Myosin light chain kinase phosphorylation in tracheal smooth muscle

Purified myosin light chain kinase from smooth muscle is phosphorylated by cyclic AMP-dependent protein kinase, protein kinase C, and the multifunctional calmodulin-dependent protein kinase II. Because phosphorylation in a specific site (site A) by any one of these kinases desensitizes myosin light chain kinase to activation by Ca2+/calmodulin, kinase phosphorylation could play an important role in regulating smooth muscle contractility. This possibility was investigated in 32P-labeled bovine tracheal smooth muscle. Treatment of tissues with carbachol, KCl, isoproterenol, or phorbol 12,13-dibutyrate increased the extent of kinase phosphorylation. Six primary phosphopeptides (A-F) of myosin light chain kinase were identified. Site A was phosphorylated to an appreciable extent only with carbachol or KCl, agents which contract tracheal smooth muscle. The extent of site A phosphorylation correlated to increases in the concentration of Ca2+/calmodulin required for activation. These results show that cyclic AMP-dependent protein kinase and protein kinase C do not affect smooth muscle contractility by phosphorylating site A in myosin light chain kinase. It is proposed that phosphorylation of myosin light chain kinase in site A in contracting tracheal smooth muscle may play a role in the reported desensitization of contractile elements to activation by Ca2+.

Okadaic acid uncouples myosin light chain phosphorylation and tension in smooth muscle

Tracheal smooth muscle precontracted with carbachol relaxes upon the addition of 3 microM okadaic acid. Although cytosolic Ca2+ concentrations decrease, myosin light chain remains highly phosphorylated (50%). In smooth muscle treated with carbachol alone or carbachol plus okadaic acid 32P is incorporated into a single peptide on myosin light chain which corresponds to the site phosphorylated by myosin light chain kinase. Treatment with okadaic acid alone does not result in myosin light chain phosphorylation or tension development. These results suggest that a cellular mechanism other than myosin light chain phosphorylation can regulate contractile tension.

Protein content and myosin light chain phosphorylation in uterine arteries during pregnancy

During pregnancy, the ovine uterine artery changes from a low- to a high-stress artery. We investigated the hypotheses that the increased stress reflects alterations in vessel wall cellularity, smooth muscle cell contractile protein contents, or activation properties. Uterine artery diameter increased during pregnancy, whereas the fractional cellular composition and thickness of the muscularis were unchanged. Results of morphometry suggest that vessel growth is associated with cell elongation. Uterine arteries from pregnant ewes had greater protein contents than those from nonpregnant ewes (104 vs. 69 mg/g, respectively); there were corresponding increases in the absolute cellular contents of actin and myosin. While the fraction of light chain phosphorylated in response to phenylephrine was unaltered, the total amount of myosin light chain phosphorylated per gram wet weight increased significantly during pregnancy. In addition, the distribution of myosin heavy chain isoforms was also altered during pregnancy. The increased stress observed in the uterine artery during ovine pregnancy reflects, in part, increases in cellular contractile protein concentrations associated with hypertrophy.

Endothelin increases cytoplasmic calcium and myosin phosphorylation in human myometrium

Endothelin, a recently discovered sarafotoxin-like peptide secreted by endothelial cells, is a potent stimulator of vascular smooth muscle contraction. We found that the action of endothelin is not restricted to the vasculature; we demonstrated that endothelin causes an increase in the concentration of intracellular Ca++ and phosphorylation of the 20 kd light chain of myosin in human uterine smooth muscle cells in culture. In the absence of Ca++ in the buffer medium of myometrial cells, the effects of endothelin on intracellular Ca++ and myosin light chain phosphorylation are attenuated but not abolished. Endothelin also increases the frequency of contraction of human uterine smooth muscle (longitudinal and circular). The contractile effects of endothelin on myometrial strips are diminished in the presence of nifedipine. We conclude that (1) human myometrium is responsive to endothelin, (2) endothelin promotes contraction in myometrium by effecting an increase in intracellular Ca++ and thus an increase in myosin light chain phosphorylation, and (3) endothelin acts in myometrium by stimulating Ca++ influx as well as Ca++ release from intracellular stores.

