Dr. Surraiya ShaikhAssistant Professor

Biochemistry SMC, DUHS 16/11/2010

CELL COMMUNICATION

Section A: An Overview of Cell Signaling

2. Communicating cells may be close together or far

apart

3. The three stages of cell signaling are reception,

transduction, and response

• Cell-to-cell communication is absolutely essential for multicellular organisms.– Cells must communicate to coordinate their

activities.• Communication between cells is also important

for many unicellular organisms.• Biologists have discovered some universal

mechanisms of cellular regulation, involving the same small set of cell-signaling mechanisms.

• Cells may receive a variety of signals, chemical signals, electromagnetic signals, and mechanical signals.

Introduction

• Multicellular organisms also release signaling molecules that target other cells.– Some transmitting cells release local regulators that

influence cells in the local vicinity.– Paracrine signaling occurs

when numerous cells can simultaneously receive and respond to growth factors produced by a single cell in their vicinity.

2. Communicating cells may be close together or far apart

Fig. 11.3a1

• In synaptic signaling, a nerve cell produces a neurotransmitter that diffuses to a single cell that is almost touching the sender.– An electrical signal passing along the nerve cell

triggers secretion of the neurotransmitter into the synapse.

– Nerve signals can travel along a series of nerve cells without unwanted responses from other cells.

Fig. 11.3a2

• Plants and animals use hormones to signal at greater distances.– In animals, specialized endocrine cells release

hormones into the circulatory system, by which they travel to target cells inother parts of the body.

– In plants, hormones may travel in vessels, but more often travel from cell to cell or by diffusion in air.

Fig. 11.3b

• Hormones and local regulators range widely in size and type.

– The plant hormone ethylene (C2H4), which promotes fruit ripening and regulates growth, is a six atom hydrocarbon.

– Insulin, which regulates sugar levels in the blood of mammals, is a protein with thousands of atoms.

• Cells may communicate by direct contact.– Signaling substances dissolved in the cytosol pass

freely between adjacent cells.

– Cells may also communicate via direct contact between substances on their surfaces.

Fig. 11.4

3. The three stages of cell signaling reception, transduction, response

• The process must involve three stages.– In reception, a chemical signal binds to a cellular

protein, typically at the cell’s surface.– In transduction, binding leads to a change in the

receptor that triggers a series of changes along a signal-transduction pathway.

– In response, the transduced signal triggers a specific cellular activity.

Fig. 11.5

CELL COMMUNICATION

Section B: Signal Reception and the Initiation of Transduction

1.A signal molecule binds to a receptor protein,

causing the protein to change shape

2. Most signal receptors are plasma membrane proteins

• A cell targeted by a particular chemical signal has a receptor protein that recognizes the signal molecule.– Recognition occurs when the signal binds to a specific site on

the receptor because it is complementary in shape.• When ligands (small molecules that bind specifically to a larger

molecule) attach to the receptor protein, the receptor typically undergoes a change in shape.– This may activate the receptor so that it can interact with other molecules.– For other receptors this leads to aggregation of receptors.

1. A signal molecule binds to a receptor protein causing the protein to change shape

• Most signal molecules are water-soluble and too large to pass through the plasma membrane.

• They influence cell activities by binding to receptor proteins on the plasma membrane.– Binding leads to change in the shape or the receptor

or to aggregation of receptors.– These trigger changes in the intracellular

environment.• Three major types of receptors are G-protein-

linked receptors, tyrosine-kinase receptors, and ion-channel receptors.

2. Most signal receptors are plasma membrane proteins

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• A G-protein-linked receptor consists of a receptor protein associated with a G-protein on the cytoplasmic side.– The receptor consists of seven alpha helices

spanning the membrane.– Effective signal

molecules include

epinephrine, other hormones, and neurotransmitters.

Fig. 11.6

• The G protein acts as an on-off switch.– If GDP is bound, the G protein is inactive.– If ATP is bound, the G protein is active.

