1
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ElSheikh A, Shyng SL. K ATP channel mutations in congenital hyperinsulinism: Progress and challenges towards mechanism-based therapies. Front Endocrinol (Lausanne) 2023; 14:1161117. [PMID: 37056678 PMCID: PMC10086357 DOI: 10.3389/fendo.2023.1161117] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/07/2023] [Accepted: 03/13/2023] [Indexed: 03/30/2023] Open
Abstract
Congenital hyperinsulinism (CHI) is the most common cause of persistent hypoglycemia in infancy/childhood and is a serious condition associated with severe recurrent attacks of hypoglycemia due to dysregulated insulin secretion. Timely diagnosis and effective treatment are crucial to prevent severe hypoglycemia that may lead to life-long neurological complications. In pancreatic β-cells, adenosine triphosphate (ATP)-sensitive K+ (KATP) channels are a central regulator of insulin secretion vital for glucose homeostasis. Genetic defects that lead to loss of expression or function of KATP channels are the most common cause of HI (KATP-HI). Much progress has been made in our understanding of the molecular genetics and pathophysiology of KATP-HI in the past decades; however, treatment remains challenging, in particular for patients with diffuse disease who do not respond to the KATP channel activator diazoxide. In this review, we discuss current approaches and limitations on the diagnosis and treatment of KATP-HI, and offer perspectives on alternative therapeutic strategies.
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Affiliation(s)
- Assmaa ElSheikh
- Department of Chemical Physiology and Biochemistry, Oregon Health & Science University, Portland, OR, United States
- Department of Medical Biochemistry, Tanta University, Tanta, Egypt
| | - Show-Ling Shyng
- Department of Chemical Physiology and Biochemistry, Oregon Health & Science University, Portland, OR, United States
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2
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Gunasekar SK, Xie L, Kumar A, Hong J, Chheda PR, Kang C, Kern DM, My-Ta C, Maurer J, Heebink J, Gerber EE, Grzesik WJ, Elliot-Hudson M, Zhang Y, Key P, Kulkarni CA, Beals JW, Smith GI, Samuel I, Smith JK, Nau P, Imai Y, Sheldon RD, Taylor EB, Lerner DJ, Norris AW, Klein S, Brohawn SG, Kerns R, Sah R. Small molecule SWELL1 complex induction improves glycemic control and nonalcoholic fatty liver disease in murine Type 2 diabetes. Nat Commun 2022; 13:784. [PMID: 35145074 PMCID: PMC8831520 DOI: 10.1038/s41467-022-28435-0] [Citation(s) in RCA: 15] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2021] [Accepted: 01/24/2022] [Indexed: 02/06/2023] Open
Abstract
Type 2 diabetes is associated with insulin resistance, impaired pancreatic β-cell insulin secretion, and nonalcoholic fatty liver disease. Tissue-specific SWELL1 ablation impairs insulin signaling in adipose, skeletal muscle, and endothelium, and impairs β-cell insulin secretion and glycemic control. Here, we show that ICl,SWELL and SWELL1 protein are reduced in adipose and β-cells in murine and human diabetes. Combining cryo-electron microscopy, molecular docking, medicinal chemistry, and functional studies, we define a structure activity relationship to rationally-design active derivatives of a SWELL1 channel inhibitor (DCPIB/SN-401), that bind the SWELL1 hexameric complex, restore SWELL1 protein, plasma membrane trafficking, signaling, glycemic control and islet insulin secretion via SWELL1-dependent mechanisms. In vivo, SN-401 restores glycemic control, reduces hepatic steatosis/injury, improves insulin-sensitivity and insulin secretion in murine diabetes. These findings demonstrate that SWELL1 channel modulators improve SWELL1-dependent systemic metabolism in Type 2 diabetes, representing a first-in-class therapeutic approach for diabetes and nonalcoholic fatty liver disease. Type 2 diabetes is associated with insulin resistance, impaired insulin secretion and liver steatosis. Here the authors report a proof-of-concept study for small molecule SWELL1 modulators as a therapeutic approach to treat diabetes and associated liver steatosis by enhancing systemic insulin-sensitivity and insulin secretion in mice.
