Human serum amyloid P component is an invariant constituent of amyloid deposits and has a uniquely homogeneous glycostructure

Proteomic characterization of novel serum amyloid P component variants from human plasma and urine

Serum amyloid P component (SAP) is a human plasma protein that has been widely studied for its influence on amyloid plaque formation and stabilization. SAP was characterized directly from human plasma and urine samples via novel affinity mass spectrometry-based proteomic technology that is able to readily discriminate between mass-altered protein variants. These analyses were able to identify several variants of SAP that have not been previously reported. These variants include microheterogeneity of the glycan structure, from the loss of one or both terminal sialic acid residues, as well as the loss of the C-terminal valine residue. Moreover, the analysis of urine allowed for the consistent identification of serum amyloid P component as a normal constituent of the urine proteome.

Glycation stimulates amyloid formation

Amyloidosis comprises a group of systemic and localized diseases with varied clinical presentations. In these diseases, amyloid forms when proteins with a largely alpha-helical structure lose their original conformation and are converted into a predominantly beta-sheet form, thereby increasing their propensity to form highly insoluble and fibrillar aggregates. Most soluble amyloid precursor proteins have substantial beta-pleated sheet secondary structure, and extensive beta-pleated sheet structure occurs in all of the deposited fibrils. The aberrant deposition of proteins as cellular inclusions or plaques in the form of amyloid fibrils is a characteristic hallmark of all amyloid diseases (or amyloidoses) and of the so-called conformational diseases. Environmental and genetic factors are known to be involved, but the mechanism by which this process happens still is poorly understood. Here we report a new finding from the Dutch group of Gebbink and colleagues, which points to the posttranslational process of glycation as a key mechanism in the formation of amyloid. These researchers showed that glycation causes albumin, a globular protein with a largely alpha-helical structure, to adopt a beta-pleated sheet structure and the quaternary structural element known as the cross-beta conformation. These are features commonly shared by all amyloids. This research is the first to show glycation as a predisposing factor for amyloidosis.

Fibrillar amyloid protein present in atheroma activates CD36 signal transduction

The self-association of proteins to form amyloid fibrils has been implicated in the pathogenesis of a number of diseases including Alzheimer's, Parkinson's, and Creutzfeldt-Jakob diseases. We recently reported that the myeloid scavenger receptor CD36 initiates a signaling cascade upon binding to fibrillar beta-amyloid that stimulates recruitment of microglia in the brain and production of inflammatory mediators. This receptor plays a key role in the pathogenesis of atherosclerosis, prompting us to evaluate whether fibrillar proteins were present in atherosclerotic lesions that could initiate signaling via CD36. We show that apolipoprotein C-II, a component of very low and high density lipoproteins, readily forms amyloid fibrils that initiate macrophage inflammatory responses including reactive oxygen production and tumor necrosis factor alpha expression. Using macrophages derived from wild type and Cd36(-/-) mice to distinguish CD36-specific events, we show that fibrillar apolipoprotein C-II activates a signaling cascade downstream of this receptor that includes Lyn and p44/42 MAPKs. Interruption of this signaling pathway through targeted deletion of Cd36 or blocking of p44/42 MAPK activation inhibits macrophage tumor necrosis factor alpha gene expression. Finally, we demonstrate that apolipoprotein C-II in human atheroma co-localizes to regions positive for markers of amyloid and macrophage accumulation. Together, these data characterize a CD36-dependent signaling cascade initiated by fibrillar amyloid species that may promote atherogenesis.

Investigating interactions of the pentraxins serum amyloid P component and C-reactive protein by mass spectrometry

The oligomeric state of human SAP (serum amyloid P component) in the absence and presence of known ligands has been investigated using nanoelectrospray ionization MS. At pH 8.0, in the absence of Ca2+, SAP has been shown to consist of pentameric and decameric forms. In the presence of physiological levels of Ca2+, SAP was observed to exist primarily as a pentamer, reflecting its in vivo state. dAMP was shown not only to promote decamerization, but also to lead to decamer stacking involving up to 30 monomers. A mechanism for this finding is proposed. CRP (C-reactive protein), a pentraxin closely related to SAP, exists as a pentamer in the presence or absence of Ca2+. Pentamers of CRP and SAP were shown to form mixed decamers in Ca2+-free buffer; however, in the presence of Ca2+, this interaction was not observed. Furthermore, no exchange of monomeric subunits was observed between the SAP and CRP oligomers, suggesting a remarkable stability of the individual pentameric complexes.

On-line immunoaffinity-liquid chromatography-mass spectrometry for identification of amyloid disease markers in biological fluids

Disease-specific alterations in proteins and peptides such as the appearance of new isoforms, changed relative concentrations of known isoforms, or changed catabolism characterize the group of protein precipitation disorders collectively known as amyloidoses. The goal of this study was to develop an approach for isolating and characterizing the pool of isoforms of a polypeptide of interest from biological fluids for use in development of diagnostic markers and elucidation of pathogenesis. For this purpose, we employed an on-line immunoaffinity-liquid chromatography-mass spectrometry (IA-LC-MS) modular approach using antibodies binding populations of protein isoforms. In this system, crude biological samples, e.g., serum, may be injected and subjected to fast hands-off analysis. The setup consists of an optional preclear column for removal of unspecific binding components, an immunoaffinity column, a short cartridgelike reversed-phase column, and an electrospray time-of-flight mass spectrometer. We have tested the system for the automated analysis of three amyloid-related polypeptides, serum amyloid P component, amyloid beta-peptide, and beta2-microglobulin, and we show the feasibility of detection of altered isoforms or determination of relative abundance of isoforms of the proteins from serum or cerebrospinal fluid samples. For each new protein investigated, the only change needed in the system is a new antibody or antibody mixture and the selection of a reversed-phase cartridge of appropriate hydrophobicity.

