Caveolin and MAL, two protein components of internal detergent-insoluble membranes, are in distinct lipid microenvironments in MDCK cells

Clostridium perfringens Epsilon Toxin Binds to and Kills Primary Human Lymphocytes

Clostridium perfringens epsilon toxin (ETX) is the third most lethal bacterial toxin and has been suggested to be an environmental trigger of multiple sclerosis, an immune-mediated disease of the human central nervous system. However, ETX cytotoxicity on primary human cells has not been investigated. In this article, we demonstrate that ETX preferentially binds to and kills human lymphocytes expressing increased levels of the myelin and lymphocyte protein MAL. Using flow cytometry, ETX binding was determined to be time and dose dependent and was highest for CD4+ cells, followed by CD8+ and then CD19+ cells. Similar results were seen with ETX-induced cytotoxicity. To determine if ETX preference for CD4+ cells was related to MAL expression, MAL gene expression was determined by RT-qPCR. CD4+ cells had the highest amount of Mal gene expression followed by CD8+ and CD19+ cells. These data indicate that primary human cells are susceptible to ETX and support the hypothesis that MAL is a main receptor for ETX. Interestingly, ETX bindings to human lymphocytes suggest that ETX may influence immune response in multiple sclerosis.

CNS endothelial derived extracellular vesicles are biomarkers of active disease in multiple sclerosis

Background

Multiple sclerosis (MS) is a complex, heterogenous disease characterized by inflammation, demyelination, and blood–brain barrier (BBB) permeability. Currently, active disease is determined by physician confirmed relapse or detection of contrast enhancing lesions via MRI indicative of BBB permeability. However, clinical confirmation of active disease can be cumbersome. As such, disease monitoring in MS could benefit from identification of an easily accessible biomarker of active disease. We believe extracellular vesicles (EV) isolated from plasma are excellent candidates to fulfill this need. Because of the critical role BBB permeability plays in MS pathogenesis and identification of active disease, we sought to identify EV originating from central nervous system (CNS) endothelial as biomarkers of active MS. Because endothelial cells secrete more EV when stimulated or injured, we hypothesized that circulating concentrations of CNS endothelial derived EV will be increased in MS patients with active disease.

Methods

To test this, we developed a novel method to identify EV originating from CNS endothelial cells isolated from patient plasma using flow cytometry. Endothelial derived EV were identified by the absence of lymphocyte or platelet markers CD3 and CD41, respectively, and positive expression of pan-endothelial markers CD31, CD105, or CD144. To determine if endothelial derived EV originated from CNS endothelial cells, EV expressing CD31, CD105, or CD144 were evaluated for expression of the myelin and lymphocyte protein MAL, a protein specifically expressed by CNS endothelial cells compared to endothelial cells of peripheral organs.

Results

Quality control experiments indicate that EV detected using our flow cytometry method are 0.2 to 1 micron in size. Flow cytometry analysis of EV isolated from 20 healthy controls, 16 relapsing–remitting MS (RRMS) patients with active disease not receiving disease modifying therapy, 14 RRMS patients with stable disease not receiving disease modifying therapy, 17 relapsing-RRMS patients with stable disease receiving natalizumab, and 14 RRMS patients with stable disease receiving ocrelizumab revealed a significant increase in the plasma concentration of CNS endothelial derived EV in patients with active disease compared to all other groups (p = 0.001). Conclusions: For the first time, we have identified a method to identify CNS endothelial derived EV in circulation from human blood samples. Results from our pilot study indicate that increased levels of CNS endothelial derived EV may be a biomarker of BBB permeability and active disease in MS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-021-00299-4.

The MAL Protein, an Integral Component of Specialized Membranes, in Normal Cells and Cancer

The MAL gene encodes a 17-kDa protein containing four putative transmembrane segments whose expression is restricted to human T cells, polarized epithelial cells and myelin-forming cells. The MAL protein has two unusual biochemical features. First, it has lipid-like properties that qualify it as a member of the group of proteolipid proteins. Second, it partitions selectively into detergent-insoluble membranes, which are known to be enriched in condensed cell membranes, consistent with MAL being distributed in highly ordered membranes in the cell. Since its original description more than thirty years ago, a large body of evidence has accumulated supporting a role of MAL in specialized membranes in all the cell types in which it is expressed. Here, we review the structure, expression and biochemical characteristics of MAL, and discuss the association of MAL with raft membranes and the function of MAL in polarized epithelial cells, T lymphocytes, and myelin-forming cells. The evidence that MAL is a putative receptor of the epsilon toxin of Clostridium perfringens, the expression of MAL in lymphomas, the hypermethylation of the MAL gene and subsequent loss of MAL expression in carcinomas are also presented. We propose a model of MAL as the organizer of specialized condensed membranes to make them functional, discuss the role of MAL as a tumor suppressor in carcinomas, consider its potential use as a cancer biomarker, and summarize the directions for future research.

Lung endothelial cells are sensitive to epsilon toxin from Clostridium perfringens

The pore-forming protein epsilon toxin (Etx) from Clostridium perfringens produces acute perivascular edema affecting several organs, especially the brain and lungs. Despite the toxin evident effect on microvasculature and endothelial cells, the underlying molecular and cellular mechanisms remain obscure. Moreover, no Etx-sensitive endothelial cell model has been identified to date. Here, we characterize the mouse lung endothelial cell line 1G11 as an Etx-sensitive cell line and compare it with the well-characterized Etx-sensitive Madin-Darby canine kidney epithelial cell line. Several experimental approaches, including morphological and cytotoxic assays, clearly demonstrate that the 1G11 cell line is highly sensitive to Etx and show the specific binding, oligomerization, and pore-forming activity of the toxin in these cells. Recently, the myelin and lymphocyte (MAL) protein has been postulated as a putative receptor for Etx. Here, we show the presence of Mal mRNA in the 1G11 cell line and the presence of the MAL protein in the endothelium of some mouse lung vessels, supporting the hypothesis that this protein is a key element in the Etx intoxication pathway. The existence of an Etx-sensitive cell line of endothelial origin would help shed light on the cellular and molecular mechanisms underlying Etx-induced edema and its consequences.

