Experimental hematology最新文献

Recently, many pieces of evidence indicate that blood cell development in the embryo is more complicated than we previously thought. Old textbooks state that the first blood cells arise in the extra-embryonic yolk sac described as transient primitive hematopoiesis, followed by definitive hematopoietic stem/progenitor emergence in the aorta-gonad mesonephros (AGM) region. These HSPCs migrate to the fetal liver and maintain fetal hematopoiesis. HSCs migrate to the bone marrow before birth and establish adult-type hematopoiesis for life. However, we and others recently reported that fetal liver hematopoiesis is supported by HSC-independent hematopoietic progenitors and the fetal-derived (HSC-independent) immune cells persist into adult life much longer than we expected. Using lineage tracing mouse models, we precisely examined the percentage of HSC-derived cells in each hematopoietic lineage and found that not 100% of cells are derived from HSCs. Instead, hematopoietic cells derived from endothelial cells in the early embryo contribute to many immune cell populations. These results were also confirmed by transplantation assays. We also identified the earliest innate lymphoid progenitors in the fetal liver. These cells arise independently of HSCs and differentiate into peritoneal B-1 cells, intestinal IgA+ cells, some T cells, and mast cells. We also examined detailed BCR of fetal-derived and HSC-derived B-1 cells.

Our data display a new paradigm in which immune cells are a mixture of cells from different origins and could function differently.

Splicing defects are a characteristic feature of myelodysplastic syndromes (MDS) and typically associate with recurrent splicing factor mutations. However, a subset of transcripts exhibit convergent abnormal splicing, occurring even in the absence of splicing-related mutations. These shared splicing events likely include common drivers of MDS hematopoietic defects, yet the functions of the resulting transcripts remain unknown. We identified a long isoform of the heterochromatin enforcer Methyl-CpG-Binding Domain 1 (MBD1), as the product of one such mutation-independent splicing event. In cord blood CD34+ cells overexpression of the MDS-associated full-length isoform (MBD1-L), containing MBD1′s 3rd CXXC domain, impaired erythroid differentiation, stalled cell cycling and promoted apoptosis while the MBD1-ΔCXXC3 isoform (MBD1-S), preferentially produced in healthy cells, did not induce these defects. Similarly, only MBD1-L impaired reconstitution capacity in vivo, particularly in the erythroid and myeloid lineages, and produced an enrichment of the MDS transcriptomic signature. We show that inclusion of the exon containing CXXC3, unique in specifically binding non-methylated CpGs, disrupts MBD1′s co-localization with heterochromatin. This triggers a striking redistribution of MBD1 from gene bodies and intergenic regions to hypomethylated promoter CpGs, resulting in widespread repression of promoter chromatin accessibility and downregulation of cell-cycle-related transcripts through its recruitment of the SETDB1:ATF7IP H3K9 methylase complex. Through knockdown or delivery of splice-switching antisense oligonucleotides targeting the CXXC3 exon into MDS cells, we confirm that targeted MBD1-L reduction inverts the quiescent, differentiation-impaired phenotype imposed by its overexpression. These findings provide evidence that mutation-independent splicing changes can drive hematopoietic dysfunction and serve as therapeutic targets in MDS.

Pediatric acute myeloid leukemia is a genetically diverse malignancy with some mutations conveying particularly high risk for relapse and death. For example, NUP98-rearranged (NUP98r) AML occurs primarily in early to mid-childhood, and it carries an overall survival of only 10-30%. It is not clear why NUP98r AML occurs disproportionately in mid-childhood or how to more effectively treat it.

We used a combination of mouse and human models to identify self-renewal programs that sustain NUP98r AML and test whether they are engaged most efficiently during neonatal or juvenile stages of life, as might be expected based on peak age of presentation. We isolated a conserved leukemia stem cell (LSC) population. The LSC signature distinguishes NUP98r AML from other pediatric AML subtypes, and it includes new candidate targets for therapy.

