Tools for Bioimaging Pancreatic β Cells in Diabetes

Indole-Based and Cyclopentenylindole-Based Analogues Containing Fluorine Group as Potential 18F-Labeled Positron Emission Tomography (PET) G-Protein Coupled Receptor 44 (GPR44) Tracers

Recently, growing evidence of the relationship between G-protein coupled receptor 44 (GPR44) and the inflammation-cancer system has garnered tremendous interest, while the exact role of GPR44 has not been fully elucidated. Currently, there is a strong and urgent need for the development of non-invasive in vivo GPR44 positron emission tomography (PET) radiotracers that can be used to aid the exploration of the relationship between inflammation and tumor biologic behavior. Accordingly, the choosing and radiolabeling of existing GPR44 antagonists containing a fluorine group could serve as a viable method to accelerate PET tracers development for in vivo imaging to this purpose. The present study aims to evaluate published (2000-present) indole-based and cyclopentenyl-indole-based analogues of the GPR44 antagonist to guide the development of fluorine-18 labeled PET tracers that can accurately detect inflammatory processes. The selected analogues contained a crucial fluorine nuclide and were characterized for various properties including binding affinity, selectivity, and pharmacokinetic and metabolic profile. Overall, 26 compounds with favorable to strong binding properties were identified. This review highlights the potential of GPR44 analogues for the development of PET tracers to study inflammation and cancer development and ultimately guide the development of targeted clinical therapies.

Acid-Resistant BODIPY Amino Acids for Peptide-Based Fluorescence Imaging of GPR54 Receptors in Pancreatic Islets

The G protein-coupled kisspeptin receptor (GPR54 or KISS1R) is an important mediator in reproduction, metabolism and cancer biology; however, there are limited fluorescent probes or antibodies for direct imaging of these receptors in cells and intact tissues, which can help to interrogate their multiple biological roles. Herein, we describe the rational design and characterization of a new acid-resistant BODIPY-based amino acid (Trp-BODIPY PLUS), and its implementation for solid-phase synthesis of fluorescent bioactive peptides. Trp-BODIPY PLUS retains the binding capabilities of both short linear and cyclic peptides and displays notable turn-on fluorescence emission upon target binding for wash-free imaging. Finally, we employed Trp-BODIPY PLUS to prepare some of the first fluorogenic kisspeptin-based probes and visualized the expression and localization of GPR54 receptors in human cells and in whole mouse pancreatic islets by fluorescence imaging.

Acid-Resistant BODIPY Amino Acids for Peptide-Based Fluorescence Imaging of GPR54 Receptors in Pancreatic Islets

The G protein-coupled kisspeptin receptor (GPR54 or KISS1R) is an important mediator in reproduction, metabolism and cancer biology; however, there are limited fluorescent probes or antibodies for direct imaging of these receptors in cells and intact tissues, which can help to interrogate their multiple biological roles. Herein, we describe the rational design and characterization of a new acid-resistant BODIPY-based amino acid (Trp-BODIPY PLUS), and its implementation for solid-phase synthesis of fluorescent bioactive peptides. Trp-BODIPY PLUS retains the binding capabilities of both short linear and cyclic peptides and displays notable turn-on fluorescence emission upon target binding for wash-free imaging. Finally, we employed Trp-BODIPY PLUS to prepare some of the first fluorogenic kisspeptin-based probes and visualized the expression and localization of GPR54 receptors in human cells and in whole mouse pancreatic islets by fluorescence imaging.

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.

Responses of INS-1 cells to glucose stimulation patterns

Diabetes has become a major public health problem in the world for many years, and it is driving us to probe into its complex mechanism of insulin secretion in pancreatic β cells. The nanoscale resolution characterization of pancreatic β cells in response to glucose led to insights into diverse mechanical and functional processes at the single cell level. Recent advances allowed the direct observations of cytoskeleton dynamics which were quantitatively determined. Here, we firstly performed the glucose stimulation with multiple physiologically relevant glucose patterns. Atomic force microscopy (AFM) produced high spatial resolution mechanical images together with the insulin secretions linking the physical interactions to the biochemical process of INS-1 cells. Altered material properties of the INS-1 cells revealed the regulation of multiple glucose stimulation patterns. Rapidly responded to high glucose (HG), INS-1 cells presented the unique meshing networks of elasticities. The decreases of Young's modulus (YM) and insulin secretion suggested that mechanical changes affected the insulin release. Furthermore, the frequency and gradient of glucose patterns induced nanomechanical and secreting changes of the INS-1 cells and gained the knowledge on the potential controllability of glucose. The relationships between the cellular mechanics and insulin secretion of INS-1 cells could contribute to establish a mechanical cell model for the study of β cells in diabetes. The results also indicated the cell mechanics as promising mechanical biomarkers for β cells, and promoted the understanding of specific mechanical mechanism of glucose regulation, which lighted on the further application of functional glucose regulation in therapy.

