Biology subjects

Sanson, B.

Publications and source records attributed to Sanson, B..

9 recordsLinked to original sources

bioRxiv2023

Drosophila axis extension is robust to an orthogonal pull by invaginating mesoderm

As tissues grow and change shape during animal development, they physically pull and push on each other and these mechanical interactions can be important for morphogenesis. During Drosophila gastrulation, mesoderm invagination temporally overlaps with the extension of the ectodermal germband; the latter is caused primarily by Myosin II-driven polarised cell intercalation. Here we investigate the impact of mesoderm invagination on ectoderm extension, examining possible mechanical and mechanotransductive effects on Myosin II recruitment and polarised cell intercalation. We find that the germband ectoderm is deformed by the mesoderm pulling in the orthogonal direction, showing mechanical coupling between these tissues. However, we do not find a significant change in Myosin II planar polarisation in response to mesoderm invagination, nor an effect on the rate of junction shrinkage leading to cell intercalation events. We find some impact on the orientation of neighbour exchange events, and an increased rate of growth of new cell junctions, but this makes little difference to the rate of cell intercalation. We conclude that the cellular mechanisms of axis extension are robust to the mechanical pull of mesoderm invagination.

developmental biology↗

bioRxiv2023

Mechanical stress combines with planar polarised patterning during metaphase to orient embryonic epithelial cell divisions

The orientation of cell division (OCD) in the plane of epithelia drives tissue morphogenesis and relaxes stresses, with errors leading to pathologies. Stress anisotropy, cell elongation and planar polarisation can all contribute to the OCD, but it is unclear how these interact in vivo. In the planar polarised Drosophila embryonic ectoderm during axis elongation, planar OCD is highly variable. We show that both a temporary reversal of tissue stress anisotropy and local compression from neighbouring dividing cells re-orient mitotic spindles during metaphase, independently of interphase cell elongation. Isotropic cells align their OCD to the anterior-posterior (AP) embryonic axis, mediated by tissue-wide planar polarised Myosin II, while the spindle of elongated cells is sterically constrained to cell long axes. Thus AP-patterning ensures that cell division combines with cell rearrangement to extend the body axis, except when strong local stress anisotropy is dissipated by cells dividing according to their elongation.

biophysics↗

bioRxiv2021

Different temporal requirements for the LRR transmembrane receptors Tartan and Toll-2 in the formation of contractile interfaces at compartmental boundaries

Compartmental boundaries physically separate groups of epithelial cells, a property fundamental for the organization of the body plan in both insects and vertebrates. In many examples, this physical separation has been shown to be the consequence of a regulated increase in contractility of the actomyosin cortex at boundary cell-cell interfaces, a property important in developmental morphogenesis beyond compartmental boundary formation. In this study, we took an unbiased screening approach to identify cell surface receptors required for actomyosin enrichment and polarisation at parasegmental boundaries (PSBs) in early Drosophila embryos, leading us to uncover different temporal requirements for two LRR receptors, Tartan and Toll-2. First, we find that Tartan is required during germband extension for actomyosin enrichment at PSBs, confirming an earlier report. Next, by following in real time the dynamics of loss of boundary straightness in tartan mutant embryos compared to wildtype and ftz mutant embryos, we show that Tartan is not required beyond germband extension. At this stage, actomyosin enrichment at PSBs becomes regulated by Wingless signalling. We find that Wingless signalling regulates Toll-2 expression and we show that Toll-2 is required for planar polarization of actomyosin after the completion of germ-band extension. Thus the formation of contractile interfaces at PSBs depends on a dynamic set of LRR receptors cues. Our study also suggests that the number of receptor cues is small and that the receptors are interchangeable.

developmental biology↗

bioRxiv2021

Embryo-scale epithelial buckling forms a propagating furrow that initiates gastrulation

