Apolipoprotein A-I localization and dipalmitoylphosphatidylcholine dynamics in reconstituted high density lipoproteins

The Composition of Reconstituted High-Density Lipoproteins (rHDL) Dictates the Degree of rHDL Cargo- and Size-Remodeling via Direct Interactions with Endogenous Lipoproteins

The application of reconstituted high-density lipoproteins (rHDL) as a drug-carrier has during the past decade been established as a promising approach for effective receptor-mediated drug delivery, and its ability to target tumors has recently been confirmed in a clinical trial. The rHDL mimics the endogenous HDL, which is known to be highly dynamic and undergo extensive enzyme-mediated remodulations. Hence, to reveal the physiological rHDL stability, a thorough characterization of the dynamics of rHDL in biologically relevant environments is needed. We employ a size-exclusion chromatography (SEC) method to evaluate the dynamics of discoidal rHDL in fetal bovine serum (FBS), where we track both the rHDL lipids (by the fluorescence from lipid-conjugated fluorophores) and apoA-I (by human apoA-I ELISA). We show by using lipoprotein depleted FBS and isolated lipoproteins that rHDL lipids can be transferred to endogenous lipoproteins via direct interactions in a nonenzymatic process, resulting in rHDL compositional- and size-remodeling. This type of dynamics could lead to misinterpretations of fluorescence-based rHDL uptake studies due to desorption of labile lipophilic fluorophores or off-target side effects due to desorption of incorporated drugs. Importantly, we show how the degree of rHDL remodeling can be controlled by the compositional design of the rHDL. Understanding the correlation between the molecular properties of the rHDL constituents and their collective dynamics is essential for improving the rHDL-based drug delivery platform. Taken together, our work highlights the need to carefully consider the compositional design of rHDL and test its stability in a biological relevant environment, when developing rHDL for drug delivery purposes.

Composition, structure and substrate properties of reconstituted discoidal HDL with apolipoprotein A-I and cholesteryl ester

To investigate the influence of lipid unsaturation and neutral lipid on the maturation of high density lipoproteins, the discoidal complexes of apoA-I, phosphatidylcholine and cholesteryl ester (CE) were prepared. Saturated dipalmitoylphosphatidylcholine (DPPC) and unsaturated palmitoyllinoleoylphosphatidylcholine (PLPC), palmitoyloleoylphosphatidylcholine (POPC), and fluorescent probe cholesteryl 1-pyrenedecanoate (CPD) that forms in a diffusion- and concentration-dependent manner short-lived dimer of unexcited and excited molecules (excimer) were used. The apoA-I/DPPC/CPD complexes were heterogeneous by size, composition and probe location. CPD molecules incorporated more efficiently into larger complexes and accumulated in a central part of the discs. The apoA-I/POPC(PLPC)/CPD were also heterogeneous, however, probe molecules distributed preferentially into smaller complexes and accumulated at disc periphery. The kinetics of CPD transfer by recombinant cholesteryl ester transfer protein (CETP) to human plasma LDL is well described by two-exponential decay, the fast component with a shorter transfer time being more populated in PLPC compared to DPPC complexes. The presence of CE molecules in discoidal HDL results in particle heterogeneity. ApoA-I influences the CETP activity modulating the properties of apolipoprotein-phospholipid interface. This may include CE molecules accumulation in the boundary lipid in unsaturated phosphatidylcholine and cluster formation in the bulk bilayer in saturated phosphatidylcholine.

Thermotropic phase transition in soluble nanoscale lipid bilayers

The role of lipid domain size and protein-lipid interfaces in the thermotropic phase transition of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) bilayers in Nanodiscs was studied using small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and generalized polarization (GP) of the lipophilic probe Laurdan. Nanodiscs are water-soluble, monodisperse, self-assembled lipid bilayers encompassed by a helical membrane scaffold protein (MSP). MSPs of different lengths were used to define the diameter of the Nanodisc lipid bilayer from 76 to 108 A and the number of DPPC molecules from 164 to 335 per discoidal structure. In Nanodiscs of all sizes, the phase transitions were broader and shifted to higher temperatures relative to those observed in vesicle preparations. The size dependences of the transition enthalpies and structural parameters of Nanodiscs reveal the presence of a boundary lipid layer in contact with the scaffold protein encircling the perimeter of the disc. The thickness of this annular layer was estimated to be approximately 15 A, or two lipid molecules. SAXS was used to measure the lateral thermal expansion of Nanodiscs, and a steep decrease of bilayer thickness during the main lipid phase transition was observed. These results provide the basis for the quantitative understanding of cooperative phase transitions in membrane bilayers in confined geometries at the nanoscale.

trans-Parinaric acid as a versatile spectroscopic label to study ligand binding properties of bovine beta-lactoglobulin

Advantageous spectroscopic properties of the plant derived polyunsaturated trans-parinaric acid (tPnA) was demonstrated in obtaining valuable data on the ligand binding characteristics of the lipocalin member bovine beta-lactoglobulin A (BLG-A). Titration of the protein with tPnA resulted in the appearance of an intense negative induced circular dichroism (CD) band and bathochromic shift of the ultraviolet (UV) peak of the ligand. The extrinsic optical activity was interpreted by the chiral contribution of the allylic axial CH bonds of tPnA to the pi-pi(*) transition of the planar tetraene chromophore. Analysis of the series of induced CD curves obtained by CD titration experiment indicated the complexation of a single ligand molecule to a uniform protein binding site. Additionally, the dramatic increase of fluorescence intensity of the lactoglobulin bound ligand suggested the hydrophobic nature of the binding site. CD and fluorescence titration data were utilized to calculate the binding constant (K(a)) of which high value ( approximately 10(6)M(-1)) refers to strong protein association of tPnA. pH dependent reversible dis- and reappearance of the induced CD signal unambigously proved the inclusion of tPnA into the central hydrophobic cavity of the lactoglobulin governed by the protonation induced conformational movement of the EF loop at the opening of the calyx. This conclusion was supported and complemented by molecular docking calculations.

