Precise pattern of recombination in serotonergic and hypothalamic neurons in a Pdx1-cre transgenic mouse line

PDX1, a transcription factor essential for organ differentiation, regulates SERCA-dependent Ca2+ homeostasis in sensory neurons

Pancreatic and duodenal homeobox 1 (PDX1) is a transcription factor required for the development and differentiation of the pancreas. Previous studies indicated that PDX1 expression was restricted to the gastrointestinal tract. Using a cre-dependent reporter, we observed PDX1-dependent expression of tdtomato (PDX1-tom) in a subpopulation of sensory nerves. Many of these PDX1-tom afferents expressed the neurofilament 200 protein and projected to the skin. Tdtomato-labeled terminals were associated with hair follicles in the form of longitudinal and circumferential lanceolate endings suggesting a role in tactile and proprioceptive perception. To begin to examine the functional significance of PDX1 in afferents, we used Fura-2 imaging to examine calcium (Ca2+) handling under naïve and nerve injury conditions. Neuropathic injury is associated with increased intracellular Ca2+ signaling that in part results from dysregulation of the sarco/endoplasmic reticulum calcium transport ATPase (SERCA). Here we demonstrate that under naïve conditions, PDX1 regulates expression of the SERCA2B isoform in sensory neurons. In response to infraorbital nerve injury, a significant reduction of PDX1 and SERCA2B expression and dysregulation of Ca2+ handling occurs in PDX1-tom trigeminal ganglia neurons. The identification of PDX1 expression in the somatosensory system and its regulation of SERCA2B and Ca2+ handling provide a new mechanism to explain pathological changes in primary afferents that may contribute to pain associated with nerve injury.

Age-associated changes in lineage composition of the enteric nervous system regulate gut health and disease

The enteric nervous system (ENS), a collection of neural cells contained in the wall of the gut, is of fundamental importance to gastrointestinal and systemic health. According to the prevailing paradigm, the ENS arises from progenitor cells migrating from the neural crest and remains largely unchanged thereafter. Here, we show that the lineage composition of maturing ENS changes with time, with a decline in the canonical lineage of neural-crest derived neurons and their replacement by a newly identified lineage of mesoderm-derived neurons. Single cell transcriptomics and immunochemical approaches establish a distinct expression profile of mesoderm-derived neurons. The dynamic balance between the proportions of neurons from these two different lineages in the post-natal gut is dependent on the availability of their respective trophic signals, GDNF-RET and HGF-MET. With increasing age, the mesoderm-derived neurons become the dominant form of neurons in the ENS, a change associated with significant functional effects on intestinal motility which can be reversed by GDNF supplementation. Transcriptomic analyses of human gut tissues show reduced GDNF-RET signaling in patients with intestinal dysmotility which is associated with reduction in neural crest-derived neuronal markers and concomitant increase in transcriptional patterns specific to mesoderm-derived neurons. Normal intestinal function in the adult gastrointestinal tract therefore appears to require an optimal balance between these two distinct lineages within the ENS.

Role of Delta/Notch-like EGF-related receptor in blood glucose homeostasis

Cell-cell interactions are necessary for optimal endocrine functions in the pancreas. β-cells, characterized by the expression and secretion of the hormone insulin, are a major constituent of functional micro-organs in the pancreas known as islets of Langerhans. Cell-cell contacts between β-cells are required to regulate insulin production and glucose-stimulated insulin secretion, which are key determinants of blood glucose homeostasis. Contact-dependent interactions between β-cells are mediated by gap junctions and cell adhesion molecules such as E-cadherin and N-CAM. Recent genome-wide studies have implicated Delta/Notch-like EGF-related receptor (Dner) as a potential susceptibility locus for Type 2 Diabetes in humans. DNER is a transmembrane protein and a proposed Notch ligand. DNER has been implicated in neuron-glia development and cell-cell interactions. Studies herein demonstrate that DNER is expressed in β-cells with an onset during early postnatal life and sustained throughout adulthood in mice. DNER loss in adult β-cells in mice (β-Dner cKO mice) disrupted islet architecture and decreased the expression of N-CAM and E-cadherin. β-Dner cKO mice also exhibited impaired glucose tolerance, defects in glucose- and KCl-induced insulin secretion, and decreased insulin sensitivity. Together, these studies suggest that DNER plays a crucial role in mediating islet cell-cell interactions and glucose homeostasis.

