The Actin Related Protein (ARP) 2/3 complex is a major regulator of the actin cytoskeleton that is implicated in cell morphogenesis in plants. However, a similar role is attributed to the endoplasmic reticulum (ER). My research explored the relationship between the two systems by using transgenic plants simultaneously expressing fluorescent proteins highlighting F-actin and ER organization in living cells. A comparison of F-actin organization in cells of wild type Arabidopsis thaliana and mutants with aberrant actin cytoskeleton suggests bundling in the distorted2 mutant but a relatively fine F-actin arrangement in klunker. These differences correlate with ER organization into cisternae, fenestrated sheets and tubules. A model relating ER-organization to the degree of actin bundling in a cell emerges and is supported by drug-induced interference in actin polymerization, altered ionic conditions and temperature. The study adds to the mechanistic understanding of cell morphogenesis in plants.

Assembly of branched actin filaments produces dynamic structures required during membrane associated processes including cell motility and endocytosis. The Actin Related Protein 2/3 (Arp2/3) complex is the only known regulator capable of nucleating actin branches. To specify the sub cellular localization and timing of actin assembly the complex is tightly regulated. Canonical activation of the Arp2/3 complex by Wiskott-Aldrich Syndrome proteins (WASP), requires preformed actin filaments, ensuring the complex nucleates new actin filaments off the sides of preformed filaments. WASP proteins can therefore propagate branch formation but cannot initiate a Y-branch without performed filaments. A key question, then, is what is the source of preformed filaments that seed branched actin network formation in cells? It is unclear how activation of Arp2/3 by multiple regulators is balanced to specify actin filament architectures that are productive in vivo. In this dissertation, we identified WISH/DIP1/SPIN90 (WDS) family proteins as activators of the Arp2/3 complex that do not require preformed filaments, and evaluated whether WDS proteins seed branching nucleation. In chapter II, we dissected the biochemical properties of WDS proteins and found they activate the Arp2/3 complex using a non-WASP like mechanism. Importantly, we discovered WDS-mediated Arp2/3 activation produces linear, unbranched filaments, and this activity is conversed from yeast to mammals. These observations highlight that WDS proteins have the biochemical capacity to seed actin branches. In chapter III, we observed WDS-generated linear filaments can seed WASP-mediated branching directly using single molecule microscopy with fluorescently labeled Dip1. We find that WDS-mediated nucleation co-opts features of branching nucleation. In chapter IV, we investigated how WDS activity is balanced with WASP. We discovered WDS proteins use a single turnover mechanism to activate Arp2/3 and this is conserved during endocytosis. In contrast, WASP-mediated activation is multi-turnover, highlighting a crucial difference between WDS proteins and WASP. Our observations explain how Arp2/3 may limit linear filament production to initiate networks and favor branches during network propagation. Finally, we use fission yeast to show that increasing Dip1 is sufficient to cause defects in actin assembly and the timing of actin patches at sites of endocytosis.

Yes / Efficient motility requires polarized cells, with pseudopods at the front and a retracting rear. Polarization is maintained by restricting the pseudopod catalyst, active Rac, to the front. Here, we show that the actin nucleation-promoting factor Wiskott-Aldrich syndrome protein (WASP) contributes to maintenance of front-rear polarity by controlling localization and cellular levels of active Rac. Dictyostelium cells lacking WASP inappropriately activate Rac at the rear, which affects their polarity and speed. WASP’s Cdc42 and Rac interacting binding (“CRIB”) motif has been thought to be essential for its activation. However, we show that the CRIB motif’s biological role is unexpectedly complex. WASP CRIB mutants are no longer able to restrict Rac activity to the front, and cannot generate new pseudopods when SCAR/WAVE is absent. Overall levels of Rac activity also increase when WASP is unable to bind to Rac. However, WASP without a functional CRIB domain localizes normally at clathrin pits during endocytosis, and activates Arp2/3 complex. Similarly, chemical inhibition of Rac does not affect WASP localization or activation at sites of endocytosis. Thus, the interaction between small GTPases and WASP is more complex than previously thought—Rac regulates a subset of WASP functions, but WASP reciprocally restricts active Rac through its CRIB motif. / Cancer Research UK grants A15672, A24450, and multidisciplinary grant A20017.

