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

Application and trend of bioluminescence imaging in metabolic syndrome research

Bioluminescence imaging is a non-invasive technology used to visualize physiological processes in animals and is useful for studying the dynamics of metabolic syndrome. Metabolic syndrome is a broad spectrum of diseases which are rapidly increasing in prevalence, and is closely associated with obesity, type 2 diabetes, nonalcoholic fatty liver disease, and circadian rhythm disorder. To better serve metabolic syndrome research, researchers have established a variety of animal models expressing luciferase, while also committing to finding more suitable luciferase promoters and developing more efficient luciferase-luciferin systems. In this review, we systematically summarize the applications of different models for bioluminescence imaging in the study of metabolic syndrome.

Critical Considerations in Bioluminescence Imaging of Transplanted Islets: Dynamic Signal Change in Early Posttransplant Phase and Signal Absorption by Tissues

OBJECTIVES

In pancreatic islet transplantation studies, bioluminescence imaging enables quantitative and noninvasive tracking of graft survival. Amid the recent heightened interest in extrahepatic sites for islet and stem cell-derived beta-like cell transplantations, proper understanding the nature of bioluminescence imaging in these sites is important.

METHODS

Islets isolated from Firefly rats ubiquitously expressing luciferase reporter gene in Lewis rats were transplanted into subcutaneous or kidney capsule sites of wild-type Lewis rats or immunodeficient mice. Posttransplant changes of bioluminescence signal curves and absorption of bioluminescence signal in transplantation sites were examined.

RESULTS

The bioluminescence signal curve dynamically changed in the early posttransplantation phase; the signal was low within the first 5 days after transplantation. A substantial amount of bioluminescence signal was absorbed by tissues surrounding islet grafts, correlating to the depth of the transplanted site from the skin surface. Grafts in kidney capsules were harder to image than those in the subcutaneous site. Within the kidney capsule, locations that minimized depth from the skin surface improved the graft detectability.

CONCLUSIONS

Posttransplant phase and graft location/depth critically impact the bioluminescence images captured in islet transplantation studies. Understanding these parameters is critical for reducing experimental biases and proper interpretation of data.

Imaging evaluation of the pancreas in diabetic patients

Diabetes mellitus (DM) is becoming a global epidemic and its diagnosis and monitoring are based on laboratory testing which sometimes have limitations. The pancreas plays a key role in metabolism and is involved in the pathogenesis of DM. It has long been known through cadaver biopsies that pancreas volume is decreased in patients with DM. With the development of different imaging modalities over the last two decades, many studies have attempted to determine whether there other changes occurred in the pancreas of diabetic patients. This review summarizes current knowledge about the use of different imaging approaches (such as CT, MR, and US) and radiomics for exploring pancreatic changes in diabetic patients. Imaging studies are expected to produce reliable information regarding DM, and radiomics could provide increasingly valuable information to identify some undetectable features and help diagnose and predict the occurrence of diabetes through pancreas imaging.

Imaging in experimental models of diabetes

Translational medicine, experimental medicine and experimental animal models, in particular mice and rats, represent a multidisciplinary field that has made it possible to achieve, in the last decades, important scientific progress. In this review, we have summarized the most frequently used imaging animal models, such as ultrasound (US), micro-CT, MRI and the optical imaging methods, and their main implications in diagnostic and therapeutic fields, with a particular focus on diabetes mellitus, a multifactorial disease extremely widespread among the general population.

Early-Phase Luciferase Signals of Islet Grafts Predicts Successful Subcutaneous Site Transplantation in Rats

PURPOSE

The transplantation of pancreatic islets is a promising cell replacement therapy for type 1 diabetes. Subcutaneous islet transplantation is currently under investigation as a means to circumvent problems associated with standard intra-hepatic islet transplantation. As modifications are being developed to improve the efficacy of subcutaneous islet transplantation, it is important to have robust methods to assess engraftment. Experimentally, ATP-dependent bioluminescence imaging using luciferase reporter genes has been effective for non-invasively tracking engraftment. However, it was heretofore unknown if the bioluminescence of subcutaneously transplanted luciferase-expressing islet grafts correlates with diabetes reversal, a primary outcome of transplantation.

PROCEDURES

A retrospective analysis was conducted using data obtained from subcutaneous islet transplantations in Lewis rats. The analysis included transplantations from our laboratory in which islet donors were transgenic rats ubiquitously expressing luciferase and recipients were wild type, streptozotocin-induced diabetic rats. Data from 79 bioluminescence scans were obtained from 27 islet transplantations during the post-transplant observation period (up to 6 weeks). The bioluminescence intensity of the subcutaneously transplanted grafts, captured after the intravenous administration of luciferin, was correlated with diabetes reversal.

RESULTS

After subcutaneous transplantation, islet bioluminescence decreased over time, dropping > 50 % from 1 to 3 weeks post-transplant. Bioluminescence intensity in the early post-transplant phase (1-2 weeks) correlated with the subsequent reversal of diabetes; based on optimized bioluminescence cutoff values, the bioluminescence intensity of islets at 1 and 2 weeks predicted successful transplantations. However, intensity in the late post-transplant phase (≥ 4 weeks) did not reflect transplantation outcomes.

CONCLUSIONS

Early-phase bioluminescence imaging of luciferase-expressing islets could serve as a useful tool to predict the success of subcutaneous islet transplantations by preceding changes in glucose homeostasis.

Bioluminescence Imaging In Vivo Confirms the Viability of Pancreatic Islets Transplanted into the Greater Omentum

PURPOSE

The liver is the most widely used site for pancreatic islet transplantation. However, several site-specific limitations impair functional success, with instant blood-mediated inflammatory reaction being the most important. The aim of this study was to develop a preclinical model for placement of the islet graft into a highly vascularized omental flap using a fibrin gel. For this purpose, we tested islet viability by bioluminescence imaging (BLI).

PROCEDURES

Pancreatic islets were isolated from luciferase-positive and luciferase-negative rats, mixed at a 1:1 ratio, placed into a plasma-thrombin bioscaffold, and transplanted in standard (10 pancreatic islets/g wt; n = 10) and marginal (4 pancreatic islets/g wt; n = 7) numbers into the omentums of syngeneic diabetic animals. For the control, 4 pancreatic islets/g were transplanted into the liver using the standard procedure (n = 7). Graft viability was tested by bioluminescence at days 14, 30, 60, and 90 post transplant. Glucose levels, intravenous glucose tolerance, and serum C-peptide were assessed regularly.

RESULTS

Nonfasting glucose levels < 10 mmol/l were restored in all animals. While islet viability in the omentum was clearly detected by stable luminescence signals throughout the whole study period, no signals were detected from islets transplanted into the liver. The bioluminescence signals were highly correlated with stimulated C-peptide levels detected at 80 days post transplant. Glucose tolerance did not differ among the 3 groups.

CONCLUSIONS

We successfully tested a preclinical model of islet transplantation into the greater omentum using a biocompatible scaffold made from autologous plasma and human thrombin. Both standard and marginal pancreatic islet numbers in a gel-form bioscaffold placed in the omentum restored glucose homeostasis in recipients with diabetes. Bioluminescence was shown promising as a direct proof of islet viability.

Intermittent normobaric oxygen inhalation enhances subcutaneous prevascularization for cell transplantation

PURPOSE

Subcutaneous tissue is a promising site for cell transplantation; advantages include minimally invasive procedures and easy post-transplant monitoring. However, limited vascularity is the major known challenge. To address this challenge, a prevascularized graft bed is prepared in recipients. We aimed to establish an improved, clinically applicable approach to promote prevascularization of the subcutaneous graft bed prior to cell transplantation.

