The tyrosine kinase connection: how GPI-anchored proteins activate T cells

GPI-Anchored Proteins and Glycosphingolipid-Rich Rafts: Platforms for Adhesion and Signaling

Glycosylphosphatidylinositol (GPI)-anchored proteins in mammalian cells play a role in adhesion and signaling. They are sorted in the trans-Golgi network into glycosphingolipid- and cholesterol-rich microdomains termed rafts. Such rafts can be isolated from many cell types including epithelial cells, neural cells, and lymphocytes. In polarized cells, the rafts segregate in distinct regions of the cell. The rafts constitute platforms for signal transduction via raft-associated srcfamily tyrosine kinases. This review compares the sorting, distribution, and signaling of GPI-anchored proteins and rafts in epithelial cells, lymphocytes, and neural cells. A possible involvement of rafts in distinct diseases is also addressed.

Signal transduction across cellular membranes can be mediated by coupling of the clustering of anchored proteins in both leaflets

One key question in signal transduction is how the signal is relayed from the outer leaflet of a cellular membrane to the inner leaflet. Using a simulation model, a mechanism for the mediation of signal transduction is proposed here in which the coupling between membrane proteins in different leaflets can be achieved by the clustering of anchored proteins, without recruiting transmembrane proteins. Depending on the hydrophobic length of the anchored proteins, three coupling patterns, including face-to-face clustering, interdigitated clustering, and weak-coupled clustering, are observed in this work. This observation provides a possible explanation of how a particular downstream signaling pathway is selected.

CD66b regulates adhesion and activation of human eosinophils

Eosinophils and their products are likely important in the pathophysiology of allergic diseases, such as bronchial asthma, and in host immunity to parasitic organisms. However, the mechanisms for proinflammatory mediator release by eosinophils are poorly understood. CD66b (CEACAM8, CGM6, NCA-95) is a single chain, GPI-anchored, highly glycosylated protein belonging to the carcinoembryonic Ag supergene family. CD66b is an activation marker for human granulocytes; however, its biological functions are largely unknown in eosinophils. We found that CD66b is highly expressed on the surface of human peripheral blood eosinophils isolated from healthy individuals. Engagement of CD66b, but not CD66a, by mAb or a natural ligand, galectin-3, activated a Src kinase family molecule, hemopoietic cell kinase (Hck), and induced cellular adhesion, superoxide production, and degranulation of eosinophils. CD66b molecules were localized in lipid rafts, and disruption of lipid rafts or removal of the GPI anchor inhibited the adhesion and activation of eosinophils. Importantly, CD66b was constitutively and physically associated with a beta2 integrin, CD11b, and cross-linking of CD66b induced a striking clustering of CD11b molecules. Thus, CD66b molecules are involved in regulating adhesion and activation of eosinophils, possibly through their localization in lipid rafts and interaction with other cell surface molecules, such as CD11b. Binding of exogenous or endogenous carbohydrate ligands(s) to CD66b may be important in the release of proinflammatory mediators by human eosinophils.

Mechanisms by which docosahexaenoic acid and related fatty acids reduce colon cancer risk and inflammatory disorders of the intestine

A growing body of epidemiological, clinical, and experimental evidence has underscored both the pharmacological potential and the nutritional value of dietary fish oil enriched in very long chain n-3 PUFAs such as docosahexaenoic acid (DHA, 22:6, n-3) and eicosapentaenoic acid (EPA, 20:5, n-3). The broad health benefits of very long chain n-3 PUFAs and the pleiotropic effects of dietary fish oil and DHA have been proposed to involve alterations in membrane structure and function, eicosanoid metabolism, gene expression and the formation of lipid peroxidation products, although a comprehensive understanding of the mechanisms of action has yet to be elucidated. In this review, we present data demonstrating that DHA selectively modulates the subcellular localization of lipidated signaling proteins depending on their transport pathway, which may be universally applied to other lipidated protein trafficking. An interesting possibility raised by the current observations is that lipidated proteins may exhibit different subcellular distribution profiles in various tissues, which contain a distinct membrane lipid composition. In addition, the current findings clearly indicate that subcellular localization of proteins with a certain trafficking pathway can be subjected to selective regulation by dietary manipulation. This form of regulated plasma membrane targeting of a select subset of upstream signaling proteins may provide cells with the flexibility to coordinate the arrangement of signaling translators on the cell surface. Ultimately, this may allow organ systems such as the colon to optimally decode, respond, and adapt to the vagaries of an ever-changing extracellular environment.

A co-clustering model involving alpha5beta1 integrin for the biological effects of GPI-anchored human carcinoembryonic antigen (CEA)

CEA functions as an intercellular adhesion molecule and is up-regulated in a wide variety of human cancers, including colon, breast and lung. Its over-expression inhibits cellular differentiation, blocks cell polarization, distorts tissue architecture, and inhibits anoikis of many different cell types. Here we report results concerning the molecular mechanism involved in these biological effects, where relatively rapid molecular changes not requiring alterations in gene expression were emphasized. Confocal microscopy experiments showed that antibody-mediated clustering of a deletion mutant of CEA (DeltaNCEA), normally incapable of self binding and clustering, led to the co-localization of integrin alpha5beta1 with patches of DeltaNCEA on the cell surface. Activation of alpha5, as defined by an anti-alpha5 mAb-sensitive increase in cell adhesion to immobilized fibronectin, and an increased binding of soluble fibronectin to cells, was also observed. This was accompanied by the recruitment of integrin-linked kinase (ILK), protein kinase B (PKB/Akt), and the mitogen-activated protein kinase (MAPK) to membrane microdomains and the phosphorylation of Akt and MAPK. Inhibition of PI3-K and ILK, but not MAPK, prevented the alpha5beta1 integrin activation. Conversely, anti-alpha5 antibody inhibited the PI3-K-mediated activation of Akt, implying the involvement of outside-in and inside-out signaling in integrin activation. Therefore we propose that CEA-mediated signaling involves clustering of CEA and co-clustering and activation of the alpha5beta1 and associated specific signaling elements on the internal surfaces of membrane microdomains. These changes may represent a molecular mechanism for the biological effects of CEA.

The melanoma tumor antigen, melanotransferrin (p97): a 25-year hallmark – from iron metabolism to tumorigenesis

Melanotransferrin (MTf) or melanoma tumor antigen p97 is a transferrin (Tf) homolog that is found predominantly bound to the cell membrane via a glycosyl phosphatidylinositol anchor. The molecule is a member of the Tf superfamily and binds iron through a single high-affinity iron(III)-binding site. Since its discovery on the plasma membrane of melanoma cells, the function of MTf has remained intriguing, particularly in relation to its role in cancer cell iron transport. In fact, considering the crucial role of iron in many metabolic pathways, e.g., DNA synthesis, it was important to understand the function of MTf in the transport of this vital nutrient. MTf has also been implicated in diverse physiological processes, such as plasminogen activation, angiogenesis and cell migration. However, recent studies using a knockout mouse and post-transcriptional gene silencing have demonstrated that MTf is not involved in iron metabolism, but plays a vital role in melanoma cell proliferation and tumorigenesis. In this review, we discuss the possible biological functions of MTf, particularly in relation to cancer.

