New roles for dopamine D2 and D3 receptors in pancreatic beta cell insulin secretion

Regulatory roles of dopamine D2 receptor in milk protein production and apoptosis in mammary epithelial cells

Dopamine D2 receptors (D2Rs) play crucial roles in regulating diverse physiological functions of the central nervous system and peripheral organs. D2Rs are also expressed in mammary glands. However, which cell types express D2Rs and whether they are involved in milk production remains unclear. The present findings revealed that D2Rs are expressed in the apical regions of the lateral membranes of mammary epithelial cells (MECs) in lactating mice. We also investigated the effects of the D2R agonist bromocriptine and/or antagonist domperidone on intracellular cAMP levels, milk protein production, and apoptosis in a lactation culture model of MECs that produce major milk components like lactating MECs in vivo. We found that bromocriptine decreased intracellular cAMP levels, whereas domperidone dose-dependently neutralized this effect. Bromocriptine also inhibited casein and lactoferrin production and suppressed activities of STAT5 and glucocorticoid receptors (GRs). Domperidone neutralized the inhibition of casein production as well as STAT5 and GR inactivation induced by bromocriptine. Furthermore, D2R activation by bromocriptine induced apoptosis and inactivated ERK, a signaling molecule responsible for promoting cell proliferation and survival. Domperidone attenuated ERK inactivation and apoptosis induced by bromocriptine. These findings suggest that D2Rs play regulatory roles in milk protein production and apoptosis in MECs.

Development of novel tools for dissection of central versus peripheral dopamine D2-like receptor signaling in dysglycemia

Dopamine (DA) D2-like receptors in both the central nervous system (CNS) and the periphery are key modulators of metabolism. Moreover, disruption of D2-like receptor signaling is implicated in dysglycemia. Yet, the respective metabolic contributions of CNS versus peripheral D2-like receptors including D2 (D2R) and D3 (D3R) receptors remain poorly understood. To address this, we developed new pharmacological tools, D2-like receptor agonists with diminished and delayed blood-brain barrier capability, to selectively manipulate D2R/D3R signaling in the periphery. We designated bromocriptine methiodide (BrMeI), a quaternary methiodide analogue of D2/3R agonist and diabetes drug bromocriptine, as our lead compound based on preservation of D2R/D3R binding and functional efficacy. We then used BrMeI and unmodified bromocriptine to dissect relative contributions of CNS versus peripheral D2R/D3R signaling in treating dysglycemia. Systemic administration of bromocriptine, with unrestricted access to CNS and peripheral targets, significantly improved both insulin sensitivity and glucose tolerance in obese, dysglycemic mice in vivo. In contrast, metabolic improvements were attenuated when access to bromocriptine was restricted either to the CNS through intracerebroventricular administration or delayed access to the CNS via BrMeI. Our findings demonstrate that the coordinated actions of both CNS and peripheral D2-like receptors are required for correcting dysglycemia. Ultimately, the development of a first-generation of drugs designed to selectively target the periphery provides a blueprint for dissecting mechanisms of central versus peripheral DA signaling and paves the way for novel strategies to treat dysglycemia.

Dopamine in the Regulation of Glucose Homeostasis, Pathogenesis of Type 2 Diabetes, and Chronic Conditions of Impaired Dopamine Activity/Metabolism: Implication for Pathophysiological and Therapeutic Purposes

Dopamine regulates several functions, such as voluntary movements, spatial memory, motivation, sleep, arousal, feeding, immune function, maternal behaviors, and lactation. Less clear is the role of dopamine in the pathophysiology of type 2 diabetes mellitus (T2D) and chronic complications and conditions frequently associated with it. This review summarizes recent evidence on the role of dopamine in regulating insular metabolism and activity, the pathophysiology of traditional chronic complications associated with T2D, the pathophysiological interconnection between T2D and chronic neurological and psychiatric disorders characterized by impaired dopamine activity/metabolism, and therapeutic implications. Reinforcing dopamine signaling is therapeutic in T2D, especially in patients with dopamine-related disorders, such as Parkinson's and Huntington's diseases, addictions, and attention-deficit/hyperactivity disorder. On the other hand, although specific trials are probably needed, certain medications approved for T2D (e.g., metformin, pioglitazone, incretin-based therapy, and gliflozins) may have a therapeutic role in such dopamine-related disorders due to anti-inflammatory and anti-oxidative effects, improvement in insulin signaling, neuroinflammation, mitochondrial dysfunction, autophagy, and apoptosis, restoration of striatal dopamine synthesis, and modulation of dopamine signaling associated with reward and hedonic eating. Last, targeting dopamine metabolism could have the potential for diagnostic and therapeutic purposes in chronic diabetes-related complications, such as diabetic retinopathy.

Trace Amine-Associated Receptors and Monoamine-Mediated Regulation of Insulin Secretion in Pancreatic Islets

Currently, metabolic syndrome treatment includes predominantly pharmacological symptom relief and complex lifestyle changes. Trace amines and their receptor systems modulate signaling pathways of dopamine, norepinephrine, and serotonin, which are involved in the pathogenesis of this disorder. Trace amine-associated receptor 1 (TAAR1) is expressed in endocrine organs, and it was revealed that TAAR1 may regulate insulin secretion in pancreatic islet β-cells. For instance, accumulating data demonstrate the positive effect of TAAR1 agonists on the dynamics of metabolic syndrome progression and MetS-associated disease development. The role of other TAARs (TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9) in the islet's function is much less studied. In this review, we summarize the evidence of TAARs' contribution to the metabolic syndrome pathogenesis and regulation of insulin secretion in pancreatic islets. Additionally, by the analysis of public transcriptomic data, we demonstrate that TAAR1 and other TAAR receptors are expressed in the pancreatic islets. We also explore associations between the expression of TAARs mRNA and other genes in studied samples and demonstrate the deregulation of TAARs' functional associations in patients with metabolic diseases compared to healthy donors.

Neurotransmitters regulate β cells insulin secretion: A neglected factor

β cells are the main cells responsible for the hypoglycemic function of pancreatic islets, and the insulin secreted by these cells is the only hormone that lowers blood glucose levels in the human body. β cells are regulated by various factors, among which neurotransmitters make an important contribution. This paper discusses the effects of neurotransmitters secreted by various sympathetic and parasympathetic nerves on β cells and summarizes the mechanisms by which various neurotransmitters regulate insulin secretion. Many neurotransmitters do not have a single source and are not only released from nerve terminals but also synthesized by β cells themselves, allowing them to synergistically regulate insulin secretion. Almost all of these neurotransmitters depend on the presence of glucose to function, and their actions are mostly related to the Ca2+ and cAMP concentrations. Although neurotransmitters have been extensively studied, many of their mechanisms remain unclear and require further exploration by researchers.

Anti-Inflammatory Effects of Peripheral Dopamine

Dopamine is synthesized in the nervous system where it acts as a neurotransmitter. Dopamine is also synthesized in a number of peripheral organs as well as in several types of cells and has organ-specific functions and, as demonstrated more recently, is involved in the regulation of the immune response and inflammatory reaction. In particular, the renal dopaminergic system is very important in the regulation of sodium transport and blood pressure and is particularly sensitive to stimuli that cause oxidative stress and inflammation. This review is focused on how dopamine is synthesized in organs and tissues and the mechanisms by which dopamine and its receptors exert their effects on the inflammatory response.

Antidepressants and type 2 diabetes: highways to knowns and unknowns

Type 2 diabetes (T2D) is a metabolic disease caused by the development of insulin resistance (IR), relative insulin deficiency, and hyperglycemia. Hyperglycemia-induced neurochemical dysregulation activates the progression of depression in T2D patients. Therefore, management of depression by antidepressant agents improves glucose homeostasis and insulin sensitivity. However, prolong use of antidepressant drugs may increase the risk for the development of T2D. However, there is strong controversy concerning the use of antidepressant drugs in T2D. Therefore, this review try to elucidate the potential effects of antidepressant drugs in T2D regarding their detrimental and beneficial effects.

