Murine oocytes suppress expression of luteinizing hormone receptor messenger ribonucleic acid by granulosa cells

This study tested the hypothesis that murine oocytes participate in the establishment of granulosa cell phenotypic heterogeneity in preovulatory follicles. In these follicles, mural granulosa cells express LH receptors (LHR) and LHR mRNA, but expression of these molecules is low or undetectable in cumulus cells. Thus, the expression of LHR mRNA is a marker of the mural granulosa cell phenotype in preovulatory follicles. Cumulus cells expressed elevated steady-state levels of LHR mRNA when oocytes were microsurgically removed from oocyte-cumulus cell complexes, and this was prevented by paracrine factor(s) secreted by isolated oocytes. These factors also suppressed FSH-induced elevation of the level of LHR mRNA expression by mural granulosa cells isolated from small antral follicles, even when expression was augmented by culturing granulosa cells on components of basal lamina. Moreover, factor(s) secreted by oocytes suppressed the expression of LHR mRNA in mural granulosa cells isolated from preovulatory follicles already expressing elevated levels of these transcripts. The ability of oocytes to suppress the LHR mRNA expression by granulosa cells was developmentally regulated. Oocytes from preantral follicles and mature (metaphase II arrested) oocytes were less effective in suppressing expression than fully grown, germinal vesicle (GV)-stage oocytes. Furthermore, two-cell-stage embryos did not suppress LHR mRNA levels. Coculture of isolated oocytes with granulosa cells affected the synthesis of very few granulosa cell proteins detected by fluorography of two-dimensional gels after S-35-methionine labeling. Thus, oocyte suppression of FSH-induced LHR mRNA expression is specific in both the suppressing cell type and the effects on granulosa cells. It is concluded that the default pathway of granulosa cell differentiation produces the mural granulosa cell phenotype, as represented by the expression of LHR mRNA. This pathway is abrogated by oocytes. Thus, oocytes play a dominant role in establishing the fundamental heterogeneity of the granulosa cell population of preovulatory follicles.