Prohormone convertase 1/3 deficiency causes obesity due to impaired proinsulin processing

Defective insulin processing is associated with obesity and diabetes. Prohormone convertase 1/3 (PC1/3) is an endopeptidase required for the processing of neurotransmitters and hormones. PC1/3 deficiency and genome-wide association studies relate PC1/3 with early onset obesity. Here, we find that deletion of PC1/3 in obesity-related neuronal cells expressing proopiomelanocortin mildly and transiently change body weight and fail to produce a phenotype when targeted to Agouti-related peptide- or nestin-expressing tissues. In contrast, pancreatic β cell-specific PC1/3 ablation induces hyperphagia with consecutive obesity despite uncontrolled diabetes with glucosuria. Obesity develops not due to impaired pro-islet amyloid polypeptide processing but due to impaired insulin maturation. Proinsulin crosses the blood-brain-barrier but does not induce central satiety. Accordingly, insulin therapy prevents hyperphagia. Further, islet PC1/3 expression levels negatively correlate with body mass index in humans. In this work, we show that impaired PC1/3-mediated proinsulin processing, as observed in human prediabetes, promotes hyperphagic obesity.

Defective insulin secretion is observed early in the development of diabetes. Here the authors report that β cell-specific deficiency of the insulin prohormone convertase 1/3 (PC1/3) leads not only to hyperglycemia, but also to hyperphagic obesity in mice.

Cilia Action in Islets: Lessons From Mouse Models

Primary cilia as a signaling organelle have garnered recent attention as a regulator of pancreatic islet function. These rod-like sensors exist on all major islet endocrine cell types and transduce a variety of external cues, while dysregulation of cilia function contributes to the development of diabetes. The complex role of islet primary cilia has been examined using genetic deletion targeting various components of cilia. In this review, we summarize experimental models for the study of islet cilia and current understanding of mechanisms of cilia regulation of islet hormone secretion. Consensus from these studies shows that pancreatic cilia perturbation can cause both endocrine and exocrine defects that are relevant to human disease. We discuss future research directions that would further elucidate cilia action in distinct groups of islet cells, including paracrine and juxtacrine regulation, GPCR signaling, and endocrine-exocrine crosstalk.

Islet vascularization is regulated by primary endothelial cilia via VEGF-A-dependent signaling

Islet vascularization is essential for intact islet function and glucose homeostasis. We have previously shown that primary cilia directly regulate insulin secretion. However, it remains unclear whether they are also implicated in islet vascularization. At eight weeks, murine Bbs4^-/-islets show significantly lower intra-islet capillary density with enlarged diameters. Transplanted Bbs4^-/- islets exhibit delayed re-vascularization and reduced vascular fenestration after engraftment, partially impairing vascular permeability and glucose delivery to β-cells. We identified primary cilia on endothelial cells as the underlying cause of this regulation, via the vascular endothelial growth factor-A (VEGF-A)/VEGF receptor 2 (VEGFR2) pathway. In vitro silencing of ciliary genes in endothelial cells disrupts VEGF-A/VEGFR2 internalization and downstream signaling. Consequently, key features of angiogenesis including proliferation and migration are attenuated in human BBS4 silenced endothelial cells. We conclude that endothelial cell primary cilia regulate islet vascularization and vascular barrier function via the VEGF-A/VEGFR2 signaling pathway.

Animal Models: Challenges and Opportunities to Determine Optimal Experimental Models of Pancreatitis and Pancreatic Cancer

At the 2018 PancreasFest meeting, experts participating in basic research met to discuss the plethora of available animal models for studying exocrine pancreatic disease. In particular, the discussion focused on the challenges currently facing the field and potential solutions. That meeting culminated in this review, which describes the advantages and limitations of both common and infrequently used models of exocrine pancreatic disease, namely, pancreatitis and exocrine pancreatic cancer. The objective is to provide a comprehensive description of the available models but also to provide investigators with guidance in the application of these models to investigate both environmental and genetic contributions to exocrine pancreatic disease. The content covers both nongenic and genetically engineered models across multiple species (large and small). Recommendations for choosing the appropriate model as well as how to conduct and present results are provided.

