In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself.[1][2][3] Cell signaling is a fundamental property of all cellular life in prokaryotes and eukaryotes.[4] Signals that originate from outside a cell (or extracellular signals) can be physical agents like mechanical pressure, voltage, temperature, light, or chemical signals (e.g., small molecules, peptides, or gas). Cell signaling can occur over short or long distances, and as a result can be classified as autocrine, juxtacrine, intracrine, paracrine, or endocrine. Signaling molecules can be synthesized from various biosynthetic pathways and released through passive or active transports, or even from cell damage.
Receptors play a key role in cell signaling as they are able to detect chemical signals or physical stimuli. Receptors are generally proteins located on the cell surface or within the interior of the cell such as the cytoplasm, organelles, and nucleus. Cell surface receptors usually bind with extracellular signals (or ligands), which causes a conformational change in the receptor that leads it to initiate enzymic activity, or to open or close ion channel activity. Some receptors do not contain enzymatic or channel-like domains but are instead linked to enzymes or transporters. Other receptors like nuclear receptors have a different mechanism such as changing their DNA binding properties and cellular localization to the nucleus.
Signal transduction begins with the transformation (or transduction) of a signal into a chemical one, which can directly activate an ion channel (ligand-gated ion channel) or initiate a second messenger system cascade that propagates the signal through the cell. Second messenger systems can amplify a signal, in which activation of a few receptors results in multiple secondary messengers being activated, thereby amplifying the initial signal (the first messenger). The downstream effects of these signaling pathways may include additional enzymatic activities such as proteolytic cleavage, phosphorylation, methylation, and ubiquitinylation.
Each cell is programmed to respond to specific extracellular signal molecules,[5] and is the basis of development, tissue repair, immunity, and homeostasis. Errors in signaling interactions may cause diseases such as cancer, autoimmunity, and diabetes.[6][7][8][9]
In many small organisms such as bacteria, quorum sensing enables individuals to begin an activity only when the population is sufficiently large. This signaling between cells was first observed in the marine bacterium Aliivibrio fischeri, which produces light when the population is dense enough.[10] The mechanism involves the production and detection of a signaling molecule, and the regulation of gene transcription in response. Quorum sensing operates in both gram-positive and gram-negative bacteria, and both within and between species.[11]
In slime moulds, individual cells known as amoebae aggregate together to form fruiting bodies and eventually spores, under the influence of a chemical signal, originally named acrasin. The individuals move by chemotaxis, i.e. they are attracted by the chemical gradient. Some species use cyclic AMP as the signal; others such as Polysphondylium violaceum use other molecules, in its case N-propionyl-gamma-L-glutamyl-L-ornithine-delta-lactam ethyl ester, nicknamed glorin.[12]