Reporter islets in the eye reveal the plasticity of the endocrine pancreas

3D-Printed Biohybrid Microstructures Enable Transplantation and Vascularization of Microtissues in the Anterior Chamber of the Eye

Hybridizing biological cells with man-made sensors enable the detection of a wide range of weak physiological responses with high specificity. The anterior chamber of the eye (ACE) is an ideal transplantation site due to its ocular immune privilege and optical transparency, which enable superior noninvasive longitudinal analyses of cells and microtissues. Engraftment of biohybrid microstructures in the ACE may, however, be affected by the pupillary response and dynamics. Here, sutureless transplantation of biohybrid microstructures, 3D printed in IP-Visio photoresin, containing a precisely localized pancreatic islet to the ACE of mice is presented. The biohybrid microstructures allow mechanical fixation in the ACE, independent of iris dynamics. After transplantation, islets in the microstructures successfully sustain their functionality for over 20 weeks and become vascularized despite physical separation from the vessel source (iris) and immersion in a low-viscous liquid (aqueous humor) with continuous circulation and clearance. This approach opens new perspectives in biohybrid microtissue transplantation in the ACE, advancing monitoring of microtissue-host interactions, disease modeling, treatment outcomes, and vascularization in engineered tissues.

Application of fluorescence lifetime imaging microscopy to monitor glucose metabolism in pancreatic islets in vivo

Glucose stimulated insulin secretion is mediated by glucose metabolism via oxidative phosphorylation generating ATP that triggers membrane depolarization and exocytosis of insulin. In stressed beta cells, glucose metabolism is remodeled, with enhanced glycolysis uncoupled from oxidative phosphorylation, resulting in the impaired glucose-mediated insulin secretion characteristic of diabetes. Relative changes in glycolysis and oxidative phosphorylation can be monitored in living cells using the 3-component fitting approach of fluorescence lifetime imaging microscopy (FLIM). We engrafted pancreatic islets onto the iris to permit in vivo FLIM monitoring of the trajectory of glucose metabolism. The results show increased oxidative phosphorylation of islet cells (∼90% beta cells) in response to hyperglycemia; in contrast red blood cells traversing the islets maintained exclusive glycolysis as expected in the absence of mitochondria.

Uncommon Transplantation Sites: Transplantation of Islets and Islet Organoids in the Anterior Chamber of the Eye of Rodents and Monkeys

The anterior chamber of the eye is a highly vascularized and innervated location that is also particularly rich in oxygen and immune privileged. This uncommon transplantation site offers unique possibilities for the observation of the transplanted material as well as for local pharmacological intervention. Transplantation of islets and islet organoids to the anterior chamber of the eye of mice and monkeys facilitates a multitude of new approaches for research into islet physiology and pathophysiology and for the treatment of diabetes. We now present a short overview of the experimental possibilities and describe an updated protocol for transplantation of islets and islet organoids into mice and monkeys.

Pericyte Control of Blood Flow in Intraocular Islet Grafts Impacts Glucose Homeostasis in Mice

The pancreatic islet depends on blood supply to efficiently sense plasma glucose levels and deliver insulin and glucagon into the circulation. Long believed to be passive conduits of nutrients and hormones, islet capillaries were recently found to be densely covered with contractile pericytes with the capacity to locally control blood flow. Here, we determined the contribution of pericyte regulation of islet blood flow to plasma insulin and glucagon levels and glycemia. Selective optogenetic activation of pericytes in intraocular islet grafts contracted capillaries and diminished blood flow. In awake mice, acute light-induced stimulation of islet pericytes decreased insulin and increased glucagon plasma levels, producing hyperglycemic effects. Interestingly, pericytes are the targets of sympathetic nerves in the islet, suggesting that sympathetic control of hormone secretion may occur in part by modulating pericyte activity and blood flow. Indeed, in vivo activation of pericytes with the sympathetic agonist phenylephrine decreased blood flow in mouse islet grafts, lowered plasma insulin levels, and increased glycemia. We further show that islet pericytes and blood vessels in living human pancreas slices responded to sympathetic input. Our findings indicate that pericytes mediate vascular responses in the islet that are required for adequate hormone secretion and glucose homeostasis. Vascular and neuronal alterations that are commonly seen in the islets of people with diabetes may impair regulation of islet blood flow and thus precipitate islet dysfunction.

Local Dexamethasone Administration Delays Allogeneic Islet Graft Rejection in the Anterior Chamber of the Eye of Non-Human Primates

Pancreatic islet transplantation into the anterior chamber of the eye (ACE) has been shown to improve glycemic control and metabolic parameters of diabetes in both murine and primate models. This novel transplantation site also allows the delivery of therapeutic agents, such as immunosuppressive drugs, locally to prevent islet graft rejection and circumvent unwanted systemic side effects. Local intravitreal administration of micronized dexamethasone implant was performed prior to allogeneic islet transplantation into the ACEs of non-human primates. Two study groups were observed namely allogeneic graft without immunosuppression (n = 4 eyes) and allogeneic graft with local immunosuppression (n = 8 eyes). Survival of islet grafts and dexamethasone concentration in the ACE were assessed in parallel for 24 weeks. Allogeneic islet grafts with local dexamethasone treatment showed significantly better survival than those with no immunosuppression (median survival time- 15 weeks vs 3 weeks, log-rank test p<0.0001). Around 73% of the grafts still survived at week 10 with a single local dexamethasone implant, where the control group showed no graft survival. Dexamethasone treated islet grafts revealed a good functional response to high glucose stimulation despite there was a transient suppression of insulin secretion from week 8 to 12. Our findings show a significant improvement of allografts survival in the ACE with local dexamethasone treatment. These results highlight the feasibility of local administration of pharmacological compounds in the ACE to improve islet graft survival and function. By eliminating the need for systemic immunosuppression, these findings may impact clinical islet transplantation in the treatment of diabetes, and the ACE may serve as a novel therapeutic islet transplantation site with high potential for local pharmacological intervention.

Recent Progress of Research Regarding the Applications of Stem Cells for Treating Diabetes Mellitus

At present, the number of diabetes patients has exceeded 537 million worldwide and this number continues to increase. Stem cell therapy represents a new direction for the treatment of diabetes; the use of stem cells overcomes some shortcomings associated with traditional therapies. Functional β-cells play an important role in the pathogenesis of diabetes. As therapeutic targets, functional β-cells are restored by a variety of stem cells, including pluripotent stem cells, mesenchymal cells, and urine-derived stem cells. Although all types of stem cells have their own characteristics, they mainly promote the repair and regeneration of β-cells through directional differentiation, immunomodulation, and paracrine signaling after homing to the injured site. However, stem cell therapy still faces many obstacles, such as low long-term cell survival rate after transplantation, low maintenance time of blood glucose homeostasis, immune rejection, and tumorigenesis. Recently, genetically edited pluripotent stem cells and the co-transplantation of mesenchymal stem cells and islet cells have made significant progress in improving the efficacy of stem cell transplantation processes, also providing powerful tools for the study of the mechanisms underlying diabetes and disease modeling. In this review, we first focused on: (1) stem cells as a pool for the differentiation of insulin-producing cells; (2) stem cells as a source for regenerative repair of damaged islets and as a potential co-transplanted population with islets; (3) the potential of combining gene editing with stem cell therapy; and (4) selection of the stem cell transplantation approach. Based on these topics, we discuss the challenges within the field of adapting stem cell-supported and stem cell-derived transplantations, and the promising routes for overcoming these problems.