Fig. 11.7a

• The G-protein system cycles between on and off.– When a G-protein-linked receptor

is activated by binding with an extracellular signal molecule, the receptor binds to an inactive G protein in membrane.

– This leads the G protein to substitute GTP for GDP.

– The G protein then binds with another membrane protein, often an enzyme, altering its activity and leading toa cellular response.

Fig. 11.7b

– The G protein can also act as a GTPase enzyme and hydrolyzes the GTP, which activated it, to GDP.

– This change turns the G protein off.– The whole system can be shut down quickly when

the extracellular signal molecule is no longer present.

Fig. 11.7c

• G-protein receptor systems are extremely widespread and diverse in their functions.

– In addition to functions already mentioned, they play an important role during embryonic development and sensory systems.

• Several human diseases are the results of activities, including bacterial infections, that interfere with G-protein function.

• The tyrosine-kinase receptor system is especially effective when the cell needs to regulate and coordinate a variety of activities and trigger several signal pathways at once.

• Extracellular growth factors often bind to tyrosine-kinase receptors.

• The cytoplasmic side of these receptors function as a tyrosine kinase, transferring a phosphate group from ATP to tyrosine on a substrate protein.

• A individual tyrosine-kinase receptors consists of several parts:– an extracellular signal-binding sites,– a single alpha helix spanning the membrane, and– an intracellular

tail with several tyrosines.

Fig. 11.8a

• When ligands bind to two receptors polypeptides, the polypeptides aggregate, forming a dimer.

• This activates the tyrosine-kinase section of both.

• These add phosphates to the tyrosine tails of the other polypeptide.

Fig. 11.8b

– One tyrosine-kinase receptor dimer may activate ten or more different intracellular proteins simultaneously.

• These activated relay proteins trigger many different transduction pathways and responses.

Fig. 11.8b

• Ligand-gated ion channels are protein pores that open or close in response to a chemical signal.– This allows or blocks ion flow,

such as Na+ or Ca2+.– Binding by a ligand to the

extracellular side changes the protein’s shape and opens the channel.

– Ion flow changes the concentration inside the cell.

– When the ligand dissociates, the channel closes.

• Ligand-gated ion channels are very important in the nervous system.

– Similar gated ion channels respond to electrical signals.

• Other signal receptors are dissolved in the cytosol or nucleus of target cells.

• The signals pass through the plasma membrane.

• These chemical messengers include the hydrophobic steroid and thyroid hormones of animals.

• Also in this group is nitric oxide (NO), a gas whose small size allows it to slide between membrane phospholipids.

• Testosterone, like other hormones, travels through the blood and enters cells throughout the body.

• In the cytosol, they bind and activate receptor proteins.

• These activated proteins enter the nucleus and turn on genes that control male sex characteristics.

Fig. 11.10

• These activated proteins act as transcription factors.– Transcription factors control which genes are

turned on - that is, which genes are transcribed into messenger RNA (mRNA).• The mRNA molecules leave the nucleus and carry

information that directs the synthesis (translation) of specific proteins at the ribosome.

• Other intracellular receptors are already in the nucleus and bind to the signal molecules there (e.g., estrogen receptors).

CELL COMMUNICATION

Section C: Signal-Transduction Pathways

1.Pathways relay signals from receptors to cellular

responses

2. Protein phosphorylation, a common mode of

regulation in cells, is a major mechanism of signal

transduction

3. Certain small molecules and ions are key components

of signaling pathways (second messengers)

• The transduction stage of signaling is usually a multistep pathway.

• These pathways often greatly amplify the signal.– If some molecules in a pathway transmit a signal to

multiple molecules of the next component, the result can be large numbers of activated molecules at the end of the pathway.

• A small number of signal molecules can produce a large cellular response.

• Also, multistep pathways provide more opportunities for coordination and regulation than do simpler systems.

Introduction

– The signal-activated receptor activates another protein, which activates another and so on, until the protein that produces the final cellular response is activated.

