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

Evolutionary dynamics of PEG10 and its interacting proteins during early and late-stage placental development in ruminants

Ruminants play a crucial role in dairy farming, pharmaceuticals, and embryonic stem cell research; thus, it is vital to prevent pregnancy loss and improve reproductive outcomes through a better understanding of placental development. Paternally-expressed gene 10 (PEG10) is a conserved gene essential for placental development in mammals, but its function in ruminants is not well understood. To develop insights into its role in placental development, this study investigated the gene structure and expression of PEG10 in cattle and goats. We found that PEG10's structure was conserved across species, and in the placenta, it retained the ability to bind to its own mRNA. Transcript analysis revealed differential expression patterns of PEG10 at early and late stages of placental development. We identified 70 proteins potentially interacting with PEG10 that were involved in biological processes like metabolism, signal transduction, cell proliferation, and immune responses. These proteins were grouped into seven clusters, associated with pathways such as amino acid degradation, the TCA cycle, longevity regulation, cardiomyopathy, proteasome function, and biosynthesis. Our findings suggest that PEG10 regulates placental development in ruminants by interacting with key proteins like CAST, ITGA6, and FTL, which are responsible for critical cellular processes in placental function.

Phospholipid flippase ATP11A brokers uterine epithelial integrity and function

Uterine adaptations driven by the steroid hormones estrogen and progesterone are pivotal for embryo implantation and, ultimately, for a successful pregnancy. Here, we show in mice that genetic ablation of the membrane lipid flippase Atp11a causes severe deficits in this hormonal response and profound defects in the morphological organization and transcriptional profile of the uterine epithelial compartment where Atp11a is expressed. Atp11a-null uterine epithelial cells lack tight junctions, and the luminal epithelium exhibits profound disruptions to cellular morphology. Interestingly, the specification of luminal epithelial cells remains incomplete as they maintain expression of the normally gland-restricted marker FOXA2. The uterine glands of Atp11a-null females are depleted for progenitor cells marked by SOX9, PAX8, LGR5, and PROM1. Collectively, these findings point to a uterine receptivity deficit that underpins the frequent failure of Atp11a-depleted females to establish a successful pregnancy. Most intriguingly, however, loss of only a single functional Atp11a allele causes a higher frequency of abnormal placental trophoblast differentiation as well as a higher incidence of developmental heart defects in wild-type embryos. These data emphasize the far-reaching impact of uterine dysfunction on reproductive outcome and highlight the importance of the maternal genotype in the etiology of developmental disorders.

Targeting of retrovirus-derived Rtl8a/8b causes late-onset obesity, reduced social response and increased apathy-like behaviour

Retrotransposon Gag-like (RTL) 8A, 8B and 8C are eutherian-specific genes derived from a certain retrovirus. They cluster as a triplet of genes on the X chromosome, but their function remains unknown. Here, we demonstrate that Rtl8a and Rtl8b play important roles in the brain: their double knockout (DKO) mice not only exhibit reduced social responses and increased apathy-like behaviour, but also become obese from young adulthood, similar to patients with late Prader-Willi syndrome (PWS), a neurodevelopmental genomic imprinting disorder. Mouse RTL8A/8B proteins are expressed in the prefrontal cortex and hypothalamus and localize to both the nucleus and cytoplasm of neurons, presumably due to the N-terminal nuclear localization signal-like sequence at the N-terminus. An RNAseq study in the cerebral cortex revealed reduced expression of several GABA type A receptor subunit genes in DKO, in particular Gabrb2, which encodes its β2 subunit. We confirmed the reduction of GABRB2 protein in the DKO cerebral cortex by western blotting. As GABRB2 has been implicated in the aetiology of several neurodevelopmental and neuropsychiatric disorders, it is likely that the reduction of GABRB2 is one of the major causes of the neuropsychiatric defects in the DKO mice.

