Teratogenicity of cobalt chloride in Xenopus laevis, assayed by the FETAX procedure

Anuran heart development and critical developmental periods: a comparative analysis of three Neotropical anuran species

The heart begins to form early during vertebrate development and is the first functional organ of the embryo. This study aimed to describe and compare the heart development in three Neotropical anuran species, Physalaemus albonotatus, Elachistocleis bicolor, and Scinax nasicus. Different Gosner Stages (GS) of embryos (GS 18-20) and premetamorphic (GS 21-25), prometamorphic (GS 26-41) and metamorphic (GS 42-46) tadpoles were analyzed using stereoscopic microscopy and Scanning Electronic Microscopy. Heart development was similar in the three analyzed species; however, some heterochronic events were identified between P. albonotatus and S. nasicus compared to E. bicolor. In addition, different patterns of melanophores arrangement were observed. During the embryonic and metamorphic periods, the main morphogenetic events occur: formation of the heart tube, regionalization of the heart compartments, development of spiral valve, onset of heartbeat, looping, and final displacement of the atrium and its complete septation. Both periods are critical for the normal morphogenesis and the correct functioning of the anuran heart. These results are useful to characterize the normal anuran heart morphology and to identify possible abnormalities caused by exposure to environmental contaminants.

Endocrine activities and cellular stress responses in the marsh frog Pelophylax ridibundus exposed to cobalt, zinc and their organic nanocomplexes

Metal-containing materials are extensively used in industry, personal care products and medicine, and their release in the environment causes concern for the potential impacts on aquatic organisms. We assessed endocrine disrupting potential of N-vinyl-2-pyrrolidone-based nanoparticles (Me-PSs) containing cobalt (Co(2+)) or zinc (Zn(2+)), using the marsh frog Pelophylax ridibundus as a model. Adult males were exposed for 14 days to waterborne Co(2+) (50μg/L), Zn(2+) (100μg/L) or corresponding concentrations of Co-PS, Zn-PS, or parental polymeric compound (PS). The indices of thyroid activity, vitellogenesis, cytochrome P450-dependent monooxygenases activity (EROD) and cytotoxicity markers were evaluated. Exposure to Co(2+) led to the elevation of serum thyrotropin (TSH) and hepatic deiodinase activities accompanied by the up-regulation of EROD activity. In contrast, the action of the polymer-containing substances (Co-PS, Zn-PS and PS) as well as free Zn(2+) caused a prominent decrease of EROD activity and a decrease in serum cortisol and TSH concentrations. Exposures to Zn(2+), Zn-PS and PS upregulated vitellogenesis in males. All exposures except Co(2+) caused neurotoxicity as indicated by the depletion of cholinesterase. These results demonstrate toxicity of Co- and Zn-containing Me-PSs and their parental compounds (Zn(2+) and PS) in frogs and indicate distinct mechanisms of Co(2+) action. Broad disruption of the hormonal pathways and reduced capacity for organic xenobiotic detoxification may have deleterious impacts on amphibian populations from habitats exposed to metallorganic pollution.

Effects of specific dosages of magnesium and zinc on the teratogenicity of cadmium, nickel, and cobalt in Xenopus embryos, as assessed by the FETAX test

The objective of this study was to determine if exposure to divalent cations, Cd(2+), Ni(2+), and Co(2+) would lead to malformations in Xenopus laevis embryos, and whether addition of Mg(2+) and Zn(2+); separately and in combination, would reduce their toxicity and teratogenicity on the embryos of Xenopus laevis as assessed by 96-h FETAX tests. Results indicate that exposure to Cd(2+), Ni(2+) or Co(2+) lead to an increase in toxicity and teratogenicity in embryos, whereas Mg(2+), Zn(2+), or a combination of them reduced the toxic and teratogenic effects of these divalent cations. Modulation of Cd(2+), Ni(2+) or Co(2+) toxicity and teratogenicity by Mg(2+) and Zn(2+), varied with the metal. Zn(2+) was observed to be a better suppressor of Co(2+) toxicity and teratogenicity than Mg(2+). In contrast, Ni(2+), and Cd(2+) teratogenicity was reduced more prominently by Mg(2+). On the other hand, combination of Mg(2+) and Zn(2+) showed potentialization effect on all divalent cation toxicity and teratogenicity. We concluded that Mg(2+) and Zn(2+) reduced the toxicity and teratogenicity of Cd(2+), Ni(2+), Co(2+).

Effects of depleted uranium on survival, growth, and metamorphosis in the African clawed frog (Xenopus laevis)

Embryos (stage 8-47, Nieuwkoop and Faber) of the African clawed frog (Xenopus laevis) were subjected to water-borne depleted uranium (DU) concentrations that ranged from 4.8 to 77.7 mg/L using an acute 96-h frog embryo teratogenesis assay-Xenopus (FETAX). In a chronic 64-d assay, X. laevis (from embryo through metamorphosis; stages 8-66) were subjected to concentrations of DU that ranged from 6.2 to 54.3 mg/L. Our results indicate DU is a non teratogenic metal. No effects on mortality, malformations, or growth were observed in the 96-h FETAX with concentrations of DU that ranged from 4.8 to 77.7 mg/L. From stage 8 to stage 47, X. laevis tadpoles do not actively feed and the gills are not well developed. Thus, uptake of DU was reduced despite exposure to elevated concentrations. The 64-d assay resulted in no concentration response for either mortality or malformations; however, a delay in metamorphosis was observed in tadpoles subjected to elevated DU concentrations (from 13.1 to 54.3 mg/L) compared to tadpoles in both the well-water control and reference. The delay in metamorphosis was likely due to increasing body burden of DU that ranged from 0.98 to 2.82 mg/kg.

A comparative study of the toxicity of mercury dichloride and methylmercury, assayed by the Frog Embryo Teratogenesis Assay--Xenopus (FETAX)

The Frog Embryo Teratogenesis Assay-Xenopus (FETAX) is a powerful and flexible bioassay that makes use of the embryos of the anuran amphibian Xenopus laevis. The FETAX can detect xenobiotics that affect embryonic development, when mortality, teratogenicity and growth inhibition are used as endpoints. The FETAX was used to compare the embryotoxic and teratogenic potentials of two chemical species of mercury, inorganic mercury(II) chloride (HgCl2) and organic methylmercury chloride (MeHgCl). A higher toxicity of MeHgCl (the estimated median lethal concentration [LC50] and median teratogenic concentration [TC50] were 0.313microM and 0.236microM, respectively) over HgCl2, with estimated LC50 and TC50 values of 0.601microM and 0.513microM, respectively). On the basis of these results, HgCl2 and MeHgCl can be classified as "slightly teratogenic compounds", as the ratio of LC50/TC50 is less than 1.5. There was a significant deviation from the commonly described monotonic behaviour of the concentration-response curves, suggesting a hormetic effect of both species of mercury. Uptake experiments, followed by neutron activation analysis, showed a higher incorporation of mercury in embryos exposed to MeHgCl compared with those exposed to HgCl2. Interestingly, Hg- exposed embryos showed a higher content of selenium and zinc than did control embryos.

In vitro comparative effect of Cd2+, Ni2+, and Co2+ on mouse postblastocyst development

Postblastocyst development of mouse preembryos was studied in vitro in order to determine direct effect of Cd2+, Ni2+, and Co2+ ions on embryogenesis during the peri-implantation stage. Uterine horns were flushed on Day 4 of pregnancy and expanded blastocysts were cultured for 4 days in the presence of micromolar Cd2+ (1.1-26.4), Ni2+ (0. 1-500) or Co2+ (1-200). Area of trophoblast outgrowth was measured and used as a quantitative toxicological endpoint. Hatching, attachment, outgrowth, and formation of inner cells mass were also registered. Significant adverse effect on the development stages were observed at 2.2 microM (Cd2+), at 10 microM (Ni2+), and at 100 microM (Co2+). Cd2+ and Co2+ decreased the area of trophoblast markedly at concentrations of 1.1 and 10 microM, respectively. Ni2+ exposure resulted in a slight increase at 10 microM followed by a marked reduction in the trophoblast area at 250 microM. Reduced proliferative ability of trophoblast cells may point to compromised invasiveness of the embryo. The lowest Cd2+ concentration (1.1 microM=0.25 microg/ml) significantly deteriorating trophoblast development was found to be lower than Cd levels ranging up to 0.512 microg/g, reported in clinical ovarian samples of occupationally nonexposed women. The morphological alteration and loss of cellular contacts in blastocysts induced by exposure to Cd2+, Ni2+, or Co2+ may adversely influence adhesion and recognition events and may disturb aggregation of mononuclear trophoblastic cells to multinucleated cells in the course of peri-implantation in vivo as well.

