Transcript Principles of Cell Communication

Principles of Cell Communication
Extracellular signal molecules bind
to specific receptors
Cells communicate by hundreds of signal molecules.
These include proteins, small peptides, amino acids,
nucleotides, steroids, retinoids, fatty acids, and dissolved
gases.
The target cell responds by means of a specific protein
called a receptor.
It binds the signal molecule and then initiates a response.
The extracellular signal molecule often act at very low
concentrations (<10^-8 M) and the receptors that bind
them have a high affinity (Ka > 10^8 L/M).
These receptors are transmembrane proteins on the target
cell surface.
When they bind an extracellular signal molecule (a ligand),
they become activated and generate a cascade of
intracellular signals that alter the behaviour of the cell.
Different cells can respond differently to the
same extracellular signal molecule
The specific way in which a cell reacts to its environment
varies.
It varies according to the set of receptor proteins the cell
possesses, which determines the particular subset of signals
it can respond to, and it varies according to the
intracellular machinery by which the cell integrates and
interprets the signals it receives.
Thus, a single signal molecule often has different effects on
different target cells.
The same signal molecule bind to identical receptor
proteins yet produces very different responses in different
types of target cells, reflecting differences in the internal
machinery to which the receptors are coupled.
Nitric oxide signals by binding directly to an
enzyme inside the target cell
Although most extracellular signals are hydrophilic
molecules that bind to receptors on the surface of the
target cell, some signal molecules are hydrophobic enough
and small enough to pass readily across the target cellplasma membrane.
NO functions to regulate smooth muscle contractions, for
nerve cells to signal to their neighbors, and is produced as
a local mediator by macrophages and neutrophils to assist
in killing microbes.
NO is made by the deamination of arginine, catalyzed by
NO synthase.
Because it passes across membranes, NO rapidly diffuses
out of the cell and into neighboring cells.
It acts locally because it has a half-life of 5-10 seconds
(converted to nitrates and nitrites).
NO binds to iron in the active site of the enzyme guanylyl
cyclase to produce cGMP inducing vasodilatation.
CO is another gas that is used as an intercellular signal.
It can act in the same way as NO, by stimulating guanylyl
cyclase.
Nuclear receptors are ligandactivated gene regulatory proteins
A number of small hydrophobic signal molecules diffuse
directly across the plasma membrane and bind to
intracellular receptor proteins.
These signal molecules include steroid hormones, thyroid
hormones, retinoids, and vitamin D.
When these signals bind to their receptors they bind to
DNA to regulate the transcription of specific genes.
These signal molecules are insoluble and are made soluble
for transport in the bloodstream by binding to carrier
proteins.
Steroid hormones persist in the blood for hours.
Therefore, water-soluble signal molecules usually mediate
responses of short duration whereas insoluble ones tend to
mediate responses that are longer lasting.
The intracellular receptors for these molecules all bind to
specific DNA sequences adjacent to the genes that the
ligand regulates.
Some receptors are located in the cytosol and enter the
nucleus after binding; others are bound directly to DNA
without a ligand.
The ligand binding also causes the receptor to bind to
coactivator proteins that induce gene transcription.
The transcription process response usually takes place in
successive steps: the direct activation of a small number of
specific genes occurs within about 30 minutes and
constitutes the primary response; the protein products of
these genes in turn activate other genes to produce a
delayed, secondary response; and so on.
The responses to steroid and thyroid hormones, vitamin D,
and retinoids, like responses to extracellular signals in
general, are determined as much by the nature of the
target cell as by the nature of the signal molecule.
The Three Largest Classes of Cell-Surface Receptor
Proteins Are Ion-Channel-linked, G-Protein-linked,
and Enzyme-linked Receptors
All water-soluble signal molecules (including
neurotransmitters and all signal proteins) bind to specific
receptor proteins on the surface of the target cells that they
influence.
These cell-surface receptor proteins act as signal
transducers.
Most cell-surface receptor proteins belong to one
of three classes.
– Ion-channel-linked receptors, also known as
transmitter-gated ion channels or ionotropic
receptors, are involved in rapid synaptic signaling
between electrically excitable cells.
– G-protein-linked receptors act indirectly to regulate the
activity of a separate plasma-membrane-bound target
protein, which can be either an enzyme or an ion
channel.
– Enzyme-linked receptors, when activated, either
function directly as enzymes or are directly associated
with enzymes that they activate.
Signaling through G-Protein-Linked CellSurface Receptors
G-protein-linked receptors form the largest family of cellsurface receptors and are found in all eukaryotes.
Despite the chemical and functional diversity of the signal
molecules that bind to them, all G-protein-linked receptors
have a similar structure.
They consist of a single polypeptide chain that threads
back and forth across the lipid bilayer seven times and are
therefore sometimes called serpentine receptors.
