Interaction of lysophospholipid/taurodeoxycholate submicellar aggregates with phospholipid bilayers

Surfactant-Mediated Assembly of Precision-Size Liposomes

Liposomes can greatly improve the pharmacokinetics of therapeutic agents due to their ability to encapsulate drugs and accumulate in target tissues. Considerable effort has been focused on methods to synthesize these nanocarriers in the past decades. However, most methods fail to controllably generate lipid vesicles at specific sizes and with low polydispersity, especially via scalable approaches suitable for clinical product manufacturing. Here, we report a surfactant-assisted liposome assembly method enabling the precise production of monodisperse liposomes with diameters ranging from 50 nm to 1 μm. To overcome scalability limitations, we used tangential flow filtration, a scalable size-based separation technique, to readily concentrate and purify the liposomal samples from more than 99.9% of detergent. Further, we propose two modes of liposome self-assembly following detergent dilution to explain the wide range of liposome size control, one in which phase separation into lipid-rich and detergent-rich phases drives the formation of large bilayer liposomes and a second where the rate of detergent monomer partitioning into solution controls bilayer leaflet imbalances that promote fusion into larger vesicles. We demonstrate the utility of controlled size assembly of liposomes by evaluating nanoparticle uptake in macrophages, where we observe a clear linear relationship between vesicle size and total nanoparticle uptake.

RIBEYE(B)-domain binds to lipid components of synaptic vesicles in an NAD(H)-dependent, redox-sensitive manner

Synaptic ribbons are needed for fast and continuous exocytosis in ribbon synapses. RIBEYE is a main protein component of synaptic ribbons and is necessary to build the synaptic ribbon. RIBEYE consists of a unique A-domain and a carboxyterminal B-domain, which binds NAD(H). Within the presynaptic terminal, the synaptic ribbons are in physical contact with large numbers of synaptic vesicle (SV)s. How this physical contact between ribbons and synaptic vesicles is established at a molecular level is not well understood. In the present study, we demonstrate that the RIBEYE(B)-domain can directly interact with lipid components of SVs using two different sedimentation assays with liposomes of defined chemical composition. Similar binding results were obtained with a SV-containing membrane fraction. The binding of liposomes to RIBEYE(B) depends upon the presence of a small amount of lysophospholipids present in the liposomes. Interestingly, binding of liposomes to RIBEYE(B) depends on NAD(H) in a redox-sensitive manner. The binding is enhanced by NADH, the reduced form, and is inhibited by NAD⁺, the oxidized form. Lipid-mediated attachment of vesicles is probably part of a multi-step process that also involves additional, protein-dependent processes.

Slow Phospholipid Exchange between a Detergent-Solubilized Membrane Protein and Lipid-Detergent Mixed Micelles: Brominated Phospholipids as Tools to Follow Its Kinetics

Membrane proteins are largely dependent for their function on the phospholipids present in their immediate environment, and when they are solubilized by detergent for further study, residual phospholipids are critical, too. Here, brominated phosphatidylcholine, a phospholipid which behaves as an unsaturated phosphatidylcholine, was used to reveal the kinetics of phospholipid exchange or transfer from detergent mixed micelles to the environment of a detergent-solubilized membrane protein, the paradigmatic P-type ATPase SERCA1a, in which Trp residues can experience fluorescence quenching by bromine atoms present on phospholipid alkyl chains in their immediate environment. Using dodecylmaltoside as the detergent, exchange of (brominated) phospholipid was found to be much slower than exchange of detergent under the same conditions, and also much slower than membrane solubilization, the latter being evidenced by light scattering changes. The kinetics of this exchange was strongly dependent on temperature. It was also dependent on the total concentration of the mixed micelles, revealing the major role for such exchange of the collision of detergent micelles with the detergent-solubilized protein. Back-transfer of the brominated phospholipid from the solubilized protein to the detergent micelle was much faster if lipid-free DDM micelles instead of mixed micelles were added for triggering dissociation of brominated phosphatidylcholine from the solubilized protein, or in the additional presence of C₁₂E₈ detergent during exchange, also emphasizing the role of the chemical nature of the micelle/protein interface. This protocol using brominated lipids appears to be valuable for revealing the possibly slow kinetics of phospholipid transfer to or from detergent-solubilized membrane proteins. Independently, continuous recording of the activity of the protein can also be used in some cases to correlate changes in activity with the exchange of a specific phospholipid, as shown here by using the Drs2p/Cdc50p complex, a lipid flippase with specific binding sites for lipids.

From interaction of lipidic vehicles with intestinal epithelial cell membranes to the formation and secretion of chylomicrons

Lipophilic drugs are carried by chylomicrons that are secreted by the small intestine and transported in lymph. This review discusses the digestion, uptake, and transport of dietary lipids and the impact that these processes have on the absorption of lipophilic drugs by the gastrointestinal tract. This chapter complements Dr. Chris Potter's chapter on the "pre-absorptive" events of drug processing and solubilization. This chapter reviews the digestion of lipids in the gastric and intestinal lumen and the role of bile salts in the solubilization of lipid digestion products for uptake by the gut. Both the passive and active uptake of lipid digestion products is discussed. How intestinal lipid transporters located at the brush border membrane may play a role in the uptake of lipids by the enterocytes is examined, as is the regulation of the absorption of cholesterol by the human ATP-binding cassette transporter-1 (ABC1). The intracellular trafficking and the resynthesis of complex lipids from lipid digestion products are explored, and the formation and secretion of chylomicrons are described.

