The Roles of Syncytin-Like Proteins in Ruminant Placentation

Epithelial-Mesenchymal Transitions Leading to Conceptus Adhesion in Ruminants: Early Pregnancy Events in Cattle

Trophoblast-endometrium interactions play a critical role in the processes of conceptus elongation, attachment, and adhesion, followed by placental development during early pregnancy in ruminants. The attachment between uterine epithelium and trophoblast cells, which is epithelial in nature, requires epithelial to mesenchymal transition (EMT), where the fetal trophoblasts come into contact with maternal epithelial cells without fully invading the maternal tissues. Understanding the early developmental period driving EMT processes in utero in ruminants is fundamental to improving fertility through the prevention of early pregnancy failure and enhancing overall reproductive efficiency in livestock. This review highlights the key events necessary for the early conceptus to progress properly towards firm adhesion with the endometrium, focusing on trophoblast-endometrium interactions. This field holds the potential to elucidate molecular and cellular mechanisms associated with trophoblast and endometrium attachment and adhesion, leading to reduced early embryonic losses and enhanced economic sustainability by developing effective reproductive management strategies.

oar-miR-29a promotes the establishment of endometrial receptivity by targeting CDC42 in sheep

Endometrial receptivity is vital for the successful implantation of embryos in sheep. Insufficient receptivity is a major cause of implantation failure, highlighting the need to understand the underlying mechanisms. MicroRNAs (miRNAs) play an essential role in the epigenetic regulation of endometrial receptivity and embryo implantation by influencing post-transcriptional processes. However, the specific mechanisms by which various miRNAs contribute to endometrial receptivity and embryo implantation in sheep remain to be fully elucidated. Our research highlights the significant role of oar-miR-29a, which promotes the migration and proliferation of sheep endometrial epithelial cells (sEECs). We found that oar-miR-29a enhances mRNA expression associated with proliferation, migration, and epithelial-mesenchymal transition (EMT) in sEECs. Additionally, this miRNA enhances the proliferation and migration capabilities of sheep trophoblast cells (sTCs). Moreover, our findings suggest that the target genes of oar-miR-29a are involved in key biological processes and pathways essential for embryo implantation. Notably, our analysis identifies cell division cycle 42 (CDC42) as a target gene of oar-miR-29a. In summary, oar-miR-29a plays a crucial role in promoting endometrial receptivity by targeting CDC42 in sheep. These findings provide insights into the miRNA-based regulatory mechanisms governing uterine physiology during the early stages of pregnancy, emphasizing the importance of this research for improving reproductive outcomes in sheep.

Endogenous Jaagsiekte sheep retrovirus envelope protein promotes sheep trophoblast cell fusion by activating PKA/MEK/ERK1/2 signaling

BACKGROUND

Endogenous Jaagsiekte sheep retrovirus envelope protein (enJSRV-Env) plays an important role in trophoblast cell fusion in sheep. However, the underlying mechanism remains unclear.

METHODS

Primary endometrial luminal epithelial cells (LECs) were isolated from the sheep uterus and cocultured with sheep trophoblast cells (STCs). Giemsa staining was conducted to count multinucleated cells in the coculture system. Gain- and loss-of-function assays were performed to explore the role of enJSRV-Env in trophoblast cell fusion in the coculture system. Co-immunoprecipitation and mass spectrometry were carried out to identify the interacting partner of enJSRV-Env in the cocultures. Western blot analysis were conducted to determine the activation of protein kinase A (PKA)/mitogen-activated extracellular signal-regulated kinase (MEK)/extracellular signal-regulated kinase 1/2 (ERK1/2) signaling.

RESULTS

Primary LECs were identified by the expression of epithelial marker cytokeratin 18. Overexpression of enJSRV-Env promoted the formation of multinucleated cells in the coculture system. enJSRV-Env activated and physically interacted with PKA, along with the activation of MEK/ERK1/2 signaling. PKA inhibition completely reversed enJSRV-Env-induced MEK/ERK1/2 activation, and ERK1/2 inhibition abolished enJSRV-Env-induced formation of multinucleated cells in the coculture system.

CONCLUSION

enJSRV-Env promotes trophoblast cell fusion in the sheep placenta by activating PKA/MEK/ERK1/2 signaling. This finding reveals a novel mechanism underlying the contribution of enJSRV-Env to trophoblast cell fusion during placental morphogenesis.

