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This version published online on February 15, 2007
Endocrinology, doi:10.1210/en.2006-1296
A more recent version of this article appeared on May 1, 2007
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Submitted on September 20, 2006
Accepted on February 6, 2007

Targeting of KV4, CaV1.2 and SNARE Proteins to Cholesterol-Rich Lipid Rafts in Pancreatic {alpha}-cells: Effects on Glucagon Stimulus-Secretion Coupling

Fuzhen Xia, Yuk M. Leung, Gregory Gaisano, Xiaodong Gao, Yi Chen, Jocelyn E. Manning Fox, Alpana Bhattacharjee, Michael B. Wheeler, Herbert Y. Gaisano, and Robert G. Tsushima*

Departments of Medicine and Physiology, University of Toronto, Toronto, Ontario M5S 1A8 Canada

* To whom correspondence should be addressed. E-mail: r.tsushima{at}utoronto.ca.

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+ (KV) and Ca2+ (CaV) 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 KV4.1/4.3 (A- type current), KV3.2/3.3 (delayed rectifier current), CaV1.2 (L-type current), CaV2.2 (N-type current), and the SNARE (SNAP-25, syntaxin 1A, VAMP-2) and SNARE-associated proteins (Munc-13-1, Munc-18a). We also detected caveolin-2, a structural protein of cholesterol-rich lipid rafts. Of these proteins, caveolin-2, KV4.1/4.3, CaV1.2, and SNARE proteins (syntaxin 1A, SNAP-25, VAMP-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 KV4.1/4.3, CaV1.2 and SNARE proteins with lipid rafts. This resulted in inhibition of A-type KV currents and enhancement of glucagon secretion from {alpha}-cells. Consistently, capacitance measurements of exocytosis of single {alpha}-cells showed enhanced exocytosis following membrane cholesterol depletion. Taken together, our results demonstrate the association of KV4, CaV1.2 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.


Key words: Lipid Rafts • {alpha}-cells • Ion channels • SNARE proteins • Cholesterol • Diabetes




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