Introduction

Mechanisms enabling one cell to influence the behavior of another almost certainly existed in the world of unicellular organisms long before multicellular organisms appeared on earth. Evidence comes from studies of some present-day unicellular eucaryotes such as yeasts. Although these cells normally lead independent lives, they can communicate and influence one another's proliferation in preparation for sexual mating. In the budding yeast Saccharomyces cerevisiae, for example, when a haploid individual is ready to mate, it secretes a peptide mating factor that signals cells of opposite mating types to stop proliferating and prepare to conjugate; the subsequent fusion of two haploid cells of the opposite mating type produces a diploid cell, which can then undergo meiosis and sporulate to generate haploid cells with new assortments of genes.

Studies of yeast mutants that are unable to mate have identified many proteins that are required in the signaling process. These proteins form a signaling network that includes cell-surface receptors, GTP-binding proteins, and protein kinases, each of which has close relatives among the proteins involved in signaling in animal cells. Through gene duplication and divergence, however, the signaling systems in animals have become much more elaborate than those in yeasts.

Extracellular Signaling Molecules Are Recognized by Specific Receptors on or in Target Cells ²

Whereas yeast cells communicate with one another for mating by secreting several kinds of small peptides, cells in higher animals communicate by means of hundreds of kinds of signaling molecules, including proteins, small peptides, amino acids, nucleotides, steroids, retinoids, fatty acid derivatives, and even dissolved gases such as nitric oxide and carbon monoxide. Most of these signaling molecules are secreted from the signaling cell by exocytosis (discussed in Chapter 13). Others are released by diffusion through the plasma membrane, while some remain tightly bound to the cell surface and influence only cells that contact the signaling cell ( Figure 15-1).

Figure 15-1

Intercellular signaling in animals. Two ways that animal cells communicate with one another are illustrated.

Regardless of the nature of the signal, the target cell responds by means of a specific protein called a receptor. It specifically binds the signaling molecule and then initiates a response in the target cell. Many of the extracellular signaling molecules act at very low concentrations (typically ≤ 10^-8 M), and the receptors that recognize them usually bind them with high affinity (affinity constant K_a ≥ 10⁸ liters/mole; see Figure3-9). In most cases the receptors are transmembrane proteins on the target-cell surface; when they bind an extracellular signaling molecule (a ligand), they become activated so as to generate a cascade of intracellular signals that alter the behavior of the cell. In some cases, however, the receptors are inside the target cell and the signaling ligand has to enter the cell to activate them: these signaling molecules therefore must be sufficiently small and hydrophobic to diffuse across the plasma membrane ( Figure 15-2).

Figure 15-2

Extracellular signaling molecules bind to either cell-surface receptors or intracellular receptors. Most signaling molecules are hydrophilic and are therefore unable to cross the plasma membrane directly; instead, they bind to cell-surface receptors, (more...)

In this chapter we concentrate mainly on the communication between animal cells that is mediated by secreted chemical signals. This emphasis reflects the state of current knowledge: secreted molecules are very much easier to study than those that are membrane-bound, and we know much more about how they work. Contact-dependent signaling via membrane-bound molecules, although harder to study and less well understood, nonetheless, is crucially important, especially during development and in immune responses; its molecular basis can be very similar to that for signaling at a distance, as we see later.

Secreted Molecules Mediate Three Forms of Signaling: Paracrine, Synaptic, and Endocrine ²

Signaling molecules that a cell secretes may be carried far afield to act on distant targets, or they may act as local mediators, affecting only cells in the immediate environment of the signaling cell. This latter process is called paracrine signaling ( Figure 15-3A). For paracrine signals to be delivered only to their proper targets, the secreted signaling molecules must not be allowed to diffuse too far; for this reason they are often rapidly taken up by neighboring target cells, destroyed by extracellular enzymes, or immobilized by the extracellular matrix.

Figure 15-3

Three forms of signaling mediated by secreted molecules. 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 (more...)

