Side-chain oxysterols modulate cholesterol accessibility through membrane remodeling.

Side-chain oxysterols, such as 25-hydroxycholesterol (25-HC), are key regulators of cholesterol homeostasis. New evidence suggests that the alteration of membrane structure by 25-HC contributes to its regulatory effects. We have examined the role of oxysterol membrane effects on cholesterol accessibility within the membrane using perfringolysin O (PFO), a cholesterol-dependent cytolysin that selectively binds accessible cholesterol, as a sensor of membrane cholesterol accessibility. We show that 25-HC increases cholesterol accessibility in a manner dependent on the membrane lipid composition. Structural analysis of molecular dynamics simulations reveals that increased cholesterol accessibility is associated with membrane thinning, and that the effects of 25-HC on cholesterol accessibility are driven by these changes in membrane thickness. Further, we find that the 25-HC antagonist LY295427 (agisterol) abrogates the membrane effects of 25-HC in a nonenantioselective manner, suggesting that agisterol antagonizes the cholesterol-homeostatic effects of 25-HC indirectly through its membrane interactions. These studies demonstrate that oxysterols regulate cholesterol accessibility, and thus the availability of cholesterol to be sensed and transported throughout the cell, by modulating the membrane environment. This work provides new insights into how alterations in membrane structure can be used to relay cholesterol regulatory signals.

The structural basis of cholesterol accessibility in membranes.

Although the majority of free cellular cholesterol is present in the plasma membrane, cholesterol homeostasis is principally regulated through sterol-sensing proteins that reside in the cholesterol-poor endoplasmic reticulum (ER). In response to acute cholesterol loading or depletion, there is rapid equilibration between the ER and plasma membrane cholesterol pools, suggesting a biophysical model in which the availability of plasma membrane cholesterol for trafficking to internal membranes modulates ER membrane behavior. Previous studies have predominantly examined cholesterol availability in terms of binding to extramembrane acceptors, but have provided limited insight into the structural changes underlying cholesterol activation. In this study, we use both molecular dynamics simulations and experimental membrane systems to examine the behavior of cholesterol in membrane bilayers. We find that cholesterol depth within the bilayer provides a reasonable structural metric for cholesterol availability and that this is correlated with cholesterol-acceptor binding. Further, the distribution of cholesterol availability in our simulations is continuous rather than divided into distinct available and unavailable pools. This data provide support for a revised cholesterol activation model in which activation is driven not by saturation of membrane-cholesterol interactions but rather by bulk membrane remodeling that reduces membrane-cholesterol affinity.

Side-chain oxysterols: from cells to membranes to molecules.

This review discusses the application of cellular biology, molecular biophysics, and computational simulation to understand membrane-mediated mechanisms by which oxysterols regulate cholesterol homeostasis. Side-chain oxysterols, which are produced enzymatically in vivo, are physiological regulators of cholesterol homeostasis and primarily serve as cellular signals for excess cholesterol. These oxysterols regulate cholesterol homeostasis through both transcriptional and non-transcriptional pathways; however, many molecular details of their interactions in these pathways are still not well understood. Cholesterol trafficking provides one mechanism for regulation. The current model of cholesterol trafficking regulation is based on the existence of two distinct cholesterol pools in the membrane: a low and a high availability/activity pool. It is proposed that the low availability/activity pool of cholesterol is integrated into tightly packing phospholipids and relatively inaccessible to water or cellular proteins, while the high availability cholesterol pool is more mobile in the membrane and is present in membranes where the phospholipids are not as compressed. Recent results suggest that oxysterols may promote cholesterol egress from membranes by shifting cholesterol from the low to the high activity pools. Furthermore, molecular simulations suggest a potential mechanism for oxysterol "activation" of cholesterol through its displacement in the membrane. This review discusses these results as well as several other important interactions between oxysterols and cholesterol in cellular and model lipid membranes. This article is part of a Special Issue entitled: Membrane protein structure and function.

25-Hydroxycholesterol increases the availability of cholesterol in phospholipid membranes.

