ALBERT

Description: Key Points Heme oxygenase-1 levels increase during erythroid differentiation. Heme oxygenase-1 actively participates in maintaining appropriate hemoglobinization rates.

Description: Transferrin receptor 1 (Tfr1) mediates uptake of circulating transferrin-bound iron to developing erythroid cells and other cell types. Its critical physiological function is highlighted by the embryonic lethal phenotype of Tfr1-knockout (Tfrc−/−) mice and the pathologies of several tissue-specific knockouts. We generated TfrcAlb-Cre mice bearing hepatocyte-specific ablation of Tfr1 to explore implications in hepatocellular and systemic iron homeostasis. TfrcAlb-Cre mice are viable and do not display any apparent liver pathology. Nevertheless, their liver iron content (LIC) is lower compared with that of control Tfrcfl/fl littermates as a result of the reduced capacity of Tfr1-deficient hepatocytes to internalize iron from transferrin. Even though liver Hamp messenger RNA (mRNA) and serum hepcidin levels do not differ between TfrcAlb-Cre and Tfrcfl/fl mice, Hamp/LIC and hepcidin/LIC ratios are significantly higher in the former. Importantly, this is accompanied by modest hypoferremia and microcytosis, and it predisposes TfrcAlb-Cre mice to iron-deficiency anemia. TfrcAlb-Cre mice appropriately regulate Hamp expression following dietary iron manipulations or holo-transferrin injection. Holo-transferrin also triggers proper induction of Hamp mRNA, ferritin, and Tfr2 in primary TfrcAlb-Cre hepatocytes. We further show that these cells can acquire 59Fe from 59Fe-transferrin, presumably via Tfr2. We conclude that Tfr1 is redundant for basal hepatocellular iron supply but essential for fine-tuning hepcidin responses according to the iron load of hepatocytes. Our data are consistent with an inhibitory function of Tfr1 on iron signaling to hepcidin via its interaction with Hfe. Moreover, they highlight hepatocellular Tfr1 as a link between cellular and systemic iron-regulatory pathways.

Description: Key Points Apotransferrin decreases TfR1 expression and alters TfR1 trafficking to normalize enucleation in β-thalassemic erythroid precursors. Decreased TfR1 upregulates hepcidin in an iron- and ERFE-independent manner, resulting in iron-restricted β-thalassemic erythropoiesis.

Description: In erythroid cells, more than 90% of transferrin-derived iron enters mitochondria where ferrochelatase inserts Fe2+ into protoporphyrin IX. However, the path of iron from endosomes to mitochondrial ferrochelatase remains elusive. The prevailing opinion is that, after its export from endosomes, the redox-active metal spreads into the cytosol and mysteriously finds its way into mitochondria through passive diffusion. An opposing view is that the highly efficient transport of iron toward ferrochelatase in erythroid cells requires a direct interaction between transferrin-endosomes and mitochondria ("kiss-and-run" hypothesis; Ponka Blood 89:1, 1997). Despite the longevity of the prevailing opinion, experimental evidence (Zhang et al. Blood 105:368, 2005; Sheftel et al. Blood 110: 125, 2007) only supports the latter hypothesis. Using 3D live confocal imaging of reticulocytes following their incubation with MitoTracker Deep Red (MTDR) and Alexa Green Transferrin (AGTf), we have demonstrated transient endosome-mitochondria interactions. We have also documented these interactions by a novel method exploiting flow sub-cytometry to analyze reticulocyte lysates labeled with MTDR and AGTf. We have thusly identified a population of particles labeled with both fluorescent markers, representing endosomes interacting with mitochondria. FACS followed by 2D confocal microscopy confirmed the association of both organelles in the double-labeled population. In the current study, we examined whether reticulocyte mitochondria interact with transferrin (Tf) in a cell-free system. Lysates of reticulocytes previously labeled with MTDR were incubated with AGTf for various time intervals. Examination of lysates by 2D confocal microscopy revealed a time-dependent increase in the number of mitochondria in contact with fluorescent Tf. This can be prevented by the presence of excess, unlabeled Fe2-Tf, but not by albumin (Fig.1). Moreover, the addition of unlabeled Fe2-Tf to reticulocyte lysates removed AGTf from mitochondria, indicating that mitochondria from reticulocyte lysates are associated with TfR that can reversibly bind Tf. In addition, we demonstrate that endosomes containing mutated recombinant holotransferrin, which cannot release iron, remain associated with mitochondria, while endosomes containing mutated recombinant apotransferrin, which cannot bind iron, are not associated with mitochondria. Our findings indicate that endosomes containing holo-Tf promote their attachment to, and drive the detachment of apo-Tf-endosomes from, mitochondria, respectively. By co-immunoprecipitation assay (from murine eryhroleukemia [MEL] cells and reticulocytes lysates), we purified the voltage-dependent anion channel 2 (VDAC2), which is located at the outer membrane of the mitochondrion (Graham, et al. Curr Top Dev Biol. 59: 87, 2004) with DMT1. We confirmed the colocalization of VDAC2 and DMT1 in MEL cells and reticulocytes by both immunofluorescence and confocal microscopy. Moreover, we found a significant decrease in the number of mitochondria in contact with Tf-endosomes after depletion of VDAC2 in MEL cells or after treatment of reticulocyte lysates with the mitochondrial uncoupler CCCP, further supporting the concept of a physical interaction between endosomes and mitochondria. To examine a possible role of DMT1-VDAC2 interactions in iron trafficking, we depleted MEL cells of VDAC2 or inhibited VADC2 using erastin (a specific VDAC2 inhibitor that alters its gating) followed by the measurement of 59Fe incorporation from 59Fe-Tf into heme. Our finding of decreased 59Fe incorporation into heme of MEL cells with silenced or inhibited VDAC2 supports the idea that this outer-membrane mitochondrial protein is involved in the interaction of endosomes with mitochondria. We are currently continuing to delineate the molecular mechanisms involved in endosome-mitochondria interactions focusing on the "signal(s)" that direct iron-carrying endosomes towards mitochondria, the players involved in the docking of endosomes to mitochondria and the "signal(s)" that determine the detachment of iron-free endosomes from mitochondria. Figure 1. Figure 1. Disclosures No relevant conflicts of interest to declare.

