Reoxygenation stress on blood-brain barrier paracellular permeability and edema in the rat

Characterizing the Linkage of Systemic Hypoxia and Angiogenesis in High-Grade Glioma to Define the Changes in Tumor Microenvironment for Predicting Prognosis

High-grade gliomas (HGG) comprising WHO grades 3 and 4 have a poor overall survival (OS) that has not improved in the past decade. Herein, markers representing four components of the tumor microenvironment (TME) were identified to define their linked expression in TME and predict the prognosis in HGG, namely, interleukin6 (IL6, inflammation), inducible nitric oxide synthase(iNOS), heat shock protein-70 (HSP70, hypoxia), vascular endothelial growth receptor (VEGF), and endothelin1 (ET1) (angiogenesis) and matrix metalloprotease-14 (MMP14) and intercellular adhesion molecule1 (ICAM1, extracellular matrix). To establish a non-invasive panel of biomarkers for precise prognostication in HGG. Eighty-six therapy-naive HGG patients with 45 controls were analyzed for the defined panel. Systemic expression of extracellular/secretory biomarkers was screened dot-immune assay (DIA), quantified by ELISA, and validated by immunocytochemistry (ICC). Expression of iNOS, HSP70, IL-6, VEGF, ET1, MMP14, and ICAM1 was found to be positively associated with grade. Quantification of circulating levels of the markers by ELISA and ICC presented a similar result. The biomarkers were observed to negatively correlate with OS (p < 0.0001). Cox-regression analysis yielded all biomarkers as good prognostic indicators and independent of confounders. On applying combination statistics, the biomarker panel achieved higher sensitivity than single markers to define survival. The intra-association of all seven biomarkers was significant, hinting of a cross-talk between the TME components and a hypoxia driven systemic inflammation upregulating the expression of other components. This is a first ever experimental study of a marker panel that can distinguish between histopathological grades and also delineate differential survival using liquid biopsy, suggesting that markers of hypoxia can be a cornerstone for personalized therapy. The panel of biomarkers of iNOS, HSP70, IL-6, VEGF, ET1, MMP14, and ICAM1 holds promise for prognostication in HGG.

Results from a first-in-human phase I safety trial to evaluate the use of a vascularized pericranial/temporoparietal fascial flap to line the resection cavity following resection of newly diagnosed glioblastoma

PURPOSE

The efficacy of systemic therapies for glioblastoma (GBM) remains limited due to the constraints of systemic toxicity and blood-brain barrier (BBB) permeability. Temporoparietal fascial flaps (TPFFs) and vascularized peri cranial flaps (PCF) are not restricted by the blood-brain barrier (BBB), as they derive their vascular supply from branches of the external carotid artery. Transposition of a vascularized TPFF or PCF along a GBM resection cavity may bring autologous tissue not restricted by the BBB in close vicinity to the tumor bed microenvironment, permit ingrowth of vascular channels fed by the external circulation, and offer a mechanism of bypassing the BBB. In addition, circulating immune cells in the vascularized flap may have better access to tumor-associated antigens (TAA) within the tumor microenvironment. We conducted a first-in-human Phase I trial assessing the safety of lining the resection cavity with autologous TPFF/PCF of newly diagnosed patients with GBM.

METHODS

12 patients underwent safe, maximal surgical resection of newly diagnosed GBMs, followed by lining of the resection cavity with a pedicled, autologous TPFF or PCF. Safety was assessed by monitoring adverse events. Secondary analysis of efficacy was examined as the proportion of patients experiencing progression-free disease (PFS) as indicated by response assessment in neuro-oncology (RANO) criteria and overall survival (OS). The study was powered to determine whether a Phase II study was warranted based on these early results. For this analysis, subjects who were alive and had not progressed as of the date of the last follow-up were considered censored and all living patients who were alive as of the date of last follow-up were considered censored for overall survival. For simplicity, we assumed that a 70% PFS rate at 6 months would be considered an encouraging response and would make an argument for further investigation of the procedure.

RESULTS

Median age of included patients was 57 years (range 46-69 years). All patients were Isocitrate dehydrogenase (IDH) wildtype. Average tumor volume was 56.6 cm3 (range 14-145 cm3). Resection was qualified as gross total resection (GTR) of all of the enhancing diseases in all patients. Grade III or above adverse events were encountered in 3 patients. No Grade IV or V serious adverse events occurred in the immediate post-operative period including seizure, infection, stroke, or tumor growing along the flap. Disease progression at the site of the original tumor was identified in only 4 (33%) patients (median 23 months, range 8-25 months), 3 of whom underwent re-operation. Histopathological analyses of those implanted flaps and tumor bed biopsy at repeat surgery demonstrated robust immune infiltrates within the transplanted flap. Importantly, no patient demonstrated evidence of tumor infiltration into the implanted flap. At the time of this manuscript preparation, only 4/12 (33%) of patients have died. Based on the statistical considerations above and including all 12 patients 10/12 (83.3%) had 6-month PFS. The median PFS was 9.10 months, and the OS was 17.6 months. 4/12 (33%) of patients have been alive for more than two years and our longest surviving patient currently is alive at 60 months.

CONCLUSIONS

This pilot study suggests that insertion of pedicled autologous TPFF/PCF along a GBM resection cavity is safe and feasible. Based on the encouraging response rate in 6-month PFS and OS, larger phase II studies are warranted to assess and reproduce safety, feasibility, and efficacy. TRIAL REGISTRATION NUMBER AND DATE OF REGISTRATION FOR PROSPECTIVELY REGISTERED TRIALS: ClinicalTrials.gov ID NCT03630289, dated: 08/02/2018.

Lung endothelium, tau, and amyloids in health and disease

Lung endothelia in the arteries, capillaries, and veins are heterogeneous in structure and function. Lung capillaries in particular represent a unique vascular niche, with a thin yet highly restrictive alveolar-capillary barrier that optimizes gas exchange. Capillary endothelium surveys the blood while simultaneously interpreting cues initiated within the alveolus and communicated via immediately adjacent type I and type II epithelial cells, fibroblasts, and pericytes. This cell-cell communication is necessary to coordinate the immune response to lower respiratory tract infection. Recent discoveries identify an important role for the microtubule-associated protein tau that is expressed in lung capillary endothelia in the host-pathogen interaction. This endothelial tau stabilizes microtubules necessary for barrier integrity, yet infection drives production of cytotoxic tau variants that are released into the airways and circulation, where they contribute to end-organ dysfunction. Similarly, beta-amyloid is produced during infection. Beta-amyloid has antimicrobial activity, but during infection it can acquire cytotoxic activity that is deleterious to the host. The production and function of these cytotoxic tau and amyloid variants are the subject of this review. Lung-derived cytotoxic tau and amyloid variants are a recently discovered mechanism of end-organ dysfunction, including neurocognitive dysfunction, during and in the aftermath of infection.

The role of oxidative stress in blood-brain barrier disruption during ischemic stroke: Antioxidants in clinical trials

Ischemic stroke is one of the leading causes of death and disability. Occlusion and reperfusion of cerebral blood vessels (i.e., ischemia/reperfusion (I/R) injury) generates reactive oxygen species (ROS) that contribute to brain cell death and dysfunction of the blood-brain barrier (BBB) via oxidative stress. BBB disruption influences the pathogenesis of ischemic stroke by contributing to cerebral edema, hemorrhagic transformation, and extravasation of circulating neurotoxic proteins. An improved understanding of mechanisms for ROS-associated alterations in BBB function during ischemia/reperfusion (I/R) injury can lead to improved treatment paradigms for ischemic stroke. Unfortunately, progress in developing ROS targeted therapeutics that are effective for stroke treatment has been slow. Here, we review how ROS are produced in response to I/R injury, their effects on BBB integrity (i.e., tight junction protein complexes, transporters), and the utilization of antioxidant treatments in ischemic stroke clinical trials. Overall, knowledge in this area provides a strong translational framework for discovery of novel drugs for stroke and/or improved strategies to mitigate I/R injury in stroke patients.

Relationship between blood-brain barrier changes and drug metabolism under high-altitude hypoxia: obstacle or opportunity for drug transport?

