Calcium- and barium-dependent exocytosis from the rat insulinoma cell line RINm5F assayed using membrane capacitance measurements and serotonin release

Serotonin Influences Insulin Secretion in Rat Insulinoma INS-1E Cells

Serotonin or 5-hydroxytryptamine (5-HT) is a monoamine that plays a critical role in insulin secretion, energy metabolism, and mitochondrial biogenesis. However, the action of serotonin in insulin production and secretion by pancreatic β cells has not yet been elucidated. Here, we investigated how exogenous nanomolar serotonin concentrations regulate insulin synthesis and secretion in rat insulinoma INS-1E cells. Nanomolar serotonin concentrations (10 and 50 nM) significantly increased insulin protein expression above the constant levels in untreated control cells and decreased insulin protein levels in the media. The reductions in insulin protein levels in the media may be associated with ubiquitin-mediated protein degradation. The levels of membrane vesicle trafficking-related proteins including Rab5, Rab3A, syntaxin6, clathrin, and EEA1 proteins were significantly decreased by serotonin treatment compared to the untreated control cells, whereas the expressions of Rab27A, GOPC, and p-caveolin-1 proteins were significantly reduced by serotonin treatment. In this condition, serotonin receptors, Gαq-coupled 5-HT2b receptor (Htr2b), and ligand-gated ion channel receptor Htr3a were significantly decreased by serotonin treatment. To confirm the serotonylation of Rab3A and Rab27A during insulin secretion, we investigated the protein levels of Rab3A and Rab27A, in which transglutaminase 2 (TGase2) serotonylated Rab3A but not Rab27A. The increases in ERK phosphorylation levels were consistent with increases in the expression of p-Akt. Also, the expression level of the Bcl-2 protein was significantly increased by 50 and 100 nM serotonin treatment compared to the untreated control cells, whereas the levels of Cu/Zn-SOD and Mn-SOD proteins decreased. These results indicate that nanomolar serotonin treatment regulates the insulin protein level but decreases this level in media through membrane vesicle trafficking-related proteins (Rab5, Rab3A, syntaxin6, clathrin, and EEA1), the Akt/ERK pathway, and Htr2b/Htr3a in INS-1E cells.

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.

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.

Increased gut serotonin production in response to bisphenol A structural analogs may contribute to their obesogenic effects

Obesogens are synthetic, environmental chemicals that can disrupt endocrine control of metabolism and contribute to the risk of obesity and metabolic disease. Bisphenol A (BPA) is one of the most studied obesogens. There is considerable evidence that BPA exposure is associated with weight gain, increased adiposity, poor blood glucose control, and nonalcoholic fatty liver disease in animal models and human populations. Increased usage of structural analogs of BPA has occurred in response to legislation banning their use in some commercial products. However, BPA analogs may also cause some of the same metabolic impairments because of common mechanisms of action. One key effector that is altered by BPA and its analogs is serotonin, however, it is unknown if BPA-induced changes in peripheral serotonin pathways underlie metabolic perturbations seen with BPA exposure. Upon ingestion, BPA and its analogs act as endocrine-disrupting chemicals in the gastrointestinal tract to influence serotonin production by the gut, where over 95% of serotonin is produced. The purpose of this review is to evaluate how BPA and its analogs alter gut serotonin regulation and then discuss how disruption of serotonergic networks influences host metabolism. We also provide evidence that BPA and its analogs enhance serotonin production in gut enterochromaffin cells. Taken together, we propose that BPA and many BPA analogs represent endocrine-disrupting chemicals that can influence host metabolism through the endogenous production of gut-derived factors, such as serotonin.

Intersections between Copper, β-Arrestin-1, Calcium, FBXW7, CD17, Insulin Resistance and Atherogenicity Mediate Depression and Anxiety Due to Type 2 Diabetes Mellitus: A Nomothetic Network Approach

Type 2 diabetes mellitus (T2DM) is frequently accompanied by affective disorders with a prevalence of comorbid depression of around 25%. Nevertheless, the biomarkers of affective symptoms including depression and anxiety due to T2DM are not well established. The present study delineated the effects of serum levels of copper, zinc, β-arrestin-1, FBXW7, lactosylceramide (LacCer), serotonin, calcium, magnesium on severity of depression and anxiety in 58 men with T2DM and 30 healthy male controls beyond the effects of insulin resistance (IR) and atherogenicity. Severity of affective symptoms was assessed using the Hamilton Depression and Anxiety rating scales. We found that 61.7% of the variance in affective symptoms was explained by the multivariate regression on copper, β-arrestin-1, calcium, and IR coupled with atherogenicity. Copper and LacCer (positive) and calcium and BXW7 (inverse) had significant specific indirect effects on affective symptoms, which were mediated by IR and atherogenicity. Copper, β-arrestin-1, and calcium were associated with affective symptoms above and beyond the effects of IR and atherogenicity. T2DM and affective symptoms share common pathways, namely increased atherogenicity, IR, copper, and β-arrestin-1, and lowered calcium, whereas copper, β-arrestin-1, calcium, LacCer, and FBXW7 may modulate depression and anxiety symptoms by affecting T2DM.

