ATP-dependent interaction of the cytosolic domains of the inwardly rectifying K+ channel Kir6.2 revealed by fluorescence resonance energy transfer

Genetically encoded ATP and NAD(P)H biosensors: potential tools in metabolic engineering

To date, many metabolic engineering tools and strategies have been developed, including tools for cofactor engineering, which is a common strategy for bioproduct synthesis. Cofactor engineering is used for the regulation of pyridine nucleotides, including NADH/NAD+ and NADPH/NADP+, and adenosine triphosphate/adenosine diphosphate (ATP/ADP), which is crucial for maintaining redox and energy balance. However, the intracellular levels of NADH/NAD+, NADPH/NADP+, and ATP/ADP cannot be monitored in real time using traditional methods. Recently, many biosensors for detecting, monitoring, and regulating the intracellular levels of NADH/NAD+, NADPH/NADP+, and ATP/ADP have been developed. Although cofactor biosensors have been mainly developed for use in mammalian cells, the potential application of cofactor biosensors in metabolic engineering in bacterial and yeast cells has received recent attention. Coupling cofactor biosensors with genetic circuits is a promising strategy in metabolic engineering for optimizing the production of biochemicals. In this review, we focus on the development of biosensors for NADH/NAD+, NADPH/NADP+, and ATP/ADP and the potential application of these biosensors in metabolic engineering. We also provide critical perspectives, identify current research challenges, and provide guidance for future research in this promising field.

A genetically encoded fluorescent biosensor for monitoring ATP in living cells with heterobifunctional aptamers

Visualizing the dynamics of ATP in living cells is key to understanding cellular energy metabolism and related diseases. However, the live-cell applications of current methods are still limited due to challenges in biological compatibility and sensitivity to pH. Herein, a novel label-free fluorescent " turn-on " biosensor for monitoring ATP in living bacterias and mammalian cells was developed. This biosensor (Broc-ATP) employed heterobifunctional aptamers to detect ATP with high sensitivity in vitro. In our system, a very useful tandem method was established by combining four Broc-ATPs with 3 × F30 three-way junction scaffold to construct an intracellular biosensor that achieves sufficient fluorescence to respond to intracellular ATP. This intracellular biosensor can be used for sensitive and specific dynamic imaging of ATP in mammalian cells. Hence, this genetically encoded biosensor provides a robust and efficient tool for the detection of intracellular ATP dynamics and 3 × F30 tandem method expands the application of heterobifunctional aptamers in mammalian cells.

Recent progress in developing fluorescent probes for imaging cell metabolites

Cellular metabolites play a crucial role in promoting and regulating cellular activities, but it has been difficult to monitor these cellular metabolites in living cells and in real time. Over the past decades, iterative development and improvements of fluorescent probes have been made, resulting in the effective monitoring of metabolites. In this review, we highlight recent progress in the use of fluorescent probes for tracking some key metabolites, such as adenosine triphosphate, cyclic adenosine monophosphate, cyclic guanosine 5'-monophosphate, Nicotinamide adenine dinucleotide (NADH), reactive oxygen species, sugar, carbon monoxide, and nitric oxide for both whole cell and subcellular imaging.

Fluorescent, Bioluminescent, and Optogenetic Approaches to Study Excitable Physiology in the Single Cardiomyocyte

This review briefly summarizes the single cell application of classical chemical dyes used to visualize cardiomyocyte physiology and their undesirable toxicities which have the potential to confound experimental observations. We will discuss, in detail, the more recent iterative development of fluorescent and bioluminescent protein-based indicators and their emerging application to cardiomyocytes. We will discuss the integration of optical control strategies (optogenetics) to augment the standard imaging approach. This will be done in the context of potential applications, and barriers, of these technologies to disease modelling, drug toxicity, and drug discovery efforts at the single-cell scale.

MoS2-LA-PEI nanocomposite carrier for real-time imaging of ATP metabolism in glioma stem cells co-cultured with endothelial cells on a microfluidic system

Stimuli-responsive carriers have extensively attracted attention in recent years. However, long-term and real-time tracking ability with stimuli-responsive carrier is still in its infant stage due to the limitations such as, low efficacy, instability and cytotoxicity in a bio-environment. In this work, we developed a reduction-sensitive carrier composed of lipoic acid-modified low molecular weight polyethyleneimine (LA-PEI) and large surface ratio MoS2 nanosheet integrated via disulfide bond to mimic a high molecular weight PEI. The positively charged carriers loading negatively charged aptamer enter the cells for a real time long-term tracking of adenosine triphosphate (ATP) metabolism in glioma stem cells (GSCs) when stimulated by TGFβ factor secreted from HUVECs. We envision that MoS₂-LA-PEI carrier has a promising potential for delivery and monitoring the changes in live cells with low cytotoxicity and high efficiency.

