Myosin isoforms in smooth muscle: how may they affect function and structure?

Modification of loop 1 affects the nucleotide binding properties of Myo1c, the adaptation motor in the inner ear

Myo1c is one of eight members of the mammalian myosin I family of actin-associated molecular motors. In stereocilia of the hair cells in the inner ear, Myo1c presumably serves as the adaptation motor, which regulates the opening and closing of transduction channels. Although there is conservation of sequence and structure among all myosins in the N-terminal motor domain, which contains the nucleotide- and actin-binding sites, some differences include the length and composition of surface loops, including loop 1, which lies near the nucleotide-binding domain. To investigate the role of loop 1, we expressed in insect cells mutants of a truncated form of Myo1c, Myo1c(1IQ), as well as chimeras of Myo1c(1IQ) with the analogous loop from other myosins. We found that replacement of the charged residues in loop 1 with alanines or the whole loop with a series of alanines did not alter the ATPase activity, transient kinetics properties, or Ca(2+) sensitivity of Myo1c(1IQ). Substitution of loop 1 with that of the corresponding region from tonic smooth muscle myosin II (Myo1c(1IQ)-tonic) or replacement with a single glycine (Myo1c(1IQ)-G) accelerated the release of ADP from A.M 2-3-fold in Ca(2+), whereas substitution with loop 1 from phasic muscle myosin II (Myo1c(1IQ)-phasic) accelerated the release of ADP 35-fold. Motility assays with chimeras containing a single alpha-helix, or SAH, domain showed that Myo1c(SAH)-tonic translocated actin in vitro twice as fast as Myo1c(SAH)-WT and 3-fold faster than Myo1c(SAH)-G. The studies show that changes induced in Myo1c via modification of loop 1 showed no resemblance to the behavior of the loop donor myosins or to the changes previously observed with similar Myo1b chimeras.

Human adipose stem cells: a potential cell source for cardiovascular tissue engineering

BACKGROUND/AIMS

A crucial step in providing clinically relevant applications of cardiovascular tissue engineering involves the identification of a suitable cell source. The objective of this study was to identify the exogenous and endogenous parameters that are critical for the differentiation of human adipose stem cells (hASCs) into cardiovascular cells.

METHODS

hASCs were isolated from human lipoaspirate samples, analyzed, and subjected to two differentiation protocols.

RESULTS

As shown by fluorescence-activated cell sorter (FACS) analysis, a population of hASCs expressed stem cell markers including CXCR4, CD34, c-kit, and ABCG2. Further, FACS and immunofluorescence analysis of hASCs, cultured for 2 weeks in DMEM-20%-FBS, showed the expression of smooth muscle cell (SMC)-specific markers including SM alpha-actin, basic calponin, h-caldesmon and SM myosin. hASCs, cultured for 2 weeks in endothelial cell growth medium-2 (EGM-2), formed a network of branched tube-like structures positive for CD31, CD144, and von Willebrand factor. The frequency of endothelial cell (EC) marker-expressing cells was passage number-dependent. Moreover, hASCs attached and formed a confluent layer on top of electrospun collagen-elastin scaffolds. Scanning electron microscopy and DAPI staining confirmed the integration of hASCs with the fibers and formation of a cell-matrix network.

CONCLUSION

Our results indicate that hASCs are a potential cell source for cardiovascular tissue engineering; however, the differentiation capacity of hASCs into SMCs and ECs is passage number- and culture condition-dependent.

(+)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.

Smooth muscle myosin: regulation and properties

The relationship of the biochemical states to the mechanical events in contraction of smooth muscle cross-bridges is reviewed. These studies use direct measurements of the kinetics of Pi and ADP release. The rate of release of Pi from thiophosphorylated cycling cross-bridges held isometric was biphasic with turnovers of 1.8 s-1 and 0.3 s-1, reflecting properties and forces directly acting on cross-bridges through mechanisms such as positive strain and inhibition by high-affinity MgADP binding. Fluorescent transients reporting release of an ADP analogue 3'-deac-edaADP were significantly faster in phasic than in tonic smooth muscles. Thiophosphorylation of myosin regulatory light chains (RLCs) increased and positive strain decreased the release rate around twofold. The rates of ADP release from rigor cross-bridges and the steady-state Pi release from cycling isometric cross-bridges are similar, indicating that the ADP-release step or an isomerization preceding it may limit the ATPase rate. Thus ADP release in phasic and tonic smooth muscles is a regulated step with strain- and dephosphorylation-dependence. High affinity of cross-bridges for ADP and slow ADP release prolong the fraction of the duty cycle occupied by strongly bound AM.ADP state(s) and contribute to the high economy of force that is characteristic of smooth muscle. RLC thiophosphorylation led to structural changes in smooth muscle cross-bridges consistent with our findings that thiophosphorylation and strain modulate product release.

