• Endothelial Bmp6 conditional knockout mice exhibit hemochromatosis, whereas hepatocyte and macrophage Bmp6 conditional knockout mice do not.• Our data support a model in which EC Bmp6 has paracrine actions on hepatocyte hemojuvelin to regulate hepcidin production.Bone morphogenetic protein 6 (BMP6) signaling in hepatocytes is a central transcriptional regulator of the iron hormone hepcidin that controls systemic iron balance. How iron levels are sensed to regulate hepcidin production is not known, but local induction of liver BMP6 expression by iron is proposed to have a critical role. To identify the cellular source of BMP6 responsible for hepcidin and iron homeostasis regulation, we generated mice with tissue-specific ablation of Bmp6 in different liver cell populations and evaluated their iron phenotype. Efficiency and specificity of Cre-mediated recombination was assessed by using Cre-reporter mice, polymerase chain reaction of genomic DNA, and quantitation of Bmp6 messenger RNA expression from isolated liver cell populations. Localization of the BMP co-receptor hemojuvelin was visualized by immunofluorescence microscopy. Analysis of the Bmp6 conditional knockout mice revealed that liver endothelial cells (ECs) expressed Bmp6, whereas resident liver macrophages (Kupffer cells) and hepatocytes did not. Loss of Bmp6 in ECs recapitulated the hemochromatosis phenotype of global Bmp6 knockout mice, whereas hepatocyte and macrophage Bmp6 conditional knockout mice exhibited no iron phenotype. Hemojuvelin was localized on the hepatocyte sinusoidal membrane immediately adjacent to Bmp6-producing sinusoidal ECs. Together, these data demonstrate that ECs are the predominant source of BMP6 in the liver and support a model in which EC BMP6 has paracrine actions on hepatocyte hemojuvelin to regulate hepcidin transcription and maintain systemic iron homeostasis. (Blood. 2017;129(4):405-414) IntroductionIron homeostasis is tightly controlled to ensure sufficient iron for erythropoiesis and other fundamental metabolic processes and to prevent the toxicity of excess iron.1 The peptide hormone hepcidin is a master regulator of systemic iron balance by mediating the degradation of the iron export protein ferroportin to limit dietary iron absorption and macrophage iron recycling.2 Hepcidin production in the liver is carefully titrated in response to changes in organismal iron content and iron utilization requirements.1 Failure to appropriately induce hepcidin in response to iron is a central feature of hereditary hemochromatosis, a genetic disorder characterized by excess iron deposits in the liver, heart, and endocrine glands, resulting in organ dysfunction. 3 The most severe juvenile-onset form of this disease is caused by mutations in the genes encoding hepcidin itself (HAMP) or hemojuvelin (HFE2, also known as HJV), which regulates hepcidin transcription in hepatocytes. 4,5 Mice with global or hepatocyte-specific deletion of these genes have a similar iron overload phenotype, 6-11 confirming the essential role of hepat...

The oral-aboral (OA) axis in the sea urchin is specified by the TGFβ family members Nodal and BMP2/4. Nodal promotes oral specification, whereas BMP2/4, despite being expressed in the oral territory, is required for aboral specification. This study explores the role of Chordin (Chd) during sea urchin embryogenesis. Chd is a secreted BMP inhibitor that plays an important role in axial and neural specification and patterning in Drosophila and vertebrate embryos. In L. variegatus embryos, Chd and BMP2/4 are functionally antagonistic. Both are expressed in overlapping domains in the oral territory prior to and during gastrulation. Perturbation shows that, surprisingly, Chd is not involved in OA axis specification. Instead, Chd is required both for normal patterning of the ciliary band at the OA boundary and for development of synaptotagmin B-positive (synB) neurons in a manner that is reciprocal with BMP2/4. Chd expression and synB-positive neural development are both downstream from p38 MAPK and Nodal, but not Goosecoid. These data are summarized in a model for synB neural development.

Anemia suppresses liver hepcidin expression to supply adequate iron for erythropoiesis. Erythroferrone mediates hepcidin suppression by anemia, but its mechanism of action remains uncertain. The bone morphogenetic protein (BMP)-SMAD signaling pathway has a central role in hepcidin transcriptional regulation. Here, we explored the contribution of individual receptor-activated SMADs in hepcidin regulation and their involvement in erythroferrone suppression of hepcidin. In Hep3B cells, or but not knockdown inhibited hepcidin () messenger RNA (mRNA) expression. Hepatocyte-specific double-knockout mice exhibited ∼90% transferrin saturation and massive liver iron overload, whereas mice or female mice with 1 functional or allele had modestly increased serum and liver iron, and single-knockout or mice had minimal to no iron loading, suggesting a gene dosage effect. mRNA was reduced in all Cre mouse livers at 12 days and in all Cre primary hepatocytes. However, only double-knockout mice continued to exhibit low liver at 8 weeks and failed to induce in response to Bmp6 in primary hepatocyte cultures. Epoetin alfa (EPO) robustly induced bone marrow erythroferrone () mRNA in control and mice but suppressed hepcidin only in control mice. Likewise, erythroferrone failed to decrease mRNA in primary hepatocytes and/ knockdown Hep3B cells. EPO and erythroferrone reduced liver Smad1/5 phosphorylation in parallel with mRNA in control mice and Hep3B cells. Thus, and have overlapping functions to govern hepcidin transcription. Moreover, erythropoietin and erythroferrone target Smad1/5 signaling and require Smad1/5 to suppress hepcidin expression.