To survive, an organism must constantly adjust its internal state to changes in the environment. To track environmental changes, the organism must receive signals. These may be in the form of chemicals, such as hormones or nutrients, or may take another form, such as light, heat, or sound. A signal itself rarely causes a simple, direct chemical change inside the cell. Instead, the signal sets off a chain of events that may involve several or even dozens of steps. The signal is thereby transduced, or changed in form. Signal transduction refers to the entire set of pathways and interactions by which environmental signals are received and responded to by single cells.
Signal transduction systems are especially important in multicellular organisms, because of the need to coordinate the activities of hundreds to trillions of cells. Multicellular organisms have developed a variety of mechanisms allowing very efficient and controlled cell-to-cell communication. Though we take it for granted, it is actually astonishing that our skin, for example, continues to grow at the right rate to replace the continuous loss of its surface every day of our lives. This tight regulation is found in every tissue of our body all of the time, and when this fine control breaks down, cancer may be the result. Clearly the molecular mechanisms behind this astounding level of control must be powerful, versatile, and sophisticated.
Signals, Receptors, and Cascades
The signals that cells use to communicate with one another are often small amino acid chains, called peptides . Depending on the cell type that releases them and the effect they have on the target cell, they may be called hormones, growth factors, neuropeptides, neurotransmitters , or cytokines . Other small molecules can also be signals, such as amino acids and steroids such as testosterone. External signals such as odorants and tastes can be carried to us in the atmosphere or in the fluids of our food and drinks. Stretch, pressure, and other mechanical effects as well as heat, pain, and light can also act as signals.
Given the huge variety of signals to which a cell is exposed, how does it know which to respond to? The answer is that signals are received by protein receptors made by the cell, and a cell is sensitive only to those signals for which it has made receptors. For instance, every cell in the body is exposed to estrogen circulating in the blood, but only a subset of them make estrogen receptors, and are therefore sensitive to its influence.
Chemical signals such as hormones bind to their receptors, usually at the surface of the cell (the plasma membrane), but sometimes within the cell. This causes a conformation (shape) change in the receptor. The conformation change typically alters the ability of the receptor to bind to another molecule in the cell, modifying that molecule's conformation, or triggering other actions.
This sequence of events triggered by the signal-receptor interaction is called a transduction cascade. A transduction cascade involves a network of enzymes that act on one another in specific ways to ultimately generate precise and appropriate responses. These responses may include alterations in cell motility or division, induction of the expression of specific genes, and the regulation of apoptosis . The molecular details of several such cascades are known, but many more undoubtedly remain to be discovered.
The value of this complex cascade of events is severalfold. First, the network of interactions provides many levels of control, so that the cell can tailor the magnitude and timing of its response very finely. Second, the many levels of interaction allow amplification of the original signal to quickly produce a strong response to a small stimulus. For example, there may be only a few hundred copies of a specific receptor on the surface of a typical cell. Activation of even a small percentage of them, acting through these amplifying enzyme cascades, can result in activation of millions of downstream target molecules. This explains how even very small amounts of signals such as growth factor can have such profound effects on appropriately receptive cells.
The Importance of Phosphorylation and Dephosphorylation
After a signal is received, signal transduction involves altering the behavior of proteins in the cascade, in effect turning them on or off like a switch. Adding or removing phosphates is a fundamental mechanism for altering the shape, and therefore the behavior, of a protein. Phosphorylation may open up an enzyme's active site, allowing it to perform chemical reactions, or it may frequently generate a binding site allowing a specific interaction (may make a bulge in one side preventing the protein from fitting together) with a molecular partner.
Enzymes that add phosphate groups to other molecules are called kinases , and the molecules the enzymes act on are called substrates. Protein kinases are a family of enzymes that use ATP to add phosphate groups on to other proteins, thereby altering the properties of these substrate proteins. Protein kinases themselves are frequently turned on or off by phosphorylation performed by other protein kinases; thus a kinase can be both enzyme and substrate.
