Metabolic regulation of stem cell function in tissue homeostasis and organismal ageing

An ERK-Dependent Feedback Mechanism Prevents Hematopoietic Stem Cell Exhaustion

Hematopoietic stem cells (HSCs) sustain hematopoiesis throughout life. HSCs exit dormancy to restore hemostasis in response to stressful events, such as acute blood loss, and must return to a quiescent state to prevent their exhaustion and resulting bone marrow failure. HSC activation is driven in part through the phosphatidylinositol 3-kinase (PI3K)/AKT/mTORC1 signaling pathway, but less is known about the cell-intrinsic pathways that control HSC dormancy. Here, we delineate an ERK-dependent, rate-limiting feedback mechanism that controls HSC fitness and their re-entry into quiescence. We show that the MEK/ERK and PI3K pathways are synchronously activated in HSCs during emergency hematopoiesis and that feedback phosphorylation of MEK1 by activated ERK counterbalances AKT/mTORC1 activation. Genetic or chemical ablation of this feedback loop tilts the balance between HSC dormancy and activation, increasing differentiated cell output and accelerating HSC exhaustion. These results suggest that MEK inhibitors developed for cancer therapy may find additional utility in controlling HSC activation.

RegenDbase: a comparative database of noncoding RNA regulation of tissue regeneration circuits across multiple taxa

Regeneration is an endogenous process of tissue repair that culminates in complete restoration of tissue and organ function. While regenerative capacity in mammals is limited to select tissues, lower vertebrates like zebrafish and salamanders are endowed with the capacity to regenerate entire limbs and most adult tissues, including heart muscle. Numerous profiling studies have been conducted using these research models in an effort to identify the genetic circuits that accompany tissue regeneration. Most of these studies, however, are confined to an individual injury model and/or research organism and focused primarily on protein encoding transcripts. Here we describe RegenDbase, a new database with the functionality to compare and contrast gene regulatory pathways within and across tissues and research models. RegenDbase combines pipelines that integrate analysis of noncoding RNAs in combination with protein encoding transcripts. We created RegenDbase with a newly generated comprehensive dataset for adult zebrafish heart regeneration combined with existing microarray and RNA-sequencing studies on multiple injured tissues. In this current release, we detail microRNA-mRNA regulatory circuits and the biological processes these interactions control during the early stages of heart regeneration. Moreover, we identify known and putative novel lncRNAs and identify their potential target genes based on proximity searches. We postulate that these candidate factors underscore robust regenerative capacity in lower vertebrates. RegenDbase provides a systems-level analysis of tissue regeneration genetic circuits across injury and animal models and addresses the growing need to understand how noncoding RNAs influence these changes in gene expression.

Hematopoietic stem cell fate through metabolic control

Hematopoietic stem cells maintain a quiescent state in the bone marrow to preserve their self-renewal capacity, but also undergo cell divisions as required. Organelles such as the mitochondria sustain cumulative damage during these cell divisions and this damage may eventually compromise the cells' self-renewal capacity. Hematopoietic stem cell divisions result in either self-renewal or differentiation, with the balance between the two affecting hematopoietic homeostasis directly; however, the heterogeneity of available hematopoietic stem cell-enriched fractions, together with the technical challenges of observing hematopoietic stem cell behavior, has long hindered the analysis of individual hematopoietic stem cells and prevented the elucidation of this process. Recent advances in genetic models, metabolomics analyses, and single-cell approaches have revealed the contributions made to hematopoietic stem cell self-renewal by metabolic cues, mitochondrial biogenesis, and autophagy/mitophagy, which have highlighted mitochondrial quality control as a key factor in the equilibrium of hematopoietic stem cells. A deeper understanding of precisely how specific modes of metabolism control hematopoietic stem cells fate at the single-cell level is therefore not only of great biological interest, but will also have clear clinical implications for the development of therapies for hematological diseases.

