A reduction in ATP demand and mitochondrial activity with neural differentiation of human embryonic stem cells

Energy metabolism and energy-sensing pathways in mammalian embryonic and adult stem cell fate

Metabolism is influenced by age, food intake, and conditions such as diabetes and obesity. How do physiological or pathological metabolic changes influence stem cells, which are crucial for tissue homeostasis? This Commentary reviews recent evidence that stem cells have different metabolic demands than differentiated cells, and that the molecular mechanisms that control stem cell self-renewal and differentiation are functionally connected to the metabolic state of the cell and the surrounding stem cell niche. Furthermore, we present how energy-sensing signaling molecules and metabolism regulators are implicated in the regulation of stem cell self-renewal and differentiation. Finally, we discuss the emerging literature on the metabolism of induced pluripotent stem cells and how manipulating metabolic pathways might aid cellular reprogramming. Determining how energy metabolism regulates stem cell fate should shed light on the decline in tissue regeneration that occurs during aging and facilitate the development of therapies for degenerative or metabolic diseases.

The effects of nuclear reprogramming on mitochondrial DNA replication

Undifferentiated mouse embryonic stem cells (ESCs) possess low numbers of mitochondrial DNA (mtDNA), which encodes key subunits associated with the generation of ATP through oxidative phosphorylation (OXPHOS). As ESCs differentiate, mtDNA copy number is regulated by the nuclear-encoded mtDNA replication factors, which initiate a major replication event on Day 6 of differentiation. Here, we examined mtDNA replication events in somatic cells reprogrammed to pluripotency, namely somatic cell-ES (SC-ES), somatic cell nuclear transfer ES (NT-ES) and induced pluripotent stem (iPS) cells, all at low-passage. MtDNA copy number in undifferentiated iPS cells was similar to ESCs whilst SC-ES and NT-ES cells had significantly increased levels, which correlated positively and negatively with Nanog and Sox2 expression, respectively. During pluripotency and differentiation, the expression of the mtDNA-specific replication factors, PolgA and Peo1, were differentially expressed in iPS and SC-ES cells when compared to ESCs. Throughout differentiation, reprogrammed somatic cells were unable to accumulate mtDNA copy number, characteristic of ESCs, especially on Day 6. In addition, iPS and SC-ES cells were also unable to regulate ATP content in a manner similar to differentiating ESCs prior to Day 14. The treatment of reprogrammed somatic cells with an inhibitor of de novo DNA methylation, 5-Azacytidine, prior to differentiation enabled iPS cells, but not SC-ES and NT-ES cells, to accumulate mtDNA copies per cell in a manner similar to ESCs. These data demonstrate that the reprogramming process disrupts the regulation of mtDNA replication during pluripotency but this can be re-established through the use of epigenetic modifiers.

Degradation of IF1 controls energy metabolism during osteogenic differentiation of stem cells

Differentiation of human mesenchymal stem cells (hMSCs) requires the rewiring of energy metabolism. Herein, we demonstrate that the ATPase inhibitory factor 1 (IF1) is expressed in hMSCs and in prostate and colon stem cells but is not expressed in the differentiated cells. IF1 inhibits oxidative phosphorylation and regulates the activity of aerobic glycolysis in hMSCs. Silencing of IF1 in hMSCs mimics the metabolic changes observed in osteocytes and accelerates cellular differentiation. Activation of IF1 degradation acts as the switch that regulates energy metabolism during differentiation. We conclude that IF1 is a stemness marker important for maintaining the quiescence state.

Novel cell-based assay reveals associations of circulating serum AhR-ligands with metabolic syndrome and mitochondrial dysfunction

