Single-Cell Transcriptomics Reveals Global Markers of Transcriptional Diversity across Different Forms of Cellular Senescence.

Cellular senescence (CS) is a state of irreversible cell cycle arrest, and the accumulation of senescent cells contributes to age-associated organismal decline. The detrimental effects of CS are due to the senescence-associated secretory phenotype (SASP), an array of signaling molecules and growth factors secreted by senescent cells that contribute to the sterile inflammation associated with aging tissues. Recent studies, both in vivo and in vitro, have highlighted the heterogeneous nature of the senescence phenotype. Single-cell transcriptomics has revealed that oncogene-induced senescence (OIS) is characterized by the presence of subpopulations of cells expressing different SASP profiles. We have generated a comprehensive dataset via single-cell transcriptional profiling of genetically homogenous clonal cell lines from different forms of senescence, including OIS, replicative senescence, and DNA damage-induced senescence. We identified subpopulations of cells that are common to all three major forms of senescence and show that the expression profiles of these subpopulations are driven by markers formerly identified in individual forms of senescence. These common signatures are characterized by chromatin modifiers, inflammation, extracellular matrix remodeling, and ribosomal protein gene expression (measured at the RNA level). The expression patterns of these subpopulations recapitulate primary and juxtacrine secondary senescence, a phenomenon where a pre-existing (primary) senescent cell induces senescence in a neighboring (secondary) cell through cell-to-cell contact. Hence, our results demonstrate that the formation of juxtacrine secondary populations of cells is common to multiple types of senescence and occurs in competition with primary senescence. Finally, we show that these subpopulations show differential susceptibility to the senolytic agent Navitoclax, suggesting that senolytic agents targeting the apoptotic pathways may be clearing only a subset of senescent cells based on their inflammatory profiles.