A bifunctional imidazolium-based cholesterol analog for the tracking of cellular cholesterol distributions and cholesterol-protein interactions.

Cholesterol is a sterol lipid found in all higher eukaryotic organisms. It is required to consolidate the basic structural integrity and dynamic principles of cellular membranes and participates in many essential cellular processes that range from signal transduction to membrane traffic and metabolism. Moreover, a growing number of clinically highly relevant diseases such as immunological disorders or cancer has been linked to changes or misfunctions in cholesterol homeostasis. Therefore, the development of molecular tools that help to further unravel the role of cholesterol in essential cellular processes is of high relevance. Herein, we report the synthesis and proof-of-concept of a novel bifunctional imidazolium-based cholesterol analog (X-CHIM) that we envision to serve as a broadly applicable tool for the simultaneous investigation of cellular cholesterol distributions as well as cholesterol-protein interactions.

CHIMs are versatile cholesterol analogs mimicking and visualizing cholesterol behavior in lipid bilayers and cells.

Cholesterol is an essential component of cellular membranes regulating the structural integrity and fluidity of biological bilayers and cellular processes such as signal transduction and membrane trafficking. However, tools to investigate the role and dynamics of cholesterol in live cells are still scarce and often show limited applicability. To address this, we previously developed a class of imidazolium-based cholesterol analogs, CHIMs. Here we confirm that CHIM membrane integration characteristics largely mimic those of cholesterol. Computational studies in simulated phospholipid bilayers and biophysical analyses of model membranes reveal that in biologically relevant systems CHIMs behave similarly to natural cholesterol. Importantly, the analogs can functionally replace cholesterol in membranes, can be readily labeled by click chemistry and follow trafficking pathways of cholesterol in live cells. Thus, CHIMs represent chemically versatile cholesterol analogs that can serve as a flexible toolbox to study cholesterol behavior and function in live cells and organisms.

Bridging of membrane surfaces by annexin A2.

The protein-mediated formation of membrane contacts is a crucial event in many cellular processes ranging from the establishment of organelle contacts to the docking of vesicles to a target membrane. Annexins are Ca2+ regulated membrane-binding proteins implicated in providing such membrane contacts; however, the molecular basis of membrane bridging by annexins is not fully understood. We addressed this central question using annexin A2 (AnxA2) that functions in secretory vesicle exocytosis possibly by providing membrane bridges. By quantitatively analyzing membrane contact formation using a novel assay based on quartz crystal microbalance recordings, we show that monomeric AnxA2 can bridge membrane surfaces Ca2+ dependently. However, this activity depends on an oxidative crosslink involving a cysteine residue in the N-terminal domain and thus formation of disulfide-linked dimers. Alkylated AnxA2 in which this cysteine residue has been modified and AnxA2 mutants lacking the N-terminal domain are not capable of bridging membrane surfaces. In contrast, a heterotetrameric complex comprising two membrane binding AnxA2 subunits linked by a S100A10 dimer can provide membrane contacts irrespective of oxidation status. Thus, monomeric AnxA2 only contains one lipid binding site and AnxA2-mediated linking of membrane surfaces under non-oxidative intracellular conditions most likely requires AnxA2-S100 complex formation.

Addressable Cholesterol Analogs for Live Imaging of Cellular Membranes.

Cholesterol is an essential component of most biological membranes and serves important functions in controlling membrane integrity, organization, and signaling. However, probes to follow the dynamic distribution of cholesterol in live cells are scarce and so far show only limited applicability. Herein, we addressed this problem by synthesizing and characterizing a class of versatile and clickable cholesterol-based imidazolium salts. We show that these cholesterol analogs faithfully mimic the biophysical properties of natural cholesterol in phospholipid mono- and bilayers, and that they integrate into the plasma membrane of cultured and primary human cells. The membrane-incorporated cholesterol analogs can be specifically labeled by click chemistry and visualized in live-cell imaging experiments that show a distribution and behavior comparable with that of endogenous membrane cholesterol. These results indicate that the cholesterol analogs can be used to reveal the dynamic distribution of cholesterol in live cells.

Delivery of cationic quantum dots using fusogenic liposomes in living cells.

Quantum dots (QDs) are fluorescent nanocrystals that present unique optical properties, especially a high photostability. However, their use for intracellular studies is still limited since their passage through the living cell membranes does not occur passively. In this work, we adapted the ethanol injection method to encapsulate cationic hydrophilic QDs into fusogenic liposomes, to deliver them in living cells. Liposomes were characterized using zeta potential, dynamic light scattering (DLS), fluorescence microscopy and transmission electron microscopy (TEM). Red blood cells (RBCs) were applied as models in this study to probe the liposome fusion with the cell membrane since RBCs do not present endocytic activity. Therefore, HeLa cells were also applied to test the QDs delivery by the liposomes. The TEM and the fluorescence microscopy confirmed the QDs encapsulation, with an efficiency of 43%, determined by UV-vis spectroscopy. Zeta potential showed that the QDs-loaded fusogenic liposomes were positively charged and presented an average size of 343nm, determined by DLS. Furthermore, fluorescence microscopy analyses of RBCs and HeLa cells confirmed the liposomes fusion with the cell membrane and suggested the release of QDs into cells. Thus, we expect that this work will contribute to improve the use of QDs as fluorescent probes to intracellular studies.

Studies on intracellular delivery of carboxyl-coated CdTe quantum dots mediated by fusogenic liposomes.

The use of Quantum Dots (QDs) as fluorescent probes for understanding biological functions has emerged as an advantageous alternative over application of conventional fluorescent dyes. Intracellular delivery of QDs is currently a specific field of research. When QDs are tracking a specific target in live cells, they are mostly applied for extracellular membrane labeling. In order to study intracellular molecules and structures it is necessary to deliver free QDs into the cell cytosol. In this work, we adapted the freeze and thaw method to encapsulate water dispersed carboxyl-coated CdTe QDs into liposomes of different compositions, including cationic liposomes with fusogenic properties. We showed that labeled liposomes were able to fuse with live human stem cells and red blood cells in an endocytic-independent way. We followed the interactions of liposomes containing QDs with the cells. The results were minutely discussed and showed that QDs were delivered, but they were not freely diffused in the cytosol of those cells. We believe that this approach has the potential to be applied as a general route for encapsulation and delivery of any membrane-impermeant material into living cells.