Structure and function of lipid droplets

Lipid droplets (LDs), evolutionarily conserved cellular organelles, are present in nearly all eukaryotic cells and some bacteria. Long regarded as inert fat storage granules, LDs have now been recognised as dynamic organelles that impact many aspects of cellular function. Besides storing neutral lipids and preventing lipotoxicity, LDs also play key roles in membrane and lipid trafficking, endoplasmic reticulum (ER) stress response, protein storage and degradation, as well as infection and immunity. Distinct from all other organelles, LDs are comprised of a hydrophobic core of triacylglycerols and/or sterol esters enclosed by a monolayer of amphipathic lipids, predominantly phospholipids. The LD surface is embedded with a large number of proteins, which are essential to LD functions. LDs originate from the ER, where most enzymes of lipid synthesis reside. The neutral lipid core of LDs expands or shrinks to meet the energy requirements of a cell. Based on size, location, protein and lipid composition, there are distinct populations of LDs in the same cell. LDs interact with most other organelles, specifically the ER and mitochondria, through contact sites. LDs are most frequently found in the cytoplasm, although nuclear LDs can form under specific conditions and/or in certain cell types. In this chapter, the current knowledge on the structure, biogenesis, growth, degradation and key functions of LDs is summarised.