Targeting of voltage-gated K+ and Ca2+ channels and soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins to cholesterol-rich lipid rafts in pancreatic alpha-cells: effects on glucagon stimulus-secretion coupling

Abstract

Pancreatic alpha-cells secrete glucagon in response to low glucose to counter insulin actions, thereby maintaining glucose homeostasis. The molecular basis of alpha-cell stimulus-secretion coupling has not been fully elucidated. We investigated the expression of voltage-gated K(+) (K(V)) and Ca(2+) (Ca(V)) channels, and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins in pancreatic alpha-cells and examined their targeting to specialized cholesterol-rich lipid rafts. In alpha-cells, we detected the expression of K(V)4.1/4.3 (A-type current), K(V)3.2/3.3 (delayed rectifier current), Ca(V)1.2 (L-type current), Ca(V)2.2 (N-type current), and the SNARE (synaptosomal-associated protein of 25 kDa, syntaxin 1A, and vesicle-associated membrane protein 2) and SNARE-associated proteins (Munc-13-1 and Munc-18a). We also detected caveolin-2, a structural protein of cholesterol-rich lipid rafts. Of these proteins, caveolin-2, K(V)4.1/4.3, Ca(V)1.2, and SNARE proteins (syntaxin 1A, synaptosomal-associated protein of 25 kDa, and vesicle-associated membrane protein 2) target to lipid raft domains on alpha-cell plasma membranes. Disruption of lipid rafts by depletion of membrane cholesterol with methyl-beta-cyclodextrin decreased the association of K(V)4.1/4.3, Ca(V)1.2, and SNARE proteins with lipid rafts. This resulted in inhibition of A-type K(V) currents and enhancement of glucagon secretion from alpha-cells. Consistently, capacitance measurements of exocytosis of single alpha-cells showed enhanced exocytosis after membrane cholesterol depletion. Taken together, our results demonstrate the association of K(V)4, Ca(V)1.2, and SNARE proteins with lipid rafts in pancreatic alpha-cells. Glucagon secretion from alpha-cells is regulated by lipid rafts, and the dissociation of SNARE proteins from cholesterol-rich lipid raft domains enhances glucagon secretion.