The Startle Disease Mutation E103K Impairs Activation of Human Homomeric α1 Glycine Receptors by Disrupting an Intersubunit Salt Bridge across the Agonist Binding Site

State-dependent protein-lipid interactions of a pentameric ligand-gated ion channel in a neuronal membrane

Pentameric ligand-gated ion channels (pLGICs) are receptor proteins that are sensitive to their membrane environment, but the mechanism for how lipids modulate function under physiological conditions in a state dependent manner is not known. The glycine receptor is a pLGIC whose structure has been resolved in different functional states. Using a realistic model of a neuronal membrane coupled with coarse-grained molecular dynamics simulations, we demonstrate that some key lipid-protein interactions are dependent on the receptor state, suggesting that lipids may regulate the receptor's conformational dynamics. Comparison with existing structural data confirms known lipid binding sites, but we also predict further protein-lipid interactions including a site at the communication interface between the extracellular and transmembrane domain. Moreover, in the active state, cholesterol can bind to the binding site of the positive allosteric modulator ivermectin. These protein-lipid interaction sites could in future be exploited for the rational design of lipid-like allosteric drugs.

Interaction between GABAA receptor α1 and β2 subunits at the N-terminal peripheral regions is crucial for receptor binding and gating

Pentameric ligand gated ion channels (pLGICs) are crucial in electrochemical signaling but exact molecular mechanisms of their activation remain elusive. So far, major effort focused on the top-down molecular pathway between the ligand binding site and the channel gate. However, recent studies revealed that pLGIC activation is associated with coordinated subunit twisting in the membrane plane. This suggests a key role of intersubunit interactions but the underlying mechanisms remain largely unknown. Herein, we investigated a "peripheral" subunit interface region of GABA_A receptor where structural modeling indicated interaction between N-terminal α₁F14 and β₂F31 residues. Our experiments underscored a crucial role of this interaction in ligand binding and gating, especially preactivation and opening, showing that the intersubunit cross-talk taking place outside (above) the top-down pathway can be strongly involved in receptor activation. Thus, described here intersubunit interaction appears to operate across a particularly long distance, affecting vast portions of the macromolecule.

The startle disease mutation α1S270T predicts shortening of glycinergic synaptic currents

KEY POINTS

Loss-of-function mutations in proteins found at glycinergic synapses, most commonly in the α1 subunit of the glycine receptor (GlyR), cause the startle disease/hyperekplexia channelopathy in man. It was recently proposed that the receptors responsible are presynaptic homomeric GlyRs, rather than postsynaptic heteromeric GlyRs (which mediate glycinergic synaptic transmission), because heteromeric GlyRs are less affected by many startle mutations than homomers. We examined the α1 startle mutation S270T, at the extracellular end of the M2 transmembrane helix. Recombinant heteromeric GlyRs were less impaired than homomers by this mutation when we measured their response to equilibrium applications of glycine. However, currents elicited by synaptic-like millisecond applications of glycine to outside-out patches were much shorter (7- to 10-fold) in all mutant receptors, both homomeric and heteromeric. Thus, the synaptic function of heteromeric receptors is likely to be impaired by the mutation.

ABSTRACT

Human startle disease is caused by mutations in glycine receptor (GlyR) subunits or in other proteins associated with glycinergic synapses. Many startle mutations are known, but it is hard to correlate the degree of impairment at molecular level with the severity of symptoms in patients. It was recently proposed that the disease is caused by disruption in the function of presynaptic homomeric GlyRs (rather than postsynaptic heteromeric GlyRs), because homomeric GlyRs are more sensitive to loss-of-function mutations than heteromers. Our patch-clamp recordings from heterologously expressed GlyRs characterised in detail the functional consequences of the α1S270T startle mutation, which is located at the extracellular end of the pore lining M2 transmembrane segment (18'). This mutation profoundly decreased the maximum single-channel open probability of homomeric GlyRs (to 0.16; cf. 0.99 for wild type) but reduced only marginally that of heteromeric GlyRs (0.96; cf. 0.99 for wild type). However, both heteromeric and homomeric mutant GlyRs became less sensitive to the neurotransmitter glycine. Responses evoked by brief, quasi-synaptic pulses of glycine onto outside-out patches were impaired in mutant receptors, as deactivation was approximately 10- and 7-fold faster for homomeric and heteromeric GlyRs, respectively. Our data suggest that the α1S270T mutation is likely to affect the opening step in GlyR activation. The faster decay of synaptic currents mediated by mutant heteromeric GlyRs is expected to reduce charge transfer at the synapse, despite the high equilibrium open probability of these mutant channels.

A Refined Open State of the Glycine Receptor Obtained via Molecular Dynamics Simulations

Pentameric ligand-gated ion channels are key players in mediating fast neurotransmission. Glycine receptors are chloride-selective members of this receptor family that mediate inhibitory synaptic transmission and are implicated in neurological disorders including autism and hyperekplexia. They have been structurally characterized by both X-ray crystallography and cryoelectron microscopy (cryo-EM) studies, with the latter giving rise to what was proposed as a possible open state. However, recent work has questioned the physiological relevance of this open state structure, since it rapidly collapses in molecular dynamics simulations. Here, we show that the collapse can be avoided by a careful equilibration protocol that reconciles the more problematic regions of the original density map and gives a stable open state that shows frequent selective chloride permeation. The protocol developed in this work provides a means to refine open-like structures of the whole pentameric ligand-gated ion channel superfamily and reconciles the previous issues with the cryo-EM structure.

The electrostatic core of the outer membrane protein X from E. coli

Electrostatic side chain contacts can contribute substantial interaction energy terms to the stability of proteins. The impact of electrostatic interactions on the structure and architecture of outer membrane proteins is however not well studied compared to soluble proteins. Here, we report the results of a systematic study of all charged side chains of the E. coli outer membrane protein X (OmpX). The data identify three distinct salt-bridge clusters in the core of OmpX that contribute significantly to protein stability in dodecylphosphocholine detergent micelles. The three clusters form an "electrostatic core" of the membrane protein OmpX, corresponding in its architectural role to the hydrophobic core of soluble proteins. This article is part of a Special Issue entitled: Molecular biophysics of membranes and membrane proteins.

Insights into channel dysfunction from modelling and molecular dynamics simulations

Developments in structural biology mean that the number of different ion channel structures has increased significantly in recent years. Structures of ion channels enable us to rationalize how mutations may lead to channelopathies. However, determining the structures of ion channels is still not trivial, especially as they necessarily exist in many distinct functional states. Therefore, the use of computational modelling can provide complementary information that can refine working hypotheses of both wild type and mutant ion channels. The simplest but still powerful tool is homology modelling. Many structures are available now that can provide suitable templates for many different types of ion channels, allowing a full three-dimensional interpretation of mutational effects. These structural models, and indeed the structures themselves obtained by X-ray crystallography, and more recently cryo-electron microscopy, can be subjected to molecular dynamics simulations, either as a tool to help explore the conformational dynamics in detail or simply as a means to refine the models further. Here we review how these approaches have been used to improve our understanding of how diseases might be linked to specific mutations in ion channel proteins. This article is part of the Special Issue entitled 'Channelopathies.'