How do Mutations in M0 of KCNJ11 Produce Diabetes?

Abstract

Many diabetogenic mutations (Endocrine Rev 2009;29:265) map to the canonical, N-terminal, submembrane (BBRC 1999;255:231) “slide” M0 helix of KCNJ11 (KIR6.2). To clarify the principal biophysical mechanism of their action, inhibiting insulin release, I analyzed effects of the first reported severe Neonatal Diabetes (ND with epilepsy and developmental delay) mutation and 18 other mutations, V59X, in M0 on macroscopic and unitary currents through ABCC8(SUR1)-containing ATP-sensitive potassium (KATP) channels, reconstituted in mammalian cells lacking endogenous SUR or KIR. Several V59X decreased, and no V59X increased or abolished, functional expression (N) of the neuroendocrine-type adenine nucleotide sensors. This a) indicated that the effect of any possible ND V59X-induced decrease in N on Vm in humans is overruled by the mean open channel probability(PO)-increasing effect, explaining insufficient insulin release due to hyperpolarization of insulin producing cells, and b) allowed complete analysis of relationships between the physical properties of the side chain in the middle of M0 and PO, its sensitivity to nucleotides, and single-channel gating kinetics. The established relationships are consistent with the results of molecular modeling and molecular dynamics simulation of severe ND KATP pores and strongly suggest that a ligand-independent stabilization of the active (burst) state with conformations without specific, micromolar affinity for inhibitory ATP, is the principal mechanism of pathogenic hyperactivity of KATP with mutations in M0, the small domain proposed to play a big role in gating of KIR6 and their relatives.I thank the National Institutes of Health for funding and G. Zhao for technical assistance.