The Nicotinic Acetylcholine Receptor: Gene Expression and Ion Channel Function

Year: 1987 DOI: 10.7907/fj5c-4h83
The nicotinic acetylcholine receptor (AChR) is a complex protein, which functions as a ligand-gated ion channel on the postsynaptic membrane at the neuromuscular junction and mediates signal transmission from neuron to muscle. Research on the AChR has had a long history and has benefited from the endeavors of scientists from many disciplines. The intensive, multidisciplinary studies have yielded valuable knowledge about this molecule, which serves as a model for the understanding of many fundamental questions in biological sciences. Chapter 1 presents a review of the AChR.

As a tissue-specific and developmental stage-specific molecule, AChR is under temporal and spatial control for its synthesis. Chapter 2 reports a qualitative and quantitative study of AChR gene activity during muscle cell differentiation, using a cDNA clone isolated from a murine muscle cell line, which codes for the γ subunit of the mouse AChR. The results indicate that the regulation of mRNA accumulation levels is a major mechanism in the differential synthesis of the AChR.

The marriage between AChR and molecular biology resulted in many cDNA clones which, after being introduced to African frogs, produced the next generation — Xenopus oocytes with exotic AChRs on them. Chapter 3 describes the attempt to localize "determinants" that specify species subunit identity in the AChR by constructing chimeric cDNA clones composed of fragments from different origins, taking advantage of the Xenopus oocyte expression system. The results from surface toxin-binding assay and two-electrode voltage-clamp recording suggested that while the species specificity can be dictated by certain subunits, the determination of subunit identity does not reside at a defined locus in the fragments tested.

Does the complex composition of multisubunits in the AChR bear any functional significance? Chapter 4 addresses this question through the study of mouse-Torpedo AChR hybrids. The complete substitution of AChR subunits between mouse and Torpedo receptors generated all 16 combinations, and a systematic analysis of these hybrids revealed an interesting pattern with respect to the voltage sensitivity in the ACh-induced response: The identity of the β subunit determines, while the interaction between the β and δ subunits modulates, the AChR voltage sensitivity. The results, therefore, suggest that different subunits of AChR may play a central role in different functional properties.

Patch-clamp technique has offered an opportunity for analyzing transmembrane current flow with the high resolution of single-channel recording. Chapter 5 describes such a study on homologous and hybrid AChRs. Voltage influence on the three parameters were evaluated, and the results indicate that the single channel conductance is independent of membrane potential and that the channel closing and opening rates together constitute the basis for the voltage sensitivity in whole-cell recording with the closing rate making the major contribution. Also investigated were the subunit roles in species specificity of channel-open duration and voltage dependence. The results are in agreement with those reported on channel duration and support the conclusions of our previous work on the subunit involvement in determining the voltage sensitivity of the AChR response.