Cystic fibrosis transmembrane conductance regulator in teleost fish

The gills and intestinal epithelia of teleost fish express cystic fibrosis transmembrane conductance regulator (CFTR), and utilize this low conductance anion channel in the apical membrane for ion secretion in seawater gill and in the basolateral membrane for ion absorption in freshwater gill. Similarly, in the intestine CFTR is present in the basolateral membrane for intestinal absorption and also in the apical membrane of secreting intestine. The expression of CFTR and the directed trafficking of the protein to the apical or basolateral membrane is salinity-dependent. The CFTR gene has been cloned and sequenced from several teleost species and although all the major elements in the human gene are present, including two nucleotide binding domains that are common to all ATP binding cassette (ABC) transporters, the sequences are divergent compared to shark or human. In euryhaline fish adapting to seawater, CFTR, localized immunocytochemically, redistributes slowly from a basolateral location to the apical membrane while ion secretory capacity increases. The facility with which teleosts regulate CFTR expression and activation during salinity adaptation make this system an appealing model for the expression and trafficking operation of this labile gene product.

Cytochrome P4501A (CYP1A) in killifish (Fundulus heteroclitus): heritability of altered expression and relationship to survival in contaminated sediments

Previous research has shown that killifish (Fundulus heteroclitus) inhabiting a creosote-contaminated site on the Elizabeth River in Virginia exhibit little induction of cytochrome P4501A (CYP1A) protein expression and activity upon exposure to typical CYP1A-inducing chemicals. We characterized the CYP1A response of first, second, and third generation laboratory-raised offspring of feral Elizabeth River killifish to exposure to sediments from the contaminated site as well as the prototypical polycyclic aromatic hydrocarbon (PAH)-type CYP1A inducers beta-naphthoflavone (BNF) and 3-methylcholanthrene (3-MC). The Elizabeth River offspring's responses were compared to those of offspring of killifish from two reference sites (King's Creek, Virginia, and Russell Creek, North Carolina). As with feral Elizabeth River killifish, the first generation embryos and larvae were refractory to CYP1A induction. However, the response observed in 3-year-old first generation adults, as well as with second and third generation fish, was much closer to that observed in reference-site fish. We suggest that the pattern of altered CYP1A response in Elizabeth River killifish, while persistent and heritable for one generation, is mostly nongenetically based. Additionally, we investigated the hypothesis that low CYP1A activity (measured as in ovo EROD activity) would correlate to increased survival in Elizabeth River sediment pore water; this hypothesis was not supported by our results.

Short- and long-term influences of heavy metals on anionic drug efflux from renal proximal tubule

We recently demonstrated in isolated killifish renal proximal tubules that two classes of nephrotoxicants, aminoglycoside antibiotics and radiocontrast agents, rapidly decrease transport mediated by multidrug resistance protein 2 (Mrp2) by causing endothelin (ET) release and signaling through an ET(B) receptor and protein kinase C (PKC). In the present study, we used killifish proximal tubules, fluorescein methotrexate, a fluorescent model substrate for Mrp2, and confocal microscopy to examine the effects of two heavy metal salts (CdCl(2) and HgCl(2)) on Mrp2 function. Three patterns of effects were seen. First, exposing tubules to 10 microM CdCl(2) or 100 nM HgCl(2) for 30 min reduced Mrp2-mediated transport. This reduction was abolished by the ET(B) receptor antagonist, RES-701-1, and by the PKC-selective inhibitor, bis-indolylmaleimide I; neither of these pharmacological tools by itself affected transport. As with aminoglycoside antibiotics and radiocontrast agents, the acute effects of 10 microM CdCl(2) or 100 nM HgCl(2) on transport were also blocked by nifedipine, suggesting that Ca(2+) also initiated cadmium and mercury action. Second, exposure to higher concentrations of CdCl(2) and HgCl(2) appeared to be toxic. Third, exposing tubules for 6 to 24 h to lower levels of CdCl(2) increased Mrp2-mediated transport and Mrp2 immunostaining at the luminal membrane of the proximal tubule cells. Together, these findings indicate that exposure of renal proximal tubules to heavy metals initially leads to reduced Mrp2 function but is followed by an induction in Mrp2-mediated transport after long-term exposure.

