All cells receive and respond to signals from their environment. Bacteria swim towards high concentration of nutrients. Many unicellular eukaryotes also respond to signaling molecules secreted by other cells, allowing cell-cell communication. Mating between yeast cells, for example, is signaled by peptides that are secreted by one cell and bind to receptors on the surface of another. In multicellular organisms, however, that cell-cell communication reaches its highest level of sophistication. That is achieved by a variety of signaling molecules that are secreted or expressed by other cells, thereby integrating and coordinating the functions of the many individual cells that make up organisms as complex as human beings.
The binding of most signaling molecules to their receptors initiates a series of intracellular reactions that regulate virtually all aspects of cell behavior, including metabolism, movement, proliferation, survival and differentiation.
Signaling molecules and their receptors
Many different kinds of molecules transmit information between the cells of multicellular organisms. Although all these molecules act as ligands that bind to receptors expressed by their target cells, there is considerable variation in the structure and function of the different types of molecules that serve as signal transmitters. Signaling molecules are generally proteins. Some signaling molecules are able to cross the plasma membrane and bind to intracellular receptors in the cytoplasm or nucleus, whereas most bind to receptors expressed on the target cell surface.
Modes of Cell-Cell signaling
The multiple varieties of signaling by secreted molecules are frequently divided into three general categories based on the distance over which signals are transmitted. In endocrine signaling, the signaling molecules (hormones0 are secreted by specialized endocrine cells and carried through the circulation to act on target cells at distant body sites. A classic example is provided by the steroid hormone estrogen, which is produced by the ovary and stimulates development and maintenance of the female reproductive system and secondary sex characteristics.
In contrast to hormones, some signaling molecules act locally to affect the behavior of nearby cells. In paracrine signaling, a molecule released by the behavior of nearby cells. In paracrine signaling, a molecule released by one cell acts on neighboring target cells. An example is provided by the action of neurotransmitters in carrying signals between nerve cells at a synapse.
Finally, some cells respond to signaling molecules they themselves produce. One important example of such autocrine signaling is the response of cells of the vertebrate immune system to foreign antigens. Certain types of T-lymphocytes respond to antigenic stimulation by synthesizing a growth factor that drives their own proliferation, thereby increasing the number of responsive T lymphocytes and amplifying the immune response.
Functions of cell-surface receptors
Most ligands responsible for cell-cell signaling (including neurotransmitters, peptide hormones, growth factors) bind to receptors on the surface of their target cells. Consequently, a major challenge in understanding cell-cell signaling is unraveling the mechanisms by which cell surface receptors transmit the signals initiated by ligand binding. Some neurotransmitter receptors are ligandgated ion channels that directly control ion flux across the plasma membrane. Other cell surface receptors, including the receptors for peptide hormones and growth factors, act instead by regulating the activity of intracellular proteins. These proteins then transmit signals from the receptor to a series of additional intracellular targets, frequently including transcription factors. Ligand binding to a receptor on the surface of the cell thus initiates a chain of intracellular reactions, ultimately reaching the target cell nucleus and resulting in programmed changes in gene expression.