Serum concentrations of environmental pollutants have been positively correlated with diabetes and metabolic syndrome in epidemiologic studies. In turn, abnormal mitochondrial function has been associated with the diseases. The relationships between these variables, however, have not been studied. We developed novel cell-based aryl hydrocarbon receptor (AhR) agonist bioassay system without solvent extraction process and analyzed whether low-dose circulating AhR ligands in human serum are associated with parameters of metabolic syndrome and mitochondrial function. Serum AhR ligand activities were measured as serum 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalent (sTCDDeq) in pM using 10 μL human sera from 97 Korean participants (47 with glucose intolerance and 50 matched controls, average age of 46.6 ± 9.9 years, 53 male and 45 female). sTCDDeq were higher in participants with glucose intolerance than normal controls and were positively associated (P < 0.01) with obesity, blood pressure, serum triglyceride, and fasting glucose, but not with HDL-cholesterol. Body mass index was in a positive linear relationship with serum AhR ligands in healthy participants. When myoblast cells were incubated with human sera, ATP generating power of mitochondria became impaired in an AhR ligand concentration-dependent manner. Our results support that circulating AhR ligands may directly reduce mitochondrial function in tissues, leading to weight gain, glucose intolerance, and metabolic syndrome. Our rapid cell-based assay using minute volume of human serum may provide one of the best monitoring systems for circulating AhR ligands, good clinical biomarkers for the progress of disease and therapeutic efficacy.

Research with parthenogenetic stem cells will help decide whether a safer clinical use is possible

The derivation and use of parthenogenetic stem cells (pESCs) are envisaged as a reliable alternative to conventional embryonic stem cells. Similar to embryonic stem cells in their proliferation, expression of pluripotency markers and capacity to multilineage differentiation, pESCs are at a lower risk of immune rejection within stem cell-based therapeutics. Moreover, pESCs represent an important model system to study the effect of paternally imprinted genes on cell differentiation. However, currently available information about the genetic and epigenetic behaviour of pESCs is limited. Thus, a detailed look at the biology of parthenogenetic (PG) embryos and PG-derived cell lines would allow gaining insight into the full potential of pESC in biotechnology. In this commentary article we review some features related to the biology of PG embryos and pESCs. In addition, novel traits on bovine pESCs (bpESCs) are discussed.

Metabolic regulation in pluripotent stem cells during reprogramming and self-renewal

Small, rapidly dividing pluripotent stem cells (PSCs) have unique energetic and biosynthetic demands compared with typically larger, quiescent differentiated cells. Shifts between glycolysis and oxidative phosphorylation with PSC differentiation or reprogramming to pluripotency are accompanied by changes in cell cycle, biomass, metabolite levels, and redox state. PSC and cancer cell metabolism are overtly similar, with metabolite levels influencing epigenetic/genetic programs. Here, we discuss the emerging roles for metabolism in PSC self-renewal, differentiation, and reprogramming.

Nampt expression increases during osteogenic differentiation of multi- and omnipotent progenitors

Despite emerging data showing that metabolic changes occur with stem cell differentiation, the cross-talk between factors governing energy metabolism and epigenetic modification is not understood. Nicotinamide adenine dinucleotide (NAD) participates in both energy metabolism and protein modification processes. Changes of the intracellular NAD concentration have been shown to correlate with differentiation of adult and embryonic stem cells. In the present study, we investigated the expression pattern of Nampt, the rate-limiting enzyme in NAD salvaging pathway, during osteogenic differentiation of the multipotent mouse fibroblast C3H10T1/2 and the omnipotent preosteoblast MC3T3-E1 cells. We found that Nampt was increasingly expressed during differentiation in both cell models. The increase of Nampt was associated with higher NAD concentration and Sirt1 activity. Knockdown of Nampt or addition of its specific inhibitor FK866 leads to lower intracellular NAD concentration and decline in osteogenesis. These findings indicate that osteogenic differentiation correlates with intracellular NAD metabolism in which Nampt plays a regulatory role.

Stem cell metabolic and spectroscopic profiling

Stem cells offer great potential for regenerative medicine because they regenerate damaged tissue by cell replacement and/or by stimulating endogenous repair mechanisms. Although stem cells are defined by their functional properties, such as the potential to proliferate, to self-renew, and to differentiate into specific cell types, their identification based on the expression of specific markers remains vague. Here, profiles of stem cell metabolism might highlight stem cell function more than the expression of single genes/markers. Thus, systematic approaches including spectroscopy might yield insight into stem cell function, identity, and stemness. We review the findings gained by means of metabolic and spectroscopic profiling methodologies, for example, nuclear magnetic resonance spectroscopy (NMRS), mass spectrometry (MS), and Raman spectroscopy (RS), with a focus on neural stem cells and neurogenesis.

