Cells have the ability to rapidly adapt to changing signals. They can respond quickly and in a graded manner to increasing hormone concentrations, and the effect of a signaling molecule can be either long- or short-lived. Cells can also adjust their sensitivity to a signal through desensitization, which occurs when 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 refers to a reduction in the response only to the signaling molecule that caused the response, such as in opioid dependence and tolerance. On the other hand, heterologous desensitization occurs when one ligand desensitizes the response to another ligand.

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

One class of receptors converts a chemical signal into an electrical signal, which then elicits a response. For instance, the ryanodine receptor, found in the membrane of the sarcoplasmic reticulum of skeletal muscle, can be activated by Ca++, caffeine, adenosine triphosphate (ATP), or metabolites of arachidonic acid to release Ca++ into the cytosol. This release of Ca++ facilitates muscle contraction. Another example is the N-methyl-D-aspartate receptor, which, when activated by glutamate in glutamergic synapses with high levels of prior synaptic activity, stimulates Ca++ influx crucial for synaptic plasticity.

There are two classes of GTP-binding proteins: low-molecular-weight, monomeric G proteins, and heterotrimeric G proteins composed of α, β, and γ subunits. GTP binding activates these proteins, while hydrolysis of GTP to GDP inactivates them. GTPases, as these proteins are called, are regulated by other proteins, including GTPase-activating proteins, which induce the hydrolysis of GTP to GDP and thereby inactivate the GTPase, and guanine nucleotide exchange factors (GEFs), which cause the GTPase to release GDP. The GDP is then rapidly replaced by GTP, activating the GTPase.

Monomeric G proteins consist of a single 20 to 40-kDa protein and can be membrane bound due to the addition of lipids after translation. They are classified into five families: Ras, Rho, Rab, Ran, and Arf. Monomeric G proteins play a central role in many enzyme-linked receptor pathways and regulate gene expression, cell proliferation, differentiation, and survival. Rho GTPases control actin cytoskeletal organization, cell cycle progression, and gene expression. The Rab GTPases regulate intravesicular transport and protein trafficking between organelles in the secretory and endocytic pathways. Ran GTPases regulate the nucleocytoplasmic transport of RNA.

In signaling pathways, there are numerous proteins and small molecules that form intricate networks within the cell. Some proteins relay the signal by passing it to another protein, while others amplify the signal by producing additional signaling molecules or activating downstream signaling proteins. Certain proteins carry the signal from one region of the cell to another. This process of signal amplification and integration is illustrated in Figure 3.7.
Cells can adjust rapidly to changing signals. 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.
This class of receptors transduces a chemical signal into an electrical signal, which elicits a response. For example, the ryanodine receptor, located in the membrane of the sarcoplasmic reticulum of skeletal muscle, is activated by Ca++ , caffeine, adenosine triphosphate (ATP), or metabolites of arachidonic acid to release Ca++ into the cytosol, which facilitates muscle contraction (see for details). In glutamergic synapses in which high levels of prior synaptic activity have led to partial membrane depolarization, activation of the N-methyl-D-aspartate receptor by glutamate stimulates Ca++ influx important for synaptic plasticity.
There are two classes of GTP-binding proteins (i.e., GTPases, which are named for their ability to hydrolyze GTP to guanosine diphosphate [GDP] and an inorganic phosphate): low-molecular-weight, monomeric G proteins and heterotrimeric G proteins composed of α, β, and γ subunits. GTP binding activates, whereas hydrolysis of GTP to GDP inactivates, GTP-binding proteins (Fig. 3.8A ). All GTPases are controlled by regulatory proteins, including GTPase-activating proteins, which induce the hydrolysis of GTP to GDP and thus inactivate the GTPase, and guanine nucleotide exchange factors (GEFs) that causes the GTPase to release GDP, which is rapidly replaced by GTP, thereby activating the GTPase (see
Fig. 3.8B
Monomeric G proteins are composed of a single 20to 40-kDa protein and can be membrane bound because of the addition of lipids posttranslationally. Monomeric G proteins have been classified into five families (Ras, Rho, Rab, Ran, and Arf), play a central role in many enzyme-linked receptor pathways, and regulate gene expression and cell proliferation, differentiation, and survival. Rho GTPases regulate actin cytoskeletal organization, cell cycle progression, and gene expression. The Rab GTPases regulate intravesicular transport and trafficking of proteins between organelles in the secretory and endocytic pathways. Ran GTPases regulate nucleocytoplasmic transport of RNA and
CHAPTER 3 Signal Transduction, Membrane Receptors, Second Messengers, and Regulation of Gene Expression 43 • Fig. 3.7 Illustration of How Intracellular Signals Are Amplified and Integrated. Signaling pathways often include dozens of proteins and small molecules that form complicated networks within the cell. Some signaling proteins relay the signal by passing the message to another protein. Many proteins amplify the signal either by producing large amounts of additional signaling molecules or by activating a large number of downstream signaling proteins. Other proteins carry the signal from one region of the cell to another. See text for more details. (Redrawn from Alberts B, et al: Molecular Biology of the Cell. 6th ed. New York: Garland Science; 2015.).