Scientific Resources

The receptor tyrosine kinase c-Kit plays a critical role in hematopoiesis,and gain-of-function mutations of the receptor are frequently seen in several malignancies,including acute myeloid leukemia,gastrointestinal stromal tumors,and testicular carcinoma. The most common mutation of c-Kit in these disorders is a substitution of the aspartic acid residue in position 816 to a valine (D816V),leading to constitutive activation of the receptor. In this study,we aimed to investigate the role of Src family kinases in c-Kit/D816V signaling. Src family kinases are necessary for the phosphorylation of wild-type c-Kit as well as of activation of downstream signaling pathways including receptor ubiquitination and the Ras/Mek/Erk pathway. Our data demonstrate that,unlike wild-type c-Kit,the phosphorylation of c-Kit/D816V is not dependent on Src family kinases. In addition,we found that neither receptor ubiquitination nor Erk activation by c-Kit/D816V required activation of Src family kinases. In vitro kinase assay using synthetic peptides revealed that c-Kit/D816V had an altered substrate specificity resembling Src and Abl tyrosine kinases. We further present evidence that,in contrast to wild-type c-Kit,Src family kinases are dispensable for c-Kit/D816V cell survival,proliferation,and colony formation. Taken together,we demonstrate that the signal transduction pathways mediated by c-Kit/D816V are markedly different from those activated by wild-type c-Kit and that altered substrate specificity of c-Kit circumvents a need for Src family kinases in signaling of growth and survival,thereby contributing to the transforming potential of c-Kit/D816V. View Publication

The number of neutrophils in the blood is tightly regulated to ensure adequate protection against microbial pathogens while minimizing damage to host tissue. Neutrophil homeostasis in the blood is achieved through a balance of neutrophil production,release from the bone marrow,and clearance from the circulation. Accumulating evidence suggests that signaling by CXCL12,through its major receptor CXCR4,plays a key role in maintaining neutrophil homeostasis. Herein,we generated mice with a myeloid lineage-restricted deletion of CXCR4 to define the mechanisms by which CXCR4 signals regulate this process. We show that CXCR4 negatively regulates neutrophil release from the bone marrow in a cell-autonomous fashion. However,CXCR4 is dispensable for neutrophil clearance from the circulation. Neutrophil mobilization responses to granulocyte colony-stimulating factor (G-CSF),CXCL2,or Listeria monocytogenes infection are absent or impaired,suggesting that disruption of CXCR4 signaling may be a common step mediating neutrophil release. Collectively,these data suggest that CXCR4 signaling maintains neutrophil homeostasis in the blood under both basal and stress granulopoiesis conditions primarily by regulating neutrophil release from the bone marrow. View Publication

The constitutive activation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway commonly occurs in cancers and is a crucial event in tumorigenesis. Chronic myelogenous leukemia (CML) is characterized by a reciprocal chromosomal translocation (9;22) that generates the Bcr-Abl fusion gene. The PI3K/Akt pathway is activated by Bcr-Abl chimera protein and mediates the leukemogenesis in CML. However,the mechanism by which Bcr-Abl activates the PI3K/Akt pathway is not completely understood. In the present study,we found that pleckstrin homology domain leucine-rich repeat protein phosphatases 1 and 2 (PHLPP1 and PHLPP2) were depleted in CML cells. We investigated the interaction between PHLPPs and Bcr-Abl in CML cell lines and Bcr-Abl+ progenitor cells from CML patients. The Abl kinase inhibitors and depletion of Bcr-Abl induced the expression of PHLPP1 and PHLPP2,which dephosphorylated Ser-473 on Akt1,-2,and -3,resulting in inhibited proliferation of CML cells. The reduction of PHLPP1 and PHLPP2 expression by short interfering RNA in CML cells weakened the Abl kinase inhibitor-mediated inhibition of proliferation. In colony-forming unit-granulocyte,erythroid,macrophage,megakaryocyte; colony-forming unit-granulocyte,macrophage; and burst-forming unit-erythroid,treatment with the Abl kinase inhibitors and depletion of Bcr-Abl induced PHLPP1 and PHLPP2 expression and inhibited colony formation of Bcr-Abl+ progenitor cells,whereas depletion of PHLPP1 and PHLPP2 weakened the inhibition of colony formation activity by the Abl kinase inhibitors in Bcr-Abl+ progenitor cells. Thus,Bcr-Abl represses the expression of PHLPP1 and PHLPP2 and continuously activates Akt1,-2,and -3 via phosphorylation on Ser-473,resulting in the proliferation of CML cells. View Publication

