There are two ways to provide evidence for a specific cellular mechanism to cause a specific age-related disease. The first, the better method, is to remove, block, or work around the mechanism, while changing as few other variables as possible. This is better because it can lead immediately to the development of a therapy if it turns out that the mechanism in question is important. The worse option is to make the mechanism more active, while changing as few other variables as possible, and see if problems happen more rapidly because of that alteration. This is worse because there is always the risk that greater activity in any biological process does cause greater harm, but is nonetheless not actually relevant to aging and age-related disease because that greater activity never happens in the normal course of matters. DNA repair deficiency is a great example of the type. Significant impairment of DNA repair produces damage, dysfunction, and accelerated disease and mortality, but really isn't all that relevant to normal aging. All that this tells us is that it is important that DNA repair functions correctly, in the same way that it is important to breathe, or important that hearts beat and blood flows. There are many ways to cause damage by breaking the operation of our biochemistry - you can hit living organisms with a hammer, for example - but very few of them tell us much about aging and age-related disease.
With that preamble out of the way, today I'll point you to an interesting open access study in which the authors uses the worse of the two methods noted above to provide evidence for senescent cells to contribute to the development of osteoarthritis, a degenerative condition in which joint tissues become inflamed and break down. This is accomplished by transplanting a sizable number of senescent cells into the joints of mice and observing the outcome over a number of months following the transplant. Senescent cells accumulate in our tissues with age, and their presence is certainly a form of damage, with plenty of evidence to link it to the development of age-related disease. Researchers have produced benefits in laboratory animals by selectively destroying senescent cells, and a variety of these approaches are under development as clinical therapies. Given that, the more conditions linked to cellular senescence, the better off we all are. For this particular study, however, the question is whether or not transplanting senescent cells into tissue is a good enough replication of the processes of aging to tell us something, or whether it is just another sophisticated way of causing damage that isn't particularly relevant to aging. The devil is in the details, but having read the details, I'm leaning towards the former position.
Senescent cells cause harm through signaling. A cell becomes senescent and ceases replication in response to reaching the Hayflick limit, or suffering damage, or finding itself in a toxic environment. Most destroy themselves or are destroyed by the immune system, but some linger. Growing numbers of these cells eventually cause serious harm. A senescent cell secretes a mix of inflammatory and other signals that cause harm to surrounding tissue structures and change the behavior of normal cells for the worse. Perhaps a few percent of all cells in our tissues are senescent by the time we are old, but that is more than enough to cause major dysfunction. Since this is largely a signaling problem, it seems fairly reasonable to suggest that researchers could reproduce the effects of senescent cells on aging via transplantation. This would be something like the reverse of the goal of a stem cell transplant, in which the transplanted cells produce benefits largely through signaling. So long as the number of transplanted senescent cells falls within the bounds of what would be expected over the course of normal aging, one can argue that this type of study can be a good, rapid test of the outcomes that cellular senescence produces. In any case, read the paper and see what you think:
Osteoarthritis (OA) is one of the leading causes of pain and disability worldwide. It can greatly increase health care costs and reduce quality of life. The key characteristics of age-related OA in humans include damage of articular cartilage with joint space narrowing and degeneration of soft tissues. Age is the leading predictor for developing OA. However, modeling age- or senescence-associated OA, which may be distinct from injury-related OA, in mice has been challenging. So far, no disease-modifying drug has been approved to treat OA other than pain reducers, partly because etiological mechanisms of age-related OA have been poorly understood to date. Potential cellular mechanisms contributing to the development of OA include low-grade inflammation, chondrocyte alteration, mitochondrial dysfunction, loss of glycosaminoglycans, and dysregulated energy metabolism. In addition, a potential contribution by senescent cells has been suggested. Cellular senescence refers to a state of stable arrest of cell proliferation in replication-competent but apoptosis-resistant cells. Senescent cells accumulate with aging in various tissues, including the articular cartilage. One key feature of senescent cells is secretion of an array of pro-inflammatory cytokines, chemokines, and growth factors, termed the senescence-associated secretory phenotype (SASP). The SASP is observed across a number of senescent cell types, including fibroblasts and mesenchymal stem cells. Although mounting evidence suggests that cellular senescence is associated with OA, whether this link is causal remains to be determined.
To test if senescent cells cause an OA-like arthropathy, we injected either senescent or control nonsenescent fibroblasts into the knee joint region of mice. We transplanted seven mice with control cells and seven with senescent cells. Three months after cell injection, senescent and nonsenescent cell-injected knees were evaluated histologically and radiologically to assess articular cartilage and overall joint structure. We found that the senescent cells induced a phenotype with features resembling OA, including articular cartilage erosion, increased pain, and impaired function. We found that Rotarod performance was significantly decreased in the mice injected with senescent cells compared with animals injected with control nonsenescent cells or those that were not injected. In addition, we found that mice injected with senescent cells moved less and traveled shorter distances than mice injected with control nonsenescent cells. To our knowledge, this is the first evidence suggesting that cellular senescence can actually cause OA. Our findings also imply that targeting senescent cells is a promising approach for preventing or treating OA.
This both provides a new model of OA and implies that clearing senescent cells with senolytics or interfering with their pro-inflammatory SASP could be a disease-modifying therapeutic option. A next step will be to test such interventions in our senescent cell-transplanted model. One of the potential mechanisms by which senescent cells could induce an OA-like phenotype is through the SASP. OA is linked to inflammation and immune cells have been found in early stage OA. IL-6, one of the key SASP components, is highly associated with OA progression. We found that the senescent cells we transplanted secreted 20 times more IL-6 than nonsenescent cells. In addition, senescent cells can directly impair progenitor function through the SASP and spread senescence to nearby cells, both of which might contribute to dysfunction of chondrocytes and therefore to OA.
The finding that cellular senescence can drive development of an OA-like state is consistent with the geroscience hypothesis - that fundamental aging mechanisms, of which cellular senescence is one, predispose to age-related disabilities and chronic diseases, such as OA. If correct, this would imply that senescent cell accumulation may not only predispose to OA, but to multiple other age-related conditions, as is increasingly appearing to be the case. We predict that senolytics or SASP inhibitors such as ruxolitinib, which decreases IL-6 secretion and effects by senescent cells and also alleviates the senescent cell-induced stem cell dysfunction caused by TGFβ-related SASP factors, will delay, prevent, or alleviate OA. Consistent with this possibility, we found that senolytics attenuate age-related loss of glycosaminoglycans, a contributor to developing OA, from the intervertebral discs of progeroid mice. Moreover, senolytics are effective when administered periodically, likely because senescent cells do not of course divide and may be slow to re-accumulate once cleared in the absence of a strong continuing insult. We predict that senolytics may have fewer side effects than the anti-inflammatory agents currently used for controlling pain.