Nuclear genome transfer in human oocytes eliminates mitochondrial DNA variants

Mitochondrial DNA mutations transmitted maternally within the oocyte cytoplasm often cause life-threatening disorders. Here we explore the use of nuclear genome transfer between unfertilized oocytes of two donors to prevent the transmission of mitochondrial mutations. Nuclear genome transfer did not reduce developmental efficiency to the blastocyst stage, and genome integrity was maintained provided that spontaneous oocyte activation was avoided through the transfer of incompletely assembled spindle-chromosome complexes. Mitochondrial DNA transferred with the nuclear genome was initially detected at levels below 1%, decreasing in blastocysts and stem-cell lines to undetectable levels, and remained undetectable after passaging for more than one year, clonal expansion, differentiation into neurons, cardiomyocytes or β-cells, and after cellular reprogramming. Stem cells and differentiated cells had mitochondrial respiratory chain enzyme activities and oxygen consumption rates indistinguishable from controls. These results demonstrate the potential of nuclear genome transfer to prevent the transmission of mitochondrial disorders in humans.

Cockayne syndrome b maintains neural precursor function

Neurodevelopmental defects are observed in the hereditary disorder Cockayne syndrome (CS). The gene most frequently mutated in CS, Cockayne Syndrome B (CSB), is required for the repair of bulky DNA adducts in transcribed genes during transcription-coupled nucleotide excision repair. CSB also plays a role in chromatin remodeling and mitochondrial function. The role of CSB in neural development is poorly understood. Here we report that the abundance of neural progenitors is normal in Csb(-/-) mice and the frequency of apoptotic cells in the neurogenic niche of the adult subependymal zone is similar in Csb(-/-) and wild type mice. Both embryonic and adult Csb(-/-) neural precursors exhibited defective self-renewal in the neurosphere assay. In Csb(-/-) neural precursors, self-renewal progressively decreased in serially passaged neurospheres. The data also indicate that Csb and the nucleotide excision repair protein Xpa preserve embryonic neural stem cell self-renewal after UV DNA damage. Although Csb(-/-) neural precursors do not exhibit altered neuronal lineage commitment after low-dose UV (1J/m(2)) in vitro, neurons differentiated in vitro from Csb(-/-) neural precursors that had been irradiated with 1J/m(2) UV exhibited defective neurite outgrowth. These findings identify a function for Csb in neural precursors.

Death receptors and mitochondria: two prime triggers of neural apoptosis and differentiation

BACKGROUND

Stem cell therapy is a strategy far from being satisfactory and applied in the clinic. Poor survival and differentiation levels of stem cells after transplantation or neural injury have been major problems. Recently, it has been recognized that cell death-relevant proteins, notably those that operate in the core of the executioner apoptosis machinery are functionally involved in differentiation of a wide range of cell types, including neural cells.

SCOPE OF REVIEW

This article will review recent studies on the mechanisms underlying the non-apoptotic function of mitochondrial and death receptor signaling pathways during neural differentiation. In addition, we will discuss how these major apoptosis-regulatory pathways control the decision between differentiation, self-renewal and cell death in neural stem cells and how levels of activity are restrained to prevent cell loss as final outcome.

MAJOR CONCLUSIONS

Emerging evidence suggests that, much like p53, caspases and Bcl-2 family members, the two prime triggers of cell death pathways, death receptors and mitochondria, may influence proliferation and differentiation potential of stem cells, neuronal plasticity, and astrocytic versus neuronal stem cell fate decision.

GENERAL SIGNIFICANCE

A better understanding of the molecular mechanisms underlying key checkpoints responsible for neural differentiation as an alternative to cell death will surely contribute to improve neuro-replacement strategies.

Mitochondrial DNA copy number is regulated in a tissue specific manner by DNA methylation of the nuclear-encoded DNA polymerase gamma A

