Over Cabrera

A novel method for the assessment of cellular composition and beta-cell viability in human islet preparations

Current methodologies to evaluate islet cell viability are largely based on tests that assess the exclusion of DNA‐binding dyes. While these tests identify cells that have lost selective membrane permeability, they do not allow us to recognize apoptotic cells, which do not yet stain with DNA‐binding dyes. Furthermore, current methods of analysis do not discriminate between cell subsets in the preparation and, in particular, they do not allow for selectively defining β‐cell viability.

Noninvasive in vivo imaging of pancreatic islet cell biology

Advanced imaging techniques have become a valuable tool in the study of complex biological processes at the cellular level in biomedical research. Here, we introduce a new technical platform for noninvasive in vivo fluorescence imaging of pancreatic islets using the anterior chamber of the eye as a natural body window. Islets transplanted into the mouse eye engrafted on the iris, became vascularized, retained cellular composition, responded to stimulation and reverted diabetes. Laser-scanning microscopy allowed repetitive in vivo imaging of islet vascularization, beta cell function and death at cellular resolution. Our results thus establish the basis for noninvasive in vivo investigations of complex cellular processes, like beta cell stimulus-response coupling, which can be performed longitudinally under both physiological and pathological conditions.

Mesenchymal Stem Cells Enhance Allogeneic Islet Engraftment in Nonhuman Primates

OBJECTIVE To test the graft-promoting effects of mesenchymal stem cells (MSCs) in a cynomolgus monkey model of islet/bone marrow transplantation. RESEARCH DESIGN AND METHODS Cynomolgus MSCs were obtained from iliac crest aspirate and characterized through passage 11 for phenotype, gene expression, differentiation potential, and karyotype. Allogeneic donor MSCs were cotransplanted intraportally with islets on postoperative day (POD) 0 and intravenously with donor marrow on PODs 5 and 11. Recipients were followed for stabilization of blood glucose levels, reduction of exogenous insulin requirement (EIR), C-peptide levels, changes in peripheral blood T regulatory cells, and chimerism. Destabilization of glycemia and increases in EIR were used as signs of rejection; additional intravenous MSCs were administered to test the effect on reversal of rejection. RESULTS MSC phenotype and a normal karyotype were observed through passage 11. IL-6, IL-10, vascular endothelial growth factor, TGF-β, hepatocyte growth factor, and galectin-1 gene expression levels varied among donors. MSC treatment significantly enhanced islet engraftment and function at 1 month posttransplant (n = 8), as compared with animals that received islets without MSCs (n = 3). Additional infusions of donor or third-party MSCs resulted in reversal of rejection episodes and prolongation of islet function in two animals. Stable islet allograft function was associated with increased numbers of regulatory T-cells in peripheral blood. CONCLUSIONS MSCs may provide an important approach for enhancement of islet engraftment, thereby decreasing the numbers of islets needed to achieve insulin independence. Furthermore, MSCs may serve as a new, safe, and effective antirejection therapy.

Glutamate is a positive autocrine signal for glucagon release.

An important feature of glucose homeostasis is the effective release of glucagon from the pancreatic alpha cell. The molecular mechanisms regulating glucagon secretion are still poorly understood. We now demonstrate that human alpha cells express ionotropic glutamate receptors (iGluRs) that are essential for glucagon release. A lowering in glucose concentration results in the release of glutamate from the alpha cell. Glutamate then acts on iGluRs of the AMPA/kainate type, resulting in membrane depolarization, opening of voltage-gated Ca(2+) channels, increase in cytoplasmic free Ca(2+) concentration, and enhanced glucagon release. In vivo blockade of iGluRs reduces glucagon secretion and exacerbates insulin-induced hypoglycemia in mice. Hence, the glutamate autocrine feedback loop endows the alpha cell with the ability to effectively potentiate its own secretory activity. This is a prerequisite to guarantee adequate glucagon release despite relatively modest changes in blood glucose concentration under physiological conditions.

ATP-gated P2X3 receptors constitute a positive autocrine signal for insulin release in the human pancreatic β cell

Extracellular ATP has been proposed as a paracrine signal in rodent islets, but it is unclear what role ATP plays in human islets. We now show the presence of an ATP signaling pathway that enhances the human β cells sensitivity and responsiveness to glucose fluctuations. By using in situ hybridization, RT-PCR, immunohistochemistry, and Western blotting as well as recordings of cytoplasmic-free Ca2+ concentration, [Ca2+]i, and hormone release in vitro, we show that human β cells express ionotropic ATP receptors of the P2X3 type and that activation of these receptors by ATP coreleased with insulin amplifies glucose-induced insulin secretion. Released ATP activates P2X3 receptors in the β-cell plasma membrane, resulting in increased [Ca2+]i and enhanced insulin secretion. Therefore, in human islets, released ATP forms a positive autocrine feedback loop that sensitizes the β cells secretory machinery. This may explain how the human pancreatic β cell can respond so effectively to relatively modest changes in glucose concentration under physiological conditions in vivo.

Interference with tissue factor prolongs intrahepatic islet allograft survival in a nonhuman primate marginal mass model.