Trimeric G Proteins Disassemble to Relay
Signals from G-Protein-linked Receptors
When extracellular signaling molecules bind to serpentine
receptors, the receptors undergo a conformational change
that enables them to activate trimeric GTP-binding
proteins (G proteins).
These G proteins are attached to the cytoplasmic face of
the plasma membrane, where they serve as relay
molecules, functionally coupling the receptors to enzymes
or ion channels in this membrane.
G proteins are composed of three protein subunits—α, β,
and γ. In the unstimulated state, the α subunit has GDP
bound and the G protein is inactive.
When stimulated by an activated receptor, the α subunit
releases its bound GDP, allowing GTP to bind in its place.
This exchange causes the trimer to dissociate into two
activated components—an α subunit and a βγ complex.
GTP binding causes a conformational change that affects
the surface of the α subunit that associates with the βγ
complex in the trimer.
This change causes the release of the βγ complex, but it
also causes and the α subunit to adopt a new shape that
allows it to interact with its target proteins.
The βγ complex does not change its conformation, but the
surface previously masked by the α subunit is now
available to interact with a second set of target proteins.
The α subunit is a GTPase, and once it hydrolyzes its
bound GTP to GDP, it reassociates with a βγ complex to reform an inactive G protein, reversing the activation
process.
The time during which the α subunit and βγ complex
remain apart and active is usually short, and it depends on
how quickly the α subunit hydrolyzes its bound GTP.
The GTPase activity of the α subunit is greatly enhanced
by the binding of a second protein, which can be either its
target protein or a specific modulator known as a regulator
of G protein signaling (RGS).
Some G Proteins Signal By Regulating the
Production of Cyclic AMP
Cyclic AMP is synthesized from ATP by a plasmamembrane-bound enzyme adenylyl cyclase, and it is
rapidly and continuously destroyed by one or more cyclic
AMP phosphodiesterases that hydrolyze cyclic AMP to
adenosine 5′-monophosphate (5′-AMP).
Many extracellular signal molecules work by increasing
cyclic AMP content, and they do so by increasing the
activity of adenylyl cyclase rather than decreasing the
activity of phosphodiesterase.
Adenylyl cyclase is a large multipass transmembrane
protein with its catalytic domain on the cytosolic side of the
plasma membrane.
All receptors that act via cyclic AMP are coupled to a
stimulatory G protein (Gs), which activates adenylyl
cyclase and thereby increases cyclic AMP concentration.
Another G protein, called inhibitory G protein (Gi),
inhibits adenylyl cyclase, but it mainly acts by directly
regulating ion channels rather than by decreasing cyclic
AMP content.
Cyclic-AMP-dependent Protein Kinase (PKA)
Mediates Most of the Effects of Cyclic AMP
Although cyclic AMP can directly activate certain types of
ion channels in the plasma membrane of some highly
specialized cells, in most animal cells it exerts its effects
mainly by activating cyclic-AMP-dependent protein kinase
(PKA).
This enzyme catalyzes the transfer of the terminal
phosphate group from ATP to specific serines or threonines
of selected target proteins, thereby regulating their activity.
In the inactive state, PKA consists of a complex of two
catalytic subunits and two regulatory subunits.
The binding of cyclic AMP to the regulatory subunits alters
their conformation, causing them to dissociate from the
complex.
The released catalytic subunits are thereby activated to
phosphorylate specific substrate protein molecules.
Some G Proteins Activate the Inositol
Phospholipid Signaling Pathway by Activating
Phospholipase C-β
Many G-protein-linked receptors exert their effects mainly
via G proteins that activate the plasma-membrane-bound
enzyme phospholipase C-β.
The phospholipase acts on an inositol phospholipid (a
phosphoinositide) called phosphatidylinositol 4,5bisphosphate [PI(4,5)P2], which is present in small
amounts in the inner half of the plasma membrane lipid
bilayer.
The activated phospholipase cleaves PI(4,5)P2 to generate
two products: inositol 1,4,5-trisphosphate and
diacylglycerol.
At this step, the signaling pathway splits into two branches.
Inositol 1,4,5-trisphosphate (IP3) is a small, water-soluble
molecule that leaves the plasma membrane and diffuses
rapidly through the cytosol.
When it reaches the endoplasmic reticulum (ER), it binds
to and opens IP3-gated Ca2+-release channels in the ER
membrane.
Ca2+ stored in the ER is released through the open
channels, quickly raising the concentration of Ca2+ in the
cytosol.
Diacylglycerol remains embedded in the membrane, where
it has two potential signaling roles.
First, it can be further cleaved to release arachidonic acid,
which can either act as a messenger in its own right or be
used in the synthesis of other small lipid messengers called
eicosanoids.
The second, and more important, function of diacylglycerol
is to activate a crucial serine/threonine protein kinase
called protein kinase C (PKC), so named because it is Ca2+dependent.