Pancreatic prolactin receptor signaling regulates maternal glucose homeostasis

Prolactin (PRL) signaling has been implicated in the regulation of glucose homeostatic adaptations to pregnancy. In this report, the PRL receptor (Prlr) gene was conditionally disrupted in the pancreas, creating an animal model which proved useful for investigating the biology and pathology of gestational diabetes including its impacts on fetal and placental development. In mice, pancreatic PRLR signaling was demonstrated to be required for pregnancy-associated changes in maternal β cell mass and function. Disruption of the Prlr gene in the pancreas resulted in fewer insulin producing cells, which failed to expand appropriately during pregnancy resulting in reduced blood insulin levels and maternal glucose intolerance. This inability to sustain normal blood glucose balance during pregnancy worsened with age and a successive pregnancy. The etiology of the insulin insufficiency was attributed to deficits in regulatory pathways controlling β cell development. Additionally, the disturbance in maternal blood glucose homeostasis, was associated with fetal overgrowth and dysregulation of inflammation and prolactin-associated transcripts in the placenta. Overall, these results indicate that the PRLR, acting within the pancreas, mediates maternal pancreatic adaptations to pregnancy and therefore its dysfunction may increase a woman's chances of becoming glucose intolerant during pregnancy.

A novel experimental model for human mixed acinar-ductal pancreatic cancer

Pancreatic cancer has remained refractory to treatment. In large part, this results from the lack of an animal model that mimics pancreatic cancer in man. We describe a novel experimental model of pancreatic cancer that shares the genetic background, histologic features and natural history of human mixed acinar–ductal carcinoma. Adult wild-type mice received an injection into the pancreatic duct of lentivirus coding two molecules, Kras^G12D mutation and shRNA p53, which recapitulate the mechanisms of pancreatic cancer in humans. The lentivirus constructs also co-expressed the luciferase gene for in vivo imaging by bioluminescence using the Xenogen IVIS imaging system. Weeks post-injection wild-type mice develop pancreatic cancer with the same histologic characteristics and metastases observed with human pancreatic mixed acinar–ductal carcinoma. This novel approach represents the first pancreatic cancer model that does not involve alterations of embryonic development, which is inherent with transgenic mice or knockout mice models. This novel experimental human pancreatic cancer model can be used to more effectively test new anti-cancer drug to inhibit tumor progression in situ and to retard metastases. Furthermore, our method of injecting lentivirus containing oncogenes and molecules implicated in the development of pancreatic can be employed in diabetic and obese mice, two common metabolic conditions characterized by an increased incidence of pancreatic cancer.

Restoration of Lepr in β cells of Lepr null mice does not prevent hyperinsulinemia and hyperglycemia

Objective

The adipose-derived hormone leptin plays an important role in regulating body weight and glucose homeostasis. Leptin receptors are expressed in the central nervous system as well as peripheral tissues involved in regulating glucose homeostasis, including insulin-producing β cells of the pancreas. Previous studies assessing the role of leptin receptors in β cells used Cre-loxP to disrupt the leptin receptor gene (Lepr) in β cells, but variable results were obtained. Furthermore, recombination of Lepr was observed in the hypothalamus or exocrine pancreas, in addition to the β cells, and Lepr in non-β cells may have compensated for the loss of Lepr in β cells, thus making it difficult to assess the direct effects of Lepr in β cells. To determine the significance of Lepr exclusively in β cells, we chose to selectively restore Lepr in β cells of Lepr null mice (Lepr^loxTB/loxTB).

Materials and methods

We used a mouse model in which endogenous expression of Lepr was disrupted by a loxP-flanked transcription blocker (Lepr^loxTB/loxTB), but was restored by Cre recombinase knocked into the Ins1 gene, which is specifically expressed in β cells (Ins1Cre). We bred Lepr^loxTB/loxTB and Ins1Cre mice to generate Lepr^loxTB/loxTB and Lepr^loxTB/loxTBIns1Cre mice, as well as Lepr^wt/wt and Lepr^wt/wtIns1Cre littermate mice. Male and female mice were weighed weekly between 6 and 11 weeks of age and fasting blood glucose was measured during this time. Oral glucose was administered to mice aged 7–12 weeks to assess glucose tolerance and insulin secretion. Relative β and α cell area and islet size were also assessed by immunostaining and analysis of pancreas sections of 12–14 week old mice.