Induced pluripotent stem cells (iPSCs) provide an unprecedented opportunity for modeling of human diseases in vitro,as well as for developing novel approaches for regenerative therapy based on immunologically compatible cells. In this study,we employed an OP9 differentiation system to characterize the hematopoietic and endothelial differentiation potential of seven human iPSC lines obtained from human fetal,neonatal,and adult fibroblasts through reprogramming with POU5F1,SOX2,NANOG,and LIN28 and compared it with the differentiation potential of five human embryonic stem cell lines (hESC,H1,H7,H9,H13,and H14). Similar to hESCs,all iPSCs generated CD34(+)CD43(+) hematopoietic progenitors and CD31(+)CD43(-) endothelial cells in coculture with OP9. When cultured in semisolid media in the presence of hematopoietic growth factors,iPSC-derived primitive blood cells formed all types of hematopoietic colonies,including GEMM colony-forming cells. Human induced pluripotent cells (hiPSCs)-derived CD43(+) cells could be separated into the following phenotypically defined subsets of primitive hematopoietic cells: CD43(+)CD235a(+)CD41a(+/-) (erythro-megakaryopoietic),lin(-)CD34(+)CD43(+)CD45(-) (multipotent),and lin(-)CD34(+)CD43(+)CD45(+) (myeloid-skewed) cells. Although we observed some variations in the efficiency of hematopoietic differentiation between different hiPSCs,the pattern of differentiation was very similar in all seven tested lines obtained through reprogramming of human fetal,neonatal,or adult fibroblasts with three or four genes. Although several issues remain to be resolved before iPSC-derived blood cells can be administered to humans for therapeutic purposes,patient-specific iPSCs can already be used for characterization of mechanisms of blood diseases and for identification of molecules that can correct affected genetic networks. View Publication

Chronic myelogenous leukemia (CML) is a hematopoietic disorder originating from p210BCR/ABL-transformed stem cells,which begins as indolent chronic phase (CP) but progresses into fatal blast crisis (BC). To investigate molecular mechanism(s) underlying disease evolution,CML-exhibiting p210BCR/ABL transgenic mice were crossed with BXH2 mice that transmit a replication-competent retrovirus. Whereas nontransgenic mice in the BXH2 background exclusively developed acute myeloid leukemia,p210BCR/ABL transgenic littermates developed nonmyeloid leukemias,in which inverse polymerase chain reaction detected 2 common viral integration sites (CISs). Interestingly,one CIS was transgene's own promoter,which up-regulated p210BCR/ABL expression. The other was the 5' noncoding region of a transcription factor,Zfp423,which induced aberrant Zfp423 expression. The cooperative activities of Zfp423 and p210BCR/ABL were demonstrated as follows: (1) introduction of Zfp423 in p210BCR/ABL transgenic bone marrow (BM) cells increased colony-forming ability,(2) suppression of ZNF423 (human homologue of Zfp423) in ZNF423-expressing,p210BCR/ABL-positive hematopoietic cells retarded cell growth,(3) mice that received a transplant of BM cells transduced with Zfp423 and p210BCR/ABL developed acute leukemia,and (4) expression of ZNF423 was found in human BCR/ABL-positive cell lines and CML BC samples. These results demonstrate that enhanced expression of p210BCR/ABL and deregulated expression of Zfp423/ZNF423 contribute to CML BC. View Publication

