Creating Supported Plasma Membrane Bilayers Using Acoustic Pressure

CaV1.3 channel clusters characterized by live-cell and isolated plasma membrane nanoscopy

A key player of excitable cells in the heart and brain is the L-type calcium channel CaV1.3. In the heart, it is required for voltage-dependent Ca2+-signaling, i.e., for controlling and modulating atrial cardiomyocyte excitation-contraction coupling. The clustering of CaV1.3 in functionally relevant channel multimers has not been addressed due to a lack of stoichiometric labeling combined with high-resolution imaging. Here, we developed a HaloTag-labeling strategy to visualize and quantify CaV1.3 clusters using STED nanoscopy to address the questions of cluster size and intra-cluster channel density. Channel clusters were identified in the plasma membrane of transfected live HEK293 cells as well as in giant plasma membrane vesicles derived from these cells that were spread on modified glass support to obtain supported plasma membrane bilayers (SPMBs). A small fraction of the channel clusters was colocalized with early and recycling endosomes at the membranes. STED nanoscopy in conjunction with live-cell and SPMB imaging enabled us to quantify CaV1.3 cluster sizes and their molecular density revealing significantly lower channel densities than expected for dense channel packing. CaV1.3 channel cluster size and molecular density were increased in SPMBs after treatment of the cells with the sympathomimetic compound isoprenaline, suggesting a regulated channel cluster condensation mechanism.

Micropatterning of functional lipid bilayer assays for quantitative bioanalysis

Interactions of the cell with its environment are mediated by the cell membrane and membrane-localized molecules. Supported lipid bilayers have enabled the recapitulation of the basic properties of cell membranes and have been broadly used to further our understanding of cellular behavior. Coupled with micropatterning techniques, lipid bilayer platforms have allowed for high throughput assays capable of performing quantitative analysis at a high spatiotemporal resolution. Here, an overview of the current methods of the lipid membrane patterning is presented. The fabrication and pattern characteristics are briefly described to present an idea of the quality and notable features of the methods, their utilizations for quantitative bioanalysis, as well as to highlight possible directions for the advanced micropatterning lipid membrane assays.

Supported Natural Membranes on Microspheres for Protein-Protein Interaction Studies

Multiple biological and pathological processes, such as signaling, cell-cell communication, and infection by various viruses, occur at the plasma membrane. The eukaryotic plasma membrane is made up of thousands of different lipids, membrane proteins, and glycolipids, and its composition is dynamic and constantly changing. Due to the central importance of membranes on the one hand and their complexity on the other, membrane model systems are instrumental for interrogating membrane-related biological processes. Here, we develop a new tool for protein-membrane interaction studies. Our method is based on natural membranes obtained from extracellular vesicles. We form membrane bilayers supported on polystyrene microspheres that can be trapped and manipulated using optical tweezers. This method allows working with membrane proteins of interest within a background of native membrane components where their correct orientation is preserved. We demonstrate our method's applicability by successfully measuring the interaction forces between the Spike protein of SARS-CoV-2 and its human receptor, ACE2. We further show that these interactions are blocked by the addition of an antibody against the receptor binding domain of the Spike protein. Our approach is versatile and broadly applicable for various membrane biology and biophysics questions.

Giant plasma membrane vesicles to study plasma membrane structure and dynamics

The plasma membrane (PM) is a highly heterogenous structure intertwined with the cortical actin cytoskeleton and extracellular matrix. This complex architecture makes it difficult to study the processes taking place at the PM. Model membrane systems that are simple mimics of the PM overcome this bottleneck and allow us to study the biophysical principles underlying the processes at the PM. Among them, cell-derived giant plasma membrane vesicles (GPMVs) are considered the most physiologically relevant system, retaining the compositional complexity of the PM to a large extent. GPMVs have become a key tool in membrane research in the last few years. In this review, I will provide a brief overview of this system, summarize recent applications and discuss the limitations.

Special Issue: Dynamics and Nano-Organization in Plasma Membranes

Cell membranes develop extraordinarily complex lipids and proteins geared to perform functions required by cells [...].

Pore-Spanning Plasma Membranes Derived from Giant Plasma Membrane Vesicles

Giant plasma membrane vesicles (GPMVs) are a highly promising model system for the eukaryotic plasma membrane. The unresolved challenge, however, is a path to surface-based structures that allows accessibility to both sides of the plasma membrane through high-resolution techniques. Such an approach would pave the way to advanced chip-based technologies for the analysis of complex cell surfaces to study the roles of membrane proteins, host-pathogen interactions, and many other bioanalytical and sensing applications. This study reports the generation of planar supported plasma membranes and for the first-time pore-spanning plasma membranes (PSPMs) derived from pure GPMVs that are spread on activated solid and highly ordered porous silicon substrates. GPMVs were produced by two different vesiculation agents and were first investigated with respect to their growth behavior and phase separation. Second, these GPMVs were spread onto silicon substrates to form planar supported plasma membrane patches. PSPMs were obtained by spreading of pure GPMVs on oxygen-plasma activated porous substrates with pore diameters of 3.5 μm. Fluorescence micrographs unambiguously showed that the PSPMs partially phase separate in a mobile ordered phase surrounded by a disordered phase, which was supported by cholesterol extraction using methyl-β-cyclodextrin.