Fox Transcription factors in lymphatic vessel development

The lymphatic vascular system is part of the circulatory system in the body and is essential for lipid absorption/transport from the digestive system, maintenance of tissue fluid homeostasis, and immune surveillance. It is involved in many pathological processes such as inflammatory diseases and tumor metastasis, as well as lymphedema, an abnormal build-up of lymphatic fluid in the interstitial tissue and swelling caused by insufficient lymphatic drainage due to inefficient lymphangiogenesis or lymphatic valve deficiency. However, the complex nature of lymphatic vessel abnormalities and diseases in humans remain poorly understood; therefore, the long-term goal of our lab is to understand the molecular and cellular mechanisms underlying the formation and function of the lymphatic vessel system during development. FoxC1 and FoxC2 are closely related members of the Forkhead/Fox transcription factor family, and they play numerous essential roles in cardiovascular development. Mutations in human FOXC2 are responsible for the autosomal dominant syndrome, Lymphedema-distichiasis, characterized by the obstruction of lymphatic drainage of the limbs and the growth of an extra set of eyelashes, as well as venous valve failure. Recent studies further demonstrate that Foxc2 is essential for lymphatic valve formation and maturation. However, the role of FoxC1 in the lymphatic system remains elusive. To determine the precise functions of Foxc1 and Foxc2 in the development of lymphatic vessels such as lymphatic cell differentiation as well as the patterning and maturation of lymphatic vessels, we recently generated lymphatic-specific single and double mutant mice for Foxc1 and Foxc2. Our extensive analyses reveal that lack of Foxc1, Foxc2, or both leads to lymphatic hyperplasia, accompanied by reduced expression of RasGAP genes and hyperactivation of ERK. Our new genome-wide RNA-seq analysis also shows that expression of Notch signaling genes such as Notch1 and Dll4 is downregulated in Foxc1/c2-mutant lymphatic endothelial cells (LECs) isolated from conditional double mutants. Thus, the objective of this application is to study the role of the transcription factors Foxc1 and Foxc2 in lymphatic vessel development. Our central hypothesis is that Foxc1 and Foxc2 are essential for lymphatic cell fate determination, lymphangiogenesis, and lymphatic valve development, and we will test our central hypothesis and accomplish the objective of this application by pursuing the following three Specific Aims: 1) Define the mechanisms by which Foxc1 and Foxc2 participate in lymphatic cell fate determination through interacting with Notch signaling. 2) Determine whether Foxc1 and Foxc2 are essential for developmental lymphangiogenesis by regulating the Ras/ERK pathway. 3) To elucidate whether Foxc1 and Foxc2 cooperatively function in lymphatic valve development. In summary, the outcome of our project is an extensive characterization of the critical roles that Foxc1 and Foxc2 play in lymphatic vessel development. Furthermore, we expect our findings to have an important positive impact on patient care, because the completion of the proposed studies will likely lead to new therapeutic strategies for the prevention and treatment of lymphedema and lymphatic disorders.