Origin and function of non-apical mitoses in epithelial-mesenchymal transition during mouse embryo gastrulation

La transition épithélio-mésenchymateuse (TEM) est un processus plastique et réversible qui permet à une cellule épithéliale de devenir motile par l’acquisition d’un phénotype mésenchymateux. La gastrulation est un processus développemental menant à la formation des trois feuillets germinatifs nécessaires à l’organogénèse. Ce processus est l’un des premiers impliquant la TEM. Dans l’embryon de souris, la gastrulation se déroule au niveau de la ligne primitive, localisée dans la région postérieure de l’épiblaste. L’épiblaste est un épithélium pseudostratifié dans lequel se produit un phénomène appelé la migration nucléaire interkinétique, qui implique le mouvement des noyaux en fonction de l’avancée dans le cycle cellulaire, les mitoses ayant toujours lieu en position apicale.Par le biais d'observations dynamiques et statiques de la division cellulaire dans l'épiblaste, nous avons démontré que les cellules de la ligne primitive subissent des mitoses plus fréquentes que les autres cellules de l'épiblaste. De plus, tandis que les mitoses sont principalement confinées au pôle apical dans les régions antérieure et postérieure de l'épiblaste, elles peuvent se produire tout le long de l'axe apico-basal dans la région de la ligne primitive.En utilisant le criblage pharmacologique, nous avons pu déterminer que la région de la ligne primitive est soumise à une régulation moins stricte de contractilité du cytosquelette d’actomyosine et de division cellulaire comparée au reste de l’épiblaste. En utilisant un modèle de simulation informatique de la dynamique de l’épiblaste, nous avons pu confirmer que la relaxation du cytosquelette d’actomyosine et l’accélération du temps de cycle observés à la ligne primitive favorisent l’apparition des mitoses non apicales. De plus, les simulations suggèrent un rôle clé pour ces changements de régulation dans la facilitation du positionnement basal des noyaux et l'extrusion basale des cellules dans la région de la ligne primitive lors de la gastrulation chez la souris.En combinant l’utilisation d’outils génétiques avec l’analyse des cribles pharmacologiques effectués in vivo et in vitro dans un modèle 3D synthétique de l’épiblaste, nous avons déterminé que les variations de fréquence et d’emplacement des mitoses dans la région de la ligne primitive n’étaient pas suffisantes pour modifier le patron de spécification du mésoderme.Doctorat en Sciences biomédicales et pharmaceutiques (Médecine)info:eu-repo/semantics/nonPublishe

Mitosis, a springboard for epithelial-mesenchymal transition?

Mitosis is a key process in development and remains critical to ensure homeostasis in adult tissues. Besides its primary role in generating two new cells, cell division involves deep structural and molecular changes that might have additional effects on cell and tissue fate and shape. Specific quantitative and qualitative regulation of mitosis has been observed in multiple morphogenetic events in different embryo models. For instance, during mouse embryo gastrulation, the portion of epithelium that undergoes epithelial to mesenchymal transition, where a static epithelial cell become mesenchymal and motile, has a higher mitotic index and a distinct localization of mitotic rounding, compared to the rest of the tissue. Here we explore the potential mechanisms through which mitosis may favor tissue reorganization in various models. Notably, we discuss the mechanical impact of cell rounding on the cell and its environment, and the modification of tissue physical parameters through changes in cell-cell and cell-matrix adhesion.info:eu-repo/semantics/publishe

Interkinetic nuclear movements promote apical expansion in pseudostratified epithelia at the expense of apicobasal elongation.

Pseudostratified epithelia (PSE) are a common type of columnar epithelia found in a wealth of embryonic and adult tissues such as ectodermal placodes, the trachea, the ureter, the gut and the neuroepithelium. PSE are characterized by the choreographed displacement of cells' nuclei along the apicobasal axis according to phases of their cell cycle. Such movements, called interkinetic movements (INM), have been proposed to influence tissue expansion and shape and suggested as culprit in several congenital diseases such as CAKUT (Congenital anomalies of kidney and urinary tract) and esophageal atresia. INM rely on cytoskeleton dynamics just as adhesion, contractility and mitosis do. Therefore, long term impairment of INM without affecting proliferation and adhesion is currently technically unachievable. Here we bypassed this hurdle by generating a 2D agent-based model of a proliferating PSE and compared its output to the growth of the chick neuroepithelium to assess the interplay between INM and these other important cell processes during growth of a PSE. We found that INM directly generates apical expansion and apical nuclear crowding. In addition, our data strongly suggest that apicobasal elongation of cells is not an emerging property of a proliferative PSE but rather requires a specific elongation program. We then discuss how such program might functionally link INM, tissue growth and differentiation

