Abstract 1415 Investigating the impact of polyglutamine expansion on the conformational dynamics of Huntingtin Exon 1 in monomeric and oligomeric states using state-of-the-art physics-based simulations

Huntingtin (Htt) is a 348 kDa multifunctional protein which is broadly involved in nervous system development and function. The exon 1 of Htt (Httex1) encodes a disordered polypeptide (91 amino acids), comprising of an N-terminal region (N17, 17 residues), a central polyglutamine (polyQ) tract (normal length∼23 amino acids) and a C-terminal proline rich domain (PRD, 51 residues). The expansion of the polyQ tract (>36 residues) coupled with subsequent oligomerization is associated with Huntington disease, a neurodegenerative condition marked by the presence of Htt inclusions (aggregates and condensates) in both nucleus and cytoplasm. The structural changes in Httex1 associated with polyQ tract expansion are of considerable interest due to its increased aggregation propensity and toxic gain-of-function in vivo. Despite extensive efforts, unraveling these structural changes has proven to be challenging due to the disordered nature of Httex1. We have performed extensive, atomistic molecular (MD) simulations of Httex1 fragments in monomeric and oligomeric states (aggregate time > 0.5 milliseconds) to investigate the secondary structural changes and inter-domain interactions related to its pathological aggregation. Using a state-of-the-art force field and water model, we generated conformational ensembles of non-pathogenic (Q16-32) and pathogenic (Q46) Httex1 fragments which showed excellent agreement with NMR-derived, α-helical fractions. Notably, with increasing polyQ length, we also observed an emergent tendency for transient β-sheet formation on the sub-microsecond timescale. Simulations of the Httex1 dimer (Q7/16) revealed a heterogeneous ensemble consisting of a variety of inter-molecular orientations (parallel, perpendicular and anti-parallel) which can interconvert on the microsecond timescale. The stability of ɑ-helical conformations in the N17 region was markedly enhanced in the dimeric state. Interestingly, an increase in polyQ length (from 7 to 16 residues) led to increased inter-conversion between the dimeric orientations and an enhancement in the rate of dimer dissociation. In conclusion, our findings provide fundamental insights into the effect of polyQ tract expansion on structural dynamics and interactions underlying Httex1 aggregation at high spatial and temporal resolution. The increase in β-sheet forming propensity of Httex1 monomers with increasing polyQ length provides a molecular-level basis for understanding the elevated aggregation propensity of pathogenic fragments. Further, our simulations indicate that the faster aggregation kinetics of pathogenic fragments may be intrinsically-linked to enhanced conformational transitions in the oligomeric state. We would like to acknowledge TAMU HPRC facility for access to computing resources used to perform MD simulations.