Serum amyloid A protein in mink during endotoxin induced inflammation and amyloidogenesis

Mink (Neovison vison) kits with pre-weaning diarrhea have elevated serum amyloid A levels and intestinal pathomorphological similarities with New Neonatal Porcine Diarrhea Syndrome

Background

Pre-weaning diarrhea (PWD) is a syndrome affecting farm-raised neonatal mink kits. Apart from diarrhea it causes greasy skin exudation, dehydration, and distressed behavior and can ultimately lead to death. No specific causative agents have been identified and the syndrome is regarded as multifactorial. The aim of the present study was to investigate a possible inflammatory state in mink kits with PWD, as indicated by raised serum concentrations of the acute phase protein serum amyloid A (SAA) and by changes in intestinal pathomorphology and intestinal contents of bacteria. Samples collected from 20 diarrheic mink kits with PWD and 20 age-matched non-diarrheic control mink kits from two commercial Danish farms during the pre-weaning period (April–May) in 2016 were analyzed.

Results

Concentrations of SAA in serum samples from mink kits with PWD were significantly higher (up to 1000-fold) compared to non-diarrheic control mink kits. Significant features of enterocytic vacuolization, atrophy and fusion of villi in jejunum and mucosal atrophy of the colon of kits with PWD were found. Moreover, attachment of coccoid bacteria to enterocytes was more often found in kits suffering from PWD, while intra-cytoplasmic eosinophil bodies were more frequently observed in control kits. Cellular infiltrations with mononuclear and neutrophil leukocytes were not associated with disease status. Bacteria from the Staphylococcus intermedius group, such as Staphylococcus delphini, were more frequently cultivated from control mink kits, whereas Enterococcus spp. dominated in mink kits with PWD. Escherichia coli was cultivated from both control and mink kits with PWD, but with a higher frequency from mink kits with PWD.

Conclusion

A significant increase in circulating concentrations of SAA was found in PWD affected mink kits from 6 to 23 days old compared to controls. The histopathological changes in PWD mink kits suggest that the type of diarrhea is secretory. Attachment of coccoid bacteria, therefore, might be responsible for an enterotoxic effect causing a loss of balance in movements of ions and water leading to the vacuolization and swelling of the enterocytes. The slight to moderate infiltrations of neutrophils irrespectively of diarrheic status and the attachment of coccoid bacteria to enterocytes are comparable to observations found in piglets suffering from New Neonatal Porcine Diarrhea Syndrome. Mechanisms behind the correlation between increased SAA levels and the observed pathological intestinal features remain obscure.

Amyloid fibrils trigger the release of neutrophil extracellular traps (NETs), causing fibril fragmentation by NET-associated elastase

The accumulation of amyloid fibrils is a feature of amyloid diseases, where cell toxicity is due to soluble oligomeric species that precede fibril formation or are formed by fibril fragmentation, but the mechanism(s) of fragmentation is still unclear. Neutrophil-derived elastase and histones were found in amyloid deposits from patients with different systemic amyloidoses. Neutrophil extracellular traps (NETs) are key players in a death mechanism in which neutrophils release DNA traps decorated with proteins such as elastase and histones to entangle pathogens. Here, we asked whether NETs are triggered by amyloid fibrils, reasoning that because proteases are present in NETs, protease digestion of amyloid may generate soluble, cytotoxic species. We show that amyloid fibrils from three different sources (α-synuclein, Sup35, and transthyretin) induced NADPH oxidase-dependent NETs in vitro from human neutrophils. Surprisingly, NET-associated elastase digested amyloid fibrils into short species that were cytotoxic for BHK-21 and HepG2 cells. In tissue sections from patients with primary amyloidosis, we also observed the co-localization of NETs with amyloid deposits as well as with oligomers, which are probably derived from elastase-induced fibril degradation (amyloidolysis). These data reveal that release of NETs, so far described to be elicited by pathogens, can also be triggered by amyloid fibrils. Moreover, the involvement of NETs in amyloidoses might be crucial for the production of toxic species derived from fibril fragmentation.

