Do Non-Replicating Cells Exhibit Senescence During the Aging Process?
Most somatic cells in the body replicate, but sizable populations do not, known as post-mitotic cells, such as varieties of neuron in the central nervous system. Cellular senescence is fundamentally a process by which cell division is halted, a reaction to DNA damage, short telomeres, or a toxic signaling environment. Cells that become senescent swell in size, as though about to divide, but instead remain large. While in that state, they secrete a potent mix of inflammatory molecules that rouse the immune system, degrade surrounding tissue structure, and alter the behavior of nearby cells. In the short term this can be useful, as a way to suppress cancer or aid in wound healing. When sustained for the long term, it is very harmful.
Senescent cells are normally quickly destroyed, but they accumulate in tissues with age, the result of a slowing of clearance processes. This is an important contributing cause of aging, and thus considerable effort is presently devoted to the development of senolytic therapies capable of selectively destroying senescent cells. In this context, one interesting question is whether or not non-replicating cells - particularly those in the brain - are capable of becoming senescent, or something like senescent. They do undergo damage, but is cell division required for cells to become harmful in this way? What does senescence look like in post-mitotic cells? Will they be destroyed by senolytic therapies? Answering this question is more complex than one might think.
Senescence-like phenotype in post-mitotic cells of mice entering middle age
There is currently no single marker with absolute specificity for senescent cells. Some markers have more universal validity while others are related to specific senescent cell types. One of the most frequently used marker of cell senescence is the activity of senescence-associated beta-galactosidase (SA-β-gal). Since 1995, the wide use of SA-β-gal to study senescence in human or mice tissues in situ has been accompanied by controversies and technical challenges. In this respect, while senescent features have been found to be activated in a range of post-mitotic cells, independent multi-marker integration and confirmation of these results is still lacking for most of them.
Here, we attempted to independently deepen this knowledge using multiple senescence markers within the same cells of wild type mice entering middle age (9 months of age). A histochemistry protocol for the pH-dependent detection of β-galactosidase activity in several tissues was used. At pH 6, routinely utilized to detect senescence-associated β-galactosidase activity, only specific cellular populations in the mouse body (including Purkinje cells and choroid plexus in the central nervous system) were detected as strongly positive for β-galactosidase activity. These post-mitotic cells were also positive for other established markers of senescence (p16, p21, and DPP4), detected by immunofluorescence, confirming a potential senescent phenotype.
Choroid plexus produces cerebrospinal fluid (CSF) and participate in brain immunosurveillance. During ageing, CSF secretion decreases as much as 50%. These modifications are concurrent with subnormal brain activity, reduced beta-amyloid clearance, and increased glycation phenomena as well as oxidative stress. The potential interplay between senescent phenotype of the choroid plexus at young/mid age and its functional decline at older age is unknown. Senescence markers have been observed in neurons in the CNS also in a pathological context, during ischemia or Alzheimer's disease.
Future research on senescent post-mitotic cells should encompass also the crucial role of mammalian target of rapamycin (mTOR) pathway. During cell cycle arrest caused by contact inhibition cells do not undergo a fully senescent phenotype. It was demonstrated that the conversion from cell cycle arrest to senescence, a phenomenon called geroconversion, requires stimulation of mTOR and downstream effectors, such as pS6K, concomitantly to p16/p21 activation. Therefore, our study thus encourages exploring the function of post-mitotic cells positive for SA-β-gal activity and other senescence markers in healthy adult or middle age organisms, by simultaneous assessment of related phenomena, to understand whether post-mitotic senescence plays a significant role as driver of ageing phenotypes.