Thrombopoietin expands erythroid progenitors, increases red cell production, and enhances erythroid recovery after myelosuppressive therapy

Thrombopoietin mimetic stimulates bone marrow vascular and stromal niches to mitigate acute radiation syndrome

BACKGROUND

Acute radiation syndrome (ARS) manifests after exposure to high doses of radiation in the instances of radiologic accidents or incidents. Facilitating regeneration of the bone marrow (BM), namely the hematopoietic stem and progenitor cells (HSPCs), is key in mitigating ARS and multi-organ failure. JNJ-26366821, a PEGylated thrombopoietin mimetic (TPOm) peptide, has been shown as an effective medical countermeasure (MCM) to treat hematopoietic-ARS (H-ARS) in mice. However, the activity of TPOm on regulating BM vascular and stromal niches to support HSPC regeneration has yet to be elucidated.

METHODS

C57BL/6J mice (9-14 weeks old) received sublethal or lethal total body irradiation (TBI), a model for H-ARS, by 137Cs or X-rays. At 24 h post-irradiation, mice were subcutaneously injected with a single dose of TPOm (0.3 mg/kg or 1.0 mg/kg) or PBS (vehicle). At homeostasis and on days 4, 7, 10, 14, 18, and 21 post-TBI with and without TPOm treatment, BM was harvested for histology, BM flow cytometry of HSPCs, endothelial (EC) and mesenchymal stromal cells (MSC), and whole-mount confocal microscopy. For survival, irradiated mice were monitored and weighed for 30 days. Lastly, BM triple negative cells (TNC; CD45-, TER-119-, CD31-) were sorted for single-cell RNA-sequencing to examine transcriptomics after TBI with or without TPOm treatment.

RESULTS

At homeostasis, TPOm expanded the number of circulating platelets and HSPCs, ECs, and MSCs in the BM. Following sublethal TBI, TPOm improved BM architecture and promoted recovery of HSPCs, ECs, and MSCs. Furthermore, TPOm elevated VEGF-C levels in normal and irradiated mice. Following lethal irradiation, mice improved body weight recovery and 30-day survival when treated with TPOm after 137Cs and X-ray exposure. Additionally, TPOm reduced vascular dilation and permeability. Finally, single-cell RNA-seq analysis indicated that TPOm increased the expression of collagens in MSCs to enhance their interaction with other progenitors in BM and upregulated the regeneration pathway in MSCs.

CONCLUSIONS

TPOm interacts with BM vascular and stromal niches to locally support hematopoietic reconstitution and systemically improve survival in mice after TBI. Therefore, this work warrants the development of TPOm as a potent radiation MCM for the treatment of ARS.

Thrombopoietin, the Primary Regulator of Platelet Production: From Mythos to Logos, a Thirty-Year Journey

Thrombopoietin, the primary regulator of blood platelet production, was postulated to exist in 1958, but was only proven to exist when the cDNA for the hormone was cloned in 1994. Since its initial cloning and characterization, the hormone has revealed many surprises. For example, instead of acting as the postulated differentiation factor for platelet precursors, megakaryocytes, it is the most potent stimulator of megakaryocyte progenitor expansion known. Moreover, it also stimulates the survival, and in combination with stem cell factor leads to the expansion of hematopoietic stem cells. All of these growth-promoting activities have resulted in its clinical use in patients with thrombocytopenia and aplastic anemia, although the clinical development of the native molecule illustrated that "it's not wise to mess with mother nature", as a highly engineered version of the native hormone led to autoantibody formation and severe thrombocytopenia. Finally, another unexpected finding was the role of the thrombopoietin receptor in stem cell biology, including the development of myeloproliferative neoplasms, an important disorder of hematopoietic stem cells. Overall, the past 30 years of clinical and basic research has yielded many important insights, which are reviewed in this paper.

Thrombopoietin mimetic stimulates bone marrow vascular and stromal niches to mitigate acute radiation syndrome

Background

Acute radiation syndrome (ARS) manifests after exposure to high doses of radiation in the instances of radiologic accidents or incidents. Facilitating the regeneration of the bone marrow (BM), namely the hematopoietic stem and progenitor cells (HSPCs), is a key in mitigating ARS and multi-organ failure. JNJ-26366821, a PEGylated thrombopoietin mimetic (TPOm) peptide, has been shown as an effective medical countermeasure (MCM) to treat hematopoietic-ARS (H-ARS) in mice. However, the activity of TPOm on regulating BM vascular and stromal niches to support HSPC regeneration has not yet been elucidated.

Methods

C57BL/6J mice (9-14 weeks old) received sublethal or lethal total body irradiation (TBI), a model for H-ARS, by 137Cs or X-rays. At 24 hours post-irradiation, mice were subcutaneously injected with a single dose of TPOm (0.3 mg/kg or 1.0 mg/kg) or PBS (vehicle). At homeostasis and on days 4, 7, 10, 14, 18, and 21 post-TBI with and without TPOm treatment, BM was harvested for histology, BM flow cytometry of HSPCs, endothelial (EC) and mesenchymal stromal cells (MSC), and whole-mount confocal microscopy. For survival, irradiated mice were monitored and weighed for 30 days. Lastly, BM triple negative cells (TNC; CD45-, TER-119-, CD31-) were sorted for single-cell RNA-sequencing to examine transcriptomics after TBI with or without TPOm treatment.

Results

At homeostasis, TPOm expanded the number of circulating platelets and HSPCs, ECs, and MSCs in the BM. Following sublethal TBI, TPOm improved BM architecture and promoted recovery of HSPCs, ECs, and MSCs. Furthermore, TPOm elevated VEGF-C levels in normal and irradiated mice. Following lethal irradiation, mice improved body weight recovery and 30-day survival when treated with TPOm after 137Cs and X-ray exposure. Additionally, TPOm reduced vascular dilation and permeability. Finally, single-cell RNA-seq analysis indicated that TPOm increased the expression of collagens in MSCs to enhance their interaction with other progenitors in BM and upregulated the regeneration pathway in MSCs.

Conclusions

TPOm interacts with BM vascular and stromal niches to locally support hematopoietic reconstitution and systemically improve survival in mice after TBI. Therefore, this work warrants the development of TPOm as a potent radiation MCM for the treatment of ARS.

Thrombopoietin treats erythropoietin resistance by correcting EPO-induced progenitorcell depletion

Recombinant human erythropoietin (rHuEPO) is a prevalent treatment for anemia in patients with chronic kidney disease. However, up to 10% of these patients exhibit EPO resistance or hyporesponsiveness, which may be caused by the depletion of erythroid progenitor cells. Thrombopoietin (TPO) has the potential to promote the growth of early progenitor cells and correct the depletion. In this study, we investigate the efficacy and the underlying mechanism of the combination therapy of TPO and EPO to EPO resistance. First, the in vivo studies suggested that intensive EPO treatment induced progenitor cell depletion in the bone marrow, where the depletion was corrected by TPO. Then, colony assays showed that EPO and TPO synergistically enhanced the burst-forming unit-erythroid (BFU-E) production but antagonistically boosted the colony-forming units of megakaryocytes (CFU-MK) production. Also, we found TPO promoted hematopoietic stem and progenitor cells (HSPCs) production, while EPO drove HSPCs toward the erythroid lineage. Additionally, EPO induced more megakaryocytic-erythroid progenitors (MEPs) toward the erythroid output. Model-based simulations indicate the efficacy of this combination therapy for treating EPO-resistant anemia in rats. In conclusion, our study demonstrated the efficacy of combination therapy in addressing EPO-resistant anemia by correcting EPO-induced erythroid progenitor depletion.

