Real-time imaging of the pancreas during development of diabetes

Hepatic conversion of acetyl-CoA to acetate plays crucial roles in energy stress

Accumulating evidence indicates that acetate is increased under energy stress conditions such as those that occur in diabetes mellitus and prolonged starvation. However, how and where acetate is produced and the nature of its biological significance are largely unknown. We observed overproduction of acetate to concentrations comparable to those of ketone bodies in patients and mice with diabetes or starvation. Mechanistically, ACOT12 and ACOT8 are dramatically upregulated in the liver to convert free fatty acid-derived acetyl-CoA to acetate and CoA. This conversion not only provides a large amount of acetate, which preferentially fuels the brain rather than muscle, but also recycles CoA, which is required for sustained fatty acid oxidation and ketogenesis. We suggest that acetate is an emerging novel 'ketone body' that may be used as a parameter to evaluate the progression of energy stress.

Live Intravital Intestine with Blood Flow Visualization in Neonatal Mice Using Two-photon Laser Scanning Microscopy

This protocol describes a novel technique to investigate the microcirculation dynamics underlying the pathology in the small intestine of neonatal mice using two-photon laser-scanning microscopy (TPLSM). Recent technological advances in multi-photon microscopy allow intravital analysis of different organs such as the liver, brain and intestine. Despite these advances, live visualization and analysis of the small intestine in neonatal rodents remain technically challenging. We herein provide a detailed description of a novel method to capture high resolution and stable images of the small intestine in neonatal mice as early as postnatal day 0. This imaging technique allows a comprehensive understanding of the development and blood flow dynamics in small intestine microcirculation.

Improved in vivo imaging method for individual islets across the mouse pancreas reveals a heterogeneous insulin secretion response to glucose

While numerous techniques can be used to measure and analyze insulin secretion in isolated islets in culture, assessments of insulin secretion in vivo are typically indirect and only semiquantitative. The CpepSfGFP reporter mouse line allows the in vivo imaging of insulin secretion from individual islets after a glucose stimulation, in live, anesthetized mice. Imaging the whole pancreas at high resolution in live mice to track the response of each individual islet over time includes numerous technical challenges and previous reports were only limited in scope and non-quantitative. Elaborating on this previous model-through the development of an improved methodology addressing anesthesia, temperature control and motion blur-we were able to track and quantify longitudinally insulin content throughout a glucose challenge in up to two hundred individual islets simultaneously. Through this approach we demonstrate quantitatively for the first time that while isolated islets respond homogeneously to glucose in culture, their profiles differ significantly in vivo. Independent of size or location, some islets respond sharply to a glucose stimulation while others barely secrete at all. This platform therefore provides a powerful approach to study the impact of disease, diet, surgery or pharmacological treatments on insulin secretion in the intact pancreas in vivo.

Functional analysis of islet cells in vitro, in situ, and in vivo

The islet of Langerhans contains at least five types of endocrine cells producing distinct hormones. In response to nutrient or neuronal stimulation, islet endocrine cells release biochemicals including peptide hormones to regulate metabolism and to control glucose homeostasis. It is now recognized that malfunction of islet cells, notably insufficient insulin release of β-cells and hypersecretion of glucagon from α-cells, represents a causal event leading to hyperglycemia and frank diabetes, a disease that is increasing at an alarming rate to reach an epidemic level worldwide. Understanding the mechanisms regulating stimulus-secretion coupling and investigating how islet β-cells maintain a robust secretory activity are important topics in islet biology and diabetes research. To facilitate such studies, a number of biological systems and assay platforms have been developed for the functional analysis of islet cells. These technologies have enabled detailed analyses of individual islets at the cellular level, either in vitro, in situ, or in vivo.

Mouse pancreatic islet macrophages use locally released ATP to monitor beta cell activity

AIMS/HYPOTHESIS

Tissue-resident macrophages sense the microenvironment and respond by producing signals that act locally to maintain a stable tissue state. It is now known that pancreatic islets contain their own unique resident macrophages, which have been shown to promote proliferation of the insulin-secreting beta cell. However, it is unclear how beta cells communicate with islet-resident macrophages. Here we hypothesised that islet macrophages sense changes in islet activity by detecting signals derived from beta cells.

METHODS

To investigate how islet-resident macrophages respond to cues from the microenvironment, we generated mice expressing a genetically encoded Ca2+ indicator in myeloid cells. We produced living pancreatic slices from these mice and used them to monitor macrophage responses to stimulation of acinar, neural and endocrine cells.

