Current status of imaging pancreatic islets

Quantitative Impact of Plasma Clearance and Down-regulation on GLP-1 Receptor Molecular Imaging

PURPOSE

Quantitative molecular imaging of beta cell mass (BCM) would enable early detection and treatment monitoring of type 1 diabetes. The glucagon-like peptide-1 (GLP-1) receptor is an attractive target due to its beta cell specificity and cell surface location. We quantitatively investigated the impact of plasma clearance and receptor internalization on targeting efficiency in healthy B6 mice.

PROCEDURES

Four exenatide-based probes were synthesized that varied in molecular weight, binding affinity, and plasma clearance. The GLP-1 receptor internalization rate and in vivo receptor expression were quantified.

RESULTS

Receptor internalization (54,000 receptors/cell in vivo) decreased significantly within minutes, reducing the benefit of a slower-clearing agent. The multimers and albumin binding probes had higher kidney and liver uptake, respectively.

CONCLUSIONS

Slow plasma clearance is beneficial for GLP-1 receptor peptide therapeutics. However, for exendin-based imaging of islets, down-regulation of the GLP-1 receptor and non-specific background uptake result in a higher target-to-background ratio for fast-clearing agents.

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.

Non-invasive bioluminescence imaging of β-cell function in obese-hyperglycemic [ob/ob] mice

Background

Type 2 diabetes results from failure of the β-cells to compensate for increased insulin demand due to abnormal levels of metabolic factors. The ob/ob(lep-/-) mouse has been extensively studied as an animal model of type 2 diabetes. Previous studies have shown a correlation between β-cell function and bioluminescent imaging in lean genetically engineered mice. The ability to noninvasively monitor β-cell function in ob/ob mice could provide new information on β-cell regulation in type 2 diabetes.

Methods

To create the B6 Albino ob/ob MIP-luc mice (ob/ob-luc), the ob/ob mouse was crossed with the CD1 MIP-luc mouse. All mice were backcrossed over multiple generations to ensure the genetic background of the transgenic mice was over 96% similar to the background of the original ob/ob mouse. Animal weight, blood glucose levels, insulin in plasma, and in vivo bioluminescence (BLI) were monitored weekly or biweekly for up to 70 weeks of age. BL imaging was performed using IVIS Spectrum (Perkin Elmer) and calculated by integrating the bioluminescence signal between 5 and 10 min after i.v. injection of D-luciferin. Insulin immunohistochemistry determined islet beta cell count and insulin secretion assay determined islet insulin function.

Results

There were significant increases in BLI and insulin levels as the ob/ob-luc mice aged while glucose levels gradually decreased. Ob/ob-luc were sacrificed at different time points to determine ex vivo BLI, islet function and total β-cell numbers using a cell counting training algorithm developed for the Vectra image analysis system (Perkin Elmer). The number of β-cells increased as the mice aged and all three ex vivo measurements correlated with BLI.

Conclusions

The ob/ob-luc mice can serve as a model of metabolic stress, similar to human type 2 diabetes using BLI as a surrogate marker for β-cell function.

Multimodal imaging of pancreatic beta cells in vivo by targeting transmembrane protein 27 (TMEM27)

AIMS/HYPOTHESIS

Non-invasive diagnostic tools specific for pancreatic beta cells will have a profound impact on our understanding of the pathophysiology of metabolic diseases such as diabetes. The objective of this study was to use molecular imaging probes specifically targeting beta cells on human samples and animal models using state-of-the-art imaging modalities (fluorescence and PET) with preclinical and clinical perspective.

METHODS

We generated a monoclonal antibody, 8/9-mAb, targeting transmembrane protein 27 (TMEM27; a surface N-glycoprotein that is highly expressed on beta cells), compared its expression in human and mouse pancreas, and demonstrated beta cell-specific binding in both. In vivo imaging was performed in mice with subcutaneous insulinomas overexpressing the human TMEM27 gene, or transgenic mice with beta cell-specific hTMEM27 expression under the control of rat insulin promoter (RIP-hTMEM27-tg), using fluorescence and radioactively labelled antibody, followed by tissue ex vivo analysis and fluorescence microscopy.

RESULTS

Fluorescently labelled 8/9-mAb showed beta cell-specific staining on human and mouse pancreatic sections. Real-time PCR on islet cDNA indicated about tenfold higher expression of hTMEM27 in RIP-hTMEM27-tg mice than in humans. In vivo fluorescence and PET imaging in nude mice with insulinoma xenografts expressing hTMEM27 showed high 8/9-mAb uptake in tumours after 72 h. Antibody homing was also observed in beta cells of RIP-hTMEM27-tg mice by in vivo fluorescence imaging. Ex vivo analysis of intact pancreas and fluorescence microscopy in beta cells confirmed these findings.

CONCLUSIONS/INTERPRETATION

hTMEM27 constitutes an attractive target for in vivo visualisation of pancreatic beta cells. Studies in mouse insulinoma models and mice expressing hTMEM27 demonstrate the feasibility of beta cell-targeted in vivo imaging, which is attractive for preclinical investigations and holds potential in clinical diagnostics.

High-resolution magnetic resonance imaging quantitatively detects individual pancreatic islets

OBJECTIVE

We studied whether manganese-enhanced high-field magnetic resonance (MR) imaging (MEHFMRI) could quantitatively detect individual islets in situ and in vivo and evaluate changes in a model of experimental diabetes.

