University of Groningen Lipid droplet autophagy in the yeast Saccharomyces cerevisiae

Cytosolic lipid droplets (LDs) are ubiquitous organelles in prokaryotes and eukaryotes that play a key role in cellular and organismal lipid homeostasis. Triacylglycerols (TAGs) and steryl esters, which are stored in LDs, are typically mobilized in growing cells or upon hormonal stimulation by LD-associated lipases and steryl ester hydrolases. Here we show that in the yeast Saccharomyces cerevisiae, LDs can also be turned over in vacuoles/ lysosomes by a process that morphologically resembles microautophagy. A distinct set of proteins involved in LD autophagy is identified, which includes the core autophagic machinery but not Atg11 or Atg20. Thus LD autophagy is distinct from endoplasmic reticulum–autophagy, pexophagy, or mitophagy, despite the close association between these organelles. Atg15 is responsible for TAG breakdown in vacuoles and is required to support growth when de novo fatty acid synthesis is compromised. Furthermore, none of the core autophagy proteins, including Atg1 and Atg8, is required for LD formation in yeast. INTRODUCTION Lipids are essential to sustain life, as they are fundamental constituents of biological membranes and metabolic energy stores and important players in many signaling pathways. The metabolic demand for lipids differs greatly in growing, differentiating, or resting cells. Therefore rapid adaptation of lipid content and composition in response to fluctuating environmental conditions is crucial to support cellular function. A key role in these lipid metabolic fluxes is played by fatty acids, which are the building blocks for membrane phospholipids and storage lipids but are subject to multiple modifications, such as elongation and desaturation, and degradation (Tehlivets et al., 2007). On the other hand, high concentrations of fatty acids are harmful and the cause of lipotoxicity, with detrimental pathological consequences (Listenberger et al., 2003; Kohlwein, 2010a). The storage of fatty acids as triacylglycerols (TAGs), which are packaged into cytosolic lipid droplets (LDs), provides a highly efficient way of dealing with fluctuating nutritional supply and physiological demand for fatty acids: fatty acids are stored as TAGs in times of excess and mobilized by lipolytic breakdown to support membrane proliferation or signaling processes in growing cells or oxidized to generate cellular energy in times of starvation (Zechner et al., 2012). Thus molecular mechanisms regulating LD formation and turnover have gained extensive biomedical attention in view of prevalent lipid-associated metabolic diseases, such as obesity and type 2 diabetes (Greenberg et al., 2011; Cusi, 2012). The neutral lipid core of LDs, consisting of TAGs and steryl esters, is delimited by a phospholipid monolayer that is decorated by a unique set of lipogenic and lipolytic enzymes and their regulators that catalyze lipid storage and degradation and interaction with other organelles (Farese and Walther, 2009; Walther and Farese, 2012; Kohlwein et al., 2013). Evidence suggests that LDs derive from the endoplasmic reticulum (ER) and may remain largely associated with this membrane after maturation, which may be functionally relevant to facilitate lipid and protein exchange and cellular dynamics (Szymanski et al., 2007; Kohlwein, 2010b; Jacquier et al., 2011; Wolinski et al., 2011). Release of fatty acids from TAG stores is controlled by LD-resident lipases and hydrolases in the cytosol, such as adipose Monitoring Editor Sandra Lemmon University of Miami Received: Aug 7, 2013 Revised: Oct 16, 2013 Accepted: Nov 14, 2013 This article was published online ahead of print in MBoC in Press (http://www .molbiolcell.org/cgi/doi/10.1091/mbc.E13-08-0448) on November 20, 2013. *Present address: Department of Pediatrics, Center for Liver, Digestive, and Metabolic Diseases, University Medical Center Groningen, University of Groningen, 9700 RB Groningen, Netherlands. The authors declare no conflict of interest. Address correspondence to: Sepp D. Kohlwein (sepp.kohlwein@uni-graz.at). © 2014 van Zutphen et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0). “ASCB®,” “The American Society for Cell Biology®,” and “Molecular Biology of the Cell®” are registered trademarks of The American Society of Cell Biology. Abbreviations used: CARS, coherent anti-Stokes Raman scattering; GFP, green fluorescent protein; LD, lipid droplet; TAG, triacylglycerol. http://www.molbiolcell.org/content/suppl/2013/11/18/mbc.E13-08-0448v1.DC1. html Supplemental Material can be found at: