Identification of functionally important residues of human thrombopoietin

The role of temperature in the binding of the disordered epitope region of human thrombopoietin to antibody: A molecular dynamics simulations study

The N-terminal domain (163 residues) of Human thrombopoietin (hTPO) is highly conserved and responsible for the receptor-binding. The crystal structure of free hTPO is not yet available, but the crystal structure of its receptor-binding domain (hTPO₁₆₃) is available in complex with the TN1-Fab antibody. According to a thermodynamic study of hTPO₁₆₃ binding to TN1-Fab Ab, the ΔH value for binding becomes more negative with an increase in temperature from 283 K to 303 K. The objective of our study is to understand how the free hTPO₁₆₃ behaves dynamically and to study the effect of temperature on the association of hTPO₁₆₃ to TN1-Fab antibody through molecular dynamics simulations. We studied the Ag-Ab interactions at two different temperatures 298 K and 303 K. The discontinuous epitope region (residues 98-115) of free hTPO₁₆₃ displays a conformational switch and it gets stabilized upon binding to the Ab at 303 K. Based on our results, it may be surmised that the epitope region 98-115 is behaving like a disordered epitope. The disordered epitopes are known to be more efficient in binding with the antibody. We also find that, there is an increase in number of hydrogen-bonding interactions and hydrophobic contacts with an increase in the temperature from 298 K to 303 K. Thus, this observation explains a possible reason behind the more negative value of ΔH at the higher temperature 303 K as compared to 298 K.

The thrombopoietin receptor: revisiting the master regulator of platelet production

Thrombopoietin (TPO) and its receptor, MPL, are the primary regulators of platelet production and critical for hematopoietic stem cell (HSC) maintenance. Since TPO was first cloned in 1994, the physiological and pathological roles of TPO and MPL have been well characterized, culminating in the first MPL agonists being approved for the treatment of chronic immune thrombocytopenia in 2008. Dysregulation of the TPO-MPL signaling axis contributes to the pathogenesis of hematological disorders: decreased expression or function results in severe thrombocytopenia progressing to bone marrow failure, while hyperactivation of MPL signaling, either by mutations in the receptor or associated Janus kinase 2 (JAK2), results in pathological myeloproliferation. Despite its importance, it was only recently that the long-running debate over the mechanism by which TPO binding activates MPL has been resolved. This review will cover key aspects of TPO and MPL structure and function and their importance in receptor activation, discuss how these are altered in hematological disorders and consider how a greater understanding could lead to the development of better-targeted and more efficacious therapies.

Origins of the Vertebrate Erythro/Megakaryocytic System

Vertebrate erythrocytes and thrombocytes arise from the common bipotent thrombocytic-erythroid progenitors (TEPs). Even though nonmammalian erythrocytes and thrombocytes are phenotypically very similar to each other, mammalian species have developed some key evolutionary improvements in the process of erythroid and thrombocytic differentiation, such as erythroid enucleation, megakaryocyte endoreduplication, and platelet formation. This brings up a few questions that we try to address in this review. Specifically, we describe the ontology of erythro-thrombopoiesis during adult hematopoiesis with focus on the phylogenetic origin of mammalian erythrocytes and thrombocytes (also termed platelets). Although the evolutionary relationship between mammalian and nonmammalian erythroid cells is clear, the appearance of mammalian megakaryocytes is less so. Here, we discuss recent data indicating that nonmammalian thrombocytes and megakaryocytes are homologs. Finally, we hypothesize that erythroid and thrombocytic differentiation evolved from a single ancestral lineage, which would explain the striking similarities between these cells.