RESULTS

Islet-resident macrophages expressed functional purinergic receptors, making them exquisite sensors of interstitial ATP levels. Indeed, islet-resident macrophages responded selectively to ATP released locally from beta cells that were physiologically activated with high levels of glucose. Because ATP is co-released with insulin and is exclusively secreted by beta cells, the activation of purinergic receptors on resident macrophages facilitates their awareness of beta cell secretory activity.

CONCLUSIONS/INTERPRETATION

Our results indicate that islet macrophages detect ATP as a proxy signal for the activation state of beta cells. Sensing beta cell activity may allow macrophages to adjust the secretion of factors to promote a stable islet composition and size.

Early sympathetic islet neuropathy in autoimmune diabetes: lessons learned and opportunities for investigation

This review outlines the current state of knowledge regarding a unique neural defect of the pancreatic islet in autoimmune diabetes, one that we have termed early sympathetic islet neuropathy (eSIN). We begin with the findings that a majority of islet sympathetic nerves are lost near the onset of type 1, but not type 2, diabetes and that this nerve loss is restricted to the islet. We discuss later work demonstrating that while the loss of islet sympathetic nerves and the loss of islet beta cells in type 1 diabetes both require infiltration of the islet by lymphocytes, their respective mechanisms of tissue destruction differ. Uniquely, eSIN requires the activation of a specific neurotrophin receptor and we propose two possible pathways for activation of this receptor during the immune attack on the islet. We also outline what is known about the functional consequences of eSIN, focusing on impairment of sympathetically mediated glucagon secretion and its application to the clinical problem of insulin-induced hypoglycaemia. Finally, we offer our view on the important remaining questions regarding this unique neural defect.

A Deeper Look into Type 1 Diabetes - Imaging Immune Responses during Onset of Disease

Cytotoxic T lymphocytes execute the killing of insulin-producing beta cells during onset of type 1 diabetes mellitus (T1D). The research community has come far in dissecting the major events in the development of this disease, but still the trigger and high-resolved information of the immunological events leading up to beta cell loss are missing. During the past decades, intravital imaging of immune responses has led to significant scientific breakthroughs in diverse models of disease, including T1D. Dynamic imaging of immune cells at the pancreatic islets during T1D onset has been made possible through the development of both advanced microscopes, and animal models that allow long-term immobilization of the pancreas. The use of these modalities has revealed a milling microenvironment at the pancreatic islets during disease onset with a plethora of active players. Clues to answering the remaining questions in this disease may lie in intravital imaging, including how key immune cells traffic to and from the pancreas, and how cells interact at this target tissue. This review highlights and discusses recent studies, models, and techniques focused to understand the immune responses during T1D onset through intravital imaging.

Deep insights: intravital imaging with two-photon microscopy

Intravital multiphoton microscopy is widely used to assess the structure and function of organs in live animals. Although different tissues vary in their accessibility for intravital multiphoton imaging, considerable progress has been made in the imaging quality of all tissues due to substantial technical improvements in the relevant imaging components, such as optics, excitation laser, detectors, and signal analysis software. In this review, we provide an overview of the technical background of intravital multiphoton microscopy. Then, we note a few seminal findings that were made through the use of multiphoton microscopy. Finally, we address the technical limitations of the method and provide an outlook for how these limitations may be overcome through future technical developments.

Comprehensive Survey of miRNA-mRNA Interactions Reveals That Ccr7 and Cd247 (CD3 zeta) are Posttranscriptionally Controlled in Pancreas Infiltrating T Lymphocytes of Non-Obese Diabetic (NOD) Mice

In autoimmune type 1 diabetes mellitus (T1D), auto-reactive clones of CD4+ and CD8+ T lymphocytes in the periphery evolve into pancreas-infiltrating T lymphocytes (PILs), which destroy insulin-producing beta-cells through inflammatory insulitis. Previously, we demonstrated that, during the development of T1D in non-obese diabetic (NOD) mice, a set of immune/inflammatory reactivity genes were differentially expressed in T lymphocytes. However, the posttranscriptional control involving miRNA interactions that occur during the evolution of thymocytes into PILs remains unknown. In this study, we postulated that miRNAs are differentially expressed during this period and that these miRNAs can interact with mRNAs involved in auto-reactivity during the progression of insulitis. To test this hypothesis, we used NOD mice to perform, for the first time, a comprehensive survey of miRNA and mRNA expression as thymocytes mature into peripheral CD3+ T lymphocytes and, subsequently, into PILs. Reconstruction of miRNA-mRNA interaction networks for target prediction revealed the participation of a large set of miRNAs that regulate mRNA targets related to apoptosis, cell adhesion, cellular regulation, cellular component organization, cellular processes, development and the immune system, among others. The interactions between miR-202-3p and the Ccr7 chemokine receptor mRNA or Cd247 (Cd3 zeta chain) mRNA found in PILs are highlighted because these interactions can contribute to a better understanding of how the lack of immune homeostasis and the emergence of autoimmunity (e.g., T1D) can be associated with the decreased activity of Ccr7 or Cd247, as previously observed in NOD mice. We demonstrate that these mRNAs are controlled at the posttranscriptional level in PILs.

