Stromal cell-derived factor-1 (SDF-1) acts together with thrombopoietin to enhance the development of megakaryocytic progenitor cells (CFU-MK)

Stromal Derived Factor-1 Gene Polymorphism in Pediatric Immune Thrombocytopenia

Immune thrombocytopenia (ITP) is a multifactorial disease in which both environmental and genetic factors have been implicated. The study aimed to investigate a possible association of single nucleotide polymorphisms (SNPs rs266085 and rs2839693) in the stromal derived factor-1 (SDF-1) gene and its association to ITP and effect on ITP severity and response to treatment. Genomic DNA was extracted from peripheral blood and polymorphism in SDF-1 gene rs266085 and rs2839693 was analyzed using PCR-restriction fragment length polymorphism technique in DNA extracted from 60 children with ITP together with 90 healthy controls. On analysis of SDF-1 rs266085 polymorphism, there was a high frequency of CC genotype in cases than controls and that difference was significant at codominant, overdominant, and dominant models (P<0.05). Furthermore, carriers of the CC genotype were more susceptible to severe ITP at onset, steroid dependency, and chronicity than carriers of other genotypes (P<0.05). Otherwise, no significant differences between ITP patients and controls as regard SDF-1 rs2839693 genotypes and alleles, and we did not find a relation between this polymorphism and ITP severity, steroid dependency, or duration. SDF-1 gene rs266085 SNP C allele is associated with susceptibility to develop ITP as well as increases the risk for severe ITP at onset, chronic ITP and steroid dependency.

Proanthocyanidin A1 promotes the production of platelets to ameliorate chemotherapy-induced thrombocytopenia through activating JAK2/STAT3 pathway

BACKGROUND

Chemotherapy-induced thrombocytopenia (CIT) is a severe adverse drug reaction, and the main reason for CIT is the destruction of megakaryocytes (MKs, precursor cells of platelet) in bone marrow by chemotherapy. Peanut skin, the seed coat of Arachis hypogaea L., is a traditional Chinese medicine commonly used to treat thrombocytopenia. However, its active compounds and the mechanisms remain unclear.

PURPOSE

This study aims to clarify the active compounds of peanut skin to exhibit thrombogenic effects against CIT and their underlying mechanisms in vitro and in vivo.

STUDY DESIGN

The bioassay-guided isolation based on the proliferation of MKs was used to explore the possible platelet-enhancing ingredients in peanut skin. HSCCC technique coupled with preparative HPLC was used to separate the active compounds. Dami cells and carboplatin-treated mice model were used to evaluate the thrombogenic effects of PS-1. Network pharmacology, molecular docking, dynamics simulation studies, kinase activity, surface plasmon resonance (SPR), cellular thermal shift assay (CETSA), isothermal dose-response fingerprint (ITDRF_CETSA) and western blot analysis were performed to investigate the mechanisms of PS-1.

RESULTS

Proanthocyanidin A1 (PS-1) and its stereoisomers (PS-2-4) were demonstrated to promote the proliferation of MKs (Dami cells), especially PS-1 (EC₅₀ = 8.58 μM). Further studies demonstrated that PS-1 could induce the differentiation of Dami cells in dose/time-dependent manner. Biological target analysis showed that PS-1 directly bound to JAK2 (KD = 2.06 μM) to exert potent activating effect (EC₅₀ = 0.66 μM). Oral administration of PS-1 (25 or 50 mg/kg) significantly improved CIT, but this effect was confirmed to be inhibited by JAK2 inhibitor AG490, indicating that PS-1 exerted its efficacy through JAK2 in vivo.

CONCLUSION

Proanthocyanins (PS-1-4) derived from peanut skin were first clarified as platelet-enhancing ingredients to improve CIT. The underlying mechanism of PS-1 was proved to promote the proliferation and differentiation of MKs via JAK2/STAT3 pathway both in vitro and in vivo.

Platelet chemokines in health and disease

In recent years, it has become clear that platelets and platelet-derived chemokines, beyond their role in thrombosis and haemostasis, are important mediators affecting a broad spectrum of (patho)physiological conditions. These biologically active proteins are released from α-granules upon platelet activation, most probably even during physiological conditions. In this review, we give a concise overview and an update on the current understanding of platelet-derived chemokines in a context of health and disease.

Note: The review process for this manuscript was fully handled by G. Y. H. Lip, Editor in Chief.

Hmga2 promotes the development of myelofibrosis in Jak2V617F knockin mice by enhancing TGF-β1 and Cxcl12 pathways

Myelofibrosis (MF) is a devastating blood disorder. The JAK2V617F mutation has been detected in ∼50% cases of MF. Elevated expression of high-mobility group AT hook 2 (HMGA2) has also been frequently observed in patients with MF. Interestingly, upregulation of HMGA2 expression has been found in association with the JAK2V617F mutation in significant cases of MF. However, the contribution of HMGA2 in the pathogenesis of MF remains elusive. To determine the effects of concurrent expression of HMGA2 and JAK2V617F mutation in hematopoiesis, we transduced bone marrow cells from Jak2^V617F knockin mice with lentivirus expressing Hmga2 and performed bone marrow transplantation. Expression of Hmga2 enhanced megakaryopoiesis, increased extramedullary hematopoiesis, and accelerated the development of MF in mice expressing Jak2^V617F Mechanistically, the data show that expression of Hmga2 enhances the activation of transforming growth factor-β1 (TGF-β1) and Cxcl12 pathways in mice expressing Jak2^V617F In addition, expression of Hmga2 causes upregulation of Fzd2, Ifi27l2a, and TGF-β receptor 2. Forced expression of Cxcl12, Fzd2, or Ifi27l2a increases megakaryocytic differentiation and proliferation in the bone marrow of Jak2^V617F mice, whereas TGF-β1 or Cxcl12 stimulation induces collagen deposition in the bone marrow mesenchymal stromal cells. Together, these findings demonstrate that expression of Hmga2 cooperates with Jak2^V617F in the pathogenesis of MF.

