Localization of the human stem cell tyrosine kinase-1 gene (FLT3) to 13q12-->q13

We have previously reported the cloning of stem cell tyrosine kinase-1 (STK-1, gene symbol FLT3), the human homolog of the mouse Flk2/Flt3 gene (Small et al., 1994). This growth factor receptor is expressed in CD34+ human bone marrow (stem/progenitor) cells. We have mapped the gene for STK-1 using PCR screening of a human-rodent somatic cell hybrid panel, and fluorescence in situ molecular hybridization (FISH) of STK-1 genomic probes to mitotic chromosomes of normal human lymphocytes. The results of both analyses show that the gene for STK-1 (FLT3) localizes to chromosome 13q12-->q13.

Cellular and molecular characterization of the role of the flk-2/flt-3 receptor tyrosine kinase in hematopoietic stem cells

The flk-2/flt-3 receptor tyrosine kinase was cloned from a hematopoietic stem cell population and is considered to play a potential role in the developmental fate of the stem cell. Using antibodies derived against the extracellular domain of the receptor, we show that stem cells from both murine fetal liver and bone marrow can express flk-2/flt-3. However, in both these tissues, there are stem cell populations that do not express the receptor. Cell cycle analysis shows that stem cells that do not express the receptor have a greater percentage of the population in G0 when compared with the flk-2/flt-3-positive population. Development of agonist antibodies to the receptor shows a proliferative role for the receptor in stem cell populations. Stimulation with an agonist antibody gives rise to an expansion of both myeloid and lymphoid cells and this effect is enhanced by the addition of kit ligand. These studies serve to further illustrate the importance of the flk-2/flt-3 receptor in the regulation of the hematopoietic stem cell.

Genomic structure of the downstream part of the human FLT3 gene: exon/intron structure conservation among genes encoding receptor tyrosine kinases (RTK) of subclass III

The FLT3 gene encodes a subclass-III receptor tyrosine kinase (RTKIII). We have determined the structural organization of the downstream part of the human FLT3 gene (also designated dsp-FLT3) that corresponds to the intracellular region of the protein. The coding region is spread over twelve exons spanning 10 kb of genomic DNA. Exon sizes range from 83 to 154 bp, while intron sizes range from 86 bp to more than 1.9 kb. Comparison with the corresponding domain of other RTKIII genes (KIT and FMS) shows that these genes share the same number of exons, which are highly conserved in size, sequence and exon/intron boundary positions. In addition, the intron phase of equivalent introns of FLT3, KIT and FMS are all identical. Our results reinforce our hypothesis based initially only on the KIT and FMS comparison showing that RTKIII genes share a common structural organization and have evolved from a common ancestor gene by cis and trans duplication. Comparison of the genomic organization of the intracellular-encoding part of RTKIII genes with that of RTKI, II and IV genes shows that subclasses III and IV are the most closely related.

Substrate specificities and identification of a putative binding site for PI3K in the carboxy tail of the murine Flt3 receptor tyrosine kinase

Flt3 is a receptor tyrosine kinase (RTK) structurally related to the CSF-1R encoded by the c-fms locus, Kit and the PDGFR which is restricted in its expression to hematopoietic precursor populations and several distinct cell types within the central nervous system. Although the ligand for Flt3 has recently been identified, the developmental function of Flt3 within these tissues has not yet been described. In order to examine the signalling properties of this receptor, we previously constructed a chimeric molecule containing the extracellular domain of CSF-1R fused to the transmembrane and cytoplasmic domain of mouse Flt3 (FF3). The ability of the FF3 to directly associate with or tyrosine phosphorylate specific cytoplasmic signalling molecules in vivo was examined. GAP, Vav, Shc, and to a lesser extent PLC gamma become tyrosine-phosphorylated but no in vivo association with the receptor was detectable. FF3 associates with PI3K activity and the SH2 domains of p85 and Grb-2. Phosphopeptide competition experiments suggest that the PI3K binding site is located outside of the kinase insert in the carboxy tail of the receptor.

