Multilamellar Liposomes as a Model for Biological Membranes: Saturation Recovery EPR Spin-Labeling Studies

5-PC as a Lipid Probe Molecule and as a Second Phospholipid in Binary Phospholipid Mixtures: Saturation Recovery EPR Studies

Mixtures of two phospholipids (PLs) with different main phase transition temperatures were investigated. Host PLs (HPLs) were represented by DMPC, DPPC, DSPC, and DMPE. The admixed PL was the spin-labeled phosphatidylcholine 5-PC(1-palmitoyl-2-(5-doxylstearoyl)phosphatidylcholine), with a unique opportunity to monitor the properties and the local environments of all admixed PL molecules using saturation recovery EPR methods. Below the HPL phase transition temperatures, 5-PC mixes with HPL to form two distinct pools with different rotational diffusion rates. The fluidity of the local environment in these two pools is very different, being more fluid for molecules with greater rotational diffusion rates. Above the HPL phase transition temperature, 5-PC mixes with HPL uniformly. This is independent of the HPL, observed for 5-PC concentrations from 0.25 mol% up to 20 mol% and for the wide temperature range. Assuminga very low concentration of 5-PC is an ideal probe molecule, we can conclude that small fluid phase domains made of HPL molecules are formed below the phase transition temperature of the HPL bilayers. In binary mixtures of HPLs with 5-PC, below the phase transition of HPL bilayers, fluid phase domains are created within the bulk gel phase of HPL lipids by the admixed second PL, namely 5-PC.

A Reliable Quantification of Cholesterol and 25-Hydroxycholesterol in Liposomal Adjuvant Formulation by Liquid Chromatography High-Resolution Tandem Mass Spectrometry

Cholesterol, as one of the major components of liposomes, plays a critical role in modulating membrane bilayer permeability, fluidity, and structural stability. Controlling these quality attributes is essential to maintaining the efficacy and fitness of the liposomes in various applications. However, during the manufacture and storage of liposomes, cholesterol has a propensity to undergo oxidative degradation. Hence, an analytical tool that is capable of determining not only the identity and quantity of cholesterol but also its associated degradants is a prerequisite to effective process control and product quality and safety assessments. In this view, a new liquid chromatography electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) method with parallel reaction monitoring (PRM) was developed and qualified to accurately quantify cholesterol and monitor the formation of 25-hydroxycholesterol degradant in liposomal drug formulations without the use of an isotopic internal standard (IS). The method was qualified according to the FDA Quality Guidance for Industry: Q2(R1). Study results showed that the method presents good specificity for cholesterol and 25-hydroxycholesterol detection in the liposomal matrix, good sensitivity characterized by LOD/LOQ in the nanomolar range, and accuracy within the range of 80 to 120%. The described method enables accurate evaluation of in-process and product release samples of Army Liposome Formulation with QS21 (ALFQ).

Immunological Regulation of Gut-Tropic Immune Cells by Extracellular Vesicles

The remarkable diversity of lymphocytes, essential components of the immune system, serves as an ingenious mechanism for maximizing the efficient utilization of limited host defense resources. While cell adhesion molecules, notably in gut-tropic T cells, play a central role in this mechanism, the counterbalancing molecular details have remained elusive. Conversely, we've uncovered the molecular pathways enabling extracellular vesicles secreted by lymphocytes to reach the gut's mucosal tissues, facilitating immunological regulation. This discovery sheds light on immune fine-tuning, offering insights into immune regulation mechanisms.

Spin-Lattice Relaxation Rates of Lipid Spin Labels as a Measure of Their Rotational Diffusion Rates in Lipid Bilayer Membranes

The spin-lattice relaxation rate (T₁^-1) of lipid spin labels obtained from saturation recovery EPR measurements in deoxygenated membranes depends primarily on the rate of the rotational diffusion of the nitroxide moiety within the lipid bilayer. It has been shown that T₁^-1 also can be used as a qualitative convenient measure of membrane fluidity that reflects local membrane dynamics; however, the relation between T₁^-1 and rotational diffusion coefficients was not provided. In this study, using data previously presented for continuous wave and saturation recovery EPR measurements of phospholipid analog spin labels, one-palmitoyl-2-(n-doxylstearoyl)phosphatidylcholine in 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine/cholesterol membranes, we show that measured T₁^-1 values are linear functions of rotational diffusion of spin labels. Thus, these linear relationships can be used to transfer T₁^-1 values into spin label rotational rates as a precise description of membrane fluidity. This linearity is independent through the wide range of conditions including lipid environment, depth in membrane, local hydrophobicity, and the anisotropy of rotational motion. Transferring the spin-lattice relaxation rates into the rotational diffusion coefficients makes the results obtained from saturation recovery EPR spin labeling easy to understand and readily comparable with other membrane fluidity data.