Ectopic Leptin Production by Intraocular Pancreatic Islet Organoids Ameliorates the Metabolic Phenotype of ob/ob Mice

The pancreatic islets of Langerhans consist of endocrine cells that secrete peptide hormones into the blood circulation in response to metabolic stimuli. When transplanted into the anterior chamber of the eye (ACE), pancreatic islets engraft and maintain morphological features of native islets as well as islet-specific vascularization and innervation patterns. In sufficient amounts, intraocular islets are able to maintain glucose homeostasis in diabetic mice. Islet organoids (pseudo-islets), which are formed by self-reassembly of islet cells following disaggregation and genetic manipulation, behave similarly to native islets. Here, we tested the hypothesis that genetically engineered intraocular islet organoids can serve as production sites for leptin. To test this hypothesis, we chose the leptin-deficient ob/ob mouse as a model system, which becomes severely obese, hyperinsulinemic, hyperglycemic, and insulin resistant. We generated a Tet-OFF-based beta-cell-specific adenoviral expression construct for mouse leptin, which allowed efficient transduction of native beta-cells, optical monitoring of leptin expression by co-expressed fluorescent proteins, and the possibility to switch-off leptin expression by treatment with doxycycline. Intraocular transplantation of islet organoids formed from transduced islet cells, which lack functional leptin receptors, to ob/ob mice allowed optical monitoring of leptin expression and ameliorated their metabolic phenotype by improving bodyweight, glucose tolerance, serum insulin, and C-peptide levels.

The Eye as a Transplantation Site to Monitor Pancreatic Islet Cell Plasticity

The endocrine cells confined in the islets of Langerhans are responsible for the maintenance of blood glucose homeostasis. In particular, beta cells produce and secrete insulin, an essential hormone regulating glucose uptake and metabolism. An insufficient amount of beta cells or defects in the molecular mechanisms leading to glucose-induced insulin secretion trigger the development of diabetes, a severe disease with epidemic spreading throughout the world. A comprehensive appreciation of the diverse adaptive procedures regulating beta cell mass and function is thus of paramount importance for the understanding of diabetes pathogenesis and for the development of effective therapeutic strategies. While significant findings were obtained by the use of islets isolated from the pancreas, in vitro studies are inherently limited since they lack the many factors influencing pancreatic islet cell function in vivo and do not allow for longitudinal monitoring of islet cell plasticity in the living organism. In this respect a number of imaging methodologies have been developed over the years for the study of islets in situ in the pancreas, a challenging task due to the relatively small size of the islets and their location, scattered throughout the organ. To increase imaging resolution and allow for longitudinal studies in individual islets, another strategy is based on the transplantation of islets into other sites that are more accessible for imaging. In this review we present the anterior chamber of the eye as a transplantation and imaging site for the study of pancreatic islet cell plasticity, and summarize the major research outcomes facilitated by this technological platform.

Pancreatic islet transplantation under the kidney capsule of mice: model of refinement for molecular and ex-vivo graft analysis

Diabetes cell therapy by human islet transplantation can restore an endogenous insulin secretion and normal glycaemic control in type 1 diabetic patients for as long as 10 years post transplantation. Before transplantation, each clinical islet preparation undergoes extensive in-vitro and in-vivo quality controls. The in-vivo quality control assay consists of transplanting human islets under the kidney capsule of immunocompromised mice. Currently, it is considered the best predictive factor to qualify clinical transplant efficiency. This chimeric model offers a wide area of study since it combines the possibility of producing not only quantitative but also a maximum of qualitative data. Today's technological advances allow us to obtain more accurate and stronger data from the animals used in research while ensuring their comfort and well-being throughout the protocol, including cage enrichment and pain treatment during and after surgery. As demonstrated in this valuable model, we are able to generate more usable results (Refine), while reducing the number of animals used (Reduce), by focusing on the development of ex-vivo analysis techniques (Replace), which clearly highlights the Burch and Russell 3Rs concept.

Mesoscopic Optical Imaging of the Pancreas-Revisiting Pancreatic Anatomy and Pathophysiology

The exocrine-endocrine multipart organization of the pancreas makes it an exceedingly challenging organ to analyze, quantitatively and spatially. Both in rodents and humans, estimates of the pancreatic cellular composition, including beta-cell mass, has been largely relying on the extrapolation of 2D stereological data originating from limited sample volumes. Alternatively, they have been obtained by low resolution non-invasive imaging techniques providing little detail regarding the anatomical organization of the pancreas and its cellular and/or molecular make up. In this mini-review, the state of the art and the future potential of currently existing and emerging high-resolution optical imaging techniques working in the mm-cm range with μm resolution, here referred to as mesoscopic imaging approaches, will be discussed regarding their contribution toward a better understanding of pancreatic anatomy both in normal conditions and in the diabetic setting. In particular, optical projection tomography (OPT) and light sheet fluorescence microscopy (LSFM) imaging of the pancreas and their associated tissue processing and computational analysis protocols will be discussed in the light of their current capabilities and future potential to obtain more detailed 3D-spatial, quantitative, and molecular information of the pancreas.

Topologically selective islet vulnerability and self-sustained downregulation of markers for β-cell maturity in streptozotocin-induced diabetes

Mouse models of Streptozotocin (STZ) induced diabetes represent the most widely used preclinical diabetes research systems. We applied state of the art optical imaging schemes, spanning from single islet resolution to the whole organ, providing a first longitudinal, 3D-spatial and quantitative account of β-cell mass (BCM) dynamics and islet longevity in STZ-treated mice. We demonstrate that STZ-induced β-cell destruction predominantly affects large islets in the pancreatic core. Further, we show that hyperglycemic STZ-treated mice still harbor a large pool of remaining β-cells but display pancreas-wide downregulation of glucose transporter type 2 (GLUT2). Islet gene expression studies confirmed this downregulation and revealed impaired β-cell maturity. Reversing hyperglycemia by islet transplantation partially restored the expression of markers for islet function, but not BCM. Jointly our results indicate that STZ-induced hyperglycemia results from β-cell dysfunction rather than β-cell ablation and that hyperglycemia in itself sustains a negative feedback loop restraining islet function recovery.

