RNA-Seq analysis on ets1 mutant embryos of Xenopus tropicalis identifies microseminoprotein beta gene 3 as an essential regulator of neural crest migration

ZSWIM4 regulates embryonic patterning and BMP signaling by promoting nuclear Smad1 degradation

The dorsoventral gradient of BMP signaling plays an essential role in embryonic patterning. Zinc Finger SWIM-Type Containing 4 (zswim4) is expressed in the Spemann-Mangold organizer at the onset of Xenopus gastrulation and is then enriched in the developing neuroectoderm at the mid-gastrula stages. Knockdown or knockout of zswim4 causes ventralization. Overexpression of zswim4 decreases, whereas knockdown of zswim4 increases the expression levels of ventrolateral mesoderm marker genes. Mechanistically, ZSWIM4 attenuates the BMP signal by reducing the protein stability of SMAD1 in the nucleus. Stable isotope labeling by amino acids in cell culture (SILAC) identifies Elongin B (ELOB) and Elongin C (ELOC) as the interaction partners of ZSWIM4. Accordingly, ZSWIM4 forms a complex with the Cul2-RING ubiquitin ligase and ELOB and ELOC, promoting the ubiquitination and degradation of SMAD1 in the nucleus. Our study identifies a novel mechanism that restricts BMP signaling in the nucleus.

Modification of maternally defined H3K4me3 regulates the inviability of interspecific Xenopus hybrids

Increasing evidence suggests that interspecific hybridization is crucial to speciation. However, chromatin incompatibility during interspecific hybridization often renders this process. Genomic imbalances such as chromosomal DNA loss and rearrangements leading to infertility have been commonly noted in hybrids. The mechanism underlying reproductive isolation of interspecific hybridization remains elusive. Here, we identified that modification of maternally defined H3K4me3 in Xenopus laevis and Xenopus tropicalis hybrids determines the different fates of the two types of hybrids as te×ls with developmental arrest and viable le×ts. Transcriptomics highlighted that the P53 pathway was overactivated, and the Wnt signaling pathway was suppressed in te×ls hybrids. Moreover, the lack of maternal H3K4me3 in te×ls disturbed the balance of gene expression between the L and S subgenomes in this hybrid. Attenuation of p53 can postpone the arrested development of te×ls. Our study suggests an additional model of reproductive isolation based on modifications of maternally defined H3K4me3.

An efficient miRNA knockout approach using CRISPR-Cas9 in Xenopus

In recent years CRISPR-Cas9 knockouts (KO) have become increasingly ultilised to study gene function. MicroRNAs (miRNAs) are short non-coding RNAs, 20-22 nucleotides long, which affect gene expression through post-transcriptional repression. We previously identified miRNAs-196a and -219 as implicated in the development of Xenopus neural crest (NC). The NC is a multipotent stem-cell population, specified during early neurulation. Following EMT, NC cells migrate to various points in the developing embryo where they give rise to a number of tissues including parts of the peripheral nervous system, pigment cells and craniofacial skeleton. Dysregulation of NC development results in many diseases grouped under the term neurocristopathies. As miRNAs are so small, it is difficult to design CRISPR sgRNAs that reproducibly lead to a KO. We have therefore designed a novel approach using two guide RNAs to effectively 'drop out' a miRNA. We have knocked out miR-196a and miR-219 and compared the results to morpholino knockdowns (KD) of the same miRNAs. Validation of efficient CRISPR miRNA KO and phenotype analysis included use of whole-mount in situ hybridization of key NC and neural plate border markers such as Pax3, Xhe2, Sox10 and Snail2, q-RT-PCR and Sanger sequencing. To show specificity we have also rescued the knockout phenotype using miRNA mimics. MiRNA-219 and miR-196a KO's both show loss of NC, altered neural plate and hatching gland phenotypes. Tadpoles show gross craniofacial and pigment phenotypes.

Xenopus chip for single-egg trapping, in vitro fertilization, development, and tadpole escape

Xenopus laevis is highly suitable as a toxicology animal model owing to its advantages in embryogenesis research. For toxicological studies, a large number of embryos must be handled simultaneously because they very rapidly develop into the target stages within a short period of time. To efficiently handle the embryos, a convenient embryo housing device is essential for fast and reliable assessment and statistical evaluation of malformation caused by toxicants. Here, we suggest 3D fabrication of single-egg trapping devices in which Xenopus eggs are fertilized in vitro, and the embryos are cultured. We used manual pipetting to insert the Xenopus eggs inside the trapping sites of the chip. By introducing a liquid circulating system, we connected a sperm-mixed solution with the chip to induce in vitro fertilization of the eggs. After the eggs were fertilized, we observed embryo development involving the formation of egg cleavage, blastula, gastrula, and tadpole. After the tadpoles grew inside the chip, we saved their lives by enabling their escape from the chip through reverse flow of the culture medium. The Xenopus chip can serve as an incubator to induce fertilization and monitor normal and abnormal development of the Xenopus from egg to tadpole.

Three-dimensional structures of avian beta-microseminoproteins: insight from the chicken egg-specific beta-microseminoprotein 3 paralog

Beta‐microseminoproteins (MSMBs) are small disulfide‐rich proteins that are conserved among vertebrates. These proteins exhibit diverse biological activities and were mainly reported to play a role in male fertility, immunity, and embryogenesis. In this work, we focused on the chicken MSMB3 protein that was previously depicted as an egg antibacterial protein. We report that MSMB3 protein is exclusively expressed in the reproductive tissues of laying hens (in contrast to chicken MSMB1 and MSMB2 paralogs), to be incorporated in the egg white during the process of egg formation. We also showed that chicken MSMB3 possesses highly conserved orthologs in bird species, including Neognathae and Palaeognathae. Chicken MSMB3 was purified from egg white using heparin affinity chromatography and was analyzed by top‐down and bottom‐up proteomics. Several proteoforms could be characterized, and a homodimer was further evidenced by NMR spectroscopy. The X‐ray structure of chicken MSMB3 was solved for the first time, revealing that this protein adopts a novel dimeric arrangement. The highly cationic MSMB3 protein exhibits a distinct electrostatic distribution compared with chicken MSMB1 and MSMB2 structural models, and with published mammalian MSMB structures. The specific incorporation of MSMB3 paralog in the egg, and its phylogenetic conservation in birds together with its peculiar homodimer arrangement and physicochemical properties, suggests that the MSMB3 protein has evolved to play a critical role during the embryonic development of avian species. These new data are likely to stimulate research to elucidate the structure/function relationships of MSMB paralogs and orthologs in the animal kingdom.