335. X-LINKED INHIBITOR OF APOPTOSIS PROTEIN (XIAP) EXPRESSION PATTERNS IN THE SHEEP OVARY

Abstract

At reproductive age, the ovary undergoes continual cyclicity of follicles due to multiple positive and negative signals that promote follicle growth and development, selection for ovulation, or atresia. The majority of follicles undergo atresia, a degenerative process involving granulosa cell apoptosis. This process is executed by caspases, which are cysteine proteases. Caspases are potently inhibited by XIAP providing a potential mechanism to control follicular atresia. The study objective was to test the hypothesis that XIAP will show elevated expression in healthy antral follicles compared to atretic antral follicles and will show an inverse relationship with active caspase-3 immunoreactivity in the same antral follicle during the sheep estrous cycle. Reproductively mature Romney ewes (n = 9) had estrous cycles synchronized with a prostaglandin F2α analogue, then tissue was collected on days 14, 15 and 16 of the subsequent natural cycle. Analysis involved determining the presence and localization of XIAP using in situ hybridization histochemistry and immunohistochemistry then comparing XIAP expression patterns with distribution of active caspase-3 protein. XIAP mRNA was not detected in primordial, primary and secondary follicles. In contrast, XIAP protein was present from the primary stage onwards. Antral follicles showed positive XIAP mRNA and protein expression in both granulosa and thecal cell layers and antral follicles on the same tissue section showed variable expression. All day 14 antral follicles were positive for XIAP mRNA expression irrespective of the level of active caspase-3 immunoreactivity, whereas an inverse relationship between active caspase-3 and XIAP was apparent in the majority of day 15 and 16 antral follicles. XIAP protein was widely expressed in active caspase-3 negative antral follicles and indicated a negative correlation with the onset of active caspase-3 expression in the majority of follicles. These results indicate that XIAP may regulate follicular atresia and act as an indicator of follicular health in the sheep ovary.