Glucose-stimulated single pancreatic islets sustain increased cytosolic ATP levels during initial Ca2+ influx and subsequent Ca2+ oscillations

Imaging of extracellular and intracellular ATP in pancreatic beta cells reveals correlation between glucose metabolism and purinergic signalling

Adenosine triphosphate (ATP) is a universal energy molecule and yet cells release it and extracellular ATP is an important signalling molecule between cells. Monitoring of ATP levels outside of cells is important for our understanding of physiological and pathophysiological processes in cells/tissues. Here, we focus on pancreatic beta cells (INS-1E) and test the hypothesis that there is an association between intra- and extracellular ATP levels which depends on glucose provision. We imaged real-time changes in extracellular ATP in pancreatic beta cells using two sensors tethered to extracellular aspects of the plasma membrane (eATeam3.10, iATPSnFR1.0). Increase in glucose induced fast micromolar ATP release to the cell surface, depending on glucose concentrations. Chronic pre-treatment with glucose increased the basal ATP signal. In addition, we co-expressed intracellular ATP sensors (ATeam1.30, PercevalHR) in the same cultures and showed that glucose induced fast increases in extracellular and intracellular ATP. Glucose and extracellular ATP stimulated glucose transport monitored by the glucose sensor (FLII12Pglu-700uDelta6). In conclusion, we propose that in beta cells there is a dynamic relation between intra- and extracellular ATP that depends on glucose transport and metabolism and these processes may be tuned by purinergic signalling. Future development of ATP sensors for imaging may aid development of novel approaches to target extracellular ATP in, for example, type 2 diabetes mellitus therapy.

Identification of eight genomic protective alleles for mitochondrial diabetes by Kinship-graph convolutional network

AIMS

Nearly 85% of maternally inherited diabetes and deafness (MIDD) are caused by the m.3243A>G mutation in the mitochondrial DNA. However, the clinical phenotypes of MIDD may also be influenced by the nuclear genome, this study aimed to investigate nuclear genome variants that influence clinical phenotypes associated with m.3243A>G mutation in MIDD based on whole-genome sequencing of the patients belonging to pedigrees.

MATERIALS AND METHODS

We analyzed a whole-genome sequencing (WGS) dataset from blood samples of 38 MIDD patients with the m.3243A > G mutation belonging to 10 pedigrees, by developing a Kinship-graph convolutional network approach, called Ki-GCN, integrated with the conventional genome-wide association study (GWAS) methods.

RESULTS

We identified eight protective alleles in the nuclear genome that have protective effects against the onset of MIDD, related deafness, and also type 2 diabetes. Based on these eight protective alleles, we constructed an effective logistic regression model to predict the early or late onset of MIDD patients.

CONCLUSIONS

There are protective alleles in the nuclear genome that are associated with the onset-age of MIDD patients and might also provide protective effects on the deafness derived from MIDD patients.

Sperm induce a secondary increase in ATP levels in mouse eggs that is independent of Ca2+ oscillations

Egg activation at fertilization in mouse eggs is caused by a series of cytosolic Ca2+ oscillations that are associated with an increase in ATP concentrations driven by increased mitochondrial activity. We have investigated the role of Ca2+ oscillations in these changes in ATP at fertilization by measuring the dynamics of ATP and Ca2+ in mouse eggs. An initial ATP increase started with the first Ca2+ transient at fertilization and then a secondary increase in ATP occurred ∼1 h later and this preceded a small and temporary increase in the frequency of Ca2+ oscillations. Other stimuli that caused Ca2+ oscillations such as PLCz1 or thimerosal, caused smaller or slower changes in ATP that failed to show the distinct secondary rise. Sperm-induced Ca2+ oscillations in the egg also triggered changes in the fluorescence of NADH which followed the pattern of Ca2+ spikes in a similar pattern to oscillations triggered by PLCz1 or thimerosal. When eggs were loaded with low concentrations of the Ca2+ chelator BAPTA, sperm triggered one small Ca2+ increase, but there were still extra phases of ATP increase that were similar to control fertilized eggs. Singular Ca2+ increases caused by thapsigargin were much less effective in elevating ATP levels. Together these data suggest that the secondary ATP increase at fertilization in mouse eggs is not caused by increases in cytosolic Ca2+. The fertilizing sperm may stimulate ATP production in eggs via both Ca2+ and by another mechanism that is independent of PLCz1 or Ca2+ oscillations.

Ca2+-Mediated Signaling Pathways: A Promising Target for the Successful Generation of Mature and Functional Stem Cell-Derived Pancreatic Beta Cells In Vitro

Diabetes mellitus is a chronic disease affecting over 500 million adults globally and is mainly categorized as type 1 diabetes mellitus (T1DM), where pancreatic beta cells are destroyed, and type 2 diabetes mellitus (T2DM), characterized by beta cell dysfunction. This review highlights the importance of the divalent cation calcium (Ca²⁺) and its associated signaling pathways in the proper functioning of beta cells and underlines the effects of Ca²⁺ dysfunction on beta cell function and its implications for the onset of diabetes. Great interest and promise are held by human pluripotent stem cell (hPSC) technology to generate functional pancreatic beta cells from diabetic patient-derived stem cells to replace the dysfunctional cells, thereby compensating for insulin deficiency and reducing the comorbidities of the disease and its associated financial and social burden on the patient and society. Beta-like cells generated by most current differentiation protocols have blunted functionality compared to their adult human counterparts. The Ca²⁺ dynamics in stem cell-derived beta-like cells and adult beta cells are summarized in this review, revealing the importance of proper Ca²⁺ homeostasis in beta-cell function. Consequently, the importance of targeting Ca²⁺ function in differentiation protocols is suggested to improve current strategies to use hPSCs to generate mature and functional beta-like cells with a comparable glucose-stimulated insulin secretion (GSIS) profile to adult beta cells.

