Blood-brain barrier dysfunction and recovery after ischemic stroke

Abstract

The blood-brain barrier (BBB) plays a vital role in regulating the trafficking of fluid, solutes and cells at the blood-brain interface and maintaining the homeostatic microenvironment of the CNS. Under pathological conditions, such as ischemic stroke, the BBB can be disrupted, followed by the extravasation of blood components into the brain and compromise of normal neuronal function. This article reviews recent advances in our knowledge of the mechanisms underlying BBB dysfunction and recovery after ischemic stroke. CNS cells in the neurovascular unit, as well as blood-borne peripheral cells constantly modulate the BBB and influence its breakdown and repair after ischemic stroke. The involvement of stroke risk factors and comorbid conditions further complicate the pathogenesis of neurovascular injury by predisposing the BBB to anatomical and functional changes that can exacerbate BBB dysfunction. Emphasis is also given to the process of long-term structural and functional restoration of the BBB after ischemic injury. With the development of novel research tools, future research on the BBB is likely to reveal promising potential therapeutic targets for protecting the BBB and improving patient outcome after ischemic stroke.

Keywords: Inflammation; Neurovascular unit; Repair; Stroke comorbidities; Tight junction.

Fig. 1. Regulation of the movement of molecules between blood and brain by microvascular endothelial cells under physiological conditions

The junctional complexes between brain microvascular endothelial cells include tight junctions and adherens junctions. Major transmembrane proteins of the junctional complexes, such as claudins and occludin (tight junction), cadherins (adherens junction), and junctional adhesion molecules (JAMs), are normally linked to the actin cytoskeleton through plasma proteins zonula occludens (ZO), cingulin, and catenin. Paracellular free diffusion of water-soluble substances along the concentration gradient is highly restricted by tight junctions. Small lipophilic substances effectively diffuse across the endothelium through transcellular pathways. Low levels of transcytosis also occur in endothelial cells, either receptor dependent or receptor independent. An array of endothelial transporters is expressed and responsible for moving nutrients, such as glucose and amino acids. In addition, various ion transporters carrying Na⁺, K⁺, Cl⁻, HCO₃⁻, Ca²⁺ and other ions are involved in maintaining brain ion homeostasis.

Fig. 2. Stepwise alterations of endothelial junctional proteins after ischemic brain injury

Ischemic insults activate a cascade of signaling in endothelial cells, leading to junctional protein alterations such as phosphorylation, translocation and degradation, each of which could increase BBB permeability and participate in the development of neurovascular injury. The phosphorylation of occludin, claudin-5 and ZO-1 can cause dissociation of the TJ complex and increase BBB permeability. Altered distribution of TJ proteins could result from increased endocytosis, as well as cellular tension transmitted through ischemia-induced actin stress fibers. Such protein translocation weakens the BBB and facilitate the degradation of TJs. TJ degradation could occur extracellularly through activity of MMPs, or intracellularly through proteasomes and lysosomes after ubiquitination. This is accompanied by the infiltration of peripheral immune cells, such as neutrophils and macrophages, which further exacerbates BBB breakdown after ischemia.

Fig. 3. Modulation of blood-brain barrier permeability by different cell types after ischemic stroke

Cell-cell interactions in the neurovascular unit dictate the brain response to ischemic injury and influence BBB damage and repair. Pericytes respond quickly to an ischemic stroke and display characteristics that may be protective or detrimental, such as detachment from the microvessel walls, constriction, migration and accumulation. Furthermore, pericytes are able to differentiate into neural and vascular lineage cells after ischemia. Astrocyte swelling is another early response in the neurovascular unit after ischemia, due to increased uptake of glutamate and water. The swelled astrocytes compress vessels in the ischemic regions and exacerbate vascular hypoperfusion. Post-ischemic astrocytes also produce VEGF and MMPs that degrade TJs and the ECM. Microglia/macrophages undergo phenotypic polarization after ischemia, developing into pro-inflammatory and anti-inflammatory phenotypes. Pro-inflammatory microglia/macrophages may promote BBB disruption, whereas anti-inflammatory microglia/macrophages can facilitate post-stroke angiogenesis and inflammation resolution. Recent studies also suggest a role of microglia/macrophages in vessel repair after injury; two mechanisms proposed being through mechanical stretching and P2RY12 signaling.

Fig. 4. Restoration of BBB permeability after stroke

In response to ischemic stroke, there is the release of multiple factors that enhance BBB permeability, e.g. ROS, inflammatory mediators and pro-angiogenic factors such as VEGF. Some of the restoration in barrier tightness with time after a stroke is related to reduced levels of those factors, but there is also an upregulation of mediators of barrier quiescence that enhance barrier properties, such as Ang-1. Those mediators can also serve (directly or indirectly) to diminish the effects of the factors enhancing barrier permeability (red lines). Progenitor cells may be directly (integration) or indirectly (mediator release) involved in barrier repair after stroke.