Pharmacological and Physicochemical Properties Optimization for Dual-Target Dopamine D3 (D3R) and μ-Opioid (MOR) Receptor Ligands as Potentially Safer Analgesics

A new generation of dual-target μ opioid receptor (MOR) agonist/dopamine D3 receptor (D3R) antagonist/partial agonists with optimized physicochemical properties was designed and synthesized. Combining in vitro cell-based on-target/off-target affinity screening, in silico computer-aided drug design, and BRET functional assays, we identified new structural scaffolds that achieved high affinity and agonist/antagonist potencies for MOR and D3R, respectively, improving the dopamine receptor subtype selectivity (e.g., D3R over D2R) and significantly enhancing central nervous system multiparameter optimization scores for predicted blood-brain barrier permeability. We identified the substituted trans-(2S,4R)-pyrrolidine and trans-phenylcyclopropyl amine as key dopaminergic moieties and tethered these to different opioid scaffolds, derived from the MOR agonists TRV130 (3) or loperamide (6). The lead compounds 46, 84, 114, and 121 have the potential of producing analgesic effects through MOR partial agonism with reduced opioid-misuse liability via D3R antagonism. Moreover, the peripherally limited derivatives could have therapeutic indications for inflammation and neuropathic pain.

Variation in the Dopamine-4-Receptor Gene in Patients with Type 1 Diabetes

INTRODUCTION

Because dopaminergic signaling pathways are one of the regulators of autoimmunity, we hypothesize that the -521C>T DRD4 gene polymorphism may associate with the risk of diabetes mellitus type 1 (DM1) and its comorbidities.

METHODS

In this case-control study, we have examined 300 patients with DM1 in comparison to 300 healthy age-matched controls. Utilizing the amplification refractory mutation system-polymerase chain reaction method, we have analyzed the -521C>T polymorphism of dopamine D4 receptor-encoding gene. Obtained results have been evaluated according to diabetes comorbidities, inflammatory markers, CD14++CD16-, and CD14+CD16+ monocyte subsets as well as lipid profile.

RESULTS

The key results of our study are as follows: (1) CC genotype and C allele are associated with a reduced risk of DM1 development (OR = 0.593, p = 0.005 and OR = 0.725, p = 0.003, respectively), whereas TT genotype and T allele are associated with a higher risk of DM1 (OR = 1.408, p = 0.04 and OR = 1.380, p = 0.003, respectively); (2) CC genotype is associated with an increased risk of dyslipidemia and retinopathy in diabetic patients (OR = 2.376, p = 0.001 and OR = 2.111, p = 0.01, respectively); (3) CC genotype and C allele carriers had the highest frequency of pro-inflammatory CD16+ monocytes (p = 2*10-4 and 0.04, respectively); (4) the DRD4 -521C>T polymorphism modifies the inflammatory status as well as lipid profile in DM1 patients.

CONCLUSION

Our data imply that the dopaminergic signaling pathways may play an important role in the etiology of DM1 as well as its comorbidities and will provide a new insight into the DM1 risk management. The -521C>T DRD4 gene polymorphism could be considered a genetic marker to predict susceptibility to DM1 as well as retinopathy and dyslipidemia progress in patients with already established disease.

Short-chain fatty acids: possible regulators of insulin secretion

The benefits of gut microbiota-derived short-chain fatty acids (SCFAs) towards health and metabolism have been emerging since the past decade. Extensive studies have been carried out to understand the mechanisms responsible in initiating the functionalities of these SCFAs towards body tissues, which greatly involves the SCFA-specific receptors free fatty acid receptor 2 (FFAR2) and free fatty acid receptor 3 (FFAR3). This review intends to discuss the potential of SCFAs particularly in regulating insulin secretion in pancreatic β-cells, by explaining the production of SCFAs in the gut, the fate of each SCFAs after their production, involvement of FFAR2 and FFAR3 signalling mechanisms and their impacts on insulin secretion. Increased secretion of insulin after SCFAs treatments were reported in many studies, but contradicting evidence also exist in several other studies. Hence, no clear consensus was achieved in determining the true potential of SCFA in regulating insulin secretion. In this review, we explore how such differences were possible and hopefully be able to shed some perspectives in understanding SCFAs-signalling behaviour and preferences.

Dopamine D3/D2 Receptor Ligands Based on Cariprazine for the Treatment of Psychostimulant Use Disorders That May Be Dual Diagnosed with Affective Disorders

Highly selective dopamine D3 receptor (D3R) partial agonists/antagonists have been developed for the treatment of psychostimulant use disorders (PSUD). However, none have reached the clinic due to insufficient potency/efficacy or potential cardiotoxicity. Cariprazine, an FDA-approved drug for the treatment of schizophrenia and bipolar disorder, is a high-affinity D3R partial agonist (Ki = 0.22 nM) with 3.6-fold selectivity over the homologous dopamine D2 receptor (D2R). We hypothesized that compounds that are moderately D3R/D2R-selective partial agonists/antagonists may be effective for the treatment of PSUD. By systematically modifying the parent molecule, we discovered partial agonists/antagonists, as measured in bioluminescence resonance energy transfer (BRET)-based assays, with high D3R affinities (Ki = 0.14-50 nM) and moderate selectivity (<100-fold) over D2R. Cariprazine and two lead analogues, 13a and 13e, decreased cocaine self-administration (FR2; 1-10 mg/kg, i.p.) in rats, suggesting that partial agonists/antagonists with modest D3R/D2R selectivity may be effective in treating PSUD and potentially comorbidities with other affective disorders.

Who Knew? Dopamine Transporter Activity Is Critical in Innate and Adaptive Immune Responses

The dopamine transporter (DAT) regulates the dimension and duration of dopamine transmission. DAT expression, its trafficking, protein-protein interactions, and its activity are conventionally studied in the CNS and within the context of neurological diseases such as Parkinson's Diseases and neuropsychiatric diseases such as drug addiction, attention deficit hyperactivity and autism. However, DAT is also expressed at the plasma membrane of peripheral immune cells such as monocytes, macrophages, T-cells, and B-cells. DAT activity via an autocrine/paracrine signaling loop regulates macrophage responses to immune stimulation. In a recent study, we identified an immunosuppressive function for DAT, where blockade of DAT activity enhanced LPS-mediated production of IL-6, TNF-α, and mitochondrial superoxide levels, demonstrating that DAT activity regulates macrophage immune responses. In the current study, we tested the hypothesis that in the DAT knockout mice, innate and adaptive immunity are perturbed. We found that genetic deletion of DAT (DAT^-/-) results in an exaggerated baseline inflammatory phenotype in peripheral circulating myeloid cells. In peritoneal macrophages obtained from DAT^-/- mice, we identified increased MHC-II expression and exaggerated phagocytic response to LPS-induced immune stimulation, suppressed T-cell populations at baseline and following systemic endotoxemia and exaggerated memory B cell expansion. In DAT^-/- mice, norepinephrine and dopamine levels are increased in spleen and thymus, but not in circulating serum. These findings in conjunction with spleen hypoplasia, increased splenic myeloid cells, and elevated MHC-II expression, in DAT^-/- mice further support a critical role for DAT activity in peripheral immunity. While the current study is only focused on identifying the role of DAT in peripheral immunity, our data point to a much broader implication of DAT activity than previously thought. This study is dedicated to the memory of Dr. Marc Caron who has left an indelible mark in the dopamine transporter field.

Recent advances in dopamine D2 receptor ligands in the treatment of neuropsychiatric disorders

Dopamine is a biologically active amine synthesized in the central and peripheral nervous system. This biogenic monoamine acts by activating five types of dopamine receptors (D_1-5 Rs), which belong to the G protein-coupled receptor family. Antagonists and partial agonists of D₂ Rs are used to treat schizophrenia, Parkinson's disease, depression, and anxiety. The typical pharmacophore with high D₂ R affinity comprises four main areas, namely aromatic moiety, cyclic amine, central linker and aromatic/heteroaromatic lipophilic fragment. From the literature reviewed herein, we can conclude that 4-(2,3-dichlorophenyl), 4-(2-methoxyphenyl)-, 4-(benzo[b]thiophen-4-yl)-1-substituted piperazine, and 4-(6-fluorobenzo[d]isoxazol-3-yl)piperidine moieties are critical for high D₂ R affinity. Four to six atoms chains are optimal for D₂ R affinity with 4-butoxyl as the most pronounced one. The bicyclic aromatic/heteroaromatic systems are most frequently occurring as lipophilic appendages to retain high D₂ R affinity. In this review, we provide a thorough overview of the therapeutic potential of D₂ R modulators in the treatment of the aforementioned disorders. In addition, this review summarizes current knowledge about these diseases, with a focus on the dopaminergic pathway underlying these pathologies. Major attention is paid to the structure, function, and pharmacology of novel D₂ R ligands, which have been developed in the last decade (2010-2021), and belong to the 1,4-disubstituted aromatic cyclic amine group. Due to the abundance of data, allosteric D₂ R ligands and D₂ R modulators from patents are not discussed in this review.