Imaging Adenosine Triphosphate (ATP)

Adenosine triphosphate (ATP) is a universal mediator of metabolism and signaling across unicellular and multicellular species. There is a fundamental interdependence between the dynamics of ATP and the physiology that occurs inside and outside the cell. Characterizing and understanding ATP dynamics provide valuable mechanistic insight into processes that range from neurotransmission to the chemotaxis of immune cells. Therefore, we require the methodology to interrogate both temporal and spatial components of ATP dynamics from the subcellular to the organismal levels in live specimens. Over the last several decades, a number of molecular probes that are specific to ATP have been developed. These probes have been combined with imaging approaches, particularly optical microscopy, to enable qualitative and quantitative detection of this critical molecule. In this review, we survey current examples of technologies available for visualizing ATP in living cells, and identify areas where new tools and approaches are needed to expand our capabilities.

CFTR silencing in pancreatic β-cells reveals a functional impact on glucose-stimulated insulin secretion and oxidative stress response

Cystic fibrosis (CF)-related diabetes (CFRD) has become a critical complication that seriously affects the clinical outcomes of CF patients. Although CFRD has emerged as the most common nonpulmonary complication of CF, little is known about its etiopathogenesis. Additionally, whether oxidative stress (OxS), a common feature of CF and diabetes, influences CFRD pathophysiology requires clarification. The main objective of this study was to shed light on the role of the cystic fibrosis transmembrane conductance regulator (CFTR) in combination with OxS in insulin secretion from pancreatic β-cells. CFTR silencing was accomplished in MIN6 cells by stable expression of small hairpin RNAs (shRNA), and glucose-induced insulin secretion was evaluated in the presence and absence of the valuable prooxidant system iron/ascorbate (Fe/Asc; 0.075/0.75 mM) along with or without the antioxidant Trolox (1 mM). Insulin output from CFTR-silenced MIN6 cells was significantly reduced (∼ 70%) at basal and at different glucose concentrations compared with control Mock cells. Furthermore, CFTR silencing rendered MIN6 cells more sensitive to OxS as evidenced by both increased lipid peroxides and weakened antioxidant defense, especially following incubation with Fe/Asc. The decreased insulin secretion in CFTR-silenced MIN6 cells was associated with high levels of NF-κB (the major participant in inflammatory responses), raised apoptosis, and diminished ATP production in response to the Fe/Asc challenge. However, these defects were alleviated by the addition of Trolox, thereby pointing out the role of OxS in aggravating the effects of CFTR deficiency. Our findings indicate that CFTR deficiency in combination with OxS may contribute to endocrine cell dysfunction and insulin secretion, which at least in part may explain the development of CFRD.

Regulation of the neuronal KCNQ2 channel by Src--a dual rearrangement of the cytosolic termini underlies bidirectional regulation of gating

Neuronal M-type K(+) channels are heteromers of KCNQ2 and KCNQ3 subunits, and are found in cell bodies, dendrites and the axon initial segment, regulating the firing properties of neurons. By contrast, presynaptic KCNQ2 homomeric channels directly regulate neurotransmitter release. Previously, we have described a mechanism for gating downregulation of KCNQ2 homomeric channels by calmodulin and syntaxin1A. Here, we describe a new mechanism for regulation of KCNQ2 channel gating that is modulated by Src, a non-receptor tyrosine kinase. In this mechanism, two concurrent distinct structural rearrangements of the cytosolic termini induce two opposing effects: upregulation of the single-channel open probability, mediated by an N-terminal tyrosine, and reduction in functional channels, mediated by a C-terminal tyrosine. In contrast, Src-mediated regulation of KCNQ3 homomeric channels, shown previously to be achieved through the corresponding tyrosine residues, involves the N-terminal-tyrosine-mediated downregulation of the open probability, rather than an upregulation. We argue that the dual bidirectional regulation of KCNQ2 functionality by Src, mediated through two separate sites, means that KCNQ2 can be modified by cellular factors that might specifically interact with either one of the sites, with potential significance in the fine-tuning of neurotransmitters release at nerve terminals.

