Cellular Interactions
465
cell. Two pivotal signaling agents that mediate several of these rapid modiFcations
in cell structure are protein kinase C (PKC) and calcium/calmodulin-dependent
protein kinase II (CaM KII). Studies indicate that elements from several of the key
signaling pathways, including PKC and CaM KII, colocalize on molecular scaffolds
(i.e. meiotic spindle) in the egg and may provide a means for these pathways to
interact.
1
Overview of Cytoplasmic Signaling
1.1
General
All cells, regardless of whether they are part
of tissue in a multicellular organism or an
individual cell functioning as an organism,
are faced with the challenge of responding
to the external environment. The ability to
make an appropriate response to external
stimuli can be critical to the functioning
of a cell and, certainly, in the case
of a single-cell organism, the ability of
a cell to reproduce. Consequently, over
evolutionary time, complex networks of
signal systems have coevolved with cells
that provide mechanisms, and probably
backup mechanisms that permit short
term, relatively rapid changes in cells
in response to external stimuli. Short-
term cytoplasmic signals are typically
mediated through kinases, phosphatases,
other enzymes, and calcium to mediate
relatively rapid changes in cells. This is
in contrast to long-term signaling events,
which are mediated by changes in gene
t
r
a
n
s
c
r
i
p
t
i
o
na
n
dc
a
nr
e
q
u
i
r
em
u
l
t
i
p
l
e
days to weeks to result in a change in
cell function.
There is a diverse array of short-term cy-
toplasmic signals, but they typically have
several features in common. They have
the ability to amplify and propagate the
signal rapidly from the site of origin,
they can be modiFed rapidly (e.g. phos-
phorylated/dephosphorylated or bound to
calmodulin/calcium), and the signal(s) can
be terminated rapidly. Such signals can
propagate globally across the entire cell, as
is the case with the calcium signals when
the egg is fertilized, or they can act in dis-
crete regions in the cell perhaps by binding
to molecular scaffolds. Intracellular signals
can be mediated through a variety of mech-
anisms such as enzyme-linked receptors,
G-proteins, gap junctions, phospholipases,
hormones, or calcium in a network of
pathways that can be highly cross-linked.
The level of free calcium in cells often
has a role in mediating some portion of
a signaling pathway. To keep the level
of intracellular-free calcium ([Ca
2
+
]
i
)low
,
cells have evolved a variety of mechanisms
including enzymes and other cell proteins
that are differentially regulated by the level
of [Ca
2
+
]
i
. These proteins have coevolved
with mechanisms that promote low lev-
els of intracellular-free calcium, so that an
elevation of free calcium will activate or
inhibit an array of calcium-dependent pro-
teins to provide a system that can affect
numerous cellular functions.
The egg, and speciFcally the mammalian
egg, is a useful system for investigating cy-
toplasmic signaling events, as this unique
cell must rapidly undergo a highly ordered
set of changes in structure and function
as a result of fusion with the sperm. If
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