Mammalian glycosylphosphatidylinositol-anchored proteins and intracellular precursors

A Soluble PrPC Derivative and Membrane-Anchored PrPC in Extracellular Vesicles Attenuate Innate Immunity by Engaging the NMDA-R/LRP1 Receptor Complex

Nonpathogenic cellular prion protein (PrPC) demonstrates anti-inflammatory activity; however, the responsible mechanisms are incompletely defined. PrPC exists as a GPI-anchored membrane protein in diverse cells; however, PrPC may be released from cells by ADAM proteases or when packaged into extracellular vesicles (EVs). In this study, we show that a soluble derivative of PrPC (S-PrP) counteracts inflammatory responses triggered by pattern recognition receptors in macrophages, including TLR2, TLR4, TLR7, TLR9, NOD1, and NOD2. S-PrP also significantly attenuates the toxicity of LPS in mice. The response of macrophages to S-PrP is mediated by a receptor assembly that includes the N-methyl-d-aspartate receptor (NMDA-R) and low-density lipoprotein receptor-related protein-1 (LRP1). PrPC was identified in EVs isolated from human plasma. These EVs replicated the activity of S-PrP, inhibiting cytokine expression and IκBα phosphorylation in LPS-treated macrophages. The effects of plasma EVs on LPS-treated macrophages were blocked by PrPC-specific Ab, by antagonists of LRP1 and the NMDA-R, by deleting Lrp1 in macrophages, and by inhibiting Src family kinases. Phosphatidylinositol-specific phospholipase C dissociated the LPS-regulatory activity from EVs, rendering the EVs inactive as LPS inhibitors. The LPS-regulatory activity that was lost from phosphatidylinositol-specific phospholipase C-treated EVs was recovered in solution. Collectively, these results demonstrate that GPI-anchored PrPC is the essential EV component required for the observed immune regulatory activity of human plasma EVs. S-PrP and EV-associated PrPC regulate innate immunity by engaging the NMDA-R/LRP1 receptor system in macrophages. The scope of pattern recognition receptors antagonized by S-PrP suggests that released forms of PrPC may have broad anti-inflammatory activity.

Staphylococcal phosphatidylinositol-specific phospholipase C potentiates lung injury via complement sensitisation

Staphylococcus aureus is frequently isolated from patients with community-acquired pneumonia and acute respiratory distress syndrome (ARDS). ARDS is associated with staphylococcal phosphatidylinositol-specific phospholipase C (PI-PLC); however, the role of PI-PLC in the pathogenesis and progression of ARDS remains unknown. Here, we showed that recombinant staphylococcal PI-PLC possesses enzyme activity that causes shedding of glycosylphosphatidylinositol-anchored CD55 and CD59 from human umbilical vein endothelial cell surfaces and triggers cell lysis via complement activity. Intranasal infection with PI-PLC-positive S. aureus resulted in greater neutrophil infiltration and increased pulmonary oedema compared with a plc-isogenic mutant. Although indistinguishable proinflammatory genes were induced, the wild-type strain activated higher levels of C5a in lung tissue accompanied by elevated albumin instillation and increased lactate dehydrogenase release in bronchoalveolar lavage fluid compared with the plc^- mutant. Following treatment with cobra venom factor to deplete complement, the wild-type strain with PI-PLC showed a reduced ability to trigger pulmonary permeability and tissue damage. PI-PLC-positive S. aureus induced the formation of membrane attack complex, mainly on type II pneumocytes, and reduced the level of CD55/CD59, indicating the importance of complement regulation in pulmonary injury. In conclusion, S. aureus PI-PLC sensitised tissue to complement activation leading to more severe tissue damage, increased pulmonary oedema, and ARDS progression.

Fluorescence correlation spectroscopy of phosphatidylinositol-specific phospholipase C monitors the interplay of substrate and activator lipid binding

