The enhanced in vitro hematopoietic activity of leridistim, a chimeric dual G-CSF and IL-3 receptor agonist

The in vitro activity of leridistim was characterized for cell proliferation, generation of colony-forming units (CFU) and differentiation of CD34+ cells. In AML-193.1.3 cells, leridistim exhibited a significant increase in potency compared to rhG-CSF, SC-65303 (an IL-3 receptor agonist) or an equimolar combination of rhG-CSF and SC-65303. CFU-GM assays demonstrated that at 50% of the maximum response, the relative potency of leridistim was 12-fold greater than the combination of rhG-CSF and rhIL-3 and 44-fold more potent than rhG-CSF alone. In multi-lineage CFU assays, a combination of erythropoietin (rhEPO) and leridistim resulted in greater numbers of BFU-E, CFU-GEMM and CFU-Mk than rhEPO alone. Ex vivo culture of peripheral blood or bone marrow CD34+ cells with leridistim substantially increased total viable cells over cultures stimulated with rhG-CSF, SC-65303, or a combination of rhG-CSF and SC-65303. Culture with leridistim, resulted in a greater increase in myeloid (CD15+/CD11b+), monocytic (CD41-/CD14+) and megakaryocytic (CD41+/CD14-) precursor cells without depleting the progenitor pool (CD34+/CD15-/CD11b-). These results demonstrate that leridistim is a more potent stimulator of hematopoietic proliferation and differentiation than the single receptor agonists (rhG-CSF and SC-65303) either alone or combined. These unique attributes suggest that leridistim may enhance hematopoietic reconstitution following myelosuppressive chemotherapy.

Receptor binding kinetics of human IL-3 variants with altered proliferative activity

The binding kinetics of native IL-3 and a set of truncated IL-3 variants to the alpha subunit of the IL-3 receptor (IL-3Ralpha) were studied using surface plasmon resonance. These variants, with amino acid substitutions at residues, 22, 42, 43, 45, 46, 113, or 116, have previously been identified to have altered capacity to stimulate cell proliferation compared to native IL-3(1-133). In this study, variants E43N and F113Y exhibited >100-fold slower association rates than IL-3(15-125) consistent with residues 43 and 113 being essential for the binding of IL-3 to the IL-3Ralpha. Variants G42A, G42D, Q45V, D46S, K116V, and K116W exhibited increased association rates (up to 15-fold relative to IL-3(15-125)) and decreased dissociation rates (up to 7-fold). The results demonstrate that both the association and dissociation rates for the binding of IL-3 to the IL-3Ralpha are altered by truncation and by amino acid substitution at individual sites. Intracellular signaling studies using K116W and E43N demonstrate that differences in the IL-3alpha binding characteristics are reflected in magnitude and kinetics of STAT5 phosphorylation.

Modulation of the binding affinity of myelopoietins for the interleukin-3 receptor by the granulocyte colony-stimulating factor receptor agonist

Myelopoietins (MPOs) are a family of recombinant chimeric proteins that are both interleukin-3 (IL-3) receptor and granulocyte colony-stimulating factor (G-CSF) receptor agonists. In this study, MPO molecules containing one of three different IL-3 receptor agonists linked with a common G-CSF receptor agonist have been examined for their IL-3 receptor binding characteristics. Binding to the alpha-subunit of the IL-3 receptor revealed that the affinity of the MPO molecules was 1.7-3.4-fold less potent than those of their individual cognate IL-3 receptor agonists. The affinity decrease was reflected in the MPO chimeras having approximately 2-fold slower dissociation rates and 2.7-5.5-fold slower association rates than the corresponding specific IL-3 receptor agonists alone. The affinity of binding of the MPO molecules to the heteromultimeric alphabeta IL-3 receptor expressed on TF-1 cells was either 3-, 10-, or 42-fold less potent than that of the individual cognate IL-3 receptor agonist. Biophysical data from nuclear magnetic resonance, near-UV circular dichroism, dynamic light scattering, analytical ultracentrifugation, and size exclusion chromatography experiments determined that there were significant tertiary structural differences between the MPO molecules. These structural differences suggested that the IL-3 and G-CSF receptor agonist domains within the MPO chimera may perturb one another to varying degrees. Thus, the differential modulation of affinity observed in IL-3 receptor binding may be a direct result of the magnitude of these interdomain interactions.

Bivalent binding and signaling characteristics of Leridistim, a novel chimeric dual agonist of interleukin-3 and granulocyte colony-stimulating factor receptors

Leridistim is a member of a novel family of engineered chimeric cytokines, myelopoietins, that contain agonists of both interleukin-3 (IL-3) receptors (IL-3R) and granulocyte colony-stimulating factor (G-CSF) receptors (G-CSFR). To more clearly understand Leridistim's function at the molecular level, binding to both IL-3R and G-CSFR and subsequent signaling characteristics have been delineated. The affinity of Leridistim for the human G-CSFR was found to be comparable to that of native G-CSF (IC(50) = 0.96 nM and 1.0 nM, respectively). Both Leridistim and G-CSF induced receptor tyrosine phosphorylation to a similar maximal level. Compared with native recombinant human IL-3 (rhIL-3), Leridistim was found to possess higher affinity for the IL-3R alpha chain (IL-3Ralpha) (IC(50) = 85 nM and 162 nM, respectively). However, the increase in Leridistim binding affinity to the functional, high-affinity heterodimeric IL-3Ralphabeta(c) receptor is lower than that observed with rhIL-3 (85 nM and 14 nM vs 162 nM and 3.5 nM, respectively). Leridistim induced tyrosine phosphorylation of beta(c) to a level comparable to native IL-3, and the level of JAK2 tyrosine phosphorylation in cells expressing both IL-3R and G-CSFR was comparable to that observed with IL-3 or G-CSF alone. The ability of Leridistim to interact with IL-3R and G-CSFR simultaneously was demonstrated using surface plasmon resonance analysis. These studies were extended to demonstrate that Leridistim exhibited a higher affinity for the IL-3R on cells that express both the IL-3Ralphabeta(c) and the G-CSFR (IC(50) = 2 nM) compared with cells that contain the IL-3Ralphabeta(c) alone (IC(50) = 14 nM). Leridistim binds to both IL-3R and G-CSFR simultaneously and has been shown to activate both receptors. The bivalent avidity may explain the unique biologic effects and unexpected potency of Leridistim in hematopoietic cells compared with rhIL-3 or G-CSF alone or in combination.

