Mechanisms of coordination of Ca2+ signals in pancreatic islet cells

The effect of forskolin and the role of Epac2A during activation, activity, and deactivation of beta cell networks

Beta cells couple stimulation by glucose with insulin secretion and impairments in this coupling play a central role in diabetes mellitus. Cyclic adenosine monophosphate (cAMP) amplifies stimulus-secretion coupling via protein kinase A and guanine nucleotide exchange protein 2 (Epac2A). With the present research, we aimed to clarify the influence of cAMP-elevating diterpene forskolin on cytoplasmic calcium dynamics and intercellular network activity, which are two of the crucial elements of normal beta cell stimulus-secretion coupling, and the role of Epac2A under normal and stimulated conditions. To this end, we performed functional multicellular calcium imaging of beta cells in mouse pancreas tissue slices after stimulation with glucose and forskolin in wild-type and Epac2A knock-out mice. Forskolin evoked calcium signals in otherwise substimulatory glucose and beta cells from Epac2A knock-out mice displayed a faster activation. During the plateau phase, beta cells from Epac2A knock-out mice displayed a slightly higher active time in response to glucose compared with wild-type littermates, and stimulation with forskolin increased the active time via an increase in oscillation frequency and a decrease in oscillation duration in both Epac2A knock-out and wild-type mice. Functional network properties during stimulation with glucose did not differ in Epac2A knock-out mice, but the presence of Epac2A was crucial for the protective effect of stimulation with forskolin in preventing a decline in beta cell functional connectivity with time. Finally, stimulation with forskolin prolonged beta cell activity during deactivation, especially in Epac2A knock-out mice.

pH-Dependence of Glucose-Dependent Activity of Beta Cell Networks in Acute Mouse Pancreatic Tissue Slice

Extracellular pH has the potential to affect various aspects of the pancreatic beta cell function. To explain this effect, a number of mechanisms was proposed involving both extracellular and intracellular targets and pathways. Here, we focus on reassessing the influence of extracellular pH on glucose-dependent beta cell activation and collective activity in physiological conditions. To this end we employed mouse pancreatic tissue slices to perform high-temporally resolved functional imaging of cytosolic Ca²⁺ oscillations. We investigated the effect of either physiological H⁺ excess or depletion on the activation properties as well as on the collective activity of beta cell in an islet. Our results indicate that lowered pH invokes activation of a subset of beta cells in substimulatory glucose concentrations, enhances the average activity of beta cells, and alters the beta cell network properties in an islet. The enhanced average activity of beta cells was determined indirectly utilizing cytosolic Ca²⁺ imaging, while direct measuring of insulin secretion confirmed that this enhanced activity is accompanied by a higher insulin release. Furthermore, reduced functional connectivity and higher functional segregation at lower pH, both signs of a reduced intercellular communication, do not necessary result in an impaired insulin release.

Glucose-dependent activation, activity, and deactivation of beta cell networks in acute mouse pancreas tissue slices

Many details of glucose-stimulated intracellular calcium changes in β cells during activation, activity, and deactivation, as well as their concentration-dependence, remain to be analyzed. Classical physiological experiments indicated that in islets, functional differences between individual cells are largely attenuated, but recent findings suggest considerable intercellular heterogeneity, with some cells possibly coordinating the collective responses. To address the above with an emphasis on heterogeneity and describing the relations between classical physiological and functional network properties, we performed functional multicellular calcium imaging in mouse pancreas tissue slices over a wide range of glucose concentrations. During activation, delays to activation of cells and any-cell-to-first-responder delays are shortened, and the sizes of simultaneously responding clusters increased with increasing glucose concentrations. Exactly the opposite characterized deactivation. The frequency of fast calcium oscillations during activity increased with increasing glucose up to 12 mM glucose concentration, beyond which oscillation duration became longer, resulting in a homogenous increase in active time. In terms of functional connectivity, islets progressed from a very segregated network to a single large functional unit with increasing glucose concentration. A comparison between classical physiological and network parameters revealed that the first-responders during activation had longer active times during plateau and the most active cells during the plateau tended to deactivate later. Cells with the most functional connections tended to activate sooner, have longer active times, and deactivate later. Our findings provide a common ground for recent differing views on β cell heterogeneity and an important baseline for future studies of stimulus-secretion and intercellular coupling.NEW & NOTEWORTHY We assessed concentration-dependence in coupled β cells, degree of functional heterogeneity, and uncovered possible specialized subpopulations during the different phases of the response to glucose at the level of many individual cells. To this aim, we combined acute mouse pancreas tissue slices with functional multicellular calcium imaging over a wide range from threshold (7 mM) and physiological (8 and 9 mM) to supraphysiological (12 and 16 mM) glucose concentrations, classical physiological, and advanced network analyses.

Membrane Potential and Calcium Dynamics in Beta Cells from Mouse Pancreas Tissue Slices: Theory, Experimentation, and Analysis

Beta cells in the pancreatic islets of Langerhans are precise biological sensors for glucose and play a central role in balancing the organism between catabolic and anabolic needs. A hallmark of the beta cell response to glucose are oscillatory changes of membrane potential that are tightly coupled with oscillatory changes in intracellular calcium concentration which, in turn, elicit oscillations of insulin secretion. Both membrane potential and calcium changes spread from one beta cell to the other in a wave-like manner. In order to assess the properties of the abovementioned responses to physiological and pathological stimuli, the main challenge remains how to effectively measure membrane potential and calcium changes at the same time with high spatial and temporal resolution, and also in as many cells as possible. To date, the most wide-spread approach has employed the electrophysiological patch-clamp method to monitor membrane potential changes. Inherently, this technique has many advantages, such as a direct contact with the cell and a high temporal resolution. However, it allows one to assess information from a single cell only. In some instances, this technique has been used in conjunction with CCD camera-based imaging, offering the opportunity to simultaneously monitor membrane potential and calcium changes, but not in the same cells and not with a reliable cellular or subcellular spatial resolution. Recently, a novel family of highly-sensitive membrane potential reporter dyes in combination with high temporal and spatial confocal calcium imaging allows for simultaneously detecting membrane potential and calcium changes in many cells at a time. Since the signals yielded from both types of reporter dyes are inherently noisy, we have developed complex methods of data denoising that permit for visualization and pixel-wise analysis of signals. Combining the experimental approach of high-resolution imaging with the advanced analysis of noisy data enables novel physiological insights and reassessment of current concepts in unprecedented detail.

Fluorescence recovery after photobleaching reveals regulation and distribution of connexin36 gap junction coupling within mouse islets of Langerhans

The pancreatic islets are central to the maintenance of glucose homeostasis through insulin secretion. Glucose‐stimulated insulin secretion is tightly linked to electrical activity in β cells within the islet. Gap junctions, composed of connexin36 (Cx36), form intercellular channels between β cells, synchronizing electrical activity and insulin secretion. Loss of gap junction coupling leads to altered insulin secretion dynamics and disrupted glucose homeostasis. Gap junction coupling is known to be disrupted in mouse models of pre‐diabetes. Although approaches to measure gap junction coupling have been devised, they either lack cell specificity, suitable quantification of coupling or spatial resolution, or are invasive. The purpose of this study was to develop fluorescence recovery after photobleaching (FRAP) as a technique to accurately and robustly measure gap junction coupling in the islet. The cationic dye Rhodamine 123 was used with FRAP to quantify dye diffusion between islet β cells as a measure of Cx36 gap junction coupling. Measurements in islets with reduced Cx36 verified the accuracy of this technique in distinguishing between distinct levels of gap junction coupling. Analysis of individual cells revealed that the distribution of coupling across the islet is highly heterogeneous. Analysis of several modulators of gap junction coupling revealed glucose‐ and cAMP‐dependent modulation of gap junction coupling in islets. Finally, FRAP was used to determine cell population specific coupling, where no functional gap junction coupling was observed between α cells and β cells in the islet. The results of this study show FRAP to be a robust technique which provides the cellular resolution to quantify the distribution and regulation of Cx36 gap junction coupling in specific cell populations within the islet. Future studies utilizing this technique may elucidate the role of gap junction coupling in the progression of diabetes and identify mechanisms of gap junction regulation for potential therapies.

