The Aryl Hydrocarbon Receptor Directs the Differentiation of Progenitor Blastomeres

Key regulatory decisions during cleavage divisions in mammalian embryogenesis determine the fate of preimplantation embryonic cells. Single-cell RNA-sequencing of early stage—2-cell, 4-cell, and 8-cell—blastomeres show that the aryl hydrocarbon receptor (AHR), traditionally considered as an environmental sensor, directs the segregation of 4-cell blastomeres. Disruption of endogenous AHR functions in Ahr knockout embryos or in embryos from dams exposed to dioxin, the prototypic xenobiotic AHR agonist, significantly impairs blastocyst formation by causing repression of OCT4 expression, decline of CDX2 expression, loss of OCT4 and CDX2 transcriptional heterogeneity, and incidence of nonspecific pluripotency downregulation. Trajectory and gene variability analyses further confirm that deregulation of OCT4 functions and changes of transcriptional heterogeneity resulting from disruption of AHR functions restrict the emergence of differentiating blastomeres within these embryos. It appears that AHR governs the differentiation of progenitor blastomeres and that interference with AHR functions significantly perturbs embryogenesis. Overall design: Dysfunctional AHR may disrupt early blastomere differentiation and cause the impaired formation of Ahr-knockout and dioxin-exposed blastocysts. Since mutual regulation between OCT4 and CDX2 is in place by the 16-cell stage, at a time when Ahr expression is undetectable, we aimed specifically at the early 2- to 8-cell stages to explore the potential role of the AHR in regulation of the transcriptome of progenitor blastomeres. To achieve this purpose, we isolated 336 single blastomeres obtained from eight embryos of each of the nine groups: Wild-type control, Ahr-knockout, and dioxin-exposed each at 2-cell, 4-cell, and 8-cell stages individually, and subjected them to Single-Cell RNA-sequencing (scRNA-seq).