A distinct mechanism for activating uncoupled nicotinic acetylcholine receptors

Abstract

The ability of the nicotinic acetylcholine receptor (nAChR) to undergo conformational transitions is exquisitely sensitive to its surrounding lipid environment. Previous work has highlighted a conformational selection mechanism, whereby different lipids stabilize different proportions of activatable resting versus nonactivatable conformations. In the absence of anionic lipids and cholesterol, the nAChR adopts an uncoupled conformation, which binds agonist with resting state-like affinity but does not usually undergo agonist-induced conformational transitions. Very slow (minutes to hours) transitions from uncoupled to coupled (resting, open and/or desensitized) conformations, however, can occur in membranes with relatively thick hydrophobic cores. Increasing membrane hydrophobic thickness 'awakens' uncoupled nAChRs by reducing the large activation energy barrier (or barriers) leading to coupled states, thus allowing conformational transitions to occur on an experimentally tractable timescale. Lipids shape activity by modulating the relative proportions of activatable versus nonactivatable conformations and by controlling the transitions between uncoupled and coupled conformations.

Intramembrane Aromatic Interactions Influence the Lipid Sensitivities of Pentameric Ligand-gated Ion Channels

“…The uncoupled nAChR exhibits more extensive peptide N-1 H/N-2 H exchange after exposure to 2 H 2 O than do coupled nAChRs (see Fig. 3B), and displays both a lower denaturation temperature and a reduced cooperativity of unfolding (11,14). Although the specific structural changes in the nAChR associated with uncoupling have yet to be defined, it has been proposed that an increased physical separation leading to weakened interactions between the agonist binding domain and TMD could account for the lack of functional "coupling" between the agonist site and channel gate.…”

“…Such studies may have broader implications because neuronal nAChRs that are functionally uncoupled have been observed in heterologous expression systems and may play a role in the response to nicotine (12,13). Lipid-dependent mechanisms for "awakening" uncoupled nAChRs have been identified and could play a role modulating synaptic communication (14). The lipid-dependent uncoupled conformation may also be germane to the interpretation of crystal structures of detergent-solubilized pLGICs (15).…”

“…Mutations along the lipid-facing surface of M4 alter gating, showing that altered M4-lipid interactions influence the allosteric pathway leading from the agonist site to the channel gate (25)(26)(27)(28)(29). One model of uncoupling proposes that lipids modulate interactions between M4 and the adjacent transmembrane ␣-helices, M1 and M3, with enhanced M4-M1/M3 interactions promoting effective coupling between the agonist binding domain and the transmembrane domain (TMD) to enhance channel gating (11,14).…”

Molecular rheology of neuronal membranes explored using a molecular rotor: Implications for receptor function

“…A possible mechanism is centered on change in membrane physical properties (such as viscosity, thickness and curvature) that would give rise to altered protein function (Brown, 2012;daCosta et al, 2013;Soubias et al, 2014). …”

Temporal evolution of helix hydration in a light-gated ion channel correlates with ion conductance

“…IR spectroscopy, however, is a label-free technique that combines notable structural sensitivity and excellent temporal resolution. In the past, steady-state IR spectroscopy has been used to obtain information about the gating mechanism of pH and ligand-gated ion channels (60,61). Technical challenges have limited so far the application of time-resolved IR spectroscopy to study the dynamics of ion channels (62).…”