Glial influence on the blood brain barrier

Abstract

The Blood Brain Barrier (BBB) is a specialized vascular structure tightly regulating central nervous system (CNS) homeostasis. Endothelial cells are the central component of the BBB and control of their barrier phenotype resides on astrocytes and pericytes. Interactions between these cells and the endothelium promote and maintain many of the physiological and metabolic characteristics that are unique to the BBB. In this review we describe recent findings related to the involvement of astroglial cells, including radial glial cells, in the induction of barrier properties during embryogenesis and adulthood. In addition, we describe changes that occur in astrocytes and endothelial cells during injury and inflammation with a particular emphasis on alterations of the BBB phenotype.

Keywords: BBB; astrocyte; astrogliosis; glial; multiple sclerosis.

FIGURE 1

Astrocyte-Endothelial cell crosstalk supporting BBB function. The BBB is formed by endothelial cells (ECs), surrounded by an endothelial and astroglial basement membranes with pericytes embedded in between. These are surrounded by astrocyte endfeet that provide support to endothelial function and establish communication with neurons. The right panel shows prototypic examples of factors secreted by astrocytes and ECs that are crucial in the establishment and maintenance of the BBB. The receptors for some of the molecules shown are included. Ang-1-2 (angiopoietin 1 and 2), ApoE3-E4 (apolipoprotein E3 and E4), AT1 (type 1 angiotensin receptor), CypA (cyclophylin A), ET (endothelin), FGF (fibroblast growth factor) GDNF (glial cell line-derived neurotrophic factor), MMP-9 (Matrix Metalloproteinase 9), Ptch1 (Patched 1), RA (retinoic acid), RARβ (RA-receptor β), Shh (Sonic Hedgehog), Smo (Smoothened), SSeCKS (Src suppressed C kinase substrate), TGFβ (transforming growth factor-β), Tie2 (receptor tyrosine kinase 2) and VEGF (Vascular endothelial growth factor). Arrow represent induction/activation; Blunted line represent inhibition/down-regulation.

FIGURE 2

BBB permeability is compromised in mice lacking Hh signalling in CNS endothelium. Wild type (WT, left panels) and Tie2-Cre; Smo^c/c (right panels) 8-week-old mice were intraperiteonally injected with Evans Blue (EB, blue). Brain cryosections from both groups were immunostained for GFAP (astrocytes, red) and laminin (Lam, basement membranes, green) (top panels), as well as for IgGs (indicators of BBB permeability, red (bottom panels)) and laminin (n = 3–5 animals). White rectangles indicate areas shown on the right of each panel. Arrowheads indicate areas showing basement membrane breakdown.

FIGURE 3

Signaling pathways and transcription factors supporting or affecting the barrier phenotype of CNS-ECs. The Wnt and Hh signaling pathways as well as the nrf-2 and ERG transcription factors appear to be playing a dominant role in promoting barrier properties at the junctional complex level. The Nf-κβ signaling pathway and the transcription factor snail seem to act as dominant negative regulators. Additional factors promoting or affecting the expression of junctional molecules and the BBB phenotype are included. Blue arrows indicate activation/stimulation and red lines denote downregulation/blockage. Akt (protein kinase B), Ang-1-2 (angiopoietin 1 and 2), ARE (antioxidant response elements), Cl-3 (claudin-3), Cl-5 (claudin-5), eNOs (endothelial nitric oxide synthase), ERG (ETS-related gene), Foxo1 (Forkhead box protein O1), Fz (frizzled), Gli (glioma-associated oncogene family zinc finger), IKK-α and -β (inhibitor of kappa B kinase α and β), LRP_5/6 (low-density lipoprotein receptor-related protein 5), NF-κβ (nuclear factor kappa B), Nkx2-1 (NK2 homeobox 1), NQO-1 (NADPH-dehydrogenase quinone-1), NR2F2 (nuclear receptor subfamily 2, group F, member 2), nrf-2 (nuclear factor (erythroid-derived 2)-like 2), occ (occludin), PI3K (Phosphatidylinositide 3-kinase), PKC (Protein kinase C), Ptch1 (Patched1), ROS (reactive oxygen species), Shh (Sonic Hegdehog), Smo (smoothened), Tie2 (receptor tyrosine kinase 2), VE-cad (VE-cadherin), VEGFR (vascular endothelial growth factor receptor), YY1 (Ying and Yang 1 protein) and ZO1 (zonula occludens 1).

FIGURE 4

GFAP expression in asymptomatic spontaneous relapsing remitting EAE mice. Astrocytes were labeled with GFAP (red) and images were acquired by confocal microscopy and using the same settings. Representative images of GFAP in the cerebellar white matter of asymptomatic sRR-EAE mice at day (d)35, d45, and d55 are shown. Asterisks denote blood vessels and white rectangles indicate the high magnification areas shown on the right of each panel. Nuclear staining in blue. Scale bar: 50 μm.

FIGURE 5

BBB changes during MS/EAE. During neuroinflammation, early BBB disturbances are associated with EC activation that is characterized by increased expression of E- and P-selectins, cell adhesion molecules (CAMs) including ALCAM, ICAM-1, ICAM-2, VCAM-1, MCAM, Ninjurin-1, VAP-1 and CD73 [1], chemokines (CCLs-CXCLs) [2] and cytokine receptors such as IFNR, TNFR, IL-17R, IL-22R, and GM-CSFR [3]. Such activation compromises the BBB phenotype by affecting junctional proteins [4] and enhances the expression of additional factors supporting the infiltration of T helper (Th) 1, Th17, and CD8 T cells as well as antigen presenting cells (APCs) [5] that accumulate in the perivascular space in a multifocal pattern. To gain access into the CNS, the infiltrating leukocytes secrete Matrix Metalloproteinases (MMPs) which target components of the astroglial basement membrane and dystroglycan (DG) at the astrocyte endfeet [6], during this process the polarity of the water channel AQP4 is perturbed leading to edema [7]. The endfeet depolarization activates the reactive gliotic program which results in increase expression of the intermediate filament GFAP [8]. Astrocytes also increase the expression of Shh [9] and ECs upregulate the Hh receptors Patched 1 (Ptch1) and Smoothened (Smo) [10] to repair the BBB and downregulate EC activation and leukocyte migration. Shh also modulates the phenotype of pathogenic Th cells by downregulating their expression of inflammatory cytokines and CAMs [11]. In the parenchymal milieu, microglia secrete IL-1β and activate the production of VEGF on astrocytes [12], which induces endothelial nitric oxidase synthase (eNOs) production and promotes junctional protein damage [13]. Finally, 50% of MS patients have autoantibodies against KIR4.1 that fix complement and induce the reactive gliosis program [14].