Roles of Pancreatic Islet Catecholamine Neurotransmitters in Glycemic Control and in Antipsychotic Drug-Induced Dysglycemia

Catecholamine neurotransmitters dopamine (DA) and norepinephrine (NE) are essential for a myriad of functions throughout the central nervous system, including metabolic regulation. These molecules are also present in the pancreas, and their study may shed light on the effects of peripheral neurotransmission on glycemic control. Though sympathetic innervation to islets provides NE that signals at local α-cell and β-cell adrenergic receptors to modify hormone secretion, α-cells and β-cells also synthesize catecholamines locally. We propose a model where α-cells and β-cells take up catecholamine precursors in response to postprandial availability, preferentially synthesizing DA. The newly synthesized DA signals in an autocrine/paracrine manner to regulate insulin and glucagon secretion and maintain glycemic control. This enables islets to couple local catecholamine signaling to changes in nutritional state. We also contend that the DA receptors expressed by α-cells and β-cells are targeted by antipsychotic drugs (APDs)-some of the most widely prescribed medications today. Blockade of local DA signaling contributes significantly to APD-induced dysglycemia, a major contributor to treatment discontinuation and development of diabetes. Thus, elucidating the peripheral actions of catecholamines will provide new insights into the regulation of metabolic pathways and may lead to novel, more effective strategies to tune metabolism and treat diabetes.

Hyperbaric oxygen exposure alleviate metabolic side-effects of olanzapine treatment and is associated with Langerhans islet proliferation in rats

Introduction: Olanzapine (OLZ) is one of the second-generation antipsychotics drugs (APDs) used to treat several psychiatric illnesses. Olanzapine treatment is often associated with many metabolic side effects in a dose dependent manner such as obesity, dyslipidemia and insulin resistance, induction of type II diabetes and acute pancreatitis in some patients. Methods: Hyperbaric Oxygen therapy (HBOT) was investigated as a tool to mitigate olanzapine metabolic side effects in rats. Thirty-six female Sprague Dawley (SD) rats were divided into 4 groups; rats on olanzapine treatment either exposed to hyperbaric oxygen therapy (HBOOLZ) or left without exposure (OLZ) then non-treated rats that either exposed to hyperbaric oxygen therapy or left without exposure (control). Rats received Hyperbaric Oxygen therapy for 35 days at 2.4 atmospheres absolute (ATA) for 2.5 h daily followed by intraperitoneal injection of olanzapine at 10 mg/kg or placebo. Results: Rats on either hyperbaric oxygen therapy or olanzapine had a significant loss in body weight. Olanzapine treatment showed a decrease in serum insulin level, triglyceride, highdensity lipoprotein (HDL) cholesterol, and lipase level but an increase in fasting blood sugar (FBS), insulin resistance index (HOMA-IR) and amylase, while rats' exposure to hyperbaric oxygen therapy reversed these effects. The Pancreatic Langerhans islets were up-regulated in both hyperbaric oxygen therapy and olanzapine treatments but the combination (HBOOLZ) doubled these islets number. Discussion: This study advocated that hyperbaric oxygen therapy can be an alternative approach to control or reverse many metabolic disorders (MDs) associatedwith olanzapine treatment. In addition, it seems that hyperbaric oxygen therapy positively affect the pancreatic Langerhans cells activity and architecture.

Gi/o protein-coupled receptor inhibition of beta-cell electrical excitability and insulin secretion depends on Na+/K+ ATPase activation

Gi/o-coupled somatostatin or α2-adrenergic receptor activation stimulated β-cell NKA activity, resulting in islet Ca2+ fluctuations. Furthermore, intra-islet paracrine activation of β-cell Gi/o-GPCRs and NKAs by δ-cell somatostatin secretion slowed Ca2+ oscillations, which decreased insulin secretion. β-cell membrane potential hyperpolarization resulting from Gi/o-GPCR activation was dependent on NKA phosphorylation by Src tyrosine kinases. Whereas, β-cell NKA function was inhibited by cAMP-dependent PKA activity. These data reveal that NKA-mediated β-cell membrane potential hyperpolarization is the primary and conserved mechanism for Gi/o-GPCR control of electrical excitability, Ca2+ handling, and insulin secretion.

The regulatory role of Gnao1 protein in diabetic encephalopathy in KK-Ay mice and streptozotocin-induced diabetic rats

AIMS

To investigate the regulation and functional role of Gnao1 in the brain of diabetic encephalopathy (DE) in various animal models.

RESULTS

Data from the biochemical and behavioral studies showed that DE models were successful induced in streptozotocin treatment animals and KK-Ay mice. Gnao1 was down regulated in the brain tissues of these two diabetes animal models with significant cognition deficiency. It suggested that the changes in DE are also related to dementia such as Alzheimer's disease (AD). Our study also showed that the expression of adrenergic α2 receptor (Adr-α2R), the upstream protein of Gnao1, was decreased in DE animal models. Furthermore, many downstream proteins of Gnao1 also altered, among which cAMP and PKA proteins were increased, CREB and BDNF proteins were decreased both in animal models and in the cell levels. In addition, Gnao1 silencing leads to the increase of reactive oxygen species (ROS) and the decreased proliferation in cultured primary astrocytes, which means that the deficiency of Gnao1 might not be benefit for DE.

CONCLUSION

Our findings demonstrated the importance of Gnao1 in DE and suggested Gnao1 as a novel marker and a promising therapeutic target for DE and dementia in animal models.

Dopamine Negatively Regulates Insulin Secretion Through Activation of D1-D2 Receptor Heteromer

There is increasing evidence that dopamine (DA) functions as a negative regulator of glucose-stimulated insulin secretion; however, the underlying molecular mechanism remains unknown. Using total internal reflection fluorescence microscopy, we monitored insulin granule exocytosis in primary islet cells to dissect the effect of DA. We found that D1 receptor antagonists rescued the DA-mediated inhibition of glucose-stimulated calcium (Ca2+) flux, thereby suggesting a role of D1 in the DA-mediated inhibition of insulin secretion. Overexpression of D2, but not D1, alone exerted an inhibitory and toxic effect that abolished the glucose-stimulated Ca2+ influx and insulin secretion in β-cells. Proximity ligation and Western blot assays revealed that D1 and D2 form heteromers in β-cells. Treatment with a D1-D2 heteromer agonist, SKF83959, transiently inhibited glucose-induced Ca2+ influx and insulin granule exocytosis. Coexpression of D1 and D2 enabled β-cells to bypass the toxic effect of D2 overexpression. DA transiently inhibited glucose-stimulated Ca2+ flux and insulin exocytosis by activating the D1-D2 heteromer. We conclude that D1 protects β-cells from the harmful effects of DA by modulating D2 signaling. The finding will contribute to our understanding of the DA signaling in regulating insulin secretion and improve methods for preventing and treating diabetes.

Antipsychotic-induced weight gain and metabolic effects show diurnal dependence and are reversible with time restricted feeding

Antipsychotic drugs (AP) are highly efficacious treatments for psychiatric disorders but are associated with significant metabolic side-effects. The circadian clock maintains metabolic homeostasis by sustaining daily rhythms in feeding, fasting and hormone regulation but how circadian rhythms interact with AP and its associated metabolic side-effects is not well-known. We hypothesized that time of AP dosing impacts the development of metabolic side-effects. Weight gain and metabolic side-effects were compared in C57Bl/6 mice and humans dosed with APs in either the morning or evening. In mice, AP dosing at the start of the light cycle/rest period (AM) resulted in significant increase in food intake and weight gain compared with equivalent dose before the onset of darkness/active period (PM). Time of AP dosing also impacted circadian gene expression, metabolic hormones and inflammatory pathways and their diurnal expression patterns. We also conducted a retrospective examination of weight and metabolic outcomes in patients who received risperidone (RIS) for the treatment of serious mental illness and observed a significant association between time of dosing and severity of RIS-induced metabolic side-effects. Time restricted feeding (TRF) has been shown in both mouse and some human studies to be an effective therapeutic intervention against obesity and metabolic disease. We demonstrate, for the first time, that TRF is an effective intervention to reduce AP-induced metabolic side effects in mice. These studies identify highly effective and translatable interventions with potential to mitigate AP-induced metabolic side effects.

