Unique functions for Notch4 in murine embryonic lymphangiogenesis

Abstract

In mice, embryonic dermal lymphatic development is a well-understood system used to study the role of genes in physiological lymphangiogenesis. The Notch signaling is an evolutionary conserved pathway that modulates cell fate decisions and shown to both inhibit and promote dermal lymphangiogenesis. Here, we demonstrate distinct roles for Notch4 signaling versus canonical Notch signaling in embryonic dermal lymphangiogenesis. At E14.5, actively growing dermal lymphatics expressed NOTCH1, NOTCH4 and DLL4, with DLL4 expression strongest and Notch active in the lymphangiogenic sprouts. Treatment of cultured LECs with VEGF-A or VEGF-C upregulated Dll4 transcripts, but differentially regulated Notch1 and Notch4 expression, and the Notch effectors of the Hes/Hey families, suggesting that VEGF-A and VEGF-C distinctly modulate Dll4/Notch signaling in the lymphatic endothelium. Mice nullizygous for Notch4 had an increase in the closure of the lymphangiogenic fronts towards the midline which correlated with reduced vessel caliber in the maturing lymphatic plexus. Activation of Notch4 suppressed lymphatic endothelial cell migration in a wounding assay significantly more then Notch1 activation, suggesting a dominant role for Notch4 in LEC migration. Unlike Notch4 nulls, inhibition of canonical Notch signaling by ectopically expressing a dominant negative form of MAML1 (DNMAML) in Prox1+ lymphatic endothelium suppressed lymphatic endothelial cell proliferation consistent with what has been described for the loss of lymphatic endothelial Notch1. Moreover, loss of Notch4 did not disrupt lymphatic endothelial canonical Notch signaling. Thus, we propose that Notch4 signaling and canonical Notch signaling have distinct functions in the coordination of embryonic dermal lymphangiogenesis.