Phosphatidylinositol-specific phospholipase C can decrease Müller cell viability and suppress its phagocytic activity by modulating PI3K/AKT signaling pathway.

Bacillus cereus endophthalmitis is a devastating eye infection that causes rapid blindness through the release of extracellular tissue-destructive exotoxins. The phagocytic and antibacterial functions of ocular cells are the keys to limiting ocular bacterial infections. In a previous study, we identified a new virulence gene, plcA-2 (different from the original plcA-1 gene), that was strongly associated with the plcA gene of Listeria monocytogenes. This plcA gene had been confirmed to play an important role in phagocytosis. However, how the Bc-phosphatidylinositol-specific phospholipase C (PI-PLC) proteins encoded by the plcA-1/2 genes affect phagocytes remains unclear in B. cereus endophthalmitis. Here, we found that the enzymatic activity of Bc-PI-PLC-A2 was approximately twofold higher than that of Bc-PI-PLC-A1, and both proteins inhibited the viability of Müller cells. In addition, PI-PLC proteins reduced phagocytosis of Müller cells by decreasing the phosphorylation levels of key proteins in the PI3K/AKT signaling pathway. In conclusion, we showed that PI-PLC proteins contribute to inhibit the viability of and suppress the phagocytosis of Müller cells, providing new insights into the pathogenic mechanism of B. cereus endophthalmitis.

Search and Subvert: Minimalist Bacterial Phosphatidylinositol-Specific Phospholipase C Enzymes.

Phosphatidylinositol-specific phospholipase C (PI-PLC) enzymes from Gram-positive bacteria are secreted virulence factors that aid in downregulating host immunity. These PI-PLCs are minimalist peripheral membrane enzymes with a distorted (ßα)8 TIM barrel fold offering a conserved and stable scaffold for the conserved catalytic amino acids while membrane recognition is achieved mostly through variable loops. Decades of experimental and computational research on these enzymes have revealed the subtle interplay between molecular mechanisms of catalysis and membrane binding, leading to a semiquantitative model for how they find, bind, and cleave their respective substrates on host cell membranes. Variations in sequence and structure of their membrane binding sites may correlate with how enzymes from different Gram-positive bacteria search for their particular targets on the membrane. Detailed molecular characterization of protein-lipid interactions have been aided by cutting-edge methods ranging from 31P field-cycling NMR relaxometry to monitor protein-induced changes in phospholipid dynamics to molecular dynamics simulations to elucidate the roles of electrostatic and cation-π interactions in lipid binding to single molecule fluorescence measurements of dynamic interactions between PI-PLCs and vesicles. This toolkit is readily applicable to other peripheral membrane proteins including orthologues in Gram-negative bacteria and more recently discovered eukaryotic minimalist PI-PLCs.

Proposed three-phenylalanine motif involved in magnetoreception signalling of an Actinopterygii protein expressed in mammalian cells.

Studies at the cellular and molecular level of magnetoreception-sensing and responding to magnetic fields-are a relatively new research area. It appears that different mechanisms of magnetoreception in animals evolved from different origins, and, therefore, many questions about its mechanisms remain left open. Here we present new information regarding the Electromagnetic Perceptive Gene (EPG) from Kryptopterus vitreolus that may serve as part of the foundation to understanding and applying magnetoreception. Using HaloTag coupled with fluorescent ligands and phosphatidylinositol specific phospholipase C we show that EPG is associated with the membrane via glycosylphosphatidylinositol anchor. EPG's function of increasing intracellular calcium was also used to generate an assay using GCaMP6m to observe the function of EPG and to compare its function with that of homologous proteins. It was also revealed that EPG relies on a motif of three phenylalanine residues to function-stably swapping these residues using site directed mutagenesis resulted in a loss of function in EPG. This information not only expands upon our current understanding of magnetoreception but may provide a foundation and template to continue characterizing and discovering more within the emerging field.

Molecular cloning of gp42, a cell-surface molecule that is selectively induced on rat natural killer cells by interleukin 2: glycolipid membrane anchoring and capacity for transmembrane signaling.