DNA methylation is an essential mechanism controlling gene expression during differentiation and development. We investigated the epigenetic regulation of the nuclear-encoded, mitochondrial DNA (mtDNA) polymerase γ catalytic subunit (PolgA) by examining the methylation status of a CpG island within exon 2 of PolgA. Bisulphite sequencing identified low methylation levels (<10%) within exon 2 of mouse oocytes, blastocysts and embryonic stem cells (ESCs), while somatic tissues contained significantly higher levels (>40%). In contrast, induced pluripotent stem (iPS) cells and somatic nuclear transfer ESCs were hypermethylated (>20%), indicating abnormal epigenetic reprogramming. Real time PCR analysis of 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC) immunoprecipitated DNA suggests active DNA methylation and demethylation within exon 2 of PolgA. Moreover, neural differentiation of ESCs promoted de novo methylation and demethylation at the exon 2 locus. Regression analysis demonstrates that cell-specific PolgA expression levels were negatively correlated with DNA methylation within exon 2 and mtDNA copy number. Finally, using chromatin immunoprecipitation (ChIP) against RNA polymerase II (RNApII) phosphorylated on serine 2, we show increased DNA methylation levels are associated with reduced RNApII transcriptional elongation. This is the first study linking nuclear DNA epigenetic regulation with mtDNA regulation during differentiation and cell specialization.

Endogenous fluorescence signatures in living pluripotent stem cells change with loss of potency

The therapeutic potential of stem cells is limited by the non-uniformity of their phenotypic state. Thus it would be advantageous to noninvasively monitor stem cell status. Driven by this challenge, we employed multidimensional multiphoton microscopy to quantify changes in endogenous fluorescence occurring with pluripotent stem cell differentiation. We found that global and cellular-scale fluorescence lifetime of human embryonic stem cells (hESC) and murine embryonic stem cells (mESC) consistently decreased with differentiation. Less consistent were trends in endogenous fluorescence intensity with differentiation, suggesting intensity is more readily impacted by nuances of species and scale of analysis. What emerges is a practical and accessible approach to evaluate, and ultimately enrich, living stem cell populations based on changes in metabolism that could be exploited for both research and clinical applications.

Mitochondrial respiration--an important therapeutic target in melanoma

The importance of mitochondria as oxygen sensors as well as producers of ATP and reactive oxygen species (ROS) has recently become a focal point of cancer research. However, in the case of melanoma, little information is available to what extent cellular bioenergetics processes contribute to the progression of the disease and related to it, whether oxidative phosphorylation (OXPHOS) has a prominent role in advanced melanoma. In this study we demonstrate that compared to melanocytes, metastatic melanoma cells have elevated levels of OXPHOS. Furthermore, treating metastatic melanoma cells with the drug, Elesclomol, which induces cancer cell apoptosis through oxidative stress, we document by way of stable isotope labeling with amino acids in cell culture (SILAC) that proteins participating in OXPHOS are downregulated. We also provide evidence that melanoma cells with high levels of glycolysis are more resistant to Elesclomol. We further show that Elesclomol upregulates hypoxia inducible factor 1-α (HIF-1α), and that prolonged exposure of melanoma cells to this drug leads to selection of melanoma cells with high levels of glycolysis. Taken together, our findings suggest that molecular targeting of OXPHOS may have efficacy for advanced melanoma.

The role of redox environment in neurogenic development

The dynamic changes of cellular redox elements during neurogenesis allow the control of specific programs for selective lineage progression. There are many redox couples that influence the cellular redox state. The shift from a reduced to an oxidized state and vice versa may act as a cellular switch mechanism of stem cell mode of action from proliferation to differentiation. The redox homeostasis ensures proper functioning of redox-sensitive signaling pathways through oxidation/reduction of critical cysteine residues on proteins involved in signal transduction. This review presents the current knowledge on the relation between changes in the cellular redox environment and stem cell programming in the course of commitment to a restricted neural lineage, focusing on in vivo neurogenesis and in vitro neuronal differentiation. The first two sections outline the main systems that control the intracellular redox environment and make it more oxidative or reductive. The last section provides the background on redox-sensitive signaling pathways that regulate neurogenesis.

Metabolic differentiation in the embryonic retina

Unlike healthy adult tissues, cancers produce energy mainly by aerobic glycolysis instead of oxidative phosphorylation. This adaptation, called the Warburg effect, may be a feature of all dividing cells, both normal and cancerous, or it may be specific to cancers. It is not known whether, in a normally growing tissue during development, proliferating and postmitotic cells produce energy in fundamentally different ways. Here we show in the embryonic Xenopus retina in vivo, that dividing progenitor cells depend less on oxidative phosphorylation for ATP production than non-dividing differentiated cells, and instead use glycogen to fuel aerobic glycolysis. The transition from glycolysis to oxidative phosphorylation is connected to the cell differentiation process. Glycolysis is indispensable for progenitor proliferation and biosynthesis, even when it is not used for ATP production. These results suggest that the Warburg effect can be a feature of normal proliferation in vivo, and that the regulation of glycolysis and oxidative phosphorylation is critical for normal development.

