A novel type of detergent-resistant membranes may contribute to an early protein sorting event in epithelial cells

Segregation of the membrane cargoes, BACE1 and amyloid precursor protein (APP) throughout the Golgi apparatus

The intracellular trafficking of β‐site amyloid precursor protein (APP) cleaving enzyme (BACE1) and APP regulates amyloid‐β production. Our previous work demonstrated that newly synthesized BACE1 and APP are segregated into distinct trafficking pathways from the trans‐Golgi network (TGN), and that alterations in their trafficking lead to an increase in Aβ production in non‐neuronal and neuronal cells. However, it is not known whether BACE1 and APP are transported through the Golgi stacks together and sorted at the TGN or segregated prior to arrival at the TGN. To address this question, we have used high‐resolution Airyscan technology followed by Huygens deconvolution to quantify the overlap of BACE1 and APP in Golgi subcompartments in HeLa cells and primary neurons. Here, we show that APP and BACE1 are segregated, on exit from the endoplasmic reticulum and in the cis‐Golgi and throughout the Golgi stack. In contrast, the transferrin receptor, which exits the TGN in AP‐1 mediated transport carriers as for BACE1, colocalizes with BACE1, but not APP, throughout the Golgi stack. The segregation of APP and BACE1 is independent of the Golgi ribbon structure and the cytoplasmic domain of the cargo. Overall, our findings reveal the segregation of different membrane cargoes early in the secretory pathway, a finding relevant to the regulation of APP processing events.

Amyloid β production along the neuronal secretory pathway: Dangerous liaisons in the Golgi?

β-amyloid peptides (Aβ) are generated in intracellular compartments of neurons and secreted to form cytotoxic fibrils and plaques. Dysfunctional membrane trafficking contributes to aberrant Aβ production and Alzheimer's disease. Endosomes represent one of the major sites for Aβ production and recently the Golgi has re-emerged also as a major location for amyloid precursor protein (APP) processing and Aβ production. Based on recent findings, here we propose that APP processing in the Golgi is finely tuned by segregating newly-synthesised APP and the β-secretase BACE1 within the Golgi and into distinct trans-Golgi network transport pathways. We hypothesise that there are multiple mechanisms responsible for segregating APP and BACE1 during transit through the Golgi, and that perturbation in Golgi morphology associated with Alzheimer's disease, and or changes in cholesterol metabolism associated with Alzheimer's disease risk factors, may lead to a loss of partitioning and enhanced Aβ production.

Rosa canina L. Can Restore Endoplasmic Reticulum Alterations, Protein Trafficking and Membrane Integrity in a Dextran Sulfate Sodium-Induced Inflammatory Bowel Disease Phenotype

Rosa canina L. is a natural polyphenol-rich medicinal plant that exhibits antioxidant and anti-inflammatory activities. Recent in vivo studies have demonstrated that a methanol extract of Rosa canina L. (RCME) has reversed an inflammatory bowel disease (IBD)-like phenotype that has been triggered by dextran sulfate sodium (DSS) in mice. In the current study, we investigated the effects of RCME on perturbations of cellular mechanisms induced by DSS-treatment of intestinal Caco-2 cells, including stress response in the endoplasmic reticulum (ER), protein trafficking and sorting as well as lipid rafts integrity and functional capacities of an intestinal enzyme. 6 days post-confluent cells were treated for 24 h with DSS (3%) or simultaneously with DSS (3%) and RCME (100 µg/mL) or exclusively with RCME (100 µg/mL) or not treated. The results obtained demonstrate the ability of RCME to counteract the substantial increase in the expression levels of several ER stress markers in DSS-treated cells. Concomitantly, the delayed trafficking of intestinal membrane glycoproteins sucrase-isomaltase (SI) and dipeptidyl peptidase 4 (DPP4) induced by DSS between the ER and the Golgi has been compromised by RCME. Furthermore, RCME restored the partially impaired polarized sorting of SI and DPP4 to the brush border membrane. An efficient sorting mechanism of SI and DPP4 is tightly associated with intact lipid rafts structures in the trans-Golgi network (TGN), which have been distorted by DSS and normalized by RCME. Finally, the enzymatic activities of SI are enhanced in the presence of RCME. Altogether, DSS treatment has triggered ER stress, impaired trafficking and function of membrane glycoproteins and distorted lipid rafts, all of which can be compromised by RCME. These findings indicate that the antioxidants in RCME act at two major sites in Caco-2 cells, the ER and the TGN and are thus capable of maintaining the membrane integrity by correcting the sorting of membrane-associated proteins.

Intermolecular Interaction between Heavy Crude Oils and Surfactants during Surfactant-Steam Flooding Process

The objective of this study is to investigate the intermolecular interactions between the surfactants and the fractions of heavy crude oils. Two possible interactions were considered; polar and ionic interactions for two heavy crude oil-surfactant systems, and 20 surfactant-steam flooding tests were conducted on these crudes by testing nine surfactants (three anionic, three cationic, and three nonionic) with different tail lengths and charged head groups. The performance differences observed in each core flood were discussed through the additional analyses. To explain polar interactions, the pseudo blends of crude oil fractions (fractionation of saturates, aromatics, resins, and asphaltenes) were exposed to the surfactant solutions under vapor and liquid water conditions and their mutual interactions were visualized under an optical microscope. To explain ionic interactions, the charges on asphaltene surfaces were analyzed by zeta potential measurements before and after core flood tests on both the produced and the residual oil asphaltenes. The addition of surfactants improved the oil recovery when compared to steam injection alone. However, different oil recoveries were obtained with different surfactants. Further analyses showed that asphaltenes are key and the interaction of asphaltenes with other crude oil fractions or surfactants determines the success of surfactant-steam processes. The polar interactions favor the emulsion formation more; hence, if the polar interactions are more dominant than the ion interactions in the overall crude oil-surfactant system, the surfactant flooding process into heavy oil reservoir became more successful.

