Phasic contractions of isolated human myometrium are associated with Rho-kinase (ROCK)-dependent phosphorylation of myosin phosphatase-targeting subunit (MYPT1)

Force generation in smooth muscle is driven by phosphorylation of myosin light chains (MYL), which is regulated by the equilibrium between the activities of myosin light chain kinase (MYLK) and myosin phosphatase (MYLP). MYLK is activated by Ca(2+)-calmodulin whereas MYLP is inhibited by phosphorylation of its myosin-binding subunit (MYPT1) by Ca(2+)-independent mechanisms, leading to generation of increased MYL phosphorylation and force for a given intracellular Ca(2+) concentration, a phenomenon known as 'calcium-sensitization'. The regulation of MYPT1 phosphorylation in human myometrium, which shows increasing phasic contractility at the onset of labour, has yet to be fully investigated. Here, we explore phosphorylation of MYPT1 at Thr696 and Thr853, alongside phosphorylation of MYL, in fresh human myometrial tissue and cultured myometrial cells. We report that pMYPT1 (Thr853) levels are dependent on the activity of Rho-associated kinase (ROCK), determined using the ROCK inhibitor g-H-1152 and siRNA-mediated knockdown of ROCK1/2, and are highly correlated to ppMYL (Thr18/Ser19) levels. Pharmacological inhibition of ROCK was associated with a decrease in oxytocin (OXT)-stimulated contractility of myometrial strips in vitro. Moreover, we have measured pMYPT1 and pMYL levels between and during spontaneous and OXT-stimulated phasic contractions by rapidly freezing contracting muscle, and demonstrate for the first time functional coupling between increases in pMYPT1 (Thr853), ppMYL (Thr18/Ser19) and phasic contractility that is ROCK-dependent. The combined approach of measuring contractility and phosphorylation has demonstrated that the phosphorylation of MYPT1 (Thr853) changes dynamically with each contraction and has elucidated a defined role for ROCK in regulating myometrial contractility through MYLP, providing new insights into uterine physiology which will stimulate further research into treatments for preterm labour.