The role of mitochondria in stem cell fate and aging

The importance of mitochondria in energy metabolism, signal transduction and aging in post-mitotic tissues has been well established. Recently, the crucial role of mitochondrial-linked signaling in stem cell function has come to light and the importance of mitochondria in mediating stem cell activity is becoming increasingly recognized. Despite the fact that many stem cells exhibit low mitochondrial content and a reliance on mitochondrial-independent glycolytic metabolism for energy, accumulating evidence has implicated the importance of mitochondrial function in stem cell activation, fate decisions and defense against senescence. In this Review, we discuss the recent advances that link mitochondrial metabolism, homeostasis, stress responses, and dynamics to stem cell function, particularly in the context of disease and aging. This Review will also highlight some recent progress in mitochondrial therapeutics that may present attractive strategies for improving stem cell function as a basis for regenerative medicine and healthy aging.

Mitochondrial and Reactive Oxygen Species Signaling Coordinate Stem Cell Fate Decisions and Life Long Maintenance

Significance: Recent discoveries in mitochondrial biology have transformed and further solidified the importance of mitochondria in development, aging, and disease. Within the realm of regenerative and stem cell research, these recent advances have brought forth new concepts that revolutionize our understanding of metabolic and redox states in the establishment of cellular identity and fate decisions. Recent Advances: Mitochondrial metabolism, morphology, and cellular redox states are dynamic characteristics that undergo shifts during stem cell differentiation. Although it was once thought that this was solely because of changing metabolic needs of differentiating cells, it is now clear that these events are driving forces in the regulation of stem cell identity and fate decisions. Critical Issues: Although recent discoveries have placed mitochondrial function and physiological reactive oxygen species (ROS) at the forefront for the regulation of stem cell self-renewal, how this may impact tissue homeostasis and regenerative capacity is poorly understood. In addition, the role of mitochondria and ROS on the maintenance of a stem cell population in many degenerative diseases and during aging is not clear, despite the fact that mitochondrial dysfunction and elevated ROS levels are commonly observed in these conditions. Future Directions: Given the newly established role for mitochondria and ROS in stem cell self-renewal capacity, special attention should now be directed in understanding how this would impact the development and progression of aging and diseases, whereby mitochondrial and ROS defects are a prominent factor. Antioxid. Redox Signal. 28, 1090-1101.

Proteostatic and Metabolic Control of Stemness

Adult stem cells, particularly those resident in tissues with little turnover, are largely quiescent and only activate in response to regenerative demands, while embryonic stem cells continuously replicate, suggesting profoundly different regulatory mechanisms within distinct stem cell types. In recent years, evidence linking metabolism, mitochondrial dynamics, and protein homeostasis (proteostasis) as fundamental regulators of stem cell function has emerged. Here, we discuss new insights into how these networks control potency, self-renewal, differentiation, and aging of highly proliferative embryonic stem cells and quiescent adult stem cells, with a focus on hematopoietic and muscle stem cells and implications for anti-aging research.

Cellular Metabolism and Aging

Metabolic changes are hallmarks of aging and genetic and pharmacologic alterations of relevant pathways can extend life span. In this review, we will outline how cellular biochemistry and energy homeostasis change during aging. We will highlight protein quality control, mitochondria, epigenetics, nutrient-sensing pathways, as well as the interplay between these systems with respect to their impact on cellular health.

Lin28a enhances in vitro osteoblastic differentiation of human periosteum-derived cells