Eugenosedin-A improves obesity-related hyperglycemia by regulating ATP-sensitive K+ channels and insulin secretion in pancreatic β cells

Eugenosedin-A (Eu-A) has been shown to protect against hyperglycemia- and hyperlipidemia-induced metabolic syndrome. We investigated the relationship of K_ATP channel activities and insulin secretion by Eu-A in vitro in pancreatic β-cells, and examined the effect of Eu-A on streptozotocin (STZ)/nicotinamide (NA)-induced type 2 diabetes mellitus (T2DM) in vivo. We isolated pancreatic islets from adult male Wistar rats (250-350 g) and identified pancreatic β-cells by the cell size, capacitance and membrane potential. Perforated patch-clamp and inside-out recordings were used to monitor the membrane potential (current-clamp mode) and channel activity (voltage-clamp mode) of β-cells. The membrane potential of β-cells was raised by Eu-A and reversed by the K_ATP channel activator diazoxide. Eu-A inhibited the K_ATP channel activity measured at - 60 mV and increased the intracellular calcium concentration ([Ca²⁺]_i), resulting in enhanced insulin secretion. Eu-A also reduced Kir6.2 protein on the cell membrane and scattered in the cytosol under normal glucose conditions (5.6 mM). In our animal study, rats were divided into normal and STZ/NA-induced T2DM groups. Normal rats fed with regular chow were divided into control and control+Eu-A (5 mg/kg/day, i.p.) groups. The STZ/NA-induced diabetic rats fed with a high-fat diet (HFD) were divided into three groups: T2DM, T2DM+Eu-A (5 mg/kg/day, i.p.), and T2DM+glibenclamide (0.5 mg/kg/day, i.p.; a K_ATP channel inhibitor). Both Eu-A and glibenclamide decreased the rats' blood glucose, prevented weight gain, and enhanced insulin secretion. We found that Eu-A blocked pancreatic β-cell K_ATP channels, caused membrane potential depolarization, and stimulated Ca²⁺ influx, thus increasing insulin secretion. Furthermore, Eu-A decreased blood glucose and increased insulin levels in T2DM rats. These results suggested that Eu-A might have clinical benefits for the control of T2DM and its complications.

Intercellular Communication in the Islet of Langerhans in Health and Disease

Blood glucose homeostasis requires proper function of pancreatic islets, which secrete insulin, glucagon, and somatostatin from the β-, α-, and δ-cells, respectively. Each islet cell type is equipped with intrinsic mechanisms for glucose sensing and secretory actions, but these intrinsic mechanisms alone cannot explain the observed secretory profiles from intact islets. Regulation of secretion involves interconnected mechanisms among and between islet cell types. Islet cells lose their normal functional signatures and secretory behaviors upon dispersal as compared to intact islets and in vivo. In dispersed islet cells, the glucose response of insulin secretion is attenuated from that seen from whole islets, coordinated oscillations in membrane potential and intracellular Ca²⁺ activity, as well as the two-phase insulin secretion profile, are missing, and glucagon secretion displays higher basal secretion profile and a reverse glucose-dependent response from that of intact islets. These observations highlight the critical roles of intercellular communication within the pancreatic islet, and how these communication pathways are crucial for proper hormonal and nonhormonal secretion and glucose homeostasis. Further, misregulated secretions of islet secretory products that arise from defective intercellular islet communication are implicated in diabetes. Intercellular communication within the islet environment comprises multiple mechanisms, including electrical synapses from gap junctional coupling, paracrine interactions among neighboring cells, and direct cell-to-cell contacts in the form of juxtacrine signaling. In this article, we describe the various mechanisms that contribute to proper islet function for each islet cell type and how intercellular islet communications are coordinated among the same and different islet cell types. © 2021 American Physiological Society. Compr Physiol 11:2191-2225, 2021.

