CD61 identifies a superior population of aged murine HSCs and is required to preserve quiescence and self-renewal.

ABSTRACT: Aging leads to a decline in function of hematopoietic stem cells (HSCs) and increases susceptibility to hematological disease. We found CD61 to be highly expressed in aged murine HSCs. Here, we investigate the role of CD61 in identifying distinct subpopulations of aged HSCs and assess how expression of CD61 affects stem cell function. We show that HSCs with high expression of CD61 are functionality superior and retain self-renewal capacity in serial transplantations. In primary transplantations, aged CD61High HSCs function similarly to young HSCs. CD61High HSCs are more quiescent than their CD61Low counterparts. We also show that in aged bone marrow, CD61High and CD61Low HSCs are transcriptomically distinct populations. Collectively, our research identifies CD61 as a key player in maintaining stem cell quiescence, ensuring the preservation of their functional integrity and potential during aging. Moreover, CD61 emerges as a marker to prospectively isolate a superior, highly dormant population of young and aged HSCs, making it a valuable tool both in fundamental and clinical research.

3-deazaadenosine (3DA) alleviates senescence to promote cellular fitness and cell therapy efficiency in mice.

Cellular senescence is a stable type of cell cycle arrest triggered by different stresses. As such, senescence drives age-related diseases and curbs cellular replicative potential. Here, we show that 3-deazaadenosine (3DA), an S-adenosyl homocysteinase (AHCY) inhibitor, alleviates replicative and oncogene-induced senescence. 3DA-treated senescent cells showed reduced global Histone H3 Lysine 36 trimethylation (H3K36me3), an epigenetic modification that marks the bodies of actively transcribed genes. By integrating transcriptome and epigenome data, we demonstrate that 3DA treatment affects key factors of the senescence transcriptional program. Remarkably, 3DA treatment alleviated senescence and increased the proliferative and regenerative potential of muscle stem cells from very old mice in vitro and in vivo. Moreover, ex vivo 3DA treatment was sufficient to enhance the engraftment of human umbilical cord blood (UCB) cells in immunocompromised mice. Together, our results identify 3DA as a promising drug enhancing the efficiency of cellular therapies by restraining senescence.

Persistent expression of microRNA-125a targets is required to induce murine hematopoietic stem cell repopulating activity.

MicroRNAs (miRs) are small noncoding RNAs that regulate gene expression posttranscriptionally by binding to the 3' untranslated regions of their target mRNAs. The evolutionarily conserved microRNA-125a (miR-125a) is highly expressed in both murine and human hematopoietic stem cells (HSCs), and previous studies have found that miR-125 strongly enhances self-renewal of HSCs and progenitors. In this study we explored whether temporary overexpression of miR-125a would be sufficient to permanently increase HSC self-renewal or, rather, whether persistent overexpression of miR-125a is required. We used three complementary in vivo approaches to reversibly enforce expression of miR-125a in murine HSCs. Additionally, we interrogated the underlying molecular mechanisms responsible for the functional changes that occur in HSCs on overexpression of miR-125a. Our data indicate that continuous expression of miR-125a is required to enhance HSC activity. Our molecular analysis confirms changes in pathways that explain the characteristics of miR-125a overexpressing HSCs. Moreover, it provides several novel putative miR-125a targets, but also highlights the complex molecular changes that collectively lead to enhanced HSC function.

Detection of chemotherapy-resistant patient-derived acute lymphoblastic leukemia clones in murine xenografts using cellular barcodes.

Clonal heterogeneity fuels leukemia evolution, therapeutic resistance, and relapse. Upfront detection of therapy-resistant leukemia clones at diagnosis may allow adaptation of treatment and prevention of relapse, but this is hampered by a paucity of methods to identify and trace single leukemia-propagating cells and their clonal offspring. Here, we tested methods of cellular barcoding analysis, to trace the in vivo competitive dynamics of hundreds of patient-derived leukemia clones upon chemotherapy-mediated selective pressure. We transplanted Nod/Scid/Il2Rγ-/- (NSG) mice with barcoded patient-derived or SupB15 acute lymphoblastic leukemia (ALL) cells and assessed clonal responses to dexamethasone, methotrexate, and vincristine, longitudinally and across nine anatomic locations. We illustrate that chemotherapy reduces clonal diversity in a drug-dependent manner. At end-stage disease, methotrexate-treated patient-derived xenografts had significantly fewer clones compared with placebo-treated mice (100 ± 10 vs. 160 ± 15 clones, p = 0.0005), while clonal complexity in vincristine- and dexamethasone-treated xenografts was unaffected (115 ± 33 and 150 ± 7 clones, p = NS). Using tools developed to assess differential gene expression, we determined whether these clonal patterns resulted from random clonal drift or selection. We identified 5 clones that were reproducibly enriched in methotrexate-treated patient-derived xenografts, suggestive of pre-existent resistance. Finally, we found that chemotherapy-mediated selection resulted in a more asymmetric distribution of leukemia clones across anatomic sites. We found that cellular barcoding is a powerful method to trace the clonal dynamics of human patient-derived leukemia cells in response to chemotherapy. In the future, integration of cellular barcoding with single-cell sequencing technology may allow in-depth characterization of therapy-resistant leukemia clones and identify novel targets to prevent relapse.

