Sortilin regulates blood–brain barrier integrity

Brain homeostasis depends on the existence of the blood–brain barrier (BBB). Despite decades of research, the factors and signalling pathways for modulating and maintaining BBB integrity are not fully elucidated. Here, we characterise the expression and function of the multifunctional receptor, sortilin, in the cells of the BBB, in vivo and in vitro. We show that sortilin acts as an important regulatory protein of the BBB’s tightness. In rats lacking sortilin, the BBB was leaky, which correlated well with relocated distribution of the localisation of zonula occludens‐1, VE‐cadherin and β‐catenin junctional proteins. Furthermore, the absence of sortilin in brain endothelial cells resulted in decreased phosphorylation of Akt signalling protein and increased the level of phospho‐ERK1/2. As a putative result of MAPK/ERK pathway activity, the junctions between the brain endothelial cells were disintegrated and the integrity of the BBB became compromised. The identified barrier differences between wild‐type and Sort1−/− brain endothelial cells can pave the way for a better understanding of sortilin’s role in the healthy and diseased BBB. Here, we demonstrated that sortilin is expressed in primary rat brain endothelial cells, astrocytes and pericytes and that the lack of sortilin compromises the integrity of the blood–brain barrier. Sortilin is known to be involved in Akt and Erk1/2 signalling, and we observed that this pathway is affected in sortilin knock‐out brain endothelial cells. This signalling pathway is known to influence the tightness of the brain endothelial barrier.