A functional link between glucokinase binding to insulin granules and conformational alterations in response to glucose and insulin

NGF effects promote the maturation of rat pancreatic beta cells by regulating GLUT2 levels and distribution, and glucokinase activity

The nerve growth factor (NGF) participates in cell survival and glucose-stimulated insulin secretion (GSIS) processes in rat adult beta cells. GSIS is a complex process in which metabolic events and ionic channel activity are finely coupled. GLUT2 and glucokinase (GK) play central roles in GSIS by regulating the rate of the glycolytic pathway. The biphasic release of insulin upon glucose stimulation characterizes mature adult beta cells. On the other hand, beta cells obtained from neonatal, suckling, and weaning rats are considered immature because they secrete low levels of insulin and do not increase insulin secretion in response to high glucose. The weaning of rats (at postnatal day 20 in laboratory conditions) involves a dietary transition from maternal milk to standard chow. It is characterized by increased basal plasma glucose levels and insulin levels, which we consider physiological insulin resistance. On the other hand, we have observed that incubating rat beta cells with NGF increases GSIS by increasing calcium currents in neonatal cells. In this work, we studied the effects of NGF on the regulation of cellular distribution and activity of GLUT2 and GK to explore its potential role in the maturation of GSIS in beta cells from P20 rats. Pancreatic islet cells from both adult and P20 rats were isolated and incubated with 5.6 mM or 15.6 mM glucose with and without NGF for 4 hours. Specific immunofluorescence assays were conducted following the incubation period to detect insulin and GLUT2. Additionally, we measured glucose uptake, glucokinase activity, and insulin secretion assays at 5.6 mM or 15.6 mM glucose concentrations. We observed an age-dependent variation in the distribution of GLUT2 in pancreatic beta cells and found that glucose plays a regulatory role in GLUT2 distribution independently of age. Moreover, NGF increases GLUT2 abundance, glucose uptake, and GSIS in P20 beta cells and GK activity in adult beta cells. Our results suggest that besides increasing calcium currents, NGF regulates metabolic components of the GSIS, thereby contributing to the maturation process of pancreatic beta cells.

Beneficial metabolic effects of recurrent periods of beta-cell rest and stimulation using stable neuropeptide Y1 and glucagon-like peptide-1 receptor agonists

AIM

To examine whether sequential administration of (d-Arg³⁵ )-sea lamprey peptide tyrosine tyrosine (1-36) (SL-PYY) and the glucagon-like peptide-1 (GLP-1) mimetic, liraglutide, has beneficial effects in diabetes.

METHODS

SL-PYY is an enzymatically stable neuropeptide Y1 receptor (NPY1R) agonist known to induce pancreatic beta-cell rest and improve overall beta-cell health. We employed SL-PYY and liraglutide to induce appropriate recurrent periods of beta-cell rest and stimulation, to assess therapeutic benefits in high fat fed (HFF) mice with streptozotocin (STZ)-induced insulin deficiency, namely HFF-STZ mice.

RESULTS

Previous studies confirm that, at a dose of 0.25 nmol/kg, liraglutide exerts bioactivity over an 8-12 hour period in mice. Initial pharmacokinetic analysis revealed that 75 nmol/kg SL-PYY yielded a similar plasma drug time profile. When SL-PYY (75 nmol/kg) and liraglutide (0.25 nmol/kg) were administered sequentially at 08:00 AM and 08:00 PM, respectively, to HFF-STZ mice for 28 days, reductions in energy intake, body weight, circulating glucose, insulin and glucagon were noted. Similarly positive, but slightly less striking, effects were also apparent with twice-daily liraglutide-only therapy. The sequential SL-PYY and liraglutide treatment also improved insulin sensitivity and glucose-induced insulin secretory responses, which was not apparent with liraglutide treatment, although benefits on glucose tolerance were mild. Interestingly, combined therapy also elevated pancreatic insulin, decreased pancreatic glucagon and enhanced the plasma insulin/glucagon ratio compared with liraglutide alone. This was not associated with an enhancement of beneficial changes in islet cell areas, proliferation or apoptosis compared with liraglutide alone, but the numbers of centrally stained glucagon-positive islet cells were reduced by sequential combination therapy.

CONCLUSION

These data show that NPY1R-induced intervals of beta-cell rest, combined with GLP-1R-stimulated periods of beta-cell stimulation, should be further evaluated as an effective treatment option for obesity-driven forms of diabetes.

Measuring Molecular Diffusion in Dynamic Subcellular Nanostructures by Fast Raster Image Correlation Spectroscopy and 3D Orbital Tracking

Here we provide demonstration that fast fluorescence fluctuation spectroscopy is a fast and robust approach to extract information on the dynamics of molecules enclosed within subcellular nanostructures (e.g., organelles or vesicles) which are also moving in the complex cellular environment. In more detail, Raster Image Correlation Spectroscopy (RICS) performed at fast timescales (i.e., microseconds) reveals the fast motion of fluorescently labeled molecules within two exemplary dynamic subcellular nanostructures of biomedical interest, the lysosome and the insulin secretory granule (ISG). The measurement of molecular diffusion is then used to extract information on the average properties of subcellular nanostructures, such as macromolecular crowding or molecular aggregation. Concerning the lysosome, fast RICS on a fluorescent tracer allowed us to quantitatively assess the increase in organelle viscosity in the pathological condition of Krabbe disease. In the case of ISGs, fast RICS on two ISG-specific secreting peptides unveiled their differential aggregation propensity depending on intragranular concentration. Finally, a combination of fast RICS and feedback-based 3D orbital tracking was used to subtract the slow movement of subcellular nanostructures from the fast diffusion of molecules contained within them and independently validate the results. Results presented here not only demonstrate the acquired ability to address the dynamic behavior of molecules in moving, nanoscopic reference systems, but prove the relevance of this approach to advance our knowledge on cell function at the subcellular scale.

It's What and When You Eat: An Overview of Transcriptional and Epigenetic Responses to Dietary Perturbations in Pancreatic Islets

Our ever-changing modern environment is a significant contributor to the increased prevalence of many chronic diseases, and particularly, type 2 diabetes mellitus (T2DM). Although the modern era has ushered in numerous changes to our daily living conditions, changes in "what" and "when" we eat appear to disproportionately fuel the rise of T2DM. The pancreatic islet is a key biological controller of an organism's glucose homeostasis and thus plays an outsized role to coordinate the response to environmental factors to preserve euglycemia through a delicate balance of endocrine outputs. Both successful and failed adaptation to dynamic environmental stimuli has been postulated to occur due to changes in the transcriptional and epigenetic regulation of pathways associated with islet secretory function and survival. Therefore, in this review we examined and evaluated the current evidence elucidating the key epigenetic mechanisms and transcriptional programs underlying the islet's coordinated response to the interaction between the timing and the composition of dietary nutrients common to modern lifestyles. With the explosion of next generation sequencing, along with the development of novel informatic and -omic approaches, future work will continue to unravel the environmental-epigenetic relationship in islet biology with the goal of identifying transcriptional and epigenetic targets associated with islet perturbations in T2DM.

The manifold roles of protein S-nitrosylation in the life of insulin

Insulin, which is released by pancreatic islet β-cells in response to elevated levels of glucose in the blood, is a critical regulator of metabolism. Insulin triggers the uptake of glucose and fatty acids into the liver, adipose tissue and muscle, and promotes the storage of these nutrients in the form of glycogen and lipids. Dysregulation of insulin synthesis, secretion, transport, degradation or signal transduction all cause failure to take up and store nutrients, resulting in type 1 diabetes mellitus, type 2 diabetes mellitus and metabolic dysfunction. In this Review, we make the case that insulin signalling is intimately coupled to protein S-nitrosylation, in which nitric oxide groups are conjugated to cysteine thiols to form S-nitrosothiols, within effectors of insulin action. We discuss the role of S-nitrosylation in the life cycle of insulin, from its synthesis and secretion in pancreatic β-cells, to its signalling and degradation in target tissues. Finally, we consider how aberrant S-nitrosylation contributes to metabolic diseases, including the roles of human genetic mutations and cellular events that alter S-nitrosylation of insulin-regulating proteins. Given the growing influence of S-nitrosylation in cellular metabolism, the field of metabolic signalling could benefit from renewed focus on S-nitrosylation in type 2 diabetes mellitus and insulin-related disorders.

