Use of anti-idiotypic antibodies to establish that monoclonal antibody 7H11D6 binds to the alpha 2-macroglobulin receptor recognition site

Internalization but not binding of thrombospondin-1 to low density lipoprotein receptor-related protein-1 requires heparan sulfate proteoglycans

The amino-terminal domain of the extracellular matrix (ECM) protein thrombospondin-1 (TSP-1) mediates binding to cell surface heparan sulfate proteoglycans (HSPG) as well as binding to the endocytic receptor, low density lipoprotein-related protein (LRP-1). We previously found that recombinant TSP-1 containing the amino-terminal residues 1-214, retained both of these interactions (Mikhailenko et al. [1997]: J Biol Chem 272:6784-6791). Here, we examined the activity of a recombinant protein containing amino-terminal residues 1-90 of TSP-1 and found that this domain did not retain high-affinity heparin-binding. The loss of heparin-binding correlated with decreased binding to the fibroblast cell surface. However, both ligand blotting and solid phase binding studies indicate that this truncated fragment of TSP-1 retained high-affinity binding to LRP-1. Consistent with this, it also retained the ability to block the uptake and degradation of (125)I-TSP-1. However, TSP-1(1-90) itself was poorly endocytosed and this truncated amino-terminal domain was considerably more effective than the full-length heparin-binding domain (HBD) of TSP-1 in blocking the catabolism of endogenously expressed TSP-1. These results indicate that TSP-1 binding to LRP-1 does not require prior or concomitant interaction with cell surface HSPG but suggest subsequent endocytosis requires high-affinity heparin-binding.

Significantly increased fractions of transformed to total α2-macroglobulin concentrations in plasma from patients with multiple sclerosis

We examined the proteinase inhibitor alpha2-macroglobulin (alpha2M) in plasma from patients with multiple sclerosis (MS); a neurological disease of the central nervous system. The plasma concentrations of native and transformed alpha2M were measured in 90 patients with clinically definite MS, 73 with relapsing-remitting and 17 with secondary progressive MS, and 132 healthy individuals. Significantly lower concentrations of native alpha2M and significantly higher concentrations of transformed alpha2M were found in MS patients. A significant correlation between the concentrations of native and transformed alpha2M was found. The fraction of transformed to total alpha2M in the MS patients was 36% higher than in the healthy individuals. The results suggest an important involvement of alpha2M in regulation of increased proteolytic activity occurring in MS disease.

Conformational variants of human alpha2-macroglobulin are reflected in a C-terminal 'switch region'

Human alpha2-macroglobulin displays extensive conformational changes when induced to transform into new quaternary structures, which are eliminated from the systemic circulation by receptor-mediated endocytosis. One major region involved in these conformational changes is located in a segment of 30 amino acids from Glu1314 to Ser1343 (-Glu-Glu-Phe-Pro-Phe-Ala-Leu-Gly-Val-Gln-Thr-Leu-Pro-Gln-Thr-Cys-Asp -Glu-Pro-Lys-Ala-His-Thr-Ser-Phe-Gln-Ile-Ser-Leu-Ser-), which we term the 'switch region' of alpha2-macroglobulin, as deduced by immunochemical techniques. Monoclonal antibodies were generated using either native, methylamine-treated or the 18-kDa C-terminal receptor-binding fragment as the immunogen. From an extensive number of obtained hybridomas, 11 mAbs were selected because of their capacity to bind to the C-terminal fragment. Irrespective of the original configuration of the antigen used for immunization, seven of the antibodies were shown to be reactive with a set of overlapping epitopes, closely positioned within the 'switch region', as confirmed by the use of synthetic peptides covering the entire C-terminal fragment. The specificities of the seven individual antibodies, as determined by ELISA and BIAcore technologies, revealed a pronounced conformational pleomorphism in the 'switch region'. The results indicate that the 'switch region' may be involved in the exposure of the receptor recognition site and can be used as an indicator region for different conformational states of alpha2-macroglobulin.

