In mammals, oocytes are kept from completing meiosis by a signal that comes from the surrounding somatic cells until oocyte cAMP levels drop in response to luteinising hormone (LH). What is the nature of the somatic cell signal?The answer, claim Nikolaev, Jaffe and colleagues on , is cGMP. The authors measure cyclic nucleotide concentrations in intact follicle-enclosed mouse oocytes using FRET-based fluorescent indicators and observe that, in meiotically arrested oocytes, cGMP passes through gap junctions into the cells. There, it inhibits cAMP hydrolysis by the phosphodiesterase PDE3A, thus maintaining high cAMP levels and blocking meiotic progression. LH acts by lowering somatic cell cGMP levels and by closing the gap junctions of the somatic cells, which results in a decrease in oocyte cGMP levels. This, in turn, allows PDE3A to hydrolyse oocyte cAMP, leading to meiotic progression. Thus, the authors conclude, somatic cell cGMP maintains meiotic arrest in oocytes, and LH causes meiotic progression by interfering with this somatic cGMP signal.
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