Tissue Biology: In Search of a New Paradigm

Animal tissues are made up of multiple cell types that are increasingly well-characterized, yet our understanding of the core principles that govern tissue organization is still incomplete. This is in part because many observable tissue characteristics, such as cellular composition and spatial patterns, are emergent properties, and as such, they cannot be explained through the knowledge of individual cells alone. Here we propose a complex systems theory perspective to address this fundamental gap in our understanding of tissue biology. We introduce the concept of cell categories, which is based on cell relations rather than cell identity. Based on these notions we then discuss common principles of tissue modularity, introducing compositional, structural, and functional tissue modules. Cell diversity and cell relations provide a basis for a new perspective on the underlying principles of tissue organization in health and disease.

Evolution of Embryo Implantation Was Enabled by the Origin of Decidual Stromal Cells in Eutherian Mammals

Mammalian pregnancy evolved in the therian stem lineage, that is, before the common ancestor of marsupials and eutherian (placental) mammals. Ancestral therian pregnancy likely involved a brief phase of attachment between the fetal and maternal tissues followed by parturition-similar to the situation in most marsupials including the opossum. In all eutherians, however, embryo attachment is followed by implantation, allowing for a stable fetal-maternal interface and an extended gestation. Embryo attachment induces an attachment reaction in the uterus that is homologous to an inflammatory response. Here, we elucidate the evolutionary mechanism by which the ancestral inflammatory response was transformed into embryo implantation in the eutherian lineage. We performed a comparative uterine transcriptomic and immunohistochemical study of three eutherians, armadillo (Dasypus novemcinctus), hyrax (Procavia capensis), and rabbit (Oryctolagus cuniculus); and one marsupial, opossum (Monodelphis domestica). Our results suggest that in the eutherian lineage, the ancestral inflammatory response was domesticated by suppressing one of its modules detrimental to pregnancy, namely, neutrophil recruitment by cytokine IL17A. Further, we propose that this suppression was mediated by decidual stromal cells, a novel cell type in eutherian mammals. We tested a prediction of this model in vitro and showed that decidual stromal cells can suppress the production of IL17A from helper T cells. Together, these results provide a mechanistic understanding of early stages in the evolution of eutherian pregnancy.

Maternal respiratory SARS-CoV-2 infection in pregnancy is associated with a robust inflammatory response at the maternal-fetal interface

BACKGROUND

Pregnant women are at increased risk for severe outcomes from coronavirus disease 2019 (COVID-19), but the pathophysiology underlying this increased morbidity and its potential effect on the developing fetus is not well understood.

METHODS

We assessed placental histology, ACE2 expression, and viral and immune dynamics at the term placenta in pregnant women with and without respiratory severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.

FINDINGS

The majority (13 of 15) of placentas analyzed had no detectable viral RNA. ACE2 was detected by immunohistochemistry in syncytiotrophoblast cells of the normal placenta during early pregnancy but was rarely seen in healthy placentas at full term, suggesting that low ACE2 expression may protect the term placenta from viral infection. Using immortalized cell lines and primary isolated placental cells, we found that cytotrophoblasts, the trophoblast stem cells and precursors to syncytiotrophoblasts, rather than syncytiotrophoblasts or Hofbauer cells, are most vulnerable to SARS-CoV-2 infection in vitro. To better understand potential immune mechanisms shielding placental cells from infection in vivo, we performed bulk and single-cell transcriptomics analyses and found that the maternal-fetal interface of SARS-CoV-2-infected women exhibited robust immune responses, including increased activation of natural killer (NK) and T cells, increased expression of interferon-related genes, as well as markers associated with pregnancy complications such as preeclampsia.

CONCLUSIONS

SARS-CoV-2 infection in late pregnancy is associated with immune activation at the maternal-fetal interface even in the absence of detectable local viral invasion.

FUNDING

NIH (T32GM007205, F30HD093350, K23MH118999, R01AI157488, U01DA040588) and Fast Grant funding support from Emergent Ventures at the Mercatus Center.

SARS-CoV-2 infection in pregnancy is associated with robust inflammatory response at the maternal-fetal interface

Pregnant women appear to be at increased risk for severe outcomes associated with COVID-19, but the pathophysiology underlying this increased morbidity and its potential impact on the developing fetus is not well understood. In this study of pregnant women with and without COVID-19, we assessed viral and immune dynamics at the placenta during maternal SARS-CoV-2 infection. Amongst uninfected women, ACE2 was detected by immunohistochemistry in syncytiotrophoblast cells of the normal placenta during early pregnancy but was rarely seen in healthy placentas at full term. Term placentas from women infected with SARS-CoV-2, however, displayed a significant increase in ACE2 levels. Using immortalized cell lines and primary isolated placental cells, we determined the vulnerability of various placental cell types to direct infection by SARS-CoV-2 in vitro. Yet, despite the susceptibility of placental cells to SARS-CoV-2 infection, viral RNA was detected in the placentas of only a subset (~13%) of women in this cohort. Through single cell transcriptomic analyses, we found that the maternal-fetal interface of SARS-CoV-2-infected women exhibited markers associated with pregnancy complications, such as preeclampsia, and robust immune responses, including increased activation of placental NK and T cells and increased expression of interferon-related genes. Overall, this study suggests that SARS-CoV-2 is associated with immune activation at the maternal-fetal interface even in the absence of detectable local viral invasion. While this likely represents a protective mechanism shielding the placenta from infection, inflammatory changes in the placenta may also contribute to poor pregnancy outcomes and thus warrant further investigation.