Dextran Sodium Sulfate-Induced Impairment of Protein Trafficking and Alterations in Membrane Composition in Intestinal Caco-2 Cell Line

A key morphological feature of inflammatory bowel disease (IBD) is the loss of the barrier function of intestinal epithelial cells. The present study investigates endoplasmic reticulum (ER) stress in addition to alterations in protein and membrane trafficking in a dextran sulfate sodium (DSS)-induced IBD-like phenotype of intestinal Caco-2 cells in culture. DSS treatment significantly reduced the transepithelial electric resistance (TEER) and increased the epithelial permeability of Caco-2 cells, without affecting their viability. This was associated with an alteration in the expression levels of inflammatory factors in addition to an increase in the expression of the ER stress protein markers, namely immunoglobulin-binding protein (BiP), C/EBP homologous protein (CHOP), activation transcription factor 4 (ATF4), and X-box binding protein (XBP1). The DSS-induced ER-stress resulted in impaired intracellular trafficking and polarized sorting of sucrase-isomaltase (SI) and dipeptidyl peptidase-4 (DPPIV), which are normally sorted to the apical membrane via association with lipid rafts. The observed impaired sorting was caused by reduced cholesterol levels and subsequent distortion of the lipid rafts. The data presented confirm perturbation of ER homeostasis in DSS-treated Caco-2 cells, accompanied by impairment of membrane and protein trafficking resulting in altered membrane integrity, cellular polarity, and hence disrupted barrier function.

Different Niemann-Pick C1 Genotypes Generate Protein Phenotypes that Vary in their Intracellular Processing, Trafficking and Localization

Niemann-Pick Type C (NP-C) is an inherited neurovisceral lysosomal storage disease characterized by a defect in the trafficking of endocytosed cholesterol. In 95% of patients the gene encoding NPC1 is affected. The correlation of the genetic background in NP-C with the clinical phenotype such as, severity and onset of liver dysfunction, ataxia, dystonia and vertical gaze palsy, has not been elucidated at the molecular level. We have designed strategies to investigate the effect of different mutations in the NPC1 gene at the protein and cellular levels. The NPC1 mutants were expressed in mammalian cells and their structural features, maturation pathways and subcellular localization elucidated. Interestingly, three classes of NPC1 mutants could be identified and further characterized. The first group comprised mutants in which the NPC1 protein revealed virtually similar structural features to the wild type species. It was trafficked to the lysosomes and colocalized with the lysosomal protein marker Lamp2. The second class of NPC1 mutants was only partially trafficked to the lysosomes, but predominantly localized to the endoplasmic reticulum (ER). In the third group with the most severe phenotype, NPC1 mutants were entirely retained in the ER, colocalizing with the ER-protein marker calnexin. In conclusion, this study relates NPC1 mutations to the trafficking behavior of the NPC1 mutants along the secretory pathway. The findings are essential for a comprehensive understanding of the pathogenesis of NP-C and propose a mutation-based personalized therapeutical approach.

Segregation in the Golgi complex precedes export of endolysosomal proteins in distinct transport carriers

Biosynthetic sorting of newly synthesized transmembrane cargos to endosomes and lysosomes is thought to occur at the TGN through recognition of sorting signals in the cytosolic tails of the cargos by adaptor proteins, leading to cargo packaging into coated vesicles destined for the endolysosomal system. Here we present evidence for a different mechanism in which two sets of endolysosomal proteins undergo early segregation to distinct domains of the Golgi complex by virtue of the proteins' luminal and transmembrane domains. Proteins in one Golgi domain exit into predominantly vesicular carriers by interaction of sorting signals with adaptor proteins, but proteins in the other domain exit into predominantly tubular carriers shared with plasma membrane proteins, independently of signal-adaptor interactions. These findings demonstrate that sorting of endolysosomal proteins begins at an earlier stage and involves mechanisms that partly differ from those described by classical models.

Establishing Structure Property Relationship in Drug Partitioning into and Release from Niosomes: Physical Chemistry Insights with Anti-Inflammatory Drugs

Understanding the physical chemistry underlying interactions of drugs with delivery formulations is extremely important in devising effective drug delivery systems. The partitioning and release kinetics of diclofenac sodium and naproxen from Brij 30 and Triton X-100 niosomal formulations have been addressed based on structural characterization, partitioning energetics, and release kinetics, thus establishing a relationship between structures and observed properties. Both the drugs partition in nonpolar regions of TX-100 niosomes via stacking of aromatic rings. The combined effects of interactions of the drugs with polar head groups and the rigidity of the niosome vesicles determine entry and partitioning of drugs into niosomes. The observed slower rate of release of the drugs from the drug encapsulated niosomes of TX-100 than those of Brij 30, suggest stable complexation of drugs in the nonpolar interior of the former. No release of drugs from the niosomes was observed until 24 h even upon varying pH conditions without SDS. However, SDS in drug loaded niosomes led to release of drugs in as early as 6 h. The sustained pattern of in vitro release kinetics of the drugs thus observed from our niosomal preparations suggest these vesicular systems to be promising for pharamaceutical applications as potential drug delivery vehicles.

Local expression of AP/AngIV/IRAP and effect of AngIV on glucose-induced epithelial transport in human jejunal mucosa

BACKGROUND

Recently it was shown that the classic renin-angiotensin system (RAS) is locally expressed in small intestinal enterocytes and exerts autocrine control of glucose transport. The aim of this study was to investigate if key components for the Angiotensin III (AngIII) and IV (AngIV) formation enzymes and the AngIV receptor, insulin-regulated aminopeptidase (IRAP), are present in the healthy jejunal mucosa. A second aim was to investigate AngIV effects on glucose-induced mucosal transport in vitro.

MATERIAL AND METHODS

Enteroscopy with mucosal biopsy sampling was performed in healthy volunteers. ELISA, Western blotting and immunohistochemistry were used to assess the protein levels and localization. The functional effect of AngIV was examined in Ussing chambers.

