Deconstructing stem cell self-renewal: genetic insights into cell-cycle regulation

Stem Cell Quiescence: Dynamism, Restraint, and Cellular Idling

Stem cells can reside in a state of reversible growth arrest, or quiescence, for prolonged periods of time. Although quiescence has long been viewed as a dormant, low-activity state, increasing evidence suggests that quiescence represents states of poised potential and active restraint, as stem cells "idle" in anticipation of activation, proliferation, and differentiation. Improved understanding of quiescent stem cell dynamics is leading to novel approaches to enhance maintenance and repair of aged or diseased tissues. In this Review, we discuss recent advances in our understanding of stem cell quiescence and techniques enabling more refined analyses of quiescence in vivo.

Quiescence, an individual journey

Quiescence is operationally characterized as a temporary and reversible proliferation arrest. There are many preconceived ideas about quiescence, quiescent cells being generally viewed as insignificant sleeping G1 cells. In fact, quiescence is central for organism physiology and its dysregulation involved in many pathologies. The quiescent state encompasses very diverse cellular situations depending on the cell type and its environment. This diversity challenges not only quiescence uniformity but also the universality of the molecular mechanisms beyond quiescence regulation. In this mini-perspective, we discuss recent advances in the concept of quiescence, and illustrate that this multifaceted cellular state is gaining increasing attention in many fields of biology.

Toward Identifying Subnetworks from FBF Binding Landscapes in Caenorhabditis Spermatogenic or Oogenic Germlines

Metazoan PUF (Pumilio and FBF) RNA-binding proteins regulate various biological processes, but a common theme across phylogeny is stem cell regulation. In Caenorhabditis elegans, FBF (fem-3 Binding Factor) maintains germline stem cells regardless of which gamete is made, but FBF also functions in the process of spermatogenesis. We have begun to "disentangle" these biological roles by asking which FBF targets are gamete-independent, as expected for stem cells, and which are gamete-specific. Specifically, we compared FBF iCLIP binding profiles in adults making sperm to those making oocytes. Normally, XX adults make oocytes. To generate XX adults making sperm, we used a fem-3(gf) mutant requiring growth at 25°; for comparison, wild-type oogenic hermaphrodites were also raised at 25°. Our FBF iCLIP data revealed FBF binding sites in 1522 RNAs from oogenic adults and 1704 RNAs from spermatogenic adults. More than half of these FBF targets were independent of germline gender. We next clustered RNAs by FBF-RNA complex frequencies and found four distinct blocks. Block I RNAs were enriched in spermatogenic germlines, and included validated target fog-3, while Block II and III RNAs were common to both genders, and Block IV RNAs were enriched in oogenic germlines. Block II (510 RNAs) included almost all validated FBF targets and was enriched for cell cycle regulators. Block III (21 RNAs) was enriched for RNA-binding proteins, including previously validated FBF targets gld-1 and htp-1 We suggest that Block I RNAs belong to the FBF network for spermatogenesis, and that Blocks II and III are associated with stem cell functions.

Prx1 Expressing Cells Are Required for Periodontal Regeneration of the Mouse Incisor

Previous studies have shown that post-natal skeletal stem cells expressing Paired-related homeobox 1 (PRX1 or PRRX1) are present in the periosteum of long bones where they contribute to post-natal bone development and regeneration. Our group also identified post-natal PRX1 expressing cells (pnPRX1+ cells) in mouse calvarial synarthroses (sutures) and showed that these cells are required for calvarial bone regeneration. Since calvarial synarthroses are similar to dentoalveolar gomphosis (periodontium) and since there is no information available on the presence or function of pnPRX1+ cells in the periodontium, the present study aimed at identifying and characterizing pnPRX1+ cells within the mouse periodontium and assess their contribution to periodontal development and regeneration. Here we demonstrated that pnPRX1+ cells are present within the periodontal ligament (PDL) of the mouse molars and of the continuously regenerating mouse incisor. By means of diphtheria toxin (DTA)-mediated conditional ablation of pnPRX1+ cells, we show that pnPRX1+ cells contribute to post-natal periodontal development of the molars and the incisor, as ablation of pnPRX1+ cells in 3-days old mice resulted in a significant enlargement of the PDL space after 18 days. The contribution of pnPRX1+ cells to periodontal regeneration was assessed by developing a novel non-critical size periodontal defect model. Outcomes showed that DTA-mediated post-natal ablation of pnPRX1+ cells results in lack of regeneration in periodontal non-critical size defects in the regeneration competent mouse incisors. Importantly, gene expression analysis of these cells shows a profile typical of quiescent cells, while gene expression analysis of human samples of periodontal stem cells (PDLSC) confirmed that Prx1 is highly expressed in human periodontium. In conclusion, pnPRX1+ cells are present within the continuously regenerating PDL of the mouse incisor, and at such location they contribute to post-natal periodontal development and regeneration. Since this study further reports the presence of PRX1 expressing cells within human periodontal ligament, we suggest that studying the mouse periodontal pnPRX1+ cells may provide significant information for the development of novel and more effective periodontal regenerative therapies in humans.

APC/CCdh1 regulates the balance between maintenance and differentiation of hematopoietic stem and progenitor cells

Hematopoietic stem and progenitor cells (HSPCs) represent the lifelong source of all blood cells and continuously regenerate the hematopoietic system through differentiation and self-renewal. The process of differentiation is initiated in the G1 phase of the cell cycle, when stem cells leave their quiescent state. During G1, the anaphase-promoting complex or cyclosome associated with the coactivator Cdh1 is highly active and marks proteins for proteasomal degradation to regulate cell proliferation. Following Cdh1 knockdown in HSPCs, we analyzed human and mouse hematopoiesis in vitro and in vivo in competitive transplantation assays. We found that Cdh1 is highly expressed in human CD34+ HSPCs and downregulated in differentiated subsets; whereas, loss of Cdh1 restricts myeloid differentiation, supports B cell development and preserves immature short-term HSPCs without affecting proliferation or viability. Our data highlight a role of Cdh1 as a regulator of balancing the maintenance of HSPCs and differentiation into mature blood cells.

Pericytes in Bone Marrow

Bone marrow environments are composed of multiple cell types, most of which are thought to be derived from mesenchymal stem cells. In mouse bone marrow, stromal cells with CD45^- Tie2^- CD90^- CD51⁺ CD105⁺ phenotype, Nestin-GFP⁺, CXCL12-abundant reticular (CAR) cells, PDGFRα⁺ Sca-1⁺ or CD51⁺ PDGFRα⁺, and Prx-1-derived CD45^- Ter119^- PDGFRα⁺ Sca-1⁺ populations select for MSC activity. There is evidence that these stromal cell populations display some significant overlap with each other and comprise important cellular constituents of the hematopoietic stem cell niche. Moreover, these mesenchymal cell populations share characteristics in their location as they all are found around bone marrow vessels (can be called "pericytes"). In this chapter, with reviewing the recent literatures, how the pericytes relate to physiological and pathological hematopoiesis is argued.

