Cell signaling

1. CELL SIGNALING
One characteristic common to all organisms is the dynamic ability tocoordinate constantly their
activities with environmental changes.The function of communicating with the environment is
achievedthrough a number of pathways that receive and process signals originatingfrom the
external environment, from other cells within theorganism and also from different regions within
the cell.
Communication is the name of the game, especially in cells; specialized receptorscalled integrins
provide vital communication links between the interior and exteriorof the cell. Integrins are
transmembrane proteins that act as mechanotransducersand signal conductors, providing a
physical link between the extracellular matrix(ECM) and the cell’s cytoskeleton. Although
integrins do not have intrinsic enzymaticactivity, they can interact with enzymes such as kinases
that have specific signalingfunctions. Integrins are involved in many cellular processes, such as
differentiation,migration, proliferation, ECM protein expression, activation of growth
factors,apoptosis, and cell survival. Interestingly, integrins work from either direction: Theycan
bind to extracellular ligands, thus triggering intracellular signal cascades, or theycan be activated
by factors from within the cell to influence the relationship of the cellwith its environment.
The formation and maintenance of the specialized tissues of multicellularorganisms depend on
the coordinated regulation of cellnumber, cell morphology, cell location and expression of
differentiatedfunctions. Such coordination results from a complex networkof communication
between cells in which signals produced affect targetcells where they are transduced into
intracellular biochemical reactionsthat dictate the physiological function of the target cell.
The basis for the coordination of the physiological functionswithin a multicellular organism is
intercellular signaling (or intercellularcommunication), which allows a single cell to influence
the behaviorof other cells in a specific manner. As compared to single-cellorganisms, where all
cells behave similarly within a broad frame,multicellular organisms contain specialized cells
Fig.1.

2. forming distinct tissuesand organs with specific functions. Therefore, higher organismshave to
coordinate a large number of physiological activitiessuch as:
– Intermediary metabolism.
– Response to external signals.
– Cell growth.
– Cell division activity.
– Differentiation and development: coordination of expressionprograms.
– Cell motility.
– Cell morphology.
Signals generated during intercellular communication must be receivedand processed in the
target cells to trigger the many intracellularbiochemical reactions that underlie the various
physiologicalfunctions of an organism. Typically, many steps are involved in theprocessing of
the signal within the cell, which is broadly describedas intracellular signaling.Importantly,both
intercellular and intracellular signaling is subjectedto regulatory mechanism that allows
thecoordination of cellular functionsin a developmental and tissue-specific manner.
In higher organisms intercellular signaling pathways have the importanttask of coordinating and
regulating cell division. The pathwaysensure that cells divide synchronously and, if necessary,
arrestcell division and enter a resting state.Cellular communication assumes great importance in
the differentiationand development of an organism. The development of an organismis based on
genetic programs that always utilize inter- andintracellular signaling pathways. Signal molecules
produced by onecell influence and change the function and morphology of othercells in the
organism.
Intercellular signaling pathways are also critical for the processingof sensory information.
External stimuli, such as optical and acousticsignals, stress, gradients of nutrients, etc.,
areregistered in sensorycells and are transmitted to other cells of the organism via
intercellularsignaling pathways.
Tools for Intercellular Signaling
We currently know of various forms of communication between cells:
–Extracellular messengers. Cells send out signals in the form of specificmessenger molecules
that the target cell transmits into a biochemicalreaction. Signaling cells can simultaneously
influencemany cells by messenger molecules so as to enable a temporallycoordinated reaction in
an organism.
–Gap junctions. Communication between bordering cells is possiblevia direct contact in
theform of “gap junctions”. Gap junctions arechannels that connect two neighboring cells to
allow a directexchange of metabolites and signaling molecules between the cells.
–Cell–cell interaction via cell surface proteins. Another form of directcommunication between
cells occurs with the help of surfaceproteins. In this process a cell surface protein of one cell
binds aspecific complementary protein on another cell. As a consequenceof the complex
formation, an intracellular signal chain is activatedwhich initiates specific biochemical reactions
in the participatingcells. Communication is then only possible upon direct contactbetween the
target cells with the surface protein of the partner cell.
–Electrical signaling. A further intercellular communication mechanismrelies on electrical
processes. The conduction of electricalimpulses by nerve cells is based on changes in the
membranepotential. The nerve cell uses these changes to communicate withother cells at
specialized nerve endings (synapses). It is central tothis type of intercellular communication that

