Signal Transduction, Membrane Receptors, Second Messengers, and Regulation of Gene Expression

Cell-to-Cell Communication

An overview of how cells communicate with each other is presented in Fig. 3.1 . Cells communicate by releasing extracellular signaling molecules (e.g., hormones and neurotransmitters ) that bind to receptor proteins located in the plasma membrane, cytoplasm, or nucleus. This signal is transduced into the activation, or inactivation, of one or more intracellular messengers by interacting with receptors. Receptors interact with a variety of intracellular signaling proteins, including kinases, phosphatases, and guanosine triphosphate (GTP)–binding proteins (G proteins) . These signaling proteins interact with and regulate the activity of target proteins and thereby modulate cellular function. Target proteins include, but are not limited to, ion channels and other transport proteins, metabolic enzymes, cytoskeletal proteins, gene regulatory proteins, and cell cycle proteins that regulate cell growth and division. Signaling pathways are characterized by (1) multiple, hierarchical steps; (2) amplification of the signal-receptor binding event, which magnifies the response; (3) activation of multiple pathways and regulation of multiple cellular functions; and (4) antagonism by constitutive and regulated feedback mechanisms, which minimize the response and provide tight regulatory control over these signaling pathways. A brief description of how cells communicate follows. Readers who desire a more in-depth presentation of this material are encouraged to consult one of the many cellular and molecular biology textbooks currently available.

Cells in higher animals release into the extracellular space hundreds of chemicals, including (1) peptides and proteins (e.g., insulin); (2) amines (e.g., epinephrine and norepinephrine); (3) steroid hormones (e.g., aldosterone, estrogen); and (4) small molecules, including amino acids, nucleotides, ions (e.g., Ca ⁺⁺ ), and gases, such as nitric oxide and carbon dioxide. Secretion of signaling molecules is cell-type specific. For example, beta cells in the pancreas release insulin, which stimulates glucose uptake into cells. The ability of a cell to respond to a specific signaling molecule depends on the expression of receptors that bind the signaling molecule with high affinity and specificity. Receptors are located in the plasma membrane, the cytosol, and the nucleus ( Fig. 3.2 ).

Signaling molecules can act over long or short distances and can require cell-to-cell contact or very close cellular proximity ( Fig. 3.3 ). Contact-dependent signaling , in which a membrane-bound signaling molecule of one cell binds directly to a plasma membrane receptor of another cell, is important during development, in immune responses, and in cancer (see Fig. 3.3A ). Molecules that are released and act locally are called paracrine (see Fig. 3.3B ) or autocrine (see Fig. 3.3C ) hormones . Paracrine signals are released by one type of cell and act on another type; they are usually taken up by target cells or rapidly degraded (within minutes) by enzymes. For example, enterochromaffin-like cells in the stomach secrete histamine, which stimulates the production of acid by neighboring parietal cells (see Chapter 27 for details). Autocrine signaling involves the release of a molecule that affects the same cell or other cells of the same type (e.g., cancer cells). In synaptic signaling (see Fig. 3.3D ), neurons transmit electrical signals along their axons and release neurotransmitters at synapses that affect the function of other neurons or cells that are distant from the neuron cell body. The close physical relationship between the nerve terminal and the target cell ensures that the neurotransmitter is delivered to a specific cell. Details on synaptic signaling are discussed in Chapter 6 . Endocrine signals are hormones that are secreted into the blood and are widely dispersed in the body (see Fig. 3.3 E ). Details on endocrine signaling are discussed in Chapter 38 .

In addition to paracrine, autocrine, endocrine, and synaptic signaling, cell-to-cell communication also occurs via gap junctions that form between adjacent cells (see Chapter 2 ). Gap junctions are specialized junctions that allow intracellular signaling molecules, generally less than 1200 daltons (Da) in size, to diffuse from the cytoplasm of one cell to an adjacent cell. The permeability of gap junctions is regulated by cytosolic [Ca ⁺⁺ ], [H ⁺ ], and cyclic adenosine monophosphate (cAMP) and by the membrane potential. Gap junctions also allow cells to be electrically coupled, which is vitally important for the coordinated activity of cardiac and smooth muscle cells (see Chapters 13 and 14).

