Lipid rafts and caveolae and their role in compartmentation of redox signaling

Human Umbilical Vein Endothelial Cells as a Versatile Cellular Model System in Diverse Experimental Paradigms: An Ultrastructural Perspective

Human umbilical vein endothelial cells (HUVECs) are primary cells isolated from the vein of an umbilical cord, extensively used in cardiovascular studies and medical research. These cells, retaining the characteristics of endothelial cells in vivo, serve as a valuable cellular model system for understanding vascular biology, endothelial dysfunction, pathophysiology of diseases such as atherosclerosis, and responses to different drugs or treatments. Transmission electron microscopy (TEM) has been a cornerstone in revealing the detailed architecture of multiple cellular model systems including HUVECs, allowing researchers to visualize subcellular organelles, membrane structures, and cytoskeletal elements. Among them, the endoplasmic reticulum, Golgi apparatus, mitochondria, and nucleus can be meticulously examined to recognize alterations indicative of cellular responses to various stimuli. Importantly, Weibel-Palade bodies are characteristic secretory organelles found in HUVECs, which can be easily distinguished in the TEM. These distinctive structures also dynamically react to different factors through regulated exocytosis, resulting in complete or selective release of their contents. This detailed review summarizes the ultrastructural features of HUVECs and highlights the utility of TEM as a pivotal tool for analyzing HUVECs in diverse research frameworks, contributing valuable insights into the comprehension of HUVEC behavior and enriching our knowledge into the complexity of vascular biology.

Caveolin-1 Autonomously Regulates Hippocampal Neurogenesis Via Mitochondrial Dynamics

The mechanisms underlying adult hippocampal neurogenesis (AHN) are not fully understood. AHN plays instrumental roles in learning and memory. Understanding the signals that regulate AHN has implications for brain function and therapy. Here we show that Caveolin-1 (Cav-1), a protein that is highly enriched in endothelial cells and the principal component of caveolae, autonomously regulates AHN. Conditional deletion of Cav-1 in adult neural progenitor cells (nestin +) led to increased neurogenesis and enhanced performance of mice in contextual discrimination. Proteomic analysis revealed that Cav-1 plays a role in mitochondrial pathways in neural progenitor cells. Importantly, Cav-1 was localized to the mitochondria in neural progenitor cells and modulated mitochondrial fission-fusion, a critical process in neurogenesis. These results suggest that Cav-1 is a novel regulator of AHN and underscore the impact of AHN on cognition.

Decrease in cholesterol in the cell membrane is essential for Nrf2 activation by quercetin

Quercetin is a flavonoid with various cytoprotective effects. We previously reported that quercetin exerts anti-allergic, anti-oxidative, and anti-fibrotic activities via the induction of heme oxygenase (HO)-1. However, the mechanisms by which quercetin induces HO-1 to exhibit cytoprotective effects are poorly understood. We focused on its action on the cell membrane, which is the first part of the cell to interact with the extracellular environment. The cell membrane contains lipid rafts and caveolae, which play important roles in cellular signaling. A recent study showed that nuclear factor E2-related factor 2 (Nrf2), a transcription factor regulating anti-oxidative enzymes including HO-1, interacts with caveolin-1 (Cav-1), a component of caveolae, to regulate cellular anti-oxidative capacity. In this study, we investigated the changes in the cell membrane that leads to the induction of HO-1 by quercetin. Quercetin decreased the amount of cholesterol in the raft fractions, which in turn promoted the induction of HO-1. It also changed the composition of the lipid rafts and decreased and increased the expression of Cav-1 in the raft and non-raft fractions, respectively. Nrf2, which was localized in the cell membrane under resting conditions, was translocated along with Cav-1 to the nucleus after exposure to quercetin. These findings indicate for the first time that the HO-1-dependent cytoprotective effects of quercetin are mediated by the structural changes in lipid rafts brought about by decreasing the amount of cholesterol in the cell membrane, which thereby results in the translocation of the Cav-1-Nrf2 complex to the nucleus and induces the expression of HO-1.

A mechanistic study on the cellular uptake, intracellular trafficking, and antisense gene regulation of bottlebrush polymer-conjugated oligonucleotides

We have developed a non-cationic transfection vector in the form of bottlebrush polymer-antisense oligonucleotide (ASO) conjugates. Termed pacDNA (polymer-assisted compaction of DNA), these agents show improved biopharmaceutical characteristics and antisense potency in vivo while suppressing non-antisense side effects. Nonetheless, there still is a lack of the mechanistic understanding of the cellular uptake, subcellular trafficking, and gene knockdown with pacDNA. Here, we show that the pacDNA enters human non-small cell lung cancer cells (NCI-H358) predominantly by scavenger receptor-mediated endocytosis and macropinocytosis and trafficks via the endolysosomal pathway within the cell. The pacDNA significantly reduces a target gene expression (KRAS) in the protein level but not in the mRNA level, despite that the transfection of certain free ASOs causes ribonuclease H1 (RNase H)-dependent degradation of KRAS mRNA. In addition, the antisense activity of pacDNA is independent of ASO chemical modification, suggesting that the pacDNA always functions as a steric blocker.

Cholesterol and Sphingomyelin Polarize at the Leading Edge of Migrating Myoblasts and Involve Their Clustering in Submicrometric Domains

Myoblast migration is crucial for myogenesis and muscular tissue homeostasis. However, its spatiotemporal control remains elusive. Here, we explored the involvement of plasma membrane cholesterol and sphingolipids in this process. In resting C2C12 mouse myoblasts, those lipids clustered in sphingomyelin/cholesterol/GM1 ganglioside (SM/chol/GM1)- and cholesterol (chol)-enriched domains, which presented a lower stiffness than the bulk membrane. Upon migration, cholesterol and sphingomyelin polarized at the front, forming cholesterol (chol)- and sphingomyelin/cholesterol (SM/chol)-enriched domains, while GM1-enriched domains polarized at the rear. A comparison of domain proportion suggested that SM/chol- and GM1-enriched domains originated from the SM/chol/GM1-coenriched domains found at resting state. Modulation of domain proportion (through cholesterol depletion, combined or not with actin polymerization inhibition, or sphingolipid synthesis inhibition) revealed that the higher the chol- and SM/chol-enriched domains, the higher the myoblast migration. At the front, chol- and SM/chol-enriched domains were found in proximity with F-actin fibers and the lateral mobility of sphingomyelin in domains was specifically restricted in a cholesterol- and cytoskeleton-dependent manner while domain abrogation impaired F-actin and focal adhesion polarization. Altogether, we showed the polarization of cholesterol and sphingomyelin and their clustering in chol- and SM/chol-enriched domains with differential properties and roles, providing a mechanism for the spatial and functional control of myoblast migration.

Expression of CD70 Modulates NO and Redox Status in Endothelial Cells

BACKGROUND

Endothelial dysfunction is a critical component in the pathogenesis of cardiovascular diseases and is closely associated with NO levels and oxidative stress. Here, we report on novel findings linking endothelial expression of CD70 with alterations in NO and reactive oxygen species.

METHODS

CD70 expression was genetically manipulated in human aortic and pulmonary artery endothelial cells. Intracellular NO and hydrogen peroxide (H₂O₂) were measured using genetically encoded biosensors, and cellular phenotypes were assessed.

RESULTS

An unbiased phenome-wide association study demonstrated that polymorphisms in CD70 associate with vascular phenotypes. Endothelial cells treated with CD70-directed short-interfering RNA demonstrated impaired wound closure, decreased agonist-stimulated NO levels, and reduced eNOS (endothelial nitric oxide synthase) protein. This was accompanied by reduced NO bioactivity, increased 3-nitrotyrosine levels, and a decrease in the eNOS binding partner heat shock protein 90. Following treatment with the thioredoxin inhibitor auranofin or with agonist histamine, intracellular H₂O₂ levels increased up to 80% in the cytosol, plasmalemmal caveolae, and mitochondria. There was increased expression of NADPH oxidase 1 complex and gp91phox; expression of copper/zinc and manganese superoxide dismutases was also elevated. CD70 knockdown reduced levels of the H₂O₂ scavenger catalase; by contrast, glutathione peroxidase 1 expression and activity were increased. CD70 overexpression enhanced endothelial wound closure, increased NO levels, and attenuated the reduction in eNOS mRNA induced by TNFα.

CONCLUSIONS

Taken together, these data establish CD70 as a novel regulatory protein in endothelial NO and reactive oxygen species homeostasis, with implications for human vascular disease.