Dissection of vertebrate hematopoiesis using zebrafish thrombopoietin

In nonmammalian vertebrates, the functional units of hemostasis are thrombocytes. Thrombocytes are thought to arise from bipotent thrombocytic/erythroid progenitors (TEPs). TEPs have been experimentally demonstrated in avian models of hematopoiesis, and mammals possess functional equivalents known as megakaryocyte/erythroid progenitors (MEPs). However, the presence of TEPs in teleosts has only been speculated. To identify and prospectively isolate TEPs, we identified, cloned, and generated recombinant zebrafish thrombopoietin (Tpo). Tpo mRNA expanded itga2b:GFP(+) (cd41:GFP(+)) thrombocytes as well as hematopoietic stem and progenitor cells (HSPCs) in the zebrafish embryo. Utilizing Tpo in clonal methylcellulose assays, we describe for the first time the prospective isolation and characterization of TEPs from transgenic zebrafish. Combinatorial use of zebrafish Tpo, erythropoietin, and granulocyte colony stimulating factor (Gcsf) allowed the investigation of HSPCs responsible for erythro-, myelo-, and thrombo-poietic differentiation. Utilizing these assays allowed the visualization and differentiation of hematopoietic progenitors ex vivo in real-time with time-lapse and high-throughput microscopy, allowing analyses of their clonogenic and proliferative capacity. These studies indicate that the functional role of Tpo in the differentiation of thrombocytes from HSPCs is well conserved among vertebrate organisms, positing the zebrafish as an excellent model to investigate diseases caused by dysregulated erythro- and thrombo-poietic differentiation.

Exome sequencing reveals a thrombopoietin ligand mutation in a Micronesian family with autosomal recessive aplastic anemia

We recently identified 2 siblings afflicted with idiopathic, autosomal recessive aplastic anemia. Whole-exome sequencing identified a novel homozygous missense mutation in thrombopoietin (THPO, c.112C>T) in both affected siblings. This mutation encodes an arginine to cysteine substitution at residue 38 or residue 17 excluding the 21-amino acid signal peptide of THPO receptor binding domain (RBD). THPO has 4 conserved cysteines in its RBD that form 2 disulfide bonds. Our in silico modeling predicts that introduction of a fifth cysteine may disrupt normal disulfide bonding to cause poor receptor binding. In functional assays, the mutant-THPO-containing media shows two- to threefold reduced ability to sustain UT7-TPO cells, which require THPO for proliferation. Both parents and a sibling with heterozygous R17C change have reduced platelet counts, whereas a sibling with wild-type sequence has normal platelet count. Thus, the R17C partial loss-of-function allele results in aplastic anemia in the homozygous state and mild thrombocytopenia in the heterozygous state in our family. Together with the recent identification of THPO receptor (MPL) mutations and the effects of THPO agonists in aplastic anemia, our results have clinical implications in the diagnosis and treatment of patients with aplastic anemia and highlight a role for the THPO-MPL pathway in hematopoiesis in vivo.

Bioinformatics prediction of overlapping frameshifted translation products in mammalian transcripts

BACKGROUND

Exceptionally, a single nucleotide sequence can be translated in vivo in two different frames to yield distinct proteins. In the case of the G-protein alpha subunit XL-alpha-s transcript, a frameshifted open reading frame (ORF) in exon 1 is translated to yield a structurally distinct protein called Alex, which plays a role in platelet aggregation and neurological processes. We carried out a novel bioinformatics screen for other possible dual-frame translated sequences, based on comparative genomics.

RESULTS

Our method searched human, mouse and rat transcripts in frames +1 and -1 for ORFs which are unusually well conserved at the amino acid level. We name these conserved frameshifted overlapping ORFs 'matreshkas' to reflect their nested character. Select findings of our analysis revealed that the G-protein coupled receptor GPR27 is entirely contained within a frame -1 matreshka, thrombopoietin contains a matreshka which spans ~70% of its length, platelet glycoprotein IIIa (ITGB3) contains a matreshka with the predicted characteristics of a secreted peptide hormone, while the potassium channel KCNK12 contains a matreshka spanning >400 amino acids.

