Mouse model of congenital polycythemia: Homologous replacement of murine gene by mutant human erythropoietin receptor gene

Rationale of Basic and Cellular Mechanisms Considered in Updating the Staging System for Diabetic Retinal Disease

Purpose

Hyperglycemia is a major risk factor for early lesions of diabetic retinal disease (DRD). Updating the DRD staging system to incorporate relevant basic and cellular mechanisms pertinent to DRD is necessary to better address early disease, disease progression, the use of therapeutic interventions, and treatment effectiveness.

Design

We sought to review preclinical and clinical evidence on basic and cellular mechanisms potentially pertinent to DRD that might eventually be relevant to update the DRD staging system.

Participants

Not applicable.

Methods

The Basic and Cellular Mechanisms Working Group (BCM-WG) of the Mary Tyler Moore Vision Initiative carefully and extensively reviewed available preclinical and clinical evidence through multiple iterations and classified these.

Main Outcome Measures

Classification was made into evidence grids, level of supporting evidence, and anticipated future relevance to DRD.

Results

A total of 40 identified targets based on pathophysiology and other parameters for DRD were grouped into concepts or evaluated as specific candidates. VEGFA, peroxisome proliferator-activated receptor-alpha related pathways, plasma kallikrein, and angiopoietin 2 had strong agreement as promising for use as biomarkers in diagnostic, monitoring, predictive, prognostic, and pharmacodynamic responses as well as for susceptibility/risk biomarkers that could underlie new assessments and eventually be considered within an updated DRD staging system or treatment, based on the evidence and need for research that would fit within a 2-year timeline. The BCM-WG found there was strong reason also to pursue the following important concepts regarding scientific research of DRD acknowledging their regulation by hyperglycemia: inflammatory/cytokines, oxidative signaling, vasoprotection, neuroprotection, mitophagy, and nutrients/microbiome.

Conclusion

Promising targets that might eventually be considered within an updated DRD staging system or treatment were identified. Although the BCM-WG recognizes that at this stage little can be incorporated into a new DRD staging system, numerous potential targets and important concepts deserve continued support and research, as they may eventually serve as biomarkers and/or therapeutic targets with measurable benefits to patients with diabetes.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Retinopathy of prematurity protection conferred by uteroplacental insufficiency through erythropoietin signaling in an experimental Murine Model

BACKGROUND

Recent clinical studies suggest that preeclampsia, characterized by uteroplacental insufficiency (UPI) and infant intrauterine growth restriction (IUGR), may be protective against retinopathy of prematurity (ROP) in preterm infants. Experimental models of UPI/IUGR have found an association of erythropoietin (EPO) with less severe oxygen-induced retinopathy (OIR); however, it is unclear if EPO/EPO receptor (EPOR) signaling was involved. We hypothesized that maternal UPI and resultant infant IUGR would protect against features of ROP through EPO/EPOR signaling.

METHODS

We compared transgenic mice with hypoactive EPOR signaling (hWtEPOR) to littermate wild-type mice (mWtEpoR) in a novel combined model of IUGR and ROP. Thromboxane A2 (TXA2) was infused into pregnant C57Bl/6J dams to produce UPI/IUGR; postnatal pups and their foster dams were subjected to a murine OIR model.

RESULTS

Following hyperoxia, hematocrits were similar between littermate wild-type (mWtEpoR) TXA2/OIR and vehicle/OIR pups. mWtEpoR TXA2/OIR had increased serum EPO, retinal EPO and VEGF, and decreased avascular retinal area (AVA) compared to vehicle/OIR pups. In comparison to the mWtEpoR TXA2/OIR pups, AVA was not reduced in hWtEPOR TXA2/OIR pups.

CONCLUSION

Our findings provide biologic evidence that UPI/OIR-induced endogenous EPOR signaling confers protection against hyperoxia-induced vascular damage that may be related to pathophysiology in ROP.

IMPACT

Maternal preeclampsia and infant growth restriction confer retinovascular protection against high oxygen-induced damage through endogenous erythropoietin signaling.

Developmental changes in iron metabolism and erythropoiesis in mice with human gain-of-function erythropoietin receptor

Iron availability for erythropoiesis is controlled by the iron-regulatory hormone hepcidin. Increased erythropoiesis negatively regulates hepcidin synthesis by erythroferrone (ERFE), a hormone produced by erythroid precursors in response to erythropoietin (EPO). The mechanisms coordinating erythropoietic activity with iron homeostasis in erythrocytosis with low EPO are not well defined as exemplified by dominantly inherited (heterozygous) gain-of-function mutation of human EPO receptor (mtHEPOR) with low EPO characterized by postnatal erythrocytosis. We previously created a mouse model of this mtHEPOR that develops fetal erythrocytosis with a transient perinatal amelioration of erythrocytosis and its reappearance at 3-6 weeks of age. Prenatally and perinatally, mtHEPOR heterozygous and homozygous mice (differing in erythrocytosis severity) had increased Erfe transcripts, reduced hepcidin, and iron deficiency. Epo was transiently normal in the prenatal life; then decreased at postnatal day 7, and remained reduced in adulthood. Postnatally, hepcidin increased in mtHEPOR heterozygotes and homozygotes, accompanied by low Erfe induction and iron accumulation. With aging, the old, especially mtHEPOR homozygotes had a decline of erythropoiesis, myeloid expansion, and local bone marrow inflammatory stress. In addition, mtHEPOR erythrocytes had a reduced lifespan. This, together with reduced iron demand for erythropoiesis, due to its age-related attenuation, likely contributes to increased iron deposition in the aged mtHEPOR mice. In conclusion, the erythroid drive-mediated inhibition of hepcidin production in mtHEPOR mice in the prenatal/perinatal period is postnatally abrogated by increasing iron stores promoting hepcidin synthesis. The differences observed in studied characteristics between mtHEPOR heterozygotes and homozygotes suggest dose-dependent alterations of downstream EPOR stimulation.

Role of Erythropoietin Receptor Signaling in Macrophages or Choroidal Endothelial Cells in Choroidal Neovascularization

Erythropoietin (EPO) has been proposed to reduce the progression of atrophic age-related macular degeneration (AMD) due to its potential role in neuroprotection. However, overactive EPO receptor (EPOR) signaling increased laser-induced choroidal neovascularization (CNV) and choroidal macrophage number in non-lasered mice, which raised the question of whether EPOR signaling increased CNV through the recruitment of macrophages to the choroid that released pro-angiogenic factors or through direct angiogenic effects on endothelial cells. In this study, we addressed the hypothesis that EPOR signaling increased CNV by direct effects on macrophages or endothelial cells. We used tamoxifen-inducible macrophage-specific or endothelial cell-specific EPOR knockout mice in the laser-induced CNV model, and cultured choroidal endothelial cells isolated from adult human donors. We found that macrophage-specific knockout of EPOR influenced laser-induced CNV in females only, whereas endothelial-specific knockout of EPOR reduced laser-induced CNV in male mice only. In cultured human choroidal endothelial cells, knockdown of EPOR reduced EPO-induced signal transducer and activator of transcription 3 (STAT3) activation. Taken together, our findings suggest that EPOR signaling in macrophages or choroidal endothelial cells regulates the development of CNV in a sex-dependent manner. Further studies regarding the role of EPO-induced signaling are required to assess EPO safety and to select or develop appropriate therapeutic approaches.

