High-resolution imaging and quantification of plasma membrane cholesterol by NanoSIMS

IL-4 Licenses B Cell Activation Through Cholesterol Synthesis

Lymphocyte activation involves a transition from quiescence and associated catabolic metabolism to a metabolic state with noted similarities to cancer cells such as heavy reliance on aerobic glycolysis for energy demands and increased nutrient requirements for biomass accumulation and cell division 1-3 . Following antigen receptor ligation, lymphocytes require spatiotemporally distinct "second signals". These include costimulatory receptor or cytokine signaling, which engage discrete programs that often involve remodeling of organelles and increased nutrient uptake or synthesis to meet changing biochemical demands 4-6 . One such signaling molecule, IL-4, is a highly pleiotropic cytokine that was first identified as a B cell co-mitogen over 30 years ago 7 . However, how IL-4 signaling mechanistically supports B cell proliferation is incompletely understood. Here, using single cell RNA sequencing we find that the cholesterol biosynthetic program is transcriptionally upregulated following IL-4 signaling during the early B cell response to influenza virus infection, and is required for B cell activation in vivo . By limiting lipid availability in vitro , we determine cholesterol to be essential for B cells to expand their endoplasmic reticulum, progress through cell cycle, and proliferate. In sum, we demonstrate that the well-known ability of IL-4 to act as a B cell growth factor is through a previously unknown rewiring of specific lipid anabolic programs, relieving sensitivity of cells to environmental nutrient availability.

Dysregulation of cellular membrane homeostasis as a crucial modulator of cancer risk

Cellular membranes serve as an epicentre combining extracellular and cytosolic components with membranous effectors, which together support numerous fundamental cellular signalling pathways that mediate biological responses. To execute their functions, membrane proteins, lipids and carbohydrates arrange, in a highly coordinated manner, into well-defined assemblies displaying diverse biological and biophysical characteristics that modulate several signalling events. The loss of membrane homeostasis can trigger oncogenic signalling. More recently, it has been documented that select membrane active dietaries (MADs) can reshape biological membranes and subsequently decrease cancer risk. In this review, we emphasize the significance of membrane domain structure, organization and their signalling functionalities as well as how loss of membrane homeostasis can steer aberrant signalling. Moreover, we describe in detail the complexities associated with the examination of these membrane domains and their association with cancer. Finally, we summarize the current literature on MADs and their effects on cellular membranes, including various mechanisms of dietary chemoprevention/interception and the functional links between nutritional bioactives, membrane homeostasis and cancer biology.

Toward Understanding the Subcellular Distributions of Cholesterol and Sphingolipids Using High-Resolution NanoSIMS Imaging

ConspectusCharacterizing the subcellular distributions of biomolecules of interest is a basic inquiry that helps inform on the potential roles of these molecules in biological functions. Presently, the functions of specific lipid species and cholesterol are not well understood, partially because cholesterol and lipid species of interest are difficult to image with high spatial resolution but without perturbing them. Because cholesterol and lipids are relatively small and their distributions are influenced by noncovalent interactions with other biomolecules, functionalizing them with relatively large labels that permit their detection may alter their distributions in membranes and between organelles. This challenge has been surmounted by exploiting rare stable isotopes as labels that may be metabolically incorporated into cholesterol and lipids without altering their chemical compositions, and the Cameca NanoSIMS 50 instrument's ability to image rare stable isotope labels with high spatial resolution. This Account covers the use of secondary ion mass spectrometry (SIMS) performed with a Cameca NanoSIMS 50 instrument for imaging cholesterol and sphingolipids in the membranes of mammalian cells. The NanoSIMS 50 detects monatomic and diatomic secondary ions ejected from the sample to map the elemental and isotopic composition at the surface of the sample with better than 50 nm lateral resolution and 5 nm depth resolution. Much effort has focused on using NanoSIMS imaging of rare isotope-labeled cholesterol and sphingolipids for testing the long-standing hypothesis that cholesterol and sphingolipids colocalize within distinct domains in the plasma membrane. By using a NanoSIMS 50 to image rare isotope-labeled cholesterol and sphingolipids in parallel with affinity-labeled proteins of interest, a hypothesis regarding the colocalization of specific membrane proteins with cholesterol and sphingolipids in distinct plasma membrane domains has been tested. NanoSIMS performed in a depth profiling mode has enabled imaging the intracellular distributions of cholesterol and sphingolipids. Important progress has also been made in developing a computational depth correction strategy for constructing more accurate three-dimensional (3D) NanoSIMS depth profiling images of intracellular component distribution without requiring additional measurements with complementary techniques or signal collection. This Account provides an overview of this exciting progress, focusing on the studies from our laboratory that shifted understanding of plasma membrane organization, and the development of enabling tools for visualizing intracellular lipids.

A basic model for the association of ligands with membrane cholesterol: application to cytolysin binding

Almost all the cholesterol in cellular membranes is associated with phospholipids in simple stoichiometric complexes. This limits the binding of sterol ligands such as filipin and Perfringolysin O (PFO) to a small fraction of the total. We offer a simple mathematical model that characterizes this complexity. It posits that the cholesterol accessible to ligands has two forms: active cholesterol, which is that not complexed with phospholipids; and extractable cholesterol, that which ligands can capture competitively from the phospholipid complexes. Simulations based on the model match published data for the association of PFO oligomers with liposomes, plasma membranes and the isolated endoplasmic reticulum. The model shows how the binding of a probe greatly underestimates cholesterol abundance when its affinity for the sterol is so weak that it competes poorly with the membrane phospholipids. Two examples are the under-staining of plasma membranes by filipin and the failure of domain D4 of PFO to label their cytoplasmic leaflets. Conversely, the exaggerated staining of endolysosomes suggests that their cholesterol, being uncomplexed, is readily available. The model is also applicable to the association of cholesterol with intrinsic membrane proteins. For example, it supports the hypothesis that the sharp threshold in the regulation of homeostatic ER proteins by cholesterol derives from the cooperativity of their binding to the sterol weakly held by the phospholipid. § Thus, the model explicates the complexity inherent in the binding of ligands like PFO and filipin to the small accessible fraction of membrane cholesterol.

