The cochaperone BAG3 coordinates protein synthesis and autophagy under mechanical strain through spatial regulation of mTORC1

Abstract

The cochaperone BAG3 is a central protein homeostasis factor in mechanically strained mammalian cells. It mediates the degradation of unfolded and damaged forms of the actin-crosslinker filamin through chaperone-assisted selective autophagy (CASA). In addition, BAG3 stimulates filamin transcription in order to compensate autophagic disposal and to maintain the actin cytoskeleton under strain. Here we demonstrate that BAG3 coordinates protein synthesis and autophagy through spatial regulation of the mammalian target of rapamycin complex 1 (mTORC1). The cochaperone utilizes its WW domain to contact a proline-rich motif in the tuberous sclerosis protein TSC1 that functions as an mTORC1 inhibitor in association with TSC2. Interaction with BAG3 results in a recruitment of TSC complexes to actin stress fibers, where the complexes act on a subpopulation of mTOR-positive vesicles associated with the cytoskeleton. Local inhibition of mTORC1 is essential to initiate autophagy at sites of filamin unfolding and damage. At the same time, BAG3-mediated sequestration of TSC1/TSC2 relieves mTORC1 inhibition in the remaining cytoplasm, which stimulates protein translation. In human muscle, an exercise-induced association of TSC1 with the cytoskeleton coincides with mTORC1 activation in the cytoplasm. The spatial regulation of mTORC1 exerted by BAG3 apparently provides the basis for a simultaneous induction of autophagy and protein synthesis to maintain the proteome under mechanical strain.

Fine-tuning of actin dynamics by the HSPB8-BAG3 chaperone complex facilitates cytokinesis and contributes to its impact on cell division

“…Here, we provide further evidence for a connection between the activity of HSPB8-BAG3 during cytokinesis and autophagy signaling by showing that treatment of HSPB8-depleted cells with rapamycin could prevent F-actin accumulation. This is in line with recent work suggesting that BAG3 modulates the spatial activity of mTORC1-the target of rapamycin and a key regulator of autophagy signaling, in addition to its direct interaction with the selective autophagic machinery via p62/SQSTM1 (Gamerdinger et al 2011;Kathage et al 2017). Overall, though the exact mechanism whereby HSPB8 and BAG3 promote disassembly of cytokinetic actin structures remains to be established, we think that we can reasonably speculate that the chaperone complex could act in the autophagic sequestration of proteins that regulate assembly of actin networks, a function that would be modulated by BAG3 mitotic phosphorylation.…”

Activation of mTORC1 signalling in rat skeletal muscle is independent of the EC‐coupling sequence but dependent on tension per se in a dose‐response relationship

“…On the contrary, possible mechanosensitive proteins could be located in structural regions of the muscle cell subjected to tensile stress during passive deformation. The most likely candidate proteins include those associated with the sarcolemma (eg, integrins or cadherins), focal adhesion (eg, FAK or actin filaments), costamere (eg, dystrophin and vinculin) and the sarcomeric Z‐line (eg, titin, phospholipase D or Filamin C unfolding‐BAG3) …”

The Role of the Multifunctional BAG3 Protein in Cellular Protein Quality Control and in Disease

“…In addition to its BAG domain, the human 575 aa BAG3 protein contains three other characteristic amino acid motifs/regions and domains: a WW (tryptophan-tryptophan) domain, two IPV (isoleucine–proline–valine) motifs and a PxxP (proline-rich) region. Its N-terminally located WW domain interacts with proline-rich repeats of proteins, such as the guanine nucleotide exchange factor 2 (PDZGEF2), the adenovirus (Ad) penton base protein, synaptopodin-2 (SYNPO2), the YAP/TAZ inhibitors LATS1/2 or AMOTL1/2 or the tuberous sclerosis 1 (TSC1) protein (Gout et al, 2010; Iwasaki et al, 2010; Ulbricht et al, 2013b; Kathage et al, 2017). Two conserved IPV motifs were identified N-terminally and in the middle of BAG3 and mediate the binding of BAG3 to the small heat shock proteins HSPB8 (HSP22) as well as HSPB6 (HSP20) and to a smaller extend also to HSPB5 (αB-crystallin) and HSPB1 (HSP27) (Carra et al, 2008a; Fuchs et al, 2009; Rauch et al, 2017).…”

“…Upon mechanical stress or muscle exercise a multi-chaperone complex consisting of BAG3, HSP70, HSPB8 and the ubiquitin ligase STUB1 targets damaged filamin for degradation to lysosomes by an autophagic process, termed chaperone-assisted selective autophagy (CASA). In addition, BAG3 seems to support autophagosome formation by interacting with synaptopodin-2 and to initiate autophagy by interfering with mTORC1 regulation (by binding to TSC1, a component of the mTORC1 inhibiting complex) (Ulbricht et al, 2013a,b, 2015; Kathage et al, 2017). Concomitantly with CASA, BAG3 also induces the transcription of filamin by engaging the YAP/TAZ inhibitors LATS1/2 and AMOTL1/2 (Ulbricht et al, 2013b).…”