Dual pancreatic adrenergic and dopaminergic signaling as a therapeutic target of bromocriptine

Bromocriptine is approved as a diabetes therapy, yet its therapeutic mechanisms remain unclear. Though bromocriptine's actions have been mainly attributed to the stimulation of brain dopamine D2 receptors (D2R), bromocriptine also targets the pancreas. Here, we employ bromocriptine as a tool to elucidate the roles of catecholamine signaling in regulating pancreatic hormone secretion. In β-cells, bromocriptine acts on D2R and α2A-adrenergic receptor (α2A-AR) to reduce glucose-stimulated insulin secretion (GSIS). Moreover, in α-cells, bromocriptine acts via D2R to reduce glucagon secretion. α2A-AR activation by bromocriptine recruits an ensemble of G proteins with no β-arrestin2 recruitment. In contrast, D2R recruits G proteins and β-arrestin2 upon bromocriptine stimulation, demonstrating receptor-specific signaling. Docking studies reveal distinct bromocriptine binding to α2A-AR versus D2R, providing a structural basis for bromocriptine's dual actions on β-cell α2A-AR and D2R. Together, joint dopaminergic and adrenergic receptor actions on α-cell and β-cell hormone release provide a new therapeutic mechanism to improve dysglycemia.

The second-generation antipsychotic drug aripiprazole modulates the serotonergic system in pancreatic islets and induces beta cell dysfunction in female mice

AIMS/HYPOTHESIS

Second-generation antipsychotic (SGA) drugs have been associated with the development of type 2 diabetes and the metabolic syndrome in patients with schizophrenia. In this study, we aimed to investigate the effects of two different SGA drugs, olanzapine and aripiprazole, on metabolic state and islet function and plasticity.

METHODS

We analysed the functional adaptation of beta cells in 12-week-old B6;129 female mice fed an olanzapine- or aripiprazole-supplemented diet (5.5-6.0 mg kg^-1 day^-1) for 6 months. Glucose and insulin tolerance tests, in vivo glucose-stimulated insulin secretion and indirect calorimetry were performed at the end of the study. The effects of SGAs on beta cell plasticity and islet serotonin levels were assessed by transcriptomic analysis and immunofluorescence. Insulin secretion was assessed by static incubations and Ca²⁺ fluxes by imaging techniques.

RESULTS

Treatment of female mice with olanzapine or aripiprazole for 6 months induced weight gain (p<0.01 and p<0.05, respectively), glucose intolerance (p<0.01) and impaired insulin secretion (p<0.05) vs mice fed a control chow diet. Aripiprazole, but not olanzapine, induced serotonin production in beta cells vs controls, likely by increasing tryptophan hydroxylase 1 (TPH1) expression, and inhibited Ca²⁺ flux. Of note, aripiprazole increased beta cell size (p<0.05) and mass (p<0.01) vs mice fed a control chow diet, along with activation of mechanistic target of rapamycin complex 1 (mTORC1)/S6 signalling, without preventing beta cell dysfunction.

CONCLUSIONS/INTERPRETATION

Both SGAs induced weight gain and beta cell dysfunction, leading to glucose intolerance; however, aripiprazole had a more potent effect in terms of metabolic alterations, which was likely a result of its ability to modulate the serotonergic system. The deleterious metabolic effects of SGAs on islet function should be considered while treating patients as these drugs may increase the risk for development of the metabolic syndrome and diabetes.

Diabetes mellitus and Parkinson's disease: dangerous liaisons between insulin and dopamine

The relationship between diabetes mellitus and Parkinson's disease has been described in several epidemiological studies over the 1960s to date. Molecular studies have shown the possible functional link between insulin and dopamine, as there is strong evidence demonstrating the action of dopamine in pancreatic islets, as well as the insulin effects on feeding and cognition through central nervous system mechanism, largely independent of glucose utilization. Therapies used for the treatment of type 2 diabetes mellitus appear to be promising candidates for symptomatic and/or disease-modifying action in neurodegenerative diseases including Parkinson's disease, while an old dopamine agonist, bromocriptine, has been repositioned for the type 2 diabetes mellitus treatment. This review will aim at reappraising the different studies that have highlighted the dangerous liaisons between diabetes mellitus and Parkinson's disease.

Insulin and glucose metabolism with olanzapine and a combination of olanzapine and samidorphan: exploratory phase 1 results in healthy volunteers

A combination of olanzapine and samidorphan (OLZ/SAM) received US Food and Drug Administration approval in May 2021 for the treatment of adults with schizophrenia or bipolar I disorder. OLZ/SAM provides the efficacy of olanzapine, while mitigating olanzapine-associated weight gain. This exploratory study characterized the metabolic profile of OLZ/SAM in healthy volunteers to gain mechanistic insights. Volunteers received once-daily oral 10 mg/10 mg OLZ/SAM, 10 mg olanzapine, or placebo for 21 days. Assessments included insulin sensitivity during an oral glucose tolerance test (OGTT), hyperinsulinemic-euglycemic clamp, other measures of glucose/lipid metabolism, and adverse event (AE) monitoring. Treatment effects were estimated with analysis of covariance. In total, 60 subjects were randomized (double-blind; placebo, n = 12; olanzapine, n = 24; OLZ/SAM, n = 24). Olanzapine resulted in hyperinsulinemia and reduced insulin sensitivity during an OGTT at day 19, changes not observed with OLZ/SAM or placebo. Insulin sensitivity, measured by hyperinsulinemic-euglycemic clamp, was decreased in all treatment groups relative to baseline, but this effect was greatest with olanzapine and OLZ/SAM. Although postprandial (OGTT) glucose and fasting cholesterol concentrations were similarly increased with olanzapine or OLZ/SAM, other early metabolic effects were distinct, including post-OGTT C-peptide concentrations and aspects of energy metabolism. Forty-nine subjects (81.7%) experienced at least 1 AE, most mild or moderate in severity. OLZ/SAM appeared to mitigate some of olanzapine's unfavorable postprandial metabolic effects (e.g., hyperinsulinemia, elevated C-peptide) in this exploratory study. These findings supplement the body of evidence from completed or ongoing OLZ/SAM clinical trials supporting its role in the treatment of schizophrenia and bipolar I disorder.

Peripheral Dopamine Directly Acts on Insulin-Sensitive Tissues to Regulate Insulin Signaling and Metabolic Function

Dopamine is a key regulator of glucose metabolism in the central nervous system. However, dopamine is also present in the periphery and may have direct effects on insulin-sensitive tissues. Dopamine receptor 2 (D2R) agonist bromocriptine is a FDA-approved drug for type 2 diabetes. Herein, we explored the role of peripheral dopamine and its receptors in regulating glucose uptake and metabolism on insulin-sensitive tissues. Peripheral dopamine effect in [3H]2-deoxyglucose uptake in insulin-sensitive tissues was tested in vivo in rats. Direct effects on [3H]2-deoxyglucose uptake, insulin receptor phosphorylation, and regulation of metabolic function were tested ex vivo in the liver, soleus muscle, and white and brown adipose tissues. Bromocriptine and the antagonists domperidone, D2R antagonist, and haloperidol, antagonist of both dopamine receptor 1 (D1R) and D2R, were used to disclose dopamine receptors’ involvement.

Peripheral dopamine increases glucose uptake in vivo. Ex vivo, only dopamine increased glucose uptake in the soleus, while bromocriptine increased it in the liver; the effects were reverted by haloperidol and domperidone, respectively. In adipose tissue, domperidone reverted dopamine- and bromocriptine-mediated potentiation of insulin-induced glucose uptake, but in turn increased the insulin receptor, Akt, AMPK, HSL, ACC, and ACL, phosphorylation. In the soleus muscle, AMPK-phosphorylation increased with bromocriptine and dopamine whose effects were suppressed by domperidone and haloperidol.