Ordered Conformation-Regulated Vesicular Membrane Permeability

In nature, the folding and conformation of proteins can control the cell or organelle membrane permeability and regulate the life activities. Here we report the first example of synthetic polypeptide vesicles that regulate their permeability via ordered transition of secondary conformations, in a manner similar to biological systems.  The polymersomes undergo a β-sheet to α-helix transition in response to reactive oxygen species (ROS), leading to wall thinning without loss of vesicular integrity. The change  of membrane structure increases the vesicular permeability and enables specific transport of payloads with different molecular weights.The change of membrane structure increases the vesicular permeability.  As a proof-of-concept, the polymersomes encapsulating enzymes could serve as nanoreactors and carries for glucose-stimulated insulin secretion in vivo inspired by human glucokinase, resulting in safe and effective treatment of type 1 diabetes mellitus in mouse models. This study will help understand the biology of biomembranes and facilitate the engineering of nanoplatforms for biomimicry, biosensing, and controlled delivery applications.

Intercellular Communication in the Islet of Langerhans in Health and Disease

Blood glucose homeostasis requires proper function of pancreatic islets, which secrete insulin, glucagon, and somatostatin from the β-, α-, and δ-cells, respectively. Each islet cell type is equipped with intrinsic mechanisms for glucose sensing and secretory actions, but these intrinsic mechanisms alone cannot explain the observed secretory profiles from intact islets. Regulation of secretion involves interconnected mechanisms among and between islet cell types. Islet cells lose their normal functional signatures and secretory behaviors upon dispersal as compared to intact islets and in vivo. In dispersed islet cells, the glucose response of insulin secretion is attenuated from that seen from whole islets, coordinated oscillations in membrane potential and intracellular Ca²⁺ activity, as well as the two-phase insulin secretion profile, are missing, and glucagon secretion displays higher basal secretion profile and a reverse glucose-dependent response from that of intact islets. These observations highlight the critical roles of intercellular communication within the pancreatic islet, and how these communication pathways are crucial for proper hormonal and nonhormonal secretion and glucose homeostasis. Further, misregulated secretions of islet secretory products that arise from defective intercellular islet communication are implicated in diabetes. Intercellular communication within the islet environment comprises multiple mechanisms, including electrical synapses from gap junctional coupling, paracrine interactions among neighboring cells, and direct cell-to-cell contacts in the form of juxtacrine signaling. In this article, we describe the various mechanisms that contribute to proper islet function for each islet cell type and how intercellular islet communications are coordinated among the same and different islet cell types. © 2021 American Physiological Society. Compr Physiol 11:2191-2225, 2021.

Basal insulin ameliorates post-breakfast hyperglycemia via suppression of post-breakfast proinsulin/C-peptide ratio and fasting serum free fatty acid levels in patients with type 2 diabetes

Background

In general, basal insulin targets fasting plasma glucose (FPG) levels, and prandial insulin targets postprandial glucose (PPG) levels. However, the effects of basal insulin on PPG levels are controversial. We investigated the effect of basal insulin on postprandial hyperglycemia using a test meal at breakfast as well as compared differences between degludec and glargine.

Methods

A total of 20 participants with type 2 diabetes were randomly assigned to degludec (n = 10) or glargine (n = 10). We initiated basal-bolus insulin therapy and titrated only basal insulin until FPG was < 6.1 mmol/L. We evaluated changes in post-breakfast glucose levels and changes in clinical parameters such as serum C-peptide (CPR), proinsulin (PI), and free fatty acids (FFA) levels between the pre- and post-titration periods. Differences between degludec and glargine in the post-titration period were also evaluated.

Results

Post-breakfast glucose levels significantly decreased by 46.1% in the post-titration period compared with the pre-titration period (n = 20, p < 0.001). These decreases correlated positively with decreases in the post-breakfast PI/CPR ratio (r = 0.692, p < 0.001) and in fasting FFA levels (r = 0.720, p < 0.001). There were no significant differences in post-breakfast glucose levels between degludec and glargine. However, the hypoglycemic rate with degludec was significantly lower than with glargine.

Conclusion

Our results suggest that basal insulin with either degludec or glargine decreases the incidence of post-breakfast hyperglycemia accompanied by decreasing the post-breakfast PI/CPR ratio and fasting FFA levels in patients with type 2 diabetes.

Functional Inactivation of Drosophila GCK Orthologs Causes Genomic Instability and Oxidative Stress in a Fly Model of MODY-2

Maturity-onset diabetes of the young (MODY) type 2 is caused by heterozygous inactivating mutations in the gene encoding glucokinase (GCK), a pivotal enzyme for glucose homeostasis. In the pancreas GCK regulates insulin secretion, while in the liver it promotes glucose utilization and storage. We showed that silencing the DrosophilaGCK orthologs Hex-A and Hex-C results in a MODY-2-like hyperglycemia. Targeted knock-down revealed that Hex-A is expressed in insulin producing cells (IPCs) whereas Hex-C is specifically expressed in the fat body. We showed that Hex-A is essential for insulin secretion and it is required for Hex-C expression. Reduced levels of either Hex-A or Hex-C resulted in chromosome aberrations (CABs), together with an increased production of advanced glycation end-products (AGEs) and reactive oxygen species (ROS). This result suggests that CABs, in GCK depleted cells, are likely due to hyperglycemia, which produces oxidative stress through AGE metabolism. In agreement with this hypothesis, treating GCK-depleted larvae with the antioxidant vitamin B6 rescued CABs, whereas the treatment with a B6 inhibitor enhanced genomic instability. Although MODY-2 rarely produces complications, our data revealed the possibility that MODY-2 impacts genome integrity.

L-Arginine prevents cereblon-mediated ubiquitination of glucokinase and stimulates glucose-6-phosphate production in pancreatic β-cells

We sought to determine a mechanism by which L-arginine increases glucose-stimulated insulin secretion (GSIS) in β-cells by finding a protein with affinity to L-arginine using arginine-immobilized magnetic nanobeads technology. Glucokinase (GCK), the key regulator of GSIS and a disease-causing gene of maturity-onset diabetes of the young type 2 (MODY2), was found to bind L-arginine. L-Arginine stimulated production of glucose-6-phosphate (G6P) and induced insulin secretion. We analyzed glucokinase mutants and identified three glutamate residues that mediate binding to L-arginine. One MODY2 patient with GCK^E442* demonstrated lower C-peptide-to-glucose ratio after arginine administration. In β-cell line, GCK^E442* reduced L-arginine-induced insulin secretion compared with GCK^WT. In addition, we elucidated that the binding of arginine protects glucokinase from degradation by E3 ubiquitin ligase cereblon mediated ubiquitination. We conclude that L-arginine induces insulin secretion by increasing G6P production by glucokinase through direct stimulation and by prevention of degradation.

Using arginine-immobilized magnetic nanobeads, Cho et al. show that glucokinase, the key regulator of glucose-stimulated insulin secretion, binds L-arginine, which protects glucokinase from ubiquitination-mediated degradation while inducing insulin secretion. This study provides mechanistic insights into how L-arginine increases insulin production.

