GM-CSF binding to its receptor induces oligomerisation of the common beta-subunit

The stoichiometry of the granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor complex is still unresolved. We have utilised a sensitive, functional assay for receptor homodimerisation to show that GM-CSF induces dimerisation of the common signalling subunit, hbeta(c). We generated a chimeric cytokine receptor in which the extracellular and transmembrane domains of hbeta(c)are fused to the cytoplasmic domain of erythropoietin receptor (EPO-R). Given that to induce EPO-R activation and mitogenic signalling there is a requirement for formation of a specific homodimeric complex, we reasoned that the cytoplasmic domain of EPO-R could be utilised as a highly sensitive reporter for functional homodimer formation. We show that, in the presence of a cytoplasmically truncated GM-CSF alpha-subunit, the hbetac-EPO receptor chimera transduces a mitogenic signal in BaF-B03 in response to GM-CSF. This is consistent with formation of a hbeta(c)homodimer following GM-CSF binding and implies that ligand stimulation induces formation of a higher order complex that contains the hbeta(c)homodimer.

An oncogenic role of sphingosine kinase

Sphingosine kinase (SphK) is a highly conserved lipid kinase that phosphorylates sphingosine to form sphingosine-1-phosphate (S1P). S1P/SphK has been implicated as a signalling pathway to regulate diverse cellular functions [1-3], including cell growth, proliferation and survival [4-8]. We report that cells overexpressing SphK have increased enzymatic activity and acquire the transformed phenotype, as determined by focus formation, colony growth in soft agar and the ability to form tumours in NOD/SCID mice. This is the first demonstration that a wild-type lipid kinase gene acts as an oncogene. Using a chemical inhibitor of SphK, or an SphK mutant that inhibits enzyme activation, we found that SphK activity is involved in oncogenic H-Ras-mediated transformation, suggesting a novel signalling pathway for Ras activation. The findings not only point to a new signalling pathway in transformation but also to the potential of SphK inhibitors in cancer therapy.

Expression of a catalytically inactive sphingosine kinase mutant blocks agonist-induced sphingosine kinase activation. A dominant-negative sphingosine kinase

Sphingosine kinase (SK) catalyzes the formation of sphingosine 1-phosphate (S1P), a lipid messenger that plays an important role in a variety of mammalian cell processes, including inhibition of apoptosis and stimulation of cell proliferation. Basal levels of S1P in cells are generally low but can increase rapidly when cells are exposed to various agonists through rapid and transient activation of SK activity. To date, elucidation of the exact signaling pathways affected by these elevated S1P levels has relied on the use of SK inhibitors that are known to have direct effects on other enzymes in the cell. Furthermore, these inhibitors block basal SK activity, which is thought to have a housekeeping function in the cell. To produce a specific inhibitor of SK activation we sought to generate a catalytically inactive, dominant-negative SK. This was accomplished by site-directed mutagenesis of Gly(82) to Asp of the human SK, a residue identified through sequence similarity to the putative catalytic domain of diacylglycerol kinase. This mutant had no detectable SK activity when expressed at high levels in HEK293T cells. Activation of endogenous SK activity by tumor necrosis factor-alpha (TNFalpha), interleukin-1beta, and phorbol esters in HEK293T cells was blocked by expression of this inactive sphingosine kinase (hSK(G82D)). Basal SK activity was unaffected by expression of hSK(G82D). Expression of hSK(G82D) had no effect on TNFalpha-induced activation of protein kinase C and sphingomyelinase activities. Thus, hSK(G82D) acts as a specific dominant-negative SK to block SK activation. This discovery provides a powerful tool for the elucidation of the exact signaling pathways affected by elevated S1P levels following SK activation. To this end we have employed the dominant-negative SK to demonstrate that TNFalpha activation of extracellular signal-regulated kinases 1 and 2 (ERK1,2) is dependent on SK activation.

