Critical role of the C-terminal segment in the maturation and export to the cell surface of the homopentameric alpha7-5HT3A receptor

Two residues determine nicotinic acetylcholine receptor requirement for RIC-3

Nicotinic acetylcholine receptors (N-AChRs) mediate fast synaptic signaling and are members of the pentameric ligand-gated ion channel (pLGIC) family. They rely on a network of accessory proteins in vivo for correct formation and transport to the cell surface. Resistance to cholinesterase 3 (RIC-3) is an endoplasmic reticulum protein that physically interacts with nascent pLGIC subunits and promotes their oligomerization. It is not known why some N-AChRs require RIC-3 in heterologous expression systems, whereas others do not. Previously we reported that the ACR-16 N-AChR from the parasitic nematode Dracunculus medinensis does not require RIC-3 in Xenopus laevis oocytes. This is unusual because all other nematode ACR-16, like the closely related Ascaris suum ACR-16, require RIC-3. Their high sequence similarity limits the number of amino acids that may be responsible, and the goal of this study was to identify them. A series of chimeras and point mutations between A. suum and D. medinensis ACR-16, followed by functional characterization with electrophysiology, identified two residues that account for a majority of the receptor requirement for RIC-3. ACR-16 with R/K159 in the cys-loop and I504 in the C-terminal tail did not require RIC-3 for functional expression. Mutating either of these to R/K159E or I504T, residues found in other nematode ACR-16, conferred a RIC-3 requirement. Our results agree with previous studies showing that these regions interact and are involved in receptor synthesis. Although it is currently unclear what precise mechanism they regulate, these residues may be critical during specific subunit folding and/or assembly cascades that RIC-3 may promote.

Structural mechanism of muscle nicotinic receptor desensitization and block by curare

Binding of the neurotransmitter acetylcholine to its receptors on muscle fibers depolarizes the membrane and thereby triggers muscle contraction. We sought to understand at the level of three-dimensional structure how agonists and antagonists alter nicotinic acetylcholine receptor conformation. We used the muscle-type receptor from the Torpedo ray to first define the structure of the receptor in a resting, activatable state. We then determined the receptor structure bound to the agonist carbachol, which stabilizes an asymmetric, closed channel desensitized state. We find conformational changes in a peripheral membrane helix are tied to recovery from desensitization. To probe mechanisms of antagonism, we obtained receptor structures with the active component of curare, a poison arrow toxin and precursor to modern muscle relaxants. d-Tubocurarine stabilizes the receptor in a desensitized-like state in the presence and absence of agonist. These findings define the transitions between resting and desensitized states and reveal divergent means by which antagonists block channel activity of the muscle-type nicotinic receptor.

Addiction-related neuroadaptations following chronic nicotine exposure

The addiction-relevant molecular, cellular, and behavioral actions of nicotine are derived from its stimulatory effects on neuronal nicotinic acetylcholine receptors (nAChRs) in the central nervous system. nAChRs expressed by dopamine-containing neurons in the ventral midbrain, most notably in the ventral tegmental area (VTA), contribute to the reward-enhancing properties of nicotine that motivate the use of tobacco products. nAChRs are also expressed by neurons in brain circuits that regulate aversion. In particular, nAChRs expressed by neurons in the medial habenula (mHb) and the interpeduncular nucleus (IPn) to which the mHb almost exclusively projects regulate the "set-point" for nicotine aversion and control nicotine intake. Different nAChR subtypes are expressed in brain reward and aversion circuits and nicotine intake is titrated to maximally engage reward-enhancing nAChRs while minimizing the recruitment of aversion-promoting nAChRs. With repeated exposure to nicotine, reward- and aversion-related nAChRs and the brain circuits in which they are expressed undergo adaptations that influence whether tobacco use will transition from occasional to habitual. Genetic variation that influences the sensitivity of addiction-relevant brain circuits to the actions of nicotine also influence the propensity to develop habitual tobacco use. Here, we review some of the key advances in our understanding of the mechanisms by which nicotine acts on brain reward and aversion circuits and the adaptations that occur in these circuits that may drive addiction to nicotine-containing tobacco products.