Reducing Glucokinase Activity to Enhance Insulin Secretion: A Counterintuitive Theory to Preserve Cellular Function and Glucose Homeostasis

Pancreatic beta-cells are the only cells in the body that can synthesize and secrete insulin. Through the process of glucose-stimulated insulin secretion, beta-cells release insulin into circulation, stimulating GLUT4-dependent glucose uptake into peripheral tissue. Insulin is normally secreted in pulses that promote signaling at the liver. Long before type 2 diabetes is diagnosed, beta-cells become oversensitive to glucose, causing impaired pulsatility and overstimulation in fasting levels of glucose. The resulting hypersecretion of insulin can cause poor insulin signaling and clearance at the liver, leading to hyperinsulinemia and insulin resistance. Continued overactivity can eventually lead to beta-cell exhaustion and failure at which point type 2 diabetes begins. To prevent or reverse the negative effects of overstimulation, beta-cell activity can be reduced. Clinical studies have revealed the potential of beta-cell rest to reverse new cases of diabetes, but treatments lack durable benefits. In this perspective, we propose an intervention that reduces overactive glucokinase activity in the beta-cell. Glucokinase is known as the glucose sensor of the beta-cell due to its high control over insulin secretion. Therefore, glycolytic overactivity may be responsible for hyperinsulinemia early in the disease and can be reduced to restore normal stimulus-secretion coupling. We have previously reported that reducing glucokinase activity in prediabetic mouse islets can restore pulsatility and enhance insulin secretion. Building on this counterintuitive finding, we review the importance of pulsatile insulin secretion and highlight how normalizing glucose sensing in the beta cell during prediabetic hyperinsulinemia may restore pulsatility and improve glucose homeostasis.

Insulin secretory granules labelled with phogrin-fluorescent proteins show alterations in size, mobility and responsiveness to glucose stimulation in living β-cells

The intracellular life of insulin secretory granules (ISGs) from biogenesis to secretion depends on their structural (e.g. size) and dynamic (e.g. diffusivity, mode of motion) properties. Thus, it would be useful to have rapid and robust measurements of such parameters in living β-cells. To provide such measurements, we have developed a fast spatiotemporal fluctuation spectroscopy. We calculate an imaging-derived Mean Squared Displacement (iMSD), which simultaneously provides the size, average diffusivity, and anomalous coefficient of ISGs, without the need to extract individual trajectories. Clustering of structural and dynamic quantities in a multidimensional parametric space defines the ISGs’ properties for different conditions. First, we create a reference using INS-1E cells expressing proinsulin fused to a fluorescent protein (FP) under basal culture conditions and validate our analysis by testing well-established stimuli, such as glucose intake, cytoskeleton disruption, or cholesterol overload. After, we investigate the effect of FP-tagged ISG protein markers on the structural and dynamic properties of the granule. While iMSD analysis produces similar results for most of the lumenal markers, the transmembrane marker phogrin-FP shows a clearly altered result. Phogrin overexpression induces a substantial granule enlargement and higher mobility, together with a partial de-polymerization of the actin cytoskeleton, and reduced cell responsiveness to glucose stimulation. Our data suggest a more careful interpretation of many previous ISG-based reports in living β-cells. The presented data pave the way to high-throughput cell-based screening of ISG structure and dynamics under various physiological and pathological conditions.

Molecular and cellular regulation of human glucokinase

Glucose metabolism in humans is tightly controlled by the activity of glucokinase (GCK). GCK is predominantly produced in the pancreas, where it catalyzes the rate-limiting step of insulin secretion, and in the liver, where it participates in glycogen synthesis. A multitude of disease-causing mutations within the gck gene have been identified. Activating mutations manifest themselves in the clinic as congenital hyperinsulinism, while loss-of-function mutations produce several diabetic conditions. Indeed, pharmaceutical companies have shown great interest in developing GCK-associated treatments for diabetic patients. Due to its essential role in maintaining whole-body glucose homeostasis, GCK activity is extensively regulated at multiple levels. GCK possesses a unique ability to self-regulate its own activity via slow conformational dynamics, which allows for a cooperative response to glucose. GCK is also subject to a number of protein-protein interactions and post-translational modification events that produce a broad range of physiological consequences. While significant advances in our understanding of these individual regulatory mechanisms have been recently achieved, how these strategies are integrated and coordinated within the cell is less clear. This review serves to synthesize the relevant findings and offer insights into the connections between molecular and cellular control of GCK.

Genetically Encoded Fluorescent Biosensors Illuminate the Spatiotemporal Regulation of Signaling Networks

Cellular signaling networks are the foundation which determines the fate and function of cells as they respond to various cues and stimuli. The discovery of fluorescent proteins over 25 years ago enabled the development of a diverse array of genetically encodable fluorescent biosensors that are capable of measuring the spatiotemporal dynamics of signal transduction pathways in live cells. In an effort to encapsulate the breadth over which fluorescent biosensors have expanded, we endeavored to assemble a comprehensive list of published engineered biosensors, and we discuss many of the molecular designs utilized in their development. Then, we review how the high temporal and spatial resolution afforded by fluorescent biosensors has aided our understanding of the spatiotemporal regulation of signaling networks at the cellular and subcellular level. Finally, we highlight some emerging areas of research in both biosensor design and applications that are on the forefront of biosensor development.

Expression of miR-206 in human islets and its role in glucokinase regulation

Inappropriate insulin secretion from β-cells is considered as an early sign of impaired glucose tolerance and type 2 diabetes (T2D). Glucokinase (GCK) is an important enzyme that regulates glucose metabolism and ensures that the normal circulating glucose concentrations are maintained. GCK expression is induced by glucose and regulated via transcription factors and regulatory proteins. Recently, microRNA-206 (miR-206) was reported to regulate GCK and alter glucose tolerance in normal and high-fat diet-fed mice. Although the study findings have implications for human diabetes, studies in human islets are lacking. Here, we analyze human islets from individuals without or with T2D, using TaqMan-based real-time qPCR at the tissue (isolated islet) level as well as at single cell resolution, to assess the relationship between miR-206 and GCK expression in normal and T2D human islets. Our data suggest that, unlike mouse islets, human islets do not exhibit any correlation between miR-206 and GCK transcripts. These data implicate the need for further studies aimed toward exploring its potential role(s) in human islets.

Nitric Oxide Activates β-Cell Glucokinase by Promoting Formation of the "Glucose-Activated" State

The release of insulin from the pancreas is tightly controlled by glucokinase (GCK) activity that couples β-cell metabolism to changes in blood sugar. Despite having only a single glucose-binding site, GCK displays positive glucose cooperativity. Ex vivo structural studies have identified several potential protein conformations with varying levels of enzymatic activity, yet it is unclear how living cells regulate GCK cooperativity. To better understand the cellular regulation of GCK activation, we developed a homotransfer Förster resonance energy transfer (FRET) GCK biosensor and used polarization microscopy to eliminate fluorescence crosstalk from FRET quantification and improve the signal-to-noise ratio. This approach enhanced sensor contrast compared to that seen with the heterotransfer FRET GCK reporter and allowed observation of individual GCK states using an automated method to analyze FRET data at the pixel level. Mutations known to activate and inhibit GCK activity produced distinct anisotropy distributions, suggesting that at least two conformational states exist in living cells. A high glucose level activated the biosensor in a manner consistent with GCK's enzymology. Interestingly, glucose-free conditions did not affect GCK biosensor FRET, indicating that there is a single low-activity state, which is counter to proposed structural models of GCK cooperativity. Under low-glucose conditions, application of chemical NO donors efficiently shifted GCK to the more active conformation. Notably, GCK activation by mutation, a high glucose level, a pharmacological GCK activator, or S-nitrosylation all shared the same FRET distribution. These data suggest a simplified model for GCK activation in living cells, where post-translational modification of GCK by S-nitrosylation facilitates a single conformational transition that enhances GCK enzymatic activity.

Regulation of Glucokinase by Intracellular Calcium Levels in Pancreatic β Cells

Glucokinase (GCK) controls the rate of glucose metabolism in pancreatic β cells, and its activity is rate-limiting for insulin secretion. Posttranslational GCK activation can be stimulated through either G protein-coupled receptors or receptor tyrosine kinase signaling pathways, suggesting a common mechanism. Here we show that inhibiting Ca(2+) release from the endoplasmic reticulum (ER) decouples GCK activation from receptor stimulation. Furthermore, pharmacological release of ER Ca(2+) stimulates activation of a GCK optical biosensor and potentiates glucose metabolism, implicating rises in cytoplasmic Ca(2+) as a critical regulatory mechanism. To explore the potential for glucose-stimulated GCK activation, the GCK biosensor was optimized using circularly permuted mCerulean3 proteins. This new sensor sensitively reports activation in response to insulin, glucagon-like peptide 1, and agents that raise cAMP levels. Transient, glucose-stimulated GCK activation was observed in βTC3 and MIN6 cells. An ER-localized channelrhodopsin was used to manipulate the cytoplasmic Ca(2+) concentration in cells expressing the optimized FRET-GCK sensor. This permitted quantification of the relationship between cytoplasmic Ca(2+) concentrations and GCK activation. Half-maximal activation of the FRET-GCK sensor was estimated to occur at ∼400 nm Ca(2+). When expressed in islets, fluctuations in GCK activation were observed in response to glucose, and we estimated that posttranslational activation of GCK enhances glucose metabolism by ∼35%. These results suggest a mechanism for integrative control over GCK activation and, therefore, glucose metabolism and insulin secretion through regulation of cytoplasmic Ca(2+) levels.

