Dynamique des filaments d'actine: de la molécule individuelle à la formation de structures organisées

Actin is one of the major constituents of the cytoskeleton. By dynamically assembling in cells, actin filaments are able to push the membrane out, and deform the cell leading to force generation and movement. In the last ten years, biochemical studies have unveiled many different biochemical pathways that lead to actin polymerization and assembly. However, the mechanism of force generation is still under debate. The main issue is how the microscopic properties of individual filaments are integrated at the scale of a cell to produce forces. In addition, little is known about the dynamic formation and disassembly of actin filaments cables (an organisation of actin filaments in parallel/or antiparrallel bundles). Recently, formins have been shown to be an essential family of proteins for the initiation of such actin-based structures. This manuscript highlights the work that we conduct to understand the mechanism of action of formins at a molecular level. Most of formins are processive nucleators; indeed they are promoting fast actin filament elongation while remaining attached to the growing end of the filament. We have shown by the original evanescent wave microscopy technique that Arabidopsis Thaliana FORMIN1 represents a new kind of formin, which moves to the side of the actin filament after nucleation. From the side of the pre-existing filament, FORMIN1 is able to nucleate a new filament, promoting the assembly of actin filaments into actin cables. We next combine biomimetic assays with TIRF microscopy, to address the mechanism of the dynamic of polymerization and depolymerization of actin filaments induced by ADF/cofilin. We visualized for the first time individual actin filament stochastic dynamics in real time, and proposed a selection process for the formation of large actin based structures initiated by formin.L'exercice de forces est indispensable au bon fonctionnement de nombreux processus cellulaires. Chez les eucaryotes, une majorité de ces phénomènes motiles est assurée par la polymérisation et la dépolymérisation spatialement et temporellement contrôlée du cytosquelette d'actine. Le cytosquelette est composé de microfilaments d'actine qui s'associent en structures complexes aux propriétés mécaniques particulières. Au cours des dix dernières années, beaucoup d'efforts ont été menés pour comprendre la dynamique des réseaux branchés de filaments d'actine initiés par le complexe Arp2/3. En revanche, peu de choses sont connues à propos de la dynamique de formation et de désassemblage des câbles de filaments d'actine. Récemment, il fut montré que les formines représentent une famille de protéines essentielles à l'initiation de ces structures.Cette thèse résume dans un premier temps le travail accompli pour comprendre le mécanisme d'action des formines à l'échelle moléculaire. La plupart des formines sont des nucléateurs processifs, c'est-à-dire qu'ils permettent la formation et l'élongation de nouveaux filaments d'actine, tout en restant liées à l'extrémité du filament qui polymérise. Nous avons montré par la technique originale de microscopie à onde évanescente que Arabidopsis Thaliana FORMIN1 représente un nouveau type de formine, qui se déplace sur le côté des filaments d'actine après les avoir formés. Depuis le côté d'un filament préexistant, FORMIN1 est capable de nucléer un autre filament, initiant la formation de câbles de filaments d'actine. Dans un deuxième temps, cette thèse s'intéresse au mécanisme moléculaire mis en jeu par l'ADF/cofiline pour accélérer la dynamique d'assemblage/désassemblage des filaments d'actine, et traite pour la première fois de la dynamique de l'actine en temps réel à l'échelle du filament individuel ou à l'intérieur de structures organisées de filaments d'actine

Dynamique des filaments d'actine.De la molécule individuelle à la formation de strcutres organisées

L'exercice de forces est indispensable au bon fonctionnement de nombreux processus cellulaires. Chez les eucaryotes, une majorité de ces phénomènes motiles est assurée par la polymérisation et la dépolymérisation spatialement et temporellement contrôlée du cytosquelette d'actine. Le cytosquelette est composé de microfilaments d'actine qui s'associent en structures complexes aux propriétés mécaniques particulières. Au cours des dix dernières années, beaucoup d'efforts ont été menés pour comprendre la dynamique des réseaux branchés de filaments d'actine initiés par le complexe Arp2/3. En revanche, peu de choses sont connues à propos de la dynamique de formation et de désassemblage des câbles de filaments d'actine. Récemment, il fut montré que les formines représentent une famille de protéines essentielles à l'initiation de ces structures. Cette thèse résume dans un premier temps le travail accompli pour comprendre le mécanisme d'action des formines à l'échelle moléculaire. La plupart des formines sont des nucléateurs processifs, c'est-à-dire qu'ils permettent la formation et l'élongation de nouveaux filaments d'actine, tout en restant liées à l'extrémité du filament qui polymérise. Nous avons montré par la technique originale de microscopie à onde évanescente que Arabidopsis Thaliana FORMIN1 représente un nouveau type de formine, qui se déplace sur le côté des filaments d'actine après les avoir formés. Depuis le côté d'un filament préexistant, FORMIN1 est capable de nucléer un autre filament, initiant la formation de câbles de filaments d'actine. Dans un deuxième temps, cette thèse s'intéresse au mécanisme moléculaire mis en jeu par l'ADF/cofiline pour accélérer la dynamique d'assemblage/désassemblage des filaments d'actine, et traite pour la première fois de la dynamique de l'actine en temps réel à l'échelle du filament individuel ou à l'intérieur de structures organisées de filaments d'actine.Actin is one of the major constituents of the cytoskeleton. By dynamically assembling in cells, actin filaments are able to push the membrane out, and deform the ceilleading to force generation and movement. ln the last ten years, biochemical studies have unveiled many different biochemical pathways that lead to actin polymerization and assembly. However, the mechanism of force generation is still under debate. The main issue is how the microscopic properties of individual filaments are integrated at the scale of a cell to produce forces. ln addition, liUle is known about the dynamic formation and disassembly of actin filaments cab les (an organisation of actin filaments in parallel/or antiparrallel bundles). Recently, formins have been shown to be al essential family of proteins for the initiation of such actin-based structures.This manuscript highlights the work that we conduct to understand the mechanism of action of formins at a molecular level. Most of formins are processive nucleators; indeed they are promoting fast actin filament elongation while remaining attached to the growing end of the filament. We have shown by the original evanescent wave microscopy technique that Arabidopsis Thaliana FORMIN1 represents a new kind of formin, which moves to the side of the actin filament after nucleation. From the side of the pre-existing filament, FORMIN1 is able to nucleate a new filament, promoting the assembly of actin filaments into actin cables. We next combine biomimetic assays with TIRF microscopy, to address the mechanism of the dynamic of polymerization and depolymerization of actin filaments induced by ADF/cofilin. We visualized for the first time individual actin filament stochastic dynamics in real time, and proposed a selection process for the formation of large actin based structures initiated by formin.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Actin Cytoskeleton: A Team Effort during Actin Assembly.

