Background

Cellular senescence exhibits a senescence-associated secretory phenotype (SASP) that can be either beneficial for acute tissue regeneration or deleterious when chronic, driving inflammation and fibrosis 1.

Recent work shows SASP secretion follows a circadian rhythm, with peak cytokine release aligned to the active phase of the organismal clock 8.

Aptamer‑based surface labeling now enables real‑time identification of senescent cells without affecting neighboring healthy cells 2.

Hypothesis

We hypothesize that administering senolytics (e.g., Dasatinib + Quercetin) at the circadian peak of SASP will preferentially clear senescent cells contributing to chronic, pathogenic SASP while sparing those engaged in transient, regenerative SASP, thereby preserving acute repair mechanisms and reducing long‑term inflammatory burden.

Mechanistic Rationale

The SASP transcriptome is under direct control of the core clock proteins BMAL1 and CLOCK, which rhythmically enhance NF‑κB‑driven transcription of IL‑6, IL‑8, and other SASP factors 9.

Consequently, senescent cells exhibit a predictable surge in secretory activity during the early subjective night in mice (or morning in humans). Clearing these cells at their secretory zenith reduces the cumulative extracellular cytokine load without eliminating the senescent cells that are required for the brief, pro‑regenerative SASP pulse that follows tissue injury.

Predictions

1. Mice receiving D+Q at the SASP peak (ZT6) will show lower circulating IL‑6 and IL‑8 levels 24 h post‑treatment compared with mice receiving the same dose at trough (ZT18) or at random times.

2. In a model of skeletal muscle injury (e.g., cardiotoxin‑induced), peak‑timed senolysis will preserve early‑phase markers of regenerative senescence (e.g., p21^high^, PDGFRα^+, transient SASP) and improve regeneration (increased MyoD^+, centrally nucleated fibers) relative to mistimed or vehicle groups.

3. Chronic inflammation readouts (fibrosis, adipose infiltration) will be significantly reduced only in the peak‑timed group after repeated dosing over 4 weeks.

Experimental Design

• Animal model: C57BL/6J mice, 8‑week‑old, subjected to unilateral tibialis anterior cardiotoxin injury.

• Groups: (a) Vehicle control, (b) D+Q administered at ZT6 (SASP peak), (c) D+Q administered at ZT18 (SASP trough), (d) D+Q given at random times (n = 8 per group).

• Intervention: Dasatinib (5 mg/kg) + Quercetin (50 mg/kg) i.p., 3x weekly for 4 weeks.

• Readouts: (i) Plasma IL‑6, IL‑8 via ELISA at 0, 4, 24 h post‑dose; (ii) Aptamer‑based fluorescent labeling of senescent cells in muscle sections at 24 h post‑dose to quantify senescent cell burden; (iii) Flow cytometry for p21^high^/PDGFRα^+ senescent cells; (iv) Regeneration metrics: MyoD^+ nuclei, centrally nucleated fibers, collagen staining (Sirius Red) at 7 days and 28 days post‑injury.

• Statistical analysis: Two‑way ANOVA with factors timing and treatment, post‑hoc Tukey.

Potential Outcomes

• If hypothesis is correct: Peak‑timed senolysis reduces SASP burden and fibrosis without impairing early regenerative senescence, leading to superior functional recovery (grip strength, treadmill endurance).

• If hypothesis is false: No difference between timing groups, or peak‑timed senolysis worsens regeneration, indicating that SASP peak cells are required for repair.

Implications

This work would provide a chronotherapeutic framework for senolytic use, aligning dosing with endogenous SASP rhythms to maximize benefit and minimize off‑target effects. It could inform clinical trial design for D+Q, fisetin, or next‑generation senolytics, suggesting that timing administration to the patient’s circadian phase (e.g., morning dosing) may improve efficacy in age‑related conditions while preserving tissue‑repair capacity.