Dysregulation of ubiquitin homeostasis and β-catenin signaling promote spinal muscular atrophy
Thomas M. Wishart, … , Brunhilde Wirth, Thomas H. Gillingwater
Thomas M. Wishart, … , Brunhilde Wirth, Thomas H. Gillingwater
Published March 3, 2014
Citation Information: J Clin Invest. 2014;124(4):1821-1834. https://doi.org/10.1172/JCI71318.
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Dysregulation of ubiquitin homeostasis and β-catenin signaling promote spinal muscular atrophy

Abstract

The autosomal recessive neurodegenerative disease spinal muscular atrophy (SMA) results from low levels of survival motor neuron (SMN) protein; however, it is unclear how reduced SMN promotes SMA development. Here, we determined that ubiquitin-dependent pathways regulate neuromuscular pathology in SMA. Using mouse models of SMA, we observed widespread perturbations in ubiquitin homeostasis, including reduced levels of ubiquitin-like modifier activating enzyme 1 (UBA1). SMN physically interacted with UBA1 in neurons, and disruption of Uba1 mRNA splicing was observed in the spinal cords of SMA mice exhibiting disease symptoms. Pharmacological or genetic suppression of UBA1 was sufficient to recapitulate an SMA-like neuromuscular pathology in zebrafish, suggesting that UBA1 directly contributes to disease pathogenesis. Dysregulation of UBA1 and subsequent ubiquitination pathways led to β-catenin accumulation, and pharmacological inhibition of β-catenin robustly ameliorated neuromuscular pathology in zebrafish, Drosophila, and mouse models of SMA. UBA1-associated disruption of β-catenin was restricted to the neuromuscular system in SMA mice; therefore, pharmacological inhibition of β-catenin in these animals failed to prevent systemic pathology in peripheral tissues and organs, indicating fundamental molecular differences between neuromuscular and systemic SMA pathology. Our data indicate that SMA-associated reduction of UBA1 contributes to neuromuscular pathogenesis through disruption of ubiquitin homeostasis and subsequent β-catenin signaling, highlighting ubiquitin homeostasis and β-catenin as potential therapeutic targets for SMA.

Authors

Thomas M. Wishart, Chantal A. Mutsaers, Markus Riessland, Michell M. Reimer, Gillian Hunter, Marie L. Hannam, Samantha L. Eaton, Heidi R. Fuller, Sarah L. Roche, Eilidh Somers, Robert Morse, Philip J. Young, Douglas J. Lamont, Matthias Hammerschmidt, Anagha Joshi, Peter Hohenstein, Glenn E. Morris, Simon H. Parson, Paul A. Skehel, Thomas Becker, Iain M. Robinson, Catherina G. Becker, Brunhilde Wirth, Thomas H. Gillingwater

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Figure 2

UBA1 physically interacts with SMN protein in vivo, and Uba1 splicing is dysregulated at late symptomatic time points in SMA mouse spinal cord.

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UBA1 physically interacts with SMN protein in vivo, and Uba1 splicing is...
(A) No change in levels of Uba1 mRNA (or a control mRNA, Fth1; similar control data using Mapt not shown) in the spinal cord of P5 severe SMA mice, quantified using qPCR (n = 3 mice per genotype; ANOVA with Tukey’s post hoc test). (B) Representative fluorescent Western blots for SMN (left lane) and UBA1 (right lane) from co-IP experiments on spinal cord extracts from WT mice, using SMN-bound beads, demonstrating that UBA1 physically interacts with SMN in vivo. (C) Graphic overview of the exon structure of Uba1. Two Uba1 splice variants are generated with unique first exons. The position of primers used to amplify each splice variant is highlighted. Note that the coding sequence of Uba1 starts in exon 2. (D–F) Bar charts showing relative expression levels of Uba1a and Uba1b, as well as the ratio of Uba1a to Uba1b, in SMA (Taiwanese) and control spinal cord at P3 (D; presymptomatic), P7 (E; early symptomatic), and P11 (late-symptomatic) (n = 3 mice per genotype, 3 independent amplifications per sample; 2-tailed, unpaired t tests). Uba1 splicing was significantly dysregulated in the late-symptomatic mice. **P < 0.01; ***P < 0.001.