Phosphatidylinositol-specific phospholipase C (PI-PLC) enzymes simultaneously interact with the substrate, PI, and with nonsubstrate lipids such as phosphatidylcholine (PC). For Bacillus thuringiensis PI-PLC these interactions are synergistic with maximal catalytic activity observed at low to moderate mole fractions of PC (X(PC)) and maximal binding occurring at low mole fractions of anionic lipids. It has been proposed that residues in alpha-helix B help to modulate membrane binding and that dimerization on the membrane surface both increases affinity for PC and activates PI-PLC, yielding the observed PI/PC synergy. Vesicle binding and activity measurements using a variety of PI-PLC mutants support many aspects of this model and reveal that while single mutations can disrupt anionic lipid binding and the anionic lipid/PC synergy, the residues important for PC binding are less localized. Interestingly, at high X(PC) mutations can both decrease membrane affinity and increase activity, supporting a model where reductions in wild-type activity at X(PC) > 0.6 result from both dilution of the substrate and tight membrane binding of PI-PLC, limiting enzyme hopping or scooting to the next substrate molecule. These results provide a direct analysis of vesicle binding and catalytic activity and shed light on how occupation of the activator site enhances enzymatic activity.

Thematic review series: lipid posttranslational modifications. GPI anchoring of protein in yeast and mammalian cells, or: how we learned to stop worrying and love glycophospholipids

Glycosylphosphatidylinositol (GPI) anchoring of cell surface proteins is the most complex and metabolically expensive of the lipid posttranslational modifications described to date. The GPI anchor is synthesized via a membrane-bound multistep pathway in the endoplasmic reticulum (ER) requiring >20 gene products. The pathway is initiated on the cytoplasmic side of the ER and completed in the ER lumen, necessitating flipping of a glycolipid intermediate across the membrane. The completed GPI anchor is attached to proteins that have been translocated across the ER membrane and that display a GPI signal anchor sequence at the C terminus. GPI proteins transit the secretory pathway to the cell surface; in yeast, many become covalently attached to the cell wall. Genes encoding proteins involved in all but one of the predicted steps in the assembly of the GPI precursor glycolipid and its transfer to protein in mammals and yeast have now been identified. Most of these genes encode polytopic membrane proteins, some of which are organized in complexes. The steps in GPI assembly, and the enzymes that carry them out, are highly conserved. GPI biosynthesis is essential for viability in yeast and for embryonic development in mammals. In this review, we describe the biosynthesis of mammalian and yeast GPIs, their transfer to protein, and their subsequent processing.

Isolation and Characterization of Glycosylphosphatidylinositol-Anchored Peptides by Hydrophilic Interaction Chromatography and MALDI Tandem Mass Spectrometry

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are posttranslationally processed proteins that become tethered to the extracellular leaflet of the plasma membrane via a C-terminal glycan-like moiety. Since the first GPI-AP was described in the 1970s, more than 500 GPI-APs have been reported in a range of species, including plants, microbes, and mammals. GPI-APs are probably involved in cell signaling, cell recognition, and cell remodeling processes, and they may potentially serve as cell surface antigens or vaccine targets in pathogenic microorganisms or transformed mammalian cells. Due to the structural complexity and physicochemical properties of GPI-APs, their identification and structural characterization is a demanding analytical task. Here, we report a simple, fast and sensitive method for isolation and structural analysis of GPI-anchors using a combination of hydrophilic interaction liquid chromatography and matrix-assisted laser desorption/ionization (MALDI) quadrupole time-of-flight tandem mass spectrometry. This method allowed analysis of GPI peptides derived from low picomole levels of the porcine kidney membrane dipeptidase. Furthermore, it allowed unambiguous assignment of the omega site via amino acid sequencing of the modified peptides. GPI-anchor-specific diagnostic ions were observed by MALDI-MS/MS at m/z 162, 286, 422, and 447, corresponding to glucosamine, mannose ethanolamine phosphate, glucosamine inositol phosphate, and mannose ethanolamine phosphate glucosamine, respectively. Thus, the methodology described herein may enable sensitive and specific detection of GPI-anchored peptides in large-scale proteomic studies of plasma membrane proteins.

Lipopolysaccharide administration increases acid and alkaline phosphatase reactivity in the cardiac muscle