Signaling Through the Erythropoietin Receptor Affects Angiogenesis in Retinovascular Disease

Purpose

Exogenous erythropoietin (EPO) is being considered for tissue protection and angiogenesis in retinal vascular diseases. However, studies are limited by insufficient tools to address signaling effects through the EPO receptor (EPOR). We used a humanized mouse model of hypoactive EPOR signaling to test the hypothesis that EPOR signaling supports angiogenesis in retinovascular diseases.

Methods

Humanized Knockin EPOR mice (hWtEPOR) with hypoactive EPOR signaling were compared to littermate wild-type mice (WT). Postnatal day (p)7 mice of each genotype were exposed to 75% oxygen for five days, followed by 21% oxygen in the oxygen-induced retinopathy model (OIR) and compared to room-air (RA)–raised pups. At time points after OIR, pups were sacrificed, and flat-mounted, lectin-stained retinas were analyzed for central avascular area or intravitreal neovascular area (IVNV). Flash-frozen retinas were analyzed for angiogenic protein (Epo, VEGF, p-Stat3) and gene (Vegfa, Kdr, Epo, Hif1α, Hif2α) expression levels.

Results

In OIR, hWtEPOR mice had increased AVA compared with WT at p8, p12, and p17, but there was no difference in IVNV between hWtEPOR and WT mice at p17. Although VEGF and p-STAT3 proteins were increased in WT at p17 OIR, there were no differences in retinal angiogenic factor expression levels between hWtEPOR and WT OIR at p17 despite similar areas of IVNV.

Conclusions

Our data support the hypothesis that EPOR signaling was associated with regrowth of vascularization following oxygen-induced capillary dropout and played a role in intravitreal angiogenesis. Additional study of EPOR signaling regulation on other angiogenic factor pathways may be considered.

Erythropoietin Receptor Signaling Supports Retinal Function after Vascular Injury

The investigation of erythropoietin (EPO) has expanded to include potential nonhematopoietic roles in neural and retinal diseases, including diabetic retinopathy. However, it remains unclear how EPO functions to support the neural retina. Transgenic mice with hypoactive EPO receptor (EPOR) signaling (hWtEPOR) were compared with littermate control mice (WT) to test the role of EPOR signaling under normal conditions and after vascular injury and regrowth into the retina. Although retinal function tested with OptoMotry and electroretinography was comparable to adult (8-week-old) littermate WT mice, hWtEPOR mice had thinner inner and outer plexiform layers and a greater number of amacrine cells. Injury and repair caused by the oxygen-induced retinopathy model reduced visual acuity thresholds, reduced electroretinography amplitudes, and thinned the outer plexiform and inner nuclear layers of both WT and hWtEPOR 8-week-old mice. In hWtEPOR compared with WT mice, scotopic a-wave amplitudes were reduced by injury, despite no change in outer nuclear layer thickness; and peripheral rod, but not cone number, was reduced. Scotopic b-waves were reduced in injured hWtEPOR mice compared with WT, and rod bipolar cell ectopic neurites were increased in both genotypes after injury, suggesting a potential reparative process to preserve connectivity and the b-wave. Normal EPOR signaling appeared important because ectopic neurites and b-waves were lower in the hWtEPOR than WT injured mice.

Naturally occurring and synthetic constitutive-active cytokine receptors in disease and therapy

Cytokines control immune related events and are critically involved in a plethora of patho-physiological processes including autoimmunity and cancer development. Mutations which cause ligand-independent, constitutive activation of cytokine receptors are quite frequently found in diseases. Many constitutive-active cytokine receptor variants have been directly connected to disease development and mechanistically analyzed. Nature's solutions to generate constitutive cytokine receptors has been recently adopted by synthetic cytokine receptor biology, with the goal to optimize immune therapeutics. Here, CAR T cell immmunotherapy represents the first example to combine synthetic biology with genetic engineering during therapy. Hence, constitutive-active cytokine receptors are therapeutic targets, but also emerging tools to improve or modulate immunotherapeutic strategies. This review gives a comprehensive insight into the field of naturally occurring and synthetic constitutive-active cytokine receptors.

Erythropoietin Signaling Increases Choroidal Macrophages and Cytokine Expression, and Exacerbates Choroidal Neovascularization

Erythropoietin (EPO) is recognized for neuroprotective and angiogenic effects and has been associated with aging and neovascular age-related macular degeneration (AMD). We hypothesized that systemic EPO facilitates the development of choroidal neovascularization (CNV). Wild type mice expressed murine EPOR (mWtEPOR) in RPE/choroids at baseline and had significantly increased serum EPO after laser treatment. To test the role of EPO signaling, we used human EPOR knock-in mice with the mWtEPOR gene replaced by either the human EPOR gene (hWtEPOR) or a mutated human EPOR gene (hMtEPOR) in a laser-induced choroidal neovascularization (LCNV) model. Loss-of-function hWtEPOR mice have reduced downstream activation, whereas gain-of-function hMtEPOR mice have increased EPOR signaling. Compared to littermate controls (mWtEPOR), hMtEPOR with increased EPOR signaling developed larger CNV lesions. At baseline, hMtEPOR mice had increased numbers of macrophages, greater expression of macrophage markers F4/80 and CD206, and following laser injury, had greater expression of cytokines CCL2, CXCL10, CCL22, IL-6, and IL-10 than mWtEPOR controls. These data support a hypothesis that injury from age- and AMD-related changes in the RPE/choroid leads to choroidal neovascularization through EPOR-mediated cytokine production.

Effects of erythropoietin receptor activity on angiogenesis, tubular injury, and fibrosis in acute kidney injury: a "U-shaped" relationship

The erythropoietin receptor (EpoR) is widely expressed but its renoprotective action is unexplored. To examine the role of EpoR in vivo in the kidney, we induced acute kidney injury (AKI) by ischemia-reperfusion in mice with different EpoR bioactivities in the kidney. EpoR bioactivity was reduced by knockin of wild-type human EpoR, which is hypofunctional relative to murine EpoR, and a renal tubule-specific EpoR knockout. These mice had lower EPO/EpoR activity and lower autophagy flux in renal tubules. Upon AKI induction, they exhibited worse renal function and structural damage, more apoptosis at the acute stage (<7 days), and slower recovery with more tubulointerstitial fibrosis at the subacute stage (14 days). In contrast, mice with hyperactive EpoR signaling from knockin of a constitutively active human EpoR had higher autophagic flux, milder kidney damage, and better renal function at the acute stage but, surprisingly, worse tubulointerstitial fibrosis and renal function at the subacute stage. Either excess or deficient EpoR activity in the kidney was associated with abnormal peritubular capillaries and tubular hypoxia, creating a "U-shaped" relationship. The direct effects of EpoR on tubular cells were confirmed in vitro by a hydrogen peroxide model using primary cultured proximal tubule cells with different EpoR activities. In summary, normal erythropoietin (EPO)/EpoR signaling in renal tubules provides defense against renal tubular injury maintains the autophagy-apoptosis balance and peritubular capillary integrity. High and low EPO/EpoR bioactivities both lead to vascular defect, and high EpoR activity overides the tubular protective effects in AKI recovery.

