FIBROBLAST GROWTH FACTOR BINDING PROTEIN 1 (FGFBP1) CONTRIBUTES IN THE ESTABLISHMENT AND MAINTENANCE OF THE BLOOD BRAIN BARRIER

The Blood Brain Barrier (BBB) is a highly specialized vascular structure whose aim is to tightly regulate the permeability between the blood flow and the Central Nervous System (CNS). To this purpose, the ECs in the brain need to present some peculiar features: the presence of high-resistance tight junctions (TJs) to block paracellular permeability, the lack of fenestrations, and the expression of some specific transmembrane transporters to selectively allow the entrance of nutrients and the exit of toxic metabolites. The high level of specialization of the brain microvasculature is obtained as a result of the interaction of the endothelial compartment with the other components of the so-called NeuroVascular Unit (NVU), such as the Basement Membrane (BM), pericytes and astrocyte end-feet. The canonical Wnt/β-catenin pathway, that is specifically activated in CNS vessels during development, regulates BBB initiation and maintenance. Moreover, inactivation of this pathway in vivo leads to angiogenic defects in the CNS and not in other vascular regions. Affymetrix analysis previously performed in our group provided a list of genes whose transcription is selectively regulated upon Wnt3a stimulation in murine primary ECs isolated from brain (bMEC). One of the most upregulated transcripts is that of Fibroblast Growth Factor Binding Protein 1 (FGFBP1) gene. FGFBP1 is a cargo protein that, after being secreted in the extracellular matrix (ECM), is able to non-covalently bind the FGF immobilized in the ECM and to mobilize it, protecting it from degradation and presenting it to FGF tyrosine-kinase receptor on the cell membrane. Given the capability of Wnt3a stimulation to selectively induce FGFBP1 expression in brain ECs, we hypothesized that this protein could be involved in the process of initiation and/or maturation of the BBB. In this work, we demonstrate in vivo in the zebrafish model that FGFBP1 knock down by morpholino presents vascular abnormalities in the brain and in the trunk, together with cerebral hemorrhages and impaired permeability. Taking advantage of the endothelial-specific FGFBP1 knock out murine model, we further demonstrate that inhibition of endothelial FGFBP1 expression affects brain vascular development, causing vascular defects and increased BBB permeability and also influencing the number of pericytes and the composition of the BM. Finally, we show in vitro that FGFBP1 absence promotes a “tip-like” phenotype and an increase in the expression of Plvap in bMECs. In conclusion, our work proposes a novel role for FGFBP1 in the maintenance of the properties of the BBB and in the regulation of the complex interactions of the endothelial compartment with the other components of the NVU.