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.

Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting

“…Interestingly, GSK-3 affects multiple pathways such as glycogen synthesis, Wnt/␤-catenin signaling, and protein synthesis, which all have been shown to play a role in skeletal muscle (157,368). Recent evidence also indicates that GSK-3 directly phosphorylates neighbor of BRCA gene 1 (NBR1) to prevent aggregation of ubiquitinated proteins and to enhance their autophagosomal degradation (324). In addition to the above activities of Akt/PKB, phosphorylation and thereby inhibition of FoxO transcription factors are most significant for the control of muscle mass (FIGURE 5A).…”

Hif-2α Promotes Degradation of Mammalian Peroxisomes by Selective Autophagy

“…For example, hypoxia-induced dephosphorylation of FUNDC1, a mitochondrial outer membrane protein, enhances its interaction with the core autophagic machinery and promotes mitophagy (Liu et al, 2012). Nbr1 phosphorylation by glycogen synthase kinase 3 prevents the aggregation of ubiquitinated proteins and their selective autophagic degradation (Nicot et al, 2014). Hif-2a could activate mRNA translation of a pexophagy receptor or a protein-modifying gene as part of its recently identified transcription-independent role in the regulation of translation (Uniacke et al, 2012).…”

Autophagy in adhesion and migration

“…Additionally, glycogen synthase kinase 3β (GSK3β) could also have a role in spatially restricting selective autophagy to leading-edge focal adhesions. GSK3β is locally inactivated at leading-edge lamella where focal adhesions turn over (Barth et al, 2008), and phosphorylation of NBR1 by GSK3β inhibits its ability to turnover ubiquitylated proteins (Nicot et al, 2014). This regulation suggests that NBR1 is preferentially activated at leading-edge focal adhesions where GSK3β is not active.…”