Uncorking gastrulation: the morphogenetic movement of bottle cells

A feather star is born: embryonic development and nervous system organization in the crinoid Antedon mediterranea

Crinoids belong to the Echinodermata, marine invertebrates with a highly derived adult pentaradial body plan. As the sister group to all other extant echinoderms, crinoids occupy a key phylogenetic position to explore the evolutionary history of the whole phylum. However, their development remains understudied compared with that of other echinoderms. Therefore, the aim here was to establish the Mediterranean feather star (Antedon mediterranea) as an experimental system for developmental biology. We first set up a method for culturing embryos in vitro and defined a standardized staging system for this species. We then optimized protocols to characterize the morphological and molecular development of the main structures of the feather star body plan. Focusing on the nervous system, we showed that the larval apical organ includes serotonergic, GABAergic and glutamatergic neurons, which develop within a conserved anterior molecular signature. We described the composition of the early post-metamorphic nervous system and revealed that it has an anterior signature. These results further our knowledge on crinoid development and provide new techniques to investigate feather star embryogenesis. This will pave the way for the inclusion of crinoids in comparative studies addressing the origin of the echinoderm body plan and the evolutionary diversification of deuterostomes.

Mathematical Modeling of Tissue Folding and Asymmetric Tissue Flow during Epithelial Morphogenesis

Recent studies have revealed that intrinsic, individual cell behavior can provide the driving force for deforming a two-dimensional cell sheet to a three-dimensional tissue without the need for external regulatory elements. However, whether intrinsic, individual cell behavior could actually generate the force to induce tissue deformation was unclear, because there was no experimental method with which to verify it in vivo. In such cases, mathematical modeling can be effective for verifying whether a locally generated force can propagate through an entire tissue and induce deformation. Moreover, the mathematical model sometimes provides potential mechanistic insight beyond the information obtained from biological experimental results. Here, we present two examples of modeling tissue morphogenesis driven by cell deformation or cell interaction. In the first example, a mathematical study on tissue-autonomous folding based on a two-dimensional vertex model revealed that active modulations of cell mechanics along the basal–lateral surface, in addition to the apical side, can induce tissue-fold formation. In the second example, by applying a two-dimensional vertex model in an apical plane, a novel mechanism of tissue flow caused by asymmetric cell interactions was discovered, which explained the mechanics behind the collective cellular movement observed during epithelial morphogenesis.

Septin-dependent remodeling of cortical microtubule drives cell reshaping during epithelial wound healing

Wounds in embryos heal rapidly through contraction of the wound edges. Despite well-recognized significance of the actomyosin purse string for wound closure, roles for other cytoskeletal components are largely unknown. Here, we report that the septin cytoskeleton cooperates with actomyosin and microtubules to coordinate circumferential contraction of the wound margin and concentric elongation of wound-proximal cells in Xenopus laevis embryos. Microtubules reoriented radially, forming bundles along lateral cell cortices in elongating wound-proximal cells. Depletion of septin 7 (Sept7) slowed wound closure by attenuating the wound edge contraction and cell elongation. ROCK/Rho-kinase inhibitor-mediated suppression of actomyosin contractility enhanced the Sept7 phenotype, whereas the Sept7 depletion did not affect the accumulation of actomyosin at the wound edge. The cortical microtubule bundles were reduced in wound-proximal cells in Sept7 knockdown (Sept7-KD) embryos, but forced bundling of microtubules mediated by the microtubule-stabilizing protein Map7 did not rescue the Sept7-KD phenotype. Nocodazole-mediated microtubule depolymerization enhanced the Sept7-KD phenotype, suggesting that Sept7 is required for microtubule reorganization during cell elongation. Our findings indicate that septins are required for the rapid wound closure by facilitating cortical microtubule reorganization and the concentric elongation of surrounding cells.

The flipflop orphan genes are required for limb bud eversion in the Tribolium embryo

Background

Unlike Drosophila but similar to other arthropod and vertebrate embryos, the flour beetle Tribolium castaneum develops everted limb buds during embryogenesis. However, the molecular processes directing the evagination of epithelia are only poorly understood.