Delayed hemoglobin switching and perinatal neocytolysis in mice with gain-of-function erythropoietin receptor

Mutations of the truncated cytoplasmic domain of human erythropoietin receptor (EPOR) result in gain-of-function of erythropoietin (EPO) signaling and a dominantly inherited polycythemia, primary familial and congenital polycythemia (PFCP). We interrogated the unexplained transient absence of perinatal polycythemia observed in PFCP patients using an animal model of PFCP to examine its erythropoiesis during embryonic, perinatal, and early postnatal periods. In this model, we replaced the murine EpoR gene (mEpoR) with the wild-type human EPOR (wtHEPOR) or mutant human EPOR gene (mtHEPOR) and previously reported that the gain-of-function mtHEPOR mice become polycythemic at 3~6 weeks of age, but not at birth, similar to the phenotype of PFCP patients. In contrast, wtHEPOR mice had sustained anemia. We report that the mtHEPOR fetuses are polycythemic, but their polycythemia is abrogated in the perinatal period and reappears again at 3 weeks after birth. mtHEPOR fetuses have a delayed switch from primitive to definitive erythropoiesis, augmented erythropoietin signaling, and prolonged Stat5 phosphorylation while the wtHEPOR fetuses are anemic. Our study demonstrates the in vivo effect of excessive EPO/EPOR signaling on developmental erythropoiesis switch and describes that fetal polycythemia in this PFCP model is followed by transient correction of polycythemia in perinatal life associated with low Epo levels and increased exposure of erythrocytes' phosphatidylserine. We suggest that neocytolysis contributes to the observed perinatal correction of polycythemia in mtHEPOR newborns as embryos leaving the hypoxic uterus are exposed to normoxia at birth.

KEY MESSAGE

Human gain-of-function EPOR (mtHEPOR) causes fetal polycythemia in knock-in mice. Wild-type human EPOR causes fetal anemia in knock-in mouse model. mtHEPOR mice have delayed switch from primitive to definitive erythropoiesis. Polycythemia of mtHEPOR mice is transiently corrected in perinatal life. mtHEPOR newborns have low Epo and increased exposure of erythrocytes' phosphatidylserine.

Epo receptors are not detectable in primary human tumor tissue samples

Erythropoietin (Epo) is a cytokine that binds and activates an Epo receptor (EpoR) expressed on the surface of erythroid progenitor cells to promote erythropoiesis. While early studies suggested EpoR transcripts were expressed exclusively in the erythroid compartment, low-level EpoR transcripts were detected in nonhematopoietic tissues and tumor cell lines using sensitive RT-PCR methods. However due to the widespread use of nonspecific anti-EpoR antibodies there are conflicting data on EpoR protein expression. In tumor cell lines and normal human tissues examined with a specific and sensitive monoclonal antibody to human EpoR (A82), little/no EpoR protein was detected and it was not functional. In contrast, EpoR protein was reportedly detectable in a breast tumor cell line (MCF-7) and breast cancer tissues with an anti-EpoR polyclonal antibody (M-20), and functional responses to rHuEpo were reported with MCF-7 cells. In another study, a functional response was reported with the lung tumor cell line (NCI-H838) at physiological levels of rHuEpo. However, the specificity of M-20 is in question and the absence of appropriate negative controls raise questions about possible false-positive effects. Here we show that with A82, no EpoR protein was detectable in normal human and matching cancer tissues from breast, lung, colon, ovary and skin with little/no EpoR in MCF-7 and most other breast and lung tumor cell lines. We show further that M-20 provides false positive staining with tissues and it binds to a non-EpoR protein that migrates at the same size as EpoR with MCF-7 lysates. EpoR protein was detectable with NCI-H838 cells, but no rHuEpo-induced phosphorylation of AKT, STAT3, pS6RP or STAT5 was observed suggesting the EpoR was not functional. Taken together these results raise questions about the hypothesis that most tumors express high levels of functional EpoR protein.

Blood Collection from Mice and Hematological Analyses on Mouse Blood

Basic phenotyping of inbred mouse strains and genetically modified mouse models usually includes the determination of blood-based parameters as a diagnostic screen for genotype effects on metabolism and organ function. A broad range of analytes, including hematological parameters, can be reliably determined in mouse blood, if appropriate samples are available. Here we describe recommended techniques for blood collection from mice and the considerations that have to be taken into account to get adequate samples for hematological investigations. Furthermore, we describe established methods used in the German Mouse Clinic (GMC) to determine hematological parameters in the mouse. Curr. Protoc. Mouse Biol. 3:101-119 © 2013 by John Wiley & Sons, Inc.

Recovery and Biodistribution of Ex Vivo Expanded Human Erythroblasts Injected into NOD/SCID/IL2Rγ mice

Ex vivo expanded erythroblasts (EBs) may serve as advanced transfusion products provided that lodgment occurs in the macrophage-niche of the marrow permitting maturation. EBs expanded from adult and cord blood expressed the receptors (CXCR4, VLA-4, and P-selectin ligand 1) necessary for interaction with macrophages. However, 4-days following transfusion to intact NOD/SCID/IL2Rγ(null) mice, CD235a(pos) EBs were observed inside CD235a(neg) splenic cells suggesting that they underwent phagocytosis. When splenectomized and intact NOD/SCID/IL2Rγ(null) mice were transfused using retrovirally labeled human EBs, human cells were visualized by bioluminescence imaging only in splenectomized animals. Four days after injection, human CD235a(pos) cells were detected in marrow and liver of splenectomized mice but only in spleen of controls. Human CD235a(pos) erythrocytes in blood remained low in all cases. These studies establish splenectomized NOD/SCID/IL2Rγ(null) mice as a suitable model for tracking and quantification of human EBs in vivo.

Up-regulation of erythropoietin receptor by nitric oxide mediates hypoxia preconditioning

Erythropoietin (Epo), known to stimulate erythroid progenitor cell survival, proliferation, and differentiation, has been shown to be neuroprotective against brain ischemia in animal models. Both Epo and Epo receptor (EpoR) are expressed in the brain and are up-regulated by hypoxia. Brain Epo signaling can stimulate neural cell survival and prevent neuron apoptosis. Neurons from EpoR null mice exhibit marked increased sensitivity to hypoxia. In endothelial cells, Epo has been shown to stimulate nitric oxide (NO) production, particularly at low pO(2). We found here that the EpoR expression on neural cells and Epo's neuroprotective effect were regulated by NO. Hypoxia increased NO production as well as EpoR expression, and inhibition of NOS activity reduced the proportion of EpoR-expressing neurons induced at low pO(2). Conversely, addition of NO donor to cultures grown under normoxia induced EpoR. Similarly, NO donor increased EpoR promoter activity in a reporter gene assay, suggesting that NO regulates EpoR at the transcription level. Preincubation of neurons with NO results in induction of EpoR, which gives rise to protection against hypoxia even in the absence of exogenous Epo, although at high concentration NO is toxic. These data provide evidence of a role for NO in Epo activity in brain and suggest links between NO production, EpoR expression, and Epo signaling in neuroprotection.

