Fibril formation from recombinant human serum amyloid A

Proteomic analysis shows that the main constituent of subepidermal localised cutaneous amyloidosis is not galectin-7

Lichen or macular localised cutaneous amyloidoses have long been described as keratinic amyloidoses and believed to be due to the deposition of cytokeratin peptides originating from epidermis in the dermal papillae. However, recently it was suggested that galectin-7 is the causative protein for this type of amyloidosis. This was based on the detection of galectin-7 in a biopsy from a patient diagnosed with Bowen's disease and localised cutaneous amyloidosis. In this study we report mass spectrometry-based proteomic analysis of the protein composition of localised cutaneous amyloid deposits from seven patients using laser microdissection and show that basal keratins are the main constituents of the amyloid deposits. Galectin-7 was not present in the dermal amyloid deposits and was only present in the overlying Congo red negative epidermis.

Morphological and primary structural consistency of fibrils from different AA patients (common variant)

Aims: To test the hypothesis that the fibril morphology and the fibril protein primary structure are conserved across different patients suffering from the common variant of systemic Amyloid A (AA) amyloidosis. Methods: Amyloid fibrils were extracted from the renal tissue of four patients. The fibril morphology was analysed in negatively stained samples with transmission electron microscopy (TEM). The fibril protein identity and fragment length were determined by using mass spectrometry. Results: The fibrils show a consistent morphology in all four patients and exhibit an average width of ∼9.6 nm and an average pitch of ∼112 nm. All fibrils are composed of polypeptide chains that can be assigned to human serum amyloid A (SAA) 1.1 protein. All fragments lack the N-terminal arginine residue and are C-terminally truncated. Differences exist concerning the exact C-terminal cleavage site. The most prominent cleavage site occurs at residues 64-67. Conclusions: Our data demonstrate that AA amyloid fibrils are consistent at the level of the protein primary structure and fibril morphology in the four analysed patients.

Serum amyloid A forms stable oligomers that disrupt vesicles at lysosomal pH and contribute to the pathogenesis of reactive amyloidosis

Serum amyloid A (SAA) is an acute-phase plasma protein that functions in innate immunity and lipid homeostasis. SAA is a protein precursor of reactive AA amyloidosis, the major complication of chronic inflammation and one of the most common human systemic amyloid diseases worldwide. Most circulating SAA is protected from proteolysis and misfolding by binding to plasma high-density lipoproteins. However, unbound soluble SAA is intrinsically disordered and is either rapidly degraded or forms amyloid in a lysosome-initiated process. Although acidic pH promotes amyloid fibril formation by this and many other proteins, the molecular underpinnings are unclear. We used an array of spectroscopic, biochemical, and structural methods to uncover that at pH 3.5-4.5, murine SAA1 forms stable soluble oligomers that are maximally folded at pH 4.3 with ∼35% α-helix and are unusually resistant to proteolysis. In solution, these oligomers neither readily convert into mature fibrils nor bind lipid surfaces via their amphipathic α-helices in a manner typical of apolipoproteins. Rather, these oligomers undergo an α-helix to β-sheet conversion catalyzed by lipid vesicles and disrupt these vesicles, suggesting a membranolytic potential. Our results provide an explanation for the lysosomal origin of AA amyloidosis. They suggest that high structural stability and resistance to proteolysis of SAA oligomers at pH 3.5-4.5 help them escape lysosomal degradation, promote SAA accumulation in lysosomes, and ultimately damage cellular membranes and liberate intracellular amyloid. We posit that these soluble prefibrillar oligomers provide a missing link in our understanding of the development of AA amyloidosis.

Influence of C-terminal truncation of murine Serum amyloid A on fibril structure

Amyloid A (AA) amyloidosis is a systemic protein misfolding disease affecting humans and other vertebrates. While the protein precursor in humans and mice is the acute-phase reactant serum amyloid A (SAA) 1.1, the deposited fibrils consist mainly of C-terminally truncated SAA fragments, termed AA proteins. For yet unknown reasons, phenotypic variations in the AA amyloid distribution pattern are clearly associated with specific AA proteins. Here we describe a bacterial expression system and chromatographic strategies to obtain significant amounts of C-terminally truncated fragments of murine SAA1.1 that correspond in truncation position to relevant pathological AA proteins found in humans. This enables us to investigate systematically structural features of derived fibrils. All fragments form fibrils under nearly physiological conditions that show similar morphological appearance and amyloid-like properties as evident from amyloid-specific dye binding, transmission electron microscopy and infrared spectroscopy. However, infrared spectroscopy suggests variations in the structural organization of the amyloid fibrils that might be derived from a modulating role of the C-terminus for the fibril structure. These results provide insights, which can help to get a better understanding of the molecular mechanisms underlying the different clinical phenotypes of AA amyloidosis.

Structure and Expression of Different Serum Amyloid A (SAA) Variants and their Concentration-Dependent Functions During Host Insults

Serum amyloid A (SAA) is, like C-reactive protein (CRP), an acute phase protein and can be used as a diagnostic, prognostic or therapy follow-up marker for many diseases. Increases in serum levels of SAA are triggered by physical insults to the host, including infection, trauma, inflammatory reactions and cancer. The order of magnitude of increase in SAA levels varies considerably, from a 10- to 100-fold during limited inflammatory events to a 1000-fold increase during severe bacterial infections and acute exacerbations of chronic inflammatory diseases. This broad response range is reflected by SAA gene duplications resulting in a cluster encoding several SAA variants and by multiple biological functions of SAA. SAA variants are single-domain proteins with simple structures and few post-translational modifications. SAA1 and SAA2 are inducible by inflammatory cytokines, whereas SAA4 is constitutively produced. We review here the regulated expression of SAA in normal and transformed cells and compare its serum levels in various disease states. At low concentrations (10-100 ng/ml), early in an inflammatory response, SAA induces chemokines or matrix degrading enzymes via Toll-like receptors and functions as an activator and chemoattractant through a G protein-coupled receptor. When an infectious or inflammatory stimulus persists, the liver continues to produce more SAA (> 1000 ng/ml) to become an antimicrobial agent by functioning as a direct opsonin of bacteria or by interference with virus infection of host cells. Thus, SAA regulates innate and adaptive immunity and this information may help to design better drugs to treat specific diseases.

Characterization of reconstituted high-density lipoprotein particles formed by lipid interactions with human serum amyloid A

The acute-phase human protein serum amyloid A (SAA) is enriched in high-density lipoprotein (HDL) in patients with inflammatory diseases. Compared with normal HDL containing apolipoprotein A-I, which is the principal protein component, characteristics of acute-phase HDL containing SAA remain largely undefined. In the present study, we examined the physicochemical properties of reconstituted HDL (rHDL) particles formed by lipid interactions with SAA. Fluorescence and circular dichroism measurements revealed that although SAA was unstructured at physiological temperature, α-helix formation was induced upon binding to phospholipid vesicles. SAA also formed rHDL particles by solubilizing phospholipid vesicles through mechanisms that are common to other exchangeable apolipoproteins. Dynamic light scattering and nondenaturing gradient gel electrophoresis analyses of rHDL after gel filtration revealed particle sizes of approximately 10nm, and a discoidal shape was verified by transmission electron microscopy. Thermal denaturation experiments indicated that SAA molecules in rHDL retained α-helical conformations at 37°C, but were almost completely denatured around 60°C. Furthermore, trypsin digestion experiments showed that lipid binding rendered SAA molecules resistant to protein degradation. In humans, three major SAA1 isoforms (SAA1.1, 1.3, and 1.5) are known. Although these isoforms have different amino acids at residues 52 and 57, no major differences in physicochemical properties between rHDL particles resulting from lipid interactions with SAA isoforms have been found. The present data provide useful insights into the effects of SAA enrichment on the physicochemical properties of HDL.

