Liver Hematopoietic Stem And Progenitor Cells Research Articles

In vitro expansion of fetal liver hematopoietic stem cells

Fetal liver hematopoietic stem and progenitor cells (HSPCs) have been considered appropriate for the management of aplastic anemia owing to their proliferative potential. Bone marrow recovery was possible in some cases; the engraftment potential of these cells, however was unsatisfactory, possibly due to the availability of a smaller number of these cells from a single fetus. The present study explores how we can expand fetal liver hematopoietic stem cells under in vitro conditions. We isolated mononuclear cells from fetal liver and hematopoietic stem cells were identified and analyzed by cell surface marker CD34. CD34+ fetal liver HSPCs cells were separated by magnetic cell sorting positive selection method. HSPCs (CD34+) were cultured by using 5 cytokines, stem cell factor (SCF), granulocyte macrophages-colony stimulating factor (GM-CSF), interleukin-6 (IL-6), Fms-related tyrosine kinase 3 (FLT-3) and erythropoietin (EPO), in 4 different combinations along with supplements, in serum-free culture media for 21 days. Cell viability continued to be greater than 90% throughout 21 days of culture. The cells expanded best in a combination of media, supplements and 5 cytokines, namely SCF, FLT-3, IL6, EPO and GM-CSF to yield a large number of total (CD34+ & CD34-) cells. Even though the total number of nucleated cells increased in culture significantly, levels of CD34 antigen expression declined steadily over this period.

Repetitive Elements Trigger RIG-I-like Receptor Signaling that Regulates the Emergence of Hematopoietic Stem and Progenitor Cells

Inflammatory signaling is required for hematopoietic stem and progenitor cell (HSPC) development. Here, we studied the involvement of RIG-I-like receptors (RLRs) in HSPC formation. Rig-I or Mda5 deficiency impaired, while Lgp2 deficiency enhanced, HSPC emergence in zebrafish embryos. Rig-I or Mda5 deficiency reduced HSPC numbers by inhibiting inflammatory signals that were in turn enhanced in Lgp2 deficient embryos. Simultaneous reduction of Lgp2 and either Rig-I or Mda5 rescued inflammatory signals and HSPC numbers. Modulating the expression of the signaling mediator Traf6 in RLR deficient embryos restored HSPC numbers. Repetitive element transcripts could be detected in hemogenic endothelial cells and HSPCs, suggesting a role as RLR ligands. Indeed, ectopic expression of repetitive elements enhanced HSPC formation in wild-type, but not in Rig-I or Mda5 deficient embryos. Manipulation of RLR expression in mouse fetal liver HSPCs indicated functional conservation among species. Thus, repetitive elements transcribed during development drive RLR-mediated inflammatory signals that regulate HSPC formation.

Fli-1 Transcriptionally Integrates Microenvironmental Cues Sensing By Self-Renewing Hematopoietic Stem and Progenitor Cells