The speed of a response to an extracellular signal depends on the mechanism of delivery. Endocrine signals are relatively slow (seconds to minutes) because time is required for diffusion and blood flow to the target cell, whereas synaptic signaling is extremely fast (milliseconds). If the response involves changes in the activity of proteins in the cell, the response may occur in milliseconds to seconds. However, if the response involves changes in gene expression and the de novo synthesis of proteins, the response may take hours to occur, and a maximal response may take days. For example, the stimulatory effect of aldosterone on sodium transport by the kidneys requires days to develop fully (see Chapter 35 ).

The response to a particular signaling molecule also depends on the ability of the molecule to reach a particular cell, on expression of the cognate receptor (i.e., receptors that recognize a particular signaling molecule or ligand with a high degree of specificity), and on the cytoplasmic signaling molecules that interact with the receptor. Thus signaling molecules frequently have many different effects that are dependent on the cell type. For example, the neurotransmitter acetylcholine stimulates contraction of skeletal muscle but decreases the force of contraction in heart muscle. This is because skeletal muscle and heart cells express different acetylcholine receptors. ^a

a The acetylcholine receptor in skeletal muscle is termed nicotinic because nicotine can mimic this action of the neurotransmitter. In contrast, the acetylcholine receptor in cardiac muscle is termed muscarinic because this effect is mimicked by muscarine, an alkaloid derived from the mushroom Amanita muscaria.

Receptors

All signaling molecules bind to specific receptors that act as signal transducers, thereby converting a ligand-receptor binding event into intracellular signals that affect cellular function. Receptors can be divided into four basic classes on the basis of their structure and mechanism of action: (1) ligand-gated ion channels , (2) G protein–coupled receptors (GPCRs) , (3) enzyme-linked receptors , and (4) nuclear receptors ( Table 3.1 ; Figs. 3.4 and 3.5 ).

Ligand-gated ion channels mediate direct and rapid synaptic signaling between electrically excitable cells (see Fig. 3.4A ). Neurotransmitters bind to receptors and either open or close ion channels, thereby changing the ionic permeability of the plasma membrane and altering the membrane potential. For examples and more details, see Chapter 6 .

GPCRs regulate the activity of other proteins, such as enzymes and ion channels (see Fig. 3.4B ). In the example in Fig. 3.4B , the interaction between the receptor and the target protein is mediated by heterotrimeric G proteins, which are composed of α, β, and γ subunits. Stimulation of G proteins by ligand-bound receptors activates or inhibits downstream target proteins that regulate signaling pathways if the target protein is an enzyme or changes membrane ion permeability if the target protein is an ion channel.

Enzyme-linked receptors either function as enzymes or are associated with and regulate enzymes (see Fig. 3.4C ). Most enzyme-linked receptors are protein kinases or are associated with protein kinases, and ligand binding causes the kinases to phosphorylate a specific subset of proteins on specific amino acids, which in turn activates or inhibits protein activity.

Nuclear receptors are small hydrophobic molecules, including steroid hormones, thyroid hormones, retinoids, and vitamin D, that have a long biological half-life (hours to days), diffuse across the plasma membrane, and bind to nuclear receptors or to cytoplasmic receptors that, once bound to their ligand, translocate to the nucleus (see Fig. 3.5 ). Some nuclear receptors, such as those that bind cortisol and aldosterone, are located in the cytosol and enter the nucleus after binding to hormone, whereas other receptors, including the thyroid hormone receptor, are located in the nucleus. In both cases, inactive receptors are bound to inhibitory proteins, and binding of hormone results in dissociation of the inhibitory complex. Hormone binding causes the receptor to bind coactivator proteins that activate gene transcription. Once activated, the hormone-receptor complex regulates the transcription of specific genes. Activation of specific genes usually occurs in two steps: an early primary response (≈30 minutes), which activates genes that stimulate other genes to produce a delayed (hours to days) secondary response (see Fig. 3.5 ). Each hormone elicits a specific response that is based on cellular expression of the cognate receptor, as well as on cell type–specific expression of gene regulatory proteins that interact with the activated receptor to regulate the transcription of a specific set of genes (see Chapter 38 for more details). In addition to steroid receptors that regulate gene expression, evidence also suggests the existence of membrane and juxtamembrane steroid receptors that mediate the rapid, nongenomic effects of steroid hormones.