Macroinvertebrate and fish populations in a restored impounded salt marsh 21 years after the reestablishment of tidal flooding

During the last two decades, the State of Connecticut has restored tidal flow to many impounded salt marshes. One of the first of these and the one most extensively studied is Impoundment One in the Barn Island Wildlife Management Area in Stonington, Connecticut. In 1990, twelve years after the re-establishment of tidal flooding, the density of the marsh snail Melampus bidentatus, the numerically dominant macroinvertebrate of the high marsh, in Impoundment One was about half that in reference marshes below the breached impoundment dike. By 1999 the densities of Melampus above and below the dike were not significantly different, but the shell-free biomass was greater above the dike as a result of the somewhat larger number and size of the snails there. Twenty-one years after the renewal of tidal flooding, three marsh macroinvertebrates (the amphipods Orchestia grillus and Uhlorchestia spartinophila and the mussel Geukensia demissa) were significantly less abundant in the previously impounded marsh than in the reference marshes, whereas another amphipod (Gammarus palustris) was more abundant above the breached dike where conditions appeared to be somewhat wetter. In 1991 the fish assemblage in a mosquito-control ditch in Impoundment One was similar to that in a ditch below the breached dike; however, the common mummichog Fundulus heteroclitus appeared to be less abundant in the restoring marsh. By 1999 the number of mummichogs caught in ditches was significantly greater in Impoundment One than in the reference marsh, but the numbers of mummichogs trapped along the tidal creek were comparable above and below the dike. The results obtained in this study and those of other restoring marshes at Barn Island indicate the full recovery of certain animal populations following the reintroduction of tidal flow to impounded marshes may require up to two or more decades. Furthermore, not only do different species recover at different rates on a single marsh, but the time required for the recovery of a particular species may vary widely from marsh to marsh, often independently of other species.

Role of NO in endothelin-regulated drug transport in the renal proximal tubule

We previously demonstrated in intact killifish renal proximal tubules that endothelin (ET), acting through an ET(B) receptor and protein kinase C (PKC), reduced transport mediated by multidrug resistance-associated protein 2 (Mrp2), i.e., luminal accumulation of fluorescein methotrexate (FL-MTX) (Masereeuw R, Terlouw SA, Van Aubel RAMH, Russel FGM, and Miller DS. Mol Pharmacol 57: 59-67, 2000). In the present study, we used confocal microscopy and quantitative image analysis to measure Mrp2-mediated transport of FL-MTX in killifish tubules as an indicator of the status of this ET-fired, intracellular signaling pathway. Exposing tubules to sodium nitroprusside (SNP), a nitric oxide (NO) donor, signaled a reduction in luminal accumulation of FL-MTX, which suggested pathway activation. N(G)-monomethyl-L-arginine (L-NMMA), an NO synthase inhibitor, blocked the action of ET-1 on transport. Because SNP effects on transport were blocked by bisindoylmaleide, a PKC-selective inhibitor, but not by RES-701-1, an ET(B)-receptor antagonist, generation of NO occurred after ET(B) receptor signaling but before PKC activation. NO generation was implicated in the actions of several nephrotoxicants, i.e., diatrizoate, gentamicin, amikacin, HgCl(2), and CdCl(2), each of which decreased Mrp2-mediated transport by activating ET signaling. For each nephrotoxicant, decreased FL-MTX transport was prevented when tubules were exposed to L-NMMA. ET-1 and each nephrotoxicant stimulated NO production by the tubules, as determined by a fluorescence-based assay. Together, the data show that NO generation follows ET binding to the basolateral ET(B) receptor and that, in activating the ET-signaling pathway, nephrotoxicants produce NO, a molecule that could contribute to subsequent toxic effects.