The search for the mechanism of early sympathetic islet neuropathy in autoimmune diabetes

This review outlines our search for the mechanism causing the early loss of islet sympathetic nerves in autoimmune diabetes. Since our previous work has documented the importance of autonomic stimulation of glucagon secretion during hypoglycaemia, the loss of these nerves may contribute to the known impairment of this specific glucagon response early in human type 1 diabetes. We therefore briefly review the contribution that autonomic activation, and sympathetic neural activation in particular, makes to the subsequent glucagon response to hypoglycaemia. We also detail evidence that animal models of autoimmune diabetes mimic both the early loss of islet sympathetic nerves and the impaired glucagon response seen in human type 1 diabetes. Using data from these animal models, we examine mechanisms by which this loss of islet nerves could occur. We provide evidence that it is not due to diabetic hyperglycaemia, but is related to the lymphocytic infiltration of the islet. Ablating the p75 neurotrophin receptor, which is present on sympathetic axons, prevents early sympathetic islet neuropathy (eSIN), but, interestingly, not diabetes. Thus, we appear to have separated the immune-related loss of islet sympathetic nerves from the immune-mediated destruction of islet β-cells. Finally, we speculate on a way to restore the sympathetic innervation of the islet.

Transdisciplinary approach to restore pancreatic islet function

The focus of our research is on islet immunobiology. We are exploring novel strategies that could be of assistance in the treatment and prevention of type 1 diabetes, as well as in the restoration of metabolic control via transplantation of insulin producing cells (i.e., islet cells). The multiple facets of diabetes and β-cell replacement encompass different complementary disciplines, such as immunology, cell biology, pharmacology, and bioengineering, among others. Through their interaction and integration, a transdisciplinary dimension is needed in order to address and overcome all aspects of the complex puzzle toward a successful clinical translation of a biological cure for diabetes.

Imaging dynamics of CD11c⁺ cells and Foxp3⁺ cells in progressive autoimmune insulitis in the NOD mouse model of type 1 diabetes

AIMS/HYPOTHESIS

The aim of this study was to visualise the dynamics and interactions of the cells involved in autoimmune-driven inflammation in type 1 diabetes.

METHODS

We adopted the anterior chamber of the eye (ACE) transplantation model to perform non-invasive imaging of leucocytes infiltrating the endocrine pancreas during initiation and progression of insulitis in the NOD mouse. Individual, ACE-transplanted islets of Langerhans were longitudinally and repetitively imaged by stereomicroscopy and two-photon microscopy to follow fluorescently labelled leucocyte subsets.

RESULTS

We demonstrate that, in spite of the immune privileged status of the eye, the ACE-transplanted islets develop infiltration and beta cell destruction, recapitulating the autoimmune insulitis of the pancreas, and exemplify this by analysing reporter cell populations expressing green fluorescent protein under the Cd11c or Foxp3 promoters. We also provide evidence that differences in morphological appearance of subpopulations of infiltrating leucocytes can be correlated to their distinct dynamic behaviour.

CONCLUSIONS/INTERPRETATION

Together, these findings demonstrate that the kinetics and dynamics of these key cellular components of autoimmune diabetes can be elucidated using this imaging platform for single cell resolution, non-invasive and repetitive monitoring of the individual islets of Langerhans during the natural development of autoimmune diabetes.

Transplantation into the anterior chamber of the eye for longitudinal, non-invasive in vivo imaging with single-cell resolution in real-time

Intravital imaging has emerged as an indispensable tool in biological research. In the process, many imaging techniques have been developed to study different biological processes in animals non-invasively. However, a major technical limitation in existing intravital imaging modalities is the inability to combine non-invasive, longitudinal imaging with single-cell resolution capabilities. We show here how transplantation into the anterior chamber of the eye circumvents such significant limitation offering a versatile experimental platform that enables non-invasive, longitudinal imaging with cellular resolution in vivo. We demonstrate the transplantation procedure in the mouse and provide representative results using a model with clinical relevance, namely pancreatic islet transplantation. In addition to enabling direct visualization in a variety of tissues transplanted into the anterior chamber of the eye, this approach provides a platform to screen drugs by performing long-term follow up and monitoring in target tissues. Because of its versatility, tissue/cell transplantation into the anterior chamber of the eye not only benefits transplantation therapies, it extends to other in vivo applications to study physiological and pathophysiological processes such as signal transduction and cancer or autoimmune disease development.

