Centrosome centering and decentering by microtubule network rearrangement.

The centrosome is positioned at the cell center by pushing and pulling forces transmitted by microtubules (MTs). Centrosome decentering is often considered to result from asymmetric, cortical pulling forces exerted in particular by molecular motors on MTs and controlled by external cues affecting the cell cortex locally. Here we used numerical simulations to investigate the possibility that it could equally result from the redistribution of pushing forces due to a reorientation of MTs. We first showed that MT gliding along cell edges and pivoting around the centrosome regulate MT rearrangement and thereby direct the spatial distribution of pushing forces, whereas the number, dynamics, and stiffness of MTs determine the magnitude of these forces. By modulating these parameters, we identified different regimes, involving both pushing and pulling forces, characterized by robust centrosome centering, robust off-centering, or "reactive" positioning. In the last-named conditions, weak asymmetric cues can induce a misbalance of pushing and pulling forces, resulting in an abrupt transition from a centered to an off-centered position. Taken together, these results point to the central role played by the configuration of the MTs on the distribution of pushing forces that position the centrosome. We suggest that asymmetric external cues should not be seen as direct driver of centrosome decentering and cell polarization but instead as inducers of an effective reorganization of the MT network, fostering centrosome motion to the cell periphery

Cell shape and contractility regulate ciliogenesis in cell cycle–arrested cells

Adhesive micropatterns show the effect of spatial confinement and actin network architecture on basal body positioning and primary cilium formation

Geometrical and mechanical properties control actin filament organization.

The different actin structures governing eukaryotic cell shape and movement are not only determined by the properties of the actin filaments and associated proteins, but also by geometrical constraints. We recently demonstrated that limiting nucleation to specific regions was sufficient to obtain actin networks with different organization. To further investigate how spatially constrained actin nucleation determines the emergent actin organization, we performed detailed simulations of the actin filament system using Cytosim. We first calibrated the steric interaction between filaments, by matching, in simulations and experiments, the bundled actin organization observed with a rectangular bar of nucleating factor. We then studied the overall organization of actin filaments generated by more complex pattern geometries used experimentally. We found that the fraction of parallel versus antiparallel bundles is determined by the mechanical properties of actin filament or bundles and the efficiency of nucleation. Thus nucleation geometry, actin filaments local interactions, bundle rigidity, and nucleation efficiency are the key parameters controlling the emergent actin architecture. We finally simulated more complex nucleation patterns and performed the corresponding experiments to confirm the predictive capabilities of the model

Type XII collagen regulates osteoblast polarity and communication during bone formation

Type XII collagen–null mice have fragile bones with disorganized collagen fiber arrangement, decreased bone matrix formation, and delayed osteoblast differentiation

A novel community driven software for functional enrichment analysis of extracellular vesicles data.

Bioinformatics tools are imperative for the in depth analysis of heterogeneous high-throughput data. Most of the software tools are developed by specific laboratories or groups or companies wherein they are designed to perform the required analysis for the group. However, such software tools may fail to capture "what the community needs in a tool". Here, we describe a novel community-driven approach to build a comprehensive functional enrichment analysis tool. Using the existing FunRich tool as a template, we invited researchers to request additional features and/or changes. Remarkably, with the enthusiastic participation of the community, we were able to implement 90% of the requested features. FunRich enables plugin for extracellular vesicles wherein users can download and analyse data from Vesiclepedia database. By involving researchers early through community needs software development, we believe that comprehensive analysis tools can be developed in various scientific disciplines

Minimal information for studies of extracellular vesicles (MISEV2023): From basic to advanced approaches

Extracellular vesicles (EVs), through their complex cargo, can reflect the state of their cell of origin and change the functions and phenotypes of other cells. These features indicate strong biomarker and therapeutic potential and have generated broad interest, as evidenced by the steady year-on-year increase in the numbers of scientific publications about EVs. Important advances have been made in EV metrology and in understanding and applying EV biology. However, hurdles remain to realising the potential of EVs in domains ranging from basic biology to clinical applications due to challenges in EV nomenclature, separation from non-vesicular extracellular particles, characterisation and functional studies. To address the challenges and opportunities in this rapidly evolving field, the International Society for Extracellular Vesicles (ISEV) updates its ‘Minimal Information for Studies of Extracellular Vesicles’, which was first published in 2014 and then in 2018 as MISEV2014 and MISEV2018, respectively. The goal of the current document, MISEV2023, is to provide researchers with an updated snapshot of available approaches and their advantages and limitations for production, separation and characterisation of EVs from multiple sources, including cell culture, body fluids and solid tissues. In addition to presenting the latest state of the art in basic principles of EV research, this document also covers advanced techniques and approaches that are currently expanding the boundaries of the field. MISEV2023 also includes new sections on EV release and uptake and a brief discussion of in vivo approaches to study EVs. Compiling feedback from ISEV expert task forces and more than 1000 researchers, this document conveys the current state of EV research to facilitate robust scientific discoveries and move the field forward even more rapidly

