Hes1 deficiency causes hematopoietic stem cell exhaustion

Crosstalk between DNA Damage Repair and Metabolic Regulation in Hematopoietic Stem Cells

Self-renewal and differentiation are two characteristics of hematopoietic stem cells (HSCs). Under steady physiological conditions, most primitive HSCs remain quiescent in the bone marrow (BM). They respond to different stimuli to refresh the blood system. The transition from quiescence to activation is accompanied by major changes in metabolism, a fundamental cellular process in living organisms that produces or consumes energy. Cellular metabolism is now considered to be a key regulator of HSC maintenance. Interestingly, HSCs possess a distinct metabolic profile with a preference for glycolysis rather than oxidative phosphorylation (OXPHOS) for energy production. Byproducts from the cellular metabolism can also damage DNA. To counteract such insults, mammalian cells have evolved a complex and efficient DNA damage repair (DDR) system to eliminate various DNA lesions and guard genomic stability. Given the enormous regenerative potential coupled with the lifetime persistence of HSCs, tight control of HSC genome stability is essential. The intersection of DDR and the HSC metabolism has recently emerged as an area of intense research interest, unraveling the profound connections between genomic stability and cellular energetics. In this brief review, we delve into the interplay between DDR deficiency and the metabolic reprogramming of HSCs, shedding light on the dynamic relationship that governs the fate and functionality of these remarkable stem cells. Understanding the crosstalk between DDR and the cellular metabolism will open a new avenue of research designed to target these interacting pathways for improving HSC function and treating hematologic disorders.

Fosl2 Deficiency Predisposes Mice to Osteopetrosis, Leading to Bone Marrow Failure

Arthritis causes Fos-like 2 (Fosl2) inactivation, and various immune cells contribute to its pathogenesis. However, little is known about the role of Fosl2 in hematopoiesis and the possible pathological role of Fosl2 inactivation in the hematopoietic system in arthritis. In this study, we show that Fosl2 maintains hematopoietic stem cell (HSC) quiescence and differentiation while controlling the inflammatory response via macrophages. Fosl2-specific deletion in the hematopoietic system caused the expansion of HSCs and myeloid cell growth while affecting erythroid and B cell differentiation. Fosl2 inactivation enhanced macrophage M1 polarization and stimulated proinflammatory cytokines and myeloid growth factors, skewing HSCs toward myeloid cell differentiation, similar to hematopoietic alterations in arthritic mice. Loss of Fosl2 mediated by Vav-iCre also displays an unexpected deletion in embryonic erythro-myeloid progenitor-derived osteoclasts, leading to osteopetrosis and anemia. The reduced bone marrow cellularity in Vav-iCreFosl2f/f mice is a consequence of the reduced bone marrow space in osteopetrotic mice rather than a direct role of Fosl2 in hematopoiesis. Thus, Fosl2 is indispensable for erythro-myeloid progenitor-derived osteoclasts to maintain the medullary cavity to ensure normal hematopoiesis. These findings improve our understanding of the pathogenesis of bone-destructive diseases and provide important implications for developing therapeutic approaches for these diseases.

Lin- PU.1dim GATA-1- defines haematopoietic stem cells with long-term multilineage reconstitution activity

Despite extensive characterization of the state and function of haematopoietic stem cells (HSCs), the use of transcription factors to define the HSC population is still limited. We show here that the HSC population in mouse bone marrow can be defined by the distinct expression levels of Spi1 and Gata1. By using a double fluorescence knock-in mouse model, PGdKI, in which the expression levels of PU.1 and GATA-1 are indicated by the expression of GFP and mCherry, respectively, we uncover that the HSCs with lymphoid and myeloid repopulating activity are specifically enriched in a Lin- PU.1dim GATA-1- (LPG) cell subset. In vivo competitive repopulation assays demonstrate that bone marrow cells gated by LPG exhibit haematopoietic reconstitution activity which is comparable to that of classical Lin- Sca1+ c-kit+ (LSK). The integrated analysis of single-cell RNA sequence data from LPG and LSK-gated cells reveals that a transcriptional network governed by core TFs contributes to regulation of HSC multipotency. These discoveries provide new clues for HSC characterization and functional study.

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.

