The GM-CSF receptor family: mechanism of activation and implications for disease

CBL linker region and RING finger mutations lead to enhanced granulocyte-macrophage colony-stimulating factor (GM-CSF) signaling via elevated levels of JAK2 and LYN

Juvenile myelomonocytic leukemia (JMML) is characterized by hypersensitivity to granulocyte-macrophage colony-stimulating factor (GM-CSF). SHP2, NF-1, KRAS, and NRAS are mutated in JMML patients, leading to aberrant regulation of RAS signaling. A subset of JMML patients harbor CBL mutations associated with 11q acquired uniparental disomy. Many of these mutations are in the linker region and the RING finger of CBL, leading to a loss of E3 ligase activity. We investigated the mechanism by which CBL-Y371H, a linker region mutant, and CBL-C384R, a RING finger mutant, lead to enhanced GM-CSF signaling. Expression of CBL mutants in the TF-1 cell line resulted in enhanced survival in the absence of GM-CSF. Cells expressing CBL mutations displayed increased phosphorylation of GM-CSF receptor βc subunit in response to stimulation, although expression of total GM-CSFR βc was lower. This suggested enhanced kinase activity downstream of GM-CSFR. JAK2 and LYN kinase expression is elevated in CBL-Y371H and CBL-C384R mutant cells, resulting in enhanced phosphorylation of CBL and S6 in response to GM-CSF stimulation. Incubation with the JAK2 inhibitor, TG101348, abolished the increased phosphorylation of GM-CSFR βc in cells expressing CBL mutants, whereas treatment with the SRC kinase inhibitor dasatinib resulted in equalization of GM-CSFR βc phosphorylation signal between wild type CBL and CBL mutant samples. Dasatinib treatment inhibited the elevated phosphorylation of CBL-Y371H and CBL-C384R mutants. Our study indicates that CBL linker and RING finger mutants lead to enhanced GM-CSF signaling due to elevated kinase expression, which can be blocked using small molecule inhibitors targeting specific downstream pathways.

The GM-CSF/IL-3/IL-5 cytokine receptor family: from ligand recognition to initiation of signaling

Granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 are members of a discrete family of cytokines that regulates the growth, differentiation, migration and effector function activities of many hematopoietic cells and immunocytes. These cytokines are involved in normal responses to infectious agents, bridging innate and adaptive immunity. However, in certain cases, the overexpression of these cytokines or their receptors can lead to excessive or aberrant initiation of signaling resulting in pathological conditions, with chronic inflammatory diseases and myeloid leukemias the most notable examples. Recent crystal structures of the GM-CSF receptor ternary complex and the IL-5 binary complex have revealed new paradigms of cytokine receptor activation. Together with a wealth of associated structure-function studies, they have significantly enhanced our understanding of how these receptors recognize cytokines and initiate signals across cell membranes. Importantly, these structures provide opportunities for structure-based approaches for the discovery of novel and disease-specific therapeutics. In addition, recent biochemical evidence has suggested that the GM-CSF/IL-3/IL-5 receptor family is capable of interacting productively with other membrane proteins at the cell surface. Such interactions may afford additional or unique biological activities and might be harnessed for selective modulation of the function of these receptors in disease.

Molecular cloning, recombinant expression and characterization of GMCSF from the rhesus monkey, Macaca mulatta

Recent studies have found that, in addition to hematopoiesis, granulocyte-macrophage colony-stimulating factor (GMCSF) plays pivotal roles in multiple immune disorders. The gene encoding Macaca mulatta GMCSF (mmGMCSF) was cloned from stimulated peripheral blood mononuclear cells (PBMCs). Concanavalin A (Con A) and mismatched allogeneic antigen-stimulation significantly increased the production of mmGMCSF by monkey PBMCs. The gene encoding mature mmGMCSF was first expressed as a soluble fusion protein in Escherichia coli, and native mmGMCSF was further prepared by protease cleavage. The recombinant mmGMCSF induced antigen-presenting dendritic cells from monkey PBMCs, suggesting a central role of mmGMCSF in the immune system of the rhesus monkey. Although the predicted mature mmGMCSF protein differs from human GMCSF (hGMCSF) at six amino acid residues, mmGMCSF showed a strong ability to support human TF-1 cell survival. Additional co-immunoprecipitation experiments revealed that mmGMCSF cross reacts with the hGMCSF receptor (hGMCSFR). In addition, the hGMCSF-neutralizing agents hGMCSFR-Fc and anti-hGMCSF antibody reduced the biological effects of mmGMCSF on TF-1 cells. These results suggest that recombinant mmGMCSF might be used for the in vitro evaluation of novel hGMCSF-neutralizing agents prior to the in vivo preclinical evaluation of these agents in the rhesus monkey model.

Th2 cytokine antagonists: potential treatments for severe asthma

INTRODUCTION

Asthma is a major disease burden worldwide. Treatment with steroids and long acting β-agonists effectively manage symptoms in many patients but do not treat the underlying cause of disease and have serious side effects when used long term and in children. Therapies targeting the underlying causes of asthma are urgently needed. T helper type 2 (Th2) cells and the cytokines they release are clinically linked to the presentation of all forms of asthma. They are the primary drivers of mild to moderate and allergic asthma. They also play a pathogenetic role in exacerbations and more severe asthma though other factors are also involved. Much effort using animal models and human studies has been dedicated to the identification of the pathogenetic roles of these cells and cytokines and whether inhibition of their activity has therapeutic benefit in asthma.

AREAS COVERED

We discuss the current status of Th2 cytokine antagonists for the treatment of asthma. We also discuss the potential for targeting Th2-inducing cytokines, Th2 cell receptors and signaling as well as the use of Th2 cell antagonists, small interfering oligonucleotides, microRNAs, and combination therapies.

EXPERT OPINION

Th2 antagonists may be most effective in particular asthma subtypes/endotypes where specific cytokines are known to be active through the analysis of biomarkers. Targeting common receptors and pathways used by these cytokines may have additional benefit. Animal models have been valuable in identifying therapeutic targets in asthma, however the results from such studies need to be carefully interpreted and applied to appropriately stratified patient cohorts in well-designed clinical studies and trials.