During adulthood and embryogenesis, fate decisions of hematopoietic stem and progenitor cells (HSPCs), such as specification, self-renewal, and differentiation are tightly regulated by their neighboring niche cells. Moreover, distinct types of niches supply differential cues to direct alternative cell fates for HSPCs. Yet, currently the intrinsic mechanisms balancing HSPC response obliqueness to microenvironmental signals are unknown. Friend Leukemia integration-1 (Fli-1), is an ETS transcription factor expressed by vascular beds and hematopoietic lineages. Fli-1 belongs to the "heptad factors" which are hypothesized to specify and sustain a hematopoietic cell fate. While Fli-1 overexpression is linked to leukemia, the functional role Fli-1 plays in HSPC specification and maintenance remains undefined. We show that inducible deletion of Fli-1 using a Rosa-CreERT2 transgenic adult mice (Fli-1ROSAΔ), results in a rapid thrombocytopenia-associated mortality. Transplantation of Fli-1ROSAΔ bone marrow (BM) cells into WT recipients, to exclude vascular-mediated defects, followed by induction of Fli-1 deletion, resulted with the same phenotype. In a set of modulated competitive transplantation experiments (differential induction time points pre- or post-transplant), we observed defective ability of Fli-1ROSAΔ HSPCs to lodge, engraft, and to sustain hematopoiesis post repopulation. Fli-1 deficient HSPCs exhibited reduced quiescent cell cycling status, a hallmark of stemness, and displayed enhanced apoptosis. Thus, Fli-1 is essential for previously unrecognized cell-autonomous HSPC functions. To determine whether Fli-1 modulates HSPC specification, Fli-1 was conditionally deleted using a developmental VE-cadherin (CDH5)-Cre transgenic model (Fli-1CDH5Δ). This resulted with premature mortality of Fli-1CDH5Δ embryos, accompanied with a hemorrhagic phenotype. Reduced numbers of hematopoietic cells were still detected in the AGM of e10.5 Fli-1CDH5Δ embryos. Conditional Fli-1 deletion using a developmental hematopoietic Vav-1 Cre transgenic model (Fli-1Vav-1Δ) resulted again with premature mortality. Reduced presence of embryonic Fli-1Vav-1Δ liver HSPCs was observed at e12.5. We also applied two in vitro co-culture systems, to study Fli-1 in endothelial to hematopoietic transition (EHT). First, isolated hemogenic endothelial cells (HEC) from WT and Fli-1ROSAΔ embryos were co-cultured with AGM-derived vascular niche. HECs isolated from Fli-1ROSAΔ AGM were still able to convert to CD45+ cells, however these cells did not expand on a vascular niche. Secondly, we have applied an endothelial to hematopoietic reprogramming system in which isolated lung ECs are virally introduced with DOX inducible FosB, Gfi1, Runx1, and Spi1 (FGRS) factors and co-cultured with vascular niche cells. Both WT and Fli-1ROSAΔ ECs were able to acquire a hemogenic like state resulting with a final capacity to convert into hematopoietic cells. Again, Fli-1ROSAΔ cells displayed lesser numbers of CD45+ cells at the end point, presumably due to impaired interaction with the vascular niche. Indeed, reduced expansion capacity was observed both for mature CD45+ and for HSPC derived from Fli-1CDH5Δ AGM region. Adult Fli-1ROSAΔ HSPCs exhibited the same niche-dependent expansion defect. Induction of Fli-1 deletion in vitro in adult HSPCs revealed loss of dependency on vascular niche inductive signals, as no additive expansion effect was observed for Fli-1ROSAΔ HSPCs in the presence of a vascular niche. Hence, Fli-1 is essential for HSPC expansion rather than hematopoietic specification. Differential RNA-seq analysis combined with epigenetic studies of expanding WT and Fli-1ROSAΔ HSPCs, revealed dysregulation of Fli-1-controlled pathways involved in transduction of microenvironmental signals for self-renewal. Unexpectedly, H3K27Ac analysis, a marker for transcriptional priming, revealed increased global acetylation of Fli-1ROSAΔ HSPCs' chromatin. Therefor, Fli-1 may not only perform as transcription activator, but foremostly as a genomic suppressor via modulation of histone acetylation status. Decrypting the mechanism(s) by which Fli-1 orchestrates HSPC self-renewal, may promote an improved expansion protocol of human HSPC pre-transplantation, and provide additional insights for microenvironmental sensing by Fli-1-dependent leukemic cells. Disclosures No relevant conflicts of interest to declare.

Characterization of a Novel JAK1 Pseudokinase Mutation in the First Case of Trisomy 21-Independent GATA1-Mutated Transient Abnormal Myelopoiesis