Regulating ENaC's gate

Epithelial Na⁺ channels (ENaCs) are members of a family of cation channels that function as sensors of the extracellular environment. ENaCs are activated by specific proteases in the biosynthetic pathway and at the cell surface and remove embedded inhibitory tracts, which allows channels to transition to higher open-probability states. Resolved structures of ENaC and an acid-sensing ion channel revealed highly organized extracellular regions. Within the periphery of ENaC subunits are unique domains formed by antiparallel β-strands containing the inhibitory tracts and protease cleavage sites. ENaCs are inhibited by Na⁺ binding to specific extracellular site(s), which promotes channel transition to a lower open-probability state. Specific inositol phospholipids and channel modification by Cys-palmitoylation enhance channel open probability. How these regulatory factors interact in a concerted manner to influence channel open probability is an important question that has not been resolved. These various factors are reviewed, and the impact of specific factors on human disorders is discussed.

On The Role of Myelin and Lymphocyte Protein (MAL) In Cancer: A Puzzle With Two Faces

Myelin and lymphocyte protein (MAL) is an integral membrane protein constituent of lipid rafts, and it is implicated in apical transport of proteins in polarized epithelial cells. However, beyond the involvement of MAL in apical sorting and as its function as a raft stabilizer, it is still not totally clear how MAL participates in cell proliferating processes. More controversial and interesting is the fact that MAL has been implicated in carcinogenesis in two opposite ways. First, this protein is overexpressed in ovarian cancer and some kinds of lymphomas where it seems to favor cancer progression. Conversely, it has been reported that downregulation of the MAL gene by promoter hypermethylation is a hallmark of several adenocarcinomas. So far, there is not enough experimental evidence to help us understand this phenomenon, and no MAL mutations or MAL isoforms have been associated with these opposite functions. This review provides an updated summary of the structure and functions of MAL, and we will discuss the possible mechanisms underlying its roles as a tumor suppressor and a tumor progression factor.

Epsilon toxin from Clostridium perfringens induces cytotoxicity in FRT thyroid epithelial cells

Epsilon toxin (Etx) is produced by Clostridium perfringens and induces enterotoxemia in ruminants. Etx crosses the blood-brain barrier, binds to myelin structures, and kills oligodendrocytes, inducing central nervous system demyelination. In addition, Etx has a cytotoxic effect on distal and collecting kidney tubules. There are few cell lines sensitive to Etx. At present, the most sensitive in vitro model for Etx is the Madin-Darby canine kidney (MDCK) cell line, where Etx oligomerizes and forms a pore with consequent ion efflux and cell death. Although the Etx receptor has not yet been fully clarified, it is known that caveolin 1 and 2 potentiate Etx cytotoxicity and oligomerization, and more recently, the myelin and lymphocyte (MAL) protein has been implicated in Etx binding and activity. Here, we studied the effect of Etx on Fischer rat thyroid cells (FRT) and observed similar effects as those seen in MDCK cells. Etx incubated with FRT cells showed binding to the plasma membrane, and western blotting assays revealed oligomeric complex formation. Moreover, cytotoxic assays on FRT cells after Etx incubation indicated cell death at a similar level as in MDCK cells. In addition, a luminescent ATP detection assay revealed ATP depletion in FRT cells after Etx exposure. Previous studies have reported that FRT cells do not express caveolins and do not form caveolae but express MAL protein in glycolipid-enriched membrane microdomains. Our results indicate that caveolins are not directly implicated in Etx cytotoxicity, supporting the notion that the MAL protein is involved in Etx action. In addition, a cell line of thyroid origin is described for the first time as a good model to study Etx action.

MYADM controls endothelial barrier function through ERM-dependent regulation of ICAM-1 expression

The endothelium maintains a barrier between blood and tissue that becomes more permeable during inflammation. Membrane rafts are ordered assemblies of cholesterol, glycolipids, and proteins that modulate proinflammatory cell signaling and barrier function. In epithelial cells, the MAL family members MAL, MAL2, and myeloid-associated differentiation marker (MYADM) regulate the function and dynamics of ordered membrane domains. We analyzed the expression of these three proteins in human endothelial cells and found that only MYADM is expressed. MYADM was confined in ordered domains at the plasma membrane, where it partially colocalized with filamentous actin and cell-cell junctions. Small interfering RNA (siRNA)-mediated MYADM knockdown increased permeability, ICAM-1 expression, and leukocyte adhesion, all of which are features of an inflammatory response. Barrier function decrease in MYADM-silenced cells was dependent on ICAM-1 expression. Membrane domains and the underlying actin cytoskeleton can regulate each other and are connected by ezrin, radixin, and moesin (ERM) proteins. In endothelial cells, MYADM knockdown induced ERM activation. Triple-ERM knockdown partially inhibited ICAM-1 increase induced by MYADM siRNA. Importantly, ERM knockdown also reduced ICAM-1 expression in response to the proinflammatory cytokine tumor necrosis factor-α. MYADM therefore regulates the connection between the plasma membrane and the cortical cytoskeleton and so can control the endothelial inflammatory response.

VIP17/MAL expression modulates epithelial cyst formation and ciliogenesis

The polarized organization of epithelial cells is required for vectorial solute transport and may be altered in renal cystic diseases. Vesicle integral protein of 17 kDa (VIP17/MAL) is involved in apical vesicle transport. VIP17/MAL overexpression in vivo results in renal cystogenesis of unknown etiology. Renal cystogenesis can occur as a consequence of defects of the primary cilium. To explore the role of VIP17/MAL in renal cystogenesis and ciliogenesis, we examined the polarization and ciliary morphology of wild-type and VIP17/MAL overexpressing Madin-Darby canine kidney renal epithelial cells grown in two-dimensional (2D) and three-dimensional (3D) cyst culture. VIP17/MAL is apically localized when expressed in cells maintained in 2D and 3D culture. VIP17/MAL overexpressing cells produce more multilumen cysts compared with controls. While the distributions of basolateral markers are not affected, VIP17/MAL expression results in aberrant sorting of the apical marker gp135 to the primary cilium. VIP17/MAL overexpression is also associated with shortened or absent cilia. Immunofluorescence analysis performed on kidney sections from VIP17/MAL transgenic mice also demonstrates fewer and shortened cilia within dilated lumens (P < 0.01). These studies demonstrate that VIP17/MAL overexpression results in abnormal cilium and cyst development, in vitro and in vivo, suggesting that VIP17/MAL overexpressing mice may develop cysts secondary to a ciliary defect.