Hahn, van Krieken et al. provide a quantitative account of β-cell mass dynamics and islet longevity in mice treated with Streptozotocin (STZ). They find that STZ-induced hyperglycemia primarily results from β-cell dysfunction rather than its ablation. This study provides insights into how the most widely used preclinical diabetes model works.

Longitudinal proteomics analysis in the immediate microenvironment of islet allografts during progression of rejection

The applicability and benefits of pancreatic islet transplantation are limited due to various issues including the need to avoid immune-mediated rejection. Here, we used our experimental platform of allogeneic islet transplant in the anterior chamber of the eye (ACE-platform) to longitudinally monitor the progress of rejection in mice and obtain aqueous humor samples representative of the microenvironment of the graft for accurately-timed proteomic analyses. LC-MS/MS-based proteomics performed on such mass-limited samples (~5 μL) identified a total of 1296 proteins. Various analyses revealed distinct protein patterns associated with the mounting of the inflammatory and immune responses and their evolution with the progression of the rejection. Pathway analyses indicated predominant changes in cytotoxic functions, cell movement, and innate and adaptive immune responses. Network prediction analyses revealed transition from humoral to cellular immune response and exacerbation of pro-inflammatory signaling. One of the proteins identified by this localized proteomics as a candidate biomarker of islet rejection, Cystatin 3, was further validated by ELISA in the aqueous humor. This study provides (1) experimental evidence demonstrating the feasibility of longitudinal localized proteomics using small aqueous humor samples and (2) proof-of-concept for the discovery of biomarkers of impending immune attack from the immediate local microenvironment of ACE-transplanted islets. SIGNIFICANCE: The combination of the ACE-platform and longitudinal localized proteomics offers a powerful approach to biomarker discovery during the various stages of immune reactions mounted against transplanted tissues including pancreatic islets. It also supports proteomics-assisted drug discovery and development efforts aimed at preventing rejection through efficacy assessment of new agents by noninvasive and longitudinal graft monitoring.

Transplantation of Islets of Langerhans into the Anterior Chamber of the Eye for Longitudinal In Vivo Imaging

Noninvasive in vivo imaging techniques are attractive tools to longitudinally study various aspects of islet of Langerhans physiology and pathophysiology. Unfortunately, most imaging modalities currently applicable for clinical use do not allow the comprehensive investigation of islet cell biology due to limitations in resolution and/or sensitivity, while high-resolution imaging technologies like laser scanning microscopy (LSM) lack the penetration depth to assess islets of Langerhans within the pancreas. Significant progress in this area was made by the combination of LSM with the anterior chamber of the mouse eye platform, utilizing the cornea as a natural body window to study cell physiology of transplanted islets of Langerhans. We here describe the transplantation and longitudinal in vivo imaging of islets of Langerhans in the anterior chamber of the mouse eye as a versatile tool to study different features of islet physiology in health and disease.

In vivo Ca2+ dynamics in single pancreatic β cells

The dynamics of cytoplasmic free Ca²⁺ concentration ([Ca²⁺]_i) in pancreatic β cells is central to our understanding of β-cell physiology and pathology. In this context, there are numerous in vitro studies available but existing in vivo data are scarce. We now critically evaluate the anterior chamber of the eye as an in vivo, non-invasive, imaging site for measuring [Ca²⁺]_i dynamics longitudinally in three dimensions and at single-cell resolution. By applying a fluorescently labeled glucose analogue 2-(N-(7-Nitrobenz-2-oxa-1,3-diazol-4-yl)Amino)-2-Deoxyglucose in vivo, we followed how glucose almost simultaneously distributes to all cells within the islet volume, resulting in [Ca²⁺]_i changes. We found that almost all β cells in healthy mice responded to a glucose challenge, while in hyperinsulinemic, hyperglycemic mice about 80% of the β cells could not be further stimulated from fasting basal conditions. This finding indicates that our imaging modality can resolve functional heterogeneity within the β-cell population in terms of glucose responsiveness. Importantly, we demonstrate that glucose homeostasis is markedly affected using isoflurane compared to hypnorm/midazolam anesthetics, which has major implications for [Ca²⁺]_i measurements. In summary, this setup offers a powerful tool to further investigate in vivo pancreatic β-cell [Ca²⁺]_i response patterns at single-cell resolution in health and disease.

Adipsin preserves beta cells in diabetic mice and associates with protection from type 2 diabetes in humans

Type 2 diabetes is characterized by insulin resistance and a gradual loss of pancreatic beta cell mass and function^1,2. Currently, there are no therapies proven to prevent beta cell loss and some, namely insulin secretagogues, have been linked to accelerated beta cell failure, thereby limiting their use in type 2 diabetes^3,4. The adipokine adipsin/complement factor D controls the alternative complement pathway and generation of complement component C3a, which acts to augment beta cell insulin secretion⁵. In contrast to other insulin secretagogues, we show that chronic replenishment of adipsin in diabetic db/db mice ameliorates hyperglycemia and increases insulin levels while preserving beta cells by blocking dedifferentiation and death. Mechanistically, we find that adipsin/C3a decreases the phosphatase Dusp26; forced expression of Dusp26 in beta cells decreases expression of core beta cell identity genes and sensitizes to cell death. In contrast, pharmacological inhibition of DUSP26 improves hyperglycemia in diabetic mice and protects human islet cells from cell death. Pertaining to human health, we show that higher concentrations of circulating adipsin are associated with a significantly lower risk of developing future diabetes among middle-aged adults after adjusting for body mass index (BMI). Collectively, these data suggest that adipsin/C3a and DUSP26-directed therapies may represent a novel approach to achieve beta cell health to treat and prevent type 2 diabetes.

Structural basis for delta cell paracrine regulation in pancreatic islets

Little is known about the role of islet delta cells in regulating blood glucose homeostasis in vivo. Delta cells are important paracrine regulators of beta cell and alpha cell secretory activity, however the structural basis underlying this regulation has yet to be determined. Most delta cells are elongated and have a well-defined cell soma and a filopodia-like structure. Using in vivo optogenetics and high-speed Ca²⁺ imaging, we show that these filopodia are dynamic structures that contain a secretory machinery, enabling the delta cell to reach a large number of beta cells within the islet. This provides for efficient regulation of beta cell activity and is modulated by endogenous IGF-1/VEGF-A signaling. In pre-diabetes, delta cells undergo morphological changes that may be a compensation to maintain paracrine regulation of the beta cell. Our data provides an integrated picture of how delta cells can modulate beta cell activity under physiological conditions.

Pancreatic islets are composed of alpha-, beta-, as well as delta-cells and appropriate regulation of glucose homeostasis relies on auto- and paracrine cellular communication. Here, the authors study the role of delta-cell filopodia in this context by employing optogenetic and calcium imaging approaches.

