Hypothesis: Senescent cardiac progenitor cells (CPCs) release exosomes enriched in miR‑21 and IL‑1α that activate Toll‑like receptor 7/8 (TLR7/8) in adjacent cardiomyocytes and fibroblasts, triggering a secondary senescence wave that amplifies fibrosis and contractile decline. Pharmacological inhibition of exosome secretion (e.g., GW4869) or neutralization of exosomal miR‑21 will break this paracrine loop, reduce p16INK4A‑positive cells, and improve cardiac function in aged mice.

Rationale: Aged hearts accumulate dysfunctional CPCs expressing p16INK4A, DNA damage markers, and a senescence‑associated secretory phenotype (SASP) that drives paracrine senescence [1]. While SASP factors act in a soluble manner, emerging evidence shows that senescent cells also disseminate signals via extracellular vesicles. In muscle, CD36‑mediated lipid uptake promotes senescence through lipid peroxidation and mitochondrial ROS [3]. CPCs in the myocardium exhibit elevated CD36 with age, leading to oxidized lipid accumulation and heightened oxidative stress. This stress biases the cargo of CPC‑derived exosomes toward microRNAs that reinforce senescence, notably miR‑21, which targets PTEN and activates AKT‑mTOR signaling, a pathway linked to the SASP [4]. Exosomal miR‑21 can be taken up by neighboring cells where it engages TLR7/8 in endosomes, triggering NF‑κB activation and a secondary SASP, thereby propagating senescence beyond the original CPC pool.

Testable predictions: 1) Isolating exosomes from senescent CPCs of aged mice and injecting them into young murine hearts will increase p16INK4A‑positive cardiomyocytes and fibroblasts, elevate fibrosis markers (collagen I/III), and reduce ejection fraction within two weeks. 2) Treating aged mice with GW4869 (an exosome release inhibitor) or with a locked‑nucleic acid antimiR‑21 will lower exosomal miR‑21 levels in cardiac tissue, decrease TLR7/8 activation in non‑CPC cells, reduce p16INK4A+ cell frequency by >40%, and improve fractional shortening compared with vehicle controls. 3) Genetic deletion of TLR7/8 specifically in cardiomyocytes will blunt the pro‑senescent effect of CPC‑derived exosomes, confirming the receptor’s role. 4) In human cardiac biopsies from older patients, exosomal miR‑21 concentration will correlate positively with CPC p16INK4A expression and inversely with left‑ventricular strain measurements obtained by echocardiography.

Falsifiable outcome: If exosome blockade fails to diminish paracrine senescence or improve functional metrics despite verified reduction in exosomal miR‑21, the hypothesis would be refuted, indicating that other SASP components or cell‑contact mechanisms dominate the senescence spread in aged myocardium.