Differences in hydrophobic properties for human alpha 2-macroglobulin and pregnancy zone protein as studied by affinity phase partitioning

Human pregnancy zone protein stabilizes misfolded proteins including preeclampsia- and Alzheimer's-associated amyloid beta peptide

Pregnancy is a unique physiological state involving biological stresses that promote protein damage (misfolding) within the maternal body. Currently, little is known regarding how the maternal body copes with elevated protein misfolding in pregnancy. This is important, because the accumulation of misfolded proteins underlies many human disorders, including preeclampsia, a serious complication of pregnancy. In this study, we show that pregnancy zone protein (PZP) efficiently inhibits the aggregation of misfolded proteins, including the amyloid beta peptide, which forms plaques in preeclampsia and in Alzheimer’s disease. We propose that up-regulation of PZP is a major maternal adaptation that helps to maintain protein homeostasis during pregnancy. Moreover, pregnancy-independent up-regulation of PZP indicates that its chaperone function could be broadly important in humans.

Protein misfolding underlies the pathology of a large number of human disorders, many of which are age-related. An exception to this is preeclampsia, a leading cause of pregnancy-associated morbidity and mortality in which misfolded proteins accumulate in body fluids and the placenta. We demonstrate that pregnancy zone protein (PZP), which is dramatically elevated in maternal plasma during pregnancy, efficiently inhibits in vitro the aggregation of misfolded proteins, including the amyloid beta peptide (Aβ) that is implicated in preeclampsia as well as with Alzheimer’s disease. The mechanism by which this inhibition occurs involves the formation of stable complexes between PZP and monomeric Aβ or small soluble Aβ oligomers formed early in the aggregation pathway. The chaperone activity of PZP is more efficient than that of the closely related protein alpha-2-macroglobulin (α₂M), although the chaperone activity of α₂M is enhanced by inducing its dissociation into PZP-like dimers. By immunohistochemistry analysis, PZP is found primarily in extravillous trophoblasts in the placenta. In severe preeclampsia, PZP-positive extravillous trophoblasts are adjacent to extracellular plaques containing Aβ, but PZP is not abundant within extracellular plaques. Our data support the conclusion that the up-regulation of PZP during pregnancy represents a major maternal adaptation that helps to maintain extracellular proteostasis during gestation in humans. We propose that overwhelming or disrupting the chaperone function of PZP could underlie the accumulation of misfolded proteins in vivo. Attempts to characterize extracellular proteostasis in pregnancy will potentially have broad-reaching significance for understanding disease-related protein misfolding.

Alpha-2-Macroglobulin Is Acutely Sensitive to Freezing and Lyophilization: Implications for Structural and Functional Studies

Alpha-2-macroglobulin is an abundant secreted protein that is of particular interest because of its diverse ligand binding profile and multifunctional nature, which includes roles as a protease inhibitor and as a molecular chaperone. The activities of alpha-2-macroglobulin are typically dependent on whether its conformation is native or transformed (i.e. adopts a more compact conformation after interactions with proteases or small nucleophiles), and are also influenced by dissociation of the native alpha-2-macroglobulin tetramer into stable dimers. Alpha-2-macroglobulin is predominately present as the native tetramer in vivo; once purified from human blood plasma, however, alpha-2-macroglobulin can undergo a number of conformational changes during storage, including transformation, aggregation or dissociation. We demonstrate that, particularly in the presence of sodium chloride or amine containing compounds, freezing and/or lyophilization of alpha-2-macroglobulin induces conformational changes with functional consequences. These conformational changes in alpha-2-macroglobulin are not always detected by standard native polyacrylamide gel electrophoresis, but can be measured using bisANS fluorescence assays. Increased surface hydrophobicity of alpha-2-macroglobulin, as assessed by bisANS fluorescence measurements, is accompanied by (i) reduced trypsin binding activity, (ii) increased chaperone activity, and (iii) increased binding to the surfaces of SH-SY5Y neurons, in part, via lipoprotein receptors. We show that sucrose (but not glycine) effectively protects native alpha-2-macroglobulin from denaturation during freezing and/or lyophilization, thereby providing a reproducible method for the handling and long-term storage of this protein.

