Exogenous factors that impact huntingtin aggregation

While expansion of a polyglutamine (polyQ) domain is the immediate cause of huntingtin (htt) aggregation associated with Huntington’s Disease (HD), other cellular factors modify aggregation. These include interactions with cellular membranes, protein biding partners, molecular crowding, and proteinaceous seeds. Here, two important factors are biophysically characterized: 1) the interaction of htt with endomembranes and 2) proteinaceous seeds obtained from a variety of htt-derived peptides. In the first project, the aggregation of htt at bilayer interfaces and in the presence of divalent cations was investigated. A major cellular factor implicated in altered htt aggregation is the binding of lipids. Furthermore, the addition of divalent cations (Ca2+ or Mg2+) greatly enhanced htt aggregation only in the presence of lipids. Divalent cations promote fibrillization directly on the bilayer surface and induce unique roughening of the disrupted membranes suggesting that the ability of divalent cations to promote fibril formation on membranes is mediated by inducing alterations to the physicochemical properties of lipid membranes. The second project explored how htt aggregation is altered in the presence of htt seeds. The first 17 N-terminal amino acids (Nt17) that precede the polyQ in htt-exon1 enhances aggregation whereas, the proline rich domain (PRD) extends past the polyQ domain and is implicated in the inhibition of htt aggregation. Htt-exon1 with polyQ domains of 20Q and 46Q was exposed to synthetic htt peptide seeds with varied flanking sequences in the presence and absence of lipids. Htt seeding readily occurred across polyQ lengths and was independent of flanking sequence. However, the addition of lipid vesicles altered the seeds’ ability to fibrillize htt. Further, exposure of C. elegans expressing a nonpathogenic N-terminal htt fragment (15-repeat glutamine residues) induced inclusion formation and resulted in decreased viability at sufficient concentrations. Collectively, these results demonstrate the complexity of htt aggregation, and the significant role cellular environment plays