Characterization of rat serum amyloid A4 (SAA4): a novel member of the SAA superfamily

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The full length rat SAA4 (rSAA4) mRNA was characterized by rapid amplification of cDNA ends.

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rSAA4 mRNA has 1830 bases including a GA dinucleotide tandem repeat in the 5′UTR.

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Three consecutive C/EBP promoter elements are crucial for transcription of rSAA4.

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rSAA4 is abundantly expressed in the liver on mRNA and protein level.

The serum amyloid A (SAA) family of proteins is encoded by multiple genes, which display allelic variation and a high degree of homology in mammals. The SAA1/2 genes code for non-glycosylated acute-phase SAA1/2 proteins, that may increase up to 1000-fold during inflammation. The SAA4 gene, well characterized in humans (hSAA4) and mice (mSaa4) codes for a SAA4 protein that is glycosylated only in humans. We here report on a previously uncharacterized SAA4 gene (rSAA4) and its product in Rattus norvegicus, the only mammalian species known not to express acute-phase SAA. The exon/intron organization of rSAA4 is similar to that reported for hSAA4 and mSaa4. By performing 5′- and 3′RACE, we identified a 1830-bases containing rSAA4 mRNA (including a GA-dinucleotide tandem repeat). Highest rSAA4 mRNA expression was detected in rat liver. In McA-RH7777 rat hepatoma cells, rSAA4 transcription was significantly upregulated in response to LPS and IL-6 while IL-1α/β and TNFα were without effect. Luciferase assays with promoter-truncation constructs identified three proximal C/EBP-elements that mediate expression of rSAA4 in McA-RH7777 cells. In line with sequence prediction a 14-kDa non-glycosylated SAA4 protein is abundantly expressed in rat liver. Fluorescence microscopy revealed predominant localization of rSAA4-GFP-tagged fusion protein in the ER.

Structural mechanism of serum amyloid A-mediated inflammatory amyloidosis

Serum amyloid A (SAA) represents an evolutionarily conserved family of inflammatory acute-phase proteins. It is also a major constituent of secondary amyloidosis. To understand its function and structural transition to amyloid, we determined a structure of human SAA1.1 in two crystal forms, representing a prototypic member of the family. Native SAA1.1 exists as a hexamer, with subunits displaying a unique four-helix bundle fold stabilized by its long C-terminal tail. Structure-based mutational studies revealed two positive-charge clusters, near the center and apex of the hexamer, that are involved in SAA association with heparin. The binding of high-density lipoprotein involves only the apex region of SAA and can be inhibited by heparin. Peptide amyloid formation assays identified the N-terminal helices 1 and 3 as amyloidogenic peptides of SAA1.1. Both peptides are secluded in the hexameric structure of SAA1.1, suggesting that the native SAA is nonpathogenic. Furthermore, dissociation of the SAA hexamer appears insufficient to initiate amyloidogenic transition, and proteolytic cleavage or removal of the C-terminal tail of SAA resulted in formation of various-sized structural aggregates containing ∼5-nm regular repeating protofibril-like units. The combined structural and functional studies provide mechanistic insights into the pathogenic contribution of glycosaminoglycan in SAA1.1-mediated AA amyloid formation.

SAA1 polymorphisms are associated with variation in antiangiogenic and tumor-suppressive activities in nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is a cancer that occurs in high frequency in Southern China. A previous functional complementation approach and the subsequent cDNA microarray analysis have identified that serum amyloid A1 (SAA1) is an NPC candidate tumor suppressor gene. SAA1 belongs to a family of acute-phase proteins that are encoded by five polymorphic coding alleles. The SAA1 genotyping results showed that only three SAA1 isoforms (SAA1.1, 1.3 and 1.5) were observed in both Hong Kong NPC patients and healthy individuals. This study aims to determine the functional role of SAA1 polymorphisms in tumor progression and to investigate the relationship between SAA1 polymorphisms and NPC risk. Indeed, we have shown that restoration of SAA1.1 and 1.3 in the SAA1-deficient NPC cell lines could suppress tumor formation and angiogenesis in vitro and in vivo. The secreted SAA1.1 and SAA1.3 proteins can block cell adhesion and induce apoptosis in the vascular endothelial cells. In contrast, the SAA1.5 cannot induce apoptosis or inhibit angiogenesis because of its weaker binding affinity to αVβ3 integrin. This can explain why SAA1.5 has no tumor-suppressive effects. Furthermore, the NPC tumors with this particular SAA1.5/1.5 genotype showed higher levels of SAA1 gene expression, and SAA1.1 and 1.3 alleles were preferentially inactivated in tumor tissues that were examined. These findings further strengthen the conclusion for the defective function of SAA1.5 in suppression of tumor formation and angiogenesis. Interestingly, the frequency of the SAA1.5/1.5 genotype in NPC patients was ~2-fold higher than in the healthy individuals (P=0.00128, odds ratio=2.28), which indicates that this SAA1 genotype is significantly associated with a higher NPC risk. Collectively, this homozygous SAA1.5/1.5 genotype appears to be a recessive susceptibility gene, which has lost the antiangiogenic function, whereas SAA1.1 and SAA1.3 are the dominant alleles of the tumor suppressor phenotype.

Effect of amino acid variations in the central region of human serum amyloid A on the amyloidogenic properties

Human serum amyloid A (SAA) is a precursor protein of the amyloid fibrils that are responsible for AA amyloidosis. Of the four human SAA genotypes, SAA1 is most commonly associated with AA amyloidosis. Furthermore, SAA1 has three major isoforms (SAA1.1, 1.3, and 1.5) that differ by single amino acid variations at two sites in their 104-amino acid sequences. In the present study, we examined the effect of amino acid variations in human SAA1 isoforms on the amyloidogenic properties. All SAA1 isoforms adopted α-helix structures at 4°C, but were unstructured at 37°C. Heparin-induced amyloid fibril formation of SAA1 was observed at 37°C, as evidenced by the increased thioflavin T (ThT) fluorescence and β-sheet structure formation. Despite a comparable increase in ThT fluorescence, SAA1 molecules retained their α-helix structures at 4°C. At both temperatures, no essential differences in ThT fluorescence and secondary structures were observed among the SAA1 isoforms. However, the fibril morphologies appeared to differ; SAA1.1 formed long and curly fibrils, whereas SAA1.3 formed thin and straight fibrils. The peptides corresponding to the central regions of the SAA1 isoforms containing amino acid variations showed distinct amyloidogenicities, reflecting their direct effects on amyloid fibril formation. These findings may provide novel insights into the influence of amino acid variations in human SAA on the pathogenesis of AA amyloidosis.

Characterization of the oligomerization and aggregation of human Serum Amyloid A

The fibrillation of Serum Amyloid A (SAA) – a major acute phase protein – is believed to play a role in the disease Amyloid A (AA) Amyloidosis. To better understand the amyloid formation pathway of SAA, we characterized the oligomerization, misfolding, and aggregation of a disease-associated isoform of human SAA – human SAA1.1 (hSAA1.1) – using techniques ranging from circular dichroism spectroscopy to atomic force microscopy, fluorescence spectroscopy, immunoblot studies, solubility measurements, and seeding experiments. We found that hSAA1.1 formed alpha helix-rich, marginally stable oligomers in vitro on refolding and cross-beta-rich aggregates following incubation at 37°C. Strikingly, while hSAA1.1 was not highly amyloidogenic in vitro, the addition of a single N-terminal methionine residue significantly enhanced the fibrillation propensity of hSAA1.1 and modulated its fibrillation pathway. A deeper understanding of the oligomerization and fibrillation pathway of hSAA1.1 may help elucidate its pathological role.