[Peculiaries of interactions between proteins of the macroglobulin family and with the endocytic receptors (a possible mechanism of transmembrane transfer)]

We have conducted a series of experiments, for specification of mechanisms which proteins of the macroglobulin family deliver regulatory substances inside of a cells. We have shown that all members of the family are not only compete for binding to proteinases, but also can interact with each other. We have confirmed that only a complex of alpha-2-macroglobulin (alpha2-MG) with proteinase is capable to react with the major endocytic receptor (low-density lipoprotein receptor-related protein, LRP). For the first time we have demonstrated, that interaction of alpha2-MG firstly with proteinase, and then with LRP provokes a progressive conformational consolidation of the multicomplex, which is accompanied by a paradoxical increase of the electrophoretic mobility in comparison with native alpha2-MG. We suggest that such stepwise conformational consolidation, together with earlier demonstrated charge neutralization (versus pI of internal environments) after interaction firstly with proteinase, and then with LRP, components of is the key moment of the mechanism of transmembrane transfer. Taking into account, that alpha2-MG transfers a broad spectrum of protein regulators, and interacts not only with LRP, but also with a signal receptor (grp78), and also can regulate (under certain conditions) both own synthesis, and synthesis of LRP and its blocker (receptor - associated protein, RAP), we suggest that this main member of the macroglobulin family plays a leading role in the regulation of intercellular interactions and in the transmission of signal inside of a cell.

Protease activation of alpha2-macroglobulin modulates a chaperone-like action with broad specificity

Alpha2-macroglobulin (alpha2M) is a major human blood glycoprotein best known for its ability to inhibit a broad spectrum of proteases by a unique trapping method. This action induces an "activated" conformation of alpha2M with an exposed binding site for the low-density lipoprotein receptor, facilitating clearance of alpha2M/protease complexes from the body. This report establishes that protease activation also modulates a potent chaperone-like action of alpha2M that has broad specificity for proteins partly unfolded as a result of heat or oxidative stress. Protease-mediated activation of alpha2M abolishes its chaperone-like activity. However, native alpha2M is able to form soluble complexes with stressed proteins and then subsequently become activated by interacting with a protease, providing a potential mechanism for the in vivo clearance of alpha2M/stressed protein/protease complexes. We propose that alpha2M is a newly discovered and unique member of a small group of abundant extracellular proteins with chaperone properties that patrol extracellular spaces for unfolded/misfolded proteins and facilitate their disposal.

Polymyxin B-Conjugated α2-Macroglobulin as an Adjunctive Therapy to Sepsis: Modes of Action and Impact on Lethality

A drug targeting both the inflammatory initiators (lipopolysaccharide; LPS) and mediators [tumor necrosis factor-alpha (TNF-alpha)] should have advantage over a "single-factor targeting strategy" in sepsis prevention trials. We have prepared conjugates of polymyxin B (PMB) and the cytokine binding protein alpha2-macroglobulin (A2M). The conjugate binds TNF-alpha as well as LPS as studied by electrophoresis and phase partitioning. Compared with free PMB, the conjugate is nontoxic to cells and does not affect the viability of human monocytes. The A2M-PMB conjugate binds to the A2M receptor (CD91/low-density lipoprotein receptor-related protein 1) with affinity similar to that of the nonmodified protein. Fluorescein isothiocyanate-labeled LPS in the presence of A2M-PMB is rapidly transported into fibroblasts for degradation via receptor-mediated endocytosis. In vitro, A2M-PMB demonstrated inhibition of LPS-induced secretion of TNF-alpha from isolated monocytes as well as in the whole blood assay. The efficacy of the drug was tested in mice after induction of acute inflammation (LPS model) and after induction of a polymicrobial sepsis by cecal ligation and puncture (CLP) model. Treatment of mice with A2M-PMB up to 250 microg/g body weight was not toxic to the animal. When the drug was administered 30 min before or 30 min after the LPS challenge, a survival rate of 90 and 70%, respectively, was obtained compared with the placebo control group (5%). A2M-PMB also protected mice after induction of polymicrobial sepsis when administered 30 min before CLP. These results support our hypothesis that A2M-PMB acts as a polyvalent drug to target different host mediators as well as sepsis inducer at the same time.

Effects of specific binding reactions on the partitioning behavior of biomaterials

Affinity partitioning is a special branch of biomaterials separations using aqueous two-phase systems. It combines the capability of diverse biomolecules to partition in aqueous two-phase systems using the principle of biorecognition. As a result, the macromolecule exhibiting affinity for a certain ligand is transferred to that phase where the ligand is present. This chapter describes the present status of the theoretical background of this approach and the properties of various natural and artificial compounds which act as affinity ligands in liquid-liquid systems. The affinity partitioning of proteins (enzymes and plasma proteins), cell membranes, cells, and nucleic acids are described as typical examples. The results are discussed in terms of theoretical understanding and practical application.

