Genetic recapitulation of human pre-eclampsia risk during convergent evolution of reduced placental invasiveness in eutherian mammals

The human placenta: new perspectives on its formation and function during early pregnancy

The placenta has evolved to support the development of the embryo and fetus during the different intrauterine periods of life. By necessity, its development must precede that of the embryo. There is now evidence that during embryogenesis and organogenesis, the development of the human placenta is supported by histotrophic nutrition secreted from endometrial glands rather than maternal blood. These secretions provide a plentiful supply of glucose, lipids, glycoproteins and growth factors that stimulate rapid proliferation and differentiation of the villous trophoblast. Furthermore, evidence from endometrial gland organoids indicates that expression and secretion of these products are upregulated following sequential exposure to oestrogen, progesterone and trophoblastic and decidual hormones, in particular prolactin. Hence, a feed-forward signalling dialogue is proposed among the trophoblast, decidua and glands that enables the placenta to stimulate its own development, independent of that of the embryo. Many common complications of pregnancy represent a spectrum of disorders associated with deficient trophoblast proliferation. Increasing evidence suggests that this spectrum is mirrored by one of impaired decidualization, potentially compromising histotroph secretion through diminished prolactin secretion and reduced gland function. Optimizing endometrial wellbeing prior to conception may therefore help to prevent common pregnancy complications, such as miscarriage, growth restriction and pre-eclampsia.

Evolutionary divergence of embryo implantation in primates

Implantation of the conceptus into the uterus is absolutely essential for successful embryo development. In humans, our understanding of this process has remained rudimentary owing to the inaccessibility of early implantation stages. Non-human primates recapitulate many aspects of human embryo development and provide crucial insights into trophoblast development, uterine receptivity and embryo invasion. Moreover, primate species exhibit a variety of implantation strategies and differ in embryo invasion depths. This review examines conservation and divergence of the key processes required for embryo implantation in different primates and in comparison with the canonical rodent model. We discuss trophectoderm compartmentalization, endometrial remodelling and embryo adhesion and invasion. Finally, we propose that studying the mechanism controlling invasion depth between different primate species may provide new insights and treatment strategies for placentation disorders in humans. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Placentation in the Human and Higher Primates

Placentation in humans is precocious and highly invasive compared to other mammals. Implantation is interstitial, with the conceptus becoming completely embedded within the endometrium towards the end of the second week post-fertilization. Villi initially form over the entire surface of the chorionic sac, stimulated by histotrophic secretions from the endometrial glands. The secondary yolk sac never makes contact with the chorion, and a choriovitelline placenta is never established. However, recent morphological and transcriptomic analyses suggest that the yolk sac plays an important role in the uptake of nutrients from the coelomic fluid. Measurements performed in vivo demonstrate that early development takes place in a physiological, low-oxygen environment that protects against teratogenic free radicals and maintains stem cells in a multipotent state. The maternal arterial circulation to the placenta is only fully established around 10-12 weeks of gestation. By then, villi have regressed over the superficial, abembryonic pole, leaving the definitive discoid placenta, which is of the villous, hemochorial type. Remodeling of the maternal spiral arteries is essential to ensure a high-volume but low-velocity inflow into the mature placenta. Extravillous trophoblast cells migrate from anchoring villi and surround the arteries. Their interactions with maternal immune cells release cytokines and proteases that are key to remodeling, and a successful pregnancy.

Accounting for diverse evolutionary forces reveals mosaic patterns of selection on human preterm birth loci

Currently, there is no comprehensive framework to evaluate the evolutionary forces acting on genomic regions associated with human complex traits and contextualize the relationship between evolution and molecular function. Here, we develop an approach to test for signatures of diverse evolutionary forces on trait-associated genomic regions. We apply our method to regions associated with spontaneous preterm birth (sPTB), a complex disorder of global health concern. We find that sPTB-associated regions harbor diverse evolutionary signatures including conservation, excess population differentiation, accelerated evolution, and balanced polymorphism. Furthermore, we integrate evolutionary context with molecular evidence to hypothesize how these regions contribute to sPTB risk. Finally, we observe enrichment in signatures of diverse evolutionary forces in sPTB-associated regions compared to genomic background. By quantifying multiple evolutionary forces acting on sPTB-associated regions, our approach improves understanding of both functional roles and the mosaic of evolutionary forces acting on loci. Our work provides a blueprint for investigating evolutionary pressures on complex traits.