Human SAA1-derived amyloid deposition in cell culture: a consistent model utilizing human peripheral blood mononuclear cells and serum-free medium

Amyloid A (AA) amyloidosis is a fatal disease caused by extracellular deposition of fibrils derived from serum AA (SAA). AA amyloid fibril formation has previously been modeled in macrophage cultures using highly amyloidogenic mouse SAA1.1, but attempts to do the same with human SAA invariably failed. Our objective was to define conditions that support human SAA-derived amyloid formation in peripheral blood mononuclear cell (PBMC) cultures. Two conditions were found to be critical - omission of fetal calf serum and use of StemPro34, a lipid-enriched medium formulated for hematopoietic progenitor cells. Cultures maintained in serum-free StemPro34 and provided with recombinant human SAA1 in the complete absence of amyloid-enhancing factor exhibited amyloid deposition within 7 d. Amyloid co-localized with cell clusters that characteristically included cells of fibrocytic/dendritic morphology as well as macrophages. These cells formed networks that appeared to serve as scaffolding within and upon which amyloid accumulated. Cells in amyloid-forming cultures demonstrated increased adherence, survival and expression of extracellular matrix components. Of the three human SAA1 isoforms, SAA1.3 showed the most extensive amyloid deposition, consistent with it being the most prevalent isoform in Japanese patients with AA amyloidosis. Attesting to the reproducibility and general applicability of this model, amyloid formation has been documented in cultures established from eight PBMC donors.

Inflammation protein SAA2.2 spontaneously forms marginally stable amyloid fibrils at physiological temperature

For nearly four decades, the formation of amyloid fibrils by the inflammation-related protein serum amyloid A (SAA) has been pathologically linked to the disease amyloid A (AA) amyloidosis. However, here we show that the nonpathogenic murine SAA2.2 spontaneously forms marginally stable amyloid fibrils at 37 °C that exhibit cross-beta structure, binding to thioflavin T, and fibrillation by a nucleation-dependent seeding mechanism. In contrast to the high stability of most known amyloid fibrils to thermal and chemical denaturation, experiments monitored by glutaraldehyde cross-linking/SDS-PAGE, thioflavin T fluorescence, and light scattering (OD(600)) showed that the mature amyloid fibrils of SAA2.2 dissociate upon incubation in >1.0 M urea or >45 °C. When considering the nonpathogenic nature of SAA2.2 and its ~1000-fold increased concentration in plasma during an inflammatory response, its extreme in vitro amyloidogenicity under physiological-like conditions suggest that SAA amyloid might play a functional role during inflammation. Of general significance, the combination of methods used here is convenient for exploring the stability of amyloid fibrils that are sensitive to urea and temperature. Furthermore, our studies imply that analogous to globular proteins, which can possess structures ranging from intrinsically disordered to extremely stable, amyloid fibrils formed in vivo might have a broader range of stabilities than previously appreciated with profound functional and pathological implications.

Serum amyloid A 2.2 refolds into a octameric oligomer that slowly converts to a more stable hexamer

Serum amyloid A (SAA) is an inflammatory protein predominantly bound to high-density lipoprotein in plasma and presumed to play various biological and pathological roles. We previously found that the murine isoform SAA2.2 exists in aqueous solution as a marginally stable hexamer at 4-20°C, but becomes an intrinsically disordered protein at 37°C. Here we show that when urea-denatured SAA2.2 is dialyzed into buffer (pH 8.0, 4°C), it refolds mostly into an octameric species. The octamer transitions to the hexameric structure upon incubation from days to weeks at 4°C, depending on the SAA2.2 concentration. Thermal denaturation of the octamer and hexamer monitored by circular dichroism showed that the octamer is ∼10°C less stable, with a denaturation mid point of ∼22°C. Thus, SAA2.2 becomes kinetically trapped by refolding into a less stable, but more kinetically accessible octameric species. The ability of SAA2.2 to form different oligomeric species in vitro along with its marginal stability, suggest that the structure of SAA might be modulated in vivo to form different biologically relevant species.

Differential affinity of serum amyloid A1 isotypes for high-density lipoprotein

Serum amyloid A (SAA), a precursor of reactive amyloid deposits, is a multigene product. SAA1, predominant both as an amyloid precursor and in plasma, consists of three allelic variants (SAA1.1, SAA1.3, and SAA1.5). Several investigations have shown that the SAA1.3 allele is associated with susceptibility to AA-amyloidosis in Japanese, and the SAA1.5 allele is related with higher serum concentrations of SAA. However, these results have not been interpreted functionally. This study assessed the affinity of SAA isotypes for high-density lipoprotein (HDL), to which SAA binds in plasma. Using a surface plasmon resonance-based apparatus (BIAcore), the affinity between immobilized recombinant human SAAs and HDL was determined. The SAA concentration was measured in fractions after ultracentrifugation (d = 1.23) of sera from patients with rheumatoid arthritis, whose SAA1 genotypes were determined. In the BIAcore analysis, as the dissociation reaction under the conditions used was too rapid to fit the typical kinetic model, the steady-state affinity model was used. The affinity (kd) of SAA1.1, SAA1.3, and SAA1.5 for HDL was 1.4 x 10(-5), 1.8 x 10(-5), and 3.7 x 10(-6), respectively. rSAA1.5 showed significantly (p < 0.05) stronger affinity than the other two. The fraction of lipid-free SAA in serum was significantly (p < 0.001) lower in the patients with larger numbers of the 1.5 allele at the SAA1 locus. These results suggest that the relatively high affinity of SAA1.5 may cause the high serum concentration and may be related to the low susceptibility to amyloidosis.

The acute-phase protein serum amyloid A3 is expressed in the bovine mammary gland and plays a role in host defence

The serum amyloid A protein is one of the major reactants in the acute-phase response. Using representational difference analysis comparing RNA from normal and involuting quarters of a dairy cow mammary gland, we found an mRNA encoding the SAA3 protein (M-SAA3). The M-SAA3 mRNA was localized to restricted populations of bovine mammary epithelial cells (MECs). It was expressed at a moderate level in late pregnancy, at a low level through lactation, was induced early in milk stasis, and expressed at high levels in most MECs during mid to late involution and inflammation/mastitis. The mature M-SAA3 peptide was expressed in Escherichia coli, antibodies made, and shown to have antibacterial activity against E. coli, Streptococcus uberis and Pseudomonas aeruginosa. These results suggest that the mammary SAA3 may have a role in protection of the mammary gland during remodelling and infection and possibly in the neonate gastrointestinal tract.

Increased susceptibility of serum amyloid A 1.1 to degradation by MMP-1: potential explanation for higher risk of type AA amyloidosis

OBJECTIVE

Genetic polymorphisms in serum amyloid A (SAA) have been shown to substantially influence the risk of developing type AA amyloidosis. Recently, a role for MMP-1 has been suggested in the pathogenesis of AA amyloidosis. Therefore, we investigated if the SAA1 isotypes are differentially degraded by MMP-1.

METHODS

Degradation of different SAA isotypes by MMP-1 was assessed by immunoblotting. MALDI-TOF mass spectrometry was used to identify degradation fragments.

RESULTS

We found that SAA1.5 is more resistant to degradation by MMP-1 than SAA1.1. This difference is caused by the capacity of MMP-1 to cleave at the site of the polymorphism at position 57.