Clonal proliferation of megakaryoblasts, called transient abnormal myelopoiesis (TAM), is a rare disease of newborns triggered by trisomy 21 (constitutional or somatic) together with acquired mutations of GATA1 resulting in the exclusive production of its short variant - GATA1s. No other TAM drivers have been described so far. We have diagnosed a unique TAM case with a typical clinical and laboratory manifestation but without the gain (or any other aberration) of chromosome 21. Thorough genomic profiling revealed 4 somatic mutations: GATA1 D65_C228del, JAK1 F636del, FN1 R2420C and SPIRE2 R471W. With respect to the generally accepted 2-hit theory, we hypothesized that this TAM arose from a collaboration of the atypical GATA1 mutation (not inducing GATA1s) with (at least) one of the other identified mutations. Unlike SPIRE2 and FN1 aberrations, various mutations of the JAK1 kinase have been previously described as leukemia drivers, suggesting JAK1 F636del as a top candidate for the second hit. Moreover, JAK1 mutations have been associated with the transformation of TAM into acute megakaryoblastic leukemia (Nikolaev et al., Blood, 2013). The aim of our project was to functionally characterize this novel JAK1 mutation. Phenylalanine 636 belongs to a phylogenetically conserved triad of amino acids suggested to control catalytic activity of JAK1 via mediating a switch between the supposedly active and inactive conformations (Toms et al., Nat Struct Mol Biol, 2013). Hence, F636 seems to be essential for JAK1 function. Surprisingly, homology modeling showed that loss of F636 is compatible with both functionally opposite conformations. Indeed, Western blot analysis of JAK/STAT signaling in transiently transformed HEK293T cells showed that catalytic activity is preserved in JAK1 F636del. However, we observed lower levels of auto- and STATs- phosphorylation compared to wild-type (wt) JAK1 suggesting decreased kinase activity of JAK1 F636del. Subsequently, we tested the oncogenic potential of JAK1 F636del in the Ba/F3 cell assay; unlike the known oncogenic JAK1 variant (JAK1 V658I), JAK1 F636del did not induce IL3-independent growth. To further assess phenotypic impact of F636del, we introduced JAK1 F636del into murine bone marrow and fetal liver hematopoietic stem and progenitor cells (HSPCs) using lentiviruses and performed colony forming assays. The number and morphology of colonies did not differ in JAK1 F636del compared to wt JAK1. Furthermore, we assessed the impact of JAK1 F636del in the context of mutated GATA1. We utilized the in-vitro model recently described by Labuhn et al. (Cancer Cell, 2019), in which the CRISPR/Cas9-mediated induction of Gata1s expression leads to the expansion and sustained proliferation of fetal liver HSPCs from embryonic day 13.5 ROSA26:Cas9-EGFPki/wt mice. Similar to wt JAK1, lentivirally introduced JAK1 F636del had no impact on the proliferation and maturation status of such Gata1-edited HPSCs irrespective of the timing of its introduction (simultaneously with Gata1 editing versus into fully established Gata1-edited culture) or of culturing conditions (fully cytokine-supplemented growth-supportive versus cytokine-depleted growth-restrictive medium). Altogether, we show that unlike known oncogenic variants, F636del identified in the first case of trisomy 21-independent TAM attenuates JAK1 kinase activity. The results of our phenotypic studies question the potential contribution of this mutation to TAM development. Interestingly, Labuhn et al. (2019) recently showed that non-activating JAK mutations occur at higher than random frequency in trisomy 21-dependent TAM. This tempts us to speculate that JAK1 mutations may still play a role in TAM. Yet, this role may significantly differ from that of known oncogenic mutations; it may result from attenuation/modulation instead of activation of downstream signaling and it may remain unrevealed utilizing the currently available sophisticated, yet still imperfect experimental models. Support: GAUK 86218, EHA Research Mobility Grant Disclosures No relevant conflicts of interest to declare.

Interferon gamma inhibits the differentiation of mouse adult liver and bone marrow hematopoietic stem cells by inhibiting the activation of notch signaling

