Blocking p62-dependent SMN degradation ameliorates spinal muscular atrophy disease phenotypes
Natalia Rodriguez-Muela, … , Rajat Singh, Lee L. Rubin
Natalia Rodriguez-Muela, … , Rajat Singh, Lee L. Rubin
Published April 19, 2018
Citation Information: J Clin Invest. 2018;128(7):3008-3023. https://doi.org/10.1172/JCI95231.
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Blocking p62-dependent SMN degradation ameliorates spinal muscular atrophy disease phenotypes

Abstract

Spinal muscular atrophy (SMA), a degenerative motor neuron (MN) disease, caused by loss of functional survival of motor neuron (SMN) protein due to SMN1 gene mutations, is a leading cause of infant mortality. Increasing SMN levels ameliorates the disease phenotype and is unanimously accepted as a therapeutic approach for patients with SMA. The ubiquitin/proteasome system is known to regulate SMN protein levels; however, whether autophagy controls SMN levels remains poorly explored. Here, we show that SMN protein is degraded by autophagy. Pharmacological and genetic inhibition of autophagy increases SMN levels, while induction of autophagy decreases these levels. SMN degradation occurs via its interaction with the autophagy adapter p62 (also known as SQSTM1). We also show that SMA neurons display reduced autophagosome clearance, increased p62 and ubiquitinated proteins levels, and hyperactivated mTORC1 signaling. Importantly, reducing p62 levels markedly increases SMN and its binding partner gemin2, promotes MN survival, and extends lifespan in fly and mouse SMA models, revealing p62 as a potential new therapeutic target for the treatment of SMA.

Authors

Natalia Rodriguez-Muela, Andrey Parkhitko, Tobias Grass, Rebecca M. Gibbs, Erika M. Norabuena, Norbert Perrimon, Rajat Singh, Lee L. Rubin

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Figure 5

SMN deficiency results in mTOR activation, which could contribute to the autophagy impairment observed in SMA cells.

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SMN deficiency results in mTOR activation, which could contribute to the...
(A) Representative immunoblot from human healthy control and SMA types I and 0 fibroblast lysates. Cells were cultured in the presence of 10% serum (S+), deprived of serum (S–) for 16 hours, or deprived of serum and then incubated for an additional 30 minutes with serum (S–/+). Phosphorylated and total levels of the mTOR targets S6K1 and S6 were measured. Quantifications are shown in Supplemental Figure 4A. (B) Representative immunoblot of mouse WT and SMA (SMNΔ7 and A2) MN lysates to measure mTOR pathway activity. Quantifications are shown on Supplemental Figure 4B. (C) Representative immunoblot from human healthy control and SMA MN lysates to measure mTOR pathway activity. (D) Quantifications of p-S6/S6 and p-S6K1/S6K1 ratios from human healthy control and SMA MN lysates. *P < 0.05 and **P < 0.01, by 2-tailed t test (n = 6 independent experiments). Data indicate the mean ± SEM, expressed relative to BJ healthy control MNs. (E) Scheme of the interplay between low SMN-p62-mTOR–defective autophagy.