In Silico Evaluation of Coding and Non-Coding nsSNPs in the Thrombopoietin Receptor (MPL) Proto-Oncogene: Assessing Their Influence on Protein Stability, Structure, and Function

The thrombopoietin receptor (MPL) gene is a critical regulator of hematopoiesis, and any alterations in its structure or function can result in a range of hematological disorders. Non-synonymous single nucleotide polymorphisms (nsSNPs) in MPL have the potential to disrupt normal protein function, prompting our investigation into the most deleterious MPL SNPs and the associated structural changes affecting protein-protein interactions. We employed a comprehensive suite of bioinformatics tools, including PredictSNP, InterPro, ConSurf, I-Mutant2.0, MUpro, Musitedeep, Project HOPE, STRING, RegulomeDB, Mutpred2, CScape, and CScape Somatic, to analyze 635 nsSNPs within the MPL gene. Among the analyzed nsSNPs, PredictSNP identified 28 as significantly pathogenic, revealing three critical functional domains within MPL. Ten of these nsSNPs exhibited high conservation scores, indicating potential effects on protein structure and function, while 14 were found to compromise MPL protein stability. Although the most harmful nsSNPs did not directly impact post-translational modification sites, 13 had the capacity to substantially alter the protein's physicochemical properties. Some mutations posed a risk to vital protein-protein interactions crucial for hematological functions, and three non-coding region nsSNPs displayed significant regulatory potential with potential implications for hematopoiesis. Furthermore, 13 out of 21 nsSNPs evaluated were classified as high-risk pathogenic variants by Mutpred2. Notably, amino acid alterations such as C291S, T293N, D295G, and W435C, while impactful on protein stability and function, were deemed non-oncogenic "passenger" mutations. Our study underscores the substantial impact of missense nsSNPs on MPL protein structure and function. Given MPL's central role in hematopoiesis, these mutations can significantly disrupt hematological processes, potentially leading to a variety of disorders. The identified high-risk pathogenic nsSNPs may hold promise as potential biomarkers or therapeutic targets for hematological diseases. This research lays the foundation for future investigations into the MPL gene's role in the realm of hematological health and diseases.

Vhl deletion in Dmp1 -expressing cells alters MEP metabolism and promotes stress erythropoiesis

In recent years, general hypoxia-inducible factor (HIF)-prolyl hydroxylase (PHD) enzyme inhibitors have been developed for the treatment of anemia due to renal disease and osteoporosis. However, it remains a challenge to target the HIF signaling pathway without dysregulating the skeletal and hematopoietic system. Here, we examined the effects of Vhl deletion in bone by performing longitudinal analyses of Vhl cKO mice at 3, 6, 10, and 24 weeks of age, where at 10 and 24 weeks of age, high bone mass and splenomegaly are present. Using flow cytometry, we observed increased frequency (%) of CD71 lo TER119 hi FSC lo orthochromatophilic erythroblasts and reticulocytes in 10- and 24-week-old Vhl cKO bone marrow (BM), which correlated with elevated erythropoietin levels in the BM and increased number of red blood cells in circulation. The absolute numbers of myeloerythroid progenitors (MEPs) in the BM were significantly reduced at 24 weeks. Bulk RNA-Seq of the MEPs showed upregulation of Epas1 ( Hif1a) and Efnb2 ( Hif2a) in Vhl cKO MEPs, consistent with a response to hypoxia, and genes involved in erythrocyte development, actin filament organization, and response to glucose. Additionally, histological analysis of Vhl cKO spleens revealed red pulp hyperplasia and the presence of megakaryocytes, both of which are features of extramedullary hematopoiesis (EMH). EMH in the spleen was correlated with the presence of mature stress erythroid progenitors, suggesting that stress erythropoiesis is occurring to compensate for the BM microenvironmental irregularities. Our studies implicate that HIF-driven alterations in skeletal homeostasis can accelerate erythropoiesis.

Key Points

• Dysregulation of HIF signaling in Dmp1+ bone cells induces stress erythropoiesis.• Skeletal homeostasis modulates erythropoiesis.

Endocytosis of the thrombopoietin receptor Mpl regulates megakaryocyte and erythroid maturation in mice

Dnm2fl/fl Pf4-Cre (Dnm2Plt-/- ) mice lacking the endocytic GTPase dynamin 2 (DNM2) in platelets and megakaryocytes (MKs) develop hallmarks of myelofibrosis. At the cellular level, the tyrosine kinase JAK2 is constitutively active but decreased in expression in Dnm2Plt-/- platelets. Additionally, Dnm2Plt-/- platelets cannot endocytose the thrombopoietin (TPO) receptor Mpl, leading to elevated circulating TPO levels. Here, we assessed whether the hyperproliferative phenotype of Dnm2Plt-/- mice was due to JAK2 constitutive activation or to elevated circulating TPO levels. In unstimulated Dnm2Plt-/- platelets, STAT3 and, to a lower extent, STAT5 were phosphorylated, but their phosphorylation was slowed and diminished upon TPO stimulation. We further crossed Dnm2Plt-/- mice in the Mpl-/- background to generate Mpl-/-Dnm2Plt-/- mice lacking Mpl ubiquitously and DNM2 in platelets and MKs. Mpl-/- Dnm2Plt-/- platelets had severely reduced JAK2 and STAT3 but normal STAT5 expression. Mpl-/- Dnm2Plt-/- mice had severely reduced bone marrow MK and hematopoietic stem and progenitor cell numbers. Additionally, Mpl-/- Dnm2Plt-/- mice had severe erythroblast (EB) maturation defects, decreased expression of hemoglobin and heme homeostasis genes and increased expression of ribosome biogenesis and protein translation genes in spleen EBs, and developed anemia with grossly elevated plasma erythropoietin (EPO) levels, leading to early fatality by postnatal day 25. Mpl-/- Dnm2Plt+/+ mice had impaired EB development at three weeks of age, which normalized with adulthood. Together, the data shows that DNM2-dependent Mpl-mediated endocytosis in platelets and MKs is required for steady-state hematopoiesis and provides novel insights into a developmentally controlled role for Mpl in normal erythropoiesis, regulating hemoglobin and heme production.

A Novel Combination Therapy of Erythropoietin and Thrombopoietin to Treat Erythropoietin-Resistance anemia

PURPOSE

Treatment with recombinant human erythropoietin (rHuEPO) may correct anemia in patients with chronic kidney disease. However, up to 10% of these patients exhibit EPO resistance or hyporesponsiveness, which may be caused by the depletion of erythroid progenitor cells. Thrombopoietin (TPO) stimulates the self-renewal of stem cells and promotes the growth of early erythroid progenitor cells. The objective of this study was to determine whether the combination of recombinant human TPO (rHuTPO) and rHuEPO could correct the depletion of erythroid precursor cells to treat EPO-resistant anemia.

METHODS

To test our hypothesis, pharmacokinetic (PK) and pharmacodynamic (PD) studies of rHuEPO and rHuTPO were performed in healthy rats. Rats received rHuEPO or rHuTPO alone or in combination. Plasma concentrations of rHuTPO and rHuEPO were measured. PD responses, including erythropoietic and thrombopoietic responses, were assessed in peripheral blood.

RESULTS

On one hand, the results demonstrated the synergistic effect of the combination of rHuEPO and rHuTPO on erythropoiesis. Compared with rHuEPO monotherapy, the combination therapies further stimulated the production of red blood cells and hemoglobin. On the other hand, rHuEPO inhibited the platelet production induced by rHuTPO and mitigate the risk of blood clots. Furthermore, we successfully developed a mechanism-based PD model to simultaneously characterize the responses of the two molecules.