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Affiliation(s)
- Susheel K Gunasekar
- Department of Internal Medicine, Cardiovascular Division, Washington University School of Medicine, St. Louis, MO, USA
| | - Litao Xie
- Department of Internal Medicine, Cardiovascular Division, Washington University School of Medicine, St. Louis, MO, USA
| | - Ashutosh Kumar
- Department of Internal Medicine, Cardiovascular Division, Washington University School of Medicine, St. Louis, MO, USA
| | - Juan Hong
- Department of Internal Medicine, Cardiovascular Division, Washington University School of Medicine, St. Louis, MO, USA
| | - Pratik R Chheda
- Department of Pharmaceutical Sciences and Experimental Therapeutics, University of Iowa, College of Pharmacy, Iowa City, IA, USA
| | - Chen Kang
- Department of Internal Medicine, Cardiovascular Division, Washington University School of Medicine, St. Louis, MO, USA
| | - David M Kern
- Department of Molecular & Cell Biology, University of California Berkeley, Berkeley, CA, USA.,Helen Wills Neuroscience Institute, University of California Berkeley, Berkeley, CA, USA
| | - Chau My-Ta
- Feinberg School of Medicine, Northwestern University, Chicago, IL, USA
| | - Joshua Maurer
- Department of Internal Medicine, Cardiovascular Division, Washington University School of Medicine, St. Louis, MO, USA
| | - John Heebink
- Department of Internal Medicine, Cardiovascular Division, Washington University School of Medicine, St. Louis, MO, USA
| | - Eva E Gerber
- Department of Molecular & Cell Biology, University of California Berkeley, Berkeley, CA, USA.,Helen Wills Neuroscience Institute, University of California Berkeley, Berkeley, CA, USA
| | - Wojciech J Grzesik
- Stead Family Department of Pediatrics, Endocrinology and Diabetes Division, Fraternal Order of Eagles Diabetes Research Center, University of Iowa, Iowa City, IA, USA
| | - Macaulay Elliot-Hudson
- Department of Internal Medicine, Cardiovascular Division, University of Iowa, Iowa City, IA, USA
| | - Yanhui Zhang
- Xiamen Cardiovascular Hospital, Xiamen University, Xiamen, China
| | - Phillip Key
- Department of Internal Medicine, Cardiovascular Division, Washington University School of Medicine, St. Louis, MO, USA
| | - Chaitanya A Kulkarni
- Department of Pharmaceutical Sciences and Experimental Therapeutics, University of Iowa, College of Pharmacy, Iowa City, IA, USA
| | - Joseph W Beals
- Center for Human Nutrition, Washington University School of Medicine, St. Louis, USA
| | - Gordon I Smith
- Center for Human Nutrition, Washington University School of Medicine, St. Louis, USA
| | - Isaac Samuel
- Department of Surgery, University of Iowa, Carver College of Medicine, Iowa City, IA, USA
| | - Jessica K Smith
- Department of Surgery, University of Iowa, Carver College of Medicine, Iowa City, IA, USA
| | - Peter Nau
- Department of Surgery, University of Iowa, Carver College of Medicine, Iowa City, IA, USA
| | - Yumi Imai
- Department of Internal Medicine, Cardiovascular Division, University of Iowa, Iowa City, IA, USA
| | - Ryan D Sheldon
- Department of Biochemistry, University of Iowa, Iowa City, IA, USA
| | - Eric B Taylor
- Department of Biochemistry, University of Iowa, Iowa City, IA, USA
| | - Daniel J Lerner
- Senseion Therapeutics Inc, BioGenerator Labs, St Louis, MO, USA
| | - Andrew W Norris
- Stead Family Department of Pediatrics, Endocrinology and Diabetes Division, Fraternal Order of Eagles Diabetes Research Center, University of Iowa, Iowa City, IA, USA
| | - Samuel Klein
- Center for Human Nutrition, Washington University School of Medicine, St. Louis, USA
| | - Stephen G Brohawn
- Department of Molecular & Cell Biology, University of California Berkeley, Berkeley, CA, USA.,Helen Wills Neuroscience Institute, University of California Berkeley, Berkeley, CA, USA
| | - Robert Kerns
- Department of Pharmaceutical Sciences and Experimental Therapeutics, University of Iowa, College of Pharmacy, Iowa City, IA, USA
| | - Rajan Sah
- Department of Internal Medicine, Cardiovascular Division, Washington University School of Medicine, St. Louis, MO, USA.
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3
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Production and purification of ATP-sensitive potassium channel particles for cryo-electron microscopy. Methods Enzymol 2021; 653:121-150. [PMID: 34099169 DOI: 10.1016/bs.mie.2021.02.008] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
ATP-sensitive potassium (KATP) channels are multimeric protein complexes made of four inward rectifying potassium channel (Kir6.x) subunits and four ABC protein sulfonylurea receptor (SURx) subunits. Kir6.x subunits form the potassium ion conducting pore of the channel, and SURx functions to regulate Kir6.x. Kir6.x and SURx are uniquely dependent on each other for expression and function. In pancreatic β-cells, channels comprising SUR1 and Kir6.2 mediate glucose-stimulated insulin secretion and are the targets of antidiabetic sulfonylureas. Mutations in genes encoding SUR1 or Kir6.2 are linked to insulin secretion disorders, with loss- or gain-of-function mutations causing congenital hyperinsulinism or neonatal diabetes mellitus, respectively. Defects in the KATP channel in other tissues underlie human diseases of the cardiovascular and nervous systems. Key to understanding how channels are regulated by physiological and pharmacological ligands and how mutations disrupt channel assembly or gating to cause disease is the ability to observe structural changes associated with subunit interactions and ligand binding. While recent advances in the structural method of single-particle cryo-electron microscopy (cryoEM) offers direct visualization of channel structures, success of obtaining high-resolution structures is dependent on highly concentrated, homogeneous KATP channel particles. In this chapter, we describe a method for expressing KATP channels in mammalian cell culture, solubilizing the channel in detergent micelles and purifying KATP channels using an affinity tag to the SURx subunit for cryoEM structural studies.
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4
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Martin GM, Sung MW, Shyng SL. Pharmacological chaperones of ATP-sensitive potassium channels: Mechanistic insight from cryoEM structures. Mol Cell Endocrinol 2020; 502:110667. [PMID: 31821855 PMCID: PMC6994177 DOI: 10.1016/j.mce.2019.110667] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/18/2019] [Revised: 11/22/2019] [Accepted: 11/22/2019] [Indexed: 02/07/2023]
Abstract
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 β-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.
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Affiliation(s)
- Gregory M Martin
- Department of Chemical Physiology and Biochemistry, Oregon Health & Science University, Portland, OR, 97239, USA
| | - Min Woo Sung
- Department of Chemical Physiology and Biochemistry, Oregon Health & Science University, Portland, OR, 97239, USA
| | - Show-Ling Shyng
- Department of Chemical Physiology and Biochemistry, Oregon Health & Science University, Portland, OR, 97239, USA.
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5
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Martin GM, Sung MW, Yang Z, Innes LM, Kandasamy B, David LL, Yoshioka C, Shyng SL. Mechanism of pharmacochaperoning in a mammalian K ATP channel revealed by cryo-EM. eLife 2019; 8:46417. [PMID: 31343405 PMCID: PMC6699824 DOI: 10.7554/elife.46417] [Citation(s) in RCA: 54] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2019] [Accepted: 07/22/2019] [Indexed: 01/03/2023] Open
Abstract
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 β-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.
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Affiliation(s)
- Gregory M Martin
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, United States
| | - Min Woo Sung
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, United States
| | - Zhongying Yang
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, United States
| | - Laura M Innes
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, United States
| | - Balamurugan Kandasamy
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, United States
| | - Larry L David
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, United States
| | - Craig Yoshioka
- Department of Biomedical Engineering, Oregon Health & Science University, Portland, United States
| | - Show-Ling Shyng
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, United States
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Martin GM, Kandasamy B, DiMaio F, Yoshioka C, Shyng SL. Anti-diabetic drug binding site in a mammalian K ATP channel revealed by Cryo-EM. eLife 2017; 6:31054. [PMID: 29035201 PMCID: PMC5655142 DOI: 10.7554/elife.31054] [Citation(s) in RCA: 99] [Impact Index Per Article: 14.1] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2017] [Accepted: 10/11/2017] [Indexed: 12/25/2022] Open
Abstract
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.
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Affiliation(s)
- Gregory M Martin
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, United States
| | - Balamurugan Kandasamy
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, United States
| | - Frank DiMaio
- Department of Biochemistry, University of Washington, Seattle, United States
| | - Craig Yoshioka
- Department of Biomedical Engineering, Oregon Health and Science University, Portland, United States
| | - Show-Ling Shyng
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, United States
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7
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Vauthier V, Housset C, Falguières T. Targeted pharmacotherapies for defective ABC transporters. Biochem Pharmacol 2017; 136:1-11. [DOI: 10.1016/j.bcp.2017.02.020] [Citation(s) in RCA: 29] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2017] [Accepted: 02/23/2017] [Indexed: 02/07/2023]
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Martin GM, Yoshioka C, Rex EA, Fay JF, Xie Q, Whorton MR, Chen JZ, Shyng SL. Cryo-EM structure of the ATP-sensitive potassium channel illuminates mechanisms of assembly and gating. eLife 2017; 6. [PMID: 28092267 PMCID: PMC5344670 DOI: 10.7554/elife.24149] [Citation(s) in RCA: 128] [Impact Index Per Article: 18.3] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2016] [Accepted: 01/11/2017] [Indexed: 12/18/2022] Open
Abstract
KATP channels are metabolic sensors that couple cell energetics to membrane excitability. In pancreatic β-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. DOI:http://dx.doi.org/10.7554/eLife.24149.001 The hormone insulin reduces blood sugar levels by encouraging fat, muscle and other body cells to take up sugar. When blood sugar levels rise following a meal, cells within the pancreas known as beta cells should release insulin. In people with diabetes, the beta cells fail to release insulin, meaning that the high blood sugar levels are not corrected. When blood sugar levels are high, beta cells generate more energy in the form of ATP molecules. The increased level of ATP causes channels called ATP-sensitive potassium (KATP) channels in the membrane of the cell to close. This triggers a cascade of events that leads to the release of insulin. Some treatments for diabetes alter how the KATP channels work. For example, a widely prescribed medication called glibenclamide (also known as glyburide in the United States) stimulates the release of insulin by preventing the flow of potassium through KATP channels. It remains unknown exactly how ATP and glibenclamide interact with the channel’s molecular structure to stop the flow of potassium ions. KATP channels are made up of two proteins called SUR1 and Kir6.2. To investigate the structure of the KATP channel, Martin et al. purified channels made of the hamster form of the SUR1 protein and the mouse form of Kir6.2, which each closely resemble their human counterparts. The channels were purified in the presence of ATP and glibenclamide and were then rapidly frozen to preserve their structure, which allowed them to be visualized individually using electron microscopy. By analyzing the images taken from many channels, Martin et al. constructed a highly detailed, three-dimensional map of the KATP channel. The structure revealed by this map shows how SUR1 and Kir6.2 work together and provides insight into how ATP and glibenclamide interact with the channel to block the flow of potassium, and hence stimulate the release of insulin. An important next step will be to improve the structure to more clearly identify where ATP and glibenclamide bind to the KATP channel. It will also be important to study the structures of channels that are bound to other regulatory molecules. This will help researchers to fully understand how KATP channels located throughout the body operate under healthy and diseased conditions. This knowledge will aid in the design of more effective drugs to treat several devastating diseases caused by defective KATP channels. DOI:http://dx.doi.org/10.7554/eLife.24149.002
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Affiliation(s)
- Gregory M Martin
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon, United States
| | - Craig Yoshioka
- Department of Biomedical Engineering, Oregon Health and Science University, Portland, Oregon, United States
| | - Emily A Rex
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon, United States
| | - Jonathan F Fay
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon, United States
| | - Qing Xie
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon, United States
| | - Matthew R Whorton
- Vollum Institute, Oregon Health and Science University, Portland, Oregon, United States
| | - James Z Chen
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon, United States
| | - Show-Ling Shyng
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon, United States
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Martin GM, Rex EA, Devaraneni P, Denton JS, Boodhansingh KE, DeLeon DD, Stanley CA, Shyng SL. Pharmacological Correction of Trafficking Defects in ATP-sensitive Potassium Channels Caused by Sulfonylurea Receptor 1 Mutations. J Biol Chem 2016; 291:21971-21983. [PMID: 27573238 DOI: 10.1074/jbc.m116.749366] [Citation(s) in RCA: 34] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2016] [Indexed: 11/06/2022] Open
Abstract
ATP-sensitive potassium (KATP) channels play a key role in mediating glucose-stimulated insulin secretion by coupling metabolic signals to β-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.
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Affiliation(s)
- Gregory M Martin
- From the Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon 97239
| | - Emily A Rex
- From the Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon 97239
| | - Prasanna Devaraneni
- From the Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon 97239
| | - Jerod S Denton
- the Department of Anesthesiology, Vanderbilt University, Nashville, Tennessee 37232, and
| | - Kara E Boodhansingh
- the Division of Endocrinology/Diabetes, Children's Hospital of Philadelphia, Philadelphia, Pennsylvania 19104
| | - Diva D DeLeon
- the Division of Endocrinology/Diabetes, Children's Hospital of Philadelphia, Philadelphia, Pennsylvania 19104
| | - Charles A Stanley
- the Division of Endocrinology/Diabetes, Children's Hospital of Philadelphia, Philadelphia, Pennsylvania 19104
| | - Show-Ling Shyng
- From the Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon 97239,
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Fu A, Robitaille K, Faubert B, Reeks C, Dai XQ, Hardy AB, Sankar KS, Ogrel S, Al-Dirbashi OY, Rocheleau JV, Wheeler MB, MacDonald PE, Jones R, Screaton RA. LKB1 couples glucose metabolism to insulin secretion in mice. Diabetologia 2015; 58:1513-22. [PMID: 25874445 DOI: 10.1007/s00125-015-3579-7] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/27/2014] [Accepted: 03/13/2015] [Indexed: 01/05/2023]
Abstract
AIMS/HYPOTHESIS Precise regulation of insulin secretion by the pancreatic beta cell is essential for the maintenance of glucose homeostasis. Insulin secretory activity is initiated by the stepwise breakdown of ambient glucose to increase cellular ATP via glycolysis and mitochondrial respiration. Knockout of Lkb1, the gene encoding liver kinase B1 (LKB1) from the beta cell in mice enhances insulin secretory activity by an undefined mechanism. Here, we sought to determine the molecular basis for how deletion of Lkb1 promotes insulin secretion. METHODS To explore the role of LKB1 on individual steps in the insulin secretion pathway, we used mitochondrial functional analyses, electrophysiology and metabolic tracing coupled with by gas chromatography and mass spectrometry. RESULTS Beta cells lacking LKB1 surprisingly display impaired mitochondrial metabolism and lower ATP levels following glucose stimulation, yet compensate for this by upregulating both uptake and synthesis of glutamine, leading to increased production of citrate. Furthermore, under low glucose conditions, Lkb1(-/-) beta cells fail to inhibit acetyl-CoA carboxylase 1 (ACC1), the rate-limiting enzyme in lipid synthesis, and consequently accumulate NEFA and display increased membrane excitability. CONCLUSIONS/INTERPRETATION Taken together, our data show that LKB1 plays a critical role in coupling glucose metabolism to insulin secretion, and factors in addition to ATP act as coupling intermediates between feeding cues and secretion. Our data suggest that beta cells lacking LKB1 could be used as a system to identify additional molecular events that connect metabolism to cellular excitation in the insulin secretion pathway.
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Affiliation(s)
- Accalia Fu
- Children's Hospital of Eastern Ontario Research Institute, 401 Smyth Road, Ottawa, K1H 8L1, Canada
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11
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Martin GM, Chen PC, Devaraneni P, Shyng SL. Pharmacological rescue of trafficking-impaired ATP-sensitive potassium channels. Front Physiol 2013; 4:386. [PMID: 24399968 PMCID: PMC3870925 DOI: 10.3389/fphys.2013.00386] [Citation(s) in RCA: 38] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2013] [Accepted: 12/09/2013] [Indexed: 12/25/2022] Open
Abstract
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 β-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 β-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.
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Affiliation(s)
- Gregory M Martin
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University Portland, OR, USA
| | - Pei-Chun Chen
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University Portland, OR, USA
| | - Prasanna Devaraneni
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University Portland, OR, USA
| | - Show-Ling Shyng
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University Portland, OR, USA
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12
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Zhou Q, Pratt EB, Shyng SL. Engineered Kir6.2 mutations that correct the trafficking defect of K(ATP) channels caused by specific SUR1 mutations. Channels (Austin) 2013; 7:313-7. [PMID: 23695995 DOI: 10.4161/chan.25003] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
KATP channels consisting of Kir6.2 and SUR1 couple cell metabolism to membrane excitability and regulate insulin secretion. The molecular interactions between SUR1 and Kir6.2 that govern channel gating and biogenesis are incompletely understood. In a recent study, we showed that a SUR1 and Kir6.2 mutation pair, E203K-SUR1 and Q52E-Kir6.2, at the SUR1/Kir6.2 interface near the plasma membrane increases the ATP-sensitivity of the channel by nearly 100-fold. Here, we report the finding that the same mutation pair also suppresses channel folding/trafficking defects caused by select SUR1 mutations in the first transmembrane domain of SUR1. Analysis of the contributions from individual mutations, however, revealed that the correction effect is attributed largely to Q52E-Kir6.2 alone. Moreover, the correction is dependent on the negative charge of the substituting amino acid at the Q52 position in Kir6.2. Our study demonstrates for the first time that engineered mutations in Kir6.2 can correct the biogenesis defect caused by specific mutations in the SUR1 subunit.
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Affiliation(s)
- Qing Zhou
- Department of Biochemistry and Molecular Biology; Oregon Health & Science University; Portland, OR USA
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Fotinou C, Aittoniemi J, de Wet H, Polidori A, Pucci B, Sansom MSP, Vénien-Bryan C, Ashcroft FM. Tetrameric structure of SUR2B revealed by electron microscopy of oriented single particles. FEBS J 2013; 280:1051-63. [PMID: 23253866 PMCID: PMC3599479 DOI: 10.1111/febs.12097] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2012] [Revised: 11/26/2012] [Accepted: 12/03/2012] [Indexed: 12/25/2022]
Abstract
The ATP-sensitive potassium (KATP) channel is a hetero-octameric complex that links cell metabolism to membrane electrical activity in many cells, thereby controlling physiological functions such as insulin release, muscle contraction and neuronal activity. It consists of four pore-forming Kir6.2 and four regulatory sulfonylurea receptor (SUR) subunits. SUR2B serves as the regulatory subunit in smooth muscle and some neurones. An integrative approach, combining electron microscopy and homology modelling, has been used to obtain information on the structure of this large (megadalton) membrane protein complex. Single-particle electron microscopy of purified SUR2B tethered to a lipid monolayer revealed that it assembles as a tetramer of four SUR2B subunits surrounding a central hole. In the absence of an X-ray structure, a homology model for SUR2B based on the X-ray structure of the related ABC transporter Sav1866 was used to fit the experimental images. The model indicates that the central hole can readily accommodate the transmembrane domains of the Kir tetramer, suggests a location for the first transmembrane domains of SUR2B (which are absent in Sav1866) and suggests the relative orientation of the SUR and Kir6.2 subunits.
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Affiliation(s)
- Constantina Fotinou
- Department of Physiology, Henry Wellcome Centre for Gene Function, University of Oxford, Oxford, UK
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Pratt EB, Zhou Q, Gay JW, Shyng SL. Engineered interaction between SUR1 and Kir6.2 that enhances ATP sensitivity in KATP channels. ACTA ACUST UNITED AC 2012; 140:175-87. [PMID: 22802363 PMCID: PMC3409095 DOI: 10.1085/jgp.201210803] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The ATP-sensitive potassium (KATP) channel consisting of the inward rectifier Kir6.2 and SUR1 (sulfonylurea receptor 1) couples cell metabolism to membrane excitability and regulates insulin secretion. Inhibition by intracellular ATP is a hallmark feature of the channel. ATP sensitivity is conferred by Kir6.2 but enhanced by SUR1. The mechanism by which SUR1 increases channel ATP sensitivity is not understood. In this study, we report molecular interactions between SUR1 and Kir6.2 that markedly alter channel ATP sensitivity. Channels bearing an E203K mutation in SUR1 and a Q52E in Kir6.2 exhibit ATP sensitivity ∼100-fold higher than wild-type channels. Cross-linking of E203C in SUR1 and Q52C in Kir6.2 locks the channel in a closed state and is reversible by reducing agents, demonstrating close proximity of the two residues. Our results reveal that ATP sensitivity in KATP channels is a dynamic parameter dictated by interactions between SUR1 and Kir6.2.
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Affiliation(s)
- Emily B Pratt
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, OR 97239, USA.
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Pratt EB, Shyng SL. ATP activates ATP-sensitive potassium channels composed of mutant sulfonylurea receptor 1 and Kir6.2 with diminished PIP2 sensitivity. Channels (Austin) 2011; 5:314-9. [PMID: 21654216 DOI: 10.4161/chan.5.4.16510] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
Abstract
ATP-sensitive potassium (K(ATP)) channels are inhibited by ATP and activated by phosphatidylinositol 4,5-bisphosphate (PIP(2)). Both channel subunits Kir6.2 and sulfonylurea receptor 1 (SUR1) contribute to gating: while Kir6.2 interacts with ATP and PIP(2), SUR1 enhances sensitivity to both ligands. Recently, we showed that a mutation, E128K, in the N-terminal transmembrane domain of SUR1 disrupts functional coupling between SUR1 and Kir6.2, leading to reduced ATP and PIP(2) sensitivities resembling channels formed by Kir6.2 alone. We show here that when E128K SUR1 was co-expressed with Kir6.2 mutants known to disrupt PIP(2) gating, the resulting channels were surprisingly stimulated rather than inhibited by ATP. To explain this paradoxical gating behavior, we propose a model in which the open state of doubly mutant channels is highly unstable; ATP binding induces a conformational change in ATP-unbound closed channels that is conducive to brief opening when ATP unbinds, giving rise to the appearance of ATP-induced stimulation.