For a large, complex multicellular organism, short-range signaling is not sufficient on its own to coordinate the behavior of the organism's cells. Sets of specialized cells have evolved with a specific role in signaling between widely separate parts of the body. The most sophisticated of these are nerve cells, or neurons, which typically extend long processes (axons) that contact target cells far away. When activated by signals from the environment or from other nerve cells, a neuron sends electrical impulses (action potentials) along its axon; when an impulse reaches the nerve terminals at the end of the axon, it stimulates the terminals to secrete a chemical signal called a neurotransmitter. The nerve terminals contact their target cell at specialized cell junctions called chemical synapses, which are designed to ensure that the neurotransmitter is delivered to the postsynaptic target cell rapidly and specifically ( Figure 15-3B). This synaptic signaling process is discussed in detail in Chapter 11 and will not be considered further here.

The other specialized signaling cells that control the behavior of the organism as a whole are endocrine cells. They secrete their signaling molecules, called hormones, into the bloodstream (of an animal) or the sap (of a plant), which carries the signal to target cells distributed widely throughout the body ( Figure 15-3C). The distinctive ways that endocrine cells and nerve cells coordinate cell behavior in animals are contrasted in Figure 15-4.

Figure 15-4

The contrast between endocrine and synaptic signaling. Endocrine cells and nerve cells work together to coordinate the diverse activities of the billions of cells in a higher animal. Endocrine cells secrete many different hormones into the blood to signal (more...)

Because endocrine signaling relies on diffusion and blood flow, it is relatively slow. Nerve cells, by contrast, can achieve much greater speed and precision. They can transmit information over long distances by electrical impulses that travel at rates of up to 100 meters per second; once released from a nerve terminal, a neurotransmitter has to diffuse less than 100 nm to the target cell, a process that takes less than a millisecond. Another difference between endocrine and synaptic signaling is that whereas hormones are greatly diluted in the bloodstream and interstitial fluid and therefore must be able to act at very low concentrations (typically < 10^-8M), neurotransmitters are diluted much less and can achieve high local concentrations. The concentration of acetylcholine in the synaptic cleft of an active neuromuscular junction, for example, is about 5 x 10^-4 M. Correspondingly, neurotransmitter receptors have a relatively low affinity for their ligand, which means that the neurotransmitter can dissociate rapidly from the receptor to terminate a response. (Neurotransmitters are quickly removed from the synaptic cleft either by specific hydrolytic enzymes or by specific membrane transport proteins that pump the neurotransmitter back into either the nerve terminal or neighboring glial cells.)

Autocrine Signaling Can Coordinate Decisions by Groups of Identical Cells ³

All of the forms of signaling discussed so far allow one cell type to influence another. By the same mechanisms, however, cells can send signals to other cells of the same type, and it follows from this that they can also send signals to themselves. In such autocrine signaling a cell secretes signaling molecules that can bind back to its own receptors. During development, for example, once a cell has been directed into a particular path of differentiation, it may begin to secrete autocrine signals that reinforce this developmental decision.

Because autocrine signaling is most effective when carried out simultaneously by neighboring cells of the same type, it may be used to encourage groups of identical cells to make the same developmental decisions ( Figure 15-5). Thus autocrine signaling is thought to be one possible mechanism underlying the "community effect" observed in early development, where a group of identical cells can respond to a differentiation-inducing signal but a single isolated cell of the same type cannot.

Figure 15-5

Autocrine signaling. A group of identical cells produces a higher concentration of a secreted signal than does a single cell.

Autocrine signaling is not confined to development, however. Eicosanoids are signaling molecules that often act in an autocrine mode in mature mammals. These fatty-acid derivatives are made by cells in all mammalian tissues. They are continuously synthesized in the plasma membrane and released to the cell exterior, where they are rapidly degraded by enzymes in extracellular fluid. Made from precursors (mainly arachidonic acid) that are cleaved from membrane phospholipids by phospholipases ( Figure 15-6), they have a wide variety of biological activities, influencing the contraction of smooth muscle and the aggregation of platelets, for example, and participating in pain and inflammatory responses. When cells are activated by tissue damage or by some types of chemical signals, the rate of eicosanoid synthesis is increased; the resulting increase in the local level of eicosanoid influences both the cells that make it and their immediate neighbors.

Figure 15-6

The synthesis of an eicosanoid. Eicosanoids are continuously synthesized in membranes from 20-carbon fatty acid chains that contain at least three double bonds, as shown for the synthesis of prostaglandin PGE₂ in (A). The subscript refers to the two carbon-carbon (more...)

