Nonmuscle myosin is regulated during smooth muscle contraction

Abstract

The participation of nonmuscle myosin in force maintenance is controversial. Furthermore, its regulation is difficult to examine in a cellular context, as the light chains of smooth muscle and nonmuscle myosin comigrate under native and denaturing electrophoresis techniques. Therefore, the regulatory light chains of smooth muscle myosin (SM-RLC) and nonmuscle myosin (NM-RLC) were purified, and these proteins were resolved by isoelectric focusing. Using this method, intact mouse aortic smooth muscle homogenates demonstrated four distinct RLC isoelectric variants. These spots were identified as phosphorylated NM-RLC (most acidic), nonphosphorylated NM-RLC, phosphorylated SM-RLC, and nonphosphorylated SM-RLC (most basic). During smooth muscle activation, NM-RLC phosphorylation increased. During depolarization, the increase in NM-RLC phosphorylation was unaffected by inhibition of either Rho kinase or PKC. However, inhibition of Rho kinase blocked the angiotensin II-induced increase in NM-RLC phosphorylation. Additionally, force for angiotensin II stimulation of aortic smooth muscle from heterozygous nonmuscle myosin IIB knockout mice was significantly less than that of wild-type littermates, suggesting that, in smooth muscle, activation of nonmuscle myosin is important for force maintenance. The data also demonstrate that, in smooth muscle, the activation of nonmuscle myosin is regulated by Ca(2+)-calmodulin-activated myosin light chain kinase during depolarization and a Rho kinase-dependent pathway during agonist stimulation.

Myosin Light Chain Kinase Is Necessary for Tonic Airway Smooth Muscle Contraction

“…Due to the small amount of force for residual contraction and atypical waveform of contraction, we could not determine whether the small contraction was more sensitive to blebbistatin, thereby indicating a primary contribution of nonmuscle myosin II. However, we expect that MLCK also catalyzes non-muscle myosin phosphorylation if it contributes to force development in tonic contraction (42). Blebbistatin is an effective inhibitor of smooth muscle actomyosin and smooth muscle contraction from mammals and chicken arteries, whereas it is not as effective for inhibiting gizzard smooth muscle contractions, consistent with the original report that blebbistatin was not effective in inhibiting avian gizzard actomyosin ATPase activity (34).…”

The Small GTPase RhoA Regulates the Contraction of Smooth Muscle Tissues by Catalyzing the Assembly of Cytoskeletal Signaling Complexes at Membrane Adhesion Sites

“…If so, our results would suggest that the RhoA-mediated phosphorylation of non-muscle myosin II light chain would be critical for excitation-contraction coupling and smooth muscle contraction. Non-muscle myosin II is expressed in cultured human airway smooth muscle cells (53) and has also been documented in vascular smooth muscle tissues (54).…”

Myosin II isoform switching mediates invasiveness after TGF-β–induced epithelial–mesenchymal transition

“…Of the three nonmuscle myosin II isoforms, myosin IIB has the highest duty ratio (34) and the highest affinity for ADP (35), suggesting that it may be tuned for prolonged force production and minimal energy consumption. Previous work has shown that myosin IIB contributes to force maintenance in aortic smooth muscle (36). The myoepithelial layer of mammary gland tissue is responsible for the contractile events during milk secretion (37) and has been suggested to be critical for proper branching architecture (38), an essential component of mammary gland development.…”