The Therapeutic Potential of Apoptotic Mesenchymal Stem Cells in Muscle Regeneration

Abstract

Macrophages are critical for muscle regeneration and have an unusually high level of responsiveness to their environment. Because of this sensitivity, their beneficial effects may be realized only upon certain activation conditions. Previous studies in our lab have shown that transplantation of classically activated macrophages and anti‐inflammatory macrophages sorted from injured muscle enhanced functional regeneration in ischemia‐reperfusion muscles. The phagocytosis of apoptotic cells that are present in injured muscle or from transplantation serves as a well‐known activation signal for macrophages to initiate a regenerative and anti‐inflammatory profile leading to enhanced muscle regeneration. The aim of this study is to investigate if the phagocytosis of apoptotic mesenchymal stem cells (MSCs) can polarize bone marrow‐derived macrophages (BMDMs) towards a regenerative and anti‐inflammatory phenotype and subsequently if apoptotic MSCs in different stages of apoptosis have differential effects. We induced apoptosis in MSCs and incubated them at different time intervals to obtain differential stages of apoptosis, which were characterized by the amount of externalized phosphatidylserine (PS) on their cell membrane. Apoptotic MSCs with different concentrations of PS were then co‐cultured with BMDMs for 48 hours. Cell surface markers and gene expression of characteristic of pro‐ and anti‐inflammatory polarized BMDMs were measured. Our results showed that apoptotic MSCs without incubation significantly increased the expression of F4/80 and CD206 on the cell surface of BMDMs. They also increased the gene expression of MRC‐1 and IL‐10 as well as decreased the expression of iNOS and IL‐1β in co‐cultured BMDMs. The extent of these effects exhibited a decreasing trend when incubation time of apoptotic MSCs increased. In conclusion, the extent of MSC apoptosis as characterized by PS expression resulted in differential anti‐inflammatory and regenerative effects on BMDMs, wherein the apoptotic MSCs in the earlier stage of apoptosis result in the greatest increase in anti‐inflammatory phenotype, which reveals their therapeutic potential in muscle regeneration.Support or Funding InformationThis project is funded by National Institutes of Health Grant 2R01EB015007‐05A1 (to Laura J. Suggs and Roger P. Farrar)