Use of genetically encoded sensors to monitor cytosolic ATP/ADP ratio in living cells

ATP is not only recognized as the universal energy "currency" in most cells but also plays a less well-known role as an intracellular and extracellular messenger. Here, we review novel approaches for measuring free ATP (or ATP/ADP ratios) in living mammalian cells by using genetically encoded sensors. We also discuss the key technical aspects of routine real-time ATP/ADP monitoring using as a model one of the last-generation fluorescent probes, a fusion protein commonly known as "Perceval." Finally, we present detailed guidelines for the simultaneous measurement of cytosolic ATP/ADP ratios and Ca(2+) concentrations alongside electrical parameters in individual pancreatic β cells, in which energy metabolism is tightly linked to plasma membrane excitability to control the secretion of insulin. With appropriate variations, this approach can be adapted to the study of cytosolic ATP/ADP ratios and Ca(2+) concentrations in malignant cells, two important aspects of oncometabolism.

Aggregation deaggregation influenced selective and sensitive detection of Cu2+ and ATP by histidine functionalized water-soluble fluorescent perylene diimide under physiological conditions and in living cells

A novel PDI-HIS probe detects Cu²⁺ to form aggregated nonfluorescent complex. Addition of 0.58 ppm ATP to this complex causes its rapid disaggregation thereby recovering the fluorescence by ∼99% in vitro and in A549 living cells.

In situ simultaneous monitoring of ATP and GTP using a graphene oxide nanosheet-based sensing platform in living cells

Here we present a detailed protocol for in situ multiple fluorescence monitoring of adenosine-5'-triphosphate (ATP) and guanosine-5'-triphosphate (GTP) in MCF-7 breast cancer cells by using graphene oxide nanosheet (GO-nS) and DNA/RNA aptamers. FAM-labeled ATP aptamer and Cy5-modified GTP aptamer are used to construct the multiple aptamer/GO-nS sensing platform through 'π-π stacking' between aptamers and GO-nS. Binding of aptamers to GO-nS guarantees the fluorescence resonance energy transfer between fluorophores and GO-nS, resulting in 'fluorescence off'. When the aptamer/GO-nS are transported inside the cells via endocytosis, the conformation of the aptamers will change on interaction with cellular ATP and GTP. On the basis of the fluorescence 'off/on' switching, simultaneous sensing and imaging of ATP and GTP in vitro and in situ have been realized through fluorescence and confocal microscopy techniques. In this protocol, we describe the synthesis of GO and GO-nS, preparation of aptamer/GO-nS platform, in vitro detection of ATP and GTP, and how to use this platform to realize intracellular ATP and GTP imaging in cultured MCF-7 cells. The preparation of GO-nS is anticipated to take 7-14 d, and assays involving microscopy imaging and MCF-7 cells culturing can be performed in 2-3 d.

Domain organization of the ATP-sensitive potassium channel complex examined by fluorescence resonance energy transfer

K(ATP) channels link cell metabolism to excitability in many cells. They are formed as tetramers of Kir6.2 subunits, each associated with a SUR1 subunit. We used mutant GFP-based FRET to assess domain organization in channel complexes. Full-length Kir6.2 subunits were linked to YFP or cyan fluorescent protein (CFP) at N or C termini, and all such constructs, including double-tagged YFP-Kir6.2-CFP (Y6.2C), formed functional K(ATP) channels. In intact COSm6 cells, background emission of YFP excited by 430-nm light was ∼6%, but the Y6.2C construct expressed alone exhibited an apparent FRET efficiency of ∼25%, confirmed by trypsin digestion, with or without SUR1 co-expression. Similar FRET efficiency was detected in mixtures of CFP- and YFP-tagged full-length Kir6.2 subunits and transmembrane domain only constructs, when tagged at the C termini but not at the N termini. The FRET-reported Kir6.2 tetramer domain organization was qualitatively consistent with Kir channel crystal structures: C termini and M2 domains are centrally located relative to N termini and M1 domains, respectively. Additional FRET analyses were performed on cells in which tagged full-length Kir6.2 and tagged SUR1 constructs were co-expressed. These analyses further revealed that 1) NBD1 of SUR1 is closer to the C terminus of Kir6.2 than to the N terminus; 2) the Kir6.2 cytoplasmic domain is not essential for complexation with SUR1; and 3) the N-terminal half of SUR1 can complex with itself in the absence of either the C-terminal half or Kir6.2.