Directed self-assembly of monodisperse phospholipid bilayer Nanodiscs with controlled size

Using a recently described self-assembly process (Bayburt, T. H.; Grinkova, Y. V.; Sligar, S. G. Nano Letters 2002, 2, 853-856), we prepared soluble monodisperse discoidal lipid/protein particles with controlled size and composition, termed Nanodiscs, in which the fragment of dipalmitoylphosphatidylcholine (DPPC) bilayer is surrounded by a helical protein belt. We have customized the size of these particles by changing the length of the amphipathic helical part of this belt, termed membrane scaffold protein (MSP). Herein we describe the design of extended and truncated MSPs, the optimization of self-assembly for each of these proteins, and the structure and composition of the resulting Nanodiscs. We show that the length of the protein helix surrounding the lipid part of a Nanodisc determines the particle diameter, as measured by HPLC and small-angle X-ray scattering (SAXS). Using different scaffold proteins, we obtained Nanodiscs with the average size from 9.5 to 12.8 nm with a very narrow size distribution (+/-3%). Functionalization of the N-terminus of the scaffold protein does not perturb their ability to form homogeneous discoidal structures. Detailed analysis of the solution scattering confirms the presence of a lipid bilayer of 5.5 nm thickness in Nanodiscs of different sizes. The results of this study provide an important structural characterization of self-assembled phospholipid bilayers and establish a framework for the design of soluble amphiphilic nanoparticles of controlled size.

Homo- and hetero-complexes of exchangeable apolipoproteins in solution and in lipid-bound form

The self-association state of human plasma apolipoprotein E (apoE) in solution and in complexes with dimyristoylphosphatidylcholine (DMPC) varying in stoichiometry was studied in sub-micromolar concentration range by gel filtration, fluorescence anisotropy, fluorescence quenching and energy transfer measurements with apolipoprotein labeled with lysine-specific fluorescent dyes. Together, these results confirm the equilibrium scheme for various apoE structures in solution: oligomer (in aged preparations) <==> 'closed' tetramer <==> 'open' tetramer ('molten globule' state) <==> native or partially denatured monomer <==> fully denatured monomer. Within DMPC:apoE discoidal complex (125:1) the apolipoprotein association state seems to be intermediate between that in solution and in larger vesicular complex (1000:1); for both complexes, the degree of exposure of fluorescein chromophores into water phase decreased. Hetero-associates of apoA-I and apoC-III-1 in solution and in the complexes with DMPC appear to behave similarly to apoE. When extrapolated to native HDL particles, 'molten globule' state seems to be a structure responsible for the interaction of exchangeable apolipoproteins with phospholipid. For a first time, the location of various apolipoprotein molecules on disc periphery was confirmed. The lysine residue(s) seems to locate closely to reacting residue(s) within apolipoprotein molecules in associates, however, with different package constraints for discoidal versus vesicular complexes with phospholipid.

Protein-lipid interactions in reconstituted high density lipoproteins: apolipoprotein and cholesterol influence

Two fluorescent probes-cis- and trans-parinaric acids were used to study the dimensions, lipid dynamics and apolipoprotein location in the reconstituted discoidal high density lipoproteins (rHDL). The rHDL particles made from apolipoprotein A-I (apoA-I), dipalmitoylphosphatidylcholine (DPPC), with or without cholesterol (Chol) were compared with the analogous particles with two other apolipoproteins-apoE and apoA-II. The data obtained for apoA-I-containing rHDL were as follows: (1) the inclusion of 8 mol.% of cholesterol did not significantly change the particle dimensions (13+/-1 nm) or the mean distance between apoA-I and the disc axis; (2) the phospholipid domains-boundary lipid region in the close vicinity to apoA-I molecule and the remaining part of the bilayer-existed at temperatures both lower and above DPPC transition temperature T(t); (3) at T<T(t) Chol molecules preferentially accumulated in the central area with a radius of 2.8 nm that conserved partially after DPPC phase transition; (4) inhomogeneous cholesterol distribution was assumed to exist within these domains. A hydrophobic matching concept was used to compare protein-lipid interactions in rHDL particles. For complexes with all three apolipoproteins studied, at T<T(t) the probe mobility in the lipid phase of rHDL was significantly higher compared to pure DPPC bilayer. After temperature-induced transition, mobility increased significantly still being lower in rHDL. The comparative study of lipid dynamics in apoA-I-, apoE- and apoA-II-containing complexes revealed the presence of boundary lipid in all three complexes without cholesterol. The degree of cholesterol exclusion from the boundary lipid region seems to increase in the order A-I<E<A-II for Chol-containing complexes, the exclusion being an inherent property of the particular apolipoprotein molecule.