Agonists to ions: Efficiency suggests a 'zipper' mechanism for nicotinic receptor activation

Agonists are classified by the strength at which they bind to their target sites (affinity) and their ability to activate receptors once bound to those sites (efficacy). Efficiency is a third fundamental agonist property that is a measure of the correlation between affinity and efficacy. Efficiency is the percent of agonist binding energy that is converted into energy for receptor activation ('gating'). In the muscle nicotinic acetylcholine receptor, agonists belong to families having discrete efficiencies of 54%, 51%, 42% or 35%. Efficiency depends on the size and composition of both the agonist and binding site, and can be estimated from, and used to interpret, concentration-response curves. A correlation between affinity and efficacy indicates that the agonist's energy changes that take place within binding and gating processes are linked. Efficiency suggests that receptors turn on and off by progressing through a sequence of energy-linked domain rearrangements, as in a zipper.

Channel opening duration in adult muscle nAChRs determined by activated external ACh binding site

We recorded currents through the cell membrane at single nAChR molecules, held at ACh or Epibatidine (Ebd) concentrations of 0.01, 0.1, 1, 10 or 100 μM. The measured current amplitudes had an absolutely fixed value of 15 pA. This was valid for different agonists at all concentrations. Binding an agonist at one or both sites in the ring of subunits allowed to open the channel, the site that initiated the opening determined the duration of the final opening of the channel. In addition, the current flow was continuously interrupted by < 3 μs shut times. The resolution of our records was optimized to reach 5 μs, but was insufficient to resolve an unknown proportion of shorter shut times. Therefore, measured durations of openings are overestimated, and cited in brackets: τo1 (3 μs) elicited by agonist-binding at the δ-site, τo2 and τo3 (40 and 183 μs) by binding at the ϵ-site, and τo4 (752 μs) by binding at the δ- and ϵ-site. Mono-liganded nAChRs trigger short bursts of 0.6 ms duration. Bi-liganded nAChRs generate long bursts that at low agonist concentrations last 12 ms. Above 10 μM ACh, long bursts are shortened, with 100 μM ACh, to 5 ms, and further at higher concentrations. While ACh was the main agonist, Ebd bound more effectively than ACh to the ϵ-site.

Structural Arrangement Produced by Concanavalin A Binding to Homomeric GluK2 Receptors

Kainate receptors are members of the ionotropic glutamate receptor family. They form cation-specific transmembrane channels upon binding glutamate that desensitize in the continued presence of agonists. Concanavalin A (Con-A), a lectin, stabilizes the active open-channel state of the kainate receptor and reduces the extent of desensitization. In this study, we used single-molecule fluorescence resonance energy transfer (smFRET) to investigate the conformational changes underlying kainate receptor modulation by Con-A. These studies showed that Con-A binding to GluK2 homomeric kainate receptors resulted in closer proximity of the subunits at the dimer–dimer interface at the amino-terminal domain as well as between the subunits at the dimer interface at the agonist-binding domain. Additionally, the modulation of receptor functions by monovalent ions, which bind to the dimer interface at the agonist-binding domain, was not observed in the presence of Con-A. Based on these results, we conclude that Con-A modulation of kainate receptor function is mediated by a shift in the conformation of the kainate receptor toward a tightly packed extracellular domain.

A de novo GRIN1 Variant Associated With Myoclonus and Developmental Delay: From Molecular Mechanism to Rescue Pharmacology

N-Methyl-D-aspartate receptors (NMDARs) are highly expressed in brain and play important roles in neurodevelopment and various neuropathologic conditions. Here, we describe a new phenotype in an individual associated with a novel de novo deleterious variant in GRIN1 (c.1595C&gt;A, p.Pro532His). The clinical phenotype is characterized with developmental encephalopathy, striking stimulus-sensitive myoclonus, and frontal lobe and frontal white matter hypoplasia, with no apparent seizures detected. NMDARs that contained the P532H within the glycine-binding domain of GluN1 with either the GluN2A or GluN2B subunits were evaluated for changes in their pharmacological and biophysical properties, which surprisingly revealed only modest changes in glycine potency but a significant decrease in glutamate potency, an increase in sensitivity to endogenous zinc inhibition, a decrease in response to maximally effective concentrations of agonists, a shortened synaptic-like response time course, a decreased channel open probability, and a reduced receptor cell surface expression. Molecule dynamics simulations suggested that the variant can lead to additional interactions across the dimer interface in the agonist-binding domains, resulting in a more open GluN2 agonist-binding domain cleft, which was also confirmed by single-molecule fluorescence resonance energy transfer measurements. Based on the functional deficits identified, several positive modulators were evaluated to explore potential rescue pharmacology.

Why does Δ9-Tetrahydrocannabinol act as a partial agonist of cannabinoid receptors?

Cannabinoid receptor 1 (CB1) is a therapeutically relevant drug target for controlling pain, obesity, and other central nervous system disorders. However, full agonists and antagonists of CB1 have been reported to cause serious side effects in patients. Therefore, partial agonists have emerged as a viable alternative to full agonists and antagonists as they avoid overstimulation and side effects. One of the key bottlenecks in the design of partial agonists is the lack of understanding of the molecular mechanism of partial agonism. In this study, we examine two mechanistic hypotheses for the origin of partial agonism in cannabinoid receptors and explain the mechanistic basis of partial agonism exhibited by Δ9-Tetrahydrocannabinol (THC). In particular, we inspect whether partial agonism emerges from the ability of THC to bind in both agonist and antagonist binding pose or from its ability to only partially activate the receptor. Extensive molecular dynamics simulations and the Markov state model capture the THC binding in both antagonist, and agonist binding poses in CB1 receptor. Furthermore, we observe that binding of THC in the agonist binding pose leads to rotation of toggle switch residues and causes partial outward movement of intracellular transmembrane helix 6 (TM6). Our simulations also suggest that the alkyl side chain of THC plays a crucial role in determining partial agonism by stabilizing the ligand in the agonist and antagonist-like poses within the pocket. This study provides us fundamental insights into the mechanistic origin of the partial agonism of THC.