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CELL COMMUNICATION
Olli-Pekka Koistinen
65195H
S-114.2500 Cellbiosystems
28.11.2007
Contents
1. Introduction
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2. Forms of cell-to-cell signalling
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3. Principles of specific cellular responding
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4. Membrane-passing signal molecules: nitric oxide
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5. Membrane-passing hormones and nuclear receptors
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6. Cell-surface receptors
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7. Intracellular signalling proteins
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8. Intracellular signalling complexes and binding domains
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9. Variability of signalling responses
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10. References
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1. Introduction
This is an exercise work for the course Cellbiosystems in Helsinki University of Technology. In
statistical physics a system is defined as a large group of identical units. When we think about the
term cellbiosystem (which by the way is not even recognized by most text editors), the first
conclusion is that we are talking about large groups of cells and their collaboration. This definition
differs from the statistical point of view, though, because animal cells aren’t exactly identical units.
As a matter of fact they are anything but identical and that – the collaboration of highly specialized
cells – is the essential reason why we are such complex beings. For about 2 500 000 000 years all
life on Earth was unicellular. Evolution of multicellular organisms was that slow, because it
demanded above all efficient and complex communication mechanisms between the cells, so that
they could govern their own behaviour for the benefit of the whole organism.
These communication mechanisms are based on extracellular signal molecules, which are produced
by cells to signal to other cells. Cells in multicellular organisms communicate by hundreds of kinds
of signal molecules, like small peptides, amino acids, proteins, nucleotides, retinoids, steroids, fatty
acid derivatives or even nitric oxide or carbon monoxide. In most cases the signal molecules get out
from the cell by exocytosis. Some molecules are released by diffusion through the plasma
membrane and some remain bound to the surface of the cell.
Whatever kind of signal we have, the target cell always responds by a specific protein called a
receptor, which binds the signal molecule and begins the response process in the target cell. The
extracellular signal molecules usually act at low concentrations (less than 0,00000001 M) and the
receptors often bind them with high affinity (affinity constant more than 100 000 000 litres/mole).
In most of the cases the receptors that recognize the signal molecules are transmembrane proteins
on the surface of the target cell. When the extracellular signal molecule (called a ligand) is attached
to them, they become active and produce a set of intracellular signals that different kinds of
intracellular signalling proteins distribute in different locations in the cell. At the ends of these
intracellular signalling pathways there are target proteins, which change the behaviour of the cell by
altering when the pathway is active. In other cases the receptors are located inside the target cell, so
that the signal molecules have to be hydrophobic and small enough to diffuse through the plasma
membrane.
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2. Forms of cell-to-cell signalling
There are basically four forms of intercellular signalling: contact-dependent signalling, paracrine
signalling, synaptic signalling and endocrine signalling. In the contact-dependent case signal
molecules remain bound to the surface of the signalling cell influencing only cells that are in
contact with it. This form of signalling is important in immune responses and during development.
Usually signal molecules are secreted, though, so they are able to travel also to distant targets.
When the secreted signal molecules affect only cells nearby them, they act as local mediators, and
the process in question is paracrine signalling. To keep the signalling area small, paracrine signal
molecules are often taken up by neighbouring cells, immobilized by the extracellular matrix or
destroyed by extracellular enzymes.
One way to transmit signals to distant places is synaptic signalling, which is based on specialized
cells called nerve cells as in neurons. When neurons are activated by signals from the environment
or other nerve cells, they send fast electric impulses called action potentials along its axon. The
axon ends at a specialized cell junction called synapse, and when the impulses gets there, they make
the nerve terminals secrete chemical signals called neurotransmitters into the synapse, where the
signal finally reaches the postsynaptic target cell. Synaptic signalling is very fast and precise, since
electrical signals can proceed at rates of up to 100 metres per second and the neurotransmitter has to
diffuse not more than 100 nanometres in the synapse.
Beside neurons, another type of specialized signalling cell is an endocrine cell, which secretes its
signal molecules, as in hormones, into bloodstream, where they can drift to different locations in the
body. Endocrine signalling is therefore much slower and less precise than synaptic signalling.
