Disruption of GLI3-ZIC3 interaction in the cadmium-induced omphalocele chick model

Presenilin genes are downregulated during somitogenesis in the cadmium-induced omphalocele chick model

PURPOSE

Although the precise pathogenesis of ventral body wall (VBW) defects is not clearly understood, it has recently postulated that disruption of somite development during early embryogenesis may cause failure of proper VBW formation. The administration of cadmium (Cd) after 60 h of incubation induces omphalocele spectrum in the chick embryo. Previous studies have shown that one of the earliest histological changes seen in this model is abnormal cell death in the somite, occurring at 4 h post treatment (4H). However, the molecular mechanism by which Cd acts in this critical period of embryogenesis still remains unclear. Presenilins are expressed in somites and play an important role in vertebrate development, including somitogenesis and thus VBW formation. We designed this study to test the hypothesis that gene expression levels of presenilin 1 (PSEN1) and presenilin 2 (PSEN2) are downregulated during the critical period of early embryogenesis in the Cd-induced omphalocele in the chick model.

METHODS

After 60 h of incubation, chick embryos were exposed to either saline or 50 μM cadmium and divided into two groups: control and Cd (n = 8 at each time point for each group). Real-time RT-PCR was performed to evaluate the relative mRNA expression levels of PSEN1 and PSEN2 in the Cd-induced omphalocele chick model. Differences between two groups at each time point were analysed statistically and the significance was accepted at p < 0.05. Immunofluorescence confocal microscopy was performed to evaluate the protein expression/distribution of presenilins in the somite of chick embryo.

RESULTS

The relative mRNA expression levels of PSEN1 and PSEN2 were significantly downregulated in the Cd group at 4H compared with controls (p < 0.005) (Table). However, there were no significant differences at the other time points. At 4H, immunofluorescence of presenilins (green) was markedly diminished in the Cd-treated embryos, whereas strong immunofluorescence of them was seen in the somite (dermomyotome) in controls (Fig. 1). 1 Immunofluorescence Confocal Microscopy for PSEN1 and PSEN2 in the dermomyotome of the somite in the trunk level of chick embryo 4H post treatment. Intensity of PSEN1 immunofluorescence (green) was markedly diminished in Cd-treated embryos, whereas strong PSEN1 expression was seen in the dermomyotome in controls. PSEN2 immunofluorescence was also decreased in the Cd-treated embryos, whereas strong PSEN2 immunofluorescence (green) was observed predominantly in the dermomyotome in controls. Immunofluorescence in orange is DNA counter staining by DAPI CONCLUSION: We provide evidence, for the first time, that gene expression of presenilins is downregulated during the narrow window of very early embryogenesis in the Cd chick model. Decreased expression of presenilins may contribute to omphalocele phenotype in Cd chick model, by disrupting somite development.

Epigenetic effect of cadmium on global de novo DNA hypomethylation in the cadmium-induced ventral body wall defect (VBWD) in the chick model

Administration of the heavy metal cadmium (Cd) induces ventral body wall defects (VBWD) in the chick embryo. In this model, the expression of most genes involved in body wall formation is altered 4h-posttreatment. However, the mechanism by which Cd results in the initiation of altered gene expression remains unclear. Epigenetic mechanisms can change genome function under exogenous influences. Moreover, Cd is one of the environmental factors that can affect epigenomic programming. De novo DNA methylation is essential for normal embryogenesis and is regulated by the DNA methyltransferases (DNMT)3A and DNMT3B. The objective of this study was to investigate the hypothesis that gene expression levels of DNMT3A/3B were altered, resulting in global DNA methylation changes during the critical period of embryogenesis in the Cd chick model. After 60-h incubation, chick embryos (n = 48) were harvested at 1, 4, and 8 h after treatment with saline or Cd, and divided into controls and Cd groups. Quantitative reverse transcription PCR was performed to evaluate the gene expression levels of DNMT3A/3B in the chick embryos and was statistically analyzed using Student's t-test. Immunohistochemistry was performed using a monoclonal antibody against 5-methylcytidine (5'MeC), which labels methyl-rich regions within the nucleus. DNMT3A/3B gene expression levels at 4 h were significantly downregulated in the Cd group compared with controls (p < 0.005/p < 0.00001, respectively). Immunoreactivity of 5'MeC was markedly diminished in the Cd group at 4 h. Our findings demonstrates for the first time that Cd impacts on the expression levels of DNMT3A/3B, which may underlie the pathogenesis of VBWD in the Cd chick model.