Deletion of SM-B, the high ATPase isoform of myosin, upregulates the PKC-mediated signal transduction pathway in murine urinary bladder smooth muscle

Division of Urology and Department of Pathobiology, University of Pennsylvania, Philadelphia, Pennsylvania; and Department of Physiology and Cell Biology, College of Medicine and Public Health, Ohio State University, Columbus, Ohio Submitted 28 March 2008 ; accepted in final form 1 December 2008 Detrusor smooth muscle (DSM) hypertrophy induced by partial bladder outlet obstruction (PBOO) is associated with changes in the NH 2 -terminal myosin heavy chain isoform from predominantly SM-B to SM-A, alteration in the Ca 2+ sensitization pathway, and the contractile characteristics from phasic to tonic in rabbits. We utilized the SM-B knockout (KO) mouse to determine whether a shift from SM-B to SM-A without PBOO is associated with changes in the signal transduction pathway mediated via PKC and CPI-17, which keeps the myosin phosphorylation (MLC 20 ) level high by inhibiting the myosin phosphatase. DSM strips from SM-B KO mice generated more force in response to electrical field stimulation, KCl, carbachol, and phorbol 12,13-dibutyrate than that of age-matched wild-type mice. There was no difference in the ED 50 for carbachol but the maximum response was greater for the SM-B KO mice. DSM from SM-B KO mice revealed increased mass and hypertrophy. The KO mice also showed an overexpression of PKC- , increased levels of phospho-CPI-17, and an elevated level of IP 3 and DAG upon stimulation with carbachol. Two-dimensional gel electrophoresis revealed an increased level of MLC 20 phosphorylation in response to carbachol. Together, these changes may be responsible for the higher level of force generation and maintenance by the DSM from the SM-B KO bladders. In conclusion, our data show that ablation of SM-B is associated with alteration of PKC-mediated signal transduction and CPI-17-mediated Ca 2+ sensitization pathway that regulate smooth muscle contraction. Interestingly, similar changes are also present in PBOO-induced DSM compensatory response in the rabbit model in which SM-B is downregulated. smooth muscle contraction; alternative splicing; CPI-17; light chain phosphorylation Address for reprint requests and other correspondence: S. Chacko, Dept. of Pathobiology and Division of Urology, 500 South Ridgeway Ave., Univ. of Pennsylvania, Glenolden, PA 19036-2307 (e-mail: chackosk{at}mail.med.upenn.edu )