Another difference between these two ways of distant signalling is that hormones must be able to
work at low concentrations (less than 0,00000001 M), whereas neurotransmitters can achieve about
0,0001 M local concentrations in the synapse.
Cells often send their signals to different kind of target cells, but target cell can also be a cell of the
same type as the signalling cell or even the signalling cell itself. In autocrine signalling a cell
secretes signal molecules that are able to bind to the cell’s own receptors. This can be utilized for
example in development: when a developmental decision is made, it can be reinforced by starting to
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secrete autocrine signals. This is much more effective, when autocrine signalling is performed
simultaneously by the whole group of identical cells that have made the same decision.
The drawback of autocrine signalling is that cancer cells can use it to overcome the normal control
on cell proliferation and survival. Using autocrine signalling they can stimulate their own survival
and proliferation and occupy places that would not be possible for normal cells.
One way to communicate with neighbouring cells is through specialized cell-cell gap junctions.
These junctions can form between closely apposed plasma membranes, connecting the cytoplasmas
of the cells directly by narrow water channels. The junctions allow only small intracellular
molecules, like cyclic AMP or calcium ions, to pass through them, but prevent the progress of
larger molecules like nucleic acids and proteins.
The specific functions and the importance of different small molecules in communication through
gap junctions are still uncertain. It has been noticed, though, that cells in a developing embryo form
and cut these connections in specific patterns, which could imply that gap junctions are important in
the signalling processes between these cells. For example, deficiency of a gap-junction protein
called connexin has caused defects in heart development in mice and humans.
3. Principles of specific cellular responding
To give some perspective, a typical cell recognizes hundreds of different signalling molecules,
which can be soluble, bound to the extracellular matrix or to the surface of a neighbouring cell and
act as millions of combinations. These combinations are responded selectively, according to the
specific character of the target cell, which has developed in the cell specialization. In addition, the
cell may respond to combinations by differentiating or multiplying or by using some specialized
function like contraction or secretion. So in principle, there are almost unlimited amount of different
signal combinations, and this enables animals to control their cells in extremely specific ways.
In most animal cells there are particular combinations of signals that simply keep the cell alive.
When lacking these survival signals, a cell death program called apoptosis is activated. This keeps
different cells in their own environments, since the combinations of survival signals are different for
different types of cells.
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The way in which the cell responds to a particular set of signals depends on both the set of receptor
proteins it possesses and the intracellular machinery by which it interprets the received signals. The
same signal molecule can affect on different target cells even in opposite ways. For example
acetylcholine decreases the force of contraction in the case of heart muscle cells but increases it in
the case of skeletal muscle cells because of their different receptor proteins. And even if the
receptor proteins were identical, the effect can still be different because of the intracellular
machinery.
Some transient signals, especially during development, cause effects that last indefinitely, but in
most of the cases in adult tissues the response ceases when the signal ends. Since the signal exerts
its effect by altering a set of unstable molecules, it is quite obvious that the speed with which the
signal removal is responded depends on the rate of destruction (and reconstruction) of the molecules
the signal affects. What is perhaps a little less obvious is that this same turnover rate defines also
the response rate when the signal begins. Let’s illustrate this with a simple example.
Assume that we have intracellular signal molecules A and B, which are maintained at concentration
of 1000 molecules per cell. The turnover rates of A and B are 100 and 10 synthesized and degraded
molecules per second, whereas their lifetimes are correspondingly 10 and 100 seconds. If a signal
affects on the cell by multiplying the turnover rates of both A and B by ten, after one second the
concentrations of A and B have increased 90 and 900 molecules per cell. Thus, the shorter lifetime
(and faster turnover rate) a molecule has, the faster is the concentration change when the turnover
rate is rapidly increased or decreased.
The same rules apply also to extracellular molecules as well as proteins. Usually some proteins
carry regulatory roles, whereupon their concentrations are easily regulated by for example covalent
modification, most commonly phosphate addition to an amino acid side chain.