Le complexe Scar/WAVE génère la formation des lamellipodes par l'intermédiaire du complexe Arp2/3 responsable de la polymérisation de réseaux d'actine branchés. Dans le but d'identifier de nouveaux régulateurs du complexe Scar/WAVE, nous avons conduit un crible en cellules de Drosophiles combinant une approche protéomique à une approche de génomique fonctionnelle. La chaîne lourde de la clathrine a été identifiée au cours de ce crible comme une protéine interagissant avec le complexe Scar/WAVE et dont la déplétion affecte la formation des lamellipodes. Ce rôle de la clathrine dans la formation des lamellipodes peut être découplé de son rôle classique dans le transport vésiculaire en utilisant différentes approches. De plus, la clathrine est localisée au lamellipode en l'absence d'adapteurs et des protéines accessoires de l'endocytose. La surexpression de la clathrine affecte le recrutement membranaire du complexe WAVE réduisant ainsi la vélocité des protrusions membranaire et la migration cellulaire. Par opposition, lorsque la clathrine est envoyée artificiellement à la membrane plasmique par une fusion à une séquence myristoylée, on observe une augmentation du recrutement membranaire du complexe Scar/WAVE, de la vélocité des protrusions membranaires et de la migration cellulaire. L'ensemble de ces résultats montrent que la clathrine envoie le complexe Scar/WAVE à la membrane plasmique et donc contrôle la formation des lamellipodes en plus de son rôle plus classique dans le traffic membranaire. / The Scar/Wave complex (SWC) generates lamellipodia through Arp2/3-dependent polymerization of branched actin networks. In order to identify new SWC regulators, we conducted a screen in Drosophila cells combining proteomics with functional genomics. This screen identified Clathrin Heavy Chain (CHC) as a protein that binds to the SWC and whose depletion affects lamellipodium formation. This role of CHC in lamellipodium formation can be uncoupled from its role in membrane traffic by several experimental approaches. Furthermore, CHC is detected in lamellipodia in the absence of the adaptor and accessory proteins of endocytosis. We found that CHC overexpression decreased membrane recruitment of the SWC, resulting in reduced velocity of protrusions and reduced cell migration. In contrast, when CHC was targeted to the membrane by fusion to a myristoylation sequence, we observed an increase in membrane recruitment of the SWC, in protrusion velocity and in cell migration. Together these data suggest that CHC brings the SWC to the plasma membrane, thereby controlling lamellipodium formation, in addition to its classical role in membrane traffic.

