Production and purification of ATP-sensitive potassium channel particles for cryo-electron microscopy

HCV affects KATP channels through GnT-IVa-mediated N-glycosylation of GLUT2 on the surface of pancreatic β-cells leading to impaired insulin secretion

PURPOSE

To explore the mechanism of insulin secretion dysfunction in pancreatic beta cells induced by N-glycosylation mediated by an infection from the hepatitis C virus (HCV).

METHODS

Min6 cell models infected with HCV and stimulated with glucose were constructed. Meanwhile, an HCV-infected animal model and a type 2 diabetes mellitus (T2DM) rat model were constructed. Glucose uptake in the Min6 cells was detected, and insulin secretion was detected by ELISA. Flow cytometry, immunofluorescence staining, Western blotting, RT-qPCR, and lectin blotting were used to detect the expression levels of related proteins and mRNA, as well as the level of N-glycosylation. HE staining was used to observe the pathological changes in the pancreatic tissue, and an oral glucose tolerance test (OGTT) was used to evaluate the glucose tolerance of the rats.

RESULTS

Compared with the NC group, the expression levels of GnT-IVa, GLUT2, galectin-9, and voltage-dependent calcium channel 1.2 (Cav1.2) were significantly downregulated in the HCV-infected group. The ATP-sensitive potassium channel (KATP) component proteins SUR1 and Kir6.2 were significantly upregulated, while intracellular glucose intake and insulin secretion decreased, N-glycosylation levels and ATP levels significantly decreased, and the overexpression of GnT-IVa reversed the effect of the HCV infection. However, treatment with the glycosylation inhibitor kifunensine (KIF) or the KATP channel activator diazine (Dia) reversed the effects of the overexpression of GnT-IVa. In the animal experiments, HE staining revealed serious pathological injuries in the pancreatic tissue of the HCV-infected rats, with decreased glucose tolerance and glycosylation levels, decreased insulin secretion, downregulated expression of GnT-IVa, GLUT2, and Cav1.2, and upregulated expression of SUR1 and Kir6.2. The overexpression treatment of GnT-IVa or the KATP channel antagonist miglinide reversed the effects of HCV.

CONCLUSION

HCV infection inhibits GLUT2 N-glycosylation on the pancreatic β cell surface by downregulating the expression of GnT-IVa and then activates the KATP pathway, which ultimately leads to disturbances in insulin secretion.

Structure of an open KATP channel reveals tandem PIP2 binding sites mediating the Kir6.2 and SUR1 regulatory interface

ATP-sensitive potassium (KATP) channels, composed of four pore-lining Kir6.2 subunits and four regulatory sulfonylurea receptor 1 (SUR1) subunits, control insulin secretion in pancreatic β-cells. KATP channel opening is stimulated by PIP2 and inhibited by ATP. Mutations that increase channel opening by PIP2 reduce ATP inhibition and cause neonatal diabetes. Although considerable evidence has implicated a role for PIP2 in KATP channel function, previously solved open-channel structures have lacked bound PIP2, and mechanisms by which PIP2 regulates KATP channels remain unresolved. Here, we report the cryoEM structure of a KATP channel harboring the neonatal diabetes mutation Kir6.2-Q52R, in the open conformation, bound to amphipathic molecules consistent with natural C18:0/C20:4 long-chain PI(4,5)P2 at two adjacent binding sites between SUR1 and Kir6.2. The canonical PIP2 binding site is conserved among PIP2-gated Kir channels. The non-canonical PIP2 binding site forms at the interface of Kir6.2 and SUR1. Functional studies demonstrate both binding sites determine channel activity. Kir6.2 pore opening is associated with a twist of the Kir6.2 cytoplasmic domain and a rotation of the N-terminal transmembrane domain of SUR1, which widens the inhibitory ATP binding pocket to disfavor ATP binding. The open conformation is particularly stabilized by the Kir6.2-Q52R residue through cation-π bonding with SUR1-W51. Together, these results uncover the cooperation between SUR1 and Kir6.2 in PIP2 binding and gating, explain the antagonistic regulation of KATP channels by PIP2 and ATP, and provide a putative mechanism by which Kir6.2-Q52R stabilizes an open channel to cause neonatal diabetes.

