In vitro modelling of anterior primitive streak patterning with human pluripotent stem cells identifies the path to notochord progenitors

Abstract

Notochord progenitors (NotoPs) represent a scarce yet crucial embryonic cell population, playing important roles in embryo patterning and eventually giving rise to the cells that form and maintain intervertebral discs. The mechanisms regulating NotoPs emergence are unclear. This knowledge gap persists due to the inherent complexity of cell fate patterning during gastrulation, particularly within the anterior primitive streak (APS), where NotoPs first arise alongside other important progenitors including neuro-mesodermal and endodermal progenitors. To gain insights into this process, we use micropatterning together with FGF and the WNT pathway activator CHIR9901, to guide the development of human embryonic stem cells into reproducible patterns of APS cell fates. We show that small variations in CHIR9901 dosage dictate the downstream dynamics of endogenous TGFbeta signalling which in turn controls cell fate decisions. We show that sustained NODAL signalling induces endoderm while NODAL inhibition is needed for NMP specification. Furthermore, we unveil a crosstalk between TGFbeta and WNT signaling pathways, wherein TGFbeta inhibition enhances WNT activity. Finally, we demonstrate that the timely inhibition of TGFbeta signalling is imperative for the emergence of NotoPs. Our work elucidates the signalling regimes underpinning NotoPs emergence and provides novel insights into the regulatory mechanisms controlling the balance of APS cell fates during gastrulation.