Structural, quantitative and functional comparison of amyloid P component in sera from patients with systemic lupus erythematosus and healthy donors

Serum amyloid P component (SAP) is a serum protein that has a function as opsonin and is known to bind nuclear material with high affinity. Quantitative and/or qualitative deficiencies in SAP may possibly lead to the impairment of normal homoeostatic mechanisms of tissue turnover. Thus, SAP knockout mice display systemic lupus erythematosus (SLE)-like manifestations such as nephritis and circulating antinuclear antibodies. In the present study, we investigated whether there are changes in the structure, function or serum levels of SAP in serum from SLE patients as compared with those from healthy donors. We found that SAP in SLE sera has the same molecular mass as that of in the sera of normal individuals, when analysed by online immunoaffinity reversed phase mass spectrometry. Also, the serum levels of SAP did not differ significantly between the two groups. Finally, as an estimate of function, SAP from SLE patients appeared to have the same affinity for heparin and nucleosomes as SAP from normal individuals, when analysed by crossed affinity immunoelectrophoresis and enzyme-linked immunosorbent capture assay (ELISA). In conclusion, the data do not support alterations in the levels, structure or function of SAP circulating in SLE patients.

Serum amyloid p component does not circulate in complex with C4-binding protein, fibronectin or any other major protein ligand

Serum amyloid P component (SAP) is a pentameric plasma protein associated with all known kinds of amyloid. The normal physiological function of the protein has not been fully elucidated but it may be involved in clearance of cellular debris and in innate immunity. An important clue to its normal function is the identity of ligands bound to SAP in the circulation. It has been reported that all SAP is complexed with C4-binding protein (C4bp) but other studies have not been able to confirm this. We here study this issue by a combination of crossed immunoelectrophoresis (CIE), size exclusion chromatography, and native polyacrylamide electrophoresis and we show that SAP in serum - analysed under native analysis conditions and free of immobilizing antibodies - does not have any major protein ligand. However, when the protein is aggregated by immobilized antibodies, C4bp and fibronectin clearly bind to SAP. If circulating SAP under normal circumstances bind any protein ligand in vivo, our results strongly suggest that this only occurs to a minor extent.

Characterization of heparin binding by a peptide from amyloid P component using capillary electrophoresis, surface plasmon resonance and isothermal titration calorimetry

Synthetic peptides based on amino-acid residues 27-38 of human serum amyloid P component represent a novel type of heparin binders as they do not contain clusters of basic amino acids or other known features associated with protein or peptide heparin binding. Here, we characterize the binding using capillary electrophoresis (CE), surface plasmon resonance (SPR), and isothermal titration calorimetry (ITC). By CE, heparin-binding activity was readily apparent for both a regular peptide and a slightly N-terminally modified form, while a sequence-scrambled peptide had no measurable binding. Dissociation constants in the 1-15 microm range were estimated, but only a minor part of the binding isotherm was covered by the experiments. SPR measurements using immobilized peptides verified heparin binding, the range of the binding constants, and the reduced binding of the sequence-scrambled peptide. Structurally defined heparin oligosaccharides were used to establish that while the tetrasaccharide is too small to exhibit strong binding, little difference in binding strength is observed between hexa- and tetradeca-saccharides. These experiments also confirmed the almost complete lack of activity of the sequence-scrambled peptide. The amino-acid sequence-dependent binding and the importance of a disulfide bond in the peptide were verified by ITC, but the experimental conditions had to be modified because of peptide precipitation and ITC yielded significantly weaker binding constants than the other methods. While the precise function of the peptide in the intact protein remains unclear, the results confirm the specificity of the glycosaminoglycan interaction with regard to peptide sequence by applying two additional biophysical techniques and showing that the N-terminal part of the peptide may be modified without changing the heparin binding capabilities.

A novel localized amyloidosis associated with lactoferrin in the cornea

We report a novel localized amyloidosis associated with lactoferrin. To elucidate the precursor protein of corneal amyloidosis associated with trichiasis, we analyzed amyloid deposits from three patients by histopathology and biochemistry. Amyloid deposits showed immunoreactivity, confirmed by electron microscopy, for only anti-human lactoferrin antibody. Electrophoresis of amyloid fibrils revealed lactoferrin with and without sugar chains; N-terminal sequence analysis revealed full-length lactoferrin and a truncated tripeptide of N-terminal amino acids, Gly-Arg-Arg. Carboxymethylated wild-type lactoferrin formed amyloid fibrils in vitro. Lactoferrin gene analysis in the three patients revealed a Glu561Asp mutation in all of the patients and a compound heterozygote of Ala11Thr and Glu561Asp mutations in one patient. A heterozygotic Glu561Asp mutation appeared in 44.8% of healthy Japanese volunteers, suggesting that the mutation may not be an essential mutation for amyloid formation (p = 0.104). Results thus suggest that lactoferrin is this precursor protein.

Targeted pharmacological depletion of serum amyloid P component for treatment of human amyloidosis

The normal plasma protein serum amyloid P component (SAP) binds to fibrils in all types of amyloid deposits, and contributes to the pathogenesis of amyloidosis. In order to intervene in this process we have developed a drug, R-1-[6-[R-2-carboxy-pyrrolidin-1-yl]-6-oxo-hexanoyl]pyrrolidine-2-carboxylic acid, that is a competitive inhibitor of SAP binding to amyloid fibrils. This palindromic compound also crosslinks and dimerizes SAP molecules, leading to their very rapid clearance by the liver, and thus produces a marked depletion of circulating human SAP. This mechanism of drug action potently removes SAP from human amyloid deposits in the tissues and may provide a new therapeutic approach to both systemic amyloidosis and diseases associated with local amyloid, including Alzheimer's disease and type 2 diabetes.

C-reactive protein, inflammation, and innate immunity

The circulating acute phase reactant C-reactive protein (CRP) has traditionally been characterized as an effector of nonclonal host resistance since it activates the classical complement cascade and mediates phagocytosis, but it is also capable of regulating inflammation. The three-dimensional structure of human CRP has revealed the molecular basis for complement activation and binding of phosphate monoesters. CRP gene expression by liver hepatocytes in response to cytokines (IL-1beta and IL-6) released in tissues requires several transcription factors which interact. Elevated levels of CRP are a prognostic marker for coronary artery disease; however, the role of CRP in atheriosclerosis remains unknown. CRP also mediates direct host protection to some microbial pathogens via its opsonic activity through certain Fcgamma-receptors. The CRP response may be one of the links between nonspecific innate immunity and specific clonal immunity.