BACKGROUND: Hematopoietic progenitors are generated in the yolk sac and aorta-gonad-mesonephros region during early mouse development. At embryonic day 10.5 the first hematopoietic stem cells emerge in the aorta-gonad-mesonephros. Subsequently,hematopoietic stem cells and progenitors are found in the fetal liver. The fetal liver is a potent hematopoietic site,playing an important role in the expansion and differentiation of hematopoietic progenitors and hematopoietic stem cells. However,little is known concerning the regulation of fetal liver hematopoietic stem cells. In particular,the role of cytokines such as interleukin-1 in the regulation of hematopoietic stem cells in the embryo has been largely unexplored. Recently,we observed that the adult pro-inflammatory cytokine interleukin-1 is involved in regulating aorta-gonad-mesonephros hematopoietic progenitor and hematopoietic stem cell activity. Therefore,we set out to investigate whether interleukin-1 also plays a role in regulating fetal liver progenitor cells and hematopoietic stem cells. DESIGN AND METHODS: We examined the interleukin-1 ligand and receptor expression pattern in the fetal liver. The effects of interleukin-1 on hematopoietic progenitor cells and hematopoietic stem cells were studied by FACS and transplantation analyses of fetal liver explants,and in vivo effects on hematopoietic stem cell and progenitors were studied in Il1r1(-/-) embryos. RESULTS: We show that fetal liver hematopoietic progenitor cells express the IL-1RI and that interleukin-1 increases fetal liver hematopoiesis,progenitor cell activity and promotes hematopoietic cell survival. Moreover,we show that in Il1r1(-/-) embryos,hematopoietic stem cell activity is impaired and myeloid progenitor activity is increased. CONCLUSIONS: The IL-1 ligand and receptor are expressed in the midgestation liver and act in the physiological regulation of fetal liver hematopoietic progenitor cells and hematopoietic stem cells. View Publication

Loss of neurofibromin or interferon consensus sequence binding protein (Icsbp) leads to a myeloproliferative disorder. Transcription of NF1 is directly controlled by ICSBP. It has been postulated that loss of NF1 expression resulting from loss of transcriptional activation by ICSBP contributes to human hematologic malignancies. To investigate the functional cooperation of these 2 proteins,we have established Icsbp-deficient mice with Nf1 haploinsufficiency. We here demonstrate that loss of Icsbp and Nf1 haploinsufficiency synergize to induce a forced myeloproliferation in Icsbp-deficient mice because of an expansion of a mature myeloid progenitor cell. Furthermore,Nf1 haploinsufficiency and loss of Icsbp contribute synergistically to progression of the myeloproliferative disorder toward transplantable leukemias. Leukemias are characterized by distinct phenotypes,which correlate with progressive genetic abnormalities. Loss of Nf1 heterozygosity is not mandatory for disease progression,but its occurrence with other genetic abnormalities indicates progressive genetic alterations in a defined subset of leukemias. These data show that loss of the 2 tumor suppressor genes Nf1 and Icsbp synergize in the induction of leukemias. View Publication

Dysregulation of the receptor tyrosine kinase fibroblast growth factor receptor 3 (FGFR3) plays a pathogenic role in a number of human hematopoietic malignancies and solid tumors. These include t(4;14) multiple myeloma associated with ectopic expression of FGFR3 and t(4;12)(p16;p13) acute myeloid leukemia associated with expression of a constitutively activated fusion tyrosine kinase,TEL-FGFR3. We recently reported that FGFR3 directly tyrosine phosphorylates RSK2 at Y529,which consequently regulates RSK2 activation. Here we identified Y707 as an additional tyrosine in RSK2 that is phosphorylated by FGFR3. Phosphorylation at Y707 contributes to RSK2 activation,through a putative disruption of the autoinhibitory alphaL-helix on the C terminus of RSK2,unlike Y529 phosphorylation,which facilitates ERK binding. Moreover,we found that FGFR3 interacts with RSK2 through residue W332 in the linker region of RSK2 and that this association is required for FGFR3-dependent phosphorylation of RSK2 at Y529 and Y707,as well as the subsequent RSK2 activation. Furthermore,in a murine bone marrow transplant assay,genetic deficiency in RSK2 resulted in a significantly delayed and attenuated myeloproliferative syndrome induced by TEL-FGFR3 as compared with wild-type cells,suggesting a critical role of RSK2 in FGFR3-induced hematopoietic transformation. Our current and previous findings represent a paradigm for tyrosine phosphorylation-dependent regulation of serine-threonine kinases. View Publication

