Species-specific vesicular monoamine transporter 2 (VMAT2) expression in mammalian pancreatic beta cells: implications for optimising radioligand-based human beta cell mass (BCM) imaging in animal models

Pancreatic acinar cells utilize tyrosine to synthesize L-dihydroxyphenylalanine

The pancreatic β cells can synthesize dopamine by taking L-dihydroxyphenylalanine, but whether pancreatic acinar cells synthesize dopamine has not been confirmed. By means of immunofluorescence, the tyrosine hydroxylase -immunoreactivity and aromatic amino acid decarboxylase (AADC)- immunoreactivity were respectively observed in pancreatic acinar cells and islet β cells. Treatment with L-dihydroxyphenylalanine, not tyrosine, caused the production of dopamine in the incubation of INS-1 cells (rat islet β cell line) and primary isolated islets, which was blocked by AADC inhibitor NSD-1015. However, only L-dihydroxyphenylalanine, but not dopamine, was detected when AR42J cells (rat pancreatic acinar cell line) were treated with tyrosine, which was blocked by tyrosine hydroxylase inhibitor AMPT. Dopamine was detected in the coculture of INS-1 cells with AR42J cells after treatment with tyrosine. In an in vivo study, pancreatic juice contained high levels of L-dihydroxyphenylalanine and dopamine. Both L-dihydroxyphenylalanine and dopamine accompanied with pancreatic enzymes and insulin in the pancreatic juice were all significantly increased after intraperitoneal injection of bethanechol chloride and their increases were all blocked by atropine. Inhibiting TH with AMPT blocked bethanechol chloride-induced increases in L-dihydroxyphenylalanine and dopamine, while inhibiting AADC with NSD-1015 only blocked the dopamine increase. Bilateral subdiaphragmatic vagotomy of rats leads to significant decreases of L-dihydroxyphenylalanine and dopamine in pancreatic juice. These results suggested that pancreatic acinar cells could utilize tyrosine to synthesize L-dihydroxyphenylalanine, not dopamine. Islet β cells only used L-dihydroxyphenylalanine, not tyrosine, to synthesize dopamine. Both L-dihydroxyphenylalanine and dopamine were respectively released into the pancreatic duct, which was regulated by the vagal cholinergic pathway. The present study provides important evidences for the source of L-dihydroxyphenylalanine and dopamine in the pancreas.

VMAT2 Safeguards β-Cells Against Dopamine Cytotoxicity Under High-Fat Diet-Induced Stress

Vesicular monoamine transporter 2 (VMAT2) uptakes cytoplasmic monoamines into vesicles for storage. VMAT2 plays a role in modulating insulin release by regulating dopamine levels in the pancreas, although the exact mechanism remains elusive. We found that VMAT2 expression in β-cells specifically increases under high blood glucose conditions. The islets isolated from β-cell-specific Vmat2 knockout (βVmat2KO) mice show elevated insulin secretion levels in response to glucose stimulation. Under prolonged high-fat diet feedings, the βVmat2KO mice exhibit impaired glucose and insulin tolerance and progressive β-cell dysfunction. Here we demonstrate VMAT2 uptake of dopamine to protect dopamine from degradation by monoamine oxidase, thereby safeguarding β-cells from excess reactive oxygen species (ROS) exposure. In the context of high demand for insulin secretion, the absence of VMAT2 leads to elevated ROS in β-cells, which accelerates β-cell dedifferentiation and β-cell loss. Therefore, VMAT2 controls the amount of dopamine in β-cells, thereby protecting pancreatic β-cells from excessive oxidative stress.

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.

EANM practice guideline/SNMMI procedure standard for dopaminergic imaging in Parkinsonian syndromes 1.0

Purpose

This joint practice guideline or procedure standard was developed collaboratively by the European Association of Nuclear Medicine (EANM) and the Society of Nuclear Medicine and Molecular Imaging (SNMMI). The goal of this guideline is to assist nuclear medicine practitioners in recommending, performing, interpreting, and reporting the results of dopaminergic imaging in parkinsonian syndromes.

