Sep 12, 2017

Shedding Light on Placental Cellular Dynamics During Pregnancy

Sheila Clark

Placental cellular dysfunction is linked to the development of disease

With the development of the placenta upon the implantation of the blastocyst into the endometrium, the fetal-maternal connection is formed. The success of the pregnancy depends on the placenta: it transfers nutrients and oxygen between the mother and fetus, facilitates the excretion of fetal waste products, confers immunity to protect to the developing fetus, and produces hormones critical for fetal development.

The placenta is a heterogeneous structure composed of many different cell types derived from either the fetus or mother. Gestational complications, such as preeclampsia (PE), have been linked to placental cellular dysfunction. Untreated, PE leads to eclampsia—the development of convulsions, and can lead to perinatal and maternal death. Patients are routinely screened for PE, but most of these tests are not specific nor definitive. Direct tissue monitoring is also not feasible because of safety concerns. Due to these current screening limitations, more specific, non-invasive testing is required.

Deciphering the cellular heterogeneity of the placenta

Studying the placenta itself is crucial for the understanding of the pathology of diseases such as PE, but because of the heterogeneous nature of the placenta and its mix of specialized and non-specialized cells, this has been a historically difficult undertaking. Additionally, a potential avenue for non-invasive screening may lie in the finding that the  placenta is a major source of circulating cell-free fetal nucleic acids in maternal blood and elevated levels of these nucleic acids have been reported in women with PE. A recent PNAS publication from the laboratory of Dennis Lo applied the use of the 10x Genomics’ Chromium™ Single Cell 3’ Solution to perform single-cell RNA-seq (scRNA-seq) and comprehensively characterize the transcriptomic heterogeneity of the human placenta—an approach that served as confirmation for the non-invasive identification of cellular dysfunction in early PE via cell-free RNA in the maternal serum.

Diverse cellular subtypes in the human placenta and placental cellular dynamics

The Chromium System and Single Cell 3’ Solution were used to provide a comprehensive profile of placental cellular heterogeneity in both normal-term and preeclamptic placentas. By profiling more than 24,000 cells, an atlas of the placenta was constructed. Single-cell transcriptomics, followed by  fetomaternal SNP ratio analysis, confirmed 11 major subtypes of placental cells, with the majority being of fetal origin.

Trophoblasts, one of the specialized cells of the placenta, play an important role in embryo implantation and interaction with the maternal uterus. Defective placental implantation has been proposed as the major pathological mechanism in early PE. The authors identified and filtered the cell signature genes of individual placental cell types by reanalyzing peripheral blood mononucleated cells (PBMCs) single-cell transcriptomic profiles and the tissue transcriptome data of leukocytes and liver from public single-cell gene expression databases, including those available from 10x Genomics. This allowed the authors to isolate and dissect the dynamic changes of both trophoblastic and non-trophoblastic cellular components in the maternal plasma, enabling the reconstruction of the trophoblast differentiation trajectory and shedding light on the mechanism underlying the regulation of trophoblast lineage development.

Uncovering cellular dysfunction in the early preeclamptic placenta

The authors also sought to uncover placental cellular dysfunction in the maternal cell-free RNA of early severe preeclamptic patients. They hypothesized that the cellular origin of the pathology can be revealed by comparing the expression levels of different cell-type–specific signatures in the maternal plasma of early severe preeclamptic patients with those of healthy pregnant controls. Using plasma RNA, they found that the extravillous trophoblast (EVTB) signature is consistently upregulated in early preeclamptic patients in two separate cohorts. Comparable cellular dynamic patterns were observed by performing scRNA-seq—transcriptional heterogeneity of genes involved in cell migration, cell death, and proliferation was significantly more variable and more highly expressed in early preeclamptic placentas. By contrast, DNA damage repair and antigen presentation ontology genes were more variable in healthy term placentas. These results suggested that EVTB in early preeclamptic placentas might have higher levels of cell death. Taken together, this provides a proof-of-concept that cell-free RNA analysis in plasma can potentially serve as a noninvasive tool in revealing placental pathology.

Increasing our understanding of placental dynamics with scRNA-seq

Applying scRNA-seq to examine the dynamics of fetal and maternal components of the placenta allowed the authors to differentiate specific pathological changes on a cell-by-cell basis. Importantly, re-constructing this cellular information from the plasma allows for a potentially noninvasive way to diagnose placental dysfunction at an early stage, enabling preventative measures to be implemented.

Additional Resources

  • Read the placental dynamics paper by Tsang et al. here.
  • Learn more about the Chromium™ Single Cell 3' Solution.
  • Read Chromium™ Single Cell 3' Solution application notes.
  • Check out our data analysis and visualization tools - Cell Ranger™ and Loupe™ Cell Browser.
  • Download Chromium™ Single Cell 3' Solution datasets.

The images used in this blog are from:

Tsang et al., "Integrative single-cell and cell-free plasma RNA transcriptomics elucidates placental cellular dynamics", PNAS, 114 (2017): E7786–E7795