Amyloids are complex tissue deposits and each type is identified by one of 22 different proteins or peptides which become re-folded into non-native conformational intermediates and then assemble into fibrils of a highly regular structure. All amyloid deposits also contain apolipoprotein E (apoE) as well as the basement membrane (BM) components, serum amyloid P and heparan sulfate proteoglycans (HSPG), perlecan or agrin. These BM components likely contribute to the overall organization of amyloid fibrils and HSPG has been further implicated in the genesis of amyloid. A growing body of evidence, summarized in this review, suggests that heparan sulfate (HS) promotes fibrillogenesis by associating with the amyloid precursors and inducing the conformational change required for their assembly into fibrils. HS also remains associated with the nascent fibrils contributing to its stability. These activities of HS are likely mediated through specific binding sites on the precursor proteins which appear to have sequence characteristics that are unique to amyloid.

Circumsporozoite protein (CSP) coats the malarial sporozoite and functions to target the liver for infection, which is the first step to developing malaria. An important tissue ligand for CSP is the glycosaminoglycan heparan sulfate (HS) found on the surface of hepatocytes and in the basement membrane of the space of Disse. To better understand this efficient targeting process, we set out to identify and characterize the HS binding site(s) of CSP. We synthesized a series of peptides corresponding to five regions of Plasmodium falciparum CSP containing basic residues, a common requirement of HS binding sites, and screened them for heparin and HS binding activity. Only one of these peptides (Pf 2), which contains a motif we have named region I-plus, demonstrated both high affinity heparin/HS binding activity and the ability to block the binding of recombinant CSP to heparin-Sepharose 4B. Analysis by isothermal titration calorimetry revealed that region I-plus has a binding constant of K d ‫؍‬ 5.0 M and a stoichiometry of n ‫؍‬ 7.8 binding sites/heparin chain. Heparin binding was dependent on the amino acid sequence of region I-plus, and the binding sites on heparin/HS are contained within a decasaccharide. Furthermore, HS oligosaccharides rich in sulfate and iduronic acid content (heparin-like) are required for efficient binding. Because liver HS is exceptionally high in both these components relative to the HS of other organs, the HS structural requirements for efficient region I-plus/HS binding are consistent with this peptide sequence functioning to target sporozoites to the liver for attachment to hepatocytes. Finally, the region I-plus heparin/HS binding site was also discovered for two other species that infect humans, Plasmodium malariae and Plasmodium vivax, further supporting the existence of a HS binding domain in the N-terminal portion of CSP.

Transthyretin (TTR) is a homotetrameric protein that transports thyroxine and retinol. Tetramer destabilization and misfolding of the released monomers result in TTR aggregation, leading to its deposition as amyloid primarily in the heart and peripheral nervous system. Over 100 mutations of TTR have been linked to familial forms of TTR amyloidosis. Considerable effort has been devoted to the study of TTR aggregation of these mutants, although the majority of TTR-related amyloidosis is represented by sporadic cases due to the aggregation and deposition of the otherwise stable wild-type (WT) protein. Heparan sulfate (HS) has been found as a pertinent component in a number of amyloid deposits, suggesting its participation in amyloidogenesis. This study aimed to investigate possible roles of HS in TTR aggregation. Examination of heart tissue from an elderly cardiomyopathic patient revealed substantial accumulation of HS associated with the TTR amyloid deposits. Studies demonstrated that heparin/HS promoted TTR fibrillization through selective interaction with a basic motif of TTR. The importance of HS for TTR fibrillization was illustrated in a cell model; TTR incubated with WT Chinese hamster ovary cells resulted in fibrillization of the protein, but not with HS-deficient cells (pgsD-677). The effect of heparin on TTR fibril formation was further demonstrated in a Drosophila model that overexpresses TTR. Heparin was colocalized with TTR deposits in the head of the flies reared on heparin-supplemented medium, whereas no heparin was detected in the nontreated flies. Heparin of low molecular weight (Klexane) did not demonstrate this effect.systemic amyloidosis | sulfated glycosaminoglycans | aging | heart failure

Serum amyloid A isoforms, apoSAA1 and apoSAA2, are apolipoproteins of unknown function that become major components of high density lipoprotein (HDL) during the acute phase of an inflammatory response. ApoSAA is also the precursor of inflammation-associated amyloid, and there is strong evidence that the formation of inflammation-associated and other types of amyloid is promoted by heparan sulfate (HS). Data presented herein demonstrate that both mouse and human apoSAA contain binding sites that are specific for heparin and HS, with no binding for the other major glycosaminoglycans detected. Cyanogen bromide-generated peptides of mouse apoSAA1 and apoSAA2 were screened for heparin binding activity. Two peptides, an apoSAA1-derived 80-mer (residues 24 -103) and a smaller carboxyl-terminal 27-mer peptide of apoSAA2 (residues 77-103), were retained by a heparin column. A synthetic peptide corresponding to the CNBr-generated 27-mer also bound heparin, and by substituting or deleting one or more of its six basic residues (