Investigating Proteolytic Processing of Ataxin 2, a Neurodegenerative Disease Associated Protein

Abstract

Ataxin 2 (ATXN2) is a ubiquitously expressed mRNA binding protein involved in the development and progression of spinocerebellar ataxia 2 (SCA2) and amyotrophic lateral sclerosis (ALS). In the context of both neurodegenerative diseases, its N-terminal polyglutamine (polyQ) domain is mutated and expanded in length. Several other polyQ proteins, such as huntingtin (Htt), ataxin 3 (ATXN3), and ataxin 7 (ATXN7), undergo proteolytic processing that produces toxic fragments containing their polyQ domains. Investigating how ATXN2 is regulated by proteolysis is hindered by the lack of available molecular biological tools such as N-terminal ATXN2 antibodies to target and analyze the endogenous N-terminus of ATXN2. To circumvent this challenge, I developed a transient overexpression model of N-terminally tagged ATXN2 in HEK293E cells. Here, I demonstrate that both wild-type and mutant ATXN2 are targets of N-terminal proteolysis. I confirmed that ATXN2 produces an independent polyQ cleavage fragment like other polyQ proteins through basic molecular biology approaches such as Western blotting and immunoprecipitation. Additionally, I identified the specific region that is both necessary and sufficient for cleavage to occur via deletion mapping with multiple truncated ATXN2 mutants and reporter constructs. Further definition of ATXN2 as a target of proteolytic cleavage aligns it with other neurodegenerative polyQ proteins, and proteolysis is currently a less explored avenue of research for ATXN2-related disease development, progression, and therapeutic modalities. This work reveals a novel site that directs cleavage of ATXN2 and provides a potential avenue of investigation for how ATXN2 posttranslational modifications contribute to the progression of SCA2 and ALS.