Cadmium uptake and distribution in mouse embryos following maternal exposure during the organogenic period: a scintillation and autoradiographic study

Gestational Cd Exposure in the CD-1 Mouse Induces Sex-Specific Hepatic Insulin Insensitivity, Obesity, and Metabolic Syndrome in Adult Female Offspring

There is compelling evidence that developmental exposure to toxic metals increases risk for obesity and obesity-related morbidity including cardiovascular disease and type 2 diabetes. To explore the hypothesis that developmental Cd exposure increases risk of obesity later in life, male, and female CD-1 mice were maternally exposed to 500 ppb CdCl2 in drinking water during a human gestational equivalent period (gestational day 0-postnatal day 10 [GD0-PND10]). Hallmark indicators of metabolic disruption, hepatic steatosis, and metabolic syndrome were evaluated prior to birth through adulthood. Maternal blood Cd levels were similar to those observed in human pregnancy cohorts, and Cd was undetected in adult offspring. There were no observed impacts of exposure on dams or pregnancy-related outcomes. Results of glucose and insulin tolerance testing revealed that Cd exposure impaired offspring glucose homeostasis on PND42. Exposure-related increases in circulating triglycerides and hepatic steatosis were apparent only in females. By PND120, Cd-exposed females were 30% heavier with 700% more perigonadal fat than unexposed control females. There was no evidence of dyslipidemia, steatosis, increased weight gain, nor increased adiposity in Cd-exposed male offspring. Hepatic transcriptome analysis on PND1, PND21, and PND42 revealed evidence for female-specific increases in oxidative stress and mitochondrial dysfunction with significant early disruption of retinoic acid signaling and altered insulin receptor signaling consistent with hepatic insulin sensitivity in adult females. The observed steatosis and metabolic syndrome-like phenotypes resulting from exposure to 500 ppb CdCl2 during the pre- and perinatal period of development equivalent to human gestation indicate that Cd acts developmentally as a sex-specific delayed obesogen.

Arsenic- and cadmium-induced toxicogenomic response in mouse embryos undergoing neurulation

Arsenic (As) and cadmium (Cd) are well-characterized teratogens in animal models inducing embryotoxicity and neural tube defects (NTDs) when exposed during neurulation. Toxicological research is needed to resolve the specific biological processes and associated molecular pathways underlying metal-induced toxicity during this timeframe in gestational development. In this study, we investigated the dose-dependent effects of As and Cd on gene expression in C57BL/6J mouse embryos exposed in utero during neurulation (GD8) to identify significantly altered genes and corresponding biological processes associated with embryotoxicity. We quantitatively examined the toxicogenomic dose-response relationship at the gene level. Our results suggest that As and Cd induce dose-dependent gene expression alterations representing shared (cell cycle, response to UV, glutathione metabolism, RNA processing) and unique (alcohol/sugar metabolism) biological processes, which serve as robust indicators of metal-induced developmental toxicity and indicate underlying embryotoxic effects. Our observations also correlate well with previously identified impacts of As and Cd on specific genes associated with metal-induced toxicity (Cdkn1a, Mt1). In summary, we have identified in a quantitative manner As and Cd induced dose-dependent effects on gene expression in mouse embryos during a peak window of sensitivity to embryotoxicity and NTDs in the sensitive C57BL/6J strain.

Cadmium-induced differential toxicogenomic response in resistant and sensitive mouse strains undergoing neurulation

Common inbred mouse strains, such as the C57BL/6 (C57) and the SWV, display differences in sensitivity to environmental teratogens during gestation. For example, the C57 is more sensitive than the SWV to cadmium (Cd) exposure during neurulation, inducing a higher incidence of neural tube defects (NTDs). Here, we report, using Cd as a model teratogen, the first large scale toxicogenomic study to compare teratogen-induced gene expression alterations in C57 and SWV embryos undergoing neurulation, identifying toxicogenomic responses that associate with developmental toxicity and differential sensitivity. Using a systems-based toxicogenomic approach, comparing Cd-exposed and control C57 and SWV embryos (12- and 24-h postinjection [p.i.] [gestational day 8.0, ip]), we examined differentially expressed genes at multiple levels (biological process, pathway, gene) using Gene Ontology (GO) analysis, pathway mapping and cross-scatter plots. In both C57 and SWV embryos, we observed several gene expression alterations linked with cell cycle-related classifications, however, only in the C57 we observed upregulation of p53-dependent mediators Ccng1 and Pmaip1, previously associated with cell cycle arrest, apoptosis and NTD formation. In addition, we also identified a greater reduction in expression of nervous system development-related genes (e.g., Zic1, En2, Neurog1, Elavl4, Metrn, Nr2f1, Nr2f2) in the C57 compared to the SWV (12-h p.i.). In summary, our results indicate that differences in Cd-induced gene expression profiles between NTD resistant and sensitive strains within enriched biological processes (including developmental and cell cycle-related categories) associate with increased sensitivity to developmental toxicity as determined by observations of increased NTD formation, mortality (resorptions) and reduced fetal growth. Such observations may provide more detailed and useful mechanistic clues for identification of differences in life-stage specific teratogenic response.

