Obstacles on the way to the clinical visualisation of beta cells: looking for the Aeneas of molecular imaging to navigate between Scylla and Charybdis

Pancreatic beta-cell mass and function and therapeutic implications of using antidiabetic medications in type 2 diabetes

Nowadays, the focus of diabetes treatment has switched from lowering the glucose level to preserving glycemic homeostasis and slowing the disease progression. The main pathophysiology of both type 1 diabetes and long-standing type 2 diabetes is pancreatic β-cell mass loss and dysfunction. According to recent research, human pancreatic β-cells possess the ability to proliferate in response to elevated insulin demands. It has been demonstrated that in insulin-resistant conditions in humans, such as obesity or pregnancy, the β-cell mass increases. This ability could be helpful in developing novel treatment approaches to restore a functional β-cell mass. Treatment strategies aimed at boosting β-cell function and mass may be a useful tool for managing diabetes mellitus and stopping its progression. This review outlines the processes of β-cell failure and detail the many β-cell abnormalities that manifest in people with diabetes mellitus. We also go over standard techniques for determining the mass and function of β-cells. Lastly, we provide the therapeutic implications of utilizing antidiabetic drugs in controlling the mass and function of pancreatic β-cells.

Cross-sectional imaging of the pancreas in diabetes

Diabetes mellitus presents a global health challenge characterized by dysregulated glucose metabolism and insulin resistance. Pancreas dysfunction contributes to the development and progression of diabetes. Cross-sectional imaging modalities have provided new insight into the structural and functional alterations of the pancreas in individuals with diabetes. This review summarizes MRI and CT studies that characterize pancreas alterations in both type 1 and type 2 diabetes and discusses future applications of these techniques.

Characterization of a S1PR2 specific 11C-labeled radiotracer in streptozotocin-induced diabetic murine model

BACKGROUND

Diabetes mellitus is a chronic progressive metabolic disorder that affects millions of people worldwide. Emerging evidence suggests the important roles of sphingolipid metabolism in diabetes. In particular, sphingosine-1-phosphate (S1P) and S1P receptor 2 (S1PR2) have important metabolic functions and are involved in several metabolic diseases. In diabetes, S1PR2 can effectively preserve β cells and improve glucose/insulin tolerance in high-fat diet induced and streptozotocin (STZ)-induced diabetic mouse models. We previously developed a group of potent and selective S1PR2 ligands and radioligands.

METHODS

In this study, we continued our efforts and characterized our leading S1PR2 radioligand, [11C]TZ34125, in a STZ-induced diabetic mouse model. [11C]TZ34125 was radiosynthesized in an automated synthesis module and in vitro saturation binding assay was performed using recombinant human S1PR2 membrane. In vitro saturation autoradiography analysis was also performed to determine the binding affinity of [11C]TZ34125 against mouse tissues. Type-1 diabetic mouse model was developed following a single high dose of STZ in C57BL/6 mice. Ex vivo biodistribution was performed to evaluate the distribution and amount of [11C]TZ34125 in tissues. In vitro autoradiography analysis was performed to compare the uptake of [11C]TZ34125 between diabetic and control animals in mouse spleen and pancreas.

RESULTS

Our in vitro saturation binding assay using [11C]TZ34125 confirmed [11C]TZ34125 is a potent radioligand to recombinant human S1PR2 membrane with a Kd value of 0.9 nM. Saturation autoradiographic analysis showed [11C]TZ34125 has a Kd of 67.5, 45.9, and 25.0 nM to mouse kidney, spleen, and liver tissues respectively. Biodistribution study in STZ-induced diabetic mice showed the uptake of [11C]TZ34125 was significantly elevated in the spleen (~2 fold higher) and pancreas (~1.4 fold higher) compared to normal controls. The increased uptake of [11C]TZ34125 was further confirmed using autoradiographic analysis in the spleen and pancreases of STZ-induced diabetic mice, indicating S1PR2 can potentially act as a biomarker of diabetes in pancreases and inflammation in spleen. Future mechanistic analysis and in vivo quantitative assessment using non-invasive PET imaging in large animal model of diabetes is worthwhile.

CONCLUSIONS

Overall, our data showed an increased uptake of our lead S1PR2-specific radioligand, [11C]TZ34125, in the spleen and pancreases of STZ-induced diabetic mice, and demonstrated [11C]TZ34125 has a great potential for preclinical and clinical usage for assessment of S1PR2 in diabetes and inflammation.

New Insights and Potential Therapeutic Interventions in Metabolic Diseases

Endocrine homeostasis and metabolic diseases have been the subject of extensive research in recent years. The development of new techniques and insights has led to a deeper understanding of the mechanisms underlying these conditions and opened up new avenues for diagnosis and treatment. In this review, we discussed the rise of metabolic diseases, especially in Western countries, the genetical, psychological, and behavioral basis of metabolic diseases, the role of nutrition and physical activity in the development of metabolic diseases, the role of single-cell transcriptomics, gut microbiota, epigenetics, advanced imaging techniques, and cell-based therapies in metabolic diseases. Finally, practical applications derived from this information are made.

Insulin-Binding Peptide Probes Provide a Novel Strategy for Pancreatic β-Cell Imaging

Type 1 diabetes develops in childhood and adolescence, with peak incidence in the early teenage years. There is an urgent need for an accurate method to detect insulin-producing β-cells in patients that is not affected by alterations in β-cell function. As part of our research program to design specific probes to measure β-cell mass, we recently developed a novel insulin-binding peptide probe (IBPP) for the detection of β-cells in vivo. Here, we applied our innovative method to show specific labeling of this IBPP to human and mouse fixed β-cells in pancreatic islets. Importantly, we showed staining of human and mouse islets in culture without any negative functional or cell viability impact. Moreover, the IBPP-stained mouse islets after tail vein injection in vivo, albeit with batch differences in staining efficiency. In conclusion, we provide evidence showing that the IBPP can be used for future accurate detection of β-cell mass in a variety of preclinical models of diabetes.

Nanobodies for Medical Imaging: About Ready for Prime Time?