Domain characterization of rabbit skeletal muscle myosin light chain kinase

Myosin light chain kinase can be divided into three distinct structural domains, an amino-terminal "tail," of unknown function, a central catalytic core and a carboxy-terminal calmodulin-binding regulatory region. We have used a combination of deletion mutagenesis and monoclonal antibody epitope mapping to define these domains more closely. A 2.95-kilobase cDNA has been isolated that includes the entire coding sequence of rabbit skeletal muscle myosin light chain kinase (607 amino acids). This cDNA, expressed in COS cells encoded a Ca2+/calmodulin-dependent myosin light chain kinase with a specific activity similar to that of the enzyme purified from rabbit skeletal muscle. Serial carboxy-terminal deletions of the regulatory and catalytic domains were constructed and expressed in COS cells. The truncated kinases had no detectable myosin light chain kinase activity. Monoclonal antibodies which inhibit the activity of the enzyme competitively with respect to myosin light chain were found to bind between residues 235-319 and 165-173, amino-terminal of the previously defined catalytic core. Thus, residues that are either involved in substrate binding or in close proximity to a light chain binding site may be located more amino-terminal than the previously defined catalytic core.

Alteration of cross-bridge kinetics by myosin light chain phosphorylation in rabbit skeletal muscle: implications for regulation of actin-myosin interaction

Myosin light chain phosphorylation in permeable skeletal muscle fibers increases isometric force and the rate of force production at submaximal levels of calcium activation; myosin light chain phosphorylation may underlie the increased rate and extent of force production associated with isometric twitch potentiation in intact fibers. To understand the mechanism by which myosin light chain phosphorylation manifests these effects, we have measured isometric force, isometric stiffness, rate of isometric force redevelopment after isotonic shortening, and isometric ATPase activity in permeabilized rabbit psoas muscle fibers. These measurements were made in the presence and absence of myosin light chain phosphorylation over a range of calcium concentrations that caused various levels of activation. The results were analyzed with a two-state cross-bridge cycle model as suggested by Brenner [Brenner, B. (1988) Proc. Natl. Acad. Sci. USA 85, 3265-3269]. The results indicate that myosin light chain phosphorylation exerts its effect on force generation and the isometric rate of force redevelopment in striated muscle through a single mechanism, namely, by increasing the rate constant describing the transition from non-force-generating cross-bridges to force-generating states (fapp). gapp, the reverse rate constant, is unaffected by phosphorylation as are the number of cycling cross-bridges. Since both calcium and myosin light chain phosphorylation increase fapp, the possibility is considered that modulation of fapp may represent a general mechanism for regulating force in actin-myosin systems.

Myosin light chain phosphorylation in human myometrial smooth muscle cells

Ca2+/calmodulin-dependent phosphorylation of the 20-kDa regulatory light chain of myosin is of signal importance in the initiation of contraction in a number of smooth muscle tissues. In this investigation, we evaluated the relationship between intracellular free Ca2+/concentration [( Ca2+]i) and the extent of myosin light chain phosphorylation in cultured human myometrial smooth muscle cells. Treatment of myometrial cells with ionomycin caused a concentration- and time-dependent increase in [Ca2+]i and phosphorylation of myosin light chain. Temporally, the increases in light chain phosphorylation and [Ca2+]i in response to ionomycin were similar. In myometrial cells treated with ionomycin (10(-5) M) for 10 s, [Ca2+]i increased from 138 to 800 nM; in these same cells, myosin light chain phosphorylation increased from 5% to a maximum value of 54%. Half-maximal phosphorylation of myosin light chain was attained at 300 nM [Ca2+]i. Treatment of myometrial smooth muscle cells with prostaglandin (PG) F2 alpha (10(-8) M) and PGE2 (10(-8) M) caused a proportionate increase in [Ca2+]i and myosin light chain phosphorylation. In addition, [Ca2+]i and myosin light chain phosphorylation increased in response to oxytocin and angiotensin II. These findings indicate that a number of uterotonic agents effect an increase in [Ca2+]i, which in turn causes phosphorylation of myosin light chain. Furthermore, the concentration of Ca2+ in the cytoplasm is a primary determinant for myosin light chain phosphorylation in human myometrial smooth muscle cells.

