VEGFR3 modulates brain microvessel branching in a mouse model of 22q11.2 deletion syndrome

The loss of a single copy of TBX1 accounts for most of the clinical signs and symptoms of 22q11.2 deletion syndrome (22q11.2DS), a common genetic disorder that is characterized by multiple congenital anomalies and brain-related clinical problems, some of which likely have vascular origins. Tbx1 mutant mice have brain vascular anomalies, thus making them a useful model to gain insights into the brain disorders associated with the human disease. Here, we found that Tbx1 has a dynamic expression pattern in brain endothelial cells (ECs), including tip cells, during early vascularization, but it is not expressed in EC progenitors. Its main morphogenetic function in the brain is to regulate negatively filopodia biogenesis and vessel branching. Because of similar phenotypes reported for Vegfr3 loss of function, we pursued a mouse genetic approach to test TBX1-VEGFR3 interaction through gain and loss of function experiments. Vegfr3 is expressed in brain ECs with extensive overlap with Tbx1 expression. We demonstrate that inactivating Vegfr3 in the Tbx1 expression domain in a Tbx1 mutant background enhances vessel branching and filopodia formation to a greater extent than that observed in the individual mutants. Furthermore, using a mouse transgenic line, we show that increasing Vegfr3 expression in the Tbx1 expression domain fully rescued the vessel branching and filopodia phenotypes caused by Tbx1 loss of function. Similar results were obtained using an in vitro model of endothelial tubulogenesis. Overall, these results provide genetic evidence that Vegfr3 is a regulator of early vessel branching and filopodia formation in the brain, and is a likely critical effector of the brain vascular phenotype caused by Tbx1 loss of function. ### Competing Interest Statement The authors have declared no competing interest.