We have previously shown that in vitro culture of rat natural killer (NK) cells in high concentrations of recombinant interleukin 2 (rIL-2) leads to the expression of a surface glycoprotein with a molecular mass of approximately 42 kD. This glycoprotein, gp42, is not induced on other lymphocytes and thus provides a lineage-specific marker for rIL-2-activated NK cells. We here present the nucleotide sequence for gp42 cDNA. The open reading frame encodes 233 amino acids with three potential sites for N-linked glycosylation. The deduced amino acid sequence lacks an apparent transmembrane domain and instead contains a hydrophobic COOH terminus that is characteristic of glycosylphosphatidylinositol (GPI)-anchored surface proteins. Consistent with this, gp42 is cleaved from the NK-like cell line, RNK-16, by phosphatidylinositol-specific phospholipase C (PI-PLC), as is gp42 expressed on CHO cells that have been transformed with gp42 cDNA. On rIL-2-activated NK cells, gp42 is resistant to PI-PLC, though our studies suggest that gp42 on these cells is still expressed as a GPI-anchored molecule. Antibody to gp42 stimulates in RNK-16 cells an increase in inositol phosphates and in intracellular calciu, signals that are associated with the activation of lymphocytes, including NK cells. rIL-2-activated NK cells, however, lack this response to gp42 as well as to other stimuli. Thus, gp42, the only NK-specific activation antigen, is a GPI-anchored surface molecule with the capacity to stimulate transmembrane signaling.

Listeria monocytogenes mutants lacking phosphatidylinositol-specific phospholipase C are avirulent.

A number of bacterial species secrete phosphatidylinositol-specific phospholipase C (PI-PLC). In this report, we show that the facultative intracellular bacterial pathogen, Listeria monocytogenes, contains a gene, plcA, predicting a polypeptide with 31% amino acid identity to a Bacillus thuringiensis PI-PLC. Accordingly, L. monocytogenes secretes PI-PLC activity, while a mutant with a transposon insertion in plcA lacks detectable PI-PLC activity. In addition, expression of plcA in B. subtilis resulted in secretion of PI-PLC activity. The L. monocytogenes PI-PLC-defective mutant was three logs less virulent for mice and failed to grow in host tissues. The mutant was also defective for in vitro growth in mouse peritoneal macrophages. These results strongly suggest that PI-PLC is an essential determinant of L. monocytogenes pathogenesis. Whether the PI-PLC acts on a bacterial or host substrate remains to be determined.

Role of virion-associated glycosylphosphatidylinositol-linked proteins CD55 and CD59 in complement resistance of cell line-derived and primary isolates of HIV-1.

This study investigates whether cell-derived glycosylphosphatidylinositol-linked complement control proteins CD55 and CD59 can be incorporated into HIV-1 virions and contribute to complement resistance. Virus was prepared by transfection of cell lines with pNL4-3, and primary isolates of HIV-1 were derived from patients' PBMCs. Virus was tested for sensitivity to complement-mediated virolysis in the presence of anti-gp160 antibody. Viral preparations from JY33 cells, which lack CD55 and CD59, were highly sensitive to complement. HIV-1 preparations from H9 and U937 cells, which express low levels of CD55 and CD59, had intermediate to high sensitivity while other cell line-derived viruses and primary isolates of HIV-1 were resistant to complement-mediated virolysis. Although the primary isolates were not lysed, they activated complement as measured by binding to a complement receptor positive cell line. While the primary isolates were resistant to lysis in the presence of HIV-specific antibody, antibody to CD59 induced lysis. Likewise, antibody to CD55 and CD59 induced lysis of cell line-derived virus. Western blot analysis of purified virus showed bands corresponding to CD55 and CD59. Phosphatidylinositol-specific phospholipase C treatment of either cell line-derived or primary isolates of HIV-1 increased sensitivity to complement while incubation of sensitive virus with purified CD55 and CD59 increased resistance to complement. These results show that CD55 and CD59 are incorporated into HIV-1 particles and function to protect virions from complement-mediated destruction, and they are the first report of host cell proteins functioning in protection of HIV-1 from immune effector mechanisms.

The extracellular domain of CD45 controls association with the CD4-T cell receptor complex and the response to antigen-specific stimulation.