CONCLUSIONS

Overall, our study indicated that a combination therapy of rHuTPO and rHuEPO could be used to treat EPO-resistant anemia and provided a quantitative basis for further optimizing the combination therapy for clinical use.

In vitro expansion of fetal liver hematopoietic stem cells

Fetal liver hematopoietic stem and progenitor cells (HSPCs) have been considered appropriate for the management of aplastic anemia owing to their proliferative potential. Bone marrow recovery was possible in some cases; the engraftment potential of these cells, however was unsatisfactory, possibly due to the availability of a smaller number of these cells from a single fetus. The present study explores how we can expand fetal liver hematopoietic stem cells under in vitro conditions. We isolated mononuclear cells from fetal liver and hematopoietic stem cells were identified and analyzed by cell surface marker CD34. CD34⁺ fetal liver HSPCs cells were separated by magnetic cell sorting positive selection method. HSPCs (CD34⁺) were cultured by using 5 cytokines, stem cell factor (SCF), granulocyte macrophages-colony stimulating factor (GM-CSF), interleukin-6 (IL-6), Fms-related tyrosine kinase 3 (FLT-3) and erythropoietin (EPO), in 4 different combinations along with supplements, in serum-free culture media for 21 days. Cell viability continued to be greater than 90% throughout 21 days of culture. The cells expanded best in a combination of media, supplements and 5 cytokines, namely SCF, FLT-3, IL6, EPO and GM-CSF to yield a large number of total (CD34⁺ & CD34^-) cells. Even though the total number of nucleated cells increased in culture significantly, levels of CD34 antigen expression declined steadily over this period.

Thrombopoietin receptor agonist is more effective than platelet transfusion for chronic liver disease with thrombocytopenia, shown by propensity score matching

AIM

The thrombopoietin receptor agonist, lusutrombopag, was recently adapted for treatment of thrombocytopenia in chronic liver disease. However, no studies have compared the effects of lusutorombopag and platelet transfusion. Therefore, we aimed to clarify the efficacy and proportion of responders of lusutrombopag compared with platelet transfusion, by propensity score matching.

METHODS

A total of 200 patients (90 lusutrombopag, 110 platelet transfusion) with thrombocytopenia were enrolled, and matched for age, liver function, renal function, platelet count, peripheral blood count, and spleen size, using the propensity score-matching method. Finally, 52 patients (26 lusutrombopag, 26 platelet transfusion) were included. The primary end-point was an increase in platelet count. Secondary end-points were the proportion of responders, duration of the sustained effect, incidence of adverse events, and predictors associated with an increase in platelet count.

RESULTS

The median increase in platelets from baseline was 48 × 10³ /μL versus 9.5 × 10³ /μL (lusutrombopag vs. transfusion, P < 0.0001). The proportion of responders (increase of >10 × 10³ /μL) was 100% versus 50.0% (P < 0.0001). Median duration of the sustained effect (increase of >50 × 10³ /μL) was 10 versus 2 days (P < 0.0001). The incidence of adverse events was 7.7% versus 50.0% (P = 0.036). Predictors associated with an increase in platelets were hemoglobin and spleen size by multiple regression analysis.

CONCLUSION

Lusutrombopag was more effective in chronic liver disease patients than platelet transfusion. The proportion of responders, effect duration, and non-incidence rate of adverse events were higher in the lusutrombopag group.

Current Advances in Red Blood Cell Generation Using Stem Cells from Diverse Sources

Blood transfusions hold an indispensable part in the modern healthcare system. Up to date, the blood supply is largely dependent on donations. Unfortunately, collecting the clinical-grade blood products has become a challenging mission due to accelerated population aging, which not only increases the need for blood transfusions but also decreases the number of healthy donors. Moreover, individuals with severe hematological abnormalities or rare blood phenotypes need alternative therapeutic approaches instead of conventional blood transfusion. In these aspects, the concept of in vitro/ex vivo production of blood cells has been emerging and many attempts have been focused on manufacturing mature erythrocytes, so-called red blood cells (RBCs), the most common and important component among the blood derivatives. In this review, we provide a general overview regarding the current strategies for generating RBCs from various stem cell sources including pluripotent stem cells (PSCs) as well as circulating blood stem cells and the remaining challenges that must be overcome prior to their practical application.

Hematopoietic Cytokine Gene Duplication in Zebrafish Erythroid and Myeloid Lineages

Hematopoiesis is a precisely orchestrated process regulated by the activity of hematopoietic cytokines and their respective receptors. Due to an extra round of whole genome duplication during vertebrate evolution in teleost fish, zebrafish have two paralogs of many important genes, including genes involved in hematopoiesis. Importantly, these duplication events brought increased level of complexity in such cases, where both ligands and receptors have been duplicated in parallel. Therefore, precise understanding of binding specificities between duplicated ligand-receptor signalosomes as well as understanding of their differential expression provide an important basis for future studies to better understand the role of duplication of these genes. However, although many recent studies in the field have partly addressed functional redundancy or sub-specialization of some of those duplicated paralogs, this information remains to be scattered over many publications and unpublished data. Therefore, the focus of this review is to provide an overview of recent findings in the zebrafish hematopoietic field regarding activity, role and specificity of some of the hematopoietic cytokines with emphasis on crucial regulators of the erythro-myeloid lineages.

Leukemic mature B-cell neoplasm observed during the treatment with romiplostim in patient with thrombocytopenia and liver cirrhosis

We report here a new adverse event of romiplostim which is a protein analog of thrombopoietin. Leukemic mature B-cell neoplasm was observed during the treatment of thrombocytopenia in a patient with liver cirrhosis. Their relationship was suggested clinically but the mechanism of leukemic expansion of lymphoma cells was not clarified.

Myeloproliferative Disorders

The emergency providers generally encounters myeloproliferative disorders (MPNs) in 1 of 2 ways: as striking laboratory abnormalities of seeming unknown consequence, or in previously diagnosed patients presenting with complications. The course of patients with MPNs is highly variable, but major complications can arise. Emergent conditions related to hyperviscosity need to be recognized early and treated aggressively. Rapid hydration, transfusion, cytoreduction, and early hematology consultation can be lifesaving. Likewise, although management is not altered, a high index of suspicion for thrombotic complications is required in patients with known MPNs as these are a significant cause of morbidity and mortality.

Thrombopoietin Receptor Agonist Mitigates Hematopoietic Radiation Syndrome and Improves Survival after Whole-Body Ionizing Irradiation Followed by Wound Trauma

Ionizing radiation combined with trauma tissue injury (combined injury, CI) results in greater mortality and H-ARS than radiation alone (radiation injury, RI), which includes thrombocytopenia. The aim of this study was to determine whether increases in numbers of thrombocytes would improve survival and mitigate H-ARS after CI. We observed in mice that WBC and platelets remained very low in surviving RI animals that were given 9.5 Gy 60Co-γ-photon radiation, whereas only lymphocytes and basophils remained low in surviving CI mice that were irradiated and then given skin wounds. Numbers of RBC and platelets, hemoglobin concentrations, and hematocrit values remained low in surviving RI and CI mice. CI induced 30-day mortality higher than RI. Radiation delayed wound healing by approximately 14 days. Treatment with a thrombopoietin receptor agonist, Alxn4100TPO, after CI improved survival, mitigated body-weight loss, and reduced water consumption. Though this therapy delayed wound-healing rate more than in vehicle groups, it greatly increased numbers of platelets in sham, wounded, RI, and CI mice; it significantly mitigated decreases in WBC, spleen weights, and splenocytes in CI mice and decreases in RBC, hemoglobin, hematocrit values, and splenocytes and splenomegaly in RI mice. The results suggest that Alxn4100TPO is effective in mitigating CI.

