Pituitary Adenylate Cyclase‐Activating Polypeptide (PACAP) Pretreatment Improves Blood–Brain Barrier (BBB) Integrity in Sepsis by Activating the Wnt3a/β‐Catenin/Claudin‐1 Signaling Pathway

ABSTRACT The augmentation of blood–brain barrier (BBB) permeability is a key pathological characteristic of sepsis‐associated encephalopathy (SAE). Pituitary adenylate cyclase‐activating polypeptide (PACAP), a well‐known neuropeptide with potent anti‐inflammatory properties, has not yet been investigated for its potential beneficial effects in SAE. In this study, we aimed to explore this aspect. First, the administration of PACAP inhibited the expression of the pro‐inflammatory cytokines interleukin‐1β (IL‐1β) and interleukin‐8 (IL‐8) and mitigated brain vascular injury by reducing the levels of intercellular adhesion molecule‐1 (ICAM‐1) and vascular cell adhesion molecule‐1 (VCAM‐1) in the brains of septic mice. Importantly, PACAP preserved BBB integrity against lipopolysaccharide (LPS)‐induced disruption in septic mice by increasing both the mRNA and protein levels of Claudin‐1. In an in vitro cell culture study, we found that PACAP alleviated the exacerbation of endothelial permeability and increased transendothelial electrical resistance (TEER) against LPS‐induced disruption in bEnd.3 brain endothelial cells, accompanied by an upregulation of Claudin‐1. Consistently, PACAP protected against the reduction of Wnt3a and β‐catenin in response to LPS treatment in bEnd.3 brain endothelial cells. Notably, the knockdown of Wnt3a abolished the beneficial effects of PACAP in reducing endothelial permeability and restoring TEER levels, suggesting that the protective actions of PACAP are mediated through the Wnt3a/β‐catenin signaling pathway. Collectively, these findings demonstrate that PACAP exerts protective effects on BBB integrity in SAE by modulating the Wnt3a/β‐catenin/Claudin‐1 signaling pathway.