Role of P-glycoprotein in attenuation of retinal amyloid-β peptide (1–40) clearance across the rat inner blood-retinal barrier induced by lipopolysaccharide

In the retina, the accumulation of amyloid-β peptide (Aβ)(1–40) has been implicated in the development of inflammation-associated retinal diseases, highlighting the importance of understanding clearance mechanisms of retinal Aβ(1–40). This study aimed to elucidate the efflux transport of Aβ(1–40) across the inner blood-retinal barrier (BRB) under normal and inflammatory conditions. To assess inner BRB-mediated efflux, transport of fluorescein-human Aβ(1–40) (FL-hAβ(1–40)) into the lumen of freshly isolated rat retinal capillaries was examined. The luminal FL-hAβ(1–40) accumulation was significantly reduced by inhibitors of P-glycoprotein (P-gp) or low-density lipoprotein receptor-related protein-1 (LRP1), suggesting involvement of these molecules in retinal Aβ(1–40) efflux across the inner BRB. Moreover, FL-hAβ(1–40) accumulation was shown to decrease in retinal capillaries isolated from in vivo lipopolysaccharide (LPS)-treated rats or normal rat retinal capillaries exposed to LPS in vitro. These results indicate that extracellular LPS causes reduction in inner BRB-mediated Aβ(1–40) efflux. Although the attenuation of luminal FL-hAβ(1–40) accumulation by in vitro LPS treatment was not significantly altered in the presence of a P-gp inhibitor, co-treatment with an LRP1 inhibitor and LPS resulted in a further reduction in FL-hAβ(1–40) transport compared with LPS only-treated retinal capillaries. These results suggest that LPS-induced reduction of Aβ(1–40) transport is mainly derived from attenuation of P-gp but not LRP1. Furthermore, the reduction in P-gp-mediated FL-hAβ(1–40) transport by LPS was restored by inhibition of toll-like receptor 4 (TLR4) and protein kinase C (PKC). Collectively, our study findings indicate that P-gp-mediated Aβ(1–40) transport is impaired via activation of the TLR4/PKC signaling pathway during LPS-induced inflammatory conditions.