烟碱型乙酰胆碱受体及其神经保护作用

新近研究证实,神经元烟碱型乙酰胆碱受体(nAChR)激动后可起到一定的神经保护作用.目前,一些作用于烟碱受体的激动剂已被作为治疗神经退行性疾病如阿尔茨海默病(AD)和帕金森病(PD)的候选药物,但是关于烟碱受体激动后如何发挥神经保护作用及其潜在的分子机制还不清楚,其中有与Ca2+相关的信号转导假说以及神经营养因子等假说.本文就烟碱型乙酰胆碱受体及其神经保护作用的研究进展予以综述.     

Temperature-Sensitive Expression of Drosophila Neuronal Nicotinic Acetylcholine Receptors

Abstract: Heterologous expression of cloned Drosophila nicotinic acetylcholine receptor (nAChR) subunits indicates that these proteins misfold when expressed in mammalian cell lines at 37°C. This misfolding can, however, be overcome either by growing transfected mammalian cells at lower temperatures or by the expression of Drosophila nAChR subunits in a Drosophila cell line. Whereas the Drosophila nAChR β subunit (SBD) cDNA, reported previously, lacked part of the SBD coding sequence, here we report the construction and expression of a full-length SBD cDNA. We have examined whether problems in expressing functional Drosophila nAChRs in either Xenopus oocytes or mammalian cell lines can be attributed to an inability of these expression systems to assemble correctly Drosophila nAChRs. Despite expression in what might be considered a more native cellular environment, we have been unable to detect functional nAChRs in a Drosophila cell line unless Drosophila nAChR subunit cDNAs are coexpressed with vertebrate nAChR subunits. Our results indicate that the folding of Drosophila nAChR subunits is temperature-sensitive and strongly suggest that the inability of these Drosophila nAChR subunits to generate functional channels in the absence of vertebrate subunits is due to a requirement for coassembly with as yet unidentified Drosophila nAChR subunits.     

Nicotinic Acetylcholine Receptors have Ligand-Specific Attachment Point Patterns

Abstract

Employing a panel of synthetic peptides as representative structural elements of the nicotinic acetylcholine receptor from Torpedo electric organ, we recently identified three sequence regions of the receptor (α55–74, α134–153 and α181–200) serving as subsites for the binding of high molecular weight antagonists of acetylcholine (Conti-Tronconi et al. 1990). The relative binding affinities to these subsites of α-bungarotoxin and three competitive antibodies varied in a ligand-specific fashion. Employing a set of homologous synthetic peptides differing from α181–200 by the exchange of single amino acid residues along the sequence, we now find that ligand binding crucially depends on the presence of particular amino acids within the subsite while others influence binding only marginally if at all. The existence of ligand-specific attachment points may account for the wide range in binding and kinetic parameters, pharmacological specificity and distinct mean open times of the receptor-integral cation channel observed for cholinergic ligands.     

Anionic Lipids Allosterically Modulate Multiple Nicotinic Acetylcholine Receptor Conformational Equilibria

Anionic lipids influence the ability of the nicotinic acetylcholine receptor to gate open in response to neurotransmitter binding, but the underlying mechanisms are poorly understood. We show here that anionic lipids with relatively small headgroups, and thus the greatest ability to influence lipid packing/bilayer physical properties, are the most effective at stabilizing an agonist-activatable receptor. The differing abilities of anionic lipids to stabilize an activatable receptor stem from differing abilities to preferentially favor resting over both uncoupled and desensitized conformations. Anionic lipids thus modulate multiple acetylcholine receptor conformational equilibria. Our data suggest that both lipids and membrane physical properties act as classic allosteric modulators influencing function by interacting with and thus preferentially stabilizing different native acetylcholine receptor conformational states.     

A Novel Agonist Binding Site on Nicotinic Acetylcholine Receptors

Abstract

This report provides evidence that physostigmine (Phy) and benzoquinonium (BZQ) are able to activate nicotinic acetylcholine receptors (nAChRs) through binding site(s) distinct from those of the natural transmitter, ACh. Such findings are in agreement with a second pathway of activation of nAChRs. Receptor activation may be modulated through the novel site, and, consequently, physiological processes involving nicotinic synapses could be controlled. Using patch clamp techniques, single channel currents activated by ACh and anatoxin were recorded from frog interosseal muscle fibers under cell-attached condition and outside-out patches excised from cultured rat hippocampal neurons. Whole cell nicotinic currents were also studied in the cultured neurons. In most of the neurons, nicotinic responses were blocked by the nicotinic antagonists methyllycaconitine (MLA) and α-bungarotoxin (α-BGT). Evaluation of the effects of Phy and BZQ on the muscle and on the α-BGT- and MLA-sensitive neuronal nAChRs demonstrated that both compounds were open channel blockers at these receptors. Furthermore, at low micromolar concentrations, Phy and BZQ activated the nAChRs of all preparations tested, such an effect being unexpectedly resistant to α-BGT or MLA. Thus, the nAChRs could be activated via two distinct binding sites: one for ACh and the other for Phy and BZQ. These findings and previous biochemical results led us to suggest that a putative endogenous ligand could bind to the new site and thereby regulate the activation of nAChRs in nicotinic synapses.     