Serum opacity factor rescues fertility among female Scarb1-/- mice by reducing HDL-free cholesterol bioavailability

Human female infertility, 20% of which is idiopathic, is a public health problem for which better diagnostics and therapeutics are needed. A novel cause of infertility emerged from studies of female mice deficient in the HDL receptor gene (Scarb1). These mice are infertile and have high plasma HDL cholesterol (C) concentrations, due to elevated HDL-free cholesterol (FC), which transfers from HDL to all tissues. Previous studies have indicated that oral delivery of probucol, an HDL-lowering drug, to female Scarb1-/- mice reduces plasma HDL-C concentrations and rescues fertility. Additionally, serum opacity factor (SOF), a bacterial virulence factor, disrupts HDL structure, and bolus SOF injection into mice reduces plasma HDL-C concentrations. Here, we discovered that delivering SOF to female Scarb1-/- mice with an adeno-associated virus (AAVSOF) induces constitutive SOF expression, reduces HDL-FC concentrations, and rescues fertility while normalizing ovary morphology. Although AAVSOF did not alter ovary-FC content, the ovary-mol% FC correlated with plasma HDL-mol% FC in a fertility-dependent way. Therefore, reversing the abnormal plasma microenvironment of high plasma HDL-mol% FC in female Scarb1-/- mice rescues fertility. These data provide the rationale to search for similar mechanistic links between HDL-mol% FC and infertility and the rescue of fertility in women by reducing plasma HDL-mol% FC.

Raddeanin A synergistically enhances the anti-tumor effect of MAP30 in multiple ways, more than promoting endosomal escape

Biomacromolecules such as proteins and nucleic acids are very attractive due to their high efficiency and specificity as cancer therapeutics. In fact, the endocytosed macromolecules are often trapped in the endosomes and cannot exhibit pharmacological effects well. Many strategies have been used to address this bottleneck, and one promising approach is to exploit the endosomal escape-promoting effect of triterpenoid saponins to aid in the release of biomacromolecules. Here, Raddeanin A (RA, an oleanane-type triterpenoid saponin) was proved to significantly promote endosomal escape as it recruited Galectin-9, an endosomal escape event reporter. As expected, RA effectively enhanced the anti-tumor effect of MAP30 (a type I ribosome-inactivating protein derived from Momordica charantia). However, based on the results of fluorescent colocalization, RA did not significantly promote MAP30 release from endosomes, suggesting that RA enhances MAP30 activity not only by promoting endosomal escape. Furthermore, it was found that the inhibitors of micropinocytosis and caveolae could almost completely inhibit the cytotoxicity of MAP30 combined with RA without affecting the cytotoxicity of MAP30 alone, indicating that RA may regulate the endocytic pathway of MAP30. Meanwhile, the effect of RA is related to the intra vesicular pH and cholesterol content on cell membrane, and is also cell-type dependent. Therefore, RA enhanced the anti-tumor effect of MAP30 in multiple ways, not just by promoting endosomal escape. Our findings will help to further decipher the possible mechanisms by which triterpenoid saponins enhance drug activity, and provide a new perspective for improving the activity of endocytosed drugs.

An iodine-containing probe as a tool for molecular detection in secondary ion mass spectrometry

We developed here an iodine-containing probe that can be used to identify the molecules of interest in secondary ion mass spectrometry (SIMS) by simple immunolabelling procedures. The immunolabelled iodine probe was readily combined with previously-developed SIMS probes carrying fluorine, to generate dual-channel SIMS data. This probe should provide a useful complement to the currently available SIMS probes, thus expanding the scope of this technology.

A novel sterol-binding protein reveals heterogeneous cholesterol distribution in neurite outgrowth and in late endosomes/lysosomes

We identified a mushroom-derived protein, maistero-2 that specifically binds 3-hydroxy sterol including cholesterol (Chol). Maistero-2 bound lipid mixture in Chol-dependent manner with a binding threshold of around 30%. Changing lipid composition did not significantly affect the threshold concentration. EGFP-maistero-2 labeled cell surface and intracellular organelle Chol with higher sensitivity than that of well-established Chol probe, D4 fragment of perfringolysin O. EGFP-maistero-2 revealed increase of cell surface Chol during neurite outgrowth and heterogeneous Chol distribution between CD63-positive and LAMP1-positive late endosomes/lysosomes. The absence of strictly conserved Thr-Leu pair present in Chol-dependent cytolysins suggests a distinct Chol-binding mechanism for maistero-2.

Shedding light on membrane rafts structure and dynamics in living cells

Cellular membranes are fundamental building blocks regulating an extensive repertoire of biological functions. These structures contain lipids and membrane proteins that are known to laterally self-aggregate in the plane of the membrane, forming defined membrane nanoscale domains essential for protein activity. Membrane rafts are described as heterogeneous, dynamic, and short-lived cholesterol- and sphingolipid-enriched membrane nanodomains (10-200 nm) induced by lipid-protein and lipid-lipid interactions. Those membrane nanodomains have been extensively characterized using model membranes and in silico methods. However, despite the development of advanced fluorescence microscopy techniques, undoubted nanoscale visualization by imaging techniques of membrane rafts in the membrane of unperturbed living cells is still uncompleted, increasing the skepticism about their existence. Here, we broadly review recent biochemical and microscopy techniques used to investigate membrane rafts in living cells and we enumerate persistent open questions to answer before unlocking the mystery of membrane rafts in living cells.

Measuring and Manipulating Membrane Cholesterol for the Study of Hedgehog Signaling

Cholesterol is an abundant lipid in mammalian plasma membranes that regulates the reception of the Hedgehog (Hh) signal in target cells. In vertebrates, cell-surface organelles called primary cilia function as compartments for the propagation of Hh signals. Recent structural, biochemical, and cell-biological studies have led to the model that Patched-1 (PTCH1), the receptor for Hh ligands, uses its transporter-like activity to lower cholesterol accessibility in the membrane surrounding primary cilia. Cholesterol restriction at cilia may represent the long-sought-after mechanism by which PTCH1 inhibits Smoothened (SMO), a cholesterol-responsive transmembrane protein of the G protein-coupled receptor superfamily that transmits the Hh signal across the membrane.Protein probes based on microbial cholesterol-binding proteins revealed that PTCH1 controls only a subset of the total cholesterol molecules, a biochemically defined fraction called accessible cholesterol. The accessible cholesterol pool coexists (and exchanges) with a pool of sequestered cholesterol, which is bound to phospholipids like sphingomyelin. In this chapter, we describe how to measure the accessible and sequestered cholesterol pools in live cells with protein-based probes. We discuss how to purify and fluorescently label these probes for use in flow cytometry and microscopy-based measurements of the cholesterol pools. Additionally, we describe how to modulate accessible cholesterol levels to determine if this pool regulates Hh signaling (or any other cellular process of interest).

Compartmentalization of Sphingolipid metabolism: Implications for signaling and therapy

Sphingolipids (SLs) are a family of bioactive lipids implicated in a variety of cellular processes, and whose levels are controlled by an interlinked network of enzymes. While the spatial distribution of SL metabolism throughout the cell has been understood for some time, the implications of this for SL signaling and biological outcomes have only recently begun to be fully explored. In this review, we outline the compartmentalization of SL metabolism and describe advances in tools for investigating and probing compartment-specific SL functions. We also briefly discuss the implications of SL compartmentalization for cell signaling and therapeutic approaches to targeting the SL network.