Recent advances in medical treatments have been revolutionary in shaping the management and treatment landscape of patients, notably cancer patients. Over the last decade, patients with diverse forms of locally advanced or metastatic cancer, such as melanoma, lung cancers, and many blood-borne malignancies, have seen their life expectancies increasing significantly. Notwithstanding these encouraging results, the present-day struggle with these treatments concerns patients who remain largely unresponsive, as well as those who experience severely toxic side effects. Gaining deeper insight into the cellular and molecular mechanisms underlying these variable responses will bring us closer to developing more effective therapeutics. To assess these mechanisms, non-invasive imaging techniques provide valuable whole-body information with precise targeting. An example of such is immuno-PET (Positron Emission Tomography), which employs radiolabeled antibodies to detect specific molecules of interest. Nanobodies, as the smallest derived antibody fragments, boast ideal characteristics for this purpose and have thus been used extensively in preclinical models and, more recently, in clinical early-stage studies as well. Their merit stems from their high affinity and specificity towards a target, among other factors. Furthermore, their small size (~14 kDa) allows them to easily disperse through the bloodstream and reach tissues in a reliable and uniform manner. In this review, we will discuss the powerful imaging potential of nanobodies, primarily through the lens of imaging malignant tumors but also touching upon their capability to image a broader variety of nonmalignant diseases.

Non-invasive Beta-cell Imaging: Visualization, Quantification, and Beyond

Pancreatic beta (β)-cell dysfunction and reduced mass play a central role in the development and progression of diabetes mellitus. Conventional histological β-cell mass (BCM) analysis is invasive and limited to cross-sectional observations in a restricted sampling area. However, the non-invasive evaluation of BCM remains elusive, and practical in vivo and clinical techniques for β-cell-specific imaging are yet to be established. The lack of such techniques hampers a deeper understanding of the pathophysiological role of BCM in diabetes, the implementation of personalized BCM-based diabetes management, and the development of antidiabetic therapies targeting BCM preservation and restoration. Nuclear medical techniques have recently triggered a major leap in this field. In particular, radioisotope-labeled probes using exendin peptides that include glucagon-like peptide-1 receptor (GLP-1R) agonist and antagonist have been employed in positron emission tomography and single-photon emission computed tomography. These probes have demonstrated high specificity to β cells and provide clear images accurately showing uptake in the pancreas and transplanted islets in preclinical in vivo and clinical studies. One of these probes, ¹¹¹indium-labeled exendin-4 derivative ([Lys¹²(¹¹¹In-BnDTPA-Ahx)]exendin-4), has captured the longitudinal changes in BCM during the development and progression of diabetes and under antidiabetic therapies in various mouse models of type 1 and type 2 diabetes mellitus. GLP-1R-targeted imaging is therefore a promising tool for non-invasive BCM evaluation. This review focuses on recent advances in non-invasive in vivo β-cell imaging for BCM evaluation in the field of diabetes; in particular, the exendin-based GLP-1R-targeted nuclear medicine techniques.

Beta Cell Imaging-From Pre-Clinical Validation to First in Man Testing

There are presently no reliable ways to quantify human pancreatic beta cell mass (BCM) in vivo, which prevents an accurate understanding of the progressive beta cell loss in diabetes or following islet transplantation. Furthermore, the lack of beta cell imaging hampers the evaluation of the impact of new drugs aiming to prevent beta cell loss or to restore BCM in diabetes. We presently discuss the potential value of BCM determination as a cornerstone for individualized therapies in diabetes, describe the presently available probes for human BCM evaluation, and discuss our approach for the discovery of novel beta cell biomarkers, based on the determination of specific splice variants present in human beta cells. This has already led to the identification of DPP6 and FXYD2ga as two promising targets for human BCM imaging, and is followed by a discussion of potential safety issues, the role for radiochemistry in the improvement of BCM imaging, and concludes with an overview of the different steps from pre-clinical validation to a first-in-man trial for novel tracers.

Targeted pancreatic beta cell imaging for early diagnosis

Pancreatic beta cells are important in blood glucose level regulation. As type 1 and 2 diabetes are getting prevalent worldwide, we need to explore new methods for early detection of beta cell-related afflictions. Using bioimaging techniques to measure beta cell mass is crucial because a decrease in beta cell density is seen in diseases such as diabetes and thus can be a new way of diagnosis for such diseases. We also need to appraise beta cell purity in transplanted islets for type 1 diabetes patients. Sufficient amount of functional beta cells must also be determined before being transplanted to the patients. In this review, indirect imaging of beta cells will be discussed. This includes membrane protein on pancreatic beta cells whereby specific probes are designed for different imaging modalities mainly magnetic resonance imaging, positron emission tomography and fluorescence imaging. Direct imaging of insulin is also explored though probes synthesized for such function are relatively fewer. The path for successful pancreatic beta cell imaging is fraught with challenges like non-specific binding, lack of beta cell-restricted targets, the requirement of probes to cross multiple lipid layers to bind to intracellular insulin. Hence, there is an urgent need to develop new imaging techniques and innovative probing constructs in the entire imaging chain of bioengineering to provide early detection of beta cell-related pathology.

Nanomedicine-Based Strategies for Diabetes: Diagnostics, Monitoring, and Treatment

Traditional methods for diabetes management require constant and tedious glucose monitoring (GM) and insulin injections, impacting quality of life. The global diabetic population is expected to increase to 439 million, with approximately US$490 billion in healthcare expenditures by 2030, imposing a significant burden on healthcare systems worldwide. Recent advances in nanotechnology have emerged as promising alternative strategies for the management of diabetes. For example, implantable nanosensors are being developed for continuous GM, new nanoparticle (NP)-based imaging approaches that quantify subtle changes in β cell mass can facilitate early diagnosis, and nanotechnology-based insulin delivery methods are being explored as novel therapies. Here, we provide a holistic summary of this rapidly advancing field compiling all aspects pertaining to the management of diabetes.