Global secretome analysis of resident cardiac progenitor cells from wild-type and transgenic heart failure mice: Why ambience matters

Resident cardiac progenitor cells (CPCs) have gained attention in cardiac regenerative medicine primarily due to their paracrine activity. In our current study we determined the role of pathological conditions such as heart failure on the autocrine-paracrine action of stem cell antigen-1 (Sca-1) expressing CPC. This comparative secretome profiling of Sca-1⁺ cells derived from transgenic heart failure (αMHC-cyclin-T1/Gαq overexpression [Cyc] cells) versus healthy (wild-type [Wt] cells) mice, achieved via mass-spectrometric quantification, enabled the identification of over 700 proteins. Our results demonstrate that the heart failure milieu caused a 2-fold enrichment of extracellular matrix proteins (ECM) like biglycan, versican, collagen XII, and angiogenic factors like heparan sulfate proteoglycan 2, plasminogen activator inhibitor 1 in the secretome. We further elucidated the direct influence of the secretome on the functional behavior of Sca-1 ⁺ cells via in vitro tube forming assay. Secreted factors present in the diseased milieu induced tube formation in Cyc cells (1.7-fold; p < 0.01) when compared with Wt cells after 24 hr of exposure. The presence of conditioned media moderately increased the proliferation of Cyc cells but had a more pronounced effect on Wt cells. Overall, these findings revealed global modifications in the secretory activity of adult Sca-1 ⁺ cells in the heart failure milieu. The secretion of ECM proteins and angiogenic factors, which are crucial for cardiac remodeling and recovery, was notably enriched in the supernatant of Cyc cells. Thus, during heart failure the microenvironment of Sca-1 ⁺ cells might favor angiogenesis and proliferation suggesting their potential to recover the damaged heart.

A transient DMSO treatment increases the differentiation potential of human pluripotent stem cells through the Rb family

The propensity for differentiation varies substantially across human pluripotent stem cell (hPSC) lines, greatly restricting the use of hPSCs for cell replacement therapy or disease modeling. Here, we investigate the underlying mechanisms and demonstrate that activation of the retinoblastoma (Rb) pathway in a transient manner is important for differentiation. In prior work, we demonstrated that pre-treating hPSCs with dimethylsulfoxide (DMSO) before directed differentiation enhanced differentiation potential across all three germ layers. Here, we show that exposure to DMSO improves the efficiency of hPSC differentiation through Rb and by repressing downstream E2F-target genes. While transient inactivation of the Rb family members (including Rb, p107, and p130) suppresses DMSO’s capacity to enhance differentiation across all germ layers, transient expression of a constitutively active (non-phosphorylatable) form of Rb increases the differentiation efficiency similar to DMSO. Inhibition of downstream targets of Rb, such as E2F signaling, also promotes differentiation of hPSCs. More generally, we demonstrate that the duration of Rb activation plays an important role in regulating differentiation capacity.

Epigenetic Erosion in Adult Stem Cells: Drivers and Passengers of Aging

In complex organisms, stem cells are key for tissue maintenance and regeneration. Adult stem cells replenish continuously dividing tissues of the epithelial and connective types, whereas in non-growing muscle and nervous tissues, they are mainly activated upon injury or stress. In addition to replacing deteriorated cells, adult stem cells have to prevent their exhaustion by self-renewal. There is mounting evidence that both differentiation and self-renewal are impaired upon aging, leading to tissue degeneration and functional decline. Understanding the molecular pathways that become deregulate in old stem cells is crucial to counteract aging-associated tissue impairment. In this review, we focus on the epigenetic mechanisms governing the transition between quiescent and active states, as well as the decision between self-renewal and differentiation in three different stem cell types, i.e., spermatogonial stem cells, hematopoietic stem cells, and muscle stem cells. We discuss the epigenetic events that channel stem cell fate decisions, how this epigenetic regulation is altered with age, and how this can lead to tissue dysfunction and disease. Finally, we provide short prospects of strategies to preserve stem cell function and thus promote healthy aging.

Constitutive Activation of NF-κB Pathway in Hematopoietic Stem Cells Causes Loss of Quiescence and Deregulated Transcription Factor Networks

Identifying physiological roles of specific signaling pathways that regulate hematopoietic stem cell (HSC) functions may lead to new treatment strategies and therapeutic interventions for hematologic disorders. Here, we provide genetic evidence that constitutive activation of NF-κB in HSCs results in reduced pool size, repopulation capacities, and quiescence of HSCs. Global transcriptional profiling and bioinformatics studies identified loss of ‘stemness’ and ‘quiescence’ signatures in HSCs with deregulated NF-κB activation. In particular, gene set enrichment analysis identified upregulation of cyclin dependent kinase- Ccnd1 and down regulation of cyclin dependent kinase inhibitor p57^kip2. Interestingly, constitutive activation of NF-κB is sufficient to alter the regulatory circuits of transcription factors (TFs) that are critical to HSC self-renewal and functions. Molecular studies identified Junb, as one of the direct targets of NF-κB in hematopoietic cells. In essence, these studies demonstrate that aberrant activation of NF-κB signals impairs HSC quiescence and functions and alters the ‘TF networks’ in HSCs.

IGFBP2 promotes neural stem cell maintenance and proliferation differentially associated with glioblastoma subtypes

Neural stem cells (NSCs) give rise to the central nervous system (CNS) and persist in certain areas of adult brains for replenishing damaged differentiated cells. The loss of the balance between NSC self-renewal and differentiation could lead to tumor formation such as the occurrence of glioblastoma (GBM), the most common and deadly human brain tumor, which could be derived from neural stem or stem-like cells. Early studies showed that insulin-like growth factor binding protein 2 (IGFBP2) mRNA levels were maintained high during the fetal brain development but decreased in the adult brains. We previously reported that IGFBP2 was frequently overexpressed in GBMs, which was correlated with GBM recurrence and poor survival and promoted glioma progression. However, the role of IGFBP2 in the CNS was not investigated yet, whose understanding will help elucidate IGFBP2 functions in GBM. In the study, we identify IGFBP2 as a critical molecule for mouse NSC maintenance. IGFBP2 is highly expressed in NSCs, and its expression exhibits an apical-basal pattern in the neural tube with a higher apical level and decreased with NSC differentiation during the CNS development. IGFBP2 promotes NSC self-renewal and proliferation but inhibits its differentiation to neurons and astrocytes. The knockdown of IGFBP2 significantly affected the expression of cell cycle, Notch pathway, and neural stemness and differentiation genes in NSCs. Further, the expression of IGFBP2-regulated cell cycle genes is significantly correlated with IGFBP2 expression in non-Mesenchymal GBM subtypes including Classical, Proneural, and Neural subtypes and of its Notch pathway genes differentially associated in the four GBM subtypes, altogether suggesting its critical and similar functions in NSCs and GBM cells.

