Aging cardiomyocytes exhibit elevated BAG3 expression and autophagic flux yet fail to clear aggregates such as lipofuscin, indicating a downstream bottleneck. We hypothesize that increased lipid droplet (LD) formation in aged hearts sequesters the BAG3/CASA complex away from its cytoskeletal targets, reducing its access to force‑unfolded sarcomeric proteins despite overall pathway upregulation. This sequestration would create a spatial uncoupling where BAG3 remains detectable in lysates but is functionally impaired at Z‑disks, explaining persistent aggregate accumulation.

Mechanistic rationale: LDs expand with age due to altered lipid metabolism and serve as platforms that bind hydrophobic proteins via phospholipid monolayers. BAG3 contains a coiled‑coil domain with affinity for lipid‑rich environments, as suggested by its interaction with SYNPO2, a protein known to associate with membranous compartments. When LDs proliferate, BAG3 may partition into these droplets, lowering its cytosolic concentration available for HSP70/HSPB8 binding and subsequent recruitment to mechanostressed substrates. Consequently, the mechanosensitive activation of CASA by cyclic strain would be blunted locally, even though global BAG3 levels rise.

Testable predictions:

1. In aged mouse cardiomyocytes, immunofluorescence will show increased colocalization of BAG3 with perilipin‑2‑positive LDs compared with young cells.
2. Pharmacological inhibition of LD synthesis (e.g., with DGAT1/2 inhibitor A922500) or genetic reduction of perilipin‑2 will decrease BAG3‑LD colocalization and restore CASA activity, measured by increased LC3‑II turnover and reduced p62/SQSTM1 puncta after acute mechanical stretch.
3. Conversely, inducing LD overload in young cardiomyocytes (via oleic acid treatment) will mimic the aged phenotype: BAG3 shifts to LDs, CASA flux declines, and force‑unfolded proteins such as filamin C accumulate.
4. Rescue of LD‑sequestered BAG3 by overexpressing a LD‑targeting‑deficient BAG3 mutant (deletion of the putative lipid‑binding motif) will improve autophagic clearance of ubiquitinated aggregates without altering total BAG3 levels.

Experimental approach: Isolate primary cardiomyocytes from young (3 mo) and aged (24 mo) mice. Treat subsets with A922500 or oleic acid, apply cyclic stretch (10% elongation, 1 Hz) for 30 min, then assess BAG3 localization (confocal microscopy with perilipin‑2 and α‑actinin markers), autophagic flux (LC3‑II/I ratio via western blot, cathepsin activity), and aggregate load (filter‑trap assay for ubiquitinated proteins, lipofuscin autofluorescence). Include BAG3‑knockout and mutant rescue controls.

Falsifiability: If LD manipulation does not alter BAG3 subcellular distribution or CASA efficiency under mechanical strain, or if BAG3‑LD colocalization remains unchanged with age, the hypothesis would be refuted. This framework links lipid homeostasis to mechanotransduction‑driven proteostasis, offering a concrete, falsifiable mechanism for the observed autophagy bottleneck in aging hearts.