Detergent-resistant Membrane Domains Research Articles

Dynamic association of the intramembrane proteases SPPL2a/b and their substrates with tetraspanin-enriched microdomains

Signal peptide peptidase-like 2a and b (SPPL2a/b) are aspartyl intramembrane proteases and cleave tail-anchored proteins as well as N-terminal fragments (NTFs) derived from type II-oriented transmembrane proteins. How these proteases recruit substrates and cleavage is regulated, is still incompletely understood. We found that SPPL2a/b localize to detergent-resistant membrane (DRM) domains with the characteristics of tetraspanin-enriched microdomains (TEMs). Based on this, association with several tetraspanins was evaluated. We demonstrate that not only SPPL2a/b but also their substrates tumor necrosis factor (TNF) and CD74 associate with tetraspanins like CD9, CD81, and CD82 and/or TEMs and analyze the stability of these complexes in different detergents. CD9 and CD81 deficiency has protease- and substrate-selective effects on SPPL2a/b function. Our findings suggest that reciprocal interactions with tetraspanins may assist protease-substrate encounters of SPPL2a/b within the membrane. Beyond SPP/SPPL proteases, this supports previous concepts that tetraspanins facilitate membrane-embedded proteolytic processes.

Analysis of Lipids, Fatty Acid, and Cholesterol in Membrane Microdomains.

The original concept that lipid and protein components are randomly distributed in cellular membranes has been challenged by evidence of compartmentalization of such components into discrete membrane microdomains (known as lipid rafts). The lipid microdomain hypothesis has generated significant controversy and rigorous inquiry to test the idea that such domains concentrate machinery to mediate cellular processes such as signaling, synaptic plasticity, and endocytosis. As such, a large number of studies have used biochemical, cell biological, and biophysical methodologies to define the composition of membrane microdomains in experimental contexts. Although biochemical preparation strategies are not without limitations (as discussed herein), the isolation of detergent-resistant and detergent-free membrane domains can provide important information about the segregation of lipids and proteins in membranes. In this chapter, we describe methodologies to isolate membranes from cell or tissue sources with biophysical/biochemical properties of membrane microdomains and also provide methods for subsequent classical or mass spectrometry-based lipid analytical approaches.

The Neuroprotective Lipocalin Apolipoprotein D Stably Interacts with Specific Subtypes of Detergent-Resistant Membrane Domains in a Basigin-Independent Manner

Accumulated evidence points to the lipocalin apolipoprotein D (ApoD), one of the few genes consistently upregulated upon brain ageing and neurodegeneration, as an endogenous controller of the redox state of cellular and extracellular lipid structures. This biochemical function has downstream consequences as apparently varied as control of glycocalyx and myelin compaction, cell viability upon oxidative stress or modulation of signalling pathways. In spite of this knowledge, it is still unclear if ApoD function requires canonical receptor-mediated transductions systems. This work aims to examine ApoD-cell membrane interaction and its dependence on a proposed ApoD receptor, Basigin. Whole and fractionated membrane preparations from the brain, primary astrocytes, glial and neuronal cell lines, reveal ApoD as a very specific component of particular subtypes of detergent-resistant microdomains (DRMs). ApoD interacts in vitro with neuronal membranes and is stably associated with astrocytic membranes. ApoD associates with DRMs with specific buoyancy properties that co-fractionate with plasma or late-endosome-lysosome markers. A mass spectrometry analysis reveals that these Triton X-114 DRMs contain both plasma membrane and endosomal-lysosomal compartment lipid raft proteins. ApoD-DRM association is maintained under metabolic and acute oxidative stress conditions. However, ApoD-membrane interaction, its internalization and its lipid-antioxidant function do not require the presence of Basigin. This work supports a stable association of ApoD with membranes, independent of Basigin, and provides the basis to fully understand ApoD antioxidant neuroprotective mechanism as a mechanism taking place in specific membrane subdomains.

CRMP4-mediated fornix development involves Semaphorin-3E signaling pathway.

