TSG-6 is concentrated in the extracellular matrix of mouse cumulus oocyte complexes through hyaluronan and inter-alpha-inhibitor binding

During development of ovarian follicles in mammals, cumulus cells and the oocyte form a mucoelastic mass that detaches itself from peripheral granulosa cell layers upon an ovulatory surge. The integrity of this cumulus-oocyte complex (COC) relies on the cohesiveness of a hyaluronan (HA)-enriched extracellular matrix (ECM). We previously identified a serum glycoprotein, inter-alpha-inhibitor (I alphaI), that is critical in organizing and stabilizing this matrix. Following an ovulatory stimulus, diffuses into the follicular fluid and becomes integrated in the ECM through its association with HA. TSG-6 (the secreted product of the tumor necrosis factor-stimulated gene 6), another HA binding protein, forms a complex with I alphaI in synovial fluid. The purpose of this study was to investigate whether TSG-6 is involved in the ECM organization of COCs. Immunolocalization of TSG-6 and I alphaI in mouse COCs at different ovulatory stages was analyzed by immunofluorescence and laser confocal microscopy. I alphaI, TSG-6, and HA colocolized in the cumulus ECM. Western blot analyses were consistent with the presence of both TSG-6 and TSG-6/I alphaI complexes in ovulated COCs. These results suggest that TSG-6 has a structural role in COC matrix formation possibly mediating cross-linking of separate HA molecules through its binding to I alphaI.