Ferroportin1 is required for normal iron cycling in zebrafish

Identification and expression analysis of hepcidin-like cDNAs from pigeon (Columba livia)

Hepcidin is a cysteine-rich, dual-function peptide with antimicrobial activity that plays a crucial role in iron homeostasis. Here, we have identified two hepcidin-like cDNA sequences from pigeon, Columba livia. The two cDNAs consist of 295 and 380 nucleotides, respectively, and were named HP1 and HP2. Sequence alignment showed that the homology between pigeon and mammals or amphibians is higher than that of pigeon and fishes. Semi-quantitative RT-PCR analysis suggested that HP1 transcripts are highly abundant in liver, abundant in spleen, less abundant in kidney and muscle, and undetectable in brain and intestine. However, HP2 are strongly expressed in the liver, spleen, kidney and muscle, weakly in the intestine, and not in the brain. After pigeon were submitted either to lipopolysaccharide (LPS) infection or iron-dextran stimulation, the hepcidin transcript levels were analyzed by a comparative RT-PCR. The results revealed that the expression of hepatic HP1 dramatically increased at 6 h post-infection of LPS injection, then gradually declined to normal levels. HP1 mRNA in the liver was 4.5-5-fold increase compared with the control animals after one week in iron-dextran injection pigeons. Interestingly, liver HP2 expression was only significant increase in the LPS infection pigeons, and not statistical change in iron-dextran stimulation ones. All these results indicate that the two hepcidins-like may have different functions in pigeon.

Functional conservation of erythropoietin signaling in zebrafish

Erythropoietin (Epo) and its cognate receptor (EpoR) are required for maintaining adequate levels of circulating erythrocytes during embryogenesis and adulthood. Here, we report the functional characterization of the zebrafish epo and epor genes. The expression of epo and epor was evaluated by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization, revealing marked parallels between zebrafish and mammalian gene expression patterns. Examination of the hypochromic mutant, weissherbst, and adult hypoxia-treated hearts indicate that zebrafish epo expression is induced by anemia and hypoxia. Overexpression of epo mRNA resulted in severe polycythemia, characterized by a striking increase in the number of cells expressing scl, c-myb, gata1, ikaros, epor, and betae1-globin, suggesting that both the erythroid progenitor and mature erythrocyte compartments respond to epo. Morpholino-mediated knockdown of the epor caused a slight decrease in primitive and complete block of definitive erythropoiesis. Abrogation of STAT5 blocked the erythropoietic expansion by epo mRNA, consistent with a requirement for STAT5 in epo signaling. Together, the characterization of zebrafish epo and epor demonstrates the conservation of an ancient program that ensures proper red blood cell numbers during normal homeostasis and under hypoxic conditions.

Channel catfish hepcidin expression in infection and anemia

Hepcidin, originally identified as a 25 amino acid antimicrobial peptide made in the liver, is a key regulator of iron balance and recycling in humans and mice. Closely related hepcidin genes and peptides have also been identified in a number of fish species and in teleosts are thought to function as endogenous antibiotics involved in host defense against infection. Here we report the transcriptional regulation of hepcidin expression by infection and anemia in the channel catfish. Changes in hepcidin expression in catfish challenged with Edwardsiella ictaluri and in fish affected by channel catfish anemia (CCA) were measured by real time quantitative PCR. Hepcidin transcript levels in the livers were increased 4, 19, and 22-fold at 4, 24, and 48h following bacterial challenge, respectively. However, augmented hepcidin expression in the intestine and olfactory sac was detected only at 48h post-infection. Hepcidin transcript levels in the livers of catfish affected by CCA were less than 14% of that present in healthy counterparts. Hepatic hepcidin transcript levels correlated significantly with serum iron concentrations (r=0.54, p<0.05) and with the percent saturation of transferrin (r=0.63, p<0.05). Similar to mammalian hepcidins, channel catfish hepcidin is an iron-responsive gene and may also play important roles in innate host defense to infection and in iron homeostasis. Mammalian hepcidins may have evolved from an antimicrobial peptide and its structure and transcriptional regulatory mechanisms have been conserved throughout vertebrate evolution.

Hepcidin ? central regulator of iron metabolism

The knowledge about mammalian iron metabolism has advanced dramatically over the past decades. Studies of genetics, biochemistry and molecular biology allowed us the identification and characterization of many of the molecules involved in regulation of iron homeostasis. Important progresses were made after the discovery in 2000 of a small peptide--hepcidin--that has been proved to play a central role in orchestration on iron metabolism also providing a link between iron metabolism and inflammation and innate immunity. Hepcidin directly interacts with ferroportin (FPN), the only known mammalian iron exporter, which is expressed by enterocytes, macrophages and hepatocytes. The direct hepcidin-FPN interaction allows an adaptative response from the body in situations that alter normal iron homeostasis (hypoxia, anemia, iron deficiency, iron overload, and inflammation).