Gap Junctions Allow Signaling Information to Be Shared by Neighboring Cells ⁴

Another way to coordinate the activities of neighboring cells is through gap junctions. These are specialized cell-cell junctions that can form between closely apposed plasma membranes, directly connecting the cytoplasms of the joined cells via narrow water-filled channels (see Figure 19-15). The channels allow the exchange of small intracellular signaling molecules ( intracellular mediators), such as Ca²⁺ and cyclic AMP, but not of macromolecules, such as proteins or nucleic acids. Thus cells connected by gap junctions can communicate with each other directly without having to deal with the barrier presented by the intervening plasma membranes ( Figure 15-7).

Figure 15-7

Signaling via gap junctions. Cells connected by gap junctions share small molecules, including small intracellular signaling molecules, and therefore can respond to extracellular signals in a coordinated way.

As discussed in Chapter 19, the pattern of gap-junction connections in a tissue can be revealed either electrically, with intracellular electrodes, or visually, after the microinjection of water-soluble dyes. Studies of this kind indicate that the cells in a developing embryo make and break gap-junction connections in specific and interesting patterns, suggesting that these junctions play an important part in the signaling processes that occur between these cells. One suspects that, like autocrine signaling described above, gap-junction communication helps adjacent cells of a similar type to coordinate their behavior. It is not known, however, which particular small molecules are important as carriers of signals through gap junctions; nor has the precise function of gap-junction communication in animal development been defined.

Each Cell Is Programmed to Respond to Specific Combinations of Signaling Molecules ⁵

Any given cell in a multicellular organism is exposed to many - perhaps hundreds - of different signals from its environment. These signals can be soluble, or bound to the extracellular matrix, or bound to the surface of a neighboring cell, and they can act in many millions of possible combinations. The cell must respond to this babel selectively, according to its own specific character, acquired through progressive cell specialization in the course of development. Thus a cell may be programmed to respond to one set of signals by differentiating, to another set by proliferating, and to yet another by carrying out some specialized function.

Most cells in higher animals, moreover, are programmed to depend on a specific set of signals simply for survival: when deprived of the appropriate signals (in a culture dish, for example), a cell will activate a suicide program and kill itself - a process called programmed cell death, which is discussed further in Chapter 21 ( Figure 15-8). Different types of cells require different sets of survival signals and so are restricted to different environments in the body.

Figure 15-8

Combinatorial signaling. Each cell type displays a set of receptors that enables it to respond to a corresponding set of signaling molecules produced by other cells. These signaling molecules work in combinations to regulate the behavior of the cell. (more...)

Because signaling molecules generally act in combinations, an animal can control the behavior of its cells in highly specific ways using a limited diversity of such molecules: hundreds of such signals can be used in millions of combinations.

Different Cells Can Respond Differently to the Same Chemical Signal ⁶

The specific way a cell reacts to its environment varies, first, according to the set of receptor proteins that the cell possesses through which it is tuned to detect a particular subset of the available signals and, second, according to the intracellular machinery by which the cell integrates and interprets the information that it receives. Thus a single signaling molecule often has different effects on different target cells. The neurotransmitter acetylcholine, for example, stimulates the contraction of skeletal muscle cells but decreases the rate and force of contraction in heart muscle cells. This is because the acetylcholine receptor proteins on skeletal muscle cells are different from those on heart muscle cells. But receptor differences are not always the explanation for the different effects. In many cases the same signaling molecule binds to identical receptor proteins and yet produces very different responses in different types of target cells, reflecting differences in the internal machinery to which the receptors are coupled ( Figure 15-9).

Figure 15-9

The same signaling molecule can induce different responses in different target cells. In some cases this is because the signaling molecule binds to different receptor proteins, as illustrated in (A) and (B). In other cases the signaling molecule binds (more...)

The Concentration of a Molecule Can Be Adjusted Quickly Only If the Lifetime of the Molecule Is Short ⁶

It is natural to think of signaling systems in terms of the changes produced when a signal is delivered. But it is just as important to consider what happens when a signal is withdrawn. During development transient signals often produce lasting effects: they can trigger a change that persists indefinitely, through cell memory mechanisms such as those discussed in Chapters 9 and 21. But in most cases, especially in adult tissues, when a signal ceases, the response fades. The signal acts on a system of molecules that is undergoing continual turnover, and when the signal is shut off, the replacement of the old molecules by new ones wipes out the traces of its action. It follows that the speed of reaction to shutting off the signal depends on the rate of turnover of the molecules that the signal affects. It may not be as obvious that this turnover rate also determines the promptness of the response when the signal is turned on.