• The original signal molecule is not passed along the pathway and may not even enter the cell.– Its information is passed on.– At each step the signal is transduced into a different

form, often by a conformational change in a protein.

1. Pathways relay signals from receptors to cellular responses

• The phosphorylation of proteins by a specific enzyme (a protein kinase) is a widespread cellular mechanism for regulating protein activity.– Most protein kinases act on other substrate proteins,

unlike the tyrosine kinases that act on themselves.• Most phosphorylation occurs at either serine or

threonine amino acids of the substrate protein.

Protein phosphorylation, a common mode of regulation in cells, is a major mechanism of signal transduction

• Many of the relay molecules in a signal-transduction pathway are protein kinases that lead to a “phosphorylation cascade”.

• Each protein phosphorylation leads to a shape change because of the interaction between the phosphate group and charged or polar amino acids.

Fig. 11.11

• Phosphorylation of a protein typically converts it from an inactive form to an active form.– The reverse (inactivation) is possible too for some

proteins.

• A single cell may have hundreds of different protein kinases, each specific for a different substrate protein.– Fully 1% of our genes may code for protein kinases.

• Abnormal activity of protein kinases can cause abnormal cell growth and contribute to the development of cancer.

• The responsibility for turning off a signal-transduction pathway belongs to protein phosphatases.– These enzymes rapidly remove phosphate groups

from proteins.– The activity of a protein regulated by

phosphorylation depends on the balance of active kinase molecules and active phosphatase molecules.

• When an extracellular signal molecule is absent, active phosphatase molecules predominate, and the signaling pathway and cellular response are shut down.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Many signaling pathways involve small, nonprotein, water-soluble molecules or ions, called second messengers.– These molecules rapidly diffuse throughout the cell.

• Second messengers participate in pathways initiated by both G-protein-linked receptors and tyrosine-kinase receptors.– Two of the most important are cyclic AMP and Ca2+.

3. Certain signal molecules and ions are key components of signaling pathways (second

messengers)

• Binding by epinephrine leads to increases in the concentration of cyclic AMP or cAMP.– This occurs because the receptor activates adenylyl

cyclase which converts ATP to cAMP.– cAMP is short-lived as phosphodiesterase converts it

to AMP.

Fig. 11.12

• More generally, many hormones and other signals trigger the formation of cAMP.– Binding by the signal to a receptor activates a G

protein that activates adenylyl cyclase in the plasma membrane.

– The cAMP from the adenylyl cyclase diffuses through the cell and activates a serine/threonine kinase, called protein kinase A which phosphorylates other proteins.

Fig. 11.13

• Other G-protein systems inhibit adenylyl cyclase.– These use a different signal molecule to activate

other receptors that activate inhibitory G proteins.• Certain microbes cause disease by disrupting

the G-protein signaling pathways.– The cholera bacterium, Vibrio cholerae, colonizes the small

intestine and produces a toxin that modifies a G protein that regulates salt and water secretion.

– The modified G protein is stuck in its active form, continuously stimulating productions of cAMP.

– This causes the intestinal cells to secrete large amounts of water and salts into the intestines, leading to profuse diarrhea and death if untreated.

• Many signal molecules in animals induce responses in their target cells via signal-transduction pathways that increase the cytosolic concentration of Ca2+.– In animal cells, increases in Ca2+ may cause

contraction of muscle cells, secretion of some substances, and cell division.

– In plant cells, increases in Ca2+ trigger responses for coping with environmental stress, including drought.

• Cells use Ca2+ as a second messenger in both G-protein pathways and tyrosine-kinase pathways.

• The Ca2+ concentration in the cytosol is typically much lower than that outside the cell, often by a factor of 10,000 or more.– Various protein pumps

transport Ca2+ outside the cell or inside the endoplasmic reticulum or other organelles.

Fig. 11.14

• Because cytosolic Ca2+ is so low, small changes in the absolute numbers of ions causes a relatively large percentage change in Ca2+ concentration.