Human Virus-Like Proteins: Implications for Gene Therapy

An analysis of mammalian genomes has revealed a significant number of DNA sequences with transposon or viral origin. Some of these elements encode functional proteins, repurposed during evolution to play significant physiological roles in certain tissues. Some human virus-like proteins, such as Peg10 and Arc/Arg3.1, structurally demonstrate significant similarity with Gag retroviral proteins, while others, like syncytins-1 and -2, resemble envelope viral proteins. In recent years, it has become clear that these proteins can be exploited for bioengineering 'humanized' capsid particles aimed at targeted mRNA delivery. Realizing this idea could provide efficient virus-like particles for gene therapy and address the problem of viral vector immunogenicity. This review provides an overview of the most-studied human proteins of viral or transposon origin and highlights their biological functions. Additionally, recent advances in exploiting these proteins for targeted mRNA delivery and prospects for their clinical application are discussed.

From the cauldron of conflict: Endogenous gene regulation by piRNA and other modes of adaptation enabled by selfish transposable elements

Transposable elements (TEs) provide a prime example of genetic conflict because they can proliferate in genomes and populations even if they harm the host. However, numerous studies have shown that TEs, though typically harmful, can also provide fuel for adaptation. This is because they code functional sequences that can be useful for the host in which they reside. In this review, I summarize the "how" and "why" of adaptation enabled by the genetic conflict between TEs and hosts. In addition, focusing on mechanisms of TE control by small piwi-interacting RNAs (piRNAs), I highlight an indirect form of adaptation enabled by conflict. In this case, mechanisms of host defense that regulate TEs have been redeployed for endogenous gene regulation. I propose that the genetic conflict released by meiosis in early eukaryotes may have been important because, among other reasons, it spurred evolutionary innovation on multiple interwoven trajectories - on the part of hosts and also embedded genetic parasites. This form of evolution may function as a complexity generating engine that was a critical player in eukaryotic evolution.

The gag-like gene RTL8 antagonizes PEG10-mediated virus like particles

PEG10 is a retroelement-derived Mart-family gene that is necessary for placentation and has been implicated in neurological disease. PEG10 resembles both retrotransposon and retroviral proteins and forms virus-like particles (VLPs) that can be purified using iodixanol ultracentrifugation. It is hypothesized that formation of VLPs is crucial to the biological roles of PEG10 in reproduction and neurological health. Here, we describe the regulation of PEG10 VLP formation and release in human cells with a role for the related Mart gene RTL8. RTL8 resembles a truncated form of PEG10 that shares homology with the N-terminal gag-like capsid domain. Alone, RTL8 is unable to form VLPs, but was incorporated into PEG10-derived particles. RTL8 co-expression decreased the abundance of PEG10 VLPs and increased intracellular levels of PEG10, suggesting a model where RTL8 inhibits PEG10 VLP formation or release. Consistent with this model, RTL8 bound to the N-terminal domain of PEG10 capsid, and modulation of RTL8 influenced PEG10-derived VLP abundance in naturally producing cells. RTL8 is broadly expressed in many of the same tissues as PEG10, including in human brain. Taken together, these results describe a novel antagonistic relationship between two human retroelement-derived genes and have implications for our understanding of PEG10 biology and disease.

RTL4, a Retrovirus-Derived Gene Implicated in Autism Spectrum Disorder, Is a Microglial Gene That Responds to Noradrenaline in the Postnatal Brain

Retrotransposon Gag-like 4 (RTL4), a gene acquired from a retrovirus, is a causative gene in autism spectrum disorder. Its knockout mice exhibit increased impulsivity, impaired short-term spatial memory, failure to adapt to novel environments, and delayed noradrenaline (NA) recovery in the frontal cortex. However, due to its very low expression in the brain, it remains unknown which brain cells express RTL4 and its dynamics in relation to NA. We addressed these issues using knock-in mice carrying endogenous Rtl4 fused to Venus, which encodes a fluorescent protein. The RTL4-Venus fusion protein was detected as a secreted protein in the midbrain, hypothalamus, hippocampus and amygdala in the postnatal brain. Its signal intensity was high during critical periods of neonatal adaptation to novel environments. It was upregulated by various stimuli, including isoproterenol administration, whereas it was decreased by anesthesia but was maintained by milnacipran administration, suggesting its highly sensitive response to stressors, possible dependence on the arousal state and involvement in the NA reuptake process. In vitro mixed glial culture experiments demonstrated that Rtl4 is a microglial gene and suggested that RTL4 secretion responds rapidly to isoproterenol. Microglial RTL4 plays an important role in the NA response and possibly in the development of the NAergic neuronal network in the brain.

tRNA-derived RNAs in human milk extracellular vesicles and associations with breastfeeding variables and maternal diet

AIMS

To describe tDRs in human milk EVs and their associations with maternal body mass index, age, dietary indices, breastfeeding frequency, season and time of milk collection in a Latina population.

