Conformational changes in alpha 7 acetylcholine receptors underlying allosteric modulation by divalent cations.

Area of Science:

• Neuroscience
• Molecular Biology
• Biochemistry

Background:

• Allosteric modulation is a key mechanism regulating membrane receptor function by endogenous and exogenous agents.
• Extracellular calcium ions physiologically modulate several nicotinic acetylcholine receptor family members.

Purpose of the Study:

• To investigate the conformational changes in the alpha 7 acetylcholine receptor (α7 nAChR) extracellular domain induced by divalent cations.
• To compare these conformational changes with those evoked by acetylcholine (ACh).

Main Methods:

• Cysteine-scanning mutagenesis was used to introduce cysteine residues at specific sites in the α7 nAChR extracellular domain.
• Rates of modification by 2-aminoethylmethane thiosulfonate were measured to probe conformational changes.
• Barium ions (Ba2+) were used as a surrogate for calcium ions (Ca2+).

Main Results:

• Divalent cations (Ba2+) decreased modification rates at M37 and M40, similar to ACh effects.
• Ba2+ had no significant effect at N52C, where ACh increased modification rates, indicating distinct conformational effects.
• Mutations in conserved glutamates (E44, E172) abolished allosteric modulation by Ba2+ but did not prevent Ba2+ effects on modification rates, suggesting a distinct binding site.

Conclusions:

• Divalent cation modulation of α7 nAChR involves significant conformational changes in the receptor's extracellular domain.
• The binding site for divalent cations appears distinct from conserved glutamate residues (E44, E172).
• Divalent cations induce some, but not all, of the conformational changes elicited by agonists like ACh.