Traumatic brain injury induced matrix metalloproteinase2 cleaves CXCL12α (stromal cell derived factor 1α) and causes neurodegeneration

Traumatic brain injury (TBI), even at mild levels, can activate matrix metalloproteinases (MMPs) and the induction of neuroinflammation that can result in blood brain barrier breakdown and neurodegeneration. MMP2 has a significant role in neuroinflammation and neurodegeneration by modulating the chemokine CXCL12α (stromal cell derived factor SDF-1α) signaling pathway and the induction of apoptosis. SDF-1α is responsible for cell proliferation and differentiation throughout the nervous system and is also implicated in various neurodegenerative illnesses. We hypothesized that TBI leads to MMP2 activation and cleavage of the N-terminal 4 amino acid residues of CXCL12α with generation of the highly neurotoxic fragment SDF-1(5-67). Using an in vitro stretch-injury model of rat neuronal cultures and the in vivo fluid percussion injury (FPI) model in rats, we found that oxidative stress has a significant role in the activation of MMP2. This is initiated by the induction of free radical generating enzyme NADPH oxidase 1 (NOX1). Induction of NOX1 correlated well with the signatures of oxidative stress marker, 4HNE in the injured neuronal cultures and cerebral cortex of rats. Further, using MMP2 siRNA and pharmacological MMP2 inhibitor, ARP100, we established the neurodegenerative role of MMP2 in cleaving SDF-1α to a neurotoxic fragment SDF-1(5-67). By immunofluorescence, western blotting and TUNEL experiments, we show the cleaved form of SDF leads to apoptotic cell death in neurons. This work identifies a new potential therapeutic target to reduce the complications of brain damage in TBI.

Novel mechanisms of platelet clearance and thrombopoietin regulation

PURPOSE OF REVIEW

The human body produces and removes 10 platelets daily to maintain a normal steady-state platelet count. Platelet production must be tightly regulated to avoid spontaneous bleeding or arterial occlusion and organ damage. Multifaceted and complex mechanisms control platelet removal and production in physiological and pathological conditions. This review will focus on different mechanisms of platelet clearance, with focus on the biological significance of platelet glycans.

RECENT FINDINGS

The Ashwell-Morrell receptor (AMR) recognizes senescent, desialylated platelets under steady state conditions. Desialylated platelets and the AMR are the physiological ligand-receptor pair regulating hepatic thrombopoietin (TPO) mRNA production, resolving the longstanding mystery of steady state TPO regulation. The AMR-mediated removal of desialylated platelets regulates TPO synthesis in the liver by recruiting JAK2 and STAT3 to increase thrombopoiesis.

SUMMARY

Inhibition of TPO production downstream of the hepatic AMR-JAK2 signaling cascade could additionally contribute to the thrombocytopenia associated with JAK1/2 treatment, which is clinically used in myeloproliferative neoplasms.

Is There a Relationship Between CXCR4 Gene Expression and Prognosis of Immune Thrombocytopenia in Children?

Immune thrombocytopenia (ITP) is a common autoimmune disorder characterized by decreased platelet count (thrombocytopenia) and bleeding symptoms due to production of autoantibodies against platelets. Chemokines are molecules inducing chemotaxis and play an important role in megakaryopoiesis, including CXCR4 chemokine receptor. CXCR4 is expressed on cells of megakaryocytic series, especially platelets, and triggers several mechanisms in these cells. The purpose of this study was to evaluate the pattern of CXCR4 gene changes upon diagnosis and after treatment and its comparison with laboratory findings in peripheral blood samples from newly diagnosed ITP patients. 35 newly diagnosed patients with ITP and 35 healthy controls were enrolled in this study. CXCR4 gene expression was investigated before and after treatment using real-time PCR. HPRT gene was used as the reference gene to calculate the expression rate of CXCR4 as CXCR4/HPRT ratio. CXCR4 gene expression upon diagnosis and after treatment in peripheral blood plasma of ITP patients showed a significant decrease in comparison with the control group while its expression did not change before and after treatment. No significant correlation was found between the expression of this gene and laboratory parameters. Due to unpredictable course of ITP in patients and the possibility of its progress to refractory form, accurate choice of a biomarker is essential for evaluating prognosis and detection of resistant forms.

Regulating billions of blood platelets: glycans and beyond

The human body produces and removes 10(11) platelets daily to maintain a normal steady state platelet count. Platelet production must be regulated to avoid spontaneous bleeding or arterial occlusion and organ damage. Multifaceted and complex mechanisms control platelet production and removal in physiological and pathological conditions. This review will focus on different mechanisms of platelet senescence and clearance with specific emphasis on the role of posttranslational modifications. It will also briefly address platelet transfusion and the role of glycans in the clearance of stored platelets.

SDF-1/CXCR4 axis in Tie2-lineage cells including endothelial progenitor cells contributes to bone fracture healing

CXC chemokine receptor 4 (CXCR4) is a specific receptor for stromal-derived-factor 1 (SDF-1). SDF-1/CXCR4 interaction is reported to play an important role in vascular development. On the other hand, the therapeutic potential of endothelial progenitor cells (EPCs) in fracture healing has been demonstrated with mechanistic insight of vasculogenesis/angiogenesis and osteogenesis enhancement at sites of fracture. The purpose of this study was to investigate the influence of the SDF-1/CXCR4 pathway in Tie2-lineage cells (including EPCs) in bone formation. We created CXCR4 gene conditional knockout mice using the Cre/loxP system and set two groups of mice: Tie2-Cre(ER) CXCR4 knockout mice (CXCR4(-/-) ) and wild-type mice (WT). We report here that in vitro, EPCs derived from of CXCR4(-/-) mouse bone marrow demonstrated severe reduction of migration activity and EPC colony-forming activity when compared with those derived from WT mouse bone marrow. In vivo, radiological and morphological examinations showed fracture healing delayed in the CXCR4(-/-) group and the relative callus area at weeks 2 and 3 was significantly smaller in CXCR4(-/-) group mice. Quantitative analysis of capillary density at perifracture sites also showed a significant decrease in the CXCR4(-/-) group. Especially, CXCR4(-/-) group mice demonstrated significant early reduction of blood flow recovery at fracture sites compared with the WT group in laser Doppler perfusion imaging analysis. Real-time RT-PCR analysis showed that the gene expressions of angiogenic markers (CD31, VE-cadherin, vascular endothelial growth factor [VEGF]) and osteogenic markers (osteocalcin, collagen 1A1, bone morphogenetic protein 2 [BMP2]) were lower in the CXCR4(-/-) group. In the gain-of-function study, the fracture in the SDF-1 intraperitoneally injected WT group healed significantly faster with enough callus formation compared with the SDF-1 injected CXCR4(-/-) group. We demonstrated that an EPC SDF-1/CXCR4 axis plays an important role in bone fracture healing using Tie2-Cre(ER) CXCR4 conditional knockout mice.