Consider, for example, two intracellular molecules X and Y, both of which are normally maintained at a concentration of 1000 molecules per cell. Molecule X has a slow turnover rate: it is synthesized and degraded at a rate of 10 molecules per second, so that each molecule has an average lifetime in the cell of 100 seconds. Molecule Y turns over 10 times as quickly: it is synthesized and degraded at a rate of 100 molecules per second, with each molecule having an average lifetime of 10 seconds. If a signal acting on the cell boosts the rates of synthesis of both X and Y tenfold without any change in the molecular lifetimes, at the end of 1 second the concentration of Y will have increased by nearly 900 molecules per cell (10 x 100 - 100) while the concentration of X will have increased by only 90 molecules per cell. In fact, after its synthesis rate has been either increased or decreased abruptly, the time required for a molecule to shift halfway from its old to its new equilibrium concentration is equal to its normal half-life - that is, it is equal to the time that would be required for its concentration to fall by half if all synthesis were stopped ( Figure 15-10).

Figure 15-10

The importance of rapid turnover. The figure shows the predicted relative rates of change in the intracellular concentrations of molecules with differing turnover times when their rates of synthesis are either decreased (A) or increased (B) suddenly by (more...)

The same principles apply to proteins as well as to small molecules and to molecules in the extracellular space as well as to those in cells. Many intracellular proteins that are rapidly degraded have short half-lives, some surviving less than 10 minutes; in most cases these are proteins with key regulatory roles, whose concentrations are rapidly regulated in the cell by changes in their rates of synthesis. Likewise, any covalent modifications of proteins that occur as part of a rapid signaling process - most commonly the addition of a phosphate group to an amino acid side chain - must be continuously removed at a rapid rate to make such signaling possible. We discuss some of these molecular events in detail later, for the case of signaling pathways that operate via cell-surface receptors. But the principles apply generally, as the next example illustrates.

Nitric Oxide Gas Signals by Binding Directly to an Enzyme Inside the Target Cell ⁷

Although most extracellular signals are mediated by hydrophilic molecules that bind to receptors on the surface of the target cell, some signaling molecules are hydrophobic enough and/or small enough to pass readily across the target-cell plasma membrane; once inside, they directly regulate the activity of specific intracellular proteins. A remarkable example is the gas nitric oxide (NO), which only recently has been recognized to act as a signaling molecule in vertebrates. When acetylcholine is released by autonomic nerves in the walls of a blood vessel, for example, it causes smooth muscle cells in the vessel wall to relax. The acetylcholine acts indirectly by inducing the endothelial cells to make and release NO, which then signals the smooth muscle cells to relax. This effect of NO on blood vessels provides an explanation for the mechanism of action of nitroglycerine, which has been used for almost 100 years to treat patients with angina (pain due to inadequate blood flow to heart muscle). The nitroglycerine is converted to NO, which relaxes blood vessels, thereby reducing the workload on the heart and, as a consequence, the oxygen requirement of the heart muscle. NO is also produced as a local mediator by activated macrophages and neutrophils to help them kill invading microorganisms. In addition, it is used by many types of nerve cells to signal neighboring cells: NO released by autonomic nerves in the penis, for example, causes the local blood vessel dilation that is responsible for penile erection.

NO is made by the enzyme NO synthase by the deamination of the amino acid arginine. Because it diffuses readily across membranes, the NO diffuses out of the cell where it is produced and passes directly into neighboring cells. It acts only locally because it has a short half-life - about 5-10 seconds - in the extracellular space before it is converted to nitrates and nitrites by oxygen and water. In many target cells, such as endothelial cells, NO reacts with iron in the active site of the enzyme guanylyl cyclase, stimulating it to produce the intracellular mediator cyclic GMP, which we discuss later. The effects of NO can be rapid, occurring within seconds, because the rate of turnover of cyclic GMP is high: rapid production from GTP by guanylyl cyclase is balanced by rapid degradation to GMP by a phosphodiesterase. There is recent evidence that carbon monoxide (CO) is also used as an intercellular signal and can act in the same way as NO, by stimulating guanylyl cyclase.