A Single Mutation in the Outer Lipid-Facing Helix of a Pentameric Ligand-Gated Ion Channel Affects Channel Function Through a Radially-Propagating Mechanism

Pentameric ligand-gated ion channels (pLGICs) mediate fast synaptic transmission and are crucial drug targets. Their gating mechanism is triggered by ligand binding in the extracellular domain that culminates in the opening of a hydrophobic gate in the transmembrane domain. This domain is made of four α-helices (M1 to M4). Recently the outer lipid-facing helix (M4) has been shown to be key to receptor function, however its role in channel opening is still poorly understood. It could act through its neighboring helices (M1/M3), or via the M4 tip interacting with the pivotal Cys-loop in the extracellular domain. Mutation of a single M4 tyrosine (Y441) to alanine renders one pLGIC-the 5-HT3A receptor-unable to function despite robust ligand binding. Using Y441A as a proxy for M4 function, we here predict likely paths of Y441 action using molecular dynamics, and test these predictions with functional assays of mutant receptors in HEK cells and Xenopus oocytes using fluorescent membrane potential sensitive dye and two-electrode voltage clamp respectively. We show that Y441 does not act via the M4 tip or Cys-loop, but instead connects radially through M1 to a residue near the ion channel hydrophobic gate on the pore-lining helix M2. This demonstrates the active role of the M4 helix in channel opening.

The functional role of the αM4 transmembrane helix in the muscle nicotinic acetylcholine receptor probed through mutagenesis and coevolutionary analyses

The activity of the muscle-type Torpedo nicotinic acetylcholine receptor (nAChR) is highly sensitive to lipids, but the underlying mechanisms remain poorly understood. The nAChR transmembrane α-helix, M4, is positioned at the perimeter of each subunit in direct contact with lipids and likely plays a central role in lipid sensing. To gain insight into the mechanisms underlying nAChR lipid sensing, we used homology modeling, coevolutionary analyses, site-directed mutagenesis, and electrophysiology to examine the role of the α-subunit M4 (αM4) in the function of the adult muscle nAChR. Ala substitutions for most αM4 residues, including those in clusters of polar residues at both the N and C termini, and deletion of up to 11 C-terminal residues had little impact on the agonist-induced response. Even Ala substitutions for coevolved pairs of residues at the interface between αM4 and the adjacent helices, αM1 and αM3, had little effect, although some impaired nAChR expression. On the other hand, Ala substitutions for Thr422 and Arg429 caused relatively large losses of function, suggesting functional roles for these specific residues. Ala substitutions for aromatic residues at the αM4-αM1/αM3 interface generally led to gains of function, as previously reported for the prokaryotic homolog, the Erwinia chrysanthemi ligand-gated ion channel (ELIC). The functional effects of individual Ala substitutions in αM4 were found to be additive, although not in a completely independent manner. Our results provide insight into the structural features of αM4 that are important. They also suggest how lipid-dependent changes in αM4 structure ultimately modify nAChR function.

An allosteric link connecting the lipid-protein interface to the gating of the nicotinic acetylcholine receptor

The mechanisms underlying lipid-sensing by membrane proteins is of considerable biological importance. A unifying mechanistic question is how a change in structure at the lipid-protein interface is translated through the transmembrane domain to influence structures critical to protein function. Gating of the nicotinic acetylcholine receptor (nAChR) is sensitive to its lipid environment. To understand how changes at the lipid-protein interface influence gating, we examined how a mutation at position 418 on the lipid-facing surface of the outer most M4 transmembrane α-helix alters the energetic couplings between M4 and the remainder of the transmembrane domain. Human muscle nAChR is sensitive to mutations at position 418, with the Cys-to-Trp mutation resulting in a 16-fold potentiation in function that leads to a congenital myasthenic syndrome. Energetic coupling between M4 and the Cys-loop, a key structure implicated in gating, do not change with C418W. Instead, Trp418 and an adjacent residue couple energetically with residues on the M1 transmembrane α-helix, leading to a reorientation of M1 that stabilizes the open state. We thus identify an allosteric link connecting the lipid-protein interface of the nAChR to altered channel function.

The M4 Transmembrane α-Helix Contributes Differently to Both the Maturation and Function of Two Prokaryotic Pentameric Ligand-gated Ion Channels

The role of the outermost transmembrane α-helix in both the maturation and function of the prokaryotic pentameric ligand-gated ion channels, GLIC and ELIC, was examined by Ala scanning mutagenesis, deletion mutations, and mutant cycle analyses. Ala mutations at the M4-M1/M3 interface lead to loss-of-function phenotypes in GLIC, with the largest negative effects occurring near the M4 C terminus. In particular, two aromatic residues at the M4 C terminus form a network of π-π and/or cation-π interactions with residues on M3 and the β6-β7 loop that is essential for both maturation and function. M4-M1/M3 interactions appear to be optimized in GLIC with even subtle structural changes at this interface leading to detrimental effects. In contrast, mutations along the M4-M1/M3 interface of ELIC typically lead to gain-of-function phenotypes, suggesting that these interactions in ELIC are not optimized for channel function. In addition, no cluster of interacting residues involving the M4 C terminus, M3, and the β6-β7 loop was found, suggesting that the M4 C terminus plays little role in ELIC maturation or function. This study shows that M4 makes distinct contributions to the maturation and gating of these two closely related homologs, suggesting that GLIC and ELIC exhibit divergent features of channel function.

