[Show abstract][Hide abstract] ABSTRACT: Insulin is secreted from the islets of Langerhans in coordinated pulses. These pulses are thought to lead to plasma insulin oscillations, which are putatively more effective in lowering blood glucose than continuous levels of insulin. Gap-junction coupling of β-cells by connexin-36 coordinates intracellular free calcium oscillations and pulsatile insulin release in isolated islets, however a role in vivo has not been shown. We test whether loss of gap-junction coupling disrupts plasma insulin oscillations and whether this impacts glucose tolerance. We characterized the connexin-36 knockout (Cx36(-/-)) mouse phenotype and performed hyperglycemic clamps with rapid sampling of insulin in Cx36(-/-) and control mice. Our results show that Cx36(-/-) mice are glucose intolerant, despite normal plasma insulin levels and insulin sensitivity. However, Cx36(-/-) mice exhibit reduced insulin pulse amplitudes and a reduction in first-phase insulin secretion. These changes are similarly found in isolated Cx36(-/-) islets. We conclude that Cx36 gap junctions regulate the in vivo dynamics of insulin secretion, which in turn is important for glucose homeostasis. Coordinated pulsatility of individual islets enhances the first-phase elevation and second-phase pulses of insulin. Because these dynamics are disrupted in the early stages of type 2 diabetes, dysregulation of gap-junction coupling could be an important factor in the development of this disease.
[Show abstract][Hide abstract] ABSTRACT: Cell-cell communication in the islet of Langerhans is important for the regulation of insulin secretion. Gap-junctions coordinate oscillations in intracellular free-calcium ([Ca(2+)](i)) and insulin secretion in the islet following elevated glucose. Gap-junctions can also ensure that oscillatory [Ca(2+)](i) ceases when glucose is at a basal levels. We determine the roles of gap-junctions and other cell-cell communication pathways in the suppression of insulin secretion under basal conditions. Metabolic, electrical and insulin secretion levels were measured from islets lacking gap-junction coupling following deletion of connexion36 (Cx36(-/-)), and these results were compared to those obtained using fully isolated β-cells. K(ATP) loss-of-function islets provide a further experimental model to specifically study gap-junction mediated suppression of electrical activity. In isolated β-cells or Cx36(-/-) islets, elevations in [Ca(2+)](i) persisted in a subset of cells even at basal glucose. Isolated β-cells showed elevated insulin secretion at basal glucose; however, insulin secretion from Cx36(-/-) islets was minimally altered. [Ca(2+)](i) was further elevated under basal conditions, but insulin release still suppressed in K(ATP) loss-of-function islets. Forced elevation of cAMP led to PKA-mediated increases in insulin secretion from islets lacking gap-junctions, but not from islets expressing Cx36 gap junctions. We conclude there is a redundancy in how cell-cell communication in the islet suppresses insulin release. Gap junctions suppress cellular heterogeneity and spontaneous [Ca(2+)](i) signals, while other juxtacrine mechanisms, regulated by PKA and glucose, suppress more distal steps in exocytosis. Each mechanism is sufficiently robust to compensate for a loss of the other and still suppress basal insulin secretion.
The Journal of Physiology 09/2011; 589(Pt 22):5453-66. · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We assayed glucose-stimulated insulin secretion (GSIS) from live, murine islets of Langerhans in microfluidic devices by the downstream formation of aqueous droplets. Zinc ions, which are cosecreted with insulin from beta-cells, were quantitatively measured from single islets with high temporal resolution using a fluorescent indicator, FluoZin-3. Real-time storage of secretions into droplets (volume of 0.470 +/- 0.009 nL) effectively preserves the temporal chemical information, allowing reconstruction of the secretory time record. The use of passive flow control within the device removes the need for syringe pumps, requiring only a single hand-held syringe. Under stimulatory glucose levels (11 mM), bursts of zinc as high as approximately 800 fg islet(-1) min(-1) were measured. Treatment with diazoxide effectively blocked zinc secretion, as expected. High temporal resolution reveals two major classes of oscillations in secreted zinc, with predominate periods at approximately 20-40 s and approximately 5-10 min. The more rapid oscillation periods match closely with those of intraislet calcium oscillations, while the slower oscillations are consistent with insulin pulses typically measured in bulk islet experiments or in the bloodstream. This droplet sampling technique should be widely applicable to time-resolved cellular secretion measurements, either in real-time or for postprocessing.