Neosurugatoxin, a Specific Antagonist of Nicotinic Acetylcholine Receptors

Neosurugatoxin (NSTX) (3 nM-30 nM), recently isolated from the Japanese ivory mollusc (Babylonia japonica) exerted a potent antinicotinic action in the isolated guinea pig ileum. Specific [3H]nicotine binding to rat forebrain membranes was saturable, reversible, and of high affinity. Nicotinic cholinergic agonists exhibited a markedly greater affinity for [3H]nicotine binding sites than a muscarinic agonist, oxotremorine. Although alpha-bungarotoxin had no effect on [3H]nicotine binding, low concentrations (1 nM-1 microM) of NSTX inhibited [3H]nicotine binding in the forebrain membranes and its IC50 value was 69 +/- 6 nM. On the other hand, NSTX did not affect muscarinic receptor binding in the brain. These data indicate that NSTX may be of appreciable interest as a neurotoxin with a selective affinity for ganglionic nicotinic receptors.     

Novel Neonicotinoid-Agarose Affinity Column for Drosophila and Musca Nicotinic Acetylcholine Receptors

Abstract: Neonicotinoids such as the insecticide imidacloprid (IMI) act as agonists at the insect nicotinic acetylcholine receptor (nAChR). Head membranes of Drosophila melanogaster and Musca domestica have a single high-affinity binding site for [³H]IMI with K _D values of 1–2 n M and B _max values of 560–850 fmol/mg of protein. Locusta and Periplaneta nAChRs isolated with an α-bungarotoxin (α-BGT)-agarose affinity column are known to be α-subunit homooligomers. This study uses 1 - [ N - (6 - chloro - 3 - pyridylmethyl) - N - ethyl]amino - 1 - amino-2-nitroethene (which inhibits [³H]IMI binding to Drosophila and Musca head membranes at 2–3 n M ) to develop a neonicotinoid-agarose affinity column. The procedure—introduction of Triton-solubilized Drosophila or Musca head membranes into this neonicotinoid-based column, elution with IMI, and analysis by lithium dodecyl sulfate-polyacrylamide gel electrophoresis—gives only three proteins (69, 66, and 61 kDa) tentatively assigned as putative subunits of the nAChR; the same three proteins are obtained with Musca using the α-BGT-agarose affinity column. Photoaffinity labeling of the Drosophila and Musca putative subunits from the neonicotinoid column with ¹²⁵I-α-BGT-4-azidosalicylic acid gives a labeled derivative of 66–69 kDa. The yield is 2–5 µg of receptor protein from 1 g of Drosophila or Musca heads. Neonicotinoid affinity chromatography to isolate native Drosophila and Musca receptors will facilitate studies on the structure and function of insect nAChRs.     

烟碱型乙酰胆碱受体及其亚单位的结构功能

烟碱型乙酰胆碱受体是配体门控的离子通道蛋白,每个受体由5个亚单位组成,每个亚单位具有4个跨膜α螺旋结构域。通道孔由5个亚单位的M2结构域(α螺旋)环绕围成,它们放射状地排列,使通道孔在膜中部逐渐变细。另外15个α螺旋相互盘绕,构成通道的外环。至今已确认了17种亚单位,它们由一个共同起源的基因家族编码,在中枢神经系统、周围神经系统和肌肉组织广泛表达。众多的亚单位,再加上组织表达的特异性导致受体类型的多样性,结果使该受体具有广泛的生理、药理特性和功能。进年来,基因敲除等现代遗传操作技术提供了在体内研究各类型受体的有效手段,使人类对该受体及亚单位的结构和功能有了长足的认识。     

Human Nicotinic Acetylcholine Receptors: Part II. Non-Neuronal Cholinergic System

Russian Journal of Bioorganic Chemistry - The role of nAChRs in functioning of central and peripheral nervous systems is widely known. Recently, expression of nAChRs in immune cells and tissues of...     