Progenipoietins: biological characterization of a family of dual agonists of fetal liver tyrosine kinase-3 and the granulocyte colony-stimulating factor receptor

The progenipoietins, a class of engineered proteins containing both fetal liver tyrosine kinase-3 and granulocyte colony-stimulating factor receptor agonist activities, were functionally characterized in vitro and in vivo. Four representative progenipoietins were evaluated for receptor binding, receptor-dependent cell proliferation, colony-forming unit activity, and their effects on hematopoiesis in the C57BL/6 mouse.The progenipoietins bound to fetal liver tyrosine kinase-3 and the granulocyte colony-stimulating factor receptor with affinities within twofold to threefold of the native ligands, and each progenipoietin bound simultaneously to both fetal liver tyrosine kinase-3 and the granulocyte colony-stimulating factor receptor. The progenipoietins exhibited different levels of activity in receptor-dependent cell proliferation assays. The fetal liver tyrosine kinase-3-dependent cell proliferation activity of three of four progenipoietins was decreased sixfold to 33-fold relative to native fetal liver tyrosine kinase-3 ligand, while granulocyte colony-stimulating factor receptor-dependent activity of the progenipoietins was within twofold to threefold of native granulocyte colony-stimulating factor. At nonsaturating concentrations, the progenipoietins stimulated colony formation to a greater extent than the equimolar combination of fetal liver tyrosine kinase-3 and granulocyte colony-stimulating factor. Treatment of mice with the progenipoietins yielded dramatic increases in peripheral blood and splenic white blood cells, polymorphonuclear leukocytes, and dendritic cells. These preclinical results demonstrate that the progenipoietins are potent hematopoietic growth factors that stimulate cells in a receptor-dependent manner. When administered in vivo, the progenipoietins effectively promote the generation of multiple cell lineages. Thus, in both in vitro and in vivo settings, the progenipoietins as single molecules exhibit the synergistic activity of the combination of fetal liver tyrosine kinase-3 and granulocyte colony-stimulating factor.

Use of combinatorial mutagenesis to select for multiply substituted human interleukin-3 variants with improved pharmacologic properties

A combinatorial mutagenesis strategy was used to create a collection of nearly 500 variants of human interleukin 3 (IL-3), each with four to nine amino acid substitutions clustered within four linear, nonoverlapping regions of the polypeptide. The variants were secreted into the periplasm of Escherichia coli and supernatants were assayed for IL-3 receptor-dependent cell proliferation activity. Sixteen percent of the variants, containing "region-restricted" substitutions, retained substantial proliferative activity through two rounds of screening. A subset of these was combined to yield variants with substitutions distributed through approximately half of the polypeptide. With one exception, "half-substituted" variants exhibited proliferative activity within 3.5-fold of native IL-3. A subset of the "half-substituted" variants was combined to yield "fully substituted" IL-3 variants having 27 or more substitutions. The combination of the substitutions resulted in a set of polypeptides, some of which exhibit increased proliferative activity relative to native IL-3. The elevated hematopoietic potency was confirmed in a methylcellulose colony-forming unit assay using freshly isolated human bone marrow cells. A subset of the multiply substituted proteins exhibited only a modest increase in inflammatory mediator (leukotriene C4) release. The molecules also exhibited 40- to 100-fold greater affinity for the alpha subunit of the IL-3 receptor and demonstrated a 10-fold faster association rate with the alpha-receptor subunit. The multiply substituted IL-3 variants described in this study provide a unique collection of molecules from which candidates for clinical evaluation may be defined and selected.

A peptidomimetic that specifically inhibits human leukocyte antigen DRB1*0401-restricted T cell proliferation

The ability of a peptidomimetic (SC-67655) to block the peptide binding site of the rheumatoid arthritis-linked human leukocyte antigen encoded by the DRB1*0401 allele was evaluated. The inhibitor bound to purified DRB1*0401 molecules with an affinity similar to that of the well-characterized peptide ligand HA307-319. Cell binding assays demonstrated that, in contrast to the promiscuous HA307-319 peptide, the peptidomimetic was highly specific for DRB1*0401. The inhibitor also blocked functional T cell responses to peptide antigens but did not block T cell proliferation in response to protein antigens. Furthermore, it did not appear to be taken up by cells. An analog of the peptidomimetic that was conjugated to a signal peptide sequence did inhibit a T cell proliferative response to protein antigen. Thus, the peptidomimetic must be taken up by cells to block the presentation of peptides derived from protein antigens. These findings have implications for the rational development of inhibitors that block the class II peptide binding groove for the treatment of autoimmune diseases.

A peptide isolated by phage display binds to ICAM-1 and inhibits binding to LFA-1

A mixed phage library containing random peptides from four to eight residues in length flanked by cysteine residues was screened using a recombinant soluble, form of human ICAM-1, which included residues 1-453, (ICAM-1(1-453)). Phage bound to immobilized ICAM-1(1-453) were eluted by three methods: (1) soluble ICAM-1(1-453), (2) neutralizing murine monoclonal antibody, (anti-ICAM-1, M174F5B7), (3) acidic conditions. After three rounds of binding and elution, a single, unique ICAM-1 binding phage bearing the peptide EWCEYLGGYLRCYA was isolated; the identical phage was selected with each method of elution. Attempts to isolate phage from non-constrained (i.e., not containing cysteines) libraries did not yield a phage that bound to ICAM-1. Phage displaying EWCEYLGGYLRCYA bound to immobilized ICAM-1(1-453) and to ICAM-1(1-185), a recombinant ICAM-1, which contains only the two amino-terminal immunoglobulin domains residing within residues 1-185. This is the region of the ICAM-1 that is bound by LFA-1. The phage did not bind to proteins other than ICAM-1. The phage bound to two ICAM-1 mutants, which contained amino acid substitutions that dramatically decreased or eliminated the binding to LFA-1. Studies were also performed with the corresponding synthetic peptide. The linear form of the synthetic EWCEYLGGYLRCYA peptide was found to inhibit LFA-1 binding to immobilized ICAM-1(1-453) in a protein-protein binding assay. By contrast, the disulfide, cyclized, form of the peptide was inactive. The EWCEYL portion of the sequence is homologous to the EWPEYL sequence found within rhinovirus coat protein 14, a nonintegrin protein that binds to ICAM-1. Taken together, the results suggests that the EWCEYLGGYLRCYA sequence is capable to binding to immobilized ICAM-1. Phage display appears to represent a new approach for the identification of peptides that interfere with ICAM-1 binding to beta 2 integrins.