Imaging of the β-cells of the islets of Langerhans

The major aim of this paper is to review the present status of the techniques for the non-invasive imaging and quantification of insulin-producing pancreatic islet β-cells. Emphasis is placed on both the expansion of prior work already considered in a prior review and novel achievements. Thus, the use of d-mannoheptulose analogs, hypoglycemic sulfonylureas and glinides, neural imaging agents, neuro-hormonal receptor ligands and nanoparticles is first dealt with. Thereafter, consideration is given on optical imaging technologies, the identification of new β-cells specific binding and target proteins, the functional imaging of islets transplanted into the eye anterior chamber and in vivo manganese-enhanced magnetic resonance imaging.

Diabetes imaging-quantitative assessment of islets of Langerhans distribution in murine pancreas using extended-focus optical coherence microscopy

Diabetes is characterized by hyperglycemia that can result from the loss of pancreatic insulin secreting β-cells in the islets of Langerhans. We analyzed ex vivo the entire gastric and duodenal lobes of a murine pancreas using extended-focus Optical Coherence Microscopy (xfOCM). To identify and quantify the islets of Langerhans observed in xfOCM tomograms we implemented an active contour algorithm based on the level set method. We show that xfOCM reveals a three-dimensional islet distribution consistent with Optical Projection Tomography, albeit with a higher resolution that also enables the detection of the smallest islets (≤ 8000 μm(3)). Although this category of the smallest islets represents only a negligible volume compared to the total β-cell volume, a recent study suggests that these islets, located at the periphery, are the first to be destroyed when type I diabetes develops. Our results underline the capability of xfOCM to contribute to the understanding of the development of diabetes, especially when considering islet volume distribution instead of the total β-cell volume only.

Pretargeting vs. direct targeting of human betalox5 islet cells subcutaneously implanted in mice using an anti-human islet cell antibody

INTRODUCTION

We previously demonstrated MORF/cMORF pretargeting of human islets and betalox 5 cells (a human beta cell line) transplanted subcutaneously in mice with the anti-human islet antibody, HPi1. We now compare pretargeting with direct targeting in the beta cell transplant model to evaluate the degree to which target/non-target (T/NT) ratios may be improved by pretargeting.

METHODS

Specific binding of an anti-human islet antibody HPi1 to the beta cells transplanted subcutaneously in mice was examined against a negative control antibody. We then compared pretargeting by MORF-HPi1 plus 111In-labeled cMORF to direct targeting by 111In-labeled HPi1.

RESULTS

HPi1 binding to betalox5 human cells in the transplant was shown by immunofluorescence. Normal organ 111In backgrounds by pretargeting were always lower, although target accumulations were similar. More importantly, the transplant to pancreas and liver ratios was, respectively, 26 and 10 by pretargeting as compared to 9 and 0.6 by direct targeting.

CONCLUSIONS

Pretargeting greatly improves the T/NT ratios, and based on the estimated endocrine to exocrine ratio within a pancreas, pretargeting may be approaching the sensitivity required for successful imaging of human islets within this organ.

Non-invasive in vivo imaging of pancreatic β-cell function and survival - a perspective

A major problem in medical research is to translate in vitro observations into the living organism. In this perspective, we discuss ongoing efforts to non-invasively image pancreatic islets/β-cells by techniques, such as magnetic resonance imaging and positron emission tomography, and present an experimental platform, which allows in vivo imaging of pancreatic β-cell mass and function longitudinally and at the single-cell level. Following transplantation of pancreatic islets into the anterior chamber of the eye of mice and rats, these islets are studied by functional microscopic imaging. This imaging platform can be utilized to address fundamental aspects of pancreatic islet cell biology in vivo in health and disease. These include the dynamics of pancreatic islet vascularization, islet cell innervation, signal-transduction, change in functional β-cell mass and immune responses. Moreover, we discuss the feasibility of studying human islet cell physiology and pathology in vivo as well as the potential of using the anterior chamber of the eye as a site for therapeutic transplantation in type 1 diabetes mellitus.

T cell-driven initiation and propagation of autoimmune diabetes

The destruction of beta cells in type 1 diabetes in humans and in autoimmune diabetes in the NOD mouse model is a consequence of chronic islet inflammation in the pancreas. The T cell-driven autoimmune response is initiated by environmental triggers which are influenced by the state of intestinal homeostasis and the microbiota. The disease process can be separated into two phases: firstly, initiation of mild, controlled, long-term infiltration and secondly, propagation of invasive inflammation which quickly progresses to beta cell deletion and autoimmune diabetes. In this review, we will discuss the cellular and molecular triggers that might be required for these two phases in the context of other issues including the unique anatomical location of pancreas, the location of T cell priming, the requirements for islet entry, and the events that ultimately drive beta cell destruction and the onset of diabetes.