4. Membrane-passing signal molecules: nitric oxide
Most extracellular signal molecules are hydrophilic and they bind to receptors on the surface of the
target cell. There are however some gases, like nitric oxide and carbon dioxide, that are small
enough to get through the plasma membrane and thus regulate directly the activity of intracellular
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proteins. Let’s have a little closer look on nitric oxide (NO), an important and common signal
molecule.
One of the functions of nitric oxide is regulation of smooth muscle contraction. For example blood
vessels are covered by smooth muscles, and the vessel walls are relaxed by the effects of nitric
oxide. The nitric oxide secretion in neighbouring endothelial cells is stimulated by acetylcholine
released by autonomic nerves of the vessel walls. Such effect of nitric acid explains that
nitroglycerine reduces the pain of angina patients: In the body, nitroglycerine breaks into nitric
oxide, which reduces the oxygen requirement of the heart muscle by relaxing blood vessels. NO is
used in many kinds of nerve cells to signal to the nearby cells. For example in penis, it causes
erection by dilating local blood vessels. They also act as local mediators by activating macrophages
and neutrophiles that kill intruder organisms.
In many target cells, nitric oxide binds to the enzyme guanylyl cyclase, and more specifically to
iron in its active site. When guanylyl cyclase is activated, it starts to produce cyclic GMP, a small
intracellular mediator. Cyclic GMP has normally fast turnover rate, so nitric oxide can affect in only
a few seconds. The turnover rate is based on fast degradation to GMP by a phosphodiesterase that is
continuously balancing the cyclic GMP generation from GTP by guanylyl cyclase. As a curiosity,
the potency drug Viagra works by inhibiting the cyclic GMP phosphodiesterase in the penis, so that
the cyclic GMP level remains elevated a longer time. Because the cyclic GMP keeps the blood
vessels relaxed, the drug helps the penis to erect.
5. Membrane-passing hormones and nuclear receptors
Nitric oxide and carbon monoxide work by stimulating intracellular enzymes (specifically guanylyl
cyclase). There are however a group of nongaseous hormones and local mediators, which are as
well small and hydrophobic, and thus pass the plasma membrane of the target cell. The difference
of them from the gases is that these molecules bind – instead of enzymes – to intracellular proteins
that directly regulate specific gene transcription by binding to DNA. Steroid hormones, thyroid
hormones, retinoids and vitamin D belong to this group.
The receptors for these molecules are part of the nuclear receptor superfamily, which contains also
some proteins activated by intracellular metabolites instead of secreted signal molecules. An
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important part of the molecules of this family are so called orphan nuclear receptors, which have
been detected only by DNA sequencing so that their ligands have remained unknown.
Steroid hormones include cortisol, the steroid sex hormones, vitamin D (in vertebrates) and an
insect moulting hormone called ecdysone. They are all made from cholesterol and they are
relaticely insoluble in water. However, steroids can be transported in extracellular fluids by carrier
proteins, which they bind to and dissociate from before passing to the target cell. Compared to
water-soluble signal molecules there are also a remarkable difference between the times they
survive in the extracellular fluids. Water-soluble hormones are often removed or broken down in
minutes and local mediators and neurotransmitters in seconds or milliseconds, whereas steroids can
persist for hours and thyroids even for days. This matter of fact accounts for the character of these
hormones water-soluble signal molecules mediating responses of short duration and water-insoluble
ones those of longer duration.
The intracellular receptors for the ligand group of steroids, thyroids, retinoids and vitamin D bind to
specific DNA sequences, which are located next to the genes the ligand regulates. Some of them,
like the receptors for cortisol, are first in the cytosol and move to the nucleus only after activated,
but some are already bound to the DNA even without activation. In both of the cases ligand binding
alters the conformation of the receptor protein, so that the inhibitory complex it is bound to is
removed. Instead, the bound ligand makes the receptor bind to specific coactivator proteins
inducing gene transcription.
