signal transduction

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Signal transduction

The transmission of molecular signals from a cell's exterior to its interior. Molecular signals are transmitted between cells by the secretion of hormones and other chemical factors, which are then picked up by different cells. Sensory signals are also received from the environment, in the form of light, taste, sound, smell, and touch. The ability of an organism to function normally is dependent on all the cells of its different organs communicating effectively with their surroundings. Once a cell picks up a hormonal or sensory signal, it must transmit this information from the surface to the interior parts of the cell—for example, to the nucleus. This occurs via signal transduction pathways that are very specific, both in their activation and in their downstream actions. Thus, the various organs in the body respond in an appropriate manner and only to relevant signals. See Cell (biology)

All signals received by cells first interact with specialized proteins in the cells called receptors, which are very specific to the signals they receive. These signals can be in various forms. The most common are chemical signals, which include all the hormones and neurotransmitters secreted within the body as well as the sensory (external) signals of taste and smell. The internal hormonal signals include steroid and peptide hormones, neurotransmitters, and biogenic amines, all of which are released from specialized cells within the various organs. The external signals of smell, which enter the nasal compartment as gaseous chemicals, are dissolved in liquid and then picked up by specialized receptors. Other external stimuli are first received by specialized receptors (for example, light receptors in the eye and touch receptors in the skin), which then convert the environmental signals into chemical ones, which are then passed on to the brain in the form of electrical impulses.

Once a receptor has received a signal, it must transmit this information effectively into the cell. This is accomplished either by a series of biochemical changes within the cell or by modifying the membrane potential by the movement of ions into 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. Receptors may be broadly classified in four groups that differ in their mode of action and in the molecules that activate them.

The largest family of receptors are the G-protein-coupled receptors (GPCRs), which depend on guanosine triphosphate (GTP) for their function. Many neurotransmitters, hormones, and small molecules bind to and activate specific G-protein-coupled receptors.

A second family of membrane-bound receptors are the receptor tyrosine kinases (RTKs). They function by phosphorylating themselves and recruiting downstream signaling components.

Ion channels are proteins open upon activation, thereby allowing the passage of ions across the membrane. Ion channels are responsive to either ligands or to voltage changes across the membrane, depending on the type of channel. The movement of ions changes the membrane potential, which in turn changes cellular function. See Biopotentials and ionic currents

Steroid receptors are located within the cell. They bind cell-permeable molecules such as steroids, thyroid hormone, and vitamin D. Once these receptors are activated by ligand, they translocate to the nucleus, where they bind specific DNA sequences to modulate gene expression. See Steroid

The intracellular component of signal propagation, also known as signal transduction, is receptor-specific. A given receptor will activate only very specific sets of downstream signaling components, thereby maintaining the specificity of the incoming signal inside the cell. In addition, signal transduction pathways amplify the incoming signal by a signaling cascade (molecule A activates several molecule B's, which in turn activate several molecule C's) resulting in an appropriate physiological response by the cell.

Several small molecules within the cell act as intracellular messengers. These include cAMP, cyclic guanosine monophosphate (cGMP), nitric oxide (NO), and Ca2+ ions. Increased levels of Ca2+ in the cell can trigger several changes, including activation of signaling pathways, changes in cell contraction and motility, or secretion of hormones or other factors, depending on the cell type. Increased levels of nitric oxide cause relaxation of smooth muscle cells and vasodilation by increasing cGMP levels within the cell. Increasing cAMP levels can modulate signaling pathways by activating the enzyme protein kinase A (PKA).

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. Signaling pathways in cells may also interact with each other and serve as signal integrators. For example, negative and positive feedback loops in pathways can modulate signals within a pathway; positive interactions between two signaling pathways can increase duration of signals; and negative interactions between pathways can block signals. See Cell nucleus, Cell organization, Endocrine system (vertebrate), Noradrenergic system

signal transduction

[′sig·nəl tranz‚dək·shən]
(cell and molecular biology)
The relaying of molecular signals (for example, as contained in a hormone) or physical signals (for example, sensory stimuli) from a cell's exterior to its intracellular response mechanisms.
References in periodicals archive ?
Disruption of signal transduction pathways can occur in other ways.
Current therapeutic targets: Signal transduction pathways, angiogenesis, pathways regulating cellular proliferation, growth, survival, differentiation, Philadelphia chromosome, histone modification, the proteasome, immune cells.
It can also provide molecular analysis of the cancers, based on the functional activity of signal transduction pathways, which modulate cancerous growth.
The aim of this proposal is to identify the role played by members of the rho-subfamily in controlling integrin-dependent signal transduction pathways active during G1 phase of the cell cycle in mammals.
CONCLUSIONS: Anthocyanins and other phenolics from bilberry upregulate the oxidative stress defense enzymes HO-1 and GST-pi in RPE, suggesting that they stimulate signal transduction pathways influencing genes controlled by the antioxidant response element.
Researchers in a number of disciplines in addition to development and toxicology contribute to discussions of such matters as signal transduction pathways as targets for teratogens, personalized nutrition and medicine in perinatal development, zebrafish as a non-mammalian model of developmental toxicology, integrating whole-animal developmental toxicity data into risk assessment, genomic approaches, and investigating drug effects in human pregnancy.
There is general agreement that arsenic does not directly interact with DNA, and that its toxic effects occur through indirect alteration of gene expression, such as via the perturbation of DNA methylation, inhibition of DNA repair, oxidative stress, and altered modulation of signal transduction pathways.
Perifosine is a novel oral anticancer agent that modulates several key signal transduction pathways, including Akt, MAPK, and JNK that have been shown to be critical for the survival of cancer cells.
The study of protein phosphorylation can provide key insights into the signal transduction pathways that are activated in cells in response to different stimuli, such as growth factor stimulation and exposure to toxicants.
Prolactin (PRL) and its receptor: actions, signal transduction pathways and phenotypes observed in PRL receptor knockout mice.
AlphaScreen([R]) SureFire([R]) assays provide a cell-based environment for allowing the measurement of endogenous proteins in cell lysates, as well as measurement of key signal transduction pathways implicated in many disease states.
This volume of the proceedings of the Academy's meeting "Cell Signaling World: Signal Transduction Pathways as Therapeutic Targets" held January 2006 in Luxembourg addresses current research on signal transduction in such topics as bacterial endotoxin, expression and suppression of cytokine signaling-3, control of human herpes virus Type 8, regulation of apolipoprotein B secretions but not lipids by tumor necrosis factors, signaling pathways used by HSV-1 and by proteinase activated receptors and melphalan's role in the reduction of the severity of experimental colitis.

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