METHODS

We applied a conventional prevascularization approach by subcutaneously implanting nylon discs into the backs of Lewis rats. After disc implantation, we treated rats with or without intermittent normobaric 100% oxygen inhalation (1 h, twice a day, for consecutive 7 days). We used histology to compare vascular density between the oxygen-treated or control groups. To assess the functional effects of prevascularization, we transplanted three hundred islets isolated from luciferase-transgenic Lewis rats into the oxygen-treated or control wild type Lewis recipients, then used bioluminescence imaging to track engraftment for 4 weeks.

RESULTS

Oxygen treatment significantly augmented prevascularization in the subcutaneous site compared to controls. Islet transplantation into prevascularized graft beds demonstrated significant improvement in engraftment efficiency in oxygen-treated recipients compared to controls at 2-4 weeks post-transplantation.

CONCLUSION

Combining intermittent normobaric 100% oxygen inhalation with a conventional vascularization approach promotes a functional vasculature within a week. A simple approach using normobaric oxygen has the potential for translation into clinical application in subcutaneous site cell transplantations.

Tools for Bioimaging Pancreatic β Cells in Diabetes

When diabetes is diagnosed, the majority of insulin-secreting pancreatic β cells are already dysfunctional or destroyed. This β cell dysfunction/destruction usually takes place over many years, making timely detection and clinical intervention difficult. For this reason, there is immense interest in developing tools to bioimage β cell mass and/or function noninvasively to facilitate early diagnosis of diabetes as well as to assist the development of novel antidiabetic therapies. Recent years have brought significant progress in β cell imaging that is now inching towards clinical applicability. We explore here the need to bioimage human β cells noninvasively in various types of diabetes, and we discuss current and emerging tools for bioimaging β cells. Further developments in this field are expected to facilitate β cell imaging in diabetes.

The oncoprotein HBXIP competitively binds KEAP1 to activate NRF2 and enhance breast cancer cell growth and metastasis

The nuclear factor E2-related factor 2 (NRF2)-Kelch-like ECH-associated protein 1 (KEAP1) signaling cascades is a key transcriptional pathway governing cellular oxidative stress and tumor development. Mammalian hepatitis B X-interacting protein (HBXIP) has critical roles in modulating cancer malignance and tumor progression. However, whether HBXIP interacts with KEAP1 and NRF2 is unclear. Here, we found that HBXIP can effectually compete with NRF2 for binding with KEAP1 protein via its highly conserved GLNLG motif. The HBXIP-mediated reduction in NRF2-KEAP1 complexes promotes NRF2 accumulation and nuclear entry, which facilities the activation of antioxidant response element (ARE)-dependent signaling cascades, thereby reducing the accumulation of endogenous cellular reactive oxygen species (ROS). We also found a strong positive correlation between HBXIP expression and NRF2 expression in breast cancer cells, tissue microarrays and clinical breast cancer tissues. Furthermore, this positive correlation was further confirmed via analysis of 1905 clinical cases of breast carcinoma provided by the cancer genomics database cBioPortal. Strikingly, disrupting the HBXIP-KEAP1 axis via mutating the GLNLG motif of HBXIP leads to potent inhibition of the malignancy of breast carcinoma both in vivo and in vitro. Our findings broaden our understanding of HBXIP as a modulation factor of cellular oxidative stress and address a novel regulatory mechanism governing redox homeostasis and the progression of breast carcinoma.

Tri-modal In vivo Imaging of Pancreatic Islets Transplanted Subcutaneously in Mice

PURPOSE

Transplantation of pancreatic islets (PIs) is a promising therapeutic approach for type 1 diabetes. The main obstacle for this strategy is that the outcome of islet engraftment depends on the engraftment site. It was our aim to develop a strategy for using non-invasive imaging techniques to assess the location and fate of transplanted PIs longitudinally in vivo.

PROCEDURES

In order to overcome the limitations of individual imaging techniques and cross-validate findings by different modalities, we have combined fluorine magnetic resonance imaging (F-19 MRI), fluorescence imaging (FLI), and bioluminescent imaging (BLI) for studying subcutaneously transplanted PIs and beta cell-like cells (INS-1E cell line) in vivo. We optimized the transduction (using lentiviral vectors) and labeling procedures (using perfluoro crown ether nanoparticles with a fluorescence dye) for PIs and INS-1E cell imaging.

RESULTS

The feasibility of using the proposed imaging methods for PI assessment was demonstrated both in vitro and in vivo. Our data suggested that F-19 MRI is suitable for high-resolution localization of transplanted cells and PIs; FLI is essential for confirmation of contrast localization by histology; and BLI is a reliable method to assess cell viability and survival after transplantation. No significant side effects on cell viability and function have been observed.

CONCLUSIONS

The proposed tri-modal imaging platform is a valuable approach for the assessment of engrafted PIs in vivo. It is potentially suitable for comparing different transplantation sites and evaluating novel strategies for improving PI transplantation technique in the future.

Molecular imaging of β-cells: diabetes and beyond

Since diabetes is becoming a global epidemic, there is a great need to develop early β-cell specific diagnostic techniques for this disorder. There are two types of diabetes (i.e., type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM)). In T1DM, the destruction of pancreatic β-cells leads to reduced insulin production or even absolute insulin deficiency, which consequently results in hyperglycemia. Actually, a central issue in the pathophysiology of all types of diabetes is the relative reduction of β-cell mass (BCM) and/or impairment of the function of individual β-cells. In the past two decades, scientists have been trying to develop imaging techniques for noninvasive measurement of the viability and mass of pancreatic β-cells. Despite intense scientific efforts, only two tracers for positron emission tomography (PET) and one contrast agent for magnetic resonance (MR) imaging are currently under clinical evaluation. β-cell specific imaging probes may also allow us to precisely and specifically visualize transplanted β-cells and to improve transplantation outcomes, as transplantation of pancreatic islets has shown promise in treating T1DM. In addition, some of these probes can be applied to the preoperative detection of hidden insulinomas as well. In the present review, we primarily summarize potential tracers under development for imaging β-cells with a focus on tracers for PET, SPECT, MRI, and optical imaging. We will discuss the advantages and limitations of the various imaging probes and extend an outlook on future developments in the field.

NF-κB is weakly activated in the NOD mouse model of type 1 diabetes

Type 1 diabetes is an autoimmune disease characterised by selective destruction of pancreatic beta cells by the immune system. The transcription factor nuclear factor-kappa B (NF-κB) regulates innate and adaptive immune responses. Using gene targeting and in vitro analysis of pancreatic islets and immune cells, NF-κB activation has been implicated in type 1 diabetes development. Here we use a non-obese diabetic (NOD) mouse model that expresses a luciferase reporter of transcriptionally active NF-κB to determine its activation in vivo during development of diabetes. Increased luciferase activity was readily detected upon treatment with Toll-like receptor ligands in vitro and in vivo, indicating activation of NF-κB. However, activated NF-κB was detectable at low levels above background in unmanipulated NOD mice, but did not vary with age, despite the progression of inflammatory infiltration in islets over time. NF-κB was highly activated in an accelerated model of type 1 diabetes that requires CD4⁺ T cells and inflammatory macrophages. These data shed light on the nature of the inflammatory response in the development of type 1 diabetes.

Isl1β Overexpression With Key β Cell Transcription Factors Enhances Glucose-Responsive Hepatic Insulin Production and Secretion

Adenoviral gene transfer of key β cell developmental regulators including Pdx1, Neurod1, and Mafa (PDA) has been reported to generate insulin-producing cells in the liver. However, PDA insulin secretion is transient and glucose unresponsive. Here, we report that an additional β cell developmental regulator, insulin gene enhancer binding protein splicing variant (Isl1β), improved insulin production and glucose-responsive secretion in PDA mice. Microarray gene expression analysis suggested that adenoviral PDA transfer required an additional element for mature β cell generation, such as Isl1 and Elf3 in the liver. In vitro promoter analysis indicated that splicing variant Isl1, or Isl1β, is an important factor for transcriptional activity of the insulin gene. In vivo bioluminescence monitoring using insulin promoter-luciferase transgenic mice verified that adenoviral PDA + Isl1β transfer produced highly intense luminescence from the liver, which peaked at day 7 and persisted for more than 10 days. Using insulin promoter-GFP transgenic mice, we further confirmed that Isl1β supplementation to PDA augmented insulin-producing cells in the liver, insulin production and secretion, and β cell‒related genes. Finally, the PDA + Isl1β combination ameliorated hyperglycemia in diabetic mice for 28 days and enhanced glucose tolerance and responsiveness. Thus, our results suggest that Isl1β is a key additional transcriptional factor for advancing the generation of insulin-producing cells in the liver in combination with PDA.

