Mechanism of Light Chain Amyloid Fibril Formation: Seeding-Nucleation and Polymerization Amyloid Formation and the Cross-Seeding Phenomenon

Abstract

Amyloid fibril formation has been described as a nucleation-polymerization reaction, whose sigmoidal kinetics is characteristic of an auto-catalytic reaction. Nascent fibrils or “nuclei” catalyze the formation of new fibrils via a positive feedback. In addition, fragments of preformed fibrils (self-seeding) can accelerate fibril formation; fibrils of proteins with different sequence (cross-seeding) are able to accelerate fibril formation, albeit with a lower efficiency. In this work, we have investigated the early events in the AL amyloid fibril formation pathway -the recruiting and addition of monomer to nascent fibrils- by dissecting the process into the nucleation and elongation process via self- and cross seeding experiments. We have tested the effect of self- and cross-seeding on the kinetics of fibril formation of five AL proteins, with a broad range of protein stabilities and amyloidogenic properties. Our data reveals that in the presence of seeds, the amyloid formation reactions become less stochastic compared to non-seeded reactions. Interestingly, the most amyloidogenic proteins showed little effects over the fibril formation kinetics. In contrast, the less amyloidogenic proteins presented a differential effect dependent of the seed used. Also, the non-amyloidogenic control has a poor efficiency in the elongation phase with homologous and heterologous seeds. Together, these results indicate that the fibril formation reaction is kinetically determined by the availability of amyloidogenic “nuclei”, and is modulated by the differential ability of proteins to elongate fibrils, while still following the same pathway and yielding the same fibril morphology.