Open to Control — New Hope for Patients with Neonatal Diabetes
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《新英格兰医药杂志》
Pancreatic beta cells, the insulin-releasing cells of the islets of Langerhans, are electrically excitable cells. They sense fluctuations in the plasma glucose concentration and convert the signal into a change in electrical activity that modulates insulin secretion. Mutations affecting the molecular machinery underlying this conversion are prime suspects in the causation of diabetes mellitus, in which an increase in the plasma glucose level is not adequately matched by increased insulin secretion. At the heart of the mechanism that changes the metabolic signal into an electrical one lies the ATP-sensitive potassium channel (KATP channel); polymorphisms in this channel have recently been linked to type 2 diabetes. In this issue of the Journal, Gloyn and colleagues (pages 1838–1849) report that a remarkable 10 out of 29 infants with neonatal diabetes had mutations in the gene encoding the pore-forming subunit of the KATP channel.
The role of KATP channels in the sensing of glucose by beta cells was established in the mid-1980s, after Neher and Sakmann developed the patch-clamp technique that permits direct electrophysiological recordings from small cells.1 Early patch-clamp studies of pancreatic beta cells identified a potassium channel in the plasma membrane that was regulated by glucose through changes in the cytoplasmic ATP concentration. When open, these channels allow potassium ions to follow their electrochemical gradient and move out of the cell, taking with them their positive charge. KATP channels are present in beta cells in such high numbers that, when even only a small percentage are open, the outward movement of potassium ions dominates other ionic fluxes and holds the membrane potential in a hyperpolarized state. This situation is observed when the plasma glucose levels are low, since KATP channels are opened by the decreasing concentrations of ATP and the increasing concentrations of magnesium–ADP complex (Mg-ADP) in the cytoplasm. At high glucose concentrations, the converse occurs, as ATP is generated metabolically from ADP, causing the KATP channels to close. Small inward currents then have greater influence on the membrane potential, triggering membrane depolarization, the opening of voltage-gated calcium channels, an increase in intracellular calcium levels, and a consequent release of insulin granules (see Figure).
Figure. Stimulus–Secretion Coupling in Pancreatic Beta Cells.
In normal beta cells (left) at high glucose concentrations, glucose metabolism results in an increase in the ATP concentration and a decrease in the concentration of magnesium–ADP complex (Mg-ADP). These changes close KATP channels, allowing small inward currents to depolarize the cells, triggering calcium entry and insulin release. Gloyn and colleagues show that many people with neonatal diabetes have mutations of the KATP channel gene (right). Mutant KATP channels are not closed by ATP even at increased glucose concentrations. The open channels allow a persistent outward potassium flux that overcomes the inward currents, thereby hyperpolarizing the cells and preventing the release of insulin.
The identification of the genes encoding the different channel subunits revolutionized research into KATP-channel physiology. KATP channels in beta cells are formed from four small subunits (Kir6.2) that surround and enclose a central pore and four larger regulatory subunits, the sulfonylurea receptors (SUR1). The eight subunits of the KATP-channel complex contain a number of interacting binding sites for nucleotides and drugs. ATP closes the channels by binding to the Kir6.2 subunit, whereas sulfonylureas, which have been used for many decades to treat type 2 diabetes, achieve the same effect by binding to SUR1.
Further evidence of the importance of the KATP channel in the sensing of glucose by the pancreas was provided by the finding that many children with congenital hyperinsulinism have mutations in subunits of the KATP channel. These mutations reduce the number of open KATP channels at resting plasma glucose concentrations, resulting in the hypersecretion of insulin and consequent hypoglycemia.2 By contrast, the mutations found in the study by Gloyn et al. seem to cause diabetes by increasing the number of open channels at the plasma membrane, thereby hyperpolarizing the beta cells and preventing the release of insulin.
The most common mutations identified in subjects with neonatal diabetes affected the arginine at position 201 of Kir6.2, a residue that is believed to lie close to the ATP-binding site and that has previously been implicated in ATP sensing. Gloyn and colleagues examined the functional effect of one of these mutations by comparing the electrophysiological properties of mutant and wild-type channels as expressed in xenopus oocytes. Converting the arginine residue to histidine markedly reduced the sensitivity of KATP channels to ATP. One would predict that with this reduction in sensitivity, more channels would be open at cytoplasmic ATP concentrations, which are believed to be in the low millimolar range in the beta cell. Although mixing mutant and wild-type Kir6.2 subunits in order to mimic the heterozygous state did not markedly shift the ATP dose–response relationship from that of the wild-type channel, free combination would result in a small proportion of channel tetramers containing only mutant subunits. These would be difficult to detect but would be predicted to produce enough open potassium channels at millimolar ATP concentrations to hyperpolarize the beta cells and inhibit insulin release (see Figure).
