Morphogenetic Mechanisms During Lymphatic Vessel Formation
Lymphatic vessels are indispensable for tissue homeostasis and implicated in the development of several pathologies including edema, hypertension, chronic inflammation and tumour metastasis. During development, the first lymphatic endothelial cells (LECs) are specified in the cardinal vein (CV), from which they emerge as non-lumenized, connected cells in a process that depends on the vascular endothelial growth factor VEGF-C and the secreted protein CCBE1. The function of CCBE1 in this process is only starting to emerge and it has been shown to facilitate VEGF-C processing to its mature, fully active form. LECs outside the CV rapidly coalesce dorsally to form the first, primordial lymphatic structures, from which all subsequently formed lymph vessels were believed to arise through radial sprouting. Most recently, however, non-venous progenitors have been identified as an alternative source for dermal and mesenterial LECs. Therefore, in addition to delamination from the CV and sprouting, local differentiation of LECs may contribute to lymph vessels formation, but the relative contribution of these processes remains unclear. Starting with their specification in the CV and through all subsequent stages of their differentiation, LECs express VE-cadherin, the central adhesion molecule assuring junctional integrity and thereby vessel stability. In a process which is unique to the LECs of lymph capillaries and not yet understood, these cells reorganize their junctions during functional vessel maturation. This remodelling results in a conversion of the so far zipper-like to button-like junctions that show discontinuous accumulations of VE-cadherin.
Since central morphogenetic steps during lymph vessel development remain unclear, we plan to investigate the early contribution of CV-derived LECs and describe their coalescence resulting in the primordial lymphatic vessels using in vivo microscopy. We will analyse biochemically and in tissue culture systems how CCBE1 helps to orchestrate the signaling complex of VEGF-C and its receptor VEGFR-3 on the surface of migrating LECs. Once these cells have coalesced and formed lumenized vessels again, we will assess in gene-deleted mice, cells and by biochemical analysis the importance of VE-cadherin for the maintenance of LEC junctions, their remodelling into the unique button type junctions and for the stability of lymph vessels. This investigation of morphogenetic processes at different stages of lymph vessel development will help us to identify mechanisms at the dynamic cell-cell or cell-matrix interface that shape LEC behaviour and thereby the lymphatic vessel system.