The CD45 tyrosine phosphatase plays an important role in regulating T lymphocyte activation, but the function of the different isoforms of CD45 is not known. T cell transfectants have been prepared that express individual CD45 isoforms in cells with a well-defined T cell receptor (TCR) from the D10 T helper 2 clone. We find that cells bearing low molecular weight CD45 isoforms are far more efficient in responding to stimulation with peptide and antigen-presenting cells compared with cells bearing high molecular weight CD45 isoforms. One hypothesis for the preferential activation of cells that express low molecular weight CD45 isoforms is that they interact with other cell surface antigens important in TCR signaling, altering their phosphorylation status and affecting the character of the signal transduction pathway. In this report, using cells expressing single isoforms, we demonstrate that low molecular weight isoforms of CD45 preferentially associate with CD4 and the TCR complex compared with high molecular weight isoforms. The molecular basis for this interaction was further examined using a glycosyl phosphatidyl inositol (GPI)-linked form of CD45Null (lacking tyrosine phosphatase domains), which preferentially associated with CD4 compared with GPI-linked CD45ABC, and cytoplasmic tail mutants of CD4, which retained the ability to coassociate. Using this panel of transfectants, it is clear that the interaction between CD4 and CD45 does not require the cytoplasmic domains of CD45, but is dependent on the specific external domain of the various isoforms: low molecular weight species were more likely to associate with the CD4-TCR complex than the higher molecular weight isoforms, and their ability to coassociate correlated with the magnitude of the response to specific antigen.

Functional and antigenic similarities between a 94-kD protein of Schistosoma mansoni (SCIP-1) and human CD59.

Schistosomiasis is a parasitic disease affecting approximately 200 million people, primarily in the third world. Schistosoma mansoni, one of the causative agents of this disease, parasitize the human mesenteric and portal blood systems while successfully evading host immune responses. During parasite penetration into the mammalian host and shortly afterwards, the larvae rapidly convert from being sensitive to being resistant to C-mediated killing. Treatment of the C-resistant parasitic forms with trypsin renders the parasite susceptible to C attack, thus indicating the presence of C inhibitory protein(s) on the parasite surface. We describe here an intrinsic schistosome C inhibitory protein (SCIP-1) that exhibits antigenic and functional similarities with the human C-inhibitor CD59. Like CD59, SCIP-1 is capable of inhibiting formation of the C membrane attack complex (MAC), probably by binding to C8 and C9 of the C terminal pathway. In addition, SCIP-1 is apparently also membrane-anchored via glycosyl phosphatidylinositol as it can be specifically released with phosphatidylinositol-specific phospholipase C. Soluble SCIP-1, partially purified from Nonidet P-40 extracts of schistosome tegument is capable of inhibiting hemolysis of sensitized sheep erythrocytes and of rabbit erythrocytes by human C. Anti-human CD59 antibodies block this activity of SCIP-1 and in addition, upon binding to intact parasites, render them vulnerable to killing by human and guinea pig C. SCIP-1 is located on the surface of C-resistant forms of the parasite, i.e., 24-h cultured mechanical schistosomula and in vivo-derived adult worms as revealed by immunofluorescence and immunogold electron microscopy studies. These results identify one of the mechanisms schistosomes use to escape immune attack.

The lymphocyte function-associated antigen 1 I domain is a transient binding module for intercellular adhesion molecule (ICAM)-1 and ICAM-3 in hydrodynamic flow.

The I domain of lymphocyte function-associated antigen (LFA)-1 contains an intercellular adhesion molecule (ICAM)-1 and ICAM-3 binding site, but the relationship of this site to regulated adhesion is unknown. To study the adhesive properties of the LFA-1 I domain, we stably expressed a GPI-anchored form of this I domain (I-GPI) on the surface of baby hamster kidney cells. I-GPI cells bound soluble ICAM-1 (sICAM-1) with a low avidity and affinity. Flow cell experiments demonstrated a specific rolling interaction of I-GPI cells on bilayers containing purified full length ICAM-1 or ICAM-3. The LFA-1 activating antibody MEM-83, or its Fab fragment, decreased the rolling velocity of I-GPI cells on ICAM-1-containing membranes. In contrast, the interaction of I-GPI cells with ICAM-3 was blocked by MEM-83. Rolling of I-GPI cells was dependent on the presence of Mg2+. Mn2+ only partially substituted for Mg2+, giving rise to a small fraction of rolling cells and increased rolling velocity. This suggests that the I domain acts as a transient, Mg2+-dependent binding module that cooperates with another Mn2+-stimulated site in LFA-1 to give rise to the stable interaction of intact LFA-1 with ICAM-1.