[Show abstract][Hide abstract] ABSTRACT: An alternative fabrication method is presented for production of masters for single- or multi-layer polymeric microfluidic devices in a standard laboratory environment, precluding the need for a cleanroom. This toner transfer masking (TTM) method utilizes an office laser printer to generate a toner pattern which is thermally transferred to a metal master to serve as a mask for etching. With master fabrication times as little as one hour (depending on channel depth) using commercially-available equipment and supplies, this approach should make microfluidic technology more widely accessible to the non-expert-even the non-scientist. The cost of fabrication consumables was estimated to be < $1 per master, over an order of magnitude decrease in consumable costs compared to standard photolithography. In addition, the use of chemical etching allows accurate control over the height of raised features (i.e., channel depths), allowing the flexibility to fabricate multiple depths on a single master with little added time. Resultant devices are shown capable of pneumatic valving, three-dimensional channel formation (using layer-connecting vias), droplet fluidics, and cell imaging and staining. The multiple-depth capabilities of the method are proven useful for cellular analysis by fabrication of handheld, disposable devices used for trapping and imaging of live murine pancreatic islets. The precise fluidic control provided by the microfluidic platform allows subsequent fixing and staining of these cells without significant movement, thus spatial correlation of imaging and staining is attainable-even with rare alpha cells that constitute only approximately 10% of the islet cells.
Lab on a Chip 04/2009; 9(8):1119-27. · 5.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The pancreatic islet is a highly coupled, multicellular system that exhibits complex spatiotemporal electrical activity in response to elevated glucose levels. The emergent properties of islets, which differ from those arising in isolated islet cells, are believed to arise in part by gap junctional coupling, but the mechanisms through which this coupling occurs are poorly understood. To uncover these mechanisms, we have used both high-speed imaging and theoretical modeling of the electrical activity in pancreatic islets under a reduction in the gap junction mediated electrical coupling. Utilizing islets from a gap junction protein connexin 36 knockout mouse model together with chemical inhibitors, we can modulate the electrical coupling in the islet in a precise manner and quantify this modulation by electrophysiology measurements. We find that after a reduction in electrical coupling, calcium waves are slowed as well as disrupted, and the number of cells showing synchronous calcium oscillations is reduced. This behavior can be reproduced by computational modeling of a heterogeneous population of beta-cells with heterogeneous levels of electrical coupling. The resulting quantitative agreement between the data and analytical models of islet connectivity, using only a single free parameter, reveals the mechanistic underpinnings of the multicellular behavior of the islet.
[Show abstract][Hide abstract] ABSTRACT: Dimethyl amiloride (DMA) enhances insulin secretion in the pancreatic beta-cell. DMA also enhances time-dependent potentiation (TDP) and enables TDP to occur in situations where it is normally absent. As we have demonstrated before, these effects are mediated in part through inhibition of neuronal nitric oxide synthase (nNOS), resulting in increased availability of arginine. Thus both DMA and arginine have the potential to correct the secretory defect in diabetes by enabling or enhancing TDP. In the current study we have demonstrated the ability of these agents to improve blood glucose homeostasis in three mouse models of type 2 diabetes. The pattern of TDP under different conditions indicates that inhibition of NOS is not the only mechanism through which DMA exerts its positive effects. Thus we also have explored another possible mechanism through which DMA enables/enhances TDP, via the activation of mitochondrial alpha-ketoglutarate dehydrogenase.
AJP Endocrinology and Metabolism 07/2008; 294(6):E1097-108. · 4.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
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.
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.
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.
[Show abstract][Hide abstract] ABSTRACT: Time-dependent potentiation (TDP) of insulin release is normally absent in mice. However, we recently demonstrated that TDP occurs in mouse islets under conditions of forced decrease of intracellular pH (pH(i)) associated with elevated NADPH+H(+) (NADPH) levels. Hence, TDP in mouse islets may be kept suppressed by neuronal nitric oxide (NO) synthase (nNOS), an NADPH-utilizing enzyme with alkaline pH optimum. To determine the role of nNOS in the suppression of TDP in mouse islets, glucose-induced TDP was monitored in mouse islets in which nNOS activity had been genetically removed or chemically inhibited and compared with the TDP response in wild-type mouse islets with and without forced intracellular acidification. Genetic deletion of nNOS was provided by an nNOS knockout (NOS-KO) mouse model, B6-129S4-Nos1(tm1Plh)/J. To explore how nNOS inhibits TDP, we compared pH(i) and NADPH levels in wild-type and NOS-KO islets and monitored TDP with various components of the nNOS reaction added. Glucose normally does not produce TDP in wild-type mouse islets except under forced intracellular acidification. Remarkably, glucose produced strong TDP in NOS-KO islets and in wild-type islets treated with nNOS inhibitors. TDP in NOS-KO islets was not inhibited by the addition of NO, and NOS-KO islets exhibited a lower pH(i) than wild-type islets. The addition of arginine to wild-type islets also enabled glucose to induce TDP. Our results show that nNOS activity contributes to the absence of TDP in mouse islets putatively through depletion of intracellular arginine.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: We generated transgenic mice expressing firefly (Photinus pyralis) luciferase (luc) under the control of the mouse insulin I promoter (MIP). The mice have normal glucose tolerance and pancreatic islet architecture. The luciferase-expressing beta cells can be readily visualized in living mice using whole-body bioluminescent imaging. The MIP-luc transgenic mice may be useful for monitoring changes in beta cell function or mass in living animals with normal or altered metabolic states.
