Identification of a P2X7 Receptor in [GH.sub.4][C.sub.1] Rat Pituitary Cells: A Potential Target for a Bioactive Substance Produced by Pfiesteria piscicida.
A biologic activity isolated from toxic Pfiesteria piscicida cultures has been determined to activitate c-fos luciferase in [GH.sub.4][C.sub.1] cells (1). P. piscicida is a heterotrophic estuarine dinoflagellate discovered in 1991 by Burkholder and co-workers (2); it has been implicated as the causative agent of major fish kills and fish disease in the two largest U.S. mainland estuaries (the Albemarle-Pamlico of North Carolina and Chesapeake Bay in Maryland and Virginia) (3). P. piscicida was first implicated as hazardous to human health following accidental exposure of laboratory workers (4). During 1993-1995, environmental exposures were anecdotally reported in North Carolina estuaries (4, 5), and in 1997 the first clinical evaluations of people shortly after environmental exposure to P. piscicida blooms were completed in Maryland (6). People who had contact with toxic P. piscicida waters or with potential toxic aerosols reported symptoms including dermal lesions and rashes, a burning sensation on contact with water, fatigue, respiratory irritation, diarrhea, severe headaches, and a neurologic syndrome characterized by learning disabilities manifested as short-term memory dysfunction and other cognitive impairment (4, 6).
[GH.sub.4][C.sub.1] cells are a rat pituitary cell line that has been used to characterize signaling pathways for a variety of first messengers (7). They have also proven useful for the investigation of algal-derived toxins including maitotoxin (8, 9) and a biologic activity produced by Pfiesteria piscicida (1). Each also caused an increase in ionic conductances and an elevation of cytosolic free calcium (9, 10). Downstream events include activation of c-fos luciferase and cytotoxicity (1, 11, 12). In vitro methods for characterization of algal-derived toxins have relied largely upon functional assays that include receptor-based assays and cell-based toxicity assays (13). Cell based assays can be further modified by changing the end point from the mitochondrial indicator for toxicity (MTT dye-based assay) to specific gene induction (11). These assays, known as reporter gene assays, use responsive cell lines that stably express reporter gene constructs.
In this study we investigated the signaling pathways that elicit the reporter gene response in [GH.sub.4][C.sub.1] cells using adenosine-5'-triphosphate (ATP) as a model compound. We identified ATP as a novel first messenger for [GH.sub.4][C.sub.1] cells that induces c-fos luciferase and cytotoxicity. Using a reporter gene assay, we conducted initial characterization of the ATP receptor through analog specificity studies using P2X receptor agonists and antagonists with differing receptor subtype selectivity. We then used two classes of P2X antagonists to examine whether putative P. piscicida toxin (pPfTx) activates a P2X pathway in these cells.
Materials and Methods
Stock cultures of stably transfected rat pituitary cells ([GH.sub.4][C.sub.1]) were maintained in Ham's F10 medium supplemented with 15% horse serum, 2.5% fetal bovine serum (FBS), and 200 [micro]g/mL neomycin antibiotic (G418; Gibco Life Technologies, Grand Island, NY). Cultures were incubated at 37 [degrees] C with 5% [CO.sub.2] and 95% air. [GH.sub.4][C.sub.1] stable transfectants were obtained by cotransfecting plasmids c-fos-luc and pSV2-neo (Richard N. Day, University of Virginia, Charlottesville, VA), as previously described (1). We purchased rabbit anti-rat P2X7 receptor antibody from Alomone Labs, Ltd., (Jerusalem, Israel).
Toxin isolation. We used an actively growing, fish-killing culture of P. piscicida for toxin isolation. Using methods described previously (14), we isolated the cultures from a fish kill and toxic P. piscicida outbreak in the Neuse Estuary in North Carolina using fish bioassays and water samples taken from the in-progress kill. P. piscicida from the fish-killing bioassays was cloned and grown with algal prey under axenic conditions (but with bacterial endosymbionts retained in the P. piscicida zoospores) (14, 15). Following Koch's postulates modified for toxic rather than infectious agents, the axenic clonal P. piscicida culture (with bacterial endosymbionts) and residual benign algal prey ([is less than] 5 cryptomonads/mL were added to cultures of healthy fish (n = 4). Control fish cultures were treated identically, except that they received similar addition of only residual cryptomonad culture without P. piscicida (n = 4, with each replicate containing three tilapia (Oreochromis mossambica). Fish death occurred and was repeated as additional live fish were added to the cultures of P. piscicida. In contrast, control fish, which had been maintained identically but with addition of benign algal prey and not P. piscicida, remained healthy (14).
