Regulation of unfolded protein response in hematopoietic stem cells

Lithium impacts the function of hematopoietic stem cells via disturbing the endoplasmic reticulum stress and Hsp90 signaling

Lithium (Li) has been widely used in clinical therapy and new Li-ion battery industry. To date, the impact of Li on the development of immune cells is largely unknown. The aim of this study was to investigate the impact of Li on hematopoiesis. C57BL/6 (B6) mice were treated with 50 ppm LiCl, 200 ppm LiCl, or the control via drinking water for 3 months, and thereafter the hematopoiesis was evaluated. Treatment with Li increased the number of mature lymphoid cells while suppressing the number of mature myeloid cells in mice. In addition, a direct action of Li on hematopoietic stem cells (HSC) suppressed endoplasmic reticulum (ER) stress to reduce the proliferation of HSC in the bone marrow (BM), thus leading to fewer HSC in mice. On the other hand, the suppression of ER stress by Li exposure increased the expression of Hsp90, which promoted the potential of lymphopoiesis but did not impact that for myelopoiesis in HSC in the BM of mice. Moreover, in vitro treatment with Li also likely disturbed the ER stress-Hsp90 signaling, suppressed the proliferation, and increased the potential for lymphopoiesis in human HSC. Our study reveals a previously unrecognized toxicity of Li on HSC and may advance our understanding for the immunotoxicology of Li.

Haploinsufficiency of the essential gene Rps12 causes defects in erythropoiesis and hematopoietic stem cell maintenance

Ribosomal protein (Rp) gene haploinsufficiency can result in Diamond-Blackfan Anemia (DBA), characterized by defective erythropoiesis and skeletal defects. Some mouse Rp mutations recapitulate DBA phenotypes, although others lack erythropoietic or skeletal defects. We generated a conditional knockout mouse to partially delete Rps12. Homozygous Rps12 deletion resulted in embryonic lethality. Mice inheriting the Rps12KO/+ genotype had growth and morphological defects, pancytopenia, and impaired erythropoiesis. A striking reduction in hematopoietic stem cells (HSCs) and progenitors in the bone marrow (BM) was associated with decreased ability to repopulate the blood system after competitive and non-competitive BM transplantation. Rps12KO/+ lost HSC quiescence, experienced ERK and MTOR activation, and increased global translation in HSC and progenitors. Post-natal heterozygous deletion of Rps12 in hematopoietic cells using Tal1-Cre-ERT also resulted in pancytopenia with decreased HSC numbers. However, post-natal Cre-ERT induction led to reduced translation in HSCs and progenitors, suggesting that this is the most direct consequence of Rps12 haploinsufficiency in hematopoietic cells. Thus, RpS12 has a strong requirement in HSC function, in addition to erythropoiesis.

Regulation of adult stem cell quiescence and its functions in the maintenance of tissue integrity

Adult stem cells are important for mammalian tissues, where they act as a cell reserve that supports normal tissue turnover and can mount a regenerative response following acute injuries. Quiescent stem cells are well established in certain tissues, such as skeletal muscle, brain, and bone marrow. The quiescent state is actively controlled and is essential for long-term maintenance of stem cell pools. In this Review, we discuss the importance of maintaining a functional pool of quiescent adult stem cells, including haematopoietic stem cells, skeletal muscle stem cells, neural stem cells, hair follicle stem cells, and mesenchymal stem cells such as fibro-adipogenic progenitors, to ensure tissue maintenance and repair. We discuss the molecular mechanisms that regulate the entry into, maintenance of, and exit from the quiescent state in mice. Recent studies revealed that quiescent stem cells have a discordance between RNA and protein levels, indicating the importance of post-transcriptional mechanisms, such as alternative polyadenylation, alternative splicing, and translation repression, in the control of stem cell quiescence. Understanding how these mechanisms guide stem cell function during homeostasis and regeneration has important implications for regenerative medicine.