Tropisetron modulates peripheral and central serotonin/insulin levels via insulin and nuclear factor kappa B/receptor for advanced glycation end products signalling to regulate type-2 diabetes in rats

Despite its known central effect, 5% of serotonin is found centrally, while around 95% is found peripherally. Serotonin is stored and co-released with insulin upon pancreatic islets stimulation by glucose. This fact raises the curiosity regarding its possible role in diabetes. Hence, in this study, we assessed the possible modulatory effects of tropisetron, a 5-HT3 receptor antagonist, on type 2 diabetes mellitus models in rats. The rats were allocated into two groups: normal and diabetic. The latter group was treated with metformin (500 mg kg-1, p.o.), tropisetron (1 and 2 mg kg-1, i.p.), and a combination of metformin and tropisetron (1 mg kg-1). The different treatment regimens corrected glucose and lipid homeostasis manifested by the decrease in serum levels of glucose, fructosamine, homeostasis model of insulin resistance, triglycerides, total cholesterol, free fatty acid, as well as receptor for advanced glycation end products. Additionally, the treatments elevated levels of insulin, serotonin, and homeostasis model of β-cell function. On the molecular level, treatments corrected the altered insulin signaling cascade (phosphorylated insulin receptor substrate 1, phosphorylated protein kinase B, and glucose transporter 4), and inhibited β-catenin and phosphorylated nuclear factor kappa B p65 in the assessed soleus skeletal muscle. A similar pattern was duplicated in the hippocampus. This study provided evidence for the role of tropisetron on type 2 diabetes mellitus via modulating the insulin signaling cascade (insulin, phosphorylated insulin receptor substrate 1, phosphorylated protein kinase B, and glucose transporter 4), improving lipid/glucose profile, decreasing inflammatory markers (receptor for advanced glycation end products, and phosphorylated nuclear factor kappa B p65), as well as increasing 5-HT and reducing β-catenin.

Serotonergic Control of Metabolic Homeostasis

New treatments are urgently needed to address the current epidemic of obesity and diabetes. Recent studies have highlighted multiple pathways whereby serotonin (5-HT) modulates energy homeostasis, leading to a renewed interest in the identification of 5-HT-based therapies for metabolic disease. This review aims to synthesize pharmacological and genetic studies that have found diverse functions of both central and peripheral 5-HT in the control of food intake, thermogenesis, and glucose and lipid metabolism. We also discuss the potential benefits of targeting the 5-HT system to combat metabolic disease.

Is There a Role for Bioactive Lipids in the Pathobiology of Diabetes Mellitus?

Inflammation, decreased levels of circulating endothelial nitric oxide (eNO) and brain-derived neurotrophic factor (BDNF), altered activity of hypothalamic neurotransmitters (including serotonin and vagal tone) and gut hormones, increased concentrations of free radicals, and imbalance in the levels of bioactive lipids and their pro- and anti-inflammatory metabolites have been suggested to play a role in diabetes mellitus (DM). Type 1 diabetes mellitus (type 1 DM) is due to autoimmune destruction of pancreatic β cells because of enhanced production of IL-6 and tumor necrosis factor-α (TNF-α) and other pro-inflammatory cytokines released by immunocytes infiltrating the pancreas in response to unknown exogenous and endogenous toxin(s). On the other hand, type 2 DM is due to increased peripheral insulin resistance secondary to enhanced production of IL-6 and TNF-α in response to high-fat and/or calorie-rich diet (rich in saturated and trans fats). Type 2 DM is also associated with significant alterations in the production and action of hypothalamic neurotransmitters, eNO, BDNF, free radicals, gut hormones, and vagus nerve activity. Thus, type 1 DM is because of excess production of pro-inflammatory cytokines close to β cells, whereas type 2 DM is due to excess of pro-inflammatory cytokines in the systemic circulation. Hence, methods designed to suppress excess production of pro-inflammatory cytokines may form a new approach to prevent both type 1 and type 2 DM. Roux-en-Y gastric bypass and similar surgeries ameliorate type 2 DM, partly by restoring to normal: gut hormones, hypothalamic neurotransmitters, eNO, vagal activity, gut microbiota, bioactive lipids, BDNF production in the gut and hypothalamus, concentrations of cytokines and free radicals that results in resetting glucose-stimulated insulin production by pancreatic β cells. Our recent studies suggested that bioactive lipids, such as arachidonic acid, eicosapentaneoic acid, and docosahexaenoic acid (which are unsaturated fatty acids) and their anti-inflammatory metabolites: lipoxin A4, resolvins, protectins, and maresins, may have antidiabetic actions. These bioactive lipids have anti-inflammatory actions, enhance eNO, BDNF production, restore hypothalamic dysfunction, enhance vagal tone, modulate production and action of ghrelin, leptin and adiponectin, and influence gut microbiota that may explain their antidiabetic action. These pieces of evidence suggest that methods designed to selectively deliver bioactive lipids to pancreatic β cells, gut, liver, and muscle may prevent type 1 and type 2 DM.