Neurodevelopmental axonal pathfinding plays a central role in correct brain wiring and subsequent cognitive abilities. Within the growth cone, various intracellular effectors transduce axonal guidance signals by remodeling the cytoskeleton. Semaphorin-3E (Sema3E) is a guidance cue implicated in development of the fornix, a neuronal tract connecting the hippocampus to the hypothalamus. Microtubule-associated protein 6 (MAP6) has been shown to be involved in the Sema3E growth-promoting signaling pathway. In this study, we identified the collapsin response mediator protein 4 (CRMP4) as a MAP6 partner and a crucial effector in Sema3E growth-promoting activity. CRMP4-KO mice displayed abnormal fornix development reminiscent of that observed in Sema3E-KO mice. CRMP4 was shown to interact with the Sema3E tripartite receptor complex within detergent-resistant membrane (DRM) domains, and DRM domain integrity was required to transduce Sema3E signaling through the Akt/GSK3 pathway. Finally, we showed that the cytoskeleton-binding domain of CRMP4 is required for Sema3E's growth-promoting activity, suggesting that CRMP4 plays a role at the interface between Sema3E receptors, located in DRM domains, and the cytoskeleton network. As the fornix is affected in many psychiatric diseases, such as schizophrenia, our results provide new insights to better understand the neurodevelopmental components of these diseases.

Detergent Resistant Membrane Domains in Broccoli Plasma Membrane Associated to the Response to Salinity Stress

Detergent-resistant membranes (DRMs) microdomains, or “raft lipids”, are key components of the plasma membrane (PM), being involved in membrane trafficking, signal transduction, cell wall metabolism or endocytosis. Proteins imbibed in these domains play important roles in these cellular functions, but there are few studies concerning DRMs under abiotic stress. In this work, we determine DRMs from the PM of broccoli roots, the lipid and protein content, the vesicles structure, their water osmotic permeability and a proteomic characterization focused mainly in aquaporin isoforms under salinity (80 mM NaCl). Based on biochemical lipid composition, higher fatty acid saturation and enriched sterol content under stress resulted in membranes, which decreased osmotic water permeability with regard to other PM vesicles, but this permeability was maintained under control and saline conditions; this maintenance may be related to a lower amount of total PIP1 and PIP2. Selective aquaporin isoforms related to the stress response such as PIP1;2 and PIP2;7 were found in DRMs and this protein partitioning may act as a mechanism to regulate aquaporins involved in the response to salt stress. Other proteins related to protein synthesis, metabolism and energy were identified in DRMs independently of the treatment, indicating their preference to organize in DMRs.

The Alzheimer's disease‐associated C99 fragment of APP regulates cellular cholesterol trafficking

The link between cholesterol homeostasis and cleavage of the amyloid precursor protein (APP), and how this relationship relates to Alzheimer's disease (AD) pathogenesis, is still unknown. Cellular cholesterol levels are regulated through crosstalk between the plasma membrane (PM), where most cellular cholesterol resides, and the endoplasmic reticulum (ER), where the protein machinery that regulates cholesterol levels resides. The intracellular transport of cholesterol from the PM to the ER is believed to be activated by a lipid‐sensing peptide(s) in the ER that can cluster PM‐derived cholesterol into transient detergent‐resistant membrane domains (DRMs) within the ER, also called the ER regulatory pool of cholesterol. When formed, these cholesterol‐rich domains in the ER maintain cellular homeostasis by inducing cholesterol esterification as a mechanism of detoxification while attenuating its de novo synthesis. In this manuscript, we propose that the 99‐aa C‐terminal fragment of APP (C99), when delivered to the ER for cleavage by γ‐secretase, acts as a lipid‐sensing peptide that forms regulatory DRMs in the ER, called mitochondria‐associated ER membranes (MAM). Our data in cellular AD models indicates that increased levels of uncleaved C99 in the ER, an early phenotype of the disease, upregulates the formation of these transient DRMs by inducing the internalization of extracellular cholesterol and its trafficking from the PM to the ER. These results suggest a novel role for C99 as a mediator of cholesterol disturbances in AD, potentially explaining early hallmarks of the disease.