Structure of an open K ATP channel reveals tandem PIP 2 binding sites mediating the Kir6.2 and SUR1 regulatory interface

ATP-sensitive potassium (K ATP ) channels, composed of four pore-lining Kir6.2 subunits and four regulatory sulfonylurea receptor 1 (SUR1) subunits, control insulin secretion in pancreatic β-cells. K ATP channel opening is stimulated by PIP 2 and inhibited by ATP. Mutations that increase channel opening by PIP 2 reduce ATP inhibition and cause neonatal diabetes. Although considerable evidence has indicated PIP 2 in K ATP channel function, previously solved open-channel structures have lacked bound PIP 2 , and mechanisms by which PIP 2 regulates K ATP channels remain unresolved. Here, we report the cryoEM structure of a K ATP channel harboring the neonatal diabetes mutation Kir6.2-Q52R, bound to natural C18:0/C20:4 long-chain PIP 2 in an open conformation. The structure reveals two adjacent PIP 2 molecules between SUR1 and Kir6.2. The first PIP 2 binding site is conserved among PIP 2 -gated Kir channels. The second site forms uniquely in K ATP at the interface of Kir6.2 and SUR1. Functional studies demonstrate both binding sites determine channel activity. Kir6.2 pore opening is associated with a twist of the Kir6.2 cytoplasmic domain and a rotation of the N-terminal transmembrane domain of SUR1, which widens the inhibitory ATP binding pocket to disfavor ATP binding. The open conformation is particularly stabilized by the Kir6.2-Q52R residue through cation-π bonding with SUR1-W51. Together, these results uncover the cooperation between SUR1 and Kir6.2 in PIP 2 binding and gating, explain the antagonistic regulation of K ATP channels by PIP 2 and ATP, and provide the mechanism by which Kir6.2-Q52R stabilizes an open channel to cause neonatal diabetes.

Vascular KATP channel structural dynamics reveal regulatory mechanism by Mg-nucleotides

Vascular K_ATP channels formed by the potassium channel Kir6.1 and its regulatory protein SUR2B maintain blood pressure in the physiological range. Overactivity of the channel due to genetic mutations in either Kir6.1 or SUR2B causes severe cardiovascular pathologies known as Cantú syndrome. The cryogenic electron microscopy structures of the vascular K_ATP channel reported here show multiple, dynamically related conformations of the regulatory subunit SUR2B. Molecular dynamics simulations reveal the negatively charged ED-domain in SUR2B, a stretch of 15 glutamate (E) and aspartate (D) residues not previously resolved, play a key MgADP-dependent role in mediating interactions at the interface between the SUR2B and Kir6.1 subunits. Our findings provide a mechanistic understanding of how channel activity is regulated by intracellular MgADP.

Vascular tone is dependent on smooth muscle K_ATP channels comprising pore-forming Kir6.1 and regulatory SUR2B subunits, in which mutations cause Cantú syndrome. Unique among K_ATP isoforms, they lack spontaneous activity and require Mg-nucleotides for activation. Structural mechanisms underlying these properties are unknown. Here, we determined cryogenic electron microscopy structures of vascular K_ATP channels bound to inhibitory ATP and glibenclamide, which differ informatively from similarly determined pancreatic K_ATP channel isoform (Kir6.2/SUR1). Unlike SUR1, SUR2B subunits adopt distinct rotational “propeller” and “quatrefoil” geometries surrounding their Kir6.1 core. The glutamate/aspartate-rich linker connecting the two halves of the SUR-ABC core is observed in a quatrefoil-like conformation. Molecular dynamics simulations reveal MgADP-dependent dynamic tripartite interactions between this linker, SUR2B, and Kir6.1. The structures captured implicate a progression of intermediate states between MgADP-free inactivated, and MgADP-bound activated conformations wherein the glutamate/aspartate-rich linker participates as mobile autoinhibitory domain, suggesting a conformational pathway toward K_ATP channel activation.