Mechanisms in blood-brain barrier opening and metabolism-challenged cerebrovascular ischemia with emphasis on ischemic stroke

Abstract

Stroke is the leading cause of disability among adults as well as the 2nd leading cause of death globally. Ischemic stroke accounts for about 85% of strokes, and currently, tissue plasminogen activator (tPA), whose therapeutic window is limited to up to 4.5 h for the appropriate population, is the only FDA approved drug in practice and medicine. After a stroke, a cascade of pathophysiological events results in the opening of the blood-brain barrier (BBB) through which further complications, disabilities, and mortality are likely to threaten the patient's health. Strikingly, tPA administration in eligible patients might cause hemorrhagic transformation and sustained damage to BBB integrity. One must, therefore, delineate upon stroke onset which cellular and molecular factors mediate BBB permeability as well as what key roles BBB rupture plays in the pathophysiology of stroke. In this review article, given our past findings of mechanisms underlying BBB opening in stroke animal models, we elucidate cellular, subcellular, and molecular factors involved in BBB permeability after ischemic stroke. The contribution of each factor to stroke severity and outcome is further discussed. Determinant factors in BBB permeability and stroke include mitochondria, miRNAs, matrix metalloproteinases (MMPs), immune cells, cytokines, chemokines, and adhesion proteins. Once these factors are interrogated and their roles in the pathophysiology of stroke are determined, novel targets for drug discovery and development can be uncovered in addition to novel therapeutic avenues for human stroke management.

Keywords: Blood-brain barrier; Cytokines; Matrix metalloproteinase(MMPs); MicroRNAs; Mitochondria; Stroke.

Figure 1.. Dynamic BBB opening following tMCAO in mice.

(A) MCA was occluded for 1 h and Evan’s blue was injected into the mice (i.v.). Transcardiac perfusion was then performed and brain images acquired as shown by representative coronal brain sections. Evan’s blue extravasation is denoted by blue arrows. Quantification of Evan’s blue extravasation in the left and right hemispheres. Data are expressed as mean ± S.D.; n=4/group; One-way ANOVA followed by post-hoc Tukey’s test, ***p<0.001; ****p <0.0001. (B) TTC–staining was performed on brain sections from the Evan’s blue extravasation assay using an additional cohort of mice (n=4/group). Evan’s blue extravasation is denoted by blue arrows. (C) Fluorescent dyes (Texas Red and rhodamine-123) were injected into tMCAO mice at different time points. Transcardiac perfusion was performed and whole brains were sectioned (20 μm). Fluorescence images were acquired as shown by representative coronal brain sections followed by staining with H&E or cresyl violet. Red arrows denote Texas Red infiltration. Yellow arrows denote rhodamine-123 extravasation. White arrows denote infarction by cresyl violet staining. Pink arrows denote pathological changes based on H&E staining (Hone et al. 2018).

Figure 2:. Cascade of events in stroke pathophysiological opening of the BBB.

After stroke, due to bioenergetics failure in insulted area, the impaired mitochondria in cerebrovascular endothelial cells leads to BBB rupture. Concurrently, upregulated expression of MMPs, exacerbate the BBB integrity by degrading the tight junction proteins. Granulocytes with assistance of adhesion proteins, cytokine and chemokine adhere to endothelial cells and then, infiltrate into the CNS and initiate inflammation. Overexpression of some miRNAs, encapsulated in exosome silenced specific genes, mediating BBB permeability and finally modulating the stroke outcome. All these cascades of events manifest their impact on the BBB permeability spatiotemporally through paracellular or transcellular breaching pathways. The transcytosis precedes the tight junction degradation so that the transcytosis occurs at 6 hours after ischemic stroke while the tight junction degradation perpetuating the BBB occurs at 40–48 hours after stroke.

Figure 3:. The interconnected factors governing the BBB opening through transcellular or paracellular permeability after ischemic stroke.

The pharmacological inhibition of mitochondrial respiratory chain reactions results in the BBB opening through cellular death (in this case apoptosis) and consequent BBB opening. The extravasation of the FITC-dextran-70 through cerebrovascular tight junction stained with ZO-1 red antibodies as well as extravasation of Evans blue dye in the epidural application (EA) model were the methods measuring BBB opening. A super-antigen, like LPS, was found to lead to BBB disintegration via mitochondrial impairment similarly to pharmacological inhibition of the mitochondrial functions (Doll et al. 2015). Mitochondrial homeostasis via mitophagy or mitochondrial biogenesis, accompanied with inter-neuronal mitochondrial transfer are increased in ischemic stroke and other neurodegenerative diseases. Some classes of MMP-2, MMP-9, and small molecule inhibitors such as minocycline were clinically documented to be effective in extension of the tPA therapeutic time window by protecting BBB (Yang et al. 2015). BBB opening is followed by the brain edema, neuronal death, and worsened physiological and behavioral outcomes.