New insights into the role of connexins in pancreatic islet function and diabetes

Multi-cellular systems require complex signaling mechanisms for proper tissue function, to mediate signaling between cells in close proximity and at distances. This holds true for the islets of Langerhans, which are multicellular micro-organs located in the pancreas responsible for glycemic control, through secretion of insulin and other hormones. Coupling of electrical and metabolic signaling between islet β-cells is required for proper insulin secretion and effective glycemic control. β-cell specific coupling is established through gap junctions composed of connexin36, which results in coordinated insulin release across the islet. Islet connexins have been implicated in both Type-1 and Type-2 diabetes; however a clear link remains to be determined. The goal of this review is to discuss recent discoveries regarding the role of connexins in regulating insulin secretion, the regulation of connexins within the islet, and recent studies which support a role for connexins in diabetes. Further studies which investigate the regulation of connexins in the islet and their role in diabetes may lead to novel diabetes therapies which regulate islet function and β-cell survival through modulation of gap junction coupling.

Functional connectivity in islets of Langerhans from mouse pancreas tissue slices

We propose a network representation of electrically coupled beta cells in islets of Langerhans. Beta cells are functionally connected on the basis of correlations between calcium dynamics of individual cells, obtained by means of confocal laser-scanning calcium imaging in islets from acute mouse pancreas tissue slices. Obtained functional networks are analyzed in the light of known structural and physiological properties of islets. Focusing on the temporal evolution of the network under stimulation with glucose, we show that the dynamics are more correlated under stimulation than under non-stimulated conditions and that the highest overall correlation, largely independent of Euclidean distances between cells, is observed in the activation and deactivation phases when cells are driven by the external stimulus. Moreover, we find that the range of interactions in networks during activity shows a clear dependence on the Euclidean distance, lending support to previous observations that beta cells are synchronized via calcium waves spreading throughout islets. Most interestingly, the functional connectivity patterns between beta cells exhibit small-world properties, suggesting that beta cells do not form a homogeneous geometric network but are connected in a functionally more efficient way. Presented results provide support for the existing knowledge of beta cell physiology from a network perspective and shed important new light on the functional organization of beta cell syncitia whose structural topology is probably not as trivial as believed so far.

Glucose-stimulated calcium dynamics in islets of Langerhans in acute mouse pancreas tissue slices

In endocrine cells within islets of Langerhans calcium ions couple cell stimulation to hormone secretion. Since the advent of modern fluorimetry, numerous in vitro studies employing primarily isolated mouse islets have investigated the effects of various secretagogues on cytoplasmic calcium, predominantly in insulin-secreting beta cells. Due to technical limitations, insights of these studies are inherently limited to a rather small subpopulation of outermost cells. The results also seem to depend on various factors, like culture conditions and duration, and are not always easily reconcilable with findings in vivo. The main controversies regard the types of calcium oscillations, presence of calcium waves, and the level of synchronized activity. Here, we set out to combine the in situ acute mouse pancreas tissue slice preparation with noninvasive fluorescent calcium labeling and subsequent confocal laser scanning microscopy to shed new light on the existing controversies utilizing an innovative approach enabling the characterization of responses in many cells from all layers of islets. Our experiments reproducibly showed stable fast calcium oscillations on a sustained plateau rather than slow oscillations as the predominant type of response in acute tissue slices, and that calcium waves are the mechanistic substrate for synchronization of oscillations. We also found indirect evidence that even a large amplitude calcium signal was not sufficient and that metabolic activation was necessary to ensure cell synchronization upon stimulation with glucose. Our novel method helped resolve existing controversies and showed the potential to help answer important physiological questions, making it one of the methods of choice for the foreseeable future.

Muscular dystrophies at different ages: metabolic and endocrine alterations

Common metabolic and endocrine alterations exist across a wide range of muscular dystrophies. Skeletal muscle plays an important role in glucose metabolism and is a major participant in different signaling pathways. Therefore, its damage may lead to different metabolic disruptions. Two of the most important metabolic alterations in muscular dystrophies may be insulin resistance and obesity. However, only insulin resistance has been demonstrated in myotonic dystrophy. In addition, endocrine disturbances such as hypogonadism, low levels of testosterone, and growth hormone have been reported. This eventually will result in consequences such as growth failure and delayed puberty in the case of childhood dystrophies. Other consequences may be reduced male fertility, reduced spermatogenesis, and oligospermia, both in childhood as well as in adult muscular dystrophies. These facts all suggest that there is a need for better comprehension of metabolic and endocrine implications for muscular dystrophies with the purpose of developing improved clinical treatments and/or improvements in the quality of life of patients with dystrophy. Therefore, the aim of this paper is to describe the current knowledge about of metabolic and endocrine alterations in diverse types of dystrophinopathies, which will be divided into two groups: childhood and adult dystrophies which have different age of onset.

Gap junction coupling and calcium waves in the pancreatic islet

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.

Effects of external ATP on Ca(2+) signalling in endothelial cells isolated from mouse islets

External ATP is believed to initiate and propagate Ca(2+) signals co-ordinating the insulin release pulses within and among the different islets in the pancreas. The possibility that islet endothelial cells participate in this process was evaluated by comparing the effects on [Ca(2+)](i) of purinoceptor activation in these cells with those in beta-cells. beta-Cell-rich pancreatic islets were isolated from ob/ob mice and dispersed into single cells/aggregates. After culture with or without endothelial cell growth supplement (ECGS), the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) was measured with ratiometric fura-2 technique. Presence of ECGS or prolongation of culture (>5 days) resulted in proliferation of endothelial cells and altered their phenotype from rounded to elongated. Endothelial cells, preliminarily identified by attachment of Dynabeads coated with the Bandeiraea simplicifolia 1 lectin (BS-1), responded in a similar way as those stained with CD31 antibodies after measurements of [Ca(2+)](i). Spontaneous transients and oscillations of [Ca(2+)](i )were seen in beta-cells, but not in endothelial cells exposed to 20 mM glucose. Addition of ATP (10 microM) resulted in pronounced and more extended rise of [Ca(2+)](i) in endothelial cells than in beta-cells. The endothelial cells differed from the beta-cells by also responding with a rise of [Ca(2+)](i) to 10 microM UTP, but not to equimolar ADP and acetylcholine. The results support the idea of mutual interactions between islet endothelium and beta-cells based on ATP-induced Ca(2+) signals. It is suggested that the endothelial cells have a tonic inhibitory action on beta-cell P2 purinoceptors resulting in impaired synchronization of the insulin release pulses.

Dexamethasone-induced insulin resistance is associated with increased connexin 36 mRNA and protein expression in pancreatic rat islets

Augmented glucose-stimulated insulin secretion (GSIS) is an adaptive mechanism exhibited by pancreatic islets from insulin-resistant animal models. Gap junction proteins have been proposed to contribute to islet function. As such, we investigated the expression of connexin 36 (Cx36), connexin 43 (Cx43), and the glucose transporter Glut2 at mRNA and protein levels in pancreatic islets of dexamethasone (DEX)-induced insulin-resistant rats. Study rats received daily injections of DEX (1 mg/kg body mass, i.p.) for 5 days, whereas control rats (CTL) received saline solution. DEX rats exhibited peripheral insulin resistance, as indicated by the significant postabsorptive insulin levels and by the constant rate for glucose disappearance (KITT). GSIS was significantly higher in DEX islets (1.8-fold in 16.7 mmol/L glucose vs. CTL, p < 0.05). A significant increase of 2.25-fold in islet area was observed in DEX vs. CTL islets (p < 0.05). Cx36 mRNA expression was significantly augmented, Cx43 diminished, and Glut2 mRNA was unaltered in islets of DEX vs. CTL (p < 0.05). Cx36 protein expression was 1.6-fold higher than that of CTL islets (p < 0.05). Glut2 protein expression was unaltered and Cx43 was not detected at the protein level. We conclude that DEX-induced insulin resistance is accompanied by increased GSIS and this may be associated with increase of Cx36 protein expression.