HLA-G is found in lipid rafts and can act as a signaling molecule

HLA-G protein is the functional homolog of Qa-2, the product of the mouse preimplantation embryo development (Ped) gene. Embryos expressing Qa-2 on the cell surface exhibit a faster rate of preimplantation cleavage and preferential survival in utero compared with Qa-2-negative embryos. Qa-2 is glycosylphosphatidylinositol (GPI) linked in the cell membrane. As a result, Qa-2 proteins cluster in cholesterol- and sphingolipid-rich lipid raft microdomains in the cell membrane and can signal via raft-associated intracellular signaling molecules. Using T cells as a model system, cross-linking of Qa-2 on the cell membrane has been shown to induce proliferation of resting cells. HLA-G, like Qa-2, lacks a cytoplasmic domain capable of transducing signals from the cell surface to the nucleus, but unlike Qa-2, HLA-G has a short six-amino acid cytoplasmic tail rather than a GPI anchor. To test whether HLA-G, like Qa-2, is located in lipid rafts and can act as a signaling molecule, we used an HLA-G transgenic mouse system. T cells were isolated and tested for HLA-G expression by immunofluorescence and for localization of HLA-G in lipid rafts by immunofluorescence and Western blotting. Next, the T cells were cross-linked with anti-HLA-G antibody to test for induction of proliferation. Our novel results show that HLA-G, like GPI-linked Qa-2, is present in lipid rafts in the cell membrane and can act as a signaling molecule to induce proliferation of resting T cells.

Molecular basis of mammalian cell invasion by Trypanosoma cruzi

Establishment of infection by Trypanosoma cruzi, the agent of Chagas' disease, depends on a series of events involving interactions of diverse parasite molecules with host components. Here we focus on the mechanisms of target cell invasion by metacyclic trypomastigotes (MT) and mammalian tissue culture trypomastigotes (TCT). During MT or TCT internalization, signal transduction pathways are activated both in the parasite and the target cell, leading to Ca2+ mobilization. For cell adhesion, MT engage surface glycoproteins, such as gp82 and gp35/50, which are Ca2+ signal-inducing molecules. In T. cruzi isolates that enter host cells in gp82-mediated manner, parasite protein tyrosine kinase as well as phospholipase C are activated, and Ca2+ is released from I P3-sensitive stores, whereas in T. cruzi isolates that attach to target cells mainly through gp35/50, the signaling pathway involving adenylate cyclase appears to be stimulated, with Ca2+ release from acidocalciosomes. In addition, T. cruzi isolate-dependent inhibitory signals, mediated by MT-specific gp90, may be triggered both in the host cell and the parasite. The repertoire of TCT molecules implicated in cell invasion includes surface glycoproteins of gp85 family, with members containing binding sites for laminin and cytokeratin 18, enzymes such as cruzipain, trans-sialidase, and an oligopeptidase B that generates a Ca2+-agonist from a precursor molecule.

Oligodendrocyte-myelin glycoprotein is present in lipid rafts and caveolin-1-enriched membranes

The oligodendrocyte-myelin glycoprotein is a ligand of the neuronal Nogo receptor and a potent inhibitor of neurite outgrowth, but its physiological function remains to be elucidated. The oligodendrocyte-myelin glycoprotein is anchored solely in the outer leaflet of the plasma membrane via its glycosylphosphatidylinositol anchor, and through its leucine-rich repeat domain, it likely interacts with other proteins. In the present study, we compare its buoyancy and detergent solubility characteristics with those of other myelin proteins. Based on its detergent solubility profile and membrane fractionation using established ultracentrifugation procedures, we conclude that the oligodendrocyte-myelin glycoprotein is a lipid raft component that is closely associated with the axolemma. Moreover, it associates with caveolin-1 and caveolin-1-enriched membranes. We postulate that, by virtue of its concentration in lipid rafts and perhaps through interactions with caveolin-1, the oligodendrocyte-myelin glycoprotein may influence signaling pathways.

What can GPI do for you?

Various functions for glycosylphosphatidylinositol (GPI) protein anchors have been described in mammalian and protozoan systems. These data suggest that some functions are common to higher and lower eukaryotes, whereas others may represent adaptations that are specifically advantageous to either unicellular or metazoan organisms. In this article, Mike Ferguson discusses the current theories of GPI function that have relevance to protozoan parasites and their mammalian hosts.

Artificially lipid-anchored proteins can elicit clustering-induced intracellular signaling events in Jurkat T-lymphocytes independent of lipid raft association

We have incorporated artificial lipid-anchored streptavidin conjugates with fully saturated or polyunsaturated lipid anchors into the plasma membranes of Jurkat T-lymphocytes to assess previous conclusions that the activation of signaling processes induced in these cells by clustering of endogenous glycosylphosphatidylinositol-anchored proteins or ganglioside GM1 depends specifically on the association of these membrane components with lipid rafts. Lipid-anchored streptavidin conjugates could be incorporated into Jurkat or other mammalian cell surfaces by inserting biotinylated phosphatidylethanolamine-polyethyleneglycols (PE-PEGs) and subsequently binding streptavidin to the cell-incorporated PE-PEGs. Saturated dipalmitoyl-PE-PEG-streptavidin conjugates prepared in this manner partitioned substantially into the detergent-insoluble membrane fraction isolated from Jurkat or fibroblast cells, whereas polyunsaturated dilinoleoyl-PE-PEG-anchored conjugates were wholly excluded from this fraction, consistent with the differences in the affinities of the two types of lipid anchors for liquid-ordered membrane domains. Remarkably, however, antibody-mediated cross-linking of either dipalmitoyl- or dilinoleoyl-PE-PEG-anchored streptavidin conjugates in Jurkat cells induced elevation of cytoplasmic calcium levels and tyrosine phosphorylation of the scaf-folding protein linker of T-cell activation in a manner similar to that observed upon cross-linking of endogenous CD59 or ganglioside GM1. The amplitude of the cross-linking-stimulated elevation of cytoplasmic calcium moreover showed an essentially identical dependence on the level of incorporated streptavidin conjugate for either type of lipid anchor. Confocal fluorescence microscopy revealed that PE-PEG-streptavidin conjugates with saturated versus polyunsaturated anchors showed very similar surface distributions vis à vis GM1 or CD59 under conditions where one or both species were cross-linked. These results indicate that cross-linking of diverse proteins anchored only to the outer leaflet of the plasma membrane can induce activation of Jurkat T-cell-signaling responses, but they appear to contradict previous suggestions that this phenomenon rests specifically on the association of such species with lipid rafts.

