Hypothesis

Circadian clock genes directly regulate the phenotypic state of vocal fold lamina propria fibroblasts, determining whether they maintain a quiescent, matrix‑preserving phenotype or transition to an activated, ECM‑remodeling state that drives presbyphonia.

Mechanistic basis

• Clock‑controlled transcription of hyaluronan synthases – BMAL1:CLOCK heterodimers bind E‑box elements in the promoters of HAS1 and HAS2, driving peak HA synthesis during the early active phase PMC11083552; Frontiers in Aging. Loss of this oscillation shifts HA production toward a constant low‑level output, favoring accumulation of low‑molecular‑weight fragments that activate TLR‑4 signaling and promote fibroblast‑to‑myofibroblast transition.
• Regulation of TGF‑β activation via integrin‑YAP/TAZ – Circadian expression of integrins (ITGAV, ITGB3) modulates latent TGF‑β bioavailability. In silico promoter analysis shows E‑box sites upstream of ITGAV, predicting rhythmic integrin levels that gate YAP/TAZ nuclear translocation PMC11083552. Disrupted integrin rhythms cause sustained YAP/TAZ activity, amplifying Smad2/3 signaling and collagen I deposition.
• Feedback through NAD⁺‑SIRT1 axis – SIRT1 deacetylates BMAL1, linking cellular redox state to clock amplitude. Age‑related NAD⁺ decline dampens SIRT1 activity, weakening BMAL1 DNA binding and further destabilizing the circadian control of fibroblast phenotype Aggietranscript; Frontiers in Aging.

Testable predictions

1. ChIP‑seq in primary vocal fold fibroblasts will show rhythmic BMAL1 occupancy at HAS2 and ITGAV promoters, peaking at zeitgeber time (ZT) 4–6 under normal light‑dark cycles; this rhythm will be flattened in Bmal1‑knockout or constant‑light conditions PMC11083552.
2. HA size distribution extracted from lamina propria explants will shift from predominantly high‑molecular‑weight (>1 MDa) to low‑molecular‑weight (<200 kDa) fractions when fibroblasts are subjected to circadian disruption (e.g., dexamethasone pulse at inappropriate phase) and this shift will correlate with increased α‑SMA expression voicescience.org.
3. Pharmacological reinforcement of clock amplitude (e.g., timed REV‑ERB agonist SR9009 administered at ZT2) will restore rhythmic integrin expression, reduce YAP/TAZ nuclear localization, and attenuate collagen cross‑linking in aged rat vocal folds, measurable by second‑harmonic generation imaging patientworthy.com; Frontiers in Neuroscience.
4. Rescue of NAD⁺ levels via nicotinamide riboside supplementation will enhance SIRT1‑BMAL1 interaction, improve clock amplitude, and preserve fibroblast quiescence, as evidenced by decreased TGF‑β‑Smad signaling and maintained HA viscosity PubMed 32882420.

Falsifiability

If any of the above predictions fail—for instance, if BMAL1 binding to HAS2/ITGAV promoters shows no circadian variation, or if HA molecular weight profiles remain unchanged despite robust clock disruption—the hypothesis that circadian regulation directly governs fibroblast phenotype and ECM homeostasis in the vocal fold would be refuted.

Broader implication

Establishing the vocal fold fibroblast clock as a gatekeeper of matrix quality would position circadian entrainment (light timing, time‑restricted feeding, pharmacologic clock agonists) as a viable preventive strategy against presbyphonia, extending the concept of the circadian rhythm as the body's primary anti‑aging firewall to the laryngeal microenvironment.