Hematopoietic Stem Cell Death Research Articles

INOS regulates hematopoietic stem and progenitor cells via mitochondrial signaling and is critical for bone marrow regeneration

Hematopoietic stem cells (HSCs) replenish blood cells under steady state and on demand, that exhibit therapeutic potential for Bone marrow failures and leukemia. Redox signaling plays key role in immune cells and hematopoiesis. However, the role of reactive nitrogen species in hematopoiesis remains unclear and requires further investigation. We investigated the significance of inducible nitric oxide synthase/nitric oxide (iNOS/NO) signaling in hematopoietic stem and progenitor cells (HSPCs) and hematopoiesis under steady-state and stress conditions. HSCs contain low levels of NO and iNOS under normal conditions, but these increase upon bone marrow stress. iNOS-deficient mice showed subtle changes in peripheral blood cells but significant alterations in HSPCs, including increased HSCs and multipotent progenitors. Surprisingly, iNOS-deficient mice displayed heightened susceptibility and delayed recovery of blood progeny following 5-Fluorouracil (5-FU) induced hematopoietic stress. Loss of quiescence and increased mitochondrial stress, indicated by elevated MitoSOX and MMPhi HSCs, were observed in iNOS-deficient mice. Furthermore, pharmacological approaches to mitigate mitochondrial stress rescued 5-FU-induced HSC death. Conversely, iNOS-NO signaling was required for demand-driven mitochondrial activity and proliferation during hematopoietic recovery, as iNOS-deficient mice and NO signaling inhibitors exhibit reduced mitochondrial activity. In conclusion, our study challenges the conventional view of iNOS-derived NO as a cytotoxic molecule and highlights its intriguing role in HSPCs. Together, our findings provide insights into the crucial role of the iNOS–NO–mitochondrial axis in regulating HSPCs and hematopoiesis.

Mlkl Mediates Age-Related Attrition of Hematopoietic Stem Cells and Ineffective Hematopoiesis in Myelodysplastic Syndrome

Aging causes anemia, perturbed immunity, and increased propensity to age-related hematological diseases such as myelodysplastic syndrome (MDS). Age-dependent functional decline in hematopoietic stem cells (HSCs), which is accompanied by morphological and functional changes in their organelles as well as sterile inflammation in the bone marrow (BM), is one of the main reasons why the hematopoietic system ages. However, molecular mechanisms by which HSCs alter their function during aging remain unclear. Necroptosis is a form of programmed necrosis that underlies sterile inflammation and can take place upon exposure to inflammatory stimuli in apoptosis-resistant cells including HSCs. MLKL, the effector molecule of necroptosis that can form channel-like structure on lipid bilayers upon activation, not only induces cell death but also injures membrane-bound organelles such as mitochondria, though its significance in HSC aging remains elusive. Our quantitative RT-PCR revealed abundant expression of MLKL in murine BM immature hematopoietic cells including HSCs (Lin-/cKit+/Sca1+/Flk2-/CD48-/CD150+), which was validated by reanalysis of the published proteome datasets (Zaro et al., eLife 2020). Using FRET-based necroptosis reporter mice and Western blotting analysis, we found that while MLKL was inactivated at the steady state, it was readily activated in HSCs upon poly I:C injection. To study the effect of MLKL activation on HSCs, we assessed cell death of HSCs isolated from poly I:C-treated MLKL-deficient (Mlkl-/-) mice. Unexpectedly, Annexin V/DAPI staining revealed no changes in the frequencies of dying and dead cells between wild-type (WT) and Mlkl-/- HSCs. Nonetheless, PVA-based HSC culture revealed that the growth suppression observed in poly I:C-treated WT HSCs largely depended on MLKL (p=0.0003). Furthermore, HSC transplantation assay revealed that reduced engraftment and skewed myeloid differentiation were observed in poly I:C-treated WT HSCs but largely alleviated by MLKL deficiency (p=0.011). These results indicate that inflammation-induced MLKL activation does not cause HSC death but impairs their growth and lymphopoietic potential. To assess the relevance of MLKL activation in HSC aging, we treated WT and Mlkl-/- mice with serial rounds of 5-FU treatment, which were shown to induce age-related alterations in HSCs (Beerman et al., Cell Stem Cell 2013). Of note, we observed marked attenuation of age-related features such as myeloid-skewed hematopoiesis and expansion of phenotypic HSCs in 5-FU-treated Mlkl-/- mice (p=0.0051). HSC transplantation experiments further revealed that MLKL deficiency alleviated 5-FU-induced decrease in engraftment and lymphopoietic potential of HSCs (p=0.013). Indeed, analysis of 18-month-old WT and Mlkl-/- mice demonstrated that MLKL deficiency largely rescued myeloid-skewed hematopoiesis and loss of BM CLPs (Lin-/cKitlo/Sca1lo/Flk2+/IL-7Rα+) (p=0.0047), and serial HSC transplantation revealed better self-renewal and lymphopoietic potential of aged Mlkl-/- HSCs (p=0.0055). Importantly, multiplex ELISA assay using BM supernatants revealed no changes in concentrations of inflammatory cytokines, indicating that the observed effects in Mlkl-/- HSCs were likely caused by cell intrinsic changes. In line with this, electron microscopy analysis revealed that age-related changes in mitochondria, such as elongated and swollen structure with disorganized cristae, were significantly attenuated in aged Mlkl-/- HSCs (p<0.0001). JC-1 dye staining further revealed that age-related accumulation of damaged mitochondria was significantly attenuated in Mlkl-/- HSCs (p=0.049). Finally, to evaluate the effect of MLKL on development of age-related hematological disease, we established MDS mouse models by transplantation of RUNX1S291fs-transduced WT and Mlkl-/- HSCs. We found that WT MDS mice mainly died of severe anemia due to ineffective hematopoiesis, whereas Mlkl-/- MDS mice showed milder anemia and better overall survival (p=0.044), indicating that MLKL promotes ineffective hematopoiesis in MDS. Taken together, our results identify MLKL as a key mediator of age-related functional decline in HSCs and ineffective hematopoiesis in MDS. They also suggest that pharmacological inhibition of MLKL activity is a promising strategy to better control aging of the hematopoietic system and age-related hematological disease.

