<p>Class A scavenger receptors are multifunctional transmembrane glycoproteins that mediate macrophage functions like phagocytosis and endocytosis. The macrophage receptor with collagenous structure (MARCO) is one such receptor. It has been shown that the extracellular cysteine-rich domain of MARCO is responsible for ligand binding, but the role of the cytoplasmic domain in ligand uptake is unclear. The aim of the studies presented in this thesis is to characterize the role of the cytoplasmic domain of MARCO and to characterize the molecular pathway of MARCO-mediated endocytosis.</p> <p>Full-length human MARCO (hMARCO) and Δ1-34hMARCO, which lacks the first thirty-four amino acids were created in order to determine whether amino acids 1-34 contained residues required for receptor internalization and surface expression. The constructs were stably expressed in HEK293T cells and found to have similar levels of surface expression and same rate of internalization without ligand. Interestingly, hMARCO, but not Δ1-34hMARCO, surface expression was up-regulated upon ligand incubation.</p> <p>In order to ascertain the importance of clathrin, dynamin and actin in MARCO-mediated endocytosis, specific endocytic inhibitors were used. MARCO-mediated ligand uptake was inhibited when clathrin and actin polymerization and, dynamin functions were impaired by these inhibitors. Furthermore, ligand uptake by Δ1-34hMARCO-expressing HEK293T was insensitive to inhibitors of clthrin and dynamin but not inhibitors of actin.</p> <p>In conclusion, MARCO mediates endocytosis via a clathrin-mediated, dynamin-dependent pathway that involves actin. Amino acids 1-34, are required clathrin and dynamin but not actin functions during MARCO-mediated endocytosis. Additionally, amino acids 1-34 might also be important for MARCO recycling but not receptor internalization or surface expression.</p> / Master of Science (MSc)

Les maladies cardiovasculaires (MCV) sont les principales causes de mortalité et de morbidité à travers le monde. En Amérique du Nord, on estime à 90 millions le nombre d’individus ayant une ou plusieurs MCV, à près de 1 million le nombre de décès reliés par année et à 525 milliards de dollars les coûts directs et indirects en 2010. En collaboration avec l’équipe du Dre. Boileau, notre laboratoire a récemment identifié, le troisième locus impliqué dans l’hypercholestérolémie familiale. Une étude publiée dans le New Engl J Med a révélé que l’absence de la convertase PCSK9 réduit de 88% le risque de MCV, corrélé à une forte réduction du taux de cholestérol plasmatique (LDL-C). Il fut démontré que PCSK9 lie directement le récepteur aux lipoprotéines de faible densité (LDLR) et, par un mécanisme méconnu, favorise sa dégradation dans les endosomes/lysosomes provoquant ainsi une accumulation des particules LDL-C dans le plasma. Dans cet ouvrage, nous nous sommes intéressés à trois aspects bien distincts : [1] Quels sont les cibles de PCSK9 ? [2] Quelle voie du trafic cellulaire est impliquée dans la dégradation du LDLR par PCSK9 ? [3] Comment peut-on inhiber la fonction de PCSK9 ? [1] Nous avons démontré que PCSK9 induit la dégradation du LDLR de même que les récepteurs ApoER2 et VLDLR. Ces deux membres de la famille du LDLR (fortes homologies) sont impliqués notamment dans le métabolisme des lipides et de la mise en place de structures neuronales. De plus, nous avons remarqué que la présence de ces récepteurs favorise l’attachement cellulaire de PCSK9 et ce, indépendamment de la présence du LDLR. Cette étude a ouvert pour la première fois le spectre d’action de PCSK9 sur d’autres protéines membranaires. [2] PCSK9 étant une protéine de la voie sécrétoire, nous avons ensuite évalué l’apport des différentes voies du trafic cellulaire, soit extra- ou intracellulaire, impliquées dans la dégradation du LDLR. À l’aide de milieux conditionnées dérivés d’hépatocytes primaires, nous avons d’abord démontré que le niveau extracellulaire de PCSK9 endogène n’a pas une grande influence sur la dégradation intracellulaire du LDLR, lorsqu’incubés sur des hépatocytes provenant de souris déficientes en PCSK9 (Pcsk9-/-). Par analyses de tri cellulaire (FACS), nous avons ensuite remarqué que la surexpression de PCSK9 diminue localement les niveaux de LDLR avec peu d’effet sur les cellules voisines. Lorsque nous avons bloqué l’endocytose du LDLR dans les cellules HepG2 (lignée de cellules hépatiques pour l’étude endogène de PCSK9), nous n’avons dénoté aucun changement des niveaux protéiques du récepteur. Par contre, nous avons pu démontrer que PCSK9 favorise la dégradation du LDLR par l’intermédiaire d’une voie intracellulaire. En effet l’interruption du trafic vésiculaire entre le réseau trans-Golgien (RTG) et les endosomes (interférence à l’ARN contre les chaînes légères de clathrine ; siCLCs) prévient la dégradation du LDLR de manière PCSK9-dépendante. [3] Par immunobuvardage d’affinité, nous avons identifié que la protéine