Hypothesis

Mechanical tension directly controls the accessibility of BAG3’s LC3‑interacting region (LIR) motifs, toggling BAG3 between a autophagy‑inhibitory state (bound to BAG1/HSP70) and an autophagy‑promoting state (exposed LIRs that recruit LC3B). In young, low‑tension cells BAG3 sequesters BAG1 and HSP70, suppressing basal autophagy; loss of BAG3 removes this brake, causing excessive flux. With aging, increased cytoskeletal tension and oxidative modifications induce a conformational shift that exposes BAG3 LIRs, enabling LC3B binding and CASA activation; BAG3 deficiency then abolishes this tension‑driven autophagy, leading to insufficient clearance.

Mechanistic Rationale

1. LIR accessibility as a tension sensor – BAG3 contains multiple LIR motifs flanking a flexible coiled‑coil domain. Applying cyclic stretch (as shown to induce BAG3‑dependent autophagosome formation [2]) could pull apart this domain, reducing intramolecular shielding of the LIRs and increasing their LC3‑binding affinity. This mirrors force‑dependent exposure seen in talin and vinculin.
2. Competing interactions – In the absence of stretch, BAG3’s BAG domain binds HSP70/HSPB8, while its N‑terminal region interacts with BAG1, forming a ternary complex that sterically blocks LIRs. The BAG1‑BAG3 switch described in aging raises the BAG3/BAG1 ratio, but only when tension is present does excess BAG3 translate into active autophagy.
3. Oxidative priming – Age‑related ROS can cysteine‑oxidize residues near the LIRs, further decreasing their affinity for intramolecular partners and favoring LC3 engagement, thus linking the redox environment to mechanosensing.

Testable Predictions

• Prediction 1: Mutating the core hydrophobic residues of each BAG3 LIR (to alanine) will abolish LC3B binding under stretch but will not affect BAG1/HSP70 interaction in relaxed cells.
• Prediction 2: A FRET‑based tension sensor inserted into BAG3’s coiled‑coil will show increased FRET efficiency (indicating extension) upon cyclic stretch, correlating with heightened LC3B co‑immunoprecipitation.
• Prediction 3: Juvenile BAG3‑KO cardiomyocytes will exhibit normalized autophagy flux when expressing a LIR‑dead BAG3 mutant that cannot bind LC3B but retains BAG1/HSP70 binding, confirming that excess autophagy stems from loss of the inhibitory complex.
• Prediction 4: In aged wild‑type hearts, pharmacological inhibition of actomyosin tension (blebbistatin) will reduce BAG3‑LC3B interaction and autophagosome formation, whereas exogenous tensile strain (via substrate stretching) will rescue autophagy in BAG3‑deficient aged cells.

Experimental Approach

1. Generate BAG3 LIR point mutants (LIR1A, LIR2A, LIR3A) and a tension‑sensor BAG3 construct; express in BAG3‑null HL‑1 cardiomyocytes.
2. Apply controlled cyclic stretch (10% elongation, 1 Hz) using a Flexcell system; measure autophagic flux (LC3‑II turnover with bafilomycin A1) and co‑IP of BAG3‑LC3B.
3. Use confocal FRET microscopy to quantify sensor extension in real time during stretch.
4. In vivo, cross BAG3‑fl/fl mice with α‑MHC‑MerCreMer for inducible KO; treat young and aged mice with blebbistatin or exogenous stretch via cardiac pressure overload (TAC) and assess autophagy markers and cardiac function.

Falsification

If LIR mutants retain normal stretch‑induced autophagy or if tension manipulation fails to alter BAG3‑LC3B binding in a manner predictive of autophagy outcomes, the hypothesis would be refuted. Conversely, confirmation would establish BAG3 as a direct mechanosensitive gatekeeper of selective autophagy, explaining its biphasic role across the lifespan.