Spontaneous in vitro formation of supramolecular beta-amyloid structures, "betaamy balls", by beta-amyloid 1-40 peptide

The concentration of beta-amyloid peptide (Abeta), x-42 or x-40 amino acids long, increases in brain with the progression Alzheimer's disease (AD). These peptides are deposited extracellularly as highly insoluble fibrils that form densities of amyloid plaques. Abeta fibrillization is a complex polymerization process preceded by the formation of oligomeric and prefibrillar Abeta intermediates. In some of our in vitro studies, in which the kinetics of intermediate steps of fibril formation were examined, we used concentrations of synthetic Abeta that exceed what is normally employed in fibrillization studies, 300-600 microM. At these concentrations, in a cell free system and under physiological conditions, Abeta 1-40 peptide (Abeta40) forms fibrils that spontaneously assemble into clearly defined spheres, "betaamy balls", with diameters of approximately 20-200 microm. These supramolecular structures show weak birefringence with Congo red staining and high stability with prolonged incubation times (at least 2 weeks) at 30 degrees C, freezing, and dilution in H(2)O. At 600 microM, they are detected after incubation for approximately 20 h. Abeta peptide 1-42 (Abeta42) lacks the ability to form betaamy balls but accelerates Abeta40 betaamy ball formation at low stoichiometric levels (1:20 Abeta42:Abeta40 ratio). Abeta42 levels above this (=10-50% w/w) impede Abeta40 betaamy ball formation. Using light (LM) and electron microscopy (EM), this study examines the gross morphology and ultrastructure of Abeta40 betaamy balls and their time course of formation, in the absence and presence of Abeta42, along with some stability measures. As spheres of a misfolded protein, betaamy balls resemble both AD Abeta senile plaques and neuronal inclusion bodies associated with other neurodegenerative diseases.

Evaluation of 99mTc-MAMA-chrysamine G as an in vivo probe for amyloidosis

To date, systemic amyloidosis is diagnosed histologically using Congo red staining or in vivo using iodine-123 labelled serum amyloid P component (123I-SAP) scintigraphy. We developed 99mTc-MAMA-CG, a 99mTc-labelled derivative of the lipophilic Congo red analogue chrysamine G (CG), as a possible alternative to 123I-SAP. In vivo 99mTc-MAMA-CG scintigraphy, performed in chickens with spontaneous joint amyloidosis, resulted as soon as 10 min after injection in scintigraphic images showing uptake of activity in amyloid-loaded organs (liver, joints). One of these chickens was studied also with 123I-SAP resulting in scintigraphic images revealing 123I-SAP binding to amyloid deposits in the liver. However, up to 11 h after injection no radioactivity was visible in the amyloid positive joints. In vitro autoradiography, performed on sections of chicken joints with Enterococcus faecalis induced amyloid arthropathy (chjAA), demonstrated the failure of 99mTc-MAMA-CG to bind significantly to amyloid deposits in the presence of 10 microM Congo red The specificity of 99mTc-MAMA-CG localisation was also established by the absence of 99mTc-MAMA-CG binding in non-amyloidotic organs in vitro and in vivo. 99mTc-MAMA-CG did not show any sign of acute toxicity. These findings establish the usefulness of 99mTc-MAMA-CG as a non-invasive in vivo diagnostic probe in chickens with amyloid arthropathy and suggest that it may also be applicable to human amyloidosis.

In vitro affinity of 99Tcm-labelled N2S2 conjugates of chrysamine G for amyloid deposits of systemic amyloidosis

To date, systemic amyloidosis is diagnosed histologically in vitro using Congo red staining or in vivo using iodine-123 serum amyloid P component (123I-SAP) scintigraphy. 99Tcm-labelled derivatives of chrysamine G (CG), a lipophilic analogue of Congo red, were synthesized as potential tracer agents for direct and quantitative scintigraphic evaluation of amyloid deposits. To determine the affinity of 99Tcm-MAMA-CG, 99Tcm-Me4MAMA-CG and 99Tcm-MAMA-CG diethyl ester for amyloid, in vitro autoradiography was performed on sections of human kidney biopsy cylinders from kidneys with amyloid deposits (types AA, Alambda and Akappa) or control kidney tissue after incubation with the respective tracer agents. The binding of 99Tcm-MAMA-CG and its tetramethyl derivative was higher to kidney biopsy material with amyloid deposits of the AA, Alambda or Akappa type compared with control kidney tissue. This higher binding was prevented by the presence of 10 microM Congo red in the incubation medium. The diethyl ester of 9Tcm-MAMA-CG did not demonstrate increased binding to Congo red-positive kidney tissue. In conclusion, 99Tcm-MAMA-CG and 99Tcm-Me4MAMA-CG localize specifically to amyloid deposits in human kidney tissue, suggesting that these tracer agents may be applicable as specific targeting agents for diagnostic purposes in clinical amyloidosis.

Impaired degradation of serum amyloid A (SAA) protein by cytokine-stimulated monocytes

Secondary amyloidosis (AA amyloidosis) is a systemic disease characterized by the extracellular tissue deposition of insoluble amyloid A (AA) protein. Aberrant metabolism of serum amyloid A (SAA) by macrophages is only one of many putative mechanisms which may be important in AA amyloidogenesis. In this study, we investigated the effects of cytokines on human monocyte-mediated SAA proteolysis. Human peripheral blood mononuclear cells (PBMC) or CD14(+) monocytes were cultured with SAA, and the culture supernatants were then subjected to anti-SAA immunoblot. CD14(+) monocytes degraded SAA completely. Whereas, when CD14(+) monocytes were pretreated with IL-1 beta or IFN-gamma, increasing amounts of SAA-related derivatives were detected in culture supernatants. These findings suggest that activation of monocytes by IL-1 beta or IFN-gamma hampers the proteolysis of a precursor protein and leads to a partial degradation of SAA. This down-regulated proteolysis of SAA protein by cytokine-stimulated monocytes may play a role in the mechanism of AA amyloid formation as well as its removal.