MATERIALS & METHODS

We sequenced small RNAs from EVs from 109 mature human milk samples collected at 1 month after delivery in the Southern California Mother's Milk Study. We grouped tDRs using hierarchical clustering and clusters were compared across tDR characteristics. We analyzed associations of tDRs with intrinsic maternal variables (body mass index, age), maternal nutrition (caloric intake, Healthy Eating Index, Dietary Inflammatory Index), and variables related to feeding and milk collection (breastfeeding frequency, season and time of milk collection) using negative binomial models.

RESULTS

We identified 338 tDRs expressed in 90% or more of milk EV samples, of which 113 were identified in all samples. tDR-1:26-Gly-CCC-1-M4 accounted for most reads (79%). Pathway analysis revealed a wide array of biological processes and disease mechanisms across the four tDR clusters. tDRs were associated with season of collection, time of collection, breastfeeding frequency, and the dietary inflammatory index.

CONCLUSIONS

tDRs are abundant in milk EVs and may be sensitive to maternal diet, seasonality, time of day, and breastfeeding frequency.

Methylation and expression of imprinted genes in circulating extracellular vesicles from women experiencing early onset preeclampsia

INTRODUCTION

Preeclampsia (PE) is a pregnancy complication marked by high blood pressure, posing risk to maternal and fetal health. "Genomic imprinting", an epigenetic phenomenon regulated by DNA methylation at Differently Methylated Regions (DMR's), influences placental development. Research on circulating extracellular vesicles (EVs) in PE suggests them as potential source for early biomarkers, but methylation status of EV-DNA in Preeclampsia is not reported yet.

METHODS

This study examines the methylation and expression profile of imprinted genes - PEG10, PEG3, MEST, and DLK1 in circulating EVs of 1st and 3rd trimester control and early onset preeclampsia (EOPE) pregnant women (n = 15) using pyrosequencing and qRT-PCR respectively.

RESULTS

In 1st trimester, PEG3 was significantly hypermethylated, whereas no significant methylation changes were noted in PEG10 and MEST in EOPE. In 3rd trimester, significant hypomethylation in PEG10, PEG3 and IGDMR was observed whereas significant hypermethyaltion noted in MEST. mRNA expression of PEG10, PEG3 and DLK1 was not affected in circulating EVs of 1st trimester EOPE. However, in 3rd trimester significant increased expression in PEG10, PEG3 and DLK1 noted. MEST expression was reduced in 3rd trimester EOPE. No correlation was observed between average DNA methylation and gene expression in PEG10 and PEG3 in 1st trimester. However, in 3rd trimester, significant negative correlation was noted in PEG10 (r = -0.426, p = 0.04), PEG3 (r = -0.496, p = 0.01), MEST (r = -0.398, p = 0.03) and DLK1 (r = -0.403, p = 0.03).

DISCUSSION

The results of our study strengthen the potential of circulating EVs from maternal serum as non-invasive indicators of placental pathophysiology, including preeclampsia.

Maternal Vitamin D3 Supplementation in an Oxidized-Oil Diet Protects Fetus from Developmental Impairment and Ameliorates Oxidative Stress in Mouse Placenta and Fetus

BACKGROUND

Fried food has increased in popularity worldwide. However, deep frying can increase the production of peroxidative toxins in food, which might be harmful to fetal development. The antioxidative effect of vitamin D3 (VD3) has been reported previously.

OBJECTIVES

This study aimed to explore how maternal VD3 supplementation in an oxidized-oil diet during gestation affects fetal antioxidative ability and development.

METHODS

Pregnant mice were randomly assigned into 3 groups: Control group (diet with fresh soybean oil), OSO group [diet with oxidized soybean oil (OSO)], and OSOV group (diet with OSO and 10,000 IU/Kg VD3). Mice were fed with the corresponding diet during gestation. On day 16.5 of gestation, the placenta and fetus were harvested to analyze antioxidative status.

