Solubilization of glycosyl-phosphatidylinositol-anchored proteins in quiescent and stimulated neutrophils

The expression of caveolin-1 and the distribution of caveolae in the murine placenta and yolk sac: parallels to the human placenta

The expression pattern of caveolin-1 and the distribution of caveolae in the murine placental labyrinth and visceral yolk sac have been determined. Immunoblot analysis demonstrates that both placenta and yolk sac express the protein caveolin-1. Immunofluorescence microscopy was used to determine which cell types in the placental labyrinth and yolk sac express caveolin-1. In yolk sac, detectable caveolin-1 was restricted to endothelial cells and smooth muscle cells of the vitelline vasculature and to mesothelial cells. Endoderm, the major cell type in the yolk sac, does not express caveolin-1 as assessed by this assay. In the labyrinth region of the placenta, endothelial cells express caveolin-1 but this protein was not detectable in any of the three trophoblast layers. These tissues were also examined by electron microscopy to determine which cell types contain the specialized plasma membrane microdomains known as caveolae. Morphologically detectable caveolae were present in endothelial and smooth muscle cells, as well as mesothelial cells of the yolk sac and in endothelial cells of the placental labyrinth. Neither endodermal cells of the yolk sac nor trophoblastic cells in the placental labyrinth contained caveolae-like structures. We conclude that caveolin-1 and caveolae have restricted distribution in the murine placenta and yolk sac and that this parallels the situation in human placenta.

The association between CD36 and Lyn protein tyrosine kinase is mediated by lipid

CD36 is a transmembrane glycoprotein receptor that engages in signal transduction implicated in important physiological and pathophysiological events. CD36 in platelets has been shown physically and functionally to associate with members of the Src family of protein tyrosine kinases, Fyn, Lyn, and Yes, but the nature of this important association has never been rigorously examined. Here, we show that CD36 does not associate with Lyn through a protein-mediated interaction. In COS cells transfected with both CD36 and Lyn these molecules did not co-precipitate, suggesting a requirement for an intermediary molecule absent from the COS cells. Yeast two-hybrid analysis confirmed that the carboxylterminal cytoplasmic tail of CD36 did not bind Lyn directly, and no Lyn binding protein bound to CD36 in a cDNA library screen. Conversely, when the CD36-Lyn association seen in platelets was analysed by biophysical parameters, dissociation occurred at 37 degrees C and also by solubilisation in octylglucoside, indicative of a lipid-mediated association. Since both CD36 and Lyn are enriched in Triton X-100-insoluble rafts at the plasma membrane, these findings point to the importance of raft-associated lipids in CD36-mediated signal transduction.

A Novel FcγR-Defined, IgG-Containing Organelle in Placental Endothelium

Placental transfer of IgG from maternal circulation to that of the fetus is crucial for fetal and newborn immunity. This process requires that IgG broach two cellular layers of the placenta. IgG transport across the first layer, the syncytiotrophoblast, is almost certainly mediated by the MHC-related FcR for IgG, FcRn. The second layer, the villus endothelium, was until recently thought to allow IgG movement nonspecifically by constitutive transcytosis in caveolae. However, we recently showed that villus endothelium expressed a separate FcR for IgG, the inhibitory motif-bearing Fc gammaRIIb2 seen most notably on macrophages and as a minor fraction of the Fc gammaRIIb expressed on B cells. Now, by quantitative microscopy, we find Fc gammaRIIb2 to be expressed abundantly in an unidentifiable and likely novel organelle of the villus endothelium, unassociated with caveolae. About half of these Fc gammaRIIb2 organelles contain IgG; the remainder lack IgG. The majority fraction (approximately 80%) of IgG-containing organelles is associated with Fc gammaRIIb. No IgG-containing organelles are associated with caveolin. These findings are compatible with Fc gammaRIIb-mediated transfer of IgG across the villus endothelium, independent of caveolae.

Activity and specificity of toxin-related mouse T cell ecto-ADP-ribosyltransferase ART2.2 depends on its association with lipid rafts

