Sulfonylureas Correct Trafficking Defects of ATP-sensitive Potassium Channels Caused by Mutations in the Sulfonylurea Receptor

Abstract

The pancreatic ATP-sensitive potassium (K ATP ) channel, a complex of four sulfonylurea receptor 1 (SUR1) and four potassium channel Kir6.2 subunits, regulates insulin secretion by linking metabolic changes to ␤-cell membrane potential. Sulfonylureas inhibit K ATP channel activities by binding to SUR1 and are widely used to treat type II diabetes. We report here that sulfonylureas also function as chemical chaperones to rescue K ATP channel trafficking defects caused by two SUR1 mutations, A116P and V187D, identified in patients with congenital hyperinsulinism. Sulfonylureas markedly increased cell surface expression of the A116P and V187D mutants by stabilizing the mutant SUR1 proteins and promoting their maturation. By contrast, diazoxide, a potassium channel opener that also binds SUR1, had no effect on surface expression of either mutant. Importantly, both mutant channels rescued to the cell surface have normal ATP, MgADP, and diazoxide sensitivities, demonstrating that SUR1 harboring either the A116P or the V187D mutation is capable of associating with Kir6.2 to form functional K ATP channels. Thus, sulfonylureas may be used to treat congenital hyperinsulinism caused by certain K ATP channel trafficking mutations. ATP-sensitive potassium (K ATP )1 channels present in the plasma membrane of pancreatic ␤-cells play a central role in mediating glucose-induced insulin secretion (1-4). The activity of K ATP channels, which regulates ␤-cell membrane potential, is determined by the relative concentrations of intracellular ATP and ADP. When the blood glucose level rises, the increased intracellular [ATP/ADP] ratio favors K ATP channel closure, resulting in membrane depolarization, Ca 2ϩ influx, and insulin secretion. When the blood glucose level falls, the above molecular events reverse, and insulin release is stopped. In the event where K ATP channels fail to open during glucose starvation, ␤-cell membrane potential remains depolarized, and insulin secretion persists, leading to severe hypoglycemia. These symptoms are found in patients suffering from congenital hyperinsulinism (5), also known as persistent hyperinsulinemia hypoglycemia of infancy (PHHI) (6). Indeed, mutations in the K ATP channel genes, sulfonylurea receptor 1 (SUR1) and the inward rectifier potassium channel Kir6.2, that lead to a loss of channel function have been shown to be major causes of PHHI (4, 6).The pancreatic K ATP channel complex consists of four poreforming Kir6.2 subunits and four regulatory SUR1 subunits (7-10). Gating of K ATP channels occurs as a result of the interplay between both channel subunits and intracellular ATP and ADP. Binding of ATP to the Kir6.2 subunit inhibits channel activity, whereas binding of Mg 2ϩ -complexed ATP or ADP to the SUR1 subunit stimulates channel activity (11)(12)(13)(14). SUR1 is a member of the ATP-binding cassette transporter family; it has three transmembrane domains: TM0, TM1, and TM2, and two large cytoplasmic nucleotide binding domains: NBD1 and NBD2 (15,16). Structure-function studies sugges...

Structurally Distinct Ligands Rescue Biogenesis Defects of the KATP Channel Complex via a Converging Mechanism

“…Interestingly, we noted that CBZ treatment significantly enhanced the core-glycosylated A116P-and V187D-SUR1 RKR3 AAA band intensity even in the absence of Kir6.2. A likely explanation is that CBZ protects the misfolded SUR1 proteins against ER-associated degradation, which would be consistent with our previous metabolic pulse-chase study showing that GBC also slows down the degradation rate of A116P-SUR1 expressed alone without Kir6.2 (20).…”

“…The FLAG tag has been shown to not interfere with channel biogenesis or function in prior studies (20,21,25). Point mutations of SUR1 were introduced into hamster SUR1 using the QuikChange site-directed mutagenesis kit (Stratagene) and confirmed by sequencing.…”

“…Using this approach, we found that the chaperoning effect of CBZ is dependent on Kir6.2. For this set of experiments, SUR1-TMD0 trafficking mutations A116P and V187D, which we have shown previously to respond to GBC and CBZ rescue (20,22,23,41,44), were used as examples. In SUR1 RKR3 AAA bearing A116P or V187D expressed without Kir6.2, both exhibited only the core-glycosylated lower band, in contrast to WT-SUR1 RKR3 AAA , which showed both lower and upper bands; treatment with CBZ failed to correct the mutant SUR1 processing defects (Fig.…”

“…Sulfonylureas (SUs), K ATP channel antagonists commonly used to treat type 2 diabetes, were the first K ATP channel pharmacological chaperones identified (20,21). Interestingly, although trafficking mutations are found throughout SUR1, SUs only correct those in TMD0.…”

ERADication of ion channels destined for the plasma membrane. Focus on “Role of ubiquitin-proteasome degradation pathway in biogenesis efficiency of β-cell ATP-sensitive potassium channels”

“…The human disease familial hyperinsulinism, also called persistent hyperinsulinemic hypoglycemia of infancy (PHHI), is caused by loss-of-function mutations of K ATP channel subunits. Recent studies from Yan et al (23) demonstrated that the PHHI mutants SUR1 A116P and V187D did not traffic normally to the plasma membrane, but their trafficking defect could be partially overcome by sulfonylurea treatment, resulting in electrophysiologically normal channels. Sulfonylurea drugs are K ATP channel inhibitors used in the treatment of type II diabetes; however, in this case they presumably act as pharmacological chaperones that bind to SUR1 subunits to promote a conformation that favors subunit assembly and is recognized as being properly folded.…”