Proteome of skeletal muscle lipid droplet reveals association with mitochondria and apolipoprotein A-I

The lipid droplet (LD) is a universal organelle governing the storage and turnover of neutral lipids. Mounting evidence indicates that elevated intramuscular triglyceride (IMTG) in skeletal muscle LDs is closely associated with insulin resistance and Type 2 Diabetes Mellitus (T2DM). Therefore, the identification of the skeletal muscle LD proteome will provide some clues to dissect the mechanism connecting IMTG with T2DM. In the present work, we identified 324 LD-associated proteins in mouse skeletal muscle LDs through mass spectrometry analysis. Besides lipid metabolism and membrane traffic proteins, a remarkable number of mitochondrial proteins were observed in the skeletal muscle LD proteome. Furthermore, imaging by fluorescence microscopy and transmission electronic microscopy (TEM) directly demonstrated that mitochondria closely adhere to LDs in vivo. Moreover, our results revealed for the first time that apolipoprotein A-I (apo A-I), the principal apolipoprotein of high density lipoprotein (HDL) particles, was also localized on skeletal muscle LDs. Further studies verified that apo A-I was expressed endogenously by skeletal muscle cells. In conclusion, we report the protein composition and characterization of skeletal muscle LDs and describe a novel LD-associated protein, apo A-I.