Establishment and Dysfunction of the Blood-Brain Barrier

Abstract

Structural and functional brain connectivity, synaptic activity, and information processing require highly coordinated signal transduction between different cell types within the neurovascular unit and intact blood-brain barrier (BBB) functions. Here, we examine the mechanisms regulating the formation and maintenance of the BBB and functions of BBB-associated cell types. Furthermore, we discuss the growing evidence associating BBB breakdown with the pathogenesis of inherited monogenic neurological disorders and complex multifactorial diseases, including Alzheimer's disease.

Figure 1. Neurovascular unit

Vessels in the Subarachnoid space: The subarachnoid space contains cerebrospinal fluid (CSF) that drains via arachnoid villi into the venous sinuses. Cerebral arteries branch into smaller pial arteries. Cerebral veins empty into dural venous sinuses. Meningeal lymphatic vessels carry CSF and immune cells to deep cervical lymph nodes. Intracerebral vessels: Pial arteries give rise to the penetrating arteries that branch into arterioles all covered by vascular smooth muscle cells (blue). The penetrating arteries are separated from brain parenchyma by the glia limitans, an astrocytic endfeet layer that forms the outer wall of the Virchow-Robin spaces containing brain interstitial fluid (ISF, white). Arterioles branch off into capillaries and the vessels enlarge, as they become venules and veins. Brain capillary unit: Endothelial cells (red) connected by tight junctions form the blood-brain barrier. Pericytes (purple) and endothelium share a common basement membrane (yellow) and connect with each other with several transmembrane junctional proteins including N-cadherin and connexins. Astrocytes (green) connect with pericytes, endothelial cells and neurons (peach). Microglia (brown) regulate immune responses. Oligodendro-cytes (aqua) support neurons with axonal myelin sheath. Integrins make connections between cellular and matrix components.

Figure 2. Blood-brain barrier development in the murine central nervous system

A. Developmental timeline. Restriction of paracellular and transcellular transport of solutes is accomplished by elimination of endothelial fenestrae and pinocytosis, formation of a continuous endothelial monolayer connected with the tight junctions, creation of highly selective endothelial transport systems, and establishment of specialized perivascular structures, including the basement membrane and the coverage of the endothelial capillary wall by pericytes and astrocytic endfeet. E, embryonic days; P, postnatal days. B. Induction and differentiation. Wnt ligands (Wnt7a/7b) secreted by neural cells bind to endothelial Frizzled receptors (FZD) and co-receptors low-density lipoprotein receptor-related protein (LRP) 5 and 6, which activate β-catenin signaling, leading to the induction of BBB specific genes. G-protein coupled receptor 124 (Gpr124) co-activates Wnt/β-catenin signaling. Endothelial cells secrete platelet-derived growth factor BB (PDGF-BB), which interacts with platelet derived growth factor receptor-β (PDGFR-β) in pericytes, inducing pericyte recruitment. Pericytes and astrocytes secrete angiopoietin-1 (Ang-1) that acts on endothelial Tie-2 receptor leading to microvascular maturation and highly stable and impermeable BBB. Pericytes are required for the expression of endothelial major facilitator superfamily domain-containing protein 2a (MFSD2a) that is critical for the BBB formation and maintenance. Astrocytes secrete sonic hedgehog (SHH) that acts on endothelial patched homolog 1 (PTC-1) receptor eliciting signaling which contributes to the BBB formation. Endothelial cells secrete vascular growth factor (VEGF) and insulin growth factor (IGF-1) contributing to proper neurovascular patterning. Additional signal transduction pathways may also participate in BBB formation. TJ, tight junction. C. Maturation and maintenance. Postnatally, brain capillaries are covered by mature pericytes sharing the basement membrane with endothelium. Astrocytic endfeet form the outer layer of the mature capillaries. Peri-cytes and astrocytes continue secreting matrix proteins (yellow) of the basement membrane. Signaling pathways mediating BBB induction and differentiation likely continue to play a role in BBB maturation and maintenance and their dysregulation may lead to BBB breakdown causing different central nervous system pathologies. AQP4, aquaporin-4 water channel.

