Pharmacological Correction of Trafficking Defects in ATP-sensitive Potassium Channels Caused by Sulfonylurea Receptor 1 Mutations.

ATP-sensitive potassium (KATP) channels play a key role in mediating glucose-stimulated insulin secretion by coupling metabolic signals to ß-cell membrane potential. Loss of KATP channel function due to mutations in ABCC8 or KCNJ11, genes encoding the sulfonylurea receptor 1 (SUR1) or the inwardly rectifying potassium channel Kir6.2, respectively, results in congenital hyperinsulinism. Many SUR1 mutations prevent trafficking of channel proteins from the endoplasmic reticulum to the cell surface. Channel inhibitors, including sulfonylureas and carbamazepine, have been shown to correct channel trafficking defects. In the present study, we identified 13 novel SUR1 mutations that cause channel trafficking defects, the majority of which are amenable to pharmacological rescue by glibenclamide and carbamazepine. By contrast, none of the mutant channels were rescued by KATP channel openers. Cross-linking experiments showed that KATP channel inhibitors promoted interactions between the N terminus of Kir6.2 and SUR1, whereas channel openers did not, suggesting the inhibitors enhance intersubunit interactions to overcome channel biogenesis and trafficking defects. Functional studies of rescued mutant channels indicate that most mutants rescued to the cell surface exhibited WT-like sensitivity to ATP, MgADP, and diazoxide. In intact cells, recovery of channel function upon trafficking rescue by reversible sulfonylureas or carbamazepine was facilitated by the KATP channel opener diazoxide. Our study expands the list of KATP channel trafficking mutations whose function can be recovered by pharmacological ligands and provides further insight into the structural mechanism by which channel inhibitors correct channel biogenesis and trafficking defects.

Structurally distinct ligands rescue biogenesis defects of the KATP channel complex via a converging mechanism.

Small molecules that correct protein misfolding and misprocessing defects offer a potential therapy for numerous human diseases. However, mechanisms underlying pharmacological correction of such defects, especially in heteromeric complexes with structurally diverse constituent proteins, are not well understood. Here we investigate how two chemically distinct compounds, glibenclamide and carbamazepine, correct biogenesis defects in ATP-sensitive potassium (KATP) channels composed of sulfonylurea receptor 1 (SUR1) and Kir6.2. We present evidence that despite structural differences, carbamazepine and glibenclamide compete for binding to KATP channels, and both drugs share a binding pocket in SUR1 to exert their effects. Moreover, both compounds engage Kir6.2, in particular the distal N terminus of Kir6.2, which is involved in normal channel biogenesis, for their chaperoning effects on SUR1 mutants. Conversely, both drugs can correct channel biogenesis defects caused by Kir6.2 mutations in a SUR1-dependent manner. Using an unnatural, photocross-linkable amino acid, azidophenylalanine, genetically encoded in Kir6.2, we demonstrate in living cells that both drugs promote interactions between the distal N terminus of Kir6.2 and SUR1. These findings reveal a converging pharmacological chaperoning mechanism wherein glibenclamide and carbamazepine stabilize the heteromeric subunit interface critical for channel biogenesis to overcome defective biogenesis caused by mutations in individual subunits.

KATP channels in focus: Progress toward a structural understanding of ligand regulation.

KATP channels are hetero-octameric complexes of four inward rectifying potassium channels, Kir6.1 or Kir6.2, and four sulfonylurea receptors, SUR1, SUR2A, or SUR2B from the ABC transporter family. This unique combination enables KATP channels to couple intracellular ATP/ADP ratios, through gating, with membrane excitability, thus regulating a broad range of cellular activities. The prominence of KATP channels in human physiology, disease, and pharmacology has long attracted research interest. Since 2017, a steady flow of high-resolution KATP cryoEM structures has revealed complex and dynamic interactions between channel subunits and their ligands. Here, we highlight insights from recent structures that begin to provide mechanistic explanations for decades of experimental data and discuss the remaining knowledge gaps in our understanding of KATP channel regulation.

Structural basis of epitope selectivity and potent protection from malaria by PfCSP antibody L9.