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Affiliation(s)
- Emily B Pratt
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, USA
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Matsumura Y, David LL, Skach WR. Role of Hsc70 binding cycle in CFTR folding and endoplasmic reticulum-associated degradation. Mol Biol Cell 2011; 22:2797-809. [PMID: 21697503 PMCID: PMC3154877 DOI: 10.1091/mbc.e11-02-0137] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
Hsc70 plays a productive role during cotranslational cystic fibrosis transmembrane conductance regulator folding that is outweighed by its dominant contribution to posttranslational targeting to the ubiquitin-proteasome system. Moreover, the outcome of Hsc70 binding appears highly sensitive to the duration of its binding cycle, which is governed by regulatory cochaperones. The Hsp/c70 cytosolic chaperone system facilitates competing pathways of protein folding and degradation. Here we use a reconstituted cell-free system to investigate the mechanism and extent to which Hsc70 contributes to these co- and posttranslational decisions for the membrane protein cystic fibrosis transmembrane conductance regulator (CFTR). Hsc70 binding to CFTR was destabilized by the C-terminal domain of Bag-1 (CBag), which stimulates client release by accelerating ADP-ATP exchange. Addition of CBag during CFTR translation slightly increased susceptibility of the newly synthesized protein to degradation, consistent with a profolding function for Hsc70. In contrast, posttranslational destabilization of Hsc70 binding nearly completely blocked CFTR ubiquitination, dislocation from the endoplasmic reticulum, and proteasome-mediated cleavage. This effect required molar excess of CBag relative to Hsc70 and was completely reversed by the CBag-binding subdomain of Hsc70. These results demonstrate that the profolding role of Hsc70 during cotranslational CFTR folding is counterbalanced by a dominant and essential role in posttranslational targeting to the ubiquitin-proteasome system. Moreover, the degradative outcome of Hsc70 binding appears highly sensitive to the duration of its binding cycle, which is in turn governed by the integrated expression of regulatory cochaperones.
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Affiliation(s)
- Yoshihiro Matsumura
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, OR 97239, USA
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Pratt EB, Tewson P, Bruederle CE, Skach WR, Shyng SL. N-terminal transmembrane domain of SUR1 controls gating of Kir6.2 by modulating channel sensitivity to PIP2. ACTA ACUST UNITED AC 2011; 137:299-314. [PMID: 21321069 PMCID: PMC3047609 DOI: 10.1085/jgp.201010557] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Functional integrity of pancreatic adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels depends on the interactions between the pore-forming potassium channel subunit Kir6.2 and the regulatory subunit sulfonylurea receptor 1 (SUR1). Previous studies have shown that the N-terminal transmembrane domain of SUR1 (TMD0) interacts with Kir6.2 and is sufficient to confer high intrinsic open probability (P(o)) and bursting patterns of activity observed in full-length K(ATP) channels. However, the nature of TMD0-Kir6.2 interactions that underlie gating modulation is not well understood. Using two previously described disease-causing mutations in TMD0 (R74W and E128K), we performed amino acid substitutions to study the structural roles of these residues in K(ATP) channel function in the context of full-length SUR1 as well as TMD0. Our results revealed that although R74W and E128K in full-length SUR1 both decrease surface channel expression and reduce channel sensitivity to ATP inhibition, they arrive there via distinct mechanisms. Mutation of R74 uniformly reduced TMD0 protein levels, suggesting that R74 is necessary for stability of TMD0. In contrast, E128 mutations retained TMD0 protein levels but reduced functional coupling between TMD0 and Kir6.2 in mini-K(ATP) channels formed by TMD0 and Kir6.2. Importantly, E128K full-length channels, despite having a greatly reduced P(o), exhibit little response to phosphatidylinositol 4,5-bisphosphate (PIP(2)) stimulation. This is reminiscent of Kir6.2 channel behavior in the absence of SUR1 and suggests that TMD0 controls Kir6.2 gating by modulating Kir6.2 interactions with PIP(2). Further supporting this notion, the E128W mutation in full-length channels resulted in channel inactivation that was prevented or reversed by exogenous PIP(2). These results identify a critical determinant in TMD0 that controls Kir6.2 gating by controlling channel sensitivity to PIP(2). Moreover, they uncover a novel mechanism of K(ATP) channel inactivation involving aberrant functional coupling between SUR1 and Kir6.2.
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Affiliation(s)
- Emily B Pratt
- Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, OR 97239, USA
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Li CJ, Zhou HL, Li J, Yao HT, Su R, Li WP. Roles of sulfonylurea receptor 1 and multidrug resistance protein 1 in modulating insulin secretion in human insulinoma. Hepatobiliary Pancreat Dis Int 2011; 10:88-94. [PMID: 21269941 DOI: 10.1016/s1499-3872(11)60013-1] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
BACKGROUND Sulfonylurea receptor 1 (SUR1) and multidrug resistance protein 1 (MRP1) are two prominent members of multidrug resistance proteins associated with insulin secretion. The aims of this study were to investigate their expression in insulinomas and their sole and synergistic effects in modulating abnormal insulin secretion. METHODS Fasting glucose, insulin and C-peptide were measured in 11 insulinoma patients and 11 healthy controls. Prolonged oral glucose tolerance tests were performed in 6 insulinoma patients. Insulin content, SUR1 and MRP1 were detected in 11 insulinoma patients by immunohistochemistry. SUR1 and MRP1 were also detected in 6 insulinoma patients by immunofluorescence. RESULTS Insulinoma patients presented the typical demonstrations of Whipple's triad. Fasting glucose of each insulinoma patient was lower than 2.8 mmol/L, and simultaneous insulin and C-peptide were increased in insulinoma patients. Prolonged oral glucose tolerance tests showed that insulin secretion in insulinoma patients were also stimulated by high glucose. Immunohistochemistry and immunofluorescence staining showed that SUR1 increased, but MRP1 decreased in insulinoma compared with the adjacent islets. CONCLUSIONS The hypersecretion of insulin in insulinomas might be, at least partially, due to the enrichment of SUR1. In contrast, MRP1, which is down-regulated in insulinomas, might reflect a negative feedback in insulin secretion.