Overexpression of Long Noncoding RNA Uc.187 Induces Preeclampsia-Like Symptoms in Pregnancy Rats

BACKGROUND

As a serious pregnancy-specific condition, preeclampsia (PE) is a serious pregnancy-specific condition characterized by insufficient trophoblastic invasion and shallow placental implantation. Long noncoding RNA uc.187, which is transcribed from an ultra-conserved region is highly expressed in the placental tissue of patients with PE, is associated with abnormal trophoblast invasion. Therefore, we aimed to further characterize the relationship between uc.187 and PE through in vitro experimental studies to find new targets to treat PE.

METHODS

In this study, we constructed PE rat models induced by lipopolysaccharide, experimented with overexpressing uc.187 and performed experiments using HTR-8/SVneo cells.

RESULTS

We found uc.187 was elevated in the placenta of PE rats. By injecting pregnant rats with a lentivirus containing the lncRNA uc.187, we successfully triggered maternal hypertension along with a series of symptoms similar to PE in humans. In vitro experiments demonstrated that high levels of uc.187 lead to decreased trophoblast invasion. In addition, our results revealed that uc.187 had high expression in PE and fetal growth restricted cells, but low expression in placental site trophoblastic tumors compared with the control groups. Results of western blot and cell immunofluorescence indicated that the aberrant biological behavior of HTR-8/SVneo cells were related to the distribution of β-catenin in the cytoplasm and nucleus.

CONCLUSIONS

Taken together, our study revealed that uc.187 was negatively correlated to trophoblastic cell invasion, and overexpression of uc.187 could induce PE-like symptoms in a pregnant rat model by affecting the distribution of β-catenin in the cytoplasm and nucleus.

Integrating evolution into medical education for women's health care practitioners

Evolution is a fundamental principle in biology; however, it has been neglected in medical education. We argue that an evolutionary perspective is especially important for women's health care providers, as selection will act strongly on reproductive parameters, and the biological costs of female reproduction are generally more resource expensive than for men (e.g. due to gestation and lactation) with greater effects on health and wellbeing. An evolutionary perspective is needed to understand antibiotic resistance, disease and health risks associated with mismatches between our evolved adaptations and current conditions, the importance of the microbiome and the maternal role in how infants acquire and develop their early-life microbiome (vaginal birth, lactation), and the importance of breastmilk as a biochemical signal from mothers to their babies. We present data that obstetrician-gynecologists' views regarding the inclusion of evolution within their training is generally positive, but many barriers are perceived. Requiring coursework in evolutionary biology with an emphasis on evolutionary medicine prior to enrollment in medical school may be a solution.

Oxidative stress in pregnancy complicated by preeclampsia

Preeclampsia is a multisystemic disorder of pregnancy that causes perinatal morbidity and mortality. Studies published in the last decade have contributed to a better understanding of physiopathogenesis through key mechanisms involved, such as altered immune response, endothelial dysfunction, oxidative stress and systemic inflammatory response, as well as genetic susceptibility. Oxidative stress (OS) plays an important role in the development of preeclampsia, since it alters placental remodeling and placental vascular endothelial dysfunction, resulting in an ischemia/reperfusion injury with an increase in xanthine oxidase activity that produces high levels of reactive oxygen species (ROS). ROS can be generated through many pathways within cells, mitochondria, endoplasmic reticulum (ER) and enzymes such as NADPH oxidase are the most important sources, causing widespread and indiscriminate damage to cells and tissues, which leads to an intravascular inflammatory response and maternal systemic endothelial dysfunction characteristic of this prenatal syndrome. Therefore, the following review aims to identify the main risk factors and the role of OS as a pathophysiological mechanism in the development of preeclampsia.

The transformative potential of an integrative approach to pregnancy

BACKGROUND

Complex traits typically involve diverse biological pathways and are shaped by numerous genetic and environmental factors. Pregnancy-associated traits and pathologies are further complicated by extensive communication across multiple tissues in two individuals, interactions between two genomes-maternal and fetal-that obscure causal variants and lead to genetic conflict, and rapid evolution of pregnancy-associated traits across mammals and in the human lineage. Given the multi-faceted complexity of human pregnancy, integrative approaches that synthesize diverse data types and analyses harbor tremendous promise to identify the genetic architecture and environmental influences underlying pregnancy-associated traits and pathologies.

METHODS

We review current research that addresses the extreme complexities of traits and pathologies associated with human pregnancy.

RESULTS

We find that successful efforts to address the many complexities of pregnancy-associated traits and pathologies often harness the power of many and diverse types of data, including genome-wide association studies, evolutionary analyses, multi-tissue transcriptomic profiles, and environmental conditions.