The initial rise in cytosolic Ca2+ induced by IP3 alters the
PKC so that it translocates from the cytosol to the
cytoplasmic face of the plasma membrane.
There it is activated by the combination of Ca2+,
diacylglycerol, and the negatively charged membrane
phospholipid phosphatidylserine.
Once activated, PKC phosphorylates target proteins that
vary depending on the cell type.
Some G Proteins Directly Regulate Ion
Channels
In some other cases, G proteins directly activate or
inactivate ion channels in the plasma membrane of the
target cell, thereby altering the ion permeability—and
hence the excitability of the membrane.
Acetylcholine released by the vagus nerve, for example,
reduces both the rate and strength of heart muscle cell
contraction.
Once activated, the α subunit of Gi inhibits adenylyl
cyclase, while the βγ complex binds to K+ channels in the
heart muscle cell plasma membrane to open them.
The opening of these K+ channels makes it harder to
depolarize the cell, which contributes to the inhibitory
effect of acetylcholine on the heart.
Other trimeric G proteins regulate the activity of ion
channels less directly, either by stimulating channel
phosphorylation (by PKA, PKC, or CaM-kinase, for
example) or by causing the production or destruction of
cyclic nucleotides that directly activate or inactivate ion
channels.
Signaling through Enzyme-Linked CellSurface Receptors
Enzyme-linked receptors are a second major type of cellsurface receptor.
They were recognized initially through their role in
responses to extracellular signal proteins that promote the
growth, proliferation, differentiation, or survival of cells in
animal tissues.
The responses to them are typically slow (on the order of
hours) and usually require many intracellular signaling
steps that eventually lead to changes in gene expression.
Enzyme-linked receptors have since been found also to
mediate direct, rapid effects on the cytoskeleton,
controlling the way a cell moves and changes its shape.
Like G-protein-linked receptors, enzyme-linked receptors
are transmembrane proteins with their ligand-binding
domain on the outer surface of the plasma membrane.
Instead of having a cytosolic domain that associates with a
trimeric G protein, however, their cytosolic domain either
has an intrinsic enzyme activity or associates directly with
an enzyme.
Activated Receptor Tyrosine Kinases
Phosphorylate Themselves
The extracellular signal proteins that act through receptor
tyrosine kinases consist of a large variety of secreted
growth factors and hormones.
Notable examples discussed include epidermal growth
factor (EGF), platelet-derived growth factor (PDGF),
fibroblast growth factors (FGFs), hepatocyte growth factor
(HGF), insulin, insulinlike growth factor-1 (IGF-1),
vascular endothelial growth factor (VEGF), macrophagecolony-stimulating factor (M-CSF), and all the
neurotrophins, including nerve growth factor (NGF).
In all cases, the binding of a signal protein to the ligandbinding domain on the outside of the cell activates the
intracellular tyrosine kinase domain.
Once activated, the kinase domain transfers a phosphate
group from ATP to selected tyrosine side chains, both on
the receptor proteins themselves and on intracellular
signaling proteins that subsequently bind to the
phosphorylated receptors.
How does the binding of an extracellular ligand activate
the kinase domain on the other side of the plasma
membrane?
For a G-protein-linked receptor, ligand binding is thought
to change the relative orientation of several of the
transmembrane α helices, thereby shifting the position of
the cytoplasmic loops relative to each other.
For the enzyme-linked receptors, two or more receptor
chains come together in the membrane, forming a dimer or
higher oligomer.
The rearrangement induced in cytosolic tails of the
receptors initiates the intracellular signaling process.
For receptor tyrosine kinases, the rearrangement enables
the neighboring kinase domains of the receptor chains to
cross-phosphorylate each other on multiple tyrosines, a
process referred to as autophosphorylation.
To activate a receptor tyrosine kinase the ligand usually
has to bind simultaneously to two adjacent receptor chains.
Autophosphorylation of the cytosolic tail of receptor
tyrosine kinases contributes to the activation process in
two ways.
First, phosphorylation of tyrosines within the kinase
domain increases the kinase activity of the enzyme.
Second, phosphorylation of tyrosines outside the kinase
domain creates high-affinity docking sites for the binding
of a number of intracellular signaling proteins in the target
cell.
Each type of signaling protein binds to a different
phosphorylated site on the activated receptor because it
contains a specific phosphotyrosine-binding domain that
recognizes surrounding features of the polypeptide chain in
addition to the phosphotyrosine.
Once bound to the activated kinase, the signaling protein
may itself become phosphorylated on tyrosines and
thereby activated; alternatively, the binding alone may be
sufficient to activate the docked signaling protein.
In summary, autophosphorylation serves as a switch to
trigger the transient assembly of a large intracellular
signaling complex, which then broadcasts signals along
multiple routes to many destinations in the cell.