Seipin Promotes Lipid Droplet Biogenesis
Hilton, Christopher Lee
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Seipin is an ER membrane protein that is required for adipogenesis in mammalians. Humans lacking functional seipin have virtually no visible adipose tissue. Seipin has been shown to be essential for the later stages of the adipogenic program in mouse pre-adipocytes. In yeast, the absence of seipin (Fld1p) leads to clusters of tiny lipid droplets or “supersized” ones, suggesting a role of the protein in droplet formation. To determine if this is true we created yeast strains that allowed us to “turn on” lipid droplet synthesis by the regulated expression of enzymes that create either triacylglycerol (TAG) or sterol ester (SE), the main neutral lipid components of droplets, in a droplet-null background with seipin (4KO) or without it (4KOfld1Δ). Using fluorescence microscopy, I showed that the number of newly formed TAG fluorescent bodies (individual droplets or clusters of unresolvable small droplets) decreased but their size increased in the absence of seipin. The large fluorescent bodies in 4KOfld1Δ were fluorescently dimmer and had an irregular perimeter compared to those in the 4KO strain, while their intracellular membranes stained with BODIPY had brighter fluorescence, suggesting that seipin is involved in the packaging of TAG. Electron microscopy showed that the TAG fluorescent bodies were clusters of small droplets. Levels of whole-cell TAG were generally similar during droplet formation, although somewhat lower at early time points. Seipin deletion had a milder effect on formation of SE fluorescent bodies. We conclude that seipin plays a direct role in normal lipid droplet assembly. Finally, in several side projects, I leaned about a possible role of seipin in droplet protein composition, the effects of different detergents on the seipin homo-oligomer, and the lack of a role of seipin in ER stress.