Vesicular monoamine transporter 2 (VMAT2) expression in hematopoietic cells and in patients with systemic mastocytosis

Multielectrode Arrays as a Means to Study Exocytosis in Human Platelets

Platelets are probably the most accessible human cells to study exocytosis by amperometry. These cell fragments accumulate biological amines, serotonin in particular, using similar if not the same mechanisms as those employed by sympathetic, serotoninergic, and histaminergic neurons. Thus, platelets have been widely recognized as a model system to study certain neurological and psychiatric diseases. Platelets release serotonin by exocytosis, a process that entails the fusion of a secretory vesicle to the plasma membrane and that can be monitored directly by classic single cell amperometry using carbon fiber electrodes. However, this is a tedious technique because any given platelet releases only 4-8 secretory δ-granules. Here, we introduce and validate a diamond-based multielectrode array (MEA) device for the high-throughput study of exocytosis by human platelets. This is probably the first reported study of human tissue using an MEA, demonstrating that they are very interesting laboratory tools to assess alterations to exocytosis in neuropsychiatric diseases. Moreover, these devices constitute a valuable platform for the rapid testing of novel drugs that act on secretory pathways in human tissues.

Dopamine, Immunity, and Disease

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

Dengue Virus Replication Is Associated with Catecholamine Biosynthesis and Metabolism in Hepatocytes

Previously, the association between the catecholamine biosynthetic enzyme L-Dopa decarboxylase (DDC) and Dengue virus (DV) replication was demonstrated in liver cells and was found to be mediated at least by the interaction between DDC and phosphoinositide 3-kinase (PI3K). Here, we show that biogenic amines production and uptake impede DV replication in hepatocytes and monocytes, while the virus reduces catecholamine biosynthesis, metabolism, and transport. To examine how catecholamine biosynthesis/metabolism influences DV, first, we verified the role of DDC by altering DDC expression. DDC silencing enhanced virus replication, but not translation, attenuated the negative effect of DDC substrates on the virus and reduced the infection related cell death. Then, the role of the downstream steps of the catecholamine biosynthesis/metabolism was analyzed by chemical inhibition of the respective enzymes, application of their substrates and/or their products; moreover, reserpine, the inhibitor of the vesicular monoamine transporter 2 (VMAT2), was used to examine the role of uptake/storage of catecholamines on DV. Apart from the role of each enzyme/transporter, these studies revealed that the dopamine uptake, and not the dopamine-signaling, is responsible for the negative effect on DV. Accordingly, all treatments expected to enhance the accumulation of catecholamines in the cell cytosol suppressed DV replication. This was verified by the use of chemical inducers of catecholamine biosynthesis. Last, the cellular redox alterations due to catecholamine oxidation were not related with the inhibition of DV replication. In turn, DV apart from its negative impact on DDC, inhibits tyrosine hydroxylase, dopamine beta-hydroxylase, monoamine oxidase, and VMAT2 expression.

PET Imaging of Vesicular Monoamine Transporter 2 in Early Diabetic Retinopathy Using [18F]FP-(+)-DTBZ

PURPOSE

Diabetic retinopathy (DR) is characterized by dopaminergic neuron loss in the retina of the eyes. [¹⁸F]fluoropropyl-(+)-dihydrotetrabenazine ([¹⁸F]FP-(+)-DTBZ) positron emission tomography (PET) has been shown to detect dopaminergic neuron loss. The study is to investigate the feasibility of PET imaging with [¹⁸F]FP-(+)-DTBZ for early diagnosis of diabetic retinopathy (DR) in diabetes mellitus (DM) rat models.

METHODS

The DM rat model was established by a single intraperitoneal injection of streptozotocin (STZ) (65 mg/kg). After 4 weeks, 8 weeks, and 12 weeks of STZ injection, the retinas of the rats were evaluated by electroretinogram (ERG), color fundus photography (CFP), fundus fluorescein angiography (FFA), and small animal PET scan with [¹⁸F]FP-(+)-DTBZ by targeting vesicular monoamine transporter 2 (VMAT2). [¹⁸F]FP-(+)-DTBZ uptake in retina was quantified as standardized uptake value (SUV). Immunofluorescence staining and Western blot were also performed to confirm the expression level of VMAT2 in retina.