A nanobody-based nuclear imaging tracer targeting dipeptidyl peptidase 6 to determine the mass of human beta cell grafts in mice

AIMS/HYPOTHESIS

Type 1 diabetes is characterised by a progressive decline in beta cell mass. This is also observed following implantation of pancreatic islet allografts, but there is no reliable information regarding the time course of beta cell loss. This is due to the limited availability of non-invasive pancreatic islet imaging techniques. We have previously described that dipeptidyl peptidase 6 (DPP6) is an alpha and beta cell-specific biomarker, and developed a camelid antibody (nanobody '4hD29') against it. We demonstrated the possibility to detect DPP6-expressing cells by single-photon emission computed tomography (SPECT)/ computed tomography (CT), but the correlation between the number of cells grafted and the SPECT signal was not assessed. Here, we investigate whether the 4hD29 nanobody allows us to detect different amounts of human pancreatic islets implanted into immune-deficient mice. In addition, we also describe the adaptation of the probe for use with positron emission tomography (PET).

METHODS

DPP6 expression was assessed in human samples using tissue arrays and immunohistochemistry. The effect of the 4hD29 nanobody on cell death and glucose-stimulated insulin secretion was measured in EndoC-βH1 cells and in human islets using Hoechst/propidium iodide staining and an anti-insulin ELISA, respectively. We performed in vivo SPECT imaging on severe combined immunodeficient (SCID) mice transplanted with different amounts of EndoC-βH1 cells (2 × 10⁶, 5 × 10⁶ and 10 × 10⁶ cells), human islets (1000 and 3000) or pancreatic exocrine tissue using ^99mTc-labelled 4hD29 nanobody. This DPP6 nanobody was also conjugated to N-chlorosuccinimide (NCS)-1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA), radiolabelled with either ⁶⁷Ga (SPECT) or ⁶⁸Ga (PET) and used in a proof-of-principle experiment to detect DPP6-expressing cells (Kelly neuroblastoma) grafted in SCID mice.

RESULTS

The DPP6 protein is mainly expressed in pancreatic islets. Importantly, the anti-DPP6 nanobody 4hD29 allows non-invasive detection of high amounts of EndoC-βH1 cells or human islets grafted in immunodeficient mice. This suggests that the probe must be further improved to detect lower numbers of islet cells. The 4hD29 nanobody neither affected beta cell viability nor altered insulin secretion in EndoC-βH1 cells and human islets. The conversion of 4hD29 nanobody into a PET probe was successful and did not alter its specificity.

CONCLUSIONS/INTERPRETATION

These findings suggest that the anti-DPP6 4hD29 nanobody may become a useful tool for the quantification of human islet grafts in mice and, pending future development, islet mass in individuals with diabetes.

A new perspective of probe development for imaging pancreatic beta cell in vivo

Beta cells assume a fundamental role in maintaining blood glucose homeostasis through the secretion of insulin, which is contingent on both beta cell mass and function, in response to elevated blood glucose levels or secretagogues. For this reason, evaluating beta cell mass and function, as well as scrutinizing how they change over time in a diabetic state, are essential prerequisites in elucidating diabetes pathophysiology. Current clinical methods to measure human beta cell mass and/or function are largely lacking, indirect and sub-optimal, highlighting the continued need for noninvasive in vivo beta cell imaging technologies such as optical imaging techniques. While numerous probes have been developed and evaluated for their specificity to beta cells, most of them are more suited to visualize beta cell mass rather than function. In this review, we highlight the distinction between beta cell mass and function, and the importance of developing more probes to measure beta cell function. Additionally, we also explore various existing probes that can be employed to measure beta cell mass and function in vivo, as well as the caveats in probe development for in vivo beta cell imaging.

Multimodal Imaging Probe Development for Pancreatic β Cells: From Fluorescence to PET

Pancreatic β cells are responsible for insulin secretion and are important for glucose regulation in a healthy body and diabetic disease patient without prelabeling of islets. While the conventional biomarkers for diabetes have been glucose and insulin concentrations in the blood, the direct determination of the pancreatic β cell mass would provide critical information for the disease status and progression. By combining fluorination and diversity-oriented fluorescence library strategy, we have developed a multimodal pancreatic β cell probe PiF for both fluorescence and for PET (positron emission tomography). By simple tail vein injection, PiF stains pancreatic β cells specifically and allows intraoperative fluorescent imaging of pancreatic islets. PiF-injected pancreatic tissue even facilitated an antibody-free islet analysis within 2 h, dramatically accelerating the day-long histological procedure without any fixing and dehydration step. Not only islets in the pancreas but also the low background of PiF in the liver allowed us to monitor the intraportal transplanted islets, which is the first in vivo visualization of transplanted human islets without a prelabeling of the islets. Finally, we could replace the built-in fluorine atom in PiF with radioactive 18F and successfully demonstrate in situ PET imaging for pancreatic islets.

PET/MRI enables simultaneous in vivo quantification of β-cell mass and function

Non-invasive imaging of β-cells represents a desirable preclinical and clinical tool to monitor the change of β-cell mass and the loss of function during pre-diabetic stages. Although it is widely accepted that manganese (Mn) ions are actively gated by voltage-dependent calcium channels (VDCC) in response to glucose metabolism, little is known on its specificity in vivo for quantification of islet β-cell function using Mn and magnetic resonance imaging (MRI). On the other hand, glucagon-like-peptide-1 receptor (GLP-1R) represents a validated target for the estimation of β-cell mass using radiolabeled exendin-4 (Ex4) and positron emission tomography (PET). However, a multiparametric imaging workflow revealing β-cell mass and function quantitatively is still missing. Methods: We developed a simultaneous PET/MRI protocol to comprehensively quantify in vivo changes in β-cell mass and function by targeting, respectively, GLP-1R and VDCC coupled with insulin secretion. Differences in the spatial distribution of Mn and radiolabeled Ex4 were monitored overtime in native and transgenic pancreata, characterized by spontaneous pancreatic neuroendocrine tumor development. Follow-up with mass spectrometry imaging (MSI) and autoradiography allowed the ex vivo validation of the specificity of Mn and PET tracer uptake and the detection of endogenous biometals, such as calcium and zinc, throughout the endocrine and exocrine pancreas. Results: Our in vivo data based on a volumetric PET/MRI readout for native pancreata and insulinomas connects uptake of Mn measured at early imaging time points to high non-specific binding by the exocrine tissue, while specific retention was only found 24 h post injection. These results are supported by cross-validation of the spatial distribution of exogenous ⁵⁵Mn and endogenous ⁴⁴Ca and ⁶⁴Zn as well with the specific internalization of the radiolabeled peptide targeting GLP-1R. Conclusion: Simultaneous PET/MR imaging of the pancreas enabled the comprehensive in vivo quantification of β-cell function and mass using Mn and radiolabeled Ex4. Most important, our data revealed that only late time-point measurements reflect the Mn uptake in the islet β-cells, while early time points detect non-specific accumulation of Mn in the exocrine pancreas.