Figure 3. The vascular triad of the blood-brain barrier

Endothelial cells are connected with each other through the tight junction (TJ), adherens junction (AJ) and gap junction (GJ) proteins. In the TJs, occludin, claudins and junctional adhesion molecules (JAMs) form a impermeable barrier to fluids and are connected to F-actin filaments by the zonula occludens ZO-1, ZO-2 and ZO-3 multi-domain scaffolding proteins of the membrane-associated guanylate kinase family. GJs formed by connexin hemichannels are specialized for direct intercellular communications. AJs are formed by homotypic binding of VE-cadherin, platelet endothelial cell adhesion molecule-1 (PECAM-1) and Nectin. Catenins link VE-cadherin to F-actin, while nectin is secured to F-actin by afadin. Pericytes communicate with the endothelial cells via growth factor-mediated signaling (unidirectional or bidirectional), adhesion via N-cadherin homotypic binding and GJs. Pericyte can modulate BBB permeability by regulating gene expression in the endothelial cells resulting in upregulation of TJ proteins, inhibition of bulk flow transcytosis and upregulation of brain endothelial specific docosahexaenoic acid (DHA) transporter, a major facilitator domain-containing protein 2A (MFSD2a). Both endothelial cells and pericytes are embedded in the basement membrane (BM) and anchored to BM via integrins. PDGF-BB, platelet-derived growth factor BB; PDGFRβ, platelet-derived growth factor receptor-β. Astrocytes regulate expression of matrix metallo-proteinase-9 (MMP-9) in pericytes by secreting apolipoprotein E (ApoE). ApoE3, but not ApoE4, binds to the low density lipoprotein receptor-related protein 1 (LRP1) in peri-cytes, which suppresses the proinflammatory cyclophilin A (CypA)-nuclear factor-κB (NFκB)-MMP-9 pathway and degradation of TJ and BM proteins causing BBB breakdown. Astrocytes signal endothelial cells by Src-suppressed C-kinase substrate (SSeCKS) to increase TJ protein expression. Neural activity-dependent glutamate release increases [Ca²⁺] in the astrocytic endfeet, which regulates vascular tone. The GJs connect the adjacent astrocytic endfeet. Glucose gets into the brain via the endothelial Glut1 transporter and is taken up by neurons via Glut3. Glucose is taken up by astrocytes mainly by Glut2 and is metabolized to lactate, which is exported to neurons by the monocarboxylic MCT1 and MCT4 transporters.

Figure 4. Vascular-mediated neurodegeneration

Aberrant pericyte-endothelial or astrocyte-pericyte signal transduction leads to BBB breakdown resulting in brain accumulation of 1) red blood cell (RBC)-derived neurotoxic hemoglobin and iron (Fe²⁺) causing production of reactive oxygen species (ROS) and oxidant stress to neurons; 2) neuronal toxic blood-derived proteins such as fibrinogen, thrombin, and plasminogen, which could be converted into plasmin that in turn degrades neuronal extracellular matrix (ECM) and leads to detachment of neurons and cell death; 3) fibrinogen that activates microglia, promotes neuroinflammation and demyelination, and prevents myelination by oligodendrocyte progenitor cells; 4) albumin that contributes to the development of vasogenic edema, capillary hypoperfusion and hypoxia. BBB breakdown can also lead to the loss of immune privilege resulting in development of anti-brain antibodies against different axonal and membrane components of neurons.

Figure 5. The neurovascular hypothesis for Alzheimer’s disease

AD genetics, vascular factors, environment and lifestyle can independently and/or synergistically lead to cere-brovascular injuries including BBB dysfunction, pericyte degeneration and cerebral blood flow reductions (oli-gemia), initiating a cascade of events that can either 1) directly cause neuronal injury and damage independently of Aβ (Hit 1, blue), and/or 2) accelerate the Aβ-dependent neurodegeneration (Hit 2, red). In the Aβ-dependent pathway, BBB dysfunction leads to faulty clearance of Aβ from brain, whereas reduced brain perfusion increases Aβ production, both causing Aβ accumulation in the brain. Reduced brain perfusion (Hit 1) and elevated levels of Aβ (Hit 2) can independently and/or synergistically lead to Tau hyperphosphorylation (p-Tau) and formation of filamentous Tau pathology. Additionally, the two hits can exacerbate neuroinflammation.

Figure 6. Alzheimer’s amyloid β-peptide clearance across the blood-brain barrier

Transvascular Aβ clearance. LRP1 binds Aβ at the abluminal side of endothelium, which recruits PICALM, resulting in PICALM/clathrin-dependent endocytosis of LRP1-Aβ complexes. Next, PICALM guides the trafficking of Aβ-LRP1 endocytic vesicles to Rab5⁺ early endosomes and then to Rab11⁺ sorting endosomes for exo-cytosis at the luminal side of the BBB, resulting in Aβ transcytosis. PICALM guides Aβ away from Rab7⁺ late endosomes and lysosomes. Apolipoprotein J (apoJ; CLU) facilitates Aβ42 clearance across the BBB via LRP2. Systemic Aβ clearance. Aβ binds to soluble LRP1 (sLRP1) in plasma. Circulating sLRP1-Aβ complexes are transported to liver and kidney for elimination from the body. Genetic risk factors. APOE2 and APOE3 carry lower risk for AD compared to APOE4. CLU variants influence risk for sporadic AD, but their effects on Aβ clearance are presently unknown. Some protective PICALM variants lead to increased PICALM expression and enhanced Aβ clearance across the BBB. PSEN1 mutations causing early autosomal dominant AD lead to increased production of Aβ, particularly Aβ42, which increases Aβ load for clearance across the BBB.