Pathogenesis, diagnosis and treatment of systemic amyloidosis

Amyloidosis is a disorder of protein folding in which normally soluble proteins are deposited as abnormal, insoluble fibrils that disrupt tissue structure and cause disease. Although about 20 different unrelated proteins can form amyloid fibrils in vivo, all such fibrils share a common cross-beta core structure. Some natural wild-type proteins are inherently amyloidogenic, form fibrils and cause amyloidosis in old age or if present for long periods at abnormally high concentration. Other amyloidogenic proteins are acquired or inherited variants, containing amino-acid substitutions that render them unstable so that they populate partly unfolded states under physiological conditions, and these intermediates then aggregate in the stable amyloid fold. In addition to the fibrils, amyloid deposits always contain the non-fibrillar pentraxin plasma protein, serum amyloid P component (SAP), because it undergoes specific calcium-dependent binding to amyloid fibrils. SAP contributes to amyloidogenesis, probably by stabilizing amyloid fibrils and retarding their clearance. Radiolabelled SAP is an extremely useful, safe, specific, non-invasive, quantitative tracer for scintigraphic imaging of systemic amyloid deposits. Its use has demonstrated that elimination of the supply of amyloid fibril precursor proteins leads to regression of amyloid deposits with clinical benefit. Current treatment of amyloidosis comprises careful maintenance of impaired organ function, replacement of end-stage organ failure by dialysis or transplantation, and vigorous efforts to control underlying conditions responsible for production of fibril precursors. New approaches under development include drugs for stabilization of the native fold of precursor proteins, inhibition of fibrillogenesis, reversion of the amyloid to the native fold, and dissociation of SAP to accelerate amyloid fibril clearance in vivo.

Role of serum amyloid P component in bacterial infection: protection of the host or protection of the pathogen

Serum amyloid P component (SAP) binds to Streptococcus pyogenes, and we show here that it also binds to Neisseria meningitidis, including a lipopolysaccharide (LPS)-negative mutant, and to rough variants of Escherichia coli. Surprisingly, this binding had a powerful antiopsonic effect both in vitro and in vivo, reducing phagocytosis and killing of bacteria. Furthermore, SAP knockout mice survived lethal infection with S. pyogenes and rough E. coli J5, organisms to which SAP binds. The susceptibility of SAP(-/-) mice was fully restored by injection of isolated human SAP. However, SAP(-/-) mice were more susceptible than wild-type animals to lethal infection with E. coli O111:B4, a smooth strain to which SAP does not bind, suggesting that SAP also has some host defense function. Although SAP binds to LPS in vitro, SAP(-/-) mice were only marginally more susceptible to lethal LPS challenge, and injection of large amounts of human SAP into wild-type mice did not affect sensitivity to LPS, indicating that SAP is not a significant modulator of LPS toxicity in vivo. In contrast, the binding of SAP to pathogenic bacteria enabled them to evade neutrophil phagocytosis and display enhanced virulence. Abrogation of this molecular camouflage is thus potentially a novel therapeutic approach, and we show here that administration to wild-type mice of (R)-1-[6-(R)-2-carboxy-pyrrolidin-1-yl]-6-oxo-hexanoyl]pyrrolidine -2- carboxylic acid, a drug that inhibits SAP binding, significantly prolonged survival during lethal infection with E. coli J5.

Novel approaches to the treatment of primary amyloidosis

Primary (AL, amyloid light-chain) amyloidosis is a plasma cell disorder in which deposits of amyloid light-chain protein cause progressive organ failure. It is important to recognise that amyloidosis is a dynamic process and chemotherapy-induced reduction of the activity of the plasma cell clone reduces the supply of the amyloid precursor protein and can result in a major regression of the deposits. The most common target organ is the kidney and renal amyloidosis manifests as proteinuria or nephrotic syndrome. Proteinuria is seen in three quarters of patients. Amyloid related nephrotic syndrome and renal failure are potentially reversible. Fatigue, congestive heart failure, hepatomegaly, peripheral neuropathy, orthostatic hypotension, carpal tunnel syndrome and macroglossia are other common features. The median survival is one to two years. Conventional-dose melphalan as standard treatment can prolong the median duration of survival by about ten months, but the clinical response rates with improvement of impaired organ function are low. Up-front high-dose chemotherapy with autologous peripheral blood stem cell transplantation is much more effective and can result in a major improvement in the clinical condition of patients. However, the toxicity related to this treatment can be relevant due to impaired organ function. Conventional-dose chemotherapy consisting of vincristine, doxorubicin and dexamethasone or high-dose dexamethasone or interferon-alpha are other possible approaches to treatment. The improvement of patient condition with an effective conventional-dose chemotherapy may increase the tolerability of high-dose chemotherapy and reduce transplantation related problems.

Alzheimer's amyloid fibrils: structure and assembly

Structural studies of Alzheimer's amyloid fibrils have revealed information about the structure at different levels. The amyloid-beta peptide has been examined in various solvents and conditions and this has led to a model by which a conformational switching occurs from alpha-helix or random coil, to a beta-sheet structure. Amyloid fibril assembly proceeds by a nucleation dependent pathway leading to elongation of the fibrils. Along this pathway small oligomeric intermediates and short fibrillar structures (protofibrils) have been observed. In cross-section the fibril appears to be composed of several subfibrils or protofilaments. Each of these protofilaments is composed of beta-sheet structure in which hydrogen bonding occurs along the length of the fibre and the beta-strands run perpendicular to the fibre axis. This hierarchy of structure is discussed in this review.

