Differential Expression Pattern of Retroviral Envelope Gene in the Equine Placenta.

Valentina Stefanetti,Stefano Capomaccio,Fabrizio Passamonti,Maria Luisa Marenzoni,Katia Cappelli,Lara Reale,Mauro Coletti,Maurizio Monaci,Martina Crociati,Luisa Pascucci,Sandra Wilsher

doi:10.3389/fvets.2021.693416

Valentina Stefanetti, Stefano Capomaccio + Show 9 more

Open Access

https://doi.org/10.3389/fvets.2021.693416

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Abstract

Endogenous retroviruses (ERVs) are proviral phases of exogenous retroviruses, which have coevolved with vertebrate genomes for millions of years. The conservation of ERV genes throughout evolution suggests their beneficial effects on their hosts' survival. An example of such positive selection is demonstrated by the syncytin gene, which encodes a protein with affinity for various mammalian placentas that is involved in the formation of syncytiotrophoblasts. Although the horse has an epitheliochorial placenta, in which the fetal trophoblasts are simply apposed to the intact uterine epithelium, we have previously demonstrated that the equine ERV (EqERV) env RNA is unexpectedly expressed in placental tissue. In the present study, we investigated the mRNA expression pattern of the EqERV env gene in different parts of the equine placenta, to gain more insight into its putative role in the fetal–maternal relationship. To this end, we used reverse transcription–quantitative PCR (RT–qPCR) and in situ hybridization assays to analyze different target areas of the equine placenta. The retroviral env gene is expressed in the equine placenta, even though there is no syncytium or erosion of the uterine endometrium. The gene is also expressed in all the sampled areas, although with some quantitative differences. We suggest that these differences are attributable to variations in the density, height, and degree of morphological complexity of the chorionic villi forming the microcotyledons. The involvement of the EqERV env gene in different functional pathways affecting the fetus–mother relationship can be hypothesized.

Highlights

Retroviruses are characterized by their ability to integrate their DNA into the genomes of their host cells, forming so-called “proviruses” [1]
The horse has an epitheliochorial placenta, in which the fetal and maternal tissues are apposed to one another, with no syncytiotrophoblast formation, we showed in a previous study that the equine endogenous retrovirus (ERV) (EqERV) env RNA is more strongly expressed in the horse placenta than in other somatic tissues, as might be expected for a candidate syncytin-like gene [20]
We proposed that the intact EqERV env gene is a candidate syncytin-like gene, as identified by Zhu et al [18], and that it belongs to the zeta lineage

Summary

Introduction

Retroviruses are characterized by their ability to integrate their DNA into the genomes of their host cells, forming so-called “proviruses” [1]. The syncytin genes, which encode the ERV envelope (Env) protein, the fusogenic activity of which is required by exogenous retroviruses to gain entry into host cells, appear to have a particular affinity for the placentas of various mammals, including Primates [7], Muridae [8], Lagomorpha [9], Carnivora [10], Ruminantia [11], Afrotherians [12], and even the distantly related Marsupialia [13] These syncytins have been well-characterized and are associated with the invasive types of placentation, where they are involved in the formation of syncytiotrophoblasts. It has been proposed that this stochastic acquisition of exogenous genes participated in creating the structural and functional diversity of the mammalian placentas [14]

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