Understanding ER homeostasis and the UPR to enhance treatment efficacy of acute myeloid leukemia

Protein biogenesis, maturation and degradation are tightly regulated processes that are governed by a complex network of signaling pathways. The endoplasmic reticulum (ER) is responsible for biosynthesis and maturation of secretory proteins. Circumstances that alter cellular protein homeostasis, determine accumulation of misfolded and unfolded proteins in the ER, a condition defined as ER stress. In case of stress, the ER activates an adaptive response called unfolded protein response (UPR), a series of pathways of major relevance for cancer biology. The UPR plays a preeminent role in adaptation of tumor cells to the harsh conditions that they experience, due to high rates of proliferation, metabolic abnormalities and hostile environment scarce in oxygen and nutrients. Furthermore, the UPR is among the main adaptive cell stress responses contributing to the development of resistance to drugs and chemotherapy. Clinical management of Acute Myeloid Leukemia (AML) has improved significantly in the last decade, thanks to development of molecular targeted therapies. However, the emergence of treatment-resistant clones renders the rate of AML cure dismal. Moreover, different cell populations that constitute the bone marrow niche recently emerged as a main determinant leading to drug resistance. Herein we summarize the most relevant literature regarding the role played by the UPR in expansion of AML and ability to develop drug resistance and we discuss different possible modalities to overturn this adaptive response against leukemia. To this aim, we also describe the interconnection of the UPR with other cellular stress responses regulating protein homeostasis. Finally, we review the newest findings about the crosstalk between AML cells and cells of the bone marrow niche, under physiological conditions and in response to therapies, discussing in particular the importance of the niche in supporting survival of AML cells by favoring protein homeostasis.

Solute Carrier Nucleoside Transporters in Hematopoiesis and Hematological Drug Toxicities: A Perspective

Simple Summary

Anticancer nucleoside analogs are promising treatments that often result in damaging toxicities and therefore ineffective treatment. Mechanisms of this are not well-researched, but cellular nucleoside transport research in mice might provide additional insight given transport’s role in mammalian hematopoiesis. Cellular nucleoside transport is a notable component of mammalian hematopoiesis due to how mutations within it relate to hematological abnormities. This review encompasses nucleoside transporters, focusing on their inherent properties, hematopoietic role, and their interplay in nucleoside drug treatment side effects. We then propose potential mechanisms to explain nucleoside transport involvement in blood disorders. Finally, we point out and advocate for future research areas that would improve therapeutic outcomes for patients taking nucleoside analog therapies.

Abstract

Anticancer nucleoside analogs produce adverse, and at times, dose-limiting hematological toxicities that can compromise treatment efficacy, yet the mechanisms of such toxicities are poorly understood. Recently, cellular nucleoside transport has been implicated in normal blood cell formation with studies from nucleoside transporter-deficient mice providing additional insights into the regulation of mammalian hematopoiesis. Furthermore, several idiopathic human genetic disorders have revealed nucleoside transport as an important component of mammalian hematopoiesis because mutations in individual nucleoside transporter genes are linked to various hematological abnormalities, including anemia. Here, we review recent developments in nucleoside transporters, including their transport characteristics, their role in the regulation of hematopoiesis, and their potential involvement in the occurrence of adverse hematological side effects due to nucleoside drug treatment. Furthermore, we discuss the putative mechanisms by which aberrant nucleoside transport may contribute to hematological abnormalities and identify the knowledge gaps where future research may positively impact treatment outcomes for patients undergoing various nucleoside analog therapies.

Sirtuins at the Service of Healthy Longevity

Sirtuins may counteract at least six hallmarks of organismal aging: neurodegeneration, chronic but ineffective inflammatory response, metabolic syndrome, DNA damage, genome instability, and cancer incidence. Moreover, caloric restriction is believed to slow down aging by boosting the activity of some sirtuins through activating adenosine monophosphate-activated protein kinase (AMPK), thus raising the level of intracellular nicotinamide adenine dinucleotide (NAD⁺) by stimulating NAD⁺ biosynthesis. Sirtuins and their downstream effectors induce intracellular signaling pathways related to a moderate caloric restriction within cells, mitigating reactive oxygen species (ROS) production, cell senescence phenotype (CSP) induction, and apoptosis as forms of the cellular stress response. Instead, it can promote DNA damage repair and survival of cells with normal, completely functional phenotypes. In this review, we discuss mechanisms of sirtuins action toward cell-conserving phenotype associated with intracellular signaling pathways related to moderate caloric restriction, as well as some tissue-specific functions of sirtuins, especially in the central nervous system, heart muscle, skeletal muscles, liver, kidneys, white adipose tissue, hematopoietic system, and immune system. In this context, we discuss the possibility of new therapeutic approaches.