Description: β-thalassemia is an inherited blood disorder caused by reduced or absence of β-globin expression which results in imbalanced globin synthesis, ineffective erythropoiesis, and anemia. How the imbalance between α- and β-globin results in ineffective erythropoiesis, if apoptosis or dysfunctional differentiation of erythroid precursors results in ineffective erythropoiesis, and whether disrupted iron regulation and / or iron overload in β-thalassemia is directly involved in the pathophysiology of ineffective erythropoiesis is incompletely understood. Iron is critical for hemoglobin synthesis and erythropoiesis is dependent on transferrin (Tf) bound iron. Tf functions as the main iron transporter in circulation, where it exists in three forms: as iron-free apo-transferrin (apoTf), monoferric Tf, or diferric Tf (holoTf). Typically, iron is bound to 30% of all Tf binding sites in circulation at which point monoferric Tf is found in the highest concentration relative to holoTf. . We have previously shown that exogenous apoTf ameliorates anemia in a mouse model of β-thalassemia intermedia (th1/th1), resulting in reduced splenomegaly, reticulocytosis, and α-globin precipitation on circulatory red blood cells (RBC). We also observe a decrease in mean corpuscular hemoglobin (MCH) and mean corpuscular volume (MCV), serum iron, Tf saturation, together suggesting that relative iron deficiency improves iron metabolism and ineffective erythropoiesis in apoTf-treated th1/th1 mice. We hypothesize that exogenous apoTf decreases cytosolic iron and heme as a consequence of increased monoferric Tf which results in less iron entering cells via Tf:TfR1 binding. Our current data reveals that in vitro incubation of purified apoTf and holoTf at 37C results in the formation of monoferric Tf, and injection of wild-type (WT) mice with a single intraperitoneal dose of apoTf (10mg) decreases holoTf (P=0.01) and increases monoferric Tf (P=0.02) in the serum 6 hours after injection. Using both calcium mobilization and anti-Tf antibodies in flow cytometry, we demonstrate that apoTf results in no TfR1 binding relative to holoTf in CHO cells (P

