Signal transduction

In biology '''signal transduction''' is any process by which a cell (biology)|cell converts one kind of signal or stimulus into another Processes referred to as signal transduction often involve a sequence of biochemistry|biochemical chemical reaction|reactions inside the cell which are carried out by enzymes and linked through second messengers Such processes take place in as little time as a millisecond or as long as a few seconds Slower processes are rarely referred to as signal transduction
In many transduction processes an increasing number of enzymes and other molecules become engaged in the events that proceed from the initial stimulus In such cases the chain of steps is referred to as a "signalling cascade" or "second messenger pathway" and the result is that a small stimulus elicits a large response
In bacteria and other one-cell organisms the variety of signal transduction processes of which the cell is capable influences how many ways it can react and respond to its environment In a less direct way the same is true of animals and plants Sensing in all forms of life depends at the cellular level on signal transduction

Overview

Stimuli

The environment of a cell may impinge on it in many ways: different kinds of molecules may buffet its surface its body may be heated or cooled it may be struck by light of various wavelengths stretched sheared or electrified (the nerves and muscles for example) Signal transduction mediates how cells respond to such stimuli
Most stimuli impinge from the outside and interact with the cell membrane Many "signaling molecules" such as the neurotransmitters that allow nerve cells to communicate across synapses bind to receptor proteins in the membrane and open portals in it

Responses

Responses triggered by signal transduction include the activation of a gene the production of metabolic energy and cell locomotion for example through remodelling of the cell skeleton
Gene activation leads to further effects since genes are expressed as proteins many of which are enzymes transcription factors or other regulators of metabolic activity Because transcription factors can activate still more genes in turn an initial stimulus can trigger via signal transduction the expression of entire suite of genes and a panoply of physiolgical events Such mass activations are often referred to as "genetic programs" one example being the sequence of events that take place when an egg is fertilized by a sperm

Types of signals

Extracellular

Signal transduction usually involves the binding of "extracellular" signaling molecules to receptors that face outwards from the membrane and trigger events inside This takes place via a change in the shape or conformation of the receptor which occurs when the signal molecule "docks" or binds Receptors typically respond only to the specific molecule or "ligand" for which they have affinity and molecules that are even only slightly different tend to have no effect or else to act as inhibitors
Most extracellular chemical signals have affinity for water and are unable to penetrate the oily barrier posed by the membrane that surrounds cells A common kind of extracellular signal is nutrient In complex organism this includes the ligands responsible for sensations of smell and taste Steroids represent an example of extracellular signals that can cross the membrane to permeate cells which they are able to do because of a partial affinity for oily surroundings (see hydrophobic)

Intracellular

Often but not always the intracellular events triggered by the external signal are considered distinct from the event of "transduction" itself which in the strictest sense refers only to the step that converts the extracellular signal to an intracellular one
Intracellular signalling molecules in eukaryotic cells include heterotrimeric G protein small GTPases cyclic nucleotides such as cyclic AMP (cAMP) and cyclic GMP (cGMP) calcium ion phophoinositide derivatives such as Phosphatidylinositol-triphosphate(PIP₃) Diacylglycerol and Inositol-triphosphate (IP₃) and various protein kinases and phosphatase Some of these are also called second messengers

Intercellular

Intercellular communication is accomplished by extracellular signalling and takes place in complex organisms that are composed of many cells Within endocrinology which is the study of intercellular signalling in animalsintercellular signalling is subdivided into the following types:

Endocrine signals are produced by endocrine cells and travel through the blood to reach all parts of the body
Paracrine signals target only cells in the vicinity of the emitting cell Neurotransmitters represent an example
Autocrine signals affect only cells that are of the same cell type as the emitting cell An example for autocrine signals is found in immune cells
Juxtacrine signals are transmitted along cell membranes via protein or lipid components integral to the membrane and are capable of affecting either the emitting cell or cells immediately adjacent

Hormones

Most of the molecules that enable signalling between the cells or tissues within an individual animal or plant are known as "hormones" Hormone-initiated signal transduction takes the following steps:

Biosynthesis of a hormone
Storage and secretion of the hormone
Transport of the hormone to the target cell
Recognition of the hormone by the hormone receptor protein leading to a conformational change
Relay and amplification of the signal that leads to defined biochemical reactions within the target cell The reactions of the target cells can in turn cause a signal to the hormone-producing cell that leads to the down-regulation of hormone production
Removal of the hormone

Hormones and other signaling molecules may exit the sending cell by exocytosis or other means of membrane transport The sending cell is typically of a specialized type Its recipents may be of one type or several as in the case of insulin which triggers diverse and systemic effects
Hormones signalling is elaborate and hard to dissect A cell can have several different receptors that recognize the same hormone but activate different signal transduction pathways; or different hormones and their receptors can invoke the same biochemical pathway Different tissue types can answer differently to the same hormone stimulus There are two classes of hormone receptors "membrane-associated receptors" and intracellular or "cytoplasmic" receptors

