Bacterial induction of Snail 1 contributes to blood-brain barrier disruption

2 4 7 3 jci.org Volume 125 Number 6 June 2015 Introduction Bacterial meningitis is the most common life-threatening infection of the CNS. To cause meningitis, blood-borne bacteria must penetrate the blood-brain barrier (BBB), which consists of an extremely specialized layer of brain microvascular endothelial cells (BMECs). The BBB separates the brain from circulating blood, and disruption of BBB integrity is a hallmark event in the pathophysiology of bacterial meningitis. This disruption is thought to result from the combined effect of bacterial entry and penetration of BMECs, direct cellular injury by bacterial cytotoxins, and/or activation of host inflammatory pathways that compromise BMEC barrier function (1). Streptococcus agalactiae, or group B Streptococcus (GBS), is a Gram-positive bacterial pathogen that is an important cause of invasive disease in newborns and a subset of adults (2). Currently, GBS is the leading cause of meningitis in the neonate (3, 4). Despite advances in intensive care management and antibiotic therapy, mortality can approach 10%, with 25% to 50% of surviving neonates exhibiting permanent neurological sequelae, including cerebral palsy, mental retardation, blindness, deafness, and seizure (2). GBS possesses many virulence factors that may contribute to the interaction with brain endothelium, including lipoteichoic acid (LTA) (5), β-hemolysin/cytolysin (β-H/C) (6), pili (7, 8), serine-rich repeat (Srr) proteins (9, 10), and HvgA (11). Recently, we have demonstrated that the GBS pilus protein PilA and Srr-1 interact with components of the host extracellular matrix (ECM) to promote BBB interactions and the development of meningitis (8, 9). Other meningeal pathogens, such as Streptococcus pneumoniae (SPN), E. coli K1, Neisseria meningitidis, and Haemophilus influenza type B (HiB, also bind ECM components and ECM receptors (e.g., integrins and laminin receptor) to mediate bacterial-BBB interactions (12–15). Given that host ECM components and receptors preferentially localize to the basolateral surface of polarized BBB endothelium (16), we hypothesized that disruption of junctional protein complexes in brain endothelium is the first step leading to bacterial access to basally expressed receptors. The BBB, composed primarily of a specialized layer of BMECs, separates the brain and its surrounding tissues from circulating blood, thereby maintaining CNS homeostasis (17). The brain endothelial cells are characterized by the presence of tight intercellular junctions that promote high transendothelial electrical resistance and therefore impede paracellular flux of macromolecules (18). In BMECs, tight junctions are composed of 4 types of integral membrane proteins: occludin, claudins, junctional adhesion molecules, and cell-selective adhesion molecules, all of which are linked through cytoplasmic proteins (zonala-occludin 1 [ZO-1], -2, -3, cingulin) to the actin cytoskeleton (19). Furthermore, tight junction integrity is important for the maintenance of apical and basal cell polarity (20). Here, we demonstrate for the first time to our knowledge that infection of brain endothelium Bacterial meningitis is a serious infection of the CNS that results when blood-borne bacteria are able to cross the blood-brain barrier (BBB). Group B Streptococcus (GBS) is the leading cause of neonatal meningitis; however, the molecular mechanisms that regulate bacterial BBB disruption and penetration are not well understood. Here, we found that infection of human brain microvascular endothelial cells (hBMECs) with GBS and other meningeal pathogens results in the induction of host transcriptional repressor Snail1, which impedes expression of tight junction genes. Moreover, GBS infection also induced Snail1 expression in murine and zebrafish models. Tight junction components ZO-1, claudin 5, and occludin were decreased at both the transcript and protein levels in hBMECs following GBS infection, and this repression was dependent on Snail1 induction. Bacteria-independent Snail1 expression was sufficient to facilitate tight junction disruption, promoting BBB permeability to allow bacterial passage. GBS induction of Snail1 expression was dependent on the ERK1/2/MAPK signaling cascade and bacterial cell wall components. Finally, overexpression of a dominant-negative Snail1 homolog in zebrafish elevated transcription of tight junction protein–encoding genes and increased zebrafish survival in response to GBS challenge. Taken together, our data support a Snail1-dependent mechanism of BBB disruption and penetration by meningeal pathogens. Bacterial induction of Snail1 contributes to blood-brain barrier disruption