Cell swelling-induced ATP release is tightly dependent on intracellular calcium elevations

Aberrations in Cell Signaling Quantified in Diabetic Murine Globes after Injury

The corneal epithelium is an avascular structure that has a unique wound healing mechanism, which allows for rapid wound closure without compromising vision. This wound healing mechanism is attenuated in diabetic patients, resulting in poor clinical outcomes and recurrent non-healing erosion. We investigated changes in cellular calcium signaling activity during the wound response in murine diabetic tissue using live cell imaging from both ex vivo and in vitro models. The calcium signaling propagation in diabetic cells was significantly decreased and displayed altered patterns compared to non-diabetic controls. Diabetic cells and tissue display distinct expression of the purinergic receptor, P2X7, which mediates the wound healing response. We speculate that alterations in P2X7 expression, interactions with other proteins, and calcium signaling activity significantly impact the wound healing response. This may explain aberrations in the diabetic wound response.

Role of the endothelial reverse mode sodium-calcium exchanger in the dilation of the rat middle cerebral artery during hypoosmotic hyponatremia

A decrease in serum sodium ion concentration below 135 mmol L^-1 is usually accompanied by a decrease in plasma osmolality (hypoosmotic hyponatremia) and leads to the disorder of intracranial homeostasis mainly due to cellular swelling. Recently, using an in vitro model of hypoosmotic hyponatremia, we have found that a decrease in sodium ion concentration in the perfusate to 121 mmol L^-1 relaxes the isolated rat middle cerebral artery (MCA). The aim of the present study was to explore the mechanism responsible for this relaxation. Isolated, pressurized, and perfused MCAs placed in a vessel chamber were subjected to a decrease in sodium ion concentration to 121 mmol L^-1. Changes in the diameter of the vessels were monitored with a video camera. The removal of the endothelium and inhibition of nitric oxide-dependent signaling or the reverse mode sodium-calcium exchanger (NCX) were used to study the mechanism of the dilation of the vessel during hyponatremia. The dilation of the MCA (19 ± 5%, p < 0.005) in a low-sodium buffer was absent after removal of the endothelium or administration of the inhibitor of the reverse mode of sodium-calcium exchange and was reversed to constriction after the inhibition of nitric oxide (NO)/cGMP signaling. The dilation of the middle cerebral artery of the rat in a 121 mmol L^-1 Na⁺ buffer depends on NO signaling and reverse mode of sodium-calcium exchange. These results suggest that constriction of large cerebral arteries with impaired NO-dependent signaling may be observed in response to hypoosmotic hyponatremia.

Impedance-Based Phenotypic Readout of Transporter Function: A Case for Glutamate Transporters

Excitatory amino acid transporters (EAAT/SLC1) mediate Na⁺-dependent uptake of extracellular glutamate and are potential drug targets for neurological disorders. Conventional methods to assess glutamate transport in vitro are based on radiolabels, fluorescent dyes or electrophysiology, which potentially compromise the cell’s physiology and are generally less suited for primary drug screens. Here, we describe a novel label-free method to assess human EAAT function in living cells, i.e., without the use of chemical modifications to the substrate or cellular environment. In adherent HEK293 cells overexpressing EAAT1, stimulation with glutamate or aspartate induced cell spreading, which was detected in real-time using an impedance-based biosensor. This change in cell morphology was prevented in the presence of the Na⁺/K⁺-ATPase inhibitor ouabain and EAAT inhibitors, which suggests the substrate-induced response was ion-dependent and transporter-specific. A mechanistic explanation for the phenotypic response was substantiated by actin cytoskeleton remodeling and changes in the intracellular levels of the osmolyte taurine, which suggests that the response involves cell swelling. In addition, substrate-induced cellular responses were observed for cells expressing other EAAT subtypes, as well as in a breast cancer cell line (MDA-MB-468) with endogenous EAAT1 expression. These findings allowed the development of a label-free high-throughput screening assay, which could be beneficial in early drug discovery for EAATs and holds potential for the study of other transport proteins that modulate cell shape.

Propidium uptake and ATP release in A549 cells share similar transport mechanisms

The lipid bilayer of eukaryotic cells' plasma membrane is almost impermeable to small ions and large polar molecules, but its miniscule basal permeability in intact cells is poorly characterized. This report describes the intrinsic membrane permeability of A549 cells toward the charged molecules propidium (Pr²⁺) and ATP^4-. Under isotonic conditions, we detected with quantitative fluorescence microscopy, a continuous low-rate uptake of Pr (∼150 × 10^-21 moles (zmol)/h/cell, [Pr]_o = 150 μM, 32°C). It was stimulated transiently but strongly by 66% hypotonic cell swelling reaching an influx amplitude of ∼1500 (zmol/h)/cell. The progressive Pr uptake with increasing [Pr]_o (30, 150, and 750 μM) suggested a permeation mechanism by simple diffusion. We quantified separately ATP release with custom wide-field-of-view chemiluminescence imaging. The strong proportionality between ATP efflux and Pr²⁺ influx during hypotonic challenge, and the absence of stimulation of transmembrane transport following 300% hypertonic shock, indicated that ATP and Pr travel the same conductive pathway. The fluorescence images revealed a homogeneously distributed intracellular uptake of Pr not consistent with high-conductance channels expressed at low density on the plasma membrane. We hypothesized that the pathway consists of transiently formed water pores evenly spread across the plasma membrane. The abolition of cell swelling-induced Pr uptake with 500 μM gadolinium, a known modulator of membrane fluidity, supported the involvement of water pores whose formation depends on the membrane fluidity. Our study suggests an alternative model of a direct permeation of ATP (and other molecules) through the phospholipid bilayer, which may have important physiological implications.

Lytic Release of Cellular ATP: Physiological Relevance and Therapeutic Applications

The lytic release of ATP due to cell and tissue injury constitutes an important source of extracellular nucleotides and may have physiological and pathophysiological roles by triggering purinergic signalling pathways. In the lungs, extracellular ATP can have protective effects by stimulating surfactant and mucus secretion. However, excessive extracellular ATP levels, such as observed in ventilator-induced lung injury, act as a danger-associated signal that activates NLRP3 inflammasome contributing to lung damage. Here, we discuss examples of lytic release that we have identified in our studies using real-time luciferin-luciferase luminescence imaging of extracellular ATP. In alveolar A549 cells, hypotonic shock-induced ATP release shows rapid lytic and slow-rising non-lytic components. Lytic release originates from the lysis of single fragile cells that could be seen as distinct spikes of ATP-dependent luminescence, but under physiological conditions, its contribution is minimal <1% of total release. By contrast, ATP release from red blood cells results primarily from hemolysis, a physiological mechanism contributing to the regulation of local blood flow in response to tissue hypoxia, mechanical stimulation and temperature changes. Lytic release of cellular ATP may have therapeutic applications, as exemplified by the use of ultrasound and microbubble-stimulated release for enhancing cancer immunotherapy in vivo.

Extracellular Vesicles in chondrogenesis and Cartilage regeneration

Extracellular vesicles (EVs), mainly exosomes and microvesicles, are bilayer lipids containing biologically active information, including nucleic acids and proteins. They are involved in cell communication and signalling, mediating many biological functions including cell growth, migration and proliferation. Recently, EVs have received great attention in the field of tissue engineering and regenerative medicine. Many in vivo and in vitro studies have attempted to evaluate the chondrogenesis potential of these microstructures and their roles in cartilage regeneration. EVs derived from mesenchymal stem cells (MSCs) or chondrocytes have been found to induce chondrocyte proliferation and chondrogenic differentiation of stem cells in vitro. Preclinical studies have shown that exosomes derived from MSCs have promising results in cartilage repair and in cell‐free therapy of osteoarthritis. This review will focus on the in vitro and in vivo chondrogenesis and cartilage regeneration of EVs as well as their potential in the treatment of osteoarthritis.