Myosin regulatory light chain phosphorylation and strain modulate adenosine diphosphate release from smooth muscle Myosin

The effects of myosin regulatory light chain (RLC) phosphorylation and strain on adenosine diphosphate (ADP) release from cross-bridges in phasic (rabbit bladder (Rbl)) and tonic (femoral artery (Rfa)) smooth muscle were determined by monitoring fluorescence transients of the novel ADP analog, 3'-deac-eda-ADP (deac-edaADP). Fluorescence transients reporting release of 3'-deac-eda-ADP were significantly faster in phasic (0.57 +/- 0.06 s(-1)) than tonic (0.29 +/- 0.03 s(-1)) smooth muscles. Thiophosphorylation of regulatory light chains increased and strain decreased the release rate approximately twofold. The calculated (k-ADP/k+ADP) dissociation constant, Kd of unstrained, unphosphorylated cross-bridges for ADP was 0.6 microM for rabbit bladder and 0.3 microM for femoral artery. The rates of ADP release from rigor bridges and reported values of Pi release (corresponding to the steady-state adenosine triphosphatase (ATPase) rate of actomyosin (AM)) from cross-bridges during a maintained isometric contraction are similar, indicating that the ADP-release step or an isomerization preceding it may be limiting the adenosine triphosphatase rate. We conclude that the strain- and dephosphorylation-dependent high affinity for and slow ADP release from smooth muscle myosin prolongs the fraction of the duty cycle occupied by strongly bound actomyosin.ADP state(s) and contributes to the high economy of force.

Isoform switching from SM-B to SM-A myosin results in decreased contractility and altered expression of thin filament regulatory proteins

We previously generated an isoform-specific gene knockout mouse in which SM-B myosin is permanently replaced by SM-A myosin. In this study, we examined the effects of SM-B myosin loss on the contractile properties of vascular smooth muscle, specifically peripheral mesenteric vessels and aorta. The absence of SM-B myosin leads to decreased velocity of shortening and increased isometric force generation in mesenteric vessels. Surprisingly, the same changes occur in aorta, which contains little or no SM-B myosin in wild-type animals. Calponin and activated mitogen-activated protein kinase expression is increased and caldesmon expression is decreased in aorta, as well as in bladder. Light chain-17b isoform (LC(17b)) expression is increased in aorta. These results suggest that the presence or absence of SM-B myosin is a critical determinant of smooth muscle contraction and that its loss leads to additional changes in thin filament regulatory proteins.

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.

Time course of airway mechanics of the (+)insert myosin isoform knockout mouse

Two smooth muscle myosin heavy chain isoforms that differ by the presence ([+]insert) or the absence ([-]insert) of a 7-amino acid insert in the motor domain have a 2-fold difference in their in vitro actin filament velocity. We hypothesized that a preferential expression of the fast (+)insert isoform in airway smooth muscle would increase the rate of bronchoconstriction. To verify our hypothesis we measured the time course of bronchoconstriction following a bolus injection of methacholine (160 microg/kg) in (+)insert isoform knockout (KO) and corresponding wild-type (WT) mice. Neither baseline airway resistance (Raw) (0.424 +/- 0.04 for WT and 0.374 +/- 0.01 cm H(2)O.s.ml(-1) for KO) nor peak Raw (4.1 +/- 0.9 for WT and 4.0 +/- 0.5 cm H(2)O.s.ml(-1) for KO) differed between groups. However, the time to peak Raw was significantly longer in the KO (17.2 +/- 0.6 s) compared with the WT (14.6 +/- 0.8 s) mice (P < 0.05). Differentiating Raw with respect to time revealed a greater rate of bronchoconstriction for the WT during the initial 4 s, presumably reflecting the faster shortening velocities under these relatively unloaded conditions. Reverse transcriptase-polymerase chain reaction analysis revealed that the (+)insert myosin isoform mRNA content in the WT airways was 47.8 +/- 5.6%. We conclude that the presence of the (+)insert myosin isoform in the airways increases the rate of bronchoconstriction.

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.

Expression of smooth muscle MyHC B in blood vessels of hypertrophied heart in experimentally hypertensive rats

We demonstrated recently a significantly lower fraction of cardiac precapillary arterioles that expressed smooth muscle myosin heavy chain (MyHC) B (SMB) in spontaneously hypertensive rats. To clarify whether this reduction of SMB expression is of genetic origin, we investigated SMB expression in cardiac precapillary arterioles of normotensive and experimentally hypertensive rats (one clip, one kidney or ANG II minipump). We observed similar SMB expression patterns in precapillary arterioles of experimentally hypertensive rats compared with normotensive controls. These observations suggest that the downregulation of SMB in spontaneously hypertensive rats is of genetic origin rather than an adaptive response to chronically enhanced blood pressure and cardiac hypertrophy.

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.

Uncoupling protein-1: involvement in a novel pathway for beta-adrenergic, cAMP-mediated intestinal relaxation

The pathway for adrenergic relaxation of smooth muscle is not fully understood. As mitochondrial uncoupling protein-1 (UCP1) expression has been reported in cells within the longitudinal smooth muscle layer of organs exhibiting peristalsis, we examined whether the absence of UCP1 affects adrenergic responsiveness. Intestinal (ileal) segments were obtained from UCP1-ablated mice and from wild-type mice (C57Bl/6, 129/SvPas, and outbred NMRI). In UCP1-containing mice, isoprenaline totally inhibited contractions induced by electrical field stimulation, but in intestine from UCP1-ablated mice, a significant residual contraction remained even at a high isoprenaline concentration; the segments were threefold less sensitive to isoprenaline. Also, when contraction was induced by carbachol, there was a residual isoprenaline-insensitive contraction. Similar results were obtained with the beta(3)-selective agonist CL-316,243 and with the adenylyl cyclase stimulator forskolin. Thus the UCP1 reported to be expressed in the longitudinal muscle layer of the mouse intestine is apparently functional, and UCP1, presumably through uncoupling, may be involved in a novel pathway leading from increased cAMP levels to relaxation in organs exhibiting peristalsis.

