R-spondin 2 is required for normal laryngeal-tracheal, lung and limb morphogenesis

Herein, we demonstrate that Lrp6-mediated R-spondin 2 signaling through the canonical Wnt pathway is required for normal morphogenesis of the respiratory tract and limbs. We show that the footless insertional mutation creates a severe hypomorphic R-spondin 2 allele (Rspo2^Tg). The predicted protein encoded by Rspo2^Tg neither bound the cell surface nor activated the canonical Wnt signaling reporter TOPFLASH. Rspo2 activation of TOPFLASH was dependent upon the second EGF-like repeat of Lrp6. Rspo2^Tg/Tg mice had severe malformations of laryngeal-tracheal cartilages, limbs and palate, and lung hypoplasia consistent with sites of Rspo2 expression. Rspo2^Tg/Tg lung defects were associated with reduced branching, a reduction in TOPGAL reporter activity, and reduced expression of the downstream Wnt target Irx3. Interbreeding the Rspo2^Tg and Lrp6^- alleles resulted in more severe defects consisting of marked lung hypoplasia and absence of tracheal-bronchial rings, laryngeal structures and all limb skeletal elements.