RESULTS

The substrate Angiotensin II, the enzymes aminopeptidases-A, B, M as well as IRAP were detected in the jejunal mucosa. Immunohistochemistry localized the enzymes to the apical brush-border membrane whereas IRAP was localized in the subapical cytosolic compartment in the enterocyte. AngIV increased the glucose-induced electrogenic transport in vitro.

CONCLUSION

The present study indicates the presence of substrates and enzymes necessary for AngIV formation as well as the receptor IRAP in the jejunal mucosa. The functional data suggest that AngIV regulates glucose uptake in the healthy human small intestine.

The membrane-associated form of α(s1)-casein interacts with cholesterol-rich detergent-resistant microdomains

Caseins, the main milk proteins, interact with colloidal calcium phosphate to form the casein micelle. The mesostructure of this supramolecular assembly markedly influences its nutritional and technological functionalities. However, its detailed molecular organization and the cellular mechanisms involved in its biogenesis have been only partially established. There is a growing body of evidence to support the concept that α(s1)-casein takes center stage in casein micelle building and transport in the secretory pathway of mammary epithelial cells. Here we have investigated the membrane-associated form of α(s1)-casein in rat mammary epithelial cells. Using metabolic labelling we show that α(s1)-casein becomes associated with membranes at the level of the endoplasmic reticulum, with no subsequent increase at the level of the Golgi apparatus. From morphological and biochemical data, it appears that caseins are in a tight relationship with membranes throughout the secretory pathway. On the other hand, we have observed that the membrane-associated form of α(s1)-casein co-purified with detergent-resistant membranes. It was poorly solubilised by Tween 20, partially insoluble in Lubrol WX, and substantially insoluble in Triton X-100. Finally, we found that cholesterol depletion results in the release of the membrane-associated form of α(s1)-casein. These experiments reveal that the insolubility of α(s1)-casein reflects its partial association with a cholesterol-rich detergent-resistant microdomain. We propose that the membrane-associated form of α(s1)-casein interacts with the lipid microdomain, or lipid raft, that forms within the membranes of the endoplasmic reticulum, for efficient forward transport and sorting in the secretory pathway of mammary epithelial cells.

Coalescence of B cell receptor and invariant chain MHC II in a raft-like membrane domain

The BCR binds antigen for processing and subsequent presentation on MHC II molecules. Polyvalent antigen induces BCR clustering and targeting to endocytic processing compartments, which are also accessed by Ii-MHC II. Here, we report that clustered BCR is able to team up with Ii-MHC II already at the plasma membrane of mouse B-lymphocytes. Colocalization of BCR and Ii-MHC II on the cell surface required clustering of both types of molecules. The clustering of only one type did not trigger the recruitment of the other. Ii-bound MIF (a ligand of Ii) also colocalized with clustered BCR upon oligomerization of MIF on the surface of the B cell. Abundant surface molecules, such as B220 or TfnR, did not cocluster with the BCR. Some membrane raft-associated molecules, such as peptide-loaded MHC II, coclustered with the BCR, whereas others, such as GM1, did not. The formation of a BCR- and Ii-MHC II-containing membrane domain by antibody-mediated clustering was independent of F-actin and led to the coendocytosis of its constituents. With a rapid Brij 98 extraction method, it was possible to capture this membrane domain biochemically as a DRM. Ii and clustered BCR were present on the same DRM, as shown by immunoisolation. The coalescence of BCR and Ii-MHC II increased tyrosine phosphorylation, indicative of enhanced BCR signaling. Our work suggests a novel role for MIF and Ii-MHC II in BCR-mediated antigen processing.

Trafficking to the apical and basolateral membranes in polarized epithelial cells

Renal epithelial cells must maintain distinct protein compositions in their apical and basolateral membranes in order to perform their transport functions. The creation of these polarized protein distributions depends on sorting signals that designate the trafficking route and site of ultimate functional residence for each protein. Segregation of newly synthesized apical and basolateral proteins into distinct carrier vesicles can occur at the trans-Golgi network, recycling endosomes, or a growing assortment of stations along the cellular trafficking pathway. The nature of the specific sorting signal and the mechanism through which it is interpreted can influence the route a protein takes through the cell. Cell type-specific variations in the targeting motifs of a protein, as are evident for Na,K-ATPase, demonstrate a remarkable capacity to adapt sorting pathways to different developmental states or physiologic requirements. This review summarizes our current understanding of apical and basolateral trafficking routes in polarized epithelial cells.

Isolation and analysis of membrane lipids and lipid rafts in common carp (Cyprinus carpio L.)

Cell membranes act as an interface between the interior of the cell and the exterior environment and facilitate a range of essential functions including cell signalling, cell structure, nutrient uptake and protection. It is composed of a lipid bilayer with integrated proteins, and the inner leaflet of the lipid bilayer comprises of liquid ordered (Lo) and liquid disordered (Ld) domains. Lo microdomains, also named as lipid rafts are enriched in cholesterol, sphingomyelin and certain types of proteins, which facilitate cell signalling and nutrient uptake. Lipid rafts have been extensively researched in mammals and the presence of functional lipid rafts was recently demonstrated in goldfish, but there is currently very little knowledge about their composition and function in fish. Therefore a protocol was established for the analysis of lipid rafts and membranous lipids in common carp (Cyprinus carpio L.) tissues. Twelve lipids were identified and analysed in the Ld domain of the membrane with the most predominant lipids found in all tissues being; triglycerides, cholesterol, phosphoethanolamine and phosphatidylcholine. Four lipids were identified in lipid rafts in all tissues analysed, triglycerides (33-62%) always found in the highest concentration followed by cholesterol (24-32%), phosphatidylcholine and sphingomyelin. Isolation of lipid rafts was confirmed by identifying the presence of the lipid raft associated protein flotillin, present at higher concentrations in the detergent resistant fraction. The data provided here build a lipid library of important carp tissues as a baseline for further studies into virus entry, protein trafficking or environmental stress analysis.