Despite a capacity for proliferation and an ability to differentiate into multiple cell types, in long-term culture and with ageing, stem cells show a reduction in growth, display a decrease in differentiation potential, and enter senescence without evidence of transformation. The Lin28a gene encodes an RNA-binding protein that plays a role in regulating stem cell activity, including self-renewal and differentiation propensity. However, the effect of the Lin28a gene on cultured human osteoprecursor cells is poorly understood. In the present study, alkaline phosphatase activity, alizarin red-positive mineralization, and calcium content, positive indicators of osteogenic differentiation, were significantly higher in cultured human periosteum-derived cells (hPDCs) with Lin28a overexpression compared with cells without Lin28a overexpression. Lin28a overexpression by hPDCs also increased mitochondrial activity, which is essential for cellular proliferation, as suggested by a reduced presence of reactive oxygen species and significantly enhanced lactate levels and ATP production. Our results suggest that, in hPDCs, the Lin28a gene enhances osteoblastic differentiation and increases mitochondrial activity. Although Lin28a is known as a marker of undifferentiated human embryogenic stem cell, there is limited evidence regarding the influence of Lin28a on osteoblastic differentiation of cultured osteoprecursor cells. This study was to examine the impact of Lin28a on osteogenic phenotypes of human periosteum-derived cells. Their phenotypes can be similar to those of mesenchymal stem cells. Our results suggest that the Lin28a gene enhances the osteoblastic differentiation of human periosteum-derived cells. In addition, the Lin28a gene increases mitochondrial activity in human periosteum-derived cells.

The impact of cellular metabolism on chromatin dynamics and epigenetics

The substrates used to modify nucleic acids and chromatin are affected by nutrient availability and the activity of metabolic pathways. Thus, cellular metabolism constitutes a fundamental component of chromatin status and thereby of genome regulation. Here we describe the biochemical and genetic principles of how metabolism can influence chromatin biology and epigenetics, discuss the functional roles of this interplay in developmental and cancer biology, and present future directions in this rapidly emerging area.

Hematopoietic stem cells under pressure

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) and progenitors are tasked with maintaining hematopoietic homeostasis in the face of numerous insults and challenges, including infection, inflammation, and exsanguination. HSCs possess the remarkable ability to reconstitute the entire hematopoietic system of an organism whose own hematopoietic system has been ablated. This ability is exploited routinely in the clinic via HSC transplantation (HSCT). Here, we focus on the physiological and molecular bottlenecks overcome by HSCs during transplantation.

RECENT FINDINGS

During transplantation, HSCs encounter a damaged bone marrow niche, characterized molecularly by increases in oxygen concentrations and an altered cytokine milieu. New mechanisms and pathways have been recently implicated during HSCT, including transplanted HSC-dependent secretion of conditioning molecules that facilitate engraftment and pathways that protect HSCs from perturbed organelle homeostasis.

SUMMARY

Better understanding the molecular processes HSCs employ to withstand the stress of transplant will illuminate novel targets for further improving conditioning regimens and engraftment during HSCT.

Regulation of Stem Cell Aging by Metabolism and Epigenetics

Stem cell aging and exhaustion are considered important drivers of organismal aging. Age-associated declines in stem cell function are characterized by metabolic and epigenetic changes. Understanding the mechanisms underlying these changes will likely reveal novel therapeutic targets for ameliorating age-associated phenotypes and for prolonging human healthspan. Recent studies have shown that metabolism plays an important role in regulating epigenetic modifications and that this regulation dramatically affects the aging process. This review focuses on current knowledge regarding the mechanisms of stem cell aging, and the links between cellular metabolism and epigenetic regulation. In addition, we discuss how these interactions sense and respond to environmental stress in order to maintain stem cell homeostasis, and how environmental stimuli regulate stem cell function. Additionally, we highlight recent advances in the development of therapeutic strategies to rejuvenate dysfunctional aged stem cells.

Mitochondrial transition ROS spike (mTRS) results from coordinated activities of complex I and nicotinamide nucleotide transhydrogenase