Regulation of myeloid and lymphoid cell development by O-glycans on Notch

Notch signaling via NOTCH1 stimulated by Delta-like ligand 4 (DLL4) is required for the development of T cells in thymus, and NOTCH2 stimulated by Notch ligand DLL1 is required for the development of marginal zone (MZ) B cells in spleen. Notch signaling also regulates myeloid cell production in bone marrow and is an essential contributor to the generation of early hematopoietic stem cells (HSC). The differentiation program in each of these cellular contexts is optimized by the regulation of Notch signaling strength by O-glycans attached to epidermal growth factor-like (EGF) repeats in the extracellular domain of Notch receptors. There are three major types of O-glycan on NOTCH1 and NOTCH2 - O-fucose, O-glucose and O-GlcNAc. The initiating sugar of each O-glycan is added in the endoplasmic reticulum (ER) by glycosyltransferases POFUT1 (fucose), POGLUT1/2/3 (glucose) or EOGT (GlcNAc), respectively. Additional sugars are added in the Golgi compartment during passage through the secretory pathway to the plasma membrane. Of particular significance for Notch signaling is the addition of GlcNAc to O-fucose on an EGF repeat by the Fringe GlcNAc-transferases LFNG, MFNG or RFNG. Canonical Notch ligands (DLL1, DLL4, JAG1, JAG2) expressed in stromal cells bind to the extracellular domain of Notch receptors expressed in hematopoietic stem cells and myeloid and lymphoid progenitors to activate Notch signaling. Ligand-receptor binding is differentially regulated by the O-glycans on Notch. This review will summarize our understanding of the regulation of Notch signaling in myeloid and lymphoid cell development by specific O-glycans in mice with dysregulated expression of a particular glycosyltransferase and discuss how this may impact immune system development and malignancy in general, and in individuals with a congenital defect in the synthesis of the O-glycans attached to EGF repeats.

Jwa participates the maintenance of intestinal epithelial homeostasis via ERK/FBXW7-mediated NOTCH1/PPARγ/STAT5 axis and acts as a novel putative aging related gene

The intestinal epithelium is a rapid self-renewal and regenerated tissue of which the structural integrity is beneficial for maintaining health. The integrity of intestinal epithelium depends on the balance of cell proliferation, differentiation, migration, and the function of intestinal stem cells, which declines due to genetic defect or aging. Jwa participates in multiple cellular processes; it also responds to oxidative stress and repairs DNA damage. However, whether Jwa plays a role in maintaining the homeostasis of intestinal renewal and regeneration is not clear. In the present study, we firstly described that the deletion of Jwa disturbed the homeostasis of intestinal epithelial renewal and regeneration. Jwa deficiency promoted NOTCH1 degradation in the ERK/FBXW7-mediated ubiquitin-proteasome pathway, thus disturbing the PPARγ/STAT5 axis. These mechanisms might partially contribute to the reduction of intestinal stem cell function and alteration of intestinal epithelial cell lineage distribution, finally suppressing the renewal and regeneration of intestinal epithelium. Moreover, our results also revealed that Jwa was a novel putative aging related gene.

Dysregulation of SIRT3 SUMOylation Confers AML Chemoresistance via Controlling HES1-Dependent Fatty Acid Oxidation

Sirtuin 3 (SIRT3) deacetylase is a key regulator for chemoresistance in acute myeloid leukemia (AML) cells due to its capability of modulating mitochondrial metabolism and reactive oxygen species (ROS). SIRT3 is de-SUMOylated by SUMO-specific peptidase 1 (SENP1), which enhances its deacetylase activity. Therefore, dysregulation of SIRT3 SUMOylation may lead to fortified chemoresistance in AML. Indeed, SIRT3 de-SUMOylation was induced by chemotherapeutic agents, which in turn, exacerbated resistance against chemotherapies in AML by activating SIRT3 via preventing its proteasome degradation. Furthermore, RNA-seq revealed that expression of a collection of genes was altered by SIRT3 de-SUMOylation including inhibition of transcription factor Hes Family BHLH Transcription Factor 1 (HES1), a downstream substrate of Notch1 signaling pathway, leading to increased fatty acids oxidation (FAO). Moreover, the SENP1 inhibitor momordin-Ic or HES1 overexpression synergized with cytarabine to eradicate AML cells in vitro and in xenograft mouse models. In summary, the current study revealed a novel role of SIRT3 SUMOylation in the regulation of chemoresistance in AML via HES1-dependent FAO and provided a rationale for SIRT3 SUMOylation and FAO targeted interventions to improve chemotherapies in AML.

