The Cytoskeletal-Chaperone Sink: Titration of Chaperone-Mediated Autophagy (CMA) by Intermediate Filament Aggregates as a Driver of Proteostatic Senescence
The Hypothesis
I suggest that the enrichment of cytoskeletal proteins—specifically filamin C and desmin—in the aging aggregome ([https://www.aging-us.com/article/101141/text]) isn't just collateral damage. I believe it’s a primary catalyst that triggers systemic proteostasis collapse. My hypothesis is that insoluble cytoskeletal aggregates function as 'molecular sinks,' sterically sequestering HSPA8 (Hsc70) and its co-chaperones, which effectively shuts down Chaperone-Mediated Autophagy (CMA) at the lysosomal membrane.
Mechanistic Reasoning
While recent research notes that chaperone sequestration adds a general burden to the cell ([https://elifesciences.org/articles/48240]), we haven't yet pinned down the exact link between cytoskeletal insolubility and CMA inhibition. I propose the following:
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The Steric Lock: Cytoskeletal proteins contain modular domains that naturally bind HSPA8. When these proteins transition into detergent-insoluble aggregates, they don't release their chaperone cargo. Instead, they lock HSPA8 into a state of 'chaperone titration,' where the dissociation rate of the chaperone-substrate complex drops significantly.
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CMA De-commissioning: CMA relies on a high concentration of free HSPA8 to recognize KFERQ-like motifs on clients and shuttle them to the LAMP2A receptor. By locking the cytosolic pool of HSPA8 into persistent cytoskeletal aggregates, the cell can’t translocate metabolic enzymes and regulatory proteins into the lysosome. This failure explains the secondary accumulation of proteins like 14-3-3 and Aβ ([https://www.frontiersin.org/journals/aging-neuroscience/articles/10.3389/fnagi.2017.00138/full]).
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Phase Separation Crosstalk: Thiol hyperoxidation, as noted by [https://doi.org/10.1101/2023.11.07.066021], likely facilitates the initial "soft" aggregation of cytoskeletal proteins. These condensates act as nucleation sites, eventually maturing into the dense, chaperone-sequestering aggregates we see in aged skeletal muscle and heart tissue.
Testable Predictions
- Kinetic Competition Assays: In aged fibroblasts, I expect that overexpressing chaperone-binding domains from filamin C will displace HSPA8 from the lysosomal membrane, mimicking the CMA impairment seen in aging.
- CMA-Rescue Intervention: If I’m right, delivering a "decoy" HSPA8-peptide—a high-affinity KFERQ-motif mimetic that doesn't sequester—should transiently restore CMA flux and lower the total aggregate load in aged cells.
- Aggregome Topology: Using proximity-labeling proteomics, I expect to find a higher stoichiometric ratio of HSPA8 trapped in insoluble cytoskeletal fractions compared to non-cytoskeletal aggregates in aged, but not young, human tissues.
This framework shifts the focus from general "chaperone exhaustion" to a specific, actionable target: selectively displacing chaperones from the cytoskeletal sink. This could potentially reactivate autophagy-lysosomal pathways without needing to fully dissolve the long-lived protein aggregates themselves.