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.

A high molar activity 18F-labeled TAK-875 derivative for PET imaging of pancreatic β-cells

Background

Results

Conclusions

Imaging of Human Insulin Secreting Cells with Gd-DOTA-P88, a Paramagnetic Contrast Agent Targeting the Beta Cell Biomarker FXYD2γa

Non-invasive imaging and quantification of human beta cell mass remains a major challenge. We performed pre-clinical in vivo validation of a peptide previously discovered by our group, namely, P88 that targets a beta cell specific biomarker, FXYD2γa. We conjugated P88 with DOTA and then complexed it with GdCl₃ to obtain the MRI (magnetic resonance imaging) contrast agent (CA) Gd-DOTA-P88. A scrambled peptide was used as a negative control CA, namely Gd-DOTA-Scramble. The CAs were injected in immunodeficient mice implanted with EndoC-βH1 cells, a human beta cell line that expresses FXYD2γa similarly to primary human beta cells. The xenograft-bearing mice were analyzed by MRI. At the end, the mice were euthanized and the CA biodistribution was evaluated on the excised tissues by measuring the Gd concentration with inductively coupled plasma mass spectrometry (ICP-MS). The MRI and biodistribution studies indicated that Gd-DOTA-P88 accumulates in EndoC-βH1 xenografts above the level observed in the background tissue, and that its uptake is significantly higher than that observed for Gd-DOTA-Scramble. In addition, the Gd-DOTA-P88 showed good xenograft-to-muscle and xenograft-to-liver uptake ratios, two potential sites of human islets transplantation. The CA shows good potential for future use to non-invasively image implanted human beta cells.

Optical clearing of the pancreas for visualization of mature β-cells and vessels in mice

Glucose metabolism is regulated by insulin, which is produced from β-cells in the pancreas. Because insulin is secreted into vessels in response to blood glucose, vascular structures of the pancreas, especially the relationship between vessels and β-cells, are important for physiological and pathological glucose metabolism. Here, we developed a system to visualize vessels surrounding mature β-cells expressing transcription factor MafA in a three-dimensional manner. Optical clearing of the pancreas prevented light scattering of fluorescence driven by the bacterial artificial chromosome (BAC)-mafA promoter in β-cells. Reconstruction of confocal images demonstrated mature β-cells and the glomerular-like structures of β-cell vasculatures labeled with DyLight 488-conjugated lectin in normal mice as well as in low-dose streptozotocin-injected diabetes model mice with reduced β-cell mass. This technological innovation of organ imaging can be used to investigate morphological changes in vascular structures during transplantation, regeneration and diabetes development.

Improved Labeling of Pancreatic Islets Using Cationic Magnetoliposomes

Pancreatic islets (PIs) transplantation is an alternative approach for the treatment of severe forms of type 1 diabetes (T1D). To monitor the success of transplantation, it is desirable to follow the location of engrafted PIs non-invasively. In vivo magnetic resonance imaging (MRI) of transplanted PIs is a feasible cell tracking method; however, this requires labeling with a suitable contrast agent prior to transplantation. We have tested the feasibility of cationic magnetoliposomes (MLs), compared to commercial contrast agents (Endorem and Resovist), by labeling insulinoma cells and freshly isolated rat PIs. It was possible to incorporate Magnetic Ressonance (MR)-detectable amounts of MLs in a shorter time (4 h) when compared to Endorem and Resovist. MLs did not show negative effects on the PIs' viability and functional parameters in vitro. Labeled islets were transplanted in the renal sub-capsular region of healthy mice. Hypointense contrast in MR images due to the labeled PIs was detected in vivo upon transplantation, while MR detection of PIs labeled with Endorem and Resovist was only possible after the addition of transfection agents. These findings indicate that MLs are suitable to image PIs, without affecting their function, which is promising for future longitudinal pre-clinical and clinical studies involving the assessment of PI transplantation.

Enhanced Specific Activity by Multichelation of Exendin-3 Leads To Improved Image Quality and In Vivo Beta Cell Imaging

Glucagon-like peptide-1 receptor (GLP-1R) targeting using radiolabeled exendin is a promising approach to noninvasively visualize and determine beta cell mass (BCM), which could help to unravel the pathophysiology of diabetes. However, saturation of the GLP-1R on beta cells occurs at low peptide doses, since the number of receptors expressed under physiological conditions is low. Therefore, tracers with high specific activities are required to sensitively image small variations in BCM. Here, we describe a novel exendin-3-based radiotracer with multiple chelators and determine its potential for in vivo beta cell imaging. Exendin-3 was modified by adding six lysine residues C-terminally conjugated with one, two, or six DTPA moieties. All compounds were labeled with ¹¹¹In and their GLP-1R affinity was determined in vitro using GLP-1R expressing cells. The in vivo behavior of the ¹¹¹In-labeled tracers was examined in BALB/c nude mice with a subcutaneous GLP-1R expressing tumor (INS-1). Brown Norway rats were used for SPECT visualization of the pancreatic BCM. Addition of six lysine and six DTPA residues (hexendin(40-45)) resulted in a 7-fold increase in specific activity (from 0.73 GBq/nmol to 5.54 GBq/nmol). IC₅₀ values varied between 5.2 and 69.5 nM. All compounds with two or six lysine and DTPA residues had a significantly lower receptor affinity than [Lys⁴⁰(DTPA)]exendin-3 (4.4 nM, p < 0.05). The biodistribution in mice revealed no significant decrease in pancreatic uptake after addition of six lysine and DTPA molecules. Hexendin(40-45) showed a 6-fold increase in absolute ¹¹¹In uptake in the pancreas of Brown Norway rats compared to [Lys⁴⁰(DTPA)]exendin-3 (182.7 ± 42.3 kBq vs 28.8 ± 6.0 kBq, p < 0.001). Visualization of the pancreas on SPECT was improved using hexendin(40-45), due to the higher count rate, achieved at the same peptide dose. In conclusion, hexendin(40-45) showed an improved visualization of the pancreas with SPECT. This tracer holds promise to sensitively and specifically detect small variations in BCM.