Correction: Quantitative distribution of patient-derived leukemia clones in murine xenografts revealed by cellular barcodes.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Donor-to-Donor Heterogeneity in the Clonal Dynamics of Transplanted Human Cord Blood Stem Cells in Murine Xenografts.

Umbilical cord blood (UCB) provides an alternative source of hematopoietic stem cells (HSCs) for allogeneic transplantation. Administration of sufficient donor HSCs is critical to restore recipient hematopoiesis and to maintain long-term polyclonal blood formation. However, due to lack of unique markers, the frequency of HSCs among UCB CD34+ cells is the subject of ongoing debate, urging for reproducible strategies for their counting. Here, we used cellular barcoding to determine the frequency and clonal dynamics of human UCB HSCs and to determine how data analysis methods affect these parameters. We transplanted lentivirally barcoded CD34+ cells from 20 UCB donors into Nod/Scid/IL2Ry-/- (NSG) mice (n = 30). Twelve recipients (of 8 UCB donors) engrafted with >1% GFP+ cells, allowing for clonal analysis by multiplexed barcode deep sequencing. Using multiple definitions of clonal diversity and strategies for data filtering, we demonstrate that differences in data analysis can change clonal counts by several orders of magnitude and propose methods to improve their consistency. Using these methods, we show that the frequency of NSG-repopulating cells was low (median ∼1 HSC/104 CD34+ UCB cells) and could vary up to 10-fold between donors. Clonal patterns in blood became increasingly consistent over time, likely reflecting initial output of transient progenitors, followed by long-term HSCs with stable hierarchies. The majority of long-term clones displayed multilineage output, yet clones with lymphoid- or myeloid-biased output were also observed. Altogether, this study uncovers substantial interdonor and analysis-induced variability in the frequency of UCB CD34+ clones that contribute to post-transplant hematopoiesis. As clone tracing is increasingly relevant, we urge for universal and transparent methods to count HSC clones during normal aging and upon transplantation.

MiR-125a enhances self-renewal, lifespan, and migration of murine hematopoietic stem and progenitor cell clones.

Expansion of hematopoietic stem cells (HSCs) is a 'holy grail' of regenerative medicine, as successful stem cell transplantations depend on the number and quality of infused HSCs. Although many attempts have been pursued to either chemically or genetically increase HSC numbers, neither clonal analysis of these expanded cells nor their ability to support mature blood lineages has been demonstrated. Here we show that miR-125a, at the single cell level, can expand murine long-term repopulating HSCs. In addition, miR-125a increases clone longevity, clone size and clonal contribution to hematopoiesis. Unexpectedly, we found that miR-125a expanded HSCs clones were highly homogenously distributed across multiple anatomical sites. Interestingly, these miR-125a overexpressing cells had enhanced mobility and were more frequently detected in the spleen. Our study reveals a novel, cell-intrinsically controlled mechanism by which HSC migration is regulated.

CBX7 Induces Self-Renewal of Human Normal and Malignant Hematopoietic Stem and Progenitor Cells by Canonical and Non-canonical Interactions.

In this study, we demonstrate that, among all five CBX Polycomb proteins, only CBX7 possesses the ability to control self-renewal of human hematopoietic stem and progenitor cells (HSPCs). Xenotransplantation of CBX7-overexpressing HSPCs resulted in increased multi-lineage long-term engraftment and myelopoiesis. Gene expression and chromatin analyses revealed perturbations in genes involved in differentiation, DNA and chromatin maintenance, and cell cycle control. CBX7 is upregulated in acute myeloid leukemia (AML), and its genetic or pharmacological repression in AML cells inhibited proliferation and induced differentiation. Mass spectrometry analysis revealed several non-histone protein interactions between CBX7 and the H3K9 methyltransferases SETDB1, EHMT1, and EHMT2. These CBX7-binding proteins possess a trimethylated lysine peptide motif highly similar to the canonical CBX7 target H3K27me3. Depletion of SETDB1 in AML cells phenocopied repression of CBX7. We identify CBX7 as an important regulator of self-renewal and uncover non-canonical crosstalk between distinct pathways, revealing therapeutic opportunities for leukemia.