Src regulates insulin secretion and glucose metabolism by influencing subcellular localization of glucokinase in pancreatic β-cells

Aims/Introduction

Src, a non‐receptor tyrosine kinase, regulates a wide range of cellular functions, and hyperactivity of Src is involved in impaired glucose metabolism in pancreatic β‐cells. However, the physiological role of Src in glucose metabolism in normal, unstressed β‐cells remains unclear. In the present study, we investigated the role of Src in insulin secretion and glucose metabolism.

Materials and Methods

Src was downregulated using small interfering ribonucleic acid in INS‐1 cells, and glucose‐induced insulin secretion, adenosine triphosphate content, intracellular calcium concentration, glucose utilization and glucokinase activity were measured. Expression levels of messenger ribonucleic acid and protein of glucokinase were examined by semiquantitative real‐time polymerase chain reaction and immunoblotting, respectively. Cells were fractionated by digitonin treatment, and subcellular localization of glucokinase was examined by immunoblotting. Interaction between glucokinase and neuronal nitric oxide synthase was estimated by immunoprecipitation.

Results

In Src downregulated INS‐1 cells, glucose‐induced insulin secretion was impaired, whereas insulin secretion induced by high K⁺ was not affected. Intracellular adenosine triphosphate content and elevation of intracellular calcium concentration by glucose stimulation were suppressed by Src downregulation. Src downregulation reduced glucose utilization in the presence of high glucose, which was accompanied by a reduction in glucokinase activity without affecting its expression. However, Src downregulation reduced glucokinase in soluble, cytoplasmic fraction, and increased it in pellet containing intaracellular organelles. In addition, interaction between glucokinase and neuronal nitric oxide synthase was facilitated by Src downregulation.

Conclusions

Src plays an important role in glucose‐induced insulin secretion in pancreatic β‐cells through maintaining subcellular localization and activity of glucokinase.

Role of Islet Glucokinase, Glucose Metabolism, and Insulin Pathway in the Enhancing Effect of Islet Neogenesis-Associated Protein on Glucose-Induced Insulin Secretion

OBJECTIVE

To demonstrate the role of islet glucokinase, glucose metabolism, and intracellular insulin mediators in the enhancing effect of islet neogenesis-associated protein pentadecapeptide (INGAP-PP) on glucose-induced insulin secretion.

METHODS

Islets from normal rats were cultured for 4 days in the absence or presence of 10 μg/mL INGAP-PP, with/without Wortmannin or LY294002. Islets were incubated with different glucose concentrations to measure insulin secretion and content, hexokinase and glucokinase activity, glucose oxidation and utilization, glucokinase, insulin receptor, insulin receptor substrate (IRS)-1/2, and PI3K concentration and phosphorylation.

RESULTS

The INGAP-PP significantly increased insulin release at high but not at low glucose concentration, glucokinase activity, glucose metabolism, glucokinase, insulin receptor, IRS-2 and PI3K protein concentration, insulin receptor and IRS-1/2 tyrosine phosphorylation, and the association of p85 with IRS-1. Wortmannin and LY294002 blocked INGAP-PP effect on insulin secretion and glucokinase protein levels in a dose-dependent manner.

CONCLUSIONS

The enhancing effect of INGAP-PP on glucose-induced insulin release could be partly ascribed to its effect on glucokinase activity and glucose metabolism and is mainly mediated by the PI3K/AKT pathway. These results, together with the low hypoglycemia risk associated with the use of INGAP-PP, offer a new alternative for diabetes prevention and treatment.

Cytoskeletal dependence of insulin granule movement dynamics in INS-1 beta-cells in response to glucose

For pancreatic β-cells to secrete insulin in response to elevated blood glucose, insulin granules retained within the subplasmalemmal space must be transported to sites of secretion on the plasma membrane. Using a combination of super-resolution STORM imaging and live cell TIRF microscopy we investigate how the organization and dynamics of the actin and microtubule cytoskeletons in INS-1 β-cells contribute to this process. GFP-labeled insulin granules display 3 different modes of motion (stationary, diffusive-like, and directed). Diffusive-like motion dominates in basal, low glucose conditions. Upon glucose stimulation no gross rearrangement of the actin cytoskeleton is observed but there are increases in the 1) rate of microtubule polymerization; 2) rate of diffusive-like motion; and 3) proportion of granules undergoing microtubule-based directed motion. By pharmacologically perturbing the actin and microtubule cytoskeletons, we determine that microtubule-dependent granule transport occurs within the subplasmalemmal space and that the actin cytoskeleton limits this transport in basal conditions, when insulin secretion needs to be inhibited.

Cholesterol-enriched membrane rafts and insulin secretion

The failure of pancreatic β‐cells to supply insulin in quantities sufficient to maintain euglycemia is a hallmark of type 2 diabetes. Perturbation of β‐cell cholesterol homeostasis, culminating in elevated intracellular cholesterol levels, impairs insulin secretion and has therefore been proposed as a mechanism contributing to β‐cell dysfunction. The manner in which this occurs, however, is unclear. Cholesterol is an essential lipid, as well as a major component of membrane rafts, and numerous proteins critical for the regulation of insulin secretion have been reported to associate with these domains. Although this suggests that alterations in membrane rafts could partially account for the reduction in insulin secretion observed when β‐cell cholesterol accumulates, this has not yet been demonstrated. In this review, we provide a brief overview of recent work implicating membrane rafts in some of the basic molecular mechanisms of insulin secretion, and discuss the insight it provides into the β‐cell dysfunction characteristic of type 2 diabetes. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2012.00200.x, 2012)

Cholesterol reduction ameliorates glucose-induced calcium handling and insulin secretion in islets from low-density lipoprotein receptor knockout mice

AIMS/HYPOTHESIS

Changes in cellular cholesterol level may contribute to beta cell dysfunction. Islets from low density lipoprotein receptor knockout (LDLR(-/-)) mice have higher cholesterol content and secrete less insulin than wild-type (WT) mice. Here, we investigated the association between cholesterol content, insulin secretion and Ca(2+) handling in these islets.

METHODS

Isolated islets from both LDLR(-/-) and WT mice were used for measurements of insulin secretion (radioimmunoassay), cholesterol content (fluorimetric assay), cytosolic Ca(2+) level (fura-2AM) and SNARE protein expression (VAMP-2, SNAP-25 and syntaxin-1A). Cholesterol was depleted by incubating the islets with increasing concentrations (0-10mmol/l) of methyl-beta-cyclodextrin (MβCD).

RESULTS

The first and second phases of glucose-stimulated insulin secretion (GSIS) were lower in LDLR(-/-) than in WT islets, paralleled by an impairment of Ca(2+) handling in the former. SNAP-25 and VAMP-2, but not syntaxin-1A, were reduced in LDLR(-/-) compared with WT islets. Removal of excess cholesterol from LDLR(-/-) islets normalized glucose- and tolbutamide-induced insulin release. Glucose-stimulated Ca(2+) handling was also normalized in cholesterol-depleted LDLR(-/-) islets. Cholesterol removal from WT islets by 0.1 and 1.0mmol/l MβCD impaired both GSIS and Ca(2+) handling. In addition, at 10mmol/l MβCD WT islet showed a loss of membrane integrity and higher DNA fragmentation.

CONCLUSION

Abnormally high (LDLR(-/-) islets) or low cholesterol content (WT islets treated with MβCD) alters both GSIS and Ca(2+) handling. Normalization of cholesterol improves Ca(2+) handling and insulin secretion in LDLR(-/-) islets.