A nanobody-based tracer targeting DPP6 for non-invasive imaging of human pancreatic endocrine cells

There are presently no reliable ways to quantify endocrine cell mass (ECM) in vivo, which prevents an accurate understanding of the progressive beta cell loss in diabetes or following islet transplantation. To address this unmet need, we coupled RNA sequencing of human pancreatic islets to a systems biology approach to identify new biomarkers of the endocrine pancreas. Dipeptidyl-Peptidase 6 (DPP6) was identified as a target whose mRNA expression is at least 25-fold higher in human pancreatic islets as compared to surrounding tissues and is not changed by proinflammatory cytokines. At the protein level, DPP6 localizes only in beta and alpha cells within the pancreas. We next generated a high-affinity camelid single-domain antibody (nanobody) targeting human DPP6. The nanobody was radiolabelled and in vivo SPECT/CT imaging and biodistribution studies were performed in immunodeficient mice that were either transplanted with DPP6-expressing Kelly neuroblastoma cells or insulin-producing human EndoC-βH1 cells. The human DPP6-expressing cells were clearly visualized in both models. In conclusion, we have identified a novel beta and alpha cell biomarker and developed a tracer for in vivo imaging of human insulin secreting cells. This provides a useful tool to non-invasively follow up intramuscularly implanted insulin secreting cells.

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

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

Synthesis and Characterization of a Promising Novel FFAR1/GPR40 Targeting Fluorescent Probe for β-Cell Imaging

Diabetes affects an increasing number of patients worldwide and is responsible for a significant rise in healthcare expenses. Imaging of β-cells bears the potential to contribute to an improved understanding, diagnosis, and development of new treatment options for diabetes. Here, we describe the first small molecule fluorescent probe targeting the free fatty acid receptor 1 (FFAR1/GPR40). This receptor is highly expressed on β-cells, and was up to now unexplored for imaging purposes. We designed a novel probe by facile modification of the selective and potent FFAR1 agonist TAK-875. Effective and specific binding of the probe was demonstrated using FFAR1 overexpressing cells. We also successfully labeled FFAR1 on MIN6 and INS1E cells, two widely used β-cell models, by applying an effective amplification protocol. Finally, we showed that the probe is capable of inducing insulin secretion in a glucose-dependent manner, thus demonstrating that functional activity of the probe was maintained. These results suggest that our probe represents a first important step to successful β-cell imaging by targeting FFAR1. The developed probe may prove to be particularly useful for in vitro and ex vivo studies of diabetic cellular and animal models to gain new insights into disease pathogenesis.

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.

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.

Islet Volume and Indexes of β-Cell Function in Humans

Islet volume and endocrine pancreas architecture (islet size distribution) may be independent determinants of β-cell function. Furthermore, the accuracy of homeostatic model assessment (HOMA) indexes in predicting β-cell mass has never been assessed. Here we investigated the relationships between islet volume, islet density, and islet size distribution, estimated after pancreatic tissue digestion, with established indexes of β-cell function in humans. We included in this study 42 patients who were candidates for islet autotransplantation and had well-characterized glucose metabolism. Indexes of insulin secretion were calculated and compared with the islet volume, as a surrogate of β-cell mass, obtained after digestion of pancreas. Islet counting analysis showed considerable interindividual variation in islet density and size. Islet volume, but not density nor size, was the only independent determinant of β-cell function assessed by insulin HOMA β-cell. Islet volume was significantly reduced in the patients with overt hyperglycemia, but not in patients with impaired fasting glucose. Insulin HOMA β-cell predicted islet volume better than other measures of fasting insulin secretion. In conclusion, the present study documented a close direct relationship between indexes of β-cell function and islet volume in humans. The insulin HOMA β-cell provides a more reliable estimate of pancreatic islet volume than fasting glucose before islet isolation.

Manganese-mediated MRI signals correlate with functional β-cell mass during diabetes progression

Diabetes diagnostic therapy and research would strongly benefit from noninvasive accurate imaging of the functional β-cells in the pancreas. Here, we developed an analysis of functional β-cell mass (BCM) by measuring manganese (Mn(2+)) uptake kinetics into glucose-stimulated β-cells by T1-weighted in vivo Mn(2+)-mediated MRI (MnMRI) in C57Bl/6J mice. Weekly MRI analysis during the diabetes progression in mice fed a high-fat/high-sucrose diet (HFD) showed increased Mn(2+)-signals in the pancreas of the HFD-fed mice during the compensation phase, when glucose tolerance and glucose-stimulated insulin secretion (GSIS) were improved and BCM was increased compared with normal diet-fed mice. The increased signal was only transient; from the 4th week on, MRI signals decreased significantly in the HFD group, and the reduced MRI signal in HFD mice persisted over the whole 12-week experimental period, which again correlated with both impaired glucose tolerance and GSIS, although BCM remained unchanged. Rapid and significantly decreased MRI signals were confirmed in diabetic mice after streptozotocin (STZ) injection. No long-term effects of Mn(2+) on glucose tolerance were observed. Our optimized MnMRI protocol fulfills the requirements of noninvasive MRI analysis and detects already small changes in the functional BCM.

