Biology subjects

Balasubramaniam, L.

Publications and source records attributed to Balasubramaniam, L..

9 recordsLinked to original sources

bioRxiv2025

Tissue spreading couples gastrulation through extracellular matrix remodelling in early avian embryos

Pre-gastrulation vertebrate embryos develop a polarised blastoderm through collective cell spreading (epiboly). The formation and role of this tissue architecture are not well understood. Here, we identify extensive reorganisation of extracellular matrix (ECM) driven by the outward migration of blastoderm edge cells during epiboly in chicken. Proliferation of inner cells and cytoskeleton dynamics of edge cells cooperatively sustain tissue expansion, which remodels the ECM to establish a basal lamina and blastoderm polarity. Blocking edge cell migration by substrate modification or disrupting the basal lamina causes tissue polarity loss and thickening across the blastoderm, prohibiting subsequent gastrulation movements. Biochemical perturbations suggest that remodelling depends on cell-ECM binding. Our findings show a vital role of the mechanical coupling between cell movement and ECM in creating and sustaining the pre-gastrulation tissue architecture.

developmental biology↗

bioRxiv2024

Force transmission is a master regulator of mechanical cell competition

Cell competition is a tissue surveillance mechanism for eliminating unwanted cells and as such is indispensable in development, infection and tumorigenesis. Although different biochemical mechanisms are proposed, due to the dearth of direct force measurements, how mechanical forces determine the competition outcome remains unclear. Here, using ex vivo tissues and different cell lines, we have discovered an unknown form of cell competition that is regulated by differences in force transmission capabilities, favoring cell types with stronger intercellular adhesion. Direct force measurements reveal increased mechanical activity at the interface of the two competing cell types in the form of large stress fluctuations which can lead to upward forces and cell elimination. We show how a winning cell type endowed with a stronger intercellular adhesion exhibits a higher resistance to elimination while benefiting from efficient force transmission to neighboring cells. This cell elimination mechanism could have broad implications of keeping strong force transmission ability for maintaining tissue boundaries and cell invasion pathology.

biophysics↗

bioRxiv2024

Mechanical stresses govern myoblast fusion and myotube growth

Myoblast fusion into multinucleated myotubes is essential for skeletal muscle development and repair, yet how tissue-scale mechanics contributes to this process remains poorly understood. Here, we show that primary myoblasts behave as an evolving active nematic system in which actomyosin-dependent stresses, extracellular-matrix (ECM) remodeling and fusion-driven myotube growth are dynamically coupled. As myoblasts fuse into elongated myotubes, orientational order increases and the nematic field is progressively reshaped. We identify a strong coupling between cellular and ECM nematic organization, whereby cytoskeleton-dependent ECM remodeling stabilizes topological defects and reinforces their associated stress patterns. Fusion events preferentially accumulate near comet-shaped +1/2 defects, which correspond to regions of high compressive stress predicted by our theoretical model. Our findings support a model in which the intrinsic fusion machinery provides fusion competence, while ECM-stabilized nematic stress patterns spatially bias the localization of fusion events. Fusion-driven myotube growth then feeds back on the mechanical landscape, increasing nematogen length and stress magnitude. Together, these results reveal a self-reinforcing biomechanical mechanism that contributes to the organization of myoblast fusion and myotube growth, with potential relevance for developmental and regenerative morphogenesis.

biophysics↗

bioRxiv2024

TiFM2.0 - Versatile mechanical measurement and actuation in live embryos

During development, spatial-temporally patterned tissue-level stresses and mechanical properties create diverse tissue shapes. To understand the mechanics of small-scale embryonic tissues, precisely controlled sensors and actuators are needed. Previously, we reported a control-based approach named tissue force microscopy (TiFM1.0), which combines dynamic positioning and imaging of an inserted cantilever probe to directly measure and impose forces in early avian embryos. Here we present an upgraded system (TiFM2.0) that utilises interferometer positioning to minimise probe holder footprint, enhancing accessibility and imaging signal. This new design enables a double-probe configuration for bidirectional stretching, compression and stress propagation experiments. As proof-of-concept, we showcase a variety of examples of TiFM2.0 applications in chicken and zebrafish embryos, including the characterization of mechanical heterogeneities important for the morphogenesis of the chicken posterior body axis. We also present simplified designs and protocols for the replication of TiFM systems with minimal custom engineering for developmental biology labs.

developmental biology↗

bioRxiv2024

Asynchronous mouse embryo polarization leads to heterogeneity in cell fate specification

The first lineage allocation in mouse and human embryos separates the inner cell mass (ICM) from the outer trophectoderm (TE). This symmetry breaking event is executed through polarization of cells at the 8-cell stage and subsequent asymmetric divisions, generating polar (TE) and apolar (ICM) cells. Here, we show that embryo polarization is unexpectedly asynchronous. Cells polarizing at the early and late 8-cell stage have distinct molecular and morphological properties that direct their following lineage specification, with early polarizing cells being biased towards producing the TE lineage. More recent studies have also implicated heterogeneities between cells prior to the 8-cell stage in the first lineage allocation: cells exhibiting reduced methyltransferase CARM1 activity at the 4-cell stage are predisposed towards the TE fate. Here, we demonstrate that reduced CARM1 activity and upregulation of its substrate BAF155 promote early polarization and TE specification. These findings provide a link between asymmetries at the 4-cell stage and polarization at the 8-cell stage, mechanisms of the first lineage allocation that had been considered separate.