Dans une cellule en migration, la polymérisation d'actine permet de projeter la membrane plasmique dans une structure appelée le lamellipode. Dans le lamellipode, l'actine est polymérisée de manière branchée par le complexe Arp2/3. L'activation du complexe Arp2/3 au lamellipode est sous le contrôle du complexe WAVE. En réponse à une cascade d’activation moléculaire, une des sous-unités du complexe WAVE expose son domaine WCA (WH2-Connecteur-Acide) qui peut alors se lier au complexe Arp2/3 et l’activer afin d'initier la formation d’un nouveau filament d’actine. La voie d’activation du complexe Arp2/3 par le complexe WAVE a été bien étudiée. Cependant la migration cellulaire est finement régulée et cette unique voie de signalisation nous semblait insuffisante. Dans le but de trouver de nouveaux régulateurs de la migration et en particulier de nouvelles protéines se liant au complexe Arp2/3, nous avons réalisé un crible bioinformatique identifiant les protéines contenant un motif Acide. Ce dernier a abouti à l’identification d’une protéine non caractérisée. In vitro, cette protéine n'active pas le complexe Arp2/3. En revanche, elle est capable d'inhiber l'activation du complexe Arp2/3 induite par le domaine WCA d'un activateur et empêche la formation de branches par le complexe Arp2/3. Nous avons appelé cette nouvelle protéine Arpin pour « Arp2/3 Inhibitor ». De manière cohérente avec son rôle inhibiteur in vitro, la déplétion d'Arpin dans différents type de cellules, induit une augmentation de la vitesse de protrusion des lamellipodes et une demi-vie augmentée des lamellipodes. Ces effets se traduisent par une migration plus rapide et plus persistante en direction. Arpin joue donc le rôle d'un frein de la migration cellulaire et permet à la cellule de tourner. Pour jouer ce rôle-là, Arpin nécessite d’être régulée rigoureusement. Dans la cellule, Arpin est inactive et nécessite d’être activée par Rac. Cependant cette régulation n'est probablement pas directe. Pour mieux comprendre la régulation d'Arpin, nous avons donc recherché des protéines partenaires. Nous avons identifié Tankyrase comme protéine interagissant avec Arpin. De façon significative, le motif d’Arpin qui permet son interaction avec Tankyrase se superpose à la séquence Acide nécessaire à son interaction avec le complexe Arp2/3. Nous avons mis en évidence in vitro une compétition entre Tankyrase et le complexe Arp2/3 sur Arpin. Ces résultats suggèrent Tankyrase inhibe la protéine inhibitrice Arpin. En conclusion, nous avons découvert une nouvelle protéine Arpin, qui inhibe le complexe Arp2/3 et qui joue un rôle régulateur important dans la migration cellulaire. Nous avons identifié une protéine régulatrice de son activité, la Tankyrase. Nous nous attendons à ce qu’Arpin soit impliquée dans des nombreux processus physiologiques ou pathologiques, où la migration cellulaire joue un rôle important, en particulier lors de la formation de métastases dans le cancer. / In migrating cells, the Arp2/3 complex generates branched actin networks that power protrusion of the leading edge in a structure called lamellipodium. The Arp2/3 complex is activated at the leading edge by the Wave complex which is itself activated by the small GTPase Rac. WAVE which is in an inactive state, then exposes its WCA domain (WH2-Connector-Acidic) that can bind to the Arp2/3 complex and activate it to trigger the formation of a new daughter actin filament. This signalling pathway of the Arp2/3 complex has been well studied. However, cell migration is a fine-tuned process that is probably regulated in a more complex manner.To identify new regulators of cell migration, especially proteins that bind to the Arp2/3 complex, we performed a bioinformatics screen to identify proteins containing an acidic motif at its C-terminus, a characteristic motif of Arp2/3 activators. By this method we retrieved an uncharacterized protein. A combination of in vitro assays revealed, however, that this protein inhibits the Arp2/3 complex by competing with the activators. We called this protein Arpin for “Arp2/3 inhibitor”. Depletion of Arpin in different kind of cells, such as mammalian cells or amoeba, induces lamellipodia to protrude faster and to last longer, consistent with its inhibitory role on Arp2/3 complex activity. These effects observed lead to an increased velocity and a more directional migration in random migration assay. The function of the Arp2/3 inhibitory protein Arpin is thus to slow down and steer cell