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

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

Serotonin- and Dopamine-Related Gene Expression in db/db Mice Islets and in MIN6 β-Cells Treated with Palmitate and Oleate

High circulating nonesterified fatty acids (NEFAs) concentration, often reported in diabetes, leads to impaired glucose-stimulated insulin secretion (GSIS) through not yet well-defined mechanisms. Serotonin and dopamine might contribute to NEFA-dependent β-cell dysfunction, since extracellular signal of these monoamines decreases GSIS. Moreover, palmitate-treated β-cells may enhance the expression of the serotonin receptor Htr2c, affecting insulin secretion. Additionally, the expression of monoamine-oxidase type B (Maob) seems to be lower in islets from humans and mice with diabetes compared to nondiabetic islets, which may lead to increased monoamine concentrations. We assessed the expression of serotonin- and dopamine-related genes in islets from db/db and wild-type (WT) mice. In addition, the effect of palmitate and oleate on the expression of such genes, 5HT content, and GSIS in MIN6 β-cell was determined. Lower Maob expression was found in islets from db/db versus WT mice and in MIN6 β-cells in response to palmitate and oleate treatment compared to vehicle. Reduced 5HT content and impaired GSIS in response to palmitate (-25%; p < 0.0001) and oleate (-43%; p < 0.0001) were detected in MIN6 β-cells. In conclusion, known defects of GSIS in islets from db/db mice and MIN6 β-cells treated with NEFAs are accompanied by reduced Maob expression and reduced 5HT content.

Serotonin promotes acinar dedifferentiation following pancreatitis-induced regeneration in the adult pancreas

The exocrine pancreas exhibits a distinctive capacity for tissue regeneration and renewal following injury. This regenerative ability has important implications for a variety of disorders, including pancreatitis and pancreatic cancer, diseases associated with high morbidity and mortality. Thus, understanding its underlying mechanisms may help in developing therapeutic interventions. Serotonin has been recognized as a potent mitogen for a variety of cells and tissues. Here we investigated whether serotonin exerts a mitogenic effect in pancreatic acinar cells in three regenerative models, inflammatory tissue injury following pancreatitis, tissue loss following partial pancreatectomy, and thyroid hormone-stimulated acinar proliferation. Genetic and pharmacological techniques were used to modulate serotonin levels in vivo. Acinar dedifferentiation and cell cycle progression during the regenerative phase were investigated over the course of 2 weeks. By comparing acinar proliferation in the different murine models of regeneration, we found that serotonin did not affect the clonal regeneration of mature acinar cells. Serotonin was, however, required for acinar dedifferentiation following inflammation-mediated tissue injury. Specifically, lack of serotonin resulted in delayed up-regulation of progenitor genes and delayed the formation of acinar-to-ductal metaplasia and defective acinar cell proliferation. We identified serotonin-dependent acinar secretion as a key step in progenitor-based regeneration, as it promoted acinar cell dedifferentiation and the recruitment of type 2 macrophages. Finally, we identified a regulatory Hes1-Ptfa axis in the uninjured adult pancreas, activated by zymogen secretion. Our findings indicated that serotonin plays a critical role in the regeneration of the adult pancreas following pancreatitis by promoting the dedifferentiation of acinar cells.

Preliminary Examination of Olanzapine and Diet Interactions On Metabolism in a Female Macaque