Xenobiotic efflux pumps in isolated fish brain capillaries

To identify specific transporters that drive xenobiotics from the central nervous system to blood, the accumulation of fluorescent drugs was studied in isolated capillaries from killifish and dogfish shark brain using confocal microscopy and quantitative image analysis. In killifish brain capillaries, luminal accumulation of fluorescent derivatives of cyclosporin A and verapamil was concentrative, specific, and energy dependent (inhibition by KCN). Transport was reduced by PSC-833, but not by leukotriene C4, indicating the involvement of P-glycoprotein. The ability of capillaries to transport the cyclosporin A derivative was unchanged over 20 h, demonstrating the long-term viability of the preparation. Luminal accumulation of the fluorescent organic anions sulforhodamine 101 and fluorescein-methotrexate was also concentrative, specific, and energy dependent. Transport of these compounds was reduced by leukotriene C4, but not by PSC-833, indicating the involvement of a multidrug resistance-associated protein (Mrp). Similar results were obtained for isolated capillaries from dogfish shark. Immunostaining localized P-glycoprotein and Mrp2 to the luminal surface of the killifish brain capillary endothelium. These findings validate a new and long-lived comparative model for studying drug transport across the blood-brain barrier and, as in mammals, implicate P-glycoprotein and Mrp2 in transport from the central nervous system to blood in fish.

Cloning and functional expression of an MIP (AQP0) homolog from killifish (Fundulus heteroclitus) lens

The major intrinsic protein (MIP) of lens fiber cells is a member of the aquaporin (AQP) water channel family. The protein is expressed at very high levels in lens fiber cells, but its physiological function is unclear. By homology to known AQPs, we have cloned a full-length cDNA encoding an MIP from the lens of killifish (Fundulus heteroclitus). The predicted protein (263 amino acids; GenBank accession no. AF191906) shows 77% identity to amphibian MIPs, 70% identity to mammalian MIPs, and 46% identity to mammalian AQP1. Expression of MIPfun in Xenopus laevis oocytes causes an approximately 40-fold increase in oocyte water permeability. This stimulation is comparable to that seen with AQP1 and substantially larger than that seen with other MIPs. The mercurials HgCl(2) and p-chloromercuribenzenesulfonate inhibit the water permeability of MIPfun by approximately 25%. MIPfun is not permeable to glycerol, urea, or formic acid but is weakly permeable to CO(2).

Interpreting nonracemic ratios of chiral organochlorines using naturally contaminated fish

Although reports of nonracemic proportions of chiral organochlorine pollutants in the environment are widespread, the interpretation of such data is not well developed. Using GC/MS and a chiral stationary phase consisting of 25% tert-butyldimethylsilylated beta-cyclodextrin in PS086 (beta-BSCD), we followed the change in the enantiomeric signature of 2-exo,3-endo,6-exo,8,9,10-hexachlorobornane (B6-923) in naturally contaminated fish maintained under toxaphene-free conditions. Whereas the enantiomeric ratio (ER) of B6-923 was near racemic at the start of the elimination experiment, it had increased severalfold by the end of 60 d. On the basis of first-order kinetics, one enantiomer of B6-923 was eliminated twice as fast as its mirror image, resulting in half-lives of 7 and 13 d, respectively. Enantioselective elimination by our test fish (Fundulus sp.) strongly suggests active biotransformation of B6-923; however, bioprocessing throughput estimates suggest a very low in situ rate of natural attenuation. These results confirm that the relatively constant ERs observed for chiral organochlorines in a given species are the result of competing processes, e.g., uptake vs elimination. Our experiments also further illustrate the utility of enantioselective analysis in characterizing the biotransformation of persistent organochlorine pollutants.