Gases such as NO and CO are not the only signaling molecules that can pass directly across the target-cell plasma membrane. A group of small, hydrophobic, nongaseous hormones and local mediators also enter target cells in this way, but instead of binding to enzymes, they bind to intracellular receptor proteins that directly regulate gene transcription.

Steroid Hormones, Thyroid Hormones, Retinoids, and Vitamin D Bind to Intracellular Receptors That Are Ligand-activated Gene Regulatory Proteins ⁸

Steroid hormones, thyroid hormones, retinoids,and vitamin Dare small hydrophobic molecules that differ greatly from one another in both chemical structure ( Figure 15-11) and function. Nonetheless, they all act by a similar mechanism. They diffuse directly across the plasma membrane of target cells and bind to intracellular receptor proteins. Ligand binding activates the receptors, which then directly regulate the transcription of specific genes. These receptors are structurally related and constitute the intracellular receptor superfamily (or steroid-hormone receptor superfamily) ( Figure 15-12).

Figure 15-11

Some signaling molecules that bind to intracellular receptors. Note that all of them are small and hydrophobic. The active, hydroxylated form of vitamin D₃is shown.

Figure 15-12

The intracellular receptor superfamily. (A) A model of an intracellular receptor protein. In its inactive state the receptor is bound to an inhibitory protein complex that contains a heat-shock protein called Hsp90 (discussed in Chapter 5). The binding (more...)

Steroid hormones, including cortisol, the steroid sex hormones, vitamin D (in vertebrates), and the moulting hormone ecdysone (in insects), are all made from cholesterol. Cortisol is produced in the cortex of the adrenal gland and influences the metabolism of many cell types. The steroid sex hormones are made in the testis and ovary and are responsible for the secondary sex characteristics that distinguish males from females. Vitamin D is synthesized in the skin in response to sunlight; after it is converted to an active form in the liver or kidneys, it functions to regulate Ca²⁺ metabolism, promoting Ca²⁺ uptake in the gut and reducing its excretion in the kidney. The thyroid hormones, which are made from the amino acid tyrosine, act to increase metabolism in a wide variety of cell types, while the retinoids, such as retinoic acid, which are made from vitamin A, play important roles as local mediators in vertebrate development. Although all of these signaling molecules are relatively insoluble in water, they are made soluble for transport in the bloodstream and other extracellular fluids by binding to specific carrier proteins, from which they dissociate before entering a target cell (see Figure 15-2).

Besides the fundamental difference in the way they signal their target cells, most water-insoluble signaling molecules differ from water-soluble ones in the length of time that they persist in the bloodstream or tissue fluids. Most water-soluble hormones are removed and/or broken down within minutes of entering the blood, and local mediators and neurotransmitters are removed from the extracellular space even faster - within seconds or milliseconds. Steroid hormones, by contrast, persist in the blood for hours and thyroid hormones for days. Consequently, water-soluble signaling molecules usually mediate responses of short duration, whereas the water-insoluble ones tend to mediate longer-lasting responses.

The intracellular receptors for the steroid and thyroid hormones, retinoids, and vitamin D all bind to specific DNA sequences adjacent to the genes that the ligand regulates. Some, such as cortisol receptors, are located primarily in the cytosol and bind to DNA only following ligand binding (see Figure 15-12); others, such as retinoid receptors, are located primarily in the nucleus and bind to DNA even in the absence of ligand. In either case, ligand binding alters the conformation of the receptor protein, which then activates (or occasionally suppresses) gene transcription. In many cases the response takes place in two steps: the direct induction of the transcription of a small number of specific genes within about 30 minutes is known as the primary response;the products of these genes in turn activate other genes and produce a delayed, secondary response. Thus a simple hormonal trigger can cause a very complex change in the pattern of gene expression ( Figure 15-13).

Figure 15-13

Early primary response (A) and delayed secondary response (B) that result from the activation of an intracellular receptor protein. The response to a steroid hormone is illustrated, but the same principles apply for all ligands that activate this family (more...)