PGC1 plays a pivotal role in renal fibrosis via regulation of fatty acid metabolism in renal tissue

Renal fibrosis is a common and irreversible pathological feature of end-stage renal disease caused by multiple etiologies. The role of inflammation in renal fibrosis tissue has been generally accepted. The latest view is that fatty acid metabolism disorder contributes to renal fibrosis. peroxisome proliferator activated receptor-gamma coactivator 1α (PGC1α) plays a key role in fatty acid metabolism, regulating fatty acid uptake and oxidized protein synthesis, preventing the accumulation of lipid in the cytoplasm, and maintaining a dynamic balanced state of intracellular lipid. In multiple animal models of renal fibrosis caused by acute or chronic kidney disease, or even age-related kidney disease, almost all of the kidney specimens show the down-regulation of PGC1α. Upregulation of PGC1α can reduce the degree of renal fibrosis in animal models, and PGC1α knockout animals exhibit severe renal fibrosis. Studies have demonstrated that AMP-activated protein kinase (AMPK), MAPK, Notch, tumor necrosis factor-like weak inducer of apoptosis (TWEAK), epidermal growth factor receptor (EGFR), non-coding RNA (ncRNAs), liver kinase B1 (LKB1), hairy and enhancer of split 1 (Hes1), and other pathways regulate the expression of PGC1α and affect fatty acid metabolism. But some of these pathways interact with each other, and the effect of the integrated pathway on renal fibrosis is not clear.

Hematopoietic stem cell regeneration through paracrine regulation of the Wnt5a/Prox1 signaling axis

The crosstalk between the BM microenvironment (niche) and hematopoietic stem cells (HSCs) is critical for HSC regeneration. Here, we show that in mice, deletion of the Fanconi anemia (FA) genes Fanca and Fancc dampened HSC regeneration through direct effects on HSCs and indirect effects on BM niche cells. FA HSCs showed persistent upregulation of the Wnt target Prox1 in response to total body irradiation (TBI). Accordingly, lineage-specific deletion of Prox1 improved long-term repopulation of the irradiated FA HSCs. Forced expression of Prox1 in WT HSCs mimicked the defective repopulation phenotype of FA HSCs. WT mice but not FA mice showed significant induction by TBI of BM stromal Wnt5a protein. Mechanistically, FA proteins regulated stromal Wnt5a expression, possibly through modulating the Wnt5a transcription activator Pax2. Wnt5a treatment of irradiated FA mice enhanced HSC regeneration. Conversely, Wnt5a neutralization inhibited HSC regeneration after TBI. Wnt5a secreted by LepR⁺CXCL12⁺ BM stromal cells inhibited β-catenin accumulation, thereby repressing Prox1 transcription in irradiated HSCs. The detrimental effect of deregulated Wnt5a/Prox1 signaling on HSC regeneration was also observed in patients with FA and aged mice. Irradiation induced upregulation of Prox1 in the HSCs of aged mice, and deletion of Prox1 in aged HSCs improved HSC regeneration. Treatment of aged mice with Wnt5a enhanced hematopoietic repopulation. Collectively, these findings identified the paracrine Wnt5a/Prox1 signaling axis as a regulator of HSC regeneration under conditions of injury and aging.

Metabolic Regulation of Stem Cells in Aging

Purpose of Review

From invertebrates to vertebrates, the ability to sense nutrient availability is critical for survival. Complex organisms have evolved numerous signaling pathways to sense nutrients and dietary fluctuations, which influence many cellular processes. Although both overabundance and extreme depletion of nutrients can lead to deleterious effects, dietary restriction without malnutrition can increase lifespan and promote overall health in many model organisms. In this review, we focus on age-dependent changes in stem cell metabolism and dietary interventions used to modulate stem cell function in aging.

Recent Findings

Over the last half-century, seminal studies have illustrated that dietary restriction confers beneficial effects on longevity in many model organisms. Many researchers have now turned to dissecting the molecular mechanisms by which these diets affect aging at the cellular level. One subpopulation of cells of particular interest are adult stem cells, the most regenerative cells of the body. It is generally accepted that the regenerative capacity of stem cells declines with age, and while the metabolic requirements of each vary across tissues, the ability of dietary interventions to influence stem cell function is striking.

