ATP-sensitive potassium (KAip) channels play a key role in coupling cell metabolism to cell excitability and govern diverse physiological processes including hormone secretion, control of vascular tone, and modulation of the activity of cardiac muscle and neurons during ischemia. The long-term goal of this project is to understand the structural basis of KATP channel gating. Towards this goal, our research has focused on the pancreatic subtype of KATP channels, which are heteromultimeric complexes each composed of four inwardly rectifying potassium channel Kir6.2 subunits and four regulatory sulfonylurea receptor 1 subunits. In pancreatic p-cells, KATP channels serve as glucose sensors to regulate insulin secretion. Mutations in either Kir6.2 or SUR1 that lead to loss of channel function are the major cause of congenital hyperinsulinism, a disease characterized by persistent insulin secretion despite low plasma glucose level. On the other hand, mutations in Kir6.2 that lead to gain of channel activity have recently been shown to cause neonatal diabetes. Several physiological molecules, including intracellular ATP, MgADP, and membrane phosphoinositides, especially PI-4,5-P2 (PIP2), regulate the activity of KATP channels. However, structural features of the channel proteins that are critical for control of channel activity by these molecules are not clearly understood. The goal of this application is to gain insight to the structure-function relationship of KATP channels using a forward genetics approach by studying how mutations identified in disease affect channel function. In the first aim, we will determine channel defects caused by nine novel Kir6.2 mutations identified in congenital hyperinsulinism using COS cells, addressing both defects in channel biogenesis/expression and gating. We will then evaluate how these mutations impact on p-cell physiology and how they respond to potential molecular or pharmacological treatments, by expressing mutant Kir6.2 in a rat pancreatic p-cell line INS-1. In the second aim, we will perform similar studies on Kir6.2 mutations recently identified in neonatal diabetes. In the third aim, we will identify intersubunit interactions in the cytoplasmic domain of Kir6.2 that are important for gating and for physical association between Kir6.2 subunits, based on our previous finding that disruption of an intersubunit ion pair in Kir6.2 impairs normal channel gating. We will focus on potential interactions that are mediated by residues that have been found mutated in congenital hyperinsulinism or neonatal diabetes. The proposed study will better our understanding of not only the structure-function relationships of KATP channels but also the molecular basis of insulin secretion diseases caused by channel mutations. Such knowledge may help identify novel structural sites for drug development and is essential for designing effective therapeutic strategies for these diseases.
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