The effect of lipopolysaccharide (LPS) administration on the in situ distribution of the reaction product of acid phosphatase (AcPase) and alkaline phosphatase (AlPase) activity was examined in the rat cardiac muscle using catalytical cytochemistry. Tissues of the heart were fixed and then incubated in reaction media for detection of AcPase and AlPase reactivity. In normal hearts, reaction product of AcPase activity was observed in lysosomes. AlPase reactivity was detected at the extracellular surface of the capillary endothelilal cells and in their caveolae. Following LPS administration, the number and the size of lysosomes possessing AcPase reactivity as well as their electron density significantly increased. Furthermore, they tended to form groups consisting of three to five lysosomes. Cytochemical reaction 2 and 24 hours after injection was similar. One week later, the reaction returned to its normal pattern. As in the case with AcPase, the first changes of the distribution of the reaction product of AlPase activity were detected 2 hours after injection. The changes included a remarkable increase of the number of enzymatically positive capillaries, intensified cytochemical reaction in endothelial cells, and an increased number of caveolae. Again, no noticeable differences in reactivity were observed 2 and 24 hours after injection and the reaction returned to normal one week later. Collectively, our data indicate that both cardiac AcPase and AlPase are affected early after injection of LPS. Although the pattern of cytochemical reaction of both phosphatases was restored one week later, it is believed that the altered distribution of their reactivity in early periods after LPS administration may be a factor contributing to the development of pathological changes in this organ at a later stage.

Prostasin is a glycosylphosphatidylinositol-anchored active serine protease

A recombinant human prostasin serine protease was expressed in several human cell lines. Subcellular fractionation showed that this serine protease is synthesized as a membrane-bound protein while a free-form prostasin is secreted into the culture medium. Prostasin was identified in nuclear and membrane fractions. Membrane-bound prostasin can be released by phosphatidylinositol-specific phospholipase C treatment, or labeled by [(3)H]ethanolamine, indicating a glycosylphosphatidylinositol anchorage. A prostasin-binding protein was identified in mouse and human seminal vesicle fluid. Both the secreted and the membrane-bound prostasin were able to form a covalently linked 82-kDa complex when incubated with seminal vesicle fluid. The complex formation between prostasin and the prostasin-binding protein was inhibited by a prostasin antibody, heparin, and serine protease inhibitors. Prostasin's serine protease activity was inhibited when bound to the prostasin-binding protein in mouse seminal vesicle fluid. This study indicates that prostasin is an active serine protease in its membrane-bound form.

Genomic organization and expression properties of the VfENOD5 gene from broad bean (Vicia faba L.)

A full-length cDNA encoding the broad bean (Vicia faba L.) early nodulin VfENOD5 was isolated from a nodule cDNA library. In addition to the ENOD5 homologues from other legumes the derived VfENOD5 amino acid sequence also displayed homologies to the phytocyanin-related nodulins GmENOD55-2, MtENOD16, and MtENOD20. A close inspection of the ENOD5 proteins from broad bean, pea and vetch indicated that all these nodulins possess a putative C-terminal GPI-anchor signal sequence. This novel finding supports the hypothesis that ENOD5 is an arabinogalactan protein. Tissue print hybridizations revealed that the broad bean ENOD5 gene was not only expressed in the central tissues of root nodules. In contrast to other legumes hybridizing transcripts were also be detected in a narrow zone within the peripheral nodule tissues. Sequence analysis of a genomic clone indicated the presence of a single intron interrupting the VfENOD5 coding region at a position precisely corresponding to the MtENOD16 and MtENOD20 introns.

Bacterial phosphatidylinositol-specific phospholipase C: structure, function, and interaction with lipids

The bacterial phosphatidylinositol-specific phospholipase C (PI-PLC) is a small, water-soluble enzyme that cleaves the natural membrane lipids PI, lyso-PI, and glycosyl-PI. The crystal structure, NMR and enzymatic mechanism of bacterial PI-PLCs are reviewed. These enzymes consist of a single domain folded as a (betaalpha)(8)-barrel (TIM barrel), are calcium-independent, and interact weakly with membranes. Sequence similarity among PI-PLCs from different bacterial species is extensive, and includes the residues involved in catalysis. Bacterial PI-PLCs are structurally similar to the catalytic domain of mammalian PI-PLCs. Comparative studies of both prokaryotic and eukaryotic isozymes have proved useful for the identification of distinct regions of the proteins that are structurally and functionally important.