Phosphorylation of smooth muscle myosin heavy and light chains. Effects of phorbol dibutyrate and agonists

A number of different protein kinases phosphorylate purified heavy chains or the 20-kDa light chain of smooth muscle myosin. The physiological significance of these phosphorylation reactions has been examined in intact smooth muscle. Myosin heavy chain was slightly phosphorylated (0.08 mol of phosphate/mol) under control conditions in bovine tracheal tissue. Treatment with carbachol, isoproterenol, or phorbol 12,13-dibutyrate resulted in no significant change. In contrast, heavy chain was phosphorylated to 0.30 mol of phosphate/mol of heavy chain in tracheal smooth muscle cells in culture. This value increased significantly with ionomycin treatment. In control tissues, 9% of the light chain was monophosphorylated with 32P in the serine site phosphorylated by myosin light chain kinase. Carbachol (0.1 microM) alone resulted in contraction and 42% monophosphorylated light chain with 32P only in the serine site phosphorylated by myosin light chain kinase. Similarly, stimulation with histamine, 5-hydroxytryptamine, or KCl resulted in 32P incorporation into only the myosin light chain kinase serine site. Phorbol 12,13-dibutyrate (1 microM) alone resulted in 22% monophosphorylated light chain. However, only 25% of the 32P was in the myosin light chain kinase serine site, whereas 75% was in a serine site phosphorylated by protein kinase C. Phorbol 12,13-dibutyrate plus carbachol resulted in 27% monophosphorylated light chain; 75% of the 32P was in the myosin light chain kinase serine site, with the remainder in the protein kinase C serine site. These results indicate that phorbol esters act to increase phosphorylation of myosin light chain by protein kinase C. However, receptor-mediated stimulation or depolarization leading to tracheal smooth muscle contraction results in phosphorylation of myosin light chain by myosin light chain kinase alone.

Cytoplasmic Ca2+ is a primary determinant for myosin phosphorylation in smooth muscle cells

Initiation of smooth muscle contraction is associated with Ca2+/calmodulin activation of myosin light chain kinase which catalyzes the phosphorylation of the 20-kDa light chain of myosin. In tracheal smooth muscle cells in culture, the extent of myosin light chain phosphorylation is less than 10% at basal cytosolic free Ca2+ concentrations of 150 nM. Stimulation of these cells with serotonin, histamine, carbachol, or the Ca2+ ionophore, ionomycin, increases free cytosolic Ca2+ concentrations and the extent of myosin light chain phosphorylation. Light chain phosphorylation reaches a maximal value of 67% at Ca2+ concentrations below 1 microM. The relationship between the extent of light chain phosphorylation and cytosolic free Ca2+ concentration is apparently independent of the source of free intracellular Ca2+ or the agent used to stimulate the cells and is not altered by pre-exposure of the contractile apparatus to high concentrations of free Ca2+. Pretreatment of cells with 8-bromo-cyclic GMP or forskolin decreases free cytosolic Ca2+ concentrations and the extent of myosin light chain phosphorylation in response to histamine or ionomycin. Pretreatment with 8-bromo-cyclic GMP also decreases the maximal extent of light chain phosphorylation. These results indicate that cytosolic free Ca2+ concentration, per se, is a primary determinant for myosin light chain phosphorylation in tracheal smooth muscle cells.

Molecular characterization of rat skeletal muscle myosin light chain kinase

A 1.85-kilobase (kb) cDNA has been isolated that encodes the catalytic and calmodulin binding domains of rat skeletal muscle myosin light chain kinase. The cDNA hybridized to a 3.3-kb RNA present in fast- and slow-twitch skeletal muscles. The reported enzymatic activity (3-fold greater in fast- than slow-twitch skeletal muscles) reflects the relative abundance of this RNA in the two types of skeletal muscle. No hybridization of the cDNA was detected to RNA isolated from smooth or nonmuscle tissues. The clone cross hybridized to a 2.2-kb RNA present in cardiac tissue. Ribonuclease protection analysis of skeletal and cardiac muscle RNA revealed major differences in the two hybridizing RNAs. Thus rat skeletal muscle contains a single myosin light chain kinase isoform, which is distinct from the cardiac, smooth, and nonmuscle forms.