Purification of hepatitis B surface antigen virus-like particles from recombinant Pichia pastoris and in vivo analysis of their immunogenic properties

Following earlier studies on high-level intracellular production of hepatitis B surface antigen (HBsAg) using recombinant Pichia pastoris, we present here in detail an enhanced method for the purification of recombinant HBsAg virus-like particles (VLPs). We have screened various detergents for their ability to promote the solubilization of recombinant intracellular HBsAg. In addition, we have analyzed the effect of cell disruption and extraction regarding their impact on the release of HBsAg. Our results show that introduction of the mild nonionic detergent Tween 20 in the initial process of cell lysis at ∼600bars by high pressure homogenization leads to the best results. The subsequent purification steps involved polyethylene glycol precipitation of host cell contaminants, hydrophobic adsorption of HBsAg to colloidal silica followed by ion-exchange chromatography and either isopycnic density ultracentrifugation or size exclusion chromatography for the recovery of the VLPs. After final KSCN treatment and dialysis, a total yield of ∼3% with a purity of >99% was reached. The pure protein was characterized by electron microscopy, showing the presence of uniform VLPs which are the pre-requisite for immunogenicity. The intramuscular co-administration of HBsAg VLPs, with either alum or a PEGylated-derivative of the toll-like receptor 2/6 agonist MALP-2, to mice resulted in the elicitation of significantly higher HBsAg-specific IgG titers as well as a stronger cellular immune response compared to mice vaccinated with a gold standard vaccine (Engerix™). These results show that P. pastoris derived HBsAg VLPs exhibit a high potential as a superior biosimilar vaccine against hepatitis B.

Lipids of plant membrane rafts

Lipids tend to organize in mono or bilayer phases in a hydrophilic environment. While they have long been thought to be incapable of coherent lateral segregation, it is now clear that spontaneous assembly of these compounds can confer microdomain organization beyond spontaneous fluidity. Membrane raft microdomains have the ability to influence spatiotemporal organization of protein complexes, thereby allowing regulation of cellular processes. In this review, we aim at summarizing briefly: (i) the history of raft discovery in animals and plants, (ii) the main findings about structural and signalling plant lipids involved in raft segregation, (iii) imaging of plant membrane domains, and their biochemical purification through detergent-insoluble membranes, as well as the existing debate on the topic. We also discuss the potential involvement of rafts in the regulation of plant physiological processes, and further discuss the prospects of future research into plant membrane rafts.

Potential of soluble CD26 as a serum marker for colorectal cancer detection

Colorectal cancer is characterized by a low survival rate even though the basis for colon cancer development, which involves the evolution of adenomas to carcinoma, is known. Moreover, the mortality rates continue to rise in economically transitioning countries although there is the opportunity to intervene in the natural history of the adenoma–cancer sequence through risk factors, screening, and treatment. Screening in particular accounted for most of the decline in colorectal cancer mortality achieved in the USA during the period 1975-2000. Patients show a better prognosis when the neoplasm is diagnosed early. Among the variety of screening strategies, the methods range from invasive and costly procedures such as colonoscopy to more low-cost and non-invasive tests such as the fecal occult blood test (guaiac and immunochemical). As a non-invasive biological serum marker would be of great benefit because of the performance of the test, several biomarkers, including cytologic assays, DNA and mRNA, and soluble proteins, have been studied. We found that the soluble CD26 (sCD26) concentration is diminished in serum of colorectal cancer patients compared to healthy donors, suggesting the potential utility of a sCD26 immunochemical detection test for early diagnosis. sCD26 originates from plasma membrane CD26 lacking its transmembrane and cytoplasmic domains. Some 90%–95% of sCD26 has been associated with serum dipeptidyl peptidase IV (DPP-IV) activity. DPP-IV, assigned to the CD26 cluster, is a pleiotropic enzyme expressed mainly on epithelial cells and lymphocytes. Our studies intended to validate this test for population screening to detect colorectal cancer and advanced adenomas are reviewed here.

AKAPs in lipid rafts are required for optimal antigen presentation by dendritic cells

Dendritic cell (DC) maturation and antigen presentation are regulated by activation of protein kinase A (PKA) signaling pathways, through unknown mechanisms. We have recently shown that interfering with PKA signaling through the use of anchoring inhibitor peptides hinders antigen presentation and DC maturation. These experiments provide evidence that DC maturation and antigen presentation are regulated by A-kinase anchoring proteins (AKAPs). Herein, we determine that the presence of AKAPs and PKA in lipid rafts regulates antigen presentation. Using a combination of western blotting and immuno-cytochemistry, we illustrate the presence of AKAP149, AKAP79, Ezrin and the regulatory subunits of PKA in DC lipid rafts. Incubation of DCs with the type II anchoring inhibitor, AKAP-in silico (AKAP-IS), removes Ezrin and RII from the lipid raft without disrupting raft formation. Addition of a lipid raft disruptor, methyl-β-cyclodextrin, blocks the efficacy of AKAP-IS, suggesting that the lipid raft must be intact for AKAP-IS to inhibit antigen presentation. Ezrin and AKAP79 are present in the lipid raft of stimulated KG1 cells, but Ezrin is not present in the lipid raft of unstimulated KG1 cells and AKAP79 levels are greatly diminished, suggesting that Ezrin and AKAP79 may be the key AKAPs responsible for regulating antigen presentation.