Adenosine diphosphate (ADP)-ribosyl-transferases (ARTs) transfer ADP-ribose from nicotinamide adenine dinucleotide (NAD) onto target proteins. T cells express ART2.2, a toxin-related, glycosylphosphatidylinositol (GPI)-anchored ecto-enzyme. After the release of NAD from cells, ART2.2 ADP-ribosylates the P2X7 purinoceptor, lymphocyte function-associated antigen (LFA-1), and other membrane. Using lymphoma transfectants expressing either ART2.2 with its native GPI anchor (ART2.2-GPI) or ART2.2 with a grafted transmembrane anchor (ART2.2-Tm), we demonstrated that ART2.2-GPI but not ART2.2-Tm associated with glycosphingolipid-enriched microdomains (lipid rafts). At limiting substrate concentrations, ART2.2-GPI exhibited more than 10-fold higher activity than ART2.2-Tm. On intact cells, ART2.2-GPI ADP-ribosylated a small number of distinct target proteins. Strikingly, the disruption of lipid rafts by cyclodextrin or membrane solubilization by Triton X-100 increased the spectrum of modified target proteins. However, ART2.2 itself was a prominent target for ADP-ribosylation only when GPI anchored. Furthermore, cholesterol depletion or detergent solubilization abolished the auto-ADP-ribosylation of ART2.2. These findings imply that ART2.2-GPI, but not ART2.2-Tm, molecules are closely associated on the plasma membrane and lend support to the hypothesis that lipid rafts exist on living cells as platforms to which certain proteins are admitted and others are excluded. Our results further suggest that raft association focuses ART2.2 on specific targets that constitutively or inducibly associate with lipid rafts.

Recruitment of the cross-linked opsonic receptor CD32A (FcgammaRIIA) to high-density detergent-resistant membrane domains in human neutrophils

We have previously shown that CD32A (or FcgammaRIIA), one of the main opsonin receptors, was rapidly insolubilized and degraded in intact neutrophils after its cross-linking. In view of these experimental difficulties, the early signalling steps in response to CD32A activation were studied in purified plasma membranes of neutrophils. After CD32A cross-linking in these fractions, the tyrosine phosphorylation of two major substrates, the receptor itself and the tyrosine kinase Syk, was observed. Phosphorylation of these two proteins was observed only in the presence of orthovanadate, indicating the presence, in the membranes, of one or more tyrosine phosphatases that maintain CD32A dephosphorylation. The tyrosine phosphorylation of these two proteins was inhibited by the Src kinase inhibitor, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2). The ligation of CD32A led to its recruitment to a previously uncharacterized subset of high-density flotillin-1-positive DRMs (detergent-resistant membranes). The changes in the solubility properties of CD32A were observed in the absence of added ATP; therefore, they were probably not secondary to the tyrosine phosphorylation of the receptor, rather they preceded it. Src kinases as well as Syk were constitutively present in DRMs of high and low density and no evident changes in their distribution were detected after cross-linking of CD32A. Pretreatment of plasma membranes with methyl-beta-cyclodextrin did not inhibit the recruitment of CD32A to DRMs, although it led to the loss of the Src kinase Lyn from these fractions. In addition, methyl-beta-cyclodextrin inhibited the tyrosine phosphorylation of CD32A and Syk induced by cross-linking of CD32A. This membrane model allowed us to observe a movement of CD32A from detergent-soluble regions of the membranes to DRMs, where it joined Src kinases and Syk and became tyrosine-phosphorylated.

Subcellular localization and mobilization of carcinoembryonic antigen-related cell adhesion molecule 8 in human neutrophils

The subcellular localization and mobilization of carcinoembryonic antigen-related cell adhesion molecule 8 (CEACAM8) was investigated quantitatively in human neutrophils. In resting neutrophils the majority of CEACAM8 was present in the secondary granules, and a small amount of CEACAM8 was present in a light membrane fraction. Stimulation of the neutrophils with phorbol 12-myristate 13-acetate caused a dramatic increase in the content of CEACAM8 in the light membrane fraction, suggesting a translocation of CEACAM8 to the plasma membrane from intracellular pools. The cellular content of CEACAM8 in the neutrophils was estimated to be 82.4 +/- 8.9 ng/10(6) cells (mean +/- SE, n = 10). Administration of granulocyte colony-stimulating factor (G-CSF) to healthy individuals resulted in an increased content of CEACAM8 in neutrophils on day 1, which decreased on day 4. However, the content of CEACAM8 in the light membrane fraction was increased on day 4, possibly due to the stimulation by induced secondary cytokines, such as tumour necrosis factor-alpha (TNF-alpha). This study establishes the secondary granules as the major intracellular pools of CEACAM8 in human neutrophils, from which it may translocate to the plasma membranes upon stimulation of the cells. The translocation of CEACAM8 seen in vivo after G-CSF administration is probably indirect and caused by cytokines such as TNF-alpha.

TFF1 is membrane-associated in breast carcinoma cell line MCF-7

Trefoil factor family (TFF) domain peptides, products of mucin-secreting epithelial cells, are thought to influence mucosal integrity. Molecular studies revealed that mammalian TFFs lack transmembrane domains. Using immunocytochemistry and FACS analysis we demonstrated the association of TFF1 with the cell membrane in MCF-7 (a breast adenocarcinoma cell line), and tested the hypothesis that glycosylphosphatidylinositol (GPI) linkage is the mechanism for this association. Cleavage of GPI anchorage using phospholipase C did not affect TFF1 binding to the cell membrane. Our results demonstrate for the first time that TFF1 is associated with the cell membrane of MCF-7 cells and is not linked via a GPI anchor.