Productive entry of type C foot-and-mouth disease virus into susceptible cultured cells requires clathrin and is dependent on the presence of plasma membrane cholesterol

We have characterized the entry leading to productive infection of a type C FMDV in two cell lines widely used for virus growth, BHK-21 and IBRS-2. Inhibition of clathrin-mediated endocytosis by sucrose treatment decreased both cell entry and virus multiplication. Evidence of a direct requirement of clathrin for productive viral entry was obtained using BHK21-tTA/anti-CHC cells, which showed a significant reduction of viral entry and infection when the synthesis and functionality of clathrin heavy chain was inhibited (Tet- cells). This was also observed for vesicular stomatitis virus (VSV) productive entry. The effect of NH(4)Cl and concanamycin A on FMDV entry and infection was consistent with the requirement of acidic compartments for decapsidation and virus replication. As expected from its higher stability at acidic pH, this requirement was higher for VSV. Since BHK-21 and IBRS-2 cells expressed caveolin-1, we explored the effect on productive virus entry of drugs that interfere with caveolae-mediated endocytosis. Treatment with nystatin did not reduce entry and infection of FMDV or VSV, while cholesterol depletion with MbetaCD significantly inhibited both steps of the FMDV cycle, indicating that plasma membrane cholesterol is required for virus productive entry.

Fas ligand is localized to membrane rafts, where it displays increased cell death–inducing activity

Fas ligand (FasL), a member of the TNF protein family, potently induces cell death by activating its matching receptor Fas. Fas-mediated killing plays a critical role in naturally and pathologically occurring cell death, including development and homeostasis of the immune system. In addition to its receptor-interacting and cell death–inducing extracellular domain, FasL has a well-conserved intracellular portion with a proline-rich SH3 domain–binding site probably involved in non-apoptotic functions. We report here that, as with the Fas receptor, a fraction of FasL is constitutively localized in rafts. These dynamic membrane microdomains, enriched in sphingolipids and cholesterol, are important for cell signaling and trafficking processes. We show that FasL is partially localized in rafts and that increased amounts of FasL are found in rafts after efficient FasL/Fas receptor interactions. Raft disorganization after cholesterol oxidase treatment and deletions within the intracellular FasL domain diminish raft partitioning and, most important, lead to decreased FasL killing. We conclude that FasL is recruited into lipid rafts for maximum Fas receptor contact and cell death–inducing potency. These findings raise the possibility that certain pathologic conditions may be treated by altering the cell death–inducing capability of FasL with drugs affecting its raft localization.

Features of influenza HA required for apical sorting differ from those required for association with DRMs or MAL

The influenza virus hemagglutinin (HA) is sorted to the apical membrane in polarized epithelial cells and associates with detergent-resistant membranes (DRMs). By systematic mutagenesis of the transmembrane residues, we show that hemagglutinin requires 10 contiguous transmembrane amino acids to enter detergent-resistant membranes and that the surface of the trimeric hemagglutinin transmembrane domain facing the lipid environment as well as that facing the interior of the trimer is important for stable association with detergent-resistant membranes. However, association with detergent-resistant membranes was not required for apical sorting. MAL/VIP17 is a protein that is required for apical transport and a small fraction of hemagglutinin co-precipitates with MAL. Mutations that prevented HA from being isolated in detergent-resistant membranes decreased co-precipitation with MAL. The hemagglutinin and MAL that co-precipitated were contained in a detergent-resistant vesicle. However, most of the co-precipitation of newly synthesized hemagglutinin with MAL occurred only after the majority of hemagglutinin reached the cell surface. Both the timing and the limited extent of co-precipitation suggest that the majority of vesicles containing hemagglutinin and MAL are not the detergent-resistant membrane transport intermediates carrying hemagglutinin from the TGN to the apical surface.

Rafts, little caves and large potholes: how lipid structure interacts with membrane proteins to create functionally diverse membrane environments

This chapter reviews how diverse lipid microdomains form in the membrane and partition proteins into different functional units that regulate cell trafficking, signalling and movement. We will concentrate upon five major issues: 1. the diversity of lipid structure that produces diverse microenvironments into which different subsets of proteins partition; 2. why ordered lipid domains exclude proteins, and the conditions required for select subsets of proteins to enter these domains; 3. the coupling of the inner and outer leaflets within ordered microdomains; 4. the effect of ordered lipid domains upon membrane properties including curvature and hydrophobicity that affect membrane fission, fusion and extension of filopodia; 5. the biological effects of these structural constraints; in particular how the properties of these domains combine to provide a very different signalling, trafficking and membrane fusion environment to that found in disordered (fluid mosaic) membrane. In addressing these problems, the review draws upon studies ranging from molecular dynamic modelling of lipid interactions, through physical studies of model membrane systems to structural and biological studies of whole cells, examining in the process problems inherent in visualising and purifying these microdomains. While the diversity of structure and function of ordered lipid microdomains is emphasised, some general roles emerge. In particular, the basis for having quite different, non-interacting ordered lipid domains on the same membrane is evident in the diversity of lipid structure and plays a key role in sorting signalling systems. The exclusion of ordered membrane from coated pits, and hence rapid endocytosis, is suggested to underlie the ability of highly ordered domains to establish stable secondary signalling systems required, for instance, in T cell receptor, insulin and neurotrophin signalling.

