Fatty acid trafficking to mitochondria and peroxisomes in Tetrahymena thermophila, a new frontier for lipid droplet biology.

Lipid droplets are increasingly recognized as necessary organelles. However, the cellular pathways that regulate lipid droplets have only been defined in select fungi, algae, plants, and animals. Our experiments expand the study of lipid droplets to an evolutionarily distinct model organism, the ciliate Tetrahymena thermophila. We identify conserved pathways that promote lipid droplet homeostasis while also uncovering features that suggest adaptation. We show that Tetrahymena accumulate lipid droplets in response to nutrient deprivation, including starvation and the stationary phase. Pulse-chase experiments with a fluorescent fatty acid analogue demonstrate lipid trafficking to lipid droplets in starved cultures. Unlike other cell types, starved Tetrahymena appear to use both peroxisomes and mitochondria (not vacuoles) for further fatty acid catabolism. We observe cooccurence of the fluorescent fatty acid analogue with markers of peroxisomes and a subpopulation of mitochondria, suggesting specialized catabolic roles for both organelles. We demonstrate a decrease in survival following starvation in the presence of inhibitors of mitochondrial fatty acid import or peroxisomal fatty acid metabolism. Together, our experiments add Tetrahymena to the expanding list of eukaryotes that increase lipid droplets in response to nutrient depletion while also uncovering important and distinct roles for mitochondrial and peroxisomal catabolism in survival pathways.