Cellular Interactions
479
it has been suggested that CaM KII is
needed for closure of the contractile ring
of the second polar body.
By activating PKC with diacylglycerol,
Colonna et al. conducted a detailed study
to determine the role of PKC on egg ac-
tivation. Their results indicate that MPF
was downregulated by PKC. Colonna et al.
demonstrated that the MPF level (moni-
tored as histone H1 kinase activity) drops
within 20 min of PKC activation, the chro-
mosomes have a normal transit from
metaphase to anaphase in a high percent-
age of eggs, pronuclei form several hours
later, and the second polar body forms.
These investigators used a concentration
of DAG that did not increase the level of
[Ca
2
+
]
i
, but they did show that the level of
[Ca
2
+
]
i
increases to about 0.5
µ
Mathigher
concentrations of DAG (i.e. 150
µ
M). This
transient increase in the level of [Ca
2
+
]
i
may have been caused by the higher con-
centration of DAG because, as the DAG
at this concentration intercalated into the
plasma membrane of the egg, it could have
affected the lipid packing in the membrane
allowing calcium to leak in from the extra-
cellular medium. The results from Moore
et al., which differ from Colonna et al., did
not detect either a formation of second
polar bodies or a decrease in MPF activity
(monitored as histone H1 kinase activity).
However, to assess the effects on MPF
activity, Colonna et al. employed DAGs,
the natural activator of PKC in cells, and
Moore et al. used phorbol esters to activate
eggs. As noted previously in this section,
phorbol esters, which are foreign to cells,
are not as likely to be properly metabolized
and regulated by cells compared to the nat-
ural activator of PKC (i.e. diacylglycerols).
The unusual results may have been in-
duced by the phorbol esters used by Moore
et al.
4.3
Remodeling through Changes in Activity of
MPF and MAP Kinase Activity in Activated
Eggs
As discussed in an earlier section, cyclin
B1 is targeted for degradation through
the ubiquitin-dependent pathway by the
rise of [Ca
2
+
]
i
in mammalian eggs, which
results
in
the
rapid
inactivation
(i.e.
within 30 min to 1 h) of MPF, while
MAP kinase activity remains elevated
for a few hours and then decreases. In
eggs pretreated with drugs that stabilize
MPF (nocodazole and cyclohexamide),
MAP kinase activity still decreased after
several hours, which suggests that MPF
degradation is not a requirement for the
eventual degradation of MAP kinase. Just
before formation of two pronuclei – one
containing the male chromatin and one
containing the female chromatin – MAP
kinase is inactivated. The activation of
MAP kinase and disassembly of the oocyte
nucleus parallels the relationship between
inactivation of MAP kinase followed by
assembly of the nuclear envelope, that is,
the nuclear envelope disassembles when
MAP kinase is active. There are three
lines of evidence that suggest that MAP
kinase activity is incompatible with the
existence of a nucleus. (1) Formation of
pronuclei is inhibited when a constitutively
active form of the upstream activator
of MAP kinase (i.e. MAP kinase kinase
referred to as MEK) were microinjected
into activated eggs. (2) Formation of
pronuclei is inhibited when a phosphatase
inhibitor (i.e. okadaic acid) is employed
to prevent dephosphorylation of MAP
kinase, a treatment that maintains MAP
kinase activity. (3) The oocyte nucleus
disassembles when MAP kinase becomes
active in oocytes, and disassembly of the
nucleus occurs rapidly when activated
previous page 1153 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 1155 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off