• Signal-transduction pathways trigger the release of Ca2+ from the cell’s ER.

• The pathways leading to release involve still other second messengers, diacylglycerol (DAG) and inositol trisphosphate (IP3).– Both molecules are produced by cleavage of

certain phospholipids in the plasma membrane.

• DAG and IP3 are created when a phospholipase cleaves a membrane phospholipid PIP2.– Phospholipase may be activated by a G protein or a

tyrosine-kinase receptor.– IP3 activates a gated-calcium channel, releasing

Ca2+.

Fig. 11.15

• Calcium ions may activate a signal-transduction pathway directly.

• Alternatively, Ca2+ binds to the protein calmodulin.– This protein is present at high levels in eukaryotes.

• When calmodulin is activated by Ca2+, calmodulin binds to other proteins, either activating or inactivating them.– These other proteins are often protein kinases and

phosphatases - relay proteins in signaling pathways

CELL COMMUNICATION

Section D: Cellular Responses to Signals

1.In response to a signal, a cell may regulate

activities in the cytoplasm or transcription in the

nucleus

2. Elaborate pathways amplify and specify the cell’s

response to signals

• Ultimately, a signal-transduction pathway leads to the regulation of one or more cellular activities.– This may be a change in an ion channel or a change in

cell metabolism.– For example, epinephrine helps regulate cellular

energy metabolism by activating enzymes that catalyze the breakdown of glycogen.

1. In response to a signal, a cell may regulate activities in the cytoplasm or transcription in

the nucleus

• The stimulation of glycogen breakdown by epinephrine involves a G-protein-linked receptor, a G Protein adenylyl cyclase and cAMP, and several protein kinases before glycogen phosphorylase is activated. Fig.

11.16

• Other signaling pathways do not regulate the activity of enzymes but the synthesis of enzymes or other proteins.

• Activated receptors may act as transcription factors that turn specific genes on or off in the nucleus.

Fig. 11.17

• Signaling pathways with multiple steps have two benefits.– They amplify the response to a signal.– They contribute to the specificity of the response.

• At each catalytic step in a cascade, the number of activated products is much greater than in the preceding step.– In the epinephrine-triggered pathway, binding by a

small number of epinephrine molecules can lead to the release of hundreds of millions of glucose molecules.

2. Elaborate pathways amplify and specify the cell’s response to signals

• Various types of cells may receive the same signal but produce very different responses.– For example, epinephrine triggers liver or striated

muscle cells to break down glycogen, but cardiac muscle cells are stimulated to contract, leading to a rapid heartbeat.

• These differences result from a basic observation:– Different kinds of cells have different collections of

proteins.

• The response of a particular cell to a signal depends on its particular collection of receptor proteins, relay proteins, and proteins needed to carry out the response.

Fig. 11.18

• Two cells that respond differently to the same signal differ in one or more of the proteins that handle and respond to the signal.– A single signal may follow a single pathway in one

cell but trigger a branched pathway in another.– Two pathways may converge to modulate a single

response.

• Branching of pathways and interactions between pathways are important for regulating and coordinating a cell’s response to incoming information.

• Rather than relying on diffusion of large relay molecules like proteins, many signal pathways are linked together physically by scaffolding proteins.– Scaffolding proteins may themselves be relay

proteins to which several other relay proteins attach.

– This hardwiring enhances the speed and accuracy of signal transfer between cells.

Fig. 11.19

• The importance of relay proteins that serve as branch or intersection points is underscored when these proteins are defective or missing.

– The inherited disorder, Wiskott-Aldrich syndrome (WAS), is due to the absence of a single relay protein.

– It leads to abnormal bleeding, eczema, and a predisposition to infections and leukemia.

– The WAS protein interacts with the microfilaments of the cytoskeleton and several signaling pathways, including those that regulate immune cell proliferation.

– When the WAS protein is absent, the cytoskeleton is not properly organized and signaling pathways are disrupted.

