Role Of Receptor Tyrosine Kinase TIE And VEGFR-3 In Blood Vascular And Lymphatic Formation And Remodeling

Mammalian vascular network originates from mesoderm during embryonic development with the initial formation of primitive network followed by further expansion into a mature vascular system,via two processes named vasculogenesis and angiogenesis..Lymphatic vessels originate from veins in mammals.The development of blood vascular and lymphatic system is a complex process,which is tightly controlled by many factors including receptor tyrosine kinases such as Tie2,Tie1 and VEGFR-3.Previous work using genetically modified mouse models demonstrated that Tie2 knockout resulted in abnormal blood vessel development and early embryonic lethality(before E10.5).Interestingly,Tie1 deficiency led to embryonic lethality before birth due to defective lymphatic and blood vascular formation.However,the mechanism underlying TIE signaling in vascular and lymphatic development is still incompletely understood.Another receptor tyrosine kinase VEGFR-3 was first identified as a marker of lymphatic endothelial cell.VEGFR-3 mediated pathway has been shown to play a vital role in lymphangiogenesis.In this study,we employed Tie2 and Tie1 conditional knockout mouse models to induce gene deletion bycrossing with the Cre ERT2 transgenic mice and characterized their vascular phenotypes.Gene deletion efficiency was confirmed by RT-PCR,immunostaining and western blot.We found that Tie2 was highly expressed in blood vessels at later stage of embryogenesis,while the expression of Tie2 in lymphatic vessels is relatively low.Induced deletion of Tie2 led to the suppression of retinal vascularization.However,Tie2 deletion did not seem to affect already formed blood vessels in skin,suggesting a stage dependent function of Tie2 in vascular development.Furthermore,we also compared the biological functions of Tie2 and Tie1 in lymphatic development.We found that postnatal knockout of Tie1 led to aberrant lymphatic network patterning and also a significant decrease in lymphatic valves.However,knockout of Tie2 in neonates did not seem to have any obvious effect on lymphatic formation and maturation.Additionally,using VEGFR-3 mutant mice with an inactivation mutation in tyrosine kinase domain,we found that insufficient signals mediated by VEGFR-3 caused a significant decrease of dermal lymphatic branches and anincrease in lymphatic diameter.However,lymphatic phenoptes in internal organs were mild compared with that in skin,and the underlying mechanism awaits further investigation.In summary,we showed that Tie2 mediated signaling was essential in retinal angiogenesis.However,it seems to be dispensable in lymphangiogenesisat least postnatally.We further showed that Tie1 was required in lymphatic network formation and maturation including valve development,while VEGFR-3 are important in lymphangiogenesis.The interaction of Tie1 and VEGFR-3 in lymphatic development is still under investigation.