Long-distance calls between cells connected by tunneling nanotubules

Long membrane tethers between cells, known as membrane nantotubes or tunneling nanotubules, create supracellular structures that allow multiple cell bodies to act in a synchronized manner. Calcium fluxes, vesicles, and cell-surface components can all traffic between cells connected by nanotubes. Thus, complex and specific messages can be transmitted between multiple cells, and the strength of signal will suffer relatively little with the distance traveled, as compared to the use of soluble factors to transmit messages. Connecting multiple antigen-presenting cells, for example, can help amplify and coordinate immune responses that are distal to an antigenic site. Conversely, because the ability of a pathogen to spread between cells is a key determinant of its capacity to multiply, pathogens may exploit nanotubes for their own transmission.

Inhibition of purinoceptors amplifies glucose-stimulated insulin release with removal of its pulsatility

External ATP has been proposed to be an autocrine regulator of glucose-stimulated insulin secretion and responsible for the synchronization of the Ca2+ rhythmicity in the beta-cells required for a pulsatile release of insulin from the pancreas. The importance of external ATP for glucose-stimulated insulin release was evaluated in rats with the aid of 2-deoxy-N-methyladenosine-3,5-bisphosphate (MRS 2179), an inhibitor of the purinoceptors known to affect the Ca2+ signaling in beta-cells. The concentration of cytoplasmic Ca2+ was measured in single beta-cells and small aggregates with ratiometric fura-2 technique and the release of insulin recorded from isolated islets and the perfused pancreas. Addition of 1 micromol/l ATP induced premature cytoplasmic Ca2+ concentration ([Ca2+]i) oscillations similar to those found in beta-cells exposed to 20 mmol/l glucose. In most experiments, the presence of 10 micromol/l MRS 2179 did not remove the glucose-induced [Ca2+]i rhythmicity in single beta-cells or the synchronization seen in coupled cells. Nevertheless, the same concentration of MRS 2179 promptly interrupted the pulsatility (frequency 0.22 +/- 0.01/min) of insulin secretion, raising the total amounts released from the pancreas. Prolonged exposure of islets to 1 and 10 micromol/l MRS 2179 enhanced insulin secretion at 20 mmol/l glucose 33% (P < 0.05) and 63% (P < 0.01), respectively, without affecting the release at 3 mmol/l glucose. The results support the idea that neural ATP signals entrain the islets into a common rhythm resulting in pulsatile release of insulin and that glucose stimulation of the secretory activity is counteracted by accumulation of inhibitory ATP around the beta-cells.

An anti-CD45RO/RB monoclonal antibody modulates T cell responses via induction of apoptosis and generation of regulatory T cells

The effects of a chimeric monoclonal antibody (chA6 mAb) that recognizes both the RO and RB isoforms of the transmembrane protein tyrosine phosphatase CD45 on human T cells were investigated. Chimeric A6 (chA6) mAb potently inhibited antigen-specific and polyclonal T cell responses. ChA6 mAb induced activation-independent apoptosis in CD4⁺CD45RO/RB^high T cells but not in CD8⁺ T cells. In addition, CD4⁺ T cell lines specific for tetanus toxoid (TT) generated in the presence of chA6 mAb were anergic and suppressed the proliferation and interferon (IFN)-γ production by TT-specific effector T cells by an interleukin-10–dependent mechanism, indicating that these cells were equivalent to type 1 regulatory T cells. Similarly, CD8⁺ T cell lines specific for the influenza A matrix protein-derived peptide (MP.58-66) generated in the presence of chA6 mAb were anergic and suppressed IFN-γ production by MP.58-66–specific effector CD8⁺ T cells. Furthermore, chA6 mAb significantly prolonged human pancreatic islet allograft survival in nonobese diabetic/severe combined immunodeficiency mice injected with human peripheral blood lymphocytes (hu-PBL-NOD/SCID). Together, these results demonstrate that the chA6 mAb is a new immunomodulatory agent with multiple modes of action, including deletion of preexisting memory and recently activated T cells and induction of anergic CD4⁺ and CD8⁺ regulatory T cells.

Loss of connexin36 channels alters beta-cell coupling, islet synchronization of glucose-induced Ca2+ and insulin oscillations, and basal insulin release

Normal insulin secretion requires the coordinated functioning of beta-cells within pancreatic islets. This coordination depends on a communications network that involves the interaction of beta-cells with extracellular signals and neighboring cells. In particular, adjacent beta-cells are coupled via channels made of connexin36 (Cx36). To assess the function of this protein, we investigated islets of transgenic mice in which the Cx36 gene was disrupted by homologous recombination. We observed that compared with wild-type and heterozygous littermates that expressed Cx36 and behaved as nontransgenic controls, mice homozygous for the Cx36 deletion (Cx36(-/-)) featured beta-cells devoid of gap junctions and failing to exchange microinjected Lucifer yellow. During glucose stimulation, islets of Cx36(-/-) mice did not display the regular oscillations of intracellular calcium concentrations ([Ca(2+)](i)) seen in controls due to the loss of cell-to-cell synchronization of [Ca(2+)](i) changes. The same islets did not release insulin in a pulsatile fashion, even though the overall output of the hormone in response to glucose stimulation was normal. However, under nonstimulatory conditions, islets lacking Cx36 showed increased basal release of insulin. These data show that Cx36-dependent signaling is essential for the proper functioning of beta-cells, particularly for the pulsatility of [Ca(2+)](i) and insulin secretion during glucose stimulation.

Islet isolation for allotransplantation: variables associated with successful islet yield and graft function

AIMS/HYPOTHESIS

Efficient islet isolation is an important prerequisite for successful clinical islet transplantation. Although progressively improved, islet yield and quality are, however, unpredictable and variable and require standardisation.

METHODS

Since 1989 we have processed 437 pancreases using the automated method. The donor characteristics, pancreas procurement, and digestion and purification procedures including a wide enzyme characterisation of these pancreases were analysed and correlated with islet yield and transplant outcome.

RESULTS

By univariate analysis, islet yield was significantly associated with donor age (r=0.16; p=0.0009), BMI (r=0.19; p=0.0004), good pancreas condition (p=0.0031) and weight (r=0.15; p=0.0056), total collagenase activity (r=0.22; p=0.0001), adjusted collagenase activity/mg (r=0.18; p=0.0002), collagenase activity/solution volume (r=0.18; p=0.0002) and neutral protease activity/solution volume (r=0.14; p=0.0029). A statistically significant contribution to the variability of islet yield in a multivariate analysis performed on donor variables was found for donor BMI (p=0.0008). In a multivariate analysis performed on pancreas variables a contribution was found for pancreas weight (p=0.0064), and for a multivariate analysis performed on digestion variables we found a contribution for digestion time (p=0.0048) and total collagenase activity (p=0.0001). Twenty-four patients with type 1 diabetes received single islet preparations from single donors. In these patients, multivariate analyses showed that the reduction in insulin requirement was significantly associated with morphological aspects of islets (p=0.0010) and that 1-month C-peptide values were associated with islet purity (p=0.0071).

CONCLUSIONS/INTERPRETATION

These data provide baseline donor, digestion and purification selection criteria for islet isolation using the automated method and indicate that the morphological aspect may be a clinically relevant measure of islets on which the decision for transplant can be based.

