Biology Reference
In-Depth Information
after refeeding
[89]
. This memory device, using in this case
an opioid receptor mechanism, triggers the signaling
beyond the hormonal levels until the appropriate energy
levels are restored.
photoactivatable derivatives of GTPases and their regula-
tors
[104]
. These molecular probes involve naturally
occurring domains that undergo large conformational
changes upon irradiation, and can be used as part of fusion
constructs to control protein function. They produce
reversible, repeatable, activation at visible wavelengths.
These probes have been very useful in characterizing the
role of the GTPases at the leading edge of the cell during
cell motility
[105]
.
To better understand the origins of microdomains from
the control of the localization of intracellular signaling
reactions, we describe a study from our laboratory by
Neves and co-workers
[106]
. Using partial differential
equations and real neuronal shapes obtained from micro-
scopic images, Neves et al. modeled how signal from the
b
-adrenergic receptor activates MAPK-1,2, through the
cAMP/PKA/b-Raf/MAPK-1,2 pathway. These studies
demonstrated that when the signal originates at the plasma
membrane and travels through the cytoplasm, the ratio of
the surface area of the plasma membrane to the cytoplasmic
volume (surface/volume (S/V) ratio) is one critical char-
acteristic that determines microdomain dynamics. For
a morphologically specialized cell such as a neuron, the
S/V is greater in the dendrites than in the cell body. Since
signals mostly start at the membrane and are dissipated in
the cytoplasm through negative regulators such as phos-
phodiesterases and phosphatases, higher S/V ratios favor
the formation of microdomains. Neves et al. conducted
simulations using PDE models and showed that in neurons
this would result in the selective formation of cAMP
microdomains in the dendrites. These predictions were then
experimentally verified.
Local activation of downstream components, such as
MAPK and PKA, indicated that spatial information is
propagated from cAMP microdomains to downstream
components. Here factors other than surface to volume ratio
also play a role. One obvious factor is diffusion coefficients
of the signaling components involved. Simulations varying
the diffusion coefficients of the different signaling compo-
nents showed that the diffusion coefficients of most
components of the signaling network did not affect micro-
domain characteristics when the signaling network was
within thin dendrites such as those found in the dendritic
arbors. The only exception was diffusion of negative regu-
lators. These results suggested that in a reaction
SIGNALING MICRODOMAINS WITHIN
CELLS
The ability of two cellular components to interact depends
not only on their mutual chemical specificity but also on
their co-localization in the various subcellular regions
within the cell. To understand the organization and func-
tional capabilities of cell signaling networks, we need to
understand how spatial separation or co-localization is
controlled to regulate the dynamic topology of cell
signaling networks. Most signals that evoke physiological
outcomes in a mammalian cell start at a cellular membrane
level, where hormones and neurotransmitters bind their
own receptors to activate the relevant intracellular signaling
pathway so that signal starts to flow through the network.
Often this involves local production near the cell surface
membrane of important signaling molecules, such as
second messengers (e.g., cAMP) as well as local activation
of protein kinases, protein phosphatases and other signaling
components. Such local production and transient elevation
of activated signaling components ensures that sufficient
signal flows through to evoke a local physiological
response. Such regions are called microdomains, and can
be described as dynamic regions of micrometer/sub-
micrometer dimensions with increased concentration of
one or more signaling components
[90]
.
A microdomain has two identifying characteristics:
a point at which the activated signaling component reaches
its highest concentration, and a gradient that details how the
concentration changes from the highest point to the
surrounding neighboring regions. The slope of the gradient
indicates the edges of the microdomain
[91]
. Typically
these are not absolute values of the activated components;
rather, they represent levels capable of transmitting signals
to downstream components that can in turn evoke physio-
logical responses. Combination of experiments together
with PDE models has identified a number of variables that
participate in controlling the nature and behavior of
microdomains. These include the role of narrow regions
with high surface to volume ratios that enable high local
concentrations of GTPases
[92,93]
. Other interdependent
factors include the topology of the signaling network, cell
shape, diffusion coefficients and reaction kinetics (for an
extended discussion see
[91]
). Advances in live cell
imaging have enabled the demonstration and importance
of microdomains
diffusion
model, using geometries obtained from real neurons, reac-
tions are dominant over diffusion in the dendritic arbors.
However, when the dendritic diameter increases, as seen for
the primary dendrites or when the cell body is considered,
the role of diffusion becomes critical for microdomain
characteristics. Thus local shape and cellular geometry are
important controllers of spatial signaling.
Simulations identified that a second important factor
controlling microdomain
e
for
several
signaling molecules,
96]
, ions such as Ca
2
รพ
[97
including cAMP
[90,94
99]
,
e
e
GTPases
[92,100]
and protein kinases
[101
103]
.Hahn
and co-workers have developed genetically encoded
e
dynamics
is
the
negative
