Spinal cord phosphoproteome of SCA2 mouse model reveals alteration of ATXN2-N-term PRM-SH3-actin interactome and of autophagy.

Abstract

Toxic polyglutamine (polyQ) expansions in ATXN2 trigger neurodegenerative processes, causing Spinocerebellar Ataxia type 2 (SCA2), and enhancing TDP-43-dependent pathology in Amyotrophic Lateral Sclerosis (ALS) / Fronto-Temporal Dementia (FTD). Primary disease events can be compensated transiently, delaying disease manifestation. To define potential therapy targets, here we studied how cells modify phosphoprotein signals, using preferentially affected nervous tissue from end-stage Atxn2-CAG100-KnockIn mice. The spinal cord phosphoproteome revealed massive hyperphosphorylations flanking the polyQ expansion in ATXN2 and for SQSTM1, and moderate hyperphosphorylations also for ALS proteins OPTN, UBQLN2, TNIP1 and TBK1-targeted TAX1BP1. Conversely, strong hypophosphorylations of WNK1, SPARCL1 and PSMD9 were found. Significant enrichments of SH3-containing proteins, autophagy / endocytosis factors, and actin modulators could be explained by N-terminal, polyQ-adjacent, proline-rich motifs (PRM) in ATXN2, suggesting that SCA2 pathogenesis is highly similar to Huntington's disease where neurotoxicity is mediated by abnormal polyQ-PRM-SH3 interactions. Validation of protein and mRNA levels were done in mouse spinal cord, and embryonic fibroblasts or patient fibroblasts after bafilomycin or arsenite treatment, observing polyQ-dependent OPTN deficiency and SQSTM1 induction impairment. Overall, this phosphoproteome profile identified and quantified the main cellular efforts in adapting autophagy pathways to the aggregation propensity of the ATXN2-N-term.