CONCLUSION

We propose that understanding of pregnancy and its pathologies will be accelerated by computational platforms that provide easy access to integrated data and analyses. By simplifying the integration of diverse data, such platforms will provide a comprehensive synthesis that transcends many of the inherent challenges present in studies of pregnancy.

Placental Origins of Chronic Disease

Epidemiological evidence links an individual's susceptibility to chronic disease in adult life to events during their intrauterine phase of development. Biologically this should not be unexpected, for organ systems are at their most plastic when progenitor cells are proliferating and differentiating. Influences operating at this time can permanently affect their structure and functional capacity, and the activity of enzyme systems and endocrine axes. It is now appreciated that such effects lay the foundations for a diverse array of diseases that become manifest many years later, often in response to secondary environmental stressors. Fetal development is underpinned by the placenta, the organ that forms the interface between the fetus and its mother. All nutrients and oxygen reaching the fetus must pass through this organ. The placenta also has major endocrine functions, orchestrating maternal adaptations to pregnancy and mobilizing resources for fetal use. In addition, it acts as a selective barrier, creating a protective milieu by minimizing exposure of the fetus to maternal hormones, such as glucocorticoids, xenobiotics, pathogens, and parasites. The placenta shows a remarkable capacity to adapt to adverse environmental cues and lessen their impact on the fetus. However, if placental function is impaired, or its capacity to adapt is exceeded, then fetal development may be compromised. Here, we explore the complex relationships between the placental phenotype and developmental programming of chronic disease in the offspring. Ensuring optimal placentation offers a new approach to the prevention of disorders such as cardiovascular disease, diabetes, and obesity, which are reaching epidemic proportions.

Open questions in the study of de novo genes: what, how and why

The study of de novo protein-coding genes is maturing from the ad hoc reporting of individual cases to the systematic analysis of extensive genomic data from several species. We identify three key challenges for this emerging field: understanding how best to identify de novo genes, how they arise and why they spread. We highlight the intellectual challenges of understanding how a de novo gene becomes integrated into pre-existing functions and becomes essential. We suggest that, as with protein sequence evolution, antagonistic co-evolution may be key to de novo gene evolution, particularly for new essential genes and new cancer-associated genes.

Evidence of positive selection associated with placental loss in tiger sharks

BACKGROUND

All vertebrates initially feed their offspring using yolk reserves. In some live-bearing species these yolk reserves may be supplemented with extra nutrition via a placenta. Sharks belonging to the Carcharhinidae family are all live-bearing, and with the exception of the tiger shark (Galeocerdo cuvier), develop placental connections after exhausting yolk reserves. Phylogenetic relationships suggest the lack of placenta in tiger sharks is due to secondary loss. This represents a dramatic shift in reproductive strategy, and is likely to have left a molecular footprint of positive selection within the genome.

RESULTS

We sequenced the transcriptome of the tiger shark and eight other live-bearing shark species. From this data we constructed a time-calibrated phylogenetic tree estimating the tiger shark lineage diverged from the placental carcharhinids approximately 94 million years ago. Along the tiger shark lineage, we identified five genes exhibiting a signature of positive selection. Four of these genes have functions likely associated with brain development (YWHAE and ARL6IP5) and sexual reproduction (VAMP4 and TCTEX1D2).

CONCLUSIONS

Our results indicate the loss of placenta in tiger sharks may be associated with subsequent adaptive changes in brain development and sperm production.

Viruses and the placenta: the essential virus first view

A virus first perspective is presented as an alternative hypothesis to explain the role of various endogenized retroviruses in the origin of the mammalian placenta. It is argued that virus-host persistence is a key determinant of host survival and the various ERVs involved have directly affected virus-host persistence.

Oxidative stress and the evolutionary origins of preeclampsia

In this speculative paper, I consider the relationship between oxidative stress and the evolution of placentation in eutherian mammals. I argue that epitheliochorial placentation, in which fetal tissues remain separated from maternal blood throughout gestation, has evolved as a protective mechanism against oxidative stress arising from pregnancy, particularly in species with unusually long gestation periods and unusually large placentas. Human beings comprise an unusual species that has the life history characteristics of an epitheliochorial species, but exhibits hemochorial placentation, in which fetal tissues come into direct contact with maternal blood. I argue that the risk of preeclampsia has arisen as a consequence of the failure of human beings to evolve epitheliochorial placentation.

