Plasma Membrane Lipid Rafts Research Articles

A Lipid-Raft Theory of Alzheimer's Disease.

I present a theory of Alzheimer's disease (AD) that explains its symptoms, pathology, and risk factors. To do this, I introduce a new theory of brain plasticity that elucidates the physiological roles of AD-related agents. New events generate synaptic and branching candidates competing for long-term enhancement. Competition resolution crucially depends on the formation of membrane lipid rafts, which requires astrocyte-produced cholesterol. Sporadic AD is caused by impaired formation of plasma-membrane lipid rafts, preventing the conversion of short- to long-term memory and yielding excessive tau phosphorylation, intracellular cholesterol accumulation, synaptic dysfunction, and neurodegeneration. Amyloid β (Aβ) production is promoted by cholesterol during the switch to competition resolution, and cholesterol accumulation stimulates chronic Aβ production, secretion, and aggregation. The theory addresses all of the major established facts known about the disease and is supported by strong evidence.

Inhibiting the cholesterol storage enzyme ACAT1/SOAT1 in aging Apolipoprotein E4 mice alter their brains inflammatory profiles.

Aging and Apolipoprotein E4 (APOE4) are the two most significant risk factors for late-onset Alzheimer's disease (LOAD). Compared to APOE3, APOE4 disrupts cholesterol homeostasis, increases cholesteryl esters (CEs), and exacerbates neuroinflammation in brain cells including microglia. Targeting CEs and neuroinflammation could be a novel strategy to ameliorate APOE4 dependent phenotypes. Toll-like receptor 4 (TLR4) is a key player in inflammation, its regulation is associated with cholesterol content of lipid rafts in cell membranes. We previously demonstrated that in normal microglia expressing APOE3, inhibiting the cholesterol storage enzyme acylCoA:cholesterol acyltransferase 1 (ACAT1/SOAT1) reduces CEs, dampened neuroinflammation via modulating the fate of TLR4. We also showed that treating myelin debris-loaded normal microglia with ACAT inhibitor F12511 reduced cellular CEs and activated ABC transporter 1 (ABCA1) for cholesterol efflux. In this study, we found that treating primary microglia expressing APOE4 with F12511 also reduces CEs, activated ABCA1, and dampened LPS dependent NFkB activation. In vivo, a two-week injections of nanoparticle F12511, which consists of DSPE-PEG 2000 , phosphatidylcholine, and F12511, to aged female APOE4 mice reduced TLR4 protein content and decreased proinflammatory cytokines including IL-1β in APOE4 mice brains. Overall, our work suggests nanoparticle F12511 is a novel agent to ameliorate LOAD.

Glucose-dependent glycosphingolipid biosynthesis fuels CD8+ T cell function and tumor control.

Glucose is essential for T cell proliferation and function, yet its specific metabolic roles in vivo remain poorly defined. Here, we identify glycosphingolipid (GSL) biosynthesis as a key pathway fueled by glucose that enables CD8+ T cell expansion and cytotoxic function in vivo. Using 13C-based stable isotope tracing, we demonstrate that CD8+ effector T cells use glucose to synthesize uridine diphosphate-glucose (UDP-Glc), a precursor for glycogen, glycan, and GSL biosynthesis. Inhibiting GSL production by targeting the enzymes UGP2 or UGCG impairs CD8+ T cell expansion and cytolytic activity without affecting glucose-dependent energy production. Mechanistically, we show that glucose-dependent GSL biosynthesis is required for plasma membrane lipid raft integrity and aggregation following TCR stimulation. Moreover, UGCG-deficient CD8+ T cells display reduced granzyme expression and tumor control in vivo. Together, our data establish GSL biosynthesis as a critical metabolic fate of glucose-independent of energy production-required for CD8+ T cell responses in vivo.