The blood-brain barrier is essential for maintaining the stability of the central nervous system and is also crucial for regulating drug metabolism, changes of blood-brain barrier's structure and function can influence how drugs are delivered to the brain. In high-altitude hypoxia, the central nervous system's function is drastically altered, which can cause disease and modify the metabolism of drugs in vivo. Changes in the structure and function of the blood-brain barrier and the transport of the drug across the blood-brain barrier under high-altitude hypoxia, are regulated by changes in brain microvascular endothelial cells, astrocytes, and pericytes, either regulated by drug metabolism factors such as drug transporters and drug-metabolizing enzymes. This article aims to review the effects of high-altitude hypoxia on the structure and function of the blood-brain barrier as well as the effects of changes in the blood-brain barrier on drug metabolism. We also hypothesized and explore the regulation and potential mechanisms of the blood-brain barrier and associated pathways, such as transcription factors, inflammatory factors, and nuclear receptors, in regulating drug transport under high-altitude hypoxia.

Hypoxia-induced blood-brain barrier dysfunction is prevented by pericyte-conditioned media via attenuated actomyosin contractility and claudin-5 stabilization

The blood-brain barrier (BBB) regulates molecular and cellular entry from the cerebrovasculature into the surrounding brain parenchyma. Many diseases of the brain are associated with dysfunction of the BBB, where hypoxia is a common stressor. However, the contribution of hypoxia to BBB dysfunction is challenging to study due to the complexity of the brain microenvironment. In this study, we used a BBB model with brain microvascular endothelial cells and pericytes differentiated from iPSCs to investigate the effect of hypoxia on barrier function. We found that hypoxia-induced barrier dysfunction is dependent upon increased actomyosin contractility and is associated with increased fibronectin fibrillogenesis. We propose a role for actomyosin contractility in mediating hypoxia-induced barrier dysfunction through modulation of junctional claudin-5. Our findings suggest pericytes may protect brain microvascular endothelial cells from hypoxic stresses and that pericyte-derived factors could be candidates for treatment of pathological barrier-forming tissues.

Blood-Brain Barrier Mechanisms in Stroke and Trauma

The brain microenvironment is tightly regulated. The blood-brain barrier (BBB), which is composed of cerebral endothelial cells, astrocytes, and pericytes, plays an important role in maintaining the brain homeostasis by regulating the transport of both beneficial and detrimental substances between circulating blood and brain parenchyma. After brain injury and disease, BBB tightness becomes dysregulated, thus leading to inflammation and secondary brain damage. In this chapter, we overview the fundamental mechanisms of BBB damage and repair after stroke and traumatic brain injury (TBI). Understanding these mechanisms may lead to therapeutic opportunities for brain injury.

In Vitro Models of the Human Blood-Brain Barrier Utilising Human Induced Pluripotent Stem Cells: Opportunities and Challenges

The blood-brain barrier (BBB) is a component of the neurovascular unit formed by specialized brain microvascular endothelial cells surrounded by astrocytes end-feet processes, pericytes, and a basement membrane. The BBB plays an important role in the maintenance of brain homeostasis and has seen a growing involvement in the pathophysiology of various neurological diseases. On the other hand, the presence of such a barrier remains an important challenge for drug delivery to treat such illnesses.Since the pioneering work describing the isolation and cultivation of primary brain microvascular cells about 50 years ago until now, the development of an in vitro model of the BBB that is scalable, capable to form tight monolayers, and predictive of drug permeability in vivo remained extremely challenging.The recent description of the use of induced pluripotent stem cells (iPSCs) as a modeling tool for neurological diseases raised momentum into the use of such cells to develop new in vitro models of the BBB. This chapter will provide an exhaustive description of the use of iPSCs as a source of cells for modeling the BBB in vitro, describe the advantages and limitations of such model, as well as describe their prospective use for disease modeling and drug permeability screening platforms.

Multiple Roles of Peripheral Immune System in Modulating Ischemia/Hypoxia-Induced Neuroinflammation

Given combined efforts of neuroscience and immunology, increasing evidence has revealed the critical roles of the immune system in regulating homeostasis and disorders of the central nervous system (CNS). Microglia have long been considered as the only immune cell type in parenchyma, while at the interface between CNS and the peripheral (meninges, choroid plexus, and perivascular space), embryonically originated border-associated macrophages (BAMs) and multiple surveilling leukocytes capable of migrating into and out of the brain have been identified to function in the healthy brain. Hypoxia-induced neuroinflammation is the key pathological procedure that can be detected in healthy people at high altitude or in various neurodegenerative diseases, during which a very thin line between a beneficial response of the peripheral immune system in maintaining brain homeostasis and a pathological role in exacerbating neuroinflammation has been revealed. Here, we are going to focus on the role of the peripheral immune system and its crosstalk with CNS in the healthy brain and especially in hypobaric or ischemic hypoxia-associated neuroinflammation.

Attenuation of cerebral edema facilitates recovery of glymphatic system function after status epilepticus

Status epilepticus (SE) is a neurological emergency usually accompanied by acute cerebral edema and long-term cognitive impairment, and is characterized by neurodegeneration and aberrant hyperphosphorylated tau protein (p-tau) aggregation. The glia-lymphatic (glymphatic) system plays a central role in facilitating the clearance of metabolic waste from the brain, but its relationship with cerebral edema and cognitive dysfunction after SE is unclear. We hypothesized that cerebral edema after SE might impair glymphatic system function through compression, thus leading to impaired removal of metabolic waste, and ultimately affecting long-term cognitive function. Our results showed that glymphatic system function was temporarily impaired, as evidenced by 2-photon imaging, MRI enhancement, imaging of brain sections, and astrocytic water channel aquaporin 4 (AQP4) protein polarization. The severity of cerebral edema on MRI correlated well with glymphatic system dysfunction within 8 days following SE. Moreover, when cerebral edema was alleviated by glibenclamide treatment or genetic deletion of Trpm4, post-SE glymphatic system function recovered earlier, along with fewer p-tau–deposited neurons and neuronal degeneration and better cognitive function. These findings suggest that SE-induced cerebral edema may cause glymphatic system dysfunction and render the post-SE brain vulnerable to p-tau aggregation and neurocognitive impairment.

Glymphatic System in the Central Nervous System, a Novel Therapeutic Direction Against Brain Edema After Stroke

Stroke is the destruction of brain function and structure, and is caused by either cerebrovascular obstruction or rupture. It is a disease associated with high mortality and disability worldwide. Brain edema after stroke is an important factor affecting neurologic function recovery. The glymphatic system is a recently discovered cerebrospinal fluid (CSF) transport system. Through the perivascular space and aquaporin 4 (AQP4) on astrocytes, it promotes the exchange of CSF and interstitial fluid (ISF), clears brain metabolic waste, and maintains the stability of the internal environment within the brain. Excessive accumulation of fluid in the brain tissue causes cerebral edema, but the glymphatic system plays an important role in the process of both intake and removal of fluid within the brain. The changes in the glymphatic system after stroke may be an important contributor to brain edema. Understanding and targeting the molecular mechanisms and the role of the glymphatic system in the formation and regression of brain edema after stroke could promote the exclusion of fluids in the brain tissue and promote the recovery of neurological function in stroke patients. In this review, we will discuss the physiology of the glymphatic system, as well as the related mechanisms and therapeutic targets involved in the formation of brain edema after stroke, which could provide a new direction for research against brain edema after stroke.

Methods used for the measurement of blood-brain barrier integrity

The blood-brain barrier (BBB) comprises the interface between blood, brain and cerebrospinal fluid. Its primary function, which is mainly carried out by tight junctions, is to stabilize the tightly controlled microenvironment of the brain. To study the development and maintenance of the BBB, as well as various roles their intrinsic mechanisms that play in neurological disorders, suitable measurements are required to demonstrate integrity and functional changes at the interfaces between the blood and brain tissue. Markers and plasma proteins with different molecular weight (MW) are used to measure the permeability of BBB. In addition, the expression changes of tight-junction proteins form the basic structure of BBB, and imaging modalities are available to study the disruption of BBB. In the present review, above mentioned methods are depicted in details, together with the pros and cons as well as the differences between these methods, which maybe benefit research studies focused on the detection of BBB breakdown.