Photolytic release of MgADP reduces rigor force in smooth muscle

Photolytic release of MgADP (25-300 microM) from caged ADP in permeabilized tonic (rabbit femoral artery-Rfa) and phasic (rabbit bladder-Rbl) smooth muscle in high-tension rigor state, in the absence of Ca(2+), caused an exponential decline (approximately 1.5% in Rfa and approximately 6% in Rbl) of rigor force, with the rate proportional to the liberated [MgADP]. The apparent second-order rate constant of MgADP binding was estimated as approximately 1.0 x 10(6) M(-1) s(-1) for both smooth muscles. In control experiments, designed to test the specificity of MgADP, photolysis of caged ADP in the absence of Mg(2+) did not decrease rigor force in either smooth muscle, but rigor force decreased after photolytic release of Mg(2+) in the presence of ADP. The effects of photolysis of caged ADP were similar in smooth muscles containing thiophosphorylated or non-phosphorylated regulatory myosin light chains. Stretching or releasing (within range of 0.1-1.2% of initial Ca(2+)-activated force) did not affect the rate or relative amplitude of the force decrease. The effect of additions of MgADP to rigor cross-bridges could result from rotation of the lever arm of smooth muscle myosin, but this need not imply that ADP-release is a significant force-producing step of the physiological cross-bridge cycle.

Myosin isoforms in female human detrusor

The aim of this study was to document the relative proportions of two isoforms of myosin heavy chain in detrusor smooth muscle of women with detrusor overactivity and in asymptomatic controls. Women aged 35-65 with documented detrusor overactivity and without a history of neurologic disease, prior incontinence surgery, elevated post-void residual urine volume, or indwelling urinary catheter were eligible for the study. Full-thickness biopsies of extraperitoneal bladder dome were obtained at the time of laparotomy in six patients with documented detrusor overactivity and in a control group of eight continent patients. Biopsies were frozen in liquid nitrogen, crushed with a frozen mortar and pestle at -80 degrees C, and homogenized in buffer, and the extracts were electrophoresed on 6% polyacrylamide sodium dodecyl sulfate gels and stained with Coomassie blue. The gels were de-stained and then the protein bands were scanned with a densitometer. The mean patient age was 48 years (range, 36-59). Seven patients were Caucasian and seven patients were African American. Detrusor smooth muscle contains a mean of 34% (range, 27-43%) SM1 and 66% (range, 57-73%) SM2 isoforms. There was no difference in isoform composition when patients were compared according to urogynecologic diagnosis or according to race. In detrusor biopsies from women, approximately 34% of myosin is of the SM1 isoform and approximately 66% is of the SM2 isoform. This ratio is relatively constant in the two races studied and unchanged in women with detrusor overactivity. Animal models utilizing outlet obstruction of the bladder to provoke detrusor instability and detrusor hypertrophy are known to alter myosin isoform distribution and may not be appropriate models of detrusor instability in human females.

Contractile properties and proteins of smooth muscles of a calponin knockout mouse

The role of h1-calponin in regulating the contractile properties of smooth muscle was investigated in bladder and vas deferens of mice carrying a targeted mutation in both alleles designed to inactivate the basic calponin gene. These calponin knockout (KO) mice displayed no detectable h1-calponin in their smooth muscles. The amplitudes of Ca2+ sensitization, force and Ca2+ sensitivity were not significantly different in permeabilized smooth muscle of KO compared with wild-type (WT) mice, nor were the delays in onset and half-times of Ca2+ sensitization, initiated by flash photolysis of caged GTPgammaS, different. The unloaded shortening velocity (Vus) of thiophosphorylated fibres was significantly (P<0.05) faster in the smooth muscle of KO than WT animals, but could be slowed by exogenous calponin to approximate WT levels; the concentration dependence of exogenous calponin slowing of Vus was proportional to its actomyosin binding in situ. Actin expression was reduced by 25-50%, relative to that of myosin heavy chain, in smooth muscle of KO mice, without any change in the relative distribution of the actin isoforms. We conclude that the faster Vus of smooth muscle of the KO mouse is consistent with, but does not prove without further study, physiological regulation of the crossbridge cycle by calponin. Our results show no detectable role of calponin in the signal transduction of the Ca2+-sensitization pathways in smooth muscle.

Smooth muscle myosin heavy chain isoforms and their role in muscle physiology

Unlike vertebrate skeletal muscle, smooth muscle myosin heavy chain isoforms are encoded by a single gene. Alternative splicing of the primary transcript from a single gene generates four smooth muscle myosin heavy chain isoforms. These isoforms differ both at the carboxyl terminus (SM1 and SM2 isoforms) and at the amino terminus (SM-A and SM-B isoforms). The smooth muscle myosin heavy chain isoforms are differentially expressed during smooth muscle development and in different smooth muscle cell types. The mechanical properties of smooth muscle may be correlated with the myosin heavy chain content/isoform expression. However, the precise function of each smooth muscle myosin heavy chain isoform to muscle contraction remains to be determined. This review mainly focuses on the molecular basis of smooth muscle myosin heavy chain isoform diversity, its expression during development and disease, and its role in muscle physiology.