Results

Male and female Lepr^loxTB/loxTB mice, lacking whole-body expression of Lepr, had a phenotype similar to db/db mice characterized by obesity, hyperinsulinemia, glucose intolerance, and impaired glucose stimulated insulin secretion. Despite restoring Lepr in β cells of Lepr^loxTB/loxTB mice, fasting insulin levels, blood glucose levels and body weight were comparable between Lepr^loxTB/loxTBIns1Cre mice and Lepr^loxTB/loxTB littermates. Furthermore, glucose tolerance and insulin secretion in male and female Lepr^loxTB/loxTBIns1Cre mice were similar to that observed in Lepr^loxTB/loxTB mice. Analysis of pancreatic insulin positive area revealed that restoration of Lepr in β cells of Lepr^loxTB/loxTB mice did not prevent hyperplasia of insulin positive cells nor did it rescue Glut-2 expression.

Conclusion

Collectively, these data suggest that direct action of leptin on β cells is insufficient to restore normal insulin secretion and glucose tolerance in mice without leptin receptor signaling elsewhere.

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Restoration of Lepr in β cells of Lepr null mice does not prevent hyperinsulinemia.

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Leptin receptors in β cells do not inhibit islet hyperplasia.

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Hyperglycemia and glucose intolerance persist despite restoration of Lepr in β cells of Lepr null mice.

Neuroanatomical circuitry between kidney and rostral elements of brain: a virally mediated transsynaptic tracing study in mice

The identity of higher-order neurons and circuits playing an associative role to control renal function is not well understood. We identified specific neural populations of rostral elements of brain regions that project multisynaptically to the kidneys in 3-6 days after injecting a retrograde tracer pseudorabies virus (PRV)-614 into kidney of 13 adult male C57BL/6J strain mice. PRV-614 infected neurons were detected in a number of mesencephalic (e.g. central amygdala nucleus), telencephalic regions and motor cortex. These divisions included the preoptic area (POA), dorsomedial hypothalamus (DMH), lateral hypothalamus, arcuate nucleus (Arc), suprachiasmatic nucleus (SCN), periventricular hypothalamus (PeH), and rostral and caudal subdivision of the paraventricular nucleus of the hypothalamus (PVN). PRV-614/Tyrosine hydroxylase (TH) double-labeled cells were found within DMH, Arc, SCN, PeH, PVN, the anterodorsal and medial POA. A subset of neurons in PVN that participated in regulating sympathetic outflow to kidney was catecholaminergic or serotonergic. PRV-614 infected neurons within the PVN also contained arginine vasopressin or oxytocin. These data demonstrate the rostral elements of brain innervate the kidney by the neuroanatomical circuitry.

Loss of Cyclin-dependent Kinase 2 in the Pancreas Links Primary β-Cell Dysfunction to Progressive Depletion of β-Cell Mass and Diabetes

The failure of pancreatic islet β-cells is a major contributor to the etiology of type 2 diabetes. β-Cell dysfunction and declining β-cell mass are two mechanisms that contribute to this failure, although it is unclear whether they are molecularly linked. Here, we show that the cell cycle regulator, cyclin-dependent kinase 2 (CDK2), couples primary β-cell dysfunction to the progressive deterioration of β-cell mass in diabetes. Mice with pancreas-specific deletion of Cdk2 are glucose-intolerant, primarily due to defects in glucose-stimulated insulin secretion. Accompanying this loss of secretion are defects in β-cell metabolism and perturbed mitochondrial structure. Persistent insulin secretion defects culminate in progressive deficits in β-cell proliferation, reduced β-cell mass, and diabetes. These outcomes may be mediated directly by the loss of CDK2, which binds to and phosphorylates the transcription factor FOXO1 in a glucose-dependent manner. Further, we identified a requirement for CDK2 in the compensatory increases in β-cell mass that occur in response to age- and diet-induced stress. Thus, CDK2 serves as an important nexus linking primary β-cell dysfunction to progressive β-cell mass deterioration in diabetes.

