Maldevelopment of dermal lymphatics in Wnt5a-knockout-mice.

Maintenance of tissue homeostasis and immune surveillance are important functions of the lymphatic vascular system. Lymphatic vessels are lined by lymphatic endothelial cells (LECs). By gene micro-array expression studies we recently compared human lymphangioma-derived LECs with umbilical vein endothelial cells (HUVECs). Here, we followed up on these studies. Besides well-known LEC markers, we observed regulation of molecules involved in immune regulation, acetylcholine degradation and platelet regulation. Moreover we identified differentially expressed WNT pathway components, which play important roles in the morphogenesis of various organs, including the blood vascular system. WNT signaling has not yet been addressed in lymphangiogenesis. We found high expression of FZD3, FZD5 and DKK2 mRNA in HUVECs, and WNT5A in LECs. The latter was verified in normal skin-derived LECs. With immunohistological methods we detected WNT5A in LECs, as well as ROR1, ROR2 and RYK in both LECs and HUVECs. In the human, mutations of WNT5A or its receptor ROR2 cause the Robinow syndrome. These patients show multiple developmental defects including the cardio-vascular system. We studied Wnt5a-knockout (ko) mouse embryos at day 18.5. We show that the number of dermal lymphatic capillaries is significantly lower in Wnt5a-null-mice. However, the mean size of individual lymphatics and the LEC number per vessel are greater. In sum, the total area covered by lymphatics and the total number of LECs are not significantly altered. The reduced number of lymphatic capillaries indicates a sprouting defect rather than a proliferation defect in the dermis of Wnt5a-ko-mice, and identifies Wnt5a as a regulator of lymphangiogenesis.