Phosphorylation of NBR1 by GSK3 modulates protein aggregation

Abstract

The autophagy receptor NBR1 (neighbor of BRCA1 gene 1) binds UB/ubiquitin and the autophagosome-conjugated MAP1LC3/LC3 (microtubule-associated protein 1 light chain 3) proteins, thereby ensuring ubiquitinated protein degradation. Numerous neurodegenerative and neuromuscular diseases are associated with inappropriate aggregation of ubiquitinated proteins and GSK3 (glycogen synthase kinase 3) activity is involved in several of these proteinopathies. Here we show that NBR1 is a substrate of GSK3. NBR1 phosphorylation by GSK3 at Thr586 prevents the aggregation of ubiquitinated proteins and their selective autophagic degradation. Indeed, NBR1 phosphorylation decreases protein aggregation induced by puromycin or by the DES/desmin N342D mutant found in desminopathy patients and stabilizes ubiquitinated proteins. Importantly, decrease of protein aggregates is due to an inhibition of their formation and not to their autophagic degradation as confirmed by data on Atg7 knockout mice. The relevance of NBR1 phosphorylation in human pathology was investigated. Analysis of muscle biopsies of sporadic inclusion body myositis (sIBM) patients revealed a strong decrease of NBR1 phosphorylation in muscles of sIBM patients that directly correlated with the severity of protein aggregation. We propose that phosphorylation of NBR1 by GSK3 modulates the formation of protein aggregates and that this regulation mechanism is defective in a human muscle proteinopathy.

Keywords: :

• autophagy
• GSK3
• NBR1
• skeletal muscle
• sIBM
• sporadic inclusion body myositis
• ubiquitinated protein aggregates

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

We thank the patients for participation in this study. We thank Dr James Woodgett for the kind gift of Gsk3a, Gsk3b^+/+, and gsk3^−/− MEFs, Drs Jocelyn Laporte, Karim Hnia, Pierre Jalinot, Christelle Morris, and Mathias Faure for kindly providing us with plasmids; Drs Yann-Gaël Gangloff, Vincent Mocquet, and April Reedy for critical reading of the manuscript. We also thank the microscopy facility (Plateau technique Imagerie/Microscopie) of the SFR Biosciences Gerland–Lyon Sud (UMS3444/US8) and its staff Christophe Chamot, Claire Lionnet, and Olivier Duc for technical assistance with imaging. This work was supported by a grant from the Association Française contre les Myopathies (AFM 16084). M Sandri was supported by a grant from the European Research Council (ERC 282310-MyoPHAGY). AS Nicot was a recipient of fellowships from AFM (14496) and from AXA Research Fund.