In Aging Cell, researchers have described the differences between primary and secondary senescent cells, comparing radiation-induced senescence to senescence induced by the senescence-associated secretory phenotype (SASP).
Heterogeneity on top of heterogeneity
It is well-known that senescent cells are highly heterogenous [1]; senescent cells from one tissue may behave differently from senescent cells in another tissue, and cells that exhibit senescent traits as the result of injury are different from cells that have been driven senescent by aging. This makes dealing with senescent cells potentially even more difficult than cancer; while removing cancer cells is universally beneficial, the same is not true of senescent cells. Similarly to cancer, however, different treatments work against different senescent cell populations [2].
One key difference involves how the cells became senescent. Replicative senescence due to telomere attrition (the “Hayflick limit”) is probably the most well-known of these, but cells can also be induced to senescence through toxic exposure or, as was done in this study, intense radiation; this fact is a core part of radiotherapy’s effectiveness as a cancer treatment. Cells driven to senescence through such methods are primary senescent cells.
Senescence can also be induced through exposure to other senescent cells’ SASP, which makes them secondary senescent cells [3]. Previous work has found that these cells behave differently from primary senescent cells, including in the way they secrete their own SASP [4]. However, the molecular biology behind these different cell types has not been fully explored in detail.
Different trajectories in the same group
These researchers used single-cell RNA sequencing (sc-RNAseq) to investigate kidney (renal) cells, first comparing a control group of quiescent cells to cells that were driven senescent through powerful radiation. The primary senescent cells exhibited all of the standard features of senescence: the telltale biomarker SA-β-gal, upregulated key senescence-associated genes, and increased expression of key inflammatory factors that make up the SASP. However, not all of the targeted cells became fully senescent.
An algorithm was able to cluster the RNA expression of these cells into two clusters associated with nonsenescence, four clusters associated with an intermediate state of senescence, and three more associated with complete senescence: C5, C6, and C8. However, even these three primary senescent groups were heterogenous: C5 strongly expressed development-related genes, while C6 and C8 each expressed stress response genes and genes related to death by apoptosis.
The researchers then used another algorithm, Slingshot, to investigate how these different types of cells had progressed towards senescence. C5 exhibited DNA damage and possible cancer, C6 exhibited increased ribosomal activity and nucleolar stress, and C8 had exhibited genes related to the SASP. The researchers described the C8 group as “consistent with a terminal senescent program characterized by elevated SASP activity and tissue remodeling features”.
Culturing cells in the SASP
This experiment was performed again, except instead of using radiation to induce senescence, the researchers cultured cells in a medium rich in SASP factors. Cells driven secondarily senescent this way exhibited the same standard senescence features that the primary senescent cells did: the increase in SA-β-gal, the decrease in proliferation, and the induction of their own SASP were all present.
However, their gene expression was not the same. Some cells that did not become senescent resisted the SASP’s effects entirely, while other non-senescent cells exhibited increased DNA damage repair markers. The secondary senescent cells exhibited fewer cancer-related genes and more DNA damage repair than the primary senescent cells did. Slingshot revealed that, while some of the final trajectories were similar to those of primary senescent cells, the stresses induced by the SASP were not the same as those induced by radiation. Notably, some cells in the secondary senescent group expressed a terminal trajectory related to hypoxia, which did not occur in the primary senescent group.
Perhaps most critically, the primary senescent cells were more likely to exhibit ECM remodeling cells as they became senescent, which the researchers noted may increase fibrosis, while the secondary senescent cells were more likely to exhibit genes related to inflammation.
This study had some key limitations. The primary senescent cells were only driven that way through radiation rather than telomere attrition or doxorubicin, which may have changed their gene expression. Only renal cells were used in this study; other cells, such as skin cells or bone marrow cells, may have exhibited different properties. However, this study provides other researchers with crucial information in dealing with different senescent cell populations and highlights the need for specific targeting when dealing with these necessary but potentially dangerous cells.
Literature
[1] Cohn, R. L., Gasek, N. S., Kuchel, G. A., & Xu, M. (2023). The heterogeneity of cellular senescence: insights at the single-cell level. Trends in cell biology, 33(1), 9-17.
[2] Gasek, N. S., Kuchel, G. A., Kirkland, J. L., & Xu, M. (2021). Strategies for targeting senescent cells in human disease. Nature aging, 1(10), 870-879.
[3] Jeon, O. H., Mehdipour, M., Gil, T. H., Kang, M., Aguirre, N. W., Robinson, Z. R., … & Conboy, I. M. (2022). Systemic induction of senescence in young mice after single heterochronic blood exchange. Nature Metabolism, 4(8), 995-1006.
[4] Acosta, J. C., Banito, A., Wuestefeld, T., Georgilis, A., Janich, P., Morton, J. P., … & Gil, J. (2013). A complex secretory program orchestrated by the inflammasome controls paracrine senescence. Nature cell biology, 15(8), 978-990.
View the article at lifespan.io