Summary

In this review, we will focus primarily on how metabolism plays a role in adult stem cell homeostasis with respect to aging, with particular emphasis on intestinal stem cells while also touching on hematopoietic, skeletal muscle, and neural stem cells. We will also discuss key metabolic signaling pathways influenced by both dietary restriction and the aging process, and will examine their role in improving tissue homeostasis and lifespan. Understanding the mechanisms behind the metabolic needs of stem cells will help bridge the divide between a basic science interpretation of stem cell function and a whole-organism view of nutrition, thereby providing insight into potential dietary or therapeutic interventions.

Pharmacological Inhibition of p38 MAPK Rejuvenates Bone Marrow Derived-Mesenchymal Stromal Cells and Boosts their Hematopoietic Stem Cell-Supportive Ability

The therapeutic value of mesenchymal stromal cells (MSCs) for various regenerative medicine applications, including hematopoietic stem cell transplantations (HSCT), has been well-established. Owing to their small numbers in vivo, it becomes necessary to expand them in vitro, which leads to a gradual loss of their regenerative capacity. Stress-induced mitogen-activated protein kinase p38 (p38 MAPK) signaling has been shown to compromise the MSC functions. Therefore, we investigated whether pharmacological inhibition of p38 MAPK signaling rejuvenates the cultured MSCs and boosts their functionality. Indeed, we found that the ex vivo expanded MSCs show activated p38 MAPK signaling and exhibit increased oxidative stress. These MSCs show a decreased ability to secrete salutary niche factors, thereby compromising their ability to support hematopoietic stem cell (HSC) self-renewal, proliferation, and differentiation. We, therefore, attempted to rejuvenate the cultured MSCs by pharmacological inhibition of p38 MAPK - a strategy broadly known as "priming of MSCs". We demonstrate that priming of MSCs with a p-38 MAPK inhibitor, PD169316, boosts their niche-supportive functions via upregulation of various HSC-supportive transcription factors. These primed MSCs expand multipotent HSCs having superior homing and long-term reconstitution ability. These findings shed light on the significance of non-cell-autonomous mechanisms operative in the hematopoietic niche and point towards the possible use of pharmacological compounds for rejuvenation of ex vivo cultured MSCs. Such approaches could improve the outcome of regenerative therapies involving in vitro cultured MSCs.

Co-culture model of B-cell acute lymphoblastic leukemia recapitulates a transcription signature of chemotherapy-refractory minimal residual disease

B-cell acute lymphoblastic leukemia (ALL) is characterized by accumulation of immature hematopoietic cells in the bone marrow, a well-established sanctuary site for leukemic cell survival during treatment. While standard of care treatment results in remission in most patients, a small population of patients will relapse, due to the presence of minimal residual disease (MRD) consisting of dormant, chemotherapy-resistant tumor cells. To interrogate this clinically relevant population of treatment refractory cells, we developed an in vitro cell model in which human ALL cells are grown in co-culture with human derived bone marrow stromal cells or osteoblasts. Within this co-culture, tumor cells are found in suspension, lightly attached to the top of the adherent cells, or buried under the adherent cells in a population that is phase dim (PD) by light microscopy. PD cells are dormant and chemotherapy-resistant, consistent with the population of cells that underlies MRD. In the current study, we characterized the transcriptional signature of PD cells by RNA-Seq, and these data were compared to a published expression data set derived from human MRD B-cell ALL patients. Our comparative analyses revealed that the PD cell population is markedly similar to the MRD expression patterns from the primary cells isolated from patients. We further identified genes and key signaling pathways that are common between the PD tumor cells from co-culture and patient derived MRD cells as potential therapeutic targets for future studies.

Lipid metabolism in focus: how the build-up and breakdown of lipids affects stem cells

Cellular metabolism has recently emerged as a key regulator of stem cell behavior. Various studies have suggested that metabolic regulatory mechanisms are conserved in different stem cell niches, suggesting a common level of stem cell regulation across tissues. Although the balance between glycolysis and oxidative phosphorylation has been shown to be distinct in stem cells and their differentiated progeny, much less is known about lipid metabolism in stem cell regulation. In this Review, we focus on how stem cells are affected by two major lipid metabolic pathways: the build-up of lipids, called de novo lipogenesis, and the breakdown of lipids, called fatty acid beta-oxidation. We cover the recent literature on hematopoietic stem cells, intestinal stem cells, neural stem/progenitor cells and cancer stem cells, where these two lipid pathways have been studied in more depth.