RESULTS

Maternal oxidized-oil diet during gestation significantly reduced placental vessel abundance, labyrinth zone area, and fetal body weight. However, dietary VD3 supplementation prevented these negative effects of oxidized-oil diet. Maternal intake of oxidized-oil diet increased serum concentrations of malondialdehyde, total-nitric oxide synthase, and inducible nitric oxide synthase, whereas VD3 supplementation showed a protection effect on it. Additionally, maternal VD3 supplementation increased the levels of antioxidative enzymes and the nuclear translocation of nuclear factor erythroid 2-related factor 2 (NRF2), thereby protecting placenta and fetus from apoptosis and oxidative stress caused by an oxidized-oil diet. The gene expression and protein levels of a fatty acid transporter solute carrier family 27 member 1 in the fetal liver were increased by maternal VD3 supplementation under oxidized-oil diet. Notably, NRF2 could be co-immunoprecipitated with the VD receptor in the placenta.

CONCLUSIONS

Maternal VD3 supplementation could protect fetus from oxidized-oil diet induced developmental impairment by alleviating oxidative stress in the placenta and fetus through the VD receptor/NRF2 pathway, at least partially. Thus, ensuring adequate levels of VD3 through supplementation is often critical during pregnancy.

A primate-specific endogenous retroviral envelope protein sequesters SFRP2 to regulate human cardiomyocyte development

Endogenous retroviruses (ERVs) occupy a significant part of the human genome, with some encoding proteins that influence the immune system or regulate cell-cell fusion in early extra-embryonic development. However, whether ERV-derived proteins regulate somatic development is unknown. Here, we report a somatic developmental function for the primate-specific ERVH48-1 (SUPYN/Suppressyn). ERVH48-1 encodes a fragment of a viral envelope that is expressed during early embryonic development. Loss of ERVH48-1 led to impaired mesoderm and cardiomyocyte commitment and diverted cells to an ectoderm-like fate. Mechanistically, ERVH48-1 is localized to sub-cellular membrane compartments through a functional N-terminal signal peptide and binds to the WNT antagonist SFRP2 to promote its polyubiquitination and degradation, thus limiting SFRP2 secretion and blocking repression of WNT/β-catenin signaling. Knockdown of SFRP2 or expression of a chimeric SFRP2 with the ERVH48-1 signal peptide rescued cardiomyocyte differentiation. This study demonstrates how ERVH48-1 modulates WNT/β-catenin signaling and cell type commitment in somatic development.

The human retroviral-like aspartic protease 1 (ASPRV1): From in vitro studies to clinical correlations

The human retroviral-like aspartic protease 1 (ASPRV1) is a retroviral-like protein that was first identified in the skin due to its expression in the stratum granulosum layer of the epidermis. Accordingly, it is also referred to as skin-specific aspartic protease. Similar to the retroviral polyproteins, the full-length ASPRV1 also undergoes self-proteolysis, the processing of the precursor is necessary for the autoactivation of the protease domain. ASPRV1's functions are well-established at the level of the skin: it is part of the epidermal proteolytic network and has a significant contribution to skin moisturization via the limited proteolysis of filaggrin; it is only natural protein substrate identified so far. Filaggrin and ASPRV1 are also specific for mammalians, these proteins provide unique features for the skins of these species, and the importance of filaggrin processing in hydration is proved by the fact that some ASPRV1 mutations are associated with skin diseases such as ichthyosis. ASPRV1 was also found to be expressed in macrophage-like neutrophil cells, indicating that its functions are not limited to the skin. In addition, differential expression of ASPRV1 was detected in many diseases, with yet unknown significance. The currently known enzymatic characteristics-that had been revealed mainly by in vitro studies-and correlations with pathogenic phenotypes imply potentially important functions in multiple cell types, which makes the protein a promising target of functional studies. In this review we describe the currently available knowledge and future perspective in regard to ASPRV1.