• As important as activating mechanisms are inactivating mechanisms.– For a cell to remain alert and capable of responding

to incoming signals, each molecular change in its signaling pathways must last only a short time.

– If signaling pathway components become locked into one state, the proper function of the cell can disrupted.

– Binding of signal molecules to receptors must be reversible, allowing the receptors to return to their inactive state when the signal is released.

– Similarly, activated signals (cAMP and phosphorylated proteins) must be inactivated by appropriate enzymes to prepare the cell for a fresh signal.

•THANK YOU

Cell Signalling

To make multicellular organisms cell must communicate. This communication is mediated by extracellular signal molecules.

Sofisticated mechanisms control which signal molecules are released from a specific type of cell, at what time and concentration they are secreted, and how these signals are interpreted by the target cells

Some signalling molecules act over long distances, some act only on the immediate neighbour cells

Most cells in higher organisms are both emiters and receivers of signals

Lecture 1: General Principles of Cell Signalling

• Signals, receptors and mediators• The prototypical pheromone signalling pathway of budding yeast• Cell surface and intracellular receptors• Types of intercellular signalling• Nuclear receptor signalling• Types of cell surface receptors• Molecular switches: signalling through GTPases and protein phosphorylation

Lecture 2: Discovery and elucidation of novel signalling pathways: a case study

Budding yeast cells responding to mating factor.

(A)The cells are normally spherical.

(B)In response to mating factor secreted by neighbouring yeast cells, they put out a protrusion toward the source of the factor in preparation for mating.

The binding of extracellular signal molecules to either cell-surface receptors or intracellular receptors.

Most signal molecules are hydrophilic and are therefore unable to cross the plasma membrane directly; instead, they bind to cell-surface receptors, which in turn generate one or more signals inside the target cell.

Some small signal molecules, by contrast, diffuse across the plasma membrane and bind to receptors inside the target celleither in the cytosol or in the nucleus (as shown here). Many of these small signal molecules are hydrophobic and nearly insoluble in aqueous solutions; they are therefore transported in the bloodstream and other extracellular fluids after binding to carrier proteins, from which they dissociate before entering the target cell.

Forms of intercellular signaling.

(A) Contact-dependent signaling requires cells to be in direct membrane-membrane contact.

(B) Paracrine signaling depends on signals that are released into the extracellular space and act locally on neighboring cells.

(C) Synaptic signaling is performed by neurons that transmit signals electrically along their axons and release neurotransmitters at synapses, which are often located far away from the cell body.

(D) Endocrine signaling depends on endocrine cells, which secrete hormones into the bloodstream that are then distributed widely throughout the body. Many of the same types of signaling molecules are used in paracrine, synaptic, and endocrine signaling; the crucial differences lie in the speed and selectivity with which the signals are delivered to their targets.

The contrast between endocrine and synaptic signaling. In complex animals, endocrine cells and nerve cells work together to coordinate the diverse activities of the billions of cells. Whereas different endocrine cells must use different hormones to communicate specifically with their target cells, different nerve cells can use the same neurotransmitter and still communicate in a highly specific manner.

(A) Endocrine cells secrete hormones into the blood, which signal only the specific target cells that recognize them. These target cells have receptors for binding a specific hormone, which the cells “pull” from the extracellular fluid.

(B) In synaptic signaling, by contrast, specificity arises from the synaptic contacts between a nerve cell and the specific target cells it signals. Usually, only a target cell that is in synaptic communication with a nerve cell is exposed to the neurotransmitter released from the nerve terminal (although some neurotransmitters act in a paracrine mode, serving as local mediators that influence multiple target cells in the area).

An animal cell's dependence on multiple extracellular signals. Each cell type displays a set of receptors that enables it to respond to a corresponding set of signal molecules produced by other cells. These signal molecules work in combinations to regulate the behaviour of the cell. As shown here, an individual cell requires multiple signals to survive (blue arrows) and additional signals to divide (red arrow) or differentiate (green arrows). If deprived of appropriate survival signals, a cell will undergo a form of cell suicide known as programmed cell death, or apoptosis.