We identified P. piscicida to species at three levels of isolation: from the fish bioassays of water collected from the Neuse fish kill, from the clonal isolate grown with algal prey, and from the subsequent mass-culture with fish. Species identification was completed from analysis of suture-swollen zoospore cells by scanning electron microscopy (15). Following standard procedure in the Burkholder/Glasgow laboratory, the species identifications were then cross-confirmed by three independent laboratories. Molecular probe analyses was conducted by D. Oldach [heteroduplex mobility assay to verify both the species identification and uni-dinoflagellate culture status (16)] and P. Rublee [fluorescent in situ hybridization rDNA probe for P. piscicida (17)]. Scanning electron microscopy was conducted by K. Steidinger and co-workers (Florida Fish and Wildlife Conservation Commission Florida Marine Research Institute, St. Petersburg, FL) and by H. Marshall and D. Seaborn (Old Dominion University, Norfolk, VA).
We mass-cultured the toxic P. piscicida isolate with live tilapia in a biohazard III facility (14, 15). They were maintained in 15-psu (practical salinity unit) sterile-filtered seawater (water source 8 km off Beaufort, NC, diluted using deionized water), or in 15-psu water made using Instant Ocean salts (Aquarium Systems, Mentor, OH) and deionized water. Toxic samples were taken from cultures that were actively killing tilapia at the time of collection. The toxic seawater medium was passed through a preparative C18 column and flushed with fresh water to remove the excess salts. The toxic material was then eluted from the column with 100% methanol. This methanol elutant was concentrated and passed through a silica gel column using an elutropic scheme of increasing polarity. We screened fractions for cytotoxicity and reporter gene activity using [GH.sub.4][C.sub.1] cells as previously described (1). The greatest P. piscicida activity was found to elute in the later, more polar fractions. These fractions were evaporated to dryness and then were placed under high vacuum to remove any remaining organic solvents that would interfere with the bioassays. The dry residue was taken up in standard volumes of methanol as the carrier solvent for further analysis. The active fraction was determined not to contain ATP by difference in chromatographic retention and by lack of ATP activity using an ATP-dependent in vitro luciferase assay, described below under "ATP Assay." Because the chemical structure of the bioactive substance has not yet been determined in the absence of a sufficient quantity of toxic culture to enable purification, the bioactive substance is referred to here as putative P. piscicida toxin (pPfTx).
Reporter gene assay. [GH.sub.4][C.sub.1] c-fos-luc cells were seeded in a 96-well clear-bottom white plate (Corning Costar, Cambridge, MA) at a density of 30,000 cells/well in 100 mL culture media and allowed to incubate overnight to ensure cell attachment. Cells treated with pPfTx were incubated for 4 hr and those with ATP were incubated for 10 hr. All incubations were performed at 37 [degrees] C with 5% [CO.sub.2] and 95% air. In experiments where oxidized-ATP (oxATP) was used, pretreatment of one group of wells with 400 mM oxATP was initiated 1 hr before cell treatment with increasing concentrations of either ATP, 2' ,3'-(4-benzoyl)benzoyl ATP (BzATP) or pPfTx. After incubation, the experimental media was removed and 20 mL cell lysis buffer [1% Triton X- 100, 5 mM Tris, 0.4 mM trans-1, 2-diaminocyclohexane-N, N, N', N'-tetraacetic acid (CDTA), 10% glycerol, pH 7.8, and 1 mM dithiothreitol (DTT)] was added to each well. Lysis was allowed to proceed at room temperature for 20 min; we then measured solubilized luciferase protein activity using a luminometer (LumiStar; BMG LabTechnologies, Durham, NC). The luminometer was programmed to inject each well with 20 [micro]L of Luciferase Assay Reagent (Promega, Madison, WI), and read the luminescence generated for 10 sec.
