Endothelin-mediated Ca2+ signals vary markedly in duration, spatial organization and temporal pattern. The elevations in Ca2+ are sustained, transient or oscillatory and occur either globally or are localized to discrete spatial domains. These different Ca2+ signals, which are dependent on the availability of specific ion channels, control distinct cellular functions. Ryanodine-sensitive Ca2+ release channels may be important in determining the organization of the Ca2+ signal. 4. Endothelin-induced Ca2+ elevations near the plasma membrane stimulate the opening of Ca(2+)-dependent K+ and Cl- channels. These channels are key regulators of membrane potential and, consequently, regulate the activity of voltage-dependent Ca2+ influx pathways. 5. Endothelin regulates the growth and differentiation of cells. It markedly potentiates the mitogenic response of other growth factors, an effect that involves activation of the mitogen-activated protein kinase cascade and induction of early response genes. 6. Finally, the vascular actions of endothelin are influenced by the relative expression of specific ion channels, the spatial and temporal pattern of the Ca2+ signal and the cellular composition of the vascular wall.

Conducting the embryonic heart: orchestrating development of specialized cardiac tissues

The heterogeneous tissues of the pacemaking and conduction system comprise the "smart components" of the heart, responsible for setting, maintaining, and coordinating the rhythmic pumping of cardiac muscle. Over the last few years, a wealth of new information has been collected about the unique genetic and phenotypic characteristics expressed by these tissues during cardiac morphogenesis. More recently, genetically modified viruses, mutational analysis, and targeted transgenesis have enabled even more precise resolution of the relationships between cell fate, gene expression, and differentiation of specialized function within developing myocardium. While some information provided by these newer approaches has supported conventional wisdom, some fresh and unexpected perspectives have also emerged. In particular, there is mounting evidence that extracardiac populations of cells migrating into the tubular heart have important morphogenetic roles in the inductive pattering and functional integration of the developing conduction system.

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.