Potentially amyloidogenic conformational intermediates populate the unfolding landscape of transthyretin: Insights from molecular dynamics simulations

Abstract

Protein aggregation into insoluble fibrillar structures known as amyloid characterizes several neurodegenerative diseases, including Alzheimer's, Huntington's and Creutzfeldt-Jakob. Transthyretin (TTR), a homotetrameric plasma protein, is known to be the causative agent of amyloid pathologies such as FAP (familial amyloid polyneuropathy), FAC (familial amyloid cardiomiopathy) and SSA (senile systemic amyloidosis). It is generally accepted that TTR tetramer dissociation and monomer partial unfolding precedes amyloid fibril formation. To explore the TTR unfolding landscape and to identify potential intermediate conformations with high tendency for amyloid formation, we have performed molecular dynamics unfolding simulations of WT-TTR and L55P-TTR, a highly amyloidogenic TTR variant. Our simulations in explicit water allow the identification of events that clearly discriminate the unfolding behavior of WT and L55P-TTR. Analysis of the simulation trajectories show that (i) the L55P monomers unfold earlier and to a larger extent than the WT; (ii) the single a-helix in the TTR monomer completely unfolds in most of the L55P simulations while remain folded in WT simulations; (iii) L55P forms, early in the simulations, aggregation-prone conformations characterized by full displacement of strands C and D from the main b-sandwich core of the monomer; (iv) L55P shows, late in the simulations, severe loss of the H-bond network and consequent destabilization of the CBEF b-sheet of the b-sandwich; (v) WT forms aggregationcompatible conformations only late in the simulations and upon extensive unfolding of the monomer. These results clearly show that, in comparison with WT, L55P-TTR does present a much higher probability of forming transient conformations compatible with aggregation and amyloid formation.

Pathogenic Mutations Induce Partial Structural Changes in the Native β-Sheet Structure of Transthyretin and Accelerate Aggregation

“…Theses results indicate that destabilization of the DA β-structure may be critical to the effective amyloid formation, as suggested by previous computational studies. 56,57 …”

“…61 Taken all together, the destabilization of the DA β-structure under amyloidogenic conditions may trigger more effective amyloid formation of the TTR variants, as suggested by other biophysical studies. 22,34,56,62 …”

“…The structural change of the strand D may accelerate amyloid formation of the pathogenic TTR variants, which is in good agreements with previous computational studies. 56,62 …”

Protein-Protein interactions leading to aggregation: Perspectives on mechanism, significance and control

“…Other studies involve model proteins such as insulin [140], lysozyme [141], and disease-related proteins including transthyrethin, microglobulin, synuclein, and the prion protein [142][143][144][145]. A number of studies take a more general approach, in order to derive conclusions as to the cause of the existing variations in fibril morphologies [146], or on the contrary, characterization of common segments (for example Q/N rich regions) that would be of general importance in fibril formation [147].…”

Localized Structural Fluctuations Promote Amyloidogenic Conformations in Transthyretin

“…Wild type and pathogenic variant TTRs have been subjected to molecular dynamics simulations to obtain insights into conformational flexibility and the transition to amyloidogenic states. The amyloidogenic mechanism has been variously related to changes in conformation and dynamics in strands F, G, and H, 20 displacement of β-strands C and D from the β-sheet, 21,22 destabilization of the CBEF β-sheet, 22 destabilization of the DAGH β-sheet, 21 and cooperative conformational transitions to α-sheet in both β-sheets of TTR. 23,24 …”