Microenvironmental control of the Wnt pathway for the establishment of barrier heterogeneity in the CNS vasculature

• Applicant: Privatdozent Stefan Liebner, Ph.D.
• Subject Area: Developmental Neurobiology ; Molecular Biology and Physiology of Neurons and Glial Cells
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 269353708

The blood-brain barrier (BBB) is formed by the neurovascular unit, comprising endothelial cells, pericytes, astrocytes, neurons and microglia, and is crucial for neural function. We have shown that Wnt/beta-catenin signalling - canonical pathway - is active in endothelial cells during brain angiogenesis, contributing to the development of the BBB by regulating barrier-associated genes. More recently, we have provided evidence that endothelial activation of canonical Wnt signalling normalizes glioma vessels by regulating Pdgfb, leading to reduced peritumoral oedema. Since recently, several other pathways like sonic hedgehog, retinoic acid have been shown be involved in the induction, maturation and maintenance of barrier properties in endothelial cells. In parallel it became evident that endothelial barrier characteristics are not identical in different brain regions. The most striking examples are the circumventricular organs (CVOs) that, due to their neurosecretory and/or neurosensory function, lack an endothelial BBB.We have collected preliminary data that support the interpretation that Wnt/beta-catenin pathway activity is specifically absent in vessels of the circumventricular organs. This suggests that in the CVOs a specific microenvironment of vascular differentiation and remodelling becomes established during embryonic development, prohibiting Wnt pathway activation.Dominant activation of endothelial beta-catenin signalling led to up-regulation of the barrier- and down-regulation of the permeability-related genes claudin-5 and MECA-32, respectively. This observation suggests that vessels in the CVOs are capable of differentiating into barrier endothelia. Although several studies describe and discuss that different areas of the CNS exhibit different BBB properties, nothing is known about the development and about the molecular cues that mediate the formation of differential endothelial barrier phenotypes. Based on our published and unpublished results, we hypothesize that vascular heterogeneity in the CNS is achieved by micro-environmental impact on Wnt/beta-catenin signalling, most evident in the CVOs. To test this hypothesis, we aim to: 1. characterize in detail the development of the CVO vessels, the expression of barrier and permeability markers, as well as Wnt pathway components and their interaction with other pathways.2. investigate the consequences of endothelial-specific gain-of-function of beta-catenin in vivo, making use of transgenic, tamoxifen-inducible animal models3. characterize the physiological and pathological consequences of beta-catenin activation, focussing on water homeostasis and on neuroinflammatory conditions.These approaches will help to understand basic principles of barrier heterogeneity, as well as functional involvement of endothelial barrier properties in the regulation of water homeostasis and immune responses in the CNS.

• DFG Programme: Research Units
• Subproject of: FOR 2325: Interactions at the Neurovascular Interface