Evidence for direct physical association between a K+ channel (Kir6.2) and an ATP-binding cassette protein (SUR1) which affects cellular distribution and kinetic behavior of an ATP-sensitive K+ channel

Expression, purification, and electrophysiological characterization of a recombinant, fluorescent Kir6.2 in mammalian cells

The inwardly rectifying K⁺ (Kir) channel, Kir6.2, plays critical roles in physiological processes in the brain, heart, and pancreas. Although Kir6.2 has been extensively studied in numerous expression systems, a comprehensive description of an expression and purification protocol has not been reported. We expressed and characterized a recombinant Kir6.2, with an N-terminal decahistidine tag, enhanced green fluorescent protein (eGFP) and deletion of C-terminal 26 amino acids, in succession, denoted eGFP-Kir6.2Δ26. eGFP-Kir6.2Δ26 was expressed in HEK293 cells and a purification protocol developed. Electrophysiological characterization showed that eGFP-Kir6.2Δ26 retains native single channel conductance (64 ± 3.3 pS), mean open times (τ₁ = 0.72 ms, τ₂ = 15.3 ms) and ATP affinity (IC₅₀ = 115 ± 25 μM) when expressed in HEK293 cells. Detergent screening using size exclusion chromatography (SEC) identified Fos-choline-14 (FC-14) as the most suitable surfactant for protein solubilization, as evidenced by maintenance of the native tetrameric structure in SDS-PAGE and western blot analysis. A two-step scheme using Co²⁺-metal affinity chromatography and SEC was implemented for purification. Purified protein activity was assessed by reconstituting eGFP-Kir6.2Δ26 in black lipid membranes (BLMs) composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG), l-α-phosphatidylinositol-4,5-bisphosphate (PIP₂) in a 89.5:10:0.5 mol ratio. Reconstituted eGFP-Kir6.2Δ26 displayed similar single channel conductance (61.8 ± 0.54 pS) compared to eGFP-Kir6.2Δ26 expressed in HEK293 membranes; however, channel mean open times increased (τ₁ = 7.9 ms, τ₂ = 61.9 ms) and ATP inhibition was significantly reduced for eGFP-Kir6.2Δ26 reconstituted into BLMs (IC₅₀ = 3.14 ± 0.4 mM). Overall, this protocol should be foundational for the production of purified Kir6.2 for future structural and biochemical studies.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

ATP-dependent potassium channels and type 2 diabetes mellitus

Diabetes mellitus is a public health problem, which affects a millions worldwide. Most diabetes cases are classified as type 2 diabetes mellitus, which is highly associated with obesity. Type 2 diabetes is considered a multifactorial disorder, with both environmental and genetic factors contributing to its development. An important issue linked with diabetes development is the failure of the insulin releasing mechanism involving abnormal activity of the ATP-dependent potassium channel, KATP. This channel is a transmembrane protein encoded by the KCNJ11 and ABCC8 genes. Furthermore, polymorphisms in these genes have been linked to type 2 diabetes because of the role of KATP in insulin release. While several genetic variations have been reported to be associated with this disease, the E23K polymorphism is most commonly associated with this pathology, as well as to obesity. Here, we review the molecular genetics of the potassium channel and discusses its most described polymorphisms and their associations with type 2 diabetes mellitus.

Insulinoma in a patient with chronic renal failure due to type 2 diabetes mellitus treated effectively with diazoxide

A 63-year-old man was diagnosed with diabetes mellitus at 42 years of age. He subsequently exhibited poor blood glucose control for a prolonged period, and his renal failure worsened. He therefore underwent hemodialysis and abdominal magnetic resonance imaging, which revealed a mass in the pancreatic tail. The immunoreactive insulin and C-peptide immunoreactivity levels were significantly elevated, and the results of a fasting test led to a diagnosis of insulinoma. The patient received treatment with oral diazoxide and continuous glucose monitoring (CGM), which resulted in the resolution of the hypoglycemia. This is a rare case of renal failure in which the CGM findings showed improvements in the blood glucose level after diazoxide administration.

The reprogrammed pancreatic progenitor-like intermediate state of hepatic cells is more susceptible to pancreatic beta cell differentiation

Induced pluripotent stem cells (iPSCs) hold great promise for cell therapy. However, their low efficiency of lineage-specific differentiation and tumorigenesis severely hinder clinical translation. We hypothesized that reprogramming of somatic cells into lineage-specific progenitor cells might allow for large-scale expansion, avoiding the tumorigenesis inherent with iPSCs and simultaneously facilitating lineage-specific differentiation. Here we aimed at reprogramming rat hepatic WB cells, using four Yamanaka factors, into pancreatic progenitor cells (PPCs) or intermediate (IM) cells that have characteristics of PPCs. IM clones were selected based on their specific morphology and alkaline phosphatase activity and stably passaged under defined culture conditions. IM cells did not have iPSC properties, could be stably expanded in large quantity, and expressed all 14 genes that are used to define the PPC developmental stage. Directed differentiation of IM and WB cells by Pdx1-Ngn3-MafA (PNM) into pancreatic beta-like cells revealed that the IM cells are more susceptible to directed beta cell differentiation because of their open chromatin configuration, as demonstrated by expression of key pancreatic beta cell genes, secretion of insulin in response to glucose stimulation, and easy access to exogenous PNM proteins at the rat insulin 1 and Pdx1 promoters. This notion that IM cells are superior to their parental cells is further supported by the epigenetic demonstration of accessibility of Pdx1 and insulin 1 promoters. In conclusion, we have developed a strategy to derive and expand PPC cells from hepatic WB cells using conventional cell reprogramming. This proof-of-principal study may offer a novel, safe and effective way to generate autologous pancreatic beta cells for cell therapy of diabetes.

Forced gating motions by a substituted titratable side chain at the bundle crossing of a potassium channel

Numerous inwardly rectifying potassium (Kir) channels possess an aromatic residue in the helix bundle crossing region, forming the narrowest pore constriction in crystal structures. However, the role of the Kir channel bundle crossing as a functional gate remains uncertain. We report a unique phenotype of Kir6.2 channels mutated to encode glutamate at this position (F168E). Despite a prediction of four glutamates in close proximity, Kir6.2(F168E) channels are predominantly closed at physiological pH, whereas alkalization causes rapid and reversible channel activation. These findings suggest that F168E glutamates are uncharged at physiological pH but become deprotonated at alkaline pH, forcing channel opening due to mutual repulsion of nearby negatively charged side chains. The potassium channel pore scaffold likely brings these glutamates close together, causing a significant pK(a) shift relative to the free side chain (as seen in the KcsA selectivity filter). Alkalization also shifts the apparent ATP sensitivity of the channel, indicating that forced motion of the bundle crossing is coupled to the ATP-binding site and may resemble conformational changes involved in wild-type Kir6.2 gating. The study demonstrates a novel mechanism for engineering extrinsic control of channel gating by pH and shows that conformational changes in the bundle crossing region are involved in ligand-dependent gating of Kir channels.

Voltage-dependent gating in a "voltage sensor-less" ion channel

The voltage sensitivity of voltage-gated cation channels is primarily attributed to conformational changes of a four transmembrane segment voltage-sensing domain, conserved across many levels of biological complexity. We have identified a remarkable point mutation that confers significant voltage dependence to Kir6.2, a ligand-gated channel that lacks any canonical voltage-sensing domain. Similar to voltage-dependent Kv channels, the Kir6.2[L157E] mutant exhibits time-dependent activation upon membrane depolarization, resulting in an outwardly rectifying current-voltage relationship. This voltage dependence is convergent with the intrinsic ligand-dependent gating mechanisms of Kir6.2, since increasing the membrane PIP2 content saturates Po and eliminates voltage dependence, whereas voltage activation is more dramatic when channel Po is reduced by application of ATP or poly-lysine. These experiments thus demonstrate an inherent voltage dependence of gating in a "ligand-gated" K+ channel, and thereby provide a new view of voltage-dependent gating mechanisms in ion channels. Most interestingly, the voltage- and ligand-dependent gating of Kir6.2[L157E] is highly sensitive to intracellular [K+], indicating an interaction between ion permeation and gating. While these two key features of channel function are classically dealt with separately, the results provide a framework for understanding their interaction, which is likely to be a general, if latent, feature of the superfamily of cation channels.

