Analysis of Two KCNJ11 Neonatal Diabetes Mutations, V59G and V59A, and the Analogous KCNJ8 I60G Substitution

Abstract

␤-Cell-type K ATP channels are octamers assembled from Kir6.2/KCNJ11 and SUR1/ABCC8. Adenine nucleotides play a major role in their regulation. Nucleotide binding to Kir6.2 inhibits channel activity, whereas ATP binding/hydrolysis on sulfonylurea receptor 1 (SUR1) opposes inhibition. Segments of the Kir6.2 N terminus are important for open-to-closed transitions, form part of the Kir ATP, sulfonylurea, and phosphoinositide binding sites, and interact with L0, an SUR cytoplasmic loop. Inputs from these elements link to the pore via the interfacial helix, which forms an elbow with the outer pore helix. Mutations that destabilize the interfacial helix increase channel activity, reduce sensitivity to inhibitory ATP and channel inhibitors, glibenclamide and repaglinide, and cause neonatal diabetes. We compared Kir6.x/SUR1 channels carrying the V59G substitution, a cause of the developmental delay, epilepsy, and neonatal diabetes syndrome, with a V59A substitution and the equivalent I60G mutation in the related Kir6.1 subunit from vascular smooth muscle. The substituted channels have increased P O values, decreased sensitivity to inhibitors, and impaired stimulation by phosphoinositides but retain sensitivity to Ba 2؉ -block. The V59G and V59A channels are either not, or poorly, stimulated by phosphoinositides, respectively. Inhibition by sequestrating phosphatidylinositol 4,5-bisphosphate with neomycin and polylysine is reduced in V59A, and abolished in V59G channels. Stimulation by SUR1 is intact, and increasing the concentration of inhibitory ATP restores the sensitivity of Val-59-substituted channels to glibenclamide. The I60G channels, strongly dependent on SUR stimulation, remain sensitive to sulfonylureas. The results suggest the interfacial helix dynamically links inhibitory inputs from the Kir N terminus to the gate and that sulfonylureas stabilize an inhibitory configuration.ATP-sensitive K ϩ channels (K ATP channels) 2 consist of four pore-forming subunits Kir6.x and four sulfonylurea receptor (SURx) subunits, which are members of the ATP-binding cassette (ABC) protein superfamily (1). Adenine nucleotides have a balanced action on K ATP channels: Mg 2ϩ -independent nucleotide binding to Kir6.x closes the channel (2-4), whereas MgADP binding to, or MgATP hydrolysis by, SURx stimulates channel openings (5-9). These results obtained with recombinant systems give an explanation for the older observation that K ATP channels are sensitive to the ratio of ADP to ATP (10, 11). There are several subtypes of K ATP channels (12) that subserve important functions in many tissues (13). The Kir6.2/SUR1 neuroendocrine channel in pancreatic ␤-cells couples insulin secretion to the plasma glucose level. K ATP channels determine the ␤-cell membrane potential and changes in the levels of ATP and ADP induced by changes in glucose metabolism determine channel activity. Antidiabetic sulfonyl-ureas such as glibenclamide (GBC) bind to SUR1 reducing its stimulatory action on the pore thus inducing channel closure that prompts insuli...

Molecular and clinical features of KATP -channel neonatal diabetes mellitus in Japan

“…Among the patients with mutations that could not be transferred to sulfonylurea, the L164P mutation of KCNJ11 has been previously reported as a sulfonylurea resistant mutation . On the contrary, V59A in patient 15 (Table ) has been reported as a mutation that could be transferred to sulfonylurea . Our patient, however, could not be switched to glibenclamide, even at the dosage of 2 mg/kg/d.…”

The Sulfonylurea Receptor 1 (Sur1)-Transient Receptor Potential Melastatin 4 (Trpm4) Channel

“… a Fusion to the N terminus of Sur1 or Trpm4 is indicated by listing the adduct first; fusion to the C terminus of Sur1 or Trpm4 is indicated by listing the adduct second. b Gift of Dr. Joseph Bryan, Pacific Northwest Diabetes Research Institute, Seattle, WA (11, 12). …”

Decomposition of Slide Helix Contributions to ATP-dependent Inhibition of Kir6.2 Channels

“…The observation of a transverse interfacial helix (the "slide helix") in Kir channel structures has provoked considerable speculation regarding its role as a mediator of coupling between the cytoplasmic ("ligand-sensing") and transmembrane ("gating") domains (22,(25)(26)(27), with obvious analogies drawn to the S4-S5 linker as a coupling element for voltage sensor movement and channel gating in voltage-gated channels (28 -30). We have taken a systematic structure-based approach to investigate the role of the slide helix and identify components that are essential for functional channel expression and ATP inhibition of Kir6.2 channels.…”