Metabolic and Functional Heterogeneity in Pancreatic β Cells

Metabolic and secretory heterogeneity are fundamental properties of pancreatic islet β cells. Emerging data suggest that stable differences in the transcriptome and proteome of individual cells may create cellular hierarchies, which, in turn, establish coordinated functional networks. These networks appear to govern the secretory activity of the whole islet and be affected in some forms of diabetes mellitus. Functional imaging, for example, of intracellular calcium dynamics, has led to the demonstration of "small worlds" behavior, and the identification of highly connected "hub" (or "leader") cells and of follower populations subservient to them. Subsequent inactivation of members of either population, for example, using optogenetic approaches or photoablation, has confirmed the importance of hub cells as possible pacemakers. Hub cells appear to be enriched for the glucose phosphorylating enzyme glucokinase and for genes encoding other enzymes involved in glucose metabolism compared to follower cells. Recent findings have shown the relevance of cellular hierarchy in islets from multiple species including human, mouse and fish, and shown that it is preserved in vivo in the context of the fully vascularized and innervated islet. Importantly, connectivity is impaired by insults, which mimic the diabetic milieu, including high glucose and/or fatty levels, and by the ablation of genes associated with type 2 diabetes risk in genome-wide association studies. We discuss here the evidence for the existence of these networks and their failure in disease settings. We also briefly survey the challenges in understanding their properties.

In vivo imaging of type 1 diabetes immunopathology using eye-transplanted islets in NOD mice

AIMS/HYPOTHESIS

Autoimmune attack against the insulin-producing beta cells in the pancreatic islets results in type 1 diabetes. However, despite considerable research, details of the type 1 diabetes immunopathology in situ are not fully understood mainly because of difficult access to the pancreatic islets in vivo.

METHODS

Here, we used direct non-invasive confocal imaging of islets transplanted in the anterior chamber of the eye (ACE) to investigate the anti-islet autoimmunity in NOD mice before, during and after diabetes onset. ACE-transplanted islets allowed longitudinal studies of the autoimmune attack against islets and revealed the infiltration kinetics and in situ motility dynamics of fluorescence-labelled autoreactive T cells during diabetes development. Ex vivo immunostaining was also used to compare immune cell infiltrations into islet grafts in the eye and kidney as well as in pancreatic islets of the same diabetic NOD mice.

RESULTS

We found similar immune infiltration in native pancreatic and ACE-transplanted islets, which established the ACE-transplanted islets as reliable reporters of the autoimmune response. Longitudinal studies in ACE-transplanted islets identified in vivo hallmarks of islet inflammation that concurred with early immune infiltration of the islets and preceded their collapse and hyperglycaemia onset. A model incorporating data on ACE-transplanted islet degranulation and swelling allowed early prediction of the autoimmune attack in the pancreas and prompted treatments to intercept type 1 diabetes.

CONCLUSIONS/INTERPRETATION

The current findings highlight the value of ACE-transplanted islets in studying early type 1 diabetes pathogenesis in vivo and underscore the need for timely intervention to halt disease progression.

What role do fat cells play in pancreatic tissue?

Background

It is now generally accepted that obesity is a major risk factor for type 2 diabetes mellitus (T2DM). Hepatic steatosis in particular, as well as visceral and ectopic fat accumulation within tissues, is associated with the development of the disease. We recently presented the first study on isolated human pancreatic adipocytes and their interaction with islets [Gerst, F., Wagner, R., Kaiser, G., Panse, M., Heni, M., Machann, J., et al., 2017. Metabolic crosstalk between fatty pancreas and fatty liver: effects on local inflammation and insulin secretion. Diabetologia 60(11):2240–2251.]. The results indicate that the function of adipocytes depends on the overall metabolic status in humans which, in turn, differentially affects islet hormone release.

Scope of Review

This review summarizes former and recent studies on factors derived from adipocytes and their effects on insulin-secreting β-cells, with particular emphasis on the human pancreas. The adipocyte secretome is discussed with a special focus on its influence on insulin secretion, β-cell survival and apoptotic β-cell death.

Major Conclusions

Human pancreatic adipocytes store lipids and release adipokines, metabolites, and pro-inflammatory molecules in response to the overall metabolic, humoral, and neuronal status. The differentially regulated adipocyte secretome impacts on endocrine function, i.e., insulin secretion, β-cell survival and death which interferes with glycemic control. This review attempts to explain why the extent of pancreatic steatosis is associated with reduced insulin secretion in some studies but not in others.

Local release of rapamycin by microparticles delays islet rejection within the anterior chamber of the eye

The anterior chamber of the eye (ACE) has emerged as a promising clinical islet transplantation site because of its multiple advantages over the conventional intra-hepatic portal site. This includes reduced surgical invasiveness and increased islet graft survival rate. It also allows for enhanced accessibility and monitoring of the islets. Although the ACE is initially an immuno-privileged site, this privilege is disrupted once the islet grafts are re-vascularized. Given that the ACE is a confined space, achieving graft immune tolerance through local immunosuppressive drug delivery is therefore feasible. Here, we show that islet rejection in the ACE of mice can be significantly suppressed through local delivery of rapamycin by carefully designed sustained-release microparticles. In this 30-day study, allogeneic islet grafts with blank microparticles were completely rejected 18 days post-transplantation into mice. Importantly, allogeneic islet grafts co-injected with rapamycin releasing microparticles into a different eye of the same recipient were preserved much longer, with some grafts surviving for more than 30 days. Hence, islet allograft survival was enhanced by a localized and prolonged delivery of an immunosuppressive drug. We envisage that this procedure will relieve diabetic transplant recipients from harsh systemic immune suppression, while achieving improved glycemic control and reduced insulin dependence.

The eye as a novel imaging site in diabetes research

Diabetes develops due to deficient functional β cell mass, insulin resistance, or both. Yet, various challenges in understanding the mechanisms underlying diabetes development in vivo remain to be overcome owing to the lack of appropriate intravital imaging technologies. To meet these challenges, we have exploited the anterior chamber of the eye (ACE) as a novel imaging site to understand diabetes basics and clinics in vivo. We have developed a technology platform transplanting pancreatic islets into the ACE where they later on can be imaged non-invasively for long time. It turns out that the ACE serves as an optimal imaging site and provides implanted islets with an oxygen-rich milieu and an immune-privileged niche where they undergo optimal engraftment, rich vascularization and dense innervation, preserve organotypic features and live with satisfactory viability and functionality. The ACE technology has led to a series of significant observations. It enables in vivo microscopy of islet cytoarchitecture, function and viability in the physiological context and intravital imaging of a variety of pathological events such as autoimmune insulitis, defects in β cell function and mass and insulin resistance during diabetes development in a real-time manner. Furthermore, application of the ACE technology in humanized mice and non-human primates verifies translational and clinical values of the technology. In this article, we describe the ACE technology in detail, review accumulated knowledge gained by means of the ACE technology and delineate prospective avenues for the ACE technology.

