Saul, D.; Jurk, D.; Doolittle, M. L.; Kosinsky, R. L.; Monroe, D.; LeBrasseur, N.; Robbins, P. D.; Niedernhofer, L.; Khosla, M.D, S.; Passos, J.

Senescent cells drive age-related tissue dysfunction via the induction of a chronic senescence-associated secretory phenotype (SASP). The cyclin-dependent kinase inhibitors p21Cip1 and p16Ink4a have long served as markers of cellular senescence. However, their individual roles remain incompletely elucidated. Thus, we conducted a comprehensive examination of multiple single-cell RNA sequencing (scRNA-seq) datasets spanning both murine and human tissues during aging. Our analysis revealed that p21Cip1 and p16Ink4a transcripts demonstrate significant heterogeneity across distinct cell types and tissues, frequently exhibiting a lack of co-expression. Moreover, we identified tissue-specific variations in SASP profiles linked to p21Cip1 or p16Ink4a expression. Our study underscores the extraordinary diversity of cellular senescence and the SASP, emphasizing that these phenomena are inherently cell- and tissue-dependent. However, a few SASP factors consistently contribute to a shared \"core\" SASP. These findings highlight the need for a more nuanced investigation of senescence across a wide array of biological contexts.