Cell Communication

FIELDS OF STUDY: Biochemistry; Molecular Biology; Genetics

ABSTRACT

The process of cell communication is defined, and its importance in biochemical processes is elaborated. Cell communication is an essential feature of nerve function and control of the metabolism of living systems, affecting the operation of the autonomic and sympathetic nervous systems.

The Basics of Cell Communication

Cell communication is also known as cell signaling or extracellular signaling. It occurs via a complex multistep process that uses several different pathways. The process begins with the synthesis of a specific signaling compound within a cell or group of cells of the same type. After synthesis, the signaling compound is released by the cell, typically crossing the cell membrane via an active-transport mechanism. The signaling compound is then transported to its target cell. At the target cell, the signaling compound is detected by a specific protein or enzyme receptor site. The detection of the signaling compound triggers a change in cellular metabolism or gene expression in the cell to which the receptor is attached. The final step in the process is the removal of the signaling compound from the receptor and its subsequent elimination from the system.

Cell Communication Functions and Methods

Cell communication or signaling serves a number of purposes, depending on the nature of the living system in which it occurs. In single-cell organisms such as bacteria and protozoa, the production of specific cell-communication compounds coordinates the organisms for cell differentiation or mating, according to which particular reproductive pathway is appropriate. For more complex life-forms, this role is carried out by compounds called pheromones, which can range from relatively simple hydrocarbons to complex proteins.

Pheromones are typically released directly into the environment, where they are detected by other members of the same species. The sense of smell plays a significant role in the detection of pheromones; in many creatures, specific anatomical structures exist for the sole purpose of detecting pheromone molecules. Moths and butterflies, for example, have receptors in their antennae that have evolved to maximize their sensitivity to specific airborne molecules. Almost all other species have a molecule-sensitive structure called the vomeronasal organ as part of their olfactory sense.

This method of cell communication is also used by plants. The production of aroma compounds and other molecular materials for the attraction of pollinators has long been known. More recent studies have also shown that plants use chemical signals to communicate with other plants and to exert some control over their environment by inhibiting the growth of competitors or promoting the growth of companion plants.

Cell communication within organisms is a more fundamental function and is responsible for a great many essential features of living biochemical systems. Intramolecular cell communication is essential to the control of metabolic processes within the cells of organisms. An extremely complex variety of chemical reactions takes place within cells, involving tens of thousands of different proteins and other biochemicals. All of these many and varied reactions are interconnected by their necessity for maintaining the existence of the organism, and a great many are more intimately related to each other as components of various cyclic biochemical mechanisms.

One function of intracellular communication is to control tissue growth. Signaling by hormones controls anabolic processes, which are processes that extract energy from adenosine triphosphate (ATP) to power the movement of various materials across cell membranes; the construction of protein molecules from amino acids; the movement of calcium into bone structures; and many other processes that build up and maintain the physical structure of the organism. The many biochemical processes that take place within a cell’s organelles (organ-like structures within the cell) and cytosol (intracellular fluid surrounding the organelles) extract energy and convert needed materials into unusable product materials that must be eliminated from the organism. Catabolic processes, also controlled by hormone signaling, are the processes that break down and eliminate by-products of metabolism and materials that are no longer required, such as signaling molecules that have already served their function.

Types of Signaling

Cell communication takes place between cells or within cells. Endocrine signaling is typical of hormonal processes in which the signaling compound is produced in one part of the organism, such as the endocrine glands, and carried in the bloodstream to receptor sites located in other parts of the organism at some distance from the signaling compound’s point of origin. Paracrine signaling, which affects receptors located on cells that are in close proximity to the originating cell, is typical of such functions as nerve-signal transmission across neural synapses and is mediated by acetylcholine and other compounds that function as neurotransmitters and neurohormones. Like paracrine signaling, juxtacrine signaling also takes place between adjacent cells; however, juxtacrine signaling relies on physical contact. In autocrine signaling, a cell responds to its own self-synthesized signaling compounds; receptors may be within the cell or located on the outer surface of the cell membrane. This method of cell communication is typical of the action of growth hormones and is especially relevant to the growth of tumors. There is some crossover in the methods of cell communication, since many compounds can signal by more than one method.

Receptors in Cell Communication

All methods of cell communication function by the interaction of a signaling compound with the corresponding receptor. Receptor proteins only bind to specific signaling compounds; their molecular structure also plays a role in their function. Lipophilic (literally, "fat-loving") receptor proteins found on the surfaces of cell and organelle membranes in the cytosol typically interact with fats, oils, and other hydrophobic ("water-fearing") signaling compounds. Hydrophilic ("water-loving") receptor proteins, normally found on cell surfaces, interact with polar or water-soluble signaling compounds. Activation of cell surface receptors often triggers the formation of a secondary signaling compound that delivers the signal into the cell. Secondary signaling compounds formed in response to such triggering include the cyclic forms of adenosine monophosphate (cAMP) and guanisine monophosphate (cGMP), inositol triphosphate (IP₃), and diacylglycerol (DAG).

Receptors are classified in one of four major categories. G-protein-coupled receptors (GPCRs) interact with compounds such as epinephrine, serotonin, and glucagon to activate G proteins that subsequently activate or inhibit a second messenger or ion channel, ultimately bringing about a change in the cell function. Ion-channel-linked receptors, typically activated by acetylcholine, change the conformation of the ion channel to allow the passage of specific ions across the cell membrane. A third class of receptor is the enzyme-linked receptors, which either behave as enzymes themselves or activate associated enzymes. The fourth class of receptor is nuclear receptors, which are found within cells rather than on the cell surface and mainly respond to steroid and thyroid hormones.

Signaling compounds that function in cell communication are many and varied in structure. The neurotransmitter acetylcholine is the principal signaling compound in nerve synapses. Other paracrine signaling compounds include dopamine, serotonin, and gamma-amino butyric acid (GABA). Endocrine signaling typically involves hormones as the signaling compounds. Most hormones are steroid compounds, based on the molecular structure of cholesterol, though the very important thyroid hormone thyroxine is not.

PRINCIPAL TERMS

• autocrine signaling: a type of cell signaling in which the signaling compound is produced within a cell and delivered to receptors on the outside of the same cell.
• endocrine signaling: a type of cell signaling in which the signaling compound is produced in one location in the body and transported to a receptor site some distance away.
• juxtacrine signaling: a type of cell signaling in which the signaling compound is produced within a cell and delivered to receptors in an adjacent cell via physical contact.
• paracrine signaling: a type of cell signaling in which the signaling compound is produced in one location in the body and delivered to receptors in a nearby cell.
• receptor: a molecule or molecular structure, typically a protein or enzyme, that interacts only with compounds that have a matching molecular structure; the interaction normally triggers a biochemical response in the cells to which the receptor is attached.
• transmitter: a biochemical compound produced to trigger a specific response at a corresponding receptor site.

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