Hyperglycaemia perturbs blood-brain barrier integrity through its effects on endothelial cell characteristics and function

Breakdown of blood-brain barrier (BBB) represents a key pathology in hyperglycemia-mediated cerebrovascular damage after an ischemic stroke. As changes in the level and nature of vasoactive agents released by endothelial cells (ECs) may contribute to BBB dysfunction, this study first explored the specific impact of hyperglycemia on EC characteristics and secretome. It then assessed whether secretome obtained from ECs subjected to normoglycaemia or hyperglycemia might regulate pericytic cytokine profile differently. Using a triple cell culture model of human BBB, composed of brain microvascular EC (BMEC), astrocytes and pericytes, this study showed that exposure to hyperglycemia (25 mM D-glucose) for 72 h impaired the BBB integrity and function as evidenced by decreases in transendothelial electrical resistance and increases in paracellular flux of sodium fluorescein. Dissolution of zonula occludens-1, a tight junction protein, and appearance of stress fibers appeared to play a key role in this pathology. Despite elevations in angiogenin, endothelin-1, interleukin-8 and basic fibroblast growth factor levels and a decrease in placental growth factor levels in BMEC subjected to hyperglycemia vs normoglycaemia (5.5 mM D-glucose), tubulogenic capacity of BMECs remained similar in both settings. Similarly, pericytes subjected to secretome obtained from hyperglycemic BMEC released higher quantities of macrophage migration inhibitory factor and serpin and lower quantities of monocyte chemoattractant protein-1, intercellular adhesion molecule, interleukin-6 and interleukin-8. Taken together these findings indicate the complexity of the mechanisms leading to BBB disruption in hyperglycemic settings and emphasize the importance of endothelial cell-pericyte axis in the development of novel therapeutic strategies.

Outgrowth Endothelial Cell Conditioned Medium Negates TNF-α-Evoked Cerebral Barrier Damage: A Reverse Translational Research to Explore Mechanisms

Improved understanding of the key mechanisms underlying cerebral ischemic injury is essential for the discovery of efficacious novel therapeutics for stroke. Through detailed analysis of plasma samples obtained from a large number of healthy volunteers (n = 90) and ischemic stroke patients (n = 81), the current study found significant elevations in the levels of TNF-α at baseline (within the first 48 h of stroke) and on days 7, 30, 90 after ischaemic stroke. It then assessed the impact of this inflammatory cytokine on an in vitro model of human blood-brain barrier (BBB) and revealed dramatic impairments in both barrier integrity and function, the main cause of early death after an ischemic stroke. Co-treatment of BBB models in similar experiments with outgrowth endothelial cell-derived conditioned media (OEC-CM) negated the deleterious effects of TNF-α on BBB. Effective suppression of anti-angiogenic factor endostatin, stress fiber formation, oxidative stress, and apoptosis along with concomitant improvements in extracellular matrix adhesive and tubulogenic properties of brain microvascular endothelial cells and OECs played an important role in OEC-CM-mediated benefits. Significant increases in pro-angiogenic endothelin-1 and monocyte chemoattractant protein-1 in OEC-CM compared to the secretomes of OEC and HBMEC, detected by proteome profiling assay, accentuate the beneficial effects of OEC-CM. In conclusion, this reverse translational study identifies TNF-α as an important mediator of post-ischemic cerebral barrier damage and proposes OEC-CM as a potential vasculoprotective therapeutic strategy by demonstrating its ability to regulate a wide range of mechanisms associated with BBB function. Clinical trial registration NCT02980354.