International audienceTwo recent studies highlight how tandems of previously described actin nucleators collaborate to produce new actin filaments. One key player in these collaborations is formin, which appears to function as a modulator of filament elongation

pH gradients guide ADF/cofilin isoforms in pollen tubes

In a recent study, Wang et al. (https://doi.org/10.1083/jcb.202206074) demonstrate that subtle differences between two ADF/cofilin isoforms allow fine spatial regulation of the actin cytoskeleton in pollen tubes. This article illustrates how two similar proteins have progressively evolved to adapt their localization and activity according to the cellular environment

Reconstitution of actin-based cellular processes: Why encapsulation changes the rules

While in vitro reconstitution of cellular processes is progressing rapidly, the encapsulation of biomimetic systems to reproduce the cellular environment is a major challenge. Here we review the difficulties, using reconstitution of processes dependent on actin polymerization as an example. Some of the problems are purely technical, due to the need for engineering strategies to encapsulate concentrated solutions in micrometer-sized compartments. However, other significant issues arise from the reduction of experimental volumes, which alters the chemical evolution of these non-equilibrium systems. Important parameters to consider for successful reconstitutions are the amount of each component, their consumption and renewal rates to guarantee their continuous availability

Building Distinct Actin Filament Networks in a Common Cytoplasm

Eukaryotic cells generate a diversity of actin filament networks in a common cytoplasm to optimally perform functions such as cell motility, cell adhesion, endocytosis and cytokinesis. Each of these networks maintains precise mechanical and dynamic properties by autonomously controlling the composition of its interacting proteins and spatial organization of its actin filaments. In this review, we discuss the chemical and physical mechanisms that target distinct sets of actin-binding proteins to distinct actin filament populations after nucleation, resulting in the assembly of actin filament networks that are optimized for specific functions

Non-linear elastic properties of actin patches to partially rescue yeast endocytosis efficiency in the absence of the cross-linker Sac6

International audienceClathrin mediated endocytosis is an essential and complex cellular process involving more than 60 proteins. In yeast, successful endocytosis requires counteracting a large turgor pressure. To this end, yeasts assemble actin patches, which accumulate elastic energy during their assembly. We investigated the material properties of reconstituted actin patches from a wild-type (WT) strain and a mutant strain lacking the cross-linker Sac6 (sac6), which has reduced endocytosis efficiency in live cells. We hypothesized that a change in the viscous properties of the actin patches, which would dissipate more mechanical energy, could explain this reduced efficiency. There was however no significant difference in the viscosity of both types of patches. However, we discovered a significantly different non-linear elastic response. While WT patches had a constant elastic modulus at different stresses, sac6 patches had a lower elastic modulus at low stresses, before stiffening at higher ones, up to values similar to WT patches. To understand the consequences of this discovery, we performed, in-vivo, a precise analysis of actin patch dynamics. Our analysis reveals that a small fraction of actin patches successfully complete endocytosis in sac6 cells, provided that those assemble an excess of actin at the membrane compared to WT. This observation indicates that non-linear elastic properties of actin networks in sac6 cells contribute to rescue endocytosis, requiring nevertheless more actin material to build-up the necessary stored elastic energy

Linking single-cell decisions to collective behaviours in social bacteria

International audienceSocial bacteria display complex behaviours whereby thousands of cells collectively and dramatically change their form and function in response to nutrient availability and changing environmental conditions. In this review, we focus on Myxococcus xanthus motility, which supports spectacular transitions based on prey availability across its life cycle. A large body of work suggests that these behaviours require sensory capacity implemented at the single-cell level. Focusing on recent genetic work on a core cellular pathway required for single-cell directional decisions, we argue that signal integration, multi-modal sensing and memory are at the root of decision making leading to multicellular behaviours. Hence, Myxococcus may be a powerful biological system to elucidate how cellular building blocks cooperate to form sensory multicellular assemblages, a possible origin of cognitive mechanisms in biological systems. This article is part of the theme issue ‘Basal cognition: conceptual tools and the view from the single cell’