developmental biology↗

bioRxiv2023

Aging and freezing of active nematic dynamics of cancer-associated fibroblasts by fibronectin matrix remodeling

Cancer-associated fibroblasts (CAFs) are one of the most abundant cell types in tumor stroma. Exhibiting an elongated morphology, CAFs align with each other, closely resembling nematic ordering in liquid crystal physics. CAFs play a pivotal role in the genesis and remodeling of the extracellular matrix (ECM), with ECM proteins, especially fibronectin, reciprocally modulating CAF alignment and coherence. Yet, the intricate feedback loops between fibronectin deposition and CAF structuring remain largely unexplored. Here, we combined CAF live imaging, traction force microscopy, ECM microfabrication, and theoretical modeling to assess how the ECM influences the dynamics of nematically ordered CAFs. We found that CAFs dynamically orchestrate a fibronectin network that mirrors their nematic ordering. Over time, this passive nematic ordering of fibronectin, in turn, steers CAF rearrangement. Contrary to most cellular systems, where defects remain dynamic at a steady state, the ECM/CAF interplay profoundly alters the behavior of both CAF and ECM nematics, leading to aging-massive slowdown, and even freezing of defect dynamics. This results in a CAFs capsule where aligned areas and defects are spatially and temporally fixed, yet active, exerting forces at the substate and transmitting forces between cells. Functionally, while defects may be permissive, it is the fibronectin loss-induced fluidization of the CAF capsule that critically undermines its barrier function.

cell biology↗

bioRxiv2022

Mechanical stress driven by rigidity sensing governs epithelial stability

Epithelia act as a barrier against environmental stress and abrasion and in vivo they are continuously exposed to environments of various mechanical properties. The impact of this environment on epithelial integrity remains elusive. By culturing epithelial cells on 2D hydrogels, we observe a loss of epithelial monolayer integrity through spontaneous hole formation when grown on soft substrates. Substrate stiffness triggers an unanticipated mechanical switch of epithelial monolayers from tensile on soft to compressive on stiff substrates. Through active nematic modelling, we find unique patterns of cell shape texture called nematic topological defects that underpin large isotropic stress fluctuations at certain locations thereby triggering mechanical failure of the monolayer and hole opening. Our results show that substrate stiffness provides feedback on monolayer mechanical state and that topological defects can trigger stochastic mechanical failure, with potential application towards a mechanistic understanding of compromised epithelial integrity in bacterial infection, tumor progression and morphogenesis.

biophysics↗

bioRxiv2020

Nature of active forces in tissues: how contractile cells can form extensile monolayers

Actomyosin machinery endows cells with contractility at a single cell level. However, at a tissue scale, cells can show either contractile or extensile behaviour based on the direction of pushing or pulling forces due to neighbour interactions or substrate interactions. Previous studies have shown that a monolayer of fibroblasts behaves as a contractile system1 while a monolayer of epithelial cells2,3 or neural crest cells behaves as an extensile system.4 How these two contradictory sources of force generation can coexist has remained unexplained. Through a combination of experiments using MDCK (Madin Darby Canine Kidney) cells, and in-silico modeling, we uncover the mechanism behind this switch in behaviour of epithelial cell monolayers from extensile to contractile as the weakening of intercellular contacts. We find that this switch in active behaviour also promotes the buildup of tension at the cell-substrate interface through an increase in actin stress fibers and higher traction forces. This in turn triggers a mechanotransductive response in vinculin translocation to focal adhesion sites and YAP (Yes-associated protein) transcription factor activation. Our studies also show that differences in extensility and contractility act to sort cells, thus determining a general mechanism for mechanobiological pattern formation during cell competition, morphogenesis and cancer progression.

biophysics↗

bioRxiv2019

Caveolae set levels of epithelial monolayer tension to eliminate tumor cells

Mechanical tension governs epithelial morphogenesis and homeostasis, but its regulation remains poorly understood. Tension is commonly contractile, arising when the actomyosin cortices of cells are mechanically coupled together by cadherin adhesion. Here we report that caveolae control levels of epithelial tension and show that this is necessary for oncogene-transfected cells to be eliminated by apical extrusion. Depletion of caveolin-1 (CAV1) in the surrounding epithelium, but not in the oncogene-expressing cells, blocked extrusion leading to the retention and proliferation of transformed cells within the monolayer. Tensile stress was aberrantly elevated in CAV1-depleted monolayers due to elevated levels of phosphoinositide-4,5-bisphosphate (PtdIns(4,5)P2) causing increased recruitment of the formin, FMNL2. Oncogenic extrusion was restored to CAV1-deficient monolayers when tension was corrected by depleting FMNL2, blocking PtdIns(4,5)P2, or disabling the interaction between FMNL2 and PtdIns(4,5)P2. Thus, by controlling lipid signalling to the actin cytoskeleton, caveolae regulate mechanical tension for epithelial homeostasis.

cell biology↗