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Drabik D, Hinc P, Stephan M, Cavalcanti RRM, Czogalla A, Dimova R. Effect of leaflet asymmetry on the stretching elasticity of lipid bilayers with phosphatidic acid. Biophys J 2024; 123:2406-2421. [PMID: 38822521 PMCID: PMC11365108 DOI: 10.1016/j.bpj.2024.05.031] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2024] [Revised: 05/03/2024] [Accepted: 05/29/2024] [Indexed: 06/03/2024] Open
Abstract
The asymmetry of membranes has a significant impact on their biophysical characteristics and behavior. This study investigates the composition and mechanical properties of symmetric and asymmetric membranes in giant unilamellar vesicles (GUVs) made of palmitoyloleoyl phosphatidylcholine (POPC) and palmitoyloleoyl phosphatidic acid (POPA). A combination of fluorescence quantification, zeta potential measurements, micropipette aspiration, and bilayer molecular dynamics simulations are used to characterize these membranes. The outer leaflet composition in vesicles is found consistent across the two preparation methods we employed, namely electroformation and inverted emulsion transfer. However, characterizing the inner leaflet poses challenges. Micropipette aspiration of GUVs show that oil residues do not substantially alter membrane elasticity, but simulations reveal increased membrane thickness and decreased interleaflet coupling in the presence of oil. Asymmetric membranes with a POPC:POPA mixture in the outer leaflet and POPC in the inner leaflet display similar stretching elasticity values to symmetric POPC:POPA membranes, suggesting potential POPA insertion into the inner leaflet during vesicle formation and suppressed asymmetry. The inverse compositional asymmetry, with POPC in the outer leaflet and POPC:POPA in the inner one yield less stretchable membranes with higher compressibility modulus compared with their symmetric counterparts. Challenges in achieving and predicting compositional correspondence highlight the limitations of phase-transfer-based methods. In addition, caution is advised when using fluorescently labeled lipids (even at low fractions of 0.5 mol %), as unexpected gel-like domains in symmetric POPC:POPA membranes were observed only with a specific type of labeled DOPE (dioleoylphosphatidylethanolamine) and the same fraction of unlabeled DOPE. The latter suggest that such domain formation may result from interactions between lipids and membrane fluorescent probes. Overall, this study underscores the complexity of factors influencing GUV membrane asymmetry, emphasizing the need for further research and improvement of characterization techniques.
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Affiliation(s)
- Dominik Drabik
- Department of Cytobiochemistry, Faculty of Biotechnology, University of Wroclaw, Wroclaw, Poland; Max Planck Institute of Colloids and Interfaces, Potsdam, Germany; Department of Biomedical Engineering, Wroclaw University of Science and Technology, Wroclaw, Poland.
| | - Piotr Hinc
- Department of Cytobiochemistry, Faculty of Biotechnology, University of Wroclaw, Wroclaw, Poland
| | - Mareike Stephan
- Max Planck Institute of Colloids and Interfaces, Potsdam, Germany
| | | | - Aleksander Czogalla
- Department of Cytobiochemistry, Faculty of Biotechnology, University of Wroclaw, Wroclaw, Poland.
| | - Rumiana Dimova
- Max Planck Institute of Colloids and Interfaces, Potsdam, Germany.
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2
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Bottacchiari M, Gallo M, Bussoletti M, Casciola CM. The diffuse interface description of fluid lipid membranes captures key features of the hemifusion pathway and lateral stress profile. PNAS NEXUS 2024; 3:pgae300. [PMID: 39114574 PMCID: PMC11304589 DOI: 10.1093/pnasnexus/pgae300] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 02/15/2024] [Accepted: 07/09/2024] [Indexed: 08/10/2024]
Abstract
Topological transitions of lipid membranes are ubiquitous in key biological processes for cell life, like neurotransmission, fertilization, morphogenesis, and viral infections. Despite this, they are not well understood due to their multiscale nature, which limits the use of molecular models and calls for a mesoscopic approach such as the celebrated Canham-Helfrich one. Unfortunately, such a model cannot handle topological transitions, hiding the crucial involved forces and the appearance of the experimentally observed hemifused intermediates. In this work, we describe the membrane as a diffuse interface preserving the Canham-Helfrich elasticity. We show that pivotal features of the hemifusion pathway are captured by this mesoscopic approach, e.g. a (meta)stable hemifusion state and the fusogenic behavior of negative monolayer spontaneous curvatures. The membrane lateral stress profile is calculated as a function of the elastic rigidities, yielding a coarse-grained version of molecular models findings. Insights into the fusogenic mechanism are reported and discussed.