Comparison of different compressed sensing algorithms for low SNR 19 F MRI applications-Imaging of transplanted pancreatic islets and cells labeled with perfluorocarbons

Transplantation of pancreatic islets is a possible treatment option for patients suffering from Type I diabetes. In vivo imaging of transplanted islets is important for assessment of the transplantation site and islet distribution. Thanks to its high specificity, the absence of intrinsic background signal in tissue and its potential for quantification, ¹⁹ F MRI is a promising technique for monitoring the fate of transplanted islets in vivo. In order to overcome the inherent low sensitivity of ¹⁹ F MRI, leading to long acquisition times with low signal-to-noise ratio (SNR), compressed sensing (CS) techniques are a valuable option. We have validated and compared different CS algorithms for acceleration of ¹⁹ F MRI acquisition in a low SNR regime using pancreatic islets labeled with perfluorocarbons both in vitro and in vivo. Using offline simulation on both in vitro and in vivo low SNR fully sampled ¹⁹ F MRI datasets of labeled islets, we have shown that CS is effective in reducing the image acquisition time by a factor of three to four without seriously affecting SNR, regardless of the particular algorithms used in this study, with the exception of CoSaMP. Using CS, signals can be detected that might have been missed by conventional ¹⁹ F MRI. Among different algorithms (SPARSEMRI, OMMP, IRWL1, Two-level and CoSAMP), the two-level l₁ method has shown the best performance if computational time is taken into account. We have demonstrated in this study that different existing CS algorithms can be used effectively for low SNR ¹⁹ F MRI. An up to fourfold gain in SNR/scan time could be used either to reduce the scan time, which is beneficial for clinical and translational applications, or to increase the number of averages, to potentially detect otherwise undetected signal when compared with conventional ¹⁹ F MRI acquisitions. Potential applications in the field of cell therapy have been demonstrated.

Reconstruction Method for In Vivo Bioluminescence Tomography Based on the Split Bregman Iterative and Surrogate Functions

PURPOSE

Bioluminescence tomography (BLT) can provide in vivo three-dimensional (3D) images for quantitative analysis of biological processes in preclinical small animal studies, which is superior than the conventional planar bioluminescence imaging. However, to reconstruct light sources under the skin in 3D with desirable accuracy and efficiency, BLT has to face the ill-posed and ill-conditioned inverse problem. In this paper, we developed a new method for BLT reconstruction, which utilized the mathematical strategies of the split Bregman iterative and surrogate functions (SBISF) method.

PROCEDURES

The proposed method considered the sparsity characteristic of the reconstructed sources. Thus, the sparsity itself was regarded as a kind of a priori information, and the sparse regularization is incorporated, which can accurately locate the position of the sources. Numerical simulation experiments of multisource cases with comparative analyses were performed to evaluate the performance of the proposed method. Then, a bead-implanted mouse and a breast cancer xenograft mouse model were employed to validate the feasibility of this method in in vivo experiments.

RESULTS

The results of both simulation and in vivo experiments indicated that comparing with the L1-norm iteration shrinkage method and non-monotone spectral projected gradient pursuit method, the proposed SBISF method provided the smallest position error with the least amount of time consumption.

CONCLUSIONS

The SBISF method is able to achieve high accuracy and high efficiency in BLT reconstruction and hold great potential for making BLT more practical in small animal studies.

Characterization of 5-(2- 18F-fluoroethoxy)-L-tryptophan for PET imaging of the pancreas

Purpose: In diabetes, pancreatic beta cell mass declines significantly prior to onset of fasting hyperglycemia. This decline may be due to endoplasmic reticulum (ER) stress, and the system L amino acid transporter LAT1 may be a biomarker of this process. In this study, we used 5-(2- ¹⁸F-fluoroethoxy)-L-tryptophan ( ¹⁸F-L-FEHTP) to target LAT1 as a potential biomarker of beta cell function in diabetes.

Procedures: Uptake of ¹⁸F-L-FEHTP was determined in wild-type C57BL/6 mice by ex vivo biodistribution. Both dynamic and static positron emission tomography (PET) images were acquired in wild-type and Akita mice, a model of ER stress-induced diabetes, as well as in mice treated with streptozotocin (STZ). LAT1 expression in both groups of mice was evaluated by immunofluorescence microscopy.

Results: Uptake of ¹⁸F-L-FEHTP was highest in the pancreas, and static PET images showed highly specific pancreatic signal. Time-activity curves showed significantly reduced ¹⁸F-L-FEHTP uptake in Akita mice, and LAT1 expression was also reduced. However, mice treated with STZ, in which beta cell mass was reduced by 62%, showed no differences in ¹⁸F-L-FEHTP uptake in the pancreas, and there was no significant correlation of ¹⁸F-L-FEHTP uptake with beta cell mass.

Conclusions: ¹⁸F-L-FEHTP is highly specific for the pancreas with little background uptake in kidney or liver. We were able to detect changes in LAT1 in a mouse model of diabetes, but these changes did not correlate with beta cell function or mass. Therefore, ¹⁸F-L-FEHTP PET is not a suitable method for the noninvasive imaging of changes in beta cell function during the progression of diabetes.

Characterization of 5-(2- 18F-fluoroethoxy)-L-tryptophan for PET imaging of the pancreas

Purpose: In diabetes, pancreatic beta cell mass declines significantly prior to onset of fasting hyperglycemia. This decline may be due to endoplasmic reticulum (ER) stress, and the system L amino acid transporter LAT1 may be a biomarker of this process. In this study, we used 5-(2- 18F-fluoroethoxy)-L-tryptophan ( 18F-L-FEHTP) to target LAT1 as a potential biomarker of beta cell function in diabetes. Procedures: Uptake of 18F-L-FEHTP was determined in wild-type C57BL/6 mice by ex vivo biodistribution. Both dynamic and static positron emission tomography (PET) images were acquired in wild-type and Akita mice, a model of ER stress-induced diabetes, as well as in mice treated with streptozotocin (STZ). LAT1 expression in both groups of mice was evaluated by immunofluorescence microscopy. Results: Uptake of 18F-L-FEHTP was highest in the pancreas, and static PET images showed highly specific pancreatic signal. Time-activity curves showed significantly reduced 18F-L-FEHTP uptake in Akita mice, and LAT1 expression was also reduced. However, mice treated with STZ, in which beta cell mass was reduced by 62%, showed no differences in 18F-L-FEHTP uptake in the pancreas, and there was no significant correlation of 18F-L-FEHTP uptake with beta cell mass. Conclusions:18F-L-FEHTP is highly specific for the pancreas with little background uptake in kidney or liver. We were able to detect changes in LAT1 in a mouse model of diabetes, but these changes did not correlate with beta cell function or mass. Therefore, 18F-L-FEHTP PET is not a suitable method for the noninvasive imaging of changes in beta cell function during the progression of diabetes.

