L'hépatite C reste un problème majeur de santé publique. Malgré la mise au point d'un modèle de réplication du virus de l'hépatite C (VHC) in vitro, les mécanismes conduisant à l'infection restent encore mal connus. Le VHC est sécrété et circule dans le sérum associé à des lipoprotéines. L'importance des lipoprotéines pour le cycle viral nous a conduits à étudier le rôle de la lipoprotéine lipase (LPL), une enzyme lipolytique, dans l'infection de la cellule par le VHC. Nous avons montré que la LPL potentialise l'attachement et l'internalisation du virus par un mécanisme similaire à la clearance hépatique des lipoprotéines. La LPL dimérique forme un pont entre les lipoprotéines associées au virus et les HSPG à la surface des cellules. Néanmoins son action conduit à une inhibition de l'infection par les souches virales JFH-1 et J6/JFH-1 produites en culture cellulaire et dans les hépatocytes humains greffés à des souris chimériques uPA-SCID. L'analyse par ultracentrifugation en gradient d'iodixanol des virus produits in vitro et in vivo a montré la présence de deux populations virales : la première, de densité très faible, est beaucoup plus infectieuse que la seconde, de densité plus élevée. L'infection in vitro par ces deux populations virales est inhibée par la LPL. La LPL représente donc un nouvel inhibiteur de l'infection par le VHC. Nous avons également démontré que la présence d'un réseau de microtubules intact et dynamique est cruciale pour l'entrée du VHC et les étapes post-fusion qui mènent à l'infection. Enfin, nous avons mis en évidence une interaction de la protéine de capside avec les tubulines α et β, conduisant à une augmentation de la polymérisation des microtubules. Ces observations suggèrent que le VHC pourrait utiliser les mécanismes de polymérisation des microtubules pour établir l'infection, et la protéine de capside jouer un rôle essentiel dans ce processus. Les nouvelles approches antivirales pourraient donc cibler les éléments du cytosquelette et/ou des lipoprotéines associées aux particules virales.

Sensory-motile cells fulfill various biological functions ranging from immune activity or wound healing to the formation of the highly complex nervous systems of vertebrates. In the case of neurons, a dynamic structure at the tip of outgrowing processes navigates towards target cells or areas during the generation of neural networks. These fan shaped growth cones are equipped with a highly complex molecular machinery able to detect various external stimuli and to translate them into directed motion. Receptor and adhesion molecules trigger signaling cascades that regulate the dynamics of an internal polymeric scaffold, the cytoskeleton. It plays a crucial role in morphology maintenance as well as in the generation and distribution of growth cone forces. The two major components, actin and microtubules (MTs) connect on multiple levels through interwoven biochemical and mechanical interactions. Actin monomers assemble into semiflexible filaments (F-actin) which in turn are either arranged in entangled networks in the flat outer region of the growth cone (lamellipodium) or in radial bundles termed filopodia. The dynamic network of actin filaments extends through polymerization at the front edge of the lamellipodium and is simultaneously moving towards the center (C-domain) of the growth cone. This retrograde flow (RF) of the actin network is driven by the polymerizing filaments themselves pushing against the cell membrane and the contractile activity of motor proteins (myosins), mainly in the more central transition zone (T-zone). Through transmembrane adhesion molecules, a fraction of the retrograde flow forces is mechanically transmitted to the cellular substrate in a clutch-like mechanism generating traction and moving the GC forward. MTs are tubular polymeric structures assembled from two types of tubulin protein subunits. They are densely bundled in the neurite and at the growth cone “neck” (where the neurite opens out into the growth cone) they splay apart entering the C-domain and more peripheral regions (P-domain). Their advancement is driven by polymerization and dynein motor protein activity. The two subsystems, an extending array of MTs and the centripetal moving actin network are antagonistic players regulating GC morphology and motility. Numerous experimental findings suggest that MTs pushing from the rear interact with actin structures and contribute to GC advancement. Nevertheless, the amount of force generated or transmitted through these rigid structures has not been investigated yet. In the present dissertation, the deformation of MTs under the influence of intracellular load is analyzed with fluorescence microscopy techniques to estimate these forces. RF mechanically couples to MTs in the GC periphery through friction and molecular cross-linkers. This leads to MT buckling which in turn allows the calculation of the underlying force. It turns out that forces of at least act on individual MT filaments in the GC periphery. Compared to the relatively low overall protrusion force of neuronal GCs, this is a substantial contribution. Interestingly, two populations of MTs buckle under different loads suggesting different buckling conditions. These could be ascribed to either the length-dependent flexural rigidity of MTs or local variations in the mechanical properties of the lamellipodial actin network. Furthermore, the relation between MT deformation levels and GC morphology and advancement was investigated. A clear trend evolves that links higher MT