Mehall, Bridget

Lipid droplets (LDs) are fat‐storage organelles that play critical roles in energy homeostasis, but can excessively accumulate in metabolic diseases such as obesity and fatty liver where hepatocytes become grossly engorged with excess lipid. The catabolism of LDs aids in disease resolution and occurs in part by lipophagy, a form of selective autophagy, but the molecular and vesicle trafficking events that facilitate this process are not fully understood. During lipophagy, autophagosomes are thought to target LDs prior to fusion with acidic lysosomes, where neutral lipids are terminally degraded by lysosomal acid lipase. Ultrastructural viewing of hepatocytes showing LDs contained within electron‐dense acidic vesicles is consistent with this concept. In addition to canonical autophagy, we find that most LDs undergoing degradation are encased within single‐membrane endo‐lysosomes, and few LDs are observed within traditional double‐membrane autophagosomes. Further, studies of LD catabolism in yeast have shown that LDs can be directly internalized by the large acidic vacuole through a process known as microlipophagy. Thus, the GOAL of this study was to test if similar microlipophagy mechanisms occur in mammalian cells such as hepatocytes, and whether this process involves autophagy. METHODS To test this, we used microscopic fatty acid tracing studies and time‐lapse confocal microscopy to determine the role of autophagosomes versus endo‐lysosomes in LD catabolism. To trace the fate of LDs into terminal degradative vesicles, cells were first loaded with the fluorescent lipid marker Bodipy C12, then treated for 24h with LAListat, an inhibitor of lysosomal acid lipase. RESULTS We observed a ~5‐fold higher incidence of LDs within endo‐lysosomes (marked with the tetraspanin CD63), compared with autophagosomes (marked by the autophagy protein LC3). Furthermore, siRNA knockdown of macroautophagy proteins (Atg5, Atg7, and Fip200) did not inhibit LD internalization by endo‐lysosomes, suggesting that autophagy is not a prerequisite for this process. In addition, time‐lapse microscopy revealed a direct internalization of cytosolic LDs by endo‐lysosomes positive for lysotracker or CD63‐mCherry. CONCLUSION These findings suggest that hepatocytes utilize a distinct form of microautophagy that entails the direct envelopment of cytoplasmic LDs by endocytic vesicles such as multivesicular bodies and lysosomes/late endosomes. In addition to lipolysis and canonical macroautophagy, this process plays an important role in the catabolic utilization of LDs by the hepatocyte. Support or Funding Information This study was supported by NIH R01DK044650 (to M.A.M.), RO1AA020735 (to M.A.M.), and the K99AA020735 (to M.B.S.).

Background and Aims Hepatocytes play a central role in storage and utilization of fat by the liver. Selective breakdown of lipid droplets (LDs) by autophagy (also called lipophagy) is a key process utilized to catabolize these lipids as an energy source. How the autophagic machinery is selectively targeted to LDs, where it mediates membrane engulfment and subsequent degradation, is unclear. Recently, we have reported that two distinct GTPases, the mechanoenzyme, dynamin2 (Dyn2), and the small regulatory Rab GTPase, Rab10, work independently at distinct steps of lipophagy in hepatocytes. Approach and Results In an attempt to understand how these proteins are regulated and recruited to autophagic organelles, we performed a nonbiased biochemical screen for Dyn2‐binding partners and found that Dyn2 actually binds Rab10 directly through a defined effector domain of Rab10 and the middle domain of Dyn2. These two GTPases can be observed to interact transiently on membrane tubules in hepatoma cells and along LD‐centric autophagic membranes. Most important, we found that a targeted disruption of this interaction leads to an inability of cells to trim tubulated cytoplasmic membranes, some of which extend from lipophagic organelles, resulting in LD accumulation. Conclusions This study identifies a functional, and direct, interaction between Dyn2 and a regulatory Rab GTPase that may play an important role in hepatocellular metabolism.