Naturally processed peptides from rheumatoid arthritis associated and non-associated HLA-DR alleles

Naturally processed peptides from immunoaffinity-purified HLA-DRB1*0401, -DRB1*0404 (rheumatoid arthritis (RA)-associated), and -DRB1*0402 (non-RA-associated) molecules were analyzed by capillary liquid chromatography and mass spectrometry. The molecular weights observed for more than 60 eluted peptides from each HLA-DR protein ranged from 788 to 3535 atomic mass units, corresponding to peptides 7 to 32 amino acids in length. Sequencing of more than 60 of the abundant peptides revealed nested sets of peptides that were derived from only 12 different proteins. The majority of these proteins were membrane-associated (HLA class I, class II, and Ig molecules). Synthetic peptides, corresponding to endogenous peptide sequences, bound with high affinity (5 to 80 nM) to the HLA-DR molecules from which they were eluted. In addition, most were promiscuous binding peptides in that they also bound to other HLA-DR molecules. Truncations of eluted peptide sequences and alanine scanning mutational analysis of a Mycobacterium leprae peptide were used to identify the peptide residues involved in binding to DRB1*0404 and DRB1*0402 molecules. Furthermore, an invariant chain peptide was eluted from the DRB1*0402 molecules but not from the RA-associated molecules. The lack of invariant chain peptides from DRB1*0401 and DRB1*0404 molecules may contribute to the loading of autoantigen peptides into these molecules and to their association with disease.

Naturally processed peptides from rheumatoid arthritis associated and non-associated HLA-DR alleles

Naturally processed peptides from immunoaffinity-purified HLA-DRB1*0401, -DRB1*0404 (rheumatoid arthritis (RA)-associated), and -DRB1*0402 (non-RA-associated) molecules were analyzed by capillary liquid chromatography and mass spectrometry. The molecular weights observed for more than 60 eluted peptides from each HLA-DR protein ranged from 788 to 3535 atomic mass units, corresponding to peptides 7 to 32 amino acids in length. Sequencing of more than 60 of the abundant peptides revealed nested sets of peptides that were derived from only 12 different proteins. The majority of these proteins were membrane-associated (HLA class I, class II, and Ig molecules). Synthetic peptides, corresponding to endogenous peptide sequences, bound with high affinity (5 to 80 nM) to the HLA-DR molecules from which they were eluted. In addition, most were promiscuous binding peptides in that they also bound to other HLA-DR molecules. Truncations of eluted peptide sequences and alanine scanning mutational analysis of a Mycobacterium leprae peptide were used to identify the peptide residues involved in binding to DRB1*0404 and DRB1*0402 molecules. Furthermore, an invariant chain peptide was eluted from the DRB1*0402 molecules but not from the RA-associated molecules. The lack of invariant chain peptides from DRB1*0401 and DRB1*0404 molecules may contribute to the loading of autoantigen peptides into these molecules and to their association with disease.

A method for measuring leukocyte rolling on the selectins

An inexpensive, high-throughput method to simulate leukocyte rolling in the microvasculature has been developed. The method utilizes a 0.22-mm-inner diameter, fused silica capillary tube, coated with E- or P-selectin. Fluorescently labeled HL-60 cells are delivered to the capillary tube at a constant flow rate, exposing the cells to wall stresses approximating those found in postcapillary venules. Cells that physically associate with the inner walls of the tube and whose rate of movement through the tube is retarded, i.e., rolling cells, are monitored by fluorescence microscopy. Images are recorded on a time-lapse videocassette recorder. Both rolling incidence and velocity were shown to be related to the concentration of selectin utilized to coat the tube. Due to the extremely small volume (50 microliters) required to fill the capillary tube, this technique is useful for testing the effect of limited quantities of potential antagonists on cell rolling. Using this technique, sLex(Glc) tetrasaccharide was shown to prevent the rolling of HL-60 cells on immobilized E-selectin while fucoidan and dextran sulfate were shown to inhibit rolling of HL-60 cells on P-selectin.

Intratracheal administration of endotoxin and cytokines. VI. Antiserum to CINC inhibits acute inflammation

Cytokine-induced neutrophil chemoattractant (CINC), a chemotactic molecule of the interleukin (IL)-8 family, is known to be induced in the rat in response to tumor necrosis factor (TNF), IL-1, and lipopolysaccharide (LPS). Intratracheal injection of endotoxin (LPS) is shown to cause CINC mRNA expression in pulmonary tissue, peaking after 2 h, and CINC protein expression in bronchoalveolar lavage (BAL) fluid, peaking after 2-4 h. Intratracheal injection of synthetic CINC causes acute inflammation that is abrogated by coinjection of antiserum to purified natural rat CINC. Intratracheal injection of antiserum to CINC inhibits intratracheal LPS- and IL-1-induced neutrophil emigration into BAL fluid by approximately 60-70%. Despite the anti-inflammatory activity of anti-CINC antiserum, TNF is elevated in the lavage fluid of rats receiving anti-CINC, suggesting that CINC may act in a negative feedback loop to downregulate TNF expression. Intratracheal injection of antiserum to CINC combined with intravenous injection of anti-E-selectin antibody inhibits intratracheal LPS- and IL-1-induced neutrophil emigration into BAL fluid by approximately 75-85%. CINC-mediated chemotactic activity and E-selectin-mediated adherence of neutrophils to endothelium contribute significantly to the pathogenesis of LPS-initiated acute inflammation.