Blood-Brain Barrier Deterioration and Hippocampal Gene Expression in Polymicrobial Sepsis: An Evaluation of Endothelial MyD88 and the Vagus Nerve

Systemic infection can initiate or exacerbate central nervous system (CNS) pathology, even in the absence of overt invasion of bacteria into the CNS. Recent epidemiological studies have demonstrated that human survivors of sepsis have an increased risk of long-term neurocognitive decline. There is thus a need for improved understanding of the physiological mechanisms whereby acute sepsis affects the CNS. In particular, MyD88-dependent activation of brain microvascular endothelial cells and a resulting loss of blood-brain barrier integrity have been proposed to play an important role in the effects of systemic inflammation on the CNS. Signaling through the vagus nerve has also been considered to be an important component of CNS responses to systemic infection. Here, we demonstrate that blood-brain barrier permeabilization and hippocampal transcriptional responses during polymicrobial sepsis occur even in the absence of MyD88-dependent signaling in cerebrovascular endothelial cells. We further demonstrate that these transcriptional responses can occur without vagus nerve input. These results suggest that redundant signals mediate CNS responses in sepsis. Either endothelial or vagus nerve activation may be individually sufficient to transmit systemic inflammation to the central nervous system. Transcriptional activation in the forebrain in sepsis may be mediated by MyD88-independent endothelial mechanisms or by non-vagal neuronal pathways.

Hypothalamic Non-AgRP, Non-POMC GABAergic Neurons Are Required for Postweaning Feeding and NPY Hyperphagia

The hypothalamus is critical for feeding and body weight regulation. Prevailing studies focus on hypothalamic neurons that are defined by selectively expressing transcription factors or neuropeptides including those expressing proopiomelanocortin (POMC) and agouti-related peptides (AgRP). The Cre expression driven by the pancreas-duodenum homeobox 1 promoter is abundant in several hypothalamic nuclei but not in AgRP or POMC neurons. Using this line, we generated mice with disruption of GABA release from a major subset of non-POMC, non-AgRP GABAergic neurons in the hypothalamus. These mice exhibited a reduction in postweaning feeding and growth, and disrupted hyperphagic responses to NPY. Disruption of GABA release severely diminished GABAergic input to the paraventricular hypothalamic nucleus (PVH). Furthermore, disruption of GABA-A receptor function in the PVH also reduced postweaning feeding and blunted NPY-induced hyperphagia. Given the limited knowledge on postweaning feeding, our results are significant in identifying GABA release from a major subset of less appreciated hypothalamic neurons as a key mediator for postweaning feeding and NPY hyperphagia, and the PVH as one major downstream site that contributes significantly to the GABA action. Significance statement: Prevalent studies on feeding in the hypothalamus focus on well characterized, selective groups neurons [e.g., proopiomelanocortin (POMC) and agouti-related peptide (AgRP) neurons], and as a result, the role of the majority of other hypothalamic neurons is largely neglected. Here, we demonstrated an important role for GABAergic projections from non-POMC non-AgRP neurons to the paraventricular hypothalamic nucleus in promoting postweaning (mainly nocturnal) feeding and mediating NPY-induced hyperphagia. Thus, these results signify an importance to study those yet to be defined hypothalamic neurons in the regulation of energy balance and reveal a neural basis for postweaning (nocturnal) feeding and NPY-mediated hyperphagia.

GABAergic projections from lateral hypothalamus to paraventricular hypothalamic nucleus promote feeding

Lesions of the lateral hypothalamus (LH) cause hypophagia. However, activation of glutamatergic neurons in LH inhibits feeding. These results suggest a potential importance for other LH neurons in stimulating feeding. Our current study in mice showed that disruption of GABA release from adult LH GABAergic neurons reduced feeding. LH GABAergic neurons project extensively to the paraventricular hypothalamic nucleus (PVH), and optogenetic stimulation of GABAergic LH → PVH fibers induced monosynaptic IPSCs in PVH neurons, and potently increased feeding, which depended on GABA release. In addition, disruption of GABA-A receptors in the PVH reduced feeding. Thus, we have identified a new feeding pathway in which GABAergic projections from the LH to the PVH promote feeding.

