Apolipoprotein E isoform-specific phase transitions in the retinal pigment epithelium drive disease phenotypes in age-related macular degeneration

The retinal pigment epithelium (RPE) is the site of initial damage leading to photoreceptor degeneration and vision loss in age-related macular degeneration (AMD). Genetic and histopathological studies implicate cholesterol dysregulation in AMD; yet mechanisms linking cholesterol to RPE injury and drusen formation remain poorly understood. Especially enigmatic are allelic variants of the cholesterol transporter APOE, major risk modifiers in Alzheimer9s disease that show reversed risk associations with AMD. Here, we investigated how ApoE isoforms modulate RPE health using live-cell imaging of primary RPE cultures and high-resolution imaging of human donor tissue. We show that the AMD-protective ApoE4 efficiently transports cholesterol and safeguards RPE homeostasis despite cellular stress. In contrast, ApoE2-expressing RPE accumulate cholesterol, which promotes autophagic deficits and complement-mediated mitochondrial fragmentation. Redox-related order-disorder phase transitions in ApoE2 drive the formation of intracellular biomolecular condensates as potential drusen precursors. Drugs that restore mitochondrial function limit condensate formation in ApoE2-RPE. Autophagic and mitochondrial defects correlate with intracellular ApoE aggregates in AMD donor RPE. Our study elucidates how AMD risk variants act as tipping points to divert the RPE from normal aging towards AMD by disrupting critical metabolic functions, and identifies mitochondrial stress-mediated aberrant phase transitions as a novel mechanism of drusen biogenesis.