Directional capacity of human mesenchymal stem cells to support hematopoietic stem cell proliferation in vitro

Hematopoietic stem cells (HSCs) reside in a specialised microenvironment in the bone marrow, which is majorly composed of mesenchymal stem cells (MSCs) and its' derivatives. This study aimed to investigate the regulatory role of MSCs to decipher the cellular and humoral communications on HSCs' proliferation, self-renewal, and differentiation at the transcriptomic level. Microarray assay was employed to analyse the gene expression profile of HSCs that imparted by MSCs during co-culture. The proliferation of human umbilical cord blood-derived HSCs (hUC-HSCs) markedly propagated when MSCs were used as the feeder layer, without disturbing the undifferentiated state of HSCs, and reduced the cell death of HSCs. Upon co-culture with MSCs, the global microarray analysis of HSCs disclosed 712 differentially expressed genes (DEGs) (561 up-regulated and 151 down-regulated). The dysregulations of various transcripts were enriched for cellular functions such as cell cycle (including CCND1), apoptosis (including TNF), and genes related to signalling pathways governing self-renewal, as well as WNT5A from the Wnt signalling pathway, MAPK, Hedgehog, FGF2 from FGF, Jak-STAT, and PITX2 from the TGF-β signalling pathway. To concur this, real-time quantitative PCR (RT-qPCR) was utilised for corroborating the microarray results from five of the most dysregulated genes. This study elucidates the underlying mechanisms of the mitogenic influences of MSCs on the propagation of HSCs. The exploitation of such mechanisms provides a potential means for achieving larger quantities of HSCs in vitro, thus obviating the need for manipulating their differentiation potential for clinical application.

Determining the role of inflammation in the selection of JAK2 mutant cells in myeloproliferative neoplasms

Myeloproliferative neoplasm (MPN) is a hematologic malignancy characterized by the clonal outgrowth of hematopoietic cells with a somatically acquired mutation most commonly in JAK2 (JAK2V617F). This mutation endows upon myeloid progenitors cytokine independent growth and consequently leads to excessive production of myeloid lineage cells. It has been previously suggested that inflammation may play a role in the clonal evolution of JAK2V617F mutants. In particular, it is possible that one or more cellular kinetic parameters of hematopoietic stem cells (HSCs) are affected by inflammation, such as division or death rates of cells, and the probability of HSC differentiation. This suggests a mechanism that can steer the outcome of the cellular competition in favor of the mutants, initiating the disease. In this paper we create a number of mathematical evolutionary models, from very abstract to more concrete, that describe cellular competition in the context of inflammation. It is possible to build a model axiomatically, where only very general assumptions are imposed on the modeling components and no arbitrary (and generally unknown) functional forms are used, and still generate a set of testable predictions. In particular, we show that, if HSC death is negligible, the evolutionary advantage of mutant cells can only be conferred by an increase in differentiation probability of HSCs in the presence of inflammation, and if death plays a significant role in the dynamics, an additional mechanism may be an increase of HSC’s division-to-death ratio in the presence of inflammation. Further, we show that in the presence of inflammation, the wild type cell population is predicted to shrink under inflammation (even in the absence of mutants). Finally, it turns out that if only the differentiation probability is affected by the inflammation, then the resulting steady state population of wild type cells will contain a relatively smaller percentage of HSCs under inflammation. If the division-to-death rate is also affected, then the percentage of HSCs under inflammation can either decrease or increase, depending on other parameters.

