A cell type-specific constitutive point mutant of the common beta-subunit of the human granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-3, and IL-5 receptors requires the GM-CSF receptor alpha-subunit for activation

Messing with βc: A unique receptor with many goals

Our understanding of the biological role of the βc family of cytokines has evolved enormously since their initial identification as bone marrow colony stimulating factors in the 1960's. It has become abundantly clear over the intervening decades that this family of cytokines has truly astonishing pleiotropic capacity, capable of regulating not only hematopoiesis but also many other normal and pathological processes such as development, inflammation, allergy and cancer. As noted in the current pandemic, βc cytokines contribute to the cytokine storm seen in acutely ill COVID-19 patients. Ongoing studies to discover how these cytokines activate their receptor are revealing insights into the fundamental mechanisms that give rise to cytokine pleiotropy and are providing tantalizing glimpses of how discrete signaling pathways may be dissected for activation with novel ligands for therapeutic benefit.

Naturally occurring and synthetic constitutive-active cytokine receptors in disease and therapy

Cytokines control immune related events and are critically involved in a plethora of patho-physiological processes including autoimmunity and cancer development. Mutations which cause ligand-independent, constitutive activation of cytokine receptors are quite frequently found in diseases. Many constitutive-active cytokine receptor variants have been directly connected to disease development and mechanistically analyzed. Nature's solutions to generate constitutive cytokine receptors has been recently adopted by synthetic cytokine receptor biology, with the goal to optimize immune therapeutics. Here, CAR T cell immmunotherapy represents the first example to combine synthetic biology with genetic engineering during therapy. Hence, constitutive-active cytokine receptors are therapeutic targets, but also emerging tools to improve or modulate immunotherapeutic strategies. This review gives a comprehensive insight into the field of naturally occurring and synthetic constitutive-active cytokine receptors.

The mechanism of GM-CSF inhibition by human GM-CSF auto-antibodies suggests novel therapeutic opportunities

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a hematopoietic growth factor that can stimulate a variety of cells, but its overexpression leads to excessive production and activation of granulocytes and macrophages with many pathogenic effects. This cytokine is a therapeutic target in inflammatory diseases, and several anti-GM-CSF antibodies have advanced to Phase 2 clinical trials in patients with such diseases, e.g., rheumatoid arthritis. GM-CSF is also an essential factor in preventing pulmonary alveolar proteinosis (PAP), a disease associated with GM-CSF malfunction arising most typically through the presence of GM-CSF neutralizing auto-antibodies. Understanding the mechanism of action for neutralizing antibodies that target GM-CSF is important for improving their specificity and affinity as therapeutics and, conversely, in devising strategies to reduce the effects of GM-CSF auto-antibodies in PAP. We have solved the crystal structures of human GM-CSF bound to antigen-binding fragments of two neutralizing antibodies, the human auto-antibody F1 and the mouse monoclonal antibody 4D4. Coordinates and structure factors of the crystal structures of the GM-CSF:F1 Fab and the GM-CSF:4D4 Fab complexes have been deposited in the RCSB Protein Data Bank under the accession numbers 6BFQ and 6BFS, respectively. The structures show that these antibodies bind to mutually exclusive epitopes on GM-CSF; however, both prevent the cytokine from interacting with its alpha receptor subunit and hence prevent receptor activation. Importantly, identification of the F1 epitope together with functional analyses highlighted modifications to GM-CSF that would abolish auto-antibody recognition whilst retaining GM-CSF function. These results provide a framework for developing novel GM-CSF molecules for PAP treatment and for optimizing current anti-GM-CSF antibodies for use in treating inflammatory disorders.

Quantification of transforming capacity and cooperation of defined genetic alterations in myeloid malignancies

OBJECTIVE

Mutations found in myeloid malignancies are qualitatively classified as conferring proliferative and survival advantages or impairing cellular differentiation. However, no suitable experimental model to quantify transforming potential of individual mutations and functional cooperation between defined genetic/epigenetic alterations has been established so far.

