The word "senescent" literally means "old". Senescent cells in your body are cells that have reached a point where they will no longer divide. This is called "cellular senescence" in biology.
Cells can split an average of about 50 times before they stop splitting (replicative senescence). This is called the Hayflick limit, which we looked at in the Telomeres chapter. The telomeres shorten every time a cell divides, until at last the cells detect that the telomeres are so short that they risk unravelling the DNA altogether, at which point senescence kicks in and the cell refuses to divide any more.
When a cell reaches senescence, it refuses to die. It is no longer very useful in general (there are some uses for senescent cells in wound healing, but that's about it), and instead, it actually starts emitting harmful messages (cytokines) which cause inflammation in the body, atherosclerosis, other age-related conditions, and can cause nearby cells to also turn senescent.
Having a few senescent cells is not a problem. The problem, though, is that the population of senescent cells grows over time. The longer you live, the more senescent cells you have, the more likely you will develop these diseases.
In March 2017, a paper was published showing that it is possible to target senescent cells and destroy them using a FOXO4 peptide. Molecules that specifically kill senescent cells are called senolytics. Others include Quercetin and Navitoclax.
In senescent cells, the FOX04 gene interacts with the p53 gene to prevent the cell from dying, but a peptide (short amino string) has been designed which can block the interaction by connecting to the FOXO4 so that it is unable to connect to the p53. This allows the cell to then proceed to apoptosis.
FOXO4 is hardly ever expressed in non-senescent cells, so interfering with it does not kill off the young cells that you want to keep.
The team that developed the treatment, led by Peter de Keizer of the Erasmus University Medical Center in The Netherlands, tested it for ten months on mice, and no side-effects were found at all.
The mice that were tested were fast-aging mice.
the mouse on the left was treated with the FOXO4 peptide
Treated mice that had missing fur started regrowing their fur back after ten days.
After three weeks, it was obvious that the treated mice were more fit, running twice the distance their non-treated friends could run.
A month after treatment, there were markers of increased health in kidney function.
Human safety studies are still in planning, it is yet to be shown that this treatment increases the lifespan of the mice, and even if you do want to buy the stuff, it's incredibly expensive for even the smallest amount, but this is still very exciting.