RESUMEN
During vertebrate embryonic development, the formation of axial structures is driven by a population of stem-like cells that reside in a region of the tailbud called the chordoneural hinge (CNH). We have compared the mouse CNH transcriptome with those of surrounding tissues and shown that the CNH and tailbud mesoderm are transcriptionally similar, and distinct from the presomitic mesoderm. Amongst CNH-enriched genes are several that are required for axial elongation, including Wnt3a, Cdx2, Brachyury/T and Fgf8, and androgen/oestrogen receptor nuclear signalling components such as Greb1 We show that the pattern and duration of tailbud Greb1 expression is conserved in mouse, zebrafish and chicken embryos, and that Greb1 is required for axial elongation and somitogenesis in zebrafish embryos. The axial truncation phenotype of Greb1 morphant embryos can be explained by much reduced expression of No tail (Ntl/Brachyury), which is required for axial progenitor maintenance. Posterior segmentation defects in the morphants (including misexpression of genes such as mespb, myoD and papC) appear to result, in part, from lost expression of the segmentation clock gene, her7.
Asunto(s)
Desarrollo Embrionario/genética , Proteínas de la Membrana/genética , Morfogénesis/genética , Proteínas de Neoplasias/genética , Proteínas de Pez Cebra/genética , Animales , Biología Computacional/métodos , Perfilación de la Expresión Génica , Regulación del Desarrollo de la Expresión Génica , Técnicas de Silenciamiento del Gen , Ratones , Fenotipo , Células Madre/citología , Células Madre/metabolismo , TranscriptomaRESUMEN
Somites are formed from the presomitic mesoderm (PSM) and give rise to the axial skeleton and skeletal muscles. The PSM is dynamic; somites are generated at the anterior end, while the posterior end is continually renewed with new cells entering from the tailbud progenitor region. Which genes control the conversion of tailbud progenitors into PSM and how is this process coordinated with cell movement? Using loss- and gain-of-function experiments and heat-shock transgenics we show in zebrafish that the transcription factor Mesogenin 1 (Msgn1), acting with Spadetail (Spt), has a central role. Msgn1 allows progression of the PSM differentiation program by switching off the progenitor maintenance genes ntl, wnt3a, wnt8 and fgf8 in the future PSM cells as they exit from the tailbud, and subsequently induces expression of PSM markers such as tbx24. msgn1 is itself positively regulated by Ntl/Wnt/Fgf, creating a negative-feedback loop that might be crucial to regulate homeostasis of the progenitor population until somitogenesis ends. Msgn1 drives not only the changes in gene expression in the nascent PSM cells but also the movements by which they stream out of the tailbud into the PSM. Loss of Msgn1 reduces the flux of cells out of the tailbud, producing smaller somites and an enlarged tailbud, and, by delaying exhaustion of the progenitor population, results in supernumerary tail somites. Through its combined effects on gene expression and cell movement, Msgn1 (with Spt) plays a key role both in genesis of the paraxial mesoderm and in maintenance of the progenitor population from which it derives.
Asunto(s)
Factores de Transcripción con Motivo Hélice-Asa-Hélice Básico/fisiología , Diferenciación Celular/genética , Movimiento Celular/genética , Células Madre Embrionarias/fisiología , Mesodermo/embriología , Proteínas de Pez Cebra/fisiología , Animales , Animales Modificados Genéticamente , Factores de Transcripción con Motivo Hélice-Asa-Hélice Básico/genética , Factores de Transcripción con Motivo Hélice-Asa-Hélice Básico/metabolismo , Rastreo Celular , Desarrollo Embrionario/genética , Células Madre Embrionarias/metabolismo , Mesodermo/citología , Mesodermo/metabolismo , Somitos/embriología , Somitos/metabolismo , Proteínas de Dominio T Box/genética , Proteínas de Dominio T Box/metabolismo , Proteínas de Dominio T Box/fisiología , Cola (estructura animal)/embriología , Torso/embriología , Pez Cebra/embriología , Pez Cebra/genética , Pez Cebra/metabolismo , Proteínas de Pez Cebra/genética , Proteínas de Pez Cebra/metabolismoRESUMEN
Regular production of somites, precursors of the axial skeleton and attached muscles is controlled by a molecular oscillator, the segmentation clock, which drives cyclic transcription of target genes in the unsegmented presomitic mesoderm (PSM). The clock is based on a negative feedback loop which generates pulses of transcription that oscillate with the same periodicity as somite formation. Mutants in several oscillating genes including the Notch pathway gene Lunatic fringe (Lfng) and the Notch target Hes7, result in defective somitogenesis and disorganised axial skeletons. Both genes encode negative regulators of Notch signalling output, but it is not yet clear if they are just secondary clock targets or if they encode components of a primary, pacemaker oscillator. In this paper, we try to identify components in the primary oscillator by manipulating delays in the feedback circuitry. We characterise recombinant mice in which Lfng and Hes7 introns are lengthened in order to delay mRNA production. Lengthening the third Hes7 intron by 10 or 20 kb disrupts accurate RNA splicing and inactivates the gene. Lfng expression and activity is normal in mice whose Lfng is lengthened by 10 kb, but no effects on segmentation are evident. We discuss these results in terms of the relative contributions of transcriptional and post-transcriptional delays towards defining the pace of segmentation, and of alternative strategies for manipulating the period of the clock.
Asunto(s)
Factores de Transcripción con Motivo Hélice-Asa-Hélice Básico/genética , Glicosiltransferasas/genética , ARN Mensajero/genética , Empalme Alternativo , Animales , Factores de Transcripción con Motivo Hélice-Asa-Hélice Básico/deficiencia , Tipificación del Cuerpo , Embrión de Mamíferos/anomalías , Embrión de Mamíferos/metabolismo , Regulación del Desarrollo de la Expresión Génica , Técnicas de Sustitución del Gen , Glicosiltransferasas/metabolismo , Ratones , Ratones de la Cepa 129 , Ratones Endogámicos C57BL , Ratones Transgénicos , Isoformas de Proteínas/deficiencia , Isoformas de Proteínas/genética , ARN Mensajero/metabolismo , Somitos/anomalías , Somitos/embriología , Somitos/metabolismoRESUMEN
A report on the Joint Meeting of the British Societies for Cell and Developmental Biology, Warwick, UK, 31 March-3 April, 2008.