Disruption of Pancreatic β-Cell Lipid Rafts Modifies Kv2.1 Channel Gating and Insulin Exocytosis

Abstract

In pancreatic β-cells, the predominant voltage-gated Ca2+ channel (CaV1.2) and K+ channel (KV2.1) are directly coupled to SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor) proteins. These SNARE proteins modulate channel expression and gating and closely associate these channels with the insulin secretory vesicles. We show that KV2.1 and CaV1.2, but not KV1.4, SUR1, or Kir6.2, target to specialized cholesterol-rich lipid raft domains on β-cell plasma membranes. Similarly, the SNARE proteins syntaxin 1A, SNAP-25, and VAMP-2, but not Munc-13-1 or n-Sec1, are associated with lipid rafts. Disruption of the lipid rafts by depleting membrane cholesterol with methyl-β-cyclodextrin shunts KV2.1, CaV1.2, and SNARE proteins out of lipid rafts. Furthermore, methyl-β-cyclodextrin inhibits KV2.1 but not CaV1.2 channel activity and enhances single-cell exocytic events and insulin secretion. Membrane compartmentalization of ion channels and SNARE proteins in lipid rafts may be critical for the temporal and spatial coordination of insulin release, forming what has been described as the excitosome complex. In pancreatic β-cells, the predominant voltage-gated Ca2+ channel (CaV1.2) and K+ channel (KV2.1) are directly coupled to SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor) proteins. These SNARE proteins modulate channel expression and gating and closely associate these channels with the insulin secretory vesicles. We show that KV2.1 and CaV1.2, but not KV1.4, SUR1, or Kir6.2, target to specialized cholesterol-rich lipid raft domains on β-cell plasma membranes. Similarly, the SNARE proteins syntaxin 1A, SNAP-25, and VAMP-2, but not Munc-13-1 or n-Sec1, are associated with lipid rafts. Disruption of the lipid rafts by depleting membrane cholesterol with methyl-β-cyclodextrin shunts KV2.1, CaV1.2, and SNARE proteins out of lipid rafts. Furthermore, methyl-β-cyclodextrin inhibits KV2.1 but not CaV1.2 channel activity and enhances single-cell exocytic events and insulin secretion. Membrane compartmentalization of ion channels and SNARE proteins in lipid rafts may be critical for the temporal and spatial coordination of insulin release, forming what has been described as the excitosome complex.