Hypothesis

Aging‑associated rise of the Hippo kinase STK38 phosphorylates BAG3 on a conserved serine within its PxxP dynein‑binding motif, weakening the BAG3‑dynein interaction and preventing microtubule‑dependent delivery of force‑unfolded cytoskeletal proteins to the MTOC. This block diminishes chaperone‑assisted selective autophagy (CASA) efficacy, leading to accumulation of degraded filamin and contractile dysfunction independent of BAG3 mutation.

Mechanistic Rationale

1. Phospho‑regulation of dynein binding – The PxxP motif of BAG3 mediates direct dynein light‑chain interaction required for retrograde transport of ubiquitinated cargo to the centrosome. Phosphorylation of adjacent serine residues by STK38 introduces negative charge that sterically hinders the SH3 domain of dynein intermediate chain, reducing binding affinity. This extends the known STK38 effect on the BAG3‑HSPB8‑SYNPO2 complex by targeting the transport step upstream of autophagosome maturation.
2. Competing phosphatase activity in skeletal muscle – Skeletal muscle expresses higher basal levels of PP2A‑B55 phosphatase, which can dephosphorylate BAG3‑PxxP, preserving dynein coupling even as STK38 rises with age. This provides a tissue‑specific explanation for why cardiac muscle shows earlier CASA decline than skeletal muscle during physiological aging.
3. Feedback amplification – Impaired aggresome formation elevates cytosolic load of misfolded filamin, which further activates STK38 via mechanosensitive Hippo signaling, creating a vicious cycle that accelerates proteostatic collapse in aged cardiomyocytes.

Testable Predictions

• Phospho‑specific antibody against BAG3‑Sxxx (the STK38 site) will show increased signal in left ventricular tissue from 24‑month‑old mice compared with 3‑month‑old controls, while skeletal muscle (tibialis anterior) will exhibit no significant change.
• Expression of a phospho‑deficient BAG3 mutant (S→A) in aged cardiomyocytes will restore dynein‑dependent transport of GFP‑filamin to the pericentriolar region, increase LC3‑II conversion, and improve fractional shortening measured by echocardiography.
• Pharmacological inhibition of STK38 (using the selective inhibitor X) in aged mice will decrease BAG3 phosphorylation, rescue SYNPO2 levels, and reduce fibrosis histology without altering Hippo pathway transcriptional targets (CTGF, CYRDI).
• Knock‑down of PP2A‑B55 in skeletal muscle will sensitize it to age‑related CASA decline, reproducing the cardiac phenotype of filamin accumulation and reduced specific force.

Experimental Approach

1. Generate phospho‑specific pBAG3 antibody; validate with peptide competition and STK38 overexpression/knockdown in HL‑1 cardiomyocytes.
2. Perform immunoblotting on cardiac and skeletal lysates from young and old mice; quantify pBAG3/total BAG3 ratio.
3. Use adenoviral AAV9 to deliver BAG3‑WT, BAG3‑S>A, or BAG3‑S>D (phospho‑mimic) into aged mouse hearts; assess dynein co‑immunoprecipitation, live‑cell tracking of GFP‑filamin–positive puncta, autophagic flux (mCherry‑GFP‑LC3), and echocardiographic function.
4. Treat cohorts with STK38 inhibitor X or vehicle for 8 weeks; evaluate fibrosis (Masson’s trichrome), SYNPO2 Western blot, and caspase‑3 activity.
5. In parallel, knockdown PP2A‑B55 tibialis anterior via AAV‑shRNA; repeat aging‑related CASA readouts.

Falsification would occur if aging does not increase BAG3 phosphorylation, if phospho‑deficient BAG3 fails to restore dynein transport or functional improvement, or if STK38 inhibition does not ameliorate CASA markers despite target engagement.