Lobe specificity of iron binding to transferrin modulates murine erythropoiesis and iron homeostasis.

Transferrin, the major plasma iron-binding molecule, interacts with cell-surface receptors to deliver iron, modulates hepcidin expression, and regulates erythropoiesis. Transferrin binds and releases iron via either or both of 2 homologous lobes (N and C). To test the hypothesis that the specificity of iron occupancy in the N vs C lobe influences transferrin function, we generated mice with mutations to abrogate iron binding in either lobe (TfN-bl or TfC-bl). Mice homozygous for either mutation had hepatocellular iron loading and decreased liver hepcidin expression (relative to iron concentration), although to different magnitudes. Both mouse models demonstrated some aspects of iron-restricted erythropoiesis, including increased zinc protoporphyrin levels, decreased hemoglobin levels, and microcytosis. Moreover, the TfN-bl/N-bl mice demonstrated the anticipated effect of iron restriction on red cell production (ie, no increase in red blood cell [RBC] count despite elevated erythropoietin levels), along with a poor response to exogenous erythropoietin. In contrast, the TfC-bl/C-bl mice had elevated RBC counts and an exaggerated response to exogenous erythropoietin sufficient to ameliorate the anemia. Observations in heterozygous mice further support a role for relative N vs C lobe iron occupancy in transferrin-mediated regulation of iron homeostasis and erythropoiesis.

Decreasing TfR1 expression reverses anemia and hepcidin suppression in ß-thalassemic mice.

Iron availability for erythropoiesis and its dysregulation in ß-thalassemia are incompletely understood. We previously demonstrated that exogenous apotransferrin leads to more effective erythropoiesis, decreasing erythroferrone (ERFE) and derepressing hepcidin in ß-thalassemic mice. Transferrin-bound iron binding to transferrin receptor 1 (TfR1) is essential for cellular iron delivery during erythropoiesis. We hypothesize that apotransferrin's effect is mediated via decreased TfR1 expression and evaluate TfR1 expression in ß-thalassemic mice in vivo and in vitro with and without added apotransferrin. Our findings demonstrate that ß-thalassemic erythroid precursors overexpress TfR1, an effect that can be reversed by the administration of exogenous apotransferrin. In vitro experiments demonstrate that apotransferrin inhibits TfR1 expression independent of erythropoietin- and iron-related signaling, decreases TfR1 partitioning to reticulocytes during enucleation, and enhances enucleation of defective ß-thalassemic erythroid precursors. These findings strongly suggest that overexpressed TfR1 may play a regulatory role contributing to iron overload and anemia in ß-thalassemic mice. To evaluate further, we crossed TfR1+/- mice, themselves exhibiting iron-restricted erythropoiesis with increased hepcidin, with ß-thalassemic mice. Resultant double-heterozygote mice demonstrate long-term improvement in ineffective erythropoiesis, hepcidin derepression, and increased erythroid enucleation in relation to ß-thalassemic mice. Our data demonstrate for the first time that TfR1+/- haploinsufficiency reverses iron overload specifically in ß-thalassemic erythroid precursors. Taken together, decreasing TfR1 expression during ß-thalassemic erythropoiesis, either directly via induced haploinsufficiency or via exogenous apotransferrin, decreases ineffective erythropoiesis and provides an endogenous mechanism to upregulate hepcidin, leading to sustained iron-restricted erythropoiesis and preventing systemic iron overload in ß-thalassemic mice.

Increased hepcidin in transferrin-treated thalassemic mice correlates with increased liver BMP2 expression and decreased hepatocyte ERK activation.