deformation in certain areas to their advancement. Interactions between RF and MTs also influence flow velocity and MT deformation. It is shown that transient RF bursts are related to higher MT deformation in the same region. An internal molecular clutch mechanism is proposed that links MT deformation to GC advancement. When focusing on GC dynamics it is often neglected that the retraction of neurites and the controlled collapse of GCs are as important for proper neural network formation as oriented outgrowth. Since erroneous connections can cause equally severe malfunctions as missing ones, the pruning of aberrant processes or the transient stalling of outgrowth at pivotal locations are common events in neuronal growth. To date, mainly short term pausing with minor cytoskeletal rearrangements or the full detachment and retraction of neurite segments were described. It is likely that these two variants do not cover the full range of possible events during neuronal pathfinding and that pausing on intermediate time scales is an appropriate means to avoid the misdetection of faint or ambiguous external signals. In the NG108-15 neuroblastoma cells investigated here, a novel type of collapse was observed. It is characterized by the degradation of actin network structures in the periphery while radial filopodia and the C-domain persist. Actin bundles in filopodia are segmented at one or multiple breaking points and subsequently fold onto the edge of the C-domain where they form an actin-rich barrier blocking MT extension. Due to this characteristic, this type of collapse was termed fold collapse. Possible molecular players responsible for this remarkable process are discussed. Throughout fold collapse, GC C-domain area and position remain stable and only the turnover of peripheral actin structures is abolished. At the same time, MT driven neurite elongation is hindered, causing the GC to stall on a time scale of several to tens of minutes. In many cases, new lamellipodial structures emerge after some time, indicating the transient nature of this collapse variant. From the detailed description of the cytoskeletal dynamics during collapse a working model including substrate contacts and contractile actin-myosin activity is derived. Within this model, the known and newly found types of GC collapse and retraction can be reduced to variations in local adhesion and motor protein activity. Altogether the results of this work indicate a more prominent role of forward directed MT-based forces in neuronal growth than previously assumed. Their regulation and distribution during outgrowth has significant impact on neurite orientation and advancement. The deformation of MT filaments is closely related to retrograde actin flow which in turn is a regulator of edge protrusion. For the stalling of GCs it is not only required that actin dynamics are decoupled from the environment but also that MT pushing is suppressed. In the case of fold collapse, this is achieved through a robust barrier assembled from filopodial actin bundles.

Microtubules are cytoskeletal protein polymers relevant to a wide range of cell functions. In order to polymerize, the constituent tubulin subunits need to bind the nucleotide GTP, but its subsequent hydrolysis to GDP in the microtubule lattice induces depolymerization. The resulting behaviour of stochastic switching between growth and shrinkage is called dynamic instability. Both dynamic instability and microtubule mechanical properties are integral to many cell functions, yet are poorly understood. The present study uses thermal fluctuation measurements of grafted microtubules with different nucleotide contents to extract stiffnesses, relaxation times, and drag coefficients with an unprecedented precision. Both the stiffness and the relaxation time data indicate that stiffness is a function of length for GDP microtubules stabilized with the chemotherapy drug taxol. By contrast, measurements on microtubules polymerized with the non-hydrolizable GTP-analogue GMPCPP show a significantly higher, but constant, stiffness. The addition of taxol is shown to not significantly affect the properties of these microtubules, but a lowering of the GMPCPP content restores the length-dependent stiffness seen for taxol microtubules. The data are interpreted on the basis of a recent biopolymer model that takes into account the anisotropic architecture of microtubules which consist of loosely coupled protofilaments arranged in a tube. Using taxol microtubules and GMPCPP microtubules as the respective analogues of the GDP and GTP state of microtubules, evidence is presented that shear coupling between neighbouring protofilaments is at least two orders of magnitude stiffer in the GTP state than in the GDP state. Previous studies of nucleotide effects on tubulin have focussed on protofilament bending, and the present study is the first to be able to show a dramatic effect on interprotofilament bonds. The finding’s profound implications for dynamic instability are discussed. In addition, internal friction is found to dominate over hydrodynamic drag for microtubules shorter than ∼ 4 μm and, like stiffness, to be affected by the bound nucleotide, but not by taxol. Furthermore, the thermal shape fluctuations of free microtubules are imaged, and the intrinsic curvatures of microtubules are shown for the first time to follow a spectrum reminiscent of thermal bending. Regarding the extraction of mechanical data, this assay, though previously described in the literature, is shown to suffer from systematic flaws.