Molecular aspects of polycythemia vera (review)

Polycythemia vera (PV) is a rare, progressive myeloproliferative disorder thought to originate from the clonal expansion of a multipotent haemopoietic stem cell. This disease is characterised by hyperproliferation of the erythroid, myeloid and megakaryocyte lineages in the early phase, anaemia and fibrosis in the spent phase, and with a significant number of patients developing acute myeloid leukaemia (AML) in the final phase. Studies investigating the growth factor requirements of committed progenitors have shown hypersensitivity to a number of haemopoietic growth factors (HGF) in vitro and several HGF receptor and signalling molecule alterations have been reported. The findings to date, however, are unable to account for the transformation of a primitive stem cell and the many alterations to growth factor responses seen in PV progenitors. Identification of the primary lesion that leads to the pathogenesis of PV is of major importance given the profound effects on regulation of the haemopoietic stem cell compartment. In this article we focus on characteristics of the disease, research findings to date and possible mechanisms to explain altered growth factor responses, receptor alterations and signalling abnormalities in PV.

The solution structure of the cytokine-binding domain of the common beta-chain of the receptors for granulocyte-macrophage colony-stimulating factor, interleukin-3 and interleukin-5

The haemopoietic cytokines, granulocyte-macrophage colony-stimulating factor, interleukin-3 and interleukin-5 bind to cell-surface receptors comprising ligand-specific alpha-chains and a shared beta-chain. The beta-chain is the critical signalling subunit of the receptor and its fourth domain not only plays a critical role in interactions with ligands, hence in receptor activation, but also contains residues whose mutation can lead to ligand-independent activation of the receptor. We have determined the NMR solution structure of the isolated human fourth domain of the beta-chain. The protein has a fibronectin type III fold with a well-defined hydrophobic core and is stabilised by an extensive network of pi-cation interactions involving Trp and Arg side-chains, including two Trp residues outside the highly conserved Trp-Ser-Xaa-Trp-Ser motif (where Xaa is any amino acid) that is found in many cytokine receptors. Most of the residues implicated in factor-independent mutants localise to the rigid core of the domain or the pi-cation stack. The loops between the B and C, and the F and G strands, that contain residues important for interactions with cytokines, lie adjacent at the membrane-distal end of the domain, consistent with their being involved cooperatively in binding cytokines. The elucidation of the structure of the cytokine-binding domain of the beta-chain provides insight into the cytokine-dependent and factor-independent activation of the receptor.

A model for assembly and activation of the GM-CSF, IL-3 and IL-5 receptors: insights from activated mutants of the common beta subunit

Granulocyte-macrophage colony stimulating factor (GM-CSF), Interleukin-3 (IL-3) and Interleukin-5 (IL-5) have overlapping, pleiotropic effects on hematopoietic cells, including neutrophils, eosinophils, monocytes and early progenitor cells. The high-affinity receptors for human GM-CSF, IL-3, and IL-5 share a common beta-subunit (hbeta(c)), which is essential for signalling and plays a major role in recruiting intracellular signalling molecules. While activation of the cytoplasmic tyrosine kinase JAK2 appears to be the initiating event for signalling, the immediate events that trigger this are still unclear. We have isolated a number of activated mutants of hbeta(c), which can be grouped into classes defined by their state of receptor phosphorylation, their requirement for alpha subunit as a cofactor, and their activities in primary cells and cell lines. We discuss these findings with regard to the stoichiometry, activation, and signalling of the normal GM-CSF/IL-3/IL-5 receptor complexes. Specifically, this work has implications for the role of the ligand-specific alpha-subunits in initiating the signalling through the beta-subunit, the role of beta subunit dimerization as a receptor trigger, and the function of receptor tyrosine phosphorylation in generating growth and survival signals. Based on the properties of the activated mutants and the recent structures of erythropoietin receptor (Epo-R) complexes, we propose a model in which (1) activation of hbeta(c) can occur via alternative states that differ with respect to stoichiometry and subunit assembly, but which all mediate proliferative responses, and (2) each of the different classes of activated mutants mimics one of these alternative states.

Cloning, characterization, and chromosomal location of a novel human K+-Cl- cotransporter