Organic Cation Transporter (OCT/OCTN) Expression at Brain Barrier Sites: Focus on CNS Drug Delivery

Therapeutic delivery to the central nervous system (CNS) continues to be a considerable challenge in the pharmacological treatment and management of neurological disorders. This is primarily due to the physiological and biochemical characteristics of brain barrier sites (i.e., blood-brain barrier (BBB), blood-cerebrospinal fluid barrier (BCSFB)). Drug uptake into brain tissue is highly restricted by expression of tight junction protein complexes and adherens junctions between brain microvascular endothelial cells and choroid plexus epithelial cells. Additionally, efflux transport proteins expressed at the plasma membrane of these same endothelial and epithelial cells act to limit CNS concentrations of centrally acting drugs. In contrast, facilitated diffusion via transporter proteins allows for substrate-specific flux of molecules across the plasma membrane, directing drug uptake into the CNS. Organic Cation Transporters (OCTs) and Novel Organic Cation Transporters (OCTNs) are two subfamilies of the solute carrier 22 (SLC22) family of proteins that have significant potential to mediate delivery of positively charged, zwitterionic, and uncharged therapeutics. While expression of these transporters has been well characterized in peripheral tissues, the functional expression of OCT and OCTN transporters at CNS barrier sites and their role in delivery of therapeutic drugs to molecular targets in the brain require more detailed analysis. In this chapter, we will review current knowledge on localization, function, and regulation of OCT and OCTN isoforms at the BBB and BCSFB with a particular emphasis on how these transporters can be utilized for CNS delivery of therapeutic agents.

Regulation of blood-brain barrier integrity by microglia in health and disease: A therapeutic opportunity

The blood-brain barrier (BBB) is a critical regulator of CNS homeostasis. It possesses physical and biochemical characteristics (i.e. tight junction protein complexes, transporters) that are necessary for the BBB to perform this physiological role. Microvascular endothelial cells require support from astrocytes, pericytes, microglia, neurons, and constituents of the extracellular matrix. This intricate relationship implies the existence of a neurovascular unit (NVU). NVU cellular components can be activated in disease and contribute to dynamic remodeling of the BBB. This is especially true of microglia, the resident immune cells of the brain, which polarize into distinct proinflammatory (M1) or anti-inflammatory (M2) phenotypes. Current data indicate that M1 pro-inflammatory microglia contribute to BBB dysfunction and vascular "leak", while M2 anti-inflammatory microglia play a protective role at the BBB. Understanding biological mechanisms involved in microglia activation provides a unique opportunity to develop novel treatment approaches for neurological diseases. In this review, we highlight characteristics of M1 proinflammatory and M2 anti-inflammatory microglia and describe how these distinct phenotypes modulate BBB physiology. Additionally, we outline the role of other NVU cell types in regulating microglial activation and highlight how microglia can be targeted for treatment of disease with a focus on ischemic stroke and Alzheimer's disease.

Theta-burst transcranial magnetic stimulation promotes stroke recovery by vascular protection and neovascularization

Rationale: The integrity and function of the blood-brain barrier (BBB) is compromised after stroke. The current study was performed to examine potential beneficial effects and underlying mechanisms of repetitive transcranial magnetic stimulation (rTMS) on angiogenesis and vascular protection, function, and repair following stroke, which are largely unknown.

Methods: Using a rat photothrombotic (PT) stroke model, continuous theta-burst rTMS was administered once daily to the infarcted hemisphere for 5 min, beginning 3 h after PT stroke. This treatment was applied for 6 days. BBB integrity, blood flow, vascular associated proteins, angiogenesis, integrity of neuronal morphology and structure, and behavioral outcome were measured and analyzed at 6 and/or 22 days after PT stroke.

Results: We report that rTMS significantly mitigated BBB permeabilization and preserved important BBB components ZO-1, claudin-5, occludin, and caveolin-1 from PT-induced degradation. Damage to vascular structure, morphology, and perfusion was ameliorated by rTMS, resulting in improved local tissue oxygenation. This was accompanied with robust protection of critical vascular components and upregulation of regulatory factors. A complex cytokine response was induced by PT, particularly at the late phase. Application of rTMS modulated this response, ameliorating levels of cytokines related to peripheral immune cell infiltration. Further investigation revealed that rTMS promoted and sustained post-ischemic angiogenesis long-term and reduced apoptosis of newborn and existing vascular endothelial cells. Application of rTMS also inhibited PT-induced excessive astrocyte-vasculature interactions and stimulated an A1 to A2 shift in vessel-associated astrocytes. Mechanistic studies revealed that rTMS dramatically increased levels of PDGFRβ associated with A2 astrocytes and their adjacent vasculature. As well, A2 astrocytes displayed marked amplification of the angiogenesis-related factors VEGF and TGFβ. PT induced a rise in vessel-associated expression of HIF-1α that was starkly intensified by rTMS treatment. Finally, rTMS preserved neuronal morphology, synaptic structure integrity and behavioral outcome.

Conclusions: These results indicate that rTMS can exert powerful protective and restorative effects on the peri-infarct microvasculature after PT stroke by, in part, promoting HIF-1α signaling and shifting vessel-associated astrocytic polarization to the A2 phenotype. This study provides further support for the potent protective effects of rTMS in the context of ischemic stroke, and these findings implicate vascular repair and protection as an important underlying phenomenon.

Structure, Function, and Regulation of the Blood-Brain Barrier Tight Junction in Central Nervous System Disorders

The blood-brain barrier (BBB) allows the brain to selectively import nutrients and energy critical to neuronal function while simultaneously excluding neurotoxic substances from the peripheral circulation. In contrast to the highly permeable vasculature present in most organs that reside outside of the central nervous system (CNS), the BBB exhibits a high transendothelial electrical resistance (TEER) along with a low rate of transcytosis and greatly restricted paracellular permeability. The property of low paracellular permeability is controlled by tight junction (TJ) protein complexes that seal the paracellular route between apposing brain microvascular endothelial cells. Although tight junction protein complexes are principal contributors to physical barrier properties, they are not static in nature. Rather, tight junction protein complexes are highly dynamic structures, where expression and/or localization of individual constituent proteins can be modified in response to pathophysiological stressors. These stressors induce modifications to tight junction protein complexes that involve de novo synthesis of new protein or discrete trafficking mechanisms. Such responsiveness of BBB tight junctions to diseases indicates that these protein complexes are critical for maintenance of CNS homeostasis. In fulfillment of this vital role, BBB tight junctions are also a major obstacle to therapeutic drug delivery to the brain. There is an opportunity to overcome this substantial obstacle and optimize neuropharmacology via acquisition of a detailed understanding of BBB tight junction structure, function, and regulation. In this review, we discuss physiological characteristics of tight junction protein complexes and how these properties regulate delivery of therapeutics to the CNS for treatment of neurological diseases. Specifically, we will discuss modulation of tight junction structure, function, and regulation both in the context of disease states and in the setting of pharmacotherapy. In particular, we will highlight how these properties can be potentially manipulated at the molecular level to increase CNS drug levels via paracellular transport to the brain.

Electrographic seizures induced by activation of ETA and ETB receptors following intrahippocampal infusion of endothelin-1 in immature rats occur by different mechanisms

We have demonstrated previously that activation of either the ET_A or ET_B receptor can induce acute electrographic seizures following the intrahippocampal infusion of endothelin-1 (ET-1) in immature (P12) rats. We also demonstrated that activation of the ET_A receptor is associated with marked focal ischemia, while activation of the ET_B receptor is not. Exploring the mechanisms underlying seizures induced by these two ET-1 receptor interactions can potentially provide insight into how focal ischemia in immature animals produces seizures and whether ischemiarelated seizures differ from seizures not associated with ischemia. To explore these seizure mechanisms we used microdialysis to determine biomarkers associated with seizures in P12 rats following the intrahippocampal infusion of two different agents: (1) ET-1, which activates both the ET_A and ET_B receptors and causes focal ischemia and (2) Ala-ET-1, which selectively activates only the ET_B receptor and does not cause ischemia. Our results show that seizures associated with combined ET_A and ET_B receptor activation (and ischemia) have a different temporal distribution and microdialysis profile from seizures associated with ET_B activation alone (and without ischemia). Seizures with combined activation peak within the first hour after infusion and the microdialysis profile is characterized by a significant increase in the ratio of glutamic acid to GABA. By contrast, seizures with activation of only the ET_B receptor peak in the second hour after infusion and microdialysis shows a significant increase in the ratio of leukotriene B4 to prostaglandin E2. These findings suggest that ischemia-related seizures in immature animals involve an imbalance of excitation and inhibition, while non-ischemiarelated seizures involve an inflammatory process resulting from an excess of leukotrienes.

Pathomechanisms of Blood-Brain Barrier Disruption in ALS

The blood-brain barrier (BBB) and the blood-spinal cord barrier (BSCB) are responsible for controlling the microenvironment within neural tissues in humans. These barriers are fundamental to all neurological processes as they provide the extreme nutritional demands of neural tissue, remove wastes, and maintain immune privileged status. Being a semipermeable membrane, both the BBB and BSCB allow the diffusion of certain molecules, whilst restricting others. In amyotrophic lateral sclerosis (ALS) and other neurodegenerative diseases, these barriers become hyperpermeable, allowing a wider variety of molecules to pass through leading to more severe and more rapidly progressing disease. The intention of this review is to discuss evidence that BBB hyperpermeability is potentially a disease driving feature in ALS and other neurodegenerative diseases. The various biochemical, physiological, and genomic factors that can influence BBB permeability in ALS and other neurodegenerative diseases are also discussed, in addition to novel therapeutic strategies centred upon the BBB.

