Receptors of this type control the fastest synaptic events in the nervous system, in which a neurotransmitter acts on the postsynaptic membrane of a nerve or muscle cell and transiently increases its permeability to particular ions. Most excitatory neurotransmitters, such as acetylcholine at the neuromuscular Junction or glutamate in the central nervous system, cause an increase in Na+ and K+s permeability. This results in a net inward current carried mainly by Na+, which depolarises the cell and increases the probability that it will generate an action potential. The action of the transmitter reaches a peak in a fraction of a millisecond,and usually decays within a few milliseconds. The sheer speed of this response implies that the coupling between the receptor and the ionic channel is a direct one, and the molecular structure of the receptor-channel complex agrees with this.In contrast to other receptor families, no intermediate biochemical steps are involved in the transduction process.
Type 2: G-protein-coupled receptors (GPCRs):- These are also known as metabotropic receptors or 7-transmembrane-spanning (heptahelical) receptors. They are membrane receptors that are coupled to intracellular effector systems via a G-protein (see below). They constitute the largest family,5 and include receptors for many hormones and slow transmitters, for example the muscarinic acetylcholine receptor (mAChR), adrenergic receptors and chemokine receptors
G-proteins and their role G-proteins comprise a family of membrane-resident proteins whose function is to recognise activated GPCRs and pass on the message to the effector systems that generate a cellular response They are the go-between proteins, but were actually called G-proteins because of their interaction with the guanine nucleotides, GTP and GDP. G-proteins consist of three subunits: α, β and γ. Guanine nucleotides bind to the α subunit, which has enzymic activity, catalysing the conversion of GTP to GDP. The β and γ subunits remain together as a βγ complex. G-proteins appear to be freely diffusible in the plane of the membrane, so a single pool of G-protein in a cell can interact with several different receptors and effectors.In the 'resting' state, the G-protein exists as an unattached αβγ trimer, with GDP occupying the site on the α subunit. When a GPCR is activated by an agonist molecule, a conformational change occurs, involving the cytoplasmic domain of the receptor, causing it to acquire high affinity for αβγ. Association of αβγ with the receptor causes the bound GDP to dissociate and to be replaced with GTP (GDP-GTP exchange), which in turn causes dissociation of the G-protein trimer, releasing α-GTP and βγ subunits; these are the 'active' forms of the G-protein, which diffuse in the membrane and can associate with various enzymes and ion channels, causing activation of the target.
Signalling is terminated when the hydrolysis of GTP to GDP occurs through the GTPase activity of the α subunit. The resulting α-GDP then dissociates from the effector, and reunites with βγ, completing the cycle.
The main targets for G-proteins, through which GPCRs control different aspects of cell function |
cAMP is a nucleotide synthesised within the cell from ATP by the action of a membrane-bound enzyme, adenylyl cyclase. It is produced continuously and inactivated by hydrolysis to 5´-AMP, by the action of a family of enzymes known as phosphodiesterases (PDEs). Many different drugs, hormones and neurotransmitters act on GPCRs and produce their effects by increasing or decreasing the catalytic activity of adenylyl cyclase, thus raising or lowering the concentration of cAMP within the cell. Cyclic AMP regulates many aspects of cellular function including, for example, enzymes involved in energy metabolism, cell division and cell differentiation, ion transport, ion channels, and the contractile proteins in smooth muscle. These varied effects are, however, all brought about by a common mechanism, namely the activation of protein kinases by cAMP. Protein kinases regulate the function of many different cellular proteins by controlling protein phosphorylation. Examples 1. Increased cAMP production in response to β-adrenoceptor activation affects enzymes involved in glycogen and fat metabolism in liver, fat and muscle cells. The result is a coordinated response in which stored energy in the form of glycogen and fat is made available as glucose to fuel muscle contraction. 2. Other examples of regulation by cAMP-dependent protein kinases include the increased activity of voltage-activated calcium channels in heart muscle cells. Phosphorylation of these channels increases the amount of Ca2+ entering the cell during the action potential, and thus increases the force of contraction of the heart. 3. In smooth muscle, cAMP-dependent protein kinase phosphorylates (thereby inactivating) another enzyme, myosin-light-chain kinase, which is required for contraction. This accounts for the smooth muscle relaxation produced by many drugs that increase cAMP production in smooth muscle 4. include certain types of mAChR (e.g. the M2 receptor of cardiac muscle; see, α2-adrenoceptors in smooth muscle, and opioid receptors