Differential display polymerase chain reaction has been used to isolate genes regulated in vascular endothelial cells by the angiogenic factor vascular endothelial cell growth factor (VEGF). Analysis of one of the bands consistently up-regulated by VEGF led us to the identification of a cDNA from a human umbilical vein endothelial cell library that is 77% identical to the human K+-Cl- cotransporter1 (KCC1). We have referred to the predicted protein as K+-Cl- cotransporter 3 (KCC3). Hydrophobicity analysis of the KCC3 amino acid sequence showed an almost identical pattern to KCC1, suggesting 12 membrane-spanning segments, a large extracellular loop with potential N-glycosylation sites, and cytoplasmic N- and C-terminal regions. The KCC3 mRNA was highly expressed in brain, heart, skeletal muscle, and kidney, showing a distinct pattern and size from KCC1 and KCC2. The KCC3 mRNA level in endothelial cells increased on treatment with VEGF and decreased with the proinflammatory cytokine tumor necrosis factor alpha, whereas KCC1 mRNA levels remained unchanged. Stable overexpression of KCC3 cDNA in HEK293 cells produced a glycoprotein of approximately 150 kDa, which was reduced to 120 kDa by glycosidase digestion. An increased initial uptake rate of 86Rb was seen in clones with high KCC3 expression, which was dependent on extracellular Cl- but not Na+ and was inhibitable by the loop diuretic agent furosemide. The KCC3 genomic localization was shown to be 15q13 by fluorescence in situ hybridization. Radiation hybrid analysis placed KCC3 within an area associated with juvenile myoclonic epilepsy. These results suggest KCC3 is a new member of the KCC family that is under distinct regulation from KCC1.

Dysregulated hematopoiesis and a progressive neurological disorder induced by expression of an activated form of the human common beta chain in transgenic mice

Previously we described activating mutations of hbetac, the common signaling subunit of the receptors for the hematopoietic and inflammatory cytokines, GM-CSF, IL-3, and IL-5. The activated mutant, hbetacFIDelta, is able to confer growth factor-independent proliferation on the murine myeloid cell line FDC-P1, and on primary committed myeloid progenitors. We have used this activating mutation to study the effects of chronic cytokine receptor stimulation. Transgenic mice were produced carrying the hbetacFIDelta cDNA linked to the constitutive promoter derived from the phosphoglycerate kinase gene, PGK-1. Transgene expression was demonstrated in several tissues and functional activity of the mutant receptor was confirmed in hematopoietic tissues by the presence of granulocyte macrophage and macrophage colony-forming cells (CFU-GM and CFU-M) in the absence of added cytokines. All transgenic mice display a myeloproliferative disorder characterized by splenomegaly, erythrocytosis, and granulocytic and megakaryocytic hyperplasia. This disorder resembles the human disease polycythemia vera, suggesting that activating mutations in hbetac may play a role in the pathogenesis of this myeloproliferative disorder. In addition, these transgenic mice develop a sporadic, progressive neurological disease and display bilateral, symmetrical foci of necrosis in the white matter of brain stem associated with an accumulation of macrophages. Thus, chronic hbetac activation has the potential to contribute to pathological events in the central nervous system.

Roles of the N and C terminal domains of the interleukin-3 receptor alpha chain in receptor function

The interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor, and IL-5 receptor alpha chains are each composed of three extracellular domains, a transmembrane domain and a short intracellular region. Domains 2 and 3 constitute the cytokine receptor module (CRM), typical of the cytokine receptor superfamily; however, the function of the N-terminal domain is not known. We have investigated the functions of the N-terminal and C-terminal domains of the IL-3 receptor (IL-3R) alpha chain. We find that cells transfected with the receptor beta chain (h beta c) and a truncated IL-3R alpha that is devoid of the intracellular region fail to proliferate or to activate STAT5 in response to human IL-3, despite binding the IL-3 with affinity indistinguishable from that of full-length receptor. In addition, IL-3-induced phosphorylation of h beta c was not detected. Thus, the IL-3R alpha intracellular region does not contribute detectably to stabilization of the receptor/ligand complex, but is essential for signal propagation. In contrast, a truncated IL-3R alpha with the N-terminal domain deleted interacts functionally with the beta chain; mouse cells transfected with these receptor chains proliferate in response to human IL-3 and STAT5 transcription factor is activated. High- and low-affinity binding sites are retained, although the affinity for IL-3 is decreased 15-fold, indicating a significant role for the N-terminal domain in IL-3 binding.