Membrane raft association of the Vpu protein of human immunodeficiency virus type 1 correlates with enhanced virus release

The Vpu protein of human immunodeficiency virus type 1 (HIV-1) is known to enhance virion release from certain cell types. To accomplish this function, Vpu interacts with the restriction factor known as bone marrow stromal cell antigen 2 (BST-2)/tetherin. In this study, we analyzed whether the Vpu protein is associated with microdomains known as lipid or membrane rafts. Our results indicate that Vpu partially partitions into detergent-resistant membrane (DRM) fractions when expressed alone or in the context of simian-human immunodeficiency virus (SHIV) infection. The ability to be partitioned into rafts was observed with both subtype B and C Vpu proteins. The use of cholesterol lowering lovastatin/M-β-cyclodextrin and co-patching experiments confirmed that Vpu can be detected in cholesterol rich regions of membranes. Finally, we present data showing that raft association-defective transmembrane mutants of Vpu have impaired enhanced virus release function, but still maintain the ability to down-regulate CD4.

A modified lipid composition in Fabry disease leads to an intracellular block of the detergent-resistant membrane-associated dipeptidyl peptidase IV

Fabry disease is an X-linked lysosomal storage disorder that leads to abnormal accumulation of glycosphingolipids due to a deficiency of alpha-galactosidase A (AGAL). The consequences of these alterations on the targeting of membrane proteins are poorly understood. Glycosphingolipids are enriched in Triton-X-100- resistant lipid rafts [detergent-resistant membranes (DRMs)] and play an important role in the transport of several membrane-associated proteins. Here, we show that In fibroblasts of patients suffering from Fabry disease, the colocalization of AGAL with the lysosomal marker LAMP2 is decreased compared with wild-type fibroblasts concomitant with a reduced transport of AGAL to lysosomes. Furthermore, overall composition of membrane lipids in the patients' fibroblasts as well as in DRMs reveals a substantial increase in the concentration of glycolipids and a slight reduction of phosphatidylethanolamine (PE). The altered glycolipid composition in Fabry fibroblasts is associated with an intracellular accumulation and impaired trafficking of the Triton-X-100 DRM-associated membrane glycoprotein dipeptidyl peptidase IV (DPPIV) in transfected Fabry cells, whereas no effect could be observed on the targeting of aminopeptidase N (ApN) that is not associated with this type of DRM. We propose that changes in the lipid composition of cell membranes in Fabry disease disturb the ordered Triton X-100 DRMs and have implications on the trafficking and sorting of DRM-associated proteins and the overall protein-lipid interaction at the cell membrane. Possible consequences could be altered signalling at the cell surface triggered by DRM-associated proteins, with implications on gene regulation and subsequent protein expression.

Rafting trips into the cell

Lipid rafts are small, heterogeneous and short-lived assemblies of cholesterol, sphingolipids and few proteins in biological membranes. They can be converted to larger and more permanent membrane domains by coalescence. Cells appear to be able to modulate the size and the longevity of lipid rafts and thus exploit the local enrichment of membrane components for processes ranging from signaling to intracellular sorting and transport. In a recent paper, we provided evidence for the internalization of MHC I and MHC II along two distinct endocytosis pathways in mouse B-lymphocytes. Both pathways were much more dependent on membrane cholesterol than the clathrin-mediated uptake of transferrin receptor, which implicated lipid rafts in the internalization of MHC molecules. Indeed, MHC I and MHC II prefer distinct raft-like membrane environments as revealed by a co-clustering analysis with the sphingolipids G(M)1 and G(M)2. Moreover, MHC I and MHC II distributed to different types of detergent resistant membranes (DRMs) prepared by a novel detergent extraction procedure. In this article addendum we discuss the relationship between DRMs, small lipid rafts and stabilized rafts/membrane domains and propose a role for membrane domains in the endocytosis of MHC proteins.

Biochemical basis of Fabry disease with emphasis on mitochondrial function and protein trafficking

Fabry disease, also known as Anderson-Fabry disease, is an X-linked lysosomal storage disorder. The clinical picture is highly variable and usually milder in females. It is a multisystemic disease involving many organs. Fabry disease is due to a deficiency of alpha-galactosidase A caused by different usually "private" mutations. Enzyme replacement therapy (ERT) has been established, other therapeutic options are at an experimental stage. Classically, mechanical deposition of storage material in blood vessels was believed to lead to decreased blood supply with consecutive organ dysfunction. Recently, however, many secondary biochemical processes have been discussed to be involved in the pathogenesis of Fabry disease. For example, compromised energy metabolism has been found both in vitro and in vivo, altered lipid composition of membranes can lead to abnormalities in trafficking and sorting of rafts-associated proteins. We discuss the role of these secondary phenomena in the pathogenesis of Fabry disease.

On the origin of serum CD26 and its altered concentration in cancer patients

Dipeptidyl peptidase IV (DPP-IV), assigned to the CD26 cluster, is expressed on epithelial cells and lymphocytes and is a multifunctional or pleiotropic protein. Its peptidase activity causes degradation of many biologically active peptides, e.g. some incretins secreted by the enteroendocrine system. DPP-IV has, therefore, become a novel therapeutic target for inhibitors that extend endogenously produced insulin half-life in diabetics, and several reviews have appeared in recent months concerning the clinical significance of CD26/DPP-IV. Biological fluids contain relatively high levels of soluble CD26 (sCD26). The physiological role of sCD26 and its relation, if any, to CD26 functions, remain poorly understood because whether the process for CD26 secretion and/or shedding from cell membranes is regulated or not is not known. Liver epithelium and lymphocytes are often cited as the most likely source of sCD26. It is important to establish which tissue or organ is the protein source as well as the circumstances that can provoke an abnormal presence/absence or altered levels in many diseases including cancer, so that sCD26 can be validated as a clinical marker or a therapeutic target. For example, we have previously reported low levels of sCD26 in the blood of colorectal cancer patients, which indicated the potential usefulness of the protein as a biomarker for this cancer in early diagnosis, monitoring and prognosis. Through this review, we envisage a role for sCD26 and the alteration of normal peptidase capacity (in clipping enteroendocrine or other peptides) in the complex crosstalk between the lymphoid lineage and, at least, some malignant tumours.

