ATP and adenosine trigger the interaction of plasma membrane IP3 receptors with protein kinase A in oviductal ciliated cells

Soil-borne fungi alter the apoplastic purinergic signaling in plants by deregulating the homeostasis of extracellular ATP and its metabolite adenosine

Purinergic signaling activated by extracellular nucleotides and their derivative nucleosides trigger sophisticated signaling networks. The outcome of these pathways determine the capacity of the organism to survive under challenging conditions. Both extracellular ATP (eATP) and Adenosine (eAdo) act as primary messengers in mammals, essential for immunosuppressive responses. Despite the clear role of eATP as a plant damage-associated molecular pattern, the function of its nucleoside, eAdo, and of the eAdo/eATP balance in plant stress response remain to be fully elucidated. This is particularly relevant in the context of plant-microbe interaction, where the intruder manipulates the extracellular matrix. Here, we identify Ado as a main molecule secreted by the vascular fungus Fusarium oxysporum. We show that eAdo modulates the plant's susceptibility to fungal colonization by altering the eATP-mediated apoplastic pH homeostasis, an essential physiological player during the infection of this pathogen. Our work indicates that plant pathogens actively imbalance the apoplastic eAdo/eATP levels as a virulence mechanism.

Nitric oxide activation by progesterone suppresses ATP-induced ciliary activity in oviductal ciliated cells

Ciliary beat frequency (CBF) regulates the oviductal transport of oocytes and embryos, which are important components of the reproductive process. Local release of ATP transiently increases CBF by increasing [Ca2+]i. Ovarian hormones also regulate ciliary activity and oviductal transport. Progesterone (P4) induces nitric oxide (NO) production and high P4 concentrations induce ciliary dysfunction. However, the mechanism by which P4 affects CBF has not been elucidated. To evaluate the role of P4 in NO production and its effect on ATP-induced increases in CBF, we measured CBF, NO concentrations and [Ca2+]i in cultures of oviductal ciliated cells treated with P4 or NO signalling-related molecules. ATP induced a [Ca2+]i peak, followed by an increase in NO concentrations that were temporally correlated with the decreased phase of the transiently increased CBF. Furthermore, P4 increased the expression of nitric oxide synthases (iNOS and nNOS) and reduced the ATP-induced increase in CBF via a mechanism that involves the NO signalling pathway. These results have improved our knowledge about intracellular messengers controlling CBF and showed that NO attenuates oviduct cell functions. Furthermore, we showed that P4 regulates neurotransmitter (ATP) actions on CBF via the NO pathway, which could explain pathologies where oviductal transport is altered and fertility decreased.

An autocrine ATP release mechanism regulates basal ciliary activity in airway epithelium

KEY POINTS

Extracellular ATP, in association with [Ca²⁺ ]_i regulation, is required to maintain basal ciliary beat frequency. Increasing extracellular ATP levels increases ciliary beating in airway epithelial cells, maintaining a sustained response by inducing the release of additional ATP. Extracellular ATP levels in the millimolar range, previously associated with pathophysiological conditions of the airway epithelium, produce a transient arrest of ciliary activity. The regulation of ciliary beat frequency is dependent on ATP release by hemichannels (connexin/pannexin) and P2X receptor activation, the blockage of which may even stop ciliary movement. The force exerted by cilia, measured by atomic force microscopy, is reduced following extracellular ATP hydrolysis. This result complements the current understanding of the ciliary beating regulatory mechanism, with special relevance to inflammatory diseases of the airway epithelium that affect mucociliary clearance.