On the mechanism and significance of ligand-induced internalization of human neutrophil chemokine receptors CXCR1 and CXCR2

It is well established that leukocyte chemotactic receptors, a subset of G protein-coupled receptors, undergo endocytosis after stimulation by ligand. However, the significance of this phenomenon to cell motility and other important leukocyte functions induced by chemoattractants has not been clearly defined. Here we show that in primary human neutrophils, the threshold levels of agonist required for endocytosis of the chemotactic receptors CXCR1 and CXCR2 were approximately 10-fold or higher than those needed for maximal chemotactic and calcium flux responses. Moreover, when stimulated by agonists at concentrations that are high enough for chemotaxis but too low for receptor endocytosis, neutrophil CXCR1 and CXCR2 could be reactivated in response to repeated application of the same agonist. Both receptors were excluded from Triton X-100-insoluble lipid rafts, and at high agonist concentrations were rapidly endocytosed by a clathrin/rab5/dynamin-dependent pathway. These data support the conclusion that neutrophil migration in response to CXCR1 or CXCR2 agonists is not dependent on endocytosis of CXCR1 or CXCR2. Rather than being integral to the process of cell migration, receptor endocytosis may be a terminal stop signal when cells reach the focus of inflammation where the chemoattractant concentrations are the highest.

Sorting and function of the human folate receptor is independent of the caveolin expression in Fisher rat thyroid epithelial cells

Caveolae are small, flask-shaped, non-clathrin coated invaginations of the plasma membrane of many mammalian cells. Caveolae have a coat that includes caveolin. They have been implicated in numerous cellular processes, including potocytosis. Since the human folate receptor (hFR) and other glycosyl-phosphatidylinositol GPI)-tailed proteins have been co-localized to caveolae, we studied the caveolin role in the hFR function by transfecting hFR and/or caveolin cDNA into Fisher rat thyroid epithelial (FRT) cells that normally do not express detectable levels of either protein. We isolated and characterized stable clones as follows: they express (1) high levels of caveolin alone, (2) hFR and caveolin, or (3) hFR alone. We discovered that hFR is correctly processed, sorted, and anchored by a GPI tail to the plasma membrane in FRT cells. No difference in the total folic acid binding or cell surface folic acid binding activity were found between the FRT cells that were transfected with hFR, or cells that were transfected with hFR and caveolin. The hFR that was expressed on the cell surface of clones that were transfected with hFR was also sensitive to phosphatidylinositol-specific phospholipase C (PI-PLC) release, and incorporated radiolabeled ethanolamine that supports the attachment of a GPI-tail on hFR. We conclude that the processing, sorting, and function of hFR is independent on the caveolin expression in FRT cells.

The endothelium but not the syncytiotrophoblast of human placenta expresses caveolae

The human placenta is a complex specialized structure that mediates the interchange of molecules, ions, and gases between maternal and foetal circulation. We have investigated the distribution and expression of caveolae/caveolin in the placenta. Using immunochemical, immunocytochemical, and ultrastructural methods, we show that the placenta expresses caveolin-1 and caveolin-2, which are marker proteins for caveolae. These proteins and caveolae were expressed at high levels in endothelium of placental capillaries and in endothelial and smooth muscle cells of larger vessels. In addition, fibroblasts in areas of the placenta with high connective tissue content also expressed caveolin. However, we were unable to detect these proteins or caveolae-like structures in the syncytiotrophoblast layer or in cytotrophoblasts. These results have important implications for further understanding placental biology and for the role of caveolae in cell regulation in this organ.

Respiratory syncytial virus assembly occurs in GM1-rich regions of the host-cell membrane and alters the cellular distribution of tyrosine phosphorylated caveolin-1

We have previously shown that respiratory syncytial virus (RSV) assembly occurs within regions of the host-cell surface membrane that are enriched in the protein caveolin-1 (cav-1). In this report, we have employed immunofluorescence microscopy to further examine the RSV assembly process. Our results show that RSV matures at regions of the cell surface that, in addition to cav-1, are enriched in the lipid-raft ganglioside GM1. Furthermore, a comparison of mock-infected and RSV-infected cells by confocal microscopy revealed a significant change in the cellular distribution of phosphocaveolin-1 (pcav-1). In mock-infected cells, pcav-1 was located at regions of the cell that interact with the extracellular matrix, termed focal adhesions (FA). In contrast, RSV-infected cells showed both a decrease in the levels of pcav-1 associated with FA and the appearance of pcav-1-containing cytoplasmic vesicles, the latter being absent in mock-infected cells. These cytoplasmic vesicles were clearly visible between 9 and 18 h post-infection and coincided with the formation of RSV filaments, although we did not observe a direct association of pcav-1 with mature virus. In addition, we noted a strong colocalization between pcav-1 and growth hormone receptor binding protein-7 (Grb7), within these cytoplasmic vesicles, which was not observed in mock-infected cells. Collectively, these findings show that the RSV assembly process occurs within specialized lipid-raft structures on the host-cell plasma membrane, induces the cellular redistribution of pcav-1 and results in the formation of cytoplasmic vesicles that contain both pcav-1 and Grb7.

