19of sperm: a hypotonic shock triggers a Ca2+ transient in the central ooplasm (Sharma
and Kinsey 2008 ). This later finding is consistent with the observation that the eggs
of zebrafish (and some other teleosts) exhibit pre-fertilization oocyte activation, on
contact with water (Sakai et al. 1997 ).
The sperm-induced Ca2+ wave in Xenopus eggs also appears to be stimulated via
IP 3 -dependent signaling (Nuccitelli et al. 1993 ). The sensitivity of the type I IP 3
receptor to IP 3 increases during oocyte maturation as the oocytes reorganize their
endoplasmic reticulum in preparation for fertilization (Machaca 2004 ). The Ca2+
signal is associated with waves of both IP 3 and PKC activation, indicating that the
IP 3 increase observed at fertilization is the result of PLC-mediated PIP 2 hydrolysis
(Larabell et al. 2004 ; Wagner et al. 2004 ). Experimental data indicate that somewhat
similar to zebrafish, the fertilizing sperm increases PLCγ activity and IP 3 production
in the egg via the stimulation of a Src-family kinase (Sato 2008 ). When the Src
kinase is activated by oolemma-resident uroplakin III (which itself is activated by a
sperm protease associated with a surface glycoprotein), it stimulates PLCγ. Ca2+
increase at fertilization is not inhibited by the microinjection of recombinant SH2
domains of PLCγ, although the expression of these domains inhibits platelet-derived
growth factor (PDGF)-stimulated Ca2+ release in eggs with exogenously expressed
PDGF receptors (that are known to recruit PLCγ through the binding of its SH2
domains). Thus, the Src-family kinase seems to stimulate PLCγ through a SH2
domain-independent mechanism (Runft et al. 1999 ). In addition, it has also been
suggested that phosphatidylinositol-3-kinase may be the upstream activator of the
Src kinase (Mammadova et al. 2009 ). In Cynops eggs the injection of IP 3 triggers a
Ca2+ transient, whereas the inhibition of the IP 3 receptors with heparin prevents the
Ca2+ waves at fertilization. This indicates that in Cynops the Ca2+ signals induced by
the fertilizing spermatozoa are probably mediated by the IP 3 receptors (Harada et al.
2011 ). In mammals, the generation of the fertilization Ca2+ transients is also medi-
ated by the phosphoinositide signaling system. The involvement of IP 3 and its
receptor in the generation of the sperm-induced Ca2+ signal is clearly demonstrated
by the fact that a monoclonal antibody raised against the IP 3 receptor or downregu-
lation of the receptor blocked the Ca2+ oscillations at fertilization (Miyazaki et al.
1992 ; Brind et al. 2000 ). In addition, sustained microinjection of IP 3 or the injection
of adenophostin, an IP 3 analog, can also induce regenerative Ca2+ rises in mamma-
lian eggs (Swann 1994 ; Jones and Nixon 2000 ).
1.3.4 How Does the Sperm Trigger the Ca2+ Signal?
Once the major components of the signaling cascade that operates at fertilization
were identified, the big unresolved question remained: how does the fertilizing sperm
trigger the rise in the cytosolic Ca2+ concentration in the egg? There have been a vari-
ety of hypotheses proposed to answer this question. One model postulated that the
sperm serves as a source of Ca2+ entry into the egg. According to the original version,
the sperm delivers a “bomb” of Ca2+ that eventually sets off a wave of Ca2+-induced
1 Egg Activation at Fertilization