Expression of the Inhibitor of T-Cell Apoptosis (ita) Gene in Hen Ovarian Follicles during Development1

Abstract

It is now well established that within the hen ovary, preovulatory follicles rarely become atretic and that granulosa cells from preovulatory follicles are relatively resistant to undergoing apoptosis in vitro. By comparison, prehierarchal (< or = 8-mm diameter) follicles are highly susceptible to becoming atretic in vivo, and approximately 70% of granulosa cells collected from 3- to 8-mm-diameter follicles rapidly undergo apoptosis when incubated for as little as 6 h in vitro in defined medium. The present studies were conducted to characterize expression of an inhibitor of apoptosis (iap) gene, inhibitor of T-cell apoptosis (ita), within hen follicle tissues at various stages of follicle development. The ita gene product has recently been shown to share homology within both the baculovirus repeat sequences of the N-terminus and the zinc ring-finger motif from the C-terminus and was originally determined to be expressed in chicken cells of T-lymphocyte lineage. In the present studies, highest levels of ita mRNA within the granulosa cell layer were found in preovulatory (atresia-resistant) follicles, with significantly lower levels detected in prehierarchal follicles. After 24 h of primary culture, ita mRNA levels increased in granulosa cells from preovulatory follicles by 3.2-fold as compared to those in freshly collected cells and were elevated by 8.9-fold in those granulosa cells from 6- to 8-mm follicles that successfully formed a primary culture monolayer. Moreover, ita mRNA levels were significantly increased in 6- to 8-mm-follicle granulosa cells after only 2 h of suspension culture, and this increase could be prevented by actinomycin D. This spontaneous increase in ita expression may serve to protect from cell death the relatively small population of prehierarchal follicle granulosa cells that survive in vitro. It is concluded from these data, taken together, that patterns of ita mRNA expression during follicle development are consistent with a potential role for this gene in protecting granulosa cells from apoptosis and thus maintaining follicle viability.