Uncaging akt

Many signal transduction pathways comprise protein kinase cascades in which the activity of each component is controlled by phosphorylation and dephosphorylation. A new layer of kinase regulation involving nucleotide binding has now been unraveled. Phosphorylation of Akt at two regulatory sites plays a major role in kinase activation. New findings show that adenosine triphosphate (ATP) binding to Akt induces an intramolecular interaction between these phosphorylation sites and other domains in the protein, creating a cage around the phosphate group and restricting the access of phosphatases to these sites. ATP hydrolysis and substrate phosphorylation open the cage, which permits dephosphorylation and inactivation of the kinase. This switchlike mechanism provides important new insights into the biology of protein kinases.

Regulation of neuronal M-channel gating in an isoform-specific manner: functional interplay between calmodulin and syntaxin 1A

Whereas neuronal M-type K(+) channels composed of KCNQ2 and KCNQ3 subunits regulate firing properties of neurons, presynaptic KCNQ2 subunits were demonstrated to regulate neurotransmitter release by directly influencing presynaptic function. Two interaction partners of M-channels, syntaxin 1A and calmodulin, are known to act presynaptically, syntaxin serving as a major protein component of the membrane fusion machinery and calmodulin serving as regulator of several processes related to neurotransmitter release. Notably, both partners specifically modulate KCNQ2 but not KCNQ3 subunits, suggesting selective presynaptic targeting to directly regulate exocytosis without interference in neuronal firing properties. Here, having first demonstrated in Xenopus oocytes, using analysis of single-channel biophysics, that both modulators downregulate the open probability of KCNQ2 but not KCNQ3 homomers, we sought to resolve the channel structural determinants that confer the isoform-specific gating downregulation and to get insights into the molecular events underlying this mechanism. We show, using optical, biochemical, electrophysiological, and molecular biology analyses, the existence of constitutive interactions between the N and C termini in homomeric KCNQ2 and KCNQ3 channels in living cells. Furthermore, rearrangement in the relative orientation of the KCNQ2 termini that accompanies reduction in single-channel open probability is induced by both regulators, strongly suggesting that closer N-C termini proximity underlies gating downregulation. Different structural determinants, identified at the N and C termini of KCNQ3, prevent the effects by syntaxin 1A and calmodulin, respectively. Moreover, we show that the syntaxin 1A and calmodulin effects can be additive or blocked at different concentration ranges of calmodulin, bearing physiological significance with regard to presynaptic exocytosis.

Quantitative glucose and ATP sensing in mammalian cells

The functioning and survival of mammalian cells requires an active energy metabolism. Metabolic dysfunction plays an important role in many human diseases, including diabetes, cancer, inherited mitochondrial disorders, and metabolic syndrome. The monosaccharide glucose constitutes a key source of cellular energy. Following its import across the plasma membrane, glucose is converted into pyruvate by the glycolysis pathway. Pyruvate oxidation supplies substrates for the ATP-generating mitochondrial oxidative phosphorylation (OXPHOS) system. To gain cell-biochemical knowledge about the operation and regulation of the cellular energy metabolism in the healthy and diseased state, quantitative knowledge is required about (changes in) metabolite concentrations under (non) steady-state conditions. This information can, for instance, be used to construct more realistic in silico models of cell metabolism, which facilitates understanding the consequences of metabolic dysfunction as well as on- and off-target effects of mitochondrial drugs. Here we review the current state-of-the-art live-cell quantification of two key cellular metabolites, glucose and ATP, using protein-based sensors. The latter apply the principle of FRET (fluorescence resonance energy transfer) and allow measurements in different cell compartments by fluorescence microscopy. We further summarize the properties and applications of the FRET-based sensors, their calibration, pitfalls, and future perspectives.