Thrombopoiesis

The production of platelets is a complex process that involves hematopoietic stem cells (HSCs), their differentiated progeny, the marrow microenvironment and hematopoietic cytokines. Much has been learned in the 110 years since James Homer Wright postulated that marrow megakaryocytes were responsible for blood platelet production, at a time when platelets were termed the "dust of the blood". In the 1980s a number of in vitro culture systems were developed that could produce megakaryocytes, followed by the identification of several cytokines that could stimulate the process in vitro. However, none of these cytokines produced a substantial thrombocytosis when injected into animals or people, nor were blood levels inversely related to platelet count, the sine qua non of a physiological regulator. A major milestone in our understanding of thrombopoiesis occurred in 1994 when thrombopoietin, the primary regulator of platelet production was cloned and initially characterized. Since that time many of the molecular mechanisms of thrombopoiesis have been identified, including the effects of thrombopoietin on the survival, proliferation, and differentiation of hematopoietic stem and progenitor cells, the development of polyploidy and proplatelet formation, the final fragmentation of megakaryocyte cytoplasm to yield blood platelets, and the regulation of this process. While much progress has been made, several outstanding questions remain, such as the nature of the signals for final platelet formation, the molecular nature of the regulation of marrow stromal thrombopoietin production, and the role of these physiological processes in malignant hematopoiesis.

SDF-1 dynamically mediates megakaryocyte niche occupancy and thrombopoiesis at steady state and following radiation injury

Megakaryocyte (MK) development in the bone marrow progresses spatially from the endosteal niche, which promotes MK progenitor proliferation, to the sinusoidal vascular niche, the site of terminal maturation and thrombopoiesis. The chemokine stromal cell-derived factor-1 (SDF-1), signaling through CXCR4, is implicated in the maturational chemotaxis of MKs toward sinusoidal vessels. Here, we demonstrate that both IV administration of SDF-1 and stabilization of endogenous SDF-1 acutely increase MK-vasculature association and thrombopoiesis with no change in MK number. In the setting of radiation injury, we find dynamic fluctuations in marrow SDF-1 distribution that spatially and temporally correlate with variations in MK niche occupancy. Stabilization of altered SDF-1 gradients directly affects MK location. Importantly, these SDF-1-mediated changes have functional consequences for platelet production, as the movement of MKs away from the vasculature decreases circulating platelets, while MK association with the vasculature increases circulating platelets. Finally, we demonstrate that manipulation of SDF-1 gradients can improve radiation-induced thrombocytopenia in a manner additive with earlier TPO treatment. Taken together, our data support the concept that SDF-1 regulates the spatial distribution of MKs in the marrow and consequently circulating platelet numbers. This knowledge of the microenvironmental regulation of the MK lineage could lead to improved therapeutic strategies for thrombocytopenia.

Homeodomain transcription factor Meis1 is a critical regulator of adult bone marrow hematopoiesis

Hematopoietic stem cells in the bone marrow have the capacity to both self-renew and to generate all cells of the hematopoietic system. The balance of these two activities is controlled by hematopoietic stem cell-intrinsic regulatory mechanisms as well as extrinsic signals from the microenvironment. Here we demonstrate that Meis1, a TALE family homeodomain transcription factor involved in numerous embryonic developmental processes, is selectively expressed in hematopoietic stem/progenitor cells. Conditional Meis1 knockout in adult hematopoietic cells resulted in a significant reduction in the hematopoietic stem/progenitor cells. Suppression of hematopoiesis by Meis1 deletion appears to be caused by impaired self-renewal activity and reduced cellular quiescence of hematopoietic stem/progenitor cells in a cell autonomous manner, resulting in stem cell exhaustion and defective long-term hematopoiesis. Meis1 deficiency down-regulated a subset of Pbx1-dependent hematopoietic stem cell signature genes, suggesting a functional link between them in the maintenance of hematopoietic stem/progenitor cells. These results show the importance of Meis1 in adult hematopoiesis.

Systems biology of megakaryocytes

The molecular pathways that regulate megakaryocyte production have historically been identified through multiple candidate gene approaches. Several transcription factors critical for generating megakaryocytes were identified by promoter analysis of megakaryocyte-specific genes, and their biological roles then verified by gene knockout studies; for example, GATA-1, NF-E2, and RUNX1 were identified in this way. In contrast, other transcription factors important for megakaryopoiesis were discovered through a systems approach; for example, c-Myb was found to be critical for the erythroid versus megakaryocyte lineage decision by genome-wide loss-of-function studies. The regulation of the levels of these transcription factors is, for the most part, cell intrinsic, although that assumption has recently been challenged. Epigenetics also impacts megakaryocyte gene expression, mediated by histone acetylation and methylation. Several cytokines have been identified to regulate megakaryocyte survival, proliferation, and differentiation, most prominent of which is thrombopoietin. Upon binding to its receptor, the product of the c-Mpl proto-oncogene, thrombopoietin induces a conformational change that activates a number of secondary messengers that promote cell survival, proliferation, and differentiation, and down-modulate receptor signaling. Among the best studied are the signal transducers and activators of transcription (STAT) proteins; phosphoinositol-3-kinase; mitogen-activated protein kinases; the phosphatases PTEN, SHP1, SHP2, and SHIP1; and the suppressors of cytokine signaling (SOCS) proteins. Additional signals activated by these secondary mediators include mammalian target of rapamycin; β(beta)-catenin; the G proteins Rac1, Rho, and CDC42; several transcription factors, including hypoxia-inducible factor 1α(alpha), the homeobox-containing proteins HOXB4 and HOXA9, and a number of signaling mediators that are reduced, including glycogen synthase kinase 3α(alpha) and the FOXO3 family of forkhead proteins. More recently, systematic interrogation of several aspects of megakaryocyte formation have been conducted, employing genomics, proteomics, and chromatin immunoprecipitation (ChIP) analyses, among others, and have yielded many previously unappreciated signaling mechanisms that regulate megakaryocyte lineage determination, proliferation, and differentiation. This chapter focuses on these pathways in normal and neoplastic megakaryopoiesis, and suggests areas that are ripe for further study.

Sequential administration of the high affinity CXCR4 antagonist BKT140 promotes megakaryopoiesis and platelet production

Platelets are the terminal differentiation product of megakaryocytes (MKs). Cytokines, such as thrombopoietin (TPO), are known to influence different steps in MK development; however, the complex differentiation and platelet localization processes are not fully understood. MKs express the receptor CXCR4 and have been shown to migrate in response to CXCL12 and to increase their platelet production. In this study, we studied the role of CXCR4 in platelet production with the high affinity CXCR4 antagonist, BKT140. Single and sequential administration of BKT140 significantly increased the number of MKs and haematopoietic progenitors (HPCs) within the bone marrow (BM). Increased megakaryopoiesis was associated with increased platelet production. Single and sequential administration of BKT140 also increased the number of HPCs in the blood. In a model of 5-fluorouracil-induced thrombocytopenia, BKT140 significantly reduced the severity and duration of thrombocytopenia and cytopenia when administered before and after chemotherapy. Our results demonstrated that the CXCR4 antagonist, BKT140, mediated unique beneficial effects by stimulating megakaryopoiesis and platelet production. These results provide evidence for the possible therapeutic use of BKT140 for modulating platelet numbers in thrombocytopenic conditions.