Super-Resolution Microscopy Using a Bioorthogonal-Based Cholesterol Probe Provides Unprecedented Capabilities for Imaging Nanoscale Lipid Heterogeneity in Living Cells

Despite more than 20 years of work since the lipid raft concept was proposed, the existence of these nanostructures remains highly controversial due to the lack of noninvasive methods to investigate their native nanorganization in living unperturbed cells. There is an unmet need for probes for direct imaging of nanoscale membrane dynamics with high spatial and temporal resolution in living cells. In this paper, a bioorthogonal-based cholesterol probe (chol-N₃ ) is developed that, combined with nanoscopy, becomes a new powerful method for direct visualization and characterization of lipid raft at unprecedented resolution in living cells. The chol-N₃ probe mimics cholesterol in synthetic and cellular membranes without perturbation. When combined with live-cell super-resolution microscopy, chol-N₃ demonstrates the existence of cholesterol-rich nanodomains of <50 nm at the plasma membrane of resting living cells. Using this tool, the lipid membrane structure of such subdiffraction limit domains is identified, and the nanoscale spatiotemporal organization of cholesterol in the plasma membrane of living cells reveals multiple cholesterol diffusion modes at different spatial localizations. Finally, imaging across thick organ samples outlines the potential of this new method to address essential biological questions that were previously beyond reach.

The Interaction of Heptakis (2,6-di-O-Methyl)-β-cyclodextrin with Mianserin Hydrochloride and Its Influence on the Drug Toxicity

One tetracyclic antidepressant, mianserin hydrochloride (MIA), has quite significant side effects on a patients’ health. Cyclodextrins, which are most commonly used to reduce the undesirable features of contained drugs within their hydrophobic interior, also have the potential to alter the toxic behavior of the drug. The present paper contains investigations and the characteristics of interaction mechanisms for MIA and the heptakis (2,6-di-O-methyl)-β-cyclodextrin (DM-β-CD) system, and evaluated the effects of the complexation on MIA cytotoxicity. In order to assess whether there was an interaction between MIA and DM-β-CD molecules, isothermal titration calorimetry (ITC) have been chosen. Electrospray ionization mass spectrometry (ESI-MS) helped to establish the complex stoichiometry, and circular dichroism spectroscopy was used to describe the process of complex formation. In order to make a wider interpretative perspective, the molecular docking results have been performed. The viability of Chinese hamster cells were investigated in the presence of DM-β-CD and its complexes with MIA in order to estimate the cytotoxicity of the drug and the conjugate with the chosen cyclodextrin. The viability of B14 cells treated with MIA+DM-β-CD is lower (the toxicity is higher) than with MIA alone, and no protective effects have been observed for complexes of MIA with DM-β-CD in any ratio.

Depth correction of 3D NanoSIMS images using secondary electron pixel intensities

Strategies that do not require additional characterization to be performed on the sample or the collection of additional secondary ion signals are needed to depth correct 3D SIMS images of cells. Here, we develop a depth correction strategy that uses the pixel intensities in the secondary electron images acquired during negative-ion NanoSIMS depth profiling to reconstruct the sample morphology. This morphology reconstruction was then used to depth correct the 3D SIMS images that show the components of interest in the sample. As a proof of concept, we applied this approach to NanoSIMS depth profiling data that show the 15N-enrichment and 18O-enrichment from 15N-sphingolipids and 18O-cholesterol, respectively, within a metabolically labeled Madin-Darby canine kidney cell. Comparison of the cell morphology reconstruction to the secondary electron images collected with the NanoSIMS revealed that the assumption of a constant sputter rate produced small inaccuracies in sample morphology after approximately 0.66 μm of material was sputtered from the cell. Nonetheless, the resulting 3D renderings of the lipid-specific isotope enrichments better matched the shapes and positions of the subcellular compartments that contained 15N-sphingolipids and 18O-cholesterol than the uncorrected 3D SIMS images. This depth correction of the 3D SIMS images also facilitated the detection of spherical cholesterol-rich compartments that were surrounded by membranes containing cholesterol and sphingolipids. Thus, we expect this approach will facilitate identifying the subcellular structures that are enriched with biomolecules of interest in 3D SIMS images while eliminating the need for correlated analyses or additional secondary ion signals for the depth correction of 3D NanoSIMS images.

Expression of Fatty Acyl-CoA Ligase Drives One-Pot De Novo Synthesis of Membrane-Bound Vesicles in a Cell-Free Transcription-Translation System

Despite the central importance of lipid membranes in cellular organization, it is challenging to reconstitute their formation de novo from minimal chemical and biological elements. Here, we describe a chemoenzymatic route to membrane-forming noncanonical phospholipids in which cysteine-modified lysolipids undergo spontaneous coupling with fatty acyl-CoA thioesters generated enzymatically by a fatty acyl-CoA ligase. Due to the high efficiency of the reaction, we were able to optimize phospholipid formation in a cell-free transcription-translation (TX-TL) system. Combining DNA encoding the fatty acyl-CoA ligase with suitable lipid precursors enabled one-pot de novo synthesis of membrane-bound vesicles. Noncanonical sphingolipid synthesis was also possible by using a cysteine-modified lysosphingomyelin as a precursor. When the sphingomyelin-interacting protein lysenin was coexpressed alongside the acyl-CoA ligase, the in situ assembled membranes were spontaneously decorated with protein. Our strategy of coupling gene expression with membrane lipid synthesis in a one-pot fashion could facilitate the generation of proteoliposomes and brings us closer to the bottom-up generation of synthetic cells using recombinant synthetic biology platforms.

Cellular cholesterol and how to find it

Cholesterol is an essential component of eukaryotic cellular membranes. Information about its subcellular localization and transport pathways inside cells are key for the understanding and treatment of cholesterol-related diseases. In this review we give an overview over the most commonly used methods that contributed to our current understanding of subcellular cholesterol localization and transport routes. First, we discuss methods that provide insights into cholesterol metabolism based on readouts of downstream effects such as esterification. Subsequently, we focus on the use of cholesterol-binding molecules as probes that facilitate visualization and quantification of sterols inside of cells. Finally, we explore different analogues of cholesterol which, when taken up by living cells, are integrated and transported in a similar fashion as endogenous sterols. Taken together, we highlight the challenges and advantages of each method such that researchers studying aspects of cholesterol transport may choose the most pertinent approach for their problem.

Normalization of cholesterol metabolism in spinal microglia alleviates neuropathic pain

Neuroinflammation is a major component in the transition to and perpetuation of neuropathic pain states. Spinal neuroinflammation involves activation of TLR4, localized to enlarged, cholesterol-enriched lipid rafts, designated here as inflammarafts. Conditional deletion of cholesterol transporters ABCA1 and ABCG1 in microglia, leading to inflammaraft formation, induced tactile allodynia in naive mice. The apoA-I binding protein (AIBP) facilitated cholesterol depletion from inflammarafts and reversed neuropathic pain in a model of chemotherapy-induced peripheral neuropathy (CIPN) in wild-type mice, but AIBP failed to reverse allodynia in mice with ABCA1/ABCG1–deficient microglia, suggesting a cholesterol-dependent mechanism. An AIBP mutant lacking the TLR4-binding domain did not bind microglia or reverse CIPN allodynia. The long-lasting therapeutic effect of a single AIBP dose in CIPN was associated with anti-inflammatory and cholesterol metabolism reprogramming and reduced accumulation of lipid droplets in microglia. These results suggest a cholesterol-driven mechanism of regulation of neuropathic pain by controlling the TLR4 inflammarafts and gene expression program in microglia and blocking the perpetuation of neuroinflammation.

Role of Lipids in Morphogenesis of T-Cell Microvilli

T cells communicate with the environment via surface receptors. Cooperation of surface receptors regulates T-cell responses to diverse stimuli. Recently, finger-like membrane protrusions, microvilli, have been demonstrated to play a role in the organization of receptors and, hence, T-cell activation. However, little is known about the morphogenesis of dynamic microvilli, especially in the cells of immune system. In this review, I focus on the potential role of lipids and lipid domains in morphogenesis of microvilli. Discussed is the option that clustering of sphingolipids with phosphoinositides at the plasma membrane results in dimpling (curved) domains. Such domains can attract phosphoinositide-binding proteins and stimulate actin cytoskeleton reorganization. This process triggers cortical actin opening and bundling of actin fibres to support the growing of microvilli. Critical regulators of microvilli morphogenesis in T cells are unknown. At the end, I suggest several candidates with a potential to organize proteins and lipids in these structures.

The use of anthrolysin O and ostreolysin A to study cholesterol in cell membranes

Cholesterol is a major component of the plasma membranes (PMs) of animal cells, comprising 35-40mol% of total PM lipids. Recent studies using cholesterol-binding bacterial toxins such as domain 4 of Anthrolysin O (ALOD4) and fungal toxins such as Ostreolysin A (OlyA) have revealed new insights into the organization of PM cholesterol. These studies have defined three distinct pools of PM cholesterol-a fixed pool that is essential for membrane integrity, a sphingomyelin (SM)-sequestered pool that can be detected by OlyA, and a third pool that is accessible and can be detected by ALOD4. Accessible cholesterol is available to interact with proteins and transport to the endoplasmic reticulum (ER), and controls many cellular signaling processes including cholesterol homeostasis, Hedgehog signaling, and bacterial and viral infection. Here, we provide detailed descriptions for the use of ALOD4 and OlyA, both of which are soluble and non-lytic proteins, to study cholesterol organization in the PMs of animal cells. Furthermore, we describe two new versions of ALOD4 that we have developed to increase the versatility of this probe in cellular studies. One is a dual His₆ and FLAG epitope-tagged version and the other is a fluorescent version where ALOD4 is fused to Neon, a monomeric fluorescent protein. These new forms of ALOD4 together with previously described OlyA provide an expanded collection of tools to sense, visualize, and modulate levels of accessible and SM-sequestered cholesterol on PMs and study the role of these cholesterol pools in diverse membrane signaling events.

Mass spectrometry imaging of untreated wet cell membranes in solution using single-layer graphene

We report a means by which atomic and molecular secondary ions, including cholesterol and fatty acids, can be sputtered through single-layer graphene to enable secondary ion mass spectrometry (SIMS) imaging of untreated wet cell membranes in solution at subcellular spatial resolution. We can observe the intrinsic molecular distribution of lipids, such as cholesterol, phosphoethanolamine and various fatty acids, in untreated wet cell membranes without any labeling. We show that graphene-covered cells prepared on a wet substrate with a cell culture medium reservoir are alive and that their cellular membranes do not disintegrate during SIMS imaging in an ultra-high-vacuum environment. Ab initio molecular dynamics calculations and ion dose-dependence studies suggest that sputtering through single-layer graphene occurs through a transient hole generated in the graphene layer. Cholesterol imaging shows that methyl-β-cyclodextrin preferentially extracts cholesterol molecules from the cholesterol-enriched regions in cell membranes.

High-resolution visualization and quantification of nucleic acid-based therapeutics in cells and tissues using Nanoscale secondary ion mass spectrometry (NanoSIMS)

Nucleic acid therapeutics (NATs) have proven useful in promoting the degradation of specific transcripts, modifying gene expression, and regulating mRNA splicing. In each situation, efficient delivery of nucleic acids to cells, tissues and intracellular compartments is crucial—both for optimizing efficacy and reducing side effects. Despite successes in NATs, our understanding of their cellular uptake and distribution in tissues is limited. Current methods have yielded insights into distribution of NATs within cells and tissues, but the sensitivity and resolution of these approaches are limited. Here, we show that nanoscale secondary ion mass spectrometry (NanoSIMS) imaging can be used to define the distribution of 5-bromo-2′-deoxythymidine (5-BrdT) modified antisense oligonucleotides (ASO) in cells and tissues with high sensitivity and spatial resolution. This approach makes it possible to define ASO uptake and distribution in different subcellular compartments and to quantify the impact of targeting ligands designed to promote ASO uptake by cells. Our studies showed that phosphorothioate ASOs are associated with filopodia and the inner nuclear membrane in cultured cells, and also revealed substantial cellular and subcellular heterogeneity of ASO uptake in mouse tissues. NanoSIMS imaging represents a significant advance in visualizing uptake and distribution of NATs; this approach will be useful in optimizing efficacy and delivery of NATs for treating human disease.

Impact of Intrinsic and Extrinsic Factors on Cellular Sphingomyelin Imaging with Specific Reporter Proteins

Sphingomyelin (SM) is a major sphingolipid in mammalian cells. Although SM is enriched in the outer leaflet of the cell plasma membrane, lipids are also observed in the inner leaflet of the plasma membrane and intracellular organelles such as endolysosomes, the Golgi apparatus and nuclei. SM is postulated to form clusters with glycosphingolipids (GSLs), cholesterol (Chol), and other SM molecules through hydrophobic interactions and hydrogen bonding. Thus, different clusters composed of SM, SM/Chol, SM/GSL and SM/GSL/Chol with different stoichiometries may exist in biomembranes. In addition, SM monomers may be located in the glycerophospholipid-rich areas of membranes. Recently developed SM-binding proteins (SBPs) distinguish these different SM assemblies. Here, we summarize the effects of intrinsic factors regulating the lipid-binding specificity of SBPs and extrinsic factors, such as the lipid phase and lipid density, on SM recognition by SBPs. The combination of different SBPs revealed the heterogeneity of SM domains in biomembranes.

Correlative light electron ion microscopy reveals in vivo localisation of bedaquiline in Mycobacterium tuberculosis-infected lungs

Correlative light, electron, and ion microscopy (CLEIM) offers huge potential to track the intracellular fate of antibiotics, with organelle-level resolution. However, a correlative approach that enables subcellular antibiotic visualisation in pathogen-infected tissue is lacking. Here, we developed correlative light, electron, and ion microscopy in tissue (CLEIMiT) and used it to identify the cell type–specific accumulation of an antibiotic in lung lesions of mice infected with Mycobacterium tuberculosis. Using CLEIMiT, we found that the anti-tuberculosis (TB) drug bedaquiline (BDQ) is localised not only in foamy macrophages in the lungs during infection but also accumulate in polymorphonuclear (PMN) cells.

This study uses correlative light, electron and ion microscopy (CLEIM) in vivo to reveal the intracellular fate of an antibiotic in lung lesions of mice infected with Mycobacterium tuberculosis, with organelle-level resolution.

Current Methods for Detecting Cell Membrane Transient Interactions

Short-lived cell membrane complexes play a key role in regulating cell signaling and communication. Many of these complexes are formed based on low-affinity and transient interactions among various lipids and proteins. New techniques have emerged to study these previously overlooked membrane transient interactions. Exciting functions of these transient interactions have been discovered in cellular events such as immune signaling, host-pathogen interactions, and diseases such as cancer. In this review, we have summarized current experimental methods that allow us to detect and analyze short-lived cell membrane protein-protein, lipid-protein, and lipid-lipid interactions. These methods can provide useful information about the strengths, kinetics, and/or spatial patterns of membrane transient interactions. However, each method also has its own limitations. We hope this review can be used as a guideline to help the audience to choose proper approaches for studying membrane transient interactions in different membrane trafficking and cell signaling events.

Aster Proteins Regulate the Accessible Cholesterol Pool in the Plasma Membrane

Recent studies have demonstrated the existence of a discrete pool of cholesterol in the plasma membranes (PM) of mammalian cells-referred to as the accessible cholesterol pool-that can be detected by the binding of modified versions of bacterial cytolysins (e.g., anthrolysin O). When the amount of accessible cholesterol in the PM exceeds a threshold level, the excess cholesterol moves to the endoplasmic reticulum (ER), where it regulates the SREBP2 pathway and undergoes esterification. We reported previously that the Aster/Gramd1 family of sterol transporters mediates nonvesicular movement of cholesterol from the PM to the ER in multiple mammalian cell types. Here, we investigated the PM pool of accessible cholesterol in cholesterol-loaded fibroblasts with a knockdown of Aster-A and in mouse macrophages from Aster-B and Aster-A/B-deficient mice. Nanoscale secondary ion mass spectrometry (NanoSIMS) analyses revealed expansion of the accessible cholesterol pool in cells lacking Aster expression. The increased accessible cholesterol pool in the PM was accompanied by reduced cholesterol movement to the ER, evidenced by increased expression of SREBP2-regulated genes. Cosedimentation experiments with liposomes revealed that the Aster-B GRAM domain binds to membranes in a cholesterol concentration-dependent manner and that the binding is facilitated by the presence of phosphatidylserine. These studies revealed that the Aster-mediated nonvesicular cholesterol transport pathway controls levels of accessible cholesterol in the PM, as well as the activity of the SREBP pathway.

Peroxidasin-mediated bromine enrichment of basement membranes

Bromine and peroxidasin (an extracellular peroxidase) are essential for generating sulfilimine cross-links between a methionine and a hydroxylysine within collagen IV, a basement membrane protein. The sulfilimine cross-links increase the structural integrity of basement membranes. The formation of sulfilimine cross-links depends on the ability of peroxidasin to use bromide and hydrogen peroxide substrates to produce hypobromous acid (HOBr). Once a sulfilimine cross-link is created, bromide is released into the extracellular space and becomes available for reutilization. Whether the HOBr generated by peroxidasin is used very selectively for creating sulfilimine cross-links or whether it also causes oxidative damage to bystander molecules (e.g., generating bromotyrosine residues in basement membrane proteins) is unclear. To examine this issue, we used nanoscale secondary ion mass spectrometry (NanoSIMS) imaging to define the distribution of bromine in mammalian tissues. We observed striking enrichment of bromine (79Br, 81Br) in basement membranes of normal human and mouse kidneys. In peroxidasin knockout mice, bromine enrichment of basement membranes of kidneys was reduced by ∼85%. Proteomic studies revealed bromination of tyrosine-1485 in the NC1 domain of α2 collagen IV from kidneys of wild-type mice; the same tyrosine was brominated in collagen IV from human kidney. Bromination of tyrosine-1485 was reduced by >90% in kidneys of peroxidasin knockout mice. Thus, in addition to promoting sulfilimine cross-links in collagen IV, peroxidasin can also brominate a bystander tyrosine. Also, the fact that bromine enrichment is largely confined to basement membranes implies that peroxidasin activity is largely restricted to basement membranes in mammalian tissues.

Measurement of Absolute Concentration at the Subcellular Scale

The concentration of a pharmaceutical drug or bioactive metabolite within the target organelle influences the effects elicited by the drug or metabolite. Although the relative concentrations of many compounds of interest within subcellular compartments have been measured, measurements of absolute concentrations in the organelle remain elusive. In this Perspective, we discuss a significant advance in using nano secondary ion mass spectrometry (nanoSIMS) to measure the absolute concentration of a ¹³C-labeled metabolite within secretory vesicles, as reported by Thomen et al. in the April issue of ACS Nano.

Interferon-mediated reprogramming of membrane cholesterol to evade bacterial toxins

Plasma membranes of animal cells are enriched for cholesterol. Cholesterol-dependent cytolysins (CDCs) are pore-forming toxins secreted by bacteria that target membrane cholesterol for their effector function. Phagocytes are essential for clearance of CDC-producing bacteria; however, the mechanisms by which these cells evade the deleterious effects of CDCs are largely unknown. Here, we report that interferon (IFN) signals convey resistance to CDC-induced pores on macrophages and neutrophils. We traced IFN-mediated resistance to CDCs to the rapid modulation of a specific pool of cholesterol in the plasma membrane of macrophages without changes to total cholesterol levels. Resistance to CDC-induced pore formation requires the production of the oxysterol 25-hydroxycholesterol (25HC), inhibition of cholesterol synthesis and redistribution of cholesterol to an esterified cholesterol pool. Accordingly, blocking the ability of IFN to reprogram cholesterol metabolism abrogates cellular protection and renders mice more susceptible to CDC-induced tissue damage. These studies illuminate targeted regulation of membrane cholesterol content as a host defense strategy.

Sphingolipids controlling ciliary and microvillar function

Cilia and microvilli are membrane protrusions that extend from the surface of many different mammalian cell types. Motile cilia or flagella are only found on specialized cells, where they control cell movement or the generation of fluid flow, whereas immotile primary cilia protrude from the surface of almost every mammalian cell to detect and transduce extracellular signals. Despite these differences, all cilia consist of a microtubule core called the axoneme. Microvilli instead contain bundled linear actin filaments and are mainly localized on epithelial cells, where they modulate the absorption of nutrients. Cilia and microvilli constitute subcellular compartments with distinctive lipid and protein repertoires and specialized functions. Here, we summarize the role of sphingolipids in defining the identity and controlling the function of cilia and microvilli in mammalian cells.

Lipid rafts and pathogens: the art of deception and exploitation

Lipid rafts, solid regions of the plasma membrane enriched in cholesterol and glycosphingolipids, are essential parts of a cell. Functionally, lipid rafts present a platform that facilitates interaction of cells with the outside world. However, the unique properties of lipid rafts required to fulfill this function at the same time make them susceptible to exploitation by pathogens. Many steps of pathogen interaction with host cells, and sometimes all steps within the entire lifecycle of various pathogens, rely on host lipid rafts. Such steps as binding of pathogens to the host cells, invasion of intracellular parasites into the cell, the intracellular dwelling of parasites, microbial assembly and exit from the host cell, and microbe transfer from one cell to another all involve lipid rafts. Interaction also includes modification of lipid rafts in host cells, inflicted by pathogens from both inside and outside the cell, through contact or remotely, to advance pathogen replication, to utilize cellular resources, and/or to mitigate immune response. Here, we provide a systematic overview of how and why pathogens interact with and exploit host lipid rafts, as well as the consequences of this interaction for the host, locally and systemically, and for the microbe. We also raise the possibility of modulation of lipid rafts as a therapeutic approach against a variety of infectious agents.

Subcellular Mass Spectrometry Imaging and Absolute Quantitative Analysis across Organelles

Mass spectrometry imaging is a field that promises to become a mainstream bioanalysis technology by allowing the combination of single-cell imaging and subcellular quantitative analysis. The frontier of single-cell imaging has advanced to the point where it is now possible to compare the chemical contents of individual organelles in terms of raw or normalized ion signal. However, to realize the full potential of this technology, it is necessary to move beyond this concept of relative quantification. Here we present a nanoSIMS imaging method that directly measures the absolute concentration of an organelle-associated, isotopically labeled, pro-drug directly from a mass spectrometry image. This is validated with a recently developed nanoelectrochemistry method for single organelles. We establish a limit of detection based on the number of isotopic labels used and the volume of the organelle of interest, also offering this calculation as a web application. This approach allows subcellular quantification of drugs and metabolites, an overarching and previously unmet goal in cell science and pharmaceutical development.

Multiplex Protein Imaging with Secondary Ion Mass Spectrometry Using Metal Oxide Nanoparticle-Conjugated Antibodies

In spite of recent developments in mass spectrometry imaging techniques, high-resolution multiplex protein bioimaging techniques are required to unveil the complex inter- and intracellular biomolecular interactions for accurate understanding of life phenomena and disease mechanisms. Herein, we report multiplex protein imaging with secondary ion mass spectrometry (SIMS) using metal oxide nanoparticle (MONP)-conjugated antibodies with <300 nm spatial resolution in the low ion dose without ion beam damage because of the high secondary ion yields of the MONPs, which can provide simultaneous imaging of several proteins, especially from cell membranes. We applied our new imaging technique for the study of hippocampal tissue samples from control and Alzheimer's disease (AD) model mice; the proximity of protein clusters in the hippocampus CA1 region showed intriguing dependence on aging and AD progress, suggesting that protein cluster proximity may be helpful for understanding pathological pathways in the microscopic cellular level.

High-Resolution Secondary Ion Mass Spectrometry Analysis of Cell Membranes

This Feature describes the use a Cameca NanoSIMS instrument for directly imaging specific lipid and protein species in the plasma membranes of mammalian cells with approximately 100 nm-lateral resolution and discusses how these analyses have already begun to transform fundamental concepts in the field of membrane biology. Secondary ion generation is discussed with emphasis on the constraints that affect the detection and identification of membrane components, and then the sample preparation methodologies and data analysis strategies that address these constraints are described.

More than one way to bind to cholesterol: atypical variants of membrane-binding domain of perfringolysin O selected by ribosome display

Herein, we report a high-throughput approach for the selection of peripheral protein domains that bind specifically to cholesterol in lipid membranes. We discovered variants of perfringolysin O, with non-conserved amino acid substitutions at regions crucial for cholesterol recognition, demonstrating an unprecedented amino acid sequence variability with binding ability for cholesterol. The developed approach provides an effective platform for a comprehensive study of protein lipid interactions.

By using developed ribosomal display, we discovered variants of perfringolysin O, a pore forming toxin from bacteria Clostridium perfringens, with non-conserved amino acid substitutions at regions crucial for cholesterol recognition.

Deciphering the Role of Lipid Droplets in Cardiovascular Disease: A Report From the 2017 National Heart, Lung, and Blood Institute Workshop

Lipid droplets (LDs) are distinct and dynamic organelles that affect the health of cells and organs. Much progress has been made in understanding how these structures are formed, how they interact with other cellular organelles, how they are used for storage of triacylglycerol in adipose tissue, and how they regulate lipolysis. Our understanding of the biology of LDs in the heart and vascular tissue is relatively primitive in comparison with LDs in adipose tissue and liver. The National Heart, Lung, and Blood Institute convened a working group to discuss how LDs affect cardiovascular diseases. The goal of the working group was to examine the current state of knowledge on the cell biology of LDs, including current methods to study them in cells and organs and reflect on how LDs influence the development and progression of cardiovascular diseases. This review summarizes the working group discussion and recommendations on research areas ripe for future investigation that will likely improve our understanding of atherosclerosis and heart function.