Protein kinase C-β distinctly regulates blood-brain barrier-forming capacity of Brain Microvascular endothelial cells and outgrowth endothelial cells

Outgrowth endothelial cells (OECs) provide an endogenous repair mechanism and thus maintain endothelial barrier integrity. As inhibition of protein kinase C-β (PKC-β) activity has been shown to attenuate endothelial damage in various pathological conditions including hyperglycaemia and ischaemic injury, the present study comparatively assessed the effect of LY333531, a PKC-β inhibitor, on the cerebral barrier integrity formed by OECs or human brain microvascular endothelial cells (HBMECs). To this end, an in vitro model of human BBB established by co-culture of astrocytes and pericytes with either OECs or HBMECs was exposed to 4 h of oxygen-glucose deprivation with/out LY333531 (0.05 µM). The inhibition of PKC-β protected the integrity and function of the BBB formed by HBMECs, as evidenced by increases in transendothelial electrical resistance and decreases in sodium fluorescein flux. It also attenuated ischaemia-evoked actin cytoskeleton remodelling, oxidative stress, and apoptosis in HBMECs. In contrast, treatments with LY333531 exacerbated the deleterious effect of ischaemia on the integrity and function of BBB formed by OECs while augmenting the levels of oxidative stress, apoptosis, and cytoskeletal reorganisation in OECs. Interestingly, the magnitude of damage in all aforementioned parameters, notably oxidative stress, was lower with low dose of LY333531 (0.01 µM). It is therefore possible that the therapeutic concentration of LY333531 (0.05 µM) may neutralise the activity of NADPH oxidase and thus trigger a negative feedback mechanism which in turn exacerbate the detrimental effects of ischaemic injury. In conclusion, targeting PKC-β signalling pathway in ischaemic settings requires close attention while using OECs as cellular therapeutic.

Treatment with outgrowth endothelial cells protects cerebral barrier against ischemic injury

BACKGROUND AND AIMS

We have previously reported that outgrowth endothelial cells (OECs) restore cerebral endothelial cell integrity through effective homing to the injury site. This study further investigates whether treatment with OECs can restore blood-brain barrier (BBB) function in settings of ischemia-reperfusion injury both in vitro and in vivo.

METHODS

An in vitro model of human BBB was established by co-culture of astrocytes, pericytes, and human brain microvascular endothelial cells (HBMECs) before exposure to oxygen-glucose deprivation alone or followed by reperfusion (OGD±R) in the absence or presence of exogenous OECs. Using a rodent model of middle cerebral artery occlusion (MCAO), we further assessed the therapeutic potential of OECs in vivo.

RESULTS

Owing to their prominent antioxidant, proliferative, and migratory properties, alongside their inherent capacity to incorporate into brain vasculature, treatments with OECs attenuated the extent of OGD±R injury on BBB integrity and function, as ascertained by increases in transendothelial electrical resistance and decreases in paracellular flux across the barrier. Similarly, intravenous delivery of OECs also led to better barrier protection in MCAO rats as evidenced by significant decreases in ipsilateral brain edema volumes on day 3 after treatment. Mechanistic studies subsequently showed that treatment with OECs substantially reduced oxidative stress and apoptosis in HBMECs subjected to ischemic damages.

CONCLUSION

This experimental study shows that OEC-based cell therapy restores BBB integrity in an effective manner by integrating into resident cerebral microvascular network, suppressing oxidative stress and cellular apoptosis.

Outgrowth endothelial progenitor cells restore cerebral barrier function following ischaemic damage: the impact of NOX2 inhibition

Disruption of blood-brain barrier (BBB), formed mainly by human brain microvascular endothelial cells (HBMECs), constitutes the major cause of mortality following ischaemic stroke. This study investigates whether OECs (outgrowth endothelial cells) can restore BBB integrity and function following ischaemic damage, and how inhibition of NOX2, a main source of vascular oxidative stress, affects the characteristics of BBB established with OECs and HBMECs in ischaemic settings. In vitro models of human BBB were constructed by co-culture of pericytes and astrocytes with either OECs or HBMECs before exposure to oxygen-glucose deprivation (OGD) alone or followed by reperfusion (OGD+R) in the absence or presence of NOX2 inhibitor, gp91ds-tat. The function and integrity of BBB were assessed by measurements of paracellular flux of sodium fluorescein (NaF) and transendothelial electrical resistance (TEER), respectively. Treatment with OECs during OGD+R effectively restored BBB integrity and function. Compared to HBMECs, OECs possessed lower NADPH oxidase activity, superoxide anion levels, and had greater total antioxidant capacity during OGD and OGD+R. Inhibition of NADPH oxidase during OGD and OGD+R restored the integrity and function of BBB established by HBMECs. This was evidenced by reductions in NADPH oxidase activity and superoxide anion levels. In contrast, treatment with gp91ds-tat aggravated ischaemic injury-induced BBB damage constructed by OECs. In conclusion, OECs are more resistant to ischaemic conditions and can effectively repair cerebral barrier following ischaemic damage. Suppression of oxidative stress through specific targeting of NOX2 requires close attention while using OECs as therapeutics.