The phospholipase C/inositol phosphate system The phosphoinositide system, an important intracellular second messenger system. PIP2 is the substrate for a membrane-bound enzyme, phospholipase Cβ (PLCβ), which splits it into DAG and inositol (1,4,5) trisphosphate (IP3), both of which function as second messengers. The activation of PLCβ by various agonists is mediated through a G-protein. After cleavage of PIP2, the status quo is restored. DAG being phosphorylated to form phosphatidic acid (PA), while the IP3 is dephosphorylated and then recoupled with PA to form PIP2 once again. Inositol phosphates and intracellular calcium Inositol (1,4,5) trisphosphate is a water-soluble mediator that is released into the cytosol and acts on a specific receptor-the IP3 receptor-which is a ligand-gated calcium channel present on the membrane of the endoplasmic reticulum. The main role of IP3, is to control the release of Ca2+ from intracellular stores. Because many drug and hormone effects involve intracellular Ca2+, this pathway is particularly important. IP3 is converted inside the cell to the (1,3,4,5) tetraphosphate, IP4, by a specific kinase. The exact role of IP4 remains unclear, but there is evidence that it too is involved in Ca2+ signalling. One possibility is that it facilitates Ca2+ entry through the plasma membrane, thus avoiding depletion of the intracellular stores as a result of the action of IP3. Diacylglycerol and protein kinase C Diacylglycerol is produced as well as IP3 whenever receptor-induced PI hydrolysis occurs. The main effect of DAG is to activate a membrane-bound protein kinase, protein kinase C (PKC), which catalyses the phosphorylation of a variety of intracellular proteins. DAG, unlike the inositol phosphates, is highly lipophilic and remains within the membrane. It binds to a specific site on the PKC molecule, which migrates from the cytosol to the cell membrane in the presence of DAG, thereby becoming activated.
Ion channels as targets for G-proteins |
G-protein-coupled receptors can control ion channel function directly by mechanisms that do not involve second messengers such as cAMP or inositol phosphates. This was first shown for cardiac muscle, but it now appears that direct G-protein-channel interaction may be quite general . In cardiac muscle, for example, mAChRs are known to enhance K+ permeability (thus hyperpolarising the cells and inhibiting electrical activity. Similar mechanisms operate in neurons, where many inhibitory drugs such as opiate analgesics reduce excitability by opening potassium channels. |
Type 3: kinase-linked and related receptors:- This is a large and heterogeneous group of membrane receptors responding mainly to protein mediators. They comprise an extracellular ligand-binding domain linked to an intracellular domain by a single transmembrane helix. In many cases, the intracellular domain is enzymic in nature (with protein kinase or guanylyl cyclase activity). Type 3 receptors include those for insulin and for various cytokines and growth factors the receptor for atrial natriuretic factor (ANF), is the main example of the guanylyl cyclase type. The two kinds are very similar structurally, even though their transduction mechanisms differ.
KINASE-LINKED AND RELATED RECEPTORS These membrane receptors are quite different in structure and function from either the ligand-gated channels or the GPCRs. They mediate the actions of a wide variety of protein mediators, including growth factors and cytokines, and hormones such as insulin and leptin, whose effects are exerted mainly at the level of gene transcription. They play a major role in controlling cell division, growth, differentiation, inflammation, tissue repair, apoptosis and immune responses, The main types are as follow
- Receptor tyrosine kinases (RTKs). These receptors have the basic structure, incorporating a tyrosine kinase moiety in the intracellular region. They include receptors for many growth factors, such as epidermal growth factor and nerve growth factor, and also the group of Toll-like receptors that recognise bacterial lipopolysaccarides and play an important role in the body's reaction to infection. The insulin receptor also belongs to the RTK class, although it has a more complex dimeric structure.