Molecular cloning of a novel receptor tyrosine kinase gene, STK, derived from enriched hematopoietic stem cells

To identify the novel receptor tyrosine kinases (RTKs) critical to the proliferation of hematopoietic stem cells, we performed polymerase chain reaction-based cloning from highly purified murine hematopoietic stem cells. Lineage marker-negative, c-KIT-positive, and Ly6A/E- or Sca-1-positive (Lin-c-KIT+Sca-1+) cells were sorted by a fluorescence-activated cell sorter. Two sets of degenerate oligonucleotide primers were directed to the conserved sequences of the catalytic domain, and were used to amplify cDNAs that encode protein tyrosine kinases (PTKs). One hundred cDNA clones were sequenced and 8 RTKs were identified, as well as 12 non-RTKs and 2 serine/threonine kinases. Sixteen cDNAs were identical to the known kinase genes (PKC beta, JAK-1, JAK-2, TYK-2, HCK, FGR, FYN, BLK, c-FES, FER, c-ABL, c-KIT, FLK-1, FLK-2, IGF1R, and ECK). Six novel cDNA sequences (stk series) were identified. However, three of them turned out to be BPK, RYK, and TEK. The remaining three showed high homology to S6 kinase II, JAK-2, and v-SEA/c-MET, respectively. Characterization of full-length cDNA sequence of the v-SEA/cMET-related gene showed that this was a novel RTK gene and we named this gene STK (stem cell-derived tyrosine kinase). We identified two distinct forms of STK cDNA; the short one encoded a putative truncated protein that lacked most of the extracellular domain. STK was expressed at various stages of hematopoietic cells, including stem cells, but we could not detect any apparent expression in other adult tissues. The expression of the truncated form of mRNA was more predominant than that of the complete form. STK was assigned by fluorescent in situ hybridization to the R-positive F1 band of chromosome 9, the same region to which hepatic growth factor-like protein has been assigned. Characterization of these PTKs, including STK, will be helpful to elucidate the molecular mechanism of the growth regulation of hematopoietic stem cells.

Cloning of the human homologue of the murine flt3 ligand: a growth factor for early hematopoietic progenitor cells

Using a fragment of the murine flt3 ligand as a probe, we have succeeded in cloning a human flt3 ligand from a human T-cell lambda gt10 cDNA library. The human and murine ligands are 72% identical at the amino acid level. Analysis of multiple cDNA clones shows that alternative splicing of the human flt3 mRNA can occur at a number of positions. A recombinant soluble form of the human flt3 ligand stimulates the proliferation and colony formation of a subpopulation of human bone marrow cells that are CD34+ and are enriched for primitive hematopoietic cells. In addition, the human flt3 ligand also stimulates the proliferation of cells expressing murine flt3 receptors. Northern blot analysis shows widespread expression of flt3 ligand mRNA transcripts in human tissues.

Fms-like tyrosine kinase 3 catalytic domain can transduce a proliferative signal in FDC-P1 cells that is qualitatively similar to the signal delivered by c-Fms

A full length clone of murine fms-like tyrosine kinase 3 [flt3, also known as fetal liver kinase 2 (flk2)] was constructed from sequences obtained from a brain complementary DNA (cDNA) library and from cDNA prepared from the cell line Tikaut. In the absence of a ligand to study the function of Flt3, a chimeric molecule was constructed comprising the extracellular domain of murine c-Fms and the transmembrane and cytoplasmic domains of Flt3. A plasmid encoding the chimeric receptor was cotransfected along with a plasmid conferring neomycin resistance into FDC-P1 cells that do not normally express c-fms or flt3 and require granulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin 3 for growth. Two types of clones were obtained following selection in GM-CSF and G418. Two of seven clones had the capacity for M-CSF-dependent colony formation in semisolid medium, indicating that the cytoplasmic domain of Flt3 can transduce a proliferative signal. From the remaining clones, M-CSF-dependent clonogenic cells could be selected by prior bulk liquid culture in M-CSF. It has been shown previously that the GM-CSF-dependent proliferative capacity is strongly inhibited by M-CSF in FDC-P1 cells engineered to express full length c-fms. This phenomenon was also observed with FD/fms-flt3 cells that were clonogenic in M-CSF. Stimulation of FD/fms or FD/fms-flt3 cells in liquid culture by M-CSF caused differentiation of a small proportion of cells along the myelomonocytic pathway which was enhanced by the combination of M-CSF and GM-CSF.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of the mitogenic pathway of the FLT3 receptor and characterization in its C terminal region of a specific binding site for the PI3' kinase