In conclusion, peripheral dopamine stimulates glucose uptake with its receptors being differentially involved in glucose uptake in insulin-sensitive tissues. Dopamine also has a role in lipid metabolism in white adipose tissue. Altogether, these results suggest that peripheral modulation of the dopaminergic system should be further evaluated as a putative therapeutic approach for metabolic disorders.

β-Cells Different Vulnerability to the Parkinsonian Neurotoxins Rotenone, 1-Methyl-4-phenylpyridinium (MPP+) and 6-Hydroxydopamine (6-OHDA)

Neurotoxins such as rotenone, 1-methyl-4-phenylpyridinium (MPP⁺) and 6-hydroxydopamine (6-OHDA) are well known for their high toxicity on dopaminergic neurons and are associated with Parkinson's disease (PD) in murine models and humans. In addition, PD patients often have glucose intolerance and may develop type 2 diabetes (T2D), whereas T2D patients have higher risk of PD compared to general population. Based on these premises, we evaluated the toxicity of these three toxins on pancreatic β-cell lines (INS-1 832/13 and MIN6) and we showed that rotenone is the most potent for reducing β-cells viability and altering mitochondrial structure and bioenergetics in the low nanomolar range, similar to that found in dopaminergic cell lines. MPP⁺ and 6-OHDA show similar effects but at higher concentration. Importantly, rotenone-induced toxicity was counteracted by α-tocopherol and partially by metformin, which are endowed with strong antioxidative and cytoprotective properties. These data show similarities between dopaminergic neurons and β-cells in terms of vulnerability to toxins and pharmacological agents capable to protect both cell types.

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.

Autism-Associated Variant in the SLC6A3 Gene Alters the Oral Microbiome and Metabolism in a Murine Model

Background: Altered dopamine (DA) signaling has been associated with autism spectrum disorder (ASD), a neurodevelopmental condition estimated to impact 1 in 54 children in the United States. There is growing evidence for alterations in both gastrointestinal function and oral microbiome composition in ASD. Recent work suggests that rare variants of the SLC6A3 gene encoding the DA transporter (DAT) identified in individuals with ASD result in structural and functional changes to the DAT. One such recently identified de novo mutation is a threonine to methionine substitution at position 356 of the DAT (DAT T356M). The DAT T356M variant is associated with ASD-like phenotypes in mice homozygous for the mutation (DAT T356M+/+), including social deficits, hyperactivity, and impaired DA signaling. Here, we determine the impact of this altered DA signaling as it relates to altered oral microbiota, and metabolic and gastrointestinal dysfunction. Methods: In the DAT T356M+/+ mouse, we determine the oral microbiota composition, metabolic function, and gastrointestinal (GI) function. We examined oral microbiota by 16S RNA sequencing. We measured metabolic function by examining glucose tolerance and we probed gastrointestinal parameters by measuring fecal dimensions and weight. Results: In the DAT T356M+/+ mouse, we evaluate how altered DA signaling relates to metabolic dysfunction and altered oral microbiota. We demonstrate that male DAT T356M+/+ mice weigh less (Wild type (WT) = 26.48 ± 0.6405 g, DAT T356M+/+ = 24.14 ± 0.4083 g) and have decreased body fat (WT = 14.89 ± 0.6206%, DAT T356M+/+ = 12.72 ± 0.4160%). These mice display improved glucose handling (WT = 32.60 ± 0.3298 kcal/g, DAT T356M+/+ = 36.97 ± 0.4910 kcal/g), and an altered oral microbiota. We found a significant decrease in Fusobacterium abundance. The abundance of Fusobacterium was associated with improved glucose handling and decreased body fat. Conclusions: Our findings provide new insights into how DAT dysfunction may alter gastrointestinal function, composition of the oral microbiota, and metabolism. Our data suggest that impaired DA signaling in ASD is associated with a number of metabolic and gastrointestinal changes which are common in individuals with ASD.

Tyrosine hydroxylase conditional KO mice reveal peripheral tissue-dependent differences in dopamine biosynthetic pathways

Dopamine (DA) exerts well-known functions in the brain as a neurotransmitter. In addition, it plays important physiological roles in peripheral organs, but it is largely unknown how and where peripheral DA is synthesized and regulated. Catecholamines in peripheral tissues are either produced within the tissue itself and/or derived from sympathetic neurons, which release neurotransmitters for uptake by peripheral tissues. To evaluate DA-producing ability of each peripheral tissue, we generated conditional KO mice (cKO mice) in which the tyrosine hydroxylase (TH) gene is ablated in the sympathoadrenal system, thus eliminating sympathetic neurons as a DA source. We then examined the alterations in the noradrenaline (NA), DA, and 3,4-dihydroxyphenylalanine (DOPA) contents in peripheral organs and performed immunohistochemical analyses of TH-expressing cells. In the heart and pancreas of cKO mice, both the TH protein and NA levels were significantly decreased, and the DA contents were decreased in parallel with NA contents, indicating that the DA supply originated from sympathetic neurons. We found TH-immunoreactive cells in the stomach and lung, where the TH protein showed a decreasing trend, but the DA levels were not decreased in cKO mice. Moreover, we found a significant correlation between the DA content in the kidney and the plasma DOPA concentration, suggesting that the kidney takes up DOPA from blood to make DA. The aforementioned data unravel differences in the DA biosynthetic pathway among tissues and support the role of sympathetic neurons as a DA supplier.

Atypical Antipsychotics and Metabolic Syndrome: From Molecular Mechanisms to Clinical Differences

Atypical antipsychotics (AAPs) are commonly prescribed medications to treat schizophrenia, bipolar disorders and other psychotic disorders. However, they might cause metabolic syndrome (MetS) in terms of weight gain, dyslipidemia, type 2 diabetes (T2D), and high blood pressure, which are responsible for reduced life expectancy and poor adherence. Importantly, there is clear evidence that early metabolic disturbances can precede weight gain, even if the latter still remains the hallmark of AAPs use. In fact, AAPs interfere profoundly with glucose and lipid homeostasis acting mostly on hypothalamus, liver, pancreatic β-cells, adipose tissue, and skeletal muscle. Their actions on hypothalamic centers via dopamine, serotonin, acetylcholine, and histamine receptors affect neuropeptides and 5'AMP-activated protein kinase (AMPK) activity, thus producing a supraphysiological sympathetic outflow augmenting levels of glucagon and hepatic glucose production. In addition, altered insulin secretion, dyslipidemia, fat deposition in the liver and adipose tissues, and insulin resistance become aggravating factors for MetS. In clinical practice, among AAPs, olanzapine and clozapine are associated with the highest risk of MetS, whereas quetiapine, risperidone, asenapine and amisulpride cause moderate alterations. The new AAPs such as ziprasidone, lurasidone and the partial agonist aripiprazole seem more tolerable on the metabolic profile. However, these aspects must be considered together with the differences among AAPs in terms of their efficacy, where clozapine still remains the most effective. Intriguingly, there seems to be a correlation between AAP's higher clinical efficacy and increase risk of metabolic alterations. Finally, a multidisciplinary approach combining psychoeducation and therapeutic drug monitoring (TDM) is proposed as a first-line strategy to avoid the MetS. In addition, pharmacological treatments are discussed as well.