CONCLUSION

These results may explain the higher risk of amyloidosis in patients with a SAA1.1/1.1 genotype vs SAA1.5/1.5 or SAA1.1/1.5 genotype. In addition, the impaired degradation of SAA1.5 by MMP-1 could also explain the higher serum SAA concentrations in persons with a SAA1.5 genotype.

Cellular events associated with the initial phase of AA amyloidogenesis: insights from a human monocyte model

Reactive amyloidosis is a systemic protein deposition disease that develops in association with chronic inflammation. The deposits are composed of extracellular, fibrillar masses of amyloid A (AA) protein, an N-terminal fragment of the acute-phase serum protein serum amyloid A (SAA). The pathogenic conversion of SAA into amyloid has been studied in two human cell culture models, peritoneal cells and peripheral blood monocytes. Human monocyte cultures proved more robust than either mouse or human peritoneal cells at initiating amyloid formation in the absence of a preformed nidus such as amyloid-enhancing factor and particularly well suited for examination of individual cells undergoing amyloid formation. Amyloid-producing monocyte cultures were stained with Congo red and Alcian blue for detection of amyloid and glycosaminglycans, respectively; immunocytochemistry was performed to identify SAA/AA, CD68, CD14, lysosomal protein Lamp-1, and early endosomal protein EEA1. SAA interaction with monocytes was also visualized directly via fluorescence confocal microscopy. Amyloid was initially detected only in intracellular vesicles, but with time was seen extracellularly. Morphologic changes in lysosomes were noted during the early phase of amyloid formation, suggesting that exocytosis of fibrils may occur via lysosome-derived vesicles. Cultures engaged in amyloid formation remained metabolically active; no cytotoxic effects were observed. Mimicking in vivo phenomena, amyloid formation was accompanied by increased glycosaminoglycan content and C-terminal processing of SAA. The ability of human monocytes to endocytose and intracellularly transform SAA into amyloid via a mechanism that requires and maintains, rather than compromises, metabolic activity distinguishes them as a useful model for probing earliest events in the disease process.

IL-22 participates in an innate anti-HIV-1 host-resistance network through acute-phase protein induction

Certain individuals are resistant to HIV-1 infection, despite repeated exposure to the virus. Although protection against HIV-1 infection in a small proportion of Caucasian individuals is associated with mutant alleles of the CCR5 HIV-1 coreceptor, the molecular mechanism underlying resistance in repeatedly HIV-1-exposed, uninfected individuals (EU) is unclear. In this study, we performed complementary transcriptome and proteome analyses on peripheral blood T cells, and plasma or serum from EU, their HIV-1-infected sexual partners, and healthy controls, all expressing wild-type CCR5. We report that activated T cells from EU overproduce several proteins involved in the innate immunity response, principally those including high levels of peroxiredoxin II, a NK-enhancing factor possessing strong anti-HIV activity, and IL-22, a cytokine involved in the production of acute-phase proteins such as the acute-phase serum amyloid A (A-SAA). Cell supernatants and serum levels of these proteins were up-regulated in EU. Moreover, a specific biomarker for EU detected in plasma was identified as an 8.6-kDa A-SAA cleavage product. Incubation of in vitro-generated myeloid immature dendritic cells with A-SAA resulted in CCR5 phosphorylation, down-regulation of CCR5 expression, and strongly decreased susceptibility of these cells to in vitro infection with a primary HIV-1 isolate. Taken together, these results suggest new correlates of EU protection and identify a cascade involving IL-22 and the acute phase protein pathway that is associated with innate host resistance to HIV infection.

Serum amyloid A (SAA)-induced remodeling of CSF-HDL

Inflammation is a risk factor for Alzheimer's disease. Serum amyloid A (SAA) is an acute phase protein that dissociates apolipoprotein AI (apoAI) from plasma HDL. In cerebrospinal fluid (CSF), the SAA concentration is much higher in subjects with Alzheimer's disease than in controls. CSF-HDL is rich in apoE, which plays an important role as a ligand for lipoprotein receptors in the central nervous system (CNS). To clarify whether SAA dissociates apoE from CSF-HDL, we added recombinant SAA to CSF and determined the apoE distribution in the CSF using native two-dimensional gel electrophoresis. We found that SAA dissociated apoE from CSF-HDL in a dose-dependent manner. This effect was more evident in apoE4 carriers than in apoE3 or apoE2 carriers. After a 24-h incubation at 37 degrees C, SAA continuously dissociated apoE from CSF-HDL. Amyloid beta (Abeta) fragments (1-42) were bound to large CSF-HDL but not to apoE dissociated by SAA. In conclusion, SAA dissociates apoE from CSF-HDL. We postulate that inflammation in the CNS may impair Abeta clearance due to the loss of apoE from CSF-HDL.

From hexamer to amyloid: marginal stability of apolipoprotein SAA2.2 leads to in vitro fibril formation at physiological temperature

Serum amyloid A (SAA) is a major acute phase reactant and a small apolipoprotein of high density lipoproteins (HDL) in the serum. In cases of prolonged inflammation, SAA may form amyloid fibrils, leading to the disease of amyloid A (AA) amyloidosis. Recently, we have shown that murine SAA2.2, a non-amyloidogenic isoform in vivo, forms a hexamer in vitro containing a putative central channel. It is reported herein that upon thermal denaturation, hexameric SAA2.2 irreversibly dissociates to a misfolded monomer at physiological temperature, formation of which coincides with a significant loss of alpha-helical and gain of beta-sheet structure. When SAA2.2 is incubated for several days at 37 degrees C, sedimentation analytical ultracentrifugation reveals the presence of soluble high molecular weight aggregates, which upon further incubation undergo subsequent self-assembly into amyloid fibrils. Limited proteolysis experiments suggest that the in vitro amyloidogenecity of SAA2.2 is related to structural alteration in its N-terminus. Our observation that SAA2.2 can form amyloid fibrils in vitro at physiological temperatures suggests that SAA2.2's inability to cause amyloidosis may be related to other factors, such as the stabilization of hexameric SAA2.2 (possibly through ligand binding), and/or the slow kinetics of aberrant misfolding and self-assembly.

Urea-induced denaturation of apolipoprotein serum amyloid A reveals marginal stability of hexamer

Serum Amyloid A (SAA) is an acute phase reactant protein that is predominantly found bound to high-density lipoprotein in plasma. Upon inflammation, the plasma concentration of SAA can increase dramatically, occasionally leading to the development of amyloid A (AA) amyloidosis, which involves the deposition of SAA amyloid fibrils in major organs. We previously found that the murine isoform SAA2.2 exists in aqueous solution as a hexamer containing a central channel. Here we show using various biophysical and biochemical techniques that the SAA2.2 hexamer can be totally dissociated into monomer by approximately 2 M urea, with the concerted loss of its alpha-helical structure. However, limited trypsin proteolysis experiments in urea showed a conserved digestion profile, suggesting the preservation of major backbone topological features in the urea-denatured state of SAA2.2. The marginal stability of hexameric SAA2.2 and the presence of residual structure in the denatured monomeric protein suggest that both forms may interconvert in vivo to exert different functions to meet the various needs during normal physiological conditions and in response to inflammatory stimuli.