New insights into the regulation of iron homeostasis

Hepcidin evolves as a potent hepatocyte-derived regulator of the body's iron distribution piloting the flow of iron via, and directly binding, to the cellular iron exporter ferroportin. The hepcidin-ferroportin axis dominates the iron egress from all cellular compartments that are critical to iron homeostasis, namely placental syncytiotrophoblasts, duodenal enterocytes, hepatocytes and macrophages of the reticuloendothelial system. The gene that encodes hepcidin expression (HAMP) is subject to regulation by proinflammatory cytokines, such as IL-6 and IL-1; excessive hepcidin production explains the relative deficiency of iron during inflammatory states, eventually resulting in the anaemia of inflammation. The haemochromatosis genes HFE, TfR2 and HJV potentially facilitate the transcription of HAMP. Disruption of each of the four genes leads to a diminished hepatic release of hepcidin consistent with both a dominant role of hepcidin in hereditary haemochromatosis and an upstream regulatory role of HFE, TfR2 and HJV on HAMP expression. The engineered generation of hepcidin agonists, mimetics or antagonists could largely broaden current therapeutic strategies to redirect the flow of iron.

Use of the zebrafish system to study primitive and definitive hematopoiesis

The zebrafish (Danio rerio) has emerged as an ideal organism for the study of hematopoiesis, the process by which all the cellular elements of the blood are formed. These elements, including erythrocytes, granulocytes, monocytes, lymphocytes, and thrombocytes, are formed through complex genetic signaling pathways that are highly conserved throughout phylogeny. Large-scale forward genetic screens have identified numerous blood mutants in zebrafish, helping to elucidate specific signaling pathways important for hematopoietic stem cells (HSCs) and the various committed blood cell lineages. Here we review both primitive and definitive hematopoiesis in zebrafish, discuss various genetic methods available in the zebrafish model for studying hematopoiesis, and describe some of the zebrafish blood mutants identified to date, many of which have known human disease counterparts.

Dual function of fish hepcidin: response to experimental iron overload and bacterial infection in sea bass (Dicentrarchus labrax)

The role of hepcidin in iron metabolism regulation and bacterial infection has been the focus of recent attention. However, in spite of the growing number of hepcidin genes known from different organisms, little is known about its putative dual function in fish. The aim of this study was to characterize the sea bass hepcidin gene and to study its role in iron metabolism and infection. The novel sea bass hepcidin gene was found to be organized into two introns and three exons with several copies present in the genome. The transcript showed a constitutive low basal expression being mainly expressed in liver and encoding a putative 85 residues long peptide. Fish were submitted either to iron status modulation or bacterial infection and the hepcidin transcript levels were analysed along with a number of other parameters. Liver hepcidin expression was found to increase in both the iron-overloaded and infected fish, while in the iron-deficient fish no alteration in expression levels was detected. These results point to the evolutionary conservation of hepcidin's dual functions.

Hepcidins in amphibians and fishes: Antimicrobial peptides or iron-regulatory hormones?

Hepcidin, originally identified as a 25 amino acid peptide antibiotic produced in the liver, is a key regulator of iron balance and recycling in humans and mice. Closely related hepcidin genes and peptides also have been identified in other mammals, amphibians, and a number of fish species. We hypothesize that hepcidin, the iron-regulatory hormone in humans, may have evolved from an antimicrobial peptide in fishes. In this review we will highlight the evidence that indicates hepcidin evolved from an antimicrobial peptide to an iron-regulatory hormone in vertebrate evolution. This evidence includes the discovery of hepcidin as an antimicrobial peptide and iron-regulatory hormone, structural comparison of mammalian hepcidins and nonmammalian hepcidins, and the cellular and molecular evidence indicating that, while some fish hepcidins may serve only as antimicrobial peptides, other fish and amphibian hepcidins may function as iron regulators.

Genetic and clinical heterogeneity of ferroportin disease

Ferroportin is encoded by the SLC40A1 gene and mediates iron export from cells by interacting with hepcidin. SLC40A1 gene mutations are associated with an autosomal type of genetic iron overload described as haemochromatosis type 4, or HFE4 (Online Mendelian Inheritance in Man number 606069), or ferroportin disease. We report three families with this condition caused by novel SLC40A1 mutations. Denaturing high-performance liquid chromatography was employed to scan for the SLC40A1 gene. A D181V (A846T) mutation in exon 6 of the ferroportin gene was detected in the affected members of an Italian family and shown to have a de novo origin in a maternal germinal line. This mutation was associated with both parenchymal and reticuloendothelial iron overload in the liver, and with reduced urinary hepcidin excretion. A G80V (G543T) mutation in exon 3 was found in the affected members of an Italian family with autosomal hyperferritinaemia,. Finally, a G267D (G1104A) mutation was identified in exon 7 in a family of Chinese descent whose members presented with isolated hyperferritinaemia. Ferroportin disease represents a protean genetic condition in which the different SLC40A1 mutations appear to be responsible for phenotypic variability. This condition should be considered not only in families with autosomal iron overload or hyperferritinaemia, but also in cases of unexplained hyperferritinaemia.