CONCLUSION

Although the in vivo existence of translated matreshkas has not been experimentally verified, this genome-wide analysis provides strong evidence that substantial overlapping coding sequences exist in a number of human and rodent transcripts.

Monoclonal anti-thrombopoietin antibodies generated by genetic immunization

Thrombopoietin (TPO) is a megakaryocyte growth and differentiation factor that is currently being investigated as a therapeutic for cancer patients undergoing myelosuppressive chemotherapy. We generated monoclonal antibodies (MAbs) specific for human thrombopoietin (hTPO) by genetic immunization using an hTPO expression plasmid and an adjuvant plasmid that encodes mouse granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4). All genetically immunized mice exhibited a high humoral immune response. Splenocytes from these mice were used to generate hybridomas. Two MAbs, designated 2B9A10 and 4C16B15 (of IgG1 and IgG3 isotypes, respectively), were subsequently selected and produced. They specifically recognized and precipitated recombinant hTPO produced by mammalian cells and were effective in sandwich enzyme-linked immunosorbent assays (ELISAs) for hTPO quantitation. Our results demonstrate that these MAbs should be useful for purification and quantitation of hTPO in clinical and laboratory settings.

Structure of the receptor-binding domain of human thrombopoietin determined by complexation with a neutralizing antibody fragment

The cytokine thrombopoietin (TPO), the ligand for the hematopoietic receptor c-Mpl, acts as a primary regulator of megakaryocytopoiesis and platelet production. We have determined the crystal structure of the receptor-binding domain of human TPO (hTPO(163)) to a 2.5-A resolution by complexation with a neutralizing Fab fragment. The backbone structure of hTPO(163) has an antiparallel four-helix bundle fold. The neutralizing Fab mainly recognizes the C-D crossover loop containing the species invariant residue Q111. Titration calorimetric experiments show that hTPO(163) interacts with soluble c-Mpl containing the extracellular cytokine receptor homology domains with 1:2 stoichiometry with the binding constants of 3.3 x 10(9) M(-1) and 1.1 x 10(6) M(-1). The presence of the neutralizing Fab did not inhibit binding of hTPO(163) to soluble c-Mpl fragments, but the lower-affinity binding disappeared. Together with prior genetic data, these define the structure-function relationships in TPO and the activation scheme of c-Mpl.

A new epitope tag from hepatitis B virus preS1 for immunodetection, localization and affinity purification of recombinant proteins

Previously, a murine monoclonal antibody (mAb) KR127 (IgG2a/kappa) that binds specifically to the preS1 of hepatitis B virus (HBV) was generated and the fine epitope was mapped to amino acids (aa) 37-45 (NSNNPDWDF). In this current study, the epitope in combination with KR127 was tested for protein tagging. Initially, to evaluate the importance of each residue of the KR127 epitope in antibody binding, alanine substitution mutants of the epitope were constructed and characterized for KR127 binding by immunoblot analysis and competition ELISA. The results showed that substitution of Ser(38) by alanine (S38A) increased the affinity to KR127. The mutated epitope (NANNPDWDF), designated S1 tag, was fused to the amino (N)- or carboxyl (C)-terminus of three human recombinant proteins, soluble B lymphocyte stimulator (sBLyS), the N-terminal domain of thrombopoietin (nTPO), and a mitochondrial ribosomal protein (CGI-113) for expression in mammalian cells, while it was fused to the N- or C-terminus of two proteins, a single-chain antibody fragment (ScFv) and the carboxyl-terminal domain (PAc) of the protective antigen of Bacillus anthracis for expression in Escherichia coli. The immunodetection, immunoprecipitation, and affinity purification of the expressed S1-tagged proteins by KR127 were successfully demonstrated. In addition, a KR127 mutant (AP2) with higher affinity, K(d) (0.9 nM), for the S1 tag compared to that (20 nM) of KR127 was obtained by mutational analysis of the heavy chain CDR3 (HCDR3) of KR127. The AP2 antibody was 4-fold more sensitive in detecting the S1-tagged protein than KR127. The S1 tag-KR127 or AP2 combination could be universally used for monitoring protein expression, localizing proteins, and protein purification, as well as studying protein interactions.