Quantitative measurement of mitochondrial membrane potential in cultured cells: calcium-induced de- and hyperpolarization of neuronal mitochondria

Mitochondrial membrane potential (ΔΨM) is a central intermediate in oxidative energy metabolism. Although ΔΨM is routinely measured qualitatively or semi-quantitatively using fluorescent probes, its quantitative assay in intact cells has been limited mostly to slow, bulk-scale radioisotope distribution methods. Here we derive and verify a biophysical model of fluorescent potentiometric probe compartmentation and dynamics using a bis-oxonol-type indicator of plasma membrane potential (ΔΨP) and the ΔΨM probe tetramethylrhodamine methyl ester (TMRM) using fluorescence imaging and voltage clamp. Using this model we introduce a purely fluorescence-based quantitative assay to measure absolute values of ΔΨM in millivolts as they vary in time in individual cells in monolayer culture. The ΔΨP-dependent distribution of the probes is modelled by Eyring rate theory. Solutions of the model are used to deconvolute ΔΨP and ΔΨM in time from the probe fluorescence intensities, taking into account their slow, ΔΨP-dependent redistribution and Nernstian behaviour. The calibration accounts for matrix:cell volume ratio, high- and low-affinity binding, activity coefficients, background fluorescence and optical dilution, allowing comparisons of potentials in cells or cell types differing in these properties. In cultured rat cortical neurons, ΔΨM is −139 mV at rest, and is regulated between −108 mV and −158 mV by concerted increases in ATP demand and Ca2+-dependent metabolic activation. Sensitivity analysis showed that the standard error of the mean in the absolute calibrated values of resting ΔΨM including all biological and systematic measurement errors introduced by the calibration parameters is less than 11 mV. Between samples treated in different ways, the typical equivalent error is ∼5 mV.

Metabolism of the preimplantation embryo: 40 years on

This review considers how our understanding of preimplantation embryo metabolism has progressed since the pioneering work on this topic in the late 1960s and early 1970s. Research has been stimulated by a desire to understand how metabolic events contribute to the development of the zygote into the blastocyst, the need for biomarkers of embryo health with which to improve the success of assisted conception technologies, and latterly by the ‘Developmental Origins of Health and Disease’ (DOHaD) concept. However, arguably, progress has not been as great as it might have been due to methodological difficulties in working with tiny amounts of tissue and the low priority assigned to fundamental research on fertility and infertility, with developments driven more by technical than scientific advances. Nevertheless, considerable progress has been made in defining the roles of the traditional nutrients: pyruvate, glucose, lactate, and amino acids; originally considered as energy sources and biosynthetic precursors, but now recognized as having multiple, overlapping functions. Other nutrients; notably lipids, are beginning to attract the attention they deserve. The pivotal role of mitochondria in early embryo development and the DOHaD concept, and in providing a cellular focus for metabolic events is now recognized. Some unifying ideas are discussed; namely ‘stress–response models’ and the ‘quiet embryo hypothesis’; the latter aiming to relate the metabolism of individual preimplantation embryos to their subsequent viability. The review concludes by updating the state of knowledge of preimplantation embryo metabolism in the early 1970s and listing some future research questions.

Label-free separation of human embryonic stem cells and their differentiating progenies by phasor fluorescence lifetime microscopy

We develop a label-free optical technique to image and discriminate undifferentiated human embryonic stem cells (hESCs) from their differentiating progenies in vitro. Using intrinsic cellular fluorophores, we perform fluorescence lifetime microscopy (FLIM) and phasor analysis to obtain hESC metabolic signatures. We identify two optical biomarkers to define the differentiation status of hESCs: Nicotinamide adenine dinucleotide (NADH) and lipid droplet-associated granules (LDAGs). These granules have a unique lifetime signature and could be formed by the interaction of reactive oxygen species and unsaturated metabolic precursor that are known to be abundant in hESC. Changes in the relative concentrations of these two intrinsic biomarkers allow for the discrimination of undifferentiated hESCs from differentiating hESCs. During early hESC differentiation we show that NADH concentrations increase, while the concentration of LDAGs decrease. These results are in agreement with a decrease in oxidative phosphorylation rate. Single-cell phasor FLIM signatures reveal an increased heterogeneity in the metabolic states of differentiating H9 and H1 hESC colonies. This technique is a promising noninvasive tool to monitor hESC metabolism during differentiation, which can have applications in high throughput analysis, drug screening, functional metabolomics and induced pluripotent stem cell generation.

