Myopathy associated BAG3 mutations lead to protein aggregation by stalling Hsp70 networks

Abstract

BAG3 is a multi-domain hub that connects two classes of chaperones, small heat shock proteins (sHSPs) via two isoleucine-proline-valine (IPV) motifs and Hsp70 via a BAG domain. Mutations in either the IPV or BAG domain of BAG3 cause a dominant form of myopathy, characterized by protein aggregation in both skeletal and cardiac muscle tissues. Surprisingly, for both disease mutants, impaired chaperone binding is not sufficient to explain disease phenotypes. Recombinant mutants are correctly folded, show unaffected Hsp70 binding but are impaired in stimulating Hsp70-dependent client processing. As a consequence, the mutant BAG3 proteins become the node for a dominant gain of function causing aggregation of itself, Hsp70, Hsp70 clients and tiered interactors within the BAG3 interactome. Importantly, genetic and pharmaceutical interference with Hsp70 binding completely reverses stress-induced protein aggregation for both BAG3 mutations. Thus, the gain of function effects of BAG3 mutants act as Achilles heel of the HSP70 machinery.

Metformin rescues muscle function in BAG3 myofibrillar myopathy models

“…Despute this, BAG3 P209L is functional and can rescue the loss of function. This is consistent with the proposal that the formation of protein aggregates results in sequestration of functional BAG3, both BAG3 P209L and BAG3 wt , ultimately causing insufficiency 26 and analyses showing BAG3 P209L is correctly folded and functional 27 . Therefore, whilst the aggregation of BAG3 P209L is the initial trigger for the disease, it does not directly cause muscle weakness.…”

The landscape of molecular chaperones across human tissues reveals a layered architecture of core and variable chaperones

“…For example, overexpression of the folding enzyme FKBP51 in a tau transgenic mouse model resulted in accumulation of tau and its toxic oligomers (19)(20)(21)(22). Likewise, chaperone downregulation or genetic aberration can cause diseases, such as mutation in the NEF co-chaperone BAG3 that leads to myopathy (23). These observations imply that the quantitative composition of the chaperone system can improve or impair its proteostatic capacity.…”

Dysregulated interactions triggered by a neuropathy-causing mutation in the IPV motif of HSP27

“…Less binding of the P182L IxI/V motif to the ACD would leave its β4/β8 groove more accessible to interactions with other parts of the protein (e.g. the NTD of itself or IxI/V of WT HSP27) or other proteins (37,75,76). Indeed, for a different IxI/V-containing protein (BAG3), our coIP data demonstrate that it binds more tightly to the P182L variant of HSP27 than the WT form, and that this interaction depends on the IPV motifs in BAG3 (Fig.…”

“…5). This co-chaperone/chaperone interaction promotes new proteinprotein interactions, in part by bringing together HSP27 with other BAG3-bound chaperones, including HSP70 (37,75). Moreover, BAG3 is recruited to stress granules via its interactions with HSPB8 where the BAG3-HSP8-HSP70 complex plays a key role in ensuring stress granule functionality (77).…”