migration.In the cell, Arpin has been shown to be inactive until it is activated by Rac, most likely by an indirect manner. We identified Tankyrase as an interactor of Arpin. Interestingly, the binding motif of Arpin to Tankyrase overlaps the acidic motif required for the binding to the Arp2/3 complex. By a biochemistry approach, we showed a competition between Tankyrase and the Arp2/3 complex for the binding to Arpin. This observation suggests that Tankyrase inhibits the inhibitory protein Arpin in the cell. To conclude, we identified a new protein, Arpin which inhibits the Arp2/3 complex and plays an important role in the control of cell migration. We identified a protein which regulated its activity, Tankyrase. Thus, we can imagine that Arpin could be implicated in numerous physiological and pathological processes where cell migration is involved, particularly during metastases formation in cancer

La motilité cellulaire est un processus intégré essentiel à de nombreux phénomènes physiologiques tels que la formation du cône de croissance et la plasticité synaptique. Des dérégulations de la motilité cellulaire peuvent être à l’origine de la formation de métastases ou de pathologies neuropsychiatriques comme la schizophrénie et l'autisme. La compréhension des mécanismes régulant la migration cellulaire est donc un enjeu majeur. La motilité cellulaire repose sur la formation de diverses structures constituées de réseaux d’actine branchés telles que le lamellipode. La formation du lamellipode nécessite l’intervention de protéines régulatrices de l’actine telles que Rac1 et les complexes Wave et Arp2/3. Grâce à l’utilisation de suivi de protéine unique, nous avons pu comprendre comment la coordination spatio-temporelle de ces régulateurs contrôle la formation et la morphologie des lamellipodes de cellules migrantes. Nous avons ainsi découvert que l’activation et la localisation du complexe Wave étaient régulées de manière enzymatique mais également mécanique. Dans une première étude, nous avons montré que la RhoGTPase Rac1 active le complexe Wave spécifiquement à l’extrémité du lamellipode. Dans une seconde étude, nous avons révélé que la localisation du complexe Wave est régulée par la dynamique des filaments des réseaux branchés d’actine. Ces données soulignent l’importance du complexe Wave dans la formation du lamellipode et révèlent l’existence d’une régulation mécanique de la localisation du complexe Wave. / Cell motility is an integrated process involved in critical phenomena such as axonal pathfinding and synaptic plasticity. Dysregulation of cell motility can induce metastasis and abnormal spine shapes observed in neuropsychiatric disorders like autism and schizophrenia. Therefore it is essential to understand how cell motility is regulated. Cell motility requires the formation of branched actin networks propelled by actin polymerization that lead to the formation of membrane protrusions such as the lamellipodium. Several actin regulatory proteins are involved in this process, such as Rac1 and the WAVE and ARP2/3 complexes. Using single protein tracking, we revealed key phenomena concerning the spatio-temporal regulation of lamellipodium formation by actin regulatory proteins. We found that the localization and activation of the WAVE complex was enzymatically regulated, but also mechanically. First, we showed that the Rac1 RhoGTPase activates the WAVE complex specifically at the tip of the lamellipodium. We also showed that WAVE complex localization is regulated by the dynamics of branched-network actin filaments. This study confirms the crucial role of the WAVE complex in lamellipodium formation and reveals the existence of a mechanical regulation of the localization of this complex in the cell.

Legionella pneumophila est une bactérie opportuniste qui émerge de l'environnement après multiplication dans des amibes et peut infecter accidentellement les macrophages alvéolaires humains, provoquant une pneumonie sévère, la légionellose. La capacité de L. pneumophila à survivre dans ses cellules hôtes est strictement dépendante du système de sécrétion de type 4 Dot/Icm, qui sécrète un large répertoire d'effecteurs dans le cytosol de l'hôte. Identifier la contribution individuelle de chaque protéine bactérienne sécrétée par le système Dot/Icm, dans le cycle infectieux de L. pneumophila reste un enjeu majeur pour comprendre les bases moléculaires de la virulence des légionelles. Mes travaux de thèse participent à cet objectif en