Mitochondria exhibit suppressed ATP production, membrane potential (∆Ψ_mt) polarization and reactive oxygen species (ROS) bursts during some cellular metabolic transitions. Although mitochondrial ROS release is influenced by ∆Ψ_mt and respiratory state, the relationship between these properties remains controversial primarily because they have not been measured simultaneously. We developed a multiparametric method for probing mitochondrial function that allowed precise characterization of the temporal relationship between ROS, ∆Ψ_mt and respiration. We uncovered a previously unknown spontaneous ROS spike - termed mitochondrial transition ROS spike (mTRS) - associated with re-polarization of ∆Ψ_mt that occurs at the transition between mitochondrial energy states. Pharmacological inhibition of complex CI (CI), nicotinamide nucleotide transhydrogenase (NNT) and antioxidant system significantly decreased the ability of mitochondria to exhibit mTRS. NADH levels followed a similar trend to that of ROS during the mTRS, providing a link between CI and NNT in mTRS regulation. We show that (i) mTRS is enhanced by simultaneous activation of CI and complex II (CII); (ii) CI is the principal origin of mTRS; (iii) NNT regulates mTRS via NADH- and ∆Ψ_mt-dependent mechanisms; (iv) mTRS is not a pH spike; and (v), mTRS changes in amplitude under stress conditions and its occurrence can be a signature of mitochondrial health. Collectively, we uncovered and characterized the biophysical properties and mechanisms of mTRS, and propose it as a potential diagnostic tool for CI-related dysfunctions, and as a biomarker of mitochondrial functional integrity.

A fruitful liaison of ZSCAN10 and ROS on the road to rejuvenation

Induced pluripotent stem cells derived from aged donors (A-iPSCs) usually show genomic instability that affects their utility and raises concerns about their safety. Now, a study highlights the importance of ZSCAN10-dependent recovery of glutathione-ROS homeostasis in counteracting the genomic defects in A-iPSCs.

Metabolic Plasticity of Stem Cells and Macrophages in Cancer

In addition to providing essential molecules for the overall function of cells, metabolism plays an important role in cell fate and can be affected by microenvironmental stimuli as well as cellular interactions. As a specific niche, tumor microenvironment (TME), consisting of different cell types including stromal/stem cells and immune cells, is characterized by distinct metabolic properties. This review will be focused on the metabolic plasticity of mesenchymal stromal/stem cells (MSC) and macrophages in TME, as well as on how the metabolic state of cancer stem cells (CSC), as key drivers of oncogenesis, affects their generation and persistence. Namely, heterogenic metabolic phenotypes of these cell populations, which include various levels of dependence on glycolysis or oxidative phosphorylation are closely linked to their complex roles in cancer progression. Besides well-known extrinsic factors, such as cytokines and growth factors, the differentiation and activation states of CSC, MSC, and macrophages are coordinated by metabolic reprogramming in TME. The significance of mutual metabolic interaction between tumor stroma and cancer cells in the immune evasion and persistence of CSC is currently under investigation.

Distinct requirements for energy metabolism in mouse primordial germ cells and their reprogramming to embryonic germ cells

Primordial germ cells (PGCs), undifferentiated embryonic germ cells, are the only cells that have the ability to become gametes and to reacquire totipotency upon fertilization. It is generally understood that the development of PGCs proceeds through the expression of germ cell-specific transcription factors and characteristic epigenomic changes. However, little is known about the properties of PGCs at the metabolite and protein levels, which are directly responsible for the control of cell function. Here, we report the distinct energy metabolism of PGCs compared with that of embryonic stem cells. Specifically, we observed remarkably enhanced oxidative phosphorylation (OXPHOS) and decreased glycolysis in embryonic day 13.5 (E13.5) PGCs, a pattern that was gradually established during PGC differentiation. We also demonstrate that glycolysis and OXPHOS are important for the control of PGC reprogramming and specification of pluripotent stem cells (PSCs) into PGCs in culture. Our findings about the unique metabolic property of PGCs provide insights into our understanding of the importance of distinct facets of energy metabolism for switching PGC and PSC status.

