Microglial Phagocytosis of Bipolar Cells Triggers Inner Retinal Degeneration in Rs1-KO Mice

X-linked retinoschisis (XLRS), caused by Retinoschisin 1 (RS1) gene mutations, leads to progressive retinal degeneration. While photoreceptor loss is a hallmark of the disease, its early pathogenic mechanisms are not well understood. Using CRISPR/Cas9-edited Rs1-exon2-knockout mice, we investigated the temporal progression of the retinal degeneration. Single-cell RNA sequencing revealed that the degeneration of bipolar cells, particularly the OFF-cone subtype, precedes photoreceptor loss. This early degeneration was driven by microglial activation and their subsequent phagocytosis of bipolar cells via phosphatidylserine exposure and C3b activation that was independent of apoptosis. These findings reveal a temporal sequence of neurodegeneration in XLRS, highlighting bipolar cells as a critical therapeutic target for early intervention. Overall design: Rs1-exon2-knockout (Rs1-/y) mice were generated on a C57BL/6J background using CRISPR/Cas9 genome editing. Hemizygous Rs1-/y males were obtained by mating heterozygous (Rs1-/+)females with wild-type C57BL/6J males, and used for experiments at 8 and 24 weeks. Retina of the experimental animals were obtained and analysed using scRNA-seq.