Biogenesis and fate of lipid droplets

International audience; Editorial Special issue "Biogenesis and Fate of Lipid Droplets" Intracellular lipid droplets (LDs) are receiving increasing interest from the scientific community, notably because of their link with metabolic diseases (obesity, diabetes) and the production of lipid-derived biofuels in microorganisms. Understanding the biogenesis and fate of LDs upon energy mobilization is essential to monitor fat storage in adipose tissue, but also to improve lipid productivity in microalgae under various conditions of growth [1]. Intracellular LDs are today considered as individual organelles since they host specific metabolic functions such as the biosynthesis of triacylglycerols (TAGs) and phospholipids [2]. In addition to the enzymes involved in these pathways, several proteins have been identified as associated with LDs and their number has been increasing with the progress of proteomics. Some of these proteins presenting hydrophobic patches and a high amphiphilicity are involved in the structure of LDs and together with polar lipids they form the membrane surrounding the neutral lipid core. Some others are involved in the budding of LDs from lipid bilayers and the interactions of LDs with membranes and other organelles. These interactions are important for the transfer of fatty acids and lipid remodelling and homeostasis. The combination of lipidomics and proteomics has led to a better description of LD composition, while microscopy has allowed the visualisation of the specific location of proteins at the surface of LDs. Nevertheless, a better knowledge of the lipid-protein interactions that lead to the biogenesis and fate of LDs is still required. In this context, cellular biologists now adopt principles and methods from biophysics and physical chemistry of lipid emulsions and colloids [3]. Emulsions made of TAGs, polar lipids and proteins can serve as models for intracellular LDs. The biodiversity of LDs in animals, plants and microorganisms and their various functions in lipid storage and transport also allows the comparison of data and knowledge from various fields that will help build a global understanding of structure-function relationships. The original articles and reviews of this special issue are a collection of topics presented at the 14th GERLI Lipidomics meeting, which was be held at St Maximin-la-Sainte Baume, France, from September 30th to October 3rd, 2018, with the aim of gathering researchers from various areas with a common interest in LDs, their physical chemistry and metabolism, their association with diseases and various applications in biotechnology, pharmacology and nutrition. The two first mini-reviews are related to lipid metabolism in microalgae. Lupette et al. cover the biosynthesis of fatty acids, acyl-glycero-lipids and sterols in diatoms and show how a metabolic intermediate common to these pathways, like acetyl-CoA, can play an essential role in directing the carbon flux from acyl-lipid to sterol biosynthesis and be determinant for the balance between TAGs and sterols. Prioretti et al. investigate how the target of rapamycin (TOR) signalling pathway could be modulated to enhance TAG production in microalgae. The use of TOR inhibitors increases TAG productivity in the marine diatom Phaeodactylum tricornutum, without stopping growth, while TAG accumulation in microalgae is usually observed under stress conditions with a low growth rate. Yuan et al. report on the characterization of three diacylglycerol acyltransferases (DGATs) from oil palm (Elaeis guineensis), the main source of vegetable oil on Earth. DGATs are key enzymes for the biosynthesis of TAGs and the formation of LDs. Using the heterologous expression of these DGATs in the yeast Saccharomyces cerevisiae, they show that two of them are true DGATs, that could restore TAG synthesis in a yeast TAG-deficient mutant, while the third one is a wax ester synthase.