Biology subjects

Belmonte, J. M.

Publications and source records attributed to Belmonte, J. M..

8 recordsLinked to original sources

bioRxiv2025

Connectivity and Contraction in Cytoskeletal Networks

The cytoskeleton"s ability to contract and propagate forces is the fundamental mechanism behind cell morphology, division and migration. This can only occur if the network is sufficiently connected, yet a rigorous description of the connectivity requirements has never been provided. In this work we focused on the polarity-sorting contraction mechanism and showed that connectivity is not determined by the spatial distribution of filaments alone, but by the interconnectivity between the dual network of filaments and motors. We developed a method to quantify filament-motor connectivity as a function of motor length, filament length distributions, and the densities of each component. Using this framework, we derived a general theory that predicts when a network is sufficiently connected to allow global or local contraction. We validated our predictions with computer simulations and introduced a novel metric to distinguish between these outcomes. Our findings show that the conditions for local and global contraction in the presence of fiber dynamics correspond, respectively, to the pulsatile and steady-state contraction behaviors observed in vivo. All results are independent of filament rigidity, making our conclusions applicable to both actin and microtubule networks. Lastly, we discuss how those outcomes are affected by the introduction of crosslinking proteins, which - despite not actively generating forces of their own - can promote global contractility at small concentrations even in networks made of short and/or rigid filaments.

cell biology↗

bioRxiv2025

Start Small: A Model for Tissue-wide Planar Cell Polarity without Morphogens

Planar cell polarity (PCP) is an important patterning mechanism in both vertebrates and invertebrates by which cells coherently polarize along the apical surface of the epithelium. This patterning mechanism acts upstream of many developmental processes, such as oriented growth, division, cell movements and orientation of body hairs. While various models have been proposed to explain PCP patterning, all rely on persistent global cues/gradients to obtain global orientation of large tissues. However, recent experimental work has shown that this process can happen independently of such global cues, challenging the current paradigm. In this work, we developed a new model for PCP using the Cellular Potts modelling framework to investigate the conditions under which global tissue orientation can be achieved without a tissue spanning morphogen. We found that a combination of a local boundary signal, a small initial tissue size and uniform proliferation can effectively establish long-range polarity without the need for global cues. We also investigated the impact of cell division planes and growth rates on final patterning. Finally, we compared the cell-autonomous and cell non-autonomous versions of our PCP model, as found in flies and mice, and found that the latter offers more robust patterning outcomes in the absence of gradients. Author summaryCells must coordinate the spatial organization of certain proteins inside them to produce aligned structures like body hairs and cilia across large distances in various organisms. This polarity of cells is also important to provide directionality for several key events during animal development. It was widely assumed that a graded signal acting across the tissue provides the global cue for the cells to attain this long-range alignment. However, the identity and existence of this global cue have recently been challenged by new experimental findings. In this work we developed a new computational model showing how coherent tissue polarity can emerge from simple, local rules in the absence of a global tissue-spanning signal. Our model proposes a mechanism where local signals polarize a small initial group of cells, and their subsequent proliferation extend this initial alignment across large scales. We extensively tested our model under different conditions and show that this proposed mechanism provides a reliable recipe for the generation of large-scale tissue polarity without the need for global signals.

developmental biology↗

bioRxiv2024

Caging of membrane-to-cortex attachment proteins can trigger cellular symmetry breaking

To migrate, divide, and change shape, cells must regulate the mechanics of their periphery. The cell surface is a complex structure that consists of a thin, contractile cortical actin network tethered to the plasma membrane by specialized membrane-to-cortex attachment (MCA) proteins. This active and constantly fluctuating system maintains a delicate mechanochemical state which permits spontaneous polarization and shape change when needed. Combining in silico, in vitro, and in vivo experiments we show how membrane viscosity and MCA protein length regulate cortical dynamics. We reveal a novel mechanism whereby caging of linker proteins in the actin cortex allows for the amplification of small changes in these key parameters, leading to major alterations of cortical contractility. In cells, this mechanism alone gives rise to symmetry breaking phenomena, suggesting that local changes in lipid composition, in combination with the choice of MCA proteins, contribute to the regulation of cellular morphogenesis and function.

biophysics↗

bioRxiv2023

Microtubule competition and cell growth recenter the nucleus after anaphase in fission yeast.

Cells actively position their nucleus based on their activity. In fission yeast, microtubule-dependent nuclear centering is critical for symmetrical cell division. After spindle disassembly at the end of anaphase, the nucleus recenters over a ~90 min period, approximately half of the duration of the cell cycle. Live cell and simulation experiments support the cooperation of two distinct mechanisms in the slow recentering of the nucleus. First, a push-push mechanism acts from spindle disassembly to septation and involves the opposing actions of the mitotic Spindle Pole Body microtubules that push the nucleus away from the ends of the cell while post-anaphase array of microtubules basket the nucleus and limit its migration toward the division plane. Second, a slow-and-grow mechanism finalizes nuclear centering in the newborn cell. In this mechanism, microtubule competition stalls the nucleus while asymmetric cell growth slowly centers it. Our work underlines how intrinsic properties of microtubules differently impact nuclear positioning according to microtubule network organization and cell size.

cell biology↗

bioRxiv2020

Mechanical competition alters the cellular interpretation of an endogenous genetic programme