Pannexin 1, a large-pore membrane channel, contributes to hypotonicity-induced ATP release in Schwann cells

Pannexin 1 (Panx 1), as a large-pore membrane channel, is highly permeable to ATP and other signaling molecules. Previous studies have demonstrated the expression of Panx 1 in the nervous system, including astrocytes, microglia, and neurons. However, the distribution and function of Panx 1 in the peripheral nervous system are not clear. Blocking the function of Panx 1 pharmacologically (carbenoxolone and probenecid) or with small interfering RNA targeting pannexins can greatly reduce hypotonicity-induced ATP release. Treatment of Schwann cells with a Ras homolog family member (Rho) GTPase inhibitor and small interfering RNA targeting Rho or cytoskeleton disrupting agents, such as nocodazole or cytochalasin D, revealed that hypotonicity-induced ATP release depended on intracellular RhoA and the cytoskeleton. These findings suggest that Panx 1 participates in ATP release in Schwann cells by regulating RhoA and the cytoskeleton arrangement. This study was approved by the Animal Ethics Committee of Nantong University, China (No. S20180806-002) on August 5, 2018.

Emerging functions and clinical prospects of connexins and pannexins in melanoma

Cellular communication through gap junctions and hemichannels formed by connexins and through channels made by pannexins allows for metabolic cooperation and control of cellular activity and signalling. These channel proteins have been described to be tumour suppressors that regulate features such as cell death, proliferation and differentiation. However, they display cancer type-dependent and stage-dependent functions and may facilitate tumour progression through junctional and non-junctional pathways. The accumulated knowledge and emerging strategies to target connexins and pannexins are providing novel clinical opportunities for the treatment of cancer. Here, we provide an updated overview of the role of connexins and pannexins in malignant melanoma. We discuss how targeting of these channel proteins may be used to potentiate antitumour effects in therapeutic settings, including through improved immune-mediated tumour elimination.

Vesicular nucleotide transporter mediates adenosine triphosphate release in compressed human periodontal ligament fibroblast cells and participates in tooth movement-induced nociception in rats

OBJECTIVE

Pain control is imperative in orthodontic treatment. Adenosine triphosphate (ATP) is a key mediator released from periodontal ligament cells that excites nociceptive nerve endings. Vesicular nucleotide transporter (VNUT), encoded by the Solute carrier family 17 member 9 (SLC17A9) gene, participates in ATP uptake into secretory vesicles; thus, it may mediate tooth movement-induced pain. In the present study, we examined whether VNUT in periodontal ligament cells participates in tooth movement-induced nociception.

DESIGN

Expression levels of SLC17A9, connexin 43, and pannexin 1 in human periodontal ligament fibroblasts (HPDLFs) were examined by quantitative reverse transcription-polymerase chain reaction. Mechanical force via centrifugation-induced ATP release was measured using an ATP bioluminescence assay. Inhibitors were used to evaluate the role of ATP transporters. Face-grooming behaviors were assessed as indicators of nociceptive responses after experimental tooth movement in rats, as well as the effects of drugs for the pain-like behavior.

RESULTS

After HPDLFs underwent mechanical stimulation by centrifugation, SLC17A9 mRNA expression in the cells was significantly upregulated. Increased ATP release from HPDLFs after mechanical stimulation was suppressed by treatment with clodronic acid, a VNUT inhibitor, at concentrations of 0.1 and 1.0 μM. In rats, face-grooming behaviors (indicators of nociception) were significantly increased on day 1 after experimental tooth movement. Increased face-grooming behaviors were suppressed by systemic administration of clodronic acid (0.1 mg/kg).

CONCLUSIONS

These results indicate that release of ATP from periodontal ligament cells via VNUT is important for nociceptive transduction during orthodontic treatment. Thus, VNUT may provide a novel drug target for tooth movement-induced pain.

Type 2 secretory cells are primary source of ATP release in mechanically stretched lung alveolar cells

Extracellular ATP and its metabolites are potent paracrine modulators of lung alveolar cell function, including surfactant secretion and fluid transport, but the sources and mechanism of intra-alveolar ATP release remain unclear. To determine the contribution of gas-exchanging alveolar type 1 (AT1) and surfactant-secreting type 2 (AT2) cells to stretch-induced ATP release, we used quantitative real-time luminescence ATP imaging and rat primary alveolar cells cultured on silicon substrate for 2-7 days. When cultured on solid support, primary AT2 cells progressively transdifferentiated into AT1-like cells with ~20% of cells showing AT1 phenotype by day 2-3 (AT2:AT1 ≈ 4:1), while on day 7, the AT2:AT1 cell ratio was reversed with up to 80% of the cells displaying characteristics of AT1 cells. Stretch (1 s, 5-35%) induced ATP release from AT2/AT1 cell cultures, and it was highest on days 2 and 3 but declined in older cultures. ATP release tightly correlated with the number of remaining AT2 cells in culture, consistent with ~10-fold lower ATP release by AT1 than AT2 cells. ATP release was unaffected by inhibitors of putative ATP channels carbenoxolone and probenecid but was significantly diminished in cells loaded with calcium chelator BAPTA. These pharmacological modulators had similar effects on stretch-induced intracellular Ca²⁺ responses measured by Fura2 fluorescence. The study revealed that AT2 cells are the primary source of stretch-induced ATP release in heterocellular AT2/AT1 cell cultures, suggesting similar contribution in intact alveoli. Our results support a role for calcium-regulated mechanism but not ATP-conducting channels in ATP release by alveolar epithelial cells.

Wide field of view quantitative imaging of cellular ATP release

Although several mechanical stressors promote ATP secretion from eukaryotic cells, few mechanosensitive pathways for ATP release have been precisely characterized and none have been clearly identified. To facilitate progress, we report here a wide field of view (∼20 × 20 mm sample area) imaging technique paired with a quantitative image analysis to accurately map the dynamics of ATP release from a cell population. The approach has been tested on A549 cells stretched at high initial strain rate (2-5 s^-1) or swelled by hypotonic shock. The amount of ATP secreted in response to a series of five graded stretch pulses (5-37% linear deformation, 1-s duration at 25°C) changed nonmonotonically with respect to strain amplitude and was inhomogeneous across the cell monolayer. In a typical experiment, extracellular ATP density averaged 250 fmol/mm², but the area of detectable signal covered only ∼40% of the cells. In some areas, ATP accumulation peaked around 900 fmol/mm², which corresponded to an estimated concentration of 4.5 µM. The total amount of ATP released from the combined stretch pulses reached 384 ± 224 pmol/million cells (n = 4). Compared with stretch, hypotonic shock (50%, 30°C) elicited a more homogeneous ATP secretion from the entire cell population but at a lower yield totaling 28 ± 12 pmol/million cells (n = 4). The quantitative extracellular ATP mapping of several thousand cells at once, with this wide field of view imaging system, will help identify ATP release pathways by providing unique insights on the dynamics and inhomogeneities of the cellular ATP secretion that are otherwise difficult to assess within the smaller field of view of a microscope.

Sustained Ca2+ mobilizations: A quantitative approach to predict their importance in cell-cell communication and wound healing

Epithelial wound healing requires the coordination of cells to migrate as a unit over the basement membrane after injury. To understand the process of this coordinated movement, it is critical to study the dynamics of cell-cell communication. We developed a method to characterize the injury-induced sustained Ca2+ mobilizations that travel between cells for periods of time up to several hours. These events of communication are concentrated along the wound edge and are reduced in cells further away from the wound. Our goal was to delineate the role and contribution of these sustained mobilizations and using MATLAB analyses, we determined the probability of cell-cell communication events in both in vitro models and ex vivo organ culture models. We demonstrated that the injury response was complex and represented the activation of a number of receptors. In addition, we found that pannexin channels mediated the cell-cell communication and motility. Furthermore, the sustained Ca2+ mobilizations are associated with changes in cell morphology and motility during wound healing. The results demonstrate that both purinoreceptors and pannexins regulate the sustained Ca2+ mobilization necessary for cell-cell communication in wound healing.

