Unveiling the Temporal Dynamics of Mesenchymal Condensations in Tracheal Development through Non-Canonical Wnt Signaling and Notum

The trachea is essential for proper airflow to the lungs for gas exchange. Frequent congenital tracheal malformations affect the cartilage, causing the collapse of the central airway during the respiratory cycle. We have shown that Notum, a Wnt ligand de-acylase that attenuates the canonical branch of the Wnt signaling pathway, is necessary for cartilaginous mesenchymal condensations. In Notum deficient tracheas, chondrogenesis is delayed, and the trachea is stenotic. It is unknown if Notum attenuates non-canonical Wnt signaling. Notably, after mesenchymal deletion of the non-canonical Wnt5a ligand, we observed premature tracheal chondrogenesis. We hypothesize that Notum and Wnt5a are required to mediate the timely formation of mesenchymal condensations, giving rise to the tracheal cartilage. Ex vivo culture of tracheal tissue shows that chemical inhibition of the Wnt non-canonical pathway promotes earlier condensations, while Notum inhibition presents delayed condensations. Furthermore, non-canonical Wnt induction prevents the formation of cartilaginous mesenchymal condensations. On the other hand, cell-cell interactions among chondroblasts increased in the absence of mesenchymal Wnt5a. By performing an unbiased analysis of the gene expression in Wnt5a and Notum deficient tracheas, we detected that mRNA of genes essential for chondrogenesis and extracellular matrix formation are upregulated by E11.5 in Wnt5a mutants. The expression profile supports the premature and delayed chondrogenesis observed in Wnt5a and Notum deficient tracheas. We conclude that Notum and Wnt5a are necessary for proper tracheal cartilage patterning by coordinating timely chondrogenesis. Thus, these studies shed light on molecular mechanisms underlying congenital anomalies of the trachea.