Proteomic analysis to characterize differential mouse strain sensitivity to cadmium-induced forelimb teratogenesis

BACKGROUND

Cadmium ion (Cd2+) is a ubiquitous environmental contaminant, and it is a potent teratogen in mice. An intraperitoneal dose of 4 mg/kg of CdCl2 at gestational day 9 causes forelimb ectrodactyly in the C57BL/6N mouse strain, but the SWV/Fnn strain is resistant. The objective of this study was to identify differentially displayed proteins in two target tissues for cadmium teratogenesis, and to derive hypotheses regarding the mechanisms involved in the murine strain difference in Cd-induced forelimb ectrodactyly.

METHODS

The global proteomics strategy used two-dimensional polyacrylamide gel electrophoresis for protein separation, and MALDI-TOF-MS and LC-MS/MS for protein identification, to compare and identify proteins in forelimb buds and yolk sacs from the two mouse strains following Cd administration.

RESULTS

More than 1,000 protein spots were detected by two-dimensional polyacrylamide gel electrophoresis in day 10.0 mouse forelimb buds and yolk sacs. Thirty-eight proteins had identifiable differences in abundance levels in Cd-treated forelimb buds between the two strains. Of those 38 proteins, 14 could be associated with the unfolded protein response process and seven are associated with actin polymerization. The proteins that were found to be differentially abundant between the strains in yolk sacs that were exposed to CdCl2 were predominantly different than the proteins detected differentially in the limb buds of the two strains with an overlap of approximately 20%.

CONCLUSIONS

These patterns of differentially displayed proteins rationalize a hypothesis that the differential murine strain response to cadmium-induced forelimb ectrodactyly is due to differences in their pathways for the unfolded protein response and/or actin polymerization.

Effects of perinatal exposure to low-dose cadmium on thyroid hormone-related and sex hormone receptor gene expressions in brain of offspring

Perinatal cadmium (Cd) exposure has been shown to alter behaviors and reduce learning ability of offspring. A few studies have shown that Cd reduced serum thyroid hormones (THs), which are important for brain development during the perinatal period. Brain specific genes, neurogranin (RC3) and myelin basic protein (BMP), are known to be regulated by TH through TH receptors (TR). It has been suggested that RC3 may play roles in memory and learning. In addition, Cd has been suggested to have estrogen-like activity. To evaluate the effects of perinatal low-dose exposure to Cd on thyroid hormone-related gene (RC3, TR-beta1, MBP, RAR-beta) and sex hormone receptor gene (ER-alpha, ER-beta and PgR) expressions in the brain and on behaviors of offspring, mice were administered with 10ppm Cd (from gestational day 1 to postnatal day 10) and/or 0.025% methimazole (MMI; anti-thyroid drug) (from gestational day 12 to postnatal day 10) in drinking water. Also, 0.1% MMI was administered as a positive control (high MMI group). RC3 mRNA expression was reduced in the female brain of combined exposure and high MMI groups and was negatively correlated with the activity in the open-field. ER-alpha, ER-beta and PgR mRNA expressions were decreased in male and female Cd, and female Cd+MMI groups, respectively; among these changes the reduced expression of PgR was opposite to estrogenic action. These results suggested that perinatal exposure to Cd disrupted the gene expressions of sex hormone receptors, which could not be considered to be a result of estrogenic action. Our study indicates that alteration in the gene expressions of RC3 and sex hormone receptors in the brain induced by perinatal Cd and MMI exposure might be one mechanism of developmental toxicity of Cd.