Simultaneous Measurement of Changes in Mitochondrial and Endoplasmic Reticulum Free Calcium in Pancreatic Beta Cells

The free calcium (Ca2+) levels in pancreatic beta cell organelles have been the subject of many recent investigations. Under pathophysiological conditions, disturbances in these pools have been linked to altered intracellular communication and cellular dysfunction. To facilitate studies of subcellular Ca2+ signaling in beta cells and, particularly, signaling between the endoplasmic reticulum (ER) and mitochondria, we designed a novel dual Ca2+ sensor which we termed DS-1. DS-1 encodes two stoichiometrically fluorescent proteins within a single plasmid, G-CEPIA-er, targeted to the ER and R-CEPIA3-mt, targeted to mitochondria. Our goal was to simultaneously measure the ER and mitochondrial Ca2+ in cells in real time. The Kds of G-CEPIA-er and R-CEPIA3-mt for Ca2+ are 672 and 3.7 μM, respectively. Confocal imaging of insulin-secreting INS-1 832/13 expressing DS-1 confirmed that the green and red fluorophores correctly colocalized with organelle-specific fluorescent markers as predicted. Further, we tested whether DS-1 exhibited the functional properties expected by challenging an INS-1 cell to glucose concentrations or drugs having well-documented effects on the ER and mitochondrial Ca2+ handling. The data obtained were consistent with those seen using other single organelle targeted probes. These results taken together suggest that DS-1 is a promising new approach for investigating Ca2+ signaling within multiple organelles of the cell.

Ameliorative effects of Portulaca oleracea L. (purslane) on the metabolic syndrome: A review

ETHNOPHARMACOLOGICAL RELEVANCE

The metabolic syndrome, which comprises hyperglycemia, dyslipidemia, abdominal obesity, and hypertension, is a worldwide problem. This disorder is a significant risk factor for insulin resistance, diabetes mellitus, cardiovascular disease incidence, and mortality. The value of herbs and natural products in the treatment of human ailments has been documented in several inquiries. An annual herbaceous plant called Portulaca oleracea L. (purslane) is used both as a traditional medicine and an edible plant to treat various ailments including gastrointestinal diseases and liver inflammation. Purslane contains a variety of secondary metabolites such as organic acids, flavonoids, terpenoids, and alkaloids.

AIM OF THE STUDY

In the current work, our team aims to shed light on the potential efficacy of purslane and its main components in treating metabolic syndrome and its complications.

STUDY DESIGN

Scopus, Google Scholar, and PubMed databases have been used to gather the most relevant in vitro, in vivo studies, and clinical trials from the time of inception up to February 2022.

RESULTS

The effects of purslane on metabolism are among its most significant pharmacological properties. In patients with metabolic syndrome, purslane could significantly lower blood glucose and balance lipid profiles. This indicates that purslane might have a potential role in the management of metabolic syndrome through different underlying mechanisms including antioxidant, anti-inflammatory, anti-hyperlipidemic, anti-obesity, and antidiabetic.

CONCLUSION

According to previous investigations, purslane can control metabolic syndrome and its complications. However, further preclinical and clinical studies are required to approve the advantageous effects of purslane on metabolic syndrome.

Transcriptome reveals the dysfunction of pancreatic islets after wound healing in severely burned mice

BACKGROUND

Severely burned patients have a higher risk of diabetes mellitus after healing, but its mechanism remains unclear. Therefore, the purpose of the study was to explore the influence of burns on pancreatic islets of mice after wound healing.

METHODS

Forty-two male C57BL/6 mice were randomized into a sham group and a burn group and subjected to sham treatment or a third-degree burn model of 30% total body surface area. Fasting blood glucose was detected weekly for 8 weeks after severe burns. Glucose-stimulated insulin secretion was measured 8 weeks post severe burns. Islets of the two groups were isolated and mRNA libraries were sequenced by the Illumina sequencing platform. The expressions of differentially expressed genes (DEGs) related to the cell cycle and the amounts of mitochondrial DNA were detected by quantitative real-time polymerase chain reaction after gene ontology, gene set enrichment analysis, and protein-protein network analysis. Hematoxylin-eosin staining of pancreatic tail tissue and adenosine triphosphate (ATP) assay of islets were performed.

RESULTS

The levels of fasting blood glucose were significantly higher within 8 weeks post severe burns. Glucose-stimulated insulin secretion was impaired at the eighth week post severe burns. Totally 128 DEGs were selected. Gene ontology and gene set enrichment analysis indicated that the pathways related to the cell cycle, protein processing, and oxidative phosphorylation were downregulated. The expressions of DEGs related to the cell cycle showed a consistent trend with mRNA sequencing data, and most of them were downregulated post severe burns. The cell mass of the burn group was less than that of the sham group. Also, the concentration of ATP and the amount of mitochondrial DNA were lower in the burn group.

CONCLUSION

In the model of severe-burned mice, disorders in glucose metabolism persist for 8 weeks after burns, which may be related to low islet cell proliferation, downregulation of protein processing, and less ATP production.

Visualizing physiological parameters in cells and tissues using genetically encoded indicators for metabolites

The study of metabolism is undergoing a renaissance. Since the year 2002, over 50 genetically-encoded fluorescent indicators (GEFIs) have been introduced, capable of monitoring metabolites with high spatial/temporal resolution using fluorescence microscopy. Indicators are fusion proteins that change their fluorescence upon binding a specific metabolite. There are indicators for sugars, monocarboxylates, Krebs cycle intermediates, amino acids, cofactors, and energy nucleotides. They permit monitoring relative levels, concentrations, and fluxes in living systems. At a minimum they report relative levels and, in some cases, absolute concentrations may be obtained by performing ad hoc calibration protocols. Proper data collection, processing, and interpretation are critical to take full advantage of these new tools. This review offers a survey of the metabolic indicators that have been validated in mammalian systems. Minimally invasive, these indicators have been instrumental for the purposes of confirmation, rebuttal and discovery. We envision that this powerful technology will foster metabolic physiology.