Combined Optical Coherence and Fluorescence Microscopy to assess dynamics and specificity of pancreatic beta-cell tracers

The identification of a beta-cell tracer is a major quest in diabetes research. However, since MRI, PET and SPECT cannot resolve individual islets, optical techniques are required to assess the specificity of these tracers. We propose to combine Optical Coherence Microscopy (OCM) with fluorescence detection in a single optical platform to facilitate these initial screening steps from cell culture up to living rodents. OCM can image islets and vascularization without any labeling. Thereby, it alleviates the need of both genetically modified mice to detect islets and injection of external dye to reveal vascularization. We characterized Cy5.5-exendin-3, an agonist of glucagon-like peptide 1 receptor (GLP1R), for which other imaging modalities have been used and can serve as a reference. Cultured cells transfected with GLP1R and incubated with Cy5.5-exendin-3 show full tracer internalization. We determined that a dose of 1 μg of Cy5.5-exendin-3 is sufficient to optically detect in vivo the tracer in islets with a high specificity. In a next step, time-lapse OCM imaging was used to monitor the rapid and specific tracer accumulation in murine islets and its persistence over hours. This optical platform represents a versatile toolbox for selecting beta-cell specific markers for diabetes research and future clinical diagnosis.

Development of a peptide-functionalized imaging nanoprobe for the targeting of (FXYD2)γa as a highly specific biomarker of pancreatic beta cells

Diabetes is characterized by a progressive decline of the pancreatic beta cell mass (BCM), which is responsible for insufficient insulin secretion and hyperglycaemia. There are currently no reliable methods to measure non-invasively the BCM in diabetic patients. Our work describes a phage display-derived peptide (P88) that is highly specific to (FXYD2)γa expressed by human beta cells and is proposed as a molecular vector for the development of functionalized imaging probes. P88 does not bind to the exocrine pancreas and is able to detect down to ~156 human pancreatic islets/mm(3) in vitro after conjugation to ultra-small particles of iron oxide (USPIO), as proven by the R2 measured on MR images. For in vivo evaluation, MRI studies were carried out on nude mice bearing Capan-2 tumours that also express (FXYD2)γa. A strong negative contrast was obtained subsequent to the injection of USPIO-P88, but not in negative controls. On human histological sections, USPIO-P88 seems to be specific to pancreatic beta cells, but not to duodenum, stomach or kidney tissues. USPIO-P88 thus represents a novel and promising tool for monitoring pancreatic BCM in diabetic patients. The quantitative correlation between BCM and R2 remains to be demonstrated in vivo, but the T2 mapping and the black pixel estimation after USPIO-P88 injection could provide important information for the future pancreatic BCM evaluation by MRI.

Targeting GLP-1 receptors for repeated magnetic resonance imaging differentiates graded losses of pancreatic beta cells in mice

AIMS/HYPOTHESIS

Non-invasive imaging of beta cells is a much-needed development but is one that faces significant biological and technological hurdles. A relevant imaging method should at least allow for an evaluation over time of the mass of beta cells under physiological and pathological conditions, and for an assessment of novel therapies. We, therefore, investigated the ability of a new MRI probe to repeatedly measure the loss of beta cells in a rodent model.

METHODS

We developed an innovative nanoparticle probe that targets the glucagon-like peptide 1 receptor, and can be used for both fluorescence imaging and MRI. Using fluorescence, we characterised the specificity and biodistribution of the probe. Using 1.5 T MRI, we longitudinally imaged the changes in insulin content in male and female mice of the RIP-DTr strain, which mimic the changes expected in type 1 and type 2 diabetes, respectively.

RESULTS

We showed that this probe selectively labelled beta cells in situ, imaged in vivo native pancreatic islets and evaluated their loss after diphtheria toxin administration, in a model of graded beta cell deletion. Thus, using clinical MRI, the probe quantitatively differentiates, in the same mouse strain, between female animals featuring a 50% loss of beta cells and the males featuring an almost complete loss of beta cells.

CONCLUSIONS/INTERPRETATION

The approach addresses several of the hurdles that have so far limited the non-invasive imaging of beta cells, including the potential to repeatedly monitor the very same animals using clinically available equipment, and to differentiate graded losses of beta cells.

Managing diabetes with nanomedicine: challenges and opportunities

Nanotechnology-based approaches hold substantial potential for improving the care of patients with diabetes. Nanoparticles are being developed as imaging contrast agents to assist in the early diagnosis of type 1 diabetes. Glucose nanosensors are being incorporated in implantable devices that enable more accurate and patient-friendly real-time tracking of blood glucose levels, and are also providing the basis for glucose-responsive nanoparticles that better mimic the body's physiological needs for insulin. Finally, nanotechnology is being used in non-invasive approaches to insulin delivery and to engineer more effective vaccine, cell and gene therapies for type 1 diabetes. Here, we analyse the current state of these approaches and discuss key issues for their translation to clinical practice.

64Cu- and 68Ga-labelled [Nle(14),Lys(40)(Ahx-NODAGA)NH2]-exendin-4 for pancreatic beta cell imaging in rats

PURPOSE

Glucagon-like peptide-1 receptor (GLP-1R) is a molecular target for imaging of pancreatic beta cells. We compared the ability of [Nle(14),Lys(40)(Ahx-NODAGA-(64)Cu)NH2]-exendin-4 ([(64)Cu]NODAGA-exendin-4) and [Nle(14),Lys(40)(Ahx-NODAGA-(68)Ga)NH2]-exendin-4 ([(68)Ga]NODAGA-exendin-4) to detect native pancreatic islets in rodents.

PROCEDURES

The stability, lipophilicity and affinity of the radiotracers to the GLP-1R were determined in vitro. The biodistribution of the tracers was assessed using autoradiography, ex vivo biodistribution and PET imaging. Estimates for human radiation dosimetry were calculated.

RESULTS

We found GLP-1R-specific labelling of pancreatic islets. However, the pancreas could not be visualised in PET images. The highest uptake of the tracers was observed in the kidneys. Effective dose estimates for [(64)Cu]NODAGA-exendin-4 and [(68)Ga]NODAGA-exendin-4 were 0.144 and 0.012 mSv/MBq, respectively.

