Immunochemical studies of phospholipids: production of antibodies to triphosphoinositide

Phosphatidylinositol 4,5-bisphosphate regulates two steps of homotypic vacuole fusion

Yeast vacuoles undergo cycles of fragmentation and fusion as part of their transmission to the daughter cell and in response to changes of nutrients and the environment. Vacuole fusion can be reconstituted in a cell free system. We now show that the vacuoles synthesize phosphoinositides during in vitro fusion. Of these phosphoinositides, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) are important for fusion. Monoclonal antibodies to PI(4,5)P(2), neomycin (a phosphoinositide ligand), and phosphatidylinositol-specific phospholipase C interfere with the reaction. Readdition of PI(4, 5)P(2) restores fusion in each case. Phosphatidylinositol 3-phosphate and PI(3,5)P(2) synthesis are not required. PI(4,5)P(2) is necessary for priming, i.e., for the Sec18p (NSF)-driven release of Sec17p (alpha-SNAP), which activates the vacuoles for subsequent tethering and docking. Therefore, it represents the kinetically earliest requirement identified for vacuole fusion so far. Furthermore, PI(4,5)P(2) is required at a step that can only occur after docking but before the BAPTA sensitive step in the latest stage of the reaction. We hence propose that PI(4,5)P(2) controls two steps of vacuole fusion.

Cross-reactions of nucleic acids with monoclonal antibodies to phosphatidylinositol phosphate and cholesterol

Four monoclonal IgM antibodies to phosphatidylinositol phosphate (PIP), four antibodies to cholesterol and one antibody to liposomes containing phosphatidylcholine, cholesterol and dicetyl phosphate were tested for reactivity with denatured DNA. Three of four antibodies to PIP cross-reacted strongly with denatured DNA. The other antibodies did react with denatured DNA but only very weakly. The binding to DNA was competed by synthetic polynucleotides. In competitive assays, one of the anti-PIP antibodies was particularly reactive with poly(dT) and another with poly(I) and poly(dG). Binding of an anti-cholesterol antibody to ssDNA was also inhibited by poly(I) and poly(dG). Two of the anti-PIP antibodies were also reactive with mononucleotides, and all four bound inositol hexaphosphate. High concns of nucleosides did not compete for binding, indicating that phosphate is involved in the binding site. Phospholipids, particularly those containing inositol phosphate, also competed for binding to DNA, but to varying extents, indicating a variable overlap in the antibody binding site for DNA and phospholipid determinants. These antibodies, induced by immunization with liposomes, showed cross-reactivity characteristics often found with certain types of autoantibodies, but they did not bear the H130 idiotype, which was identified on IgM anti-DNA autoantibodies from MRL-lpr/lpr mice.

Production and characterization of monoclonal antibodies that bind to phosphatidylinositol 4,5-bisphosphate

We established a series of monoclonal antibodies (MAbs) that bound to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] which is confidently believed to play an important role in cellular signal transduction. We used complement dependent liposome lysis assay for screening MAbs and antibodies that bind to PI(4,5)P2 but not to phosphatidylinositol 4-monophosphate [PI(4)P] were selected. The reactivity of the MAbs was analyzed by complement dependent liposome lysis assay, enzyme-linked immunosorbent assay (ELISA) and inhibition of liposome lysis by soluble haptens. The MAbs exhibited three distinct reactivity profiles measured by three different assay systems and the results obtained from three typical MAbs designated AM-2, AM-7 and AM-212 are described. On liposome lysis assay, all three MAbs were highly specific with PI(4,5)P2 and no cross-reaction with other acidic phospholipids such as PI(4)P, phosphatidylinositol (PI), phosphatidic acid (PA), phosphatidylserine (PS), phosphatidylethanolamine (PE) and cardiolipin (CL) was observed. All three MAbs were able to react with as low as 0.1 mol% of PI(4,5)P2 embedded in the liposomal membrane. On ELISA, AM-2 and AM-212 bound only to PI(4,5)P2 and did not cross-react with other acidic phospholipids including PI(4)P. In contrast, AM-7 showed considerable binding to other phospholipids such as PI(4)P, PA and PE on ELISA. The reactivities of the MAbs with water soluble haptens such as inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] were examined by inhibition of liposome lysis. Among three clones examined, only AM-212 showed considerable reactivity with Ins(1,4,5)P3 while also showing weak cross-reactivity with fructose 1,6-diphosphate and inositol 1,4-bisphosphate. No cross-reaction with other structural analogs such as inositol 1-monophosphate and inositol 2-monophosphate was observed.

Phosphate-binding specificities of monoclonal antibodies against phosphoinositides in liposomes

Monoclonal antibodies against phosphatidylinositol phosphate were produced after injecting a mouse with liposomes containing dimyristoylphosphatidylcholine, cholesterol, phosphatidylinositol phosphate and lipid A. The antibodies raised were IgM (kappa) and their activities were assayed by complement-dependent damage to liposomes lacking lipid A but containing the rest of original immunizing mixture of lipids. Three of the four antibodies selected cross-reacted with liposomes containing phosphatidylinositol instead of phosphatidylinositol phosphate; and two of the antibodies cross-reacted with liposomes containing phosphatidylinositol diphosphate. Each of the antibodies had a phosphate-binding specificity. Each also cross-reacted with liposomes containing sulfogalactosyl ceramide, but not with liposomes containing galactosyl ceramide, or gangliosides or with liposomes containing lipid A but lacking phosphoinositides. Recognition of sulfogalactosyl ceramide probably occurred because the chemical characteristics of the sulfate group were sufficiently similar to those of phosphate to allow recognition by the antibody. The phosphate-binding specificity was further confirmed by inhibition by phosphocholine, inositol hexaphosphate, ATP, AMP and even sodium phosphate, but not by choline or inositol.

Monoclonal immunoglobulin M lambda coagulation inhibitor with phospholipid specificity. Mechanism of a lupus anticoagulant

Prolongation of all phospholipid-dependent coagulation tests was found in a patient with macroglobulinemia, despite absence of bleeding manifestations. The purified monoclonal IgM lambda protein and its Fabmu tryptic fragment induced similar changes in normal plasma. Patient IgM and Fabmu completely inhibited Ca++-dependent binding of radiolabeled prothrombin and Factor X to mixed phospholipid micelles. The patient's IgM lambda paraprotein reacted with phosphatidylserine and, to a lesser extent, with phosphatidylinositol and phosphatidic acid, but not with phosphatidylcholine or phosphatidylethanolamine. Prior incubation of phospholipid with patient Fabmu blocked the positive reactions. Substitution of washed platelets for phospholipid led to normalization of patient coagulation tests and corrected all abnormalities produced in normal plasma by patient IgM. Furthermore, binding of 125I-Factor Xa to thrombin-treated platelets was entirely normal in the presence of patient IgM. These studies support the concept that platelets, rather than phospholipid micelles, are the primary locus of prothrombin and Factor X activation in normal hemostasis.

Antibodies to cerebroside sulfate inhibit the effects of morphine and beta-endorphin

Morphine and beta-endorphin inhibit the shaking response of pentobarbital-anesthetized rats to ice water. Stereotaxically guided administration of antibodies to cerebroside sulfate into the periaqueductal gray region, the most sensitive brain region in which to demonstrate inhibition of this response, antagonizes the effect of morphine and beta-endorphin. These results suggest that cerebroside sulfate may be an integral component of an opiate receptor in rat brain.