Rapid acquisition of microorganisms and microbial genes can help explain punctuated evolution

The punctuated mode of evolution posits that evolution occurs in rare bursts of rapid evolutionary change followed by long periods of genetic stability (stasis). The accepted cause for the rapid changes in punctuated evolution is special ecological circumstances – selection forces brought about by changes in the environment. This article presents a complementary explanation for punctuated evolution by the rapid formation of genetic variants in animals and plants by the acquisition of microorganisms from the environment into microbiomes and microbial genes into host genomes by horizontal gene transfer. Several examples of major evolutionary events driven by microorganisms are discussed, including the formation of the first eukaryotic cell, the ability of some animals to digest cellulose and other plant cell-wall complex polysaccharides, dynamics of root system architecture, and the formation of placental mammals. These changes by cooperation were quantum leaps in the evolutionary development of complex bilolgical systems and can contribute to an understanding of the mechanisms underlying punctuated evolution.

Ruminant conceptus-maternal interactions: interferon-tau and beyond

Embryonic or fetal loss in cattle is associated with problems that occur during oocyte maturation, early embryonic development, conceptus elongation, maternal recognition of pregnancy (MRP), and/or placental attachment and implantation. Many of these problems manifest as inadequate or asynchronous communication between the developing conceptus and endometrium, resulting in pregnancy failure. This review will provide an overview of how various conceptus-endometrial paracrine signaling systems control the fate of early pregnancy in cattle and other ruminants. We begin by summarizing the actions of interferon-tau, the classic MRP signal in ruminates, and then explore how other secretory factors derived from either the conceptus or endometrium influence establishment and maintenance of pregnancy. Insight into how the endometrium responds to male vs. female conceptuses or conceptuses produced by in vitro methods will also be described. Specific focus will be placed on describing how "omic" technologies and other cutting-edge techniques have assisted with identifying novel conceptus and/or endometrial factors and their functions. Recent findings indicate that the endometrial transcriptome and histotroph are altered by conceptus sex, quality, and origin, suggesting that the endometrium is a sensor of conceptus biochemistry. Although the endometrium has a certain level of flexibility in terms of conceptus-maternal interactions, this interplay is not sufficient to retain some pregnancies. However, new information inspires us to learn more and will help develop technologies that mitigate early embryonic loss and reproductive failure in ruminants and other animals.

The Ruminant Placental Trophoblast Binucleate cell - an evolutionary breakthrough

Viviparity and the development of a placenta are two of the major reasons for the success of the mammals in colonizing all habitats, both terrestrial and aquatic. The placenta is an apposition of fetal to maternal tissue which serves two main, but competing functions: to maximize oxygen transfer and the acquisition of nutrients from the mother, but to minimize immunological rejection by the maternal immune system. This has resulted in the evolution of four main types differing in the degree of loss of the maternal uterine epithelial (UE) barrier: epitheliochorial, synepitheliochorial, endotheliochorial, and hemochorial, all providing a successful safe balance between the needs of mother and fetus. Epitheliochorial is the least invasive, a simple apposition and microvillar interdigitation of the apices of uterine epithelium and trophoblast. It is suggested to have evolved as a response to the increase in the size of the animal to provide a sufficiently long gestation to produce a single altricial (run/swim-soon-as-born) neonate as in the Cetartiodactyla. The mother needs to have good control of the fetal demands so the UE barrier is maintained. However, in the synepitheliochorial placenta, characteristic of all ruminants, the fetus has evolved a means of increasing, or at least maintaining, demand without the need for invasion. This has been achieved by the development of the trophoblast binucleate cell which, uniquely, can fuse with a UE cell to form fetomaternal hybrid tissue. This can maintain some maternal barrier function but also deliver fetally synthesized immunomodulatory and metabolic messages to the maternal circulation. This review provides the evidence for this remarkable evolutionary step and also considers an alternative explanation for the formation of the structure of the ruminant placenta.

