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At fertilization, eggs unite with sperm to initiate developmental programs that give rise to development of the embryo. Defining the molecular mechanism of this fundamental process at the beginning of life has been a key question in cell and developmental biology. In this review, we examine sperm-induced signal transduction events that lead to release of intracellular Ca(2+), a pivotal trigger of developmental activation, during fertilization in Xenopus laevis. Recent data demonstrate that metabolism of inositol 1,4,5-trisphosphate (IP(3)), a second messenger for Ca(2+) release, is carefully regulated and involves phospholipase C (PLC) and the tyrosine kinase Src. Roles of other potential regulators in this pathway, such as phosphatidylinositol 3-kinase, heterotrimeric GTP-binding protein, phospholipase D (PLD) and phosphatidic acid (PA) are also discussed. Finally, we address roles of egg lipid/membrane microdomains or 'rafts' as a platform for the sperm-egg membrane interaction and subsequent signaling events of egg activation.
Fig. 1. A model for the signaling network that triggers intracellular Ca2+ release in fertilized Xenopus eggs. Transient Ca2+ release, depicted as ‘Ca2+’ in the figure, in the fertilized Xenopus egg involves two major processes: IP3-induced Ca2+ release (IICR) (1) and Ca2+-induced Ca2+ release (CICR) (2). IP3 and DG (3) are produced by the hydrolysis of PIP2 that is catalyzed at least in part by PLCγ, a member of the PLC family of enzymes. Minutes after sperm–egg interaction, PLCγ undergoes translocation from the cytoplasm to lipid/membrane microdomains called ‘rafts’. Src then phosphorylates PLCγ on tyrosine and activates the lipase (4). Some portion of tyrosine-phosphorylated PLCγ reenters the eggcytoplasm (5) and may catalyze IP3 production in membranes within the cytoplasm of the fertilized egg. Increased concentration of intracellular-free Ca2+ ions, as promoted by IICR and CICR, may also be responsible for further activation of PLC in the eggcytoplasm (6). It has been demonstrated that the SH2 domain of PLCγ, which can play a major role in PLCγ membrane-translocation and activation by tyrosine phosphorylation, is not required in Xenopus egg fertilization. Thus, raft-translocation of PLCγ should utilize an alternative mechanism: the PH domain of PLCγ may bind to PIP2 and/or PIP3 in the membrane (7). PIP3 is an enzymatic product of PI 3-kinase. The 85-kDa subunit of PI 3-kinase is shown to translocate from the non-raft egg membrane to the raft membranes within minutes of sperm–egg interaction (8) and a pharmacological PI 3-kinase inhibitor, LY294002, inhibits sperm-induced PLCγ activation and the Ca2+ transient. As discussed in the text, it is unlikely that PLCζ-like sperm-derived factors are responsible for the Ca2+ transient (9), although we suggest that another contribution from sperm may play a role (i.e., PA and protease; see below). Sperm-induced activation of Src, an upstream event of PLCγ activation, occurs in the raft membranes (10). Src is concentrated in the raft membranes of unfertilized eggs and its activation can be reconstituted in vitro by the addition of sperm to the isolated rafts. Sperm-induced Src activation may involve three pathways: activation of heterotrimeric GTP-binding protein(s) (Gαβγ) (11) that would connect the extracellular proteolysis of UPIII by the sperm-derived protease activity (9) and the intracellular enzymatic up-regulation of Src. Secondly, PIP3, a possible trigger for raft-translocation of PLCγ, could also be a trigger for Src activation (12). Third, involvement of PLD activity and its enzymatic product, PA, both of which increase after sperm–egg interaction, may activate Src (PA may also bind and directly activate PLCγ) (13). The origin of stimulatory PA (from sperm or zygote or both?) is now under investigation. One candidate molecule for a sperm receptor is UPIII (14), a single-transmembrane protein that is predominantly tyrosine-phosphorylated (possibly by Src) in the rafts of fertilized eggs (15). The function of the tyrosine-phosphorylated form of UPIII is unknown. A specific antibody against the extracellular domain of UPIII can inhibit normal fertilization and sperm-derived tryptic protease(s), which are thought to be important in initiating egg activation and partial proteolysis of UPIII (9). Sperm–egg interaction and egg activation signaling may also involve a spermdisintegrin-egg integrin interaction (9). UPIII interacts with a tetraspanin partner, UPIb (16), to provide proper molecular organization in the egg rafts. Note that the thick grey arrows in the figure represent activation events and that the thin black arrows represent translocation events.