The unfolded protein response as regulator of cancer stemness and differentiation: Mechanisms and implications for cancer therapy

The unfolded protein response (UPR) is an adaptive mechanism that regulates protein and cellular homeostasis. Three endoplasmic reticulum (ER) membrane localized stress sensors, IRE1, PERK and ATF6, coordinate the UPR in order to maintain ER proteostasis and cell survival, or induce cell death when homeostasis cannot be restored. However, recent studies have identified alternative functions for the UPR in developmental biology processes and cell fate decisions under both normal and cancerous conditions. In cancer, increasing evidence points towards the involvement of the three UPR sensors in oncogenic reprogramming and the regulation of tumor cells endowed with stem cell properties, named cancer stem cells (CSCs), that are considered to be the most malignant cells in tumors. Here we review the reported roles and underlying molecular mechanisms of the three UPR sensors in regulating stemness and differentiation, particularly in solid tumor cells, processes that have a major impact on tumor aggressiveness. Mainly PERK and IRE1 branches of the UPR were found to regulate CSCs and tumor development and examples are provided for breast cancer, colon cancer and aggressive brain tumors, glioblastoma. Although the underlying mechanisms and interactions between the different UPR branches in regulating stemness in cancer need to be further elucidated, we propose that PERK and IRE1 targeted therapy could inhibit self-renewal of CSCs or induce differentiation that is predicted to have therapeutic benefit. For this, more specific UPR modulators need to be developed with favorable pharmacological properties that together with patient stratification will allow optimal evaluation in clinical studies.

Revisiting the question of nucleated versus enucleated erythrocytes in birds and mammals

Erythrocyte enucleation is thought to have evolved in mammals to support their energetic cost of high metabolic activities. However, birds face similar selection pressure yet possess nucleated erythrocytes. Current hypotheses on the mammalian erythrocyte enucleation claim that the absence of cell organelles allows erythrocytes to 1) pack more hemoglobin into the cells to increase oxygen carrying capacity and 2) decrease erythrocyte size for increased surface area-to-volume ratio, and improved ability to traverse small capillaries. In this article, we first empirically tested current hypotheses using both conventional and phylogenetically informed analysis comparing literature values of mean cell hemoglobin concentration (MCHC) and mean cell volume (MCV) between 181 avian and 194 mammalian species. We found no difference in MCHC levels between birds and mammals using both conventional and phylogenetically corrected analysis. MCV was higher in birds than mammals according to conventional analysis, but the difference was lost when we controlled for phylogeny. These results suggested that avian and mammalian erythrocytes may employ different strategies to solve a common problem. To further investigate existing hypotheses or develop new hypothesis, we need to understand the functions of various organelles in avian erythrocytes. Consequently, we covered potential physiological functions of various cell organelles in avian erythrocytes based on current knowledge, while making explicit comparisons to their mammalian counterparts. Finally, we proposed by taking an integrative and comparative approach, using tools from molecular biology to evolutionary biology, would allow us to better understand the fundamental physiological functions of various components of avian and mammalian erythrocytes.

Metabolic and Redox Regulation of Cardiovascular Stem Cell Biology and Pathology

Significance: Cardiovascular stem cells are important for regeneration and repair of damaged tissue. Recent Advances: Pluripotent stem cells have a unique metabolism, which is adopted for their energetic and biosynthetic demand as rapidly proliferating cells. Stem cell differentiation requires an exceptional metabolic flexibility allowing for metabolic remodeling between glycolysis and oxidative phosphorylation. Critical Issues: Respiration is associated with the generation of reactive oxygen species (ROS) by the mitochondrial respiratory chain. But also the membrane-bound protein nicotinamide adenine dinucleotide phosphate oxidase (NADPH oxidase, NOX) contributes to ROS levels. ROS not only play a significant role in stem cell differentiation and tissue renewal but also cause senescence and contribute to tissue aging. Future Directions: For utilization of stem cells in therapeutic approaches, a deep understanding of the molecular mechanisms how metabolism and the cellular redox state regulate stem cell differentiation is required. Modulating the redox state of stem cells using antioxidative agents may be suitable to enhance activity of endothelial progenitor cells. Antioxid. Redox Signal. 35, 163-181.