P2Y receptors play a critical role in epithelial cell communication and migration

Cellular injury induces a complex series of events that involves Ca2+ signaling, cell communication, and migration. One of the first responses following mechanical injury is the propagation of a Ca2+ wave (Klepeis et al. [2001] J Cell Sci 114(Pt 23):4185-4195). The wave is generated by the extracellular release of ATP, which also induces phosphorylation of ERK (Yang et al. [2004] J Cell Biochem 91(5):938-950). ATP and other nucleotides, which bind to and activate specific purinergic receptors were used to mimic injury. Our goal was to determine which of the P2Y purinergic receptors are expressed and stimulated in corneal epithelial cells and which signaling pathways are activated leading to changes in cell migration, an event critical for wound closure. In this study, we demonstrated that the P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11 receptors were present in corneal epithelial cells. A potency profile was determined by Ca2+ imaging for nucleotide agonists as follows: ATP > or = UTP > ADP > or = UDP. In contrast, negligible responses were seen for beta,gamma-meATP, a general P2X receptor agonist and adenosine, a P1 receptor agonist. Homologous desensitization of the Ca2+ response was observed for the four nucleotides. However, P2Y receptor internalization and degradation was not detected following stimulation with ATP, which is in contrast to EGFR internalization observed in response to EGF. ATP induced cell migration was comparable to that of EGF and was maximal at 1 microM. Cells exposed to ATP, UTP, ADP, and UDP demonstrated a rapid twofold increase in phosphorylation of paxillin at Y31 and Y118, however, there was no activation elicited by beta,gamma-meATP or adenosine. Additional studies demonstrated that wound closure was inhibited by reactive blue 2. These results indicate that P2Y receptors play a critical role in the injury repair process.

Bone marrow mesenchymal stem cells express a restricted set of functionally active chemokine receptors capable of promoting migration to pancreatic islets

Bone marrow-derived mesenchymal stem cells (BM-MSCs) are stromal cells with the ability to proliferate and differentiate into many tissues. Although they represent powerful tools for several therapeutic settings, mechanisms regulating their migration to peripheral tissues are still unknown. Here, we report chemokine receptor expression on human BM-MSCs and their role in mediating migration to tissues. A minority of BM-MSCs (2% to 25%) expressed a restricted set of chemokine receptors (CXC receptor 4 [CXCR4], CX3C receptor 1 [CX3CR1], CXCR6, CC chemokine receptor 1 [CCR1], CCR7) and, accordingly, showed appreciable chemotactic migration in response to the chemokines CXC ligand 12 (CXCL12), CX3CL1, CXCL16, CC chemokine ligand 3 (CCL3), and CCL19. Using human pancreatic islets as an in vitro model of peripheral tissue, we showed that islet supernatants released factors able to attract BM-MSCs in vitro, and this attraction was principally mediated by CX3CL1 and CXCL12. Moreover, cells with features of BM-MSCs were detected within the pancreatic islets of mice injected with green fluorescent protein (GFP)-positive BM. A population of bona fide MSCs that also expressed CXCR4, CXCR6, CCR1, and CCR7 could be isolated from normal adult human pancreas. This study defines the chemokine receptor repertoire of human BM-MSCs that determines their migratory activity. Modulation of homing capacity may be instrumental for harnessing the therapeutic potential of BM-MSCs.

Three roads to islet bursting: emergent oscillations in coupled phantom bursters

Glucose-induced membrane potential and Ca(2+) oscillations in isolated pancreatic beta-cells occur over a wide range of frequencies, from >6/min (fast) to <1/min (slow). However, cells within intact islets generally oscillate with periods of 10-60 s (medium). The phantom bursting concept addresses how beta-cells can generate such a wide range of frequencies. Here, we explore an updated phantom bursting model to determine how heterogeneity in a single parameter can explain both the broad frequency range observed in single cells and the rarity of medium oscillations. We then incorporate the single-cell model into an islet model with parameter heterogeneity. We show that strongly coupled islets must be composed of predominantly medium oscillating single cells or a mixture of fast and slow cells to robustly produce medium oscillations. Surprisingly, we find that this constraint does not hold for moderate coupling, and that robustly medium oscillating islets can arise from populations of single cells that are essentially all slow or all fast. Thus, with coupled phantom bursters, medium oscillating islets can be constructed out of cells that are either all fast, all slow, or a combination of the two.

Mitochondrial metabolism reveals a functional architecture in intact islets of Langerhans from normal and diabetic Psammomys obesus

The cells within the intact islet of Langerhans function as a metabolic syncytium, secreting insulin in a coordinated and oscillatory manner in response to external fuel. With increased glucose, the oscillatory amplitude is enhanced, leading to the hypothesis that cells within the islet are secreting with greater synchronization. Consequently, non-insulin-dependent diabetes mellitus (NIDDM; type 2 diabetes)-induced irregularities in insulin secretion oscillations may be attributed to decreased intercellular coordination. The purpose of the present study was to determine whether the degree of metabolic coordination within the intact islet was enhanced by increased glucose and compromised by NIDDM. Experiments were performed with isolated islets from normal and diabetic Psammomys obesus. Using confocal microscopy and the mitochondrial potentiometric dye rhodamine 123, we measured mitochondrial membrane potential oscillations in individual cells within intact islets. When mitochondrial membrane potential was averaged from all the cells in a single islet, the resultant waveform demonstrated clear sinusoidal oscillations. Cells within islets were heterogeneous in terms of cellular synchronicity (similarity in phase and period), sinusoidal regularity, and frequency of oscillation. Cells within normal islets oscillated with greater synchronicity compared with cells within diabetic islets. The range of oscillatory frequencies was unchanged by glucose or diabetes. Cells within diabetic (but not normal) islets increased oscillatory regularity in response to glucose. These data support the hypothesis that glucose enhances metabolic coupling in normal islets and that the dampening of oscillatory insulin secretion in NIDDM may result from disrupted metabolic coupling.

Local dissipation and coupling properties of cellular oscillators: a case study on calcium oscillations

Synchronised signal transduction between cells is crucial, since it assures fast and immutable information processing, which is vital for flawless functioning of living organisms. The question arises how to recognise the ability of a cell to be easily coupled with other cells. In the present paper, we investigate the system properties that determine best coupling abilities and assure the most efficient signal transduction between cells. A case study is done for intercellular calcium oscillations. For a particular diffusion-like coupled system of cellular oscillators, we determined the minimal gap-junctional permeability that is necessary for synchronisation of initially asynchronous oscillators. Our results show that dissipation is a crucial system property that determines the coupling ability of cellular oscillators. We found that low dissipation assures synchronisation of coupled cells already at very low gap-junctional permeability, whereas highly dissipative oscillators require much higher gap-junctional permeability in order to synchronise. The results are discussed in the sense of their biological importance for systems where the synchronous responses of cells were recognised to be indispensable for appropriate physiological functioning of the tissue.

Pulses of external ATP aid to the synchronization of pancreatic beta-cells by generating premature Ca(2+) oscillations

Pancreatic beta-cells respond to glucose stimulation with increase of the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), manifested as membrane-derived slow oscillations sometimes superimposed with transients of intracellular origin. The effect of external ATP on the oscillatory Ca(2+) signal for pulsatile insulin release was studied by digital imaging of fura-2 loaded beta-cells and small aggregates isolated from islets of ob/ob-mice. Addition of ATP (0.01-100 microM) to media containing 20mM glucose temporarily synchronized the [Ca(2+)](i) rhythmicity in the absence of cell contact by eliciting premature oscillations. External ATP triggered premature [Ca(2+)](i) oscillations also when the sarcoendoplasmic reticulum Ca(2+)-ATPase was inhibited with 50 microM cyclopiazonic acid and phospholipase C inhibited with 10 microM U-73122. The effect of ATP was mimicked by other activators of cytoplasmic phospholipase A(2) (10nM acetylcholine, 0.1-1 micro M of the C-terminal octapeptide of cholecystokinin and 2 microg/ml melittin) and suppressed by an inhibitor of the enzyme (50 microM p-amylcinnamoylanthranilic acid). Premature oscillations generated by pulses of ATP sometimes triggered subsequent oscillations. However, prolonged exposure to high concentrations of the nucleotide (10-100 microM) had a suppressive action on the beta-cell rhythmicity. The early effects of ATP included generation of transients induced by inositol (1,4,5) trisphosphate and superimposed on the premature oscillation or on an ordinary oscillation induced by glucose. The results support the idea that purinergic activation of phospholipase A(2) has a co-ordinating effect on the beta-cell rhythmicity by triggering premature [Ca(2+)](i) oscillations mediated by closure of ATP-sensitive K(+) channels.

