Activation of ER stress mediated by HspB4/Cryaa reduction contributes to photoreceptor apoptosis in rd9 mouse model of X-linked Retinitis Pigmentosa

X-linked Retinitis Pigmentosa (XLRP) is an inherited retinopathy predominantly caused by RPGR mutations, ultimately leading to photoreceptor apoptosis and vision loss. Although the genetic basis is well established, the downstream pathogenic and molecular mechanisms remain inadequately elucidated. The aim of this study is to explore the pathogenesis from the perspective of endoplasmic reticulum (ER) stress, which was previously underexplored. Using rd9 mouse retinas and mouse embryonic fibroblast (MEF) cells, we confirmed the presence of ER stress, activation of the unfolded protein response (UPR), and photoreceptor apoptosis through Western blot and immunofluorescence (IF) analyses. Furthermore, knockdown of Cryaa in 661W cells directly induced ER stress and UPR activation. Transmission electron microscopy (TEM) photomicrographs revealed significant ultrastructural alterations in ER and mitochondria within rd9 retinas and MEF cells. Proteomic analyses of rd9 retinas identified significantly enriched pathways including UPR, protein folding, and unfolded protein binding, associated with Cryaa downregulation. Collectively, our results identify Cryaa as a critical regulator of ER stress and demonstrate that its reduction promotes UPR activation and subsequent photoreceptor apoptosis in the rd9 model. These findings reveal, for the first time, a key role for Cryaa in XLRP pathology and may provide a novel therapeutic perspective for this disease.