Free Cholesterol Affects the Function and Localization of Human Na+/Taurocholate Cotransporting Polypeptide (NTCP) and Organic Cation Transporter 1 (OCT1)

Non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) are associated with obesity. They are accompanied by increased levels of free cholesterol in the liver. Most free cholesterol resides within the plasma membrane. We assessed the impact of adding or removing free cholesterol on the function and localization of two hepatocellular uptake transporters: the Na⁺/taurocholate cotransporting polypeptide (NTCP) and the organic cation transporter 1 (OCT1). We used a cholesterol-MCD complex (cholesterol) to add cholesterol and methyl-β-cyclodextrin (MCD) to remove cholesterol. Our results demonstrate that adding cholesterol decreases NTCP capacity from 132 ± 20 to 69 ± 37 µL/mg/min and OCT1 capacity from 209 ± 66 to 125 ± 26 µL/mg/min. Removing cholesterol increased NTCP and OCT1 capacity to 224 ± 65 and 279 ± 20 µL/mg/min, respectively. In addition, adding cholesterol increased the localization of NTCP within lipid rafts, while adding or removing cholesterol increased OCT1 localization in lipid rafts. These results demonstrate that increased cholesterol levels can impair NTCP and OCT1 function, suggesting that the free cholesterol content of the liver can alter bile acid and drug uptake into the liver. This could explain the increased plasma bile acid levels in NAFLD and NASH patients and potentially lead to altered drug disposition.

MAL2 mediates the formation of stable HER2 signaling complexes within lipid raft-rich membrane protrusions in breast cancer cells

The lipid raft-resident protein, MAL2, has been implicated as contributing to the pathogenesis of several malignancies, including breast cancer, but the underlying mechanism for its effects on tumorigenesis is unknown. Here, we show that MAL2-mediated lipid raft formation leads to HER2 plasma membrane retention and enhanced HER2 signaling in breast cancer cells. We demonstrate physical interactions between HER2 and MAL2 in lipid rafts using proximity ligation assays. Super-resolution structured illumination microscopy imaging displays the structural organization of the HER2/Ezrin/NHERF1/PMCA2 protein complex. Formation of this protein complex maintains low intracellular calcium concentrations in the vicinity of the plasma membrane. HER2/MAL2 protein interactions in lipid rafts are enhanced in trastuzumab-resistant breast cancer cells. Our findings suggest that MAL2 is crucial for lipid raft formation, HER2 signaling, and HER2 membrane stability in breast cancer cells, suggesting MAL2 as a potential therapeutic target.

Jeong et al. show that the formation of MAL2-mediated lipid raft-rich membrane protrusions is crucial for HER2 signaling in breast cancer cells. MAL2 is required for the formation of HER2/Ezrin/NHERF1/PMCA2 protein complexes. Formation of these protein complexes leads to a low calcium environment in the plasma membrane

Melatonin: Regulation of Biomolecular Condensates in Neurodegenerative Disorders

Biomolecular condensates are membraneless organelles (MLOs) that form dynamic, chemically distinct subcellular compartments organizing macromolecules such as proteins, RNA, and DNA in unicellular prokaryotic bacteria and complex eukaryotic cells. Separated from surrounding environments, MLOs in the nucleoplasm, cytoplasm, and mitochondria assemble by liquid-liquid phase separation (LLPS) into transient, non-static, liquid-like droplets that regulate essential molecular functions. LLPS is primarily controlled by post-translational modifications (PTMs) that fine-tune the balance between attractive and repulsive charge states and/or binding motifs of proteins. Aberrant phase separation due to dysregulated membrane lipid rafts and/or PTMs, as well as the absence of adequate hydrotropic small molecules such as ATP, or the presence of specific RNA proteins can cause pathological protein aggregation in neurodegenerative disorders. Melatonin may exert a dominant influence over phase separation in biomolecular condensates by optimizing membrane and MLO interdependent reactions through stabilizing lipid raft domains, reducing line tension, and maintaining negative membrane curvature and fluidity. As a potent antioxidant, melatonin protects cardiolipin and other membrane lipids from peroxidation cascades, supporting protein trafficking, signaling, ion channel activities, and ATPase functionality during condensate coacervation or dissolution. Melatonin may even control condensate LLPS through PTM and balance mRNA- and RNA-binding protein composition by regulating N6-methyladenosine (m6A) modifications. There is currently a lack of pharmaceuticals targeting neurodegenerative disorders via the regulation of phase separation. The potential of melatonin in the modulation of biomolecular condensate in the attenuation of aberrant condensate aggregation in neurodegenerative disorders is discussed in this review.

Complex Formation of Heme Oxygenase-2 with Heme Is Competitively Inhibited by the Cytosolic Domain of Caveolin-1

The mechanism and physiological functions of heme oxygenase-2 (HO-2)-mediated carbon monoxide (CO) production, accompanied by heme metabolism, have been studied intensively in recent years. The enzymatic activity of constitutively expressed HO-2 must be strictly controlled in terms of the toxicity and chemical stability of CO. In this study, the molecular interaction between HO-2 and caveolin-1 and its effect on HO action were evaluated. An enzyme kinetics assay with residues 82-101 of caveolin-1, also called the caveolin scaffold domain, inhibited HO-2 activity in a competitive manner. Analytical ultracentrifugation and a hemin titration assay suggested that the inhibitory effect was generated by direct binding of caveolin-1 to aromatic residues, which were defined as components of the caveolin-binding motif in the HO-2 heme pocket. Herein, we developed a HO-2-based fluorescence bioprobe, namely EGFP-Δ19/D159H, which was capable of quantifying heme binding by HO-2 as the initial step in the CO production. The fluorescence of EGFP-Δ19/D159H decreased in accordance with 5-aminolevulinic acid-facilitated heme biosynthesis in COS-7 cells. In contrast, expression of the N-terminal cytosolic domain of caveolin-1 (residues 1-101) increased the probe fluorescence, suggesting that the cytosolic domain of caveolin-1 potently inhibits the binding of heme to the heme pocket of EGFP-Δ19/D159H. Taken together, our results suggest that caveolin-1 is a negative regulator of HO-2 enzymatic action. Moreover, our bioprobe EGFP-Δ19/D159H represents a powerful tool for use in future studies addressing HO-2-mediated CO production.

Anandamide Exerts a Differential Effect on Human Placenta Before and After the Onset of Labor

The onset of labor involves the action of multiple factors and recent reports have postulated the endocannabinoid system as a new regulator of this process. Our objective was to study the role of anandamide, one of the main endocannabinoids, on the regulation of placental molecules that contribute to the onset of labor at term. Placental samples were obtained from patients with laboring vaginal deliveries and from non-laboring elective cesarean sections. Vaginal delivery placentas produced higher prostaglandins levels than cesarean section samples. Besides, no differences were observed in NOS basal activity between groups. Incubation of vaginal delivery placentas with anandamide increased prostaglandins concentration and decreased NOS activity. Antagonism of type-1cannabinoid receptor (CB1) did not alter the effect observed on NOS activity. Conversely, incubation of cesarean section placentas with anandamide reduced prostaglandins levels and enhanced NOS activity, the latter involving the participation of CB1. Furthermore, we observed a differential expression of the main components of the endocannabinoid system between placental samples, being the change in CB1 localization the most relevant finding. Our results suggest that anandamide acts as a modulator of the signals that regulate labor, exerting differential actions depending on CB1 localization in laboring or non-laboring term placentas.

FXYD1 Is Protective Against Vascular Dysfunction

[Figure: see text].

SOD3 Is a Non-Mutagenic Growth Regulator Affecting Cell Migration and Proliferation Signal Transduction

Superoxide dismutase (SOD) family isoenzymes, SOD1, SOD2, and SOD3, synthesize hydrogen peroxide (H₂O₂), which regulates the signal transduction. H₂O₂ is a second messenger able to enter into the cells through aquaporin 3 cell membrane channels and to modify protein tyrosine phosphatase activity. SOD3 has been shown to activate signaling pathways in tissue injuries, inflammation, and cancer models. Similar to the H₂O₂ response in the cells, the cellular response of SOD3 is dose-dependent; even a short supraphysiological concentration reduces the cell survival and activates the growth arrest and apoptotic signaling, whereas the physiological SOD3 levels support its growth and survival. In the current work, we studied the signaling networks stimulated by SOD3 overexpression demonstrating a high diversity in the activation of signaling cascades. The results obtained suggest that SOD3, although inducing cell growth and affecting various biological processes, does not cause detectable long-term DNA aberrations. Therefore, according to the present data, SOD3 is not a mutagen. Additionally, we compared SOD3-driven immortalized mouse embryonic fibroblasts to SV40 immortalized NIH3T3 cells, demonstrating a marked difference in the activation of cellular kinases. The data presented may contain important druggable targets to abrogate unwanted cell growth.

