Distinct senotypes in p16- and p21-positive cells across human and mouse aging tissues

Senescent cells drive age-related tissue dysfunction via the induction of a chronic senescence-associated secretory phenotype (SASP). The cyclin-dependent kinase inhibitors p21^Cip1 and p16^Ink4a have long served as markers of cellular senescence. However, their individual roles remain incompletely elucidated, particularly in vivo. Thus, we conducted a comprehensive examination of multiple single-cell RNA sequencing datasets spanning both murine and human tissues during aging. Our analysis revealed that p21^Cip1 and p16^Ink4a 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 p21^Cip1 or p16^Ink4a expression. Using RNA velocity and pseudotime analyses, we discovered that p21+ and p16+ cells follow independent trajectory dynamics, with no evidence of direct transitions between these two states. Despite this heterogeneity, we identified a limited set of shared "core" SASP factors that may drive common senescence-related functions. Our study underscores the substantial diversity of cellular senescence and the SASP, emphasizing that these phenomena are inherently cell- and tissue-dependent.