Ligation of low-density lipoprotein receptor-related protein with antibodies elevates intracellular calcium and inositol 1,4, 5-trisphosphate in macrophages

We have probed the signaling characteristics of the macrophage low-density lipoprotein receptor-related protein (LRP) with monoclonal antibody 8G1, its Fab and F(ab')(2) fragments directed against the ligand binding heavy chain, and monoclonal antibody 5A6 directed against the membrane-spanning light chain of LRP. Ligation of LRP with 8G1, its Fab and F(ab')(2) fragments, or 5A6 increased intracellular Ca(2+) levels two- to threefold. Prior ligation of LRP with 8G1 did not affect the increase in [Ca(2+)](i) observed on subsequent ligation of LRP with lactoferrin, P. exotoxin A, or lipoprotein lipase. Binding to LRP by 8G1, its Fab and F(ab')(2) fragments, or 5A6 increased inositol 1,4,5-trisphosphate (IP(3)) levels by 50 to 100%. Incubation of macrophages with guanosine 5', 3'-O(thio)-triphosphate (GTP-gamma-S) before treatment with antibody potentiated and sustained the 8G1-induced increase in IP(3) levels. Treatment of macrophages with guanyl-5'-yl thiophosphate prior to GTP-gamma-S treatment abolished the GTP-gamma-S-potentiated increase in IP(3) levels in 8G1-treated macrophages. Antibody-induced increases in IP(3) and [Ca(2+)](i) in macrophages on ligation of LRP were pertussis toxin sensitive. Binding of 8G1 or its Fab or F(ab')(2) fragments to LRP stimulated macrophage protein kinase C (PKC) activity as evaluated by histone IIIs phosphorylation by about two- to sevenfold. Staurosporin inhibited the anti-LRP antibody-induced increase in PKC activity. Ligation of LRP with 8G1 increased cellular cAMP levels about twofold. Preincubation of macrophage with the LRP-binding protein receptor-associated protein suppressed the 8G1-induced increase in cAMP levels. Thus, binding of antibodies directed against either chain of LRP triggers complex signaling cascades.

Nickel inhibits binding of alpha2-macroglobulin-methylamine to the low-density lipoprotein receptor-related protein/alpha2-macroglobulin receptor but not the alpha2-macroglobulin signaling receptor

A previous study demonstrated that activated alpha2-macroglobulin (alpha2M*) binding to the low-density receptor-related protein/alpha2-macroglobulin receptor (LRP/alpha2MR) is blocked by Ni2+ [Hussain, M. M., et al. (1995) Biochemistry 34, 16074-16081]. We now report that the effect of Ni2+ is on a region of the alpha2M molecule upstream of the carboxyl terminal receptor recognition domain. This observation is consistent with previous observations from this laboratory suggesting that alpha2M* binding to LRP/alpha2MR involves a region of the alpha2M molecule immediately upstream of the receptor recognition domain [Enghild, J. J., et al. (1989) Biochemistry 28, 1406-1412]. We further demonstrate that Ni2+ has no effect on the binding of alpha2M* or a cloned and expressed receptor binding fragment (RBF) to the recently described alpha2M signaling receptor as assessed by direct binding and signal transduction studies.

Structural aspects of the interaction of peptidyl-glycylleucine-carboxyamide, a highly potent antimicrobial peptide from frog skin, with lipids