Biochemical Analysis of Lipid Rafts to Study Pathogenic Mechanisms of Neural Diseases.

The discovery of dynamic platforms in cell membranes, called lipid rafts or detergent resistant membrane domains, opened a new chapter on studies of membrane cell biology. Indeed, the analysis of lipid rafts enabled innovative ways to understand cellular and molecular mechanisms regulating normal and pathological processes. Lipid rafts have been studied in most cell types, where they work by providing transient and fluid architectural scaffolding platforms regulating a spectrum of important signaling pathways, including receptor activities, protein-protein interactions, posttranslational modifications of proteins and lipids and the function of ion channels. In this chapter, we will explain how to isolate these membrane domains from neural tissue samples and perform further analysis of proteins and lipids.

Development and Characterization of an Avirulent Leishmania major Strain.

Leishmania major causes cutaneous leishmaniasis. An antileishmanial vaccine for humans is unavailable. In this study, we report development of two attenuated L. major strains-5ASKH-HP and LV39-HP-by continuous culture (high passage) of the corresponding virulent strains (low passage). Both avirulent strains showed similar changes in proteome profiles when analyzed by surface-enhanced laser desorption ionization mass spectrometry. Liquid chromatography-mass spectrometry and microarray characterization of 5ASKH strains revealed substantially altered gene and protein expression profiles, respectively. Both virulent and avirulent L. major strains grew comparably in culture, but the avirulent strain survived significantly less in BALB/c-derived peritoneal macrophages. Both attenuated strains failed to infect BALB/c mice and elicited IFN-γ, but not IL-4 and IL-10, responses. 5ASKH-HP parasites failed to induce significant infection even in severely immunocompromised- SCID or inducible NO synthase-, CD40-, or IL-12-deficient mice, indicating attenuation. The avirulent strain induced less IL-10, but higher IL-12, in macrophages. The avirulent strain failed to reduce CD40 relocation to the detergent-resistant membrane domain and to inhibit CD40-induced phosphorylation of the kinases Lyn and protein kinase C-β and MAPKs MKK-3/6 and p38MAPK or to upregulate MEK-1/2 and ERK-1/2 in BALB/c-derived peritoneal macrophages. The virulent and the avirulent strains reciprocally modulated CD40-induced Ras-mediated signaling through PI-3K and Raf-1. Avirulent 5ASKH-primed BALB/c mice were protected against virulent L. major challenge infection. The loss of virulence accompanied by substantially altered proteome profiles and the elicitation of host-protective immune responses indicate plausibly irreversible attenuation of the L. major strain and its potential use as a vaccine strain.

The Function of Hepatocellular Uptake Transporters is Affected by Free Cholesterol

BackgroundWe have previously shown that several transporters located in the basolateral membrane of human hepatocytes e.g. NTCP, OCT1, OATP1B1, and OATP1B3 interact with each other. These interactions are due in part to their co‐localization in detergent resistant membrane domains and impact both function and expression. Understanding how transporter interaction and altered membrane cholesterol content impact the function of these drug uptake transporters has implications regarding pharmacological studies, drug development, and drug‐drug interaction predictions.AimWe wanted to test how both the addition and removal of free cholesterol to and from the plasma membrane alters the function and localization of drug transporters.MethodsThe function of NTCP or OCT1 was measured using HEK293 Flp‐In cells stably expressing either transporter. We treated the cells with methyl‐b‐cyclodextrin (MCD) to remove cholesterol or with a cholesterol‐MCD complex to add cholesterol. Lipid rafts were isolated from HEK293 Flp‐In cells or frozen human liver samples by homogenization in 1% Triton X‐100 and sucrose gradient centrifugation. Gradient fractions were analyzed by western blotting using positive and negative lipid raft markers as well as transporter specific antibodies.ResultsTreatment with MCD stimulated uptake by NTCP while inhibiting uptake by OCT1. Subsequent treatment of the MCD treated cells with the cholesterol‐MCD complex restored function in part for both NTCP and OCT1. Removal and restoration of membrane free cholesterol also impacted the co‐localization of the transporters within the detergent resistant membrane domains. We confirmed the differential localization of transporters in detergent resistant membranes comparing samples of normal and fatty human livers.Summary and ConclusionBoth adding or removing cholesterol alters transport function in a transporter‐dependent way. Furthermore, the localization of transporters in lipid rafts is different in normal and in fatty human livers. These results suggest that drug disposition is affected by fatty liver disease and that some drugs may be cleared faster or slower in patients with increased free cholesterol in the liver, due to NAFLD or NASH.Support or Funding InformationR01GM077336 and P30GM118247

