Placental single cell transcriptomics: Opportunities for endocrine disrupting chemical toxicology

Abstract

The placenta performs essential biologic functions for fetal development throughout pregnancy. Placental dysfunction is at the root of multiple adverse birth outcomes such as intrauterine growth restriction, preeclampsia, and preterm birth. Exposure to endocrine disrupting chemicals during pregnancy can cause placental dysfunction, and many prior human studies have examined molecular changes in bulk placental tissues. Placenta-specific cell types, including cytotrophoblasts, syncytiotrophoblasts, extravillous trophoblasts, and placental resident macrophage Hofbauer cells play unique roles in placental development, structure, and function. Toxicant-induced changes in relative abundance and/or impairment of these cell types likely contribute to placental pathogenesis. Although gene expression insights gained from bulk placental tissue RNA-sequencing data are useful, their interpretation is limited because bulk analysis can mask the effects of a chemical on individual populations of placental cells. Cutting-edge single cell RNA-sequencing technologies are enabling the investigation of placental cell-type specific responses to endocrine disrupting chemicals. Moreover, in situ bioinformatic cell deconvolution enables the estimation of cell type proportions in bulk placental tissue gene expression data. These emerging technologies have tremendous potential to provide novel mechanistic insights in a complex heterogeneous tissue with implications for toxicant contributions to adverse pregnancy outcomes.

Keywords: Cellular deconvolution; Endocrine disrupting chemicals; Placenta; Single cell analyses; Villous tissue.

Figure 1.. Human term placental tissue cellular heterogeneity.

A. Major cell types of the placenta include the resident macrophage Hofbauer cells (green), mesenchymal cells including fibroblasts (brown), and endothelial cells that line the fetal vasculature (pink). The outermost layer is the multinucleated semi-continuous syncytiotrophoblast (purple), which is formed by the syncytialization of cytotrophoblasts (light blue). The syncytiotrophoblast is the direct barrier between the placental villous tissue and the intervillous space perfused with maternal blood (dark red). Created with BioRender.com. B. Human term placenta villous tree excised from human placenta in Harris Lab. C. Cross-section of human term villous tree isolated by Harris Lab and visualized using hematoxylin and eosin stain.

Figure 2.. Recommendations for placental RNA-sequencing implementation based on sample size and research question.

Single-cell RNA-sequencing data is most amenable to in vitro studies and studies with small sample sizes, and can produce cell specific signatures of EDC disruption. Large cohort studies may find it more challenging to deploy single-cell sequencing, but can leverage single cell analyses to adjust for cellular heterogeneity and examine cell proportions within their data. *Syncytiotrophoblasts are multi-nucleated and may be challenging to capture using standardized single-cell approaches. EDC=Endocrine Disrupting Chemicals, SC=Single Cell, SN=Single Nuclear, SCB=Syncytiotrophoblast, EVT=Extra villous trophoblast, CYT=Cytotrophoblast, dNK=decidual natural killer cell, HB=hofbauer cell, MC=macrophage”