Association with nitric oxide synthase on insulin secretory granules regulates glucokinase protein levels

Glucokinase (GCK) association with insulin-secretory granules is controlled by interaction with nitric oxide synthase (NOS) and is reversed by GCK S-nitrosylation. Nonetheless, the function of GCK sequestration on secretory granules is unknown. Here we report that the S-nitrosylation blocking V367M mutation prevents GCK accumulation on secretory granules by inhibiting association with NOS. Expression of this mutant is reduced compared with a second S-nitrosylation blocking GCK mutant (C371S) that accumulates to secretory granules and is expressed at levels greater than wild type. Even so, the rate of degradation for wild type and mutant GCK proteins were not significantly different from one another, and neither mutation disrupted the ability of GCK to be ubiquitinated. Furthermore, gene silencing of NOS reduced endogenous GCK content but did not affect β-actin content. Treatment of GCK(C371S) expressing cells with short interfering RNA specific for NOS also blocked accumulation of this protein to secretory granules and reduced expression levels to that of GCK(V367M). Conversely, cotransfection of catalytically inactive NOS increased GCK-mCherry levels. Expression of GCK(C371S) in βTC3 cells enhanced glucose metabolism compared with untransfected cells and cells expressing wild type GCK, even though this mutant has slightly reduced enzymatic activity in vitro. Finally, molecular dynamics simulations revealed that V367M induces conformational changes in GCK that are similar to S-nitrosylated GCK, thereby suggesting a mechanism for V367M-inhibition of NOS association. Our findings suggest that sequestration of GCK on secretory granules regulates cellular GCK protein content, and thus cellular GCK activity, by acting as a storage pool for GCK proteins.

Ultramarine, a chromoprotein acceptor for Förster resonance energy transfer

We have engineered a monomeric blue non-fluorescent chromoprotein called Ultramarine (fluorescence quantum yield, 0.001; ε _585nm, 64,000 M⁻¹. cm⁻¹) for use as a Förster resonance energy transfer acceptor for a number of different donor fluorescent proteins. We show its use for monitoring activation of caspase 3 in live cells using fluorescence lifetime imaging. Ultramarine has the potential to increase the number of cellular parameters that can be imaged simultaneously.

Plant-based FRET biosensor discriminates environmental zinc levels

Heavy metal accumulation in the environment poses great risks to flora and fauna. However, monitoring sites prone to accumulation poses scale and economic challenges. In this study, we present and test a method for monitoring these sites using fluorescent resonance energy transfer (FRET) change in response to zinc (Zn) accumulation in plants as a proxy for environmental health. We modified a plant Zn transport protein by adding flanking fluorescent proteins (FPs) and deploying the construct into two different species. In Arabidopsis thaliana, FRET was monitored by a confocal microscope and had a 1.4-fold increase in intensity as the metal concentration increased. This led to a 16.7% overall error-rate when discriminating between a control (1μm Zn) and high (10mm Zn) treatment after 96h. The second host plant (Populus tremula×Populu salba) also had greater FRET values (1.3-fold increase) when exposed to the higher concentration of Zn, while overall error-rates were greater at 22.4%. These results indicate that as plants accumulate Zn, protein conformational changes occur in response to Zn causing differing interaction between FPs. This results in greater FRET values when exposed to greater amounts of Zn and monitored with appropriate light sources and filters. We also demonstrate how this construct can be moved into different host plants effectively including one tree species. This chimeric protein potentially offers a method for monitoring large areas of land for Zn accumulation, is transferable among species, and could be modified to monitor other specific heavy metals that pose environmental risks.

Glucagon-like peptide 1 stimulates post-translational activation of glucokinase in pancreatic beta cells

Glucagon-like peptide 1 (GLP-1) potentiates glucose-stimulated insulin secretion from pancreatic β cells, yet does not directly stimulate secretion. The mechanisms underlying this phenomenon are incompletely understood. Here, we report that GLP-1 augments glucose-dependent rises in NAD(P)H autofluorescence in both βTC3 insulinoma cells and islets in a manner consistent with post-translational activation of glucokinase (GCK). GLP-1 treatment increased GCK activity and enhanced GCK S-nitrosylation in βTC3 cells. A 2-fold increase in S-nitrosylated GCK was also observed in mouse islets. Furthermore, GLP-1 activated a FRET-based GCK reporter in living cells. Activation of this reporter was sensitive to inhibition of nitric-oxide synthase (NOS), and incorporating the S-nitrosylation-blocking V367M mutation into this sensor prevented activation by GLP-1. GLP-1 potentiation of the glucose-dependent increase in islet NAD(P)H autofluorescence was also sensitive to a NOS inhibitor, whereas NOS inhibition did not affect the response to glucose alone. Expression of the GCK(V367M) mutant also blocked GLP-1 potentiation of the NAD(P)H response to glucose in βTC3 cells, but did not significantly affect metabolism of glucose in the absence of GLP-1. Co-expression of WT or mutant GCK proteins with a sensor for insulin secretory granule fusion also revealed that blockade of post-translational GCK S-nitrosylation diminished the effects of GLP-1 on granule exocytosis by ∼40% in βTC3 cells. These results suggest that post-translational activation of GCK is an important mechanism for mediating the insulinotropic effects of GLP-1.

Endogenous activation of glucokinase by 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase is glucose dependent

Glucokinase (GK) plays a crucial role as glucose sensor in glucose-induced insulin secretion in pancreatic β-cells. The bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) acts as an endogenous GK activator. Therefore, the goal of this study was the analysis of GK-PFK-2/FBPase-2 complex formation and its effect on metabolic stimulus-secretion coupling in β-cells in dependence upon glucose. The interaction between GK and PFK-2/FBPase-2 was analyzed in insulin-secreting MIN6 cells with a new fluorescence-based mammalian two-hybrid system. In contrast to the commonly used mammalian two-hybrid systems that require sampling before detection, the system used allows monitoring of the effects of environmental changes on protein-protein interactions on the single-cell level. Increasing the glucose concentration in the cell culture medium from 3 to 10 and 25 mmol/liter amplified the interaction between the enzymes stepwise. Importantly, in line with these results, overexpression of PFK-2/FBPase-2 in MIN6 cells evoked only at 10 and 25 mmol/liter, an increase in insulin secretion. Furthermore, a PFK-2/FBPase-2 mutant with an abolished GK-binding motif neither showed a glucose-dependent GK binding nor was able to increase insulin secretion. The results obtained with the mammalian two-hybrid system could be confirmed by fluorescence resonance energy transfer experiments in COS cells. Furthermore, the established interaction between GK and the liver GRP served in all experiments as a control. Thus, this study clearly showed that binding and activation of GK by PFK-2/FBPase-2 in β-cells is promoted by glucose, resulting in an enhancement of insulin secretion at stimulatory glucose concentrations, without affecting basal insulin secretion.

Naturally occurring glucokinase mutations are associated with defects in posttranslational S-nitrosylation

Posttranslational activation of glucokinase (GCK) through S-nitrosylation has been recently observed in the insulin-secreting pancreatic beta-cell; however, the function of this molecular mechanism in regulating the physiology of insulin secretion is not well understood. To more fully understand the function of posttranslational regulation of GCK, we examined two naturally occurring GCK mutations that map to residues proximal to the S-nitrosylated cysteine and cause mild fasting hyperglycemia (maturity-onset diabetes of the young; subtype glucokinase). The kinetics of recombinantly generated GCK-R369P and GCK-V367M were assessed in vitro. The GCK-R369P protein has greatly reduced catalytic activity (relative activity index 0.05 vs. 1.00 for wild type), whereas the GCK-V367M has near normal kinetics (relative activity index 1.26 vs. 1.00 for wild type). Quantitative imaging and biochemical assays were used to assess the effect of these mutants on the metabolic response to glucose, GCK activation, and S-nitrosylation of GCK in betaTC3 insulinoma cells. Expression of either mutant in betaTC3 cells did not affect the metabolic response to 5 mM glucose. However, expression of either mutant blocked the effects of insulin on glucose-stimulated nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate reduction, suggesting defects in posttranslational regulation of GCK. Each of these mutations blocked GCK activation, and prevented posttranslational cysteine S-nitrosylation. Our findings link defects in hormone-regulated GCK S-nitrosylation to hyperglycemia and support a role for posttranslational regulation of GCK S-nitrosylation as a vital regulatory mechanism for glucose-stimulated insulin secretion.

