Spemann organizer gene Goosecoid promotes delamination of neuroblasts from the otic vesicle.

Neurons of the Statoacoustic Ganglion (SAG), which innervate the inner ear, originate as neuroblasts in the floor of the otic vesicle and subsequently delaminate and migrate toward the hindbrain before completing differentiation. In all vertebrates, locally expressed Fgf initiates SAG development by inducing expression of Neurogenin1 (Ngn1) in the floor of the otic vesicle. However, not all Ngn1-positive cells undergo delamination, nor has the mechanism controlling SAG delamination been elucidated. Here we report that Goosecoid (Gsc), best known for regulating cellular dynamics in the Spemann organizer, regulates delamination of neuroblasts in the otic vesicle. In zebrafish, Fgf coregulates expression of Gsc and Ngn1 in partially overlapping domains, with delamination occurring primarily in the zone of overlap. Loss of Gsc severely inhibits delamination, whereas overexpression of Gsc greatly increases delamination. Comisexpression of Ngn1 and Gsc induces ectopic delamination of some cells from the medial wall of the otic vesicle but with a low incidence, suggesting the action of a local inhibitor. The medial marker Pax2a is required to restrict the domain of gsc expression, and misexpression of Pax2a is sufficient to block delamination and fully suppress the effects of Gsc The opposing activities of Gsc and Pax2a correlate with repression or up-regulation, respectively, of E-cadherin (cdh1). These data resolve a genetic mechanism controlling delamination of otic neuroblasts. The data also elucidate a developmental role for Gsc consistent with a general function in promoting epithelial-to-mesenchymal transition (EMT).

Xenopus Zic3 controls notochord and organizer development through suppression of the Wnt/ß-catenin signaling pathway.

Zic3 controls neuroectodermal differentiation and left-right patterning in Xenopus laevis embryos. Here we demonstrate that Zic3 can suppress Wnt/ß-catenin signaling and control development of the notochord and Spemann's organizer. When we overexpressed Zic3 by injecting its RNA into the dorsal marginal zone of 2-cell-stage embryos, the embryos lost mesodermal dorsal midline structures and showed reduced expression of organizer markers (Siamois and Goosecoid) and a notochord marker (Xnot). Co-injection of Siamois RNA partially rescued the reduction of Xnot expression caused by Zic3 overexpression. Because the expression of Siamois in the organizer region is controlled by Wnt/ß-catenin signaling, we subsequently examined the functional interaction between Zic3 and Wnt signaling. Co-injection of Xenopus Zic RNAs and ß-catenin RNA with a reporter responsive to the Wnt/ß-catenin cascade indicated that Zic1, Zic2, Zic3, Zic4, and Zic5 can all suppress ß-catenin-mediated transcriptional activation. In addition, co-injection of Zic3 RNA inhibited the secondary axis formation caused by ventral-side injection of ß-catenin RNA in Xenopus embryos. Zic3-mediated Wnt/ß-catenin signal suppression required the nuclear localization of Zic3, and involved the reduction of ß-catenin nuclear transport and enhancement of ß-catenin degradation. Furthermore, Zic3 co-precipitated with Tcf1 (a ß-catenin co-factor) and XIC (I-mfa domain containing factor required for dorsoanterior development). The findings in this report produce a novel system for fine-tuning of Wnt/ß-catenin signaling.

Embryonic organizer formation disorder leads to multiorgan dysplasia in Down syndrome.

Despite the high prevalence of Down syndrome (DS) and early identification of the cause (trisomy 21), its molecular pathogenesis has been poorly understood and specific treatments have consequently been practically unavailable. A number of medical conditions throughout the body associated with DS have prompted us to investigate its molecular etiology from the viewpoint of the embryonic organizer, which can steer the development of surrounding cells into specific organs and tissues. We established a DS zebrafish model by overexpressing the human DYRK1A gene, a highly haploinsufficient gene located at the "critical region" within 21q22. We found that both embryonic organizer and body axis were significantly impaired during early embryogenesis, producing abnormalities of the nervous, heart, visceral, and blood systems, similar to those observed with DS. Quantitative phosphoproteome analysis and related assays demonstrated that the DYRK1A-overexpressed zebrafish embryos had anomalous phosphorylation of ß-catenin and Hsp90ab1, resulting in Wnt signaling enhancement and TGF-ß inhibition. We found an uncovered ectopic molecular mechanism present in amniocytes from fetuses diagnosed with DS and isolated hematopoietic stem cells (HSCs) of DS patients. Importantly, the abnormal proliferation of DS HSCs could be recovered by switching the balance between Wnt and TGF-ß signaling in vitro. Our findings provide a novel molecular pathogenic mechanism in which ectopic Wnt and TGF-ß lead to DS physical dysplasia, suggesting potential targeted therapies for DS.