MATERIALS AND METHODS

Based on cytokine-independent proliferation as a marker for cellular transformation, we used limiting dilution and clonal expansion of retrovirally transduced cells in the presence or absence of cytokines to quantify the transformation potential of constitutively active receptor mutants and short hairpin RNAs (shRNA) targeting transcription factors by RNA interference. Interleukin-3-dependent 32D cells were transduced with betaGMR-I374N, c-KitV558D, or c-MplS368C, and cloning efficiencies were normalized to viral integration numbers as determined by quantitative polymerase chain reaction.

RESULTS

In this assay, c-KitV558D and c-MplS368C were about 25-fold more effective than betaGMR-I374N. To study cooperation of defined genetic/epigenetic aberrations, receptor mutants were coexpressed with shRNAs targeting PU.1 and p53. In p53-hypomorphic, but not in 32D wild-type cells, RNA interference against PU.1 significantly enhances transformation efficacy by c-KitV558D, but not by c-MplS368C, as compared to control shRNA. These data demonstrate nonredundant, receptor-specific and p53-dependent responses to reduced PU.1 expression in 32D cells.

CONCLUSION

This cell culture model represents a useful tool to quantify hematopoietic cell transformation by defined genetic and epigenetic alterations.

The structure of the GM-CSF receptor complex reveals a distinct mode of cytokine receptor activation

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a pleiotropic cytokine that controls the production and function of blood cells, is deregulated in clinical conditions such as rheumatoid arthritis and leukemia, yet offers therapeutic value for other diseases. Its receptors are heterodimers consisting of a ligand-specific alpha subunit and a betac subunit that is shared with the interleukin (IL)-3 and IL-5 receptors. How signaling is initiated remains an enigma. We report here the crystal structure of the human GM-CSF/GM-CSF receptor ternary complex and its assembly into an unexpected dodecamer or higher-order complex. Importantly, mutagenesis of the GM-CSF receptor at the dodecamer interface and functional studies reveal that dodecamer formation is required for receptor activation and signaling. This unusual form of receptor assembly likely applies also to IL-3 and IL-5 receptors, providing a structural basis for understanding their mechanism of activation and for the development of therapeutics.

Forced retinoic acid receptor alpha homodimers prime mice for APL-like leukemia

RARA becomes an acute promyelocytic leukemia (APL) oncogene by fusion with any of five translocation partners. Unlike RARalpha, the fusion proteins homodimerize, which may be central to oncogenic activation. This model was tested by replacing PML with dimerization domains from p50NFkappaB (p50-RARalpha) or the rapamycin-sensitive dimerizing peptide of FKBP12 (F3-RARalpha). The X-RARalpha fusions recapitulated in vitro activities of PML-RARalpha. For F3-RARalpha, these properties were rapamycin sensitive. Although in vivo the artificial fusions alone are poor initiators of leukemia, p50-RARalpha readily cooperates with an activated mutant CDw131 to induce APL-like disease. These results demonstrate that the dimerization interface of RARalpha fusion partners is a critical element in APL pathogenesis while pointing to other features of PML for enhancing penetrance and progression.

Cooperation of cytokine signaling with chimeric transcription factors in leukemogenesis: PML-retinoic acid receptor alpha blocks growth factor-mediated differentiation

We utilized a mouse model of acute promyelocytic leukemia (APL) to investigate how aberrant activation of cytokine signaling pathways interacts with chimeric transcription factors to generate acute myeloid leukemia. Expression in mice of the APL-associated fusion, PML-RARA, initially has only modest effects on myelopoiesis. Whereas treatment of control animals with interleukin-3 (IL-3) resulted in expanded myelopoiesis without a block in differentiation, PML-RARA abrogated differentiation that normally characterizes the response to IL-3. Retroviral transduction of bone marrow with an IL-3-expressing retrovirus revealed that IL-3 and promyelocytic leukemia-retinoic acid receptor alpha (PML-RARalpha) combined to generate a lethal leukemia-like syndrome in <21 days. We also observed that a constitutively activated mutant IL-3 receptor, beta(c)V449E, cooperated with PML-RARalpha in leukemogenesis, whereas a different activated mutant, beta(c)I374N, did not. Analysis of additional mutations introduced into beta(c)V449E showed that, although tyrosine phosphorylation of beta(c) is necessary for cooperation, the Src homology 2 domain-containing transforming protein binding site is dispensable. Our results indicate that chimeric transcription factors can block the differentiative effects of growth factors. This combination can be potently leukemogenic, but the particular manner in which these types of mutations interact determines the ability of such combinations to generate acute myeloid leukemia.