Regionally specific levels and patterns of keratin 8 expression in the mouse embryo visceral endoderm emerge upon anterior-posterior axis determination

The mechanical properties of the different germ layers of the early mammalian embryo are likely to be critical for morphogenesis. Cytoskeleton components (actin and myosin, microtubules, intermediate filaments) are major determinants of epithelial plasticity and resilience to stress. Here, we take advantage of a mouse reporter for Keratin 8 to record the pattern of the keratin intermediate filaments network in the first epithelia of the developing mouse embryo. At the blastocyst stage, Keratin 8 is strongly expressed in the trophectoderm, and undetectable in the inner cell mass and its derivatives, the epiblast and primitive endoderm. Visceral endoderm cells that differentiate from the primitive endoderm at the egg cylinder stage display apical Keratin 8 filaments. Upon migration of the Anterior Visceral Endoderm and determination of the anterior-posterior axis, Keratin 8 becomes regionally distributed, with a stronger expression in embryonic, compared to extra-embryonic, visceral endoderm. This pattern emerges concomitantly to a modification of the distribution of Filamentous (F)-actin, from a cortical ring to a dense apical shroud, in extra-embryonic visceral endoderm only. Those regional characteristics are maintained across gastrulation. Interestingly, for each stage and region of the embryo, adjacent germ layers display contrasted levels of keratin filaments, which may play a role in their adaptation to growth and morphological changes.info:eu-repo/semantics/publishe

Asymmetry in the frequency and position of mitosis in the mouse embryo epiblast at gastrulation.

At gastrulation, a subpopulation of epiblast cells constitutes a transient posteriorly located structure called the primitive streak, where cells that undergo epithelial-mesenchymal transition make up the mesoderm and endoderm lineages. Mouse embryo epiblast cells were labelled ubiquitously or in a mosaic fashion. Cell shape, packing, organization and division were recorded through live imaging during primitive streak formation. Posterior epiblast displays a higher frequency of rosettes, some of which associate with a central cell undergoing mitosis. Cells at the primitive streak, in particular delaminating cells, undergo mitosis more frequently than other epiblast cells. In pseudostratified epithelia, mitosis takes place at the apical side of the epithelium. However, mitosis is not restricted to the apical side of the epiblast, particularly on its posterior side. Non-apical mitosis occurs specifically in the streak even when ectopically located. Posterior non-apical mitosis results in one or two daughter cells leaving the epiblast layer. Cell rearrangement associated with mitotic cell rounding in posterior epiblast, in particular when non-apical, might thus facilitate cell ingression and transition to a mesenchymal phenotype.info:eu-repo/semantics/publishe

An asymmetry in the frequency and position of mitosis in the epiblast precedes gastrulation and suggests a role for mitotic rounding in cell delamination during primitive streak epithelial-mesenchymal transition

info:eu-repo/semantics/publishedin revision a EMBO Report

Keratin filaments mediate the expansion of extra-embryonic membranes in the post-gastrulation mouse embryo.

Mesoderm arises at gastrulation and contributes to both the mouse embryo proper and its extra-embryonic membranes. Two-photon live imaging of embryos bearing a keratin reporter allowed recording filament nucleation and elongation in the extra-embryonic region. Upon separation of amniotic and exocoelomic cavities, keratin 8 formed apical cables co-aligned across multiple cells in the amnion, allantois, and blood islands. An influence of substrate rigidity and composition on cell behavior and keratin content was observed in mesoderm explants. Embryos lacking all keratin filaments displayed a deflated extra-embryonic cavity, a narrow thick amnion, and a short allantois. Single-cell RNA sequencing of sorted mesoderm cells and micro-dissected amnion, chorion, and allantois, provided an atlas of transcriptomes with germ layer and regional information. It defined the cytoskeleton and adhesion expression profile of mesoderm-derived keratin 8-enriched cells lining the exocoelomic cavity. Those findings indicate a novel role for keratin filaments in the expansion of extra-embryonic structures and suggest mechanisms of mesoderm adaptation to the environment.info:eu-repo/semantics/publishe