EXPRESSION OF THE INHIBITOR OF T-CELL APOPTOSIS (ITA) GENE IN HEN OVARIAN FOLLICLES DURING DEVELOPMENT

It is now well established that within the hen ovary, preovulatory fol licles rarely become atretic and that granulosa cells from preovulator y follicles are relatively resistant to undergoing apoptosis in vitro. By comparison, prehierarchal (less than or equal to 8-mm diameter) fo llicles are highly susceptible to becoming atretic in vivo, and approx imately 70% of granulosa cells collected from 3- to 8-mm-diameter foll icles rapidly undergo apoptosis when incubated for as little as 6 h in vitro in defined medium. The present studies were conducted to charac terize expression of an inhibitor of apoptosis (iap) gene, inhibitor o f T-cell apoptosis (ifa), within hen follicle tissues at various stage s of follicle development. The ifa gene product has recently been show n to share homology within both the baculovirus repeat sequences of th e N-terminus and the zinc ring-finger motif from the C-terminus and wa s originally determined to be expressed in chicken cells of T-lymphocy te lineage. In the present studies, highest levels of ifa 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 i n 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 t hose granulosa cells from 6- to 8-mm follicles that successfully forme d a primary culture monolayer. Moreover, ifa mRNA levels were signific antly increased in 6- to 8-mm-follicle granulosa cells after only 2 h of suspension culture, and this increase could be prevented by actinom ycin D. This spontaneous increase in ita expression may serve to prote ct from cell death the relatively small population of prehierarchal fo llicle granulosa cells that survive in vitro, It is concluded from the se data, taken together, that patterns of ita mRNA expression during f ollicle development are consistent with a potential role for this gene in protecting granulosa cells from apoptosis and thus maintaining fol licle viability.