Differential expression of glycosylphosphatidylinositol-anchored proteins in a murine T cell hybridoma mutant producing limiting amounts of the glycolipid core. Implications for paroxysmal nocturnal hemoglobinuria

Hypomorphic promoter mutation in PIGM causes inherited glycosylphosphatidylinositol deficiency

Attachment to the plasma membrane by linkage to a glycosylphosphatidylinositol (GPI) anchor is a mode of protein expression highly conserved from protozoa to mammals. As a clinical entity, deficiency of GPI has been recognized as paroxysmal nocturnal hemoglobinuria, an acquired clonal disorder associated with somatic mutations of the X-linked PIGA gene in hematopoietic cells. We have identified a novel disease characterized by a propensity to venous thrombosis and seizures in which deficiency of GPI is inherited in an autosomal recessive manner. In two unrelated kindreds, a point mutation (c --> g) at position -270 from the start codon of PIGM, a mannosyltransferase-encoding gene, disrupts binding of the transcription factor Sp1 to its cognate promoter motif. This mutation substantially reduces transcription of PIGM and blocks mannosylation of GPI, leading to partial but severe deficiency of GPI. These findings indicate that biosynthesis of GPI is essential to maintain homeostasis of blood coagulation and neurological function.

Activation-induced aggregation and processing of the human Fas antigen. Detection with cytoplasmic domain-specific antibodies

Fas (APO1/CD95) is a type 1 transmembrane protein critically involved in receptor-mediated apoptosis. Previous studies have shown that Fas exists in monomeric form in resting cells and aggregates upon cross-linking to form a complex that serves to recruit additional signaling molecules to the cell membrane. To study the molecular fate of the Fas antigen following receptor activation, a monoclonal antibody specific for the cell death domain of Fas has been generated. This monoclonal antibody (3D5) could be used in Western blot analysis using total cell lysates to identify different forms of Fas antigens without immunoprecipitation. High molecular mass (>200 kDa), SDS- and beta-mercaptoethanol-resistant Fas aggregates were formed immediately following receptor cross-linking, and a 97-kDa band (p97) was detected about 2 h later. p97 could be detected by antibodies against either the death domain or the C terminus. However, p97 could not be precipitated by antiextracellular domain antibodies. Thus, p97 most likely represents a processed form of the high molecular weight Fas aggregates. Although p97 generation followed a similar time course as CPP32 activation and poly(ADP-ribose) polymerase cleavage, it could not be inhibited by cysteine protease, calpain, or proteasome inhibitors.

Paroxysmal nocturnal hemoglobinuria as a molecular disease

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired, clonal disorder of hematopoietic cells caused by somatic mutation in the X-linked PIGA gene encoding a protein involved in the synthesis of the glycosylphosphatidylinositol (GPI) anchor by which many proteins are attached to the membrane of cells. About 15 proteins have been found to be lacking or markedly deficient on the abnormal blood cells. These defects result in a clinical syndrome that includes intravascular hemolysis mediated by complement, unusual venous thromboses, deficits of hematopoiesis, and other manifestations. Therapy is presently directed mainly at the consequences of the disorder rather than its basic causes and includes replacement of iron, folic acid, and whole blood; hormonal modulation (prednisone, androgens); anticoagulation; and bone marrow transplantation. PNH is a chronic disease with more than half of adult patients surviving 15 years or more; prognosis is less good in children.

Accumulation of glucosaminyl(acyl)phosphatidylinositol in an S3 HeLa subline expressing normal dolicholphosphomannose synthase activity

Glucosaminyl(acyl)phosphatidylinositol [GlcN(acyl)PI], the third intermediate in the mammalian glycosylphosphatidylinositol (GPI) anchor pathway, is undetectable in most cells. This intermediate was previously shown to accumulate, however, in murine lymphoma mutant E and in yeast mutant dpm1, both of which lack dolicholphosphomannose synthase activity. Here we report that a mammalian HeLa S3 subline, denoted D, produces large amounts of GlcN(acyl)PI. The level of GlcN(acyl)PI in this subline is twice that in the murine lymphoma mutant E and 4 times that in the parental S3 line. This HeLa D subline differs from the previously reported mutants that accumulate GlcN(acyl)PI because no defects in the synthesis or utilization of dolicholphosphomannose were found. Kinetic analysis indicated that in this HeLa subline there is an increased rate of synthesis of GlcN(acyl)PI, whereas the rate of metabolism for this GPI is comparable to that in wild-type cells. Furthermore, HeLa D cells accumulate GlcN(acyl)PI without a block in the synthesis of the downstream mannosylated GPI anchor precursors and GPI-anchored proteins. These findings might be relevant for understanding the regulation of the GPI pathway.

