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

SUMMARYMitochondrial DNA (mtDNA) mutations frequently cause neurological diseases. Modeling of these defects has been difficult because of the challenges associated with engineering mtDNA. We show here that neural progenitor cells (NPCs) derived from human induced pluripotent stem cells (iPSCs) retain the parental mtDNA profile and exhibit a metabolic switch toward oxidative phosphorylation. NPCs derived in this way from patients carrying a deleterious homoplasmic mutation in the mitochondrial gene MT-ATP6 (m.9185T>C) showed defective ATP production and abnormally high mitochondrial membrane potential (MMP), plus altered calcium homeostasis, which represents a potential cause of neural impairment. High-content screening of FDA-approved drugs using the MMP phenotype highlighted avanafil, which we found was able to partially rescue the calcium defect in patient NPCs and differentiated neurons. Overall, our results show that iPSC-derived NPCs provide an effective model for drug screening to target mtDNA disorders that affect the nervous system.

Mitochondria are organelles playing pivotal roles in a range of diverse cellular functions, from energy generation to redox homeostasis and apoptosis regulation. Their loss of functionality may indeed contribute to the development of aging and age-related neurodegenerative disorders. Recently, mitochondria have been shown to exhibit peculiar features in pluripotent stem cells (PSCs). Moreover, an extensive restructuring of mitochondria has been observed during the process of cellular reprogramming, i.e. the conversion of somatic cells into induced pluripotent stem cells (iPSCs). These transformation events impact mitochondrial number, morphology, activity, cellular metabolism, and mtDNA integrity. PSCs retain the capability to self-renew indefinitely and to give rise to virtually any cell type of the body and thus hold great promise in medical research. Understanding the mitochondrial properties of PSCs, and how to modulate them, may thus help to shed light on the features of stemness and possibly increase our knowledge on cellular identity and differentiation pathways. Here, we review these recent findings and discuss their implications in the context of stem cell biology, aging research, and regenerative medicine.