BAG3 functions as a redox‑sensitive mechanotransducer that couples oxidative strain to CASA activation via cysteine S‑glutathionylation

Hypothesis

BAG3 directly senses mechanical and oxidative stress through conserved cysteine residues in its BAG domain. Oxidative‑induced S‑glutathionylation of these cysteines modulates BAG3’s affinity for HSP70, promoting CASA complex assembly at sites of mechanical strain. This redox‑mechanosensing explains why aging elevates BAG3 levels yet CASA activity declines: chronic oxidative stress leads to over‑glutathionylation, locking BAG3 in a low‑affinity state that impairs HSP70 cycling and autophagosome formation.

Mechanistic Basis

• Redox switch: Cys‑14 and Cys‑31 (human BAG3) lie within the HSP70‑binding interface. S‑glutathionylation adds a bulky glutathione moiety, sterically hindering HSP70 nucleotide‑domain interaction and reducing ATP hydrolysis rates.
• Mechanical coupling: Stretch‑induced ROS production at focal adhesions activates glutathione‑transferase‑like enzymes (e.g., GSTP1) that catalyze BAG3 glutathionylation locally, concentrating the modified BAG3 at strained cytoskeleton (Z‑disks, postsynaptic densities).
• Functional outcome: Under acute strain, transient glutathionylation fine‑tunes BAG3‑HSP70 dynamics to boost CASA flux; persistent oxidative load causes hyper‑glutathionylation, diminishing HSP70 recruitment, slowing ubiquitination by CHIP, and causing CASA complex sequestration (as seen with P209L/Y233X mutants).

Testable Predictions

1. Mutagenesis: BAG3 C14S/C31S (non‑glutathionylable) mutants will show heightened HSP70 ATPase activity and increased LC3‑II puncta under basal conditions, but fail to further increase autophagy when cells are subjected to cyclic stretch or H2O2.
2. Redox biosensor: A roGFP2‑based BAG3 sensor will reveal rapid, strain‑dependent oxidation that correlates with p62‑positive autophagosome formation at Z‑disks in adult cardiomyocytes.
3. Pharmacological blockade: Inhibiting glutathione synthesis (BSO) or glutathionylation (with monochlorobimane) will blunt stretch‑induced CASA activation, while mimicking glutathionylation (using GST‑fusion peptides) will suppress it even under mechanical load.
4. Aging model: In aged mouse hearts, BAG3 will exhibit higher baseline glutathionylation, reduced HSP70 binding, and rescued CASA flux upon treatment with glutaredoxin‑1 overexpression or NAC supplementation.

Experimental Plan

• Cell models: Primary neonatal rat cardiomyocytes and differentiated human iPSC‑neurons expressing WT or mutant BAG3‑GFP.
• Stress application: Cyclic tensile strain (10% elongation, 1 Hz) ± 200 µM H2O2 for 30 min.
• Readouts: Co‑immunoprecipitation of BAG3‑HSP70, ATPase assays, LC3‑II/ p62 immunoblotting, puncta quantification via confocal microscopy, contractility (edge detection in cardiomyocytes) or synaptic vesicle recycling (FM‑4‑64 in neurons).
• In vivo validation: AAV9‑mediated cardiac delivery of BAG3 C14S/C31S in aged mice; assess ejection fraction, fibrosis, and ubiquitinated sarcomeric proteins.

Falsifiability

If BAG3 cysteine glutathionylation does not alter HSP70 binding, autophagy flux, or mechanoprotective outcomes under the described conditions, the hypothesis is refuted. Conversely, confirmation would establish a novel redox‑mechanostat linking extracellular stress to selective autophagy, offering a mechanistic explanation for age‑related CASA decline and a target for therapeutic intervention.