Protein kinases can be broadly divided into two groups based on the amino acids to which they add phosphate groups. The serine/threonine kinases (ser/thr) are found in all eukaryotic cells and tend to be involved in regulation of metabolic and cytoskeletal activity. The tyrosine (tyr) kinases are found in all animals but not in yeast, protozoa, or plants, and appear to have evolved specifically to deal with the complex challenges of signaling in animals.
Molecular switches are useful only if they can also be flipped back to their original state. This is achieved by specific protein phosphatases, which can remove phosphate groups from kinase substrates.
Signal Transduction: The RTK Pathway
How does the cell "know" when a particular receptor molecule in the membrane is occupied, and how is that information chemically translated into actions within the cell? Let us examine the signaling pathway for the receptor tyrosine kinases (RTK). The RTKs are a very powerful and important family of signaling molecules and include receptors for potent growth factors and such hormones as insulin, epidermal growth factor, and nerve growth factor.
In this system, the extracellular "ligand " (growth factor or hormone) must crosslink two receptor molecules in order to begin the transduction cascade. The interaction of the two intracellular domains of the receptors then initiates a signaling response.
The simplest RTKs have three parts: a ligand binding site outside the cell, a single membrane-spanning domain, and a tyrosine kinase domain inside the cell. The ligand is typically a diffusible peptide or small protein produced elsewhere in the organism, and in this case is the specific growth factor recognized by the RTK. In the absence of its specific growth factor, this receptor remains unbound to a second receptor, and is inactive. Growth factor, when it arrives, binds to two receptors, cross-linking them. This causes the two tyrosine kinase domains to come into contact with one another. Each kinase now has a substrate, formed by the other receptor, and so each phosphorylates the other on multiple tyrosine sites.
The receptors may now bind with one or more other proteins (called SH2 proteins) that specifically recognize their phosphorylated tyrosines. Many of these are enzymes, while others are adapter molecules that in turn attract and bind other enzymes. Often these enzymes are inactive until they join the receptor complex, because their substrates are found only in the membrane. The products of these enzymes may act on yet other molecules, thus continuing the signaling cascade, or they may be used in cell metabolism for growth or other responses.
Signal Transduction: The GPCR Pathway and G Proteins
The G protein coupled receptors (GPCR) illustrate another way to receive and interpret a signal, in which the ligand binds to a single transmembrane protein, causing a conformation change and activating "G proteins" inside the cell. G proteins are so named because they have a binding site for guanine nucleotide, either a diphosphate (GDP) or a triphosphate (GTP). Like ATP, GTP is the high-energy form of the nucleotide, while GDP is the low-energy form.
The GPCR family of proteins has at least 300 members in humans. They are specific receptors for numerous neurotransmitters, hormones, peptides and other substances. A large subgroup of these are odorant receptors directly responsible for our senses of taste and smell.
Surprisingly, a further group of GPCRs is responsible for our sense of vision. Opsin molecules, including rhodopsin, are actually GPCRs. Instead of a ligand binding site, rhodopsin has a molecule of retinal bound in the same relative position. Light alters the conformation of the retinal, and rhodopsin responds to this by altering its protein conformation. This causes G protein activation in a similar manner to the other GPCRs.
Each GPCR is associated with a particular type of G protein inside the cell, and there are dozens of different G proteins known. G proteins are generally inactive in the GDP-bound form. They are activated when GDP departs and is replaced by GTP. GTP is found in excess in cells, and so GDP departure is followed rapidly by GTP binding. This causes a profound conformational change, allowing the G protein to interact with and influence numerous target molecules.
Each GPCR associated G protein consists of three parts, the α, β, and γ subunits. The α subunit binds either GDP or GTP. The β and γ subunits are always found together in a complex called Gβγ. In unstimulated cells the whole three-part complex is found in the plasma membrane , with GDP in the binding site.
Binding of ligand to the GPCR causes a conformational change that is transmitted to the cytoplasmic region of the receptor, which interacts with the G protein to dissociate the GDP. This results in GTP binding, which alters the structure of the α subunit, freeing it from Gβγ. Both parts, the Gα(GTP) and the Gβγ, now diffuse away from the receptor and separately interact with and influence many other molecules in the cell.