The molecular and cellular biology of thrombopoietin: the primary regulator of platelet production

The term thrombopoietin (TPO) was first coined in 1958 and used to describe the humoral substance responsible for causing the platelet count to rise in response to thrombocytopenic stimuli. Despite much progress during the 1980s in the purification and characterization of the humoral regulators of lymphocyte, erythrocyte, monocyte and granulocyte production, the successful search to purify and molecularly clone thrombopoietin did not begin until the oncogene v-mpl was discovered in 1990. Since that time the proto-oncogene c-mpl was identified and, based on homology arguments, believed to encode a hematopoietic cytokine receptor, a hypothesis later proven when the cytoplasmic domain was linked to the ligand binding domain of the IL-4 receptor and shown to support the IL-4 induced growth of hematopoietic cells (Skoda et al., 1993). Finally, two different strategies using c-mpl lead to the identification of a novel ligand for the receptor in 1994 (de Sauvage et al., 1994; Lok et al., 1994; Bartley et al., 1994), a protein that displays all the biologic properties of TPO. This review attempts to distill what has been learned of the molecular and cellular biology of TPO and its receptor during the past several years, and links this information to several new insights into human disease and its treatment.

Thrombopoietin: a pan-hematopoietic cytokine

The recent discovery of thrombopoietin has enhanced our understanding of both hematopoiesis and platelet production. Thrombopoietin supports hematopoietic stem cell survival and expansion as well as promoting all aspects of megakaryocyte development. The hormone displays many structural similarities to other members of the hematopoietic cytokine family and some notable differences, and regulation of its expression requires both receptor-mediated removal and other mechanisms. Thrombopoietin induces receptor dimerization and tyrosine phosphorylation, and a series of signaling events including activation of JAK/STAT, Shc/Ras/MAPK and PI3K/Akt; these pathways overlap with those induced by other cytokines, but the differences that lead to the unique biological effects of the hormone are gradually being uncovered. Our growing appreciation of how cytokine signaling pathways are translated into megakaryocyte development is discussed.

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.

Native thrombopoietin: structure and function

Thrombopoietin (TPO), the c-Mpl ligand, is produced constitutively in liver and other organs, circulates in the bloodstream, and is delivered to bone marrow, where it stimulates the early development of multiple hematopoietic lineages and megakaryocytopoiesis. The concentration of TPO in blood is regulated by c-Mpl mass on platelets and megakaryocytes. In addition to regulation by the number of TPO molecules, including the possible modulation of TPO mRNA abundance in bone marrow, megakaryocytopoiesis and platelet production may be regulated as a result of modulation of TPO activity by proteolytic processing that generates truncated forms of the molecule. Characterization of TPO partially purified from human plasma, however, revealed that the full-length molecule was the predominant form in the blood of both normal individuals and thrombocytopenic patients, although small amounts of truncated species were detected. Thus, truncation of TPO, at least that in the circulation examined, does not appear to contribute to the direct regulation of platelet production in response to increased demand. Given that native TPO isolated from the plasma of thrombocytopenic animals comprises truncated forms, the truncation of TPO is likely of physiological importance in the life history of this molecule.

Expression, purification, and characterization of human recombinant thrombopoietin in Chinese hamster ovary cells

Thrombopoietin (TPO) is a primary regulator of megakaryocytopoiesis, a process through which megakaryocytes proliferate and mature into platelets. Recombinant human TPO (rhTPO) was expressed in Chinese hamster ovary (CHO) cells and purified from the culture medium. The cDNA encoding full-length TPO, including the native signal peptide sequence, was amplified by PCR from a human fetal liver cDNA library. The product was cloned into a mammalian expression vector under the control of the SV40 early promoter and enhancer. Secreted rhTPO was purified in three conventional chromatography steps. It migrates on SDS-PAGE as a broad band, characteristic of a heavily glycosylated protein, with an average molecular mass of 85 kDa. rhTPO expressed in CHO cells is biologically active in vitro as demonstrated by its ability to stimulate the proliferation of a megakaryocytic cell line and to trigger the JAK/STAT signal transduction pathway. rhTPO also shows activity in vivo as judged by the elevation of platelet count in treated mice.