Diet-induced β-cell insulin resistance results in reversible loss of functional β-cell mass

Although convincing in genetic models, the relevance of β-cell insulin resistance in diet-induced type 2 diabetes (T2DM) remains unclear. Exemplified by diabetes-prone, male, C57B1/6J mice being fed different combinations of Western-style diet, we show that β-cell insulin resistance occurs early during T2DM progression and is due to a combination of lipotoxicity and increased β-cell workload. Within 8 wk of being fed a high-fat, high-sucrose diet, mice became obese, developed impaired insulin and glucose tolerances, and displayed noncompensatory insulin release, due, at least in part, to reduced expression of syntaxin-1A. Through reporter islets transplanted to the anterior chamber of the eye, we demonstrated a concomitant loss of functional β-cell mass. When mice were changed from diabetogenic diet to normal chow diet, the diabetes phenotype was reversed, suggesting a remarkable plasticity of functional β-cell mass in the early phase of T2DM development. Our data reinforce the relevance of diet composition as an environmental factor determining different routes of diabetes progression in a given genetic background. Employing the in vivo reporter islet–monitoring approach will allow researchers to define key times in the dynamics of reversible loss of functional β-cell mass and, thus, to investigate the underlying, molecular mechanisms involved in the progression toward T2DM manifestation.—Paschen, M., Moede, T., Valladolid-Acebes, I., Leibiger, B., Moruzzi, N., Jacob, S., García-Prieto, C. F., Brismar, K., Leibiger, I. B., Berggren, P.-O. Diet-induced β-cell insulin resistance results in reversible loss of functional β-cell mass.

Imaging endocrinology in animal models of endocrine disease

Endocrine organs secrete a variety of hormones involved in the regulation of a multitude of body functions. Although pancreatic islets were discovered at the turn of the 19th century, other endocrine glands remained commonly described as diffuse endocrine systems. Over the last two decades, development of new imaging techniques and genetically-modified animals with cell-specific fluorescent tags or specific hormone deficiencies have enabled in vivo imaging of endocrine organs and revealed intricate endocrine cell network structures and plasticity. Overall, these new tools have revolutionized our understanding of endocrine function. The overarching aim of this Review is to describe the current mechanistic understanding that has emerged from imaging studies of endocrine cell network structure/function relationships in animal models, with a particular emphasis on the pituitary gland and the endocrine pancreas.

Pancreatic Islet Blood Flow Dynamics in Primates

Blood flow regulation in pancreatic islets is critical for function but poorly understood. Here, we establish an in vivo imaging platform in a non-human primate where islets transplanted autologously into the anterior chamber of the eye are monitored non-invasively and longitudinally at single-cell resolution. Engrafted islets were vascularized and innervated and maintained the cytoarchitecture of in situ islets in the pancreas. Blood flow velocity in the engrafted islets was not affected by increasing blood glucose levels and/or the GLP-1R agonist liraglutide. However, islet blood flow was dynamic in nature and fluctuated in various capillaries. This was associated with vasoconstriction events resembling a sphincter-like action, most likely regulated by adrenergic signaling. These observations suggest a mechanism in primate islets that diverts blood flow to cell regions with higher metabolic demand. The described imaging technology applied in non-human primate islets may contribute to a better understanding of human islet pathophysiology.

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Monkey islets transplanted autologously into the anterior chamber of the eye (ACE)

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Monkey ACE islets imaged in vivo, longitudinally, and at single-cell resolution

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Monkey islet blood flow is dynamic and unaffected by glucose/liraglutide treatment

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Directional blood flow may be explained by islet structure-function relationship

Inositol hexakisphosphate kinase 1 is a metabolic sensor in pancreatic β-cells

Diphosphoinositol pentakisphosphate (IP₇) is critical for the exocytotic capacity of the pancreatic β-cell, but its regulation by the primary instigator of β-cell exocytosis, glucose, is unknown. The high K_m for ATP of the IP₇-generating enzymes, the inositol hexakisphosphate kinases (IP6K1 and 2) suggests that these enzymes might serve as metabolic sensors in insulin secreting β-cells and act as translators of disrupted metabolism in diabetes. We investigated this hypothesis and now show that glucose stimulation, which increases the ATP/ADP ratio, leads to an early rise in IP₇ concentration in β-cells. RNAi mediated knock down of the IP6K1 isoform inhibits both glucose-mediated increase in IP₇ and first phase insulin secretion, demonstrating that IP6K1 integrates glucose metabolism and insulin exocytosis. In diabetic mouse islets the deranged ATP/ADP levels under both basal and glucose-stimulated conditions are mirrored in both disrupted IP₇ generation and insulin release. Thus the unique metabolic sensing properties of IP6K1 guarantees appropriate concentrations of IP₇ and thereby both correct basal insulin secretion and intact first phase insulin release. In addition, our data suggest that a specific cell signaling defect, namely, inappropriate IP₇ generation may be an essential convergence point integrating multiple metabolic defects into the commonly observed phenotype in diabetes.

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Glucose increases IP7 levels transiently through IP6K1 in pancreatic β-cells.

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IP6K1 decodes glucose-driven increases in ATP/ADP ratio into 1st phase insulin release.

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IP7 production and insulin release mirror perturbed metabolism in diabetic islets.

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IP6K1 acts as a β-cell metabolic sensor under normal and pathological conditions.

Kinetics of functional beta cell mass decay in a diphtheria toxin receptor mouse model of diabetes

Functional beta cell mass is an essential biomarker for the diagnosis and staging of diabetes. It has however proven technically challenging to study this parameter during diabetes progression. Here we have detailed the kinetics of the rapid decline in functional beta cell mass in the RIP-DTR mouse, a model of hyperglycemia resulting from diphtheria toxin induced beta cell ablation. A novel combination of imaging modalities was employed to study the pattern of beta cell destruction. Optical projection tomography of the pancreas and longitudinal in vivo confocal microscopy of islets transplanted into the anterior chamber of the eye allowed to investigate kinetics and tomographic location of beta cell mass decay in individual islets as well as at the entire islet population level. The correlation between beta cell mass and function was determined by complementary in vivo and ex vivo characterizations, demonstrating that beta cell function and glucose tolerance were impaired within the first two days following treatment when more than 50% of beta cell mass was remaining. Our results illustrate the importance of acquiring quantitative functional and morphological parameters to assess the functional status of the endocrine pancreas.