Role of STAT3 and C/EBP in cytokine-dependent expression of the mouse serum amyloid P-component (SAP) and C-reactive protein (CRP) genes

Inflammation is accompanied by a rapid increase in blood levels of acute phase proteins synthesized by hepatocytes in response to cytokines. Although C-reactive protein (CRP) levels increase dramatically in most mammals, the major acute phase protein in the mouse is the homologous pentraxin, serum amyloid P-component (SAP), whereas CRP is a minor acute phase reactant. The molecular basis for the pronounced difference in SAP and CRP gene expression in the mouse is unknown. Transfection of ++/Li mouse hepatoma cells with CAT-reporter constructs containing the 5'-flanking region of the mouse CRP gene indicated that transcription was stimulated by either IL-6, or IL-6 plus IL-1, when > or =360 bp of the 5'-proximal DNA was present. Examination of the 5'-flanking region of the mouse SAP gene revealed that the region between -433 and -397 from the transcription start site responded to IL-1 and IL-6 by binding both STAT3 and C/EBPbeta. This responsive region consisted of two adjacent C/EBPbeta consensus sites that overlap with two STAT3 consensus sites and was found to bind C/EBPbeta at an upstream site of -427 to -409 and STAT3 at a downstream site of -415 to -397. By contrast, the 360 bp promoter of the CRP gene was bound only by STAT3 at consensus sites at -93, -142, -173, and -287 from the start site; however, a single consensus site for C/EBP at -75 was not recognized. STAT3 appears to be necessary for both mouse SAP and CRP gene transcription since overexpression of an inactive, deletion mutant of STAT3 inhibited transcription of both genes. The results indicate that both STAT3 and C/EBPbeta participate in mouse SAP gene expression, whereas only STAT3 is involved in mouse CRP gene expression. The findings for mouse SAP gene expression are consistent with the reported interaction between these two transcription factors for human CRP gene transcription.

[Mechanisms of amyloidosis and the proteins involved]

INTRODUCTION

Recent data in amyloid research have shed light on the amyloid substance and have broadened our knowledge on the mechanism of amyloid deposition.

CURRENT KNOWLEDGE AND KEY POINTS

Despite uniform physical properties relating to the presence of beta-pleates, amyloid deposits are chemically heterogeneous and have different origins; additional types will probably be described in the future. Immunohistochemical techniques using specific antisera for each of the major protein present in fibrils could help greatly to subclassify these disorders. In most circumstances, a circulating precursor protein may result from overproduction of either intact or aberrant molecule, a reduction in its degradation or excretion, or genetic abnormalities associated with variant proteins. The cleavage of protein precursor molecules of the protein component of amyloid fibrils characterizes amyloidogenesis, though it is not necessary for some amyloidosis forms. This review summarizes advances in the understanding of the nature of amyloid substances, the mechanism of amyloid deposition and the principal pathogenic hypothesis.

FUTURE PROSPECTS AND PROJECTS

SAP component is common in all amyloidosis and may be the target for future therapy.

Microscale characterization of the structure-activity relationship of a heparin-binding glycopeptide using affinity capillary electrophoresis and immobilized enzymes

A heparin-binding glycopeptide (T3) from human serum amyloid P component was characterized by taking advantage of two important features of capillary electrophoresis: the low sample consumption and the possibility of doing on-line binding studies. Incubations with neuraminidase and proteolytic enzymes were carried out with enzymes immobilized on paramagnetic microbeads. Affinity capillary electrophoresis subsequently was used to characterize T3 and its fragments with respect to heparin binding. We find that an intact glycan moiety makes the C-terminal part of T3 relatively resistant to chymotryptic clevage. This protection is lost upon desialylation. Also, the C-terminus of T3 is involved in heparin binding while the N-terminal part of the molecule has no appreciable binding activity. The micromethods presented here make it feasible to perform structure-function studies even on the small amounts of analytes that are typically available when working with glycopeptides from natural sources.

Isoforms of murine and human serum amyloid P component

Isoelectric focusing (IEF) and immunofixation of murine serum amyloid P component (SAP), purified and in serum, showed a distinct and strain-dependent isoform pattern with up to seven bands (pI 5.1-5.7). Neuraminidase treatment caused a shift of the isoforms to more basic pI values, but did not affect their number. When the acute-phase response was analysed in three mouse strains, CBA/J and C3H/HeN initially showed seven SAP isoforms in serum and C57BL/6 J three or four. The responses in all three strains peaked at day 2 and were normalized within 14 days. On days 2 and 4, CBA/J and C3H/HeN mice showed one more acidic isoform and an increase in the concentration of the most basic isoform. C57BL/6 J mice exhibited two to three new isoforms during the acute-phase response. This appears to be the first demonstration of the physiological existence of SAP isoforms. In contrast, demonstration of isoforms of human SAP required the presence of urea and higher SAP concentrations. TEF and immunofixation of SAP monomers showed five to eight isoforms, ranging from pI 4.7-5.7. IEF of SAP in human serum resulted in a less distinct pattern and more acidic isoforms. As with murine SAP, neuraminidase treatment caused a shift of the isoforms, but no reduction in isoform number. Two-dimensional gel electrophoresis confirmed the existence of multiple isoforms of human SAP monomers.

A heparin-binding peptide from human serum amyloid P component characterized by affinity capillary electrophoresis

Affinity capillary electrophoresis (CE) was used for a detailed characterization of the binding between heparin and a peptide isolated from the heparin-binding serum protein amyloid P component (SAP). The peptide corresponds to a tryptic fragment (T3) comprising amino acids 14-38 of SAP. By including ligands in the electrophoresis buffer various glycosaminoglycans could be screened for binding of T3 using one sample aliquot. The binding was found to be highly specific for heparin and heparin fragments down to tetramers and appeared strongest at a slightly alkaline pH while no binding could be demonstrated with heparan sulfate, chondroitin sulfate, desulfated heparin, mannose 6-phosphate and phosphotyrosine. The T3-heparin complexes were sufficiently stable to perform quantitative measurements of the binding using preequilibration of samples prior to a CE-mediated separation of bound and free T3-peptide. Plots based on quantitation of analyte peaks corresponding to free and complexed T3 yielded a dissociation constant of 1.5 microM for the interaction with heparin. The results indicate that a specific subfraction of the heparin molecules is active in binding interactions with the peptide. The affinity CE approach proved to be useful for these studies because of its sensitivity to complex formation involving charged ligands and the possibility of achieving separations under native conditions. Also advantageous is the low sample consumption and the ability to analyze unlabeled reactants in solution.

