Protein quality control during aging involves recruitment of the macroautophagy pathway by BAG3

Abstract

The Hsc/Hsp70 co-chaperones of the BAG (Bcl-2-associated athanogene) protein family are modulators of protein quality control. We examined the specific roles of BAG1 and BAG3 in protein degradation during the aging process. We show that BAG1 and BAG3 regulate proteasomal and macroautophagic pathways, respectively, for the degradation of polyubiquitinated proteins. Moreover, using models of cellular aging, we find that a switch from BAG1 to BAG3 determines that aged cells use more intensively the macroautophagic system for turnover of polyubiquitinated proteins. This increased macroautophagic flux is regulated by BAG3 in concert with the ubiquitin-binding protein p62/SQSTM1. The BAG3/BAG1 ratio is also elevated in neurons during aging of the rodent brain, where, consistent with a higher macroautophagy activity, we find increased levels of the autophagosomal marker LC3-II as well as a higher cathepsin activity. We conclude that the BAG3-mediated recruitment of the macroautophagy pathway is an important adaptation of the protein quality control system to maintain protein homeostasis in the presence of an enhanced pro-oxidant and aggregationprone milieu characteristic of aging.

BAG3 induction is required to mitigate proteotoxicity via selective autophagy following inhibition of constitutive protein degradation pathways

“…Of note, the conversion of LC3-I to LC3-II was not impaired in ST80/ Bortezomib-cotreated cells (Figure 2b BAG3 is upregulated upon ST80/Bortezomib cotreatment As ST80/Bortezomib cotreatment triggers accumulation of potentially toxic protein aggregates, we hypothesized that the cotreatment simultaneously engages a rescue mechanism in surviving cells to mitigate proteotoxic stress, thereby allowing cells to recover. As the co-chaperone BAG3 has been reported to be upregulated when proteasomal function declines during aging, 8 we analyzed BAG3 expression levels. Intriguingly, ST80/ Bortezomib cotreatment strongly co-operated to increase BAG3 mRNA and protein levels in the subpopulation of RMS cells that survived ST80/Bortezomib cotreatment (Figures 3a-c, Supplementary Figure S2b).…”

“…9 Also, BAG3 interacts with dynein and stimulates substrate transfer from HSP70 to the dynein motor complex via its nucleotide-exchange factor activity, thereby promoting the retrograde transport of misfolded proteins along microtubules to the microtubule organizing center to form insoluble protein aggregates and aggresomes. 8 Furthermore, BAG3 binds to the selective autophagy receptor p62 that links ubiquitinated substrates to the autophagy marker protein LC3 on autophagosomal membranes, thereby targeting misfolded proteins to autophagosomes for their degradation via selective autophagy. 8 Also, BAG3 has recently been reported to mediate selective autophagy of misfolded proteins independently of substrate ubiquitination.…”

“…7 For example, during aging when the activity of the proteasome declines, a transcriptional switch from Bcl-2-associated athanogene 1 (BAG1)-regulated protein degradation via the proteasome to BAG3-dependent selective autophagy has recently been described. 8 BAG3 is a co-chaperone of the BAG family (BAG1-6) and binds to chaperone proteins such as HSP70 and HSPB8, which allows the recognition and binding to misfolded substrates. 9 Furthermore, BAG3 stimulates substrate transfer from HSP70 to the dynein motor complex by acting as a nucleotide-exchange factor, thereby facilitating the dyneinmediated retrograde transport of misfolded proteins to the microtubule organizing center to form aggresomes.…”

“…9 Furthermore, BAG3 stimulates substrate transfer from HSP70 to the dynein motor complex by acting as a nucleotide-exchange factor, thereby facilitating the dyneinmediated retrograde transport of misfolded proteins to the microtubule organizing center to form aggresomes. 8 In addition, BAG3 promotes selective autophagy via its interaction with p62. 8 Given the importance of proper PQC to maintain cell viability, these pathways have been exploited as therapeutic targets in oncology.…”

“…8 In addition, BAG3 promotes selective autophagy via its interaction with p62. 8 Given the importance of proper PQC to maintain cell viability, these pathways have been exploited as therapeutic targets in oncology. 10 For example, Bortezomib, a reversible proteasome inhibitor, demonstrated potent in vitro and in vivo antitumor activity against various hematological malignancies and solid cancers, and is approved for the treatment of certain tumors, for example, multiple myeloma.…”