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 signaling molecule. Even when different types of cells have the identical intracellular receptor, the set of genes that the receptor regulates is different. This is because more than one type of gene regulatory protein generally must bind to a eucaryotic gene in order to activate its transcription. An intracellular receptor can activate a gene, therefore, only if the right combination of other gene regulatory proteins is also present, and some of these are cell-type specific. Thus thyroid hormone, vitamin D, and each steroid hormone and retinoid induces a characteristic set of responses in an animal because (1) only certain types of cells have receptors for it and (2) each of these cell types contains a different combination of other cell-type-specific gene regulatory proteins that collaborate with the activated receptor to influence the transcription of specific sets of genes. The molecular details of how intracellular receptors and other gene regulatory proteins control specific gene transcription are discussed in Chapter 9.

There Are Three Known Classes of Cell-Surface Receptor Proteins: Ion-Channel-linked, G-Protein-linked, and Enzyme-linked ⁹

Recombinant DNA techniques have revolutionized the study of the receptors and intracellular proteins involved in cell signaling. Because these proteins often constitute less than 0.01% of the total mass of protein in the cell, it has been extremely difficult to purify them. Cloning the DNA sequences that encode the proteins has greatly accelerated the process of characterization, and most of the signaling proteins discussed in this chapter have been characterized in this way. A major contribution of these DNA-cloning and -sequencing studies has been to reveal that the bewildering diversity of known receptor proteins can be reduced to a much smaller number of large families. The intracellular receptors that we have just discussed constitute one such family. We now consider the family groups that can be identified within the other, larger, class of signal receptors - those located on the cell surface.

All water-soluble signaling molecules (including neurotransmitters, protein hormones, and protein growth factors), as well as some lipid-soluble ones, bind to specific receptor proteins on the surface of the target cells they influence. These cell-surface receptor proteins act as signal transducers: they bind the signaling ligand with high affinity and convert this extracellular event into one or more intracellular signals that alter the behavior of the target cell.

Most cell-surface receptor proteins belong to one of three classes, defined by the transduction mechanism used. Ion-channel-linked receptors, also known as transmitter-gated ion channels, are involved in rapid synaptic signaling between electrically excitable cells. This type of signaling is mediated by a small number of neurotransmitters that transiently open or close the ion channel formed by the protein to which they bind, briefly changing the ion permeability of the plasma membrane and thereby the excitability of the postsynaptic cell ( Figure 15-14A). The ion-channel-linked receptors belong to a family of homologous, multipass transmembrane proteins. They are discussed in Chapter 11and will not be considered further here.

Figure 15-14

Three classes of cell-surface receptors. Although many enzyme-linked receptors have intrinsic enzyme activity as shown in (C), many others rely on associated enzymes (not shown).

G-protein-linked receptors act indirectly to regulate the activity of a separate plasma-membrane-bound target protein, which can be an enzyme or an ion channel. The interaction between the receptor and the target protein is mediated by a third protein, called a trimeric GTP-binding regulatory protein (G protein) ( Figure 15-14B). The activation of the target protein either alters the concentration of one or more intracellular mediators (if the target protein is an enzyme) or alters the ion permeability of the plasma membrane (if the target protein is an ion channel). The intracellular mediators act in turn to alter the behavior of yet other proteins in the cell. All of the G-protein-linked receptors belong to a large superfamily of homologous, seven-pass transmembrane proteins.

Enzyme-linked receptors, when activated, either function directly as enzymes or are associated with enzymes ( Figure 15-14C). Most are single-pass transmembrane proteins, with their ligand-binding site outside the cell and their catalytic site inside. Compared with the other two classes, enzyme-linked receptors are heterogeneous, although the great majority are protein kinases, or are associated with protein kinases, that phosphorylate specific sets of proteins in the target cell.

Activated Cell-Surface Receptors Trigger Phosphate-Group Additions to a Network of Intracellular Proteins ^{9, 10}

Much of the remainder of this chapter is concerned with how G-protein-linked receptors and enzyme-linked receptors operate. Signals received at the surface of a cell by both of these classes of receptors are often relayed to the nucleus, where they alter the expression of specific genes and thereby alter the behavior of the cell. Elaborate sets of intracellular signaling proteins form the relay systems. The majority of these proteins are of one of two kinds: proteins that become phosphorylated by protein kinases, and proteins that are induced to bind GTP when the signal arrives. In both cases the proteins gain one or more phosphates in their activated state and lose the phosphates when the signal decays ( Figure 15-15). These proteins in turn generally cause the phosphorylation of downstream proteins as part of a phosphorylation cascade.