Umbilical cord blood-derived stromal cells regulate megakaryocytic proliferation and migration through SDF-1/PECAM-1 pathway

We have previously reported that human umbilical cord blood-derived stromal cells (hUCBDSCs) are able to enhance the expansion of CFU-Meg in vitro, particularly promote the megakaryocytic lineage recovery, and effectively protect the survival of irradiated mice. In this study, we demonstrated that hUCBDSCs secreted SDF-1 to stimulate PECAM-1 expression in HEL cells (MK cell line), and consequently promoted the proliferation and migration of HEL cells. On the other hand, SDF-1 knock down in hUCBDSCs or PECAM-1 knock down in HEL cells diminished or abrogated the above effect. In addition, SDF-1/PECAM-1 probably activated PI3K/Akt and MAPK/ERK1/2 pathways. This report for the first time defines a SDF-1/PECAM-1 signaling pathway in the proliferation and migration of MKs, which provides supportive evidence for the clinical applications of hUCBDSCs in the treatment of megakaryocytic injury.

VEGFR1 stimulates a CXCR4-dependent translocation of megakaryocytes to the vascular niche, enhancing platelet production in mice

It has previously been reported that VEGF-A stimulates megakaryocyte (MK) maturation in vitro. Here we show that treatment of mice with the isoform VEGF-A165 resulted in a significant increase in circulating numbers of platelets. Using specific VEGFR1 and VEGFR2 blocking mAbs and selective VEGFR1 and 2 agonists, PlGF-2 and VEGF-E, respectively, we show directly that stimulation of VEGFR1, but not VEGFR2, increases circulating platelet numbers in vivo. Using flow cytometric analysis of harvested MKs, we show that while PlGF does not change the absolute numbers of MKs present in the bone marrow and the spleen, it increases both their maturation and cell-surface expression of CXCR4 in the bone marrow. Histology of the bone marrow revealed a redistribution of MKs from the endosteal to the vascular niche in response to both VEGF-A165 and PlGF-2 treatment in vivo. Antagonism of CXCR4 suppressed both the VEGFR1-stimulated redistribution of megakyocytes within the bone marrow compartment and the VEGF-A165–induced thrombocytosis. In conclusion, we define a novel proinflammatory VEGFR1-mediated pathway that stimulates the maturation and up-regulation of CXCR4 on megakaryocytes, leading to their redistribution within the bone marrow environment, thereby enhancing platelet production in vivo.

HemaMax™, a recombinant human interleukin-12, is a potent mitigator of acute radiation injury in mice and non-human primates

HemaMax, a recombinant human interleukin-12 (IL-12), is under development to address an unmet medical need for effective treatments against acute radiation syndrome due to radiological terrorism or accident when administered at least 24 hours after radiation exposure. This study investigated pharmacokinetics, pharmacodynamics, and efficacy of m-HemaMax (recombinant murine IL-12), and HemaMax to increase survival after total body irradiation (TBI) in mice and rhesus monkeys, respectively, with no supportive care. In mice, m-HemaMax at an optimal 20 ng/mouse dose significantly increased percent survival and survival time when administered 24 hours after TBI between 8–9 Gy (p<0.05 Pearson's chi-square test). This survival benefit was accompanied by increases in plasma interferon-γ (IFN-γ) and erythropoietin levels, recovery of femoral bone hematopoiesis characterized with the presence of IL-12 receptor β2 subunit–expressing myeloid progenitors, megakaryocytes, and osteoblasts. Mitigation of jejunal radiation damage was also examined. At allometrically equivalent doses, HemaMax showed similar pharmacokinetics in rhesus monkeys compared to m-HemaMax in mice, but more robustly increased plasma IFN-γ levels. HemaMax also increased plasma erythropoietin, IL-15, IL-18, and neopterin levels. At non-human primate doses pharmacologically equivalent to murine doses, HemaMax (100 ng/Kg and 250 ng/Kg) administered at 24 hours after TBI (6.7 Gy/LD_50/30) significantly increased percent survival of HemaMax groups compared to vehicle (p<0.05 Pearson's chi-square test). This survival benefit was accompanied by a significantly higher leukocyte (neutrophils and lymphocytes), thrombocyte, and reticulocyte counts during nadir (days 12–14) and significantly less weight loss at day 12 compared to vehicle. These findings indicate successful interspecies dose conversion and provide proof of concept that HemaMax increases survival in irradiated rhesus monkeys by promoting hematopoiesis and recovery of immune functions and possibly gastrointestinal functions, likely through a network of interactions involving dendritic cells, osteoblasts, and soluble factors such as IL-12, IFN-γ, and cytoprotectant erythropoietin.

Hematopoietic stromal cells and megakaryocyte development

The hematopoietic microenvironment, and in particular the hematopoietic stromal cell element, are intimately involved in megakaryocyte development. The process of megakaryocytopoiesis occurs within a complex bone marrow microenvironment where adhesive interactions, chemokines, as well as cytokines play a pivotal role. Here we review the effect of stromal cells and cytokines on megakaryocytopoiesis with the aim of exploring new therapeutic strategies for platelet recovery after hematopoietic stem cell transplantation (HSCT).

In vivo expansion of the megakaryocyte progenitor cell population in adult CD26-deficient mice

OBJECTIVE

Megakaryopoiesis involves commitment of hematopoietic stem cells (HSC) toward the myeloid lineage in combination with the proliferation, maturation, and terminal differentiation of progenitors into megakaryocytes. The exact mechanism of megakaryocyte development from HSC is unknown, but growth factors such as thrombopoietin have been identified as critical. Additionally, it has been suggested that the chemokine CXCL12/stromal-cell derived factor-1α has a role in regulating megakaryopoiesis and thrombopoiesis. We recently reported the importance of the extracellular protease CD26 (dipeptidylpeptidase IV) in regulating HSC responses to CXCL12, as well as modulating HSC trafficking into and out of the bone marrow. However, the importance of CD26 for megakaryopoiesis has not been reported. We therefore compared megakaryocyte development between CD26-deficient (CD26(-/-)) mice and C57BL/6 control mice.