Comparison between intact and desialylated human serum amyloid P component by laser photo CIDNP (chemically induced dynamic nuclear polarization) technique: an indication for a conformational impact of sialic acid

The human pentraxin serum amyloid P component (SAP) exhibits no microheterogeneity in its complex di-antennary glycan. To elucidate whether the removal of sialic acids from this glycoprotein might affect the accessibility of certain amino acid residues of the protein we employed the laser photo CIDNP approach as a sensitive tool. The CIDNP effect is generated by the interaction of a photoexcited dye with reactive amino acids and results in enhanced absorption- or emission-signals which can be observed for the three aromatic amino acids histidine, tryptophan, and tyrosine if they are accessible to the dye. Therefore, this technique can be applied to explore surface exposure of these amino acid residues. The respective spectra of SAP and enzymatically desialylated SAP were determined. Six tryptophan/histidine signals and one tyrosine signal are present in the aromatic part of the CIDNP difference spectrum of SAP. The corresponding spectrum of desialylated SAP shows remarkable alterations. The chemical shift of one Trp/His-characteristic signal is decreased by 0.1 ppm. One Trp/His-signal disappeared and a new one was formed in the CIDNP difference spectrum of desialylated SAP, while the other signals were unaffected. The Tyr signal has a clearly enhanced intensity in desialylated SAP. Therefore, the removal of sialic acid moieties from the single N-glycan of each monomer apparently affects surface presentation of distinct CIDNP-reactive amino acids of SAP [1]. A conformational change of the protein part of SAP in relation with a different orientation of the desialylated oligosaccharide chain in comparison to the complete one is a possible explanation of our CIDNP results.

Pentameric and decameric structures in solution of serum amyloid P component by X-ray and neutron scattering and molecular modelling analyses

Human serum amyloid P component (SAP) is a normal plasma glycoprotein and the precursor of amyloid P component which is a universal constituent of the abnormal tissue deposits in amyloidosis. X-ray and neutron scattering data showed that pentameric or decameric ring structures for SAP in solution are readily distinguished. Further neutron data collection showed that SAP pentamers were reproducibly obtained in the presence of Ca2+ at pH 5.5 or in the presence of methyl 4,6-O-(1-carboxyethylidene)-beta-d-galactopyranoside (MObetaDG) and Ca2+ at pH 6.0 to 8.0, while SAP decamers were obtained in the presence of EDTA between pH 5.5 and 8.0. SAP pentamers have a mean X-ray RG of 3.99(+/-0.11) nm and a mean neutron RG of 3.69(+/-0.12) nm in 100% 2H2O. SAP decamers have a mean X-ray RG of 4.23(+/-0.12) nm and a mean neutron RG of 4.09(+/-0.14) nm in 100% 2H2O. The absorption coefficients of SAP pentamers and decamers differ by 10%. If we infer that the two alpha-helical A-faces are in contact with each other in the SAP decamer, the lack of structural change of the decamer with pH may be explained by the absence of His residues from the A-face of the SAP pentamer, and the change in absorption coefficients is compatible with the presence of Trp residues at this A-face. The rigid ring structure of pentameric SAP provided a test of scattering curves calculated from crystal structures. The only structural unknown is the orientation of the five chemically homogeneous oligosaccharide chains relative to the protein, but extended oligosaccharide structures were found to account for its scattering curve. X-ray scattering curves were best calculated using a hydrated structure, while neutron scattering curves were best calculated using an unhydrated structure. The outcome of these analyses was used to model the structure of decameric SAP. The evaluation of 640 structures for two SAP pentamers brought face-to-face to form SAP decamers gave better curve fits for structures in which the two A-faces were in contact with each other, in which it is likely that the two pentamers were out of alignment by a rotation of 36 degrees and the oligosaccharide chains were extended.

Amyloid deposition is delayed in mice with targeted deletion of the serum amyloid P component gene

The tissue amyloid deposits that characterize systemic amyloidosis, Alzheimer's disease and the transmissible spongiform encephalopathies always contain serum amyloid P component (SAP) bound to the amyloid fibrils. We have previously proposed that this normal plasma protein may contribute to amyloidogenesis by stabilizing the deposits. Here we show that the induction of reactive amyloidosis is retarded in mice with targeted deletion of the SAP gene. This first demonstration of the participation of SAP in pathogenesis of amyloidosis in vivo confirms that inhibition of SAP binding to amyloid fibrils is an attractive therapeutic target in a range of serious human diseases.

Crystal structure of a decameric complex of human serum amyloid P component with bound dAMP

Serum amyloid P component (SAP) is a glycoprotein that binds in a calcium-dependent fashion to a variety of ligands including other proteins, glycosaminoglycans and DNA. SAP is universally associated with the amyloid deposits in all forms of amyloidoses including Alzheimer's disease. Small-molecule ligands that displace SAP from amyloid fibrils and thereby expose the fibrils to proteolytic clearance mechanisms hold potential as drugs for the prevention and treatment of amyloidosis. We have carried out a screen for novel SAP ligands and have identified 2'-deoxyadenosine-5'-monophosphate (dAMP) as a ligand. The crystal structure of the SAP-dAMP complex determined at 2.8 A resolution (R = 0.232, R(free) = 0.252) reveals a decamer in which all interactions between SAP pentamers are mediated by the ligand. The stability of the decamer in solution has been demonstrated by gel filtration chromatography. The two calcium ions of SAP are bridged by the dAMP phosphate group and five hydrogen bonds are formed between the protein and the ligand, including specific interactions made by the adenine base. This mode of dAMP binding is not compatible with the nucleotide being part of double-helical DNA. The SAP-dAMP decamer is stabilized mainly by base-stacking of adjacent ligand molecules and possibly by electrostatic interactions involving the dAMP phosphate groups; decamerization buries 1000 A2 (2.6%) of the pentamer solvent-accessible surface. Ligand-induced decamerization of SAP, which utilizes the high cooperativity of a multiple-site interaction, may be a strategy to overcome the problems for drug design associated with the rather modest affinities of SAP for small-molecule ligands.

