Abstract Aims/hypothesis The two-pore domain K + channel TWIK1-related alkalinisation-activated K + channel 2 (TALK-2) is encoded by KCNK17 , which is one of the most abundant beta cell K + channel transcripts that also shows high islet expression specificity. Polymorphisms that increase islet KNCK17 expression or result in TALK-2 gain-of-function are associated with a predisposition for developing type 2 diabetes. However, there is a gap in knowledge of the beta cell function(s) of TALK-2. As K + channels typically control beta cell Ca 2+ handling, we aimed to examine the TALK-2 channel control of beta cell Ca 2+ homeostasis and the resulting impact on insulin secretion. Methods Localisation of TALK-2 was evaluated with immunofluorescent staining as well as TALK-2-GFP construct co-expressed with intracellular markers. TALK-2 function was evaluated by measuring changes in cytoplasmic Ca 2+ (Ca 2+ C ), endoplasmic reticulum Ca 2+ (Ca 2+ ER ), ER membrane potential ( V m ), K + currents and insulin secretion in a TALK-2 inducible cell line and/or primary human beta cells with adenoviral-mediated shRNA knockdown (KD) of TALK-2 or scramble shRNA. Results TALK-2 protein localised to the plasma membrane and ER membrane, and formed functional channels on the ER membrane. Ca 2+ ER release was accelerated by TALK-2 (slope for TALK-2-expressing cells vs controls: 14.8 ± 0.7 vs 8.9 ± 1.3, respectively, shown as mean ± SE), which reduced Ca 2+ ER storage (ΔCa 2+ ER amplitude: TALK-2-expressing cells reduced by 25 ± 5%) and increased basal relative Ca 2+ C (fold increase by 12 ± 2%). Furthermore, TALK-2 diminished ER membrane hyperpolarisation following Ca 2+ ER release (Accelerated Sensor of Action Potentials ASAP3 ER amplitude decreased by 20 ± 0.8% in TALK-2-expressing cells), suggesting that TALK-2 strengthens the electrical driving force for Ca 2+ ER leak. In human beta cells, TALK-2-KD increased Ca 2+ ER stores by reducing Ca 2+ ER leak (2.30 ± 0.12 vs controls 2.65 ± 0.14). Moreover, TALK-2-KD reduced beta cell Ca 2+ C at euglycaemic conditions (2.88 ± 0.36 vs controls 3.16 ± 0.36) and increased beta cell Ca 2+ C influx in response to hyperglycaemic conditions (4.07 ± 0.55 vs controls 3.45 ± 0.48). Human pseudoislets with beta cell-specific TALK-2-KD displayed reduced basal insulin secretion (0.266 ± 0.065 vs controls 0.432 ± 0.073) and enhanced glucose-stimulated insulin secretion (GSIS; 85.01 ± 13.96 vs controls 42.53 ± 5.52). Conclusions/interpretation These data support the notion that TALK-2 functions on the human beta cell ER membrane to increase the electrical driving force for beta cell Ca 2+ ER release, reduces glucose-stimulated Ca 2+ influx and limits GSIS. Furthermore, TALK-2-mediated amplification of Ca 2+ ER leak likely enhances basal insulin secretion by increasing Ca 2+ C . Therefore, polymorphisms in KCNK17 that increase TALK-2 activity or expression would be predicted to increase type 2 diabetes risk by blunting beta cell glucose-stimulated Ca 2+ influx, limiting GSIS, promoting Ca 2+ ER leak and elevating basal insulin secretion. Graphical Abstract
Dobson et al. (Wed,) studied this question.