Dissection Of Akt Signaling Pathway In Lymphatic Vascular Development

The lymphatic system plays essential roles in the maintenance of tissue fluid homeostasis, transportation of macromolecules as well as immune cells into the blood circulation in mammals. Lymphatic development is controlled by many genes, which regulates various events including lymphatic endothelial cell functions, the formation of a well-organized mature lymphatic vasculature with smooth muscle cell coverage and valves. Great progress has been made during the last two decades with the advancement of genetic manipulation techniques. As a result, a number of genes and related signaling pathways were found to participate in lymphatic development. The serine/threonine protein kinase Akt (also named protein kinase B, PKB) mediated signaling pathway is one of the most important pathways for a variety of cellular processes like cell survival, migration, differentiation and also proliferation. Akt has 3 isoforms (Aktl/Akt2/Akt3) in mammals which were encoded by different genes but possess high sequence identity. It was previously reported that Akt mediated signaling is critically involved in blood endothelial cell functions and also in blood vascular development of zebrafish and chicken by mediating growth factor induced signals. However, the function of Akt in lymphatic vascular development was unknown.Here we have investigated the role of Akt in lymphatic growth using Akt deficient mice. Firstly, we found that lymphangiogenesis occurred in Akt1-/-, Akt2-/- or Akt3-/-mice. However, the diameter of lymphatic capillaries is significantly less in Akt1-/-mice than that of wild type and there was only slight change in Akt2-/- or Akt3-/- mice. There was also a dramatic decrease of the cell number in lymphatic capillaries. Secondly, valves present in the small collecting lymphatics in the superficial dermal layer of ear skin were rarely observed in Akt1-/- mice, although they could be detected in the large collecting lymphatics in the deep layer of skin tissues. Fluorescence microlymphangiography assay showed that skin lymphatic network in Akt1-/- mice was functional but with a delay in draining FITC-dextran. There was abnormal enlargement of collecting lymphatic vessels, and further analysis showed that smooth muscle cell coverage with collecting lymphatics became much sparser in Akt1 deficient mice than that in wild type control. Finally we showed that lymphatic vessels were detected in compound Akt null mice, and that lymphangiogenesis could be induced by VEGF-C delivered via adenoviral vectors in adult mice lacking Aktl. These results indicate Akt is functionally involved during lymphatic development. In spite of the compensatory roles of other Akt isoforms, we dissected out that Aktl is more critically required in the lymphatic development.