Lipid Droplets Embedded in a Model Cell Membrane Create a Phospholipid Diffusion Barrier

Abstract

the endoplasmic reticulum (ER) membrane where neutral lipids are synthesized. While the molecular mechanisms of this process are unknown, it appears that LD biogenesis is following a threestage process: neutral lipid synthesis, lens formation (via intra-membrane lipid accumulation), and finally LD formation. [3] At relatively low concentrations, neutral lipids accumulate between the two leaflets of the ER bilayer to eventually form a lipid reservoir with a lens shape. [3,4] Numerical modeling suggests that such lenses could exist in the ER membrane with a size of tens of nanometers. [3,5,6] Upon the continuous production of neutral lipids, they accumulate into the reservoir which leads to a lipid lens growth. Above a certain size, this lipid lens becomes unstable and can lead to a spontaneous budding of the oily reservoir. [3,7] Ultimately, the droplet pinchoff from the membrane leads to its release into the cytosol. [8][9][10] LDs dynamically adapt to metabolic changes in the cell and balance uptake of free fatty acids by their esterification and storage as triglycerides, and consumption of triglycerides by the release of fatty acids under catabolic conditions. These essential metabolic functions of LDs are executed by proteins on the LD surface, many of which dynamically partition from the ER bilayer to the LD monolayer membrane. [11,12] Aberrant LD functions are implicated in numerous metabolic diseases such as obesity, diabetes,The ORCID identification number(s) for the author(s) of this article can be found under

Phospholipids Diffusion on the Surface of Model Lipid Droplets

“…Unsaturated Phospholipids: DOPC, DOPE, N-DOPE, POPC, LPC. A relevant biophysical composition for a phospholipid monolayer covering LDs localized in the ER membrane can be approximated by a DOPC/DOPE mixture with a molar ratio between 1:1 and 3:1 [7,8,17,18]. For completeness, we vary the composition from pure DOPC to a pure DOPE monolayer.…”

“…Many of the biophysical properties of LDs have been investigated over the last decade using model systems [6]. As example, droplet interface bilayers (DiB), advanced free-standing bilayer (AFB), and giant unilamellar vesicles (GUVs) have been extensively used to reconstitute artificial and natural LDs in such lipid membranes [16, 17, 18, 19, 20, 21]. After successful reconstitution of LDs in a bilayer, the morphology of LDs was studied as function of the phospholipidic composition of the DiBs, GUVs or AFB systems [18, 19, 20].…”

“…In consequence, there are few experimental studies in the literature that address diffusion [24]. Interestingly, this problem is not limited to experimental studies but, also, occurs in theoretical studies with numerical simulations [18, 25, 29].…”

“…Many of the biophysical properties of LDs have been investigated over the last decade using model systems [6]. As example, droplet interface bilayers (DiB), advanced free-standing bilayer (AFB), and giant unilamellar vesicles (GUVs) have been extensively used to reconstitute artificial and natural LDs in such lipid membranes [16,17,18,19,20,21].…”

“…After successful reconstitution of LDs in a bilayer, the morphology of LDs was studied as function of the phospholipidic composition of the DiBs, GUVs or AFB systems [18, 19, 20]. It has been shown that the shape of LDs in a bilayer is determined by the balance of surface forces at the contact line between the embedded-LDs and the bilayer [18, 19, 20]. Thus, the associated contact angle of LDs can be predicted using wetting theory [11, 22].…”