Repetitive magnetic stimulation promotes the proliferation of neural progenitor cells via modulating the expression of miR-106b

Increasing evidence shows that repetitive transcranial magnetic stimulation (rTMS) promotes neurogenesis and the expression of microRNA (miR)‑106b. The present study investigated whether rTMS promotes the proliferation of neural progenitor cells (NPCs) and whether the effect is associated with the expression of miR‑106b. NPCs were cultured from the rat hippocampus and exposed to rTMS daily, comprising 1,000 stimuli for 3 days at 10 Hz, with 1.75 T output. The proliferation ability of the NPCs was revealed by EdU staining, and the levels of miR‑106b and downstream gene p21 in the NPCs were measured by reverse transcription‑quantitative polymerase chain reaction and western blot analyses. For analysis of the mechanism, the NPCs were transfected with Lenti‑miR‑106b or small interfering RNAs prior to rTMS. The results showed that: i) rTMS increased NPC proliferation, as revealed by the increased proportion of EdU‑positive cells; ii) rTMS was able to upregulate the expression of miR‑106b and downregulate the level of p21 in NPCs; iii) overexpression of miR‑106b further enhanced the effects of rTMS, whereas knockdown of miR‑106b had the opposite effects. Taken together, these data indicated that rTMS can promote NPC proliferation by upregulating the expression of miR‑106b and possibly inhibiting the expression of p21.

Influence of diet and metabolism on hematopoietic stem cells and leukemia development following ionizing radiation exposure

PURPOSE

The review aims to discuss the prominence of dietary and metabolic regulators in maintaining hematopoietic stem cell (HSC) function, long-term self-renewal, and differentiation.

RESULTS

Most adult stem cells are preserved in a quiescent, nonmotile state in vivo which acts as a "protective state" for stem cells to reduce endogenous stress provoked by DNA replication and cellular respiration as well as exogenous environmental stress. The dynamic balance between quiescence, self-renewal and differentiation is critical for supporting a functional blood system throughout life of an organism. Stress-conditions, for example ionizing radiation exposure can trigger the blood forming HSCs to proliferate and migrate through extramedullary tissues to expand the number of HSCs and increase hematopoiesis. In addition, a wealth of investigation validated that deregulation of this balance plays a critical pathogenic role in various different hematopoietic diseases including the leukemia development.

CONCLUSION

The review summarizes the current knowledge on how alterations in dietary and metabolic factors could alter the risk of leukemia development following ionizing radiation exposure by inhibiting or even reversing the leukemic progression. Understanding the influence of diet, metabolism, and epigenetics on radiation-induced leukemogenesis may lead to the development of practical interventions to reduce the risk in exposed populations.

Omega 3 polyunsaturated fatty acids inhibit cell proliferation by regulating cell cycle in fad3b transgenic mouse embryonic stem cells

Background

The consumption of omega 3 polyunsaturated fatty acids (PUFAs) is important for human health and is closely associated with cell proliferation and differentiation. This study aimed to investigate the influence of omega 3 PUFAs on embryonic stem cell (ESC) proliferation and explore potential mechanisms that mediate these effects.

Methods

In this study, we isolated ESCs from fad3b-expressing transgenic mice. We detected the fatty-acid composition of ESCs using gas chromatography-mass spectroscopy, analyzed cell-cycle phases using flow cytometry, and detected gene expression using real-time polymerase chain reaction (PCR) and western blots.

Results

The amount of omega 3 PUFAs significantly increased in fad3b versus control ESCs. However, the growth of fad3b ESCs was slower than that of control cells, and most fad3b ESCs were in a prolonged G0/G1 phase after being passaged for 18 h. Therefore, we hypothesized that fad3b expression inhibited the cell cycle in ESCs by increasing the expression of P21, which then decreased the expression of cyclin-dependent kinase 4 (Cdk4). We found that pretreatment of fad3b ESCs with PD0325901, a P21 inhibitor, clearly attenuated the inhibitory effects of P21 on Cdk4, and resumed the cell cycle.

Conclusions

Expression of the fad3b gene in ESCs increased the omega 3 PUFA content, which inhibited cell proliferation by prolonging the G1 phase but did not arrest the G0-to-G1 or G1-to-S transitions. The prolonged G1 phase in fad3b ESCs was probably induced by downregulation of Cdk4 expression via p21 upregulation. These results suggest that accumulation of omega 3 PUFAs in vivo may beneficially affect ESC differentiation and that fad3b ESCs may be a useful tool for investigating related mechanisms.

Electronic supplementary material

The online version of this article (10.1186/s12944-018-0862-x) contains supplementary material, which is available to authorized users.

Rehmannia glutinosa exhibits anti-aging effect through maintaining the quiescence and decreasing the senescence of hematopoietic stem cells

BACKGROUND

The time-related decline in regenerative capacity and organ homeostasis is a major feature of aging. Rehmannia glutinosa and Astragalus membranaceus have been used as traditional Chinese herbal medicines for enhanced immunity and prolonged life. However, the mechanism by which this herbal medicine slows aging is unknown. In this study, we investigated the mechanism of the herbal anti-aging effect.

METHODS

Mice were fed diets supplemented with R. glutinosa or A. membranaceus for 10 months; the control group was fed a standard diet. The phenotypes were evaluated using a grading score system and survival analysis. The percentages of the senescence phenotypes of hematopoietic stem cells (HSCs) were determined by fluorescence-activated cell sorting analysis. The function and the mechanism of HSCs were analyzed by clonogenic assay and the real-time polymerase chain reaction.

RESULTS

The anti-aging effect of R. glutinosa is due to the enhanced function of HSCs. Mice fed with R. glutinosa displayed characteristics of a slowed aging process, including decreased senescence and increased rate of survival. Flow cytometry analysis showed decreased numbers of Lin-Sca1+c-kit- (LSK) cells, long-term HSCs (LT-HSCs) and short-term HSCs (ST-HSCs) in the R. glutinosa group. In vitro, clonogenic assays showed increased self-renewal ability of LT-HSCs from the R. glutinosa group as well as maintaining LSK quiescence through upregulated p18 expression. The R. glutinosa group also showed decreased reactive oxygen species levels and the percentage of β-gal+ cells through downregulation of the cellular senescence-associated protein p53 and p16.

CONCLUSION

Rehmannia glutinosa exerts anti-aging effects by maintaining the quiescence and decreasing the senescence of HSCs.

UHRF1 Licensed Self-Renewal of Active Adult Neural Stem Cells

Adult neurogenesis in the brain continuously seeds new neurons throughout life, but how homeostasis of adult neural stem cells (NSCs) is maintained is incompletely understood. Here, we demonstrate that the DNA methylation adapter ubiquitin-like, containing PHD and RING finger domains-1 (UHRF1) is expressed in, and regulates proliferation of, the active but not quiescent pool of adult neural progenitor cells. Mice with a neural stem cell-specific deficiency in UHRF1 exhibit a massive depletion of neurogenesis resulting in a collapse of formation of new neurons. In the absence of UHRF1, NSCs unexpectedly remain in the cell cycle but with a 17-fold increased cell cycle length due to a failure of replication phase entry caused by promoter demethylation and derepression of Cdkn1a, which encodes the cyclin-dependent kinase inhibitor p21. UHRF1 does not affect the proportion progenitor cells active within the cell cycle but among these cells, UHRF1 is critical for licensing replication re-entry. Therefore, this study shows that a UHRF1-Cdkn1a axis is essential for the control of stem cell self-renewal and neurogenesis in the adult brain. Stem Cells 2018;36:1736-1751.

