Anterior-enriched filopodia create the appearance of asymmetric membrane microdomains in polarizing C. elegans zygotes

International audienceThe association of molecules within membrane microdomains is critical for the intracellular organization of cells. During polarization of the C. elegans zygote, both polarity proteins and actomyosin regulators associate within dynamic membrane-associated foci. Recently, a novel class of asymmetric membrane-associated structures was described that appeared to be enriched in phosphatidylinositol 4,5-bisphosphate (PIP 2), suggesting that PIP 2 domains could constitute signaling hubs to promote cell polarization and actin nucleation. Here, we probe the nature of these domains using a variety of membrane-and actin cortex-associated probes. These data demonstrate that these domains are filopodia, which are stimulated transiently during polarity establishment and accumulate in the zygote anterior. The resulting membrane protrusions create local membrane topology that quantitatively accounts for observed local increases in the fluorescence signal of membrane-associated molecules, suggesting molecules are not selectively enriched in these domains relative to bulk membrane and that the PIP 2 pool as revealed by PH PLCδ1 simply reflects plasma membrane localization. Given the ubiquity of 3D membrane structures in cells, including filopodia, microvilli and membrane folds, similar caveats are likely to apply to analysis of membrane-associated molecules in a broad range of systems

Cofilin tunes the nucleotide state of actin filaments and severs at bare and decorated segment boundaries.

International audienceActin-based motility demands the spatial and temporal coordination of numerous regulatory actin-binding proteins (ABPs) [1], many of which bind with affinities that depend on the nucleotide state of actin filament. Cofilin, one of three ABPs that precisely choreograph actin assembly and organization into comet tails that drive motility in vitro [2], binds and stochastically severs aged ADP actin filament segments of de novo growing actin filaments [3]. Deficiencies in methodologies to track in real time the nucleotide state of actin filaments, as well as cofilin severing, limit the molecular understanding of coupling between actin filament chemical and mechanical states and severing. We engineered a fluorescently labeled cofilin that retains actin filament binding and severing activities. Because cofilin binding depends strongly on the actin-bound nucleotide, direct visualization of fluorescent cofilin binding serves as a marker of the actin filament nucleotide state during assembly. Bound cofilin allosterically accelerates P(i) release from unoccupied filament subunits, which shortens the filament ATP/ADP-P(i) cap length by nearly an order of magnitude. Real-time visualization of filament severing indicates that fragmentation scales with and occurs preferentially at boundaries between bare and cofilin-decorated filament segments, thereby controlling the overall filament length, depending on cofilin binding density

Dynamique des réseaux d'actine d'architecture contrôlée.

During my thesis I have developed different projects in order to tackle the problem of actin network dynamics and organization as well as the molecular mechanism at the origin of force production in biomimetic reconstituted systems. My first interest concerned the spatiotemporal organization of actin networks and actin-binding proteins during actin based motility of nucleation promoting factor-coated particles (Achard et al, Current Biology, 2010 and Reymann et al, accepted at MBOC). I observed in real time the incorporation of two actin regulators (Capping protein et ADF/cofilin) and showed that their biochemical control of actin dynamics also governs its mechanical properties. To further characterize the mechanical properties of expanding actin networks, I used an innovative micropatterning set-up allowing a reproducible spatial control of actin nucleation sites. It allowed me to show that geometrical boundaries, such as those encountered in cells, affect the dynamic formation of highly ordered actin structures and hence control the location of force production (Reymann et al, Nature Materials, 2010). Finally the addition of molecular motors on this tunable system allowed me to study implications for myosin-induced contractility. In particular, HMM-MyosinVI selectively interact with the different actin network architectures (parallel, anti-parallel organization or entangled networks) and leads to a selective three-phase process of tension, deformation of actin networks tightly coupled to massive filament disassembly. This phenomenon being highly dependent on actin network architecture could therefore play an essential role in the spatial regulation of expanding and contracting regions of actin cytoskeleton in cells.Mon travail de thèse fut de développer différents projets en vue de mieux comprendre la dynamique et l'organisation des réseaux d'actine, ainsi que les mécanismes moléculaires à l'origine de la production de force grâce à différents systèmes reconstitués biomimétiques. Dans un premier temps, je me suis intéressée à l'étude de l'organisation spatiotemporelle des réseaux dynamiques d'actine et de ses protéines associées durant la propulsion de particules recouvertes de promoteurs de nucléation des filaments d'actine (Achard et al, Current Biology, 2010 et Reymann et al, accepté à MBoC). J'ai notamment suivi en temps réel l'incorporation de deux régulateurs de l'actine (Capping protein, protéine de coiffe et ADF/cofilin, protéine de fragmentation) et montré que leurs actions conjuguées assurent un contrôle biochimique de l'assemblage d'un réseau complexe d'actine, mais gouvernent également les propriétés mécaniques de ce réseau. Par ailleurs, afin de mieux caractériser les propriétés mécaniques de ces réseaux d'actine en expansion, j'ai développé un système biomimétique novateur utilisant la procédure de micropatrons ou "micropatterning" qui permet un contrôle spatial reproductible des sites de nucléation d'actine. Cela m'a permis de montrer comment des barrières géométriques, semblables à celles trouvées dans les cellules, peuvent influencer la formation dynamique de réseaux organisés d'actine et ainsi contrôler la localisation de la production de forces. (Reymann et al, Nature Materials, 2010). De plus, l'incorporation de moteurs moléculaires dans ce système versatile, nous a permis d'étudier la contraction induite par des myosines. En particulier, j'ai pu montrer que les myosines VI HMM interagissent de manière sélective avec différentes architectures d'actine (organisation parallèle ou antiparallèle, réseau enchevêtré), aboutissant à un processus en trois phases: tension, puis déformation des réseaux d'actine fortement couplée à un désassemblage massif des filaments. Aussi, ce phénomène de désassemblage massif induit par la myosine est intimement dépendant de l'architecture du réseau d'actine et pourrait, de ce fait, jouer un rôle essentiel dans la régulation spatiale des zones d'expansion et de contraction du cytosquelette in vivo

Dynamics of controlled actin network's architecture

Mon travail fut de développer différents projets en vue de mieux comprendre la dynamique et l'organisation des réseaux d'actine et les mécanismes moléculaires à l'origine de la production de force, cela en systèmes reconstitués bio-mimétiques. Dans un premier temps je me suis intéressée à l'étude de l'organisation spatio-temporelle des réseaux d'actine et de ses protéines associées durant la motilité de particules recouverte de promoteurs de nucléation (Achard et al, Current Biology, 2010 et Reymann et al, sous presse à MBOC). J'ai suivi en temps réel l'incorporation de deux régulateurs de l'actine (capping protein et ADF/cofiline) et montré que leur contrôle biochimique sur l'actine gouverne également ces propriétés mécaniques. Afin de mieux caractériser les propriétés mécaniques de ces réseaux d'actine en expension, j'ai ensuite développé un système biomimétique novateur utilisant un set-up de micro-patterning permettant un contrôle spatial reproductible des sites de nucléation d'actine. Cela m'a permis de montrer comment des barrières géométriques, semblables à celles trouvées dans les cellules, peuvent influencer la formation dynamique de réseaux organisés d'actine et ainsi contrôler la localisation de la production de forces. (Reymann et al, Nature Materials, 2010). De plus l'addition de moteurs moléculaires sur ce système versatile nous a permis d'étudier la contraction induite par des myosines. En particulier les myosines VI-HMM interagissent de manière sélective sur différentes architectures d'actine (organisation parallèle ou antiparallèle, réseau enchevêtré), aboutissant à un processus en trois phase : tension puis déformation des réseaux d'actine fortement couplé à un désassemblage massif des filaments. Ce phénomène est intimement dépendant de l'architecture du réseau d'actine et pourrait donc jouer un rôle essentiel dans la régulation spatiale des zones d'expansion et de contraction du cytosquelette in vivo. (Travail en cours d'écriture).I have developed different projects in order to tackle the problem of actin network dynamics and organization as well as the molecular mechanism at the origin of force production in biomimetic reconstituted systems. My first interest concerned the spatiotemporal organization of actin networks and actin-binding proteins during actin based motility of nucleation promoting factor-coated particles (Achard et al, Current Biology, 2010 and Reymann et al, in press at MBOC). I tracked in real time the incorporation of two actin regulators and showed that their biochemical control of actin dynamics also governs its mechanical properties. To further characterize mechanical properties of expanding actin networks, I used an innovative micro-patterning set-up allowing a reproducible spatial control of actin nucleation sites. It allowed me to show that geometrical boundaries, such as those encountered in cells, affect the dynamic formation of highly ordered actin structures and hence control the location of force production (Reymann et al, Nature Materials, 2010). Finally the addition of molecular motors on this tunable system allowed me to study implications for myosin-induced contractility. In particular, HMM-MyosinVI selectively interact with the different actin network architectures (parallel, anti-parallel organization or entangled networks) and leads to a selective three-phase process of tension, deformation of actin networks tightly coupled to massive filament disassembly. This phenomenon being highly dependent on actin network architecture could therefore play an essential role in the spatial regulation of expanding and contracting regions of actin cytoskeleton in cells. (Work in writing process)