Multi-Electrode Array Analysis Identifies Complex Dopamine Responses and Glucose Sensing Properties of Substantia Nigra Neurons in Mouse Brain Slices

Dopaminergic (DA) midbrain neurons within the substantia nigra (SN) display an autonomous pacemaker activity that is crucial for dopamine release and voluntary movement control. Their progressive degeneration is a hallmark of Parkinson's disease. Their metabolically demanding activity-mode affects Ca²⁺ homeostasis, elevates metabolic stress, and renders SN DA neurons particularly vulnerable to degenerative stressors. Accordingly, their activity is regulated by complex mechanisms, notably by dopamine itself, via inhibitory D2-autoreceptors and the neuroprotective neuronal Ca²⁺ sensor NCS-1. Analyzing regulation of SN DA neuron activity-pattern is complicated by their high vulnerability. We studied this activity and its control by dopamine, NCS-1, and glucose with extracellular multi-electrode array (MEA) recordings from midbrain slices of juvenile and adult mice. Our tailored MEA- and spike sorting-protocols allowed high throughput and long recording times. According to individual dopamine-responses, we identified two distinct SN cell-types, in similar frequency: dopamine-inhibited and dopamine-excited neurons. Dopamine-excited neurons were either silent in the absence of dopamine, or they displayed pacemaker-activities, similar to that of dopamine-inhibited neurons. Inhibition of pacemaker-activity by dopamine is typical for SN DA neurons, and it can undergo prominent desensitization. We show for adult mice, that the number of SN DA neurons with desensitized dopamine-inhibition was increased (~60–100%) by a knockout of NCS-1, or by prevention of NCS-1 binding to D2-autoreceptors, while time-course and degrees of desensitization were not altered. The number of neurons with desensitized D2-responses was also higher (~65%) at high glucose-levels (25 mM), compared to lower glucose (2.5 mM), while again desensitization-kinetics were unaltered. However, spontaneous firing-rates were significantly higher at high glucose-levels (~20%). Moreover, transient glucose-deprivation (1 mM) induced a fast and fully-reversible pacemaker frequency reduction. To directly address and quantify glucose-sensing properties of SN DA neurons, we continuously monitored their electrical activity, while altering extracellular glucose concentrations stepwise from 0.5 mM up to 25 mM. SN DA neurons were excited by glucose, with EC₅₀ values ranging from 0.35 to 2.3 mM. In conclusion, we identified a novel, common subtype of dopamine-excited SN neurons, and a complex, joint regulation of dopamine-inhibited neurons by dopamine and glucose, within the range of physiological brain glucose-levels.

Dopamine regulates pancreatic glucagon and insulin secretion via adrenergic and dopaminergic receptors

Dopamine (DA) and norepinephrine (NE) are catecholamines primarily studied in the central nervous system that also act in the pancreas as peripheral regulators of metabolism. Pancreatic catecholamine signaling has also been increasingly implicated as a mechanism responsible for the metabolic disturbances produced by antipsychotic drugs (APDs). Critically, however, the mechanisms by which catecholamines modulate pancreatic hormone release are not completely understood. We show that human and mouse pancreatic α- and β-cells express the catecholamine biosynthetic and signaling machinery, and that α-cells synthesize DA de novo. This locally-produced pancreatic DA signals via both α- and β-cell adrenergic and dopaminergic receptors with different affinities to regulate glucagon and insulin release. Significantly, we show DA functions as a biased agonist at α_2A-adrenergic receptors, preferentially signaling via the canonical G protein-mediated pathway. Our findings highlight the interplay between DA and NE signaling as a novel form of regulation to modulate pancreatic hormone release. Lastly, pharmacological blockade of DA D₂-like receptors in human islets with APDs significantly raises insulin and glucagon release. This offers a new mechanism where APDs act directly on islet α- and β-cell targets to produce metabolic disturbances.

Differential Morphological Diagnosis of Various Forms of Congenital Hyperinsulinism in Children

INTRODUCTION

Congenital hyperinsulinism (CHI) has diffuse (CHI-D), focal (CHI-F) and atypical (CHI-A) forms. Surgical management depends on preoperative [18F]-DOPA PET/CT and intraoperative morphological differential diagnosis of CHI forms. Objective: to improve differential diagnosis of CHI forms by comparative analysis [18F]-DOPA PET/CT data, as well as cytological, histological and immunohistochemical analysis (CHIA).

MATERIALS AND METHODS

The study included 35 CHI patients aged 3.2 ± 2.0 months; 10 patients who died from congenital heart disease at the age of 3.2 ± 2.9 months (control group). We used PET/CT, CHIA of pancreas with antibodies to ChrA, insulin, Isl1, Nkx2.2, SST, NeuroD1, SSTR2, SSTR5, DR1, DR2, DR5; fluorescence microscopy with NeuroD1/ChrA, Isl1/insulin, insulin/SSTR2, DR2/NeuroD1 cocktails.

RESULTS

Intraoperative examination of pancreatic smears showed the presence of large nuclei, on average, in: 14.5 ± 3.5 cells of CHI-F; 8.4 ± 1.1 of CHI-D; and 4.5 ± 0.7 of control group (from 10 fields of view, x400). The percentage of Isl1+ and NeuroD1+endocrinocytes significantly differed from that in the control for all forms of CHI. The percentage of NeuroD1+exocrinocytes was also significantly higher than in the control. The proportion of ChrA+ and DR2+endocrinocytes was higher in CHI-D than in CHI-F, while the proportion of insulin+cells was higher in CHI-A. The number of SST+cells was significantly higher in CHI-D and CHI-F than in CHI-A.

CONCLUSION

For intraoperative differential diagnosis of CHI forms, in addition to frozen sections, quantitative cytological analysis can be used. In quantitative immunohistochemistry, CHI forms differ in the expression of ChrA, insulin, SST and DR2. The development of a NeuroD1 inhibitor would be advisable for targeted therapy of CHI.

Metabolomic Profiles Predict Diabetes Remission after Bariatric Surgery

Background: Amino acid metabolites (AAMs) have been linked to glucose homeostasis and type 2 diabetes (T2D). We investigated whether (1) baseline AAMs predict T2D remission 12 months after bariatric surgery and (2) whether AAMs are superior for predicting T2D remission postoperatively compared with existing prediction models. Methods: Among 24 participants undergoing bariatric surgery, 16 diabetes-related AAMs were quantified at baseline and postoperative 3 and 12 months. Existing prediction models included the ABCD, DiaRem, and IMS models. Results: Baseline L-dihydroxyphenylalanine (L-DOPA) (areas under receiver operating characteristic curves (AUROC), 0.92; 95% confidence interval (CI), 0.75 to 1.00) and 3-hydroxyanthranilic acid (3-HAA) (AUROC, 0.85; 95% CI, 0.67 to 1.00) better predicted T2D remission 12 months postoperatively than the ABCD model (AUROC, 0.81; 95% CI, 0.54 to 1.00), which presented the highest AUROC value among the three models. The superior prognostic performance of L-DOPA (AUROC at 3 months, 0.97; 95% CI, 0.91 to 1.00) and 3-HAA (AUROC at 3 months, 0.86; 95% CI, 0.63 to 1.00) continued until 3 months postoperatively. Conclusions: The AAM profile predicts T2D remission after bariatric surgery more effectively than the existing prediction models.

Tetraspanin-7 regulation of L-type voltage-dependent calcium channels controls pancreatic β-cell insulin secretion

KEY POINTS

Tetraspanin (TSPAN) proteins regulate many biological processes, including intracellular calcium (Ca2+ ) handling. TSPAN-7 is enriched in pancreatic islet cells; however, the function of islet TSPAN-7 has not been identified. Here, we characterize how β-cell TSPAN-7 regulates Ca2+ handling and hormone secretion. We find that TSPAN-7 reduces β-cell glucose-stimulated Ca2+ entry, slows Ca2+ oscillation frequency and decreases glucose-stimulated insulin secretion. TSPAN-7 controls β-cell function through a direct interaction with L-type voltage-dependent Ca2+ channels (CaV 1.2 and CaV 1.3), which reduces channel Ca2+ conductance. TSPAN-7 slows activation of CaV 1.2 and accelerates recovery from voltage-dependent inactivation; TSPAN-7 also slows CaV 1.3 inactivation kinetics. These findings strongly implicate TSPAN-7 as a key regulator in determining the set-point of glucose-stimulated Ca2+ influx and insulin secretion.