A secretory Golgi bypass route to the apical surface domain of epithelial MDCK cells

Proteins leave the endoplasmic reticulum (ER) for the plasma membrane via the classical secretory pathway, but routes bypassing the Golgi apparatus have also been observed. Apical and basolateral protein secretion in epithelial Madin-Darby canine kidney (MDCK) cells display differential sensitivity to Brefeldin A (BFA), where low concentrations retard apical transport, while basolateral transport still proceeds through intact Golgi cisternae. We now describe that BFA-mediated retardation of glycoprotein and proteoglycan transport through the Golgi apparatus induces surface transport of molecules lacking Golgi modifications, possessing those acquired in the ER. Low concentrations of BFA induces apical Golgi bypass, while higher concentrations were required to induce basolateral Golgi bypass. Addition of the KDEL ER-retrieval sequence to model protein cores allowed observation of apical Golgi bypass in untreated MDCK cells. Basolateral Golgi bypass was only observed after the addition of BFA or upon cholesterol depletion. Thus, in MDCK cells, an apical Golgi bypass route can transport cargo from pre-Golgi organelles in untreated cells, while the basolateral bypass route is inducible.

Domains in biological membranes

Our current view on biological membranes has gradually evolved from the influential fluid mosaic model of the early 1970s to a distinctively more complex picture. Biological membranes are now assumed to encompass multiple membrane domains and a plethora of protein-lipid and protein-protein interactions that compartmentalize and temporarily order what has originally been envisioned to be mostly random. In this minireview, we will first highlight some structural principles that govern membrane domain formation and permit a classification of membrane domains. We will then focus on the still controversial issue of lipid-based membrane domains, or lipid rafts, and discuss recent advances in detecting these enigmatic structures in living cells. Finally we will evaluate biochemical approaches to characterize lipid rafts and discuss their contribution to the emerging topic of lipid raft diversity.

Impact of glycosylation and detergent-resistant membranes on the function of intestinal sucrase-isomaltase

Sucrase-isomaltase (SI) is a highly N- and O-glycosylated intestinal brush border membrane protein. SI is sorted with high fidelity to the apical membrane via O-linked glycans that mediate its association with lipid rafts or detergent-resistant membranes (DRMs). Here, we show that N- and O-glycosylation and DRMs are implicated in the regulation of the function of SI in intestinal Caco-2 cells. The activities of sucrase (SUC) and isomaltase (IM) increase substantially in DRMs when N- and O-glycosylation patterns are intact. Disruption of DRMs by solubilization with Triton X-100 at 37°C substantially reduces the activities of SUC and IM. Furthermore, modulation of O-glycosylation by benzyl-2-acetamido-2-deoxy-α-d-galactopyranoside and N-glycosylation by deoxymannojirimycin is linked to a decreased capacity of SI to associate with DRMs, with a subsequent reduction of the enzymatic activities of SUC and IM. This is the first report that reveals a direct role of N- and O-glycans in association with DRMs in regulating the function of a membrane glycoprotein.

Endocytosis of MHC molecules by distinct membrane rafts

In B-lymphocytes, endocytosis of MHC I and MHC II molecules is important for the cross-priming and presentation of labile antigens, respectively. Here, we report that MHC I and MHC II were internalized by separate endocytic carriers that lacked transferrin receptor. Cholera toxin B was co-internalized with MHC II, but not with MHC I, suggesting that the CLIC/GEEC pathway is involved in the uptake of MHC II. Endocytosis of MHC I and MHC II was inhibited by filipin, but only MHC II showed a strong preference for a membrane raft environment in a co-clustering analysis with G(M)1. By using a novel method for the extraction of detergent-resistant membranes (DRMs), we observed that MHC I and MHC II associate with two distinct types of DRMs. These differ in density, protein content, lipid composition, and ultrastructure. The results of cell surface biotinylation and subsequent DRM isolation show that precursors for both DRMs coexist in the plasma membrane. Moreover, clustering of MHC proteins at the cell surface resulted in shifts of the respective DRMs, revealing proximity-induced changes in the membrane environment. Our results suggest that the preference of MHC I and MHC II for distinct membrane rafts directs them to different cellular entry points.

Neutralization of endomembrane compartments in epithelial MDCK cells affects proteoglycan synthesis in the apical secretory pathway

PGs (proteoglycans) are proteins acquiring long, linear and sulfated GAG (glycosaminoglycan) chains during Golgi passage. In MDCK cells (Madin–Darby canine kidney cells), most of the CS (chondroitin sulfate) PGs are secreted apically, whereas most of the HS (heparan sulfate) PGs are secreted basolaterally. The apical and basolateral secretory routes differ in their GAG synthesis, since a protein core that traverses both routes acquires shorter chains, but more sulfate, in the basolateral pathway than in the apical counterpart [Tveit, Dick, Skibeli and Prydz (2005) J. Biol. Chem. 280, 29596–29603]. Golgi cisternae and the trans-Golgi network have slightly acidic lumens. We therefore investigated how neutralization of endomembrane compartments with the vacuolar H+-ATPase inhibitor Baf A1 (bafilomycin A1) affected GAG synthesis and PG sorting in MDCK cells. Baf A1 induced a slight reduction in basolateral secretion of macromolecules, which was compensated by an apical increase. More dramatic changes occurred to PG synthesis in the apical pathway on neutralization. The difference in apical and basolateral PG sulfation levels observed for control cells was abolished, due to enhanced sulfation of apical CS-GAGs. In addition, a large fraction of apical HS-GAGs was elongated to longer chain lengths. The differential sensitivity of the apical and basolateral secretory pathways to Baf A1 indicates that the apical pathway is more acidic than the basolateral counterpart in untreated MDCK cells. Neutralization gave an apical GAG output that was more similar to that of the basolateral pathway, suggesting that neutralization made the luminal environments of the two pathways more similar.