Niche Cadherins Control the Quiescence-to-Activation Transition in Muscle Stem Cells

Many adult stem cells display prolonged quiescence, promoted by cues from their niche. Upon tissue damage, a coordinated transition to the activated state is required because non-physiological breaks in quiescence often lead to stem cell depletion and impaired regeneration. Here, we identify cadherin-mediated adhesion and signaling between muscle stem cells (satellite cells [SCs]) and their myofiber niche as a mechanism that orchestrates the quiescence-to-activation transition. Conditional removal of N-cadherin and M-cadherin in mice leads to a break in SC quiescence, with long-term expansion of a regeneration-proficient SC pool. These SCs have an incomplete disruption of the myofiber-SC adhesive junction and maintain niche residence and cell polarity, yet show properties of SCs in a state of transition from quiescence toward full activation. Among these is nuclear localization of β-catenin, which is necessary for this phenotype. Injury-induced perturbation of niche adhesive junctions is therefore a likely first step in the quiescence-to-activation transition.

COPS2 Antagonizes OCT4 to Accelerate the G2/M Transition of Mouse Embryonic Stem Cells

Proper regulation of the cell cycle is essential to safeguard the genomic integrity of embryonic stem cells (ESCs) while maintaining the fast proliferation rate. The pluripotency factor OCT4 has been shown to inhibit CDK1 activation, thus preventing mitotic entry and facilitating the maintenance of genomic integrity. Yet, how ESCs enter mitosis in the presence of OCT4 remains unclear. We previously reported that COPS2 promotes the progression through the G2/M phase of mouse ESCs. In this study, through co-immunoprecipitation and mass spectrometric analysis, we found that COPS2 interacts with OCT4 and CDK1. We further demonstrated that COPS2 stimulates the activity of CDK1/CYCLIN B only when OCT4 is present. Consistently, COPS2 promotes the G2/M transition only in the presence of OCT4 in HeLa cells. Mechanistically, COPS2 attenuates the interaction between OCT4 and CDK1 by sequestering OCT4 and forming a COPS2/CDK1 complex, thus blocking the inhibitory effect of OCT4 on CDK1 activation.

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COPS2 is required for the rapid G2/M transition in mouse embryonic stem cells

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COPS2 counteracts the inhibitory effect of OCT4 on CDK1 activation

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COPS2 accelerates the G2/M transition only in the presence of OCT4

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COPS2 competes with OCT4 in binding to CDK1

Immunomodulatory drugs downregulate IKZF1 leading to expansion of hematopoietic progenitors with concomitant block of megakaryocytic maturation

The immunomodulatory drugs, lenalidomide and pomalidomide yield high response rates in multiple myeloma patients, but are associated with a high rate of thrombocytopenia and increased risk of secondary hematologic malignancies. Here, we demonstrate that the immunomodulatory drugs induce self-renewal of hematopoietic progenitors and upregulate megakaryocytic colonies by inhibiting apoptosis and increasing proliferation of early megakaryocytic progenitors via down-regulation of IKZF1. In this process, the immunomodulatory drugs degrade IKZF1 and subsequently down-regulate its binding partner, GATA1. This results in the decrease of GATA1 targets such as ZFPM1 and NFE2, leading to expansion of megakaryocytic progenitors with concomitant inhibition of maturation of megakaryocytes. The down-regulation of GATA1 further decreases CCND1 and increases CDKN2A expression. Overexpression of GATA1 abrogated the effects of the immunomodulatory drugs and restored maturation of megakaryocytic progenitors. Our data not only provide the mechanism for the immunomodulatory drugs induced thrombocytopenia but also help to explain the higher risk of secondary malignancies and long-term cytopenia induced by enhanced cell cycling and subsequent exhaustion of the stem cell pool.

Transcriptional Profiling of Quiescent Muscle Stem Cells In Vivo

Muscle stem cells (MuSCs) persist in a quiescent state and activate in response to specific stimuli. The quiescent state is both actively maintained and dynamically regulated. However, analyses of quiescence have come primarily from cells removed from their niche. Although these cells are still quiescent, biochemical changes certainly occur during the isolation process. Here, we analyze the transcriptome of MuSCs in vivo utilizing MuSC-specific labeling of RNA. Notably, labeling transcripts during the isolation procedure revealed very active transcription of specific subsets of genes. However, using the transcription inhibitor α-amanitin, we show that the ex vivo transcriptome remains largely reflective of the in vivo transcriptome. Together, these data provide perspective on the molecular regulation of the quiescent state at the transcriptional level, demonstrate the utility of these tools for probing transcriptional dynamics in vivo, and provide an invaluable resource for understanding stem cell state transitions.

ICAM-1 Deficiency in the Bone Marrow Niche Impairs Quiescence and Repopulation of Hematopoietic Stem Cells

The bone marrow niche plays a critical role in controlling the fate of hematopoietic stem cells (HSCs) by integrating intrinsic and extrinsic signals. However, the molecular events in the HSC niche remain to be investigated. Here, we report that intercellular adhesion molecule-1 (ICAM-1) maintains HSC quiescence and repopulation capacity in the niche. ICAM-1-deficient mice (ICAM-1^−/−) displayed significant expansion of phenotypic long-term HSCs with impaired quiescence, as well as favoring myeloid cell expansion. ICAM-1-deficient HSCs presented normal reconstitution capacity during serial transplantation; however, reciprocal transplantation experiments showed that ICAM-1 deficiency in the niche impaired HSC quiescence and repopulation capacity. In addition, ICAM-1 deletion caused failure to retain HSCs in the bone marrow and changed the expression profile of stroma cell-derived factors, possibly representing the mechanism for defective HSCs in ICAM-1^−/− mice. Collectively, these observations identify ICAM-1 as a regulator in the bone marrow niche.

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ICAM-1 deficiency expands HSC^−LT with impaired quiescence and repopulation

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The defects characterizing HSC^−LT in ICAM-1^−/− mice are niche cell dependent

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ICAM-1^−/− niche brings about impaired bone marrow retention and homing of HSC^−LT

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ICAM-1 in human stroma cells might affect the progression of myelocytic leukemia

Neural stem cell heterogeneity in the mammalian forebrain

The brain was long considered an organ that underwent very little change after development. It is now well established that the mammalian central nervous system contains neural stem cells that generate progeny that are capable of making new neurons, astrocytes, and oligodendrocytes throughout life. The field has advanced rapidly as it strives to understand the basic biology of these precursor cells, and explore their potential to promote brain repair. The purpose of this review is to present current knowledge about the diversity of neural stem cells in vitro and in vivo, and highlight distinctions between neural stem cell populations, throughout development, and within the niche. A comprehensive understanding of neural stem cell heterogeneity will provide insights into the cellular and molecular regulation of neural development and lifelong neurogenesis, and will guide the development of novel strategies to promote regeneration and neural repair.