Glucose-Stimulated Calcium Dynamics in Beta Cells From Male C57BL/6J, C57BL/6N, and NMRI Mice: A Comparison of Activation, Activity, and Deactivation Properties in Tissue Slices

Although mice are a very instrumental model in islet beta cell research, possible phenotypic differences between strains and substrains are largely neglected in the scientific community. In this study, we show important phenotypic differences in beta cell responses to glucose between C57BL/6J, C57BL/6N, and NMRI mice, i.e., the three most commonly used strains. High-resolution multicellular confocal imaging of beta cells in acute pancreas tissue slices was used to measure and quantitatively compare the calcium dynamics in response to a wide range of glucose concentrations. Strain- and substrain-specific features were found in all three phases of beta cell responses to glucose: a shift in the dose-response curve characterizing the delay to activation and deactivation in response to stimulus onset and termination, respectively, and distinct concentration-encoding principles during the plateau phase in terms of frequency, duration, and active time changes with increasing glucose concentrations. Our results underline the significance of carefully choosing and reporting the strain to enable comparison and increase reproducibility, emphasize the importance of analyzing a number of different beta cell physiological parameters characterizing the response to glucose, and provide a valuable standard for future studies on beta cell calcium dynamics in health and disease in tissue slices.

Lactate is an energy substrate for rodent cortical neurons and enhances their firing activity

Glucose is the mandatory fuel for the brain, yet the relative contribution of glucose and lactate for neuronal energy metabolism is unclear. We found that increased lactate, but not glucose concentration, enhances the spiking activity of neurons of the cerebral cortex. Enhanced spiking was dependent on ATP-sensitive potassium (KATP) channels formed with KCNJ11 and ABCC8 subunits, which we show are functionally expressed in most neocortical neuronal types. We also demonstrate the ability of cortical neurons to take-up and metabolize lactate. We further reveal that ATP is produced by cortical neurons largely via oxidative phosphorylation and only modestly by glycolysis. Our data demonstrate that in active neurons, lactate is preferred to glucose as an energy substrate, and that lactate metabolism shapes neuronal activity in the neocortex through KATP channels. Our results highlight the importance of metabolic crosstalk between neurons and astrocytes for brain function.

Mitochondrial clearance of calcium facilitated by MICU2 controls insulin secretion

OBJECTIVE

Transport of Ca2+ into pancreatic β cell mitochondria facilitates nutrient-mediated insulin secretion. However, the underlying mechanism is unclear. Recent establishment of the molecular identity of the mitochondrial Ca2+ uniporter (MCU) and associated proteins allows modification of mitochondrial Ca2+ transport in intact cells. We examined the consequences of deficiency of the accessory protein MICU2 in rat and human insulin-secreting cells and mouse islets.

METHODS

siRNA silencing of Micu2 in the INS-1 832/13 and EndoC-βH1 cell lines was performed; Micu2-/- mice were also studied. Insulin secretion and mechanistic analyses utilizing live confocal imaging to assess mitochondrial function and intracellular Ca2+ dynamics were performed.

RESULTS

Silencing of Micu2 abrogated GSIS in the INS-1 832/13 and EndoC-βH1 cells. The Micu2-/- mice also displayed attenuated GSIS. Mitochondrial Ca2+ uptake declined in MICU2-deficient INS-1 832/13 and EndoC-βH1 cells in response to high glucose and high K+. MICU2 silencing in INS-1 832/13 cells, presumably through its effects on mitochondrial Ca2+ uptake, perturbed mitochondrial function illustrated by absent mitochondrial membrane hyperpolarization and lowering of the ATP/ADP ratio in response to elevated glucose. Despite the loss of mitochondrial Ca2+ uptake, cytosolic Ca2+ was lower in siMICU2-treated INS-1 832/13 cells in response to high K+. It was hypothesized that Ca2+ accumulated in the submembrane compartment in MICU2-deficient cells, resulting in desensitization of voltage-dependent Ca2+ channels, lowering total cytosolic Ca2+. Upon high K+ stimulation, MICU2-silenced cells showed higher and prolonged increases in submembrane Ca2+ levels.

CONCLUSIONS

MICU2 plays a critical role in β cell mitochondrial Ca2+ uptake. β cell mitochondria sequestered Ca2+ from the submembrane compartment, preventing desensitization of voltage-dependent Ca2+ channels and facilitating GSIS.

Mitochondrial Calcium Signaling in Pancreatic β-Cell

Accumulation of calcium in energized mitochondria of pancreatic β-cells is emerging as a crucial process for pancreatic β-cell function. β-cell mitochondria sense and shape calcium signals, linking the metabolism of glucose and other secretagogues to the generation of signals that promote insulin secretion during nutrient stimulation. Here, we describe the role of mitochondrial calcium signaling in pancreatic β-cell function. We report the latest pharmacological and genetic findings, including the first mitochondrial calcium-targeted intervention strategies developed to modulate pancreatic β-cell function and their potential relevance in the context of diabetes.

Mitochondrial calcium in command of juggling myriads of cellular functions

The puzzling traits related to the evolutionary aspect of mitochondria, still positions the mitochondrion at the center of the research. The theory of endosymbiosis popularized by Lynn Margulis in 1967 gained prominence wherein the mitochondrion is believed to have emerged as a prokaryote and later integrated into the eukaryotic system. This semi-autonomous organelle has bagged two responsible but perilous cellular functions: a) energy metabolism, and b) calcium buffering, though both are interdependent. While most of the mitochondrial functions are saliently regulated by calcium ions, the calcium buffering role of mitochondria decides the cellular fate. Though calcium overload in few mitochondria makes them dysfunctional at the early stage of cellular stress, this doesn't lead to sudden cell death due to critical checkpoints like mitophagy, mitochondrial fusion, etc. Thus, mitochondrion juggles with multiple crucial cellular functions with its calcium buffering skill.

Augmented mitochondrial energy metabolism is an early response to chronic glucose stress in human pancreatic beta cells

AIMS/HYPOTHESIS

In islets from individuals with type 2 diabetes and in islets exposed to chronic elevated glucose, mitochondrial energy metabolism is impaired. Here, we studied early metabolic changes and mitochondrial adaptations in human beta cells during chronic glucose stress.

METHODS

Respiration and cytosolic ATP changes were measured in human islet cell clusters after culture for 4 days in 11.1 mmol/l glucose. Metabolomics was applied to analyse intracellular metabolite changes as a result of glucose stress conditions. Alterations in beta cell function were followed using insulin secretion assays or cytosolic calcium signalling after expression of the calcium probe YC3.6 specifically in beta cells of islet clusters.