CDK4, pRB and E2F1: connected to insulin

Pancreatic beta-cells are metabolic sensors involved in the control of glucose homeostasis. This particular cell type controls insulin secretion through a fine-tuned process, which dregulation have important pathological consequences, such as observed during type 2 diabetes. We recently implicated E2F1 in the control of glucose homeostasis. First we showed that E2f1-/- mice have decreased pancreatic size, as the result of impaired postnatal pancreatic growth. We observed in this study that E2F1 was highly expressed in non-proliferating pancreatic beta-cells, suggesting that E2F1, besides the control of beta-cell number could have a role in pancreatic beta-cell function. We demonstrate in our recent study, both in vitro and in vivo that E2F1 directly regulates the expression of Kir6.2, a key component of the KATP channel involved in the regulation of glucose-induced insulin secretion in pancreatic beta-cells. Expression of Kir6.2 is lost in pancreas of E2f1-/- mice, resulting in insulin secretion defects in these mice. Furthermore, we demonstrated by in tissue chromatin immunoprecipitation analysis that regulation of Kir6.2 expression by E2F1 follows the same regulatory pathway that the classical E2F1 target genes, implicating the participation of CDK4 and retinoblastoma protein. Moreover, in this context, E2F1 transcriptional activity is regulated by glucose and insulin through the CDK4-dependent inactivation of the pRB protein. In summary we provide evidence that the CDK4-pRB-E2F1 regulatory pathway is involved in glucose homeostasis. In our recent study we decipher a new function for these factors in the control of insulin secretion and open up new avenues for the treatment of metabolic diseases, in particular type 2 diabetes.

The CDK4-pRB-E2F1 pathway controls insulin secretion

CDK4-pRB-E2F1 cell-cycle regulators are robustly expressed in non-proliferating beta cells, suggesting that besides the control of beta-cell number the CDK4-pRB-E2F1 pathway has a role in beta-cell function. We show here that E2F1 directly regulates expression of Kir6.2, which is a key component of the K(ATP) channel involved in the regulation of glucose-induced insulin secretion. We demonstrate, through chromatin immunoprecipitation analysis from tissues, that Kir6.2 expression is regulated at the promoter level by the CDK4-pRB-E2F1 pathway. Consistently, inhibition of CDK4, or genetic inactivation of E2F1, results in decreased expression of Kir6.2, impaired insulin secretion and glucose intolerance in mice. Furthermore we show that rescue of Kir6.2 expression restores insulin secretion in E2f1(-/-) beta cells. Finally, we demonstrate that CDK4 is activated by glucose through the insulin pathway, ultimately resulting in E2F1 activation and, consequently, increased expression of Kir6.2. In summary we provide evidence that the CDK4-pRB-E2F1 regulatory pathway is involved in glucose homeostasis, defining a new link between cell proliferation and metabolism.

Review. SUR1: a unique ATP-binding cassette protein that functions as an ion channel regulator

SUR1 is an ATP-binding cassette (ABC) transporter with a novel function. In contrast to other ABC proteins, it serves as the regulatory subunit of an ion channel. The ATP-sensitive (KATP) channel is an octameric complex of four pore-forming Kir6.2 subunits and four regulatory SUR1 subunits, and it links cell metabolism to electrical activity in many cell types. ATPase activity at the nucleotide-binding domains of SUR results in an increase in KATP channel open probability. Conversely, ATP binding to Kir6.2 closes the channel. Metabolic regulation is achieved by the balance between these two opposing effects. Precisely how SUR1 talks to Kir6.2 remains unclear, but recent studies have identified some residues and domains that are involved in both physical and functional interactions between the two proteins. The importance of these interactions is exemplified by the fact that impaired regulation of Kir6.2 by SUR1 results in human disease, with loss-of-function SUR1 mutations causing congenital hyperinsulinism and gain-of-function SUR1 mutations leading to neonatal diabetes. This paper reviews recent data on the regulation of Kir6.2 by SUR1 and considers the molecular mechanisms by which SUR1 mutations produce disease.

Effects of sodium azide, barium ion, d-amphetamine and procaine on inward rectifying potassium channel 6.2 expressed in Xenopus oocytes

BACKGROUND/PURPOSE

Inward rectifying potassium channel 6.2 (Kir6.2DelataC26 channel) is closely related to ATP-sensitive potassium channels. Whether sodium azide, barium ion, d-amphetamine or procaine acts directly on the Kir6.2DeltaC26 channel remains unclear. We studied the effects of these compounds on Kir6.2DeltaC26 channel expressed in Xenopus oocytes.

METHODS

The coding sequence of a truncated form of mouse Kir6.2 (GenBank accession number NP_034732.1), Kir6.2(1-364) (i.e. Kir6.2DeltaC26), was subcloned into the pET20b(+) vector. Plasmid containing the correct T7 promoter-Kir6.2(1-364) cDNA fragment [Kir6.2/pET20b(+)] was then subject to NotI digestion to generate the templates for in vitro run-off transcriptions. The channel was expressed in Xenopus laevis oocytes. Two-electrode voltage clamping was used to measure the effects of sodium azide, barium ion, d-amphetamine and procaine on Kir6.2DeltaC26 channel current.

RESULTS

Sodium azide activated and barium ion and d-amphetamine inhibited the Kir6.2DeltaC26 channel. Procaine did not have any significant effect on the Kir6.2DeltaC26 channel.

CONCLUSION

Kir6.2DeltaC26 channel expressed in Xenopus oocytes can be used as a pharmacological tool for the study of inward rectifying potassium channels.

A rare case of adult-onset nesidioblastosis treated successfully with diazoxide

A 54-year-old man was admitted to our hospital for evaluation of hypoglycemia. He had frequent episodes of loss of concentration before dinner. The ratio of IRI to plasma glucose (PG) was 0.8-1.0. Abdominal CT revealed no pancreatic tumor, and angiography of splenic artery showed no definite tumor stain within the pancreas. Based on the results of selective arterial calcium stimulation and hepatic venous sampling (ASVS), the provisional diagnosis was a small insulinoma in the pancreatic body. The patient underwent subtotal distal pancreatectomy. However, histopathological and immunohistochemical examinations of the resected tissue showed hypertrophy of islets of Langerhans islands and beta cells around pancreatic ducts. The final diagnosis was adult-onset nesidioblastosis. Postoperatively, the patient continued to exhibit hyperinsulinemia and nighttime hypoglycemia. Octreotide, voglibose and diet therapies failed to improve the nocturnal hypoglycemia. However, treatment with diazoxide at a starting dose of 200 mg/day resulted in immediate amelioration of nocturnal hypoglycemia. This is the first Japanese adult case of nesidioblastosis treated successfully with diazoxide. This case report suggests that diazoxide may be effective for adult-onset nesidioblastosis in a manner similar to that described for pediatric cases.

Ligand-dependent linkage of the ATP site to inhibition gate closure in the KATP channel

Major advances have been made on the inhibition gate and ATP site of the K(ir)6.2 subunit of the K(ATP) channel, but little is known about conformational coupling between the two. ATP site mutations dramatically disrupt ATP-dependent gating without effect on ligand-independent gating, observed as interconversions between active burst and inactive interburst conformations in the absence of ATP. This suggests that linkage between site and gate is conditionally dependent on ATP occupancy. We studied all substitutions at position 334 of the ATP site in K(ir)6.2deltaC26 that express in Xenopus oocytes. All substitutions disrupted ATP-dependent gating by 10-fold or more. Only positive-charged arginine or lysine at 334, however, slowed ligand-independent gating from the burst, and this was in some but not all patches. Moreover, the polycationic peptide protamine reversed the slowed gating from the burst of 334R mutant channels, and speeded the slow gating from the burst of wild-type SUR1/K(ir)6.2 in the absence of ATP. Our results support a two-step ligand-dependent linkage mechanism for K(ir)6.2 channels in which ATP-occupied sites function to electrostatically dissociate COOH-terminal domains from the membrane, then as in all K(ir) channels, free COOH-terminal domains and inner M2 helices transit to a lower energy state for gate closure.