Methods

Currently nuclear medicine investigations can assess both presynaptic and postsynaptic function of dopaminergic synapses. To date both EANM and SNMMI have published procedural guidelines for dopamine transporter imaging with single photon emission computed tomography (SPECT) (in 2009 and 2011, respectively). An EANM guideline for D2 SPECT imaging is also available (2009). Since the publication of these previous guidelines, new lines of evidence have been made available on semiquantification, harmonization, comparison with normal datasets, and longitudinal analyses of dopamine transporter imaging with SPECT. Similarly, details on acquisition protocols and simplified quantification methods are now available for dopamine transporter imaging with PET, including recently developed fluorinated tracers. Finally, [¹⁸F]fluorodopa PET is now used in some centers for the differential diagnosis of parkinsonism, although procedural guidelines aiming to define standard procedures for [¹⁸F]fluorodopa imaging in this setting are still lacking.

Conclusion

All these emerging issues are addressed in the present procedural guidelines for dopaminergic imaging in parkinsonian syndromes.

News ways of understanding the complex biology of diabetes using PET

The understanding of metabolic disease and diabetes on a molecular level has increased significantly due to the recent advances in molecular biology and biotechnology. However, in vitro studies and animal models do not always translate to the human disease, perhaps illustrated by the failure of many drug candidates in the clinical phase. Non-invasive biomedical imaging techniques such as Positron Emission Tomography (PET) offer tools for direct visualization and quantification of molecular processes in humans. Developments in this area potentially enable longitudinal in vivo studies of receptors and processes involved in diabetes guiding drug development and diagnosis in the near future. This mini-review focuses on describing the overall perspective of how PET can be used to increase our understanding and improve treatment of diabetes. The methodological aspects and future developments and challenges are highlighted.

Decreased striatal vesicular monoamine transporter 2 (VMAT2) expression in a type 1 diabetic rat model: A longitudinal study using micro-PET/CT

AIMS

Diabetes mellitus is a risk factor for Parkinson's disease. These diseases share similar pathogenic pathways, such as mitochondrial dysfunction, inflammation, and altered metabolism. Despite these similarities, the pathogenic relationship between these two diseases is unclear. [¹⁸F]FP-(+)-DTBZ is a promising radiotracer targeting VMAT2, which has been used to measure β-cell mass and to diagnose Parkinson's disease. The aim of this study was to examine the effect of type 1 diabetes on VMAT2 expression in the striatum using [¹⁸F]FP-(+)-DTBZ.

MATERIALS AND METHODS

A longitudinal study of type 1 diabetic rats was established by intraperitoneally injecting male Wistar rats with streptozotocin. Rats injected with saline were used as the control group. Glucose level, body weight, and [¹⁸F]FP-(+)-DTBZ uptake in the striatum and pancreas were evaluated at 0.5, 1, 4, 6 and 12 months after STZ or saline injection.

RESULTS

At one-half month post-STZ injection, the glucose levels in these rats increased and then returned to a normal level at 6 months. Along with increased glucose levels, body weight was also decreased significantly and returned slowly to a normal level. β-Cell mass and striatal [¹⁸F]FP-(+)-DTBZ uptake were impaired significantly at 2 weeks post-STZ injection in type 1 diabetic rats and returned to a normal level at 6 and 4 months post-STZ injection.

CONCLUSIONS

Due to increased glucose levels and decreased β-cell mass, decreased [¹⁸F]FP-(+)-DTBZ uptake in the striatum was observed in type 1 diabetic rats. Decreased BCM and increased glucose levels were correlated with VMAT2 expression in the striatum which indicated DM is a risk factor for PD.

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.

Life-long impairment of glucose homeostasis upon prenatal exposure to psychostimulants

Maternal drug abuse during pregnancy is a rapidly escalating societal problem. Psychostimulants, including amphetamine, cocaine, and methamphetamine, are amongst the illicit drugs most commonly consumed by pregnant women. Neuropharmacology concepts posit that psychostimulants affect monoamine signaling in the nervous system by their affinities to neurotransmitter reuptake and vesicular transporters to heighten neurotransmitter availability extracellularly. Exacerbated dopamine signaling is particularly considered as a key determinant of psychostimulant action. Much less is known about possible adverse effects of these drugs on peripheral organs, and if in utero exposure induces lifelong pathologies. Here, we addressed this question by combining human RNA-seq data with cellular and mouse models of neuroendocrine development. We show that episodic maternal exposure to psychostimulants during pregnancy coincident with the intrauterine specification of pancreatic β cells permanently impairs their ability of insulin production, leading to glucose intolerance in adult female but not male offspring. We link psychostimulant action specifically to serotonin signaling and implicate the sex-specific epigenetic reprogramming of serotonin-related gene regulatory networks upstream from the transcription factor Pet1/Fev as determinants of reduced insulin production.