The interaction of PGLa (peptidyl-glycylleucine-carboxyamide), a 21-amino-acid residue cationic peptide, isolated from the skin of the South African clawed frog, Xenopus laevis, with model membrane systems was investigated. Our studies focussed on the importance of the difference in the phospholipid composition of bacterial and erythrocyte membranes. This is of particular interest to gain information on the specificity of membranolysis exhibited by this peptide against bacteria but not against erythrocytes. In phosphate buffer at physiological pH, as well as in the presence of the zwitterionic phosphatidylcholine and sphingomyelin. the peptide had a random structure but it adopted an alpha-helical conformation in the presence of negatively charged lipids. Furthermore, calorimetric experiments showed that PGLa had no effects on the thermotropic phase behavior of liposomes composed of the choline phosphatides, while separation of a distinct peptide-rich domain was observed for phosphatidylglycerol liposomes. In addition to the main transition of pure 1,2-dipalmitoylglycerophosphoglycerol at 40 degrees C a second transition owing to the peptide-perturbed lipid domains was found at 41 degrees C. This conclusion is supported by X-ray diffraction experiments which indicated that PGLa penetrates into the hydrophobic core of the bilayer inducing an untilting of the hydrocarbon chains as observed in the gel phase of the pure lipid. These results demonstrate that this antibacterial peptide specifically interacts with negatively charged lipid membranes, which are characteristic of bacterial membranes. This can be explained based on the structural features of PGLa.

The binding of receptor-recognized alpha2-macroglobulin to the low density lipoprotein receptor-related protein and the alpha2M signaling receptor is decoupled by oxidation

Receptor-recognized forms of alpha2-macroglobulin (alpha2M*) bind to two classes of cellular receptors, a high affinity site comprising approximately 1500 sites/cell and a lower affinity site comprising about 60,000 sites/cell. The latter class has been identified as the so-called low density lipoprotein receptor-related protein (LRP). Ligation of receptors distinct from LRP activates cell signaling pathways. Strong circumstantial evidence suggests that the high affinity binding sites are responsible for cell signaling induced by alpha2M*. Using sodium hypochlorite, a powerful oxidant produced by the H2O2-myeloperoxidase-Cl- system, we now demonstrate that binding to the high affinity sites correlates directly with activation of the signaling cascade. Oxidation of alpha2M* using 200 microM hypochlorite completely abolishes its binding to LRP without affecting its ability to activate the macrophage signaling cascade. Scatchard analysis shows binding to a single class of high affinity sites (Kd - 71 +/- 12 pM). Surprisingly, oxidation of native alpha2-macroglobulin (alpha2M) with 125 microM hypochlorite results in the exposure of its receptor-binding site to LRP, but the ligand is unable to induce cell signaling. Scatchard analysis shows binding to a single class of lower affinity sites (Kd - 0.7 +/- 0.15 nM). Oxidation of a cloned and expressed carboxyl-terminal 20-kDa fragment of alpha2M (RBF), which is capable of binding to both LRP and the signaling receptor, results in no significant change in its binding Kd, supporting our earlier finding that the oxidation-sensitive site is predominantly outside of RBF. Attempts to understand the mechanism responsible for the selective exposure of LRP-binding sites in oxidized native alpha2M suggest that partial protein unfolding may be the most likely mechanism. These studies provide strong evidence that the high affinity sites (Kd - 71 pM) are the alpha2M* signaling receptor.

Cellular internalization and degradation of thrombospondin-1 is mediated by the amino-terminal heparin binding domain (HBD). High affinity interaction of dimeric HBD with the low density lipoprotein receptor-related protein

Thrombospondin-1 (TSP-1) is a large modular trimeric protein that has been proposed to play a diverse role in biological processes. Newly synthesized TSP-1 either is incorporated into the matrix or binds to the cell surface where it is rapidly internalized and degraded. TSP-1 catabolism is mediated by the low density lipoprotein receptor-related protein (LRP), a large endocytic receptor that is a member of the low density lipoprotein receptor family. Using adenovirus-mediated gene transfer experiments, we demonstrate that the very low density lipoprotein receptor can also bind and internalize TSP-1. An objective of the current investigation was to identify the portion of TSP-1 that binds to these endocytic receptors. The current studies found that the amino-terminal heparin binding domain (HBD, residues 1-214) of mouse TSP-1, when prepared as a fusion protein with glutathione S-transferase (GST), bound to purified LRP with an apparent KD ranging from 10 to 25 nM. Recombinant HBD (rHBD) purified following proteolytic cleavage of GST-HBD, also bound to purified LRP, but with an apparent KD of 830 nM. The difference in affinity was attributed to the fact that GST-HBD exists in solution as a dimer, whereas rHBD is a monomer. Like TSP-1, 125I-labeled GST-HBD or 125I-labeled rHBD were internalized and degraded by wild type fibroblasts that express LRP, but not by fibroblasts that are genetically deficient in LRP. The catabolism of both 125I-labeled GST-HBD and rHBD in wild type fibroblast was blocked by the 39-kDa receptor-associated protein, an inhibitor of LRP function. GST-HBD and rHBD both completely blocked catabolism of 125I-labeled TSP-1 in a dose-dependent manner, as did antibodies prepared against the HBD. Taken together, these data provide compelling evidence that the amino-terminal domain of TSP-1 binds to LRP and thus the recognition determinants on TSP-1 for both LRP and for cell surface proteoglycans reside within the same TSP-1 domain. Further, high affinity binding of TSP-1 to LRP likely results from the trimeric structure of TSP-1.

