Photoreceptor outer segments promote the accumulation of lipid droplets in microglia by reprogramming lipid metabolism

Abstract

Retinal degeneration (RD) ultimately leads to blindness owing to progressive photoreceptor loss. Previous studies have shown that microglia play a key role in photoreceptor degeneration through multiple phenotypes. However, the determinants of their phenotypic regulation remain unclear. As microglia migrate into the subretinal space and engulf photoreceptor outer segments (POSs), we aim to investigate how POSs affect microglial activation and the relevant pathways. A classic mouse microglial cell line (BV2) was used to explore the effect of POS exposure on microglia activation. Transcriptomic and lipidomic analyses were used to investigate the mechanisms by which POS treatment influenced microglial lipid metabolism. A classic RD model induced by sodium iodate (SI) was used to confirm the microglial phenotype activated by POS in vivo. Using a mouse microglial cell line (BV2), we found that POS exposure induced a novel microglial state distinguished by a distinctive morphological feature, a specific transcriptional signature, and a distinct lipidomic profile compared with the LPS and untreated groups. BV2 cells with prolonged POS exposure showed significantly different lipid metabolic pathways, particularly genes associated with cholesterol metabolism. This lipidomic analysis further identified a parallel increase in free fatty acid (FFA) and cholesterol ester (CE) levels, with FFA-22:6 (DHA) and CE-22:6 being the most upregulated, a key metabolic transition that might play a crucial role in promoting lipid droplet (LD) formation in POS-treated microglia. Consistent with these findings, LD accumulation was also observed in the microglia located in the photoreceptor layer of the SI-induced retinal degeneration model. Functionally, POS-treated microglia showed the suppression of interferon-responsive transcriptional programs and anti-inflammatory effects, and exhibited impaired engulfment capabilities while maintaining normal lysosomal function. Taken together, these findings reveal a lipid-mediated mechanism that drives microglial phenotypic switching and provide a more physiologically relevant in vivo model to mimic microglial activation in degenerative retinas than LPS stimulation.