Identification of genes involved in resistance to interferon-alpha in cutaneous T-cell lymphoma

Interferon-alpha therapy has been shown to be active in the treatment of mycosis fungoides although the individual response to this therapy is unpredictable and dependent on essentially unknown factors. In an effort to better understand the molecular mechanisms of interferon-alpha resistance we have developed an interferon-alpha resistant variant from a sensitive cutaneous T-cell lymphoma cell line. We have performed expression analysis to detect genes differentially expressed between both variants using a cDNA microarray including 6386 cancer-implicated genes. The experiments showed that resistance to interferon-alpha is consistently associated with changes in the expression of a set of 39 genes, involved in signal transduction, apoptosis, transcription regulation, and cell growth. Additional studies performed confirm that STAT1 and STAT3 expression and interferon-alpha induction and activation are not altered between both variants. The gene MAL, highly overexpressed by resistant cells, was also found to be expressed by tumoral cells in a series of cutaneous T-cell lymphoma patients treated with interferon-alpha and/or photochemotherapy. MAL expression was associated with longer time to complete remission. Time-course experiments of the sensitive and resistant cells showed a differential expression of a subset of genes involved in interferon-response (1 to 4 hours), cell growth and apoptosis (24 to 48 hours.), and signal transduction.

Proteins of peripheral myelin are associated with glycosphingolipid/cholesterol-enriched membranes

A characteristic feature of the vertebrate nervous system is the ensheathment of axons by myelin, a multilamellar membrane specialization produced by polarized glial cells. Although the main protein and lipid components of the myelin sheath are well characterized, relatively little is known about the mechanisms of their intracellular distribution to the respective sites of assembly within the myelin sheath. To analyze whether peripheral myelin protein trafficking is mediated by glycosphingolipid/cholesterol-enriched membranes (GEMs), we studied the association of established myelin proteins, peripheral myelin protein 22 (PMP22), protein zero (P0), plasmolipin, and myelin basic protein (MBP), with these membrane microdomains. To examine the association of the selected peripheral myelin proteins with detergent-insoluble GEMs, purified myelin from sciatic nerve of adult rat was extracted with Triton X-100 at 4 degrees C and 37 degrees C and, in additional experiments, was pretreated with the cholesterol chelator methyl-beta-cyclodextrin. The material was then centrifuged to equilibrium in sucrose gradients, and fractions were analyzed by Western blotting. Here we demonstrate for the first time that PMP22, P0, and plasmolipin prepared from purified peripheral myelin are associated with GEMs. To characterize whether the association of these proteins is a specialized feature of myelinating Schwann cells, we studied the distribution of PMP22, P0, and plasmolipin in transiently transfected HeLa cells. These experiments confirm the specific association of these proteins with GEMs in both neural and nonneural cell types.

MAL mediates apical transport of secretory proteins in polarized epithelial Madin-Darby canine kidney cells

The MAL proteolipid is an integral membrane protein identified as a component of the raft machinery for apical sorting of membrane proteins in Madin-Darby canine kidney (MDCK) cells. Previous studies have implicated lipid rafts in the transport of exogenous thyroglobulin (Tg), the predominant secretory protein of thyroid epithelial cells, to the apical surface in MDCK cells. We have examined the secretion of recombinant Tg and gp80/clusterin, a major endogenous secretory protein not detected in Triton X-100 insoluble rafts, for the investigation of the involvement of MAL in the constitutive apical secretory pathway of MDCK cells. We show that MAL depletion impairs apical secretion of Tg and causes its accumulation in the Golgi. Cholesterol sequestration, which blocks apical secretion of Tg, did not alter the levels of MAL in rafts but created a block proximal to Tg entrance into rafts. Apical secretion of gp80/clusterin was also inhibited by elimination of endogenous MAL. Our results suggest a role for MAL in the transport of both endogenously and exogenously expressed apical secretory proteins in MDCK cells.

Cell biology of caveolae and caveolin

Originally described in the 1950s caveolae are morphologically identifiable as small omega-shaped plasma membrane invaginations present in most cell types. Caveolae are particularly abundant in adipocytes, fibroblasts, type 1 pneumocytes, endothelial and epithelial cells as well as in smooth and striated muscle cells. The first proposed function for caveolae was that of mediating the internalisation and transendothelial trafficking of solutes. Caveolae have been the object of intense research since the discovery of a biochemical marker protein, caveolin, in the early 1990s. Three genes encoding for caveolins have been characterised in mammals. Caveolins (18-24 kDa) are integral membrane proteins that constitute the major protein component of caveolar membrane in vivo. In addition to a structural role of caveolins in the formation of caveolae, caveolin protein interacts directly, and in a regulated manner, with a number of signalling molecules. We present here a general overview of the current knowledge on the structural role of caveolin in caveolae formation, and implication of caveolin in the control of cell signalling.

BENE, a novel raft-associated protein of the MAL proteolipid family, interacts with caveolin-1 in human endothelial-like ECV304 cells

The MAL proteolipid, an integral protein present in glycolipid- and cholesterol-enriched membrane (GEM) rafts, is an element of the machinery necessary for apical sorting in polarized epithelial Madin-Darby canine kidney cells. MAL was the first member identified of an extended family of proteins that have significant overall sequence identity. In this study we have used a newly generated monoclonal antibody to investigate an unedited member of this family, named BENE, which was found to be expressed in endothelial-like ECV304 cells and normal human endothelium. Human BENE was characterized as a proteolipid protein predominantly present in GEM rafts in ECV304 cells. Coimmunoprecipitation experiments revealed that BENE interacted with caveolin-1. Confocal immunofluorescence and electron microscopic analyses indicated that BENE mainly accumulated into intracellular vesicular/tubular structures that partially colocalize with internal caveolin-1. In response to cell surface cholesterol oxidation, BENE redistributed to the dilated vesicular structures that concentrate most of the caveolin-1 originally on the cell surface. After cessation of cholesterol oxidation, a detectable fraction of the BENE molecules migrated to the plasmalemma accompanying caveolin-1 and then returned progressively to its steady state distribution. Together, these features highlight the BENE proteolipid as being an element of the machinery for raft-mediated trafficking in endothelial cells.