Dynamique des réseaux d'actine d'architecture contrôlée

Mon travail fut de développer différents projets en vue de mieux comprendre la dynamique et l'organisation des réseaux d'actine et les mécanismes moléculaires à l'origine de la production de force, cela en systèmes reconstitués bio-mimétiques. Dans un premier temps je me suis intéressée à l'étude de l'organisation spatio-temporelle des réseaux d'actine et de ses protéines associées durant la motilité de particules recouverte de promoteurs de nucléation (Achard et al, Current Biology, 2010 et Reymann et al, sous presse à MBOC). J'ai suivi en temps réel l'incorporation de deux régulateurs de l'actine (capping protein et ADF/cofiline) et montré que leur contrôle biochimique sur l'actine gouverne également ces propriétés mécaniques. Afin de mieux caractériser les propriétés mécaniques de ces réseaux d'actine en expension, j'ai ensuite développé un système biomimétique novateur utilisant un set-up de micro-patterning permettant un contrôle spatial reproductible des sites de nucléation d'actine. Cela m'a permis de montrer comment des barrières géométriques, semblables à celles trouvées dans les cellules, peuvent influencer la formation dynamique de réseaux organisés d'actine et ainsi contrôler la localisation de la production de forces. (Reymann et al, Nature Materials, 2010). De plus l'addition de moteurs moléculaires sur ce système versatile nous a permis d'étudier la contraction induite par des myosines. En particulier les myosines VI-HMM interagissent de manière sélective sur différentes architectures d'actine (organisation parallèle ou antiparallèle, réseau enchevêtré), aboutissant à un processus en trois phase : tension puis déformation des réseaux d'actine fortement couplé à un désassemblage massif des filaments. Ce phénomène est intimement dépendant de l'architecture du réseau d'actine et pourrait donc jouer un rôle essentiel dans la régulation spatiale des zones d'expansion et de contraction du cytosquelette in vivo. (Travail en cours d'écriture).I have developed different projects in order to tackle the problem of actin network dynamics and organization as well as the molecular mechanism at the origin of force production in biomimetic reconstituted systems. My first interest concerned the spatiotemporal organization of actin networks and actin-binding proteins during actin based motility of nucleation promoting factor-coated particles (Achard et al, Current Biology, 2010 and Reymann et al, in press at MBOC). I tracked in real time the incorporation of two actin regulators and showed that their biochemical control of actin dynamics also governs its mechanical properties. To further characterize mechanical properties of expanding actin networks, I used an innovative micro-patterning set-up allowing a reproducible spatial control of actin nucleation sites. It allowed me to show that geometrical boundaries, such as those encountered in cells, affect the dynamic formation of highly ordered actin structures and hence control the location of force production (Reymann et al, Nature Materials, 2010). Finally the addition of molecular motors on this tunable system allowed me to study implications for myosin-induced contractility. In particular, HMM-MyosinVI selectively interact with the different actin network architectures (parallel, anti-parallel organization or entangled networks) and leads to a selective three-phase process of tension, deformation of actin networks tightly coupled to massive filament disassembly. This phenomenon being highly dependent on actin network architecture could therefore play an essential role in the spatial regulation of expanding and contracting regions of actin cytoskeleton in cells. (Work in writing process).SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Geometrical control of actin assembly and contractility

International audienceThe actin cytoskeleton is a fundamental player in many cellular processes. Ultrastructural studies have revealed its extremely complex organization, where actin filaments self-organize into defined and specialized structures of distinct functions and, yet, are able to selectively recruit biochemical regulators that are available in the entire cell volume. To overcome this extraordinary complexity, simplified reconstituted systems significantly improve our understanding of actin dynamics and self-organization. However, little is known regarding physical rules governing actin networks organization and to which extent network structure may direct and regulate selective interactions with specific regulators. Here, we describe the first method to direct actin filament assembly to specific 2D motifs with a finely tuned geometry and relative distribution. This method enables the study of how geometrical confinement governs actin network structural organization and how, in return, structural cues can control selective contraction by myosin motor. The protocol relies on the use of surface micropatterning and functionalization procedures in order to selectively direct actin filament assembly to specific sites of nucleation

Nucleation geometry governs ordered actin networks structures.

