β-cell serotonin production is associated with female sex, old age, and diabetes-free condition

Monoamines' role in islet cell function and type 2 diabetes risk

The two monoamines serotonin and melatonin have recently been highlighted as potent regulators of islet hormone secretion and overall glucose homeostasis in the body. In fact, dysregulated signaling of both amines are implicated in β-cell dysfunction and development of type 2 diabetes mellitus (T2DM). Serotonin is a key player in β-cell physiology and plays a role in expansion of β-cell mass. Melatonin regulates circadian rhythm and nutrient metabolism and reduces insulin release in human and rodent islets in vitro. Herein, we focus on the role of serotonin and melatonin in islet physiology and the pathophysiology of T2DM. This includes effects on hormone secretion, receptor expression, genetic variants influencing β-cell function, melatonin treatment, and compounds that alter serotonin availability and signaling.

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.

Lactation improves pancreatic β cell mass and function through serotonin production

Pregnancy imposes a substantial metabolic burden on women through weight gain and insulin resistance. Lactation reduces the risk of maternal postpartum diabetes, but the mechanisms underlying this benefit are unknown. Here, we identified long-term beneficial effects of lactation on β cell function, which last for years after the cessation of lactation. We analyzed metabolic phenotypes including β cell characteristics in lactating and non-lactating humans and mice. Lactating and non-lactating women showed comparable glucose tolerance at 2 months after delivery, but after a mean of 3.6 years, glucose tolerance in lactated women had improved compared to non-lactated women. In humans, the disposition index, a measure of insulin secretory function of β cells considering the degree of insulin sensitivity, was higher in lactated women at 3.6 years after delivery. In mice, lactation improved glucose tolerance and increased β cell mass at 3 weeks after delivery. Amelioration of glucose tolerance and insulin secretion were maintained up to 4 months after delivery in lactated mice. During lactation, prolactin induced serotonin production in β cells. Secreted serotonin stimulated β cell proliferation through serotonin receptor 2B in an autocrine and paracrine manner. In addition, intracellular serotonin acted as an antioxidant to mitigate oxidative stress and improved β cell survival. Together, our results suggest that serotonin mediates the long-term beneficial effects of lactation on female metabolic health by increasing β cell proliferation and reducing oxidative stress in β cells.

Pancreatic β-Cells Communicate With Vagal Sensory Neurons

BACKGROUND AND AIMS

Destroying visceral sensory nerves impacts pancreatic islet function, glucose metabolism, and diabetes onset, but how islet endocrine cells interact with sensory neurons has not been studied.

METHODS

We characterized the anatomical pattern of pancreatic sensory innervation by combining viral tracing, immunohistochemistry, and reporter mouse models. To assess the functional interactions of β-cells with vagal sensory neurons, we recorded Ca²⁺ responses in individual nodose neurons in vivo while selectively stimulating β-cells with chemogenetic and pharmacologic approaches.

RESULTS

We found that pancreatic islets are innervated by vagal sensory axons expressing Phox2b, substance P, calcitonin-gene related peptide, and the serotonin receptor 5-HT₃R. Centrally, vagal neurons projecting to the pancreas terminate in the commissural nucleus of the solitary tract. Nodose neurons responded in vivo to chemogenetic stimulation of β-cells and to pancreas infusion with serotonin, but were not sensitive to insulin. Responses to chemogenetic and pharmacologic stimulation of β-cells were blocked by a 5-HT₃R antagonist and were enhanced by increasing serotonin levels in β-cells. We further confirmed directly in living pancreas slices that sensory terminals in the islet were sensitive to serotonin.

CONCLUSIONS

Our study establishes that pancreatic β-cells communicate with vagal sensory neurons, likely using serotonin signaling as a transduction mechanism. Serotonin is coreleased with insulin and may therefore convey information about the secretory state of β-cells via vagal afferent nerves.

Serotonin Regulates Adult β-Cell Mass by Stimulating Perinatal β-Cell Proliferation

A sufficient β-cell mass is crucial for preventing diabetes, and perinatal β-cell proliferation is important in determining the adult β-cell mass. However, it is not yet known how perinatal β-cell proliferation is regulated. Here, we report that serotonin regulates β-cell proliferation through serotonin receptor 2B (HTR2B) in an autocrine/paracrine manner during the perinatal period. In β-cell-specific Tph1 knockout (Tph1 βKO) mice, perinatal β-cell proliferation was reduced along with the loss of serotonin production in β-cells. Adult Tph1 βKO mice exhibited glucose intolerance with decreased β-cell mass. Disruption of Htr2b in β-cells also resulted in decreased perinatal β-cell proliferation and glucose intolerance in adulthood. Growth hormone (GH) was found to induce serotonin production in β-cells through activation of STAT5 during the perinatal period. Thus, our results indicate that GH-GH receptor-STAT5-serotonin-HTR2B signaling plays a critical role in determining the β-cell mass by regulating perinatal β-cell proliferation, and defects in this pathway affect metabolic phenotypes in adults.

Integrating the inputs that shape pancreatic islet hormone release

The pancreatic islet is a complex mini organ composed of a variety of endocrine cells and their support cells, which together tightly control blood glucose homeostasis. Changes in glucose concentration are commonly regarded as the chief signal controlling insulin-secreting beta cells, glucagon-secreting alpha cells and somatostatin-secreting delta cells. However, each of these cell types is highly responsive to a multitude of endocrine, paracrine, nutritional and neural inputs, which collectively shape the final endocrine output of the islet. Here, we review the principal inputs for each islet-cell type and the physiological circumstances in which these signals arise, through the prism of the insights generated by the transcriptomes of each of the major endocrine-cell types. A comprehensive integration of the factors that influence blood glucose homeostasis is essential to successfully improve therapeutic strategies for better diabetes management.

Ectopic serotonin production in β-cell specific transgenic mice

Genetically modified mice have been widely used in the field of β-cell research. However, analysis of results gathered using genetically modified organisms should be interpreted carefully as the results may be confounded by several factors. Here, we showed the ectopic serotonin (5-HT) production in β-cells of RIP-Cre^Mgn, MIP-GFP, and MIP-Cre/ERT mice. These mice contained a human growth hormone (hGH) cassette to enhance transgene expression and hGH expression and Stat5 phosphorylation were detected in pancreatic islets of these mice. The expression level of tryptophan hydroxylase 1 (Tph1) was upregulated in pancreatic islets of transgenic mice with an hGH cassette but not in transgenic mice without an hGH cassette. Ectopic 5-HT production was not observed in β-cell-specific prolactin receptor (Prlr) knockout mice or Stat5 knockout mice crossed with RIP-Cre^Mgn. We further confirmed that 5-HT production in β-cells of several transgenic mice was induced by hGH expression followed by the activation of the Prlr-Stat5-Tph1 pathway. These findings indicate that results obtained using transgenic mice containing the hGH cassette should be interpreted with care.