Side-chain oxysterols are enzymatically generated oxidation products of cholesterol that serve a central role in mediating cholesterol homeostasis. Recent work has shown that side-chain oxysterols, such as 25-hydroxycholesterol (25-HC), alter membrane structure in very different ways from cholesterol, suggesting a possible mechanism for how these oxysterols regulate cholesterol homeostasis. Here we extend our previous work by using molecular-dynamics simulations of 25-HC and cholesterol mixtures in 1-palmitoyl-2-oleoyl-phosphatidylcholine bilayers to examine the combined effects of 25-HC and cholesterol in the same bilayer. 25-HC causes larger changes in membrane structure when added to cholesterol-containing membranes than when added to cholesterol-free membranes. We also find that the presence of 25-HC changes the position, orientation, and solvent accessibility of cholesterol, shifting it into the water interface and thus increasing its availability to external acceptors. This is consistent with experimental results showing that oxysterols can trigger cholesterol trafficking from the plasma membrane to the endoplasmic reticulum. These effects provide a potential mechanism for 25-HC-mediated regulation of cholesterol trafficking and homeostasis through modulation of cholesterol availability.

Perturbations of membrane structure by cholesterol and cholesterol derivatives are determined by sterol orientation.

Cholesterol is essential for proper function and regulation of eukaryotic membranes, and significant amounts of metabolic energy are dedicated to controlling cellular cholesterol levels. Oxidation products of cholesterol, the oxysterols, are enzymatically produced molecules that play a major role in mediating cholesterol homeostasis through mechanisms which have not yet been fully elucidated. Certain oxysterols are known to have direct effects on membrane permeability and structure, effects that are strikingly different from that of cholesterol. We use molecular dynamics simulations of these oxysterols in 1-palmitoyl 2-oleoyl phosphatidylcholine (POPC) bilayers to explain the structural origins for the differing effects of cholesterol and 25-hydroxycholesterol on bilayer properties. In particular, we demonstrate that the source for these differing perturbations is the much wider range of molecular orientations accessible to 25-hydroxycholesterol when compared to cholesterol. This study shows that direct membrane perturbation by side-chain oxysterols is significant and suggests that these membrane perturbations may play a role in the oxysterol regulation of cholesterol homeostasis.

Massively parallel single-cell chromatin landscapes of human immune cell development and intratumoral T cell exhaustion.

Understanding complex tissues requires single-cell deconstruction of gene regulation with precision and scale. Here, we assess the performance of a massively parallel droplet-based method for mapping transposase-accessible chromatin in single cells using sequencing (scATAC-seq). We apply scATAC-seq to obtain chromatin profiles of more than 200,000 single cells in human blood and basal cell carcinoma. In blood, application of scATAC-seq enables marker-free identification of cell type-specific cis- and trans-regulatory elements, mapping of disease-associated enhancer activity and reconstruction of trajectories of cellular differentiation. In basal cell carcinoma, application of scATAC-seq reveals regulatory networks in malignant, stromal and immune cells in the tumor microenvironment. Analysis of scATAC-seq profiles from serial tumor biopsies before and after programmed cell death protein 1 blockade identifies chromatin regulators of therapy-responsive T cell subsets and reveals a shared regulatory program that governs intratumoral CD8+ T cell exhaustion and CD4+ T follicular helper cell development. We anticipate that scATAC-seq will enable the unbiased discovery of gene regulatory factors across diverse biological systems.

Epitope mapping using mRNA display and a unidirectional nested deletion library.

In vitro selection targeting an anti-polyhistidine monoclonal antibody was performed using mRNA display with a random, unconstrained 27-mer peptide library. After six rounds of selection, epitope-like peptides were identified that contain two to five consecutive, internal histidines and are biased for arginine residues, without any other identifiable consensus. The epitope was further refined by constructing a high-complexity, unidirectional fragment library from the final selection pool. Selection by mRNA display minimized the dominant peptide from the original selection to a 15-residue functional sequence (peptide Cmin: RHDAGDHHHHHGVRQ; K(D) = 38 nM). Other peptides recovered from the fragment library selection revealed a separate consensus motif (ARRXA) C-terminal to the histidine track. Kinetics measurements made by surface plasmon resonance, using purified Fab (antigen-binding fragment) to prevent avidity effects, demonstrate that the selected peptides bind with 10- to 75-fold higher affinities than a hexahistidine peptide. The highest affinity peptides (K(D) approximately 10 nM) encode both a short histidine track and the ARRXA motif, suggesting that the motif and other flanking residues make important contacts adjacent to the core polyhistidine-binding site and can contribute >2.5 kcal/mol of binding free energy. The fragment library construction methodology described here is applicable to the development of high-complexity protein or cDNA expression libraries for the identification of protein-protein interaction domains.