Clinical data suggest that atypical antipsychotics such as olanzapine (OLZ) induce significant metabolic changes that are serious side effects of their primary use. Since controlled human studies are problematic and rodent data may be poorly translatable, we have initiated development of a macaque model of OLZ-induced metabolic disease. In this preliminary feasibility study, we examined some metabolic effects of OLZ in a female macaque in the context of a standard low-calorie/fat monkey chow diet followed by a high-fat/sugar Western-style diet (WSD). A female Japanese macaque was administered OLZ (1.25 mg/day) for 6 months, with dietary changes at 2-month intervals as follows: OLZ+Restricted chow, OLZ+Unrestricted chow, OLZ+WSD, and placebo+WSD. Weight was assessed weekly. Glucose tolerance tests (GTT) and Dexascans were performed at baseline and every 2 months. Omental (OM) and subcutaneous (SQ) adipose tissue biopsies were obtained at baseline, after OLZ+Unrestricted chow and after OLZ+WSD to evaluate adipocyte size, lipolysis and insulin-stimulated free fatty acid uptake (FFA). A separate trial was conducted on 2 monkeys with 5 days of OLZ- or no-treatment followed by RT-PCR on rostral and medial basal hypothalamus. Weight increased on OLZ+Restricted chow and stabilized on OLZ+Unrestricted chow. OLZ+WSD did not significantly change the weight plateau. Weight declined upon withdrawal of OLZ with continued WSD. Body fat increased from 14% at baseline to 22%, 30%, 28% and 19% at 2, 4, 6 and 8 mo, respectively, indicating that body fat was elevated on OLZ regardless of diet and declined upon OLZ removal. Glucose tolerance and the insulin response during GTT were normal with OLZ+Restricted chow or OLZ+Unrestricted chow. Addition of WSD with OLZ impaired glucose clearance during GTT. Insulin remained in the normal range, but first phase insulin secretion was reduced. After removal of OLZ, but continued WSD, glucose clearance returned to normal, but this was associated with hyperinsulinemia. Adipocyte diameter was increased in OM and SQ fat by OLZ+chow and OLZ+WSD to a similar extent. (p<0.01, 2-way ANOVA). In OM, isoproterenol-stimulated lipolysis occurred at baseline. In both depots, isoproterenol-stimulated lipolysis occurred with OLZ+chow, but it was significantly blunted by addition of WSD (ANOVA p<0.0001; posthoc p<0.05). Insulin increased FFA uptake at baseline. OLZ +chow or OLZ+WSD increased basal FFA uptake and insulin-induced FFA uptake was blunted in both depots (posthoc p<0.05). There was a marked decrease in POMC gene expression, and increased AgRP and NPY expression in the hypothalamus. There was also a clear increase in serotonin (5HT) 2C, melanocortin (MCR4), and Leptin (LepR) receptor gene expression. These data support the hypotheses that OLZ acts on peripheral tissues as well as in the CNS; that changes in hypothalamic gene expression occur very rapidly and precede increased fat accumulation; that adipose tissue exhibits insulin resistance prior to alterations in GTT; that addition of WSD to OLZ precipitates hyperglycemia without an obvious insulin response; and that removal of OLZ and continued WSD resulted in normalized glucose clearance and elevated insulin. These data suggest complex and early responses to OLZ that may be exacerbated by WSD.

Atypical antipsychotics and effects of adrenergic and serotonergic receptor binding on insulin secretion in-vivo: an animal model

Atypical antipsychotics (AAPs) are associated with several metabolic sequelae including increased risk of type 2 diabetes. Growing evidence points to a direct drug effect of these compounds on glucose homeostasis, independent of weight gain. While the responsible mechanisms have yet to be elucidated, the heterogeneous binding profiles of AAPs likely include receptors involved in glucose metabolism. This study aimed to clarify weight-gain independent mechanisms of AAP-induced alterations in insulin secretion. Deconstruction of the receptor binding profiles of these agents was done using representative antagonists. Healthy rats were pre-treated with a single subcutaneous dose of prazosin 0.25mg/kg (n = 16), a selective α1 antagonist; idazoxan 0.5mg/kg (n = 10), a selective α2 antagonist; SB242084 0.5mg/kg (n = 10), a selective 5HT2C antagonist; WAY100635 0.1mg/kg (n = 10), a selective 5HT1A antagonist; MDL100907 0.5mg/kg (n = 8), a selective 5HT2A antagonist; or vehicle: 0.9% NaCl saline (n = 8), DMSO (n = 8), or cyclodextrin (n = 5). Hyperglycemic clamps were employed following injection, providing an index of secretory capacity of pancreatic β-cells. Treatment with prazosin and MDL100907 resulted in significant decreases in both insulin and C-peptide secretion compared to their respective controls, DMSO and saline. These findings were corroborated with decreased glucose infusion rate and disposition index in the prazosin group. Results suggest that α1 and 5HT2A receptor antagonism may be involved in glucose dysregulation with AAP treatment, however, the exact mechanisms involved remain unknown.

Life without peripheral serotonin: insights from tryptophan hydroxylase 1 knockout mice reveal the existence of paracrine/autocrine serotonergic networks

Since its identification, 75 years ago, the monoamine serotonin (5-HT) has attracted considerable attention toward its role as a neurotransmitter in the central nervous system. Yet, increasing evidence, from a growing number of research groups, substantiates the fact that 5-HT regulates important nonneuronal functions. Peripheral 5-HT, synthesized by the enzyme tryptophan hydroxyase (Tph) in intestinal cells, was assumed to be distributed throughout the entire body by blood platelets and to behave as a pleiotropic hormone. A decade ago, generation of a mouse model devoid of peripheral 5-HT lead to the discovery of a second isoform of the enzyme Tph and also suggested that 5-HT might act as a local regulator in various organs. The objective of this review is to highlight the newly discovered functions played by the monoamine using the Tph1 KO murine model and to outline current findings that led to the discovery of complete serotonergic systems in unexpected organs. Within an organ, both the presence of local Tph enzymatic activity and serotonergic components are of particular importance as they support the view that 5-HT meets the criteria to be qualified as a monoamine with a paracrine/autocrine function.

