XB-ART-41412
Curr Biol
April 27, 2010;
20
(8):
738-43.
The nodal inhibitor Coco is a critical target of leftward flow in Xenopus.
Abstract
Vertebrate laterality, which is manifested by asymmetrically placed organs [1], depends on asymmetric activation of the
Nodal signaling cascade in the
left lateral plate
mesoderm [2]. In fish, amphibians, and mammals, a
cilia-driven leftward flow of extracellular fluid acts upstream of the
Nodal cascade [3-6]. The direct target of flow has remained elusive. In Xenopus, flow occurs at the
gastrocoel roof plate (GRP) in the dorsal midline of the
embryo [4, 7]. The GRP is bordered by a second, bilaterally symmetrical
Nodal expression domain [8]. Here we identify the
Nodal inhibitor
Coco as a critical target of flow.
Coco and Xenopus
Nodal-related 1 (
Xnr1) are coexpressed in the lateralmost ciliated GRP cells.
Coco becomes downregulated on the
left side of the GRP as a direct readout of flow. Ablation of flow prevented
Coco repression, whereas
Xnr1 expression was independent of flow. Loss of flow-induced laterality defects were rescued by knockdown of
Coco on the
left side. Parallel knockdown of
Coco and
Xnr1 in GRP cells restored laterality defects in flow-impaired embryos, demonstrating that
Coco acted through GRP-expressed
Xnr1.
Coco thus acts as a critical target of flow, suggesting that symmetry is broken by flow-mediated
left-asymmetric release of
Nodal repression at the midline.
PubMed ID:
20381352
Article link:
Curr Biol
Species referenced:
Xenopus laevis
Genes referenced:
dand5
dnah9
myod1
nodal
nodal1
Morpholinos:
nodal1 MO5
Article Images:
[+] show captions
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Figure 1. Bilateral Xnr1 in Somitic Gastrocoel Roof Plate Cells Acts Downstream of Flow(Aa–Bb) Xnr1 expression during flow (stage 17) in 1–3 rows on the lateral margins of the gastrocoel roof plate (GRP) in prospective somitic cells. In situ hybridization was performed with probes specific for Xnr1 (Aa) and MyoD (Ba and Bb). Histological sections at comparable levels (indicated in Aa) identify Xnr1 (Ab and Ac) cells as MyoD-positive somitic cells (Ba and Bb). For assignment of boundaries (broken lines), please see Figure S1A.(Ca–Cc) Xnr1 cells harbor unpolarized monocilia. The image shows an overlay of in situ hybridization signal and scanning electron micrograph of same specimen (Ca). Higher magnification in (Cb) reveals central cilia (yellow) on Xnr1-positive cells, as opposed to polarized cilia (white) at the center of the GRP. Evaluation of the entire width of GRP (Cc) demonstrates the unpolarized nature of cilia on Xnr1 cells on either side (green dot represents a cilium with unclear polarization).(Da) Wild-type (WT) leftward flow in morphants injected bilaterally with Xnr1-MO (cf. Movie S1). Particle movement in a representative specimen is visualized by gradient time trails (GTTs; cf. Experimental Procedures for details), in which the color gradient from green to red represents 25 s (cf. color bar). Targeted area is indicated by red lines (corresponding to coinjected lineage tracer rhodamine-B dextran).(Db) Quantification of results from 8 embryos, representing 2103 particles. Morphants display a robust leftward flow (3.2 ± 1.9 μm/s), as demonstrated by the distribution of mean particle directionality (wind rose) and WT p = 0.8 [38].The following abbreviations are used: a, anterior; bp, blastopore; d, dorsal; e, endoderm; hyGRP, prospective hypochordal cells; l, left; LECs, lateral endodermal crest cells; no, notochord; p, posterior; r, right; s, presomitic mesoderm; SEM, scanning electron micrograph; sGRP, prospective somitic cells, v, ventral. The color gradient in (Da) represents 25 s. Boxes in (Ab), (Ba), (Ca), and (Cb) indicate areas of higher magnification in (Ac), (Bb), (Cb), and (Cc). Scale bars in (Ca), (Cb), and (Da) represent 100 μm, 10 μm, and 50 μm, respectively.
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Figure 2. Expression Patterns of Xnr1 Are Independent of Leftward Flow(A) Schematic representation of dorsal explant, shown in ventral view. The blue box indicates the region shown in (B). The following abbreviations are used: cbc, circumblastoporal collar; LECs, lateral endodermal crest cells.(B) Representative examples of Xnr1 mRNA expression patterns in the GRP of stage 17 embryos. Following in situ hybridization, dorsal explants were prepared and classified into right bias (R > L), equal distribution (R = L), or left bias (R <L) of Xnr1 signals at the GRP.(C) Xnr1 expression patterns are indistinguishable in pre- and postflow stages (left bars) and are unaltered in embryos, in which flow was ablated (right bars) by dnah9 knockdown (dnah9-MO) or by methylcellulose (MC) injections. Numbers in bars represent percentages; numbers of embryos analyzed are indicated in brackets.
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Figure 3. Expression Patterns of the Nodal Inhibitor Coco Vary with Leftward Flow(Aa–Ac) Coco is coexpressed with Xnr1 in lateral somitic GRP cells (cf. Xnr1 in Figures 1Ab and 1Ac).(Aa) Dorsal explant of a whole-mount in situ hybridized embryo at stage 17 with a Coco-specific antisense probe.(Ab and Ac) Histological analysis of expression domain in somitic GRP cells.(B) Representative examples of Coco mRNA expression patterns in the GRP of stage 19 embryos. Following in situ hybridization, dorsal explants were prepared and classified into right bias (R > L), equal distribution (R = L), or left bias (R <L) of Coco signals at the GRP.(C) Increase of right bias Coco expression pattern in postflow stages (left bars) is dependent on flow (right bars).(Da and Db) Wild-type leftward flow (3.4 ± 1.8 μm/s; p = 0.78) in morphants injected uni- or bilaterally with Coco-MO (cf. Movie S1), as depicted in GTT blot of representative explant (Da) and summary wind rose display of 9 embryos representing 2677 particles (Db).The following abbreviations are used: bp, blastopore; d, dorsal; e, endoderm; hyGRP, prospective hypochordal cells; l, left; LECs, lateral endodermal crest cells; no, notochord; r, right; s, presomitic mesoderm; sGRP, prospective somitic cells; v, ventral. The box in (Ab) indicates the area of higher magnification in (Ac). Color gradient in (Da) represents 25 s. Scale bar represents 50 μm.
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