Understanding the factors that direct tissue organization during development is one of the most fundamental goals in developmental biology. Various hypotheses explain cell sorting and tissue organization on the basis of the adhesive and mechanical properties of the constituent cells. However, validating these hypotheses has been difficult due to the lack of appropriate tools to measure these parameters. Here we use atomic force microscopy (AFM) to quantify the adhesive and mechanical properties of individual ectoderm, mesoderm and endoderm progenitor cells from gastrulating zebrafish embryos. Combining these data with tissue self-assembly in vitro and the sorting behaviour of progenitors in vivo, we have shown that differential actomyosin-dependent cell-cortex tension, regulated by Nodal/TGFbeta-signalling (transforming growth factor beta), constitutes a key factor that directs progenitor-cell sorting. These results demonstrate a previously unrecognized role for Nodal-controlled cell-cortex tension in germ-layer organization during gastrulation.

Collective cell migration, the simultaneous movement of multiple cells that are connected by cell-cell adhesion, is ubiquitous in development, tissue repair, and tumor metastasis [1, 2]. It has been hypothesized that the directionality of cell movement during collective migration emerges as a collective property [3, 4]. Here we determine how movement directionality is established in collective mesendoderm migration during zebrafish gastrulation. By interfering with two key features of collective migration, (1) having neighboring cells and (2) adhering to them, we show that individual mesendoderm cells are capable of normal directed migration when moving as single cells but require cell-cell adhesion to participate in coordinated and directed migration when moving as part of a group. We conclude that movement directionality is not a de novo collective property of mesendoderm cells but rather a property of single mesendoderm cells that requires cell-cell adhesion during collective migration.

Unfertilized oocytes have the intrinsic capacity to remodel sperm and the nuclei of somatic cells. The discoveries that cells can change their phenotype from differentiated to embryonic state using oocytes or specific transcription factors have been recognized as two major breakthroughs in the biomedical field. Here, we show that ASF1A, a histone-remodeling chaperone specifically enriched in the metaphase II human oocyte, is necessary for reprogramming of human adult dermal fibroblasts (hADFs) into undifferentiated induced pluripotent stem cell. We also show that overexpression of just ASF1A and OCT4 in hADFs exposed to the oocyte-specific paracrine growth factor GDF9 can reprogram hADFs into pluripotent cells. Our Report underscores the importance of studying the unfertilized MII oocyte as a means to understand the molecular pathways governing somatic cell reprogramming.

Bacterial physiology and adaptation are influenced by the exopolysaccharides (EPS) they produce. These polymers are indispensable for the assembly of the biofilm extracellular matrix in multiple bacterial species. In a previous study, we described the profound gene expression changes leading to biofilm assembly in B. cereus ATCC14579 (CECT148). We found that a genomic region putatively dedicated to the synthesis of a capsular polysaccharide (eps2) was overexpressed in a biofilm cell population compared to in a planktonic population, while we detected no change in the transcript abundance from another genomic region (eps1) also likely to be involved in polysaccharide production. Preliminary biofilm assays suggested a mild role for the products of the eps2 region in biofilm formation and no function for the products of the eps1 region. The aim of this work was to better define the roles of these two regions in B. cereus multicellularity. We demonstrate that the eps2 region is indeed involved in bacterial adhesion to surfaces, cell-to-cell interaction, cellular aggregation and biofilm formation, while the eps1 region appears to be involved in a kind of social bacterial motility. Consistent with these results, we further demonstrate using bacterial-host cell interaction experiments that the eps2 region is more relevant to the adhesion to human epithelial cells and the zebrafish intestine, suggesting that this region encodes a bacterial factor that may potentiate gut colonization and enhance pathogenicity against humans.

Mouse and cell-based studies have shown that macroH2A histone variants predominantly associate with heterochromatin. Functional studies found that macroH2As are involved in gene repression, inhibiting the acquisition of pluripotency and preserving cell differentiation. However, only a few studies have analysed the role of macroH2A during early embryo development. We report the development of transgenic zebrafish lines expressing macroH2A isoforms (mH2A1 and mH2A2) fusion proteins (with GFP) under identified endogenous promoters. We found that mH2A1 and mH2A2 have different spatial and temporal expression patterns during embryonic development. mH2A1 is expressed mostly in the extraembryonic Yolk Syncytial Layer (YSL) starting before shield stage and decreasing once morphogenesis is completed. mH2A2 expression lags behind mH2A1, becoming evident at 24 hpf, within the whole body of the embryo proper. Our ChIP-seq analysis showed that mH2A1 and mH2A2 bind to different DNA regions, changing dramatically after gastrulation. We further analysed RNA-seq data and showed that there is not a general/unspecific repressing function of mH2A1 or mH2A2 associated with heterochromatin but a fine regulation depending on cell types and stage of development. mH2A1 downregulates DNA expression in specific cells and embryo stages and its effect is independent of heterochromatin formation but it is correlated with nucleus quiescence instead. Whereas mH2A2 DNA association correlates with upregulation of differentially expressed genes between 75% epiboly and 24 hpf stages. Our data provide information for underlying molecules that participate in crucial early developmental events, and open new venues to explore mH2A related mechanisms that involve cell proliferation, differentiation, migration and metabolism.