Age greatly influences the capacity of pre-leukemic progenitors to self-renew, transform and give rise to LSCs. Specifically, we found that the fetal state confers an unanticipated layer of protection against NUP98r AML. NUP98::HOXA9 induction in fetal progenitors causes precocious erythroid differentiation. In contrast, NUP98::HOXA9 induction in postnatal progenitors hyperactivates self-renewal programs while preserving an otherwise normal hematopoietic differentiation trajectory. NUP98::HOXA9-expressing neonatal progenitors self-renew, form colonies and give rise to AML far more efficiently than fetal progenitors. The fetal state confers similar protection against KMT2A::MLLT1-driven AML, another high-risk subtype. Active fetal leukemia suppression may explain why fetal leukemias are exceedingly rare even when leukemogenic mutations arise before birth.

Interestingly, fetal protection does not extend to all pediatric AML oncoproteins. The infant AML driver, MNX1, causes marked expansion of fetal progenitors that dissipates almost entirely after birth. Thus, ontogeny has mutation-specific effects on self-renewal and leukemogenic potential.

Lens Epithelium Derived Growth Factor/p75 (LEDGF/p75) is a chromatin-associated protein involved in multiple malignancies. It tethers the MLL/KMT2A fusion protein to the chromatin and plays a critical role in the initiation and maintenance of MLL-r leukemia which mostly affects pediatric patients and is linked to a high rate of relapse and resistance to conventional chemotherapy. Moreover, LEDGF/p75 is overexpressed in AML and prostate cancer patients who are resistant to chemotherapy. We have previously shown that reduced LEDGF/p75 expression sensitizes proliferation and survival of KMT2A-r Thp1 cells to cytarabine treatment through the sphingosine-1 pathway. Here, we studied modulation of chemoresistance by LEDGF/p75 in various types of leukemia. At first, we corroborated the results in Thp1 and expanded these findings by ectopically expressing LEDGF/p75 in the KMT2A-r Molm13 cells that naturally express low levels of LEDGF/p75. Overexpression of LEDGF/p75 in those cells resulted in increased proliferation and reduced apoptosis in the presence of cytarabine. A similar result was obtained after LEDGF/p75 depletion in K562 (CML, KMT2A WT) since a 3-fold increase in sensitivity to vincristine compared to the control was found. Vincristine treated K562 LEDGF/p75 KD cells show more apoptosis and increased caspase3 expression. Interestingly, LEDGF/p75 depletion in SEM cells (ALL, KMT2A-r) resulted in 4-fold more proliferation and less apoptosis upon cytarabine treatment compared to cells expressing mock miRNA. In conclusion, LEDGF/p75 induces chemoresistance in Thp1 and K562 cells to cytarabine and vincristine respectively but sensitizes SEM cells to cytarabine. Our findings highlight the opposing role of LEDGF/p75 in modulating chemoresistance across leukemias. Targeting LEDGF/p75 may be a promising strategy to enhance chemotherapy efficacy in specific leukemic subtypes.

Red blood cells (RBCs) comprise a critical component of the cardiovascular network, which constitutes the first functional organ system of the developing mammalian embryo. Examination of circulating blood cells in mammalian embryos revealed two distinct types of erythroid cells: large, nucleated “primitive” erythroblasts followed by smaller, enucleated “definitive” erythrocytes. This review describes the current understanding of primitive and definitive erythropoiesis gleaned from studies of mouse and human embryos and induced pluripotent stem cells (iPSCs). Primitive erythropoiesis in the mouse embryo comprises a transient wave of committed primitive erythroid progenitors (primitive erythroid colony-forming cells, EryP-CFC) in the early yolk sac that generates a robust cohort of precursors that mature in the bloodstream and enucleate. In contrast, definitive erythropoiesis has two distinct developmental origins. The first comprises a transient wave of definitive erythroid progenitors (burst-forming units erythroid, BFU-E) that emerge in the yolk sac and seed the fetal liver where they terminally mature to provide the first definitive RBCs. The second comprises hematopoietic stem cell (HSC)-derived BFU-E that terminally mature at sites colonized by HSCs particularly the fetal liver and subsequently the bone marrow. Primitive and definitive erythropoiesis are derived from endothelial identity precursors with distinct developmental origins. Although they share prototypical transcriptional regulation, primitive and definitive erythropoiesis are also characterized by distinct lineage-specific factors. The exquisitely timed, sequential production of primitive and definitive erythroid cells is necessary for the survival and growth of the mammalian embryo.

