Pathogenic Serum Amyloid A 1.1 Shows a Long Oligomer-rich Fibrillation Lag Phase Contrary to the Highly Amyloidogenic Non-pathogenic SAA2.2

Saipraveen Srinivasan,Sanket Patke,Yun Wang,Zhuqiu Ye,Jeffrey Litt,Sunit K Srivastava,Maria M Lopez,Dmitry Kurouski,Igor K Lednev,Ravi S Kane,Wilfredo Colón

doi:10.1074/jbc.m112.394155

Abstract

Serum amyloid A (SAA) is best known for being the main component of amyloid in the inflammation-related disease amyloid A (AA) amyloidosis. Despite the high sequence identity among different SAA isoforms, not all SAA proteins are pathogenic. In most mouse strains, the AA deposits mostly consist of SAA1.1. Conversely, the CE/J type mouse expresses a single non-pathogenic SAA2.2 protein that is 94% identical to SAA1.1. Here we show that SAA1.1 and SAA2.2 differ in their quaternary structure, fibrillation kinetics, prefibrillar oligomers, and fibril morphology. At 37 °C and inflammation-related SAA concentrations, SAA1.1 exhibits an oligomer-rich fibrillation lag phase of a few days, whereas SAA2.2 shows virtually no lag phase and forms small fibrils within a few hours. Deep UV resonance Raman, far UV-circular dichroism, atomic force microscopy, and fibrillation cross-seeding experiments suggest that SAA1.1 and SAA2.2 fibrils possess different morphology. Both the long-lived oligomers of pathogenic SAA1.1 and the fleeting prefibrillar oligomers of non-pathogenic SAA2.2, but not their respective amyloid fibrils, permeabilized synthetic bilayer membranes in vitro. This study represents the first comprehensive comparison between the biophysical properties of SAA isoforms with distinct pathogenicities, and the results suggest that structural and kinetic differences in the oligomerization-fibrillation of SAA1.1 and SAA2.2, more than their intrinsic amyloidogenicity, may contribute to their diverse pathogenicity.

Highlights

Murine SAA1.1 is pathogenic and SAA2.2 is non-pathogenic in amyloid A (AA) amyloidosis
It is tempting to assume that pathogenic Serum amyloid A (SAA) are highly amyloidogenic and that non-pathogenic SAAs are amyloid-resistant, we have shown that the non-pathogenic murine SAA2.2 rapidly forms amyloid fibrils in vitro at 37 °C, making it one of the most inherently amyloidogenic proteins known to date [9]
SAA1.1 refolded into a mixture of oligomers with retention times in size exclusion chromatography (SEC) that were consistent with dodecamer (12.4 ml), tetramer (15.7 ml), and monomer (17.9 ml) (Fig. 1b)

Summary

Background

Murine SAA1.1 is pathogenic and SAA2.2 is non-pathogenic in AA amyloidosis. Results: SAA1.1 and SAA2.2 exhibit different biophysical properties, including fibrillation kinetics and fibril morphology. Deep UV resonance Raman, far UV-circular dichroism, atomic force microscopy, and fibrillation cross-seeding experiments suggest that SAA1.1 and SAA2.2 fibrils possess different morphology Both the long-lived oligomers of pathogenic SAA1.1 and the fleeting prefibrillar oligomers of nonpathogenic SAA2.2, but not their respective amyloid fibrils, permeabilized synthetic bilayer membranes in vitro. This study represents the first comprehensive comparison between the biophysical properties of SAA isoforms with distinct pathogenicities, and the results suggest that structural and kinetic differences in the oligomerization-fibrillation of SAA1.1 and SAA2.2, more than their intrinsic amyloidogenicity, may contribute to their diverse pathogenicity. Our results suggest possible structural and kinetic reasons that may contribute to the distinct pathogenic tendencies of SAA proteins

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION