Differentiation of the brain vasculature: the answer came blowing by the Wnt

Abstract

Vascularization of the vertebrate brain takes place during embryonic development from a preformed perineural vascular plexus. As a consequence of the intimate contact with neuroectodermal cells the vessels, which are entering the brain exclusively via sprouting angiogenesis, acquire and maintain unique barrier properties known as the blood-brain barrier (BBB). The endothelial BBB depends upon the close association of endothelial cells with pericytes, astrocytes, neurons and microglia, which are summarized in the term neuro-vascular unit. Although it is known since decades that the CNS tissue provides the cues for BBB induction and differentiation in endothelial cells, the molecular mechanism remained obscure.Only recently, the canonical Wnt/beta-catenin pathway and the Wnt7a/7b growth factors have been implicated in brain angiogenesis on the one hand and in BBB induction on the other. This breakthrough in understanding the differentiation of the brain vasculature prompted us to review these findings embedded in the emerging concepts of Wnt signaling in the vasculature. In particular, interactions with other pathways that are crucial for vascular development such as VEGF, Notch, angiopoietins and Sonic hedgehog are discussed. Finally, we considered the potential role of the Wnt pathway in vascular brain pathologies in which BBB function is hampered, as for example in glioma, stroke and Alzheimer's disease.

Figure 1

Schematic junctional organization and β-catenin signaling in CNS ECs: comparison of brain and retina. A In the brain and spinal cord mainly Wnt7a and Wnt7b growth factors act on an uncharacterized Fzd-LRP receptor complex to elicit β-catenin target gene transcription. The positively regulated target genes identified so far are the TJ protein claudin-3, Glut-1 and ABCB1/MDR1. Conversely, the expression of Meca-32/Plvap becomes suppressed by β-catenin transcription via an unknown mechanism. B in the retina Norrin is the predominant ligand activating β-catenin signaling downstream of the Fzd4-LRP5-TSPAN12 receptor complex. So far only the repression of Meca-32/Plvap was identified as target of this signaling. To date the effect of Norrin signaling on claudin-3, Glut-1 or other barrier-related genes is not know. See the grey supported area and arrows for the "canonical" Wnt pathway.

Figure 2

Scheme of known Wnt pathways. A Canonical Wnt/β-catenin pathway. In the "off-state" no Wnt proteins are present or are inhibited by factors like WIF, sFRPs and Dkk. Cytosolic β-catenin is targeted to proteolytic degradation through phosphorylation by the APC-Axin-GSK3β-CK1á complex and ubiquitination by the βTrCP-dependent E3 ubiquitin ligase. In the "on-state" stimulation of Fzd receptors and their co-receptors Lrp5/6 by Wnt ligands, leads to recruitment of Dvl and Axin to Fzd, thereby inhibiting the degradation complex. Consequently, β-catenin accumulates in the cytoplasm and enters the nucleus, activating target gene transcription through association with Lef1/TCF. B Non-canonical Wnt/Ca²⁺ pathway. Interaction of non-canonical Wnt ligands with Fzd receptors can lead to G-Protein mediated phosphorylation of Dsh, thereby activating PLC and increasing intracellular calcium levels. These will activate CAMKII and PKC, as well as the transcription factor NFAT. Additionally, Fzd receptors in association with Kny, Ror2 or Ryk receptors can activate JNK, promoting target gene expression through AP-1. C Non-canonical Wnt/PCP pathway. This pathway is characterized by an asymmetric distribution of Fzds, the cadherin Flamingo and VANGL2, resulting in cell polarization. Wnt signaling promoted by either Wnt or an interaction of Fzd with VANGL2 activates RhoA/B, Cdc42 or Rac1. Dsh activates Rac1, while RhoA/B and Cdc42 need the participation of Daam1 downstream of Dsh. Rac1 can also activate JNK, resulting in the NFAT pathway. All three down stream pathways are key players in cytoskeletal rearrangement and cellular polarity. The signaling of the Flamingo/Fzd interaction is still obscure.