Expression of the membrane tetraspanin claudin 18 on cancer cells promotes T lymphocyte infiltration and antitumor immunity in pancreatic cancer

Tumors weakly infiltrated by T lymphocytes poorly respond to immunotherapy. We aimed to unveil malignancy-associated programs regulating Tcell entrance, arrest, and activation in the tumor environment. Differential expression of cell adhesion and tissue architecture programs, particularly the presence of the membrane tetraspanin claudin (CLDN)18 as a signature gene, demarcated immune-infiltrated from immune-depleted mouse pancreatic tumors. In human pancreatic ductal adenocarcinoma (PDAC) and non-small cell lung cancer, CLDN18 expression positively correlated with more differentiated histology and favorable prognosis. CLDN18 on the cell surface promoted accrual of cytotoxic T lymphocytes (CTLs), facilitating direct CTL contacts with tumor cells by driving the mobilization of the adhesion protein ALCAM to the lipid rafts of the tumor cell membrane through actin. This process favored the formation of robust immunological synapses (ISs) between CTLs and CLDN18-positive cancer cells, resulting in increased Tcell activation. Our data reveal an immune role for CLDN18 in orchestrating Tcell infiltration and shaping the tumor immune contexture.

Cellular Uptake and Cytotoxicity of Clostridium perfringens Iota-Toxin.

Clostridium perfringens iota-toxin is composed of two separate proteins: a binding protein (Ib) that recognizes a host cell receptor and promotes the cellular uptake of a catalytic protein and (Ia) possessing ADP-ribosyltransferase activity that induces actin cytoskeleton disorganization. Ib exhibits the overall structure of bacterial pore-forming toxins (PFTs). Lipolysis-stimulated lipoprotein receptor (LSR) is defined as a host cell receptor for Ib. The binding of Ib to LSR causes an oligomer formation of Ib in lipid rafts of plasma membranes, mediating the entry of Ia into the cytoplasm. Ia induces actin cytoskeleton disruption via the ADP-ribosylation of G-actin and causes cell rounding and death. The binding protein alone disrupts the cell membrane and induces cytotoxicity in sensitive cells. Host cells permeabilized by the pore formation of Ib are repaired by a Ca2+-dependent plasma repair pathway. This review shows that the cellular uptake of iota-toxin utilizes a pathway of plasma membrane repair and that Ib alone induces cytotoxicity.

Rafting on the Evidence for Lipid Raft-like Domains as Hubs Triggering Environmental Toxicants' Cellular Effects.

The plasma membrane lipid rafts are cholesterol- and sphingolipid-enriched domains that allow regularly distributed, sub-micro-sized structures englobing proteins to compartmentalize cellular processes. These membrane domains can be highly heterogeneous and dynamic, functioning as signal transduction platforms that amplify the local concentrations and signaling of individual components. Moreover, they participate in cell signaling routes that are known to be important targets of environmental toxicants affecting cell redox status and calcium homeostasis, immune regulation, and hormonal functions. In this work, the evidence that plasma membrane raft-like domains operate as hubs for toxicants' cellular actions is discussed, and suggestions for future research are provided. Several studies address the insertion of pesticides and other organic pollutants into membranes, their accumulation in lipid rafts, or lipid rafts' disruption by polychlorinated biphenyls (PCBs), benzo[a]pyrene (B[a]P), and even metals/metalloids. In hepatocytes, macrophages, or neurons, B[a]P, airborne particulate matter, and other toxicants caused rafts' protein and lipid remodeling, oxidative changes, or amyloidogenesis. Different studies investigated the role of the invaginated lipid rafts present in endothelial cells in mediating the vascular inflammatory effects of PCBs. Furthermore, in vitro and in vivo data strongly implicate raft-localized NADPH oxidases, the aryl hydrocarbon receptor, caveolin-1, and protein kinases in the toxic mechanisms of occupational and environmental chemicals.

B cell receptor-induced protein dynamics and the emerging role of SUMOylation revealed by proximity proteomics.

Successful B cell activation, which is critical for high-affinity antibody production, is controlled by the B cell antigen receptor (BCR). However, we still lack a comprehensive protein-level view of the very dynamic multi-branched cellular events triggered by antigen binding. Here, we employed APEX2 proximity biotinylation to study antigen-induced changes, 5-15 min after receptor activation, at the vicinity of the plasma membrane lipid rafts, wherein BCR enriches upon activation. The data reveals dynamics of signaling proteins, as well as various players linked to the subsequent processes, such as actin cytoskeleton remodeling and endocytosis. Interestingly, our differential expression analysis identified dynamic responses in various proteins previously not linked to early B cell activation. We demonstrate active SUMOylation at the sites of BCR activation in various conditions and report its functional role in BCR signaling through the AKT and ERK1/2 axes.