PEG10 viral aspartic protease domain is essential for the maintenance of fetal capillary structure in the mouse placenta

The therian-specific gene paternally expressed 10 (Peg10) plays an essential role in placenta formation: Peg10 knockout mice exhibit early embryonic lethality as a result of severe placental defects. The PEG10 protein exhibits homology with long terminal repeat (LTR) retrotransposon GAG and POL proteins; therefore, we generated mice harboring a mutation in the highly conserved viral aspartic protease motif in the POL-like region of PEG10 because this motif is essential for the life cycle of LTR retrotransposons/retroviruses. Intriguingly, frequent perinatal lethality, not early embryonic lethality, was observed with fetal and placental growth retardation starting mid-gestation. In the mutant placentas, severe defects were observed in the fetal vasculature, where PEG10 is expressed in the three trophoblast cell layers that surround fetal capillary endothelial cells. Thus, Peg10 has essential roles, not only in early placenta formation, but also in placental vasculature maintenance from mid- to late-gestation. This implies that along the feto-maternal placenta interface an interaction occurs between two retrovirus-derived genes, Peg10 and retrotransposon Gag like 1 (Rtl1, also called Peg11), that is essential for the maintenance of fetal capillary endothelial cells.

Summary: Disruption of the highly conserved viral aspartic protease domain in PEG10 causes placental abnormality leading to perinatal lethality in mice.

Endogenous retroviruses drive lineage-specific regulatory evolution across primate and rodent placentae

In mammals, the placenta mediates maternal-fetal nutrient and waste exchange and acts in an immunomodulatory way to facilitate maternal-fetal tolerance. The placenta is highly diverse across mammalian species, yet the molecular mechanisms that distinguish the placenta of human from other mammals are not fully understood. Using an interspecies transcriptomic comparison of human, macaque, and mouse late-gestation placentae, we identified hundreds of genes with lineage-specific expression-including dozens that are placentally-enriched and potentially related to pregnancy. We further annotated the enhancers for different human tissues using epigenomic data and demonstrate that the placenta and chorion are unique in that their enhancers display the least conservation. We identified numerous lineage-specific human placental enhancers and found they highly overlap with specific families of endogenous retroviruses (ERVs), including MER21A, MER41A/B and MER39B that were previously linked to immune response and placental function. Among these ERV families, we further demonstrate that MER41A/B insertions create dozens of lineage-specific Serum Response Factor (SRF) binding loci in human, including one adjacent to FBN2, a placenta-specific gene with increased expression in humans that produces the peptide hormone placensin to stimulate glucose secretion and trophoblast invasion. Overall, our results demonstrate the prevalence of lineage-specific placental enhancers which are frequently associated with ERV insertions and likely facilitate the lineage-specific evolution of the mammalian placenta.

Microbial driven genetic variation in holobionts

Genetic variation in holobionts, (host and microbiome), occurring by changes in both host and microbiome genomes, can be observed from two perspectives: observable variations and the processes that bring about the variation. The observable includes the enormous genetic diversity of prokaryotes, which gave rise to eukaryotic organisms. Holobionts then evolved a rich microbiome with a stable core containing essential genes, less so common taxa, and a more diverse non-core enabling considerable genetic variation. The result being that, the human gut microbiome, for example, contains 1,000 times more unique genes than are present in the human genome. Microbial driven genetic variation processes in holobionts include: (1) Acquisition of novel microbes from the environment, which bring in multiple genes in one step, (2) amplification/reduction of certain microbes in the microbiome, that contribute to holobiont` s adaptation to changing conditions, (3) horizontal gene transfer between microbes and between microbes and host, (4) mutation, which plays an important role in optimizing interactions between different microbiota and between microbiota and host. We suggest that invertebrates and plants, where microbes can live intracellularly, have a greater chance of genetic exchange between microbiota and host, thus a greater chance of vertical transmission and a greater effect of microbiome on evolution of host than vertebrates. However, even in vertebrates the microbiome can aid in environmental fluctuations by amplification/reduction and by acquisition of novel microorganisms.