Roles of blood-brain barrier integrins and extracellular matrix in stroke

Ischemicstroke is a leading cause of death and disability in the United States, but recent advances in treatments [i.e., endovascular thrombectomy and tissue plasminogen activator (t-PA)] that target the stroke-causing blood clot, while improving overall stroke mortality rates, have had much less of an impact on overall stroke morbidity. This may in part be attributed to the lack of therapeutics targeting reperfusion-induced injury after the blood clot has been removed, which, if left unchecked, can expand injury from its core into the surrounding at risk tissue (penumbra). This occurs in two phases of increased permeability of the blood-brain barrier, a physical barrier that under physiologic conditions regulates brain influx and efflux of substances and consists of tight junction forming endothelial cells (and transporter proteins), astrocytes, pericytes, extracellular matrix, and their integrin cellular receptors. During, embryonic development, maturity, and following stroke reperfusion, cerebral vasculature undergoes significant changes including changes in expression of integrins and degradation of surrounding extracellular matrix. Integrins, heterodimers with α and β subunits, and their extracellular matrix ligands, a collection of proteoglycans, glycoproteins, and collagens, have been modestly studied in the context of stroke compared with other diseases (e.g., cancer). In this review, we describe the effect that various integrins and extracellular matrix components have in embryonic brain development, and how this changes in both maturity and in the poststroke environment. Particular focus will be on how these changes in integrins and the extracellular matrix affect blood-brain barrier components and their potential as diagnostic and therapeutic targets for ischemic stroke.

Diplomats' Mystery Illness and Pulsed Radiofrequency/Microwave Radiation

Importance: A mystery illness striking U.S. and Canadian diplomats to Cuba (and now China) "has confounded the FBI, the State Department and US intelligence agencies" (Lederman, Weissenstein, & Lee, 2017). Sonic explanations for the so-called health attacks have long dominated media reports, propelled by peculiar sounds heard and auditory symptoms experienced. Sonic mediation was justly rejected by experts. We assessed whether pulsed radiofrequency/microwave radiation (RF/MW) exposure can accommodate reported facts in diplomats, including unusual ones. Observations: (1) Noises: Many diplomats heard chirping, ringing or grinding noises at night during episodes reportedly triggering health problems. Some reported that noises were localized with laser-like precision or said the sounds seemed to follow them (within the territory in which they were perceived). Pulsed RF/MW engenders just these apparent "sounds" via the Frey effect. Perceived "sounds" differ by head dimensions and pulse characteristics and can be perceived as located behind in or above the head. Ability to hear the "sounds" depends on high-frequency hearing and low ambient noise. (2) Signs/symptoms: Hearing loss and tinnitus are prominent in affected diplomats and in RF/MW-affected individuals. Each of the protean symptoms that diplomats report also affect persons reporting symptoms from RF/MW: sleep problems, headaches, and cognitive problems dominate in both groups. Sensations of pressure or vibration figure in each. Both encompass vision, balance, and speech problems and nosebleeds. Brain injury and brain swelling are reported in both. (3) Mechanisms: Oxidative stress provides a documented mechanism of RF/MW injury compatible with reported signs and symptoms; sequelae of endothelial dysfunction (yielding blood flow compromise), membrane damage, blood-brain barrier disruption, mitochondrial injury, apoptosis, and autoimmune triggering afford downstream mechanisms, of varying persistence, that merit investigation. (4) Of note, microwaving of the U.S. embassy in Moscow is historically documented. Conclusions and relevance: Reported facts appear consistent with pulsed RF/MW as the source of injury in affected diplomats. Nondiplomats citing symptoms from RF/MW, often with an inciting pulsed-RF/MW exposure, report compatible health conditions. Under the RF/MW hypothesis, lessons learned for diplomats and for RF/MW-affected civilians may each aid the other.

Blood-brain barrier dysfunction in ischemic stroke: targeting tight junctions and transporters for vascular protection

The blood-brain barrier (BBB) is a physical and biochemical barrier that precisely controls cerebral homeostasis. It also plays a central role in the regulation of blood-to-brain flux of endogenous and exogenous xenobiotics and associated metabolites. This is accomplished by molecular characteristics of brain microvessel endothelial cells such as tight junction protein complexes and functional expression of influx and efflux transporters. One of the pathophysiological features of ischemic stroke is disruption of the BBB, which significantly contributes to development of brain injury and subsequent neurological impairment. Biochemical characteristics of BBB damage include decreased expression and altered organization of tight junction constituent proteins as well as modulation of functional expression of endogenous BBB transporters. Therefore, there is a critical need for development of novel therapeutic strategies that can protect against BBB dysfunction (i.e., vascular protection) in the setting of ischemic stroke. Such strategies include targeting tight junctions to ensure that they maintain their correct structure or targeting transporters to control flux of physiological substrates for protection of endothelial homeostasis. In this review, we will describe the pathophysiological mechanisms in cerebral microvascular endothelial cells that lead to BBB dysfunction following onset of stroke. Additionally, we will utilize this state-of-the-art knowledge to provide insights on novel pharmacological strategies that can be developed to confer BBB protection in the setting of ischemic stroke.

Simultaneous functional photoacoustic microscopy and electrocorticography reveal the impact of rtPA on dynamic neurovascular functions after cerebral ischemia

The advance of thrombolytic therapy has been hampered by the lack of optimization of the therapy during the hyperacute phase of focal ischemia. Here, we investigate neurovascular dynamics using a custom-designed hybrid electrocorticography (ECoG)-functional photoacoustic microscopy (fPAM) imaging system during the hyperacute phase (first 6 h) of photothrombotic ischemia (PTI) in male Wistar rats following recombinant tissue plasminogen activator (rtPA)-mediated thrombolysis. We reported, for the first time, the changes in neural activity and cerebral hemodynamic responses following rtPA infusion at different time points post PTI. Interestingly, very early administration of rtPA (< 1 h post PTI) resulted in only partial recovery of neurovascular dynamics (specifically , neural activity recovered to 71 ± 3.5% of baseline and hemodynamics to only 52 ± 2.6% of baseline) and late administration of rtPA (> 4 h post PTI) resulted in the deterioration of neurovascular function. A therapeutic window between 1 and 3 h post PTI was found to improve recovery of neurovascular function (i.e. significant restoration of neural activity to 93 ± 4.2% of baseline and hemodynamics to 81 ± 2.1% of baseline, respectively). The novel combination of fPAM and ECoG enables direct mapping of neurovascular dynamics and serves as a platform to evaluate potential interventions for stroke.

Functional Expression of P-glycoprotein and Organic Anion Transporting Polypeptides at the Blood-Brain Barrier: Understanding Transport Mechanisms for Improved CNS Drug Delivery?

Drug delivery to the central nervous system (CNS) is greatly limited by the blood-brain barrier (BBB). Physical and biochemical properties of the BBB have rendered treatment of CNS diseases, including those with a hypoxia/reoxygenation (H/R) component, extremely difficult. Targeting endogenous BBB transporters from the ATP-binding cassette (ABC) superfamily (i.e., P-glycoprotein (P-gp)) or from the solute carrier (SLC) family (i.e., organic anion transporting polypeptides (OATPs in humans; Oatps in rodents)) has been suggested as a strategy that can improve delivery of drugs to the brain. With respect to P-gp, direct pharmacological inhibition using small molecules or selective regulation by targeting intracellular signaling pathways has been explored. These approaches have been largely unsuccessful due to toxicity issues and unpredictable pharmacokinetics. Therefore, our laboratory has proposed that optimization of CNS drug delivery, particularly for treatment of diseases with an H/R component, can be achieved by targeting Oatp isoforms at the BBB. As the major drug transporting Oatp isoform, Oatp1a4 has demonstrated blood-to-brain transport of substrate drugs with neuroprotective properties. Furthermore, our laboratory has shown that targeting Oatp1a4 regulation (i.e., TGF-β signaling mediated via the ALK-1 and ALK-5 transmembrane receptors) represents an opportunity to control Oatp1a4 functional expression for the purpose of delivering therapeutics to the CNS. In this review, we will discuss limitations of targeting P-gp-mediated transport activity and the advantages of targeting Oatp-mediated transport. Through this discussion, we will also provide critical information on novel approaches to improve CNS drug delivery by targeting endogenous uptake transporters expressed at the BBB.