caractérisant la voie cellulaire ciblée par la protéine kinase LegK2. Des tests d'interaction et de phosphorylation ont identifié le complexe nucléateur d'actine ARP2/3 comme cible de LegK2. Suite à l'adressage de LegK2 à la surface de la vacuole après sa translocation dans le cytosol de l'hôte, l'interaction LegK2-ARP2/3 inhibe la polymérisation d'actine sur le phagosome. Cette inhibition permet à Legionella de diminuer le trafic des endosomes tardifs et/ou des lysosomes vers le phagosome et favorise ainsi l'évasion du phagosome à la voie de dégradation endocytique. L'interaction LegK2-ARP2/3 met en évidence un mécanisme original de virulence dans lequel le remodelage local du cytosquelette d'actine de la cellule hôte permet à la bactérie de manipuler le trafic vésiculaire pour échapper aux défenses de l'hôte / Legionella pneumophila is an opportunistic bacterium that emerges from the environment after multiplication in protozoans and can accidentally infect human alveolar macrophages leading to a severe pneumonia, the legionellosis. The L. pneumophila ability to survive within host-cells is strictly dependent on the Dot/Icm Type 4 Secretion System that translocates a large repertoire of effectors into the host cell cytosol. Deciphering the individual contribution of each bacterial protein translocated by the Dot/Icm system in the L. pneumophila infectious cycle remains a major challenge to understand the molecular basis of Legionella virulence. My works contribute to this objective by characterizing the cellular pathway targeted by the protein kinase LegK2. Interaction and phosphorylation assays identified the actin nucleator ARP2/3 complex as the target of LegK2. Following the LegK2 addressing to the vacuole surface after its translocation into host cytosol, LegK2- ARP2/3 interplay inhibits the actin polymerization on the phagosome. This inhibition allows Legionella to decrease the late endosome/lysosome trafficking towards the phagosome and promotes the phagosome evasion from endocytic degradation pathway. LegK2-ARP2/3 interplay highlights an original mechanism of virulence wherein the local actin cytoskeleton remodeling of host cell allows bacteria to hijack the vesicles trafficking in order to escape host-cell defenses

Le complexe Arp2/3 génère des réseaux d’actine branchés, qui produisent une forcée de poussée permettant à la cellule de remodeler ses membranes. Le complexe WASH active le complexe Arp2/3 à la surface des endosomes et facilite ainsi la scission membranaire des intermédiaires de transports contenants des récepteurs internalisés tels que les intégrines α5β1. De ce fait, le complexe WASH en favorisant le recyclage des intégrines, joue un rôle crucial dans l’invasion des cellules tumorales durant la progression tumorale. Cependant, le mécanisme d’assemblage du complexe WASH est inconnu. Dans cette étude, nous rapportons l’identification du premier facteur d’assemblage du complexe WASH. Nous avons identifié la protéine HSBP1 grâce à un crible des protéines qui se lient aux formes précurseurs des sous-unités mais plus au complexe une fois assemblé. La reconstitution biochimique et la modélisation moléculaire nous a permis de montrer que HSBP1 est associé avec le précurseur trimérique CCDC53, le dissocie et forme un hétérotrimère qui va éventuellement libérer une forme monomérique de CCDC53 pour l’assemblage du complexe WASH. Le rôle de HSBP1 dans l’assemblage du complexe WASH est conservé. En effet, WASH est déstabilisé dans des cellules mammaires par le knock-down de HSBP1 et dans l’amibe Dictyostelium par le knock-out de HSBP1. La déstabilisation du complexe WASH par le knock-out de HSBP1 phénocopie la déplétion de WASH dans l’amibe Dictyostelium. Dans des cellules humaines de carcinomes mammaires l’inhibition de l’expression de HSBP1 altère le recyclage des intégrines à la membrane plasmidique. Il en résulte des adhésions focales défectueuses et des capacités invasives réduites. De plus, HSBP1 est localisé aux centrosomes et est requis pour la polarité des cellules lors de la migration. Enfin, nous avons trouvé que la surexpression de HSBP1 dans des tumeurs mammaires est associée à une augmentation des niveaux du complexe WASH et à un mauvais pronostic pour les patientes atteintes de cancer du sein. En conclusion, HSBP1 est un facteur d’assemblage conservé qui contrôle les niveaux du complexe WASH. / The Arp2/3 complex generates branched actin networks, which produces a pushing force that helps the cell to remodel