Ectopic miR-125a Expression Induces Long-Term Repopulating Stem Cell Capacity in Mouse and Human Hematopoietic Progenitors.

Umbilical cord blood (CB) is a convenient and broadly used source of hematopoietic stem cells (HSCs) for allogeneic stem cell transplantation. However, limiting numbers of HSCs remain a major constraint for its clinical application. Although one feasible option would be to expand HSCs to improve therapeutic outcome, available protocols and the molecular mechanisms governing the self-renewal of HSCs are unclear. Here, we show that ectopic expression of a single microRNA (miRNA), miR-125a, in purified murine and human multipotent progenitors (MPPs) resulted in increased self-renewal and robust long-term multi-lineage repopulation in transplanted recipient mice. Using quantitative proteomics and western blot analysis, we identified a restricted set of miR-125a targets involved in conferring long-term repopulating capacity to MPPs in humans and mice. Our findings offer the innovative potential to use MPPs with enhanced self-renewal activity to augment limited sources of HSCs to improve clinical protocols.

Tracing dynamics and clonal heterogeneity of Cbx7-induced leukemic stem cells by cellular barcoding.

Accurate monitoring of tumor dynamics and leukemic stem cell (LSC) heterogeneity is important for the development of personalized cancer therapies. In this study, we experimentally induced distinct types of leukemia in mice by enforced expression of Cbx7. Simultaneous cellular barcoding allowed for thorough analysis of leukemias at the clonal level and revealed high and unpredictable tumor complexity. Multiple LSC clones with distinct leukemic properties coexisted. Some of these clones remained dormant but bore leukemic potential, as they progressed to full-blown leukemia after challenge. LSC clones could retain multilineage differentiation capacities, where one clone induced phenotypically distinct leukemias. Beyond a detailed insight into CBX7-driven leukemic biology, our model is of general relevance for the understanding of tumor dynamics and clonal evolution.

MicroRNA-125 family members exert a similar role in the regulation of murine hematopoiesis.

MicroRNAs (miRNAs) are crucial for proper functioning of hematopoietic stem and progenitor cells (HSPCs). Members of the miRNA-125 family (consisting of miR-125a, miR-125b1, and miR-125b2) are known to confer a proliferative advantage on cells upon overexpression, to decrease the rate of apoptosis by targeting proapoptotic genes, and to promote differentiation toward the myeloid lineage in mice. However, many distinct biological effects of the three miR-125 species have been reported as well. In the current study, we set out to assess whether the three miRNA-125s that carry identical seed sequences could be functionally different. Our data show that overexpression of each of the three miR-125 family members preserves HSPCs in a primitive state in vitro, results in a competitive advantage upon serial transplantation, and promotes skewing toward the myeloid lineage. All miR-125 family members decreased the pool of phenotypically defined Lin(-)Sca(+)Kit(+)CD48(-)CD150(+) long-term hematopoietic stem cells, simultaneously increasing the self-renewal activity upon secondary transplantation. The downregulation of miR-125s in hematopoietic stem cells abolishes these effects and impairs long-term contribution to blood cell production. The introduction of a point mutation within the miRNA-125 seed sequence abolishes all abovementioned effects and leads to the restoration of normal hematopoiesis. Our results show that all miR-125 family members are similar in function, they likely operate in a seed-sequence-dependent manner, and they induce a highly comparable hematopoietic phenotype.

Asymmetry in skeletal distribution of mouse hematopoietic stem cell clones and their equilibration by mobilizing cytokines.

Hematopoietic stem cells (HSCs) are able to migrate through the blood stream and engraft bone marrow (BM) niches. These features are key factors for successful stem cell transplantations that are used in cancer patients and in gene therapy protocols. It is unknown to what extent transplanted HSCs distribute throughout different anatomical niches in the BM and whether this changes with age. Here we determine the degree of hematopoietic migration at a clonal level by transplanting individual young and aged mouse HSCs labeled with barcoded viral vector, followed by assessing the skeletal distribution of hundreds of HSC clones. We detected highly skewed representation of individual clones in different bones at least 11 mo after transplantation. Importantly, a single challenge with the clinically relevant mobilizing agent granulocyte colony-stimulating factor (G-CSF) caused rapid redistribution of HSCs across the skeletal compartments. Old and young HSC clones showed a similar level of migratory behavior. Clonal make-up of blood of secondary recipients recapitulates the barcode composition of HSCs in the bone of origin. These data demonstrate a previously unanticipated high skeletal disequilibrium of the clonal composition of HSC pool long-term after transplantation. Our findings have important implications for experimental and clinical and stem cell transplantation protocols.