Binding of sialyl Lewis x to E-selectin as measured by fluorescence polarization

Fluorescence polarization has been used to directly measure the binding of the tetrasaccharide sialyl Lewisx (sLe(x)[Glc], or NeuAc alpha 2-3Gal beta 1-4[Fuc alpha 1-3]Glc) to a soluble form of E-selectin, a member of the class of adhesion molecules that plays an important role in immune-cell response to inflammation. The experiments utilized a fluorescent derivative of sLe(x)[Glc] with fluorescein attached directly to the glucose residue through a beta-glycosidic linkage. The resulting fluorescent sLe(x) was shown to inhibit binding of HL60 cells to immobilized E-selectin and exhibited fluorescence polarization enhancement in the presence of a monovalent form of a recombinant soluble E-selectin-Fc chimera. Thermodynamic dissociation constants of 107 +/- 26 and 120 +/- 31 microM were obtained for the fluorescent sLe(x)[Glc] and the free sLe(x)[Glc] sugars, respectively. These results demonstrate that E-selectin interacts weakly with the minimal carbohydrate recognition determinant sLe(x). Additional binding interactions through the action of the authentic coreceptor or via clustering of the ligand and E-selectin molecules on the respective neutrophil and endothelial cell surfaces may also play a role in the overall cellular binding strength. However, the basic interaction between carbohydrate and protein appears weak, consistent with other carbohydrate-protein interactions studied to date.

Studies on selectin-carbohydrate interactions

Recruitment of neutrophils to sites of inflammation is now believed to occur through an initial rolling interaction at the luminal surface of activated endothelium and is mediated by a class of mammalian lectins referred to as the selectins. Selectins recognize carbohydrate determinants on co-receptors. It is generally believed that many selectin molecules must bind to many carbohydrate receptor molecules i.e. multivalent binding, to enable sufficient binding strength to elicit the rolling response between the neutrophil and the endothelial cell. One of the approaches to the generation of more potent molecular antagonists of the selectin-mediated cell-cell interaction is to mimic the multivalent interaction in a single compound. Recent experiments utilising conjugated forms of sialyl Lewisx-BSA have explored this feasibility (Welply et al., 1994). In that study, monovalent sLex (sialic acid alpha 2-3Gal beta 1-4(Fuc alpha 1-3)GlcNAc), the minimum binding determinant for E-selectin, as well as monovalent sialyllactosamine (sialic acid alpha 2-3Gal beta 1-4GlcNAc), a non-binding structure, and the corresponding multivalent BSA-conjugated forms were tested for their ability to inhibit binding of HL-60 cells to immobilised E-selectin. As expected, only sLex and sLex-BSA were found to do so. sLex16-BSA (16 mol tetrasaccharide/mol BSA) showed a dose-dependent inhibition of HL-60 binding with a measured IC50 of 1 microM; demonstrating close to a three-order of magnitude enhancement of inhibitory activity compared to free sLex. This result indicated that multivalent forms of sLex are capable of binding to E-selectin with higher affinity than do monovalent glycans. In another study, fluorescent forms of monovalent sLex were synthesized and used to measure a true thermodynamic dissociation constant for the monovalent sLex:E-selectin interaction of 120 +/- 31 microM (Jacob et.al., 1995).

Multivalent sialyl-LeX: potent inhibitors of E-selectin-mediated cell adhesion; reagent for staining activated endothelial cells

Free, monovalent, SLeX (Neu5Ac alpha 2-3Gal beta 1-4(Fuc alpha 1-3)-GlcNAc), SLn (Neu5Ac alpha 2-3Gal beta 1-4GlcNAc) and corresponding BSA-conjugated forms--displaying different ratios of SLeX and SLn to protein--were tested for their ability to inhibit binding of HL-60 cells to immobilized E-selectin. Free SLeX and conjugated SLeX-BSA inhibited cell binding in a dose-dependent manner. SLn and SLn-BSA did not inhibit binding. SLeX16BSA (16 mol tetrasaccharide/mol BSA) and monovalent SLeX inhibited cell binding with measured inhibitory concentrations (IC50S) of 1 microM and 1 mM, respectively, demonstrating a three-order-of-magnitude enhancement of inhibitory activity with the multivalent form of SLeX. A SLex7BSA conjugate was 10-fold less potent than those with 11 or 16 mol SLeX/mol BSA. An assay which measured neutrophil rolling on interleukin (IL)-1 beta-activated human umbilical vein endothelial cells (HUVECs) showed 50% reduction in the number of rolling neutrophils in the presence of 1 microM SLeX16BSA, whereas the level of free, monovalent SLeX oligosaccharide required to produce the same effect was approximately 0.3 mM. SLeX-BSA was found to be an excellent reagent for staining endothelial cells expressing E-selectin. Biotinylated SLeX-BSA in conjunction with Texas red avidin-stained lipopolysaccharide (LPS)-activated HUVECs, and co-incubation of activated cells with anti-E-selectin, specifically blocked staining. The distribution of E-selectin, as determined by binding of SLeX-BSA, was virtually identical with that obtained by binding of anti-E-selectin antibody.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and oligosaccharide structure analysis of rhodopsin glycoforms containing galactose and sialic acid

The N-linked oligosaccharides of frog (Rana pipiens) rhodopsin were analysed by sequential exoglycosidase digestion and gel filtration chromatography, following reductive tritiation. In addition, selected tryptic glycopeptides obtained from frog retinal rod outer segment membranes were examined by electrospray mass spectrometry (ES-MS), fast atom bombardment mass spectrometry (FAB-MS), amino acid sequence and composition analysis, and carbohydrate composition analysis. The amino acid sequence data demonstrated that the glycopeptides were derived from rhodopsin and confirmed the presence of two N-glycosylation sites, at residues Asn2 and Asn15. The predominant glycan (approximately 60% of total) had the structure GlcNAc beta 1-2Man alpha 1-3(Man alpha 1-6) Man beta 1-4GlcNAc beta 1-4GlcNAc-(Asn), with the remaining structures containing 1-3 additional hexose residues, as reported previously for bovine rhodopsin. Unlike bovine rhodopsin, however, a sizable fraction of the total glycans of frog rhodopsin also contained sialic acid (NeuAc), with the sialylated oligosaccharides being present exclusively at the Asn2 site. FAB-MS analysis of oligosaccharides released from the Asn2 site gave, among other signals, an abundant quasimolecular ion corresponding to a glycan of composition NeuAc1Hex6HexNAc3 (where Hex is hexose and HexNAc is N-acetylhexosamine), consistent with a hybrid structure. The potential biological implications of these results are discussed in the context of rod outer segment membrane renewal.

