Today, we want to point out a new study showing how senescent cells poison their healthy neighbors and that the more there are, the faster they make other cells become senescent.
What are senescent cells?
As you age, increasing numbers of your cells enter into a state known as senescence. Senescent cells do not divide or support the tissues of which they are part; instead, they emit a range of potentially harmful chemical signals that encourage nearby healthy cells to enter the same senescent state, which is known as the “bystander effect”. Their presence causes many problems: they reduce tissue repair, increase chronic inflammation, and can even eventually raise the risk of cancer and other age-related diseases.
Senescent cells normally destroy themselves via a programmed process called apoptosis, and they are also removed by the immune system; however, the immune system weakens with age, and increasing numbers of senescent cells escape this process and begin to accumulate in all the tissues of the body.
By the time people reach old age, significant numbers of these senescent cells have built up, causing chronic inflammation and damage to surrounding cells and tissue. These senescent cells are one of the hallmarks of aging and a key process in the progression of aging.
Senescent cells only make up a small number of total cells in the body, but they secrete pro-inflammatory cytokines, chemokines, and extracellular matrix proteases, which, together, form the senescence-associated secretory phenotype, or SASP. The SASP is thought to significantly contribute to aging and cancer; thus, targeting senescent cells and removing them has been suggested as a potential solution to this problem.
The more there are, the worse it gets
A new study shows just how significant the bystander effect is and how the more senescent cells there are, the faster they build up . These researchers took senescent cells from aged mice and transplanted them into NSG mice, which are special mice engineered to have a compromised immune system. Within three weeks, the skeletal muscle and skin cells near injection sites showed multiple senescent cell biomarkers. The adjacent cells showed an increase of senescence markers similar to those observed between the ages of 8 and 32 months; in other words, healthy cells near the injected senescent cells rapidly aged and took on the same senescent cell markers as the initial senescent cells.
Senescent cells accumulate with age in multiple tissues and may cause age‐associated disease and functional decline. In vitro, senescent cells induce senescence in bystander cells. To see how important this bystander effect may be for accumulation of senescent cells in vivo, we xenotransplanted senescent cells into skeletal muscle and skin of immunocompromised NSG mice. 3 weeks after the last transplantation, mouse dermal fibroblasts and myofibres displayed multiple senescence markers in the vicinity of transplanted senescent cells, but not where non‐senescent or no cells were injected. Adjacent to injected senescent cells, the magnitude of the bystander effect was similar to the increase in senescence markers in myofibres between 8 and 32 months of age. The age‐associated increase of senescence markers in muscle correlated with fibre thinning, a widely used marker of muscle aging and sarcopenia. Senescent cell transplantation resulted in borderline induction of centrally nucleated fibres and no significant thinning, suggesting that myofibre aging might be a delayed consequence of senescence‐like signalling. To assess the relative importance of the bystander effect versus cell‐autonomous senescence, we compared senescent hepatocyte frequencies in livers of wild‐type and NSG mice under ad libitum and dietary restricted feeding. This enabled us to approximate cell‐autonomous and bystander‐driven senescent cell accumulation as well as the impact of immunosurveillance separately. The results suggest a significant impact of the bystander effect for accumulation of senescent hepatocytes in liver and indicate that senostatic interventions like dietary restriction may act as senolytics in immunocompetent animals.
These findings further reinforce the fact that even a small number of senescent cells in tissue can cause a huge amount of problems by producing harmful secretions and making cells around them senescent just like them. The case for therapies that remove them is, therefore, very strong, given their strong association with multiple age-related diseases. Fortunately, the first senescent cell removal therapy is in human trials right now.
 da Silva, P. F., Ogrodnik, M., Kucheryavenko, O., Glibert, J., Miwa, S., Cameron, K., … & von Zglinicki, T. (2018). The bystander effect contributes to the accumulation of senescent cells in vivo. Aging Cell, e12848.