Differential sorting and Golgi export requirements for raft-associated and raft-independent apical proteins along the biosynthetic pathway

Sorting signals for apically destined proteins are highly diverse and can be present within the luminal, membrane-associated, and cytoplasmic domains of these proteins. A subset of apical proteins partition into detergent-resistant membranes, and the association of these proteins with glycolipid-enriched microdomains or lipid rafts may be important for their proper targeting. Recently, we observed that raft-associated and raft-independent apical proteins take different routes to the apical surface of polarized Madin-Darby canine kidney cells (Cresawn, K. O., Potter, B. A., Oztan, A., Guerriero, C. J., Ihrke, G., Goldenring, J. R., Apodaca, G., and Weisz, O. A. (2007) EMBO J. 26, 3737-3748). Here we reconstituted in vitro the export of raft-associated and raft-independent markers staged intracellularly at 19 degrees C. Surprisingly, whereas release of the raft-associated protein influenza hemagglutinin was dependent on the addition of an ATP-regenerating system and cytosol, release of a yellow fluorescent protein (YFP)-tagged raft-independent protein (the 75-kDa neurotrophin receptor; YFP-p75) was efficient even in the absence of these constituents. Subsequent studies suggested that YFP-p75 is released from the trans-Golgi network in fragile tubules that do not withstand isolation procedures. Moreover, immunofluorescence analysis revealed that hemagglutinin and YFP-p75 segregate into distinct subdomains of the Golgi complex at 19 degrees C. Our data suggest that raft-associated and raft-independent proteins accumulate at distinct intracellular sites upon low temperature staging, and that upon warming, they exit these compartments in transport carriers that have very different membrane characteristics and morphologies.

Different glycoforms of prostate-specific membrane antigen are intracellularly transported through their association with distinct detergent-resistant membranes

Hormone-refractory prostate carcinomas as well as the neovasculature of different tumours express high levels of PSMA (prostate-specific membrane antigen). PSMA is a type II-transmembrane glycoprotein and a potential tumour marker for both diagnosis and passive immunotherapy. Here, we report on the association of PSMA with DRMs (detergent-resistant membranes) at different stages of the protein maturation pathway in human prostate carcinoma LNCaP cells. At least three PSMA glycoforms were biochemically identified based on their extractability behaviour in different non-ionic detergents. In particular, one precursor glycoform of PSMA is associated with Tween 20-insoluble DRMs, whereas the complex glycosylated protein segregates into membrane structures that are insoluble in Lubrol WX and display a different lipid composition. Association of PSMA with these membranes occurs in the Golgi compartment together with the acquisition of a native conformation. PSMA homodimers reach the plasma membrane of LNCaP cells in Lubrol WX-insoluble lipid/protein complexes. At the steady state, the majority of PSMA remains within these membrane microdomains at the cell surface. We conclude that the intracellular transport of PSMA occurs through populations of DRMs distinct for each biosynthetic form and cellular compartment.

Detergent resistance as a tool in membrane research

The biological membrane is a complicated matrix wherein different lipid environments are thought to exist. The more ordered or raft environment has been perceived biochemically accessible via its relative resistance to detergent. This paper outlines the protocols developed in our laboratory for the analysis of such detergent-resistant membranes (DRMs). We stress the fact that DRMs are artifactual in nature and should not be equivocated to lipid rafts, their usefulness being limited to assigning raft-association potential most convincingly when changes in DRM composition are induced by biochemically/physiologically relevant events. These protocols are completed in 1-2 d.

Endoplasmic reticulum stress in the proapoptotic action of edelfosine in solid tumor cells

The endoplasmic reticulum (ER) has been posited as a potential anticancer target. The synthetic antitumor alkyl-lysophospholipid analogue edelfosine accumulates in the ER of solid tumor cells. This ER accumulation of the drug leads to the inhibition of phosphatidylcholine and protein synthesis, G(2)-M arrest, depletion of ER-stored Ca(2+), Bax up-regulation and activation, transcriptional factor growth arrest and DNA damage-inducible gene 153 up-regulation, caspase-4 and caspase-8 activation, and eventually to apoptosis. Edelfosine prompted ER stress apoptotic signaling, but not the survival unfolded protein response. Edelfosine also induced persistent c-Jun NH(2)-terminal kinase (JNK) activation. Gene transfer-mediated overexpression of apoptosis signal-regulating kinase 1, which plays a crucial role in ER stress, enhanced edelfosine-induced JNK activation and apoptosis. Inhibition of JNK, caspase-4, or caspase-8 activation diminished edelfosine-induced apoptosis. Edelfosine treatment led to the generation of the p20 caspase-8 cleavage fragment of BAP31, directing proapoptotic signals between the ER and the mitochondria. bax(-/-)bak(-/-) double-knockout cells fail to undergo edelfosine-induced ER-stored Ca(2+) release and apoptosis. Wild-type and bax(-/-)bak(-/-) cells showed similar patterns of phosphatidylcholine and protein synthesis inhibition, despite their differences in drug sensitivity. Thus, edelfosine-induced apoptosis is dependent on Bax/Bak-mediated ER-stored Ca(2+) release, but phosphatidylcholine and protein synthesis inhibition is not critical. Transfection-enforced expression of Bcl-X(L), which localizes specifically in mitochondria, prevented apoptosis without inhibiting ER-stored Ca(2+) release. These data reveal that edelfosine induces an ER stress response in solid tumor cells, providing novel insights into the edelfosine-mediated antitumor activity. Our data also indicate that mitochondria are indispensable for this edelfosine-induced cell death initiated by ER stress.