Analysis of amyloid deposition in a transgenic mouse model of homozygous familial amyloidotic polyneuropathy

Amyloid fibrils derived from the Japanese, Portuguese, and Swedish types of familial amyloidotic polyneuropathy all consist of a variant transthyretin (TTR) with a substitution of methionine for valine at position 30 (TTR Met 30). In an attempt to establish an animal model of TTR Met-30-associated homozygous familial amyloidotic polyneuropathy and to study the structural and functional properties of human TTR Met 30, we generated a mouse line carrying a null mutation at the endogenous ttr locus (ttr-/-) and the human mutant ttr gene (6.0-hMet 30) as a transgene. In these mice, human TTR Met-30-derived amyloid deposits were first observed in the esophagus and stomach when the mice were 11 months of age. With advancing age, amyloid deposits extended to various other tissues. Because no significant difference was detected in the onset, progression, and tissue distribution of amyloid deposition between the ttr-/- and ttr+/+ transgenic mice expressing 6.0-hMet 30, endogenous normal mouse TTR probably does not affect the deposition of human TTR Met-30-derived amyloid in mice. TTR is a tetramer composed of four identical subunits that binds thyroxine (T4) and plasma retinol-binding protein. The introduction of 6.0-hMet 30 into the ttr-/- mice significantly increased their depressed serum levels of T4 and retinol-binding protein, suggesting that human TTR Met 30 binds T4 and retinol-binding protein in vivo. The T4-binding ability of human TTR Met 30 was confirmed by the analysis of T4-binding proteins in the sera of ttr-/- transgenic mice expressing 6.0-hMet 30. The T4-binding studies also demonstrated the presence of hybrid tetramers between mouse and human TTR subunits in the ttr+/+ transgenic mice expressing 6.0-hMet 30.

Biology and therapy of immunoglobulin deposition diseases

All forms of MIDD represent pathologic deposition of immunoglobulin as amorphous casts, crystals, congophilic fibrils (in AL amyloid), or punctate noncongophilic deposits (in LCDD/HCDD/LHCDD). Diagnosis is based on identification and immunohistochemical characterization of deposits and Congo red staining. Current information including development of novel in vitro and in vivo models suggests a contributory role of both protein and host factors in the pathogenesis of these disorders. In particular, primary structural features of the VL portions of the light chain molecule may affect not only the extent but also the morphologic type of protein deposits. Thus, certain types of light chains may be particularly pathogenic, although the nature or extent of proteolysis/processing involved in the pathogenesis of these deposits is yet unclear. Recent data also point to the importance of accessory molecules, cytokines, and host factors in this process. Newer therapeutic approaches using high-dose therapy with cytotoxic agents or dexamethasone appear promising, although these data need to be confirmed in a larger number of patients. The serendipitous discovery of I-DOX as an agent capable of promoting amyloid resorption provides another novel approach in patients with AL amyloidosis. Continued research on the mechanisms of deposition and resorption of these immunoglobulin deposits should provide important information that can be used to design strategies for more effective therapy and, ultimately, prevention of MIDD.

Glycosylation: heterogeneity and the 3D structure of proteins

Glycoproteins generally exist as populations of glycosylated variants (glycoforms) of a single polypeptide. Although the same glycosylation machinery is available to all proteins that enter the secretory pathway in a given cell, most glycoproteins emerge with characteristic glycosylation patterns and heterogeneous populations of glycans at each glycosylation site. The factors that control the composition of the glycoform populations and the role that heterogeneity plays in the function of glycoproteins are important questions for glycobiology. A full understanding of the implications of glycosylation for the structure and function of a protein can only be reached when a glycoprotein is viewed as a single entity. Individual glycoproteins, by virtue of their unique structures, can selectively control their own glycosylation by modulating interactions with the glycosylating enzymes in the cell. Examples include protein-specific glycosylation within the immunoglobulins and immunoglobulin superfamily and site-specific processing in ribonuclease, Thy-1, IgG, tissue plasminogen activator, and influenza A hemagglutinin. General roles for the range of sugars on glycoproteins such as the leukocyte antigens include orientating the molecules on the cell surface. A major role for specific sugars is in recognition by lectins, including chaperones involved in protein folding. In addition, the recognition of identical motifs in different glycans allows a heterogeneous population of glycoforms to participate in specific biological interactions.

Ligand-binding sites in human serum amyloid P component

Amyloid P component (AP) is a naturally occurring glycoprotein that is found in serum and basement membranes. AP is also a component of all types of amyloid, including that found in individuals who suffer from Alzheimer's disease and Down's syndrome. Because AP has been found to bind strongly and specifically to certain glycosaminoglycans that are components of amyloid deposits, AP may play an important role in the maintenance of amyloid. In the present work, we isolated and identified two proteolytic fragments of AP that are responsible for its heparin-binding activity. Neither fragment corresponds to published heparin-binding sequences. The structural requirements for activity of the peptides (amino acid residues 27-38 and 192-203 of AP) were examined by means of solid-phase inhibition assays with synthetic peptides. AP-(192-203)-peptide inhibits the Ca(2+)-dependent binding of AP to heparin with an IC50 of 25 microM, while the IC50 of AP-(27-38)-peptide and AP-(33-38)-peptide are 10 microM and 2 microM, respectively. The understanding of the structure and function of active AP peptides will be useful for development of amyloid-targeted diagnostics and therapeutics.

Hereditary hepatic and systemic amyloidosis caused by a new deletion/insertion mutation in the apolipoprotein AI gene

We report a Spanish family with autosomal-dominant non-neuropathic hereditary amyloidosis with a unique hepatic presentation and death from liver failure, usually by the sixth decade. The disease is caused by a previously unreported deletion/insertion mutation in exon 4 of the apolipoprotein AI (apoAI) gene encoding loss of residues 60-71 of normal mature apoAI and insertion at that position of two new residues, ValThr. Affected individuals are heterozygous for this mutation and have both normal apoAI and variant molecules bearing one extra positive charge, as predicted from the DNA sequence. The amyloid fibrils are composed exclusively of NH2-terminal fragments of the variant, ending mainly at positions corresponding to residues 83 and 92 in the mature wild-type sequence. Amyloid fibrils derived from the other three known amyloidogenic apoAI variants are also composed of similar NH2-terminal fragments. All known amyloidogenic apoAI variants carry one extra positive charge in this region, suggesting that it may be responsible for their enhanced amyloidogenicity. In addition to causing a new phenotype, this is the first deletion mutation to be described in association with hereditary amyloidosis and it significantly extends the value of the apoAI model for investigation of molecular mechanisms of amyloid fibrillogenesis.

