Cross-linking of CD59 and of other glycosyl phosphatidylinositol-anchored molecules on neutrophils triggers cell activation via tyrosine kinase

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins II: Intercellular Transfer of Matter (Inheritance?) That Matters

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of the plasma membrane (PM) bilayer by covalent linkage to a typical glycolipid and expressed in all eukaryotic organisms so far studied. Lipolytic release from PMs into extracellular compartments and intercellular transfer are regarded as the main (patho)physiological roles exerted by GPI-APs. The intercellular transfer of GPI-APs relies on the complete GPI anchor and is mediated by extracellular vesicles such as microvesicles and exosomes and lipid-free homo- or heteromeric aggregates, and lipoprotein-like particles such as prostasomes and surfactant-like particles, or lipid-containing micelle-like complexes. In mammalian organisms, non-vesicular transfer is controlled by the distance between donor and acceptor cells/tissues; intrinsic conditions such as age, metabolic state, and stress; extrinsic factors such as GPI-binding proteins; hormones such as insulin; and drugs such as anti-diabetic sulfonylureas. It proceeds either "directly" upon close neighborhood or contact of donor and acceptor cells or "indirectly" as a consequence of the induced lipolytic release of GPI-APs from PMs. Those displace from the serum GPI-binding proteins GPI-APs, which have retained the complete anchor, and become assembled in aggregates or micelle-like complexes. Importantly, intercellular transfer of GPI-APs has been shown to induce specific phenotypes such as stimulation of lipid and glycogen synthesis, in cultured human adipocytes, blood cells, and induced pluripotent stem cells. As a consequence, intercellular transfer of GPI-APs should be regarded as non-genetic inheritance of (acquired) features between somatic cells which is based on the biogenesis and transmission of matter such as GPI-APs and "membrane landscapes", rather than the replication and transmission of information such as DNA. Its operation in mammalian organisms remains to be clarified.

The Missing Protein: Is T-Cadherin a Previously Unknown GPI-Anchored Receptor on Platelets?

The membrane of platelets contains at least one uncharacterized glycosylphosphatidylinositol (GPI)-anchored protein according to the literature. Moreover, there is not enough knowledge on the receptor of low-density lipoproteins (LDL) mediating rapid Ca²⁺ signaling in platelets. Coincidentally, expression of a GPI-anchored protein T-cadherin increases LDL-induced Ca²⁺ signaling in nucleated cells. Here we showed evidence that supports the hypothesis about the presence of T-cadherin on platelets. The presence of T-cadherin on the surface of platelets and megakaryocytes was proven using antibodies whose specificity was tested on several negative and positive control cells by flow cytometry and confocal microscopy. Using phosphatidylinositol-specific phospholipase C, the presence of glycosylphosphatidylinositol anchor in the platelet T-cadherin form as well as in other known forms was confirmed. We showed by immunoblotting that the significant part of T-cadherin was detected in specific membrane domains (detergent Triton X-114 resistant) and the molecular weight of this newly identified protein was greater than that of T-cadherin from nucleated cells. Nevertheless, polymerase chain reaction data confirmed only the presence of isoform-1 of T-cadherin in platelets and megakaryocytes, which was also present in nucleated cells. We observed the redistribution of this newly identified protein after the activation of platelets, but only further work may explain its functional importance. Thus, our data described T-cadherin with some post-translational modifications as a new GPI-anchored protein on human platelets.

Role of the Glycosylphosphatidylinositol-Anchored Protein TEX101 and Its Related Molecules in Spermatogenesis

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) on the plasma membrane are involved in several cellular processes, including sperm functions. Thus far, several GPI-APs have been identified in the testicular germ cells, and there is increasing evidence of their biological significance during fertilization. Among GPI-APs identified in the testis, this review focuses on TEX101, a germ cell-specific GPI-AP that belongs to the lymphocyte antigen 6/urokinase-type plasminogen activator receptor superfamily. This molecule was originally identified as a glycoprotein that contained the antigen epitope for a specific monoclonal antibody; it was produced by immunizing female mice with an allogenic testicular homogenate. This review mainly describes the current understanding of the biochemical, morphological, and physiological characteristics of TEX101. Furthermore, future avenues for the investigation of testicular GPI-Aps, including their potential role as regulators of ion channels, are discussed.

Membrane insertion and intercellular transfer of glycosylphosphatidylinositol-anchored proteins: potential therapeutic applications

Anchorage of a subset of cell surface proteins in eukaryotic cells is mediated by a glycosylphosphatidylinositol (GPI) moiety covalently attached to the carboxy-terminus of the protein moiety. Experimental evidence for the potential of GPI-anchored proteins (GPI-AP) of being released from cells into the extracellular environment has been accumulating, which involves either the loss or retention of the GPI anchor. Release of GPI-AP from donor cells may occur spontaneously or in response to endogenous or environmental signals. The experimental evidence for direct insertion of exogenous GPI-AP equipped with the complete anchor structure into the outer plasma membrane bilayer leaflets of acceptor cells is reviewed as well as the potential underlying molecular mechanisms. Furthermore, promiscuous transfer of certain GPI-AP between plasma membranes of different cells in vivo under certain (patho)physiological conditions has been reported. Engineering of target cell surfaces using chimeric GPI-AP with complete GPI anchor may be useful for therapeutic applications.

CD59 is a potential biomarker of esophageal squamous cell carcinoma radioresistance by affecting DNA repair

Radiation therapy is an important treatment modality for esophageal cancer. However, acquisition of radioresistance ultimately results in esophageal cancer relapse. CD59, a membrane-bound complement regulatory protein, can transduce signals via a Src kinase in the lipid raft, thus playing a complement-independent role. However, the effect of CD59 on the esophageal cancer response to ionizing radiation remains unclear. In this study, we found that the expression level of CD59 was positively correlated with the radioresistance of esophageal cancer cell lines and clinical specimens. High CD59 expression indicated poor overall survival (OS) and disease-free survival (DFS) in esophageal squamous cell carcinoma (ESCC) patients who received radiotherapy. Genetic alteration of CD59 expression modulated the radiosensitivity of esophageal cancer cells to ionizing radiation. CD59 deficiency exacerbated DNA damage, hindered cell proliferation, and induced G2/M cell cycle arrest and cellular senescence, leading to an impaired DNA damage repair ability. In addition, CD59 deficiency almost completely reduced the phosphorylation of Src at Y416 despite ionizing radiation. A Src inhibitor saracatinib sensitized esophageal cancer cells to irradiation. Therefore, CD59 may be a potential biomarker for predicting the radioresistance of ESCC to radiotherapy.

