CONOTOXINS AND HOMERIC NICOTINIC ACETYLCHOLINE RECEPTORS

烟碱型乙酰胆碱受体(NAChRs)是五聚体配体门控离子通道复合体；在哺乳动物中，大约有16个不同的基因编码nAChR亚单位。NAChR基因家族的一个独特分支编码形成同源nAChRs的亚基；哺乳动物的alpha7 nAChR亚基就是一个例子。大多数其他哺乳动物的nAChR亚基只有在存在不止一种类型的亚基时才形成功能受体。我们最近发现了一类以同源nAChR亚单位为靶标的Conus多肽，即α4/3芋螺毒素亚家族。 该项目的总体目标是研究甲型4/3芋螺毒素与它们的同源nAChR靶标之间的相互作用；提出了三项一般性倡议。第一个涉及两种密切相关的α-4/3螺毒素，即α-螺毒素IML和IM11。这两者在功能上都抑制α7烟碱型乙酰胆碱受体，但显然是在不同的位置。与标准的竞争性拮抗剂(如α-银环蛇毒素)不同，α-芋螺毒素IM11似乎通过一个独特的部位起作用。一个目标是提供这个新结合位点的分子定义。第二组实验检查了以软体动物扫描乙酰胆碱结合蛋白为靶点的α4/3芋螺毒素，这是nAChR配体结合域的模型。最近在确定AChBP结构方面的突破为任何配体门控离子通道提供了第一幅配体结合位置的详细图片；一个长期目标是确定AChBP是否可以与结合的α4/3结晶 芋螺毒素。另一个目标是确定软体动物系统中各种甲型4/3芋螺毒素的靶标。最后一组实验目标是检查甲型4/3芋螺毒素对秀丽线虫等模式生物的影响。初步工作表明，α-芋螺毒素IML可以阻断这种生物体中的nAChR。在许多无脊椎动物系统中，同源亚基组成的nAChR亚基比异构亚单位所占的比例要大得多；因此，对α4/3芋螺毒素亚家族的研究可能为这些生物体中不同的尼古丁受体光谱提供有效的神经药理学基础。

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channel complexes; in mammals, ca. 16 different genes encode nAChR subunits. A distinct branch of the nAChR gene family encodes subunits that form homomeric nAChRs; an example is the mammalian alpha7 nAChR subunit. Most other mammalian nAChR subunits form functional receptors only when more than one type of subunit is present. We recently discovered a class of Conus peptides, the alpha4/3 conotoxin subfamily, that appears to target homomeric nAChR subunits. The broad goal of the project is to investigate interactions between the alpha4/3 conotoxins and their homomeric nAChR targets; there are three general initiatives proposed. The first concerns two closely-related alpha4/3 conotoxins, alpha-conotoxins Iml and Imll. Both of these functionally inhibit the alpha7 nicotinic acetylcholine receptor, but apparently at different sites. Alpha-conotoxin Imll appears to act through a unique site, distinct from that of the standard competitive antagonists (such as alpha-bungarotoxin). One goal is to provide a molecular definition of this novel binding site. A second set of experiments examines the alpha4/3 conotoxins that target molluscan acetylcholine binding proteins, which are models for nAChR ligand binding domains. The recent breakthrough in determining the structure of AChBPs provided the first detailed picture of a ligand binding site for any ligand-gated ion channel; a long-term goal is to determine whether the AChBP can be crystallized with a bound alpha4/3 conotoxin. Another goal is to define the targets of various alpha4/3 conotoxins in molluscan systems. A final set of experimental objectives is to examine the effects of alpha4/3 conotoxins in model organisms such as C. elegans. Preliminary work has shown that alpha-conotoxin Iml blocks an nAChR in this organism. In many invertebrate systems, the homomeric subunits comprise a much greater fraction of nAChR subunits than the heteromeric subunits; the proposed study of the alpha4/3 conotoxin subfamily may therefore provide the basis for an effective neuropharmacology for the spectrum of lifferent nicotinic receptors in these organisms.