Hormonal regulation and cellular distribution of connexin 32.2. and connexin 32.7 RNAs in the ovary of Atlantic croaker

The in vitro effects of human chorionic gonadotropin (hCG) on ovarian conne xin (Cx) 32.2 and 32.7 RNA levels and ovarian follicle maturation were asse ssed, and the cellular distribution of Cx transcripts in the ovary was dete rmined. hCG caused a concentration-dependent induction of Cx32.2 RNA, which peaked coincidentally with the appearance of morphological indices of oocy te maturational competence (OMC). Cx32.2 RNA levels declined thereafter in all treatment groups, although this decline was not accompanied by the onse t of germinal vesicle breakdown (GVBD) at the lowest hCG concentration used . The levels of Cx32.7 RNA initially declined and subsequently increased to preincubation values after hCG treatment, but these changes were not depen dent on hCG concentration. In a separate experiment, the decline in Cx32.7 RNA occurred in the presence or absence of hCG and was prevented by low (ph ysiological) concentrations of estradiol-17 beta (E2) or by protein kinase C (PKC) inhibitor, but was enhanced in the presence of high E2 concentratio ns or of PKC activator. These changes in Cx32.7 RNA abundance were not asso ciated with any indices of oocyte maturation. In situ hybridization of tiss ue sections showed the presence of Cx32.2 and Cx32.7 RNA in somatic cells o f the ovarian follicle but not in oocytes. Cx32.2 RNA seemed to be present in granulosa and thecal cells, but the assay resolution was insufficient to reliably determine the distribution of Cx32.7 transcript by somatic cell t ype. In view of earlier findings that Cx32.2-based (but not Cx32.7-based) c onnexons can form functional homotypic channels, these results indicate tha t Cx32.2 gene expression in granulosa cells is sufficient for the formation of homologous gap junctions (GJ). Northern blot of RNA extracts from ovula ted eggs, which are free of follicle cells, showed the presence of relative ly low levels of both Cx RNAs. Thus, it is possible that Cx32.2 is present in oocytes and that it participates in heterologous (homotypic) GJ formatio n between the oocyte and the granulosa cells. In conclusion, Cx32.2 RNA lev els in somatic cells of the ovarian follicle correlated positively with mor phological indices of OMC acquisition, but subsequently declined during GVB D. These changes in Cx32.2 RNA may function in the regulation of GJ contact s during follicular maturation. (C) 2000 Academic Press.