Leukemia arising out of paroxysmal nocturnal hemoglobinuria

In paroxysmal nocturnal hemoglobinuria (PNH), one or more hematopoietic stem cells that are defective in GPI anchor assembly as a result of mutation in the PIG-A gene preferentially expand in the bone marrow and give rise to peripheral blood elements that are deficient in GPI anchored protein expression. According to current concepts, 5-15% of PNH patients develop leukocyte dyscrasias which invariably are acute myelogenous leukemia (AML). In this review, the literature from 1962 to the present is analyzed regarding the type of leukocyte dyscrasia, incidence, and cytogenetic features of the abnormal cells that have been reported. Among a total of 119 cases that are well-documented, 104 myeloid dyscrasias involving several categories in addition to AML, as well as 15 lymphoid dyscrasias are described. Of 1,760 patients in 15 series that contain 20 or more patients, 16 (1%) are reported as having developed "acute leukemia." However, of 288 listed as having died, 13 (5%) are recorded as having had "acute leukemia." In 32 of the patients with hematological dyscrasias where karyotypes were analyzed, 7 were found to be normal and 25 found to harbor various alterations with the +8 abnormality present in 8. In 5 of 7 instances evidence indicates that the dyscratic cell arises from the PNH clone. Processes potentially involved in the evolution of the dyscratic cells from PNH clones are discussed.

Genetic complexity, structure, and characterization of highly active bovine intestinal alkaline phosphatases

Mammalian alkaline phosphatases (APs) display 10-100-fold higher kcat values than do bacterial APs. To begin uncovering the critical residues that determine the catalytic efficiency of mammalian APs, we have compared the sequence of two bovine intestinal APs, i.e. a moderately active isozyme (bovine intestinal alkaline phosphatase, bIAP I, approximately 3,000 units/mg) previously cloned in our laboratory, and a highly active isozyme (bIAP II, approximately 8, 000 units/mg) of hitherto unknown sequence. An unprecedented level of complexity was revealed for the bovine AP family of genes during our attempts to clone the bIAP II cDNA from cow intestinal RNAs. We cloned and characterized two novel full-length IAP cDNAs (bIAP III and bIAP IV) and obtained partial sequences for three other IAP cDNAs (bIAP V, VI, and VII). Moreover, we identified and partially cloned a gene coding for a second tissue nonspecific AP (TNAP-2). However, the cDNA for bIAP II, appeared unclonable. The sequence of the entire bIAP II isozyme was determined instead by a classical protein sequencing strategy using trypsin, carboxypeptidase, and endoproteinase Lys-C, Asp-N, and Glu-C digestions, as well as cyanogen bromide cleavage and NH2-terminal sequencing. A chimeric bIAP II cDNA was then constructed by ligating wild-type and mutagenized fragments of bIAP I, III, and IV to build a cDNA encoding the identified bIAP II sequence. Expression and enzymatic characterization of the recombinant bIAP I, II, III, and IV isozymes revealed average kcat values of 1800, 5900, 4200, and 6100 s-1, respectively. Comparison of the bIAP I and bIAP II sequences identified 24 amino acid positions as likely candidates to explain differences in kcat. Site-directed mutagenesis and kinetic studies revealed that a G322D mutation in bIAP II reduced its kcat to 1300 s-1, while the converse mutation, i.e. D322G, in bIAP I increased its kcat to 5800 s-1. Other mutations in bIAP II had no effect on its kinetic properties. Our data clearly indicate that residue 322 is the major determinant of the high catalytic turnover in bovine IAPs. This residue is not directly involved in the mechanism of catalysis but is spatially sufficiently close to the active site to influence substrate positioning and hydrolysis of the phosphoenzyme complex.

CD24 mediates rolling of breast carcinoma cells on P-selectin

P-selectin mediates rolling of neutrophils and other leukocytes on activated endothelial cells and platelets through binding to P-selectin glycoprotein ligand-1 (PSGL-1). Certain PSGL-1 negative tumor cell lines can bind P-selectin under static conditions through the GPI-linked surface mucin, CD24, but the physiological significance of this interaction and whether it can occur under flow conditions is not known. Here, we show that CD24+ PSGL-1- KS breast carcinoma cells attach to and roll on recombinant P-selectin under a continuous wall shear stress, although at a lower density and higher velocity than CD24+ PSGL-1+ cells, such as HL-60. Adding excess soluble CD24 or removing CD24 from the cell surface with phosphatidylinositol-phospholipase C (PI-PLC) significantly reduced KS cell rolling on P-selectin. The ability of KS cells to roll on P-selectin was positively correlated with the CD24 expression level. Comparison with three other CD24+ cell lines established that expression of sialyl-Lewis(x) antigen was also necessary for CD24-mediated rolling on P-selectin. CD24 purified from KS cells supported rolling of P-selectin transfectants, but not L-selectin transfectants. Finally, KS cells rolled on vascular endothelium in vivo in a P-selectin-dependent manner. Together our data show that CD24 serves as a ligand for P-selectin under physiological flow conditions. Interaction of tumor cells with P-selectin via CD24 may be an important adhesion pathway in cancer metastasis.