Epitope mapping by screening of phage display libraries of a monoclonal antibody directed against the receptor binding domain of human alpha2-macroglobulin

The human proteinase inhibitor, alpha2-macroglobulin (a2-M), inhibits a large number of proteinases. Alpha2-M-proteinase complexes are rapidly cleared from the circulation by binding to a cellular receptor (alpha2-M-R/LRP) via the receptor binding domain (RBD) which is made up of a 20 kDa C-terminal stretch of the 180 kDa monomer of the inhibitor. A monoclonal antibody (mab alpha-1) has been described which reacts with the receptor-recognizable form of the inhibitor, the so called transformed alpha2-M (a2-Mt). By screening of a phage display library an epitope in the RBD of the inhibitor was identified that reacts with mab alpha-1. Out of 25 phage clones a heptapeptide sequence (S-x1-x2-D-x3-x4-K) was obtained containing identical amino acids in three positions. A consensus peptide (S-R-S-D-P-P-K) was synthesized and found to displace alpha2-Mt from binding to mab alpha-1 and to receptor. The specificity of competition was demonstrated by a reversed peptide and a control antibody. By structural comparison it was found that the consensus heptapeptide mimics a discontinuous conformationally constrained epitope present in the RBD of the inhibitor. This is the first report describing the detection of discontinuous epitopes by phage display using a short linear peptide.

A procedure for human pregnancy zone protein (and human alpha 2-macroglobulin) purification using hydrophobic interaction chromatography on phenyl-sepharose CL-4B column

In the present work we describe a procedure for the purification of human pregnancy zone protein (PZP) from pooled late pregnancy plasma by using hydrophobic interaction chromatography (HIC) on a phenyl-Sepharose column. The HIC step allowed the complete isolation of haptoglobins and the partial separation of human alpha 2-macroglobulin (alpha 2-M) from a protein fraction containing PZP previously obtained by a DEAE-Sephacel chromatography. Pure and native PZP, with a recovery of nearly 25% and biological activity of protease-binding, was obtained by two definitive final steps consisting of zinc-chelate and size-filtration chromatographies. Moreover, we further present an alternative procedure for the purification of alpha 2-M from the same pregnancy plasma, based on the differential elution of PZP and alpha 2-M from the HIC. This purification step gave rise to a highly purified product with a recovery of 10%. This differential elution could be explained by differences in surface hydrophobicity observed between both proteins. In addition, considering the different hydrophobic properties exhibited by native PZP and PZP-protease complexes, HIC on phenyl-Sepharose column could also be used for separating both conformational states of PZP.

The contact zones in human alpha2-macroglobulin--functional domains important for the regulation of the trapping mechanism

A functional domain termed the contact zone, which is the region of a subunit interacting with another non-covalently bound subunit, is suggested to play a decisive role in the trapping mechanism of human alpha2-macroglobulin. Tetrameric alpha2-macroglobulin can be dissociated into stable dimers with intact thiol esters by sodium thiocyanate, whereby the contact zones are disrupted. The dissociation leads to significant conformational changes, as studied by ultraviolet-difference spectroscopy, CD, fluorescence and affinity partitioning. The conformation of the dimers is similar to that of MeNH2-treated alpha2-macroglobulin, in which the thiol esters are cleaved, a conformational state with a closed trap occurs, and receptor-recognition sites are exposed. The receptor-binding domain is at least partly exposed in the dimer, as judged by binding of specific mAbs. The bait region in the dimers can be cleaved by proteases, and activation of the thiol esters ensues without binding of the protease. When the dimers were treated with MeNH2, no conformational changes could be detected by ultraviolet-difference spectroscopy or CD. The conformational changes occurring on dissociation into dimers are suggested to be related to trap closure and receptor-recognition-site exposure without cleavage of the thiol esters. The model presented here suggests that two separate conformational changes occur in alpha2-macroglobulin upon activation. The first involves changes at the contact zones as a result of the thiol-ester cleavage, and the second causes exposure of the receptor-recognition sites and closure of the trap.

Purification and characterization of a tetrameric alpha-macroglobulin proteinase inhibitor from the gastropod mollusc Biomphalaria glabrata

The alpha-macroglobulin proteinase inhibitors (alpha Ms) are a family of proteins with the unique ability to inhibit a broad spectrum of proteinases. Whereas monomeric, dimeric and tetrameric alpha Ms have been identified in vertebrates, all invertebrate alpha Ms characterized so far have been dimeric. This paper reports the isolation and characterization of a tetrameric alpha M from the tropical planorbid snail Biomphalaria glabrata. The sequence of 18 amino acids at the N-terminus indicates homology with other alpha Ms. The subunit mass of approx. 200 kDa was determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and SDS/PAGE. The quaternary structure was determined by sedimentation equilibrium centrifugation and native pore-limit electrophoresis. Evidence for a thioester is provided by the fact that methylamine treatment prevents the autolytic cleavage of the snail alpha M subunit and results in the release of 4 mol of thiols per mol of snail alpha M. The snail alpha M inhibited the serine proteinase trypsin, the cysteine proteinase bromelain and the metalloproteinase thermolysin. The spectrum of proteinases inhibited, together with the demonstration of steric protection of the proteinase active site and a "slow to fast' conformational change after reacting with trypsin, all suggest that the inhibitory mechanism of the snail alpha M is similar to the "trap mechanism' of human alpha 2-macroglobulin.