Airborne fine particles drive H1N1 viruses deep into the lower respiratory tract and distant organs.

Mounting data suggest that environmental pollution due to airborne fine particles (AFPs) increases the occurrence and severity of respiratory virus infection in humans. However, it is unclear whether and how interactions with AFPs alter viral infection and distribution. We report synergetic effects between various AFPs and the H1N1 virus, regulated by physicochemical properties of the AFPs. Unlike infection caused by virus alone, AFPs facilitated the internalization of virus through a receptor-independent pathway. Moreover, AFPs promoted the budding and dispersal of progeny virions, likely mediated by lipid rafts in the host plasma membrane. Infected animal models demonstrated that AFPs favored penetration of the H1N1 virus into the distal lung, and its translocation into extrapulmonary organs including the liver, spleen, and kidney, thus causing severe local and systemic disorders. Our findings revealed a key role of AFPs in driving viral infection throughout the respiratory tract and beyond. These insights entail stronger air quality management and air pollution reduction policies.

Phase Separation Driven by Active Flows.

We extend the continuum theories of active nematohydrodynamics to model a two-fluid mixture with separate velocity fields for each fluid component, coupled through a viscous drag. The model is used to study an active nematic fluid mixed with an isotropic fluid. We find microphase separation, and argue that this results from an interplay between active anchoring and active flows driven by concentration gradients. The results may be relevant to cell sorting and the formation of lipid rafts in cell membranes.

Effects of statin therapy in hospitalized adult COVID-19 patients: a systematic review and meta-analysis of randomized controlled trials.

COVID-19 is associated with endothelial activation and systemic inflammation; consequently, statins can be used in its treatment as they have anti-inflammatory, antithrombotic, and profibrinolytic properties and may interfere with COVID-19 viral entry into cells through disruption of cell membrane lipid rafts. We performed a meta-analysis of randomized clinical trials that compared statin therapy to placebo or to standard care in adult patients hospitalized for COVID-19. We searched the MEDLINE, EMBASE, and Cochrane Library databases for all-cause mortality, hospitalization duration, and admission to the intensive care unit. Of the 228 studies reviewed, four studies were included, with a total of 1,231 patients, of whom 610 (49.5%) were treated with statins. There was no significant difference in all-cause mortality (odds ratio [OR] 0.96; 95% confidence interval [95%CI]: 0.61-1.51; p=0.86; I2=13%), duration of hospitalization (mean difference [MD] 0.21; 95%CI: -1.74-2.16; p=0.83; I2=92%), intensive care unit admission (OR= 3.31; 95%CI: 0.13-87.1; p=0.47; I2=84%), need for mechanical ventilation (OR= 1.03; 95%CI: 0.36-2.94; p=0.95; I2=0%), or increase in liver enzyme levels (OR= 0.58; 95%CI: 0.27-1.25; p=0.16; I2=0%) between patients treated with or without statin therapy. Our findings suggest that in adult patients hospitalized with COVID-19, statin therapy results in no difference in clinical outcomes when compared to outcomes by placebo or standard of care. Prospero database registration: (www.crd.york.ac.uk/prospero) under the number CRD42022338283.

CD40 induces selective routing of Ras isoforms to subcellular compartments.

Ras GTPases are central to cellular signaling and oncogenesis. The three loci of the Ras gene encode for four protein isoforms namely Harvey-Ras (H-Ras), Kirsten-Ras (K-Ras 4A and 4B), and Neuroblastoma-Ras (N-Ras) which share ~ 80% sequence similarity and used to be considered functionally redundant. The small molecule inhibitors of Ras lack specificity for the isoforms leading to widespread toxicity in Ras-targeted therapeutics. Ras isoforms' tissue-specific expression and selective association with carcinogenesis, embryonic development, and infection suggested their non-redundancy. We show that CD40, an antigen-presenting cell (APC)-expressed immune receptor, induces selective relocation of H-Ras, K-Ras, and N-Ras to the Plasma membrane (PM) lipid rafts, mitochondria, endoplasmic reticulum (ER), but not to the Golgi complex (GC). The two palmitoylated Ras isoforms-H-Ras and N-Ras-have a similar pattern of colocalization into the lipid-rich raft microdomain of the PM at early time points when compared to non-palmitoylated K-Ras (4B) with polylysine residues. CD40-induced trafficking of H-Ras and K-Ras to mitochondria and ER was found to be similar but different from that of N-Ras. Trafficking of all the Ras isoforms to the GC was independent of CD40 stimulation. The receptor-driven trafficking and spatial segregation of H-Ras, K-Ras, and N-Ras imply isoform-specific subcellular signaling platforms for the functional non-redundancy of Ras isoforms. PDB structures have been modified to illustrate various signaling proteins.