Iron overload results in significant morbidity and mortality in ß-thalassemic patients. Insufficient hepcidin is implicated in parenchymal iron overload in ß-thalassemia and approaches to increase hepcidin have therapeutic potential. We have previously shown that exogenous apo-transferrin markedly ameliorates ineffective erythropoiesis and increases hepcidin expression in Hbb(th1/th1) (thalassemic) mice. We utilize in vivo and in vitro systems to investigate effects of exogenous apo-transferrin on Smad and ERK1/2 signaling, pathways that participate in hepcidin regulation. Our results demonstrate that apo-transferrin increases hepcidin expression in vivo despite decreased circulating and parenchymal iron concentrations and unchanged liver Bmp6 mRNA expression in thalassemic mice. Hepatocytes from apo-transferrin-treated mice demonstrate decreased ERK1/2 pathway and increased serum BMP2 concentration and hepatocyte BMP2 expression. Furthermore, hepatocyte ERK1/2 phosphorylation is enhanced by neutralizing anti-BMP2/4 antibodies and suppressed in vitro in a dose-dependent manner by BMP2, resulting in converse effects on hepcidin expression, and hepatocytes treated with MEK/ERK1/2 inhibitor U0126 in combination with BMP2 exhibit an additive increase in hepcidin expression. Lastly, bone marrow erythroferrone expression is normalized in apo-transferrin treated thalassemic mice but increased in apo-transferrin injected wild-type mice. These findings suggest that increased hepcidin expression after exogenous apo-transferrin is in part independent of erythroferrone and support a model in which apo-transferrin treatment in thalassemic mice increases BMP2 expression in the liver and other organs, decreases hepatocellular ERK1/2 activation, and increases nuclear Smad to increase hepcidin expression in hepatocytes.

Extrahepatic sources of factor VIII potentially contribute to the coagulation cascade correcting the bleeding phenotype of mice with hemophilia A.

A large fraction of factor VIII in blood originates from liver sinusoidal endothelial cells although extrahepatic sources also contribute to plasma factor VIII levels. Identification of cell-types other than endothelial cells with the capacity to synthesize and release factor VIII will be helpful for therapeutic approaches in hemophilia A. Recent cell therapy and bone marrow transplantation studies indicated that Küpffer cells, monocytes and mesenchymal stromal cells could synthesize factor VIII in sufficient amount to ameliorate the bleeding phenotype in hemophilic mice. To further establish the role of blood cells in expressing factor VIII, we studied various types of mouse and human hematopoietic cells. We identified factor VIII in cells isolated from peripheral and cord blood, as well as bone marrow. Co-staining for cell type-specific markers verified that factor VIII was expressed in monocytes, macrophages and megakaryocytes. We additionally verified that factor VIII was expressed in liver sinusoidal endothelial cells and endothelial cells elsewhere, e.g., in the spleen, lungs and kidneys. Factor VIII was well expressed in sinusoidal endothelial cells and Küpffer cells isolated from human liver, whereas by comparison isolated human hepatocytes expressed factor VIII at very low levels. After transplantation of CD34(+) human cord blood cells into NOD/SCIDγNull-hemophilia A mice, fluorescence activated cell sorting of peripheral blood showed >40% donor cells engrafted in the majority of mice. In these animals, plasma factor VIII activity 12 weeks after cell transplantation was up to 5% and nine of 12 mice survived after a tail clip-assay. In conclusion, hematopoietic cells, in addition to endothelial cells, express and secrete factor VIII: this information should offer further opportunities for understanding mechanisms of factor VIII synthesis and replenishment.

Comprehensive analysis of the editing window of C-to-T TALE base editors.

One of the most recent advances in the genome editing field has been the addition of "TALE Base Editors", an innovative platform for cell therapy that relies on the deamination of cytidines within double strand DNA, leading to the formation of an uracil (U) intermediate. These molecular tools are fusions of transcription activator-like effector domains (TALE) for specific DNA sequence binding, split-DddA deaminase halves that will, upon catalytic domain reconstitution, initiate the conversion of a cytosine (C) to a thymine (T), and an uracil glycosylase inhibitor (UGI). We developed a high throughput screening strategy capable to probe key editing parameters in a precisely defined genomic context in cellulo, excluding or minimizing biases arising from different microenvironmental and/or epigenetic contexts. Here we aimed to further explore how target composition and TALEB architecture will impact the editing outcomes. We demonstrated how the nature of the linker between TALE array and split DddAtox head allows us to fine tune the editing window, also controlling possible bystander activity. Furthermore, we showed that both the TALEB architecture and spacer length separating the two TALE DNA binding regions impact the target TC editing dependence by the surrounding bases, leading to more restrictive or permissive editing profiles.

Iron chelation improves ineffective erythropoiesis and iron overload in myelodysplastic syndrome mice.

