METALLOTHIONEIN GENE-EXPRESSION AND METAL REGULATION DURING PREIMPLANTATION MOUSE EMBRYO DEVELOPMENT (MT MESSENGER-RNA DURING EARLY DEVELOPMENT)

In order to provide information concerning gene expression and regulation in the preimplantation mammalian embryo, and to explore the roles of metallothionein (MT) during this period of development, the constitutive and metal-induced MT mRNA levels in mouse ova, preimplantation embryos, and oviducts were determined. These results were correlated with the effects of transient exposure to high levels of metals (zinz (Zn) or cadmium (Cd)) on the continued development of preimplantation embryos into blastocysts in culture. RNA from preimplantation mouse embryos at different stages of development (Days 1 through 4 of gestation; D1 = vaginal plug) was analyzed using the reverse transcriptase-polymerase chain reaction (RT-PCR) to specifically amplify MT-I and MT-II mRNA transcripts. MT-I mRNA in ova, preimplantation embryos, and oviducts was detected using in situ hybridization. This mRNA in the oviduct was also analyzed by Northern blotting. The results establish that the mouse MT genes are coordinately and constitutively expressed at low basal levels in ova and preimplantation mouse embryos. In unfertilized (ova), fertilized (one-cell) eggs, and two-cell embryos, the MT-I gene was not detectably responsive to metal ions, whereas in later cleavage stage embryos (four- and eight-cell) the MT-I gene was detectably responsive to metals in some blastomeres of some of the embryos. In contrast, after the third cleavage this gene was highly metal-inducible in essentially all cells of the embryo (morula/blastocyst). Surprisingly, the appearance of metal responsiveness of the MT genes during development correlated with decreased Zn toxicity and increased Cd toxicity; two-cell embryos were Zn-sensitive and Cd-resistant, whereas eight-cell and older embryos were Zn-resistant and Cd-sensitive. In the oviduct, MT-I mRNA was not abundant in total RNA, but was detected specifically in the epithelial cells of the isthmus region and was elevated in these cells on D3 and D4 of gestation. In the oviduct, only isthmus epithelial cells responded to metals (Zn or Cd) by increased accumulation of this mRNA. These studies suggest that preimplantation mouse embryo develops the capacity to respond to metals in the environmental milieu by induction of MT gene expression at about the third cleavage. Whether the lack of responsiveness of these genes before this stage reflects transcriptional repression or attenuated metal ion influx and/or enhanced efflux remains to be determined. Sensitivity and resistance of preimplantation embryos to acute metal toxicity involve mechanisms other than MT gene expression in preimplantation mouse embryos. The cell-specific expression of MT genes in the isthmus epithelia of the oviduct suggests that these cells play a role in metal ion homeostasis on D3 and D4 of pregnancy, perhaps by creating a local environment rich in the essential metals Zn and copper and/or by protecting from the toxic effects of Cd. © 1991.