The Bile Acid-Phospholipid Conjugate Ursodeoxycholyl-Lysophosphatidylethanolamide (UDCA-LPE) Disintegrates the Lipid Backbone of Raft Plasma Membrane Domains by the Removal of the Membrane Phospholipase A2

The bile acid-phospholipid conjugate ursodeoxycholyl-lysophosphatidylethanolamide (UDCA-LPE) was shown to have anti-inflammatory, antisteatotic, and antifibrotic properties, rendering it as a drug targeting non-alcoholic steatohepatitis (NASH). On a molecular level, it disrupted the heterotetrameric fatty acid uptake complex localized in detergent-resistant membrane domains of the plasma membrane (DRM-PM). However, its mode of action was unclear. Methodologically, UDCA-LPE was incubated with the liver tumor cell line HepG2 as well as their isolated DRM-PM and all other cellular membranes (non-DRM). The membrane cholesterol and phospholipids were quantified as well as the DRM-PM protein composition by Western blotting. The results show a loss of DRM-PM by UDCA-LPE (50 µM) with a 63.13 ± 7.14% reduction of phospholipids and an 81.94 ± 8.30% reduction of cholesterol in relation to mg total protein. The ratio of phospholipids to cholesterol changed from 2:1 to 4:1, resembling those of non-DRM fractions. Among the members of the fatty acid uptake complex, the calcium-independent membrane phospholipase A2 (iPLA2β) abandoned DRM-PM most rapidly. As a consequence, the other members of this transport system disappeared as well as the DRM-PM anchored fibrosis regulating proteins integrin β-1 and lysophospholipid receptor 1 (LPAR-1). It is concluded that UDCA-LPE executes its action by iPLA2β removal from DRM-PM and consequent dissolution of the raft lipid platform.

Detergent-resistant domains in Spodoptera frugiperda midgut microvillar membranes and their relation to microapocrine secretion.

The midgut from lepidopteran insects has a particular way to release proteins to the lumen, named microapocrine secretion that could be an adaptation to release secretory contents into the lumen at water absorbing regions. In this process small vesicles (microapocrine vesicles) bud from the midgut microvilli as double membrane vesicles, where the inner membrane comes from the secretion vesicle and the outer one from the microvillar membrane. The molecular machinery associated with this process may be recruited by specific midgut microvilli membrane domains. To address to this, Spodoptera frugiperda midgut microvillar membranes, prepared by magnesium treatment and free from cytoskeleton with the hyperosmotic Tris procedure, were submitted to detergent extraction and fractionated by density gradient ultracentrifugation. Detergent-resistant membrane domains (DRM) were recovered and their proteins identified by proteomics. Microapocrine vesicles were isolated by washing the luminal surface of the midgut epithelium, followed by freezing and thawing plus centrifugation to recover only membranes. Proteins from purified microvillar membranes and microapocrine vesicle membranes were identified by proteomics. Comparison of the two populations suggests that the budding of microapocrine vesicles surrounded by microvillar membrane is not a random process, because only around 50% of the microvillar membrane proteins are in the microapocrine vesicles. From the 16 proteins from DRM, 14 were enriched in the microapocrine membrane vesicles. These results suggest that on budding, the microapocrine vesicle membrane is enclosed by DRM and a surrounding area of the microvillar membrane. It is proposed that the DRMs somehow recruit the proteins composing the secretory machinery.