Abstract 6259: Novel inhibitors of StAR related lipid transfer protein 5 inhibitors target plasma membrane lipid raft growth signaling

Abstract Background: Treatment of colorectal cancer (CRC) presents a major challenge due to chemotherapeutic resistance and recurrence, which are attributed to the presence of cancer stem cells (CSCs). Cholesterol metabolism is a critical regulator of CSCs. Lipid Rafts are cholesterol-rich microdomains in the plasma membrane (PM) that anchor various growth receptors tyrosine kinases (RTKs). STARD5 is a novel protein that helps transport cholesterol from the endoplasmic reticulum to the PM. We have identified specific small molecule inhibitors of the STARD5 which disrupt the lipid rafts, resulting in the inhibition of growth signaling pathways. Methods and Results: In-silico modeling studies using STARD5 crystal structure revealed 8 putative inhibitors (SD5i (A-H)). In vitro binding studies to determine secondary structural changes using circular dichroism spectroscopy revealed SD5iA and SD5iB as excellent STARD5 ligands. Both SD5iA and SD5iB caused a potent inhibition (IC50 0.3 -18 µM) of CSC-enriched 3D spheroids in 9 CRC cell lines representative of 3 consensus molecular subtypes. However, SD5iA but not SD5iB showed toxicity towards normal murine intestinal organoids at effective doses against CRC CSCs. Also, SD5iB enhanced cytotoxicity by oxaliplatin and panitumumab against CRC CSCs in HT-29 spheroids. Hence, SD5iB was advanced into in vivo studies. SD5iB caused no discernible toxicity in doses as high as 100 mg/kg in NCr nude mice at gross, serum chemistry, organ viability, or intestinal stem cell level (organoid formation). Subsequently, we generated s.c. xenograft using dual CSC marker sorted HT-29 cells and enriched them further with chemotherapy (5-fluorouracil and oxaliplatin weekly x 3) treatment followed by randomization to vehicle or SD5iB (100mg/kg i.p. 3/wk x 3 wks.). We observed xenograft growth inhibition and LGR5 expression in SD5iB-treated animals compared to vehicle controls. Mechanistically, we observed a decrease in free accessible PM cholesterol by SD5i measured by fluorescently tagged cholesterol-binding bacterial protein fALOD4 in HT-29 cells. Moreover, lipid raft staining (cholera toxin B staining) was significantly reduced in SD5iB-treated HT-29 cells compared to vehicle control using immunofluorescence. Further, lipid raft isolation was done by sucrose gradient ultracentrifugation to study the changes in the phosphorylation of RTKs SD5iB treatment in HT-29 spheroids resulted in decreased phosphorylation of EGFR, IGF1Rβ, and FGFR1 in isolated lipid rafts. Consequently, we observed a robust inhibition of ERK1/2 activation following SD5iB treatment compared to vehicle control. Conclusion: SD5i shows anti-CSC activity by targeting lipid-raft-driven growth factor signaling. Hence, STARD5 inhibitors represent a novel class of targeted therapy that will enhance the efficacy of FDA-approved therapies in CRC. Citation Format: Shoja Hajira, Chetna Sharon, Daniel RodriguezAgudo, Rio Boothelllo, Glenn Kellogg, Gregorio Gil, William Pandak, Bhaumik B. Patel. Novel inhibitors of StAR related lipid transfer protein 5 inhibitors target plasma membrane lipid raft growth signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6259.