The interaction between apolipoprotein serum amyloid A and high-density lipoprotein

Serum amyloid A (SAA) is a small apolipoprotein that binds to high-density lipoproteins (HDLs) via its N-terminus. The murine isoform SAA2.2 forms a hexamer in solution and the N-terminus is shielded from the solvent. Therefore, it is unclear how the SAA2.2 hexamer might bind HDL. In this study, the binding of SAA2.2 to murine HDL was investigated by glutaraldehyde cross-linking and polyacrylamide gel electrophoresis. The hexamer did not bind HDL significantly at 20 degrees C. However, at temperatures between 25-30 degrees C, SAA2.2 became destabilized and its monomeric form bound to HDL. SAA2.2 binding did not significantly replace Apo A-I in HDL particles. At 37-45 degrees C SAA2.2 binds less to HDL, suggesting that its binding is weak and sensitive to physiological and pathological temperatures, and thereby, potentially modulated, in vivo, by other factors.

Slower clearance of human SAA1.5 in mice: implications for allele specific variation of SAA concentration in human

Serum amyloid A (SAA), the serum precursor of the fibrillar component (AA proteins) in reactive amyloid deposits, is a multigene product. SAA1, the prominent acute phase isotype in serum and also the dominant fibril precursor, has several allelic variants. In Japan, each of the three major alleles (1.1, 1.3 and 1.5) appears with approximately equal frequency. Recent research suggested that allele 1.5 has a positive influence on the serum SAA concentration. To clarify this, in the present study, recombinant human SAA1.1, SAA1.3 and SAA1.5 were produced in an E. coli expression system and those species of reconstituted high density lipoprotein were injected into mice to examine plasma clearance. SAA1.5 disappeared from plasma more slowly than the other two isotypes. This may account for the positive influence of allele 1.5 on the serum SAA concentration.

Murine apolipoprotein serum amyloid A in solution forms a hexamer containing a central channel

Serum amyloid A (SAA) is a small apolipoprotein that binds to high-density lipoproteins in the serum. Although SAA seems to play a role in host defense and lipid transport and metabolism, its specific functions have not been defined. Despite the growing implications that SAA plays a role in the pathology of various diseases, a high-resolution structure of SAA is lacking because of limited solubility in the high-density lipoprotein-free form. In this study, complementary methods including glutaraldehyde cross-linking, size-exclusion chromatography, and sedimentation-velocity analytical ultracentrifugation were used to show that murine SAA2.2 in aqueous solution exists in a monomer-hexamer equilibrium. Electron microscopy of hexameric SAA2.2 revealed that the subunits are arranged in a ring forming a putative central channel. Limited trypsin proteolysis and mass spectrometry analysis identified a significantly protease-resistant SAA2.2 region comprising residues 39-86. The isolated 39-86 SAA2.2 fragment did not hexamerize, suggesting that part of the N terminus is involved in SAA2.2 hexamer formation. Circular-dichroism spectrum deconvolution and secondary-structure prediction suggest that SAA2.2 contains approximately 50% of its residues in alpha-helical conformation and <10% in beta-structure. These findings are consistent with the recent discovery that human SAA1.1 forms a membrane channel and have important implications for understanding the 3D structure, multiple functions, and pathological roles of this highly conserved protein.

Silent mutations in secondary Shine-Dalgarno sequences in the cDNA of human serum amyloid A4 promotes expression of recombinant protein in Escherichia coli

The serum amyloid A (SAA) superfamily comprises a number of differentially expressed genes with a high degree of homology in mammalian species. SAA4, an apolipoprotein constitutively expressed only in humans and mice, is associated almost entirely with lipoproteins of the high-density range. The presence of SAA4 mRNA and protein in macrophage-derived foam cells of coronary and carotid arteries suggested a specific role of human SAA4 during inflammation including atherosclerosis. Here we underline the importance of ribosome binding site (rbs)-like sequences (also known as Shine-Dalgarno sequences) in the SAA4 cDNA for expression of recombinant SAA4 protein in Escherichia coli. In contrast to rbs sequences coded by the expression vectors, rbs-like sequences in the cDNA of target protein(s) are known to interfere with protein translation via binding to the small 16S ribosome subunit, yielding low or even no expression. Here we show that PCR mutations of two rbs-like sequences in the human SAA4 cDNA promote expression of considerable amounts of recombinant SAA4 in E.coli.

A putative role for cathepsin K in degradation of AA and AL amyloidosis

The aims of this study were to investigate the role of cathepsin K in the pathology of amyloidosis by demonstrating its presence in multinucleated giant cells (MGCs) adjacent to amyloid deposits, and determining its ability to degrade amyloid fibril proteins in vitro. The study was performed using autopsy and biopsy specimens from patients with AA or AL amyloidosis. In six (55%) patients with AA amyloidosis and seven (58%) patients with AL amyloidosis, variable numbers of CD68-immunoreactive MGCs were found adjacent to amyloid deposits. In each case strong cytoplasmic immunostaining for cathepsin K was found in MGCs; immunostaining of amyloid deposits was present in five (45%) patients with AA amyloidosis and three (25%) patients with AL amyloidosis. In vitro degradation experiments showed that recombinant cathepsin K completely degraded AA amyloid fibril proteins at pH 5.5 as shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting. Less effective degradation took place at pH 7.4 and there was no degradation in the presence of a general cysteine protease inhibitor (E64) or in the absence of cathepsin K. This is the first study to show that cathepsin K is expressed in MGCs adjacent to amyloid deposits and to demonstrate its ability to degrade amyloid fibril proteins.

Expression of mouse apolipoprotein SAA1.1 in CE/J mice: isoform-specific effects on amyloidogenesis

Amyloid A (AA) amyloid deposition in mice is dependent upon isoform-specific effects of the serum amyloid A (SAA) protein. In type A mice, SAA1.1 and SAA2.1 are the major apolipoprotein-SAA isoforms found on high-density lipoproteins. During inflammation, both isoforms are increased 1000-fold, but only SAA1.1 is selectively deposited into amyloid fibrils. Previous studies showed that the CE/J mouse strain is resistant to amyloid induction. This resistance is not due to a deficiency in SAA synthesis, but is probably related to the unusual SAA isoform present. The CE/J mouse has a single acute-phase SAA protein (SAA2.2), which is a composite of the SAA1.1 and SAA2.1, with an amino terminus similar to the nonamyloidogenic SAA2.1. Recently, genetic experiments suggested that the SAA2.2 isoform might provide protection from amyloid deposition. To determine the amyloidogenic potential of the CE/J mouse, we generated SAA adenoviral vectors to express the various isoforms in vitro and in vivo. Purified recombinant SAA proteins demonstrated that SAA1.1 was fibrillogenic in vitro, whereas SAA2.2 was unable to form fibrils. Incubation of increasing concentrations of the nonamyloidogenic SAA2.2 protein with the amyloidogenic SAA1.1 did not inhibit the fibrillogenic nature of SAA1.1, or alter its ability to form extensive fibrils. Injection of the mouse SAA1.1 or SAA2.2 adenoviral vectors into mice resulted in isoform-specific expression of the SAA proteins. Amyloid induction after viral expression of the SAA1.1 protein resulted in the deposition of amyloid fibrils in the CE/J mouse, whereas SAA2.2 expression had no effect. Similar expression of the SAA2.2 protein in C57BL/6 mice did not alter amyloid deposition. These data demonstrate that the failure of the CE/J mouse to deposit amyloid is due to the structural inability of the SAA2.2 to form amyloid fibrils. This mouse provides a unique system to test the amyloidogenic potential of altered SAA proteins and to determine the important structural features of the protein.

