Design and expression of human alpha7 nicotinic acetylcholine receptor extracellular domain mutants with enhanced solubility and ligand-binding properties

Serum autoantibodies against α7-nicotinic receptors in subgroups of patients with bipolar disorder or schizophrenia: clinical features and link with peripheral inflammation

There is growing evidence that autoantibodies (AAbs) against proteins expressed in the brain are playing an important role in neurological and psychiatric disorders. Here, we explore the presence and the role of peripheral AAbs to the α7-nicotinic acetylcholine receptor (nAChR) in inflammatory subgroups of psychiatric patients with bipolar disorder (BD) or schizophrenia (SCZ) and healthy controls. We have identified a continuum of AAb levels in serum when employing a novel ELISA technique, with a significant elevation in patients compared to controls. Using unsupervised two-step clustering to stratify all the subjects according to their immuno-inflammatory background, we delineate one subgroup consisting solely of psychiatric patients with severe symptoms, high inflammatory profile, and significantly increased levels of anti-nAChR AAbs. In this context, we have used monoclonal mouse anti-human α7-nAChR antibodies (α7-nAChR-mAbs) and shown that TNF-α release was enhanced upon LPS stimulation in macrophages pre-incubated with α7-nAChR-mAbs compared to the use of an isotype control. These findings provide a basis for further study of circulating nicotinic AAbs, and the inflammatory profile observed in patients with major mood and psychotic disorders.

The expression of soluble functional α7-nicotinic acetylcholine receptors in E. coli and its high-affinity binding to neonicotinoid pesticides

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels mediating fast cholinergic synaptic transmission in nervous system. In insects, nAChRs are the target sites for several naturally occurring and synthetic compounds, including the neonicotinoid insecticides. So far, one of the major strategies to explore the interaction of nAChR and ligands is based on the heterologous expression of nAChR, which is tough, and needs to be explored. In this study, we expressed and purified extracellular domain of rat a7 subunit (Rα7-ECD), the binding site of the ligands in E. coli and determined the interactions and kinetic constants of neonicotinoid insecticides with Rα7-ECD. The recombinant Rα7-ECD is water-soluble and appears to be correctly folded. The interactions of three neonicotinoid pesticides with Rα7-ECD were assessed by surface plasmon resonance (SPR) biosensor. The results revealed a fast association and fast disassociation binding mode of Rα7-ECD/pesticides complexes with the KD value of clothianidin (6.414E-9 M) > imidacloprid (9.030E-9 M) > acetamiprid (2.874E-6 M), respectively. This study demonstrated that the nAChR expressed from E. coli was functional, and SPR biosensor technology would be a good alternative for characterizing members of nAChR receptor family.

N-Stearoylethanolamine protects the brain and improves memory of mice treated with lipopolysaccharide or immunized with the extracellular domain of α7 nicotinic acetylcholine receptor

Neuroinflammation is an important risk factor for neurodegenerative disorders like Alzheimer's disease. Nicotinic acetylcholine receptors of α7 subtype (α7 nAChRs) regulate inflammatory processes in various tissues, including the brain. N-stearoylethanolamine (NSE) is a biologically active cell membrane component with anti-inflammatory and membrane-protective properties. Previously we found that mice injected with bacterial lipopolysaccharide (LPS) or immunized with recombinant extracellular domain (1-208) of α7 nAChR subunit possessed decreased α7 nAChR levels, accumulated pathogenic amyloid-beta peptide Aβ(1-42) in the brain and demonstrated impaired episodic memory compared to non-treated mice. Here we studied the effect of NSE on behavior and brain components of LPS- treated or α7(1-208)-immunized mice. NSE, given per os, non-significantly decreased LPS-stimulated interleukin-6 elevation in the brain, slowed down the α7(1-208)-specific IgG antibody production and prevented the antibody penetration into the brain of mice. NSE prevented the loss of α7 nAChRs and accumulation of α7-bound Aβ(1-42) in the brain and brain mitochondria of LPS-treated or α7(1-208)-immunized mice and supported mitochondria resistance to apoptosis by attenuating Ca²⁺-stimulated cytochrome c release. Finally, NSE significantly improved episodic memory of mice impaired by either LPS treatment or immunization with α7(1-208). The results of our study demonstrate a therapeutic potential of NSE for prevention of cognitive disfunction caused by neuroinflammation or autoimmune reaction that allows suggesting this drug as a candidate for the treatment or prophylaxis of Alzheimer's pathology.

Crystal structure of a human neuronal nAChR extracellular domain in pentameric assembly: Ligand-bound α2 homopentamer

In this study we report the X-ray crystal structure of the extracellular domain (ECD) of the human neuronal α2 nicotinic acetylcholine receptor (nAChR) subunit in complex with the agonist epibatidine at 3.2 Å. Interestingly, α2 was crystallized as a pentamer, revealing the intersubunit interactions in a wild type neuronal nAChR ECD and the full ligand binding pocket conferred by two adjacent α subunits. The pentameric assembly presents the conserved structural scaffold observed in homologous proteins, as well as distinctive features, providing unique structural information of the binding site between principal and complementary faces. Structure-guided mutagenesis and electrophysiological data confirmed the presence of the α2(+)/α2(-) binding site on the heteromeric low sensitivity α2β2 nAChR and validated the functional importance of specific residues in α2 and β2 nAChR subunits. Given the pathological importance of the α2 nAChR subunit and the high sequence identity with α4 (78%) and other neuronal nAChR subunits, our findings offer valuable information for modeling several nAChRs and ultimately for structure-based design of subtype specific drugs against the nAChR associated diseases.