Design of Staphylococcus aureus New Vaccine Candidates with B and T Cell Epitope Mapping, Reverse Vaccinology, and Immunoinformatics.

An effective vaccine against Staphylococcus aureus infection is a major planetary heath priority, particularly with increasing antibiotic resistance worldwide. Previous efforts for a highly effective S. aureus vaccine were largely unsuccessful, in part, because the vaccine designs have tended to target mainly the B cell immunity and development of opsonic antibodies. In contrast, recent observations suggest that cell mediated immunity may be critical for protection against S. aureus. In addition, the S. aureus surface proteins are among the key immunodominant antigens because they are the first molecules to interact with the host organism cells and tissues. We report here an original vaccinomics study in which we used a reverse vaccinology and immunoinformatics in silico strategy integrated with genomics. After analyzing 2767 proteins, we defined 16 proteins of S. aureus as promising subunit vaccine candidates. Phosphatidylinositol phosphodiesterase (Plc) is secreted by extracellular pathogens such as S. aureus. We mapped the B and T cell epitopes for the Plc protein, tested the reactivity of the synthesized epitopes by Western blotting, and verified our findings in a pilot study of 10 patients with S. aureus infection. The peptides were then tested for their protective effect in groups of mice challenged with pathogenic S. aureus strain, which showed high protection level. These findings warrant further translational research for development of novel vaccines against S. aureus infection. Reverse vaccinology is an advanced approach that can be applied to identify new vaccine candidates against a host of microorganisms, including S. aureus.

A function for a specific zinc metalloprotease of African trypanosomes.

The Trypanosoma brucei genome encodes three groups of zinc metalloproteases, each of which contains approximately 30% amino acid identity with the major surface protease (MSP, also called GP63) of Leishmania. One of these proteases, TbMSP-B, is encoded by four nearly identical, tandem genes transcribed in both bloodstream and procyclic trypanosomes. Earlier work showed that RNA interference against TbMSP-B prevents release of a recombinant variant surface glycoprotein (VSG) from procyclic trypanosomes. Here, we used gene deletions to show that TbMSP-B and a phospholipase C (GPI-PLC) act in concert to remove native VSG during differentiation of bloodstream trypanosomes to procyclic form. When the four tandem TbMSP-B genes were deleted from both chromosomal alleles, bloodstream B (-/-) trypanosomes could still differentiate to procyclic form, but VSG was removed more slowly and in a non-truncated form compared to differentiation of wild-type organisms. Similarly, when both alleles of the single-copy GPI-PLC gene were deleted, bloodstream PLC (-/-) cells could still differentiate. However, when all the genes for both TbMSP-B and GPI-PLC were deleted from the diploid genome, the bloodstream B (-/-) PLC (-/-) trypanosomes did not proliferate in the differentiation medium, and 60% of the VSG remained on the cell surface. Inhibitors of cysteine proteases did not affect this result. These findings demonstrate that removal of 60% of the VSG during differentiation from bloodstream to procyclic form is due to the synergistic activities of GPI-PLC and TbMSP-B.

Identification of a lipid-anchored heparan sulfate proteoglycan in Schwann cells.

Schwann cells synthesize both hydrophobic and peripheral cell surface heparan sulfate proteoglycans (HSPGs). Previous analysis of the kinetics of radiolabeling suggested the peripheral HSPGs are derived from the membrane-anchored forms (Carey, D., and D. Evans. 1989. J. Cell Biol. 108:1891-1897). Peripheral cell surface HSPGs were purified from phytic acid extracts of cultured neonatal rat sciatic nerve Schwann cells by anion exchange, gel filtration, and laminin-affinity chromatography. Approximately 250 micrograms of HSPG protein was obtained from 2 X 10(9) cells with an estimated recovery of 23% and an overall purification of approximately 2000-fold. SDS-PAGE analysis indicated the absence of non-HSPG proteins in the purified material. Analysis of heparinase digestion products revealed the presence of at least six core protein species ranging in molecular weight from 57,000 to 185,000. The purified HSPGs were used to produce polyclonal antisera in rabbits. The antisera immunoprecipitated a subpopulation of 35SO4-labeled HSPGs that were released from Schwann cells by incubation in medium containing phosphatidylinositol-specific phospholipase C (PI-PLC); smaller amounts of immunoprecipated HSPGs were also present in phytic acid extracts. In the presence of excess unlabeled PI-PLC-released proteins, immunoprecipitation of phytic acid-solubilized HSPGs was inhibited. SDS-PAGE analysis of proteins immunoprecipitated from extracts of [35S]methionine labeled Schwann cells demonstrated that the antisera precipitated an HSPG species that was present in the pool of proteins released by PI-PLC, with smaller amounts present in phytic acid extracts. Nitrous acid degradation of the immunoprecipitated proteins produced a single 67,000-Mr core protein. When used for indirect immunofluorescence labeling, the antisera stained the external surface of cultured Schwann cells. Preincubation of the cultures in medium containing PI-PLC but not phytic acid significantly reduced the cell surface staining. The antisera stained the outer ring of Schwann cell membrane in sections of adult rat sciatic nerve but did not stain myelin or axonal membranes. This localization suggests the HSPG may play a role in binding the Schwann cell plasma membrane to the adjacent basement membrane surrounding the individual axon-Schwann cell units.