Expanded LUXendin Color Palette for GLP1R Detection and Visualization In Vitro and In Vivo

The glucagon-like peptide-1 receptor (GLP1R) is expressed in peripheral tissues and the brain, where it exerts pleiotropic actions on metabolic and inflammatory processes. Detection and visualization of GLP1R remains challenging, partly due to a lack of validated reagents. Previously, we generated LUXendins, antagonistic red and far-red fluorescent probes for specific labeling of GLP1R in live and fixed cells/tissues. We now extend this concept to the green and near-infrared color ranges by synthesizing and testing LUXendin492, LUXendin551, LUXendin615, and LUXendin762. All four probes brightly and specifically label GLP1R in cells and pancreatic islets. Further, LUXendin551 acts as a chemical beta cell reporter in preclinical rodent models, while LUXendin762 allows noninvasive imaging, highlighting differentially accessible GLP1R populations. We thus expand the color palette of LUXendins to seven different spectra, opening up a range of experiments using wide-field microscopy available in most labs through super-resolution imaging and whole animal imaging. With this, we expect that LUXendins will continue to generate novel and specific insights into GLP1R biology.

The Current State of Beta-Cell-Mass PET Imaging for Diabetes Research and Therapies

Diabetes is a chronic metabolic disease affecting over 400 million people worldwide and one of the leading causes of death, especially in developing nations. The disease is characterized by chronic hyperglycemia, caused by defects in the insulin secretion or action pathway. Current diagnostic methods measure metabolic byproducts of the disease such as glucose level, glycated hemoglobin (HbA1c), insulin or C-peptide levels, which are indicators of the beta-cell function. However, they inaccurately reflect the disease progression and provide poor longitudinal information. Beta-cell mass has been suggested as an alternative approach to study disease progression in correlation to beta-cell function, as it behaves differently in the diabetes physiopathology. Study of the beta-cell mass, however, requires highly invasive and potentially harmful procedures such as pancreatic biopsies, making diagnosis and monitoring of the disease tedious. Nuclear medical imaging techniques using radiation emitting tracers have been suggested as strong non-invasive tools for beta-cell mass. A highly sensitive and high-resolution technique, such as positron emission tomography, provides an ideal solution for the visualization of beta-cell mass, which is particularly essential for better characterization of a disease such as diabetes, and for estimating treatment effects towards regeneration of the beta-cell mass. Development of novel, validated biomarkers that are aimed at beta-cell mass imaging are thus highly necessary and would contribute to invaluable breakthroughs in the field of diabetes research and therapies. This review aims to describe the various biomarkers and radioactive probes currently available for positron emission tomography imaging of beta-cell mass, as well as highlight the need for precise quantification and visualization of the beta-cell mass for designing new therapy strategies and monitoring changes in the beta-cell mass during the progression of diabetes.

68Ga-labelled-exendin-4: New GLP1R targeting agents for imaging pancreatic β-cell and insulinoma

OBJECTIVE

Glucagon-like peptide-1 receptor (GLP1R) specifically expressed on the surface of pancreatic β-cells and insulinoma, is a potential biomarker for imaging β-cell mass (BCM). In this study, two new 68Ga-labelled GLP1R targeting agents were prepared and their biological properties for imaging BCM and insulinoma were evaluated.

METHODS

[68Ga]Ga-HBED-CC-MAL-Cys39-exendin-4 ([68Ga]Ga-4) and its dimer ([68Ga]Ga-5) were synthesized from corresponding precursors. Cell uptake studies were evaluated in INS-1 cells. Biodistribution and microPET studies were performed in male normal Sprague-Dawley rats, diabetic rats and insulinoma xenograft NOD/SCID mice.

