Cellular Interactions
477
perturbations even if the phorbol ester was
applied for a brief period.
Mouse eggs also have been used to
demonstrate the involvement of PKC in
activation. In the study by Gallicano et al.,
the role of PKC appeared to be identical
to that described above for hamster eggs
but in addition to application of DAG,
sperm inseminated eggs were utilized. It
was shown that sperm have the same effect
as other PKC agonists, that is, after sperm
penetration PKC in the egg translocates
to the cell periphery. In the Gallicano
etal
.study
,theinvestigatorsusedanother
method, a biochemical assay, to conFrm
that PKC was translocating to the cell
periphery at the same time that the activity
ofthekinaseincreased
.Inaddition
,atthe
site of second polar body initiation, PKC
was shown to be enriched. Raz et al. used
rat eggs to demonstrate that speciFc PKC
isotypes translocate to the cell periphery
in mammalian eggs. Upon activation of
the kinase in the rat egg, PKC isotypes
α
,
β
,an
d
γ
translocate to the plasma
membrane. These results Ft well with
the model that proposes that PKC acts
downstream of the fertilization-induced
elevation in [Ca
2
+
]
i
since, noted previously
PKC
α
,
β
,and
γ
are calcium-dependent
isotypes. This observation supports the
earlier work of Gallicano et al. who, using
a PKC reporter dye that detects several
PKC isotypes, showed, as a result of
egg activation and fertilization that PKC
became active and translocated to the
plasma membrane of mammalian eggs. In
addition, a more recent study also supports
this model. Luria et al. have examined PKC
α
,
β
1, and
β
2 in mouse eggs and report
that PKC
α
translocates to the plasma
membrane as a result of egg activation
and fertilization. Two other laboratories
did not detect either PKC
β
1o
rPKC
β
2
isotypes at the protein or RNA levels in
mouse eggs.
Activated PKC drives the egg into an in-
terphase state of the cell cycle and has been
shown to be involved with cortical gran-
ule exocytosis in fertilization-competent
r
a
te
g
g
s
.I
nt
h
es
t
u
d
yb
yR
a
ze
t
a
l
.
,
PKC was activated by both phorbol esters
and DAGs and showed that activation of
PKC induced both of these events. Cor-
tical granule exocytosis in mouse eggs
has also been reported after PKC is ac-
tivated. However, since PKC inhibitors
cannot block the fertilization-induced exo-
cytosis of cortical granules, this indicates
that the PKC pathway and other calcium-
dependent signaling pathways also induce
cortical granule exocytosis (Ducibella and
Le±evre, 1997; Raz et al., 1998c). There
is a report that inhibition of CaM KII
blocks cortical granule exocytosis, but an-
other study claims that CaM KII has no
effect on cortical granule exocytosis. In
mammals, the cortical granule exocyto-
sis pathway becomes linked to calcium
as the oocyte proceeds through meiosis
to become the fertilization-competent egg.
As mentioned above, PKC is known to
translocate to the cell periphery near the
plasma membrane and since the cortical
granules are located in this region, PKC
may serve as one of the regulators of corti-
cal granule exocytosis. Gallicano et al. have
proposed that PKC regulates the remodel-
ingo
ftheeggin
tothezygo
teinave
ry
precise temporal and spatial fashion. In
that model, active PKC translocates to the
plasma membrane, which is induced by
the fertilization-induced rise in [Ca
2
+
]
i
.
Once PKC becomes active, it is tethered to
the cytoplasmic face of the plasma mem-
brane and can phosphorylate substrates
in that vicinity. It may serve to trigger a
number of events including cortical gran-
ule exocytosis. One of the mechanisms
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