Human toxicity of cobalt-containing dust and experimental studies on the mechanism of interstitial lung disease (hard metal disease)

In the industry, the potential for exposure to cobalt metal dust is particularly important during the production of cobalt powder and the processing and use of hard metals and other cobalt-containing alloys. The different adverse health effects reported in these workers are reviewed. One of the main target organs is the respiratory tract, and this article concentrates on the lung parenchymal reactions induced by cobalt-containing dust. Clinical and epidemiological data indicate that this manifestation is rarely, if ever, induced by pure cobalt metal dust alone, but requires the concomitant inhalation of other compounds such as tungsten carbide in the hard metal industry (hard metal disease). Experimental studies demonstrate that cobalt metal and metallic carbides interact to produce an elective lung toxicity. Recent work on the mechanism of this interaction, which is based on the production of activated oxygen species, is reviewed. A practical implication in industrial hygiene should be that permissible exposure levels to Co dust might have to be different when exposure is to pure Co particles or an association with carbides.

Characterization of pNiXa, a serpin of Xenopus laevis oocytes and embryos, and its histidine-rich, Ni(II)-binding domain

A Ni(II)-binding serpin, pNiXa, is abundant in Xenopus oocytes and embryos. Kinetic assays show that purified pNiXa strongly inhibits bovine alpha-chymotrypsin (Ki = 3 mM), weakly inhibits porcine elastase (K1 = 0.5 microM), and does not inhibit bovine trypsin. The reversible, slow-binding inhibition of alpha-chymotrypsin by pNiXa is unaffected by Ni(II). Ovochymase in egg exudates is inhibited by pNiXa, but to a limited extent, even at high pNiXa concentrations. An octadecapeptide that models the His-rich domain (-HRHRHEQQGHHDSAKHGH-) of pNiXa forms six-coordinate, octahedral Ni(II)-complexes when the N-terminus is acetylated, and a square-planar Ni(II)-complex when the N-terminus is unblocked. Spectroscopy reveals two distinct types of octahedral Ni(II)-coordination to the N-acetylated octadecapeptide, involving, respectively, 3-4 and 5-6 imidazole nitrogens; the octadecapeptide undergoes partial, reversible precipitation in pH- and Ni(II)-dependent fashion, suggesting an insoluble, Ni(II)-coupled (Hx)n-dimer. Such (Hx)n-peptide interaction is confirmed by an enzyme-linked biotinavidin assay with N-biotin-KHRHRHE-amide and N-acetyl-KHRHRHE-resin beads, which become coupled after adding Ni(II) or Zn(II). H2O2 oxidation of 2'-deoxyguanosine to mutagenic 8-hydroxy-2'-deoxyguanosine is enhanced by the octahedral Ni(II)-octadecapeptide complex, although the effect is more intense with the square-planar Ni(II)-octadecapeptide complex. Immunoperoxidase staining of whole mounts with pNiXa antibody shows that pNiXa is distributed throughout gastrula-stage embryos and is localized during organogenesis in the brain, eye, spinal cord, myotomes, craniofacial tissues, and other sites of Ni(II)-induced anomalies. Patterns of pNiXa staining are similar in controls and Ni(II)-exposed embryos. Binding of Ni(II) to pNiXa may cause embryotoxicity by enhancing oxidative reactions that produce tissue injury and genotoxicity. Although the natural target proteinases for pNiXa inhibition have not been established, pNiXa may be an important regulator of proteolysis during embryonic development.