Essential role of FBXL5-mediated cellular iron homeostasis in maintenance of hematopoietic stem cells

Hematopoietic stem cells (HSCs) are maintained in a hypoxic niche to limit oxidative stress. Although iron elicits oxidative stress, the importance of iron homeostasis in HSCs has been unknown. Here we show that iron regulation by the F-box protein FBXL5 is required for HSC self-renewal. Conditional deletion of Fbxl5 in mouse HSCs results in cellular iron overload and a reduced cell number. Bone marrow transplantation reveals that FBXL5-deficient HSCs are unable to reconstitute the hematopoietic system of irradiated recipients as a result of stem cell exhaustion. Transcriptomic analysis shows abnormal activation of oxidative stress responses and the cell cycle in FBXL5-deficient mouse HSCs as well as downregulation of FBXL5 expression in HSCs of patients with myelodysplastic syndrome. Suppression of iron regulatory protein 2 (IRP2) accumulation in FBXL5-deficient mouse HSCs restores stem cell function, implicating IRP2 as a potential therapeutic target for human hematopoietic diseases associated with FBXL5 downregulation.

Oxidative stress management in the hair follicle: Could targeting NRF2 counter age-related hair disorders and beyond?

Widespread expression of the transcription factor, nuclear factor (erythroid-derived 2)-like 2 (NRF2), which maintains redox homeostasis, has recently been identified in the hair follicle (HF). Small molecule activators of NRF2 may therefore be useful in the management of HF pathologies associated with redox imbalance, ranging from HF greying and HF ageing via androgenetic alopecia and alopecia areata to chemotherapy-induced hair loss. Indeed, NRF2 activation has been shown to prevent peroxide-induced hair growth inhibition. Multiple parameters can increase the levels of reactive oxygen species in the HF, for example melanogenesis, depilation-induced trauma, neurogenic and autoimmune inflammation, toxic drugs, environmental stressors such as UV irradiation, genetic defects and aging-associated mitochondrial dysfunction. In this review, the potential mechanisms whereby NRF2 activation could prove beneficial in treatment of redox-associated HF disorders are therefore discussed.

Understanding the mTOR signaling pathway via mathematical modeling

The mechanistic target of rapamycin (mTOR) is a central regulatory pathway that integrates a variety of environmental cues to control cellular growth and homeostasis by intricate molecular feedbacks. In spite of extensive knowledge about its components, the molecular understanding of how these function together in space and time remains poor and there is a need for Systems Biology approaches to perform systematic analyses. In this work, we review the recent progress how the combined efforts of mathematical models and quantitative experiments shed new light on our understanding of the mTOR signaling pathway. In particular, we discuss the modeling concepts applied in mTOR signaling, the role of multiple feedbacks and the crosstalk mechanisms of mTOR with other signaling pathways. We also discuss the contribution of principles from information and network theory that have been successfully applied in dissecting design principles of the mTOR signaling network. We finally propose to classify the mTOR models in terms of the time scale and network complexity, and outline the importance of the classification toward the development of highly comprehensive and predictive models. WIREs Syst Biol Med 2017, 9:e1379. doi: 10.1002/wsbm.1379 For further resources related to this article, please visit the WIREs website.

Metabolic regulation of hematopoietic and leukemic stem/progenitor cells under homeostatic and stress conditions

Hematopoietic stem cells (HSCs) exhibit multilineage differentiation and self-renewal activities that maintain the entire hematopoietic system during an organism's lifetime. These abilities are sustained by intrinsic transcriptional programs and extrinsic cues from the microenvironment or niche. Recent studies using metabolomics technologies reveal that metabolic regulation plays an essential role in HSC maintenance. Metabolic pathways provide energy and building blocks for other factors functioning at steady state and in stress. Here we review recent advances in our understanding of metabolic regulation in HSCs relevant to cell cycle quiescence, symmetric/asymmetric division, and proliferation following stress and lineage commitment, and discuss the therapeutic potential of targeting metabolic factors or pathways to treat hematological malignancies.