RESULTS

[68Ga]Ga-4 and [68Ga]Ga-5 were efficiently radiolabelled by a simple one-step reaction without purification leading to high radiochemical yields and radiochemical purities (both >95%, decay corrected, n = 6, molar activity 15 GBq/μmol). They both showed excellent stability (~95%) in phosphate-buffered saline, pH 7.4, and in rat serum (~90%) for 2 h. Biodistribution studies and small animal PET/CT imaging showed that [68Ga]Ga-4 displayed specific uptake in rat pancreas and mouse insulinoma, and a reduced uptake in the pancreas of diabetic rat was observed (~62% reduction). Notably, it exhibited a rapid time-to-peak pancreatic uptake (0.96 ± 0.19%ID/g in 15 min) and fast clearance from the kidney (42% clearance in 30 min). Results suggested a favorable in vivo kinetics for human imaging studies.

CONCLUSIONS

[68Ga]Ga-4 targeting GLP1R of pancreatic β-cells may be a potentially useful PET agent and a suitable candidate for further structural modification studies. This agent has demonstrated several advantages, rapid time-to-peak pancreatic uptake and faster clearance from the kidney, factors may enhance diagnosis of diabetes and insulinoma.

The Human Islet: Mini-Organ With Mega-Impact

This review focuses on the human pancreatic islet-including its structure, cell composition, development, function, and dysfunction. After providing a historical timeline of key discoveries about human islets over the past century, we describe new research approaches and technologies that are being used to study human islets and how these are providing insight into human islet physiology and pathophysiology. We also describe changes or adaptations in human islets in response to physiologic challenges such as pregnancy, aging, and insulin resistance and discuss islet changes in human diabetes of many forms. We outline current and future interventions being developed to protect, restore, or replace human islets. The review also highlights unresolved questions about human islets and proposes areas where additional research on human islets is needed.

Elucidating Gold-MnO2 Core-Shell Nanoenvelope for Real Time SERS-Guided Photothermal Therapy on Pancreatic Cancer Cells

Pancreatic cancer represents one of the most aggressive in nature with a miserable prognosis that warrants efficient diagnostic and therapeutic interventions. Herein, a MnO₂ overlaid gold nanoparticle (AuNPs) based photothermal theranostic nanoenvelope (PTTNe:MnO₂@AuNPs) was fabricated to substantiate surface-enhanced Raman spectroscopy (SERS) guided real-time monitoring of photothermal therapy (PTT) in pancreatic cancer cells. A sharp enhancement of the fingerprint Raman signature of MnO₂ at 569 cm^-1 exhibited as a marker peak for the first time to elucidate the intracellular PTT event. In this strategic design, the leftover bare AuNPs after the degradation of the MnO₂ layer from the nanoenvelope in the presence of intracellular H₂O₂ enabled real-time tracking of biomolecular changes of Raman spectral variations during PTT. Moreover, the surface of the as-synthesized nanoenvelope was functionalized with a pancreatic cancer cell targeting peptide sequence for cholecystokinin fashioned the PTTNe with admirable stability and biocompatibility. Finally, the precise cell death mechanism was explicitly assessed by SERS spectral analysis as a complementary technique. This targeted phototheranostic approach demonstrated in pancreatic cancer cells presented a therapeutically viable prototype for futuristic personalized cancer nanomedicine.

PET Imaging of GPR44 by Antagonist [11C]MK-7246 in Pigs

A validated imaging marker for beta-cell mass would improve understanding of diabetes etiology and enable new strategies in therapy development. We previously identified the membrane-spanning protein GPR44 as highly expressed and specific to the beta cells of the pancreas. The selective GPR44 antagonist MK-7246 was radiolabeled with carbon-11 and the resulting positron-emission tomography (PET) tracer [¹¹C]MK-7246 was evaluated in a pig model and in vitro cell lines. The [¹¹C]MK-7246 compound demonstrated mainly hepatobiliary excretion with a clearly defined pancreas, no spillover from adjacent tissues, and pancreatic binding similar in magnitude to the previously evaluated GPR44 radioligand [¹¹C]AZ12204657. The binding could be blocked by preadministration of nonradioactive MK-7246, indicating a receptor-binding mechanism. [¹¹C]MK-7246 showed strong potential as a PET ligand candidate for visualization of beta-cell mass (BCM) and clinical translation of this methodology is ongoing.