Ligand interaction of human alpha 2-macroglobulin-alpha 2-macroglobulin receptor studied by partitioning in aqueous two-phase systems

Alpha 2-macroglobulin (alpha 2-M) is a major proteinase inhibitor in human blood and tissue. Besides its antiproteolytic potential, alpha 2-M was found to modulate antigen- and mitogen-driven immune responses and cell growth by binding and transporting distinct cytokines, growth factors and hormones. The inhibitor is cleared from circulation by binding to a multifunctional cellular receptor present on different cell types. Alpha 2-M, as well as its receptor, are capable of binding a variety of ligands. In the present study we have applied aqueous two-phase systems to analyze the interaction of IL-1 beta and alpha 2-M receptor to different forms of alpha 2-M. The partition of IL-1 beta was changed by addition of transformed alpha 2-M to the two-phase systems rather than by the native inhibitor. The interaction between IL-1 beta and alpha 2-M was enhanced by divalent cations. In addition, the complex formation between 125I-labelled receptor and alpha 2-M could clearly be demonstrated by partitioning. In the presence of divalent cations, transformed alpha 2-M, in contrast to the native inhibitor, effectively changed the partition of the receptor. However, the observed alteration of the partition coefficient was found to be less compared with the values obtained by partitioning of the receptor in the presence of whole plasma containing the inhibitor in equivalent concentrations. The results indicate that other components of the plasma exist which competitively bind to the receptor but independent of Ca2+-ions.

Use of hydrophobic affinity partitioning as a method for studying various conformational states of the human alpha-macroglobulins

The serum proteins alpha 2-macroglobulin and pregnancy zone protein undergo major conformational changes when complexed with proteinases. It is shown that the changes in delta log Kmax determined by hydrophobic affinity partitioning is a measure of the extent of changes in the conformation of these alpha-macroglobulins. We introduce a new term for the changes of surface hydrophobicity in a protein as delta log Kacc. This defines the difference of delta log Kmax between a modified and an unmodified conformational state of a specific protein and can be useful as a parameter to describe the apparent conformational changes in the protein.

Comparison of conformational changes of pregnancy zone protein and human alpha 2-macroglobulin, a study using hydrophobic affinity partitioning

Conformational changes of human alpha 2-macroglobulin (alpha 2M) and pregnancy zone protein (PZP), reflected in changes in surface hydrophobicity, have been studied. The results show that the conformation of alpha 2M is governed by the degree of 'trapping'. Thus, cleavage in the bait region and of the thiol ester by proteinase treatment causes a two-fold increase in surface hydrophobicity of alpha 2M. However, the increase is still higher (three-fold) when the thiol esters in alpha 2M alone are cleaved by methylamine. Cyanylation of the thiol groups exposed upon methylamine treatment yields a derivative with the same hydrophobicity as native alpha 2M. Treatment of this derivative with chymotrypsin restores the hydrophobicity to that of methylamine-treated alpha 2M. Since the C-terminal 18 kDa fragment of alpha 2M exhibits no hydrophobicity, the change in hydrophobicity seems not to reside in the receptor binding site. In contrast to alpha 2M, modification of both native and methylamine-treated PZP with chymotrypsin gives a reduction (about 40%) in hydrophobicity. The change in hydrophobicity is insignificant on treatment with methylamine alone. Furthermore, hydrophobic interactions appear not to contribute to tetramerization of PZP. The present study indicates major differences in the conformational states of alpha 2M and PZP as reflected in the hydrophobic surfaces exhibited.

Liquid-liquid partition chromatography as a method to examine surface properties of antibodies and antigen-antibody complexes

We demonstrate liquid-liquid partition chromatography in aqueous two-phase systems (LLPC) as a simple method for examining the surface properties of immunoglobulins and antigen-antibody complexes in solution. LLPC separates molecules with respect to the properties of the exposed surfaces. As an example, the method may be used to detect changes in the conformation of IgG following chemical modification like acylation or iodination. We have studied the partitioning of antibodies and antigen-antibody complexes, modelled by rabbit antibodies against three human serum proteins, in aqueous polyethylene glycol/dextran two-phase systems at pH 7. Analysis of both polyclonal and monoclonal antibodies against various antigens suggested that the partition properties of immunoglobulins are related mainly to their antigen specificity and not to subclass-specific structures. Furthermore, experiments indicated that changes in the surface properties of antigen and/or antibody following complexation may be detected. Thus, LLPC may prove to be a new way of studying the relation between antibody structure and function in solution.

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.