Some membrane proteins do not fit the classic definition of receptors, but they subserve a receptor-like function in that they recognize extracellular signals and transduce the signals into an intracellular second messenger that has a biological effect. For example, on activation by a ligand, some membrane proteins undergo regulated intramembrane proteolysis (RIP) , which elaborates a cytosolic peptide fragment that enters the nucleus and regulates gene expression ( Fig. 3.6 ). In this signaling pathway, binding of ligand to a plasma membrane receptor leads to ectodomain shedding, facilitated by members of the metalloproteinase-disintegrin family, and produces a carboxy-terminal fragment that is the substrate for γ-secretase. γ-Secretase induces RIP, thereby causing the release of an intracellular domain of the protein that enters the nucleus and regulates transcription (see Fig. 3.6 ). The best characterized example of RIP is the sterol regulatory element–binding protein (SREBP), a transmembrane protein expressed in the membrane of the endoplasmic reticulum. When cellular cholesterol levels are low, SREBP undergoes RIP, and the proteolytically cleaved fragment is translocated into the nucleus, where it transcriptionally activates genes that promote cholesterol biosynthesis.

Receptors and Signal Transduction Pathways

When hormones bind to plasma membrane receptors, signals are relayed to effector proteins via intracellular signaling pathways. When hormones bind to nuclear or cytosolic receptors, they relay signals primarily through regulation of gene expression. Signaling pathways can amplify and integrate signals but can also downregulate and desensitize signals, reducing or terminating the response, even in the continued presence of hormone.

Intracellular signaling molecules—so-called second messengers (the first messenger of the signal is the ligand that binds to the receptor)—include small molecules such as cAMP, cGMP, Ca ⁺⁺ , and diacylglycerol. Signaling pathways often include dozens of small molecules that form complicated networks within the cell ( Fig. 3.7 ). Some proteins in the intracellular signaling pathways relay the signal by passing the message directly to another protein (e.g., by phosphorylating a target, or by binding and causing an allosteric change). Such intracellular signaling proteins act as reversible molecular switches : When a signal is received, they switch from an inactive to an active form or vice versa, until another signaling molecule reverses the process. This principle of reversibility is central to many signaling pathways. In many cases, activation is achieved by reversing inhibition: For example, the thyroid hormone receptor is bound to an inhibitory protein in the absence of signal.

Signaling complexes, composed of multiple proteins that interact physically, enhance the speed, efficiency, and specificity of signaling. Many proteins, usually enzymes or ion channels, transduce the signal into a different chemical form and simultaneously amplify the signal either by producing large amounts of additional signaling molecules or by activating a large number of downstream signaling proteins. For example, adenylyl cyclase, the enzyme that makes cAMP, transduces a signal (receptor activation of G proteins) and amplifies the signal by generating large amounts of cAMP. Other types of signaling proteins include those that integrate multiple signals. Other proteins carry the signal from one region of the cell to another: for example, by translocating from the cytosol to the nucleus.

Cells can respond quickly and in a graded manner to increasing concentrations of hormone, and the effect of a signaling molecule can be either long- or short-lived. Cells can also adjust their sensitivity to a signal by desensitization, whereby prolonged exposure to a hormone decreases the cell’s response over time. Desensitization is a reversible process that can involve a reduction in the number of receptors expressed in the plasma membrane, inactivation of receptors, or changes in signaling proteins that mediate the downstream effect of the receptors. Homologous desensitization involves a reduction in the response only to the signaling molecule that caused the response (e.g., opioid dependence and tolerance), whereas heterologous desensitization is when one ligand desensitizes the response to another ligand.

Table 3.1 summarizes the four general classes of receptors and provides a few examples of the signal transduction pathways associated with each class of receptors.