Types of receptors

Transmembrane receptors

Transmembrane receptors are proteins that span the thickness of the plasma membrane of the cell with one end of the receptor outside (extracellular domain) and one inside (intracellular domain) the cell When the extracellular domain recognizes the hormone the whole receptor undergoes a structural shift that affects the intracellular domain leading to further action In this case the hormone itself does not pass through the plasma membrane into the cell

Hormone recognition by transmembrane receptors

The recognition of the chemical structure of a hormone by the hormone receptor uses the same (non-covalent) mechanisms such as hydrogen bonds electrostatic forces hydrophobe and Van der Waals forces The equivalent between receptor-bound and free hormone equals [1] + [2] <-> [3] with

K_d = { {[4] * [5]} over {[6]} }

[7]=receptor; [8]=free hormone; [9]=receptor-bound hormone
The important value for the strength of the signal relayed by the receptor is the concentration of the hormone-receptor complex which is defined by the affinity of the hormone for the receptor the concentration of the hormone and of course the concentration of the receptor The concentration of the circulating hormone is the key value for the strength of the signal since the other two values are constant For fast reaction the hormone-producing cells can store prehormones and quickly modify and release them if necessary Also the recipient cell can modify the sensitivity of the receptor for example by phosphorylation; also the variation of the number of receptors can vary the total signal strength in the recipient cell

Signal transduction of transmembrane receptors by structural changes

Signal transduction across the plasma membrane is possible only by many components working together First the receptor has to recognize the hormone with the extracellular domain then activate other proteins within the cytosol with its cytoplasmic domain which the protein does through a shift in conformation The activated effector proteins usually stay close to the membrane or are anchored within the membrane by lipid anchor a posttranslational modification (see myristoilation palmitorylation farnesylation geranylation and the glycosyl-phosphatidyl-inositol-anchor) Many membrane-associated proteins can be activated in turn or come together to form a multi-protein complex that finally sends a signal via a soluble molecule into the cell

Signal transduction of transmembrane receptors that are ion channels

A ligand-activated ion channel will recognize its ligand and then undergo a structural change that opens a gap (channel) in the plasma membrane through which ions can pass These ions will then relay the signal An example for this mechanism is found in the receiving cell of a synapse

Signal transduction of transmembrane receptors on change of transmembrane potential

An ion channel can also open when the receptor is activated by a change in cell potential that is, the difference of the electrical charge on both sides of the membrane If such a change occurs the ion channel of the receptor will open and let ions pass through In neurons this mechanism underlies the action potential impulses that travel along nerves

Nuclear receptors

Nuclear (or cytoplasmic) receptors are soluble proteins localized within the cytoplasm or the nucleoplasm The hormone has to pass through the plasma membrane usually by passive diffusion to reach the receptor and initiate the signal cascade The nuclear receptors are ligand-activated transcription activators; on binding with the ligand (the hormone) they will pass through the nuclear membrane into the nucleus and enable the production of a certain gene and thus the production of a protein

The typical ligands for nuclear receptors are lipophilic hormones with steroid hormones (for example testosterone progesterone and cortisol) and derivatives of vitamin A and D among them These hormones play a key role in the regulation of metabolism organ function developmental processes and cell differentiation The key value for the signal strength is the hormone concentration which is regulated by :

Biosynthesis and secretion of hormones in the endocrine tissue As an example the hypothalamus receives information both electrical and chemical It produces releasing factors that affect the hypophysis and make it produce glandotrope hormones which in turn activate endocrine organs so that they finally produce hormones for the target tissues This hierarchical system allows for the amplification of the original signal that reached the hypothalamus The released hormones dampen the production of these hormones by feedback inhibition to avoid overproduction
Availability of the hormone in the cytosol Several hormones can be converted into a storage form by the target cell for later use This reduces the amount of available hormone
Modification of the hormone in the target tissue Some hormones can be modified by the target cell so they no longer trigger the hormone receptor (or at least not the same one) effectively reducing the amount of available hormone

The nuclear receptors that were activated by the hormones attach at the DNA at receptor-specific Hormone Responsive Elements (HREs) DNA sequences that are located in the promoter region of the genes that are activated by the hormone-receptor complex As this enables the transcription of the according gene these hormones are also called inductors of gene expression The activation of gene transcription is much slower than signals that directly affect existing proteins As a consequence the effects of hormones that use nuclearic receptors are usually long-term Although the signal transduction via these soluble receptors involves only a few proteins the details of gene regulation are yet not well understood The nuclearic receptors all have a similar modular structure:

N-AAAABBBBCCCCDDDDEEEEFFFF-C

where CCCC is the DNA-binding domain that contains zinc fingers and EEEE the ligand-binding domain The latter is also responsible for dimerization of most nuclearic receptors prior to DNA binding As a third function it contains structural elements that are responsible for transactivation used for communication with the translational apparatus The zinc fingers in the DNA-binding domain stabilize DNA binding by holding contact to the phosphate backbone of the DNA The DNA sequences that match the receptor are usually hexameric repeats either normal inverted or everted The sequences are quite similar but their orientation and distance are the parameters by which the DNA-binding domains of the receptors can tell them apart

Steroid receptors

Steroid receptors are a subclass of nuclear receptors located primarily within the cytosol In the absence of steroid hormone the receptors cling together in a complex called aporeceptor complex which also contains chaperone proteins (also known as heatshock proteins or Hsps) The Hsps are necessary to activate the receptor by assisting the protein to fold in a way such that the signal sequence which enables its passage into the nucleus is accessible
Steroid receptors can also have a repressive effect on gene expression when their transactivation domain is hidden so it cannot activate transcription Furthermore steroid receptor activity can be enhanced by phosphorylation of serine residues at their N-terminal end as a result of another signal transduction pathway for example a by a growth factor This behaviour is called crosstalk

RXR- and orphan-receptors

These nucleric receptors can be activated by

a classic endocrine-synthesized hormone that entered the cell by diffusion
a hormone that was build within the cell (for example retinol) from a precursor or prohormone which can be brought to the cell through the bloodstream
a hormone that was completely synthesized within the cell for example prostaglandin

These receptors are located in the nucleus and are not accompanied by chaperone proteins In the absence of hormone they bind to their specific DNA sequence repressing the gene Upon activation by the hormone they activate the transcription of the gene they were repressing

Signal amplification

A principle of signal transduction is the signal amplification The binding of one or a few neurotransmitter molecules can enable the entry of millions of ions The binding of one or just a few hormone molecules can induce an enzymatic reaction that affect many substrates The amplification can occur at several points of the signal pathway

Signal amplification at the transmembrane hormone receptor

A receptor that has been activated by a hormone can activate many downstream effector proteins For example a rhodopsin molecule in the plasma membrane of a retina cell in the eye that was activated by a photon can activate up to 2000 effector molecules (in this case transducin) per second The total strength of signal amplification by a receptor is determined by:

The lifetime of the hormone-receptor-complex The more stable the hormone-receptor-complex is, the less likely the hormone dissociates from the receptor the longer the receptor will remain active thus activate more effector proteins
The amount and lifetime of the receptor-effector protein-complex The more effector protein is available to be activated by the receptor and the faster the activated effector protein can dissociate from the receptor the more effector protein will be activates in the same amount of time
Deactivation of the activated receptor A receptor that is engaged in a hormone-receptor-complex can be deactivated either by covalent modification (for example phosphorylation) or by internalization (see ubiquitin)

Intracellular signal transduction

Intracellular signal transduction is largely carried out by second messenger molecules

Ca²⁺ as a second messenger

Ca²⁺ acts as a signal molecule within the cell This works by tightly limiting the time and space when Ca²⁺ is free (and thus active) Therefore the concentration of free Ca²⁺ within the cell is usually very low; it is stored within organelles usually the endoplasmic reticulum (sarcoplasmic reticulum in muscle cells) where it is bound to molecules like calreticulin

Activation of Ca²⁺

To become active Ca²⁺ has to be released from the endoplasmic reticulum into the cytosol There are two combined receptor/ion channel proteins that perform the task of controlled transport of Ca²⁺:

The InsP₃-receptor will transport Ca²⁺ upon interaction with inositol triphosphate (thus the name) on its cytosolic side It consists of four identical subunits
The ryanodine-receptor is named after the plant alkaloid ryanodine It is similar to the InsP₃ receptor and stimulated to transport Ca²⁺ into the cytosol by recognizing Ca²⁺ on its cytosolic side thus establishing a feedback mechanism; a small amount of Ca²⁺ in the cytosol near the receptor will cause it to release even more Ca²⁺ It is especially important in neurons and muscle cells In heart and pancreas cells another second messenger (cyclic ADP ribose) takes part in the receptor activation

The localized and time-limited activity of Ca²⁺ in the cytosol is also called a Ca²⁺ wave The building of the wave is done by

the feedback mechanism of the ryanodine receptor and
the activation of phospholipase C by Ca²⁺ which leads to the production of inositol triphosphate which in turn activates the InsP₃ receptor

Function of Ca²⁺

Ca²⁺ is used in a multitude of processes among them muscle contraction release of neurotransmitter from nerve endings vision in retina cells proliferation secretion cytoskeleton management cell motion gene expression and metabolism The three main pathways that lead to Ca²⁺ activation are :