Release of cholesterol-rich particles from the macrophage plasma membrane during movement of filopodia and lamellipodia

Cultured mouse peritoneal macrophages release large numbers of ~30-nm cholesterol-rich particles. Here, we show that those particles represent fragments of the plasma membrane that are pulled away and left behind during the projection and retraction of filopodia and lamellipodia. Consistent with this finding, the particles are enriched in proteins found in focal adhesions, which attach macrophages to the substrate. The release of particles is abolished by blocking cell movement (either by depolymerizing actin with latrunculin A or by inhibiting myosin II with blebbistatin). Confocal microscopy and NanoSIMS imaging studies revealed that the plasma membrane-derived particles are enriched in 'accessible cholesterol' (a mobile pool of cholesterol detectable with the modified cytolysin ALO-D4) but not in sphingolipid-sequestered cholesterol [a pool detectable with ostreolysin A (OlyA)]. The discovery that macrophages release cholesterol-rich particles during cellular locomotion is likely relevant to cholesterol efflux and could contribute to extracellular cholesterol deposition in atherosclerotic plaques.

Investigation of psoriasis skin tissue by label-free multi-modal imaging: a case study on a phototherapy-treated patient

Background: Psoriasis is a systemic inflammatory disease characterized by epidermal proliferation in the skin. Altered lipid metabolism is considered to be a central factor in the psoriatic etiopathogenesis. Thus, it is necessary to visualize chemical specificity of the samples for better medical diagnosis and treatment. Here, we investigate its role in the development of psoriatic lesions, before and after ultraviolet phototherapy, in a case study.

Methods: The distribution and morphology of different lipids and fibrous proteins in psoriatic (lesional) tissues were visualized by two complementary label-free imaging techniques: 1) non-linear microscopy (NLM), providing images of lipids/proteins throughout the skin layers at submicrometer resolution; and 2) mass spectrometry imaging (MSI), offering high chemical specificity and hence the detection of different lipid species in the epidermal and dermal regions. A conventional method of histological evaluation was performed on the tissues, with no direct comparison with NLM and MSI.

Results: Psoriatic tissues had a higher lipid content, mainly in cholesterol, in both the epidermal and dermal regions, compared to healthy tissues. Moreover, the collagen and elastin fibers in the psoriatic tissues had a tendency to assemble as larger bundles, while healthy tissues showed smaller fibers more homogeneously spread. Although phototherapy significantly reduced the cholesterol content, it also increased the amounts of collagen in both lesional and non-lesional tissues.

Conclusion: This study introduces NLM and MSI as two complementary techniques which are chemical specific and can be used to assess and visualize the distribution of lipids, collagen, and elastin in a non-invasive and label-free manner.

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Learning Modality-Specific Representations for Visible-Infrared Person Re-Identification

Traditional person re-identification (re-id) methods perform poorly under changing illuminations. This situation can be addressed by using dual-cameras that capture visible images in a bright environment and infrared images in a dark environment. Yet, this scheme needs to solve the visible-infrared matching issue, which is largely under-studied. Matching pedestrians across heterogeneous modalities is extremely challenging because of different visual characteristics. In this paper, we propose a novel framework that employ modality-specific networks to tackle with the heterogeneous matching problem. The proposed framework utilizes the modality-related information and extracts modality-specific representations (MSR) by constructing an individual network for each modality. In addition, a cross-modality Euclidean constraint is introduced to narrow the gap between different networks. We also integrate the modality-shared layers into modality-specific networks to extract shareable information and use a modality-shared identity loss to facilitate the extraction of modality-invariant features. Then a modality-specific discriminant metric is learned for each domain to strengthen the discriminative power of MSR. Eventually, we use a view classifier to learn view information. The experiments demonstrate that the MSR effectively improves the performance of deep networks on VI-REID and remarkably outperforms the state-of-the-art methods.

GPIHBP1 expression in gliomas promotes utilization of lipoprotein-derived nutrients

GPIHBP1, a GPI-anchored protein of capillary endothelial cells, binds lipoprotein lipase (LPL) within the subendothelial spaces and shuttles it to the capillary lumen. GPIHBP1-bound LPL is essential for the margination of triglyceride-rich lipoproteins (TRLs) along capillaries, allowing the lipolytic processing of TRLs to proceed. In peripheral tissues, the intravascular processing of TRLs by the GPIHBP1-LPL complex is crucial for the generation of lipid nutrients for adjacent parenchymal cells. GPIHBP1 is absent from the capillaries of the brain, which uses glucose for fuel; however, GPIHBP1 is expressed in the capillaries of mouse and human gliomas. Importantly, the GPIHBP1 in glioma capillaries captures locally produced LPL. We use NanoSIMS imaging to show that TRLs marginate along glioma capillaries and that there is uptake of TRL-derived lipid nutrients by surrounding glioma cells. Thus, GPIHBP1 expression in gliomas facilitates TRL processing and provides a source of lipid nutrients for glioma cells.

Functional link between plasma membrane spatiotemporal dynamics, cancer biology, and dietary membrane-altering agents

The cell plasma membrane serves as a nexus integrating extra- and intracellular components, which together enable many of the fundamental cellular signaling processes that sustain life. In order to perform this key function, plasma membrane components assemble into well-defined domains exhibiting distinct biochemical and biophysical properties that modulate various signaling events. Dysregulation of these highly dynamic membrane domains can promote oncogenic signaling. Recently, it has been demonstrated that select membrane-targeted dietary bioactives (MTDBs) have the ability to remodel plasma membrane domains and subsequently reduce cancer risk. In this review, we focus on the importance of plasma membrane domain structural and signaling functionalities as well as how loss of membrane homeostasis can drive aberrant signaling. Additionally, we discuss the intricacies associated with the investigation of these membrane domain features and their associations with cancer biology. Lastly, we describe the current literature focusing on MTDBs, including mechanisms of chemoprevention and therapeutics in order to establish a functional link between these membrane-altering biomolecules, tuning of plasma membrane hierarchal organization, and their implications in cancer prevention.

Comment on 'Orthogonal lipid sensors identify transbilayer asymmetry of plasma membrane cholesterol'

The plasma membrane in mammalian cells is rich in cholesterol, but how the cholesterol is partitioned between the two leaflets of the plasma membrane remains a matter of debate. Recently, Liu et al. used domain 4 (D4) of perfringolysin O as a cholesterol sensor to argue that cholesterol is mostly in the exofacial leaflet (Liu et al., 2017). This conclusion was made by interpreting D4 binding in live cells using in vitro calibrations with liposomes. However, liposomes may be unfaithful in mimicking the plasma membrane, as we demonstrate here. Also, D4 binding is highly sensitive to the presence of cytosolic proteins. In addition, we find that a D4 variant, which requires >35 mol% cholesterol to bind to liposomes in vitro, does in fact bind to the cytoplasmic leaflet of the plasma membrane in a cholesterol-dependent manner. Thus, we believe, based on the current evidence, that it is unlikely that there is a significantly higher proportion of cholesterol in the exofacial leaflet of the plasma membrane compared to the cytosolic leaflet.

