Serum amyloid A-induced blood-brain barrier dysfunction associated with decreased claudin-5 expression in rat brain endothelial cells and its inhibition by high-density lipoprotein in vitro

The blood-brain barrier (BBB) is the multicellular interface located between the peripheral circulation and the brain parenchyma. BBB dysfunction is reported in many CNS diseases, such cognitive impairment, depression, Alzheimer's disease (AD), and multiple sclerosis (MS). Emerging evidence indicates that liver-derived inflammatory mediators are upregulated in neurological diseases with BBB dysfunction. Serum amyloid A (SAA), an acute phase protein secreted by hepatocytes, could be a candidate inflammatory signaling molecule transmitted from the liver to the brain; however, its contribution to BBB dysfunction is poorly understood. The present study aimed to elucidate the involvement of SAA in BBB impairment in an in vitro BBB model using rat brain microvascular endothelial cells (RBECs). We demonstrated that Apo-SAA significantly decreased transendothelial electrical resistance (TEER) and increased sodium fluorescein (Na-F) permeability in RBEC monolayers. Apo-SAA also decreased claudin-5 expression levels in RBECs. Furthermore, the Apo-SAA-mediated impairment of the BBB with decreased claudin-5 expression was inhibited by the addition of a high-density lipoprotein (HDL) related to SAA in plasma. These findings suggest that HDL counteracts the effects of SAA on BBB function. Therefore, the functional imbalance between SAA and HDL may induce BBB impairment, thereby triggering development of neuroinflammation. SAA could be a significant endogenous mediator in the liver-to-brain inflammation axis.