International audienceActin filaments constitute one of the main components of cell cytoskeleton. Assembled into bundles in filopodia or in stress fibres, they play a pivotal role in eukaryotes during cell morphogenesis, adhesion and motility. The bundle emergence has been extensively related to specific actin regulators in vivo. Such dynamic modulation was also highlighted by biochemical reconstitution of the actin-network assembly, in bulk solution or with biomimetic devices. However, the question of how geometrical boundaries, such as those encountered in cells, affect the dynamic formation of highly ordered actin structures remains poorly studied. Here we demonstrate that the nucleation geometry in itself can be the principal determinant of actin-network architecture. We developed a micropatterning method that enables the spatial control of actin nucleation sites for in vitro assays. Shape, orientation and distance between nucleation regions control filament orientation and length, filament-filament interactions and filopodium-like bundle formation. Modelling of filament growth and interactions demonstrates that basic mechanical and probabilistic laws govern actin assembly in higher-order structures

A "primer"-based mechanism underlies branched actin filament network formation and motility.

International audienceCells use actin assembly to generate forces for membrane protrusions during movement [1] or, in the case of pathogens, to propel themselves in the host cells, in crude extracts [2], or in mixtures of actin and other purified proteins [3]. Significant progress has been made in understanding the mechanism of actin-based motility at a macroscopic level by using biomimetic systems in vitro [4-6]. Here, we combined such a system with evanescent wave microscopy to visualize Arp2/3-mediated actin network formation at single-actin-filament resolution. We found that actin filaments that we call "primers" determine the origin of the autocatalytic and propagative formation of the actin network. In the presence of capping protein, multiple "primers" generate independent networks that merge around the object to form an outer "shell" made of entangled and capped filaments. Simultaneously, newly created filaments on the surface of the particle initiate mechanical stress, which develops until symmetry breaking. Our results and extensive modeling support that the stress, which releases into propulsive forces [7], is controlled not by any specific orientation of actin filaments toward the nucleation sites but only by new monomers added near the load surface

Rapid assembly of a polar network architecture drives efficient actomyosin contractility

S ummary Actin network architecture and dynamics play a central role in cell contractility and tissue morphogenesis. Pulsed contractions driven by RhoA represent a generic mode of actomyosin contractility, but the mechanisms underlying (1) how their specific architecture emerges, and (2) how this architecture supports the contractile function of the network, remain unclear. Here, we combine quantitative microscopy, single-molecule imaging, numerical simulations and simple mathematical modelling, to explore the dynamic network architecture underlying pulsed contraction. We show that during pulsed contractions, two subpopulations of formins are recruited by RhoA from the cytoplasm and bind to the cell surface in the early C. elegans embryo: recruited formins, a functionally inactive population, and elongating formins, which actively participate in actin filaments elongation. Focusing on formin dynamics during pulses, we show that minority elongating formins precede recruited formins, a kinetic dynamics compatible with formins capturing and rapidly saturating barbed ends available for filament elongation. We then show that these elongating formins assemble a polar network of actin, with barbed ends pointing out of the pulse, pointing to a kinetic rather than mechanical control of network architecture. Finally, our numerical simulations demonstrate that this geometry favors rapid network contraction. Our results thus show that formins saturate available actin filaments barbed ends and convert a local, biochemical gradient of RhoA activity into a polar network architecture, thereby driving rapid and efficient network contractility, an important evolutionary feature in a metazoan with a rapid embryonic cell cycles. H ighlights The formin CYK-1 drives actin network assembly during RhoA-driven pulses The process is extremely rapid, with a formin-based actin elongation rate higher than 1.3 μm·s -1 A barbed-end saturation mechanism allows for responsive F-actin assembly Rapid and responsive F-actin elongation results in the assembly of aster-like polar actin networks Numerical simulations show network polarity drives very efficient network contractilit