ABSTRACT

Extracellular nucleotides, including ATP, are locally released by the airway epithelium and stimulate ciliary activity in a [Ca²⁺ ]_i -dependent manner after mechanical stimulation of ciliated cells. However, it is unclear whether the ATP released is involved in regulating basal ciliary activity and mediating changes in ciliary activity in response to chemical stimulation. In the present study, we evaluated ciliary beat frequency (CBF) and ciliary beating forces in primary cultures from mouse tracheal epithelium, using videomicroscopy and atomic force microscopy (AFM), respectively. Extracellular ATP levels and [Ca²⁺ ]_i were measured by luminometric and fluorimetric assays, respectively. Uptake of ethidium bromide was measured to evaluate hemichannel functionality. We show that hydrolysis of constitutive extracellular ATP levels with apyrase (50 U ml^-1 ) reduced basal CBF by 45% and ciliary force by 67%. The apyrase effect on CBF was potentiated by carbenoxolone, a hemichannel inhibitor, and oxidized ATP, an antagonist used to block P2X7 receptors, which reduced basal CBF by 85%. Additionally, increasing extracellular ATP levels (0.1-100 μm) increased CBF, maintaining a sustained response that was suppressed in the presence of carbenoxolone. We also show that high levels of ATP (1 mm), associated with inflammatory conditions, lowered basal CBF by reducing [Ca²⁺ ]_i and hemichannel functionality. In summary, we provide evidence indicating that airway epithelium ATP release is the molecular autocrine mechanism regulating basal ciliary activity and is also the mediator of the ciliary response to chemical stimulation.

Low physiological levels of prostaglandins E2 and F2α improve human sperm functions

Prostaglandins (PGs) have been reported to be present in the seminal fluid and cervical mucus, affecting different stages of sperm maturation from spermatogenesis to the acrosome reaction. This study assessed the effects of low physiological PGE2 and PGF2α concentrations on human sperm motility and on the ability of the spermatozoa to bind to the zona pellucida (ZP). Human spermatozoa were isolated from seminal samples with normal concentration and motility parameters and incubated with 1 μM PGE2, 1 μM PGF2α or control solution to determine sperm motility and the ability to bind to human ZP. The effects of both PGs on intracellular calcium levels were determined. Incubation for 2 or 18 h with PGE2 or PGF2α resulted in a significant (P < 0.05) increase in the percentage of spermatozoa with progressive motility. In contrast with PGF2α, PGE2 alone induced an increase in sperm intracellular calcium levels; however, the percentage of sperm bound to the human ZP was doubled for both PGs. These results indicate that incubation of human spermatozoa with low physiological levels of PGE2 or PGF2α increases sperm functions and could improve conditions for assisted reproduction protocols.

Purinergic signalling in the reproductive system in health and disease

There are multiple roles for purinergic signalling in both male and female reproductive organs. ATP, released as a cotransmitter with noradrenaline from sympathetic nerves, contracts smooth muscle via P2X1 receptors in vas deferens, seminal vesicles, prostate and uterus, as well as in blood vessels. Male infertility occurs in P2X1 receptor knockout mice. Both short- and long-term trophic purinergic signalling occurs in reproductive organs. Purinergic signalling is involved in hormone secretion, penile erection, sperm motility and capacitation, and mucous production. Changes in purinoceptor expression occur in pathophysiological conditions, including pre-eclampsia, cancer and pain.

Epithelial cell culture from human adenoids: a functional study model for ciliated and secretory cells

Background. Mucociliary transport (MCT) is a defense mechanism of the airway. To study the underlying mechanisms of MCT, we have both developed an experimental model of cultures, from human adenoid tissue of ciliated and secretory cells, and characterized the response to local chemical signals that control ciliary activity and the secretion of respiratory mucins in vitro. Materials and Methods. In ciliated cell cultures, ciliary beat frequency (CBF) and intracellular Ca²⁺ levels were measured in response to ATP, UTP, and adenosine. In secretory cultures, mucin synthesis and secretion were identified by using immunodetection. Mucin content was taken from conditioned medium and analyzed in the presence or absence of UTP. Results. Enriched ciliated cell monolayers and secretory cells were obtained. Ciliated cells showed a basal CBF of 10.7 Hz that increased significantly after exposure to ATP, UTP, or adenosine. Mature secretory cells showed active secretion of granules containing different glycoproteins, including MUC5AC. Conclusion. Culture of ciliated and secretory cells grown from adenoid epithelium is a reproducible and feasible experimental model, in which it is possible to observe ciliary and secretory activities, with a potential use as a model to understand mucociliary transport control mechanisms.

