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Cell biology of lipid droplets
Role of BSCL2-seipin in the development of lipodystrophies [H. Wolinski]
Mutations in the human BSCL2/seipin gene are associated with the most severe form of congenital generalized lipodystrophy (CGL), characterized by a near total loss of adipose tissue and the disposal of excess dietary neutral lipids in other organs such as in the liver and in muscles. As a consequence, malfunctions of BSCL2/Seipin lead to complications such as insulin resistance, fatty liver, and muscular hypertrophy. Although seipin has been studied for more than a decade in different model systems including the yeast Saccharomyces cerevisiae, the molecular function of this protein is unknown. Phosphatidic acid (PA) is an essential intermediate in the synthesis of membrane-forming glycerophospholipids as well as of triacylglycerol (TAG) that is stored in lipid droplets (LD). We recently showed that malfunctions of the yeast Sei1-Ldb16 complex, resembling human seipin, results in abnormal localization of the mammalian lipin-1 ortholog Pah1 at a subdomain of the nuclear endoplasmic reticulum (nER). Of note, this specific fraction of the nuclear envelope is tightly associated with the major bulk of LD. Pah1 is responsible for the dephosphorylation of PA to diacylglycerol (DAG), which is further acylated to storage TAG. Notably, aberrant distribution of Pah1 is accompanied by the accumulation of PA at such sites as indicated by PA-marker proteins such as Opi1. In an upcoming project we will follow this thread and will address the role of BSCL2 in PA metabolisms both in yeast and mammalian cells.
Remodeling of lipid droplets (LDs) during neutral lipid degradation and growth in murine and human adipocytes [H. Wolinski].
Using 4D live cell imaging and label-free coherent anti-Stokes Raman scattering (CARS) microscopy we found that LDs in murine and human adipocytes grow by a transfer of lipids between closely associated LDs. This phenonemon is not a rapid and spontaneous process but rather occurs over several hours and does not appear to require physical interaction over large LD surface areas. These data indicate that LD growth is a highly regulated process leading to the heterogeneous LD size distribution within and between individual cells. During review of this work basically the same process of LD growth was found by Gong et al. and Jambunathan et al. Both research groups identified Fsp27 as a factor promoting LD clustering and lipid transfer in 3T3-L1 cells and confirm our hypothesis that LD growth is regulated by a protein machinery.
In addition, we show that stimulation of lipolysis in 3T3-L1 adipocytes causes progressive shrinkage and almost complete degradation of all cellular LDs but without generation of micro LD and fragmentation as suggested earlier. However, we show that micro LD are formed de novo in response to cellular fatty acid overload caused by the absence of BSA which acts as a fatty acid scavanger. We suggest that this phenomenon represents a mechansim to prevent lipotoxicity.
This study demonstrates the impact of advanced imaging techniques such as 4D live cell imaging and CARS microscopy for studying basic cell biological aspects of lipid metabolism.
Heterogeneous lipid droplet size in murine adipocytes. CARS image acquired at 2840cm-1. Maximum-intensity projection.
Fast degradation of neutral lipids upon lipolytic stimulation in 3T3-L1 cells.
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