Zn(2+)-induction of metallothionein in myotomal cell nuclei during somitogenesis of Xenopus laevis

The localization of metallothionein in control and Zn-exposed embryos of Xenopus laevis was studied by whole-mount immunohistochemical staining. The embryos were grown according to the FETAX (Frog Embryo Teratogenesis Assay: Xenopus) protocol from N/F stage 8 to stage 47, with or without addition of ZnCl2 (300 microM) to the medium. At stages 27, 38, 42, 45 and 47, control and Zn-exposed embryos were fixed in buffered formalin, and whole mounts were stained by an immunoperoxidase technique, using monoclonal murine antibody to equine metallothionein. Staining of metallothionein was evident in myotomal cell nuclei of developing somites by stage 27, stomatodeum, oropharynx, and gills by stage 38, developing kidneys (mesonephros) by stage 45, and liver by stage 47. The staining of metallothionein at these sites was more intense in Zn-exposed embryos than controls. The central nervous system (especially the spinal cord) and the yolk mass were faintly stained for metallothionein in controls and Zn-exposed embryos. Staining of metallothionein in myotomal cell nuclei was most prominent at stage 38, diminished at stages 42 and 45, and practically disappeared by stage 47. This is the first report that metallothionein is expressed in myotomal cell nuclei of Xenopus embryos during normal somitogenesis and becomes increased when the embryos are exposed to teratogenic levels of Zn2+.

Xenopus lipovitellin 1 is a Zn(2+)- and Cd(2+)-binding protein

This report discusses the identification of a Zn(2+)- and Cd(2+)-binding protein of Xenopus laevis that is abundant in vitellogenic oocytes and in embryos from fertilization to stage 46. Oocyte or embryo homogenates were fractionated by SDS-PAGE, blotted onto nitrocellulose, and probed with 65Zn2+ or 109Cd2+. The resulting autoradiograms showed binding of both radionuclides to a protein, designated pCdZn. Freon extraction of oocyte and embryo homogenates showed pCdZn to be a yolk protein. When pCdZn was isolated from oocyte homogenates by ammonium sulfate precipitation, delipidation, and chromatography, it co-purified with lipovitellin 1. The amino acid composition of pCdZn closely resembled the reported composition of lipovitellin 1 and the molecular weight of purified pCdZn (approximately 115 kD) corresponded to reported values for lipovitellin 1 (111-121 kD). Amino acid sequence analyses of five peptides derived from pCdZn yielded 94% identity to the reported sequence of lipovitellin 1, deduced from the DNA sequence of the Xenopus vitellogenin A2 precursor gene. Based on these findings, pCdZn was identified as lipovitellin 1. This study suggests that lipovitellin 1 is the major storage protein for zinc in mature oocytes and developing embryos of Xenopus laevis.

Effects of teratogenic exposures to Zn2+, Cd2+, Ni2+, Co2+, and Cu2+ on metallothionein and metallothionein-mRNA contents of Xenopus embryos

Xenopus laevis embryos were analyzed for metallothionein by silver-saturation assay and metallothionein-mRNA by reverse transcriptase/polymerase chain reaction following exposures to the following metal chlorides at levels that caused > 95% malformations and < 7% mortality: Zn2+ (300 microM); Cd2+ (18 microM); Ni2+ (56 microM); Co2+ (1,800 microM); and Cu2+ (5.6 microM). At the beginning of the exposure (stages 8), metallothionein-mRNA and metallothionein levels averaged 2.0 x 10(6) copies/embryo and 19 pmol/embryo, respectively. In control embryos at stages 26, 36, 42, and 46, metallothionein-mRNA content averaged 9, 37, 104, and 97 copies x 10(6)/embryo, and metallothionein content averaged 6, 11, 15, and 18 pmol/embryo. In Zn(2+) -exposed embryos at the same stages, metallothionein-mRNA content averaged 116*, 11,400*, 3,210*, and 14 copies x 10(6)/embryo and metallothionein content averaged 10, 18*, 46*, and 90* pmol/embryo; in Cd(2+)-exposed embryos, metallothionein-mRNA content averaged 22, 7,170*, 1,783*, and 240 copies x 10(6)/embryo and metallothionein content averaged 8, 14, 33*, and 56* pmol/embryo, respectively (*P < 0.05 versus controls). Exposure-response curves (Cd2+, 1-18 microM; Zn2+, 3-300 microM) indicated that Cd2+ was 3- to 5-times more potent than Zn2+, based on metallothionein-mRNA response at stage 36 and metallothionein response at stage 46. In Ni(2+)-, Co(2+)-, or Cu(2+)-exposed embryos, metallothionein-mRNA and metallothionein contents did not differ significantly from controls.(ABSTRACT TRUNCATED AT 250 WORDS)