A primary objective in malaria vaccine design is the generation of high-quality antibody responses against the circumsporozoite protein of the malaria parasite, Plasmodium falciparum (PfCSP). To enable rational antigen design, we solved a cryo-EM structure of the highly potent anti-PfCSP antibody L9 in complex with recombinant PfCSP. We found that L9 Fab binds multivalently to the minor (NPNV) repeat domain, which is stabilized by a unique set of affinity-matured homotypic, antibody-antibody contacts. Molecular dynamics simulations revealed a critical role of the L9 light chain in integrity of the homotypic interface, which likely impacts PfCSP affinity and protective efficacy. These findings reveal the molecular mechanism of the unique NPNV selectivity of L9 and emphasize the importance of anti-homotypic affinity maturation in protective immunity against P. falciparum.

Affinity-matured homotypic interactions induce spectrum of PfCSP structures that influence protection from malaria infection.

The generation of high-quality antibody responses to Plasmodium falciparum (Pf) circumsporozoite protein (PfCSP), the primary surface antigen of Pf sporozoites, is paramount to the development of an effective malaria vaccine. Here we present an in-depth structural and functional analysis of a panel of potent antibodies encoded by the immunoglobulin heavy chain variable (IGHV) gene IGHV3-33, which is among the most prevalent and potent antibody families induced in the anti-PfCSP immune response and targets the Asn-Ala-Asn-Pro (NANP) repeat region. Cryo-electron microscopy (cryo-EM) reveals a remarkable spectrum of helical antibody-PfCSP structures stabilized by homotypic interactions between tightly packed fragments antigen binding (Fabs), many of which correlate with somatic hypermutation. We demonstrate a key role of these mutated homotypic contacts for high avidity binding to PfCSP and in protection from Pf malaria infection. Together, these data emphasize the importance of anti-homotypic affinity maturation in the frequent selection of IGHV3-33 antibodies and highlight key features underlying the potent protection of this antibody family.

Profound structural conservation of chemically cross-linked HIV-1 envelope glycoprotein experimental vaccine antigens.

Chemical cross-linking is used to stabilize protein structures with additional benefits of pathogen and toxin inactivation for vaccine use, but its use has been restricted by the potential for local or global structural distortion. This is of particular importance when the protein in question requires a high degree of structural conservation for inducing a biological outcome such as the elicitation of antibodies to conformationally sensitive epitopes. The HIV-1 envelope glycoprotein (Env) trimer is metastable and shifts between different conformational states, complicating its use as a vaccine antigen. Here we have used the hetero-bifunctional zero-length reagent 1-Ethyl-3-(3-Dimethylaminopropyl)-Carbodiimide (EDC) to cross-link two soluble Env trimers, selected well-folded trimer species using antibody affinity, and transferred this process to good manufacturing practice (GMP) for experimental medicine use. Cross-linking enhanced trimer stability to biophysical and enzyme attack. Cryo-EM analysis revealed that cross-linking retained the overall structure with root-mean-square deviations (RMSDs) between unmodified and cross-linked Env trimers of 0.4-0.5 Å. Despite this negligible distortion of global trimer structure, we identified individual inter-subunit, intra-subunit, and intra-protomer cross-links. Antigenicity and immunogenicity of the trimers were selectively modified by cross-linking, with cross-linked ConS retaining bnAb binding more consistently than ConM. Thus, the EDC cross-linking process improves trimer stability whilst maintaining protein folding, and is readily transferred to GMP, consistent with the more general use of this approach in protein-based vaccine design.

Pharmacological chaperones of ATP-sensitive potassium channels: Mechanistic insight from cryoEM structures.

ATP-sensitive potassium (KATP) channels are uniquely evolved protein complexes that couple cell energy levels to cell excitability. They govern a wide range of physiological processes including hormone secretion, neuronal transmission, vascular dilation, and cardiac and neuronal preconditioning against ischemic injuries. In pancreatic ß-cells, KATP channels composed of Kir6.2 and SUR1, encoded by KCNJ11 and ABCC8, respectively, play a key role in coupling blood glucose concentration to insulin secretion. Mutations in ABCC8 or KCNJ11 that diminish channel function result in congenital hyperinsulinism. Many of these mutations principally hamper channel biogenesis and hence trafficking to the cell surface. Several small molecules have been shown to correct channel biogenesis and trafficking defects. Here, we review studies aimed at understanding how mutations impair channel biogenesis and trafficking and how pharmacological ligands overcome channel trafficking defects, particularly highlighting recent cryo-EM structural studies which have shed light on the mechanisms of channel assembly and pharmacological chaperones.