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Affiliation(s)
- Matteo Bottacchiari
- Department of Basic and Applied Sciences for Engineering, Sapienza University of Rome, via Antonio Scarpa 16, Rome 00161, Italy
- Department of Mechanical and Aerospace Engineering, Sapienza University of Rome, via Eudossiana 18, Rome 00184, Italy
| | - Mirko Gallo
- Department of Mechanical and Aerospace Engineering, Sapienza University of Rome, via Eudossiana 18, Rome 00184, Italy
| | - Marco Bussoletti
- Department of Mechanical and Aerospace Engineering, Sapienza University of Rome, via Eudossiana 18, Rome 00184, Italy
| | - Carlo M Casciola
- Department of Mechanical and Aerospace Engineering, Sapienza University of Rome, via Eudossiana 18, Rome 00184, Italy
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3
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Zabala-Ferrera O, Liu P, Beltramo PJ. Determining the Bending Rigidity of Free-Standing Planar Phospholipid Bilayers. MEMBRANES 2023; 13:129. [PMID: 36837632 PMCID: PMC9959114 DOI: 10.3390/membranes13020129] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 12/30/2022] [Revised: 01/14/2023] [Accepted: 01/17/2023] [Indexed: 06/18/2023]
Abstract
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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4
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Basham CM, Spittle S, Sangoro J, El-Beyrouthy J, Freeman E, Sarles SA. Entrapment and Voltage-Driven Reorganization of Hydrophobic Nanoparticles in Planar Phospholipid Bilayers. ACS APPLIED MATERIALS & INTERFACES 2022; 14:54558-54571. [PMID: 36459500 DOI: 10.1021/acsami.2c16677] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/17/2023]
Abstract
Engineered nanoparticles (NPs) possess diverse physical and chemical properties, which make them attractive agents for targeted cellular interactions within the human body. Once affiliated with the plasma membrane, NPs can become embedded within its hydrophobic core, which can limit the intended therapeutic functionality and affect the associated toxicity. As such, understanding the physical effects of embedded NPs on a plasma membrane is critical to understanding their design and clinical use. Here, we demonstrate that functionalized, hydrophobic gold NPs dissolved in oil can be directly trapped within the hydrophobic interior of a phospholipid membrane assembled using the droplet interface bilayer technique. This approach to model membrane formation preserves lateral lipid diffusion found in cell membranes and permits simultaneous imaging and electrophysiology to study the effects of embedded NPs on the electromechanical properties of the bilayer. We show that trapped NPs enhance ion conductance and lateral membrane tension in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) bilayers while lowering the adhesive energy of the joined droplets. Embedded NPs also cause changes in bilayer capacitance and area in response to applied voltage, which are nonmonotonic for DOPC bilayers. This electrophysical characterization can reveal NP entrapment without relying on changes in membrane thickness. By evaluating the energetic components of membrane tension under an applied potential, we demonstrate that these nonmonotonic, voltage-dependent responses are caused by reversible clustering of NPs within the unsaturated DOPC membrane core; aggregates form spontaneously at low voltages and are dispersed by higher transmembrane potentials of magnitude similar to those found in the cellular environment. These findings allow for a better understanding of lipid-dependent NP interactions, while providing a platform to study relationships between other hydrophobic nanomaterials and organic membranes.
