Ataxin-1 regulates B cell function and the severity of autoimmune experimental encephalomyelitis Alessandro Didonnaa,1, Ester Canto Puiga, Qin Maa, Atsuko Matsunagaa, Brenda Hoa, Stacy J. Cailliera, Hengameh Shamsa, Nicholas Leea, Stephen L. Hausera, Qiumin Tan (谭秋敏)b, Scott S. Zamvila,c, and Jorge R. Oksenberga aWeill Institute for Neurosciences, Department of Neurology, University of California, San Francisco, CA 94158; bDepartment of Cell Biology, University of Alberta, Edmonton, AB T6G 2H7, Canada and cProgram in Immunology, University of California, San Francisco, CA 94158 Edited by Lawrence Steinman, Stanford University School of Medicine, Stanford, CA, and approved August 5, 2020 (received for review February 28, 2020) Ataxin-1 (ATXN1) is a ubiquitous polyglutamine protein expressed ataxin-1 as a potential contributor to Alzheimer’s disease (AD) primarily in the nucleus where it binds chromatin and functions as risk (9). a transcriptional repressor. Mutant forms of ataxin-1 containing Multiple sclerosis (MS) is a chronic disorder of the central expanded glutamine stretches cause the movement disorder spi- nervous system (CNS) characterized by focal lymphocyte infil- nocerebellar ataxia type 1 (SCA1) through a toxic gain-of-function tration, breakdown of myelin sheaths, proliferation of astrocytes, mechanism in the cerebellum. Conversely, ATXN1 loss-of-function and axonal degeneration. Although the primary etiology remains is implicated in cancer development and Alzheimer’s disease (AD) unknown, the interplay between genetic and environmental pathogenesis. ATXN1 was recently nominated as a susceptibility factors is believed to initiate MS pathogenesis (10). In a recent locus for multiple sclerosis (MS). Here, we show that Atxn1-null large-scale genomic effort, the locus containing the ATXN1 gene mice develop a more severe experimental autoimmune encepha- was found associated with MS susceptibility (11). Here, we build lomyelitis (EAE) course compared to wildtype mice. The aggravated on this discovery to characterize the role of ataxin-1 in the phenotype is mediated by increased T helper type 1 (Th1) cell po- context of CNS autoimmunity. By employing the MS model ex- larization, which in turn results from the dysregulation of B cell ac- perimental autoimmune encephalomyelitis (EAE) in Atxn1-null tivity. Ataxin-1 ablation in B cells leads to aberrant expression of key mice, we show that ataxin-1 exerts a B cell-mediated protective costimulatory molecules involved in proinflammatory T cell differ- effect on autoimmune demyelination, enabled by native inter- IMMUNOLOGY AND INFLAMMATION entiation, including cluster of differentiation (CD)44 and CD80. In actions with the transcriptional repressor capicua (CIC). addition, comprehensive phosphoflow cytometry and transcrip- tional profiling link the exaggerated proliferation of ataxin-1 defi- Results cient B cells to the activation of extracellular signal-regulated kinase Ataxin-1 Exerts a Protective Activity on Autoimmune Demyelination. (ERK) and signal transducer and activator of transcription (STAT) pathways. Lastly, selective deletion of the physiological binding The known genetic contributions to MS risk consist of 232 au- partner capicua (CIC) demonstrates the importance of ATXN1 native tosomal risk variants (11). This map includes a region in chro- interactions for correct B cell functioning. Altogether, we report a mosome 6p22.3 (position 16672769 in hg19), in which the marker immunomodulatory role for ataxin-1 and provide a functional de- rs719316 in the third intron of the ATXN1 gene represents the = × −13 = scription of the ATXN1 locus genetic association with MS risk. strongest association (P 1.62 10 ,oddsratio(OR) 1.072) multiple sclerosis | autoimmunity | antigen presentation | B cells | ataxin-1 Significance he discovery of the ATXN1 gene and the subsequent mo- Over 200 genomic loci have been found associated with the Tlecular characterization of its protein product ataxin-1 have risk of developing multiple sclerosis (MS). Despite this impor- taken place mostly in the context of spinocerebellar ataxia type 1 tant body of data, limited information exists on the cellular (SCA1) (1). Ataxin-1 contains an unstable polyglutamine (polyQ) pathways and molecular mechanisms underlying MS genetic domain, which can undergo pathological expansion and cause the complexity. In this study, we report the functional character- selective neurodegeneration of cerebellar Purkinje cells—the ization of the ataxin-1 encoding ATXN1 susceptibility locus. principal site of SCA1 pathology—resulting in progressive motor Ataxin-1 is a polyglutamine protein that is classically associated incoordination. Mutant ataxin-1 escapes the standard cellular with the neurodegenerative