Sites phosphorylated in myosin light chain in contracting smooth muscle

Purified smooth muscle myosin light chain can be phosphorylated at multiple sites by myosin light chain kinase and protein kinase C. We have determined the sites phosphorylated on myosin light chain in intact bovine tracheal smooth muscle. Stimulation with 10 microM carbachol resulted in 66 +/- 5% monophosphorylated and 11 +/- 2% diphosphorylated myosin light chain after 1 min, and 47 +/- 4% monophosphorylated and 5 +/- 2% diphosphorylated myosin light chain after 30 min. Myosin heavy chain contained 0.06 +/- 0.01 mol of phosphate/mol of protein which did not change with carbachol. At both 1 and 30 min the monophosphorylated myosin light chain contained only phosphoserine whereas the diphosphorylated myosin light chain contained both phosphoserine and phosphothreonine. Two-dimensional peptide mapping of tryptic digests of monophosphorylated and diphosphorylated myosin light chain obtained from carbachol-stimulated tissue was similar to the peptide maps of purified light chain monophosphorylated and diphosphorylated, respectively, by myosin light chain kinase; these maps were distinct from the map obtained with tracheal light chain phosphorylated by protein kinase C. Phosphorylation of tracheal smooth muscle myosin light chain by myosin light chain kinase yields the tryptic phosphopeptide ATSNVFAMFDQSQIQEFK with S the phosphoserine in the monophosphorylated myosin light chain and TS the phosphotreonine and phosphoserine in the diphosphorylated myosin light chain. Thus, stimulation of tracheal smooth muscle with a high concentration of carbachol results in formation of both monophosphorylated and diphosphorylated myosin light chain although the amount of diphosphorylated light chain is substantially less than monophosphorylated light chain. In the intact muscle, myosin light chain is phosphorylated at sites corresponding to myosin light chain kinase phosphorylation.

Calcium dependence of myosin light chain phosphorylation in smooth muscle cells

Smooth muscle cells grown in culture may provide a model system for studying the Ca2+ dependence of myosin light chain phosphorylation. Tracheal smooth muscle cells in culture had 60% of the myosin content of tracheal tissue. Western analysis with appropriate antibodies demonstrated one 20-kDa light chain and the presence of a 150-kDa myosin light chain kinase in both tracheal smooth muscle tissue and cells. Moreover, tracheal cells contained 74% of the myosin light chain kinase activity measured in tissue. Similar types of analyses of nonmuscle cells showed a much lower myosin and myosin light chain kinase content. Carbachol (10 microM) or ionomycin (10 microM) stimulation of fura-2-containing cells resulted in a rapid increase in cytosolic free Ca2+ concentration and in the extent of myosin light chain phosphorylation. Maximal increases in Ca2+ concentrations were greater with ionomycin than with carbachol (4400 versus 492 nM). Light chain phosphorylation increased after the Ca2+ concentration exceeded 200 nM from control values of 165 nM. Half-maximal phosphorylation (33%) occurred at 260 nM Ca2+. There was a similar relationship between free cytosolic Ca2+ concentrations and the extent of myosin light chain phosphorylation in carbachol- and ionomycin-stimulated cells. This relationship had a Hill coefficient of 2.7. These observations indicate that small changes in Ca2+ concentrations stimulate myosin light chain phosphorylation and thus presumably contraction in smooth muscle cells.

Biochemical events associated with activation of smooth muscle contraction

Biochemical events associated with activation of smooth muscle contraction were studied in neurally stimulated bovine tracheal smooth muscle. A latency period of 500 ms preceded increases in isometric force and myosin light chain phosphorylation. However, stimulation resulted in the rapid hydrolysis of inositol phospholipids as demonstrated by increases in inositol phosphates by 500 ms. Inositol trisphosphate increased 2-fold with no significant change in inositol tetrakisphosphate. The apparent activation state of myosin light chain kinase was assessed indirectly through measurements of the fractional activation of a second calmodulin-dependent enzyme, cyclic nucleotide phosphodiesterase. The fractional activation of cyclic nucleotide phosphodiesterase increased after neural stimulation to a maximal extent by 500 ms and remained at this level for at least 4 s. The monophosphorylation of myosin light chain increased after 500 ms and reached a maximum value by 2 s. Diphosphorylation also occurred but to a much lesser extent. Fractional activation of cyclic nucleotide phosphodiesterase and myosin light chain phosphorylation both decreased after 10 min continuous stimulation, although the force response remained at a maximal level. These observations demonstrate that inositol trisphosphate formation and activation of cyclic nucleotide phosphodiesterase (and hence most likely myosin light chain kinase) by calmodulin precede myosin light chain phosphorylation and that these events are sufficiently rapid to mediate the contractile response of neurally stimulated tracheal smooth muscle.