Distinct membrane localization and kinase association of the two isoforms of CD58

The adhesion molecule CD58 is involved in intercellular adhesion and in signal transduction. It is natively expressed in both a transmembrane form and a glycosylphosphatidylinositol (GPI)-anchored form, and hence provides a model for the study of two distinct membrane-anchored forms of the same protein in the same cell. We demonstrate here that the two isoforms of CD58 are localized in distinct membrane compartments. The GPI-anchored form localizes in lipid rafts, while the transmembrane form resides in nonraft domains. In addition to distinct membrane localization, the two isoforms of CD58 differ in their association with protein kinases. GPI-anchored CD58, residing in raft domains, is constitutively associated with protein kinases. However, cross-linking mediates a substantial increase in kinase activity which is predominantly associated with the transmembrane CD58 in nonraft membrane domains. The extensive inducible kinase activity, associated with transmembrane CD58, is demonstrated in wild-type cells as well as in GPI-deficient variant cells. Thus, although the transmembrane CD58 is excluded from rafts, it may trigger signaling independently of the GPI-linked isoform.

Human Doppel and prion protein share common membrane microdomains and internalization pathways

Doppel is the first identified homologue of the prion protein (PrPc) implicated in prion disease. Doppel is considered an N-truncated form of PrPc, and shares with PrPc several structural and biochemical features. When over expressed in the brain of some PrP knockout animals, it provokes cerebellar ataxia. As this phenotype is rescued by reintroducing the PrP gene, it has been suggested that Doppel and PrPc have antagonistic functions and may compete for a common ligand. However, a direct interaction between the two proteins has recently been observed. To investigate whether the neuronal environment is suitable for such possibility, human Doppel and PrPc were expressed separately, or together, in neuroblastoma cells, and then studied by biochemical and immunomicroscopic tools, as well as in intact cells expressing fluorescent fusion constructs. The results demonstrate that Doppel and PrPc co-patch extensively at the plasma membrane, and get internalized together after ganglioside cross-linking by cholera toxin or addition of an antibody against only one of the proteins. These processes no longer occur if the integrity of rafts is disrupted. We also show that, whereas each protein expressed alone occupies Triton X-100-insoluble membrane microdomains, co-transfected Doppel and PrPc redistribute together into a less ordered lipidic environment. All these features are consistent with interactions occurring between Doppel and PrPc in our neuronal cell model.

The Netrin family of guidance factors: emphasis on Netrin-1 signalling

During the development of the nervous system, neurons respond to the coordinated action of a variety of attractive and repulsive signals from the embryonic environment. Netrins form a family of extracellular proteins that regulate the migration of neurons and axonal growth cones. These proteins are bifunctional signals that are chemoattractive for some neurons and chemorepellent for others. Netrins mainly interact with the specific receptors DCC and UNC-5 family. To date, several Netrins have been described in mouse and humans: Netrin-1, -3/NTL2, -4/beta and G-Netrins. Netrin-1 is the most studied member of the family. It is involved in the development many projections of the nervous system. When Netrin-1 interacts with its specific receptors, a cascade of local cytoplasmic events is triggered. Several signal transduction pathways and effector molecules have been implicated in the response to Netrin-1: small Rho-GTPases, MAP-Kinases, second messengers and the Microtubule Associated Protein 1B (MAP1B).

Membrane domains in lymphocytes - from lipid rafts to protein scaffolds

Lateral compartmentalization of the plasma membrane into domains is a key feature of immune cell activation and subsequent immune effector functions. Here, we will review the high diversity of membrane domains, ranging from elementary lipid rafts, envisioned as dynamic and small domains (in the tens of nm), to relatively stable microm-scale membrane domains, which form the immunologic synapse of T lymphocytes. We will discuss the relationship between these different types of plasma membrane domains and how raft lipid- and protein-controlled interactions and cell biological processes cooperate to generate functional domains that mediate lymphocyte activity.

Lipid rafts in cytokine signaling

Lipid rafts are established as critical structures for a variety of cellular processes, including immune cell activation. Beyond their importance for initial immune cell activation at the immunological synapse, lipid rafts are now also being recognized as important sites for cytokine and growth factor signal transduction, both in immune cells as part of secondary regulatory processes, and in non-immune cells. This review summarizes current knowledge regarding the roles of rafts in cytokine signaling and emphasizes the need for measures to better standardize the study of rafts.

CEACAM6 is a determinant of pancreatic adenocarcinoma cellular invasiveness

Pancreatic adenocarcinoma is among the most aggressively invasive malignancies. The immunoglobulin superfamily member carcinoembryonic antigen-related cell adhesion molecule 6 (CEACAM6) is emerging as an important determinant of the malignant phenotype in a range of cancers. We sought to define the role of CEACAM6 in pancreatic adenocarcinoma cellular invasiveness. CEACAM6 was stably overexpressed in Capan2 cells, which inherently express low levels of CEACAM6. Retrovirally mediated RNA interference was used to silence CEACAM6 expression in BxPC3 cells, which inherently overexpress CEACAM6. Cellular invasiveness was quantified using a modified Boyden chamber assay. Overexpression of CEACAM6 increased Capan2 cellular invasiveness, whereas CEACAM6 knockdown attenuated BxPC3 invasiveness. A role for the c-Src tyrosine kinase in mediating CEACAM6-dependent invasiveness was defined using constitutively active and dominant-negative c-Src expression constructs. c-Src-dependent modulation of matrix metalloproteinase-9 activity contributes significantly to the increased cellular invasiveness induced by CEACAM6 overexpression. Levels of CEACAM6 expression can modulate pancreatic adenocarcinoma cellular invasiveness in a c-Src-dependent manner. This pathway warrants further investigation as a target for therapy.

Enhanced responses of glycosylphosphatidylinositol anchor-deficient T lymphocytes

The functions of GPI-anchored proteins in T lymphocyte activation have been controversial. This issue was addressed by studying the responses of T lymphocytes from T lymphocyte-specific GPI anchor-deficient mice to different stimuli that normally allow coligation of TCR and GPI-anchored proteins. Stimulation of GPI anchor-deficient T lymphocytes with ConA induced 2-fold higher proliferative responses than did normal cells. In response to allogeneic stimulation, proliferation of GPI anchor-deficient T lymphocytes was enhanced 2- to 3-fold. The response to ConA of a GPI anchor-deficient anti-OVA T lymphocyte clone generated from these mice was approximately 3-fold higher than that of cells from the same clone in which GPI anchor expression was restored by retroviral transduction. The response of the GPI anchor-deficient cloned anti-OVA T lymphocytes to antigenic stimulation was similar to that of the retrovirally restored cells. These results indicate that coligation with GPI-anchored proteins counteracts the response to TCR stimulation by ConA or alloantigen but not protein Ag.

