The Hedgehog Pathway Promotes Blood-Brain Barrier Integrity and CNS Immune Quiescence

Abstract

The blood-brain barrier (BBB) is composed of tightly bound endothelial cells (ECs) and perivascular astrocytes that regulate central nervous system (CNS) homeostasis. We showed that astrocytes secrete Sonic hedgehog and that BBB ECs express Hedgehog (Hh) receptors, which together promote BBB formation and integrity during embryonic development and adulthood. Using pharmacological inhibition and genetic inactivation of the Hh signaling pathway in ECs, we also demonstrated a critical role of the Hh pathway in promoting the immune quiescence of BBB ECs by decreasing the expression of proinflammatory mediators and the adhesion and migration of leukocytes, in vivo and in vitro. Overall, the Hh pathway provides a barrier-promoting effect and an endogenous anti-inflammatory balance to CNS-directed immune attacks, as occurs in multiple sclerosis.

JAML mediates monocyte and CD8 T cell migration across the brain endothelium

“…Informed consent and ethic approval were given prior to surgery (HD04.046). Primary cultures of BBB‐ECs were established as previously described 6, 7…”

“…Protein from resting and inflamed human BBB‐ECs was isolated as previously described 6. 20 μ g of proteins were run on Bio‐Rad 4–20% Mini‐PROTEAN ® TGX ™ gels (Hercules, CA) under reducing conditions.…”

“…Monocytes were isolated by positive selection according to the manufacturer's protocol (Miltenyi). Activated CD8 T cells and monocytes (10 6 cells per well) were added to the transwell chamber containing anti‐JAML (R&D systems, 20 μ g/mL) or control immunoglobulins and migrated for 18 h. Migrated cells were imaged using DIC microscopy (Leica MZ10F; Leica Microsystems, Wetzlar ‐ Germany), and were collected and counted as previously described 6, 7, 8…”

The Blood–Brain Barrier

“…Interestingly, transendothelial electrical resistance (TEER) and permeability experiments showed that activation of the Hh pathway induced expression of junctional proteins and promoted a BBB phenotype. In addition, mice genetically engineered to lose the signal transducer Smo on ECs had a significant increase in BBB permeability that correlated with a decrease in junctional protein expression and disturbed BMs (Alvarez et al 2011a(Alvarez et al , 2013, supporting the concept that the Hh pathway has a significant influence on BBB function.…”

“…In particular, Wnt/bcatenin signaling through Lef1, as well as Sonic hedgehog signaling through Gli have been shown to be important for regulating the formation and function of the BBB (Liebner et al 2008;Stenman et al 2008;Daneman et al 2009;Alvarez et al 2011a). In addition, vascular metabolism has been implicated in regulating barrier properties of CNS vasculature by metabolizing potential toxins or altering the properties of molecules (Fig.…”