Calcium control of smooth muscle contractility

Ca2+ is a primary second messenger that binds to an intracellular receptor protein, calmodulin. Increases in cytosolic Ca2+ concentration mediated by activation of cell surface receptors result in the formation of a Ca2+ calmodulin complex that regulates many Ca2+-dependent cellular processes. In smooth muscle, Ca2+/calmodulin activates myosin light chain kinase, which phosphorylates the regulatory light chain of myosin. This phosphorylation reaction increases the actin-activated MgATPase activity of myosin and is associated with increases in contractile properties, including force, stiffness, and maximal shortening velocity. These biochemical and biomechanical responses occur rapidly (seconds) in response to physiological stimulation involving neurotransmitter activation of smooth muscle cells. Thus, the Ca2+-dependent phosphorylation of the myosin light chain is a primary event in activation of smooth muscle contraction.

Atrial natriuretic peptide inhibits the agonist-induced increase in extent of myosin light chain phosphorylation in aortic smooth muscle

The effect of atrial natriuretic peptide (ANP) on angiotensin II- and histamine-induced contraction and muscle light chain phosphorylation was examined in strips of rabbit aorta smooth muscle. Preincubation of strips with 10(-7) M ANP prior to addition of either agonist inhibits both the increase in extent of myosin light chain phosphorylation and the contractile response to either 5 x 10(-8) M angiotensin II or 10(-5) M histamine without inhibiting the agonist-induced increase in the intracellular free Ca2+ concentration. Furthermore, in muscle strips precontracted with either angiotensin II or histamine, addition of ANP leads to a prompt relaxation and a prompt decrease in the extent of myosin light chain phosphorylation. These data argue that ANP uncouples the initial agonist-induced Ca2+ transient from the increase in extent of myosin light chain phosphorylation either by inhibiting the Ca2+-dependent activation of myosin light chain kinase or stimulating the activity of a phosphoprotein phosphatase capable of bringing about the rapid dephosphorylation of phosphorylated myosin light chains.

Properties of a monoclonal antibody directed to the calmodulin-binding domain of rabbit skeletal muscle myosin light chain kinase

A synthetic peptide representing the calmodulin-binding domain of rabbit skeletal muscle myosin light chain kinase (K-R-R-W-K-K-N-F-I-A-V-S-A-A-N-R-F-K-K-I-S-S-S-G-A-L) was used as an antigen to produce a monoclonal antibody. The antibody (designated MAb RSkCBP1, of the IgM class) reacted with similar affinity (KD approximately 20 nM) by competitive enzyme-linked immunoassay (ELISA) with the antigen peptide and intact rabbit skeletal muscle myosin light chain kinase. MAb RSkCBP1 inhibited rabbit skeletal muscle myosin light chain kinase activity competitively with respect to calmodulin (Ki = 20 nM). The antibody also inhibited myosin light chain kinase activity in extracts of skeletal muscle from several mammalian species (rabbit, sheep, and bovine) and an avian species (chicken). The concentration of MAb RSKCBP1 required for 50% inhibition of enzyme activity was similar for the mammalian species (80 nM) but was significantly higher for the avian species (1.2 microM). A competitive ELISA protocol was used to analyze weak cross-reactivity to other calmodulin-binding peptides and proteins. This assay demonstrated no cross-reactivity with the venom peptides melittin or mastoparan; smooth muscle myosin light chain kinases from hog carotid, bovine trachea, or chicken gizzard; bovine brain calmodulin-dependent calcineurin; or rabbit skeletal muscle troponin I. These data support the contention that the synthetic peptide used as the antigen represents the calmodulin-binding domain of rabbit skeletal muscle myosin light chain kinase and that the calmodulin-binding domains of different calmodulin-regulated proteins may have distinct primary and/or higher order structures.