WOOD, AW*; MATSUMOTO, JA; VAN DER KRAAK, G; University of Guelph; University of Guelph; University of Guelph: Identification of an oocyte cysteine protease inhibitor cDNA in rainbow trout: differential expression during follicular development and atresia
Oocyte atresia in mammals is characterized by widespread apoptosis in the granulosa cells of the follicle. In contrast to the mammalian model, our recent studies show little evidence of granulosa cell apoptosis during oocyte atresia in teleost fish. As an alternative, we have hypothesized that yolk-protein hydrolysis plays a leading role in mediating oocyte regression. Yolk-protein hydrolysis is mediated by cathepsins, a diverse family of cysteine and aspartic proteases that are sequestered in yolk platelets during vitellogenesis. We currently have little understanding of mechanisms by which these proteases are regulated. Using reverse transcription-PCR (RT-PCR), we have identified a cDNA from rainbow trout ovary that encodes a novel, oocyte-specific cysteine protease inhibitor (OCPIshort). The predicted protein is dominated by a cysteine-rich thyroglobulin type-1 motif, a domain characterized by potent inhibitory activity against papain-like proteases and cathepsins. Using RT-PCR, we have shown that rainbow trout OCPIshort is specifically expressed in early and mid-vitellogenic oocytes, is absent from previtellogenic follicles, follicular epithelium and all somatic tissues, and is significantly down-regulated in atretic oocytes. The expression profile of this novel transcript suggests its involvement in regulating the proteolytic activity of oocyte hydrolases directly involved in vitellogenin processing, and yolk storage protein hydrolysis during follicular atresia. We thus have evidence from an oviparous vertebrate for a novel mechanism of atresia that differs markedly from the well-described mammalian model involving granulosa cell apoptosis.