C-reactive protein and serum amyloid A protein in neonatal infections

In this study, we examine C-reactive protein (CRP) and serum amyloid protein A (SAA). Although the former is the best known and most commonly used indicator of inflammation, certain considerations underline the inadequacy of CRP determination alone for the early diagnosis of infection. In fact symptoms often precede the CRP elevation. SAA protein comprises a family of polymorphic apolipoproteins produced mainly by the liver, and several studies have stressed its importance in the diagnosis and monitoring of various diseases. Pathological SAA values are often detected in association with normal CRP concentrations. SAA rises earlier and more sharply than CRP. Finally, contrary to CRP, SAA presents the same trend in viral as well as bacterial infections. Although the data available on SAA in neonates are currently very limited, it is possible to postulate a role of primary importance for SAA in the management of neonatal infections.

Serum amyloid A, the major vertebrate acute-phase reactant

The serum amyloid A (SAA) family comprises a number of differentially expressed apolipoproteins, acute-phase SAAs (A-SAAs) and constitutive SAAs (C-SAAs). A-SAAs are major acute-phase reactants, the in vivo concentrations of which increase by as much as 1000-fold during inflammation. A-SAA mRNAs or proteins have been identified in all vertebrates investigated to date and are highly conserved. In contrast, C-SAAs are induced minimally, if at all, during the acute-phase response and have only been found in human and mouse. Although the liver is the primary site of synthesis of both A-SAA and C-SAA, extrahepatic production has been reported for most family members in most of the mammalian species studied. In vitro, the dramatic induction of A-SAA mRNA in response to pro-inflammatory stimuli is due largely to the synergistic effects of cytokine signaling pathways, principally those of the interleukin-1 and interleukin-6 type cytokines. This induction can be enhanced by glucocorticoids. Studies of the A-SAA promoters in several mammalian species have identified a range of transcription factors that are variously involved in defining both cytokine responsiveness and cell specificity. These include NF-kappaB, C/EBP, YY1, AP-2, SAF and Sp1. A-SAA is also post-transcriptionally regulated. Although the precise role of A-SAA in host defense during inflammation has not been defined, many potential clinically important functions have been proposed for individual SAA family members. These include involvement in lipid metabolism/transport, induction of extracellular-matrix-degrading enzymes, and chemotactic recruitment of inflammatory cells to sites of inflammation. A-SAA is potentially involved in the pathogenesis of several chronic inflammatory diseases: it is the precursor of the amyloid A protein deposited in amyloid A amyloidosis, and it has also been implicated in the pathogenesis of atheroscelerosis and rheumatoid arthritis.

Characterization of monoclonal antibodies to human protein 1/Clara cell 10 kilodalton protein

Human protein 1/Clara cell Mr 10,000 protein consists of two identical subunits of seventy amino acid residues each. In the present study, eight clones of monoclonal antibodies against native protein 1 were prepared and their respective epitopes were immunochemically and immunohistochemically characterized using native protein 1, truncated recombinant protein 1 and synthesized peptides. Among the clones, three designated as TY-5, TY-7 and TY-8 recognized amino acid residues 7-16, residues 19-28, and residues 39-46, respectively, all of which comprise the hydrophobic cavity of protein 1, possibly associated with chemical binding function. With the exception of TY-4, the remaining clones recognized residues 61-68 which are exposed to solvent. The epitope of TY-4 remains undetermined. Proper selection and combination of clones and recombinant protein 1 may be useful for fundamental and clinical studies of protein 1.

Fourier transform infrared spectroscopic investigation of temperature- and pressure-induced disaggregation of amyloid A

The conformation-sensitive amide I band in the Fourier transform infrared (FTIR) spectra of amyloid A suspensions in D2O was examined as a function of temperature (25-95 degrees C) and applied hydrostatic pressure (1-12 kbar) to assess the stability of the peptide. The principal changes observed upon heating were a significant loss of intermolecular beta-sheet structure, and an increase in the broad band centred at 1644 cm(-1) assigned to unordered structure and alpha-helices of the dissociated species. Application of hydrostatic pressure at ambient temperature resulted in a limited degree of aggregate dissociation. These structural changes were partially reversible with cooling or release of the applied pressure. Dissolving the aggregated peptide in alkaline solution (pH 12) also resulted in disaggregation. Dissociation of organ-deposited amyloid substance bears clinical relevance. The present data indicate that residual amounts of undissociated amyloid in the milieu at physiological and acidic pH may act as nucleating foci rendering dissociated amyloid to reaggregate into organized amyloid.

Serum amyloid A (SAA): a concise review of biology, assay methods and clinical usefulness

Serum amyloid A (SAA) is a family of proteins encoded in a multigene complex. Acute phase isotypes SAA1 and SAA2 are synthesized in response to inflammatory cytokines. SAA and C-reactive protein (CRP) are now the most sensitive indicators for assessing inflammatory activity. In viral infection and kidney allograft rejection, SAA proved more useful than CRP. Development of convenient assay methods for SAA will facilitate its use in clinical laboratories.

SAA1 alleles as risk factors in reactive systemic AA amyloidosis

SAA1 is the predominant isoform of acute phase human SAA deposited as AA amyloid fibrils in reactive systemic amyloidosis. It has recently been reported that in the Japanese population, in whom the SAA1 gamma allele occurs with a frequency of 37%, possession of, and especially, homozygosity for this allele is a significant risk factor for AA amyloidosis in adult patients with rheumatoid arthritis (RA). In contrast we report here that in a control sample of 95 healthy adult male Caucasians the SAA1 gamma allele occurs at the much lower frequency of 5.3% and that, among 41 patients with juvenile chronic arthritis (JCA) and AA amyloidosis, there was a highly significantly increased frequency of the SAA1 alpha allele (90.2%), and particularly homozygosity for this allele (80.5%), compared both to the healthy controls (75.8% and 57.9% respectively) and to 8 JCA cases without amyloid (56.3% and 12.5%). A similar trend with respect to frequency of the SAA1 alpha allele and homozygosity for it was observed among 26 adult Caucasian RA patients with AA amyloid and 26 such cases without amyloid, although it did not reach statistical significance. These results suggest that there is probably differential amyloidogenicity amongst the different SAA1 isoforms and indicate that homozygosity for SAA1 alpha and SAA1 gamma in the different populations is a significant risk factor for development of AA amyloidosis. In Caucasian patients with JCA, the presence of the homozygous SAA1 alpha genotype indicates high risk of amyloidosis and should encourage early and aggressive anti-inflammatory therapy to keep circulating SAA levels as low as possible.

In vitro amyloid fibril formation by synthetic peptides corresponding to the amino terminus of apoSAA isoforms from amyloid-susceptible and amyloid-resistant mice