A colorimetric assay for determination of cell viability in algal cultures

In this work, we propose the determination of cell viability in algal cultures by using a colorimetric assay widely used for estimation of cell proliferation in animal cell cultures. The method is based on in vivo reduction by metabolically active cells of a tetrazolium compound (MTS=3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenil)-2H-tetrazolium, inner salt) to a colored formazan, with maximal absorbance at 490 nm, that is released to the culture medium. For this purpose, we have tested two microalgae with high commercial value (Dunaliella and Spirulina) and two seaweeds with different morphology (Ulva and Gracilaria). Color development in this assay is directly proportional to the number of viable cells, to the incubation time in the presence of the assay solution, and to the incubation temperature. A direct significant correlation was found between algal photosynthesis rate and color development in all species used through this work. Moreover, the intensity of absorbance at 490 nm was significantly lower in stressed cells (e.g. in nutrient-limited cultures, in the presence of toxic substances, and in osmotically-stressed cultures). We conclude that cell viability of algal cultures can be rapidly and easily estimated through colorimetric determination of the reduction of MTS to formazan.

Perverted responses of the human granulocyte-macrophage colony-stimulating factor receptor in mouse cell lines due to cross-species beta-subunit association

Transfected murine cell lines are commonly used to study the function of many human cytokine or receptor mutants. This study reports the inappropriate activation of the human granulocyte-macrophage colony-stimulating factor (hGM-CSF) receptor by the human GM-CSF antagonist, E21R, when the human receptor is introduced into the murine cell line BaF-B03. E21R-induced proliferation of the BaF-B03 cells is dependent on transfection with both hGM-CSF receptor alpha and beta(c) subunits. Studies on the underlying mechanism revealed constitutive association between human and mouse beta(c) and GM-CSF receptor-alpha, tyrosine phosphorylation of mouse and human beta(c), and association of phosphorylated mouse beta(c) into an activated human GM-CSF receptor complex in response to E21R and GM-CSF. This interspecies receptor cross-talk of receptor signaling subunits may produce misleading results and emphasizes the need to use cell lines devoid of the cognate endogenous receptors for functional analysis of ligand and receptor mutants.

Peptide insertions in domain 4 of hbeta(c), the shared signalling receptor subunit for GM-CSF, IL3 and IL5, induce ligand-independent activation

A mutant form of the common beta-subunit of the GM-CSF, interleukin-3 (IL3) and IL5 receptors is activated by a 37 residue duplicated segment which includes the WSXWS motif and an adjacent, highly conserved, aliphatic/basic element. Haemopoietic expression of this mutant, hbeta(c)FIDelta, in mice leads to myeloproliferative disease. To examine the mechanism of activation of this mutant we targetted the two conserved motifs in each repeat for mutagenesis. Here we show that this mutant exhibits constitutive activity in BaF-B03 cells in the presence of mouse or human GM-CSF receptor alpha-subunit (GMRalpha) and this activity is disrupted by mutations of the conserved motifs in the first repeat. In the presence of these mutations the receptor reverts to an alternative conformation which retains responsiveness to human IL3 in a CTLL cell line co-expressing the human IL3 receptor alpha-subunit (hIL3Ralpha). Remarkably, the activated conformation is maintained in the presence of substitutions, deletions or replacement of the second repeat. This suggests that activation occurs due to insertion of extra sequence after the WSXWS motif and is not dependent on the length or specific sequence of the insertion. Thus hbeta(c) displays an ability to fold into functional receptor conformations given insertion of up to 37 residues in the membrane-proximal region. Constitutive activation most likely results from a specific conformational change which alters a dormant, inactive receptor complex, permitting functional association with GMRalpha and ligand-independent mitogenic signalling.

