Biology Reference
In-Depth Information
SFK
NAADP
Ca
2
+
PLC
ζ
PLC
γ
Exocytosis
cADPr
InsP
3
Ca
2
+
cGMP
NO
Egg activation
Figure 3.2
Src-family kinases (SFK) activate PLCγ to produce InsP
3
and trigger the calcium
waves (blue pathway); in sea urchins. In mammals, sperm-egg fusion introduces PLCζ into
the egg cytoplasm, producing InsP
3
(yellow pathway). Sperm-egg fusion may also introduce
NAADP in echinoderms; NAADP activates plasma membrane calcium channels (red
pathway). In ascidians, NAADP deactivates plasma membrane channels, while cADPr triggers
local calcium release to trigger cortical granule exocytosis (red pathway). In sea urchins,
calcium activates nitric oxide production, which generates cADPr via cGMP (green pathway).
Abbreviations
: PLC, phosphoinositide phospholipase C; cADPr, cyclic ADP-ribose; cGMP,
cyclic guanosine monophosphate; InsP
3
, inositol trisphosphate.
Source
: From
Whitaker (2008)
.
fertilization-induced calcium signals are widely shared throughout the animal
kingdom, in some groups the egg experiences a single fertilization Ca
2+
wave,
while others experience more than one.
In
Xenopus
, while the genome is still quiescent, fertilization of the egg induces
rotation of the egg cortex relative to the cytoplasm, pushing the dorsalizing maternal
cytoplasmic factors to the part of the egg cortex that is opposite to the sperm entry
point (
Figure 3.3
). A maternal transcript, Fatvg, plays the key role in the process
of cortical rotation (
Chan et al., 2007
). Just before gastrulation, the epigenetically
driven cortical rotation leads to formation of the Spemann organizer. The cortical
rotation performs the so-called slow transport of the dorsalizing factors. The slow
transport is complemented by the fast dorsalward transport of maternal factors along
the polymerizing microtubules. A special role is played by the maternal β-catenin, a
component of the Wnt pathway (
Moon and Kimelman, 1998
): it induces the expres-
sion of a number of downstream genes, including its own gene, leading to formation
of the dorsoventral and anteroposterior axes of the embryo (
Weaver and Kimelman,
2004
).
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