G protein regulated pathways
Pathways regulated by receptor-tyrosine kinases
Ligand- or current-regulated ion channels

There are two different ways in which Ca²⁺ can regulate proteins:

A direct recognition of Ca²⁺ by the protein
Binding of Ca²⁺ in the active center of an enzyme

One of the best studied interactions of Ca²⁺ with a protein is the regulation of calmodulin by Ca²⁺ Calmodulin itself can regulate other proteins or be part of a larger protein (for example phosphorylase kinase) The Ca²⁺/calmodulin complex plays an important role in proliferation mitosis and neural signal transduction

Lipophilic second messenger molecules

One group of lipophilic second messenger molecules consists of inositol triphosphate and diacylglycerol Others are ceramide and lysophosphatic acid

Nitric oxide (NO) as second messenger

The gas nitric oxide is a free radical which diffuses through the plasma membrane and affects nearby cells NO is made from arginine and oxygen by the enzyme NO synthase with citrulline as a by-product NO works mainly through activation of its target receptor the enzyme soluble guanylate cyclase which when activated produces the second messenger cyclic guanosine monophosphate (cGMP) NO can also act through covalent modification of proteins or their metal cofactors Some of these modifications are reversible and work through a redox mechanism In high concentrations NO is toxic and is thought to be responsible for some damage after a stroke NO serves three main functions:

Relaxation of blood vessels
Regulation of exocytosis of neurotransmitters
Cellular immune response

Research questions

When considering signal transduction pathways and networks outstanding questions researchers are addressing include:

Why do so many different signal transduction pathways share common chemicals?
How does the cell keep its messages from getting crossed?
Are the different pathways spatially segregated or do they use the same chemicals in different ways or perhaps just in different amounts?

Further information

Bibliography

Non-technical

Werner R. Loewenstein The Touchstone of : Molecular Information Cell Communication and the Foundations of Life Oxford University Press 1999 ISBN 0195140575

Technical

Gerhard Krauss Biochemistry of Signal Transduction and Regulation Wiley-VCH 1999 ISBN 3527303782
John T. Hancock Cell Signalling Addison-Wesley 1998 ISBN 0582312671

Sources used in article (or earlier version)

Cosma Shalizi's "Signal transduction" Notebook from 2003-01-20 used under the GFDL with permission

External links

Signal Transduction - The Virtual Library of Biochemistry and Cell Biology

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In [[biology]] '''signal transduction''' is any process by which a [[cell (biology)|cell]] converts one kind of signal or stimulus into another Processes referred to as [[signal transduction]]  often involve a [[sequence]] of [[biochemistry|biochemical]] [[chemical reaction|reactions]] inside the cell which are carried out by [[enzyme]]s and linked through [[second messenger]]s Such processes take [[place]] in as little [[time]] as a [[millisecond]] or as long as a few seconds Slower processes are rarely referred to as signal transduction

In many transduction processes an increasing [[number]] of enzymes and [[other]] molecules become engaged in the events that proceed from the initial stimulus In such cases the chain of steps is referred to as a "[[Signalling cascade (biochemistry)|signalling cascade]]" or "second messenger pathway" and the [[result]] is that a small stimulus elicits a large response

In [[bacterium|bacteria]] and [[other]] one-cell [[organism]]s the variety of [[signal transduction]]  processes of which the cell is capable influences how many ways it can react and [[respond]] to its environment In a less direct way the same is true of [[animal]]s and [[plant]]s [[sense|Sensing]] in all forms of [[life]] depends at the cellular level on signal transduction

==Overview==

===Stimuli===

Most stimuli impinge from the outside and interact with the [[cell membrane]] Many "signaling molecules" such as the [[neurotransmitter]]s that allow [[neuron|nerve cells]] to communicate across [[synapse]]s bind to [[transmembrane receptor|receptor proteins]] in the membrane and open [[ion channel|portals]] in it

===Responses===

Responses triggered by [[signal transduction]]  include the [[transcription|activation]] of a [[gene]] the [[cell metabolism|production of metabolic energy]] and cell [[locomotion]] for example through remodelling of the [[cytoskeleton|cell skeleton]]

Gene activation leads to further effects since genes are expressed as [[protein]]s many of which are enzymes [[transcription factor]]s or [[other]] regulators of metabolic activity Because transcription factors can activate [[still]] more genes in turn an initial stimulus can trigger via [[signal transduction]]  the expression of entire suite of genes and a panoply of physiolgical events Such [[mass]] activations are often referred to as "[[genetic program]]s" one example being the [[sequence]] of events that take [[place]] when an [[Ovum|egg]] is [[fertilization|fertilized]] by a [[sperm]]

