The blood-brain barrier (BBB), a critical guardian of communication between the periphery and the brain, is frequently compromised in neurological diseases such as multiple sclerosis (MS), resulting in the inappropriate passage of molecules and leukocytes into the brain. Here we show that the glucocorticoid anti-inflammatory messenger annexin A1 (ANXA1) is expressed in brain microvascular endothelial cells, where it regulates BBB integrity. In particular, ANXA1 −/− mice exhibit significantly increased BBB permeability as a result of disrupted interendothelial cell tight junctions, essentially related to changes in the actin cytoskeleton, which stabilizes tight and adherens junctions. This situation is reminiscent of early MS pathology, a relationship confirmed by our detection of a selective loss of ANXA1 in the plasma and cerebrovascular endothelium of patients with MS. Importantly, this loss is swiftly restored by i.v. administration of human recombinant ANXA1. Analysis in vitro confirms that treatment of cerebrovascular endothelial cells with recombinant ANXA1 restores cell polarity, cytoskeleton integrity, and paracellular permeability through inhibition of the small G protein RhoA. We thus propose ANXA1 as a critical physiological regulator of BBB integrity and suggest it may have utility in the treatment of MS, correcting BBB function and hence ameliorating disease.T he presence of narrow and dense tight junctions between adjacent endothelial cells is peculiar to the cerebral vasculature, and their integrity is essential for the maintenance of correct blood-brain barrier (BBB) function as the primary regulator of cross-talk between the brain and the rest of the body (1). Increasing evidence indicates that the integrity of this structural and functional barrier is compromised in neurological conditions such as multiple sclerosis (MS), Alzheimer's, and Parkinson diseases, leading to the failure of the normal mechanisms controlling passage of substances into the brain (2) and to the sensitization and/or worsening of pathologic conditions. Pharmacological intervention to prevent or correct BBB alteration in such diseases is a difficult task, but potential therapeutic leads can be gained from the study of endogenous mediators regulating barrier integrity.Annexin A1 (ANXA1) is an important anti-inflammatory protein, principally known as a regulator of peripheral leukocyte migration and a promoter of macrophage phagocytosis (3). ANXA1 is expressed in several cell types within the brain, including ependyma and microglia, but in particular in the endothelium of the brain microvasculature (4), although its role in these cells remains obscure. We have previously shown glucocorticoids to up-regulate expression of ANXA1 in the cerebral endothelium (5), and, given that glucocorticoids enhance BBB tightness (6), we hypothesized that ANXA1 may play a role in the regulation of BBB permeability. Through combined in vitro and in vivo approaches, we have identified a dual role for ANXA1 in organizing the interendothelial cell...

Sex differences have been widely reported in neuroinflammatory disorders, focusing on the contributory role of estrogen. The microvascular endothelium of the brain is a critical component of the blood-brain barrier (BBB) and it is recognised as a major interface for communication between the periphery and the brain. As such, the cerebral capillary endothelium represents an important target for the peripheral estrogen neuroprotective functions, leading us to hypothesise that estrogen can limit BBB breakdown following the onset of peripheral inflammation.Comparison of male and female murine responses to peripheral LPS challenge revealed a short-term inflammation-induced deficit in BBB integrity in males that was not apparent in young females, but was notable in older, reproductively senescent females. Importantly, ovariectomy and hence estrogen loss recapitulated an aged phenotype in young females, which was reversible upon estradiol replacement. Using a well-established model of human cerebrovascular endothelial cells we investigated the effects of estradiol upon key barrier features, namely paracellular permeability, transendothelial electrical resistance, tight junction integrity and lymphocyte transmigration under basal and inflammatory conditions, modelled by treatment with TNFα and IFNγ. In all cases estradiol prevented inflammationinduced defects in barrier function, action mediated in large part through up-regulation of the central coordinator of tight junction integrity, annexin A1. The key role of this protein was then further confirmed in studies of human or murine annexin A1 genetic ablation models.

Annexin A1 is a potent anti-inflammatory molecule that has been extensively studied in the peripheral immune system, but has not as yet been exploited as a therapeutic target/agent. In the last decade, we have undertaken the study of this molecule in the central nervous system (CNS), focusing particularly on the primary interface between the peripheral body and CNS: the blood–brain barrier. In this review, we provide an overview of the role of this molecule in the brain, with a particular emphasis on its functions in the endothelium of the blood–brain barrier, and the protective actions the molecule may exert in neuroinflammatory, neurovascular and metabolic disease. We focus on the possible new therapeutic avenues opened up by an increased understanding of the role of annexin A1 in the CNS vasculature, and its potential for repairing blood–brain barrier damage in disease and aging.