Continuous Pore-Spanning Lipid Bilayers on Silicon Oxide-Coated Porous Substrates

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.

Pore-spanning membranes as a tool to investigate lateral lipid membrane heterogeneity

Over the years, it has become more and more obvious that lipid membranes show a very complex behavior. This behavior arises in part from the large number of different kinds of lipids and proteins and how they dynamically interact with each other. In vitro studies using artificial membrane systems have shed light on the heterogeneity based on lipid-lipid interactions in multicomponent bilayer mixtures. Inspired by the raft hypothesis, the coexistence of liquid-disordered (ld) and liquid-ordered (lo) phases has drawn much attention. It was shown that ternary lipid mixtures containing low- and high-melting temperature lipids and cholesterol can phase separate into a lo phase enriched in the high-melting lipids and cholesterol and a ld phase enriched in the low-melting lipids. Depending on the model membrane system under investigation, different domain sizes, shapes, and mobilities have been found. Here, we describe how to generate phase-separated lo/ld phases in model membrane systems termed pore-spanning membranes (PSMs). These PSMs are prepared on porous silicon substrates with pore sizes in the micrometer regime. A proper functionalization of the top surface of the substrates is required to achieve the spreading of giant unilamellar vesicles (GUVs) to obtain PSMs. Starting with lo/ld phase-separated GUVs lead to membrane heterogeneities in the PSMs. Depending on the functionalization strategy of the top surface of the silicon substrate, different membrane heterogeneities are observed in the PSMs employing fluorescence microscopy. A quantitative analysis of the heterogeneity as well as the dynamics of the lipid domains is described.

From LUVs to GUVs─How to Cover Micrometer-Sized Pores with Membranes

Pore-spanning membranes (PSMs) are a versatile tool to investigate membrane-confined processes in a bottom-up approach. Pore sizes in the micrometer range are most suited to visualize PSMs using fluorescence microscopy. However, the preparation of these PSMs relies on the spreading of giant unilamellar vesicles (GUVs). GUV production faces several limitations. Thus, alternative ways to generate PSMs starting from large or small unilamellar vesicles that are more reproducibly prepared are highly desirable. Here we describe a method to produce PSMs obtained from large unilamellar vesicles, making use of droplet-stabilized GUVs generated in a microfluidic device. We analyzed the lipid diffusion in the free-standing and supported parts of the PSMs using z-scan fluorescence correlation spectroscopy and fluorescence recovery after photobleaching experiments in combination with finite element simulations. Employing atomic force indentation experiments, we also investigated the mechanical properties of the PSMs. Both lipid diffusion constants and lateral membrane tension were compared to those obtained on PSMs derived from electroformed GUVs, which are known to be solvent- and detergent-free, under otherwise identical conditions. Our results demonstrate that the lipid diffusion, as well as the mechanical properties of the resulting PSMs, is almost unaffected by the GUV formation procedure but depends on the chosen substrate functionalization. With the new method in hand, we were able to reconstitute the syntaxin-1A transmembrane domain in microfluidic GUVs and PSMs, which was visualized by fluorescence microscopy.

The Interaction of Gb3 Glycosphingolipids with ld and lo Phase Lipids in Lipid Monolayers Is a Function of Their Fatty Acids

The glycosphingolipid Gb₃ is a specific receptor of the bacterial Shiga toxin (STx). Binding of STx to Gb₃ is a prerequisite for its internalization into the host cells, and the ceramide's fatty acid of Gb₃ has been shown to influence STx binding. In in vitro studies on liquid ordered (l_o)/liquid disordered (l_d) coexisting artificial membranes, Shiga toxin B (STxB) binds solely to l_o domains, thus harboring Gb₃ concomitant with an observed lipid redistribution process. These findings raise the question of how the molecular structure of the fatty acid of Gb₃ influences the interaction of Gb₃ with the different lipids preferentially either found in the l_o phase, namely, sphingomyelin and cholesterol, or in the l_d phase. We addressed this question by using a series of synthetically available and unlabeled Gb₃ glycosphingolipids carrying different long chain C₂₄ fatty acids (saturated, monounsaturated, and α-hydroxylated). In conjunction with surface tension experiments on Langmuir monolayers, we quantified the excess of free energy of mixing of the different Gb₃ species in monolayers composed of either sphingomyelin or cholesterol or composed of a fluid phase lipid (DOPC). From a calculation of the total free energy of mixing, we conclude that mixing of the saturated Gb₃ species with the l_d lipid DOPC is energetically less favorable than all other combinations, while the unsaturated species mix equally well with the l_o phase lipids sphingomyelin and cholesterol and the l_d phase lipid DOPC. Furthermore, we found that STxB partially penetrates in mixed lipid monolayers (DOPC/sphingomyelin/cholesterol) containing the Gb₃ sphingolipid with a saturated or a monounsaturated C₂₄ fatty acid. The maximum insertion pressure, as a measure for protein insertion, is >30 mN/m for both Gb₃ molecules and is not significantly different for the two Gb₃ species.