==Types of signals==

===Extracellular===
Signal transduction usually involves the binding of "extracellular" signaling molecules to receptors that face outwards from the membrane and trigger events inside This takes [[place]] via a change in the shape or [[Chemical conformation|conformation]] of the receptor which occurs when the signal [[molecule]] "docks" or binds Receptors typically [[respond]] only to the specific [[molecule]] or "[[ligand]]" for which they have affinity and molecules that are even only slightly different tend to have no effect or else to act as [[inhibitor]]s

Most extracellular chemical signals have affinity for [[water]] and are unable to penetrate the oily barrier posed by the membrane that surrounds cells  A common kind of extracellular signal is [[nutrient]] In complex organism this includes the ligands responsible for sensations of smell and taste [[Steroid]]s represent an example of extracellular signals that can cross the membrane to permeate cells which they are able to do because of a partial affinity for oily surroundings (see [[hydrophobic]])

===Intracellular===

Often but not always the [[intracellular]] events triggered by the external signal are considered [[distinct]] from the event of "transduction" itself which in the strictest sense refers only to the step that converts the extracellular signal to an ''intra''cellular one

Intracellular signalling molecules in [[Eukaryote|eukaryotic]] cells include heterotrimeric [[G protein]] [[small GTPase]]s [[cyclic nucleotide]]s such as [[cyclic AMP]] (cAMP) and [[cyclic GMP]] (cGMP) [[calcium]] [[ion]] [[phophoinositide]] derivatives such as [[Phosphatidylinositol-triphosphate]]([[PIP3|PIP3]]) [[Diacylglycerol|Diacylglycerol (DAG)]] and [[Inositol triphosphate|Inositol-triphosphate]] ([[IP3|IP3]]) and various [[protein]] [[kinase]]s and [[protein phosphatase|phosphatase]]s Some of these are also called [[second messenger]]s

===Intercellular===
''Inter''cellular communication is accomplished by extracellular signalling and takes [[place]] in complex organisms that are composed of many cells Within [[endocrinology]] which is the study of intercellular signalling in animals
intercellular signalling is subdivided into the following types: 
* ''Endocrine'' signals are produced by [[endocrine cell]]s and travel through the [[circulatory system|blood]] to reach all parts of the body 
* ''Paracrine'' signals target only cells in the vicinity of the emitting cell Neurotransmitters represent an example 
* ''Autocrine'' signals affect only cells that are of the same cell type as the emitting cell An example for autocrine signals is found in [[immune cell]]s 
* ''Juxtacrine'' signals are transmitted along cell membranes via [[protein]] or lipid components [[integral]] to the membrane and are capable of affecting either the emitting cell or cells immediately adjacent

===Hormones===

Most of the molecules that enable signalling between the cells or tissues within an individual animal or plant are known as "[[hormone]]s" Hormone-initiated [[signal transduction]]  takes the following steps:
#''[[Protein biosynthesis|Biosynthesis]]'' of a hormone 
#''Storage'' and ''[[Cellular secretion|secretion]]'' of the hormone 
#''Transport'' of the hormone to the target cell 
#''Recognition'' of the hormone by the hormone [[receptor (biochemistry)|receptor]] [[protein]] leading to a conformational change 
#''Relay'' and ''amplification'' of the signal that leads to defined biochemical reactions within the target cell The reactions of the target cells can in turn cause a signal to the hormone-producing cell that leads to the down-regulation of hormone production 
#''Removal'' of the hormone

Hormones and [[other]] signaling molecules may exit the sending cell by [[exocytosis]] or [[other]] means of [[cell membrane|membrane transport]] The sending cell is typically of a specialized type Its recipents may be of one type or several as in the case of [[insulin]] which triggers diverse and systemic effects

Hormones signalling is elaborate and hard to dissect A cell can have several different receptors that recognize the same hormone but activate different [[signal transduction]]  pathways; or different hormones and their receptors can invoke the same biochemical pathway Different tissue types can [[answer]] differently to the same hormone [[stimulus]] There are two classes of hormone receptors "[[membrane protein|membrane-associated]] receptors" and [[intracellular]] or "[[cytoplasm|cytoplasmic]]" receptors

==Types of receptors==

=== Transmembrane receptors ===
[[Transmembrane receptor]]s are proteins that span the thickness of the [[plasma membrane]] of the cell with one end of the receptor outside (''extracellular domain'') and one inside (''intracellular domain'') the cell When the extracellular domain recognizes the hormone the whole receptor undergoes a structural shift that affects the [[intracellular]] domain leading to further action In this case the hormone itself does ''not'' pass through the plasma membrane into the cell

