The senolytic molecule 25 hydroxycholesterol (25HC) targets aged, non-proliferating (senescent) cells for clearance to rejuvenate aged muscle in mice.
Aged cells enter a stage of permanent cell cycle arrest called senescence, releasing molecules known as the senescence associated secretory phenotype (SASP) into the surrounding cellular milieu. In a snowball effect, the SASP molecules drive surrounding cells to enter senescence. Research has provided compelling evidence that senescent cell buildup leads to the decline of tissue health with age in crucial organs like the heart, brain, and skeletal muscle. Along those lines, research has been geared toward identifying agents that may eliminate aged, non-proliferating senescent cells, known as senolytics.
Published in iScience, Melov and colleagues from the Buck Institute show that injecting 50 mg/kg of the metabolite 25HC, a naturally-occurring molecule in our bodies, into aged mice restores muscle mass by an astounding 23% by clearing senescent cells. The Buck Institute-based team ran single cell analyses showing that the gene CRYAB displays substantially elevated activity after experimentally inducing senescence in muscle cells from mice. The research team then went on to identify 25HC as an senolytic metabolite that targets muscle cells from mice and human heart, liver, kidney, and cartilage cells. These findings demonstrate that 25HC may become a promising candidate for a new, potent senolytic agent to prevent age-related tissue decline across multiple tissue types.
“Senescent cells arise in only a small fraction of cells within a tissue as we age, and do not occur in all cells within an organ or tissue type,” said Melov in a press release. “But because these cells are misbehaving through production of inflammatory factors, they’re having an impact on the surrounding tissue in a way which we can measure with this technology. Measuring this impact is particularly important, not to mention the fact that we can actually identify, characterize, and quantify such cells within the tissue itself.”
To determine if the senolytic 25HC can rejuvenate tissue by killing senescent cells in aged animals, Melov and colleagues examined the muscles of aged mice. They found that after injecting 25HC into the mice for five days, muscle mass was restored by an astounding 23%. What’s more, they validated that the muscle tissue was rejuvenated by killing senescent cells. Specifically, a metabolite produced when senescent cells die, 15-d-PGJ2 (15-Deoxy-Delta-12,14-prostaglandin J2), displayed substantially elevated levels in the urine of the mice treated with 25HC. These findings demonstrate that 25HC restores aged muscle in mice by eliminating senescent cells.
“This is a really important breakthrough for evaluating the efficacy of candidate senolytic drugs in vivo,” said Melov in a press release referring to the use of the marker 15-d-PGJ2. “Otherwise, how do you know whether the intervention is having any effect unless you’ve got some massive effect on functional outcomes? In aging research, we don’t often have massive improvements when it comes to function, but even small effects can be beneficial.”
Melov and colleagues induced senescence in muscle cells and measured changes in gene activity, finding that CRYAB was one of the genes that increased the most. They next sought to identify senolytic agents to target cells that display increased activation of CRYAB. The research team looked at a database for commercially available inhibitors of CRYAB and tested the effects of treating mouse and human muscle cells with these molecules. They found that the most efficacious of the chemical agents targeting CRYAB was the naturally-occurring metabolite 25HC.
Melov and colleagues next tested the senolytic effects of the newly identified metabolite. They found that 25HC eliminates senescent cells in mouse muscle cells, confirming this capability. To figure out whether 25HC has the same effect on other tissues in human cells, Melov and colleagues treated human heart, liver, kidney, and cartilage cells with 25HC after inducing senescence. They found that 25HC eliminates senescent cells in these cell lineages from four types of human tissues. These findings demonstrate that 25HC eliminates senescent cells from multiple types of tissue from mice and humans.
Interestingly, 25HC levels have been found to increase in people infected by the SARS-CoV-2 virus, indicating that it may play a role in the cellular defense mechanism against the virus. “In fact, it’s been advocated to be a type of therapeutic for COVID infection. And there’s been two or three studies on this, showing that the molecule seems to be effective in reducing the viral load,” said Melov. He went on to say that 25HC’s mechanism of action against SARS-CoV-2 may have something to do with alleviating the burden of senescent cells.
A few limitations exist for the study. Notably, only an indirect method for confirming the death of senescent cells was performed with mice administered 25HC by measuring their urinary 15-dPGJ2 content. Directly measuring senescent cell death in aged mice would confirm that 25HC rejuvenates muscle by killing senescent cells. Another important aspect was that no functional improvement for muscle was measured in the mice, such as increased endurance. It will also be interesting to see whether 25HC can rejuvenate other tissues, aside from muscle, in aged animals in future studies.
25HC is currently unavailable for purchase as a senolytic agent for everyday use. However, one can buy 5 mg of 25HC for experimental use for $50. This means that effective doses of 25HC have yet to be determined in humans, and when it does become available on the market, the cost will likely be pricey. In their study, Melov and colleagues compared the efficacy of 25HC against ABT263, a chemotherapeutic with known senolytic effects, and found that 25HC is the most potent at eliminating senescent cells. Therefore, the tantalizing benefits of 25HC await further research to figure out whether people can use it to restore tissue health during aging. Whether 25HC is more potent than the commonly used senolytic combo quercetin and dasatinib also remains to be investigated.