Disruption of maternal vascular remodeling by a fetal endoretrovirus-derived gene in preeclampsia

BACKGROUND

Preeclampsia, one of the most lethal pregnancy-related diseases, is associated with the disruption of uterine spiral artery remodeling during placentation. However, the early molecular events leading to preeclampsia remain unknown.

RESULTS

By analyzing placentas from preeclampsia, non-preeclampsia, and twin pregnancies with selective intrauterine growth restriction, we show that the pathogenesis of preeclampsia is attributed to immature trophoblast and maldeveloped endothelial cells. Delayed epigenetic reprogramming during early extraembryonic tissue development leads to generation of excessive immature trophoblast cells. We find reduction of de novo DNA methylation in these trophoblast cells results in selective overexpression of maternally imprinted genes, including the endoretrovirus-derived gene PEG10 (paternally expressed gene 10). PEG10 forms virus-like particles, which are transferred from the trophoblast to the closely proximate endothelial cells. In normal pregnancy, only a low amount of PEG10 is transferred to maternal cells; however, in preeclampsia, excessive PEG10 disrupts maternal vascular development by inhibiting TGF-beta signaling.

CONCLUSIONS

Our study reveals the intricate epigenetic mechanisms that regulate trans-generational genetic conflict and ultimately ensure proper maternal-fetal interface formation.

The Diverse Evolutionary Histories of Domesticated Metaviral Capsid Genes in Mammals

Selfish genetic elements comprise significant fractions of mammalian genomes. In rare instances, host genomes domesticate segments of these elements for function. Using a complete human genome assembly and 25 additional vertebrate genomes, we re-analyzed the evolutionary trajectories and functional potential of capsid (CA) genes domesticated from Metaviridae, a lineage of retrovirus-like retrotransposons. Our study expands on previous analyses to unearth several new insights about the evolutionary histories of these ancient genes. We find that at least five independent domestication events occurred from diverse Metaviridae, giving rise to three universally retained single-copy genes evolving under purifying selection and two gene families unique to placental mammals, with multiple members showing evidence of rapid evolution. In the SIRH/RTL family, we find diverse amino-terminal domains, widespread loss of protein-coding capacity in RTL10 despite its retention in several mammalian lineages, and differential utilization of an ancient programmed ribosomal frameshift in RTL3 between the domesticated CA and protease domains. Our analyses also reveal that most members of the PNMA family in mammalian genomes encode a conserved putative amino-terminal RNA-binding domain (RBD) both adjoining and independent from domesticated CA domains. Our analyses lead to a significant correction of previous annotations of the essential CCDC8 gene. We show that this putative RBD is also present in several extant Metaviridae, revealing a novel protein domain configuration in retrotransposons. Collectively, our study reveals the divergent outcomes of multiple domestication events from diverse Metaviridae in the common ancestor of placental mammals.

Evolution of the nervous system by acquisition of retrovirus-derived genes in mammals

In the course of evolution, the most highly developed organ is likely the brain, which has become more complex over time and acquired diverse forms and functions in different species. In particular, mammals have developed complex and high-functioning brains, and it has been reported that several genes derived from retroviruses were involved in mammalian brain evolution, that is, generating the complexity of the nervous system. Especially, the sushi-ichi-related retrotransposon homolog (SIRH)/retrotransposon gag-like (RTL) genes have been suggested to play a role in the evolutionary processes shaping brain morphology and function in mammals. Genetic mutation and altered expression of genes are linked to neurological disorders, highlighting how the acquisition of virus-derived genes in mammals has both driven brain evolution and imposed a susceptibility to diseases. This review provides an overview of the functions, diversity, evolution and diseases associated with SIRH/RTL genes in the nervous system. The contribution of retroviruses to brain evolution is an important research topic in evolutionary biology and neuroscience, and further insights are expected to be gained through future studies.