Human induced pluripotent stem cells harbor homoplasmic and heteroplasmic mitochondrial DNA mutations while maintaining human embryonic stem cell-like metabolic reprogramming

Human induced pluripotent stem cells (iPSCs) have been recently found to harbor genomic alterations. However, the integrity of mitochondrial DNA (mtDNA) within reprogrammed cells has yet to be investigated. mtDNA mutations occur at a high rate and contribute to the pathology of a number of human disorders. Furthermore, the lack of mtDNA integrity may alter cellular bioenergetics and limit efficient differentiation. We demonstrated previously that the derivation of iPSCs is associated with mitochondrial remodeling and a metabolic switch towards glycolysis. Here, we have discovered that alterations of mtDNA can occur upon the induction of pluripotency. Massively parallel pyrosequencing of mtDNA revealed that human iPSCs derived from young healthy donors harbored single base mtDNA mutations (substitutions, insertions, and deletions), both homoplasmic (in all mtDNA molecules) and heteroplasmic (in a fraction of mtDNAs), not present in the parental cells. mtDNA modifications were mostly common variants and not disease related. Moreover, iPSC lines bearing different mtDNA mutational loads maintained a consistent human embryonic stem cell-like reprogramming of energy metabolism. This involved the upregulation of glycolytic enzymes, increased glucose-6-phosphate levels, and the over-expression of pyruvate dehydrogenase kinase 1 protein, which reroutes the bioenergetic flux toward glycolysis. Hence, mtDNA mutations within iPSCs may not necessarily impair the correct establishment of pluripotency and the associated metabolic reprogramming. Nonetheless, the occurrence of pathogenic mtDNA modifications might be an important aspect to monitor when characterizing iPSC lines. Finally, we speculate that this random rearrangement of mtDNA molecules might prove beneficial for the derivation of mutation-free iPSCs from patients with mtDNA disorders.

The mitochondrial electron transport chain is dispensable for proliferation and differentiation of epidermal progenitor cells

Tissue stem cells and germ line or embryonic stem cells were shown to have reduced oxidative metabolism, which was proposed to be an adaptive mechanism to reduce damage accumulation caused by reactive oxygen species. However, an alternate explanation is that stem cells are less dependent on specialized cytoplasmic functions compared with differentiated cells, therefore, having a high nuclear-to-cytoplasmic volume ratio and consequently a low mitochondrial content. To determine whether stem cells rely or not on mitochondrial respiration, we selectively ablated the electron transport chain in the basal layer of the epidermis, which includes the epidermal progenitor/stem cells (EPSCs). This was achieved using a loxP-flanked mitochondrial transcription factor A (Tfam) allele in conjunction with a keratin 14 Cre transgene. The epidermis of these animals (Tfam(EKO)) showed a profound depletion of mitochondrial DNA and complete absence of respiratory chain complexes. However, despite a short lifespan due to malnutrition, epidermal development and skin barrier function were not impaired. Differentiation of epidermal layers was normal and no proliferation defect or major increase of apoptosis could be observed. In contrast, mice with an epidermal ablation of prohibitin-2, a scaffold protein in the inner mitochondrial membrane, displayed a dramatic phenotype observable already in utero, with severely impaired skin architecture and barrier function, ultimately causing death from dehydration shortly after birth. In conclusion, we here provide unequivocal evidence that EPSCs, and probably tissue stem cells in general, are independent of the mitochondrial respiratory chain, but still require a functional dynamic mitochondrial compartment.