Experimental approaches for high-resolution in vivo imaging of islet of Langerhans biology

Under physiological conditions and in the pathogenesis of diabetes mellitus systemic influences play a substantial role for function and survival of cells of the islet of Langerhans. Therefore, in vivo studies to understand islet biology are indispensible and imaging techniques are increasingly used for this purpose. Among the diverse imaging modalities currently only laser scanning microscopy (LSM) allows resolution and visualization of individual cells and cellular processes. To overcome limited tissue penetration and working distance of LSM and enable in vivo investigations of islet cell physiology, various experimental approaches have been developed. Especially, the recently developed imaging platforms have significantly improved the possibility to study islets at a cellular level in vivo, and provided novel insight into islet biology in health and disease. The various approaches, their applications, and reported results, as well as their limitations are reviewed in this article.

Immunological perspective of self versus tumor antigens: insights from the RIP-gp model

Self-reactive T cells in the body are controlled by mechanisms of peripheral tolerance that limit their activation and induction of immune pathology. Our understanding of these mechanisms has been advanced by the use of tissue-specific promoters to express neo-self-antigens. Here, we present findings using the RIP-gp (rat insulin promoter-glycoprotein) transgenic mouse, which expresses the lymphocytic choriomeningitis virus glycoprotein (LCMV-gp) specifically in the pancreatic β islet cells. T cells responsive to this antigen remain ignorant of the LCMV-gp expressed by the islets, and breaking tolerance is dependent upon the maturation status of antigen-presenting cells, the avidity of the T-cell receptor ligation, and the level of major histocompatibility complex expression in the pancreas. Furthermore, decreased activity of Casitas B-lineage lymphoma b, a negative regulator of T-cell receptor signaling, can allow recognition and destruction of the pancreatic islets. This review discusses the roles of these factors in the context of anti-tissue responses, both in the setting of autoimmunity and in anti-tumor immunity.

High-resolution, noninvasive longitudinal live imaging of immune responses

Intravital imaging emerged as an indispensible tool in biological research, and a variety of imaging techniques have been developed to noninvasively monitor tissues in vivo. However, most of the current techniques lack the resolution to study events at the single-cell level. Although intravital multiphoton microscopy has addressed this limitation, the need for repeated noninvasive access to the same tissue in longitudinal in vivo studies remains largely unmet. We now report on a previously unexplored approach to study immune responses after transplantation of pancreatic islets into the anterior chamber of the mouse eye. This approach enabled (i) longitudinal, noninvasive imaging of transplanted tissues in vivo; (ii) in vivo cytolabeling to assess cellular phenotype and viability in situ; (iii) local intervention by topical application or intraocular injection; and (iv) real-time tracking of infiltrating immune cells in the target tissue.

Accurate measurement of pancreatic islet beta-cell mass using a second-generation fluorescent exendin-4 analog

The hallmark of type 1 diabetes is autoimmune destruction of the insulin-producing β-cells of the pancreatic islets. Autoimmune diabetes has been difficult to study or treat because it is not usually diagnosed until substantial β-cell loss has already occurred. Imaging agents that permit noninvasive visualization of changes in β-cell mass remain a high-priority goal. We report on the development and testing of a near-infrared fluorescent β-cell imaging agent. Based on the amino acid sequence of exendin-4, we created a neopeptide via introduction of an unnatural amino acid at the K(12) position, which could subsequently be conjugated to fluorophores via bioorthogonal copper-catalyzed click-chemistry. Cell assays confirmed that the resulting fluorescent probe (E4(×12)-VT750) had a high binding affinity (~3 nM). Its in vivo properties were evaluated using high-resolution intravital imaging, histology, whole-pancreas visualization, and endoscopic imaging. According to intravital microscopy, the probe rapidly bound to β-cells and, as demonstrated by confocal microscopy, it was internalized. Histology of the whole pancreas showed a close correspondence between fluorescence and insulin staining, and there was an excellent correlation between imaging signals and β-cell mass in mice treated with streptozotocin, a β-cell toxin. Individual islets could also be visualized by endoscopic imaging. In short, E4(×12)-VT750 showed strong and selective binding to glucose-like peptide-1 receptors and permitted accurate measurement of β-cell mass in both diabetic and nondiabetic mice. This near-infrared imaging probe, as well as future radioisotope-labeled versions of it, should prove to be important tools for monitoring diabetes, progression, and treatment in both experimental and clinical contexts.