MATERIALS AND METHODS

Adult CD26(-/-) mice and C57BL/6 control mice were evaluated using blood differentials, histological analysis, flow cytometric analysis, and progenitor colony assays.

RESULTS

Bone marrow from CD26(-/-) mice has a significantly expanded megakaryocyte and megakaryocyte progenitor population compared to control C57BL/6 mice bone marrow.

CONCLUSIONS

Our results indicate that endogenous CD26 normally suppresses megakaryopoiesis and that loss of CD26 activity results in expansion of the megakaryocyte progenitor population in vivo. This suggests the potential use of CD26 inhibitors to improve megakaryocyte progenitor function and/or reconstitution of the megakaryocyte cell population.

Platelet-derived chemokines: pathophysiology and therapeutic aspects

The identification of chemokines in blood platelets has strengthened our view of these cells as participants in immune host defense. Platelet chemokines representing prestored and rapidly releasable proteins may play a major role as first-line inflammatory mediators. This is evident from their capability to recruit early inflammatory cells such as neutrophil granulocytes and monocytes and even to exhibit direct antimicrobial activity. However, insight is growing that platelet chemokines may be also long-term regulators, e.g., by activating T lymphocytes, by modulating the formation of endothelium and even thrombocytopoiesis itself. This review deals with the individual and cooperative functionality of platelet chemokines, as well as their potential as a basis for therapeutic intervention in the pathology of inflammation, infection, allergy and tumors. Within this context, therapeutic strategies based on the use of antibodies, modified chemokines, chemokine-binding proteins and chemokine receptor antagonists as well as first clinical studies will be addressed.

Stem cells and the niche: a dynamic duo

Stem cell niches are dynamic microenvironments that balance stem cell activity to maintain tissue homeostasis and repair throughout the lifetime of an organism. The development of strategies to monitor and perturb niche components has provided insight into the responsive nature of the niche and offers a framework to uncover how disruption of normal stem cell niche function may contribute to aging and disease onset and progression. Additional work in the identification of genetic factors that regulate the formation, activity, and size of stem cell niches will facilitate incorporation of the niche into stem cell-based therapies and regenerative medicine.

Thrombocytopenia in Patients With Chronic Hepatitis C: A Possible Role of HCV on Platelet Progenitor Cell Maturation

A total of 30 patients with chronic hepatitis C (HCV) thrombocytopenia (TP) and 20 healthy controls were studied. Both groups were subjected to complete medical history, clinical examination in addition to assessment of hepatitis markers: level of thrombopoietin (Tpo), Geimsa-stained bone marrow smears, and in vitro short-term megakaryocytic progenitors culture (CFU-MK). Serum Tpo level was significantly elevated in patients with TP HCV. Short-term CFU-MK showed an evident depression in the colony-forming unit—megakaryocyte (CFU-meg). There is a positive correlation between the number of CFU-meg and the platelet count and between serum Tpo level and prothrombin time, transaminase, albumin, and the Child Pugh score of liver disease; a negative correlation between serum Tpo level and the number of CFU-meg and between serum Tpo level and the platelet count. Thus, the level of Tpo could be an indicator of intact functional response of the hepatocytes.

Regulation of megakaryocyte maturation and platelet formation

Each day in every human, approximately 1 x 10(11) platelets are produced by the cytoplasmic fragmentation of megakaryocytes (MK), their marrow precursor cells. Platelets are the predominating factor in the process of hemostasis and thrombosis. Recent studies have shown that platelets also play a hitherto unsuspected role in several other processes such as inflammation, innate immunity, neoangiogenesis and tumor metastasis. The late phases of MK differentiation identified by polyploidization, maturation and organized fragmentation of the cytoplasm leading to the release of platelets in the blood stream represent a unique model of differentiation. The molecular and cellular mechanisms regulating platelet biogenesis are better understood and may explain several platelet disorders. This review focuses on MK polyploidization, and platelet formation, and discusses their alteration in some platelet disorders.

Pathobiology of secondary immune thrombocytopenia

Primary immune thrombocytopenic purpura (ITP) remains a diagnosis of exclusion both from nonimmune causes of thrombocytopenia and immune thrombocytopenia that develops in the context of other disorders (secondary immune thrombocytopenia). The pathobiology, natural history, and response to therapy of the diverse causes of secondary ITP differ from each other and from primary ITP, so accurate diagnosis is essential. Immune thrombocytopenia can be secondary to medications or to a concurrent disease, such as an autoimmune condition (eg, systemic lupus erythematosus [SLE], antiphospholipid antibody syndrome [APS], immune thyroid disease, or Evans syndrome), a lymphoproliferative disease (eg, chronic lymphocytic leukemia or large granular T-lymphocyte lymphocytic leukemia), or chronic infection, eg, with Helicobacter pylori, human immunodeficiency virus (HIV), or hepatitis C virus (HCV). Response to infection may generate antibodies that cross-react with platelet antigens (HIV, H pylori) or immune complexes that bind to platelet Fcγ receptors (HCV), and platelet production may be impaired by infection of megakaryocyte (MK) bone marrow–dependent progenitor cells (HCV and HIV), decreased production of thrombopoietin (TPO), and splenic sequestration of platelets secondary to portal hypertension (HCV). Sudden and severe onset of thrombocytopenia has been observed in children after vaccination for measles, mumps, and rubella or natural viral infections, including Epstein-Barr virus, cytomegalovirus, and varicella zoster virus. This thrombocytopenia may be caused by cross-reacting antibodies and closely mimics acute ITP of childhood. Proper diagnosis and treatment of the underlying disorder, where necessary, play an important role in patient management.

Restoration and reversible expansion of the osteoblastic hematopoietic stem cell niche after marrow radioablation

Adequate recovery of hematopoietic stem cell (HSC) niches after cytotoxic conditioning regimens is essential to successful bone marrow transplantation. Yet, very little is known about the mechanisms that drive the restoration of these niches after bone marrow injury. Here we describe a profound disruption of the marrow microenvironment after lethal total body irradiation of mice that leads to the generation of osteoblasts restoring the HSC niche, followed by a transient, reversible expansion of this niche. Within 48 hours after irradiation, surviving host megakaryocytes were observed close to the endosteal surface of trabecular bone rather than in their normal parasinusoidal site concomitant with an increased stromal-derived factor-1 level. A subsequent increase in 2 megakaryocyte-derived growth factors, platelet-derived growth factor-beta and basic fibroblast growth factor, induces a 2-fold expansion of the population of N-cadherin-/osteopontin-positive osteoblasts, relative to the homeostatic osteoblast population, and hence, increases the number of potential niches for HSC engraftment. After donor cell engraftment, this expanded microenvironment reverts to its homeostatic state. Our results demonstrate the rapid recovery of osteoblastic stem cell niches after marrow radioablation, provide critical insights into the associated mechanisms, and suggest novel means to manipulate the bone marrow microenvironment to promote HSC engraftment.