RESULTS

ERG dysfunction was observed at 8 weeks in STZ-diabetic rats, evidenced by smaller amplitudes of oscillatory potentials (OPs) when compared with OPs in normal rats. CFP and FFA showed no significant difference in vascular leakage and neovascularization between STZ-diabetic retinas and normal ones until 8 weeks. PET imaging revealed that the SUV of [¹⁸F]FP-(+)-DTBZ was significantly lower in the STZ-diabetic retinas compared with the normal ones as early as of week 4. The results from immunofluorescence staining and Western blots confirmed the early findings in PET imaging studies.

CONCLUSIONS

Early DR can be non-invasively detected with PET imaging using [¹⁸F]FP-(+)-DTBZ targeting VMAT2. The expression level of VMAT2 in retina may act as a new biomarker for early DR diagnosis.

Histamine receptors, agonists, and antagonists in health and disease

Histamine in the brain is produced by a group of tuberomamillary neurons in the posterior hypothalamus and a limited number of mast cells in different parts of the brain. Four G-protein-coupled receptors mediate the effects of histamine. Two of these receptors, H3 and H4 receptors, are high-affinity receptors in the brain and immune system, respectively. The two classic histamine receptors, H1 receptor and H2 receptor, are well known as drug targets for allergy and gastric ulcer, respectively. These receptors have lower affinity for histamine than the more recently discovered H3 and H4 receptors. The H1 and H2 receptors are important postsynaptic receptors in the brain, and they mediate many of the central effects of histamine on, e.g., alertness and wakefulness. H3 receptor is a pre- and postsynaptic receptor, which regulates release of histamine and several other neurotransmitters, including serotonin, GABA, and glutamate. H4 receptor is found in cerebral blood vessels and microglia, but its expression in neurons is not yet well established. Pitolisant, a H3 receptor antagonist, is used to treat narcolepsy and hypersomnia. H1 receptor antagonists have been used to treat insomnia, but its use requires precautions due to potential side effects. H2 receptor antagonists have shown efficacy in treatment of schizophrenia, but they are not in widespread clinical use. H4 receptor ligands may in the future be tested for neuroimmunological disorders and potentially neurodegenerative disorders in which inflammation plays a role, but clinical tests have not yet been initiated.

Dopamine: an immune transmitter

The dopaminergic system controls several vital central nervous system functions, including the control of movement, reward behaviors and cognition. Alterations of dopaminergic signaling are involved in the pathogenesis of neurodegenerative and psychiatric disorders, in particular Parkinson’s disease, which are associated with a subtle and chronic inflammatory response. A substantial body of evidence has demonstrated the non-neuronal expression of dopamine, its receptors and of the machinery that governs synthesis, secretion and storage of dopamine across several immune cell types. This review aims to summarize current knowledge on the role and expression of dopamine in immune cells. One of the goals is to decipher the complex mechanisms through which these cell types respond to dopamine, in order to address the impact this has on neurodegenerative and psychiatric pathologies such as Parkinson’s disease. A further aim is to illustrate the gaps in our understanding of the physiological roles of dopamine to encourage more targeted research focused on understanding the consequences of aberrant dopamine production on immune regulation. These highlights may prompt scientists in the field to consider alternative functions of this important neurotransmitter when targeting neuroinflammatory/neurodegenerative pathologies.

Alpha synuclein in hematopoiesis and immunity

Parkinson's disease (PD) is the second most common neurodegenerative condition and intracellular deposition of Lewy bodies in the substantia nigra (SN), which can cause dopaminergic neuronal death, is the hallmark of this syndrome. α-synuclein (syn) is a small protein expressed mainly in neurons but can also be found in a number of tissues. It can be present as a soluble monomer under normal physiological conditions, but can be toxic in its oligomeric or fibrillary forms. Most of the available literature has focused on the effects of α-syn pathology in the mechanisms leading to PD. However, the normal functions of α-syn still remain to be fully elucidated. Notably, α-syn in the hematopoietic system seems to mediate important functions as indicated by anemia and incomplete cell maturation when this protein is absent. This review will summarize basic genetic and structural findings, and critical information that suggests an essential role of α-syn in the development and activation of the hematopoietic system and immunity.