BackgroundThe paradigm of hematopoietic stem and progenitor cells (HSPCs) has become accepted ever since the discovery of adult mouse liver hematopoietic stem cells and their multipotent characteristics that give rise to all blood cells. However, differences between bone marrow (BM) and liver hematopoietic stem cells and the hematopoietic microenvironment remain poorly understood. In addition, the regulation of the liver hematopoietic system remains unknown.MethodsClone formation assays were used to confirm that the proliferation of adult mouse liver and bone marrow HSPCs. Model mice with different interferon gamma (IFN-γ) levels and a co-culture system were used to detect the differentiation of liver HSPCs. The γ-secretase inhibitor (GSI) and the JAK/STAT inhibitor ruxolitinib and cell culture assays were used to explore the molecular mechanism by which IFN-γ impairs HSPC proliferation and differentiation.ResultsThe colony-forming activity of liver and bone marrow HSPCs was inhibited by IFN-γ. Model mice with different IFN-γ levels showed that the differentiation of liver HSPCs was impaired by IFN-γ. Using a co-culture system comprising liver HSPCs, we found that IFN-γ inhibited the development of liver hematopoietic stem cells into γδT cells. We then demonstrated that IFN-γ might impair liver HSPC differentiation by inhibiting the activation of the notch signaling via the JAK/STAT signaling pathway.ConclusionsIFN-γ inhibited the proliferation of liver-derived HSPCs. IFN-γ also impaired the differentiation of long-term hematopoietic stem cells (LT-HSCs) into short-term hematopoietic stem cells (ST-HSCs) and multipotent progenitors (MPPs) and the process from LSK (Lineage−Sca-1+c-Kit+) cells to γδT cells. Importantly, we proposed that IFN-γ might inhibit the activation of notch signaling through the JAK/STAT signaling pathway and thus impair the differentiation process of mouse adult liver and BM hematopoietic stem cells.

Kupffer Cells Promote the Differentiation of Adult Liver Hematopoietic Stem and Progenitor Cells into Lymphocytes via ICAM-1 and LFA-1 Interaction.

It has been reported that the adult liver contains hematopoietic stem and progenitor cells (HSPCs), which are associated with long-term hematopoietic reconstitution activity. Hepatic hematopoiesis plays an important role in the generation of cells involved in liver diseases. However, how the progenitors differentiate into functional myeloid cells and lymphocytes in the liver microenvironment remains unknown. In the present study, HSPC transplantation experiments were used to confirm that adult murine liver HSPCs differentiate into both myeloid cells and lymphocytes (preferentially T cells) compared with bone marrow HSPCs. Using a coculture system comprised of kupffer cells and HSPCs, we found that kupffer cells promote adult liver HSPCs to primarily generate T cells and B cells. We then demonstrated that kupffer cells can also promote HSPC expansion. A blockade of intercellular cell adhesion molecule-1 (ICAM-1) in a liver HSPC and kupffer cell coculture system impaired the adhesion, expansion, and differentiation of HSPCs. These results suggest a critical role of kupffer cells in the maintenance and promotion of adult mouse liver hematopoiesis. These findings provide important insight into understanding liver extramedullary hematopoiesis and its significance, particularly under the state of some liver diseases, such as hepatitis, nonalcoholic fatty liver disease (NAFLD), and hepatocellular carcinoma (HCC).