DNA damage and tissue repair: What we can learn from planaria

Faithful renewal of aging and damaged tissues is central to organismal lifespan. Stem cells (SCs) generate the cellular progeny that replenish adult tissues across the body but this task becomes increasingly compromised over time. The age related decline in SC-mediated tissue maintenance is a multifactorial event that commonly affects genome integrity. The presence of DNA damage in SCs that are under continuous demand to divide poses a great risk for age-related disorders such as cancer. However, performing analysis of SCs with genomic instability and the DNA damage response during tissue renewal present significant challenges. Here we introduce an alternative experimental system based on the planaria flatworm Schmidtea mediterranea to address at the organismal level studies intersecting SC-mediated tissue renewal in the presence of genomic instability. Planaria have abundant SCs (neoblasts) that maintain high rates of cellular turnover and a variety of molecular tools have been developed to induce DNA damage and dissect how neoblasts respond to this stressor. S. mediterranea displays high evolutionary conservation of DNA repair mechanisms and signaling pathways regulating adult SCs. We describe genetically induced-DNA damage models and highlight body-wide signals affecting cellular decisions such as survival, proliferation, and death in the presence of genomic instability. We also discuss transcriptomic changes in the DNA damage response during injury repair and propose DNA repair as key component of tissue regeneration. Additional studies using planaria will provide insights about mechanisms regulating survival and growth of cells with DNA damage during tissue renewal and regeneration.

Stretching the limits: from homeostasis to stem cell plasticity in wound healing and cancer

Stem cells (SCs) govern tissue homeostasis and wound repair. They reside within niches, the special microenvironments within tissues that control SC lineage outputs. Upon injury or stress, new signals emanating from damaged tissue can divert nearby cells into adopting behaviours that are not part of their homeostatic repertoire. This behaviour, known as SC plasticity, typically resolves as wounds heal. However, in cancer, it can endure. Recent studies have yielded insights into the orchestrators of maintenance and lineage commitment for SCs belonging to three mammalian tissues: the haematopoietic system, the skin epithelium and the intestinal epithelium. We delineate the multifactorial determinants and general principles underlying the remarkable facets of SC plasticity, which lend promise for regenerative medicine and cancer therapeutics.

Understanding the Cellular Origin of the Mononuclear Phagocyte System Sheds Light on the Myeloid Postulate of Immune Paralysis in Sepsis

Sepsis, in essence, is a serious clinical condition that can subsequently result in death as a consequence of a systemic inflammatory response syndrome including febrile leukopenia, hypotension, and multiple organ failures. To date, such life-threatening organ dysfunction remains one of the leading causes of death in intensive care units, with an increasing incidence rate worldwide and particularly within the rapidly growing senior population. While most of the clinical trials are aimed at dampening the overwhelming immune response to infection that spreads through the bloodstream, based on several human immunological investigations, it is now widely accepted that susceptibility to nosocomial infections and long-term sepsis mortality involves an immunosuppressive phase that is characterized by a decrease in some subsets of dendritic cells (DCs). Only recently substantial advances have been made in terms of the origin of the mononuclear phagocyte system that is now likely to allow for a better understanding of how the paralysis of DCs leads to sepsis-related death. Indeed, the unifying view of each subset of DCs has already improved our understanding of the pivotal pathways that contribute to the shift in commitment of their progenitors that originate from the bone marrow. It is quite plausible that this anomaly in sepsis may occur at the single level of DC-committed precursors, and elucidating the immunological basis for such a derangement during the ontogeny of each subset of DCs is now of particular importance for restoring an adequate cell fate decision to their vulnerable progenitors. Last but not least, it provides a direct perspective on the development of sophisticated myelopoiesis-based strategies that are currently being considered for the treatment of immunosenescence within different tissue microenvironments, such as the kidney and the spleen.

Systemic Virus Infections Differentially Modulate Cell Cycle State and Functionality of Long-Term Hematopoietic Stem Cells In Vivo

Quiescent long-term hematopoietic stem cells (LT-HSCs) are efficiently activated by type I interferon (IFN-I). However, this effect remains poorly investigated in the context of IFN-I-inducing virus infections. Here we report that both vesicular stomatitis virus (VSV) and murine cytomegalovirus (MCMV) infection induce LT-HSC activation that substantially differs from the effects triggered upon injection of synthetic IFN-I-inducing agents. In both infections, inflammatory responses had to exceed local thresholds within the bone marrow to confer LT-HSC cell cycle entry, and IFN-I receptor triggering was not critical for this activation. After resolution of acute MCMV infection, LT-HSCs returned to phenotypic quiescence. However, non-acute MCMV infection induced a sustained inflammatory milieu within the bone marrow that was associated with long-lasting impairment of LT-HSC function. In conclusion, our results show that systemic virus infections fundamentally affect LT-HSCs and that also non-acute inflammatory stimuli in bone marrow donors can affect the reconstitution potential of bone marrow transplants.

T-cell Immunoglobulin and ITIM Domain Contributes to CD8+ T-cell Immunosenescence

Aging is associated with immune dysfunction, especially T-cell defects, which result in increased susceptibility to various diseases. Previous studies showed that T cells from aged mice express multiple inhibitory receptors, providing evidence of the relationship between T-cell exhaustion and T-cell senescence. In this study, we showed that T-cell immunoglobulin and immunoreceptor tyrosine-based inhibitory motif (ITIM) domain (TIGIT), a novel co-inhibitory receptor, was upregulated in CD8+ T cells of elderly adults. Aged TIGIT+ CD8+ T cells expressed high levels of other inhibitory receptors including PD-1 and exhibited features of exhaustion such as downregulation of the key costimulatory receptor CD28, representative intrinsic transcriptional regulation, low production of cytokines, and high susceptibility to apoptosis. Importantly, their functional defects associated with aging were reversed by TIGIT knockdown. CD226 downregulation on aged TIGIT+ CD8+ T cells is likely involved in TIGIT-mediated negative immune suppression. Collectively, our findings indicated that TIGIT acts as a critical immune regulator during aging, providing a strong rationale for targeting TIGIT to improve dysfunction related to immune system aging.

Microtechnology applied to stem cells research and development

Microfabrication and microfluidics contribute to the research of cellular functions of cells and their interaction with their environment. Previously, it has been shown that microfluidics can contribute to the isolation, selection, characterization and migration of cells. This review aims to provide stem cell researchers with a toolkit of microtechnology (mT) instruments for elucidating complex stem cells functions which are challenging to decipher with traditional assays and animal models. These microdevices are able to investigate about the differentiation and niche interaction, stem cells transcriptomics, therapeutic functions and the capture of their secreted microvesicles. In conclusion, microtechnology will allow a more realistic assessment of stem cells properties, driving and accelerating the translation of regenerative medicine approaches to the clinic.

Division-independent differentiation mandates proliferative competition among stem cells

Cancer-initiating gatekeeper mutations that arise in stem cells would be especially potent if they stabilize and expand an affected stem cell lineage. It is therefore important to understand how different stem cell organization strategies promote or prevent variant stem cell amplification in response to different types of mutation, including those that activate proliferation. Stem cell numbers can be maintained constant while producing differentiated products through individually asymmetrical division outcomes or by population asymmetry strategies in which individual stem cell lineages necessarily compete for niche space. We considered alternative mechanisms underlying population asymmetry and used quantitative modeling to predict starkly different consequences of altering proliferation rate: A variant, faster proliferating mutant stem cell should compete better only when stem cell division and differentiation are independent processes. For most types of stem cells, it has not been possible to ascertain experimentally whether division and differentiation are coupled. However, Drosophila follicle stem cells (FSCs) provided a favorable system with which to investigate population asymmetry mechanisms and also for measuring the impact of altered proliferation on competition. We found from detailed cell lineage studies that division and differentiation of an individual FSC are not coupled. We also found that FSC representation, reflecting maintenance and amplification, was highly responsive to genetic changes that altered only the rate of FSC proliferation. The FSC paradigm therefore provides definitive experimental evidence for the general principle that relative proliferation rate will always be a major determinant of competition among stem cells specifically when stem cell division and differentiation are independent.

