Université de Montréal L’IMPORTANCE DE LA VOIE DE SIGNALISATION AU NIVEAU DE LA BARRIÈRE HÉMATO-ENCÉPHALIQUE THE IMPORTANCE OF THE HEDGEHOG SIGNALING PATHWAY AT THE LEVEL OF THE BLOOD-BRAIN BARRIER par

During embryogenesis, vascular development is characterized by sequential steps of vasculogenesis, angiogenesis, and in some organs, barriergenesis. In the CNS, the bloodbrain barrier (BBB) is composed of tightly bound endothelial cells (ECs) which regulate the entry of blood-borne molecules and immune cells into the CNS. Perivascular astrocytes are known to regulate BBB permeability and quiescence by secreting essential factors, the identity of which remains unclear. Our study shows that human astrocytes express and secrete Sonic Hh (Shh) and conversely, that human BBB-ECs bear the Hh receptor Patched-1 (Ptch-1), the signal transducer Smoothened (Smo) as well as transcription factors of the Gli family. Furthermore, we show that activation of the Hh pathway in BBB-ECs restricts the passage of soluble tracers in vitro and in vivo. Blocking Hh signaling in vitro and using Shh knock-out (-/-) embryonic mice, we demonstrate a reduced expression of TJ molecules claudin-5, occludin and ZO-1. Hh activation also decreases the surface expression of cell adhesion molecules ICAM-1 and VCAM-1, and decreases BBB-ECs secretion of pro-inflammatory cytokines IL-8/CXCL8 and monocytes chemoattractant protein 1 MCP-1/CCL2, resulting in a reduction of migrating CD4 lymphocytes across human BBB-EC monolayers. In vitro treatment with inflammatory cytokines TNF-α and IFN-γ, upregulates the production of astrocytic Shh and the BBB-EC surface expression of Ptch-1 and Smo. In active Multiple Sclerosis (MS) lesions, in which the BBB is disrupted, Shh expression is drastically upregulated in hypertrophic astrocytes, while Ptch-1 and Smo expression is down-regulated or left unchanged, suggesting that a deregulation in the Hh signaling pathway may prevent the barrier stabilizing properties of Hh. Our data demonstrate an anti-inflammatory and BBBpromoting effect of astrocyte-secreted Hh and suggest that a pro-inflammatory environment disrupt the BBB by impacting, at least in part, on Hh signaling in brain ECs.