[Show abstract][Hide abstract] ABSTRACT: Amiloride derivatives, commonly used for their diuretic and antihypertensive properties, can also cause a sustained but reversible decrease of intracellular pH (pHi). Using dimethyl amiloride (DMA) on normal rodent pancreatic islets, we previously demonstrated the critical influence of islet pHi on insulin secretion. Nutrient-stimulated insulin secretion (NSIS) requires a specific pHi-range, and is dramatically enhanced by forced intracellular acidification with DMA. Furthermore, DMA can enable certain non-secretagogues to stimulate insulin secretion, and induce time-dependent potentiation (TDP) of insulin release in mouse islets where this function is normally absent. The present study was performed to determine whether pHi-manipulation could correct the secretory defect in islets isolated from mice with type 2 diabetes.
Using two mouse models of type 2 diabetes, we compared a) pHi-regulation, and b) NSIS with and without treatment with amiloride derivatives, in islets isolated from diabetic mice and wild type mice.
A majority of the islets from the diabetic mice showed a slightly elevated basal pHi and/or poor recovery from acid/base load. DMA treatment produced a significant increase of NSIS in islets from the diabetic models. DMA also enabled glucose to induce TDP in the islets from diabetic mice, albeit to a lesser degree than in normal islets.
Islets from diabetic mice show some mis-regulation of intracellular pH, and their secretory capacity is consistently enhanced by DMA/amiloride. Thus, amiloride derivatives show promise as potential therapeutic agents for type 2 diabetes.
[Show abstract][Hide abstract] ABSTRACT: We present a convenient method for monitoring pancreatic beta cell development in real-time, through in vitro culture of embryonic pancreatic explants from transgenic mice with a genetic tag for insulin-producing beta cells.
[Show abstract][Hide abstract] ABSTRACT: The pancreatic islet is a functional microorgan involved in maintaining normoglycemia through regulated secretion of insulin and other hormones. Extracellular glucose stimulates insulin secretion from islet beta cells through an increase in redox state, which can be measured by NAD(P)H autofluorescence. Glucose concentrations over approximately 7 mM generate synchronous oscillations in beta cell intracellular Ca2+ concentration ([Ca2+]i), which lead to pulsatile insulin secretion. Prevailing models assume that the pancreatic islet acts as a functional syncytium, and the whole islet [Ca2+]i response has been modeled in terms of islet bursting and pacemaker models. To test these models, we developed a microfluidic device capable of partially stimulating an islet, while allowing observation of the NAD(P)H and [Ca2+]i responses. We show that beta cell [Ca2+]i oscillations occur only within regions stimulated with more than approximately 6.6 mM glucose. Furthermore, we show that tolbutamide, an antagonist of the ATP-sensitive K+ channel, allows these oscillations to travel farther into the nonstimulated regions of the islet. Our approach shows that the extent of Ca2+ propagation across the islet depends on a delicate interaction between the degree of coupling and the extent of ATP-sensitive K+-channel activation and illustrates an experimental paradigm that will have utility for many other biological systems.
Proceedings of the National Academy of Sciences 09/2004; 101(35):12899-903. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Glucose-stimulated insulin secretion is a multistep process dependent on beta-cell metabolic flux. Our previous studies on intact pancreatic islets used two-photon NAD(P)H imaging as a quantitative measure of the combined redox signal from NADH and NADPH (referred to as NAD(P)H). These studies showed that pyruvate, a non-secretagogue, enters beta-cells and causes a transient rise in NAD(P)H. To further characterize the metabolic fate of pyruvate, we have now developed one-photon flavoprotein microscopy as a simultaneous assay of lipoamide dehydrogenase (LipDH) autofluorescence. This flavoprotein is in direct equilibrium with mitochondrial NADH. Hence, a comparison of LipDH and NAD(P)H autofluorescence provides a method to distinguish the production of NADH, NADPH, or both. Using this method, the glucose dose response is consistent with an increase in both NADH and NADPH. In contrast, the transient rise in NAD(P)H observed with pyruvate stimulation is not accompanied by a significant change in LipDH, which indicates that pyruvate raises cellular NADPH without raising NADH. In comparison, methyl pyruvate stimulated a robust NADH and NADPH response. These data provide new evidence that exogenous pyruvate does not induce a significant rise in mitochondrial NADH. This inability likely results in its failure to produce the ATP necessary for stimulated secretion of insulin. Overall, these data are consistent with either a restricted pyruvate dehydrogenase-dependent metabolism or a buffering of the NADH response by other metabolic mechanisms.
Journal of Biological Chemistry 08/2004; 279(30):31780-7. · 4.65 Impact Factor