Human islet cell MORF/cMORF pretargeting in a xenogeneic murine transplant model

Noninvasive measurement of human islet cell mass in pancreas or following islet transplantation by nuclear imaging has yet to be achieved. It has been shown using mouse tumor models that pretargeting imaging strategies are sensitive and can greatly increase target to nontarget signal ratios. The objective now is to demonstrate the specific pretargeting of human islet cells in mice. Our pretargeting strategy uses an anti-human islet cell antibody HPi1, conjugated to a phosphorodiamidate morpholino oligomer (MORF) that binds specifically to a (99m)Tc labeled complementary MORF (cMORF). Sensitivity and specificity of the pretargeting were first validated in culture using a human beta cell line (betalox5) and a negative control human cell line (HEK293). Pretargeting was then used to target and visualize these two cell lines and human islets transplanted subcutaneously in NOD-scid IL2rγ(null) mice. In culture, (99m)Tc accumulation on the betalox5 cells pretargeted by MORF-HPi1 was 100-fold higher than on untreated betalox5 cells or following treatment with native HPi1 and much higher than on the MORF-HPi1 pretargeted control HEK293 cells. Small animal imaging readily localized the transplanted betalox5 cells and human islets, but not the HEK293 cells. Ex vivo counting demonstrated 3-fold higher (99m)Tc accumulation in the transplanted betalox5 cells and human islets than in the control HEK293 cells. The target accumulation was also shown to increase linearly with increased numbers of the implanted betalox5 cells. These results demonstrate specific binding of radioactivity and successful imaging of human betalox5 cells and human islets transplanted in mice. Thus MORF/cMORF pretargeting may be useful to measure noninvasively human islet cell mass within the pancreas or following islet transplantation.

A novel technique for the in vivo imaging of autoimmune diabetes development in the pancreas by two-photon microscopy

Type 1 diabetes (T1D) is characterized by the immune-mediated destruction of beta cells in the pancreas. Little is known about the in vivo dynamic interactions between T cells and beta cells or the kinetic behavior of other immune cell subsets in the pancreatic islets. Utilizing multiphoton microscopy we have designed a technique that allows for the real-time visualization of diabetogenic T cells and dendritic cells in pancreatic islets in a live animal, including their interplay with beta cells and the vasculature. Using a custom designed stage, the pancreas was surgically exposed under live conditions so that imaging of islets under intact blood pressure and oxygen supply became possible. We demonstrate here that this approach allows for the tracking of diabetogenic leukocytes as well as vascularization phenotype of islets and accumulation of dendritic cells in islets during diabetes pathogenesis. This technique should be useful in mapping crucial kinetic events in T1D pathogenesis and in testing the impact of immune based interventions on T cell migration, extravasation and islet destruction.

Near-infrared fluorescent probe for imaging of pancreatic beta cells

The ability to image and ultimately quantitate beta-cell mass in vivo will likely have far reaching implications in the study of diabetes biology, in the monitoring of disease progression or response to treatment, and for drug development. Here, using animal models, we report on the synthesis, characterization, and intravital microscopic imaging properties of a near-infrared fluorescent exendin-4 analogue with specificity for the GLP-1 receptor on beta cells (E4(K12)-Fl). The agent demonstrated subnanomolar EC(50) binding concentrations, with high specificity and binding that could be inhibited by GLP-1R agonists. Following intravenous administration to mice, pancreatic islets were readily distinguishable from exocrine pancreas, achieving target-to-background ratios within the pancreas of 6:1, as measured by intravital microscopy. Serial imaging revealed rapid accumulation kinetics (with initial signal within the islets detectable within 3 min and peak fluorescence within 20 min of injection), making this an ideal agent for in vivo imaging.

Expression level of a pancreatic neo-antigen in beta cells determines degree of diabetes pathogenesis

It is not fully understood how the expression level of autoantigens in beta cells impacts autoimmune diabetes (T1D) development. Earlier studies using ovalbumin and also insulin had shown that secreted antigens could enhance diabetes development through facilitated presentation by antigen presenting cells. Here we sought to determine how the expression level of a membrane bound, non-secreted or cross-presented neo-antigen, the glycoprotein (GP) of lymphocytic choriomeningitis virus (LCMV), would influence T1D. We found that an RIP-LCMV transgenic mouse line exhibiting higher levels of beta cell GP expression developed more severe diabetes after LCMV infection or transfer of high numbers of activated autoreactive T cells. Importantly, all beta cells were lost and a significant increase in morbidity and mortality from T1D was noted. Insulitis and accumulation of autoaggressive CD8 cells was more profound in the RIP-LCMV-GP high-expressor line. Interestingly, the additional introduction of neo-antigen-specific CD4(+) helper or regulatory T cells was able to influence diabetogenesis positively or negatively. We conclude that a higher degree of autoantigen expression results in increased diabetes susceptibility. Therefore, autoantigens such as insulin that are expressed at higher levels in beta cells might have a more profound impact on diabetes pathogenesis.