Immunostaining. We performed immunostaining for P2X7 receptors using cell homogenates of [GH.sub.4][C.sub.1] cells on Western transfers. [GH.sub.4][C.sub.1] cells were removed from 100-mm dishes with PBS-EDTA and resuspended in PBS containing protease inhibitors (4 [micro]M phenylmethylsulfonyl fluoride and 2 [micro]g/mL each of pepstatin, leupeptin, trypsin inihibitor, and aprotinin). Cells were lysed by freeze-thawing and then sonicated at 50 W, three pulses of 20 sec on ice. The lysates were centrifuged at 10,000 x g for 20 min, and supernatants were separated by 7.5% SDS-polyacrylamide gel electrophoresis. Separated proteins were then transferred to nitrocellulose and incubated with 1:200 rabbit anti-rat P2X7 antibody (18). Transfers were washed in TBS 0.1% Triton-X100 between antibody incubations. The detection was electrogenerated chemiluminscence according to the manufacturer (Amersham, Buckinghamshire, UK) for 5 min. The transfers were then exposed to Hyperfilm-ECL (Sigma, St. Louis, MO) for 60 sec and developed with an X-ray processor. The corresponding blocking peptide (P2X7 576-595 peptide) at 10 [micro]g/mL was incubated with the same antibody solution for 1 hr at 23 [degrees] C. Transfers were then probed and developed as described above.
ATP assay. We used the ATP Bioluminescent Assay Kit (Sigma, St. Louis, MO) to determine the amount of ATP present in pPfTx samples. The assay kit contained ATP Standard (2.0 [micro]mol ATP), ATP Assay Mix Dilution Buffer (Mg[SO.sub.4], DTT, EDTA, bovine serum albumin, tricine buffer salts), and ATP Assay Mix (luciferase, luciferin, Mg[SO.sub.4], DTT, EDTA, bovine serum albumin, tricine buffer salts). We made serial dilutions from the ATP Standard after it had been diluted to a concentration of 40 [micro]M in double-distilled [H.sub.2]O. We plated 40 [micro]L of each serial dilution in triplicate in a 96-well plate, pPfTx was plated in triplicate alongside the ATP Standard. We added 40 [micro]L ATP Assay Mix diluted 1:25 with ATP Assay Dilution Buffer to each well; the luminescence was generated by the catalyzing activity and measured by a luminometer.
Effect of ATP and pPfTx on c-fos luciferase. We examined ATP for its ability to mimic the action of pPfTx in [GH.sub.4][C.sub.1] cells. A biphasic luciferase response was generated from the induction of the [GH.sub.4][C.sub.1] cells with increasing concentrations of ATP (Figure 1). We observed a half-maximal effect at 30 mM, with a maximal effect occurring at 200 mM. Concentrations of ATP that exceeded 200 mM caused a concentration-dependent inhibition of c-fos luciferase. This decrease is associated with cytotoxicity as determined by MTT cytotoxicity assay. A similar biphasic c-fos luciferase response was generated by the addition of serial dilutions of pPfTx to [GH.sub.4][C.sub.1] cells (Figure 2). These results indicate that pPfTx mimics the action of ATP to induce c-fos luciferase and cytoxtoxicity in [GH.sub.4][C.sub.1] cells and lead us to conduct preliminary characterization of the ATP receptor on [GH.sub.4][C.sub.1] cells.
Analog characterization of the ATP receptor in [GH.sub.4][C.sub.1] cells. The preliminary characterization of the ATP receptor was determined by conducting analog selectivity studies. The primary analogs tested that were effective in this study are shown in Figure 3. We first tested the moderately selective P2 antagonist, pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid (PPADS). PPADS caused concentration-dependent inhibition of c-fos luciferase in the presence and absence of added ATP (Figure 4). The inhibition of c-fos luciferase by PPADS was not associated with cytotoxicity. We next examined the P2X1 and P2X3 subtype selective agonist [Alpha], [Beta]-methyleneadenosine 5'-triphosphate ([Alpha], [Beta]-MeATP). [Alpha], [Beta]-MeATP failed to increase or decrease c-fos luciferase activity (Figure 5). Taken together, these results indicate that if P2X receptors mediate the effects of ATP on c-fos luciferase in [GH.sub.4][C.sub.1] cells, the receptor is not of the P2X1 or P2X3 subtype.