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Affiliation(s)
- Cheng-Jiang Li
- Department of Endocrinology, First Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou 310003, China
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Zhou Q, Garin I, Castaño L, Argente J, Muñoz-Calvo MT, Perez de Nanclares G, Shyng SL. Neonatal diabetes caused by mutations in sulfonylurea receptor 1: interplay between expression and Mg-nucleotide gating defects of ATP-sensitive potassium channels. J Clin Endocrinol Metab 2010; 95:E473-8. [PMID: 20810569 PMCID: PMC2999977 DOI: 10.1210/jc.2010-1231] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
CONTEXT ATP-sensitive potassium (KATP) channels regulate insulin secretion by coupling glucose metabolism to β-cell membrane potential. Gain-of-function mutations in the sulfonylurea receptor 1 (SUR1) or Kir6.2 channel subunit underlie neonatal diabetes. OBJECTIVE The objective of the study was to determine the mechanisms by which two SUR1 mutations, E208K and V324M, associated with transient neonatal diabetes affect KATP channel function. DESIGN E208K or V324M mutant SUR1 was coexpressed with Kir6.2 in COS cells, and expression and gating properties of the resulting channels were assessed biochemically and electrophysiologically. RESULTS Both E208K and V324M augment channel response to MgADP stimulation without altering sensitivity to ATP4- or sulfonylureas. Surprisingly, whereas E208K causes only a small increase in MgADP response consistent with the mild transient diabetes phenotype, V324M causes a severe activating gating defect. Unlike E208K, V324M also impairs channel expression at the cell surface, which is expected to dampen its functional impact on β-cells. When either mutation was combined with a mutation in the second nucleotide binding domain of SUR1 previously shown to abolish Mg-nucleotide response, the activating effect of E208K and V324M was also abolished. Moreover, combination of E208K and V324M results in channels with Mg-nucleotide sensitivity greater than that seen in individual mutations alone. CONCLUSION The results demonstrate that E208K and V324M, located in distinct domains of SUR1, enhance transduction of Mg-nucleotide stimulation from the SUR1 nucleotide binding folds to Kir6.2. Furthermore, they suggest that diabetes severity is determined by interplay between effects of a mutation on channel expression and channel gating.
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Affiliation(s)
- Qing Zhou
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, 3181 S.W. Sam Jackson Park Road, Portland, Oregon 97239, USA
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Mendre C, Mouillac B. [Pharmacological chaperones: a potential therapeutic treatment for conformational diseases]. Med Sci (Paris) 2010; 26:627-35. [PMID: 20619166 DOI: 10.1051/medsci/2010266-7627] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Many genetic and neurodegenerative diseases in humans result from protein misfolding and/or aggregation. These diseases are named conformational diseases. As a result, the misfolded non functional proteins are rejected and misrouted by the cellular quality control system, and cannot play their endogenous physiological roles. Specific compounds (ligands, substrates or inhibitors) known as pharmacological chaperones are able to bind and stabilize these misfolded proteins. Their interaction allows the target proteins to escape the quality control system and to be functionally rescued. These pharmacochaperones may possess different intrinsic activity: they can be antagonists (inhibitors), agonists (activators) or allosteric modulators of the target receptors, ionic channels or enzymes. Pharmacological chaperones have obviously a therapeutic potential to treat rare diseases like cystic fibrosis, retinitis pigmentosa, nephrogenic diabetes insipidus, Fabry disease, Gaucher disease, but also for cancers and more frequent and highly invalidant neurodegenerative disorders such as Alzheimer's disease or Parkinson's disease.
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Affiliation(s)
- Christiane Mendre
- Universités Montpellier 1 et 2, 141, 34094 Montpellier Cedex 05, France.
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Gong B, Zhang LY, Lam DSC, Pang CP, Yam GHF. Sodium 4-phenylbutyrate ameliorates the effects of cataract-causing mutant gammaD-crystallin in cultured cells. Mol Vis 2010; 16:997-1003. [PMID: 20577655 PMCID: PMC2890554] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2010] [Accepted: 06/01/2010] [Indexed: 10/25/2022] Open
Abstract
PURPOSE gammaD-Crystallin (CRYGD) is a major structural lens crystallin and its mutations result in congenital cataract formation. In this study, we attempted to correct the altered protein features of G165fsX8 CRYGD protein with small chemical molecules. METHODS Recombinant FLAG-tagged mutants (R15C, R15S, P24T, R61C, and G165fsX8) of CRYGD were expressed in COS-7 cells and treated with small chemical molecules with reported protein chaperoning properties (sodium 4-phenylbutyrate [4-PBA], trimethylamine N-oxide [TMAO], and glycerol and DMSO [DMSO]). Protein solubility in 0.5% Triton X-100 and subcellular distribution was examined by western blotting and immunofluorescence, respectively. Apoptosis was assayed as the percentage of fragmented nuclei in transfected cells. Expression of heat-shock proteins (Hsp70 and Hsp90) was examined by reverse transcription-polymerase chain reaction analysis. RESULTS Unlike WT and most mutants (R15C, R15S, P24T, and R61C) of CRYGD, G165fsX8 CRYGD was significantly insoluble in 0.5% Triton X-100. This insolubility was alleviated by dose-dependent 4-PBA treatment. The treatment relieved the mislocalization of G165fsX8 CRYGD from the nuclear envelope. Also, 4-PBA treatment reduced cell apoptosis and caused an upregulation of Hsp70. CONCLUSIONS 4-PBA treatment reduced the defective phenotype of mutant G165fsX8 CRYGD and rescued the affected cells from apoptosis. This could be a potential treatment for lens structural protein and prevent lens opacity in cataract formation.