Maturational differences between vascular and bladder smooth muscle during ovine development

Maturation rates of vascular and visceral smooth muscle (SM) during ovine development were compared by quantifying contractile protein, myosin heavy chain (MHC) isoform contents, and contractile properties of aortas and bladders from female fetal (n = 19) and postnatal (n = 21) sheep. Actin, myosin, and protein contents rose progressively throughout development in both tissues (P </= 0.003); however, expression patterns differed. During the last trimester, i. e., 101-130 days (term approximately 145 days), bladder actin and MHC contents were approximately twofold greater (P < 0.04) than those in the aorta. Although the fractional content of 204-kDa SM1 MHC in the bladder decreased from 74 +/- 3% at midgestation to 48 +/- 2% 3 mo postnatal, the aorta exhibited an increase from 30 +/- 2% to 65 +/- 2%. Bladder MHC (MHC-B) migrating at 200 kDa contained only SM2 throughout development. In contrast, 200-kDa MHC in the aorta was predominantly nonmuscle MHC-B at midgestation, which was gradually replaced by SM2 as development progressed. Along with its early expression of SM2, bladder muscle obtained maximal stress generating capacity (1.7 x 10(5) N/m(2)) by term gestation, whereas the aorta exhibited no contractions until after birth. We conclude that whereas aortic SM maturation is delayed until after birth, bladder SM matures biochemically and functionally during prenatal development, thus supporting early requirements for micturition.

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.

Phosphorylation regulates the ADP-induced rotation of the light chain domain of smooth muscle myosin

We have observed the effects of MgADP and thiophosphorylation on the conformational state of the light chain domain of myosin in skinned smooth muscle. Electron paramagnetic resonance (EPR) spectroscopy was used to monitor the orientation of spin probes attached to the myosin regulatory light chain (RLC). Two spectral states were seen, termed here "intermediate" and "final", that are distinguished by a approximately 24 degrees axial rotation of spin probes attached to the RLC. The two observed conformations are similar to those found previously for smooth muscle myosin S1; the final state corresponds to the major conformation of S1 in the absence of ADP, while the intermediate state corresponds to the conformation of S1 with ADP bound. Light chain domain orientation was observed as a function of the MgADP concentration and the extent of RLC thiophosphorylation. In rigor (no MgADP), LC domains were distributed equally between the intermediate state and the final state; upon addition of saturating (3.5 mM) MgADP, about one-third of the LC domains in the final state rotated approximately 20 degrees axially to the intermediate state. The progression of the change in populations was fit to a simple binding equation, yielding an apparent dissociation constant of approximately 110 microM for skinned smooth muscle fibers and approximately 730 microM for thiophosphorylated, skinned smooth muscle fibers. These observations suggest a model that explains the behavior of "latch bridges" in smooth muscle.

Smooth-muscle myosin heavy-chain SM-B isoform expression in developing and adult rat lung

The smooth-muscle cells composing the vasculature and airways of the lung display a variety of contractile protein phenotypes. To date, however, it has remained unclear how these phenotypes might contribute differentially to contractile activity. To address this issue, we made monospecific rabbit polyclonal antibodies against the difference peptide for the SM-B smooth-muscle myosin heavy chain (SMMHC) and used these to investigate the distribution of the SM-B isoform in lung. SM-B has a seven-amino acid insert in the head region that is known to result in a higher actin-activated adenosine triphosphatase activity and in vitro motility. During development, reactivity is first seen in the trachea and bronchi of saccular lung at the time of birth, when other SMMHC isoforms also are present. Immunoreactivity spreads distally through the airways as development proceeds, reaching the level of alveolar septae in the adult. Although the smaller vessels of the pulmonary vasculature react strongly with the SM-B antibody, reactivity is infrequently observed in large pulmonary vessels. Adult tracheal smooth muscle is highly and more uniformly reactive, commensurate with its relatively high maximal velocity of shortening. The differential expression of the SM-B isoform in vascular and airway smooth muscles demonstrated in this study may provide the molecular basis for functional differences between these smooth-muscle cell types and may provide one mechanism for adapting contractility in response to physiologic stresses in the lung.

Pharmacomechanical coupling: the role of calcium, G-proteins, kinases and phosphatases

The concept of pharmacomechanical coupling, introduced 30 years ago to account for physiological mechanisms that can regulate contraction of smooth muscle independently of the membrane potential, has since been transformed from a definition into what we now recognize as a complex of well-defined, molecular mechanisms. The release of Ca2+ from the SR by a chemical messenger, InsP3, is well known to be initiated not by depolarization, but by agonist-receptor interaction. Furthermore, this G-protein-coupled phosphatidylinositol cascade, one of many processes covered by the umbrella of pharmacomechanical coupling, is part of complex and general signal transduction mechanisms also operating in many non-muscle cells of diverse organisms. It is also clear that, although the major contractile regulatory mechanism of smooth muscle, phosphorylation/dephosphorylation of MLC20, is [Ca2+]-dependent, the activity of both the kinase and the phosphatase can also be modulated independently of [Ca2+]i. Sensitization to Ca2+ is attributed to inhibition of SMPP-1M, a process most likely dominated by activation of the monomeric GTP-binding protein RhoA that, in turn, activates Rho-kinase that phosphorylates the regulatory subunit of SMPP-1M and inhibits its myosin phosphatase activity. It is likely that the tonic phase of contraction activated by a variety of excitatory agonists is, at least in part, mediated by this Ca(2+)-sensitizing mechanism. Desensitization to Ca2+ can occur either through inhibitory phosphorylation of MLCK by other kinases or autophosphorylation and by activation of SMPP-1M by cyclic nucleotide-activated kinases, probably involving phosphorylation of a phosphatase activator. Based on our current understanding of the complexity of the many cross-talking signal transduction mechanisms that operate in cells, it is likely that, in the future, our current concepts will be refined, additional mechanisms of pharmacomechanical coupling will be recognized, and those contributing to the pathologenesis diseases, such as hypertension and asthma, will be identified.