FANCD2 and HES1 suppress inflammation-induced PPARɣ to prevent haematopoietic stem cell exhaustion

The Fanconi anaemia protein FANCD2 suppresses PPARƔ to maintain haematopoietic stem cell's (HSC) function; however, the underlying mechanism is not known. Here we show that FANCD2 acts in concert with the Notch target HES1 to suppress inflammation-induced PPARƔ in HSC maintenance. Loss of HES1 exacerbates FANCD2-KO HSC defects. However, deletion of HES1 does not cause more severe inflammation-mediated HSC defects in FANCD2-KO mice, indicating that both FANCD2 and HES1 are required for limiting detrimental effects of inflammation on HSCs. Further analysis shows that both FANCD2 and HES1 are required for transcriptional repression of inflammation-activated PPARg promoter. Inflammation orchestrates an overlapping transcriptional programme in HSPCs deficient for FANCD2 and HES1, featuring upregulation of genes in fatty acid oxidation (FAO) and oxidative phosphorylation. Loss of FANCD2 or HES1 augments both basal and inflammation-primed FAO. Targeted inhibition of PPARƔ or the mitochondrial carnitine palmitoyltransferase-1 (CPT1) reduces FAO and ameliorates HSC defects in inflammation-primed HSPCs deleted for FANCD2 or HES1 or both. Finally, depletion of PPARg or CPT1 restores quiescence in these mutant HSCs under inflammatory stress. Our results suggest that this novel FANCD2/HES1/PPARƔ axis may constitute a key component of immunometabolic regulation, connecting inflammation, cellular metabolism and HSC function.

Persistent DNA damage-induced NLRP12 improves hematopoietic stem cell function

NOD-like receptor 12 (NLRP12) is a member of the nucleotide-binding domain and leucine-rich repeat containing receptor inflammasome family that plays a central role in innate immunity. We previously showed that DNA damage upregulated NLRP12 in hematopoietic stem cells (HSCs) of mice deficient in the DNA repair gene Fanca. However, the role of NLRP12 in HSC maintenance is not known. Here, we show that persistent DNA damage-induced NLRP12 improves HSC function in both mouse and human models of DNA repair deficiency and aging. Specifically, treatment of Fanca-/- mice with the DNA cross-linker mitomycin C or ionizing radiation induces NLRP12 upregulation in phenotypic HSCs. NLRP12 expression is specifically induced by persistent DNA damage. Functionally, knockdown of NLRP12 exacerbates the repopulation defect of Fanca-/- HSCs. Persistent DNA damage-induced NLRP12 was also observed in the HSCs from aged mice, and depletion of NLRP12 in these aged HSCs compromised their self-renewal and hematopoietic recovery. Consistently, overexpression of NLRP12 substantially improved the long-term repopulating function of Fanca-/- and aged HSCs. Finally, persistent DNA damage-induced NLRP12 maintains the function of HSCs from patients with FA or aged donors. These results reveal a potentially novel role of NLRP12 in HSC maintenance and suggest that NLRP12 targeting has therapeutic potential in DNA repair disorders and aging.

Hes1 deficiency causes hematopoietic stem cell exhaustion

The transcriptional repressor Hairy Enhancer of Split 1 (HES1) plays an essential role in the development of many organs by promoting the maintenance of stem/progenitor cells, controlling the reversibility of cellular quiescence, and regulating both cell fate decisions. Deletion of Hes1 in mice results in severe defects in multiple organs and is lethal in late embryogenesis. Here we have investigated the role of HES1 in hematopoiesis using a hematopoietic lineage‐specific Hes1 knockout mouse model. We found that while Hes1 is dispensable for steady‐state hematopoiesis, Hes1‐deficient hematopoietic stem cells (HSCs) undergo exhaustion under replicative stress. Loss of Hes1 upregulates the expression of genes involved in PPARγ signaling and fatty acid metabolism pathways, and augments fatty acid oxidation (FAO) in Hes1^f/fVav1Cre HSCs and progenitors. Functionally, PPARγ targeting or FAO inhibition ameliorates the repopulating defects of Hes1^f/fVav1Cre HSCs through improving quiescence in HSCs. Lastly, transcriptome analysis reveals that disruption of Hes1 in hematopoietic lineage alters expression of genes critical to HSC function, PPARγ signaling, and fatty acid metabolism. Together, our findings identify a novel role of HES1 in regulating stress hematopoiesis and provide mechanistic insight into the function of HES1 in HSC maintenance.