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The mechanism and long-term consequences of early blood–brain barrier (BBB) disruption after cerebral ischaemic/reperfusion (I/R) injury are poorly understood. Here we discover that I/R induces subtle BBB leakage within 30–60 min, likely independent of gelatinase B/MMP-9 activities. The early BBB disruption is caused by the activation of ROCK/MLC signalling, persistent actin polymerization and the disassembly of junctional proteins within microvascular endothelial cells (ECs). Furthermore, the EC alterations facilitate subsequent infiltration of peripheral immune cells, including MMP-9-producing neutrophils/macrophages, resulting in late-onset, irreversible BBB damage. Inactivation of actin depolymerizing factor (ADF) causes sustained actin polymerization in ECs, whereas EC-targeted overexpression of constitutively active mutant ADF reduces actin polymerization and junctional protein disassembly, attenuates both early- and late-onset BBB impairment, and improves long-term histological and neurological outcomes. Thus, we identify a previously unexplored role for early BBB disruption in stroke outcomes, whereby BBB rupture may be a cause rather than a consequence of parenchymal cell injury., Matrix metalloproteinases (MMPs) released from infiltrating immune cells are a major contributor to blood-brain barrier (BBB) breakdown following stroke. Here, the authors identify an early, MMP-independent BBB breakdown mechanism caused by rapid cytoskeletal rearrangements in endothelial cells, which could be inhibited by ADF.

The mechanism and long-term consequences of early blood–brain barrier (BBB) disruption after cerebral ischaemic/reperfusion (I/R) injury are poorly understood. Here we discover that I/R induces subtle BBB leakage within 30–60 min, likely independent of gelatinase B/MMP-9 activities. The early BBB disruption is caused by the activation of ROCK/MLC signalling, persistent actin polymerization and the disassembly of junctional proteins within microvascular endothelial cells (ECs). Furthermore, the EC alterations facilitate subsequent infiltration of peripheral immune cells, including MMP-9-producing neutrophils/macrophages, resulting in late-onset, irreversible BBB damage. Inactivation of actin depolymerizing factor (ADF) causes sustained actin polymerization in ECs, whereas EC-targeted overexpression of constitutively active mutant ADF reduces actin polymerization and junctional protein disassembly, attenuates both early- and late-onset BBB impairment, and improves long-term histological and neurological outcomes. Thus, we identify a previously unexplored role for early BBB disruption in stroke outcomes, whereby BBB rupture may be a cause rather than a consequence of parenchymal cell injury.