The gene transcription response happens usually in a few phases. The primary response, a direct
activation of a small number of specific genes, can take about 30 minutes. These genes, in turn,
produce proteins that activate other genes that cause the secondary response, and so on. Thus a
simple hormonal stimulation can well trigger even a complex change in gene expression. The
process becomes even more complex and competent, when we grasp the fact that a eucaryotic gene
usually needs more than one type of gene regulatory protein binding to it to activate the
transcription. Because only certain cell types have receptor proteins for a particular hormone and
each of these types have different combination of other cell-type-specific gene regulatory proteins
to work with it, the hormone has different effects on different animal cell types even if they both
had the same receptor for it.
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6. Cell-surface receptors
All water-soluble signal molecules work by binding to cell-surface receptor proteins. These
receptors are so called signal transducers, as in they convert the extracellular ligand binding into
intracellular signals. The largest classes of such cell-surface receptor proteins are ion-channellinked receptors, G-protein-linked receptors and enzyme-linked receptors.
Ion-channel-linked receptors (or transmitter-gated ion channels or ionotropic receptors) are used for
fast synaptic signalling between electrically excitable cells. They are a large family of homologous,
multipass transmembrane proteins acting as ion-channels temporarily opened and closed by a small
amount of neurotransmitters binding to them. This changes the ion permeability of the plasma
membrane as well as the excitability of the postsynaptic cell.
G-protein-linked receptors use a separate cell-surface protein, like an enzyme or an ion channel, to
regulate the cell behaviour. There is a third protein, called a trimeric GTP-binding protein (G
protein), that mediates the interaction between these two plasma-membrane-bound proteins. All the
G-protein-linked receptors belong to a large family of homologous, seven-pass transmembrane
proteins.
Enzyme-linked receptors, in turn, either act as enzymes or are associated with enzymes activating
them. They are single pass transmembrane proteins, the ligand-binding site outside and the catalytic
or enzyme-binding site inside the cell. The majority of them are protein kinases or at least
associated with them, which means that when ligand binds to them, a specific combination of
proteins phosphorylates. Enzyme-linked receptors are not, though, as homologous in structure as
the two first classes of cell-surface receptor proteins.
Outside these three classes there are still some cell-surface receptor proteins, some of which are
dependent on intracellular proteolytic events.
7. Intracellular signalling proteins
Both G-protein-linked and enzyme-linked receptors send the received signals into the cell interior
by a chain of intracellular signalling events. These chains contain both small intracellular mediators
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or second messengers and large intracellular signalling proteins. The small second messengers are
produced in large amounts and they are fast to diffuse and carry the signal to other parts of the cell.
The large signalling proteins, in turn, can work by producing small intracellular mediators or
relaying the signal to another signalling protein. These proteins can be roughly divided to following
seven groups according to their functions:
1. Relay proteins move the signal to the next member in the chain.
2. Messenger proteins transport the signal between different parts of the cell.
3. Amplifier proteins increase the signal by activating large amounts of signal molecules.
4. Transducer proteins change the form of the signal.
5. Bifurcation proteins spread the signal to other pathways.
6. Integrator proteins unite signals from two or more pathways.
7. Latent gene regulatory proteins are activated at the cell surface and move to the nucleus to trigger
gene transcription.
In addition, there are modulator proteins (to modify signalling protein activity), adaptor proteins
that bind signalling proteins together), anchoring proteins (to keep specific signalling proteins still)
and scaffold proteins (to both bind signalling proteins together and keep them still), which are also
important in intracellular signalling.
Some intracellular signalling proteins act as molecular switches. Some signal makes them active
and another turns them off. The switches operate in two different ways. The majority of them is
activated or inactivated by the addition and removal of phosphate groups. Phosphorylation is caused
by protein kinase and the reverse process by phosphatase. Many of these signalling proteins are
themselves protein kinases and they often act as parts of phosphorylation cascade, where the signal
is relayed forward between consecutive protein kinases. In the process signal may be amplified or
spread to other pathways.
The other class of signalling proteins that act as molecular switches are GTP-binding proteins as in
G proteins. They are also activated by the influence of phosphate groups, but the phosphate appears
in GTP and GDP bound to the protein. When a GTP-binding protein is activated, it has intrinsic
GTPase activity and it shut itself off by hydrolyzing the GTP to GDP.
