Pathways for nicotinic receptor desensitization

Abstract

Nicotinic acetylcholine receptors (AChRs) are ligand-gated ion channels that generate transient currents by binding agonists and switching rapidly between closed- and open-channel conformations. Upon sustained exposure to ACh, the cell response diminishes slowly because of desensitization, a process that shuts the channel even with agonists still bound. In liganded receptors, the main desensitization pathway is from the open-channel conformation, but after agonists dissociate the main recovery pathway is to the closed-channel conformation. In this Viewpoint, I discuss two mechanisms that can explain the selection of different pathways, a question that has puzzled the community for 60 yr. The first is based on a discrete-state model (the "prism"), in which closed, open, and desensitized conformational states interconnect directly. This model predicts that 5% of unliganded AChRs are desensitized. Different pathways are taken with versus without agonists because ligands have different energy properties (φ values) at the transition states of the desensitization and recovery reactions. The second is a potential energy surface model (the "monkey saddle"), in which the states connect indirectly at a shared transition state region. Different pathways are taken because agonists shift the position of the gating transition state relative to the point where gating and desensitization conformational trajectories intersect. Understanding desensitization pathways appears to be a problem of kinetics rather than of thermodynamics. Other aspects of the two mechanisms are considered, as are experiments that may someday distinguish them.

Figure 1.

Prism: pathway selection by φ. (A) Vertices, stable C, O, and D conformations; superscript A, agonist; lowercase letters, free energy changes (direction of arrow); dashed line, unused pathway. Independent energy changes are g, unliganded gating; b, agonist binding; d, desensitization. Binding energy is the same to D and O (twice that to C); agonists influence a reaction only by a binding energy change; other energies set by microscopic reversibility. In adult mammalian AChRs g = +8.3, d = −6.5 kcal/mol, so (g + d) = +1.8 kcal/mol (5% of unliganded receptors are D). Red, main path with a bound agonist: ^AD is connected to ^AO because here φ ∼ 1; blue, main recovery path without agonists; D is connected to C because here φ ∼ 0. (B) Landscape analogy. Top: Before the earthquake (the agonist), a hiker in the valley of D takes the easier, east trail to C (blue arrow). Bottom: An earthquake collapses only the west side of the range, to lower ^AD (and ^AO) valleys and west pass. The trail from ^AD to ^AO is the same as before the quake, but the unaltered east trail to ^AC is now disfavored. Not shown. The quake also levels the once-high pass between ^AC and ^AO (behind the central peak). After reaching ^AO, the hiker can cross readily back and forth along this “gating” trail, eventually to revisit ^AD by the easier west trail from ^AO (red arrow).

Figure 2.

Monkey saddle: pathway selection by a soft transition state. (A) Top: Three-well potential energy surface (PES). The conformational trajectories for gating (white) and desensitization (green) intersect at a short-lived intermediate state (blue dot). The bifurcation is on the ^AO-side of the gating committor (‡), so in experiments, ^AD appears to be connected to ^AO. Not depicted: Without agonists, ‡ moves closer to O to put the bifurcation on the C-side, so D appears to be connected to C. Bottom: Discrete-state model of the monkey-saddle landscape (^AX, the intermediate configuration at the bifurcation). Top modified from https://en.wikipedia.org/wiki/Monkey_saddle. (B) Cross sections through the gating PES. Left: C and O energy wells as parabolas (the D well is out of the plane). Top: Without agonists, the bifurcation to D is on the wall of the C parabola. Bottom: Agonists lower the relative free energy of O and move the bifurcation onto the wall of the O parabola. Right: Gating transition state region as a rugged landscape. Short-lived intermediates (wells) reflect fractional rearrangements of the global gating isomerization (ECD, extracellular domain; TMD, transmembrane domain). The position of ‡ relative to the bifurcation is malleable and depends on the tilt of the “roof line” that connects the barrier tops (Zhou et al., 2005). Without agonists, D appears to connect to C (top), but with agonists, ^AD appears to connect to ^AO (bottom).