Description: Thalassemias are a heterogeneous group of red blood cell disorders ranging from a clinically severe phenotype requiring life-saving transfusions (thalassemia major) to a relatively moderate symptomatic disorder, sometimes requiring transfusions (thalassemia intermedia). Thalassemia minor, the least severe form of the disorder, is characterized by minimal to mild symptoms. While thalassemia minor and intermedia are vastly more prevalent than thalassemia major, the latter is often fatal when not treated. Though considered a major cause of morbidity and mortality worldwide, there is still no universally available cure for this severe form of thalassemia. A reason for this is at least in part due to the lack of full understanding of pathophsyiology of thalassemia. The underlying cause of pathology in thalassemia is the premature apoptotic destruction of erythroblasts causing ineffective erythropoeisis. Normally, the assembly of adult hemoglobin (consisting of a tetramer of two α- and two β-globin chains) features a very tight coordination of α- and β-globin chain synthesis. However, in β-thalassemia, β-globin synthesis is decelerated causing α-globin accumulation; while in α-thalassemia the opposite scenario occurs. Unpaired globin chains that accumulate in thalassemic erythroblasts are bound to heme. In addition, in β-thalassemia an erythroid specific protease destroys excess α-globin chains, likely leading to the generation of a pool of “free” heme in erythroblasts. “Free” heme is toxic, but this toxicity will likely be augmented, if heme oxygenase 1 (HO-1) can release iron from heme. To date, virtually no information about the expression of HO-1 in erythroblasts has been produced; however, we have recently provided unequivocal evidence that this enzyme is present in several model erythroid cells1. Based on this novel and important finding, we hypothesize that in β-thalassemic erythroblasts HO-1 mediated release of iron from heme is the major culprit responsible for cellular damage. To test this hypothesis we exploited the mouse model of β-thalassemia, th3/th3. Thus far, our data indicates that HO-1 expression is increased in liver, spleen and kidney of β-thalassemic mice compared to wild type mice. Importantly, we observed that Epo-mediated erythroid differentiation of fetal liver (FL) cells isolated from β-thalassemic fetuses, display increased levels of HO-1 at mRNA and protein levels as well as decreased phosphorylated eiF2-α. Ferritin levels are also increased in these cells suggesting increased heme catabolism and iron release. Altogether, these results indicate that β-thalassemic erythroblasts have inappropriately high levels of unbound heme that is continuously degraded by HO-1. Further research is needed to determine whether HO-1 liberated iron is responsible for the damage of β-thalassemic erythroblasts. 1Garcia-Santos D, et al. Heme oxygenase 1 is expressed in murine erythroid cells where it controls the level of regulatory heme. Blood 123 (14): 2269-77, 2014. Disclosures No relevant conflicts of interest to declare.

Description: Thalassemias are a heterogeneous group of red blood cell (RBC) disorders ranging from a clinically severe phenotype requiring lifesaving transfusions (thalassemia major) to a relatively moderate symptomatic disorder, sometimes requiring transfusions (thalassemia intermedia). Though considered a major cause of morbidity and mortality worldwide, there is still no universally available cure for thalassemia major. The reason for this is, at least in part, due to the lack of full understanding of pathophysiology of thalassemia. The underlying basis of thalassemia pathology is the premature apoptotic destruction of erythroblasts causing ineffective erythropoeisis. In β-thalassemia, β-globin synthesis is diminished causing α-globin accumulation. Unpaired globin chains that accumulate in thalassemic erythroblasts are bound to heme. Moreover, in β-thalassemia an erythroid-specific protease destroys excess α-globin chains, likely leading to the generation of a pool of "free" heme in erythroblasts. Physiologically, heme can be degraded only via heme oxygenases (HO). Circulating erythrocytes contain the majority of heme destined for catabolism; this process takes place primarily in splenic and hepatic macrophages following erythrophagocytosis of senescent RBC. Heme oxygenase, in particular its heme-inducible isoform HO1, has been extensively studied in hepatocytes and many other non-erythroid cells. Recently, we have provided unequivocal evidence that this enzyme is present in erythroid progenitors as well as their differentiated progenies.1 "Unshielded" heme is toxic, but this toxicity will likely be augmented, if HO1 releases iron from heme. We hypothesize that in β-thalassemic erythroblasts HO1-mediated release of iron from heme is the major culprit responsible for cellular damage. Additionally, it has been shown that prevention of heme-derived iron release from splenic and hepatic macrophages improves β-thalassemia phenotype2. Therefore, suppression of HO1-mediated heme catabolism from senescent RBC could be beneficial in reversing thalassemic phenotype. To test this hypothesis, we exploited the mouse model of β-thalassemia known as th3/th3; we obtained these mice from Dr. Stefano Rivella. Our data indicates that HO1 expression is increased in the liver of β-thalassemic mice as compared to wild type mice. Importantly, we observed that erythropoietin-mediated erythroid differentiation of fetal liver (FL) cells from β-thalassemic fetuses increased HO1 mRNA and protein levels to a higher degree than in their wild type counterparts. Ferritin levels were increased in β-thalassemic FL cells suggesting increased heme catabolism and iron release from the tetrapyrrole macrocycle. To investigate the contribution of HO1 to the pathology associated with β-thalassemia, wild type and thalassemic (th3/+) mice were injected intraperitoneally with 40 µmoles/kg/d of tin-protoporphyrin IX (SnPP, HO inhibitor) during a 4-weeks, 3-times a week. Our results show that β-thalassemic mice injected with SnPP have increased hemoglobin levels and red blood cell counts, and display a decrease in the spleen index, reticulocyte counts and liver iron content when compared to PBS-injected β-thalassemic mice. Furthermore, while hepcidin levels remain unchanged, liver ferroportin expression decreases in SnPP-injected β-thalassemic mice. Our results indicate that β-thalassemic erythroblasts have high levels of HO1, which would be expected to degrade any "free" heme. Further research is needed to determine whether iron liberated from heme by HO1 is directly responsible for the damage of β-thalassemic erythroblasts. 1GarciaSantos D, et al. Heme oxygenase 1 is expressed in murine erythroid cells where it controls the level of regulatory heme. Blood 123 (14): 226977, 2014. 2Nai A, et al. Deletion of TMPRSS6 attenuates the phenotype in a mouse model of β-thalassemia. Blood 119 (21): 5021, 2012. Disclosures No relevant conflicts of interest to declare.