Preparation and characterization of a C-terminal fragment of pregnancy zone protein corresponding to the receptor-binding peptide from human alpha 2-macroglobulin

Digestion of the pregnancy zone protein with papain at pH 4.5 yields an 18 kDa C-terminal fragment. This fragment consists of the 145 C-terminal amino-acid residues cleaved at Asn-1288 Ile and is homologous to the C-terminal receptor binding fragment of human alpha 2-macroglobulin obtained by cleavage with papain. The fragment contains an intrachain disulfide bond between 1308Cys and 1423Cys corresponding to that between 1304Cys and 1419Cys in alpha 2-macroglobulin. An oligosaccharide chain, is present in the C-terminal fragment of pregnancy zone protein as in human alpha 2-macroglobulin. The PZP C-terminal fragment was demonstrated to bind to the LRP/alpha 2M-receptor. Both the pregnancy zone protein and alpha 2-macroglobulin fragments bind three mAb's (alpha 1:1, R35, and 7H11D6) generated against alpha 2-macroglobulin. The mAb 7H11D6 was generated against the alpha 2-macroglobulin-proteinase complex (Isaacs, I.J., Steiner, J.P., Roche, P.A., Pizzo, S.V. and Strickland, D.K. (1988) J. Biol. Chem. 263, 6709-6714) and the binding of this to the C-terminal fragments of both pregnancy zone protein and alpha 2-macroglobulin indicates that both proteins use the same receptor recognition site for binding to the LRP/alpha 2M-receptor.

Production of conformation-specific monoclonal antibodies against alpha 2 macroglobulin and their use for quantitation of total and transformed alpha 2 macroglobulin in human blood

Monoclonal antibodies against the human proteinase inhibitor, alpha 2 macroglobulin, have been produced by immunizing BALB/c mice with alpha 2 macroglobulin reacted with methylamine. Two antibodies have been characterized in detail with respect to their binding to native alpha 2 macroglobulin and to different derivatives of the inhibitor. The antibody alpha-1 was found to recognize only those forms of the inhibitor which were transformed by reaction with different proteinases or with methylamine. Binding of alpha-1 was mapped to a specific epitope localized within a distance of 138 amino acid residues from the C terminal end of alpha 2 macroglobulin. The C terminal end is assumed to be exposed during the transformation of the inhibitor and harbours the receptor recognition site. The monoclonal antibody alpha-11 was found to bind to all forms of the inhibitor indicating that its epitope is located in a region not involved in major conformational changes of the inhibitor. On the basis of the different reactivity patterns of alpha-1 and alpha-11 two enzyme-linked immunosorption assays were established for quantitation of total and transformed alpha 2 macroglobulin in human blood. The concentration of the two forms have been determined in a population of 114 healthy individuals giving values of 254 +/- 6.6 mg/dl (mean +/- SEM) of total alpha 2 macroglobulin and 1.07 +/- 0.05 mg/dl (mean +/- SEM) of the transformed inhibitor.