ABSTRACT

Glucose-stimulated insulin secretion (GSIS) is regulated by calcium (Ca2+ ) entry into pancreatic β-cells through voltage-dependent Ca2+ (CaV ) channels. Tetraspanin (TSPAN) transmembrane proteins control Ca2+ handling, and thus they may also modulate GSIS. TSPAN-7 is the most abundant islet TSPAN and immunostaining of mouse and human pancreatic slices shows that TSPAN-7 is highly expressed in β- and α-cells; however, the function of islet TSPAN-7 has not been determined. Here, we show that TSPAN-7 knockdown (KD) increases glucose-stimulated Ca2+ influx into mouse and human β-cells. Additionally, mouse β-cell Ca2+ oscillation frequency was accelerated by TSPAN-7 KD. Because TSPAN-7 KD also enhanced Ca2+ entry when membrane potential was clamped with depolarization, the effect of TSPAN-7 on CaV channel activity was examined. TSPAN-7 KD enhanced L-type CaV currents in mouse and human β-cells. Conversely, heterologous expression of TSPAN-7 with CaV 1.2 and CaV 1.3 L-type CaV channels decreased CaV currents and reduced Ca2+ influx through both channels. This was presumably the result of a direct interaction of TSPAN-7 and L-type CaV channels because TSPAN-7 coimmunoprecipitated with both CaV 1.2 and CaV 1.3 from primary human β-cells and from a heterologous expression system. Finally, TSPAN-7 KD in human β-cells increased basal (5.6 mM glucose) and stimulated (45 mM KCl + 14 mM glucose) insulin secretion. These findings strongly suggest that TSPAN-7 modulation of β-cell L-type CaV channels is a key determinant of β-cell glucose-stimulated Ca2+ entry and thus the set-point of GSIS.

Exome Sequencing Identifies A Nonsense Variant in DAO Associated With Reduced Energy Expenditure in American Indians

BACKGROUND

Obesity and energy expenditure (EE) are heritable and genetic variants influencing EE may contribute to the development of obesity. We sought to identify genetic variants that affect EE in American Indians, an ethnic group with high prevalence of obesity.

METHODS

Whole-exome sequencing was performed in 373 healthy Pima Indians informative for 24-hour EE during energy balance. Genetic association analyses of all high-quality exonic variants (≥5 carriers) was performed, and those predicted to be damaging were prioritized.

RESULTS

Rs752074397 introduces a premature stop codon (Cys264Ter) in DAO and demonstrated the strongest association for 24-hour EE, where the Ter allele associated with substantially lower 24-hour EE (mean lower by 268 kcal/d) and sleeping EE (by 135 kcal/d). The Ter allele has a frequency = 0.5% in Pima Indians, whereas is extremely rare in most other ethnic groups (frequency < 0.01%). In vitro functional analysis showed reduced protein levels for the truncated form of DAO consistent with increased protein degradation. DAO encodes D-amino acid oxidase, which is involved in dopamine synthesis which might explain its role in modulating EE.

CONCLUSION

Our results indicate that a nonsense mutation in DAO may influence EE in American Indians. Identification of variants that influence energy metabolism may lead to new pathways to treat human obesity.

CLINICAL TRIAL REGISTRATION NUMBER

NCT00340132.

Dopamine D2 receptor signaling modulates pancreatic beta cell circadian rhythms

Antipsychotic drugs (APD) have clinically important, adverse effects on metabolism that limit their therapeutic utility. Pancreatic beta cells produce dopamine and express the D₂ dopamine receptor (D2R). As D2R antagonists, APDs alter glucose-stimulated insulin secretion, indicating that dopamine likely plays a role in APD-induced metabolic dysfunction. Insulin secretion from beta cells is also modulated by the circadian clock. Disturbed circadian rhythms cause metabolic disturbances similar to those observed in APD-treated subjects. Given the importance of dopamine and circadian rhythms for beta cells, we hypothesized that the beta cell dopamine system and circadian clock interact and dually regulate insulin secretion, and that circadian manipulations may alter the metabolic impact of APDs. We measured circadian rhythms, insulin release, and the impact of dopamine upon these processes in beta cells using bioluminescent reporters. We then assessed the impact of circadian timing on weight gain and metabolic outcomes in mice treated with the APD sulpiride at the onset of light or dark. We found that molecular components of the dopamine system were rhythmically expressed in beta cells. D2R stimulation by endogenous dopamine or the agonist bromocriptine reduced circadian rhythm amplitude, and altered the temporal profile of insulin secretion. Sulpiride caused greater weight gain and hyperinsulinemia in mice when given in the dark phase compared to the light phase. D2R-acting drugs affect circadian-dopamine interactions and modulate beta cell metabolic function. These findings identify circadian timing as a novel and important mechanism underlying APD-induced metabolic dysfunction, offering new possibilities for therapeutic interventions.

D2-Like Receptors Mediate Dopamine-Inhibited Insulin Secretion via Ion Channels in Rat Pancreatic β-Cells

Dopamine (DA) has a vital role in the central nervous system and also modulates lipid and glucose metabolism. The present study aimed to investigate the effect of dopamine on insulin secretion and the underlying mechanisms in rat pancreatic β-cells. Data from the radioimmunoassay indicated that dopamine inhibited insulin secretion in a glucose- and dose-dependent manner. This inhibitory effect of dopamine was mediated mainly by D2-like receptors, but not D1-like receptors. Whole-cell patch-clamp recordings showed that dopamine decreased voltage-dependent Ca²⁺ channel currents, which could be reversed by inhibition of the D2-like receptor. Dopamine increased voltage-dependent potassium (K_V) channel currents and shortened action potential duration, which was antagonized by inhibition of D2-like receptors. Further experiments showed that D2-like receptor activation by quinpirole increased K_V channel currents. In addition, using calcium imaging techniques, we found that dopamine reduced intracellular Ca²⁺ concentration, which was also reversed by D2-like receptor antagonists. Similarly, quinpirole was found to decrease intracellular Ca²⁺ levels. Taken together, these findings demonstrate that dopamine inhibits insulin secretion mainly by acting on D2-like receptors, inhibiting Ca²⁺ channels, and activating Kv channels. This process results in shortened action potential duration and decreased intracellular Ca²⁺ levels in β-cells. This work offers new insights into a glucose-dependent mechanism whereby dopamine regulates insulin secretion.

Second-Generation Antipsychotics and Dysregulation of Glucose Metabolism: Beyond Weight Gain

Second-generation antipsychotics (SGAs) are the cornerstone of treatment for schizophrenia because of their high clinical efficacy. However, SGA treatment is associated with severe metabolic alterations and body weight gain, which can increase the risk of type 2 diabetes and cardiovascular disease, and greatly accelerate mortality. Several underlying mechanisms have been proposed for antipsychotic-induced weight gain (AIWG), but some studies suggest that metabolic changes in insulin-sensitive tissues can be triggered before the onset of AIWG. In this review, we give an outlook on current research about the metabolic disturbances provoked by SGAs, with a particular focus on whole-body glucose homeostasis disturbances induced independently of AIWG, lipid dysregulation or adipose tissue disturbances. Specifically, we discuss the mechanistic insights gleamed from cellular and preclinical animal studies that have reported on the impact of SGAs on insulin signaling, endogenous glucose production, glucose uptake and insulin secretion in the liver, skeletal muscle and the endocrine pancreas. Finally, we discuss some of the genetic and epigenetic changes that might explain the different susceptibilities of SGA-treated patients to the metabolic side-effects of antipsychotics.

PET Imaging of Pancreatic Dopamine D2 and D3 Receptor Density with 11C-(+)-PHNO in Type 1 Diabetes