3. electrical signals canbe transformed into chemical signals. This type of communicationwill not
be discussed in this book.
Steps of Intercellular Signaling:
In the communication between cells of an organism, the signals(messengers such as hormones)
are produced in specialized cells.The signal-producing function of these cells is itself regulated,
sothat the signal is only produced upon a particular stimulus. In thisway signaling pathways can
be coupled to one another and coordinated.
The following steps are involved in intercellular communication.
1. Formation of a Signal in the Signal-producing Cell as a Result of an External
Trigger:
Most extracellular messengers are produced in response to externaltriggers and are released by
exocytosis. Physical stimuli like electricalsignals, changes in ion concentration or, most
frequently, other extracellularsignaling molecules serve as a trigger to increase the amountof the
messenger available for extracellular communication. The mechanisms by which the external
trigger signals increase the amountof extracellular messenger are diverse, and include stimulation
of thebiosynthesis of the messenger, increased production of the maturemessenger from
precursors and release of the messenger from astore form. The latter mechanism is extensively
used in the releaseof hormones of the neural system (neurotransmitters) in responseto electrical
signals, e.g. at synapses.
2. Transport of the Signal to the Target Cell:
The extracellular signal produced may be distributed via the circulationor it may reach the target
cell simply by diffusion. In long-rangesignaling via the circulation, the extracellular messenger is
oftenbound to specific carrier proteins or incorporated into larger proteincomplexes. This may
serve to prevent degradation in the extracellularmedium or to provide for docking to specific
cells only. Furthermore,processing or metabolization of a messenger during transport
mayconvert it from an inactive form to an active form.
3. Registration of the Signal in the Target Cell:
A target cell that receives a signal within the framework of intercellularcommunication transmits
the signal in intracellular pathways thattrigger distinct biochemical activities in a cell-type-
specific mannerand determine the response of the target cell. Specialized proteins,termed
receptors, are utilized for the reception of signals in the targetcell. Only those cells that carry the
appropriate receptor will be activatedfor further transduction of the signal into the interior of the
cell.
There are two principal ways by which target cells can process incoming signals:
– Cell surface receptors receive the signal (e.g. a messenger substance)at the outside of the cell,
become activated and initiate asignaling chain in the interior of the cell. In such signaling
pathways,the membrane-bound receptor transduces the signal at thecell membrane so that it is
not necessary for the signal to actuallyenter the cell.
– The messenger enters into the target cell and binds and activatesthe receptor localized in the
cytosol or nucleus.The result of communication between the signaling and receivingcells is a
defined biochemical reaction in the target cell. The natureand extent of this reaction depends on
many individual reactions thatparticipate either directly or indirectly in signal transduction.
Beginning with the hormone-producing cell, the following processesare all contributing factors
for hormonal signal transduction in higher organisms (Fig. 2)

