Developmental and protein kinase-dependent regulation of ovarian connexin mRNA and oocyte maturational competence in Atlantic croaker

The acquisition of oocyte maturational competence (OMC) in ovarian follicle s of Atlantic croaker is associated with increased gap junction (GJ) contac ts and increased levels of ovarian connexin (Cx) 32.2 mRNA. However, the de velopmental control of ovarian Cx gene expression and the mechanisms of OMC acquisition are unknown. Ovarian Cx32.2 and Cx32.7 mRNA levels were determ ined in fish with gonadosomatic indices (GSI; gonad weight-to-body weight r atio) ranging from 0.1 to 13%. The mRNA level for both Cx increased from a low level in previtellogenic ovaries (GSI, <1%) to a peak level during the midstage of ovarian growth (GSI, 6-7%). Levels of Cx32.2 mRNA, but not Cx32 .7 mRNA, declined markedly during late ovarian vitellogenic growth (GSI, 7- 13%), and increased again upon stimulation of OMC by human chorionic gonado tropin (hCG). These changes in ovarian Cx32.2 mRNA seem to parallel previou sly reported changes in the incidence of oocyte-granulosa cell GJ during fo llicular growth and early maturation. In vitro treatment with hCG and prote in kinase A (PKA) activators (dbcAMP and forskolin) induced ovarian Cx32.2 mRNA levels and OMC. The effects of hCG were blocked by PKA inhibitors (H89 , H7). Protein kinase C (PKC) inhibitors (GF 109207X) had little effect on hCG-induced Cx32.2 mRNA or OMC, whereas PKC activators (PMA) blocked both e vents. There was no association between changes in Cx32.7 mRNA levels and O MC status in these experiments. In conclusion, changes in Cx32.2 gene expre ssion seem to be involved in the regulation of oocyte-granulosa cell GJ dur ing growth and differentiation of the croaker ovarian follicle. Also, the s timulation of OMC and Cx32.2 mRNA levels by hCG is mediated by PKA-dependen t pathways and antagonized by PKC-dependent mechanisms.