Sodium butyrate alleviates deoxynivalenol-induced hepatic cholesterol metabolic dysfunction via RORγ-mediated histone acetylation modification in weaning piglets

BackgroundCholesterol is an essential component of lipid rafts in cell plasma membrane, which exerts a hepatoprotective role against mycotoxin exposure in pigs, and cholesterol metabolism is vulnerable to epigenetic histone acetylation. Therefore, our present study aimed to investigate whether a histone deacetylase inhibitor (sodium butyrate [NaBu]) could protect the porcine liver from deoxynivalenol (DON) exposure by modulating cholesterol metabolism. Herein, we randomly divided 28 pigs into four groups, which were fed an uncontaminated basal diet, contaminated diet (4 mg DON/kg), uncontaminated diet supplemented with 0.2% NaBu or 4 mg/kg DON contaminated diet (4 mg DON/kg) supplemented with 0.2% NaBu for 28 d.ResultsWe found that the serum alanine transaminase (ALT), aspartate transaminase (AST), and alkaline phosphatase (ALP) were all increased in pigs exposed to DON, indicative of significant liver injury. Furthermore, the cholesterol content in the serum of DON-exposed pigs was significantly reduced, compared to the healthy Vehicle group. Transcriptome analysis of porcine liver tissues revealed that the cholesterol homeostasis pathway was highly enriched due to DON exposure. In which we validated by qRT-PCR and western blotting that the cholesterol program was markedly activated. Importantly, NaBu effectively restored parameters associated with liver injury, along with the cholesterol content and the expression of key genes involved in the cholesterol biosynthesis pathway. Mechanistically, we performed a ChIP-seq analysis of H3K27ac and showed that NaBu strongly diminished DON-increased H3K27ac genome-wide enrichment. We further validated that the elevated H3K27ac and H3K9ac occupancies on cholesterol biosynthesis genes were both decreased by NaBu, as determined by ChIP-qPCR analysis. Notably, nuclear receptor RORγ, a novel regulator of cholesterol biosynthesis, was found in the hyperacetylated regions. Again, a remarkable increase of RORγ at both mRNA and protein levels in DON-exposed porcine livers was drastically reduced by NaBu. Consistent with RORγ expression, NaBu also hindered RORγ transcriptional binding enrichments on these activated cholesterol biosynthesis genes like HMGCR, SQLE, and DHCR24. Furthermore, we conducted an in vitro luciferase reporter assay to verify that porcine RORγ directly bonds to the promoters of the above target genes.ConclusionsCollectively, our results demonstrate the utility of the natural product NaBu as a potential anti-mycotoxin nutritional strategy for regulating cholesterol metabolism via RORγ-mediated histone acetylation modification.

Mechanisms underlying Nrf2 nuclear translocation by non-lethal levels of hydrogen peroxide: p38 MAPK-dependent neutral sphingomyelinase2 membrane trafficking and ceramide/PKCζ/CK2 signaling.

Hydrogen peroxide is an aerobic metabolite playing a central role in redox signaling and oxidative stress. H2O2 could activate redox sensitive transcription factors, such as Nrf2, AP-1 and NF-κB by different manners. In some cells, treatment with non-lethal levels of H2O2 induces rapid activation of Nrf2, which upregulates expression of a set of genes involved in glutathione (GSH) synthesis and defenses against oxidative damage. It depends on two steps, the rapid translational activation of Nrf2 and facilitation of Nrf2 nuclear translocation. We review the molecular mechanisms by which H2O2 induces nuclear translocation of Nrf2 in cultured cells by highlighting the role of neutral sphingomyelinase 2 (nSMase2), a GSH sensor. H2O2 enters cells through aquaporin channels in the plasma membrane and is rapidly reduced to H2O by GSH peroxidases to consume cellular GSH, resulting in nSMase2 activation to generate ceramide. H2O2 also activates p38 MAP kinase, which enhances transfer of nSMase2 from perinuclear regions to plasma membrane lipid rafts to accelerate ceramide generation. Low levels of ceramide activate PKCζ, which then activates casein kinase 2 (CK2). These protein kinases are able to phosphorylate Nrf2 to stabilize and activate it. Notably, Nrf2 also binds to caveolin-1 (Cav1), which protects Nrf2 from Keap1-mediated degradation and limits Nrf2 nuclear translocation. We propose that Cav1serves as a signaling hub for the control of H2O2-mediated phosphorylation of Nrf2 by kinases, which results in release of Nrf2 from Cav1 to facilitate nuclear translocation. In summary, H2O2 induces GSH depletion which is recovered by Nrf2 activation dependent on p38/nSMase2/ceramide signaling.