Mechanosensitive ATP release in the lungs: New insights from real-time luminescence imaging studies

Extracellular ATP and other nucleotides are important autocrine/paracrine mediators that stimulate purinergic receptors and regulate diverse processes in the normal lungs. They are also associated with pathogenesis of a number of respiratory diseases and clinical complications including acute respiratory distress syndrome and ventilator induced lung injury. Mechanical forces are major stimuli for cellular ATP release but precise mechanisms responsible for this release are still debated. The present review intends to provide the current state of knowledge of the mechanisms of ATP release in the lung. Putative pathways of the release, including the contribution of cell membrane injury and cell lysis are discussed addressing their strength, weaknesses and missing evidence that requires future study. We also provide an overview of the recent technical advances in studying cellular ATP release in vitro and ex vivo. Special attention is given to new insights into lung ATP release obtained with the real-time luminescence ATP imaging. This includes recent data on stretch-induced mechanosensitive ATP release in a model and primary cells of lung alveoli in vitro as well as inflation-induced ATP release in airspaces and pulmonary blood vessels of lungs, ex vivo.

Elevated hydrostatic pressure stimulates ATP release which mediates activation of the NLRP3 inflammasome via P2X4 in rat urothelial cells

Partial bladder outlet obstruction (pBOO) is a prevalent urological condition commonly accompanied by increased intravesical pressure, inflammation, and fibrosis. Studies have demonstrated that pBOO results in increased NLRP3 inflammasome and caspase-1 activation and that ATP is released from urothelial cells in response to elevated pressure. In the present study, we investigated the role of elevated pressure in triggering caspase-1 activation via purinergic receptors activation in urothelial cells. Rat urothelial cell line, MYP3 cells, was subjected to hydrostatic pressures of 15 cmH₂O for 60 min, or 40 cmH₂O for 1 min to simulate elevated storage and voiding pressure conditions, respectively. ATP concentration in the supernatant media and intracellular caspase-1 activity in cell lysates were measured. Pressure experiments were repeated in the presence of antagonists for purinergic receptors to determine the mechanism for pressure-induced caspase-1 activation. Exposure of MYP3 cells to both pressure conditions resulted in an increase in extracellular ATP levels and intracellular caspase-1 activity. Treatment with P2X₇ antagonist led to a decrease in pressure-induced ATP release by MYP3 cells, while P2X₄ antagonist had no effect but both antagonists inhibited pressure-induced caspase-1 activation. Moreover, when MYP3 cells were treated with extracellular ATP (500 µM), P2X₄ antagonist inhibited ATP-induced caspase-1 activation, but not P2X₇ antagonist. We concluded that pressure-induced extracellular ATP in urothelial cells is amplified by P2X₇ receptor activation and ATP-induced-ATP release. The amplified ATP signal then activates P2X₄ receptors, which mediate activation of the caspase-1 inflammatory response.

Search for Upstream Cell Volume Sensors: The Role of Plasma Membrane and Cytoplasmic Hydrogel

The plasma membrane plays a prominent role in the regulation of cell volume by mediating selective transport of extra- and intracellular osmolytes. Recent studies show that upstream sensors of cell volume changes are mainly located within the cytoplasm that displays properties of a hydrogel and not in the plasma membrane. Cell volume changes occurring in anisosmotic medium as well as in isosmotic environment affect properties of cytoplasmic hydrogel that, in turn, trigger rapid regulatory volume increase and decrease (RVI and RVD). The downstream signaling pathways include reorganization of 2D cytoskeleton and altered composition of polyphosphoinositides located on the inner surface of the plasma membrane. In addition to its action on physico-chemical properties of cytoplasmic hydrogel, cell volume changes in anisosmotic conditions affect the ionic strength of the cytoplasm and the [Na⁺]_i/[K⁺]_i ratio. Elevated intracellular ionic strength evoked by long term exposure of cells to hypertonic environment resulted in the activation of TonEBP and augmented expression of genes controlling intracellular organic osmolyte levels. The role of Na⁺_i/K⁺_i -sensitive, Ca²⁺_i -mediated and Ca²⁺_i-independent mechanisms of excitation-transcription coupling in cell volume-adjustment remains unknown.

Vesicular nucleotide transporter mediates ATP release and migration in neutrophils

Neutrophils migrate to sites infected by pathogenic microorganisms. This migration is regulated by neutrophil-secreted ATP, which stimulates neutrophils in an autocrine manner through purinergic receptors on the plasma membrane. Although previous studies have shown that ATP is released through channels at the plasma membrane of the neutrophil, it remains unknown whether it is also released through alternate secretory systems involving vesicular mechanisms. In this study, we investigated the possible involvement of vesicular nucleotide transporter (VNUT), a key molecule for vesicular storage and nucleotide release, in ATP secretion from neutrophils. RT-PCR and Western blotting analysis indicated that VNUT is expressed in mouse neutrophils. Immunohistochemical analysis indicated that VNUT mainly colocalized with matrix metalloproteinase-9 (MMP-9), a marker of tertiary granules, which are secretory organelles. In mouse neutrophils, ATP release was inhibited by clodronate, which is a potent VNUT inhibitor. Furthermore, neutrophils from VNUT^-/- mice did not release ATP and exhibited significantly reduced migration in vitro and in vivo These findings suggest that tertiary granule-localized VNUT is responsible for vesicular ATP release and subsequent neutrophil migration. Thus, these findings suggest an additional mechanism through which ATP is released by neutrophils.

Calcimycin mediates mycobacterial killing by inducing intracellular calcium-regulated autophagy in a P2RX7 dependent manner

Phenotypic screening led to the identification of calcimycin as a potent inhibitor of Mycobacterium bovis BCG (M. bovis BCG) growth in vitro and in THP-1 cells. In the present study, we aim to decipher the mechanism of antimycobacterial activity of calcimycin. We noticed that treatment with calcimycin led to up-regulation of different autophagy markers like Beclin-1, autophagy-related gene (Atg) 7, Atg 3 and enhanced microtubule-associated protein 1A/1B-light chain 3-I (LC3-I) to LC3-II conversion in macrophages. This calcimycin-mediated killing of intracellular M. smegmatis and M. bovis BCG was abrogated in the presence of 3-methyladenine (3-MA). We also demonstrate that calcimycin binding with purinergic receptor P2X7 (P2RX7) led to increase in intracellular calcium level that regulates the extracellular release of ATP. ATP was able to regulate calcimycin-induced autophagy through P2RX7 in an autocrine fashion. Blocking of either P2RX7 expression by 1-[N,O-bis(5-Isoquinolinesulfonyl)-N-methyl-l-tyrosyl]-4-phenylpiperazine (KN-62) or reducing intracellular calcium levels by 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra (acetoxy-methyl) ester (BAPTA-AM) abrogated the antimycobacterial activity of calcimycin. Taken together, these results showed that calcimycin exerts its antimycobacterial effect by regulating intracellular calcium-dependent ATP release that induces autophagy in a P2RX7 dependent manner.

ATP release during cell swelling activates a Ca2+-dependent Cl- current by autocrine mechanism in mouse hippocampal microglia

Microglia cells, resident immune cells of the brain, survey brain parenchyma by dynamically extending and retracting their processes. Cl⁻ channels, activated in the cellular response to stretch/swelling, take part in several functions deeply connected with microglia physiology, including cell shape changes, proliferation, differentiation and migration. However, the molecular identity and functional properties of these Cl⁻ channels are largely unknown. We investigated the properties of swelling-activated currents in microglial from acute hippocampal slices of Cx3cr1^+/GFP mice by whole-cell patch-clamp and imaging techniques. The exposure of cells to a mild hypotonic medium, caused an outward rectifying current, developing in 5–10 minutes and reverting upon stimulus washout. This current, required for microglia ability to extend processes towards a damage signal, was carried mainly by Cl⁻ ions and dependent on intracellular Ca²⁺. Moreover, it involved swelling-induced ATP release. We identified a purine-dependent mechanism, likely constituting an amplification pathway of current activation: under hypotonic conditions, ATP release triggered the Ca²⁺-dependent activation of anionic channels by autocrine purine receptors stimulation. Our study on native microglia describes for the first time the functional properties of stretch/swelling-activated currents, representing a key element in microglia ability to monitor the brain parenchyma.