Role of Transporters in Central Nervous System Drug Delivery and Blood-Brain Barrier Protection: Relevance to Treatment of Stroke

Ischemic stroke is a leading cause of morbidity and mortality in the United States. The only approved pharmacologic treatment for ischemic stroke is thrombolysis via recombinant tissue plasminogen activator (r-tPA). A short therapeutic window and serious adverse events (ie, hemorrhage, excitotoxicity) greatly limit r-tPA therapy, which indicates an essential need to develop novel stroke treatment paradigms. Transporters expressed at the blood-brain barrier (BBB) provide a significant opportunity to advance stroke therapy via central nervous system delivery of drugs that have neuroprotective properties. Examples of such transporters include organic anion–transporting polypeptides (Oatps) and organic cation transporters (Octs). In addition, multidrug resistance proteins (Mrps) are transporter targets in brain microvascular endothelial cells that can be exploited to preserve BBB integrity in the setting of stroke. Here, we review current knowledge on stroke pharmacotherapy and demonstrate how endogenous BBB transporters can be targeted for improvement of ischemic stroke treatment.

Hypoxic Stress and Inflammatory Pain Disrupt Blood-Brain Barrier Tight Junctions: Implications for Drug Delivery to the Central Nervous System

A functional blood-brain barrier (BBB) is necessary to maintain central nervous system (CNS) homeostasis. Many diseases affecting the CNS, however, alter the functional integrity of the BBB. It has been shown that various diseases and physiological stressors can impact the BBB's ability to selectively restrict passage of substances from the blood to the brain. Modifications of the BBB's permeability properties can potentially contribute to the pathophysiology of CNS diseases and result in altered brain delivery of therapeutic agents. Hypoxia and/or inflammation are central components of a number of diseases affecting the CNS. A number of studies indicate hypoxia or inflammatory pain increase BBB paracellular permeability, induce changes in the expression and/or localization of tight junction proteins, and affect CNS drug uptake. In this review, we look at what is currently known with regard to BBB disruption following a hypoxic or inflammatory insult in vivo. Potential mechanisms involved in altering tight junction components at the BBB are also discussed. A more detailed understanding of the mediators involved in changing BBB functional integrity in response to hypoxia or inflammatory pain could potentially lead to new treatments for CNS diseases with hypoxic or inflammatory components. Additionally, greater insight into the mechanisms involved in TJ rearrangement at the BBB may lead to novel strategies to pharmacologically increase delivery of drugs to the CNS.

Nrf2 signaling increases expression of ATP-binding cassette subfamily C mRNA transcripts at the blood-brain barrier following hypoxia-reoxygenation stress

Background

Strategies to maintain BBB integrity in diseases with a hypoxia/reoxygenation (H/R) component involve preventing glutathione (GSH) loss from endothelial cells. GSH efflux transporters include multidrug resistance proteins (Mrps). Therefore, characterization of Mrp regulation at the BBB during H/R is required to advance these transporters as therapeutic targets. Our goal was to investigate, in vivo, regulation of Abcc1, Abcc2, and Abcc4 mRNA expression (i.e., genes encoding Mrp isoforms that transport GSH) by nuclear factor E2-related factor (Nrf2) using a well-established H/R model.

Methods

Female Sprague–Dawley rats (200–250 g) were subjected to normoxia (Nx, 21% O₂, 60 min), hypoxia (Hx, 6% O₂, 60 min) or H/R (6% O₂, 60 min followed by 21% O₂, 10 min, 30 min, or 1 h) or were treated with the Nrf2 activator sulforaphane (25 mg/kg, i.p.) for 3 h. Abcc mRNA expression in brain microvessels was determined using quantitative real-time PCR. Nrf2 signaling activation was examined using an electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) respectively. Data were expressed as mean ± SD and analyzed via ANOVA followed by the post hoc Bonferroni t test.

Results

We observed increased microvascular expression of Abcc1, Abcc2, and Abcc4 mRNA following H/R treatment with reoxygenation times of 10 min, 30 min, and 1 h and in animals treated with sulforaphane. Using a biotinylated Nrf2 probe, we observed an upward band shift in brain microvessels isolated from H/R animals or animals administered sulforaphane. ChIP studies showed increased Nrf2 binding to antioxidant response elements on Abcc1, Abcc2, and Abcc4 promoters following H/R or sulforaphane treatment, suggesting a role for Nrf2 signaling in Abcc gene regulation.

Conclusions

Our data show increased Abcc1, Abcc2, and Abcc4 mRNA expression at the BBB in response to H/R stress and that Abcc gene expression is regulated by Nrf2 signaling. Since these Mrp isoforms transport GSH, these results may point to endogenous transporters that can be targeted for BBB protection during H/R stress. Experiments are ongoing to examine functional implications of Nrf2-mediated increases in Abcc transcript expression. Such studies will determine utility of targeting Mrp isoforms for BBB protection in diseases with an H/R component.

Cerebral Vascular Disease and Neurovascular Injury in Ischemic Stroke

The consequences of cerebrovascular disease are among the leading health issues worldwide. Large and small cerebral vessel disease can trigger stroke and contribute to the vascular component of other forms of neurological dysfunction and degeneration. Both forms of vascular disease are driven by diverse risk factors, with hypertension as the leading contributor. Despite the importance of neurovascular disease and subsequent injury after ischemic events, fundamental knowledge in these areas lag behind our current understanding of neuroprotection and vascular biology in general. The goal of this review is to address select key structural and functional changes in the vasculature that promote hypoperfusion and ischemia, while also affecting the extent of injury and effectiveness of therapy. In addition, as damage to the blood-brain barrier is one of the major consequences of ischemia, we discuss cellular and molecular mechanisms underlying ischemia-induced changes in blood-brain barrier integrity and function, including alterations in endothelial cells and the contribution of pericytes, immune cells, and matrix metalloproteinases. Identification of cell types, pathways, and molecules that control vascular changes before and after ischemia may result in novel approaches to slow the progression of cerebrovascular disease and lessen both the frequency and impact of ischemic events.

Blood-brain barrier breakdown and neovascularization processes after stroke and traumatic brain injury

PURPOSE OF REVIEW

Angiogenesis or vascular reorganization plays a role in recovery after stroke and traumatic brain injury (TBI). In this review, we have focused on two major events that occur during stroke and TBI from a vascular perspective - what is the process and time course of blood-brain barrier (BBB) breakdown? and how does the surrounding vasculature recover and facilitate repair?

RECENT FINDINGS

Despite differences in the primary injury, the BBB changes overlap between stroke and TBI. Disruption of BBB involves a series of events: formation of caveolae, trans and paracellular disruption, tight junction breakdown and vascular disruption. Confounding factors that need careful assessment and standardization are the severity, duration and extent of the stroke and TBI that influences BBB disruption. Vascular repair proceeds through long-term neovascularization processes: angiogenesis, arteriogenesis and vasculogenesis. Enhancing each of these processes may impart beneficial effects in endogenous recovery.

SUMMARY

Our understanding of BBB breakdown acutely after the cerebrovascular injury has come a long way; however, we lack a clear understanding of the course of BBB disruption and BBB recovery and the evolution of individual cellular events associated with BBB change. Neovascularization responses have been widely studied in stroke for their role in functional recovery but the role of vascular reorganization after TBI in recovery is much less defined.

High-Field MRI Reveals a Drastic Increase of Hypoxia-Induced Microhemorrhages upon Tissue Reoxygenation in the Mouse Brain with Strong Predominance in the Olfactory Bulb

Human pathophysiology of high altitude hypoxic brain injury is not well understood and research on the underlying mechanisms is hampered by the lack of well-characterized animal models. In this study, we explored the evolution of brain injury by magnetic resonance imaging (MRI) and histological methods in mice exposed to normobaric hypoxia at 8% oxygen for 48 hours followed by rapid reoxygenation and incubation for further 24 h under normoxic conditions. T2*-, diffusion-weighted and T2-relaxometry MRI was performed before exposure, immediately after 48 hours of hypoxia and 24 hours after reoxygenation. Cerebral microhemorrhages, previously described in humans suffering from severe high altitude cerebral edema, were also detected in mice upon hypoxia-reoxygenation with a strong region-specific clustering in the olfactory bulb, and to a lesser extent, in the basal ganglia and cerebral white matter. The number of microhemorrhages determined immediately after hypoxia was low, but strongly increased 24 hours upon onset of reoxygenation. Histologically verified microhemorrhages were exclusively located around cerebral microvessels with disrupted interendothelial tight junction protein ZO-1. In contrast, quantitative T2 and apparent-diffusion-coefficient values immediately after hypoxia and after 24 hours of reoxygenation did not show any region-specific alteration, consistent with subtle multifocal but not with regional or global brain edema.