Activation of KATP channels by H2S in rat insulin-secreting cells and the underlying mechanisms

H2S is an important gasotransmitter, generated in mammalian cells from L-cysteine metabolism. As it stimulates K(ATP) channels in vascular smooth muscle cells, H2S may also function as an endogenous opener of K(ATP) channels in INS-1E cells, an insulin-secreting cell line. In the present study, K(ATP) channel currents in INS-1E cells were recorded using the whole-cell and single-channel recording configurations of the patch-clamp technique. K(ATP) channels in INS-1E cells have a single-channel conductance of 78 pS. These channels were activated by diazoxide and inhibited by gliclazide. ATP (3 mm) in the pipette solution inhibited K(ATP) channels in INS-1E cells. Significant amount of H2S was produced from INS-1E cells in which the expression of cystathinonie gamma-lyase (CSE) was confirmed. After INS-1E cells were transfected with CSE-targeted short interfering RNA (CSE-siRNA) or treated with DL-propargylglycine (PPG; 1-5 mm) to inhibit CSE, endogenous production of H2S was abolished. Increase in extracellular glucose concentration significantly decreased endogenous production of H2S in INS-1E cells, and increased insulin secretion. After transfection of INS-1E cells with adenovirus containing the CSE gene (Ad-CSE) to overexpress CSE, high glucose-stimulated insulin secretion was virtually abolished. Basal K(ATP) channel currents were significantly reduced after incubating INS-1E cells with a high glucose concentration (16 mm) or lowering endogenous H2S level by CSE-siRNA transfection. Under these conditions, exogenously applied H2S significantly increased whole-cell K(ATP) channel currents at concentrations equal to or lower than 100 microm. H2S (100 microm) markedly increased open probability by more than 2-fold of single K(ATP) channels (inside-out recording) in native INS-1E cells (n = 4, P < 0.05). Single-channel conductance and ATP sensitivity of K(ATP) channels were not changed by H2S. In conclusion, endogenous H2S production from INS-1E cells varies with in vivo conditions, which significantly affects insulin secretion from INS-1E cells. H2S stimulates K(ATP) channels in INS-1E cells, independent of activation of cytosolic second messengers, which may underlie H2S-inhibited insulin secretion from these cells. Interaction among H2S, glucose and the K(ATP) channel may constitute an important and novel mechanism for the fine control of insulin secretion from pancreatic beta-cells.

Insulin receptor (IR) and glucose transporter 2 (GLUT2) proteins form a complex on the rat hepatocyte membrane

The hepatic glucose transporter, GLUT2, facilitates bidirectional glucose transport across the hepatocyte plasma membrane under insulin regulation. We studied the interactions of IR and GLUT2 proteins to determine whether they are physically coupled in a receptor-transporter complex. By comparing endosome and plasma membrane IR and GLUT2 ratios before and after feeding, it was determined that IR and GLUT2 are internalized in a fixed ratio. When solubilized hepatocytes were immunoprecipitated with antibodies against either IR or GLUT2, both proteins co-precipitated. The association of IR and GLUT2 was further assessed by confocal microscopy. Sections of fed liver were incubated with fluorescein-tagged anti-GLUT2 or Texas Red-tagged anti-IR. Colocalization was observed both at the plasma membrane and in the cytosol. Fluorescence-resonance energy transfer studies further confirmed this association. We conclude that IR and GLUT2 form a receptor-transporter complex in hepatocytes, which forms a mechanism of insulin-mediated hepatic glucose regulation.

Modeling regulation of cardiac KATP and L-type Ca2+ currents by ATP, ADP, and Mg2+

Changes in cytosolic free Mg(2+) and adenosine nucleotide phosphates affect cardiac excitability and contractility. To investigate how modulation by Mg(2+), ATP, and ADP of K(ATP) and L-type Ca(2+) channels influences excitation-contraction coupling, we incorporated equations for intracellular ATP and MgADP regulation of the K(ATP) current and MgATP regulation of the L-type Ca(2+) current in an ionic-metabolic model of the canine ventricular myocyte. The new model: 1), quantitatively reproduces a dose-response relationship for the effects of changes in ATP on K(ATP) current, 2), simulates effects of ADP in modulating ATP sensitivity of K(ATP) channel, 3), predicts activation of Ca(2+) current during rapid increase in MgATP, and 4), demonstrates that decreased ATP/ADP ratio with normal total Mg(2+) or increased free Mg(2+) with normal ATP and ADP activate K(ATP) current, shorten action potential, and alter ionic currents and intracellular Ca(2+) signals. The model predictions are in agreement with experimental data measured under normal and a variety of pathological conditions.

Cardiac KATP channels in health and disease

ATP-sensitive potassium (K(ATP)) channels are evolutionarily conserved plasma-membrane protein complexes, widely represented in tissue beds with high metabolic activity. There, they are formed through physical association of the inwardly rectifying potassium channel pore, most typically Kir6.2, and the regulatory sulfonylurea receptor subunit, an ATP-binding cassette protein. Energetic signals, received via tight integration with cellular metabolic pathways, are processed by the sulfonylurea receptor subunit that in turn gates the nucleotide sensitivity of the channel pore thereby controlling membrane potential dependent cellular functions. Recent findings, elicited from genetic disruption of channel proteins, have established in vivo the requirement of intact K(ATP) channels in the proper function of cardiac muscle under stress. In the heart, where K(ATP) channels were originally discovered, channel ablation compromises cardioprotection under ischemic insult. New data implicate the requirement of intact K(ATP) channels for the cardiac adaptive response to acute stress. K(ATP) channels have been further implicated in the adaptive cardiac response to chronic (patho)physiologic hemodynamic load, with K(ATP) channel deficiency affecting structural remodeling, rendering the heart vulnerable to calcium-dependent maladaptation and predisposing to heart failure. These findings are underscored by the identification in humans that defective K(ATP) channels induced by mutations in ABCC9, the gene encoding the cardiac sulfonylurea receptor subunit, confer susceptibility to dilated cardiomyopathy. Thus, in parallel with the developed understanding of the molecular identity and mode of action of K(ATP) channels since their discovery, there is now an expanded understanding of their critical significance in the cardiac stress response in health and disease.

ATP-sensitive K+ channel channel/enzyme multimer: metabolic gating in the heart

Cardiac ATP-sensitive K(+) (K(ATP)) channels, gated by cellular metabolism, are formed by association of the inwardly rectifying potassium channel Kir6.2, the potassium conducting subunit, and SUR2A, the ATP-binding cassette protein that serves as the regulatory subunit. Kir6.2 is the principal site of ATP-induced channel inhibition, while SUR2A regulates K(+) flux through adenine nucleotide binding and catalysis. The ATPase-driven conformations within the regulatory SUR2A subunit of the K(ATP) channel complex have determinate linkage with the states of the channel's pore. The probability and life-time of ATPase-induced SUR2A intermediates, rather than competitive nucleotide binding alone, defines nucleotide-dependent K(ATP) channel gating. Cooperative interaction, instead of independent contribution of individual nucleotide binding domains within the SUR2A subunit, serves a decisive role in defining K(ATP) channel behavior. Integration of K(ATP) channels with the cellular energetic network renders these channel/enzyme heteromultimers high-fidelity metabolic sensors. This vital function is facilitated through phosphotransfer enzyme-mediated transmission of controllable energetic signals. By virtue of coupling with cellular energetic networks and the ability to decode metabolic signals, K(ATP) channels set membrane excitability to match demand for homeostatic maintenance. This new paradigm in the operation of an ion channel multimer is essential in providing the basis for K(ATP) channel function in the cardiac cell, and for understanding genetic defects associated with life-threatening diseases that result from the inability of the channel complex to optimally fulfill its physiological role.