The nuclear receptor superfamily. All nuclear hormone receptors bind to DNA as either homodimers or heterodimers, but for simplicity we show them as monomers here. (A) The receptors all have a related structure. The short DNA-binding domain in each receptor is shown in green. (B) A receptor protein in its inactive state is bound to inhibitory proteins. Domain-swap experiments suggest that many of the ligand-binding, transcription-activating, and DNA-binding domains in these receptors can function as interchangeable modules. (C) The binding of ligand to the receptor causes the ligand-binding domain of the receptor to clamp shut around the ligand, the inhibitory proteins to dissociate, and coactivator proteins to bind to the receptor's transcription-activating domain, thereby increasing gene transcription. (D) The three-dimensional structure of a ligand-binding domain with (right) and without (left) ligand bound. Note that the blue α helix acts as a lid that snaps shut when the ligand (shown in red) binds, trapping the ligand in place.

Responses induced by the activation of a nuclear hormone receptor. (A) Early primary response and (B) delayed secondary response. The figure shows the responses to a steroid hormone, but the same principles apply for all ligands that activate this family of receptor proteins. Some of the primary-response proteins turn on secondary-response genes, whereas others turn off the primary-response genes. The actual number of primary- and secondary-response genes is greater than shown. As expected, drugs that inhibit protein synthesis suppress the transcription of secondary-response genes but not primary-response genes, allowing these two classes of gene transcription responses to be readily distinguished.

Three classes of cell-surface receptors.

(A) Ion-channel-linked receptors

(B) G-protein-linked receptors

(C) enzyme-linked receptors

Although many enzyme-linked receptors have intrinsic enzyme activity, as shown on the left, many others rely on associated enzymes

Different kinds of intracellular signaling proteins along a signaling pathway from a cell-surface receptor to the nucleus.

In this example, a series of signaling proteins and small intracellular mediators relay the extracellular signal into the cell, causing a change in gene expression.

The signal is amplified, altered (transduced), and distributed en route. Many of the steps can be modulated by other extracellular and intracellular signals, so that the final result of one signal depends on other factors affecting the cell.

Ultimately, the signaling pathway activates (or inactivates) target proteins that alter cell behavior. In this example, the target is a gene regulatory protein.

Two types of intracellular signaling proteins that act as molecular switches. In both cases, a signaling protein is activated by the addition of a phosphate group and inactivated by the removal of the phosphate. (A) The phosphate is added covalently to the signaling protein by a protein kinase. (B) A signaling protein is induced to exchange its bound GDP for GTP. To emphasize the similarity in the two mechanisms, ATP is shown as APPP, ADP as APP, GTP as GPPP, and GDP as GPP.

Extracellular signals A and B both activate a different series of protein phosphorylations, each of which leads to the phosphorylation of protein Y but at different sites on the protein. Protein Y is activated only when both of these sites are phosphorylated, and therefore it becomes active only when signals A and B are simultaneously present. For this reason, integrator proteins are sometimes called coincidence detectors.

Signal integration

Intracellular signalling complexes enhance the speed, the efficiency, and the specificity of the response

A specific signalling complex can be formed using modular interaction domains

Signalling through G-protein-coupled cell-surface receptors

Cyclic AMP is synthesized by the adenylyl cyclase from ATP. It is a cyclization reaction that removes two phosphates as pyrophosphate.

Cyclic AMP is short-lived. It is rapidly hydrolyzed by phosphodiesterases to give 5’-AMP as shown on the figure.

The pyrophosphate is hydrolyzed to inorganic phosphates. This reaction is the thermodynamic driver for the synthesis of cAMP.

A cultured nerve cell responding to the neurotransmiter serotonin. Serotonin acts through a GPCR and activates cAMP synthesis. The cells express a fluorescent proteins that changes its fluorescence upon binding of cAMP. Blue indicate low concentration of cAMP, yellow – intermediate and, red a high concentration of cAMP.