- Serine/threonine kinases. This smaller class is similar in structure to RTKs but phosphorylate serine and/or threonine residues rather than tyrosine. The main example is the receptor for transforming growth factor (TGF).
- Cytokine receptors. These receptors lack intrinsic enzyme activity. When occupied, they associate with, and activate, a cytosolic tyrosine kinase, such as Jak (the Janus kinase) or other kinases. Ligands for these receptors include cytokines such as interferons and colony-stimulating factors involved in immunological responses.
- Guanylyl cyclase-linked receptors. These are similar in structure to RTKs, but the enzymic moiety is guanylyl cyclase and they exert their effects by stimulating cGMP formation. The main example is the receptor for ANF.
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In many cases, ligand binding to the receptor leads to dimerisation. The association of the two intracellular kinase domains allows a mutual autophosphorylation of intracellular tyrosine residues to occur. The phosphorylated tyrosine residues then serve as high-affinity docking sites for other intracellular proteins that form the next stage in the signal transduction cascade. One important group of such 'adapter' proteins is known as the SH2 domain proteins (standing for Src homology, because it was first identified in the Src oncogene product).
 Two well-defined signal transduction pathways are summarised in The Ras/Raf pathway mediates the effect of many growth factors and mitogens. Ras, which is a proto-oncogene product, functions like a G-protein, and conveys the signal (by GDP/GTP exchange) from the SH2 domain protein, Grb, which is phosphorylated by the RTK. Activation of Ras in turn activates Raf, which is the first of a sequence of three serine/threonine kinases, each of which phosphorylates, and activates, the next in line. The last of these, mitogen-activated protein (MAP) kinase, phosphorylates one or more transcription factors that initiate gene expression, resulting in a variety of cellular responses, including cell division.
Many SH2 domain proteins are enzymes, such as protein kinases or phospholipases. Some growth factors activate a specific subtype of phospholipase C (PLCγ), thereby causing phospholipid breakdown, IP3 formation and Ca2+ release. Other SH2-containing proteins couple phosphotyrosine-containing proteins with a variety of other functional proteins, including many that are involved in the control of cell division and differentiation. The end result is to activate or inhibit, by phosphorylation, a variety of transcription factors that migrate to the nucleus and suppress or induce the expression of particular genes. A second pathway, the Jak/Stat pathway is involved in responses to many cytokines. Dimerisation of these receptors occurs when the cytokine binds, and this attracts a cytosolic tyrosine kinase unit (Jak) to associate with, and phosphorylate, the receptor dimer. Jaks belong to a family of proteins, different members having specificity for different cytokine receptors. Among the targets for phosphorylation by Jak are a family of transcription factors (Stats). These are SH2 domain proteins that bind to the phosphotyrosine groups on the receptor-Jak complex, and are themselves phosphorylated. Thus activated, Stat migrates to the nucleus and activates gene expression. The membrane-bound form of guanylyl cyclase, the enzyme responsible for generating the second messenger cGMP in response to the binding of peptides such as atrial natriuretic peptide, resembles the tyrosine kinase family and is activated in a similar way by dimerisation when the agonist is bound. Type 4: nuclear receptors:- These are receptors that regulate gene transcription. The term nuclear receptors is something of a misnomer, because some are actually located in the cytosol and migrate to the nuclear compartment when a ligand is present. They include receptors for steroid hormones, thyroid hormone, and other agents such as retinoic acid and vitamin D.
NUCLEAR RECEPTORS Receptors for steroid hormones such as oestrogen and the glucocorticoids were present in the cytoplasm of cells and translocated into the nucleus after binding with their steroid partner. Other hormones, such as the thyroid hormone T3 and the fat-soluble vitamins D and A (retinoic acid) and their derivatives that regulate growth and development, were found to act in a similar fashion. |
