Cholesterol Induces Specific Spatial and Orientational Order in Cholesterol/Phospholipid Membranes

In lipid bilayers, cholesterol facilitates the formation of the liquid-ordered phase and enables the formation of laterally ordered structures such as lipid rafts. While these domains have an important role in a variety of cellular processes, the precise atomic-level mechanisms responsible for cholesterol's specific ordering and packing capability have remained unresolved

Cholesterol Induces Specific Spatial and Orientational Order in Cholesterol/Phospholipid Membranes

In lipid bilayers, cholesterol facilitates the formation of the liquid-ordered phase and enables the formation of laterally ordered structures such as lipid rafts. While these domains have an important role in a variety of cellular processes, the precise atomic-level mechanisms responsible for cholesterol's specific ordering and packing capability have remained unresolved

An In Vitro System for Studying Murid Herpesvirus-4 Latency and Reactivation

The narrow species tropisms of Epstein-Barr Virus (EBV) and the Kaposi's Sarcoma -associated Herpesvirus (KSHV) have made Murid Herpesvirus-4 (MuHV-4) an important tool for understanding how gammaherpesviruses colonize their hosts. However, while MuHV-4 pathogenesis studies can assign a quantitative importance to individual genes, the complexity of in vivo infection can make the underlying mechanisms hard to discern. Furthermore, the lack of good in vitro MuHV-4 latency/reactivation systems with which to dissect mechanisms at the cellular level has made some parallels with EBV and KSHV hard to draw. Here we achieved control of the MuHV-4 lytic/latent switch in vitro by modifying the 5′ untranslated region of its major lytic transactivator gene, ORF50. We terminated normal ORF50 transcripts by inserting a polyadenylation signal and transcribed ORF50 instead from a down-stream, doxycycline-inducible promoter. In this way we could establish fibroblast clones that maintained latent MuHV-4 episomes without detectable lytic replication. Productive virus reactivation was then induced with doxycycline. We used this system to show that the MuHV-4 K3 gene plays a significant role in protecting reactivating cells against CD8+ T cell recognition

Role of Lipids in Spheroidal High Density Lipoproteins

We study the structure and dynamics of spherical high density lipoprotein (HDL) particles through coarse-grained multi-microsecond molecular dynamics simulations. We simulate both a lipid droplet without the apolipoprotein A-I (apoA-I) and the full HDL particle including two apoA-I molecules surrounding the lipid compartment. The present models are the first ones among computational studies where the size and lipid composition of HDL are realistic, corresponding to human serum HDL. We focus on the role of lipids in HDL structure and dynamics. Particular attention is paid to the assembly of lipids and the influence of lipid-protein interactions on HDL properties. We find that the properties of lipids depend significantly on their location in the particle (core, intermediate region, surface). Unlike the hydrophobic core, the intermediate and surface regions are characterized by prominent conformational lipid order. Yet, not only the conformations but also the dynamics of lipids are found to be distinctly different in the different regions of HDL, highlighting the importance of dynamics in considering the functionalization of HDL. The structure of the lipid droplet close to the HDL-water interface is altered by the presence of apoA-Is, with most prominent changes being observed for cholesterol and polar lipids. For cholesterol, slow trafficking between the surface layer and the regimes underneath is observed. The lipid-protein interactions are strongest for cholesterol, in particular its interaction with hydrophobic residues of apoA-I. Our results reveal that not only hydrophobicity but also conformational entropy of the molecules are the driving forces in the formation of HDL structure. The results provide the first detailed structural model for HDL and its dynamics with and without apoA-I, and indicate how the interplay and competition between entropy and detailed interactions may be used in nanoparticle and drug design through self-assembly

Particle simulations of efficient fast electron generation near the cutoff layer of an electrostatic wave

Fast electron generation near the cutoff of an electrostatic plasma wave is investigated by particle-in-cell simulations and test particle calculations. Intense electron plasma waves which are excited in an underdense plasma region propagate up the density gradient until they are reflected from the cutoff layer. The density gradient affects the fast electron generation by the wave considerably. At low densities, the phase velocity is fairly close to the thermal distribution, which leads to wave-particle interactions with a large electron population. The trapped electrons are accelerated by the electron plasma wave with increasing phase velocity resulting in a very large and energetic population behind the cutoff layer. Since the accelerating electrons receive energy, the wave must be damped. A simple model based on the conservation of the energy of the wave and the trapped electrons is developed to describe the damping mechanism.</p

From Discoidal to Spheroidal HDL particles through Coarse Grained and All Atom Molecular Dynamics Simulations

In vivo high density lipoproteins (HDL) originate as discoidal complexes of apolipoprotein (apo) A-I, phospholipids (PL) and cholesterol. These nascent HDL complexes are remodeled by the enzyme Lecithin Cholesterol Acyl Transferase (LCAT), that catalyses the transition from discoidal (PL-rich) to spherical HDL (the form of circulating HDL) by generating cholesteryl esters (CE) and lyso-PC. The phase separation of neutral lipids, CE and triglycerides (TG), creates a hydrophobic core encapsulated by the protein and amphipathic lipid molecules. To investigate the conformational change of apoA-I in the transition from PL-rich to CE-rich HDL particles, we initially performed all atom (AA) and coarse grained (CG) molecular dynamics (MD) simulations at 310 K on two starting model discoidal HDL particles containing palmitoyloleoylphosphatidylcholine (POPC), cholesterol (UC) and full length apoA-I molecules with molar ratios of 160:24:2 and 160:64:2, respectively. In the 100 ns coarse grained structures a fraction of the cholesterol molecules was mutated to cholesteryl oleate (CO) molecules and an equivalent number of POPC molecules were removed. The main goal was to mimic the LCAT activity in silico by simulating model CE-rich HDL particles representing small HDL2 particles with a cholesterol concentration similar to that of circulating HDL. Then, the two mutated structures containing POPC:CO:UC:apoA-I molar ratios of 142:18:6:2 and 105:55:9:2, respectively, were subjected to a 100 ns CG MD simulation at 310K. In both CG MD simulations CO molecules form a hydrophobic core in the 100 ns time scale, indicating that hydrophobic interactions play an active role in the stabilization of spheroidal HDL particles. It is also interesting to note the separation of UC molecules, as observed experimentally, into two distinct environments: free and bound to the protein. This work was supported by the National Institute of Health