Intraocular in vivo imaging of pancreatic islet cell physiology/pathology

BACKGROUND

Diabetes mellitus has reached epidemic proportions and requires new strategies for treatment. Unfortunately, the efficacy of treatment regimens on maintaining/re-gaining functional beta cell mass can, at the present, only be determined indirectly. Direct monitoring of beta cell mass is complicated by the anatomy of the endocrine pancreas, which consists of thousands to a million of discrete micro-organs, i.e. islets of Langerhans, which are scattered throughout the pancreas.

SCOPE OF REVIEW

Here, we review the progress made over the last years using the anterior chamber of the eye as a transplantation site for functional imaging of pancreatic islet cells in the living organism. Islets engrafted on the iris are vascularized and innervated and the cornea, serving as a natural body-window, allows for microscopic, non-invasive, longitudinal evaluation of islet/beta cell function and survival with single-cell resolution in health and disease.

MAJOR CONCLUSIONS

Data provided by us and others demonstrate the high versatility of this imaging platform. The use of 'reporter islets' engrafted in the eye, reporting on the status of in situ endogenous islets in the pancreas of the same animal, allows the identification of key-events in the development and progression of diabetes. This will not only serve as a versatile research tool but will also lay the foundation for a personalized medicine approach and will serve as a screening platform for new drugs and/or treatment protocols. 'Metabolic' islet transplantation, in which islets engrafted in the eye replace the endogenous beta cells, will allow for the establishment of islet-specific transgenic models and 'humanized' mouse models as well as serving as the basis for a new clinical transplantation site for the cure of diabetes.

Biochemical profiling of diabetes disease progression by multivariate vibrational microspectroscopy of the pancreas

Despite the dramatic increase in the prevalence of diabetes, techniques for in situ studies of the underlying pancreatic biochemistry are lacking. Such methods would facilitate obtaining mechanistic understanding of diabetes pathophysiology and aid in prognostic and/or diagnostic assessments. In this report we demonstrate how a multivariate imaging approach (orthogonal projections to latent structures - discriminant analysis) can be applied to generate full vibrational microspectroscopic profiles of pancreatic tissues. These profiles enable extraction of known and previously unrecorded biochemical alterations in models of diabetes, and allow for classification of the investigated tissue with regards to tissue type, strain and stage of disease progression. Most significantly, the approach provided evidence for dramatic alterations of the pancreatic biochemistry at the initial onset of immune-infiltration in the Non Obese Diabetic model for type 1 diabetes. Further, it enabled detection of a previously undocumented accumulation of collagen fibrils in the leptin deficient ob/ob mouse islets. By generating high quality spectral profiles through the tissue capsule of hydrated human pancreata and by in vivo Raman imaging of pancreatic islets transplanted to the anterior chamber of the eye, we provide critical feasibility studies for the translation of this technique to diagnostic assessments of pancreatic biochemistry in vivo.

Label-free fast 3D coherent imaging reveals pancreatic islet micro-vascularization and dynamic blood flow

In diabetes, pancreatic β-cells play a key role. These cells are clustered within structures called islets of Langerhans inside the pancreas and produce insulin, which is directly secreted into the blood stream. The dense vascularization of islets of Langerhans is critical for maintaining a proper regulation of blood glucose homeostasis and is known to be affected from the early stage of diabetes. The deep localization of these islets inside the pancreas in the abdominal cavity renders their in vivo visualization a challenging task. A fast label-free imaging method with high spatial resolution is required to study the vascular network of islets of Langerhans. Based on these requirements, we developed a label-free and three-dimensional imaging method for observing islets of Langerhans using extended-focus Fourier domain Optical Coherence Microscopy (xfOCM). In addition to structural imaging, this system provides three-dimensional vascular network imaging and dynamic blood flow information within islets of Langerhans. We propose our method to deepen the understanding of the interconnection between diabetes and the evolution of the islet vascular network.

Intra-islet lesions and lobular variations in β-cell mass expansion in ob/ob mice revealed by 3D imaging of intact pancreas

The leptin deficient ob/ob mouse is a widely used model for studies on initial aspects of metabolic disturbances leading to type 2 diabetes, including insulin resistance and obesity. Although it is generally accepted that ob/ob mice display a dramatic increase in β-cell mass to compensate for increased insulin demand, the spatial and quantitative dynamics of β-cell mass distribution in this model has not been assessed by modern optical 3D imaging techniques. We applied optical projection tomography and ultramicroscopy imaging to extract information about individual islet β-cell volumes throughout the volume of ob/ob pancreas between 4 and 52 weeks of age. Our data show that cystic lesions constitute a significant volume of the hyperplastic ob/ob islets. We propose that these lesions are formed by a mechanism involving extravasation of red blood cells/plasma due to increased islet vessel blood flow and vessel instability. Further, our data indicate that the primary lobular compartments of the ob/ob pancreas have different potentials for expanding their β-cell population. Unawareness of the characteristics of β-cell expansion in ob/ob mice presented in this report may significantly influence ex vivo and in vivo assessments of this model in studies of β-cell adaptation and function.

Alterations in β-Cell Calcium Dynamics and Efficacy Outweigh Islet Mass Adaptation in Compensation of Insulin Resistance and Prediabetes Onset

Emerging insulin resistance is normally compensated by increased insulin production of pancreatic β-cells, thereby maintaining normoglycemia. However, it is unclear whether this is achieved by adaptation of β-cell function, mass, or both. Most importantly, it is still unknown which of these adaptive mechanisms fail when type 2 diabetes develops. We performed longitudinal in vivo imaging of β-cell calcium dynamics and islet mass of transplanted islets of Langerhans throughout diet-induced progression from normal glucose homeostasis, through compensation of insulin resistance, to prediabetes. The results show that compensation of insulin resistance is predominated by alterations of β-cell function, while islet mass only gradually expands. Hereby, functional adaptation is mediated by increased calcium efficacy, which involves Epac signaling. Prior to prediabetes, β-cell function displays decreased stimulated calcium dynamics, whereas islet mass continues to increase through prediabetes onset. Thus, our data reveal a predominant role of islet function with distinct contributions of triggering and amplifying pathway in the in vivo processes preceding diabetes onset. These findings support protection and recovery of β-cell function as primary goals for prevention and treatment of diabetes and provide insight into potential therapeutic targets.

Clinical intraocular islet transplantation is not a number issue

It is now well established that beta cell replacement through pancreatic islet transplantation results in significant improvement in the quality-of-life of type 1 diabetes (T1D) patients. This is achieved through improved control and prevention of severe drops in blood sugar levels. Islet transplant therapy is on the verge of becoming standard-of-care in the USA. Yet, as with other established transplantation therapies, there remain hurdles to overcome to bring islet transplantation to full fruition as a long-lasting therapy of T1D. One of these hurdles is establishing reliable new sites, other than the liver, where durable efficacy and survival of transplanted islets can be achieved. In this article, we discuss the anterior chamber of the eye as a new site for clinical islet transplantation in the treatment of T1D. We specifically focus on the common conceptions, and preconceptions, on the requirements of islet mass, and whether or not the anterior chamber can accommodate sufficient islets to achieve meaningful efficacy and significant impact on hyperglycemia in clinical application.