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.

Is it time for a new classification of mast cells? What do we know about mast cell heterogeneity?

Mast cells (MCs) are derived from committed precursors that leave the hematopoietic tissue, migrate in the blood, and colonize peripheral tissues where they terminally differentiate under microenvironment stimuli. They are distributed in almost all vascularized tissues where they act both as immune effectors and housekeeping cells, contributing to tissue homeostasis. Historically, MCs were classified into 2 subtypes, according to tryptic enzymes expression. However, MCs display a striking heterogeneity that reflects a complex interplay between different microenvironmental signals delivered by various tissues, and a differentiation program that decides their identity. Moreover, tissue-specific MCs show a trained memory, which contributes to shape their function in a specific microenvironment. In this review, we summarize the current state of our understanding of MC heterogeneity that reflects their different tissue experiences. We describe the discovery of unique cell molecules that can be used to distinguish specific MC subsets in vivo, and discuss how the improved ability to recognize these subsets provided new insights into the biology of MCs. These recent advances will be helpful for the understanding of the specific role of individual MC subsets in the control of tissue homeostasis, and in the regulation of pathological conditions such as infection, autoimmunity, and cancer.

Serotonergic regulation of insulin secretion

The exact physiological role for the monoamine serotonin (5-HT) in modulation of insulin secretion is yet to be fully understood. Although the presence of this monoamine in islets of Langerhans is well established, it is only with recent advances that the complex signalling network in islets involving 5-HT is being unravelled. With more than fourteen different 5-HT receptors expressed in human islets and receptor-independent mechanisms in insulin-producing β-cells, our understanding of 5-HT's regulation of insulin secretion is increasing. It is now widely accepted that failure of the pancreatic β-cell to release sufficient amounts of insulin is the main cause of type 2 diabetes (T2D), an ongoing global epidemic. In this context, 5-HT signalling may be of importance. In fact, 5-HT may serve an essential role in regulating the release of insulin and glucagon, the two main hormones that control glucose and lipid homoeostasis. In this review, we will discuss past and current understanding of 5-HT's role in the endocrine pancreas.

Radiomanganese PET Detects Changes in Functional β-Cell Mass in Mouse Models of Diabetes

The noninvasive measurement of functional β-cell mass would be clinically valuable for monitoring the progression of type 1 and type 2 diabetes as well as the viability of transplanted insulin-producing cells. Although previous work using MRI has shown promise for functional β-cell mass determination through voltage-dependent Ca²⁺ channel (VDCC)-mediated internalization of Mn²⁺, the clinical utility of this technique is limited by the cytotoxic levels of the Mn²⁺ contrast agent. Here, we show that positron emission tomography (PET) is advantageous for determining functional β-cell mass using ⁵²Mn²⁺ (t_1/2: 5.6 days). We investigated the whole-body distribution of ⁵²Mn²⁺ in healthy adult mice by dynamic and static PET imaging. Pancreatic VDCC uptake of ⁵²Mn²⁺ was successfully manipulated pharmacologically in vitro and in vivo using glucose, nifedipine (VDCC blocker), the sulfonylureas tolbutamide and glibenclamide (K_ATP channel blockers), and diazoxide (K_ATP channel opener). In a mouse model of streptozotocin-induced type 1 diabetes, ⁵²Mn²⁺ uptake in the pancreas was distinguished from healthy controls in parallel with classic histological quantification of β-cell mass from pancreatic sections. ⁵²Mn²⁺-PET also reported the expected increase in functional β-cell mass in the ob/ob model of pretype 2 diabetes, a result corroborated by histological β-cell mass measurements and live-cell imaging of β-cell Ca²⁺ oscillations. These results indicate that ⁵²Mn²⁺-PET is a sensitive new tool for the noninvasive assessment of functional β-cell mass.