CONCLUSION

[(64)Cu]NODAGA-exendin-4 might be more effective for labelling islets than [(68)Ga]NODAGA-exendin-4. This is probably due to the lower specific radioactivity of [(68)Ga]NODAGA-exendin-4 compared to [(64)Cu]NODAGA-exendin-4. The radiation dose in the kidneys may limit the use of [(64)Cu]NODAGA-exendin-4 as a clinical tracer.

Positron emission tomography ligand [11C]5-hydroxy-tryptophan can be used as a surrogate marker for the human endocrine pancreas

In humans, a well-developed serotonin system is localized to the pancreatic islets while being absent in exocrine pancreas. Assessment of pancreatic serotonin biosynthesis could therefore be used to estimate the human endocrine pancreas. Proof of concept was tested in a prospective clinical trial by comparisons of type 1 diabetic (T1D) patients, with extensive reduction of β-cells, with healthy volunteers (HVs). C-peptide-negative (i.e., insulin-deficient) T1D subjects (n = 10) and HVs (n = 9) underwent dynamic positron emission tomography with the radiolabeled serotonin precursor [(11)C]5-hydroxy-tryptophan ([(11)C]5-HTP). A significant accumulation of [(11)C]5-HTP was obtained in the pancreas of the HVs, with large interindividual variation. A substantial and highly significant reduction (66%) in the pancreatic uptake of [(11)C]5-HTP in T1D subjects was observed, and this was most evident in the corpus and caudal regions of the pancreas where β-cells normally are the major constituent of the islets. [(11)C]5-HTP retention in the pancreas was reduced in T1D compared with nondiabetic subjects. Accumulation of [(11)C]5-HTP in the pancreas of both HVs and subjects with T1D was in agreement with previously reported morphological observations on the β-cell volume, implying that [(11)C]5-HTP retention is a useful noninvasive surrogate marker for the human endocrine pancreas.

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.

Beta cell imaging - a key tool in optimized diabetes prevention and treatment

The prevalence of diabetes is 382 million worldwide, and is expected to rise to 592 million in 2035 (http://www.idf.org/diabetesatlas); 2.5-15% of national annual healthcare budgets are related to diabetes care, potentially increasing to 40% in high-prevalence countries. Beta cell dysfunction and death are central events in diabetes pathogenesis, but the natural history of beta cell loss remains unknown. Clinical imaging of beta cells will play a pivotal role in developing strategies for optimized diabetes prevention and treatment.

Grg3/TLE3 and Grg1/TLE1 induce monohormonal pancreatic β-cells while repressing α-cell functions

In the pancreas, α- and β-cells possess a degree of plasticity. In vitro differentiation of pluripotent cells yields mostly α- and polyhormonal β-like cells, indicating a gap in understanding of how functional monohormonal β-cells are formed and of the endogenous repressive mechanisms used to maintain β-cell identity. We show that the corepressor Grg3 is expressed in almost all β-cells throughout embryogenesis to adulthood. However, Grg3 is expressed in fewer nascent α-cells and is progressively lost from α-cells as endocrine cells mature into adulthood. We show that mouse Grg3(+/-) β-cells have increased α-specific gene expression, and Grg3(+/-) pancreata have more α-cells and more polyhormonal cells, indicating that Grg3 is required for the physiologic maintenance of monohormonal β-cell identity. Ectopic expression of Grg3 in α-cells represses glucagon and Arx, and the addition of Pdx1 induces Glut2 expression and glucose-responsive insulin secretion. Furthermore, we found that Grg1 is the predominant Groucho expressed in human β-cells but acts functionally similarly to Grg3. Overall, we find that Grg3 and Grg1 establish a monohormonal β-cell identity, and Groucho family members may be useful tools or markers for making functional β-cells.

GLP-1R-targeting magnetic nanoparticles for pancreatic islet imaging

Noninvasive assessment of pancreatic β-cell mass would tremendously aid in managing type 1 diabetes (T1D). Toward this goal, we synthesized an exendin-4 conjugated magnetic iron oxide-based nanoparticle probe targeting glucagon-like peptide 1 receptor (GLP-1R), which is highly expressed on the surface of pancreatic β-cells. In vitro studies in βTC-6, the β-cell line, showed specific accumulation of the targeted probe (termed MN-Ex10-Cy5.5) compared with nontargeted (termed MN-Cy5.5). In vivo magnetic resonance imaging showed a significant transverse relaxation time (T2) shortening in the pancreata of mice injected with the MN-Ex10-Cy5.5 probe compared with control animals injected with the nontargeted probe at 7.5 and 24 h after injection. Furthermore, ΔT2 of the pancreata of prediabetic NOD mice was significantly higher than that of diabetic NOD mice after the injection of MN-Ex10-Cy5.5, indicating the decrease of probe accumulation in these animals due to β-cell loss. Of note, ΔT2 of prediabetic and diabetic NOD mice injected with MN-Cy5.5 was not significantly changed, reflecting the nonspecific mode of accumulation of nontargeted probe. We believe our results point to the potential for using this agent for monitoring the disease development and response of T1D to therapy.

Osteocalcin promotes β-cell proliferation during development and adulthood through Gprc6a

Expanding β-cell mass through β-cell proliferation is considered a potential therapeutic approach to treat β-cell failure in diabetic patients. A necessary step toward achieving this goal is to identify signaling pathways that regulate β-cell proliferation in vivo. Here we show that osteocalcin, a bone-derived hormone, regulates β-cell replication in a cyclin D1-dependent manner by signaling through the Gprc6a receptor expressed in these cells. Accordingly, mice lacking Gprc6a in the β-cell lineage only are glucose intolerant due to an impaired ability to produce insulin. Remarkably, this regulation occurs during both the perinatal peak of β-cell proliferation and in adulthood. Hence, the loss of osteocalcin/Gprc6a signaling has a profound effect on β-cell mass accrual during late pancreas morphogenesis. This study extends the endocrine role of osteocalcin to the developmental period and establishes osteocalcin/Gprc6a signaling as a major regulator of β-cell endowment that can become a potential target for β-cell proliferative therapies.

