The research community has been devoting more time and energy to the investigation of extracellular vesicles of late. These membrane-bound packages of proteins and other molecules are an important facet of the way in which cells communicate with one another. Signaling between cells is itself very significant, a potential point of intervention for many classes of therapy. For example, most current stem cell therapies appear to work largely due to the signaling provided by transplanted cells - given sufficient understanding of the signaling, the cells could be dispensed with and the signals applied directly.
As another example, the growing presence of cellular senescence with age has a large detrimental impact on tissue function, despite the comparatively small numbers of senescent cells present even in older individuals, because these negative effects are mediated by signaling. In this way, a handful of errant cells can put the entire local environment into disarray. On that topic, the open access paper here takes a short tour of what is known about extracellular vesicles in the context of cellular senescence.
Cellular senescence prevents the proliferation of cells exposed to potentially oncogenic stresses, such as DNA-damaging reagents, irradiation, telomere shortening, and oncogene activation. Mutations in genes essential for the senescence-induced cell cycle arrest predispose cells to immortalization and shorten lifespan by increasing cancer incidence. However, cellular senescence not only arrests the cell cycle but also changes how the cell impacts its microenvironment. The way in which senescent cells influence their microenvironment is highly context dependent. It promotes tumor development in many cases, but can also be tumor suppressive in certain circumstances. Removal of senescent cells that accumulated in the body during aging alleviates atherosclerosis, hepatic steatosis, tumor development, and functional declines of heart, kidney, and fat tissues, resulting in prolonged healthspan and lifespan. These effects may be attributable to so-called , whereby cells secrete high levels of inflammatory cytokines, chemokines, growth factors, and metalloproteinases.
Although the involvement of typical secretory proteins in the non-cell-autonomous effects of senescent cells has been well studied, the functions of membrane-enclosed vesicles secreted by senescent cells have not been studied until recently. These extracellular vesicles (EVs) were once thought to be cellular trash, but now it is clear that they are critical mediators in intercellular communication. Emerging evidence indicates that EVs also play important roles in cellular senescence and aging. This field is rapidly advancing especially since it was reported that EVs deliver functional RNA to the recipient cells. Extracellular vesicles contain a huge variety of proteins and nucleic acids in a cell type-dependent manner.
Senescence-associated increase in EV secretion seems to be a general feature of cellular senescence and has been observed in fibroblasts, epithelial cells, and cancer cells. This increase is at least partially mediated by p53 and one of its targets, TSAP6, although the mechanism whereby TSAP6 regulates EV secretion is not well understood. It is known that EV secretion contributes to the clearance of harmful molecules in cells, such as cytoplasmic DNA. It has been shown that EV-mediated removal of cytoplasmic DNA is essential for the survival of senescent cells, which may explain why EV secretion is increased in senescent cells.
Recent findings implicate senescent cell EVs in cancer development, vascular calcification, and age-related decline in bone formation. Increased secretion of EV-associated DNA from senescent cells is likely to be pro-inflammatory and may contribute to age-related chronic inflammation. Whether senescent cell EVs promote or suppress cancer development may be context dependent. Despite this progress, it should be noted that the functions of senescent cell EVs are still understudied, at least partially due to inadequate understanding of EVs themselves. This research field is immature and the methods used are not sufficiently standardized yet. Nevertheless, EVs are now shown to be critical players in cellular senescence and aging, and more functions will be revealed in the future as the EV research field matures.