Targets and probes for non-invasive imaging of β-cells

β-cells, located in the islets of the pancreas, are responsible for production and secretion of insulin and play a crucial role in blood sugar regulation. Pathologic β-cells often cause serious medical conditions affecting blood glucose level, which severely impact life quality and are life-threatening if untreated. With 347 million patients, diabetes is one of the most prevalent diseases, and will continue to be one of the largest socioeconomic challenges in the future. The diagnosis still relies mainly on indirect methods like blood sugar measurements. A non-invasive diagnostic imaging modality would allow direct evaluation of β-cell mass and would be a huge step towards personalized medicine. Hyperinsulinism is another serious condition caused by β-cells that excessively secrete insulin, like for instance β-cell hyperplasia and insulinomas. Treatment options with drugs are normally not curative, whereas curative procedures usually consist of the resection of affected regions for which, however, an exact localization of the foci is necessary. In this review, we describe potential tracers under development for targeting β-cells with focus on radiotracers for PET and SPECT imaging, which allow the non-invasive visualization of β-cells. We discuss either the advantages or limitations for the various tracers and modalities. This article concludes with an outlook on future developments and discuss the potential of new imaging probes including dual probes that utilize functionalities for both a radioactive and optical moiety as well as for theranostic applications.

Human Beta Cells Produce and Release Serotonin to Inhibit Glucagon Secretion from Alpha Cells

In the pancreatic islet, serotonin is an autocrine signal increasing beta cell mass during metabolic challenges such as those associated with pregnancy or high-fat diet. It is still unclear whether serotonin is relevant for regular islet physiology and hormone secretion. Here, we show that human beta cells produce and secrete serotonin when stimulated with increases in glucose concentration. Serotonin secretion from beta cells decreases cyclic AMP (cAMP) levels in neighboring alpha cells via 5-HT1F receptors and inhibits glucagon secretion. Without serotonergic input, alpha cells lose their ability to regulate glucagon secretion in response to changes in glucose concentration, suggesting that diminished serotonergic control of alpha cells can cause glucose blindness and the uncontrolled glucagon secretion associated with diabetes. Supporting this model, pharmacological activation of 5-HT1F receptors reduces glucagon secretion and has hypoglycemic effects in diabetic mice. Thus, modulation of serotonin signaling in the islet represents a drug intervention opportunity.

In vivo imaging of beta cells with radiotracers: state of the art, prospects and recommendations for development and use

Radiotracer imaging is characterised by high in vivo sensitivity, with a detection limit in the lower picomolar range. Therefore, radiotracers represent a valuable tool for imaging pancreatic beta cells. High demands are made of radiotracers for in vivo imaging of beta cells. Beta cells represent only a small fraction of the volume of the pancreas (usually 1-3%) and are scattered in the tiny islets of Langerhans throughout the organ. In order to be able to measure a beta cell-specific signal, one has to rely on highly specific tracer molecules because current in vivo imaging technologies do not allow the resolution of single islets in humans non-invasively. Currently, a considerable amount of preclinical data are available for several radiotracers and three are under clinical evaluation. We summarise the current status of the evaluation of these tracer molecules and put forward recommendations for their further evaluation.

Gastrointestinal dopamine as an anti-incretin and its possible role in bypass surgery as therapy for type 2 diabetes with associated obesity

The objective of this review was to summarize and integrate specific clinical observations from the field of gastric bypass surgery and recent findings in beta cell biology. When considered together, these data sets suggest a previously unrecognized physiological mechanism which may explain how Roux-en-Y gastric bypass (RYGB) surgery mediates the early rapid reversal of hyperglycemia, observed before weight loss, in certain type 2 diabetes mellitus (T2DM) patients. The novel mechanism is based on a recently recognized inhibitory circuit of glucose stimulated insulin secretion driven by DA stored in β-cell vesicles and the gut. We propose that DA and glucagon-like peptide 1 (GLP-1) represent two opposing arms of a glucose stimulated insulin secretion (GSIS) regulatory system and hypothesize that dopamine represents the "anti-incretin" hypothesized to explain the beneficial effects of bariatric surgery on T2DM. These new hypotheses and the research driven by them may directly impact our understanding of: 1) the mechanisms underlying improved glucose homeostasis seen before weight loss following bariatric surgery; and 2) the regulation of glucose stimulated insulin secretion within islets. On a practical level, these studies may result in the development of novels drugs to modulate insulin secretion and/or methods to quantitatively asses in real time beta cell function and mass.

