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We are pleased with the keen interest in our recent work published in Gastroenterology on signaling to smooth muscle myosin regulatory light chain (RLC) phosphorylation in myosin phosphatase target subunit knockout mice.1He W.Q. et al.Gastroenterology. 2013; 144: 1456-1465Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar Smooth muscle contractile responses converge on the regulation of the contractile machinery, involving phosphorylation of the myosin RLC subunit by the Ca2+-dependent myosin light chain kinase (MLCK).2Kamm K.E. et al.J Biol Chem. 2001; 276: 4527-4530Abstract Full Text Full Text PDF PubMed Scopus (468) Google Scholar, 3Somlyo A.P. et al.Physiol Rev. 2003; 83: 1325-1358Crossref PubMed Scopus (1646) Google Scholar, 4Grassie M.E. et al.Arch Biochem Biophys. 2011; 510: 147-159Crossref PubMed Scopus (168) Google Scholar This phosphorylation allows the myosin motor head to bind to actin filaments to initiate cell shortening and force development. The key element in smooth muscle contractile responses, including tonic and phasic gastrointestinal smooth muscles, is thus related to the extent of RLC phosphorylation, which depends on the ratio of MLCK to myosin light chain phosphatase (MLCP) activity. Both MLCK and MLCP activities are regulated in a dynamic manner with integrated signaling modules impinging on both MLCK and MLCP. We had previously shown in different smooth muscles, including intestinal smooth muscles, that knockout of MLCK, resulted in contractile failure and death in vivo and markedly attenuated contractile responses of isolated smooth muscle segments in vitro.5He W.Q. et al.Gastroenterology. 2008; 135: 610-620Abstract Full Text Full Text PDF PubMed Scopus (130) Google Scholar, 6Zhang W.C. et al.J Biol Chem. 2010; 285: 5522-5531Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar, 7He W.Q. et al.Am J Physiol Heart Circ Physiol. 2011; 301: H584-H591Crossref PubMed Scopus (50) Google Scholar, 8Gao N. et al.J Biol Chem. 2013; 288: 7596-7605Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar Because MLCP is composed of a protein phosphatase type 1 catalytic subunit bound stoichiometrically to the myosin phosphatase target subunit (MYPT1) and this binding to myosin is thought to be crucial for RLC dephosphorylation,3Somlyo A.P. et al.Physiol Rev. 2003; 83: 1325-1358Crossref PubMed Scopus (1646) Google Scholar, 4Grassie M.E. et al.Arch Biochem Biophys. 2011; 510: 147-159Crossref PubMed Scopus (168) Google Scholar we anticipated that the knockout of MYPT1 would lead to marked phenotypic responses, perhaps similar in principle with the MLCK knockout with contractile failure and death in vivo. Thus, we were most surprised to observe a mild phenotype in the MYPT1 knockout mice with relatively modest changes in contractile responses of isolated tissues.1He W.Q. et al.Gastroenterology. 2013; 144: 1456-1465Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar Rattan proposes that some alterations may be revealed with additional studies involving a larger number of animals or a more extensive analysis of the entire gut from the esophagus to the anorectum. Additional signaling studies are also proposed in relation to Ca2+-sensitization responses involving region specific responses, including pathophysiology of certain motility disorders with dysfunctional inhibitory and excitatory neurotransmissions. We certainly believe these types of studies are needed to understand the specific roles of Ca2+ sensitization under different physiologic and pathophysiologic conditions in different regions of the gut. This is why we are continuing these types of investigations beyond our initial report on MYPT1 knockout in smooth muscles of mice. We appreciate the enthusiasm Rattan exhibits for such studies. Smooth Muscle–Specific Myosin Phosphatase Target Subunit 1 (MYPT1): An Important Piece of the PuzzleGastroenterologyVol. 145Issue 6PreviewWe have read with a great interest a recent article by He et al in the June issue of Gastroenterology.1 The studies provide strong evidence in favor of the concept that smooth muscle–specific myosin phosphatase target subunit 1 (MYPT1) of myosin light chain phosphatase (MLCP) plays a critical role in the agonist-induced contraction/relaxation of the smooth muscle. This was shown in their studies using animals with knocked out MYPT1–/–. The investigators employed the Cre-loxP system in which they used the promoter region and exon 1 of Mypt1 flanked by 2 loxP sites to establish Mypt1-floxed mice. Full-Text PDF