The glycoprotease of Pasteurella haemolytica A1 eliminates binding of myeloid cells to P-selectin but not to E-selectin

HL-60 cells and neutrophils treated with the glycoprotease from Pasteurella haemolytica A1, an enzyme which is specific for O-sialoglycoproteins, were found to be incapable of binding P-selectin but still bound E-selectin. Comparative analysis of [35-S] cysteine labeled proteins from HL-60 cells by 2-dimensional electrophoresis indicated that two major proteins with M(r) 100 and 115 kd were significantly removed from cells which had been treated.

Characterization of N-linked oligosaccharides by electrospray and tandem mass spectrometry

Electrospray and tandem mass spectrometry are used to characterize underivatized oligosaccharides that have been digested from asparagine side chains of glycoproteins. Oligosaccharides that contain sialic acids were detected with the best sensitivity in the negative-ion detection mode whereas those that do not contain sialic acid were detected with the best sensitivity in the positive-ion detection mode. The positive-ion abundances of oligosaccharides were greatly enhanced in electrospray mass spectra by adding 10 mM sodium acetate or ammonium acetate to the sample solvent. Tandem mass spectrometry was used to determine primary structural features of the oligosaccharides. Methodology that has been developed on branched high-mannose, hybrid, and complex carbohydrate standards was applied to a mixture of oligosaccharides that were digested with N-glycanase from the glycoprotein, ovalbumin. The composition and relative abundances of individual oligosaccharides obtained from the electrospray mass spectrum compare favorably to those obtained by anion-exchange chromatography/pulsed amperometric detection and by gel permeation chromatography of the oligosaccharides after radiolabelling the reducing end of the carbohydrates. The oligosaccharide content of ovalbumin was independently determined from the heterogeneity observed in the electrospray mass spectrum of the intact 44-kDa glycoprotein. Comparison of the oligosaccharide compositions determined before and after enzymatic digestion shows a selective digestion of high-mannose and low molecular weight oligosaccharides by N-glycanase.

Oligosaccharides at each glycosylation site make structure-dependent contributions to biological properties of human tissue plasminogen activator

The effect of altering oligosaccharide structures at sites 184 and 448 of tissue plasminogen activator (tPA) has been examined. Alteration to high-mannose forms at sites 184 and 448 was accomplished by the growth of cells in the presence of deoxymannojirimycin (dMM). Modification to neutral, unsialylated forms at these sites was achieved by neuraminidase treatment of control preparations of tPA. Oligosaccharides at site 117 were not markedly affected by either treatment because structures at this site are high-mannose and not sialylated in untreated preparations. The effect on enzymatic activity and on a related property, lysine affinity, was determined. dMM treatment was found to increase both the lysine affinity and catalytic activity of tPA. Neuraminidase treatment increased enzyme activity, but was without effect on affinity for lysine. To evaluate the effects of alterations at site 184 and site 448, the catalytic activity and lysine affinity of type I and type II tPA were monitored individually. In the dMM-treated sample, type I tPA (with sugars at sites 117, 184 and 448) was found to have 2- to 3-fold increased catalytic activity and an affinity for lysine which was greater than that of type I from untreated preparations, but less than that of control type II tPA (containing sugar only at sites 117 and 448). In neuraminidase-treated type I, catalytic activity was also enhanced but lysine affinity remained unchanged. Type II from dMM- and neuraminidase-treated preparations had catalytic activity that was increased approximately 1.5-fold compared to untreated controls, whereas affinity for lysine was unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of glycosylated bovine placental lactogen and the effect of enzymatic deglycosylation on receptor binding and biological activity

Bovine placental lactogen (bPL) is a glycoprotein hormone that has both somatogenic and lactogenic properties. Purified preparations of the hormone contain many isoforms that are separated by isoelectric focusing. The sequence for bPL contains one consensus site for an N-linked oligosaccharide and many potential sites for O-linked sugars. To determine whether the isoforms are the result of differences in glycosylation, the oligosaccharide portion of bPL was partially characterized. In addition, a number of the isoforms were isolated and enzymatically deglycosylated to determine the effect of O- and N-linked glycosylation on biological activity. Biological activity was assessed in a somatotropin radioreceptor assay and also in the Nb2 lymphoma lactogenic bioassay. The structure of N-linked oligosaccharide was found to be sialylated and triantennary and appeared to be the same for all of the different charge isomers. Compositional analysis suggested that O-linked oligosaccharides were also present. Treatment of the intact hormone with neuraminidase resulted in the loss of some, but not all, of the isoforms, suggesting that a large degree of the charge heterogeneity is due to posttranslational modifications unrelated to glycosylation. Enzymatic removal of N-linked oligosaccharides from native bPL resulted in a 1.2-2.3-fold increase in binding to the somatotropin receptor, whereas receptor binding was unaffected by enzymatic removal of O-linked oligosaccharide. Lactogenic activity was affected very little by the removal of either type of oligosaccharide. The data suggests that glycosylation of bPL may have a small effect on receptor specificity, but that overall its presence does not dramatically affect receptor binding or biological activity.