Effects of Acute Systemic Hypoxia and Hypercapnia on Brain Damage in a Rat Model of Hypoxia-Ischemia

Therapeutic hypercapnia has the potential for neuroprotection after global cerebral ischemia. Here we further investigated the effects of different degrees of acute systemic hypoxia in combination with hypercapnia on brain damage in a rat model of hypoxia and ischemia. Adult wistar rats underwent unilateral common carotid artery (CCA) ligation for 60 min followed by ventilation with normoxic or systemic hypoxic gas containing 11%O2,13%O2,15%O2 and 18%O2 (targeted to PaO2 30-39 mmHg, 40-49 mmHg, 50-59 mmHg, and 60-69 mmHg, respectively) or systemic hypoxic gas containing 8% carbon dioxide (targeted to PaCO2 60-80 mmHg) for 180 min. The mean artery pressure (MAP), blood gas, and cerebral blood flow (CBF) were evaluated. The cortical vascular permeability and brain edema were examined. The ipsilateral cortex damage and the percentage of hippocampal apoptotic neurons were evaluated by Nissl staining and terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate-biotin nick end labeling (TUNEL) assay as well as flow cytometry, respectively. Immunofluorescence and western blotting were performed to determine aquaporin-4 (AQP4) expression. In rats treated with severe hypoxia (PaO2 < 50 mmHg), hypercapnia augmented the decline of MAP with cortical CBF and damaged blood-brain barrier permeability (p < 0.05). In contrast, in rats treated with mild to moderate hypoxia (PaO2 > 50 mmHg), hypercapnia protected against these pathophysiological changes. Moreover, hypercapnia treatment significantly reduced brain damage in the ischemic ipsilateral cortex and decreased the percentage of apoptotic neurons in the hippocampus after the CCA ligated rats were exposed to mild or moderate hypoxemia (PaO2 > 50 mmHg); especially under mild hypoxemia (PaO2 > 60 mmHg), hypercapnia significantly attenuated the expression of AQP4 protein with brain edema (p < 0.05). Hypercapnia exerts beneficial effects under mild to moderate hypoxemia and augments detrimental effects under severe hypoxemia on brain damage in a rat model of hypoxia-ischemia.

Melatonin reduces excitotoxic blood-brain barrier breakdown in neonatal rats

The blood-brain barrier (BBB) is a complex structure that protects the central nervous system from peripheral insults. Understanding the molecular basis of BBB function and dysfunction holds significant potential for future strategies to prevent and treat neurological damage. The aim of our study was (1) to investigate BBB alterations following excitotoxicity and (2) to test the protective properties of melatonin. Ibotenate, a glutamate analog, was injected intracerebrally in postnatal day 5 (P5) rat pups to mimic excitotoxic injury. Animals were than randomly divided into two groups, one receiving intraperitoneal (i.p.) melatonin injections (5mg/kg), and the other phosphate buffer saline (PBS) injections. Pups were sacrificed 2, 4 and 18 h after ibotenate injection. We determined lesion size at 5 days by histology, the location and organization of tight junction (TJ) proteins by immunohistochemical studies, and BBB leakage by dextran extravasation. Expression levels of BBB genes (TJs, efflux transporters and detoxification enzymes) were determined in the cortex and choroid plexus by quantitative PCR. Dextran extravasation was seen 2h after the insult, suggesting a rapid BBB breakdown that was resolved by 4h. Extravasation was significantly reduced in melatonin-treated pups. Gene expression and immunohistochemical assays showed dynamic BBB modifications during the first 4h, partially prevented by melatonin. Lesion-size measurements confirmed white matter neuroprotection by melatonin. Our study is the first to evaluate BBB structure and function at a very early time point following excitotoxicity in neonates. Melatonin neuroprotects by preventing TJ modifications and BBB disruption at this early phase, before its previously demonstrated anti-inflammatory, antioxidant and axonal regrowth-promoting effects.

Targeting transporters: promoting blood-brain barrier repair in response to oxidative stress injury

The blood-brain barrier (BBB) is a physical and biochemical barrier that precisely regulates the ability of endogenous and exogenous substances to accumulate within brain tissue. It possesses structural and biochemical features (i.e., tight junction and adherens junction protein complexes, influx and efflux transporters) that work in concert to control solute permeation. Oxidative stress, a critical component of several diseases including cerebral hypoxia/ischemia and peripheral inflammatory pain, can cause considerable injury to the BBB and lead to significant CNS pathology. This suggests a critical need for novel therapeutic approaches that can protect the BBB in diseases with an oxidative stress component. Recent studies have identified molecular targets (i.e., putative membrane transporters, intracellular signaling systems) that can be exploited for optimization of endothelial drug delivery or for control of transport of endogenous substrates such as the antioxidant glutathione (GSH). In particular, targeting transporters offers a unique approach to protect BBB integrity by promoting repair of cell-cell interactions at the level of the brain microvascular endothelium. This review summarizes current knowledge in this area and emphasizes those targets that present considerable opportunity for providing BBB protection and/or promoting BBB repair in the setting of oxidative stress. This article is part of a Special Issue entitled SI: Cell Interactions In Stroke.

The blood-brain barrier

Blood vessels are critical to deliver oxygen and nutrients to all of the tissues and organs throughout the body. The blood vessels that vascularize the central nervous system (CNS) possess unique properties, termed the blood-brain barrier, which allow these vessels to tightly regulate the movement of ions, molecules, and cells between the blood and the brain. This precise control of CNS homeostasis allows for proper neuronal function and also protects the neural tissue from toxins and pathogens, and alterations of these barrier properties are an important component of pathology and progression of different neurological diseases. The physiological barrier is coordinated by a series of physical, transport, and metabolic properties possessed by the endothelial cells (ECs) that form the walls of the blood vessels, and these properties are regulated by interactions with different vascular, immune, and neural cells. Understanding how these different cell populations interact to regulate the barrier properties is essential for understanding how the brain functions during health and disease.

Biphasic modulation of paracellular claudin-5 expression in mouse brain endothelial cells is mediated through the phosphoinositide-3-kinase/AKT pathway

Blood-brain barrier (BBB) integrity is compromised in many central nervous system disorders. Complex astrocyte and vascular endothelial cell interactions that regulate BBB integrity may be disturbed in these disorders. We previously showed that systemic administration of 3-chloropropanediol [(S)-(+)-3-chloro-1,2-propanediol] induces a transitory glial fibrillary acidic protein-astrocyte loss, reversible loss of tight junction complexes, and BBB integrity disruption. However, the intracellular signaling mechanisms that induce BBB integrity marker loss are unclear. We hypothesize that 3-chloropropanediol-induced modulation of tight junction protein expression is mediated through the phosphoinositide-3-kinase (PI3K)/AKT pathway. To test this hypothesis, we used a mouse brain endothelial cell line (bEnd.3) exposed to 3-chloropropanediol for up to 3 days. Results showed early reversible loss of sharp paracellular claudin-5 expression 90, 105, and 120 minutes after 3-chloropropanediol (500 μM) treatment. Sharp paracellular claudin-5 profiles were later restored, but lost again by 2 and 3 days after 3-chloropropanediol treatment. Western blot and immunofluorescence studies showed increased p85-PI3K expression and transitory increased AKT (Thr308) phosphorylation at 15 and 30 minutes after 3-chloropropanediol administration. PI3K inhibitors LY294002 [2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one hydrochloride; 2.5-25 μM] and PI-828 [2-(4-morpholinyl)-8-(4-aminopheny)l-4H-1-benzopyran-4-one; 0.1-10 μM] prevented the 3-chloropropanediol-induced AKT (Thr308) phosphorylation and both early and late loss of paracellular claudin-5. However, AKT inhibitors only prevented the early changes in claudin-5 expression. This mechanistic study provides a greater understanding of the intracellular signaling pathways mediating tight junction protein expression and supports a hypothesis that two independent pathways triggered by PI3K mediate early and late loss of paracellular claudin-5 expression.