Detection of Serum Amyloid A Isoforms in Cattle

Two-dimensional electrophoresis and immunoblotting were used to characterize 7 serum amyloid A (SAA) isoforms in cows with amyloidosis or chronic inflammation. Five SAA isoforms (isoelectric point [pI] 5.6, 6.1, 6.4, 6.8, and 7.7) were detected in all 10 amyloidosis serum samples, while 2 isoforms, pI 5.2 and 8.6, were detected in 4 and 9 of the samples, respectively. The same 7 isoforms were also detected in the serum of cows with chronic inflammation, but SAA pI 5.2 and 8.6 were detected in only 1 and 2 of 10 samples, respectively. It was concluded that although an amyloid-specific SAA isoform was not detected, examination of SAA isoform detection patterns may help identify bovine amyloidosis.

Rapid induction of experimental AA amyloidosis in mink by intravenous injection of amyloid enhancing factor

Studies of amyloid enhancing factor (AEF)-induced amyloidosis are commonly performed in mice. In mink, earlier studies of amyloid A (AA) amyloidosis showed that the predeposition phase was highly variable. Thus, the aim of the study was to establish an AEF-induced AA amyloidosis model in mink to facilitate studies of early amyloid deposition in a species with prominent ellipsoids, anatomical structures lacking in mice but present in most other mammals. AEF was extracted from mink spleens containing AA. Mink received one intravenous injection of AEF and repeated subcutaneous injections of lipopolysaccharide (LPS) as an inflammatory stimulus. On day 4, small amounts of amyloid were detected in the marginal zone in the spleen. On day 7, considerable amyloid deposition was detected in the ellipsoids and marginal zones in the spleen and in the space of Disse in the liver. By immunohistochemistry, the deposits were identified as AA amyloid. Immunolabeling was also detected in lymphoid follicles and the red pulp of some animals. Control animals receiving only AEF were negative. Control animals receiving only LPS were negative except for one of three animals which had small amounts of amyloid in the spleen. The mink AEF model is a suitable tool to study the development of AA amyloidosis in a species with a spleen containing both well-developed ellipsoids and marginal zone.

Splenic ellipsoids: an early target for deposition of AA amyloid induced in mink

The spleen is the primary target for spontaneous as well as experimental AA amyloidosis in animals such as mice and mink, and is therefore a valuable organ for study of the initial phases of amyloid fibrillogenesis and deposition. We have investigated splenic amyloid AA deposits induced in the mink, and we demonstrate a novel target for AA, namely the splenic ellipsoids. We show presence of amyloid P component (AP), glycosaminoglycans (GAGs) and apolipoprotein E (apoE), all well-known common elements of amyloid, co-localizing with AA. In addition, apolipoprotein AI (apoAI) was seen co-localized to the AA deposits in the ellipsoids. We hypothesize that the ellipsoids may be important splenic structures for initial AA formation. The apoAI in the ellipsoids could displace SAA from acute phase HDL at this site, thereby making SAA available for amyloid formation and deposition.

Enrichment of serum amyloid proteins by hydrophobic interaction chromatography combined with two-dimensional electrophoresis with immobilised pH gradients

Serum amyloid A protein was subjected to one-step octyl-Sepharose extraction in three different dimensions. Elution was performed partly without UV recording, and with urea or guanidine-based buffers. The eluent was applied directly to denaturing two-dimensional electrophoresis with immobilised pH gradient, or octyl-Sepharose extracted fractions were pooled and lyophilised before application. Proteins were characterised by N-terminal analysis or mass spectrometry. In most of the species that were studied, previously undescribed serum amyloid proteins were detected. Compared to conventional strategies, the presented techniques are more rational and yield more comprehensive information. The presented data also provide a basis for novel perspectives regarding certain inflammatory conditions.