LPS-induced clustering of CD14 triggers generation of PI(4,5)P2

Bacterial lipopolysaccharide (LPS) induces strong pro-inflammatory reactions after sequential binding to CD14 protein and TLR4 receptor. Here, we show that CD14 controls generation of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] in response to LPS binding. In J774 cells and HEK293 cells expressing CD14 exposed to 10-100 ng/ml LPS, the level of PI(4,5)P2 rose in a biphasic manner with peaks at 5-10 min and 60 min. After 5-10 min of LPS stimulation, CD14 underwent prominent clustering in the plasma membrane, accompanied by accumulation of PI(4,5)P2 and type-I phosphatidylinositol 4-phosphate 5-kinase (PIP5K) isoforms Iα and Iγ (encoded by Pip5k1a and Pip5k1c, respectively) in the CD14 region. Clustering of CD14 with antibodies, without LPS and TLR4 participation, was sufficient to trigger PI(4,5)P2 elevation. The newly generated PI(4,5)P2 accumulated in rafts, which also accommodated CD14 and a large portion of PIP5K Iα and PIP5K Iγ. Silencing of PIP5K Iα and PIP5K Iγ, or application of drugs interfering with PI(4,5)P2 synthesis and availability, abolished the LPS-induced PI(4,5)P2 elevation and inhibited downstream pro-inflammatory reactions. Taken together, these data indicate that LPS induces clustering of CD14, which triggers PI(4,5)P2 generation in rafts that is required for maximal pro-inflammatory signaling of TLR4.

Expressed glycosylphosphatidylinositol-anchored horseradish peroxidase identifies co-clustering molecules in individual lipid raft domains

Lipid rafts that are enriched in glycosylphosphatidylinositol (GPI)-anchored proteins serve as a platform for important biological events. To elucidate the molecular mechanisms of these events, identification of co-clustering molecules in individual raft domains is required. Here we describe an approach to this issue using the recently developed method termed enzyme-mediated activation of radical source (EMARS), by which molecules in the vicinity within 300 nm from horseradish peroxidase (HRP) set on the probed molecule are labeled. GPI-anchored HRP fusion proteins (HRP-GPIs), in which the GPI attachment signals derived from human decay accelerating factor and Thy-1 were separately connected to the C-terminus of HRP, were expressed in HeLa S3 cells, and the EMARS reaction was catalyzed by these expressed HRP-GPIs under a living condition. As a result, these different HRP-GPIs had differences in glycosylation and localization and formed distinct clusters. This novel approach distinguished molecular clusters associated with individual GPI-anchored proteins, suggesting that it can identify co-clustering molecules in individual raft domains.

Lipid rafts generate digital-like signal transduction in cell plasma membranes

Lipid rafts are meso-scale (5-200 nm) cell membrane domains where signaling molecules assemble and function. However, due to their dynamic nature, it has been difficult to unravel the mechanism of signal transduction in lipid rafts. Recent advanced imaging techniques have revealed that signaling molecules are frequently, but transiently, recruited to rafts with the aid of protein-protein, protein-lipid, and/or lipid-lipid interactions. Individual signaling molecules within the raft are activated only for a short period of time. Immobilization of signaling molecules by cytoskeletal actin filaments and scaffold proteins may facilitate more efficient signal transmission from rafts. In this review, current opinions of how the transient nature of molecular interactions in rafts generates digital-like signal transduction in cell membranes, and the benefits this phenomenon provides, are discussed.

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.

Stimulation of Ly-6G on neutrophils in LPS-primed mice induces platelet-activating factor (PAF)-mediated anaphylaxis-like shock

Previously, two anti-Ly-6G mAb-RB6-8C5 and 1A8-have been used to deplete neutrophils in mice and to clarify their involvement in immune responses. During the course of experiments on neutrophil depletion, we noticed that i.v. injection of RB6-8C5 or 1A8 induced anaphylaxis-like shock in mice pretreated i.v. with LPS. Signs of shock, such as hypothermia, appeared within a few minutes, and the mice died of shock within 20 min of the antibody injection. In vivo experiments, including depletion of various cell types, indicated that neutrophils and macrophages (but not platelets, basophils, or mast cells) are involved in the shock. Experiments using various drugs and gene-targeted mice demonstrated that PAF is the central mediator of the shock. Optimal LPS priming required at least 1 h, and the priming was associated with neutrophil accumulation within pulmonary and hepatic blood vessels. Consistently, following 1A8 injection into LPS-pretreated mice, the mRNA for LysoPAFAT (a PAF biosynthetic enzyme) was markedly up-regulated in neutrophils accumulated in the lung but not in macrophages. These results suggest that (1) stimulation of Ly-6G on LPS-primed neutrophils induces PAF-mediated anaphylaxis-like shock in mice, (2) neutrophils are primed by LPS during and/or after their accumulation in lung and liver to rapidly induce LysoPAFAT, and (3) macrophages may play a pivotal role in the priming phase and/or in the challenge phase by unknown mechanisms. These findings may be related to adult respiratory distress syndrome, although the natural ligand for Ly-6G remains to be identified.

Alemtuzumab induction of intracellular signaling and apoptosis in malignant B lymphocytes

The molecular changes induced by alemtuzumab following binding of CD52 on B tumor cells were investigated. Alemtuzumab alone had no detectable impact on cell signaling but cross-linking of alemtuzumab on the surface of B tumor lines with anti-human Fc antibodies induced a transient Ca(2+) flux followed by phosphorylation of several kinases involved in stress and survival pathways, and expression of associated proteins including TNF-α. Cross-linking of alemtuzumab also induced capping and caspase-dependent apoptosis of the tumor lines. When using primary cells from B-CLL patients, alemtuzumab alone was capable of inducing protein phosphorylation and apoptosis through the cross-linking of alemtuzumab by FcγRIIb receptors on B-CLL cells. Apoptosis was prevented by blocking of FcγRIIb receptors with anti-CD32 antibody. Overall, our results indicate that cross-linking of alemtuzumab on B tumor cells can occur naturally through Fc receptor interaction and leads to the activation of specific cellular pathways and induction of apoptosis.

Inhibition of lipolysis by adiposomes containing glycosylphosphatidylinositol-anchored Gce1 protein in rat adipocytes

Small membrane vesicles released from large adipocytes and harbouring the glycosylphosphatidylinositol-anchored (GPI-) AMP-degrading protein CD73 have previously been demonstrated to stimulate the signal-induced esterification of free fatty acids into neutral lipids suggesting a role of these so-called adiposomes (ADIP) in the paracrine regulation of lipid metabolism in the adipose tissue. Here the involvement of another constituent GPI-protein of ADIP, the cAMP-degrading protein Gce1 in the signal-induced inhibition of lipolysis was investigated in primary rat adipocytes. Incubation of small, and to a lower degree, large adipocytes with ADIP inhibited lipolysis and increased its sensitivity toward inhibition by H(2)O(2), the anti-diabetic drug glimepiride and palmitate. This was accompanied by the transfer of Gce1 from the ADIP to detergent-insoluble glycolipid-enriched plasma membrane microdomains (DIGs) and its subsequent translocation to cytoplasmic lipid droplets (LD) of the acceptor adipocytes. The translocation from DIGs to LD rather than the transfer from ADIP to DIGs of Gce1 was stimulated by H(2)O(2) > glimepiride > palmitate. Both transfer and translocation led to salt- and carbonate-resistant association of Gce1 with DIGs and LD, respectively, and relied on the structural integrity of the DIGs and GPI anchor of Gce1. In conclusion, the trafficking of GPI-proteins from ADIP of donor adipocytes via DIGs to LD of acceptor adipocytes mediates paracrine regulation of lipolysis within adipose tissue.

