Single‐Cell Fluorescence Imaging Reveals Heterogeneity in Senescence Biomarkers and Identifies Rapamycin‐Responsive Sub‐Populations

ABSTRACT Cellular senescence is a state of irreversible cell cycle arrest accompanied by a distinctive inflammatory secretory profile known as the senescence‐associated secretory phenotype (SASP). While various biomarkers, such as senescence‐associated beta‐galactosidase (SA‐βgal), EdU incorporation, p21 and p16, are used to identify senescent cells, no single biomarker universally defines cellular senescence and current methods often fail to address heterogeneity in biomarker expression levels. This study leverages single‐cell fluorescence imaging to assess multiple senescence markers including SA‐βgal enzymatic activity, p21 and IL‐6 expression and nuclear and cell area in chemotherapy‐induced (mitomycin C) and oxidative stress‐induced (D‐galactose) senescence models in human fibroblasts. Our findings reveal significant heterogeneity in SA‐βgal activity and distinct sub‐populations within senescent cells. Nuclear and cell area measurements emerged as robust indicators of cellular senescence, displaying similar variability across individual cells. Importantly, we identified specific nuclear area sub‐populations that strongly correlate with IL‐6 expression levels, demonstrating a relationship between the heterogeneous expression of senescence biomarkers and the SASP. To address this heterogeneity, we introduced an induction threshold method to more accurately quantify the percentage of cells expressing senescence biomarkers. Furthermore, in both senescence models, we observed that rapamycin, a well‐known senomorphic agent, selectively targets specific biomarker‐expressing sub‐populations. This study underscores the value of assessing cellular heterogeneity in senescence research and provides an improved approach for analysing senescence markers in diverse cellular contexts.