How important is the role of compact denatured states on amyloid formation by transthyretin?

Amyloid fibril formation and deposition have been associated with a series of diseases, including Alzheimer ́s, Spongiform encephalopathies, and several systemic amyloidosis. In most of these amyloid diseases, it has been shown that the normal precursor protein, due to proteolysis, mutation or molecular environment stress, undergoes misfolding, leading to molecular species with a high tendency for ordered aggregation into amyloid. However, the structural nature of these amyloidogenic intermediates is the subject of debate. Transthyretin (TTR) is a homotetrameric plasma protein with a high percentage of beta-sheet. TTR has been implicated in diseases such as Familial Amyloid Polyneuropathy (FAP) and Senil Systemic Amyloidosis (SSA). The current view on the mechanism of amyloid formation by TTR implies tetramer dissociation and monomer partial unfolding (for a recent review see (1)). However, very little is known about the structure of the amyloidogenic intermediates or the extent of monomer unfolding required for amyloid formation. Limited proteolysis in the fibrillar state and monoclonal antibody binding to highly amyloidogenic TTR mutants suggested loss of structure in beta-strands C and D and also in loop DE (2). Here, based on molecular dynamics (MD) unfolding simulations of TTR, we propose that compact denatured states may play a central role as amyloidogenic species. From our simulations it is not clear how partially or locally unfolded species could provide the framework for protofibril formation. Alternatively, our simulations seem to indicate that in order to unfold beta-strands C and D and the loop DE, and expose beta-strands A and B for subunit interaction and aggregation, a global unfolding event is required. Thus, amyloid formation by TTR does not seem to be mediated by a local structural fluctuation or a local partial unfolding event, but by a global denaturing process of the subunit beta-sandwich. If in fact, compact denatured states play a central role on amyloid formation, the good agreement between amyloidogenic potential and protein conformational stability, observed for several proteins, could be more easily explained.