High-throughput screening system for dynamic monitoring of exocytotic vesicle trafficking in mast cells

Mast cells, in addition to endocrine cells and neurons, are typical secretory cells. Their function in allergic inflammation is to secrete inflammatory mediators from secretory vesicles. Intracellular synthesized inflammatory mediators are transported by vesicular monoamine transporters (VMATs) to vesicles where they are stored. After stimulation, the contents of the secretory vesicles are released via exocytosis. This study established a high throughput imaging screening system to monitor the functions of secretory vesicles in mast cells, including molecular uptake via VMAT2 and the exocytotic process, by using a novel fluorescent probe, FFN206, which was developed as a VMAT2 substrate. After loading with FFN206, the rapid uptake of FFN206 was observed and secretory vesicles in mouse bone marrow derived mast cells and a cultured mast cell line were clearly visualized. FFN206 uptake by secretory vesicles was time-dependent and was blocked by reserpine. Furthermore, exocytotic trafficking was monitored dynamically by real-time high-throughput fluorescence quantitation. In the present study, we verified the application of FFN206 for the monitoring of functional vesicles. This high-throughput screening system may benefit instinctive drug evaluation.

Immunochemical localization of vesicular monoamine transporter 2 (VMAT2) in mouse brain

Vesicular monoamine transporter 2 (VMAT2, SLC18A2) is a transmembrane transporter protein that packages dopamine, serotonin, norepinephrine, and histamine into vesicles in preparation for neurotransmitter release from the presynaptic neuron. VMAT2 function and related vesicle dynamics have been linked to susceptibility to oxidative stress, exogenous toxicants, and Parkinson's disease. To address a recent depletion of commonly used antibodies to VMAT2, we generated and characterized a novel rabbit polyclonal antibody generated against a 19 amino acid epitope corresponding to an antigenic sequence within the C-terminal tail of mouse VMAT2. We used genetic models of altered VMAT2 expression to demonstrate that the antibody specifically recognizes VMAT2 and localizes to synaptic vesicles. Furthermore, immunohistochemical labeling using this VMAT2 antibody produces immunoreactivity that is consistent with expected VMAT2 regional distribution. We show the distribution of VMAT2 in monoaminergic brain regions of mouse brain, notably the midbrain, striatum, olfactory tubercle, dopaminergic paraventricular nuclei, tuberomammillary nucleus, raphe nucleus, and locus coeruleus. Normal neurotransmitter vesicle dynamics are critical for proper health and functioning of the nervous system, and this well-characterized VMAT2 antibody will be a useful tool in studying neurodegenerative and neuropsychiatric conditions characterized by vesicular dysfunction.

Evaluation of pancreatic VMAT2 binding with active and inactive enantiomers of 18F-FP-DTBZ in baboons

INTRODUCTION

¹⁸F-Fluoropropyl-(+)-dihydrotetrabenazine (¹⁸F-FP-(+)-DTBZ) is a vesicular monoamine transporter type 2 (VMAT2) radiotracer for positron emission tomography (PET) imaging to quantify human β-cell mass. Renal cortex and spleen have been suggested as reference regions, however, little is known about ¹⁸F-FP-(+)-DTBZ binding in these regions including the fraction of radiometabolite. We compared the kinetics of ¹⁸F-FP-(+)-DTBZ and its inactive enantiomer ¹⁸F-FP-(-)-DTBZ in baboons, estimated the non-displaceable binding (V_ND) of the tracers, and used ex vivo studies to measure radiometabolite fractions.

METHODS

PET scans were conducted for up to 4h with (+) and (-) enantiomers. Displacement experiments using unlabeled (+) and (-) enantiomers of FP-DTBZ and fluvoxamine (to evaluate sigma-1 receptor binding) were performed. SUV curves were used to calculate displacement values in the pancreas, renal cortex, and spleen. Distribution volumes (V_T) were computed, and three approaches for calculation of V_ND were compared: (1) ¹⁸F-FP-(+)-DTBZ reference V_T, (2) ¹⁸F-FP-(-)-DTBZ pancreatic V_T, and (3) a scaled ¹⁸F-FP-(+)-DTBZ reference V_T values. Ex vivo study was conducted to measure radiometabolite fraction in homogenized tissue samples from baboons at 90min post-injection.

RESULTS

Spleen uptake was lowest for both tracers. Highest uptake was in the pancreas with ¹⁸F-FP-(+)-DTBZ and renal cortex with ¹⁸F-FP-(-)-DTBZ. Substantial displacement effect was observed only with unlabeled FP-(+)-DTBZ in the ¹⁸F-FP-(+)-DTBZ studies. Radiometabolite fraction was higher in the renal cortex than the spleen. Approaches (1) and (3) with spleen to estimate V_ND provided lowest inter-subject variability of BP_ND.

