PKA phosphorylation of ATAXIN1 in Purkinje cells modulates early onset of ataxia

Spinocerebellar ataxia type 1 (SCA1) is a fatal adult-onset, autosomal dominant ataxia characterized in part by dysfunction and degeneration of Purkinje cells of the cerebellum. The fundamental basis of pathology is an aberration in the regulation of RNA splicing and gene transcription. SCA1 is caused by an unstable CAG trinucleotide repeat mutation in the ATXN1 gene that codes for a toxic ATXN1 protein with an abnormal polyglutamine repeat. Decreasing mutant ATXN1 can reverse disease phenotypes in SCA1 mouse models. Phosphorylation of ATXN1 at Serine 776 (S776) is critical for disease and this modification influences ATXN1 protein levels and protein-protein interactions, which can exacerbate toxicity. Previous in vitro studies implicated PKA, cAMP protein kinase, in phosphorylation of ATXN1 at S776. The hypothesis being tested is that PKA-mediated ATXN1-S776 phosphorylation stabilizes ATXN1 and drives pathogenic pathways involved in disease. SCA1 mouse models expressing wild type human ATXN1[30Q] or mutant human ATXN1[82Q] were crossed to a PKA mutant mice that exhibit attenuated PKA activity. I found that PKA hypofunction leads to a decrease of phospho-S776-ATXN1 and total ATXN1 expressed in cerebellar Purkinje neurons. Mouse Atxn1 protein expressed in other cerebellar cell types was unchanged, pointing to cell specificity. In order to evaluate the disease relevance of these effects, I tested SCA1 disease metrics in the affected model, including motor behavior, histopathology and gene expression changes. Motor performance was improved to wild type levels early in disease, but progressive Purkinje cell atrophy was not averted. These results hinted at a dissociation between mechanisms causing ataxia versus Purkinje cell degeneration. Indeed, RNA sequencing studies revealed transcriptional changes linked to motor dysfunction that are distinct from those associated with progressive pathology. This work suggests ATXN1 is phosphorylated by PKA in Purkinje neurons early in disease and drives pathways that underlie early onset ataxia that are independent of pathways promoting progressive neurodegeneration.