Synchronization and entrainment of cytoplasmic Ca2+ oscillations in cell clusters prepared from single or multiple mouse pancreatic islets

In contrast to pancreatic islets, isolated beta-cells stimulated by glucose display irregular and asynchronous increases in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)). Here, clusters of 5-30 cells were prepared from a single mouse islet or from pools of islets, loaded with fura-2, and studied with a camera-based system. [Ca(2+)](i) oscillations were compared in pairs of clusters by computing the difference in period and a synchronization index lambda. During perifusion with 12 mM glucose, the clusters exhibited regular [Ca(2+)](i) oscillations that were quasi-perfectly synchronized (Delta period of 1.4% and index lambda close to 1.0) between cells of each cluster. In contrast, separate clusters were not synchronized, even when prepared from one single islet. Pairs of clusters neighboring on the same coverslip were not better synchronized than pairs of clusters examined separately (distinct coverslips). We next attempted to synchronize clusters perifused with 12 mM glucose by applying external signals. A single pulse of 20 mM glucose, 10 mM amino acids, or 10 microM tolbutamide transiently altered [Ca(2+)](i) oscillations but did not reset the clusters to oscillate synchronously. On a background of 12 mM glucose, repetitive applications (1 min/5 min) of 10 microM tolbutamide, but not of 20 mM glucose, synchronized separate clusters. Our results identify a level of beta-cell heterogeneity intermediate between single beta-cells and the whole islet. They do not support the idea that substances released by islet cells serve as paracrine synchronizers. However, synchronization can be achieved by an external signal, if this signal has a sufficient strength to overwhelm the intrinsic rhythm of glucose-induced oscillations and is repetitively applied.

Pancreatic beta-cells communicate via intermittent release of ATP

The role of external ATP for intercellular communication was studied in glucose-stimulated pancreatic beta-cells isolated from ob/ob mice. Digital image analyses with fura-2 revealed spontaneous transients of cytoplasmic Ca2+ appearing in synchrony in the absence of cell contacts. After removal of slow oscillations with methoxyverapamil, addition of ATP (0.1-100 microM) resulted in prompt firing of a transient, followed by suppression of the generation and synchronization of spontaneously occurring transients. It was possible to trigger transients during the suppressive phase by raising the concentration of ATP. The dual action of ATP was mimicked by ADP or 2-methylthio-ATP but not by AMP or UTP. The number of spontaneous transients and their synchronization were reduced in the presence of the dephosphorylating agent apyrase. Additional evidence that intermittent release of ATP participates in the generation of spontaneous Ca2+ transients was obtained from the suppression observed from use of antagonists of the purinoceptors [suramin (0.3-30 microM), pyridoxalphosphate-6-azophenyl-2,4-disulfonic acid (PPADS; 10-30 microM) and 2-deoxy-N-methyladenosine (MRS 2179; 0.3-30 microM)] or from counteracting beta-cell release of ATP by inhibiting exocytosis with 100 nM epinephrine, 100 nM somatostatin, or lowering the temperature below 30 degrees C. The data indicate that ATP has time-dependent actions (prompt stimulation followed by inhibition) on the generation of Ca2+ transients mediated by P2Y receptors. It is proposed that beta-cells both receive a neural ATP signal with coordinating effects on their Ca2+ oscillations and propagate this message to adjacent cells via intermittent release of ATP combined with gap junction coupling.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Carbon monoxide stimulates insulin release and propagates Ca2+ signals between pancreatic beta-cells

A key question for understanding the mechanisms of pulsatile insulin release is how the underlying beta-cell oscillations of the cytoplasmic Ca2+ concentration ([Ca2+]i) are synchronized within and among the islets in the pancreas. Nitric oxide has been proposed to coordinate the activity of the beta-cells by precipitating transients of [Ca2+]i. Comparing ob/ob mice and lean controls, we have now studied the action of carbon monoxide (CO), another neurotransmitter with stimulatory effects on cGMP production. A strong immunoreactivity for the CO-producing constitutive heme oxygenase (HO-2) was found in ganglionic cells located in the periphery of the islets and in almost all islet endocrine cells. Islets from ob/ob mice had sixfold higher generation of CO (1 nmol.min-1.mg protein-1) than the lean controls. This is 100-fold the rate for their constitutive production of NO. Moreover, islets from ob/ob mice showed a threefold increase in HO-2 expression and expressed inducible HO (HO-1). The presence of an excessive islet production of CO in the ob/ob mouse had its counterpart in a pronounced suppression of the glucose-stimulated insulin release from islets exposed to the HO inhibitor Zn-protoporhyrin (10 microM) and in a 16 times higher frequency of [Ca2+]i transients in their beta-cells. Hemin (0.1 and 1.0 microM), the natural substrate for HO, promoted the appearance of [Ca2+]i transients, and 10 microM of the HO inhibitors Zn-protoporphyrin and Cr-mesoporphyrin had a suppressive action both on the firing of transients and their synchronization. It is concluded that the increased islet production of CO contributes to the hyperinsulinemia in ob/ob mice. In addition to serving as a positive modulator of glucose-stimulated insulin release, CO acts as a messenger propagating Ca2+ signals with coordinating effects on the beta-cell rhythmicity.

Synchronization of pancreatic beta-cell rhythmicity after glucagon induction of Ca2+ transients

Pancreatic beta-cells are biological oscillators requiring a coupling force for the synchronization of the cytoplasmic Ca(2+) oscillations responsible for pulsatile insulin release. Testing the idea that transients, superimposed on the oscillations, are important for this synchronization, the concentration of cytoplasmic Ca(2+) ([Ca(2+)](i)) was measured with ratiometric fura-2 technique in single beta-cells and small aggregates prepared from islets isolated from ob/ob-mice. Image analyses revealed asynchronous [Ca(2+)](i) oscillations in adjacent beta-cells lacking physical contact. The addition of glucagon stimulated the firing of [Ca(2+)](i) transients, which appeared in synchrony in adjacent beta-cells. Moreover, the presence of glucagon promoted synchronization of the [Ca(2+)](i) oscillations in beta-cells separated by a distance <100 microm but not in those >200 microm apart. The results support the proposal that the repolarizing effect of [Ca(2+)](i) transients provides a coupling force for co-ordinating the pulses of insulin release generated by pancreatic beta-cells.

Contribution of abnormal insulin secretion and insulin resistance to the pathogenesis of type 2 diabetes in myotonic dystrophy

OBJECTIVE

Myotonic dystrophy (MyD), the most common adult form of muscular dystrophy, is often complicated by diabetes. MyD is dominantly inherited and is due to heterozygosity for a trinucleotide repeat expansion mutation in a protein kinase gene able to induce derangement of RNA metabolism responsible of an aberrant insulin receptor expression.

RESEARCH DESIGN AND METHODS

To assess insulin sensitivity and secretion before the onset of diabetes, we studied 10 MyD patients, 10 offspring of type 2 diabetes (OFF), and 10 healthy subjects with no family history of diabetes (control subjects) with dual X-ray energy absorption, euglycemic-hyperinsulinemic clamp (40 mU/[m(2). min]) combined with infusion of [6,6-D(2)]-glucose and oral glucose tolerance test (OGTT).