Methods for cell volume measurement

Volume is an essential characteristic of a cell, and this review describes the main methods of its measurement that have been used in the past several decades. The discussed methods include various implementations of light scattering, estimates based on one or two cell dimensions, surface scanning, fluorescence confocal and transmission slice-by-slice imaging, intracellular volume markers, displacement of extracellular solution, quantitative phase imaging, radioactive methods, and some others. Suitability of these methods to some typical samples and applications is discussed. © 2017 International Society for Advancement of Cytometry.

Cell culture: complications due to mechanical release of ATP and activation of purinoceptors

There is abundant evidence that ATP (adenosine 5′-triphosphate) is released from a variety of cultured cells in response to mechanical stimulation. The release mechanism involved appears to be a combination of vesicular exocytosis and connexin and pannexin hemichannels. Purinergic receptors on cultured cells mediate both short-term purinergic signalling of secretion and long-term (trophic) signalling such as proliferation, migration, differentiation and apoptosis. We aim in this review to bring to the attention of non-purinergic researchers using tissue culture that the release of ATP in response to mechanical stress evoked by the unavoidable movement of the cells acting on functional purinergic receptors on the culture cells is likely to complicate the interpretation of their data.

MSC exosome as a cell-free MSC therapy for cartilage regeneration: Implications for osteoarthritis treatment

Mesenchymal stem cell (MSC) therapies have demonstrated efficacy in cartilage repair in animal and clinical studies. The efficacy of MSC-based therapies which was previously predicated on the chondrogenic potential of MSC is increasingly attributed to the paracrine secretion, particularly exosomes. Exosomes are thought to function primarily as intercellular communication vehicles to transfer bioactive lipids, nucleic acids (mRNAs and microRNAs) and proteins between cells to elicit biological responses in recipient cells. For MSC exosomes, many of these biological responses translated to a therapeutic outcome in injured or diseased cells. Here, we review the current understanding of MSC exosomes, discuss the possible mechanisms of action in cartilage repair within the context of the widely reported immunomodulatory and regenerative potency of MSC exosomes, and provide new perspectives for development of an off-the-shelf and cell-free MSC therapy for treatment of cartilage injuries and osteoarthritis.

Calcium is not required for triggering volume restoration in hypotonically challenged A549 epithelial cells

Maintenance of cell volume is a fundamental housekeeping function in eukaryotic cells. Acute cell swelling activates a regulatory volume decrease (RVD) process with poorly defined volume sensing and intermediate signaling mechanisms. Here, we analyzed the putative role of Ca²⁺ signaling in RVD in single substrate-adherent human lung epithelial A549 cells. Acute cell swelling was induced by perfusion of the flow-through imaging chamber with 50 % hypotonic solution at a defined fluid turnover rate. Changes in cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and cell volume were monitored simultaneously with ratiometric Fura-2 fluorescence and 3D reconstruction of stereoscopic single-cell images, respectively. Hypotonic challenge caused a progressive swelling peaking at ∼20 min and followed, during the next 20 min, by RVD of 60 ± 7 % of the peak volume increase. However, at the rate of swelling used in our experiments, these processes were not accompanied by a measurable increment of [Ca²⁺]_i. Loading with intracellular Ca²⁺ chelator BAPTA slightly delayed peak of swelling but did not prevent RVD in 82 % of cells. Further, electrophysiology whole-cell patch-clamp experiments showed that BAPTA did not block activation of volume-regulated anion channel (VRAC) measured as swelling-induced outwardly rectifying 5-nitro-2-(3-phenylpropyl-amino) benzoic acid sensitive current. Together, our data suggest that intracellular Ca²⁺-mediated signaling is not essential for VRAC activation and subsequent volume restoration in A549 cells.

Real-time imaging of inflation-induced ATP release in the ex vivo rat lung

Extracellular ATP and other nucleotides are important autocrine/paracrine mediators that regulate diverse processes critical for lung function, including mucociliary clearance, surfactant secretion, and local blood flow. Cellular ATP release is mechanosensitive; however, the impact of physical stimuli on ATP release during breathing has never been tested in intact lungs in real time and remains elusive. In this pilot study, we investigated inflation-induced ATP release in rat lungs ex vivo by real-time luciferin-luciferase (LL) bioluminescence imaging coupled with simultaneous infrared tissue imaging to identify ATP-releasing sites. With LL solution introduced into air spaces, brief inflation of such edematous lung (1 s, ∼20 cmH₂O) induced transient (<30 s) ATP release in a limited number of air-inflated alveolar sacs during their recruitment/opening. Released ATP reached concentrations of ∼10^-6 M, relevant for autocrine/paracrine signaling, but it remained spatially restricted to single alveolar sacs or their clusters. ATP release was stimulus dependent: prolonged (100 s) inflation evoked long-lasting ATP release that terminated upon alveoli deflation/derecruitment while cyclic inflation/suction produced cyclic ATP release. With LL introduced into blood vessels, inflation induced transient ATP release in many small patchlike areas the size of alveolar sacs. Findings suggest that inflation induces ATP release in both alveoli and the surrounding blood capillary network; the functional units of ATP release presumably consist of alveolar sacs or their clusters. Our study demonstrates the feasibility of real-time ATP release imaging in ex vivo lungs and provides the first direct evidence of inflation-induced ATP release in lung air spaces and in pulmonary blood capillaries, highlighting the importance of purinergic signaling in lung function.

Actinin-1 binds to the C-terminus of A2B adenosine receptor (A2BAR) and enhances A2BAR cell-surface expression

A2BAR (A2B adenosine receptor) has been implicated in several physiological conditions, such as allergic or inflammatory disorders, vasodilation, cell growth and epithelial electrolyte secretion. For mediating the protein-protein interactions of A2BAR, the receptor's C-terminus is recognized to be crucial. In the present study, we unexpectedly found that two point mutations in the A2BAR C-terminus (F297A and R298A) drastically impaired the expression of A2BAR protein by accelerating its degradation. Thus we tested the hypothesis that these two point mutations disrupt A2BAR's interaction with a protein essential for A2BAR stability. Our results show that both mutations disrupted the interaction of A2BAR with actinin-1, an actin-associated protein. Furthermore, actinin-1 binding stabilized the global and cell-surface expression of A2BAR. By contrast, actinin-4, another non-muscle actinin isoform, did not bind to A2BAR. Thus our findings reveal a previously unidentified regulatory mechanism of A2BAR abundance.

Tension force-induced ATP promotes osteogenesis through P2X7 receptor in osteoblasts

Orthodontic tooth movement induces alveolar bone resorption and formation by mechanical stimuli. Force exerted on the traction side promotes bone formation. Adenosine triphosphate (ATP) is one of the key mediators that respond to bone cells by mechanical stimuli. However, the effect of tension force (TF)-induced ATP on osteogenesis is inadequately understood. Accordingly, we investigated the effect of TF on ATP production and osteogenesis in MC3T3-E1 cells. Cells were incubated in the presence or absence of P2X7 receptor antagonist A438079, and then stimulated with or without cyclic TF (6% or 18%) for a maximum of 24 h using Flexercell Strain Unit 3000. TF significantly increased extracellular ATP release compared to control. Six percent TF had maximum effect on ATP release compared to 18% TF and control. Six percent TF induced the expression of Runx2 and Osterix. Six percent TF also increased the expression of extracellular matrix proteins (ECMPs), ALP activity, and the calcium content in ECM. A438079 blocked the stimulatory effect of 6% TF on the expression of Runx2, Osterix and ECMPs, ALP activity, and calcium content in ECM. This study indicated that TF-induced extracellular ATP is released in osteoblasts, suggesting that TF-induced ATP promotes osteogenesis by autocrine action through P2X7 receptor in osteoblasts. J. Cell. Biochem. 116: 12–21, 2015. © 2014 The Authors. Journal of Cellular Biochemistry published by Wiley Periodicals, Inc.