Extracellular truncations of h beta c, the common signaling subunit for interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF), and IL-5, lead to ligand-independent activation

The hypothesis that extracellular truncation of the common receptor subunit for interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor, and IL-5 (h beta c) can lead to ligand-independent activation was tested by infecting factor-dependent hematopoietic cell lines with retroviruses encoding truncated forms of h beta c. A truncation, resembling that in v-Mpl, and retaining 45 h beta c-derived extracellular residues, led to constitutive activation in the murine myeloid cell line, FDC-P1. However, infection of cells with retrovirus encoding a more severely truncated receptor, retaining only 7 h beta c-derived extracellular residues, did not confer factor independence on these cells. These experiments show that truncation activates the receptor and define a 37-amino acid segment of h beta c (H395-A431) which contains two motifs conserved throughout the cytokine receptor superfamily (consensus Y/H XX R/Q VR and WSXWS), as essential for factor-independent signaling. The mechanism of activation was also investigated in less severe truncations. A receptor that retains the entire membrane-proximal domain (domain 4) also conferred factor independent growth on FDC-P1 cells; however, a retrovirus encoding a truncated form of h beta c having two intact membrane proximal domains did not have this ability, suggesting that domain 3 may have an inhibitory role in h beta c. The ability of these receptors to confer factor independence was cell specific as demonstrated by their inability to confer factor-independent growth when introduced into the murine IL-3-dependent pro-B cell line BaF-B03. These results are consistent with a model in which activation requires unmasking of an interactive receptor surface in domain 4 and association with a myeloid-specific receptor or accessory component. We suggest that in the absence of ligand intramolecular interactions prevent inappropriate signaling.

Cytokine receptor genes: structure, chromosomal location, and involvement in human disease

Haemopoietic cytokines regulate haemopoietic cell function via specific cell surface receptors. These receptors are members of a large superfamily of transmembrane proteins and are characterised by a 200 amino acid extracellular sequence encoding the ligand binding domain. Several of the genes for members of this superfamily have now been characterised at the molecular level revealing a highly conserved organisation and a number of these genes have been localised cytogenetically. The recent finding that genes for the IL-3 and GM-CSF receptor alpha chain subunits colocalise to a small region of the pseudoautosomal region and the observation that the LIF receptor locus is present in a cluster of receptor genes on chromosome 5 suggest the possibility that subsets of cytokine receptor genes may be organised into clusters. This possibility is discussed and the potential significance of cytokine receptor gene clusters is assessed. Several of the receptor genes are known to be involved in inherited disorders and there is evidence to suggest lesions in cytokine receptor genes could have a role in leukaemia. We review the gene organisation, localisation and involvement in disease for the known cytokine receptor loci. This large family of receptors is expanding with the steady discovery of new members--all of which have the potential to be involved in human disorders.

The three rows gene of Drosophila melanogaster encodes a novel protein that is required for chromosome disjunction during mitosis

Zygotic expression of the three rows (thr) gene of Drosophila melanogaster is required for normal cell proliferation during embryogenesis. Mitotic defects in thr mutant embryos begin during mitosis 15, and all subsequent divisions are disrupted. Chromosome disjunction and consequently cytokinesis fail during these defective mitoses, although the initial mitotic processes (chromosome condensation, spindle assembly, metaphase plate formation, and cyclin degradation) are not affected. Despite the failure of chromosome disjunction and cytokinesis, later mitotic events (chromosome decondensation) and subsequent cell cycle progression continue. The thr gene has been isolated and shown to encode a 1209 amino acid protein that shares no extended sequence similarity with known proteins. thr mRNA is present as maternal mRNA that degrades at the time of cellularization. At this and all subsequent times during embryogenesis, zygotic expression correlates with mitotic proliferation. These observations, together with the observation that the zygotic phenotype of thr mutant embryos is influenced by the maternal genotype, suggest that the embryonic phenotype results from exhaustion of the maternal thr contribution and does not reflect a developmentally restricted requirement for thr function. Our results indicate that the novel thr product is required specifically for chromosome disjunction during all mitoses.

Cloning and bacterial expression of the CYS3 gene encoding cystathionine gamma-lyase of Saccharomyces cerevisiae and the physicochemical and enzymatic properties of the protein