==== Hormone recognition by transmembrane receptors ====
The recognition of the [[chemical structure]] of a hormone by the hormone receptor uses the same (non-[[covalent]]) mechanisms such as [[hydrogen bond]]s [[electrostatic]] forces [[hydrophobic|hydrophobe]] and [[Van der Waals force]]s The equivalent between receptor-bound and free hormone equals '''[H] + [R] <-> [HR]''' with

<math>K_d = { { [H] * [R] } over { [HR] } }</math> 
[R]=receptor; [H]=free hormone; [HR]=receptor-bound hormone

The important value for the strength of the signal relayed by the receptor is the [[concentration]] of the hormone-receptor complex which is defined by the [[affinity]] of the hormone for the receptor the [[concentration]] of the hormone and of course the [[concentration]] of the receptor The [[concentration]] of the circulating hormone is the key value for the strength of the signal since the [[other]] two values are constant For fast reaction the hormone-producing cells can store [[prehormone]]s and quickly modify and release them if necessary Also the recipient cell can modify the sensitivity of the receptor for example by [[phosphorylation]]; also the variation of the [[number]] of receptors can vary the total [[signal strength]]  in the recipient cell

==== [[Signal transduction]]  of transmembrane receptors by structural changes ====
Signal transduction across the plasma membrane is possible only by many components working together First the receptor has to recognize the hormone with the extracellular domain then activate [[other]] proteins within the cytosol with its cytoplasmic domain which the [[protein]] does through a shift in conformation The activated effector proteins usually stay close to the membrane or are anchored within the membrane by [[Lipid anchored protein|lipid anchor]]s a [[posttranslational modification]] (see [[myristoilation]] [[palmitorylation]] [[farnesylation]] [[geranylation]] and the [[glycosyl-phosphatidyl-inositol-anchor]]) Many membrane-associated proteins can be activated in turn or come together to form a multi-protein complex that finally sends a signal via a soluble [[molecule]] into the cell

==== [[Signal transduction]]  of transmembrane receptors that are [[ion channel]]s ====
A ''ligand-activated ion channel'' will recognize its [[ligand]] and then undergo a [[structural change]]  that opens a gap (channel) in the plasma membrane through which ions can pass These ions will then relay the signal An example for this mechanism is found in the receiving cell of a [[synapse]] 
==== [[Signal transduction]]  of transmembrane receptors on change of [[transmembrane potential]] ====
An ion channel can also open when the receptor is activated by a change in [[cell potential]] that is, the difference of the [[Electric charge|electrical charge]] on both sides of the membrane If such a change occurs the ion channel of the receptor will open and let ions pass through In [[neuron]]s this mechanism underlies the [[action potential]] impulses that travel along [[nerve]]s

=== Nuclear receptors ===
Nuclear (or cytoplasmic) receptors are soluble proteins localized within the [[cytoplasm]] or the [[nucleoplasm]] The hormone has to pass through the plasma membrane usually by passive diffusion to reach the receptor and initiate the signal [[cascade]] The nuclear receptors are [[ligand]]-activated [[transcription]] activators; on binding with the [[ligand]] (the hormone) they will pass through the [[nuclear membrane]] into the [[cell nucleus|nucleus]] and enable the production of a certain [[gene]] and thus the production of a protein 
The typical ligands for nuclear receptors are [[lipophilic]] hormones with [[steroid]] hormones (for example [[testosterone]] [[progesterone]] and [[cortisol]]) and derivatives of [[vitamin]] A and D among them These hormones play a key role in the [[regulation]] of [[metabolism]] [[organ (anatomy)|organ]] function [[developmental biology|developmental processes]] and [[cell differentiation]] The key value for the [[signal strength]] is the hormone [[concentration]] which is regulated by : 
*''[[Biosynthesis]] and [[secretion]] of hormones in the [[endocrine tissue]]'' As an example the [[hypothalamus]] receives information both electrical and chemical It produces ''releasing factors'' that affect the [[hypophysis]] and make it produce ''glandotrope hormones'' which in turn activate endocrine organs so that they finally produce hormones for the target tissues This hierarchical system allows for the ''amplification'' of the original signal that reached the hypothalamus The released hormones dampen the production of these hormones by ''feedback inhibition'' to avoid [[overproduction]] 
* ''Availability of the hormone in the [[cytosol]]'' Several hormones can be converted into a storage form by the target cell for later use This reduces the amount of available hormone 
* ''Modification of the hormone in the target tissue'' Some hormones can be modified by the target cell so they no longer trigger the hormone receptor (or at least not the same one) effectively reducing the amount of available hormone

