Department of Physiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA.
Department of Physiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA
J Gen Physiol. 2018 Jul 2;150(7):969-976. doi: 10.1085/jgp.201812017. Epub 2018 May 29.
The adenosine triphosphate (ATP)-sensitive (K) channels in pancreatic β cells couple the blood glucose level to insulin secretion. K channels in pancreatic β cells comprise the pore-forming Kir6.2 and the modulatory sulfonylurea receptor 1 (SUR1) subunits. Currently, there is no high-affinity and relatively specific inhibitor for the Kir6.2 pore. The importance of developing such inhibitors is twofold. First, in many cases, the lack of such an inhibitor precludes an unambiguous determination of the Kir6.2's role in certain physiological and pathological processes. This problem is exacerbated because Kir6.2 knockout mice do not yield the expected phenotypes of hyperinsulinemia and hypoglycemia, which in part, may reflect developmental adaptation. Second, mutations in Kir6.2 or SUR1 that increase the K current cause permanent neonatal diabetes mellitus (PNDM). Many patients who have PNDM have been successfully treated with sulphonylureas, a common class of antidiabetic drugs that bind to SUR1 and indirectly inhibit Kir6.2, thereby promoting insulin secretion. However, some PNDM-causing mutations render K channels insensitive to sulphonylureas. Conceptually, because these mutations are located intracellularly, an inhibitor blocking the Kir6.2 pore from the extracellular side might provide another approach to this problem. Here, by screening the venoms from >200 animals against human Kir6.2 coexpressed with SUR1, we discovered a small protein of 54 residues (SpTx-1) that inhibits the K channel from the extracellular side. It inhibits the channel with a dissociation constant value of 15 nM in a relatively specific manner and with an apparent one-to-one stoichiometry. SpTx-1 evidently inhibits the channel by primarily targeting Kir6.2 rather than SUR1; it inhibits not only wild-type Kir6.2 coexpressed with SUR1 but also a Kir6.2 mutant expressed without SUR1. Importantly, SpTx-1 suppresses both sulfonylurea-sensitive and -insensitive, PNDM-causing Kir6.2 mutants. Thus, it will be a valuable tool to investigate the channel's physiological and biophysical properties and to test a new strategy for treating sulfonylurea-resistant PNDM.
三磷酸腺苷（ATP）敏感性（K）通道在胰腺β细胞中将血糖水平与胰岛素分泌偶联。胰腺β细胞中的 K 通道由形成孔的 Kir6.2 和调节磺酰脲受体 1（SUR1）亚基组成。目前，还没有针对 Kir6.2 孔的高亲和力和相对特异性抑制剂。开发此类抑制剂的重要性有两个方面。首先，在许多情况下，缺乏此类抑制剂会妨碍对 Kir6.2 在某些生理和病理过程中作用的明确确定。这个问题更加严重，因为 Kir6.2 敲除小鼠不能产生高胰岛素血症和低血糖症的预期表型，这在一定程度上可能反映了发育适应。其次，Kir6.2 或 SUR1 的突变会增加 K 电流，导致永久性新生儿糖尿病（PNDM）。许多患有 PNDM 的患者已成功接受磺酰脲类药物治疗，磺酰脲类药物是一类常见的抗糖尿病药物，与 SUR1 结合并间接抑制 Kir6.2，从而促进胰岛素分泌。然而，一些导致 PNDM 的突变使 K 通道对磺酰脲类药物不敏感。从概念上讲，由于这些突变位于细胞内，从细胞外侧面阻断 Kir6.2 孔的抑制剂可能为解决这个问题提供另一种方法。在这里，我们通过筛选超过 200 种动物的毒液与 SUR1 共表达的人 Kir6.2，发现了一种 54 个残基的小蛋白（SpTx-1），它从细胞外侧面抑制 K 通道。它以 15 nM 的解离常数值以相对特异性方式抑制通道，并且具有明显的一一化学计量关系。SpTx-1 显然主要通过靶向 Kir6.2 而不是 SUR1 来抑制通道；它不仅抑制与 SUR1 共表达的野生型 Kir6.2，还抑制没有 SUR1 表达的 Kir6.2 突变体。重要的是，SpTx-1 抑制了磺酰脲敏感和不敏感的、导致 PNDM 的 Kir6.2 突变体。因此，它将成为研究通道生理和生物物理特性以及测试治疗磺酰脲类药物抵抗型 PNDM 的新策略的有价值工具。