Lipid exchange between mixed micelles of phospholipid and triton X-100

If phospholipase catalyzed hydrolysis of phospholipid dissolved in a detergent mixed micelle is limited to the phospholipid carried by a single micelle, then hydrolysis ceases upon exhaustion of that pool. However, if the rate of phospholipid exchange between micelles exceeds the catalytic rate then all of the phospholipid is available for hydrolysis. To determine phospholipid availability we studied the exchange of 1,2-dioleoyl-sn-glycero-3-phosphocholine between mixed micelles of phospholipid and non-ionic Triton detergents by both stopped-flow fluorescence-recovery and nuclear magnetic resonance-relaxation techniques. Stopped-flow analysis was performed by combining mixed micelles of Triton and phospholipid with mixed micelles that contained the fluorescent phospholipid 1-palmitoyl-2-(12-[{7-nitro-2-1, 3-benzoxadiazo-4-yl}amino]dodecanoyl)-sn-glycero-3-phosphocholine (P-2-NBD-PC). The concentration dependence of fluorescence recovery suggested a second-order exchange mechanism that was saturable. The true second-order rate constant depends on the specific mechanism for exchange, which was not determined in this study, but the rate constant will be on the order of 106 to 107 M-1s-1. Incorporation of 1-palmitoyl-2-(16-doxylstearoyl)phosphatidylcholine into micelles increased the rate of proton relaxation and gave a limiting relaxation time of 1.3 ms. The results demonstrate that phospholipid exchange was rapid and that the phospholipid content of a single micelle did not limit the rate of phospholipid hydrolysis by phospholipases.

Time-resolved fluorescence anisotropy of fluorescent-labeled lysophospholipid and taurodeoxycholate aggregates

Previous work from this laboratory demonstrated that the environment-sensitive lysolipid N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)- monomyristoylphosphatidylethanolamine (N-NBD-MPE), at concentrations below its critical micelle concentration (CMCN-NBD-MPE = 4 microM), reached maximum fluorescence yield upon the addition of taurodeoxycholate (TDC) at concentrations well below its CMC (CMCTDC = 2.5 mM). These data indicated the formation of micellar aggregates of the two amphiphiles at concentrations below both of their CMCs. In the present study, fluorescence lifetime and differential polarization measurements were made to determine the size of these aggregates. In the absence of TDC and at 0.5 mM TDC a single lifetime (tau) and rotational correlation time (phi) were measured for N-NBD-MPE at the submicellar concentration of 2 microM, indicating a lack of interaction between the two molecules at this concentration. Above 0.5 mM TDC, two discrete lifetimes were resolved. Based on these lifetimes, two distinct rotational correlation times were established through polarization measurements. The shorter phi(0.19-0.73 ns) was ascribed to local probe motions, whereas the longer phi was in a time range expected for global rotation of aggregates the size of simple bile salt micelles (3-6.5 ns). From the longer phi, molecular volume and hydrodynamic radii were calculated, ranging from approximately 15 A at 1 mM to approximately 18 A at 5 mM TDC. These data support the conclusion that monomeric lysolipids in solution seed the aggregation of numerous TDC molecules (aggregation number = 16 at 1 mM TDC) to form a TDC micelle with a lysolipid core at concentrations below which they both self-aggregate.

Movement of fatty acids, fatty acid analogues, and bile acids across phospholipid bilayers

How lipophilic acids move across membranes, either model or biological, is the subject of controversy. We describe experiments which better define the mechanism and rates in protein-free phospholipid bilayers. The transbilayer movement of lipophilic acids [fatty acids (FA), covalently-labeled FA, bile acids, and retinoic acid] was monitored by entrapping pyranin, a water-soluble, pH-sensitive fluorescent molecule to measure pH inside unilamellar vesicles [Kamp, F., & Hamilton, J.A. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11367-11370]. Equations for the pseudo-unimolecular rate constants for transbilayer movement of un-ionized (kappa FAH) and ionized (kappa FA-) acids are derived. All FA studied (octanoic, lauric, myristic, palmitic, stearic, oleic, elaidic, linoleic, linolelaidic, and arachidonic) and retinoic acid exhibited rapid transbilayer movement (t 1/2 < 1 s) via the un-ionized form across small unilamellar egg phosphatidylcholine (PC) vesicles. FA produced by phospholipase A2 in the outer leaflet of PC vesicles equilibrated rapidly to the inner leaflet. Ionized FA showed enhanced transbilayer movement (kappa FA- = 0.029 s-1) in the presence of equimolar valinomycin. The three FA analogues [12-(9-anthroyloxy)stearic acid, 5-doxylstearic acid, and 1-pyrenenonanoic acid] moved across PC bilayers via the un-ionized form; except for the anthroyloxy FA (kappa FAH = 4.8 x 10(-3) s-1), the rates were too fast to measure (t 1/2 < 1 s). The rate for cholic acid (CA) transbilayer movement was slow (kappa CAH = 0.056 s-1) compared to that of the more hydrophobic bile acids, deoxy- and chenodeoxycholic acid (t 1/2 < 1 s). The taurine conjugates of the three bile acids did not cross the bilayer (t 1/2 > 1 h). A further application of the pyranin method was to measure the partitioning of FA and bile acids among water, albumin, and PC vesicles. Our results show that the ability of lipophilic acids to permeate a PC bilayer rapidly is dependent on the presence of the un-ionized acid in the membrane interface. Considering the fast unfacilitated movement of FA across protein-free phospholipid bilayers, it is unlikely that there is a universal need for a transport protein to enhance movement of FA across membrane bilayers. Physiological implications of proton movement accompanying fast movement of un-ionized lipophilic acids (and the consequent generation of a pH gradient) are discussed.