Expansion of beta-cell mass in response to pregnancy

Inadequate beta-cell mass can lead to insulin insufficiency and diabetes. During times of prolonged metabolic demand for insulin, the endocrine pancreas can respond by increasing beta-cell mass, both by increasing cell size and by changing the balance between beta-cell proliferation and apoptosis. In this paper, we review recent advances in our understanding of the mechanisms that control the adaptive expansion of beta-cell mass, focusing on the islet's response to pregnancy, a physiological state of insulin resistance. Functional characterization of factors controlling both beta-cell proliferation and survival might not only lead to the development of successful therapeutic strategies to enhance the response of the beta-cell to increased metabolic loads, but also improve islet transplantation regimens.

Intracellular serotonin modulates insulin secretion from pancreatic beta-cells by protein serotonylation

Non-neuronal, peripheral serotonin deficiency causes diabetes mellitus and identifies an intracellular role for serotonin in the regulation of insulin secretion.

While serotonin (5-HT) co-localization with insulin in granules of pancreatic β-cells was demonstrated more than three decades ago, its physiological role in the etiology of diabetes is still unclear. We combined biochemical and electrophysiological analyses of mice selectively deficient in peripheral tryptophan hydroxylase (Tph1−/−) and 5-HT to show that intracellular 5-HT regulates insulin secretion. We found that these mice are diabetic and have an impaired insulin secretion due to the lack of 5-HT in the pancreas. The pharmacological restoration of peripheral 5-HT levels rescued the impaired insulin secretion in vivo. These findings were further evidenced by patch clamp experiments with isolated Tph1−/− β-cells, which clearly showed that the secretory defect is downstream of Ca²⁺-signaling and can be rescued by direct intracellular application of 5-HT via the clamp pipette. In elucidating the underlying mechanism further, we demonstrate the covalent coupling of 5-HT by transglutaminases during insulin exocytosis to two key players in insulin secretion, the small GTPases Rab3a and Rab27a. This renders them constitutively active in a receptor-independent signaling mechanism we have recently termed serotonylation. Concordantly, an inhibition of such activating serotonylation in β-cells abates insulin secretion. We also observed inactivation of serotonylated Rab3a by enhanced proteasomal degradation, which is in line with the inactivation of other serotonylated GTPases. Our results demonstrate that 5-HT regulates insulin secretion by serotonylation of GTPases within pancreatic β-cells and suggest that intracellular 5-HT functions in various microenvironments via this mechanism in concert with the known receptor-mediated signaling.

Diabetes is the most prevalent metabolic disease and one that affects individuals of every social and economic status. The disease can arise as a result of reduced secretion of insulin from pancreatic β-cells or reduced action of insulin on its target organs. Therefore, understanding how to prevent and treat diabetes requires an extensive knowledge of the regulation of insulin secretion. In this study, we identify the hormone serotonin as a new regulator of insulin secretion and thereby attribute a function to the co-localization of serotonin and insulin in pancreatic β-cells that was first observed 30 years ago but until now not understood. We first demonstrate that a lack of serotonin in β-cells of transgenic mice leads to reduced insulin secretion and diabetes mellitus and that pharmacological replenishment of serotonin rescues insulin secretion in these mice. Interestingly, serotonin mainly acts not as an intercellular signaling molecule via its traditional surface receptors but intracellularly via regulation of the activity of target proteins through covalent coupling of serotonin to them. This coupling, called serotonylation, activates specific small GTPases, which in turn promote glucose-mediated insulin secretion. Adding this receptor-independent signaling mechanism to the multifarious regulatory functions of serotonin, we hypothesize that protein serotonylation modulates physiological secretion processes in all serotonin-containing tissues.

3H-serotonin as a marker of oscillatory insulin secretion in clonal β-cells (INS-1)

Serotonin release from preloaded pancreatic beta-cells has been used as a marker for insulin release in studying exocytosis from single cells using the amperometric technique. We found that single pancreatic beta-cells exhibited oscillations in exocytosis with a period of 1-1.5 min as measured amperometrically by serotonin release. We also show that 3H-serotonin can be used to monitor exocytosis from intact and streptolysin-O permeabilized clonal insulin-secreting cells preloaded with labeled serotonin and that serotonin release correlated with insulin secretion in the same cells. The use of 3H-serotonin provides a real-time indicator of exocytosis from populations of clonal insulin-secreting cells.

