Mitochondrial dynamics and degradation in the oleaginous yeast Lipomyces starkeyi

Growth conditions inducing G1 cell cycle arrest enhance lipid production in the oleaginous yeast Lipomyces starkeyi

Lipid droplets are cytoplasmic organelles that store lipids for energy and membrane synthesis. The oleaginous yeast Lipomyces starkeyi is one of the most promising lipid producers and has attracted attention as a biofuel source. It is known that the expansion of lipid droplets is enhanced under nutrient-poor conditions. Therefore, we prepared a novel nitrogen-depleted medium (N medium) in which to culture L. starkeyi cells. Lipid accumulation was rapidly induced, and this was reversed by the addition of ammonium. In this condition, cell proliferation stopped and cells with giant lipid droplets were arrested in G1 phase. We investigated whether cell cycle arrest at a specific phase is required for lipid accumulation. Lipid accumulation was repressed in hydroxyurea-synchronized S phase cells and was increased in nocodazole-arrested G2/M phase cells. Moreover, the enrichment of G1 phase cells by rapamycin induced massive lipid accumulation. From these results, we conclude that L. starkeyi cells store lipids from G2/M phase and then arrest cell proliferation in the subsequent G1 phase, where lipid accumulation is enhanced. Cell cycle control is an attractive approach for biofuel production.

Links between autophagy and lipid droplet dynamics

Autophagy is a catabolic process in which cytoplasmic components are delivered to vacuoles or lysosomes for degradation and nutrient recycling. Autophagy-mediated degradation of membrane lipids provides a source of fatty acids for the synthesis of energy-rich, storage lipid esters such as triacylglycerol (TAG). In eukaryotes, storage lipids are packaged into dynamic subcellular organelles, lipid droplets. In times of energy scarcity, lipid droplets can be degraded via autophagy in a process termed lipophagy to release fatty acids for energy production via fatty acid β-oxidation. On the other hand, emerging evidence suggests that lipid droplets are required for the efficient execution of autophagic processes. Here, we review recent advances in our understanding of metabolic interactions between autophagy and TAG storage, and discuss mechanisms of lipophagy. Free fatty acids are cytotoxic due to their detergent-like properties and their incorporation into lipid intermediates that are toxic at high levels. Thus, we also discuss how cells manage lipotoxic stresses during autophagy-mediated mobilization of fatty acids from lipid droplets and organellar membranes for energy generation.