Specification of embryonic mesoderm by Pinhead signaling

Among the three embryonic germ layers, mesoderm plays a central role in the establishment of the vertebrate body plan. Mesoderm is specified by secreted signaling proteins from the FGF, Nodal, BMP and Wnt families. No new classes of extracellular mesoderm-inducing factors have been identified in more than two decades. Here we show that the Xenopus pinhead (pnhd) gene encodes a novel secreted protein that activates specific mesodermal markers in presumptive ectoderm. We demonstrate that Pnhd is required for the specification of a subset of embryonic mesoderm in vivo. RNA sequencing revealed that many transcriptional targets of Pnhd are shared with those of the FGF pathway. Pnhd signaling involves active FGF but not Wnt receptors and is accompanied by Erk1 phosphorylation. We propose that Pnhd operates in the marginal zone to specify mesodermal progenitors via an FGF receptor-dependent mechanism. Gain of function and loss of function study of Pinhead, a novel growth factor. Duplicate samples for control ectoderm explants (Cocaps1, Cocaps2) and Flag-Pinhead-expressing caps (FlagPin1, FlagPin2) at stage 11, gastrula were analyzed for Pinhead gain-of-function by RNAseq. For Pinhead loss-of-function, duplicate samples for control marginal zone explants (Co1, Co2) and Pinhead-depleted (Pinhead SpMo) marginal zone explants (PhSpMo1, PhSpMo2) were studied at st.11 by RNAseq.