CD59 but not DAF deficiency accelerates atherosclerosis in female ApoE knockout mice

Although the complement system has been implicated in atherosclerosis, the influence of membrane-bound complement regulators in this process has not been well understood. We studied the role of two membrane complement regulators, decay-accelerating factor (DAF) and CD59, in a murine model of atherosclerosis. DAF(-/-) and CD59(-/-) mice were crossed with apolipoprotein E (ApoE)-deficient mice to generate DAF(-/-)ApoE(-/-) and CD59(-/-)ApoE(-/-) mice. Mice were fed a high fat diet (HFD) for 8 or 16 weeks. En face analysis showed that CD59 deficiency led to more extensive lesions in female ApoE(-/-) mice both at 8 weeks (2.07+/-0.27% vs.1.34+/-0.21%, P=0.06) and 16 weeks (17.13+/-1.14% vs. 9.72+/-1.14%, P<0.001). Similarly, lesions measured by aortic root sectioning were larger in female CD59(-/-)ApoE(-/-) mice than in controls at 8 weeks of HFD feeding (20.74+/-1.33% vs. 13.12+/-1.46%, P<0.005). On the other hand, DAF deficiency did not significantly influence atherosclerosis in ApoE(-/-) mice. Immunohistochemistry revealed more abundant membrane attack complex (MAC) deposition and more collagen staining in the aortic roots of CD59(-/-)ApoE(-/-) mice. Unexpectedly, total plasma cholesterol levels in female CD59(-/-)ApoE(-/-) mice were found to be elevated compared with CD59(+/+)ApoE(-/-) mice. We conclude that CD59 but not DAF offered protection in atherosclerosis in the context of ApoE deficiency. The protective role of CD59 was gender-biased and most likely involved prevention of MAC-mediated vascular injury, with possible contribution from an undefined effect on plasma cholesterol homeostasis.

CD160 inhibits activation of human CD4+ T cells through interaction with herpesvirus entry mediator

CD160, a glycosylphosphatidylinositol-anchored member of the immunoglobulin superfamily, is expressed on both cytolytic lymphocytes and some unstimulated CD4+ T cells. Here we show that CD160 expression was increased after activation of human CD4+ T cells and that crosslinking CD160 with monoclonal antibody strongly inhibited CD3- and CD28-mediated activation. We found that herpesvirus entry mediator (HVEM) was a ligand of CD160 that acted as a 'bidirectional switch' for T cell activation, producing a positive or negative outcome depending on the engagement of HVEM by CD160 and known HVEM ligands such as B and T lymphocyte attenuator (BTLA) and the T lymphocyte receptor LIGHT. Inhibition of CD4+ T cell activation by HVEM-transfected cells was dependent on CD160 and BTLA; when the cysteine-rich domain 1 of HVEM was deleted, this inhibition was lost, resulting in strong T cell activation. CD160 thus serves as a negative regulator of CD4+ T cell activation through its interaction with HVEM.

GPI-anchored receptor clusters transiently recruit Lyn and G alpha for temporary cluster immobilization and Lyn activation: single-molecule tracking study 1

The signaling mechanisms for glycosylphosphatidylinositol-anchored receptors (GPI-ARs) have been investigated by tracking single molecules in living cells. Upon the engagement or colloidal gold–induced cross-linking of CD59 (and other GPI-ARs) at physiological levels, CD59 clusters containing three to nine CD59 molecules were formed, and single molecules of Gαi2 or Lyn (GFP conjugates) exhibited the frequent but transient (133 and 200 ms, respectively) recruitment to CD59 clusters, via both protein–protein and lipid–lipid (raft) interactions. Each CD59 cluster undergoes alternating periods of actin-dependent temporary immobilization (0.57-s lifetime; stimulation-induced temporary arrest of lateral diffusion [STALL], inducing IP₃ production) and slow diffusion (1.2 s). STALL of a CD59 cluster was induced right after the recruitment of Gαi2. Because both Gαi2 and Lyn are required for the STALL, and because Lyn is constitutively recruited to CD59 clusters, the STALL of CD59 clusters is likely induced by the Gαi2 binding to, and its subsequent activation of, Lyn within the same CD59 cluster.

Dynamic recruitment of phospholipase C gamma at transiently immobilized GPI-anchored receptor clusters induces IP3-Ca2+ signaling: single-molecule tracking study 2

Clusters of CD59, a glycosylphosphatidylinositol-anchored receptor (GPI-AR), with physiological sizes of approximately six CD59 molecules, recruit Gαi2 and Lyn via protein–protein and raft interactions. Lyn is activated probably by the Gαi2 binding in the same CD59 cluster, inducing the CD59 cluster's binding to F-actin, resulting in its immobilization, termed stimulation-induced temporary arrest of lateral diffusion (STALL; with a 0.57-s lifetime, occurring approximately every 2 s). Simultaneous single-molecule tracking of GFP-PLCγ2 and CD59 clusters revealed that PLCγ2 molecules are transiently (median = 0.25 s) recruited from the cytoplasm exclusively at the CD59 clusters undergoing STALL, producing the IP₃–Ca²⁺ signal. Therefore, we propose that the CD59 cluster in STALL may be a key, albeit transient, platform for transducing the extracellular GPI-AR signal to the intracellular IP₃–Ca²⁺ signal, via PLCγ2 recruitment. The prolonged, analogue, bulk IP₃–Ca²⁺ signal, which lasts for more than several minutes, is likely generated by the sum of the short-lived, digital-like IP₃ bursts, each created by the transient recruitment of PLCγ2 molecules to STALLed CD59.

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.

Structural and functional comparison of native pentameric, denatured monomeric and biotinylated C-reactive protein

There are many controversies surrounding the biological activities of native C-reactive protein (nCRP) and its various modified forms such as monomerized and biotinylated CRP (mCRP and bCRP). No simple methods have been described to distinguish among these forms. By adapting established electrophoresis methods, we have developed a useful quality control method with which we have investigated the structural and functional characteristics of these forms of CRP. Under all electrophoresis conditions, biotinylation altered the electrophoretic mobility of CRP. nCRP was sensitive to sodium dodecyl sulphate (SDS)-induced monomerization, and only mCRP was susceptible to digestion by trypsin or neutrophil-derived serine proteases. bCRP and mCRP but not nCRP bound to cells, suggesting that chemical modification by biotin and denaturation had altered the structural integrity of CRP. Neither nCRP nor mCRP had the ability to induce secretion of chemokines, nor did they increase intracellular adhesion molecule 1 (ICAM-1) expression in endothelial cells.