RESULTS

MyD had reduced lean body mass, but peripheral insulin sensitivity was not different to that of control subjects in contrast to OFF, which showed insulin resistance. Insulin secretion, obtained by deconvolution of OGTT data, was also shown to be comparable with that of OFF and control subjects (index of beta-cell function = Phi; P = 0.91) even if increased parameters of insulin secretion were found during the first 30 min (Phi(30); P = 0.05) of the oral glucose challenge. Fasting plasma proinsulin concentrations (P = 0.01) and the ratio to insulin (P = 0.01) were increased in MyD patients. The proinsulin levels also failed to be suppressed during the clamp and showed exaggerated response after the OGTT. Increased proinsulin levels were shown to be peculiar of MyD patients when compared with OFF.

CONCLUSIONS

In nondiabetic, young MyD patients, insulin sensitivity was preserved, and an increased early secretory response to oral glucose was detected. Abnormal plasma proinsulin levels in the fasting state, during the clamp, and during the OGTT were shown to be secretory dysfunctions peculiar of MyD patients and may be more important than insulin resistance in determining the high risk to develop diabetes in these patients.

Connexin 36 controls synchronization of Ca2+ oscillations and insulin secretion in MIN6 cells

Cx36 is the predominant connexin isoform expressed by pancreatic beta-cells. However, little is known about the role of this protein in the functioning of insulin-secreting cells. To address this question, we searched for a cell line expressing Cx36 and having glucose-induced insulin secretion comparable to that of primary beta-cells. By evaluating Cx36 expression in MIN6, betaTC3, RIN2A, INS1, and HIT cell lines, which differ in their sensitivity to glucose, we found that wild-type MIN6 cells fit these requirements. Therefore, we stably transfected MIN6 cells with a cDNA coding for a Cx36 antisense sequence to study the role of Cx36 in these cells. Independent clones of MIN6 cells were obtained that had a markedly reduced Cx36 expression. Loss of Cx36 decreased functional gap junctional conductance in these clones. This alteration impaired the synchronization of glucose-induced [Ca(2+)](i) oscillations and insulin secretion in response to glucose, to secretagogues that increase [cAMP](i), and to depolarizing conditions. These data provide the first evidence that Cx36-made channels 1) mediate functional coupling in MIN6 cells, 2) provide for synchronous [Ca(2+)](i) oscillations, and 3) are necessary for proper insulin secretion in response to metabolizable and nonmetabolizable secretagogues.

Stretch activation of Ca2+ transients in pancreatic beta cells by mobilization of intracellular stores

INTRODUCTION

Nonadrenergic, noncholinergic neurons have been proposed to synchronize pulsatile insulin release from the islets in the pancreas by triggering transient increases of the cytoplasmic Ca2+ concentration ([Ca2+]i) in beta-cells via an inositol trisphoshate-dependent mechanism.

AIMS

To test whether pancreatic beta-cells respond to stretch activation with similar types of transients and whether these Ca signals propagate to other beta-cells in the presence and absence of cell contacts.

METHODOLOGY

Single cells and small aggregates were prepared from beta-cell-rich islets from mice. After 2-5 days of culture, [Ca2+]i was measured with digital imaging and the indicator fura-2 during superfusion with a medium containing 20 mmol/L glucose and 50 micromol/L methoxyverapamil. Membrane stretch was induced by osmotic swelling or focal touch stimulation.

RESULTS

Lowering the medium osmolarity with 100-102 mOSM/L by removal of sucrose or by dilution resulted in a 2-3-fold increase in the number of transients during an initial 5-minute period. Sucrose omission was stimulatory also after isosmolar replacement with readily penetrating urea. The intracellular Ca2+-ATPase inhibitor thapsigargin suppressed both the spontaneously occurring transients and those initiated by volume expansion. Touch stimuli induced [Ca2+]i transients, which rapidly propagated to cells within the same aggregate or lacking contact.

CONCLUSION

The observations support the idea that beta-cells both receive and regenerate extracellular signals triggering [Ca2+]i transients. Touch stimulation is a useful tool for investigating the propagation of [Ca2+]i signals between pancreatic beta-cells lacking physical contact.

Intercellular calcium signaling mediated by point-source burst release of ATP

Calcium signaling, manifested as intercellular waves of rising cytosolic calcium, is, in many cell types, the result of calcium-induced secretion of ATP and activation of purinergic receptors. The mechanism by which ATP is released has hitherto not been established. Here, we show by real-time bioluminescence imaging that ATP efflux is not uniform across a field of cells but is restricted to brief, abrupt point-source bursts. The ATP bursts emanate from single cells and manifest the transient opening of nonselective membrane channels, which admits fluorescent indicators of < or = 1.5 kDa. These observations challenge the existence of regenerative ATP release, because ATP efflux is finite and restricted to a point source. Transient efflux of cytosolic nucleotides from a subset of cells may represent a conserved pathway for coordinating local activity of electrically nonexcitable cells, because identical patterns of ATP release were identified in human astrocytes, endothelial cells, and bronchial epithelial cells.

A model for glucose-induced wave propagation in pancreatic islets of Langerhans

A reaction-diffusion type model is constructed, describing the spatio-temporal dynamics of the basic intracellular variables assumed to be involved in the initiation of the insulin secretion process by beta -cells in the pancreatic islets of Langerhans. The model includes equations for the electric membrane potential of the cells, with respective kinetics for ionic currents, for concentrations of both free and stored intracellular calcium, and for the intra- and extracellular concentrations of glucose. An empirical expression connecting the equation for the intracellular glucose concentration to the electrical equation is introduced. The model reproduces the events observed in experiments in vitro upon external glucose application to the islets of Langerhans, such as usual bursting oscillations of the membrane potential and corresponding oscillations of the intracellular calcium concentration. It also allows simulation of electric wave propagation through the islet, initiated by the spatial gradient of glucose concentration within the islet. The gradient emerges due to glucose diffusing into the islets from the external medium, being high at the edges. The latter results show that glucose diffusion presents a means for wave initiation in the islets, which supports our previous assumption (Aslanidi et al., 2001).

Control mechanisms of the oscillations of insulin secretion in vitro and in vivo

The mechanisms driving the pulsatility of insulin secretion in vivo and in vitro are still unclear. Because glucose metabolism and changes in cytosolic free Ca(2+) ([Ca(2+)](c)) in beta-cells play a key role in the control of insulin secretion, and because oscillations of these two factors have been observed in single isolated islets and beta-cells, pulsatile insulin secretion could theoretically result from [Ca(2+)](c) or metabolism oscillations. We could not detect metabolic oscillations independent from [Ca(2+)](c) changes in beta-cells, and imposed metabolic oscillations were poorly effective in inducing oscillations of secretion when [Ca(2+)](c) was kept stable, which suggests that metabolic oscillations are not the direct regulator of the oscillations of secretion. By contrast, tight temporal and quantitative correlations between the changes in [Ca(2+)](c) and insulin release strongly suggest that [Ca(2+)](c) oscillations are the direct drivers of insulin secretion oscillations. Metabolism may play a dual role, inducing [Ca(2+)](c) oscillations (via changes in ATP-sensitive K(+) channel activity and membrane potential) and amplifying the secretory response by increasing the efficiency of Ca(2+) on exocytosis. The mechanisms underlying the oscillations of insulin secretion by the isolated pancreas and those observed in vivo remain elusive. It is not known how the functioning of distinct islets is synchronized, and the possible role of intrapancreatic ganglia in this synchronization requires confirmation. That pulsatile insulin secretion is beneficial in vivo, by preventing insulin resistance, is suggested by the greater hypoglycemic effect of exogenous insulin when it is infused in a pulsatile rather than continuous manner. The observation that type 2 diabetic patients have impaired pulsatile insulin secretion has prompted the suggestion that such dysregulation contributes to the disease and justifies the efforts toward understanding of the mechanism underlying the pulsatility of insulin secretion both in vitro and in vivo.

