RPG: Iodide Channels in the Thyroid

氯离子通道是生物体中表征最广泛的一类阴离子通道。这些通道参与各种各样的任务，从高度专门化的任务，如盐和水的跨上皮移动，到一般的内务功能，如调节细胞体积。膜片钳记录技术的应用在很大程度上有助于阐明通道的行为和调节。在一些情况下，分子生物学工具已经导致了初级结构数据，从而允许确定潜在的功能特征的结构。这种通道的一个决定性性质是它们传导氯离子的能力。然而，大量的氯离子通道已经被证明比氯离子更好地传导碘。在大多数体系中，这种选择性不存在问题，因为与碘化物相比，氯化物的存在严重过剩。一个值得注意的例外是甲状腺，它将碘从血液向外输送到滤泡腔。在这里，碘选择性通道将对正常生理有很大贡献。因此，甲状腺滤泡细胞可以代表一个理想的模型系统，最终克隆一个碘离子选择性阴离子通道，从而有助于阐明决定卤化物选择性的结构特征。为了获得对确定这些长期目标的可行性具有重要意义的初步数据，本研究建议使用斑片记录技术：1)确定碘传导通道是否存在于滤泡甲状腺细胞的顶膜中；2)研究这种通道的生物物理性质，特别是卤化物的选择性。高选择性药物的问世和一级结构信息的提供促进了对阳离子通道选择性通透性的全面研究，而阴离子通道的通透性却因缺乏特定的药理探针而受到影响。此外，与阳离子通道相比，分子技术在阴离子通道的克隆和结构功能研究中的应用还处于起步阶段。然而，后者提供的信息表明，这些通道的门控不一定是由类似于阳离子通道的机制引起的。因此，通过实现这一建议的目标所获得的信息可能是对管理阴离子通道的离子选择性的新机制的第一次洞察。

Chloride channels represent the most extensively characterized class of anion channels in living organisms. These channels participate in a wide variety of tasks, ranging from highly specialized ones, such as transepithelial movement of salts and water, to general housekeeping functions, such as regulation of cell volume. The elucidation of channel behavior and regulation has been aided largely by the application of patch clamp recording techniques. In several instances, molecular biological tools have led to primary structural data, permitting the determination of the structures underlying functional characteristics. A defining property of such channels is their ability to conduct chloride ions. However, a significant number of chloride channels have been demonstrated to conduct iodide better than chloride. In most systems, this selectivity presents no problems, since chloride is present in tremendous excess compared to iodide. A notable exception is the thyroid gland, which vectorially transports iodide from the blood and into the follicular lumen. Here, an iodide selective channel would contribute substantially to normal physiology. Thus, the thyroid follicular cell could represent an ideal model system for eventually cloning an iodide selective anion channel, thereby facilitating elucidation of structural features that determine halide selectivity. In order to obtain preliminary data that will be important in determining the feasibility of such long term goals, this study proposes to use patch recording techniques: 1) to determine if an iodide conducting channel resides in the apical membrane of the follicular thyrocyte and 2) to study biophysical properties, especially the halide selectivity, of such a channel. Whereas comprehensive studies of cation channel selective permeability have been facilitated by the availability of both highly selective drugs as well as primary structural information, that of anion channels has suffered from the lack of specific pharmacological probes. M oreover, compared to that of cation channels, the application of molecular techniques in cloning and structure-function studies of anion channels is in its infancy. The latter has provided, however, information suggesting that the gating of these channels does not necessarily result from mechanisms analogous to those of cation channels. Thus, information gained from achieving the goals of this proposal may represent the first insights into novel mechanisms governing ionic selectivity of anion channels.