Background. Tissue factor (TF) expression on islets can result in an instant blood-mediated inflammatory reaction (IBMIR) that contributes to early islet loss. We tested whether peritransplant protection of islets from IBMIR with a monoclonal anti-TF antibody (CNTO859) would enhance engraftment in our nonhuman primate marginal mass model. Methods. Each of six pairs of cynomolgus monkeys (CM) with streptozotocin-induced diabetes was closely matched for metabolic control and was transplanted with 5,000 IEQ/kg allogeneic, ABO-compatible islets from the same donor under the cover of steroid-free immunosuppression. For each pair, experimental animals received islets cultured with 20 μg/mL anti-TF and were dosed with 6 mg/kg anti-TF intravenously, 10–25 min before islet infusion; control monkeys received an equal number of islets from the same preparation cultured without anti-TF and no in vivo treatment. Results. Early fasting C-peptide (CP) values were different between (P<0.01), but not within, pairs and correlated with in vitro functional capacity of islets as assessed by perifusion (r=0.60; P=0.022). Compared to their matched controls, experimental animals had decreased posttransplant markers of coagulation, higher fasting CP levels (1 month posttransplant and end of study) and prolonged graft function. Conclusions. These data suggest that pretreatment of islets and the recipient with anti-TF may limit the effects of IBMIR, thereby enhancing islet engraftment and survival.

Human, nonhuman primate, and rat pancreatic islets express erythropoietin receptors.

Background. Erythropoietin (EPO) promotes survival in a variety of cells by mediating antiapoptotic signals through the EPO receptor (R). The authors examined pancreatic islets for the presence of EPO-R to determine whether these cells are protected by EPO from cytokine-induced apoptosis. Methods. Reverse-transcriptase polymerase chain reaction, immunohistology, and Western blots were used to establish the presence and localization of EPO-R on rat, nonhuman primate, and human islets. Islets were exposed to cytokines in the presence and absence of recombinant EPO and apoptosis was measured using a terminal deoxynucleotide transferase-mediated dUTP nick-end labeling assay followed by fluorescence-activated cell sorter analysis. Glucose stimulation indices were measured to assess the effect of EPO on islet function. Results. The presence of EPO-R was demonstrated on islets regardless of species. Recombinant EPO protected islets in culture from cytokine-induced apoptosis in a dose-dependent manner. Furthermore, the presence of EPO in the media does not adversely affect islet function. Conclusions. This is the first demonstration that pancreatic islets express EPO-R and that EPO may prevent islet-cell apoptosis in culture. In vivo trials to evaluate the potential of long-term expression of EPO to augment islet survival in transplantation are underway.

Noninvasive in vivo model demonstrating the effects of autonomic innervation on pancreatic islet function

The autonomic nervous system is thought to modulate blood glucose homeostasis by regulating endocrine cell activity in the pancreatic islets of Langerhans. The role of islet innervation, however, has remained elusive because the direct effects of autonomic nervous input on islet cell physiology cannot be studied in the pancreas. Here, we used an in vivo model to study the role of islet nervous input in glucose homeostasis. We transplanted islets into the anterior chamber of the eye and found that islet grafts became densely innervated by the rich parasympathetic and sympathetic nervous supply of the iris. Parasympathetic innervation was imaged intravitally by using transgenic mice expressing GFP in cholinergic axons. To manipulate selectively the islet nervous input, we increased the ambient illumination to increase the parasympathetic input to the islet grafts via the pupillary light reflex. This reduced fasting glycemia and improved glucose tolerance. These effects could be blocked by topical application of the muscarinic antagonist atropine to the eye, indicating that local cholinergic innervation had a direct effect on islet function in vivo. By using this approach, we found that parasympathetic innervation influences islet function in C57BL/6 mice but not in 129X1 mice, which reflected differences in innervation densities and may explain major strain differences in glucose homeostasis. This study directly demonstrates that autonomic axons innervating the islet modulate glucose homeostasis.

Automated, High-Throughput Assays for Evaluation of Human Pancreatic Islet Function

An important challenge in pancreatic islet transplantation in association with type 1 diabetes is to define automatic high-throughput assays for evaluation of human islet function. The physiological techniques presently used are amenable to small-scale experimental samples and produce descriptive results. The postgenomic era provides an opportunity to analyze biological processes on a larger scale, but the transition to high-throughput technologies is still a challenge. As a first step to implement high-throughput assays for the study of human islet function, we have developed two methodologies: multiple automated perifusion to determine islet hormone secretion and high-throughput kinetic imaging to examine islet cellular responses. Both technologies use fully automated devices that allow performing simultaneous experiments on multiple islet preparations. Our results illustrate that these technologies can be applied to study the functional status and explore the pharmacological profiles of islet cells. These methodologies will enable functional characterization of human islet preparations before transplantation and thereby provide the basis for the establishment of predictive tests for β-cell potency.

Mitochondrial GTP Insensitivity Contributes to Hypoglycemia in Hyperinsulinemia Hyperammonemia by Inhibiting Glucagon Release

Mitochondrial GTP (mtGTP)-insensitive mutations in glutamate dehydrogenase (GDHH454Y) result in fasting and amino acid–induced hypoglycemia in hyperinsulinemia hyperammonemia (HI/HA). Surprisingly, hypoglycemia may occur in this disorder despite appropriately suppressed insulin. To better understand the islet-specific contribution, transgenic mice expressing the human activating mutation in β-cells (H454Y mice) were characterized in vivo. As in the humans with HI/HA, H454Y mice had fasting hypoglycemia, but plasma insulin concentrations were similar to the controls. Paradoxically, both glucose- and glutamine-stimulated insulin secretion were severely impaired in H454Y mice. Instead, lack of a glucagon response during hypoglycemic clamps identified impaired counterregulation. Moreover, both insulin and glucagon secretion were impaired in perifused islets. Acute pharmacologic inhibition of GDH restored both insulin and glucagon secretion and normalized glucose tolerance in vivo. These studies support the presence of an mtGTP-dependent signal generated via β-cell GDH that inhibits α-cells. As such, in children with activating GDH mutations of HI/HA, this insulin-independent glucagon suppression may contribute importantly to symptomatic hypoglycemia. The identification of a human mutation causing congenital hypoglucagonemic hypoglycemia highlights a central role of the mtGTP–GDH–glucagon axis in glucose homeostasis.