One target for certain Gα(GTP) types is the enzyme adenyl cyclase. This enzyme uses ATP to generate cyclic AMP (cAMP). The cAMP molecule is a "second messenger," one of a family of small diffusible substances that powerfully induce cytoplasmic responses.
Cyclic AMP exerts much of its effects by activating the cAMP-dependent protein kinase (PKA), a ser/thr kinase that can phosphorylate and influence many cellular proteins. For example, PKA phosphorylates CREB (cyclic AMP response element binding protein), which is found attached to the promoters of many genes. Phosphorylation of CREB by PKA can thus regulate the expression of these genes.
Another Gα(GTP) target is an enzyme, phospholipase C, which cleaves a membrane lipid called PIP-2. This produces two products: DAG and IP-3. DAG stays in the membrane and binds all members of the ser/thr protein kinase C (PKC) family of enzymes, which may then become activated and phosphorylate and regulate a host of metabolic and structural enzymes. IP-3, another second messenger, rapidly diffuses to IP-3 receptors in the endoplasmic reticulum membrane. When IP-3 binds, it opens channels in the membrane, releasing stored calcium into the cytoplasm. Calcium is normally kept at very low levels in the cytoplasm, and even small increases cause numerous major effects, so that calcium is also regarded as a powerful second messenger. These effects include the activation of various calcium binding proteins such as calmodulin and its many relatives. The calcium-bound versions of these proteins regulate many other enzymes. For example, calcium activates all members of a large and important family of ser/thr kinases called calcium/calmodulin dependent protein kinases (CAM kinases), which themselves regulate the activity of numerous important substrate molecules.
Just as protein kinases need to be turned off, so too do G proteins. This occurs when the GTP on the Gα is cleaved to generate GDP, thus favoring the reformation of the three-part inactive protein.
Interacting Pathways, Defective Signaling, and Treatments for Disease
The GPCR and RTK pathways do not necessarily remain separate, either from each other or from other signaling pathways. One of the most important pathways is directly downstream of RTKs and also many GPCRs, and is called the mitogen activated protein (MAP) kinase cascade. MAP kinase is an abundant ser/thr kinase that, when activated, phosphorylates and powerfully affects the activity of a large number of cytoskeletal, signaling, and nuclear proteins, including an important family of transcription factors , thus directly influencing gene expression.
Both pathways also help regulate a particularly important process called apoptosis . Many cells need specific growth factors to stay alive; the growth factors trigger pathways involving various ser/thr protein kinases that ultimately inactivate molecules that would otherwise promote apoptosis.
The entire signal transduction system normally works astonishingly well, but serious problems can occur. Cancer is unregulated cell growth and occurs when the machinery tightly regulating cell growth breaks down. It is often easy to see how this has occurred. Mutations in growth factor receptors, G proteins, MAP kinases, and other molecules frequently contribute to cancer, and generally result in these molecules losing their normal switching function, staying in the activated form and therefore inappropriately stimulating these important enzyme cascades.
The complexity of the signaling system makes for challenging research, but once understood it holds the promise for better treatments for cancer and other diseases. This is because each step in each pathway provides one or more targets for drugs. Designing a drug that could quiet the excess signaling caused by defective MAP kinase, for example, might provide a promising cancer treatment.
The examples given thus far provide only an outline of how signal transduction cascades work and an overview of a few of the most important enzymes. The actual process is much more complex, and there is much about the process that remains mysterious. Perhaps the biggest mystery is how the cell makes sense of all of the input from different growth factors, hormones, extracellular substrates, and so on to produce an appropriate response. The solution to this problem will result from a complete understanding and computer modeling of the biochemical and kinetic properties of the components of all these signaling cascades.
see also Apoptosis; Cell, Eukaryotic; Cell Cycle; Gene; Gene Expression: Overview of Control; Pharmacogenetics and Pharmacogenomics.
Alberts, Bruce., et al. Molecular Biology of the Cell, 4th ed. New York: Garland Science, 2002.
Scott, John D., and Tony Pawson. "Cell Communication: The Inside Story." Scientific American (June 2000).
Special Issue on Mapping Cellular Signaling. Science 296, no. 5573 (May 31, 2002).