Arabinogalactan-proteins from Nicotiana alata and Pyrus communis contain glycosylphosphatidylinositol membrane anchors

Arabinogalactan-proteins (AGPs) are a class of proteoglycans found in cell secretions and plasma membranes of plants. Attention is currently focused on their structure and their potential role in growth and development. We present evidence that two members of a major class of AGPs, the classical AGPs, AGPNa1 from styles of Nicotiana alata and AGPPc1 from cell suspension cultures of Pyrus communis, undergo C-terminal processing involving glycosylphosphatidylinositol membrane anchors. The evidence is that (i) the transmembrane helix at the C terminus predicted from the cDNA encoding these proteins is not present-the C-terminal amino acid is Asn87 and Ser97 for AGPNa1 and AGPPc1, respectively; (ii) both AGP protein backbones are substituted with ethanolamine at the C-terminal amino acid; and (iii) inositol, glucosamine, and mannose are present in the native AGPs. An examination of the deduced amino acid sequences of other classical AGP protein backbones shows that glycosylphosphatidylinositol-anchors may be a common feature of this class of AGPs.

Glycosylphosphatidylinositol anchors of membrane glycoproteins are binding determinants for the channel-forming toxin aerolysin

Cells that are sensitive to the channel-forming toxin aerolysin contain surface glycoproteins that bind the toxin with high affinity. Here we show that a common feature of aerolysin receptors is the presence of a glycosylphosphatidylinositol anchor, and we present evidence that the anchor itself is an essential part of the toxin binding determinant. The glycosylphosphatidylinositol (GPI)-anchored T-lymphocyte protein Thy-1 is an example of a protein that acts as an aerolysin receptor. This protein retained its ability to bind aerolysin when it was expressed in Chinese hamster ovary cells, but could not bind the toxin when expressed in Escherichia coli, where the GPI anchor is absent. An unrelated GPI-anchored protein, the variant surface glycoprotein of trypanosomes, was shown to bind aerolysin with similar affinity to Thy-1, and this binding ability was significantly reduced when the anchor was removed chemically. Cathepsin D, a protein with no affinity for aerolysin, was converted to an aerolysin binding form when it was expressed as a GPI-anchored hybrid in COS cells. Not all GPI-anchored proteins bind aerolysin. In some cases this may be due to differences in the structure of the anchor itself. Thus the GPI-anchored proteins procyclin of Trypanosoma congolense and gp63 of Leishmania major did not bind aerolysin, but when gp63 was expressed with a mammalian GPI anchor in Chinese hamster ovary cells, it bound the toxin.

Phosphatidylinositol hydrolysis by Trypanosoma brucei glycosylphosphatidylinositol phospholipase C

Detergent-solubilized glycosylphosphatidylinositol (GPI)-anchored structures can be cleaved by C-type phospholipases isolated from peanuts and bloodstream cells of the African trypanosome, Trypanosoma brucei. The two enzymes differ in their reported ability to hydrolyze phosphatidylinositol (PI); while the peanut enzyme readily hydrolyzes PI in vitro, the T. brucei enzyme was reported to be virtually inactive against PI and consequently named GPI-specific phospholipase C (GPI-PLC). In this paper, we describe experiments in which we reinvestigated the substrate specificity of T. brucei GPI-PLC by incubating the purified enzyme with Triton X-100/PI-mixed micelles and by studying PI hydrolysis. We found that PI hydrolysis occurred in a detergent-dependent fashion over the range of concentrations tested (5 microM to 1 mM PI). At 5 microM PI, hydrolysis was maximal at 0.005% Triton X-100, whereas at 1 mM PI, maximal hydrolysis required 0.05% Triton X-100. Hydrolysis of both PI and GPI was strongly affected by the presence of phospholipids. Endogenous PI was hydrolyzed during osmotic and detergent lysis of trypanosomes under conditions used to obtain quantitative hydrolysis of the GPI-anchored trypanosome variant surface glycoprotein. PI hydrolysis in the lysates was inhibited by sodium p-chloromercuriphenylsulfonate but unaffected by EGTA, consistent with the proposal that hydrolysis is due to GPI-PLC. These results suggest that the function of T. brucei GPI-PLC may be to regulate PI as well as (or instead of) GPI levels.