Thanks to an interdisciplinary and exciting collaboration between the iBV and the CBI in Toulouse, the Rauzi and the Wang teams unveiled a Cdc42 dependent mechanical process responsible for tissue elongation in the developing Drosophila egg chamber. By engineering new optogenetic techniques and infra-red laser based surgery, Anna Popkova and others show that a polarized supracellular actomyosin network, working as a molecular corset, generates tissue scale forces directing egg chamber elongation. Finally, this study opens new avenues to better understand how supra-cellular cytoskeletal networks emerge to drive embryo-scale morphogenesis during development. This work was published in Nature Communications.
Understanding the mechanisms that are responsible to shape tissues and organs is an important and exciting challenge in developmental biology. The actomyosin cortex often plays a key role in generating the forces necessary to shape individual cells. Nevertheless, the actomyosin architecture can extend beyond the size of a single cell: cytoskeletal networks emerge at the scale of a tissue. These supra-cellular contractile structures can generate large forces that can rapidly shape epithelia.
In this study, scientists from the Rauzi and the Wang lab use the Drosophila egg chamber as a model system to study the origin and the mechanics of supra-cellular actomyosin networks. Popkova and others show that a supra-cellular array of parallel actin bundles, emerging from interdigitating filopodia, envelopes the follicle tissue. Filopodia radiate in a polarized way from basal stress fibers and extend by penetrating the neighboring cell cortexes. Filopodia can be mechanosensitive and function as anchors between cells. The small GTPase Cdc42 governs the formation of intercellular filopodia and stress fibers in the follicular cells. Thus, a Cdc42-dependent supracellular cytoskeletal network provides a scaffold integrating local oscillatory actomyosin contractions at the tissue scale to drive global polarized forces and tissue elongation.
Actin filaments assemble into diverse protrusive and contractile networks to generate forces in diverse cellular processes. Stress fibers are contractile higher order cytoskeletal structures composed of actomyosin bundles. Stress fibers play a key role in generating forces along the fiber direction and have important implication in cell adhesion to the extracellular matrix.
Filopodia are dynamic, finger-like plasma membrane protrusions that act as antennae to sense the mechanical and chemical environment. They are often regarded as “sensory organelles”. Filopodia are involved in many biological processes, such as growth cone guidance, cell migration, wound closure, and macrophage-induced cell invasion. These thin membrane protrusions are 60–200 nm in diameter and contain parallel bundles of 10–30 actin filaments held together by actin-binding proteins.
Laser dissection is a useful tool in developmental biology to probe the mechanical forces from the subcellular to the tissue/embryo scale. During tissue morphogenesis, cells are equipped with actomyosin networks generating forces. In this study, researchers used near-infrared (NIR) femtosecond (fs) pulsed laser surgery to dissect the actomyosin cytoskeleton with subcellular precision. This technique allows to selectively ablate actomyosin networks while preserving the cell plasma membrane. The resulting recoil of the remaining network, after laser dissection, is imaged and analyzed to deduce local forces responsible for tissue morphogenesis.
Optogenetics allows to control, via light stimulation, protein conformation changes and thus protein activity with spatial and temporal specificity. By using molecular engineering, proteins of interest are fused to photo-activatable proteins than can be expressed in specific cells. Using the optogenetic tool PA-Cdc42 which leads to the expression of a light-activatable Cdc42 protein, researchers were able to precisely determine the role of Cdc42 in follicular cells.
To read more:
A Cdc42-mediated supracellular network drives polarized forces and Drosophila egg chamber extension.
Popkova A, Stone OJ, Chen L, Qin X, Liu C, Liu J, Belguise K, Montell DJ, Hahn KM, Rauzi M@, Wang X@.
Nat Commun. 2020 Apr 21;11(1):1921. doi: 10.1038/s41467-020-15593-2.
Press release : Actualités scientifiques de l’INSB
Movie: Time-lapse of a representative mCD8GFP-expressing egg chamber labelled with MyoII-mCherry. Laser dissection of the supra-cellular actomyosin network was performed along the AP axis of the egg chamber. Scale bar 10 μm.
DOCTORAL POSITION AT THE UNIVERSITY CÔTE D’AZUR
4D MORPHOMETRIC STUDY OF CELL AND TISSUE SHAPE CHANGES TO COMPUTATIONALLY
UNRAVEL THE PROCESS OF SEA URCHIN EMBRYO GASTRULATION
Computational morphometric analysis has become an essential tool in modern biology to better understand how cell and tissue change shape during embryo development. Therefore, exciting collaborations among computer scientists and biologists have arisen to pierce the mystery of how life take shape. In recent years, new microscopy techniques (e.g., SPIM) have enabled the digital image acquisition of developing embryos with unprecedented 3D spatial and temporal resolution allowing a fine reconstruction of all the morphogenetic processes concurring to shape the embryo .
The acquisition of 3D+t high resolved image series results in huge amount of data (also referred to as BIG data sets). Basic image processing approaches fail to provide the necessary tools for multi-dimensional image analysis. We have developed sophisticated image analysis tools to extract multi-dimensional information from BIG data sets . Our image analysis tools were successfully applied to embryos constituted of tens of cells . Very recently, in a joint collaboration between the Morpheme team and the Rauzi team, this computational tool has been extended to temporal 3D image series of the developing sea urchin embryo constituted by more than 1000 cells. These BIG data sets together with our image analysis tools give access for the first time to detailed morphometric information of shape changes of each single cell (tracked over time in 3D) forming the sea urchin embryo .