Acquisition and Exaptation of Endogenous Retroviruses in Mammalian Placenta

Endogenous retroviruses (ERVs) are retrovirus-like sequences that were previously integrated into the host genome. Although most ERVs are inactivated by mutations, deletions, or epigenetic regulation, some remain transcriptionally active and impact host physiology. Several ERV-encoded proteins, such as Syncytins and Suppressyn, contribute to placenta acquisition, a crucial adaptation in mammals that protects the fetus from external threats and other risks while enabling the maternal supply of oxygen, nutrients, and antibodies. In primates, Syncytin-1 and Syncytin-2 facilitate cell-cell fusion for placental formation. Suppressyn is the first ERV-derived protein that inhibits cell fusion by binding to ASCT2, the receptor for Syncytin-1. Furthermore, Syncytin-2 likely inserted into the genome of the common ancestor of Anthropoidea, whereas Syncytin-1 and Suppressyn likely inserted into the ancestor of catarrhines; however, they were inactivated in some lineages, suggesting that multiple exaptation events had occurred. This review discusses the role of ERV-encoded proteins, particularly Syncytins and Suppressyn, in placental development and function, focusing on the integration of ERVs into the host genome and their contribution to the genetic mechanisms underlying placentogenesis. This review provides valuable insights into the molecular and genetic aspects of placentation, potentially shedding light on broader evolutionary and physiological processes in mammals.

Retrovirus-Derived RTL9 Plays an Important Role in Innate Antifungal Immunity in the Eutherian Brain

Retrotransposon Gag-like (RTL) genes play a variety of essential and important roles in the eutherian placenta and brain. It has recently been demonstrated that RTL5 and RTL6 (also known as sushi-ichi retrotransposon homolog 8 (SIRH8) and SIRH3) are microglial genes that play important roles in the brain's innate immunity against viruses and bacteria through their removal of double-stranded RNA and lipopolysaccharide, respectively. In this work, we addressed the function of RTL9 (also known as SIRH10). Using knock-in mice that produce RTL9-mCherry fusion protein, we examined RTL9 expression in the brain and its reaction to fungal zymosan. Here, we demonstrate that RTL9 plays an important role, degrading zymosan in the brain. The RTL9 protein is localized in the microglial lysosomes where incorporated zymosan is digested. Furthermore, in Rtl9 knockout mice expressing RTL9ΔC protein lacking the C-terminus retroviral GAG-like region, the zymosan degrading activity was lost. Thus, RTL9 is essentially engaged in this reaction, presumably via its GAG-like region. Together with our previous study, this result highlights the importance of three retrovirus-derived microglial RTL genes as eutherian-specific constituents of the current brain innate immune system: RTL9, RTL5 and RTL6, responding to fungi, viruses and bacteria, respectively.

Roles of retrovirus-derived PEG10 and PEG11/RTL1 in mammalian development and evolution and their involvement in human disease

PEG10 and PEG11/RTL1 are paternally expressed, imprinted genes that play essential roles in the current eutherian developmental system and are therefore associated with developmental abnormalities caused by aberrant genomic imprinting. They are also presumed to be retrovirus-derived genes with homology to the sushi-ichi retrotransposon GAG and POL, further expanding our comprehension of mammalian evolution via the domestication (exaptation) of retrovirus-derived acquired genes. In this manuscript, we review the importance of PEG10 and PEG11/RTL1 in genomic imprinting research via their functional roles in development and human disease, including neurodevelopmental disorders of genomic imprinting, Angelman, Kagami-Ogata and Temple syndromes, and the impact of newly inserted DNA on the emergence of newly imprinted regions. We also discuss their possible roles as ancestors of other retrovirus-derived RTL/SIRH genes that likewise play important roles in the current mammalian developmental system, such as in the placenta, brain and innate immune system.

Retrovirus-Derived RTL/SIRH: Their Diverse Roles in the Current Eutherian Developmental System and Contribution to Eutherian Evolution

Eutherians have 11 retrotransposon Gag-like (RTL)/sushi-ichi retrotransposon homolog (SIRH) genes presumably derived from a certain retrovirus. Accumulating evidence indicates that the RTL/SIRH genes play a variety of roles in the current mammalian developmental system, such as in the placenta, brain, and innate immune system, in a eutherian-specific manner. It has been shown that the functional role of Paternally Expressed 10 (PEG10) in placental formation is unique to the therian mammals, as are the eutherian-specific roles of PEG10 and PEG11/RTL1 in maintaining the fetal capillary network and the endocrine regulation of RTL7/SIRH7 (aka Leucine Zipper Down-Regulated in Cancer 1 (LDOCK1)) in the placenta. In the brain, PEG11/RTL1 is expressed in the corticospinal tract and hippocampal commissure, mammalian-specific structures, and in the corpus callosum, a eutherian-specific structure. Unexpectedly, at least three RTL/SIRH genes, RTL5/SIRH8, RTL6/SIRH3, and RTL9/SIRH10, play important roles in combating a variety of pathogens, namely viruses, bacteria, and fungi, respectively, suggesting that the innate immunity system of the brain in eutherians has been enhanced by the emergence of these new components. In this review, we will summarize the function of 10 out of the 11 RTL/SIRH genes and discuss their roles in eutherian development and evolution.