Complement regulator CD59 deficiency fails to augment susceptibility to actively induced experimental autoimmune myasthenia gravis

Complement deficient mice are resistant to experimental autoimmune myasthenia gravis (EAMG), suggesting a pivotal role for the membrane attack complex (MAC) in EAMG pathogenesis. To test the significance of MAC regulation in EAMG pathogenesis, CD59 KO and wild type mice were immunized with acetylcholine receptor (AChR). Interestingly, deletion of CD59, the regulator of MAC assembly, failed to augment EAMG susceptibility. The CD59 KO mice had reduced serum anti-AChR IgG1, IgG2b and complement levels. Their lymph node cell IL-2 production and lymphocyte proliferation response to AChR were reduced. The data challenge the current paradigm that CD59 is solely involved in MAC regulation and suggest a role for this molecule in antigen-driven T cell and B cell activation.

Complement regulatory proteins and autoimmunity

The complement system is known to be involved in autoimmunity at several levels. Activated complement contributes to the inflammatory tissue injury characteristic of many autoimmune disease settings. On the other hand, early components of the classical pathway, including C1q, C4 and C2, are thought to be important for disposing apoptotic cellular autoantigens and/or the induction of B cell tolerance in the bone marrow, and their deficiency is a strong risk factor for systemic autoimmunity. Recent studies using transgenic mice have revealed membrane complement regulatory proteins as important modulators in the pathogenesis and manifestation of autoimmune injury. Available evidence suggests that these regulatory proteins may act to suppress autoimmunity via both complement-dependent and -independent mechanisms.

The complement regulatory proteins CD55 (decay accelerating factor) and CD59 are expressed on the inner acrosomal membrane of human spermatozoa as well as CD46 (membrane cofactor protein)

The complement regulatory proteins CD55 and CD59 are expressed on the plasma membrane of human spermatozoa, whereas CD46 is only on the inner acrosomal membrane (IAM) which becomes surfaced exposed after the acrosome reaction when sperm assume fertilisation-competence. CD55 & CD59, two glycosylphosphatidylinositol (GPI)-anchored proteins, have been detected previously in some studies also in the acrosomal region of chemically fixed spermatozoa but never demonstrated at this site on unfixed spermatozoa. Dual labelling immunofluorescence and confocal microscopy on fresh unfixed spermatozoa, with minimal subsequent time to fixation, has shown CD55 to be markedly expressed on the IAM, more than on the plasma membrane. However, unlike for CD46, CD55 displayed patchy staining over the acrosome, with some variation between individual spermatozoa. All IAM-associated CD55 was localised within GM1-containing lipid rafts. CD59 was expressed also on the IAM, but in a pronounced granular pattern with more variation observed from one spermatozoa to another. Both CD55 & CD59 were released from the IAM by PI-PLC, demonstrating them to be GPI-anchored. Analysis of acrosome-reacted spermatozoal CD55 by Western blotting revealed a novel single 55 kDa protein lacking significant oligosaccharides susceptible to glycosidases. Antibody-induced membrane rafting and release of CD55 & CD59 in vitro may have influenced previous results. Significant coexpression of CD55 & CD46 on the IAM suggests some functional cooperation at this site.

Identification of a Naegleria fowleri membrane protein reactive with anti-human CD59 antibody

Naegleria fowleri, the causative agent of primary amebic meningoencephalitis, is resistant to complement lysis. The presence of a complement regulatory protein on the surface of N. fowleri was investigated. Southern blot and Northern blot analyses demonstrated hybridization of a radiolabeled cDNA probe for CD59 to genomic DNA and RNA, respectively, from pathogenic N. fowleri. An 18-kDa immunoreactive protein was detected on the membrane of N. fowleri by Western immunoblot and immunofluorescence analyses with monoclonal antibodies for human CD59. Complement component C9 immunoprecipitated with the N. fowleri "CD59-like" protein from amebae incubated with normal human serum. In contrast, a gene or protein similar to CD59 was not detected in nonpathogenic, complement-sensitive N. gruberi amebae. Collectively, our studies suggest that a protein reactive with antibodies to human CD59 is present on the surface of N. fowleri amebae and may play a role in resistance to lysis by cytolytic proteins.

Shedding and uptake of gangliosides and glycosylphosphatidylinositol-anchored proteins

Gangliosides and glycosylphosphatidylinositol (GPI)-anchored proteins have very different biosynthetic origin, but they have one thing in common: they are both comprised of a relatively large hydrophilic moiety tethered to a membrane by a relatively small lipid tail. Both gangliosides and GPI-anchored proteins can be actively shed from the membrane of one cell and taken up by other cells by insertion of their lipid anchors into the cell membrane. The process of shedding and uptake of gangliosides and GPI-anchored proteins has been independently discovered in several disciplines during the last few decades, but these discoveries were largely ignored by people working in other areas of science. By bringing together results from these, sometimes very distant disciplines, in this review, we give an overview of current knowledge about shedding and uptake of gangliosides and GPI-anchored proteins. Tumor cells and some pathogens apparently misuse this process for their own advantage, but its real physiological functions remain to be discovered.

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.

GPI-80, a beta2 integrin associated glycosylphosphatidylinositol-anchored protein, concentrates on pseudopodia without association with beta2 integrin during neutrophil migration

Previously, we identified a glycosylphosphatidylinositol (GPI)-anchored protein, designated GPI-80, present on human neutrophils and monocytes. GPI-80 is physically associated with beta2 integrin on the surface of human neutrophils and may be a regulator of neutrophil adherence and migration. However, it is not yet known how GPI-80 regulates cell adhesion and migration. To investigate the physiological role(s) of GPI-80, we examined the topological relationship of GPI-80 and the beta2 integrin subunit (CD18) on resting and migrating human neutrophils by confocal laser microscopy. On resting neutrophils, GPI-80 was evenly distributed on the cell surface and was associated with CD18. On the other hand, during the early phase of migration (5 - 30 minutes), GPI-80 was detected on cell bodies and also on pseudopodia, but CD18 was detected only on cell bodies, where it was associated with GPI-80. In the late phase of migration (60 minutes), GPI-80 was detected only on pseudopodia and its association with CD18 was hardly observed. Furthermore, some of the GPI-80 on pseudopodia of migrating neutrophils during the late phase was associated with urokinase-type plasminogen activator receptor (uPAR), a regulator of beta2 integrin-dependent adherence and migration. The distribution of GPI-80 on cell surfaces is similar to that of uPAR. These observations suggest that GPI-80 belongs to the beta2 integrin-associated GPI-anchored protein family, which has regulatory activity in cell adherence.

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.

Calreticulin is at the surface of circulating neutrophils and uses CD59 as an adaptor molecule

Calreticulin, which has been proposed to be a C1q receptor on neutrophils, has neither a transmembrane domain nor a GPI-anchor attachment site and must utilize an adaptor molecule to attach to the plasma membrane. The expression of ecto-calreticulin on purified human neutrophils did not result from contamination by soluble or intracellular calreticulin released during cell fractionation because it was expressed on circulating neutrophils, and the expression did not increase significantly with neutrophil isolation. All neutrophils expressed calreticulin with a unimodal distribution. Treatment of neutrophils with either a cholesterol-binding agent or phosphatidylinositol-specific phospholipase C dramatically decreased ecto-calreticulin expression indicating that the adaptor molecule(s) are located in lipid rafts and have a GPI-anchor. Analysis for the co-expression of specific GPI-anchored proteins and ecto-calreticulin in cells that were deficient in specific GPI-anchored proteins, indicated that ecto-calreticulin was best associated with CD59. Calreticulin reciprocally immunoprecipited with CD59, which provided direct evidence that CD59 is an adaptor for ecto-calreticulin. Immunofluorescence and confocal microscopy demonstrated that ecto-calreticulin co-localized with a fraction of CD59 at the cell surface. Cross-linking ecto-calreticulin with antibodies induced a Ca2+ flux, which suggests that ecto-calreticulin is capable of signaling following ligand binding. Ecto-calreticulin has been associated with diverse cellular functions. An appreciation that the adaptors for ecto-calreticulin are GPI-anchored will provide a framework for understanding any common features underlying ecto-calreticulin ligation.