disorder spinocerebellar ataxia systems of protein degradation and accumulates within the nuclear type 1 (SCA1). Here, we show that ataxin-1 also exerts a pro- compartment of neuronal cells, where it exerts its pathogenic ac- tective activity against autoimmune demyelination in a pre- tivity through a toxic gain-of-function mechanism (2). Ataxin-1 is clinical model of MS. This function is associated with an able to bind chromatin and interacts with a number of known immunomodulatory role mainly targeting the B cell compart- transcriptional repressors, indicating a role in the regulation of ment. Altogether, these findings expand our current knowl- gene expression (3). However, the full spectrum of ataxin-1 edge on both MS pathogenesis and ataxin-1 biology. functions is far from being fully described. Author contributions: A.D., S.L.H., S.S.Z., and J.R.O. designed research; A.D., E.C.P., Q.M., Mice lacking ataxin-1 are viable, fertile, and do not show any A.M., B.H., S.J.C., N.L., and Q.T. performed research; Q.T. contributed new reagents/ana- evidence of ataxia or neurodegeneration. Notwithstanding the lytic tools; A.D., E.C.P., Q.M., H.S., S.S.Z., and J.R.O. analyzed data; and A.D., S.L.H., Q.T., lack of gross phenotypes, Atxn1-null mice display decreased ex- S.S.Z., and J.R.O. wrote the paper. ploratory behavior and impaired performance on the rotarod The authors declare no competing interest. apparatus and water maze test (4). A body of experimental ev- This article is a PNAS Direct Submission. idence also supports a role for ataxin-1 in neurodevelopment, This open access article is distributed under Creative Commons Attribution-NonCommercial- adult neurogenesis, and extracellular matrix remodeling (5–7). In NoDerivatives License 4.0 (CC BY-NC-ND). addition, ataxin-1 controls cell proliferation and the epithelial- 1To whom correspondence may be addressed. Email: [email protected]. mesenchymal transition (EMT) of cancer cells (8). More recently, This article contains supporting information online at https://www.pnas.org/lookup/suppl/ ataxin-1 loss-of-function was shown to elevate beta secretase 1 doi:10.1073/pnas.2003798117/-/DCSupplemental. (BACE1) expression and aggravate Aβ pathology, proposing www.pnas.org/cgi/doi/10.1073/pnas.2003798117 PNAS Latest Articles | 1of9 Downloaded by guest on September 30, 2021 in both fixed- and random-effects models (Fig. 1A). Subsequent cells (APCs) influences T cell differentiation. To discern between analysis in African Americans possibly refined the association, these two scenarios, we measured the propensity of isolated naïve with the top single nucleotide polymorphism (SNP) rs932411 CD4+ T cells to differentiate toward Th1, Th17, and Treg lineages mapping within the seventh intron of ATXN1 (P = 0.0022) (12). in response to nonselective T cell receptor stimulation via anti- However, seven genes map to the ATXN1 locus, each one CD3/anti-CD28 engagement. Under these conditions, no differ- representing a potential candidate that could explain the asso- ences were detected between naïve knockout and wildtype T cells ciation with MS susceptibility. To discern among them, we ap- (Fig. 2D), suggesting that the elevated Th1 differentiation post- − − plied a recently developed in silico approach, computing the immunization and exacerbated phenotype of the Atxn1 / strain regulatory potential of rs719316 to all of the neighboring genes could result from an indirect effect on APC function. To test this in the extended haplotype block in the context of cell-specific hypothesis, we analyzed by flow cytometry the cell surface ex- protein networks (13). ATXN1 showed the highest scores in all of pression of CD44 and CD80 in splenic T cells (CD4+ and CD8+), the cell types analyzed (Fig. 1B), adding support to consider B cells, and monocytes. Both proteins are critical costimulatory ATXN1 as the most plausible disease risk gene within the locus. molecules in T cell activation and promote Th1 differentiation (16, Concurrently, expression quantitative trait locus (eQTL) analysis 17). Basal expression of both markers in naïve mice was increased in both brain and immune tissues [Genotype-Tissue Expression in knockout B cells while CD44 was decreased in knockout T cells (GTEx) Portal] excluded long-range effects targeting genes (Fig. 2E and SI Appendix,TableS1). A similar increase in CD44 outside the locus. Therefore, we decided to functionally validate and CD80 levels was also measured in knockout B cells and this prediction in vivo exploring the role of ATXN1 in EAE, a monocytes upon MOG peptide stimulation (Fig. 2F and SI Ap- murine disease that recapitulates several clinical, immunological, pendix,TableS1). B cells participate in Th1 reactivation in and histopathological features of