Specific proteins of the apolipoprotein serum amyloid (apoSAA) family that are synthesized in large quantities during the acute, early phase of inflammation can serve as the proteinaceous precursors for amyloid fibrils. To model fibrillogenesis in such inflammatory diseases, we have used electron microscopy and X-ray diffraction to examine the structures formed by synthetic peptides corresponding in sequence to the 11 amino-terminal amino acids of murine apoSAA1, apoSAAcej, and apoSAA2 and to the 15 amino-terminal amino acids of apoSAA2. This region is reported to be the major fibrillogenic determinant of apoSAA isoforms. Both in 1 mM Tris buffer and in 35% acetonitrile, 0.1% trifluoracetic acid (ACN/TFA), all of the peptides formed macromolecular assemblies consisting of twisted, approximately 40- to 60-A-thick ribbons, which varied in width from around 40-70 A (for 11-mer apoSAA2 in Tris) up to 900 A (for the other peptides). X-ray diffraction patterns recorded from lyophilized peptides, vapor-hydrated samples, and solubilized/dried samples showed hydrogen bonding and intersheet reflections typical of a beta-pleated sheet conformation. The coherent lengths measured from the breadths of the X-ray reflections indicated that with hydration the growth of the assemblies in the intersheet stacking direction was comparable to that in the hydrogen-bonding direction, and analysis of oriented samples showed that the beta-strands were oriented perpendicular to both the long axis and the face of the assemblies. These X-ray results are consistent with the ribbon- or plate-like morphology of the individual aggregates and emphasize the polymorphic nature of amyloidogenic peptides. Our findings demonstrate that X-ray diffraction measurements on vapor-hydrated or solubilized/dried versus lyophilized, amyloidogenic peptides are a good indicator of their fibrillogenic potential. For example, from the highest to the lowest potential, the peptides examined here were ranked as: Abeta1-28 > Abeta1-40 > apoSAA1 approximately apoSAAcej > apoSAA2 > Abeta17-42. Experiments in which the three different 11-mer apoSAA isoforms were solubilized in ACN/TFA and then combined as binary mixtures showed that the ribbon morphology was not affected but that the extent of hydrogen bonding in the assemblies was substantially reduced. Our observations on the in vitro assembly of apoSAA analogs emphasize that amyloid fibril formation and morphology depend on primary sequence, length of polypeptide chain, the presence of additional fibrillogenic polypeptides, and solvent conditions.

A unique amyloidogenic apolipoprotein serum amyloid A (apoSAA) isoform expressed by the amyloid resistant CE/J mouse strain exhibits higher affinity for macrophages than apoSAA1 and apoSAA2 expressed by amyloid susceptible CBA/J mice

CBA/J and other inbred strains of mice that express the amyloidogenic apolipoprotein serum amyloid A (apoSAA) apoSAA2, together with apoSAA1, are susceptible to amyloid A (AA) amyloidosis, whereas CE/J mice that express a single unique isoform, apoSAACEJ, are resistant. Studies indicate that CBA/JxCE/J hybrid mice that express apoSAA2 in the presence of apoSAACEJ are protected from amyloidogenesis. To define a mechanism by which expression of apoSAACEJ may protect from AA formation in the presence of apoSAA2, binding of recombinant apoSAA (r-apoSAA) isoforms, validated by N-terminal sequencing, to a murine macrophage cell line was investigated. Maximal specific binding occurred after incubation of radiolabeled apoSAA with IC-21 macrophages (1x105 cells/ml) for 30 min at 4 degreesC. The binding of 125I-r-apoSAA1, 125I-r-apoSAA2 and 125I-r-apoSAACEJ was specific and saturable, with an affinity (Kd) of about 2.8, 3.2 and 1.3 nM, respectively, and approximately 2-4x106 sites per cell. Competitive binding experiments indicate apoSAACEJ binds with higher affinity to macrophages than does either apoSAA1 or apoSAA2. We suggest that greater cellular affinity of apoSAACEJ compared to apoSAA2 may contribute to protection from AA amyloid in certain CBA/JxCE/J hybrid mice by interfering with interaction of apoSAA2 by macrophages and hence either membrane associated or intracellular degradation.

Catabolism of lipid-free recombinant apolipoprotein serum amyloid A by mouse macrophages in vitro results in removal of the amyloid fibril-forming amino terminus

Serum amyloid A fibrils are formed when the normally rapid catabolism of the acute-phase reactant apolipoprotein serum amyloid A (apoSAA) is incomplete; thus amyloidosis may be viewed as a condition of dysregulated proteolysis. There is evidence that apoSAA is dissociated from plasma high-density lipoprotein (HDL) prior to fibril formation. The objective of this study was to investigate degradation of lipid-free apoSAA by tissue macrophages derived from amyloid-susceptible CBA/J mice in vitro. Peritoneal macrophages derived from untreated (normal) mice converted apoSAA (12 kDa) to a single 4 kDa C-terminal peptide while splenic macrophages converted apoSAA to 10, 7 and 4 kDa C-terminal peptides and a 4 kDa peptide that lacked the C- and N-terminal regions. Similar patterns of proteolysis occurred when peritoneal and splenic macrophages from amyloidotic CBA/J mice were used. Conditioned medium prepared from peritoneal, but not splenic macrophages, degraded apoSAA. Specific sites of cleavage indicated activity of cathepsin G- and elastase-like neutral proteases. The data indicate that lipid-free apoSAA can be degraded by secreted or cell-associated neutral proteases that are generated by macrophages to yield peptides that lack fibrillogenic potential.

Adenoviral expression of murine serum amyloid A proteins to study amyloid fibrillogenesis

Serum amyloid A (SAA) proteins are one of the most inducible acute-phase reactants and are precursors of secondary amyloidosis. In the mouse, SAA1 and SAA2 are induced in approximately equal quantities in response to amyloid induction models. These two isotypes differ in only 9 of 103 amino acid residues; however, only SAA2 is selectively deposited into amyloid fibrils. SAA expression in the CE/J mouse species is an exception in that gene duplication did not occur and the CE/J variant is a hybrid molecule sharing features of SAA1 and SAA2. However, even though it is more closely related to SAA2 it is not deposited as amyloid fibrils. We have developed an adenoviral vector system to overexpress SAA proteins in cell culture to determine the ability of these proteins to form amyloid fibrils, and to study the structural features in relation to amyloid formation. Both the SAA2 and CE/J SAA proteins were synthesized in large quantities and purified to homogeneity. Electron microscopic analysis of the SAA proteins revealed that the SAA2 protein was capable of forming amyloid fibrils, whereas the CE/J SAA was incapable. Radiolabelled SAAs were associated with normal or acute-phase high-density lipoproteins (HDLs); we examined them for their clearance from the circulation. In normal mice, SAA2 had a half-life of 70 min and CE/J SAA had a half-life of 120 min; however, in amyloid mice 50% of the SAA2 cleared in 55 min, compared with 135 min for the CE/J protein. When the SAA proteins were associated with acute-phase HDLs, SAA2 clearance was decreased to 60 min in normal mice compared with 30 min in amyloidogenic mice. Both normal and acute-phase HDLs were capable of depositing SAA2 into preformed amyloid fibrils, whereas the CE/J protein did not become associated with amyloid fibrils. This established approach opens the doors for large-scale SAA production and for the examination of specific amino acids involved in the fibrillogenic capability of the SAA2 molecule in vitro and in vivo.

Effect of serum amyloid A on cellular affinity of low density lipoprotein

Serum amyloid A, an apolipoprotein of high density lipoproteins, is also present to a lesser degree in low density lipoproteins and is co-localized with apolipoprotein B in atherosclerotic lesions. This study examined the effect of serum amyloid A on cellular affinity of low density lipoprotein in vitro. 125I-labelled low density lipoprotein, when loaded with recombinant serum amyloid A1 (acute phase isotype) or recombinant serum amyloid A4 (constitutive isotype), had enhanced binding to both human skin fibroblasts and a murine macrophage cell line, J774, while its degradation was slightly increased in both cells. The binding of oxidized low density lipoprotein to J774 cells was also enhanced by addition of recombinant serum amyloid A1 or serum amyloid A4, and degradation of oxidized low density lipoprotein was moderately enhanced by recombinant serum amyloid A1. The effects of recombinant serum amyloid A on cellular binding of labelled low density lipoprotein were not competed by non-labelled low density lipoprotein and were diminished in the presence of high density lipoprotein. These findings suggest that serum amyloid A in low density lipoprotein may promote association of low density lipoprotein with cells by non-specific adsorption, and high density lipoprotein may prevent such interactions by removal of serum amyloid A.