Transitions between cell progenitors and progeny depend on precise transcriptional mechanisms to adjust gene expression programs. The sterile alpha motif-containing 1 (SAMD1) gene encodes a transcription factor which coordinates histone modifications during embryonic stem cell exit from pluripotency. SAMD1 is expressed throughout many biological systems, but its role in hematopoiesis is unknown. SAMD1 prefers to bind chromatin at unmethylated CpG islands (CGIs), where it acts primarily as a transcriptional repressor. SAMD1 interacts with and promotes the function of lysine demethylase LSD1, which blocks terminal erythropoiesis. Samd1 knockout is embryonic lethal in mice. To test Samd1 in hematopoiesis, we performed competitive transplant experiments in mice using shRNA knockdown HSCs. Samd1 knockdown versus control HSCs revealed an increase in HSC repopulation with 3.9-fold more CD45.2+ after 8 weeks. We conducted scRNA-seq and chromatin occupancy profiling in Samd1 knockdown and knockout cells, revealing that Samd1 regulated a genetic network consistent with a role in stem cell self-renewal, including the repression of erythroid-specific genes. Ongoing experiments are testing whether SAMD1 functions in partnership with the lysine demethylase LSD1 during erythropoiesis. Both SAMD1 and LSD1 are commonly upregulated in acute myeloid leukemia (AML), and high expression is correlated with poor prognosis. These mechanisms may be exploitable to improve HSC expansion ex vivo. Linking Samd1 function to signaling, transcription, or other cellular functions opens the door to translational avenues for studying the contribution of Samd1 in hematologic pathologies.

Macrophages maintain hematopoietic stem cell (HSC) quality by assessing cell surface Calreticulin (Calr), an "eat-me" signal induced by reactive oxygen species (ROS). Using zebrafish genetics, we identified Beta-2-microglobulin (B2m) as a crucial "don't eat-me" signal on blood stem cells. A chemical screen revealed inducers of surface Calr that promoted HSC proliferation without triggering ROS or macrophage clearance. Whole genome CRISPR-Cas9 screening showed that Tlr3 signaling regulated b2m expression. Targeting b2m or Tlr3 reduced the HSC clonality. Elevated B2m levels correlated with high expression of repetitive elements (RE) transcripts. Overall, our data suggest that RE-associated dsRNA could interact with TLR3 to stimulate surface expression of B2m on HSPCs. These findings suggest that the balance of Calr and B2m regulates macrophage-HSC interactions and defines hematopoietic clonality.

One of the breakthroughs in hematopoietic stem cell (HSC) field over the last decade is the discovery of two alternative differentiation routes from primitive HSCs to mature megakaryocytes: one through the stepwise hematopoietic hierarchy (stepwise route), and the other by direct differentiation (direct route). This raises a fundamental question of the physiological importance of two alternative differentiation routes for megakaryopoiesis. A major challenge in addressing this question is the lack of fate-mapping systems that distinguish the two differentiation routes.

This work was initiated by designing genetic systems that distinguished the direct and stepwise differentiation routes for hematopoiesis. We found that Cd48-Dre specifically and constitutively marked all haematopoietic cells on the stepwise differentiation route. A combination of KitcreER, Cd48dre and Rosa26loxp-STOP-loxp-rox-loxp-ZsGreen-STOP-rox-tdTomato allowed inducible and simultaneous fate-mapping of haematopoietic stem and progenitor cells on the direct and stepwise differentiation routes.