Diverse Antibody Responses to Conserved Structural Motifs in Plasmodium falciparum Circumsporozoite Protein.

Malaria vaccine candidate RTS,S/AS01 is based on the central and C-terminal regions of the circumsporozoite protein (CSP) of P. falciparum. mAb397 was isolated from a volunteer in an RTS,S/AS01 clinical trial, and it protects mice from infection by malaria sporozoites. However, mAb397 originates from the less commonly used VH3-15 germline gene compared to the VH3-30/33 antibodies generally elicited by RTS,S to the central NANP repeat region of CSP. The crystal structure of mAb397 with an NPNA4 peptide shows that the central NPNA forms a type I ß-turn and is the main recognition motif. In most anti-NANP antibodies studied to date, a germline-encoded Trp is used to engage the Pro in NPNA ß-turns, but here the Trp interacts with the first Asn. This "conserved" Trp, however, can arise from different germline genes and be located in the heavy or the light chain. Variation in the terminal ψ angles of the NPNA ß-turns results in different dispositions of the subsequent NPNA and, hence, different stoichiometries and modes of antibody binding to rsCSP. Diverse protective antibodies against NANP repeats are therefore not limited to a single germline gene response or mode of binding.

Mechanism of pharmacochaperoning in a mammalian KATP channel revealed by cryo-EM.

ATP-sensitive potassium (KATP) channels composed of a pore-forming Kir6.2 potassium channel and a regulatory ABC transporter sulfonylurea receptor 1 (SUR1) regulate insulin secretion in pancreatic ß-cells to maintain glucose homeostasis. Mutations that impair channel folding or assembly prevent cell surface expression and cause congenital hyperinsulinism. Structurally diverse KATP inhibitors are known to act as pharmacochaperones to correct mutant channel expression, but the mechanism is unknown. Here, we compare cryoEM structures of a mammalian KATP channel bound to pharmacochaperones glibenclamide, repaglinide, and carbamazepine. We found all three drugs bind within a common pocket in SUR1. Further, we found the N-terminus of Kir6.2 inserted within the central cavity of the SUR1 ABC core, adjacent the drug binding pocket. The findings reveal a common mechanism by which diverse compounds stabilize the Kir6.2 N-terminus within SUR1's ABC core, allowing it to act as a firm 'handle' for the assembly of metastable mutant SUR1-Kir6.2 complexes.

Molecular signatures of retinal ganglion cells revealed through single cell profiling.

Retinal ganglion cells can be classified into more than 40 distinct subtypes, whether by functional classification or transcriptomics. The examination of these subtypes in relation to their physiology, projection patterns, and circuitry would be greatly facilitated through the identification of specific molecular identifiers for the generation of transgenic mice. Advances in single cell transcriptomic profiling have enabled the identification of molecular signatures for cellular subtypes that are only rarely found. Therefore, we used single cell profiling combined with hierarchical clustering and correlate analyses to identify genes expressed in distinct populations of Parvalbumin-expressing cells and functionally classified RGCs. RGCs were manually isolated based either upon fluorescence or physiological distinction through cell-attached recordings. Microarray hybridization and RNA-Sequencing were employed for the characterization of transcriptomes and in situ hybridization was utilized to further characterize gene candidate expression. Gene candidates were identified based upon cluster correlation, as well as expression specificity within physiologically distinct classes of RGCs. Further, we identified Prph, Ctxn3, and Prkcq as potential candidates for ipRGC classification in the murine retina. The use of these genes, or one of the other newly identified subset markers, for the generation of a transgenic mouse would enable future studies of RGC-subtype specific function, wiring, and projection.

Anti-diabetic drug binding site in a mammalian KATP channel revealed by Cryo-EM.