snoRNA U17 regulates cellular cholesterol trafficking.

Cholesterol is required for the growth and viability of mammalian cells and is an obligate precursor for steroid hormone synthesis. Using a loss-of-function screen for mutants with defects in intracellular cholesterol trafficking, a Chinese hamster ovary cell mutant with haploinsufficiency of the U17 snoRNA was isolated. U17 is an H/ACA orphan snoRNA, for which a function other than ribosomal processing has not previously been identified. Through expression profiling, we identified hypoxia-upregulated mitochondrial movement regulator (HUMMR) mRNA as a target that is negatively regulated by U17 snoRNA. Upregulation of HUMMR in U17 snoRNA-deficient cells promoted the formation of ER-mitochondrial contacts, decreasing esterification of cholesterol and facilitating cholesterol trafficking to mitochondria. U17 snoRNA and HUMMR regulate mitochondrial synthesis of steroids in vivo and are developmentally regulated in steroidogenic tissues, suggesting that the U17 snoRNA-HUMMR pathway may serve a previously unrecognized, physiological role in gonadal tissue maturation.

Improved Coarse-Grained Modeling of Cholesterol-Containing Lipid Bilayers.

Cholesterol trafficking, which is an essential function in mammalian cells, is intimately connected to molecular-scale interactions through cholesterol modulation of membrane structure and dynamics and interaction with membrane receptors. Since these effects of cholesterol occur on micro- to millisecond timescales, it is essential to develop accurate coarse-grained simulation models that can reach these timescales. Cholesterol has been shown experimentally to thicken the membrane and increase phospholipid tail order between 0-40% cholesterol, above which these effects plateau or slightly decrease. Here, we showed that the published MARTINI coarse-grained force-field for phospholipid (POPC) and cholesterol fails to capture these effects. Using reference atomistic simulations, we systematically modified POPC and cholesterol bonded parameters in MARTINI to improve its performance. We showed that the corrections to pseudo-bond angles between glycerol and the lipid tails and around the oleoyl double bond particle (the "angle-corrected model") slightly improves the agreement of MARTINI with experimentally measured thermal, elastic, and dynamic properties of POPC membranes. The angle-corrected model improves prediction of the thickening and ordering effects up to 40% cholesterol but overestimates these effects at higher cholesterol concentration. In accordance with prior work that showed the cholesterol rough face methyl groups are important for limiting cholesterol self-association, we revised the coarse-grained representation of these methyl groups to better match cholesterol-cholesterol radial distribution functions from atomistic simulations. In addition, by using a finer-grained representation of the branched cholesterol tail than MARTINI, we improved predictions of lipid tail order and bilayer thickness across a wide range of concentrations. Finally, transferability testing shows that a model incorporating our revised parameters into DOPC outperforms other CG models in a DOPC/cholesterol simulation series, which further argues for its efficacy and generalizability. These results argue for the importance of systematic optimization for coarse-graining biologically important molecules like cholesterol with complicated molecular structure.

The puromycin route to assess stereo- and regiochemical constraints on peptide bond formation in eukaryotic ribosomes.

We synthesized a series of puromycin analogues to probe the chemical specificity of the ribosome in an intact eukaryotic translation system. These studies reveal that both d-enantiomers and beta-amino acid analogues can be incorporated into protein, and provide a quantitative means to rank natural and unnatural residues. Modeling of a d-amino acid analogue into the 50S ribosomal subunit indicates that steric clash may provide part of the chiral discrimination. The data presented provide one metric of the chiral and regiospecificity of mammalian ribosomes.