Ataxin-3 protein and RNA toxicity in spinocerebellar ataxia type 3: current insights and emerging therapeutic strategies

Abstract

Ataxin-3 is a ubiquitously expressed deubiqutinating enzyme with important functions in the proteasomal protein degradation pathway and regulation of transcription. The C-terminus of the ataxin-3 protein contains a polyglutamine (PolyQ) region that, when mutationally expanded to over 52 glutamines, causes the neurodegenerative disease spinocerebellar ataxia 3 (SCA3). In spite of extensive research, the molecular mechanisms underlying the cellular toxicity resulting from mutant ataxin-3 remain elusive and no preventive treatment is currently available. It has become clear over the last decade that the hallmark intracellular ataxin-3 aggregates are likely not the main toxic entity in SCA3. Instead, the soluble PolyQ containing fragments arising from proteolytic cleavage of ataxin-3 by caspases and calpains are now regarded to be of greater influence in pathogenesis. In addition, recent evidence suggests potential involvement of a RNA toxicity component in SCA3 and other PolyQ expansion disorders, increasing the pathogenic complexity. Herein, we review the functioning of ataxin-3 and the involvement of known protein and RNA toxicity mechanisms of mutant ataxin-3 that have been discovered, as well as future opportunities for therapeutic intervention.

Fig. 1

Schematic representation of the ATXN3 gene, exon–intron structure and protein product showing protein functional domains, posttranslational modifications and binding domains of the main interacting partners. a The ATXN3 gene (Ensembl transcript ID ENST00000393287) consists of 11 exons with the start codon in exon 1 and the CAG repeat in exon 10. The shape of the boxes depict the reading frame, nt nucleotides. The height of the introns are relative to their actual length. b The ataxin-3 protein consists of 361 amino acids (aa) with a Josephin domain in the N-terminal part that contains crucial amino acids for its isopeptidase activity [cysteine 14 (C), histidine 119 (H), and asparagine 134 (N)] and two nuclear export signals (NES). The C-terminal part contains three ubiquitin interacting motifs (UIM 1 to 3), a nuclear localisation signal (NLS) and the polyglutamine (polyQ) repeat. Specific amino acids known to undergo posttranslational modifications are indicated as follows: yellow circles phosphorylation (P), purple eclipse ubiquitination (Ub), orange triangle calpain cleavage site, pink triangle caspase cleavage motif. c Binding domains of the main interacting partners: ubiquitin; VCP/p97 valosin-containing protein, hHR23A and hHR23B human homologues of yeast protein RAD23, and DNA

Fig. 2

Schematic representation of cellular pathogenesis in SCA3. The ATXN3 gene can be transcribed into mRNA containing 11 exons. The expanded CAG repeat is located in the penultimate exon and the transcript is translated into a mutant polyQ repeat containing protein. This polyQ repeat triggers conformational changes, resulting in abnormally folded mutant ataxin-3. Mutant ataxin-3 can be proteolytically cleaved, giving rise to C-terminal fragments and possibly N-terminal fragments (dashed line) that are aggregation-prone. Full length and cleaved forms of ataxin-3 form soluble monomers, oligomers or large insoluble aggregates, both in the nucleus and in the cytoplasm that cause toxicity. Other cellular disturbances resulting from mutant ataxin-3 presence involved in SCA3 pathogenesis include transcriptional deregulation, impaired autophagy, mitochondrial dysfunction, proteasomal impairment, and compromised axonal transport. Next to mutant polyQ-induced toxicity, there is likely also an RNA toxicity component involved in the disease pathogenesis