Isolation and partial characterization of heparan sulphate proteoglycans from human hepatic amyloid

Serum Amyloid A Binding to Glycosaminoglycans is Synergistic with Amyloid Formation: Therapeutic Targeting in the Inflammation-linked Amyloidosis

Serum amyloid A (SAA), a small lipophilic plasma protein elevated in inflammation, is a precursor of amyloid A (AA) amyloidosis, the major life-threatening complication of chronic inflammation in animals and humans. Although heparan sulfate (HS) is a potent amyloid agonist, particularly in AA amyloidosis, therapeutic targeting of SAA-HS interactions using a small-molecule HS/heparin decoy was unsuccessful. To understand molecular underpinnings, we used recombinant lipid-free human and murine SAA1 and human SAA2 to explore their interactions with various glycosaminoglycans at pH 5.5-7.4 during amyloid formation, from native protein to amyloid oligomers and fibrils. Effects of pH and glycosaminoglycan sulfation/charge supported by prior computational studies indicate electrostatic origin of SAA-glycosaminoglycan interactions. HS/heparin can promote amyloidogenesis by inducing non-native β-sheet and apparently causing liquid droplet formation in SAA in solution. Structural and binding studies by spectroscopy and ELISA reveal previously unknown synergy between amyloid formation and heparin/HS binding by SAA. We propose that this synergy potentially extends to other protein amyloids and stems from longitudinal binding of HS polyanions to basic residue arrays on amyloid oligomers or fibrils. This binding mode explains our finding that a minimal heparin chain length exceeding 20 monosaccharides is necessary to compete with HS for binding to amyloid oligomers. The results help explain prior failure of a small-molecule drug in targeting of SAA-HS interactions and consider alternative HS-targeting approaches for AA and, potentially, other amyloid diseases.

Glycosaminoglycans are part of amyloid fibrils: ultrastructural evidence in avian AA amyloid stained with cuprolinic blue and labeled with immunogold

In domestic brown layer fowl, reactive amyloidosis of internal organs, such as liver and spleen, and of the joints is a common disorder. In a variety of amyloid types including the AA-amyloid of the chicken, in addition to amyloid fibrils, proteoglycans and glycosaminoglycans (GAGs) are found on immunohistochemistry or after extraction. The aim of the present report is to study amyloid fibrils for the ultrastructural location of GAGs by cuprolinic blue staining and immunogold labeling. Rabbit antichicken AA antiserum was used for the immunogold labeling on conventionally embedded and cryoembedded liver tissue and revealed similar results. Therefore conventional blocks could be used for further analysis. Cuprolinic blue staining was performed on blocks of joint tissue in which clearly discernable rod-shaped glycoproteins were encountered in between collagen fibrils. Moreover, it appeared to stain larger deposits which might represent amyloid. Postlabeling with the immunogold method of the cuprolinic blue-stained tissue proved that cuprolinic blue positive fibrils represented AA-amyloid fibrils. Therefore, it was concluded that the GAGs which appeared to colocalize with the fibrillar microanatomy of amyloid, represent a structural part of the amyloid fibrils and that the avian amyloid fibrils may be considered as a pathological proteoglycan.

Characterization of proteoglycans and glycosaminoglycans in splenic AA amyloid induced in mink

Amyloidosis of the protein AA type is readily induced in mink using repeated injections of bacterial lipopolysaccharide (LPS). We have characterized splenic proteoglycans/glycosaminoglycans (PGs/GAGs) in mink during amyloidogenesis. Moderate to rich amounts of amyloid exhibiting green birefringence was demonstrated by polarization microscopy of the splenic section stained with Congo red in seven out of eight minks after 10 weeks of LPS-treatment, and a significant increase in the total amount of PGs and GAGs in AA amyloid spleens was observed (two to eight times that in unstimulated animals). Intact PGs as well as free GAGs were extracted, and heparan sulfate (HS) was the most abundant GAG in the amyloid as well as in the control spleens. The GAGs showing the most pronounced increase in the amyloid spleens was of the chondroitin sulfate/dermatan sulfate (CS/DS) type and these were extracted in the form of free GAG chains. We conclude that there is a selective enrichment of PGs/GAGs in extracted splenic amyloid in the mink, which confirms to previous observations in human amyloid as well as in other animal species, supporting their pathogenic significance in the formation of AA amyloid.

Vascular basement membrane pathology and Alzheimer's disease

We have previously demonstrated that the capillary vascular basement membrane (VBM) is pathologically altered in Alzheimer's disease (AD). This microangiopathy is highlighted by the immunocytochemical localization of the three principal intrinsic VBM components: heparan sulfate proteoglycan, collagen type IV, and laminin. These three VBM components also immunolable amyloid deposits and senile plaque-associated glial processes. The present study examines the ultrastructure of the VBM in one brain region severely affected (temporal gyrus) and one relatively spared (cerebellum) from the lesions of AD in both AD and neurological control cases. The cross-sectional area as well as the width of the VBM were found to be greater in AD cortical capillaries. In addition, we found ultrastructural evidence for the activation of microglial-related perivascular cells, and their apparent extravasation through the VBM, findings consistent with the hypothesis that these cells are being recruited as part of a disease-related immune response. The recruitment of these "resting" microglial-like cells from their intra-VBM location to plaques and tangles in AD may explain (1) the thickening and vacuolization of the VBM; (2) the specificity of this VBM alteration to brain regions where there are plaques and tangles; and (3) the source of some of the large number of activated microglia in these affected areas. Thus, while VBM alterations may not be specific to AD, these changes appear to be specifically related to the disease process.