Hypoxia. 5. Hypoxia and hematopoiesis

Our understanding of organismal responses to hypoxia has stemmed from studies of erythropoietin regulation by hypoxia that led to the discovery of the master regulator of the hypoxic response, i.e., hypoxia-inducible factor (HIF). This is a transcription factor that is now known to induce the expression of a battery of genes in response to hypoxia. HIF-1 and HIF-2 regulate many genes that are involved in erythropoiesis and iron metabolism, which are essential for tissue oxygen delivery.

EPO receptor gain-of-function causes hereditary polycythemia, alters CD34 cell differentiation and increases circulating endothelial precursors

Background

Gain-of-function of erythropoietin receptor (EPOR) mutations represent the major cause of primary hereditary polycythemia. EPOR is also found in non-erythroid tissues, although its physiological role is still undefined.

Methodology/Principal Findings

We describe a family with polycythemia due to a heterozygous mutation of the EPOR gene that causes a G→T change at nucleotide 1251 of exon 8. The novel EPOR G1251T mutation results in the replacement of a glutamate residue by a stop codon at amino acid 393. Differently from polycythemia vera, EPOR G1251T CD34⁺ cells proliferate and differentiate towards the erythroid phenotype in the presence of minimal amounts of EPO. Moreover, the affected individuals show a 20-fold increase of circulating endothelial precursors. The analysis of erythroid precursor membranes demonstrates a heretofore undescribed accumulation of the truncated EPOR, probably due to the absence of residues involved in the EPO-dependent receptor internalization and degradation. Mutated receptor expression in EPOR-negative cells results in EPOR and Stat5 phosphorylation. Moreover, patient erythroid precursors present an increased activation of EPOR and its effectors, including Stat5 and Erk1/2 pathway.

Conclusions/Significance

Our data provide an unanticipated mechanism for autosomal dominant inherited polycythemia due to a heterozygous EPOR mutation and suggest a regulatory role of EPO/EPOR pathway in human circulating endothelial precursors homeostasis.

Advances in understanding the pathogenesis of primary familial and congenital polycythaemia

Primary familial and congenital polycythemia (PFCP) is an autosomal-dominant proliferative disorder characterized by erythrocytosis and hypersensitivity of erythroid progenitors to erythropoietin (Epo). Several lines of evidence suggest a causal role of truncated erythropoietin receptor (EpoR) in this disease. In this review, we discuss PFCP in the context of erythrocytosis and EpoR signalling. We focus on recent studies describing mechanisms underlying Epo-dependent EpoR down-regulation. One mechanism depends on internalization mediated through the p85 regulatory subunit of the Phosphoinositide 3-Kinase, and the other utilizes ubiquitin-based proteasomal degradation. Truncated PFCP EpoRs are not properly down-regulated upon stimulation, underscoring the importance of these mechanisms in the pathogenesis of PFCP.

Erythropoietin receptor signaling regulates both erythropoiesis and megakaryopoiesis in vivo

Transgenic expression of a gain-of-function truncated mouse erythropoietin receptor gene (EpoR) leads to expansion of the HSC pool in response to human erythropoietin (Epo). We have re-examined this observation using a knock-in mouse model, wherein the mouse EpoR gene was replaced in its proper genetic locus by a single copy of either a wild-type human or a polycythemia-inducing truncated human EPOR gene. Bone marrow cells obtained from knock-in mice were transplanted together with competitor bone marrow cells in a model that allows tracking of erythroid, platelet, and leukocyte contributions by each genotype. Secondary transplants were also performed. Stem/progenitor cells were identified phenotypically and isolated for colony-forming assays to evaluate cytokine responsiveness by cells with the wild-type human or truncated human EPOR gene. Augmented Epo signaling increased erythroid repopulation post-transplant as expected, but had no effect on short-term or long-term leukocyte repopulation. However, the wild-type human EPOR knock-in mouse showed decreases in both erythroid and platelet repopulation compared to marrow cells from the mutant human EPOR knock-in mouse or normal B6 animals. These results provide evidence supporting a role for Epo signaling in megakaryopoiesis in vivo and suggest a role for Epo signaling early in hematopoietic development.

The glial cell response is an essential component of hypoxia-induced erythropoiesis in mice

A key adaptation to environmental hypoxia is an increase in erythropoiesis, driven by the hormone erythropoietin (EPO) through what is traditionally thought to be primarily a renal response. However, both neurons and astrocytes (the largest subpopulation of glial cells in the CNS) also express EPO following ischemic injury, and this response is known to ameliorate damage to the brain. To investigate the role of glial cells as a component of the systemic response to hypoxia, we created astrocyte-specific deletions of the murine genes encoding the hypoxia-inducible transcription factors HIF-1alpha and HIF-2alpha and their negative regulator von Hippel-Lindau (VHL) as well as astrocyte-specific deletion of the HIF target gene Vegf. We found that loss of the hypoxic response in astrocytes does not cause anemia in mice but is necessary for approximately 50% of the acute erythropoietic response to hypoxic stress. In accord with this, erythroid progenitor cells and reticulocytes were substantially reduced in number in mice lacking HIF function in astrocytes following hypoxic stress. Thus, we have demonstrated that the glial component of the CNS is an essential component of hypoxia-induced erythropoiesis.

Erythroid cells in vitro: from developmental biology to blood transfusion products

PURPOSE OF REVIEW

Red blood cells (RBCs) transfusion plays a critical role in numerous therapies. Disruption of blood collection by political unrest, natural disasters and emerging infections and implementation of restrictions on the use of erythropoiesis-stimulating agents in cancer may impact blood availability in the near future. These considerations highlight the importance of developing alternative blood products.

RECENT FINDINGS

Knowledge about the processes that control RBC production has been applied to the establishment of culture conditions allowing ex-vivo generation of RBCs in numbers close to those (2.5 x 10 cells/ml) present in a transfusion, from cord blood, donated blood units or embryonic stem cells. In addition, experimental studies demonstrate that such cells protect mice from lethal bleeding. Therefore, erythroid cells generated ex vivo may be suitable for transfusion provided they can be produced safely in adequate numbers. However, much remains to be done to translate a theoretical production of approximately 2.5 x 10 RBCs in the laboratory into a 'clinical grade production process'.

SUMMARY

This review summarizes the state-of-the-art in establishing ex-vivo culture conditions for erythroid cells and discusses the most compelling issues to be addressed to translate this progress into a clinical grade transfusion product.

