Impaired Oligodendrocyte Maturation Is an Early Feature in SCA3 Disease Pathogenesis

Abstract

Spinocerebellar ataxia Type 3 (SCA3), the most common dominantly inherited ataxia, is a polyglutamine neurodegenerative disease for which there is no disease-modifying therapy. The polyglutamine-encoding CAG repeat expansion in the ATXN3 gene results in expression of a mutant form of the ATXN3 protein, a deubiquitinase that causes selective neurodegeneration despite being widely expressed. The mechanisms driving neurodegeneration in SCA3 are unclear. Research to date, however, has focused almost exclusively on neurons. Here, using equal male and female age-matched transgenic mice expressing fulllength human mutant ATXN3, we identified early and robust transcriptional changes in selectively vulnerable brain regions that implicate oligodendrocytes in disease pathogenesis. We mapped transcriptional changes across early, mid, and late stages of disease in two selectively vulnerable brain regions: the cerebellum and brainstem. The most significant disease-associated module through weighted gene coexpression network analysis revealed dysfunction in SCA3 oligodendrocyte maturation. These results reflect a toxic gain-of-function mechanism, as ATXN3 KO mice do not exhibit any impairments in oligodendrocyte maturation. Genetic crosses to reporter mice revealed a marked reduction in mature oligodendrocytes in SCA3-disease vulnerable brain regions, and ultrastructural microscopy confirmed abnormalities in axonal myelination. Further study of isolated oligodendrocyte precursor cells from SCA3 mice established that this impairment in oligodendrocyte maturation is a cell-autonomous process. We conclude that SCA3 is not simply a disease of neurons, and the search for therapeutic strategies and disease biomarkers will need to account for non-neuronal involvement in SCA3 pathogenesis.

Drosophila as a Model of Unconventional Translation in Spinocerebellar Ataxia Type 3

“…Utilizing the versatility of Drosophila genetics along with the Gal4-UAS binary system of expression, we can express either the homomeric or the alternating polyQ repeat of pathogenic ataxin-3 transgenes individually in specific tissues [ 39 , 40 , 41 , 42 , 43 , 45 , 47 , 51 , 70 , 72 , 73 ]. We focused our investigations on the type of tissue that is primarily impacted in SCA3/MJD, neuronal and glial [ 14 , 15 , 18 , 74 ]. We used Gal4 drivers that express constructs in all neurons (elav-Gal4) or in all glial cells (except midline glia; repo-Gal4), and employed the same constructs as in Figure 1 , except that the Kozak sequences were optimized for fly-based translation.…”

“…The experiments conducted in Figure 2 B,D were conducted with flies collected during development from lines expressing either construct in glia. Although we were unable to study differences in adult longevity between our two lines in glia, the high levels of toxicity in these cells compared to neurons points to an interesting role for glia in SCA3/MJD pathogenesis and is an active field of investigation; a recent study identified impaired oligodendrocyte maturation as a contributor to pathogenesis in a transgenic mouse model of SCA3/MJD [ 74 ].…”

ASOs are an effective treatment for disease-associated oligodendrocyte signatures in premanifest and symptomatic SCA3 mice

“…Indeed, in the brainstem, cerebellum, and spinal cord, many of the representative OL maturation transcripts were expressed at levels comparable to WT levels through 4 weeks of age ( Fig 2F-I ). Our previous study showed that changes in mature OL RNA expression translated to reduced mature OL protein expression 21 , though not until 8 weeks of age in Q84 mice. Due to the study design, we are unable to evaluate the downstream translational effects at 4 weeks.…”

“…Dysregulation in mature myelinating OL transcripts begins between 3 and 4 weeks of age in Q84 mice 21 , therefore we addressed if the onset of this impairment could be impeded with premanifest ASO treatment. Indeed, in the brainstem, cerebellum, and spinal cord, many of the representative OL maturation transcripts were expressed at levels comparable to WT levels through 4 weeks of age ( Fig 2F-I ).…”

“…To determine how GO term myelination pathways in treated and nontreated symptomatic mice are affected throughout the mouse brain, we transcriptionally To understand the cellular implications of regional OL transcript and protein expression rescue, we examined mature OL cell counts utilizing the BAC transgenic Mobp-eGFP reporter mouse 30 crossed with SCA3 mice. We previously used this mouse model cross to show that in SCA3 vulnerable brain regions (pons and DCN), mature Mobp-eGFP positive OLs are significantly reduced in a dose-dependent disease manner 21 . Thus, we questioned if mature OL cell counts could be rescued with ASO-5 treatment at a timepoint in which behavior is also rescued 18 .…”