Modifications of the Ca2+ release mechanisms of mouse oocytes by fertilization and by sperm factor

Abstract

A cytosolic factor from sperm (SF) is thought to be responsible for the generation of intracellular calcium oscillations ([Ca2+]i) associated with fertilization in mammalian oocytes. Whether or not mouse oocytes injected with SF exhibit modifications of their Ca2+ release mechanisms similar to those observed in fertilized oocytes is not known and this was investigated here by injecting porcine SF (pSF). First, pSF-activated oocytes injected with CaCl2 showed persistent sensitization of the Ca2+-induced Ca2+ release mechanism, but this sensitization was absent in SrCl2-activated oocytes. Second, pSF-injected oocytes re-initiated oscillations when fused with untreated oocytes, although the Ca2+ responses were short-lived compared to those initiated by fertilization. Likewise, in the presence of colcemid, pSF-initiated oscillations were prolonged but ceased in advance of those in fertilized zygotes. Also, pronuclear envelope breakdown induced by okadaic acid was not associated with Ca2+ release in pSF-generated zygotes, whereas it was observed in fertilized zygotes. Finally, roscovitine, an inhibitor of maturation promoting factor, blocked pSF-induced [Ca2+]i oscillations. Together, these results show that pSF-induced [Ca2+]i responses exhibit properties similar to those triggered by the sperm, although the SF's Ca2+ active component(s) may be less stable or more susceptible to degradation, resulting in shorter modification of the oocyte's Ca2+ release mechanisms.