RESULTS

At early stages of glucose stress, mitochondrial energy metabolism was augmented in contrast to the previously described mitochondrial dysfunction in beta cells from islets of diabetic donors. Following chronic glucose stress, mitochondrial respiration increased (by 52.4%, p < 0.001) and, as a consequence, the cytosolic ATP/ADP ratio in resting human pancreatic islet cells was elevated (by 27.8%, p < 0.05). Because of mitochondrial overactivation in the resting state, nutrient-induced beta cell activation was reduced. In addition, chronic glucose stress caused metabolic adaptations that resulted in the accumulation of intermediates of the glycolytic pathway, the pentose phosphate pathway and the TCA cycle; the most strongly augmented metabolite was glycerol 3-phosphate. The changes in metabolites observed are likely to be due to the inability of mitochondria to cope with continuous nutrient oversupply. To protect beta cells from chronic glucose stress, we inhibited mitochondrial pyruvate transport. Metabolite concentrations were partially normalised and the mitochondrial respiratory response to nutrients was markedly improved. Furthermore, stimulus-secretion coupling as assessed by cytosolic calcium signalling, was restored.

CONCLUSION/INTERPRETATION

We propose that metabolic changes and associated mitochondrial overactivation are early adaptations to glucose stress, and may reflect what happens as a result of poor blood glucose control. Inhibition of mitochondrial pyruvate transport reduces mitochondrial nutrient overload and allows beta cells to recover from chronic glucose stress. Graphical abstract.

Cellular and systemic mechanisms for glucose sensing and homeostasis

Glucose is a major source of energy in animals. Maintaining blood glucose levels within a physiological range is important for facilitating glucose uptake by cells, as required for optimal functioning. Glucose homeostasis relies on multiple glucose-sensing cells in the body that constantly monitor blood glucose levels and respond accordingly to adjust its glycemia. These include not only pancreatic β-cells and α-cells that secrete insulin and glucagon, but also central and peripheral neurons regulating pancreatic endocrine function. Different types of cells respond distinctively to changes in blood glucose levels, and the mechanisms involved in glucose sensing are diverse. Notably, recent studies have challenged the currently held views regarding glucose-sensing mechanisms. Furthermore, peripheral and central glucose-sensing cells appear to work in concert to control blood glucose level and maintain glucose and energy homeostasis in organisms. In this review, we summarize the established concepts and recent advances in the understanding of cellular and systemic mechanisms that regulate glucose sensing and its homeostasis.

Investigation of Ultrasound-Mediated Intracellular Ca2+ Oscillations in HIT-T15 Pancreatic β-Cell Line

In glucose-stimulated insulin secretion (GSIS) of pancreatic β-cells, the rise of free cytosolic Ca2+ concentration through voltage-gated calcium channels (VGCCs) triggers the exocytosis of insulin-containing granules. Recently, mechanically induced insulin secretion pathways were also reported, which utilize free cytosolic Ca2+ ions as a direct regulator of exocytosis. In this study, we aimed to investigate intracellular Ca2+ responses on the HIT-T15 pancreatic β-cell line upon low-intensity pulsed ultrasound (LIPUS) stimulation and found that ultrasound induces two distinct types of intracellular Ca2+ oscillation, fast-irregular and slow-periodic, from otherwise resting cells. Both Ca2+ patterns depend on the purinergic signaling activated by the rise of extracellular ATP or ADP concentration upon ultrasound stimulation, which facilitates the release through mechanosensitive hemichannels on the plasma membrane. Further study demonstrated that two subtypes of purinergic receptors, P2X and P2Y, are working in a competitive manner depending on the level of glucose in the cell media. The findings can serve as an essential groundwork providing an underlying mechanism for the development of a new therapeutic approach for diabetic conditions with further validation.

The small fiber neuropathy NaV1.7 I228M mutation: impaired neurite integrity via bioenergetic and mitotoxic mechanisms, and protection by dexpramipexole

Gain-of-function variants in voltage-gated sodium channel NaV1.7 that increase firing frequency and spontaneous firing of dorsal root ganglion (DRG) neurons have recently been identified in 5–10% of patients with idiopathic small fiber neuropathy (I-SFN). Our previous in vitro observations suggest that enhanced sodium channel activity can contribute to a decrease in length of peripheral sensory axons. We have hypothesized that sustained sodium influx due to the expression of SFN-associated sodium channel variants may trigger an energetic deficit in neurons that contributes to degeneration and loss of nerve fibers in SFN. Using an ATP FRET biosensor, we now demonstrate reduced steady-state levels of ATP and markedly faster ATP decay in response to membrane depolarization in cultured DRG neurons expressing an SFN-associated variant NaV1.7, I228M, compared with wild-type neurons. We also observed that I228M neurons show a significant reduction in mitochondrial density and size, indicating dysfunctional mitochondria and a reduced bioenergetic capacity. Finally, we report that exposure to dexpramipexole, a drug that improves mitochondrial energy metabolism, increases the neurite length of I228M-expressing neurons. Our data suggest that expression of gain-of-function variants of NaV1.7 can damage mitochondria and compromise cellular capacity for ATP production. The resulting bioenergetic crisis can consequently contribute to loss of axons in SFN. We suggest that, in addition to interventions that reduce ionic disturbance caused by mutant NaV1.7 channels, an alternative therapeutic strategy might target the bioenergetic burden and mitochondrial damage that occur in SFN associated with NaV1.7 gain-of-function mutations.

NEW & NOTEWORTHY Sodium channel NaV1.7 mutations that increase dorsal root ganglion (DRG) neuron excitability have been identified in small fiber neuropathy (SFN). We demonstrate reduced steady-state ATP levels, faster depolarization-evoked ATP decay, and reduced mitochondrial density and size in cultured DRG neurons expressing SFN-associated variant NaV1.7 I228M. Dexpramipexole, which improves mitochondrial energy metabolism, has a protective effect. Because gain-of-function NaV1.7 variants can compromise bioenergetics, therapeutic strategies that target bioenergetic burden and mitochondrial damage merit study in SFN.

