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 3

Genetic and pharmacological suppression of uba1 in zebrafish leads to dose-dependent motor axon pathology.

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Genetic and pharmacological suppression of uba1 in zebrafish leads to do...
(A) Representative fluorescence micrographs of motor axons growing out from the spinal cord in a control zebrafish 34 hours after fertilization, and in animals injected with either 4 ng or 6 ng of a MO suppressing uba1 levels (see Supplemental Figure 4). (B) Representative higher-magnification confocal micrographs showing abnormal sprouts and axonal extensions in motor axons from MO-treated zebrafish. Scale bars: 50 μm. (C) Dose-dependent increase in the occurrence of abnormal branching in MO-treated zebrafish (Kruskal-Wallis test with Dunn’s post hoc test; uninjected, n = 310 motor neurons, n = 31 animals; 4 ng, n = 360, n = 36 animals; 6 ng, n = 360, n = 36 animals). Only motor axons with modest (type 2; see Supplemental Figure 4) or severely abnormal branching (type 3; see Supplemental Figure 4) were quantified as having abnormal branching. (D) Representative confocal micrographs showing perturbations in motor axon morphology in Tg(hb9:gfp) zebrafish embryos treated with 50 μM of the UBA1 inhibitor UBEI-41. Note the presence of a “double-exit” motor axon (right hand side of image) in the UBEI-41 example, with the axon branch emerging on the right side of the pair showing stunted outgrowth. Scale bars: 100 μm; 30 μm (B, D). (E) Levels of abnormal motor axon branching and axon outgrowth in UBEI-41–treated zebrafish. Note the dose-dependent increase of numbers of aberrant motor axons in the UBEI-41 group compared with DMSO controls (10 μM UBEI-41 n = 258 nerves, n = 11 animals; 50 μM UBEI-41 n = 280 nerves n = 12 animals; Kruskal-Wallis test with Dunn’s post hoc test). ***P < 0.001.