Inhibition of Fas-mediated apoptotic cell death of murine T lymphocytes in a mouse model of immunosenescence in linkage to deterioration in cell membrane raft function

We previously developed a transgenic mouse line into which a rabbit protein kinase Calpha (PKCalpha) gene fused to a human CD2 promoter/enhancer was introduced, and we found that immunosenescence was facilitated in these transgenic mice. In this study, we found that along with age-dependent increase in the level of protein expression of PKCalpha and its translocation to the membrane, activated T cells became less sensitive to apoptosis-inducing anti-Fas antibody. The capacity of T cells to express Fas antigen on their surfaces in response to anti-CD3 and interleukin-2 was impaired in PKCalpha-transgenic mice of relatively advanced age, although background Fas expression levels on T cells from those mice were high. We then found that out of proportion to a high level of cell surface Fas expression the density of cholera toxin B (CTx)-binding raft elements decreased in PKCalpha-transgenic mice of relatively advanced age and to a lesser extent in normal mice of advanced age. Correspondingly, the expression level of raft-associating Lck was decreased in these mice. These findings suggest for the first time that immunosenescence of T cells involves a decrease in density of cell surface CTx-binding raft elements, which might underlie a deterioration in T-cell signal pathway for either cell death or cell activation.

Lipid raft domains and protein networks in T-cell receptor signal transduction

Activation of the T-cell antigen receptor (TCR) is a key event in triggering the physiological responses of T lymphocytes to antigen. The earliest TCR-evoked signalling steps, such as tyrosine phosphorylations, ras activation and induction of Ca(2+) fluxes, are initiated in the T-cell plasma membrane. It has been implicated that cholesterol- and sphingolipid-rich membrane domains, termed lipid rafts, form platforms for the regulation and transduction of TCR signals at the plasma membrane; however, recent experiments have now differentiated distinct roles for lipid-raft-mediated and protein-mediated interactions in the formation of TCR signalling membrane domains.

CEACAM6 cross-linking induces caveolin-1-dependent, Src-mediated focal adhesion kinase phosphorylation in BxPC3 pancreatic adenocarcinoma cells

Despite lacking transmembrane or intracellular domains, glycosylphosphatidylinositol-anchored proteins can modulate intracellular signaling events, in many cases through aggregation within membrane "lipid raft" microdomains. CEACAM6 is a glycosylphosphatidylinositol-linked cell surface protein of importance in the anchorage-independent survival and metastasis of pancreatic adenocarcinoma cells. We examined the effects of antibody-mediated cross-linking of CEACAM6 on intracellular signaling events and anchorage-independent survival of the CEACAM6-overexpressing pancreatic ductal adenocarcinoma cell line, BxPC3. CEACAM6 cross-linking increased c-Src activation and induced tyrosine phosphorylation of p125(FAK) focal adhesion kinase. Focal adhesion kinase phosphorylation was dependent on c-Src kinase activation, for which caveolin-1 was required. CEACAM6 cross-linking induced a significant increase in cellular resistance to anoikis. These observations represent the first characterization of the mechanism through which this important cell surface oncoprotein influences intracellular signaling events and hence malignant cellular behavior.

Lipid rafts: elusive or illusive?

There has been considerable recent interest in the possibility that the plasma membrane contains lipid "rafts," microdomains enriched in cholesterol and sphingolipids. It has been suggested that such rafts could play an important role in many cellular processes including signal transduction, membrane trafficking, cytoskeletal organization, and pathogen entry. However, rafts have proven difficult to visualize in living cells. Most of the evidence for their existence and function relies on indirect methods such as detergent extraction, and a number of recent studies have revealed possible problems with these methods. Direct studies of the distribution of raft components in living cells have not yet reached a consensus on the size or even the presence of these microdomains, and hence it seems that a definitive proof of raft existence has yet to be obtained.

CEACAM6 gene silencing impairs anoikis resistance and in vivo metastatic ability of pancreatic adenocarcinoma cells

Anoikis is the apoptotic response induced in normal cells by inadequate or inappropriate adhesion to substrate. It is postulated that resistance to anoikis facilitates tumorigenesis and metastasis. Carcinoembryonic antigen-related cell adhesion molecule 6 (CEACAM6) is an immunoglobulin superfamily member overexpressed in a number of human cancers and implicated in anoikis resistance. We tested the effect of CEACAM6 gene silencing on anoikis in pancreatic adenocarcinoma cell lines. Anoikis was induced in PANC1, Capan2, MiaPaCa2 and Mia(AR) (a MiaPaCa2-derived anoikis-resistant subline) by culture in poly-2-hydroxyethylmethacrylate-coated wells. Anoikis was quantified by YO-PRO-1/propidium iodide staining and flow cytometry. The role of caspase activation was determined using fluorometric profiling and the caspase inhibitor Z-Val-Ala-Asp-fluoromethyl ketone (Z-VAD-fmk). CEACAM6 expression was suppressed by RNA interference. Using a nude mouse orthotopic xenograft model, we assessed the effect of this treatment on in vivo metastatic ability. Anoikis resistance was associated with increased CEACAM6 expression. CEACAM6-specific short interfering ribonucleic acid (siRNA), but not control siRNA, increased susceptibility to caspase-mediated anoikis, an effect abrogated by Z-VAD-fmk, and decreased Akt phosphorylation (Ser-473) under anchorage-independent conditions. CEACAM6 gene silencing reversed the acquired anoikis resistance of Mia(AR) and inhibited its in vivo metastatic ability. CEACAM6 warrants further investigation as a novel therapeutic target for the treatment of pancreatic adenocarcinoma.

Cholesterol requirement of hepatitis B surface antigen (HBsAg) secretion

Hepatitis B virus-infected patients secrete enormous quantities (50-300 microg/ml) of hepatitis B surface antigen (HBsAg) in their serum. One hypothesis for this synthetic effort is that these lipoprotein particles serve to adsorb neutralizing antisurface antibodies. We have shown that insulin suppresses the expression of HBsAg in human hepatoma cell Hep3B cells. We further studied the signaling pathway of insulin on the inhibition of HBsAg. Using a fungal metabolite, lovastatin, to block the p21Ras signaling pathway of insulin, we found that lovastatin inhibited the secretion of HBsAg into culture medium in Hep3B cells; however, the involvement of p21Ras-MAPKs was excluded in this effect. The cholesterol depletion from the membrane, leading to the destabilization of rafts, was the mechanism for the lovastatin inhibition of HBsAg secretion. However, lovastatin has no effect on the secretion of infectious viral Dane particles. Herein, we show for the first time that cholesterol is required for HBsAg secretion.