Role of placental metallothionein in maternal to fetal transfer of cadmium in genetically altered mice

The role of placental metallothionein (MT) as a barrier for maternal to fetal transfer of cadmium (Cd) was investigated using mice which overexpressed the MT-1 isoform (MT-1*), mice which did not express the MT-1 and 2 isoforms (MT-null) and control mice (C57BL/6). In addition, immunohistochemical localization of MT in the placenta was determined in these mice. Two days prior to parturition, the mice were injected with radioactive 109Cd chloride (4 microCi, 0.6 ng Cd/mouse) and killed 24 h later. Organs and fetuses were collected and radioactivity, MT and metal levels were measured. Cd accumulated mainly in the liver and kidney (80% of administered dose) with very low levels (0.1-0.3%) detected in fetuses. When analyzed on a per organ or per gram basis, MT-null fetuses accumulated significantly more Cd (3-10-fold) than the control fetuses and there was no significant difference in fetal Cd accumulation in the MT-1* and control fetuses. As expected, MT and zinc levels were higher in MT-1* than C57BL/6 mice and no MT was detected in MT-null mice. Most striking was the high hepatic MT levels in MT-1* dams (4 mg/g). Immunohistochemical analysis showed MT staining in spongiotrophoblasts, glycogen cells, visceral yolk sac, trophoblast giant cells and maternal decidual cells with the MT-1* placenta staining much more intensely as compared to control placenta. The results suggest that placental MT reduces maternal to fetal Cd transfer, however the low doses of Cd administered in the present experiment resulted in high levels of Cd accumulation in liver and kidney in all groups of mice with a low concentration of Cd reaching the placenta. Thus, the role of placental MT as a barrier for Cd is inconclusive.

Teratogenic effects and distribution of cadmium (Cd2+) administered via osmotic minipumps to gravid CF-1 mice

Studies to identify the mechanisms underlying the teratogenic effects of cadmium (Cd2+) have been complicated by the inherent difficulties of chronically and subchronically administering specific doses of Cd2+ to gravid animals under strictly controlled conditions. The objective of the present study was to develop a relatively simple animal model for examining the teratogenic effects of subchronic Cd2+ exposure. Cd2+ was administered to gravid CF-1 mice by subcutaneously implanted Alzet osmotic minipumps, which released fixed amounts of Cd2+ over a 14-day period between days 5 and 18 of gestation. The results showed that Cd2+ administered in this manner produced fetal anomalies and that the patterns of Cd2+ distribution and the specific developmental defects were similar to those that have been reported for other routes of Cd2+ administration. These findings indicate that osmotic minipumps may serve as useful tools in long-term studies of Cd2+ teratogenicity. They would appear to be especially useful in teratogenic evaluations where minimizing maternal stress and administering precise doses of Cd2+ are important.

Cadmium teratogenicity and its relationship with metallothionein gene expression in midgestation mouse embryos

As an approach toward understanding the mechanisms by which cadmium (Cd) exerts its teratogenic effects, the expression and metal regulation of the metallothionein (MT) genes in midgestation mouse embryos were studied by Northern blot and in situ hybridization. Maternal injection of a teratogenic dosage of Cd (50 mumol Cd/kg body wt) did not induce MT mRNA in day 10 (D10) CD-1 mouse embryos, whereas zinc (Zn) (50 mumol/kg was an effective inducer. In contrast, Cd was about 10-fold more potent than Zn at rapidly inducing MT mRNA in D10 embryos incubated in vitro in medium containing micromolar concentrations of these metals. This suggests that following maternal injection, Cd but not Zn is prevented from reaching the D10 embryo and establishes that the embryonic MT genes are not refractory to metal induction, which might have explained the sensitivity of the embryo to Cd. MT mRNA was detected at high levels only in the extraembryonic membranes of D9 embryos exposed to Cd in vivo. On days 9 and 10, no embryonic cell types contained detectable levels of MT mRNA. This mRNA was detected first at low levels in hepatocytes on D11, soon after formation of liver and these levels increased dramatically by D12. Therefore, Cd teratogenicity was not associated with high levels of cell type-specific expression of the MT genes in Cd-sensitive regions of the embryo (neural tube, limb bud), that might have served to target Cd to these cells. Taken together, the results of this study suggest that Cd teratogenicity reflects damage to maternal or extraembryonic tissues. However, the results cannot exclude the possibility that certain cells in the embryo are exceptionally sensitive to low levels of Cd.