Genetic rescue of lineage-balanced blood cell production reveals a crucial role for STAT3 antiinflammatory activity in hematopoiesis

Blood cell formation must be appropriately maintained throughout life to provide robust immune function, hemostasis, and oxygen delivery to tissues, and to prevent disorders that result from over- or underproduction of critical lineages. Persistent inflammation deregulates hematopoiesis by damaging hematopoietic stem and progenitor cells (HSPCs), leading to elevated myeloid cell output and eventual bone marrow failure. Nonetheless, antiinflammatory mechanisms that protect the hematopoietic system are understudied. The transcriptional regulator STAT3 has myriad roles in HSPC-derived populations and nonhematopoietic tissues, including a potent antiinflammatory function in differentiated myeloid cells. STAT3 antiinflammatory activity is facilitated by STAT3-mediated transcriptional repression of Ube2n, which encodes the E2 ubiquitin-conjugating enzyme Ubc13 involved in proinflammatory signaling. Here we demonstrate a crucial role for STAT3 antiinflammatory activity in preservation of HSPCs and lineage-balanced hematopoiesis. Conditional Stat3 removal from the hematopoietic system led to depletion of the bone marrow lineage- Sca-1+ c-Kit+ CD150+ CD48- HSPC subset (LSK CD150+ CD48- cells), myeloid-skewed hematopoiesis, and accrual of DNA damage in HSPCs. These responses were accompanied by intrinsic transcriptional alterations in HSPCs, including deregulation of inflammatory, survival and developmental pathways. Concomitant Ube2n/Ubc13 deletion from Stat3-deficient hematopoietic cells enabled lineage-balanced hematopoiesis, mitigated depletion of bone marrow LSK CD150+ CD48- cells, alleviated HSPC DNA damage, and corrected a majority of aberrant transcriptional responses. These results indicate an intrinsic protective role for STAT3 in the hematopoietic system, and suggest that this is mediated by STAT3-dependent restraint of excessive proinflammatory signaling via Ubc13 modulation.

Replication stress in hematopoietic stem cells in mouse and man

Life-long blood regeneration relies on a rare population of self-renewing hematopoietic stem cells (HSCs). These cells' nearly unlimited self-renewal potential and lifetime persistence in the body signifies the need for tight control of their genome integrity. Their quiescent state, tightly linked with low metabolic activity, is one of the main strategies employed by HSCs to preserve an intact genome. On the other hand, HSCs need to be able to quickly respond to increased blood demands and rapidly increase their cellular output in order to fight infection-associated inflammation or extensive blood loss. This increase in proliferation rate, however, comes at the price of exposing HSCs to DNA damage inevitably associated with the process of DNA replication. Any interference with normal replication fork progression leads to a specialized molecular response termed replication stress (RS). Importantly, increased levels of RS are a hallmark feature of aged HSCs, where an accumulating body of evidence points to causative relationships between RS and the aging-associated impairment of the blood system's functional capacity. In this review, we present an overview of RS in HSCs focusing on its causes and consequences for the blood system of mice and men.

Imaging-Based Screen Identifies Laminin 411 as a Physiologically Relevant Niche Factor with Importance for i-Hep Applications

Use of hepatocytes derived from induced pluripotent stem cells (i-Heps) is limited by their functional differences in comparison with primary cells. Extracellular niche factors likely play a critical role in bridging this gap. Using image-based characterization (high content analysis; HCA) of freshly isolated hepatocytes from 17 human donors, we devised and validated an algorithm (Hepatocyte Likeness Index; HLI) for comparing the hepatic properties of cells against a physiological gold standard. The HLI was then applied in a targeted screen of extracellular niche factors to identify substrates driving i-Heps closer to the standard. Laminin 411, the top hit, was validated in two additional induced pluripotent stem cell (iPSC) lines, primary tissue, and an in vitro model of α1-antitrypsin deficiency. Cumulatively, these data provide a reference method to control and screen for i-Hep differentiation, identify Laminin 411 as a key niche protein, and underscore the importance of combining substrates, soluble factors, and HCA when developing iPSC applications.

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iPSC-derived hepatocytes (i-Heps) are functionally limited compared with primary cells

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Factors within the extracellular niche likely play a role in bridging this gap

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Laminin 411 was shown to be an important niche factor for i-Heps

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High content image analysis (HCA) can help development of i-Hep applications

Transient CDK4/6 inhibition protects hematopoietic stem cells from chemotherapy-induced exhaustion

Conventional cytotoxic chemotherapy is highly effective in certain cancers but causes dose-limiting damage to normal proliferating cells, especially hematopoietic stem and progenitor cells (HSPCs). Serial exposure to cytotoxics causes a long-term hematopoietic compromise ("exhaustion"), which limits the use of chemotherapy and success of cancer therapy. We show that the coadministration of G1T28 (trilaciclib), which is a small-molecule inhibitor of cyclin-dependent kinases 4 and 6 (CDK4/6), contemporaneously with cytotoxic chemotherapy protects murine hematopoietic stem cells (HSCs) from chemotherapy-induced exhaustion in a serial 5-fluorouracil treatment model. Consistent with a cell-intrinsic effect, we show directly preserved HSC function resulting in a more rapid recovery of peripheral blood counts, enhanced serial transplantation capacity, and reduced myeloid skewing. When administered to healthy human volunteers, G1T28 demonstrated excellent in vivo pharmacology and transiently inhibited bone marrow (BM) HSPC proliferation. These findings suggest that the combination of CDK4/6 inhibitors with cytotoxic chemotherapy should provide a means to attenuate therapy-induced BM exhaustion in patients with cancer.

Icariin improves Fanconi anemia hematopoietic stem cell function through SIRT6-mediated NF-kappa B inhibition

Icariin (ICA) is a flavonoid glucoside derived from the Epimedium plant genus, which has potent regenerative properties and is used in western medicine to treat impotence. Recently, ICA has generated great interest in improving hepatic stellate cell function and cardiac rejuvenation. However, how this natural component functions in hematopoiesis remains unexplored. Here we have examined the role of ICA on hematopoietic stem cells (HSCs) using the cancer-prone disease model of Fanconi anemia (FA), an inherited bone marrow failure syndrome with extremely high risk of leukemic predisposition. We show that ICA reverses the less quiescent status of HSCs deficient for the Fanca or Fancd2 gene, and improves the ability of these mutant stem cells to form colony formation units (CFU) in vitro and reconstitutes hematopoiesis in transplanted recipients. Further analysis reveals that ICA upregulates enzyme activity of the chromatin binding protein SIRT6 in Fanca^-/- and Fancd2^-/- HSCs, both of which have an intrinsic low SIRT6 activity. Furthermore, forced expression of SIRT6 blocks the natural decline of quiescent HSCs in Fanca^-/- or Fancd2^-/- mice and improves the repopulating capacity of these mutant HSCs in irradiated recipients. Mechanistically, ICA enhances SIRT6-mediated H3K9 deacetylation on the promoter of NF-κB and represses the expression of NF-κB target genes. Together, our findings indicate that ICA improves the function of HSCs by stimulating SIRT6 activity and contributes to the regenerative effect of ICA.