The mitochondrial respiratory chain is essential for haematopoietic stem cell function

Adult and fetal hematopoietic stem cells (HSCs) display a glycolytic phenotype, which is required for maintenance of stemness; however, whether mitochondrial respiration is required to maintain HSC function is not known. Here we report that loss of the mitochondrial complex III subunit Rieske iron sulfur protein (RISP) in fetal mouse HSCs allows them to proliferate but impairs their differentiation, resulting in anemia and prenatal death. RISP null fetal HSCs displayed impaired respiration resulting in a decreased NAD+/NADH ratio. RISP null fetal HSCs and progenitors exhibited an increase in both DNA and histone methylation associated with increases in 2-hydroxyglutarate (2-HG), a metabolite known to inhibit DNA and histone demethylases. RISP inactivation in adult HSCs also impaired respiration resulting in loss of quiescence concomitant with severe pancytopenia and lethality. Thus, respiration is dispensable for adult or fetal HSC proliferation, but essential for fetal HSC differentiation and maintenance of adult HSC quiescence.

Programmed mitophagy is essential for the glycolytic switch during cell differentiation

Retinal ganglion cells (RGCs) are the sole projecting neurons of the retina and their axons form the optic nerve. Here, we show that embryogenesis-associated mouse RGC differentiation depends on mitophagy, the programmed autophagic clearance of mitochondria. The elimination of mitochondria during RGC differentiation was coupled to a metabolic shift with increased lactate production and elevated expression of glycolytic enzymes at the mRNA level. Pharmacological and genetic inhibition of either mitophagy or glycolysis consistently inhibited RGC differentiation. Local hypoxia triggered expression of the mitophagy regulator BCL2/adenovirus E1B 19-kDa-interacting protein 3-like (BNIP3L, best known as NIX) at peak RGC differentiation. Retinas from NIX-deficient mice displayed increased mitochondrial mass, reduced expression of glycolytic enzymes and decreased neuronal differentiation. Similarly, we provide evidence that NIX-dependent mitophagy contributes to mitochondrial elimination during macrophage polarization towards the proinflammatory and more glycolytic M1 phenotype, but not to M2 macrophage differentiation, which primarily relies on oxidative phosphorylation. In summary, developmentally controlled mitophagy promotes a metabolic switch towards glycolysis, which in turn contributes to cellular differentiation in several distinct developmental contexts.

DNA-damage response in hematopoietic stem cells: an evolutionary trade-off between blood regeneration and leukemia suppression

Self-renewing and multipotent hematopoietic stem cells (HSCs) maintain lifelong hematopoiesis. Their enormous regenerative potential coupled with lifetime persistence in the body, in contrast with the Progenitors, demand tight control of HSCs genome stability. Indeed, failure to accurately repair DNA damage in HSCs is associated with bone marrow failure and accelerated leukemogenesis. Recent observations exposed remarkable differences in several DNA-damage response (DDR) aspects between HSCs and Progenitors, especially in their DNA-repair capacities and susceptibility to apoptosis. Human HSCs in comparison with Progenitors exhibit delayed DNA double-strand break rejoining, persistent DDR signaling activation, higher sensitivity to the cytotoxic effects of ionizing radiation and attenuated expression of DNA-repair genes. Importantly, the distinct DDR of HSCs was also documented in mouse models. Nevertheless, physiological significance and the molecular basis of the HSCs-specific DDR features are only partially understood. Taking radiation-induced DDR as a paradigm, this review will focus on the current advances in understanding the role of cell-intrinsic DDR regulators and the cellular microenvironment in balancing stemness with genome stability. Pre-leukemia HSCs and clonal hematopoiesis evolvement will be discussed as an evolutionary compromise between the need for lifelong blood regeneration and DDR. Uniquely for this review, we outline the differences in HSCs-related DDR as highlighted by various experimental systems and attempt to provide their critical analysis.

Beyond growth signaling: Paneth cells metabolically support ISCs

Single Lgr5 intestinal stem cells (ISCs) can be expanded in vitro into epithelial organoids or "mini-guts", self-organizing cellular structures that recreate the intestinal differentiation program; Paneth cells, which constitute the intestinal stem cell niche, secrete stem cell growth signals, and are thus essential for this process. In a recent paper published in Nature, Rodríguez-Colman et al. describe how Paneth cells may be supporting the metabolic state of ISCs.