Purinergic Mechanisms and Pain

There is a brief introductory summary of purinergic signaling involving ATP storage, release, and ectoenzymatic breakdown, and the current classification of receptor subtypes for purines and pyrimidines. The review then describes purinergic mechanosensory transduction involved in visceral, cutaneous, and musculoskeletal nociception and on the roles played by receptor subtypes in neuropathic and inflammatory pain. Multiple purinoceptor subtypes are involved in pain pathways both as an initiator and modulator. Activation of homomeric P2X3 receptors contributes to acute nociception and activation of heteromeric P2X2/3 receptors appears to modulate longer-lasting nociceptive sensitivity associated with nerve injury or chronic inflammation. In neuropathic pain activation of P2X4, P2X7, and P2Y12 receptors on microglia may serve to maintain nociceptive sensitivity through complex neural-glial cell interactions and antagonists to these receptors reduce neuropathic pain. Potential therapeutic approaches involving purinergic mechanisms will be discussed.

ATP release, generation and hydrolysis in exocrine pancreatic duct cells

Extracellular adenosine triphosphate (ATP) regulates pancreatic duct function via P2Y and P2X receptors. It is well known that ATP is released from upstream pancreatic acinar cells. The ATP homeostasis in pancreatic ducts, which secrete bicarbonate-rich fluid, has not yet been examined. First, our aim was to reveal whether pancreatic duct cells release ATP locally and whether they enzymatically modify extracellular nucleotides/sides. Second, we wished to explore which physiological and pathophysiological factors may be important in these processes. Using a human pancreatic duct cell line, Capan-1, and online luminescence measurement, we detected fast ATP release in response to pH changes, bile acid, mechanical stress and hypo-osmotic stress. ATP release following hypo-osmotic stress was sensitive to drugs affecting exocytosis, pannexin-1, connexins, maxi-anion channels and transient receptor potential cation channel subfamily V member 4 (TRPV4) channels, and corresponding transcripts were expressed in duct cells. Direct stimulation of intracellular Ca(2+) and cAMP signalling and ethanol application had negligible effects on ATP release. The released ATP was sequentially dephosphorylated through ecto-nucleoside triphosphate diphosphohydrolase (NTPDase2) and ecto-5'-nucleotidase/CD73 reactions, with respective generation of adenosine diphosphate (ADP) and adenosine and their maintenance in the extracellular medium at basal levels. In addition, Capan-1 cells express counteracting adenylate kinase (AK1) and nucleoside diphosphate kinase (NDPK) enzymes (NME1, 2), which contribute to metabolism and regeneration of extracellular ATP and other nucleotides (ADP, uridine diphosphate (UDP) and uridine triphosphate (UTP)). In conclusion, we illustrate a complex regulation of extracellular purine homeostasis in a pancreatic duct cell model involving: ATP release by several mechanisms and subsequent nucleotide breakdown and ATP regeneration via counteracting nucleotide-inactivating and nucleotide-phosphorylating ecto-enzymes. We suggest that extracellular ATP homeostasis in pancreatic ducts may be important in pancreas physiology and potentially in pancreas pathophysiology.

Heat induces adenosine triphosphate release from mast cells in vitro: a putative mechanism for moxibustion

OBJECTIVE

To investigate the role of adenosine tri-phosphate (ATP) purinergic signaling in mast cells (MCs) modulated by heat to further understand the molecular mechanisms of moxibustion.

METHODS

Skin temperatures induced by monkshood cake moxibustion were evaluated by measuring the Neiguan acupoint (PC 6) from 31 participants with a digital thermocouple thermometer. Temperatures of 43 °C and 52 °C were applied to cultured human leukemia mast cell line HMC-1 in vitro. Calcium fluorescence was applied to detect intracellular Ca2+ ([Ca2+]). Extracellular ATP contents were measured by luciferin-luciferase assay.

RESULTS

Maximum skin temperatures mostly ranged from 40-45 °C , but some reached up to 50 °C. Both 43 °C and 52 °C induced MC degranulation, which was accompanied by an increase in [Ca2+] and ATP release. Complexing extracellular Ca2+ with 5 mM ethylene glycol-bis (β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) inhibited the noxious, heat-induced elevation of [Ca2+]i and prevented the enhanced ATP secretion by those. cells at 52 °C, but not 43 °C.

CONCLUSION

Monkshood cake moxibustion can generate heat sufficient to trigger cellular events of MCs, including degranulation, [Ca2+]i elevation, and ATP release, suggesting that purinergic signals originating from MCs are possibly the initiating response of acupoints to moxibustion.

P2X7 Receptor Mediates Spinal Microglia Activation of Visceral Hyperalgesia in a Rat Model of Chronic Pancreatitis

BACKGROUND & AIMS

Molecular mechanisms underlying the activated spinal microglia in association with the pain in chronic pancreatitis (CP) remain unknown. We tested whether P2X7R on spinal microglia mediates the pathogenesis of visceral pain using a CP rat model.

METHODS

The CP model was induced via intraductal injection of 2% trinitrobenzene sulfonic acid into male Sprague-Dawley rats. Hyperalgesia was assessed based on the mechanical sensitivity to Von-Frey filaments (VFFs), and nocifensive behaviors were measured in response to electrical stimulation of the pancreas. Three weeks after CP induction, spinal cord samples were harvested for immunostaining, immunoblot, and real-time polymerase chain reaction analyses of the P2X7R. Changes in nocifensive behaviors and associated molecular effectors were assessed by blocking spinal cord P2X7R pharmacologically using the selective P2X7R antagonist brilliant blue G (BBG) or genetically using short interfering RNA (siRNA).

RESULTS

CP induced a significant up-regulation of spinal P2X7R expression, which colocalized with a microglial marker (OX-42). Intrathecal administration of BBG significantly attenuated CP-related visceral hyperalgesia in response to VFF-mediated or electrical stimulation of the pancreas, which was associated with suppressed spinal expression of P2X7R and inhibited activation of spinal microglia. Intrathecal injection of siRNA to knock down P2X7R expression in the spinal cord would suppress the nociceptive behaviors in CP rats.

CONCLUSIONS

Spinal microglia P2X7R mediates central sensitization of chronic visceral pain in CP. BBG may represent an effective drug for the treatment of chronic pain in CP patients.

Evidence for Extracellular ATP as a Stress Signal in a Single-Celled Organism

ATP is omnipresent in biology and acts as an extracellular signaling molecule in mammals. Information regarding the signaling function of extracellular ATP in single-celled eukaryotes is lacking. Here, we explore the role of extracellular ATP in cell volume recovery during osmotic swelling in the amoeba Dictyostelium. Release of micromolar ATP could be detected during cell swelling and regulatory cell volume decrease (RVD) phases during hypotonic challenge. Scavenging ATP with apyrase caused profound cell swelling and loss of RVD. Apyrase-induced swelling could be rescued by 100 μM βγ-imidoATP. N-Ethylmalemide (NEM), an inhibitor of vesicular exocytosis, caused heightened cell swelling, loss of RVD, and inhibition of ATP release. Amoebas with impaired contractile vacuole (CV) fusion (drainin knockout [KO] cells) displayed increased swelling but intact ATP release. One hundred micromolar Gd(3+) caused cell swelling while blocking any recovery by βγ-imidoATP. ATP release was 4-fold higher in the presence of Gd(3+). Cell swelling was associated with an increase in intracellular nitric oxide (NO), with NO-scavenging agents causing cell swelling. Swelling-induced NO production was inhibited by both apyrase and Gd(3+), while NO donors rescued apyrase- and Gd(3+)-induced swelling. These data suggest extracellular ATP released during cell swelling is an important signal that elicits RVD. Though the cell surface receptor for ATP in Dictyostelium remains elusive, we suggest ATP operates through a Gd(3+)-sensitive receptor that is coupled with intracellular NO production.

Deciphering physiological role of the mechanosensitive TRPV4 channel in the distal nephron

Long-standing experimental evidence suggests that epithelial cells in the renal tubule are able to sense osmotic and pressure gradients caused by alterations in ultrafiltrate flow by elevating intracellular Ca(2+) concentration. These responses are viewed as critical regulators of a variety of processes ranging from transport of water and solutes to cellular growth and differentiation. A loss in the ability to sense mechanical stimuli has been implicated in numerous pathologies associated with systemic imbalance of electrolytes and to the development of polycystic kidney disease. The molecular mechanisms conferring mechanosensitive properties to epithelial tubular cells involve activation of transient receptor potential (TRP) channels, such as TRPV4, allowing direct Ca(2+) influx to increase intracellular Ca(2+) concentration. In this review, we critically analyze the current evidence about signaling determinants of TRPV4 activation by luminal flow in the distal nephron and discuss how dysfunction of this mechanism contributes to the progression of polycystic kidney disease. We also review the physiological relevance of TRPV4-based mechanosensitivity in controlling flow-dependent K(+) secretion in the distal renal tubule.

Modulation of extracellular ATP content of mast cells and DRG neurons by irradiation: studies on underlying mechanism of low-level-laser therapy

Low-level-laser therapy (LLLT) is an effective complementary treatment, especially for anti-inflammation and wound healing in which dermis or mucus mast cells (MCs) are involved. In periphery, MCs crosstalk with neurons via purinergic signals and participate in various physiological and pathophysiological processes. Whether extracellular ATP, an important purine in purinergic signaling, of MCs and neurons could be modulated by irradiation remains unknown. In this study, effects of red-laser irradiation on extracellular ATP content of MCs and dorsal root ganglia (DRG) neurons were investigated and underlying mechanisms were explored in vitro. Our results show that irradiation led to elevation of extracellular ATP level in the human mast cell line HMC-1 in a dose-dependent manner, which was accompanied by elevation of intracellular ATP content, an indicator for ATP synthesis, together with [Ca²⁺]_i elevation, a trigger signal for exocytotic ATP release. In contrast to MCs, irradiation attenuated the extracellular ATP content of neurons, which could be abolished by ARL 67156, a nonspecific ecto-ATPases inhibitor. Our results suggest that irradiation potentiates extracellular ATP of MCs by promoting ATP synthesis and release and attenuates extracellular ATP of neurons by upregulating ecto-ATPase activity. The opposite responses of these two cell types indicate complex mechanisms underlying LLLT.

Low-intensity pulsed ultrasound-induced ATP increases bone formation via the P2X7 receptor in osteoblast-like MC3T3-E1 cells

Low-intensity pulsed ultrasound (LIPUS) is used for bone healing in orthopedics and dentistry. It has been shown that LIPUS induces the secretion of extracellular adenosine triphosphate (ATP), a key mediator of osteoblast response to mechanical stimuli. However, the detailed mechanism of LIPUS-induced osteogenesis has been elusive. In this study, we investigated the role of the P2X7 receptor in LIPUS-induced osteogenesis. LIPUS induced the release of extracellular ATP, differentiation of osteoblasts and osteogenesis via the P2X7 receptor, without affecting the activity of alkaline phosphatase (ALPase). These results suggest that LIPUS-induced extracellular ATP promotes bone formation via the osteoblast P2X7 receptor independently of ALPase.

Understanding the roles of the P2X7 receptor in solid tumour progression and therapeutic perspectives

P2X7 is an intriguing ionotropic receptor for which the activation by extracellular ATP induces rapid inward cationic currents and intracellular signalling pathways associated with numerous physiological processes such as the induction of the inflammatory cascade, the survival and proliferation of cells. In contrast, long-term stimulation of P2X7 is generally associated with membrane permeabilisation and cell death. Recently, P2X7 has attracted great attention in the cancer field, and particularly in the neoplastic transformation and the progression of solid tumours. A growing number of studies were published; however they often appeared contradictory in their results and conclusions. As such, the involvement of P2X7 in the oncogenic process remains unclear so far. The present review aims to discuss the current knowledge and hypotheses on the involvement of the P2X7 receptor in the development and progression of solid tumours, and highlight the different aspects that require further clarification in order to decipher whether P2X7 could be considered as a cancer biomarker or as a target for pharmacological intervention. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Signalling molecules in the urothelium

The urothelium was long considered to be a silent barrier protecting the body from the toxic effects of urine. However, today a number of dynamic abilities of the urothelium are well recognized, including its ability to act as a sensor of the intravesical environment. During recent years several pathways of these urothelial abilities have been proposed and a major part of these pathways includes release of signalling molecules. It is now evident that the urothelium represents only one part of the sensory web. Urinary bladder signalling is finely tuned machinery of signalling molecules, acting in autocrine and paracrine manner, and their receptors are specifically distributed among different types of cells in the urinary bladder. In the present review the current knowledge of the formation, release, and signalling effects of urothelial acetylcholine, ATP, adenosine, and nitric oxide in health and disease is discussed.

Hemolysis is a primary ATP-release mechanism in human erythrocytes

The hypothesis that regulated ATP release from red blood cells (RBCs) contributes to nitric oxide-dependent control of local blood flow has sparked much interest in underlying release mechanisms. Several stimuli, including shear stress and hypoxia, have been found to induce significant RBC ATP release attributed to activation of ATP-conducting channels. In the present study, we first evaluated different experimental approaches investigating stimulated RBC ATP release and quantifying hemolysis. We then measured ATP and free hemoglobin in each and every RBC supernatant sample to directly assess the contribution of hemolysis to ATP release. Hypotonic shock, shear stress, and hypoxia, but not cyclic adenosine monophosphate agonists, significantly enhanced ATP release. It tightly correlated, however, with free hemoglobin in RBC supernatants, indicating that lysis was responsible for most, if not all, ATP release. Luminescence ATP imaging combined with simultaneous infrared cell imaging showed that ATP was released exclusively from lysing cells with no contribution from intact cells. In summary, with all stimuli tested, we found no evidence of regulated ATP release from intact RBCs other than by cell lysis. Such a release mechanism might be physiologically relevant in vivo, eg, during exercise and hypoxia where intravascular hemolysis, predominantly of senescent cells, is augmented.

TRPV Channels in Mast Cells as a Target for Low-Level-Laser Therapy

Low-level laser irradiation in the visible as well as infrared range is applied to skin for treatment of various diseases. Here we summarize and discuss effects of laser irradiation on mast cells that leads to degranulation of the cells. This process may contribute to initial steps in the final medical effects. We suggest that activation of TRPV channels in the mast cells forms a basis for the underlying mechanisms and that released ATP and histamine may be putative mediators for therapeutic effects.

Involvement of P2Y11 receptor in silica nanoparticles 30-induced IL-6 production by human keratinocytes

We have previously reported that P2Y11 receptor mediates IFN-γ-induced IL-6 production in human keratinocytes, suggesting the importance of purinergic signaling in skin inflammatory diseases. In this study, the involvement of various P2 receptors in IL-6 production induced by silica nanoparticle 30 (SNP30) was examined in a human keratinocyte cell line, HaCaT. Exposure to SNP30 increased IL-6 production in the cells. Ecto-nucleotidase (apyrase), a non-selective antagonist of P2Y receptors (suramin), and a selective P2Y11 receptor antagonist (NF157) all inhibited IL-6 production. Nucleotides such as ATP and UTP themselves also significantly increased IL-6 production in the cells. It was further confirmed that ATP was released from HaCaT cells exposed to SNP30. These results support the possible role of ATP in SNP30-induced IL-6 production by HaCaT cells. In conclusion, these data demonstrate that P2Y11 receptor also mediates SNP30-induced IL-6 production in human keratinocytes, confirming that the ATP-P2Y11 purinergic signaling is a common pathway of IL-6 production leading to induction of skin inflammatory diseases.

