Key Points ERFE suppresses BMP/SMAD signaling in vitro and in vivo. ERFE inhibits hepcidin induction by BMP5, BMP6, and BMP7.
During hindbrain development, facial branchiomotor neurons (FBM neurons) migrate from medial rhombomere (r) 4 to lateral r6. In zebrafish, mutations in planar cell polarity genes celsr2 and frizzled3a block caudal migration of FBM neurons. Here, we investigated the role of cadherins Celsr1-3, and Fzd3 in FBM neuron migration in mice. In Celsr1 mutants (knock-out and Crash alleles), caudal migration was compromised and neurons often migrated rostrally into r2 and r3, as well as laterally. These phenotypes were not caused by defects in hindbrain patterning or neuronal specification. Celsr1 is expressed in FBM neuron precursors and the floor plate, but not in FBM neurons. Consistent with this, conditional inactivation showed that the function of Celsr1 in FBM neuron migration was non-cell autonomous. In Celsr2 mutants, FBM neurons initiated caudal migration but moved prematurely into lateral r4 and r5. This phenotype was enhanced by inactivation of Celsr3 in FBM neurons and mimicked by inactivation of Fzd3. Furthermore, Celsr2 was epistatic to Celsr1. These data indicate that Celsr1-3 differentially regulate FBM neuron migration. Celsr1 helps to specify the direction of FBM neuron migration, whereas Celsr2 and 3 control its ability to migrate.
During development, facial branchiomotor (FBM) neurons, which innervate muscles in the vertebrate head, migrate caudally and radially within the brainstem to form a motor nucleus at the pial surface. Several components of the Wnt/planar cell polarity (PCP) pathway, including the transmembrane protein Vangl2, regulate caudal migration of FBM neurons in zebrafish, but their roles in neuronal migration in mouse have not been investigated in detail. Therefore, we analyzed FBM neuron migration in mouse looptail (Lp) mutants, in which Vangl2 is inactivated. In Vangl2 Lp/+ and Vangl2 Lp/Lp embryos, FBM neurons failed to migrate caudally from rhombomere (r) 4 into r6. Although caudal migration was largely blocked, many FBM neurons underwent normal radial migration to the pial surface of the neural tube. In addition, hindbrain patterning and FBM progenitor specification were intact, and FBM neurons did not transfate into other non-migratory neuron types, indicating a specific effect on caudal migration. Since loss-of-function in some zebrafish Wnt/PCP genes does not affect caudal migration of FBM neurons, we tested whether this was also the case in mouse. Embryos null for Ptk7, a regulator of PCP signaling, had severe defects in caudal migration of FBM neurons. However, FBM neurons migrated normally in Dishevelled (Dvl) 1/2 double mutants, and in zebrafish embryos with disrupted Dvl signaling, suggesting that Dvl function is essentially dispensable for FBM neuron caudal migration. Consistent with this, loss of Dvl2 function in Vangl2 Lp/+ embryos did not exacerbate the Vangl2 Lp/+ neuronal migration phenotype. These data indicate that caudal migration of FBM neurons is regulated by multiple components of the Wnt/PCP pathway, but, importantly, may not require Dishevelled function. Interestingly, genetic-interaction experiments suggest that rostral FBM neuron migration, which is normally suppressed, depends upon Dvl function.
Interactions between a neuron and its environment play a major role in neuronal migration. We show here that the cell adhesion molecule Transient Axonal Glycoprotein (Tag1) is necessary for the migration of the facial branchiomotor neurons (FBMNs) in the zebrafish hindbrain. In tag1 morphant embryos, FBMN migration is specifically blocked, with no effect on organization or patterning of other hindbrain neurons. Furthermore, using suboptimal morpholino doses and genetic mutants, we found that tag1, lamininα1 (lama1) and stbm, which encodes a transmembrane protein Vangl2, exhibit pairwise genetic interactions for FBMN migration. Using time-lapse analyses, we found that FBMNs are affected similarly in all three single morphant embryos, with an inability to extend protrusions in a specific direction, and resulting in the failure of caudal migration. These data suggest that tag1, lama1 and vangl2 participate in a common mechanism that integrates signaling between the FBMN and its environment to regulate migration.
Decreased hepcidin mobilizes iron, which facilitates erythropoiesis, but excess iron is pathogenic in beta-thalassemia and other iron-loading anaemias. Erythropoietin (EPO) enhances erythroferrone (ERFE) synthesis by erythroblasts, and ERFE suppresses hepatic hepcidin production, through an unknown mechanism. The BMP/SMAD pathway in the liver is critical for control of hepcidin, and we show that EPO suppressed hepcidin and other BMP target genes in vivo in a partially ERFE-dependent manner. Furthermore, recombinant ERFE suppressed the hepatic BMP/SMAD pathway independently of changes in serum and liver iron, and in vitro, ERFE decreased SMAD 1/5/8 phosphorylation and inhibited expression of BMP target genes in hepatoma cells. ERFE specifically abrogated the induction of hepcidin by BMP5, BMP6 and BMP7, but had no or very little effect on hepcidin induction by BMP2, 4, 9 or Activin B. A neutralising anti-ERFE antibody prevented the ability of ERFE to inhibit hepcidin induction by BMP5, 6 and 7. Cell-free Homogeneous Time Resolved Fluorescence assays showed that BMP5, BMP6 and BMP7 competed with anti-ERFE for binding to ERFE. Biacore analysis showed that ERFE binds to BMP6 with a higher affinity compared to its binding to BMP2, BMP4 or Activin B, and does not bind to GDF15. We propose that ERFE suppresses hepcidin by inhibiting hepatic BMP/SMAD signaling via preferentially binding and impairing the function of an evolutionarily closely related BMP sub-group consisting of BMP5, BMP6 and BMP7. These findings indicate that ERFE can act as a natural ligand trap generated by stimulated erythropoiesis in order to regulate availability of iron. Disclosures Arezes: Pfizer: Research Funding. Foy:Pfizer: Employment. McHugh:Pfizer: Research Funding. Sawant:Pfizer: Employment. Benard:Pfizer: Employment. Quinkert:Pfizer: Research Funding. Terraube:Pfizer: Employment. Brinth:Pfizer: Employment. Tam:Pfizer: Employment. LaVallie:Pfizer: Employment. Cunningham:Pfizer: Employment. Lambert:Pfizer: Employment. Draper:Pfizer: Research Funding. Jasuja:Pfizer: Employment. Drakesmith:La Jolla Pharmaceutical Company: Research Funding; Pfizer: Research Funding; Alnylam: Consultancy; Kymab: Membership on an entity's Board of Directors or advisory committees.
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