The beta 1----2-D-xylose and alpha 1----3-L-fucose substituted N-linked oligosaccharides from Erythrina cristagalli lectin. Isolation, characterisation and comparison with other legume lectins

The carbohydrate moieties of Erythrina cristagalli lectin were released as oligosaccharides by hydrazinolysis, followed by N-acetylation and reduction with NaB3H4. Fractionation of the tritium-labelled oligosaccharide mixture by Bio-Gel P-4 column chromatography and high-voltage borate electrophoresis revealed that it is composed of five neutral oligosaccharides. Structural studies by sequential exoglycosidase digestion in combination with methylation analysis and two-dimensional 1H-NMR showed that the major component was the fucose-containing heptasaccharide Man alpha 3(Man alpha 6)(Xyl beta 2)Man beta 4GlcNAc beta 4(Fuc alpha 3)GlcNAcol. This is the first report of such a structure in plant lectins. Small amounts of the corresponding afucosyl hexasaccharide were also identified, as well as three other minor components. The structure of the heptasaccharide shows the twin characteristics of a newly established family of N-linked glycans, found to date only in plants. The characteristics are substitution of the common pentasaccharide core [Man alpha 3(Man alpha 6)Man beta 4GlcNAc beta 4GlcNAc] by a D-xylose residue linked beta 1----2 to the beta-mannosyl residue and an L-fucose residue linked alpha 1----3 to the reducing terminal N-acetylglucosamine residue. The oligosaccharide heterogeneity pattern for Erythrina cristagalli lectin was also found for the lectins from four other Erythrina species and the lectins of two other legumes, Sophora japonica and Lonchocarpus capassa.

Oligosaccharyltransferase activity is markedly increased during differentiation of a nonfusing myoblast cell line

We have studied several aspects of glycoprotein synthesis in myoblast differentiation by using a nonfusing myoblast cell line, BC3H1. Previous studies showed that transfer of proliferating undifferentiated BC3H1 cells to mitogen-depleted medium results in the cells' withdrawal from the cell cycle and induction of a variety of muscle-specific gene products [E. N. Olson, L. Glaser, J. P. Merlie, R. Sebane, and J. Lindstrom (1983) J. Biol. Chem. 258, 13946-13953]. Because cell surface glycoproteins have been implicated in myoblast differentiation, in the present study we measured the amount of oligosaccharyltransferase in microsomes isolated from BC3H1 cells at various stages of differentiation. By using an acceptor peptide containing the sequence-Asn-Leu-Thr-, enzyme activity was measured by formation of [3H]glycopeptide. In addition, active enzyme protein was measured with a 125I-labeled photoreactive derivative of the acceptor tripeptide. Both of these independent assay methods revealed a marked increase in oligosaccharyltransferase when differentiation was induced by serum depletion. Moreover, mitogenic stimulation of differentiated cells resulted in a return of oligosaccharyltransferase to near basal levels. This reversible increase in this key enzyme in protein glycosylation occurred despite the fact that both total protein and glycoprotein synthesis were depressed during differentiation. These data indicate that during myogenesis the level of oligosaccharyltransferase is regulated in parallel with a number of muscle-specific gene products. These results are discussed in the context of regulation of the pathway of glycoprotein synthesis.

Studies on properties of membrane-associated oligosaccharyltransferase using an active site-directed photoaffinity probe

Previous attempts in several laboratories, including ours, to purify oligosaccharyl-transferase have met with limited success because of the lability of the membrane-associated enzyme after solubilization with detergents. In an effort to identify the enzyme in face of this lability, we recently developed a photoaffinity reagent to label the active site [J. K. Welply, P. Shenbagamurthi, F. Naider, H. R. Park, and W. J. Lennarz (1985) J. Biol. Chem. 260, 6459-6465]. In this report, the preparations of a more sensitive selective labeling probe, 125I-labeled N alpha-3-(4-hydroxyphenylpropionyl)-Asn-Lys-(N epsilon-p-azidobenzoyl)-Thr-NH2, is described. Using this new probe, we have confirmed, independently of catalytic activity, that hen oviduct oligosaccharyltransferase is tightly associated with the endoplasmic reticulum membrane. The 125I-labeled oligosaccharyltransferase was released from the membrane by detergent and strong alkali treatments but not by sonication, high salt, or hypotonic shock. However, all procedures that released the enzyme from the membrane resulted in a dramatic loss of enzyme activity. Treatment of sealed microsomal membrane vesicles with phospholipase A resulted in nearly complete enzyme inactivation; in contrast, phospholipase C or D had moderate or little effect, respectively. Taken together, these results suggest that the hydrophobic environment of the membrane is required for oligosaccharyltransferase activity. Trypsin treatment of intact vesicles diminished enzyme activity by nearly 70%, but it had no effect on the binding affinity of the enzyme for the 125I-labeled photoaffinity probe. This result suggests that the polypeptide acceptor portion of oligosaccharyltransferase is lumenally disposed, and that a trypsin-sensitive, cytoplasmically oriented domain or another subunit binds the carbohydrate donor, dolichol-PP-oligosaccharide.

Regulation of N-linked glycoprotein assembly in uteri by steroid hormones

The effects of the steroid hormones 17 beta-estradiol (E2) and progesterone on N-linked glycoprotein assembly in ovariectomized mice have been examined. Both priming and nidatory E2 markedly stimulate [3H]mannose incorporation (3- to 6-fold) into uterine glycoproteins, whereas uterine bulk protein synthesis is not stimulated under the same conditions. Progesterone alone stimulates glycoprotein synthesis modestly (1.5-fold) over that in oil-injected controls, but antagonizes the action of E2 when coinjected with the estrogen. The E2 effect is not systemic, because livers from these same animals do not display an increase in glycoprotein synthesis. When mice were injected with tamoxifen or clomiphene, two drugs that mimic E2 actions in uteri without inducing the full extent of cell proliferation that normally accompanies E2 treatment, a similar enhancement of uterine glycoprotein synthesis was observed. Although mannosylphosphoryldolichol synthase activity rose in parallel with glycoprotein synthesis during E2 priming, the apparent activities of two other enzymes involved in the assembly of N-linked glycoproteins, namely chitobiosylpyrophosphoryldolichol synthase and oligosaccharyltransferase, remained relatively unchanged. Furthermore, the apparent in vivo rate of dolichol phosphorylation was not altered during E2 priming. Supplementation of uterine tissue slices with dolichylphosphate failed to enhance the rate of protein glycosylation in vivo. In addition, changes in the pool sizes of GDP-mannose did not correlate with changes in the in vivo rate of glycoprotein synthesis. Collectively, these observations indicate that the E2-dependent increase in glycoprotein synthesis is not likely to be due to increased enzyme activities for oligosaccharide assembly or transfer to protein, increased dolichylphosphate availability, or increased sugar nucleotide availability. To study the effects of E2 on the production of specific glycoproteins, the pattern of [3H]mannose-labeled glycoproteins produced as a function of days of E2 priming was examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Estrogen priming induced the secretion of 9-11 [3H]mannose-labeled glycoproteins by uteri; however, the pattern of tissue-associated glycoproteins remained constant throughout this interval. It appears, therefore, that estrogen priming induces the secretion of a few specific glycoproteins while generally enhancing the production of most tissue-associated glycoproteins. Most (70%) of the [3H]mannose-labeled oligosaccharide chains of these glycoproteins were of the polymannose type.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of yolk platelets isolated from developing embryos of Arbacia punctulata