Enrichment of Sca1+ hematopoietic progenitors in polycythemic mice inhibits leukemogenesis

Polycythemia vera (PV) is a myeloproliferative disorder characterized by a pronounced increase in the number of erythroid cells. However, despite this aberrant proliferation, the incidence of erythroleukemia is paradoxically rare in PV patients. In this study, we show that the progression of Friend virus-induced erythroleukemia is delayed in a mouse model of primary familial congenital polycythemia in which the wild-type Epo-receptor (EpoR) gene is replaced with a truncated human EPOR gene. Herein, we show that these mice exhibit enrichment of Sca1(+)/cKit(-) progenitors and several mature immune cells, such as dendritic cells and macrophages. In cotransplantation experiments, Sca1(+)/cKit(-) progenitors inhibit the tumorigenicity of Sca1(-)/cKit(+) erythroleukemic cells. A cell line established from Sca1(+)/cKit(-) progenitors is also capable of inhibiting leukemic proliferation in culture and in mice. This phenomenon of leukemic inhibition, also detected in the serum of PV patients, is partially attributed to increased nitric oxide secretion. In addition, the administration of erythropoietin into leukemic mice induces a polycythemia-like state associated with the expansion of Sca1(+)/cKit(-) progenitors and derivative immune cells, thereby inhibiting leukemia progression. This study indicates that a combination therapy incorporating the enrichment of Sca1(+)/cKit(-) progenitors may serve as a novel approach for the treatment of leukemia.

Ligand-induced EpoR internalization is mediated by JAK2 and p85 and is impaired by mutations responsible for primary familial and congenital polycythemia

Epo-induced endocytosis of EpoR plays important roles in the down-regulation of EpoR signaling and is the primary means that regulates circulating Epo concentrations. Here we show that cell-surface EpoR is internalized via clathrin-mediated endocytosis. Both JAK2 kinase activity and EpoR cytoplasmic tyrosines are important for ligand-dependent EpoR internalization. Phosphorylated Y429, Y431, and Y479 in the EpoR cytoplasmic domain bind p85 subunit of PI3 kinase on Epo stimulation and individually are sufficient to mediate Epo-dependent EpoR internalization. Knockdown of p85alpha and p85beta or expression of their dominant-negative forms, but not inhibition of PI3 kinase activity, dramatically impaired EpoR internalization, indicating that p85alpha and p85beta may recruit proteins in the endocytic machinery on Epo stimulation. Furthermore, mutated EpoRs from primary familial and congenital polycythemia (PFCP) patients lacking the 3 important tyrosines do not bind p85 or internalize on stimulation. Addition of residues encompassing Y429 and Y431 to these truncated receptors restored p85beta binding and Epo sensitivity. Our results identify a novel PI3 kinase activity-independent function of p85 in EpoR internalization and support a model that defects of internalization in truncated EpoRs from PFCP patients contribute to Epo hypersensitivity and prolonged signaling.

Pathophysiology of anemia and erythrocytosis

An increasing understanding of the process of erythropoiesis raises some interesting questions about the pathophysiology, diagnosis and treatment of anemia and erythrocytosis. The mechanisms underlying the development of many of the erythrocytoses, previously characterised as idiopathic, have been elucidated leading to an increased understanding of oxygen homeostasis. Characterisation of anemia and erythrocytosis in relation to serum erythropoietin levels can be a useful addition to clinical diagnostic criteria and provide a rationale for treatment with erythropoiesis stimulating agents (ESAs). Recombinant human erythropoietin as well as other ESAs are now widely used to treat anemias associated with a range of conditions, including chronic kidney disease, chronic inflammatory disorders and cancer. There is also heightened awareness of the potential abuse of ESAs to boost athletic performance in competitive sport. The discovery of erythropoietin receptors outside of the erythropoietic compartment may herald future applications for ESAs in the management of neurological and cardiac diseases. The current controversy concerning optimal hemoglobin levels in chronic kidney disease patients treated with ESAs and the potential negative clinical outcomes of ESA treatment in cancer reinforces the need for cautious evaluation of the pleiotropic effects of ESAs in non-erythroid tissues.

Endogenous erythropoietin signaling is required for normal neural progenitor cell proliferation

Erythropoietin (Epo) and its receptor (EpoR), critical for erythropoiesis, are expressed in the nervous system. Prior to death in utero because of severe anemia EpoR-null mice have fewer neural progenitor cells, and differentiated neurons are markedly sensitive to hypoxia, suggesting that during development Epo stimulates neural cell proliferation and prevents neuron apoptosis by promoting oxygen delivery to brain or by direct interaction with neural cells. Here we present evidence that neural progenitor cells express EpoR at higher levels compared with mature neurons; that Epo stimulates proliferation of embryonic neural progenitor cells; and that endogenous Epo contributes to neural progenitor cell proliferation and maintenance. EpoR-null mice were rescued with selective EpoR expression driven by the endogenous EpoR promoter in hematopoietic tissue but not in brain. Although these mice exhibited normal hematopoiesis and erythrocyte production and survived to adulthood, neural cell proliferation and viability were affected. Embryonic brain exhibited increased neural cell apoptosis, and neural cell proliferation was reduced in the adult hippocampus and subventricular zone. Neural cells from these animals were more sensitive to hypoxia/glutamate neurotoxicity than normal neurons in culture and in vivo. These observations demonstrate that endogenous Epo/EpoR signaling promotes cell survival in embryonic brain and contributes to neural cell proliferation in adult brain in regions associated with neurogenesis. Therefore, Epo exerts extra-hematopoietic function and contributes directly to brain development, maintenance, and repair by promoting cell survival and proliferation independent of insult, injury, or ischemia.

Molecular insights into stress erythropoiesis

PURPOSE OF REVIEW

In addition to its essential role in baseline erythropoiesis, the hormone erythropoietin drives the erythropoietic response to hypoxic stress. A mechanistic understanding of stress erythropoiesis would benefit multiple clinical settings, and may aid in understanding leukemogenesis.

RECENT FINDINGS

The spectrum of progenitors targeted by the erythropoietin receptor is broader during stress than during baseline erythropoiesis. Further, the requirement for erythropoietin receptor signaling is more stringent during stress. However, erythropoietin receptor signaling has been mostly studied in vitro, where it is difficult to relate signaling events to stress-dependent changes in erythroid homeostasis. Here we review advances in flow cytometry that allow the identification and study of murine erythroid precursors in hematopoietic tissue as they are responding to stress in vivo. The death receptor Fas and its ligand, FasL, are coexpressed by early splenic erythroblasts, suppressing erythroblast survival and erythropoietic rate. During stress, erythropoietin receptor signaling downregulates erythroblast Fas and FasL, consequently increasing erythropoietic rate.

SUMMARY

Erythropoietic rate is regulated at least in part through the erythropoietin receptor-mediated survival of splenic early erythroblasts. Future research will delineate how multiple antiapoptotic pathways, potentially activated by the erythropoietin receptor, interact to produce the remarkable dynamic range of erythropoiesis.

beta-Trcp mediates ubiquitination and degradation of the erythropoietin receptor and controls cell proliferation

Control of intensity and duration of erythropoietin (Epo) signaling is necessary to tightly regulate red blood cell production. We have recently shown that the ubiquitin/proteasome system plays a major role in the control of Epo-R signaling. Indeed, after Epo stimulation, Epo-R is ubiquitinated and its intracellular part is degraded by the proteasome, preventing further signal transduction. The remaining part of the receptor and associated Epo are internalized and degraded by the lysosomes. We show that beta-Trcp is responsible for Epo-R ubiquitination and degradation. After Epo stimulation, beta-Trcp binds to the Epo-R. This binding, like Epo-R ubiquitination, requires Jak2 activation. The Epo-R contains a typical DSG binding sequence for beta-Trcp that is highly conserved among species. Interestingly, this sequence is located in a region of the Epo-R that is deleted in patients with familial polycythemia. Mutation of the serine residue of this motif to alanine (Epo-RS462A) abolished beta-Trcp binding, Epo-R ubiquitination, and degradation. Epo-RS462A activation was prolonged and BaF3 cells expressing this receptor are hypersensitive to Epo, suggesting that part of the hypersensitivity to Epo in familial polycythemia could be the result of the lack of beta-Trcp recruitment to the Epo-R.

