Identification of p58^IPK as a novel neuroprotective factor for retinal neurons

PURPOSE. Endoplasmic reticulum (ER)-resident chaperone protein p58^IPK plays a vital role in regulation of protein folding and biosynthesis. The goal of this study was to examine the role of p58^IPK in retinal neuronal cells under normal and stressed conditions. METHODS. Retinal expression of p58^IPK, retinal morphology, apoptosis, ER stress, and apoptotic gene expression were examined in p58^IPK knockout (KO) and/or wild-type (WT) mice with or without intravitreal injection of N-methyl-D-aspartic acid (NMDA). In in vitro experiments, differentiated R28 retinal neuronal cells transduced with adenovirus encoding p58^IPK (Ad-p58^IPK) or control virus (Ad-LacZ) were exposed to tunicamycin (TM) or hydrogen peroxide (H₂O₂). Levels of ER stress, apoptosis, and cell survival were evaluated. RESULTS. Chaperone protein p58^IPK is expressed predominantly in retinal ganglion cells (RGC), inner retinal neurons, and the photoreceptor inner segments. Mice lacking p58^IPK exhibited increased CHOP expression and loss of RGCs with aging (8–10 months). Intravitreal injection of NMDA induced retinal ER stress and increased p58^IPK expression in WT mice; this resulted in greater ER stress and enhanced RGC apoptosis in p58^IPK KO mice. In cultured R28 cells, overexpression of p58^IPK significantly reduced eIF2a phosphorylation, decreased CHOP expression, and alleviated the activation of caspase-3 and PARP. Overexpression of p58^IPK also protected against oxidative and ER stress-induced cell apoptosis. Furthermore, p58^IPK downregulated the proapoptotic gene Bax and upregulated the antiapoptotic gene Bcl-2 expression in stressed R28 cells. CONCLUSIONS. Our study has demonstrated a protective role of p58^IPK in retinal neurons, which may act in part through a mechanism involving modulation of ER homeostasis and apoptosis, particularly under conditions of cellular stresses.