TBX1 mutation causes most of the physical features of 22q11.2 deletion syndrome (22q11.2DS), in humans and mice, but it is not known whether it contributes to the behavioral disorders and psychiatric diseases that characterize this syndrome. It has been hypothesized that some neuroanatomical defects in 22q11.2DS have a vascular basis. The purpose of this study was to determine in mice whether brain vascular development is affected by Tbx1 mutation.Methods
We evaluated gene and protein expression by quantitative RTPCR and western blotting respectively on histological sections of whole brains or on brain endothelial cells (ECs) isolated from Tbx1 mutants. Brain vessel density was measured by the reconstruction of confocal images from histological sections where brain ECs were identified by Glut1 immunostaining. Vessel permeability and perfusion were evaluated by the infusion of appropriate tracer molecules. For experiments performed on cultured ECs, Tbx1 expression was knocked down by RNA-interference.Results
We show that in embryonic and adult mouse brain, Tbx1 is expressed in ECs and cell-fate mapping indicates that most brain vessels derive from Tbx1-expressing cells. Loss of Tbx1 causes brain vascular abnormalities consisting mainly of a widespread increase in vessel numbers, increased angiogenic sprouting and branching and vessel disorganization. This phenotypic picture is reproducible in 3D matrigel cultures of ECs after Tbx1 knockdown by siRNA, where there is increased microtubule branching. Thus, the in vivo vascular phenotype is likely to be cell-autonomous. Tbx1 mutant brain vessels have normal permeability, but in EC-specific Tbx1 null embryos some brain vessels are poorly perfused. At the molecular level, we show that in ECs TBX1 regulates Delta-like ligand 4 (DLL4), a transmembrane ligand for the Notch family of receptors that plays a key role in angiogenesis. Like Tbx1, Dll4 is anti-angiogenic, and Dll4 + /- mice have a striking hypervascularization phenotype that is similar to that of Tbx1 null embryos.Conclusions
We show that Tbx1 is required in brain endothelial cells to establish vascular patterning. Our data suggest that a newly identified molecular pathway, Tbx1-Dll4/Notch1, may regulate brain angiogenesis in mice. Our study should encourage clinicians to look for brain microvasculature defects in 22q11.2DS patients and, if found, seek correlations with 22q11.2DS-related neuroanatomical defects.