Nicotinic acetylcholine receptor-lipid interactions: Mechanistic insight and biological function

Membrane lipids are potent modulators of the nicotinic acetylcholine receptor (nAChR) from Torpedo. Lipids influence nAChR function by both conformational selection and kinetic mechanisms, stabilizing varying proportions of activatable versus non-activatable conformations, as well as influencing the transitions between these conformational states. Of note, some membranes stabilize an electrically silent uncoupled conformation that binds agonist but does not undergo agonist-induced conformational transitions. The uncoupled nAChR, however, does transition to activatable conformations in relatively thick lipid bilayers, such as those found in lipid rafts. In this review, we discuss current understanding of lipid-nAChR interactions in the context of increasingly available high resolution structural and functional data. These data highlight different sites of lipid action, including the lipid-exposed M4 transmembrane α-helix. Current evidence suggests that lipids alter nAChR function by modulating interactions between M4 and the adjacent transmembrane α-helices, M1 and M3. These interactions have also been implicated in both the folding and trafficking of nAChRs to the cell surface. We review current mechanistic understanding of lipid-nAChR interactions, and highlight potential biological roles for lipid-nAChR interactions in modulating the synaptic response. This article is part of a Special Issue entitled: Lipid-protein interactions.

The role of the M4 lipid-sensor in the folding, trafficking, and allosteric modulation of nicotinic acetylcholine receptors

With the availability of high resolution structural data, increasing attention has focused on the mechanisms by which drugs and endogenous compounds allosterically modulate nicotinic acetylcholine receptor (nAChR) function. Lipids are potent modulators of the nAChR from Torpedo. Membrane lipids influence nAChR function by both conformational selection and kinetic mechanisms, stabilizing varying proportions of pre-existing resting, open, desensitized, and uncoupled conformations, as well as influencing the transitions between these conformational states. Structural and functional data highlight a role for the lipid-exposed M4 transmembrane α-helix of each subunit in lipid sensing, and suggest that lipids influence gating by altering the binding of M4 to the adjacent transmembrane α-helices, M1 and M3. M4 has also been implicated in both the folding and trafficking of nAChRs to the cell surface, as well as in the potentiation of nAChR gating by neurosteroids. Here, we discuss the roles of M4 in the folding, trafficking, and allosteric modulation of nAChRs. We also consider the hypothesis that variable chemistry at the M4-M1/M3 transmembrane α-helical interface in different nAChR subunits governs the capacity for potentiation by activating lipids. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Functional evaluation of key interactions evident in the structure of the eukaryotic Cys-loop receptor GluCl

The publication of the first high-resolution crystal structure of a eukaryotic Cys-loop receptor, GluClα, has provided valuable structural information on this important class of ligand-gated ion channels (LGIC). However, limited functional data exist for the GluCl receptors. Before applying the structural insights from GluCl to mammalian Cys-loop receptors such as nicotinic acetylcholine and GABA receptors, it is important to ensure that established functional features of mammalian Cys-loop receptors are present in the more distantly related GluCl receptors. Here, we seek to identify ligand-binding interactions that are generally associated with Cys-loop receptors, including the frequently observed cation−π interaction. Our studies were performed on the highly homologous GluClβ receptor, because GluClα is not activated by glutamate in Xenopus laevis oocytes. Mutagenesis of the signal peptide and pore lining helix was performed to enhance functional expression and sensitivity to applied ligand, respectively. Conventional and unnatural amino acid mutagenesis indicate a strong cation−π interaction between Y206 and the protonated amine of glutamate, as well as other important ionic and hydrogen bond interactions between the ligand and the binding site, consistent with the crystal structure.

Gating of pentameric ligand-gated ion channels: structural insights and ambiguities

Pentameric ligand-gated ion channels (pLGICs) mediate fast synaptic communication by converting chemical signals into an electrical response. Recently solved agonist-bound and agonist-free structures greatly extend our understanding of these complex molecular machines. A key challenge to a full description of function, however, is the ability to unequivocally relate determined structures to the canonical resting, open, and desensitized states. Here, we review current understanding of pLGIC structure, with a focus on the conformational changes underlying channel gating. We compare available structural information and review the evidence supporting the assignment of each structure to a particular conformational state. We discuss multiple factors that may complicate the interpretation of crystal structures, highlighting the potential influence of lipids and detergents. We contend that further advances in the structural biology of pLGICs will require deeper insight into the nature of pLGIC-lipid interactions.

