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 ß-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.

Astrocytic expression of ZIP14 (SLC39A14) is part of the inflammatory reaction in chronic neurodegeneration with iron overload.

Neurodegeneration is associated with inflammation and mismanaged iron homeostasis, leading to increased concentration of non-transferrin-bound iron (NTBI) in the brain. NTBI can be taken up by cells expressing Zrt-, Irt-like protein-14 (ZIP14), which is regulated by iron overload and pro-inflammatory cytokines, for example, interleukin-1ß (IL-1ß) and IL-6. Here, we focus on the astrocytic involvement and regulation of ZIP14 in an experimental model of chronic neurodegeneration with inflammation and iron overload. Rats were unilaterally injected with ibotenic acid in striatum resulting in excitotoxicity-induced neuronal loss in substantia nigra pars reticulata (SNpr). ZIP14 expression was measured in SNpr using immunohistochemistry, western blotting, and RT-qPCR. Cultures of primary astrocytes were examined for Zip14 mRNA expression after stimulation with ferric ammonium citrate (FAC), IL-6, or IL-1ß. To study the involvement of ZIP14 in astrocytic iron uptake, uptake of 59 Fe was investigated after treatment with IL-1ß and siRNA-mediated ZIP14 knockdown. In the lesioned SNpr, reactive astrocytes, but not microglia, revealed increased ZIP14 expression with a main confinement to cell bodies and cellular processes. In astrocyte cultures, FAC and IL-1ß stimulation increased Zip14 expression and IL-1ß stimulation increased uptake of 59 Fe. Increased 59 Fe uptake was also observed after siRNA-mediated ZIP14 knockdown suggesting that lowering of ZIP14 impaired the balance between astrocytic uptake and export of iron. We conclude that astrocytes increase ZIP14 expression in response to inflammation and iron exposure and that ZIP14 seems pertinent for iron uptake in astrocytes and plays a role for a balanced astrocytic iron homeostasis.

The vascular basement membrane in the healthy and pathological brain.

The vascular basement membrane contributes to the integrity of the blood-brain barrier (BBB), which is formed by brain capillary endothelial cells (BCECs). The BCECs receive support from pericytes embedded in the vascular basement membrane and from astrocyte endfeet. The vascular basement membrane forms a three-dimensional protein network predominantly composed of laminin, collagen IV, nidogen, and heparan sulfate proteoglycans that mutually support interactions between BCECs, pericytes, and astrocytes. Major changes in the molecular composition of the vascular basement membrane are observed in acute and chronic neuropathological settings. In the present review, we cover the significance of the vascular basement membrane in the healthy and pathological brain. In stroke, loss of BBB integrity is accompanied by upregulation of proteolytic enzymes and degradation of vascular basement membrane proteins. There is yet no causal relationship between expression or activity of matrix proteases and the degradation of vascular matrix proteins in vivo. In Alzheimer's disease, changes in the vascular basement membrane include accumulation of Aß, composite changes, and thickening. The physical properties of the vascular basement membrane carry the potential of obstructing drug delivery to the brain, e.g. thickening of the basement membrane can affect drug delivery to the brain, especially the delivery of nanoparticles.