La forme native des hétérodimères de tubulines, indispensable à leur incorporation aux microtubules, est obtenue après prise en charge des tubulines par différentes protéines chaperonnes. Arl2 (ADP Ribosylation Factor-like Protein 2) est une protéine qui régule l'action de certaines protéines chaperonnes des tubulines. En utilisant des modèles de cancer du sein, nous avons montré dans ce travail que des modifications de l'expression d'Arl2 induisaient (1) des perturbations importantes de la dynamique microtubulaire et de la mitose ; (2) une chimiorésistance à différents agents anticancéreux par un mécanisme dépendant de la phosphatase PP2A et associé au facteur de transcription p53 ; (3) des modifications de la croissance tumorale. Les résultats obtenus ont permis d'établir, pour la première fois, un lien entre des événements régulant la production de tubulines fonctionnelles et des perturbations au niveau des phénotypes microtubulaires et cellulaires de cellules cancéreuses humaines. Ce travail ouvre des perspectives nouvelles dans l'identification de facteurs influençant à la fois la chimiorésistance et le développement tumoral de cellules cancéreuses mammaires.

Trypanosomes are hemoflagellated protozoan parasites causing chagas disease in South America, Leishmaniasis throughout the world, and African sleeping sickness in humans and nagana in animals in Africa. About 55 million people and 25 million cattle have been estimated to be at risk of contracting African sleeping sickness or nagana respectively. Once injected into the blood stream via the bite of a tsetse fly, the parasite evades the host's immune response by repeatedly changing its surface antigens, thus making the development of a vaccine seem impossible. Furthermore, chemotherapy existing today can be toxic, suggesting that novel methods to prevent diseases caused by trypanosomes are essential. All parasites of the Trypanosomatidae family contain unique microtubular structures called the subpellicular microtubules. Microtubules are made of tubulin and of microtubule associated proteins (MAPs). Unlike other microtubules, the subpellicular microtubules are crosslinked to one another and to the plasma membrane. The unique structure of the subpellicular microtubules has been attributed to unique trypanosome subpellicular MAPs which stabilize the microtubule polymers and crosslink them to one another. Three unique types of subpellicular MAPs have been identified: MARP, which is a high molecular mass MAP that stabilizes microtubules, p52 that is a 52kDa MAP which crosslinks microtubules, and pI5, which is a I5kDa protein which bundles microtubules. Because trypanosome MAPs have been shown to be unique to these parasites, these molecules could serve as useful target sites for therapy. In this study pI5 was cloned and sequenced and shown to contain highly organized, nearly identical tandem repeats with a periodicity of 10 amino acids, rich in positively charged and in hydrophobic amino acids. It was shown that pI5 can also bind phospholipids, suggesting that it may not only bundle the microtubule polymer through its positively charged amino acids but may also crosslink the microtubules to the plasma membrane through its hydrophobic regions, thus contributing to the stable structure of the subpellicular microtubules. To test for the efficiency of pI5 as a vaccine candidate, the recombinant pI5 was cloned into an adenovirus, which was used as a vaccine delivery system for pI5. Mice were vaccinated with the native purified pI5, with the expressed recombinant pI5 and with the adenovirus containing the recombinant pI5 gene (Ad-pI5). The results indicated that pI5 protected 100% of the animals vaccinated with the recombinant molecule (8/8), and 87% of the animals vaccinated with the native protein (7/S), while none of the control animals were protected. Animals that were vaccinated with the Ad-pI5 were protected but so were the control animals vaccinated with an adenovirus containing the lacZ gene. We have shown that vaccination with the adenovirus is associated with an elevated CDS+ T cell response which is known to be trypanostatic (S6), suggesting that animals vaccinated with Ad-pIS may have been protected not only by the specific anti-plS response but also by non specific immunity that was induced by the adenovirus itself. The source of the native and recombinant pI5 was from a different strain of T. brucei that was used for challenge. Since the subpellicular microtubules are common to all members of the Trypanosomatidae family, pI5 may ultimately serve as a common target for therapy to all types of diseases caused by trypanosomes. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2001.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

