The role of retinoic acid in lymphatic vasculature development

First steps in embryonic lymph vessel development involve reprogramming of blood vessel endothelial cells, after which novel lymphatic endothelial cells migrate to form jugular lymph sacs. Several factors have been described to affect reprogramming of blood endothelial cells and to be involved in establishing lymphatic endothelial cell fate. However, it is unknown how this is initiated, why reprogramming always occurs at specific sites, and how migration is regulated. Retinoic acid is involved in lymphatic differentiation and initiation of lymph node development. To study its specific role in initiation of lymphatic endothelial differentiation, we will reprogram embryonic blood endothelial cells in vitro and will make use of specific mutant mouse models for retinoic acid signaling. Moreover, we aim to identify genes regulated by retinoic acid which are involved in lymphatic endothelial cell differentiation, using gene expression profiling. These candidate genes will be analyzed by morpholinos injected into zebrafish embryos and in available mouse mutant models. We will furthermore determine the source of retinoic acid by using mouse mutants with a cell specific defect in retinoic acid synthesis, and analyze the effect on the formation and location of the jugular lymph sacs. Since lymph sacs always develop in proximity of cranial nerves, we hypothesize that established nerve fibers control jugular lymph sac formation possibly by synthesis of retinoic acid. This multi-disciplinary approach will enable us to clarify what the initiation signals for lymphatic development are and whether there is indeed a role for nerves in establishing the location of these first events.