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Affiliation(s)
- Colin M Basham
- Mechanical Aerospace and Biomedical Engineering, University of Tennessee, Knoxville, Tennessee37996, United States
| | - Stephanie Spittle
- Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee37996, United States
| | - Joshua Sangoro
- Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee37996, United States
| | - Joyce El-Beyrouthy
- School of Environmental, Civil, Agricultural, and Mechanical Engineering, University of Georgia, Athens, Georgia30602, United States
| | - Eric Freeman
- School of Environmental, Civil, Agricultural, and Mechanical Engineering, University of Georgia, Athens, Georgia30602, United States
| | - Stephen A Sarles
- Mechanical Aerospace and Biomedical Engineering, University of Tennessee, Knoxville, Tennessee37996, United States
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5
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Scheidegger L, Stricker L, Beltramo PJ, Vermant J. Domain Size Regulation in Phospholipid Model Membranes Using Oil Molecules and Hybrid Lipids. J Phys Chem B 2022; 126:5842-5854. [PMID: 35895895 PMCID: PMC9377339 DOI: 10.1021/acs.jpcb.2c02862] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/25/2022] [Revised: 07/06/2022] [Indexed: 11/29/2022]
Abstract
The formation of domains in multicomponent lipid mixtures has been suggested to play a role in moderating signal transduction in cells. Understanding how domain size may be regulated by both hybrid lipid molecules and impurities is important for understanding real biological processes; at the same time, developing model systems where domain size can be regulated is crucial to enable systematic studies of domain formation kinetics and thermodynamics. Here, we perform a model study of the effects of oil molecules, which swell the bilayer, and line-active hybrid phospholipids using a thermally induced liquid-solid phase separation in planar, free-standing lipid bilayers consisting of DOPC and DPPC (1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, respectively). The experiments show that the kinetics of domain growth are significantly affected by the type and molecular structure of the oil (squalene, hexadecane, or decane), with the main contributing factors being the degree of swelling of the bilayer and the changes in line tension induced by the different oils, with smaller domains resulting from systems with smaller values of the line tension. POPC (1-palmitoyl-sn-2-oleoyl-glycero-3-phosphocholine), on the other hand, acts as a line-active hybrid lipid, reducing the domain size when added in small amounts and slowing down domain coarsening. Finally, we show that despite the regulation of domain size by both methods, the phase transition temperature is influenced by the presence of oil molecules but not significantly by the presence of hybrid lipids. Overall, our results show how to regulate domain size in binary membrane model systems, over a wide range of length scales, by incorporating oil molecules and hybrid lipids.
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Affiliation(s)
- Laura Scheidegger
- Department
of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, 8093 Zürich, Switzerland
| | - Laura Stricker
- Department
of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, 8093 Zürich, Switzerland
| | - Peter J. Beltramo
- Department
of Chemical Engineering, University of Massachusetts
Amherst, Amherst, Massachusetts 01003, United States
| | - Jan Vermant
- Department
of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, 8093 Zürich, Switzerland
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6
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Wurl A, Ott M, Plato E, Meister A, Hamdi F, Kastritis PL, Blume A, Ferreira TM. Filling the Gap with Long n-Alkanes: Incorporation of C20 and C30 into Phospholipid Membranes. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2022; 38:8595-8606. [PMID: 35786894 DOI: 10.1021/acs.langmuir.2c00872] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
Investigating how hydrophobic molecules mix with phospholipid bilayers and how they affect membrane properties is commonplace in biophysics. Despite this, a molecular-level empirical description of a membrane model as simple as a phospholipid bilayer with long linear hydrophobic chains incorporated is still missing. Here, we present an unprecedented molecular characterization of the incorporation of two long n-alkanes, n-eicosane (C20) and n-triacontane (C30) with 20 and 30 carbons, respectively, in phosphatidylcholine (PC) bilayers using a combination of experimental techniques (2H NMR, 31P NMR, 1H-13C dipolar recoupling solid-state NMR, X-ray scattering, and cryogenic electron microscopy) and atomistic molecular dynamics (MD) simulations. At low hydration, deuterated C20 and C30 yield 2H NMR spectra evidencing anisotropic-motion, which demonstrates their miscibility in PC membranes up to a critical alkane-to-acyl-chain volume fraction, ϕc. The acquired 2H NMR spectra of C20 and C30 have notably different lineshapes. At low alkane volume fractions below ϕc, CHARMM36 MD simulations predict such 2H NMR spectra qualitatively and thus enable an atomistic-level interpretation of the spectra. Above ϕc, the 2H NMR lineshapes become characteristic of motions in the intermediate-regime that, together with the MD simulation results, suggest the onset of immiscibility between the alkane molecules and the acyl chains. For all the systems investigated, the phospholipid molecular structure is unperturbed by the presence of the alkanes. However, at conditions of excess hydration and at surprisingly low alkane fractions below ϕc, a peak characteristic of isotropic motion is observed in both the 2H spectra of the alkanes and 31P spectra of the phospholipids, strongly indicating that the incorporation of the alkanes induces a reduction on the average radius of the lipid vesicles.
