Single-cell response to Wnt activation in human embryonic stem cells reveals uncoupling of Wnt target gene expression

Wnt signaling drives nuclear translocation of {beta}-catenin and its subsequent association with the DNA-bound TCF/LEF transcription factors, which dictate target gene specificity by recognizing Wnt responsive elements across the genome. {beta}-catenin target genes are therefore thought to be collectively activated upon Wnt pathway stimulation. However, this appears in contrast with the non-overlapping patterns of Wnt target gene expression in several contexts, including early mammalian embryogenesis. Here we followed Wnt target gene expression in human embryonic stem cells after Wnt pathway stimulation at a single-cell resolution. Cells changed gene expression program over time consistent with three key developmental events: i) loss of pluripotency, ii) induction of Wnt target genes, and iii) mesoderm specification. Contrary to our expectation, not all cells displayed equal amplitude of Wnt target gene activation; rather, they distributed in a continuum from strong to weak responders when ranked based on the expression of the target AXIN2. Moreover, high AXIN2 did not always correspond to elevated expression of other Wnt targets, which were activated in different proportions in individual cells. This uncoupling of Wnt target gene expression, which was also identified in single colorectal cancer cells with hyper-activated Wnt signaling, underlines the necessity to identify additional mechanisms that explain the heterogeneity of the Wnt/{beta}-catenin-mediated transcriptional outputs in single cells.