Deletion of β-Arrestin2 in Mice Limited Pancreatic β-Cell Expansion under Metabolic Stress through Activation of the JNK Pathway

β-Arrestin2 (βarr2) is an adaptor protein that interacts with numerous signaling molecules and regulates insulin sensitivity. We reported previously that βarr2 was abundantly expressed in mouse pancreatic β-cells, and loss of βarr2 leads to impairment of acute- and late-phase insulin secretion. In the present study, we examined the dynamic changes of β-cell mass in βarr2-deficient (βarr2^-/-) mice in vivo and explored the underlying mechanisms involved. βarr2^-/- mice with exclusively luciferase overexpression in β-cells were generated and fed a high-fat diet (HFD). β-Cell mass was determined by in vivo noninvasive bioluminescence imaging from 4 to 20 wks of age. Proliferation was measured by 5-bromo-2-deoxyuridine (BrdU) incorporation and fluorescence-activated cell sorter analysis. Quantitative real-time polymerase chain reaction (qRT-PCR) and immunoblotting were conducted for gene and protein expression. We found that β-cell mass was reduced dramatically in βarr2^-/- mice at 12 wks old compared with that of their respective HFD-fed controls. The percentage of BrdU- and Ki67-positive cells reduced in islets from βarr2^-/- mice. Exposure of βarr2^-/- islets to high levels of glucose and free fatty acids (FFAs) exacerbated cell death, which was associated with upregulation of the JNK pathway in these islets. Conversely, overexpression of βarr2 amplified β-cell proliferation with a concomitant increase in cyclinD2 expression and a decrease in p21 expression and protected β-cells from glucose- and FFA-induced cell death through JNK-activation inhibition. In conclusion, βarr2 plays roles in regulation of pancreatic β-cell mass through the modulation of cell cycle regulatory genes and the inhibition of JNK activation induced by glucolipotoxity, which implicates a role for βarr2 in the development of type 2 diabetes.

Dual-Reporter β-Cell-Specific Male Transgenic Rats for the Analysis of β-Cell Functional Mass and Enrichment by Flow Cytometry

Mouse β-cell-specific reporter lines have played a key role in diabetes research. Although the rat provides several advantages, its use has lagged behind the mouse due to the relative paucity of genetic models. In this report we describe the generation and characterization of transgenic rats expressing a Renilla luciferase (RLuc)-enhanced yellow fluorescent protein (YFP) fusion under control of a 9-kb genomic fragment from the rat ins2 gene (RIP7-RLuc-YFP). Analysis of RLuc luminescence and YFP fluorescence revealed that reporter expression is restricted to β-cells in the adult rat. Physiological characteristics including body weight, fat and lean mass, fasting and fed glucose levels, glucose and insulin tolerance, and β-cell mass were similar between two RIP7-RLuc-YFP lines and wild-type littermates. Glucose-induced insulin secretion in isolated islets was indistinguishable from controls in one of the lines, whereas surprisingly, insulin secretion was defective in the second line. Consequently, subsequent studies were limited to the former line. We asked whether transgene activity was responsive to glucose as shown previously for the ins2 gene. Exposing islets ex vivo to high glucose (16.7 mM) or in vivo infusion of glucose for 24 hours increased luciferase activity in islets, whereas the fraction of YFP-positive β-cells after glucose infusion was unchanged. Finally, we showed that fluorescence-activated cell sorting of YFP-positive islet cells can be used to enrich for β-cells. Overall, this transgenic line will enable for the first time the application of both fluorescence and bioluminescence/luminescence-based approaches for the study of rat β-cells.

Chemiluminescence Imaging of Superoxide Anion Detects Beta-Cell Function and Mass

Superoxide anion is produced during normal cellular respiration and plays key roles in cellular physiology with its dysregulation being associated with a variety of diseases. Superoxide anion is a short-lived molecule and, therefore, its homeostatic regulation and role in biology and disease requires dynamic quantification with fine temporal resolution. Here we validated coelenterazine as a reporter of intracellular superoxide anion concentration and used it as a dynamic measure both in vitro and in vivo. Chemiluminescence was dependent upon superoxide anion levels, including those produced during cellular respiration, and concentrations varied both kinetically and temporally in response to physiologically relevant fluctuations in glucose levels. In vivo imaging with coelenterazine revealed that beta cells of the pancreas have increased levels of superoxide anion, which acted as a measure of beta-cell function and mass and could predict the susceptibility of mice to diabetes mellitus. Glucose response and regulation are key elements of cellular physiology and organismal biology, and superoxide anion appears to play a fundamental and dynamic role in both of these processes.

Pancreatic β-cell imaging in humans: fiction or option?

Diabetes mellitus is a growing worldwide epidemic disease, currently affecting 1 in 12 adults. Treatment of disease complications typically consumes ∼10% of healthcare budgets in developed societies. Whilst immune-mediated destruction of insulin-secreting pancreatic β cells is responsible for Type 1 diabetes, both the loss and dysfunction of these cells underly the more prevalent Type 2 diabetes. The establishment of robust drug development programmes aimed at β-cell restoration is still hampered by the absence of means to measure β-cell mass prospectively in vivo, an approach which would provide new opportunities for understanding disease mechanisms and ultimately assigning personalized treatments. In the present review, we describe the progress towards this goal achieved by the Innovative Medicines Initiative in Diabetes, a collaborative public-private consortium supported by the European Commission and by dedicated resources of pharmaceutical companies. We compare several of the available imaging methods and molecular targets and provide suggestions as to the likeliest to lead to tractable approaches. Furthermore, we discuss the simultaneous development of animal models that can be used to measure subtle changes in β-cell mass, a prerequisite for validating the clinical potential of the different imaging tracers.

Signal Transducer and Activator of Transcription 3, Mediated Remodeling of the Tumor Microenvironment Results in Enhanced Tumor Drug Delivery in a Mouse Model of Pancreatic Cancer

BACKGROUND & AIMS

A hallmark of pancreatic ductal adenocarcinoma (PDAC) is the presence of a dense desmoplastic reaction (stroma) that impedes drug delivery to the tumor. Attempts to deplete the tumor stroma have resulted in formation of more aggressive tumors. We have identified signal transducer and activator of transcription (STAT) 3 as a biomarker of resistance to cytotoxic and molecularly targeted therapy in PDAC. The purpose of this study is to investigate the effects of targeting STAT3 on the PDAC stroma and on therapeutic resistance.

METHODS

Activated STAT3 protein expression was determined in human pancreatic tissues and tumor cell lines. In vivo effects of AZD1480, a JAK/STAT3 inhibitor, gemcitabine or the combination were determined in Ptf1a(cre/+);LSL-Kras(G12D/+);Tgfbr2(flox/flox) (PKT) mice and in orthotopic tumor xenografts. Drug delivery was analyzed by matrix-assisted laser desorption/ionization imaging mass spectrometry. Collagen second harmonic generation imaging quantified tumor collagen alignment and density.

RESULTS

STAT3 activation correlates with decreased survival and advanced tumor stage in patients with PDAC. STAT3 inhibition combined with gemcitabine significantly inhibits tumor growth in both an orthotopic and the PKT mouse model of PDAC. This combined therapy attenuates in vivo expression of SPARC, increases microvessel density, and enhances drug delivery to the tumor without depletion of stromal collagen or hyaluronan. Instead, the PDAC tumors demonstrate vascular normalization, remodeling of the tumor stroma, and down-regulation of cytidine deaminase.

CONCLUSIONS

Targeted inhibition of STAT3 combined with gemcitabine enhances in vivo drug delivery and therapeutic response in PDAC. These effects occur through tumor stromal remodeling and down-regulation of cytidine deaminase without depletion of tumor stromal content.

Generation and Characterization of Transgenic Mice Expressing Mouse Ins1 Promoter for Pancreatic β-Cell-Specific Gene Overexpression and Knockout

The technologies for pancreatic β-cell-specific gene overexpression or knockout are fundamental for investigations of functional genes in vivo. Here we generated the Ins1-Cre-Dsred and Ins1-rtTA mouse models, which expressed the Cre recombinase or reverse tetracycline regulatable transactivator (rtTA) without hGH minigene under the control of mouse Ins1 promoter. Our data showed that the Cre-mediated recombination and rtTA-mediated activation could be efficiently detected at embryonic day 13.5 when these models were crossed with the reporter mice (ROSA(mT/mG) or tetO-HIST1H2BJ/GFP). The Cre and rtTA expression was restricted to β-cells without leakage in the brain and other tissues. Moreover, both the transgenic lines showed normal glucose tolerance and insulin secretion. These results suggested that the Ins1-Cre-Dsred and Ins1-rtTA mice could be used to knock out or overexpress target genes in embryos and adults to facilitate β-cell researches.