Hypercapnic hypoxia compromises bactericidal activity of fish anterior kidney cells against opportunistic environmental pathogens

Acute hypoxia can cause massive fish and shellfish mortality. Less clear is the role that chronic sublethal hypoxia might play in aquatic animal health. This study tested whether production of reactive oxygen species (ROS) and bactericidal activity of fish phagocytic cells are suppressed under the conditions of decreased oxygen and pH and increased carbon dioxide which occur in the blood and tissue of animals exposed to sublethal hypoxia. Anterior head kidney (AHK) cells of the mummichog, Fundulus heteroclitus, were exposed in parallel to normoxic (pO2=45 torr, pCO2=3.8 torr, pH=7.6) or hypoxic (pO2=15 torr, pCO2=8.0 torr, pH=7.0) conditions and stimulated with a yeast cell wall extract, zymosan. or live Vibrio parahaemolyticus. Hypercapnic hypoxia suppressed zymosan-stimulated ROS production by 76.0% as measured in the chemiluminescence assay and by 58.5% in the nitroblue tetrazolium (NBT) assay. The low O2, high CO2 and low pH conditions also suppressed superoxide production by 75.0 and 47.3% as measured by the NBT assay at two different challenge ratios of cells:bacteria (1:1 and 1:10, respectively). In addition to its effects on ROS production, hypercapnic hypoxia also reduced bactericidal activity by 23.6 and 72.5% at the 1:1 and 1:10 challenge ratios, respectively. Low oxygen levels alone (pO2=15 torr, pCO2=0.76 torr, pH=7.6) did not significantly compromise the killing activity of cells challenged with equal numbers of V. parahaemolyticus. At the higher 1:10 AHK:bacteria challenge ratio, low oxygen caused a small (26.3%) but significant suppression of bactericidal activity as compared to aerial conditions (pO2=155 torr, pCO2=0.76 torr, pH=7.6). This study demonstrates that while hypoxia alone has detrimental effects on immune function, suppression of phagocytic cell activity is compounded by naturally occurring conditions of hypercapnia and low pH, creating conditions that might be exploited by opportunistic pathogens. These results indicate that the adverse health effects of chronic hypercapnic hypoxia might greatly exceed the effects of low oxygen alone.

3,3',4,4'-Tetrachlorobiphenyl oxidation in fish, bird and reptile species: relationship to cytochrome P450 1A inactivation and reactive oxygen production

Previously we showed that the polychlorinated biphenyl 3,3',4,4'-tetrachlorobiphenyl (TCB) caused a release of reactive oxygen species (ROS) from cytochrome P450 1A (CYP1A) of the fish scup (Stenotomus chrysops), and from rat and human CYP1A1. This was linked to a TCB- and NADPH-dependent oxidative inactivation of the enzyme, which in scup and rat was inversely related to the rates of TCB oxidation. We examined the relationship between rates of TCB oxidation, CYP1A inactivation and ROS production in liver microsomes from additional vertebrate species, including skate (Raja erinacea), eel (Anguilla rostrata), killifish (Fundulus heteroclitus), winter flounder (Pleuronectes americanus), chicken (Gallus domesticus), cormorant (Phalacrocorax auritus), gull (Larus argentatus), and turtle (Chrysemys picta picta). TCB oxidation rates were induced in all fish and birds treated with aryl hydrocarbon receptor agonists. Induced rates of TCB oxidation were <1 pmol/min/mg microsomal protein in all fish, and 6-14 pmol/min/mg in the birds. In all species but one, TCB oxidation rates correlated positively with EROD rates, indicating likely involvement of CYP1A in TCB oxidation. Incubation of liver microsomes of most species with TCB+NADPH resulted in an immediate (TCB-dependent) inhibition of EROD, and a progressive loss of EROD capacity, indicating an oxidative inactivation of CYP1A like that in scup. NADPH stimulated production of ROS (H(2)O(2) and/or O(2)(-*)) by liver microsomes, slightly in some species (eel) and greatly in others (chicken, turtle). Among the birds and the fish, NADPH-stimulated ROS production correlated positively with EROD activity. TCB caused a significant stimulation of ROS production by liver microsomes of flounder, killifish, cormorant and gull, as well as scup. The stimulation of CYP1A inactivation and ROS generation indicates an uncoupling of CYP1A by TCB in many species, and when compared between species, the rates of CYP1A inactivation correlated inversely with rates of TCB oxidation. Some feature(s) of binding/active site topology may hinder TCB oxidation, enhancing the likelihood for attack of an oxidizing species in the active site.

