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A brain-specific angiogenic mechanism enabled by tip cell specialization

par Schevenels, Giel
Président du jury Vanhamme, Luc
Promoteur Vanhollebeke, Benoît
Publication Non publié, 2024-04-22

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Résumé :

During vertebrate development, a vascular network branches throughout the body to ensure oxygen and nutrient supply and waste drainage. In contrast to the apparent uniformity of their ancestral functions, blood vessels specialize to meet the local physiological requirements of the tissues they irrigate. Along the vascular tree, endothelial cells form the inner lining of blood vessels: the endothelium. In response to signals emitted by neurons and glial cells of the brain, the highly malleable endothelial cells acquire a set of properties, which together form a barrier to molecular exchange between the blood and the brain: the blood-brain barrier (BBB). Upon binding Wnt7a/b ligands, endothelial cells undergo Wnt/β-catenin signaling, which drives the acquisition of BBB properties. Intriguingly, the barrier-inducing Wnt7a/b signals are also required for brain vascularization and thereby couple the two processes. This is unexpected, as the molecular regulation of vessel sprouting is deemed universal across tissues. Therefore, in this project we embarked on elucidating why Wnt/β-catenin signaling is required only for vascularization of the brain and not any other organ. To address this question, we assessed which behaviors are activated by Wnt/β-catenin signaling in the zebrafish perineural endothelium, using transcriptomics. We found that endothelial cells, under the control of Wnt/β-catenin signaling, express high levels of mmp25 before sprouting into the brain. CRISPR/Cas9 mutagenesis of mmp25 resulted in angiogenic defects selectively at the level of the brain. Combining multiple genetic approaches, high resolution imaging and in vitro experiments, we uncovered that endothelial tip cells, leading the vessel sprouts, use Mmp25 to penetrate the embryonic meningeal basement membrane (BM). This specialized protein layer lines the surface of the brain and is deposited by meningeal fibroblasts, from early embryonic stages onwards. More precisely, Mmp25 cleaves α5/6 trimers of collagen IV, which are not usually interposed between the endothelium and the surrounding tissue. Upon fragilizing the meningeal BM by genetic inactivation of Col4a5/6, Wnt/β-catenin signaling-deficient endothelial cells successfully invaded the brain, suggesting that the identified Mmp25 effector function at least in part explains the brain-specific requirement for Wnt/β-catenin signaling. As a consequence of the absence of Wnt/β-catenin signaling, the restored cerebrovasculature does not form a BBB and is leaky. Thus, the meningeal BM, by acting as a migration barrier, restricts brain access to properly maturating vessel sprouts.This work identifies Mmp25 as a novel regulator of brain-specific angiogenesis, and thereby uncovers an unanticipated level of tip cell functional heterogeneity across organs. Furthermore, it illustrates how organs, by imposing local constraints on tip cell function, can select blood vessels that match their specific physiological requirements.