Perspective of PLCζ
The PLCζ gene has been cloned in mouse, rat, dog, pig, cow, monkey, and human. The Ca2+ oscillationinducing activity of sperm extract is compatible among mammalian species, although the grade of the activity seems distinct. Besides soluble cytosolic sperm factor, an insoluble component bound to the perinuclear matrix of the sperm has the egg-activating activity. A recent assay of the Ca2+ oscillation-inducing activity coupled to tandem mass spectrometry has shown that the activity of the component from the perinuclear matrix corresponded to PLCζ (27). It is important to elucidate in the near future whether PLCζ is the COIP that operates during physiological fertilization of mammalian eggs. The experiment using transgenic RNA interference of PLCζ has shown both reduction of the PLCζ content in spermatozoa of the transgenic mice and reduction of the number of Ca2+ spikes in eggs inseminated with the sperm (28). Since the number of Ca2+ spikes tends to be affected by experimental conditions of in vitro fertilization, it is desirable to show complete block of Ca2+ oscillations at fertilization by knocking out the PLCζ gene.
It is also interesting to examine the sperm factor from the phylogenic standpoint. As to fertilization signaling in non-mammalian eggs, the activation of PLCγ via Src-family PTK is involved at fertilization of sea urchin, starfish, ascidia, and frog (12) because Ca2+ responses at fertilization are prevented by pre-injection of the SH2 domain of PTK or PLCγ on the basis of a dominantnegative experiment. The mode of stimulation of the sperm is still unknown in these species. At present, evidence for the sperm factor hypothesis is accumulated in ascidia (2). Further studies are also necessary to examine the activation and modification mechanism of PLCζ on the basis of the molecular structure because PLCζ might be inactivated in the sperm to prevent [Ca2+]i rises before it is introduced into the egg cytoplasm. In this context, PLCζ bound to the perinuclear matrix might be important. PLCζ driven into the egg cytoplasm might require any aid of egg factor(s); for example, the accession to the membrane might be promoted by an egg factor, since PLCζ lacks the PH domain.
The nuclear translocation of PLCζ might have biological significance other than regulation of Ca2+ oscillations. The PI pathway exists in the nucleus, and PLC plays biological roles in the nucleus (29). For example, PLCβ1 translocates into the nucleus during G2/M transition in immature mouse oocytes and participates in germinal vesicle breakdown (30). It is predicted that diacylglycerol produced by PLCβ1 attracts PKCβII from the cytoplasm and PKCβII causes lamin phosphorylation leading to nuclear-envelope breakdown (29). PLCδ4 is predominantly present in the nucleus and increases dramatically at G1/ S transition in response to mitogenic stimulation (31). It is interesting to address whether PLCζ accumulated in the PN affects cell proliferation and differentiation in early embryonic development. PLCζ may be applied for artificial egg activation for stockbreeding. PLCζ could be utilized as an assisted reproduction technology in clinical medicine.