Type 1 diabetes mellitus (T1DM) has traditionally been characterized by a complete destruction of β-cell mass (BCM); however, there is growing evidence of possible residual BCM present in T1DM. Given the absence of in vivo tools to measure BCM, routine clinical measures of β-cell function (e.g., C-peptide release) may not reflect BCM. We previously demonstrated the potential utility of PET imaging with the dopamine D₂ and D₃ receptor agonist 3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol (¹¹C-(+)-PHNO) to differentiate between healthy control (HC) and T1DM individuals. Methods: Sixteen individuals participated (10 men, 6 women; 9 HCs, 7 T1DMs). The average duration of diabetes was 18 ± 6 y (range, 14-30 y). Individuals underwent PET/CT scanning with a 120-min dynamic PET scan centered on the pancreas. One- and 2-tissue-compartment models were used to estimate pancreas and spleen distribution volume. Reference region approaches (spleen as reference) were also investigated. Quantitative PET measures were correlated with clinical outcome measures. Immunohistochemistry was performed to examine colocalization of dopamine receptors with endocrine hormones in HC and T1DM pancreatic tissue. Results: C-peptide release was not detectable in any T1DM individuals, whereas proinsulin was detectable in 3 of 5 T1DM individuals. Pancreas SUV ratio minus 1 (SUVR-1) (20-30 min; spleen as reference region) demonstrated a statistically significant reduction (-36.2%) in radioligand binding (HCs, 5.6; T1DMs, 3.6; P = 0.03). Age at diagnosis correlated significantly with pancreas SUVR-1 (20-30 min) (R ² = 0.67, P = 0.025). Duration of diabetes did not significantly correlate with pancreas SUVR-1 (20-30 min) (R ² = 0.36, P = 0.16). Mean acute C-peptide response to arginine at maximal glycemic potentiation did not significantly correlate with SUVR-1 (20-30 min) (R ² = 0.57, P = 0.05), nor did mean baseline proinsulin (R ² = 0.45, P = 0.10). Immunohistochemistry demonstrated colocalization of dopamine D₃ receptor and dopamine D₂ receptor in HCs. No colocalization of the dopamine D₃ receptor or dopamine D₂ receptor was seen with somatostatin, glucagon, or polypeptide Y. In a separate T1DM individual, no immunostaining was seen with dopamine D₃ receptor, dopamine D₂ receptor, or insulin antibodies, suggesting that loss of endocrine dopamine D₃ receptor and dopamine D₂ receptor expression accompanies loss of β-cell functional insulin secretory capacity. Conclusion: Thirty-minute scan durations and SUVR-1 provide quantitative outcome measures for ¹¹C-(+)-PHNO, a dopamine D₃ receptor-preferring agonist PET radioligand, to differentiate BCM in T1DM and HCs.

Dopamine outside the brain: The eye, cardiovascular system and endocrine pancreas

Dopamine (DA) and DA receptors (DR) have been extensively studied in the central nervous system (CNS), but their role in the periphery is still poorly understood. Here we summarize data on DA and DRs in the eye, cardiovascular system and endocrine pancreas, three districts where DA and DA-related drugs have been studied and the expression of DR documented. In the eye, DA modulates ciliary blood flow and aqueous production, which impacts on intraocular pressure and glaucoma. In the cardiovascular system, DA increases blood pressure and heart activity, mostly through a stimulation of adrenoceptors, and induces vasodilatation in the renal circulation, possibly through D1R stimulation. In pancreatic islets, beta cells store DA and co-release it with insulin. D1R is mainly expressed in beta cells, where it stimulates insulin release, while D2R is expressed in both beta and delta cells (in the latter at higher level), where it inhibits, respectively, insulin and somatostatin release. The formation of D2R-somatostatin receptor 5 heteromers (documented in the CNS), might add complexity to the system. DA may exert both direct autocrine effects on beta cells, and indirect paracrine effects through delta cells and somatostatin. Bromocriptine, an FDA approved drug for diabetes, endowed with both D1R (antagonistic) and D2R (agonistic) actions, may exert complex effects, resulting from the integration of direct effects on beta cells and paracrine effects from delta cells. A full comprehension of peripheral DA signaling deserves further studies that may generate innovative therapeutic drugs to manage conditions such as glaucoma, cardiovascular diseases and diabetes.

The Significance of Chirality in Drug Design and Synthesis of Bitopic Ligands as D3 Receptor (D3R) Selective Agonists

Because of the large degree of homology among dopamine D₂-like receptors, discovering ligands capable of discriminating between the D₂, D₃, and D₄ receptor subtypes remains a significant challenge. Previous work has exemplified the use of bitopic ligands as a powerful strategy in achieving subtype selectivity for agonists and antagonists alike. Inspired by the potential for chemical modification of the D₃ preferential agonists (+)-PD128,907 (1) and PF592,379 (2), we synthesized bitopic structures to further improve their D₃R selectivity. We found that the (2S,5S) conformation of scaffold 2 resulted in a privileged architecture with increased affinity and selectivity for the D₃R. In addition, a cyclopropyl moiety incorporated into the linker and full resolution of the chiral centers resulted in lead compound 53 and eutomer 53a that demonstrate significantly higher D₃R binding selectivities than the reference compounds. Moreover, the favorable metabolic stability in rat liver microsomes supports future studies in in vivo models of dopamine system dysregulation.

Where Is Dopamine and how do Immune Cells See it?: Dopamine-Mediated Immune Cell Function in Health and Disease

Dopamine is well recognized as a neurotransmitter in the brain, and regulates critical functions in a variety of peripheral systems. Growing research has also shown that dopamine acts as an important regulator of immune function. Many immune cells express dopamine receptors and other dopamine related proteins, enabling them to actively respond to dopamine and suggesting that dopaminergic immunoregulation is an important part of proper immune function. A detailed understanding of the physiological concentrations of dopamine in specific regions of the human body, particularly in peripheral systems, is critical to the development of hypotheses and experiments examining the effects of physiologically relevant dopamine concentrations on immune cells. Unfortunately, the dopamine concentrations to which these immune cells would be exposed in different anatomical regions are not clear. To address this issue, this comprehensive review details the current information regarding concentrations of dopamine found in both the central nervous system and in many regions of the periphery. In addition, we discuss the immune cells present in each region, and how these could interact with dopamine in each compartment described. Finally, the review briefly addresses how changes in these dopamine concentrations could influence immune cell dysfunction in several disease states including Parkinson's disease, multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease, as well as the collection of pathologies, cognitive and motor symptoms associated with HIV infection in the central nervous system, known as NeuroHIV. These data will improve our understanding of the interactions between the dopaminergic and immune systems during both homeostatic function and in disease, clarify the effects of existing dopaminergic drugs and promote the creation of new therapeutic strategies based on manipulating immune function through dopaminergic signaling. Graphical Abstract.

A role for foregut tyrosine metabolism in glucose tolerance

Objective

We hypothesized that DA and L-DOPA derived from nutritional tyrosine and the resultant observed postprandial plasma excursions of L-DOPA and DA might affect glucose tolerance via their ability to be taken-up by beta cells and inhibit glucose-stimulated β-cell insulin secretion.

Methods

To investigate a possible circuit between meal-stimulated 3,4-dihydroxy-L-phenylalanine (L-DOPA) and dopamine (DA) production in the GI tract and pancreatic β-cells, we: 1) mapped GI mucosal expression of tyrosine hydroxylase (TH) and aromatic amino acid decarboxylase (AADC); 2) measured L-DOPA and DA content of GI mucosal tissues following meal challenges with different L-tyrosine (TYR) content, 3) determined whether meal TYR content impacts plasma insulin and glucose excursions; and 4) characterized postprandial plasma excursions of L-DOPA and DA in response to meal tyrosine content in rodents and a population of bariatric surgery patients. Next, we characterized: 1) the metabolic transformation of TYR and L-DOPA into DA in vitro using purified islet tissue; 2) the metabolic transformation of orally administrated stable isotope labeled TYR into pancreatic DA, and 3) using a nuclear medicine technique, we studied endocrine beta cells in situ release and binding of DA in response to a glucose challenge.

Results

We demonstrate in rodents that intestinal content and circulatory concentrations L-DOPA and DA, plasma glucose and insulin are responsive to the tyrosine (TYR) content of a test meal. Intestinal expression of two enzymes, Tyrosine hydroxylase (TH) and Aromatic Amino acid Decarboxylase (AADC), essential to the transformation of TYR to DA was mapped and the metabolism of metabolism of TYR to DA was traced in human islets and a rodent beta cell line in vitro and from gut to the pancreas in vivo. Lastly, we show that β cells secrete and bind DA in situ in response to glucose stimulation.

Conclusions

We provide proof-of-principle evidence for the existence of a novel postprandial circuit of glucose homeostasis dependent on nutritional tyrosine. DA and L-DOPA derived from nutritional tyrosine may serve to defend against hypoglycemia via inhibition of glucose-stimulated β-cell insulin secretion as proposed by the anti-incretin hypothesis.

•

Nutritional tyrosine is metabolized to L DOPA and DA in the foregut.

•

Postprandial L-DOPA and DA plasma concentrations rise in response to tyrosine.

•

Oral stable isotope labeled tyrosine is found postprandially in the pancreas as DA.

•

L-DOPA and DA are inhibitors of beta cell glucose-stimulated insulin secretion.

•

Postprandial L-DOPA and DA excursions are muted in certain bariatric surgery patients.