Molecular Misreading: The Frameshift Species as Modulating Agents of Aggregation and Neurodegenerative Phenotype of Polyglutamine Proteins

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Macroscopically discernible inclusions, containing detergent-insoluble fibrillar aggregates of proteins with expanded polyglutamine (polyQ) tract are the major histological hallmark of the polyglutamine-repeat diseases, e. g., Huntington’s disease and several spinocerebellar ataxias. The polyQ stretches are exclusively encoded by the CAG codon, which is intrinsically highly prone to translational frameshifting. A shift of the reading frame within the polyQ stretch will lead to GCA encoded polyalanine (polyA) or AGC encoded polyserine (polyS) stretches, both of which compared the polyQ repeats posses a higher tendency to aggregate. Here, using a broad spectrum of in vivo and ex vivo approaches we aim to explore the impact of the stochastic frameshifting within the polyQ stretches on the disease onset and the role of frameshift products in modulating the toxicity and re-shaping the aggregation pathway of the parental polyQ protein. Furthermore, we will address whether changes in the environmental factors (e.g., cognate glutaminyl-tRNA availability) dependent on the cell stage or aging can alter frameshifting frequency and thus the polyQ aggregation pattern. The research on the cell-type specificity in polyQ diseases will additionally focus on elongation factor 1A (eEF1A) and has been found to be recruited in pathologic polyQ-aggregates. The two cell-type specific isoforms of eEF1A (eEF1A1 and eEF1A2) possess largely non-overlapping expression patterns: whereas EF1A1 is nearly ubiquitously expressed, EF1A2 is restricted to long lasting postmitotic cells. Thereby, we seek to understand whether expression of eEF1A2 can be related to an increased frameshift rate, which in turn could explain the cell-type specificity of polyQ-mediated toxicity and the predominant manifestation of polyQ diseases in postmitotic cells.This new direction of our research seeks to fill the remarkable void in current understanding of the link between the molecular events of aggregate formation and key features of the clinical histopathology (e.g., cell-type specificity, heterogeneity of the aggregation process and time of onset). This proposal is a logical extension of our current research program; it allows us to build on our expertise on structural studies on the aggregation mechanism of polyQ proteins, and we have established the experimental procedures necessary to technically accomplish the goals of this proposal.