[摘要]：Mammalian oocytes are arrested in meiotic prophase by an inhibitory signal from the surrounding somatic cells in the ovarian follicle. In response to luteinizing hormone (LH), which binds to receptors on the somatic cells, the oocyte proceeds to second metaphase, where it can be fertilized. Here we investigate how the somatic cells regulate the prophase-to-metaphase transition in the oocyte, and show that the inhibitory signal from the somatic cells is cGMP. Using FRET-based cyclic nucleotide sensors in follicle-enclosed mouse oocytes, we find that cGMP passes through gap junctions into the oocyte, where it inhibits the hydrolysis of cAMP by the phosphodiesterase PDE3A. This inhibition maintains a high concentration of cAMP and thus blocks meiotic progression. LH reverses the inhibitory signal by lowering cGMP levels in the somatic cells (from similar to 2 mu M to similar to 80 nM at 1 hour after LH stimulation) and by closing gap junctions between the somatic cells. The resulting decrease in oocyte cGMP ( from similar to 1 mu M to similar to 40 nM) relieves the inhibition of PDE3A, increasing its activity by similar to 5-fold. This causes a decrease in oocyte cAMP ( from similar to 700 nM to similar to 140 nM), leading to the resumption of meiosis.
