Signal Transduction

Signal Transduction Pathways

Transmission is continued either by a series of biochemical changes within the cell or by modification of the cell membrane potential by the movement of ions in or out of the cell. Receptors that initiate biochemical changes can do so either directly via intrinsic enzymatic activities within the receptor or by activating intracellular messenger molecules.

Signal transducing receptors are of four general classes:

1. Receptors that penetrate the plasma membrane and have intrinsic enzymatic activity or are enzyme associated (Enzyme-linked Receptors)
2. Receptors that are coupled, inside the cell, to G proteins (7-TM Receptors)
3. Receptors that are found intracellularly and upon ligand binding directly alter gene transcription (Nuclear Receptors)
4. Ligand-gated ion channels

The intracellular component of signal transduction is highly receptor specific, thereby maintaining the specificity of the incoming signal inside the cell. Signal transduction pathways amplify the incoming signal by a signaling cascade using a network of enzymes that act on one another in specific ways to ultimately generate a precise and appropriate physiological response by the cell. 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. Several small molecules within the cell act as intracellular messengers (also known as second messengers). These include cAMP, cGMP, nitric oxide, lipids and Ca²⁺ ions. Activated receptors stimulate second messenger production, which in turn activate other enzymes and so the cascade continues.

Physiology and Disease

One of the most important functions of cell signaling is to control and maintain normal physiological balance within the body. Activation of different signaling pathways leads to diverse physiological responses, such as cell proliferation, death, differentiation, and metabolism. In the last few years signal transduction therapy has become one of the most important areas of modern drug research. In a healthy organism, the processes of cellular growth and differentiation are tightly controlled, but in the pathological state, are uncoupled in such a way as to result in further damage-causing signals, or the growth of the malfunctioning cells. Proliferation of damaged or malfunctioning cells is often a key factor in the generation of disorders such as cancer, infectious diseases, inflammation, arteriosclerosis, arthritis, and neurodegenerative diseases.