Novel insights into the development and maintenance of the blood-brain barrier

Abstract

The blood-brain barrier (BBB) is essential for maintaining homeostasis within the central nervous system (CNS) and is a prerequisite for proper neuronal function. The BBB is localized to microvascular endothelial cells that strictly control the passage of metabolites into and out of the CNS. Complex and continuous tight junctions and lack of fenestrae combined with low pinocytotic activity make the BBB endothelium a tight barrier for water soluble moleucles. In combination with its expression of specific enzymes and transport molecules, the BBB endothelium is unique and distinguishable from all other endothelial cells in the body. During embryonic development, the CNS is vascularized by angiogenic sprouting from vascular networks originating outside of the CNS in a precise spatio-temporal manner. The particular barrier characteristics of BBB endothelial cells are induced during CNS angiogenesis by cross-talk with cellular and acellular elements within the developing CNS. In this review, we summarize the currently known cellular and molecular mechanisms mediating brain angiogenesis and introduce more recently discovered CNS-specific pathways (Wnt/β-catenin, Norrin/Frizzled4 and hedgehog) and molecules (GPR124) that are crucial in BBB differentiation and maturation. Finally, based on observations that BBB dysfunction is associated with many human diseases such as multiple sclerosis, stroke and brain tumors, we discuss recent insights into the molecular mechanisms involved in maintaining barrier characteristics in the mature BBB endothelium.

Fig. 1

Differentiation of the blood–brain barrier (BBB). Left Angiogenesis phase. Vascular sprouts radially invade the embryonic neuroectoderm toward a concentration gradient of vascular endothelial cell growth factor-A (VEGF), which is produced by neuroectodermal cells located in the ventricular layer (EC endothelial cell, NPC neuronal precursor cell, dpf days post-fertilization). Growth factors such as the endothelial-cell-specific receptor tyrosine kinase Tie-2 and its ligands angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2) are involved in angiogenic sprouting early during embryogenesis, together with the morphogens Wnt7a/7b/3a, Norrin and potentially sonic hedgehog (Shh). Middle Differentiation phase. The phenotype of cerebral endothelial cells changes such that they down-regulate their expression of the MECA-32 antigen. In this phase, the anti-angiogenic barrier-inducing signals start to overrule the pro-angiogenic signals. Shh, Norrin and Ang-1 are produced by differentiating astrocytes. Tie2- and neuropilin 1 (Nrp1)-expressing myeloid cells promote anastomosis of tip cells to establish vascular circuits (PC pericyte, AC astrocyte, N neuronal cell). Right Maturation phase. Despite the cerebral endothelial cells forming the barrier proper, close contact with PCs, ACs and possibly Ns is required for the maintenance of the BBB (MG microglial cell, P4 postnatal day 4). The molecular mechanisms involved in this cross-talk required for BBB maintenance in the mature central nervous system (CNS) remain largely unknown but Norrin/Frizzled4 signaling, in particular, seems to be important, at least in specific regions of the CNS (olfactory bulb, cerebellum, retina; see Fig. 2)

Fig. 2

Expression of the described barrier inducing factors in the adult mouse brain. Representation of the available global expression data of barrier-inducing factors in the adult mouse brain. Note the minimal overlap but the rather distinct regional expression of the individual factors suggesting site-specific induction/maintenance cues for endothelial barrier properties in the CNS (SVZ subventricular zone, Shh sonic hedgehog, GPR124 G-protein-coupled receptor)