Determinants of platelet number and regulation of thrombopoiesis

Our understanding of thrombopoiesis has improved greatly in the last two decades with the availability of in vitro assays of megakaryocyte progenitor cell growth, with the cloning and characterization of stem cell factor (SCF) and thrombopoietin (Tpo), the latter the primary humoral regulator of this process, and with the generation of genetically altered murine models of thrombopoietic failure and excess. While SCF affects developmentally early aspects of megakaryocyte growth, Tpo affects nearly all aspects of platelet production, from hematopoietic stem cell (HSC) self-renewal and expansion, through stimulation of megakaryocyte progenitor cell proliferation, to supporting their maturation into platelet-producing cells. The molecular and cellular mechanisms through which the marrow microenvironment and humoral mediators affect platelet production provide new insights into the interplay between intrinsic and extrinsic influences on hematopoiesis, and highlight new opportunities to translate basic biology into clinical advances.

SH2-inositol phosphatase 1 negatively influences early megakaryocyte progenitors

Background

The SH2-containing-5′inositol phosphatase-1 (SHIP) influences signals downstream of cytokine/chemokine receptors that play a role in megakaryocytopoiesis, including thrombopoietin, stromal-cell-derived-Factor-1/CXCL-12 and interleukin-3. We hypothesize that SHIP might control megakaryocytopoiesis through effects on proliferation of megakaryocyte progenitors (MKP) and megakaryocytes (MK).

Methodology and Principal Findings

Herein, we report the megakaryocytic phenotype and MK functional assays of hematopoietic organs of two strains of SHIP deficient mice with deletion of the SHIP promoter/first exon or the inositol phosphatase domain. Both SHIP deficient strains exhibit a profound increase in MKP numbers in bone marrow (BM), spleen and blood as analyzed by flow cytometry (Lin⁻c-Kit⁺CD41⁺) and functional assays (CFU-MK). SHIP deficient MKP display increased phosphorylation of Signal Transducers and Activators of Transcription 3 (STAT-3), protein kinase B (PKB/AKT) and extracellular signal-regulated kinases (ERKs). Despite increased MKP content, total body number of mature MK (Lin⁻c-kit⁻CD41⁺) are not significantly changed as SHIP deficient BM contains reduced MK while spleen MK numbers are increased. Reduction of CXCR-4 expression in SHIP deficient MK may influence MK localization to the spleen instead of the BM. Endomitosis, process involved in MK maturation, was preserved in SHIP deficient MK. Circulating platelets and red blood cells are also reduced in SHIP deficient mice.

Conclusions/Significance

SHIP may play an important role in regulation of essential signaling pathways that control early megakaryocytopoiesis in vivo.

Hypermethylation of CXCR4 promoter in CD34+ cells from patients with primary myelofibrosis

Constitutive mobilization of CD34(+) cells in patients with primary myelofibrosis (PMF) has been attributed to proteolytic disruption of the CXCR4/SDF-1 axis and reduced CXCR4 expression. We document here that the number of circulating CD34(+)/CXCR4(+) cells in PMF patients, as well as the cellular CXCR4 expression, was directly related to CXCR4 mRNA level and that reduced CXCR4 mRNA level was not due to SDF-1-induced downregulation. To address whether epigenetic regulation contributes to defective CXCR4 expression, we studied the methylation status of the CXCR4 promoter using methylation-specific polymerase chain reaction and methylation-specific sequencing in the JAK2V617F-positive HEL cell line and in CD34(+) cells. We found that CD34(+) cells from PMF patients, unlike those from normal subjects, presented hypermethylation of CXCR4 promoter CpG island 1. Following incubation with the demethylating agent 5-Aza-2'-deoxycytidine (5-AzaD), the percentage of PMF CD34(+) cells expressing CXCR4 increased 3-10 times, whereas CXCR4 mRNA level increased approximately 4 times. 5-AzaD-treated PMF CD34(+) cells displayed almost complete reversal of CpG1 island 1 hypermethylation and showed enhanced migration in vitro in response to SDF-1. These data point to abnormal methylation of the CXCR4 promoter as a mechanism contributing to constitutive migration of CD34(+) cells in PMF. Disclosure of potential conflicts of interest is found at the end of this article.

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.

Altered SDF-1/CXCR4 axis in patients with primary myelofibrosis and in the Gata1 low mouse model of the disease

OBJECTIVE

To assess whether alterations in the stromal cell-derived factor-1 (SDF-1)/CXCR4 occur in patients with primary myelofibrosis (PMF) and in Gata1 low mice, an animal model for myelofibrosis, and whether these abnormalities might account for increased stem/progenitor cell trafficking.

MATERIALS AND METHODS

In the mouse, SDF-1 mRNA levels were assayed in liver, spleen, and marrow. SDF-1 protein levels were quantified in plasma and marrow and CXCR4 mRNA and protein levels were evaluated on stem/progenitor cells and megakaryocytes purified from the marrow. SDF-1 protein levels were also evaluated in plasma and in marrow biopsy specimens obtained from normal donors and PMF patients.

RESULTS

In Gata1 low mice, the plasma SDF-1 protein was five times higher than normal in younger animals. Furthermore, SDF-1 immunostaining of marrow sections progressively increased with age. Similar abnormalities were observed in PMF patients. In fact, plasma SDF-1 levels in PMF patients were significantly higher (by twofold) than normal (p < 0.01) and SDF-1 immunostaining of marrow biopsy specimens demonstrated increased SDF-1 deposition in specific areas. In two of the patients, SDF-1 deposition was normalized by curative therapy with allogenic stem cell transplantation. Similar to what already has been reported for PMF patients, the marrow from Gata1 low mice contained fewer CXCR4 pos CD117 pos cells and these cells expressed low levels of CXCR4 mRNA and protein.

CONCLUSION

Similar abnormalities in the SDF-1/CXCR4 axis are observed in PMF patients and in the Gata1 low mice model of myelofibrosis. We suggest that these abnormalities contribute to the increased stem/progenitor cell trafficking observed in this mouse model as well as patients with PMF.