A new library of HEMET model: Insulin effects on hepatic metabolism

Prediction and simulation of cell culture behaviour, under different chemical and physical stimuli by a mathematical model, represent an innovative way to create a virtual cell laboratory, where it is possible to perform and optimize experimental protocol, saving time and money. In silico experiments permit to reproduce pathological and physiological situations and make toxicological tests. In this paper we introduce a new library of HEMET (HEpatocyte METabolism) software that allows the insulin effects on hepatic metabolism to be simulated. This new set of nonlinear differential equations, derived from biochemical reactions which involve this pancreatic hormone, allows the catabolites concentration in hepatic cell culture after insulin infusion to be predicted. The validation procedures were carried out using data obtained from specifically designed cell experiments and from literature. A user friendly interface allows to easily change model parameters, rate constants and inputs simulating a wide range of physiological and pathological scenarios.

Insulin signaling in the pancreatic beta-cell

The appropriate function of insulin-producing pancreatic beta-cells is crucial for the regulation of glucose homeostasis, and its impairment leads to diabetes mellitus, the most common metabolic disorder in man. In addition to glucose, the major nutrient factor, inputs from the nervous system, humoral components, and cell-cell communication within the islet of Langerhans act together to guarantee the release of appropriate amounts of insulin in response to changes in blood glucose levels. Data obtained within the past decade in several laboratories have revitalized controversy over the autocrine feedback action of secreted insulin on beta-cell function. Although insulin historically has been suggested to exert a negative effect on beta-cells, recent data provide evidence for a positive role of insulin in transcription, translation, ion flux, insulin secretion, proliferation, and beta-cell survival. Current insights on the role of insulin on pancreatic beta-cell function are discussed.

The fluorescent protein color palette

Advances in fluorescent protein development over the past 10 years have led to fine-tuning of the Aequorea victoria jellyfish color palette in the emission color range from blue to yellow, while a significant amount of progress has been achieved with reef coral species in the generation of monomeric fluorescent proteins emitting in the orange to far-red spectral regions. It is not inconceivable that near-infrared fluorescent proteins loom on the horizon. Expansion of the fluorescent protein family to include optical highlighters and FRET biosensors further arms this ubiquitous class of fluorophores with biological probes capable of photoactivation, photoconversion, and detection of molecular interactions beyond the resolution limits of optical microscopy. The success of these endeavors certainly suggests that almost any biological parameter can be investigated using the appropriate fluorescent protein-based application.

Negative and positive feedback regulation of insulin in glucose-stimulated Ca2+ response in pancreatic beta cells

Secreted insulin from pancreatic beta cells exerts autocrine and paracrine effects within the islets. The present study has evaluated how exogenous insulin participates in cytosolic Ca(2+) response to high glucose, according to glucose concentration at which insulin is applied. When 100 nM insulin was pretreated to the bath solution containing islet cells in the presence of basal level of glucose, the elevation of cytosolic Ca(2+) concentration ([Ca(2+)](c)) by subsequently applied 10mM glucose was remarkably attenuated. In contrast, the glucose-stimulated [Ca(2+)](c) elevation was more potentiated when insulin was superimposed on the high glucose stimulation. These insulin actions were modestly inhibited by the application of LY294002, the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, but not completely, suggesting that another mechanism is also involved. By 100 nM insulin, phosphorylated form of AMP-activated protein kinases (p-AMPK) was dramatically decreased in basal glucose but increased in high glucose, when compared with their reciprocal controls. These results may suggest that the extent of AMPK activation may be a tool for insulin receptors to monitor blood glucose level, with which insulin-induced insulin receptor activation determines the way to go negatively or positively toward [Ca(2+)](c).

Direct effect of cholesterol on insulin secretion: a novel mechanism for pancreatic beta-cell dysfunction

OBJECTIVE

Type 2 diabetes is often accompanied by abnormal blood lipid and lipoprotein levels, but most studies on the link between hyperlipidemia and diabetes have focused on free fatty acids (FFAs). In this study, we examined the relationship between cholesterol and insulin secretion from pancreatic beta-cells that is independent of the effects of FFAs.

RESEARCH DESIGN AND METHODS

Several methods were used to modulate cholesterol levels in intact islets and cultured beta-cells, including a recently developed mouse model that exhibits elevated cholesterol but normal FFA levels. Acute and metabolic alteration of cholesterol was done using pharmacological reagents.

RESULTS

We found a direct link between elevated serum cholesterol and reduced insulin secretion, with normal secretion restored by cholesterol depletion. We further demonstrate that excess cholesterol inhibits secretion by downregulation of metabolism through increased neuronal nitric oxide synthase dimerization.

CONCLUSIONS

This direct effect of cholesterol on beta-cell metabolism opens a novel set of mechanisms that may contribute to beta-cell dysfunction and the onset of diabetes in obese patients.

Allosteric Activation of Human Glucokinase by Free Polyubiquitin Chains and Its Ubiquitin-dependent Cotranslational Proteasomal Degradation

Human glucokinase (hGK) is a monomeric enzyme highly regulated in pancreatic beta-cells (isoform 1) and hepatocytes (isoforms 2 and 3). Although certain cellular proteins are known to either stimulate or inhibit its activity, little is known about post-translational modifications of this enzyme and their possible regulatory functions. In this study, we have identified isoforms 1 and 2 of hGK as novel substrates for the ubiquitin-conjugating enzyme system of the rabbit reticulocyte lysate. Both isoforms were polyubiquitinated on at least two lysine residues, and mutation analysis indicated that multiple lysine residues functioned as redundant acceptor sites. Deletion of its C-terminal alpha-helix, as part of a ubiquitin-interacting motif, affected the polyubiquitination at one of the sites and resulted in a completely inactive enzyme. Evidence is presented that poly/multiubiquitination of hGK in vitro serves as a signal for proteasomal degradation of the newly synthesized protein. Moreover, the recombinant hGK was found to interact with and to be allosterically activated up to approximately 1.4-fold by purified free pentaubiquitin chains at approximately 100 nm (with an apparent EC(50) of 93 nm), and possibly also by unidentified polyubiquitinated proteins assigned to their equilibrium binding to the ubiquitin-interacting motif site. The affinity of pentaubiquitin binding to hGK is regulated by the ligand (d-glucose)-dependent conformational state of the site. Both ubiquitination of hGK and its activation by polyubiquitin chains potentially represent physiological regulatory mechanisms for glucokinase-dependent insulin secretion in pancreatic beta-cells.

Cell biology assessment of glucokinase mutations V62M and G72R in pancreatic beta-cells: evidence for cellular instability of catalytic activity

Mutations in the glucokinase (GK) gene cause defects in blood glucose homeostasis. In some cases (V62M and G72R), the phenotype cannot be explained by altered enzyme kinetics or protein instability. We used transient and stable expression of green fluorescent protein (GFP) GK chimaeras in MIN6 beta-cells to study the phenotype defect of V62M and G72R. GK activity in lysates of MIN6 cell lines stably expressing wild-type or mutant GFP GK showed the expected affinity for glucose and response to pharmacological activators, indicating the expression of catalytically active enzymes. MIN6 cells stably expressing GFP V62M or GFP G72R had a lower GK activity-to-GK immunoreactivity ratio and GK activity-to-GK mRNA ratio but not GK immunoreactivity-to-GK mRNA ratio than wild-type GFP GK. Heterologous expression of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2/FDP2) in cell lines increased GK activity for wild-type GK and V62M but not for G72R, whereas expression of liver GK regulatory protein (GKRP) increased GK activity for wild type but not V62M or G72R. Lack of interaction of these mutants with GKRP was also evident in hepatocyte transfections from the lack of nuclear accumulation. These results suggest that cellular loss of GK catalytic activity rather than impaired translation or enhanced protein degradation may account for the hyperglycemia in subjects with V62M and G72R mutations.

Modulation of mitochondrial membrane permeability in pathogenesis, autophagy and control of metabolism

The mitochondrial inner and outer membranes have contrasting permeability characteristics. The outer membrane is non-specifically permeable to all low-molecular-weight solutes, whereas the inner membrane is impermeable except through specific transporters. After stresses and sometimes in normal physiology, the permeability of the two membranes can reverse. In the inner membrane, permeability transition pores open to cause the mitochondrial permeability transition (MPT). As the MPT involves more and more mitochondria, autophagy, apoptosis and necrosis progressively develop linked to the proportion of mitochondria injured and the extent of adenosine triphosphate (ATP) depletion, a phenomenon of necrapoptosis. By contrast, the outer membrane may decrease its permeability after certain stresses via closure of voltage-dependent anion channels (VDAC). The VDAC closure globally suppresses mitochondrial function to prevent futile ATP hydrolysis in hypoxia-ischemia and possibly the release of toxic superoxide under conditions of oxidative stress. The VDAC closure may also facilitate selective oxidation of acetaldehyde after ethanol exposure and promote aerobic glycolysis in cancer cells. By contrast, VDAC opening is proposed to stimulate oxidative phosphorylation and promote insulin release by glucose-stimulated pancreatic beta cells. Thus, VDAC serves as a global regulator, or governator, of mitochondrial function. Understanding of how these mitochondrial membrane permeability changes are themselves regulated remains incomplete and requires future study.

Novel insights into the regulation of the bound and diffusible glucokinase in MIN6 beta-cells

A stable MIN6 beta-cell clone overexpressing glucokinase as an enhanced cyan fluorescent protein (ECFP) fusion construct was generated for analysis of glucokinase regulation in these glucose-responsive insulin-secreting cells. A higher glucokinase enzyme activity accompanied by an improved glucose-induced insulin secretion indicated the integration of ECFP-glucokinase into the functional pool of glucokinase protein in MIN6-ECFP-glucokinase cells. Fluorescence recovery after photobleaching experiments of MIN6-ECFP-glucokinase cells and photoactivation of a transiently transfected photoswitchable cyan fluorescent protein (PS-CFP)-glucokinase construct in MIN6 cells indicate a higher motility of the diffusible glucokinase fraction at high glucose concentrations. In agreement with previous studies, we observed significant binding of ECFP-glucokinase to insulin secretory granules. Using fluorescence lifetime imaging, we obtained evidence for an association between glucokinase and alpha-tubulin in MIN6-ECFP-glucokinase cells. Furthermore, immunohistochemistry and fluorescence resonance energy transfer analysis by acceptor photobleaching showed distinct association between endogenous glucokinase and alpha-tubulin as well as beta-tubulin in MIN6 cells. Interestingly, glucokinase was also colocalized with kinesin, a motor protein involved in insulin secretory granule movement. Therefore, we suggest a role of a bound glucokinase protein fraction in the regulation of insulin granule movement along tubulin filaments.

Improved metabolic stimulus for glucose-induced insulin secretion through GK and PFK-2/FBPase-2 coexpression in insulin-producing RINm5F cells

The glucose sensor enzyme glucokinase plays a pivotal role in the regulation of glucose-induced insulin secretion in pancreatic beta-cells. Activation of glucokinase represents a promising concept for the treatment of type 2 diabetes. Therefore, we analyzed the glucokinase activation through its physiological interaction partner, the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) and the resulting effect on glucose metabolism in insulin-producing cells. In RINm5F-GK-PFK-2/FBPase-2 cells stably overexpressing glucokinase plus islet PFK-2/FBPase-2, colocalization between both enzymes as well as elevation of glucokinase activity were significantly increased at a stimulatory glucose concentration of 10 mmol/liter. RINm5F-GK-PFK-2/FBPase-2 cells showed under this culture condition a significant increase in glucose utilization and in the ATP/ADP ratio compared with RINm5F-GK cells, which only overexpress glucokinase. Also glucose-induced insulin secretion was elevated in RINm5F-GK-PFK-2/FBPase-2 cells in comparison to RINm5F-GK cells. Furthermore, pyruvate accumulation and lactate production in RINm5F-GK-PFK-2/FBPase-2 cells were significantly lower at both 10 and 30 mmol/liter glucose than in RINm5F-GK and RINm5F cells. The significant improvement of glucose metabolism after PFK-2/FBPase-2 overexpression is apparently not exclusively the result of high glucokinase enzyme activity. Stabilization of the closed glucokinase conformation by PFK-2/FBPase-2 may not only activate the enzyme but also improve metabolic channeling in beta-cells.

Time-dependent mechanisms in beta-cell glucose sensing

The relation between plasma glucose and insulin release from pancreatic beta-cells is not stationary in the sense that a given glucose concentration leads to a specific rate of insulin secretion. A number of time-dependent mechanisms appear to exist that modify insulin release both on a short and a longer time scale. Typically, two phases are described. The first phase, lasting up to 10 min, is a pulse of insulin release in response to fast changes in glucose concentration. The second phase is a more steady increase of insulin release over minutes to hours, if the elevated glucose concentration is sustained. The paper describes the glucose sensing mechanism via the complex dynamics of the key enzyme glucokinase, which controls the first step in glucose metabolism: phosphorylation of glucose to glucose-6-phosphate. Three time-dependent phenomena (mechanisms) are described. The fastest, corresponding to the first phase, is a delayed negative feedback regulating the glucokinase activity. Due to the delay, a rapid glucose increase will cause a burst of activity in the glucose sensing system, before the glucokinase is down-regulated. The second mechanism corresponds to the translocation of glucokinase from an inactive to an active form. As the translocation is controlled by the product(s) of the glucokinase reaction rather than by the substrate glucose, this mechanism gives a positive, but saturable, feedback. Finally, the release of the insulin granules is assumed to be enhanced by previous glucose exposure, giving a so-called glucose memory to the beta-cells. The effect depends on the insulin release of the cells, and this mechanism constitutes a second positive, saturable feedback system. Taken together, the three phenomena describe most of the glucose sensing behaviour of the beta-cells. The results indicate that the insulin release is not a precise function of the plasma glucose concentration. It rather looks as if the beta-cells just increase the insulin production, until the plasma glucose has returned to normal. This type of integral control has the advantage that the precise glucose sensitivity of the beta-cells is not important for normal glucose homeostasis.

From clinicogenetic studies of maturity-onset diabetes of the young to unraveling complex mechanisms of glucokinase regulation

Glucokinase functions as a glucose sensor in pancreatic beta-cells and regulates hepatic glucose metabolism. A total of 83 probands were referred for a diagnostic screening of mutations in the glucokinase (GCK) gene. We found 11 different mutations (V62A, G72R, L146R, A208T, M210K, Y215X, S263P, E339G, R377C, S453L, and IVS5 + 1G>C) in 14 probands. Functional characterization of recombinant glutathionyl S-transferase-G72R glucokinase showed slightly increased activity, whereas S263P and G264S had near-normal activity. The other point mutations were inactivating. S263P showed marked thermal instability, whereas the stability of G72R and G264S differed only slightly from that of wild type. G72R and M210K did not respond to an allosteric glucokinase activator (GKA) or the hepatic glucokinase regulatory protein (GKRP). Mutation analysis of the role of glycine at position 72 by substituting E, F, K, M, S, or Q showed that G is unique since all these mutants had very low or no activity and were refractory to GKRP and GKA. Structural analysis provided plausible explanations for the drug resistance of G72R and M210K. Our study provides further evidence that protein instability in combination with loss of control by a putative endogenous activator and GKRP could be involved in the development of hyperglycemia in maturity-onset diabetes of the young, type 2. Furthermore, based on data obtained on G264S, we propose that other and still unknown mechanisms participate in the regulation of glucokinase.