Cell mechanics: Wave of migration

International audienc

contrôle de la polarité des cellules adhérentes

we used fibronectin micro-patterns imposing cells to spread upon adhesive and non-adhesive areas. The relative positioning of these areas was used to impose asymmetric adhesive environment to cells.Therefore we showed that cells develop large stress fibres upon non-adhesive areas and protrusions upon adhesive ones. This polarity of the actin cytoskeleton induced a specific recruitment of APC, a molecules interacting with both actin and microtubules. Thereby the polarity of the actin network was transmitted to the microtubules which keep growing along stress fibres whereas they stop when contacting adhesive edges. The internal polarity of cells as revealed by the relative positioning of nucleus and centrosome was thus harmonised with the geometry of the extracellular matrix. Furthermore, we showed that this geometry impinge on spindle orientation and cell division axis independently of cell shape.Les travaux effectués au cours de cette thèse et présentés dans ce manuscrit consistent en l'utilisation de micro-patrons adhésifs pour l'étude de l'organisation spatiale des organites intracellulaires. Nous avions comme modèle de travail la compartimentation et la polarisation des cellules au sein des tissus. Dans cette situation, les cellules n'adhèrent pas de façon homogène et isotrope à leur environnement. Au contraire, elles s'attachent à leur voisines et à la matrice extracellulaire, en des endroits particuliers, grâce à des complexes transmembranaires, les adhésions. Ces adhésions sont une des bases structurales de la construction du cytosquelette. Leur distribution spatiale peut être anisotrope, et parfois asymétrique, ce qui guide la polarité intrinsèque des cellules. Nous avons utilisé la technique d'impression par micro-contact pour manipuler la forme des cellules et la distribution de leurs adhésions. Les cellules adoptent l'enveloppe convexe du patron adhésif quelle que soit sa géométrie. Si, par exemple, les cellules sont contraintes de s'attacher à un T ou un V, sans pouvoir établir de contact en dehors de ce patron, elles adopteront dans les deux cas la même forme triangulaire. Nous avons utilisé cette propriété pour imposer aux cellules des formes identiques sur des patrons adhésifs différents afin d'analyser le rôle spécifique de la distribution des adhésions sur l'organisation du cytosquelette et des compartiments intracellulaires. Nos mesures montrent que les cellules développent des tensions élevées dans les filaments d'actine, assemblés en fibres de stress, au-dessus des zones non adhésives. Sur les zones adhésives, la tension est plus faible, et la polymérisation de l'actine en un réseau branché induit la formation de protrusions membranaires. La localisation des protrusions est donc complémentaire de celle des zones contractiles.Cette polarisation du système actine dirige le recrutement de certaines protéines dont l'activité influence la dynamique des microtubules. La croissance des microtubules en contact avec le cortex cellulaire est modulée différemment selon que l'actine forme des fibres de stress ou un réseau branché. Cependant, quel que soit le comportement des extrémités du réseau de microtubules à la périphérie, le centre du réseau, le centrosome, se maintient toujours au centre de la cellule. Le noyau est exclu de ce centre et se positionne vers les zones de contraction. Ainsi, à l'intérieur de la cellule, l'orientation de l'axe noyau-centrosome répond à l'asymétrie établie en périphérie.La polarité du cortex est conservée pendant la division cellulaire ou mitose. Le corps cellulaire, qui s'est arrondi à l'entrée en mitose, est maintenu en contact avec le substrat adhésif par des fibres de rétraction riches en actine. Les mesures expérimentales de l'orientation des divisions, sur différents patrons adhésifs, révèlent que la distribution spatiale des ancrages de ces fibres sur la cellule guide l'orientation du fuseau mitotique, et par conséquent, le plan de division des cellules. En effet, les pôles du fuseau se positionnent en face des zones corticales où sont arrimées les fibres de rétraction, indépendamment de la forme qu'avait la cellule avant l'entrée en mitose. Il existe des moteurs moléculaires ancrés dans le cortex des cellules et capables de tirer sur les microtubules astraux émanant des pôles du fuseau. En faisant l'hypothèse que ces moteurs ne sont activés que dans les zones où se situent les fibres de rétraction, on peut établir un modèle physique permettant de rendre compte de toutes les observations expérimentales effectuées