Changes in P2Y 2 receptor localization on adrenaline‐ and noradrenaline‐containing chromaffin cells in the rat adrenal gland during development and aging

Purinergic signalling during development and ageing

Extracellular purines and pyrimidines play major roles during embryogenesis, organogenesis, postnatal development and ageing in vertebrates, including humans. Pluripotent stem cells can differentiate into three primary germ layers of the embryo but may also be involved in plasticity and repair of the adult brain. These cells express the molecular components necessary for purinergic signalling, and their developmental fates can be manipulated via this signalling pathway. Functional P1, P2Y and P2X receptor subtypes and ectonucleotidases are involved in the development of different organ systems, including heart, blood vessels, skeletal muscle, urinary bladder, central and peripheral neurons, retina, inner ear, gut, lung and vas deferens. The importance of purinergic signalling in the ageing process is suggested by changes in expression of A1 and A2 receptors in old rat brains and reduction of P2X receptor expression in ageing mouse brain. By contrast, in the periphery, increases in expression of P2X3 and P2X4 receptors are seen in bladder and pancreas.

Impairment of vesicular ATP release affects glucose metabolism and increases insulin sensitivity

Neuroendocrine cells store ATP in secretory granules and release it along with hormones that may trigger a variety of cellular responses in a process called purinergic chemical transmission. Although the vesicular nucleotide transporter (VNUT) has been shown to be involved in vesicular storage and release of ATP, its physiological relevance in vivo is far less well understood. In Vnut knockout (Vnut(-/-)) mice, we found that the loss of functional VNUT in adrenal chromaffin granules and insulin granules in the islets of Langerhans led to several significant effects. Vesicular ATP accumulation and depolarization-dependent ATP release were absent in the chromaffin granules of Vnut(-/-) mice. Glucose-responsive ATP release was also absent in pancreatic β-cells in Vnut(-/-) mice, while glucose-responsive insulin secretion was enhanced to a greater extent than that in wild-type tissue. Vnut(-/-) mice exhibited improved glucose tolerance and low blood glucose upon fasting due to increased insulin sensitivity. These results demonstrated an essential role of VNUT in vesicular storage and release of ATP in neuroendocrine cells in vivo and suggest that vesicular ATP and/or its degradation products act as feedback regulators in catecholamine and insulin secretion, thereby regulating blood glucose homeostasis.

Expression of the P2Y₂ receptor on the rat ocular surface during a 1-year rearing period

PURPOSE

P2Y2 receptors are expressed on ocular surface tissues. Diquafosol ophthalmic solution (DIQUAS(®) ophthalmic solution 3 %; Santen Pharmaceutical Co., Ltd.) acts on these receptors and promotes the secretion of water and mucin. It has been shown to be an efficient dry eye treatment. If P2Y2 receptor expression on the ocular surface decreases with age, the effect of diquafosol may be reduced in elderly persons. In this study, we investigated the changes in P2Y2 receptor expression on the rat ocular surface over an extended period of time.

METHODS

P2Y2 receptor expression in the conjunctiva, cornea, meibomian gland and lacrimal glands of male and female Sprague-Dawley rats was examined from 5 weeks until 53 weeks of age using immunostaining and quantitative-PCR.

RESULTS

In the immunohistological examinations, P2Y2 receptor expression was observed in the conjunctival epithelium containing goblet cells, corneal epithelium, meibomian gland ductal epithelium and lacrimal gland ductal epithelium. However, its expression was not significantly different between each age group or between sexes. Regarding P2Y2 receptor mRNA expression, there was an age-related increase in the bulbar conjunctiva. In particular, a significant increase was observed in the 53-week-old age group as compared to the 5-week-old female age group. However, age-related changes in expression were not observed in the cornea or meibomian gland in males or females.

CONCLUSIONS

We observed no significant age-related decrease was observed for P2Y2 receptor protein and mRNA expression on rat ocular surface tissues.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Selective catecholamine recognition with NeuroSensor 521: a fluorescent sensor for the visualization of norepinephrine in fixed and live cells

A method for the selective labeling and imaging of catecholamines in live and fixed secretory cells is reported. The method integrates a tailored approach using a novel fluorescence-based turn-on molecular sensor (NeuroSensor 521) that can exploit the high concentration of neurotransmitters and acidic environment within secretory vesicles for the selective recognition of norepinephrine and dopamine. The utility of the method was demonstrated by selectively labeling and imaging norepinephrine in secretory vesicles such that discrimination between norepinephrine- and epinephrine-enriched populations of chromaffin cells was observed. This method was validated in fixed cells by co-staining with an anti-PNMT antibody.

Functional distribution of Ca2+-coupled P2 purinergic receptors among adrenergic and noradrenergic bovine adrenal chromaffin cells

BACKGROUND

Adrenal chromaffin cells mediate acute responses to stress through the release of epinephrine. Chromaffin cell function is regulated by several receptors, present both in adrenergic (AD) and noradrenergic (NA) cells. Extracellular ATP exerts excitatory and inhibitory actions on chromaffin cells via ionotropic (P2X) and metabotropic (P2Y) receptors. We have taken advantage of the actions of the purinergic agonists ATP and UTP on cytosolic free Ca2+ concentration ([Ca2+]i) to determine whether P2X and P2Y receptors might be asymmetrically distributed among AD and NA chromaffin cells.

