Hypothesis

BAG3 functions as a mechanosensitive node that integrates extracellular matrix stiffness signals via focal adhesion kinase (FAK) to regulate the balance between chaperone‑mediated autophagy (CMA) and macroautophagy in muscle stem cells (MuSCs). We propose that mechanical strain activates FAK, which directly phosphorylates BAG3 on serine‑xxx residues. This phosphorylation reduces BAG3’s affinity for Hsp70 while increasing its interaction with p62/SQSTM1, thereby shifting the Hsp70‑BAG3 complex from a CMA‑promoting state to a macroautophagy‑promoting state. In young MuSCs cultured on compliant matrices, low FAK activity keeps BAG3 predominantly in the Hsp70‑bound conformation, supporting efficient CMA of glycolytic enzymes and preserving regenerative capacity. With age, increased matrix cross‑linking raises basal FAK activity, leading to chronic BAG3 phosphorylation, excessive macroautophagic flux, and selective depletion of CMA substrates. The resulting metabolic imbalance impairs MuSC proliferation and contributes to the regenerative decline observed in aged skeletal muscle.

Testable predictions

1. Phospho‑specific detection – Antibodies against phospho‑BAG3 (Ser‑xxx) will show low signal in young MuSCs on soft hydrogels and elevated signal in aged MuSCs or young MuSCs cultured on stiff (>15 kPa) substrates.
2. FAK dependence – Pharmacological inhibition of FAK (PF‑573228) or CRISPR‑mediated FAK knockout will decrease phospho‑BAG3 levels, restore Hsp70‑BAG3 interaction, and rescue CMA flux (measured by KFERQ‑based reporter) in aged MuSCs.
3. Mutational analysis – Expression of a phospho‑deficient BAG3 mutant (S→A) in aged MuSCs will maintain high Hsp70 binding, sustain CMA of glycolytic enzymes (e.g., GAPDH, PKM2), and improve proliferation and myotube formation compared with wild‑type BAG3. Conversely, a phospho‑mimetic mutant (S→D) will phenocopy the aged state even on soft matrices.
4. Metabolic rescue – Restoring CMA in phospho‑mimetic BAG3‑expressing MuSCs by overexpressing LAMP2A will replenish glycolytic enzyme levels and partially rescue proliferative capacity, linking the mechanotransduction‑autophagy axis to metabolic dysfunction.
5. In vivo validation – Mice bearing a knock‑in phospho‑deficient BAG3 allele will exhibit delayed onset of age‑related muscle weakness, higher MuSC regenerative capacity after injury, and reduced fibrosis compared with wild‑type littermates.

Mechanistic insight beyond existing data

While prior work established BAG3 as a proteotoxicity sensor and mechanotransduction hub 3 4, it did not specify how mechanical signals are transduced into changes in BAG3’s client‑protein selectivity. By positioning FAK as the upstream kinase that directly modifies BAG3, we bridge the gap between extracellular stiffness sensing and the observed shift from BAG1‑mediated proteasomal degradation to BAG3‑mediated autophagy during aging 1. Moreover, this model explains why glycolytic metabolism is particularly affected in aged MuSCs 2: CMA preferentially degrades glycolytic enzymes, and their loss shifts MuSCs toward a less proliferative, more oxidative state. Finally, the hypothesis offers a therapeutic angle: targeting the FAK‑BAG3 phosphorylation node could rebalance autophagy flux without globally altering BAG3 levels, addressing the unresolved question of whether optimal BAG1/BAG3 ratios or absolute BAG3 levels are more relevant 1.

Falsifiability – If phospho‑BAG3 levels do not change with matrix stiffness or FAK activity, or if manipulating BAG3 phosphorylation fails to affect CMA flux or MuSC regeneration, the core premise would be refuted.