Adipocyte Metabolic Pathways Regulated by Diet Control the Female Germline Stem Cell Lineage in Drosophila melanogaster

Nutrients affect adult stem cells through complex mechanisms involving multiple organs. Adipocytes are highly sensitive to diet and have key metabolic roles, and obesity increases the risk for many cancers. How diet-regulated adipocyte metabolic pathways influence normal stem cell lineages, however, remains unclear. Drosophila melanogaster has highly conserved adipocyte metabolism and a well-characterized female germline stem cell (GSC) lineage response to diet. Here, we conducted an isobaric tags for relative and absolute quantification (iTRAQ) proteomic analysis to identify diet-regulated adipocyte metabolic pathways that control the female GSC lineage. On a rich (relative to poor) diet, adipocyte Hexokinase-C and metabolic enzymes involved in pyruvate/acetyl-CoA production are upregulated, promoting a shift of glucose metabolism toward macromolecule biosynthesis. Adipocyte-specific knockdown shows that these enzymes support early GSC progeny survival. Further, enzymes catalyzing fatty acid oxidation and phosphatidylethanolamine synthesis in adipocytes promote GSC maintenance, whereas lipid and iron transport from adipocytes controls vitellogenesis and GSC number, respectively. These results show a functional relationship between specific metabolic pathways in adipocytes and distinct processes in the GSC lineage, suggesting the adipocyte metabolism-stem cell link as an important area of investigation in other stem cell systems.

Mechanistic Investigations of the Mitochondrial Complex I Inhibitor Rotenone in the Context of Pharmacological and Safety Evaluation

Inhibitors of the mitochondrial respiratory chain complex I are suggested to exert anti-tumor activity on those tumors relying on oxidative metabolism and are therefore of interest to oncology research. Nevertheless, the safety profile of these inhibitors should be thoroughly assessed. Rotenone, a proven complex I inhibitor, has shown anti-carcinogenic activity in several studies. In this context rotenone was used in this study as a tool compound with the aim to identify suitable biomarker candidates and provide enhanced mechanistic insights into the molecular and cellular effects of complex I inhibitors. Rats were treated with 400 ppm rotenone daily for 1, 3 or 14 consecutive days followed by necropsy. Classical clinical endpoints, including hematology, clinical chemistry and histopathology with supporting investigations (FACS-analysis, enzymatic activity assays) were examined as well as gene expression analysis. Through these investigations, we identified liver, bone marrow and bone as target organs amongst approx. 40 organs evaluated at least histopathologically. Our results suggest blood analysis, bone marrow parameters, assessment of lactate in serum and glycogen in liver, and especially gene expression analysis in liver as useful parameters for an experimental model to help to characterize the profile of complex I inhibitors with respect to a tolerable risk-benefit balance.

Interaction between epigenetic and metabolism in aging stem cells

Aging is accompanied by a decline in tissue function, regeneration, and repair. A large part of this decline is caused by the deterioration of tissue stem cell function. Understanding the mechanisms that drive stem cell aging and how to counteract them is a critical step for enhancing tissue repair and maintenance during aging. Emerging evidence indicates that epigenetic modifiers and metabolism regulators interact to impact lifespan, suggesting that this mechanism may also affect stem cell function with age. This review focuses on the interaction between chromatin and metabolism in the regulation of tissue stem cells during aging. We also discuss how these mechanisms integrate environmental stimuli such as nutrient stress to regulate stem cell function. Finally, this review examines new perspectives for regeneration, rejuvenation, and treatment of age-related decline of stem cell function.

Autophagy maintains the metabolism and function of young and old stem cells

With age, hematopoietic stem cells (HSCs) lose their ability to regenerate the blood system, and promote disease development. Autophagy is associated with health and longevity, and is critical for protecting HSCs from metabolic stress. Here, we show that loss of autophagy in HSCs causes accumulation of mitochondria and an activated metabolic state, which drives accelerated myeloid differentiation mainly through epigenetic deregulations, and impairs HSC self-renewal activity and regenerative potential. Strikingly, the majority of HSCs in aged mice share these altered metabolic and functional features. However, ~ 1/3 of aged HSCs exhibit high autophagy levels and maintain a low metabolic state with robust long-term regeneration potential similar to healthy young HSCs. Our results demonstrate that autophagy actively suppresses HSC metabolism by clearing active, healthy mitochondria to maintain quiescence and stemness, and becomes increasingly necessary with age to preserve the regenerative capacity of old HSCs.

The microenvironment in human myeloid malignancies: emerging concepts and therapeutic implications

Similar to their healthy counterpart, malignant hematopoietic stem cells in myeloid malignancies, such as myeloproliferative neoplasms, myelodysplastic syndromes, and acute myeloid leukemia, reside in a highly complex and dynamic cellular microenvironment in the bone marrow. This environment provides key regulatory signals for and tightly controls cardinal features of hematopoietic stem cells (HSCs), including self-renewal, quiescence, differentiation, and migration. These features are essential to maintaining cellular homeostasis and blood regeneration throughout life. A large number of studies have extensively addressed the composition of the bone marrow niche in mouse models, as well as the cellular and molecular communication modalities at play under both normal and pathogenic situations. Although instrumental to interrogating the complex composition of the HSC niche and dissecting the niche remodeling processes that appear to actively contribute to leukemogenesis, these models may not fully recapitulate the human system due to immunophenotypic, architectural, and functional inter-species variability. This review summarizes several aspects related to the human hematopoietic niche: (1) its anatomical structure, composition, and function in normal hematopoiesis; (2) its alteration and functional relevance in the context of chronic and acute myeloid malignancies; (3) age-related niche changes and their suspected impact on hematopoiesis; (4) ongoing efforts to develop new models to study niche-leukemic cell interaction in human myeloid malignancies; and finally, (5) how the knowledge gained into leukemic stem cell (LSC) niche dependencies might be exploited to devise novel therapeutic strategies that aim at disrupting essential niche-LSC interactions or improve the regenerative ability of the disease-associated hematopoietic niche.

Transcriptional Regulation of Stem Cell and Cancer Stem Cell Metabolism

PURPOSE OF REVIEW

Metabolism is increasingly recognized as a major player in control of stem cell function and fate. How stem cell metabolism is established, maintained, and regulated is a fundamental question of biology and medicine. In this review, we discuss major metabolic programs in stem cells and cancer stem cells, with a focus on key transcription factors that shape the stem cell metabolic phenotype.

RECENT FINDINGS

Cancer stem cells primarily use oxidative phosphorylation for energy generation, in contrast to normal stem cells, which rely on glycolytic metabolism with the exception of mouse embryonic stem cells. Transcription factors control the metabolic phenotype of stem cells by modulating the expression of enzymes and thus the activity of metabolic pathways. It is evident that HIF1α and PGC1α function as master regulators of glycolytic and mitochondrial metabolism, respectively.

SUMMARY

Transcriptional regulation is a key mechanism for establishing specific metabolic programs in stem cells and cancer stem cells.

Redox status in a model of cancer stem cells

Reversible oxidation of Cys residues is a crucial element of redox homeostasis and signaling. According to a popular concept in oxidative stress signaling, the oxidation of targets of signals can only take place following an overwhelming of the cellular antioxidant capacity. This concept, however, ignores the activation of feedback mechanisms possibly leading to a paradoxical effect. In a model of cancer stem cells (CSC), stably overexpressing the TAZ oncogene, we observed that the increased formation of oxidants is associated with a globally more reduced state of proteins. Redox proteomics revealed that several proteins, capable of undergoing reversible redox transitions, are indeed more reduced while just few are more oxidized. Among the proteins more oxidized, G6PDH emerges as both more expressed and activated by oxidation. This accounts for the observed more reduced state of the NADPH/NADP⁺ couple. The dynamic redox flux generating this apparently paradoxical effect is rationalized in a computational system biology model highlighting the crucial role of G6PDH activity on the rate of redox transitions eventually leading to the reduction of reversible redox switches.