WNT signaling memory is required for ACTIVIN to function as a morphogen

It has now become clear that the process of fate specification during early embryogenesis is mediated by a handful of key signaling pathways. However, how the temporal and spatial integration of these signals plays out to give rise to self-organization of tissues remains obscure. Here we use artificial human gastruloids and quantitative single-cell analysis to dissect the temporal integration of two key pathways WNT and ACTIVIN that along with BMP control gastrulation and primitive streak patterning in model systems. We showed that ACTIVIN elicits a transient signaling response, as well as a transient induction of differentiation. However, unlike BMP and WNT, ACTIVIN cannot induce stable primitive streak formation and mesodermal patterning. Pre-exposure to WNT switches the response of cells to ACTIVIN whereby it becomes a concentration dependent morphogen. This provides evidence for WNT signaling memory that occurs at the transcriptional level and not as a modifier of ACTIVIN signaling dynamics. Time series RNAseq analysis of the effect of the addition of activin to a hESC culture. The study consists of 3 control samples (0, 6, 12h) and 5 activin-treated samples (collected at 1, 2.5, 4, 8, 12h). There are no replicates per timepoint/condition.