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Vertebrate laterality, which is manifested by asymmetrically placed organs [1], depends on asymmetric activation of the Nodal signaling cascade in the leftlateral 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.
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.
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.
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.