Role of Cholesterol and Lipid Rafts in Cancer Signaling: A Promising Therapeutic Opportunity?

Cholesterol is a lipid molecule that plays an essential role in a number of biological processes, both physiological and pathological. It is an essential structural constituent of cell membranes, and it is fundamental for biosynthesis, integrity, and functions of biological membranes, including membrane trafficking and signaling. Moreover, cholesterol is the major lipid component of lipid rafts, a sort of lipid-based structures that regulate the assembly and functioning of numerous cell signaling pathways, including those related to cancer, such as tumor cell growth, adhesion, migration, invasion, and apoptosis. Considering the importance of cholesterol metabolism, its homeostasis is strictly regulated at every stage: import, synthesis, export, metabolism, and storage. The alterations of this homeostatic balance are known to be associated with cardiovascular diseases and atherosclerosis, but mounting evidence also connects these behaviors to increased cancer risks. Although there is conflicting evidence on the role of cholesterol in cancer development, most of the studies consistently suggest that a dysregulation of cholesterol homeostasis could lead to cancer development. This review aims to discuss the current understanding of cholesterol homeostasis in normal and cancerous cells, summarizing key findings from recent preclinical and clinical studies that have investigated the role of major players in cholesterol regulation and the organization of lipid rafts, which could represent promising therapeutic targets.

High intraluminal pressure promotes vascular inflammation via caveolin-1

The aetiology and progression of hypertension involves various endogenous systems, such as the renin angiotensin system, the sympathetic nervous system, and endothelial dysfunction. Recent data suggest that vascular inflammation may also play a key role in the pathogenesis of hypertension. This study sought to determine whether high intraluminal pressure results in vascular inflammation. Leukocyte adhesion was assessed in rat carotid arteries exposed to 1 h of high intraluminal pressure. The effect of intraluminal pressure on signaling mechanisms including reactive oxygen species production (ROS), arginase expression, and NFĸB translocation was monitored. 1 h exposure to high intraluminal pressure (120 mmHg) resulted in increased leukocyte adhesion and inflammatory gene expression in rat carotid arteries. High intraluminal pressure also resulted in a downstream signaling cascade of ROS production, arginase expression, and NFĸB translocation. This process was found to be angiotensin II-independent and mediated by the mechanosensor caveolae, as caveolin-1 (Cav1)-deficient endothelial cells and mice were protected from pressure-induced vascular inflammatory signaling and leukocyte adhesion. Cav1 deficiency also resulted in a reduction in pressure-induced glomerular macrophage infiltration in vivo. These findings demonstrate Cav1 is an important mechanosensor in pressure-induced vascular and renal inflammation.

Lipid Raft Association Stabilizes VEGF Receptor 2 in Endothelial Cells

The binding of vascular endothelial growth factor A (VEGF) to VEGF receptor-2 (VEGFR-2) stimulates angiogenic signaling. Lipid rafts are cholesterol-dense regions of the plasma membrane that serve as an organizational platform for biomolecules. Although VEGFR2 has been shown to colocalize with lipid rafts to regulate its activation, the effect of lipid rafts on non-activated VEGFR2 has not been explored. Here, we characterized the involvement of lipid rafts in modulating the stability of non-activated VEGFR2 in endothelial cells using raft disrupting agents: methyl-β-cyclodextrin, sphingomyelinase and simvastatin. Disrupting lipid rafts selectively decreased the levels of non-activated VEGFR2 as a result of increased lysosomal degradation. The decreased expression of VEGFR2 translated to reduced VEGF-activation of the extracellular signal-regulated protein kinases (ERK). Overall, our results indicate that lipid rafts stabilize VEGFR2 and its associated signal transduction activities required for angiogenesis. Thus, modulation of lipid rafts may provide a means to regulate the sensitivity of endothelial cells to VEGF stimulation. Indeed, the ability of simvastatin to down regulate VEGFR2 and inhibit VEGF activity suggest a potential mechanism underlying the observation that this drug improves outcomes in the treatment of certain cancers.

Caveolin-1 is Involved in Regulating the Biological Response of Cells to Nanosecond Pulsed Electric Fields

Nanosecond pulsed electric fields (nsPEFs) induce changes in the plasma membrane (PM), including PM permeabilization (termed nanoporation), allowing free passage of ions into the cell and, in certain cases, cell death. Recent studies from our laboratory show that the composition of the PM is a critical determinant of PM nanoporation. Thus, we hypothesized that the biological response to nsPEF exposure could be influenced by lipid microdomains, including caveolae, which are specialized invaginations of the PM that are enriched in cholesterol and contain aggregates of important cell signaling proteins, such as caveolin-1 (Cav1). Caveolae play a significant role in cellular signal transduction, including control of calcium influx and cell death by interaction of Cav1 with regulatory signaling proteins. Present results show that depletion of Cav1 increased the influx of calcium, while Cav1 overexpression produced the opposite effect. Additionally, Cav1 is known to bind and sequester important cell signaling proteins within caveolae, rendering the binding partners inactive. Imaging of the PM after nsPEF exposure showed localized depletion of PM Cav1 and results of co-immunoprecipitation studies showed dissociation of two critical Cav1 binding partners (transient receptor potential cation channel subfamily C1 (TRPC₁) and inositol trisphosphate receptor (IP₃R)) after exposure to nsPEFs. Release of TRPC₁ and IP₃R from Cav1 would activate downstream signaling cascades, including store-operated calcium entry, which could explain the influx in calcium after nsPEF exposure. Results of the current study establish a significant relationship between Cav1 and the activation of cell signaling pathways in response to nsPEFs.

Redistribution of Membrane Raft Microdomains by Apolipoprotein A-I In Mouse Aortic Endothelial Cells

Apolipoprotein A-I (apoAI) upregulates ATP-binding cassette transport A1 (ABCA1) in various cell types. ABCA1 has been shown to induce the redistribution of raft-associated proteins and lipids to the non-raft membrane. This report investigated the effect of apoAI on ABCA1 expression and raft cholesterol and protein distribution, as well as the effect of ABCA1 knockdown on apoAI-induced changes in mouse aortic endothelial cells (MAECs). Our data demonstrated that ABCA1 was distributed in both the lipid raft and non-raft membranes and was coimmunoprecipitated with caveolin-1 (CAV1). ApoAI treatment significantly increased the mRNA and protein levels of ABCA1 and reduced the percentage of ABCA1 in the raft membrane. Our data also showed that free cholesterol (FC) and CAV1 were concentrated in the raft-like detergent-resistant membranes (DRMs) under the control conditions. ApoAI treatment did not alter the cellular level of FC and CAV1 significantly but reduced the percentage of FC and CAV1 in the DRMs. Knockdown of ABCA1 attenuated apoAI-induced redistribution of FC and CAV1. The percentage of FC and CAV1 in the DRMs was correlated inversely with the cellular level of ABCA1, suggesting that apoAI induces relocation of CAV1 and FC from the raft to the non-rail membrane via a mechanism involving upregulation of ABCA1.

LDL receptors, caveolae and cholesterol in endothelial dysfunction: oxLDLs accomplices or victims?

Oxidized LDLs (oxLDLs) and oxysterols play a key role in endothelial dysfunction and the development of atherosclerosis. The loss of vascular endothelium function negatively impacts vasomotion, cell growth, adhesiveness and barrier functions. While for some of these disturbances, a reasonable explanation can be provided from a mechanistic standpoint, for many others, the molecular mediators that are involved are unknown. Caveolae, specific plasma membrane domains, have recently emerged as targets and mediators of oxLDL-induced endothelial dysfunction. Caveolae and their associated protein caveolin-1 (Cav-1) are involved in oxLDLs/LDLs transcytosis, mainly through the scavenger receptor class B type 1 (SR-B1 or SCARB1). In contrast, oxLDLs endocytosis is mediated by the lectin-like oxidized LDL receptor 1 (LOX-1), whose activity depends on an intact caveolae system. In addition, LOX-1 regulates the expression of Cav-1 and vice versa. On the other hand, oxLDLs may affect cholesterol plasma membrane content/distribution thus influencing caveolae architecture, Cav-1 localization and the associated signalling. Overall, the evidence indicate that caveolae have both active and passive roles in oxLDL-induced endothelial cell dysfunction. First, as mediators of lipid uptake and transfer in the subendothelial space and, later, as targets of changes in composition/dynamics of plasma membrane lipids resulting from increased levels of circulating oxLDLs. Gaining a better understanding of how oxLDLs interact with endothelial cells and modulate caveolae-mediated signalling pathways, leading to endothelial dysfunction, is crucial to find new targets for intervention to tackle atherosclerosis and the related clinical entities. LINKED ARTICLES: This article is part of a themed issue on Oxysterols, Lifelong Health and Therapeutics. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.16/issuetoc.

