A dual function of FGF signaling in Xenopus left-right axis formation

R-Spondin 2 governs Xenopus left-right body axis formation by establishing an FGF signaling gradient

Establishment of the left-right (LR, sinistral, dextral) body axis in many vertebrate embryos relies on cilia-driven leftward fluid flow within an LR organizer (LRO). A cardinal question is how leftward flow triggers symmetry breakage. The chemosensation model posits that ciliary flow enriches a signaling molecule on the left side of the LRO that promotes sinistral cell fate. However, the nature of this sinistralizing signal has remained elusive. In the Xenopus LRO, we identified the stem cell growth factor R-Spondin 2 (Rspo2) as a symmetrically expressed, sinistralizing signal. As predicted for a flow-mediated signal, Rspo2 operates downstream of leftward flow but upstream of the asymmetrically expressed gene dand5. Unexpectedly, in LR patterning, Rspo2 acts as an FGF receptor antagonist: Rspo2 via its TSP1 domain binds Fgfr4 and promotes its membrane clearance by Znrf3-mediated endocytosis. Concordantly, we find that at flow-stage, FGF signaling is dextralizing and forms a gradient across the LRO, high on the dextral- and low on the sinistral side. Rspo2 gain- and loss-of function equalize this FGF signaling gradient and sinistralize and dextralize development, respectively. We propose that leftward flow of Rspo2 produces an FGF signaling gradient that governs LR-symmetry breakage.

FGF-mediated establishment of left-right asymmetry requires Rab7 function in the dorsal mesoderm in Xenopus

Gastrulation denotes a very important developmental process, which includes significant structural tissue rearrangements and patterning events that shape the emerging vertebrate organism. At the end of gastrulation, the three body axes are spatially defined while the left-right axis still lacks any molecular or morphological polarity. In most vertebrates, this is established during neurulation by a symmetry breaking LR organizer. However, this mesoderm-derived structure depends on proper induction and specification of the mesoderm, which in turn requires involvement of several signaling pathways. Endocytosis and the endosomal machinery offer manifold platforms for intracellular pathway regulation, especially late endosomes claim increasing attention. The late endosomal regulator Rab7 has been linked to mesoderm specification during gastrulation. Distinct axial defects due to compromised dorsal mesoderm development in rab7-deficient Xenopus embryos suggested a requirement of Rab7 for FGF-dependent mesoderm patterning and LR asymmetry. Here we specifically addressed such a role of Rab7, demonstrating a functional requirement for LR organizer development and symmetry breakage. Using different FGF/MAPK pathway components we show that Rab7 participates in dorsal mesoderm patterning. We suggest a hierarchical classification of Rab7 upstream of MAPK-dependent mesoderm specification, most probably at the level of the small GTPase Ras. Thus, this study affords an insight on how the Rab7-regulated endosomal machinery could participate in signal transduction to enable correct mesoderm specification and left-right asymmetry.

dmrt2 and myf5 Link Early Somitogenesis to Left-Right Axis Determination in Xenopus laevis

The vertebrate left-right axis is specified during neurulation by events occurring in a transient ciliated epithelium termed left-right organizer (LRO), which is made up of two distinct cell types. In the axial midline, central LRO (cLRO) cells project motile monocilia and generate a leftward fluid flow, which represents the mechanism of symmetry breakage. This directional fluid flow is perceived by laterally positioned sensory LRO (sLRO) cells, which harbor non-motile cilia. In sLRO cells on the left side, flow-induced signaling triggers post-transcriptional repression of the multi-pathway antagonist dand5. Subsequently, the co-expressed Tgf-β growth factor Nodal1 is released from Dand5-mediated repression to induce left-sided gene expression. Interestingly, Xenopus sLRO cells have somitic fate, suggesting a connection between LR determination and somitogenesis. Here, we show that doublesex and mab3-related transcription factor 2 (Dmrt2), known to be involved in vertebrate somitogenesis, is required for LRO ciliogenesis and sLRO specification. In dmrt2 morphants, misexpression of the myogenic transcription factors tbx6 and myf5 at early gastrula stages preceded the misspecification of sLRO cells at neurula stages. myf5 morphant tadpoles also showed LR defects due to a failure of sLRO development. The gain of myf5 function reintroduced sLRO cells in dmrt2 morphants, demonstrating that paraxial patterning and somitogenesis are functionally linked to LR axis formation in Xenopus.

Rab7 is required for mesoderm patterning and gastrulation in Xenopus

Early embryogenesis requires tightly controlled temporal and spatial coordination of cellular behavior and signaling. Modulations are achieved at multiple levels, from cellular transcription to tissue-scale behavior. Intracellularly, the endolysosomal system emerges as an important regulator at different levels, but in vivo studies are rare. In the frog Xenopus, little is known about the developmental roles of endosomal regulators, or their potential involvement in signaling, especially for late endosomes. Here, we analyzed a hypothesized role of Rab7 in this context, a small GTPase known for its role as a late endosomal regulator. First, rab7 showed strong maternal expression. Following localized zygotic transcript enrichment in the mesodermal ring and neural plate, it was found in tailbud-stage neural ectoderm, notochord, pronephros, eyes and neural crest tissues. Inhibition resulted in strong axis defects caused by a requirement of rab7 for mesodermal patterning and correct gastrulation movements. To test a potential involvement in growth factor signaling, we analyzed early Wnt-dependent processes in the mesoderm. Our results suggest a selective requirement for ligand-induced Wnt activation, implicating a context-dependent role of Rab7.

Summary: The late endosomal regulator Rab7 is required for gastrulation movements and axis elongation in Xenopus by regulating early mesoderm patterning.

Combinatorial transcription factor activities on open chromatin induce embryonic heterogeneity in vertebrates

During vertebrate gastrulation, mesoderm is induced in pluripotent cells, concomitant with dorsal‐ventral patterning and establishing of the dorsal axis. We applied single‐cell chromatin accessibility and transcriptome analyses to explore the emergence of cellular heterogeneity during gastrulation in Xenopus tropicalis. Transcriptionally inactive lineage‐restricted genes exhibit relatively open chromatin in animal caps, whereas chromatin accessibility in dorsal marginal zone cells more closely reflects transcriptional activity. We characterized single‐cell trajectories and identified head and trunk organizer cell clusters in early gastrulae. By integrating chromatin accessibility and transcriptome data, we inferred the activity of transcription factors in single‐cell clusters and tested the activity of organizer‐expressed transcription factors in animal caps, alone or in combination. The expression profile induced by a combination of Foxb1 and Eomes most closely resembles that observed in the head organizer. Genes induced by Eomes, Otx2, or the Irx3‐Otx2 combination are enriched for maternally regulated H3K4me3 modifications, whereas Lhx8‐induced genes are marked more frequently by zygotically controlled H3K4me3. Taken together, our results show that transcription factors cooperate in a combinatorial fashion in generally open chromatin to orchestrate zygotic gene expression.

Right, left and cilia: How asymmetry is established

The initial breaking of left-right (L-R) symmetry in the embryo is controlled by a motile-cilia-driven leftward fluid flow in the left-right organiser (LRO), resulting in L-R asymmetric gene expression flanking the LRO. Ultimately this results in left- but not right-sided activation of the Nodal-Pitx2 pathway in more lateral tissues. While aspects of the initial breaking event clearly vary between vertebrates, events in the Lateral Plate Mesoderm (LPM) are conserved through the vertebrate lineage. Evidence from model systems and humans highlights the role of cilia both in the initial symmetry breaking and in the ability of more lateral tissues to exhibit asymmetric gene expression. In this review we concentrate on the process of L-R determination in mouse and humans.