Metabolic oscillations in beta-cells

Whereas the mechanisms underlying oscillatory insulin secretion remain unknown, several models have been advanced to explain if they involve generation of metabolic oscillations in beta-cells. Evidence, including measurements of oxygen consumption, glucose consumption, NADH, and ATP/ADP ratio, has accumulated to support the hypothesis that energy metabolism in beta-cells can oscillate. Where simultaneous measurements have been made, these oscillations are well correlated with oscillations in intracellular [Ca(2+)] and insulin secretion. Considerable evidence has been accumulated to suggest that entry of Ca(2+) into cells can modulate metabolism both positively and negatively. The main positive effect of Ca(2+) is an increase in oxygen consumption, believed to involve activation of mitochondrial dehydrogenases. Negative feedback by Ca(2+) includes decreases in glucose consumption and decreases in the mitochondrial membrane potential. Ca(2+) also provides negative feedback by increasing consumption of ATP. The negative feedback provided by Ca(2+) provides a mechanism for generating oscillations based on a model in which glucose stimulates a rise in ATP/ADP ratio that closes ATP-sensitive K(+) (K(ATP)) channels, thus depolarizing the cell membrane and allowing Ca(2+) entry through voltage-sensitive channels. Ca(2+) entry reduces the ATP/ADP ratio and allows reopening of the K(ATP) channel.

Human pancreatic islets produce and secrete MCP-1/CCL2: relevance in human islet transplantation

We investigated the capacity of human islets to produce monocyte chemoattractant protein-1 (MCP-1). Primary cultures of pancreatic islets expressed and secreted MCP-1, as determined by Northern blot, immunohistochemistry, in situ hybridization, and enzyme-linked immunosorbent assay. The produced MCP-1 was biologically active as it attracted monocytes in chemotaxis assay, and chemotactic activity was almost abrogated by a neutralizing anti-MCP-1 monoclonal antibody. Expression of MCP-1 was increased by primary inflammatory cytokines (interleukin-1 beta, tumor necrosis factor-alpha) and lipopolysaccharide at both the mRNA and protein levels but not by glucose. However, MCP-1 did not modulate insulin secretion. MCP-1 secreted by pancreatic islets plays a relevant role in the clinical outcome of islet transplant in patients with type 1 diabetes. In fact, low MCP-1 secretion resulted as the most relevant factor for long-lasting insulin independence. This finding opens new approaches in the management of human islet transplantation. Finally, the finding that MCP-1 appears constitutively present in normal human islet beta-cells (immunohistochemistry and in situ hybridization), in the absence of an inflammatory infiltrate, suggests that this chemokine could have functions other than monocyte recruitment and opens a new link between the endocrine and immune systems.

Effects of noise on the off rate of Ca(2+) binding proteins in a coupled biochemical cell system

We investigated numerically the kinetic properties of calcium binding proteins by using a three-calcium store model and discussed the response of a two-way coupled biochemical cell system, whose subsystems were coupled via diffusion-like cytosolic calcium transfer through gap junctions, to the external stimulation. When we used noise to modulate the off rate of the Ca(2+) binding proteins in simulation, an SR-like phenomenon of synchronous oscillations is observed. In addition, the interaction involving noise, coupling and the dynamics of Ca(2+) binding proteins is discussed.

Growth factors but not gap junctions play a role in injury-induced Ca2+ waves in epithelial cells

This paper characterizes the early responses of epithelial cells to injury. Ca2+ is an important early messenger that transiently increases in the cytoplasm of cells in response to external stimuli. Its elevation leads to the regulation of signaling pathways responsible for the downstream events important for wound repair, such as cell migration and proliferation. Live cell imaging in combination with confocal laser scanning microscopy of fluo-3 AM loaded cells was performed. We found that mechanical injury in a confluent region of cells creates an elevation in Ca2+ that is immediately initiated at the wound edge and travels as a wave to neighboring cells, with [Ca2+]i returning to background levels within two minutes. Addition of epidermal growth factor (EGF), but not platelet-derived growth factor-BB, resulted in increased [Ca2+]i, and EGF specifically enhanced the amplitude and duration of the injury-induced Ca2+ wave. Propagation of the Ca2+ wave was dependent on intracellular Ca2+ stores, as was demonstrated using both thapsigargin and Ca2+ chelators (EGTA and BAPTA/AM). Injury-induced Ca2+ waves were not mediated via gap junctions, as the gap-junction inhibitors 1-heptanol and 18α-glycyrrhetinic acid did not alter wave propagation, nor did the cells recover in photobleaching experiments. Additional studies also demonstrated that the wave could propagate across an acellular region. The propagation of the injury-induced Ca2+ wave occurs via diffusion of an extracellular mediator, most probably via a nucleotide such as ATP or UTP, that is released upon cell damage.

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Nitric oxide induces synchronous Ca2+ transients in pancreatic beta cells lacking contact

AIMS

To evaluate the role of nitric oxide (NO) in the coordination of the Ca2+ signals generating pulsatile insulin release in pancreatic beta cells isolated from ob/ob mice.

METHODOLOGY

Using ratiometric fura-2 technique for recording glucose-induced cytoplasmic Ca2+ transients, it was possible to demonstrate a synchronization of beta cells lacking contact.

RESULTS

The frequency of the transients increased 10-fold in the presence of 20 n M glucagon. Additional increase in frequency with maintenance of synchronization was observed when the beta cells were exposed to 100 microM of the NO donors sodium nitroprusside and hydroxylamine. Bolus additions of 0.1-10 microM gaseous NO resulted in prompt appearance of cytoplasmic Ca2+ transients. An activator of soluble guanylate cyclase (mesoporphyrin) increased the frequency of the transients, and inhibition of this enzyme with 1H-(1,2,4) oxadiazolo [4,3-a] quinoxalin-1-one had the opposite effect.

CONCLUSION

The results support the idea that nitrergic nerves generate beta-cell transients of Ca2+ synchronizing the activity of the numerous islets in the pancreas.

Enhancement of connexin 43 expression increases proliferation and differentiation of an osteoblast-like cell line

Bone cells form a functional syncytium as they are coupled by gap junctions composed mainly of connexin 43 (Cx43). To further understand the role of Cx43 in bone cell growth and differentiation, we stably transfected Cx45-expressing UMR 106-01 cells with Cx43 using an expression vector containing rat Cx43 cDNA. Three stably transfected clones were analyzed, all of which showed altered expression of Cx43 and/or Cx45 as was obvious from immunocytochemistry and Northern blotting. Double whole-cell patch clamping revealed single-channel conductances of 20 (Cx45) and 60 pS (Cx43). The overexpression of Cx43 led to an increase in dye coupling concomitant with elevated gap-junctional conductance. The phenotype of the transfected clones was characterized by an increased proliferation (4- to 7-fold) compared to controls. Moreover, a transfectant clone with 10- to 12-fold enhanced Cx43 expression showed a significantly increased calcium content of the extracellular matrix and enlarged mineralization nodules, while alkaline phosphatase was moderately increased. We conclude that enhanced gap-junctional coupling via Cx43 significantly promotes proliferation and differentiation of UMR cells.