KATP channels are also found in a range of other tissues such as muscle and brain, where they are believed to open during ischemia, thereby hyperpolarizing the cells and reducing electrical and mechanical activity as the oxygen concentration decreases. At a molecular level, they primarily contain Kir6.2, but it is often coupled to an alternative sulfonylurea receptor (SUR2 isoforms in skeletal, cardiac, and smooth muscle and some neurons). It is therefore interesting that a subgroup of the subjects with neonatal diabetes and Kir6.2 mutations have extrapancreatic signs that include muscular weakness, epilepsy, developmental delays, and dysmorphic features. Surprisingly, however, this multiorgan involvement was observed only in a subgroup of the patients who did not have mutations at residue R201.
It is unclear why some mutations appear to be silent in extrapancreatic tissues whereas others have such profound effects, but this variability might indicate that certain residues in Kir6.2 are more critical for coupling to SUR1 than to SUR2. However, the extrapancreatic effects in some persons might teach us important lessons about the elusive role of the KATP channel in muscle and the nervous system. Questions about the function of KATP channels in extrapancreatic tissues are by no means purely academic. Many of the sulfonylureas that are used routinely to treat type 2 diabetes affect all types of KATP channels, giving rise to concern that they may cause rare but potentially life-threatening side effects.
On the other hand, one benefit of this study for subjects with neonatal diabetes is that sulfonylureas, unlike glucose, stimulated insulin release in the patients who were tested and also closed the mutant KATP channels in vitro, which might allow affected persons to switch to oral medication. If the same is found in subjects with extrapancreatic involvement, there may be a basis for giving a trial of nonselective sulfonylureas not only to treat the diabetes, but also for the associated musculoskeletal and neurologic symptoms. Ironically, potential side effects of one of the classic medications for type 2 diabetes — effects on extrapancreatic KATP channels — may actually become a major goal of treatment.
Source Information
From the Department of Clinical Biochemistry, University of Cambridge, Cambridge, United Kingdom.
References
Hamill OP, Marty A, Neher E, Sakmann B, Sigworth FJ. Improved patch-clamp techniques for high-resolution current recording from cells and cell-free membrane patches. Pflugers Arch 1981;391:85-100.
Thomas PM, Cote GJ, Wohllk N, et al. Mutations in the sulfonylurea receptor gene in familial persistent hyperinsulinemic hypoglycemia of infancy. Science 1995;268:426-429.(Fiona M. Gribble, B.M., B)
The role of KATP channels in the sensing of glucose by beta cells was established in the mid-1980s, after Neher and Sakmann developed the patch-clamp technique that permits direct electrophysiological recordings from small cells.1 Early patch-clamp studies of pancreatic beta cells identified a potassium channel in the plasma membrane that was regulated by glucose through changes in the cytoplasmic ATP concentration. When open, these channels allow potassium ions to follow their electrochemical gradient and move out of the cell, taking with them their positive charge. KATP channels are present in beta cells in such high numbers that, when even only a small percentage are open, the outward movement of potassium ions dominates other ionic fluxes and holds the membrane potential in a hyperpolarized state. This situation is observed when the plasma glucose levels are low, since KATP channels are opened by the decreasing concentrations of ATP and the increasing concentrations of magnesium–ADP complex (Mg-ADP) in the cytoplasm. At high glucose concentrations, the converse occurs, as ATP is generated metabolically from ADP, causing the KATP channels to close. Small inward currents then have greater influence on the membrane potential, triggering membrane depolarization, the opening of voltage-gated calcium channels, an increase in intracellular calcium levels, and a consequent release of insulin granules (see Figure).
Figure. Stimulus–Secretion Coupling in Pancreatic Beta Cells.
In normal beta cells (left) at high glucose concentrations, glucose metabolism results in an increase in the ATP concentration and a decrease in the concentration of magnesium–ADP complex (Mg-ADP). These changes close KATP channels, allowing small inward currents to depolarize the cells, triggering calcium entry and insulin release. Gloyn and colleagues show that many people with neonatal diabetes have mutations of the KATP channel gene (right). Mutant KATP channels are not closed by ATP even at increased glucose concentrations. The open channels allow a persistent outward potassium flux that overcomes the inward currents, thereby hyperpolarizing the cells and preventing the release of insulin.