Conceptus-induced, interferon tau-dependent gene expression in bovine endometrial epithelial and stromal cells†

Bovine endometrium consists of epithelial and stromal cells that respond to conceptus interferon tau (IFNT), the maternal recognition of pregnancy (MRP) signal, by increasing expression of IFN-stimulated genes (ISGs). Endometrial epithelial and stromal-cell-specific ISGs are largely unknown but hypothesized to have essential functions during pregnancy establishment. Bovine endometrial epithelial cells were cultured in inserts above stromal fibroblast (SF) cells for 6 h in medium alone or with IFNT. The epithelial and SF transcriptomic response was analyzed separately using RNA sequencing and compared to a list of 369 DEGs recently identified in intact bovine endometrium in response to elongating bovine conceptuses and IFNT. Bovine endometrial epithelial and SF shared 223 and 70 DEGs in common with the list of 369 endometrial DEGs. Well-known ISGs identified in the epithelial and SF were ISG15, MX1, MX2, and OAS2. DEGs identified in the epithelial but not SF included a number of IRF molecules (IRF1, IRF2, IRF3, and IRF8), mitochondria SLC transporters (SLC25A19, SLC25A28, and SLC25A30), and a ghrelin receptor. Expression of ZC3HAV1, an anti-retroviral gene, increased specifically within the SF. Gene ontology analysis identified the type I IFN signaling pathway and activation of nuclear factor kappa B transcription factors as biological processes associated with the epithelial cell DEGs. This study has identified biologically relevant IFNT-stimulated genes within specific endometrial cell types. The findings provide critical information regarding the effects of conceptus IFNT on specific endometrial compartments during early developmental processes in cattle.

Origins and evolutionary consequences of ancient endogenous retroviruses

Retroviruses infect a broad range of vertebrate hosts that includes amphibians, reptiles, fish, birds and mammals. In addition, a typical vertebrate genome contains thousands of loci composed of ancient retroviral sequences known as endogenous retroviruses (ERVs). ERVs are molecular remnants of ancient retroviruses and proof that the ongoing relationship between retroviruses and their vertebrate hosts began hundreds of millions of years ago. The long-term impact of retroviruses on vertebrate evolution is twofold: first, as with other viruses, retroviruses act as agents of selection, driving the evolution of host genes that block viral infection or that mitigate pathogenesis, and second, through the phenomenon of endogenization, retroviruses contribute an abundance of genetic novelty to host genomes, including unique protein-coding genes and cis-acting regulatory elements. This Review describes ERV origins, their diversity and their relationships to retroviruses and discusses the potential for ERVs to reveal virus-host interactions on evolutionary timescales. It also describes some of the many examples of cellular functions, including protein-coding genes and regulatory elements, that have evolved from ERVs.

DNA methylation of the endogenous retrovirus Fematrin-1 in fetal placenta is associated with survival rate of cloned calves

INTRODUCTION

The expression of retroviral envelope proteins in the placenta facilitates generation of the multinuclear syncytiotrophoblast as an outer cellular layer of the placenta by fusion of the trophoblastic cells. This process is essential for placenta development in eutherians and for successful pregnancy.

METHODS

We tested the hypothesis that alterations in DNA methylation and gene expression profiles of the endogenous retroviruses (ERVs) and genes related to epigenetic reprogramming in placenta of cloned calves result in abnormal offspring phenotypes. The fetal cotyledons in 13 somatic cell nuclear transfer (SCNT) pregnancies were collected. DNA methylation level of Fematrin-1 was analyzed using bisulfite PCR and mRNA levels of Fematrin-1, Syncytin-Rum1, DNMT1, DNMT3A, DNMT3B, TET1, TET2 and TET3 measured by RT-qPCR.

RESULTS

Methylation of Fematrin-1 in placenta of control animals produced by artificial insemination (AI) was similar to live SCNT-produced calves, but hypermethylated than dead SCNT-produced calves. The levels of mRNA differed between SCNT-produced calves and AI animals for all genes, except TET3. However, no differences were observed between the live and dead cloned calves for all genes. Moreover, no differences were found between mRNA levels of Fematrin-1 and Syncytin-Rum1.

DISCUSSION

Our results suggest that this altered DNA methylation, deregulation in the expression of ERVs and in the genes of epigenetic machinery in fetal cotyledons of cloned calves may be associated with abnormal placentogenesis found in SCNT-produced animals. Further studies characterizing other mechanisms involved in the regulation of ERVs are important to support the development of new strategies to improve the efficiency of cloning.

