A sequential dimerization mechanism for erythropoietin receptor activation

A non-peptide compound which can mimic the effect of thrombopoietin via c-Mpl

Thrombopoietin (TPO) is a cytokine which plays a central role in megakaryopoiesis and platelet production by binding to its cell surface receptor, termed c-Mpl. In the present study, two benzodiazepinones that compete with the binding of TPO to the extracellular region of c-Mpl were identified, and one of them stimulated the proliferation of a human TPO-dependent megakaryocytic cell line, UT-7/TPO. It stimulated the activation of signal transducer and activator of transcription 5 in UT-7/TPO cells. These results suggest that a non-peptide compound can mimic the effect of TPO via c-Mpl.

Homodimerization restores biological activity to an inactive erythropoietin mutant

Erythropoietin (Epo) is believed to transduce a signal by bringing two Epo receptors into close proximity, enabling cross-phosphorylation. We compared monomeric Epos with homodimers in which two Epo monomers are linked by polyglycine. Monomeric Epo mutant R103A is unable to support Epo-dependent cell growth or trigger Janus kinase 2 and STAT5 activation, even at concentrations greater than 7,000 times that sufficient for wild-type Epo activity. In contrast, R103A homodimer induces proliferation and transduces signal at concentrations similar to that of wild-type Epo monomer and homodimer. These experiments show that two discrete domains on Epo are required for receptor binding and activation. Our results also suggest that the EpoR can be dimerized by different forms and sizes of molecules, as long as two recognition motifs are provided in the same molecule. Design of other dimeric molecules may enhance our understanding of cytokine specificity and signal transduction.

Surrogate thrombopoietin

The extracellular domain of human c-Mpl, the receptor for thrombopoietin (TPO), was expressed as a chimeric protein with the interleukin-2 receptor alpha chain on the surface of murine B cell-line B300-19. BALB/c mice were immunized with cells expressing the chimeric protein. The IgG purified from the resulting immune serum immunoprecipitated human c-Mpl. The immune IgG supported proliferation of both stable transfectant Ba/F3 cells expressing whole c-Mpl molecules (c-Mpl-Ba/F3 No. 9) and UT7/TPO cells bearing naturally occurring c-Mpl, whereas it did not support the growth of the untransfected parental Ba/F3 cells. Cell growth was induced using 3 to 100 microg/ml of immune IgG in a dose-dependent manner, but this induction was decreased at doses higher than 100 microg/ml. Non-immune IgG did not affect cell growth of c-Mpl-Ba/F3 No. 9 cells. Although the Fab fragment of immune IgG also immunoprecipitated c-Mpl, it did not support cell growth at concentrations as high as 180 microg/ml, implying that the bivalent binding of receptors by antibodies is essential for cell proliferation. These results suggest that antibodies against human c-Mpl stimulate the proliferation and differentiation of megakaryocytes by their bivalent binding to receptors like TPO.

Mutational analysis of thrombopoietin for identification of receptor and neutralizing antibody sites

Thrombopoietin (TPO) is a hematopoietin important for megakaryocyte proliferation and production of blood platelets. We sought to characterize how TPO binds and activates its receptor, myeloproliferative leukemia virus receptor. The erythropoietin-like domain of TPO (TPO1-153) has been fused to the gIII coat protein of M13 bacteriophage. Forty residues were chosen for mutation to alanine using the criteria that they were charged residues or predicted to be solvent-exposed, based on a homology model. Phage enzyme-linked immunosorbent assay was used to determine affinities for binding to both the TPO receptor and five anti-TPO1-153 monoclonal antibodies. Mutations at mostly positively charged residues (Asp8, Lys14, Lys52, Lys59, Lys136, Lys138, Arg140) caused the greatest reduction in receptor-binding affinity. Most of these residues mapped to helices-1 and -4 and a loop region between helix-1 and helix-2. Two of the monoclonal antibodies that blocked TPO binding and bioactivity had determinants in helix-4. In contrast, the other three monoclonal antibodies, which were effective at blocking TPO activity but did not block initial binding of TPO to its receptor, had epitopes predominantly on helix or 3. These results suggest that TPO has two distinct receptor-binding sites that function to dimerize TPO receptors in a sequential fashion.

Characterization of the receptor binding determinants of granulocyte colony stimulating factor

We performed a series of experiments using alanine-scanning mutagenesis to locate side chains within human granulocyte colony-stimulating factor (G-CSF) that are involved in human G-CSF receptor binding. We constructed a panel of 28 alanine mutants that examined all surface exposed residues on helices A and D, as well as all charged residues on the surface of G-CSF. The G-CSF mutants were expressed in a transiently transfected mammalian cell line and quantitated by a sensitive biosensor method. We measured the activity of mutant proteins using an in vitro proliferation assay and an ELISA binding competition assay. These studies show that there is a region of five charged residues on helices A and C employed by G-CSF in binding its receptor, with the most important residue in this binding patch being Glu 19. Both wild-type G-CSF and the E19A mutant were expressed in E. coli. The re-folded proteins were found to have proliferative activities similar to the analogous proteins from mammalian cells: furthermore, biophysical analysis indicated that the E19A mutation does not cause gross structural perturbations in G-CSF. Although G-CSF is likely to signal through receptor homo-dimerization, we found no compelling evidence for a second receptor binding region. We also found no evidence of self-antagonism at high G-CSF concentrations, suggesting that, in contrast to human growth hormone (hGH) and erythropoietin (EPO), G-CSF probably does not signal via a pure 2:1 receptor ligand complex. Thus, G-CSF, while having a similar tertiary structure to hGH and EPO, uses different areas of the four helix bundle for high-affinity interaction with its receptor.

Homodimerization of Erythropoietin Receptor by a Bivalent Monoclonal Antibody Triggers Cell Proliferation and Differentiation of Erythroid Precursors

Erythropoietin (EPO) stimulates proliferation and differentiation of erythroid progenitor cells. Several lines of evidence indicate that the most likely mechanism of EPO receptor (EPO-R) activation by EPO is homodimerization of the receptor on the surface of erythrocyte precursors. Therefore, we argued that it should be possible to raise EPO-R monoclonal antibodies (MoAbs) that would activate the receptor by dimerization and thus mimic EPO action. We have identified such an agonist MoAb (MoAb34) directed against the extracellular EPO binding domain of the EPO-R. This bivalent IgG antibody triggers the proliferation of EPO-dependent cell lines and induces differentiation of erythroid precursors in vitro. In contrast, the monovalent Fab fragment, which cannot dimerize the receptor, is completely inactive. The mechanism of receptor activation by homodimerization implies that at high ligand concentrations the formation of 1:1 receptor/ligand complexes is favored over 2:1 complexes, thereby turning the ligand agonist into an antagonist. Thus, EPO and MoAb34 should self-antagonize at high concentrations in both cell proliferation and differentiation assays. Our data indeed demonstrate that EPO and MoAb34 antagonize ligand-dependent cell proliferation with IC50 values of approximately 20 and 2 μmol/L, respectively. Erythroid colony formation (BFUe) is inhibited at MoAb34 concentrations above 1 μmol/L. Furthermore, we analyzed the MoAb34:EPO-R interaction using a mathematic model describing antibody-mediated receptor dimerization. The data for proliferation and differentiation activity were consistent with the receptor dimer formation on the cell surface predicted by the model.