Mitochondrial morphology transitions and functions: implications for retrograde signaling?

ABSTRACT. Stem cells from human exfoliated deciduous teeth (SHED) are isolated from the dental pulp tissue of primary teeth and can differentiate into neuronal cells. Although SHED are a desirable type of stem cells for transplantation therapy and for the study of neurological diseases, a large part of the neuronal differentiation machinery of SHED remains unclear. Recent studies have suggested that mitochondrial activity is involved in the differentiation of stem cells. In the present work, we investigated the neuronal differentiation machinery of SHED by focusing on mitochondrial activity. During neuronal differentiation of SHED, we observed increased mitochondrial membrane potential, increased mitochondrial DNA, and elongated mitochondria. Furthermore, to examine the demand for mitochondrial activity in neuronal differentiation, we then differentiated SHED into neuronal cells in the presence of rotenone, an inhibitor of mitochondrial respiratory chain complex I, and carbonyl cyanide m-chlorophenyl hydrazone (CCCP), a mitochondrial uncoupler, and found that neuronal differentiation was inhibited by treatment with rotenone and CCCP. These results indicated that increased mitochondrial activity was crucial for the neuronal differentiation of SHED.

A B S T R A C TMitochondria-shaping proteins control the dynamic equilibrium between fusion and fission of the mitochondrial network. Their balance is strictly required to regulate various processes, including the quality of mitochondria, cell metabolism, cell death, proliferation and cell migration. Alterations in these processes are frequently encountered in cancer, during both its onset and later progression, as evidence emerge connecting alterations in mitochondrial dynamics with cancer development. In recent years, novel therapeutic approaches to fight against different human tumors aim at exploiting the immune system's ability to specifically recognize tumor antigens, thus killing malignant cells in a process named immune-surveillance. Interestingly, data are accumulating on the role that mitochondrial dynamics play also for the correct function of both the innate and the adaptive immune system. By this review, we overview how mitochondrial dynamics can affect various processes during cancer development, acting directly on tumor cells or indirectly on cells responsible for tumor aggression and defence.

Running title: Regulation and quantification of mitochondrial morphology and content. Word count: 15,454 (total)Characters: 107,091 (including spaces); 92,067 (excluding spaces) Keywords: mitochondrial biogenesis, mitochondrial dynamics, mitochondrial degradation, living cells, TMRM, microscopy, segmentation, image analysis.Page 2 of 37 ABSTRACTMitochondria play a key role in signal transduction, redox homeostasis and cell survival, which extends far beyond their classical functioning in ATP production and energy metabolism. In living cells, mitochondrial content ("mitochondrial mass") depends on the cell-controlled balance between mitochondrial biogenesis and degradation. These processes are intricately linked to changes in net mitochondrial morphology and spatiotemporal positioning ("mitochondrial dynamics"), which are governed by mitochondrial fusion, fission and motility. It is becoming increasingly clear that mitochondrial mass and dynamics, as well as its ultrastructure and volume, are mechanistically linked to mitochondrial function and the cell. This means that proper quantification of mitochondrial morphology and content is of prime importance in understanding mitochondrial and cellular physiology in health and disease. This review first presents how cellular mitochondrial content is regulated at the level of mitochondrial biogenesis, degradation and dynamics. Next we discuss how mitochondrial dynamics and content can be analyzed with a special emphasis on quantitative live-cell microscopy strategies.