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.

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.

Molecular mechanics of two smooth muscle heavy meromyosin constructs that differ by an insert in the motor domain

Phasic and tonic smooth muscles express two myosin heavy chain isoforms that differ by only a seven amino acid insert in a flexible surface loop located near the nucleotide-binding site. The inserted isoform found predominantly in phasic muscle has two times the actin-activated ATPase activity and in vitro actin filament velocity as the non-insert isoform found mainly in tonic muscle (Kelley, C.A., Takahashi, M., Yu, J.H. & Adelstein, R.S. 1993. J Biol Chem 268, 12848, Rovner, A.S., Freyzon, Y. & Trybus, K.M. 1997. J Musc Res Cell Motil 18, 103). We used a laser trap to characterize the molecular mechanics of the inserted isoform [(+)insert] and of a mutant lacking the insert [(-)insert], which is analogous to the isoform found in tonic muscles. The constructs were expressed in the baculovirus/insect cell system. Unitary displacements (Duni) were similar for both the constructs (approximately 10 nm) but the attachment time (ton) for the (-)insert was two times that of the (+)insert. These data suggest that the insert in the nucleotide-binding loop does not affect the inherent mechanics of the myosin molecule but rather the kinetics of the cross-bridge cycle.

Myosin essential light chain isoforms modulate the velocity of shortening propelled by nonphosphorylated cross-bridges

The differential effects of essential light chain isoforms (LC17a and LC17b) on the mechanical properties of smooth muscle were determined by exchanging recombinant for endogenous LC17 in permeabilized smooth muscle treated with trifluoperazine (TFP). Co-precipitation with endogenous myosin heavy chain verified that 40-60% of endogenous LC17a could be exchanged for recombinant LC17a or LC17b. Upon addition of MgATP in Ca2+-free solution, recombinant LC17 exchange induced slow contractions unaccompanied by regulatory light chain (RLC) phosphorylation only in TFP-treated, but not in untreated, permeabilized smooth muscle; the shortening velocity and rate of force development were approximately 1.5 and 2 times faster, respectively, in response to LC17a than LC17b. Additional incubation with recombinant, thiophosphorylated RLC increased the shortening velocity, independent of the LC17 isoform exchanged. The LC17-induced contractions of TFP-treated muscles were abolished by prior addition of nonphosphorylated RLC. We suggest that LC17 stiffens the lever arm of myosin and, in the absence of regulation by RLC, permits cross-bridge cycling without requiring RLC phosphorylation. Our results are compatible with nonphosphorylated RLC acting as a repressor and with LC17 isoforms modulating the MgADP affinity and, consequently, rate of cooperative cycling of nonphosphorylated cross-bridges.

A 7-amino-acid insert in the heavy chain nucleotide binding loop alters the kinetics of smooth muscle myosin in the laser trap

Two smooth muscle myosin heavy chain isoforms differ by a 7-amino-acid insert in a flexible surface loop located near the nucleotide binding site. The non-inserted isoform is predominantly found in tonic muscle, while the inserted isoform is mainly found in phasic muscle. The inserted isoform has twice the actin-activated ATPase activity and actin filament velocity in the in vitro motility assay as the non-inserted isoform. We used the laser trap to characterize the molecular mechanics and kinetics of the inserted isoform ((+)insert) and of a mutant lacking the insert ((-)insert), analogous to the isoform found in tonic muscle. The constructs were expressed as heavy meromyosin using the baculovirus/insect cell system. Unitary displacement (d) was similar for both constructs (approximately 10 nm) but the attachment time (t(on) for the (-)insert was twice as long as for the (+)insert regardless of the [MgATP]. Both the relative average isometric force (Favg(-insert)/Favg(+insert) = 1.1 +/- 0.2 (mean +/- SE) using the in vitro motility mixture assay, and the unitary force (F approximately 1 pN) using the laser trap, showed no difference between the two constructs. However, as under unloaded conditions, t(on) under loaded conditions was longer for the (-)insert compared with the (+)insert construct at limiting [MgATP]. These data suggest that the insert in this surface loop does not affect the mechanics but rather the kinetics of the cross-bridge cycle. Through comparisons of t(on) from d measurements to various [MgATP], we conclude that the insert affects two specific steps in the cross-bridge cycle, that is, MgADP release and MgATP binding.

Transforming growth factor-beta dynamically regulates vascular smooth muscle differentiation in vivo

Variations in the levels of smooth muscle-specific isoforms of contractile proteins have been reported to occur in many different vascular diseases. However, although much work has been done in vitro to investigate the regulation of smooth muscle cell differentiation, the molecular mechanisms which regulate the differentiation of vascular smooth muscle tissue in vivo are unknown. Using quantitative immunofluorescence, we show that in rat arteries levels of smooth muscle differentiation markers correlate with the levels of the cytokine TGF-beta. In young mice with one allele of the TGF-beta1 gene deleted, the levels of both TGF-beta1 and smooth muscle differentiation markers are reduced compared to wild-type controls. This regulation of smooth muscle differentiation by TGF-beta during post-natal development also occurs dynamically in the adult animal. Following various pharmacological or surgical interventions, including treatment of mice with tamoxifen and balloon injury of rat carotid arteries, there is a strong correlation between the changes in the levels of TGF-beta and changes in the levels of smooth muscle differentiation markers (r=0. 9, P&lt;0.0001 for n=26 experiments). We conclude that TGF-beta dynamically regulates smooth muscle differentiation in rodent arteries in vivo.