RESEARCH DESIGN AND METHODS

Whole pancreata from untreated (n = 3), MnCl(2) and glucose-injected mice (n = 6), and mice injected with either streptozotocin (STZ; n = 4) or citrate buffer (n = 4) were imaged ex vivo for unambiguous evaluation of islets. Exteriorized pancreata of MnCl(2) and glucose-injected mice (n = 6) were imaged in vivo to directly visualize the gland and minimize movements. In all cases, MR images were acquired in a 14.1 Tesla scanner and correlated with the corresponding (immuno)histological sections.

RESULTS

In ex vivo experiments, MEHFMRI distinguished different pancreatic tissues and evaluated the relative abundance of islets in the pancreata of normoglycemic mice. MEHFMRI also detected a significant decrease in the numerical and volume density of islets in STZ-injected mice. However, in the latter measurements the loss of β-cells was undervalued under the conditions tested. The experiments on the externalized pancreata confirmed that MEHFMRI could visualize native individual islets in living, anesthetized mice.

CONCLUSIONS

Data show that MEHFMRI quantitatively visualizes individual islets in the intact mouse pancreas, both ex vivo and in vivo.

In vitro phage display in a rat beta cell line: a simple approach for the generation of a single-chain antibody targeting a novel beta cell-specific epitope

AIMS/HYPOTHESIS

The aim of the present study was to evaluate in vitro phage display in a beta cell line as a novel strategy for the isolation of beta cell-specific agents/biomarkers.

METHODS

A single-chain antibody (SCA) library was pre-incubated with AR42J cells in order to eliminate SCAs with exocrine binding properties. It was then panned against INS-1 cells to select beta cell-targeted antibodies.

RESULTS

By these means, we isolated a novel antibody, SCA B5, that binds rapidly (6.0 min) and with a 450-fold higher specificity to beta cells relative to exocrine cells. We estimated for SCA B5 a binding affinity in the low micromol/l range and 858 binding sites per beta cell. Confocal microscopy showed binding to the beta cell surface and confirmed subsequent internalisation. Moreover, staining of rat and human pancreatic tissue sections with SCA B5 suggests that the target epitope is presented in pancreatic beta cells of different origins. Infrared imaging revealed that labelling of beta cells with tracer SCA B5 is strictly dependent on beta cell mass. With competition assays we excluded insulin, glutamate decarboxylase, C-peptide and islet amyloid polypeptide as SCA B5 targets. In accordance with these predictions, SCA B5 homed in vivo highly selectively to normal beta cells and dysfunctional beta cells of diabetic rats. Moreover, accumulation of radioactively labelled SCA B5 in the pancreas was reduced by 80% after pre-injection with unlabelled SCA B5, thereby confirming the specific uptake in the pancreas.

CONCLUSIONS/INTERPRETATION

We report a simple strategy for the generation of an SCA targeting a novel beta cell-specific epitope.

Bioluminescence imaging in mouse models quantifies beta cell mass in the pancreas and after islet transplantation

PURPOSE

We developed a mouse model that enables non-invasive assessment of changes in beta cell mass.

PROCEDURES

We generated a transgenic mouse expressing luciferase under control of the mouse insulin I promoter [mouse insulin promoter-luciferase-Vanderbilt University (MIP-Luc-VU)] and characterized this model in mice with increased or decreased beta cell mass and after islet transplantation.

RESULTS

Streptozotocin-induced, diabetic MIP-Luc-VU mice had a progressive decline in bioluminescence that correlated with a decrease in beta cell mass. MIP-Luc-VU animals fed a high-fat diet displayed a progressive increase in bioluminescence that reflected an increase in beta cell mass. MIP-Luc-VU islets transplanted beneath the renal capsule or into the liver emitted bioluminescence proportional to the number of islets transplanted and could be imaged for more than a year.

CONCLUSIONS

Bioluminescence in the MIP-Luc-VU mouse model is proportional to beta cell mass in the setting of increased and decreased beta cell mass and after transplantation.

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.

Pancreatic glucose uptake in vivo in men with newly diagnosed type 1 diabetes

CONTEXT

Due to the restricted accessibility of pancreatic tissue in living man, direct analysis of the events preceding development of autoimmune changes in the pancreas has been problematic. In vivo imaging of insulitis might markedly increase understanding of the events and timing of the events that are necessary for the progression toward overt type 1 diabetes.

DESIGN

To evaluate possibilities to visualize insulitis in man in vivo with positron emission tomography, we studied 12 male patients (age 26 +/- 7 yr) with newly diagnosed type 1 diabetes (duration range 0-7 months) and nine age- and sex-matched healthy controls after an overnight fast using 2-[(18)F]fluoro-2-deoxy-D-glucose and [(11)C]methionine. For definition of the regions of interest, pancreas was localized with magnetic resonance imaging or computed tomography-positron emission tomography.

RESULTS

Glucose uptake to the pancreas was markedly higher in the patients with type 1 diabetes than in the healthy controls (22.9 +/- 6.4 vs. 17.8 +/- 6.0 micromol/kg.min, P = 0.039). Glucose uptake to the pancreas of the patients was inversely associated with the duration of diabetes (r = -0.58; P = 0.024), so that in patients with newly diagnosed type 1 diabetes, glucose uptake was higher than in the healthy controls or patients with long duration of diabetes. Methionine uptake to the pancreas of the patients was similar as in the controls (3.7 +/- 1.9 vs. 4.6 +/- 2.4 micromol/kg.min, P = 0.21).

CONCLUSIONS

In patients with type 1 diabetes, glucose uptake to the pancreas is enhanced at or soon after the time of diagnosis.