Differentiation of SH-SY5Y cells to a neuronal phenotype changes cellular bioenergetics and the response to oxidative stress

Cell differentiation is associated with changes in metabolism and function. Understanding these changes during differentiation is important in the context of stem cell research, cancer, and neurodegenerative diseases. An early event in neurodegenerative diseases is the alteration of mitochondrial function and increased oxidative stress. Studies using both undifferentiated and differentiated SH-SY5Y neuroblastoma cells have shown distinct responses to cellular stressors; however, the mechanisms remain unclear. We hypothesized that because the regulation of glycolysis and oxidative phosphorylation is modulated during cellular differentiation, this would change bioenergetic function and the response to oxidative stress. To test this, we used retinoic acid (RA) to induce differentiation of SH-SY5Y cells and assessed changes in cellular bioenergetics using extracellular flux analysis. After exposure to RA, the SH-SY5Y cells had an increased mitochondrial membrane potential, without changing mitochondrial number. Differentiated cells exhibited greater stimulation of mitochondrial respiration with uncoupling and an increased bioenergetic reserve capacity. The increased reserve capacity in the differentiated cells was suppressed by the inhibitor of glycolysis 2-deoxy-d-glucose. Furthermore, we found that differentiated cells were substantially more resistant to cytotoxicity and mitochondrial dysfunction induced by the reactive lipid species 4-hydroxynonenal or the reactive oxygen species generator 2,3-dimethoxy-1,4-naphthoquinone. We then analyzed the levels of selected mitochondrial proteins and found an increase in complex IV subunits, which we propose contributes to the increase in reserve capacity in the differentiated cells. Furthermore, we found an increase in MnSOD that could, at least in part, account for the increased resistance to oxidative stress. Our findings suggest that profound changes in mitochondrial metabolism and antioxidant defenses occur upon differentiation of neuroblastoma cells to a neuron-like phenotype.

Mitochondria determine the differentiation potential of cardiac mesoangioblasts

An understanding of cardiac progenitor cell biology would facilitate their therapeutic potential for cardiomyocyte restoration and functional heart repair. Our previous studies identified cardiac mesoangioblasts as precommitted progenitor cells from the postnatal heart, which can be expanded in vitro and efficiently differentiated in vitro and in vivo to contribute new myocardium after injury.Based on their proliferation potential in culture, we show here that two populations of mesoangioblasts can be isolated from explant cultures of mouse and human heart.Although both populations express similar surface markers, together with a panel of instructive cardiac transcription factors, they differ significantly in their cellular content of mitochondria. Slow dividing (SD) cells, containing many mitochondria, can be efficiently differentiated with 5-azacytidine (5-aza) to generate cardiomyocytes expressing mature structural markers. In contrast, fast dividing (FD) mesoangioblasts, which contain decreased quantities of mitochondria, do not respond to 5-aza treatment.Notably, increasing mitochondrial numbers using pharmacological nitric oxide (NO) donors reverses the differentiation block in FD mesoangioblasts and leads to a progressive maturation to cardiomyocytes; conversely decreasing mitochondrial content, using respiratory chain inhibitors and chloramphenicol, perturbs cardiomyocyte differentiation in SD populations. Furthermore, isolated cardiac mesoangioblasts from aged mouse and human hearts are found to be almost exclusively mitochondria low FD populations, which are permissive for cardiomyocyte differentiation only after NO treatment. Taken together,this study illustrates a key role for mitochondria in cardiac mesoangioblast differentiation and raises the interesting possibility that treatments, which increase cellular mitochondrial content, may have utility for cardiac stem cell therapy.

Assessing mitochondrial dysfunction in cells

Assessing mitochondrial dysfunction requires definition of the dysfunction to be investigated. Usually, it is the ability of the mitochondria to make ATP appropriately in response to energy demands. Where other functions are of interest, tailored solutions are required. Dysfunction can be assessed in isolated mitochondria, in cells or in vivo, with different balances between precise experimental control and physiological relevance. There are many methods to measure mitochondrial function and dysfunction in these systems. Generally, measurements of fluxes give more information about the ability to make ATP than do measurements of intermediates and potentials. For isolated mitochondria, the best assay is mitochondrial respiratory control: the increase in respiration rate in response to ADP. For intact cells, the best assay is the equivalent measurement of cell respiratory control, which reports the rate of ATP production, the proton leak rate, the coupling efficiency, the maximum respiratory rate, the respiratory control ratio and the spare respiratory capacity. Measurements of membrane potential provide useful additional information. Measurement of both respiration and potential during appropriate titrations enables the identification of the primary sites of effectors and the distribution of control, allowing deeper quantitative analyses. Many other measurements in current use can be more problematic, as discussed in the present review.

