In addition, SF appears to sensitize Ca2+-induced Ca2+ release (CICR), a mechanism shown to be stimulated in recently fertilized eggs and thought to be mediated by the IP3R. Furthermore, fertilized mammalian eggs exhibit downregulation of the IP3R, and SF signals a comparable degradation of this receptor. Collectively, these data demonstrate that the sperm and SF initiate [Ca2+]i oscillations in mammalian eggs by activating a similar signaling pathway that is mediated by the IP3R.
In sea urchin egg homogenates, addition of SF induced IP3 production and Ca2+ release, and these responses were blocked by preincubation of SF with U73122, a PLC activation inhibitor, suggesting that SF stimulates the PI pathway. Whether or not SF stimulates this signaling pathway in mammalian eggs has not been determined nor has it been shown that SF injection induces IP3 production in single eggs as opposed to ho-mogenates.
In the present study we investigated whether SF stimulates the Pi pathway in mouse eggs by incubating eggs and SF with U73122 prior to injection of SF. We then determined, at the single cell level using Xenopus oocytes, whether injection of SF was able to trigger IP3 production. We then tested whether PLC71, PLC72, and PLC84 and/or its splice variants, which have been identified in sperm/ testis and/or eggs, are the active IP3-producing component(s) in our extracts. We also determined whether increased intracellular concentrations of an IP3R agonist in mouse eggs were sufficient to sensitize CICR, a hallmark of fertilization.