Recent Advancement in Elimination Strategies and Potential Rejuvenation Targets of Senescence

Cellular senescence is a state of exiting the cell cycle, resisting apoptosis, and changing phenotype. Senescent cells (SCs) can be identified by large, distorted morphology and irreversible inability to replicate. In early development, senescence has beneficial roles like tissue patterning and wound healing, where SCs are cleared by the immune system. However, there is a steep rise in SC number as organisms age. The issue with SC accumulation stems from the loss of cellular function, alterations of the microenvironment, and secretions of pro-inflammatory molecules, consisting of cytokines, chemokines, matrix metalloproteinases (MMPs), interleukins, and extracellular matrix (ECM)-associated molecules. This secreted cocktail is referred to as the senescence-associated secretory phenotype (SASP), a hallmark of cellular senescence. The SASP promotes inflammation and displays a bystander effect where paracrine signaling turns proliferating cells into senescent states. To alleviate age-associated diseases, researchers have developed novel methods and techniques to selectively eliminate SCs in aged individuals. Although studies demonstrated that selectively killing SCs improves age-related disorders, there are drawbacks to SC removal. Considering favorable aspects of senescence in the body, this paper reviews recent advancements in elimination strategies and potential rejuvenation targets of senescence to bring researchers in the field up to date.

Soft substrate maintains proliferative and adipogenic differentiation potential of human mesenchymal stem cells on long-term expansion by delaying senescence

Human mesenchymal stem cells (hMSCs), during in vitro expansion, gradually lose their distinct spindle morphology, self-renewal ability, multi-lineage differentiation potential and enter replicative senescence. This loss of cellular function is a major roadblock for clinical applications which demand cells in large numbers. Here, we demonstrate a novel role of substrate stiffness in the maintenance of hMSCs over long-term expansion. When serially passaged for 45 days from passage 3 to passage 18 on polyacrylamide gel of Young's modulus E=5 kPa, hMSCs maintained their proliferation rate and showed nine times higher population doubling in comparison to their counterparts cultured on plastic Petri-plates. They did not express markers of senescence, maintained their morphology and other mechanical properties such as cell stiffness and cellular traction, and were significantly superior in adipogenic differentiation potential. These results were demonstrated in hMSCs from two different sources, umbilical cord and bone marrow. In summary, our result shows that a soft gel is a suitable substrate to maintain the stemness of mesenchymal stem cells. As preparation of polyacrylamide gel is a well-established, and well-standardized protocol, we propose that this novel system of cell expansion will be useful in therapeutic and research applications of hMSCs.