Sulfonylurea Receptor 1 Mutations That Cause Opposite Insulin Secretion Defects with Chemical Chaperone Exposure

Abstract

The ␤-cell ATP-sensitive potassium (K ATP ) channel composed of sulfonylurea receptor SUR1 and potassium channel Kir6.2 serves a key role in insulin secretion regulation by linking glucose metabolism to cell excitability. Mutations in SUR1 or Kir6.2 that decrease channel function are typically associated with congenital hyperinsulinism, whereas those that increase channel function are associated with neonatal diabetes. Here we report that two hyperinsulinism-associated SUR1 missense mutations, R74W and E128K, surprisingly reduce channel inhibition by intracellular ATP, a gating defect expected to yield the opposite disease phenotype neonatal diabetes. Under normal conditions, both mutant channels showed poor surface expression due to retention in the endoplasmic reticulum, accounting for the loss of channel function phenotype in the congenital hyperinsulinism patients. This trafficking defect, however, could be corrected by treating cells with the oral hypoglycemic drugs sulfonylureas, which we have shown previously to act as small molecule chemical chaperones for K ATP channels. The R74W and E128K mutants thus rescued to the cell surface paradoxically exhibited ATP sensitivity 6-and 12-fold lower than wild-type channels, respectively. Further analyses revealed a nucleotide-independent decrease in mutant channel intrinsic open probability, suggesting the mutations may reduce ATP sensitivity by causing functional uncoupling between SUR1 and Kir6.2. In insulin-secreting cells, rescue of both mutant channels to the cell surface led to hyperpolarized membrane potentials and reduced insulin secretion upon glucose stimulation. Our results show that sulfonylureas, as chemical chaperones, can dictate manifestation of the two opposite insulin secretion defects by altering the expression levels of the disease mutants.The ␤-cell ATP-sensitive potassium (K ATP ) 3 channels are essential for triggering glucose-stimulated insulin secretion as they couple metabolic signals to electrical signals (1). Each channel is a complex of four regulatory sulfonylurea receptor 1 (SUR1) subunits, encoded by ABCC8, and four pore-forming Kir6.2 inwardly rectifying potassium channel subunits, encoded by KCNJ11 (1). Mutations in ABCC8 or KCNJ11 that perturb the expression and/or gating of the channel lead to channel dysfunction and insulin secretion disorders. Whereas a net loss in channel activity results in congenital hyperinsulinism (CHI), a net gain of channel function causes permanent neonatal diabetes mellitus (PNDM) (2). SUR1 and Kir6.2 co-assemble in the endoplasmic reticulum (ER) to form channel complexes; successful assembly overcomes the arginine-lysine-arginine (RKR) ER retention motif in SUR1 and Kir6.2 to permit channel trafficking to the plasma membrane (3, 4). Quality surveillance mechanisms are in place to prevent misfolded or unassembled channel subunits from exiting the ER where the retained proteins are eventually degraded by the ubiquitin-proteasome pathway (5). In the plasma membrane, channel activities are regulated by i...

LKB1 couples glucose metabolism to insulin secretion in mice

“…The central role of the K ATP channel in beta cell function is highlighted by deficiencies in both first and second phase GSIS in islets from sulfonylurea 1 (SUR1) −/− and ATP-sensitive inward rectifier potassium channel (KIR6.2) −/− mice [10]. In humans, inappropriate inhibition or activation of K ATP channel activity arising from mutations in KIR6.2 and SUR1 underlie congenital hyperinsulinaemia or neonatal diabetes, respectively [11][12][13].…”

Impact of Disease-causing SUR1 Mutations on the KATP Channel Subunit Interface Probed with a Rhodamine Protection Assay

“…The hyperinsulinism E128K mutation disrupts channel trafficking and reduces channel function through poor surface expression (16). After rescue to the cell surface by a chemical chaperone, the E128K mutant channels are hyperactive with a lower than normal ATP sensitivity like the F132L mutants, although they exhibit an abnormally low P o (25). The F132L and E128K mutations cause therefore an abnormally high activity through different mechanisms.…”

“…Estimation from single-channel records (supplemental Fig. S4) yielded P o ϭ 0.073 Ϯ 0.02 (n ϭ 7) for SUR2A E126A , significantly lower than for wild type and equivalent to Kir6.2⌬C36 (6,7,23,25).…”

ATP activates ATP-sensitive potassium channels composed of mutant sulfonylurea receptor 1 and Kir6.2 with diminished PIP2 sensitivity

“…E128K mutation prevents efficient surface expression of the channel; we have previously shown that this defect is partially corrected by treating cells with a K ATP channel antagonist, tolbutamide, which acts as a chemical chaperone. 15 Cells expressing the E128K mutation were therefore pre-treated with 300 μM tolbutamide overnight to facilitate surface expression. Tolbutamide was then washed out for 2 hours prior to recording to remove its inhibitory effect on channel activity.…”