Embryotoxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD): Theembryonic vasculature is a physiological target for TCDD-induced DNA damage and apoptotic cell death in medaka (Orizias latipes)

Combination of genomic DNA fingerprinting into the medaka specific-locus test system for studying environmental germ-line mutagenesis

A specific-locus test (SLT) system has been established using the medaka fish (Oryzias latipes), where recessive visible mutations (at the b, lf or gu loci) detected during early embryonic stages (TM) or after hatching (VM), and dominant lethals (DL) can be examined in the same individual F1 progeny of treated parents. It was found that approximately 90% of the F1 embryos with gamma-ray-induced specific-locus mutations were concomitantly accompanied by dominant lethals irrespective of doses and germ-cell stages at the time of exposure, suggesting that DNA alterations in such mutants might include both the marker loci and region(s) responsible for dominant lethals. In contrast, embryonic lethality of the ENU (ethylnitorosourea)-induced specific-locus mutants considerably varied among ENU concentrations as well as germ-cell stages treated. Further, synergistic effect of combined treatments with gamma-rays and ENU on induction of mutations were suggested in postmeiotic male germ cells, while in spermatogonia no synergistic effect was found. DNA alterations at the 87 arbitrarily primed polymerase chain reaction (AP-PCR) markers spread over the genome were examined for individual dominant lethal embryos from 4.75 Gy-irradiated sperm or spermatids. It was found that, 14 out of 20 dominant lethal embryos lost more than one AP-PCR markers, including multiple markers located on the identical linkage group (average genetic distances, approximately 11 cM). Also found was that frequency of loss of the AP-PCR markers in the severely malformed dominant lethal embryos was higher (approximately 4.5%) than that in the slightly malformed lethal embryos (approximately 1.6%). Here, results of these studies, including previously unpublished work, are presented to illustrate the potential usefulness of the medaka SLT system for monitoring environmental mutagens.

Correlation of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced apoptotic cell death in the embryonic vasculature with embryotoxicity

Vertebrate embryos are particularly sensitive to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Identification of tissues that are susceptible to the adverse effects of TCDD is requisite for understanding the embryo toxic effects of TCDD. The objective of the present study was to quantitate the temporal appearance of and dose dependence of apoptosis in TCDD-exposed medaka embryos (Oryzias latipes). A fluorescent-based DNA end-labeling assay provided a sensitive method for detection of TCDD-induced apoptosis in tissue sections of medaka embryos. Apoptotic cells were readily apparent in the medial yolk vein at all observed embryonic stages in TCDD-exposed embryos. Slope-comparison analysis indicated that TCDD-induced programmed cell death in the embryonic medial yolk vein was mechanistically linked to embryo mortality. These data are consistent with the hypothesis that vascular damage contributes to the acute embryo toxic effects of TCDD. However, as sublethal concentrations of dioxin-like compounds are more typical of environmental exposures, tissue damage was also assessed in medaka fry that were exposed to low doses of TCDD during embryonic development. Cell death was detected in gill and digestive tissues in visibly healthy medaka fry that had been exposed to low doses of TCDD during embryonic development. Increased expression of cytochrome P450 1A is a major biochemical consequence of TCDD exposure and is often used as a biomarker for exposure to dioxin-like compounds. Therefore, we compared the tissue distribution of TCDD-induced P450 1A expression and TCDD-induced programmed cell death. TCDD-induced programmed cell death co-localized with TCDD-induced P450 1A expression in both embryos and in visibly healthy post-hatch fry. Our results suggest that aberrant programmed cell death may be a suitable marker for exposure of feral organisms to dioxin-like compounds.