H2S protects against fatal myelosuppression by promoting the generation of megakaryocytes/platelets

Background

Our previous pilot studies aimed to examine the role of hydrogen sulfide (H₂S) in the generation of endothelial progenitor cells led to an unexpected result, i.e., H₂S promoted the differentiation of certain hematopoietic stem/progenitor cells in the bone marrow. This gave rise to an idea that H₂S might promote hematopoiesis.

Methods

To test this idea, a mice model of myelosuppression and cultured fetal liver cells were used to examine the role of H₂S in hematopoiesis.

Results

H₂S promoted the generation of megakaryocytes, increased platelet levels, ameliorate entorrhagia, and improved survival. These H₂S effects were blocked in both in vivo and in vitro models with thrombopoietin (TPO) receptor knockout mice (c-mpl^−/− mice). In contrast, H₂S promoted megakaryocytes/platelets generation in both in vivo and in vitro models with TPO knockout mice (TPO^−/− mice).

Conclusions

H₂S is a novel promoter for megakaryopoiesis by acting on the TPO receptors but not TPO to generate megakaryocytes/platelets.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-016-0244-7) contains supplementary material, which is available to authorized users.

Evolving insights into the synergy between erythropoietin and thrombopoietin and the bipotent erythroid/megakaryocytic progenitor cell

Although the synergy between erythropoietin and thrombopoietin has previously been pointed out, the clonal demonstration of a human bipotent erythroid/megakaryocytic progenitor (MEP) was first published in Experimental Hematology (Papayannopoulou T, Brice M, Farrer D, Kaushansky K. Exp Hematol. 1996;24:660-669) and later in the same year in Blood (Debili N, Coulombel L, Croisille L, et al. Blood. 1996;88:1284-1296). This demonstration, and the fact that both bipotent and monopotent erythroid or megakaryocytic progenitors co-express markers of both lineages and respond to both lineage-specific transcription factors, has provided a background for the extensive use of MEP assessment by fluorescence-activated cell sorting in many subsequent studies. Beyond this, the demonstration of shared regulatory elements and the presence of single mutations affecting both lineages have inspired further studies to decipher how the shift in transcription factor networks occurs from one lineage to the other. Furthermore, in addition to shared effects, erythropoietin and thrombopoietin have additional independent effects. Most notable for thrombopoietin is its effect on hematopoietic stem cells illustrated by in vitro and in vivo approaches.

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.

Erythropoiesis from human embryonic stem cells through erythropoietin-independent AKT signaling

Unlimited self renewal capacity and differentiation potential make human pluripotent stem cells (PSC) a promising source for the ex vivo manufacture of red blood cells (RBCs) for safe transfusion. Current methods to induce erythropoiesis from PSC suffer from low yields of RBCs, most of which are immature and contain embryonic and fetal rather than adult hemoglobins. We have previously shown that homodimerization of the intracellular component of MPL (ic-MPL) induces erythropoiesis from human cord blood progenitors. The goal of this study was to investigate the potential of ic-MPL dimerization to induce erythropoiesis from human embryonic stem cells (hESCs) and to identify the signaling pathways activated by this strategy. We present here the evidence that ic-MPL dimerization induces erythropoietin (EPO)-independent erythroid differentiation from hESC by inducing the generation of erythroid progenitors and by promoting more efficient erythroid maturation with increased RBC enucleation as well as increased gamma:epsilon globin ratio and production of beta-globin protein. ic-MPL dimerization is significantly more potent than EPO in inducing erythropoiesis, and its effect is additive to EPO. Signaling studies show that dimerization of ic-MPL, unlike stimulation of the wild type MPL receptor, activates AKT in the absence of JAK2/STAT5 signaling. AKT activation upregulates GATA-1 and FOXO3 transcriptional pathways with resulting inhibition of apoptosis, modulation of cell cycle, and enhanced maturation of erythroid cells. These findings open up potential new targets for the generation of therapeutically relevant RBC products from hPSC.

Myeloproliferative disorders

The emergency providers generally encounters myeloproliferative disorders (MPNs) in 1 of 2 ways: as striking laboratory abnormalities of seeming unknown consequence, or in previously diagnosed patients presenting with complications. The course of patients with MPNs is highly variable, but major complications can arise. Emergent conditions related to hyperviscosity need to be recognized early and treated aggressively. Rapid hydration, transfusion, cytoreduction, and early hematology consultation can be lifesaving. Likewise, although management is not altered, a high index of suspicion for thrombotic complications is required in patients with known MPNs as these are a significant cause of morbidity and mortality.

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.

Preliminary nonclinical toxicity, pharmacokinetics, and pharmacodynamics of ALXN4100TPO, a thrombopoietin receptor agonist, in CD2F1 mice

ALXN4100TPO, a thrombopoietin (TPO) receptor agonist, increases platelets, abrogates radiation-induced thrombocytopenia and affords significant survival benefit to lethally irradiated mice. This preliminary nonclinical safety study assessed effects of a single subcutaneous (sc) administration of ALXN4100TPO in CD2F1 mice randomized into naïve, control antibody (ALXN4200, 100 mg/kg), low (1 mg/kg), medium (10 mg/kg), or high (100 mg/kg) ALXN4100TPO doses. End points included clinical observations, body weight changes, hematology, histopathology, pharmacokinetics, pharmacodynamics by measuring platelet counts, and endogenous TPO (eTPO) levels. Salient findings were prominent increase in platelet counts and end cells of myeloid and lymphoid lineages; elevated megakaryopoiesis in bone marrow; and extramedullary hematopoiesis in spleen and liver. Serum ALXN4100TPO levels were maximum 24 hours after administration, with a half-life of 13 days. Endogenous TPO levels were elevated in 10 and 100 mg/kg ALXN4100TPO-treated groups. In conclusion, ALXN4100TPO (1-100 mg/kg, sc) treatment in CD2F1 mice resulted in profound pharmacological changes in the hematopoietic tissue; however, no life-threatening adverse events were observed.

The biology of thrombopoietin and thrombopoietin receptor agonists

Thrombopoietin (TPO) is the major physiological regulator of platelet production. TPO binds the TPO receptor, activates JAK and STAT pathways, thus stimulating megakaryocyte growth and platelet production. There is no "sensor" of the platelet count; rather TPO is produced in the liver at a constant rate and cleared by TPO receptors on platelets. TPO levels are inversely proportional to the rate of platelet production. Early recombinant TPO molecules were potent stimulators of platelet production and increased platelets in patients with immune thrombocytopenia, chemotherapy-induced thrombocytopenia, myelodysplastic syndromes and platelet apheresis donors. Neutralizing antibodies formed against one recombinant protein and ended their development. A second generation of TPO receptor agonists, romiplostim and eltrombopag, has been developed. Romiplostim is an IgG heavy chain into which four TPO agonist peptides have been inserted. Eltrombopag is an oral small molecule. These activate the TPO receptor by different mechanisms to increase megakaryocyte growth and platelet production. After administration of either to healthy volunteers, there is a delay of 5 days before the platelet count rises and subsequently reaches a peak after 12-14 days. Both have been highly effective in treating ITP and hepatitis C thrombocytopenia. Studies in a wide variety of other thrombocytopenic conditions are underway.