Multifunctional in vivo imaging of pancreatic islets during diabetes development

Pancreatic islet dysfunction leading to insufficient glucose-stimulated insulin secretion triggers the clinical onset of diabetes. How islet dysfunction develops is not well understood at the cellular level, partly owing to the lack of approaches to study single islets longitudinally in vivo Here, we present a noninvasive, high-resolution system to quantitatively image real-time glucose metabolism from single islets in vivo, currently not available with any other method. In addition, this multifunctional system simultaneously reports islet function, proliferation, vasculature and macrophage infiltration in vivo from the same set of images. Applying our method to a longitudinal high-fat diet study revealed changes in islet function as well as alternations in islet microenvironment. More importantly, this label-free system enabled us to image real-time glucose metabolism directly from single human islets in vivo for the first time, opening the door to noninvasive longitudinal in vivo studies of healthy and diabetic human islets.

Noninvasive in vivo imaging of embryonic β-cell development in the anterior chamber of the eye

The fetal environment plays a decisive role in modifying the risk for developing diabetes later in life. Developing novel methodology for noninvasive imaging of β-cell development in vivo under the controlled physiological conditions of the host can serve to understand how this environment affects β-cell growth and differentiation. A number of culture models have been designed for pancreatic rudiment but none match the complexity of the in utero or even normal physiological environment. Speier et al. recently developed a platform of noninvasive in vivo imaging of pancreatic islets using the anterior chamber of the eye where islets get vascularized, grow and respond to physiological changes. The same methodology was adapted for the study of pancreatic development. E13.0, still undifferentiated rudiments with fluorescent lineage tracing were implanted in the AC of the eye, allowing the longitudinal study of their growth and differentiation. Within 48 h the anlages get vascularized and grow but their mesenchyme displays a selective growth advantage. The resulting imbalance leads to alteration in the differentiation pattern of the progenitors. Reducing the mesenchyme to its bare minimum before implantation allows the restoration of a proper balance and a development that mimics the normal pancreatic development. These groundbreaking observations demonstrate that the anterior chamber of the eye provides a good system for noninvasive in vivo fluorescence imaging of the developing pancreas under the physiology of the host and can have important implications for designing strategies to prevent or reverse the deleterious effects of hyperglycemia on altering β-cell function later in life.

Non-invasive cell type selective in vivo monitoring of insulin resistance dynamics

Insulin resistance contributes to the development of cardio-vascular disease and diabetes. An important but unresolved task is to study the dynamics of insulin resistance in selective cell types of insulin target tissues in vivo. Here we present a novel technique to monitor insulin resistance dynamics non-invasively and longitudinally in vivo in a cell type-specific manner, exemplified by the pancreatic β-cell situated within the micro-organ the islet of Langerhans. We utilize the anterior chamber of the eye (ACE) as a transplantation site and the cornea as a natural body-window to study the development and reversibility of insulin resistance. Engrafted islets in the ACE that express a FoxO1-GFP-based biosensor in their β-cells, report on insulin resistance measured by fluorescence microscopy at single-cell resolution in the living mouse. This technique allows monitoring of cell type specific insulin sensitivity/resistance in real-time in the context of whole body insulin resistance during progression and intervention of disease.

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.

New insights into the architecture of the islet of Langerhans: a focused cross-species assessment

The human genome project and its search for factors underlying human diseases has fostered a major human research effort. Therefore, unsurprisingly, in recent years we have observed an increasing number of studies on human islet cells, including disease approaches focusing on type 1 and type 2 diabetes. Yet, the field of islet and diabetes research relies on the legacy of rodent-based investigations, which have proven difficult to translate to humans, particularly in type 1 diabetes. Whole islet physiology and pathology may differ between rodents and humans, and thus a comprehensive cross-species as well as species-specific view on islet research is much needed. In this review we summarise the current knowledge of interspecies islet cytoarchitecture, and discuss its potential impact on islet function and future perspectives in islet pathophysiology research.

PI3K-C2α Knockdown Results in Rerouting of Insulin Signaling and Pancreatic Beta Cell Proliferation

Insulin resistance is a syndrome that affects multiple insulin target tissues, each having different biological functions regulated by insulin. A remaining question is to mechanistically explain how an insulin target cell/tissue can be insulin resistant in one biological function and insulin sensitive in another at the same time. Here, we provide evidence that in pancreatic β cells, knockdown of PI3K-C2α expression results in rerouting of the insulin signal from insulin receptor (IR)-B/PI3K-C2α/PKB-mediated metabolic signaling to IR-B/Shc/ERK-mediated mitogenic signaling, which allows the β cell to switch from a highly glucose-responsive, differentiated state to a proliferative state. Our data suggest the existence of IR-cascade-selective insulin resistance, which allows rerouting of the insulin signal within the same target cell. Hence, factors involved in the rerouting of the insulin signal represent tentative therapeutic targets in the treatment of insulin resistance.

Distinct roles of β-cell mass and function during type 1 diabetes onset and remission

Cure of type 1 diabetes (T1D) by immune intervention at disease onset depends on the restoration of insulin secretion by endogenous β-cells. However, little is known about the potential of β-cell mass and function to recover after autoimmune attack ablation. Using a longitudinal in vivo imaging approach, we show how functional status and mass of β-cells adapt in response to the onset and remission of T1D. We demonstrate that infiltration reduces β-cell mass prior to onset and, together with emerging hyperglycemia, affects β-cell function. After immune intervention, persisting hyperglycemia prevents functional recovery but promotes β-cell mass increase in mouse islets. When blood glucose levels return to normoglycemia β-cell mass expansion stops, and subsequently glucose tolerance recovers in combination with β-cell function. Similar to mouse islets, human islets exhibit cell exhaustion and recovery in response to transient hyperglycemia. However, the effect of hyperglycemia on human islet mass increase is minor and transient. Our data demonstrate a major role of functional exhaustion and recovery of β-cells during T1D onset and remission. Therefore, these findings support early intervention therapy for individuals with T1D.