We are at present interested in better understating the mechanisms driving gastrulation in the sea urchin embryo. During this developmental phase, the tissue located at the vegetal pole buckles initiating gut formation. Such changes in tissue shape are driven by stereotypic cell shape and topological changes. The goal of this doctoral project is to characterize in 4D the cell shape/topology and cytoskeletal protein variations, and to perform cell population analysis to eventually unveil the key stereotypic processes driving tissue buckling.
We are seeking a very motivated and talented candidate with advanced expertise in computer science, mathematics or physics. We expect the candidate to have skills in several of the following fields: Image Processing and Analysis, Data Sciences, and Machine Learning. S/he should be proficient in programming in C/C++ and Python languages. Previous experience in biological or medical imaging will be considered as an asset.
Applicants must send as soon as possible a CV, a statement of interest, and 2 or 3 reference letters to Grégoire Malandain (firstname.lastname@example.org) and to Matteo Rauzi (email@example.com).
Deadline: 17th of May 2020
Location: Inria-I3S Morpheme team, I3S, Sophia-Antipolis, France
 PJ Keller, “Imaging Morphogenesis: Technological Advances and Biological Insights,” Science, vol. 340, no. 6137, pp. 1234168+, June 2013.
 R Fernandez, P Das, V Mirabet, E Moscardi, J Traas, JL Verdeil, G Malandain, and C Godin, “Imaging plant growth in 4-d: robust tissue reconstruction and lineaging at cell resolution,”Nat Meth, vol. 7, pp. 547–553, 2010.
 Guignard, L., Fiuza, U.-M., Leggio, B., Laussu, J., Faure, E., Michelin, G., Biasuz, K., Hufnagel, L., Malandain, G., Godin, C., and Lemaire, P. (2020). Contact-area dependent cell communications and the morphological invariance of ascidian embryogenesis. Science, accepted for publication.
 Moullet, A. (2020) “Automated segmentation of sea urchin embryos”, Ms thesis.
Doctoral position at the Côte d’Azur & Sorbonne University
Studying the molecular control and the mechanics of sea urchin gastrulation: a model for epithelial folding
Epithelial folding is a key process in the life of all animals. During embryogenesis, this process takes place notably during gastrulation. In the laboratory, we use the sea urchin embryo to study the mechanisms, mechanics and molecular control of epithelial folding. Sea urchin gastrulae combine a number of outstanding features making this model system a unique opportunity to study folding: (i) external development and tissue simplicity enabling both experimentation and modeling approaches; (ii) cell transparency allowing in toto light sheet imaging; (iii) known key signaling factors and available methods for functional analyses; and (iv) mechanically accessible tissues permitting direct measurements of tissue mechanical properties. Given these features, the sea urchin gastrula is thus a perfect playground for both biologists and biophysicists. The aim is to provide new insights on the fundamental process of epithelial folding both at the mechanical and molecular levels.
The proposed project is a collaborative work between two laboratories (Rauzi M. at iBV in Nice and Croce J. at the LBDV at the marine station in Villefranche-sur-Mer) that gather people from different backgrounds (biology, informatics, physics, and engineering) to generate an interdisciplinary and synergistic group in an international environment.
Seeking a talented and very motivated candidate.
Deadline: April 27th, 2020. Starting date: October 2020.
Send as soon as possible a CV, a motivation letter, master scores/ranking and reference letters to: firstname.lastname@example.org and email@example.com
CROCE EVOINSIDE / RAUZI LAB
Postdoctoral position at the University Côte d’Azur
Studying the mechanisms and mechanics of sea urchin gastrulation: a model for epithelial folding
During embryo gastrulation one of the key morphogenetic processes is tissue folding. We use the sea urchin embryo, a spherical monolayer epithelium that folds its vegetal plate forming a tube: the future gut. What are the mechanisms and the mechanical foces driving folding is still not well understood.The sea urchin gastrula combines a number of outstanding features making this model system a unique opportunity to study the mechanisms and mechanics of tissue folding: (1) simplicity; (2) transparencey for in toto light sheet imaging; (3) known key signaling factors; (4) the gastrula is a mechanically accessible tissue: it can be partitioned, cells can be transplanted, micro-indentation and micro-pipetting techniques can be applied to measure tissue mechanical properties. The sea urchin gastrula is thus a perfect playground for biologists and biophysicists.
The projects developed in the lab gather people from different backgrounds (biology, informatics, physics, and engineering) to generate an interdisciplinary and synergistic group in an international environment.
Seeking talanted candidates preferibly with expereince in micro pipetting, nano/μ indentation, live imaging, light sheet microscopy or marine model systems.
Deadline: May 10th, 2020. Starting date: Before the end of 2020.
Send a CV, motivation letter and reference letters to firstname.lastname@example.org
RAUZI LAB: http://ibv.unice.fr/research-team/rauzi
iBV is a member of Université Cote d’Azur (UCA), a cluster of Research and Higher Education on the French Riviera. Each year UCA organises a special award ceremony recognising the talent and accomplishments of researchers, students and artists who have been awarded prestigious prizes for their work.
This year, 7 iBV members will take part to the ceremony for their excellent scientific contributions:
The ceremony will take place at the "Galet" amphitheater, at the Pasteur Hospital Center in Nice on Monday December 10th at 6pm.
Congratulations to All Awardees !