CONCLUSIONS

V_T differences among organs and between enantiomers indicated that scaling of reference region values is needed for quantification of VMAT2 binding in the pancreas with ¹⁸F-FP-(+)-DTBZ. Since the kidney PET signal has greater partial volume averaging and more radiometabolites, the spleen was considered a more practical candidate for use as a scaled-reference region in the quantification of ¹⁸F-FP-(+)-DTBZ in the pancreas.

Mast cell mediators: their differential release and the secretory pathways involved

Mast cells (MC) are widely distributed throughout the body and are common at mucosal surfaces, a major host-environment interface. MC are functionally and phenotypically heterogeneous depending on the microenvironment in which they mature. Although MC have been classically viewed as effector cells of IgE-mediated allergic diseases, they are also recognized as important in host defense, innate and acquired immunity, homeostatic responses, and immunoregulation. MC activation can induce release of pre-formed mediators such as histamine from their granules, as well as release of de novo synthesized lipid mediators, cytokines, and chemokines that play diverse roles, not only in allergic reactions but also in numerous physiological and pathophysiological responses. Indeed, MC release their mediators in a discriminating and chronological manner, depending upon the stimuli involved and their signaling cascades (e.g., IgE-mediated or Toll-like receptor-mediated). However, the precise mechanisms underlying differential mediator release in response to these stimuli are poorly known. This review summarizes our knowledge of MC mediators and will focus on what is known about the discriminatory release of these mediators dependent upon diverse stimuli, MC phenotypes, and species of origin, as well as on the intracellular synthesis, storage, and secretory processes involved.

Glucagon effects on 3H-histamine uptake by the isolated guinea-pig heart during anaphylaxis

We estimated the influence of acute glucagon applications on ³H-histamine uptake by the isolated guinea-pig heart, during a single ³H-histamine passage through the coronary circulation, before and during anaphylaxis, and the influence of glucagon on level of histamine, NO, O₂⁻, and H₂O₂ in the venous effluent during anaphylaxis. Before anaphylaxis, glucagon pretreatment does not change ³H-histamine Umax and the level of endogenous histamine. At the same time, in the presence of glucagon, ³H-histamine Unet is increased and backflux is decreased when compared to the corresponding values in the absence of glucagon. During anaphylaxis, in the presence of glucagon, the values of ³H-histamine Umax and Unet are significantly higher and backflux is significantly lower in the presence of glucagon when compared to the corresponding values in the absence of glucagon. The level of endogenous histamine during anaphylaxis in the presence of glucagon (6.9–7.38 × 10⁻⁸  μM) is significantly lower than the histamine level in the absence of glucagon (10.35–10.45 × 10⁻⁸  μM). Glucagon pretreatment leads to a significant increase in NO release (5.69 nmol/mL) in comparison with the period before glucagon administration (2.49 nmol/mL). Then, in the presence of glucagon, O₂⁻ level fails to increase during anaphylaxis. Also, our results show no significant differences in H₂O₂ levels before, during, and after anaphylaxis in the presence of glucagon, but these values are significantly lower than the corresponding values in the absence of glucagon. In conclusion, our results show that glucagon increases NO release and prevents the increased release of free radicals during anaphylaxis, and decreases histamine level in the venous effluent during cardiac anaphylaxis, which may be a consequence of decreased histamine release and/or intensified histamine capturing by the heart during anaphylaxis.

Drug induced increases in CNS dopamine alter monocyte, macrophage and T cell functions: implications for HAND

Central nervous system (CNS) complications resulting from HIV infection remain a major public health problem as individuals live longer due to the success of combined antiretroviral therapy (cART). As many as 70 % of HIV infected people have HIV associated neurocognitive disorders (HAND). Many HIV infected individuals abuse drugs, such as cocaine, heroin or methamphetamine, that may be important cofactors in the development of HIV CNS disease. Despite different mechanisms of action, all drugs of abuse increase extracellular dopamine in the CNS. The effects of dopamine on HIV neuropathogenesis are not well understood, and drug induced increases in CNS dopamine may be a common mechanism by which different types of drugs of abuse impact the development of HAND. Monocytes and macrophages are central to HIV infection of the CNS and to HAND. While T cells have not been shown to be a major factor in HIV-associated neuropathogenesis, studies indicate that T cells may play a larger role in the development of HAND in HIV infected drug abusers. Drug induced increases in CNS dopamine may dysregulate functions of, or increase HIV infection in, monocytes, macrophages and T cells in the brain. Thus, characterizing the effects of dopamine on these cells is important for understanding the mechanisms that mediate the development of HAND in drug abusers.