Regulation of caveolae through cholesterol-depletion-dependent tubulation mediated by PACSIN2

The membrane-shaping ability of PACSIN2 (also known as syndapin II), which is mediated by its F-BAR domain, has been shown to be essential for caveolar morphogenesis, presumably through the shaping of the caveolar neck. Caveolar membranes contain abundant cholesterol. However, the role of cholesterol in PACSIN2-mediated membrane deformation remains unclear. Here, we show that the binding of PACSIN2 to the membrane can be negatively regulated by cholesterol. We prepared reconstituted membranes based on the lipid composition of caveolae. The reconstituted membrane with cholesterol had a weaker affinity for the F-BAR domain of PACSIN2 than a membrane without cholesterol. Consistent with this, upon depletion of cholesterol from the plasma membrane, PACSIN2 localized at tubules that had caveolin-1 at their tips, suggesting that cholesterol inhibits membrane tubulation mediated by PACSIN2. The tubules induced by PACSIN2 could be representative of an intermediate of caveolae endocytosis. Consistent with this, the removal of caveolae from the plasma membrane upon cholesterol depletion was diminished in the PACSIN2-deficient cells. These data suggest that PACSIN2-mediated caveolae internalization is dependent on the amount of cholesterol, providing a mechanism for cholesterol-dependent regulation of caveolae.This article has an associated First Person interview with the first author of the paper.

Coronavirus Interplay With Lipid Rafts and Autophagy Unveils Promising Therapeutic Targets

Coronaviruses are enveloped, single-stranded, positive-sense RNA viruses that can infect animal and human hosts. The infection induces mild or sometimes severe acute respiratory diseases. Nowadays, the appearance of a new, highly pathogenic and lethal coronavirus variant, SARS-CoV-2, responsible for a pandemic (COVID-19), represents a global problem for human health. Unfortunately, only limited approaches are available to treat coronavirus infections and a vaccine against this new coronavirus variant is not yet available. The plasma membrane microdomain lipid rafts have been found by researchers to be involved in the replication cycle of numerous viruses, including coronaviruses. Indeed, some pathogen recognition receptors for coronaviruses as for other viruses cluster into lipid rafts, and it is therefore conceivable that the first contact between virus and host cells occurs into these specialized regions, representing a port of cell entry for viruses. Recent data highlighted the peculiar pro-viral or anti-viral role played by autophagy in the host immune responses to viral infections. Coronaviruses, like other viruses, were reported to be able to exploit the autophagic machinery to increase their replication or to inhibit the degradation of viral products. Agents known to disrupt lipid rafts, such as metil-β-cyclodextrins or statins, as well as autophagy inhibitor agents, were shown to have an anti-viral role. In this review, we briefly describe the involvement of lipid rafts and autophagy in coronavirus infection and replication. We also hint how lipid rafts and autophagy may represent a potential therapeutic target to be investigated for the treatment of coronavirus infections.

The caveolar-mitochondrial interface: regulation of cellular metabolism in physiology and pathophysiology

The plasma membrane is an important cellular organelle that is often overlooked in terms of a primary factor in regulating physiology and pathophysiology. There is emerging evidence to suggest that the plasma membrane serves a greater purpose than a simple barrier or transporter of ions. New paradigms suggest that the membrane serves as a critical bridge to connect extracellular to intracellular communication particularly to regulate energy and metabolism by forming physical and biochemical associations with intracellular organelles. This review will focus on the relationship of a particular membrane microdomain - caveolae - with mitochondria and the particular implication of this to physiology and pathophysiology.

The Emerging Roles of Nicotinamide Adenine Dinucleotide Phosphate Oxidase 2 in Skeletal Muscle Redox Signaling and Metabolism

Significance: Skeletal muscle is a crucial tissue to whole-body locomotion and metabolic health. Reactive oxygen species (ROS) have emerged as intracellular messengers participating in both physiological and pathological adaptations in skeletal muscle. A complex interplay between ROS-producing enzymes and antioxidant networks exists in different subcellular compartments of mature skeletal muscle. Recent evidence suggests that nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) are a major source of contraction- and insulin-stimulated oxidants production, but they may paradoxically also contribute to muscle insulin resistance and atrophy. Recent Advances: Pharmacological and molecular biological tools, including redox-sensitive probes and transgenic mouse models, have generated novel insights into compartmentalized redox signaling and suggested that NOX2 contributes to redox control of skeletal muscle metabolism. Critical Issues: Major outstanding questions in skeletal muscle include where NOX2 activation occurs under different conditions in health and disease, how NOX2 activation is regulated, how superoxide/hydrogen peroxide generated by NOX2 reaches the cytosol, what the signaling mediators are downstream of NOX2, and the role of NOX2 for different physiological and pathophysiological processes. Future Directions: Future research should utilize and expand the current redox-signaling toolbox to clarify the NOX2-dependent mechanisms in skeletal muscle and determine whether the proposed functions of NOX2 in cells and animal models are conserved into humans.

P2X7 Interactions and Signaling - Making Head or Tail of It

Extracellular adenine nucleotides play important roles in cell-cell communication and tissue homeostasis. High concentrations of extracellular ATP released by dying cells are sensed as a danger signal by the P2X7 receptor, a non-specific cation channel. Studies in P2X7 knockout mice and numerous disease models have demonstrated an important role of this receptor in inflammatory processes. P2X7 activation has been shown to induce a variety of cellular responses that are not usually associated with ion channel function, for example changes in the plasma membrane composition and morphology, ectodomain shedding, activation of lipases, kinases, and transcription factors, as well as cytokine release and apoptosis. In contrast to all other P2X family members, the P2X7 receptor contains a long intracellular C-terminus that constitutes 40% of the whole protein and is considered essential for most of these effects. So far, over 50 different proteins have been identified to physically interact with the P2X7 receptor. However, few of these interactions have been confirmed in independent studies and for the majority of these proteins, the interaction domains and the physiological consequences of the interactions are only poorly described. Also, while the structure of the P2X7 extracellular domain has recently been resolved, information about the organization and structure of its C-terminal tail remains elusive. After shortly describing the structure and assembly of the P2X7 receptor, this review gives an update of the identified or proposed interaction domains within the P2X7 C-terminus, describes signaling pathways in which this receptor has been involved, and provides an overlook of the identified interaction partners.

Deletion of caveolin scaffolding domain alters cancer cell migration

Caveolin-1 (Cav-1) is an integral membrane protein that plays an important role in proliferative and terminally differentiated cells. As a structural component of Caveolae, Cav-1 interacts with signaling molecules via a caveolin scaffolding domain (CSD) regulating cell signaling. Recent reports have shown that Cav-1 is a negative regulator in tumor metastasis. Therefore, we hypothesize that Cav-1 inhibits cell migration through its CSD. HeLa cells were engineered to overexpress Cav-1 (Cav-1 OE), Cav-1 without a functional CSD (∆CSD), or enhanced green fluorescent protein (EGFP) as a control. HeLa cell migration was suppressed in Cav-1 OE cells while ∆CSD showed increased migration, which corresponded to a decrease in the tight junction protein, zonula occludens (ZO-1). The migration phenotype was confirmed in multiple cancer cell lines. Phosphorylated STAT-3 was decreased in Cav-1 OE cells compared to control and ∆CSD cells; reducing STAT-3 expression alone decreased cell migration. ∆CSD blunted HeLa proliferation by increasing the number of cells in the G2/M phase of the cell cycle. Overexpressing the CSD peptide alone suppressed HeLa cell migration and inhibited pSTAT3. These findings suggest that Cav-1 CSD may be critical in controlling the dynamic phenotype of cancer cells by facilitating the interaction of specific signal transduction pathways, regulating STAT3 and participating in a G2/M checkpoint. Modulating the CSD and targeting specific proteins may offer potential new therapies in the treatment of cancer metastasis.

