Glucocorticoid receptor-dependent therapeutic efficacy of tauroursodeoxycholic acid in preclinical models of Spinocerebellar ataxia type 3
Spinocerebellar ataxia type 3 (SCA3) is an adult-onset neurodegenerative disease caused by a polyglutamine expansion in the ataxin-3 (ATXN3) gene.  No effective treatment is available for this disorder, other than symptomatic approaches. Bile acids have shown therapeutic efficacy in neurodegenerative disease models. Here, we pinpointed tauroursodeoxycholic acid (TUDCA) as an efficient therapeutic, improving the motor and neuropathological phenotype of SCA3 nematode and mouse models. Surprisingly, transcriptomic and functional in vivo data showed that TUDCA acts in neuronal tissue through the glucocorticoid receptor (GR), but independently of its canonical receptor, the FXR. TUDCA was also predicted to bind to the GR, similarly to corticosteroid molecules. GR levels were decreased in disease-affected brain regions, likely due to increased protein degradation as a consequence of ATXN3 dysfunction, being restored by TUDCA treatment. Lastly, based on results of a cohort of pre-symptomatic and symptomatic SCA3 patients, we demonstrated that peripheral expression of GR and FKBP5 might be indicators of clinical conversion and disease progression, respectively. We have established a novel in vivo mechanism for the neuroprotective effects of TUDCA in SCA3, and propose this readily available drug for a clinical trial in SCA3 patients. Overall design: RNA was extracted from flash-frozen brainstem of 34-week-old mice using the AllPrep® DNA/RNA/miRNA kit (Qiagen, Hilden, Germany), following the manufacturer's instructions. A total of 3 mice per group (WT, TG and 7 day TUDCA-treated TG mice) was evaluated. Library preparation was carried out by targeted amplicon amplification for AmpliSeq, as previously described (https://doi.org/10.1186/s12864-015-2270-1). The quality of assembled libraries was assessed using the Agilent Bioanalyzer High Sensitivity chip. Library amplification was performed by emulsion PCR in an Ion Chef Instrument (Thermo Scientific, Waltham, Massachusetts), and cDNA quality was assessed with the Agilent TapeStation High Sensitivity tape. Sequencing was performed in the Ion S5TM XL sequencer (Thermo Scientific, Waltham, Massachusetts), and reads were aligned to the AmpliSeq Mouse Transcriptome v1, obtaining a total of 23,930 transcripts.