The nuclear receptors that were activated by the hormones attach at the DNA at receptor-specific ''Hormone Responsive Elements'' (''HREs'') DNA sequences that are located in the [[promoter]] [[region]] of the genes that are activated by the hormone-receptor complex As this enables the transcription of the according [[gene]] these hormones are also called ''inductors of [[gene expression]]'' The activation of [[gene]] transcription is much slower than signals that directly affect existing proteins As a consequence the effects of hormones that use nuclearic receptors are usually long-term Although the [[signal transduction]] via these soluble receptors involves only a few proteins the details of [[gene]] [[regulation]] are yet not well understood The nuclearic receptors all have a similar modular structure: 
:N-AAAABBBBCCCCDDDDEEEEFFFF-C 
where CCCC is the DNA-binding domain that contains [[zinc finger]]s and EEEE the ligand-binding domain The latter is also responsible for [[dimerization]] of most nuclearic receptors prior to DNA binding As a third function it contains structural elements that are responsible for [[transactivation]] used for communication with the translational apparatus The [[zinc]] fingers in the DNA-binding domain stabilize DNA binding by holding contact to the phosphate backbone of the [[DNA]] The DNA sequences that match the receptor are usually hexameric repeats either normal inverted or everted The sequences are quite similar but their orientation and [[distance]] are the parameters by which the DNA-binding domains of the receptors can [[tell]] them apart

==== Steroid receptors ==== 
''[[Steroid receptor]]s'' are a subclass of nuclear receptors located primarily within the cytosol In the absence of steroid hormone the receptors cling together in a complex called ''aporeceptor complex'' which also contains [[chaperone protein]]s (also known as ''[[heatshock protein]]s'' or ''Hsp''s) The ''Hsp''s are necessary to activate the receptor by assisting the [[protein]] to [[protein folding|fold]] in a way such that the [[signal sequence]] which enables its passage into the nucleus is accessible 
Steroid receptors can also have a ''repressive'' effect on [[gene]] expression when their [[transactivation]] domain is hidden so it cannot activate transcription Furthermore steroid receptor activity can be enhanced by [[phosphorylation]] of [[serine]] residues at their N-terminal end as a [[result]] of another [[signal transduction]] pathway for example a by a [[growth factor]] This behaviour is called ''[[crosstalk]]''

==== RXR- and orphan-receptors ====
These nucleric receptors can be activated by 
* a ''classic'' endocrine-synthesized hormone that entered the cell by diffusion 
* a hormone that was build within the cell (for example [[retinol]]) from a [[precursor]] or [[prohormone]] which can be brought to the cell through the bloodstream 
* a hormone that was completely synthesized within the cell for example [[prostaglandin]] 
These receptors are located in the nucleus and are ''not'' accompanied by ''chaperone proteins'' In the absence of hormone they bind to their specific DNA [[sequence]] repressing the [[gene]] Upon activation by the hormone they activate the transcription of the [[gene]] they were repressing

== Signal amplification ==
A principle of [[signal transduction]]  is the ''signal amplification'' The binding of one or a few neurotransmitter molecules can enable the entry of millions of ions The binding of one or just a few hormone molecules can induce an enzymatic reaction that affect many substrates The amplification can occur at several points of the signal pathway

=== Signal amplification at the transmembrane hormone receptor ===
A receptor that has been activated by a hormone can activate many ''downstream'' effector proteins For example a [[rhodopsin]] [[molecule]] in the plasma membrane of a [[retina]] cell in the [[eye]] that was activated by a [[photon]] can activate up to 2000 effector molecules (in this case [[transducin]]) per second The total strength of signal amplification by a receptor is determined by:  
* ''The lifetime of the hormone-receptor-complex'' The more [[stable]] the hormone-receptor-complex is, the less likely the hormone dissociates from the receptor the longer the receptor will remain active thus activate more effector proteins   
* ''The amount and lifetime of the receptor-effector protein-complex'' The more effector [[protein]] is available to be activated by the receptor and the faster the activated effector [[protein]] can dissociate from the receptor the more effector [[protein]] will be activates in the same amount of [[time]]  
* ''Deactivation of the activated receptor'' A receptor that is engaged in a hormone-receptor-complex can be deactivated either by covalent modification (for example phosphorylation) or by internalization (see [[ubiquitin]])

== [[Intracellular]] [[signal transduction]] == 
Intracellular [[signal transduction]] is largely carried out by [[second messenger]] molecules 
=== [[calcium|Ca2+]] as a second messenger === 
Ca2+ acts as a signal [[molecule]] within the cell This works by tightly limiting the [[time]] and space when Ca2+ is free (and thus active) Therefore the [[concentration]] of free Ca2+ within the cell is usually very low; it is stored within [[organelle]]s usually the [[endoplasmic reticulum]] ([[sarcoplasmic reticulum]] in muscle cells) where it is bound to molecules like [[calreticulin]]