Nicotinic Acetylcholine Receptors in HIV: Possible Roles During HAND and Inflammation

Infection with the human immunodeficiency virus (HIV) remains a threat to global health. Since its discovery, many efforts have been directed at understanding the mechanisms and consequences of infection. Although there have been substantial advances since the advent of antiretroviral therapy, there are still complications that significantly compromise the health of infected patients, particularly, chronic inflammation and HIV-associated neurocognitive disorders (HAND). In this review, a new perspective is addressed in the field of HIV, where the alpha7 nicotinic acetylcholine receptor (α7-nAChR) is the protagonist. We comprehensively discuss the available evidence implicating α7-nAChRs in the context of HIV and provide possible explanations about its role in HAND and inflammation in both the central nervous system and the periphery.     

Initial Characterization of the Nicotinic Acetylcholine Receptors in Rat Hippocampal Neurons

Abstract

The properties of the neuronal nicotinic acetylcholine receptor in primary cultures of hippocampal cells from fetal rats (17–18 days gestation) were studied using the whole-cell patch-clamp technique in Na⁺-external, Cs⁺-internal and nominally Mg²⁺-free solutions. The nicotinic agonists acetylcholine, (+)anatoxin-a, and (-) and (+)nicotine all evoked inward whole-cell currents in hippocampal neurons that were voltage clamped near their resting potentials. Sensitivity to (+)anatoxin-a was first detected at around day 6, and thereafter the magnitude of the response increased as a function of number of days in culture up to about 40 days. The whole-cell current waveforms consisted of more than one peak whose relative amplitude depended on the agonist concentration. These currents were reversibly blocked by micromolar concentrations of d-tubocurarine, mecamylamine, and dihydro-β-erythroidine. At nanomolar concentrations, neuronal bungarotoxin, α-bungarotoxin and α-cobratoxin caused an irreversible blockade of the currents but they were unaffected by tetrodotoxin, atropine, DL-2-amino-5-phosphonovaleric acid, Mg²⁺, and 6,7-dinitroquinoxaline-2,3-dione. In addition, the currents were also blocked in a reversible manner by methyllycaconitine at picomolar concentration. The current-voltage plots elicited by both (+)anatoxin-a and acetylcholine revealed larger inward currents and smaller or no outward currents. The present results demonstrate the existence of an inwardly rectifying, snake neurotoxin-sensitive functional nicotinic acetylcholine receptor ion channel in rat hippocampal neurons.     

(+)-Anatoxin-a Is a Potent Agonist at Neuronal Nicotinic Acetylcholine Receptors

Abstract: The effects of the nicotinic agonist (+)-anatoxin-a have been examined in four different preparations, representing at least two classes of neuronal nicotinic receptors. (+)-Anatoxin-a was most potent (EC₅₀= 48 n M ) in stimulating ⁸⁶Rb⁺ influx into M10 cells, which express the nicotinic receptor subtype comprising α4 and β2 subunits. A presynaptic nicotinic receptor mediating acetylcholine release from hippocampal synaptosomes was similarly sensitive to (+)-anatoxin-a (EC₅₀= 140 n M ). α-Bungarotoxin-sensitive neuronal nicotinic receptors, studied using patch-clamp recording techniques, required slightly higher concentrations of this alkaloid for activation: Nicotinic currents in hippocampal neurons were activated by (+)-anatoxin-a with an EC₅₀ of 3.9 γ M , whereas α7 homooligomers reconstituted in Xenopus oocytes yielded an EC₅₀ value of 0.58 γ M for (+)-anatoxin-a. In these diverse preparations, (+)-anatoxin-a was between three and 50 times more potent than (–)-nicotine and ˜20 times more potent than acetylcholine, making it the most efficacious nicotinic agonist thus far described.     

Potency of Agonists and Competitive Antagonists on Adult- and Fetal-Type Nicotinic Acetylcholine Receptors

1. The potency of agonists and competitive antagonists on the two expressed forms of the nicotinic acetylcholine receptor (adult or junctional subtype, -AChR; fetal or extrajunctional subtype, -AChR) have not previously been compared systematically in homogeneous receptor preparations.2. Each subtype of the receptor was expressed separately in Xenopus oocytes by cytoplasmic injection of combinations of RNA transcribed in vitro. The presence of each type of receptor was confirmed by single-channel recordings. Expressing oocytes were assayed using discontinuous, single-electrode voltage clamp by measuring peak currents in response to test compounds.3. The extrajunctional subtype was more potently activated by the nicotinic agonist dimethylphenyl piperazinium iodide (DMPP) than was the junctional form. There was no statistically significant difference in potency between the two subtypes for other nicotinic agonists (nicotine, cytisine and succinylcholine). The rank order of potency for -AChR was succinylcholine>cytisine>DMPP>nicotine, and that for -AChR was DMPP>cytisine>succinylcholine>nicotine.4. Two agonists (cytisine and succinylcholine) displayed six- to eight-fold greater intrinsic activity in activating -AChR over -AChR. There was no difference between the two forms of receptor in efficacy for nicotine.5. The extrajunctional form was much more potently inhibited by the steroidal competitive antagonist pancuronium than was the junctional receptor. However, there was no significant difference in potency of inhibition by the curariform drug atracurium.6. Contrary to previous reports, there is no consistent relation between the effect of agonists and antagonists and the subtype of receptor. These data suggest that the resistance or sensitivity to these agents seen in various clinical settings are related to other cellular factors.     