Lactoferrin perturbs intracellular trafficking, disrupts cholesterol-rich lipid rafts and inhibits glycolysis of highly metastatic cancer cells harbouring plasmalemmal V-ATPase

The milk-derived bovine lactoferrin (bLf) is an iron-binding glycoprotein with remarkable selective anticancer activity towards highly metastatic cancer cells displaying the proton pump V-ATPase at the plasma membrane. As studies aiming to dissect the bLf mechanisms of action are critical to improve its efficacy and boost its targeted clinical use, herein we sought to further uncover the molecular basis of bLf anticancer activity. We showed that bLf co-localizes with V-ATPase and cholesterol-rich lipid rafts at the plasma membrane of highly metastatic cancer cells. Our data also revealed that bLf perturbs cellular trafficking, induces intracellular accumulation of cholesterol and lipid rafts disruption, downregulates PI3K, and AKT or p-AKT and inhibits glycolysis of cancer cells harbouring V-ATPase at the plasma membrane lipid rafts. Altogether, our results can lay the foundation for future bLf-based targeted anticancer strategies as they unravel a novel cascade of molecular events that explains and further reinforces bLf selectivity for cancer cells displaying plasmalemmal V-ATPase.

Zika Virus Exploits Lipid Rafts to Infect Host Cells

Several flaviviruses such as Hepatitis C virus, West Nile virus, Dengue virus and Japanese Encephalitis virus exploit the raft platform to enter host cells whereas the involvement of lipid rafts in Zika virus–host cell interaction has not yet been demonstrated. Zika virus disease is caused by a flavivirus transmitted by Aedes spp. Mosquitoes, although other mechanisms such as blood transfusion, sexual and maternal–fetal transmission have been demonstrated. Symptoms are generally mild, such as fever, rash, joint pain and conjunctivitis, but neurological complications, including Guillain-Barré syndrome, have been associated to this viral infection. During pregnancy, it can cause microcephaly and other congenital abnormalities in the fetus, as well as pregnancy complications, representing a serious health threat. In this study, we show for the first time that Zika virus employs cell membrane lipid rafts as a portal of entry into Vero cells. We previously demonstrated that the antifungal drug Amphotericin B (AmphB) hampers a microbe–host cell interaction through the disruption of lipid raft architecture. Here, we found that Amphotericin B by the same mechanism of action inhibits both Zika virus cell entry and replication. These data encourage further studies on the off-label use of Amphotericin B in Zika virus infections as a new and alternate antiviral therapy.

Lipid Raft Facilitated Receptor Organization and Signaling: A Functional Rheostat in Embryonic Development, Stem Cell Biology and Cancer.

Molecular views of plasma membrane organization and dynamics are gradually changing over the past fifty years. Dynamics of plasma membrane instigate several signaling nexuses in eukaryotic cells. The striking feature of plasma membrane dynamics is that, it is internally transfigured into various subdomains of clustered macromolecules. Lipid rafts are nanoscale subdomains, enriched with cholesterol and sphingolipids, reside as floating entity mostly on the exoplasmic leaflet of the lipid bilayer. In terms of functionality, lipid rafts are unique among other membrane subdomains. Herein, advances on the roles of lipid rafts in cellular physiology and homeostasis are discussed, precisely, on how rafts dynamically harbor signaling proteins, including GPCRs, catalytic receptors, and ionotropic receptors within it and orchestrate multiple signaling pathways. In the developmental proceedings signaling are designed for patterning of overall organism and they differ from the somatic cell physiology and signaling of fully developed organisms. Some of the developmental signals are characteristic in maintenance of stemness and activated during several types of tumor development and cancer progression. The harmony between extracellular signaling and lineage specific transcriptional programs are extremely important for embryonic development. The roles of plasma membrane lipid rafts mediated signaling in lineage specificity, early embryonic development, stem cell maintenance are emerging. In view of this, we have highlighted and analyzed the roles of lipid rafts in receptor organization, cell signaling, and gene expression during embryonic development; from pre-implantation through the post-implantation phase, in stem cell and cancer biology.