RESULTS

The [Ca2+]i and the [Na+]i were recorded from immunolabeled bovine chromaffin cells by single-cell fluorescence imaging. Among the ATP-sensitive cells ~40% did not yield [Ca2+]i responses to ATP in the absence of extracellular Ca2+ (Ca2+o), indicating that they expressed P2X receptors and did not express Ca2+- mobilizing P2Y receptors; the remainder expressed Ca2+-mobilizing P2Y receptors. Relative to AD-cells approximately twice as many NA-cells expressed P2X receptors while not expressing Ca2+- mobilizing P2Y receptors, as indicated by the proportion of cells lacking [Ca2+]i responses and exhibiting [Na+]i responses to ATP in the absence and presence of Ca2+o, respectively. The density of P2X receptors in NA-cells appeared to be 30-50% larger, as suggested by comparing the average size of the [Na+]i and [Ca2+]i responses to ATP. Conversely, approximately twice as many AD-cells expressed Ca2+-mobilizing P2Y receptors, and they appeared to exhibit a higher (~20%) receptor density. UTP raised the [Ca2+]i in a fraction of the cells and did not raise the [Na+]i in any of the cells tested, confirming its specificity as a P2Y agonist. The cell density of UTP-sensitive P2Y receptors did not appear to vary among AD- and NA-cells.

CONCLUSION

Although neither of the major purinoceptor types can be ascribed to a particular cell phenotype, P2X and Ca2+-mobilizing P2Y receptors are preferentially located to noradrenergic and adrenergic chromaffin cells, respectively. ATP might, in addition to an UTP-sensitive P2Y receptor, activate an UTP-insensitive P2Y receptor subtype. A model for a short-loop feedback interaction is presented whereby locally released ATP acts upon P2Y receptors in adrenergic cells, inhibiting Ca2+ influx and contributing to terminate evoked epinephrine secretion.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

P2 receptor web: Complexity and fine-tuning

The present review offers a new perspective on a family of receptors, termed P2 receptors, specific for nucleoside tri- and diphosphates of purines/pyrimidines. We emphasize here that while decoding the inputs of various related extracellular ligands, P2 receptors are a clear example of increasing biological complexity. They are represented by 7 ionotropic P2X and 8 metabotropic P2Y receptors; they have very heterogeneous ligands and binding characteristics, molecular properties, transduction mechanisms, cellular localization and protein-protein interactions. While the reason for this sophistication is unknown, a few compelling issues emerge while looking at such a rich variety. We ask, for instance, why so many different receptor subtypes are necessary for triggering biological properties and functions, and if these receptors are more than the sum of their single entities. A first possibility is that newly synthesized P2 proteins are casually located on the cell surface (stochastic hypothesis). Alternatively, distinct subunits are engaged on different cell phenotypes by genetic control (genetic determinism) and/or selective recruitment under physiopathological conditions and epigenetic stimuli (epigenetic determinism). Nevertheless, an appropriate way to both dissect the vast biological scenario and molecular complexity among P2 receptors and to integrate and upgrade their assortment is to regard them as a "combinatorial receptor web", that is, a dynamic architecture of P2 proteins demonstrating economic efficiency and involving a process of "fine-tuning", a mechanism which endorses the dynamic nature of all biological reactions. In the present analysis, we stimulate a scientific query about what contributes to such a vast P2 receptor sophistication.

Adenine nucleotides inhibit proliferation of the human lung adenocarcinoma cell line LXF-289 by activation of nuclear factor kappaB1 and mitogen-activated protein kinase pathways

Extracellular nucleotides have a profound role in the regulation of the proliferation of diseased tissue. We studied how extracellular nucleotides regulate the proliferation of LXF-289 cells, the adenocarcinoma-derived cell line from human lung bronchial tumor. ATP and ADP strongly inhibited LXF-289 cell proliferation. The nucleotide potency profile was ATP = ADP = ATPgammaS > > UTP, UDP, whereas alpha,beta-methylene-ATP, beta,gamma-methylene-ATP, 2',3'-O-(4-benzoylbenzoyl)-ATP, AMP and UMP were inactive. The nucleotide potency profile and the total blockade of the ATP-mediated inhibitory effect by the phospholipase C inhibitor U-73122 clearly show that P2Y receptors, but not P2X receptors, control LXF-289 cell proliferation. Treatment of proliferating LXF-289 cells with 100 microm ATP or ADP induced significant reduction of cell number and massive accumulation of cells in the S phase. Arrest in S phase is also indicated by the enhancement of the antiproliferative effect of ATP by coapplication of the cytostatic drugs cisplatin, paclitaxel and etoposide. Inhibition of LXF-289 cell proliferation by ATP was completely reversed by inhibitors of extracellular signal related kinase-activating kinase/extracellular signal related kinase 1/2 (PD98059, U0126), p38 mitogen-activated protein kinase (SB203508), phosphatidylinositol-3-kinase (wortmannin), and nuclear factor kappaB1 (SN50). Western blot analysis revealed transient activation of p38 mitogen-activated protein kinase, extracellular signal-related kinase 1/2, and nuclear factor kappaB1 and possibly new formation of p50 from its precursor p105. ATP-induced attenuation of LXF-289 cell proliferation was accompanied by transient translocation of p50 nuclear factor kappaB1 and extracellular signal-related kinase 1/2 to the nucleus in a similar time period. In summary, inhibition of LXF-289 cell proliferation is mediated via P2Y receptors by activation of multiple mitogen-activated protein kinase pathways and nuclear factor kappaB1, arresting the cells in the S phase.