Modulation of ovarian follicle maturation and effects on apoptotic cell death in Holtzman rats exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in utero and lactationally

Recent reports have described the reproduction-modulating and endocrine-disrupting effects following exposure to toxic substances such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Herein, we set out (1) to determine whether TCDD exposure exerts detrimental effects on follicle maturation in the Holtzman rat ovary and (2) to determine whether the effects of TCDD are mediated in part via apoptotic cell death. In certain species, dioxin exposure is correlated with reduced fecundity, reduced ovulatory rate, an increased incidence of endometriosis, and various reproductive cancers. Although some of the effects of TCDD are mediated via the hypothalamic-pituitary axis, direct effects on the ovary have also been observed. In the present study, an oral dose of 1 microgram TCDD/kg maternal body weight was administered on Day 15 of gestation. Female pups were sacrificed on Postnatal Day 21/22, and the ovaries were excised, fixed for histologic analysis, and analyzed in a double-blind paradigm. The analysis included a count and measurement and classification of preantral and antral follicles throughout the entire ovary. The contralateral ovary from each animal was analyzed for DNA fragmentation indicative of apoptotic cell death. The results indicate that TCDD treatment significantly reduced the number of antral follicles in the size classes 50,000 to 74,999 microns2 and > 100,000 microns2. We also observed a reduction in the number of preantral follicles less than 50,000 microns2. No difference was observed in the degree of apoptotic cell death in antral (50,000 to > 100,000 microns2) and preantral follicles (50,000 microns2 to > 75,000 microns2) between TCDD-treated and control-treated tissues. These data support the hypothesis that TCDD results in a diminution in the number of antral and preantral follicles of certain size classes in animals exposed during critical periods of development, but that apoptosis does not appear to be the underlying mechanism in these particular follicles. This does not preclude apoptosis occurring in pools of smaller precursor follicles.

Correlation of 2,3,7,8-tetrachlorodibenzo-p-dioxin induction of cytochrome P4501A in vascular endothelium with toxicity in early life stages of lake trout

Edema and cardiovascular dysfunction occur in vertebrates exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during early development. This study examined cytochrome P4501A (CYP1A) induction in endothelium and its possible association with mortality due to the edema and vascular effects of TCDD in lake trout early life stages. Lake trout (Salvelinus namaycush) eggs were injected at 24-50 hr postfertilization with 0.2 microl of 50 mM phosphatidylcholine liposomes or liposomes containing TCDD to give seven doses ranging from 11 to 176 pg TCDD/g egg. Doses of TCDD greater than 44 pg/g egg elicited hemorrhages; yolk sac, pericardial, and meningial edema; craniofacial malformations; regional ischemia; growth retardation; and mortality at the sac fry stage of development. Expression of CYP1A was assessed at four developmental stages, by immunohistochemical analysis of serial sections of individual fish with monoclonal antibody 1-12-3 to teleost CYP1A. CYP1A staining occurred in endothelial cells of many organs of TCDD-exposed but not vehicle-exposed embryos at 1 week prehatch and sac fry at 2 weeks posthatch. Earlier developmental stages examined were negative for CYP1A expression at any dose of TCDD. The strongest response occurred in sac fry at TCDD doses greater than 88 pg TCDD/g egg but was detected at doses as low as 22 pg TCDD/g egg. CYP1A staining in endothelium appeared at lower doses and was stronger than that in other cell types, in both prehatch embryos and posthatch sac fry. Thus, the vascular system is a major initial site affected by TCDD in lake trout early life stages, and the vascular endothelium is a cell type uniquely sensitive to induction of CYP1A in these developing animals. Based on an index of immunohistochemical staining of CYP1A, endothelial CYP1A induction in sac fry by TCDD occurred with an ED50 of 64-69 pg TCDD/g egg, similar to the dose-response for mortality occurring during the sac fry stage of development (LD50 = 47 pg TCDD/g egg). The correlations seen here suggest that CYP1A or aryl hydrocarbon receptor (AhR) in the endothelium may be linked to early lesions that result in TCDD-induced vascular derangements leading to yolk sac, pericardial, and meningial edema that is associated with lake trout sac fry mortality, but the precise mechanism remains to be determined.