Current understanding of K ATP channels in neonatal diseases: focus on insulin secretion disorders

ATP-sensitive potassium (K(ATP)) channels are cell metabolic sensors that couple cell metabolic status to electric activity, thus regulating many cellular functions. In pancreatic beta cells, K(ATP) channels modulate insulin secretion in response to fluctuations in plasma glucose level, and play an important role in glucose homeostasis. Recent studies show that gain-of-function and loss-of-function mutations in K(ATP) channel subunits cause neonatal diabetes mellitus and congenital hyperinsulinism respectively. These findings lead to significant changes in the diagnosis and treatment for neonatal insulin secretion disorders. This review describes the physiological and pathophysiological functions of K(ATP) channels in glucose homeostasis, their specific roles in neonatal diabetes mellitus and congenital hyperinsulinism, as well as future perspectives of K(ATP) channels in neonatal diseases.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Visualization of ATP levels inside single living cells with fluorescence resonance energy transfer-based genetically encoded indicators

Adenosine 5'-triphosphate (ATP) is the major energy currency of cells and is involved in many cellular processes. However, there is no method for real-time monitoring of ATP levels inside individual living cells. To visualize ATP levels, we generated a series of fluorescence resonance energy transfer (FRET)-based indicators for ATP that were composed of the epsilon subunit of the bacterial F(o)F(1)-ATP synthase sandwiched by the cyan- and yellow-fluorescent proteins. The indicators, named ATeams, had apparent dissociation constants for ATP ranging from 7.4 muM to 3.3 mM. By targeting ATeams to different subcellular compartments, we unexpectedly found that ATP levels in the mitochondrial matrix of HeLa cells are significantly lower than those of cytoplasm and nucleus. We also succeeded in measuring changes in the ATP level inside single HeLa cells after treatment with inhibitors of glycolysis and/or oxidative phosphorylation, revealing that glycolysis is the major ATP-generating pathway of the cells grown in glucose-rich medium. This was also confirmed by an experiment using oligomycin A, an inhibitor of F(o)F(1)-ATP synthase. In addition, it was demonstrated that HeLa cells change ATP-generating pathway in response to changes of nutrition in the environment.

Rearrangements in the relative orientation of cytoplasmic domains induced by a membrane-anchored protein mediate modulations in Kv channel gating

Interdomain interactions between intracellular N and C termini have been described for various K(+) channels, including the voltage-gated Kv2.1, and suggested to affect channel gating. However, no channel regulatory protein directly affecting N/C interactions has been demonstrated. Most Kv2.1 channel interactions with regulatory factors occur at its C terminus. The vesicular SNARE that is also present at a high concentration in the neuronal plasma membrane, VAMP2, is the only protein documented to affect Kv2.1 gating by binding to its N terminus. As its binding target has been mapped near a site implicated in Kv2.1 N/C interactions, we hypothesized that VAMP2 binding to the N terminus requires concomitant conformational changes in the C terminus, which wraps around the N terminus from the outside, to give VAMP2 access. Here, we first determined that the Kv2.1 N terminus, although crucial, is not sufficient to convey functional interaction with VAMP2, and that, concomitant to its binding to the "docking loop" at the Kv2.1 N terminus, VAMP2 binds to the proximal part of the Kv2.1 C terminus, C1a. Next, using computational biology approaches (ab initio modeling, docking, and molecular dynamics simulations) supported by molecular biology, biochemical, electrophysiological, and fluorescence resonance energy transfer analyses, we mapped the interaction sites on both VAMP2 and Kv2.1 and found that this interaction is accompanied by rearrangements in the relative orientation of Kv2.1 cytoplasmic domains. We propose that VAMP2 modulates Kv2.1 inactivation by interfering with the interaction between the docking loop and C1a, a mechanism for gating regulation that may pertain also to other Kv channels.

Whole-cell recording using the perforated patch clamp technique

Many ion channels, particularly potassium channels, are regulated by intracellular substances, such as nucleotides or Ca(2+). These modulators are washed out of the cell during standard whole-cell patch clamp recordings, or maintained at a particular concentration if they are included in the pipette solution. Perforated patch clamp recording permits electrical access between the cell and the patch pipette using pore-forming antibiotics such as nystatin or amphotericin B. These are permeable to small monovalent ions but present a physical barrier to the larger impermeable ions and molecules. This maintains the integrity of many cytoplasmic components including soluble second messengers, and also helps to prevent channel "run down".

