The Effect of Glycosaminoglycans (GAGs) on Amyloid Aggregation and Toxicity

Abstract

Amyloidosis is a protein folding disorder in which normally soluble proteins are deposited extracellularly as insoluble fibrils, impairing tissue structure and function. Charged polyelectrolytes such as glycosaminoglycans (GAGs) are frequently found associated with the proteinaceous deposits in tissues of patients affected by amyloid diseases. Experimental evidence indicate that they can play an active role in favoring amyloid fibril formation and stabilization. Binding of GAGs to amyloid fibrils occurs mainly through electrostatic interactions involving the negative polyelectrolyte charges and positively charged side chains residues of aggregating protein. Similarly to catalyst for reactions, GAGs favor aggregation, nucleation and amyloid fibril formation functioning as a structural templates for the self-assembly of highly cytotoxic oligomeric precursors, rich in β-sheets, into harmless amyloid fibrils. Moreover, the GAGs amyloid promoting activity can be facilitated through specific interactions via consensus binding sites between amyloid polypeptide and GAGs molecules. We review the effect of GAGs on amyloid deposition as well as proteins not strictly related to diseases. In addition, we consider the potential of the GAGs therapy in amyloidosis.

How Glycosaminoglycans Promote Fibrillation of Salmon Calcitonin

“…8C, bottom panel, lanes 1-3). When treating soluble sCT with trypsin, no peptide was retained in the gel (data not shown), most likely reflecting the small size of tryptic products sCT(1-11), sCT (12)(13)(14)(15)(16)(17)(18), sCT (19 -24), and sCT (25)(26)(27)(28)(29)(30)(31)(32). For sCT-OH, trypsin digestion of fibrils gave results identical to those presented here for sCT-NH 2 (data not shown).…”

“…Over this pH interval, the charge of sCT will change by up to 1.3 units (see below), and this will affect interactions with the highly anionic GAG molecules. To investigate the role of charge in these interactions, we used the following three variants of sCT: 1) the natural form, which is C-terminally amidated (sCT-NH 2 ) and goes from a net charge of approximately ϩ3.9 at pH 5.5 to approximately ϩ2.6 at pH 7.4 based on conventional pK a value calculations; 2) a form with a free C-terminal carboxylate group (sCT-OH), which will be anionic above approximately pH 4, going from a net charge of ϩ2.9 at pH 5.5 to ϩ1.6 at pH 7.4; and 3) the mutant His 17 3 Ala (sCTH17A-OH; with a free C-terminal carboxylate group), which removes the only side chain that titrates over the pH interval 7.4 -5.5 so that the net charge of this mutant only decreases by 0.6 unit from ϩ2.1 at pH 5.5 to ϩ1.5 at pH 7.4.…”

“…Besides promoting fibrillation, GAGs hinder cytotoxicity induced by prefibrillar oligomers of other proteins such as islet amyloid polypeptide (15) and amyloid-␤ (16). Furthermore, non-fibril-inducing GAG mimetics are a growing field of interest for therapeutic applications (17). Thus it remains unresolved whether GAGs primarily hinder or promote unwanted aggregation in vivo (17)(18)(19).…”

“…Furthermore, non-fibril-inducing GAG mimetics are a growing field of interest for therapeutic applications (17). Thus it remains unresolved whether GAGs primarily hinder or promote unwanted aggregation in vivo (17)(18)(19). Recently GAGs have been suggested to be involved in the storage of protein hormones in granular vesicles by promoting aggregation of protein hormones (20,21).…”

Modulation of amyloid assembly by glycosaminoglycans: from mechanism to biological significance

“…These linear and unbranched polysaccharides are ubiquitously associated with amyloid deposits D r a f t 5 linked to protein misfolding diseases (Iannuzzi et al 2015), including those from TTR (Inoue et al 1998), Aβ (DeWitt et al 1993, IAPP (Young et al 1992), β2-microglobulin (Ohishi et al 1990) and immunoglobulin light chain (Stenstad et al 1991). These studies have suggested a causative role of sulfated GAGs in the fibrillogenesis and deposition of a variety of amyloidogenic polypeptides and have prompted biophysical investigations.…”