Growth of erythroid cells from thawed unseparated cord blood in vitro without exogenous erythropoietin

INTRODUCTION

Previous erythroid cell cultures have depended on added serum or erythropoietin. In this paper, the growth of erythroid cells from thawed unseparated cord blood units in vitro without serum or exogenous erythropoietin is reported.

METHODS

Thawed volume-reduced cord blood was cultured in conditions designed to support the megakaryocytic lineage, with thrombopoietin and interleukins 3 and 6. Erythroid cells were detected with glycophorin A (GlyA), CD71, and benzidine (flow cytometry and immunocytochemistry).

RESULTS

Nucleated and anucleated GlyA-positive, as well as benzidine-positive cells were observed from day 9. In flow cytometry, at days 0 and 9, 5.9% and 14% of all events were GlyA+, and 14% and 53% were CD71+, respectively. At days 0 and 9, 4.5% and 12% of the events were double-positive for GlyA and CD71, respectively. By day 14, the percentages of GlyA+, CD71+ and double-positive events had started to decrease (9.7%, 35%, and 5.3%, respectively).

CONCLUSIONS

Erythroid cells were generated from thawed unseparated cord blood units without exogenous erythropoietin. Thawed cord blood possesses the potential for erythroid growth in vitro in a culture medium designed for other cell types.

Thrombopoietin modulates the proliferation, migration and cytokine profile of decidual cell subsets during early gestation

During early gestation, a considerable increase in different leukocyte subsets can be observed in the decidualized endometrium concomitantly to the invasion of cytotrophoblast cells (CTB). To date, it is still in question which factors induce this accumulation of immune cells and whether it is evoked by an in situ proliferation or by a migratory process. Studies on hepatoblastoma cells identified thrombopoietin (TPO) as a novel factor, which elicits dose-dependent chemotactic and chemokinetic effects. However, the impact and function of TPO on decidual cells has not been clarified yet. This study analyses the expression and function of TPO and its receptor c-Mpl in decidua during early gestation. Applying western blot analysis, we detected that TPO is expressed by decidual immune cells (uNK cells and CD14+ monocytes) as well as CTB and decidual stromal cells (DSCs). Expression of the different isoforms of c-Mpl was found in uNK cells, CD14+ monocytes and DSC. Studying the signalling pathway proteins in the uNK cells, an activation of STAT3/Tyr by TPO, was detected. The investigation of the proliferative effects of TPO on the decidual cell subsets revealed that TPO enhances the proliferation of uNK cells and CTB. No change of the proliferative activity after TPO incubation was found in DSC and even a decrease in CD14+ monocytes. In addition, TPO was observed to induce significantly the migratory activity of uNK cells, CD14+ monocytes and CTB. Investigating the effects of TPO on the cytokine profile of the isolated decidual cells, we observed a decrease in the secretion of IL-8, IL-10 and IL-1β of isolated uNK cells, CD14+ monocytes and CTB, although these changes did not reach statistical significance. Thus, we here identified TPO as a novel factor modulating the proliferation, migration and possibly cytokine secretion of decidual cell subsets.

Novel pathways to erythropoiesis induced by dimerization of intracellular C-Mpl in human hematopoietic progenitors

The cytokine thrombopoietin (Tpo) plays a critical role in hematopoiesis by binding to the extracellular domain and inducing homodimerization of the intracellular signaling domain of its receptor, c-Mpl. Mpl homodimerization can also be accomplished by binding of a synthetic ligand to a constitutively expressed fusion protein F36VMpl consisting of a ligand binding domain (F36V) and the intracellular signaling domain of Mpl. Unexpectedly, in contrast to Tpo stimulation, robust erythropoiesis is induced after dimerization of F36VMpl in human CD34+ progenitor cells. The goal of this study was to define the hematopoietic progenitor stages at which dimerization of intracellular Mpl induces erythropoiesis and the downstream molecular events that mediate this unanticipated effect. Dimerization (in the absence of erythropoietin and other cytokines) in human common myeloid progenitors and megakaryocytic erythroid progenitors caused a significant increase in CD34+ cells (p < .01) and induced all stages of erythropoiesis including production of enucleated red blood cells. In contrast, erythropoiesis was not seen with Tpo stimulation. CD34+ cell expansion was the result of increased cell cycling and survival (p < .05). Microarray profiling of CD34+ cells demonstrated that a unique transcriptional pattern is activated in progenitors by F36VMpl dimerization. Ligand-inducible dimerization of intracellular Mpl in human myeloerythroid progenitors induces progenitor expansion and erythropoiesis through molecular mechanisms that are not shared by Tpo stimulation of endogenous Mpl.

Mouse models of diseases of megakaryocyte and platelet homeostasis

Platelets are the small anuclear blood cells that are the product of megakaryocytopoiesis, the process of hematopoietic stem cell commitment to megakaryocyte production and the differentiation and maturation of these cells for platelet release. Deregulation or disruption of megakaryocytopoiesis can result in platelet deficiencies, the thrombocytopenias, with attendant risk of hemorrhage or thrombocytosis, a pathological excess of platelet numbers. Mouse models, particularly those engineered to carry genetic alterations modeling mutations associated with human disease, have provided important insights into megakaryocytopoiesis and deregulation of this process in disease. This review focuses on mouse models of diseases of altered megakaryocyte and platelet number, illustrating the profound contribution of these models in validating suspected roles of disease-associated genetic alterations, promoting discovery of new links between genetic mutations and specific diseases, and providing unique tools for better understanding of disease pathophysiology and progression, as well as resources to define drug action or develop new therapeutic strategies.

Epigenetics: prenatal exposure to genistein leaves a permanent signature on the hematopoietic lineage

Recent studies demonstrate that maternal diet during pregnancy results in long-lasting effects on the progeny. Supplementation of maternal diet with genistein, a phytoestrogen ubiquitous in the daily diet, altered coat color of agouti mice due to epigenetic changes. We studied hematopoiesis of mice prenatally exposed to genistein (270 mg/kg feed) compared with that of mice prenatally exposed to phytoestrogen-poor feed and observed a significant increase in granulopoiesis, erythropoiesis, and mild macrocytosis at the adult age of 12 wk. Genistein exposure was associated with hypermethylation of certain repetitive elements, which coincided with a significant down-regulation of estrogen-responsive genes and genes involved in hematopoiesis in bone marrow cells of genistein-exposed mice, as assessed by microarray technology. Although genistein exposure did not affect global methylation in fetal liver of fetuses at embryonic day 14.5, it accelerated the switch from primitive to definitive erythroid lineage. Taken together, our data demonstrate that prenatal exposure to genistein affects fetal erythropoiesis and exerts lifelong alterations in gene expression and DNA methylation of hematopoietic cells.

Gene therapy of MPL deficiency: challenging balance between leukemia and pancytopenia

Signaling of the thrombopoietin (THPO) receptor MPL is critical for the maintenance of hematopoietic stem cells (HSCs) and megakaryocytic differentiation. Inherited loss-of-function mutations of MPL cause severe thrombocytopenia and aplastic anemia, a syndrome called congenital amegakaryocytic thrombocytopenia (CAMT). With the aim to assess the toxicity of retroviral expression of Mpl as a basis for further development of a gene therapy for this disorder, we expressed Mpl in a murine bone marrow transplantation (BMT) model. Treated mice developed a profound yet transient elevation of multilineage hematopoiesis, which showed morphologic features of a chronic myeloproliferative disorder (CMPD) with progressive pancytopenia. Ten percent of mice (3/27) developed erythroleukemia, associated with insertional activation of Sfpi1 and Fli1. The majority of transplanted mice developed a progressive pancytopenia with histopathological features of a myelodysplastic syndrome (MDS)-like disorder. To avoid these adverse reactions, improved retroviral vectors were designed that mediate reduced and more physiological Mpl expression. Self-inactivating gamma-retroviral vectors were constructed that expressed Mpl from the phosphoglycerate kinase (PGK) or the murine Mpl promoter. Mice that received BM cells expressing Mpl from the Mpl promoter were free of any previously observed adverse reactions.