New hyperekplexia mutations provide insight into glycine receptor assembly, trafficking, and activation mechanisms

Hyperekplexia is a syndrome of readily provoked startle responses, alongside episodic and generalized hypertonia, that presents within the first month of life. Inhibitory glycine receptors are pentameric ligand-gated ion channels with a definitive and clinically well stratified linkage to hyperekplexia. Most hyperekplexia cases are caused by mutations in the α1 subunit of the human glycine receptor (hGlyR) gene (GLRA1). Here we analyzed 68 new unrelated hyperekplexia probands for GLRA1 mutations and identified 19 mutations, of which 9 were novel. Electrophysiological analysis demonstrated that the dominant mutations p.Q226E, p.V280M, and p.R414H induced spontaneous channel activity, indicating that this is a recurring mechanism in hGlyR pathophysiology. p.Q226E, at the top of TM1, most likely induced tonic activation via an enhanced electrostatic attraction to p.R271 at the top of TM2, suggesting a structural mechanism for channel activation. Receptors incorporating p.P230S (which is heterozygous with p.R65W) desensitized much faster than wild type receptors and represent a new TM1 site capable of modulating desensitization. The recessive mutations p.R72C, p.R218W, p.L291P, p.D388A, and p.E375X precluded cell surface expression unless co-expressed with α1 wild type subunits. The recessive p.E375X mutation resulted in subunit truncation upstream of the TM4 domain. Surprisingly, on the basis of three independent assays, we were able to infer that p.E375X truncated subunits are incorporated into functional hGlyRs together with unmutated α1 or α1 plus β subunits. These aberrant receptors exhibit significantly reduced glycine sensitivity. To our knowledge, this is the first suggestion that subunits lacking TM4 domains might be incorporated into functional pentameric ligand-gated ion channel receptors.

Molecular mechanisms of acetylcholine receptor-lipid interactions: from model membranes to human biology

Lipids are potent modulators of the Torpedo nicotinic acetylcholine receptor. Lipids influence nicotinic receptor function by allosteric mechanisms, stabilizing varying proportions of pre-existing resting, open, desensitized, and uncoupled conformations. Recent structures reveal that lipids could alter function by modulating transmembrane α-helix/α-helix packing, which in turn could alter the conformation of the allosteric interface that links the agonist-binding and transmembrane pore domains-this interface is essential in the coupling of agonist binding to channel gating. We discuss potential mechanisms by which lipids stabilize different conformational states in the context of the hypothesis that lipid-nicotinic receptor interactions modulate receptor function at biological synapses.

Structure-function study of the fourth transmembrane segment of the GABAρ1 receptor

The Cys-loop family of receptors mediates synaptic neurotransmission in the central nervous system of vertebrates. These receptors share several structural characteristics and assemble in the plasma membrane as multimers with fivefold symmetry. Of these, the ionotropic GABA receptors are key players in the pathogenesis of diseases like epilepsy, anxiety, and schizophrenia. Different experimental approaches have shed some light on the mechanisms behind the function of these receptors; but little is known about their structure at high resolution. Sequence homology with the nicotinic acetylcholine receptor predicts that ionotropic GABA receptors possess four transmembrane segments (TM1-4) and that TM2 forms the wall of the ion channel. However, the role of the other three segments is unclear. The GABAρ1 receptor plays a fundamental role in the regulation of neurotransmission along the visual pathway, is highly sensitive to GABA, and exhibits little desensitization. In our recent investigations of the role of TM4 in receptor function, a key residue in this domain (W475) was found to be involved in activation of the receptor. Here we have generated a structural model of the GABAρ1 receptor in silico and assessed its validity by electrophysiologically testing nine amino acid substitutions of W475 and deletions of the neighboring residues (Y474 and S476). The results identify a critical linkage between the ligand-binding domain and the TM4 domain and provide a framework for more detailed structure-function analyses of ionotropic GABA receptors.