STAT1 promotes megakaryopoiesis downstream of GATA-1 in mice

Thrombocytosis is associated with inflammation, and certain inflammatory cytokines, including IFN-gamma, stimulate megakaryocyte and platelet production. However, the roles of IFN-gamma and its downstream effector STAT1 in megakaryocyte development are poorly understood. We previously reported that STAT1 expression was significantly downregulated in Gata1-knockdown murine megakaryocytes, which also have impaired terminal maturation. Here, we show that ectopic expression of STAT1, or its target effector IRF-1, rescued multiple defects in Gata1-deficient megakaryopoiesis in mice, inducing polyploidization and expression of a subset of platelet-expressing genes. Enforced expression of STAT1, IRF-1, or GATA-1 enhanced phosphorylation of STAT1, STAT3, and STAT5 in cultured Gata1-deficient murine megakaryocytes, with concomitant megakaryocyte maturation. In contrast, enhanced thrombopoietin signaling, conferred by enforced expression of constitutively active JAK2 or c-MPL, induced phosphorylation of STAT3 and STAT5, but not STAT1, and failed to rescue megakaryocyte maturation. Finally, megakaryocytes from Stat1(-/-) mice were defective in polyploidization. Together, these findings reveal a unique role for STAT1 in megakaryopoiesis and provide new insights into how GATA-1 regulates this process. Our studies elucidate potential mechanisms by which various inflammatory disorders can cause elevated platelet counts.

Review article: the pathophysiology of thrombocytopenia in hepatitis C virus infection and chronic liver disease

BACKGROUND

The pathophystology of thrombocytopenia in patients with chronic liver disease resulting from hepatitis C virus (HCV) infection is complex and involves several complementary mechanisms that likely act in concert.

AIM

To summarize the available data on the etiology of thrombocytopenia in patients with chronic liver disease.

RESULTS

In patients with untreated hepatitis C, both prevalence and severity of thrombocytopenia increase in parallel with the extent of disease, usually becoming clinically relevant when patients develop extensive fibrosis and/or cirrhosis. Pathogenetic mechanisms include hypersptenism secondary to portal hypertension, bone marrow suppression resulting from either HCV itself or interferon treatment, aberrations of the immune system resulting in the formation of anti-platelet antibodies and/or immune-complexes that bind to platelets and facilitate their premature clearance, development of immunologically-mediated extrahepatic manifestations including mixed cryoglobulinemia with or without associated joint, renal, or cutaneous involvement, and thrombopoietin (TPO) deficiency secondary to liver dysfunction. In chronic liver disease, the natural inverse relationship between TPO and platelet levels is not maintained; therefore, blood TPO levels fail to have clinical relevance or predictive value in assessing the thrombocytopenic status of a given patient.

CONCLUSIONS

The development of thrombocytopenisa in patients with chronic liver disease is complex and multifactorial.

H4 histamine receptor mediates optimal migration of mast cell precursors to CXCL12

BACKGROUND

CXCL12, a constitutive chemokine (ligand of CXCR4 and CXCR7), is expressed in the skin and airway epithelium and plays a significant role in allergic airway diseases. The pleiotropic effects of CXCL12 are enhanced by cofactors specific to the target cell.

OBJECTIVE

We hypothesized that histamine, a major mediator of allergic reactions, could interact with CXCL12 to promote human mast cell (MC) migration.

METHODS

The chemotactic effects of CXCL12 alone or in combination with histamine were evaluated on human precursor and mature MCs by using in vitro migration assays.

RESULTS

CXCL12 exerts a chemotactic activity on both precursor and fully mature MCs. Histamine and supernatants from IgE-activated MCs enhanced CXCL12 chemotactic activity on the precursor MC population. The synergy between histamine and CXCL12 was not observed with mature MCs, CD4(+) T cells, and monocytes. Inhibition of histamine receptors pharmacologically or with specific small interfering RNA (siRNA) indicated that synergy between histamine and CXCL12 required the H(4) receptor.

CONCLUSION

Histamine released by allergen-activated mature MCs might promote MC-rich allergic inflammation by enhancing recruitment of their precursors in tissues constitutively expressing CXCL12, including skin and airways.

CLINICAL IMPLICATIONS

This work highlights a novel role of the H(4) receptor in the perpetuation of allergic responses and provides evidence for the utility of H(4) receptor antagonists in the treatment of allergic diseases.

Chemokine stromal cell-derived factor-1 induction by C/EBPβ activation is associated with all-trans-retinoic acid-induced leukemic cell differentiation

Stromal cell-derived factor-1 (SDF-1/CXCL12) is one of the essential chemokines, which mediates hematopoietic differentiations. However, the mechanism by which SDF-1 expression is regulated in granulocyte differentiation is poorly understood. Here, we suggest a novel mechanism by which all-trans-retinoic acid (ATRA) induces the expression of SDF-1 during the differentiation of promyelomonocytic leukemic U937 cells. Moreover, we also demonstrate that activation of transcription factor C/EBPbeta by ATRA regulates SDF-1 expression in U937 cells. In addition, we show that the cyclin-dependent kinase inhibitors p21(WAF1/CIP1) and Pyk2 are up-regulated by SDF-1 and increased markedly by the costimulation of ATRA and SDF-1. Furthermore, ATRA and SDF-1alpha additively induce U937 cell differentiation. Indeed, silencing the expression of SDF-1 inhibits ATRA-induced granulocyte differentiation significantly. Taken together, these results indicate that SDF-1alpha is involved in granulocyte differentiation in response to ATRA, mediated by the activation of the transcription factor C/EBPbeta.

Homeostatic chemokines increase survival of B-chronic lymphocytic leukemia cells through inactivation of transcription factor FOXO3a

B-chronic lymphocytic leukemia (B-CLL) cell is characterized by the accumulation of long-lived CD5+ B lymphocytes, whose survival in vivo is in part dependent on exogenous factors such as cytokines and/or extracellular matrix proteins. Homeostatic chemokines are critical mediators of lymphoid cell trafficking. However, how they function in cell signaling and survival remains ill-defined. In this study, we have investigated the role of the homeostatic chemokines, CXCL12, CCL21, CCL19 and CXCL13, in B-CLL cell survival. Using primary leukemic cells isolated from 26 patients, we observed that each chemokine enhances cell survival. Chemokines induced the phosphorylation of ERK1/2 and p90RSK, and of Akt and its effectors GSK3 and FOXO3a. Consistently, inhibitors against mitogen-activated protein kinase/extracellular signal-regulated kinase and phosphatidylinositol 3-kinase inhibited chemokine-induced survival. Moreover, using a constitutively active mutated form of FOXO3a or siRNAs against FOXO3a in transfection experiments performed in primary B-CLL cells, we directly demonstrated the critical role of FOXO3a in both spontaneous and chemokine-induced B-CLL cell survival. Overall, our data support the notion that homeostatic chemokines contribute to B-CLL resistance to cell death through inactivation of the transcription factor FOXO3a, which may represent a novel therapeutic target in this hematopoietic malignancy.