By screening a yeast genomic library, we isolated and characterized a gene rescuing the cysteine requirement in a "cys1" strain of Saccharomyces cerevisiae. Except for four residues in the open reading frame composed of 1,182 nucleotides, the DNA sequence was the same as that for the CYS3 (CYI1) gene, encoding cystathionine gamma-lyase (EC 4.4.1.1), and isolated previously as a cycloheximide-induced gene (B. Ono, K. Tanaka, K. Naito, C. Heike, S. Shinoda, S. Yamamoto, S. Ohmori, T. Oshima, and A. Toh-e, J. Bacteriol. 174:pp.3339-3347, 1992). S. cerevisiae "cys1" strains carry two closely linked mutations; one (cys1) causes a defect in serine O-acetyltransferase (EC 2.3.1.30), and another, designated cys3, impairs cystathionine gamma-lyase activity. Rescue of the cysteine requirement by the gene encoding cystathionine gamma-lyase is consistent with both defects being responsible for the cysteine auxotrophy. In an effort to further determine the physicochemical and enzymatic properties of this enzyme, a coding fragment was cloned into an Escherichia coli expression plasmid, and the protein was produced in the bacteria. The induced protein was extracted by sonication and purified to homogeneity through one course of DEAE-cellulose column chromatography. The yield of the protein was approximately 150 mg from cells cultured in 1 liter of L broth. The protein showed molecular weights of approximately 194,000 and 48,000 (for the subunit), suggesting a tetrameric structure. An s20,w value of 8.8 was estimated by centrifugation in a sucrose concentration gradient. No sulfhydryl groups were detected, which is consistent with the absence of cysteine residues in the coding sequence. The isoelectric point was at pH 5.2. The protein showed a number of cystathionine-related activities, i.e., cystathionine beta-lyase (EC 4.4.1.8), cystathionine gamma-lyase, and cystathionine gamma-synthase (EC 4.2.99.9) with L-homoserine as substrate. In addition, we demonstrated L-homoserine sulfhydrylase (adding H2S) activity but could find no detectable serine O-acteyltransferease activity. In this paper, we compare the enzymatic properties of the protein with those of homologous enzymes previously reported and discuss the possibility that this enzyme has a physiological role as cystathionine Beta-lyase and cystathionine gamma-synthase in addition to its previously described role as cystathionine gamma-lyase.

A cytokine receptor gene cluster in the X-Y pseudoautosomal region?

The receptors for interleukin-3 (IL-3), IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF) are heterodimers comprised of ligand specific alpha chains and a common beta chain. The genes encoding the IL-5 receptor alpha chain and the common beta chain reside on chromosome 3 and 22 respectively, while the GM-CSF receptor alpha chain gene (CSF2RA) has been mapped to the pseudoautosomal region (PAR) of the sex chromosomes, which is a 2.6-Mb stretch of homologous sequence at the tips of the short arms within which a single obligatory recombination occurs during male meiosis. We have mapped the gene encoding the IL-3 receptor alpha chain (IL3RA) to the sex chromosomes by polymerase chain reaction (PCR) analysis of human-mouse or human-chinese hamster cell hybrids, and to Yp13.3 and Xp22.3 using fluorescence in situ hybridization. To explore the possibility that IL3RA is located within the pseudoautosomal region we screened the Centre d'Etude du Polymorphisme Humain (CEPH) pedigrees for an informative-restriction fragment-length polymorphism (RFLP) that showed male meiotic recombination. Two informative CEPH pedigrees were identified that displayed this phenomenon, confirming the psuedoautosomal location of IL3RA. Using long-range restriction mapping we have found that IL3RA maps to the same 190-kb restriction fragment as CSF2RA, suggesting that a cytokine receptor gene cluster may reside in the PAR.

Chromosomal organization of chicken histone genes: preferred associations and inverted duplications

We present a detailed picture of the disposition of core and H1 histone genes in the chicken genome. Forty-two genes were located within four nonoverlapping regions totalling approximately 175 kilobases and covered by three cosmid clones and a number of lambda clones. The genes for the tissue-specific H5 histone and other variant histones were not found in these regions. The longest continuous region mapped was 67 kilobases and contained 21 histone genes in five dissimilar clusters. No long-range repeat was evident, but there were preferred associations, such as H1 genes with paired, divergently transcribed H2A-H2B genes and H3-H4 associations. However, there were exceptions, and even when associations such as H1-H2A-H2B we maintained, the order of those genes within a cluster may not have been. Another feature was the presence of three (unrelated) clusters in which genes were symmetrically ordered around central H3 genes; in one such cluster, the boundaries of a duplicated H2A-H4 gene pair contained related repeat sequences. Despite the dispersed nature of chicken histone genes, the number of each type was approximately equal, being represented as follows: 6 H1, 10 H2A, 8 H2B, 10 H3, and 8 H4.