NADPH oxidase-mediated induction of reactive oxygen species and extracellular matrix deposition by insulin-like growth factor binding protein-5

Insulin-like growth factor binding protein-5 (IGFBP-5) induces production of the extracellular matrix (ECM) components collagen and fibronectin both in vitro and in vivo and is overexpressed in patients with fibrosing lung diseases, such as idiopathic pulmonary fibrosis (IPF) and systemic sclerosis (SSc). However, the mechanism by which IGFBP-5 exerts its fibrotic effect is incompletely understood. Recent reports have shown a substantial role of reactive oxygen species (ROS) in fibrosis; thus we hypothesized that IGFBP-5 induces production of ROS to mediate the profibrotic process. In vitro analyses revealed that ROS production was induced by recombinant and adenoviral vector-mediated IGFBP-5 (AdBP5) in a dose- and time-dependent manner, regulated through MEK/ERK and JNK signaling, and primarily mediated by NADPH oxidase (Nox). Silencing IGFBP-5 in SSc and IPF fibroblasts reduced ROS production. The antioxidants diphenyleneiodonium and N-acetylcysteine blocked IGFBP-5-stimulated ECM production in normal, SSc, and IPF human primary lung fibroblasts. In murine fibroblasts lacking critical components of the Nox machinery, AdBP5-stimulated ROS production and fibronectin expression were reduced compared with wild-type fibroblasts. IGFBP-5 stimulated transcriptional expression of Nox3 in human fibroblasts while selective knockdown of Nox3 reduced ROS production by IGFBP-5. Thus IGFBP-5 mediates fibrosis through production of ROS in a Nox-dependent manner.

Contrasting roles of oxidized lipids in modulating membrane microdomains

Lipid rafts display a lateral heterogeneity forming membrane microdomains that hold a fundamental role on biological membranes and are indispensable to physiological functions of cells. Oxidative stress in cellular environments may cause lipid oxidation, changing membrane composition and organization, thus implying in effects in cell signaling and even loss of homeostasis. The individual contribution of oxidized lipid species to the formation or disruption of lipid rafts in membranes still remains unknown. Here, we investigate the role of different structures of oxidized phospholipids on rafts microdomains by carefully controlling the membrane composition. Our experimental approach based on fluorescence microscopy of giant unilamellar vesicles (GUV) enables the direct visualization of the impact of hydroperoxidized POPC lipid (referred to as POPCOOH) and shortened chain lipid PazePC (1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine) on phase separation. We found that the molecular structure of oxidized lipid is of paramount importance on lipid mixing and/or demixing. The hydrophobic mismatch promoted by POPCOOH coupled to its cylindrical molecular shape favor microdomains formation. In contrast, the conical shape of PazePC causes disarrangement of lipid 2D organized platforms. Our findings contribute to better unraveling how oxidized phospholipids can trigger formation or disruption of lipid rafts. As a consequence, phospholipid oxidation may indirectly affect association or dissociation of key biomolecules in the rafts thus altering cell signaling and homeostasis.

The Plasma Membrane: A Platform for Intra- and Intercellular Redox Signaling

Membranes are of outmost importance to allow for specific signal transduction due to their ability to localize, amplify, and direct signals. However, due to the double-edged nature of reactive oxygen species (ROS)—toxic at high concentrations but essential signal molecules—subcellular localization of ROS-producing systems to the plasma membrane has been traditionally regarded as a protective strategy to defend cells from unwanted side-effects. Nevertheless, specialized regions, such as lipid rafts and caveolae, house and regulate the activated/inhibited states of important ROS-producing systems and concentrate redox targets, demonstrating that plasma membrane functions may go beyond acting as a securing lipid barrier. This is nicely evinced by nicotinamide adenine dinucleotide phosphate (NADPH)-oxidases (NOX), enzymes whose primary function is to generate ROS and which have been shown to reside in specific lipid compartments. In addition, membrane-inserted bidirectional H₂O₂-transporters modulate their conductance precisely during the passage of the molecules through the lipid bilayer, ensuring time-scaled delivery of the signal. This review aims to summarize current evidence supporting the role of the plasma membrane as an organizing center that serves as a platform for redox signal transmission, particularly NOX-driven, providing specificity at the same time that limits undesirable oxidative damage in case of malfunction. As an example of malfunction, we explore several pathological situations in which an inflammatory component is present, such as inflammatory bowel disease and neurodegenerative disorders, to illustrate how dysregulation of plasma-membrane-localized redox signaling impacts normal cell physiology.

Comparative analysis of the Pocillopora damicornis genome highlights role of immune system in coral evolution

Comparative analysis of the expanding genomic resources for scleractinian corals may provide insights into the evolution of these organisms, with implications for their continued persistence under global climate change. Here, we sequenced and annotated the genome of Pocillopora damicornis, one of the most abundant and widespread corals in the world. We compared this genome, based on protein-coding gene orthology, with other publicly available coral genomes (Cnidaria, Anthozoa, Scleractinia), as well as genomes from other anthozoan groups (Actiniaria, Corallimorpharia), and two basal metazoan outgroup phlya (Porifera, Ctenophora). We found that 46.6% of P. damicornis genes had orthologs in all other scleractinians, defining a coral ‘core’ genome enriched in basic housekeeping functions. Of these core genes, 3.7% were unique to scleractinians and were enriched in immune functionality, suggesting an important role of the immune system in coral evolution. Genes occurring only in P. damicornis were enriched in cellular signaling and stress response pathways, and we found similar immune-related gene family expansions in each coral species, indicating that immune system diversification may be a prominent feature of scleractinian coral evolution at multiple taxonomic levels. Diversification of the immune gene repertoire may underlie scleractinian adaptations to symbiosis, pathogen interactions, and environmental stress.

oxLDL-mediated cellular senescence is associated with increased NADPH oxidase p47phox recruitment to caveolae

Atherosclerosis develops as a consequence of inflammation and cell senescence. In critical factors involved in the atherosclerotic changes, reactive oxygen species (ROS) generation is considered a leading cause. While NADPH oxidases, particularly NOX2, are the main sources of ROS, how they are regulated in the disease is incompletely understood. In addition, how caveolae, the membrane structure implicated in oxLDL deposition under vascular endothelia, is involved in the oxLDL-mediated ROS production remains mostly elusive. We report here that macrophages exposed to oxLDL up-regulate its caveolin-1 expression, and the latter in turn up-regulates NOX2 p47phox level. This combination effect results in increased cellular senescence. Interestingly, oxLDL treatment causes the p47phox residing in the cytosol to translocate to the caveolae. Immunoprecipitation assays confirms that cavelin-1 is in high degree association with p47phox. These results suggest caveolin-1 may serve as the membrane target for p47phox and as a switch for ROS production following oxLDL exposure. Our results reveal a previously unknown molecular event in oxLDL-mediated cellular ageing, and may provide a target for clinical intervention for atherosclerosis.

Lipids in the cell: organisation regulates function

Lipids are fundamental building blocks of all cells and play important roles in the pathogenesis of different diseases, including inflammation, autoimmune disease, cancer, and neurodegeneration. The lipid composition of different organelles can vary substantially from cell to cell, but increasing evidence demonstrates that lipids become organised specifically in each compartment, and this organisation is essential for regulating cell function. For example, lipid microdomains in the plasma membrane, known as lipid rafts, are platforms for concentrating protein receptors and can influence intra-cellular signalling. Lipid organisation is tightly regulated and can be observed across different model organisms, including bacteria, yeast, Drosophila, and Caenorhabditis elegans, suggesting that lipid organisation is evolutionarily conserved. In this review, we summarise the importance and function of specific lipid domains in main cellular organelles and discuss recent advances that investigate how these specific and highly regulated structures contribute to diverse biological processes.