====Activation of Ca2+====
To become active Ca2+ has to be released from the endoplasmic reticulum into the [[cytosol]] There are two combined receptor/ion channel proteins that perform the task of controlled transport of Ca2+: 
* The ''InsP3-receptor'' will transport Ca2+ upon interaction with [[inositol triphosphate]] (thus the name) on its cytosolic side It consists of four identical subunits 
* The ''ryanodine-receptor'' is named after the [[plant]] [[alkaloid]] [[ryanodine]] It is similar to the InsP3 receptor and stimulated to transport Ca2+ into the cytosol by ''recognizing Ca2+ on its cytosolic side'' thus establishing a feedback mechanism; a small amount of Ca2+ in the cytosol near the receptor will cause it to release even more Ca2+ It is especially important in [[neuron]]s and [[muscle cell]]s In [[heart]] and [[pancreas]] cells another second messenger ([[cyclic ADP ribose]]) takes part in the receptor activation 
The localized and time-limited activity of Ca2+ in the cytosol is also called a ''Ca2+ wave'' The [[building]] of the [[wave]] is done by

* the feedback mechanism of the ryanodine receptor and 
* the activation of [[phospholipase]] C by Ca2+ which leads to the production of [[inositol triphosphate]] which in turn activates the InsP3 receptor 
==== Function of Ca2+ ====
Ca2+ is used in a multitude of processes among them [[muscle]] contraction release of neurotransmitter from nerve endings [[vision]] in [[retina]] cells [[proliferation]] [[secretion]] [[cytoskeleton]] management [[cell motion]] [[gene expression]] and [[metabolism]] The three main pathways that [[lead]] to Ca2+ activation are : 
# [[G protein]] regulated pathways 
# Pathways regulated by receptor-[[tyrosine kinase]]s 
# [[Ligand]]- or current-regulated [[ion channel]]s 
There are two different ways in which Ca2+ can regulate proteins: 
# A direct recognition of Ca2+ by the [[protein]] 
# Binding of Ca2+ in the [[active center]] of an [[enzyme]] 
One of the best studied interactions of Ca2+ with a [[protein]] is the [[regulation]] of [[calmodulin]] by Ca2+ Calmodulin itself can regulate [[other]] proteins or be part of a larger [[protein]] (for example [[phosphorylase kinase]]) The Ca2+/calmodulin complex plays an important role in proliferation [[mitosis]] and neural [[signal transduction]]

=== [[lipophile|Lipophilic]] second messenger molecules === 
One group of lipophilic second messenger molecules consists of [[inositol triphosphate]] and [[diacylglycerol]] Others are [[ceramide]] and [[lysophosphatic acid]]

=== Nitric oxide (NO) as second messenger === 
The gas [[nitric oxide]] is a [[free radical]] which diffuses through the plasma membrane and affects nearby cells  NO is made from [[arginine]] and [[oxygen]] by the [[enzyme]] [[NO synthase]] with [[citrulline]] as a by-product  NO works mainly through activation of its target receptor the [[enzyme]] [[soluble guanylate cyclase]] which when activated produces the second messenger cyclic guanosine monophosphate (cGMP)  NO can also act through [[covalent]] modification of proteins or their metal cofactors Some of these modifications are reversible and work through a [[redox]] mechanism In high concentrations NO is toxic and is thought to be responsible for some damage after a [[stroke]] NO serves three main functions: 
# Relaxation of [[blood vessel]]s 
# [[Regulation]] of [[exocytosis]] of [[neurotransmitter]]s 
# Cellular [[immune system|immune response]]

==Research questions==

When considering [[signal transduction]]  pathways and networks outstanding questions researchers are addressing include:
*Why do so many different [[signal transduction]]  pathways share common chemicals? 
*How does the cell keep its messages from getting [[crosstalk|crossed]]? 
*Are the different pathways spatially segregated or do they use the same chemicals in different ways or perhaps just in different amounts?

==Further information==

===See also===
*[[G protein-coupled receptor]] -- [[GTPase]]s -- [[Protein phosphatase]]

===Bibliography===

''Non-technical''
*Werner R. Loewenstein ''The [[Touchstone]] of [[Life:]]: Molecular Information Cell Communication and the Foundations of Life'' Oxford University Press [[1999]] ISBN 0195140575

''Technical''
*Gerhard Krauss ''Biochemistry of Signal Transduction and Regulation'' Wiley-VCH [[1999]] ISBN 3527303782
*John T. Hancock ''Cell Signalling'' Addison-Wesley [[1998]] ISBN 0582312671

===Sources used in article (or earlier version)===
*[http://bactraorg/notebooks/signal-transductionhtml Cosma Shalizi's "Signal transduction" Notebook from 2003-01-20 used under the GFDL with permission]

=== External links ===
*[http://wwwbiochemweborg/signalingshtml Signal Transduction - The Virtual Library of [[Biochemistry]] and Cell Biology]

[[Category:Biochemistry]][[Category:Cell biology]][[Category:Signal transduction]]
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