Yolk platelets, a major organelle of sea urchin eggs and embryos, were isolated from Arbacia punctulata and biochemically characterized over the course of development to the pluteus stage. Fractionation by sucrose gradient centrifugation revealed yolk platelets in two major density classes. The low-density yolk platelet fraction could be obtained as a very homogeneous preparation and was highly enriched in acid phosphatase activity, while depleted of mitochondrial (cytochrome c oxidase) and plasma membrane (phosphodiesterase) marker enzymes. The chemical composition of low-density yolk platelets prepared from eggs and embryos at various stages of development remained unchanged in terms of phospholipid, triglyceride, hexose, sialic acid, RNA, and protein. However, analysis of the major yolk platelet glycoproteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a number of stage-specific changes. These glycoproteins were found to be major glycoproteins of crude embryo lysates and were predominantly of the polymannose, N-linked type. The predominance of polymannose-type glycoproteins in yolk platelets was further demonstrated by their staining with concanavalin A-colloidal gold in Lowicryl-embedded sections of embryos. These studies represent the first systematic biochemical characterization of intact yolk platelets and the changes in them during early embryonic development.

Calcitonin is a substrate for oligosaccharyltransferase in vitro

Tumor-derived, large molecular weight forms of calcitonin, have been postulated to result from glycosylation of the hormone. To address this question we have examined the glycosylation of calcitonin in vitro and in cultured thyroidal C-cells. We show that native, undenatured calcitonin is an active substrate for oligosaccharyltransferase and that glycosylation of calcitonin by the transferase is inhibited by tunicamycin. In addition, calcitonin is an effective competitive inhibitor of the glycosylation of a known peptide substrate for oligosaccharyltransferase. Pulse-labelling of cultured medullary thyroid carcinoma cells with [3H]-mannose indicate that detectable quantities of carbohydrate-containing forms of calcitonin are produced in these cells. These data indicate that glycosylation of calcitonin is one mechanism whereby tumor cells could produce higher molecular weight forms of the hormone.

Active site-directed photoaffinity labeling and partial characterization of oligosaccharyltransferase

Oligosaccharyltransferase, the enzyme that catalyzes the transfer of the oligosaccharide chain of dolichol-P-P-GlcNAc2Man9Glc3 to asparagine residues in -Asn-X-Thr/Ser- sites within polypeptides, has been radiolabeled using a photoactivatable azido tripeptide acceptor, N alpha-[3H]Ac-Asn-Lys(N epsilon-p-azidobenzoyl)-Thr-NH2. As determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the molecular mass of the oligosaccharyltransferase polypeptide from hen oviduct microsomes is 60 kDa. Radiolabeling of the 60-kDa polypeptide was completely dependent upon photolysis of hen oviduct endoplasmic reticulum preparations in the presence of the azido peptide and Mn2+, which is required for enzymatic activity. Labeling of the enzyme was not inhibited in the presence of a 10-fold excess of the nonacceptor peptides, unacetylated Asn-Lys(N epsilon-p-azidobenzoyl)-Thr-NH2 or unacetylated Asn-Leu-Thr-NH2, whereas it was completely abolished by the presence of a 10-fold excess of the competing acceptor peptide, N alpha-Bz-Asn-Leu-Thr-NH2. Thermal inactivation of oligosaccharyltransferase was achieved by heating endoplasmic reticulum preparations to 60 degrees C. This loss of enzyme activity at 60 degrees C paralleled a comparable decrease in radiolabeling of the 60-kDa polypeptide, whereas temperatures of 50 degrees C and lower had no effect on either process. Oligosaccharyltransferase itself may be an N-linked glycoprotein, because the 60-kDa radiolabeled polypeptide binds to concanavalin A-agarose and is susceptible to digestion by beta-endohexosaminidase H.

Developmental regulation of glycosyltransferases involved in synthesis of N-linked glycoproteins in sea urchin embryos

Previous in vivo studies using drugs that inhibit the N-glycosylation of proteins have demonstrated that newly synthesized N-linked glycoproteins are required for gastrulation in embryos of two species of sea urchins, Strongylocentrotus purpuratus and Arbacia punctulata. To understand the biochemical events regulating glycoprotein synthesis during gastrulation in S. purpuratus embryos, we examined the in vitro activities of enzymes catalyzing several of the early steps in N-linked glycoprotein synthesis. The activities of glycosyl transferases responsible for production of N,N-diacetylchitobiosylpyrophosphoryldolichol and glucosylphosphoryldolichol, two intermediates in the formation of oligosaccharylpyrophosphoryldolichol (the carbohydrate donor for N-glycosylation), were low but detectable in membranes from eggs. After fertilization these activities remained constant or increased slowly up to the blastula stage and thereafter increased rapidly at gastrulation. In agreement with these in vitro findings, in vivo labeling experiments revealed that the rate of incorporation of [3H]Man into oligosaccharylpyrophosphoryldolichol and into protein increased three- to fourfold prior to gastrulation and then slightly more at the prism stage. In contrast, in vitro activity of mannosylphosphoryldolichol synthase, another enzyme in the pathway of N-linked glycosylation, was maximal in membranes from egg and embryos in the early stages of development and declined prior to gastrulation. Furthermore, the level of this activity was at least 100-fold greater than that for enzymes involved in the formation of the chitobiosyl and glucosyl lipids. With the exception of mannosylphosphoryldolichol synthase activity, these data indicate that there is a general activation of the glycosylation apparatus before gastrulation in sea urchin embryos. Possible explanations for the decrease in mannosylphosphoryldolichol synthase activity are discussed.