Ketones and lactate increase cancer cell "stemness," driving recurrence, metastasis and poor clinical outcome in breast cancer: achieving personalized medicine via Metabolo-Genomics

Previously, we showed that high-energy metabolites (lactate and ketones) "fuel" tumor growth and experimental metastasis in an in vivo xenograft model, most likely by driving oxidative mitochondrial metabolism in breast cancer cells. To mechanistically understand how these metabolites affect tumor cell behavior, here we used genome-wide transcriptional profiling. Briefly, human breast cancer cells (MCF7) were cultured with lactate or ketones, and then subjected to transcriptional analysis (exon-array). Interestingly, our results show that treatment with these high-energy metabolites increases the transcriptional expression of gene profiles normally associated with "stemness," including genes upregulated in embryonic stem (ES) cells. Similarly, we observe that lactate and ketones promote the growth of bonafide ES cells, providing functional validation. The lactate- and ketone-induced "gene signatures" were able to predict poor clinical outcome (including recurrence and metastasis) in a cohort of human breast cancer patients. Taken together, our results are consistent with the idea that lactate and ketone utilization in cancer cells promotes the "cancer stem cell" phenotype, resulting in significant decreases in patient survival. One possible mechanism by which these high-energy metabolites might induce stemness is by increasing the pool of Acetyl-CoA, leading to increased histone acetylation, and elevated gene expression. Thus, our results mechanistically imply that clinical outcome in breast cancer could simply be determined by epigenetics and energy metabolism, rather than by the accumulation of specific "classical" gene mutations. We also suggest that high-risk cancer patients (identified by the lactate/ketone gene signatures) could be treated with new therapeutics that target oxidative mitochondrial metabolism, such as the anti-oxidant and "mitochondrial poison" metformin. Finally, we propose that this new approach to personalized cancer medicine be termed "Metabolo-Genomics," which incorporates features of both 1) cell metabolism and 2) gene transcriptional profiling. Importantly, this powerful new approach directly links cancer cell metabolism with clinical outcome, and new therapeutic strategies for inhibiting the TCA cycle and mitochondrial oxidative phosphorylation in cancer cells.

The adaptation of human embryonic stem cells to different feeder-free culture conditions is accompanied by a mitochondrial response

The mitochondrial contribution to the maintenance of human embryonic stem cell (hESC) pluripotency and culture homeostasis remains poorly understood. Here, we sought to determine whether hESC adaptation to different feeder-free culture conditions is linked to a mitochondrial adaptation. The expression of ESC pluripotency factors and parameters of mitochondrial contribution including mitochondrial membrane potential, mtDNA content, and the expression of master mitochondrial genes implicated in replication, transcription, and biogenesis were determined in 8 hESC lines maintained in 2 distinct human feeders-conditioned media (CM): human foreskin fibroblast-CM (HFF-CM) and mesenchymal stem cell-CM (MSC-CM). We show a robust parallel trend between the expression of ESC pluripotency factors and the mitochondrial contribution depending on the culture conditions employed to maintain the hESCs, with those in MSC-CM consistently displaying increased levels of pluripotency markers associated to an enhanced mitochondrial contribution. The differences in the mitochondrial status between hESCs maintained in MSC-CM versus HFF-CM respond to coordinated changes in mitochondrial gene expression and biogenesis. Importantly, the culture conditions determine the mitochondrial distribution within the stage-specific embryonic antigen 3 positive (SSEA3(+)) and negative (SSEA3(-)) isolated cell subsets. hESC colonies in MSC-CM display an "intrinsic" high mitochondrial status which may suffice to support undifferentiated growth, whereas hESC colonies maintained in HFF-CM show low mitochondrial status, possibly relying on the production of autologous niche with higher mitochondrial status to support pluripotency and culture homeostasis. Pluripotency markers and mitochondrial status are concomitantly reverted on changing the culture conditions, supporting an unrecognized role of the mitochondria in response to hESC culture adaptation. We provide the first evidence supporting that hESCs adaptation to different feeder-free culture systems relies on a mitochondrial response.