The regulation of myosin phosphatase in pregnant human myometrium

Abstract

Myometrial smooth muscle contractility is regulated predominantly through the reversible phosphorylation of MYLs (myosin light chains), catalysed by MYLK (MYL kinase) and MYLP (MYL phosphatase) activities. MYLK is activated by Ca2+-calmodulin, and most uterotonic agonists operate through myometrial receptors that increase [Ca2+]i (intracellular Ca2+ concentration). Moreover, there is substantial evidence for Ca2+-independent inhibition of MYLP in smooth muscle, leading to generation of increased MYL phosphorylation and force for a given [Ca2+]i, a phenomenon known as 'Ca2+-sensitization'. ROCK (Rho-associated kinase)-mediated phosphorylation and inhibition of MYLP has been proposed as a mechanism for Ca2+-sensitization in smooth muscle. However, it is unclear to date whether the mechanisms that sensitize the contractile machinery to Ca2+ are important in the myometrium, as they appear to be in vascular and respiratory smooth muscle. In the present paper, we discuss the signalling pathways regulating MYLP activity and the involvement of ROCK in myometrial contractility, and present recent data from our laboratory which support a role for Ca2+-sensitization in human myometrium.

Inhibition of Uterine Contractility by Thalidomide Analogs via Phosphodiesterase-4 Inhibition and Calcium Entry Blockade

“…Phosphorylation of MLCPT1 is achieved by several protein kinases, including Rho-associated kinase (ROCK) activated by the small G protein RhoA [43,45,46], which, in turn, is positively regulated by Rho-guanine nucleotide exchange factors (RhoGEFs) but negatively regulated by Rho-guanine nucleotide dissociation inhibitors (RhoGDIs) [47,48]. Several experiments conducted in vascular smooth muscle have demonstrated that ligand binding to metabotropic receptors (often G protein-coupled receptors) are capable of producing greater increases in force than KCl for a given [Ca 2+ ] i ; indeed, contractions induced by KCl are completely driven by membrane depolarization and Ca 2+ entry through voltage-operated Ca 2+ channels [8,26,27,28]. Therefore, although myometrial phasic contractions may be stronger than tonic contractions, they seem to be more sensitive to fine control; in fact, substantial differences have been found between uterine phasic and vascular tonic contractions due to the regulation of ROCK-induced phosphorylation of MLCP and MLC [8,43,46].…”

“…Several experiments conducted in vascular smooth muscle have demonstrated that ligand binding to metabotropic receptors (often G protein-coupled receptors) are capable of producing greater increases in force than KCl for a given [Ca 2+ ] i ; indeed, contractions induced by KCl are completely driven by membrane depolarization and Ca 2+ entry through voltage-operated Ca 2+ channels [8,26,27,28]. Therefore, although myometrial phasic contractions may be stronger than tonic contractions, they seem to be more sensitive to fine control; in fact, substantial differences have been found between uterine phasic and vascular tonic contractions due to the regulation of ROCK-induced phosphorylation of MLCP and MLC [8,43,46]. Furthermore, several crucial regulatory proteins involved in contraction are susceptible to phosphorylation by PKAs activated by elevated levels of cAMP; for example, ROCK1 and ROCK2 possess 45 and 43 sites, respectively, which are capable of being phosphorylated, some of which might inhibit their activity [47].…”

“…The uterus also responds to many agonists; thus, changes in the expression of receptors and the activation of signaling pathways in myocytes may be involved in the regulation of uterine contractility [1,7]. The uterine quiescence maintained during pregnancy and the increased activity during the spontaneous onset of parturition may be associated with functional changes in myometrial receptors [8]. Uterine infections induce the production and release of proinflammatory cytokines, such as tumor necrosis factor (TNF)-α, interleukin (IL)-6 and IL-1β, which, in turn, activate phospholipid metabolism, arachidonic acid release and the signal transduction system involving nuclear factor (NF)-κB to increase the expression of cyclooxygenase (COX)-2, which provokes the production of prostaglandin (PG)-E2 and PG-F2α, both of which stimulate contractility by activating the phospholipase-C/Ca 2+ (PLC) pathway.…”

“…Uterine infections induce the production and release of proinflammatory cytokines, such as tumor necrosis factor (TNF)-α, interleukin (IL)-6 and IL-1β, which, in turn, activate phospholipid metabolism, arachidonic acid release and the signal transduction system involving nuclear factor (NF)-κB to increase the expression of cyclooxygenase (COX)-2, which provokes the production of prostaglandin (PG)-E2 and PG-F2α, both of which stimulate contractility by activating the phospholipase-C/Ca 2+ (PLC) pathway. Cytokines and eicosanoids interact to accelerate normal parturition, resulting in preterm labor [1,3,4,5,6,8,9]. However, the second messenger adenosine 3′,5′-cyclic monophosphate (cAMP) inhibits cytokine production and promotes the relaxation of the myometrium by activating cAMP-dependent protein kinase (PKA), which phosphorylates the myosin light–chain (MLC) kinase (MLCK), decreasing the affinity of this enzyme for the Ca 2+ –calmodulin complex.…”

Inverted formin 2 regulates intracellular trafficking, placentation, and pregnancy outcome

“…Several formin family genes have previously been associated with pregnancy and reproductive phenotypes, including preterm birth ( Cruickshank et al, 2013 ; Elovitz et al, 2014 ; Montenegro et al, 2009 ). Furthermore, there is evidence of increased expression of a formin activator, RhoA-GTP, during pregnancy ( Hudson and Bernal, 2012 ).…”