Serum amyloid P component prevents proteolysis of the amyloid fibrils of Alzheimer disease and systemic amyloidosis

Extracellular deposition of amyloid fibrils is responsible for the pathology in the systemic amyloidoses and probably also in Alzheimer disease [Haass, C. & Selkoe, D. J. (1993) Cell 75, 1039-1042] and type II diabetes mellitus [Lorenzo, A., Razzaboni, B., Weir, G. C. & Yankner, B. A. (1994) Nature (London) 368, 756-760]. The fibrils themselves are relatively resistant to proteolysis in vitro but amyloid deposits do regress in vivo, usually with clinical benefit, if new amyloid fibril formation can be halted. Serum amyloid P component (SAP) binds to all types of amyloid fibrils and is a universal constituent of amyloid deposits, including the plaques, amorphous amyloid beta protein deposits and neurofibrillary tangles of Alzheimer disease [Coria, F., Castano, E., Prelli, F., Larrondo-Lillo, M., van Duinen, S., Shelanski, M. L. & Frangione, B. (1988) Lab. Invest. 58, 454-458; Duong, T., Pommier, E. C. & Scheibel, A. B. (1989) Acta Neuropathol. 78, 429-437]. Here we show that SAP prevents proteolysis of the amyloid fibrils of Alzheimer disease, of systemic amyloid A amyloidosis and of systemic monoclonal light chain amyloidosis and may thereby contribute to their persistence in vivo. SAP is not an enzyme inhibitor and is protective only when bound to the fibrils. Interference with binding of SAP to amyloid fibrils in vivo is thus an attractive therapeutic objective, achievement of which should promote regression of the deposits.

Glycosylation of IgG during potentially arthritogenic lentiviral infections

Agalactosyl IgG [Gal(0)] was first discovered in patients with rheumatoid arthritis (RA). However, the proportion of this glycoform is also raised in tuberculosis and leprosy. This has helped reinforce the suggestion that RA may be triggered by a mycobacterium-like slow bacterial infection. On the other hand, arthritis can occur in mycobacterial diseases, so raised Gal(0) could be associated with a tendency to arthritis, rather than with a particular type of infection. Therefore, we wished to find out whether the percentage of Gal(0) [%Gal(0)] is increased in sheep and goats following infection with maedi visna virus or caprine arthritis encephalitis virus (CAEV), both of which can lead to inflammatory synovitis. We found that the normal level of Gal(0) in these species is much lower than in humans. Goats infected with CAEV or Mycobacterium paratuberculosis (used as a control mycobacterial infection) had a significant increase in %Gal(0), though it was still below the level seen in normal humans. Studies by Western blot confirmed the presence of terminal N-acetylglucosamine on heavy chains, and percentages of Gal(0) comparable to those seen in human RA could be generated by exposing goat IgG to streptococcal beta-galactosidase. The rise in %Gal(0) was greatest in members of infected herds that were just starting to manifest arthritis, and tended to be lower in those in which severe carpitis had developed at the time of bleeding, implying the possibility that raise %Gal(0) may be an early or predisposing event for the development of arthritis.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of serum amyloid P component from human aortic atherosclerotic lesions

Serum amyloid P component (SAP) is a glycoprotein in human plasma. We recently showed the localization of SAP in human atherosclerotic lesions by immunohistochemical staining. In this study, the presence of SAP in atherosclerotic lesions was confirmed, and the biochemical character of SAP in atherosclerotic intima was investigated and compared with that of native SAP. Atherosclerotic intima was sequentially extracted with 2 mmol/L CaCl2-Tris-buffered saline (TBS), 10 mmol/L EDTA-TBS, 3 mol/L guanidine-TBS, and collagenase digestion. The character of SAP in each extract was studied with double immunodiffusion, electroimmunoassay, crossed immunoelectrophoresis, and Western immunoblotting. The total amount of SAP in atherosclerotic intima was 190 +/- 64 micrograms/g wet tissue with an SAP-albumin ratio of 1:22.7, which is 44 times higher than the relative plasma ratio of 1:1000. This suggests that SAP is specifically localized in atherosclerotic lesions. SAP from the intima was indistinguishable from plasma or purified SAP with respect to immunological character and molecular weight. However, electrophoretic mobility and the binding of SAP to atherosclerotic intima appeared heterogeneous. Of total extractable SAP, about 43% appeared in the CaCl2-TBS fraction, 25% in the EDTA-TBS fraction, and 32% in the collagenase digestion fraction. SAP is one of the two pentraxins in human plasma; the other is C-reactive protein, which has also been reported to locate in atherosclerotic lesions. Our findings suggest a role for SAP in atherogenesis and encourage efforts to determine more precisely the physiological contributions of the pentraxin family to the development of atherosclerosis.

Structure and function of the pentraxins

Over the past two years, the three-dimensional structure of the serum amyloid P component was defined by X-ray diffraction, the first such visualization of a pentraxin. Binding sites for calcium, ligands and complement were identified. New fusion proteins with amino acid sequence homology to the pentraxins were described, and new insights were gained into pentraxin phylogeny, biosynthesis, ligands, complement activation, leukocyte reactivity and biological functions in vivo.

The pentraxins, C-reactive protein and serum amyloid P component, are cleared and catabolized by hepatocytes in vivo

The cellular sites of clearance and degradation of the pentraxin plasma proteins, C-reactive protein, the classical acute phase reactant, and serum amyloid P component (SAP), a universal constituent of amyloid deposits, were sought using the ligand 125I-tyramine cellobiose (TC) which is substantially retained within the cells in which catabolism takes place. Pentraxins labeled with 125I-TC showed the same in vitro and in vivo ligand binding and the same in vivo plasma t1/2 as the directly iodinated proteins and the native unlabeled pentraxins, indicating that their mode of clearance was likely to be physiological. After intravenous injection into mice and rabbits of human C-reactive protein, human SAP, and mouse SAP, each labeled with 125I-TC, most of the radioactivity remaining in the body at 24 h was located in hepatocytes. None was detected in other liver cells, and only traces were present in other viscera; the rest was in the carcass, representing intact pentraxins in the blood and extravascular compartment, and escaped label which had not yet been excreted. Hepatocytes are thus the single major site of pentraxin clearance and catabolism in vivo. This is consistent with the observation that SAP that has localized to amyloid deposits persists there and is not degraded.