The fine structure of neuromuscular junctions and the sarcoplasmic reticulum of extrinsic eye muscles of Fundulus heteroclitus

The extrinsic eye muscles of the killifish (F. heteroclitus) were fixed in OSO₄ (pH 7.6) and subsequently dehydrated, embedded, and sectioned for electron microscopy. The fine structures of neuromuscular junctions and of sarcoplasmic reticulum were then observed. The neuromuscular junction consists of the apposition of axolemma (60 to 70 Å) and sarcolemma (90 to 100 Å), with an intervening cleft space of 200 to 300 Å, forming a synaptolemma 400 to 500 Å thick. The terminal axons contain synaptic vesicles, mitochondria, and agranular reticulum. The subsynaptic sarcolemma lacks the infolding arrangement characteristic of neuromuscular junctions from other vertebrate skeletal muscle, making them more nearly like that of insect neuromuscular junctions. A comparison between the folded and non-folded subsynaptic membrane types is made and discussed in terms of comparative rates of acetylcholine diffusion from the synaptic cleft and resistances of the clefts and subsynaptic membranes. The sarcoplasmic reticulum consists of segmentally arranged, membrane-limited vesicles and tubular and cisternal elements which surround individual myofibrils in a sleeve-like arrangement. Triadic differentiation occurs at or near the A-I junction. Unit sleeves span the A and I bands alternately and consist of closed terminal cisternae interconnected across the A and I bands by tubular cisternae. The thickness of the sarcoplasmic membranes increases from 30 to 40 Å in intertriadic regions to 50 to 70 Å at the triads. The location of the triads is compared with previously described striated muscle from Ambystoma larval myotomes, cardiac and sartorius muscles of the albino rat, mouse limb muscle, chameleon lizard muscle, and insect muscle, with reference to their possible role in intracellular impulse conduction.

FINE STRUCTURE OF CHLORIDE CELLS FROM THREE SPECIES OF FUNDULUS

A morphological basis for osmoregulation in the teleosts was studied by comparing the fine structure of chloride cells found in epithelia of the gills of three species of fish: Fundulus heteroclitus which can survive in a wide range of salinities, and F. similis and F. chrysotus which are usually restricted to salt water and fresh water environments, respectively. Gills were removed from F. heteroclitus which had been laboratory adapted to either sea water or pond water. For a comparison, gills were also removed from the marine F. similis and the fresh water F. chrysotus which had been adapted to their natural environments. Gill-filaments were fixed in Millonig's phosphate buffered (pH 7.4), 1 per cent osmium tetroxide and were embedded in Epon. Thin sections of filaments were stained with lead hydroxide. The cytoplasm of chloride cells of all three species of Fundulus is heavily populated with mitochondria and is filled with tubules of the agranular endoplasmic reticulum (ER). An orderly secretory cycle was indicated for chloride cells of salt water adapted F. heteroclitus and the marine F. similis. An amorphous material is observed in the agranular ER. Its density increases towards the apical end of the cell. In the apical cytoplasm, tubules of the agranular ER appear to converge and to discharge the amorphous material into an apical cavity. Except for the actual opening of the apical cavity, the distal end of salt water adapted chloride cells is characteristically shielded from the hypertonic environment by thin cytoplasmic flanges projecting from the neighboring epithelial cells. Chloride cells of the fresh water F. chrysotus resemble chloride cells of pond water adapted F. heteroclitus, in that these cells do not have apical cavities with the functional appearance of those in the sea water adapted forms. The distal end of fresh water adapted chloride cells is typically exposed to the free surface of the gill-filament. The possible function of the cell type is discussed.