Desmin Aggresome Formation as a Proteostatic Adaptation Preserving Contractile Function in Aging Vocal Folds

Hypothesis

In aging vocal folds, desmin upregulation does not merely reflect a stress response but drives the formation of cytoplasmic desmin‑based aggresomes that sequester misfolded sarcomeric proteins, thereby maintaining thyroarytenoid contractility despite global proteostatic decline.

Mechanistic Rationale

• Desmin, a type III intermediate filament, polymerizes into stable filaments that can serve as a scaffold for aggresome formation when chaperone capacity is overwhelmed [https://pmc.ncbi.nlm.nih.gov/articles/PMC9677652/].
• Aggresomes concentrate ubiquitinated misfolded clients near the microtubule‑organizing center, facilitating autophagic clearance and reducing cytotoxic diffuse aggregates [https://elifesciences.org/articles/48240].
• In the thyroarytenoid, desmin‑rich aggresomes could physically tether degraded myosin heavy chains and troponin I precursors, preserving their local concentration for reassembly when proteasome activity rebounds, thus mitigating the >1.3‑fold loss of contractile proteins observed with age [https://www.pnas.org/doi/10.1073/pnas.1906592116].
• This model reframes desmin aggregation not as a marker of damage but as a controlled, thermodynamically favorable "last attempt at order" that converts dangerous soluble oligomers into inert, sequestered deposits.

Testable Predictions

1. Spatial correlation: Aged vocal folds with high‑molecular‑weight desmin aggregates (detected by filter‑trap assay) will show lower levels of diffuse ubiquitin‑positive inclusions compared with age‑matched folds lacking such aggregates.
2. Functional preservation: Specimens exhibiting quantified desmin aggresomes will retain greater specific tension (measured via ex vivo muscle strip physiology) than those with equivalent total desmin but diffuse distribution.
3. Dependence on aggresome formation: Pharmacological disruption of microtubule‑dependent aggresome transport (e.g., with nocodazole low dose) in aged vocal fold explants will increase diffuse misfolded protein load and accelerate functional decline.
4. Autophagy linkage: Enhancing autophagic flux (e.g., via spermidine treatment) will reduce aggresome size without increasing diffuse aggregates, indicating a dynamic equilibrium rather than irreversible accumulation.

Experimental Approach

• Sample collection: Harvest thyroarytenoid from young (3 mo) and aged (24 mo) rabbits; isolate single fibers and whole‑mount tissue.
• Aggregate quantification: Use sequential extraction (soluble → SDS‑insoluble → urea‑soluble) followed by Western blot for desmin, myosin heavy chain, troponin I, p62, and ubiquitin; confirm aggregate morphology by immunofluorescence and electron microscopy.
• Functional assay: Mount vocal fold strips in a perfusion chamber; measure passive tension, active twitch, and tetanic force under standardized calcium stimulation.
• Intervention groups: Treat aged explants with (a) nocodazole (0.1 µM) to inhibit aggresome transport, (b) spermidine (50 µM) to boost autophagy, (c) vehicle control.
• Analysis: Correlate desmin aggresome burden (percentage of total desmin in SDS‑insoluble fraction) with specific tension across conditions; apply linear regression and ANOVA to test significance.

Falsifiability

If desmin‑rich aggresomes are merely epiphenomena, then (i) disrupting aggresome formation will not worsen diffuse misfolded protein levels or contractile loss, and (ii) enhancing autophagy will not selectively reduce aggresome size without affecting overall protein homeostasis. Demonstrating either outcome would refute the protective aggresome hypothesis and support the view that desmin aggregates are detrimental byproducts of proteostatic collapse in presbyphonia.