The identification of the genes encoding the different channel subunits revolutionized research into KATP-channel physiology. KATP channels in beta cells are formed from four small subunits (Kir6.2) that surround and enclose a central pore and four larger regulatory subunits, the sulfonylurea receptors (SUR1). The eight subunits of the KATP-channel complex contain a number of interacting binding sites for nucleotides and drugs. ATP closes the channels by binding to the Kir6.2 subunit, whereas sulfonylureas, which have been used for many decades to treat type 2 diabetes, achieve the same effect by binding to SUR1.
Further evidence of the importance of the KATP channel in the sensing of glucose by the pancreas was provided by the finding that many children with congenital hyperinsulinism have mutations in subunits of the KATP channel. These mutations reduce the number of open KATP channels at resting plasma glucose concentrations, resulting in the hypersecretion of insulin and consequent hypoglycemia.2 By contrast, the mutations found in the study by Gloyn et al. seem to cause diabetes by increasing the number of open channels at the plasma membrane, thereby hyperpolarizing the beta cells and preventing the release of insulin.
The most common mutations identified in subjects with neonatal diabetes affected the arginine at position 201 of Kir6.2, a residue that is believed to lie close to the ATP-binding site and that has previously been implicated in ATP sensing. Gloyn and colleagues examined the functional effect of one of these mutations by comparing the electrophysiological properties of mutant and wild-type channels as expressed in xenopus oocytes. Converting the arginine residue to histidine markedly reduced the sensitivity of KATP channels to ATP. One would predict that with this reduction in sensitivity, more channels would be open at cytoplasmic ATP concentrations, which are believed to be in the low millimolar range in the beta cell. Although mixing mutant and wild-type Kir6.2 subunits in order to mimic the heterozygous state did not markedly shift the ATP dose–response relationship from that of the wild-type channel, free combination would result in a small proportion of channel tetramers containing only mutant subunits. These would be difficult to detect but would be predicted to produce enough open potassium channels at millimolar ATP concentrations to hyperpolarize the beta cells and inhibit insulin release (see Figure).
KATP channels are also found in a range of other tissues such as muscle and brain, where they are believed to open during ischemia, thereby hyperpolarizing the cells and reducing electrical and mechanical activity as the oxygen concentration decreases. At a molecular level, they primarily contain Kir6.2, but it is often coupled to an alternative sulfonylurea receptor (SUR2 isoforms in skeletal, cardiac, and smooth muscle and some neurons). It is therefore interesting that a subgroup of the subjects with neonatal diabetes and Kir6.2 mutations have extrapancreatic signs that include muscular weakness, epilepsy, developmental delays, and dysmorphic features. Surprisingly, however, this multiorgan involvement was observed only in a subgroup of the patients who did not have mutations at residue R201.
It is unclear why some mutations appear to be silent in extrapancreatic tissues whereas others have such profound effects, but this variability might indicate that certain residues in Kir6.2 are more critical for coupling to SUR1 than to SUR2. However, the extrapancreatic effects in some persons might teach us important lessons about the elusive role of the KATP channel in muscle and the nervous system. Questions about the function of KATP channels in extrapancreatic tissues are by no means purely academic. Many of the sulfonylureas that are used routinely to treat type 2 diabetes affect all types of KATP channels, giving rise to concern that they may cause rare but potentially life-threatening side effects.
On the other hand, one benefit of this study for subjects with neonatal diabetes is that sulfonylureas, unlike glucose, stimulated insulin release in the patients who were tested and also closed the mutant KATP channels in vitro, which might allow affected persons to switch to oral medication. If the same is found in subjects with extrapancreatic involvement, there may be a basis for giving a trial of nonselective sulfonylureas not only to treat the diabetes, but also for the associated musculoskeletal and neurologic symptoms. Ironically, potential side effects of one of the classic medications for type 2 diabetes — effects on extrapancreatic KATP channels — may actually become a major goal of treatment.
Source Information
From the Department of Clinical Biochemistry, University of Cambridge, Cambridge, United Kingdom.
References
Hamill OP, Marty A, Neher E, Sakmann B, Sigworth FJ. Improved patch-clamp techniques for high-resolution current recording from cells and cell-free membrane patches. Pflugers Arch 1981;391:85-100.
Thomas PM, Cote GJ, Wohllk N, et al. Mutations in the sulfonylurea receptor gene in familial persistent hyperinsulinemic hypoglycemia of infancy. Science 1995;268:426-429.(Fiona M. Gribble, B.M., B)