We next examined a second antagonist, oxATP, which is an irreversible P2X antagonist with moderate selectivity for P2X7 receptors. Pretreatment with 400 [micro]m oxATP inhibited the majority of the effect of ATP to increase c-fos luciferase, and fully inhibited the effects of ATP to decrease luciferase (Figure 6). oxATP, unlike PPADS, did not decrease c-fos luciferase activity. We next tested the action of an agonist, BzATP, that shows selectivity for P2X7 receptors. BzATP did not increase c-fos luciferase activity, but it caused concentration-dependent inhibition of c-fos luciferase activity (Figure 7). The half-maximal effect of BzATP was nearly 10 times lower than the half-maximal effect of ATP to inhibit c-fos luciferase. This is consistent with an action on P2X7 subtype purinoreceptors. The failure of BzATP to induce c-fos luciferase was unexpected. It is possible that BzATP only affected the second component of the biphasic response (i.e., cytotoxicty but not induction of the reporter gene). An alternative possibility is that BzATP induces both responses but has greater efficacy for cytotoxicity. We addressed this question by testing BzATP at the shorter incubation period of 4 hr and found that BzATP did cause a biphasic response (data not shown). We also examined the effect of oxATP on the action of BzATP. Pretreatment of 400 mM oxATP fully inhibited the effect of BzATP to decrease luciferase (Figure 7), which is consistent with an effect mediated by purinogenic receptors of the P2X7 class.
Identification of the P2X7 receptor by immunoblotting. We examined the presence of the P2X7 receptor by immunostaining Western transfers of [GH.sub.4][C.sub.1] cell membranes. [GH.sub.4][C.sub.1] cells expressed an approximate 70-kDa band that was immunoreactive to a rabbit antibody directed to the intracellular carboxyl terminal domain, unique to the P2X7 class of purinergic receptors (Figure 8). We examined the specificity of the staining of the 70-kDa band by preabsorption of the primary antiserum with 10 [micro]g/mL of the carboxyl terminal peptide sequence 575-595. No immunostaining of the 70-kDa band was evident under matched conditions (data not shown). This result provides an additional line of evidence for the presence of P2X7 receptors on [GH.sub.4][C.sub.1] cells.
Testing pPfTx for ATP activity. We examined the role of P2X7 receptors in the action of the pPfTx. We sought to determine whether the pPfTx contained any activity attributable to ATP. We used an ATP-dependent luciferase assay to quantify ATP. The sensitivity of the assay was 40 nM; 4 [micro]m ATP generated a 100-fold increase in response (Figure 9). pPfTx, given in an amount that caused a maximal induction of c-fos luciferase in the reporter gene assay, failed to mimic any effect of ATP to activate the luciferase enzyme directly. The pPfTx contained [is less than] 40 nM ATP, but by the reporter gene assay, it contained 200 [micro]m ATP equivalents, indicating that the effect of pPfTx in the reporter gene assay is not attributable to ATP.
Effect of P2X antagonists on pPfTx induction of c-fos luciferase. We used two P2 antagonists of differing selectivity to examine the role of P2X receptors in the action of pPfTx to induce c-fos luciferase and cytotoxicity. PPADS, a P2 agonist, given at 200 [micro]m inhibited both the activity of pPfTx and ATP (Figure 10). oxATP, an irreversible P2X antagonist that has selectivity for P2X7 receptors, was added at a concentration of 400 [micro]m as a pretreatment 1 hr before the addition of increasing concentrations of both ATP and pPfTx (Figure 11). oxATP inhibited the luciferase induction of both ATP and pPfTx, suggesting that both substances induce c-fos luciferase and cytotoxicity by a common mechanism involving a P2X7 subtype receptor.
