The Characterization of Liquid-Liquid Phase Separation in a Spinocerebellar Ataxia Type I Model
Spinocerebellar ataxia type I (SCA1) is a neurodegenerative disease caused by an expansion of glutamine residues within the ataxin-1 protein. Ataxin-1 is a nuclear protein with unknown function, although it is known to have the ability to bind RNA. RNA-binding proteins, such as ataxin-1, require mechanisms to localize RNA interactions within the nucleus. One such mechanism is liquid-liquid phase separation (LLPS), whereby RNA-binding proteins undergo a phase transition in the nucleus. This forms a membraneless, dynamic liquid droplet, which assists in concentrating the RNA-binding protein interactions. Due to preliminary evidence suggesting ataxin-1 undergoes LLPS, as well as the implications of LLPS in other neurodegenerative disorders, we aimed to characterize LLPS in a SCA1 model.
Ataxin-1 forms distinct nuclear inclusions when transfected and over expressed in human cells, which appear similar to liquid droplets formed by other LLPS proteins. To characterize ataxin-1 nuclear inclusions as liquid droplets, live-cell imaging was used to analyze the liquid-like properties of both wild type and disease-causing (mutant) ataxin-1. We show that both wild type and mutant ataxin-1 nuclear inclusions are mobile and can coalesce. Further analyses suggestthat mutant ataxin-1 is more mobile between nuclear inclusions compared to wild type ataxin-1. This implies that mutant ataxin-1 is more liquid-like than wild type ataxin-1. Examining differences in LLPS properties of wild type and mutant ataxin-1 could provide insight into SCA1 pathogenic mechanisms, although further characterization is required.