Critical role of gap junction coupled KATP channel activity for regulated insulin secretion

Pancreatic beta-cells secrete insulin in response to closure of ATP-sensitive K+ (KATP) channels, which causes membrane depolarization and a concomitant rise in intracellular Ca2+ (Cai). In intact islets, beta-cells are coupled by gap junctions, which are proposed to synchronize electrical activity and Cai oscillations after exposure to stimulatory glucose (>7 mM). To determine the significance of this coupling in regulating insulin secretion, we examined islets and beta-cells from transgenic mice that express zero functional KATP channels in approximately 70% of their beta-cells, but normal KATP channel density in the remainder. We found that KATP channel activity from approximately 30% of the beta-cells is sufficient to maintain strong glucose dependence of metabolism, Cai, membrane potential, and insulin secretion from intact islets, but that glucose dependence is lost in isolated transgenic cells. Further, inhibition of gap junctions caused loss of glucose sensitivity specifically in transgenic islets. These data demonstrate a critical role of gap junctional coupling of KATP channel activity in control of membrane potential across the islet. Control via coupling lessens the effects of cell-cell variation and provides resistance to defects in excitability that would otherwise lead to a profound diabetic state, such as occurs in persistent neonatal diabetes mellitus.

Regulation of two insulin granule populations within the reserve pool by distinct calcium sources

Insulin granule trafficking is a key step of glucose-stimulated insulin secretion from pancreatic beta cells. Using quantitative live cell imaging, we examined insulin granule movements within the reserve pool upon secretory stimulation in betaTC3 cells. For this study, we developed a custom image analysis program that permitted automatic tracking of the individual motions of over 20,000 granules. This analysis of a large sample size enabled us to study micro-populations of granules that were not quantifiable in previous studies. While over 90% of the granules depend on Ca2+ efflux from the endoplasmic reticulum for their mobilization, a small and fast-moving population of granules responds to extracellular Ca2+ influx after depolarization of the plasma membrane. We show that this differential regulation of the two granule populations is consistent with localized Ca2+ signals, and that the cytoskeletal network is involved in both types of granule movement. The fast-moving granules are correlated temporally and spatially to the replacement of the secreted insulin granules, which supports the hypothesis that these granules are responsible for replenishing the readily releasable pool. Our study provides a model by which glucose and other secretory stimuli can regulate the readily releasable pool through the same mechanisms that regulate insulin secretion.

Voltage-dependent anion channel (VDAC) as mitochondrial governator--thinking outside the box

Despite a detailed understanding of their metabolism, mitochondria often behave anomalously. In particular, global suppression of mitochondrial metabolism and metabolite exchange occurs in apoptosis, ischemia and anoxia, cytopathic hypoxia of sepsis and multiple organ failure, alcoholic liver disease, aerobic glycolysis in cancer cells (Warburg effect) and unstimulated pancreatic beta cells. Here, we propose that closure of voltage-dependent anion channels (VDAC) in the mitochondrial outer membrane accounts for global mitochondrial suppression. In anoxia, cytopathic hypoxia and ethanol treatment, reactive oxygen and nitrogen species, cytokines, kinase cascades and increased NADH act to inhibit VDAC conductance and promote selective oxidation of membrane-permeable respiratory substrates like short chain fatty acids and acetaldehyde. In cancer cells, highly expressed hexokinase binds to and inhibits VDAC to suppress mitochondrial function while stimulating glycolysis, but an escape mechanism intervenes when glucose-6-phosphate accumulates and dissociates hexokinase from VDAC. Similarly, glucokinase binds mitochondria of insulin-secreting beta cells, possibly blocking VDAC and suppressing mitochondrial function. We propose that glucose metabolism leads to glucose-6-phosphate-dependent unbinding of glucokinase, relief of VDAC inhibition, release of ATP from mitochondria and ATP-dependent insulin release. In support of the overall proposal, ethanol treatment of isolated rat hepatocytes inhibited mitochondrial respiration and accessibility to adenylate kinase in the intermembrane space, effects that were overcome by digitonin permeabilization of the outer membrane. Overall, these considerations suggest that VDAC is a dynamic regulator, or governator, of global mitochondrial function both in health and disease.

Insights into the structure and regulation of glucokinase from a novel mutation (V62M), which causes maturity-onset diabetes of the young

Glucokinase (GCK) serves as the pancreatic glucose sensor. Heterozygous inactivating GCK mutations cause hyperglycemia, whereas activating mutations cause hypoglycemia. We studied the GCK V62M mutation identified in two families and co-segregating with hyperglycemia to understand how this mutation resulted in reduced function. Structural modeling locates the mutation close to five naturally occurring activating mutations in the allosteric activator site of the enzyme. Recombinant glutathionyl S-transferase-V62M GCK is paradoxically activated rather than inactivated due to a decreased S0.5 for glucose compared with wild type (4.88 versus 7.55 mM). The recently described pharmacological activator (RO0281675) interacts with GCK at this site. V62M GCK does not respond to RO0281675, nor does it respond to the hepatic glucokinase regulatory protein (GKRP). The enzyme is also thermally unstable, but this lability is apparently less pronounced than in the proven instability mutant E300K. Functional and structural analysis of seven amino acid substitutions at residue Val62 has identified a non-linear relationship between activation by the pharmacological activator and the van der Waals interactions energies. Smaller energies allow a hydrophobic interaction between the activator and glucokinase, whereas larger energies prohibit the ligand from fitting into the binding pocket. We conclude that V62M may cause hyperglycemia by a complex defect of GCK regulation involving instability in combination with loss of control by a putative endogenous activator and/or GKRP. This study illustrates that mutations that cause hyperglycemia are not necessarily kinetically inactivating but may exert their effects by other complex mechanisms. Elucidating such mechanisms leads to a deeper understanding of the GCK glucose sensor and the biochemistry of beta-cells and hepatocytes.

Neuronal glucosensing: what do we know after 50 years?

Glucosensing neurons are specialized cells that use glucose as a signaling molecule to alter their action potential frequency in response to variations in ambient glucose levels. Glucokinase (GK) appears to be the primary regulator of most neuronal glucosensing, but other regulators almost certainly exist. Glucose-excited neurons increase their activity when glucose levels rise, and most use GK and an ATP-sensitive K(+) channel as the ultimate effector of glucose-induced signaling. Glucose-inhibited (GI) neurons increase their activity at low glucose levels. Although many use GK, it is unclear what the final pathway of GI neuronal glucosensing is. Glucosensing neurons are located in brain sites and respond to and integrate a variety of hormonal, metabolic, transmitter, and peptide signals involved in the regulation of energy homeostasis and other biological functions. Although it is still uncertain whether daily fluctuations in blood glucose play a specific regulatory role in these physiological functions, it is clear that large decreases in glucose availability stimulate food intake and counterregulatory responses that restore glucose levels to sustain cerebral function. Finally, glucosensing is altered in obesity and after recurrent bouts of hypoglycemia, and this altered sensing may contribute to the adverse outcomes of these conditions. Thus, although much is known, much remains to be learned about the physiological function of brain glucosensing neurons.

Glucokinase is an integral component of the insulin granules in glucose-responsive insulin secretory cells and does not translocate during glucose stimulation

The association of glucokinase with insulin secretory granules has been shown by cell microscopy techniques. We used MIN6 insulin-secretory cells and organelle fractionation to determine the effects of glucose on the subcellular distribution of glucokinase. After permeabilization with digitonin, 50% of total glucokinase remained bound intracellularly, while 30% was associated with the 13,000g particulate fraction. After density gradient fractionation of the organelles, immunoreactive glucokinase was distributed approximately equally between dense insulin granules and low-density organelles that cofractionate with mitochondria. Although MIN6 cells show glucose-responsive insulin secretion, glucokinase association with the granules and low-density organelles was not affected by glucose. Subfractionation of the insulin granule components by hypotonic lysis followed by sucrose gradient centrifugation showed that glucokinase colocalized with the granule membrane marker phogrin and not with insulin. PFK2 (6-phosphofructo-2-kinase-2/fructose-2,6-bisphosphatase)/FDPase-2, a glucokinase-binding protein, and glyceraldehyde phosphate dehydrogenase, which has been implicated in granule fusion, also colocalized with glucokinase after hypotonic lysis or detergent extaction of the granules. The results suggest that glucokinase is an integral component of the granule and does not translocate during glucose stimulation.