ATP depletion inhibits Ca2+ release, influx and extrusion in pancreatic acinar cells but not pathological Ca2+ responses induced by bile

Here, we describe novel mechanisms limiting a toxic cytosolic Ca(2+) rise during adenosine 5'-triphosphate (ATP) depletion. We studied the effect of ATP depletion on Ca(2+) signalling in mouse pancreatic acinar cells. Measurements of ATP in isolated cells after adenovirus-mediated expression of firefly luciferase revealed that the cytosolic ATP concentration fell from approximately 1 mM to near zero after treatment with oligomycin plus iodoacetate. ATP depletion resulted in the inhibition of Ca(2+) extrusion, which was accompanied by a remarkably synchronous inhibition of store-operated Ca(2+) influx. Alternative inhibition of Ca(2+) extrusion by carboxyeosin had a much smaller effect on Ca(2+) influx. The coordinated metabolic inhibition of Ca(2+) influx and extrusion suggests the existence of a common ATP-dependent master regulator of both processes. ATP-depletion also suppressed acetylcholine (ACh)-induced Ca(2+) oscillations, which was due to the inhibition of Ca(2+) release from internal stores. This could be particularly important for limiting Ca(2+) toxicity during periods of hypoxia. In contrast, metabolic control of Ca(2+) influx and Ca(2+) release from internal stores spectacularly failed to prevent large toxic Ca(2+) responses induced by bile acids-activators of acute pancreatitis (a frequent and often fatal disease of the exocrine pancreas). The bile acids taurolithocholic acid 3-sulphate (TLC-S), taurochenodeoxycholic acid (TCDC) and taurocholic acid (TC) were used in our experiments. Neither Ca(2+) release from internal stores nor Ca(2+) influx triggered by bile acids were inhibited by ATP depletion, emphasising the danger of these pathological mechanisms.

Subunit-stoichiometric evidence for kir6.2 channel gating, ATP binding, and binding-gating coupling

ATP-sensitive K(+) channels are gated by intracellular ATP, allowing them to couple intermediary metabolism to cellular excitability, whereas the gating mechanism remains unclear. To understand subunit stoichiometry for the ATP-dependent channel gating, we constructed tandem-multimeric Kir6.2 channels by selective disruption of the binding or gating mechanism in certain subunits. Stepwise disruptions of channel gating caused graded losses in ATP sensitivity and increases in basal P(open), with no effect on maximum ATP inhibition. Prevention of ATP binding lowered the ATP sensitivity and maximum inhibition without affecting basal P(open). The ATP-dependent gating required a minimum of two functional subunits. Two adjacent subunits are more favorable for ATP binding than two diagonal ones. Subunits showed negative cooperativity in ATP binding and positive cooperativity in channel gating. Joint disruptions of the binding and gating mechanisms in the same or alternate subunits of a concatemer revealed that both intra- and intersubunit couplings contributed to channel gating, although the binding-gating coupling preferred the intrasubunit to intersubunit configuration within the C terminus. No such preference was found between the C and N termini. These phenomena are well-described with the operational model used widely for ligand-receptor interactions.

Depletion of ATP and release of presynaptic inhibition in the CA1 region of hippocampal slices during hypoglycemic hypoxia

Transient recovery (TR) of evoked synaptic potentials and ATP depletion during the late stage of hypoxic hypoglycemic insults were investigated in rat hippocampal slices. TR was observed not only in the late stage of insult, but also during recovery. The concentration of ATP corresponded to the appearance (27% of control) and disappearance (15% of control) of TR. Paired pulse studies showed the presynaptic nature of the release of inhibition of synaptic transmission during TR. Both N- and P/Q-type voltage-dependent calcium channels were involved in the appearance of TR. This evidence suggests that underlying mechanisms of TR appearance during hypoxic hypoglycemic insult might be related to ATP depletion and release of A1 adenosine receptor mediated inhibition of presynaptic voltage-dependent calcium channels.

ATP sensitivity of the ATP-sensitive K+ channel in intact and permeabilized pancreatic beta-cells

ATP-sensitive K(+) channels (K(ATP) channels) couple cell metabolism to electrical activity and thereby to physiological processes such as hormone secretion, muscle contraction, and neuronal activity. However, the mechanism by which metabolism regulates K(ATP) channel activity, and the channel sensitivity to inhibition by ATP in its native environment, remain controversial. Here, we used alpha-toxin to permeabilize single pancreatic beta-cells and measure K(ATP) channel ATP sensitivity. We show that the channel ATP sensitivity is approximately sevenfold lower in the permeabilized cell than in the inside-out patch and that this is caused by interaction of Mg-nucleotides with the nucleotide-binding domains of the SUR1 subunit of the channel. The ATP sensitivity observed in permeabilized cells accounts quantitatively for K(ATP) channel activity in intact cells. Thus, our results show that the principal metabolic regulators of K(ATP) channel activity are MgATP and MgADP.