Overexpression of ZNF703 facilitates tumorigenesis and predicts unfavorable prognosis in patients with cholangiocarcinoma

Background

NET (NocA/Nlz, Elbow, Tlp-1) family members have recently emerged as important players in the development of human cancers. Zinc finger protein 703 (ZNF703), locating on chromosome 8 (8p11.23), a member of the NET/Nlz family of zinc finger transcription factors, had been demonstrated to be a much novel oncogene of several malignancies. This study aimed to investigate the expression of ZNF703 in cholangiocarcinoma (CCA) and attempted to elucidate its biological effects in CCA progression.

Methods

The correlation between ZNF703 expression and clinicopathological characteristics of CCA was evaluated through analyzing 85 cases. The biological effects of ZNF703 were investigated both in vitro and in vivo in which proliferation, migration, and invasive potential were mainly explored. Statistical software SPSS 16.0 was used for statistical analyses.

Results

ZNF703 was overexpressed in CCA tissues with subcellular localizations mainly in the nucleus and partly in the cytoplasm or membrane. High expression of ZNF703 was related to tumor location (P=0.002), pathological grading (P=0.024), depth of invasion (P=0.002), distant metastasis (P=0. 011) and AJCC stage (P=0.008). Both in vitro and in vivo studies demonstrated that ZNF703 could potently promote proliferation, migration and invasion throughout the progression of CCA.

Conclusion

ZNF703 can potently facilitate tumor growth and metastasis in many respects throughout the progression of CCA, which may act as an oncogene in CCA and can be considered as a novel potential therapeutic target.

Stimulatory effect of icariin on the proliferation of neural stem cells from rat hippocampus

Background

Icariin (ICA), a major ingredient of Epimediumbrevicornum, has various pharmacological activities including central nervous system protective functions such as the improvement of learning and memory function in mice models of Alzheimer’s disease. It has been reported that ICA can promote regeneration of peripheral nerve and functional recovery. The purpose of this study was to investigate the potentiating effect of ICA on the proliferation of rat hippocampal neural stem cells, and explore the possible mechanism involved.

Methods

Primary neural stem cells were prepared from the hippocampus of newly born SD rats, and cells were cultured in special stem cell culture medium. Neural stem cells were confirmed by immunofluorescence detection of nestin, NSE and GFAP expression. The effect of ICA on the growth and proliferation of the neural stem cells was evaluated by 5-ethynyl-2-deoxyuridine (EdU) labeling of proliferating cells, and photomicrographic images of the cultured neural stem cells. Further, the mechanism of ICA-induced cell proliferation of neural stem cells was investigated by analyzing the gene and protein expression of cell cycle related genes cyclin D1 and p21.

Results

The present study showed that icariin promotes the growth and proliferation of neural stem cells from rat hippocampus in a dose-dependent manner. Incubation of cells with icariin resulted in significant increase in the number of stem cell spheres as well as the increased incorporation of EdU when compared with cells exposed to control vehicle. In addition, it was found that icariin-induced effect on neural stem cells is associated with increased mRNA and protein expression of cell cycle genes cyclin D1 and p21.

Conclusions

This study evidently demonstrates the potentiating effect of ICA on neural stem cell growth and proliferation, which might be mediated through regulation of cell cycle gene and protein expression promoting cell cycle progression.

Electronic supplementary material

The online version of this article (10.1186/s12906-018-2095-y) contains supplementary material, which is available to authorized users.

Proliferation and survival of human amniotic epithelial cells during their hepatic differentiation

Stem cells derived from placental tissues are an attractive source of cells for regenerative medicine. Amniotic epithelial cells isolated from human amnion (hAECs) have desirable and competitive characteristics that make them stand out between other stem cells. They have the ability to differentiate toward all three germ layers, they are not tumorigenic and they have immunosuppressive properties. Although liver transplantation is the best way to treat acute and chronic hepatic failure patients, there are several obstacles. Recently, stem cells have been spotlighted as alternative source of hepatocytes because of their potential for hepatogenic differentiation. In this work, we aimed to study the proliferation and survival of the hAECs during their hepatic differentiation. We have also analyzed the changes in pluripotency and hepatic markers. We differentiated amniotic cells applying a specific hepatic differentiation (HD) protocol. We determined by qRT-PCR that hAECs express significant levels of SOX-2, OCT-4 and NANOG during at least 15 days in culture and these pluripotent markers diminish during HD. SSEA-4 expression was reduced during HD, measured by immunofluorescence. Morphological characteristics became more similar to hepatic ones in differentiated cells and representative hepatic markers significantly augmented their expression, measured by qRT-PCR and Western blot. Cells achieved a differentiation efficiency of 75%. We observed that HD induced proliferation and promoted survival of hAECs, during 30 days in culture, evaluated by ³H-thymidine incorporation and MTT assay. HD also promoted changes in hAECs cell cycle. Cyclin D1 expression increased, while p21 and p53 levels were reduced. Immunofluorescence analysis showed that Ki-67 expression was upregulated during HD. Finally, ERK 1/2 phosphorylation, which is intimately linked to proliferation and cell survival, augmented during all HD process and the inhibition of this signaling pathway affected not only proliferation but also differentiation. Our results suggest that HD promotes proliferation and survival of hAECs, providing important evidence about the mechanisms governing their hepatic differentiation. We bring new knowledge concerning some of the optimal transplantation conditions for these hepatic like cells.

Mammalian sterile 20 kinase 1 and 2 are important regulators of hematopoietic stem cells in stress condition

The mammalian Hippo signaling pathway has been implicated in the self-renewal and differentiation of stem and progenitor cells. MST1 and MST2 (MST1/2) are core serine-threonine kinases in the Hippo signaling pathway, one of which, MST1, has been extensively investigated for its role in T cell and myeloid cell function. These studies have identified MST1 as a promising therapeutic target in immunological disease. However, the roles of MST1/2 in hematopoietic stem cell (HSC) function in vivo are not fully understood. Here, we report that mice with a conditional deletion of Mst1/2 exhibit impaired hematopoietic stem and progenitor cell (HSPC) function under stress condition. Furthermore, Mst1/2 deletion markedly altered mature cell output. Therefore, MST1/2 are indispensable for maintenance as well as function of stem and progenitor cells under steady state conditions and with transplantation stress.