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

ATP-sensitive K+ channel knockout compromises the metabolic benefit of exercise training, resulting in cardiac deficits

Exercise training elicits a metabolic and cardiovascular response that underlies fitness. The molecular mechanisms that orchestrate this adaptive response and secure the wide-ranging gains of a regimented exercise program are poorly understood. Formed through association of the Kir6.2 pore and the sulfonylurea receptor, the stress-responsive ATP-sensitive K(+) channels (K(ATP) channels), with their metabolic-sensing capability and broad tissue expression, are potential candidates for integrating the systemic adaptive response to repetitive exercise. Here, the responses of mice lacking functional Kir6.2-containing K(ATP) channels (Kir6.2-KO) were compared with wild-type controls following a 28-day endurance swimming protocol. While chronic aquatic training resulted in lighter, leaner, and fitter wild-type animals, the Kir6.2-KO manifested less augmentation in exercise capacity and lacked metabolic improvement in body fat composition and glycemic handling with myocellular defects. Moreover, the repetitive stress of swimming unmasked a survival disadvantage in the Kir6.2-KO, associated with pathologic calcium-dependent structural damage in the heart and impaired cardiac performance. Thus, Kir6.2-containing K(ATP) channel activity is required for attainment of the physiologic benefits of exercise training without injury.

The gasotransmitter role of hydrogen sulfide

A novel concept of "gasotransmitter" arrived recently. Gasotransmitters are small molecules of endogenous gases with important physiological functions. Their production and metabolism are enzymatically regulated, and their effects are not dependent on specific membrane receptors. Following the identification of nitric oxide and carbon monoxide as gasotransmitters, hydrogen sulfide (H(2)S) may be qualified as the third gasotransmitter. Recent studies have shown that H(2)S is generated from vascular smooth muscle cells (SMCs), catalyzed by specific H(2)S-generating enzyme. At physiologically relevant concentrations, H(2)S relaxes vascular tissues, an effect mediated by the activation of ATP-sensitive K(+) (K(ATP)) channels in vascular SMCs. H(2)S directly alters the activity of K(ATP) channels without the involvement of second messengers. Furthermore, the endogenous production of H(2)S in the cardiovascular system is likely regulated by nitric oxide, whereas the vasorelaxant effect of nitric oxide is inhibited by H(2)S. It is anticipated that future studies will better reveal the molecular mechanisms underlying the effect of H(2)S on K(ATP) channel proteins, the interaction of H(2)S and other gasotransmitters in cardiovascular system, the endogenous stimulators and inhibitors of H(2)S metabolism, the role of H(2)S in the regulation of heart function, and the abnormal H(2)S production and action under various pathophysiological conditions.

TRPC5 is a regulator of hippocampal neurite length and growth cone morphology

Growth cone motility is regulated by both fast voltage-dependent Ca2+ channels and by unknown receptor-operated Ca2+ entry mechanisms. Transient receptor potential (TRP) homomeric TRPC5 ion channels are receptor-operated, Ca2+-permeable channels predominantly expressed in the brain. Here we show that TRPC5 is expressed in growth cones of young rat hippocampal neurons. Our results indicate that TRPC5 channel subunits interact with the growth cone-enriched protein stathmin 2, are packaged into vesicles and are carried to newly forming growth cones and synapses. Once in the growth cone, TRPC5 channels regulate neurite extension and growth-cone morphology. Dominant-negative TRPC5 expression allowed significantly longer neurites and filopodia to form. We conclude that TRPC5 channels are important components of the mechanism controlling neurite extension and growth cone morphology.

N-terminal transmembrane domain of the SUR controls trafficking and gating of Kir6 channel subunits

The sulfonylurea receptor (SUR), an ATP-binding cassette (ABC) protein, assembles with a potassium channel subunit (Kir6) to form the ATP-sensitive potassium channel (K(ATP)) complex. Although SUR is an important regulator of Kir6, the specific SUR domain that associates with Kir6 is still unknown. All functional ABC proteins contain two transmembrane domains but some, including SUR and MRP1 (multidrug resistance protein 1), contain an extra N-terminal transmembrane domain called TMD0. The functions of any TMD0s are largely unclear. Using Xenopus oocytes to coexpress truncated SUR constructs with Kir6, we demonstrated by immunoprecipitation, single-oocyte chemiluminescence and electrophysiological measurements that the TMD0 of SUR1 strongly associated with Kir6.2 and modulated its trafficking and gating. Two TMD0 mutations, A116P and V187D, previously correlated with persistent hyperinsulinemic hypoglycemia of infancy, were found to disrupt the association between TMD0 and Kir6.2. These results underscore the importance of TMD0 in K(ATP) channel function, explaining how specific mutations within this domain result in disease, and suggest how an ABC protein has evolved to regulate a potassium channel.

Hydroxyl-substituted sulfonylureas as potent inhibitors of specific [3H]glyburide binding to rat brain synaptosomes

We are seeking to discover potent CNS-active sulfonylureas with structural features that allow for the formation of several types of prodrugs. We report herein the syntheses of compounds comprising an initial series of hydroxyl-substituted analogues of the potent ATP-sensitive potassium channel blockers glyburide (glibenclamide) and gliquidone. Somewhat unexpectedly, several of the compounds were found to be comparably potent to glyburide as inhibitors of specific [(3)H]glyburide binding in rat brain preparations.

K(+) channels as therapeutic drug targets

K(+) channels play critical roles in a wide variety of physiological processes, including the regulation of heart rate, muscle contraction, neurotransmitter release, neuronal excitability, insulin secretion, epithelial electrolyte transport, cell volume regulation, and cell proliferation. As such, K(+) channels have been recognized as potential therapeutic drug targets for many years. Unfortunately, progress toward identifying selective K(+) channel modulators has been severely hampered by the need to use native currents and primary cells in the drug-screening process. Today, however, more than 80 K(+) channel and K(+) channel-related genes have been identified, and an understanding of the molecular composition of many important native K(+) currents has begun to emerge. The identification of these molecular K(+) channel drug targets should lead to the discovery of novel drug candidates. A summary of progress is presented.

Ageing is associated with a decrease in the number of sarcolemmal ATP-sensitive K+ channels in a gender-dependent manner

The opening of sarcolemmal K(ATP) channels is considered to be an important endogenous cardioprotective mechanism. On the other hand, age-dependent changes in the myocardial susceptibility to ischemia and hypoxia have been observed in different species, including humans. Here, we have hypothesized that aging might be associated with the changes in sarcolemmal K(ATP) channels. Therefore, the main objective of the present study was to establish whether aging changes expression of cardiac sarcolemmal ATP-sensitive K+ (K(ATP)) channels. RT-PCR using primers specific for K(ATP) channel subunits, Kir6.2, Kir6.1 and SUR2A subunits was performed using total RNA from guinea-pig ventricular tissue. Whole cell electrophysiology was done on isolated guinea-pig ventricular cardiomyocytes. Western blotting using anti-Kir6.2 and anti-SUR2A antibodies was performed on cardiac membrane fraction. Tissue and cells were harvested from young and old, male and female guinea-pigs. RT-PCR analysis did not reveal significant age-related changes in levels of Kir6.1 or Kir6.2 mRNAs. However, levels of SUR2A were significantly lower in old than in young females. Such age-differences were not observed with cardiac tissue from male animals. In both old and young males, pinacidil (100 microM) induced outward currents. The difference between current density of pinacidil-sensitive component in females, but not males, was statistically significant. Western blotting analysis revealed higher levels of Kir6.2 and SUR2A proteins in cardiac membrane fraction from young than old females. The present study demonstrates that in females, but not males, aging is associated with decrease in number of cardiac K(ATP) channels which is due to decrease in levels of the SUR2A subunit.