In vitro clinical-grade generation of red blood cells from human umbilical cord blood CD34+ cells

BACKGROUND

There is no appropriate alternative source of red blood cells (RBCs) to relieve the worsening shortage of blood available for transfusion. Therefore, in vitro generation of clinically available RBCs from hematopoietic stem cells could be a promising new source to supplement the blood supply. However, there have been few studies about the generation of clinical-grade RBCs by coculture on human mesenchymal stem cells (MSCs) and various cytokine supplements, even though the production of pure RBCs requires coculture on stromal cells and proper cytokine supplements.

STUDY DESIGN AND METHODS

Umbilical cord blood (CB) CD34+ cells were cultured in serum-free medium supplemented with two cytokine sets of stem cell factor (SCF) plus interleukin-3 (IL-3) plus erythropoietin (EPO) and SCF plus IL-3 plus EPO plus thrombopoietin (TPO) plus Flt-3 for 1 week, followed by coculture upon MSCs derived from bone marrow (BM) or CB for 2 weeks.

RESULTS

Almost pure clinical-grade RBCs could be generated by coculturing with CB-MSCs but not BM-MSCs. Expansion fold and enucleation rate were significantly higher in coculture with CB-MSCs than BM-MSCs. Despite a 2.5-fold expansion of erythroblasts in the presence of TPO and Flt-3 for 8 days, the final RBC count was higher without TPO and Flt-3.

CONCLUSIONS

This study is the first report on generating clinical-grade RBCs by in vitro culture with human MSCs and compared effectiveness of several cytokines for RBC production. This provides a useful basis for future production of clinically available RBCs and a model of erythropoiesis that is analogous to the in vivo system.

Historical review: megakaryopoiesis and thrombopoiesis

The study of thrombopoiesis has evolved greatly since an era when platelets were termed "the dust of the blood," only about 100 years ago. During this time megakaryocytes were identified as the origin of blood platelets; marrow-derived megakaryocytic progenitor cells were functionally defined and then purified; and the primary regulator of the process, thrombopoietin, was cloned and characterized and therapeutic thrombopoietic agents developed. During this journey we continue to learn that the physiologic mechanisms that drive proplatelet formation can be recapitulated in cell-free systems and their biochemistry evaluated; the molecular underpinnings of endomitosis are being increasingly understood; the intracellular signals sent by engagement of a large number of megakaryocyte surface receptors have been defined; and many of the transcription factors that drive megakaryocytic fate determination have been identified and experimentally manipulated. While some of these biologic processes mimic those seen in other cell types, megakaryocytes and platelets possess enough unique developmental features that we are virtually assured that continued study of thrombopoiesis will yield innumerable clinical and scientific insights for many decades to come.

A New Role of Thrombopoietin Enhancing ex Vivo Expansion of Endothelial Precursor Cells Derived from AC133-positive Cells

We previously reported that CD31(bright) cells, which were sorted from cultured AC133(+) cells of adult peripheral blood cells, differentiated more efficiently into endothelial cells than CD31(+) cells or CD31(-) cells, suggesting that CD31(bright) cells may be endothelial precursor cells. In this study, we found that CD31(bright) cells have a strong ability to release cytokines. The mixture of vascular endothelial growth factor (VEGF), thrombopoietin (TPO), and stem cell factor stimulated ex vivo expansion of the total cell number from cultured AC133(+) cells of adult peripheral blood cells and cord blood cells, resulting in incrementation of the adhesion cells, in which endothelial nitric oxide synthase and kinase insert domain-containing receptor were positive. Moreover, the mixture of VEGF and TPO increased the CD31(bright) cell population when compared with VEGF alone or the mixture of VEGF and stem cell factor. These data suggest that TPO is an important growth factor that can promote endothelial precursor cells expansion ex vivo.

Thrombopoietin Enhances Generation of CD34+ Cells from Human Embryonic Stem Cells

The role of thrombopoietin (TPO) in adult hematopoiesis is well-established. A recent report suggests that TPO and vascular endothelial growth factor (VEGF) play a role in promoting formation of early erythropoietic progenitors in a nonhuman primate embryonic stem cell (ES) model. No such report exists for human ES cells as yet. Because TPO may become an important factor promoting human ES cell-derived hematopoiesis, we sought to investigate whether TPO in combination with VEGF can enhance human ES-derived hematopoiesis in an EB-derived culture system. The emphasis of this work was to demonstrate the molecular mechanisms involved in this process, specifically the role of c-mpl and its ligand TPO. Human ES cells were cultured to the EB state, and EB-derived secondary cultures supporting hematopoietic differentiation were established: condition 1, control (stem cell factor [SCF] and Flt3 ligand [Flt3L]); condition 2, SCF, Flt3L, and TPO; and condition 3, SCF, Flt3L, TPO, and VEGF. Cells were harvested daily, starting at day 2 and continuing until day 8, for reverse transcription-polymerase chain reaction and Western blot. There was no evidence of expression of c-mpl and VEGF receptor on the gene or protein level until day 8, when the formation of well-established hematopoietic colonies began. This correlated with the formation of CD34+/CD31- negative progenitors, mostly found in blast-forming units-erythroid-like colonies. We concluded that TPO and VEGF play an important synergistic role in the formation of early ES-derived hematopoietic progenitors that occurs through the c-mpl and VEGF receptors. Disclosure of potential conflicts of interest is found at the end of this article.

The megakaryocyte lineage originates from hemangioblast precursors and is an integral component both of primitive and of definitive hematopoiesis

In the adult, platelets are derived from unipotential megakaryocyte colony-forming cells (Meg-CFCs) that arise from bipotential megakaryocyte/erythroid progenitors (MEPs). To better define the developmental origin of the megakaryocyte lineage, several aspects of megakaryopoiesis, including progenitors, maturing megakaryocytes, and circulating platelets, were examined in the murine embryo. We found that a majority of hemangioblast precursors during early gastrulation contains megakaryocyte potential. Combining progenitor assays with immunohistochemical analysis, we identified 2 waves of MEPs in the yolk sac associated with the primitive and definitive erythroid lineages. Primitive MEPs emerge at E7.25 along with megakaryocyte and primitive erythroid progenitors, indicating that primitive hematopoiesis is bilineage in nature. Subsequently, definitive MEPs expand in the yolk sac with Meg-CFCs and definitive erythroid progenitors. The first GP1bbeta-positive cells in the conceptus were identified in the yolk sac at E9.5, while large, highly reticulated platelets were detected in the embryonic bloodstream beginning at E10.5. At this time, the number of megakaryocyte progenitors begins to decline in the yolk sac and expand in the fetal liver. We conclude that the megakaryocyte lineage initially originates from hemangioblast precursors during early gastrulation and is closely associated both with primitive and with definitive erythroid lineages in the yolk sac prior to the transition of hematopoiesis to intraembryonic sites.