Identical amyloid precursor proteins in two breeds of chickens which differ in susceptibility to develop amyloid arthropathy

Chickens (Gallus gallus domesticus) can suffer from AA amyloidosis featuring the joints as major targets of amyloid accumulation. Analysis of post-mortem recordings from commercial chickens revealed that amyloid arthropathy frequently occurred in brown layer chickens, but never in white layers. The suspected higher susceptibility of brown layers was confirmed experimentally by inducing amyloidosis with an arthropathic and amyloidogenic strain of E. faecalis. Sequence analysis of cDNA coding for SAA, the amyloid precursor protein, revealed that the SAA proteins are identical in both breeds, although some nucleotide differences existed in the untranslated regions of the mRNAs. The chicken SAA gene was found to be a single copy gene which comprises 4 exons. The first of these exons apparently occupies a conserved position and is not translated. Investigation of the affected joints using in situ hybridization demonstrated local SAA gene expression. It is concluded that the likelihood of an E. faecalis induced arthritis to progress to amyloidosis is breed-dependent, but is not a consequence of a more amyloidogenic SAA.

Amyloid arthropathy in chickens

The present paper presents an overview of current knowledge of amyloid arthropathy in chickens, and covers the pathogenesis of amyloidosis in general and in birds, field cases reported, and the studies performed to assess the amyloidogenicity of various agents compared to that of Enterococcus faecalis. An animal model of amyloid arthropathy is presented, as are studies on the pathogenesis of arthropathic and amyloidogenic E. faecalis infections in brown layers. The review concludes with a description of the pathology of amyloid arthropathy, the biochemical characterization of the chicken joint amyloid protein as being of the AA type, investigation of the serum amyloid A (SAA) gene involved, and local SAA mRNA expression in joint and liver.

Transgenic mouse model of AA amyloidosis

AA amyloidosis can be induced in mice experimentally through injection of certain chemical or biological compounds. However, the usefulness of this approach is limited by its dependence on exogenous inflammatory agents that stimulate cytokines to increase the synthesis of precursor serum amyloid A (SAA) protein and the transitory nature of the pathological fibrillar deposits. We now report that transgenic mice carrying the human interleukin 6 gene under the control of the metallothionein-I promoter had markedly increased concentrations of SAA and developed amyloid in the spleen, liver, and kidneys by 3 months of age. At the time of death about 6 months later, organs obtained from these animals had extensive amyloid deposits. This disease process was apparent radiographically using small-animal computer axial tomography and magnetic resonance imaging equipment. The AA nature of the amyloid was evidenced immunohistochemically and was unequivocally established by sequence analysis of protein extracted from the fibrils. The availability of this unique in vivo experimental model of AA amyloidosis provides the means to assess the therapeutic efficacy of agents designed to reduce or prevent the fibrillar deposits found in AA and other types of amyloid-associated disease.

Acceleration of amyloid protein A amyloidosis by amyloid-like synthetic fibrils

Amyloid protein A (AA) amyloidosis is a consequence of some long-standing inflammatory conditions, and subsequently, an N-terminal fragment of the acute phase protein serum AA forms beta-sheet fibrils that are deposited in different tissues. It is unknown why only some individuals develop AA amyloidosis. In the mouse model, AA amyloidosis develops after approximately 25 days of inflammatory challenge. This lag phase can be shortened dramatically by administration of a small amount of amyloid extract containing an as yet undefined amyloid-enhancing factor. In the present study, we show that preformed amyloid-like fibrils made from short synthetic peptides corresponding to parts of several different amyloid fibril proteins exert amyloidogenic enhancing activity when given i.v. to mice at the induction of inflammation. We followed i.v. administered, radiolabeled, heterologous, synthetic fibrils to the lung and to the perifollicular area in the spleen and found that new AA-amyloid fibrils developed on these preformed fibrils. Our findings thus show that preformed, synthetic, amyloid-like fibrils have an in vivo nidus activity and that amyloid-enhancing activity may occur, at least in part, through this mechanism. Our findings also show that fibrils of a heterologous chemical nature exert amyloid-enhancing activity.

