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Review
. 2018;337:83-110.
doi: 10.1016/bs.ircmb.2017.12.007. Epub 2018 Feb 12.

Lipid Droplets as Organelles

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Free PMC article
Review

Lipid Droplets as Organelles

Sarah Cohen. Int Rev Cell Mol Biol. .
Free PMC article

Abstract

Long considered inert fat storage depots, it has become clear that lipid droplets (LDs) are bona fide organelles. Like other organelles, they have a characteristic complement of proteins and lipids, and undergo a life cycle that includes biogenesis, maturation, interactions with other organelles, and turnover. I will discuss recent insights into mechanisms governing the life cycle of LDs, and compare and contrast the LD life cycle with that of other metabolic organelles such as mitochondria, peroxisomes, and autophagosomes, highlighting open questions in the field.

Keywords: Autophagy; Cell biology; Lipid droplets; Lipolysis; Membrane contact sites; Organelle biogenesis; Organelles.

Figures

Figure 1:
Figure 1:. The LD Lifecycle.
LDs undergo a complex lifecycle that begins with biogenesis from the ER (a). LDs undergo a maturation process involving the sequential recruitment of different proteins, such as perilipins (b); the change from green to red indicates the sequential recruitment of proteins. Under certain conditions LDs may undergo fusion (c) or fission (d), and LDs may make contacts with a number of other organelles, including the ER, mitochondria, and peroxisomes (e). Arrows indicate lipid and/or protein transfer between organelles, although in many cases the types of lipid being transferred and direction of transfer are as yet unknown. Finally, LDs can be turned over via lipolysis involving cytoplasmic lipases, or via macroautophagy, referred to as lipophagy (f).
Figure 2:
Figure 2:. LD Biogenesis.
COS-1 cells were transfected to express a model peptide consisting of the hydrophobic domain of the methyl transferase ALDI, fused to the LD-targeting signal of caveolin. This model peptide is initially targeted to the ER, and accumulates in microdomains where LD biogenesis will occur. Transfected cells were treated with oleic acid and a fluorescent fatty acid analogue (FA-BODIPY). During LD biogenesis, the FA-BODIPY accumulated in an ER microdomain, forming a nascent LD. Numbers indicate time in minutes after the addition of oleic acid and FA-BODIPY. Modified with permission from Kassan et al., 2013.
Figure 3:
Figure 3:. LD-organelle contacts in space.
Examples of LD-organelle contacts in segmented multispectral lattice light-sheet images. Shown are contacts between LDs (blue) and ER (yellow), mitochondria (green), Golgi (red), lysosomes (magenta), or peroxisomes (cyan). Scale bar, 2 μm. Image credit: Sarah Cohen, Alex Valm, Wesley Legant, Eric Betzig, and Jennifer Lippincott-Schwartz. Images acquired as described in (Valm et al., 2017).
Figure 4:
Figure 4:. LD-organelle contacts over time.
Micrographs of a COS-7 cell expressing fusion proteins targeted to the ER (yellow), mitochondria (green), Golgi (red), lysosomes (magenta), and peroxisomes (cyan), and labeled with a lipophilic dye to label LDs (blue). The LDs outlined in white were tracked, and their interorganelle contacts mapped over time. A blue line indicates that the LD was successfully tracked, while colored lines indicate that the LD was within 1 pixel (97 nm) of the indicated organelle at the specified time point. Numbers on the micrographs represent time (s). Scale bar, 5 μm. Modified with permission from Valm et al., 2017.
Figure 5:
Figure 5:. LD turnover by lipophagy.
Electron micrographs of cultured hepatocytes treated with excess oleic acid or methionine/choline-deficient medium. Shown are autophagic vacuoles containing only lipids, other cargo, or mixed (lipids plus other cargo). Lipids are indicated by stars. Modified with permission from Singh et al., 2009.

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