Enhanced ultrasonography using a nano/microbubble contrast agent for islet transplantation

Recent basic and clinical studies have assessed the use of highly sensitive imaging modalities for visualizing transplanted islets. We investigated the utility of enhanced ultrasonography, combined with fluorescent acoustic liposome nano/microbubbles (FALs), for evaluating angiogenesis and the endocrine function of transplanted islets. BALB/c mice were classified into three groups: Diabetic mice that underwent syngeneic islet transplantation into the subrenal capsule and achieved normoglycemia (Tx group); those that failed to achieve normoglycemia (Tx-DM group); and those not receiving any treatment (DM group). Mice were examined by FAL-enhanced high frequency ultrasonography. The echogenicity of the islets increased rapidly within the first minute after injection of FALs and remained at a higher level in the Tx group, while small increases were observed in the other two groups. In histological assessments, fluorescently stained erythrocytes could be seen in and around the transplanted islets, indicating that the transplanted islets were enhanced by infusion of FALs via vessel networks between the engrafted islets and tissue. Furthermore, the echogenicity correlated significantly with endocrine parameters, including blood glucose (BG), serum insulin, and the BG change in the glucose tolerance test. In conclusion, the echogenicity of the islets under FAS-enhanced ultrasonosonography correlated with the endocrine status of transplanted islets.

Autoreactive T cells induce necrosis and not BCL-2-regulated or death receptor-mediated apoptosis or RIPK3-dependent necroptosis of transplanted islets in a mouse model of type 1 diabetes

AIMS/HYPOTHESIS

Type 1 diabetes results from T cell-mediated destruction of pancreatic beta cells. The mechanisms of beta cell destruction in vivo, however, remain unclear. We aimed to test the relative roles of the main cell death pathways: apoptosis, necrosis and necroptosis, in beta cell death in the development of CD4(+) T cell-mediated autoimmune diabetes.

METHODS

We altered expression levels of critical cell death proteins in mouse islets and tested their ability to survive CD4(+) T cell-mediated attack using an in vivo graft model.

RESULTS

Loss of the B cell leukaemia/lymphoma 2 (BCL-2) homology domain 3-only proteins BIM, PUMA or BID did not protect beta cells from this death. Overexpression of the anti-apoptotic protein BCL-2 or combined deficiency of the pro-apoptotic multi-BCL2 homology domain proteins BAX and BAK also failed to prevent beta cell destruction. Furthermore, loss of function of the death receptor Fas or its essential downstream signalling molecule Fas-associated death domain (FADD) in islets was also not protective. Using electron microscopy we observed that dying beta cells showed features of necrosis. However, islets deficient in receptor-interacting serine/threonine protein kinase 3 (RIPK3), a critical initiator of necroptosis, were still normally susceptible to CD4(+) T cell-mediated destruction. Remarkably, simultaneous inhibition of apoptosis and necroptosis by combining loss of RIPK3 and overexpression of BCL-2 in islets did not protect them against immune attack either.

CONCLUSIONS/INTERPRETATION

Collectively, our data indicate that beta cells die by necrosis in autoimmune diabetes and that the programmed cell death pathways apoptosis and necroptosis are both dispensable for this process.

Generation of insulin-producing cells from the mouse liver using β cell-related gene transfer including Mafa and Mafb

Recent studies on the large Maf transcription factors have shown that Mafb and Mafa have respective and distinctive roles in β-cell development and maturation. However, whether this difference in roles is due to the timing of the gene expression (roughly, expression of Mafb before birth and of Mafa after birth) or to the specific function of each gene is unclear. Our aim was to examine the functional differences between these genes that are closely related to β cells by using an in vivo model of β-like cell generation. We monitored insulin gene transcription by measuring bioluminescence emitted from the liver of insulin promoter-luciferase transgenic (MIP-Luc-VU) mice. Adenoviral gene transfers of Pdx1/Neurod/Mafa (PDA) and Pdx1/Neurod/Mafb (PDB) combinations generated intense luminescence from the liver that lasted for more than 1 week and peaked at 3 days after transduction. The peak signal intensities of PDA and PDB were comparable. However, PDA but not PDB transfer resulted in significant bioluminescence on day 10, suggesting that Mafa has a more sustainable role in insulin gene activation than does Mafb. Both PDA and PDB transfers ameliorated the glucose levels in a streptozotocin (STZ)-induced diabetic model for up to 21 days and 7 days, respectively. Furthermore, PDA transfer induced several gene expressions necessary for glucose sensing and insulin secretion in the liver on day 9. However, a glucose tolerance test and liver perfusion experiment did not show glucose-stimulated insulin secretion from intrahepatic β-like cells. These results demonstrate that bioluminescence imaging in MIP-Luc-VU mice provides a noninvasive means of detecting β-like cells in the liver. They also show that Mafa has a markedly intense and sustained role in β-like cell production in comparison with Mafb.

Generation and characterization of Ins1-cre-driver C57BL/6N for exclusive pancreatic beta cell-specific Cre-loxP recombination

Cre/loxP system-mediated site-specific recombination is utilized to study gene function in vivo. Successful conditional knockout of genes of interest is dependent on the availability of Cre-driver mice. We produced and characterized pancreatic β cell-specific Cre-driver mice for use in diabetes mellitus research. The gene encoding Cre was inserted into the second exon of mouse Ins1 in a bacterial artificial chromosome (BAC). Five founder mice were produced by microinjection of linearized BAC Ins1-cre. The transgene was integrated between Mafa and the telomere on chromosome 15 in one of the founders, BAC Ins1-cre25. To investigate Cre-loxP recombination, BAC Ins1-cre25 males were crossed with two different Cre-reporters, R26R and R26GRR females. On gross observation, reporter signal after Cre-loxP recombination was detected exclusively in the adult pancreatic islets in both F₁ mice. Immunohistological analysis indicated that Cre-loxP recombination-mediated reporter signal was colocalized with insulin in pancreatic islet cells of both F₁ mice, but not with glucagon. Moreover, Cre-loxP recombination signal was already observed in the pancreatic islets at E13.5 in both F₁ fetuses. Finally, we investigated ectopic Cre-loxP recombination for Ins1, because the ortholog Ins2 is also expressed in the brain, in addition to the pancreas. However, there was no Cre-loxP recombination-mediated reporter signal in the brain of both F₁ mice. Our data suggest that BAC Ins1-cre25 mice are a useful Cre-driver C57BL/6N for pancreatic β cell-specific Cre-loxP recombination, except for crossing with knock-in mice carrying floxed gene on chromosome 15.

Generation and characterization of MafA-Kusabira Orange mice

MafA and MafB are basic leucine zipper transcription factors expressed in mature pancreatic β- and α-cells, respectively. MafA is not only an insulin gene transcription factor but is also critical for the maturation and maintenance of β-cell function, whereas MafB is expressed in immature β-cells during development and in compromised β-cells in diabetes. In this study, we developed a mouse model to easily trace the promoter activity of MafA in β-cells as a tool for studying β-cell differentiation, maturation, regeneration and function using the expression of the fluorescent protein Kusabira Orange (KOr) driven by the BAC-mafA promoter. The expression of KOr was highly restricted to β-cells in the transgenic pancreas. By crossing MafA-KOr mice with MafB(GFP/+) reporter mice, simultaneous monitoring of MafA and MafB expressions in the isolated islets was successfully performed. This system can be a useful tool for examining dynamic changes in the differentiation and function of pancreatic islets by visualizing the expressions of MafA and MafB.