An intact dilysine-like motif in the carboxyl terminus of MAL is required for normal apical transport of the influenza virus hemagglutinin cargo protein in epithelial Madin-Darby canine kidney cells

The MAL proteolipid, a component of the integral protein sorting machinery, has been demonstrated as being necessary for normal apical transport of the influenza virus hemagglutinin (HA) and the overall apical membrane proteins in Madin-Darby canine kidney (MDCK) cells. The MAL carboxy terminus ends with the sequence Arg-Trp-Lys-Ser-Ser (RWKSS), which resembles dilysine-based motifs involved in protein sorting. To investigate whether the RWKSS pentapeptide plays a role in modulating the distribution of MAL and/or its function in apical transport, we have expressed MAL proteins with distinct carboxy terminus in MDCK cells whose apical transport was impaired by depletion of endogenous MAL. Apical transport of HA was restored to normal levels by expression of MAL with an intact but not with modified carboxyl terminal sequences bearing mutations that impair the functioning of dilysine-based sorting signals, although all the MAL proteins analyzed incorporated efficiently into lipid rafts. Ultrastructural analysis indicated that compared with MAL bearing an intact RWKSS sequence, a mutant with lysine -3 substituted by serine showed a twofold increased presence in clathrin-coated cytoplasmic structures and a reduced expression on the plasma membrane. These results indicate that the carboxyl-terminal RWKSS sequence modulates the distribution of MAL in clathrin-coated elements and is necessary for HA transport to the apical surface.

Characterization of GFP-MAL expression and incorporation in rafts

During myelin formation, membrane-associated proteins have to be sorted and transported in specified membrane regions such as compact and non-compact myelin membranes. One protein that may be involved in such a process is the Myelin and Lymphocyte protein MAL (VIP17/ MVP17). MAL was identified as a novel myelin membrane component expressed by oligodendrocytes and Schwann cells. Since MAL has been shown to be important in the apical sorting machinery of polarized cells, we have started to investigate the possible functional role of MAL in sorting myelin membrane-associated molecules. In this study, we have generated cDNA constructs with green fluorescent protein (GFP) either at the N- or C-terminus of MAL. Transfection experiments showed that GFP-MAL expression resembles that of normal MAL, whereas the MAL-GFP fusion construct was not properly transported within the cell. Furthermore, we could demonstrate that GFP-MAL is enriched in detergent insoluble glycolipid-enriched microdomains as already seen for untagged MAL. As a prerequisite for the generation of transgenic mice expressing GFP-MAL under the control of its own regulatory elements, we have generated a cDNA construct with an 8-kb MAL promotor fragment fused to GFP-MAL. Transfection experiments of the Oli-neu oligodendrocyte cell line showed that GFP-MAL was expressed, but only in cells, which were stimulated for differentiation with cAMP. In summary, the results confirm that the fusion protein GFP-MAL is incorporated into detergent-insoluble complexes and the 8-kb MAL promotor fragment is sufficient to be activated in oligodendrocytes.

Genetic alterations in primary mediastinal B-cell lymphoma: an update

Primary mediastinal B-cell lymphoma (PMBL) is a distinct clinical entity among non-Hodgkin's lymphoma. The malignancy has received little attention from a standpoint of basic research due in part to its rarity. However, based on recent studies consistent trends are beginning to emerge regarding the molecular and chromosomal alterations commonly observed in this disease. By both CGH and AP-PCR, genetic gains involving chromosomes 2, 5, 7, 9p, 12, and Xq are among the most frequently observed events. From a molecular standpoint, alterations in the c-myc, p16(INK4) and p53 genes have been observed in up to 30% of cases. This information along with the well-established histological, immunological, and clinical features should convince the few remaining disbelievers that PMBL is a distinct pathological entity among non-Hodgkin's lymphomas.

Interaction of shrimp ras protein with mammalian caveolin-1

BALB/3T3 cells were transformed by transfection with DNA encoding the mutated ras(Q(61)K) from shrimp Penaeus japonicus (Huang and Chuang. 1999. J Exp Zool 283:510-521). The caveolin-1 in the membrane fraction extractable with 2% octyl glucoside was significant reduced, compared to untransformed cells. To understand this in more detail, the interaction of S-Ras with caveolin was investigated using caveolin-1 purified from rat lungs. The purified caveolin-1 binds c-Src, suppressing its autophosphorylation. It also binds to phosphatidylserine-cholesterol liposomes. These reconstituted caveolin-phosphatidylserine-cholesterol vesicles, which act as a model of caveolae, recruit both bacterially expressed S-Ras and rat K(B)-Ras proteins, as demonstrated on western blots with antibodies against caveolin-1 and Ras. Caveolin-1 suppressed the intrinsic GTPase activity of S-Ras, sustaining it in the active GTP bound form. By contrast, caveolin-1 enhanced the intrinsic GTPase activity of K(B)-Ras, to convert it into the inactive GDP-bound form. These events suggest that caveolin may act as a docking site for Ras proteins and may be able to either maintain or alter their activity state. These events may be associated with the ability of S-ras(Q(61)K) to successfully transform cells.

The MAL proteolipid is necessary for the overall apical delivery of membrane proteins in the polarized epithelial Madin-Darby canine kidney and fischer rat thyroid cell lines

The MAL proteolipid has been recently demonstrated as being necessary for correct apical sorting of the transmembrane influenza virus hemagglutinin (HA) in Madin-Darby canine kidney (MDCK) cells. The fact that, in contrast to MDCK cells, Fischer rat thyroid (FRT) cells target the majority of glycosylphosphatidylinositol (GPI)-anchored proteins to the basolateral membrane provides us with the opportunity to determine the role of MAL in apical transport of membrane proteins under conditions in which the majority of GPI-anchored proteins are (MDCK cells) or are not (FRT cells) targeted to the apical surface. Using an antisense oligonucleotide-based strategy to deplete endogenous MAL, we have observed that correct transport of apical transmembrane proteins associated (HA) or not (exogenous neurotrophin receptor and endogenous dipeptidyl peptidase IV) with lipid rafts, as well as that of the bulk of endogenous apical membrane, takes place in FRT cells by a pathway that requires normal MAL levels. Even transport of placental alkaline phosphatase, a GPI-anchored protein that is targeted apically in FRT cells, was dependent on normal MAL levels. Similarly, in addition to the reported effect of MAL on HA transport, depletion of MAL in MDCK cells caused a dramatic reduction in the apical delivery of the GPI-anchored gD1-DAF protein, neurotrophin receptor, and the bulk of membrane proteins. These results suggest that MAL is necessary for the overall apical transport of membrane proteins in polarized MDCK and FRT cells.