Valproic Acid Expands the Numbers of HSCs from Cord Blood CD34+ Cells By Linking Epigenetic Modifications to Mitochondrial Remodeling and p53 Upregulation

Umbical cord blood (UCB) is an important source of hematopoietic stem cells (HSCs) for allogeneic transplantation of patients suffering from hematological malignancies and genetic disorders. Its use, however, as an HSC graft for adults has been restricted due to the limited numbers of HSCs within a single UCB unit. We have previously reported that UCB CD34+ cell numbers can be expanded over a seven-day period of incubation by ex vivo treatment with a cytokine combination and the histone deacetylase inhibitor, valproic acid (VPA) (Chaurasia et al. J Clin Invest. 2014:124(6): 2378-2395). In this study, we addressed the mechanism underling this ex-vivo expansion of UCB-derived CD34+cells. We found that VPA triggered two distinct phases of HSC behavior. The first phase (days 1-4) was characterized by a prompt elevation of the expression of stem cell phenotypic markers (CD90 and CD49f), which was associated with acetylation of histone H3, and expression of pluripotency genes including OCT4, NANOG, and SOX2. During this period, the cells underwent a limited number of cell divisions. The second phase (days 4-7) was characterized by a greater number of cell divisions that resulted in a drastic increase in the absolute number of phenotypically defined HSCs. Both acetylation of H3 and expression of the pluripotency genes were transient events since they were no longer evident during the second phase. During the entire incubation period, VPA-induced HSC cells maintained a distinct mitochondrial metabolic profile characterized by low mitochondrial potential, ROS levels and mass. Remarkably, the p38 activity, a downstream target of ROS, was also suppressed in VPA-expanded HSCs. Given that both ROS and p38 activity lead to exhaustion and limit the lifespan of the pool of HSCs, our findings suggest that the VPA-expanded HSCs are equipped with mechanisms that monitor and ensure their fitness.In addition, removal of VPA from the culture after 24, 48 and 72hrs of incubation reversed the acetylation of H3 and the expression of the pluripotency genes. These events were further accompanied by the rapid loss of stem cell marker expression (24hrs after removal) leading to decreased HSC numbers. Importantly, upon VPA removal, the expanded cells exhibited an increase in mitochondrial ROS, mass and potential suggesting that the epigenetic reprograming and mitochondrial remodeling triggered by VPA were linked and were both reversible events.Both transcript and protein levels of p53 were also increased during the first phase but not during the second phase of the ex-vivo expansion of CD34+cell. Importantly, removal of VPA diminished the up-regulation of p53 during the first phase. We further assessed the role of p53 in VPA stem cell expansion with pifithrin-α, a small molecule inhibitor of p53 function. P53 inhibition led to a significant elevation of the mitochondrial ROS without affecting cellular viability. Moreover, treatment with pifithrin-α reduced both the percentage and the absolute number of HSCs induced by VPA. Conversely, co-treatment with VPA and a low concentration of nutlin (1μM), an inhibitor of p53 degradation decreased further mitochondrial ROS levels and augmented the percentage of HSCs induced by VPA. By contrast, treatment with higher concentrations of nutlin (10μM) led to HSC cell death. These findings raise the possibility that VPA induces a moderate up-regulation of p53 as a stress-response mechanism to cope with oxidative stress. It seems plausible that p53 along with alterations in mitochondrial mass and potential are among the mechanisms required to repress oxidative stress and thus, are implicated in the HSC expansion by VPA.Collectively, our data establishes that VPA expands ex-vivo the pool of HSCs from CD34+ CB cells by triggering epigenetic modifications that are tightly linked to mitochondrial remodeling. Notably, these events are reversible and do not result in the malignant transformation of the CD34+ cells. Moreover, these data indicate that VPA modulates and maintains a critical threshold of p53 that is required for HSCs expansion but is not sufficient to trigger the exhaustion and death of HSCs. In this context, VPA also represses p38 activity that if up-regulated can limit the lifespan of HSCs. These studies demonstrate that effective ex vivo HSC expansion requires the coordination and retention of a number of critical biological events that define an HSC DisclosuresNo relevant conflicts of interest to declare.