Results

Here we show that the newly discovered genes Tc-flipflop1 and Tc-flipflop2 are involved in regulating the directional budding of appendages. RNAi-knockdown of Tc-flipflop results in a variety of phenotypic traits. Most prominently, embryonic limb buds frequently grow inwards rather than out, leading to the development of inverted appendages inside the larval body. Moreover, affected embryos display dorsal closure defects. The Tc-flipflop genes are evolutionarily non-conserved, and their molecular function is not evident. We further found that Tc-RhoGEF2, a highly-conserved gene known to be involved in actomyosin-dependent cell movement and cell shape changes, shows a Tc-flipflop-like RNAi-phenotype.

Conclusions

The similarity of the inverted appendage phenotype in both the flipflop- and the RhoGEF2 RNAi gene knockdown led us to conclude that the Tc-flipflop orphan genes act in a Rho-dependent pathway that is essential for the early morphogenesis of polarised epithelial movements. Our work describes one of the few examples of an orphan gene playing a crucial role in an important developmental process.

Electronic supplementary material

The online version of this article (10.1186/s12983-017-0234-9) contains supplementary material, which is available to authorized users.

Rho GTPases in mammalian spinal neural tube closure

Neural tube closure is an important morphogenetic event that involves dramatic reshaping of both neural and non-neural tissues. Rho GTPases are key cytoskeletal regulators involved in cell motility and in several developmental processes, and are thus expected to play pivotal roles in neurulation. Here, we discuss 2 recent studies that shed light on the roles of distinct Rho GTPases in different tissues during neurulation. RhoA plays an essential role in regulating actomyosin dynamics in the neural epithelium of the elevating neural folds, while Rac1 is required for the formation of cell protrusions in the non-neural surface ectoderm during neural fold fusion.

Xenopus Pkdcc1 and Pkdcc2 Are Two New Tyrosine Kinases Involved in the Regulation of JNK Dependent Wnt/PCP Signaling Pathway

Protein Kinase Domain Containing, Cytoplasmic (PKDCC) is a protein kinase which has been implicated in longitudinal bone growth through regulation of chondrocytes formation. Nevertheless, the mechanism by which this occurs remains unknown. Here, we identified two new members of the PKDCC family, Pkdcc1 and Pkdcc2 from Xenopus laevis. Interestingly, our knockdown experiments revealed that these two proteins are both involved on blastopore and neural tube closure during gastrula and neurula stages, respectively. In vertebrates, tissue polarity and cell movement observed during gastrulation and neural tube closure are controlled by Wnt/Planar Cell Polarity (PCP) molecular pathway. Our results showed that Pkdcc1 and Pkdcc2 promote the recruitment of Dvl to the plasma membrane. But surprisingly, they revealed different roles in the induction of a luciferase reporter under the control of Atf2 promoter. While Pkdcc1 induces Atf2 expression, Pkdcc2 does not, and furthermore inhibits its normal induction by Wnt11 and Wnt5a. Altogether our data show, for the first time, that members of the PKDCC family are involved in the regulation of JNK dependent Wnt/PCP signaling pathway.

Apical constriction: themes and variations on a cellular mechanism driving morphogenesis

Apical constriction is a cell shape change that promotes tissue remodeling in a variety of homeostatic and developmental contexts, including gastrulation in many organisms and neural tube formation in vertebrates. In recent years, progress has been made towards understanding how the distinct cell biological processes that together drive apical constriction are coordinated. These processes include the contraction of actin-myosin networks, which generates force, and the attachment of actin networks to cell-cell junctions, which allows forces to be transmitted between cells. Different cell types regulate contractility and adhesion in unique ways, resulting in apical constriction with varying dynamics and subcellular organizations, as well as a variety of resulting tissue shape changes. Understanding both the common themes and the variations in apical constriction mechanisms promises to provide insight into the mechanics that underlie tissue morphogenesis.