The FLT3 receptor tyrosine kinase (RTK) belongs to the class III subfamily which includes PDGF, CSF1 and SLF receptors. The recent cloning of the FLT3 ligand suggesting its important role in the differentiation and proliferation of the hematopoietic stem cells, has confirmed the initial expression analysis showing restricted pattern of receptor expression within the primitive hematopoietic population. To better understand the function of the FLT3 receptor and its relationship with the other hematopoietic RTKs, we analyzed the mitogenic pathway and substrate specificity of this receptor. The construction of a chimeric receptor called FF3, between the extracellular region of the CSF1 receptor fused with the transmembrane and the cytoplasmic regions of FLT3, has allowed an analysis in the absence of FLT3 ligand. We have shown in previous studies that FF3 is able to transduce the signal induced by CSF1, to induce tyrosine phosphorylation and/or association of several cytoplasmic proteins. We show here that this new receptor is fully functional in Ba/F3 hematopoietic cells, inducing a CSF1 dependence when expressed at the surface of this IL3 dependent cell line. The PI3' Kinase interacts with the FF3 receptor through SH2 domains and its binding site is localized on the tyrosine residue 958 in the C terminal part of the receptor.

Ligand for FLT3/FLK2 receptor tyrosine kinase regulates growth of haematopoietic stem cells and is encoded by variant RNAs

The FLT3/FLK2 receptor tyrosine kinase is closely related to two receptors, c-Kit and c-Fms, which function with their respective ligands, Kit ligand and macrophage colony-stimulating factor to control differentiation of haematopoietic and non-haematopoietic cells. FLT3/FLK2 is thought to be present on haematopoietic stem cells and found in brain, placenta and testis. We have purified to homogeneity and partially sequenced a soluble form of the FLT3/FLK2 ligand produced by mouse thymic stromal cells. We isolated several mouse and human complementary DNAs that encode polypeptides with identical N termini and different C termini. Some variants contain hydrophobic transmembrane segments, suggesting that processing may be required to release soluble ligand. The purified ligand enhances the response of mouse stem cells and a primitive human progenitor cell population to other growth factors such as interleukins IL-3 and IL-6 and to granulocyte-macrophage colony-stimulating factor, and also stimulates fetal thymocytes.

The expression of FMS, KIT and FLT3 in hematopoietic malignancies

Three receptor molecules, belonging to the class III of receptor tyrosine kinases, namely the receptors for colony-stimulating factor 1, CSF1R (product of the FMS proto-oncogene) and Steel factor, SLFR (product of the KIT proto-oncogene), as well as the recently identified FLT3/FLK2 gene product, appear to play distinct roles in normal hematopoietic differentiation. Their potential role in leukemic hematopoiesis has been approached by expression studies in hematopoietic malignancies, especially in acute leukemias of the myeloid and lymphoid lineages. We present here a review of available data, and discuss the possible significance and potential applications of these results.

