Oocyte regulation of kit ligand expression in mouse ovarian follicles

Kit ligand (KL), a product of granulosa cells in ovarian follicles, is a putative regulator of oocyte development. However, the factors that regulate KL mRNA levels in granulosa cells remain unclear. This study tested the hypothesis that oocytes regulate granulosa cell steady-state KL mRNA expression levels and that the characteristics of this regulation are dependent on the stage of growth and development of both oocytes and follicles. Levels of mRNA for the KL splice variants (KL-1 and KL-2) were shown to be high in granulosa cells from preantral follicles and then decline after follicular antrum formation. Preovulatory follicular development was associated with a dramatic increase in steady-state levels of KL-1 mRNA in mural granulosa but not cumulus cells. Regulation of these changes was examined in vitro using partly grown oocytes isolated from preantral follicles and fully grown oocytes isolated from preovulatory follicles. FSH increased the steady-state KL mRNA levels in preantral granulosa cells in vitro. Partly grown oocytes either increased or decreased KL mRNA levels in preantral granulosa cells depending on the absence or presence of FSH stimulation, respectively. Fully grown oocytes reduced the KL mRNA level in preantral granulosa cells and increased the ratio of KL-1 to KL-2 mRNA. In mural granulosa cell culture, FSH augmented testosterone-dependent elevation of the steady-state KL mRNA level, but had no effect alone. Fully grown oocytes reduced KL-2 but not KL-1 mRNA levels in mural granulosa cells treated with testosterone plus FSH, whereas fully grown oocytes reduced levels of both KL transcripts in cumulus cell culture. These effects of oocytes on steady-state KL mRNA expression levels in vitro explain the changes in granulosa cell KL mRNA levels observed during follicle development in vivo. The results therefore support the hypothesis that oocytes regulate granulosa cell kit ligand mRNA levels in a way that is characteristic of the stage of growth and development of the oocyte. Moreover, the results suggest that oocytes play a major role in promoting dynamic changes in gene expression by granulosa cells appropriate to the stage of follicular development.