Role of non-polyQ regions on the aggregation of polyQ proteins into amyloid fibrils triggered by polyQ expansions

Ten neurodegenerative diseases, referred to as polyglutamine (polyQ) diseases, are associated
with the aggregation into amyloid fibrils of ten different proteins containing a polyQ
expansion higher than a pathological threshold comprised between 35 to 45Q (1, 2). A large
body of evidence indicate that the polyQ expansion is the critical determinant for the
aggregation of these polyQ proteins. The aggregation process of polyQ proteins is, however,
still not well understood. To better understand this mechanism at a molecular level, we have
characterized model polyQ proteins made of the β-lactamase BlaP from Bacillus licheniformis
749/C and a polyQ tract of 0 to 79Q inserted either at position 197 or position 216 of BlaP.
Those chimeras recapitulate the same aggregation behaviours than that of disease-associated
polyQ proteins: there is a glutamine threshold for the aggregation into amyloid fibrils and the
anticipation phenomenon. Most importantly, the threshold critically depends on the structural
integrity of BlaP (3) which would impose some conformational and/or sterical constraints to
the polyQ tract. Moreover the position of the polyQ insertion into BlaP modifies the
aggregation propensity of BlaP chimeras. The present work aims to further investigate (i) how
the protein context affects the different phases of the aggregation phenomenon (i.e. the
nucleation and elongation phases) and (ii) the role of the oligomers formed during the early
time of the aggregation process. The techniques used are mainly (1) quartz crystal
microbalance (QCM) and atomic force microscopy (AFM) to study the elongation step of
amyloid fibril formation and (2) dynamic light scattering (DLS) to study the evolution of the
different populations formed during the aggregation time course. The results of these
experiments indicate that the native conformation of BlaP197(Gln)55 interferes mainly with the
nucleation but not with the elongation step of amyloid fibril formation. Moreover, these
results demonstrate that the sequences flanking the polyQ tract significantly influence its
propensity to elongate amyloid fibrils. Finally, they clearly indicate that the oligomers of
BlaP197(Gln)79 observed at the early stage of the aggregation process are on the pathway of
amyloid fibril formation, and likely constitute the aggregation nucleus.