Possible contribution of pannexin-1 to ATP release in human upper airway epithelia

Pannexins are a family of transmembrane nonselective channel proteins that participate in the release of ATP into extracellular space. Previous studies have suggested that pannexin-1 (Panx1) may constitute a local autocrine/paracrine system via transmitter ATP in association with the purinergic P2X7 receptor. In this study, we investigate the expressions of Panx1 and P2X7 in human nasal mucosa, together with hypotonic stress-induced ATP release from this tissue. Twenty men and one woman ranging in age from 10 to 82 years with an average age of 44.2 ± 4.4 years participated in the study. Inferior turbinates were collected from patients with chronic hypertrophic rhinitis during endoscopic endonasal surgery. The expressions of Panx1 and P2X7 were examined by fluorescence immunohistochemistry and quantitative reverse transcription-polymerase chain reaction (qRT-PCR). We also examined hypotonic stress-induced ATP release from the turbinate mucosa and the effects of channel blockers in an ex vivo experiment. Substantial expressions of both proteins were observed in human nasal mucosa. The immunoreactivity for Panx1 was stronger than that for P2X7. The presence of the transcripts of Panx1 and P2X7 was also shown by qRT-PCR. Ten and 100 μmol/L carbenoxolone (a Panx1 channel blocker) significantly inhibited the ATP release from the nasal mucosa, but flufenamic acid (a connexin channel blocker) and gadolinium (a stretch-activated channel blocker) did not. These results indicate the coexistence of Panx1 and P2X7 in, and Panx1-dependent ATP release from, the human nasal mucosa, suggesting the possible participation of these molecules in the physiological functions of the upper airway.

Renal epithelial cells can release ATP by vesicular fusion

Renal epithelial cells have the ability to release nucleotides as paracrine factors. In the intercalated cells of the collecting duct, ATP is released by connexin30 (cx30), which is selectively expressed in this cell type. However, ATP is released by virtually all renal epithelia and the aim of the present study was to identify possible alternative nucleotide release pathways in a renal epithelial cell model. We used MDCK (type1) cells to screen for various potential ATP release pathways. In these cells, inhibition of the vesicular H⁺-ATPases (bafilomycin) reduced both the spontaneous and hypotonically (80%)-induced nucleotide release. Interference with vesicular fusion using N-ethylamide markedly reduced the spontaneous nucleotide release, as did interference with trafficking from the endoplasmic reticulum to the Golgi apparatus (brefeldin A1) and vesicular transport (nocodazole). These findings were substantiated using a siRNA directed against SNAP-23, which significantly reduced spontaneous ATP release. Inhibition of pannexin and connexins did not affect the spontaneous ATP release in this cell type, which consists of ~90% principal cells. TIRF-microscopy of either fluorescently-labeled ATP (MANT-ATP) or quinacrine-loaded vesicles, revealed that spontaneous release of single vesicles could be promoted by either hypoosmolality (50%) or ionomycin. This vesicular release decreased the overall cellular fluorescence by 5.8 and 7.6% respectively. In summary, this study supports the notion that spontaneous and induced ATP release can occur via exocytosis in renal epithelial cells.

Real-time luminescence imaging of cellular ATP release

Extracellular ATP and other purines are ubiquitous mediators of local intercellular signaling within the body. While the last two decades have witnessed enormous progress in uncovering and characterizing purinergic receptors and extracellular enzymes controlling purinergic signals, our understanding of the initiating step in this cascade, i.e., ATP release, is still obscure. Imaging of extracellular ATP by luciferin-luciferase bioluminescence offers the advantage of studying ATP release and distribution dynamics in real time. However, low-light signal generated by bioluminescence reactions remains the major obstacle to imaging such rapid processes, imposing substantial constraints on its spatial and temporal resolution. We have developed an improved microscopy system for real-time ATP imaging, which detects ATP-dependent luciferin-luciferase luminescence at ∼10 frames/s, sufficient to follow rapid ATP release with sensitivity of ∼10 nM and dynamic range up to 100 μM. In addition, simultaneous differential interference contrast cell images are acquired with infra-red optics. Our imaging method: (1) identifies ATP-releasing cells or sites, (2) determines absolute ATP concentration and its spreading manner at release sites, and (3) permits analysis of ATP release kinetics from single cells. We provide instrumental details of our approach and give several examples of ATP-release imaging at cellular and tissue levels, to illustrate its potential utility.

ATP release from mast cells by physical stimulation: a putative early step in activation of acupuncture points

In Chinese medicine acupuncture points are treated by physical stimuli to counteract various diseases. These stimuli include mechanical stress as applied during the needle manipulation or tuina, high temperatures as applied during moxibustion, and red laser light applied during laser acupuncture. This study aimed to investigate cellular responses to stimuli that might occur in the tissue of acupuncture points. Since they have a characteristically high density of mast cells that degranulate in response to acupuncture, we asked whether these processes lead to ATP release. We tested in in vitro experiments on mast cells of the human mast-cell line HMC-1 the effects of the physical stimuli; mechanical stress was applied by superfusion of the cells with hypotonic solution, heat was applied by incubation of the cells at 52°C, and red laser light of 657 nm was used for irradiation. We demonstrate that all the stimuli induce ATP release from model human mast HMC-1 cells, and this release is associated with an intracellular free Ca(2+) rise. We hypothesize that ATP released from mast cells supplements the already known release of ATP from keratinocytes and, by acting on P2X receptors, it may serve as initial mediator of acupuncture-induced analgesia.

Kv7/M-type potassium channels in rat skin keratinocytes

Skin keratinocytes fulfil important signalling and protective functions. Immunocytochemical experiments revealed the unexpected presence of immunoreactivity for the M-type potassium channel subunit Kv7.2 in the keratinocyte layer of intact rat paw skin and in keratinocytes isolated from the skin of 1-day-old rats and cultured in vitro for 3–10 days. Application of the M-channel enhancer retigabine (3–10 μM) to isolated cultured rat keratinocytes: (a) increased outward membrane currents recorded under voltage clamp, (b) produced ~3 mV hyperpolarization at rest, (c) enhanced ~3-fold the release of ATP induced by the TRPV3 agonist carvacrol (1 mM) and (d) increased the amplitude of the carvacrol-induced intracellular Ca²⁺ transient measured with Fura-2. The effect of retigabine on ATP release was prevented by the M-channel blocking agent XE991. We conclude that rat skin keratinocytes possess M-channels that, when activated, can modify their physiological properties, with potential significance for their sensory and other biological functions.

Vascular endothelial cells mediate mechanical stimulation-induced enhancement of endothelin hyperalgesia via activation of P2X2/3 receptors on nociceptors

Endothelin-1 (ET-1) is unique among a broad range of hyperalgesic agents in that it induces hyperalgesia in rats that is markedly enhanced by repeated mechanical stimulation at the site of administration. Antagonists to the ET-1 receptors, ET(A) and ET(B), attenuated both initial as well as stimulation-induced enhancement of hyperalgesia (SIEH) by endothelin. However, administering antisense oligodeoxynucleotide to attenuate ET(A) receptor expression on nociceptors attenuated ET-1 hyperalgesia but had no effect on SIEH, suggesting that this is mediated via a non-neuronal cell. Because vascular endothelial cells are both stretch sensitive and express ET(A) and ET(B) receptors, we tested the hypothesis that SIEH is dependent on endothelial cells by impairing vascular endothelial function with octoxynol-9 administration; this procedure eliminated SIEH without attenuating ET-1 hyperalgesia. A role for protein kinase Cε (PKCε), a second messenger implicated in the induction and maintenance of chronic pain, was explored. Intrathecal antisense for PKCε did not inhibit either ET-1 hyperalgesia or SIEH, suggesting no role for neuronal PKCε; however, administration of a PKCε inhibitor at the site of testing selectively attenuated SIEH. Compatible with endothelial cells releasing ATP in response to mechanical stimulation, P2X(2/3) receptor antagonists eliminated SIEH. The endothelium also appears to contribute to hyperalgesia in two ergonomic pain models (eccentric exercise and hindlimb vibration) and in a model of endometriosis. We propose that SIEH is produced by an effect of ET-1 on vascular endothelial cells, sensitizing its release of ATP in response to mechanical stimulation; ATP in turn acts at the nociceptor P2X(2/3) receptor.

