Myosin Light Chain Kinase and Myosin Phosphorylation Effect Frequency-Dependent Potentiation of Skeletal Muscle Contraction

• Published in: Proceedings of the National Academy of Sciences - PNAS 2005-11, Vol.102 (48), p.17519-17524
• Main Authors: Zhi, Gang, Ryder, Jeffrey W., Huang, Jian, Peiguo Ding, Chen, Yue, Zhao, Yingming, Kamm, Kristine E., Stull, James T., Korn, Edward D.
• Format: Article
• Language: English
• Subjects: Animals; Biochemistry; Biological Sciences; Blotting, Southern; Blotting, Western; DNA; DNA Primers; Electric Stimulation; Gene Expression; Genotype; Knockout mice; Messenger RNA; Mice; Mice, Knockout; Mice, Mutant Strains; Muscle Contraction - physiology; Muscle, Skeletal - metabolism; Muscle, Skeletal - physiology; Muscles; Muscular system; Myocardium; Myosin-Light-Chain Kinase - genetics; Myosin-Light-Chain Kinase - metabolism; Myosins - metabolism; Phosphorylation; Polymerase Chain Reaction; Signal transduction; Signal Transduction - physiology; Skeletal muscle; Smooth muscle; Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; Tetanus; Urinary bladder
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0506846102

Repetitive stimulation potentiates contractile tension of fast-twitch skeletal muscle. We examined the role of myosin regulatory light chain (RLC) phosphorylation in this physiological response by ablating$Ca^{2+}/calmodulin-dependent$skeletal muscle myosin light chain kinase (MLCK) gene expression. Western blot and quantitative-PCR showed that MLCK is expressed predominantly in fast-twitch skeletal muscle fibers with insignificant amounts in heart and smooth muscle. In contrast smooth muscle MLCK had a more ubiquitous tissue distribution, with the greatest expression observed in smooth muscle tissue. Ablation of the MYLK2 gene in mice resulted in loss of skeletal muscle MLCK expression, with no change in smooth muscle MLCK expression. In isolated fast-twitch skeletal muscles from these knockout mice, there was no significant increase in RLC phosphorylation in response to repetitive electrical stimulation. Furthermore, isometric twitch-tension potentiation after a brief tetanus (posttetanic twitch potentiation) or low-frequency twitch potentiation (staircase) was attenuated relative to responses in muscles from wild-type mice. Interestingly, the site of phosphorylation of the small amount of monophospho-rylated RLC in the knockout mice was the same site phosphorylated by MLCK, indicating a potential alternative signaling pathway affecting contractile potentiation. Loss of skeletal muscle MLCK expression had no effect on cardiac RLC phosphorylation. These results identify myosin light chain phosphorylation by the dedicated skeletal muscle$Ca^{2+}/calmodulin-dependent$MLCK as a primary biochemical mechanism for tension potentiation due to repetitive stimulation in fast-twitch skeletal muscle.