Lipid Raft Integrity and Cellular Cholesterol Homeostasis Are Critical for SARS-CoV-2 Entry into Cells.

Lipid rafts in cell plasma membranes play a critical role in the life cycle of many viruses. However, the involvement of membrane cholesterol-rich lipid rafts in the entry of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) into target cells is not well known. In this study, we investigated whether the presence of cholesterol-rich microdomains is required for the entry of SARS-CoV-2 into host cells. Our results show that depletion of cholesterol in the rafts by methyl-beta-cyclodextrin (MβCD) treatment impaired the expression of the cell surface receptor angiotensin-converting enzyme 2 (ACE2), resulting in a significant increase in SARS-CoV-2 entry into cells. The effects exerted by MβCD could be substantially reversed by exogenous cholesterol replenishment. In contrast, disturbance of intracellular cholesterol homeostasis by statins or siRNA knockdown of key genes involved in the cholesterol biosynthesis and transport pathways reduced SARS-CoV-2 entry into cells. Our study also reveals that SREBP2-mediated cholesterol biosynthesis is involved in the process of SARS-CoV-2 entry in target cells. These results suggest that the host membrane cholesterol-enriched lipid rafts and cellular cholesterol homeostasis are essential for SARS-CoV-2 entry into cells. Pharmacological manipulation of intracellular cholesterol might provide new therapeutic strategies to alleviate SARS-CoV-2 entry into cells.

Cholesterol Protects the Liquid-Ordered Phase of Raft Model Membranes from the Destructive Effect of Ionic Liquids.

Ionic liquids (ILs), although being a class of promising green solvents, have received many reports on the toxicity to living organisms. In this work, aiming at elucidating the disruptive effect of ILs to cell membrane lipid rafts, we investigated the effect of three 1-octylimidazolium-based ILs on the properties of the liquid ordered phase (Lo, a commonly used lipid raft model) of egg sphingomyelin (SM)-cholesterol model membrane. We found that, in the absence of cholesterol, a very low IL:SM molar ratio of 0.01:1 could disrupt the integrity of the bilayer structure. In sharp contrast, the presence of cholesterol in lipid bilayers helps the Lo phase resist the damaging effect of the ILs. For the role of the IL headgroup, we found that the mono- and trisubstituted species show a stronger destructive effect on the structures of the model rafts than the commonly used disubstituted counterpart.

Interactions of polycyclic aromatic hydrocarbons and their nitro derivatives with bilayer and monolayer models of fungal membranes

Polycyclic aromatic hydrocarbons (PAH) are ubiquitous persistent organic pollutants forming as byproducts in combustion processes. These compounds can become nitrated in the environment forming even more toxic nitro derivatives. Both PAHs and nitro-PAHs accumulate in the soils which can lead to the pollution of vast areas of arable land. The best means of their elimination from the environment is bioremediation that is the degradation by soil microorganisms. Important key players with high prospects are non-ligninolytic fungi, which can incorporate PAHs and their derivatives into their plasma membranes, oxidize and metabolize to nontoxic products. The accumulation of PAHs in the membrane can, however, be toxic to the fungal cell, but the mechanisms of the PAH-membrane lipid interactions and how they influence the membrane phase state and properties are still poorly understood. To shed light on these questions we employed fungal plasma membrane models and studied their interactions with selected PAHs and nitro-PAHs. Two complementary model systems were investigated: giant unilamellar vesicles (GUV) and Langmuir monolayers. We find that the native PAHs induce phase separation and the formation of solid ordered phase in the GUVs, whereas the nitro-derivatives do not disturb the liquid crystalline state of the membranes. The Langmuir monolayer studies indicate that some PAHs preferably partition in the liquid-expanded phase while others in the liquid-condensed domains. The possibility of the incorporation of PAHs into the condensed domains implies that these compounds can influence the order in the lipid bilayer and affect the function of the lipid rafts in plasma membranes.