Cell signaling
[citation needed] In biology, signals are mostly chemical in nature, but can also be physical cues such as pressure, voltage, temperature, or light.), lipids (e.g. steroid, prostaglandin), peptides (e.g. insulin, ACTH), carbohydrates, glycosylated proteins (proteoglycans), nucleic acids, etc.On the other hand, liposoluble chemicals such as steroid hormones, can diffuse passively across the plasma membrane and interact with intracellular receptors.Cell signaling can occur over short or long distances, and can be further classified as autocrine, intracrine, juxtacrine, paracrine, or endocrine.Receptors have the ability to detect a signal either by binding to a specific chemical or by undergoing a conformational change when interacting with physical agents.The intracellular domain is responsible for the initiation of a cascade of chemical reactions which ultimately triggers the specific cellular function controlled by the receptor.The downstream effects of these signaling pathways may include additional enzymatic activities such as proteolytic cleavage, phosphorylation, methylation, and ubiquitinylation.Signaling molecules can be synthesized from various biosynthetic pathways and released through passive or active transports, or even from cell damage.Each cell is programmed to respond to specific extracellular signal molecules, and is the basis of development, tissue repair, immunity, and homeostasis.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.[6] In slime molds, individual cells aggregate together to form fruiting bodies and eventually spores, under the influence of a chemical signal, known as an acrasin.Signaling molecules can belong to several chemical classes: lipids, phospholipids, amino acids, monoamines, proteins, glycoproteins, or gases.Signaling molecules binding surface receptors are generally large and hydrophilic (e.g. TRH, Vasopressin, Acetylcholine), while those entering the cell are generally small and hydrophobic (e.g. glucocorticoids, thyroid hormones, cholecalciferol, retinoic acid), but important exceptions to both are numerous, and the same molecule can act both via surface receptors or in an intracrine manner to different effects.[10] Hydrogen sulfide is produced in small amounts by some cells of the human body and has a number of biological signaling functions.[12] In exocytosis, membrane-bound secretory vesicles are carried to the cell membrane, where they dock and fuse at porosomes and their contents (i.e., water-soluble molecules) are secreted into the extracellular environment.There are three types: Additionally, in unicellular organisms such as bacteria, juxtacrine signaling means interactions by membrane contact.Juxtacrine signalling via direct membrane contacts is also present between neuronal cell bodies and motile processes of microglia both during development,[15] and in the adult brain.For example, epinephrine and norepinephrine can function as hormones when released from the adrenal gland and are transported to the heart by way of the blood stream.Binding of a paracrine factor to its respective receptor initiates signal transduction cascades, eliciting different responses.[23] Cell surface receptors play an essential role in the biological systems of single- and multi-cellular organisms and malfunction or damage to these proteins is associated with cancer, heart disease, and asthma.The ligands that bind and activate these receptors include light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters, and vary in size from small molecules to peptides to large proteins.The long ago discovered association between GPCRs and many endogenous and exogenous substances, resulting in e.g. analgesia, is another dynamically developing field of pharmaceutical research.[citation needed] Examples of the enzymatic activity include: Intracellular receptors exist freely in the cytoplasm, nucleus, or can be bound to organelles or membranes.They are generally intracellular receptors (typically cytoplasmic or nuclear) and initiate signal transduction for steroid hormones which lead to changes in gene expression over a time period of hours to days.The multistep process of the transduction stage is often composed of the activation of proteins by addition or removal of phosphate groups or even the release of other small molecules or ions that can act as messengers.The entire set of cell changes induced by receptor activation is called a signal transduction mechanism or pathway.Many cellular proteins are activated downstream of the growth factor receptors (such as EGFR) that initiate this signal transduction pathway.[45] In unicellular organisms such as bacteria, signaling can be used to 'activate' peers from a dormant state, enhance virulence, defend against bacteriophages, etc.[46] In quorum sensing, which is also found in social insects, the multiplicity of individual signals has the potentiality to create a positive feedback loop, generating coordinated response.
Cellular SignallingbiologyBritish Englishprocesscellular lifeprokaryoteseukaryotespressurevoltagetemperaturereceptorautocrineintracrinejuxtacrineparacrineendocrineAutocrine signalingReceptorscytoplasmorganellesnucleusCell membrane receptorsG-protein receptorsEnzyme-linked receptorstransmembrane proteinsIntracellular receptorssignal transductionligand-gated ion channelsecond messenger systemdownstream effectsproteolytic cleavagephosphorylationmethylationubiquitinylationpassiveactive transportscell damagedevelopmenttissue repairimmunityhomeostasiscancerautoimmunitydiabetesbacteriaquorum sensingAliivibrio fischeriproduces lightslime moldsacrasinchemotaxiscyclic AMPPolysphondylium violaceumdipeptideextracellular spaceparacrine signalingendocrine signalingjuxtacrine signalingnotch signalingSynapticchemical synapseselectrical synapsesneuronslipidsphospholipidsamino acidsmonoaminesproteinsglycoproteinshydrophilicVasopressinAcetylcholinehydrophobicglucocorticoidsthyroid hormonescholecalciferolretinoic acidnitric oxideExocytosismoleculesneurotransmittersactive transportendocytosischemical substancescell membranepassive transportporosomesvesiclessecretionneurotransmissionsynaptic vesiclessynaptic cleftreverse transportmembrane transport proteinsextracellular matrixmulticellularligandproteinoligosaccharidemembrane proteininteractjunctionglycoproteinunicellulargrowth factorscytokinechemokineimmune responsemicrogliaendocrine factorscirculatory systemjuxtacrine interactionsextracellularepinephrinenorepinephrineadrenal glandEstrogenorgansfibroblast growth factorHedgehogTGF-β superfamilyhormonesglandsorganismvertebrateshypothalamus