Hypoxia inhibits lymphatic thoracic duct formation in zebrafish

Hypoxia promotes blood vessel growth through up-regulation of pro-angiogenic pathways but its role on the lymphatic system remains unclear. The homeobox transcription factor Prox1 is a master control gene for generating lymphatic endothelial cells (LECs) and is up-regulated by hypoxia-inducible factors in mammals. While vascular endothelial growth factor A (VEGFA) is critical for angiogenesis, VEGFC and its receptor VEGF receptor-3 (VEGFR-3) are essential for the initial sprouting and directed migration as well as for the subsequent survival of LECs. The aim of this study was to determine the effects of hypoxia on the development of the lymphatic system in zebrafish. Zebrafish embryos were obtained from Tg(SAGFF27C; UAS:GFP) animals carrying a lymphatic reporter gene coupled to green fluorescent protein (GFP). Exposure of 1-day old zebrafish embryos to hypoxic conditions (5% O2) for 24 h inhibited thoracic duct formation (-27%, p < 0.0001). Hypoxia inhibited the expression of pro-lymphangiogenic factors prox1a, vegfc and vegfr-3. This inhibition was relieved after re-oxygenation. On the other hand, hypoxia increased the expression of vegfa, a pro-angiogenic factor. In conclusion, hypoxia has opposite effects on vascular development in zebrafish, inhibiting the development of the lymphatic vascular system while promoting the development of the blood vascular system.