Changes in structure of alpha 2-macroglobulin upon reaction with trypsin as assessed by light scattering and differential scanning calorimetry

Employing a combination of static and dynamic light scattering, as well as differential scanning calorimetry (DSC), the structural changes which appear in alpha 2-macroglobulin (alpha 2M) upon trypsin binding have been further characterized. Light-scattering measurements suggest that a 15% reduction in both the hydrodynamic radius and radius of gyration occurs when two molecules of trypsin complex to alpha 2M. Approx. 85% of this trypsin-induced compaction results from the binding of the first proteinase. A complementary result was obtained from DSC measurements in which the major fraction of the trypsin-induced conversion of alpha 2M to a single more thermally stable form results from interaction with the first proteinase molecule. These observations support a functionally asymmetric model of trypsin binding to alpha 2M in which the significant reduction in size of the complex is primarily due to the initial interaction of alpha 2M with a single proteinase molecule.

Differences in the proteinase inhibition mechanism of human alpha 2-macroglobulin and pregnancy zone protein

Different conformational states of human alpha 2-macroglobulin (alpha 2M) and pregnancy zone protein (PZP) were investigated following modifications of the functional sites, i.e. the 'bait' regions and the thiol esters, by use of chymotrypsin, methylamine and dinitrophenylthiocyanate. Gel electrophoresis, mAb (7H11D6 and alpha 1:1) and in vivo plasma clearance were used to describe different molecular states in the proteinase inhibitors. In alpha 2M, in which the thiol ester is broken by binding of methylamine and the 'trap' is closed, cyanylation of the liberated thiol group from the thiol ester modulates reopening of the 'trap' and the 'bait' regions become available for cleavage again. The trapping of proteinases in the cyanylated derivative indicates that the trap functions as in native alpha 2M. In contrast, cyanylation has no effect on proteinase-treated alpha 2M. As demonstrated by binding to mAb, the methylamine and dinitrophenylthiocyanate-treated alpha 2M exposes the receptor-recognition site, but the derivative is not cleared from the circulation in mice. The trap is not functional in PZP. In native PZP and PZP treated with methylamine, the conformational states seem similar. The receptor-recognition sites are not exposed and removal from the circulation in vivo is not seen for these as for the PZP-chymotrypsin complex. Tetramers are only formed when proteinases can be covalently bound to the PZP. Conformational changes are not detected in PZP derivatives in which the thiol ester is treated with methylamine and dinitrophenylthiocyanate. The results suggest that the conformational changes in alpha 2M are generated by mechanisms different to these in PZP. The key structure gearing the conformational changes in alpha 2M is the thiol ester, by which the events 'trapping' and exposure of the receptor-recognition site can be separated. In PZP, the crucial step for the conformational changes is the cleavage of the 'bait' region, since cleavage of the thiol ester does not lead to any detectable conformational changes by the methods used.

Ultrastructure of alpha 2-macroglobulins

New results concerning the ultrastructure of human alpha 2-macroglobulin (alpha 2M) molecules are presented in connection and comparison with the historical, the current and our own most recent, even unpublished results on the structure and function of alpha 2M and related proteins. The electron microscopic approach uses classical negative staining, combined with the new imaging mode "Electron Energy Loss Spectroscopy", which provides unusual contrast, resolution and readability of the electron micrographs. Immuno- and cryoelectron microscopy, as well as image processing has provided new data necessary to the building of tentative 3D models of the molecule. A model for the native tetrameric alpha 2M is described for the first time, and tries to explain and gather the various observations, sometimes contradictory, taken from different laboratories. A revised version for a model of the methylamine- and proteinase-transformed forms of alpha 2M is also shown. The probable positions of the bait regions and the thiol esters are given on both models. We confirm that alpha 2M is a twin trap capable of inactivating one or two proteinases by partial immobilization. Preliminary results on the production of crystals of alpha 2M-chymotrypsin complexes are also presented. A critical analysis of our models is presented in comparison with others. The technical limitations reached with some techniques and some possible extensions of future research in the field are also presented.