Complement-mediated lysis by anti-CD20 mAb correlates with segregation into lipid rafts

Despite the clinical success of anti-CD20 monoclonal antibody (mAb) in the treatment of lymphoma, there remains considerable uncertainty about its mechanism of action. Here we show that the ability of mAbs to translocate CD20 into low-density, detergent-insoluble membrane rafts appears to control how effectively they mediate complement lysis of lymphoma cells. In vitro studies using a panel of anti-B-cell mAbs revealed that the anti-CD20 mAbs, with one exception (B1), are unusually effective at recruiting human complement. Differences in complement recruitment could not be explained by the level of mAb binding or isotype but did correlate with the redistribution of CD20 in the cell membrane following mAb ligation. Membrane fractionation confirmed that B1, unlike 1F5 and rituximab, was unable to translocate CD20 into lipid rafts. In addition, we were able to drive B1 and a range of other anti-B-cell mAbs into a detergent-insoluble fraction of the cell by hyper-cross-linking with an F(ab')(2) anti-Ig Ab, a treatment that also conferred the ability to activate lytic complement. Thus, we have shown that an important mAb effector function appears to be controlled by movement of the target molecule into membrane rafts, either because a raft location favors complement activation by mAbs or because rafts are more sensitive to complement penetration.

GPI-anchored GFP signals Ca2+ but is homogeneously distributed on the cell surface

Glycosyl-phosphatidylinositol (GPI)-anchored proteins are unique in that they penetrate only the outer leaflet of the plasma membrane but are still able to mediate intracellular signalling events following antibody-induced ligation. Detergent solubilisation studies suggest that microdomains exist at the cell surface within which are sequestered GPI-linked proteins. Here we report the construction and expression of a fluorescent GPI anchor on the surface of CHO, EL4, and U937 cells by fusing green fluorescent protein (GFP) to the GPI-attachment site of CD59. The resultant GFP-GPI has properties comparable to that of endogenously expressed GPI-anchored molecules as shown by Triton X-114 partitioning. However, sucrose gradient floatation showed that GFP-GPI was only partially resistant to detergent solubilisation. Furthermore confocal scanning laser microscopy revealed a homogeneous distribution of GFP-GPI at the cell surface, which only became clustered following cross-linking of the GPI anchor via an anti-GFP antibody. Surprisingly, GFP-GPI signalled Ca2+ change upon cross-linking demonstrating its signalling competence. Our results suggest that the GPI-anchor itself does not confer a clustered distribution to molecules but that clustering occurs following ligation with antibody, which allows the protein to become Ca2+ signalling competent.

Low density detergent-insoluble membrane of Xenopus eggs: subcellular microdomain for tyrosine kinase signaling in fertilization

Protein-tyrosine phosphorylation plays an important role in egg activation signaling at fertilization. We show that in Xenopus, fertilization stimulates a rapid and transient tyrosine phosphorylation of egg proteins, as revealed by immunoblotting with anti-phosphotyrosine antibody. Immunofluorescent microscopic analysis demonstrated that the phosphorylation occurs in cortical area of the egg animal hemisphere. To further characterize subcellular compartment for fertilization-dependent tyrosine kinase signaling, we isolated low density detergent-insoluble membrane (LD-DIM) fraction from Xenopus eggs. The egg LD-DIM was enriched in cholesterol and GM1 ganglioside. It also contained signaling molecules such as Xyk (Xenopus egg Src), Gqα, Ras, integrin β1 and CD9. Fertilization stimulated tyrosine phosphorylation of Xyk and some other LD-DIM proteins. Remarkably, sperm stimulated tyrosine phosphorylation of the LD-DIM proteins in vitro. The sperm-dependent phosphorylation was sensitive to the tyrosine kinase inhibitors PP2 and genistein. We found that pretreatment of eggs with methyl-β-cyclodextrin, a cholesterol-binding substance, led to a decrease in cholesterol, Xyk and sperm-induced tyrosine phosphorylation in LD-DIM. In methyl-β-cyclodextrin-treated eggs, sperm-induced Ca2+ transient and first cell division were also inhibited. These findings suggest that the egg LD-DIM might serve as subcellular microdomain for tyrosine kinase signaling in Xenopus egg fertilization.

Urokinase-type plasminogen activator up-regulates its own expression by endothelial cells and monocytes via the u-PAR pathway

Signal transduction by urokinase-type plasminogen activator (u-PA) bound to its cell receptor has been well established. In the present study, we found, for the first time to our knowledge, that u-PA promotes its own synthesis by endothelial cells and monocytes. This phenomenon was characterized and shown to involve the u-PA receptor (u-PAR) pathway. The finding may be of general importance, since most cells that express u-PAR also produce u-PA. Human umbilical vein endothelial cells (HUVECs), U937 monocytes, and human peripheral blood monocytes (PFMCs) were incubated with diisopropylfluorophosphate (DFP)-pretreated u-PA, the amino-terminal fragment (ATF) of u-PA, or the kringle domain. A threefold up-regulation of u-PA secretion and synthesis by u-PA or ATF was found. The predominant effect was expressed in HUVECs, in which u-PA mRNA was also up-regulated. The u-PA kringle domain had no effect on u-PA synthesis, leading to the conclusion that the EGF domain was responsible. This was also consistent with the additional finding that the u-PAR, to which the EGF domain binds, was necessary for the up-regulation. The results indicate that u-PA up-regulates itself via its EGF domain and u-PAR. The possibilities that the results were related to displacement of receptor-bound u-PA or the blocking of u-PA incorporation into the cells were excluded. A modest up-regulation of u-PAR was also associated with this phenomenon.

