NBD-lipid Uptake Assay for Mammalian Cell Lines

Reconstitution of ATP-dependent lipid transporters: gaining insight into molecular characteristics, regulation, and mechanisms

Lipid transporters play a crucial role in supporting essential cellular processes such as organelle assembly, vesicular trafficking, and lipid homeostasis by driving lipid transport across membranes. Cryo-electron microscopy has recently resolved the structures of several ATP-dependent lipid transporters, but functional characterization remains a major challenge. Although studies of detergent-purified proteins have advanced our understanding of these transporters, in vitro evidence for lipid transport is still limited to a few ATP-dependent lipid transporters. Reconstitution into model membranes, such as liposomes, is a suitable approach to study lipid transporters in vitro and to investigate their key molecular features. In this review, we discuss the current approaches for reconstituting ATP-driven lipid transporters into large liposomes and common techniques used to study lipid transport in proteoliposomes. We also highlight the existing knowledge on the regulatory mechanisms that modulate the activity of lipid transporters, and finally, we address the limitations of the current approaches and future perspectives in this field.

Influenza A Virus Infection Alters Lipid Packing and Surface Electrostatic Potential of the Host Plasma Membrane

The pathogenesis of influenza A viruses (IAVs) is influenced by several factors, including IAV strain origin and reassortment, tissue tropism and host type. While such factors were mostly investigated in the context of virus entry, fusion and replication, little is known about the viral-induced changes to the host lipid membranes which might be relevant in the context of virion assembly. In this work, we applied several biophysical fluorescence microscope techniques (i.e., Förster energy resonance transfer, generalized polarization imaging and scanning fluorescence correlation spectroscopy) to quantify the effect of infection by two IAV strains of different origin on the plasma membrane (PM) of avian and human cell lines. We found that IAV infection affects the membrane charge of the inner leaflet of the PM. Moreover, we showed that IAV infection impacts lipid-lipid interactions by decreasing membrane fluidity and increasing lipid packing. Because of such alterations, diffusive dynamics of membrane-associated proteins are hindered. Taken together, our results indicate that the infection of avian and human cell lines with IAV strains of different origins had similar effects on the biophysical properties of the PM.

Visualizing Loss of Plasma Membrane Lipid Asymmetry Using Annexin V Staining

Loss of plasma membrane lipid asymmetry contributes to many cellular functions and responses, including apoptosis, blood coagulation, and cell fusion. In this protocol, we describe the use of fluorescently labeled annexin V to detect loss of lipid asymmetry in the plasma membrane of adherent living cells by fluorescence microscopy. The approach provides a simple, sensitive, and reproducible method to detect changes in lipid asymmetry but is limited by low sample throughput. The protocol can also be adapted to other fluorescently labeled lipid-binding proteins or peptide probes. To validate the lipid binding properties of such probes, we additionally describe here the preparation and use of giant unilamellar vesicles as simple model membrane systems that have a size comparable to cells. Key features Monitoring loss of lipid asymmetry in the plasma membrane via confocal microscopy. Protocol can be applied to any type of cell that is adherent in culture, including primary cells. Assay can be adapted to other fluorescently labeled lipid-binding proteins or peptide probes. Giant unilamellar vesicles serve as a tool to validate the lipid binding properties of such probes. Graphical overview Imaging the binding of fluorescent annexin V to adherent mammalian cells and giant vesicles by confocal microscopy. Annexin V labeling is a useful method for detecting a loss of plasma membrane lipid asymmetry in cells (top image, red); DAPI can be used to identify nuclei (top image, blue). Giant vesicles are used as a tool to validate the lipid binding properties of annexin V to anionic lipids (lower image, red).

Visualizing NBD-lipid Uptake in Mammalian Cells by Confocal Microscopy

Eukaryotic cells use a series of membrane transporters to control the movement of lipids across their plasma membrane. Several tools and techniques have been developed to analyze the activity of these transporters in the plasma membrane of mammalian cells. Among them, assays based on fluorescence microscopy in combination with fluorescent lipid probes are particularly suitable, allowing visualization of lipid internalization in living cells. Here, we provide a step-by-step protocol for mammalian cell culture, lipid probe preparation, cell labeling, and confocal imaging to monitor lipid internalization by lipid flippases at the plasma membrane based on lipid probes carrying a fluorophore at a short-chain fatty acid. The protocol allows studying a wide range of mammalian cell lines, to test the impact of gene knockouts on lipid internalization at the plasma membrane and changes in lipid uptake during cell differentiation. Key features Visualization and quantification of lipid internalization by lipid flippases at the plasma membrane based on confocal microscopy. Assay is performed on living adherent mammalian cells in culture. The protocol can be easily modified to a wide variety of mammalian cell lines.