its membranes. The WASH complex activates the Arp2/3 complex at the surface of endosomes and thereby, facilitates the membrane scission of the transport intermediates containing internalized receptors such as α5β1 integrins. Hence, by promoting integrin recycling, the WASH complex plays a crucial role in tumor cell invasion during cancer progression. However, how cells assemble the WASH complex at first is unknown. Here we report the identification of the first assembly factor of the WASH complex. We identified HSBP1 in a proteomics screen for proteins binding to precursor forms of subunits, but not to the fully assembled WASH complex. Through biochemical reconstitution and molecular modeling, we found that HSBP1 associates with the precursor CCDC53 trimer, dissociates it and forms a heterotrimer that will eventually contribute a single CCDC53 molecule to the assembling WASH complex. The role of HSBP1 in WASH complex assembly is well conserved since WASH is similarly destabilized upon HSBP1 knock-down in mammalian cells or upon HSBP1 knock-out in Dictyostelium amoeba. In line with the defective assembly of the WASH complex, the HSBP1 knock-out closely phenocopies WASH knock-out in amoeba. In human mammary carcinoma cells, HSBP1 depletion results in impaired integrin recycling to the plasma membrane leading to the defective development of focal adhesions and reduced invasion abilities. Moreover, HSBP1 was found to localize at the centrosome and was required for the polarization associated with the migration. On the other end, in mammary breast tumors, we found that HSBP1 was often overexpressed and that its overexpression was associated with increased levels of the WASH complex and with poor prognosis for breast cancer patients. Hence, HSBP1 is a conserved assembly factor that controls the levels of the WASH complex.

<p>    </p> <p>When mammalian eggs are fertilized, they undergo an egg-to-embryo transition during which different egg activation events take place. Egg activation events include the establishment of blocks to polyspermy, which prevent multiple sperm from fertilizing an egg. One of these blocks to polyspermy occurs at the level of the egg plasma membrane (the membrane block to polyspermy). Previous work in our lab provides evidence that the mammalian membrane block to polyspermy is mediated by sperm-induced calcium signaling and the egg’s actomyosin cytoskeleton (McAvey et al., 2002). This thesis research builds upon this foundation, testing hypotheses about two specific effector molecules, one involved in calcium signaling and one with the actin cytoskeleton, and also developing the use of an actin probe for live-cell imaging, with the goal of imaging actin dynamics in eggs undergoing fertilization. Specifically, we examined the calcium effector molecule Ca2+/Calmodulin-dependent-protein kinase IIg (<strong>CaMKII</strong>g), based on previous studies showing that CaMKII plays a role in the membrane block (Gardner et al., 2007) and that the g isoform of CaMKII is necessary and sufficient for eggs to complete meiosis (Backs et al., 2010). We tested the hypothesis that CaMKIIg would mediate the membrane block to polyspermy but found that egg activation driven by expression of a constitutively active form of CaMKIIg was not sufficient to establish the membrane block. Our studies of the actin cytoskeleton focused on the Arp2/3 complex as a candidate. We tested the hypothesis that Arp2/3, which mediates actin filament branching, was involved in membrane block establishment, building on the finding that disruption of actin with the drug cytochalasin D impairs the membrane block (McAvey et al., 2022). These studies used the Arp2/3 inhibitor CK666, predicting that we would see increased sperm incorporation in CK666-treated eggs. However, an assay of sperm incorporation over time indicated that Arp2/3 may not play a significant role in the membrane block to polyspermy, although follow-up studies will be beneficial. Lastly, the actin probe SiR- Actin was assessed for use on oocytes undergoing live-cell imaging during meiosis I and II. Oocytes were treated with differing concentrations of SiR-Actin and live cell imaged while maturing through meiosis I or completing meiosis II. Higher doses and longer exposure to SiR- Actin caused abnormalities in oocytes during meiosis I but not in eggs completing meiosis II. Together, this work sets the stage of a range of future studies into the mammalian membrane block to polyspermy. </p>