Abstract
Homeostasis of the retinal pigment epithelium (RPE) is essential for the health and proper function of the retina. Regulation of RPE homeostasis is, however, largely unexplored, yet dysfunction of this process may lead to retinal degenerative diseases, including age-related macular degeneration (AMD). Here, we report that chemokine receptor CXCR5 regulates RPE homeostasis through PI3K/AKT signaling and by suppression of FOXO1 activation. We used primary RPE cells isolated from CXCR5-deficient mice and wild type controls, as well as ex vivo RPE-choroidal-scleral complexes (RCSC) to investigate the regulation of homeostasis. CXCR5 expression in mouse RPE cells was diminished by treatment with hydrogen peroxide. Lack of CXCR5 expression leads to an abnormal cellular shape, pigmentation, decreased expression of the RPE differentiation marker RPE65, an increase in the undifferentiated progenitor marker MITF, and compromised RPE barrier function, as well as compromised cell-to-cell interaction. An increase in epithelial-mesenchymal transition (EMT) markers (αSMA, N-cadherin, and vimentin) was noted in CXCR5-deficient RPE cells both in vitro and in age-progression specimens of CXCR5-/- mice (6, 12, 24-months old). Deregulated autophagy in CXCR5-deficient RPE cells was observed by decreased LC3B-II, increased p62, abnormal autophagosomes, and impaired lysosome enzymatic activity as shown by GFP-LC3-RFP reporter plasmid. Mechanistically, deficiency in CXCR5 resulted in the downregulation of PI3K and AKT signaling, but upregulation and nuclear localization of FOXO1. Additionally, inhibition of PI3K in RPE cells resulted in an increased expression of FOXO1. Inhibition of FOXO1, however, reverts the degradation of ZO-1 caused by CXCR5 deficiency. Collectively, these findings suggest that CXCR5 maintains PI3K/AKT signaling, which controls FOXO1 activation, thereby regulating the expression of genes involved in RPE EMT and autophagy deregulation.