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TheE4variant ofAPOEstrongly predisposes individuals to late-onset Alzheimer’s disease. We demonstrate that in response to neutral lipid synthesis, apolipoprotein E (APOE) in astrocytes can avoid translocation into the ER lumen and traffic to lipid droplets (LDs) via membrane bridges at ER-LD contacts.APOEknockdown promotes fewer, larger LDs containing more unsaturated triglyceride. This LD size distribution phenotype was rescued by chimeric APOE that targets only LDs. APOE4-expressing astrocytes also form a small number of large LDs enriched in unsaturated triglyceride. Additionally, the larger LDs inAPOE4cells exhibit impaired turnover and increased sensitivity to lipid peroxidation. Our data indicate that APOE plays a previously unrecognized role as an LD surface protein that regulates LD size and composition.APOE4is a toxic gain of function variant that causes aberrant LD composition and morphology. We propose thatAPOE4astrocytes with large, unsaturated LDs are sensitized to lipid peroxidation or lipotoxicity, which could contribute to Alzheimer’s disease risk.SummaryWindhamet al. discover that APOE in astrocytes can traffic to lipid droplets (LDs), where it modulates LD composition and size. Astrocytes expressing the Alzheimer’s risk variant APOE4 form large LDs with impaired turnover and increased peroxidation sensitivity.

Rhabdomyolysis is a clinical emergency characterized by severe muscle damage resulting in the release of intracellular muscle components leading to myoglobinuria and in severe cases, acute kidney failure. Rhabdomyolysis is caused by genetic factors that are linked to increased disease susceptibility in response to extrinsic triggers. Recessive mutations inTANGO2result in episodic rhabdomyolysis, metabolic crises, encephalopathy and cardiac arrhythmia, the underlying mechanism contributing to disease onset in response to specific triggers remains unclear. To address these challenges, we created a zebrafish model of Tango2 deficiency. Here we show that loss of Tango2 in zebrafish results in growth defects, early lethality and increased susceptibility of muscle defects similar toTANGO2patients. Detailed analyses of skeletal muscle revealed defects in the sarcoplasmic reticulum and mitochondria at the onset of disease development. The sarcoplasmic reticulum (SR) constitutes the primary lipid biosynthesis site and regulates calcium handling in skeletal muscle to control excitation-contraction coupling. Tango2 deficient SR exhibits increased sensitivity to calcium release that was partly restored by inhibition of Ryr1-mediated Ca2+release in skeletal muscle. Using lipidomics, we identified alterations in the glycerolipid state oftango2mutants which is critical for membrane stability and energy balance. Therefore, these studies provide insight into key disease processes in Tango2 deficiency and increased our understanding of how specific defects can predispose to environmental triggers in TANGO2-related disorders.