Metabolic correction of defects in the lipid anchoring of Thy-1 in lymphoma mutants

Ethanolamine phosphate linked to the first mannose residue of glycosylphosphatidylinositol (GPI) lipids is a major feature of the GPI structure that is recognized by human GPI transamidase

Glycosylphosphatidylinositol (GPI) anchoring of proteins is catalyzed by GPI transamidase (GPIT), a multisubunit, endoplasmic reticulum (ER)-localized enzyme. GPIT recognizes ER-translocated proteins that have a GPI-directing C-terminal signal sequence and replaces this sequence with a preassembled GPI anchor. Although the GPI signal sequence has been extensively characterized, little is known about the structural features of the GPI lipid substrate that enable its recognition by GPIT. In a previous study we showed that mature GPIs could be co-immunoprecipitated with GPIT complexes containing functional subunits (Vainauskas, S., and Menon, A. K. (2004) J. Biol. Chem. 279, 6540-6545). We now use this approach, as well as a method that reconstitutes the interaction between GPIs and GPIT, to define the basis of the interaction between GPI and human GPIT. We report that (i) human GPIT can interact with GPI biosynthetic intermediates, not just mature GPIs competent for transfer to protein, (ii) the ethanolamine phosphate group on the third mannose residue of the GPI glycan is not critical for GPI recognition by GPIT, (iii) the ethanolamine phosphate residue linked to the first mannose of the GPI structure is a major feature of GPIs that is recognized by human GPIT, and (iv) the simplest GPI recognized by human GPIT is EtN-P-2Manalpha1-4GlcN-(acyl)-phosphatidyl-inositol. These studies define the molecular characteristics of GPI that are recognized by GPIT and open the way to identifying GPIT subunits that are involved in this process.

Down-regulation of glycosylphosphatidylinositol-specific phospholipase D induced by lipopolysaccharide and oxidative stress in the murine monocyte- macrophage cell line RAW 264.7

Serum glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) activity is reduced over 75% in systemic inflammatory response syndrome. To investigate the mechanism of this response, expression of the GPI-PLD gene was studied in the mouse monocyte-macrophage cell line RAW 264.7 stimulated with lipopolysaccharide (LPS; 0.5 to 50 ng/ml). GPI-PLD mRNA was reduced approximately 60% in a time- and dose-dependent manner. Oxidative stress induced by 0.5 mM H(2)O(2) or 50 microM menadione also caused a greater than 50% reduction in GPI-PLD mRNA. The antioxidant N-acetyl-L-cysteine attenuated the down-regulatory effect of H(2)O(2) but not of LPS. Cotreatment of the cells with actinomycin D inhibited down-regulation induced by either LPS or H(2)O(2). The half-life of GPI-PLD mRNA was not affected by LPS, or decreased slightly with H(2)O(2), indicating that the reduction in GPI-PLD mRNA is due primarily to transcriptional regulation. Stimulation with tumor necrosis factor alpha (TNF-alpha) resulted in approximately 40% reduction in GPI-PLD mRNA in human A549 alveolar carcinoma cells but not RAW 264.7 cells, suggesting that alternative pathways could exist in different cell types for down-regulating GPI-PLD expression during an inflammatory response and the TNF-alpha autocrine signaling mechanism alone is not sufficient to recapitulate the LPS-induced reduction of GPI-PLD in macrophages. Sublines of RAW 264.7 cells with reduced GPI-PLD expression exhibited increased cell sensitivity to LPS stimulation and membrane-anchored CD14 expression on the cell surface. Our data suggest that down-regulation of GPI-PLD could play an important role in the control of proinflammatory responses.

Biosynthesis of glycosylphosphatidylinositols in mammals and unicellular microbes

Membrane anchoring of cell surface proteins via glycosylphosphatidylinositol (GPI) occurs in all eukaryotic organisms. In addition, GPI-related glycophospholipids are important constituents of the glycan coat of certain protozoa. Defects in GPI biosynthesis can retard, if not abolish growth of these organisms. In humans, a defect in GPI biosynthesis can cause paroxysmal nocturnal hemoglobinuria (PNH), a severe acquired bone marrow disorder. Here, we review advances in the characterization of GPI biosynthesis in parasitic protozoa, yeast and mammalian cells. The GPI core structure as well as the major steps in its biosynthesis are conserved throughout evolution. However, there are significant biosynthetic differences between mammals and microbes. First indications are that these differences could be exploited as targets in the design of novel pharmacotherapeutics that selectively inhibit GPI biosynthesis in unicellular microbes.