Probing the activation-promoted structural rearrangements in preassembled receptor-G protein complexes

Activation of heterotrimeric G proteins by their cognate seven transmembrane domain receptors is believed to involve conformational changes propagated from the receptor to the G proteins. However, the nature of these changes remains unknown. We monitored the conformational rearrangements at the interfaces between receptors and G proteins and between G protein subunits by measuring bioluminescence resonance energy transfer between probes inserted at multiple sites in receptor-G protein complexes. Using the data obtained for the alpha(2A)AR-G alpha(i1) beta1gamma2 complex and the available crystal structures of G alpha(i1) beta1gamma2, we propose a model wherein agonist binding induces conformational reorganization of a preexisting receptor-G protein complex, leading the G alpha-G betagamma interface to open but not dissociate. This conformational change may represent the movement required to allow nucleotide exit from the G alpha subunit, thus reflecting the initial activation event.

Gating of the ATP-sensitive K+ channel by a pore-lining phenylalanine residue

ATP-sensitive K(+) (K(ATP)) channels are gated by intracellular ATP, proton and phospholipids. The pore-forming Kir6.2 subunit has all essential machineries for channel gating by these ligands. It is known that channel gating involves the inner helix bundle of crossing in which a phenylalanine residue (Phe168) is found in the TM2 at the narrowest region of the ion-conduction pathway in the Kir6.2. Here we present evidence that Phe168-Kir6.2 functions as an ATP- and proton-activated gate via steric hindrance and hydrophobic interactions. Site-specific mutations of Phe168 to a small amino acid resulted in losses of the ATP- and proton-dependent gating, whereas the channel gating was well maintained after mutation to a bulky tryptophan, supporting the steric hindrance effect. The steric hindrance effect, though necessary, was insufficient for the gating, as mutating Phe168 to a bulky hydrophilic residue severely compromised the channel gating. Single-channel kinetics of the F168W mutant resembled the wild-type channel. Small residues increased P(open), and displayed long-lasting closures and long-lasting openings. Kinetic modeling showed that these resulted from stabilization of the channel to open and long-lived closed states, suggesting that a bulky and hydrophobic residue may lower the energy barrier for the switch between channel openings and closures. Thus, it is likely that the Phe168 acts as not only a steric hindrance gate but also potentially a facilitator of gating transitions in the Kir6.2 channel.

Mutation of critical GIRK subunit residues disrupts N- and C-termini association and channel function

The subfamily of G-protein-linked inwardly rectifying potassium channels (GIRKs) is coupled to G-protein receptors throughout the CNS and in the heart. We used mutational analysis to address the role of a specific hydrophobic region of the GIRK1 subunit. Deletion of the GIRK1 C-terminal residues 330-384, as well as the point mutation I331R, resulted in a decrease in channel function when coexpressed with GIRK4 in oocytes and in COS-7 cells. Surface protein expression of GIRK1 I331R coexpressed with GIRK4 was comparable with wild type, indicating that subunits assemble and are correctly localized to the membrane. Subsequent mutation of homologous residues in both the GIRK4 subunit and Kir2.1 (Gbetagamma-independent inward rectifier) also resulted in a decrease in channel function. Intracellular domain associations resulted in the coimmunoprecipitation of the GIRK1 N and C termini and GIRK4 N and C termini. The point mutation I331R in the GIRK1 C terminus or L337R in the GIRK4 C terminus decreased the association between the N and C termini. Mutation of a GIRK1 N-terminal hydrophobic residue, predicted structurally to interact with the C-terminal domain, also resulted in a decrease in channel function and termini association. We hypothesize that the hydrophobic nature of this GIRK1 subunit region is critical for interaction between adjacent termini and is permissive for channel gating. In addition, the homologous mutation in cytoplasmic domains of Kir2.1 (L330R) did not disrupt association, suggesting that the overall structural integrity of this region is critical for inward rectifier function.

