Nicotinic acetylcholine receptor

 ( Ion Channels ) 

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.

A number of electron microscopy and x-ray crystallography studies have provided very high resolution structural information for muscle and neuronal nAChRs and their binding domains.

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-terminal end.

The nAChR is a non-selective cation channel, meaning that several different positively charged ions can cross through. It is permeable to Na⁺ and K⁺, with some subunit combinations that are also permeable to Ca²⁺. 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. The channel usually opens rapidly and tends to remain open until the agonist diffusion away, which usually takes about 1 millisecond. AChRs can spontaneously open with no ligands bound or can spontaneously close with ligands bound, and mutations in the channel can shift the likelihood of either event. Therefore, ACh binding changes the probability of pore opening, which increases as more ACh binds.

The nAChR is unable to bind ACh when bound to any of the snake venom Alpha-neurotoxin. These α- antagonistically bind tightly and noncovalently to nAChRs of skeletal muscles and in neurons, thereby blocking the action of ACh at the postsynaptic membrane, inhibiting ion flow and leading to paralysis and death. The nAChR contains two binding sites for snake venom neurotoxins. Progress in discovering the dynamics of binding action of these sites has proved difficult, although recent studies using normal mode dynamics 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.

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 and PKC, 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.

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. In mammals, nAchR subunits have been found to be encoded by 17 genes, and of these, nine genes encoding α-subunits and three encoding β-subunits are expressed in the brain. β₂ subunit-containing nAChRs (β₂nAChRs) and α₇nAChRs are widely expressed in the brain, whereas other nAChR subunits have more restricted expression. 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.