Development of an optimized 5-stage protocol for the in vitro preparation of insulin-secreting cells from mouse ES cells

In order to produce insulin-secreting cells with a high value of glucose-stimulated insulin secretion (GSIS) from mouse embryonic stem cells, we have developed an optimized 5-stage protocol by referring to culture conditions so far reported elsewhere. This protocol is characterized by 4 points: (1) use of an activin-free medium in the first stage, (2) use of gelatin/fibronectin coated culture dishes in 1-4 stages throughout, (3) removal of undifferentiated cells by cell sorter at the end of 4th stage, and (4) sedimental culture in the 5th stage. GSIS value of the produced cells reached 2.4, that was at a higher rank of those so far reported. The produced cells were transplanted in diabetes model mice but no remedy effect was observed. Then transplantation was conducted in pre-diabetes model mice, in which GSIS was impaired without affecting insulin producing function. The transplantation of 5 × 10(6) cells resulted in a marked improvement of glucose tolerance within 20 days. This effect decreased but was still observed at 120 days post-transplantation. This demonstrates the feasibility of the novel optimized protocol.

Mapping the target localization and biodistribution of non-radiolabeled VMAT2 ligands in rat brain

Imaging targeting vesicular monoamine transporter (VMAT2) alterations is a sensitive tool for early diagnosis of Parkinson's disease. Our group has reported several novel 2-amino-DTBZ derivatives as potential VMAT2 imaging agents. The objective of this paper is to develop a non-radiolabeled methodology to screen the candidate compounds for accelerating the drug discovery process. 9-[(18)F]fluoropropyl-(+)-dihydrotetrabenazine ([(18)F]AV-133) is a PET imaging agent targeting VMAT2 binding sites in the brain. Nonradioactive AV-133 was injected (iv) into rats, at the end of the allotted time, the animals were killed and six regions of brain and plasma from each animal were processed for quantitative measurement of AV-133 by LC-MS/MS. These data were converted to the percentage injected dose per gram tissue weight (%ID/g tissue) and the brain target tissue to background ratios to allow direct comparison with data obtained by gamma counting of the injected radioactive [(18)F]AV-133. The %ID/g and the brain target tissue to background ratios calculated using the LC-MS/MS method were highly correlated to the values obtained by standard radioactivity measurements of [(18)F]AV-133. The pattern of AV-133 in rat brain was consistent with the known distribution of VMAT2. The concordance indicated that high-sensitivity LC-MS/MS is an indispensable tool in evaluating the quantity of administered chemical in tissue as part of the development of new molecular imaging probes. Furthermore, several novel 2-amino-DTBZ derivatives were detected using this methodology, and their biodistribution data in rat brain were obtained. The information about target engagements of candidates was provided.

New perspectives of vesicular monoamine transporter 2 chemical characteristics in mammals and its constant expression in type 1 diabetes rat models

Vesicular monoamine transporter 2 (VMAT2) has been exploited as a biomarker of β-cell mass in human islets. However, a current report suggested no immunoreactivity of VMAT2 in the β cells of rat islets. To investigate the cellular localization of VMAT2 in islets further, the pancreatic tissues from monkeys and humans were compared with those of rats and mice. The study was performed using among-species comparisons and a type 1 diabetes model (T1DM) for rats by Western blotting, double-label immunofluorescence, and confocal laser scanning microscopy. We found that VMAT2-immunoreactivity (IR) was distributed peripherally in the islets of rodents, but was widely scattered throughout the islets of primates. Consistent with rodent islets, VMAT2-IR did not exist in insulin (INS)-IR cells but was abundantly present in glucagon (GLU)-IR and pancreatic polypeptide (PP)-IR cells in monkey and human islets. VMAT2-IR had no colocalization with INS-IR in any part of the rat pancreas (head, body, and tail). INS-IR cells were reduced dramatically in T1DM rat islets, but no significant alteration in the proportion of VMAT2-IR cells and GLU-IR cells was observed. Furthermore, a strong colocalization of VMAT2-IR with GLU-IR was distributed in the peripheral regions of diabetic islets. For the first time, the current study demonstrates the presence of VMAT2 in α cells and PP cells but not in β cells in the islets of monkeys and humans. This study provides convinced morphologic evidence that VMAT2 is not present in β cells. There needs to be studies for new markers for β cell mass.