A GATA-2-GPR65 Regulatory Module Controls Hematopoietic Stem Cell Generation

The generation of hematopoietic stem cells (HSCs) via endothelial-to-hematopoietic transition within the aorta-gonad-mesonephros (AGM) region of the mammalian embryo is crucial for development of the adult hematopoietic system. Many questions remain unanswered regarding the molecular program in hemogenic endothelium that promotes the budding of hematopoietic cell clusters containing HSCs. We demonstrated that a deletion of a Gata2 cis-element reduced GATA-2 levels in the AGM and abrogated the capacity of hemogenic endothelium to generate HSCs. Consistent with the defective HSC generator, the mutant fetal livers were deficient in hematopoietic stem and progenitor cells (HSPCs). Using an ex vivo intact AGM culture system, we demonstrated that retrovirus-mediated GATA-2 expression in the +9.5-/- AGM rescues its hematopoietic defect. Thus, the reduced GATA-2 levels in the +9.5-/- AGM cause the HSC generation defect, and this rescue assay provides a unique system to decipher the downstream genetic network. To discover novel druggable regulators in the GATA-2 pathway to promote HSC generation, we profiled the expression pattern of all G-protein-coupled-receptors, which represent the most successful class of pharmaceutical targets, in the AGM using our RNA-seq dataset (+9.5+/+ vs. +9.5-/- AGM). This global GPCR analysis revealed four GATA-1 and GATA-2 co-regulated genes, Adora3, Gpr65, Ltb4r1, and Adora2b. Database mining revealed that only the Gpr65 expression pattern resembled that of Gata2. To evaluate GPR65 functions during HSC generation, we conducted an shRNA-based loss-of-function analysis in the AGM. While downregulating Gpr65 did not alter the abundance of the CD31+ c-Kit+ hematopoietic cell population, it significantly increased the CD31+ c-Kit+ Sca1+ HSC-containing cell population (1.4 fold, p<0.05), indicating that GPR65 suppresses HSC generation. To validate the involvement of GPR65 during the HSC generation process in vivo, we conducted a morpholino oligonucleotide (MO)-based loss-of-function study in zebrafish. In situ hybridization analysis revealed high Runx1/c-Myb expression (labeling definitive HSCs and progenitors) in 48% of embryos injected with Gpr65 MOs compared with 11% of wild type embryos. Consistent with the ex vivo AGM analysis, this increase in Runx1/c-Myb expression upon Gpr65 MO treatment suggests GPR65 is a negative regulator of HSC emergence in vivo. To dissect the molecular mechanism governing GPR65-suppressed HSC generation, we tested whether lowering Gpr65 levels altered the expression of key HSC regulators. Quantitative RT-PCR analysis revealed that downregulating Gpr65 by 60-70% in AGM CD31+ c-Kit- endothelialcells increased Gata2 mRNA by 2.9 fold (p<0.05), Gata2 primary transcripts by 3.9 fold (p<0.05), and elevated expression of the GATA-2 target gene Runx1 (2.9 fold, p<0.05). These results support a mechanism whereby GPR65-mediated Gata2 repression is an important determinant of GPR65-suppressed HSC generation. In addition to this important function in the AGM, Gpr65 knockdown studies in primary fetal liver HSPCs revealed GPR65 suppression of Gata2 transcription to the same magnitude as in the AGM. To determine if GPR65-mediated Gata2 repression requires the +9.5 site, we infected freshly isolated HSPCs from fetal livers heterozygous for the +9.5 site with retrovirus expressing shRNA targeting Gpr65. Quantitative RT-PCR with allele-specific primers revealed that Gpr65 knockdown significantly upregulates Gata2 primary transcripts from the wild type (3.1 fold, p<0.01), but not the 9.5 mutant, allele. These results establish a requirement of the +9.5 site for GPR65 to repress Gata2 transcription. As we reported that SetD8, the only enzyme known to monomethylate H4K20, represses Gata2 expression via the +9.5 site, we tested whether GPR65 represses Gata2 expression through SetD8. H4K20me1 ChIP revealed that downregulating Gpr65 significantly reduces H4K20me1 levels at the +9.5 site by 30% (p<0.005), suggesting that GPR65 repression of Gata2 transcription involves SetD8. Our studies indicate that a G-protein coupled receptor, GPR65, is negative regulator of HSC generation and establish a GATA-2-GPR65 Type Iincoherent feedforward loop that controls HSC generation, providing a foundation to develop new targets for expanding HSCs for transplantation therapies and a new druggable target to treat hematologic disorders. DisclosuresZon:FATE Therapeutics: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Other: Founder; Scholar Rock: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Other: Founder.

GATA1-Centered Genetic Network on Chromosome 21 Drives Down Syndrome Acute Megakaryoblastic Leukemia

