Studies on Pathogenesis of Experimental AA Amyloidosis : Effects of Amyloid Enhancing Factor and Amyloid-Like Fibrils in Rapid Amyloid Induction

Sammanfattning: Amyloidosis is a group of diseases, caused by an extracellular deposition of a characteristic proteinaceous material, amyloid, in various organs. Fibril formation occurs in all of amyloid related diseases, making it a crucial mechanism to understand.Experimental inflammatozy-induced amyloidosis (AA amyloidosis) is proposed to be a nucleation dependent process developing after a lag phase of weeks. The lag phase may be shortened to days by administration of a material extracted from amyloid-loaded tissues. This material is referred to as amyloid enhancing factor (AEF), and is supposed to contain a nucleus that starts fibril formation. However, the nature of this nucleus has not been definitely established.We have established a murine model of accelerated AA amyloidosis. In this model we have studied amyloid enhancing effects of preparations containing fibrillazy structures extracted from murine amyloid and from amyloid-like fibrils produced in vitro.Our results show that the murine AEF preparation contains no components other than AA amyloid fibrils and is active infemtomolar doses. This AEF preparation is active when administered orally and retains its activity in animals for months after administration. Amyloid fibrils prepared in vitro from amyloidogenic peptides and certain non amyloidogenic proteins have AEF effect as well. Denaturation of the AA protein in AEF abolishes itsamyloidogenic effect. Nonfibrillazy preparation of amyloidogenic peptide has no AEF effect. Radioiodinated amyloid-like fibrils can be detected in newly formed splenic amyloid, and co-localization of such fibrils with AN/SAA is demonstrated.Therefore we propose that the active component in AEF is the amyloid fibril itself. The mechanism of nucleation is considered to be similar to the seeded nucleation proposed forprion propagation, in which fibrils, small fibril fragments, or oligomers of scrapie prion protein (PrP) induce profound conformational change in cellular PrP. We propose that experimental AA amyloidosis belongs to the transmissible amyloidoses. The finding that amyloidlike fibrils from naturally occurring nonamyloidogenic proteins act as AEF is of great interest. Ingestion or inhalation of such fibrils may introduce seeds that can start the nucleation process in individuals with elevated SAA levels. Hypothetically, this may explain why only a fraction of patients with longstanding inflammatozy conditions develop amyloid deposits and may implicate environmental factors as important risk factors for AA amyloidosis.