Carlo Brugnara

Positional cloning of zebrafish ferroportin1 identifies a conservedvertebrate iron exporter

Defects in iron absorption and utilization lead to iron deficiency and overload disorders. Adult mammals absorb iron through the duodenum, whereas embryos obtain iron through placental transport. Iron uptake from the intestinal lumen through the apical surface of polarized duodenal enterocytes is mediated by the divalent metal transporter, DMT1 (refs 1,2,3). A second transporter has been postulated to export iron across the basolateral surface to the circulation. Here we have used positional cloning to identify the gene responsible for the hypochromic anaemia of the zebrafish mutant weissherbst. The gene, ferroportin1, encodes a multiple-transmembrane domain protein, expressed in the yolk sac, that is a candidate for the elusive iron exporter. Zebrafish ferroportin1 is required for the transport of iron from maternally derived yolk stores to the circulation and functions as an iron exporter when expressed in Xenopus oocytes. Human Ferroportin1 is found at the basal surface of placental syncytiotrophoblasts, suggesting that it also transports iron from mother to embryo. Mammalian Ferroportin1 is expressed at the basolateral surface of duodenal enterocytes and could export cellular iron into the circulation. We propose that Ferroportin1 function may be perturbed in mammalian disorders of iron deficiency or overload.

Fetal Anemia and Apoptosis of Red Cell Progenitors in Stat5a−/−5b−/− Mice: A Direct Role for Stat5 in Bcl-XL Induction

The erythropoietin receptor (EpoR) is essential for production of red blood cells; a principal function of EpoR is to rescue committed erythroid progenitors from apoptosis. Stat5 is rapidly activated following EpoR stimulation, but its function in erythropoiesis has been unclear since adult Stat5a-/-5b-/- mice have normal steady-state hematocrit. Here we show that Stat5 is essential for the high erythropoietic rate during fetal development. Stat5a-/-5b-/- embryos are severely anemic; erythroid progenitors are present in low numbers, show higher levels of apoptosis, and are less responsive to Epo. These findings are explained by a crucial role for Stat5 in EpoRs antiapoptotic signaling: it mediates the immediate-early induction of Bcl-X(L) in erythroid cells through direct binding to the Bcl-X promoter.

Iron Deficiency and Erythropoiesis: New Diagnostic Approaches

Iron deficiency anemia is one of the most common diseases worldwide. In the majority of cases, the presence of hypochromic microcytic anemia and biochemical evidence for depletion of body iron stores makes the diagnosis relatively straightforward. However, in several clinical conditions, classic biochemical indices such as serum iron, transferrin saturation, and ferritin may not be informative or may not change rapidly enough to reflect transient iron-deficient states (functional iron deficiency), such as the ones that develop during recombinant human erythropoietin (r-HuEPO) therapy. The identification and treatment of iron deficiency in settings such as r-HuEPO therapy, anemia of chronic disease, and iron deficiency of early childhood may be improved by the use of red cell and reticulocyte cellular indices, which reflect in almost real time the development of iron deficiency and the response to iron therapy. In the anemia of chronic disease, measurements of plasma cytokines and iron metabolism regulators such as hepcidin (when available) may be helpful in the characterization of the pathophysiologic basis of this condition. The ratio of serum transferrin receptor (sTfR) to serum ferritin (R/F ratio) has been shown to have excellent performance in estimating body iron stores, but it cannot be used widely because of the lack of standardization for sTfR assays. The combination of hematologic markers such as reticulocyte hemoglobin content, which decreases with iron deficiency, and R/F ratio may allow for a more precise classification of anemias.

Inhibition of Ca(2+)-dependent K+ transport and cell dehydration in sickle erythrocytes by clotrimazole and other imidazole derivatives.