Traffic, polarity, and detergent solubility of a glycosylphosphatidylinositol-anchored protein after LDL-deprivation of MDCK cells.

Glycosylphosphatidylinositol-anchored proteins, GPI-proteins, are selectively delivered to the apical surfaces of many types of morphologically polarized epithelial cells. It has been proposed that the unit for targeting GPI-proteins to the apical surface is a membrane lipid domain. This sorting domain or molecular cluster has been equated to detergent (Triton X-100)-insoluble membrane fractions that are enriched in enriched in GPI-proteins, glycosphingolipids, and cholesterol. To determine the role of cholesterol in the formation of sorting domains and to examine its importance in the intracellular traffic and membrane polarity of GPI-proteins, we studied the behavior of a model GPI-protein, gD1-DAF, in MDCK cells cultured for 3 or 14 d without their principal source of cholesterol, serum LDL. LDL deprivation affects the intracellular traffic of gD1-DAF. Surface expression of gD1-DAF is reduced in LDL-deprived cells; this reduction is most marked after 3 d of LDL deprivation. We also find a great reduction in the fraction of gD1-DAF that is detergent-insoluble in these cells and a change in its membrane milieu defined by susceptibility to cleavage with PI-specific phospholipase C. Despite these changes, the surface polarity of gD1-DAF is no different in LDL-deprived cells than in control cells.

A glycosylphosphatidylinositol (GPI)-negative phenotype produced in Leishmania major by GPI phospholipase C from Trypanosoma brucei: topography of two GPI pathways.

The major surface macromolecules of the protozoan parasite Leishmania major, gp63 (a metalloprotease), and lipophosphoglycan (a polysaccharide), are glycosylphosphatidylinositol (GPI) anchored. We expressed a cytoplasmic glycosylphosphatidylinositol phospholipase C (GPI-PLC) in L. major in order to examine the topography of the protein-GPI and polysaccharide-GPI pathways. In L. major cells expressing GPI-PLC, cell-associated gp63 could not be detected in immunoblots. Pulse-chase analysis revealed that gp63 was secreted into the culture medium with a half-time of 5.5 h. Secreted gp63 lacked anti-cross reacting determinant epitopes, and was not metabolically labeled with [3H]ethanolamine, indicating that it never received a GPI anchor. Further, the quantity of putative protein-GPI intermediates decreased approximately 10-fold. In striking contrast, lipophosphoglycan levels were unaltered. However, GPI-PLC cleaved polysaccharide-GPI intermediates (glycoinositol phospholipids) in vitro. Thus, reactions specific to the polysaccharide-GPI pathway are compartmentalized in vivo within the endoplasmic reticulum, thereby sequestering polysaccharide-GPI intermediates from GPI-PLC cleavage. On the contrary, protein-GPI synthesis at least up to production of Man(1 alpha 6)Man(1 alpha 4)GlcN-(1 alpha 6)-myo-inositol-1-phospholipid is cytosolic. To our knowledge this represents the first use of a catabolic enzyme in vivo to elucidate the topography of biosynthetic pathways. GPI-PLC causes a protein-GPI-negative phenotype in L. major, even when genes for GPI biosynthesis are functional. This phenotype is remarkably similar to that of some GPI mutants of mammalian cells: implications for paroxysmal nocturnal hemoglobinuria and Thy-1-negative T-lymphoma are discussed.