Effect of cellular elements on pressure-velocity relationship in mice

The effect of cellular elements in the blood on peripheral vascular function in mice was evaluated using the pressure-velocity relationships in the iliac arteries of 5 wild type (WT) and 3 polycythemic (MH) mice. Pressure was obtained using a fluid filled catheter in the left iliac artery and blood velocity was measured in the right iliac artery using a 20 MHz pulsed Doppler probe. The proximal aorta was then occluded for one minute to allow flow velocity to decay to zero. The pressure-velocity relationship in the diastolic phase was determined before and after aortic occlusion. In both groups the pressure-velocity relationship was almost linear and the slopes were similar. However, the extrapolated zero-velocity intercept was significantly higher for the MH than WT mice before (55.4 +/- 4.0 vs. 36.2 +/- 4.1 mmHg, p<0.01) and after occlusion (50.7 +/- 5.5 vs. 23.8 +/- 3.1 mmHg, p<0.01). Hematocrits were 41%+/-3 in WT and 59%+/-3 in MH mice. These data show that cellular elements in the blood alter the pressure-velocity relationships in peripheral vessels of mice.

Signals for stress erythropoiesis are integrated via an erythropoietin receptor-phosphotyrosine-343-Stat5 axis

Anemia due to chronic disease or chemotherapy often is ameliorated by erythropoietin (Epo). Present studies reveal that, unlike steady-state erythropoiesis, erythropoiesis during anemia depends sharply on an Epo receptor-phosphotyrosine-343-Stat5 signaling axis. In mice expressing a phosphotyrosine-null (PY-null) Epo receptor allele (EpoR-HM), severe and persistent anemia was induced by hemolysis or 5-fluorouracil. In short-term transplantation experiments, donor EpoR-HM bone marrow cells also failed to efficiently repopulate the erythroid compartment. In each context, stress erythropoiesis was rescued to WT levels upon the selective restoration of an EpoR PY343 Stat5-binding site (EpoR-H allele). As studied using a unique primary culture system, EpoR-HM erythroblasts exhibited marked stage-specific losses in Epo-dependent growth and survival. EpoR-H PY343 signals restored efficient erythroblast expansion, and the selective Epo induction of the Stat5 target genes proviral integration site-1 (Pim-1) and oncostatin-M. Bcl2-like 1 (Bcl-x), in contrast, was not significantly induced via WT-EpoR, EpoR-HM, or EpoR-H alleles. In Kit+ CD71+ erythroblasts, EpoR-PY343 signals furthermore enhanced SCF growth effects, and SCF modulation of Pim-1 kinase and oncostatin-M expression. In maturing Kit- CD71+ erythroblasts, oncostatin-M exerted antiapoptotic effects that likewise depended on EpoR PY343-mediated events. Stress erythropoiesis, therefore, requires stage-specific EpoR-PY343-Stat5 signals, some of which selectively bolster SCF and oncostatin-M action.

Human adenosine A(3) receptor leads to intracellular Ca(2+) mobilization but is insufficient to activate the signaling pathway via phosphoinositide 3-kinase gamma in mice

Selective antagonists for the adenosine A(3) receptor (A3AR), a member of the G protein-coupled receptors, have been indicated as potential drugs for anti-asthma or anti-inflammation. However, potent antagonists for the rodent A3AR have not been identified. To evaluate the pharmacological effects of human A3AR antagonists in mice, we here generated A3AR-humanized mice, in which the mouse A3AR gene was replaced by its human counterpart. The expression levels of human A3AR in the A3AR-humanized mice were equivalent to those of mouse A3AR in wild-type mice. Elevation of the intracellular Ca(2+) concentration induced by an A3AR agonist was observed in bone marrow-derived mast cells from the A3AR-humanized mice and this Ca(2+) mobilization was completely antagonized by a human A3AR antagonist. However, antigen-dependent degranulation was not potentiated by the A3AR agonist in the mast cells from A3AR-humanized mice. The agonist-stimulated human A3AR did not lead to the phosphorylation of either extracellular signal-regulated kinase 1/2 or protein kinase B in A3AR-humanized mice. The rate of human A3AR internalization in the mast cells was also markedly decreased compared with that of mouse A3AR in the mast cells. These results demonstrate that the human A3AR is insufficient to activate phosphoinositide 3-kinase gamma-dependent signaling pathways in mice, probably due to the uncoupling of member(s) of the G proteins, which are capable of activating phosphoinositide 3-kinase gamma, to the human A3AR, despite the mouse G protein(s) responsible for the Ca(2+) elevation are coupled with the human A3AR.

Erythropoietin hypersensitivity in primary familial and congenital polycythemia: Role of tyrosines Y285 and Y344 in erythropoietin receptor cytoplasmic domain

Erythropoietin receptor (EPOR) gene mutations leading to truncations of the cytoplasmic, carboxy-terminal region of EPOR have been described in some patients with primary familial and congenital polycythemia (PFCP), a disorder characterized by isolated erythrocytosis and increased sensitivity of erythroid progenitors to Epo. We studied the role of EPOR in the pathogenesis of PFCP and the requirement for intracytoplasmic tyrosine residues Y285 and Y344 in generation of Epo hypersensitivity phenotype. Interleukin-3-dependent hematopoietic cells were engineered to express variant human EPORs using retrovirus-mediated gene transfer. We introduced tyrosine to phenylalanine substitutions in EPOR-ME, a naturally occurring, mutant human EPOR (G5881T), truncated by 110 carboxy-terminal amino acids and associated with autosomal dominantly inherited PFCP. Cells expressing EPOR-ME exhibited increased Epo sensitivity compared to cells expressing wild type EPOR. Mutation of Y285 alone had a relatively minor effect on Epo hypersensitivity whereas mutation of Y344 resulted in loss of increased Epo sensitivity. Expression of a tyrosine-null truncated EPOR conferred further decrease of Epo-mediated proliferation suggesting that both Y285 and Y344 may contribute to proliferation signals. In the context of EPOR-ME, Y344 was required for Epo-induced Stat5 tyrosine phosphorylation. The positive effect of either Y285 or Y344 on cellular proliferation was associated with Epo-induced tyrosine phosphorylation of Stat1. These findings suggest that both tyrosine residues Y285 and Y344 in the cytoplasmic domain of EPOR-ME may contribute to increased Epo sensitivity that is characteristic of PFCP phenotype.

