Estrogen receptor alpha isoform ERdelta7 in myometrium modulates uterine quiescence during pregnancy

Background: Circulating estrogen (E2) levels are high throughout pregnancy and increase towards term, however its local tissue specific actions vary across gestation. For example, myometrial E2 regulated uterotonic action is disabled until term, whereas it's proliferative function is maintained in the breast. We have identified gestationally regulated splicing events, mediated by hnRNPG and modulated by E2 that generate alternatively spliced estrogen receptor alpha (ER alpha) variants (ER Delta 7 and ER alpha 46) in the myometrium. These variants allow for differential, gestationally regulated, modulation of the uterotonic action of E2. Methods: Human myometrium isolated from preterm and term non-laboring and laboring pregnant women were analyzed for ER alpha isoforms and splice factor levels. Lentiviral mediated shRNA knockdown of hnRNPG and overexpression of ER Delta 7 were performed in human myometrial (hTERT-HM) cells. Functional 3D collagen contraction assays were executed. Findings: ER Delta 7 acts as a dominant negative repressor of the uterotonic action of ER alpha 66 and ER alpha 46 isoforms through the regulation of the myometrial gap junction protein GJA1. Elimination of hnRNPG inhibits the generation of ER Delta 7 while overexpression of ER Delta 7 inhibited GJA1 expression. Moreover in vivo human myometrial hnRNPG levels decline at term in an E2 dependent manner resulting in a withdrawal of ER Delta 7 levels and its tocolytic action at term. Interpretation: Our findings implicate the unique role of ER.7 as a modulator of myometrial quiescence and define the mechanism of ER.7 generation, through hormonally regulated splicing events. Fund: This study was supported by NIH OPRU U01 supplement (HD047905), University of Pittsburgh and Wayne State University Perinatal Research Initiative (USA). (c) 2018 The Authors. Published by Elsevier B.V.