Myelodysplastic syndrome (MDS) is a heterogeneous group of bone marrow stem cell disorders characterized by ineffective hematopoiesis and cytopenias, most commonly anemia. Red cell transfusion therapy for anemia in MDS results in iron overload, correlating with reduced overall survival. Whether the treatment of iron overload benefits MDS patients remains controversial. We evaluate underlying iron-related pathophysiology and the effect of iron chelation using deferiprone on erythropoiesis in NUP98-HOXD13 transgenic mice, a highly penetrant well-established MDS mouse model. Our results characterize an iron overload phenotype with aberrant erythropoiesis in these mice which was reversed by deferiprone-treatment. Serum erythropoietin levels decreased while erythroblast erythropoietin receptor expression increased in deferiprone-treated MDS mice. We demonstrate, for the first time, normalized expression of the iron chaperones Pcbp1 and Ncoa4 and increased ferritin stores in late-stage erythroblasts from deferiprone-treated MDS mice, evidence of aberrant iron trafficking in MDS erythroblasts. Importantly, erythroblast ferritin is increased in response to deferiprone, correlating with decreased erythroblast ROS. Finally, we confirmed increased expression of genes involved in iron uptake, sensing, and trafficking in stem and progenitor cells from MDS patients. Taken together, our findings provide evidence that erythroblast-specific iron metabolism is a novel potential therapeutic target to reverse ineffective erythropoiesis in MDS.

Efficient multitool/multiplex gene engineering with TALE-BE.

TALE base editors are a recent addition to the genome editing toolbox. These molecular tools are fusions of a transcription activator-like effector domain (TALE), split-DddA deaminase halves, and an uracil glycosylase inhibitor (UGI) that have the distinct ability to directly edit double strand DNA, converting a cytosine (C) to a thymine (T). To dissect the editing rules of TALE-BE, we combined the screening of dozens of TALE-BE targeting nuclear genomic loci with a medium/high throughput strategy based on precise knock-in of TALE-BE target site collections into the cell genome. This latter approach allowed us to gain in depth insight of the editing rules in cellulo, while excluding confounding factors such as epigenetic and microenvironmental differences among different genomic loci. Using the knowledge gained, we designed TALE-BE targeting CD52 and achieved very high frequency of gene knock-out (up to 80% of phenotypic CD52 knock out). We further demonstrated that TALE-BE generate only insignificant levels of Indels and byproducts. Finally, we combined two molecular tools, a TALE-BE and a TALEN, for multiplex genome engineering, generating high levels of double gene knock-out (∼75%) without creation of translocations between the two targeted sites.

The hepcidin regulator erythroferrone is a new member of the erythropoiesis-iron-bone circuitry.

Background: Erythroblast erythroferrone (ERFE) secretion inhibits hepcidin expression by sequestering several bone morphogenetic protein (BMP) family members to increase iron availability for erythropoiesis. Methods: To address whether ERFE functions also in bone and whether the mechanism of ERFE action in bone involves BMPs, we utilize the Erfe-/- mouse model as well as ß-thalassemic (Hbbth3/+) mice with systemic loss of ERFE expression. In additional, we employ comprehensive skeletal phenotyping analyses as well as functional assays in vitro to address mechanistically the function of ERFE in bone. Results: We report that ERFE expression in osteoblasts is higher compared with erythroblasts, is independent of erythropoietin, and functional in suppressing hepatocyte hepcidin expression. Erfe-/- mice display low-bone-mass arising from increased bone resorption despite a concomitant increase in bone formation. Consistently, Erfe-/- osteoblasts exhibit enhanced mineralization, Sost and Rankl expression, and BMP-mediated signaling ex vivo. The ERFE effect on osteoclasts is mediated through increased osteoblastic RANKL and sclerostin expression, increasing osteoclastogenesis in Erfe-/- mice. Importantly, Erfe loss in Hbbth3/+mice, a disease model with increased ERFE expression, triggers profound osteoclastic bone resorption and bone loss. Conclusions: Together, ERFE exerts an osteoprotective effect by modulating BMP signaling in osteoblasts, decreasing RANKL production to limit osteoclastogenesis, and prevents excessive bone loss during expanded erythropoiesis in ß-thalassemia. Funding: YZG acknowledges the support of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) (R01 DK107670 to YZG and DK095112 to RF, SR, and YZG). MZ acknowledges the support of the National Institute on Aging (U19 AG60917) and NIDDK (R01 DK113627). TY acknowledges the support of the National Institute on Aging (R01 AG71870). SR acknowledges the support of NIDDK (R01 DK090554) and Commonwealth Universal Research Enhancement (CURE) Program Pennsylvania.