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.

Characterization and function of the human macrophage dopaminergic system: implications for CNS disease and drug abuse

Background

Perivascular macrophages and microglia are critical to CNS function. Drugs of abuse increase extracellular dopamine in the CNS, exposing these cells to elevated levels of dopamine. In rodent macrophages and human T-cells, dopamine was shown to modulate cellular functions through activation of dopamine receptors and other dopaminergic proteins. The expression of these proteins and the effects of dopamine on human macrophage functions had not been studied.

Methods

To study dopaminergic gene expression, qRT-PCR was performed on mRNA from primary human monocyte derived macrophages (MDM). Expression and localization of dopaminergic proteins was examined by immunoblotting isolated plasma membrane, total membrane and cytosolic proteins from MDM. To characterize dopamine-mediated changes in cytokine production in basal and inflammatory conditions, macrophages were treated with different concentrations of dopamine in the presence or absence of LPS and cytokine production was assayed by ELISA. Statistical significance was determined using two-tailed Students’ T-tests or Wilcoxen Signed Rank tests.

Results

These data show that MDM express mRNA for all five subtypes of dopamine receptors, and that dopamine receptors 3 and 4 are expressed on the plasma membrane. MDM also express mRNA for the dopamine transporter (DAT), vesicular monoamine transporter 2 (VMAT2), tyrosine hydroxylase (TH) and aromatic amino acid decarboxylase (AADC). DAT is expressed on the plasma membrane, VMAT2 on cellular membranes and TH and AADC are in the cytosol. Dopamine also alters macrophage cytokine production in both untreated and LPS-treated cells. Untreated macrophages show dopamine mediated increases IL-6 and CCL2. Macrophages treated with LPS show increased IL-6, CCL2, CXCL8 and IL-10 and decreased TNF-α.

Conclusions

Monocyte derived macrophages express dopamine receptors and other dopaminergic proteins through which dopamine may modulate macrophage functions. Thus, increased CNS dopamine levels due to drug abuse may exacerbate the development of neurological diseases including Alzheimer’s disease and HIV associated neurological disorders.

Decreased defluorination using the novel beta-cell imaging agent [18F]FE-DTBZ-d4 in pigs examined by PET

Background

Fluorine-18 dihydrotetrabenazine [DTBZ] analogues, which selectively target the vesicular monoamine transporter 2 [VMAT2], have been extensively studied for in vivo quantification of beta cell mass by positron-emission tomography [PET]. This study describes a novel deuterated radioligand [¹⁸F]fluoroethyl [FE]-DTBZ-d4, aimed to increase the stability against in vivo defluorination previously observed for [¹⁸F]FE-DTBZ.

Methods

[¹⁸F]FE-DTBZ-d4 was synthesized by alkylation of 9-O-desmethyl-(+)-DTBZ precursor with deuterated [¹⁸F]FE bromide ([¹⁸F]FCD₂CD₂Br). Radioligand binding potential [BP] was assessed by an in vitro saturation homogenate binding assay using human endocrine and exocrine pancreatic tissues. In vivo pharmacokinetics and pharmacodynamics [PK/PD] was studied in a porcine model by PET/computed tomography, and the rate of defluorination was quantified by compartmental modeling.

Results

[¹⁸F]FE-DTBZ-d4 was produced in reproducible good radiochemical yield in 100 ± 20 min. Radiochemical purity of the formulated product was > 98% for up to 5 h with specific radioactivities that ranged from 192 to 529 GBq/μmol at the end of the synthesis. The in vitro BP for VMAT2 in the islet tissue was 27.0 ± 8.8, and for the exocrine tissue, 1.7 ± 1.0. The rate of in vivo defluorination was decreased significantly (k_{defluorination} = 0.0016 ± 0.0007 min^-1) compared to the non-deuterated analogue (k_{defluorination} = 0.012 ± 0.002 min^-1), resulting in a six fold increase in half-life stability.