Smad4 in osteoblasts exerts a differential impact on HSC fate depending on osteoblast maturation stage.

Osteoblasts (OBs) are indispensable for the maintenance of hematopoietic stem cells (HSCs) in the bone marrow microenvironment. Here we investigated how Smad4 modulates HSC fate at distinct stages of OB development. For this, we conditionally knocked out Smad4 in cells expressing type I collagen (Col1a1) and osteocalcin (OC), respectively. Col1a1-expressing OBs were widely present in both the trabecular and cortical compartment, whereas OC-expressing OBs were predominantly located in the cortical compartment. HSCs from Col1a1 mutants displayed senescence-associated phenotypes. OC mutants did not exhibit HSC senescence-related phenotypes, but instead showed preferential HSC death. Of note, stromal cell-derived factor 1 expression was lower in Col1a1 mutants than control littermates, suggesting potential impairment of CXCR4-CXCL12-mediated HSC retention. Disruption of the CXCR4-CXCL12 axis by AMD3100 administration led to an increase in the senescence-associated β-galactosidase activity and low competitive potential. Collectively, our findings indicate that deletion of Smad4 in OBs differentially modulates HSC fate in a stage-dependent manner.

Replication stress in MLL-rearrangements.

Hematopoietic stem cells (HSC) are the only cells capable of self-renewal throughout the individual's lifetime and generate the whole spectrum of blood cells. Therefore genome aberrations in HSC can result in hematopoiesis failure or leukemic transformation. Chromosomal translocations, inversions, amplifications and complex rearrangements at the 11q human genomic locus encoding mixed lineage leukemia gene (MLL) are the hallmark of several blood malignancies including infant, therapy-induced, donor - and de novo leukemias. The vast majority of these 11q aberrations fall within a 7.3kb MLL breakpoint cluster region (MLLbcr) with a particular hotspot at the intron11-exon12 boundary [1]. Intriguingly, a large variety of genotoxic, cytotoxic and biological stimuli were connected with MLLbcr breakage pointing to the existence of several DNA cleavage and repair mechanisms acting at this locus [1, 2]. From the broad spectrum of stimuli triggering cleavage in concert with diverse mutagenic outcomes at the locus it is tempting to seek for a common molecular process engaged. Based on our and others’ experimental evidences, we postulate that replication stress in HSC can be responsible for MLL rearrangements (Figure ​(Figure1).1). Thus, our data revealed MLLbcr breakage upon mere replication blockage via DNA polymerase inhibition or upon exposure to the nucleoside analog 5-fluorouracil [2]. Induction of HSC's specific replication stress can be linked to many agents and conditions implicated in MLL leukemias. Normally, quiescence of HSC with only rare replication cycles accompanied by low metabolic activity and ROS levels contributes to minimize the mutational load under homeostatic conditions [3, 4]. In contrast, forcing HSC into excessive cycling by chronic stimulation with physiological triggers mimicking inflammation, bleeding or cytopenia provokes a robust DDR that drives both HSC death and mutagenesis of the survivors. Thus, Walter et al. [4] detected DDR markers associated with replication fork stalling and collapse such as DNA breaks and nuclear γ H2AX, 53BP1 and FANCD2 foci upon enforced HSC exit from quiescence. Transplantation induces rapid cycling of normally dormant HSC that can be exacerbated by donor immunosuppression, damaged microenvironment and altered cytokine profile. Signs of endogenous DNA damage upon serial transplantation of HSC are well documented in both humans and mice with evidence for altered DNA replication dynamics, chromosome gaps and breaks indicative of replication stress [3, 5]. We suggest that exhaustion or failure of replication stress-associated high fidelity repair pathways under transplantation challenge can be implicated in donor cell-derived acute leukemia with MLL translocations in patients who received HSC transplant [6]. Given the fact that replication stress in HSC is associated with aging [3] one can hypothesize that MLL rearrangements, particularly amplifications often associated with complex rearrangements [7], observed in de novo AML in the elderly are the consequence of replication stress associated DNA repair failures. Figure 1 MLL rearrangements can result from failure to correctly resolve replication stress in HSC Induction of replication stress in HSC can also be linked to agents and conditions associated with common solid cancer therapies. Indeed, topoisomerase II inhibitors and cytostatics with a different mode-of-action such as alkylating agents or 5-fluorouracil [1, 8] but all implicated in the etiology of therapy-induced leukemia or MLL rearrangements can recruit dormant HSC into the replication cycle as a result of chemotherapy-associated cytopenia. Moreover, fetal HSC which are the infant leukemia cell-of-origin are highly cycling populations and thus collision of replication forks with lesions can cause overwhelming replicative stress. Altogether, replication stress may represent the integrating signal in HSC following genotoxic exposure of the fetus and of patients undergoing radio- or chemotherapy, in HSC undergoing excessive self-renewal in the fetus or upon transplantation as well as in HSC from aged individuals suffering from the exhaustion of replication factors [3]. The extraordinary susceptibility of the MLLbcr to replication stress-induced breakage may stem from its secondary structure resulting in the collision of transcription and replication machineries recruiting nucleases such as Endonuclease G in decondensed chromatin [1](Figure ​](Figure11). To summarize, replication stress response plays a key role in regulating HSC function. We anticipate that deeper understanding of associated molecular mechanisms responsible for MLLbcr cleavage and subsequent repair in HSC can hold the key for future chemoprevention and anti-aging modalities.