Annexine A2 (AnxA2) interagit spécifiquement avec le domaine C-terminal de PCSK9, important pour son action sur le LDLR. Plus spécifiquement, nous avons cartographié le domaine R1 (acides aminés 34 à 108) comme étant responsable de l’interaction PCSK9AnxA2 qui, jusqu’à présent, n’avait aucune fonction propre. Finalement, nous avons démontré que l’ajout d’AnxA2 prévient la dégradation du LDLR induite par PCSK9. En somme, nos travaux ont pu identifier que d’autres membres de la famille du LDLR, soit ApoER2 et VLDLR, sont sensibles à la présence de PCSK9. De plus, nous avons mis en évidence que l’intégrité du trafic intracellulaire est critique à l’action de PCSK9 sur le LDLR et ce, de manière endogène. Finalement, nous avons identifié l’Annexine A2 comme unique inhibiteur naturel pouvant interférer avec la dégradation du LDLR par PCSK9. Il est indéniable que PCSK9 soit une cible de premier choix pour contrer l’hypercholestérolémie afin de prévenir le développement de MCV. Cet ouvrage apporte donc des apports considérables dans notre compréhension des voies cellulaires impliquées, des cibles affectées et ouvre directement la porte à une approche thérapeutique à fort potentiel. / Cardiovascular disease (CVD) is the primary cause of death and morbidity worldwide, claiming about 900 000 lives yearly in North America alone. A high level of circulating LDL-cholesterol is a major risk factor positively correlated with premature development of complex CVD mainly due to a rapid buildup of lipid deposition in the arteries. In collaboration with Dre Boileau, we recently discovered that the convertase PCSK9 is the third locus of familial hypercholesterolemia. A study published in the New Eng J Med revealed that the absence of PCSK9 reduces the risk of CVD by ~88%, resulting from a strong reduction of cholesterol in the bloodstream (LDL-C). It has been shown that PCSK9 directly binds the low-density lipoprotein receptor (LDLR) and by an unknown mechanism, reroutes it towards degradation in late endosomes/lysosomes, resulting in the accumulation of LDL-C particles in plasma. In this thesis, we addressed three different aspects of PCSK9 biology: [1] What are the targets of PCSK9? [2] Which cellular trafficking components are involved in PCSK9-induced LDLR degradation? [3] How can we inhibit the function of PCSK9? [1] We first demonstrated that PCSK9 induces the degradation of the LDLR and two of its closest family members. These include the very-low-density-lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2) implicated in neuronal development and lipid metabolism. In addition, we demonstrated that these receptors enhance the cellular association of PCSK9 independently of the presence of the LDLR. This study represents the first evidence that PCSK9 could target other proteins for degradation, reinforcing its role as a key regulator of some members of the LDLR family. [2] Since PCSK9 is a secreted protein, we decided to investigate the contributions of both the intra- and extracellular trafficking pathways in LDLR degradation. Using conditioned media derived from mice primary hepatocytes, we showed that endogenously secreted PCSK9 was not able to influence LDLR levels of PCSK9-deficient primary hepatocytes (Pcsk9-/-). By flow cytometry (FACS), we observed that overexpression of the gain-of-function PCSK9-D374Y, but not wild type PCSK9, decreases cell surface LDLR on adjacent cells suggesting that its spectrum of action is local. We also noticed that blockade of endocytosis in HepG2 cells (commonly used to study endogenous LDLR degradation by PCSK9) does not affect total LDLR protein levels. In contrast, disruption of the intracellular trafficking between the trans-Golgi network (TGN) and endosomes (siRNAs against clathrin light chains; CLCs) prevented LDLR degradation in a PCSK9-specific manner. [3] By Far Western blotting, we identified that Annexin A2 (AnxA2) specifically interacts with the C-terminal domain of PCSK9, which is crucial for its function in LDLR degradation. Moreover, we determined that the R1 domain (amino acids 34 to 108) is responsible for the PCSK9AnxA2 interaction, which confers a new function for this protein. Finally, we showed that addition of AnxA2 prevents PCSK9-induced LDLR degradation. In summary, this work allowed us to identify that PCSK9 induces the degradation of the LDLR and its closest family members, ApoER2 and VLDLR. We also highlighted that the integrity of the intracellular trafficking pathway is crucial for endogenous PCSK9-induced LDLR degradation. Furthermore, we discovered that AnxA2 is a unique, natural inhibitor capable of interfering with the action of PCSK9 in LDLR degradation. It is undeniable that PCSK9 is a genetically validated target to reduce circulating LDL-cholesterol and prevent CVD. This thesis brings forth important contributions in our understanding of the cellular pathways involved and opens the door for novel therapeutic approaches.

