Title: Notch Signaling Determines Lymphatic Cell Fate and Regulates Sprouting Lymphangiogenesis
Description: The lymphatic vascular system is necessary for physiological regulation of tissue fluid homeostasis and absorption of dietary fat. Lymphatics also function in the inflammatory response and are involved in pathological conditions such as wound healing and cancer. We show that the Notch signaling pathway is a regulator of both developmental and pathological lymphangiogenesis. Notch1 and Notch4 are expressed by the lymphatic endothelium, and Delta-like ligand 4 (Dll4) is the predominantly expressed Notch ligand in the developing lymphatic vessels of the embryonic dermis and pathological lymphatic vessels of the wounded cornea. Dll4 was able to induce Notch activation in human dermal lymphatic endothelial cells (HDLECs), whereas Jagged1 (Jag1) was not. In HDLECs, Notch signaling is activated in response to Vascular Endothelial Growth Factor (VEGF) or Vascular Endothelial Growth Factor-C (VEGF-C) stimulation. In vitro assays demonstrated that Notch activation inhibits HDLEC proliferation, migration, and capillary network formation; these effects were coincident with increased levels of HEY1 and HEY2, biphasic regulation of VEGFR-3, and decreased levels of VEGFR-2.

Using genetic intervention of Notch signaling, we demonstrated that Notch regulates developmental sprouting lymphangiogenesis by restricting growth and sprouting of lymphatics in the murine embryonic dermis. Using pharmacological intervention of Notch signaling, we found that Notch restricted pathological sprouting lymphangiogenesis in the corneal suture assay, which models inflammation-induced lymphangiogenesis. However, pharmacological intervention of Notch signaling did not measurably affect pathological sprouting lymphangiogenesis in an orthotopic tumor model of human breast cancer. Our data from analysis of HDLECs, dermis, and sutured cornea support a role for Dll4-driven Notch signaling in restricting sprouting lymphangiogenesis. Lymphatic specification/separation requires a venous endothelial cell to become a lymphatic endothelial cell, and lymphatic valve formation requires a duct endothelial cell to become a valve endothelial cell. Through analysis of genes regulated by Notch in HDLECs, we demonstrated that Notch determines lymphatic endothelial cell fates.

Notch inhibits genes critical for lymphatic specification and separation (PROX1, PDPN), and induces genes important for lymphatic valve formation (FNEIIIA, ITGA9, CX37). We conclude that Notch is a context-dependent regulator of lymphangiogenesis. Notch functions in the tip/stalk, venous to lymphatic, and duct endothelial to valve endothelial cell fate decisions in lymphatic vasculature. Given the critical functions of the lymphatic vasculature in multiple physiological and pathological settings, understanding Notch functions in the lymphatic vasculature is critical to design treatments for conditions caused by lymphatic malfunction.