Figure 15-15

Two major intracellular signaling mechanisms share common features. In both cases a signaling protein is activated by the addition of a phosphate group and inactivated by the removal of the phosphate. In (A) the phosphate is added covalently to the signaling (more...)

The phosphorylation cascades are mediated by two main types of protein kinases: serine/threonine kinases, which phosphorylate proteins on serines and (less often) threonines, and tyrosine kinases, which phosphorylate proteins on tyrosines. An occasional kinase can do both. It is estimated that about 1% of our genes encode protein kinases and that a single mammalian cell may contain more than 100 distinct kinds of these enzymes, most of which are serine/threonine kinases. Although fewer than 0.1% of the phosphorylated proteins in cells contain phosphotyrosine, we shall see that this small minority plays a crucial part in signaling by most enzyme-linked receptors.

As discussed previously, complex cell behaviors, such as survival or proliferation, are generally stimulated by specific combinations of signals rather than by a single signal acting alone (see Figure 15-8). The cell has to integrate the information coming from separate signals so as to make a proper responseto live or die, or to proliferate or stay quiescent. The integration seems to depend on interactions between the various protein phosphorylation cascades that are activated by different extracellular signals. In particular, some of the signaling proteins in the cascades function as integrating devices, equivalent to micro-processors in a computer: in response to multiple signal inputs, they produce an output that is calibrated to cause the desired biological effect. Two examples of how such integrating proteins could operate are illustrated in Figure 15-16.

Figure 15-16

Signal integration. In (A) signals A and B activate different cascades 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 (more...)

The complexity of such signal-response systems, with multiple interacting relay chains of signaling proteins, is daunting. But recombinant DNA technology, combined with classical genetic analyses in Drosophila, the nematode C. elegans, and yeasts, as well as more conventional biochemical and pharmacological methods, is rapidly uncovering the intricate details of these mechanisms by which activated receptor proteins change the behavior of the cell.

Summary

Each cell in a multicellular animal is programmed during development to respond to a specific set of signals that act in various combinations to regulate the behavior of the cell and to determine whether the cell lives or dies and whether it proliferates or stays quiescent. Most of these signals mediate paracrine signaling, in which local mediators are rapidly taken up, destroyed, or immobilized, so that they act only on neighboring cells. In addition, centralized control is exerted both by endocrine signaling, in which hormones secreted by endocrine cells are carried in the blood to target cells throughout the body, and by synaptic signaling, in which neurotransmitters secreted by nerve cells act locally on the postsynaptic cells that their axons contact.

Cell signaling requires both extracellular signaling molecules and a complementary set of receptor proteins in each cell that enable it to bind and respond to them in a programmed and characteristic way. Some small hydrophobic signaling molecules, including the steroid and thyroid hormones and the retinoids, diffuse across the plasma membrane of the target cell and activate intracellular receptor proteins, which directly regulate the transcription of specific genes. Some dissolved gases, such as nitric oxide and carbon monoxide, act as local mediators by diffusing across the plasma membrane of the target cell and activating an intracellular enzyme - usually guanylyl cyclase, which produces cyclic GMP in the target cell. But most extracellular signaling molecules are hydrophilic and are able to activate receptor proteins only on the surface of the target cell; these receptors act as signal transducers, converting the extracellular binding event into intracellular signals that alter the behavior of the target cell. There are three main families of cell-surface receptors, each of which transduces extracellular signals in a different way. Ion-channel-linked receptors are transmitter-gated ion channels that open or close briefly in response to the binding of a neurotransmitter. G-protein-linked receptors indirectly activate or inactivate plasma-membrane-bound enzymes or ion channels via trimeric GTP-binding proteins (G proteins). Enzyme-linked receptors either act directly as enzymes or are associated with enzymes; the enzymes are usually protein kinases that phosphorylate specific proteins in the target cell. Through cascades of highly regulated protein phosphorylations, elaborate sets of interacting proteins relay most signals from the cell surface to the nucleus, thereby altering the cell's pattern of gene expression and, as a consequence, its behavior. Cross-talk between different signaling cascades enables a cell to integrate information from the multiple signals that it receives.