ATP has a dual role as both an energy source for enzymatic reactions and as a first messenger for several classes of G protein-coupled receptors and ligand-gated ion channels. ATP was found to induce c-fos luciferase in [GH.sub.4][C.sub.1] cells with a characteristic biphasic response, pPfTx also induced c-fos luciferase in a similar manner. These results suggest that the pPfTx is either ATP or an ATP agonist. We examined whether pPfTx was ATP using two lines of evidence. The first was that ATP and pPfTx do not share common chromatographic retention properties (data not shown). The second was that pPfTx could not mimic the action of ATP to catalyze isolated luciferase enzyme in the presence of cofactors. Taken together, these results indicate that pPfTx is not ATP, but rather an ATP agonist.
ATP activates receptors of the purinogenic P2 class (19). P2 receptors are divided into two classes--P2X and P2Y--based on molecular structure. P2X receptors are ATP-activated ion channels, and P2Y receptors are ATP-activated G protein-coupled receptors. We began by examining analog selectivity for induction of c-fos luciferase by ATP using P2 agonists and antagonists of differing selectivity. PPADS, a P2 antagonist of moderate selectivity for P2X receptors (19), caused concentration-dependent inhibition of ATP induction of c-fos luciferase in [GH.sub.4][C.sub.1] cells with a half-maximal effect (100 [micro]M), which is consistent with an action on P2 receptors. Further examination with [Alpha], [Beta]-methylene ATP ([Alpha], [Beta]-MeATP), both a P2X1- and P2X3-selective ATP agonist (19, 20), indicated that [Alpha], [Beta]-MeATP did not affect c-fos luciferase, providing additional evidence that the pathway of activation was neither a P2X1 or P2X3 receptor. A second antagonist, oxATP, which is irreversible and more selective for P2X receptors (21), inhibited nearly fully the actions of ATP on c-fos luciferase in [GH.sub.4][C.sub.1] cells. Because oxATP has been reported to have some degree of selectivity for P2X7 receptors, we tested an agonist, BzATP, which has selectivity for P2X7 receptors (21-23). BzATP failed to increase c-fos luciferase in [GH.sub.4][C.sub.1] cells, but it did cause a concentration-dependent inhibition of luciferase activity. BzATP was nearly 10 times more potent at inhibiting c-fos luciferase than ATP. BzATP induces greater maximal ion conductance than ATP in cells expressing P2X7 receptors; this may be the basis for the greater efficacy of BzATP for cytotoxicity (21). The action of BzATP was fully inhibited by oxATP. Taken together, these analog selectivity experiments with BzATP and oxATP are consistent with the presence of P2X7 receptors on [GH.sub.4][C.sub.1] cells. The presence of P2X7 receptors on [GH.sub.4][C.sub.1] cells was additionally supported by the immunostaining of Western transfers of [GH.sub.4][C.sub.1] cell membranes using an antibody specific to the unique carboxy terminal domain of the P2X7 receptor.
ATP-activated ion channel receptors were originally identified in mast cells (24) and later found in other myeloid-derived cells, including macrophages and microglia (25, 26). These channels, designated P2Z, were subsequently found to be of the P2X class and renamed P2X7 (27). Although the P2X7 receptor subtype is found largely in cells of immune origin, it has also been identified in cell lines and primary cell cultures of nonimmune origin (18, 28, 29). The role for a P2X7 receptor in [GH.sub.4][C.sub.1] cells has not been determined; however, it is well known that this cell lineage (growth hormone-producing cells) mediates local inflammatory responses in the pituitary gland and may modulate the hypothalamic pituitary axis during systemic inflammatory reactions (30).
