ATP-gated P2X ₃ receptors constitute a positive autocrine signal for insulin release in the human pancreatic β cell

Abstract

Extracellular ATP has been proposed as a paracrine signal in rodent islets, but it is unclear what role ATP plays in human islets. We now show the presence of an ATP signaling pathway that enhances the human β cell's sensitivity and responsiveness to glucose fluctuations. By using in situ hybridization, RT-PCR, immunohistochemistry, and Western blotting as well as recordings of cytoplasmic-free Ca 2+ concentration, [Ca 2+ ] i , and hormone release in vitro, we show that human β cells express ionotropic ATP receptors of the P2X 3 type and that activation of these receptors by ATP coreleased with insulin amplifies glucose-induced insulin secretion. Released ATP activates P2X 3 receptors in the β-cell plasma membrane, resulting in increased [Ca 2+ ] i and enhanced insulin secretion. Therefore, in human islets, released ATP forms a positive autocrine feedback loop that sensitizes the β cell's secretory machinery. This may explain how the human pancreatic β cell can respond so effectively to relatively modest changes in glucose concentration under physiological conditions in vivo.extracellular ATP | human pancreatic β cell | insulin secretion | P2X receptor | positive autocrine feedback G lucose homeostasis is tightly controlled by hormone secretion from the endocrine pancreas, the islets of Langerhans. Even small physiological deviations (e.g., 10%) in plasma glucose are effectively counteracted by sharp (e.g., 3-fold) increases in the secretion of the islet hormones insulin and glucagon (1). Intraislet autocrine and paracrine signaling are pivotal mechanisms for proper function of the islet, making islet cells extremely sensitive and responsive to plasma glucose fluctuations. The roles of different compounds such as GABA, glutamate, Zn 2+ , insulin, and ATP as autocrine and paracrine regulators of islet hormone release have been examined extensively (2-8). Extracellular ATP seems important because it is present in insulin-containing secretory granules and is released during glucose stimulation in sufficient amounts to stimulate ATP receptors (9-12).Extracellular ATP is an important neurotransmitter signal in the brain as well as in vascular, immune, and endocrine cells (13-15). The purinergic system comprises receptors for extracellular ATP and adenosine, the P2 and P1 receptors, respectively. P2 purinergic receptors can be divided into metabotropic P2Y receptors (G protein coupled) and ionotropic P2X receptors (ligand-gated ion channels) (16). The ionotropic P2X family comprises seven subtypes named P2X 1 -P2X 7 that regulate cell function by opening cation channels permeable to Na + , K + , and Ca 2+ (15,17). Activation of these channels regulates the release of neurotransmitters and hormones, either through direct Ca 2+ influx or by promoting membrane depolarization and thereby inducing action potentials (18-21).The role of ATP signaling in the physiology of pancreatic islets has been studied in rodent models, but the results in the literature are conflicting (22-28). In rat islets, purinergic agonists...

Purinergic Autocrine Regulation of Mechanosensitivity and Serotonin Release in a Human EC Model

“…Our study focused on BON cells and hECs. BON cells display an EC phenotype 2,5–7,18–21,46 relevant to hECs and are the only hEC model shown to be suitable for mechanosensitivity studies in single cells. 1,2,8,19 Our study proved that ATP release is a critical regulator of mechanosensitivity, Ca 2+ signaling, and 5-HT release.…”

Extracellular ATP and other nucleotides—ubiquitous triggers of intercellular messenger release

“…In a study on human β cells, ATP signaling enhanced the sensitivity and responsiveness of β cells to glucose fluctuations [278]. Human β cells expressed ionotropic P2X3Rs and activation of these receptors through ATP co-released with insulin amplified glucose-induced insulin secretion.…”

“…In mouse pancreatic α cells ATP inhibited Ca 2+ signaling and glucagon secretion [325] whereas ATP stimulated small increases in glucagon secretion in human, monkey, and mouse islets, whereby a subset of human α cells expressed P2X4Rs [278]. In addition to mononucleotides, the dinucleoside polyphosphate diadenosine tetraphosphate (Ap 4 A), which can act on a variety of nucleotide receptors [326], stimulated both glucagon and insulin release from rat pancreas perfused with varying glucose concentrations [280].…”

Exocytosis from pancreatic β-cells: mathematical modelling of the exit of low-molecular-weight granule content

“…Whether this process can be regulated remains to be examined. Nevertheless, it is possible that selective release of GABA and ATP via the fusion pore may allow these compounds to exert a local paracrine effect on neighbouring islet cells [11,37,38].…”

“…This method involves the 'engineering' of the b-cells to express P2X 2 (P2X 2 R) or GABA A (GABA A R) membrane receptors. Normally, these receptors are not expressed at high levels in the rat b-cells (but see [10,11] for the expression of these receptors in human b-cells). P2X 2 R and GABA A R belong to the class of ionotropic receptors and represent ligand-gated ion channels that are activated by extracellular ATP (P2X 2 R) or GABA (GABA A R), respectively.…”