STK-1, the human homolog of Flk-2/Flt-3, is selectively expressed in CD34+ human bone marrow cells and is involved in the proliferation of early progenitor/stem cells

We cloned the cDNA for stem cell tyrosine kinase 1 (STK-1), the human homolog of murine Flk-2/Flt-3, from a CD34+ hematopoietic stem cell-enriched library and investigated its expression in subsets of normal human bone marrow. The cDNA encodes a protein of 993 aa with 85% identity and 92% similarity to Flk-2/Flt-3. STK-1 is a member of the type III receptor tyrosine kinase family that includes KIT (steel factor receptor), FMS (colony-stimulating factor 1R), and platelet-derived growth factor receptor. STK-1 expression in human blood and marrow is restricted to CD34+ cells, a population greatly enriched for stem/progenitor cells. Anti-STK-1 antiserum recognizes polypeptides of 160 and 130 kDa in several STK-1-expressing cell lines and in 3T3 cells transfected with a STK-1 expression vector. Antisense oligonucleotides directed against STK-1 sequences inhibited hematopoietic colony formation, most strongly in long-term bone marrow cultures. These data suggest that STK-1 may function as a growth factor receptor on hematopoietic stem and/or progenitor cells.

Molecular cloning of a ligand for the flt3/flk-2 tyrosine kinase receptor: a proliferative factor for primitive hematopoietic cells

Cloning of a ligand for the murine flt3/flk-2 tyrosine kinase receptor was undertaken using a soluble form of the receptor to identify a source of ligand. A murine T cell line, P7B-0.3A4, was identified that appeared to express a cell surface ligand for this receptor. A cDNA clone was isolated from an expression library prepared from these cells that was capable, when transfected into cells, of conferring binding to a soluble form of the flt3/flk-2 receptor. The cDNA for this ligand encodes a type I transmembrane protein that stimulates the proliferation of cells transfected with the flt3/flk-2 receptor. A soluble form of the ligand stimulates the proliferation of defined subpopulations of murine bone marrow and fetal liver cells as well as human bone marrow cells that are highly enriched for hematopoietic stem cells and primitive uncommitted progenitor cells.

Developmental expression of flt3 mRNA in the mouse brain

The expression and localization of flt3 mRNA were investigated using Northern blotting and in situ hybridization during the developmental process of the mouse brain. By Northern blotting, the expression of the flt3 gene was not detected in embryonic, neonatal, or early postnatal brains, but was markedly increased with age. In adult mice, flt3 was abundantly expressed in the cerebellum, moderately in the pons, and faintly in the thalamus and the cerebral cortex. By in situ hybridization, some neurons that expressed flt3 formed synaptic connections with each other. The present findings suggest that the flt3 gene expression is related to the cell type and the developmental process.

Mitogenic signalling and substrate specificity of the Flk2/Flt3 receptor tyrosine kinase in fibroblasts and interleukin 3-dependent hematopoietic cells

Flk2/Flt3 is a recently identified receptor tyrosine kinase expressed in brain, placenta, testis, and primitive hematopoietic cells. The mitogenic signalling potential and biochemical properties of Flk2/Flt3 have been analyzed by using a chimeric receptor composed of the extracellular domain of the human colony-stimulating factor 1 receptor and the transmembrane and cytoplasmic domains of murine Flk2/Flt3. We demonstrate that colony-stimulating factor 1 stimulation of the Flk2/Flt3 kinase in transfected NIH 3T3 fibroblasts leads to a transformed phenotype and generates a full proliferative response in the absence of other growth factors. In transfected interleukin 3 (IL-3)-dependent Ba/F3 lymphoid cells, activation of the chimeric receptor can abrogate IL-3 requirement and sustain long-term proliferation. We show that phospholipase C-gamma 1, Ras GTPase-activating protein, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, Vav, Fyn, and Src are components of the Flk2/Flt3 signal transduction pathway. In addition, we demonstrate that phospholipase C-gamma 1, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, and Src family tyrosine kinases, but not Ras GTPase-activating protein, Vav, or Nck, physically associate with the Flk2/Flt3 cytoplasmic domain. Cell-type-specific differences in tyrosine phosphorylation of p85 and Shc are observed. A comparative analysis of the Flk2/Flt3 signal cascade with those of the endogenous platelet-derived growth factor and IL-3 receptors indicates that Flk2/Flt3 displays specific substrate preferences. Furthermore, tyrosine phosphorylation of p85 and Shc is similarly affected by totally different growth factors in the same cellular background.