Creatine kinase is physically associated with the cardiac ATP-sensitive K+ channel in vivo

Cardiac sarcolemmal ATP-sensitive K+ (KATP) channels, composed of Kir6.2 and SUR2A subunits, couple the metabolic status of cells with the membrane excitability. Based on previous functional studies, we have hypothesized that creatine kinase (CK) may be a part of the sarcolemmal KATP channel protein complex. The inside-out and whole cell patch clamp electrophysiology applied on guinea pig cardiomyocytes showed that substrates of CK regulate KATP channels activity. Following immunoprecipitation of guinea-pig cardiac membrane fraction with the anti-SUR2 antibody, Coomassie blue staining revealed, besides Kir6.2 and SUR2A, a polypeptide at approximately 48 kDa. Western blotting analysis confirmed the nature of putative Kir6.2 and SUR2A, whereas matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis identified p48 kDa as a muscle form of CK. In addition, the CK activity was found in the anti-SUR2A immunoprecipitate and the cross reactivity between an anti-CK antibody and the anti-SUR2A immunoprecipitate was observed as well as vice verse. Further results obtained at the level of recombinant channel subunits demonstrated that CK is directly physically associated with the SUR2A, but not the Kir6.2, subunit. All together, these results suggest that the CK is associated with SUR2A subunit in vivo, which is an integral part of the sarcolemmal KATP channel protein complex.

Gender-specific difference in cardiac ATP-sensitive K(+) channels

OBJECTIVES

The main objective of this study was to establish whether gender regulates expression and/or properties of cardiac ATP-sensitive K(+) (K(ATP)) channels.

BACKGROUND

Recently, evidence has been provided that differing cardiac responses in males and females to metabolic stress may result from gender-specific difference(s) in the efficiency of endogenous cardioprotective mechanism(s) such as K(ATP) channels.

METHODS

A reverse transcription polymerase chain reaction (RT-PCR) using primers specific for Kir6.2, Kir6.1 and SUR2A subunits was performed on total RNA from guinea pig ventricular tissue. Western blotting using anti-Kir6.2 and anti-SUR2A antibodies was performed on cardiac membrane fraction. Whole-cell, single-channel electrophysiology and digital epifluorescent Ca(2+) imaging were performed on isolated guinea pig ventricular cardiomyocytes.

RESULTS

The RT-PCR revealed higher levels of SUR2A, but not Kir6.1 and Kir6.2, messenger RNA in female tissue relative to male tissue, while much higher levels of both Kir6.2 and SUR2A proteins in cardiac membrane fraction in female tissue compared with male tissue were found. In both male and female tissue, pinacidil (100 microM), a K(ATP) channel opener, induced outward whole-cell currents. The current density of the pinacidil-sensitive component was significantly higher in female tissue than it was in male tissue, while no differences in single K(ATP) channel properties between genders were observed. Ischemia-reperfusion challenge induced significant intracellular Ca(2+) loading in male, but not female, cardiomyocytes. To test the hypothesis that SUR2A expression is the limiting factor in K(ATP) channel formation, we took different volumes of Kir6.2 and SUR2A complementary DNA (cDNA) from the same cDNA pool and subjected them to PCR. In order to obtain a band having 50% of the maximal intensity, a volume of SUR2a cDNA approximately 20 times the volume of Kir6.2 cDNA was required.

CONCLUSIONS

This study has demonstrated that female tissue expresses higher levels of functional cardiac K(ATP) channels than male tissue due to the higher expression of the SUR2A subunit, which has an impact on cardiac response to ischemia-reperfusion challenge.

Reciprocal regulation of expression of pore-forming KATP channel genes by hypoxia

The ATP-sensitive potassium (KATP) channel is thought to play an important role in the protection of heart and brain against tissue hypoxia. The genetic regulation of the components of the channel by hypoxia has not been previously described. Here, we investigated the regulation of the two pore-forming channel proteins, Kir6.1 and Kir6.2, in response to hypoxia in vivo and in vitro. We find that these two structurally-related inwardly-rectifying potassium channel proteins are reciprocally regulated by hypoxia in vivo, with upregulation of Kir6.1 and down-regulation of Kir6.2, thereby resulting in a significant change in the composition of the channel complex in response to hypoxia. In vitro we describe neuronal and cardiac cell lines in which Kir6.1 is up-regulated by hypoxia, demonstrating that Kir6.1 is a hypoxia-inducible gene. We conclude that the heart and brain display genetic plasticity in response to hypoxic stress through specific genetic reprograming of cytoprotective channel genes.

Signaling in channel/enzyme multimers: ATPase transitions in SUR module gate ATP-sensitive K+ conductance

ATP-sensitive potassium (K(ATP)) channels are bifunctional multimers assembled by an ion conductor and a sulfonylurea receptor (SUR) ATPase. Sensitive to ATP/ADP, K(ATP) channels are vital metabolic sensors. However, channel regulation by competitive ATP/ADP binding would require oscillations in intracellular nucleotides incompatible with cell survival. We found that channel behavior is determined by the ATPase-driven engagement of SUR into discrete conformations. Capture of the SUR catalytic cycle in prehydrolytic states facilitated pore closure, while recruitment of posthydrolytic intermediates translated in pore opening. In the cell, channel openers stabilized posthydrolytic states promoting K(ATP) channel activation. Nucleotide exchange between intrinsic ATPase and ATP/ADP-scavenging systems defined the lifetimes of specific SUR conformations gating K(ATP) channels. Signal transduction through the catalytic module provides a paradigm for channel/enzyme operation and integrates membrane excitability with metabolic cascades.

Adenylate kinase phosphotransfer communicates cellular energetic signals to ATP-sensitive potassium channels

Transduction of energetic signals into membrane electrical events governs vital cellular functions, ranging from hormone secretion and cytoprotection to appetite control and hair growth. Central to the regulation of such diverse cellular processes are the metabolism sensing ATP-sensitive K+ (K(ATP)) channels. However, the mechanism that communicates metabolic signals and integrates cellular energetics with K(ATP) channel-dependent membrane excitability remains elusive. Here, we identify that the response of K(ATP) channels to metabolic challenge is regulated by adenylate kinase phosphotransfer. Adenylate kinase associates with the K(ATP) channel complex, anchoring cellular phosphotransfer networks and facilitating delivery of mitochondrial signals to the membrane environment. Deletion of the adenylate kinase gene compromised nucleotide exchange at the channel site and impeded communication between mitochondria and K(ATP) channels, rendering cellular metabolic sensing defective. Assigning a signal processing role to adenylate kinase identifies a phosphorelay mechanism essential for efficient coupling of cellular energetics with K(ATP) channels and associated functions.

Molecular structure of the glibenclamide binding site of the beta-cell K(ATP) channel

We have investigated the structure of the glibenclamide binding site of pancreatic beta-cell ATP-sensitive potassium (K(ATP)) channels. K(ATP) channels are a complex of four pore-forming Kir6.2 subunits and four sulfonylurea receptor (SUR1) subunits. SUR1 (ABCC8) belongs to the ATP binding cassette family of proteins and has two nucleotide binding domains (NBD1 and NBD2) and 17 putative transmembrane (TM) sequences. Co-expression in a baculovirus expression system of two parts of SUR1 between NBD1 and TM12 leads to restoration of glibenclamide binding activity, whereas expression of either individual N- or C-terminal part alone gave no glibenclamide binding activity, confirming a bivalent structure of the glibenclamide binding site. By using N-terminally truncated recombinant proteins we have shown that CL3 - the cytosolic loop between TM5 and TM6 - plays a key role in formation of the N-terminal component of the glibenclamide binding site. Analysis of deletion variants of the C-terminal part of SUR1 showed that CL8 - the cytosolic loop between TM15 and TM16 - is the only determinant for the C-terminal component of the glibenclamide binding site. We suggest that in SUR1 in the native K(ATP) channel close proximity of CL3 and CL8 leads to formation of the glibenclamide binding site.

