Claudin-1-Dependent Destabilization of the Blood–Brain Barrier in Chronic Stroke

Recent evidence suggests that blood brain barrier (BBB) recovery and reestablishment of BBB impermeability after stroke is incomplete. This could influence stroke recovery, increase the risk of repeat stroke and be a solid substrate for developing vascular dementia. While accumulating evidence has defined morphological alterations and underlying mechanisms of tight junction (TJ) changes during BBB breakdown in acute stroke, very little is known about the type of alterations and mechanisms in BBB “leakage" found sub-acutely or chronically. The current study examined BBB structural alterations during the “BBB leakage” associated with the chronic phase of stroke in male mice and both genders human. We found significant upregulation of claudin-1 mRNA and protein, a nonspecific claudin for blood vessels, and downregulation in claudin-5 expression. Morphological and biochemical as well as FRET and FRAP analysis of post-ischemic brain endothelial cells and cells overexpressing claudin-1 indicated that newly synthesized claudin-1 was present on the cell membrane (∼45%), incorporated into the TJ complex with established interaction with ZO-1 and building homophilic cis- and trans-interactions. The appearance of claudin-1 in the TJ complex reduced claudin-5 strands (homophilic claudin-5 cis- and trans-interactions) and claudin-5/ZO-1 interaction affecting claudin-5 incorporation into the TJ complex. Moreover, claudin-1 induction was associated with an endothelial proinflammatory phenotype. Targeting claudin-1 with a specific C1C2 peptide improved brain endothelial barrier permeability and functional recovery in chronic stroke condition. This study highlights a potential “defect” in post-ischemic barrier formation, which may underlie prolonged vessel leakiness. Significance statement While rarely expressed at the normal blood-brain barrier (BBB), claudin-1 is expressed in pathological conditions. Analyzing post-stroke human and mouse blood microvessels we have identified that claudin-1 is highly expressed in leaky brain microvessels. Our results reveal that claudin-1 is incorporated in BBB tight junction complex, impeding BBB recovery and causing BBB leakiness during post-stroke recovery. Targeting claudin-1 with a claudin-1 peptide improves brain endothelial barrier permeability and consequently functional neurological recovery after stroke.