The role and regulation of urokinase-type plasminogen activator receptor gene expression in cancer invasion and metastasis

This article reviews the role of urokinase-type plasminogen activator receptor (uPAR) and its protein, mRNA, cDNA, genomic organization, promoter, transcription activation factors, and signal transduction. The uPAR has been implicated in several biological processes including angiogenesis, monocyte migration, cancer metastasis, trophoblast implantation, and wound healing. It is a specific cell surface receptor for its ligand uPA which catalyzes the formation of plasmin from plasminogen to generate the proteolytic cascade that contributes to the breakdown of extracellular matrix, a key step in cancer metastasis. The uPAR is a 55-60 kDa glycoprotein organized as three homologous cysteine-rich domains. It attaches to the plasma membrane via a covalent linkage to a glycosyl-phosphatidylinositol (GPI) moiety and appears to play an important role in transmembrane signalling. The 1.4-kb human uPAR cDNA and 21.23-kb genomic DNA have been cloned and the gene contains seven exons. The uPAR promoter region was defined in a 188 bp fragment between bases -141 and +47 relative to the transcription start site. Binding of transcription factors (Sp1, AP-2, NFkappaB and two AP-1) to the uPAR promoter region activates the basal transcription of the gene. There is a strong correlation between uPAR expression and the invasive cancer cell phenotype. uPAR may play a critical role in the process of cancer invasion and metastasis, as antisense uPAR mRNA can inhibit cancer spread in vitro and in vivo. These studies may provide a novel therapeutic target for blocking cancer invasion and metastasis.

Glycosylphosphatidylinositol-anchored proteins are not required for crosslinking-mediated endocytosis or transfection of avidin bioconjugates into biotinylated cells

Even though glycosylphosphatidylinositol (GPI)-anchored proteins lack direct structural contact with the intracellular space, these ubiquitously expressed surface receptors activate signaling cascades and endocytosis when crosslinked by extracellular ligands. Such properties may be due to their association with membrane microdomains composed of glycosphingolipids, cholesterol and some signaling proteins. In this study, we hypothesize that GPI proteins may be required for crosslinking-mediated endocytosis of extracellular bioconjugates. To test this hypothesis, we first biotinylated the surface membranes of native K562 erythroleukemia cells versus K562 cells incapable of surface GPI protein expression. We then compared the entry of fluorescently labeled avidin or DNA condensed on polyethylenimine-avidin bioconjugates into the two biotinylated cell populations. Using fluorescence microscopy, nearly 100% efficiency of fluorescent avidin endocytosis was demonstrated in both cell types over a 24 h period. Surprisingly, plasmid DNA transfer was slightly more efficient among the biotinylated GPI-negative cells as measured by the expression of green fluorescence protein. Our findings that GPI proteins are not required for the endocytosis of avidin bioconjugates into biotinylated cells suggest that endocytosis associated with general membrane crosslinking may be due to overall reorganization of the membrane domains rather than GPI protein-specific interactions.

The CD45 tyrosine phosphatase regulates Campath-1H (CD52)-induced TCR-dependent signal transduction in human T cells

Campath-1H, a humanized mAb undergoing clinical trials for treatment of leukemia, transplantation and autoimmune diseases, produces substantial lymphocyte depletion in vivo. The antibody binds to CD52, a highly glycosylated molecule attached to the membrane by a glycosylphosphatidylinositol anchor. Cross-linked Campath-1H is known to activate T cells in vitro. We have investigated the molecular basis for these effects by comparing the protein tyrosine phosphorylation signals induced by Campath-1H and the CD3 mAb OKT3 in primary T cells, and in CD45(+)TCR(+), CD45(-)TCR(+) and CD45(+)TCR(-) Jurkat subclones transfected with CD52. Our results show that Campath-1H triggers similar tyrosine phosphorylation events as OKT3 in both primary T cells and in the CD45(+)TCR(+) Jurkat sub-clone, albeit at quantitatively lower levels. However, no phospholipase C gamma 1 activation nor calcium signals were detected in response to CD52 ligation. The CD52-mediated induction of protein tyrosine phosphorylation was absolutely dependent upon the expression of both the TCR and the CD45 phosphotyrosine phosphatase at the cell surface. Cross-linking of Campath-1H was essential for signal transduction in all cells investigated. Fluorescence resonance energy transfer was used to demonstrate CD52 homo-association at the cell surface in Jurkat T cells in a TCR- and CD45-independent manner, and CD52-TCR association in CD45(+)TCR(+) cells. We propose a model to explain the activating effects of Campath-1H in which CD52 mAb cross-linking causes the trapping of TCR polypeptides within molecular complexes at the cell surface, thereby inducing signals via the TCR by a process which depends on the CD45-mediated regulation of the p56(lck) and p59(fyn) tyrosine kinases.

Convergence of Fc gamma receptor IIA and Fc gamma receptor IIIB signaling pathways in human neutrophils

Human neutrophils (PMNs) express two receptors for the Fc domain of IgG: the transmembrane FcgammaRIIA, whose cytosolic sequence contains an immunoreceptor tyrosine-based activation motif, and the GPI-anchored FcgammaRIIIB. Cross-linking of FcgammaRIIIB induces cell activation, but the mechanism is still uncertain. We have used mAbs to cross-link selectively each of the two receptors and to assess their signaling phenotypes and functional relation. Cross-linking of FcgammaRIIIB induces intracellular Ca2+ release and receptor capping. The Ca2+ response is blocked by wortmannin and by N,N-dimethylsphingosine, inhibitors of phosphatidylinositol 3-kinase and sphingosine kinase, respectively. Identical dose-response curves are obtained for the Ca2+ release stimulated by cross-linking FcgammaRIIA, implicating these two enzymes in a common signaling pathway. Wortmannin also inhibits capping of both receptors, but not receptor endocytosis. Fluorescence microscopy in double-labeled PMNs demonstrates that FcgammaRIIA colocalizes with cross-linked FcgammaRIIIB. The signaling phenotypes of the two receptors diverge only under frustrated phagocytosis conditions, where FcgammaRIIIB bound to substrate-immobilized Ab does not elicit cell spreading. We propose that FcgammaRIIIB signaling is conducted by molecules of FcgammaRIIA that are recruited to protein/lipid domains induced by clustered FcgammaRIIIB and, thus, are brought into juxtaposition for immunoreceptor tyrosine-based activation motif phosphorylation and activation of PMNs.

Specificity in signal transduction among glycosylphosphatidylinositols of Plasmodium falciparum, Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp

Glycosylphosphatidylinositols (GPIs) and related glycoconjugates of parasite origin have been shown to regulate both the innate and acquired immune systems of the host. This is achieved through the activation of novel GPI-dependent signalling pathways in macrophages, lymphocytes and other cell types. Parasite GPIs impart at least two distinct signals to host cells through the structurally distinct inositolphosphoglycan (IPG) and fatty acid domains. Binding of IPG to as yet uncharacterized cell surface receptor(s) leads to activation of src-family protein tyrosine kinases: depending upon structure, GPI-derived fatty acids can either activate or antagonize protein kinase C, and may enter the sphingomyelinase pathway. The degree of fatty acid saturation may also contribute to signalling activity. Thus, variation in structure of parasite GPIs imparts different properties of signal transduction upon this class of glycolipid. The divergent activities of GPIs from various protozoal taxa reflect global aspects of the host/parasite relationship, suggesting that GPI signalling is a central determinant of disease in malaria, leishmaniasis and both American and African trypanosomiases.