We have investigated the interaction of clotrimazole (CLT) and related compounds with the erythroid Ca(2+)-activated K+ channel, a mediator of sickle cell dehydration. We measured K+ transport, membrane potential, and cell volume upon activation of this pathway in sickle erythrocytes. CLT blocked almost completely Ca(2+)-activated K+ transport in homozygous hemoglobin S cells, with IC50 values of 29 +/- 15 nM in isotonic 20 mM salt solution and 51 +/- 15 nM in normal saline (n = 3). The inhibition of K+ transport by CLT was caused by a specific interaction with the Ca(2+)-activated K+ channel of human red cells, since it displaced bound 125I-Charybdotoxin, a specific ligand of the Gardos channel, with an IC50 (12 +/- 4 nM in isotonic 20 mM) similar to the IC50 values for flux inhibition. When homozygous hemoglobin S cells were dehydrated by incubation in the presence of 100 microM CaCl2 and the ionophore A23187, or by exposure to cycles of oxygenation and deoxygenation, CLT effectively inhibited cell dehydration and K+ loss. The IC50 of CLT for inhibition of Ca(2+)-activated K+ transport in sickle cells is significantly lower than plasma concentrations of CLT achievable after nontoxic oral doses. We therefore propose that oral administration of CLT may prevent red cell dehydration in patients with sickle cell anemia.

ANION EXCHANGER 1 (BAND 3) IS REQUIRED TO PREVENT ERYTHROCYTE MEMBRANE SURFACE LOSS BUT NOT TO FORM THE MEMBRANE SKELETON

The red blood cell (RBC) membrane protein AE1 provides high affinity binding sites for the membrane skeleton, a structure critical to RBC integrity. AE1 biosynthesis is postulated to be required for terminal erythropoiesis and membrane skeleton assembly. We used targeted mutagenesis to assess AE1 function in vivo. RBCs lacking AE1 spontaneously shed membrane vesicles and tubules, leading to severe spherocytosis and hemolysis, but the levels of the major skeleton components, the synthesis of spectrin in mutant erythroblasts, and skeletal architecture are normal or nearly normal. The results indicate that AE1 does not regulate RBC membrane skeleton assembly in vivo but is essential for membrane stability. We postulate that stabilization is achieved through AE1-lipid interactions and that loss of these interactions is a key pathogenic event in hereditary spherocytosis.

Positional cloning of the zebrafish sauternes gene: a model for congenital sideroblastic anaemia.

Many human anaemias are caused by defects in haemoglobin synthesis. The zebrafish mutant sauternes (sau) has a microcytic, hypochromic anaemia, suggesting that haemoglobin production is perturbed. During embryogenesis, sau mutants have delayed erythroid maturation and abnormal globin gene expression. Using positional cloning techniques, we show that sau encodes the erythroid-specific isoform of δ-aminolevulinate synthase (ALAS2; also known as ALAS-E), the enzyme required for the first step in haem biosynthesis. As mutations in ALAS2 cause congenital sideroblastic anaemia (CSA) in humans, sau represents the first animal model of this disease.

Foxo3 is required for the regulation of oxidative stress in erythropoiesis

Erythroid cells accumulate hemoglobin as they mature and as a result are highly prone to oxidative damage. However, mechanisms of transcriptional control of antioxidant defense in erythroid cells have thus far been poorly characterized. We observed that animals deficient in the forkhead box O3 (Foxo3) transcription factor died rapidly when exposed to erythroid oxidative stress-induced conditions, while wild-type mice showed no decreased viability. In view of this striking finding, we investigated the potential role of Foxo3 in the regulation of ROS in erythropoiesis. Foxo3 expression, nuclear localization, and transcriptional activity were all enhanced during normal erythroid cell maturation. Foxo3-deficient erythrocytes exhibited decreased expression of ROS scavenging enzymes and had a ROS-mediated shortened lifespan and evidence of oxidative damage. Furthermore, loss of Foxo3 induced mitotic arrest in erythroid precursor cells, leading to a significant decrease in the rate of in vivo erythroid maturation. We identified ROS-mediated upregulation of p21(CIP1/WAF1/Sdi1) (also known as Cdkn1a) as a major contributor to the interference with cell cycle progression in Foxo3-deficient erythroid precursor cells. These findings establish an essential nonredundant function for Foxo3 in the regulation of oxidative stress, cell cycle, maturation, and lifespan of erythroid cells. These results may have an impact on the understanding of human disorders in which ROS play a role.