Co‐localization of Hepatocellular Uptake Transporters in Detergent Resistant Membranes (Lipid Rafts)

BackgroundWe have previously shown that several transporters located in the basolateral membrane of human hepatocytes (OCT1, NTCP, OATP1B1, and OATP1B3) interact with each other. These interactions affect both surface expression and function. Currently, transporter studies are performed using cell lines that express a single transporter at a time without considering how transporter interactions will affect function. Determining how transporters co‐localize, e.g. in detergent resistant lipid rafts or via direct protein‐protein interactions, has far‐reaching implications for drug metabolism and pharmacological studies.AimThe purpose of this study is to test the hypothesis that these transporters co‐localize with well‐known lipid raft marker proteins.MethodsFollowing homogenization in 1% Triton X‐100 on ice, lipid rafts were isolated from HEK293 cells expressing NTCP, OCT1, OATP1B1, and OATP1B3, or from frozen human liver samples. Homogenates in 40% sucrose were overlaid with a discontinuous 5% to 35% sucrose gradient. After centrifugation for 18 hours at 197,000 × gav, gradients were fractionated, and aliquots were analyzed using western blotting. The blots were probed with antibodies against positive (caveolin and Na+/K+ ATPase) and negative (clathrin) lipid raft markers, as well as with transporter‐specific antibodies.ResultsIn human liver, caveolin, Na+/K+ ATPase, OATP1B1, OATP1B3, and OCT1 were detected in similar fractions. OCT1 also was found in clathrin‐positive fractions towards the bottom of the gradient. In HEK293 cells, NTCP and OCT1 co‐localized with caveolin in the middle of the gradient and again, OCT1 was also found towards the bottom of the gradient.ConclusionThe detection of NTCP, OCT1, OATP1B1, and OATP1B3 along with the positive lipid raft markers caveolin and Na+/K+ ATPase indicates that co‐localization of these transporters occurs, at least in part, because of their residence in detergent resistant membrane domains. Future experiments are aimed at identifying the functional consequences of this co‐localization.Support or Funding InformationP20 GM103549, P30 GM118247, and GM077336This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Cholesterol trafficking and raft-like membrane domain composition mediate scavenger receptor class B type 1-dependent lipid sensing in intestinal epithelial cells

Scavenger receptor Class B type 1 (SR-B1) is a lipid transporter and sensor. In intestinal epithelial cells, SR-B1-dependent lipid sensing is associated with SR-B1 recruitment in raft-like/ detergent-resistant membrane domains and interaction of its C-terminal transmembrane domain with plasma membrane cholesterol. To clarify the initiating events occurring during lipid sensing by SR-B1, we analyzed cholesterol trafficking and raft-like domain composition in intestinal epithelial cells expressing wild-type SR-B1 or the mutated form SR-B1-Q445A, defective in membrane cholesterol binding and signal initiation. These features of SR-B1 were found to influence both apical cholesterol efflux and intracellular cholesterol trafficking from plasma membrane to lipid droplets, and the lipid composition of raft-like domains. Lipidomic analysis revealed likely participation of d18:0/16:0 sphingomyelin and 16:0/0:0 lysophosphatidylethanolamine in lipid sensing by SR-B1. Proteomic analysis identified proteins, whose abundance changed in raft-like domains during lipid sensing, and these included molecules linked to lipid raft dynamics and signal transduction. These findings provide new insights into the role of SR-B1 in cellular cholesterol homeostasis and suggest molecular links between SR-B1-dependent lipid sensing and cell cholesterol and lipid droplet dynamics.