We mapped the turnover rates and differentiation kinetics of each branch of the hematopoietic hierarchy. We found that megakaryocytes were produced through the two routes with comparable kinetics and quantity under homeostasis. Single-cell RNA-sequencing of the fate-mapped megakaryocytes revealed that the direct and stepwise routes contributed to the niche-supporting and immune megakaryocytes respectively, but contributed to the platelet-producing megakaryocytes together. Consistent with this, megakaryocytes generated through different routes displayed different activities in vitro and in vivo. Chemotherapy preferentially enhanced megakaryopoiesis through the direct route, whereas inflammation preferentially enhanced megakaryopoiesis through the stepwise route. In summary, our work links the differentiation route to the cellular heterogeneity of adult megakaryocytes. Alternative differentiation routes result in variable combinations of functionally distinct megakaryocyte subpopulations poised for different physiological demands.

A majority of studies have focused on understanding how hemogenic endothelial cells (HECs) mature into hematopoietic stem and progenitor cells (HSPCs). HECs and nascent HSPCs can be identified by the co-expression of endothelial and hematopoietic markers. However, most of the inductive signals that initiate the switch from endothelial to hematopoietic fate must occur in a subset of endothelial cells prior hematopoietic genes being expressed. Difficulties labelling the endothelial ancestors of HECs in vivo have resulted in uncharacterized endothelial cell populations that serve has the foundation for hematopoietic development. In addition, the spatial coexistence of these developmental intermediates in the mammalian embryo further challenges their characterization. Here, utilizing the rapid embryonic development of the zebrafish, which results in synchronized developmental transitions, we segregated and characterized these intermediates. Using this system, in conjunction with novel reporter lines, we traced back the endothelial ancestry of HECs. Unexpectedly, we found that these endothelial precursors have distinct molecular characteristics from their surrounding endothelial counterparts, indicating that HECs derive from a specific endothelial pool that differs from the rest of developing endothelial cells (ECs). Quiescence was one of the hallmarks of these HEC precursors, and we showed that p65 activation critically mediated it both in vivo and in vitro. In addition, when quiescence was lost by p65 ablation, HECs failed to specify. On the contrary, enforced quiescence increased the pool of HEC precursors available to transdifferentiate. Our work uncovers in vivo the previously enigmatic biology of the ECs that serve as the foundation for the hematopoietic system. This knowledge could be used to optimize in vitro protocols of HSPC generation and their derivatives.

Hematopoietic stem cells (HSCs) establish hematopoiesis and maintain regeneration of blood and immune cells to meet shifting demands for blood cell production during development and throughout life. The protein homeostasis (proteostasis) network is uniquely configured in adult HSCs to preserve stem cell fitness and longevity. However, how proteostasis is regulated in developing HSCs is largely unexplored. Here, we comprehensively analyzed proteostasis network activity throughout fetal and neonatal development. Fetal HSCs exhibited up to 7-fold higher protein synthesis rates than their adult counterparts but contained similarly low amounts of unfolded and misfolded proteins as adult HSCs without substantial shifts in protein degradation activity. These data suggested that fetal and adult HSCs utilize distinct mechanisms to preserve proteostasis. We found that fetal HSCs preferentially activated Hsf1, a key proteostasis sensor, and preferentially expressed multiple Hsf1 target genes. Deletion of Hsf1 in the developing hematopoietic system altered HSC ontogeny during the transition from the fetal liver to the bone marrow. Strikingly, HSCs exhibited a dramatic spike in unfolded protein abundance at birth, raising the question of whether proteostasis disruption impairs the function of temporally analogous human umbilical cord blood-derived HSCs. To test this, cord blood CD34+ cells were sorted based on unfolded protein content. Cord blood hematopoietic stem and progenitor cells with low unfolded protein content formed up to 9-fold more colonies than cells with high unfolded protein, which also exhibited diminished reconstituting activity in vivo. Overall, distinct regulation of proteostasis is a key developmental feature of HSCs that could be leveraged to optimize HSC-based therapeutics.