Human FLT3/FLK2 gene: cDNA cloning and expression in hematopoietic cells

The human FLT3 cDNA was cloned from a pre-B cell line and characterized. The deduced amino acid sequence shows that FLT3 codes for a receptor-type tyrosine kinase of 993 residues, presenting a strong similarity with the corresponding mouse FLT3/FLK2 protein as well as with the receptors for colony-stimulating factor 1 (CSF1R/FMS) and steel locus factor (SLFR/KIT). An analysis of the expression of the gene using amplification of reverse transcribed FLT3 mRNA by polymerase chain reaction shows that FLT3 is expressed in various lymphohematopoietic cells and tissues, including a series of immature cell lines and leukemias of lymphocytic origin.

Purification and characterization of heterogeneous pluripotent hematopoietic stem cell populations expressing high levels of c-kit receptor

Mouse pluripotent hematopoietic stem cells (PHSC) were fractionated based on size and density using counterflow centrifugal elutriation (CCE). These heterogeneous PHSC populations were further enriched by subtraction of cells with lineage-specific markers (Lin-) followed by positive sorting for c-kit expression. The cells were characterized for their functional and biochemical properties. We defined a subpopulation of c-kit-positive cells that expressed high numbers of c-kit receptors (c-kitBR). One hundred c-kitBR cells from either low- or higher-density fractions were sufficient to repopulate the lymphohematopoietic system in WBB6F1-W/Wv (W/Wv) recipients, whereas no PHSC were found in cells with low (c-kitDULL) or no (c-kitNEG) c-kit expression. Lin- c-kitBR cells were separated into RhoDULL and RhoBR subsets based on their ability to efflux rhodamine 123 (Rho). The PHSC were concentrated in Lin- c-kitBR RhoDULL cells and the number of Lin- c-kitBR RhoBR cells correlated directly with the number of day 12 colony-forming unit-spleen (CFU-S12) in each fraction. We were not able to enrich further for PHSC using monoclonal antibodies to the cell-surface markers AA4.1 or CD4, which have been used by others to isolate PHSC. The small, low-density Lin- c-kitBR subset contained PHSC and few CFU-S12. This enabled us to assay PHSC for expression of the flk-2 gene, which encodes a tyrosine kinase receptor present on fetal liver PHSC. Purified RNA from the low-density Lin- c-kitBR subset did not contain flk-2 mRNA. We suggest that AA4.1, CD4 and flk-2 are expressed as stage-specific markers on PHSC in cell cycle.

The expression of cytokine receptors by purified hemopoietic stem cells

Sorted fractions from mouse bone marrow containing highly purified hemopoietic stem and progenitor cells were studied for the expression of growth factor receptors. With the use of rhodamine 123 WGA+, 15-1.1-, low density cells were separated into quiescent pluripotent stem cells and committed progenitor cells. RNA was extracted and cDNA was prepared by reverse transcription. Using primers specific for growth factor receptors, the cDNA of each sorted fraction was amplified by polymerase chain reaction (PCR). The quiescent rhodamine 123 dull stem cell fraction was found to express the interleukin 3 (IL-3) receptor beta unit and c-kit, but not the granulocyte-macrophage colony stimulating factor (GM-CSF) receptor beta unit nor flk-2. The rhodamine 123 bright fraction with activated stem cells and mostly committed progenitor cells similarly expressed the IL-3-R beta, and c-kit. However, this fraction also expressed flk-2 and GM-CSF-R beta. Since the expression of c-kit in the stem cell fraction does not correspond with the poor response to the kit-ligand stem cell factor (SCF) by these cells, we further analyzed the fractions with respect to their binding of biotinylated SCF. The SCF-binding cells were found to be all rhodamine 123 bright. This indicates that the expression of c-kit is not sufficient to yield a functional receptor for SCF; c-kit probably needs a partner molecule to form a functional high-affinity binding site for SCF. Similar to the beta unit of the GM-CSF receptor, this partner is then not expressed in the stem cell fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical characterization and analysis of the transforming potential of the FLT3/FLK2 receptor tyrosine kinase

