AA amyloid fibrils from diseased tissue are structurally different from in vitro formed SAA fibrils

Abstract

Systemic AA amyloidosis is a world-wide occurring protein misfolding disease of humans and animals. It arises from the formation of amyloid fibrils from serum amyloid A (SAA) protein. Using cryo electron microscopy we here show that amyloid fibrils which were purified from AA amyloidotic mice are structurally different from fibrils formed from recombinant SAA protein in vitro. Ex vivo amyloid fibrils consist of fibril proteins that contain more residues within their ordered parts and possess a higher β-sheet content than in vitro fibril proteins. They are also more resistant to proteolysis than their in vitro formed counterparts. These data suggest that pathogenic amyloid fibrils may originate from proteolytic selection, allowing specific fibril morphologies to proliferate and to cause damage to the surrounding tissue.

Protease Resistance of ex vivo Amyloid Fibrils implies the proteolytic Selection of disease-associated Fibril Morphologies

“…The first one attributes encountered differences in proteolytic susceptibility to differences in the fibril structure, which is supported by studies revealing structural differences between ex vivo and in vitro fibrils [13–16,18–20]. The different fibril structures may be associated with different thermodynamic stabilities of the fibrils, although protease resistant mouse AA amyloid fibrils were recently found not to be very stable to guanidine denaturation [18]. The second one is associated with the fact that ex vivo amyloid fibrils can contain non-fibril components of amyloid tissue deposits, such as SAP, which was previously found to protect amyloid fibrils from being degraded in vitro [33] and in vivo [32].…”

“…Several studies previously showed that disease-associated amyloid fibrils from patient tissue are structurally different from fibrils formed from the same polypeptide chain in vitro [13–16,18–20]. In two cases, namely murine SAA1.1 protein and human Aβ peptide, it was additionally found that ex vivo fibrils are more protease resistant than in vitro fibrils [16,18]. These observations led to the hypothesis that disease-associated amyloid fibrils were selected inside the body by their proteolytic stability [18].…”

“…In two cases, namely murine SAA1.1 protein and human Aβ peptide, it was additionally found that ex vivo fibrils are more protease resistant than in vitro fibrils [16,18]. These observations led to the hypothesis that disease-associated amyloid fibrils were selected inside the body by their proteolytic stability [18]. That is, protease stable fibrils have a higher chance to escape endogenous clearance mechanisms, to proliferate and to accumulate in a native cellular environment.…”

Challenges in sample preparation and structure determination of amyloids by cryo-EM

“…The observed amplified sample is thus not directly taken from human tissue but is a more indirect representation of invivo structures than ex-vivo structures. Interestingly, none of the few known exvivo fibril structures have been observed either in-vitro or in amplified patientderived samples, yet (102,107). Nonetheless, in-vitro structures have broadened our understanding of amyloid architecture in general and revealed common structural features such as the amyloid key, which has been observed invitro and in-vivo and they furthermore help to interpret aggregation kinetics (108,109).…”

Amyloid Structural Changes Studied by Infrared Microspectroscopy in Bigenic Cellular Models of Alzheimer’s Disease

“…It has been suggested that amyloid proteins such as Aβ and αSyn can form oligomerssupramolecular structures consisting of several noncovalent assemblies [19] and characterized by structural diversity [20] due to the partial enrichment of β-sheet structures, which may define the neurotoxicity of oligomers [21]. Still, the relative contribution of amyloid-specific secondary structures to neurotoxicity remains unknown.…”