Children with trisomy 21 (Down syndrome, DS) are predisposed to develop acute megakaryoblastic leukemia (DS-AMKL) as well as the antecedent transient leukemia (DS-TL). Mutations in the transcription factor GATA1 -leading to the exclusive expression of the shorter GATA1 isoform (GATA1s)- are present in nearly all children with DS-AMKL and DS-TL. GATA1s is both essential and sufficient to cause DS-TL in synergy with trisomy 21.To elucidate how the presence of an extra copy of chromosome 21 (hsa21) perturbs fetal hematopoiesis to provide a GATA1s-sensitive background during trisomy 21-associated leukemogenesis, we integrated an RNAi viability screening (512 shRNAmirs against 210 genes on hsa21) and a proteomics approach creating an hsa21 oncogenic network centered on GATA1s. shRNA-mediated knock-down of 42 genes conferred a profound selective growth disadvantage in DS-AMKL cell lines (CMK and CMY). A secondary functional validation screening confirmed 8 genes to specifically affect proliferation, cell viability, apoptosis or differentiation in GATA1s/trisomy-associated leukemia; whereas expression of 9 genes was also essential for proliferation and survival of erythroleukemia (K562) and non-DS-AMKL (M07) cell lines.Gain- and loss-of-function studies of 12 selected candidates (8 GATA1s/trisomy-specific oncogenes plus 4 global oncogenes) in CD34+ hematopoietic stem and progenitor cells (HSPCs) uncovered their regulatory function during megakaryopoiesis, erythropoiesis and myelopoiesis. Knockdown of four genes (USP25, BACH1, U2AF1 and C21orf33) inhibited megakaryocytic and erythroid in vitro differentiation, while enhancing myeloid differentiation. Inversely, ectopic expression of six genes (C21orf33, CHAF1B, IFNGR2, WDR4, RUNX1 or GABPA) resulted in a switch from erythroid to megakaryocytic differentiation. These 12 candidate genes acted synergistically to enhance the self-renewal efficiency of murine fetal liver cells in vitro. Pooled transduction of these genes increased the replating efficiency (more than 5 rounds) of fetal liver HSPCs whereas the colony-forming capacity was lost after second replating in the empty vector control. Further, 9 out of 12 candidate genes were overexpressed in DS-AMKL patient samples (n=23) compared to non-DS-AMKL (n=37; 1.3-fold to 2-fold) underscoring their relevance for the pathogenesis of DS-AMKL.Using an in vivo biotinylation approach to study the protein-protein interaction in DS-AMKL cells, we showed that bioGATA1 is associated with protein-complexes of 10 different hsa21-oncogenes, which are involved in splicing, deubiquitination and transcriptional regulation. Direct interactions with several of these factors are perturbed in N-terminal truncated GATA1s.Thus, we deciphered a complex interactive network on hsa21 around GATA1 positively regulating megakaryopoiesis. Deregulation of this network results in synergistic effects on hematopoietic differentiation, which can promote transformation of GATA1s-mutated fetal hematopoietic progenitor cells. DisclosuresNo relevant conflicts of interest to declare.

Metabolic Programming of the Fetal Liver Hematopoietic Stem and Progenitor Cell Pool