Visualization of detergent solubilization of membranes: implications for the isolation of rafts

Although different detergents can give rise to detergent-resistant membranes of different composition, it is unclear whether this represents domain heterogeneity in the original membrane. We compared the mechanism of action of five detergents on supported lipid bilayers composed of equimolar sphingomyelin, cholesterol, and dioleoylphosphatidylcholine imaged by atomic force microscopy, and on raft and nonraft marker proteins in live cells imaged by confocal microscopy. There was a marked correlation between the detergent solubilization of the cell membrane and that of the supported lipid bilayers. In both systems Triton X-100 and CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate) distinguished between the nonraft liquid-disordered (l(d)) and raft liquid ordered (l(o)) lipid phases by selectively solubilizing the l(d) phase. A higher concentration of Lubrol was required, and not all the l(d) phase was solubilized. The solubilization by Brij 96 occurred by a two-stage mechanism that initially resulted in the solubilization of some l(d) phase and then progressed to the solubilization of both l(d) and l(o) phases simultaneously. Octyl glucoside simultaneously solubilized both l(o) and l(d) phases. These data show that the mechanism of membrane solubilization is unique to an individual detergent. Our observations have significant implications for using different detergents to isolate membrane rafts from biological systems.

Differential detergent resistance of the apical and basolateral NPPases: relationship with polarized targeting

Targeting of glycosylphosphatidylinositol-anchored proteins to the apical surface of epithelial cells involves clustering in Triton X-100-resistant membrane microdomains or rafts. The role of these microdomains in sorting transmembrane proteins is more questionable because, unlike glycosylphosphatidylinositol-anchored proteins, apical transmembrane proteins are rather soluble in Triton X-100. They are, however, resistant to milder detergents such as Lubrol WX or Tween 20. It has been proposed that specific membrane microdomains, defined by resistance to these detergents, would carry transmembrane proteins to the apical surface. We have used MDCK cells stably transfected with the apical and basolateral pyrophosphatases/phosphodiesterases, NPP3 and NPP1, to examine the relationship between detergent resistance and apical targeting. The apically expressed wild-type NPP3 was insoluble in Lubrol WX whereas wild-type NPP1, which is expressed basolaterally, was essentially soluble. By using tail mutants and chimeric constructs that combine the cytoplasmic, transmembrane and extracellular domains of NPP1 and NPP3, we show that there is not a strict correlation between detergent resistance and apical targeting. Lubrol resistance is an intrinsic property of NPP3, which is acquired early during the biosynthetic process irrespective of its final destination, and depends on positively charged residues in its cytoplasmic tail.

Functional symmetry of endomembranes

In higher eukaryotic cells pleiomorphic compartments composed of vacuoles, tubules and vesicles move from the endoplasmic reticulum (ER) and the plasma membrane to the cell center, operating in early biosynthetic trafficking and endocytosis, respectively. Besides transporting cargo to the Golgi apparatus and lysosomes, a major task of these compartments is to promote extensive membrane recycling. The endocytic membrane system is traditionally divided into early (sorting) endosomes, late endosomes and the endocytic recycling compartment (ERC). Recent studies on the intermediate compartment (IC) between the ER and the Golgi apparatus suggest that it also consists of peripheral ("early") and centralized ("late") structures, as well as a third component, designated here as the biosynthetic recycling compartment (BRC). We propose that the ERC and the BRC exist as long-lived "mirror compartments" at the cell center that also share the ability to expand and become mobilized during cell activation. These considerations emphasize the functional symmetry of endomembrane compartments, which provides a basis for the membrane rearrangements taking place during cell division, polarization, and differentiation.

Polarized biosynthetic traffic in renal epithelial cells: sorting, sorting, everywhere

The maintenance of apical and basolateral membrane domains with distinct protein and lipid compositions is necessary for the proper function of polarized epithelial cells. Delivery of cargo to the basolateral surface is thought to be mediated by the interaction of cytoplasmically disposed sorting signals with sorting receptors, whereas apically destined cargoes are sorted via mechanisms dependent on cytoplasmic, glycan-mediated, or lipid-interacting sorting signals. Apical and basolateral cargo are delivered to the surface in discrete tubular and vesicular carriers that bud from the trans-Golgi network (TGN). While it has long been thought that the TGN is the primary compartment in which apical and basolateral cargoes are segregated, recent studies suggest that sorting may begin earlier along the biosynthetic pathway. Moreover, rather than being delivered directly from the TGN to the cell surface, at least a subset of biosynthetic cargo appears to transit recycling endosomes en route to the plasma membrane. The implications and limitations of these challenges to the conventional model for how proteins are sorted and trafficked along the biosynthetic pathway are discussed.

Differences in the apical and basolateral pathways for glycosaminoglycan biosynthesis in Madin–Darby canine kidney cells

Serglycin with a green fluorescent protein tag (SG-GFP) expressed in epithelial Madin-Darby canine kidney cells is secreted mainly (85%) into the apical medium, but the glycosaminoglycan (GAG) chains on the SG-GFP protein core secreted basolaterally (15%) carry most of the sulfate added during biosynthesis (Tveit et al. (2005) J. Biol. Chem., 280, 29596-29603). Here we report further differences in apical and basolateral GAG synthesis. The less intensely sulfated chondroitin sulfate (CS) chains on apically secreted SG-GFP are longer than CS chains attached to basolateral SG-GFP, whereas the heparan sulfate (HS) chains are of similar lengths. When the supply of 3'-phosphoadenosine-5'-phosphosulfate (PAPS) is limited by chlorate treatment, the synthesis machinery maintains sulfation of HS chains on basolateral SG-GFP until it is inhibited at 50 mM chlorate, whereas basolateral CS chains lose sulfate already at 12.5 mM chlorate and become longer. Apically, incorporation of 35S-sulfate into CS is reduced to a lesser extent at higher chlorate concentrations than basolateral CS, although apical CS is less intensely sulfated than basolateral CS in control cells. Similar to what was found for basolateral HS, sulfation of apical HS was not reduced at chlorate concentrations below 50 mM. Also, protein-free, xyloside-based GAG chains secreted basolaterally are more intensely sulfated than their apical counterpart, supporting the view that separate apical and basolateral pathways exist for GAG synthesis and sulfation. Introduction of benzyl beta-d-xyloside (BX) to the GAG synthesis machinery reduces the apical secretion of SG-GFP dramatically and also the modification of SG-GFP by HS.