Conclusions

[¹⁸F]FE-DTBZ-d4 has similar PK and PD properties for VMAT2 imaging as its non-deuterated analogue [¹⁸F]FE-DTBZ in addition to gaining significantly increased stability against defluorination. [¹⁸F]FE-DTBZ-d4 is a prime candidate for future preclinical and clinical studies on focal clusters of beta cells, such as in intramuscular islet grafts.

Imaging the islet graft by positron emission tomography

Clinical islet transplantation is being investigated as a permanent cure for type 1 diabetes mellitus (T1DM). Currently, intraportal infusion of islets is the favoured procedure, but several novel implantation sites have been suggested. Noninvasive longitudinal methodologies are an increasingly important tool for assessing the fate of transplanted islets, their mass, function and early signs of rejection. This article reviews the approaches available for islet graft imaging by positron emission tomography and progress in the field, as well as future challenges and opportunities.

Polycythemia from mast cell activation syndrome: lessons learned

A middle-aged woman presented with fatigue and mild increases in hematocrit and red cell mass. Polycythemia vera was diagnosed. She underwent therapeutic phlebotomy but clinically worsened. On reevaluation, other problems were noted including episodic malaise, nausea, rash and vasomotor issues. The JAK2V617F mutation was absent; paraneoplastic erythrocytosis was investigated. Serum tryptase and urinary N-methylhistamine were normal, but urinary prostaglandin D2 was elevated. Skin and marrow biopsies showed no mast cell abnormalities. Extensive other evaluation was negative. Gastrointestinal tract biopsies were histologically normal but revealed increased, aberrant mast cells on immunohistochemistry; the KITD816V mutation was absent. Mast cell activation syndrome, recently identified as a clonal disorder involving assorted KIT mutations, was diagnosed. Imatinib 200 mg/d rapidly effected complete, sustained response. Diagnosis of mast cell activation syndrome is hindered by multiple factors, but existing therapies for mast cell disease are usually achieve significant benefit, highlighting the importance of early diagnosis. Multiple important aspects of clinical reasoning are illustrated by the case.

Methamphetamine toxicity and its implications during HIV-1 infection

Over the past two decades methamphetamine (MA) abuse has seen a dramatic increase. The abuse of MA is particularly high in groups that are at higher risk for HIV-1 infection, especially men who have sex with men (MSM). This review is focused on MA toxicity in the CNS as well as in the periphery. In the CNS, MA toxicity is comprised of numerous effects, including, but not limited to, oxidative stress produced by dysregulation of the dopaminergic system, hyperthermia, apoptosis, and neuroinflammation. Multiple lines of evidence demonstrate that these effects exacerbate the neurodegenerative damage caused by CNS infection of HIV perhaps because both MA and HIV target the frontostriatal regions of the brain. MA has also been demonstrated to increase viral load in the CNS of SIV-infected macaques. Using transgenic animal models, as well as cultured cells, the HIV proteins Tat and gp120 have been demonstrated to have neurotoxic properties that are aggravated by MA. In addition, MA has been shown to exhibit detrimental effects on the blood-brain barrier (BBB) that have the potential to increase the probability of CNS infection by HIV. Although the effects of MA in the periphery have not been as extensively studied as have the effects on the CNS, recent reports demonstrate the potential effects of MA on HIV infection in the periphery including increased expression of HIV co-receptors and increased expression of inflammatory cytokines.

VMAT2: a dynamic regulator of brain monoaminergic neuronal function interacting with drugs of abuse

The monoaminergic neuron, in particular the dopaminergic neuron, is central to mediating the hedonic and addictive properties of drugs of abuse. The effects of amphetamine (AMPH) and cocaine (COC), for example, depend on the ability to increase dopamine in the synapse, by effects on either the plasma membrane transporter DAT or the vesicular transporter for monoamine storage, VMAT2. The potential role of DAT as a target for AMPH and COC has been reviewed extensively. Here, we present VMAT2 as a target that enables the rewarding and addictive actions of these drugs, based on imaging, neurochemical, biochemical, cell biological, genetic, and immunohistochemical evidence. The presence of VMAT2 in noradrenergic, serotoninergic, histaminergic, and potentially trace aminergic neurons invites consideration of a wider role for aminergic neurotransmission in AMPH and COC abuse and addiction.