Polycomb Cbx family members mediate the balance between haematopoietic stem cell self-renewal and differentiation.

The balance between self-renewal and differentiation of adult stem cells is essential for tissue homeostasis. Here we show that in the haematopoietic system this process is governed by polycomb chromobox (Cbx) proteins. Cbx7 is specifically expressed in haematopoietic stem cells (HSCs), and its overexpression enhances self-renewal and induces leukaemia. This effect is dependent on integration into polycomb repressive complex-1 (PRC1) and requires H3K27me3 binding. In contrast, overexpression of Cbx2, Cbx4 or Cbx8 results in differentiation and exhaustion of HSCs. ChIP-sequencing analysis shows that Cbx7 and Cbx8 share most of their targets; we identified approximately 200 differential targets. Whereas genes targeted by Cbx8 are highly expressed in HSCs and become repressed in progenitors, Cbx7 targets show the opposite expression pattern. Thus, Cbx7 preserves HSC self-renewal by repressing progenitor-specific genes. Taken together, the presence of distinct Cbx proteins confers target selectivity to PRC1 and provides a molecular balance between self-renewal and differentiation of HSCs.

Genetic screen identifies microRNA cluster 99b/let-7e/125a as a regulator of primitive hematopoietic cells.

Hematopoietic stem/progenitor cell (HSPC) traits differ between genetically distinct mouse strains. For example, DBA/2 mice have a higher HSPC frequency compared with C57BL/6 mice. We performed a genetic screen for micro-RNAs that are differentially expressed between LSK, LS(-)K(+), erythroid and myeloid cells isolated from C57BL/6 and DBA/2 mice. This analysis identified 131 micro-RNAs that were differentially expressed between cell types and 15 that were differentially expressed between mouse strains. Of special interest was an evolutionary conserved miR cluster located on chromosome 17 consisting of miR-99b, let-7e, and miR-125a. All cluster members were most highly expressed in LSKs and down-regulated upon differentiation. In addition, these microRNAs were higher expressed in DBA/2 cells compared with C57BL/6 cells, and thus correlated with HSPC frequency. To functionally characterize these microRNAs, we overexpressed the entire miR-cluster 99b/let-7e/125a and miR-125a alone in BM cells from C57BL/6 mice. Overexpression of the miR-cluster or miR-125a dramatically increased day-35 CAFC activity and caused severe hematopoietic phenotypes upon transplantation. We showed that a single member of the miR-cluster, namely miR-125a, is responsible for the majority of the observed miR-cluster overexpression effects. Finally, we performed genome-wide gene expression arrays and identified candidate target genes through which miR-125a may modulate HSPC fate.

Expression quantitative trait loci are highly sensitive to cellular differentiation state.

Genetical genomics is a strategy for mapping gene expression variation to expression quantitative trait loci (eQTLs). We performed a genetical genomics experiment in four functionally distinct but developmentally closely related hematopoietic cell populations isolated from the BXD panel of recombinant inbred mouse strains. This analysis allowed us to analyze eQTL robustness/sensitivity across different cellular differentiation states. Although we identified a large number (365) of "static" eQTLs that were consistently active in all four cell types, we found a much larger number (1,283) of "dynamic" eQTLs showing cell-type-dependence. Of these, 140, 45, 531, and 295 were preferentially active in stem, progenitor, erythroid, and myeloid cells, respectively. A detailed investigation of those dynamic eQTLs showed that in many cases the eQTL specificity was associated with expression changes in the target gene. We found no evidence for target genes that were regulated by distinct eQTLs in different cell types, suggesting that large-scale changes within functional regulatory networks are uncommon. Our results demonstrate that heritable differences in gene expression are highly sensitive to the developmental stage of the cell population under study. Therefore, future genetical genomics studies should aim at studying multiple well-defined and highly purified cell types in order to construct as comprehensive a picture of the changing functional regulatory relationships as possible.

Fibroblast growth factor-1 and -2 preserve long-term repopulating ability of hematopoietic stem cells in serum-free cultures.