Gαi/o-coupled receptor signaling restricts pancreatic β-cell expansion

Gi-GPCRs, G protein-coupled receptors that signal via Gα proteins of the i/o class (Gαi/o), acutely regulate cellular behaviors widely in mammalian tissues, but their impact on the development and growth of these tissues is less clear. For example, Gi-GPCRs acutely regulate insulin release from pancreatic β cells, and variants in genes encoding several Gi-GPCRs--including the α-2a adrenergic receptor, ADRA2A--increase the risk of type 2 diabetes mellitus. However, type 2 diabetes also is associated with reduced total β-cell mass, and the role of Gi-GPCRs in establishing β-cell mass is unknown. Therefore, we asked whether Gi-GPCR signaling regulates β-cell mass. Here we show that Gi-GPCRs limit the proliferation of the insulin-producing pancreatic β cells and especially their expansion during the critical perinatal period. Increased Gi-GPCR activity in perinatal β cells decreased β-cell proliferation, reduced adult β-cell mass, and impaired glucose homeostasis. In contrast, Gi-GPCR inhibition enhanced perinatal β-cell proliferation, increased adult β-cell mass, and improved glucose homeostasis. Transcriptome analysis detected the expression of multiple Gi-GPCRs in developing and adult β cells, and gene-deletion experiments identified ADRA2A as a key Gi-GPCR regulator of β-cell replication. These studies link Gi-GPCR signaling to β-cell mass and diabetes risk and identify it as a potential target for therapies to protect and increase β-cell mass in patients with diabetes.

Pancreas-specific Cre driver lines and considerations for their prudent use

Cre/LoxP has broad utility for studying the function, development, and oncogenic transformation of pancreatic cells in mice. Here we provide an overview of the Cre driver lines that are available for such studies. We discuss how variegated expression, transgene silencing, and recombination in undesired cell types have conspired to limit the performance of these lines, sometimes leading to serious experimental concerns. We also discuss preferred strategies for achieving high-fidelity driver lines and remind investigators of the continuing need for caution when interpreting results obtained from any Cre/LoxP-based experiment performed in mice.

Metabolic pitfalls of CNS Cre-based technology

Nestin-Cre mice have a significant metabolic phenotype that is hard to discern from current literature. Indeed, the Cre-lox system has numerous problems that can affect physiological parameters, and these are missed when the correct control strains are not used. Despite the increasing use of the Cre-lox system, these issues were not visible to the scientific community previously and may have affected published work. This makes it important to highlight the issues and raise awareness of the pitfalls of the Cre-lox system. Therefore, this perspective will discuss the impact of CNS and peripheral "off-target" Cre recombination on metabolic systems and describe the development of new approaches to obviate the difficulties.

Loss of Foxd3 results in decreased β-cell proliferation and glucose intolerance during pregnancy

A complete molecular understanding of β-cell mass expansion will be useful for the improvement of therapies to treat diabetic patients. During normal periods of metabolic challenges, such as pregnancy, β-cells proliferate, or self-renew, to meet the new physiological demands. The transcription factor Forkhead box D3 (Foxd3) is required for maintenance and self-renewal of several diverse progenitor cell lineages, and Foxd3 is expressed in the pancreatic primordium beginning at 10.5 d postcoitum, becoming localized predominantly to β-cells after birth. Here, we show that mice carrying a pancreas-specific deletion of Foxd3 have impaired glucose tolerance, decreased β-cell mass, decreased β-cell proliferation, and decreased β-cell size during pregnancy. In addition, several genes known to regulate proliferation, Foxm1, Skp2, Ezh2, Akt2, and Cdkn1a, are misregulated in islets isolated from these Foxd3 mutant mice. Together, these data place Foxd3 upstream of several pathways critical for β-cell mass expansion in vivo.

Convergence of the insulin and serotonin programs in the pancreatic β-cell

OBJECTIVE

Despite their origins in different germ layers, pancreatic islet cells share many common developmental features with neurons, especially serotonin-producing neurons in the hindbrain. Therefore, we tested whether these developmental parallels have functional consequences.

RESEARCH DESIGN AND METHODS

We used transcriptional profiling, immunohistochemistry, DNA-binding analyses, and mouse genetic models to assess the expression and function of key serotonergic genes in the pancreas.

RESULTS

We found that islet cells expressed the genes encoding all of the products necessary for synthesizing, packaging, and secreting serotonin, including both isoforms of the serotonin synthetic enzyme tryptophan hydroxylase and the archetypal serotonergic transcription factor Pet1. As in serotonergic neurons, Pet1 expression in islets required homeodomain transcription factor Nkx2.2 but not Nkx6.1. In β-cells, Pet1 bound to the serotonergic genes but also to a conserved insulin gene regulatory element. Mice lacking Pet1 displayed reduced insulin production and secretion and impaired glucose tolerance.

CONCLUSIONS

These studies demonstrate that a common transcriptional cascade drives the differentiation of β-cells and serotonergic neurons and imparts the shared ability to produce serotonin. The interrelated biology of these two cell types has important implications for the pathology and treatment of diabetes.