Generation and phenotypic analysis of a transgenic line of rabbits secreting active recombinant human erythropoietin in the milk

Production of recombinant human erythropoietin (rhEPO) for therapeutic purposes relies on its expression in selected clones of transfected mammalian cells. Alternatively, this glycoprotein can be produced by targeted secretion into the body fluid of transgenic mammals. Here, we report on the generation of a transgenic rabbits producing rhEPO in the lactating mammary gland. Transgenic individuals are viable, fertile and transmit the rhEPO gene to the offspring. Northern blot data indicated that the expression of the transgene in the mammary gland is controlled by whey acidic protien (WAP) regulatory sequences during the period of lactation. While the hybridization with total RNA revealed the expression only in the lactating mammary gland, the highly sensitive combinatory approach using RT-PCR/hybridization technique detected a minor ectopic expression. The level of rhEPO secretion in the founder female, measured in the period of lactation, varied in the range of 60-178 and 60-162 mIU/ml in the milk and blood plasma, respectively. Biological activity of the milk rhEPO was confirmed by a standard [3H]-thymidine incorporation test. Thus, we describe the model of a rhEPO-transgenic rabbit, valuable for studies of rhEPO glycosylation and function, which can be useful for the development of transgenic approaches designed for the preparation of recombinant proteins by alternative biopharmaceutical production.

Polycythemia vera and other primary polycythemias

PURPOSE OF REVIEW

Diagnosis and therapy of polycythemia vera are controversial since the molecular basis of polycythemia vera remains unknown. Distinguishing between polycythemia vera and other polycythemic disorders can be very challenging. The purpose of this review is to discuss the recent progress in this area and critically review the published data in context of our knowledge of other polycythemic disorders.

RECENT FINDINGS

Erythropoietin is the principal regulator of regulator of erythropoiesis; its production is regulated by the degree of hypoxia. Our knowledge of cellular responses to hypoxia has recently exploded and led to the elucidation of the molecular basis of a polycythemia caused by augmentation of hypoxic sensing, Chuvash polycythemia. Similar progress in understanding the molecular basis of polycythemia vera has been elusive. A simple, readily available laboratory test to establish a diagnosis of polycythemia vera would be highly desirable; however, none exists. The value of quantization of neutrophil PRV-1 mRNA, platelet c-mpl expression, in vitro assays of erythroid progenitor cells, serum erythropoietin levels, establishing clonality in female subjects using assays employing X-chromosome-based polymorphism assays, and the progress in the chromosomal location of the gene is discussed. Integration of this information underlies the complexity of the molecular biology of polycythemia vera and indicates likely interaction of multiple genetic events in the genesis of polycythemia vera.

SUMMARY

The existence of family clustering of PV may facilitate the search for PV molecular basis. Only collaborative interaction of clinical researchers and laboratory scientists will lead to meaningful progress in determining the molecular basis of PV.

Polycythemia and oxygen sensing

Polycythemias can be differentiated based on the responsiveness of erythroid progenitors to circulating cytokines. Primary polycythemias are characterized by an augmented response due to acquired somatic or inherited germ-line mutations that are expressed within hematopoietic progenitors causing increased proliferation or decreased apoptosis and resulting in accumulation of red blood cells. In terms of oxygen requirements, primary polycythemias can be viewed as the production of hemoglobin fully dissociated from the tissue oxygen needs and from the oxygen sensing pathway. Polycythemia vera (PV) is the most common primary polycythemia. PV bone marrow progenitors cells can form erythroid colonies in the absence of exogenous erythropoietin in vitro. These endogenous erythroid colonies (EEC) are useful in differentiating PV and secondary polycythemias. They also can differentiate PV where this feature is independent of Epo signalling from primary familial and congenital polycythemia. In this autosomal dominant primary polycythemia, at variance with PV, EEC formation is abolished by anti-Epo and anti-Epo receptor neutralising antibodies. Mutations of the EPOR have been described and resulted in nine cases in truncated EPORs lacking the cytoplasmic carboxy-terminal of the receptor which possesses a negative growth regulatory domain. However, recent data suggest that different mutations may cause PFCP in most cases. Secondary polycythemia can be viewed as either physiological response to satisfy the oxygen needs of the tissues, resulting for instance from high affinity hemoglobins or BPG mutase deficiency, or as the result of germ-line or somatic mutations disturbing the oxygen sensing pathway or its target: Epo. Chuvash polycythemia is a frequently symptomatic disorder with an autosomal recessive inheritance and inappropriately high Epo levels. The erythroid progenitors are hypersensitive to Epo linking this condition to both primary and secondary polycythemia. A germline missense mutation at nucleotide 598 in both alleles of the von Hippel-Lindau gene results in increased hypoxia inducible factor-1 (HIF-1) expression in normoxic conditions. HIF-1 controls the expression of many genes including Epo. Identifying causal defects in other situations like post-renal transplant erythrocytosis and cases of autosomal dominant polycythemia with high Epo levels will help further understanding of the regulation of erythropoiesis.

Hematopoietic cell survival signals are elicited through non-tyrosine-containing sequences in the membrane-proximal region of the erythropoietin receptor (EPOR) by a Stat5-dependent pathway

OBJECTIVE

Erythropoietin is essential for red blood cell development in vivo and is also an important therapeutic agent to treat anemia resulting from kidney failure or bone marrow suppression. The erythropoietin receptor (EPOR) elicits both positive and negative regulatory signaling pathways, primarily through phosphorylated tyrosine residues in the cytoplasmic domain of the activated receptor complex. Surprisingly, however, EPOR tyrosine residues are dispensable for in vivo erythropoiesis under nonstress conditions. One of the key signaling molecules elicited by the EPOR is the Stat5 transcription factor. Stat5 activation has been mapped to tyrosines 343 and 401 in the EPOR cytoplasmic region, although non-tyrosine-containing sequences in the EPOR cytoplasmic region can also stimulate Stat5. To test the functional role of non-tyrosine-containing sequences in the EPOR, we analyzed a series of mutant EPOR isoforms in cell survival and proliferation assays.

METHODS

The IL-3-dependent 32D cell line was stably transfected with cDNAs encoding the wild-type EPOR or mutant EPORs containing or lacking intracellular tyrosines, in the absence or presence of a dominant inhibitory Stat5 isoform. EPO-dependent cell signaling, survival, and proliferation were evaluated.

RESULTS

EPOR isoforms lacking intracellular tyrosine residues elicit an important survival signal in 32D cells. Stat5 function is critical for EPO-dependent cell survival mediated by these non-tyrosine-containing receptor sequences. Interestingly, EPO-dependent survival does not require the presence of fetal calf serum (FCS) in the culture medium, yet FCS is important for 32D cell proliferation in response to EPO.

CONCLUSION

Our results elucidate a previously unrecognized survival pathway elicited by the EPOR. They demonstrate that this pathway requires Stat5 and is serum independent. These findings contribute significantly to our understanding of the complexity by which the EPOR functions in hematopoietic cells.

Classification and molecular biology of polycythemias (erythrocytoses) and thrombocytosis

In this article, polycythemic disorders are classified based on the current understanding of biology of erythropoieses and divided into primary and secondary polycythemias. Special emphasis is given to recently uncovered molecular bases of newly described congenital polycythemic disorders. This clarification of the pathophysiology of some of the congenital polycythemic states has obvious utility for more accurate diagnosis and rational prognostic determination. The molecular basis of congenital thrombocytoses is only beginning to be uncovered. In contrast, the molecular bases of polycythemia vera and essential thrombocythemia remain unknown, thus their diagnostic criteria are imprecise and their treatment remains largely empirical. The central premise of this article is that deciphering the molecular basis of human diseases leads to improved understanding of hematopoiesis, precise diagnosis, and the potential for development of a specific therapy.