Embryotoxicity and in vivo cytogenetic changes following maternal exposure to cadmium chloride in mice

Cadmium is a well-known teratogen in laboratory animals and a widespread environmental pollutant. The frequencies of sister chromatid exchanges (SCEs), nucleolar organizing regions (NORs) and chromosomal aberrations were analysed in maternal bone marrow and fetal liver and/or lung cells of mice, following maternal treatment with cadmium chloride, on gestational days 8 through 10. The embryotoxic effects and morphological changes on day 18 fetuses were also studied. Cadmium chloride is readily transferred across the placenta and significant levels were detected in both the placenta and fetus. No significant changes in the frequencies of SCEs or NORs in maternal and fetal cells were observed following exposure to cadmium chloride. Fetal tissues showed mitotic inhibition at the highest dose levels (8.4 and 11.4 mg/kg, b.w.). Maternal treatment with cadmium chloride increased embryonic resorptions and fetal lethality, as well as reduced placental weight; however, it did not produce significant chromosomal changes except at the highest dose level (11.4 mg/kg).

Chronic cadmium exposure during pregnancy in the mouse: influence of exposure levels on fetal and maternal uptake

The uptake and distribution of orally administered cadmium-109 was studied in pregnant mice. Female outbred QS mice were given cadmium (Cd) supplemented drinking water for 1 mo before pregnancy and for the duration of pregnancy. The water contained either 0.0015 ppm Cd, 0.24 ppm Cd, or 40 ppm Cd. For the duration of pregnancy, 1.48 micrograms Cd/l (0.0015 ppm) in each solution was in the form of 109Cd (1 mCi/l). Control mice were given distilled/deionized water. On the day before term the mice were killed and a variety of adult and fetal tissues were examined in a gamma counter to determine their 109Cd concentrations. For each group the 109Cd concentration was highest in the maternal gastrointestinal tract, liver, and kidneys and lowest in the central nervous system (CNS) and blood. In general, the 109Cd concentrations in each organ were similar for each group of mice and were therefore independent of the overall oral Cd dose. A notable exception was the lower level in the duodenum in the 40 ppm group. In the fetal unit the chorioallantoic placenta contained the highest concentration of 109Cd. Concentrations in the fetuses were very low, comparable to those in the adult CNS. The 109Cd levels in the fetuses from group A were about fivefold greater than those of the fetuses from group C. There was no statistically significant evidence of specific localization in the fetal brain, kidney, or liver.

Stromal stem cells (CFU-f) in yolk sac, liver, spleen and bone marrow of pre- and postnatal mice

Our results demonstrate that in yolk sac, liver, spleen and femoral bone marrow of mice at ages ranging between 11 d of gestation and adult life, important changes in the stromal stem cell population (CFU-f assay) occur which are correlated with haemopoiesis. In the liver, spleen and bone marrow, high numbers of CFU-f precede high haemopoietic stem cell values. As haemopoiesis starts in the spleen, CFU-f numbers in fetal liver are low. Similarly, CFU-f numbers decrease in the spleen as bone marrow haemopoiesis starts. This suggests the existence of a migration stream of stromal stem cells. In spleen and bone marrow, CFU-f numbers decrease towards adult life as these organs maintain a stable haemopoietic activity.

Teratogenicity of ionic cadmium in the Wistar rat

In rats of the present (re-derived) Wistar-Porton strain that are dosed either intravenously (i.v.), or intraperitoneally (i.p.) with Cd (1.25 mg/kg body weight) on day 12 of gestation (gd 12), foetal uptake of Cd is at least 6-fold greater than that reported in an earlier study (Webb and Samarawickrama 1981). Higher doses (1.5 and 2.0 mg/kg body weight) are lethal to the maternal animal when administered i.v., but not if given ip. The foetotoxicity of i.p. injected Cd, however, increases with the dose over the range 1.25-2.0 mg Cd/kg body weight. The teratogenic response, which is also wider than that observed previously, is maximal after the injection of 1.25 mg Cd/kg body weight i.v. on gd 10 and i.p. on gd 12. Whilst the incidences of hydrocephalus, urogenital abnormalities, cleft palate and other less common defects are similar after dosing by both routes, the incidence, range and severity of skeletal malformations are greater after i.p. than after i.v. administration of Cd on gd 12. This difference in response is unlikely to be explained by a difference in either foetal, or placental uptake of the metallic ion since, at 4 h after i.p. dosing, the foetal concentration of Cd is not significantly different from that after i.v. injection, whilst the placental concentration is about 33% less. It is suggested that damage to the maternal liver, which is more severe after the i.v. injection of the optimum dose, may be an additional factor that, in conjunction with the inhibition of transport in the placenta and biosynthetic processes in the embryo/foetus, contributes to the teratogenic effects of Cd in the pregnant rat.

