McNiven, Mark A.

ABSTRACT Lipophagy is a central cellular process for providing the cell with a readily utilized, high energy source of neutral lipids. Since its discovery over a decade ago, we are just starting to understand the molecular components that drive lipophagy, how it is activated in response to nutrient availability, and its potential as a therapeutic target in disease. In this Cell Science at a Glance article and the accompanying poster, we first provide a brief overview of the different structural and enzymatic proteins that comprise the lipid droplet (LD) proteome and reside within the limiting phospholipid monolayer of this complex organelle. We then highlight key players in the catabolic breakdown of LDs during the functionally linked lipolysis and lipophagy processes. Finally, we discuss what is currently known about macro- and micro-lipophagy based on findings in yeast, mammalian and other model systems, and how impairment of these important functions can lead to disease states.

Neutral lipids are a critical source for cellular energy and are stored within specialized organelles known as lipid droplets (LD). Degradation of LDs for energy utilization is accomplished by two principal pathways: lipolysis that is activated by cAMP to recruit adipose triglyceride lipase (ATGL) to the LD surface, and lipophagy that utilizes the autophagic machinery to target and traffic LDs for degradation by lysosomal acid lipase (LAL). However, the mechanisms underlying synergy between these seemingly distinct pathways are not well understood. Previous studies show that autpohagic membrane cannot effectively engulf large organelles, but is size‐restricted to small cargo. Based on this, we HYPOTHESIZE that lipolysis and lipophagy are sequential pathways whereby lipolysis first reduces the size of large LDs to allow for subsequent autophagic engulfment of small LDs for lipophagy. Methods Using light and electron microscopy combined with biochemical approaches, we explored LD size and the sequential nature of lipolysis and lipophagy in primary rat and AML12 hepatocytes. Live‐cell microscopy was used for the first time to monitor LDs transitioning from lipolysis in the cytoplasm to lipophagy within endolysosomal vesicles. Results Confocal microscopy of endogenous ATGL revealed a preferential association with large LDs with an average size of 4.0 μm 2 , seven‐fold larger than LDs not associated with ATGL. In striking comparison, LDs associated with components of the autophagic machinery (LC3, Lamp3) were 50% smaller in size (0.39 μm 2 ). Consistent with this observation, siRNA knockdown of ATGL resulted in the persistence of large LDs (2.8 μm 2 ), whereas knockdown of LAL left cells filled with numerous small LDs (1.5 μm 2 ). These observations were mimicked by inhibitors of ATGL (atglistatin) and LAL (lalistat). In support of these findings, differential centrifugation of AML12 homogenates allowed for separation of two distinct populations of LDs: a buoyant fraction of large LDs enriched in the cytosolic lipase ATGL, and a second collection of smaller, less buoyant LDs enriched in lysosomal enzymes such as Lamp2A, the autophagosome marker LC3‐II, and the endosomal marker Tsg101. Most interesting are time lapse movies of live cells showing engulfment of small LDs by a Lamp3‐positive late endosomal compartment, suggesting that these organelles play an important role in the direct targeting and engulfment of LDs for lipophagy. Conclusion Hepatocytes appear to catabolize LDs in a tandem stepwise process that first utilizes cytosolic lipases to substantially reduce LD size, thereby allowing for subsequent engulfment of small LDs by autophagosomes as well as late endosomes and lysosomes. Support or Funding Information Supported by NIAAA RO1AA020735 to MAM; NIDDK award T32DK007352 to MBS and RJS, and the Mayo Clinic Kogod Aging Fellowship to MBS. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