La cryptogéine est un éliciteur protéique des réactions de défense chez le tabac sécrété par l’oomycète Phytophthora cryptogea. Son interaction avec un récepteur encore non identifié de la membrane plasmique déclenche une cascade d’événements de signalisation qui conduisent à une reprogrammation génique, à la diffusion d’un signal conférant une résistance systémique et, ultimement, à la mort des cellules directement exposées à cet éliciteur. Notre équipe avait précédemment mis en évidence une stimulation de la formation de vésicules d’endocytose quelques minutes après l’élicitation. Des études de microscopie électronique et d’inhibition pharmacologique avaient permis d’émettre l’hypothèse que cette endocytose était dépendante de la clathrine (CME). Ce processus essentiel des cellules eucaryotes concoure au maintien de l’homéostasie du plasmalemme. L’endocytose joue également deux rôles aux effets antagonistes lorsqu’elle est induite par un stimulus de l’environnement. Elle permet d’une part la production d’endosomes de signalisation qui vont délivrer le stimulus au cœur de la cellule, et assure d’autre part la désensibilisation de la membrane afin de préparer la cellule à percevoir d’autres stimuli.Mon travail de thèse avait pour objectif de confirmer que l’endocytose induite après traitement de cellules de tabac par la cryptogéine est bien dépendante de la clathrine et d’essayer de déterminer si elle est impliquée, directement ou indirectement, dans la transduction du signal d’élicitation et dans le développement des réactions de défense. Afin de visualiser in vivo la dynamique endocytaire, j’ai établi une suspension cellulaire de tabac exprimant une chaine légère de clathrine fusionnée à la GFP. La caractérisation de cette suspension par des approches biochimiques et par microscopie a confirmé l’induction d’une endocytose dépendante de la clathrine suite à l’élicitation par la cryptogéine. J’ai également développé une stratégie d’inhibition de la CME faisant appel à l’expression d’une version tronquée de la CHC, appelée hub, dont la propriété est d’empêcher la formation du manteau de clathrine à la membrane plasmique. La caractérisation d’une lignée cellulaire co-exprimant le marqueur d’endocytose GFP-CLC et le hub a montré qu’il était possible d’empêcher l’endocytose à clathrine induite par la cryptogéine sans altérer de façon significative l’endocytose constitutive. L’utilisation de cette stratégie d’inhibition sélective a ainsi démontré que des événements précoces induits à la membrane plasmique par la cryptogéine, telles l’alcalinisation du milieu extracellulaire et la production de formes actives de l’oxygène, ne sont pas dépendants de la CME. L’impact de l’invalidation de l’endocytose induite sur le déclenchement des réponses tardives a été étudié sur des cellules en suspension et sur des plants de tabac. Mes travaux ont révélé que l’endocytose contribuait de façon réduite à la reprogrammation du transcriptome et au déclenchement de la mort cellulaire programmée. Des travaux préliminaires effectués sur des plantes exposées à divers pathogènes du tabac ont montré que l’expression du hub affecte la sensibilité des plantes à certains de ces pathogènes. L’ensemble de ces travaux ouvrent la voie à une étude plus intégrative du rôle de l’endocytose dans l’interaction tabac-cryptogéine / Cryptogein, a protein secreted by the oomycete Phytophthora cryptogea, is an elicitor of defense reaction in tobacco. Cryptogein binding to an unidentified receptor of the plasma membrane triggers a signaling cascade that leads to changes in gene expression, production of a systemic acquired resistance signal, and cell death. Our lab previously reported a stimulation of endocytosis a few minutes after elicitation. Electron microscopy and pharmacological studies evidenced that this endocytosis is clathrin-mediated. Clathrin-mediated endocytosis (CME) is a fundamental eukaryotic cell process that ensures plasma membrane homeostasis. It also plays two antagonistic roles in extracellular signal transduction either by producing endosomes that convey the signal into the heart of the cell, or by downregulating plasma membrane receptors to attenuate cellular responsiveness and prepare the cell for subsequent signals.The aim of my thesis was to confirm clathrin dependence of cryptogein-induced endocytosis and to find out whether endocytosis is involved in cryptogein signaling and defense reactions. I established a tobacco cell suspension expressing clathrin light chain fused to GFP to follow CME in living cells. Biochemical and microscopic characterization of the cell suspension confirmed that cryptogein-induced endocytosis is clathrin-mediated. I also developed a dominant-negative strategy to inhibit CME by expressing a truncated form of clathrin heavy