Urokinase activates calcium-dependent potassium channels in U937 cells via calcium release from intracellular stores

The urokinase receptor (uPAR) is highly expressed in the human promyelocytic cell line U937 and contributes to transmembrane signalling. However, the signalling mechanisms are poorly understood. We used the patch-clamp technique to demonstrate that urokinase (uPA) binds to uPAR and thereby stimulates Ca(2+)-activated K+ channels in U937 cells. uPA transiently increased K+ currents within 30 s. The K+ currents were pertussis toxin-sensitive and were also observed in Ca(2+)-free solution. However, when cells were dialysed with EGTA, uPA did not affect K+ currents. The intracellular Ca2+ response to uPA was independent of extracellular Ca2+, was pertussis toxin-sensitive, and was blocked by both thapsigargin and the phospholipase C inhibitor U-73122. The uPA-induced increase in intracellular Ca2+ was independent of uPA proteolytic activity. Furthermore, uPA initiated a rapid formation of inositol 1,4, 5-trisphosphate [Ins(1,4,5)P3]. The amino-terminal uPA fragment and uPA inactivated with diisopropyl fluorophosphate or with inhibitory antibody, elicited the same Ca2+ signal. On the other hand, Ca2+ signalling required the intact uPAR because the effects were abrogated by PtdIns-specific phospholipase C, which removes the uPAR from the cell surface. The prevention of glycosyl phosphatidylinositol moiety synthesis and interference with uPAR anchoring to the cell surface using mannosamine also abolished Ca2+ signals. Taken together, our findings indicate that uPA binds to uPAR and stimulates the production of Ins(1,4,5)P3 via a G-protein- and phospholipase C-dependent mechanism. Ins(1,4,5)P3 in turn liberates Ca2+ from intracellular stores, which leads to the stimulation of Ca(2+)-activated K+ channels.

Microdomain-dependent regulation of Lck and Fyn protein-tyrosine kinases in T lymphocyte plasma membranes

Src family protein-tyrosine kinases are implicated in signaling via glycosylphosphatidylinositol (GPI)-anchored receptors. Both kinds of molecules reside in opposite leaflets of the same sphingolipid-enriched microdomains in the lymphocyte plasma membrane without making direct contact. Under detergent-free conditions, we isolated a GPI-enriched plasma membrane fraction, also containing transmembrane proteins, selectively associated with sphingolipid microdomains. Nonionic detergents released the transmembrane proteins, yielding core sphingolipid microdomains, limited amounts of which could also be obtained by detergent-free subcellular fractionation. Protein-tyrosine kinase activity in membranes containing both GPI-anchored and transmembrane proteins was much lower than in core sphingolipid microdomains but was strongly reactivated by nonionic detergents. The inhibitory mechanism acting on Lck and Fyn kinases in these membranes was independent of the protein-tyrosine phosphatase CD45 and was characterized as a mixed, noncompetitive one. We propose that in lymphocyte plasma membranes, Lck and Fyn kinases exhibit optimal activity when juxtaposed to the GPI- and sphingolipid-enriched core microdomains but encounter inhibitory conditions in surrounding membrane areas that are rich in glycerophospholipids and contain additional transmembrane proteins.

Protection of cells from complement-mediated attack: CD59 receptor clustering for the entry of macromolecules into hematopoietic cells

CD59 is a glycosylphosphatidylinositol anchored protein (GPI-protein) that is expressed on surface membranes to protect host cells from complement-mediated attack. CD59 may also serve as a receptor for the endocytosis of macromolecules into nucleated cells. Here we investigate the effects of primary clustering of CD59 with anti-CD59 monoclonal antibody and the secondary clustering of biotinylated anti-CD59 with avidin on red blood cells and erythroleukemic K562 cells. On red blood cells, CD59-targeted antibodies remained evenly distributed on the external membranes. In contrast, clustering, capping and endocytosis of the CD59-targeted complexes was detected on K562 cells. Secondary clustering appeared more efficient and resulted in endosomal localization of the fluorescently labeled complexes within 2 hours. The endocytosis of CD59-bound complexes did not affect K562 cell viability or growth and the surface level of CD59 was constant during the process. These result suggest clustering and subsequent endocytosis of CD59 may enable the entry of macromolecules to the endosomal compartments of hematopoietic cells.

Functions of lipid rafts in biological membranes

Recent studies showing that detergent-resistant membrane fragments can be isolated from cells suggest that biological membranes are not always in a liquid-crystalline phase. Instead, sphingolipid and cholesterol-rich membranes such as plasma membranes appear to exist, at least partially, in the liquid-ordered phase or a phase with similar properties. Sphingolipid and cholesterol-rich domains may exist as phase-separated "rafts" in the membrane. We discuss the relationship between detergent-resistant membranes, rafts, caveolae, and low-density plasma membrane fragments. We also discuss possible functions of lipid rafts in membranes. Signal transduction through the high-affinity receptor for IgE on basophils, and possibly through related receptors on other hematopoietic cells, appears to be enhanced by association with rafts. Raft association may also aid in signaling through proteins anchored by glycosylphosphatidylinositol, particularly in hematopoietic cells and neurons. Rafts may also function in sorting and trafficking through the secretory and endocytic pathways.

High level transcription of the complement regulatory protein CD59 requires an enhancer located in intron 1

CD59 is a complement regulatory protein and may also act as a signal-transducing molecule. CD59 transgenic mice have been generated using a CD59 minigene (CD59 minigene-1). Although this minigene contained a 4.6-kilobase pair 5'-flanking region from the human CD59 gene as a promoter, the expression levels of the CD59 mRNA were substantially lower than those observed in humans, suggesting that CD59 gene expression might also require other transcriptional regulatory elements such as an enhancer. To investigate the transcriptional regulation of the CD59 gene, we used three cell lines that express CD59 at different levels. We have identified DNase I-hypersensitive sites in intron 1 in HeLa cells, which express CD59 at high levels, but not in Jurkat (intermediate level) or Raji cells (low level). Furthermore, cell line-specific enhancer activity was detected in a fragment containing these DNase I-hypersensitive sites. The CD59 enhancer was mapped to between -1155 and -888 upstream of the 5'-end of exon 2. To investigate the enhancer activity in vivo, a new CD59 minigene was constructed by the addition of the enhancer fragment into CD59 minigene-1. High expressor CD59 transgenic mice were generated using the new minigene.

Role of plasmalemmal caveolae in signal transduction

Caveolae are specialized plasmalemmal microdomains originally studied in numerous cell types for their involvement in the transcytosis of macromolecules. They are enriched in glycosphingolipids, cholesterol, sphingomyelin, and lipid-anchored membrane proteins, and they are characterized by a light buoyant density and resistance to solubilization by Triton X-100 at 4 degreesC. Once the identification of the marker protein caveolin made it possible to purify this specialized membrane domain, it was discovered that caveolae also contain a variety of signal transduction molecules. This includes G protein-coupled receptors, G proteins and adenylyl cyclase, molecules involved in the regulation of intracellular calcium homeostasis, and their effectors including the endothelial isoform of nitric oxide synthase, multiple components of the tyrosine kinase-mitogen-activated protein kinase pathway, and numerous lipid signaling molecules. More recent work has indicated that caveolae further serve to compartmentalize, modulate, and integrate signaling events at the cell surface. This specialized plasmalemmal domain warrants direct consideration in future investigations of both normal and pathological signal transduction in pulmonary cell types.

