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.

Huntington's disease (HD) is a neurodegenerative disorder caused by an expanded CAG trinucleotide repeat in the huntingtin gene (). Molecular chaperones have been implicated in suppressing or delaying the aggregation of mutant Htt. Using and assays, we have identified a trimeric chaperone complex (Hsc70, Hsp110, and J-protein) that completely suppresses fibrilization of HttExon1Q The composition of this chaperone complex is variable as recruitment of different chaperone family members forms distinct functional complexes. The trimeric chaperone complex is also able to resolubilize Htt fibrils. We confirmed the biological significance of these findings in HD patient-derived neural cells and on an organismal level in Among the proteins in this chaperone complex, the J-protein is the concentration-limiting factor. The single overexpression of DNAJB1 in HEK293T cells is sufficient to profoundly reduce HttExon1Q aggregation and represents a target of future therapeutic avenues for HD.

The maintenance of cellular identity requires continuous adaptation to environmental changes. This process is particularly critical for stem cells, which need to preserve their differentiation potential over time. Among the mechanisms responsible for regulating cellular homeostatic responses, mitochondria are emerging as key players. Given their dynamic and multifaceted role in energy metabolism, redox, and calcium balance, as well as cell death, mitochondria appear at the interface between environmental cues and the control of epigenetic identity. In this review, we describe how mitochondria have been implicated in the processes of acquisition and loss of stemness, with a specific focus on pluripotency. Dissecting the biological functions of mitochondria in stem cell homeostasis and differentiation will provide essential knowledge to understand the dynamics of cell fate modulation, and to establish improved stem cell-based medical applications.

RNAi is widely applied to inhibit expression of specific genes, but it is limited by variable efficiency and specificity of empirically designed siRNA or shRNA constructs. This complicates studies targeting individual genes and significantly impairs large-scale screens using genome-wide knockdown libraries. Here, we show that ectopic expression of the RISC slicer Argonaute-2 (Ago2, eIF2C2) dramatically enhances RNAi specifically for mRNA targets with perfectly matched binding sites. This effect depends on its endonuclease activity and is uncoupled from its regulation of microRNA expression. To model the application of Ago2 coexpression with shRNA knockdown, we targeted the EGF receptor (EGFR) in lung cancer cells exhibiting oncogene addiction to EGFR. Whereas multiple empirically designed shRNA constructs exhibited highly divergent efficiencies in mediating EGFR knockdown and cell killing, coexpression of Ago2 resulted in uniform and highly specific target gene suppression and apoptosis in EGFR-dependent cells. Codelivery of Ago2 with shRNA constructs or siRNA duplexes thus provides a strategy to enhance the efficacy and the specificity of RNAi in experimental and potentially therapeutic settings.microRNA ͉ RNAi screening ͉ shRNA ͉ siRNA ͉ Ago2