Dhx15 regulates zebrafish definitive hematopoiesis through the unfolded protein response pathway

Gene alterations are recognized as important events in acute myeloid leukemia (AML) progression. Studies on hematopoiesis of altered genes contribute to a better understanding on their roles in AML progression. Our previous work reported a DEAH box helicase 15 (DHX15) R222G mutation in AML patients, and we showed DHX15 overexpression is associated with poor prognosis in AML patients. In this work, we further study the role of dhx15 in zebrafish developmental hematopoiesis by generating dhx15^−/− zebrafish using transcription activator‐like effector nuclease technology. Whole‐mount in situ hybridization (WISH) analysis showed hematopoietic stem/progenitor cells were dramatically perturbed when dhx15 was deleted. Immunofluorescence staining indicated inhibited hematopoietic stem/progenitor cell (HSPC) proliferation instead of accelerated apoptosis were detected in dhx15^−/− zebrafish. Furthermore, our data showed that HSPC defect is mediated through the unfolded protein response (UPR) pathway. DHX15 R222G mutation, a recurrent mutation identified in AML patients, displayed a compromised function in restoring HSPC failure in dhx15^−/−; Tg (hsp: DHX15 R222G) zebrafish. Collectively, this work revealed a vital role of dhx15 in the maintenance of definitive hematopoiesis in zebrafish through the unfolded protein respone pathway. The study of DHX15 and DHX15 R222G mutation could hold clinical significance for evaluating prognosis of AML patients with aberrant DHX15 expression.

Induction of blood-circulating bile acids supports recovery from myelosuppressive chemotherapy

Chemotherapeutic agents can reduce bone marrow (BM) activity, causing myelosuppression, a common life-threatening complication of cancer treatment. It is challenging to predict the patients in whom prolonged myelosuppression will occur, resulting in a delay or discontinuation of the treatment protocol. An early indicator of recovery from myelosuppression would thus be highly beneficial in clinical settings. In this study, bile acids (BAs) were highly increased in the systemic circulation as a natural response during recovery from myelosuppression, supporting regeneration of BM cells. BA levels in the blood of pediatric cancer patients and mice treated with chemotherapeutic agents were increased, in synchrony with early proliferation of BM cells and recovery from myelosuppression. In a mouse model of altered BA composition, Cyp8b1 knockout mice, a subset of mice recovered poorly after chemotherapy. The poor recovery correlated with low levels and changes in composition of BAs in the liver and systemic circulation. Conversely, BA supplementation in chemotherapy-treated wild-type mice resulted in significantly improved recovery. The results suggest that part of the mechanism by which BAs support recovery is the suppression of endoplasmic reticulum stress pathways in expanding and recovering hematopoietic cells. The findings propose a novel role of BAs as early markers of recovery and active components of the recovery process after chemotherapy.

Distinct effects of chondroitin sulfate on hematopoietic cells and the stromal microenvironment in bone marrow hematopoiesis

