The cardiac neural crest gene MafB ectopically directs CXCR4 expression in the trunk neural crest.

The cardiac neural crest is a subpopulation of cells arising from the caudal hindbrain. The delaminated cardiac neural crest cells migrate to the heart using the CXCR/SDF1 chemokine signaling system. These cells contribute to the formation of the cardiovascular system, including the septation of the outflow tract, which is unique to these cells. Here, we investigated the effect of ectopic expression of the cardiac neural crest gene MafB on trunk neural crest cells. First, we found that MafB has the potential to activate its own cis-regulatory element in enteric and trunk neural crest cells but not in cranial neural crest cells. Forced expression of two cardiac neural crest genes, Ets1 and Sox8, together with or without MafB, induced ectopic Sox10E2 enhancer activity in the trunk region. Finally, we uncovered that the expression of MafB, Ets1 and Sox8 can induce ectopic CXCR4 expression in the trunk neural crest cells, resulting in acquisition of responsiveness to the SDF1 signal. These results demonstrate that MafB, Ets1 and Sox8 are critical components for generation of the identity of the cardiac neural crest, especially the cell migration property.

Identification of regulatory elements for MafB expression in the cardiac neural crest.

Cardiac neural crest cells arise in the caudal hindbrain and then migrate to the heart through the pharyngeal arches. These cells contribute to the formation of the heart, including the outflow tract, and are unique to this neural crest population. MafB is a transcription factor expressed specifically in early migrating cardiac neural crest cells as well as in rhombomeres (r) 5 and 6. Here, we identified the regulatory region in the chicken genome controlling the expression of endogenous MafB transcripts and used these essential elements to express MafB in the cardiac neural crest in reporter assays. A reporter driven by this regulatory region was employed to trace the migration of these cells into the pharyngeal arches. This regulatory region demonstrated transcriptional activity in the cardiac neural crest but not in other neural crest cell subpopulations, such as the cranial and trunk cells. This study provides insights into the gene regulatory mechanisms that specify cardiac neural crest cells among neural crest cell populations.

Transcriptome profiling of the cardiac neural crest reveals a critical role for MafB.

The cardiac neural crest originates in the caudal hindbrain, migrates to the heart, and contributes to septation of the cardiac outflow tract and ventricles, an ability unique to this neural crest subpopulation. Here we have used a FoxD3 neural crest enhancer to isolate a pure population of cardiac neural crest cells for transcriptome analysis. This has led to the identification of transcription factors, signaling receptors/ligands, and cell adhesion molecules upregulated in the early migrating cardiac neural crest. We then functionally tested the role of one of the upregulated transcription factors, MafB, and found that it acts as a regulator of Sox10 expression specifically in the cardiac neural crest. Our results not only reveal the genome-wide profile of early migrating cardiac neural crest cells, but also provide molecular insight into what makes the cardiac neural crest unique.

Developmental mechanisms of migratory muscle precursors in medaka pectoral fin formation.

Limb muscles are formed from migratory muscle precursor cells (MMPs) that delaminate from the ventral region of dermomyotomes and migrate into the limb bud. MMPs remain undifferentiated during migration, commencing differentiation into skeletal muscle after arrival in the limb. However, it is still unclear whether the developmental mechanisms of MMPs are conserved in teleost fishes. Here, we investigate the development of pectoral fin muscles in the teleost medaka Oryzias latipes. Expression of the MMP marker lbx1 is first observed in several somites prior to the appearance of fin buds. lbx1-positive cells subsequently move anteriorly and localize in the prospective fin bud region to differentiate into skeletal muscle cells. To address the developmental mechanisms underlying fin muscle formation, we knocked down tbx5, a gene that is required for fin bud formation. tbx5 morphants showed loss of fin buds, whereas lbx1 expression initiated normally in anterior somites. Unlike in normal embryos, expression of lbx1 was not maintained in migrating fin MMPs or within the fin buds. We suggest that fin MMPs appear to undergo two phases in their development, with an initial specification of MMPs occurring independent of fin buds and a second fin bud-dependent phase of MMP migration and proliferation. Our results showed that medaka fin muscle is composed of MMPs. It is suggested that the developmental mechanism of fin muscle formation is conserved in teleost fishes including medaka. Through this study, we also propose new insights into the developmental mechanisms of MMPs in fin bud formation.

DND protein functions as a translation repressor during zebrafish embryogenesis.

Germline and somatic cell distinction is regulated through a combination of microRNA and germ cell-specific RNA-binding proteins in zebrafish. An RNA-binding protein, DND, has been reported to relieve the miR-430-mediated repression of some germ plasm mRNAs such as nanos3 and tdrd7 in primordial germ cells (PGCs). Here, we showed that miR-430-mediated repression is not counteracted by the overexpression of DND protein in somatic cells. Using a λN-box B tethering assay in the embryo, we found that tethering of DND to reporter mRNA results in translation repression without affecting mRNA stability. Translation repression by DND was not dependent on another germline-specific translation repressor, Nanos3, in zebrafish embryos. Moreover, our data suggested that DND represses translation of nanog and dnd mRNAs, whereas an RNA-binding protein DAZ-like (DAZL) promotes dnd mRNA translation. Thus, our study showed that DND protein functions as a translation repressor of specific mRNAs to control PGC development in zebrafish.