Interaction of phosphoinositolglycan(-peptides) with plasma membrane lipid rafts of rat adipocytes

Insulin receptor-independent activation of the insulin signal transduction cascade in insulin-responsive target cells by phosphoinositolglycans (PIG) and PIG-peptides (PIG-P) is accompanied by redistribution of glycosylphosphatidylinositol (GPI)-anchored plasma membrane proteins (GPI proteins) and dually acylated nonreceptor tyrosine kinases from detergent/carbonate-resistant glycolipid-enriched plasma membrane raft domains of high-cholesterol content (hcDIGs) to rafts of lower cholesterol content (lcDIGs). Here we studied the nature and localization of the primary target of PIG(-P) in isolated rat adipocytes. Radiolabeled PIG-P (Tyr-Cys-Asn-NH-(CH(2))(2)-O-PO(OH)O-6Manalpha1(Manalpha1-2)-2Manalpha1-6Manalpha1-4GluN1-6Ino-1,2-(cyclic)-phosphate) prepared by chemical synthesis or a radiolabeled lipolytically cleaved GPI protein from Saccharomyces cerevisiae, which harbors the PIG-P moiety, bind to isolated hcDIGs but not to lcDIGs. Binding is saturable and abolished by pretreatment of intact adipocytes with trypsin followed by NaCl or with N-ethylmaleimide, indicating specific interaction of PIG-P with a cell surface protein. A 115-kDa polypeptide released from the cell surface by the trypsin/NaCl-treatment is labeled by [(14)C]N-ethylmaleimide. The labeling is diminished upon incubation of adipocytes with PIG-P which can be explained by direct binding of PIG-P to the 115-kDa protein and concomitant loss of its accessibility to N-ethylmaleimide. Binding of PIG-P to hcDIGs is considerably increased after pretreatment of adipocytes with (glycosyl)phosphatidylinositol-specific phospholipases compatible with lipolytic removal of endogenous ligands, such as GPI proteins/lipids. These data demonstrate that in rat adipocytes synthetic PIG(-P) as well as lipolytically cleaved GPI proteins interact specifically with hcDIGs. The interaction depends on the presence of a trypsin/NaCl/NEM-sensitive 115-kDa protein located at hcDIGs which thus represents a candidate for a binding protein for exogenous insulin-mimetic PIG(-P) and possibly endogenous GPI proteins/lipids.

Complementary expression and neurite outgrowth activity of netrin-G subfamily members

Classical members of the UNC6/netrin family are secreted proteins which play a role as long-range cues for directing growth cones. We here identified in mice a novel member netrin-G2 which constitute a subfamily with netrin-G1 among the UNC6/netrin family. Both of these netrin-Gs are characterized by glycosyl phosphatidyl-inositol linkage onto cells, molecular variants presumably generated by alternative splicing and lack of any appreciable affinity to receptors for classical netrins. These genes are preferentially expressed in the central nervous system with complementary distribution in most brain areas, that is netrin-G1 in the dorsal thalamus, olfactory bulb and inferior colliculus, and netrin-G2 in the cerebral cortex, habenular nucleus and superior colliculus. Consistently, immunohistochemical analysis revealed that netrin-G1 molecules are present on thalamocortical but not corticothalamic axons. Thalamic and neocortical neurons extended long neurites on immobilized recombinant netrin-G1 or netrin-G2 in vitro. Immobilized anti-netrin-G1 antibodies altered shapes of cultured thalamic neurons. We propose that netrin-Gs provide short-range cues for axonal and/or dendritic behavior through bi-directional signaling.

The dual functions of GPI-anchored PH-20: hyaluronidase and intracellular signaling

The ovulated mammalian oocyte is surrounded by the "cumulus ECM", composed of cells embedded in an extracellular matrix that is rich in hyaluronic acid (HA). The cumulus ECM is a viscoelastic gel that sperm must traverse prior to fertilization. Mammalian sperm have a GPI-anchored hyaluronidase which is known as PH-20 and also as SPAM 1. PH-20 is located on the sperm surface, and in the lysosome-derived acrosome, where it is bound to the inner acrosomal membrane. PH-20 appears to be a multifunctional protein; it is a hyaluronidase, a receptor for HA-induced cell signaling, and a receptor for the zona pellucida surrounding the oocyte. The zona pellucida recognition function of PH-20 was discovered first. This function is ascribed to the inner acrosomal membrane PH-20, which appears to differ biochemically from the PH-20 on the sperm surface. Later, when bee venom hyaluronidase was cloned, a marked cDNA sequence homology with PH-20 was recognized, and it is now apparent that PH-20 is the hyaluronidase of mammalian sperm. PH-20 is unique among the hyaluronidases in that it has enzyme activity at both acid and neutral pH, and these activities appear to involve two different domains in the protein. The neutral enzyme activity of plasma membrane PH-20 is responsible for local degradation of the cumulus ECM during sperm penetration. Plasma membrane PH-20 mediates HA-induced sperm signaling via a HA binding domain that is separate from the hyaluronidase domains. This signaling is associated with an increase in intracellular calcium and as a consequence, the responsiveness of sperm to induction of the acrosome reaction by the zona pellucida is increased. There is extensive evidence that GPI-anchored proteins are involved in signal transduction initiated by a diverse group of cell surface receptors. GPI-anchored proteins involved in signaling are often associated with signaling proteins bound to the cytoplasmic leaflet of the plasma membrane, typically Src family, non-receptor protein tyrosine kinases. PH-20 appears to initiate intracellular signaling by aggregating in the plasma membrane, and a 92-kDa protein may be the cell signaling molecule linked to PH-20.

The role of lipid rafts in signalling and membrane trafficking in T lymphocytes

Combinatorial association of different lipid species generates microheterogeneity in biological membranes. The association of glycosphingolipids with cholesterol forms membrane microdomains – lipid rafts – that are involved in specialised pathways of protein/lipid transport and signalling. Lipid rafts are normally dispersed in cellular membranes and appear to require specialised machinery to reorganise them to operate. Caveolin-1 and MAL are members of two different protein families involved in reorganisation of lipid rafts for signalling and/or intracellular transport in epithelial cells. T cell activation induces a rapid compartmentalisation of signalling machinery into reorganised rafts that are used as platforms for the assembly of the signalling complex. Costimulatory molecules participate in this process by providing signals that mobilise raft lipids and proteins, and remodel the cytoskeleton to the contact site. As in epithelial cells, rafts are used also as vesicular carriers for membrane trafficking in T lymphocytes. Furthermore, there are potential similarities between the specialised protein machinery underlying raft-mediated processes in T lymphocytes and polarised epithelial cells.

Glycosylphosphatidyl inositol-anchored proteins and fyn kinase assemble in noncaveolar plasma membrane microdomains defined by reggie-1 and -2

Using confocal laser scanning and double immunogold electron microscopy, we demonstrate that reggie-1 and -2 are colocalized in < or =0.1-microm plasma membrane microdomains of neurons and astrocytes. In astrocytes, reggie-1 and -2 do not occur in caveolae but clearly outside these structures. Microscopy and coimmunoprecipitation show that reggie-1 and -2 are associated with fyn kinase and with the glycosylphosphatidyl inositol-anchored proteins Thy-1 and F3 that, when activated by antibody cross-linking, selectively copatch with reggie. Jurkat cells, after cross-linking of Thy-1 or GM1 (with the use of cholera toxin), exhibit substantial colocalization of reggie-1 and -2 with Thy-1, GM1, the T-cell receptor complex and fyn. This, and the accumulation of reggie proteins in detergent-resistant membrane fractions containing F3, Thy-1, and fyn imparts to reggie-1 and -2 properties of raft-associated proteins. It also suggests that reggie-1 and -2 participate in the formation of signal transduction centers. In addition, we find reggie-1 and -2 in endolysosomes. In Jurkat cells, reggie-1 and -2 together with fyn and Thy-1 increase in endolysosomes concurrent with a decrease at the plasma membrane. Thus, reggie-1 and -2 define raft-related microdomain signaling centers in neurons and T cells, and the protein complex involved in signaling becomes subject to degradation.