Addition of G418 and other aminoglycoside antibiotics to mammalian cells results in the release of GPI-anchored proteins

Resistance to the neomycin analogue G418 forms the basis of a dominant marker selection system for mammalian cells transfected with the bacterial neomycin gene. We found that COS-1 cells stably transfected with the neomycin resistance gene had a greater than 50% reduction in cell-associated glycosylphosphatidylinositol (GPI)-anchored alkaline phosphatase (AP). A similarly reduced amount of AP was also observed in wild-type COS-1 cells incubated in the presence of G418 or other aminoglycoside antibiotics. The AP was released from cells into the culture supernatant in its GPI-anchored form. Our data suggest that the G418-induced reduction of AP involves a vesiculation process of COS-1 cells.

Phosphatidylinositol phospholipase C is activated allosterically by the aminoglycoside G418. 2-deoxy-2-fluoro-scyllo-inositol-1-O-dodecylphosphonate and its analogs inhibit glycosylphosphatidylinositol phospholipase C

Phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus cereus is inhibited by myo-inositol-1-O-dodecylphosphonate (Ins-1-O-dodecylphosphonate) (Morris, J. C., Ping-Sheng, L., Shen, T. Y., and Mensa-Wilmot, K.(1995) J. Biol. Chem. 270, 2517-2524). A set of novel fluorinated 2-deoxy-Ins-1-O-dodecylphosphonates were tested against PI-PLC, with potent competitive inhibition by 2-deoxy-2-fluoro-scyllo-Ins-1-O-dodecylphosphonate (VP-616L) (Xi(50) = 0.09). 2-Deoxy-2-fluoro-myo-Ins-1-O-dodecylphosphonate and 2-deoxy-2,2-difluoro-myo-Ins-1-O-dodecylphosphonate were 8.3-fold and 4.8-fold less effective, respectively, than VP-616L. Methyl 2-deoxy-2,2-difluoro-myo-Ins-1-O-dodecylphosphonate was inactive. Also, a hundredfold less PI-PLC is required to cleave a glycosylphosphatidylinositol (GPI) than is needed to cleave PI. Implied in these observations are the following: (i) in powerful inhibitors an active site residue probably interacts with the equatorially oriented fluoro substituent; (ii) substrate recognition requires a negative charge on the phosphoryl at the Ins-1 position, and (iii) a GPI is better substrate than PI, for PI-PLC. Aminoglycoside antibiotics kanamycin A, gentamycin, and G418 stimulated PI-PLC cleavage of the GPI anchor of variant surface glycoprotein (VSG) from Trypanosoma brucei 2- to 4-fold. G418, which appears to act on the enzyme.substrate complex, increased kcat and Km 6.4-fold and 9.9-fold, respectively. PI-PLC was activated by G418 even in the presence of the inhibitor VP-616L. In control experiments, the lectin concanavalin A (ConA), which probably acts by substrate sequestration, inhibited both PI-PLC (Xi(50) = 0.00025) and GPI-specific phospholipase D (Xi(50) = 0.00018). G418 failed to activate PI-PLC when ConA was present. These observations indicate that G418 is an allosteric activator of Bacillus cereus PI-PLC. Since G418 stimulates a purified enzyme that is not involved in aminoglycoside metabolism, we propose that binding of aminoglycosides to cellular proteins could contribute to the development of the nephrotoxicity associated with the use of these aminoglycoside antibiotics.