CD41+/CD45+Cells Without Acetylcholinesterase Activity Are Immature and a Major Megakaryocytic Population in Murine Bone Marrow

Murine megakaryocytes (MKs) are defined by CD41/CD61 expression and acetylcholinesterase (AChE) activity; however, their stages of differentiation in bone marrow (BM) have not been fully elucidated. In murine lineage-negative (Lin(-))/CD45(+) BM cells, we found CD41(+) MKs without AChE activity (AChE(-)) except for CD41(++) MKs with AChE activity (AChE(+)), in which CD61 expression was similar to their CD41 level. Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs could differentiate into AChE(+), with an accompanying increase in CD41/CD61 during in vitro culture. Both proplatelet formation (PPF) and platelet (PLT) production for Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs were observed later than for Lin(-)/CD41(++)/CD45(+)/AChE(+) MKs, whereas MK progenitors were scarcely detected in both subpopulations. GeneChip and semiquantitative polymerase chain reaction analyses revealed that the Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs are assigned at the stage between the progenitor and PPF preparation phases in respect to the many MK/PLT-specific gene expressions, including beta1-tubulin. In normal mice, the number of Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs was 100 times higher than that of AChE(+) MKs in BM. When MK destruction and consequent thrombocytopenia were caused by an antitumor agent, mitomycin-C, Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs led to an increase in AChE(+) MKs and subsequent PLT recovery with interleukin-11 administration. It was concluded that MKs in murine BM at least in part consist of immature Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs and more differentiated Lin(-)/CD41(++)/CD45(+)/AChE(+) MKs. Immature Lin(-)/CD41(+)/CD45(+)/AChE(-) MKs are a major MK population compared with AChE(+) MKs in BM and play an important role in rapid PLT recovery in vivo.

Cleavage fragments of the third complement component (C3) enhance stromal derived factor-1 (SDF-1)-mediated platelet production during reactive postbleeding thrombocytosis

We hypothesized that the third complement component (C3) cleavage fragments (C3a and (des-Arg)C3a) are involved in stress/inflammation-related thrombocytosis, and investigated their potential role in reactive thrombocytosis induced by bleeding. We found that platelet counts are lower in C3-deficient mice in response to excessive bleeding as compared to normal littermates and that C3a and (des-Arg)C3a enhance stromal-derived factor-1 (SDF-1)-dependent megakaryocyte (Megs) migration, adhesion and platelet shedding. At the molecular level, C3a stimulates in Megs MAPKp42/44 phosphorylation, and enhances incorporation of CXCR4 into membrane lipid rafts increasing the responsiveness of Megs to SDF-1. We found that perturbation of lipid raft formation by statins decreases SDF-1/C3a-dependent platelet production in vitro and in an in vivo model statins ameliorated post-bleeding thrombocytosis. Thus, inhibition of lipid raft formation could find potential clinical application as a means of ameliorating some forms of thrombocytosis.

A novel role for PECAM-1 in megakaryocytokinesis and recovery of platelet counts in thrombocytopenic mice

During thrombopoiesis, maturing megakaryocytes (MKs) migrate within the complex bone marrow stromal microenvironment from the proliferative osteoblastic niche to the capillary-rich vascular niche where proplatelet formation and platelet release occurs. This physiologic process involves proliferation, differentiation, migration, and maturation of MKs before platelet production occurs. In this study, we report a role for the glycoprotein PECAM-1 in thrombopoiesis. We show that following induced thrombocytopenia, recovery of the peripheral platelet count is impaired in PECAM-1-deficient mice. Whereas MK maturation, proplatelet formation, and platelet production under in vitro conditions were unaffected, we identified a migration defect in PECAM-1-deficient MKs in response to a gradient of stromal cell-derived factor 1 (SDF1), a major chemokine regulating MK migration within the bone marrow. This defect could be explained by defective PECAM-1(-/-) MK polarization of the SDF1 receptor CXCR4 and an increase in adhesion to immobilized bone marrow matrix proteins that can be explained by an increase in integrin activation. The defect of migration and polarization was confirmed in vivo with demonstration of altered spatial localization of MKs within the bone marrow in PECAM-1-deficient mice, following immune-induced thrombocytopenia. This study identifies a novel role for PECAM-1 in regulating MK migration and thrombopoiesis.

Transcriptional regulation of megakaryopoiesis: thrombopoietin signaling and nuclear factors

PURPOSE OF REVIEW

Thrombopoietin, the primary regulator of megakaryopoiesis, acts by modulating several nuclear transcription factors. This review focuses on recent studies that have provided new insights into the functional roles of these proteins.

RECENT FINDINGS

Among the transcription factors responsible for megakaryopoiesis, important roles for the signal transducer and activator of transcription and Forkhead box proteins have been defined. In addition, in the past several years, two new groups of transcription factors have been found to be affected by thrombopoietin signaling, homeodomain-containing HOX proteins and hypoxia-inducible factor. The HOX transcription factors, which play key roles in body pattern development, are also expressed in adult hematopoietic stem cells and play pivotal roles in their proliferation. Thrombopoietin controls the levels of expression and modifies the function of homeodomain proteins in thrombopoietin-responsive leukemic cells and in primary immature hematopoietic cells. Levels of hypoxia-inducible factor, a master transcription factor required for the adaptation to hypoxic conditions, is also affected by thrombopoietin in these cells.

SUMMARY

The discovery that a diverse range of transcription factors are downstream effectors of thrombopoietin helps to explain the molecular mechanisms by which the hormone affects hematopoiesis.

Cbf beta-SMMHC induces distinct abnormal myeloid progenitors able to develop acute myeloid leukemia

The acute myeloid leukemia (AML)-associated CBF beta-SMMHC fusion protein impairs hematopoietic differentiation and predisposes to leukemic transformation. The mechanism of leukemia progression, however, is poorly understood. In this study, we report a conditional Cbfb-MYH11 knockin mouse model that develops AML with a median latency of 5 months. Cbf beta-SMMHC expression reduced the multilineage repopulation capacity of hematopoietic stem cells (HSCs) while maintaining their numbers under competitive conditions. The fusion protein induced abnormal myeloid progenitors (AMPs) with limited proliferative potential but leukemic predisposition similar to that of HSCs in transplanted mice. In addition, Cbf beta-SMMHC blocked megakaryocytic maturation at the CFU-Meg to megakaryocyte transition. These data show that a leukemia oncoprotein can inhibit differentiation and proliferation while not affecting the maintenance of long-term HSCs.