Therapy with oral clotrimazole induces inhibition of the Gardos channel and reduction of erythrocyte dehydration in patients with sickle cell disease.

Pathologic water loss from sickle erythrocytes concentrates the abnormal hemoglobin and promotes sickling. The Ca2+-activated K+ channel (Gardos channel) contributes to this deleterious dehydration in vitro, and blockade of K+ and water loss via this channel could be a potential therapy in vivo. We treated five subjects who have sickle cell anemia with oral clotrimazole, a specific Gardos channel inhibitor. Patients were started on a dose of 10 mg clotrimazole/kg/d for one week. Protocol design allowed the daily dose to be escalated by 10 mg/kg each week until significant changes in erythrocyte density and K+ transport were achieved. Blood was sampled three times a week for hematological and chemical assays, erythrocyte density, cation content, and K+ transport. At dosages of 20 mg clotrimazole/kg/d, all subjects showed Gardos channel inhibition, reduced erythrocyte dehydration, increased cell K+ content, and somewhat increased hemoglobin levels. Adverse effects were limited to mild/moderate dysuria in all subjects, and a reversible increase in plasma alanine transaminase and aspartic transaminase levels in two subjects treated with 30 mg clotrimazole/kg/d. This is the first in vivo evidence that the Gardos channel causes dehydration of sickle erythrocytes, and that its pharmacologic inhibition provides a realistic antisickling strategy.

cDNA Cloning and Functional Characterization of the Mouse Ca2+-gated K+ Channel, mIK1 ROLES IN REGULATORY VOLUME DECREASE AND ERYTHROID DIFFERENTIATION

We have cloned from murine erythroleukemia (MEL) cells, thymus, and stomach the cDNA encoding the Ca2+-gated K+ (KCa) channel, mIK1, the mouse homolog of hIK1 (Ishii, T. M., Silvia, C., Hirschberg, B., Bond, C. T., Adelman, J. P., and Maylie, J. (1997) Proc. Natl. Acad. Sci.(U. S. A. 94, 11651–11656). mIK1 mRNA was detected at varied levels in many tissue types. mIK1 KCa channel activity expressed inXenopus oocytes closely resembled the Kca of red cells (Gardos channel) and MEL cells in its single channel conductance, lack of voltage-sensitivity of activation, inward rectification, and Ca2+ concentration dependence. mIK1 also resembled the erythroid channel in its pharmacological properties, mediating whole cell and unitary currents sensitive to low nm concentrations of both clotrimazole (CLT) and its des-imidazolyl metabolite, 2-chlorophenyl-bisphenyl-methanol, and to low nm concentrations of iodocharybdotoxin. Whereas control oocytes subjected to hypotonic swelling remained swollen, mIK1 expression conferred on oocytes a novel, Ca2+-dependent, CLT-sensitive regulatory volume decrease response. Hypotonic swelling of voltage-clamped mIK1-expressing oocytes increased outward currents that were Ca2+-dependent, CLT-sensitive, and reversed near the K+ equilibrium potential. mIK1 mRNA levels in ES cells increased steadily during erythroid differentiation in culture, in contrast to other KCa mRNAs examined. Low nanomolar concentrations of CLT inhibited proliferation and erythroid differentiation of peripheral blood stem cells in liquid culture.

E2F4 Is Essential for Normal Erythrocyte Maturation and Neonatal Viability

The retinoblastoma protein (pRB) plays a key role in the control of normal development and proliferation through the regulation of the E2F transcription factors. We generated a mutant mouse model to assess the in vivo role of the predominant E2F family member, E2F4. Remarkably, loss of E2F4 had no detectable effect on either cell cycle arrest or proliferation. However, E2F4 was essential for normal development. E2f4-/- mice died of an increased susceptibility to opportunistic infections that appeared to result from craniofacial defects. They also displayed a variety of erythroid abnormalities that arose from a cell autonomous defect in late stage maturation. This suggests that E2F4 makes a major contribution to the control of erythrocyte development by the pRB tumor suppressor.