Measuring the Impact of Bile Acids on the Membrane Order of Primary Hepatocytes and Isolated Mitochondria by Fluorescence Imaging and Spectroscopy

Cholestasis is characterized by impaired secretion of bile flow that can result in the accumulation of highly abnormal levels of bile acids causing hepatocyte and biliary injury. As amphipathic molecules, bile acids can intercalate in lipid membranes, and pathophysiologic concentrations of bile acids have the potential to induce marked changes in the biophysical properties of biomembranes, including membrane ordering. These effects, particularly on the mitochondrial and plasma membranes, have been proposed to trigger toxicity of bile acids. This chapter details different fluorescence-based methods (fluorescence polarization, and spectroscopy/imaging of solvatochromic dyes) to evaluate the impact of different bile acids on membrane order. Protocols are described for the application of these methods to biomimetic vesicles, isolated mitochondria, and hepatocytes, following a bottom-up approach.

Linker length in fluorophore–cholesterol conjugates directs phase selectivity and cellular localisation in GUVs and live cells

Lipid membrane fluorescent probes that are both domain-selective and compatible with demanding microscopy methods are crucial to elucidate the presence and function of rafts and domains in cells and biophysical models. Whereas targeting fluorescent probes to liquid-disordered (L_d) domains is relatively facile, it is far more difficult to direct probes with high selectivity to liquid-ordered (L_o) domains. Here, a simple, one-pot approach to probe–cholesterol conjugation is described using Steglich esterification to synthesise two identical BODIPY derivatives that differ only in the length of the aliphatic chain between the dye and cholesterol. In the first, BODIPY-Ar-Chol, the probe and cholesterol were directly ester linked and in the second BODIPY-Ahx-Chol, a hexyl linker separated probe from cholesterol. Uptake and distribution of each probe was compared in ternary, phase separated giant unilamellar vesicles (GUVs) using a commercial L_d marker as a reference. BODIPY-Ar-Chol targets almost exclusively the L_d domains with selectivity of >90% whereas by contrast introducing the C₆ linker between the probe and cholesterol drove the probe to L_o with excellent selectivity (>80%). The profound impact of the linker length extended also to uptake and distribution in live mammalian cells. BODIPY-Ahx-Chol associates strongly with the plasma membrane where it partitioned preferably into opposing micron dimensioned do-mains to a commercial L_d marker and its concentration at the membrane was reduced by cyclodextrin treatment of the cells. By contrast the BODIPY-Ahx-Chol permeated the membrane and localised strongly to lipid droplets within the cell. The data demonstrates the profound influence of linker length in cholesterol bioconjugates in directing the probe.

By inserting a hexyl linker between a BODIPY probe and cholesterol pendant, the localization of the probe at ternary phase separated GUVs switches from L_d to L_o domains with high specificity.

Phase partitioning, solvent-switchable BODIPY probes for high contrast cellular imaging and FCS

Lipophilic BODIPY fluorphores, in which the BODIPY core bears pendant dipyrido[3,2-a:2′,3′-c]phenazine (Dppz) or naphthyridyl and cholesterol substituents were designed and prepared as lipid probes for both liposomes and live cell imaging.

Membrane Order Is a Key Regulator of Divalent Cation-Induced Clustering of PI(3,5)P2 and PI(4,5)P2

Although the evidence for the presence of functionally important nanosized phosphorylated phosphoinositide (PIP)-rich domains within cellular membranes has accumulated, very limited information is available regarding the structural determinants for compartmentalization of these phospholipids. Here, we used a combination of fluorescence spectroscopy and microscopy techniques to characterize differences in divalent cation-induced clustering of PI(4,5)P₂ and PI(3,5)P₂. Through these methodologies we were able to detect differences in divalent cation-induced clustering efficiency and cluster size. Ca²⁺-induced PI(4,5)P₂ clusters are shown to be significantly larger than the ones observed for PI(3,5)P₂. Clustering of PI(4,5)P₂ is also detected at physiological concentrations of Mg²⁺, suggesting that in cellular membranes, these molecules are constitutively driven to clustering by the high intracellular concentration of divalent cations. Importantly, it is shown that lipid membrane order is a key factor in the regulation of clustering for both PIP isoforms, with a major impact on cluster sizes. Clustered PI(4,5)P₂ and PI(3,5)P₂ are observed to present considerably higher affinity for more ordered lipid phases than the monomeric species or than PI(4)P, possibly reflecting a more general tendency of clustered lipids for insertion into ordered domains. These results support a model for the description of the lateral organization of PIPs in cellular membranes, where both divalent cation interaction and membrane order are key modulators defining the lateral organization of these lipids.

Imidazolium Salts Mimicking the Structure of Natural Lipids Exploit Remarkable Properties Forming Lamellar Phases and Giant Vesicles

Tailor-made ionic liquids based on imidazolium salts have recently attracted a large amount of attention because of their extraordinary properties and versatile functionality. An intriguing ability to interact with and stabilize membranes has already been reported for 1,3-dialkylimidazolium compounds. We now reveal further insights into the field by investigating 1,3-dimethyl-4,5-dialkylimidazolium (C_n-IMe·HI, n = 7, 11, 15) and 1,3-dibenzyl-4,5-dialkylimidazolium (C_n-IBn·HBr, n = 7, 11, 15) salts. Diverse alkyl chain lengths and headgroups differing in their steric demand were employed for the membrane interface interaction with bilayer membranes imitating the cellular plasma membrane. Membrane hydration properties and domain fluidization were analyzed by fluorescent bilayer probes in direct comparison to established model membranes in a buffered aqueous environment, which resembles the salt content and pH of the cytosol of living cells. Membrane binding and insertion was analyzed via a quartz crystal microbalance and confocal laser scanning microscopy. We show that short-chain 4,5-dialkylimidazolium salts with a bulky headgroup were able to disintegrate membranes. Long-chain imidazolium salts form bilayer membrane vesicles spontaneously and autonomously without the addition of other lipids. These 4,5-dialkylimidazolium salts are highly eligible for further biochemical engineering and drug delivery.