Lipid-exchange Rate Assay for Lipid Droplet Fusion in Live Cells

Lipid droplets (LDs) are central organelles in maintaining lipid homeostasis. Defective LD growth often results in the development of metabolic disorders. LD fusion and growth mediated by cell death-inducing DNA fragmentation factor alpha (DFFA)-like effector (CIDE) family proteins are crucial for various biological processes including unilocular LD formation in the adipocytes, lipid storage in the liver, milk lipid secretion in the mammary epithelia cells, and lipid secretion in the skin sebocytes. Previous methodology by Gong et al. (2011) first reported a lipid-exchange rate assay to evaluate the fusion ability of each LD pair in the cells mediated by CIDE family proteins and their regulators, but photobleaching issue remains a problem and a detailed procedure was not provided. Here, we provide an improved and detailed protocol for the lipid-exchange rate measurement. The three key steps for this assay are cell preparation, image acquisition, and data analysis. The images of the fluorescence recovery are acquired after photobleaching followed by the measurement of the intensity changes in the LD pair. The difference in fluorescent intensity is used to obtain the lipid exchange rate between the LDs. The accuracy and repetitiveness of the calculated exchange rates are assured with three-cycle of photobleaching process and the linear criteria in data fitting. With this quantitative assay, we are able to identify the functional roles of the key proteins and the effects of their mutants on LD fusion.