Three-dimensional optical method for integrated visualization of mouse islet microstructure and vascular network with subcellular-level resolution

Microscopic visualization of islets of Langerhans under normal and diabetic conditions is essential for understanding the pathophysiology of the disease. The intrinsic opacity of pancreata, however, limits optical accessibility for high-resolution light microscopy of islets in situ. Because the standard microtome-based, 2-D tissue analysis confines visualization of the islet architecture at a specific cut plane, 3-D representation of image data is preferable for islet assessment. We applied optical clearing to minimize the random light scattering in the mouse pancreatic tissue. The optical-cleared pancreas allowed penetrative, 3-D microscopic imaging of the islet microstructure and vasculature. Specifically, the islet vasculature was revealed by vessel painting-lipophilic dye labeling of blood vessels-for confocal microscopy. The voxel-based confocal micrographs were digitally processed with projection algorithms for 3-D visualization. Unlike the microtome-based tissue imaging, this optical method for penetrative imaging of mouse islets yielded clear, continuous optical sections for an integrated visualization of the islet microstructure and vasculature with subcellular-level resolution. We thus provide a useful imaging approach to change our conventional planar view of the islet structure into a 3-D panorama for better understanding of the islet physiology.

A genomic-based approach identifies FXYD domain containing ion transport regulator 2 (FXYD2)gammaa as a pancreatic beta cell-specific biomarker

AIMS/HYPOTHESIS

Non-invasive imaging of the pancreatic beta cell mass (BCM) requires the identification of novel and specific beta cell biomarkers. We have developed a systems biology approach to the identification of promising beta cell markers.

METHODS

We followed a functional genomics strategy based on massive parallel signal sequencing (MPSS) and microarray data obtained in human islets, purified primary rat beta cells, non-beta cells and INS-1E cells to identify promising beta cell markers. Candidate biomarkers were validated and screened using established human and macaque (Macacus cynomolgus) tissue microarrays.

RESULTS

After a series of filtering steps, 12 beta cell-specific membrane proteins were identified. For four of the proteins we selected or produced antibodies targeting specifically the human proteins and their splice variants; all four candidates were confirmed as islet-specific in human pancreas. Two splice variants of FXYD domain containing ion transport regulator 2 (FXYD2), a regulating subunit of the Na(+)-K(+)-ATPase, were identified as preferentially present in human pancreatic islets. The presence of FXYD2gammaa was restricted to pancreatic islets and selectively detected in pancreatic beta cells. Analysis of human fetal pancreas samples showed the presence of FXYD2gammaa at an early stage (15 weeks). Histological examination of pancreatic sections from individuals with type 1 diabetes or sections from pancreases of streptozotocin-treated Macacus cynomolgus monkeys indicated a close correlation between loss of FXYD2gammaa and loss of insulin-positive cells.

CONCLUSIONS/INTERPRETATION

We propose human FXYD2gammaa as a novel beta cell-specific biomarker.

Prolonged survival and improved glycemia in BioBreeding diabetic rats after early sustained exposure to glucagon-like peptide 1

BACKGROUND

Type 1 diabetes (T1D) in both humans and BioBreeding (BB) rats is an autoimmune disease that results in complete destruction of islets and insulin dependency for life. Glucagon-like peptide 1 (GLP-1) promotes beta cell proliferation and neogenesis and has a potent insulinotropic effect. We hypothesized that the expression of GLP-1 before disease onset would increase islet mass, delay diabetes and prolong survival of BB rats.

METHODS

Vascular smooth muscle cells retrovirally transduced to secrete GLP-1 were seeded into TheraCyte encapsulation devices, implanted subcutaneously, and rats were monitored for diabetes.

RESULTS

In untreated control rats, plasma GLP-1 levels were 34.5-39.5 pmol/l, whereas, in treated rats, plasma levels were elevated, in the range 90-250.4 pmol/l. Hypoglycemia was not detected and this was anticipated from the glucose-regulated action of GLP-1. Diabetes onset (mean + or - SEM) in untreated rats occurred at 56.5 + or - 0.6 days (n = 6) and, in GLP-1-treated rats, was delayed until 76.4 + or - 3.3 days (n = 5) (p < 0.001). After disease onset, untreated control rats showed a rapid weight loss and elevated blood glucose (>650 mg/dl) and did not survive beyond 11 days. At 5 days after diabetes onset, insulin-secreting islets were absent in untreated rats. By contrast, treated rats maintained weight for up to 143 days of age and showed insulin-secreting beta cells.