Roles for an Epo receptor Tyr-343 Stat5 pathway in proliferative co-signaling with kit

Erythroid progenitor cell expansion depends upon co-signaling by Epo receptor (EpoR) and Kit, but underlying mechanisms are incompletely understood. To quantitatively analyze EpoR contributions to co-signaling, phosphotyrosine (Tyr(P)) mutants were expressed as human epidermal growth factor (hEGF) receptor-mEpoR EE chimeras at matched and physiological levels in FDCW2 hematopoietic progenitor cells and were assayed for proliferative activities in the absence or presence of endogenous Kit stimulation. Two Tyr(P)-null (but Jak2-coupled) EpoR forms each retained <or=25% of the wild-type activity, whereas the add-back of single Tyr(P) sites in the EpoR forms EE-T-Y343 (Stat5 binding site), EE-Y479 (p85/phosphatidylinositol 3-kinase binding site), or EE-Y464 (Src kinase binding site) significantly enhanced activities (to 100, 95, and 50% of EE-WT (wild type) levels, respectively). EE-Y343&Y401 and EEF343&F401 double add-back and deletion constructs were also prepared and were shown to possess 90 and <or=50% of wild-type activity. In contrast, efficient Kit co-signaling activity was retained only by EE-T-Y343 and EE-Y343&Y401 EpoR forms. EE-T-Y343 together with EE-T-Y343F and EE-WT EpoR forms were also analyzed in embryonic stem cell-derived erythroid G1E-2 cells with highly comparable outcomes, including the ability of EE-T-Y343 (but not EE-T-Y-343F) to synergize with Kit. Despite specific connection of EE-T-Y343 to Stat5, the contributions of Kit to EpoR-dependent proliferation did not involve Kit effects on Stat5 activation (but was limited by the mutation of Kit Tyr(P)-567 and Tyr(P)-569 Src kinase recruitment sites). Instead, co-signaling appears to depend upon the downstream integration of Kit signals with the targets of an EpoR/Jak2/Y343/Stat 5 response axis.

Hemostasis and coagulation at a hematocrit level of 0.85: functional consequences of erythrocytosis

We have generated a transgenic mouse line that reaches a hematocrit concentration of 0.85 due to constitutive overexpression of human erythropoietin in an oxygen-independent manner. Unexpectedly, this excessive erythrocytosis did not lead to thrombembolic complications in all investigated organs at any age. Thus, we investigated the mechanisms preventing thrombembolism in this mouse model. Blood analysis revealed an age-dependent elevation of reticulocyte numbers and a marked thrombocytopenia that matched the reduced megakaryocyte numbers in the bone marrow. However, platelet counts were not different from wild-type controls, when calculations were based on the distribution (eg, plasma) volume, thereby explaining why thrombopoietin levels did not increase in transgenic mice. Nevertheless, bleeding time was significantly increased in transgenic animals. A longitudinal investigation using computerized thromboelastography revealed that thrombus formation was reduced with increasing age from 1 to 8 months in transgenic animals. We observed that increasing erythrocyte concentrations inhibited profoundly and reversibly thrombus formation and prolonged the time of clot development, most likely due to mechanical interference of red blood cells with clot-forming platelets. Transgenic animals showed increased nitric oxide levels in the blood that could inhibit vasoconstriction and platelet activation. Finally, we observed that plasmatic coagulation activity in transgenic animals was significantly decreased. Taken together, our findings suggest that prevention of thrombembolic disease in these erythrocytotic transgenic mice was due to functional consequences inherent to increased erythrocyte concentrations and a reduction of plasmatic coagulation activity, the cause of which remains to be elucidated.

A novel mutation in the erythropoietin receptor gene is associated with familial erythrocytosis

Primary familial erythrocytosis (familial polycythemia) is a rare myeloproliferative disorder with an autosomal dominant mode of inheritance. We studied a new kindred with autosomal dominantly inherited familial erythrocytosis. The molecular basis for the observed phenotype of isolated erythrocytosis is heterozygosity for a novel nonsense mutation affecting codon 399 in exon 8 of the erythropoietin receptor (EPOR) gene, encoding an EpoR peptide that is truncated by 110 amino acids at its C-terminus. The new EPOR gene mutation 5881G>T was found to segregate with isolated erythrocytosis in the affected family and this mutation represents the most extensive EpoR truncation reported to date, associated with familial erythrocytosis. Erythroid progenitors from an affected individual displayed Epo hypersensitivity in in vitro methylcellulose cultures, as indicated by more numerous erythroid burst-forming unit-derived colonies in low Epo concentrations compared to normal controls. Expression of mutant EpoR in interleukin 3-dependent hematopoietic cells was associated with Epo hyperresponsiveness compared to cells expressing wild-type EpoR.

New insights into erythropoiesis

Commitment of hematopoietic cells to the erythroid lineage involves the actions of several transcription factors, including TAL1, LMO2, and GATA-2. The differentiation of committed erythroid progenitor cells involves other transcription factors, including NF-E2 and EKLF. Upon binding erythropoietin, the principal regulator of erythropoiesis, cell surface erythropoietin receptors dimerize and activate specific intracellular kinases, including Janus family tyrosine protein kinase 2, phosphoinositol-3 kinase, and mitogen-activated protein kinase. Important substrates of these kinases are tyrosines in the erythropoietin receptors themselves and the signal transducer and transcription activator proteins. Erythropoietin prevents erythroid cell apoptosis. Some of the apoptotic tendency of erythroid cells can be attributed to proapoptotic molecules produced by hematopoietic cells, macrophages, and stromal cells. Cell divisions accompanying terminal erythroid differentiation are finely controlled by cell cycle regulators, and disruption of these terminal divisions causes erythroid cell apoptosis. In reticulocyte maturation, regulated degradation of internal organelles involves a lipoxygenase, whereas survival requires the antiapoptotic protein Bcl-x.

The distal region and receptor tyrosines of the Epo receptor are non-essential for in vivo erythropoiesis

The erythropoietin receptor (EpoR) is required for the proliferation and survival of committed erythroid lineage cells. Previous studies have utilized receptor mutations to show the requirement for the distal half of the cytoplasmic domain of the EpoR and receptor tyrosines for activation of signaling pathways potentially critical to Epo function. To extend these studies to in vivo erythropoiesis, we have created two mutant strains of mice. One strain (H) contains a truncation of the distal half of the cytoplasmic domain, while the second strain (HM) contains the same truncation as well as the mutation of the residual tyrosine (Y(343)) to a phenylalanine. Strikingly, both strains of mice are viable, with only slight alterations in constitutive erythropoiesis or in in vitro assays of red cell lineage function. Challenging H mutant mice with continuous injections of Epo results in an erythrocytosis that is not seen in HM mice. The results demonstrate that neither the distal region nor receptor tyrosines are essential for in vivo EpoR function, but contribute to receptor function in a subtle manner.