Expression and characterization of bioactive human thrombopoietin in the milk of transgenic mice

Human thrombopoietin (hTPO) is the primary physiological regulator of platelet production and plays a pivotal role in promoting the proliferation and maturation of megakaryocytic progenitor cells and megakaryocytes. In this study, transgenic mice were produced harboring either full-length or the erythropoietin (EPO)-like amino-terminal domain of hTPO cDNA sequences fused to the regulatory elements of the bovine beta-casein gene. The transgene RNA was expressed exclusively in the mammary glands of eight transgenic mice, and a trace amount of the transgene was also found in the lungs of one mouse. The full-length form induced efficient expression of the protein with the highest expression level of 1500 microg/ml; however, the EPO-like domain alone expressed the protein at <0.1 microg/ml. The proteins from the two recombinant cDNAs have apparent molecular weights of about 74 and 17 kDa, due to glycosylation in the case of the full-length cDNA. Cell proliferation assay in vitro indicated that both of the recombinant forms stimulated proliferation of the TPO-dependent BaF3-Mpl cells. A positive correlation appeared between the amount of TPO in the milk of lactating animals and their blood platelet levels. About a twofold increase in platelet numbers in the blood was observed after direct subcutaneous injection of the recombinant hTPO at the level of 30 microg/kg of body weight. On the basis of these results, we anticipate that the recombinant hTPO produced efficiently in milk of transgenic mice will have the same activities as the native hTPO in a few in vivo as well as in vitro biochemical aspects.

(R)evolutionary considerations in hematopoietic development

Evolutionary aspects of three characteristics of the mammalian hematopoietic system are considered in the context of both established and recent data. First, the lineage relationships among early members of the hematopoietic hierarchy are reconsidered in a tripartite model proposing lineage segregation based on vascular function, innate immunity, and acquired immunity on an evolutionary time scale. Second, the observation of two stem cell populations that differ in cell cycle status is considered as an evolved mechanism to enhance survival of the species in response to exposure to environmental toxins. Finally, the mobilization of hematopoietic stem cells into the peripheral circulation is proposed to be a mechanism for rapid dissemination of myeloid function during acute bacterial infections. These revolutionary hypotheses challenge some conventional concepts of stem cell biology, and provide an evolutionary context for considering mammalian hematopoiesis.

Native thrombopoietin: structure and function

Thrombopoietin (TPO), the c-Mpl ligand, is produced constitutively in liver and other organs, circulates in the bloodstream, and is delivered to bone marrow, where it stimulates the early development of multiple hematopoietic lineages and megakaryocytopoiesis. The concentration of TPO in blood is regulated by c-Mpl mass on platelets and megakaryocytes. In addition to regulation by the number of TPO molecules, including the possible modulation of TPO mRNA abundance in bone marrow, megakaryocytopoiesis and platelet production may be regulated as a result of modulation of TPO activity by proteolytic processing that generates truncated forms of the molecule. Characterization of TPO partially purified from human plasma, however, revealed that the full-length molecule was the predominant form in the blood of both normal individuals and thrombocytopenic patients, although small amounts of truncated species were detected. Thus, truncation of TPO, at least that in the circulation examined, does not appear to contribute to the direct regulation of platelet production in response to increased demand. Given that native TPO isolated from the plasma of thrombocytopenic animals comprises truncated forms, the truncation of TPO is likely of physiological importance in the life history of this molecule.