TALK-1 channels control β cell endoplasmic reticulum Ca ²⁺ homeostasis

Abstract

Ca2+ handling by the endoplasmic reticulum (ER) serves critical roles controlling pancreatic β-cell function and becomes perturbed during the pathogenesis of diabetes. ER Ca2+ homeostasis is determined by ion movements across the ER membrane, including K+ flux through K+ channels. Here, we demonstrated that K+ flux through ER-localized TALK-1 channels facilitated Ca2+ release from the ER in mouse and human β-cells. We found that β-cells from mice lacking TALK-1 exhibited reduced basal cytosolic Ca2+ and increased ER Ca2+ concentrations, suggesting reduced ER Ca2+ leak. These changes in Ca2+ homeostasis were presumably due to TALK-1-mediated ER K+ flux, because we recorded K+ currents mediated by functional TALK-1 channels on the nuclear membrane, which is continuous with the ER. Moreover, overexpression of K+-impermeable TALK-1 channels in HEK293 cells did not reduce ER Ca2+ stores. Reduced ER Ca2+ content in β-cells is associated with ER stress and islet dysfunction in diabetes, and islets from TALK-1-deficient mice fed a high-fat diet showed reduced signs of ER stress, suggesting that TALK-1 activity exacerbated ER stress. Our data establish TALK-1 channels as key regulators of β-cell ER Ca2+, and suggest that TALK-1 may be a therapeutic target to reduce ER Ca2+ handling defects in β-cells during the pathogenesis of diabetes.

A novel mutation inKCNK16causing a gain-of-function in the TALK-1 potassium channel: a new cause of maturity onset diabetes of the young

“…TALK-1 is expressed on both the β-cell plasma membrane and the ER membrane 16 . Ca 2+ ER release is balanced by negative charge on the luminal ER membrane; this charge is dissipated by ER TALK-1 K + influx leading to enhanced Ca 2+ ER release 16 .…”

“…TALK-1 is expressed on both the β-cell plasma membrane and the ER membrane 16 . Ca 2+ ER release is balanced by negative charge on the luminal ER membrane; this charge is dissipated by ER TALK-1 K + influx leading to enhanced Ca 2+ ER release 16 . Thus overactive TALK-1 channels (e.g., TALK-1 Ala277Glu) increase Ca 2+ ER release, whereas TALK-1 ablation reduces Ca 2+ ER release 16 .…”

“…Inhibition of SERCAs with CPA resulted in significantly less Ca 2+ ER release in β-cells expressing TALK-1 Leu114Pro compared to TALK-1 WT (62.6% decrease; Fig. 3E, 3F), suggesting reduced Ca 2+ ER storage with TALK-1 Leu114Pro 16 . β-cells expressing TALK-1 Leu114Pro also showed elevated basal [Ca 2+ cyto] compared to β-cells expressing TALK-1 WT (28.8% increase in AUC; Fig.…”

“…Ca 2+ imaging was performed as previously described 9 , switching from 2mM glucose to 20mM glucose. For endoplasmic reticulum (ER) Ca 2+ [Ca 2+ ER]: Islets were perfused in Krebs-Ringer-HEPES buffer without extracellular Ca 2+ and 100 M diazoxide and monitored for Ca 2+ ER release mediated through blockade of the sarco(endo)plasmic reticulum Ca 2+ -ATPase (SERCA) with 50 M cyclopiazonic acid (CPA, Alomone labs), as previously described 16 .…”

“…3A, 3C, and 3D). TALK-1 has been previously shown to modulate Ca 2+ ER homeostasis by providing a countercurrent for Ca 2+ ER release 16 ; thus TALK-1 Leu114Pro control of Ca 2+ ER storage was also examined. Inhibition of SERCAs with CPA resulted in significantly less Ca 2+ ER release in β-cells expressing TALK-1 Leu114Pro compared to TALK-1 WT (62.6% decrease; Fig.…”