We recently cloned an additional member of the receptor type tyrosine kinase class III. This new gene, called Flt3 by our group [Rosnet, O., Matteï, M.G., Marchetto, S. & Birnbaum, D. (1991). Genomics, 9, 380-385; Rosnet, O., Marchetto, S., deLapeyriere, O. & Birnbaum, D. (1991). Oncogene, 6, 1641-1650] and Flk2 by others [Matthews, W., Jordan, C.T., Wieg, G.W., Pardoll, D. & Lemischka, I.R. (1991). Cell, 65, 1143-1152] is strongly related to the important developmental genes Kit, Fms and Pdgfr. The murine 3.2-kb full-length cDNA, when introduced into COS-1 cells, shows the expression of two polypeptides with apparent molecular weights of 155 kDa and 132 kDa. Treatment of cells with N-linked glycosylation inhibitors results in the expression of a 110-kDa protein. We have shown that FLT3 contains an intrinsic tyrosine kinase activity. A point mutation in a highly conserved residue within the phosphoryltransferase domain inactivates the catalytic function of this receptor, whereas activation by way of a chimeric molecule between the ligand-binding domain of colony-stimulating factor type 1 (CSF-1) receptor (CSF-1R) and the kinase domain of FLT3 results, in the presence of CSF-1, in the development of the transforming activity of this receptor as shown by anchorage-independent cell growth. Finally, expression analysis of the FLT3 protein shows that, in addition to the hematopoietic system, FLT3 is strongly expressed in neural, gonadal, hepatic and placental tissues in the mouse.

Characterization of the protein encoded by the flt3 (flk2) receptor-like tyrosine kinase gene

We have developed rabbit polyclonal antibodies to the C-terminus of the flt3-encoded protein, which is a member of the receptor tyrosine kinase family. Immunoprecipitation using this antiserum brings down two protein bands, a major band of 143 kDa and a less abundant, more diffuse, band of 158 kDa. Pulse-chase analysis of flt3 protein from transfected COS-7 cells shows that the larger band is derived from the smaller one and presumably represents maturation of the protein from a glycosylated high-mannose form to a complex carbohydrate form. N-glycosidase F digestion confirmed the presence of N-linked carbohydrates, and cell-surface labeling of flt3-transfected cells indicated that the 158-kDa glycoprotein is the species found on the cell surface. A mutated form of the flt3 protein that was defective in its glycosylational processing was identified. Western blotting of the immunoprecipitated flt3 protein showed that it is heavily phosphorylated on tyrosine, and that this phosphorylation probably occurs in the absence of ligand. In this regard, the flt3 protein resembles the c-erbB2 protein, which is also highly phosphorylated in the absence of ligand. These data suggest that the flt3 receptor regulates the growth and differentiation of cells via an as yet unknown ligand.

A receptor tyrosine kinase specific to hematopoietic stem and progenitor cell-enriched populations

To elucidate the molecular biology of the hematopoietic stem cell, we have begun to isolate genes from murine cell populations enriched in stem cell activity. One such cDNA encodes a novel receptor tyrosine kinase, designated fetal liver kinase-2 or flk-2, which is related to the W locus gene product c-kit. Expression analyses suggest an extremely restricted distribution of flk-2. It is expressed in populations enriched for stem cells and primitive uncommitted progenitors, and is absent in populations containing more mature cells. Therefore, this receptor may be a key signal transducing component in the totipotent hematopoietic stem cell and its immediate self-renewing progeny.