Drowning stars: reassessing the role of astrocytes in brain edema

Edema formation frequently complicates brain infarction, tumors, and trauma. Despite the significant mortality of this condition, current treatment options are often ineffective or incompletely understood. Recent studies have revealed the existence of a brain-wide paravascular pathway for cerebrospinal (CSF) and interstitial fluid (ISF) exchange. The current review critically examines the contribution of this 'glymphatic' system to the main types of brain edema. We propose that in cytotoxic edema, energy depletion enhances glymphatic CSF influx, whilst suppressing ISF efflux. We also argue that paravascular inflammation or 'paravasculitis' plays a critical role in vasogenic edema. Finally, recent advances in diagnostic imaging of glymphatic function may hold the key to defining the edema profile of individual patients, and thus enable more targeted therapy.

Hypoxia selectively disrupts brain microvascular endothelial tight junction complexes through a hypoxia-inducible factor-1 (HIF-1) dependent mechanism

The blood-brain barrier (BBB) constitutes a critical barrier for the maintenance of central nervous system homeostasis. Brain microvascular endothelial cells line the vessel walls and express tight junction (TJ) complexes that restrict paracellular passage across the BBB, thereby fulfilling a crucial role in ensuring brain function. Hypoxia, an impaired O(2) delivery, is known to cause BBB dysfunction but the mechanisms that drive this disruption remain unclear. This study discloses the relevance of the master regulator of the hypoxic response, hypoxia-inducible factor-1 (HIF-1), in hypoxia-induced barrier disruption using the rat brain endothelial cell line RBE4. Hypoxic exposure rapidly induced stabilization of the HIF-1 oxygen-dependent alpha subunit (HIF-1α) concomitantly with BBB impairment and TJ disruption mainly through delocalization and increased tyrosine phosphorylation of TJ proteins. Similar observations were obtained by normoxic stabilization of HIF-1α using CoCl(2), deferoxamine, and dimethyloxalylglycine underlining the involvement of HIF-1 in barrier dysfunction particularly via TJ alterations. In agreement inhibition of HIF-1 stabilization by 2-methoxyestradiol and YC-1 improved barrier function in hypoxic cells. Overall our data suggests that activation of HIF-1-mediated signaling disrupts TJ resulting in increased BBB permeability.

Cell-specific blood-brain barrier regulation in health and disease: a focus on hypoxia

The blood-brain barrier (BBB) is a complex vascular structure consisting of microvascular endothelial cells that line the vessel wall, astrocyte end-feet, pericytes, as well as the basal lamina. BBB cells act in concert to maintain the characteristic impermeable and low paracellular flux of the brain vascular network, thus ensuring a homeostatic neuronal environment. Alterations in BBB stability that occur during injury have dire consequences on disease progression and it is clear that BBB cell-specific responses, positive or negative, must make a significant contribution to injury outcome. Reduced oxygenation, or hypoxia, is a characteristic of many brain diseases that significantly increases barrier permeability. Recent data suggest that hypoxia-inducible factor (HIF-1), the master regulator of the hypoxic response, probably mediates many hypoxic effects either directly or indirectly via its target genes. This review discusses current knowledge of physiological cell-specific regulation of barrier function, their responses to hypoxia as well as consequences of hypoxic- and HIF-1-mediated mechanisms on barrier integrity during select brain diseases. In the final sections, the potential of current advances in targeting HIF-1 as a therapeutic strategy will be overviewed.

Stroke-induced brain parenchymal injury drives blood-brain barrier early leakage kinetics: a combined in vivo/in vitro study

The disappointing clinical outcomes of neuroprotectants challenge the relevance of preclinical stroke models and data in defining early cerebrovascular events as potential therapeutic targets. The kinetics of blood-brain barrier (BBB) leakage after reperfusion and the link with parenchymal lesion remain debated. By using in vivo and in vitro approaches, we conducted a kinetic analysis of BBB dysfunction during early reperfusion. After 60 minutes of middle cerebral artery occlusion followed by reperfusion times up to 24 hours in mice, a non-invasive magnetic resonance imaging method, through an original sequence of diffusion-weighted imaging, determined brain water mobility in microvascular compartments (D*) apart from parenchymal compartments (apparent diffusion coefficient). An increase in D* found at 4 hours post reperfusion concurred with the onset of both Evans blue/Dextran extravasations and in vitro BBB opening under oxygen-glucose deprivation and reoxygenation (R). The BBB leakage coincided with an emerging cell death in brain tissue as well as in activated glial cells in vitro. The co-culture of BBB endothelial and glial cells evidenced a recovery of endothelium tightness when glial cells were absent or non-injured during R. Preserving the ischemic brain parenchymal cells within 4 hours of reperfusion may improve therapeutic strategies for cerebrovascular protection against stroke.

Endothelial Von Willebrand factor promotes blood-brain barrier flexibility and provides protection from hypoxia and seizures in mice

OBJECTIVE

Aberrant blood-brain barrier (BBB) permeability is a hallmark pathology of many central nervous system diseases. von Willebrand factor (VWF) is stored in endothelial Weibel-Palade bodies from where it is released on activation into plasma and basement membrane. The role of VWF in endothelial homeostasis is unclear. The goal of this study was to assess the role of VWF in disease models associated with increased BBB permeability.

APPROACH AND RESULTS

We did not find any differences in BBB permeability to Evans blue dye at baseline between wild-type and VWF(-/-) animals. We next used 2 models presenting with increased BBB permeability, hypoxia/reoxygenation and pilocarpine-induced status epilepticus, to assess the response of VWF(-/-) animals. In both models, VWF(-/-) mice maintained a tighter BBB than wild-type mice. VWF(-/-) mice fared worse in both conditions, with ≈ 100% of VWF(-/-) mice dying within 120 minutes after pilocarpine administration, whereas >80% of wild-type animals survived. Investigation into the status of tight junction proteins revealed that VWF(-/-) mice expressed more claudin-5 at baseline. In vitro work confirmed that the presence of subendothelial VWF is inhibitory to claudin-5 expression.

CONCLUSIONS

VWF deficiency confers partial preservation of BBB integrity after hypoxia/reoxygenation and seizures. Surprisingly, this decrease in BBB permeability did not result in protection of animals because they demonstrated more severe pathology in both models compared with wild-type animals. These data suggest that a rigid BBB is detrimental (to the organism) during certain disease states and that VWF release may provide desired flexibility under stress.

Venous hemodynamics in neurological disorders: an analytical review with hydrodynamic analysis

Venous abnormalities contribute to the pathophysiology of several neurological conditions. This paper reviews the literature regarding venous abnormalities in multiple sclerosis (MS), leukoaraiosis, and normal-pressure hydrocephalus (NPH). The review is supplemented with hydrodynamic analysis to assess the effects on cerebrospinal fluid (CSF) dynamics and cerebral blood flow (CBF) of venous hypertension in general, and chronic cerebrospinal venous insufficiency (CCSVI) in particular.CCSVI-like venous anomalies seem unlikely to account for reduced CBF in patients with MS, thus other mechanisms must be at work, which increase the hydraulic resistance of the cerebral vascular bed in MS. Similarly, hydrodynamic changes appear to be responsible for reduced CBF in leukoaraiosis. The hydrodynamic properties of the periventricular veins make these vessels particularly vulnerable to ischemia and plaque formation.Venous hypertension in the dural sinuses can alter intracranial compliance. Consequently, venous hypertension may change the CSF dynamics, affecting the intracranial windkessel mechanism. MS and NPH appear to share some similar characteristics, with both conditions exhibiting increased CSF pulsatility in the aqueduct of Sylvius.CCSVI appears to be a real phenomenon associated with MS, which causes venous hypertension in the dural sinuses. However, the role of CCSVI in the pathophysiology of MS remains unclear.

Triggers and effectors of oxidative stress at blood-brain barrier level: relevance for brain ageing and neurodegeneration

As fundamental research advances, it is becoming increasingly clear that a clinically expressed disease implies a mixture of intertwining molecular disturbances. Oxidative stress is one of such pathogenic pathways involved in virtually all central nervous system pathologies, infectious, inflammatory, or degenerative in nature. Since brain homeostasis largely depends on integrity of blood-brain barrier (BBB), many studies focused lately on BBB alteration in a wide spectrum of brain diseases. The proper two-way molecular transfer through BBB depends on several factors, including the functional status of its tight junction (TJ) complexes of proteins sealing neighbour endothelial cells. Although there is abundant experimental work showing that oxidative stress associates BBB permeability alteration, less is known about its implications, at molecular level, in TJ protein expression or TJ-related cell signalling. In this paper, oxidative stress is presented as a common pathway for different brain pathogenic mechanisms which lead to BBB dysregulation. We revise here oxidative-induced molecular mechanisms of BBB disruption and TJ protein expression alteration, in relation to ageing and neurodegeneration.