Cell apical constriction driven by actomyosin contraction forces is a conserved mechanism during tissue folding in embryo development. While much effort has been made to better understand the molecular mechanisms responsible for apical constriction, it is still not clear if apical actomyosin contraction forces are necessary or sufficient per se to drive tissue folding. To tackle this question, we use the Drosophila embryo model system that forms a furrow on the ventral side, initiating mesoderm internalization. Past computational models support the idea that cell apical contraction forces may not be sufficient and that active or passive cell apico-basal forces may be necessary to drive cell wedging and tissue furrowing. By using 3D computational modelling and in toto embryo image analysis and manipulation, we now challenge this idea and show that embryo-scale force balance of the tissue surface, rather than cell-autonomous shape changes, is necessary and sufficient to drive a buckling of the epithelial surface forming a furrow which propagates and initiates embryo gastrulation.

developmental biology↗

bioRxiv2021

Adhesion dynamics regulate cell intercalation behaviour in an active tissue

Cell intercalation is a key cell behaviour of morphogenesis and wound healing, where local cell neighbour exchanges can cause dramatic tissue deformations such as body axis extension. Here, we develop a mechanical model to understand active cell intercalation behaviours in the context of an epithelial tissue. Extending existing descriptions, such as vertex models, the junctional actomyosin cortex of every cell is modelled as a continuum morphoelastic rod, explicitly representing cortices facing each other at bicellular junctions. Cells are described directly in terms of the key subcellular constituents that drive dynamics, with localised stresses from the contractile actomyosin cortex and adhesion molecules coupling apposed cortices. This multi-scale apposed-cortex formulation reveals key behaviours that drive tissue dynamics, such as cell-cell shearing and flow of junctional material past cell vertices. We show that cell neighbour exchanges can be driven by purely junctional mechanisms. Active contractility and viscous turnover in a single bicellular junction are sufficient to shrink and remove a junction. Next, the 4-way vertex is resolved and a new, orthogonal junction extends passively. The adhesion timescale defines a frictional viscosity that is an important regulator of these dynamics, modulating tension transmission in the tissue as well as the speeds of junction shrinkage and growth. The model additionally predicts that rosettes, which form when a vertex becomes common to many cells, are likely to occur in active tissues with high adhesive friction. SIGNIFICANCECell intercalation, or neighbour exchange, is a crucial behaviour that can drive tissue deformations, dissipate stress and facilitate wound healing. Substantial experimental work has identified the key molecular players facilitating intercalation, but there remains a lack of consensus and understanding of their physical roles. Existing biophysical models that represent cell-cell contacts with single edges cannot study the continuous dynamics of intercalation, involving shear between coupled cell cortices. Deriving a continuum description of the cell cortex, explicitly coupling neighbouring cortices with adhesions, we define the biophysical conditions required for successful neighbour exchanges. Furthermore, we show how the turnover of adhesion molecules specifies a viscous friction that regulates active tissue dynamics.

biophysics↗

bioRxiv2019

The tricellular vertex-specific adhesion molecule Sidekick facilitates polarised cell intercalation during Drosophila axis extension

In epithelia, tricellular vertices are emerging as important sites for the regulation of epithelial integrity and function. Compared to bicellular contacts, however, much less knowledge is available. In particular, resident proteins at tricellular vertices were identified only at occluding junctions, with none known at adherens junctions. In a previous study, we discovered that in Drosophila embryos, the adhesion molecule Sidekick (Sdk), well known in invertebrates and vertebrates for its role in the visual system, localises at tricellular vertices at the level of adherens junctions. Here, we survey a wide range of Drosophila epithelia and establish that Sdk is a resident protein at tricellular adherens junctions, the first of its kind. Clonal analysis suggests that pair-wise homophilic adhesion is necessary and sufficient for Sdk tricellular vertex localisation. Super-resolution imaging using structured illumination reveals that Sdk proteins form string-like structures at vertices. Postulating that Sdk may have a role in epithelia where adherens junctions are actively remodelled, we analysed the phenotype of sdk null mutant embryos during Drosophila axis extension, using quantitative methods. We find that apical cell shapes are strikingly abnormal in sdk mutants. Moreover, adhesion at apical vertices is compromised in rearranging cells, with holes forming and persisting throughout axis extension. Finally, we show that polarized cell intercalation is decreased and abnormal in sdk mutants. Mathematical modeling of the cell behaviours supports the conclusion that the T1 transitions of polarized cell intercalation are delayed in sdk mutants. We propose that this delay, in combination with a change in the mechanical properties of the converging and extending tissue, causes the striking cell shape phenotype of sdk mutant embryos.