GLP-1 peptide analogs for targeting pancreatic beta cells

Loss or dysfunction of the pancreatic beta cells or insulin receptors leads to diabetes mellitus (DM). This usually occurs over many years; therefore, the development of methods for the timely detection and clinical intervention are vital to prevent the development of this disease. Glucagon-like peptide-1 receptor (GLP-1R) is the receptor of GLP-1, an incretin hormone that causes insulin secretion in a glucose-dependent manner. GLP-1R is highly expressed on the surface of pancreatic beta cells, providing a potential target for bioimaging. In this review, we provide an overview of various strategies, such as the development of GLP-1R agonists (e.g., exendin-4), and GLP-1 sequence modifications for GLP-1R targeting for the diagnosis and treatment of pancreatic beta cell disorders. We also discuss the challenges of targeting pancreatic beta cells and strategies to address such challenges.

Metabolic roles of G protein-coupled receptor signaling in obesity and type 2 diabetes

The incidence of obesity and type 2 diabetes (T2D) has been increasing steadily worldwide. It is estimated that by 2045 more than 800 million people will be suffering from diabetes. Despite the advancements in modern medicine, more effective therapies for treating obesity and T2D are needed. G protein-coupled receptors (GPCRs) have emerged as important drug targets for various chronic diseases, including obesity, T2D, and liver diseases. During the past two decades, many laboratories worldwide focused on understanding the role of GPCR signaling in regulating glucose metabolism and energy homeostasis. The information gained from these studies can guide the development of novel therapeutic agents. In this review, we summarize recent studies providing insights into the role of GPCR signaling in peripheral, metabolically important tissues such as pancreas, liver, skeletal muscle, and adipose tissue, focusing primarily on the use of mutant animal models and human data.

β-Cell Dysfunction, Hepatic Lipid Metabolism, and Cardiovascular Health in Type 2 Diabetes: New Directions of Research and Novel Therapeutic Strategies

Cardiovascular disease (CVD) remains a major problem for people with type 2 diabetes mellitus (T2DM), and dyslipidemia is one of the main drivers for both metabolic diseases. In this review, the major pathophysiological and molecular mechanisms of β-cell dysfunction and recovery in T2DM are discussed in the context of abnormal hepatic lipid metabolism and cardiovascular health. (i) In normal health, continuous exposure of the pancreas to nutrient stimulus increases the demand on β-cells. In the long term, this will not only stress β-cells and decrease their insulin secretory capacity, but also will blunt the cellular response to insulin. (ii) At the pre-diabetes stage, β-cells compensate for insulin resistance through hypersecretion of insulin. This increases the metabolic burden on the stressed β-cells and changes hepatic lipoprotein metabolism and adipose tissue function. (iii) If this lipotoxic hyperinsulinemic environment is not removed, β-cells start to lose function, and CVD risk rises due to lower lipoprotein clearance. (iv) Once developed, T2DM can be reversed by weight loss, a process described recently as remission. However, the precise mechanism(s) by which calorie restriction causes normalization of lipoprotein metabolism and restores β-cell function are not fully established. Understanding the pathophysiological and molecular basis of β-cell failure and recovery during remission is critical to reduce β-cell burden and loss of function. The aim of this review is to highlight the link between lipoprotein export and lipid-driven β-cell dysfunction in T2DM and how this is related to cardiovascular health. A second aim is to understand the mechanisms of β-cell recovery after weight loss, and to explore new areas of research for developing more targeted future therapies to prevent T2DM and the associated CVD events.

A century later, still fighting back: antigen-specific immunotherapies for type 1 diabetes

Type 1 diabetes (T1D) is a chronic metabolic disease caused by the autoimmune destruction of insulin-producing β-cells. Ever since the 1920s, the fate of patients suffering from T1D was dramatically improved owing to the isolation and production of insulin, and the scientific field has largely progressed as a result of the evidence gathered about its underpinnings and mechanisms. The last years have seen this knowledge transformed into actual antigen-specific immunotherapies with potential to restore selectively the breach of tolerance to β-cell autoantigens and halt the autoimmune aggression. However, so far, the results of both prevention and reversion trials in T1D have been rather discouraging, so there is still an urgent need to optimize those immunotherapies and their associated factors, for example, posology and administration patterns, route and timing. In this review, we look back on what has been achieved in the last century and identify the main autoantigens driving the autoimmune attack in T1D. Then, we take a deep dive into the numerous antigen-specific immunotherapies trialed and the ones still at a preclinical phase, ranging from peptides, proteins and agent combinations to gene transfer, nanoparticles, cell-based strategies and novel approaches exploiting naturally occurring tolerogenic processes. Finally, we provide insight into the several features to be considered in a T1D clinical trial, the ideal time point for intervention and the biomarkers needed for monitoring the successful regulatory effect of the antigen-specific immunotherapy. Although further research and optimization remain imperative, the development of a therapeutic armamentarium against T1D autoimmunity is certainly advancing with a confident step.