The IplA Ca2+ channel of Dictyostelium discoideum is necessary for chemotaxis mediated through Ca2+, but not through cAMP, and has a fundamental role in natural aggregation

During aggregation of Dictyostelium discoideum, nondissipating, symmetrical, outwardly moving waves of cAMP direct cells towards aggregation centers. It has been assumed that the spatial and temporal characteristics of the front and back of each cAMP wave regulate both chemokinesis and chemotaxis. However, during the period preceding aggregation, cells acquire not only the capacity to chemotax in a spatial gradient of cAMP, but also in a spatial gradient of Ca(2+). The null mutant of the putative IplA Ca(2+) channel gene, iplA(-), undergoes normal chemotaxis in spatial gradients of cAMP and normal chemokinetic responses to increasing temporal gradients of cAMP, both generated in vitro. However, iplA(-) cells lose the capacity to undergo chemotaxis in response to a spatial gradient of Ca(2+), suggesting that IplA is either the Ca(2+) chemotaxis receptor or an essential component of the Ca(2+) chemotaxis regulatory pathway. In response to natural chemotactic waves generated by wild-type cells, the chemokinetic response of iplA(-) cells to the temporal dynamics of the cAMP wave is intact, but the capacity to reorient in the direction of the aggregation center at the onset of each wave is lost. These results suggest that transient Ca(2+) gradients formed between cells at the onset of each natural cAMP wave augment reorientation towards the aggregation center. If this hypothesis proves correct, it will provide a more complex contextual framework for interpreting D. discoideum chemotaxis.

Misoprostol elevates intracellular calcium in Neuro-2a cells via protein kinase A

Misoprostol, a prostaglandin type E analogue, has been implicated in a number of neurodevelopmental disorders. However, its mode of action in the nervous system is not well understood. Misoprostol acts on the same receptors as prostaglandin E(2) (PGE(2)), a natural lipid-derived compound, which mediates important physiological functions in the nervous system via activation of four EP receptors (EP1-4). In this study we use a ratiometric calcium imaging with fura-2 AM as a calcium indicator to show that misoprostol alters intracellular calcium levels in mouse neuroblastoma (Neuro-2a) cells via similar mechanisms as PGE(2). We demonstrate that the misoprostol-induced increase in calcium is mediated by a protein kinase A (PKA)-dependent mechanism and that the EP4 receptor signaling pathway may play an inhibitory role on calcium regulation. Overall, this study provides further support for the involvement of PGE(2) signaling in calcium homeostasis and suggests its important role in the nervous system.

Functional ryanodine receptors in the plasma membrane of RINm5F pancreatic beta-cells

Ryanodine receptors (RyR) are Ca²⁺ channels that mediate Ca²⁺ release from intracellular stores in response to diverse intracellular signals. In RINm5F insulinoma cells, caffeine, and 4-chloro-m-cresol (4CmC), agonists of RyR, stimulated Ca²⁺ entry that was independent of store-operated Ca²⁺ entry, and blocked by prior incubation with a concentration of ryanodine that inactivates RyR. Patch-clamp recording identified small numbers of large-conductance (γ_K = 169 pS) cation channels that were activated by caffeine, 4CmC or low concentrations of ryanodine. Similar channels were detected in rat pancreatic β-cells. In RINm5F cells, the channels were blocked by cytosolic, but not extracellular, ruthenium red. Subcellular fractionation showed that type 3 IP₃ receptors (IP₃R3) were expressed predominantly in endoplasmic reticulum, whereas RyR2 were present also in plasma membrane fractions. Using RNAi selectively to reduce expression of RyR1, RyR2, or IP₃R3, we showed that RyR2 mediates both the Ca²⁺ entry and the plasma membrane currents evoked by agonists of RyR. We conclude that small numbers of RyR2 are selectively expressed in the plasma membrane of RINm5F pancreatic β-cells, where they mediate Ca²⁺ entry.