Altered affinity of the platelet vesicular monoamine transporter 2 to dihydrotetrabenazine in children with major depression

Platelet vesicular monoamine transporter (VMAT2) binding characteristics were assessed, using high affinity dihydrotetrabenazine ([(3)H]TBZOH) binding, in 14 children with major depression (MDD) and 16 matched controls. All participants underwent a thorough diagnostic evaluation and the levels of depression and anxiety were measured. K (d) values were significantly lower in children with MDD versus controls (2.93 ± 0.84 vs. 3.63 ± 0.56 nM, respectively, t = 2.4, df = 18.4, p = 0.025). B (max) values did not differ significantly. This preliminary finding indicates a possible structural change in platelet VMAT2 in children with MDD.

Vesicular monoamine transporter 2 mRNA levels are reduced in platelets from patients with Parkinson's disease

Despite advances in neuroimaging, the diagnosis of idiopathic Parkinson's disease (PD) remains clinical. The identification of biological markers for an early diagnosis is of great interest to start a neuroprotective therapy aimed at slowing, blocking or reversing the disease progression. Vesicular monoamine transporter 2 (VMAT2) sequesters cytoplasmic dopamine into synaptic vesicles for storage and release. Thus, VMAT2 impairment can regulate intra- and extracellular dopamine levels, influencing oxidative stress and neuronal death. Because in vivo imaging studies have demonstrated a VMAT2 reduction in PD patients greater than would be explained by neuronal loss alone, as an exploratory study we assessed VMAT2 mRNA and protein levels in platelets from 39 PD patients, 39 healthy subjects and 10 patients with vascular parkinsonism (VP) to identify a possible peripheral biomarker for PD. A significant reduction (p < 0.05) of VMAT2 mRNA levels was demonstrated in PD patients versus healthy controls. Patients with VP showed VMAT2 mRNA levels similar to controls. No difference in VMAT2 mRNA levels was found in untreated versus treated patients. No correlation was observed between mRNA levels and demographic or clinical characteristics. Furthermore, eight SNPs tagging the VMAT2 gene did not show effects on VMAT2 mRNA levels. Western blot analysis did not allow the quantification of VMAT2 protein expression in blood platelets. Although further studies in a greater number of cases are needed to confirm our data, the reduction in VMAT2 mRNA in platelets from PD patients suggests the existence of a systemic impairment of this transporter possibly contributing to PD pathology.

In vivo imaging of vesicular monoamine transporter 2 in pancreas using an (18)F epoxide derivative of tetrabenazine

OBJECTIVES

Development of imaging agents for pancreatic beta cell mass may provide tools for studying insulin-secreting beta cells and their relationship with diabetes mellitus. In this paper, a new imaging agent, [(18)F](+)-2-oxiranyl-3-isobutyl-9-(3-fluoropropoxy)-10-methoxy-2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinoline [(18)F](+)4, which displays properties targeting vesicular monoamine transporter 2 (VMAT2) binding sites of beta cells in the pancreas, was evaluated as a positron emission tomography (PET) agent for estimating beta cell mass in vivo. The hydrolyzable epoxide group of (+)4 may provide a mechanism for shifting biodistribution from liver to kidney, thus reducing the background signal.

METHODS

Both (18)F- and (19)F-labeled (+) and (-) isomers of 4 were synthesized and evaluated. Organ distribution was carried out in normal rats. Uptake of [(18)F](+)4 in pancreas of normal rats was measured and correlated with blocking studies using competing drugs, (+)dihydrotetrabenazine [(+)-DTBZ] or 9-fluoropropyl-(+)dihydro tetrabenazine [FP-(+)-DTBZ, (+)2].

RESULTS

In vitro binding study of VMAT2 using rat brain striatum showed a K(i) value of 0.08 and 0.15 nM for the (+)4 and (+/-)4, respectively. The in vivo biodistribution of [(18)F](+)4 in rats showed the highest uptake in the pancreas (2.68 %ID/g at 60 min postinjection). In vivo competition experiments with cold FP-(+)-DTBZ, (+)2, (3.5 mg/kg, 5 min iv pretreatment) led to a significant reduction of pancreas uptake (85% blockade at 60 min). The inactive isomer [(18)F](-)4 showed significantly lower pancreas uptake (0.22 %ID/g at 30 min postinjection). Animal PET imaging studies of [(18)F](+)4 in normal rats demonstrated an avid pancreatic uptake in rats.

