Mesenchymal stem cell mitochondrial fission and fusion a new pathway towards senescence

Mitochondria are highly dynamic organelles with their own life cycle. In response to a cell¿s bio-energetic state they continuously undergo structural remodeling through fission and fusion processes, mitochondrial dynamics. These tightly regulated events are believed to be an integral part in many cellular events such as apoptosis, senescence, and age-related diseases. As such, mitochondrial free radical production has been proposed as a theory of aging. Thus, oxidative damage can be associated with mitochondrial fission and fusion events, possibly responsible for mesenchymal stromal/stem cells senescence. Adult tissue derived mesenchymal stromal/stem cells present a therapeutic potential. However, to obtain an adequate mesenchymal stromal/stem cell number for clinical use extensive in vitro expansion is required. Unfortunately, these cells undergo replicative senescence rapidly. Therefore, we proposed to study the mitochondrial structural remodeling process during mesenchymal stromal/stem cell in vitro culture. Our objective was to determine mitochondrial phenotypic changes associated with mesenchymal stromal/stem cell senescence. We compared cells at population doubling two with cells at population doubling seven. With increased cell passage we observed a significant change in morphology. We confirmed senescence-associated ß-galactosidase activity. Cells at population doubling seven had increased mitochondrial mass, augmented superoxide production, and a decrease in mitochondrial membrane potential. Protein characterization evidenced MFN1 was in markedly expressed in senescent mesenchymal cells compared with early passage cells. Contrary, young cells had increased DRP1, associated with mitochondrial membrane fission. Collectively, our results demonstrate a senescent phenotype was observed for cells cultured at population doubling seven, characterized by changes in mitochondrial phenotype.