Differential effects of recombinant thrombopoietin and bone marrow stromal-conditioned media on neonatal versus adult megakaryocytes

Umbilical cord blood (CB) is a valuable source of stem cells for transplantation, but CB transplantations are frequently complicated by delayed platelet engraftment. The reasons underlying this are unclear. We hypothesized that CB- and peripheral-blood (PB)-derived megakaryocytes (MKs) respond differently to the adult hematopoietic microenvironment and to thrombopoietin (Tpo). To test this, we cultured CB- and PB-CD34(+) cells in adult bone marrow stromal conditioned media (CM) or unconditioned media (UCM) with increasing concentrations of recombinant Tpo and compared the effects of these conditions on CB-versus PB-MKs. PB-MKs reached highest ploidy in response to UCM + 100 ng/mL rTpo, and the addition of CM inhibited their maturation. In contrast, CB-MKs reached highest ploidy in CM without rTpo, and high rTpo concentrations (> 0.1 ng/mL) inhibited their maturation. This is the first evidence that human neonatal and adult MKs have substantially different biologic responses to Tpo and potentially to other cytokines.

Isolation and characterization of human myeloid progenitor populations--TpoR as discriminator between common myeloid and megakaryocyte/erythroid progenitors

OBJECTIVE

The common myeloid progenitor (CMP) and its progeny, the granulocyte/monocyte progenitor (GMP) and megakaryocyte/erythrocyte progenitor (MEP), have been isolated based on the surface expression of CD34, IL-3Ralpha, and CD45RA. However, high resolution of IL-3Ralpha- and IL-3Ralphalo cells required to adequately separate the CMP and MEP populations is difficult to achieve. The aim of this study was to find a complementary surface marker to obtain a better separation of these two populations and to further characterize the acquired progenitor populations.

MATERIALS AND METHODS

To evaluate the thrombopoietin receptor (TpoR) as a candidate marker, CD19-/CD34+/IL-3Ralphalo/-/CD45RA-/TpoR- (CMP), CD19-/CD34+/IL-3Ralphalo/CD45RA+/TpoR- (GMP), and CD19-/CD34+/IL-3Ralphalo/-/CD45RA-/TpoR+ (MEP) cells from human bone marrow were sorted to semisolid cultures for colony assays, and in addition analyzed for their surface expression of other growth factor receptors (GFRs) and sorted to real-time RT-PCR for gene expression analysis.

RESULTS

The colony-formation and gene expression assays showed that inclusion of TpoR as a marker gave a distinct and reproducible separation of the myeloid progenitors. Furthermore, most GFR surface expression correlated to gene expression, but there were also striking discrepancies, in particular for the common beta-chain of the IL-3R, GM-CSFR, and IL-5R and for TpoR.

CONCLUSION

Our data establish the TpoR as an important tool for isolation of the myeloid progenitors and demonstrate that the surface expression of GFRs cannot be predicted by their gene expression. Importantly, the refined isolation of CMPs will allow more detailed studies of regulatory mechanisms steering CMPs towards erythropoiesis vs granulopoiesis in steady state and response to peripheral demands.

Blocking the alpha 4 integrin-paxillin interaction selectively impairs mononuclear leukocyte recruitment to an inflammatory site

Antagonists to alpha4 integrin show promise for several autoimmune and inflammatory diseases but may exhibit mechanism-based toxicities. We tested the capacity of blockade of alpha4 integrin signaling to perturb functions involved in inflammation, while limiting potential adverse effects. We generated and characterized mice bearing a Y991A mutation in alpha4 integrin [alpha4(Y991A) mice], which blocks paxillin binding and inhibits alpha4 integrin signals that support leukocyte migration. In contrast to the embryonic-lethal phenotype of alpha4 integrin-null mice, mice bearing the alpha4(Y991A) mutation were viable and fertile; however, they exhibited defective recruitment of mononuclear leukocytes into thioglycollate-induced peritonitis. Alpha4 integrins are essential for definitive hematopoiesis; however, the alpha4(Y991A) mice had intact lymphohematopoiesis and, with the exception of reduced Peyer's patches, normal architecture and cellularity of secondary lymphoid tissues. We conclude that interference with alpha4 integrin signaling can selectively impair mononuclear leukocyte recruitment to sites of inflammation while sparing vital functions of alpha4 integrins in development and hematopoiesis.

Malignant hematopoietic cell lines: in vitro models for the study of erythroleukemia

A panel of leukemia cell lines has been assembled over the last 30 years representing a spectrum of erythroid cells arrested at various stages of differentiation. The oldest cell line is K-562 which is one of the most prolific in use. Most cell lines have been established from acute myeloid leukemia M6 or chronic myeloid leukemia in blast crisis and generally express immunoprofiles typically seen in immature erythroid cells. Several cell lines are constitutively growth factor-dependent, responding proliferatively to a variety of cytokines. The predominant cytogenetic abnormalities are the t(9;22)(q34;q11) found exclusively in CML-derived cell lines, and rearrangements of chromosomes 5 and 7 which occur in all disease subtypes. Ph+ve cell lines consistently displayed structural and numerical changes associated with disease evolution, including +8, -17/17p-/i(17q), and +19. It is striking that many cell lines though committed to either the erythroid or megakaryocytic lineage tend to co-express features of the other lineage, consistent with the concept of a common erythroid-megakaryocytic progenitor. Several cell lines may be induced to differentiate along the erythroid, megakaryocytic or monocytic pathway by treatment with pharmacological or physiological reagents. Notable functional features include expression of various globin chains or the complete hemoglobins as erythroid attributes. Taken together, this class of cell lines is relatively well characterized and affords useful model systems for immature erythroid cells.

Targeting c-Mpl for revival of human immunodeficiency virus type 1-induced hematopoietic inhibition when CD34+ progenitor cells are re-engrafted into a fresh stromal microenvironment in vivo

The inhibition of multilineage hematopoiesis which occurs in the severe combined immunodeficiency mouse with transplanted human fetal thymus and liver tissues (SCID-hu Thy/Liv) due to human immunodeficiency virus type 1 (HIV-1) infection is also accompanied by a severe loss of c-Mpl expression on these progenitor cells. Inhibition of colony-forming activity (CFA) of the CD34(+) progenitor cells is partially revived to about 40% of mock-infected Thy/Liv implants, following reconstitution of the CD34(+) cells that were exposed to HIV-1 infection, in a new Thy/Liv stromal microenvironment of irradiated secondary SCID-hu recipients at 3 weeks post-re-engraftment. In addition, in these reconstituted animals, the proportion of c-Mpl(+) CD34(+) cells relative to c-Mpl(-) CD34(+) cells increased by about 25%, to 35% of mock-infected implants, suggesting a reacquirement of c-Mpl phenotype by the c-Mpl(-) CD34(+) cells. These results suggest a correlation between c-Mpl expression and multilineage CFA of the human CD34(+) progenitor cells that have experienced the effects of HIV-1 infection. Treatment of the secondary-recipient animals with the c-Mpl ligand, thrombopoietin (Tpo), further increased c-Mpl expression and CFA of re-engrafted CD34(+) cells previously exposed to virus in the primary implants to about 50 to 70% over that of those re-engrafted CD34(+) cells derived from implants of untreated animals. Blocking of c-Mpl with anti-c-Mpl monoclonal antibody in vivo by injecting the SCID-hu animals resulted in the reduction or loss of CFA. Thus, inhibition, absence, or loss of c-Mpl expression as in the c-Mpl(-) CD34(+) subset of cells is the likely cause of CFA inhibition. Further, CFA of the CD34(+) cells segregates with their c-Mpl expression. Therefore, c-Mpl may play a role in hematopoietic inhibition during HIV-1 infection, and control of its expression levels may aid in hematopoietic recovery and thereby reduce the incidence of cytopenias occurring in infected individuals.