Description: Inactivating mutations in divalent metal transporter 1 (DMT1) are associated with a severe defect in erythroid iron utilization and cause moderate to severe hypochromic microcytic anemia in human patients and two rodent models. We have previously shown that DMT1 deficiency impairs erythroid differentiation, induces apoptosis of erythroid precursors and causes the suppression of colony-forming capacity of erythroid progenitors. Using in vitro cultures of fetal liver cells we were able to recapitulate this in vivo defect. We confirmed abnormal pattern of erythroid differentiation and increased apoptosis (2.5-times) of DMT1-mutant erythroblasts when compared to wild-type (wt) fetal liver erythroblats. Determination of 2’,7’-Dichlorofluorescein diacetate-dependent intensity of fluorescence, which is proportional to the concentration of reactive oxygen species (ROS), revealed elevated levels of ROS in DMT1-mutant erythroblats when compared to wt erythroblast. This result suggests that oxidative stress contributes to the apoptosis in DMT1-mutant cells. We also observed that the defective erythroid differentiation of DMT1-mutant erythroblasts is marked by a blunted induction of heme oxygenase-1, an enzyme that co-regulates erythroid differentiation by controlling the heme regulatory pool in erythroid cells (Garcia-Santos et al., Blood, 2014, 123 (14): 2269-77). In further studies we focused on mature red blood cells (RBC), because it is known that nutritional iron deficiency and certain types of congenital hypochromic anemia are associated with increased levels of ROS and shortened life span of RBC that can be at least partially attributed to a programmed cell death of erythrocytes, so called eryptosis (Lang et al., Int J Biochem Cell Biol, 2012, 44 (8): 1236-43). Using labeling with carboxyfluorescein diacetate succinimidyl ester, we observed an accelerated clearance of DMT1-mutant RBC from circulating blood when compared to wild-type RBC. In vitro, DMT1-mutant RBC exposed to hyperosmotic shock or glucose depletion showed significantly increased levels of phosphatidylserine on the membrane detected by Annexin V binding. Together, these results confirmed eryptosis of DMT1-mutant RBC. As eryptosis is proposed to be triggered via activation of Ca2+ cation channels, we next measured the concentration of cytosolic Ca2+ using Fluo3/AM fluorescent dye and found significantly elevated content of intracellular Ca2+ in DMT1-mutant RBC when compared to wt RBC. In addition, DMT1-mutant RBC had higher levels of ROS than wt RBC despite significantly increased activity of anti-oxidative defense enzymes; glutathione peroxidase (1.6-times), catalase (1.9-times) and methemoglobin reductase (1.9-times). This indicates that exaggerated anti-oxidative defense in DMT1-mutant RBC is not sufficient to eliminate ROS effectively. Furthermore, DMT1-mutant RBC also showed accelerated anaerobic glycolysis as detected by increased activities of hexokinase (2.5-times), pyruvate kinase (2.4-times), glucose-phosphate isomerase (3.2-times). This result together with reduced ATP/ADP (1.6-times) ratio in DMT1-mutant RBC when compared to wt RBC suggests an increased demand for ATP in DMT1-mutant erythrocytes. In conclusion we propose that increased oxidative stress and accelerated destruction of RBC contribute to the pathophysiology of anemia caused by DMT1-deficiency. Grant support: Czech Grant Agency, grant No. P305/11/1745; Ministry of Health Czech Republic, grant No. NT13587, Education for Competitiveness Operational Program, CZ.1.07/2.3.00/20.0164, Internal Grant of Palacky University Olomouc, LF_2014_011 and in part by the Canadian Institutes of Health Research (D.G-S., P.P.). Disclosures No relevant conflicts of interest to declare.