Single-Stranded DNA-Binding Transcriptional Regulator FUBP1 Is Essential for Fetal and Adult Hematopoietic Stem Cell Self-Renewal.

The ability of hematopoietic stem cells (HSCs) to self-renew is a prerequisite for the establishment of definitive hematopoiesis and life-long blood regeneration. Here, we report the single-stranded DNA-binding transcriptional regulator far upstream element (FUSE)-binding protein 1 (FUBP1) as an essential factor of HSC self-renewal. Functional inactivation of FUBP1 in two different mouse models resulted in embryonic lethal anemia at around E15.5 caused by severely diminished HSCs. Fetal and adult HSCs lacking FUBP1 revealed an HSC-intrinsic defect in their maintenance, expansion, and long-term blood reconstitution, but could differentiate into all hematopoietic lineages. FUBP1-deficient adult HSCs exhibit significant transcriptional changes, including upregulation of the cell-cycle inhibitor p21 and the pro-apoptotic Noxa molecule. These changes caused an increase in generation time and death of HSCs as determined by video-microscopy-based tracking. Our data establish FUBP1 and its recognition of single-stranded genomic DNA as an important element in the transcriptional regulation of HSC self-renewal.

Ddx46 Is Required for Multi-Lineage Differentiation of Hematopoietic Stem Cells in Zebrafish

Balanced and precisely controlled processes between self-renewal and differentiation of hematopoietic stem cells (HSCs) into all blood lineages are critical for vertebrate definitive hematopoiesis. However, the molecular mechanisms underlying the maintenance and differentiation of HSCs have not been fully elucidated. Here, we show that zebrafish Ddx46, encoding a DEAD-box RNA helicase, is expressed in HSCs of the caudal hematopoietic tissue (CHT). The number of HSCs expressing the molecular markers cmyb or T-cell acute lymphocytic leukemia 1 (tal1) was markedly reduced in Ddx46 mutants. However, massive cell death of HSCs was not detected, and proliferation of HSCs was normal in the CHT of the mutants at 48 h postfertilization. We found that myelopoiesis occurred, but erythropoiesis and lymphopoiesis were suppressed, in Ddx46 mutants. Consistent with these results, the expression of spi1, encoding a regulator of myeloid development, was maintained, but the expression of gata1a, encoding a regulator of erythrocyte development, was downregulated in the mutants. Taken together, our results provide the first genetic evidence that zebrafish Ddx46 is required for the multilineage differentiation of HSCs during development, through the regulation of specific gene expressions.

Hematopoietic Stem Cells and HIV Infection

Hematopoietic Stem Cells and HIV Infection

Deletion of Puma protects hematopoietic stem cells and confers long-term survival in response to high-dose γ-irradiation

AbstractMolecular paradigms underlying the death of hematopoietic stem cells (HSCs) induced by ionizing radiation are poorly defined. We have examined the role of Puma (p53 up-regulated mediator of apoptosis) in apoptosis of HSCs after radiation injury. In the absence of Puma, HSCs were highly resistant to γ-radiation in a cell autonomous manner. As a result, Puma-null mice or the wild-type mice reconstituted with Puma-null bone marrow cells were strikingly able to survive for a long term after high-dose γ-radiation that normally would pose 100% lethality on wild-type animals. Interestingly, there was no increase of malignancy in the exposed animals. Such profound beneficial effects of Puma deficiency were likely associated with better maintained quiescence and more efficient DNA repair in the stem cells. This study demonstrates that Puma is a unique mediator in radiation-induced death of HSCs. Puma may be a potential target for developing an effective treatment aimed to protect HSCs from lethal radiation.