Nonopsonic phagocytosis of Mycobacterium kansasii by human neutrophils depends on cholesterol and is mediated by CR3 associated with glycosylphosphatidylinositol-anchored proteins

Receptors involved in the phagocytosis of microorganisms under nonopsonic conditions have been little studied in neutrophils. Complement receptor type 3 (CR3) is a pattern recognition receptor able to internalize zymosan and C3bi-coated particles. We report that Abs directed against CR3 strongly inhibited nonopsonic phagocytosis of Mycobacterium kansasii in human neutrophils. In these cells CR3 has been found associated with several GPI-anchored proteins localized in cholesterol-rich microdomains (rafts) of the plasma membrane. Cholesterol sequestration by nystatin, filipin, or beta-cyclodextrin as well as treatment of neutrophils with phosphatidylinositol phospholipase C to remove GPI-anchored proteins from the cell surface markedly inhibited phagocytosis of M. kansasii, without affecting phagocytosis of zymosan or serum-opsonized M. kansasii. Abs directed against several GPI-anchored proteins inhibited phagocytosis of M. kansasii, but not of zymosan. N:-acetyl-D-glucosamine, which is known to disrupt interactions between CR3 and GPI proteins, also strongly diminished phagocytosis of these mycobacteria. In conclusion, phagocytosis of M. kansasii involved CR3, GPI-anchored receptors, and cholesterol. In contrast, phagocytosis of zymosan or opsonized particles involved CR3, but not cholesterol or GPI proteins. We propose that CR3, when associated with a GPI protein, relocates in cholesterol-rich domains where M. kansasii are internalized. When CR3 is not associated with a GPI protein, it remains outside of these domains and mediates phagocytosis of zymosan and opsonized particles, but not of M. kansasii.

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.

Determination of the non-ionic detergent insolubility and phosphoprotein associations of glycosylphosphatidylinositol-anchored proteins expressed on T cells

Glycosylphosphatidylinositol (GPI)-anchored proteins are poorly solublized in non-ionic detergents such as Triton X-100 and Nonidet P40, but are easily solublized by detergents with high critical micelle concentrations such as octylglucoside. This solubility profile has been suggested to be due to the localization of GPI-anchored proteins to lipid microdomains rich in cholesterol and sphingolipids. Additionally, GPI-anchored proteins expressed on haemopoietic cells have been shown to associate with src-family tyrosine kinases and heterotrimeric G proteins. Despite these observations, the non-ionic detergent insolubility of GPI-anchored proteins on haemopoietic cells has not been quantified nor has a relationship between the non-ionic detergent insolubility of these proteins and their association with signal-transduction molecules been identified. Here we show that GPI-anchored proteins found on T-cell tumours and activated T cells, although significantly more insoluble then transmembrane proteins, are not uniform in their detergent insolubility. Whereas CD59 was between 4% and 13% soluble, CD48 was between 13% and 25% soluble, CD55 was between 20% and 30% soluble, and CD109 was between 34% and 75% soluble. The ability of these GPI-anchored proteins to associate with phosphoproteins was correlated with their detergent insolubility: the more detergent-insoluble that a GPI-anchored protein was, the greater the level of phosphoprotein associations. These experiments reveal a relationship between non-ionic detergent insolubility and association with signal-transduction molecules and suggest a cause-and-effect relationship between these two properties. In total, these experiments support the hypothesis that the association of GPI-anchored proteins with signalling molecules is due to their sorting to lipid microdomains.

Freeze-fracture cytochemistry: a new method combining immunocytochemistry and enzyme cytochemistry on replicas

We describe a new freeze-fracture cytochemical technique consisting of combined immunocytochemistry and enzyme cytochemistry. This technique reveals the relationship between molecules in biological membranes by double labeling with two different cytochemical markers (i.e., immunogold probes and cerium). In this method, antigens were detected with specific primary antibodies and appropriate secondary immunoprobes. Subsequently, alkaline phosphates activity was detected with cerium as the capture agent on the same replicas. Octyl-glucoside (OG) digestion before the cytochemical reactions was crucial to the success of this combined method. OG is an efficient detergent and OG digestion can preserve both immunocytochemical antigenicity and enzyme activity on replicas. As an initial examination, we applied this technique to the study of glycosyl-phosphatidyl-inositol-anchored proteins and adhesion molecules in human neutrophils. The method described here should serve as a unique additional approach for the study of topology and dynamics of molecules in biomembranes.