Characterization of a 150 kDa accessory receptor for TGF-beta 1 on keratinocytes: direct evidence for a GPI anchor and ligand binding of the released form

Transforming growth factor-beta (TGF-beta) is a key modulator of epidermal development and homeostasis, and has been shown to potently regulate keratinocyte migration and function during wound repair. There are three cloned TGF-beta receptors termed type I, type II, and type III that are found on most cell types. The types I and II are the signaling receptors, while the type III is believed to facilitate TGF-beta binding to the types I and II receptors. Recently, we reported that in addition to these receptors, human keratinocytes express a 150 kDa TGF-beta 1 binding protein (r150) which forms a heteromeric complex with the TGF-beta signaling receptors. This accessory receptor was described as glycosyl phosphatidylinositol-specific anchored based on its sensitivity to phosphatidylinositol phospholipase C (PIPLC). In the present study, we demonstrate that the GPI-anchor is contained in r150 itself and not on a tightly associated protein and that it binds TGF-beta 1 with an affinity similar to those of the types I and II TGF-beta signaling receptors. Furthermore, the PIPLC released (soluble) form of this protein is capable of binding TGF-beta 1 independently from the signaling receptors. In addition, we provide evidence that r150 is released from the cell surface by an endogenous phospholipase C. Our observation that r150 interacts with the TGF-beta signaling receptors, together with the finding that the soluble r150 binds TGF-beta 1 suggest that r150 in either its membrane anchored or soluble form may potentiate or antagonize TGF-beta signaling. Elucidating the mechanism by which r150 functions as an accessory molecule in TGF-beta signaling may be critical to understanding the molecular mechanisms underlying the regulation of TGF-beta action in keratinocytes.

Cell type-specific expression of fasciclin II isoforms reveals neuronal-glial interactions during peripheral nerve growth

During the formation of the insect peripheral nervous system (PNS), the cell adhesion receptor fasciclin II has been shown to play a prominent role in axonal fasciculation and synapse formation during motor neuron outgrowth. In the moth Manduca, fasciclin II (MFas II) is expressed both as a transmembrane isoform (TM-MFas II) and a glycosyl phosphatidylinositol-linked isoform (GPI-MFas II). By using RNA and antibody probes, we have shown that these two isoforms are expressed in nonoverlapping patterns: TM-MFas II is expressed exclusively by neurons and becomes localized to their most motile regions, while GPI-MFas II is expressed primarily by the glial cells that ensheath the peripheral nerves. This cell-type specificity of expression allowed us to monitor the nature of neuronal-glial interactions during PNS development. The outgrowth of TM-MFas II-positive axons in many regions preceded the arrival of GPI-MFas II-expressing glial processes that enwrapped them. In a few key locations, however, GPI-MFas II-positive glial cells differentiated before the arrival of the first axons and prefigured their subsequent trajectories. Prior inhibition of GPI-MFas II expression disrupted the subsequent outgrowth of axons at these locations but not elsewhere in the PNS. Our results suggest that the two isoforms of MFas II play distinct roles with respect to cellular motility and nerve formation.

An anti-inflammatory drug, propagermanium, may target GPI-anchored proteins associated with an MCP-1 receptor, CCR2

Monocyte chemoattractant protein-1 (MCP-1) promotes the migration and activation of monocytes and plays a pivotal role in the development of chronic inflammation. Propagermanium (3-oxygermylpropionic acid polymer) has been used as a therapeutic agent against chronic hepatitis B in Japan. We report here that propagermanium specifically inhibits in vitro chemotactic migration of monocytes by MCP-1. Propagermanium did not inhibit binding of MCP-1 to a human monocytic cell line, THP-1 cells, or affect intracellular Ca(2+) mobilization or the cAMP concentration in MCP-1-treated THP-1 cells. The effect of propagermanium seems to require glycosylphosphatidylinositol (GPI)-anchored proteins, as cleavage of GPI anchors by phosphatidylinositol-phospholipase C (PI-PLC) eliminated the inhibitory activity of propagermanium. Anti-GPI-anchored protein antibodies, such as anti-CD55 and anti-CD59, reduced staining of C-C chemokine receptor 2 (CCR2) with an anti-CCR2 antibody against the N-terminus of CCR2 in a flow cytometric analysis, and these antibodies also selectively inhibited MCP-1-induced migration of THP-1 cells. Furthermore, under fluorescence microscopy, GPI-anchored proteins colocalized with CCR2 on THP-1 cells. These results suggest that propagermanium may target GPI-anchored proteins that are closely associated with CCR2 to selectively inhibit the MCP-1-induced chemotaxis, thus providing a mechanistic basis for the anti-inflammatory effects of the drug.

The carboxyl terminus of Dictyostelium discoideum protein 1I encodes a functional glycosyl-phosphatidylinositol signal sequence

The 1I gene is expressed in the prespore cells of culminating Dictyostelium discoideum. The open reading frame of 1I cDNA encodes a protein of 155 amino acids with hydrophobic segments at both its NH(2)- and COOH-termini that are indicative of a glycosyl-phosphatidylinositol (GPI)-anchored protein. A hexaHis-tagged form of 1I expressed in D. discoideum cells appeared on Western blot analysis as a doublet of 27 and 24 kDa, with a minor polypeptide of 22 kDa. None of the polypeptides were released from the cell surface with bacterial phosphatidylinositol-specific phospholipase C, although all three were released upon nitrous acid treatment, indicating the presence of a phospholipase-resistant GPI anchor. Further evidence for the C-terminal sequence of 1I acting as a GPI attachment signal was obtained by replacing the GPI anchor signal sequence of porcine membrane dipeptidase with that from 1I. Two constructs of dipeptidase with the 1I GPI signal sequence were constructed, one of which included an additional six amino acids in the hydrophilic spacer. Both of the resultant constructs were targeted to the surface of COS cells and were GPI-anchored as shown by digestion with phospholipase C, indicating that the Dictyostelium GPI signal sequence is functional in mammalian cells. Site-specific antibodies recognising epitopes either side of the expected GPI anchor attachment site were used to determine the site of GPI anchor attachment in the constructs. These parallel approaches show that the C-terminal signal sequence of 1I can direct the addition of a GPI anchor.

