Biology subjects

Nestor-Bergmann, A.

Publications and source records attributed to Nestor-Bergmann, A..

6 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↗

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

Dysregulated wild-type cell division at the interface between host and oncogenic epithelium

Epithelial tissues are highly sensitive to anisotropies in mechanical force, with cells altering fundamental behaviours such as cell adhesion, migration and cell division [1-5]. It is well known that in the later stages of carcinoma (epithelial cancer), the presence of tumours alters the mechanical properties of a host tissue and that these changes contribute to disease progression [6-9]. However, in the earliest stages of carcinoma, when a clonal cluster of oncogene-expressing cells first establishes in the epithelium, the extent to which mechanical changes alter cell behaviour in the tissue as a whole remains unclear. This is despite knowledge that many common oncogenes, such as oncogenic Ras, alter cell stiffness and contractility [10-13]. Here, we investigate how mechanical changes at the cellular level of an oncogenic cluster can translate into the generation of anisotropic strain across an epithelium, altering cell behaviour in neighbouring host tissue. We generated clusters of oncogene-expressing cells within otherwise normal in vivo epithelium, using Xenopus laevis embryos. We find that cells in kRasV12, but not cMYC, clusters have increased contractility, which introduces radial stress in the tissue and deforms surrounding host cells. The strain imposed by kRasV12 clusters leads to increased cell division and altered division orientation in the neighbouring host tissue, effects that can be rescued by reducing actomyosin contractility specifically in the kRasV12 cells. Our findings indicate that some oncogenes can alter the mechanical and proliferative properties of host tissue from the very earliest stages of cancer development, changes which have the potential to contribute to tumorigenesis.

cell biology↗

bioRxiv2018

A mechanosensitive RhoA pathway that protects epithelia against acute tensile stress.

Adherens junctions are tensile structures that couple epithelial cells together. Junctional tension can arise from cell-intrinsic application of contractility or from the cell-extrinsic forces of tissue movement. In all these circumstances, it is essential that epithelial integrity be preserved despite the application of tensile stress. In this study, we identify junctional RhoA as a mechanosensitive signaling pathway that responds to epithelial stress. The junctional specificity of this response is mediated by the heterotrimeric protein G12, which is recruited by E-cadherin and, in turn, recruits p114 RhoGEF to activate RhoA. Further, we identify Myosin VI as a key mechanosensor, based on its intrinsic capacity to anchor E-cadherin to F-actin when exposed to tensile load. Tension-activated RhoA signaling was necessary to preserve epithelial integrity, which otherwise undergoes fracture when monolayer stress is acutely increased by calyculin. Paradoxically, this homeostatic RhoA signaling pathway increases junctional actomyosin, a contractile response that might be expected to itself promote fracture. Simulations of a vertex-based model revealed that the protective effect of RhoA signaling can be explained through increased yield limit at multicellular vertices, where experiments showed p114 RhoGEF was necessary to increase E-cadherin and promote actin assembly and organization.

cell biology↗

bioRxiv2017

Whole-tissue stretching reveals a shape-dependent mechanism of orienting the mitotic spindle and a role for mechanical stress in cueing mitosis

Distinct mechanisms involving cell shape and mechanical force are known to influence the rate and orientation of division in cultured cells. However, uncoupling the impact of shape and force in tissues remains challenging. Combining stretching of Xenopus laevis tissue with a novel method of inferring relative mechanical stress, we find separate roles for cell shape in orientating division and mechanical stress in cueing division. We demonstrate that division orientation is best predicted by an axis of cell shape defined by the position of tricellular junctions (TCJs), which aligns exactly with the principal axis of local cell stress rather than the tissue-level stress. The alignment of division to cell shape requires functional cadherin and the localisation of the spindle orientation protein, LGN, to TCJs, but is not sensitive to relative cell stress magnitude. In contrast, cell proliferation rate is more directly regulated by mechanical stress, being correlated with relative isotropic stress, and can be decoupled from cell shape when myosin II is depleted.

cell biology↗