4. – Biosynthesis of the hormone. The enzymes involved in biosynthesisof a hormone can, for
example, be controlled by other signal transductionpathways. There may be feedback
mechanisms that couplethe activity of the biosynthetic enzymes to the concentration of the
circulating enzyme via allosteric mechanisms.
– Degradation and modification of the hormone. The active hormone maybe inactivated by
metabolization or inactive hormone precursors maybe converted into the active hormone by
enzymatic transformation.
– Storage and secretion of the hormone. There are signals (electricalsignals, Ca2+ signals) to
trigger the secretion of stored hormones.
– Transport of the hormone to the target cell. The distribution of ahormone via the circulation
contributes to the accessibility of thathormone at a particular location of an organism.
– Reception of the signal by the hormone receptor. The hormonereceptors are primarily
responsible for the registration of the signaland the further transduction of the signal in
intracellular signalingpaths.
Hormones in Intercellular Signaling:
Signaling molecules for the communication between cells are knownas hormones. Hormones
that are proteins and regulate cell proliferationare known as growth factors. The hormones are
produced in specializedcells (the hormone-producing cells) or they may be introducedinto the
organism as inactive precursors (e.g. vitamins) thatrequire metabolic activation for generation of
the active form. Examplesof the latter type include vitamin D and retinoic acid. Typically,the
.The individual steps
of intercellular communication.
Upon reception of a triggering
stimulus, the signal is
transformed into a chemical
messenger within the
signalingcell. The messenger is
secretedand transported to the
targetcell, where the signal is
registered,transmitted further
and finally converted into a
biochemical reaction.
Not shown are processes of
termination or regulation of
communication which can act at
any of the above steps.
(Fig.2)

5. hormone-producing cells contain biosynthetic pathways that areresponsible for the production of
the hormone. Furthermore, hormonesmay be specifically inactivated by modifying enzymes.
Endocrine, Paracrine and Autocrine Signaling:
Various forms of intercellular communication by hormones can bediscerned based on the range
of the signal transmission (Fig.2.1)
Endocrine Signaling:In endocrine signaling, the hormone messenger is synthesized
inspecific signaling, or endocrine, cells and exported via exocytosisinto the extracellular medium
(e.g. blood or lymphatic fluid in animals).The hormone is then distributed throughout the entire
bodyvia the circulatory system so that remote regions of an organismcan be reached. Only those
cells or tissues elicit a hormonal response which contains the appropriate receptors for the
hormone.
Paracrine Signaling: Paracrine signal transduction occurs over the medium range.
Thehormone reaches the target cells from the hormone-producing cellby passive diffusion. The
producing cell must be found in the vicinityof the receiving cells for this type of communication.
The signalingis rather local and the participating signaling molecules are sometimes termed
tissue hormones or local mediators. A special caseof paracrine signal transduction is synaptic
neurotransmission inwhich a nerve cell communicates with either another nerve cell orwith a
muscle cell.
Autocrine Signaling: In autocrine signaling, cells of the same type communicate with
oneanother. The hormone produced by the signaling cell affects a cell ofthe same type by
binding to receptors on these cells, initiating an intracellularsignal cascade. If an autocrine
(a)Endocrinal signal
transduction:
The hormone is formedin the
specialized endocrinal tissue is
released into the
extracellularmedium and
transported via
the circulatory system to the target
cells.
(Fig.3)

6. hormone issecreted simultaneouslyby many cells then a strong response occurs in the
cells.Autocrine mechanisms are of particular importance in the immuneresponse.
(Fig.4) (b) Paracrinal signaltransduction: the hormonereaches the target cell, which
isfound in closejuxtaposition tothe hormone producing cell, viadiffusion.
(c) Autocrinal signaltransduction: the hormone actson the same cell type as theone in which it is
produced.
Intracellular Signaling: Basics
External signals such as hormones or sensory signals are specificallyrecognized by receptors that
transduce the external signal into an intracellularsignaling chain. The intracellular signaling paths
controlall functions of the cell such as intermediary metabolism, cell divisionactivity,
morphology and the transcription programme.
Reception of External Signals:
Extracellular signal molecules generally fall into two classes: The first and the largestclass
ofsignals consist of molecules that are too large or too hydrophilic to cross theplasma membrane
of the target cell.They rely on the receptors on the surface of the target cellto relay their massage
across the membrane.
The second class of signals consists ofmolecules that are small enough or hydrophobic enough to
slip easily through theplasma membrane. Once inside, these signal molecules either activate
intracellularenzymes or bind to intracellular receptors proteins that regulate gene expression.
Cells use two principal ways for transducing external signals into intracellularsignaling paths. In
one way, the signal receipt and signaltransduction occurs at the cell membrane bytransmembrane