Rufy3 is an adapter protein for small GTPases that activates a Rac guanine nucleotide exchange factor to control neuronal polarity

RUN and FYVE domain-containing 3 (Rufy3) is an adapter protein for small GTPase proteins and is bound to activated Rap2, a Ras family protein in the developing neuron. Previously, we reported the presence of a rapid cell polarity determination mechanism involving Rufy3, which is likely required for in vivo neuronal development. However, the molecular details of this mechanism are unclear. To this end, here we produced Rufy3 knock-out (Rufy3-KO) mice to study the role of Rufy3 in more detail. Examining Rufy3-KO neurons, we found that Rufy3 is recruited via glycoprotein M6A to detergent-resistant membrane domains, which are biochemically similar to lipid rafts. We also clarified that Rufy3, as a component of a ternary complex, induces the assembly of Rap2 in the axonal growth cone, whereas in the absence of Rufy3, the accumulation of a Rac guanine nucleotide exchange factor, T-cell lymphoma invasion and metastasis 2 (Tiam2/STEF), is inhibited downstream of Rap2. We also found that Rufy3 regulates the cellular localization of Rap2 and Tiam2/STEF. Taken together, we conclude that Rufy3 is a physiological adapter for Rap2 and activates Tiam2/STEF in glycoprotein M6A-regulated neuronal polarity and axon growth.

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.

HS-AFM and SERS Analysis of Murine Norovirus Infection: Involvement of the Lipid Rafts

Studies on human norovirus are severely hampered by the absence of a cell culture system until the discovery of murine norovirus (MNV). The cell membrane domains called lipid rafts have been defined as a port of entry for viruses. This study is conducted to investigate murine norovirus binding on the mouse leukemic monocyte macrophage cell line. Lipid raft related structures are extracted from cells by detergent treatment resulting detergent-resistant membrane (DRMs) domains. The real-time polymerase chain reaction technique is performed to detect the viral genome, thereby the MNV binding on the DRMs. The interactions between MNV and DRMs are investigated by high-speed atomic force microscopy (HS-AFM) combined with surface-enhanced Raman spectroscopy (SERS). The inoculation of the virus onto cells results in the aggregations of detergent-resistant membrane domains significantly. The characteristic Raman band of MNV is found in inoculated samples. To be sure that these results are originated from specific interactions between DRM and MNV, methyl-β-cyclo-dextrin (MβCD) is applied to disrupt lipid rafts. The MNV binding on DRMs is precluded by the MβCD treatment. The cholesterols chains are defined as a key factor in the interactions between norovirus and DRMs. The authors conclude that the MNV binding involves the presence of DRMs and cholesterol dependent.

Over-expression of stomatin causes syncytium formation in nonfusogenic JEG-3 choriocarcinoma placental cells.

Placental trophoblast differentiation involves the continuous fusion of mononuclear cytotrophoblasts. However, except for syncytin, little is known about the detailed mechanisms underlying trophoblast fusion. A previous study indicated that lipid rafts play an important role in HTLV-1 syncytium formation. To identify proteins that may be involved in placental trophoblast differentiation, we examined stomatin, an important lipid-raft protein that localizes to detergent-resistant membrane domains. The syncytium and human chorionic gonadotropin (β-hCG; a marker of placental trophoblast differentiation) were visualized by immunofluorescence staining. We found that overexpression of stomatin in the nonfusogenic JEG-3 cell line caused syncytium formation and increased the fusion index of cells. Treating these cells with N(6) ,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate further increased cell fusion by stomatin. β-hCG was found in a few JEG-3 cells overexpressing stomatin at 48 h, and its levels increased dramatically at 72 h along with the formation of the multinuclear syncytium. RNA interference was used to decrease stomatin expression in BeWo cells, a fusogenic human choriocarcinoma cell line. After knockdown for 72 h, stomatin levels decreased by almost 95%. The fusion indexes of control and stomatin-knockdown cells at 72 h were 9.4 and 6.5%, respectively. Our data indicated that stomatin could trigger syncytium formation and upregulate β-hCG for cell fusion in nonfusogenic JEG-3 cells. Downregulation of stomatin slightly inhibited the fusion index of fusogenic BeWo cells. Thus, these data suggested that stomatin plays an important role in trophoblast differentiation.

