BAG3 regulates the specificity of the recognition of specific MAPT species by NBR1 and SQSTM1

ABSTRACT

Macroautophagy/autophagy receptors are essential for the recognition and clearance of specific cargos by selective autophagy, which is essential for maintaining MAPT proteostasis. Previous studies have implicated different autophagy receptors in directing distinct species of MAPT to autophagy, but the underlying mechanisms have not been fully investigated. Here we examine how the autophagy receptors NBR1 and SQSTM1 differentially associate with specific forms of MAPT. In primary neurons depletion of NBR1, unlike depletion of SQSTM1, significantly increased phosphorylated MAPT levels. The specificity of the interactions was confirmed using in vitro binding assays with purified proteins. We provide direct evidence that the co-chaperone BAG3 promotes the preferential association of NBR1 with monomeric MAPT and SQSTM1 with oligomeric MAPT. Using an in vitro affinity-isolation assay, we show that SQSTM1 only binds to monomeric MAPT when BAG3 is absent and fails to bind when BAG3 is present. The opposite is true of NBR1; its association with monomeric MAPT was dependent on the presence of BAG3. Interestingly, in Alzheimer disease brain the association of NBR1 with BAG3 was significantly decreased. In a mouse model, ablation of BAG3 in neural cells disrupted the association of NBR1 with phosphorylated MAPT and led to increased levels of phosphorylated and oligomeric MAPT. Overall, our results uncover a novel role for BAG3 in regulating the specificity of selective autophagy receptors in targeting different species of MAPT and provide compelling evidence that BAG3 plays a key role in maintaining MAPT proteostasis.

Abbreviations: AD: Alzheimer disease; BAG3: BCL2-associated athanogene 3; BSA: bovine serum albumin; CERAD: Consortium to Establish a Registry for Alzheimer’s Disease; ESCRT: endosomal sorting complexes required for transport; GST: glutathione S-transferases; MAPT: microtubule-associated protein tau; NBR1: NBR1, autophagy cargo receptor; NFT: neurofibrillary tangles; PMI: postmortem interval; SQSTM1: sequestosome 1.

KEYWORDS:

• Autophagy receptor
• Alzheimer’s disease
• BAG3
• MAPT
• NBR1
• SQSTM1

Acknowledgements

We thank Dr. C. Pröschel, University of Rochester, for providing us with the pHUUG and FigB vectors; Dr. P. Davies for the gift of phospho-MAPT (Ser396/404) (PHF1) antibodies; and Dr. P. Dolan for the gift of phospho-MAPT (Ser262) antibody. Dr. N. Kanaan, Michigan State University for the gift of TOC1 antibody. T. Johansen, University of Tromso for MYC-SQSTM1 plasmid. Dr. Jeff Kuret, Ohio State University for pT7-htau40 vector. Brain samples were obtained from the University of Florida Neuromedicine Human Brain and Tissue Bank (UF HBTB) with informed consent of the patients or their relatives and the approval of the local institutional review boards. The UF HBTB is supported by the Florida Alzheimer’s Disease Research Center (P30AG066506). We would also thank Sudarshan Ramanan for assistance with some of the immunoblots.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/15548627.2023.2276622

Additional information

Funding

The work was supported by the Alzheimer’s Association [AARF-21-721039]; National Institutes of Health [R01 AG073121].