Evidence in several organ systems demonstrates that pregnancy presents a window of vulnerability for establishing a foundation for health or chronic disease. Overnutrition and the complex metabolic changes that can accompany it can result in permanent phenotypic changes and a predisposition to metabolic syndrome, inflammatory or immune-mediated diseases. We previously reported that prenatal overnutrition stunted fetal liver size. Herein, we hypothesize that this might perturb hematopoietic stem and progenitor cell (HSPC) expansion.To test the effects of a high-fat diet (HFD) and maternal obesity on offspring hematopoiesis, we used a mouse model of diet-induced obesity, feeding female mice a HFD or control diet starting at 5-7 weeks of age and keeping them on the respective diet during subsequent breeding and pregnancy. We then studied offspring at gestational day 14.5 by immunophenotyping, gene expression analysis, qRT-PCR, and transplantation. Fetal livers from HFD offspring had 51% fewer c-Kit+ Sca-1+ Linlo/- and 27% fewer AA4.1+ Sca-1+ Linlo/- (ASL) hematopoietic stem and progenitor cells (HSPC) than controls. This restriction in HSPC numbers was not due to apoptosis or increased reactive oxygen species, as tested by flow cytometry.To determine whether there might be an increase in hematopoietic differentiation to account for relative HSPC deficiencies in HFD livers, we examined hematopoietic lineage subsets. HFD fetal livers had a relative increase in myeloid (Gr-1+/Ter119+) and B220+ lymphoid cells, with comparable proportion of CD3+ cells to controls. Taken together, these results suggest that chronic HFD fetal programming skews fetal liver HSPCs toward differentiation.When we examined global gene expression of male HFD fetal livers versus controls by RNA-seq, we found differential expression of 125 genes. Among the upregulated transcripts, several were involved in hematopoietic regulation, stress response, and HSPC migration. We then used qRT-PCR to test for expression of several of these genes, along with genes critically involved in fetal HSPC self-renewal, within an HSPC-enriched (Sca-1+) population of chronic HFD fetal liver cells. As in RNA-seq, Matrixmetalloproteinase-8 and 9 (Mmp8, Mmp9), which are involved in cell mobilization, were upregulated in HFD-programmed cells. Early growth response-1 (Egr-1) was downregulated as well, further suggesting premature migration of HSPCs from HFD fetal liver. Hmga2, which is implicated in fetal stem cell self-renewal, and its direct target, Igf2bp2, were significantly downregulated in chronic HFD Sca-1+ cells. Along with the immunophenotyping data, these findings suggest that maternal obesity and HFD bias HSPCs toward differentiation, at the expense of self-renewal. To dissect the direct metabolic impact, we studied fetal livers from timed pregnancy cohorts after acute HFD exposure or diet reversal in obese dams, which partially ameliorated several molecular and immunophenotypic endpoints.Finally, we performed a functional test of chronic HFD fetal liver cells by transplantation. A non-competitive transplant into irradiated male recipients yielded no difference in chimerism between HFD or control fetal liver-engrafted animals. Next, we preconditioned a cohort of female and male animals on HFD (or control diet) for 11 weeks, irradiated them, and then competitively transplanted them with a 1:1 ratio of HFD and control fetal liver cells. HFD-programmed fetal liver HSPCs engrafted HFD-conditioned male recipients at significantly lower rates than in HFD-conditioned female or control recipients of either sex. HFD-programmed donor cells retained the significant bias toward the myeloid (Gr-1+/Mac-1+) lineage, noted in the primary graft cells, and away from the B220+ B cell lineage in HFD-conditioned males.In aggregate, prenatal HFD and maternal obesity suppress self-renewal in favor of HSPC differentiation during a time of critical developmental expansion. This suggests an HSPC defect that appears at least partly specified by the stem cell microenvironment. Our work is the first to demonstrate metabolic vulnerability of the hematopoietic stem and progenitor cell compartment and establishes the hematopoietic system as a target for in utero developmental programming. DisclosuresNo relevant conflicts of interest to declare.

Micro RNA Regulate Developmental Deficits In The Hematopoietic Stem Cell Compartment In Fanconi Anemia