Fatty liver disease affects 80–100 million Americans and is a precursor to irreversible and sometimes fatal liver damage. The initial stages of fatty liver disease are marked by the hepatocellular accumulation of fat‐storage organelles known as lipid droplets (LDs) that are degraded by two pathways: lipolysis that involves the recruitment of cytosolic lipases (ATGL, HSL) to the LD surface, and lipophagy that relies on autophagic membranes to target and traffic LDs for lysosomal degradation. Importantly, the mechanisms underlying a cooperation between these seemingly distinct pathways are not well understood. Previous studies have shown that the autophagic engulfment of cytosolic cargo is size‐restricted. Because of this, we HYPOTHESIZE that lipolysis and lipophagy occur as sequential processes initiated by cytosolic lipases to reduce LD size and allow for subsequent engulfment by autophagic membranes. Methods In AML12 hepatocytes, immunofluorescence microscopy and biochemical approaches were used to determine the enrichment of lipolysis vs lipophagy machinery on large vs. small LDs. Live‐cell microscopy was used for the first time to monitor LDs transitioning from lipolysis to lipophagic engulfment. Results Confocal microscopy of AML12 cells immunolabeled for adipose triglyceride lipase (ATGL) revealed a preferential association with large LDs with an average area of 4.0 μm 2 , seven‐fold larger than LDs not associated with ATGL. In striking comparison, LDs associated with components of the autophagic machinery (LC3) were two‐fold smaller in area (0.39 μm 2 ). Consistent with this observation was the finding that siRNA knockdown of ATGL resulted in the persistence of large LDs (2.8 μm 2 ), whereas knockdown of lysosomal acid lipase (LAL) left cells filled with many LDs of a smaller size (1.5 μm 2 ). These observations were mimicked by inhibitors of ATGL (atglistatin) and lysosomal acidification (chloroquine). In support of these findings, differential centrifugation of AML12 homogenates allowed for separation of two distinct populations of LDs: a buoyant fraction of large LDs enriched in the cytosolic lipase ATGL, and a second collection of smaller, less buoyant LDs enriched in lysosomal enzymes such as Lamp2A and the autophagosome marker LC3‐II. Most interesting are time lapse movies of live cells showing engulfment of small LDs by a LAMP3‐positive late endosomal/lysosomal compartment, suggesting that these organelles play an important role in the direct targeting and engulfment of LDs for lipophagy. Conclusion Hepatocytes appear to catabolize LDs in a tandem stepwise process that first utilizes cytosolic lipases to substantially reduce LD size, thereby allowing for subsequent engulfment by autophagosomes as well as late endosomes and lysosomes. Support or Funding Information This study was generously supported by funding from NIAAA R01AA020735, NIDDK T32DK007352, and the Mayo Clinic Robert and Arlene Kogod Center on Aging. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

SPOP mutation disables 53BP1 ubiquitination and exclusion from DNA double-strand break sites and causes genome instability.

Macropinocytosis is an essential mechanism by which pancreatic tumor cells internalize extracellular material to be used as an energy source to fuel proliferation. We describe the mechanism by which the actin cross-linking α-actinin proteins mediate membrane ruffling and macropinosome delivery to the lysosome during this process.

ABSTRACT The Golgi apparatus is a dynamic membranous structure, which has been observed to alter its location and morphology during the cell cycle and after microtubule disruption. These dynamics are believed to be supported by a close structural interaction of the Golgi with the microtubule cytoskeleton and associated motor enzymes. One micro-tubule-dependent motor enzyme, kinesin, has been implicated in Golgi movement and function although direct evidence supporting this interaction is lacking. In this study, we utilized two well-characterized kinesin antibodies in conjunction with subcellular fractionation techniques, immunoblot analysis and immunofluorescence microscopy to conduct a detailed study on the association of kinesin with the Golgi and other membranous organelles in a polarized epithelial cell, the primary rat hepatocyte. We found that kinesin represents ∼0.3% of total protein in rat liver homogenates, with ∼30% membrane-associated and the remainder in the cytosol. Among membrane fractions, kinesin was concentrated markedly in Golgi-enriched fractions, which were prepared using two independent techniques. Kinesin was also abundant in fractions enriched in transcytotic carriers and secretory vesicles, with lower levels detected on fractions enriched in endosomes, endoplasmic reticulum, lysosomes and mito-chondria. Immunofluorescence microscopy showed that kinesin is concentrated on Golgi-like structures in both primary cultured hepatocytes and rat hepatocyte-derived clone 9 cells. Double-label immunofluorescence demonstrated that kinesin staining colocalizes with the Golgi marker, α-mannosidase II, in both cell types. These results provide compelling evidence showing that kinesin is associated with the Golgi complex in cells and implicate this motor enzyme in Golgi structure, function and dynamics.