MAL, a proteolipid in glycosphingolipid enriched domains: functional implications in myelin and beyond

The myelin and lymphocyte protein MAL (VIP17/MVP17) is a proteolipid of 17 kD with a hydrophobicity pattern that indicates a four transmembrane domain structure. The MAL cDNA has been cloned from human T-cells, rat oligodendrocytes and the Madin-Darby canine kidney (MDCK) cell line. In the nervous system both myelinating cells, oligodendrocytes and Schwann cells, express MAL protein. MAL expression parallels myelin formation, and MAL is predominantly localized in compact myelin. Prior to myelin formation MAL is also found in immature Schwann cells. Outside the nervous system MAL expression is found in T-cells and in distinct epithelial cells, e.g. in kidney, stomach and thyroid gland, where MAL is localised in the apical plasma membrane. Specific glycosphingolipids, e.g. galactosylceramide and sulfatide, are enriched in such apical kidney and stomach membranes as well as in myelin. MAL copurifies with these glycosphingolipids in detergent insoluble domains, indicating a close association and possible functional interactions of MAL with glycosphingolipids in these tissues. Moreover, recent reports point to additional functions of MAL-glycosphingolipid complexes in signalling, cell differentiation and apical sorting. The role of MAL in the formation, stabilisation and maintenance of glycosphingolipid-enriched membrane microdomains and its contribution to specific membrane properties in myelin and epithelial cells are discussed.

Incorporation of MAL, an integral protein element of the machinery for the glycolipid and cholesterol-mediated apical pathway of transport, into artificial membranes requires neither of these lipid species

The MAL proteolipid, an integral membrane protein with selective residence in glycolipid- and cholesterol-enriched membrane (GEM) microdomains, has recently been identified as being an element of the integral protein machinery necessary for apical transport in MDCK cells. With the use of a recombinant baculovirus, we have expressed and purified polyhistidine-tagged MAL to determine whether MAL has special lipid requirements for becoming incorporated into membranes. In contrast with caveolin-1, a component of GEMs that requires cholesterol for its integration into artificial membranes, MAL incorporation took place with dimyristoylphosphatidylcholine as the only lipid component. The presence of cholesterol, sphingomyelin, or galactocerebrosides did not affect the efficiency of this process. These results indicated that MAL is compatible with membranes containing either only phospholipids or also glycolipids and cholesterol and are consistent with the reported requirement of a sorting event for the specific targeting of MAL to GEM microdomains.

MAL, an integral element of the apical sorting machinery, is an itinerant protein that cycles between the trans-Golgi network and the plasma membrane

The MAL proteolipid is a nonglycosylated integral membrane protein found in glycolipid-enriched membrane microdomains. In polarized epithelial Madin-Darby canine kidney cells, MAL is necessary for normal apical transport and accurate sorting of the influenza virus hemagglutinin. MAL is thus part of the integral machinery for glycolipid-enriched membrane-mediated apical transport. At steady state, MAL is predominantly located in perinuclear vesicles that probably arise from the trans-Golgi network (TGN). To act on membrane traffic and to prevent their accumulation in the target compartment, integral membrane elements of the protein-sorting machinery should be itinerant proteins that cycle between the donor and target compartments. To establish whether MAL is an itinerant protein, we engineered the last extracellular loop of MAL by insertion of sequences containing the FLAG epitope or with sequences containing residues that became O-glycosylated within the cells or that displayed biotinylatable groups. The ectopic expression of these modified MAL proteins allowed us to investigate the surface expression of MAL and its movement through different compartments after internalization with the use of a combination of assays, including surface biotinylation, surface binding of anti-FLAG antibodies, neuraminidase sensitivity, and drug treatments. Immunofluorescence and flow cytometric analyses indicated that, in addition to its Golgi localization, MAL was also expressed on the cell surface, from which it was rapidly internalized. This retrieval implies transport through the endosomal pathway and requires endosomal acidification, because it can be inhibited by drugs such as chloroquine, monensin, and NH(4)Cl. Resialylation experiments of surface MAL treated with neuraminidase indicated that approximately 30% of the internalized MAL molecules were delivered to the TGN, probably to start a new cycle of cargo transport. Together, these observations suggest that, as predicted for integral membrane members of the late protein transport machinery, MAL is an itinerant protein cycling between the TGN and the plasma membrane.

Substitution of the two carboxyl-terminal serines by alanine causes retention of MAL, a component of the apical sorting machinery, in the endoplasmic reticulum

MAL, a selective resident of glycolipid-enriched membranes (GEMs), is an integral membrane protein necessary for apical transport and accurate sorting of the influenza virus hemagglutinin in MDCK cells. The carboxyl-terminal end of MAL has the sequence Phe-Ser-Leu-Ile-Arg-Trp-Lys-Ser-Ser (FSLIRWKSS), which includes the LIRW motif necessary for sorting MAL to GEMs, and whose last five amino acids resemble dilysine-based signals involved in endoplasmic reticulum (ER) retention. We have addressed the influence of the carboxyl-terminal serines in both MAL distribution and incorporation into GEMs. Substitution of the serines by alanine impeded the access of MAL to GEMs and changed its distribution from a perinuclear distribution to an ER pattern. The RWKSS sequence appended to the carboxyl-terminus of CD4 caused retention of the chimera in the ER. Thus, although this pentapeptide can function producing ER retention in other protein context, the presence of the carboxyl-terminal serines in the intact MAL molecule prevents its use as an ER-retention signal.