CONCLUSIONS

Sustained GLP-1 expression delivered by encapsulated cells before diabetes onset in BB rats showed an improved clinical outcome, suggesting the potential for treating patients using long lasting GLP-1 analogs.

Approaches for imaging islets: recent advances and future prospects

The establishment of improved technologies for imaging of the pancreas is a key element in addressing several aspects of diabetes pathogenesis. In this respect, the development of a protocol that allows for non-invasive scoring of human islets, or islet beta-cells, is of particular importance. The development of such a technology would have profound impact on both clinical and experimental medicine, ranging from early diagnosis of diabetes to the evaluation of therapeutic regimes. Another important task is the development of modalities for high-resolution imaging of experimental animal models for diabetes. Rodent models for diabetes research have for decades been instrumental to the diabetes research community. The ability to image, and to accurately quantify, key players of diabetogenic processes with molecular specificity will be of great importance for elucidating mechanistic aspects of the disease. This chapter aims to overview current progress within these research areas.

In vivo imaging of murine endocrine islets of Langerhans with extended-focus optical coherence microscopy

AIMS/HYPOTHESIS

Structural and functional imaging of the islets of Langerhans and the insulin-secreting beta cells represents a significant challenge and a long-lasting objective in diabetes research. In vivo microscopy offers a valuable insight into beta cell function but has severe limitations regarding sample labelling, imaging speed and depth, and was primarily performed on isolated islets lacking native innervations and vascularisation. This article introduces extended-focus optical coherence microscopy (xfOCM) to image murine pancreatic islets in their natural environment in situ, i.e. in vivo and in a label-free condition.

METHODS

Ex vivo measurements on excised pancreases were performed and validated by standard immunohistochemistry to investigate the structures that can be observed with xfOCM. The influence of streptozotocin on the signature of the islets was investigated in a second step. Finally, xfOCM was applied to make measurements of the murine pancreas in situ and in vivo.

RESULTS

xfOCM circumvents the fundamental physical limit that trades lateral resolution for depth of field, and achieves fast volumetric imaging with high resolution in all three dimensions. It allows label-free visualisation of pancreatic lobules, ducts, blood vessels and individual islets of Langerhans ex vivo and in vivo, and detects streptozotocin-induced islet destruction.

CONCLUSIONS/INTERPRETATION

Our results demonstrate the potential value of xfOCM in high-resolution in vivo studies to assess islet structure and function in animal models of diabetes, aiming towards its use in longitudinal studies of diabetes progression and islet transplants.

Mouse Models for Type 1 Diabetes

Our understanding of the genetics, aetiology and pathogenesis of Type 1 Diabetes (T1D) was propelled by the discovery of animal models of T1D in the late 1970s and early 1980s, particularly the non-obese diabetic (NOD) mouse. Since then, transgenic and gene-targeting technologies allowed the generation of many models with reduced genetic and pathogenic complexity. These models allowed researchers to zoom in on specific aspects of this complex disease. In this review, we provide an overview of currently available mouse models for T1D.

Noninvasive high-resolution in vivo imaging of cell biology in the anterior chamber of the mouse eye

There is clearly a demand for an experimental platform that enables cell biology to be studied in intact vascularized and innervated tissue in vivo. This platform should allow observations of cells noninvasively and longitudinally at single-cell resolution. For this purpose, we use the anterior chamber of the mouse eye in combination with laser scanning microscopy (LSM). Tissue transplanted to the anterior chamber of the eye is rapidly vascularized, innervated and regains function. After transplantation, LSM through the cornea allows repetitive and noninvasive in vivo imaging at cellular resolution. Morphology, vascularization, cell function and cell survival are monitored longitudinally using fluorescent proteins and dyes. We have used this system to study pancreatic islets, but the platform can easily be adapted for studying a variety of tissues and additional biological parameters. Transplantation to the anterior chamber of the eye takes 25 min, and in vivo imaging 1-5 h, depending on the features monitored.

Imaging the pancreas: from ex vivo to non-invasive technology

While many recently published reviews have covered non-invasive nuclear imaging techniques, the aim of this review is to focus on current developments in optical imaging technologies for investigating the pancreas. Several of these modalities are being developed into non-invasive, real-time monitoring routines for pancreatic diseases. However, they also provide pre-clinical ex vivo and/or intravital tools for three-dimensional quantitative assessments of cellular and molecular events, with levels of specificity and resolution difficult to achieve with other currently available modalities.