Dissecting the Role of the Lipodystrophy Protein Seipin in the Biogenesis of the Lipid Droplet Organelle
Cartwright, Bethany Rose
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Long thought to be little more than inert storage depots, lipid droplets have recently become recognized as unique, dynamic, regulated organelles that play an essential role in fat storage. Despite this increased interest, much remains unknown. Lipid droplets have been observed to emerge from the endoplasmic reticulum, but the available models for lipid droplet biogenesis are largely conceptual, with little to no evidence for specific mechanisms of droplet formation. Debate even continues within the field as to whether lipid droplet formation is a spontaneous process, driven by physicochemical and hydrophobic forces, or a regulated process driven by protein factors. The Goodman laboratory previously found evidence to suggest that seipin, mutated in the most severe cases of congenital generalized lipodystrophy, may be a key factor in the early stages of lipid droplet formation. Seipin resides at the junction between lipid droplets and the endoplasmic reticulum, and deletion of seipin results in both a drastic impediment to de novo droplet formation and a striking disorganization of droplet morphology. For my thesis work, I have explored several aspects of seipin’s role at the lipid droplet. I have studied the effects of seipin deletion on protein targeting to abnormal lipid droplets, through which I identified a unique effect of seipin on the regulation of lipase targeting. I have also analyzed the topology of the seipin complex itself through a series of deletion mutants, identifying regions that contribute to the localization, membrane association, and stability of the seipin complex. Furthermore, these studies have led to novel insights on the function of seipin, through the characterization of a remarkable N-terminal seipin mutation that presents with defects in droplet initiation but homogenous droplet morphology. I have therefore concluded that seipin plays two dissectible roles in lipid droplet formation: 1) promoting lipid droplet initiation and 2) regulating subsequent droplet morphology. Finally, I suggest hypotheses on the mechanisms by which seipin exerts these effects, proposing that the N-terminus of seipin may regulate lipin, a mouse lipodystrophy protein, to effect droplet initiation, while the bulk of the protein may serve to regulate the access of phospholipids to the lipid droplet surface.