269. CD44 signaling in mouse ovulatory cumulus - oocyte complexes

Abstract

Oocytes grow and develop within ovarian follicles, providing a nurturing environment before their release (ovulation) into the oviduct for fertilisation. For ovulation to occur the ovarian follicle responds to LH from the pituitary, leading to a cascade of regulated gene expression and formation of the hyaluronan rich cumulus matrix around the oocyte. This COC (cumulus oocyte complex) matrix is composed of high concentrations of the ECM glycosaminoglycan hyaluronan (HA) cross-linked by several HA-binding proteins. Several of the COC matrix components are essential for ovulation, since null gene mutations in mice lead to ovulation defects. Mechanisms by which the COC matrix controls ovulation however, are unknown. We have investigated cellular signalling and cellular phenotypes that occur as part of the formation of the COC matrix. The transmembrane HA receptor CD44 was significantly upregulated in cumulus cells from 6 h after hCG (LH analogue) treatment (9.8 ± 1.5 fold) until ovulation at 12 h post hCG (11.8 ± 2.9-fold). In many cell types CD44 activates the intracellular Rho-family GTPase Rac1 and its activator, the guanine exchange factor Tiam1, pleiotropic regulators of cytoskeletal function, cell-cell adhesion and migration. We found both Rac1 and Tiam1 were strongly detected in cumulus cells, but not regulated by hCG. These observations show that at the time of ovulation a macro-molecular complex associated with cell motility is assembled through the extracellular interaction of the COC matrix and cell surface proteins. We investigated the migratory and invasive activity of COCs from hormonally stimulated mice. Migration of cumulus cells from hCG treated mice was significantly increased compared with untreated COCs. Furthermore the hCG-stimulated cumulus cells were able to invade a range of ECM substrates including collagen and laminin. These results suggest the cumulus cells in the expanded COC transition to a motile cell phenotype that may play a key role in promoting ovulation.