Cellular senescence occurs in response to stress, damage, and cells hitting the Hayflick limit on replication. Senescent cells cease replication, and begin to secrete a pro-inflammatory, pro-growth mix of signals. In cell cultures, senescence is a common phenomenon. Researchers are beginning to realize that the presence versus absence of senescent cells in culture may explain a sizable fraction of the variation in outcomes resulting from stem cell therapies. Slightly different methodologies of production of stem cells for transplantation may result in sizable differences in the proportion of those cells that are senescent, and thus the ability of the treatment to produce benefits to a patient.
Another consideration is the age of the donor. In today's open access paper, researchers demonstrate that mesenchymal stem cells taken from bone marrow are more senescent in culture and less capable of inducing regeneration following transplantation when the source is older mice versus younger mice. This problem can be mostly fixed by applying the senolytic combination of dasatinib and quercetin briefly to cultured cells, destroying many of the senescent cells, prior to transplantion. Senolytic treatment of cells to be used in cell therapies may well become commonplace in the years ahead.
Bone marrow mesenchymal stem cells (BMSCs) are a population of multipotent progenitor cells that have regenerative potential of various tissues. With aging, the function of BMSCs declines dramatically, and limited interventions exist to rejuvenate BMSC population from aged donors. The underlying mechanisms of age-related changes in BMSCs are not fully characterized. With aging, a portion of BMSCs might become senescent like other MSCs, and contribute to these changes. Cellular senescence refers to the stable arrest of cellular proliferation, and might impact aged BMSCs function by both intrinsic and extrinsic mechanisms.
Recently, various senolytic drugs have been developed to specifically kill senescent cells. The cocktail of dasatinib and quercetin (D + Q) is the first reported senolytic agents, and eliminates senescent cells by transiently suppressing senescence-associated anti-apoptotic pathways which are highly activated in senescent cells. In this study, an in vitro cell culture and in vivo transplantation model was used to assess the effect of D + Q on inherent osteogenic potential of BMSCs derived from old donors.
BMSCs cultures were established from young (3 month-old) and old (27 month-old) male mice, and were treated with vehicle (V) or D + Q for 24 hours. At the end of the 24 hour treatment period, an ATP-based assay revealed that D + Q-treated old BMSC cultures had 20-30% lower cell number than V-treated cells while the reduction of cell number in young BMSCs by D + Q was only 10%. These results indicate that D + Q has age-preferential killing effects on certain cell populations in BMSCs. The cultures were stained with senescence associated beta-galactosidase (SABG), a classic biomarker for cellular senescence in vitro. Old BMSCs contained more SABG + cells (20%) than young BMSCs (10%) and D + Q reduced SABG + cells in old (10%) but not young BMSCs.
The effect of D + Q on the bone forming potential of old BMSCs in vivo was assessed using the calvarial defect model. D + Q- or V-treated old BMSCs as well as young BMSC cultures were loaded into collagen-hydroxyapatite (HA) scaffolds and implanted into calvarial defects in 3-month-old immunodeficient mice. The area of newly formed mineralized tissue relative to the defect size was significantly lower in calvaria transplanted with V-treated old BMSCs (50%) compared to both young (90%) and D + Q-treated old cells (80%).
In summary, D + Q treatment improved the osteogenic capacity of old BMSCs, and resulted in bone organoids with restored bone remodeling and an enlarged and functional bone marrow space. D + Q does appear to be beneficial for restoring the bone and bone marrow forming potential of old BMSCs. These agents along with other senolytic compounds hold promise for improving BMSC function in aged populations.