On the role of ATP release, ectoATPase activity, and extracellular ADP in the regulatory volume decrease of Huh-7 human hepatoma cells

Hypotonicity triggered in human hepatoma cells (Huh-7) the release of ATP and cell swelling, followed by volume regulatory decrease (RVD). We analyzed how the interaction between those processes modulates cell volume. Cells exposed to hypotonic medium swelled 1.5 times their basal volume. Swelling was followed by 41% RVD(40) (extent of RVD after 40 min of maximum), whereas the concentration of extracellular ATP (ATP(e)) increased 10 times to a maximum value at 15 min. Exogenous apyrase (which removes di- and trinucleotides) did not alter RVD, whereas exogenous Na(+)-K(+)-ATPase (which converts ATP to ADP in the extracellular medium) enhanced RVD(40) by 2.6 times, suggesting that hypotonic treatment alone produced a basal RVD, whereas extracellular ADP activated RVD to achieve complete volume regulation (i.e., RVD(40) ≈100%). Under hypotonicity, addition of 2-(methylthio)adenosine 5'-diphosphate (2MetSADP; ADP analog) increased RVD to the same extent as exposure to Na(+)-K(+)-ATPase and the same analog did not stimulate RVD when coincubated with MRS2211, a blocker of ADP receptor P2Y(13). RT-PCR and Western blot analysis confirmed the presence of P2Y(13). Cells exhibited significant ectoATPase activity, which according to RT-PCR analysis can be assigned to ENTPDase2. Both carbenoxolone, a blocker of conductive ATP release, and brefeldin A, an inhibitor of exocytosis, were able to partially decrease ATP(e) accumulation, pointing to the presence of at least two mechanisms for ATP release. Thus, in Huh-7 cells, hypotonic treatment triggered the release of ATP. Conversion of ATP(e) to ADP(e) by ENTPDase 2 activity facilitates the accumulated ADP(e) to activate P2Y(13) receptors, which mediate complete RVD.

Purinergic signalling - a possible mechanism for KCNQ1 channel response to cell volume challenges

AIM

A number of K(+) channels are regulated by small, fast changes in cell volume. The mechanisms underlying cell volume sensitivity are not known, but one possible mechanism could be purinergic signalling. Volume activated ATP release could trigger signalling pathways that subsequently lead to ion channel stimulation and cell volume back-regulation. Our aim was to investigate whether volume sensitivity of the voltage-gated K(+) channel, KCNQ1, is dependent on ATP release and regulation by purinergic signalling.

METHODS

We used Xenopus oocytes heterologously expressing human KCNQ1, KCNE1, water channels (AQP1) and P2Y2 receptors. ATP release was monitored by a luciferin-luciferase assay and ion channel conductance was recorded by two-electrode voltage clamp.

RESULTS

The luminescence assay showed that oocytes released ATP in response to mechanical, hypoosmotic stimuli and hyperosmotic stimuli. Basal ATP release was approx. three times higher in the KCNQ1 + AQP1 and KCNQ1 injected oocytes compared to the non-injected ones. Exogenously added ATP (0.1 mm) did not have any substantial effect on volume-induced KCNQ1 currents. Nevertheless, apyrase decreased all currents by about 50%. Suramin inhibited about 23% of the KCNQ1 volume sensitivity. Expression of P2Y2 receptors stimulated endogenous Cl(-) channels, but it also led to 68% inhibition of the KCNQ1 currents. Adenosine (0.1 mm) also inhibited the KCNQ1 currents by about 56%.

CONCLUSION

Xenopus oocytes release ATP in response to mechanical stimuli and cell volume changes. Purinergic P2 and P1 receptors confer some of the KCNQ1 channel volume sensitivity, although endogenous adenosine receptors and expressed P2Y2 receptors do so in the negative direction.

Involvement of P2Y₁₁ receptor in IFN-γ-induced IL-6 production in human keratinocytes

Extracellular ATP and P2 receptors are reported to be involved in interleukin-6 (IL-6) production by human keratinocytes, but the role of extracellular ATP in cytokine-induced IL-6 production remains unclear. In this study, we investigated the involvement of various P2 receptors in IL-6 production induced by the Th1 cytokine interferon-gamma (IFN-γ) in a human keratinocyte cell line, HaCaT. IFN-γ increased IL-6 production in HaCaT cells. A non-selective antagonist of P2Y receptors (suramin), a selective P2Y11 receptor antagonist (NF157), ecto-nucleotidase (apyrase), and a soluble adenylate cyclase inhibitor (KH7) all inhibited IL-6 production. It was further confirmed that ATP was released from HaCaT cells stimulated with IFN-γ. These results suggest that extracellular ATP and P2Y11 receptor are involved in IFN-γ-induced IL-6 production. Knockdown of P2Y11 receptor suppressed IL-6 production, strongly supporting this idea. In conclusion, these data demonstrate that P2Y11 receptor mediates IFN-γ-induced IL-6 production in human keratinocytes, and suggest the importance of purinergic signaling in IFN-γ-induced skin inflammatory conditions, such as psoriasis.

Imaging and characterization of stretch-induced ATP release from alveolar A549 cells

Abstract  Mechano-transduction at cellular and tissue levels often involves ATP release and activation of the purinergic signalling cascade. In the lungs, stretch is an important physical stimulus but its impact on ATP release, the underlying release mechanisms and transduction pathways are poorly understood. Here, we investigated the effect of unidirectional stretch on ATP release from human alveolar A549 cells by real-time luciferin-luciferase bioluminescence imaging coupled with simultaneous infrared imaging, to monitor the extent of cell stretch and to identify ATP releasing cells. In subconfluent (<90%) cell cultures, single 1 s stretch (10-40%)-induced transient ATP release from a small fraction (1.5%) of cells that grew in number dose-dependently with increasing extent of stretch. ATP concentration in the proximity (150 μm) of releasing cells often exceeded 10 μm, sufficient for autocrine/paracrine purinoreceptor stimulation of neighbouring cells. ATP release responses were insensitive to the putative ATP channel blockers carbenoxolone and 5-nitro-2-(3-phenylpropyl-amino) benzoic acid, but were inhibited by N-ethylmaleimide and bafilomycin. In confluent cell cultures, the maximal fraction of responding cells dropped to <0.2%, but was enhanced several-fold in the wound/scratch area after it was repopulated by new cells during the healing process. Fluo8 fluorescence experiments revealed two types of stretch-induced intracellular Ca(2+) responses, rapid sustained Ca(2+) elevations in a limited number of cells and delayed secondary responses in neighbouring cells, seen as Ca(2+) waves whose propagation was consistent with extracellular diffusion of released ATP. Our experiments revealed that a single >10% stretch was sufficient to initiate intercellular purinergic signalling in alveolar cells, which may contribute to the regulation of surfactant secretion and wound healing.

Primary cilium-dependent sensing of urinary flow and paracrine purinergic signaling

During the last 10 years or so, the renal research community has set the primary cilium into the lime light. From being viewed as a possible evolutionary rudiment, today the primary cilium has achieved the noble status of a physiologically relevant and necessary cellular structure. Its prime function in renal epithelium appears to be its ability to sense urinary flow. Much is still lacking to understand how the primary cilium senses flow. Transducer proteins, such as specific mechano-sensory ion channels, have been identified and are necessary for flow-dependent increases of epithelial [Ca(2+)](i). Other ciliary receptor proteins have been suggested, which may open the field of primary cilia sensing to become an even more dynamic topic of research. A flow-induced increase of [Ca(2+)](i) has been observed in all renal and other ciliated epithelial cells. Work over the last 5 years has addressed the mechanism underlying the flow-induced increase of [Ca(2+)](i). It has become apparent that an initial Ca(2+) influx triggers a global increase of epithelial [Ca(2+)](i). Eventually, it also became clear that mechanical stimulation of the epithelial cells triggers the release of ATP. Intriguingly, ATP is an auto- and paracrine signaling molecule that regulates electrolyte and water transport in the nephron by binding to apical and basolateral purinergic receptors. ATP inhibits transport at almost all sites from the proximal to the distal tubule and thus elicits a diuretic response. In the perspective of this review, the primary cilium is a sensory structure and the adequate stimulus is the mechanical deflection. The output signal is the released ATP, a paracrine factor that ultimately modulates the main function of the kidney, i.e. the enormous task of absorbing some 180 L of filtrate every day.