Acetylcholine Nicotinic Channel Assays
Background of Nicotinic Acetylcholine Receptors (AChRs)
AChRs are a family of acetylcholine-gated cation channels that form the major excitatory neurotransmitter receptors on muscles and nerves in the peripheral nervous system. They are traditionally classified into "muscle" and "neuronal" types.
Structure and Functions of AChRs
AChRs are thought to be formed from five homologous subunits oriented like barrel staves around a central cation channel. All subunits of AChRs and other receptors in this superfamily have a characteristic pattern of structural features extending from their N-terminus to their C-terminus. The primary function of the AChRs is to mediate signal transmission at the postsynaptic level.
Fig.1 Structure of the nicotinic acetylcholine receptors. (Karlin, 2002)
Roles of AChRs in Diseases
AChRs are associated with a growing list of diseases. The disease first and best characterized with the direct involvement of AChRs is autoimmune myasthenia gravis, in which an antibody-mediated autoimmune response to muscle AChRs impairs neuromuscular transmission. Among the many changes in Alzheimer's disease (AD) patients' brains is a large loss of high-affinity nicotine binding sites, suggesting that loss of some AChR subtypes occurs in the course of AD. A dopamine hypothesis of schizophrenia suggests that it is caused by excess dopamine. Some similar symptoms can also be caused by drugs like PCP that act as channel blockers for glutamate receptors and AChRs. Other diseases caused by mutations in neuronal AChRs will probably be found in the future, and these will help reveal the functional roles of the AChRs involved.
Published Data
| Paper Title | Structure and gating mechanism of the α7 nicotinic acetylcholine receptor |
| Journal | Cell |
| Published | 2021 |
| Abstract | The α7 nicotinic acetylcholine receptor plays an important role in the central nervous system and cholinergic inflammatory pathways. This ligand-gated ion channel, assembled as a homopentamer, is particularly permeable to Ca2+ and desensitized faster than any other Cys-loop receptor. The scientists demonstrated the cryo-electron microscopic structure of the resting, activating, and desensitizing state of the human α7 nicotine receptor in a lipid environment, elucidating the main steps of the gated cycle. These structures also reveal elements that contribute to its function, including C-terminal latches that allow channels to open, and anionic rings in the extracellular vestibule that contribute to its high conductivity and calcium permeability. The comparison between α7 structures provides a basis for mapping the gating cycle and revealing differences in the gating mechanisms of the Cys-loop receptor superfamily. |
| Result |
The researchers describe the structure of human α7 nicotinic acetylcholine receptors in three basic gated cyclic conformations: a closed, resting state; the state of being open or active; and a closed, desensitized state. This structure generates hypotheses that they test using electrophysiology and in combination with experiments to understand channel function. They report a component located in the extracellular domain that contributes to the high calcium permeability of α7 and may regulate the relative permeability of the broader Cys-loop receptor family. The discovery of the C-terminal latch extended the role of the C-terminal in binding the coupling agonist to the channel opening. In the desensitization state, hydrophobic shrinkage of pore 9' may contribute to the rapid occurrence of α7 desensitization. These studies reveal the gated circulation of nicotine receptors, and their mechanisms compared to glycine receptors, and open up new avenues of exploration for studying this important family of signaling proteins.
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Fig.2 Global conformational changes during the nicotinic receptor gating cycle. (Noviello, 2021)
References
- Karlin A. Emerging structure of the nicotinic acetylcholine receptors. Nature Reviews Neuroscience. 2002, 3(2): 102-114.
- Noviello, C.M.; et al. Structure and gating mechanism of the α7 nicotinic acetylcholine receptor. Cell. 2021, 184(8): 2121-2134. e13.