Cytological and immunocytochemical characterization of the insulin secreting insulinoma cell line RINm5F

The rat insulinoma cell line RINm5F, an insulin secreting pancreatic beta cell line, has been used as an attractive model for basic studies of the mechanisms of insulin secretion and, more recently, as a model for the development of alternative methods for the treatment of diabetes. To elucidate the cytological properties and expression patterns of hormones of the gastro-entero-pancreatic system, suspensions of RINm5F cells were investigated by various methods including immunocytochemistry on serial semithin sections, quantitative immunocytochemistry, routine electron microscopy, immuno-electron microscopy, in situ hybridization, and TUNEL technique. At the ultrastructural level, several phenotypes of RIm5F cells were characterized by differences in the number, shape, size, and density of their secretory granules. The most common type contained a mixture of round granules varying in size and electron density. A second type predominantly contained relatively large, moderately dense granules. Moreover, a minority of cells was characterized by the occurrence of polymorphous electron dense granules or the complete absence of any secretory granules. The immunohistochemical data showed that, among the established islet hormones, insulin was present in more than 50% of cells, whereas glucagon and somatostatin occurred only sporadically. Though cells positive for pancreatic polypeptide (PP) were not found, PP-related peptides (NPY and PYY) however could be detected in a minority of cells. The great majority of RINm5F cells were immunoreactive for chromogranin B (CgB), followed by insulin, chromogranin A (CgA), and serotonin (5-HT). In addition to intercellular differences in the density of immunostaining, numerous colocalizations of immunoreactivities were found, suggesting that RINm5F cells represent a mixture of subtypes concerning the individual pattern of hormone expression. The present results reveal a wide range of heterogeneity with respect to the morphology and especially the hormone content between individual RINm5F cells.

Enhancement of the dense-core vesicle secretory cycle by glucocorticoid differentiation of PC12 cells: characteristics of rapid exocytosis and endocytosis

The secretory cycle of dense-core vesicles (DCVs) in physiologically stimulated patch-clamped PC12 cells was analyzed using both amperometry and capacitance measurements. Untreated cells had low or undetectable Ca currents and sparse secretory responses to short depolarizations. Dexamethasone (5 microM) treatment for 5-7 d tripled Ca current magnitude and dramatically increased quantal secretion in response to depolarization with action potentials. Such cells expressed L-, N-, and P-type Ca channels, and depolarization evoked rapid catecholamine secretion recorded as amperometric spikes; the average latency was approximately 50 msec. These spikes were much smaller and shorter than those of primary adrenal chromaffin cells, reflecting the smaller size of DCVs in PC12 cells. Depolarizing pulse trains also elicited a rapid increase in membrane capacitance corresponding to exocytosis in differentiated but not in naïve cells. On termination of stimulation, membrane capacitance declined within 20 sec to baseline indicative of rapid endocytosis (RE). RE did not take place when secretion was stimulated in the presence of Ba or Sr, indicating that RE is Ca-specific. RE was blocked when either anti-dynamin antibodies or the pleckstrin homology domain of dynamin-1 was loaded into the cell via the patch pipette. These studies indicate that neuroendocrine differentiation of PC12 cells with glucocorticoids enhances the development of the excitable membrane and increases the coupling between Ca channels and vesicle release sites, leading to rapid exocytosis and endocytosis. Slow catecholamine secretion in undifferentiated cells may be caused in part by a lack of localized secretory machinery rather than being an intrinsic property of dense-core vesicles.

Modulation of N-type calcium channels translocation in RINm5F insulinoma cells

An intracellular pool of N-type voltage-operated calcium channels has recently been described in both IMR32 human neuroblastoma and PC12 rat pheochromocytoma cells. These channels were found to be accumulated in subcellular fractions where the chromogranin B-containing secretory granules were also enriched. Upon exocytosis N-type calcium channels were reversibly inserted in the plasma membrane. We have now extended this study to RINm5F rat insulinoma cells, and characterized the parallelism between the 'regulated' secretion of serotonin and the recruitment of surface calcium channels. Exocytosis was stimulated by different means, such as depolarization with high KCl, high Ba(2+)alone or protein kinase C activation; on the other hand exocytosis was inhibited with the non-selective calcium channel antagonist Cd(2+)or with noradrenaline. Stimulated release was always accompanied, with parallel kinetics, by calcium channel recruitment, while inhibition of secretion blocked calcium channel recruitment too. During repetitive depolarizations we revealed a potentiation of [Ca(2+)]()i transients in single Fura-2 loaded RINm5F cells, that was accompanied by an increase in surface VOCCs, suggesting a physiological role for the newly recruited channels. 2000 Academic Press@p$hr

