480
Cellular Interactions
MAPk
in
a
s
ei
sin
j
e
c
t
edin
t
oth
eo
o
c
y
t
e
nucleus. Thus, MAP kinase may serve
to
regulate
both
the
organization
of
microtubules and the state of the nucleus;
when microtubules are in an M-phase
conFguration and nuclei are absent, MAP
kinase is active, whereas nuclei form
and an interphase array of microtubules
assemble when MAP kinase activity is
reduced or absent.
One of the hallmarks of egg activation
and functions to prevent polyspermy is
cortical granule exocytosis, which is not
under the control of MAP kinase or MP±.
This was demonstrated by inhibiting pro-
tein synthesis in eggs arrested at meiotic
metaphase II. It has been shown that a
decrease in MP± activity is caused by a de-
crease in cyclin B1 concentration, which
can be decreased by inhibiting protein
synthesis that can also slowly decrease
MAP kinase activity. Consequently, the
egg will be released from arrest at meiotic
metaphase II after inhibition of protein
synthesis. However, cortical granule exo-
cytosis did not occur after a decrease in
M±P and MAP kinase activity, suggesting
that the regulation of cortical granule ex-
ocytosis is not under the control of M±P
or MAP kinase. Since a rise in [Ca
2
+
]
i
that
accompanies fertilization or egg activation
triggers cortical granule exocytosis, this
suggests that a calcium-dependent compo-
nent of the cell other than MP± regulates
this event. In support of this assertion, the
activation of another kinase, PKC (whose
conventional isotypes are calcium depen-
dent) induces cortical granule exocytosis
as discussed in Sect. 4.2.
Although c-mos is reported to be an
important component of cytostatic factor,
an activity present in metaphase II eggs
thought to arrest the egg cell cycle at
meiotic metaphase II, the activity of c-
mos remains elevated well after the time
that MP± is degraded and the cell has
proceeded into telophase. However, when
using double-stranded RNA interference
(RNAi) to destroy c-mos in eggs, the eggs
activate indicating c-mos clearly has a role
in egg activation.
5
Concluding Remarks
Signaling pathways trigger speciFc chan-
ges in the conversion of the egg into
the zygote. This review highlighted several
signaling pathways that have the potential
to interact on molecular scaffolds within
theegg
.Theeggmayusethesesca
f
fo
lds
to localize components of the various
signal pathways and to allow cross talk
between different pathways. Scaffolds also
provide a means to promote rapid changes
in the functioning and modulation of
the signaling pathways. In the egg, the
meiotic spindle was described as a major
molecular scaffold for localizing elements
of the signaling pathways to coordinate
early developmental events downstream
of the calcium signal.
The signaling agents that trigger many
of the early events after fertilization have
been identiFed, although clearly more
work is needed to deFne other pathways.
Much less is known of the signaling com-
ponents that trigger events occurring sev-
eral hours after fertilization and affecting
such processes as pronuclear transcription
or embryonic genome activation. These
will likely be the focus of many research
efforts in the future. Investigating the pu-
tative interactions between the signaling
pathways can lead to improvements in
cloning technology applied to agricultural
animals. By studying these signaling fac-
tors, the success rates of nuclear transfer
can be improved, which would make this
biotechnology approach more robust.
previous page 1154 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 1156 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off