Expression of endogenous retroviruses in pre‐implantation stages of bovine embryo

Progressive Exaptation of Endogenous Retroviruses in Placental Evolution in Cattle

Viviparity is made possible by the placenta, a structure acquired relatively recently in the evolutionary history of eutherian mammals. Compared to oviparity, it increases the survival rate of the fetus, owing to the eutherian placenta. Questions such as "How was the placenta acquired?" and "Why is there diversity in placental morphology among mammalian species?" remain largely unsolved. Our present understanding of the molecules regulating placental development remains unclear, owing in no small part to the persistent obscurity surrounding the molecular mechanisms underlying placental acquisition. Numerous genes associated with the development of eutherian placental morphology likely evolved to function at the fetal-maternal interface in conjunction with those participating in embryogenesis. Therefore, identifying these genes, how they were acquired, and how they came to be expressed specifically at the fetal-maternal interface will shed light on some crucial molecular mechanisms underlying placental evolution. Exhaustive studies support the hypothesis that endogenous retroviruses (ERVs) could be evolutional driving forces for trophoblast cell fusion and placental structure in mammalian placentas including those of the bovine species. This review focuses on bovine ERVs (BERVs) and their expression and function in the placenta.

Fasting and overfeeding affect the expression of the immunity- or inflammation-related genes in the liver of poultry via endogenous retrovirus

It is known that nutrition and immunity are connected, but the mechanism is not very clear. Endogenous retroviruses (ERV) account for 8 to 10% of the human and mouse genomes and play an important role in some biological processes of animals. Recent studies indicate that the activation of ERV can affect the expression of the immunity- or inflammation-related genes, and the activities of ERV are subjected to regulation of many factors including nutritional factors. Therefore, we hypothesize that nutritional status can affect the expression of the immunity- or inflammation-related genes via ERV. To verify this hypothesis, the nutritional status of animals was altered by fasting or overfeeding, and the expression of intact ERV (ERVK18P, ERVK25P) and immunity- or inflammation-related genes (DDX41, IFIH1, IFNG, IRF7, STAT3) in the liver was determined by quantitative PCR, followed by overexpressing ERVK25P in goose primary hepatocytes and determining the expression of the immunity- or inflammation-related genes. The data showed that compared with the control group (no fasting), the expression of ERV and the immunity- or inflammation-related genes was increased in the liver of the fasted chickens but decreased in the liver of the fasted geese. Moreover, compared with the control group (routinely fed), the expression of ERV and the immunity- or inflammation-related genes was increased in the liver of the overfed geese. In addition, overexpression of ERVK25P in goose primary hepatocytes can induce the expression of the immunity- or inflammation-related genes. In conclusion, these findings suggest that ERV mediate the effects of fasting and overfeeding on the expression of the immunity- or inflammation-related genes, the mediation varied with poultry species, and ERV and the immunity- or inflammation-related genes may be involved in the development of goose fatty liver. This study provides a potential mechanism for the connection between nutrition and immunity.

Viruses in the reproductive tract: On their way to the germ line?

Studies of vertebrate genomes have indicated that all species contain in their chromosomes stretches of DNA with sequence similarity to viral genomes. How such 'endogenous' viral elements (EVEs) ended up in host genomes is usually explained in general terms such as 'they entered the germ line at some point during evolution'. This seems a correct statement, but is also rather imprecise. The vast number of endogenous viral sequences suggest that common routes to the 'germ line' may exist, as relying on chance alone may not easily explain the abundance of EVEs in modern mammalian genomes. An increasing number of virus types have been detected in human semen and a growing number of studies have reported on viral infections that cause male infertility or subfertility and on viral infections that threaten in vitro fertilisation practices. Thus, it is timely to survey the pathway(s) that viruses can use to gain access to the human germ line. Embryo transfer and semen quality studies in livestock form another source of relevant information because virus infection during reproduction is clearly unwanted, as is the case for the human situation. In this review, studies on viruses in the male and female reproductive tract and in the early embryo will be discussed to propose a plausible viral route to the mammalian germ line.

The necessity of ZSCAN4 for preimplantation development and gene expression of bovine embryos

Zinc finger and SCAN domain containing 4 (Zscan4) is a gene that is specifically expressed during zygotic genome activation (ZGA) in mouse preimplantation embryos, and a reduction of Zscan4 transcripts leads to developmental failure. In mouse embryonic stem cells (ESCs), Zscan4 is expressed transiently in as little as 1–5% of the cell population. Zscan4 has also been shown to enhance the efficiency of mouse induced pluripotent stem cells (iPSCs) generation and their quality. Although ZSCAN4 plays important roles in murine embryos and stem cells, its expression and role in bovine embryos is unknown. This study examines ZSCAN4 transcripts in bovine embryos at various developmental stages and attempts to elucidate the functions of ZSCAN4 during bovine preimplantation development. ZSCAN4 transcripts were found to be upregulated at the 8- and 16-cell stages. We next attempted ZSCAN4 downregulation in bovine early embryos by RNA interference and evaluated developmental competency and transcripts levels of genes involved in ZGA and iPSCs generation. Although the bovine embryos injected with ZSCAN4-siRNA could develop to the 8-cell stage, very few were developing beyond the 16-cell stage. PIWIL2 expression was reduced in ZSCAN4 downregulated embryos. It is possible that ZSCAN4 downregulated embryos fail to regulate gene expression during ZGA. Our results indicate that ZSCAN4 is an important factor for the preimplantation development of bovine embryos.

Endogenous Retroviruses Function as Gene Expression Regulatory Elements During Mammalian Pre-implantation Embryo Development

Pre-implantation embryo development encompasses several key developmental events, especially the activation of zygotic genome activation (ZGA)-related genes. Endogenous retroviruses (ERVs), which are regarded as “deleterious genomic parasites”, were previously considered to be “junk DNA”. However, it is now known that ERVs, with limited conservatism across species, mediate conserved developmental processes (e.g., ZGA). Transcriptional activation of ERVs occurs during the transition from maternal control to zygotic genome control, signifying ZGA. ERVs are versatile participants in rewiring gene expression networks during epigenetic reprogramming. Particularly, a subtle balance exists between ERV activation and ERV repression in host–virus interplay, which leads to stage-specific ERV expression during pre-implantation embryo development. A large portion of somatic cell nuclear transfer (SCNT) embryos display developmental arrest and ZGA failure during pre-implantation embryo development. Furthermore, because of the close relationship between ERV activation and ZGA, exploring the regulatory mechanism underlying ERV activation may also shed more light on the enigma of SCNT embryo development in model animals.