The MAL proteolipid is necessary for normal apical transport and accurate sorting of the influenza virus hemagglutinin in Madin-Darby canine kidney cells

The MAL (MAL/VIP17) proteolipid is a nonglycosylated integral membrane protein expressed in a restricted pattern of cell types, including T lymphocytes, myelin-forming cells, and polarized epithelial cells. Transport of the influenza virus hemagglutinin (HA) to the apical surface of epithelial Madin-Darby canine kidney (MDCK) cells appears to be mediated by a pathway involving glycolipid- and cholesterol- enriched membranes (GEMs). In MDCK cells, MAL has been proposed previously as being an element of the protein machinery for the GEM-dependent apical transport pathway. Using an antisense oligonucleotide-based strategy and a newly generated monoclonal antibody to canine MAL, herein we have approached the effect of MAL depletion on HA transport in MDCK cells. We have found that MAL depletion diminishes the presence of HA in GEMs, reduces the rate of HA transport to the cell surface, inhibits the delivery of HA to the apical surface, and produces partial missorting of HA to the basolateral membrane. These effects were corrected by ectopic expression of MAL in MDCK cells whose endogenous MAL protein was depleted. Our results indicate that MAL is necessary for both normal apical transport and accurate sorting of HA.

Targeting of MAL, a putative element of the apical sorting machinery, to glycolipid-enriched membranes requires a pre-golgi sorting event

The MAL proteolipid is a nonglycosylated polytopic membrane protein with specific residence in glycolipid-enriched membrane (GEM) microdomains. MAL has been proposed as an element of the machinery for apical transport in polarized epithelial cells. Previous work demonstrated that MAL requires its four carboxyl-terminal amino acids to be targeted to GEMs. In the present work, we have engineered MAL with N-glycosylation consensus sequences to delimit the site at which commitment of MAL to access into GEMs takes place. Comparison of engineered MAL proteins bearing either an intact or a truncated carboxyl terminus revealed that whereas the former acquired endo H-sensitive and endo H-resistant mature glycosylation, the protein with a deleted carboxyl terminus did not. These results indicate that although MAL incorporation into GEMs takes place mainly in the Golgi, commitment of MAL to enter GEMs is a pre-Golgi event.

CD4 segregates into specific detergent-resistant T-cell membrane microdomains

In T cells, glycolipids, glycoproteins attached to the membrane via a glycosylphosphatidylinositol (GPI) anchor, and Src-like tyrosine kinases are highly enriched in a membrane fraction resistant to solubilization by nonionic detergents. We have investigated the distribution of CD4 in T-cell membranes and found that approximately 10% of the CD4 co-receptor is associated with detergent-insoluble membrane microdomains, whilst the remaining 90% is in soluble membranes. Moreover, approximately 60% of the "insoluble CD4" is present in membrane microdomains containing GPI-anchored proteins and high glycolipid-dependent kinase activity, whereas the remaining 40% displays no association with GPI-anchored proteins and lacks glycolipid-associated kinase activity These results indicate that CD4 segregates at least into three different membrane microenvironments: 1) soluble membranes; 2) insoluble membrane microdomains containing GPI-anchored proteins; and 3) insoluble membrane microdomains devoid of GPI-anchored proteins. The level of CD4 in insoluble membranes was not modified upon triggering activation by T-cell receptor-crosslinking but detectable amounts of CD3 subunits were recruited into these specialized membranes under those conditions. The physical separation of CD4 into different membrane microenvironments raises the possibility of that some of the multiple functions of CD4 might segregate into distinct types of lipid microenvironment. The fact that components of T-cell receptor/CD3 complex were recruited into insoluble membranes upon stimulation is consistent with the CD4 present in this membrane fraction might participate in T-cell receptor-triggered activation events.

MAL, a novel integral membrane protein of human T lymphocytes, associates with glycosylphosphatidylinositol-anchored proteins and Src-like tyrosine kinases

A large fraction of glycosylphosphatidylinositol (GPI)-anchored proteins and Src-like kinases are confined to glycolipid-enriched membrane (GEM) microdomains. The particular membrane topology of GPI-anchored proteins has led to the postulation of the existence of integral membrane proteins linking extracellular stimuli with cytosolic machinery for endocytosis and signaling. The human MAL cDNA was identified during a search for novel genes differentially expressed during T cell development, and encodes a multispanning membrane protein displaying lipid-like properties. To address the biochemical characterization of endogenous MAL and to analyze its possible association with other proteins, we have generated a monoclonal antibody (mAb) specific to the MAL molecule. Using this mAb, we have identified MAL in GEM microdomains of both the HPB-ALL T cell line and human peripheral blood lymphocytes. Co-immunoprecipitation experiments with antibodies to the MAL molecule or to the GPI-anchored CD59 antigen indicated specific association of MAL with GPI-anchored proteins and Src-like tyrosine kinases. In addition, both MAL and the Src-like kinase Lck were identified in GEM obtained from an endosomal-enriched membrane fraction. These features of MAL closely match some of the properties expected for the hypothetical integral membrane linker proteins acting in specialized GEM-mediated functions.

The caveolae membrane system

The cell biology of caveolae is a rapidly growing area of biomedical research. Caveolae are known primarily for their ability to transport molecules across endothelial cells, but modern cellular techniques have dramatically extended our view of caveolae. They form a unique endocytic and exocytic compartment at the surface of most cells and are capable of importing molecules and delivering them to specific locations within the cell, exporting molecules to extracellular space, and compartmentalizing a variety of signaling activities. They are not simply an endocytic device with a peculiar membrane shape but constitute an entire membrane system with multiple functions essential for the cell. Specific diseases attack this system: Pathogens have been identified that use it as a means of gaining entrance to the cell. Trying to understand the full range of functions of caveolae challenges our basic instincts about the cell.

rMAL is a glycosphingolipid-associated protein of myelin and apical membranes of epithelial cells in kidney and stomach

rMAL, the rat myelin and lymphocyte protein, is a small hydrophobic protein of 17 kDa with four putative transmembrane domains and is expressed in oligodendrocytes and Schwann cells, the myelinating cells of the nervous system. In addition, transcript expression has been found in kidney, spleen, and intestine. Confocal microscopy and immunoelectron microscopy with an affinity-purified antibody localized rMAL to compact myelin in a pattern similar to the structural myelin proteins: myelin basic protein and proteolipid protein. In kidney and stomach epithelia, rMAL is located almost exclusively on the apical (luminal) membranes of the cells lining distal tubuli in kidney and the glandular part of the stomach. Biochemical analysis of plasma membranes isolated from spinal cord and kidney demonstrated that rMAL is a proteolipid that is present in detergent insoluble complexes typical for proteins associated with glycosphingolipids. Lipid and protein analysis showed a co-enrichment of glycosphingolipids and rMAL protein within these complexes, indicating a close association of rMAL to glycosphingolipids in myelin and in kidney in vivo. We conclude that specific rMAL-glycosphingolipid interactions may lead to the formation and maintenance of stable protein-lipid microdomains in myelin and apical epithelial membranes. They may contribute to specific properties of these highly specialized plasma membranes.