The antagonists PPADS and oxATP, which we used to characterize the P2X receptor in [GH.sub.4][C.sub.1] cells, were also useful in examining the action of pPfTx. The pathway(s) leading to the biphasic effect on c-fos luciferase of pPfTx in [GH.sub.4][C.sub.1] cells appears to be mediated by the same receptor that mediates the response to ATP. Both PPADS and oxATP inhibited pPfTx induction of c-fos luciferase. These results are consistent with pPfTx acting as a P2X7 receptor agonist. Although these results do not prove that P2X7 is the initial cellular target for the putative toxin, they do indicate that this receptor is necessary in the signal transduction pathway. At this point it is not possible to exclude effects of pPfTx on additional receptors, including other P2X receptor subtypes. This may be most readily determined using expression systems for various cloned receptors.
It remains to be determined whether the P2X7 agonist activity isolated from P. piscicida is responsible for the wildlife effects associated with this organism. Macrophages and mast cells express P2X7 receptors, which have been suggested to have a role in inflammation. The entry of monocytes into peripherial tissues precedes their differentiation into activated macrophages, a process that involves the action of interferon-[Gamma], which in turn leads to expression of P2X7 receptors (31). In activated macrophages, P2X7 receptors mediate chronic inflammatory responses normally driven by ATP. The responses include fusion of macrophages into multinucleated giant cells and several inflammatory responses that result from production of interleukin 1-[Beta], including release of prostaglandins, production of matrix, and chemoattraction of neutophils (32-34). These responses are characteristic of granulatomatous lesions found in fish that are associated with toxic P. piscicida (9, 35). Because P. piscicida has the capacity to phagocytize blood cells and cause tissue injury (2), it may initiate an acute inflammation that is potentiated to a chronic response by pPfTx, behaving as a potent ATP mimic at P2X7 receptors on activated macrophages.
Whether the P2X7 agonist activity isolated from P. piscicida contributes to the human neurocognitive effects associated with this organism is less obvious. P2X7 receptors in the central nervous system have been best characterized in microglia (31, 36). Microglia are the central nervous system counterpart to tissue macrophages and they normally provide a defensive inflammatory response to infections and tissue damage (37). However, inappropriate activation of microglia can elicit neurotoxic effects that may include release of excitotoxic amino acids and cytolytic and inflammatory agents (37). One approach used to study the effects of Pfiesteria on neurocognitive impairment is a rat model using radial arm-maze testing (38).
Our results indicate that the cytotoxic effect originally described for a putative P. piscicida toxin is mediated by a P2X7 receptor. Based on the current understanding of the role of P2X7 receptors in disease and the observed effects directly attributable to exposure to P. piscicida toxins, P2X7 receptor-mediated chronic inflammation may provide a basis to better understand the animal and human toxicity associated with this organism.
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Karen L. Kimm-Brinson, (1) Peter D. R. Moeller,(1) Michele Barbier,(1) Howard Glasgow Jr.,(2) JoAnn M. Burkholder,(2) and John S. Ramsdell(1)
(1) Marine Biotoxins Program, Center for Coastal Environmental Health and Biomolecular Research, National Oceanic and Atmospheric Administration-National Ocean Service, Charleston, South Carolina, USA; (2)Department of Botany, North Carolina State University, Raleigh, North Carolina, USA
Address correspondence to J. S. Ramsdell, Chief, Coastal Research Branch Center for Coastal Environmental Health and Biomolecular Research, NOAA-National Ocean Service, 219 Fort Johnson Road, Charleston, SC 29412 USA. Telephone: (843) 762-8510. Fax: (843) 762-8700. E-mail: firstname.lastname@example.org
This work was funded by the National Oceanic and Atmospheric Administration (NOAA-NOS) and by the North Carolina General Assembly, the Z. Smith Reynolds Foundation, and an anonymous foundation (grants to coauthors J.M. Burkholder and H. Glasgow). The National Ocean Service (NOS) does not approve, recommend, or endorse any proprietary product or material mentioned in this publication. No reference shall be made to NOS, or to this publication furnished by NOS, in any advertising or sales promotion that would indicate or imply that NOS approves, recommends, or endorses any proprietary product or proprietary material mentioned herein or that has as its purpose any intent to cause directly or indirectly the advertised product to be used or purchased because of NOS publication.
Received 1 September 2000; accepted 14 November 2000.
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|Author:||Ramsdell, John S.|
|Publication:||Environmental Health Perspectives|
|Date:||May 1, 2001|
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