Kimiko Takebayashi-Suzuki

Wnt signaling is essential for tissue and organ regeneration in animals such as amphibians, and several Wnt ligands are highly expressed in regenerating tissues. However, little is known about the specific role played by each ligand and their relative contribution to regeneration processes. Here, we show that Wnt5a plays critical roles in cell proliferation and differentiation during Xenopus tadpole tail regeneration. CRISPR/Cas9‐mediated knock‐out (KO) of the wnt5a gene in Xenopus tadpoles caused a delay in tail regeneration. Expression of wnt5a is induced after wound healing in regenerating tails and is positively regulated by the early injury response genes junb and inhba , which are required for the initiation of regenerative cell proliferation. Interestingly, in wnt5a KO tadpoles, cell proliferation initiated by JunB and InhibinβA was reduced at late stages of tail regeneration, suggesting an essential role of Wnt5a in the maintenance of regenerative cell proliferation. Moreover, differentiation of the notochord and neural tissue was also affected in the regenerating tails of wnt5a KO tadpoles. Our findings demonstrate that Wnt5a acts downstream of JunB and InhibinβA to maintain cell proliferation and that it also promotes differentiation in Xenopus tadpole tail regeneration.

During embryonic development in vertebrates, morphogens play an important role in cell fate determination and morphogenesis. Bone morphogenetic proteins (BMPs) belonging to the transforming growth factor-β (TGF-β) family control the dorsal–ventral (DV) patterning of embryos, whereas other morphogens such as fibroblast growth factor (FGF), Wnt family members, and retinoic acid (RA) regulate the formation of the anterior–posterior (AP) axis. Activation of morphogen signaling results in changes in the expression of target genes including transcription factors that direct cell fate along the body axes. To ensure the correct establishment of the body plan, the processes of DV and AP axis formation must be linked and coordinately regulated by a fine-tuning of morphogen signaling. In this review, we focus on the interplay of various intracellular regulatory mechanisms and discuss how communication among morphogen signaling pathways modulates body axis formation in vertebrate embryos.

Neural induction and patterning in vertebrates are regulated during early development by several morphogens, such as bone morphogenetic proteins (BMPs) and fibroblast growth factors (FGFs). Ventral ectoderm differentiates into epidermis in response to BMPs, whereas BMP signaling is tightly inhibited in the dorsal ectoderm which develops into neural tissues. Here, we show that Cdc2‐like kinase 2 (Clk2) promotes early neural development and inhibits epidermis differentiation in Xenopus embryos. clk2 is specifically expressed in neural tissues along the anterior‐posterior axis during early Xenopus embryogenesis. When overexpressed in ectodermal explants, Clk2 induces the expression of both anterior and posterior neural marker genes. In agreement with this observation, overexpression of Clk2 in whole embryos expands the neural plate at the expense of epidermal ectoderm. Interestingly, the neural‐inducing activity of Clk2 is increased following BMP inhibition and activation of the FGF signaling pathway in ectodermal explants. Clk2 also downregulates the level of p‐Smad1/5/8 in cooperation with BMP inhibition, in addition to increasing the level of activated MAPK together with FGF. These results suggest that Clk2 plays a role in early neural development of Xenopus possibly via modulation of morphogen signals such as the BMP and FGF pathways.

During early vertebrate embryogenesis, bone morphogenetic proteins (BMPs) belonging to the transforming growth factor‐β (TGF‐β) family of growth factors play a central role in dorsal–ventral (DV) patterning of embryos, while other growth factors such as Wnt and fibroblast growth factor (FGF) family members regulate formation of the anterior–posterior (AP) axis. Although the establishment of body plan is thought to require coordinated formation of the DV and AP axes, the mechanistic details underlying this coordination are not well understood. Here, we show that a Xenopus homologue of zbtb14 plays an essential role in the regulation of both DV and AP patterning during early Xenopus development. We show that overexpression of Zbtb14 promotes neural induction and inhibits epidermal differentiation, thereby regulating DV patterning. In addition, Zbtb14 promotes the formation of posterior neural tissue and suppresses anterior neural development. Consistent with this, knock‐down experiments show that Zbtb14 is required for neural development, especially for the formation of posterior neural tissues. Mechanistically, Zbtb14 reduces the levels of phosphorylated Smad1/5/8 to suppress BMP signaling and induces an accumulation of β‐Catenin to promote Wnt signaling. Collectively, these results suggest that Zbtb14 plays a crucial role in the formation of DV and AP axes by regulating both the BMP and Wnt signaling pathways during early Xenopus embryogenesis.

Impulse-conducting Purkinje cells differentiate from myocytes during embryogenesis. In the embryonic chicken heart, this conversion of contractile myocytes into conduction cells occurs subendocardially and periarterially. The unique sites of Purkinje fibre differentiation suggest that a shear stress-induced paracrine signal from the endocardium and arterial beds may induce adjacent myocytes to differentiate into conduction cells. Consistent with this model, Purkinje fibre marker genes can be induced in cultured embryonic myocytes by endothelin (ET), an endothelial cell-derived signalling peptide. This inductive response is, however, gradually lost from myocytes as embryos develop, and mature myocytes express only genes characteristic of hypertrophy in response to ET. In vivo, active ET is produced, through proteolytic processing, from its precursor by ET-converting enzyme 1 (ECE1) and triggers signalling by binding to its receptors, ET_A and ET_B. In the embryonic heart, the expression of these ET signalling components changes dynamically, defining the site and timing of Purkinje fibre differentiation within the ventricular myocardium during chick embryogenesis.