In this study, we demonstrate that extended culture of unfractionated mouse bone marrow (BM) cells, in serum-free medium, supplemented only with fibroblast growth factor (FGF)-1, FGF-2, or FGF-1 +2 preserves long-term repopulating hematopoietic stem cells (HSCs). Using competitive repopulation assays, high levels of stem cell activity were detectable at 1, 3, and 5 weeks after initiation of culture. FGFs as single growth factors failed to support cultures of highly purified Lin(-)Sca-1(+)c-Kit(+)(LSK) cells. However, cocultures of purified CD45.1 LSK cells with whole BM CD45.2 cells provided high levels of CD45.1 chimerism after transplant, showing that HSC activity originated from LSK cells. Subsequently, we tested the reconstituting potential of cells cultured in FGF-1 + 2 with the addition of early acting stimulatory molecules, stem cell factor +interleukin-11 + Flt3 ligand. The addition of these growth factors resulted in a strong mitogenic response, inducing rapid differentiation and thereby completely overriding FGF-dependent stem cell conservation. Importantly, although HSC activity is typically rapidly lost after short-term culture in vitro, our current protocol allows us to sustain stem cell repopulation potential for periods up to 5 weeks.

The Polycomb group gene Ezh2 prevents hematopoietic stem cell exhaustion.

The molecular mechanism responsible for a decline of stem cell functioning after replicative stress remains unknown. We used mouse embryonic fibroblasts (MEFs) and hematopoietic stem cells (HSCs) to identify genes involved in the process of cellular aging. In proliferating and senescent MEFs one of the most differentially expressed transcripts was Enhancer of zeste homolog 2 (Ezh2), a Polycomb group protein (PcG) involved in histone methylation and deacetylation. Retroviral overexpression of Ezh2 in MEFs resulted in bypassing of the senescence program. More importantly, whereas normal HSCs were rapidly exhausted after serial transplantations, overexpression of Ezh2 completely conserved long-term repopulating potential. Animals that were reconstituted with 3 times serially transplanted control bone marrow cells all died due to hematopoietic failure. In contrast, similarly transplanted Ezh2-overexpressing stem cells restored stem cell quality to normal levels. In a "genetic genomics" screen, we identified novel putative Ezh2 target or partner stem cell genes that are associated with chromatin modification. Our data suggest that stabilization of the chromatin structure preserves HSC potential after replicative stress.

Uncovering regulatory pathways that affect hematopoietic stem cell function using 'genetical genomics'.

We combined large-scale mRNA expression analysis and gene mapping to identify genes and loci that control hematopoietic stem cell (HSC) function. We measured mRNA expression levels in purified HSCs isolated from a panel of densely genotyped recombinant inbred mouse strains. We mapped quantitative trait loci (QTLs) associated with variation in expression of thousands of transcripts. By comparing the physical transcript position with the location of the controlling QTL, we identified polymorphic cis-acting stem cell genes. We also identified multiple trans-acting control loci that modify expression of large numbers of genes. These groups of coregulated transcripts identify pathways that specify variation in stem cells. We illustrate this concept with the identification of candidate genes involved with HSC turnover. We compared expression QTLs in HSCs and brain from the same mice and identified both shared and tissue-specific QTLs. Our data are accessible through WebQTL, a web-based interface that allows custom genetic linkage analysis and identification of coregulated transcripts.

Impaired hematopoietic stem cell functioning after serial transplantation and during normal aging.

Adult somatic stem cells possess extensive self-renewal capacity, as their primary role is to replenish aged and functionally impaired tissues. We have previously shown that the stem cell pool in short-lived DBA/2 (D2) mice is reduced during aging, in contrast to long-lived C57BL/6 (B6) mice. This suggests the existence of a genetically determined mitotic clock operating in stem cells, which possibly limits organismal aging. In the study reported here, unfractionated bone marrow (BM) cells or highly purified Lin(-)Sca-1(+)c-kit(+) (LSK) cells were serially transplanted in lethally irradiated D2 and B6 mice. In both strains, serial transplantation resulted in a substantial loss of stem cell activity. However, as we estimate that in B6 mice, the maximum number of population doublings of primitive cells is approximately 30, in D2 mice this is only approximately 20, resulting in a 1,000-fold difference in expansion potential, irrespective of whether whole bone marrow or purified hematopoietic stem cells (HSCs) were transplanted. Interestingly, recipients reconstituted with serially transplanted BM cells were able to accept a freshly isolated graft without any further conditioning. Finally, we show that whereas transplantation of BM cells into healthy, nonconditioned, young B6 recipients does not lead to engraftment, young BM cells do engraft and provide multilineage reconstitution in nonirradiated aged mice. Our data clearly establish the relevance of an intrinsic, genetically controlled program associated with impaired stem cell functioning during aging.