Inflammation decreases the level of alpha7 nicotinic acetylcholine receptors in the brain mitochondria and makes them more susceptible to apoptosis induction

α7 nicotinic acetylcholine receptors (α7 nAChRs) are involved in regulating inflammatory reactions, as well as the cell viability. They are expressed in both the plasma membrane and mitochondria of eukaryotic cells. Previously we found that neuroinflammation resulted in the decrease of α7 nAChR density in the brain of mice and was accompanied by accumulation of amyloid-beta (Aβ) peptides and memory impairment. In the present paper, it is shown that inflammation induced by either regular bacterial lipopolysaccharide (LPS) injections or immunizations with α7 nAChR extracellular domain (1-208) affected also the brain cell mitochondria. Using various modifications of sandwich ELISA, we observed the decrease of α7 nAChRs and accumulation of Aβ(1-40) and Aβ(1-42) in mitochondria of immunized or LPS-treated mice compared to control ones. Mitochondria of treated mice responded with cytochrome c release to lower Ca(2+) concentrations than mitochondria of control mice and were less sensitive to its attenuation with α7 nAChR agonist PNU282987. It is concluded that inflammation decreases α7 nAChR expression in both mitochondria and cell plasma membrane and makes mitochondria more susceptible to apoptosis induction.

α7 Nicotinic acetylcholine receptor-specific antibody induces inflammation and amyloid β42 accumulation in the mouse brain to impair memory

Nicotinic acetylcholine receptors (nAChRs) expressed in the brain are involved in regulating cognitive functions, as well as inflammatory reactions. Their density is decreased upon Alzheimer disease accompanied by accumulation of β-amyloid (Aβ42), memory deficit and neuroinflammation. Previously we found that α7 nAChR-specific antibody induced pro-inflammatory interleukin-6 production in U373 glioblastoma cells and that such antibodies were present in the blood of humans. We raised a hypothesis that α7 nAChR-specific antibody can cause neuroinflammation when penetrating the brain. To test this, C57Bl/6 mice were either immunized with extracellular domain of α7 nAChR subunit α7(1-208) or injected with bacterial lipopolysaccharide (LPS) for 5 months. We studied their behavior and the presence of α3, α4, α7, β2 and β4 nAChR subunits, Aβ40 and Aβ42 and activated astrocytes in the brain by sandwich ELISA and confocal microscopy. It was found that either LPS injections or immunizations with α7(1-208) resulted in region-specific decrease of α7 and α4β2 and increase of α3β4 nAChRs, accumulation of Aβ42 and activated astrocytes in the brain of mice and worsening of their episodic memory. Intravenously transferred α7 nAChR-specific-antibodies penetrated the brain parenchyma of mice pre-injected with LPS. Our data demonstrate that (1) neuroinflammation is sufficient to provoke the decrease of α7 and α4β2 nAChRs, Aβ42 accumulation and memory impairment in mice and (2) α7(1-208) nAChR-specific antibodies can cause inflammation within the brain resulting in the symptoms typical for Alzheimer disease.

Crystal structures of free and antagonist-bound states of human α9 nicotinic receptor extracellular domain

We determined the X-ray crystal structures of the extracellular domain (ECD) of the monomeric state of human neuronal α9 nicotinic acetylcholine receptor (nAChR) and of its complexes with the antagonists methyllycaconitine and α-bungarotoxin at resolutions of 1.8 Å, 1.7 Å and 2.7 Å, respectively. The structure of the monomeric α9 ECD superimposed well with the structures of homologous proteins in pentameric assemblies, denoting native folding, despite the absence of a complementary subunit and transmembrane domain. The interaction motifs of both antagonists were similar to those in the complexes with homologous pentameric proteins, thus highlighting the major contribution of the principal side of α9 ECD to their binding. The structures revealed a functionally important β7-β10 strand interaction in α9-containing nAChRs, involving their unique Thr147, a hydration pocket similar to that of mouse α1 ECD and a membrane-facing network coordinated by the invariant Arg210.

End-plate acetylcholine receptor: structure, mechanism, pharmacology, and disease

The synapse is a localized neurohumoral contact between a neuron and an effector cell and may be considered the quantum of fast intercellular communication. Analogously, the postsynaptic neurotransmitter receptor may be considered the quantum of fast chemical to electrical transduction. Our understanding of postsynaptic receptors began to develop about a hundred years ago with the demonstration that electrical stimulation of the vagus nerve released acetylcholine and slowed the heart beat. During the past 50 years, advances in understanding postsynaptic receptors increased at a rapid pace, owing largely to studies of the acetylcholine receptor (AChR) at the motor endplate. The endplate AChR belongs to a large superfamily of neurotransmitter receptors, called Cys-loop receptors, and has served as an exemplar receptor for probing fundamental structures and mechanisms that underlie fast synaptic transmission in the central and peripheral nervous systems. Recent studies provide an increasingly detailed picture of the structure of the AChR and the symphony of molecular motions that underpin its remarkably fast and efficient chemoelectrical transduction.

