271.3.5 Repetitive Ca2+ Oscillations
It is not entirely clear what makes the Ca2+ signal oscillate. Most signals that are
generated by Ca2+ mobilization from intracellular stores have a predisposition for
oscillation (Berridge and Galione 1988 ). Both IP 3 and ryanodine receptors show
Ca2+-induced Ca2+ release that is the basis of oscillatory activity in many cell types,
and high cytosolic Ca2+ inhibits further Ca2+ release through both receptor types.
These features are sufficient to elicit the oscillatory pattern. Nevertheless, store
depletion adds an additional negative feedback constituent, and because PLCζ
shows very high sensitivity to Ca2+ (Kouchi et al. 2004 ), the Ca2+-promoted produc-
tion of IP 3 provides one more positive feedback for the IP 3 receptor.
The oscillations are probably controlled by the basic feedback properties of the
IP 3 receptor (Adkins and Taylor 1999 ). According to one popular model of Ca2+
oscillations, Ca2+-dependent IP 3 production by PLCζ (i.e., the nonlinear feedback
loop of Ca2+ on PLCζ activity) leads to oscillating IP 3 levels, which accounts for the
repetitive nature of the fertilization Ca2+ signal (Dupont and Dumollard 2004 ).
However, injection of the mammalian sperm factor into frog eggs causes only one
Ca2+ transient (Wu et al. 2001 ), indicating that the feedback of Ca2+ on PLCζ cannot
in itself explain repetitiveness. Another plausible model argues that instead of con-
trolling PLCζ activity, Ca2+ may act directly on the IP 3 receptors (De Young and
Keizer 1992 ). In this version, IP 3 concentrations do not oscillate, but instead IP 3 at
a constant level provides continuous stimulation to its receptor. The receptor opens
when intracellular Ca2+ is low and closes when Ca2+ concentration at the receptor
rises above a threshold level. Observations that IP 3 levels do not oscillate in HeLa
cells during the repetitive Ca2+ signal induced by metabotropic glutamate receptor
stimulation (Matsu-ura et al. 2006 ) and that providing a sustained IP 3 supply can
trigger Ca2+ oscillations in mouse eggs (Jones and Nixon 2000 ) support this model.
Measuring IP 3 at fertilization could help in distinguishing between the different
models; unfortunately, due to inherent difficulties, the data available can be inter-
preted in various ways (Shirakawa et al. 2006 ). It has also been suggested that in
mammalian eggs the two mechanisms may coexist. In unfertilized eggs, the IP 3
receptor alone is responsible for the Ca2+ oscillations seen after sustained injection
of IP 3. Then at gamete fusion, the sperm introduces PLCζ into the ooplasm after
which a new mechanism, regenerative IP 3 production, regulates the oscillatory Ca2+
signal (Swann and Yu 2008 ).
The Ca2+ oscillations also seem to be dependent on a Ca2+ influx across the
plasma membrane. In mouse eggs, the sperm-induced Ca2+ spikes stop or slow
down significantly upon the removal of extracellular Ca2+ (Kline and Kline 1992 ;
Shiina et al. 1993 ). In addition, incubation in the presence of thapsigargin stimulates
Ca2+ entry in mouse, pig, and human eggs (Kline and Kline 1992 ; Machaty et al.
2002 ; Martín- Romero et al. 2008 ). Thapsigargin is an inhibitor of the SERCA
pumps. Ca2+ slowly leaks out of the endoplasmic reticulum, the blocked pumps are
not able to reload Ca2+, and the stores become depleted. The fact that store depletion
triggers extracellular Ca2+ influx indicates that a mechanism known as store-oper-
1 Egg Activation at Fertilization