Quantitative analysis of cellular senescence in culture and in Vivo

Cellular senescence refers to the irreversible growth arrest of normally dividing cells in response to various types of stress. Cellular senescence is induced by telomere shortening due to repeated cell division, which causes a DNA damage response, as well as genotoxic, oxidative, and inflammatory stress. Strong mitogenic signaling, such as oncogene activation, also drives cells into a senescent state. Senescent cells express a specific subset of genes, termed the senescence-associated secretory phenotype (SASP), including proinflammatory factors, growth factors, and matrix metalloproteinases, which together promote non-cell autonomous, secondary senescence. Clearance of senescent cells that accumulate with age improves health span, implicating cellular senescence as a contributing factor to the aging process. Thus, there is a need for methods to identify and quantify cellular senescence, both in cultured cells and in vivo. Here, methods for the most well-characterized and widely used senescent assays are described, from cell morphology and senescenceassociated β-galactosidase (SA-βgal) staining to nuclear biomarkers, SASP, and altered levels of tumor suppressors.