The 40 kDa 63Ni(2+)-binding protein (pNiXc) on western blots of Xenopus laevis oocytes and embryos is the monomer of fructose-1,6-bisphosphate aldolase A

A Ni(2+)-binding protein (pNiXc, 40 kDa), present in Xenopus laevis oocytes and embryos, was isolated from mature oocytes by chromatography on DEAE-cellulose and cellulose phosphate, followed by FPLC on Ni-iminodiacetate-Agarose, or reverse-phase HPLC on a C-4 column. Size-exclusion HPLC showed that intact pNiXc is approximately 155 kDa, consistent with tetrameric structure. After cleavage with Lys-C proteinase or cyanogen bromide, six peptides were separated by HPLC and sequenced by Edman degradation, providing sequence data for 83 residues. Data-base search showed similarity of pNiXc to eukaryotic aldolases, with 96% identity to human aldolase A. pNiXc demonstrated aldolase activity with fructose 1,6-bisphosphate as substrate (Km, 30 microM Vmax 26 mumol min-1 mg-1); the aldolase activity was inhibited non-competitively by Cu2+, Cd2+, Co2+, or Ni2+. Equilibrium dialysis showed high affinity binding (Kd, 7 microM) of 1 mole of Ni per mole of 40 kDa subunit. Based on metal-blot competition assays, the abilities of metals to compete with 63Ni2+ for binding to pNiXc were ranked: Cu2+ >> Zn2+ > Cd2+ > Co2+. This study identifies pNiXc as the monomer of fructose-1,6-bisphosphate aldolase A, and raises the possibility that aldolase A is a target enzyme for metal toxicity.

Lipovitellin 2 beta is the 31 kD Ni(2+)-binding protein (pNiXb) in Xenopus oocytes and embryos

An Ni(2+)-binding protein (pNiXb, 31 kD) present in mature Xenopus laevis oocytes and in embryos from fertilization in N/F stage 42, was isolated and characterized. After oocytes or embryos were fractionated by PAGE, electroblotted onto nitrocellulose, and probed with 63Ni2+, pNiXb was detected by autoradiography. pNiXb, a yolk protein located in the embryonic gut, was purified from yolk platelets by ammonium sulfate precipitation, delipidation, gel filtration chromatography, and HPLC analysis. During these steps, pNiXb copurified with lipovitellin 2. The N-terminal sequence of purified pNiXb exactly matched that of Xenopus lipovitellin 2 beta, deduced from the DNA sequence of the Xenopus vitellogenin A2 precursor gene. Since pNiXb and lipovitellin 2 beta agree in N-terminal sequence, amino acid composition, and apparent molecular weight, they appear to be identical. Based on a metal-blot competition assay, the abilities of metal ions to compete with 63Ni2+ for binding to pNiXb were ranked: Zn2+ approximately Cu2+ approximately Co2+ > Cd2+ approximately Mn2+ > Sn2+. This study shows that Xenopus lipovitellin 2 beta is a metal-binding protein in vitro, and raises the possibility that it may function similarly in vivo.

Malformations persist after metamorphosis of Xenopus laevis tadpoles exposed to Ni2+, Co2+, or Cd2+ in FETAX assays

This study was performed to determine whether malformations induced in Xenopus laevis embryos by exposures to divalent nickel, cobalt, or cadmium chlorides in FETAX assays persist after the tadpoles undergo metamorphosis to juvenile frogs. Embryos were exposed for four days to EC50 concentrations of Ni2+, Co2+, or Cd2+ under the standard conditions of FETAX assays; thereafter, the exposures were discontinued and the tadpoles were kept in aquaria through metamorphosis. Controls were treated similarly, without exposure to metals. At 13 weeks of age, surviving frogs were killed and examined for malformations. Control and metal-exposed groups of Xenopus did not differ significantly in their median ages at metamorphosis, mean body weights, or survival at 13 weeks. Overall incidences of malformations found in Ni(2+)-, Co(2+)-, or Cd(2+)-exposed frogs at 13 weeks of age were 55, 40, and 51%, respectively (P < 0.01 vs. 3% in controls). The malformations of metal-exposed frogs included retinal depigmentation, diastematomyelia, scoliosis, kyphosis, phocomelia, sacro-pelvic and hind-limb deformities, and dysplasia of the heart, kidney, ovary and gut.