Spinocerebellar ataxia type 1 (SCA1) is caused by the expansion of a trinucleotide repeat that encodes a polyglutamine tract in ataxin-1 (ATXN1). The expanded polyglutamine in ATXN1 increases the protein's stability and results in its accumulation and toxicity. Previous studies have demonstrated that decreasing ATXN1 levels ameliorates SCA1 phenotypes and pathology in mouse models. We rationalized that reducing ATXN1 levels through pharmacological inhibition of its modulators could provide a therapeutic avenue for SCA1. Here, through a forward genetic screen in Drosophila we identified, p21-activated kinase 3 (Pak3) as a modulator of ATXN1 levels. Loss-of-function of fly Pak3 or Pak1, whose mammalian homologs belong to Group I of PAK proteins, reduces ATXN1 levels, and accordingly, improves disease pathology in a Drosophila model of SCA1. Knockdown of PAK1 potently reduces ATXN1 levels in mammalian cells independent of the well-characterized S776 phosphorylation site (known to stabilize ATXN1) thus revealing a novel molecular pathway that regulates ATXN1 levels. Furthermore, pharmacological inhibition of PAKs decreases ATXN1 levels in a mouse model of SCA1. To explore the potential of using PAK inhibitors in combination therapy, we combined the pharmacological inhibition of PAK with MSK1, a previously identified modulator of ATXN1, and examined their effects on ATXN1 levels. We found that inhibition of both pathways results in an additive decrease in ATXN1 levels. Together, this study identifies PAK signaling as a distinct molecular pathway that regulates ATXN1 levels and presents a promising opportunity to pursue for developing potential therapeutics for SCA1.
Bibliographical noteFunding Information:
This project was supported by the National Institutes of Health/ National Institute of Neurological Disorders and Stroke (NIH/ NINDS) 2R37NSO27699 (H.Y.Z.); NIH/NINDS F32 NS083091 (Q.T.); Baylor College of Medicine IDDRC grant 1 U54HD083092 from the Eunice Kennedy Shriver National Institute of Child Health & Human Development. The content is solely the responsibility of the authors and does not necessarily represents the official views of the National Institutes of Health (administrative and microscopy cores). H.Y.Z. is an investigator of the Howard Hughes Medical Institute. The authors would like to thank Rituraj Pal and Maxime W. C. Rousseaux for experimental suggestions and critical reading of the article.