ATP-binding affinity of the ε subunit of thermophilic F1-ATPase under label-free conditions

The ε subunits of several bacterial F₁-ATPases bind ATP. ATP binding to the ε subunit has been shown to be involved in the regulation of F₁-ATPase from thermophilic Bacillus sp. PS3 (TF₁). We previously reported that the dissociation constant for ATP of wild-type ε subunit of TF₁ at 25 °C is 4.3 μM by measuring changes in the fluorescence of the dye attached to the ε subunit (Kato, S. et al. (2007) J. Biol. Chem.282, 37618). However, we have recently noticed that this varies with the dye used. In this report, to determine the affinity for ATP under label-free conditions, we have measured the competitive displacement of 2′(3′)-O-N′-methylaniloyl-aminoadenosine-5′-triphosphate (Mant-ATP), a fluorescent analog of ATP, by ATP. The dissociation constant for ATP of wild-type ε subunit of TF₁ at 25 °C was determined to be 0.29 μM, which is one order of magnitude higher affinity than previously reported values.

•

The ε subunit of F₁-ATPase from Bacillus PS3 specifically binds ATP.

•

Fluorescent labeling of ε subunit for the measurement may affect binding affinity.

•

ATP binding affinity under label-free conditions was determined.

•

Affinity of ε subunit for ATP is revised.

Endocytosis of KATP Channels Drives Glucose-Stimulated Excitation of Pancreatic β Cells

Insulin secretion from pancreatic β cells in response to high glucose (HG) critically depends on the inhibition of K_ATP channel activity in HG. It is generally believed that HG-induced effects are mediated by the increase in intracellular ATP, but here, we showed that, in INS-1 cells, endocytosis of K_ATP channel plays a major role. Upon HG stimulation, resting membrane potential depolarized by 30.6 mV (from -69.2 to -38.6 mV) and K_ATP conductance decreased by 91% (from 0.243 to 0.022 nS/pF), whereas intracellular ATP was increased by only 47%. HG stimulation induced internalization of K_ATP channels, causing a significant decrease in surface channel density, and this decrease was completely abolished by inhibiting endocytosis using dynasore, a dynamin inhibitor, or a PKC inhibitor. These drugs profoundly inhibited HG-induced depolarization. Our results suggest that the control of K_ATP channel surface density plays a greater role than ATP-dependent gating in regulating β cell excitability.

Cellular stress is a prerequisite for glucose-induced mitochondrial matrix alkalinization in pancreatic β-cells

Changes in mitochondrial and cytosolic pH alter the chemical gradient across the inner mitochondrial membrane. The proton chemical gradient contributes to mitochondrial ATP synthesis as well as the uptake and release of metabolites and ions from the organelle. Here mitochondrial pH and ΔpH were studied for the first time in human pancreatic β-cells. Adenoviruses were used for rat insulin promoter dependent expression of the pH sensor SypHer targeted to either the mitochondrial matrix or the cytosol. The matrix pH in resting human β-cells is low (pH = 7.50 ± SD 0.17) compared to published values in other cell types. Consequently, the ΔpH of β-cells mitochondria is small. Glucose stimulation consistently resulted in acidification of the matrix pH in INS-1E insulinoma cells and β-cells in intact human islets or islet monolayer cultures. We registered acidification with similar kinetics but of slightly smaller amplitude in the cytosol of β-cells, thus glucose stimulation further reduced the ΔpH. Infection of human islets with high levels of adenoviruses caused the mitochondrial pH to increase. The apoptosis inducer and broad-spectrum kinase inhibitor staurosporine had similar effects on pH homeostasis. Although staurosporine alone does not affect the mitochondrial pH, glucose slightly increases the matrix pH of staurosporine treated cells. These two cellular stressors alter the normal mitochondrial pH response to glucose in pancreatic β-cells.

Mitochondrial toxicity induced by a thiourea gold(i) complex: mitochondrial permeability transition and respiratory deficit

Gold(i) complexes have been widely used as antibacterial and antitumor agents because of their excellent biological activities. However, there are few reports on the study of gold(i) complexes at the subcellular level. Herein, we investigated the toxicity of a gold(i) complex (N,N'-disubstituted cyclic thiourea ligand) - AuTuCl - to isolated mitochondria via various methods. The results showed that AuTuCl induced mitochondrial swelling, elevated ROS generation and triggered collapse of the membrane potential, which indicated the induction of mitochondrial permeability transition (MPT). It also enhanced the permeability of H+ and K+ of the inner membrane and declined membrane fluidity, which might be the result of MPT. Moreover, AuTuCl impaired the mitochondrial respiratory chain and suppressed the activities of complexes II and IV in the respiratory chain. It also triggered the deficiency of ATP and the effusion of Cyt c, which were strictly related to respiration and apoptosis. These results indicated that AuTuCl severely affected the structure and function of mitochondria. It was proposed that MPT and impairment of the respiratory chain were responsible for the mitotoxicity of AuTuCl, thus causing energy deficiency and even apoptosis. This conceivable mechanism can serve as a clue for better understanding of the toxicology of AuTuCl.

Individual islet respirometry reveals functional diversity within the islet population of mice and human donors

OBJECTIVE

Islets from the same pancreas show remarkable variability in glucose sensitivity. While mitochondrial respiration is essential for glucose-stimulated insulin secretion, little is known regarding heterogeneity in mitochondrial function at the individual islet level. This is due in part to a lack of high-throughput and non-invasive methods for detecting single islet function.

METHODS

We have developed a novel non-invasive, high-throughput methodology capable of assessing mitochondrial respiration in large-sized individual islets using the XF96 analyzer (Agilent Technologies).

RESULTS

By increasing measurement sensitivity, we have reduced the minimal size of mouse and human islets needed to assess mitochondrial respiration to single large islets of >35,000 μm2 area (∼210 μm diameter). In addition, we have measured heterogeneous glucose-stimulated mitochondrial respiration among individual human and mouse islets from the same pancreas, allowing population analyses of islet mitochondrial function for the first time.

CONCLUSIONS

We have developed a novel methodology capable of analyzing mitochondrial function in large-sized individual islets. By highlighting islet functional heterogeneity, we hope this methodology can significantly advance islet research.