CONCLUSION

The preliminary results suggest that the epoxide, [(18)F](+)4, is highly selective in binding to VMAT2 and it has an excellent uptake in the pancreas of rats. The liver uptake was significantly reduced through the use of the epoxide group. Therefore, it may be potentially useful for imaging beta cell mass in the pancreas.

Clinical correlates of blood serotonin levels in patients with mastocytosis

BACKGROUND

Mastocytosis is a clonal disorder associated with an increased mast cell burden. We have recently demonstrated the ability of human mast cells to express and be activated through multiple serotonin receptors; to synthesize and release serotonin; and that mastocytosis patients may have abnormal serotonin levels. As serotonin has been implicated in the genesis of clinical symptoms found in association with some chronic diseases, we have now determined the whole blood serotonin levels in 29 patients diagnosed with mastocytosis, and correlated these levels with multiple clinical and laboratory parameters.

MATERIALS AND METHODS

Patients with mastocytosis were categorized according to disease variant. Blood serotonin values were determined and correlated with values reported for normal subjects; and clinical and laboratory features of the disease.

RESULTS

Total blood serotonin levels followed a bimodal distribution in line with our earlier report, unlike the normal distribution reported for normal individuals. Serotonin levels did not correlate with platelet numbers, liver function tests or serum tryptase levels. Patients with lower serotonin values had greater rates of fatigue (P = 0.0001), migraine headaches (P = 0.0028), psychiatric symptoms (P = 0.0001), diarrhoea (P = 0.0407), flushing (0.0085), and abdominal and bone pain (P = 0.0001).

CONCLUSIONS

Our study suggests that low blood serotonin levels help define a sub-group of patients with mastocytosis that are more likely to present with neurological and gastrointestinal complaints, and suggests that the use of pharmacologic agents that alter blood serotonin levels could be explored in selected patients.

Relationship between pancreatic vesicular monoamine transporter 2 (VMAT2) and insulin expression in human pancreas

Vesicular monoamine transporter 2 (VMAT2) is expressed in pancreatic beta cells and has recently been proposed as a target for measurement of beta cell mass in vivo. We questioned, (1) What proportion of beta cells express VMAT2? (2) Is VMAT2 expressed by other pancreatic endocrine or non-endocrine cells? (3) Is the relationship between VMAT2 and insulin expression disturbed in type 1 (T1DM) or type 2 diabetes (T2DM)? Human pancreas (7 non-diabetics, 5 T2DM, 10 T1DM) was immunostained for insulin, VMAT2 and other pancreatic hormones. Most beta cells expressed VMAT2. VMAT2 expression was not changed by the presence of diabetes. In tail of pancreas VMAT2 immunostaining closely correlated with insulin staining. However, VMAT2 was also expressed in some pancreatic polypeptide (PP) cells. Although VMAT2 was not excluded as a target for beta cell mass measurement, expression of VMAT2 in PP cells predicts residual VMAT2 expression in human pancreas even in the absence of beta cells.

VMAT2 gene expression and function as it applies to imaging beta-cell mass

Diabetes mellitus is a metabolic disorder characterized by hyperglycemia. The two main forms of the disease are distinguished by different pathogenesis, natural histories, and population distributions and indicated as either type 1 (T1DM) or type 2 diabetes mellitus (T2DM). It is well established that T1DM is an autoimmune disease whereby beta-cells of pancreatic islets are destroyed leading to loss of endogenous insulin production. Albeit less dramatic, beta-cell mass (BCM) also drops in T2DM. Therefore, it is realistic to expect that noninvasive measures of BCM might provide useful information in the diabetes-care field. Preclinical studies have demonstrated that BCM measurements by positron emission tomography scanning, using the vesicular monoamine transporter type 2 (VMAT2) as a tissue-specific surrogate marker of insulin production and [11C] Dihydrotetrabenazine (DTBZ) as the radioligand specific for this molecule, is feasible in animal models. Unfortunately, the mechanisms underlying beta-cell-specific expression of VMAT2 are still largely unexplored, and a much better understanding of the regulation of VMAT2 gene expression and of its function in beta-cells will be required before the full utility of this technique in the prediction and treatment of individuals with diabetes can be understood. In this review, we summarize much of what is understood about the regulation of VMAT2 and identify questions whose answers may help in understanding what measurements of VMAT2 density mean in the context of diabetes.