The hallmarks of cell-cell fusion

Cell-cell fusion is essential for fertilization and organ development. Dedicated proteins known as fusogens are responsible for mediating membrane fusion. However, until recently, these proteins either remained unidentified or were poorly understood at the mechanistic level. Here, we review how fusogens surmount multiple energy barriers to mediate cell-cell fusion. We describe how early preparatory steps bring membranes to a distance of ∼10 nm, while fusogens act in the final approach between membranes. The mechanical force exerted by cell fusogens and the accompanying lipidic rearrangements constitute the hallmarks of cell-cell fusion. Finally, we discuss the relationship between viral and eukaryotic fusogens, highlight a classification scheme regrouping a superfamily of fusogens called Fusexins, and propose new questions and avenues of enquiry.

IFPA award in placentology lecture: Phylogenomic origins and evolution of the mammalian placenta

The placenta has had the most dynamic evolutionary history of all mammalian organs. It has undergone massive shifts in anatomy, physiology, and the way in which uterine and fetal tissue interact with one another during pregnancy. The human placenta is arguably the best studied amongst mammals, yet much about its function during pregnancy is not understood. The purpose of this paper is to outline the evolutionary history of the placenta, and to point out major gaps in the current state of knowledge. I also propose novel theoretical, experimental, and computational approaches that are likely to provide insight into the normal process of placentation and the role the placenta plays in the great obstetrical syndromes.

Not so bad after all: retroviruses and long terminal repeat retrotransposons as a source of new genes in vertebrates

Viruses and transposable elements, once considered as purely junk and selfish sequences, have repeatedly been used as a source of novel protein-coding genes during the evolution of most eukaryotic lineages, a phenomenon called 'molecular domestication'. This is exemplified perfectly in mammals and other vertebrates, where many genes derived from long terminal repeat (LTR) retroelements (retroviruses and LTR retrotransposons) have been identified through comparative genomics and functional analyses. In particular, genes derived from gag structural protein and envelope (env) genes, as well as from the integrase-coding and protease-coding sequences, have been identified in humans and other vertebrates. Retroelement-derived genes are involved in many important biological processes including placenta formation, cognitive functions in the brain and immunity against retroelements, as well as in cell proliferation, apoptosis and cancer. These observations support an important role of retroelement-derived genes in the evolution and diversification of the vertebrate lineage.

RD-114 virus story: from RNA rumor virus to a useful viral tool for elucidating the world cats' journey

RD-114 virus is a feline endogenous retrovirus (ERV) isolated from human rhabdomyosarcoma in 1971 and classified as endogenous gammaretrovirus in domestic cats (Felis catus). Based on the previous reports in 70's, it has been considered that a horizontal, infectious event occurred to transfer the virus from ancient baboon species to ancient cat species, whereupon it became endogenous in the cat species about several million years ago in Mediterranean Basin. Although it has been believed that all domestic cats harbor infectious RD-114 provirus in their genome, we revealed that cats do not have infectious RD-114 viral loci, but infectious RD-114 virus is resurrected by recombination between uninfectious RD-114 virus-related ERVs [here we designated them as RD-114-related sequences (RDRSs)]. Further, we also revealed the RDRSs which would potentially be resurrected as RD-114 virus (here we refer to them as ''new'' RDRSs) had entered the genome of the domestic cat after domestication of the cat around 10 thousand years ago. The fractions and positions of RDRSs in the cat genome differed in Western and Eastern cat populations and cat breeds. Our study revealed that RDRS would be a useful tool for elucidating the world travel routes of domestic cats after domestication.

Recent advances in the study of active endogenous retrovirus envelope glycoproteins in the mammalian placenta

Endogenous retroviruses (ERVs) are a component of the vertebrate genome and originate from exogenous infections of retroviruses in the germline of the host. ERVs have coevolved with their hosts over millions of years. Envelope glycoproteins of endogenous retroviruses are often expressed in the mammalian placenta, and their potential function has aroused considerable research interest, including the manipulation of maternal physiology to benefit the fetus. In most mammalian species, trophoblast fusion in the placenta is an important event, involving the formation of a multinucleated syncytiotrophoblast layer to fulfill essential fetomaternal exchange functions. The key function in this process derives from the envelope genes of endogenous retroviruses, namely syncytins, which show fusogenic properties and placenta-specific expression. This review discusses the important role of the recognized endogenous retrovirus envelope glycoproteins in the mammalian placenta.