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Affiliation(s)
- Anika Wurl
- NMR Group - Institute of Physics, Martin Luther University Halle-Wittenberg, 06099 Halle (Saale), Germany
| | - Maria Ott
- Department of Biotechnology and Biochemistry, Martin Luther University Halle-Wittenberg, 06099 Halle, Saale, Germany
| | - Eric Plato
- NMR Group - Institute of Physics, Martin Luther University Halle-Wittenberg, 06099 Halle (Saale), Germany
| | - Annette Meister
- Department of Biotechnology and Biochemistry, Martin Luther University Halle-Wittenberg, 06099 Halle, Saale, Germany
| | - Farzad Hamdi
- Department of Biotechnology and Biochemistry, Martin Luther University Halle-Wittenberg, 06099 Halle, Saale, Germany
| | - Panagiotis L Kastritis
- Department of Biotechnology and Biochemistry, Martin Luther University Halle-Wittenberg, 06099 Halle, Saale, Germany
| | - Alfred Blume
- Insitute of Chemistry, Martin Luther University Halle-Wittenberg, 06099 Halle, Saale, Germany
| | - Tiago M Ferreira
- NMR Group - Institute of Physics, Martin Luther University Halle-Wittenberg, 06099 Halle (Saale), Germany
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7
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Kataoka-Hamai C, Kawakami K. Hydrocarbon Penetration into Phospholipid Monolayers Formed at Hydrocarbon-Water Interfaces. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2022; 38:3720-3728. [PMID: 35289166 DOI: 10.1021/acs.langmuir.1c03269] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
Phospholipid monolayers formed at oil-water interfaces are used for various biological applications. However, monolayer structures are not well understood. Herein, we investigated hydrocarbon partitioning in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine monolayers formed at hydrocarbon-water interfaces using fluorescence microscopy and pendant drop tensiometry. The monolayers strongly interacted with squalene, n-hexadecane, n-tetradecane, n-dodecane, n-decane, and n-butylcyclohexane. These alkane and alkylcyclohexane molecules remained within the monolayers during area compression. In contrast, the monolayers interacted weakly with n-pentylbenzene and n-butylbenzene. These alkylbenzenes were gradually removed from the monolayers upon area compression and were completely expelled at an area per lipid of ∼70 Å2. Surface pressure analysis indicated that the ability of hydrocarbons to penetrate the monolayers was enhanced in the order of n-butylbenzene < n-pentylbenzene < n-butylcyclohexane < n-hexadecane.
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Affiliation(s)
- Chiho Kataoka-Hamai
- Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
| | - Kohsaku Kawakami
- Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
- Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan
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8
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Abstract
Dynamic processes on membrane surfaces are essential for biological function. Traditionally, quantitative measurements of lipid/protein motion have been interpreted in the framework of membrane hydrodynamics. However, some recent single-molecule tracking studies have proven difficult to interpret via hydrodynamic arguments. Does this suggest a failure of hydrodynamic theory or simply highlight the dangers in attempting to extend hydrodynamic arguments down to molecular scales? Intermolecular correlations are superior to single-molecule observations for studying hydrodynamics due to the longer length scales involved. The current work reports dynamic pair correlations of lipids in model membranes. Submicron distance-dependent correlations are well resolved, and complementary numerical calculations indicate that hydrodynamic theory can predict membrane dynamics over distances of tens of nanometers and longer. Lipid membranes are complex quasi–two-dimensional fluids, whose importance in biology and unique physical/materials properties have made them a major target for biophysical research. Recent single-molecule tracking experiments in membranes have caused some controversy, calling the venerable Saffman–Delbrück model into question and suggesting that, perhaps, current understanding of membrane hydrodynamics is imperfect. However, single-molecule tracking is not well suited to resolving the details of hydrodynamic flows; observations involving correlations between multiple molecules are superior for this purpose. Here dual-color molecular tracking with submillisecond time resolution and submicron spatial resolution is employed to reveal correlations in the Brownian motion of pairs of fluorescently labeled lipids in membranes. These correlations extend hundreds of nanometers in freely floating bilayers (black lipid membranes) but are severely suppressed in supported lipid bilayers. The measurements are consistent with hydrodynamic predictions based on an extended Saffman–Delbrück theory that explicitly accounts for the two-leaflet bilayer structure of lipid membranes.
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