MgADP promotes a catch-like state developed through force-calcium hysteresis in tonic smooth muscle

Tonic rabbit femoral artery and phasic rabbit ileum smooth muscles permeabilized with Triton X-100 were activated either by increasing [Ca2+] from pCa > 8.0 to pCa 6.0 (calcium-ascending protocol) or contracted at pCa 6.0 before lowering [Ca2+] (calcium-descending protocol). The effects of, respectively, high [MgATP]/low [MgADP] [10 mM MgATP + creatine phosphate (CP) + creatine kinase (CK)] or low [MgATP]/[MgADP] (2 mM MgATP, 0 CP, 0 CK) on the "force-[Ca]" relationships were determined. In femoral artery at low, but not at high, [MgATP]/[MgADP] the force and the ratio of stiffness/force at pCa 7.2 were significantly higher under the calcium-descending than calcium-ascending protocols (54% vs. 3% of Po, the force at pCa 6.0) (force hysteresis); the levels of regulatory myosin light chain (MLC20) phosphorylation (9 +/- 2% vs. 10 +/- 2%) and the velocities of unloaded shortening V0 (0.02 +/- 0.004 l/s with both protocols) were not significantly different. No significant force hysteresis was detected in rabbit ileum under either of these experimental conditions. [MgADP], measured in extracts of permeabilized femoral artery strips by two methods, was 130-140 microM during maintained force under the calcium-descending protocol. Exogenous CP (10 mM) applied during the descending protocol reduced endogenous [MgADP] to 46 +/- 10 microM and abolished force hysteresis: residual force at low [Ca2+] was 17 +/- 5% of maximal force. We conclude that the proportion of force-generating nonphosphorylated (AMdp) relative to phosphorylated cross-bridges is higher on the Ca2+-descending than on the Ca2+-ascending force curve in tonic smooth muscle, that this population of positively strained dephosphorylated cross-bridges has a high affinity for MgADP, and that the dephosphorylated AMdp . MgADP state makes a significant contribution to force maintenance at low levels of MLC20 phosphorylation.

Lung smooth muscle differentiation

The vascular and visceral smooth muscle tissues of the lung perform a number of tasks that are critical to pulmonary function. Smooth muscle function often is compromised as a result of lung disease. Though a great deal is known about regulation of smooth muscle cell replication and cell and tissue contractility, much less is understood regarding the phenotype of the contractile protein machinery of lung smooth muscle cells. This review focuses on the expression of cytoskeletal and contractile proteins of lung vascular and airway smooth muscle cells during development, in the adult and during vascular and airway remodeling. Emphasis is placed on the expression of the heavy chain of smooth muscle myosin, as well as the regulation of its gene. Important areas for future research are discussed.

Expression of smooth muscle myosin heavy chain B in cardiac vessels of normotensive and hypertensive rats

We investigated expression of the 5'-spliced isoform of smooth muscle myosin heavy chain (SM-MHC-B) in smooth muscle cells of cardiac vessels of the left ventricle of normotensive (Wistar-Kyoto) and spontaneously hypertensive rats of the stroke-prone strain by immunofluorescence microscopy. In parallel, liver and bladder were studied for characterization of the nature of vessels expressing SM-MHC-B and for semiquantitative evaluation of its abundance. Smooth muscle cells were detected by staining with a monoclonal antibody specific for alpha-smooth muscle actin. Abundance of the SM-MHC-B isoform in these cells was evaluated by using an antibody raised against the seven-amino acid insert at the 25K/50K junction of the myosin head (a25K/50K) that specifically recognized SM-MHC-B. In the ventricle, a25K/50K immunoreactivity was observed in smooth muscle cells of precapillary arterioles but not in larger vessels or aorta. The a25K/50K immunoresponse of those vessels with the highest expression level of SM-MHC-B closely resembled the signal observed in the smooth muscle layer of urinary bladder known to preferentially express SM-MHC-B. Interestingly, in left ventricles of stroke-prone spontaneously hypertensive rats, there was a significantly reduced fraction of a25K/50K-positive precapillary arterioles compared with normotensive control rats.