The effect of prednisolone on the foetotoxicity of cadmium in pregnant mice

The effects of prednisolone was investigated on the foetotoxicity of cadmium. CFLP female mice were given a single i.p. dose of 2.5 mg cadmium (Cd) per kg body weight on day 5 or 9 of gestation. This treatment significantly decreased both the number of live foetuses and foetal weights on day 18 of gestation. Prednisolone (0.1 mg kg-1) given daily from the day of cadmium treatment death, prevented the effects of cadmium on foetal weights in both groups, and on the number of live foetuses when cadmium was given on day 9 of gestation. When Cd was given on day 13 of gestation similar treatment with prednisolone did not influence either litter size or the weights of 1-day-old pups.

Histological changes induced in developing limb buds of C57BL mouse embryos submitted in utero to the combined influence of acetazolamide and cadmium sulphate

Microscopic defects in limb buds of C57BL mouse embryos after the combined teratogenic action of acetazolamide plus cadmium sulphate administered on day 9 of gestation were studied in serial sections. Postaxial deficiencies observed in 12-15-day embryos and affecting preferentially the right forelimbs were classified in nine morphological types according to increasing amounts of missing parts. Type X defect consists of a nearly complete amelia in which all four limbs are represented only by the girdle and proximal end of the stylopod. Type XI abnormality appears as an intermediate reduction affecting the area of digit IV. In addition to modifications of the forelimb bud shape detected from the 10-day stage onwards, observations made 24 and 48 hr after treatment confirmed that the postaxial defects result from an absolute lack of postaxial mesoderm occurring without cell necrosis as a consequence of a postaxial shortening of the apical ectodermal ridge (aer). In 10-day embryos, the latter appears shortened and hypertrophied; it is later fragmented into alternate thick and thin portions in 11-day affected limb buds. These ectodermal changes might account for the genesis of all types of defects observed. Untreated 9-day embryos with 12-25 pairs of somites display a number of asymmetries between their right and left forelimb territories: Until the 19-somite stage, the vascular supply to that area is provided exclusively by the umbilical vein, which is larger on the right side; the initial amount of somatopleural limb mesoderm is greater in the right rudiment and the genesis of its aer is slightly protracted as compared to the left one. These asymmetries might contribute to the right side predominance of the forelimb defects induced by acetazolamide and cadmium.

The ontogeny of expression of murine metallothionein: comparison with the alpha-fetoprotein gene

The ontogeny of expression of mouse metallothionein was studied by RNA dot and Northern blot hybridization using a cloned cDNA probe. In some instances the synthesis of metallothionein was analyzed by cell-free translation of RNA as well as pulse-labeling of proteins in short-term organ cultures followed by polyacrylamide gel electrophoresis. Interesting parallels between metallothionein and alpha-fetoprotein gene expression during development were noted. Like alpha-fetoprotein mRNA ( Dziadek and Andrews, 1983), metallothionein mRNA was found to be abundant in developing liver as well as in visceral yolk sac endoderm. In addition, metallothionein mRNA was abundant in parietal yolk sac. During liver development metallothionein and alpha-fetoprotein mRNAs were abundant by Day 12 of gestation, increasing to maximal levels on Day 16 and decreasing during late fetal and neonatal life to basal levels in adult. Metallothionein mRNA increased in maternal liver and was also abundant in certain hepatomas. Synthesis of metallothionein and levels of metallothionein mRNA in visceral yolk sac increased from Day 9 of gestation to maximal levels on Days 11-12 and then decreased abruptly after Day 15. RNA from differentiated teratocarcinoma cells with primitive, parietal or visceral endoderm characteristics each contained high levels of metallothionein mRNA, whereas, levels of this mRNA varied widely among embryonal carcinoma stem cell lines. alpha-Fetoprotein mRNA was not detected in embryonal carcinoma cells but was expressed in visceral endoderm-like differentiated cells. These results indicate that parietal and visceral endoderm cells actively express the metallothionein gene and further suggest that expression may be initiated at the earlier stage of primitive endoderm.