7. (TM) receptorsthat register the signal at the cell membrane. In the other way,the messenger
passes the cell membrane and binds to the receptorthat is localized in the cytosol or in the
nucleus. Uponreceiving a signal, the receptor becomes activated to transmit the signalfurther.
The activated receptor passes the signal onto components,usually proteins, further downstream in
the intracellular signalingpathway, which then become activated themselves for further
signaltransmission. Depending on the nature of the external stimulus, distinctsignaling paths are
activated. Finally, specific biochemical processesare triggered in the cell, whichrepresent the
endpoints ofthe signaling pathway.
(Fig.5)
Activation and Deactivation of Signaling Proteins:
Intracellular signal transduction usually comprises many componentsoften acting in a sequential
manner where one componentpasses the signal on to the next component. The key functions in
intracellularsignaling are performed by proteins that have the abilityto specifically recognizes,
process and transduce signals. The majorsignal transducers are:
– Receptors.
– Signaling enzymes.
– Regulatory GTPases.
In the absence of the signal, the signal transducers exist in an inactiveor less-active ground state.
Upon receipt of the signal, the signal transducersbecome activated and transit into the activestate.

8. Only if in theactive state is further transmission of the signal to the next signalingcomponent
possible. The active state is then terminated after some time by deactivation
processes and the signal transducer transitsback into the inactive state from which it may start
another round ofactivation and deactivation. A multitude of mechanismsare used to activate the
signaling proteins. The major mechanisms are:
– Binding of other signaling molecules.
– Conformational transitions.
– Covalent modifications.
– Membrane targeting.
– Compartmentalization.
Following activation, the signaling protein must be deactivated inorder to terminate or attenuate
signaling. By restraining the lifetimeof the activated state with the help of specific deactivation
mechanisms,the signal flow can be controlled and fine-tuned, and it canbe coordinated with
signaling through other signaling paths. Themechanisms for deactivation are variable. The
deactivation mechanismmay be intrinsic to the signaling protein and may be enhancedby specific
accessory proteins like GTPases. Otherdeactivation mechanisms use signal-directed inhibitory
modificationsof the signaling protein such as phosphorylation. The removalof activating
modifications by specific enzyme systems is anotherway for terminating signaling. The many
ways of activating and inactivatingsignaling proteins are illustrated best by the example ofthe
protein kinases.
(Fig.6)
Activation and
deactivation
of signaling proteins.
Activating signals trigger
the transition from the
inactive ground state into
the active state from
which signals are passed
on to the next signaling
component.
Deactivating orregulatory
signals limit the lifetime
of the activated state and
induce return into the
ground state.

9. Intracellular Receptors:
The most prominent class of the intracellular or cytoplasmic receptorscomprises the nuclear
receptors that are found in the cytosoland/or in the nucleus. To activate the nuclear receptors,the
hormone must penetrate the target cell by passive diffusion. Thenuclear receptors can be
classified as ligand-controlled transcriptionactivators. The hormone acts as the activating ligand;
the activatedreceptor stimulates the transcriptional activity of genes which carryDNA elements
specific for the receptor.
Interaction between Hormone and Receptor:
Receptors are the specific binding partners for signaling molecules;the former are able to
recognize and specifically bind the latter basedon their chemical structure. The binding and
recognition are governedby the same principles and the same non-covalent interactionsas those
for the binding of a substrate to an enzyme, i.e. hydrogenbonds, electrostatic interactions
(including dipole–dipole interactions),van der Waals interactions and hydrophobic interactions.
Signalingmolecules bind their cognate receptors with an affinity greaterthan that usually
observed for an enzyme and substrate.
The binding of a hormone to a receptor can in most cases be describedby the simple reaction
scheme:
[H] + [R] [HR], with KD = [H]·[R]/[HR]
Where [H] is the concentration of free hormone, [R] is the concentrationof the free receptor and
[HR] is the concentration of hormone-receptor complex. The value for the equilibrium constant
of dissociation,KD, usually lies in the range of 10–6 to 10–12 M. This simpleformalism is only
applicable to cytoplasmic receptors. For membrane-bound receptors, a quantitative treatment of
the binding equilibriumis much more difficult.
In contrast to the steroid and thyroid hormones, the vast majority of signalmolecules are too large
or too hydrophilic to cross the plasma membrane of the targetcell. Most cell surface receptor
proteins belong to one of three large families:
–Ion channel-linked receptors
–G-protein-linked receptors
–Enzyme-linked receptors
These families differ in the nature of the intracellular signal that they generate whenextracellular
signal molecules binds to them.
•For ion-channel-linked receptors, the resulting signal is a flow of ions across themembrane,
which produces electrical current.