Creating an α7 nicotinic acetylcholine recognition domain from the acetylcholine-binding protein: crystallographic and ligand selectivity analyses

Determining the structure of the ligand-binding domain of the nicotinic acetylcholine receptor (nAChR) has been a long standing goal in the design of selective drugs useful in implicated diseases for this prevalent receptor family. Acetylcholine-binding proteins have proven to be valuable surrogates with structural similarity and sequence identity to the extracellular domain of the nicotinic receptor, yet these soluble proteins have their unique features and do not serve as exact replicates of the nAChRs of interest. Here we systematically modify the sequence of these proteins toward the homomeric human α7 nAChR. These chimeric proteins exhibit a shift in affinities to reflect α7 binding characteristics yet maintain expression levels and stability conducive for crystallization. We also present a pentameric humanoid nAChR extracellular domain with the structural determination of the α7 nAChR glycosylation site.

Ligand-binding domain of an α7-nicotinic receptor chimera and its complex with agonist

The α(7) acetylcholine receptor (AChR) mediates pre- and postsynaptic neurotransmission in the central nervous system and is a potential therapeutic target in neurodegenerative, neuropsychiatric and inflammatory disorders. We determined the crystal structure of the extracellular domain of a receptor chimera constructed from the human α(7) AChR and Lymnaea stagnalis acetylcholine binding protein (AChBP), which shares 64% sequence identity and 71% similarity with native α(7). We also determined the structure with bound epibatidine, a potent AChR agonist. Comparison of the structures revealed molecular rearrangements and interactions that mediate agonist recognition and early steps in signal transduction in α(7) AChRs. The structures further revealed a ring of negative charge within the central vestibule, poised to contribute to cation selectivity. Structure-guided mutational studies disclosed distinctive contributions to agonist recognition and signal transduction in α(7) AChRs. The structures provide a realistic template for structure-aided drug design and for defining structure-function relationships of α(7) AChRs.

Expression of water-soluble, ligand-binding concatameric extracellular domains of the human neuronal nicotinic receptor alpha4 and beta2 subunits in the yeast Pichia pastoris: glycosylation is not required for ligand binding

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation channels that are responsible for cell communication via the neurotransmitter acetylcholine. The predominant nAChR subtype in the mammalian brain with a high affinity for nicotine is composed of α4 and β2 subunits. This nAChR subtype is responsible for addiction to nicotine and is thought to be implicated in Alzheimer and Parkinson diseases and therefore presents an important target for drug design. In an effort to obtain water-soluble, ligand-binding domains of the human α4β2 nAChR for structural studies, we expressed the extracellular domains (ECDs) of these subunits in the eukaryotic expression system Pichia pastoris. The wild-type ECDs and their mutants containing the more hydrophilic Cys-loop from the snail acetylcholine-binding protein (individually expressed or coexpressed) did not demonstrate any specific interaction with ligands. We then linked the mutated ECDs with the 24-amino acid peptide (AGS)(8) and observed that the β2-24-α4 ECD concatamer, but not the α4-24-β2 one, exhibited very satisfactory water solubility and ligand binding properties. The (125)I-epibatidine and [(3)H]nicotine bound to β2-24-α4 with dissociation constants (K(d)) of 0.38 and 19 nm, respectively, close to the published values for the intact α4β2 AChR. In addition, (125)I-epibatidine binding was blocked by nicotine, cytisine, acetylcholine, and carbamylcholine with inhibition constants (K(i)) of 20.64, 3.24, 242, and 2,254 nm, respectively. Interestingly, deglycosylation of the concatamer did not affect its ligand binding properties. Furthermore, the deglycosylated β2-24-α4 ECD existed mainly in monomeric form, thus forming an appropriate material for structural studies and possibly for pharmacological evaluation of novel α4β2 nAChR-specific agonists.

In pursuit of the high-resolution structure of nicotinic acetylcholine receptors

The nicotinic acetylcholine receptor (nAChR) has been studied extensively for well over four decades because of its important physiological roles and medical relevance. A large body of data from biochemical and biophysical studies are now available. The structural information, which is needed to integrate existing data to address the mechanism and function of nAChRs, started to emerge in recent years. Structural studies of acetylcholine binding proteins (AChBPs) have greatly facilitated the study of nAChRs. The recently determined crystal structures of the prokaryotic homologues of nAChRs will probably have similar impact over time. However, a direct structural model of nAChRs at high resolution will be important for mechanistic studies and drug development. Here we will review some of the recent efforts in this area and use the high-resolution structure of the extracellular domains of nAChR alpha1 to illustrate the potential insights one may gain at higher resolution.