Substrate recognition by oligosaccharyl transferase. Inhibition of co-translational glycosylation by acceptor peptides

In a microsome system rendered competent in protein translation by the addition of rabbit reticulocyte lysate, co-translational insertion and glycosylation of N-linked glycoproteins is observed when the appropriate mRNA is supplied. We have utilized this system to examine the ability of acceptor tripeptides of the type Asn-X-Thr/Ser to inhibit co-translational glycosylation. Using endogenous oligosaccharide-lipid as the carbohydrate donor, dog pancreas microsomes efficiently glycosylated N alpha-[3H]Ac-Asn-Leu-Thr-NHCH3 (apparent Km = 100 microM). Glycopeptide formation was essentially complete within 20 min. In the presence of mRNA from vesicular stomatitis virus or chicken ovalbumin, a similar tripeptide, N alpha-Ac-Asn-Leu-Thr-NH2, inhibited co-translational glycosylation. Translocation of the nascent chains was not affected. Thus, in the absence of peptide, all translated G protein was glycosylated and found within the microsomes, whereas in the presence of the peptide a mixture of glycosylated and nonglycosylated G protein was sequestered. Inhibition of nascent chain glycosylation was competitive and not merely the result of oligosaccharide lipid depletion, because preincubation of the microsomes with the peptide followed by its removal did not affect subsequent glycosylation of ovalbumin or G protein. Six derivatives of Asn-Leu-Thr-NH2, three of which were acceptors and three of which were not, were tested for their ability to inhibit co-translational glycosylation. The three acceptor peptides, N alpha-Ac-Asn-Leu-Thr-NH2, N alpha-Oc-Asn-Leu-Thr-NH2, and N alpha-Bz-Asn-Leu-Thr-NH2, effectively inhibited nascent chain glycosylation. In contrast, the three nonacceptors, N alpha-Ac-Gln-Leu-Thr-NH2, N alpha-Ac-Asn(N beta-Me)-Leu-Thr-NH2, and Asn-Leu-Thr-NH2, had no effect. Taken together, these data indicate that the inhibition of co-translational glycosylation by a peptide is dependent on its ability to compete for the active site of the oligosaccharyl transferase.

Substrate recognition by oligosaccharyltransferase. Studies on glycosylation of modified Asn-X-Thr/Ser tripeptides

The minimum primary structural requirement for N-glycosylation of proteins is the sequence -Asn-X-Thr/Ser-. In the present study, NH2-terminal derivatives of Asn-Leu-Thr-NH2 and peptides with asparagine replacements have been tested as substrates or inhibitors of N-glycosylation. The glycosylation of a known acceptor, N alpha-[3H]Ac-Asn-Leu-Thr-NHCH3, was optimized in chicken oviduct microsomes. The reaction was shown to be dependent upon Mn2+ and linear for 10 min at 30 degrees C; the apparent Km for the peptide was found to be 10 microM. N alpha-Acyl derivatives of Asn-Leu-Thr-NH2 (N-acetyl, N-benzoyl, N-octanoyl, or N-t-butoxycarbonyl) inhibited the glycosylation of N alpha-[3H] Ac-Asn-Leu-Thr-NHCH3 in a dose-dependent manner; additional experiments demonstrated that these compounds were alternative substrates rather than true inhibitors. The benzoyl and octanoyl derivatives were 10 times as effective as N alpha-Ac-Asn-Leu-Thr-NH2 in inhibiting glycosylation. In contrast, peptides containing asparagine modifications or substitutions were neither substrates nor inhibitors of N-glycosylation. They did not compete for glycosylation of 3H-peptide at 100-fold greater concentrations, and did not deplete endogenous pools of oligosaccharide-lipid. Thus, the asparagine side chain is an absolute requirement for recognition by the transferase. The majority of the glycosylated product (61%), but only 1% of the unglycosylated peptide, remained associated with the microsomes after high speed centrifugation. A large 41-amino acid residue acceptor peptide, alpha-lac17-58, was a poor substitute for glycosylation unless detergent was added to the microsomes. In contrast, glycosylation of tripeptide acceptors was not stimulated by detergent. Both of these findings suggest that the tripeptides are freely permeable to the microsomal membrane and support the earlier conclusion that glycosylation of proteins occurs at the luminal face of the microsomes.

beta-Galactosidase alpha-complementation. Overlapping sequences

Enzyme activity is restored to two defective beta-galactosidase molecules (M15 protein lacking amino acid residues 11-41 and M112 protein lacking residues 23-31) by incubation with CNBr2 (residues 3-92 of beta-galactosidase). M15 and M112 proteins (alpha-acceptors) are dimers. Complemented enzyme, like wild type, has a tetrameric structure. Cleavage of CNBr2 with glutamic acid-specific protease yielded a much smaller alp ha-donor (3-41 peptide) which was also effective in complementation, indicating that the M15 protein can supply all of the residues from 42-92 for the structure of complemented enzyme. Treatment of M112 protein/3-41 peptide complemented enzyme with trypsin under very mild conditions followed by examination of the products demonstrated that the alpha-donor pep]tide supplies the NH2-terminal segment of complemented enzyme. Similar trypsin treatment of M15 protein/CNBr2 indicated that in this complemented enzyme the polypeptide region beyond those residues missing in the alpha-acceptor can be provided by either the alpha-donor or the alpha-acceptor. Both M15 protein and M112 protein are more susceptible to mild tryptic proteolysis than complemented enzyme, indicating a more open structure. Several antipeptide antibodies that react with these two proteins do not react with beta-galactosidase. M112 protein, like M15 protein, can be activated by anti-beta-galactosidase but to a much higher level.

Positions of early nonsense and deletion mutations in lacZ

The positions of three Escherichia coli lacZ operator-proximal nonsense mutations and one deletion mutation have been determined. The nonsense mutations were suppressed with supF, resulting in the production of active beta-galactosidase by each strain. Amino acid sequencing identified the positions of the tyrosine residues inserted by supF, and thereby established that nonsense mutations lacZ2, lacZ2246, and lacZU131 are at sites corresponding to amino acids 23, 36, and 41 of beta-galactosidase, respectively. The deletion mutant, lacZM112, produced a dimeric beta-galactosidase protein missing amino acid residues 23 through 31 of the native enzyme.