A short peptide motif at the carboxyl terminus is required for incorporation of the integral membrane MAL protein to glycolipid-enriched membranes

The MAL (VIP17, MVP17) proteolipid, an integral membrane protein with specific residence in glycolipid-enriched membrane (GEM) microdomains, has been recently proposed as a component of the protein machinery for GEM vesiculation. In this work, we have searched the COOH terminus of MAL for sorting determinants responsible for targeting to GEMs. This has allowed the identification of the sequence Leu-Ile-Arg-Trp (LIRW) as necessary for the access of MAL to GEMs. This motif requires at least one additional amino acid at its COOH end for full effectiveness. The arginine within the LIRW motif is the most crucial residue for targeting to GEMs, tryptophan replacement affects targeting to a lesser extent, and the leucine-isoleucine pair tolerates substitution by valine, but not by alanine, without effect on targeting. Pulse-chase experiments indicate that the LIRW tetrapeptide is required for access to GEMs early after MAL biosynthesis. Interestingly, the loss of the capacity of the MAL protein to be incorporated into GEMs correlated with the loss of its response to brefeldin A treatment. This is the first identification of a juxtamembrane peptide motif required for incorporation of an integral membrane protein into GEMs.

Expression of the MAL gene in the thyroid: the MAL proteolipid, a component of glycolipid-enriched membranes, is apically distributed in thyroid follicles

The MAL proteolipid, an integral membrane protein expressed in T lymphocytes, polarized epithelial MDCK cells, and myelin-forming cells, has been identified as a component of internal glycolipid-enriched membrane (GEM) microdomains. On the basis of its ability to induce vesicle formation by ectopic expression, MAL has been recently proposed as a component of the machinery for GEM vesiculation. Taking into account the proposed role of GEMs in polarized transport, we have investigated the expression of the MAL gene in thyroid cells. Interestingly, MAL messenger RNA species were detected in the human thyroid, whereas they were undetectable in other endocrine glands tested. Moreover, epithelial FRT cells, a polarized rat cell line of thyroid origin, also expressed MAL transcripts. Immunohistochemical analysis of thyroid follicles, with a newly developed anti-MAL monoclonal antibody, indicates that MAL distribution is restricted to the apical zone of thyroid epithelial cells. Biochemical analyses, using FRT cells, indicate exclusive residence of MAL in GEM microdomains, and these analyses allowed the identification of MAL as a major protein component of the GEM fraction in this cell line. Our results are consistent with a role for MAL as a component of GEM microdomains in thyroid epithelial cells.

Association of Src family tyrosine kinase Lyn with ganglioside GD3 in rat brain. Possible regulation of Lyn by glycosphingolipid in caveolae-like domains

Association of gangliosides with specific proteins in the central nervous system was examined by co-immunoprecipitation with anti-ganglioside antibody. Protein kinase activity was detected in precipitates with monoclonal antibody to ganglioside GD3 (R24) from membranal fraction of rat brain. Using in vitro kinase assay, several phosphorylated proteins of 40, 53, 56, and 80 kDa were isolated by gel electrophoresis. Of these proteins, the proteins of 53 and 56 kDa (p53/56) were identified as two isoforms of Src family tyrosine kinase Lyn, based on co-migration during gel electrophoresis, comparative peptide mapping, and sequential immunoprecipitation with anti-Lyn antibody. The identification was confirmed using a cDNA expression system in Chinese hamster ovary (CHO) cells, which express solely ganglioside GM3, the enzymatic substrate of GD3 synthase. In co-transfection with GD3 synthase and Lyn expression plasmids, R24 immunoprecipitated Lyn and anti-Lyn antibody immunoprecipitated GD3. R24 treatment of rat primary cerebellar cultures induced Lyn activation and rapid tyrosine phosphorylation of several substrates including mitogen-activated protein kinases. Furthermore, sucrose density gradient analysis showed that Lyn of cerebellum and CHO transfectants were detected in a low density light-scattering band, i.e. the caveolae membrane fraction. R24 immunoprecipitated caveolin from Triton X-100 extract of CHO transfectants. These observations suggest that GD3 may regulate Lyn in a caveolae-like domain on brain cell membranes.

Recombinant expression of the MAL proteolipid, a component of glycolipid-enriched membrane microdomains, induces the formation of vesicular structures in insect cells

The MAL proteolipid has been identified as a component of glycolipid-enriched membrane microdomains resistant to detergent solubilization in epithelial Madin-Darby canine cells, as well as in T lymphocytes and in myelin-forming cells. To study the function of the MAL proteolipid we have ectopically expressed a tagged form of MAL in both mammalian and insect cellular backgrounds. Immunofluorescence analysis in transiently transfected COS-7 cells showed the presence of MAL in large vesicular structures, and biochemical analysis identified MAL in the fraction of membranes resistant to Triton X-100 solubilization. Electron microscopic analysis showed that the expression of MAL in Sf21 cells morphologically resulted in the intracellular accumulation of large vesicles with a diameter from 200 to greater than 700 nm that were absent in uninfected or control infected cultures. Thus, ectopic expression of MAL in this heterologous expression system was sufficient to drive the formation of vesicles with a size similar to that of the vesicles detected in mammalian cells. These vesicles were clearly different from the caveolae-like vesicles induced by caveolin expression, as evidenced by co-infection experiments using a recombinant caveolin baculovirus. Taken together, these results suggest that the MAL proteolipid might play a role as a component of the machinery of vesiculation of glycolipid-enriched membranes.