Cholesterol esters form supercooled lipid droplets whose nucleation is facilitated by triacylglycerols

Abstract

Cellular cholesterol can be metabolized to its fatty acid esters, cholesteryl esters (CEs), to be stored in lipid droplets (LDs). With triacylglycerols (TGs), CEs represent the main neutral lipids in LDs. However, while TG melts at ~4 °C, CE melts at ~44 °C, raising the question of how CE-rich LDs form in cells. Here, we show that CE forms supercooled droplets when the CE concentration in LDs is above 20% to TG and, in particular, liquid-crystalline phases when the fraction of CEs is above 90% at 37 °C. In model bilayers, CEs condense and nucleate droplets when the CE/phospholipid ratio reaches over 10-15%. This concentration is reduced by TG pre-clusters in the membrane that thereby facilitate CE nucleation. Accordingly, blocking TG synthesis in cells is sufficient to strongly dampen CE LD nucleation. Finally, CE LDs emerged at seipins, which cluster and nucleate TG LDs in the ER. However, when TG synthesis is inhibited, similar numbers of LDs are generated in the presence and absence of seipin, suggesting that seipin controls CE LD formation via its TG clustering capacity. Our data point to a unique model whereby TG pre-clusters, favorable at seipins, catalyze the nucleation of CE LDs.

Discrimination of lipid composition and cellular localization in human liver tissues by stimulated Raman scattering microscopy

“…We discovered a high spatial correlation between the birefringent crystals and saturated CE by comparing SRS images and polarization images of the same tissues. This finding is supported by previous studies reporting that CE molecules can form liquid crystalline lattices in lipid droplets 33 . Indeed, we have observed the emergence of liquid crystals with typical “Maltese crosses” in liver biopsies of animal models of NASH preceding the onset of inflammation and fibrosis 32 .…”

“…This finding is supported by previous studies reporting that CE molecules can form liquid crystalline lattices in lipid droplets. 33 Indeed, we have observed the emergence of liquid crystals with typical "Maltese crosses" in liver biopsies of animal models of NASH preceding the onset of inflammation and fibrosis. 32 The strong spatial correlation indicates that the main composition of these birefringent liquid crystals is saturated CE, suggesting that the presence and the fate of saturated CE (e.g., lipolysis, phase equilibrium, and phase transition of cholesterol) may also play a vital role in NASH progression.…”

Lymph-targeted high-density lipoprotein-mimetic nanovaccine for multi-antigenic personalized cancer immunotherapy

“…2C ), which is because the DCs preferentially internalize foreign structures with stronger hydrophobic cores and longer alkyl chains ( 43 ). Moreover, the melting point of CO (~50°C) is above body temperature and much higher than that of TO (~0°C) ( 44 ), suggesting that the CO-containing LNP-M has preferable stability. Together, LNP-M with CO-formed lipid core was selected for further studies in consideration of high stability, appreciable lymphatic targeting, and favorable DC uptake.…”

Binding of perilipin 3 to membranes containing diacylglycerol is mediated by conserved residues within its PAT domain

“…The phospholipids were hydrated with 100 μl of buffer B (20 mM Hepes, 100 mM KCl, 2 mM MgCl 2 , pH 7.4) and mixed with neutral lipid (triolein or cholesteryl oleate) in 1:2 M ratio. Cholesteryl oleate was heated to 50 °C to liquify the suspension ( 63 ). A crude lipid-buffer emulsion was generated by 24 cycles of vortexing, followed by 2 min of sonication.…”