Exploring the biophysical properties of phytosterols in the plasma membrane for novel cancer prevention strategies

Cancer is a global problem with no sign that incidences are reducing. The great costs associated with curing cancer, through developing novel treatments and applying patented therapies, is an increasing burden to developed and developing nations alike. These financial and societal problems will be alleviated by research efforts into prevention, or treatments that utilise off-patent or repurposed agents. Phytosterols are natural components of the diet found in an array of seeds, nuts and vegetables and have been added to several consumer food products for the management of cardio-vascular disease through their ability to lower LDL-cholesterol levels. In this review, we provide a connected view between the fields of structural biophysics and cellular and molecular biology to evaluate the growing evidence that phytosterols impair oncogenic pathways in a range of cancer types. The current state of understanding of how phytosterols alter the biophysical properties of plasma membrane is described, and the potential for phytosterols to be repurposed from cardio-vascular to oncology therapeutics. Through an overview of the types of biophysical and molecular biology experiments that have been performed to date, this review informs the reader of the molecular and biophysical mechanisms through which phytosterols could have anti-cancer properties via their interactions with the plasma cell membrane. We also outline emerging and under-explored areas such as computational modelling, improved biomimetic membranes and ex vivo tissue evaluation. Focus of future research in these areas should improve understanding, not just of phytosterols in cancer cell biology but also to give insights into the interaction between the plasma membrane and the genome. These fields are increasingly providing meaningful biological and clinical data but iterative experiments between molecular biology assays, biosynthetic membrane studies and computational membrane modelling improve and refine our understanding of the role of different sterol components of the plasma membrane.

The Role of Peroxiredoxins in the Transduction of H2O2 Signals

Hydrogen peroxide (H₂O₂) is produced on stimulation of many cell surface receptors and serves as an intracellular messenger in the regulation of diverse physiological events, mostly by oxidizing cysteine residues of effector proteins. Mammalian cells express multiple H₂O₂-eliminating enzymes, including catalase, glutathione peroxidase (GPx), and peroxiredoxin (Prx). A conserved cysteine in Prx family members is the site of oxidation by H₂O₂. Peroxiredoxins possess a high-affinity binding site for H₂O₂ that is lacking in catalase and GPx and which renders the catalytic cysteine highly susceptible to oxidation, with a rate constant several orders of magnitude greater than that for oxidation of cysteine in most H₂O₂ effector proteins. Moreover, Prxs are abundant and present in all subcellular compartments. The cysteines of most H₂O₂ effectors are therefore at a competitive disadvantage for reaction with H₂O₂. Recent Advances: Here we review intracellular sources of H₂O₂ as well as H₂O₂ target proteins classified according to biochemical and cellular function. We then highlight two strategies implemented by cells to overcome the kinetic disadvantage of most target proteins with regard to H₂O₂-mediated oxidation: transient inactivation of local Prx molecules via phosphorylation, and indirect oxidation of target cysteines via oxidized Prx. Critical Issues and Future Directions: Recent studies suggest that only a small fraction of the total pools of Prxs and H₂O₂ effector proteins localized in specific subcellular compartments participates in H₂O₂ signaling. Development of sensitive tools to selectively detect phosphorylated Prxs and oxidized effector proteins is needed to provide further insight into H₂O₂ signaling. Antioxid. Redox Signal. 28, 537-557.

Caveolins as Regulators of Stress Adaptation

Caveolins have been recognized over the past few decades as key regulators of cell physiology. They are ubiquitously expressed and regulate a number of processes that ultimately impact efficiency of cellular processes. Though not critical to life, they are central to stress adaptation in a number of organs. The following review will focus specifically on the role of caveolin in stress adaptation in the heart, brain, and eye, three organs that are susceptible to acute and chronic stress and that show as well declining function with age. In addition, we consider some novel molecular mechanisms that may account for this stress adaptation and also offer potential to drive the future of caveolin research.

Gaseous Signaling Molecules in Cardiovascular Function: From Mechanisms to Clinical Translation

Carbon monoxide (CO), hydrogen sulfide (H₂S), and nitric oxide (NO) constitute endogenous gaseous molecules produced by specific enzymes. These gases are chemically simple, but exert multiple effects and act through shared molecular targets to control both physiology and pathophysiology in the cardiovascular system (CVS). The gases act via direct and/or indirect interactions with each other in proteins such as heme-containing enzymes, the mitochondrial respiratory complex, and ion channels, among others. Studies of the major impacts of CO, H₂S, and NO on the CVS have revealed their involvement in controlling blood pressure and in reducing cardiac reperfusion injuries, although their functional roles are not limited to these conditions. In this review, the basic aspects of CO, H₂S, and NO, including their production and effects on enzymes, mitochondrial respiration and biogenesis, and ion channels are briefly addressed to provide insight into their biology with respect to the CVS. Finally, potential therapeutic applications of CO, H₂S, and NO with the CVS are addressed, based on the use of exogenous donors and different types of delivery systems.

Myosin 1g Contributes to CD44 Adhesion Protein and Lipid Rafts Recycling and Controls CD44 Capping and Cell Migration in B Lymphocytes

Cell migration and adhesion are critical for immune system function and involve many proteins, which must be continuously transported and recycled in the cell. Recycling of adhesion molecules requires the participation of several proteins, including actin, tubulin, and GTPases, and of membrane components such as sphingolipids and cholesterol. However, roles of actin motor proteins in adhesion molecule recycling are poorly understood. In this study, we identified myosin 1g as one of the important motor proteins that drives recycling of the adhesion protein CD44 in B lymphocytes. We demonstrate that the lack of Myo1g decreases the cell-surface levels of CD44 and of the lipid raft surrogate GM1. In cells depleted of Myo1g, the recycling of CD44 was delayed, the delay seems to be caused at the level of formation of recycling complex and entry into recycling endosomes. Moreover, a defective lipid raft recycling in Myo1g-deficient cells had an impact both on the capping of CD44 and on cell migration. Both processes required the transportation of lipid rafts to the cell surface to deliver signaling components. Furthermore, the extramembrane was essential for cell expansion and remodeling of the plasma membrane topology. Therefore, Myo1g is important during the recycling of lipid rafts to the membrane and to the accompanied proteins that regulate plasma membrane plasticity. Thus, Myosin 1g contributes to cell adhesion and cell migration through CD44 recycling in B lymphocytes.

Myocyte membrane and microdomain modifications in diabetes: determinants of ischemic tolerance and cardioprotection

Cardiovascular disease, predominantly ischemic heart disease (IHD), is the leading cause of death in diabetes mellitus (DM). In addition to eliciting cardiomyopathy, DM induces a ‘wicked triumvirate’: (i) increasing the risk and incidence of IHD and myocardial ischemia; (ii) decreasing myocardial tolerance to ischemia–reperfusion (I–R) injury; and (iii) inhibiting or eliminating responses to cardioprotective stimuli. Changes in ischemic tolerance and cardioprotective signaling may contribute to substantially higher mortality and morbidity following ischemic insult in DM patients. Among the diverse mechanisms implicated in diabetic impairment of ischemic tolerance and cardioprotection, changes in sarcolemmal makeup may play an overarching role and are considered in detail in the current review. Observations predominantly in animal models reveal DM-dependent changes in membrane lipid composition (cholesterol and triglyceride accumulation, fatty acid saturation vs. reduced desaturation, phospholipid remodeling) that contribute to modulation of caveolar domains, gap junctions and T-tubules. These modifications influence sarcolemmal biophysical properties, receptor and phospholipid signaling, ion channel and transporter functions, contributing to contractile and electrophysiological dysfunction, cardiomyopathy, ischemic intolerance and suppression of protective signaling. A better understanding of these sarcolemmal abnormalities in types I and II DM (T1DM, T2DM) can inform approaches to limiting cardiomyopathy, associated IHD and their consequences. Key knowledge gaps include details of sarcolemmal changes in models of T2DM, temporal patterns of lipid, microdomain and T-tubule changes during disease development, and the precise impacts of these diverse sarcolemmal modifications. Importantly, exercise, dietary, pharmacological and gene approaches have potential for improving sarcolemmal makeup, and thus myocyte function and stress-resistance in this ubiquitous metabolic disorder.

