title: Investigating the role of cellular senescence in skeletal muscle and sarcopenia
Creators: name: Shigdar, Shahjahan
Publisher: name: University of Liverpool Repository
Year: 2021

description: Senescence was first described by the pioneering work of Hayflick whereby human diploid fibroblasts underwent repeated population doublings, and the cells demonstrated a finite replicative limit in culture. It is now recognised that most replicative cells have the capability to undergo senescence. More recently senescence-like properties have been described in terminally differentiated cells, including skeletal muscle, although the nature of this senescence remains unclear. There is limited data examining the role of senescence in the determination of terminally differentiated tissues such as skeletal muscle. The overall hypothesis of this thesis was that senescent cells accumulate in old mice, either locally to skeletal muscle or distally, and that they produce an inflammatory Senescence Associated Secretory Phenotype (SASP) which may contribute at least in part to the chronic NF-κB activation and the loss of muscle mass and function seen in the muscles of old compared with adult wild type (WT) mice. This programme of work tested this hypothesis by utilising different models of senescence in vitro and in vivo and using a senolytic and transgenic intervention. Initial studies examined the potential for mouse skeletal muscle myoblasts (C2C12 myoblasts) to undergo senescence when treated with the DNA damaging agent, etoposide. Treatment of cells with etoposide resulted in cell cycle arrest, increased levels of p21, increased SA-β-Gal activity, and altered morphology, providing proof-of-principle that mononuclear myoblasts can become senescent. Further, the senescent myoblasts produced a proinflammatory SASP detectable in the cell media that was able to increase NF-κB transcription factor activity in mature C2C21 myotubes. Interpretation and comparison of data from senescent cells and control cells proved a challenge since under normal conditions, control myoblasts continue to divide until reaching confluence and contact inhibition occurs. At this point muscle cells initiate differentiation, with expression of some indices common to senescence. Examination of muscles of old WT mice showed no evidence of the presence of classical senescent cells in muscles of old mice although old mice showed evidence of increased levels of cytokines and chemokines in the plasma. Data suggest that muscle may partially contribute to the increase in some of these plasma cytokines/chemokines by a non-senescent mechanism. In support of this conclusion, studies using either a transgenic or a pharmacological approach to remove whole body senescent cells in old mice had some minor effects on plasma cytokine/chemokine levels but provided no protection against the age-related loss of muscle mass and function. Evidence for the presence of senescence cells in old mice remains unclear but some studies have suggested that removal of senescent cells provides protection against age-related muscle loss. One potential reason for the difference with the current study is the age of the mice used whereby studies that have demonstrated senescence within skeletal muscle have commonly made use of geriatric (32-months-old) mice suggesting that senescence is only involved in the very late stages of sarcopenia.