Clusterin Protects Granulosa Cells from Apoptotic Cell Death during Follicular Atresia

Abstract

Clusterin expression is associated with programmed cell death (apoptosis) in many cell types but its exact role has not yet been defined. This study was carried out to determine the cellular localization of clusterin in the ovary and its functional role in the apoptotic cell death of ovarian follicles. A homogenous population of healthy and atretic follicles was obtained by treating immature rats with pregnant mare serum gonadotropin (PMSG). Apoptotic cell death was evaluated by TUNEL. Clusterin expression in the healthy and atretic follicles was examined by immunohistochemical and Western blot analyses, and gene expression was examined by Northen blot analysis. Clusterin protein and its mRNA are only expressed in granulosa cells of atretic follicles obtained from PMSG-treated rats on day 5 of the treatment. Healthy follicles from PMSG-treated rats on day 2 of the treatment do not express clusterin. Theca and stroma cells of both healthy and atretic follicles showed no signs of apoptosis and did not express clusterin. Withdrawal of trophic support from granulosa cells in cultures to induce apoptosis resulted in a dramatic increase in the levels of clusterin and its mRNA compared to cells cultured in serum-supplemented medium. In an attempt to establish the functional role of clusterin in the apoptotic cell death of ovarian follicles, the biosynthesis of clusterin in granulosa cells of healthy follicles was blocked by treatment of cells with antisense oligonucleotide to its cDNA. Treatment of granulosa cells with the antisense oligonucleotide resulted in an increase in the apoptotic cell death compared to the control. These findings indicate that depletion of clusterin can lead to the programmed cell death in ovary, suggesting a functional role for this protein in follicular atresia.