Nicotinic acetylcholine receptor

Nicotinic receptors, with a molecular mass of 290 kDa, are made up of five subunits, arranged symmetrically around a central pore. In vertebrates, nicotinic receptors are broadly classified into two subtypes based on their primary sites of expression: muscle-type nicotinic receptors and neuronal-type nicotinic receptors. In the muscle-type receptors, found at the neuromuscular junction, receptors are either the embryonic form, composed of α1, β1, γ, and δ subunits in a 2:1:1:1 ratio ((α1)2β1γδ), or the adult form composed of α1, β1, δ, and ε subunits in a 2:1:1:1 ratio ((α1)2β1δε). The neuronal subtypes are various homomeric (all one type of subunit) or heteromeric (at least one α and one β) combinations of twelve different nicotinic receptor subunits: α2−α10 and β2−β4. Examples of the neuronal subtypes include: (α4)3(β2)2, (α4)2(β2)3, (α3)2(β4)3, α4α6β3(β2)2, (α7)5, and many others. In both muscle-type and neuronal-type receptors, the subunits are very similar to one another, especially in the hydrophobic regions. ==Binding==

As with all ligand-gated ion channels, opening of the nAChR channel pore requires the binding of a chemical messenger. Several different terms are used to refer to the molecules that bind receptors, such as ligand, agonist, or transmitter. As well as the endogenous agonist acetylcholine, agonists of the nAChR include nicotine, epibatidine, and choline. Nicotinic antagonists that block the receptor include mecamylamine, dihydro-β-erythroidine, α-bungarotoxin, and hexamethonium. In muscle-type nAChRs, the acetylcholine binding sites are located at the α and either ε or δ subunits interface. In neuronal nAChRs, the binding site is located at the interface of an α and a β subunit or between two α subunits in the case of α7 receptors. The binding site is located in the extracellular domain near the N terminus. When an agonist binds to the site, all present subunits undergo a conformational change and the channel is opened and a pore with a diameter of about 0.65 nm opens. ==Channel opening==

Nicotinic AChRs may exist in different interconvertible conformational states. Binding of an agonist stabilizes the open and desensitized states. In normal physiological conditions, the receptor needs exactly two molecules of ACh to open. Opening of the channel allows positively charged ions to move across it; in particular, sodium enters the cell and potassium exits. The net flow of positively charged ions is inward. The nAChR is a non-selective cation channel, meaning that several different positively charged ions can cross through. The amount of sodium and potassium the channels allow through their pores (their conductance) varies from 50 to 110 pS, with the conductance depending on the specific subunit composition as well as the permeant ion. Many neuronal nAChRs can affect the release of other neurotransmitters. have aided in predicting the nature of both the binding mechanisms of snake toxins and of ACh to nAChRs. These studies have shown that a twist-like motion caused by ACh binding is likely responsible for pore opening, and that one or two molecules of α-bungarotoxin (or other long-chain α-neurotoxin) suffice to halt this motion. The toxins seem to lock together neighboring receptor subunits, inhibiting the twist and therefore, the opening motion. ==Effects==

The activation of receptors by nicotine modifies the state of neurons through two main mechanisms. On one hand, the movement of cations causes a depolarization of the plasma membrane (which results in an excitatory postsynaptic potential in neurons) leading to the activation of voltage-gated ion channels. On the other hand, the entry of calcium acts, either directly or indirectly, on different intracellular cascades. This leads, for example, to the regulation of activity of some genes or the release of neurotransmitters. ==Regulation==

Desensitization Ligand-bound desensitization of receptors was first characterized by Katz and Thesleff in the nicotinic acetylcholine receptor. Prolonged or repeated exposure to a stimulus often results in decreased responsiveness of that receptor toward a stimulus, termed desensitization. nAChR function can be modulated by phosphorylation by the activation of second messenger-dependent protein kinases. PKA as well as tyrosine kinases, have been shown to phosphorylate the nAChR resulting in its desensitization. It has been reported that, after prolonged receptor exposure to the agonist, the agonist itself causes an agonist-induced conformational change in the receptor, resulting in receptor desensitization. Desensitized receptors can revert to a prolonged open state when an agonist is bound in the presence of a positive allosteric modulator, for example PNU-120,596. Also, there is evidence that indicates specific chaperone molecules have regulatory effects on these receptors. ==Roles==