Conformational dynamics of the ligand-binding domain of inward rectifier K channels as revealed by molecular dynamics simulations: toward an understanding of Kir channel gating

Inward rectifier (Kir) potassium channels are characterized by two transmembrane helices per subunit, plus an intracellular C-terminal domain that controls channel gating in response to changes in concentration of various ligands. Based on the crystal structure of the tetrameric C-terminal domain of Kir3.1, it is possible to build a homology model of the ATP-binding C-terminal domain of Kir6.2. Molecular dynamics simulations have been used to probe the dynamics of Kir C-terminal domains and to explore the relationship between their dynamics and possible mechanisms of channel gating. Multiple simulations, each of 10 ns duration, have been performed for Kir3.1 (crystal structure) and Kir6.2 (homology model), in both their monomeric and tetrameric forms. The Kir6.2 simulations were performed with and without bound ATP. The results of the simulations reveal comparable conformational stability for the crystal structure and the homology model. There is some decrease in conformational flexibility when comparing the monomers with the tetramers, corresponding mainly to the subunit interfaces in the tetramer. The beta-phosphate of ATP interacts with the side chain of K185 in the Kir6.2 model and simulations. The flexibility of the Kir6.2 tetramer is not changed greatly by the presence of bound ATP, other than in two loop regions. Principal components analysis of the simulated dynamics suggests loss of symmetry in both the Kir3.1 and Kir6.2 tetramers, consistent with "dimer-of-dimers" motion of subunits in C-terminal domains of the corresponding Kir channels. This is suggestive of a gating model in which a transition between exact tetrameric symmetry and dimer-of-dimers symmetry is associated with a change in transmembrane helix packing coupled to gating of the channel. Dimer-of-dimers motion of the C-terminal domain tetramer is also supported by coarse-grained (anisotropic network model) calculations. It is of interest that loss of exact rotational symmetry has also been suggested to play a role in gating in the bacterial Kir homolog, KirBac1.1, and in the nicotinic acetylcholine receptor channel.

Determinants of voltage-gated potassium channel surface expression and localization in Mammalian neurons

Neurons strictly regulate expression of a wide variety of voltage-dependent ion channels in their surface membranes to achieve precise yet dynamic control of intrinsic membrane excitability. Neurons also exhibit extreme morphological complexity that underlies diverse aspects of their function. Most ion channels are preferentially targeted to either the axonal or somatodendritic compartments, where they become further localized to discrete membrane subdomains. This restricted accumulation of ion channels enables local control of membrane signaling events in specific microdomains of a given compartment. Voltage-dependent K+ (Kv) channels act as potent modulators of diverse excitatory events such as action potentials, excitatory synaptic potentials, and Ca2+ influx. Kv channels exhibit diverse patterns of cellular expression, and distinct subtype-specific localization, in mammalian central neurons. Here we review the mechanisms regulating the abundance and distribution of Kv channels in mammalian neurons and discuss how dynamic regulation of these events impacts neuronal signaling.

Functional analysis of a structural model of the ATP-binding site of the KATP channel Kir6.2 subunit

ATP-sensitive potassium (KATP) channels couple cell metabolism to electrical activity by regulating K+ flux across the plasma membrane. Channel closure is mediated by ATP, which binds to the pore-forming subunit (Kir6.2). Here we use homology modelling and ligand docking to construct a model of the Kir6.2 tetramer and identify the ATP-binding site. The model is consistent with a large amount of functional data and was further tested by mutagenesis. Ligand binding occurs at the interface between two subunits. The phosphate tail of ATP interacts with R201 and K185 in the C-terminus of one subunit, and with R50 in the N-terminus of another; the N6 atom of the adenine ring interacts with E179 and R301 in the same subunit. Mutation of residues lining the binding pocket reduced ATP-dependent channel inhibition. The model also suggests that interactions between the C-terminus of one subunit and the 'slide helix' of the adjacent subunit may be involved in ATP-dependent gating. Consistent with a role in gating, mutations in the slide helix bias the intrinsic channel conformation towards the open state.

Ligand-induced rearrangement of the dimeric metabotropic glutamate receptor 1alpha

The extracellular domain of the metabotropic glutamate receptor 1alpha (mGluR1alpha) forms a dimer and the ligand, glutamate, induces a structural rearrangement in this domain. However, the conformational change in the cytoplasmic domain, which is critical for mGluR1alpha's interaction with G proteins, remains unclear. Here we investigated the ligand-induced conformational changes in the cytoplasmic domain by fluorescence resonance energy transfer (FRET) analysis of mGluR1alpha labeled with fluorescent protein(s) under total internal reflection field microscopy. Upon ligand binding, the intersubunit FRET efficiency between the second loops increased, whereas that between first loops decreased. In contrast, the intrasubunit FRET did not change clearly. These results show that ligand binding does not change the structure of each subunit, but does change the dimeric allocation of the cytoplasmic regions, which may underlie downstream signaling.