TSG-6 Is Concentrated in the Extracellular Matrix of Mouse Cumulus Oocyte Complexes Through Hyaluronan and Inter-Alpha-Inhibitor Binding1

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 (IαI), that is critical in organizing and stabilizing this matrix. Following an ovulatory stimulus, IαI 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 IαI 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 IαI in mouse COCs at different ovulatory stages was analyzed by immunofluorescence and laser confocal microscopy. IαI, TSG-6, and HA colocolized in the cumulus ECM. Western blot analyses were consistent with the presence of both TSG-6 and TSG-6/IαI 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 IαI.