The ability of stem cells to propagate indefinitely is believed to occur via the fine modulation of pathways commonly involved in cellular senescence, including the telomerase, the p53, and the mitochondrial/oxidative stress pathways. Induced pluripotent stem cells (iPSCs) are a novel stem cell population obtained from somatic cells through forced expression of a set of genes normally expressed in embryonic stem cells (ESCs). These reprogrammed cells acquire self-renewal properties and appear almost undistinguishable from ESCs in terms of morphology, gene expression, and differentiation potential. Accordingly, iPSCs exhibit alterations of the senescence-related telomerase and p53 signaling pathways. However, although treatments with antioxidants have been recently shown to enhance cellular reprogramming, detailed information regarding the state of the mitochondrial/oxidative stress pathway in iPSCs is still lacking. Mitochondria undergo specific changes during organismal development and aging. Thus, addressing whether somatic mitochondria within iPSCs acquire ESC-like features or retain the phenotype of the parental cell is an unanswered but relevant question. Herein, we demonstrate that somatic mitochondria within human iPSCs revert to an immature ESC-like state with respect to organelle morphology and distribution, expression of nuclear factors involved in mitochondrial biogenesis, content of mitochondrial DNA, intracellular ATP level, oxidative damage, and lactate generation. Upon differentiation, mitochondria within iPSCs and ESCs exhibited analogous maturation and anaerobic-to-aerobic metabolic modifications. Overall, the data highlight that human iPSCs and ESCs, although not identical, share similar mitochondrial properties and suggest that cellular reprogramming can modulate the mitochondrial/oxidative stress pathway, thus inducing a rejuvenated state capable of escaping cellular senescence.

STEM CELLS® is a monthly publication, it has been published continuously since 1983. The genetics and genomics; translational and clinical research; technology development. 2007;25:500 -510

Human stromal (mesenchymal) stem cells (hMSCs) are multipotent stem cells with ability to differentiate into mesoderm-type cells e.g. osteoblasts and adipocytes and thus they are being introduced into clinical trials for tissue regeneration. Traditionally, hMSCs have been isolated from bone marrow, but the number of cells obtained is limited. Here, we compared the MSC-like cell populations, obtained from alternative sources for MSC: adipose tissue and skin, with the standard phenotype of human bone marrow MSC (BM-MSCs). MSC from human adipose tissue (human adipose stromal cells (hATSCs)) and human skin (human adult skin stromal cells, (hASSCs) and human new-born skin stromal cells (hNSSCs)) grew readily in culture and the growth rate was highest in hNSSCs and lowest in hATSCs. Compared with phenotype of hBM-MSC, all cell populations were CD34−, CD45−, CD14−, CD31−, HLA-DR−, CD13+, CD29+, CD44+, CD73+, CD90+,and CD105+. When exposed to in vitro differentiation, hATSCs, hASSCs and hNSSCs exhibited quantitative differences in their ability to differentiate into adipocytes and to osteoblastic cells. Using a microarray-based approach we have unveiled a common MSC molecular signature composed of 33 CD markers including known MSC markers and several novel markers e.g. CD165, CD276, and CD82. However, significant differences in the molecular phenotype between these different stromal cell populations were observed suggesting ontological and functional differences. In conclusion, MSC populations obtained from different tissues exhibit significant differences in their proliferation, differentiation and molecular phenotype, which should be taken into consideration when planning their use in clinical protocols.Electronic supplementary materialThe online version of this article (doi:10.1007/s12015-012-9365-8) contains supplementary material, which is available to authorized users.

Reprogramming somatic cells to a pluripotent state drastically reconfigures the cellular anabolic requirements, thus potentially inducing cancer-like metabolic transformation. Accordingly, we and others previously showed that somatic mitochondria and bioenergetics are extensively remodeled upon derivation of induced pluripotent stem cells (iPSCs), as the cells transit from oxidative to glycolytic metabolism. In the attempt to identify possible regulatory mechanisms underlying this metabolic restructuring, we investigated the contributing role of hypoxia-inducible factor one alpha (HIF1a), a master regulator of energy metabolism, in the induction and maintenance of pluripotency. We discovered that the ablation of HIF1a function in dermal fibroblasts dramatically hampers reprogramming efficiency, while small molecule-based activation of HIF1a significantly improves cell fate conversion. Transcriptional and bioenergetic analysis during reprogramming initiation indicated that the transduction of the four factors is sufficient to upregulate the HIF1a target pyruvate dehydrogenase kinase (PDK) one and set in motion the glycolytic shift. However, additional HIF1a activation appears critical in the early upregulation of other HIF1a-associated metabolic regulators, including PDK3 and pyruvate kinase (PK) isoform M2 (PKM2), resulting in increased glycolysis and enhanced reprogramming. Accordingly, elevated levels of PDK1, PDK3, and PKM2 and reduced PK activity could be observed in iPSCs and human embryonic stem cells in the undifferentiated state. Overall, the findings suggest that the early induction of HIF1a targets may be instrumental in iPSC derivation via the activation of a glycolytic program. These findings implicate the HIF1a pathway as an enabling regulator of cellular reprogramming. STEM CELLS 2014;32:364-376