Thioredoxin (Trx1) regulates CD4 membrane domain localization and is required for efficient CD4-dependent HIV-1 entry

BackgroundCD4 is a glycoprotein expressed on the surfaces of certain immune cells. On lymphocytes, an important function of CD4 is to co-engage Major Histocompatibility Complex (MHC) molecules with the T Cell Receptor (TCR), a process that is essential for antigen-specific activation of T cells. CD4 localizes dynamically into distinct membrane microdomains, an important feature of its immunoregulatory function that has also been shown to influence the efficiency of HIV replication. However, the mechanism by which CD4 localization is regulated and the biological significance of this is incompletely understood. MethodsIn this study, we used confocal microscopy, density-gradient centrifugation and flow cytometry to analyze dynamic redox-dependent effects on CD4 membrane domain localization. ResultsBlocking cell surface redox exchanges with both a membrane-impermeable sulfhydryl blocker (DTNB) and specific antibody inhibitors of Thioredoxin-1 (Trx1) induces translocation of CD4 into detergent-resistant membrane domains (DRM). In contrast, Trx1 inactivation does not change the localization of the chemokine receptor CCR5, suggesting that this effect is targeted. Moreover, DTNB treatment and Trx1 depletion coincide with strong inhibition of CD4-dependent HIV entry, but only moderate reductions in the infectivity of a CD4-independent HIV pseudovirion. ConclusionsChanges in the extracellular redox environment, potentially mediated by allosteric consequences of functional disulfide bond oxidoreduction, may represent a signal for translocation of CD4 into DRM clusters, and this sequestration, another potential mechanism by which the anti-HIV effects of cell surface oxidoreductase inhibition are exerted. General significanceExtracellular redox conditions may regulate CD4 function by potentiating changes in its membrane domain localization.

Characteristics of morphological differences of detergent-resistant membrane domains isolated from different cells and investigated by atomic force microscopy

Planar rafts and caveolae are specific membrane clusters that contain high concentrations of cholesterol and lipids consisting of saturated fatty acids. These clusters are resistant to detergents and are known as “detergent-resistant membrane domains” (DRMs). Their morphology and size were studied by atomic force microscopy (AFM). Planar rafts extracted by Lubrol WX from monocytes of healthy donors are 150.6 ± 68.6 nm in diameter and 5.7 ± 2.9 nm in height, while caveolae are 87.3 ± 46.1 nm in diameter and 9.4 ± 5.4 nm in height. Significant differences in size and morphology were found between DRMs isolated from monocytes of healthy donors and patients with myocardial infarction, as well as between DRMs of monocytes and endothelial cells. The morphology dynamics of the isolated planar rafts and caveolae indicates that they quickly aggregate during storage; therefore, in order to assess the actual DRM size and morphology it is necessary to investigate them immediately after isolation.

Cholesterol in brain disease: sometimes determinant and frequently implicated.

Cholesterol is essential for neuronal physiology, both during development and in the adult life: as a major component of cell membranes and precursor of steroid hormones, it contributes to the regulation of ion permeability, cell shape, cell-cell interaction, and transmembrane signaling. Consistently, hereditary diseases with mutations in cholesterol-related genes result in impaired brain function during early life. In addition, defects in brain cholesterol metabolism may contribute to neurological syndromes, such as Alzheimer's disease (AD), Huntington's disease (HD), and Parkinson's disease (PD), and even to the cognitive deficits typical of the old age. In these cases, brain cholesterol defects may be secondary to disease-causing elements and contribute to the functional deficits by altering synaptic functions. In the first part of this review, we will describe hereditary and non-hereditary causes of cholesterol dyshomeostasis and the relationship to brain diseases. In the second part, we will focus on the mechanisms by which perturbation of cholesterol metabolism can affect synaptic function.