Mammalian oocyte activation by the synergistic action of discrete sperm head components: Induction of calcium transients and involvement of proteolysis

Sperm-borne oocyte-activating factor (SOAF) elicits activation sufficient for full development and originates from sperm head submembrane matrices. SOAF comprises discrete, heat-sensitive and -stable components (referred to here respectively as SOAF-I and -II) which are each necessary but not sufficient to activate oocytes. The heat-sensitive SOAF component, SOAF-I-m, becomes solubilized from the perinuclear matrix under reducing conditions (the SOAF transition) to generate SOAF-I-s. Although calcium transients likely play an important role in oocyte activation at fertilization, the question is open as to whether demembranated heads or SOAF-I-s and/or SOAF-II can induce calcium transients. We now report that injection of demembranated sperm heads into mouse oocytes efficiently induced Ca2+ oscillations. When injected independently, SOAF-I-s and demembranated heads heated to 48 degrees C failed to generate Ca2+ oscillations. However, co-injection of SOAF-I-s and 48 degrees C-heated heads induced oscillations, mirroring their synergistic ability to activate oocytes. This suggests that SOAF-mediated activation proceeds via pathways resembling those at fertilization and provides the first direct evidence that multiple sperm components are required to induce Ca2+ oscillations. We probed the SOAF-I-s liberation at the center of this activation and show that in vitro it was sensitive to a profile of serine protease inhibitors. These findings support a model in which mammalian oocyte activation, including the induction of calcium transients, involves proteolytic processing of SOAF from sperm head submembrane compartments.