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.

Quantitative assessment of Pdx1 promoter activity in vivo using a secreted luciferase reporter system

The luciferase reporter system is useful for the assessment of various biological processes in vivo. The transcription factor pancreatic and duodenal homeobox 1 (Pdx1) is critical for the formation and the function of pancreatic β-cells. A novel reporter system using secreted Gaussia princeps luciferase (GLuc) under the control of a Pdx1 promoter was generated and activated in rat and mouse β-cell lines. This Pdx1-GLuc construct was used as a transgene for the generation of reporter mice to monitor Pdx1 promoter activity in vivo via the measurement of secreted GLuc activity in a small aliquot of blood. Significantly increased plasma GLuc activity was observed in Pdx1-GLuc mice. Analysis of Pdx1-GLuc mice by bioluminescence imaging, GLuc reporter assays using homogenates of various organs, and immunohistochemistry revealed that GLuc expression and activity were exponentially higher in pancreatic β-cells than in pancreatic non-β-cells, the duodenum, and other organs. In addition, GLuc activity secreted into the culture medium from islets isolated from Pdx1-GLuc mice correlated with the number of islets. The transplantation of Pdx1-GLuc islets into severe combined immunodeficiency mice elevated their plasma GLuc activity. Conversely, a partial pancreatectomy in Pdx1-GLuc mice reduced plasma GLuc activity. These results suggest that a secreted luciferase reporter system in vivo enables not only the monitoring of promoter activity but also a quantitative and minimally invasive assessment of physiological and pathological changes in small cell masses, such as pancreatic β-cells.

Feasibility of baculovirus-mediated reporter gene delivery for efficient monitoring of islet transplantation in vivo

OBJECTIVE

The objective of this study was to explore the feasibility of baculovirus vector-mediated sodium iodide symporter (NIS) gene delivery to monitor islet transplantation.

METHODS

Baculovirus vectors expressing green fluorescent protein (GFP) or NIS (Bac-GFP and Bac-NIS) were established using the Bac-to-Bac baculovirus expression system. The GFP expression of Bac-GFP-infected rat islets was observed in vitro by fluorescence microscopy. Iodine uptake and inhibition of iodine uptake by NaClO4 in Bac-NIS-infected islets were dynamically monitored in vitro. Bac-GFP- or Bac-NIS-infected islets were implanted into the left axillary cavity of NOD-SCID mice, and fluorescence imaging and (125)I NanoSPECT/CT imaging were subsequently performed in vivo.

RESULTS

Bac-GFP efficiently infected rat islets (over 95% infected at MOI=40), and the expression of GFP lasted approximately two weeks. NaClO4 could inhibit iodine uptake by Bac-NIS-infected islets. In vivo imaging revealed that the fluorescence intensity of the transplant sites in Bac-GFP-infected groups was significantly higher than in the non-infected group. Grafts could be clearly observed by (125)I NanoSPECT/CT imaging for up to 8 h.

CONCLUSION

Baculovirus vectors are powerful vehicles for studying rat islets in gene delivery. It is feasible to use a baculovirus vector to delivery an NIS gene for non-invasive monitoring transplanted islets in vivo by the expression of the target gene.

Label-free detection of insulin and glucagon within human islets of Langerhans using Raman spectroscopy

Intrahepatic transplantation of donor islets of Langerhans is a promising therapy for patients with type 1 diabetes. It is of critical importance to accurately monitor islet quality before transplantation, which is currently done by standard histological methods that are performed off-line and require extensive sample preparation. As an alternative, we propose Raman spectroscopy which is a non-destructive and label-free technique that allows continuous real-time monitoring of the tissue to study biological changes as they occur. By performing Raman spectroscopic measurements on purified insulin and glucagon, we showed that the 520 cm(-1) band assigned to disulfide bridges in insulin, and the 1552 cm(-1) band assigned to tryptophan in glucagon are mutually exclusive and could therefore be used as indirect markers for the label-free distinction between both hormones. High-resolution hyperspectral Raman imaging for these bands showed the distribution of disulfide bridges and tryptophan at sub-micrometer scale, which correlated with the location of insulin and glucagon as revealed by conventional immunohistochemistry. As a measure for this correlation, quantitative analysis was performed comparing the Raman images with the fluorescence images, resulting in Dice coefficients (ranging between 0 and 1) of 0.36 for insulin and 0.19 for glucagon. Although the use of separate microscope systems with different spatial resolution and the use of indirect Raman markers cause some image mismatch, our findings indicate that Raman bands for disulfide bridges and tryptophan can be used as distinctive markers for the label-free detection of insulin and glucagon in human islets of Langerhans.

Implanted islets in the anterior chamber of the eye are prone to autoimmune attack in a mouse model of diabetes

AIMS/HYPOTHESIS

Type 1 diabetes is an autoimmune disease resulting from the destruction of insulin-producing beta cells. Along with advances in generating replacement beta cells for treating diabetes, there is also increasing demand for non-invasive tools to evaluate the recurrence of autoimmune attack on transplanted tissue. Here, we examined the anterior chamber of the eye as a potential islet transplant site, and also evaluated whether in vivo imaging of the islets transplanted in the eye could enable real-time visualisation of autoimmune processes underway in the pancreas.

METHODS

Syngeneic islet equivalents were transplanted into the eye or kidney capsule of streptozotocin-induced diabetic C57BL/6 mice to compare islet dose (25-125 islet equivalents) and function across transplant sites. Autoimmune attack of syngeneic islets was evaluated in the pancreas and eye tissues of NOD and NOD-severe combined immunodeficient (SCID) mice given diabetogenic splenocytes.

RESULTS

Islet transplantation in the eye decreased fasting plasma glucose levels and increased weight gain and survival in an islet-dose-dependent manner. Even 50 islets in the eye reduced blood glucose levels, whereas ≥ 200 islets were required in the kidney for a similar effect. Autoimmune destruction of pancreatic islets in the eye mirrored that in the pancreas and could be visualised in real time by non-invasive imaging.

CONCLUSIONS/INTERPRETATION

We found that far fewer islets were required to restore normoglycaemia when transplanted into the anterior chamber of the eye vs the kidney capsule. However, our results suggest that islets are not protected against autoimmune attack in the eye, making this a suitable site for visualising autoimmune processes against transplanted tissue.

Imaging of transplanted islets by positron emission tomography, magnetic resonance imaging, and ultrasonography

While islet transplantation is considered a useful therapeutic option for severe diabetes mellitus (DM), the outcome of this treatment remains unsatisfactory. This is largely due to the damage and loss of islets in the early transplant stage. Thus, it is important to monitor the condition of the transplanted islets, so that a treatment can be selected to rescue the islets from damage if needed. Recently, numerous trials have been performed to investigate the efficacy of different imaging modalities for visualizing transplanted islets. Positron emission tomography (PET) and magnetic resonance imaging (MRI) are the most commonly used imaging modalities for this purpose. Some groups, including ours, have also tried to visualize transplanted islets by ultrasonography (US). In this review article, we discuss the recent progress in islet imaging.

Knockdown of aberrantly upregulated aryl hydrocarbon receptor reduces tumor growth and metastasis of MDA-MB-231 human breast cancer cell line

The aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor that belongs to the basic-helix-loop-helix (bHLH)-Per-ARNT-Sim (PAS) superfamily of transcription factors, mediates toxic response induced by environmental chemicals such as polycyclic aromatic hydrocarbons (PAH). AhR is expressed at high levels in several human breast carcinoma cell lines in direct correlation with the degree of their malignancy. Recent studies suggest a possible role for AhR in cancer independent of PAH. Therefore, we established stable AhR knockdown cells of the human breast cancer cell line MDA-MB-231 and analyzed their tumorigenic properties in in vitro and in vivo model systems. In addition we analyzed their response to radiation and chemotherapeutic treatment. AhR knockdown attenuated these cells tumorigenic properties in vitro including proliferation, anchorage independent growth, migration and apoptosis and reduced orthotopic xenograft tumor growth and lung metastasis in vivo. Notably, we observed that AhR knockdown enhanced radiation-induced apoptosis as well as significantly decreased cell clonogenic survival. Furthermore, AhR knockdown in MDA-MB-231 cells sensitized them to paclitaxel treatment, evident by a decrease in the required cytotoxic dose. Subsequent analysis revealed AhR knockdown significantly reduced phosphorylation of AKT, which impacts cell proliferation and survival. Apoptosis-focused gene expression analyses revealed an altered expression of genes regulating apoptosis in MDA-MB-231 cells. Collectively, our data identify AhR as a potential novel therapeutic target in the treatment of metastatic breast cancer.

Enhancing pancreatic Beta-cell regeneration in vivo with pioglitazone and alogliptin

Aims/Hypothesis

Pancreatic beta-cells retain limited ability to regenerate and proliferate after various physiologic triggers. Identifying therapies that are able to enhance beta-cell regeneration may therefore be useful for the treatment of both type 1 and type 2 diabetes.

Methods

In this study we investigated endogenous and transplanted beta-cell regeneration by serially quantifying changes in bioluminescence from beta-cells from transgenic mice expressing firefly luciferase under the control of the mouse insulin I promoter. We tested the ability of pioglitazone and alogliptin, two drugs developed for the treatment of type 2 diabetes, to enhance beta-cell regeneration, and also defined the effect of the immunosuppression with rapamycin and tacrolimus on transplanted islet beta mass.

Results

Pioglitazone is a stimulator of nuclear receptor peroxisome proliferator-activated receptor gamma while alogliptin is a selective dipeptidyl peptidase IV inhibitor. Pioglitazone alone, or in combination with alogliptin, enhanced endogenous beta-cell regeneration in streptozotocin-treated mice, while alogliptin alone had modest effects. In a model of syngeneic islet transplantation, immunosuppression with rapamycin and tacrolimus induced an early loss of beta-cell mass, while treatment with insulin implants to maintain normoglycemia and pioglitazone plus alogliptin was able to partially promote beta-cell mass recovery.

Conclusions/Interpretation

These data highlight the utility of bioluminescence for serially quantifying functional beta-cell mass in living mice. They also demonstrate the ability of pioglitazone, used either alone or in combination with alogliptin, to enhance regeneration of endogenous islet beta-cells as well as transplanted islets into recipients treated with rapamycin and tacrolimus.

Bioluminescence imaging of β cells and intrahepatic insulin gene activity under normal and pathological conditions

In diabetes research, bioluminescence imaging (BLI) has been applied in studies of β-cell impairment, development, and islet transplantation. To develop a mouse model that enables noninvasive imaging of β cells, we generated a bacterial artificial chromosome (BAC) transgenic mouse in which a mouse 200-kbp genomic fragment comprising the insulin I gene drives luciferase expression (Ins1-luc BAC transgenic mouse). BLI of mice was performed using the IVIS Spectrum system after intraperitoneal injection of luciferin, and the bioluminescence signal from the pancreatic region analyzed. When compared with MIP-Luc-VU mice [FVB/N-Tg(Ins1-luc)VUPwrs/J] expressing luciferase under the control of the 9.2-kbp mouse insulin I promoter (MIP), the bioluminescence emission from Ins1-luc BAC transgenic mice was enhanced approximately 4-fold. Streptozotocin-treated Ins1-luc BAC transgenic mice developed severe diabetes concomitant with a sharp decline in the BLI signal intensity in the pancreas. Conversely, mice fed a high-fat diet for 8 weeks showed an increase in the signal, reflecting a decrease or increase in the β-cell mass. Although the bioluminescence intensity of the islets correlated well with the number of isolated islets in vitro, the intensity obtained from a living mouse in vivo did not necessarily reflect an absolute quantification of the β-cell mass under pathological conditions. On the other hand, adenovirus-mediated gene transduction of β-cell-related transcription factors in Ins1-luc BAC transgenic mice generated luminescence from the hepatic region for more than 1 week. These results demonstrate that BLI in Ins1-luc BAC transgenic mice provides a noninvasive method of imaging islet β cells and extrapancreatic activity of the insulin gene in the liver under normal and pathological conditions.

Bioluminescence imaging reveals dynamics of beta cell loss in the non-obese diabetic (NOD) mouse model

We generated a mouse model (MIP-Luc-VU-NOD) that enables non-invasive bioluminescence imaging (BLI) of beta cell loss during the progression of autoimmune diabetes and determined the relationship between BLI and disease progression. MIP-Luc-VU-NOD mice displayed insulitis and a decline in bioluminescence with age which correlated with beta cell mass, plasma insulin, and pancreatic insulin content. Bioluminescence declined gradually in female MIP-Luc-VU-NOD mice, reaching less than 50% of the initial BLI at 10 weeks of age, whereas hyperglycemia did not ensue until mice were at least 16 weeks old. Mice that did not become diabetic maintained insulin secretion and had less of a decline in bioluminescence than mice that became diabetic. Bioluminescence measurements predicted a decline in beta cell mass prior to the onset of hyperglycemia and tracked beta cell loss. This model should be useful for investigating the fundamental processes underlying autoimmune diabetes and developing new therapies targeting beta cell protection and regeneration.

Noninvasive monitoring of β-cell mass and fetal β-cell genesis in mice using bioluminescence imaging

Bioluminescence imaging (BLI) has been applied in gene therapy and research to screen for transgene expression, progression of infection, tumor growth and metastasis, and transplantation. It enables real-time and relatively noninvasive localization and serial quantification of biological processes in experimental animals. In diabetes research, BLI has been employed for the quantification of β-cell mass, monitoring of islet graft survival after transplantation, and detection of reporter gene expression. Here, we explore the use of BLI in a transgenic mouse expressing luciferase under the control of the mouse insulin 1 promoter (MIP-Luc-VU). A previous report on MIP-Luc-VU mice showed luminescence intensities emitted from the islets correlated well with the number of islets in vitro and in vivo. In this study, we showed MIP-Luc-VU mice fed a high fat diet for 8 weeks gave rise to a greater bioluminescent signal than mice fed a regular diet for the same period of time. Conversely, there was a strong reduction in the signal observed in diabetic Mafa-deficient/Mafk-transgenic mutant mice and streptozotocin-treated mice, reflecting the loss of β-cells. Furthermore, we were able to monitor fetal β-cell genesis in MIP-Luc-VU mice during the late gestational stage in a noninvasive and repetitive manner. In summary, we show that bioluminescence imaging of mice expressing a β-cell specific reporter allows detection of changes in β-cell mass and visualization of fetal β-cell neogenesis in uteri.

Imaging free zinc levels in vivo - what can be learned?

Our ever-expanding knowledge about the role of zinc in biology includes its role in redox modulation, immune response, neurotransmission, reproduction, diabetes, cancer, and Alzheimers disease is galvanizing interest in detecting and monitoring the various forms of Zn(II) in biological systems. This paper reviews reported strategies for detecting and tracking of labile or "free" unchelated Zn(II) in tissues. While different bound structural forms of Zn(II) have been identified and studied in vitro by multiple techniques, very few molecular imaging methods have successfully tracked the ion in vivo. A number of MRI and optical strategies have now been reported for detection of free Zn(II) in cells and tissues but only a few have been applied successfully in vivo. A recent report of a MRI sensor for in vivo tracking of Zn(II) released from pancreatic β-cells during insulin secretion exemplifies the promise of rational design of new Zn(II) sensors for tracking this biologically important ion in vivo. Such studies promise to provide new insights into zinc trafficking in vivo and the critical role of this ion in many human diseases.