Pleckstrin-2 is essential for erythropoiesis in ß-thalassemic mice, reducing apoptosis and enhancing enucleation.

Erythropoiesis involves complex interrelated molecular signals influencing cell survival, differentiation, and enucleation. Diseases associated with ineffective erythropoiesis, such as ß-thalassemias, exhibit erythroid expansion and defective enucleation. Clear mechanistic determinants of what make erythropoiesis effective are lacking. We previously demonstrated that exogenous transferrin ameliorates ineffective erythropoiesis in ß-thalassemic mice. In the current work, we utilize transferrin treatment to elucidate a molecular signature of ineffective erythropoiesis in ß-thalassemia. We hypothesize that compensatory mechanisms are required in ß-thalassemic erythropoiesis to prevent apoptosis and enhance enucleation. We identify pleckstrin-2-a STAT5-dependent lipid binding protein downstream of erythropoietin-as an important regulatory node. We demonstrate that partial loss of pleckstrin-2 leads to worsening ineffective erythropoiesis and pleckstrin-2 knockout leads to embryonic lethality in ß-thalassemic mice. In addition, the membrane-associated active form of pleckstrin-2 occurs at an earlier stage during ß-thalassemic erythropoiesis. Furthermore, membrane-associated activated pleckstrin-2 decreases cofilin mitochondrial localization in ß-thalassemic erythroblasts and pleckstrin-2 knockdown in vitro induces cofilin-mediated apoptosis in ß-thalassemic erythroblasts. Lastly, pleckstrin-2 enhances enucleation by interacting with and activating RacGTPases in ß-thalassemic erythroblasts. This data elucidates the important compensatory role of pleckstrin-2 in ß-thalassemia and provides support for the development of targeted therapeutics in diseases of ineffective erythropoiesis.

Dysregulated iron metabolism in polycythemia vera: etiology and consequences.

Polycythemia vera (PV) is a chronic myeloproliferative neoplasm. Virtually all PV patients are iron deficient at presentation and/or during the course of their disease. The co-existence of iron deficiency and polycythemia presents a physiological disconnect. Hepcidin, the master regulator of iron metabolism, is regulated by circulating iron levels, erythroblast secretion of erythroferrone, and inflammation. Both decreased circulating iron and increased erythroferrone levels, which occur as a consequence of erythroid hyperplasia in PV, are anticipated to suppress hepcidin and enable recovery from iron deficiency. Inflammation which accompanies PV is likely to counteract hepcidin suppression, but the relatively low serum ferritin levels observed suggest that inflammation is not a major contributor to the dysregulated iron metabolism. Furthermore, potential defects in iron absorption, aberrant hypoxia sensing and signaling, and frequency of bleeding to account for iron deficiency in PV patients have not been fully elucidated. Insufficiently suppressed hepcidin given the degree of iron deficiency in PV patients strongly suggests that disordered iron metabolism is an important component of the pathobiology of PV. Normalization of hematocrit levels using therapeutic phlebotomy is the most common approach for reducing the incidence of thrombotic complications, a therapy which exacerbates iron deficiency, contributing to a variety of non-hematological symptoms. The use of cytoreductive therapy in high-risk PV patients frequently works more effectively to reverse PV-associated symptoms in iron-deficient relative to iron-replete patients. Lastly, differences in iron-related parameters between PV patients and mice with JAK2 V617F and JAK2 exon 12 mutations suggest that specific regions in JAK2 may influence iron metabolism by nuanced changes of erythropoietin receptor signaling. In this review, we comprehensively discuss the clinical consequences of iron deficiency in PV, provide a framework for understanding the potential dysregulation of iron metabolism, and present a rationale for additional therapeutic options for iron-deficient PV patients.