UBXD4, a UBX-containing protein, regulates the cell surface number and stability of alpha3-containing nicotinic acetylcholine receptors

Adaptor proteins are likely to modulate spatially and temporally the trafficking of a number of membrane proteins, including neuronal nicotinic acetylcholine receptors (nAChRs). A yeast two-hybrid screen identified a novel UBX-containing protein, UBXD4, as one of the cytosolic proteins that interact directly with the alpha3 and alpha4 nAChR subunits. The function of UBX-containing proteins is largely unknown. Immunoprecipitation and confocal microscopy confirmed the interaction of UBXD4 with alpha3-containing nAChRs (alpha3* nAChRs) expressed in HEK293 cells, PC12 cells, and rat cortical neurons. Overexpression of UBXD4 in differentiated PC12 cells (dPC12) increased nAChR cell surface expression, especially that of the alpha3beta2 subtype. These findings were corroborated by electrophysiology, immunofluorescent staining, and biotinylation of surface receptors. Silencing of UBXD4 led to a significant reduction of alpha3* nAChRs in rat cortical neurons and dPC12 cells. Biochemical and immunofluorescence studies of endogenous UBXD4 showed that the protein is located in both the ER and cis-Golgi compartments. Our investigations also showed that the alpha3 subunit is ubiquitinated and that UBXD4 can interfere with its ubiquitination and consequent degradation by the proteasome. Our data suggest that UBXD4 modulates the distribution of alpha3* nAChRs between specialized intracellular compartments and the plasma membrane. This effect is achieved by controlling the stability of the alpha3 subunit and, consequently, the number of receptors at the cell surface.

A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of neurotransmitter-gated ion channels, a family that also includes receptors for gamma-aminobutyric acid, glycine and 5-hydroxytryptamine. In humans, nAChRs have been implicated in several neurological and psychiatric disorders and are major targets for pharmaceutical drug discovery. In addition, nAChRs are important targets for neuroactive pesticides in insects and in other invertebrates. Historically, nAChRs have been one of the most intensively studied families of neurotransmitter receptors. They were the first neurotransmitter receptors to be biochemically purified and the first to be characterized by molecular cloning and heterologous expression. Although much has been learnt from studies of native nAChRs, the expression of recombinant nAChRs has provided dramatic advances in the characterization of these important receptors. This review will provide a brief history of the characterization of nAChRs by heterologous expression. It will focus, in particular, upon studies of recombinant nAChRs, work that has been conducted by many hundreds of scientists during a period of almost 30 years since the molecular cloning of nAChR subunits in the early 1980s.

Importance of the C-terminus of the human 5-HT3A receptor subunit

Amongst the family members of Cys-loop LGICs, the atypical ability of the 5-HT3A subunit to form functional homomeric receptors allowed a direct investigation of the role of the C-terminus. Deletion of the three C-terminal amino acids (DeltaGln453-DeltaTyr454-DeltaAla455) from the h5-HT3A subunit prevented formation of a specific radioligand binding site as well as expression within the cell membrane. Removal of merely the C-terminal residue (DeltaAla455) reduced specific radioligand binding (to 4+/-1% relative to the wild-type; cells grown at 37 degrees C) and also cell membrane expression; these reductions were less evident when the DeltaAla455 expressing cells were grown at 27 degrees C (specific radioligand binding levels 27+/-5% relative to wild-type also grown at 27 degrees C). Mutation of the h5-HT3A C-terminal amino acid, alanine, for either glycine (Ala455Gly), valine (Ala455Val) or leucine (Ala455Leu) reduced specific radioligand binding levels by 24+/-23%, 32+/-12% and 88+/-1%, respectively; the latter mutant also displaying reduced membrane expression. In contrast, mutation to alanine of the two amino acids preceding the C-terminal alanine (Gln453Ala and Tyr454Ala) had no detrimental effects on specific radioligand binding or cell membrane expression levels. The present study demonstrates an important role for the C-terminus in the formation of the functional h5-HT3A receptor. The partial restoration of 5-HT3 receptor binding and cell membrane expression when cells expressing C-terminal mutant 5-HT3A subunits were grown at a lower temperature (27 degrees C) suggests that the C-terminus stabilises the 5-HT3 receptor allowing subunit folding and subsequent maturation.

Nicotine enhances intracellular nicotinic receptor maturation: A novel mechanism of neural plasticity?

Nicotine addiction, the primary cause of tobacco consumption, is mediated through nicotine binding to brain nicotinic acetylcholine receptor (nAChRs). Upon chronic exposure, nicotine elicits a cascade of events, starting with nAChR activation and desensitization, followed by a long term up-regulation that corresponds to an increase in the number of the high affinity nAChRs, a paradoxical process that occurs in the brain of smokers. Recent investigation of the maturation and trafficking of the major brain alpha4beta2 nAChR demonstrates that up-regulation is initiated in the endoplasmic reticulum soon after protein translation. The data thus far accumulated provide evidence that nicotine elicits up-regulation by promoting maturation of nAChR precursors that would otherwise be degraded. This "maturational enhancer" action of nicotine probably contributes to the long term effect of chronic nicotine, and suggests a novel mechanism of neuronal plasticity through an yet unknown endogenous substance which would modulate the receptor expression under physiological conditions.