DYRK1A Is a Regulator of S-Phase Entry in Hepatic Progenitor Cells

Hepatic progenitor cells (HPCs) are adult liver stem cells that act as second line of defense in liver regeneration. They are normally quiescent, but in case of severe liver damage, HPC proliferation is triggered by external activation mechanisms from their niche. Although several important proproliferative mechanisms have been described, it is not known which key intracellular regulators govern the switch between HPC quiescence and active cell cycle. We performed a high-throughput kinome small interfering RNA (siRNA) screen in HepaRG cells, a HPC-like cell line, and evaluated the effect on proliferation with a 5-ethynyl-2'-deoxyuridine (EdU) incorporation assay. One hit increased the percentage of EdU-positive cells after knockdown: dual specificity tyrosine phosphorylation regulated kinase 1A (DYRK1A). Although upon DYRK1A silencing, the percentage of EdU- and phosphorylated histone H3 (pH3)-positive cells was increased, and total cell numbers were not increased, possibly through a subsequent delay in cell cycle progression. This phenotype was confirmed with chemical inhibition of DYRK1A using harmine and with primary HPCs cultured as liver organoids. DYRK1A inhibition impaired Dimerization Partner, RB-like, E2F, and multivulva class B (DREAM) complex formation in HPCs and abolished its transcriptional repression on cell cycle progression. To further analyze DYRK1A function in HPC proliferation, liver organoid cultures were established from mBACtgDyrk1A mice, which harbor one extra copy of the murine Dyrk1a gene (Dyrk+++). Dyrk+++ organoids had both a reduced percentage of EdU-positive cells and reduced proliferation compared with wild-type organoids. This study provides evidence for an essential role of DYRK1A as balanced regulator of S-phase entry in HPCs. An exact gene dosage is crucial, as both DYRK1A deficiency and overexpression affect HPC cell cycle progression.

Cyclin D/CDK4/6 activity controls G1 length in mammalian cells

The length of the G1 phase in the cell cycle shows significant variability in different cell types and tissue types. To gain insights into the control of G1 length, we generated an E2F activity reporter that captures free E2F activity after dissociation from Rb sequestration and followed its kinetics of activation at the single-cell level, in real time. Our results demonstrate that its activity is precisely coordinated with S phase progression. Quantitative analysis indicates that there is a pre-S phase delay between E2F transcriptional dynamic and activity dynamics. This delay is variable among different cell types and is strongly modulated by the cyclin D/CDK4/6 complex activity through Rb phosphorylation. Our findings suggest that the main function of this complex is to regulate the appropriate timing of G1 length.

Efficient Generation of Glucose-Responsive Beta Cells from Isolated GP2+ Human Pancreatic Progenitors

Stem cell-based therapy for type 1 diabetes would benefit from implementation of a cell purification step at the pancreatic endoderm stage. This would increase the safety of the final cell product, allow the establishment of an intermediate-stage stem cell bank, and provide a means for upscaling β cell manufacturing. Comparative gene expression analysis revealed glycoprotein 2 (GP2) as a specific cell surface marker for isolating pancreatic endoderm cells (PECs) from differentiated hESCs and human fetal pancreas. Isolated GP2⁺ PECs efficiently differentiated into glucose responsive insulin-producing cells in vitro. We found that in vitro PEC proliferation declines due to enhanced expression of the cyclin-dependent kinase (CDK) inhibitors CDKN1A and CDKN2A. However, we identified a time window when reducing CDKN1A or CDKN2A expression increased proliferation and yield of GP2⁺ PECs. Altogether, our results contribute tools and concepts toward the isolation and use of PECs as a source for the safe production of hPSC-derived β cells.

Phenotypic Screen Identifies a Small Molecule Modulating ERK2 and Promoting Stem Cell Proliferation

Stem cells display a fundamentally different mechanism of proliferation control when compared to somatic cells. Uncovering these mechanisms would maximize the impact in drug discovery with a higher translational applicability. The unbiased approach used in phenotype-based drug discovery (PDD) programs can offer a unique opportunity to identify such novel biological phenomenon. Here, we describe an integrated phenotypic screening approach, employing a combination of in vitro and in vivo PDD models to identify a small molecule increasing stem cell proliferation. We demonstrate that a combination of both in vitro and in vivo screening models improves hit identification and reproducibility of effects across various PDD models. Using cell viability and colony size phenotype measurement we characterize the structure activity relationship of the lead molecule, and identify that the small molecule inhibits phosphorylation of ERK2 and promotes stem cell proliferation. This study demonstrates a PDD approach that employs combinatorial models to identify compounds promoting stem cell proliferation.

The mitochondrial metabolic checkpoint and aging of hematopoietic stem cells

PURPOSE OF REVIEW

Cell-cycle checkpoints are surveillance mechanisms in eukaryotic cells that monitor the condition of the cell, repair cellular damages, and allow the cell to progress through the various phases of the cell cycle when conditions become favorable. We review recent advances in hematopoietic stem cell (HSC) biology, highlighting a mitochondrial metabolic checkpoint that is essential for HSCs to return to the quiescent state.

RECENT FINDINGS

As quiescent HSCs enter the cell cycle, mitochondrial biogenesis is induced, which is associated with increased mitochondrial protein folding stress and mitochondrial oxidative stress. Mitochondrial unfolded protein response and mitochondrial oxidative stress response are activated to alleviate stresses and allow HSCs to exit the cell cycle and return to quiescence. Other mitochondrial maintenance mechanisms include mitophagy and asymmetric segregation of aged mitochondria.

SUMMARY

Because loss of HSC quiescence results in the depletion of the HSC pool and compromised tissue regeneration, deciphering the molecular mechanisms that regulate the mitochondrial metabolic checkpoint in HSCs will increase our understanding of hematopoiesis and how it becomes dysregulated under pathological conditions and during aging. More broadly, this knowledge is instrumental for understanding the maintenance of cells that convert between quiescence and proliferation to support their physiological functions.

The Role of Cellular Proliferation in Adipogenic Differentiation of Human Adipose Tissue-Derived Mesenchymal Stem Cells

Mitotic clonal expansion has been suggested as a prerequisite for adipogenesis in murine preadipocytes, but the precise role of cell proliferation during human adipogenesis is unclear. Using adipose tissue-derived human mesenchymal stem cells as an in vitro cell model for adipogenic study, a group of cell cycle regulators, including Cdk1 and CCND1, were found to be downregulated as early as 24 h after adipogenic initiation and consistently, cell proliferation activity was restricted to the first 48 h of adipogenic induction. Cell proliferation was either further inhibited using siRNAs targeting cell cycle genes or enhanced by supplementing exogenous growth factor, basic fibroblast growth factor (bFGF), at specific time intervals during adipogenesis. Expression knockdown of Cdk1 at the initiation of adipogenic induction resulted in significantly increased adipocytes, even though total number of cells was significantly reduced compared to siControl-treated cells. bFGF stimulated proliferation throughout adipogenic differentiation, but exerted differential effect on adipogenic outcome at different phases, promoting adipogenesis during mitotic phase (first 48 h), but significantly inhibiting adipogenesis during adipogenic commitment phase (days 3-6). Our results demonstrate that cellular proliferation is counteractive to adipogenic commitment in human adipogenesis. However, cellular proliferation stimulation can be beneficial for adipogenesis during the mitotic phase by increasing the population of cells capable of committing to adipocytes before adipogenic commitment.