Glycan requirements of glycosylphosphatidylinositol phospholipase C from Trypanosoma brucei. Glucosaminylinositol derivatives inhibit phosphatidylinositol phospholipase C

Glycosylphosphatidylinositol phospholipase C (GPI-PLC) from Trypanosoma brucei and phosphatidylinositol phospholipase C (PI-PLC) from Bacillus sp. both cleave glycosylphosphatidylinositols (GPIs). However, phosphatidylinositol, which is efficiently cleaved by PI-PLC, is a very poor substrate for GPI-PLC. We examined GPI-PLC substrate requirements using glycoinositol analogs of GPI components as potential inhibitors. Glucosaminyl (alpha 1-->6)-D-myo-inositol (GlcN(alpha 1-->6)Ins), GlcN(alpha 1-->6)Ins 1,2-cyclic phosphate, GlcN(alpha 1-->6)-2-deoxy-Ins, and GlcN(alpha 1-->6)Ins 1-dodecyl phosphonate inhibited GPI-PLC. GlcN(alpha 1-->6)Ins was as effective as Man-(alpha 1-->4)GlcN(alpha 1-->6)Ins; we surmise that GlcN(alpha 1-->6)Ins is the crucial glycan motif for GPI-PLC recognition. Inhibition by GlcN(alpha 1-->6)Ins 1,2-cyclic phosphate suggests product inhibition since GPIs cleaved by GPI-PLC possess a GlcN(alpha 1-->6)Ins 1,2-cyclic phosphate at the terminus of the residual glycan. The effectiveness of GlcN(alpha 1-->6)-2-deoxy-Ins indicates that the D-myo-inositol (Ins) 2-hydroxyl is not required for substrate recognition, although it is probably essential for catalysis. GlcN(alpha 1-->6)-2-deoxy-L-myo-inositol, unlike GlcN(alpha 1-->6)-2- deoxy-Ins, had no effect on GPI-PLC; hence, GPI-PLC can distinguish between the two enantiomers of Ins. Surprisingly, GlcN(alpha 1-->6)Ins 1,2-cyclic phosphate was not a potent inhibitor of Bacillus cereus PI-PLC, and GlcN(alpha 1-->6)Ins had no effect on the enzyme. However, both GlcN(alpha 1-->6)Ins 1-phosphate and GlcN(alpha 1-->6)Ins 1-dodecyl phosphonate were competitive inhibitors of PI-PLC. These observations suggest an important role for a phosphoryl group at the Ins 1-position in PI-PLC recognition of GPIs. Other studies indicate that abstraction of a proton from the Ins 2-hydroxyl is not an early event in PI-PLC cleavage of GPIs. Furthermore, both GlcN(alpha 1-->6)-2-deoxy-Ins 1-phosphate and GlcN(alpha 1-->6)-2-deoxy-L- myo-inositol inhibited PI-PLC without affecting GPI-PLC. Last, the aminoglycoside G418 stimulated PI-PLC, but had no effect on GPI-PLC. Thus, these enzymes represent mechanistic subclasses of GPI phospholipases C, distinguishable by their sensitivity to GlcN(alpha 1-->6)Ins derivatives and aminoglycosides. Possible allosteric regulation of PI-PLC by GlcN(alpha 1-->6)Ins analogs is discussed.

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.

Sodium butyrate causes reexpression of three membrane proteins on glycolipid-anchoring mutants

Murine Thy-1-negative lymphoma mutants synthesize membrane proteins that normally bear glycolipid anchors but do not express these proteins on the cell surface. This phenotype may reflect altered regulation of gene(s) required for anchor biosynthesis. Since tissue culture cells treated with sodium butyrate transcribe new DNA sequences and since these transcripts are translated, it was of interest to determine whether butyrate treatment could restore surface expression of lipid-anchored proteins. When Thy-1-negative lymphoma mutants (complementation groups A-C, E, F, and H) were cultured for three days in 1.5 mM butyrate, a small percentage of the class H cells acquired phosphatidylinositol-specific phospholipase C-releasable surface Thy-1 and J11d. Membrane-associated Thy-1 was not observed before 24 h of treatment. Induction was reversible. Cell fusion studies have shown that murine LM (TK-) fibroblasts can be assigned to the class H lymphoma complementation group. Although these cells synthesize Ly-6, this normally lipid-anchored protein is absent from the cell surface. When LM (TK-) cells were cultured for three days in butyrate, 10% of the cells reversibly expressed Ly-6. In addition, LM (TK-) cells transfected with a plasmid encoding Thy-1 do not express Thy-1, but could be induced to express both Ly-6 and Thy-1 by butyrate treatment. Northern analysis of total RNA from Ly-6/Thy-1-expressing cells indicates that increased steady-state transcript levels cannot account for surface expression of these proteins. We conclude that the lack of expression of three proteins at the surface of class H mutant and the LM (TK-) cells is not due to gross structural lesions in genes along the anchor biosynthetic pathway.