Mechanical alterations in smooth muscle from mice lacking desmin

Mice with a null mutation introduced in the desmin gene were used to study the mechanical role of intermediate filaments in smooth muscle cells. Vas deferens (VD), urinary bladder (UB) and portal vein (PV) preparations were obtained from adult animals lacking desmin (Des -/-) and from age- and weight-matched wild-type animals (Des +/+). Active force per cross-sectional area was decreased in the smooth muscle of the Des -/- compared with Des +/+ mice (VD to 42%; UB to 34%). Quantitative gel electrophoresis suggests a marginally lower cellular content of myosin, but the organization of the contractile apparatus appeared unchanged by electron microscopy. A similar reduction in stress was measured in Des -/- skinned fibres showing that altered activation mechanisms were not involved. The results indicate that the reduced active force is caused by low intrinsic force generation of the contractile filaments or subtle modifications in the coupling between the contractile elements and the cytoskeleton. The relationship between length and passive stress was less steep in the Des -/- samples and a second length force curve after maximal extension revealed a loss of passive stress. The maximal shortening velocity was reduced in Des -/- skinned VD and UB preparations by approximately 25-40%. This was associated with an increased relative content of the basic essential myosin light chain, suggesting that alterations in the contractile system towards a slower, more economical muscle had occurred. PV preparations showed no difference in mechanical properties in Des +/+ and Des -/- animals, a result that was consistent with the predominance of vimentin instead of desmin in this vascular tissue. In conclusion, the results show that, although intermediate filaments in smooth muscle are not required for force generation or maintenance of passive tension, they have a role in cellular transmission of both active and passive force.

Myosin heavy chain isoforms and dynamic contractile properties: skeletal versus smooth muscle

Myosin, one of the primary contractile muscle proteins, displays molecular, enzymatic, structural, functional and regulatory variability. This variability has been shown to account for a significant amount of the functional uniqueness of skeletal and smooth muscle. However, the universal generation of force and/or shortening by these two muscle types belies the ever-increasing number of known distinct differences that bring this about. Thus, the notion that the functional roles of skeletal and smooth muscle, their development and regulation, all appear to be uniquely applicable for their physiological purpose no longer appears heretical. This manuscript presents a cursory overview of the numerous ways in which these two types of muscle use a host of myosin molecules to bring about a common result, force generation and/or shortening.

Myosin heavy chain transitions during development. Functional implications for the respiratory musculature

The myosin heavy chain (MHC) exists as multiple isoforms that are encoded for by a family of genes. The respiratory musculature demonstrates muscle-specific and temporally-dependent changes in MHC isoform expression during maturation. Developmental expression of MHC isoforms correlate well with postnatal changes in actomyosin ATPase activity, specific force generation (P0/CSA), maximum unloaded velocity of shortening (V0) and and fatigue resistance. More specifically, as the expression of MHCneonatal declines and MHC2A, MHC2X, and MHC2B increase, actomyosin ATPase activity, P0/CSA, V0, and muscle fatigability increase. The increase in actomyosin ATPase activity with maturation is partially offset by a postnatal increase in oxidative capacity; however, as fatigue resistance declines with development it is apparent that the energy costs of contraction are not fully matched by an increase in energy production. Developmental transitions in smooth muscle MHC phenotype also occur although their functional importance remains unclear.

Myosin gene expression and cell phenotypes in vascular smooth muscle during development, in experimental models, and in vascular disease

In the aortic wall of mammalian species, the maturation phase of smooth muscle cell (SMC) lineage is characterized by two temporally correlated but opposite regulatory processes of gene expression: upregulation of SM type SM2 myosin isoform and downregulation of brain (myosin heavy chain B)- and platelet (myosin heavy chain A(pla))-type nonmuscle myosins. Using the myosin isoform approach to study vascular SMC biology, we have shown (1) a marked SMC heterogeneity in adult arterial vessels, ie, coexistence of an "immature" and a fully differentiated SMC population; and (2) the propensity of the immature type SMC population to be activated in experimental models and human vascular diseases that are characterized by proliferation and migration of medial SMCs into the subendothelial space.

Myosin isoforms and functional diversity in vertebrate smooth muscle

The expression of fast and slow myosin isoforms in individual cells is associated with differences in shortening velocities and power output in fully differentiated vertebrate striated muscle. This paradigm in which shortening velocity is determined by the myosin isoform (and load) is inappropriate for smooth muscle. Smooth muscle tissues express multiple myosin heavy and light chain isoforms, and it is not currently possible to separate and identify chemically distinct native myosin hexamers (i.e., isoforms). It is not known if different isoforms are localized in subpopulations of cells or in specific cellular domains nor whether they combine preferentially to form a small number of native myosin hexamer isoforms. Potentially, thick filaments are aggregates of many different combinations of heavy and light chain isoforms that may or may not exhibit different kinetics. Shortening velocities in smooth muscle are regulated by Ca(2+)-dependent crossbridge phosphorylation of the myosin regulatory light chains. Much of the observed diversity in power output in smooth muscle may be attributed to regulatory mechanisms modulating crossbridge cycling rates rather than contractile protein isoform expression.

Cryo-atomic force microscopy of smooth muscle myosin

The motor and regulatory domains of the head and the 14-nm pitch of the alpha-helical coiled-coil of the tail of extended (6S) smooth-muscle myosin molecules were imaged with cryo atomic force microscopy at 80-85 K, and the effects of thiophosphorylation of the regulatory light chain were examined. The tail was 4 nm shorter in thiophosphorylated than in nonphosphorylated myosin. The first major bend was invariant, at approximately 51 nm from the head-tail junction (H-T), coincident with low probability in the paircoil score. The second major bend was 100 nm from the H-T junction in nonphosphorylated and closer to a skip residue than the bend (at 95 nm) in thiophosphorylated molecules. The shorter tail and distance between the two major bends induced by thiophosphorylation are interpreted to result from melting of the coiled-coil. An additional bend not previously reported occurred, with a lower frequency, approximately 24 nm from the H-T. The range of separation between the two heads was greater in thiophosphorylated molecules. Occasional high-resolution images showed slight unwinding of the coiled-coil of the base of the heads. We suggest that phosphorylation of MLC20 can affect the structure of extended, 6S myosin.