The bone marrow (BM) microenvironment, known as the BM niche, regulates hematopoiesis but is also affected by interactions with hematopoietic cells. Recent evidence indicates that extracellular matrix components are involved in these interactions. Chondroitin sulfate (CS), a glycosaminoglycan, is a major component of the extracellular matrix; however, it is not known whether CS has a physiological role in hematopoiesis. Here, we analyzed the functions of CS in hematopoietic and niche cells. CSGalNAcT1, which encodes CS N-acetylgalactosaminyltransferase-1 (T1), a key enzyme in CS biosynthesis, was highly expressed in hematopoietic stem and progenitor cells (HSPCs) and endothelial cells (ECs), but not in mesenchymal stromal cells (MSCs) in BM. In T1 knockout (T1KO) mice, a greater number of HSPCs existed compared with the wild-type (WT), but HSPCs from T1KO mice showed significantly impaired repopulation in WT recipient mice on serial transplantation. RNA sequence analysis revealed the activation of IFN-α/β signaling and endoplasmic reticulum stress in T1KO HSPCs. In contrast, the number of WT HSPCs repopulated in T1KO recipient mice was larger than that in WT recipient mice after serial transplantation, indicating that the T1KO niche supports repopulation of HSPCs better than the WT niche. There was no obvious difference in the distribution of vasculature and MSCs between WT and T1KO BM, suggesting that CS loss alters vascular niche functions without affecting its structure. Our results revealed distinct roles of CS in hematopoietic cells and BM niche, indicating that crosstalk between these components is important to maintain homeostasis in BM.

Taurine represses age-associated gut hyperplasia in Drosophila via counteracting endoplasmic reticulum stress

As they age, adult stem cells become more prone to functional decline, which is responsible for aging‐associated tissue degeneration and diseases. One goal of aging research is to identify drugs that can repair age‐associated tissue degeneration. Multiple organ development‐related signaling pathways have recently been demonstrated to have functions in tissue homeostasis and aging process. Therefore, in this study, we tested several chemicals that are essential for organ development to assess their ability to delay intestinal stem cell (ISC) aging and promote gut function in adult Drosophila. We found that taurine, a free amino acid that supports neurological development and tissue metabolism in humans, represses ISC hyperproliferation and restrains the intestinal functional decline seen in aged animals. We found that taurine represses age‐associated ISC hyperproliferation through a mechanism that eliminated endoplasmic reticulum (ER) stress by upregulation of the target genes of unfolded protein response in the ER (UPR^ER) and inhibiting the c‐Jun N‐terminal kinase (JNK) signaling. Our findings show that taurine plays a critical role in delaying the aging process in stem cells and suggest that it may be used as a natural compound for the treatment of age‐associated, or damage‐induced intestinal dysfunction in humans.

The complex interplay between endoplasmic reticulum stress and the NLRP3 inflammasome: a potential therapeutic target for inflammatory disorders

Inflammation is the result of a complex network of cellular and molecular interactions and mechanisms that facilitate immune protection against intrinsic and extrinsic stimuli, particularly pathogens, to maintain homeostasis and promote tissue healing. However, dysregulation in the immune system elicits excess/abnormal inflammation resulting in unintended tissue damage and causes major inflammatory diseases including asthma, chronic obstructive pulmonary disease, atherosclerosis, inflammatory bowel diseases, sarcoidosis and rheumatoid arthritis. It is now widely accepted that both endoplasmic reticulum (ER) stress and inflammasomes play critical roles in activating inflammatory signalling cascades. Notably, evidence is mounting for the involvement of ER stress in exacerbating inflammasome-induced inflammatory cascades, which may provide a new axis for therapeutic targeting in a range of inflammatory disorders. Here, we comprehensively review the roles, mechanisms and interactions of both ER stress and inflammasomes, as well as their interconnected relationships in inflammatory signalling cascades. We also discuss novel therapeutic strategies that are being developed to treat ER stress- and inflammasome-related inflammatory disorders.

ER Stress and Unfolded Protein Response in Leukemia: Friend, Foe, or Both?

The unfolded protein response (UPR) is an evolutionarily conserved adaptive signaling pathway triggered by a stress of the endoplasmic reticulum (ER) lumen compartment, which is initiated by the accumulation of unfolded proteins. This response, mediated by three sensors-Inositol Requiring Enzyme 1 (IRE1), Activating Transcription Factor 6 (ATF6), and Protein Kinase RNA-Like Endoplasmic Reticulum Kinase (PERK)—allows restoring protein homeostasis and maintaining cell survival. UPR represents a major cytoprotective signaling network for cancer cells, which frequently experience disturbed proteostasis owing to their rapid proliferation in an usually unfavorable microenvironment. Increased basal UPR also participates in the resistance of tumor cells against chemotherapy. UPR activation also occurs during hematopoiesis, and growing evidence supports the critical cytoprotective role played by ER stress in the emergence and proliferation of leukemic cells. In case of severe or prolonged stress, pro-survival UPR may however evolve into a cell death program called terminal UPR. Interestingly, a large number of studies have revealed that the induction of proapoptotic UPR can also strongly contribute to the sensitization of leukemic cells to chemotherapy. Here, we review the current knowledge on the consequences of the deregulation of UPR signaling in leukemias and their implications for the treatment of these diseases.

