Lipid droplets (LDs) are organelles that play a major role in regulating the storage of neutral lipids. Dysregulation of LDs is associated with metabolic disorders, such as fatty liver diseases, obesity, diabetes, and atherosclerosis. We have developed LD-selective small-molecule fluorescence probes (probes 3 and 4) that are available for both one-and two-photon microscopy, employing live or fixed cells. We found that probes 3 and 4 sensitively detect the increased LDs in response to oleic acid or endoplasmic reticulum stress, both in cells and tissues of the liver. The narrow absorption and emission bands of probes 3 and 4 allow multicolor imaging for the study of the role of LDs in pathophysiology and LD-associated signaling by the coapplication of the probes for different organelles or antibodies against specific proteins. In addition, we show here, for the first time, that two-photon microscopy imaging using our LD-selective probes with LysoTracker provides a novel method for screening drugs to potentially induce steatosis and/or phospholipidosis.L ipid droplets (LDs), known as lipid bodies or adiposomes, consist of neutral lipid composed mainly of triacylglycerol and cholesterylester, surrounded by a phospholipid monolayer. 1 LDs are produced from the triglycerides in the endoplasmic reticulum (ER), the primary site of lipid metabolism. 2 Previously, LDs were considered as inert and static aggregates of neutral lipids; however, they are now regarded as the organelles involved in many physiological processes, including membrane synthesis, trafficking, and protein degradation, beyond lipid storage. 3 LDs can associate with most cellular organelles, and the interactions between LDs and other organelles, as well as LD biogenesis and degradation, are tightly coupled to cellular metabolism. 4 Dysregulation of LDs is considered to be associated with metabolic disorders, such as fatty liver diseases (FLDs), obesity, diabetes, and atherosclerosis. 4 Recent studies have shown that pharmacological ER stress inducers, including tunicamycin and thapsigargin, increase de novo lipogenesis and LD formation in hepatocytes. 5 In addition, unresolved ER stress induces the dysregulation of hepatic lipid metabolism, suggesting that ER stress and lipid metabolism are tightly associated. 6 Small-molecule fluorescent probes are an effective approach for the tracking of LDs. However, currently used fluorescent probes for LDs, including Nile Red, LipidTox, and BODIPY 493/503, have limitations. Nile Red nondiscriminately labels cellular organelles, leading to low LD selectivity. In addition, Nile Red is not suitable for multicolor imaging because of its broad emission spectrum in the cells. 7 LipidTox is optimized for fixed cells, rather than live cells. 8 BODIPY 493/503 is also limited in the accurate imaging of LDs owing to its short Stokes shift, significant background signal, and low photostability. 8,9 Two-photon microscopy (TPM) indicates the abilities of biological samples to minimize autofluorescence and photodamage, while yie...
