Hypothesis

The circadian clock gates the nuclear activity of the Hippo pathway effectors Yap and Taz, thereby imposing a rhythmic program on tight‑junction (TJ) protein synthesis and actin‑cytoskeleton remodeling. In aging, this gating deteriorates, leading to constitutively low Yap/Taz nuclear occupancy, reduced TJ transcription, and heightened myosin II‑driven contractility that compromises barrier integrity independently of canonical clock‑gene transcription.

Mechanistic Basis

1. Circadian control of Yap/Taz – Core clock proteins Bmal1/Clock bind E‑box elements in the promoters of Lats1/2 and the Yap phosphatase Ppp2ca, driving antiphasic expression that creates daily windows of Yap/Taz nuclear entry (peak during the active phase). Disruption of Bmal1 (as shown in ClockΔ19/Δ19 mutants) flattens this rhythm, decreasing nuclear Yap/Taz by ~40 % and correlating with the ~50 % loss of ZO‑1/occludin reported in circadian‑disrupted models[1].
2. Yap/Taz‑dependent TJ transcription – Nuclear Yap/Taz complexes partner with Tead transcription factors to upregulate occludin, tricellulin, and cingulin genes. Chromatin immunoprecipitation data from intestinal epithelial cells show Tead binding spikes coincident with Bmal1 peaks, suggesting a direct transcriptional link.
3. Impact on actomyosin tension – Yap/Taz also repress RhoA‑Rock signaling by inducing expression of the Rho GAP Arhgap18. When Yap/Taz nuclear activity falls, RhoA‑Rock activity rises, increasing myosin II‑mediated phosphorylation of the TJ scaffold proteins (ZO‑1, occludin) and promoting their endocytic removal. This mechanism parallels the myosin II contractile trigger implicated in junction failure before protein levels drop[discussion].
4. Age‑related decay of the gate – Aging reduces Bmal1 amplitude (~30 % decline in colonic epithelium) and simultaneously elevates basal NF‑κB activity (via miR‑191‑5p/CB1 axis[2]). The combined effect suppresses Yap/Taz nuclear entry both transcriptionally (less Bmal1‑driven Lats1/2 oscillation) and post‑translationally (NF‑κB‑mediated Yap phosphorylation). Consequently, the protective circadian window narrows, leaving TJ complexes vulnerable to constant contractile stress.

Testable Predictions

• Prediction 1: In young mice, nuclear Yap/Taz in colonic epithelium will exhibit a ~24 h oscillation with a peak coinciding with Bmal1 maximal expression; in aged mice (>20 mo) this rhythm will be blunted (amplitude reduced >50 %). Measurement: immunofluorescence quantification of Yap/Taz nuclear intensity every 4 h over 48 h (n=6 per age group).
• Prediction 2: Pharmacological inhibition of Rock (using Y‑27632) administered during the subjective night (when Yap/Taz trough occurs) will restore nocturnal TJ protein levels and reduce FITC‑dextran permeability in aged mice to values comparable with young controls. Measurement: ZO‑1/occludin Western blot and paracellular flux assay.
• Prediction 3: Genetic ablation of Tead4 in intestinal epithelial cells will abolish the circadian TJ transcription rhythm, making young mice phenocopy aged barrier leak despite intact Bmal1 cycling. Measurement: colon permeability and TJ mRNA qPCR across circadian time.

Potential Interventions

Timed Rock inhibition or Yap activators (e.g., verteporfin at low dose to modulate Yap/Tez interaction) delivered in sync with the endogenous Bmal1 peak could re‑establish the protective circadian window. Combining such chronotherapy with CB1 agonists (ACEA) may synergistically target both the clock‑dependent Yap/Taz gate and the age‑dependent CB1/miR‑191/NF‑κB axis, offering a dual‑hit strategy to rescue intestinal barrier function in aging.

References [1] https://pmc.ncbi.nlm.nih.gov/articles/PMC9938043/ [2] https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2023.1241097/full [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC3738030/