Session N07: The Physics of Cell Membranes I: From Simplified Models to Complex Functionality

Are cell membranes electrorheological fluids?

Living cells maintain transmembrane electric potential difference across the plasma membrane as well as the membranes of various organelles. Using giant vesicles as an in-vitro membrane model system, we explore the effects of electric field on membrane mechanical properties. We discover that bilayers may not behave as simple two-dimensional fluids as commonly assumed but their viscosity may be modulated by electric fields.  We find that membrane viscosity decreases with increasing electric field magnitude, while membrane electric polarization has the opposite effect. The potential impact of coupled endogenous electric fields and membrane fluidity on biological function opens an exciting new direction to explore.   

Electromechanics of Lipid Bilayers: A Dimensionally Reduced Theory

From experiments, it is well known that electric fields affect the physics of lipid bilayers. For instance, lipid vesicles change their morphologies under an external electric field, and under physiological conditions lipid bilayers are immersed in electrolytes and are thus exposed to electric fields as well. Therefore, understanding the electromechanics of lipid bilayers is essential for many biologically relevant processes. While the continuum theory of the mechanics of lipid bilayers is well established, a unified theory that captures the coupling between electric fields and mechanical deformations is currently missing.

Lipid bilayers are commonly modeled as a two-dimensional manifold. However, in order to capture the effects of an electric field, the potential must be resolved through the thickness of the lipid bilayer, rendering existing approaches inadequate. To derive an electromechanical theory for lipid bilayers, we propose a new dimension reduction approach. This allows us to derive a surface theory for the electrostatics of thin films. Furthermore, we recover the well-known equations governing the mechanics of thin films. Finally, we combine the surface description of the electrostatics and mechanics with appropriate constitutive models to formulate a new theory for the electromechanics of lipid bilayers.   

Applications of dimensionally reduced theory of lipid bilayers: Fluctuation spectra and shear-induced hydrodynamic instabilities

Lipid bilayers are fundamental to a plethora of cellular phenomena, often displaying unique physics owing to their behaving as in-plane fluids and out-of-plane elastic solids. Typically, a direct two-dimensional approach is used in order to obtain a computationally and analytically tractable theory. However, such an approach cannot capture some experimentally and computationally observed phenomena such as Kelvin-Helmholtz-like instabilities or electromechanical coupling in lipid bilayers. Here, we apply an “effective” two-dimensional framework that explicitly includes membrane thickness while retaining the amenability of previous theories. We find that the membrane thickness acts to slow the linear response of lipid bilayers to external perturbations. This novel dependence on thickness alters the behavior of the fluctuation spectrum in the short wavelength regime, where bending effects are relevant. We also study the effects of thickness on the development of shear-induced Kelvin-Helmholtz-like hydrodynamic instabilities.   

Functional and functionalized membranes

Cell membranes are complex dynamic structures, and their composition and structure are major determinants of pathology. It is now commonly accepted that the membranes’ physical properties, such as fluidity and thickness, are determining factors for permeability, partitioning of drug molecules, and protein aggregation. Membrane-interacting molecules can in some instances be expected to have a greater therapeutic potential than traditional therapies targeting receptors or enzymes. I will provide a perspective on the basic mechanisms how physical membrane properties can affect diseases, and the therapeutic potential of changing membrane properties to target certain diseases. We developed red blood cell based hybrid liposomes for targeted drug delivery with antiviral and antibiotic properties that show great therapeutic potential because of their biocompatibility. We also use these ideas and techniques in our start-up (www.synth-med.com ) that develops smart membrane-based sensors for the detection of pathogens in water and food.   

Electrostatic interactions in symmetric and asymmetric charged free-standing membranes

Phospholipid bilayer elastic properties govern numerous biological processes, ranging from the deformation necessary during endocytosis and exocytosis, to the lateral diffusion of membrane proteins.  In this presentation, we will discuss recent advances in our lab applying a novel technique to create free-standing charge asymmetric membranes with controlled composition on either membrane leaflet.  Capacitance measurements characterize the degree of charge asymmetry, indicating transmembrane potentials approaching the biological case of 100 mV.   Electrostriction experiments detail the effect of charge on the membrane Young's modulus, showing that as the overall charge in the membrane increases so does the membrane stiffness. However, asymmetric membranes are less stiff than their symmetric counterparts. The measured membrane Young's modulus is then related to the membrane bending rigidity by applying thin plate theory in the framework of linear elasticity.  We critically examine our results in comparison with measurements of bending rigidity using other methods (supported lipid bilayers and giant unilamellar vesicles), with a particular focus on the impact of phospholipid composition, preparation method, and asymmetry.    