Establishment of an In Vitro Model of Human Blood-Brain Barrier to Study the Impact of Ischemic Injury

The blood-brain barrier (BBB), mainly composed of brain microvascular endothelial cells, astrocyte end-feet, and pericytes, serves as a physical and biochemical barrier that selectively limits the passage of circulating molecules into the brain parenchyma. The disruption of its integrity and function is a major cause of increased mortality and disability among ischemic stroke patients. Hence, scrutiny of the cellular and molecular mechanisms that alter BBB permeability following an ischemic injury remains of paramount importance. In this context, establishment of an in vitro model of BBB that closely simulates human cerebral barrier may offer an easy, inexpensive, and straightforward approach to identify signaling pathways involved in BBB breakdown and may help to discover new therapeutic targets to restore its damage. This chapter describes a sequential method pertaining to establishment of a triple culture model of human BBB consisting of the three main cellular components of the cerebral barrier. It also documents how the integrity and function of this barrier are evaluated through measurements of transendothelial electrical resistance (TEER) and paracellular flux of permeability marker and sodium fluorescein (NaF, 376 Da), respectively, both in normal and experimental conditions mimicking ischemic injury.

Therapeutic hypothermia augments the restorative effects of PKC-β and Nox2 inhibition on an in vitro model of human blood-brain barrier

To investigate whether therapeutic hypothermia augments the restorative impact of protein kinase C-β (PKC-β) and Nox2 inhibition on an in vitro model of human blood-brain barrier (BBB). Cells cultured in normoglycaemic (5.5 mM) or hyperglycaemic (25 mM, 6 to 120 h) conditions were treated with therapeutic hypothermia (35 °C) in the absence or presence of a PKC-β inhibitor (LY333531, 0.05 μM) or a Nox2 inhibitor (gp91ds-tat, 50 μM). BBB was established by co-culture of human brain microvascular endothelial cells (HBMECs) with astrocytes (HAs) and pericytes. BBB integrity and function were assessed via transendothelial electrical resistance (TEER) and paracellular flux of sodium fluorescein (NaF, 376 Da). Nox activity (lucigenin assay), superoxide anion production (cytochrome-C reduction assay), cellular proliferative capacity (wound scratch assay) and actin cytoskeletal formation (rhodamine-phalloidin staining) were assessed both in HBMECs and HAs using the specific methodologies indicated in brackets. Therapeutic hypothermia augmented the protective effects of PKC-β or Nox2 inhibition on BBB integrity and function in experimental setting of hyperglycaemia, as evidenced by increases in TEER and concomitant decreases in paracellular flux of NaF. The combinatory approaches were more effective in repairing physical damage exerted on HBMEC and HA monolayers by wound scratch and in decreasing Nox activity and superoxide anion production compared to sole treatment regimen with either agent. Similarly, the combinatory approaches were more effective in suppressing actin stress fibre formation and maintaining normal cytoskeletal structure. Therapeutic hypothermia augments the cerebral barrier-restorative capacity of agents specifically targeting PKC-β or Nox2 pathways.

