Triglyceride Lenses at the Air−Water Interface as a Model System for Studying the Initial Stage in the Biogenesis of Lipid Droplets

B. CARUSO; WILKE, NATALIA; M.A. PERILLO

LANGMUIR

AMER CHEMICAL SOC

Lugar: Washington; Año: 2021 vol. 31 p. 10958 - 10970

0743-7463

Lipid droplets (LD) are intracellular structures consisting of anapolar lipid core, composed mainly of triglycerides (TG) and steryl esters,coated by a lipid−protein mixed monolayer. The mechanisms underlying LDbiogenesis at the endoplasmic reticulum membrane are a matter of manycurrent investigations. Although models explaining the budding-off ofprotuberances of phase-segregated TG inside bilayers have been proposedrecently, the assumption of such initial blisters needs further empiricalsupport. Here, we study mixtures of egg phosphatidylcholine (EPC) and TGat the air−water interface in order to describe some physical properties andtopographic stability of TG bulk structures in contact with interfaces. Brewsterangle microscopy images revealed the appearance of microscopic collapsedstructures (CS) with highly reproducible lateral size (∼1 μm lateral radius)not varying with lateral packing changes and being highly stable at surfacepressures (π) beyond collapse. By surface spectral fluorescence microscopy, we were able to characterize the solvatochromism ofNile Red both in monolayers and inside CS. This allowed to conclude that CS corresponded to a phase of liquid TG and tocharacterize them as lenses forming a three-phase (oil−water−air) system. Thereby, the thicknesses of the lenses could bedetermined, observing that they were dramatically flattened when EPC was present (6−12 nm compared to 30−50 nm for lenses onEPC/TG and TG films, respectively). Considering the shape of lenses, the interfacial tensions, and the Neumann?s triangle, thisexperimental approach allows one to estimate the oil−water interfacial tension acting at each individual microscopic lens and atvarying compression states of the surrounding monolayer. Thus, lenses formed on air−water Langmuir films can serve to assessvariables of relevance to the initial step of LD biogenesis, such as the degree of dispersion of excluded-TG phase and shape, spatialdistribution, and oil−water interfacial tension of lenses.