FANCD2 modulates the mitochondrial stress response to prevent common fragile site instability

Common fragile sites (CFSs) are genomic regions frequently involved in cancer-associated rearrangements. Most CFSs lie within large genes, and their instability involves transcription- and replication-dependent mechanisms. Here, we uncover a role for the mitochondrial stress response pathway in the regulation of CFS stability in human cells. We show that FANCD2, a master regulator of CFS stability, dampens the activation of the mitochondrial stress response and prevents mitochondrial dysfunction. Genetic or pharmacological activation of mitochondrial stress signaling induces CFS gene expression and concomitant relocalization to CFSs of FANCD2. FANCD2 attenuates CFS gene transcription and promotes CFS gene stability. Mechanistically, we demonstrate that the mitochondrial stress-dependent induction of CFS genes is mediated by ubiquitin-like protein 5 (UBL5), and that a UBL5-FANCD2 dependent axis regulates the mitochondrial UPR in human cells. We propose that FANCD2 coordinates nuclear and mitochondrial activities to prevent genome instability.

Protein quality control of cell stemness

Protein quality control (PQC) systems play essential roles in the recognition, refolding and clearance of aberrant proteins, thus ensuring cellular protein homeostasis, or proteostasis. Especially, continued proliferation and differentiation of stem cells require a high rate of translation; therefore, accurate PQC systems are essential to maintain stem cell function. Growing evidence suggested crucial roles of PQC systems in regulating the stemness and differentiation of stem cells. This review focuses on current knowledge regarding the components of the proteostasis network in stem cells, and the importance of proteostasis in maintaining stem cell identity and regenerative functions. A complete understanding of this process might uncover potential applications in aging intervention and aging-related diseases.

Evaluating endoplasmic reticulum stress and unfolded protein response through the lens of ecology and evolution

Considerable progress has been made in understanding the physiological basis for variation in the life-history patterns of animals, particularly with regard to the roles of oxidative stress and hormonal regulation. However, an underappreciated and understudied area that could play a role in mediating inter- and intraspecific variation of life history is endoplasmic reticulum (ER) stress, and the resulting unfolded protein response (UPR^ER ). ER stress response and the UPR^ER maintain proteostasis in cells by reducing the intracellular load of secretory proteins and enhancing protein folding capacity or initiating apoptosis in cells that cannot recover. Proper modulation of the ER stress response and execution of the UPR^ER allow animals to respond to intracellular and extracellular stressors and adapt to constantly changing environments. ER stress responses are heritable and there is considerable individual variation in UPR^ER phenotype in animals, suggesting that ER stress and UPR^ER phenotype can be subjected to natural selection. The variation in UPR^ER phenotype presumably reflects the way animals respond to ER stress and environmental challenges. Most of what we know about ER stress and the UPR^ER in animals has either come from biomedical studies using cell culture or from experiments involving conventional laboratory or agriculturally important models that exhibit limited genetic diversity. Furthermore, these studies involve the assessment of experimentally induced qualitative changes in gene expression as opposed to the quantitative variations that occur in naturally existing populations. Almost all of these studies were conducted in controlled settings that are often quite different from the conditions animals experience in nature. Herein, we review studies that investigated ER stress and the UPR^ER in relation to key life-history traits including growth and development, reproduction, bioenergetics and physical performance, and ageing and senescence. We then ask if these studies can inform us about the role of ER stress and the UPR^ER in mediating the aforementioned life-history traits in free-living animals. We propose that there is a need to conduct experiments pertaining to ER stress and the UPR^ER in ecologically relevant settings, to characterize variation in ER stress and the UPR^ER in free-living animals, and to relate the observed variation to key life-history traits. We urge others to integrate multiple physiological systems and investigate how interactions between ER stress and oxidative stress shape life-history trade-offs in free-living animals.