ENTH domain-dependent membrane remodelling

Cellular membranes are anything but flat structures. They display a wide variety of complex and beautiful shapes, most of which have evolved for a particular physiological reason and are adapted to accommodate certain cellular demands. In membrane trafficking events, the dynamic remodelling of cellular membranes is apparent. In clathrin-mediated endocytosis for example, the plasma membrane undergoes heavy deformation to generate and internalize a highly curved clathrin-coated vesicle. This process has become a model system to study proteins with the ability to sense and induce membrane curvature and over the last two decades numerous membrane remodelling molecules and molecular mechanisms have been identified in this process. In this review, we discuss the interaction of epsin1 ENTH domain with membranes, which is one of the best-studied examples of a peripheral and transiently membrane bending protein important for clathrin-mediated endocytosis.

Phase separation in pore-spanning membranes induced by differences in surface adhesion

A porous scaffold providing different adhesion energies alters the behaviour of coexisting phases in lipid membranes considerably.

Shiga toxin binding alters lipid packing and the domain structure of Gb3-containing membranes: a solid-state NMR study

We studied the influence of globotriaosylceramide (Gb3) lipid molecules on the properties of phospholipid membranes composed of a liquid ordered (lo)/liquid disordered (ld) phase separated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/N-palmitoyl-d-erythro-sphingosylphosphorylcholine (PSM)/cholesterol mixture (40/35/20, mol/mol/mol) supplemented with 5 mol% of either short acyl chain palmitoyl-Gb3 or long acyl chain lignoceryl-Gb3 using 2H solid-state NMR spectroscopy. To this end, both globotriaosylceramides were chemically synthesized featuring a perdeuterated lipid acyl chain. The solid-state 2H NMR spectra support the phase separation into a POPC-rich ld phase and a PSM/cholesterol-rich lo phase. The long chain lignoceryl-Gb3 showed a rather unusual order parameter profile of the acyl chain, which flattens out for the last ∼6 methylene segments. Such an odd chain conformation can be explained by partial chain interdigitation and/or a very fluid midplane region of the membrane. Possibly, the Gb3 molecules may thus preferentially be localized at the lo/ld phase boundary. In contrast, the short chain palmitoyl-Gb3 was well associated with the PSM/cholesterol-rich lo phase. Gb3 molecules act as membrane receptors for the Shiga toxin (STx) produced by Shigella dysenteriae and by enterohemorrhagic strains of Escherichia coli (EHEC). The B-subunits of STx (STxB) forming a pentameric structure were produced recombinantly and incubated with the membrane mixtures leading to alterations in the lipid packing properties and lateral organization of the membranes. Typically, STxB binding led to a decrease in lipid chain order in agreement with partial immersion of protein segments into the lipid-water interface of the membrane. In the presence of STxB, Gb3 preferentially partitioned into the lo membrane phase. In particular the short acyl chain palmitoyl-Gb3 showed very similar chain order parameters to PSM. In the presence of STxB, all lipid species showed isotropic contributions to the 2H NMR powder spectra; this was most pronounced for the Gb3 molecules. Such isotropic contributions are caused by highly curved membrane structures, which have previously been detected as membrane invaginations in fluorescence microscopy. Our analysis estimated that STxB induced highly curved membrane structures with a curvature radius of less than ∼10 nm likely related to the insertion of STxB segments into the lipid-water interface of the membrane.