Novel Strategies to Protect and Visualize Pancreatic β Cells in Diabetes

A common feature in the pathophysiology of different types of diabetes is the reduction of β cell mass and/or impairment of β cell function. Diagnosis and treatment of type 1 and type 2 diabetes is currently hampered by a lack of reliable techniques to restore β cell survival, to improve insulin secretion, and to quantify β cell mass in patients. Current new approaches may allow us to precisely and specifically visualize β cells in vivo and provide viable therapeutic strategies to preserve, recover, and regenerate β cells. In this review, we discuss recent protective approaches for β cells and the advantages and limitations of current imaging probes in the field.

Current Progress and Perspective: Clinical Imaging of Islet Transplantation

Islet transplantation has great potential as a cure for type 1 diabetes. At present; the lack of a clinically validated non-invasive imaging method to track islet grafts limits the success of this treatment. Some major clinical imaging modalities and various molecular probes, which have been studied for non-invasive monitoring of transplanted islets, could potentially fulfill the goal of understanding pathophysiology of the functional status and viability of the islet grafts. In this current review, we summarize the recent clinical studies of a variety of imaging modalities and molecular probes for non-invasive imaging of transplanted beta cell mass. This review also includes discussions on in vivo detection of endogenous beta cell mass using clinical imaging modalities and various molecular probes, which will be useful for longitudinally detecting the status of islet transplantation in Type 1 diabetic patients. For the conclusion and perspectives, we highlight the applications of multimodality and novel imaging methods in islet transplantation.

Lycopene in protection against obesity and diabetes: A mechanistic review

Lycopene, a natural pigment that mainly exists in the mature fruit of tomatoes, has gained increasing attention due to its protective effects against obesity and diabetes. The aim of this review is to summarize the potential mechanisms in which lycopene exerts protection against obesity and diabetes, along with highlighting its bioavailability, synthesis and safety. Literature sources used in this review were from the PubMed Database, China Knowledge Resource Integrated Database, China Science and Technology Journal Database, National Science and Technology Library, Wanfang Data, and the Web of Science. For the inquiries, keywords such as lycopene, properties, synthesis, diabetes, obesity, and safety were used in various combinations. About 200 articles and reviews were evaluated. Lycopene exhibits anti-obesity and anti-diabetic activities in different organs and/or tissues, including adipose tissue, liver, kidney, pancreas, brain, ovaries, intestine, and eyes. The underlying mechanism may be attributed to its anti-oxidant and anti-inflammatory properties and through its ability to regulate of AGE/RAGE, JNK/MAPK, PI3K/Akt, SIRT1/FoxO1/PPARγ signaling pathways and AchE activity. The epidemiological investigations support that lycopene consumption may contribute to lowering the risk of obesity and diabetes. The cis-isomers of lycopene are more bioavailable and better absorbed than trans-lycopene, and mainly distribute in liver and adipose tissue. Lycopene exhibits a good margin of safety and can be obtained by plant extraction, chemical synthesis and microbial fermentation. In summary, lycopene consumption beneficially contributes to protecting against diabetes and obesity in animal studies and epidemiological investigations, which supports the potential of this compound as a preventive/therapeutic agent against these disorders. Well-designed, prospective clinical studies are warranted to evaluate the potential therapeutic effect of lycopene against common metabolic diseases.

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.

Advances in GLP-1 receptor targeting radiolabeled agent development and prospective of theranostics

In the light of theranostics/radiotheranostics and prospective of personalized medicine in diabetes and oncology, this review presents prior and current advances in the development of radiolabeled imaging and radiotherapeutic exendin-based agents targeting glucagon-like peptide-1 receptor. The review covers chemistry, preclinical, and clinical evaluation. Such critical aspects as structure-activity-relationship, stability, physiological potency, kidney uptake, and dosimetry are discussed.