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1. Determining the Bending Rigidity of Free-Standing Planar Phospholipid Bilayers

   https://doi.org/10.3390/membranes13020129  

   Zabala-Ferrera, Oscar ; Liu, Paige ; Beltramo, Peter J. ( February 2023 , Membranes)

   We describe a method to determine membrane bending rigidity from capacitance measurements on large area, free-standing, planar, biomembranes. The bending rigidity of lipid membranes is an important biological mechanical property that is commonly optically measured in vesicles, but difficult to quantify in a planar, unsupported system. To accomplish this, we simultaneously image and apply an electric potential to free-standing, millimeter area, planar lipid bilayers composed of DOPC and DOPG phospholipids to measure the membrane Young’s (elasticity) modulus. The bilayer is then modeled as two adjacent thin elastic films to calculate bending rigidity from the electromechanical response of the membrane to the applied field. Using DOPC, we show that bending rigidities determined by this approach are in good agreement with the existing work using neutron spin echo on vesicles, atomic force spectroscopy on supported lipid bilayers, and micropipette aspiration of giant unilamellar vesicles. We study the effect of asymmetric calcium concentration on symmetric DOPC and DOPG membranes and quantify the resulting changes in bending rigidity. This platform offers the ability to create planar bilayers of controlled lipid composition and aqueous ionic environment, with the ability to asymmetrically alter both. We aim to leverage this high degree of compositional and environmental control, along with the capacity to measure physical properties, in the study of various biological processes in the future. 

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2. Membrane poration, wrinkling, and compression: deformations of lipid vesicles induced by amphiphilic Janus nanoparticles

   https://doi.org/10.1039/D0NR05355D  

   Wiemann, Jared T. ; Shen, Zhiqiang ; Ye, Huilin ; Li, Ying ; Yu, Yan ( October 2020 , Nanoscale)

   null (Ed.)

   Building upon our previous studies on interactions of amphiphilic Janus nanoparticles with glass-supported lipid bilayers, we study here how these Janus nanoparticles perturb the structural integrity and induce shape instabilities of membranes of giant unilamellar vesicles (GUVs). We show that 100 nm amphiphilic Janus nanoparticles disrupt GUV membranes at a threshold particle concentration similar to that in supported lipid bilayers, but cause drastically different membrane deformations, including membrane wrinkling, protrusion, poration, and even collapse of entire vesicles. By combining experiments with molecular simulations, we reveal how Janus nanoparticles alter local membrane curvature and collectively compress the membrane to induce shape transformation of vesicles. Our study demonstrates that amphiphilic Janus nanoparticles disrupt vesicle membranes differently and more effectively than uniform amphiphilic particles. 

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3. Quantifying Solute Partitioning in Phosphatidylcholine Membranes

   Gobrogge, Christine A. Walker ( October 2017 , Analytical chemistry)

   Time-resolved fluorescence measurements were used to characterize and quantify solute partitioning into 1,2- dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid vesicles as a function of solute concentration and temperature. The solutes, coumarin 152 (C152) and coumarin 461 (C461), both belong to a family of 7-aminocoumarin dyes that have distinctive fluorescence lifetimes in different solvation environments. The two solutes differ in the 4-position where C152 has a trifluoromethyl group in place of C461’s -CH3 group. In vesicle containing solutions, multiexponential fluorescence decays imply separate solute populations in the aqueous buffer, solvated in the vesicle headgroup region and solvated in the acyl chain bilayer interior, respectively. Fluorescence amplitudes, corrected for differences in radiative rates, are used to calculate absolute partition coefficients and average number of solutes per vesicle as a function of coumarin:lipid ratio and average number of solutes per vesicle. Results show that C152 has an ∼10-fold greater affinity than C461 for lipid bilayers, despite both solutes having similar hydrophobicities as inferred from their log(P) values. Temperature-dependent partitioning data are used to calculate enthalpies and entropies of C152 partitioning as a function of concentration. These values are used to extrapolate to the infinitely dilute limit. Above and below the lipid gel−liquid crystalline temperature, partitioning is exothermic with negative changes in entropy. In the vicinity of the transition temperature, these quantities change sign with ΔHpart becoming endothermic (+70 kJ/mol) and entropically favored (ΔSpart = +240 J/(mol·K)). 

