Packaging of Fat: An Evolving Model of Lipid Droplet Assembly and Expansion

Dawn L Brasaemle,Nathan E Wolins

doi:10.1074/jbc.r111.309088

Abstract

Lipid droplets (LDs) are organelles found in most types of cells in the tissues of vertebrates, invertebrates, and plants, as well as in bacteria and yeast. They differ from other organelles in binding a unique complement of proteins and lacking an aqueous core but share aspects of protein trafficking with secretory membrane compartments. In this minireview, we focus on recent evidence supporting an endoplasmic reticulum origin for LD formation and discuss recent findings regarding LD maturation and fusion.

Highlights

Eukaryotic cells produce and traffic membranes that maintain intracellular compartments that facilitate regulation of metabolism by controlling the subcellular localization of metabolites, the concentrations of enzymes and substrates to optimize enzyme reactions, and the isolation of toxic metabolites
The expression of perilipin 1 is limited to adipocytes of white and brown adipose tissue and steroidogenic cells of the adrenal cortex, testes, and ovaries; subcellular localization of perilipin 1 is essentially restricted to lipid droplets (LDs)
Perilipin 3 is stable in the cytoplasm of cells but rapidly associates with nascent LDs when cells are incubated with fatty acids to promote triacylglycerol (TAG) synthesis and storage

Summary

Enzymes for Neutral Lipid Synthesis Reside in ER

The majority of cellular membranes comprise the secretory pathway; membrane flow to maintain these compartments originates from the ER. Deletion of caveolin-1 reduces TAG deposition in the LDs of adipose tissue in mice [25] while reducing the localization of mitochondria to the proximity of LDs [26] It is unclear whether the decreased TAG storage is a consequence of decreased substrate import and esterification or is due to rearrangements in membrane contact sites between cellular organelles and LDs that usually facilitate TAG transfer and packaging into LDs. a recent report shows that C-terminal sequences of perilipin 5 form contact sites between mitochondria and LDs [27]; perilipin 5 may facilitate the exchange of lipids between the two compartments

ER Proteins Are Required for LD Formation and Expansion

Maturation of LDs and LD Fusion

Concluding Remarks