Brd4 BD1 Domain Antagonism of MS436 Preserves Blood‐Brain Barrier Integrity via Rnf43/β‐Catenin Signaling Pathway

Blood-brain barrier (BBB) disruption is central to neurodegenerative and cerebrovascular diseases, but its causal role and therapeutic targeting remain challenging. Bromodomain and extraterminal domain (BET) proteins, particularly Brd4, regulate transcription via dual bromodomains (BD1, BD2). While BD1 antagonists are explored for tumors and BD2 antagonists for inflammation, their impact on cerebrovascular integrity is unclear. Crucially, opposing BBB effects of domain-specific BET antagonism: the BD2 antagonist RVX208 disrupt the BBB, whereas the BD1 antagonist MS436 significantly reduces leakage and improves neurological outcomes, revealing therapeutic potential is demonstrated. Mechanistically, endothelial Brd4 critically suppresses BBB stability. Its ablation upregulates tight junction (TJ) proteins. Brd4, acting exclusively through its BD1 domain, destabilizes TJs via Rnf43 is identified, which promotes β-catenin and TJ protein degradation. This defines a novel Brd4 BD1/Rnf43/β-catenin axis essential for BBB integrity. The findings establish Brd4 BD1 inhibition as a novel BBB-protective strategy and position MS436 for repurposing in cerebrovascular diseases, necessitating reevaluation of domain-specific BET targeting for neurovascular pathologies.