Sulfonylureas are anti-diabetic medications that act by inhibiting pancreatic KATP channels composed of SUR1 and Kir6.2. The mechanism by which these drugs interact with and inhibit the channel has been extensively investigated, yet it remains unclear where the drug binding pocket resides. Here, we present a cryo-EM structure of a hamster SUR1/rat Kir6.2 channel bound to a high-affinity sulfonylurea drug glibenclamide and ATP at 3.63 Å resolution, which reveals unprecedented details of the ATP and glibenclamide binding sites. Importantly, the structure shows for the first time that glibenclamide is lodged in the transmembrane bundle of the SUR1-ABC core connected to the first nucleotide binding domain near the inner leaflet of the lipid bilayer. Mutation of residues predicted to interact with glibenclamide in our model led to reduced sensitivity to glibenclamide. Our structure provides novel mechanistic insights of how sulfonylureas and ATP interact with the KATP channel complex to inhibit channel activity.

Cryo-EM structure of the ATP-sensitive potassium channel illuminates mechanisms of assembly and gating.

KATP channels are metabolic sensors that couple cell energetics to membrane excitability. In pancreatic ß-cells, channels formed by SUR1 and Kir6.2 regulate insulin secretion and are the targets of antidiabetic sulfonylureas. Here, we used cryo-EM to elucidate structural basis of channel assembly and gating. The structure, determined in the presence of ATP and the sulfonylurea glibenclamide, at ~6 Å resolution reveals a closed Kir6.2 tetrameric core with four peripheral SUR1s each anchored to a Kir6.2 by its N-terminal transmembrane domain (TMD0). Intricate interactions between TMD0, the loop following TMD0, and Kir6.2 near the proposed PIP2 binding site, and where ATP density is observed, suggest SUR1 may contribute to ATP and PIP2 binding to enhance Kir6.2 sensitivity to both. The SUR1-ABC core is found in an unusual inward-facing conformation whereby the two nucleotide binding domains are misaligned along a two-fold symmetry axis, revealing a possible mechanism by which glibenclamide inhibits channel activity.

Single cell transcriptome profiling of developing chick retinal cells.

The vertebrate retina is a specialized photosensitive tissue comprised of six neuronal and one glial cell types, each of which develops in prescribed proportions at overlapping timepoints from a common progenitor pool. While each of these cells has a specific function contributing to proper vision in the mature animal, their differential representation in the retina as well as the presence of distinctive cellular subtypes makes identifying the transcriptomic signatures that lead to each retinal cell's fate determination and development challenging. We have analyzed transcriptomes from individual cells isolated from the chick retina throughout retinogenesis. While we focused our efforts on the retinal ganglion cells, our transcriptomes of developing chick cells also contained representation from multiple retinal cell types, including photoreceptors and interneurons at different stages of development. Most interesting was the identification of transcriptomes from individual mixed lineage progenitor cells in the chick as these cells offer a window into the cell fate decision-making process. Taken together, these data sets will enable us to uncover the most critical genes acting in the steps of cell fate determination and early differentiation of various retinal cell types.

Onecut1 and Onecut2 play critical roles in the development of the mouse retina.

The entire repertoire of intrinsic factors that control the cell fate determination process of specific retinal neurons has yet to be fully identified. Single cell transcriptome profiling experiments of retinal progenitor cells revealed considerable gene expression heterogeneity between individual cells, especially among different classes of transcription factors. In this study, we show that two of those factors, Onecut1 and Onecut2, are expressed during mouse retinal development. Using mice that are deficient for each of these transcription factors, we further demonstrate a significant loss (∼70-80%) of horizontal cells in the absence of either of these proteins, while the other retinal cells appear at normal numbers. Microarray profiling experiments performed on knockout retinas revealed defects in horizontal cell genes as early as E14.5. Additional profiling assays showed an upregulation of several stress response genes in the adult Onecut2 knockout, suggesting that the integrity of the retina is compromised in the absence of normal numbers of horizontal cells. Interestingly, melanopsin, the gene coding for the photopigment found in photosensitive ganglion cells, was observed to be upregulated in Onecut1 deficient retinas, pointing to a possible regulatory role for Onecut1. Taken together, our data show that similar to Onecut1, Onecut2 is also necessary for the formation of normal numbers of horizontal cells in the developing retina.

