Expression of Metallothionein Genes in Preimplantation Embryos, Decidua and Placentae

Abstract

Metallothioneins (MT) are cysteine rich, heavy metal binding proteins whose expression can be regulated by a variety of factors, such as metal ions, glucocorticoids, and cytokines. Although little is known about expression of MT genes during early mammalian development, substantial variations in expression of these genes in the fetal and neonatal period occur which has lead to the concept that MT may play important roles during development, such as maintenance of zinc and copper homeostasis and protection from heavy metal toxicity. However, the precise functions of MT are unknown. We have examined expression and regulation of metallothionein genes from the time of fertilization to midgestation. Utilizing the preimplantation rabbit embryo as a model it was determined that MT is constitutively expressed at low levels in the blastocyst (D6), and zinc can induce MT as early as day-4 of gestation just after the morula to blastocyst transition. The mouse embryo also develops the capacity to respond to metal ions by the late morula stage. Therefore, the preimplantation mammalian embryo can alter expression of the genome in response to changes in the levels of metal ions in the oviductal and uterine milieu just prior to implantation. In mice, MT mRNA is present at low levels in the uterine luminal epithelium just before implantation (D4), but upon implantation, MT gene expression is specifically and dramatically elevated in the deciduum (D5-10). In the midgestation placenta, constitutively high levels of MT mRNA are present in the outer placental spongiotrophoblasts. At this stage of gestation, the visceral yolk sac endoderm also actively expresses the MT genes. These results provide the basis for the concept that from the time of implantation to late in gestation, the mouse embryo is surrounded by cells, interposed between the maternal and embryonic environments, which actively express the MT genes. This suggests that MT, and perhaps other stress related proteins, play an important role in the establishment and maintenance of normal pregnancy. Studies of the regulation of MT genes in decidua suggest that embryo derived factors are not involved, but rather that these genes may be regulated in an autocrine/paracrine manner, perhaps in response to the pro-inflammatory process of implantation and decidualization.