Compositional analysis of glucosaminyl(acyl)phosphatidylinositol accumulated in HeLa S3 cells

GlcN(acyl)PtdIns, a derivative of phosphatidylinositol (PtdIns) in which glucosamine and a fatty acid are linked to inositol hydroxyl groups, has been proposed to be an intermediate in the mammalian biosynthetic pathway for glycosylphosphatidylinositol (glycosyl-PtdIns) anchors of membrane proteins. In this report, GlcN(acyl)PtdIns metabolically labeled with [3H]inositol is shown to accumulate in a HeLa S3 cell subline. The amount of GlcN(acyl)PtdIns in these HeLa S3 cells is about 10(7) molecules/cell, a level comparable to those of the most abundant glycosyl-PtdIns-containing molecules reported to date. GlcN(acyl)PtdIns was purified by a two-step procedure involving octyl-Sepharose and thin-layer chromatography. Octyl-Sepharose separated phospholipids according to their number of hydrocarbon chains: one in 2-lysoPtdIns, two in PtdIns, and three in GlcN(acyl)PtdIns. Purification also was aided by prior treatment of lipid extracts with bee venom phospholipase A2, an enzyme that did not cleave GlcN(acyl)PtdIns. The GlcN-inositol head group in purified GlcN(acyl)PtdIns was confirmed by a number of procedures, including cation-exchange chromatography and mass spectrometry; after radiomethylation, an equal molar ratio of GlcN(Me)2/inositol was measured. Fatty acid analysis indicated an overall stoichiometry of 2.3 mol fatty acid/mol inositol with palmitic (16:0), stearic (18:0) and oleic (18:1) acids being predominant. Analysis of GlcN(acyl)inositol produced by HF fragmentation showed that palmitate was the acyl group attached to inositol and indicated that stearic and oleic acids were in the glycerolipid. Base methanolysis revealed that about 15% of the purified GlcN(acyl)PtdIns contained alkylglycerol. A substantial conversion of GlcN(acyl)PtdIns to a slightly more polar lipid occurred after overnight incubation in even mildly alkaline buffers. Although the current data do not allow proposal of a structure for this lipid, its formation from GlcN(acyl)PtdIns may be important because the conversion appeared to occur in vivo.

Biosynthesis of glycosylphosphatidylinositol membrane anchors

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Paroxysmal nocturnal hemoglobinuria and the glycosylphosphatidylinositol anchor

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Multifunctional roles of glycosyl-phosphatidylinositol lipids

The results presented here indicate that GPI lipids are a structurally and functionally diverse molecular family. Despite new detailed information on the structures of GPI-anchored proteins, there is relatively scant information on the structure of free-GPI. Thus, little is known of the relationships between GPI structures and the mechanism of their biological effects. For example, there is no distinction at the structural level between hormone-sensitive free-GPI and those that serve as precursors for protein-GPI. Nor is there precise biochemical data on the mechanism and importance of free-GPI in hormone signaling, or the signaling roles that GPI anchors play in protein function. The T-cell activation cascade is an ideal system for studying both forms of GPI and their derivatives. The study of GPI molecules in T lymphocytes offers the exciting possibility of addressing questions on the structure, function, genesis, and regulation of both free- and protein-GPI molecules in a single cell type. The detection of multiple protein-GPI and free-GPI forms, and of hormone-sensitive GPI, provides the first approach to these issues. For the moment, the potential for biochemical signaling by intact GPI or its metabolites is enormous. If significant progress is to be made, the structures of hormone sensitive free-GPI must be elucidated. Only then can we precisely define the roles of these molecules in the regulation of cell metabolism and proliferation.

Induction of Fc gamma R-III (CD16) expression on neutrophils affected by paroxysmal nocturnal haemoglobinuria by administration of granulocyte colony-stimulating factor

The inducibility of glycosyl-phosphatidylinositol (GPI)-anchored proteins on affected paroxysmal nocturnal haemoglobinuria (PNH) neutrophils (PMN) after both in vitro and in vivo stimulation was investigated. Fc gamma R-III (CD16), decay-accelerating factor (DAF/CD55) and 20 kD homologous restriction factor (HRF20/CD59) were demonstrated to be concurrently deficient on unstimulated defective PNH PMN. Upon in vitro stimulation with either N-formyl-methionyl-leucyl-phenylalanine (fMLP), zymosan-activated serum (ZAS), or recombinant human granulocyte colony-stimulation factor (G-CSF), neither CD16 nor CD55 expression was induced on defective PNH PMN. G-CSF was administered to two patients with PNH when their conditions were complicated by bacterial infections, or to prevent infections associated with the extraction of teeth or cataract surgery. CD16 expression was induced on the defective PNH PMN in both cases during the administration of G-CSF, but the expression of CD55 and CD59 was not. CD16, induced on the defective PNH PMN during the administration of G-CSF, was phosphatidylinositol-specific phospholipase C (PIPLC)-sensitive, implying that it had GPI-linkage to the membranes. The patients treated with G-CSF recovered from infection or evaded infection. These observations suggest that a deficiency of GPI-anchored proteins is not always seen in defective PNH blood cells, at least under certain stimulation conditions.