GEneSTATION 1.0: a synthetic resource of diverse evolutionary and functional genomic data for studying the evolution of pregnancy-associated tissues and phenotypes

Mammalian gestation and pregnancy are fast evolving processes that involve the interaction of the fetal, maternal and paternal genomes. Version 1.0 of the GEneSTATION database (http://genestation.org) integrates diverse types of omics data across mammals to advance understanding of the genetic basis of gestation and pregnancy-associated phenotypes and to accelerate the translation of discoveries from model organisms to humans. GEneSTATION is built using tools from the Generic Model Organism Database project, including the biology-aware database CHADO, new tools for rapid data integration, and algorithms that streamline synthesis and user access. GEneSTATION contains curated life history information on pregnancy and reproduction from 23 high-quality mammalian genomes. For every human gene, GEneSTATION contains diverse evolutionary (e.g. gene age, population genetic and molecular evolutionary statistics), organismal (e.g. tissue-specific gene and protein expression, differential gene expression, disease phenotype), and molecular data types (e.g. Gene Ontology Annotation, protein interactions), as well as links to many general (e.g. Entrez, PubMed) and pregnancy disease-specific (e.g. PTBgene, dbPTB) databases. By facilitating the synthesis of diverse functional and evolutionary data in pregnancy-associated tissues and phenotypes and enabling their quick, intuitive, accurate and customized meta-analysis, GEneSTATION provides a novel platform for comprehensive investigation of the function and evolution of mammalian pregnancy.

Integrating Diverse Types of Genomic Data to Identify Genes that Underlie Adverse Pregnancy Phenotypes

Progress in understanding complex genetic diseases has been bolstered by synthetic approaches that overlay diverse data types and analyses to identify functionally important genes. Pre-term birth (PTB), a major complication of pregnancy, is a leading cause of infant mortality worldwide. A major obstacle in addressing PTB is that the mechanisms controlling parturition and birth timing remain poorly understood. Integrative approaches that overlay datasets derived from comparative genomics with function-derived ones have potential to advance our understanding of the genetics of birth timing, and thus provide insights into the genes that may contribute to PTB. We intersected data from fast evolving coding and non-coding gene regions in the human and primate lineage with data from genes expressed in the placenta, from genes that show enriched expression only in the placenta, as well as from genes that are differentially expressed in four distinct PTB clinical subtypes. A large fraction of genes that are expressed in placenta, and differentially expressed in PTB clinical subtypes (23–34%) are fast evolving, and are associated with functions that include adhesion neurodevelopmental and immune processes. Functional categories of genes that express fast evolution in coding regions differ from those linked to fast evolution in non-coding regions. Finally, there is a surprising lack of overlap between fast evolving genes that are differentially expressed in four PTB clinical subtypes. Integrative approaches, especially those that incorporate evolutionary perspectives, can be successful in identifying potential genetic contributions to complex genetic diseases, such as PTB.

Uterine artery blood flow, fetal hypoxia and fetal growth

Evolutionary trade-offs required for bipedalism and brain expansion influence the pregnancy rise in uterine artery (UtA) blood flow and, in turn, reproductive success. We consider the importance of UtA blood flow by reviewing its determinants and presenting data from 191 normotensive (normal, n = 125) or hypertensive (preeclampsia (PE) or gestational hypertension (GH), n = 29) Andean residents of very high (4100-4300 m) or low altitude (400 m, n = 37). Prior studies show that UtA blood flow is reduced in pregnancies with intrauterine growth restriction (IUGR) but whether the IUGR is due to resultant fetal hypoxia is unclear. We found higher UtA blood flow and Doppler indices of fetal hypoxia in normotensive women at high versus low altitude but similar fetal growth. UtA blood flow was markedly lower in early-onset PE versus normal high-altitude women, and their fetuses more hypoxic as indicated by lower fetal heart rate, Doppler indices and greater IUGR. We concluded that, despite greater fetal hypoxia, fetal growth was well defended by higher UtA blood flows in normal Andeans at high altitude but when compounded by lower UtA blood flow in early-onset PE, exaggerated fetal hypoxia caused the fetus to respond by decreasing cardiac output and redistributing blood flow to help maintain brain development at the expense of growth elsewhere. We speculate that UtA blood flow is not only an important supply line but also a trigger for stimulating the metabolic and other processes regulating feto-placental metabolism and growth. Studies using the natural laboratory of high altitude are valuable for identifying the physiological and genetic mechanisms involved in human reproductive success.