Measurements of cytoplasmic Ca2+ in islet cell clusters show that glucose rapidly recruits beta-cells and gradually increases the individual cell response

The proportion of isolated single beta-cells developing a metabolic, biosynthetic, or secretory response increases with glucose concentration (recruitment). It is unclear whether recruitment persists in situ when beta-cells are coupled. We therefore measured the cytoplasmic free Ca2+ correction ([Ca2+]i) (the triggering signal of glucose-induced insulin secretion) in mouse islet single cells or clusters cultured for 1-2 days. In single cells, the threshold glucose concentration ranged between 6 and 10 mmol/l, at which concentration a maximum of approximately 65% responsive cells was reached. Only 13% of the cells did not respond to glucose plus tolbutamide. The proportion of clusters showing a [Ca2+]i rise increased from approximately 20 to 95% between 6 and 10 mmol/l glucose, indicating that the threshold sensitivity to glucose differs between clusters. Within responsive clusters, 75% of the cells were active at 6 mmol/l glucose and 95-100% at 8-10 mmol/l glucose, indicating that individual cell recruitment is not prominent within clusters; in clusters responding to glucose, all or almost all cells participated in the response. Independently of cell recruitment, glucose gradually augmented the magnitude of the average [Ca2+]i rise in individual cells, whether isolated or associated in clusters. When insulin secretion was measured simultaneously with [Ca2+]i, a good temporal and quantitative correlation was found between both events. However, beta-cell recruitment was maximal at 10 mmol/l glucose, whereas insulin secretion increased up to 15-20 mmol/l glucose. In conclusion, beta-cell recruitment by glucose can occur at the stage of the [Ca2+]i response. However, this type of recruitment is restricted to a narrow range of glucose concentrations, particularly when beta-cell association decreases the heterogeneity of the responses. Glucose-induced insulin secretion by islets, therefore, cannot entirely be ascribed to recruitment of beta-cells to generate a [Ca2+]i response. Modulation of the amplitude of the [Ca2+]i response and of the action of Ca2+ on exocytosis (amplifying actions of glucose) may be more important.

Excitation wave propagation as a possible mechanism for signal transmission in pancreatic islets of Langerhans

In response to glucose application, beta-cells forming pancreatic islets of Langerhans start bursting oscillations of the membrane potential and intracellular calcium concentration, inducing insulin secretion by the cells. Until recently, it has been assumed that the bursting activity of beta-cells in a single islet of Langerhans is synchronized across the whole islet due to coupling between the cells. However, time delays of several seconds in the activity of distant cells are usually observed in the islets of Langerhans, indicating that electrical/calcium wave propagation through the islets can occur. This work presents both experimental and theoretical evidence for wave propagation in the islets of Langerhans. Experiments with Fura-2 fluorescence monitoring of spatiotemporal calcium dynamics in the islets have clearly shown such wave propagation. Furthermore, numerical simulations of the model describing a cluster of electrically coupled beta-cells have supported our view that the experimentally observed calcium waves are due to electric pulses propagating through the cluster. This point of view is also supported by independent experimental results. Based on the model equations, an approximate analytical expression for the wave velocity is introduced, indicating which parameters can alter the velocity. We point to the possible role of the observed waves as signals controlling the insulin secretion inside the islets of Langerhans, in particular, in the regions that cannot be reached by any external stimuli such as high glucose concentration outside the islets.

Expression of connexin 30 in Xenopus embryos and its involvement in hatching gland function

Connexins are a family of proteins that assemble to form gap junction channels. Cell-cell communication through gap junctions mediates many important events in embryogenesis, including limb patterning, lens physiology, neuronal function, left-right asymmetry, and secretion from gland tissue. We studied the expression of connexin 30 (Cx30) in the Xenopus embryo and find that it is expressed in the developing hatching gland and pronephros. To determine whether its expression plays a functional role in the activity of the hatching gland, we exposed pre-hatching embryos to drugs that block gap junctional communication. This resulted in a continuation of normal growth and development but specifically abolished hatching. The treatment did not affect Cx30 or Xenopus hatching enzyme transcription, suggesting a post-transcriptional effect on Cx30 gap junctions. We conclude that junctional communication, possibly mediated by Cx30, is involved in secretion of hatching enzyme in Xenopus.

Metabolic control of beta-cell function

Glucose-induced insulin secretion is pulsatile. Glucose metabolism generates oscillations in the ATP/ADP ratio which lead to opening and closing of ATP-sensitive K(+)-channels producing subsequent oscillations in membrane potential, cytoplasmic calcium and insulin release. Metabolic signals derived from glucose can also stimulate insulin release independent of their effects on ATP-sensitive K(+)-channels. The ATP/ADP ratio may mediate both ATP-sensitive K(+)-channel-dependent and -independent pathways of secretion. Glucose metabolism also results in an increase in long-chain acyl-CoA, which is proposed to act as an effector molecule in the beta -cell. Long-chain acyl-CoA has a variety of effects in the beta -cell that may effect insulin secretion including opening ATP-sensitive K(+)-channels, activating endoplasmic reticulum Ca(2+)-ATPases and stimulating classical protein kinase C activity. In addition to stimulating insulin release, nutrients also effect gene expression, protein synthesis and beta -cell proliferation. Gene expression is effected by nutrient induction of a variety of immediate early response genes. Glucose stimulates proinsulin biosynthesis both at the translational and transcriptional level. beta -cell proliferation, as a result of insulin-like growth factor and growth hormone mitogenic pathways, is also glucose dependent. Thus, many beta -cell functions in addition to secretion are controlled by nutrient metabolism.

Junctional communication of pancreatic beta cells contributes to the control of insulin secretion and glucose tolerance

Proper insulin secretion requires the coordinated functioning of the numerous beta cells that form pancreatic islets. This coordination depends on a network of communication mechanisms whereby beta cells interact with extracellular signals and adjacent cells via connexin channels. To assess whether connexin-dependent communication plays a role in vivo, we have developed transgenic mice in which connexin 32 (Cx32), one of the vertebrate connexins found in the pancreas, is expressed in beta cells. We show that the altered beta-cell coupling that results from this expression causes reduced insulin secretion in response to physiologically relevant concentrations of glucose and abnormal tolerance to the sugar. These alterations were observed in spite of normal numbers of islets, increased insulin content, and preserved secretory response to glucose by individual beta cells. Moreover, glucose-stimulated islets showed improved electrical synchronization of these cells and increased cytosolic levels of Ca(2+). The results show that connexins contribute to the control of beta cells in vivo and that their excess is detrimental for insulin secretion.

Synchronization of Ca(2+)-signals within insulin-secreting pseudoislets: effects of gap-junctional uncouplers

The secretory response of the intact islet is greater than the response of individual beta-cells in isolation, and functional coupling between cells is critical in insulin release. The changes in intracellular Ca(2+)([Ca(2+)](i)) which initiate insulin secretory responses are synchronized between groups of cells within the islet, and gap-junctions are thought to play a central role in coordinating signalling events. We have used the MIN6 insulin-secreting cell line, to examine whether uncoupling gap-junctions alters the synchronicity of nutrient- and non-nutrient-evoked Ca(2+)oscillations, or affects insulin secretion. MIN6 cells express mRNA species that can be amplified using PCR primers for connexin 36. A commonly used gap-junctional inhibitor, heptanol, inhibited glucose- and tolbutamide-induced Ca(2+)-oscillations to basal levels in MIN6 cell clusters at concentrations of 0.5 mM and greater, and it had similar effects in pseudoislets when used at 2.5 mM. Lower heptanol concentrations altered the frequency of Ca(2+)transients without affecting their synchronicity, in both monolayers and pseudoislets. Heptanol also had effects on insulin secretion from MIN6 pseudoislets such that 1 mM enhanced secretion while 2.5 mM was inhibitory. These data suggest that heptanol has multiple effects in pancreatic beta-cells, none of which appears to be related to uncoupling of synchronicity of Ca(2+)signalling between cells. A second gap-junction uncoupler, 18 alpha-glycyrrhetinic acid, also failed to uncouple synchronized Ca(2+)-oscillations, and it had no effect on insulin secretion. These data provide evidence that Ca(2+)signalling events occur simultaneously across the bulk mass of the pseudoislet, and suggest that gap-junctions are not required to coordinate the synchronicity of these events, nor is communication via gap junctions essential for integrated insulin secretory responses.