Lateral diffusion and retrograde movements of individual cell surface components on single motile cells observed with Nanovid microscopy

A recently introduced extension of video-enhanced light microscopy, called Nanovid microscopy, documents the dynamic reorganization of individual cell surface components on living cells. 40-microns colloidal gold probes coupled to different types of poly-L-lysine label negative cell surface components of PTK2 cells. Evidence is provided that they bind to negative sialic acid residues of glycoproteins, probably through nonspecific electrostatic interactions. The gold probes, coupled to short poly-L-lysine molecules (4 kD) displayed Brownian motion, with a diffusion coefficient in the range 0.1-0.2 micron2/s. A diffusion coefficient in the 0.1 micron2/s range was also observed with 40-nm gold probes coupled to an antibody against the lipid-linked Thy-1 antigen on 3T3 fibroblasts. Diffusion of these probes is largely confined to apparent microdomains of 1-2 microns in size. On the other hand, the gold probes, coupled to long poly-L-lysine molecules (240 kD) molecules and bound to the leading lamella, were driven rearward, toward the boundary between lamelloplasm and perinuclear cytoplasm at a velocity of 0.5-1 micron/min by a directed ATP-dependent mechanism. This uniform motion was inhibited by cytochalasin, suggesting actin microfilament involvement. A similar behavior on MO cells was observed when the antibody-labeled gold served as a marker for the PGP-1 (GP-80) antigen. These results show that Nanovid microscopy, offering the possibility to observe the motion of individual specific cell surface components, provides a new and powerful tool to study the dynamic reorganization of the cell membrane during locomotion and in other biological contexts as well.

Correction of a defect in mammalian GPI anchor biosynthesis by a transfected yeast gene

Glycosylphosphatidylinositol (GPI) serves as a membrane anchor for a large number of eukaryotic proteins. A genetic approach was used to investigate the biosynthesis of GPI anchor precursors in mammalian cells. T cell hybridoma mutants that cannot synthesize dolichol-phosphate-mannose (Dol-P-Man) also do not express on their surface GPI-anchored proteins such as Thy-1 and Ly-6A. These mutants cannot form mannose-containing GPI precursors. Transfection with the yeast Dol-P-Man synthase gene rescues the synthesis of both Dol-P-Man and mannose-containing GPI precursors, as well as the surface expression of Thy-1 and Ly-6A, suggesting that Dol-P-Man is the donor of at least one mannose residue in the GPI core.

Suppression of focus formation by bovine papillomavirus-transformed cells by contact with non-transformed cells: involvement of sugar(s) and phosphorylation

Focus formation by bovine papilloma virus-transformed C127 cells was inhibited by direct contact with non-transformed C127 cells. The suppressive capacity of C127 cells was abolished by introduction of the neomycin resistance gene (neor) but not by that of the hygromycin resistance gene (hygrr). Though both genes code for phosphotransferase which inactivates the aminoglycoside antibiotics, their substrates are different, i.e., there is no cross-resistance between them. As the neomycin phosphotransferase phosphorylates the specific hydroxyl group of the sugar in the aminoglycosides, such as 3'OH of the glucose residue of kanamycin A, some specific sugar(s) on the molecules exposed on the cell surface must be responsible for the suppressive signal and their phosphorylation must have resulted in the loss of that signal. The sugar must have the structure shared by kanamycin, neomycin or G418 but not by hygromycin B. Involvement of sugar was also suggested by the observation that concanavalin A partially abrogated the suppressive capacity of C127 cells.