The required beta cell research for improving treatment of type 2 diabetes

In healthy individuals, insulin resistance is associated with physiological conditions such as pregnancy or body weight gain and triggers an increase in beta cell number and insulin secretion capacity to preserve normoglycaemia. Failure of this beta cell compensation capacity is a fundamental cause of diabetic hyperglycaemia. Incomplete understanding of the molecular mechanisms controlling the plasticity of adult beta cells mechanisms and how these cells fail during the pathogenesis of diabetes strongly limits the ability to develop new beta cell-specific therapies. Here, current knowledge of the signalling pathways controlling beta cell plasticity is reviewed, and possible directions for future research are discussed.

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.

Imaging beta-cell mass and function in situ and in vivo

Glucose-stimulated insulin secretion (GSIS) from pancreatic beta-cells is critical to the maintenance of blood glucose homeostasis in animals. Both decrease in pancreatic beta-cell mass and defects in beta-cell function contribute to the onset of diabetes, although the underlying mechanisms remain largely unknown. Molecular imaging techniques can help beta-cell study in a number of ways. High-resolution fluorescence imaging techniques provide novel insights into the fundamental mechanisms underlying GSIS in isolated beta-cells or in situ in pancreatic islets, and dynamic changes of beta-cell mass and function can be noninvasively monitored in vivo by imaging techniques such as positron emission tomography and single-photon emission computed tomography. All these techniques will contribute to the better understanding of the progression of diabetes and the search for the optimized therapeutic measures that reverse deficits in beta-cell mass and function.

Revisiting the notion of type 1 diabetes being a T-cell-mediated autoimmune disease

PURPOSE OF REVIEW

Type 1 diabetes (T1D) research is at present in a critical period of development and during the past few years several large phase III studies targeting T-cell autoimmunity in recent-onset patients with T1D failed to reach the primary endpoint.

RECENT FINDINGS

Cause and pathogenesis of T1D remain largely unknown. In humans, insulitis is discrete, affects few islets and is present only in about one-third of patients with recent-onset T1D. The rapid increase in incidence of T1D argues against a decisive role for genetic factors and instead for the hypothesis that infectious agents, possibly entering the pancreas via the ductal compartment, are involved in disease pathogenesis. Repeated episodes of bacteria or virus-induced innate inflammations affecting only certain lobes of the pancreas fit well with the reported heterogeneity of the disease within the pancreas as well as with the slow progression over many years.

SUMMARY

In humans there is limited support for T1D being primarily an autoimmune disease; instead available findings support the view that T1D can be regarded as an innate inflammatory disease affecting the entire pancreas, but with its main clinical manifestations emanating from the loss of the insulin-producing cells.

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.

Theranostic magnetic resonance imaging of type 1 diabetes and pancreatic islet transplantation

Type 1 diabetes mellitus results in impaired insulin production by pancreatic islets due to autoimmunity. Islet transplantation has recently emerged as a promising treatment for this disease. To visualize and monitor endogenous and transplanted islets, non-invasive strategies are currently being developed. These include strategies for in vivo magnetic resonance imaging of microvascular changes during diabetes development, tracking the recruitment of diabetogenic T cells to the pancreas, and imaging of endogenous and transplanted islet mass. The combination of MR imaging agents with therapy is a novel state-of-the-art theranostic approach that has a tremendous potential for type 1 diabetes management. Though still in its infancy, theranostic MR imaging has shown certain encouraging progress. Here we provide an overview of the latest accomplishments in this area as it applies to changes in islet vasculature during diabetes development, monitoring autoimmune attack mediated by T cells, and imaging of transplanted islets. Future challenges and opportunities in the area of theranostic MRI are discussed as well.

In vivo tracking techniques for cellular regeneration, replacement, and redirection

Cellular therapy can be defined as the transplantation of living cells for the treatment of medical conditions. Three main objectives of cellular therapy are regeneration of damaged tissue, replacement of function by secretion of biologically active molecules, and redirection of aberrant processes. Given the complex nature of these approaches, in vivo tracking of the transplanted cells is critical to evaluate their potential benefit and to optimize treatment strategies. Recent advances are reviewed that enable in vivo cell tracking as an important adjunct to implement cellular therapy in clinical practice.

Vesicular monoamine transporter, type 2 (VMAT2) expression as it compares to insulin and pancreatic polypeptide in the head, body and tail of the human pancreas

The vesicular monoamine transporter, type 2 (VMAT2) is responsible for sequestering monoamine neurotransmitters into exocytic vesicles in neurons, enterochromaffin-like cells of the stomach and cells arising from the common myeloid progenitor. VMAT2 is also present in the pancreas and is expressed by insulin producing β cells, but not by glucagon or somatostatin expressing islet cells. Positron emission tomography (PET) targeting of VMAT2 is currently being evaluated as a non-invasive tool to measure β cell mass (BCM) in living humans. In recent trials, PET measurements of VMAT2 in the pancreas overestimated BCM in type 1 diabetes (T1D) patients predicted to have little to no BCM by metabolic measures. Recently, tissue immunohistochemistry studies suggested that VMAT2 staining may also co-localize with pancreatic polypeptide (PP) staining cells in pancreas tissue, but these studies were not quantitative. In this report, we evaluated VMAT2 specificity for β cells in sub-regions of the human pancreas using antibodies targeting VMAT2, insulin and PP by double-label immunofluorescence. Immunostaining for VMAT2 and insulin demonstrated 89 ± 8% overlap in the body and tail of the pancreas. However, 44 ± 12% and 53 ± 15% of VMAT2 cells co-stained with PP- and insulin-staining cells, respectively in the pancreatic head. Significant co-staining for VMAT2 and PP cells in the head of the pancreas may partly explain the apparent overestimation of BCM in T1D by PET. Specific targeting of the pancreatic body and tail using VMAT2 PET scanning may reflect BCM more accurately.