10. •G-protein-linked receptors activate a class of membrane bound proteins.
•Enzyme-linked receptors act as an enzyme or are associated with enzymesinside the cell.
Ion-channel-linked Receptors:
Of all the cell-surface types, ion channel-linked receptors function in the simplest anddirect
way.These receptors are responsible for the rapid transmissions of signalsacross synapses in the
nervous system.They transduce a chemical signals in the form of a pulse of
neurotransmitterdelivered to the outside of the target cell.When neurotransmitter binds, this type
of receptor alters its conformation so as toopen or close channel for the flow of specific types of
ions (such as Na+, K+, Ca2+)
G-Protein-Linked Receptors:
G-protein-linked receptors form the largest family of cell-surface receptors, with hundredsof
members already identified in mammalian cells.They mediate response to anenormous diversity
of extracellular signal molecules, including hormones, localmediators, and
neurotransmitters.These signal molecules are as varied in structureas they are in function: they
can be proteins, small peptides, or derivatives of aminoacids or fatty acids, and for each one of
them there is a different receptor.Despite thediversity of signal molecules that bind to them, all
G-protein-linked receptors that havebeen analyzed possess a similar structure: each made of a
single polypeptide chainthreads back and forth cross the lipid bilayer seven times.
ENZYME-LINKED RECEPTORS:
Enzyme-linked receptors are membrane bound proteins that display their ligandbinding domains
on the outer surface of the plasma membrane. Instead of associatingwith a G protein, however,
the cytoplasmic domain of the receptor acts as an enzymeor forms a complex with another
protein that acts as an enzyme. Enzyme-linkedreceptors play major role in cell differentiation
growth and cell survival. They are alsomediate direct, rapid reconfigurations of the cytoskeleton,
controlling the way a cellmoves and changes its shape.
Signaling Enzymes:
Signaling enzymes are at the heart of intracellular signaling. Enzymescan be regulated by
allosteric transitions in response to bindingof effector molecules, by covalent modifications such
as phosphorylationor by membrane targeting. These mechanisms serve toinduce the transition of
enzymes from an inactive or low activestate into the active state, making enzymes the ideal
instrumentfor the receipt and transmission of signals. The most prominent signalingenzymes are
the protein kinases and protein phosphatases,the enzymes involved in synthesis and degradation
of second messengersand the regulatory GTPases. We know of different ways bywhich enzymes
participate in signaling:
– Signaling enzymes modify other enzymes or proteins to carry the signal on or to terminate
signaling. The most frequently used tool for signaltransmission in a cell is the reversible
modification of proteins byphosphorylation that serves to activate or inactivate signaling
proteins.The phosphorylation status of a protein is controlled by theactivity of protein kinases
and protein phosphatases.Both classes of enzymes are elementary components of