TRPC1 and TRPC5 form a novel cation channel in mammalian brain

TRP proteins are cation channels responding to receptor-dependent activation of phospholipase C. Mammalian (TRPC) channels can form hetero-oligomeric channels in vitro, but native TRPC channel complexes have not been identified to date. We demonstrate here that TRPC1 and TRPC5 are subunits of a heteromeric neuronal channel. Both TRPC proteins have overlapping distributions in the hippocampus. Coexpression of TRPC1 and TRPC5 in HEK293 cells resulted in a novel nonselective cation channel with a voltage dependence similar to NMDA receptor channels, but unlike that of any reported TRPC channel. TRPC1/TRPC5 heteromers were activated by G(q)-coupled receptors but not by depletion of intracellular Ca(2+) stores. In contrast to the more common view of the TRP family as comprising store-operated channels, we propose that many TRPC heteromers form diverse receptor-regulated nonselective cation channels in the mammalian brain.

ATPase activity of the sulfonylurea receptor: a catalytic function for the KATP channel complex

ATP-sensitive K+ (KATP) channels are unique metabolic sensors formed by association of Kir6.2, an inwardly rectifying K+ channel, and the sulfonylurea receptor SUR, an ATP binding cassette protein. We identified an ATPase activity in immunoprecipitates of cardiac KATP channels and in purified fusion proteins containing nucleotide binding domains NBD1 and NBD2 of the cardiac SUR2A isoform. NBD2 hydrolyzed ATP with a twofold higher rate compared to NBD1. The ATPase required Mg2+ and was insensitive to ouabain, oligomycin, thapsigargin, or levamisole. K1348A and D1469N mutations in NBD2 reduced ATPase activity and produced channels with increased sensitivity to ATP. KATP channel openers, which bind to SUR, promoted ATPase activity in purified sarcolemma. At higher concentrations, openers reduced ATPase activity, possibly through stabilization of MgADP at the channel site. K1348A and D1469N mutations attenuated the effect of openers on KATP channel activity. Opener-induced channel activation was also inhibited by the creatine kinase/creatine phosphate system that removes ADP from the channel complex. Thus, the KATP channel complex functions not only as a K+ conductance, but also as an enzyme regulating nucleotide-dependent channel gating through an intrinsic ATPase activity of the SUR subunit. Modulation of the channel ATPase activity and/or scavenging the product of the ATPase reaction provide novel means to regulate cellular functions associated with KATP channel opening.

Investigation of the molecular assembly of beta-cell K(ATP) channels

We have investigated the protein interactions involved in the assembly of pancreatic beta-cell ATP-sensitive potassium channels. The channels are a heterooligomeric complex of pore-forming Kir6.2 subunits and sulfonylurea receptor (SUR1) subunits. SUR1 belongs to the ATP binding cassette (ABC) family of proteins and has two nucleotide binding domains (NBD1 and NBD2) and 17 putative transmembrane (TM) sequences. Previously we showed that co-expression in a baculovirus expression system of two parts of SUR1 divided at Pro1042 between TM12 and 13 leads to restoration of glibenclamide binding activity, whereas expression of either individual N- or C-terminal domain alone gave no glibenclamide binding activity [M.V. Mikhailov and S.J.H. Ashcroft (2000) J. Biol. Chem. 275, 3360-3364]. Here we show that the two half-molecules formed by division of SUR1 between NBD1 and TM12 or between TM13 and 14 also self-assemble to give glibenclamide binding activity. However, deletion of NBD1 from the N-part of SUR1 abolished SUR1 assembly, indicating a critical role for NBD1 in SUR1 assembly. We found that differences in glibenclamide binding activity obtained after co-expression of different half-molecules are attributable to different amounts of binding sites, but the binding affinities remained nearly the same. Simultaneous expression of Kir6.2 resulted in enhanced glibenclamide binding activity only when the N-half of SUR1 included TM12. We conclude that TM12 and 13 are not essential for SUR1 assembly whereas TM12 takes part in SUR1 Kir6.2 interaction. This interaction is specific for Kir 6.2 because no enhancement of glibenclamide binding was observed when half-molecules were expressed together with Kir4.1. We propose a model of K(ATP) channel organisation based on these data.

The I182 region of k(ir)6.2 is closely associated with ligand binding in K(ATP) channel inhibition by ATP

The ATP-inhibited potassium (K(ATP)) channel is assembled from four inward rectifier potassium (K(ir)6.x) subunits and four sulfonylurea receptor (SURx) subunits. The inhibitory action of ATP is mediated by at least two distinct functional domains within the C-terminal cytoplasmic tail of K(ir)6.2. The G334D mutation of K(ir)6.2 virtually eliminates ATP-dependent gating with no effect on ligand-independent gating, suggesting a role in linkage of the site to the gate or in the ATP binding site, itself. The T171A mutation of K(ir)6.2 strongly disrupts both ATP-dependent and ligand-independent gating, suggesting a role for T171 in the gating step. A neighboring mutation, I182Q, virtually eliminates ATP inhibition, but its effect on ligand-independent gating remained unknown. We have now characterized both the K(i) values for inhibition by ATP and the ligand-independent gating kinetics of 15 substitutions at position 182. All substitutions decreased ATP-dependent inhibition gating as measured by the K(i), many profoundly so, yet had little or no effect on ligand-independent gating kinetics. Thus, substitutions at position 182 are unlikely to act by disrupting inhibition gate movement. Our results indicate an indispensable role for I182 in a step of the ATP binding mechanism, the linkage mechanism coupling the ATP binding site to the inhibition gate, or both.

Rat homolog of sulfonylurea receptor 2B determines glibenclamide sensitivity of ROMK2 in Xenopus laevis oocyte

Recent studies showed that coexpression of Kir6.1 or Kir6.2 with the sulfonylurea receptor (SUR1, SUR2A, or SUR2B) reconstituted an inwardly rectifying, ATP-sensitive K(+) channel that was inhibited by glibenclamide (2, 15-17). Here we report the isolation of a rat homolog of mouse SUR2B (denoted rSUR2B) from a rat kidney cDNA library. The rSUR2B sequence contains a 4,635-bp open reading frame that encodes a 1,545-amino acid polypeptide, showing 67% shared identity with SUR1 (a pancreatic beta-cell isoform) and 98% with both SUR2A (a brain isoform) and SUR2B (a vascular smooth muscle isoform). Consistent with the predicted structures of other members of the ATP-binding cassette (ABC) superfamily, the sequence of rSUR2B contains 17 putative membrane-spanning segments. Also, predicted Walker A and B consensus binding motifs, present in other ABC members, are conserved in the rSUR2B sequence. RT-PCR revealed that rSUR2B is widely expressed in various rat tissues including brain, colon, heart, kidney, liver, skeletal muscle, and spleen. The intrarenal distribution of the rSUR2B transcript was investigated using RT-PCR and Southern blot of microdissected tubules. The rSUR2B transcript was detected in proximal tubule, cortical thick ascending limb, distal collecting tubule, cortical collecting duct, and outer medullary collecting duct, but not medullary thick ascending limb. This distal distribution overlaps with that of ROMK. Coexpression of rSUR2B with ROMK2 cRNA (in 1:10 ratio) in Xenopus laevis oocytes resulted in whole cell Ba(2+)-sensitive K(+) currents that were inhibited by glibenclamide (50% inhibition with 0.2 mM glibenclamide). In contrast, rSUR2B did not confer significant glibenclamide sensitivity to oocytes coinjected with ROMK1 or ROMK3. The interaction between ROMK2 and rSUR2B was further studied by coimmunoprecipitation of in vitro translated rSUR2B and ROMK2. In agreement with the functional data, the rSUR2B protein was coimmunoprecipitated with ROMK2 in the ROMK2-rSUR2B cotranslated samples. Our data demonstrate that ROMK2, but not ROMK1 and ROMK3, can interact with rSUR2B to confer a sulfonylurea-sensitive K(+) channel, implicating SUR proteins in forming and regulating renal ATP-sensitive K(+) channels. The ROMK isoform specificity of glibenclamide effects suggests that the NH(2) terminus of the ROMK protein mediates rSUR2B-ROMK2 interactions.