Unfolded Protein Response in Leukemia: From Basic Understanding to Therapeutic Opportunities

Understanding genetic and epigenetic changes that underlie abnormal proliferation of hematopoietic stem and progenitor cells is critical for development of new approaches to monitor and treat leukemia. The unfolded protein response (UPR) is a conserved adaptive signaling pathway that governs protein folding, secretion, and energy production and serves to maintain protein homeostasis in various cellular compartments. Deregulated UPR signaling, which often occurs in hematopoietic stem cells and leukemia, defines the degree of cellular toxicity and perturbs protein homeostasis, and at the same time, offers a novel therapeutic target. Here, we review current knowledge related to altered UPR signaling in leukemia and highlight possible strategies for exploiting the UPR as treatment for this disease.

Divide and conquer: two stem cell populations in squamous epithelia, reserves and the active duty forces

Stem cells are of great interest to the scientific community due to their potential role in regenerative and rejuvenative medicine. However, their role in the aging process and carcinogenesis remains unclear. Because DNA replication in stem cells may contribute to the background mutation rate and thereby to cancer, reducing proliferation and establishing a relatively quiescent stem cell compartment has been hypothesized to limit DNA replication-associated mutagenesis. On the other hand, as the main function of stem cells is to provide daughter cells to build and maintain tissues, the idea of a quiescent stem cell compartment appears counterintuitive. Intriguing observations in mice have led to the idea of separated stem cell compartments that consist of cells with different proliferative activity. Some epithelia of short-lived rodents appear to lack quiescent stem cells. Comparing stem cells of different species and different organs (comparative stem cell biology) may allow us to elucidate the evolutionary pressures such as the balance between cancer and longevity that govern stem cell biology (evolutionary stem cell biology). The oral mucosa and its stem cells are an exciting model system to explore the characteristics of quiescent stem cells that have eluded biologists for decades.

Quiescence Entry, Maintenance, and Exit in Adult Stem Cells

Cells of unicellular and multicellular eukaryotes can respond to certain environmental cues by arresting the cell cycle and entering a reversible state of quiescence. Quiescent cells do not divide, but can re-enter the cell cycle and resume proliferation if exposed to some signals from the environment. Quiescent cells in mammals and humans include adult stem cells. These cells exhibit improved stress resistance and enhanced survival ability. In response to certain extrinsic signals, adult stem cells can self-renew by dividing asymmetrically. Such asymmetric divisions not only allow the maintenance of a population of quiescent cells, but also yield daughter progenitor cells. A multistep process of the controlled proliferation of these progenitor cells leads to the formation of one or more types of fully differentiated cells. An age-related decline in the ability of adult stem cells to balance quiescence maintenance and regulated proliferation has been implicated in many aging-associated diseases. In this review, we describe many traits shared by different types of quiescent adult stem cells. We discuss how these traits contribute to the quiescence, self-renewal, and proliferation of adult stem cells. We examine the cell-intrinsic mechanisms that allow establishing and sustaining the characteristic traits of adult stem cells, thereby regulating quiescence entry, maintenance, and exit.

Guest editorial: Regulatory signaling in normal and abnormal hematopoiesis

Stem cells are characterized by their unique ability to both self-renew and differentiate along multiple cellular lineages. Self-renewal and differentiation must be tightly controlled to ensure an appropriate stem cell pool in tissue over the lifetime of an organism. Elucidating the mechanisms controlling stem cell fate and maintenance remains a key challenge in stem cell biology. Hematopoietic stem cells (HSCs) are responsible for the lifelong production of multiple blood cell lineages. To remain functional, these cells must interact with a particular microenvironment, known as the stem cell niche. HSC niches provide various factors, including cytokines, extracellular matrices, nutrients, hormones, and metabolites. These niche factors modulate cell-intrinsic molecular regulatory networks in HSCs. Niche signals also play crucial roles in the induction of HSCs from pluripotent stem cells or vascular endothelial cells. The Progress in Hematology review series in the current issue highlights some critical regulators of HSC maintenance and production.