När ett blad gulnar genomgår det både morfologiska och metaboliska förändringar. Denna process benämns senescence och en förbättrad förståelse av dess mekanismer är viktiga både ur ett grundvetenskapligt perspektiv och för potentiella bioteknologiska applikationer. Denna avhandling rapporterar om flera viktiga aspekter relaterade till de cellulära och metaboliska mekanismer som sker under senescencen med tonvikt på mitokondriernas bidrag till denna process. I ett första steg utvecklade vi metoder för att isolera antingen mycket funktionella mitokondrier eller mycket rena mitokondrier från blad av Arabidopsis thaliana. Dessa metoder användes sedan till för att studera mitokondriella bidrag till cellens redox balans och att uppskatta mitokondriernas kapacitet under senescence-processen. Framför allt jämfördes induktionen av senescencen berodende på olika mörkerbehandlingar av Arabidopsis. Jämförelse mellan individuellt mörklagda blad med hela mörklagda växter visade en betydande skillnad i metabolisk strategi mellan de två mörkerbehandlingarna. Genom att integrera data från mätningar av fotosyntes, respiration och konfokal mikroskopi med transcriptomics- och metabolomics-profiler föreslår vi att metabolismen hos blad från helt mörklagda växter antar ett ”stand-by läge” för att kunna bibehålla fotosynteskapaciteten så länge som möjligt. I kontrast till detta visar mitokondrier från individuellt mörklagda blad en hög aktivitet och kan därmed producera energi och kolskelett för degraderingen av cellkomponenter, vilket möjliggör återvinning av näringsämnen. Vi har även studerat dynamiken av det mikrotubulibaserade cytoskelettet under mörkerindicerad senescence. Mitokondriernas rörlighet påverkades av en tidig nedbrytning av mikrotubuli hos individuellt mörklagda blad men inte hos blad där hela växten mörkerbehandlats. Dessutom verkade ett flertal mikrotubuliassocierade proteiner (MAP’s) att vara involverade i buntningen av mikrotubuli runt kloroplasterna. Sammanfattningsvis belyser det arbete som presenteras i denna avhandling ett flertal viktiga steg med avseende på metabolisk anpassning och andra cellulär mekanismer i Arabidopsisblad som utsätts för långvarig mörkerbehendling. Specifikt föreslår vi att mitokondrierna bidrar med speciella och viktiga funktioner under bladens senescence eftersom mitokondriernas roll under långvarig mörkerbehandling av blad verkar bero på den totala statusen av metabolismen hos växten. / When switching from green to yellow, a leaf undergoes both morphological and metabolic changes. This process is known as senescence and improved understanding of its mechanisms is important both from a fundamental scientific perspective but also for biotechnological applications. The present thesis reports on several important aspects regarding the cellular and metabolic mechanisms occurring during leaf senescence with an emphasis on the mitochondrial contribution to this process. As a first step, we developed methods to isolate either highly functional crude mitochondria or highly purified mitochondria from leaves of Arabidopsis thaliana. These methods were further used to study mitochondrial contributions to cellular redox homeostasis and to estimate the mitochondrial capacities in leaves undergoing senescence. In particular, we compared the induction of senescence by different dark treatments in Arabidopsis. The comparison between individually darkened leaves and leaves from whole darkened plants revealed different metabolic strategies in response to darkness. Integrating data from measurements of photosynthesis, respiration and confocal laser microscopy with transcriptomics and metabolomics profiling, we suggested that metabolism in leaves of the whole darkened plants enter a “stand-by mode” with low mitochondrial activity in order to maintain the photosynthetic machinery for as long as possible. In contrast, mitochondria from individually darkened leaves are more active and may provide energy and carbon skeletons for the degradation of cell constituents, facilitating the retrieval of nutrients. We also investigated the dynamic of the microtubular cytoskeleton during dark-induced senescence. Mitochondrial mobility was affected by an early disruption of the microtubules in individually darkened leaves but not in whole darkened plants. In addition, several microtubules associated proteins (MAPs) seemed to be involved in the bundling of the microtubules around the chloroplasts. Altogether, the work presented in this thesis highlights several important steps regarding the metabolic adjustments and the cellular mechanisms in Arabidopsis leaves submitted to prolonged darkness. In particular, we suggest the mitochondria to fulfill specific and important functions during leaf senescence since the role of mitochondria in leaves experiencing prolonged darkness appears very dependant on the whole metabolic status of the plant.