The diverse evolutionary histories of domesticated metaviral capsid genes in mammals

Selfish genetic elements and their remnants comprise at least half of the human genome. Active transposons duplicate by inserting copies at new sites in a host genome. Following insertion, transposons can acquire mutations that render them inactive; the accrual of additional mutations can render them unrecognizable over time. However, in rare instances, segments of transposons become useful for the host, in a process called gene domestication. Using the first complete human genome assembly and 25 additional vertebrate genomes, we analyzed the evolutionary trajectories and functional potential of genes domesticated from the capsid genes of Metaviridae, a retroviral-like retrotransposon family. Our analysis reveals four families of domesticated capsid genes in placental mammals with varied evolutionary outcomes, ranging from universal retention to lineage-specific duplications or losses and from purifying selection to lineage-specific rapid evolution. The four families of domesticated capsid genes have divergent amino-terminal domains, inherited from four distinct ancestral metaviruses. Structural predictions reveal that many domesticated genes encode a previously unrecognized RNA-binding domain retained in multiple paralogs in mammalian genomes both adjacent to and independent from the capsid domain. Collectively, our study reveals diverse outcomes of domestication of diverse metaviruses, which led to structurally and evolutionarily diverse genes that encode important, but still largely-unknown functions in placental mammals. (207).

Paternal Expressed Gene 10 (PEG10) is decreased in early-onset preeclampsia

BACKGROUND

Preeclampsia is a severe complication of pregnancy which is attributed to placental dysfunction. The retrotransposon, Paternal Expressed Gene 10 (PEG10) harbours critical placental functions pertaining to placental trophoblast cells. Limited evidence exists on whether PEG10 is involved in preeclampsia pathogenesis. This study characterised the expression and regulation of PEG10 in placentas from patients with early-onset preeclampsia compared to gestation-matched controls.

METHODS

PEG10 expression was measured in plasma and placentas collected from patients with early-onset preeclampsia (< 34 weeks') and gestation-matched controls using ELISA (protein) and RT-qPCR (mRNA). First-trimester human trophoblast stem cells (hTSCs) were used for in vitro studies. PEG10 expression was measured during hTSC differentiation and hTSC exposure to hypoxia (1% O₂) and inflammatory cytokines (IL-6 and TNFα) using RT-qPCR. Functional studies used PEG10 siRNA to measure the effect of reduced PEG10 on canonical TGF-[Formula: see text] signalling and proliferation using luciferase and xCELLigence assays, respectively.

RESULTS

PEG10 mRNA expression was significantly reduced in placentas from patients with early-onset preeclampsia (< 34 weeks' gestation) relative to controls (p = 0.04, n = 78 vs n = 18 controls). PEG10 protein expression was also reduced in preeclamptic placentas (p = 0.03, n = 5 vs n = 5 controls, blinded assessment of immunohistochemical staining), but neither PEG10 mRNA nor protein could be detected in maternal circulation. PEG10 was most highly expressed in hTSCs, and its expression was reduced as hTSCs differentiated into syncytiotrophoblasts (p < 0.0001) and extravillous trophoblasts (p < 0.001). Trophoblast differentiation was not altered when hTSCs were treated with PEG10 siRNA (n = 5 vs n = 5 controls). PEG10 was significantly reduced in hTSCs exposed to hypoxia (p < 0.01). PEG10 was also reduced in hTSCs treated with the inflammatory cytokine TNF [Formula: see text] (p < 0.01), but not IL-6. PEG10 knocked down (siRNA) in hTSCs showed reduced activation of the canonical TGF-β signalling effector, the SMAD binding element (p < 0.05) relative to controls. PEG10 knockdown in hTSCs however was not associated with any significant alterations in proliferation.

