1741-P: The Type 2 Diabetes–Associated K+ Channel TALK-2 Is Localized to the Endoplasmic Reticulum and Modulates Beta Cell Ca2+ Handling and Insulin Secretion

Introduction and Objective: KCNK17, which encodes TALK-2, is one of the most abundant β-cell K⁺ channel transcripts. Polymorphisms in KCNK17 are associated with an increased predisposition for developing T2D. Therefore, our objective was to determine the β-cell functions of TALK-2.

Methods: Localization of TALK-2 was investigated with immunofluorescent staining and TALK-2-GFP constructs. TALK-2 function was tested with Ca²⁺ imaging of inducible TALK-2 cells, adenoviral KCNK17 shRNA mediated knockdown (KD) of dispersed human islets, and β-cell TALK-2 KD pseudoislets. Insulin secretion was performed with pseudoislets.

Results: TALK-2 protein displayed ER localization in β-cells within human pancreatic sections. Additionally, TALK-2-GFP colocalized with an ER-RFP marker and a plasmalemma dye in HEK293 cells. Overexpression of TALK-2 in HEK293 cells accelerated ER Ca²⁺ release (by 62±13%), reduced ER Ca²⁺ storage (by 67±12%) and increased basal cytosolic Ca²⁺ (by 11±0.5%). In human β-cells TALK-2 KD increased ER Ca²⁺ storage (by 16±5%). Moreover, TALK-2 KD increased Ca²⁺ influx at 11mM glucose in human β-cells that were dispersed (by 20±6%) and in pseudoislet preps (by 208±64%). Finally, β-cell TALK-2 KD pseudoislets showed reduced insulin secretion at 1mM (by 36±10%).

Conclusion: These data support that ER and plasmalemma localized TALK-2 channels function to reduce β-cell ER Ca²⁺ stores and limit glucose-induced Ca²⁺ influx respectively. Furthermore, TALK-2-mediated augmentation of ER Ca²⁺ leak likely enhanced basal insulin secretion by increasing cytosolic Ca²⁺. Therefore, polymorphisms in KCNK17 that increase TALK-2 activity or expression would be predicted to diminish β-cell ER Ca²⁺ stores, impair glucose-stimulated Ca²⁺ influx, elevate basal insulin secretion, and thus enhance insulin resistance.