Impaired AQP2 trafficking in Fxyd1 knockout mice: A role for FXYD1 in regulated vesicular transport

The final adjustment of urine volume occurs in the inner medullary collecting duct (IMCD), chiefly mediated by the water channel aquaporin 2 (AQP2). With vasopressin stimulation, AQP2 accumulation in the apical plasma membrane of principal cells allows water reabsorption from the lumen. We report that FXYD1 (phospholemman), better known as a regulator of Na,K-ATPase, has a role in AQP2 trafficking. Daytime urine of Fxyd1 knockout mice was more dilute than WT despite similar serum vasopressin, but both genotypes could concentrate urine during water deprivation. FXYD1 was found in IMCD. In WT mice, phosphorylated FXYD1 was detected intracellularly, and vasopressin induced its dephosphorylation. We tested the hypothesis that the dilute urine in knockouts was caused by alteration of AQP2 trafficking. In WT mice at baseline, FXYD1 and AQP2 were not strongly co-localized, but elevation of vasopressin produced translocation of both FXYD1 and AQP2 to the apical plasma membrane. In kidney slices, baseline AQP2 distribution was more scattered in the Fxyd1 knockout than in WT. Apical recruitment of AQP2 occurred in vasopressin-treated Fxyd1 knockout slices, but upon vasopressin washout, there was more rapid reversal of apical AQP2 localization and more heterogeneous cytoplasmic distribution of AQP2. Notably, in sucrose gradients, AQP2 was present in a detergent-resistant membrane domain that had lower sedimentation density in the knockout than in WT, and vasopressin treatment normalized its density. We propose that FXYD1 plays a role in regulating AQP2 retention in apical membrane, and that this involves transfers between raft-like membrane domains in endosomes and plasma membranes.

Cellular uptake and intracellular localization of poly (acrylic acid) nanoparticles in a rainbow trout (Oncorhynchus mykiss) gill epithelial cell line, RTgill-W1

The ever-growing production of engineered nanoparticles (NPs) for use in many agricultural, commercial, consumer, and industrial applications will lead to their accidental or intentional release into the environment. Potential routes of environmental exposure include manufacturing or transport spills, disposal of NP-containing products down the drain and/or in landfills, as well as direct usage on agricultural land. Therefore, NPs will inevitably contaminate aquatic environments and interact with resident organisms. However, there is limited information regarding the mechanisms that regulate NP transport into fish from the environment. Thus, our primary objective was to elucidate the mechanism(s) underlying cellular uptake and intracellular fate of 3-9nm poly (acrylic acid) NPs loaded with the fluorescent dye Nile red using a rainbow trout (Oncorhynchus mykiss) gill epithelial cell line (RTgill-W1). In vitro measurements with NP-treated RTgill-W1 cells were carried out using a combination of laser scanning confocal microscopy, flow cytometry, fluorescent biomarkers (transferrin, cholera toxin B subunit, and dextran), endocytosis inhibitors (chlorpromazine, genistein, and wortmannin), and stains (4', 6-diamidino-2-phenylindole, Hoechst 33342, CellMask Deep Red, and LysoTracker Yellow). Clathrin-mediated endocytosis (CME), caveolae-mediated endocytosis and macropinocytosis pathways were active in RTgill-W1 cells, and these pathways were exploited by the non-cytotoxic NPs to enter these cells. We have demonstrated that NP uptake by RTgill-W1 cells was impeded when clathrin-coated pit formation was blocked by chlorpromazine. Furthermore, colocalization analysis revealed a moderate positive relationship between NPs and LysoTracker Yellow-positive lysosomal compartments indicating that CME was the dominant operative mechanism involved in NP internalization by RTgill-W1 cells. Overall, our results clearly show that fish gill epithelial cells internalized NPs via energy-dependent endocytotic processes. This study enhances our understanding of complex NP-cell interactions and the results obtained in vitro imply a potential risk to aquatic organisms.

Na/K-ATPase regulates bovine sperm capacitation through raft- and non-raft-mediated signaling mechanisms

Highly dynamic lipid microdomains (rafts) in the sperm plasma membrane contain several signaling proteins that regulate sperm capacitation. Na/K-ATPase isoforms (testis-specific isoform ATP1A4 and ubiquitous isoform ATP1A1) are abundant in bovine sperm plasma membrane. We previously reported that incubation of bovine sperm with ouabain, a specific Na/K-ATPase ligand, induced tyrosine phosphorylation of several sperm proteins during capacitation. The objective of this study was to investigate the roles of lipid rafts and non-rafts in Na/K-ATPase enzyme activity and signaling during bovine sperm capacitation. Content of ATP1A4 and, to a lesser extent, ATP1A1 was increased in raft and non-raft fractions of capacitated sperm, although non-raft enzyme activities of both isoforms were higher than the corresponding activities in rafts from capacitated sperm. Yet, ATP1A4 was the predominant isoform responsible for total Na/K-ATPase activity in both rafts and non-rafts. A comparative increase in phosphorylation of signaling molecules was observed in both raft (CAV1) and non-raft (EGFR and ERK1/2) membrane fractions during capacitation. Although SRC was phosphorylated in both membrane fractions, the non-raft fraction possessed more of this activated form. We also inferred, by immunoprecipitation, that ATP1A4 interacted with CAV1 and EGFR in the raft fraction, whereas interactions of ATP1A4 with SRC, EGFR, and ERK1/2 occurred in the non-raft fraction of ouabain-capacitated sperm; conversely, ATP1A1 interacted only with CAV1 in both fractions of uncapacitated and capacitated sperm. In conclusion, both raft and non-raft cohorts of Na/K-ATPase isoforms contributed to phosphorylation of signaling molecules during bovine sperm capacitation.

Redox modification of caveolar proteins in the cardiovascular system- role in cellular signalling and disease

Rapid and coordinated release of a variety of reactive oxygen species (ROS) such as superoxide (O₂^.-), hydrogen peroxide (H₂O₂) and peroxynitrite, in specific microdomains, play a crucial role in cell signalling in the cardiovascular system. These reactions are mediated by reversible and functional modifications of a wide variety of key proteins. Dysregulation of this oxidative signalling occurs in almost all forms of cardiovascular disease (CVD), including at the very early phases. Despite the heavily publicized failure of "antioxidants" to improve CVD progression, pharmacotherapies such as those targeting the renin-angiotensin system, or statins, exert at least part of their large clinical benefit via modulating cellular redox signalling. Over 250 proteins, including receptors, ion channels and pumps, and signalling proteins are found in the caveolae. An increasing proportion of these are being recognized as redox regulated-proteins, that reside in the immediate vicinity of the two major cellular sources of ROS, nicotinamide adenine dinucleotide phosphate oxidase (Nox) and uncoupled endothelial nitric oxide synthase (eNOS). This review focuses on what is known about redox signalling within the caveolae, as well as endogenous protective mechanisms utilized by the cell, and new approaches to targeting dysregulated redox signalling in the caveolae as a therapeutic strategy in CVD.

The NRF2 activator DH404 attenuates adverse ventricular remodeling post-myocardial infarction by modifying redox signalling

BACKGROUND

The novel synthetic triterpenoid, bardoxolone methyl, has the ability to upregulate cytoprotective proteins via induction of the nuclear factor erythroid-2-related factor 2 (Nrf2) pathway. This makes it a promising therapeutic agent in disease states characterized by dysregulated oxidative signalling. We have examined the effect of a Nrf2 activator, dihydro-CDDO-trifluoroethyl amide (DH404), a derivative of bardoxolone methyl, on post-infarct cardiac remodeling in rats.

METHODS/RESULTS

DH404, administered from day 2 post myocardial infarction (MI: 30min transient ischemia followed by reperfusion) resulted in almost complete protection against adverse ventricular remodeling as assessed at day 28 (left ventricular end-systolic area: sham 0.14±0.01cm², MI vehicle 0.29±0.04cm² vs. MI DH404 0.18±0.02cm², P<0.05); infarct size (21.3±3.4% MI vehicle vs. 10.9±2.3% MI DH404, P<0.05) with associated benefits on systolic function (fractional shortening: sham 71.9±2.6%, MI vehicle 36.2±1.9% vs. MI DH404 58.6±4.0%, P<0.05). These structural and functional benefits were associated with lower myocardial expression of atrial natriuretic peptide (ANP, P<0.01 vs. MI vehicle), and decreased fibronectin (P<0.01 vs. MI vehicle) in DH404-treated MI rats at 28 days. MI increased glutathionylation of endothelial nitric oxide synthase (eNOS) in vitro - a molecular switch that uncouples the enzyme, increasing superoxide production and decreasing nitric oxide (NO) bioavailability. MI-induced eNOS glutathionylation was substantially ameliorated by DH404. An associated increase in glutaredoxin 1 (Grx1) co-immunoprecipitation with eNOS without a change in expression was mechanistically intriguing. Indeed, in parallel in vitro experiments, silencing of Grx1 abolished the protective effect of DH404 against Angiotensin II-induced eNOS uncoupling.

CONCLUSION

The bardoxolone derivative DH404 significantly attenuated cardiac remodeling post MI, at least in part, by re-coupling of eNOS and increasing the functional interaction of Grx1 with eNOS. This agent may have clinical benefits protecting against post MI cardiomyopathy.