Molecular Colocalization Using Massive Gold Cluster Secondary Ion Mass Spectrometry

We report on the ion emission from impacts of hypervelocity massive gold clusters for use in secondary ion mass spectrometry. Two massive gold clusters are considered, 520 keV Au₄₀₀⁴⁺ and 1040 keV Au₂₈₀₀⁸⁺. The emission of fragment ions and molecular ions is evaluated for a series of neat samples, glycine, phenylalanine, arginine, and gramicidin S. A 2 to 4-fold increase of molecular ion emission is observed from impacts of 1040 keV Au₂₈₀₀⁸⁺ versus 520 keV Au₄₀₀⁴⁺. Compared to impacts of 20 keV Ar₂₀₀₀⁺ and 20 keV (H₂O)₇₀₀₀⁺ in static conditions, impacts of 1040 keV Au₂₈₀₀⁸⁺ display a 6 to 9-fold increase in the number of detected molecular ions per projectile impact. To explain the increased emission of molecular species, we examine the size of the impact craters and calculate the ratio of molecular ions to fragment ions. The characterization of Au₂₈₀₀⁸⁺ and the operating conditions of the gold liquid metal ion source are presented.

NanoSIMS imaging reveals unexpected heterogeneity in nutrient uptake by brown adipocytes

Heterogeneity in the metabolic properties of adipocytes in white adipose tissue has been well documented. We sought to investigate metabolic heterogeneity in adipocytes of brown adipose tissue (BAT), focusing on heterogeneity in nutrient uptake. To explore the possibility of metabolic heterogeneity in brown adipocytes, we used nanoscale secondary ion mass spectrometry (NanoSIMS) to quantify uptake of lipids in adipocytes interscapular BAT and perivascular adipose tissue (PVAT) after an intravenous injection of triglyceride-rich lipoproteins (TRLs) containing [²H]triglycerides (²H-TRLs). The uptake of deuterated lipids into brown adipocytes was quantified by NanoSIMS. We also examined ¹³C enrichment in brown adipocytes after administering [¹³C]glucose or ¹³C-labeled mixed fatty acids by gastric gavage. The uptake of ²H-TRLs-derived lipids into brown adipocytes was heterogeneous, with ²H enrichment in adjacent adipocytes varying by more than fourfold. We also observed substantial heterogeneity in ¹³C enrichment in adjacent brown adipocytes after administering [¹³C]glucose or [¹³C]fatty acids by gastric gavage. The uptake of nutrients by adjacent brown adipocytes within a single depot is variable, suggesting that there is heterogeneity in the metabolic properties of brown adipocytes.

Roles of membrane lipids in the organization of epithelial cells: Old and new problems

Epithelial cells have characteristic membrane domains. Identification of membrane proteins playing an important role in these membrane domains has progressed and numerous studies have been performed on the functional analysis of these membrane proteins. On the other hand, the precise roles of membrane lipids in the organization of these membrane domains are largely unknown. Historically, the concept of lipid raft arose from the analysis of lipid composition of the apical membrane, and it can be said that epithelial cells are an optimal experimental model for elucidating the functions of lipids. In this review, I discuss the role of lipids in the formation of epithelial polarity and in the formation of cell membrane structures of epithelial cells such as microvilli in the apical domain, cell-cell adhesion apparatus in the lateral domain and cell-matrix adhesion in the basal domain.

Macrophages release plasma membrane-derived particles rich in accessible cholesterol

Macrophages are generally assumed to unload surplus cholesterol through direct interactions between ABC transporters on the plasma membrane and HDLs, but they have also been reported to release cholesterol-containing particles. How macrophage-derived particles are formed and released has not been clear. To understand the genesis of macrophage-derived particles, we imaged mouse macrophages by EM and nanoscale secondary ion mass spectrometry (nanoSIMS). By scanning EM, we found that large numbers of 20- to 120-nm particles are released from the fingerlike projections (filopodia) of macrophages. These particles attach to the substrate, forming a "lawn" of particles surrounding macrophages. By nanoSIMS imaging we showed that these particles are enriched in the mobile and metabolically active accessible pool of cholesterol (detectable by ALO-D4, a modified version of a cholesterol-binding cytolysin). The cholesterol content of macrophage-derived particles was increased by loading the cells with cholesterol or by adding LXR and RXR agonists to the cell-culture medium. Incubating macrophages with HDL reduced the cholesterol content of macrophage-derived particles. We propose that release of accessible cholesterol-rich particles from the macrophage plasma membrane could assist in disposing of surplus cholesterol and increase the efficiency of cholesterol movement to HDL.

Optimizing colormaps with consideration for color vision deficiency to enable accurate interpretation of scientific data

Color vision deficiency (CVD) affects more than 4% of the population and leads to a different visual perception of colors. Though this has been known for decades, colormaps with many colors across the visual spectra are often used to represent data, leading to the potential for misinterpretation or difficulty with interpretation by someone with this deficiency. Until the creation of the module presented here, there were no colormaps mathematically optimized for CVD using modern color appearance models. While there have been some attempts to make aesthetically pleasing or subjectively tolerable colormaps for those with CVD, our goal was to make optimized colormaps for the most accurate perception of scientific data by as many viewers as possible. We developed a Python module, cmaputil, to create CVD-optimized colormaps, which imports colormaps and modifies them to be perceptually uniform in CVD-safe colorspace while linearizing and maximizing the brightness range. The module is made available to the science community to enable others to easily create their own CVD-optimized colormaps. Here, we present an example CVD-optimized colormap created with this module that is optimized for viewing by those without a CVD as well as those with red-green colorblindness. This colormap, cividis, enables nearly-identical visual-data interpretation to both groups, is perceptually uniform in hue and brightness, and increases in brightness linearly.

Mechanisms of cellular cholesterol compartmentalization: recent insights

This review discusses advances in understanding how the controlled delivery of cholesterol between subcellular compartments is achieved and what novel experimental strategies are being employed to address this fundamental question. Recent work has focused on cholesterol-binding proteins that can facilitate directional cholesterol transfer between contacts of the ER and Golgi or late endosomal membranes. Increasing structural information on cholesterol-binding proteins, new modules engineered from them as well as improved imaging and gene editing techniques are providing valuable insights. There is also mounting information on how the crosstalk between cholesterol transport and nutrient signaling is orchestrated and how cellular fatty acid metabolism and cholesterol homeostasis are intertwined.