Towards PET imaging of intact pancreatic beta cell mass: a transgenic strategy

PURPOSE

We have generated transgenic mouse lines expressing the positron emission tomography (PET) reporter gene, sr39tk, under the control of the mouse insulin I promoter (MIP-sr39tk) to image endogenous islets using PET.

PROCEDURES

The MIP-sr39tk transgene was constructed using the 8.3 kb fragment of the mouse insulin I promoter and the sr39tk coding sequence from the mrfp-hrl-ttk trifusion construct. Expression of sr39TK in beta cells was confirmed by fluorescence immunohistochemistry of pancreatic sections. Histological sections were used to determine beta cell mass, islet area and islet number. Beta cell function was determined using intraperitoneal glucose tolerance tests. For ex vivo biodistrubution, mice were injected i.v. with 9.25 MBq [(18)F]fluorohydroxymethyl-butyl-guanine (FHBG), euthanized 1 h later and pancreata and other organs were collected and counted. For PET scans, mice were injected i.v. with 9.25 MBq [(18)F]FHBG, and dynamic scans were conducted for 1 h, followed by a 30 min static acquisition. To induce type 1 diabetes-like symptoms, MIP-sr39tk mice were injected i.p. with 40 mg/kg streptozotocin (STZ) once per day for five consecutive days, and biodistribution and PET scans were conducted thereafter.

RESULTS

Ex vivo quantification of [(18)F]FHBG uptake in the pancreas showed a 4.5-fold difference in transgenic vs. non-transgenics, confirming that expression of sr39TK results in the retention of the PET tracer. In STZ-treated MIP-sr39tk mice, impairments in glucose tolerance and decreases in beta cell mass correlated significantly with a diminishment in [(18)F]FHBG uptake before fasting hyperglycemia became apparent.

CONCLUSIONS

The MIP-sr39tk mouse demonstrates that PET imaging can detect changes in beta cell mass that precede the onset of diabetes.

Multimodal image coregistration and inducible selective cell ablation to evaluate imaging ligands

We combined multimodal imaging (bioluminescence, X-ray computed tomography, and PET), tomographic reconstruction of bioluminescent sources, and two unique, complementary models to evaluate three previously synthesized PET radiotracers thought to target pancreatic beta cells. The three radiotracers {[(18)F]fluoropropyl-(+)-dihydrotetrabenazine ([(18)F]FP-DTBZ), [(18)F](+)-2-oxiranyl-3-isobutyl-9-(3-fluoropropoxy)-10-methoxy-2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinoline ((18)F-AV-266), and (2S,3R,11bR)-9-(3-fluoropropoxy)-2-(hydroxymethyl)-3-isobutyl-10-methoxy-2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinolin-2-ol ((18)F-AV-300)} bind vesicular monoamine transporter 2. Tomographic reconstruction of the bioluminescent signal in mice expressing luciferase only in pancreatic beta cells was used to delineate the pancreas and was coregistered with PET and X-ray computed tomography images. This strategy enabled unambiguous identification of the pancreas on PET images, permitting accurate quantification of the pancreatic PET signal. We show here that, after conditional, specific, and rapid mouse beta-cell ablation, beta-cell loss was detected by bioluminescence imaging but not by PET imaging, given that the pancreatic signal provided by three PET radiotracers was not altered. To determine whether these ligands bound human beta cells in vivo, we imaged mice transplanted with luciferase-expressing human islets. The human islets were imaged by bioluminescence but not with the PET ligands, indicating that these vesicular monoamine transporter 2-directed ligands did not specifically bind beta cells. These data demonstrate the utility of coregistered multimodal imaging as a platform for evaluation and validation of candidate ligands for imaging islets.

Imaging of pancreatic islet cells

At present, the onset and progress of diabetes, and the efficacy of potential treatments, can only be assessed through indirect means, i.e. blood glucose, insulin, or C-peptide measurements. The development of non-invasive and reliable methods for (1) quantification of pancreatic beta islet cell mass in vivo, (2) determining endogenous islet function and survival, and (3) visualizing the biodistribution, survival, and function of transplanted exogenous islets are critical to further advance both basic science research and islet cell therapy in diabetes. Islet cell imaging using magnetic resonance, bioluminescence, positron emission tomography, or single photon emission computed tomography may provide us with a direct means to interrogate islet cell distribution, survival, and function. Current state-of-the-art strategies for beta-cell imaging are discussed and reviewed here in context of their clinical relevance.

Noninvasive MRI of β-cell function using a Zn2+-responsive contrast agent

Elevation of postprandial glucose stimulates release of insulin from granules stored in pancreatic islet β-cells. We demonstrate here that divalent zinc ions coreleased with insulin from β-cells in response to high glucose are readily detected by MRI using the Zn(2+)-responsive T(1) agent, GdDOTA-diBPEN. Image contrast was significantly enhanced in the mouse pancreas after injection of a bolus of glucose followed by a low dose of the Zn(2+) sensor. Images of the pancreas were not enhanced by the agent in mice without addition of glucose to stimulate insulin release, nor were images enhanced in streptozotocin-treated mice with or without added glucose. These observations are consistent with MRI detection of Zn(2+) released from β-cells only during glucose-stimulated insulin secretion. Images of mice fed a high-fat (60%) diet over a 12-wk period and subjected to this same imaging protocol showed a larger volume of contrast-enhanced pancreatic tissue, consistent with the expansion of pancreatic β-cell mass during fat accumulation and progression to type 2 diabetes. This MRI sensor offers the exciting potential for deep-tissue monitoring of β-cell function in vivo during development of type 2 diabetes or after implantation of islets in type I diabetic patients.

Molecular imaging: a promising tool to monitor islet transplantation

Replacement of insulin production by pancreatic islet transplantation has great potential as a therapy for type 1 diabetes mellitus. At present, the lack of an effective approach to islet grafts assessment limits the success of this treatment. The development of molecular imaging techniques has the potential to fulfill the goal of real-time noninvasive monitoring of the functional status and viability of the islet grafts. We review the application of a variety of imaging modalities for detecting endogenous and transplanted beta-cell mass. The review also explores the various molecular imaging strategies for assessing islet delivery, the metabolic effects on the islet grafts as well as detection of immunorejection. Here, we highlight the use of combined imaging and therapeutic interventions in islet transplantation and the in vivo monitoring of stem cells differentiation into insulin-producing cells.

Evolution of β-Cell Replacement Therapy in Diabetes Mellitus: Islet Cell Transplantation

Diabetes mellitus remains one of the leading causes of morbidity and mortality worldwide. According to the Centers for Disease Control and Prevention, approximately 23.6 million people in the United States are affected. Of these individuals, 5 to 10% have been diagnosed with Type 1 diabetes mellitus (T1DM), an autoimmune disease. Although it often appears in childhood, T1DM may manifest at any age, leading to significant morbidity and decreased quality of life. Since the 1960s, the surgical treatment for diabetes mellitus has evolved to become a viable alternative to insulin administration, beginning with pancreatic transplantation. While islet cell transplantation has emerged as another potential alternative, its role in the treatment of T1DM remains to be solidified as research continues to establish it as a truly viable alternative for achieving insulin independence. In this paper, the historical evolution, procurement, current status, benefits, risks, and ongoing research of islet cell transplantation are explored.

Noninvasive imaging techniques in islet transplantation

Since the Edmonton trials, insulin independence can reproducibly be achieved after islet transplantation. However, a majority of patients resume insulin treatment in the first 5 years after transplantation. Several mechanisms have been proposed but are difficult to pinpoint in one particular patient. Current tools for the metabolic monitoring of islet grafts indicate islet dysfunction when it is too late to take action. Noninvasive imaging of transplanted islets could be used to study β-cell mass and β-cell function just after infusion, during vascularization or autoimmune and alloimmune attacks. This review will focus on the most recent advances in various imaging techniques (bioluminescence imaging, fluorescence optical imaging, MRI, and positron emission tomography). Emphasis will be placed on pertinent approaches for translation to human practice.

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.