Raft membrane domains: from a liquid-ordered membrane phase to a site of pathogen attack

While the existence of cholesterol/sphingolipid (raft) membrane domains in the plasma membrane is now supported by strong experimental evidence, the structure of these domains, their size, their dynamics, and their molecular composition remain to be understood. Raft domains are thought to represent a specific physical state of lipid bilayers, the liquid-ordered phase. Recent observations suggest that in the mammalian plasma membrane small raft domains in ordered lipid phases are in a dynamic equilibrium with a less ordered membrane environment. Rafts may be enlarged and/or stabilized by protein-mediated cross-linking of raft-associated components. These changes of plasma membrane structure are perceived by the cells as signals, most likely an important element of immunoreceptor signalling. Pathogens abuse raft domains on the host cell plasma membrane as concentration devices, as signalling platforms and/or entry sites into the cell. Elucidation of these interactions requires a detailed understanding raft structure and dynamics.

Listeriolysin O-induced stimulation of mucin exocytosis in polarized intestinal mucin-secreting cells: evidence for toxin recognition of membrane-associated lipids and subsequent toxin internalization through caveolae

Lysteriolysin O (LLO) induces a microtubule-dependent activation of mucin exocytosis in the human mucin-secreting HT29-MTX. Cholesterol inhibits the LLO-induced mucin exocytosis, whereas the oxidized form of cholesterol had no inhibitory effect. LLO-induced mucin exocytosis inhibited by cholesterol can be restored by enzymatic treatment with cholesterol oxidase. Inhibition of cholesterol synthesis in HT29-MTX cells results in a decrease in the LLO-induced mucin exocytosis. Other lipids such as gangliosides are able to inhibit the LLO-induced mucin exocytosis, suggesting that the binding of the toxin occurs at a multiplicity of membrane-associated lipids acting as receptors. Incubation of the toxin with lipids such as cholesterol or gangliosides does not decrease binding of LLO to target membranes. The present work also provides evidence that the LLO-induced mucin exocytosis develops independently of the pore-forming activity of the toxin. Finally, we demonstrated that the toxin associates with detergent-insoluble glycolipid microdomains (DIGs) containing VIP/21 caveolin, allowing internalization of the toxin and subsequent activation of the mucin exocytosis.

Netrin-G1: a novel glycosyl phosphatidylinositol-linked mammalian netrin that is functionally divergent from classical netrins

UNC-6/netrins compose a small phylogenetically conserved family of proteins that act as axon guidance cues. With a signal sequence trap method, we isolated a cDNA encoding a novel member of the UNC-6/netrin family, which we named netrin-G1. Unlike classical netrins, netrin-G1 consists of at least six isoforms of which five were predominantly anchored to the plasma membrane via glycosyl phosphatidyl-inositol linkages. Netrin-G1 transcripts were first detected in midbrain and hindbrain regions by embryonic day 12 and reached highest levels at perinatal stages in various brain regions, including olfactory bulb mitral cells, thalamus, and deep cerebellar nuclei. Its expression was primarily restricted to the CNS. Interestingly, netrin-G1 proteins did not show appreciable affinity to any netrin receptor examined. Unlike netrin-1, a secreted form of netrin-G1 consistently failed to attract circumferentially growing axons from the cerebellar plate. Our findings suggest that netrin-G1 and its putative receptors have coevolved independently from the classical netrins. The expression pattern of netrin-G1 and its predicted neuronal membrane localization suggest it may also have novel signaling functions in nervous system development.

The specificity for the differentiation blocking activity of carcinoembryonic antigen resides in its glycophosphatidyl-inositol anchor

Ectopic expression of various members of the human carcinoembryonic antigen (CEA) family of intercellular adhesion molecules in murine myoblasts either blocks (CEA, CEACAM6) or allows (CEACAM1) myogenic differentiation. These surface glycoproteins form a subset of the immunoglobulin (Ig) superfamily and are very closely related, but differ in the precise sequence of their external domains and in their mode of anchorage to the cell membrane. CEA and CEACAM6 are glycophosphatidyl-inositol (GPI) anchored, whereas CEACAM1 is transmembrane (TM) anchored. Overexpression of GPI-linked neural cell adhesion molecule (NCAM) p125, also an adhesion molecule of the Ig superfamily, accelerates myogenic differentiation. The molecular requirements for the myogenic differentiation block were investigated using chimeric constructs in which the COOH-terminal hydrophobic domains of CEA, CEACAM1, and NCAM p125 were exchanged. The presence of the GPI signal sequence specifically from CEA in the chimeras was sufficient to convert both CEACAM1 and NCAM into differentiation-blocking proteins. Conversely, CEA could be converted into a neutral protein by exchanging its GPI anchor for the TM anchor of CEACAM1. Since the external domains of CEA, CEACAM1, and NCAM can all undergo homophilic interactions, and mutations in the self-adhesive domains of CEA abrogate its differentiation-blocking activity, the structural requirements for differentiation-inhibition are any self-adhesive domains attached to the specific GPI anchor derived from CEA. We therefore suggest that biologically significant functional information resides in the processed extreme COOH terminus of CEA and in the GPI anchor that it determines.

Engagement of the human pre-B cell receptor generates a lipid raft-dependent calcium signaling complex

Pre-B cell receptor (pre-BCR) expression is critical for B lineage development. The signaling events initiated by the pre-BCR, however, remain poorly defined. We demonstrate that lipid rafts are the major functional compartment for human pre-B cell activation. A fraction of pre-BCR was constitutively raft associated, and receptor engagement enhanced this association. These events promoted Lyn activation and Igbeta phosphorylation and led to the generation of a raft-associated signaling module composed of tyrosine phosphorylated Lyn, Syk, BLNK, PI3K, Btk, VAV, and PLCgamma2. Formation of this module was essential for pre-BCR calcium signaling. Together, these observations directly link the previously identified genetic requirement for the components of this module in B lineage development with theirfunctional role(s) in human preBCR signaling.

Paramecium GPI proteins: variability of expression and localization

In Paramecium primaurelia, the two major classes of cell surface proteins, the surface antigen (SAg) and the surface GPI proteins (SGPs), are linked to the plasma membrane through a glycosylphosphatidylinositol (GPI) anchor. In the present study, we have characterized the expression of the SGPs in several geographical strains of P. primaurelia and P. tetraurelia at different temperatures, 23 degrees C and 32 degrees C. The identification of the expressed SGPs was performed on purified cilia, by establishing the SGP SDS-PAGE profiles under four different conditions: with or without their anchoring lipid, cleaved with a Bacillus thuringiensis phosphatidylinositol-specific phospholipase C (PI-PLC), and either in a reduced or in an unreduced state. This screening revealed the existence of specific sets of ciliary SGPs, as a function of temperature and the geographical origin of the strains. The SGPs the most abundant at 23 degrees C and 32 degrees C displayed a rapid turnover. We also looked for the presence of PI-PLC releasable proteins in purified cortices. In addition to the SAg and SGPs, the cortical fraction was shown to contain other PI-PLC releasable proteins, not found in the ciliary fraction, thus localized exclusively in the interciliary region.