The secretome of endothelial progenitor cells: a potential therapeutic strategy for ischemic stroke

Ischemic stroke continues to be a leading cause of mortality and morbidity in the world. Despite recent advances in the field of stroke medicine, thrombolysis with recombinant tissue plasminogen activator remains as the only pharmacological therapy for stroke patients. However, due to short therapeutic window (4.5 hours of stroke onset) and increased risk of hemorrhage beyond this point, each year globally less than 1% of stroke patients receive this therapy which necessitate the discovery of safe and efficacious therapeutics that can be used beyond the acute phase of stroke. Accumulating evidence indicates that endothelial progenitor cells (EPCs), equipped with an inherent capacity to migrate, proliferate and differentiate, may be one such therapeutics. However, the limited availability of EPCs in peripheral blood and early senescence of few isolated cells in culture conditions adversely affect their application as effective therapeutics. Given that much of the EPC-mediated reparative effects on neurovasculature is realized by a wide range of biologically active substances released by these cells, it is possible that EPC-secretome may serve as an important therapeutic after an ischemic stroke. In light of this assumption, this review paper firstly discusses the main constituents of EPC-secretome that may exert the beneficial effects of EPCs on neurovasculature, and then reviews the currently scant literature that focuses on its therapeutic capacity.

MicroRNA: An Emerging Predictive, Diagnostic, Prognostic and Therapeutic Strategy in Ischaemic Stroke

Stroke continues to be the third-leading cause of death and disability worldwide. The limited availability of diagnostic tools approved therapeutics and biomarkers that help monitor disease progression or predict future events remain as the major challenges in the field of stroke medicine. Hence, attempts to discover safe and efficacious therapeutics and reliable biomarkers are of paramount importance. MicroRNAs (miRNAs) are a class of non-coding RNAs that play important roles in regulating gene expression. Since miRNAs also play important roles in key mechanisms associated with the pathogenesis of stroke, including energy failure, inflammation and cell death, it is possible that miRNAs may serve as reliable blood-based markers for risk prediction, diagnosis and prognosis of ischaemic stroke. Discovery of better neurological outcome and smaller cerebral infarcts in animal models of ischaemic stroke treated with miRNA agomirs or antagomirs indicate that miRNAs may also play a cerebrovascular protective role after an ischaemic stroke. Nonetheless, further evidences on the optimum time for treatment and route of administration are required before effective translation of these findings into clinical practice. Bearing these in mind, this paper reviews the current literature discussing the involvement of miRNAs in major pathologies associated with ischaemic stroke and evaluates their value as reliable biomarkers and therapeutics for ischaemic stroke.

Outgrowth endothelial cells form a functional cerebral barrier and restore its integrity after damage

Breakdown of blood-brain barrier, formed mainly by brain microvascular endothelial cells (BMECs), represents the major cause of mortality during early phases of ischemic strokes. Hence, discovery of novel agents that can effectively replace dead or dying endothelial cells to restore blood-brain barrier integrity is of paramount importance in stroke medicine. Although endothelial progenitor cells (EPCs) represent one such agents, their rarity in peripheral blood severely limits their adequate isolation and therapeutic use for acute ischemic stroke which necessitate their ex vivo expansion and generate early EPCs and outgrowth endothelial cells (OECs) as a result. Functional analyses of these cells, in the present study, demonstrated that only OECs endocytosed DiI-labelled acetylated low-density lipoprotein and formed tubules on matrigel, prominent endothelial cell and angiogenesis markers, respectively. Further analyses by flow cytometry demonstrated that OECs expressed specific markers for stemness (CD34), immaturity (CD133) and endothelial cells (CD31) but not for hematopoietic cells (CD45). Like BMECs, OECs established an equally tight in vitro model of human BBB with astrocytes and pericytes, suggesting their capacity to form tight junctions. Ischemic injury mimicked by concurrent deprivation of oxygen and glucose (4 hours) or deprivation of oxygen and glucose followed by reperfusion (20 hours) affected both barrier integrity and function in a similar fashion as evidenced by decreases in transendothelial electrical resistance and increases in paracellular flux, respectively. Wound scratch assays comparing the vasculoreparative capacity of cells revealed that, compared to BMECs, OECs possessed a greater proliferative and directional migratory capacity. In a triple culture model of BBB established with astrocytes, pericytes and BMEC, exogenous addition of OECs effectively repaired the damage induced on endothelial layer in serum-free conditions. Taken together, these data demonstrate that OECs may effectively home to the site of vascular injury and repair the damage to maintain (neuro)vascular homeostasis during or after a cerebral ischemic injury.