The GPI anchor of cell-surface proteins is synthesized on the cytoplasmic face of the endoplasmic reticulum.

Glycosylphosphatidylinositol (GPI) membrane protein anchors are synthesized from sugar nucleotides and phospholipids in the ER and transferred to newly synthesized proteins destined for the cell surface. The topology of GPI synthesis in the ER was investigated using sealed trypanosome microsomes and the membrane-impermeant probes phosphatidylinositol-specific phospholipase C, Con A, and proteinase K. All the GPI biosynthetic intermediates examined were found to be located on the external face of the microsomal vesicles suggesting that the principal steps of GPI assembly occur in the cytoplasmic leaflet of the ER. Protease protection experiments showed that newly GPI-modified trypanosome variant surface glycoprotein was primarily oriented towards the ER lumen, consistent with eventual expression at the cell surface. The unusual topographical arrangement of the GPI assembly pathway suggests that a biosynthetic intermediate, possibly the phosphoethanolamine-containing anchor precursor, must be translocated across the ER membrane bilayer in the process of constructing a GPI anchor.

Correctly sorted molecules of a GPI-anchored protein are clustered and immobile when they arrive at the apical surface of MDCK cells.

Glycosyl-phosphatidylinositol (GPI)-anchored proteins are sorted to the apical surface of many epithelial cell types. To better understand the mechanism for apical segregation of these proteins, we analyzed the lateral mobility and molecular associations of a model GPI-anchored protein, herpes simplex virus gD1 fused to human decay accelerating factor (gD1-DAF) (Lisanti, M. P., I. W. Caras, M. A. Davitz, and E. Rodriguez-Boulan. 1989. J. Cell Biol. 109:2145-2156) shortly after arrival and after long-term residence at the surface of confluent, polarized MDCK cells. FRAP measurements of lateral diffusion showed that the mobile fraction of newly arrived gD1-DAF molecules was much less than the mobile fraction of long-term resident molecules (40 vs. 80-90%). Fluorescence resonance energy transfer measurements showed that the newly arrived molecules were clustered, while resident molecules were not. Newly delivered gD1-DAF molecules were clustered but not immobilized in mutant, Concanavalin A-resistant MDCK cells that failed to sort gD1-DAF. Our results indicate that GPI-anchored proteins in MDCK cells are clustered before delivery to the surface. However, clustering alone does not target molecules for apical delivery. The immobilization observed when gD1-DAF is correctly sorted suggests that the clusters must associate some component of the cell's cytoplasm.

Heparan sulfate expression in polarized epithelial cells: the apical sorting of glypican (GPI-anchored proteoglycan) is inversely related to its heparan sulfate content.

Several processes that occur in the luminal compartments of the tissues are modulated by heparin-like polysaccharides. To identify proteins responsible for the expression of heparan sulfate at the apex of polarized cells, we investigated the polarity of the expression of the cell surface heparan sulfate proteoglycans in CaCo-2 cells. Domain-specific biotinylation of the apical and basolateral membranes of these cells identified glypican, a GPI-linked heparan sulfate proteoglycan, as the major source of apical heparan sulfate. Yet, most of this proteoglycan was expressed at the basolateral surface, an unexpected finding for a glypiated protein. Metabolic labeling and chase experiments indicated that sorting mechanisms, rather than differential turnover, accounted for this bipolar expression of glypican. Chlorate treatment did not affect the polarity of the expression of glypican in CaCo-2 cells, and transfectant MDCK cells expressed wild-type glypican and a syndecan-4/glypican chimera also in an essentially unpolarized fashion. Yet, complete removal of the heparan sulfate glycanation sites from the glypican core protein resulted in the nearly exclusive apical targeting of glypican in the transfectants, whereas two- and one-chain mutant forms had intermediate distributions. These results indicate that glypican accounts for the expression of apical heparan sulfate, but that glycanation of the core protein antagonizes the activity of the apical sorting signal conveyed by the GPI anchor of this proteoglycan. A possible implication of these findings is that heparan sulfate glycanation may be a determinant of the subcellular expression of glypican. Alternatively, inverse glycanation-apical sorting relationships in glypican may insure near constant deliveries of HS to the apical compartment, or "active" GPI-mediated entry of heparan sulfate into apical membrane compartments may require the overriding of this antagonizing effect of the heparan sulfate chains.