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4. A Tunable Microfluidic Device Enables Cargo Encapsulation by Cell‐ or Organelle‐Sized Lipid Vesicles Comprising Asymmetric Lipid Bilayers

   https://doi.org/10.1002/adbi.201900010  

   Romanov, Valentin ; McCullough, John ; Gale, Bruce K. ; Frost, Adam ( May 2019 , Advanced Biosystems)

   Cellular membranes play host to a wide variety of morphologically and chemically complex processes. Although model membranes, like liposomes, are already widely used to reconstitute and study these processes, better tools are needed for making model bilayers that faithfully mimic cellular membranes. Existing methods for fabricating cell‐sized (µm) or organelle‐sized (tens to hundreds of nanometers) lipid vesicles have distinctly different requirements. Of particular note for biology, it remains challenging for any technique to efficiently encapsulate fragile cargo molecules or to generate liposomes with stable, asymmetric lipid leaflets within the bilayer. Here a tunable microfluidic device and protocol for fabricating liposomes with desired diameters ranging from ≈10 µm to ≈100 nm are described. Lipid vesicle size is templated by the simple inclusion of a polycarbonate filter within the microfluidic system and tuned with flow rate. It is shown that the vesicles made with this device are stable, unilamellar, lipid asymmetric, and capable of supporting transmembrane protein assembly, peripheral membrane protein binding, as well as soluble cargo encapsulation (including designer nanocages for biotechnology applications). These fabricated vesicles provide a new platform for studying the biophysically rich processes found within lipid–lipid and lipid–protein systems typically associated with cellular membranes.

    

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5. Paclitaxel loading in cationic liposome vectors is enhanced by replacement of oleoyl with linoleoyl tails with distinct lipid shapes

   https://doi.org/10.1038/s41598-021-86484-9  

   Zhen, Yuhong ; Ewert, Kai K. ; Fisher, William S. ; Steffes, Victoria M. ; Li, Youli ; Safinya, Cyrus R. ( March 2021 , Scientific Reports)

   Lipid carriers of hydrophobic paclitaxel (PTX) are used in clinical trials for cancer chemotherapy. Improving their loading capacity requires enhanced PTX solubilization. We compared the time-dependence of PTX membrane solubility as a function of PTX content in cationic liposomes (CLs) with lipid tails containing one (oleoyl; DOPC/DOTAP) or two (linoleoyl; DLinPC/newly synthesized DLinTAP)cisdouble bonds by using microscopy to generate kinetic phase diagrams. The DLin lipids displayed significantly increased PTX membrane solubility over DO lipids. Remarkably, 8 mol% PTX in DLinTAP/DLinPC CLs remained soluble for approximately as long as 3 mol% PTX (the solubility limit, which has been the focus of most previous studies and clinical trials) in DOTAP/DOPC CLs. The increase in solubility is likely caused by enhanced molecular affinity between lipid tails and PTX, rather than by the transition in membrane structure from bilayers to inverse cylindrical micelles observed with small-angle X-ray scattering. Importantly, the efficacy of PTX-loaded CLs against prostate cancer cells (their IC50 of PTX cytotoxicity) was unaffected by changing the lipid tails, and toxicity of the CL carrier was negligible. Moreover, efficacy was approximately doubled against melanoma cells for PTX-loaded DLinTAP/DLinPC over DOTAP/DOPC CLs. Our findings demonstrate the potential of chemical modifications of the lipid tails to increase the PTX membrane loading while maintaining (and in some cases even increasing) the efficacy of CLs. The increased PTX solubility will aid the development of liposomal PTX carriers that require significantly less lipid to deliver a given amount of PTX, reducing side effects and costs.

    

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