Syntaxin 1 interacts with the L(D) subtype of voltage-gated Ca(2+) channels in pancreatic beta cells

Interaction of syntaxin 1 with the alpha(1D) subunit of the voltage-gated L type Ca(2+) channel was investigated in the pancreatic beta cell. Coexpression of the enhanced green fluorescent protein-linked alpha(1D) subunit with the enhanced blue fluorescent protein-linked syntaxin 1 and Western blot analysis together with subcellular fractionation demonstrated that the alpha(1D) subunit and syntaxin 1 were colocalized in the plasma membrane. Furthermore, the alpha(1D) subunit was coimmunoprecipitated efficiently by a polyclonal antibody against syntaxin 1. Syntaxin 1 also played a central role in the modulation of L type Ca(2+) channel activity because there was a faster Ca(2+) current run-down in cells incubated with antisyntaxin 1 compared with controls. In parallel, antisyntaxin 1 markedly reduced insulin release in both intact and permeabilized cells, subsequent to depolarization with K(+) or exposure to high Ca(2+). Exchanging Ca(2+) for Ba(2+) abolished the effect of antisyntaxin 1 on both Ca(2+) channel activity and insulin exocytosis. Moreover, antisyntaxin 1 had no significant effects on Ca(2+)-independent insulin release trigged by hypertonic stimulation. This suggests that there is a structure-function relationship between the alpha(1D) subunit of the L type Ca(2+) channel and the exocytotic machinery in the pancreatic beta cell.

Barium triggers rapid endocytosis in calf adrenal chromaffin cells

1. Changes in cell capacitance were monitored in whole-cell patch-clamp recordings from calf adrenal chromaffin cells using a software-based phase-tracking technique. Rapid endocytosis and exocytosis were observed in extracellular solutions containing either Ca2+ or Ba2+. 2. There was no significant difference in the magnitude or the time course of rapid endocytosis of cells stimulated in Ca2+ as compared to Ba2+. When cells were pretreated with caffeine and thapsigargin in order to deplete intracellular Ca2+ stores, rapid endocytosis in Ba2+ was not affected. This indicates that Ba2+ itself is capable of supporting rapid endocytosis. 3. The application of the calmodulin inhibitor calmidazolium via the intracellular pipette solution did not inhibit rapid endocytosis. Although our findings are inconsistent with an immediate requirement for calmodulin in rapid endocytosis, they do not rule out an involvement on a longer time scale. 4. While rapid endocytosis was not affected by the substitution of Ca2+ with Ba2+, the maximum rate of exocytosis was higher in cells stimulated in Ca2+ than in Ba2+. Since Ba2+ currents were much larger than Ca2+ currents during depolarizations to +10 mV (the test potential used in these experiments), Ba2+ appears to be less efficient at promoting exocytosis than Ca2+.

Vesicle recycling revisited: rapid endocytosis may be the first step

Synaptic vesicle recycling is a critical feature of neuronal communication as it ensures a constant supply of releasable transmitter at the nerve terminal. Physiological studies predict that vesicle recycling is rapid and recent studies with fluorescent dyes have confirmed that the entire process may occur in less than a minute. Two competing hypotheses have been proposed for the first step in the process comprising endocytosis of vesicular membrane. The coated vesicle model proposes that vesicular membrane components merge with the plasma membrane and are subsequently recovered and possibly sorted in coated pits. These pinch off as coated vesicles that either fuse with a sorting endosome from which new vesicles emerge or uncoat to become synaptic vesicles directly. The alternative "kiss-and-run" model proposes that "empty" vesicles are retrieved intact from the plasma membrane after secretion occurs via a fusion pore; they are then immediately refilled with transmitter and re-enter the secretion-competent pool. This article summarizes the data for both models and focusses on new information that supports the kiss-and-run model. In particular, the phenomenon of rapid endocytosis, which may represent the key endocytotic step in recycling, is discussed. Rapid endocytosis has time-constants in the order of a few seconds, thus is temporally consistent with the rate of vesicle recycling. Moreover, rapid endocytosis appears to be clathrin-independent, thus does not involve the coated vesicle pathway. We present a model that accommodates both types of endocytosis, which appear to coexist in many secretory tissues including neurons. Rapid endocytosis may reflect the principal mechanism operative under normal physiological rates of stimulation while coated vesicles may come into play at higher rates of stimulation. These two processes may feed into different populations of vesicles corresponding to distinct pools defined by studies of the kinetics of transmitter release.