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Yan FF, Pratt EB, Chen PC, Wang F, Skach WR, David LL, Shyng SL. Role of Hsp90 in biogenesis of the beta-cell ATP-sensitive potassium channel complex. Mol Biol Cell 2010; 21:1945-54. [PMID: 20427569 PMCID: PMC2883939 DOI: 10.1091/mbc.e10-02-0116] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
The study identifies Hsp90 as a molecular chaperone for KATP channels. Inhibition of Hsp90 function reduces, whereas overexpression of Hsp90 enhances, channel expression at the cell surface. Hsp90 facilitates channel biogenesis by targeting the SUR1 subunit. Up-regulation of Hsp90 also enhances expression of some SUR1 mutants with folding defects. The pancreatic β-cell ATP-sensitive potassium (KATP) channel is a multimeric protein complex composed of four inwardly rectifying potassium channel (Kir6.2) and four sulfonylurea receptor 1 (SUR1) subunits. KATP channels play a key role in glucose-stimulated insulin secretion by linking glucose metabolism to membrane excitability. Many SUR1 and Kir6.2 mutations reduce channel function by disrupting channel biogenesis and processing, resulting in insulin secretion disease. To better understand the mechanisms governing KATP channel biogenesis, a proteomics approach was used to identify chaperone proteins associated with KATP channels. We report that chaperone proteins heat-shock protein (Hsp)90, heat-shock cognate protein (Hsc)70, and Hsp40 are associated with β-cell KATP channels. Pharmacologic inhibition of Hsp90 function by geldanamycin reduces, whereas overexpression of Hsp90 increases surface expression of wild-type KATP channels. Coimmunoprecipitation data indicate that channel association with the Hsp90 complex is mediated through SUR1. Accordingly, manipulation of Hsp90 protein expression or function has significant effects on the biogenesis efficiency of SUR1, but not Kir6.2, expressed alone. Interestingly, overexpression of Hsp90 selectively improved surface expression of mutant channels harboring a subset of disease-causing SUR1 processing mutations. Our study demonstrates that Hsp90 regulates biogenesis efficiency of heteromeric KATP channels via SUR1, thereby affecting functional expression of the channel in β-cell membrane.
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Affiliation(s)
- Fei-Fei Yan
- Department of Biochemistry and Molecular Biology, Oregon Health & Science University, Portland, OR 97239, USA
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Akrouh A, Halcomb SE, Nichols CG, Sala-Rabanal M. Molecular biology of K(ATP) channels and implications for health and disease. IUBMB Life 2009; 61:971-8. [PMID: 19787700 DOI: 10.1002/iub.246] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
The ATP-sensitive potassium (K(ATP)) channel is expressed in most excitable tissues and plays a critical role in numerous physiological processes by coupling intracellular energetics to electrical activity. The channel is comprised of four Kir6.x subunits associated with four regulatory sulfonylurea receptors (SUR). Intracellular ATP acts on Kir6.x to inhibit channel activity, while MgADP stimulates channel activity through SUR. Changes in the cytosolic [ATP] to [ADP] ratio thus determine channel activity. Multiple mutations in Kir6.x and SUR genes have implicated K(ATP) channels in various diseases ranging from diabetes and hyperinsulinism to cardiac arrhythmias and cardiovascular disease. Continuing studies of channel physiology and pathology will bring new insights to the molecular basis of K(ATP) channel function, leading to a better understanding of the role that K(ATP) channels play in both health and disease.
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Affiliation(s)
- Alejandro Akrouh
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, MO 63110, USA
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Hosy E, Dupuis JP, Vivaudou M. Impact of disease-causing SUR1 mutations on the KATP channel subunit interface probed with a rhodamine protection assay. J Biol Chem 2009; 285:3084-91. [PMID: 19933268 DOI: 10.1074/jbc.m109.043307] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The function of the ATP-sensitive potassium (K(ATP)) channel relies on the proper coupling between its two subunits: the pore-forming Kir6.2 and the regulator SUR. The conformation of the interface between these two subunits can be monitored using a rhodamine 123 (Rho) protection assay because Rho blocks Kir6.2 with an efficiency that depends on the relative position of transmembrane domain (TMD) 0 of the associated SUR (Hosy, E., Dérand, R., Revilloud, J., and Vivaudou, M. (2007) J. Physiol. 582, 27-39). Here we find that the natural and synthetic K(ATP) channel activators MgADP, zinc, and SR47063 induced a Rho-insensitive conformation. The activating mutation F132L in SUR1, which causes neonatal diabetes, also rendered the channel resistant to Rho block, suggesting that it stabilized an activated conformation by uncoupling TMD0 from the rest of SUR1. At a nearby residue, the SUR1 mutation E128K impairs trafficking, thereby reducing surface expression and causing hyperinsulinism. To augment channel density at the plasma membrane to investigate the effect of mutating this residue on channel function, we introduced the milder mutation E126A at the matching residue of SUR2A. Mutation E126A imposed a hypersensitive Rho phenotype indicative of a functional uncoupling between TMD0 and Kir6.2. These results suggest that the TMD0-Kir6.2 interface is mobile and that the gating modes of Kir6.2 correlate with distinct positions of TMD0. They further demonstrate that the second intracellular loop of SUR, which contains the two residues studied here, is a key structural element of the TMD0-Kir6.2 interface.
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Affiliation(s)
- Eric Hosy
- Institut de Biologie Structurale (CEA, CNRS, UJF), Laboratoire des Protéines Membranaires, 41 Rue Jules Horowitz, 38027 Grenoble, France
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