CONCLUSIONS

Placental PEG10 is reduced in patients with early-onset preeclampsia. In vitro studies suggest that hypoxia and inflammation may contribute to PEG10 downregulation. Reduced PEG10 alters canonical TGF-[Formula: see text] signalling, and thus may be involved in trophoblast dysfunction associated with this pathway.

The people behind the papers - Masahito Irie, Fumitoshi Ishino and Tomoko Kaneko-Ishino

Viral-derived genes have had a huge impact during mammalian evolution, with many of the exapted genes being expressed in the placenta. Now, new research published in Development describes the importance of two genes with retroviral origins in microglia, the innate immune cells of the brain, which are derived from another extra-embryonic tissue, the yolk sac. We caught up with the first author, Masahito Irie, and the corresponding authors, Fumitoshi Ishino, Professor at Tokyo Medical and Dental University, and Tomoko Kaneko-Ishino, Professor at Tokai University, to hear about more about their research.

Mammalian genome innovation through transposon domestication

Since the discovery of transposons, their sheer abundance in host genomes has puzzled many. While historically viewed as largely harmless 'parasitic' DNAs during evolution, transposons are not a mere record of ancient genome invasion. Instead, nearly every element of transposon biology has been integrated into host biology. Here we review how host genome sequences introduced by transposon activities provide raw material for genome innovation and document the distinct evolutionary path of each species.

Endogenous Retroviruses and Placental Evolution, Development, and Diversity

The main roles of placentas include physical protection, nutrient and oxygen import, export of gasses and fetal waste products, and endocrinological regulation. In addition to physical protection of the fetus, the placentas must provide immune protection throughout gestation. These basic functions are well-conserved; however, placentas are undoubtedly recent evolving organs with structural and cellular diversities. These differences have been explained for the last two decades through co-opting genes and gene control elements derived from transposable elements, including endogenous retroviruses (ERVs). However, the differences in placental structures have not been explained or characterized. This manuscript addresses the sorting of ERVs and their integration into the mammalian genomes and provides new ways to explain why placental structures have diverged.

The Evolutionary Advantage in Mammals of the Complementary Monoallelic Expression Mechanism of Genomic Imprinting and Its Emergence From a Defense Against the Insertion Into the Host Genome

In viviparous mammals, genomic imprinting regulates parent-of-origin-specific monoallelic expression of paternally and maternally expressed imprinted genes (PEGs and MEGs) in a region-specific manner. It plays an essential role in mammalian development: aberrant imprinting regulation causes a variety of developmental defects, including fetal, neonatal, and postnatal lethality as well as growth abnormalities. Mechanistically, PEGs and MEGs are reciprocally regulated by DNA methylation of germ-line differentially methylated regions (gDMRs), thereby exhibiting eliciting complementary expression from parental genomes. The fact that most gDMR sequences are derived from insertion events provides strong support for the claim that genomic imprinting emerged as a host defense mechanism against the insertion in the genome. Recent studies on the molecular mechanisms concerning how the DNA methylation marks on the gDMRs are established in gametes and maintained in the pre- and postimplantation periods have further revealed the close relationship between genomic imprinting and invading DNA, such as retroviruses and LTR retrotransposons. In the presence of gDMRs, the monoallelic expression of PEGs and MEGs confers an apparent advantage by the functional compensation that takes place between the two parental genomes. Thus, it is likely that genomic imprinting is a consequence of an evolutionary trade-off for improved survival. In addition, novel genes were introduced into the mammalian genome via this same surprising and complex process as imprinted genes, such as the genes acquired from retroviruses as well as those that were duplicated by retropositioning. Importantly, these genes play essential/important roles in the current eutherian developmental system, such as that in the placenta and/or brain. Thus, genomic imprinting has played a critically important role in the evolutionary emergence of mammals, not only by providing a means to escape from the adverse effects of invading DNA with sequences corresponding to the gDMRs, but also by the acquisition of novel functions in development, growth and behavior via the mechanism of complementary monoallelic expression.