Single‐Cell <scp>RNA</scp>‐Seq Reveals Aging‐Related Impairment of Microglial Efferocytosis Contributing to Apoptotic Cells Accumulation After Retinal Injury

Aging is associated with increased retinal cell apoptosis, which contributes to decreases in retinal function. Apoptotic retinal cell clearance relies on microglial efferocytosis, but the impact of aging on this process has not been fully elucidated. In this study, we aimed to shed light on this by using single-cell RNA sequencing (sc-RNA-seq) to compare young and aged mouse retinal transcriptional profiles, in which 74,412 retinal cells from young and aged mice were classified into 10 transcriptionally distinct retinal cell types, and differentially expressed genes between young versus aged retinas were mainly associated with cellular senescence and apoptosis. Furthermore, ligand-receptor interactions (e.g., AXL-GAS6, MERTK-GAS6) between microglia and other retinal cells were strengthened in aged, compared to young retinas. Additionally, among microglia, Subcluster 4 was found under partial clustering to be associated with efferocytosis, of which aged microglia had downregulated efferocytosis-associated genes. The impact of aging on microglial efferocytosis was further verified in vitro by doxorubicin (DOX)-induced senescent BV2 microglia, and in vivo by a retinal ischemia/reperfusion (I/R) injury mouse model. In vitro, DOX-treated BV2 microglia had significantly lowered efferocytosis, as well as efferocytosis-related MerTK and Axl protein expression; this was also present in vivo in aged retinas post-I/R injury, with increased co-localization of ionized calcium-binding adapter molecule 1⁺ microglia with apoptotic retinal cells, along with reduced efferocytosis-related protein expression. Overall, microglial efferocytosis of apoptotic cells decreased with aging, suggesting that modulating this process could serve as a possible therapeutic target for age-related retinal diseases.