Background: Processing of pre-miRNAs by Dicer is essential for miRNA biogenesis. Results: The nuclear-cytoplasmic dynamics of TAZ and YAP control Dicer levels and activity through regulation of the LIN28/Let-7 axis. Conclusion: The Hippo pathway effectors TAZ and YAP control miRNA biogenesis. Significance: Our work provides crucial insight into the poorly understood signaling mechanisms controlling miRNA biogenesis.

During vertebrate craniofacial development, neural crest cells (NCCs) contribute to most of the craniofacial pharyngeal skeleton. Defects in NCC specification, migration and differentiation resulting in malformations in the craniofacial complex are associated with human craniofacial disorders including Treacher-Collins Syndrome, caused by mutations in TCOF1. It has been hypothesized that perturbed ribosome biogenesis and resulting p53 mediated neuroepithelial apoptosis results in NCC hypoplasia in mouse Tcof1 mutants. However, the underlying mechanisms linking ribosome biogenesis and NCC development remain poorly understood. Here we report a new zebrafish mutant, fantome (fan), which harbors a point mutation and predicted premature stop codon in zebrafish wdr43, the ortholog to yeast UTP5. Although wdr43 mRNA is widely expressed during early zebrafish development, and its deficiency triggers early neural, eye, heart and pharyngeal arch defects, later defects appear fairly restricted to NCC derived craniofacial cartilages. Here we show that the C-terminus of Wdr43, which is absent in fan mutant protein, is both necessary and sufficient to mediate its nucleolar localization and protein interactions in metazoans. We demonstrate that Wdr43 functions in ribosome biogenesis, and that defects observed in fan mutants are mediated by a p53 dependent pathway. Finally, we show that proper localization of a variety of nucleolar proteins, including TCOF1, is dependent on that of WDR43. Together, our findings provide new insight into roles for Wdr43 in development, ribosome biogenesis, and also ribosomopathy-induced craniofacial phenotypes including Treacher-Collins Syndrome.

The iron regulatory hormone hepcidin is transcriptionally up-regulated in response to iron loading, but the mechanisms by which iron levels are sensed are not well understood. Large-scale genetic screens in the zebrafish have resulted in the identification of hypochromic anemia mutants with a range of mutations affecting conserved pathways in iron metabolism and heme synthesis. We hypothesized that transferrin plays a critical role both in iron transport and in regulating hepcidin expression in zebrafish embryos. Here we report the identification and characterization of the zebrafish hypochromic anemia mutant, gavi, which exhibits transferrin deficiency due to mutations in transferrin-a. Morpholino knockdown of transferrin-a in wild-type embryos reproduced the anemia phenotype and decreased somite and terminal gut iron staining, while coinjection of transferrin-a cRNA partially restored these defects. Embryos with transferrin-a or transferrin receptor 2 (TfR2) deficiency exhibited low levels of hepcidin expression, however anemia, in the absence of a defect in the transferrin pathway, failed to impair hepcidin expression. These data indicate that transferrin-a transports iron and that hepcidin expression is regulated by a transferrin-a-dependent pathway in the zebrafish embryo. IntroductionTransferrin, a serum protein with 2 iron binding sites, is the major carrier of ferric iron in human plasma and is required to deliver iron to developing erythroid and other cells. Iron-bound holotransferrin binds transferrin receptor 1 (TfR1) on the surface of the cell, resulting in endocytosis of an endosome containing both transferrin and TfR1 (reviewed in Aisen 1 ). After acidification of the endosome, ferrous iron exits the endosome via the divalent metal transporter (DMT1). 2,3 Transferrin receptor 2 (TfR2), a homolog of TfR1, with restricted expression to hepatocytes and erythroid cells, 4 binds transferrin at lower affinity than TfR1 5,6 and is involved in the regulation of iron homeostasis.As humans have no active means of iron excretion, maintaining iron homeostasis requires adjustment of iron absorption to reflect the body's iron stores. Hepcidin is a peptide hormone, primarily expressed in the liver, 7 which modulates iron absorption and iron delivery to erythrocytes by binding the iron transporter ferroportin (fpn), resulting in internalization and degradation of fpn. 8 In mammalian models, hepcidin is transcriptionally up-regulated in response to inflammation 9,10 or iron overload 11 and down-regulated in response to anemia, iron deficiency, or hypoxia. 9 Although mutations in HFE, 12 transferrin receptor 2 (TFR2), 13 hepcidin (HAMP), 14 or hemojuvelin (HJV or HFE2) 15 have each been associated with hemochromatosis and impaired hepcidin expression, 16 the mechanisms by which iron overload is sensed and hepcidin is regulated are not completely understood.The zebrafish, Danio rerio, provides an excellent system for the identification and analysis of genes involved in iron metabolism and erythroid development. La...

Hemojuvelin (Hjv), a member of the repulsive-guidance molecule (RGM) family, upregulates transcription of the iron regulatory hormone hepcidin by activating the bone morphogenetic protein (BMP) signaling pathway in mammalian cells. Mammalian models have identified furin, neogenin, and matriptase-2 as modifiers of Hjv's function. Using the zebrafish model, we evaluated the effects of hjv and its interacting proteins on hepcidin expression during embryonic development. We found that hjv is strongly expressed in the notochord and somites of the zebrafish embryo and that morpholino knockdown of hjv impaired the development of these structures. Knockdown of hjv or other hjv-related genes, including zebrafish orthologs of furin or neogenin, however, failed to decrease hepcidin expression relative to liver size. In contrast, overexpression of bmp2b or knockdown of matriptase-2 enhanced the intensity and extent of hepcidin expression in zebrafish embryos, but this occurred in an hjv-independent manner. Furthermore, we demonstrated that zebrafish hjv can activate the human hepcidin promoter and enhance BMP responsive gene expression in vitro, but is expressed at low levels in the zebrafish embryonic liver. Taken together, these data support an alternative mechanism for hepcidin regulation during zebrafish embryonic development, which is independent of hjv.