Autosomal recessive osteopetrosis (ARO) is a group of genetic disorders that involve defects that preclude the normal function of osteoclasts,which differentiate from hematopoietic precursors. In half of human cases,ARO is the result of mutations in the TCIRG1 gene,which codes for a subunit of the vacuolar proton pump that plays a fundamental role in the acidification of the cell-bone interface. Functional mutations of this pump severely impair the resorption of bone mineral. Although postnatal hematopoietic stem cell transplantation can partially rescue the hematological phenotype of ARO,other stigmata of the disease,such as secondary neurological and growth defects,are not reversed. For this reason,ARO is a paradigm for genetic diseases that would benefit from effective prenatal treatment. Using the oc/oc mutant mouse,a murine model whose osteopetrotic phenotype closely recapitulates human TCIRG1-dependent ARO,we report that in utero transplantation of adult bone marrow hematopoietic stem cells can correct the ARO phenotype in a limited number of mice. Here we report that in utero injection of allogeneic fetal liver cells,which include hematopoietic stem cells,into oc/oc mouse fetuses at 13.5 days post coitum produces a high level of engraftment,and the oc/oc phenotype is completely rescued in a high percentage of these mice. Therefore,oc/oc pathology appears to be particularly sensitive to this form of early treatment of the ARO genetic disorder. View Publication

The basic helix-loop-helix transcription factor stem cell leukemia gene (Scl) is a master regulator for hematopoiesis essential for hematopoietic specification and proper differentiation of the erythroid and megakaryocyte lineages. However,the critical downstream targets of Scl remain undefined. Here,we identified a novel Scl target gene,transcription factor myocyte enhancer factor 2 C (Mef2C) from Scl(fl/fl) fetal liver progenitor cell lines. Analysis of Mef2C(-/-) embryos showed that Mef2C,in contrast to Scl,is not essential for specification into primitive or definitive hematopoietic lineages. However,adult VavCre(+)Mef2C(fl/fl) mice exhibited platelet defects similar to those observed in Scl-deficient mice. The platelet counts were reduced,whereas platelet size was increased and the platelet shape and granularity were altered. Furthermore,megakaryopoiesis was severely impaired in vitro. Chromatin immunoprecipitation microarray hybridization analysis revealed that Mef2C is directly regulated by Scl in megakaryocytic cells,but not in erythroid cells. In addition,an Scl-independent requirement for Mef2C in B-lymphoid homeostasis was observed in Mef2C-deficient mice,characterized as severe age-dependent reduction of specific B-cell progenitor populations reminiscent of premature aging. In summary,this work identifies Mef2C as an integral member of hematopoietic transcription factors with distinct upstream regulatory mechanisms and functional requirements in megakaryocyte and B-lymphoid lineages. View Publication

Regulation of cellular metabolism by the citric acid cycle occurs in the mitochondria. However,the citric acid cycle intermediate succinate was shown recently to be a ligand for the G-protein-coupled receptor GPR91. Here,we describe a role for succinate and its receptor in the stimulation of hematopoietic progenitor cell (HPC) growth. GPR91 mRNA and protein expression were detected in human bone marrow CD34+ progenitor cells,as well as in erythroid and megakaryocyte cultures and the erythroleukemic cell line TF-1. Treatment of these cell cultures with succinate resulted in increased proliferation rates. The proliferation response of TF-1 cells was pertussis toxin (PTX)-sensitive,suggesting a role for Gi signaling. Proliferation was also blocked when TF-1 cells were transfected with small interfering RNA specific for GPR91. Succinate stimulated activation of the Erk MAPK pathway and inositol phosphate accumulation in a PTX-sensitive manner. Pretreatment of TF-1 cells with the Erk1/2 kinase (MEK) inhibitor PD98059 blocked the proliferation response. Succinate treatment additionally protected TF-1 cells from cell death induced by serum deprivation. Finally,in vivo administration of succinate was found to elevate the levels of hemoglobin,platelets,and neutrophils in a mouse model of chemotherapy-induced myelosuppression. These results suggest that succinate-GPR91 signaling is capable of promoting HPC development. View Publication

Scl and Lyl1 encode two related basic-helix-loop-helix transcription factors implicated in T cell acute lymphoblastic leukemia. Previous studies showed that Scl is essential for embryonic and adult erythropoiesis,while Lyl1 is important for B cell development. Single-knockout mice have not revealed an essential function for Scl or Lyl1 in adult hematopoietic stem cells (HSCs). To determine if maintenance of HSCs in single-knockout mice is due to functional redundancy,we generated Lyl1;Scl-conditional double-knockout mice. Here,we report a striking genetic interaction between the two genes,with a clear dose dependence for the presence of Scl or Lyl1 alleles for HSC function. Bone marrow repopulation assays and analyses demonstrated rapid loss of hematopoietic progenitors due to apoptosis. The function of HSCs could be rescued by a single allele of Lyl1 but not Scl. These results show that expression of at least one of these factors is essential for maintenance of adult HSC function. View Publication