Intracellular Enhancement of BMP Signaling by LIM-Domain Protein FHL3 Controls Spatiotemporal Emergence of the Neural Crest Driven by WNT Signaling

Abstract

How multiple morphogens are coordinated in space and time to position key embryonic tissues remains elusive. During neural crest formation, bone morphogenetic protein (BMP), fibroblast growth factor (FGF) and WNT signaling cooperate by acting either on the paraxial mesoderm or directly on the neural border ectoderm, but how each tissue interprets this complex information remains poorly understood. Here we show that Fhl3, a scaffold LIM domain protein of previously unknown developmental function, is essential for neural crest formation by linking BMP and WNT signaling thereby positioning the neural crest-inducing signaling center in the paraxial mesoderm. During gastrulation, Fhl3 promotes Smad phosphorylation and Smad-dependent wnt8 activation in the paraxial mesoderm, thus enhancing the response of mesoderm compared to ectoderm although both germ layers are subjected to a common extracellular BMP gradient. This promotes a non-cell autonomous WNT signaling ensuring neural border ectoderm specification by the underlying mesoderm. During neurulation, in turn, neural crest inducers activate fhl3 in the ectoderm, promoting BMP/Smad-dependent WNT activity required for neural crest specification. We show that Fhl3 binds Smads downstream of BMP signals and promotes Smad binding to wnt8 regulatory elements. Our findings highlight how Fhl3, acting cell-autonomously, ensures a fine spatial and temporal coordination of BMP and WNT signaling at several steps of neural crest development.