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8. Intracellular signalling complexes and binding domains
Even if we have a single extracellular signal influencing through a single type of G-protein-linked
or enzyme-linked receptor, they usually trigger several signalling chains and thus cause several kind
of changes in the behaviour of the cell. It is even usual that these two cell-surface receptor protein
types activate some same pathways. It is still not fully understood how cells are able to respond so
specifically to such a diversity of extracellular signals, even though many of them can bind to the
same receptor type and activate same pathways.
One answer to clear this problem is offered by scaffold proteins, which organizes signalling
proteins into signalling complexes. By avoiding unwanted cross-talk between different pathways,
scaffold proteins may enhance the precision, speed and efficiency of the signal. Signalling
complexes can also form only temporarily, when signalling proteins gather around the
phosphorylated cytoplasmic tail of the activated receptor. In another transient case, receptor
activation causes the adjacent plasma membrane to produce modified phospholipids that collects
specific intracellular signal proteins at them.
Intracellular signalling proteins have certain binding domains, compact protein modules that bind to
particular motif in another molecule or a lipid. These modular domains allow signalling proteins to
bind to one another in numerous combinations and even form three-dimensional networks of
interactions. Further combining of these interactions also makes it easy for new signalling pathways
to evolve.
Important binding domains are for example src homology 2 (SH2) domains and phosphotyrosinebinding (PTB) domains that bind to phosphorylated tyrosines in a particular peptide sequence and
src homology 3 (SH3) domains that bind to short proline-rich amino acid sequence. Pleckstrin
homology (PH) domains, in turn, bind to specific phosphorylated inositol phospholipids produced
by plasma membrane, and so they are used for anchoring the protein on the membrane. PDZ
domains often appear on scaffold proteins, where each PDZ domain in the same protein usually has
a specific motif. Some proteins in the signalling pathway basically consist solely of binding
domains. These proteins only combine two other signalling proteins together.
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9. Variability of signalling responses
Some responses that occur by cells, like the primary responses to steroid hormones, depend quite
linearly on the concentration of the signal molecule. Many responses, though, are much more
abrupt, when the concentration increases properly, and some even seem to follow an all-or-nothing
principle. The explanation for this is that these responses require more than one intracellular
molecule or complex to simultaneously bind to the same target macromolecule. Such responses
sharpen when the amount of cooperating molecules increases, and at some point they start to look
like an all-or-nothing curve.
True all-or-nothing responses can be produced by another mechanism, using so called positive
feedback. Action potentials generated by nerve and muscle cells in response to neurotransmitters
represent this kind of pattern. The mechanism is based on a net influx of sodium ions, first caused
by the activation of ion-channel-linked acetylcholine receptors. The influx depolarizes the
membrane locally, which causes the neighbouring voltage-gated sodium channels to open. This
strengthens the influx of sodium ions, which depolarizes the membrane even more and opens even
more sodium ion channels. If the initial depolarization is strong enough, the process explodes and
spreads to the whole membrane.
Cells can detect various kinds of stimuli and changes in their signal intensities over a wide range.
This leads to the fact that the target cells must be able to adapt their sensitivity reversibly. The
adaptation mechanism is based on negative feedback that occurs delayed. For example in chemical
signalling, relatively equal changes in ligand concentrations can be responded similarly in spite of
the actual concentrations. The decensitization towards the signal molecule can be caused by ligandinduced receptor endocytosis, by fast inactivation of the receptors, by a change in some protein that
affects on the transduction process of the signal or by an inhibitor production that prevents the
transduction process to go on.
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10. References
Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter:
Molecular Biology of the Cell, chapter 15.
Fourth Edition. Garland Science, Taylor & Francis Group. 2002.
ISBN 0-8153-4072-9 (Paperback)
Jyrki Heino, Matti Vuento:
Biokemian ja solubiologian perusteet, chapter 11.
First Edition. WSOY Oppimateriaalit Oy. 2007.
ISBN 978-951-0-32563-6
Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Cell_signaling, 24.11.2007.
http://en.wikipedia.org/wiki/Image:MAPKpathway.png, 27.11.2007.
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