Functional identification of secondary mutations inducing autonomous growth in synergy with a truncated interleukin-3 receptor Implications for multi-step oncogenesis

OBJECTIVE

A truncated common beta chain (Deltabeta(C)) of the interleukin-3 (IL-3) receptor complex was previously identified as a key factor in inducing autonomous growth of IL-3-independent mutants. Expression of Deltabeta(C) in IL-3-dependent hematopoietic cells does not result in immediate factor-independent growth, but increases the frequency of obtaining autonomous mutants by three to four orders of magnitude. This study was designed to delineate the mechanisms by which Deltabeta(C) increases the frequency to autonomous growth.

DESIGN AND METHODS

Retroviral vectors were used to express Deltabeta(C) into IL-3-dependent myeloid cells, which were then tested for factor-independent growth. To determine if secondary genetic events were required for conversion to autonomous growth, elements of the Cre-loxP recombinant system were used to excise Deltabeta(C) in factor-independent clones.

RESULTS

Excision of Deltabeta(C) in factor-independent clones revealed two types of phenotypes: reversion to factor-dependent growth (1/8) or continued IL-3-dependent growth (7/8). Analysis of cells that remained factor independent revealed constitutive activation of STAT5, not observed in factor-dependent revertants. Analysis of revertant cells demonstrated the presence of interacting secondary mutations that synergize with Deltabeta(C)-induced proliferation. A cysteine residue within the truncated extracellular domain of Deltabeta(C) was also found to be required for its oncogenic potential, supporting a model of dimerization for receptor activation.

CONCLUSIONS

The high incidence of obtaining factor-independent mutants from cells expressing Deltabeta(C) results from the selection of mutations that either complement Deltabeta(C) expression to promote proliferation or that singly or in synergy with other secondary mutations negate the requirement of Deltabeta(C) expression for proliferation.

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.

The role of disulfide-linked dimerization in interleukin-3 receptor signaling and biological activity

Cysteine residues 86 and 91 of the beta subunit of the human interleukin (hIL)-3 receptor (hbetac) participate in disulfide-linked receptor subunit heterodimerization. This linkage is essential for receptor tyrosine phosphorylation, since the Cys-86 --> Ala (Mc4) and Cys-91 --> Ala (Mc5) mutations abolished both events. Here, we used these mutants to examine whether disulfide-linked receptor dimerization affects the biological and biochemical activities of the IL-3 receptor. Murine T cells expressing hIL-3Ralpha and Mc4 or Mc5 did not proliferate in hIL-3, whereas cells expressing wild-type hbetac exhibited rapid proliferation. However, a small subpopulation of cells expressing each mutant could be selected for growth in IL-3, and these proliferated similarly to cells expressing wild-type hbetac, despite failing to undergo IL-3-stimulated hbetac tyrosine phosphorylation. The Mc4 and Mc5 mutations substantially reduced, but did not abrogate, IL-3-mediated anti-apoptotic activity in the unselected populations. Moreover, the mutations abolished IL-3-induced JAK2, STAT, and AKT activation in the unselected cells, whereas activation of these molecules in IL-3-selected cells was normal. In contrast, Mc4 and Mc5 showed a limited effect on activation of Erk1 and -2 in unselected cells. These data suggest that whereas disulfide-mediated cross-linking and hbetac tyrosine phosphorylation are normally important for receptor activation, alternative mechanisms can bypass these requirements.