Common hydrogen bond interactions in diverse phosphoryl transfer active sites.

Phosphoryl transfer reactions figure prominently in energy metabolism, signaling, transport and motility. Prior detailed studies of selected systems have highlighted mechanistic features that distinguish different phosphoryl transfer enzymes. Here, a top-down approach is developed for comparing statistically the active site configurations between populations of diverse structures in the Protein Data Bank, and it reveals patterns of hydrogen bonding that transcend enzyme families. Through analysis of large samples of structures, insights are drawn at a level of detail exceeding the experimental precision of an individual structure. In phosphagen kinases, for example, hydrogen bonds with the O3ß of the nucleotide substrate are revealed as analogous to those in unrelated G proteins. In G proteins and other enzymes, interactions with O3ß have been understood in terms of electrostatic favoring of the transition state. Ground state quantum mechanical calculations on model compounds show that the active site interactions highlighted in our database analysis can affect substrate phosphate charge and bond length, in ways that are consistent with prior experimental observations, by modulating hyperconjugative orbital interactions that weaken the scissile bond. Testing experimentally the inference about the importance of O3ß interactions in phosphagen kinases, mutation of arginine kinase Arg280 decreases kcat, as predicted, with little impact upon KM.

Carbamazepine inhibits ATP-sensitive potassium channel activity by disrupting channel response to MgADP.

In pancreatic ß-cells, K(ATP) channels consisting of Kir6.2 and SUR1 couple cell metabolism to membrane excitability and regulate insulin secretion. Sulfonylureas, insulin secretagogues used to treat type II diabetes, inhibit K(ATP) channel activity primarily by abolishing the stimulatory effect of MgADP endowed by SUR1. In addition, sulfonylureas have been shown to function as pharmacological chaperones to correct channel biogenesis and trafficking defects. Recently, we reported that carbamazepine, an anticonvulsant known to inhibit voltage-gated sodium channels, has profound effects on K(ATP) channels. Like sulfonylureas, carbamazepine corrects trafficking defects in channels bearing mutations in the first transmembrane domain of SUR1. Moreover, carbamazepine inhibits the activity of K(ATP) channels such that rescued mutant channels are unable to open when the intracellular ATP/ADP ratio is lowered by metabolic inhibition. Here, we investigated the mechanism by which carbamazepine inhibits K(ATP) channel activity. We show that carbamazepine specifically blocks channel response to MgADP. This gating effect resembles that of sulfonylureas. Our results reveal striking similarities between carbamazepine and sulfonylureas in their effects on K(ATP) channel biogenesis and gating and suggest that the 2 classes of drugs may act via a converging mechanism.

Pharmacological rescue of trafficking-impaired ATP-sensitive potassium channels.

ATP-sensitive potassium (KATP) channels link cell metabolism to membrane excitability and are involved in a wide range of physiological processes including hormone secretion, control of vascular tone, and protection of cardiac and neuronal cells against ischemic injuries. In pancreatic ß-cells, KATP channels play a key role in glucose-stimulated insulin secretion, and gain or loss of channel function results in neonatal diabetes or congenital hyperinsulinism, respectively. The ß-cell KATP channel is formed by co-assembly of four Kir6.2 inwardly rectifying potassium channel subunits encoded by KCNJ11 and four sulfonylurea receptor 1 subunits encoded by ABCC8. Many mutations in ABCC8 or KCNJ11 cause loss of channel function, thus, congenital hyperinsulinism by hampering channel biogenesis and hence trafficking to the cell surface. The trafficking defects caused by a subset of these mutations can be corrected by sulfonylureas, KATP channel antagonists that have long been used to treat type 2 diabetes. More recently, carbamazepine, an anticonvulsant that is thought to target primarily voltage-gated sodium channels has been shown to correct KATP channel trafficking defects. This article reviews studies to date aimed at understanding the mechanisms by which mutations impair channel biogenesis and trafficking and the mechanisms by which pharmacological ligands overcome channel trafficking defects. Insight into channel structure-function relationships and therapeutic implications from these studies are discussed.