Résumé Les protéines XMAP215/ch-TOG appartiennent à une famille de protéines associées aux microtubules (MAPs), bien conservée tout au long de l'évolution, la famille XMAP215/Dis1. Cette famille joue un rôle dans la régulation du cytosquelette des microtubules (MT), en particulier pendant la division cellulaire. Chez l'humain, ch-TOG est la protéine surexprimée dans les tumeurs du colon et du foie, une protéine qui provient de cellules blastiques et de plusieurs formes de cancer. Certaines protéines XMAP215/ch-TOG ont été retrouvées dans différentes localisations cellulaires, toujours reliées aux MTs, donnant origine à une activité spécifique. Cependant, la localisation exacte de XMAP215/ch-TOG ainsi que son activité restait à être déterminées. Dans ce contexte scientifique, nous avons développé une série d'anticorps monoclonaux (mcAB) qui nous ont permis d'identifier deux populations différentes de la famille des protéines XMAP215/Dis1. Les images de microscopie confocale des cellules fixées ont montré une première localisation, la colocalisation bien connue XMAP215-microtubulaire (MT-XMAP215) qui s'observe pendant l'interphase et pendant la mitose (fuseau mitotique). Une deuxième localisation a été identifiée sur le bout plus des MTs, donnant XMAP215/ch-TOG comme faisant parti de la famille des protéines de bout plus (+TIPs). Cette deuxième colocalisation a été identifiée comme +TIP XMAP215/ch-TOG. La +TIP XMAP215 est la protéine la plus distale du bout des MTs. La hiérarchie a été établie en faisant la comparaison de la localisation de XMAP215/ch-TOG avec les protéines les plus connues du bout plus, telles qu'EB1, CLIP170 et p150Glued. Dans l'extrait mitotique de Xenopus laevis, les images obtenues in vivo par la microscopie de fluorescence par réflexion totale interne (TIRF) ont permis d'identifier une +TIP XMAP215 présente au bout des MTs qui polymérisent et dépolymérisent. Les images de microscopie cryo-électronique (Cryo-EM) ont montré une activité spécifique de la population +TIP XMAP215. Dans les solutions de tubuline pure, XMAP215 induit la formation de structures au bout des MTs, cette activité est compatible avec les mécanismes de croissance des MTs. Sur la base de nos résultats, nous proposons un modèle où XMAP215 se charge des dimères de tubuline en devenant une structure de type protofilament. Cette structure se lie au bout du MT en utilisant son domaine C-terminal, en rajoutant les dimères de tubuline et aussi certainement en participant à la fermeture de la structure microtubulaire même. La protéine interviendrait donc dans la dépolymérisation et aurait un rôle dans le mécanisme de dépolymérisation contrôlée. Une fois que l'addition de tubuline a eu lieu, la +TIP XMAP215 pourrait évoluer en MT-XMAP215, la forme la plus connue de la protéine qui a été associée au trafic des granules d'ARN. Mots cles : XMAP215, ch-TOG, microtubules, anticorps monoclonaux.

As major effector cells of the innate immune response, macrophages must adeptly migrate from blood to infected tissues. Endothelial transmigration is accomplished by matrix metalloproteinase (MMP)-induced degradation of basement membrane and extracellular matrix components. The classical activation of macrophages with LPS and IFN-γ causes enhanced microtubule stabilization and secretion of MMPs. Macrophages upregulate MMP-9 expression and secretion upon immunological challenge, and require its activity for migration during inflammatory response. However, the dynamics of MMP-9 production and intracellular distribution, as well as the mechanisms responsible for its trafficking, are unknown. Using immunofluorescent imaging, we localized intracellular MMP-9 to small Golgi-derived cytoplasmic vesicles that contain calreticulin and PDI, in activated macrophages. Vesicular organelles of MMP-9 aligned along stable subsets of microtubules and colocalized with the anterograde molecular motor protein, kinesin. We demonstrated a functional contribution of stable MTs in the enhanced trafficking of MMP-9 extracellularly, and showed that heterogeneity exists in macrophage cell populations with respect to MMP-9 production.

As major effector cells of the innate immune response, macrophages must adeptly migrate from blood to infected tissues. Endothelial transmigration is accomplished by matrix metalloproteinase (MMP)-induced degradation of basement membrane and extracellular matrix components. The classical activation of macrophages with LPS and IFN-γ causes enhanced microtubule stabilization and secretion of MMPs. Macrophages upregulate MMP-9 expression and secretion upon immunological challenge, and require its activity for migration during inflammatory response. However, the dynamics of MMP-9 production and intracellular distribution, as well as the mechanisms responsible for its trafficking, are unknown. Using immunofluorescent imaging, we localized intracellular MMP-9 to small Golgi-derived cytoplasmic vesicles that contain calreticulin and PDI, in activated macrophages. Vesicular organelles of MMP-9 aligned along stable subsets of microtubules and colocalized with the anterograde molecular motor protein, kinesin. We demonstrated a functional contribution of stable MTs in the enhanced trafficking of MMP-9 extracellularly, and showed that heterogeneity exists in macrophage cell populations with respect to MMP-9 production.