Novel murine myeloid cell lines that exhibit a differentiation switch in response to IL-3 or GM-CSF, or to different constitutively active mutants of the GM-CSF receptor β subunit

Several activating mutations have recently been described in the common β subunit for the human interleukin(IL)-3, IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors (hβc). Two of these, FIΔ and I374N, result, respectively, in a 37–amino acid duplication and an isoleucine-to-asparagine substitution in the extracellular domain. A third, V449E, leads to valine-to–glutamic acid substitution in the transmembrane domain. Previous studies have shown that when expressed in murine hemopoietic cells in vitro, the extracellular mutants can confer factor independence on only the granulocyte-macrophage lineage while the transmembrane mutant can do so to all cell types of the myeloid and erythroid compartments. To further study the signaling properties of the constitutively active hβc mutants, we have used novel murine hemopoietic cell lines, which we describe in this report. These lines, FDB1 and FDB2, proliferate in murine IL-3 and undergo granulocyte-macrophage differentiation in response to murine GM-CSF. We find that while the transmembrane mutant, V449E, confers factor-independent proliferation on these cell lines, the extracellular hβc mutants promote differentiation. Hence, in addition to their ability to confer factor independence on distinct cell types, transmembrane and extracellular activated hβc mutants deliver distinct signals to the same cell type. Thus, the FDB cell lines, in combination with activated hβc mutants, constitute a powerful new system to distinguish between signals that determine hemopoietic proliferation or differentiation. (Blood. 2000;95:120-127)

Novel murine myeloid cell lines that exhibit a differentiation switch in response to IL-3 or GM-CSF, or to different constitutively active mutants of the GM-CSF receptor β subunit

Several activating mutations have recently been described in the common β subunit for the human interleukin(IL)-3, IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors (hβc). Two of these, FIΔ and I374N, result, respectively, in a 37–amino acid duplication and an isoleucine-to-asparagine substitution in the extracellular domain. A third, V449E, leads to valine-to–glutamic acid substitution in the transmembrane domain. Previous studies have shown that when expressed in murine hemopoietic cells in vitro, the extracellular mutants can confer factor independence on only the granulocyte-macrophage lineage while the transmembrane mutant can do so to all cell types of the myeloid and erythroid compartments. To further study the signaling properties of the constitutively active hβc mutants, we have used novel murine hemopoietic cell lines, which we describe in this report. These lines, FDB1 and FDB2, proliferate in murine IL-3 and undergo granulocyte-macrophage differentiation in response to murine GM-CSF. We find that while the transmembrane mutant, V449E, confers factor-independent proliferation on these cell lines, the extracellular hβc mutants promote differentiation. Hence, in addition to their ability to confer factor independence on distinct cell types, transmembrane and extracellular activated hβc mutants deliver distinct signals to the same cell type. Thus, the FDB cell lines, in combination with activated hβc mutants, constitute a powerful new system to distinguish between signals that determine hemopoietic proliferation or differentiation. (Blood. 2000;95:120-127)

Myeloproliferative disorder and leukaemia in mice induced by different classes of constitutive mutants of the human IL-3/IL-5/GM-CSF receptor common beta subunit

Several constitutively active mutant forms of the common beta subunit of the human IL-3, IL-5 and GM-CSF receptors (hbetac), which enable it to signal in the absence of ligand, have recently been described. Two of these, V449E and I374N, are amino acid substitutions in the transmembrane and extracellular regions of hbetac, respectively. A third, FIDelta, contains a 37 amino acid duplication in the extracellular domain. We have shown previously that when expressed in primary murine haemopoietic cells, the extracellular mutants confer factor-independence on cells of the neutrophil and monocyte lineages only, whereas V449E does so on all cell types of the myeloid and erythroid compartments. To study the in vivo effects and leukaemic potential of these mutants, we have expressed all three in mice by bone marrow reconstitution using retrovirally infected donor cells. Expression of the extracellular mutants leads to an early onset, chronic myeloproliferative disorder marked by elevations in the neutrophil, monocyte, erythrocyte and platelet lineages. In contrast, expression of V449E leads to an acute leukaemia-like syndrome of anaemia, thrombocytopaenia and blast cell expansion. These data support the possibility that activating mutations in hbetac are involved in haemopoietic disorders in man.