Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder that primarily affects cerebellar Purkinje cells and brain stem neurons, causing progressive ataxia, motor impairment and eventual death. SCA1 is one of nine polyglutamine neurodegenerative diseases; we have shown that in SCA1, the expanded CAG repeat tract in ataxin-1 (Atx-1) causes the protein to gain some toxic function and misfold and/or accumulate in neurons. Our ataxin-1 null mice have a mild learning deficit, but the normal function of this protein has proven elusive. Yet it is becoming increasingly clear that we need to understand the normal function of this protein to gain deeper insight into the pathogenic mechanisms of SCA1. For example, we found that overexpression of wild-type (WT) Atx-1 produces a mild SCA1 phenotype and that both WT and mutant Atx-1 share interacting proteins and genetic modifiers. Both forms are phosphorylated by Akt at S776, and Atx-1-induced degeneration can be suppressed by either eliminating this phosphorylation site or decreasing Akt activity. We recently learned that the represser Gfi-1 and its Drosophila homolog Senseless are physical and genetic interactors of mammalian and Drosophila Atx-1 homologs, respectively, which further implicates the normal function of Atx-1 in pathogenesis. Along with the fact that Gfi-1 null mice develop Purkinje cell degeneration, these data led us to propose that a component of the gain-of-function mechanism in SCA1 is enhanced normal function and/or interactions of Atx-1. In this proposal we therefore seek to determine the function and genetic and physical interactions of Atx-1 to gain insight into SCA1 pathogenesis. We will first generate Atx-1 null mice that also lack the Atx-1 paralog/interactor (Atx-1L) to eliminate functional redundancy and study the consequences of loss of both proteins. We will determine if heterozygosity for loss of Atx-1, Atx-1L, or Gfi-1 modify the phenotypes of our authentic model of SCA1, the Scal 154Q/+ knockin mice. We will study the gene expression profiles in Purkinje cells of Scal 154Q/+, Gfi-1, Atx-1, Atx-1L, double Atx-1;Atx-1L null mice, and control mice; we will identify proteins that physically interact with Atx-1, Atx-1L, and Gfi-1. Data from phenotypic and genetic interaction studies together with the transcriptome and interactome data will allow us to build molecular networks relevant to SCA1 pathogenesis and Atx-1 function. This multidisciplinary approach is very powerful for revealing therapeutic targets, as we have illustrated for the Akt pathway. We will thus also pursue preclinical trials targeting the Akt pathway as well as novel pathways revealed by the proposed studies