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.

Antibody cross-linking of the glycosylphosphatidylinositol-linked protein CD59 on hematopoietic cells induces signaling pathways resembling activation by complement

CD59 is a glycosylphosphatidylinositol-anchored cell surface glycoprotein involved in protecting cells from host-mediated complement attack. Studies have shown that antibody cross-linking of CD59 induces a series of intracellular signaling events including the activation of protein-tyrosine kinases (PTK). To further characterize these events, antibodies and complement 8, one of the natural ligands of CD59, were used to activate CD59. Antibody-induced cross-linking of CD59 on the surface of THP-1 and U937 hematopoietic cell lines as well as exposure to complement 8 induces a rapid increase in the tyrosine phosphorylation of several proteins within the cell. Consistent with an early role for the Src family PTKs in these signaling events, we found that transient activation of Hck- and CD59-mediated signaling was abrogated in the presence of the Src family PTK-selective inhibitor PP1. Although the molecular mechanism by which CD59 communicates to Hck is unknown, cellular fractionation studies indicated that both CD59 and Hck are compartmentalized in plasma membrane microdomains. We also detected tyrosine phosphorylation of the adaptor proteins p120 and Shc, and the cytoplasmic nonreceptor tyrosine kinase Syk. The identification of CD59-mediated signaling events may help explain why paroxysmal nocturnal hemoglobinuria patients, who are deficient in glycosylphosphatidylinositol-linked proteins including CD59, are susceptible to proliferative disorders.

Signal transduction in leucocytes via GPI-anchored proteins: an experimental artefact or an aspect of immunoreceptor function?

Membrane proteins anchored in the membrane via a glycolipid glycosylphosphatidylinositol (GPI) as well as some glycolipids are able to transduce signals and induce diverse functional responses in cells upon their cross-linking via antibodies or natural ligands. In some cases this signaling capacity seems to be due to associations of these molecules with specific transmembrane proteins. GPI-anchored proteins are components of membrane microdomains enriched in glycosphingolipids and cholesterol and devoid of most transmembrane proteins. These membrane specializations are relatively resistant to solubilization in solutions of some mild detergents at low temperatures. These 'GPI-microdomains' contain also cytoplasmic signaling molecules such as Src-family protein tyrosine kinases and trimeric G-proteins. Thus, at least some signaling elicited upon cross-linking of GPI-anchored proteins and glycolipids may be due to perturbation of the signaling molecules associated with these microdomains. It is suggested that these specialized areas of the membrane rich in signaling molecules interact with immunoreceptors (TCR, BCR, Fc receptors) cross-linked upon their interactions with ligands and importantly contribute to initiation of proximal phases of their signaling pathways.

Activation of human neutrophils by soluble immune complexes: role of Fc gamma RII and Fc gamma RIIIb in stimulation of the respiratory burst and elevation of intracellular Ca2+

Activation of control, unprimed neutrophils with soluble immune complexes fails to generate a respiratory burst. However, if the cells are primed with either tumor necrosis factor-alpha or granulocyte-macrophage colony-stimulating factor prior to addition of soluble immune complexes, then a rapid and transient burst of reactive oxidant secretion is observed. In unprimed neutrophils the soluble immune complexes stimulate an intracellular Ca2+ transient that arises from the mobilization of intracellular Ca2+. However, in primed cells, an "extra" intracellular Ca2+ signal is observed that arises from Ca2+ influx. After removal of Fc gamma RIIIb by treatment with pronase or PI-PLC, the soluble immune complexes fail to activate a respiratory burst in unprimed neutrophils and the "extra" Ca2+ signal is not observed. These results indicate that during priming Fc gamma RIIIb becomes functionally activated and thence its ligation leads to stimulated Ca2+ influx and the generation of intracellular signals that lead to NADPH oxidase activation. Experiments using Fab/F(ab')2 fragments to specifically crosslink either Fc gamma RII or Fc gamma RIIIb and experiments with neutrophils from an individual with Fc gamma RIIIb gene deficiency confirm this important function for Fc gamma RIIIb in neutrophil activation.

HDL3 binds to glycosylphosphatidylinositol-anchored proteins to activate signalling pathways

Previous studies have indicated that in HepG2 cells HDL3-signalling involves glycosylphosphatidylinositol (GPI) anchored proteins. HDL3-binding to HepG2 cells was found to be enhanced by cellular preincubation with PI-PLC inhibitors and sensitive to a cellular preincubation with exogenous PI-PLC, suggesting that HDL3 binds directly on GPI-anchored proteins to initiate signaling. Moreover HDL3-binding was found to be partly inhibited by antibodies against the HDL-binding protein (AbHBP). HDL3, when binding to HepG2 cells, promoted the release in the culture medium of a 110 kDa protein that binds AbHBP, while a cellular preincubation with antibodies against the inositol-phosphoglycan (IPG) moiety of GPI-anchor (AbIPG), used to block lipolytic cleavage of the GPI-anchor, inhibits HDL3-induced release of the 110 kDa protein in the culture medium. In [3H]-PC prelabeled HepG2 cells, AbHBP were found to stimulate PC-hydrolysis and DAG generation within 5 min as did HDL3 stimulation. Cellular preincubation with AbIPG was found to inhibit only the HDL3-signal and not the AbHBP-signal, while a prior cellular pretreatment with PI-PLC from Bacillus cereus was found to inhibit the HDL3-and AbHBP-signal. Moreover cellular preincubation with AbHBP for 1 h at 37 degrees C was found to inhibit HDL3-signalling pathways. Our results suggest that in HepG2 cells a 110 kDa protein, which could be HBP, can be anchored to the membrane via GPI, and can function in HDL3-signalling pathways as binding sites.

Signal transduction via glycosyl phosphatidylinositol-anchored proteins in T cells is inhibited by lowering cellular cholesterol

Glycosylphosphatidylinositol (GPI)-anchored proteins can deliver costimulatory signals to lymphocytes, but the exact pathway of signal transduction involved is not yet characterized. GPI-anchored proteins are fixed to the cell surface solely by a phospholipid moiety and are clustered in distinct membrane domains that are formed by an unique lipid composition requiring cholesterol. To elucidate the role of membrane lipids for signal transduction via GPI-anchored proteins, we studied the influence of reduced cellular cholesterol content on calcium signaling via GPI-anchored CD59 and CD48 in Jurkat T cells. Lowering cholesterol by different inhibitors of cellular cholesterol synthesis suppressed calcium response via GPI-anchored proteins by about 50%, whereas stimulation via CD3 was only minimally affected (<10%). The decrease in overall calcium response via GPI-anchored proteins was reflected by inhibition of calcium release from intracellular stores. Cell surface expression of GPI-anchored proteins was not changed quantitatively by lowering cellular cholesterol, and neither was the pattern of immunofluorescence in microscopic examination. In addition, the distribution of GPI-anchored proteins in detergent-insoluble complexes remained unaltered. These results suggest that cellular cholesterol is an important prerequisite for signal transduction via GPI-anchored proteins beyond formation of membrane domains.