Sp Transcription Factors Establish the Signaling Environment in the Neuromesodermal Progenitor Niche During Axial Elongation.

Neuromesodermal competent progenitors (NMCs) are located in the caudal epiblast near the node and primitive streak and give rise to spinal cord and somitic mesoderm during trunk and tail elongation. Their self-renewal depends on an autoregulatory loop involving Wnt3a and Fgf signaling, and the Tbxt and Cdx transcription factors, but the mechanisms underlying loop formation and the establishment of the niche are poorly understood. Here, we identify the zinc-finger transcription factors Sp5 and Sp8 (Sp5/8) as essential regulators of NMC maintenance. Sp5/8 expression is controlled by Wnt, Fgf and retinoic acid signaling, and they cooperate with Tbxt, Tcf7 and Cdx2 to sustain a robust autoregulatory network that promotes high Wnt/Fgf and low retinoic acid activity in the niche. These factors bind a novel enhancer essential for Wnt3a expression and feedback-loop integrity. Mechanistically, Sp5/8 regulate the dynamic exchange of activating and repressive Tcf complexes at Wnt-responsive enhancers. Our findings define a transcriptional module centered on Sp5/8 that stabilizes niche signaling and transcriptional circuitry essential for NMC fate decisions and trunk development.