VMAT2 identified as a regulator of late-stage β-cell differentiation

Cell replacement therapy for diabetes mellitus requires cost-effective generation of high-quality, insulin-producing, pancreatic β cells from pluripotent stem cells. Development of this technique has been hampered by a lack of knowledge of the molecular mechanisms underlying β-cell differentiation. The present study identified reserpine and tetrabenazine (TBZ), both vesicular monoamine transporter 2 (VMAT2) inhibitors, as promoters of late-stage differentiation of Pdx1-positive pancreatic progenitor cells into Neurog3 (referred to henceforth as Ngn3)-positive endocrine precursors. VMAT2-controlled monoamines, such as dopamine, histamine and serotonin, negatively regulated β-cell differentiation. Reserpine or TBZ acted additively with dibutyryl adenosine 3',5'-cyclic AMP, a cell-permeable cAMP analog, to potentiate differentiation of embryonic stem (ES) cells into β cells that exhibited glucose-stimulated insulin secretion. When ES cell-derived β cells were transplanted into AKITA diabetic mice, the cells reversed hyperglycemia. Our protocol provides a basis for the understanding of β-cell differentiation and its application to a cost-effective production of functional β cells for cell therapy.

Genetically engineered pig models for diabetes research

Diabetes mellitus (DM) has emerged into a steadily increasing health problem and the predicted future dimension of the global DM epidemic is alarming: an increase from currently 346 million to over 400 million affected people worldwide by the year 2030 was extrapolated. Thus concerted research efforts are imperative to gain insight into disease mechanisms and to expand the basis for development of preventive and therapeutic strategies. Diabetic rodent models have traditionally been used to follow these goals, but have limitations for translational research. The pig is another classical animal model for diabetes research. Genetic engineering now facilitates tailoring pig models which mimic human disease mechanisms at the molecular level. This article reviews the existing genetically engineered pig models for diabetes research and their current and future applications. Further, the potential role of the pig as donor of pancreatic islets for xenotransplantation or as host for growing human pancreas is outlined.

Sympathetic innervation during development is necessary for pancreatic islet architecture and functional maturation

Sympathetic neurons depend on target-derived neurotrophic cues to control their survival and growth. However, whether sympathetic innervation contributes reciprocally to the development of target tissues is less clear. Here, we report that sympathetic innervation is necessary for the formation of the pancreatic islets of Langerhans and for their functional maturation. Genetic or pharmacological ablation of sympathetic innervation during development resulted in altered islet architecture, reduced insulin secretion, and impaired glucose tolerance in mice. Similar defects were observed with pharmacological blockade of β-adrenergic signaling. Conversely, the administration of a β-adrenergic agonist restored islet morphology and glucose tolerance in deinnervated animals. Furthermore, in neuron-islet cocultures, sympathetic neurons promoted islet cell migration in a β-adrenergic-dependent manner. This study reveals that islet architecture requires extrinsic inductive cues from neighboring tissues such as sympathetic nerves and suggests that early perturbations in sympathetic innervation might underlie metabolic disorders.

Minireview: Dopaminergic regulation of insulin secretion from the pancreatic islet

Exogenous dopamine inhibits insulin secretion from pancreatic β-cells, but the lack of dopaminergic neurons in pancreatic islets has led to controversy regarding the importance of this effect. Recent data, however, suggest a plausible physiologic role for dopamine in the regulation of insulin secretion. We review the literature underlying our current understanding of dopaminergic signaling that can down-regulate glucose-stimulated insulin secretion from pancreatic islets. In this negative feedback loop, dopamine is synthesized in the β-cells from circulating L-dopa, serves as an autocrine signal that is cosecreted with insulin, and causes a tonic inhibition on glucose-stimulated insulin secretion. On the whole animal scale, L-dopa is produced by cells in the gastrointestinal tract, and its concentration in the blood plasma increases following a mixed meal. By reviewing the outcome of certain types of bariatric surgery that result in rapid amelioration of glucose tolerance, we hypothesize that dopamine serves as an "antiincretin" signal that counterbalances the stimulatory effect of glucagon-like peptide 1.

Localization and expression of VMAT2 aross mammalian species: a translational guide for its visualization and targeting in health and disease

VMAT2 is the vesicular monoamine transporter that allows DA, NE, Epi, His, and 5-HT uptake into neurons and endocrine cells. A second isoform, VMAT1, has similar structure and function, but does not recognize histamine as a substrate. VMAT1 is absent from neurons, and its major function appears to be in endocrine cells, that is, enterochromaffin cells, which scavenge 5-HT, but not histamine, from dietary sources. This chapter provides an update on the neuroanatomical distribution of VMAT2 across various mammalian species, including human, primate, pig, rat, and mouse. When necessary, VMAT1 expression is provided as a contrast. The main purpose of this chapter is to allow clinicians, in particular endocrinologists and diagnosing neuroradiologists and neuropathologists, an acquaintanceship with the possibilities for VMAT2 as a target for in vivo imaging, and drug development, based on this updated information.