Endogenous retroviruses (ERVs) are fixed and abundant in the genomes of vertebrates. Circumstantial evidence suggests that ERVs play a role in mammalian reproduction, particularly placental morphogenesis, because intact ERV envelope genes were found to be expressed in the syncytiotrophoblasts of human and mouse placenta and to elicit fusion of cells in vitro. We report here in vivo and in vitro experiments finding that the envelope of a particular class of ERVs of sheep, endogenous Jaagsiekte sheep retroviruses (enJSRVs), regulates trophectoderm growth and differentiation in the periimplantation conceptus (embryo͞fetus and associated extraembryonic membranes). The enJSRV envelope gene is expressed in the trophectoderm of the elongating ovine conceptus after day 12 of pregnancy. Loss-of-function experiments were conducted in utero by injecting morpholino antisense oligonucleotides on day 8 of pregnancy that blocked enJSRV envelope protein production in the conceptus trophectoderm. This approach retarded trophectoderm outgrowth during conceptus elongation and inhibited trophoblast giant binucleate cell differentiation as observed on day 16. Pregnancy loss was observed by day 20 in sheep receiving morpholino antisense oligonucleotides. In vitro inhibition of the enJSRV envelope reduced the proliferation of mononuclear trophectoderm cells isolated from day 15 conceptuses. Consequently, these results demonstrate that the enJSRV envelope regulates trophectoderm growth and differentiation in the periimplantation ovine conceptus. This work supports the hypothesis that ERVs play fundamental roles in placental morphogenesis and mammalian reproduction.development ͉ placenta ͉ sheep ͉ trophectoderm T he sheep genome contains Ϸ20 copies of endogenous retroviruses (ERVs) highly related to the exogenous and pathogenic Jaagsiekte sheep retrovirus (JSRV) (1-3). Endogenous JSRVs (enJSRVs) are abundantly expressed in the epithelia of the female genital tract (4). In the placenta, enJSRVs are expressed in the mononuclear trophectoderm cells of the conceptus (embryo͞fetus and associated extraembryonic membranes) and are most abundant in the trophoblast giant binucleate cells (BNCs) and multinucleated syncytial plaques of the placentomes (5-7). The temporal expression of the enJSRV envelope (env) gene in the trophectoderm is coincident with key events in the development of the sheep conceptus (8). enJSRV env mRNAs are first detected at day 12 (5), when the blastocyst begins the process of elongation, involving the intense proliferation and outgrowth of mononuclear trophectoderm cells producing IFN-, the antiluteolytic signal for pregnancy recognition in ruminants (9, 10).In sheep, trophoblast giant BNCs differentiate from mononuclear trophectoderm cells beginning on day 14, migrate, and then fuse with the uterine luminal epithelium, as well as each other, to form multinucleated syncytial plaques that ultimately form the cotyledonary portions of the placenta (11). The BNCs derive from the mononuclear trophectoderm cells by a poorly ch...

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Ornithine decarboxylase (ODC1) is considered the rate-controlling enzyme for the classical de novo biosynthesis of polyamines (putrescine, spermidine, and spermine) in mammals. However, metabolism of arginine to agmatine via arginine decarboxylase (ADC) and conversion of agmatine to polyamines via agmatinase (AGMAT) is an alternative pathway long recognized in lower organisms, but only recently suggested for neurons and liver cells of mammals. We now provide evidence for a functional ADC/AGMAT pathway for the synthesis of polyamines in mammalian reproductive tissue for embryonic survival and development. We first investigated cellular functions of polyamines by in vivo knockdown of translation of mRNA for ODC1 in ovine conceptus trophectoderm using morpholino antisense oligonucleotides (MAOs) and found that one-half of the conceptuses were morphologically and functionally either normal or abnormal. Furthermore, we found that increases in ADC/AGMAT mRNA levels and in the translation of AGMAT mRNA among conceptuses in MAO-ODC1 knockdown compensated for the loss of ODC1, supporting polyamine synthesis from arginine and accounting for the normal and abnormal phenotypes of conceptuses. We conclude that the majority of polyamine synthesis is by the conventional ODC1-dependent pathway (arginine-ornithine-putrescine) and that deficiencies in ODC1 result in increased activity of the rescue ADC/AGMAT-dependent pathway (arginine-agmatine-putrescine) for production of polyamines. The presence of an alternative ADC/AGMAT pathway for converting arginine into putrescine is functionally important for supporting survival and development of mammalian conceptuses.

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There is a dialogue between the developing conceptus (embryo-fetus and associated placental membranes) and maternal uterus which must be established during the peri-implantation period for pregnancy recognition signaling, implantation, regulation of gene expression by uterine epithelial and stromal cells, placentation and exchange of nutrients and gases. The uterus provide a microenvironment in which molecules secreted by uterine epithelia or transported into the uterine lumen represent histotroph required for growth and development of the conceptus and receptivity of the uterus to implantation. Pregnancy recognition signaling mechanisms sustain the functional lifespan of the corpora lutea (CL) which produce progesterone, the hormone of pregnancy essential for uterine functions that support implantation and placentation required for a successful outcome of pregnancy. It is within the peri-implantation period that most embryonic deaths occur due to deficiencies attributed to uterine functions or failure of the conceptus to develop appropriately, signal pregnancy recognition and/or undergo implantation and placentation. With proper placentation, the fetal fluids and fetal membranes each have unique functions to ensure hematotrophic and histotrophic nutrition in support of growth and development of the fetus. The endocrine status of the pregnant female and her nutritional status are critical for successful establishment and maintenance of pregnancy. This review addresses the complexity of key mechanisms that are characteristic of successful reproduction in sheep and pigs and gaps in knowledge that must be the subject of research in order to enhance fertility and reproductive health of livestock species.