The subunits of the nicotinic receptors belong to a multigene family (16 members in humans) and the assembly of combinations of subunits results in a large number of different receptors (for more information see the Ligand-Gated Ion Channel database). These receptors, with highly variable kinetic, electrophysiological and pharmacological properties, respond to nicotine differently, at very different effective concentrations. This functional diversity allows them to take part in two major types of neurotransmission. Classical synaptic transmission (wiring transmission) involves the release of high concentrations of neurotransmitter, acting on immediately neighboring receptors. In contrast, paracrine transmission (volume transmission) involves neurotransmitters released by axon terminals, which then diffuse through the extra-cellular medium until they reach their receptors, which may be distant. Nicotinic receptors can also be found in different synaptic locations; for example the muscle nicotinic receptor always functions post-synaptically. The neuronal forms of the receptor can be found both post-synaptically (involved in classical neurotransmission) and pre-synaptically where they can influence the release of multiple neurotransmitters. Nicotine addiction arises from nAChR-mediated dopamine release in the mesolimbic pathway. ==Subunits==

17 vertebrate nAChR subunits have been identified, which are divided into muscle-type and neuronal-type subunits. Although an α8 subunit/gene is present in avian species such as the chicken, it is not present in human or mammalian species. The nAChR subunits have been divided into four subfamilies (I–IV) based on similarities in protein sequence. In addition, subfamily III has been further divided into three types. • α genes: (muscle), (neuronal), , , , , , , , • β genes: (muscle), (neuronal), , • Other genes: (delta), (epsilon), (gamma) Neuronal nAChRs are transmembrane proteins that form pentameric structures assembled from a family of subunits composed of α2–α10 and β2–β4. These subunits were discovered from the mid-1980s through the early 1990s, when cDNAs for multiple nAChR subunits were cloned from rat and chicken brains, leading to the identification of eleven different genes (twelve in chickens) that code for neuronal nAChR subunits; The subunit genes identified were named α2–α10 (α8 only found in chickens) and β2–β4. It has also been discovered that various subunit combinations could form functional nAChRs that could be activated by acetylcholine and nicotine, and the different combinations of subunits generate subtypes of nAChRs with diverse functional and pharmacological properties. When expressed alone, α7, α8, α9, and α10 are able to form functional receptors, but other α subunits require the presence of β subunits to form functional receptors. The pentameric assembly of nAChRs is subjected to the subunits that are produced in various cell types such as in the human lung where epithelial and muscular pentamers largely differ. CHRNA5/A3/B4 An important nAchR gene cluster (CHRNA5/A3/B4) contains the genes encoding for the α5, α3 and β4 subunits. Genetic studies have identified single nucleotide polymorphisms (SNPs) in the chromosomal locus encoding these three nAChR genes as risk factors for nicotine dependence, lung cancer, chronic obstructive pulmonary disease, alcoholism, and peripheral arterial disease. The CHRNA5/A3/B4 genes are co-expressed in many cell types and the transcriptional activities of the promoter regions of the three genes are regulated by many of the same transcription factors, demonstrating that their clustering may reflect control of gene expression. Nicotinic receptors containing α6 or β3 subunits expressed in brain regions, especially in the ventral tegmental area and substantia nigra, are important for drug behaviors due to their role in dopamine release. Genetic variation in these genes can alter sensitivity to drugs of abuse in numerous ways, including changing the amino acid structure of the protein or cause alterations in transcriptional and translational regulation. Both of these nAChR subunits are present in the brain and the occurrence of mutations in these two subunits cause a focal type of epilepsy. Examples include the CHRNA4 insertion mutation 776ins3 that is associated with nocturnal seizures and psychiatric disorders, and the CHRNB2 mutation I312M that seems to cause not only epilepsy but also very specific cognitive deficits, such as deficits in learning and memory. There is naturally occurring genetic variation between these two genes and analysis of single nucleotide polymorphisms (SNPs) and other gene modifications show a higher variation in the CHRNA4 gene than in the CHRNB2 gene, implying that nAChR β2, the protein encoded by CHRNB2, associates with more subunits than α4. CHRNA2 has also been reported as a third candidate for nocturnal frontal lobe seizures. Additionally, smoking rates are significantly higher in those with schizophrenia, implying that smoking nicotine may be a form of self-medicating. CHRNA7 has also been shown to modulate immune responses though cholinergic anti-inflammatory pathway (CAP). Notable variations Nicotinic receptors are pentamers of these subunits; i.e., each receptor contains five subunits. Thus, there is immense potential of variation of these subunits, some of which are more commonly found than others. The most broadly expressed subtypes include (α1)2β1δε (adult muscle-type), (α3)2(β4)3 (ganglion-type), (α4)2(β2)3 (CNS-type) and (α7)5 (another CNS-type). A comparison follows: == See also ==