Assessment of Caveolae/Lipid Rafts in Isolated Cells

This chapter outlines protocols to evaluate protein localization, recruitment or phosphorylation levels in cholesterol/sphingolipids-enriched cell membrane domains and recommends experimental designs with pharmacological tolls to evaluate potential cell functions associated with these domains. We emphasize the need for the combination of several approaches towards understanding the protein components and cellular functions attributed to these distinct microdomains.

Cytochrome P450 enzymes but not NADPH oxidases are the source of the NADPH-dependent lucigenin chemiluminescence in membrane assays

Measuring NADPH oxidase (Nox)-derived reactive oxygen species (ROS) in living tissues and cells is a constant challenge. All probes available display limitations regarding sensitivity, specificity or demand highly specialized detection techniques. In search for a presumably easy, versatile, sensitive and specific technique, numerous studies have used NADPH-stimulated assays in membrane fractions which have been suggested to reflect Nox activity. However, we previously found an unaltered activity with these assays in triple Nox knockout mouse (Nox1-Nox2-Nox4^-/-) tissue and cells compared to wild type. Moreover, the high ROS production of intact cells overexpressing Nox enzymes could not be recapitulated in NADPH-stimulated membrane assays. Thus, the signal obtained in these assays has to derive from a source other than NADPH oxidases. Using a combination of native protein electrophoresis, NADPH-stimulated assays and mass spectrometry, mitochondrial proteins and cytochrome P450 were identified as possible source of the assay signal. Cells lacking functional mitochondrial complexes, however, displayed a normal activity in NADPH-stimulated membrane assays suggesting that mitochondrial oxidoreductases are unlikely sources of the signal. Microsomes overexpressing P450 reductase, cytochromes b5 and P450 generated a NADPH-dependent signal in assays utilizing lucigenin, L-012 and dihydroethidium (DHE). Knockout of the cytochrome P450 reductase by CRISPR/Cas9 technology (POR^-/-) in HEK293 cells overexpressing Nox4 or Nox5 did not interfere with ROS production in intact cells. However, POR^-/- abolished the signal in NADPH-stimulated assays using membrane fractions from the very same cells. Moreover, membranes of rat smooth muscle cells treated with angiotensin II showed an increased NADPH-dependent signal with lucigenin which was abolished by the knockout of POR but not by knockout of p22phox.

IN CONCLUSION

the cytochrome P450 system accounts for the majority of the signal of Nox activity chemiluminescence based assays.

Mitigation of indomethacin-induced gastrointestinal damages in fat-1 transgenic mice via gate-keeper action of ω-3-polyunsaturated fatty acids

Non-steroidal anti-inflammatory drugs (NSAIDs) damage the gastrointestinal (GI) epithelial cell membranes by inducing several signals through lipid raft organization after membrane incorporation, whereas ω-3 polyunsaturated fatty acids (PUFAs) relieve inflammation, reduce oxidative stress, and provide cytoprotection, consequent to lipid raft disorganization. Therefore, we hypothesized that ω-3 PUFAs can protect the GI from NSAID-induced damages by initiating the gatekeeper action of cell membranes, subsequent to anti-inflammatory and anti-oxidative actions. Administration of indomethacin (IND) leads to the formation of lipid rafts and activation of caveolin-1; however, no such observations were made upon co-administration of eicosapentaenoic acid (EPA) and IND. In addition, the EPA-induced lipid raft disorganization, caveolin-1 inactivation, and cellular cytotoxicity were inhibited when target cells were knocked-out using G-protein coupled receptor 120 (GPR 120). EPA significantly attenuated IND-induced oxidative damage and apoptosis. IND administration induced significant ulceration, bleeding, and oedema in the stomach or small intestine of wild-type (WT) mice; however, such severe damages to the GI significantly decreased in fat-1 transgenic (TG) mice (P < 0.001), which exhibited decreased cyclooxygenase-2 expression and apoptosis, decreased interleukin-1β and FAS concentrations, and increased heme oxygenase-1 concentration. Our study indicates that the gatekeeper function of ω-3 PUFAs improves GI safety when administered with NSAID.

Ischemic conditions and ß-secretase activation: The impact of membrane cholesterol enrichment as triggering factor in rat brain endothelial cells

Among harmful conditions damaging the blood–brain barrier, cerebral stroke and reperfusion injuries were proposed as contributing factors to Alzheimer's disease etiology. Indeed it was reported that ischemic conditions promote β-amyloid peptide production in brain endothelial cells, although implicated mechanisms are yet not fully understood.Oxidative injury related to ischemia affects membrane-lipids profile by altering their biochemical properties and structural dynamics, which are also believed to play significant role in the amyloid precursor protein processing, suggesting a link between alterations in lipid membrane composition and β-amyloid peptide production enhancement.Using brain microvascular endothelial cells, here we demonstrate how oxygen and glucose deprivation followed by normal conditions restoration, mimicking ischemic environment, increases cell cholesterol amount (+20%), reduces membrane fluidity and results in strong activation (+40%) of β-secretase 1 enzymatic activity. Moreover, we observed an increase of amyloid precursor protein and β-secretase 1 protein levels with altered localization in non-discrete (Triton X-100 soluble) membrane domains, leading to an enhanced production of amyloid precursor protein β-carboxyl-terminal fragment. Therefore, lipid alterations induced by oxygen and glucose deprivation enhance β-secretase 1 activity, favor its proximity to amyloid precursor protein and may concur to increased amyloidogenic cleavage. The latter, represents a detrimental event that may contribute to β-amyloid homeostasis impairment in the brain and to Alzheimer's disease-related BBB dysfunctions.

Caveolin-1 Regulates the P2Y2 Receptor Signaling in Human 1321N1 Astrocytoma Cells

Damage to the CNS can cause a differential spatio-temporal release of multiple factors, such as nucleotides, ATP and UTP. The latter interact with neuronal and glial nucleotide receptors. The P2Y2 nucleotide receptor (P2Y2R) has gained prominence as a modulator of gliotic responses after CNS injury. Still, the molecular mechanisms underlying these responses in glia are not fully understood. Membrane-raft microdomains, such as caveolae, and their constituent caveolins, modulate receptor signaling in astrocytes; yet, their role in P2Y2R signaling has not been adequately explored. Hence, this study evaluated the role of caveolin-1 (Cav-1) in modulating P2Y2R subcellular distribution and signaling in human 1321N1 astrocytoma cells. Recombinant hP2Y2R expressed in 1321N1 cells and Cav-1 were found to co-fractionate in light-density membrane-raft fractions, co-localize via confocal microscopy, and co-immunoprecipitate. Raft localization was dependent on ATP stimulation and Cav-1 expression. This hP2Y2R/Cav-1 distribution and interaction was confirmed with various cell model systems differing in the expression of both P2Y2R and Cav-1, and shRNA knockdown of Cav-1 expression. Furthermore, shRNA knockdown of Cav-1 expression decreased nucleotide-induced increases in the intracellular Ca(2+) concentration in 1321N1 and C6 glioma cells without altering TRAP-6 and carbachol Ca(2+) responses. In addition, Cav-1 shRNA knockdown also decreased AKT phosphorylation and altered the kinetics of ERK1/2 activation in 1321N1 cells. Our findings strongly suggest that P2Y2R interaction with Cav-1 in membrane-raft caveolae of 1321N1 cells modulates receptor coupling to its downstream signaling machinery. Thus, P2Y2R/Cav-1 interactions represent a novel target for controlling P2Y2R function after CNS injury.

The plasma membrane as a capacitor for energy and metabolism

When considering which components of the cell are the most critical to function and physiology, we naturally focus on the nucleus, the mitochondria that regulate energy and apoptotic signaling, or other organelles such as the endoplasmic reticulum, Golgi, ribosomes, etc. Few people will suggest that the membrane is the most critical element of a cell in terms of function and physiology. Those that consider the membrane critical will point to its obvious barrier function regulated by the lipid bilayer and numerous ion channels that regulate homeostatic gradients. What becomes evident upon closer inspection is that not all membranes are created equal and that there are lipid-rich microdomains that serve as platforms of signaling and a means of communication with the intracellular environment. In this review, we explore the evolution of membranes, focus on lipid-rich microdomains, and advance the novel concept that membranes serve as "capacitors for energy and metabolism." Within this framework, the membrane then is the primary and critical regulator of stress and disease adaptation of the cell.