Cross talk of pp125(FAK) and pp59(Lyn) non-receptor tyrosine kinases to insulin-mimetic signaling in adipocytes

Signaling molecules downstream from the insulin receptor, such as the insulin receptor substrate protein 1 (IRS-1), are also activated by other receptor tyrosine kinases. Here we demonstrate that the non-receptor tyrosine kinases, focal adhesion kinase pp125(FAK) and Src-class kinase pp59(Lyn), after insulin-independent activation by phosphoinositolglycans (PIG), can cross talk to metabolic insulin signaling in rat and 3T3-L1 adipocytes. Introduction by electroporation of neutralizing antibodies against pp59(Lyn) and pp125(FAK) into isolated rat adipocytes blocked IRS-1 tyrosine phosphorylation in response to PIG but not insulin. Introduction of peptides encompassing either the major autophosphorylation site of pp125(FAK), tyrosine 397, or its regulatory loop with the twin tyrosines 576 and 577 inhibited PIG-induced IRS-1 tyrosine phosphorylation and glucose transport. PIG-induced pp59(Lyn) kinase activation and pp125(FAK) tyrosine phosphorylation were impaired by the former and latter peptide, respectively. Up-regulation of pp125(FAK) by integrin clustering diminished PIG-induced IRS-1 tyrosine phosphorylation and glucose transport in nonadherent but not adherent adipocytes. In conclusion, PIG induced IRS-1 tyrosine phosphorylation by causing (integrin antagonized) recruitment of IRS-1 and pp59(Lyn) to the common signaling platform molecule pp125(FAK), where cross talk of PIG-like structures and extracellular matrix proteins to metabolic insulin signaling may converge, possibly for the integration of the demands of glucose metabolism and cell architecture.

Channel formation by the glycosylphosphatidylinositol-anchored protein binding toxin aerolysin is not promoted by lipid rafts

Glycosylphosphatidylinositol-anchored proteins may be concentrated in membrane microdomains (lipid rafts) that are also enriched in cholesterol and sphingolipids. The glycosyl anchor of these proteins is a specific, high affinity receptor for the channel-forming protein aerolysin. We wished to determine if the presence of rafts promotes the activity of aerolysin. Treatment of T lymphocytes with methyl-beta-cyclodextrin, which destroys lipid rafts by sequestering cholesterol, had no measurable effect on the sensitivity of the cells to aerolysin; nor did similar treatment of erythrocytes decrease the rate at which they were lysed by the toxin. We also studied the rate of aerolysin-induced channel formation in liposomes containing glycosylphosphatidylinositol-anchored placental alkaline phosphatase, which we show is a receptor for aerolysin. In liposomes containing sphingolipids as well as glycerophospholipids and cholesterol, most of the enzyme was Triton X-100-insoluble, indicating that it was localized in rafts, whereas in liposomes prepared without sphingolipids, all of the enzyme was soluble. Aerolysin was no more active against liposomes containing rafts than against those that did not. We conclude that lipid rafts do not promote channel formation by aerolysin.

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.

Signaling through sphingolipid microdomains of the plasma membrane: the concept of signaling platform

Transmembrane signaling requires modular interactions between signaling proteins, phosphorylation or dephosphorylation of the interacting protein partners and temporary elaboration of supramolecular structures, to convey the molecular information from the cell surface to the nucleus. Such signaling complexes at the plasma membrane are instrumental in translating the extracellular cues into intracellular signals for gene activation. In the most straightforward case, ligand binding promotes homodimerization of the transmembrane receptor which facilitates modular interactions between the receptor's cytoplasmic domains and intracellular signaling and adaptor proteins. For example, most growth factor receptors contain a cytoplasmic protein tyrosine kinase (PTK) domain and ligand-mediated receptor dimerization leads to cross phosphorylation of tyrosines in the receptor's cytoplasmic domains, an event that initiates the signaling cascade. In other signaling pathways where the receptors have no intrinsic kinase activity, intracellular nonreceptor PTKs (i.e. Src family PTKs, JAKs) are recruited to the cytoplasmic domain of the engaged receptor. Execution of these initial phosphorylations and their translation into efficient cellular stimulation requires concomitant activation of diverse signaling pathways. Availability of stable, preassembled matrices at the plasma membrane would facilitate scaffolding of a large array of receptors, coreceptors, tyrosine kinases and other signaling and adapter proteins, as it is the case in signaling via the T cell antigen receptor. The concept of the signaling platform has gained usage to characterize the membrane structure where many different membrane-bound components need to be assembled in a coordinated manner to carry out signaling. The structural basis of the signaling platform lies in preferential assembly of certain classes of lipids into distinct physical and functional compartments within the plasma membrane. These membrane microdomains or rafts (Figure 1) serve as privileged sites where receptors and proximal signaling molecules optimally interact. In this review, we shall discuss first how signaling platforms are assembled and how receptors and their signaling machinery could be functionally linked in such structures. The second part of our review will deal with selected examples of raft-based signaling pathways in T lymphocytes and NK cells to illustrate the ways in which rafts may facilitate signaling.

Signal transduction induced in Trypanosoma cruzi metacyclic trypomastigotes during the invasion of mammalian cells

Penetration of Trypanosoma cruzi into mammalian cells depends on the activation of the parasite's protein tyrosine kinase and on the increase in cytosolic Ca2+ concentration. We used metacyclic trypomastigotes, the T. cruzi developmental forms that initiate infection in mammalian hosts, to investigate the association of these two events and to identify the various components of the parasite signal transduction pathway involved in host cell invasion. We have found that i) both the protein tyrosine kinase activation, as measured by phosphorylation of a 175-kDa protein (p175), and Ca2+ mobilization were induced in the metacyclic forms by the HeLa cell extract but not by the extract of T. cruzi-resistant K562 cells; ii) treatment of parasites with the tyrosine kinase inhibitor genistein blocked both p175 phosphorylation and the increase in cytosolic Ca2+ concentration; iii) the recombinant protein J18, which contains the full-length sequence of gp82, a metacyclic stage surface glycoprotein involved in target cell invasion, interfered with tyrosine kinase and Ca2+ responses, whereas the monoclonal antibody 3F6 directed at gp82 induced parasite p175 phosphorylation and Ca2+ mobilization; iv) treatment of metacyclic forms with phospholipase C inhibitor U73122 blocked Ca2+ signaling and impaired the ability of the parasites to enter HeLa cells, and v) drugs such as heparin, a competitive IP3-receptor blocker, caffeine, which affects Ca2+ release from IP3-sensitive stores, in addition to thapsigargin, which depletes intracellular Ca2+ compartments and lithium ion, reduced the parasite infectivity. Taken together, these data suggest that protein tyrosine kinase, phospholipase C and IP3 are involved in the signaling cascade that is initiated on the parasite cell surface by gp82 and leads to Ca2+ mobilization required for target cell invasion.