Cognitive Deficits in Schizophrenia: Focus on Neuronal Nicotinic Acetylcholine Receptors and Smoking

Patients with schizophrenia present with deficits in specific areas of cognition. These are quantifiable by neuropsychological testing and can be clinically observable as negative signs. Concomitantly, they self-administer nicotine in the form of cigarette smoking. Nicotine dependence is more prevalent in this patient population when compared to other psychiatric conditions or to non-mentally ill people. The target for nicotine is the neuronal nicotinic acetylcholine receptor (nAChR). There is ample evidence that these receptors are involved in normal cognitive operations within the brain. This review describes neuronal nAChR structure and function, focusing on both cholinergic agonist-induced nAChR desensitization and nAChR up-regulation. The several mechanisms proposed for the nAChR up-regulation are examined in detail. Desensitization and up-regulation of nAChRs may be relevant to the physiopathology of schizophrenia. The participation of several subtypes of neuronal nAChRs in the cognitive processing of non-mentally ill persons and schizophrenic patients is reviewed. The role of smoking is then examined as a possible cognitive remediator in this psychiatric condition. Finally, pharmacological strategies focused on neuronal nAChRs are discussed as possible therapeutic avenues that may ameliorate the cognitive deficits of schizophrenia.     

Subunit Composition of Nicotinic Acetylcholine Receptors in the Neurons of Autonomic Ganglia

The distribution of subunits in different parts of the autonomic nervous system and the correlation between physiological functions of autonomic ganglion neurons and subunit composition of the nAChR are the subjects of their analysis. In particular, it was found that functionally different autonomic ganglia differ from each other in the subunit composition of their nAChR, and the sensitivity of the nAChR to specific antibodies is markedly variable even within the same ganglion.     

Polyarginine Peptides As a New Class of Ligands of Nicotinic Acetylcholine Receptors

Arginine-containing peptides R3, R8, and R16 were obtained by solid-phase peptide synthesis, and their binding to nicotinic acetylcholine receptors (nAChRs) of muscle and neuronal (α7) types was studied by competitive radioligand assay with the use of ¹²⁵I-α-bungarotoxin. The resulting peptides exhibited a significantly greater binding activity with respect to the muscle-type nAChRs than to the α7 receptor. Thus, we have discovered a new class of nAChR ligands. The affinity of the synthesized oligoarginines for nAChR depended on the number of amino acid residues in the chain. The highest affinity was exhibited by the R16 peptide, which contained 16 arginine residues.     

Imbalances in Mobilization and Activation of Pro-Inflammatory and Vascular Reparative Bone Marrow-Derived Cells in Diabetic Retinopathy

Diabetic retinopathy is a sight-threatening complication of diabetes, affecting 65% of patients after 10 years of the disease. Diabetic metabolic insult leads to chronic low-grade inflammation, retinal endothelial cell loss and inadequate vascular repair. This is partly due to bone marrow (BM) pathology leading to increased activity of BM-derived pro-inflammatory monocytes and impaired function of BM-derived reparative circulating angiogenic cells (CACs). We propose that diabetes has a significant long-term effect on the nature and proportion of BM-derived cells that circulate in the blood, localize to the retina and home back to their BM niche. Using a streptozotocin mouse model of diabetic retinopathy with GFP BM-transplantation, we have demonstrated that BM-derived circulating pro-inflammatory monocytes are increased in diabetes while reparative CACs are trapped in the BM and spleen, with impaired release into circulation. Diabetes also alters activation of splenocytes and BM-derived dendritic cells in response to LPS stimulation. A majority of the BM-derived GFP cells that migrate to the retina express microglial markers, while others express endothelial, pericyte and Müller cell markers. Diabetes significantly increases infiltration of BM-derived microglia in an activated state, while reducing infiltration of BM-derived endothelial progenitor cells in the retina. Further, control CACs injected into the vitreous are very efficient at migrating back to their BM niche, whereas diabetic CACs have lost this ability, indicating that the in vivo homing efficiency of diabetic CACs is dramatically decreased. Moreover, diabetes causes a significant reduction in expression of specific integrins regulating CAC migration. Collectively, these findings indicate that BM pathology in diabetes could play a role in both increased pro-inflammatory state and inadequate vascular repair contributing to diabetic retinopathy.