The blood-brain barrier (BBB) is the specialized system of brain microvascular endothelial cells (BMVEC) that shields the brain from toxic substances in the blood, supplies brain tissues with nutrients, and filters harmful compounds from the brain back to the bloodstream. The close interaction between BMVEC and other components of the neurovascular unit (astrocytes, pericytes, neurons, and basement membrane) ensures proper function of the central nervous system (CNS). Transport across the BBB is strictly limited through both physical (tight junctions) and metabolic barriers (enzymes, diverse transport systems). A functional polarity exists between the luminal and abluminal membrane surfaces of the BMVEC. As a result of restricted permeability, the BBB is a limiting factor for the delivery of therapeutic agents into the CNS. BBB breakdown or alterations in transport systems play an important role in the pathogenesis of many CNS diseases (HIV-1 encephalitis, Alzheimer's disease, ischemia, tumors, multiple sclerosis, and Parkinson's disease). Proinflammatory substances and specific disease-associated proteins often mediate such BBB dysfunction. Despite seemingly diverse underlying causes of BBB dysfunction, common intracellular pathways emerge for the regulation of the BBB structural and functional integrity. Better understanding of tight junction regulation and factors affecting transport systems will allow the development of therapeutics to improve the BBB function in health and disease.

Traumatic brain injury (TBI) is a major cause of death in the young age group and leads to persisting neurological impairment in many of its victims. It may result in permanent functional deficits because of both primary and secondary damages. This review addresses the role of oxidative stress in TBI-mediated secondary damages by affecting the function of the vascular unit, changes in blood-brain barrier (BBB) permeability, posttraumatic edema formation, and modulation of various pathophysiological factors such as inflammatory factors and enzymes associated with trauma. Oxidative stress plays a major role in many pathophysiologic changes that occur after TBI. In fact, oxidative stress occurs when there is an impairment or inability to balance antioxidant production with reactive oxygen species (ROS) and reactive nitrogen species (RNS) levels. ROS directly downregulate proteins of tight junctions and indirectly activate matrix metalloproteinases (MMPs) that contribute to open the BBB. Loosening of the vasculature and perivascular unit by oxidative stress-induced activation of MMPs and fluid channel aquaporins promotes vascular or cellular fluid edema, enhances leakiness of the BBB, and leads to progression of neuroinflammation. Likewise, oxidative stress activates directly the inflammatory cytokines and growth factors such as IL-1β, tumor necrosis factor-α (TNF-α), and transforming growth factor-beta (TGF-β) or indirectly by activating MMPs. In another pathway, oxidative stress-induced degradation of endothelial vascular endothelial growth factor receptor-2 (VEGFR-2) by MMPs leads to a subsequent elevation of cellular/serum VEGF level. The decrease in VEGFR-2 with a subsequent increase in VEGF-A level leads to apoptosis and neuroinflammation via the activation of caspase-1/3 and IL-1β release.

We investigate the hypothesis that oxidative damage of the cerebral vascular barrier interface (the blood brain barrier, BBB) causes the development of mild traumatic brain injury (mTBI) during primary blast wave spectrum. The underlying biochemical and cellular mechanisms of this vascular layer-structure injury are examined in a novel animal model of shock tube. We first established that low frequency (123 kPa) single or repeated shock wave causes BBB/brain injury through biochemical activation by acute mechanical force that occurs at 6–24 hrs after the exposure. This biochemical damage of the cerebral vasculature is initiated by the induction of free radical generating enzymes NADPH oxidase (NOX1) and inducible nitric oxide synthase (iNOS). Induction of these enzymes by shock wave exposure correlated well with the signatures of oxidative and nitrosative damage (4HNE/3NT) and reduction of the BBB tight junction (TJ) proteins occludin, claudin-5 and zonula occluden 1 (ZO-1) in the brain microvessel. In parallel with TJ protein disruption, the perivascular unit was significantly diminished by single or repeated shock wave exposure coinciding with the kinetic profile. Loosening of the vasculature and perivascular unit was mediated by oxidative stress-induced activation of matrix metalloproteinases and fluid channel aquaporin-4, promoting vascular fluid cavitation/edema, enhanced leakiness of the BBB and progression of neuroinflammation. The BBB leakiness and neuroinflammation were functionally demonstrated in an in vivo model by enhanced permeability of Na-Fl/EB low molecular weight tracers and the infiltration of immune cells across the BBB. The detection of brain cell matters NSE/S100β in the blood samples validated the neuro-astroglial injury in shock wave TBI. Our hypothesis that cerebral vascular injury occurring prior to the development of neurological disorders in mild TBI was further confirmed by the activation of caspase-3 and cell apoptosis mostly around the perivascular region. Thus, induction of oxidative stress and MMPs activation by shock wave underlies the mechanisms of cerebral vascular BBB leakage and neuroinflammation.

Brain microvascular endothelial cells (BMVEC) connected by tight junctions (TJ) form a tight monolayer at the blood-brain barrier (BBB). We investigated the idea that BBB dysfunction seen in alcohol abuse is associated with oxidative stress stemming from ethanol (EtOH) metabolism in BMVEC. Exposure to EtOH induced catalytic activity/expression of EtOH-metabolizing enzymes, which paralleled enhanced generation of reactive oxygen species (ROS). EtOH-mediated oxidative stress led to activation of myosin light chain (MLC) kinase, phosphorylation of MLC and TJ proteins, decreased BBB integrity, and enhanced monocyte migration across BBB. Acetaldehyde or ROS donors mimicked changes induced by EtOH in BMVEC. Thus, oxidative stress resulting from alcohol metabolism in BMVEC can lead to BBB breakdown in alcohol abuse, serving as an aggravating factor in neuroinflammatory disorders.

The blood-brain barrier (BBB) formed by brain microvascular endothelial cells (BMVEC) regulates the passage of molecules and leukocytes in and out of the brain. Oxidative stress is a major underlying cause of neurodegenerative and neuroinflammatory disorders and BBB injury associated with them. Using human BMVEC grown on porous membranes covered with basement membrane (BM) matrix (BBB models), we demonstrated that reactive oxygen species (ROS) augmented permeability and monocyte migration across BBB. ROS activated matrix metalloproteinases (MMP-1, -2, and -9) and decreased tissue inhibitors of MMPs (TIMP-1 and -2) in a protein tyrosine kinase (PTK)-dependent manner. Increase in MMPs and PTK activities paralleled degradation of BM protein and enhanced tyrosine phosphorylation of tight junction (TJ) protein. These effects and enhanced permeability/monocyte migration were prevented by inhibitors of MMPs, PTKs, or antioxidant suggesting that oxidative stress caused BBB injury via degradation of BM protein by activated MMPs and by PTKmediated TJ protein phosphorylation. These findings point to new therapeutic interventions ameliorating BBB dysfunction in neurological disorders such as stroke or neuroinflammation. Keywords: blood-brain barrier, brain endothelial cell, matrix metalloproteinases, oxidative stress, protein tyrosine kinase, protein tyrosine phosphatase.