Beyond Intracellular Signaling: The Ins and Outs of Second Messengers Microdomains

A typical characteristic of eukaryotic cells compared to prokaryotes is represented by the spatial heterogeneity of the different structural and functional components: for example, most of the genetic material is surrounded by a highly specific membrane structure (the nuclear membrane), continuous with, yet largely different from, the endoplasmic reticulum (ER); oxidative phosphorylation is carried out by organelles enclosed by a double membrane, the mitochondria; in addition, distinct domains, enriched in specific proteins, are present in the plasma membrane (PM) of most cells. Less obvious, but now generally accepted, is the notion that even the concentration of small molecules such as second messengers (Ca²⁺ and cAMP in particular) can be highly heterogeneous within cells. In the case of most organelles, the differences in the luminal levels of second messengers depend either on the existence on their membrane of proteins that allow the accumulation/release of the second messenger (e.g., in the case of Ca²⁺, pumps, exchangers or channels), or on the synthesis and degradation of the specific molecule within the lumen (the autonomous intramitochondrial cAMP system). It needs stressing that the existence of a surrounding membrane does not necessarily imply the existence of a gradient between the cytosol and the organelle lumen. For example, the nuclear membrane is highly permeable to both Ca²⁺ and cAMP (nuclear pores are permeable to solutes up to 50 kDa) and differences in [Ca²⁺] or [cAMP] between cytoplasm and nucleoplasm are not seen in steady state and only very transiently during cell activation. A similar situation has been observed, as far as Ca²⁺ is concerned, in peroxisomes.

A Portulaca oleracea L. extract promotes insulin secretion via a K+ATP channel dependent pathway in INS-1 pancreatic β-cells

BACKGROUND/OBJECTIVE

This study was designed to investigate how a Portulaca oleracea L. extract (POE) stimulates insulin secretion in INS-1 pancreatic β-cells.

MATERIALS/METHOD

INS-1 pancreatic β-cells were incubated in the presence of various glucose concentrations: 1.1 or 5.6, 16.7 mM glucose. The cells were treated with insulin secretagogues or insulin secretion inhibitor for insulin secretion assay using an insulin ELISA kit. In order to quantify intracellular influx of Ca²⁺ caused by POE treatment, the effect of POE on intracellular Ca²⁺ in INS-1 pancreatic β-cells was examined using Fluo-2 AM dye.

RESULTS

POE at 10 to 200 µg/mL significantly increased insulin secretion dose-dependently as compared to the control. Experiments at three glucose concentrations (1.1, 5.6, and 16.7 mM) confirmed that POE significantly stimulated insulin secretion on its own as well as in a glucose-dependent manner. POE also exerted synergistic effects on insulin secretion with secretagogues, such as L-alanine, 3-isobutyl-1-methylxanthine, and especially tolbutamide, and at a depolarizing concentration of KCl. The insulin secretion caused by POE was significantly attenuated by treatment with diazoxide, an opener of the K⁺_ATP channel (blocking insulin secretion) and by verapamil (a Ca²⁺ channel blocker). The insulinotropic effect of POE was not observed under Ca²⁺-free conditions in INS-1 pancreatic β-cells. When the cells were preincubated with a Ca²⁺ fluorescent dye, Fluo-2 (acetoxymethyl ester), the cells treated with POE showed changes in fluorescence in red, green, and blue tones, indicating a significant increase in intracellular Ca²⁺, which closely correlated with increases in the levels of insulin secretion.

CONCLUSIONS

These findings indicate that POE stimulates insulin secretion via a K⁺_ATP channel-dependent pathway in INS-1 pancreatic β-cells.

Application of FRET-Based Biosensor "ATeam" for Visualization of ATP Levels in the Mitochondrial Matrix of Living Mammalian Cells

Genetically encoded biosensors utilizing the Förster resonance energy transfer (FRET) are powerful tools for live cell imaging of various cellular processes. Our group has previously developed a series of FRET-based biosensors, named "ATeam," for visualization of ATP levels inside a single living cell. ATeam not only provides a window of insight into a single cell but also allows for visualization of ATP levels in mitochondrial matrix of a single living cell. This novel tool is able to monitor alterations in cellular ATP in response to various treatments in real time. Here we present a method for the evaluation of ATP levels in mitochondria in living cells by using ATeam. At the end of this chapter, an example of experimental results is described for a better understanding of the presented procedure.

Red fluorescent cAMP indicator with increased affinity and expanded dynamic range

cAMP is one of the most important second messengers in biological processes. Cellular dynamics of cAMP have been investigated using a series of fluorescent indicators; however, their sensitivity was sub-optimal for detecting cAMP dynamics at a low concentration range, due to a low ligand affinity and/or poor dynamic range. Seeking an indicator with improved detection sensitivity, we performed insertion screening of circularly permuted mApple, a red fluorescent protein, into the cAMP-binding motif of PKA regulatory subunit Iα and developed an improved cAMP indicator named R-FlincA (Red Fluorescent indicator for cAMP). Its increased affinity (Kd = 0.3 μM) and expanded dynamic range (860% at pH 7.2) allowed the detection of subtle changes in the cellular cAMP dynamics at sub-μM concentrations, which could not be easily observed with existing indicators. Increased detection sensitivity also strengthened the advantages of using R-FlincA as a red fluorescent indicator, as it permits a series of applications, including multi-channel/function imaging of multiple second messengers and combinatorial imaging with photo-manipulation. These results strongly suggest that R-FlincA is a promising tool that accelerates cAMP research by revealing unobserved cAMP dynamics at a low concentration range.

The Pancreatic β-Cell: A Bioenergetic Perspective

The pancreatic β-cell secretes insulin in response to elevated plasma glucose. This review applies an external bioenergetic critique to the central processes of glucose-stimulated insulin secretion, including glycolytic and mitochondrial metabolism, the cytosolic adenine nucleotide pool, and its interaction with plasma membrane ion channels. The control mechanisms responsible for the unique responsiveness of the cell to glucose availability are discussed from bioenergetic and metabolic control standpoints. The concept of coupling factor facilitation of secretion is critiqued, and an attempt is made to unravel the bioenergetic basis of the oscillatory mechanisms controlling secretion. The need to consider the physiological constraints operating in the intact cell is emphasized throughout. The aim is to provide a coherent pathway through an extensive, complex, and sometimes bewildering literature, particularly for those unfamiliar with the field.

Mitochondrial dysfunction induces dendritic loss via eIF2α phosphorylation

Mitochondria are key contributors to the etiology of diseases associated with neuromuscular defects or neurodegeneration. How changes in cellular metabolism specifically impact neuronal intracellular processes and cause neuropathological events is still unclear. We here dissect the molecular mechanism by which mitochondrial dysfunction induced by Prel aberrant function mediates selective dendritic loss in Drosophila melanogaster class IV dendritic arborization neurons. Using in vivo ATP imaging, we found that neuronal cellular ATP levels during development are not correlated with the progression of dendritic loss. We searched for mitochondrial stress signaling pathways that induce dendritic loss and found that mitochondrial dysfunction is associated with increased eIF2α phosphorylation, which is sufficient to induce dendritic pathology in class IV arborization neurons. We also observed that eIF2α phosphorylation mediates dendritic loss when mitochondrial dysfunction results from other genetic perturbations. Furthermore, mitochondrial dysfunction induces translation repression in class IV neurons in an eIF2α phosphorylation-dependent manner, suggesting that differential translation attenuation among neuron subtypes is a determinant of preferential vulnerability.

BTeam, a Novel BRET-based Biosensor for the Accurate Quantification of ATP Concentration within Living Cells

ATP levels may represent fundamental health conditions of cells. However, precise measurement of intracellular ATP levels in living cells is hindered by the lack of suitable methodologies. Here, we developed a novel ATP biosensor termed “BTeam”. BTeam comprises a yellow fluorescent protein (YFP), the ATP binding domain of the ε subunit of the bacterial ATP synthase, and an ATP-nonconsuming luciferase (NLuc). To attain emission, BTeam simply required NLuc substrate. BTeam showed elevated bioluminescence resonance energy transfer efficiency upon ATP binding, resulted in the emission spectra changes correlating with ATP concentrations. By using values of YFP/NLuc emission ratio to represent ATP levels, BTeam achieved steady signal outputs even though emission intensities were altered. With this biosensor, we succeeded in the accurate quantification of intracellular ATP concentrations of a population of living cells, as demonstrated by detecting the slight distribution in the cytosol (3.7–4.1 mM) and mitochondrial matrix (2.4–2.7 mM) within some cultured cell lines. Furthermore, BTeam allowed continuous tracing of cytosolic ATP levels of the same cells, as well as bioluminescent imaging of cytosolic ATP dynamics within individual cells. This simple and accurate technique should be an effective method for quantitative measurement of intracellular ATP concentrations.

Cucurbita ficifolia Bouché increases insulin secretion in RINm5F cells through an influx of Ca(2+) from the endoplasmic reticulum

ETHNOPHARMACOLOGICAL IMPORTANCE

Cucurbita ficifolia Bouché(C. ficifolia) is a plant used in Mexican traditional medicine to control type 2 diabetes (T2D). The hypoglycemic effect of the fruit of C. ficifolia has been demonstrated in different experimental models and in T2D patients. It has been proposed that D-chiro-inositol (DCI) is the active compound of the fruit. Additionally, it has been reported that C. ficifolia increases the mRNA expression of insulin and Kir 6.2 (a component of the ATP-sensitive potassium (K(+)ATP) channel, which is activated by sulphonylurea) in RINm5F cells. However, it remains unclear whether C. ficifolia and DCI causes the secretion of insulin by increasing the concentration of intracellular calcium ([Ca(2+)]i) through K(+)ATP channel blockage or from the reservoir in the endoplasmic reticulum (ER).

MATERIAL AND METHODS

The aqueous extract of C. ficifolia was obtained and standardized with regard to its DCI content. RINm5F pancreatic β-cells were incubated with different concentrations (50, 100, 200 and 400μM) of DCI alone or C. ficifolia (9, 18, 36 and 72µg of extract/mL), and the [Ca(2+)]i of the cells was quantified. The cells were preloaded with the Ca(2+) fluorescent dye fluo4-acetoxymethyl ester (AM) and visualized by confocal microscopy. Insulin secretion was measured by an ELISA method. Subsequently, the effect of C. ficifolia on the K(+)ATP channel was evaluated. In this case, the blocker activator diazoxide was used to inhibit the C. ficifolia-induced calcium influx. In addition, the inositol 1,4,5-trisphosphate (IP3)-receptor-selective inhibitor 2-amino-thoxydiphenylborate (2-APB) was used to inhibit the influx of calcium from the ER that was induced by C. ficifolia.

RESULTS

It was found that DCI alone did not increase [Ca(2+)]i or insulin secretion. In contrast, treatment with C. ficifolia increased [Ca(2+)]i 10-fold compared with the control group. Insulin secretion increased by 46.9%. In the presence of diazoxide, C. ficifolia decreased [Ca(2+)]i by 50%, while insulin secretion increased by 36.4%. In contrast, in the presence of 2-APB, C. ficifolia increased [Ca(2+)]i 18-fold, while insulin secretion remained constant, indicating an additive effect. Therefore, C. ficifolia was not found to block the K(+)ATP channel. However, it did exert an effect by increasing [Ca(2+)]i from the ER, which may partly explain the insulin secretion observed following treatment with C. ficifolia.

CONCLUSIONS

The hypoglycemic properties of C. ficifolia can be explained in part by its effect as a secretagogue for insulin through an increase in [Ca(2+)]i from the calcium reservoir in the ER. Therefore, the mechanism of action of C. ficifolia is different to those of the currently used hypoglycemic drugs, such as sulfonylureas. These results support that C. ficifolia may be a potential natural resource for new agents to control T2D.

