Structural determinants for interaction of partial agonists with acetylcholine binding protein and neuronal α7 nicotinic acetylcholine receptor

The pentameric acetylcholine-binding protein (AChBP) is a soluble surrogate of the ligand binding domain of nicotinic acetylcholine receptors. Agonists bind within a nest of aromatic side chains contributed by loops C and F on opposing faces of each subunit interface. Crystal structures of Aplysia AChBP bound with the agonist anabaseine, two partial agonists selectively activating the α7 receptor, 3-(2,4-dimethoxybenzylidene)-anabaseine and its 4-hydroxy metabolite, and an indole-containing partial agonist, tropisetron, were solved at 2.7–1.75 Å resolution. All structures identify the Trp 147 carbonyl oxygen as the hydrogen bond acceptor for the agonist-protonated nitrogen. In the partial agonist complexes, the benzylidene and indole substituent positions, dictated by tight interactions with loop F, preclude loop C from adopting the closed conformation seen for full agonists. Fluctuation in loop C position and duality in ligand binding orientations suggest molecular bases for partial agonism at full-length receptors. This study, while pointing to loop F as a major determinant of receptor subtype selectivity, also identifies a new template region for designing α7-selective partial agonists to treat cognitive deficits in mental and neurodegenerative disorders

Functional Magnetic Resonance Imaging of Effects of a Nicotinic Agonist in Schizophrenia

3-(2,4-Dimethoxybenzylidene)-anabaseine (DMXB-A) is a partial agonist at α7-nicotinic acetylcholine receptors and is now in early clinical development for treatment of deficits in neurocognition and sensory gating in schizophrenia. During its initial phase 2 test, functional magnetic resonance imaging (fMRI) studies were conducted to determine whether the drug had its intended effect on hippocampal inhibitory interneurons. Increased hemodynamic activity in the hippocampus in schizophrenia is found during many tasks, including smooth pursuit eye movements, and may reflect inhibitory dysfunction. Placebo and two doses of drug were administered in a random, double-blind crossover design. After the morning drug/placebo ingestion, subjects underwent fMRI while performing a smooth pursuit eye movement task. Data were analyzed from 16 nonsmoking patients, including 7 women and 9 men. The 150-mg dose of DMXB-A, compared with placebo, diminished the activity of the hippocampus during pursuit eye movements, but the 75-mg dose was ineffective. The effect at the 150-mg dose was negatively correlated with plasma drug levels. The findings are consistent with the previously established function of α7-nicotinic receptors on inhibitory interneurons in the hippocampus and with genetic evidence for deficits in these receptors in schizophrenia. Imaging of drug response is useful in planning future clinical tests of this compound and other nicotinic agonists for schizophrenia

Identification of hemolytic and neuroactive fractions in the venom of the sea anemone Bunodosoma cangicum

Sea anemones are a rich source of biologically active substances. In crayfish muscle fibers, Bunodosoma cangicum whole venom selectively blocks the I K(Ca) currents. In the present study, we report for the first time powerful hemolytic and neuroactive effects present in two different fractions obtained by gel-filtration chromatography from whole venom of B. cangicum. A cytolytic fraction (Bcg-2) with components of molecular mass ranging from 8 to 18 kDa elicited hemolysis of mouse erythrocytes with an EC50 = 14 µg/ml and a maximum dose of 22 µg/ml. The effects of the neuroactive fraction, Bcg-3 (2 to 5 kDa), were studied on isolated crab nerves. This fraction prolonged the compound action potentials by increasing their duration and rise time in a dose-dependent manner. This effect was evident after the washout of the preparation, suggesting the existence of a reversible substance that was initially masking the effects of an irreversible one. In order to elucidate the target of Bcg-3 action, the fraction was applied to a tetraethylammonium-pretreated preparation. An additional increase in action potential duration was observed, suggesting a blockade of a different population of K+ channels or of tetraethylammonium-insensitive channels. Also, tetrodotoxin could not block the action potentials in a Bcg-3-pretreated preparation, suggesting a possible interaction of Bcg-3 with Na+ channels. The present data suggest that B. cangicum venom contains at least two bioactive fractions whose activity on cell membranes seems to differ from the I K(Ca) blockade described previously

Partial activation of alpha 7 nicotinic acetylcholine receptors: insights from molecular dynamics simulations

Nicotinic acetylcholine receptors (nAChRs) are drug targets for neuronal disorders and diseases. Partial agonists for nAChRs are currently being developed as drugs for the treatment of neurological diseases for their relative safety originated from reduced excessive stimulation. In the current study, molecular docking, molecular dynamics simulations and binding energy calculations were performed to theoretically investigate the interactions between the partial agonists, 4-OH-DMXBA and tropisetron with α7-nAChR. The results suggest that the partial agonists 4-OH-DMXBA and tropisetron bind with α7-nAChR in a binding mode similar to that with AChBP. The non-conserved residues in the binding sites contribute to the orientation deviation of these partial agonists from their orientation in AChBP. Energy calculation and decomposition using MM-GB/SA suggests that the van der Waals term (ΔE) is the main driving force for the binding of the partial agonists to α7-nAChR. The molecular dynamics simulations showed that the opening of the C-loop binding with the partial agonists is in-between the openings for the binding with the full agonist and in the apo state. This conformation difference for the C-loop sheds light on the partial agonism of nAChR