Endometrial recognition of pregnancy occurs in the grey short-tailed opossum ( Monodelphis domestica)

In human pregnancy, recognition of an embryo within the uterus is essential to support the fetus through gestation. In most marsupials, such as the opossums, pregnancy is shorter than the oestrous cycle and the steroid hormone profile during pregnancy and oestrous cycle are indistinguishable. For these reasons, it was assumed that recognition of pregnancy, as a trait, evolved in the eutherian (placental) stem lineage and independently in wallabies and kangaroos. To investigate whether uterine recognition of pregnancy occurs in species with pregnancy shorter than the oestrous cycle, we examined reproduction in the short-tailed opossum ( Monodelphis domestica), a marsupial with a plesiomorphic mode of pregnancy. We examined the morphological and gene expression changes in the uterus of females in the non-pregnant oestrous cycle and compared these to pregnancy. We found that the presence of an embryo did not alter some aspects of uterine development but increased glandular activity. Transcriptionally, we saw big differences between the uterus of pregnant and cycling animals. These differences included an upregulation of genes involved in transport, inflammation and metabolic-activity in response to the presence of a fetus. Furthermore, transcriptional differences reflected protein level differences in transporter abundance. Our results suggest that while the uterus exhibits programmed changes after ovulation, its transcriptional landscape during pregnancy responds to the presence of a fetus and upregulates genes that may be essential for fetal support. These results are consistent with endometrial recognition of pregnancy occurring in the opossum. While the effects on maternal physiology appear to differ, recognition of pregnancy has now been observed in eutherian mammals, as well as, Australian and American marsupials.

The mammalian decidual cell evolved from a cellular stress response

Among animal species, cell types vary greatly in terms of number and kind. The number of cell types found within an organism differs considerably between species, and cell type diversity is a significant contributor to differences in organismal structure and function. These observations suggest that cell type origination is a significant source of evolutionary novelty. The molecular mechanisms that result in the evolution of novel cell types, however, are poorly understood. Here, we show that a novel cell type of eutherians mammals, the decidual stromal cell (DSC), evolved by rewiring an ancestral cellular stress response. We isolated the precursor cell type of DSCs, endometrial stromal fibroblasts (ESFs), from the opossum Monodelphis domestica. We show that, in opossum ESFs, the majority of decidual core regulatory genes respond to decidualizing signals but do not regulate decidual effector genes. Rather, in opossum ESFs, decidual transcription factors function in apoptotic and oxidative stress response. We propose that rewiring of cellular stress responses was an important mechanism for the evolution of the eutherian decidual cell type.

Genetic Associations with Gestational Duration and Spontaneous Preterm Birth

BACKGROUND

Despite evidence that genetic factors contribute to the duration of gestation and the risk of preterm birth, robust associations with genetic variants have not been identified. We used large data sets that included the gestational duration to determine possible genetic associations.

METHODS

We performed a genomewide association study in a discovery set of samples obtained from 43,568 women of European ancestry using gestational duration as a continuous trait and term or preterm (<37 weeks) birth as a dichotomous outcome. We used samples from three Nordic data sets (involving a total of 8643 women) to test for replication of genomic loci that had significant genomewide association (P<5.0×10^-8) or an association with suggestive significance (P<1.0×10^-6) in the discovery set.

RESULTS

In the discovery and replication data sets, four loci (EBF1, EEFSEC, AGTR2, and WNT4) were significantly associated with gestational duration. Functional analysis showed that an implicated variant in WNT4 alters the binding of the estrogen receptor. The association between variants in ADCY5 and RAP2C and gestational duration had suggestive significance in the discovery set and significant evidence of association in the replication sets; these variants also showed genomewide significance in a joint analysis. Common variants in EBF1, EEFSEC, and AGTR2 showed association with preterm birth with genomewide significance. An analysis of mother-infant dyads suggested that these variants act at the level of the maternal genome.

CONCLUSIONS

In this genomewide association study, we found that variants at the EBF1, EEFSEC, AGTR2, WNT4, ADCY5, and RAP2C loci were associated with gestational duration and variants at the EBF1, EEFSEC, and AGTR2 loci with preterm birth. Previously established roles of these genes in uterine development, maternal nutrition, and vascular control support their mechanistic involvement. (Funded by the March of Dimes and others.).

The Transcriptomic Evolution of Mammalian Pregnancy: Gene Expression Innovations in Endometrial Stromal Fibroblasts

The endometrial stromal fibroblast (ESF) is a cell type present in the uterine lining of therian mammals. In the stem lineage of eutherian mammals, ESF acquired the ability to differentiate into decidual cells in order to allow embryo implantation. We call the latter cell type "neo-ESF" in contrast to "paleo-ESF" which is homologous to eutherian ESF but is not able to decidualize. In this study, we compare the transcriptomes of ESF from six therian species: Opossum (Monodelphis domestica; paleo-ESF), mink, rat, rabbit, human (all neo-ESF), and cow (secondarily nondecidualizing neo-ESF). We find evidence for strong stabilizing selection on transcriptome composition suggesting that the expression of approximately 5,600 genes is maintained by natural selection. The evolution of neo-ESF from paleo-ESF involved the following gene expression changes: Loss of expression of genes related to inflammation and immune response, lower expression of genes opposing tissue invasion, increased markers for proliferation as well as the recruitment of FOXM1, a key gene transiently expressed during decidualization. Signaling pathways also evolve rapidly and continue to evolve within eutherian lineages. In the bovine lineage, where invasiveness and decidualization were secondarily lost, we see a re-expression of genes found in opossum, most prominently WISP2, and a loss of gene expression related to angiogenesis. The data from this and previous studies support a scenario, where the proinflammatory paleo-ESF was reprogrammed to express anti-inflammatory genes in response to the inflammatory stimulus coming from the implanting conceptus and thus paving the way for extended, trans-cyclic gestation.