The acute phase serum amyloid A protein (SAA) in the horse: isolation and characterization of three isoforms

Serum amyloid A (SAA) from acute phase horse serum was isolated using hydrophobic interaction chromatography, gel filtration and ion exchange chromatography. Three SAA isoforms with different isoelectric points, i.e. SAA pI 8.0, SAA pI 9.0 and SAA pI 9.7, were identified by two-dimensional electrophoresis and further characterized with amino acid sequence analysis. These isoforms were found in similar concentrations in all animals investigated, with SAA pI 9.7 constituting about half of the total SAA content. Partial amino acid sequence analysis verified the previously published heterogeneous SAA sequence. SAA pI 8.0 was found to have isoleucine in Position 16, glutamine in Position 44 and glycine in Position 59. SAA pI 9.0 had leucine, glutamine and alanine in the corresponding positions. In SAA pI 9.7 leucine, lysine and alanine were detected. The three isoforms characterized in this study are all acute phase SAAs. SAA pI 9.0 and 9.7 correspond to amyloid A protein variants previously isolated from amyloid deposits of equine liver, while there are no reports on an amyloid A variant corresponding to SAA pI 8.0.

Isolation of serum amyloid A protein by small-scale hydrophobic interaction chromatography and two-dimensional electrophoresis

A recently introduced technique to isolate serum amyloid A protein is hydrophobic interaction chromatography combined with two-dimensional electrophoresis with immobilized pH gradients. A modification of the original version of this technique is presented. Mouse serum was subjected to hydrophobic interaction chromatography on a small scale, and the eluate was applied directly to two-dimensional electrophoresis. Simple electropherogramss with optimal resolution of serum amyloid A protein were obtained. The presented technique facilitates isolation of serum amyloid A protein from small blood volumes, and might also be adapted to alternative applications.

Characterization of human serum amyloid A protein isoforms separated by two-dimensional electrophoresis by liquid chromatography/electrospray ionization tandem mass spectrometry

A detailed structural analysis of the serum amyloid A proteins (SAA) of an individual with highly active, chronic rheumatoid arthritis is reported. SAA isoforms were separated by high-resolution two dimensional (2-D) gel electrophoresis. Peptide mapping by reverse-phase chromatography/electrospray ionization tandem mass spectrometry was applied to correlate the protein(s) contained in each spot with their respective coding gene and to study the post-translational processing and modification events which might result in differential electrophoretic mobility. Nine protein spots were analyzed. The six major spots corresponded to the Arg and des-Arg forms of SAA1 alpha and SAA2 alpha, respectively, and to the glycosylated and nonglycosylated form of constitutive serum amyloid A protein (C-SAA). Two minor spots were identified as SAA1 alpha isoforms containing post-translational modifications. We suggest that these variants contained a gamma-N, N'-dimethylasparagine residue at position 83 and that one of them was additionally oxidized at Trp53 and Trp85. The ninth spot was shown to contain a mixture of SAA1 alpha and SAA2 alpha. To our knowledge, this is the first report in which analysis of peptides has been used to verify the presence of C-SAA in acute-phase serum. Furthermore, the data illustrate that extensive post-translational processing results in a structurally diverse class of acute-phase SAA proteins, which are derived from a small number of genes. Finally, the fast and conclusive technology used in this study promises to be generally useful for the comprehensive investigation of proteins at the level of the primary structure.

Serum amyloid A protein in humans and four animal species: a comparison by two dimensional electrophoresis

Two-dimensional electrophoresis and N-terminal analysis were used to study serum amyloid A protein (SAA) from humans, mink, fox, goat and rabbit. Previously uncharacterized SAA variants were demonstrated in fox, goat and rabbit, and considerable interspecies homology was seen. In rabbit, two novel SAAs were characterized, and SAA1 and SAA2 were demonstrated in mink and rabbit sera. The results confirm previous cDNA studies and indicate that SAA do possess an important function also in fox and goat.