Binding of perlecan to transthyretin in vitro

Transthyretin is one of two specific proteins involved in the transport of thyroid hormones in plasma; it possesses two binding sites for serum retinol-binding protein. In the present study we demonstrate that transthyretin also interacts in vitro with [35S]sulphate-labelled material from the medium of HepG2 cells. By using the same strategy as for purifying serum retinol-binding protein, [35S]sulphate-labelled medium was specifically eluted from a transthyretin-affinity column. Ion-exchange chromatography showed that the material was highly polyanionic, and its size and alkali susceptibility suggested that it was a proteoglycan. Structural analyses with chondroitinase ABC lyase and nitrous acid revealed that approx. 20% was chondroitin sulphate and 80% heparan sulphate. Immunoprecipitation showed that the [35S]sulphate-labelled material contained perlecan. Further analysis by binding studies revealed specific and saturable binding of 125I-transthyretin to perlecan-enriched Matrigel. Because inhibition of sulphation by treating HepG2 cells with sodium chlorate increased the affinity of the perlecan for transthyretin, and [3H]heparin was not retained by the transthyretin affinity column, the binding is probably mediated by the core protein and is not a protein-glycosaminoglycan interaction. Because perlecan is released from transthyretin in water, the binding might be due to hydrophobic interactions.

Characterization of proteoglycans associated with mouse splenic AA amyloidosis

We here report for the first time on the chemical characteristics of proteoglycans associated with mouse splenic reactive AA amyloid. Amyloid was induced in CBA/J mice by two different procedures; conventional casein treatment and by employing Freund's complete adjuvant, accelerated by Trypan Blue. Pulse-labelling was employed at distinct stages during amyloid development, followed by [35S]proteoglycan characterization of organ extracts. Repetitive 35S injections were also administered during the phase where amyloid deposition occurred most rapidly. Proteoglycans were extracted with guanidine in the presence of protease inhibitors and purified. The results showed that the production of proteoglycans is dramatically enhanced during amyloidogenesis, the glycosaminoglycan and proteoglycan accumulation being not only dependent on alterations in proteoglycan catabolism, but rather on increased synthesis. The increment could be demonstrated even at the stage before microscopic detection of amyloid deposits, clearly suggesting that the upregulation of proteoglycan expression precedes amyloid fibril formation. Two major proteoglycans were found to accumulate in advanced splenic amyloid; one a heparan sulphate proteoglycan of approx. 200 kDa with a core protein of 70 kDa, the other a chondroitin sulphate proteoglycan of smaller size. Moreover, free dermatan sulphate chains seemed to specifically accumulate in the organs during amyloid fibrillogenesis. We suggest that free glycosaminoglycans may be a specific feature of amyloidosis and that different proteoglycans and glycosaminoglycans play a role in formation and stabilization of amyloid fibrils in vivo.

On the association between amyloid fibrils and glycosaminoglycans; possible interactive role of Ca2+ and amyloid P-component

We have previously reported the specific association of glycosaminoglycans (GAG) and proteoglycans (PG) with amyloid fibrils and characterized the polysaccharides directly extracted from amyloid-laden tissues. In the present study we further elucidate the association between purified amyloid fibrils and GAG/PG with special reference to those GAG/PG associated with amyloid P-component (AP) and the interactive role of Ca2+ ions. Amyloid fibrils were isolated from human hepatic AA amyloid employing water extraction with and without preceding removal of AP, an extrafibrillar protein component of all amyloids, using sodium citrate. GAG/PG co-isolated with the amyloid extracts, with and without AP, were isolated and characterized. Agarose-affinity chromatography of extracts containing AP was performed, and the GAG associated with this extrafibrillary protein were characterized as well. Several different GAG/PG populations were demonstrated in the various extracts. The abolition of calcium-dependent binding markedly influenced the amount of GAG/PG recovered in the fibril extracts, as well as the total amount of amyloid material obtained. Thus, it seems that calcium plays an important role in the association between the fibrils and the sugar moieties, and that a significant fraction of the GAG found in amyloid exhibits a Ca(2+)-dependent fibril-GAG interaction. No significant difference in the proportion between galactosaminoglycans and glucosamines was, however, disclosed when the two extraction protocols were compared, suggesting that no particular GAG species has a higher affinity for the fibrils themselves. Both dermatan/chondroitin sulphate and heparan sulphate identified in the present study exhibited a Ca(2+)-dependent interaction with AP, supporting previous findings. However, the amyloid-associated galactosaminoglycans found, especially the large PG appearing in small amounts, seemed to have a higher affinity for the extrafibrillar AP than the other GAG.