Understanding the Effect of Foreign Particles on Membrane Elasticity in Lipid Vesicles

Phospholipid vesicles are excellent model systems for investigating the biophysical effect of various foreign particles on cell membranes. They are tunable self-assembled membrane structures that emulate real-life scenarios, and are thus useful to understand fundamental biophysical membrane properties and their pharmaceutical applications. Foreign particles can be ionic (salt), polymeric, or complex drug molecules. These particles interact with lipid membranes, affecting their nanoscopic structure and dynamics and simultaneously controlling their mesoscopic elastic properties. At salt concentrations identical to oceanic conditions experienced by marine life, we observed an increase in bending elasticity of model lipid membranes along with an increase in bilayer thickness. In the presence of short surfactant-like polymers, we observed the formation of multilayered structures and partial disruption of lipid bilayer causing a reduction in bending elasticity. However, for small drug molecules like Acetaminophen, quite often used as a common pain reliever, we observed an almost 50% reduction in membrane elasticity along with overall deformation of the vesicle structures. We have used a combination of neutron spin-echo (NSE), dynamic light scattering (DLS), small-angle scattering (SANS/SAXS), and cryo-TEM to bridge the gap between molecular lipid assemblies and their mesoscopic elastic properties. Our studies reveal new scaling behaviors to understand the vesicle dynamics with possible applications in topical drugs or nutraceutical formulations.   

X-ray Studies of Free Floating and Supported Bilayers Formed from Cholesterol Phospholipid Mixtures

Phospholipid bilayers form the structural scaffolding around which the plasma membranes of eukaryotic cell membranes are built.  As a consequence, the structural and mechanical properties of these bilayers provide important input for understanding functions of the membrane such as transport, trafficking and protein-membrane interactions.  We have used small angle x-ray scattering (SAXS) and x-ray reflectivity (XRF) to study the structure of bilayers comprised of varying mixtures of DPPC and Cholesterol in both the supported geometry (on top of single crystal Si-111 substrates) and as free floating liposomes.  Analysis of the scattering provides information on head group spacing, roughness, the liquid-gel transition, leaflet asymmetries and the differences between supported and free bilayers.  These results may provide insight into how cholesterol content is related to membrane function, and why certain membranes, such as the mammalian eye-lens membranes have extraordinarily high cholesterol content.    

A Dynamic Model for Cell Membrane Growth in a Genetically Minimal Cell

With 493 genes and 452 protein coding genes the synthetically engineered J. Craig Venter Institute (JCVI) Syn3A Minimal Cell provides a unique platform to study the fundamental processes of a living cell. In addition to determining the production of membrane components (phosphatidylglycerol, glycolipids, transmembrane proteins etc.) required for the cell to double from the expected cell biomass, we have developed a lipid biomass based on lipidomic measurements obtained by the Saenz Research Group (TU-Dresden). After developing these expected "production goals" of the cellular economy, we have constructed a dynamic kinetic model linking lipid metabolism and membrane protein insertion via the Sec system to cell membrane growth utilizing theoretical and experimentally determined values for lipid headgroup membrane surface area contribution. Simulating this model via multi-scale simulation methodologies that we have developed allows us to predict the variation in cell doubling times observed across populations of hundreds of cells, ranging from 90-110 minutes, that are determined by and responsive to other cellular processes such as gene expression and metabolism of key biochemical moieties, such as CTP used in phospholipid synthesis and UTP used in glycolipid synthesis.   

Non-ideality in lipid mixtures, a molecular dynamics study

Biological membranes are complex environments characterized by multicomponent lipid mixtures[1]. We investigate in this work binary lipid bilayers using the SPICA coarse-grained molecular dynamics model.  Adapting the Kirkwood-Buff theory of liquid mixtures [2] to finite wavelegth density fluctuations statistics, we compare various practical approaches for determining the interaction parameters in a theory of regular solution description of these numerical lipid mixtures.

[1] Ole G. Mouritsen, L. A. Bagatolli. Life - As A Matter Of Fat  Springer-Verlag GmbH, 2015

[2] A. Ben-Naim, Water and Aqueous Solutions: Introduction to a Molecular Theory Plenum Press, 1974   

Red Blood Cell membrane in low Reynolds flow and a vorticity-based characterization of shapes

While some existing numerical methods have successfully simulated the coupling between the fluid and the red blood cell they usually are highly complex to implement. We introduce an alternative phase-field model formulation of two-dimensional cells that solves the vorticity and the stream function that simplifies the numerical implementation [1]. We integrate the red blood cell dynamics immersed in a Poiseuille flow and reproduce previously reported morphologies (slippers or parachutes).

In the case of flow in a very wide channel, we discover a new metastable shape referred to as 'anti-parachute' that evolves into a horizontal slipper centered on the channel. This sort of metastable morphology may contribute to the dynamical response of the blood. We can also use the influence of the cell on the vorticity to characterise the stability or meta-stability of a given shape.

[1] Soft Matter, 2021, DOI: 10.1039/D1SM00559F