Neurotoxins distinguish between different neuronal nicotinic acetylcholine receptor subunit combinations

Amino acid residues forming the interface of a neuronal nicotinic acetylcholine receptor with kappa-bungarotoxin: a study using single residue substituted peptide analogs

kappa-Bungarotoxin is a high affinity antagonist of neuronal nicotinic acetylcholine receptors of the alpha 3 subtype. Three sequence segments of the alpha 3 subunit that contribute to forming the binding site for kappa-bungarotoxin were previously located using synthetic peptides corresponding to the complete alpha 3 subunit, i.e., alpha 3(1-18), alpha 3(50-71) and alpha 3(180-201). Here we use single residue substituted peptide analogs of the alpha 3(50-71) sequence, in which amino acids are sequentially replaced by Gly, to determine which residues are important for kappa-bungarotoxin binding activity. Although no single substitution obliterated kappa-bungarotoxin binding, several amino acid substitutions lowered the affinity for kappa-bungarotoxin--i.e., two negatively charged residues (Glu51 and Asp62), and several aliphatic and aromatic residues (Leu54, Leu56, and Tyr63). These results indicate that the interface of the alpha 3 subunit with kappa-bungarotoxin involves primarily hydrophobic interactions, and a few negatively charged residues.

Identification of cultured cells expressing ligand-gated cationic channels

We have identified cultured cells that express ligand-gated cation channels using a simple method which may also be applied to the screening of chemical agents for their use as agonists or antagonists. This assay is based upon the observation that many ligand-gated cation channels are permeable to lithium and agonists induce the flux of lithium into the cells which contain them. Since the accumulation of intracellular lithium can alter the cell cycle, the measurement of [3H]thymidine ([3H]thy) incorporation should reflect this occurrence. This expectation was realized using the PC12 cell line which expresses neuronal-like nicotinic acetylcholine receptor (nAChR). When cholinergic agonists are applied to PC12 cells in the presence of lithium-containing buffer and cells are subsequently pulsed with [3H]thy, the radiolabel incorporation into these cells relative to controls is reduced. If cholinergic antagonists are included or if the concentration of agonist either rapidly desensitizes receptors or is insufficient to induce channel opening, the reduction in [3H]thy incorporation is not observed. This method also provides a rapid way to screen cultured cell lines for those that express ligand-gated cation channels. This assay offers the potential to be automated for the low cost screening of drugs which act upon ligand-gated ion channels.

Pharmacological and functional diversity of neuronal nicotinic acetylcholine receptors

Recent molecular cloning studies have identified several genes encoding alpha and beta subunits of the nicotinic acetylcholine receptor. These genes have distinct, although overlapping, patterns of expression in the brain and peripheral ganglia. Multiple nicotinic receptors with distinct pharmacological and functional properties can be made in oocytes by pairwise combination of different alpha-type subunits with different beta-type subunits. Both alpha and beta subunits contribute to the pharmacological and functional diversity. Evan Deneris and colleagues explain how oocyte expression studies, in concert with immunological and electrophysiological analysis in vivo, are beginning to reveal the subunit compositions of different neuronal nicotinic receptor subtypes.

The postsynaptic 43K protein clusters muscle nicotinic acetylcholine receptors in Xenopus oocytes

Nicotinic acetylcholine receptors (AChRs) are localized at high concentrations in the postsynaptic membrane of the neuromuscular junction. A peripheral membrane protein of Mr 43,000 (43K protein) is closely associated with AChRs and has been proposed to anchor receptors at postsynaptic sites. We have used the Xenopus oocyte expression system to test the idea that the 43K protein clusters AChRs. Mouse muscle AChRs expressed in oocytes after injection of RNA encoding receptor subunits are uniformly distributed in the surface membrane. Coinjection of AChR RNA and RNA encoding the mouse muscle 43K protein causes AChRs to form clusters of 0.5-1.5 microns diameter. AChR clustering is not a consequence of increased receptor expression in the surface membrane or nonspecific clustering of all membrane proteins. The 43K protein is colocalized with AChRs in clusters when the two proteins are expressed together and forms clusters of similar size even in the absence of AChRs. These results provide direct evidence that the 43K protein causes clustering of AChRs and suggest that regulation of 43K protein clustering may be a key step in neuromuscular synaptogenesis.

Intracellular recording in avian brain of a nicotinic response that is insensitive to K-bungarotoxin

We examined nicotinic acetylcholine receptors in the avian brain using a combination of autoradiographic and intracellular electrophysiological techniques. We found that the lateral spiriform nucleus (SPL) in the mesencephalon has a very high density of 3H-nicotine binding sites but no detectable 125I-K-bungarotoxin (125I-K-BuTx) or 125I-alpha-bungarotoxin (125I-alpha-BuTx) bindings sites. Intracellular recordings in brain slices revealed that SPL neurons depolarize in response to nicotine and carbachol (in the presence of atropine). These depolarizations were blocked by the classic nicotinic antagonists d-tubocurarine and dihydro-beta-erythroidine. As predicted for nicotinic receptors with a high affinity for nicotine, neither K-BuTx nor alpha-BuTx blocked these nicotinic responses. Thus, although the existence of high-affinity 3H-nicotine binding sites has been known for some time, we now report the in situ detection of a functional nicotinic receptor that has a high affinity for nicotine and is K-BuTx-insensitive.