On the ATP binding site of the ε subunit from bacterial F-type ATP synthases

F-type ATP synthases are reversible machinery that not only synthesize adenosine triphosphate (ATP) using an electrochemical gradient across the membrane, but also can hydrolyze ATP to pump ions under certain conditions. To prevent wasteful ATP hydrolysis, subunit ε in bacterial ATP synthases changes its conformation from the non-inhibitory down- to the inhibitory up-state at a low cellular ATP concentration. Recently, a crystal structure of the ε subunit in complex with ATP was solved in a non-biologically relevant dimeric form. Here, to derive the functional ATP binding site motif, we carried out molecular dynamics simulations and free energy calculations. Our results suggest that the ATP binding site markedly differs from the experimental resolved one; we observe a reorientation of several residues, which bind to ATP in the crystal structure. In addition we find that an Mg(2+) ion is coordinated by ATP, replacing interactions of the second chain in the crystal structure. Thus we demonstrate more generally the influence of crystallization effects on ligand binding sites and their respective binding modes. Furthermore, we propose a role for two highly conserved residues to control the ATP binding/unbinding event, which have not been considered before. Additionally our results provide the basis for the rational development of new biosensors based on subunit ε, as shown previously for novel sensors measuring the ATP concentration in cells.

High affinity nucleotide-binding mutant of the ε subunit of thermophilic F1-ATPase

Specific ATP binding to the ε subunit of thermophilic F1-ATPase has been utilized for the biosensors of ATP in vivo. I report here that the ε subunit containing R103A/R115A mutations can bind ATP with a dissociation constant at 52 nM, which is two orders of magnitude higher affinity than the wild type. The mutant retained specificity for ATP; ADP and GTP bound to the mutant with dissociation constants 16 and 53 μM, respectively. Thus, the mutant would be a good platform for various types of nucleotide biosensor with appropriate modifications.

Spatiotemporal quantification of subcellular ATP levels in a single HeLa cell during changes in morphology

The quantitative relationship between change in cell shape and ATP consumption is an unsolved problem in cell biology. In this study, a simultaneous imaging and image processing analysis allowed us to observe and quantify these relationships under physiological conditions, for the first time. We focused on two marginal regions of cells: the microtubule-rich 'lamella' and the actin-rich 'peripheral structure'. Simultaneous imaging and correlation analysis revealed that microtubule dynamics cause lamellar shape change accompanying an increase in ATP level. Also, image processing and spatiotemporal quantification enabled to visualize a chronological change of the relationships between the protrusion length and ATP levels, and it suggested they are influencing each other. Furthermore, inhibition of microtubule dynamics diminished motility in the peripheral structure and the range of fluctuation of ATP level in the lamella. This work clearly demonstrates that cellular motility and morphology are regulated by ATP-related cooperative function between microtubule and actin dynamics.

Calcium modulation of exocytosis-linked plasma membrane potential oscillations in INS-1 832/13 cells

Oscillations in plasma membrane potential initiated by substrate-dependent blockade of ATP-sensitive K+ channels in insulin-secreting INS-1 832/13 are differentially linked to distinct voltage-activated Ca2+ channels and drive exocytosis. Ca2+ feeds back to control oscillation frequency, amplitude and prevalence.

The role of mitochondrially derived ATP in synaptic vesicle recycling

Synaptic mitochondria are thought to be critical in supporting neuronal energy requirements at the synapse, and bioenergetic failure at the synapse may impair neural transmission and contribute to neurodegeneration. However, little is known about the energy requirements of synaptic vesicle release or whether these energy requirements go unmet in disease, primarily due to a lack of appropriate tools and sensitive assays. To determine the dependence of synaptic vesicle cycling on mitochondrially derived ATP levels, we developed two complementary assays sensitive to mitochondrially derived ATP in individual, living hippocampal boutons. The first is a functional assay for mitochondrially derived ATP that uses the extent of synaptic vesicle cycling as a surrogate for ATP level. The second uses ATP FRET sensors to directly measure ATP at the synapse. Using these assays, we show that endocytosis has high ATP requirements and that vesicle reacidification and exocytosis require comparatively little energy. We then show that to meet these energy needs, mitochondrially derived ATP is rapidly dispersed in axons, thereby maintaining near normal levels of ATP even in boutons lacking mitochondria. As a result, the capacity for synaptic vesicle cycling is similar in boutons without mitochondria as in those with mitochondria. Finally, we show that loss of a key respiratory subunit implicated in Leigh disease markedly decreases mitochondrially derived ATP levels in axons, thus inhibiting synaptic vesicle cycling. This proves that mitochondria-based energy failure can occur and be detected in individual neurons that have a genetic mitochondrial defect.

Inactivation of corticotropin-releasing hormone-induced insulinotropic role by high-altitude hypoxia

We have shown that hypoxia reduces plasma insulin, which correlates with corticotropin-releasing hormone (CRH) receptor 1 (CRHR1) in rats, but the mechanism remains unclear. Here, we report that hypobaric hypoxia at an altitude of 5,000 m for 8 h enhances rat plasma CRH, corticosterone, and glucose levels, whereas the plasma insulin and pancreatic ATP/ADP ratio is reduced. In islets cultured under normoxia, CRH stimulated insulin release in a glucose- and CRH-level-dependent manner by activating CRHR1 and thus the cAMP-dependent protein kinase pathway and calcium influx through L-type channels. In islets cultured under hypoxia, however, the insulinotropic effect of CRH was inactivated due to reduced ATP and cAMP and coincident loss of intracellular calcium oscillations. Serum and glucocorticoid-inducible kinase 1 (SGK1) also played an inhibitory role. In human volunteers rapidly ascended to 3,860 m, plasma CRH and glucose levels increased without a detectable change in plasma insulin. By contrast, volunteers with acute mountain sickness (AMS) exhibited a marked decrease in HOMA insulin sensitivity (HOMA-IS) and enhanced plasma CRH. In conclusion, hypoxia may attenuate the CRH-insulinotropic effect by reducing cellular ATP/ADP ratio, cAMP and calcium influx, and upregulated SGK1. Hypoxia may not affect HOMA-IS in healthy volunteers but reduces it in AMS volunteers.

Mathematical models of electrical activity of the pancreatic β-cell: a physiological review

Mathematical modeling of the electrical activity of the pancreatic β-cell has been extremely important for understanding the cellular mechanisms involved in glucose-stimulated insulin secretion. Several models have been proposed over the last 30 y, growing in complexity as experimental evidence of the cellular mechanisms involved has become available. Almost all the models have been developed based on experimental data from rodents. However, given the many important differences between species, models of human β-cells have recently been developed. This review summarizes how modeling of β-cells has evolved, highlighting the proposed physiological mechanisms underlying β-cell electrical activity.