Hematopoietic failure is the predominant manifestation of Fanconi anemia (FA), a recessively inherited disorder in DNA double strand break (DSB) repair. The etiology underlying the progressive loss of hematopoietic stem and progenitor cells (HSPC) in FA remains to be fully clarified, but is widely considered to be mediated by the “stress activation” of p53. In a murine model of FA (Fancc), we recently showed that the onset of hematopoietic failure precedes the developmentally timed HSPC migration to the bone marrow (BM), corroborating prior reports of reduced cord blood progenitor frequency and work in human ES cell lines. Comparison of genotype frequencies at E14.5 and postnatally (25% vs 19%; p<0.004) further indicated that hematopoietic failure coincides with fetal loss during late gestation. This data has now been replicated in Fancd2 animals. The quantitative and qualitative severity of fetal HSPC pool defects in Fancc and Fancd2 mice suggested to us the possibility of a unique developmental phenotype. Fetal liver HSPC are characterized by specific molecular and functional features to support the high self-renewal capacity during developmental pool expansion. To determine the molecular basis of fetal HSPC losses in FA, we systematically studied progenitor cell populations from Fancc E14.5 fetal liver and postnatal bone marrow for the expression of candidate genes involved in HSPC function. Results confirmed in utero genotype-specific p53 pathway activation and differences in key cell cycle regulators (p21, p27 and p57). Among a group of developmentally dysregulated genes, we found the non-histone chromatin remodeling factor Hmga2 substantially upregulated in FA versus WT E14.5 cells. As an important developmental regulator of self-renewal activity, Hmga2 is not expressed postnatally, which we confirmed in FA bone marrow cells. To dissect the FA phenotype further, we investigated the expression of other components of the developmental signaling cascade, in particular micro RNA (miR) let7b, the major let7 family member accounting for translational suppression of Hmga2 during late gestation. Results indicate that let7b is downregulated in E14.5 Fancc and Fancd2 progenitor cells. To reconcile the in utero activation of a major pathway promoting self-renewal and HSPC expansion with the functional and numeric defects we found in fetal FA HSPC, we investigated other upstream components. Systematic dissection of the signaling cascade revealed an 80% decline by q RT-PCR of miR-125b, a stem cell regulatory miRNA and nodal point that targets let7b (indirectly via Lin28 suppression), but also prevents inappropriate p53 activation. A relatively more profound decrease in miR-125b expression in Sca-1+ cells substantiate this as a stem/progenitor cell phenotype. The data are further consistent with our results showing the upregulation of p21 in E14.5 FL cells (as an indicator of p53 pathway activation), suggesting that the loss of miR-125b expression is a proximal mechanism for p53 upregulation in FA HSPC. Finally, when we tested for evidence of DNA damage, we observed gamma-H2AX foci in sorted HSPC as well as increased transcriptional activation of DNA repair via non-homologous end-joining (Ku70, Ku80, Prkdc transcripts) in Sca-1 enriched Fancc and Fancd2 FL cells. Based on the results to-date, we propose a model whereby relatively decreased expression of miR-125b serves as a key regulatory step for the activation of a critical self-renewal pathway (let7b-Hmga2), but also dysregulates p53 signaling during FA hematopoietic development. Accruing and unresolved DNA damage in repair-deficient fetal FA HSPC curtails their net expansion. The functional compromise in the murine HSPC pool reveals a regulatory role for miRNA in FA HSPC failure and provides a unique model for the mechanistic study of HSPC loss under physiologic conditions and without the experimental induction via exogenous cytokines, aldehydes or alkylating agents. Disclosures:No relevant conflicts of interest to declare.

Distinct Roles of Integrins α6 and α4 in Fetal Liver Hematopoietic Stem and Progenitor Cell Homing.

During development and after transplantation, intravenously injected hematopoietic stem and progenitor cells (HSPCs) selectively transmigrate through the sinusoidal walls into the bone marrow (BM) niches to engraft and reconstitute hematopoiesis. The rate of reconstitution following transplantation varies depending on the source of hematopoietic stem cells (HSCs) (To, et al 1992). However, the molecular pathways that control the homing of HSCs, in particular, of fetal HSCs are still not well understood. In the present study we studied the contribution of α6 and α4 integrins in homing of fetal liver HSPCs into adult BM by using function-blocking antibodies and an integrin α6 knockout mouse model. We found an ubiquitous expression of both integrin α6 and α4 receptors on fetal liver Lin−Sca-1+c-kit+ (LSK) HSPCs. Genetic ablation of integrin α6 resulted in reduced homing of fetal liver progenitors (HPCs) to BM of lethally irradiated adult recipients. In agreement with this, the integrin α6 antibody inhibited homing of fetal liver HPCs into BM and spleen. The role of integrin a6 in homing and engraftment of fetal liver HSCs was studied by a competitive repopulation assay by using integrin α6−/− or α6+/+ fetal liver cells. Absence of α6 integrin in fetal liver cells did not cause any engraftment defect or mobilization hypersensitivity as compared to wild-type cells. In agreement with this, anti-integrin α6 antibody did not either inhibit BM homing of short-term or long-term HSCs. In contrast, homing of fetal liver HSCs and HPCs to BM was virtually abrogated after treatment with integrin α4 antibody. Our results show that the α6 integrin receptors are functional during homing of fetal liver HPCs, but not multilineage repopulating HSC in vivo. Furthermore, we show the critical role of integrin α4 receptor for homing of both fetal liver HPCs and multilineage repopulating HSCs to BM, indicating distinct developmentally regulated functions for integrin α6 and α4 receptors during fetal hematopoiesis