Differential plasma clearance of murine acute-phase serum amyloid A proteins SAA1 and SAA2

Serum amyloid A (SAA) proteins SAA1 and SAA2 are prominent acute-phase reactants which circulate in association with the high-density-lipoprotein (HDL) fraction of plasma. Plasma levels of SAA1 and SAA2 increase dramatically, by as much as 1000-fold, within 24 h of tissue injury and then rapidly decrease with cessation of the inflammatory stimulus, suggesting that SAA clearance and/or catabolism is important to the re-establishment of homoeostasis. In this context, aberrant SAA catabolism has long been considered a potential factor in the pathogenesis of reactive amyloidosis. To initiate studies aimed at understanding the differential regulation of SAA metabolism, we have produced 35S-labelled murine SAA1 and SAA2 in Escherichia coli, bound them individually to HDL, and then compared the plasma-clearance characteristics of SAA1 and SAA2 under normal and acute-phase conditions. When bound to normal HDL, SAA2 [half-life (t1/2) = 30 min] was cleared significantly faster than SAA1 (t1/2 = 75 min). Clearance of SAA1 and SAA2 was significantly slower when each was bound to acute-phase HDL as opposed to normal HDL, when clearance rates were determined in acute-phase mice versus normal mice, and when normal HDL was remodelled to contain both recombinant isotypes rather than just one of the isotypes. Thus it appears that an increased amount of SAA on HDL, or possibly the combined presence of both isotypes on HDL, is associated with a prolongation in the plasma half-life of SAA.

Instability, unfolding and aggregation of human lysozyme variants underlying amyloid fibrillogenesis

Tissue deposition of soluble proteins as amyloid fibrils underlies a range of fatal diseases. The two naturally occurring human lysozyme variants are both amyloidogenic, and are shown here to be unstable. They aggregate to form amyloid fibrils with transformation of the mainly helical native fold, observed in crystal structures, to the amyloid fibril cross-beta fold. Biophysical studies suggest that partly folded intermediates are involved in fibrillogenesis, and this may be relevant to amyloidosis generally.

Generation of soluble recombinant human acute phase serum amyloid A2 (A-SAA2) protein and its use in development of an A-SAA specific ELISA

Human acute phase serum amyloid A (the A-SAA2 isoform) was expressed at high levels using the pGEX bacterial expression system. A-SAA2 protein was expressed in E. coli NM544 as part of a fusion protein facilitating rapid purification. A-SAA2 was cleaved from the fusion moiety in the presence of a non-ionic detergent (Triton X-100) to release a soluble A-SAA2. Further purification using ion exchange chromatography yielded a pure A-SAA2 (3 mg per litre of culture). Antibodies generated against recombinant A-SAA2 were specific for the acute phase SAAs, A-SAA1 and A-SAA2 and showed no cross-reactivity with the constitutively expressed SAA (C-SAA). These antibodies were used to develop a rapid enzyme-linked immunosorbent assay (ELISA) specific for the measurement of A-SAA in serum.

Both acute phase and constitutive serum amyloid A are present in atherosclerotic lesions

The polymorphic protein, serum amyloid A (SAA), consists of acute phase isotypes and a constitutive isotype. Both are associated mostly with high density lipoproteins (HDL) in the circulation. In the present study, both SAA isotypes were detected by immunohistochemistry and immunoblotting using monoclonal antibodies in atherosclerotic lesions. As the distribution of SAA was identical with that of apolipoprotein B and SAA is known to be associated also with low density lipoproteins (LDL), SAA may also be delivered to the artery wall by LDL.

Expression of recombinant human serum amyloid A in mammalian cells and demonstration of the region necessary for high-density lipoprotein binding and amyloid fibril formation by site-directed mutagenesis

Site-directed mutagenesis of the acute-phase human serum amyloid A (SAA1 alpha) protein was used to evaluate the importance of the N-terminal amino acid residues, namely RSFFSFLGEAF The full-length cDNA clone of SAA1 alpha (pA1.mod.) was used to create two mutations, namely Gly-8 to Asp-8 and an 11 amino acid truncation between Arg-1 and Phe-11 respectively. Wild-type and mutant cDNAs were expressed in Chinese hamster ovary (CHO) cells under the control of the human cytomegalovirus promoter, which resulted in the secretion of the processed proteins into the culture media. Wild-type recombinant human SAA (rSAA) protein was shown to have pI values of 6.0 and 6.4, similar to the human SAA isoform SAA1 alpha and SAA1 alpha desArg found in acute-phase plasma. N-terminal sequencing of 56 residues confirmed its identity with human SAA1 alpha. The total yield of wild-type rSAA measured by ELISA was between 3.5 and 30 mg/l. The two mutations resulted in reduced expression levels of the mutant SAA proteins (3-10 mg/l). Further measurements of rSAA concentration in lipid fractions of culture medium collected at a density of 1.21 g/ml (high-density liporotein; HDL) and 1.063-1.18 g/ml (very-low-density lipoprotein/low-density lipoprotein; VLDL/LDL) showed that 76% of the wild-type protein was found in the HDL fraction and the remaining 24% in the infranatant non-lipid fraction. In contrast the relative concentration of mutant rSAA in HDL and infranatant fractions was reversed. This is consistent with the previously proposed involvement of the 11 amino acid peptide in anchoring. SAA protein on to HDL3 [Turnell, Sarra, Glover, Baum, Caspi, Baltz and Pepys (1986) Mol. Biol. Med. 3, 387-407]. Wild-type rSAA protein was shown to from amyloid fibrils in vitro under acidic conditions as shown by electron microscopy, and stained positive with Congo Red and exhibited apple-green birefringence when viewed under polarized light. Under the same conditions mutSAA(G8D) and mutSAA delta 1-11 did not form amyloid fibrils. In conclusion, replacement of Gly-8 by Asp-8 or deletion of the first 11 amino acid residues at the N-terminus of rSAA diminishes its capacity to bind to HDL and decreases amyloid fibril formation.

In vitro degradation of serum amyloid A by cathepsin D and other acid proteases: possible protection against amyloid fibril formation

The effects of acid proteases on degradation of serum amyloid A protein (SAA) were investigated in vitro. Human recombinant SAA1 (rSAA1), when incubated with human spleen extracts at pH 3.2, was degraded in the amino-terminal portion of the molecule. This reaction was inhibited by an acid protease inhibitor, pepstatin. The degraded SAA molecules lacking nine or more amino-terminal residues, when exposed to in vitro fibril-forming conditions, failed to form Congo red positive precipitates and did not show amyloid fibril-like structure by electron microscopy. This suggests that the amino-terminal portion of SAA is essential for fibril formation. Cathepsin D, one of the lysosomal enzymes, also initiated degradation of rSAA1 at the amino-terminus. Cathepsin D immunoreactivity was detected in marginal areas of amyloid deposits in spleens from patients with reactive amyloidosis. These findings suggest that cathepsin D or similar acid proteases may be involved in SAA catabolism and may protect against amyloid formation.

Cathepsin B generates the most common form of amyloid A (76 residues) as a degradation product from serum amyloid A

Amyloid A protein (AA), the chief constituent of reactive amyloid deposits, is derived from serum amyloid A (SAA) and most commonly corresponds to the amino-terminal 76 residues (AA76). Digestion of recombinant human SAA1 with a lysosomal thiol protease, cathepsin B, and analysis of the products by SDS-PAGE and amino-terminal sequencing revealed that AA76 was generated as a minor and transient degradation product. Digestion with neutrophil elastase generated intermediates different from AA76. This finding suggests that cathepsin B may play an important role in amyloid fibrilogenesis by converting SAA to AA.