The modulating actions of sulfonylurea on atrial natriuretic peptide release in experimental acute heart failure

OBJECTIVES

This study defined the modulating actions of sulfonylurea on acute release of atrial natriuretic peptide (ANP) in experimental acute heart failure.

BACKGROUND

Sulfonylurea drugs, blockers of cardioprotective ATP-sensitive K(+) (K(ATP)) channels, may increase the risk of early cardiovascular mortality. In cardiovascular diseases such as acute heart failure, early release of ANP is essential for cardiorenal homeostasis. Although K(ATP) channels regulate secretion of hormones, such as insulin, it is unknown whether sulfonylureas interfere with ANP release in acute heart failure.

METHODS

The effects of acute administration of glyburide (0.3 mg/kg), a prototype sulfonylurea, on ANP release and sodium excretion were measured in vivo in a canine model of pacing-induced acute heart failure characterized by acute atrial stretch. Immunoreactivity, in atrial tissue, for ANP and the K(ATP) channel subunit, Kir6.2, was determined using specific antibodies.

RESULTS

With increased left atrial pressure in heart failure, plasma levels of ANP increased rapidly and peaked within 25+/-3 min. Glyburide delayed the time required for peak plasma ANP secretion to 48+/-5 min. This resulted in reduced natriuresis from 84+/-17 microEq/min in the absence of glyburide, to 34+/-9 microEq/min in the presence of glyburide. However, glyburide did not alter the renal natriuretic responsiveness to exogenously administered ANP in normal dogs. In atrial tissue, both ANP and the K(ATP) channel subunit, Kir6.2, displayed strong immunoreactivity and co-localization.

CONCLUSIONS

Glyburide delays release of ANP in acute heart failure resulting in impaired natriuresis. This cannot be ascribed to an antinatriuretic effect on the kidney, but rather may be due to interference with K(ATP) channel-dependent ANP secretion from the atrium. Such adverse outcome of sulfonylurea drug use could reduce the compensatory capacity to preserve cardiorenal homeostasis in acute heart failure.

Interactions of the sulfonylurea receptor 1 subunit in the molecular assembly of beta-cell K(ATP) channels

We have investigated protein interactions involved in pancreatic beta-cell ATP-sensitive potassium channel assembly. These channels, which are of key importance for control of insulin release, are a hetero-oligomeric complex of pore-forming Kir6.2 subunits and sulfonylurea receptor (SUR1) subunits with two nucleotide-binding domains (NBD1 and NBD2). We divided SUR1 into two halves at Pro-1042. Expression of either the individual N- or C-terminal domain in a baculovirus expression system did not lead to glibenclamide binding activity, although studies with green fluorescent protein fusion proteins showed that both half-molecules were inserted into the plasma membrane. However, significant glibenclamide binding activity was observed when the half-molecules were co-expressed (even when NBD2 was deleted from the C-terminal half-molecule). Simultaneous expression of Kir6.2 resulted in enhanced glibenclamide binding activity. We conclude that the glibenclamide-binding site includes amino acid residues from both halves of the molecule, that there is strong interaction between different regions of SUR1, that NBD2 is not essential for glibenclamide binding, and that interactions between Kir6.2 and SUR1 participate in ATP-sensitive potassium channel assembly. Investigation of NBD1-green fluorescent protein fusion protein distribution inside insect cells expressing C-terminal halves of SUR1 demonstrated strong interaction between NBD1 and NBD2. We also expressed and purified NBD1 from Escherichia coli. Purified NBD1 was found to exist as a tetramer indicating strong homomeric attractions and a possible role for NBD1 in SUR1 assembly.

Channelopathies of inwardly rectifying potassium channels

Mutations in genes encoding ion channels have increasingly been identified to cause disease conditions collectively termed channelopathies. Recognizing the molecular basis of an ion channel disease has provided new opportunities for screening, early diagnosis, and therapy of such conditions. This synopsis provides an overview of progress in the identification of molecular defects in inwardly rectifying potassium (Kir) channels. Structurally and functionally distinct from other channel families, Kir channels are ubiquitously expressed and serve functions as diverse as regulation of resting membrane potential, maintenance of K(+) homeostasis, control of heart rate, and hormone secretion. In humans, persistent hyperinsulinemic hypoglycemia of infancy, a disorder affecting the function of pancreatic beta cells, and Bartter's syndrome, characterized by hypokalemic alkalosis, hypercalciuria, increased serum aldosterone, and plasma renin activity, are the two major diseases linked so far to mutations in a Kir channel or associated protein. In addition, the weaver phenotype, a neurological disorder in mice, has also been associated with mutations in a Kir channel subtype. Further genetic linkage analysis and full understanding of the consequence that a defect in a Kir channel would have on disease pathogenesis are among the priorities in this emerging field of molecular medicine.

Pharmacological plasticity of cardiac ATP-sensitive potassium channels toward diazoxide revealed by ADP

The pharmacological phenotype of ATP-sensitive potassium (K(ATP)) channels is defined by their tissue-specific regulatory subunit, the sulfonylurea receptor (SUR), which associates with the pore-forming channel core, Kir6.2. The potassium channel opener diazoxide has hyperglycemic and hypotensive properties that stem from its ability to open K(ATP) channels in pancreas and smooth muscle. Diazoxide is believed not to have any significant action on cardiac sarcolemmal K(ATP) channels. Yet, diazoxide can be cardioprotective in ischemia and has been found to bind to the presumed cardiac sarcolemmal K(ATP) channel-regulatory subunit, SUR2A. Here, in excised patches, diazoxide (300 microM) activated pancreatic SUR1/Kir6.2 currents and had little effect on native or recombinant cardiac SUR2A/Kir6.2 currents. However, in the presence of cytoplasmic ADP (100 microM), SUR2A/Kir6.2 channels became as sensitive to diazoxide as SUR1/Kir6. 2 channels. This effect involved specific interactions between MgADP and SUR, as it required Mg(2+), but not ATP, and was abolished by point mutations in the second nucleotide-binding domain of SUR, which impaired channel activation by MgADP. At the whole-cell level, in cardiomyocytes treated with oligomycin to block mitochondrial function, diazoxide could also activate K(ATP) currents only after cytosolic ADP had been raised by a creatine kinase inhibitor. Thus, ADP serves as a cofactor to define the responsiveness of cardiac K(ATP) channels toward diazoxide. The present demonstration of a pharmacological plasticity of K(ATP) channels identifies a mechanism for the control of channel activity in cardiac cells depending on the cellular ADP levels, which are elevated under ischemia.

The molecular assembly of ATP-sensitive potassium channels. Determinants on the pore forming subunit

ATP-sensitive potassium channels form a link between membrane excitability and cellular metabolism. These channels are important in physiological processes such as insulin release and they are an important site of drug action. They are an octomeric complex comprised of four sulfonylurea receptors, a member of the ATP-binding cassette family of proteins, and four Kir 6.0 subunits from the inward rectifier family of potassium channels. We have investigated the nature of the interaction between SUR1 and Kir 6.2 and the domains on the channel responsible for the biochemical and functional manifestations of coupling. The results point to the proximal C terminus determining biochemical interaction in a region that also largely governs homotypic and heterotypic interaction between different Kir family members. While this domain may be necessary for functional communication between the two proteins, it is not sufficient since relative modifications of either the N or C terminus are able to disrupt many aspects of functional coupling mediated by the sulfonylurea receptor.