11. signalingpathways and their activity is subject to manifold regulation. Theimportance of the
protein kinases for cellular functions is illustratedby the large number (more than 500) of
different protein kinasesencoded in the human genome.
– Signaling enzymes can catalyze the formation, degradation or release of small-molecule
effectors – the second messengers. The enzymes involvedin the formation or degradation of
second messengers,such as the phospholipases or the adenylyl cyclases, are majorcomponents of
signaling pathways, and are reversiblyactivated and inactivated during signal transduction.
–Regulatory GTPases switch between active and inactive conformations, depending on the
binding of GDP or GTP. The regulatory GTPasesfunction as switches that can exist in an active,
GTPboundstate or the inactive, GDP-bound state. In the active state,the GTPases can transmit
signals to downstream components inthe signaling chain. In the inactive state, signal
transmission isrepressed and an activating upstream signal in the form ofexchange of bound
GDP by GTP is required in order to activatethe GTPase for further signal transmission.
Adaptors and Scaffolding Proteins:
Adaptor proteins do not harbor enzyme activities. Rather,adaptor proteins mediate the signal
transmission between proteinsof a signaling chain by bringing these proteins together. They
function as clamps to colonialize proteins for an effective and specific signaling.Furthermore,
adaptor proteins help to target signaling proteinsto specific subcellular sites and to recruit
signaling moleculesinto multi-protein signaling complexes.In the latter case, the adaptor proteins
may function as a scaffoldor docking site for assembly of different signaling molecules at
distinctsites. The proteins are then also termed docking or scaffoldingproteins. Typically,
scaffolding proteins contain several binding domainswith distinct binding specificities for
complementary siteson the target proteins. Furthermore, adaptor proteins are often subjectedto
regulatory modifications, e.g. phosphorylations, which providesignal-directed docking sites for
signaling proteins.
Diffusible Intracellular Messengers: Second Messengers
The intracellular activation of enzymes in a signaling chain can leadto the formation of diffusible
small signaling molecules in the cell.These intracellular signaling molecules are also termed
“secondmessengers”. The second messenger molecules activateand recruit cognate enzymes for
the further signal transduction.The following properties are important for the function of
diffusibleintracellular messengers:
–Second messengers may be rapidly formed from precursors by enzymatic reactions.
Typically, the enzymes involved in formation of secondmessengers are parts of signaling
pathways and are activated duringsignaling to produce the second messenger in a
regulatedmanner. Often, these enzymes have high turnover numbers andcan form a large amount
of second messenger leading to highlocal concentrations.
–Second messengers may be rapidly released from intracellular stores.For example, the second
messenger Ca2+ is stored in specificcompartments and is released from the storage upon a
regulatorysignal. This mechanism provides for the fast and locally controlledproduction of the
second messenger.

12. –Second messengers may be rapidly inactivated or stored in specific compartments. To allow
for a termination of the second messengerfunction, the messengers are degraded by specific
enzymes or theyare removed by storage or transport into the extracellular medium. As for the
messenger-producing enzymes, themessenger degrading enzymes can be part of signaling
pathsand are regulated by distinct signals.
–Second messengers may activate different effector proteins. Bindingsites for a particular
second messenger (Ca2+, cAMP) may occur ondifferent signaling proteins. This property allows
a given secondmessenger to regulate multiple target proteins which leads to adiversification and
variability of second messenger signaling.
–Second messengers allow amplification of signals. The enzymaticproduction of large amounts
of a messenger makes an importantcontribution to the amplification of signals. Typically, the
enzymesinvolved in the metabolism of the second messengers are activatedby upstream signals.
During the lifetime of the activated state, theenzymes may produce large amounts of the second
messengerallowing for the activation of a large number of further downstreammessenger targets.
Reference Books:
-Biochemistry of Signal Transduction and Regulation
By Gerhard Krauss
-Molecular Biology of the Cell
By Alberts • Johnson • Lewis • Raff • Roberts • Walter
-Cell Communication: The inside story
By John D.Scott & Tony Pawson
-Basic Concepts in Biochemistry
By Hiram F. Gilbert