Association of single nucleotide polymorphisms in the thrombopoietin-receptor gene, but not the thrombopoietin gene, with differences in platelet count

Little is known about the mechanisms explaining the wide variation in platelet counts (PLT) and other hematologic parameters in humans. We previously showed that the sex-based difference in hematocrit was associated with nucleotide variation in the erythropoietin receptor gene (EPOR). We sought to identify new polymorphisms of the human thrombopoietin (TPO) and thrombopoietin receptor (TPOR) genes to determine any associations with blood PLT counts. We screened TPO and TPOR for polymorphisms using single-strand conformation polymorphism (SSCP) and DNA sequencing. Association of polymorphisms was studied in 304 normal subjects with low or high PLT counts. Distribution of allelic frequency was analyzed by the Chi-square statistic. Single nucleotide polymorphisms (SNPs) with two alleles were found in TPO and TPOR. The TPO SNP was a G to A transition at nucleotide 5753, and the TPOR SNP was a C to A transversion at position 550 in the 5'-promoter area. The allelic frequencies were 0.54 for G and 0.46 for A of TPO, and 0.62 for C and 0.38 for A of TPOR in a Caucasian population. The frequency of the TPOR allele "C" was significantly higher in subjects with high PLT count (>258 k/mm3) versus low PLT count (<224 k/mm3) and in males with high PLT count (>258 k/mm3) versus males with low PLT count (<212 k/mm3). In contrast, the frequency of the TPO alleles was not related to blood PLT counts. An association of TPO and TPOR allele distribution to red and white blood cell parameters was seen. These new SNPs found for the human TPO and TPOR genes help explain variations in blood PLT counts and may be useful in patient studies related to the roles of TPO and/or TPOR in disease.

Induction of megakaryocytopoiesis and thrombocytopoiesis by JTZ-132, a novel small molecule with thrombopoietin mimetic activities

We report in this paper that a novel small molecule, JTZ-132, induced growth and differentiation of megakaryocytic progenitor cells and improved thrombocytopenia in myelosuppressed mice. JTZ-132 stimulated proliferation of UT-7/TPO cells, a cell line highly sensitive to thrombopoietin (TPO), and exhibited full efficacy comparable to TPO with an approximate EC50 (median effective concentration) value of 0.43 μM, whereas little proliferation was observed in a TPO-insensitive cell line, UT-7/EPO, and human carcinoma cell line, HCT116. Signal transduction studies revealed that JTZ-132 induced tyrosine phosphorylation of c-Mpl, Janus kinase-2 (JAK2), and signal transducers and activators of transcription 5 (STAT5) in UT-7/TPO cells as well as TPO. JTZ-132 increased the number of megakaryocyte-specific marker, CD61+ and CD41+, cells in cultures of mouse and human bone marrow cells, respectively, and the colonyforming unit megakaryocytes in mouse bone marrow cells. In vivo experiments in x-ray irradiation– or busulfan injection–induced myelosuppressed mice demonstrated that subcutaneously injected JTZ-132 at 30 mg/kg showed significantly higher platelet number at nadir and accelerated platelet recovery without affecting white blood cell number. These data suggest that JTZ-132 is a novel stimulator of megakaryocytopoiesis and thrombocytopoiesis in vitro and in vivo with TPO mimetic activities and that it is useful for the treatment of thrombocytopenia.

Synergistic effects on erythropoiesis, thrombopoiesis, and stem cell competitiveness in mice deficient in thrombopoietin and steel factor receptors

The degree of redundancy between thrombopoietin (Tpo) and steel factor (SF) cytokine pathways in the regulation of hematopoiesis was investigated by generating mice lacking both c-Mpl and fully functional c-Kit receptors. Double-mutant c-Mpl(-/-)Kit(Wv/Wv) mice exhibited reduced viability, making up only 2% of the offspring from c-Mpl(-/-)Kit(Wv/)(+) intercrosses. The thrombocytopenia and megakaryocytopenia characteristic of c-Mpl(-/-) mice was unchanged in c-Mpl(-/-)Kit(Wv/Wv) mice. However, the number of megakaryocytic colony forming units (CFU-Mks) was significantly reduced, particularly in the spleen. While Kit(Wv/Wv) mice, but not c-Mpl(-/-) mice, are anemic, the anemia was more severe in double-mutant c-Mpl(-/-)Kit(Wv/Wv) mice, indicating redundancy between Tpo and SF in erythropoiesis. At the primitive cell level, c-Mpl(-/-) and Kit(Wv/Wv) mice have similar phenotypes, including reduced progenitors, colony forming units-spleen (CFU-Ss), and repopulating activities. All of these parameters were exacerbated in double-mutant mice. c-Mpl(-/-)Kit(Wv/Wv) mice had 8-fold fewer clonogenic progenitor cells and at least 28-fold fewer CFU-Ss. c-Mpl(-/-) mice also demonstrated a reduced threshold requirement for nonmyeloablative transplant repopulation, a trait previously associated only with Kit(W) mice, and the level of nonmyeloablative engraftment was significantly greater in c-Mpl(-/-) Kit(Wv/Wv) double mutants. Thus, c-Mpl(-/-) Kit(Wv/Wv) mice reveal nonredundant and synergistic effects of Tpo and SF on primitive hematopoietic cells.

A functional erythropoietin receptor is necessary for the action of thrombopoietin on erythroid cells lacking c-mpl

OBJECTIVE

We hypothesized that thrombopoietin (TPO) exerts its mitogenic effects on erythroid cells, at least in part, via an interaction of TPO with the cells' erythropoietin receptor (EPO-R).

METHODS

We used BaF3 cells stably transfected with EPO-R to demonstrate that TPO alone is sufficient to support the long-term growth and proliferation of BaF3/EPO-R cells and to develop a TPO-dependent variant, BaF3/EPO-R(T), which is highly sensitive to and dependent on TPO for its proliferation. Northern analysis and RT-PCR were used to verify that both BaF3/EPO-R and BaF3/EPO-R(T) cells express EPO-R but lack c-mpl, the TPO receptor. To confirm that TPO responsiveness of BaF3/EPO-R(T) is due to TPO's interaction with EPO-R, EPO-R was downregulated by antisense mRNA.

RESULTS

Downregulation of EPO-R in BaF3/EPO-R(T) cells abolishes responsiveness to both EPO and TPO. Viability of EPO-treated transfectants decreased from 95% to 36%, while that of TPO-treated transfectants decreased from 95% to 9% by 48 hours. Nontransfected BaF3/EPO-R(T), and BaF3/EPO-R(T) transfected with vector alone, remained viable and grew in either EPO or TPO.

CONCLUSION

Our results suggest a functional EPO-R may be necessary and sufficient for TPO to exert its mitogenic effects on erythroid cells.

Angiotensin II receptor type 1 expression in erythroid progenitors: implications for the pathogenesis of postrenal transplant erythrocytosis

Under normal physiological conditions red blood cell production is controlled primarily by erythropoietin, although multiple additional stimulatory factors are likely to be involved. One of these factors, angiotensin II, can modulate erythropoiesis directly via its type 1 receptor, as well as indirectly through multiple secondary mediators. We propose that angiotensin II exerts its stimulatory effect during the early stages of erythropoiesis, and that this effect serves as an important compensatory mechanism if erythropoietin production is chronically inadequate. We speculate that if this compensatory stimulation continues to be abnormally high after restoration of erythropoietin production following renal transplantation, erythrocytosis ensues.