developmental biology↗

bioRxiv2019

Joint Motion Estimation and Source Identification using Convective Regularisation with an Application to the Analysis of Laser Nanoablations

We propose a variational method for joint motion estimation and source identification in one-dimensional image sequences. The problem is motivated by fluorescence microscopy data of laser nanoablations of cell membranes in live Drosophila embryos, which can be conveniently--and without loss of significant information--represented in space-time plots, so called kymographs. Based on mechanical models of tissue formation, we propose a variational formulation that is based on the nonhomogenous continuity equation and investigate the solution of this ill-posed inverse problem using convective regularisation. We show existence of a minimiser of the minimisation problem, derive the associated Euler-Lagrange equations, and numerically solve them using a finite element discretisation together with Newtons method. Based on synthetic data, we demonstrate that source estimation can be crucial whenever signal variations can not be explained by advection alone. Furthermore, we perform an extensive evaluation and comparison of various models, including standard optical flow, based on manually annotated kymographs that measure velocities of visible features. Finally, we present results for data generated by a mechanical model of tissue formation and demonstrate that our approach reliably estimates both a velocity and a source.

biophysics↗

bioRxiv2018

Actomyosin-driven tension at compartmental boundaries orients cell division independently of cell geometry in vivo

During animal development, planar polarization of the actomyosin cytoskeleton underlies key morphogenetic events such as axis extension and boundary formation. Actomyosin is enriched along compartment boundaries during segmentation of the Drosophila embryo, forming supracellular contractile cables that keep cells segregated at boundaries. Here, we show that these contractile actomyosin cables bias the orientation of division in cells in contact with compartment boundaries. By decreasing actomyosin cable tension locally using laser ablation or, conversely ectopically increasing tension using laser wounding, we demonstrate that localised subcellular force is necessary and sufficient to orient mitoses in vivo. Moreover this bias is independent of cell geometry and involves capture of the spindle pole by the actomyosin cortex.

developmental biology↗

bioRxiv2017

Par3/Baz levels control epithelial folding at actomyosin-enriched compartmental boundaries

Epithelial folding is crucial to shape embryos and tissues during development. Here we investigate the coupling between epithelial folding and actomyosin-rich boundaries. The mechanistic relationship between the two is unclear, since actomyosin-rich boundaries can be either associated with folds or not, while epithelial folding has been found to be either dependent or independent of actomyosin contractility. Here we investigate the shallow folds that form at compartmental parasegment boundaries (PSBs) in the early Drosophila embryo. First, we demonstrate that formation of these folds is dependent on the contractility of supracellular actomyosin cables. When the Myosin II phosphatase Flawing is depleted at the PSBs, actomyosin contractility increases, resulting in deeper folds. Conversely, in wingless mutants, actomyosin enrichment and increased contractility at PSBs are lost and this correlates with an absence of folding. Furthermore, when we make ectopic PSBs by expressing Wingless ubiquitously, the ectopic boundaries become enriched in actomyosin and epithelial folds form. Ectopic PSB folds, however, are much deeper than endogenous ones, indicating that epithelial folding is normally under inhibitory control. We present evidence that depletion of Bazooka/Par-3 levels at PSB cell-cell contacts, which is under Wingless signaling control, is responsible for this inhibition. Bazooka is found depleted at endogenous but not ectopic PSBs. In embryos overexpressing Bazooka, endogenous PSB folds form earlier and are much deeper. To ask how local signaling at the boundaries control Bazooka levels at cell-cell contacts, we examined embryos that ectopically expressed Wingless in an hedgehog mutant background. In these embryos, inhibition of folding is rescued, with ectopic PSBs now forming shallow folds as endogenous PSBs. Bazooka is depleted at these ectopic PSBs in absence of Hedgehog, suggesting an opposite effect of Wingless and Hedgehog signaling on Bazooka levels at PSB cell-cell contacts. This uncovers a new role of Bazooka in controlling fold formation at actomyosin-rich compartmental boundaries.

cell biology↗