The intrinsic genetic programme of a cell is not sufficient to explain all of the cells activities. External mechanical stimuli are increasingly recognized as determinants of cell behaviour. In the epithelial folding event that constitutes the beginning of gastrulation in Drosophila, the genetic programme of the future mesoderm leads to the establishment of a contractile actomyosin network that triggers apical constriction of cells, and thereby, tissue folding. However, some cells do not constrict but instead stretch, even though they share the same genetic programme as their constricting neighbours. We show here that tissue-wide interactions force these cells to expand even when an otherwise sufficient amount of apical, active actomyosin is present. Models based on contractile forces and linear stress-strain responses do not reproduce experimental observations, but simulations in which cells behave as ductile materials with non-linear mechanical properties do. Our models show that this behaviour is a general emergent property of actomyosin networks [in a supracellular context, in accordance with our experimental observations of actin reorganisation within stretching cells.

developmental biology↗

bioRxiv2020

Cellular mechanisms of chick limb bud morphogenesis

Although some of the molecular pathways involved in limb bud morphogenesis have been identified, the cellular basis of the process is not yet understood. Proposed cell behaviours include active cell migration and oriented cell division, but ultimately, these questions can only be resolved by watching individual mesenchymal cells within a completely normal developmental context. We developed a minimally-invasive in ovo two-photon technique, to capture high quality time-lapse sequences up to 100 microns deep in the unperturbed growing chick limb bud. Using this technique, we characterized cell shapes and other oriented behaviours throughout the limb bud, and found that cell intercalation drives tissue movements, rather than oriented cell divisions or migration. We then developed a 3D cell-based computer simulation of morphogenesis, in which cellular extensions physically pull cells towards each other, with directional bias controlled by molecular gradients from the ectoderm (Wnts) and the Apical Ectodermal Ridge (FGFs). We defined the initial and target shapes of the chick limb bud in 3D by OPT scanning, and explored which orientations of mesenchymal intercalation correctly explain limb morphogenesis. The model made a couple of predictions: Firstly, that elongation can only be explained when cells intercalate along the direction towards the nearest ectoderm. This produces a general convergence of tissue towards the central proximo-distal (PD) axis of the limb, and a resultant extension of the tissue along the PD axis. Secondly, the correct in silico morphology can only be achieved if the contractile forces of mesenchymal cells in the very distal region (under the Apical Ectodermal Ridge) have shorter life times than in the rest of the limb bud, effectively making the tissue more fluid by augmenting the rate of cell rearrangement. We argue that this less-organised region of mesenchyme is necessary to prevent PD-oriented intercalation events in the distal tip that would otherwise inhibit outgrowth.

developmental biology↗

bioRxiv2019

A Mechanical Model of Early Somite Segmentation

The clock-and-wavefront model (CW) hypothesizes that the formation of somites in vertebrate embryos results from the interplay of molecular oscillations with a wave traveling along the body axis. This model however does not explain how molecular information is interpreted by cells to modulate their rearrangement into somites. Here we performed Scanning Electron Microscopy (SEM) on the pre-somitic mesoderm (PSM) of chicken embryos at stages 11-12 to describe in detail the cell shape changes occurring along the axis of the PSM. This reveals a wave of epithelialization of the dorsal PSM that precedes somite segmentation. Signs of spatially periodic apical constriction appear in this layer starting at least 3-4 somite lengths caudal to the most recently formed somite. The sizes of these clusters correspond to the typical diameter of chicken somites. We propose that a mechanical instability process leads to the separation of cells into these structures and positions the future inter-somite boundaries. We present a model in which a wave of apical constriction leads to increasing tension and periodic failure of adhesion junctions within the dorsal epithelial layer of the PSM, thus positioning somite boundaries. This model can produce spatially periodic segments whose size depends on the speed of the contraction wave (W) and the rate of increase of apical contractility ({Lambda}). The {Lambda}/W ratio determines whether this mechanism produces spatially and temporally regular or irregular segments, and whether segment sizes increase with the wave speed (scaling) as in the CW model. We discuss the limitations of a purely mechanical model of somite segmentation and the role of biomechanics along with CW during somitogenesis.

developmental biology↗

bioRxiv2018

Polarity sorting drives remodeling of actin-myosin networks

Cytoskeletal networks of actin filaments and myosin motors drive many dynamic cell processes. A key characteristic of these networks is their contractility. Despite intense experimental and theoretical efforts, it is not clear what mechanism favors network contraction over expansion. Recent work points to a dominant role for the nonlinear mechanical response of actin filaments, which can withstand stretching but buckle upon compression. Here we present an alternative mechanism. We study how interactions between actin and myosin-2 at the single filament level translate into contraction at the network scale by performing time-lapse imaging on reconstituted quasi-2D-networks mimicking the cell cortex. We observe myosin end-dwelling after it runs processively along actin filaments. This leads to transport and clustering of actin filament ends and the formation of transiently stable bipolar structures. Further we show that myosin-driven polarity sorting leads to polar actin aster formation, which act as contractile nodes that drive contraction in crosslinked networks. Computer simulations comparing the roles of the end-dwelling mechanism and a buckling-dependent mechanism show that the relative contribution of end-dwelling contraction increases as the network mesh-size decreases.

biophysics↗