A transmembrane domain of the sulfonylurea receptor mediates activation of ATP-sensitive K(+) channels by K(+) channel openers

ATP-sensitive K(+) (K(ATP)) channels are a complex of an ATP-binding cassette transporter, the sulfonylurea receptor (SUR), and an inward rectifier K(+) channel subunit, Kir6.2. The diverse pharmacological responsiveness of K(ATP) channels from various tissues are thought to arise from distinct SUR isoforms. Thus, when assembled with Kir6. 2, the pancreatic beta cell isoform SUR1 is activated by the hyperglycemic drug diazoxide but not by hypotensive drugs like cromakalim, whereas the cardiac muscle isoform SUR2A is activated by cromakalim and not by diazoxide. We exploited these differences between SUR1 and SUR2A to pursue a chimeric approach designed to identify the structural determinants of SUR involved in the pharmacological activation of K(ATP) channels. Wild-type and chimeric SUR were coexpressed with Kir6.2 in Xenopus oocytes, and we studied the resulting channels with the patch-clamp technique in the excised inside-out configuration. The third transmembrane domain of SUR is found to be an important determinant of the response to cromakalim, which possibly harbors at least part of its binding site. Contrary to expectations, diazoxide sensitivity could not be linked specifically to the carboxyl-terminal end (nucleotide-binding domain 2) of SUR but appeared to involve complex allosteric interactions between transmembrane and nucleotide-binding domains. In addition to providing direct evidence for the structure-function relationship governing K(ATP) channel activation by potassium channel-opening drugs, a family of drugs of the highest therapeutic interest, these findings delineate the determinants of ligand specificity within the modular ATP-binding cassette-transporter architecture of SUR.

Sulfonylurea and K(+)-channel opener sensitivity of K(ATP) channels. Functional coupling of Kir6.2 and SUR1 subunits

The sensitivity of K(ATP) channels to high-affinity block by sulfonylureas and to stimulation by K(+) channel openers and MgADP (PCOs) is conferred by the regulatory sulfonylurea receptor (SUR) subunit, whereas ATP inhibits the channel through interaction with the inward rectifier (Kir6.2) subunit. Phosphatidylinositol 4, 5-bisphosphate (PIP(2)) profoundly antagonized ATP inhibition of K(ATP) channels expressed from cloned Kir6.2+SUR1 subunits, but also abolished high affinity tolbutamide sensitivity. By stabilizing the open state of the channel, PIP(2) drives the channel away from closed state(s) that are preferentially affected by high affinity tolbutamide binding, thereby producing an apparent loss of high affinity tolbutamide inhibition. Mutant K(ATP) channels (Kir6. 2[DeltaN30] or Kir6.2[L164A], coexpressed with SUR1) also displayed an "uncoupled" phenotype with no high affinity tolbutamide block and with intrinsically higher open state stability. Conversely, Kir6. 2[R176A]+SUR1 channels, which have an intrinsically lower open state stability, displayed a greater high affinity fraction of tolbutamide block. In addition to antagonizing high-affinity block by tolbutamide, PIP(2) also altered the stimulatory action of the PCOs, diazoxide and MgADP. With time after PIP(2) application, PCO stimulation first increased, and then subsequently decreased, probably reflecting a common pathway for activation of the channel by stimulatory PCOs and PIP(2). The net effect of increasing open state stability, either by PIP(2) or mutagenesis, is an apparent "uncoupling" of the Kir6.2 subunit from the regulatory input of SUR1, an action that can be partially reversed by screening negative charges on the membrane with poly-L-lysine.

Gene delivery of Kir6.2/SUR2A in conjunction with pinacidil handles intracellular Ca2+ homeostasis under metabolic stress

Metabolic injury is a complex process affecting various tissues, with intracellular Ca2+ loading recognized as a common precipitating event leading to cell death. We have recently observed that cells overexpressing recombinant ATP-sensitive K+ (KATP) channel subunits may acquire resistance against metabolic stress. To examine whether, under metabolic challenge, intracellular Ca2+ homeostasis can be maintained by an activator of channel proteins, we delivered Kir6.2 and SUR2A genes, which encode KATP channel subunits, into a somatic cell line lacking native KATP channels. Hypoxia-reoxygenation was simulated by application and removal of the mitochondrial poison 2,4 dinitrophenol. Under such metabolic stress, Ca2+ loading was induced by Ca2+ influx during hypoxia and release of Ca2+ from intracellular stores during reoxygenation. Delivery of Kir6.2/SUR2A genes, in conjunction with the KATP channel activator pinacidil, prevented intracellular Ca2+ loading irrespective of whether the channel opener was applied throughout the duration of hypoxia-reoxygenation or transiently during the hypoxic or reoxygenation stage. In all stages of injury, the effect of pinacidil was inhibited by the selective antagonist of KATP channel, 5-hydroxydecanoate. The present study provides evidence that combined use of gene delivery and pharmacological targeting of recombinant proteins can handle intracellular Ca2+ homeostasis under hypoxia-reoxygenation irrespective of the stage of the metabolic insult.

Characterization of low-affinity binding sites for glibenclamide on the Kir6.2 subunit of the beta-cell KATP channel

The ATP-sensitive K+ channel, an octameric complex of two structurally unrelated types of subunits, SUR1 and Kir6.2, plays a central role in the physiological regulation of insulin secretion. The sulfonylurea glibenclamide, which trigger insulin secretion by blocking the ATP-sensitive K+ channel, interacts with both high and low affinity binding sites present on beta-cells. The high affinity binding site has been localized on SUR1 but the molecular nature of the low affinity site is still uncertain. In this study, we analyzed the pharmacology of glibenclamide in a transformed COS-7 cell line expressing the rat Kir6.2 cDNA and compared with that of the MIN6 beta cell line expressing natively both the Kir6.2 and the SUR1 subunits. Binding studies and Scatchard analysis revealed the presence of a single class of low affinity binding sites for glibenclamide on the COS/Kir6.2 cells with characteristics similar to that observed for the low affinity site of the MIN6 beta cells.

ATP-sensitive potassium channels: a model of heteromultimeric potassium channel/receptor assemblies

ATP-sensitive K+ channels (KATP channels) play important roles in many cellular functions by coupling cell metabolism to electrical activity. By cloning members of the novel inwardly rectifying K+ channel subfamily Kir6.0 (Kir6.1 and Kir6.2) and the receptors for sulfonylureas (SUR1 and SUR2), researchers have clarified the molecular structure of KATP channels. KATP channels comprise two subunits: a Kir6.0 subfamily subunit, which is a member of the inwardly rectifying K+ channel family; and a SUR subunit, which is a member of the ATP-binding cassette (ABC) protein superfamily. KATP channels are the first example of a heteromultimeric complex assembled with a K+ channel and a receptor that are structurally unrelated to each other. Since 1995, molecular biological and molecular genetic studies of KATP channels have provided insights into the structure-function relationships, molecular regulation, and pathophysiological roles of KATP channels.

A new ER trafficking signal regulates the subunit stoichiometry of plasma membrane K(ATP) channels

Proper ion channel function often requires specific combinations of pore-forming alpha and regulatory beta subunits, but little is known about the mechanisms that regulate the surface expression of different channel combinations. Our studies of ATP-sensitive K+ channel (K(ATP)) trafficking reveal an essential quality control function for a trafficking motif present in each of the alpha (Kir6.1/2) and beta (SUR1) subunits of the K(ATP) complex. We show that this novel motif for endoplasmic reticulum (ER) retention/retrieval is required at multiple stages of K(ATP) assembly to restrict surface expression to fully assembled and correctly regulated octameric channels. We conclude that exposure of a three amino acid motif (RKR) can explain how assembly of an ion channel complex is coupled to intracellular trafficking.