Blood-brain barrier integrity and glial support: mechanisms that can be targeted for novel therapeutic approaches in stroke

The blood-brain barrier (BBB) is a critical regulator of brain homeostasis. Additionally, the BBB is the most significant obstacle to effective CNS drug delivery. It possesses specific charcteristics (i.e., tight junction protein complexes, influx and efflux transporters) that control permeation of circulating solutes including therapeutic agents. In order to form this "barrier," brain microvascular endothelial cells require support of adjacent astrocytes and microglia. This intricate relationship also occurs between endothelial cells and other cell types and structures of the CNS (i.e., pericytes, neurons, extracellular matrix), which implies existence of a "neurovascular unit." Ischemic stroke can disrupt the neurovascular unit at both the structural and functional level, which leads to an increase in leak across the BBB. Recent studies have identified several pathophysiological mechanisms (i.e., oxidative stress, activation of cytokine-mediated intracellular signaling systems) that mediate changes in the neurovascular unit during ischemic stroke. This review summarizes current knowledge in this area and emphasizes pathways (i.e., oxidative stress, cytokine-mediated intracellular signaling, glial-expressed receptors/targets) that can be manipulated pharmacologically for i) preservation of BBB and glial integrity during ischemic stroke and ii) control of drug permeation and/or transport across the BBB. Targeting these pathways present a novel opportunity for optimization of CNS delivery of therapeutics in the setting of ischemic stroke.

Histone deacetylase inhibitors as therapeutic agents for acute central nervous system injuries

Histone deacetylase (HDAC) inhibitors are emerging as a novel class of potentially therapeutic agents for treating acute injuries of the central nervous system (CNS). In this review, we summarize data regarding the effects of HDAC inhibitor administration in models of acute CNS injury and discuss issues warranting clinical trials. We have previously shown that the pan-HDAC inhibitor ITF2357, a compound shown to be safe and effective in humans, improves functional recovery and attenuates tissue damage when administered as late as 24 h after injury. Using a well-characterized, clinically relevant mouse model of closed head injury, we demonstrated that a single dose of ITF2357 administered 24 h after injury improves neurobehavioral recovery and reduces tissue damage. ITF2357-induced functional improvement was found to be sustained up to 14 d after trauma and was associated with augmented histone acetylation. Single postinjury administration of ITF2357 also attenuated injury-induced inflammatory responses, as indicated by reduced glial accumulation and activation as well as enhanced caspase-3 expression within microglia/macrophages after treatment. Because no specific therapeutic intervention is currently available for treating brain trauma patients, the ability to affect functional outcome by postinjury administration of HDAC inhibitors within a clinically feasible timeframe may be of great importance. Furthermore, a growing body of evidence indicates that HDAC inhibitors are beneficial for treating various forms of acute CNS injury including ischemic and hemorrhagic stroke. Because HDAC inhibitors are currently approved for other use, they represent a promising new avenue of treatment, and their use in the setting of CNS injury warrants clinical evaluation.

Protein kinase C activation modulates reversible increase in cortical blood-brain barrier permeability and tight junction protein expression during hypoxia and posthypoxic reoxygenation

Hypoxia (Hx) is a component of many disease states including stroke. Ischemic stroke occurs when there is a restriction of cerebral blood flow and oxygen to part of the brain. During the ischemic, and subsequent reperfusion phase of stroke, blood-brain barrier (BBB) integrity is lost with tight junction (TJ) protein disruption. However, the mechanisms of Hx and reoxygenation (HR)-induced loss of BBB integrity are not fully understood. We examined the role of protein kinase C (PKC) isozymes in modifying TJ protein expression in a rat model of global Hx. The Hx (6% O(2)) induced increased hippocampal and cortical vascular permeability to 4 and 10 kDa dextran fluorescein isothiocyanate (FITC) and endogenous rat-IgG. Cortical microvessels revealed morphologic changes in nPKC-θ distribution, increased nPKC-θ and aPKC-ζ protein expression, and activation by phosphorylation of nPKC-θ (Thr538) and aPKC-ζ (Thr410) residues after Hx treatment. Claudin-5, occludin, and ZO-1 showed disrupted organization at endothelial cell margins, whereas Western blot analysis showed increased TJ protein expression after Hx. The PKC inhibition with chelerythrine chloride (5 mg/kg intraperitoneally) attenuated Hx-induced hippocampal vascular permeability and claudin-5, PKC (θ and ζ) expression, and phosphorylation. This study supports the hypothesis that nPKC-θ and aPKC-ζ signaling mediates TJ protein disruption resulting in increased BBB permeability.

Acetylcholine receptor agonist reduces brain damage induced by hypoxia-ischemia in newborn rats

OBJECTIVE

The newborn rat model has been developed to elucidate the mechanism and management of perinatal brain damage. Our study hypothesis is that an acetylcholine receptor agonist (carbachol) reduces hypoxia-ischemia (HI)-induced brain damage in a well-established newborn rat model.

STUDY DESIGN

7-day-old Wistar rats were divided into 3 groups at random: carbachol preinjection and HI (Carb/HI), saline preinjection and HI (Saline/HI), and only HI (HI). Rats were subjected to left carotid artery ligation followed by 2 hours of hypoxia (8% oxygen). We injected carbachol or saline before hypoxic loading. After 7 days, we checked for brain damage.

RESULTS

In the cerebral cortex, 25% of the Carb/HI group showed mild neural damage, and the remaining 75% showed no damage. In contrast, more than 80% of the Saline/HI and HI groups had severe neural damage. Similarly, neural damage significantly decreased in Carb/HI compared with Saline/HI and HI for CA1, CA2, CA3, and the dentate gyrus of hippocampal regions.

CONCLUSION

Acetylcholine receptor agonist has a potent effect by reducing perinatal brain damage induced by HI in newborn rats.

Oxygen resuscitation does not ameliorate neonatal hypoxia/ischemia-induced cerebral edema

Neonatal hypoxia/ischemia (HI) is a common cause of cognitive and behavioral deficits in children with hyperoxia treatment (HHI) being the current therapy for newborn resuscitation. HI induces cerebral edema that is associated with poor neurological outcomes. Our objective was to characterize cerebral edema after HI and determine the consequences of HHI (40% or 100% O(2)). Dry weight analyses showed cerebral edema 1 to 21 days after HI in the ipsilateral cortex; and 3 to 21 days after HI in the contralateral cortex. Furthermore, HI increased blood-brain barrier (BBB) permeability 1 to 7 days after HI, leading to bilateral cortical vasogenic edema. HHI failed to prevent HI-induced increase in BBB permeability and edema development. At the molecular level, HI increased ipsilateral, but not contralateral, AQP4 cortical levels at 3 and up to 21 days after HI. HHI treatment did not further affect HI-induced changes in AQP4. In addition, we observed developmental increases of AQP4 accompanied by significant reduction in water content and increase permeability of the BBB. Our results suggest that the ipsilateral HI-induced increase in AQP4 may be beneficial and that its absence in the contralateral cortex may account for edema formation after HI. Finally, we showed that HI induced impaired motor coordination 21 days after the insult and HHI did not ameliorate this behavioral outcome. We conclude that HHI treatment is effective as a resuscitating therapy, but does not ameliorate HI-induced cerebral edema and impaired motor coordination.

Oxidative stress increases blood-brain barrier permeability and induces alterations in occludin during hypoxia-reoxygenation

The blood-brain barrier (BBB) has a critical role in central nervous system homeostasis. Intercellular tight junction (TJ) protein complexes of the brain microvasculature limit paracellular diffusion of substances from the blood into the brain. Hypoxia and reoxygenation (HR) is a central component to numerous disease states and pathologic conditions. We have previously shown that HR can influence the permeability of the BBB as well as the critical TJ protein occludin. During HR, free radicals are produced, which may lead to oxidative stress. Using the free radical scavenger tempol (200 mg/kg, intraperitoneal), we show that oxidative stress produced during HR (6% O(2) for 1 h, followed by room air for 20 min) mediates an increase in BBB permeability in vivo using in situ brain perfusion. We also show that these changes are associated with alterations in the structure and localization of occludin. Our data indicate that oxidative stress is associated with movement of occludin away from the TJ. Furthermore, subcellular fractionation of cerebral microvessels reveals alterations in occludin oligomeric assemblies in TJ associated with plasma membrane lipid rafts. Our data suggest that pharmacological inhibition of disease states with an HR component may help preserve BBB functional integrity.