An insert in the motor domain determines the functional properties of expressed smooth muscle myosin isoforms

Smooth muscle myosin isoforms of the heavy chain and the essential light chain have been hypothesized to contribute to the different shortening velocities of phasic and tonic smooth muscles, and to their different affinities for MgADP. We used the baculovirus/insect cell system to express homogeneous heavy meromyosin molecules differing only in seven amino acid insert (QGPSFSY) in the motor domain near the active site, or in the type of essential light chain isoform. Myosin from tonic rabbit uterine smooth muscle lacks the heavy chain insert, while myosin from phasic chicken gizzard contains it. The properties of a mutant uterine heavy meromyosin with added insert, and a mutant gizzard heavy meromyosin with the insert deleted, were compared with their wild type progenitors. Phosphorylated heavy meromyosins with the insert have a twofold higher enzymatic activity and in vitro motility han heavy meromyosins without the insert. These functional properties were not altered by the essential light chain isoforms. The altered motility caused by the insert implies that it modulates the rate of ADP release, the molecular step believed to limit shortening velocity. The insert may thus account in part for both the lower sensitivity to MgADP and the higher shortening velocity of phasic compared to tonic smooth muscles.

Alternative splicing and cycling kinetics of myosin change during hypertrophy of human smooth muscle cells

We investigated in vivo expression of myosin heavy chain (MHC) isoforms, 17 kDa myosin light chain (MLC17), and phosphorylation of the 20 kDa MLC (MLC20) as well as mechanical performance of chemically skinned fibers of normal and hypertrophied smooth muscle (SM) of human myometrium. According to their immunological reactivity, we identified three MHC isoenzymes in the human myometrium: two SM-MHC (SM1 with 204 kDa and SM2 with 200 kDa), and one non-muscle specific MHC (NM with 196 kDa). No cross-reactivity was detected with an antibody raised against a peptide corresponding to a seven amino acid insert at the 25K/50K junction of the myosin head (a-25K/50K) in both normal and hypertrophied myometrium. In contrast, SM-MHC of human myomatous tissue strongly reacted with a-25K/50K. Expression of SM1/SM2/NM (%) in normal myometrium was 31.7/34.7/33.6 and 35.1/40.9/24 in hypertrophied myometrium. The increased SM2 and decreased NM expression in the hypertrophied state was statistically significant (P < 0.05). MHC isoform distribution in myomatous tissue was similar to normal myometrium (36.3/35.3/29.4). In vivo expression of MLC17a increased from 25.5% in normal to 44.2% in hypertrophied (P < 0.001) myometrium. Phosphorylation levels of MLC20 upon maximal Ca(2+)-calmodulin activation of skinned myometrial fibers were the same in normal and hypertrophied myometrial fibers. Maximal force of isometric contraction of skinned fibers (pCa 4.5, slack-length) was 2.85 mN/mm2 and 5.6 mN/mm2 in the normal and hypertrophied state, respectively (P < 0.001). Apparent maximal shortening velocity (Vmax(appt), extrapolated from the force-velocity relation) of myometrium rose from 0.13 muscle length s-1 (ML/s) in normal to 0.24 ML/s in hypertrophied fibers (P < 0.001).

Myosin-based cortical tension in Dictyostelium resolved into heavy and light chain-regulated components

Cortical tension in most nonmuscle cells is due largely to force production by conventional myosin (myosin II) assembled into the cytoskeleton. Cytoskeletal contraction in smooth muscle and nonmuscle cells is influenced by the degree of myosin filament assembly, and by activation of myosin motor function via regulatory light chain phosphorylation. Recombinant Dictyostelium discoideum cell lines have been generated bearing altered myosin heavy chains, resulting in either constitutive motor function or constitutive assembly into the cytoskeleton. Analysis of these cells allowed stiffening responses to agonists, measured on single cells, to be resolved into an regulatory light chain-mediated component reflecting activation of motor function, and a myosin heavy chain phosphorylation-regulated component reflecting assembly of filaments into the cytoskeleton. These two components can account for all of the cortical stiffening response seen during tested in vivo contractile events.

Alternative splicing of smooth muscle myosin heavy chains and its functional consequences

The aim of our study was to determine the relation between alternatively spliced myosin heavy chain (MHC) isoforms and the contractility of smooth muscle. The relative amount of MHC with an alternatively spliced insert in the 5' (amino terminal) domain was determined on the protein level using a peptide-directed antibody (a25K/50K) raised against the inserted sequence (QGPSFAY). Smooth muscle MHC isoforms of both bladder and myometrium but not nonmuscle MHC reacted with a25/50K. Using a quantitative Western-blot approach the amount of 5'-inserted MHC in rat bladder was detected to be about eightfold higher than in normal rat myometrium. The amount of heavy chain with insert was found to be decreased by about 50% in the myometrium of pregnant rats. Although bladder contained significantly more 5'-inserted MHC than myometrium, apparent maximal shortening velocities (Vmax) were comparable, being 0.138 +/- 0.012 and 0.114 +/- 0.023 muscle length per second of skinned bladder and normal myometrium fibers, respectively. Phosphorylation of myosin light chain 20 induced by maximal Ca2+/calmodulin activation was the same in bladder and myometrial fibers. These results suggest that the amount of 5'-inserted MHC is not necessarily associated with contractile properties of smooth muscle.