Phosphorylation of alpha B-Crystallin Involves Interleukin-1 beta-Mediated Intracellular Retention in Retinal Muller Cells: A New Mechanism Underlying Fibrovascular Membrane Formation

PURPOSE. Chronic inflammation plays a pivotal role in the pathology of proliferative diabetic retinopathy (PDR), in which biological alterations of retinal glial cells are one of the key elements. The phosphorylation of alpha B-crystallin/CRYAB modulates its molecular dynamics and chaperone activity, and attenuates alpha B-crystallin secretion via exosomes. In this study, we investigated the effect of phosphorylated alpha B-crystallin in retinal Muller cells on diabetic mimicking conditions, including interleukin (IL)-1 beta stimuli. METHODS. Human retinal Muller cells (MIO-M1) were used to examine gene and protein expressions with real-time quantitative PCR, enzyme linked immunosorbent assay (ELISA), and immunoblot analyses. Cell apoptosis was assessed by Caspase-3/7 assay and TdT-mediated dUTP nick-end labeling staining. Retinal tissues isolated from the Spontaneously Diabetic Torii (SDT) fatty rat, a type 2 diabetic animal model with obesity, and fibrovascular membranes from patients with PDR were examined by double-staining immunofluorescence. RESULTS. CRYAB mRNA was downregulated in MIO-M1 cells with the addition of 10 ng/mL IL-1 beta; however, intracellular alpha B-crystallin protein levels were maintained. The alpha B-crystallin serine 59 (Ser59) residue was phosphorylated with IL-1 beta application in MIO-M1 cells. Cell apoptosis in MIO-M1 cells was induced by CRYAB knockdown. Immunoreactivity for Ser59-phosphorylated alpha B-crystallin and glial fibrillary acidic protein was colocalized in glial cells of SDT fatty rats and fibrovascular membranes. CONCLUSIONS. The Ser59 phosphorylation of alpha B-crystallin was modulated by IL-1 beta in Muller cells under diabetic mimicking inflammatory conditions, suggesting that alpha B-crystallin contributes to the pathogenesis of PDR through an anti-apoptotic effect.