The proliferative state, graft-site and contact-time of competent chick ectoblast determine the quality and quantity of neural induction by Hensen's node.

We have assessed the type and amount of neural tissue induced in the chick gastrula ectoblast with increasing duration of contact with the Hensen node inducer. At least 4 h contact is necessary to induce the ectoblast in the area pellucida, 9 h in the area opaca and even longer at the margin of overgrowth. In the area pellucida, the inductive response shifts from archencephalic type at 4-6 h contact to deuterencephalic type after 9 h contact. The induced tissue volume and cell number increased as graft-host contact increased from 4-9 h, and then decreased with longer contact. We suggest that in addition to the period of contact and the grafting position on competent ectoblast, the rate of cell proliferation may control the axial specificity and morphological organization of the induced neural tissue.

Chromosomal anomaly spectrum in early pregnancy loss in relation to presence or absence of an embryonic pole.

OBJECTIVE: To compare the cytogenetic findings in a series of missed miscarriages evaluated by chorionic villus sampling, in relation to embryonic pole presence (embryonic or anembryonic). DESIGN: Prospective cross-sectional study. SETTING: Tertiary referral hospital. PATIENT(S): Women presenting with a missed miscarriage. INTERVENTION(S): Transcervical chorionic villus sampling and cytogenetic studies in the chorionic villi with use of the semidirect method. MAIN OUTCOME MEASURES(S): Embryonic pole presence or absence assessed by transvaginal ultrasound examination. Type of chromosomal anomalies found in both subgroups. RESULT(S): Although the chromosomal abnormality rate was similar for miscarriages with absent or present embryo (61% vs. 68% respectively), frequencies for viable autosomal trisomies (2.3% vs. 19%) and monosomy X (0% vs. 9.2%) were significantly lower when no embryonic pole was seen. CONCLUSION(S): Viable autosomal trisomies and monosomies X appear not to be a common cause of miscarriage with an early fetal demise (anembryonic miscarriage).

Foregut endoderm is specified early in avian development through signal(s) emanating from Hensen's node or its derivatives.

In this study, the initial specification of foregut endoderm in the chick embryo was analyzed. A fate map constructed for the area pellucida endoderm at definitive streak-stage showed centrally-located presumptive cells of foregut-derived organs around Hensen's node. Intracoelomic cultivation of the area pellucida endoderm at this stage combined with somatic mesoderm resulted in the differentiation predominantly into intestinal epithelium, suggesting that this endoderm may not yet be regionally specified. In vitro cultivation of this endoderm for 1-1.5 day combined with Hensen's node or its derivatives but not with other embryonic structures/tissues elicited endodermal expression of cSox2 but not of cHoxb9, which is characteristic of specified foregut endoderm. When the anteriormost or posteriormost part of the area pellucida endoderm at this stage, whose fate is extraembryonic, was combined with Hensen's node or its derivatives for 1 day, then enwrapped with somatic mesoderm and cultivated for a long period intracoelomically, differentiation of various foregut organ epithelia was observed. Such epithelia never appeared in the endoderm associated with other embryonic structures/tissues and cultured similarly. Thus, Hensen's node and its derivatives that lie centrally in the presumptive endodermal area of the foregut are likely to play an important role in the initial specification of the foregut. Chordin-expressing COS cells or noggin-producing CHO cells transplanted into the anteriormost area pellucida of the definitve streak-stage embryo could induce endodermal expression of cSox2 but not of cHoxb9, suggesting that chordin and noggin that emanate from Hensen's node and its derivatives, may be involved in this process.