Light scattering as an intrinsic indicator for pancreatic islet cell mass and secretion

The pancreatic islet of Langerhans is composed of endocrine cells producing and releasing hormones from secretory granules in response to various stimuli for maintenance of blood glucose homeostasis. In order to adapt to a variation in functional demands, these islets are capable of modulating their hormone secretion by increasing the number of endocrine cells as well as the functional response of individual cells. A failure in adaptive mechanisms will lead to inadequate blood glucose regulation and thereby to the development of diabetes. It is therefore necessary to develop tools for the assessment of both pancreatic islet mass and function, with the aim of understanding cellular regulatory mechanisms and factors guiding islet plasticity. Although most of the existing techniques rely on the use of artificial indicators, we present an imaging methodology based on intrinsic optical properties originating from mature insulin secretory granules within endocrine cells that reveals both pancreatic islet mass and function. We demonstrate the advantage of using this imaging strategy by monitoring in vivo scattering signal from pancreatic islets engrafted into the anterior chamber of the mouse eye, and how this versatile and noninvasive methodology permits the characterization of islet morphology and plasticity as well as hormone secretory status.

Human Serum Versus Human Serum Albumin Supplementation in Human Islet Pretransplantation Culture: In Vitro and In Vivo Assessment

There is conflicting evidence favoring both the use of human serum (HS) and of human serum albumin (HSA) in human islet culture. We evaluated the effects of HS versus HSA supplementation on 1) in vitro β-cell viability and function and 2) in vivo islet graft revascularization, islet viability, β-cell death, and metabolic outcome after transplantation. Islets isolated from 14 cadaveric organ donors were cultured for 3 days in CMRL 1066 medium supplemented with HS or HSA. After 3 days in culture, β-cell apoptosis was lower in HS group (1.41 ± 0.27 vs. 2.38 ± 0.39%, p = 0.029), and the recovery of islets was 77 ± 11% and 54 ± 1% in HS- and HSA-cultured groups, respectively. Glucose-stimulated insulin secretion (GSIS) was higher in HS group (29.4, range 10.4-99.9, vs. 22.3, range 8.7-70.6, p = 0.031). In vivo viability and revascularization was determined in HS- and HSA-cultured islets transplanted into the anterior chamber of the eye of Balb/c mice (n = 14), and β-cell apoptosis in paraffin-embedded mouse eyes. Islet viability and β-cell apoptosis were similar in both groups. Revascularization was observed in one graft (HS group) on day 10 after transplantation. Islet function was determined in streptozotocin (STZ)-diabetic nude mice (n = 33) transplanted with 2,000 IEQs cultured with HS or HSA that showed similar blood glucose levels and percentage of normoglycemic animals over time. In conclusion, human islets cultured in medium supplemented with HS showed higher survival in vitro, as well as islet viability and function. The higher in vitro survival increased the number of islets available for transplantation. However, the beneficial effect on viability and function did not translate into an improved metabolic evolution when a similar number of HSA- and HS-cultured islets was transplanted.

Beta cell specific probing with fluorescent exendin-4 is progressively reduced in type 2 diabetic mouse models

Probes based on GLP-1R agonist exendin-4 have shown promise as in vivo β cell tracers. However, questions remain regarding the β cell specificity of exendin-4 probes, and it is unclear if the expression levels of the GLP-1R are affected in a type 2 diabetic state. Using in vivo probing followed by ex vivo imaging we found fluorescent exendin-4 probes to distinctly label the pancreatic islets in mice in a Glp-1r dependent manner. Furthermore, a co-localization study revealed a near 100 percent β cell specificity with less than one percent probing in other analyzed cell types. We then tested if probing was affected in models of type 2 diabetes using the Lepr(db/db) (db/db) and the Diet-Induced Obese (DIO) mouse. Although nearly all β cells continued to be probed, we observed a progressive decline in probing intensity in both models with the most dramatic reduction seen in db/db mice. This was paralleled by a progressive decrease in Glp-1r protein expression levels. These data confirm β cell specificity for exendin-4 based probes in mice. Furthermore, they also suggest that GLP-1R targeting probes may provide a tool to monitor β cell function rather than mass in type 2 diabetic mouse models.

β-Cell Ca(2+) dynamics and function are compromised in aging

Defects in pancreatic β-cell function and survival are key components in type 2 diabetes (T2D). An age-dependent deterioration in β-cell function has also been observed, but little is known about the molecular mechanisms behind this phenomenon. Our previous studies indicate that the regulation of cytoplasmic free Ca(2+) concentration ([Ca(2+)]i) may be critical and that this is dependent on the proper function of the mitochondria. The [Ca(2+)]i dynamics of the pancreatic β-cell are driven by an interplay between glucose-induced influx of extracellular Ca(2+) via voltage-dependent Ca(2+) channels and the inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-mediated liberation of Ca(2+) from intracellular stores. Our previous work has indicated a direct relationship between disruption of Ins(1,4,5)P3-mediated Ca(2+) regulation and loss of β-cell function, including disturbed [Ca(2+)]i dynamics and compromised insulin secretion. To investigate these processes in aging we used three mouse models, a premature aging mitochondrial mutator mouse, a mature aging phenotype (C57BL/6) and an aging-resistant phenotype (129). Our data suggest that age-dependent impairment in mitochondrial function leads to modest changes in [Ca(2+)]i dynamics in mouse β-cells, particularly in the pattern of [Ca(2+)]i oscillations. These changes are driven by modifications in both PLC/Ins(1,4,5)P3-mediated Ca(2+) mobilization from intracellular stores and decreased β-cell Ca(2+) influx over the plasma membrane. Our findings underscore an important concept, namely that even relatively small, time-dependent changes in β-cell signal-transduction result in compromised insulin release and in a diabetic phenotype.

Connexin 36 mediates blood cell flow in mouse pancreatic islets

The insulin-secreting β-cells are contained within islets of Langerhans, which are highly vascularized. Blood cell flow rates through islets are glucose-dependent, even though there are no changes in blood cell flow within in the surrounding exocrine pancreas. This suggests a specific mechanism of glucose-regulated blood flow in the islet. Pancreatic islets respond to elevated glucose with synchronous pulses of electrical activity and insulin secretion across all β-cells in the islet. Connexin 36 (Cx36) gap junctions between islet β-cells mediate this synchronization, which is lost in Cx36 knockout mice (Cx36(-/-)). This leads to glucose intolerance in these mice, despite normal plasma insulin levels and insulin sensitivity. Thus, we sought to investigate whether the glucose-dependent changes in intraislet blood cell flow are also dependent on coordinated pulsatile electrical activity. We visualized and quantified blood cell flow using high-speed in vivo fluorescence imaging of labeled red blood cells and plasma. With the use of a live animal glucose clamp, blood cell flow was measured during either hypoglycemia (∼50 mg/dl) or hyperglycemia (∼300 mg/dl). In contrast to the large glucose-dependent islet blood velocity changes observed in wild-type mice, only minimal differences are observed in both Cx36(+/-) and Cx36(-/-) mice. This observation supports a novel model where intraislet blood cell flow is regulated by the coordinated electrical activity in the islet β-cells. Because Cx36 expression and function is reduced in type 2 diabetes, the resulting defect in intraislet blood cell flow regulation may also play a significant role in diabetic pathology.