ABSTRACT Suprastimulation of pancreatic acinar cells with specific agonists inhibits zymogen secretion and induces the formation of large basolateral blebs. Currently the molecular mechanisms that mediate this dramatic morphologic response are undefined. Further, it is unclear if blebbing represents a terminal or reversible event. Using computer-enhanced video microscopy of living acini we have found that these large blebs form rapidly (within 2-3 minutes) and exhibit ameboid undulations. They are induced by small increases in agonist concentration and require an energy-dependent phosphorylation event. Remarkably, the blebs are rapidly absorbed when agonist levels are reduced, indicating that blebbing is a reversible response to a physiological stimulus, not a terminal event. Morphological methods show that these dramatic changes in cell shape are accompanied by a marked reorganization of actin and myosin II at the basolateral domain. During 30 minutes of suprastimulation, both basolateral actin and myosin II gradually increase to form a ring centered at the necks of the blebs. Immunocytochemical and biochemical studies with a phospho-specific antibody to the myosin regulatory light chain reveal an activation of myosin II in suprastimulated acini that is completely absent in resting cells. Studies using cytoskeletal antagonistic drugs indicate that bleb formation and motility require actin remodeling concomitant with an activation of myosin II. This aberrant activation and reorganization of the actin-myosin cytoskeleton is likely to have detrimental effects on acinar cell function. Additionally, this mechanism of bleb formation may be conserved among other forms of physiological blebbing events.

Currently, the hepatocellular trafficking pathways that are used by the hepatitis B virus (HBV) during viral infection and shedding are poorly defined. It is known that the HBV uses late endosomal and multivesicular body (MVB) compartments for assembly and release. The intraluminal vesicles (ILVs) generated within MVBs have also been implicated in the late synthesis stages of a variety of pathogenic viruses. We recently observed that the HBV within infected hepatocytes appears to associate with the tetraspanin protein CD63, known to be a prominent and essential component of ILVs. Immunofluorescence microscopy of HBV‐expressing cells showed that CD63 colocalized with HBV proteins (large hepatitis B surface antigens [LHBs] and hepatitis B core) and labeled an exceptionally large number of secreted extracellular vesicles of uniform size. Small interfering RNA (siRNA)–mediated depletion of CD63 induced a substantial accumulation of intracellular LHBs protein but did not alter the levels of either intracellular or extracellular HBV DNA, nor pregenomic RNA. Consistent with these findings, we found that markedly less LHBs protein was associated with the released HBV particles from CD63 siRNA‐treated cells. Importantly, the HBV viral particles that were shed from CD63‐depleted cells were substantially less infective than those collected from control cells with normal CD63 levels. Conclusion: These findings implicate the tetraspanin protein CD63 as a marker and an important component in the formation and release of infectious HBV particles.

Significance Lipid droplets (LDs) are specialized fat-storage organelles that can be used as a fuel source by many types of cells when nutrients are scarce. One mechanism used by hepatocytes (the functional cells of the liver) for the catabolism of these energy reserves is the lysosome-directed process of autophagy. Traditionally, autophagy necessitates the enclosure of cargo within a double-membrane autophagosome before delivery to the lysosome for degradation. Here, we use live-cell and electron microscopy to demonstrate that stable contacts between LDs and lysosomes in hepatocytes can result in the transfer of both proteins and lipids from LDs directly into the lysosome in the absence of an autophagosomal intermediate. These findings reveal a mechanism used for lipid homeostasis in the liver.