A soluble form of the F3 neuronal cell adhesion molecule promotes neurite outgrowth.

The F3 molecule is a member of the immunoglobulin superfamily anchored to membranes by a glycane-phosphatidylinositol, and is predominantly expressed on subsets of axons of the central and peripheral nervous system. In a previous paper (Gennarini, G., P. Durbec, A. Boned, G. Rougon, and C. Goridis. 1991. Neuron. 6:595-606), we have established that F3 fulfills the operational definition of a cell adhesion molecule and that it stimulates neurite outgrowth when presented to sensory neurons as a surface component of transfected CHO cells. In the present study the question as to whether soluble forms of F3 would be functionally active was addressed in vitro on cultures of mouse dorsal root ganglion neurons. We observed that preparations enriched in soluble F3 had no effect on neuron attachment but enhanced neurite initiation and neurite outgrowth in a dose-dependent manner. By contrast, soluble NCAM-120 does not have any measurable effect on these phenomena. Addition of anti-F3 monovalent antibodies reduced the number of process-bearing neurons and the neuritic output per neuron to control values. Addition of cerebrospinal fluid, a natural source of soluble F3, also stimulated neurite extension, and this effect was partially blocked by anti-F3 antibodies. Our results suggest that the soluble forms of adhesive proteins with neurite outgrowth-promoting properties could act at a distance from their site of release in a way reminiscent of growth and trophic factors.

Differential regulation and function of CD73, a glycosyl-phosphatidylinositol-linked 70-kD adhesion molecule, on lymphocytes and endothelial cells.

CD73, otherwise known as ecto-5'-nucleotidase, is a glycosyl-phosphatidylinositol-linked 70-kD molecule expressed on different cell types, including vascular endothelial cells (EC) and certain subtypes of lymphocytes. There is strong evidence for lymphocyte CD73 having a role in several immunological phenomena such as lymphocyte activation, proliferation, and adhesion to endothelium, but the physiological role of CD73 in other cell types is less clear. To compare the biological characteristics of CD73 in different cell types, we have studied the structure, function, and surface modulation of CD73 on lymphocytes and EC. CD73 molecules on lymphocytes are shed from the cell surface as a consequence of triggering with an anti-CD73 mAb, mimicking ligand binding. In contrast, triggering of endothelial CD73 does not have any effect on its expression. Lymphocyte CD73 is susceptible to phosphatidylinositol phospholipase, whereas only a small portion of CD73 on EC could be removed by this enzyme. Furthermore, CD73 on EC was unable to deliver a tyrosine phosphorylation inducing signal upon mAb triggering, whereas triggering of lymphocyte CD73 can induce tyrosine phosphorylation. Despite the functional differences, CD73 molecules on lymphocytes and EC were practically identical structurally, when studied at the protein, mRNA, and cDNA level. Thus, CD73 is an interesting example of a molecule which lacks structural variants but yet has a wide diversity of biological functions. We suggest that the ligand-induced shedding of lymphocyte CD73 represents an important and novel means of controlling lymphocyte-EC interactions.

An internally positioned signal can direct attachment of a glycophospholipid membrane anchor.

All known glycophosphatidylinositol (GPI)-anchored membrane proteins contain a COOH-terminal hydrophobic domain necessary for signalling anchor attachment. To examine the requirement that this signal be at the COOH terminus of the protein, we constructed a chimeric protein, DAFhGH, in which human growth hormone (hGH) was fused to the COOH terminus of decay accelerating factor (DAF) (a GPI-anchored protein), thereby placing the GPI signal in the middle of the chimeric protein. We show that the fusion protein appears to be processed at the normal DAF processing site in COS cells, producing GPI-anchored DAF on the cell surface. This result indicates that the GPI signal does not have to be at the COOH terminus to direct anchor addition, suggesting that the absence of a hydrophilic COOH-terminal extension (beyond the hydrophobic domain) is not a necessary requirement for GPI anchoring. A similar DAFhGH fusion, containing an internal GPI signal in which the DAF hydrophobic domain was replaced with the signal peptide of hGH, also produced GPI-anchored cell surface DAF. The signal for GPI attachment thus exhibits neither position specificity nor sequence specificity. In addition, mutant DAF or DAFhGH constructs lacking an NH2-terminal signal peptide failed to produce GPI-anchored protein, suggesting that membrane translocation is necessary for anchor addition.