chain, the hub domain, which prevents clathrin-coated pit formation at the plasma membrane. Characterization of a cell line co-expressing GFP-CLC and the hub domain showed that it is possible to hinder cryptogein-induced CME without significantly altering constitutive endocytosis. This selective inhibition strategy revealed that cryptogein-induced early signaling events such as alkalinisation of the extracellular medium and reactive oxygen species production are CME-independent.Consequences of induced-CME inhibition on later responses to cryptogein were studied in cell suspension and in tobacco plants. My results showed that endocytosis contributes in a minor way to transcriptome reprogramming and cell death induction. Moreover, preliminary results suggested that hub expression increases the plant’s sensitivity to several pathogens. Altogether these results open up the prospect of addressing the role of CME during tobacco-cryptogein interaction in a more integrative view

Much of our knowledge of cellular biology arises from direct observation of active cellular functions. Tools and techniques have steadily developed over the past several hundreds of years to aid in our understanding and control of the nanoworld and are referred to as nanotechnologies. In the context of nanotechnology, DNA is not used as a carrier for genetic information (as it is in cell), but as a construction material. DNA offers unprecedented control over the construction of simplified biomimetic models for the study of biological processes. This thesis first introduces and defines the field of DNA nanotechnology, with particular emphasis on the interaction of snthetic DNA nanostructures with biological membranes. Inspired by the protein clathrin, three-fold symmetric DNA tile made of eight, short DNA strands and capable of polymerising is described and studied, with the aim to interact with and controllably bend a membrane bilayer. This structure presented challenges during construction so an enhanced three-armed DNA structure built with DNA origami was designed. The succesful assembly of a rigid and functionalisable nanostructure is described. This origami structure was polymerised into large constructs in solution and on a supported lipid membrane. The shape of the structure was modulated to vary its curvature and apply a bending force to a lipid vesicle when anchored to it. Following the conclusion of this study, we present the construction of a small, unique DNA structure for enhanced electron microscopy imaging in cell lysate. This project is part of a developing technique to couple the interaction specificity of dyes in super-resolution microscopy and the high-resolution output of electron microscopy. Finally, the optimisation procedures and recommendations for TEM imaging of samples of DNA origami and lipid structures are discussed.

In mammalian cells, homeostasis and fate during development relies on the proper transport of membrane-bound cargoes to their designated cellular locations. The hetero-tetrameric adaptor protein complexes (APs) are required for sorting and concentration of cargo at donor membranes, a crucial step during targeted transport. AP2, which functions at the plasma membrane during clathrin-mediated endocytosis, is well characterized. In contrast, AP1 a clathrin adaptor mediating the delivery of lysosomal hydrolases via mannose 6-phosphate receptors (MPRs) and AP3 an adaptor ensuring the proper targeting of lysosomal membrane protein are difficult to study by classic cell biology tools. To gain new insights on these APs, our lab has previously designed an in vitro system. Reconstituted liposomes were modified with small peptides mimicking the cytosolic domains of bona fide cargoes for AP1 and AP3 respectively and thereby enabling the selective recruitment of these APs and the identification of the interacting protein network. In the study at hand we utilize above-described liposomes to generate supported lipid bilayers and Giant Unilamellar Vesicles (GUVs), large-scale membrane systems suited for analysis by fluorescence microscopy. By using cytosol containing fluorescently-tagged subunits, we visualized clathrin coats on artificial membranes under near physiological conditions for the first time. Moreover, we demonstrated clathrin-independent recruitment of AP3 coats on respective GUVs. Presence of active ARF1 was sufficient for the selective assembly of AP1-dependent clathrin coats and AP3 coats on GUVs. By using dye-conjugated ARF1, we show that ARF1 colocalized with AP3 coats on GUVs and that increased association of ARF1 with GUVs coincided with AP1-dependent clathrin coats. Our previous study identified members of the septin family together with AP3 coats on liposomes. Here we show on GUVs, that active ARF1 stimulated the assembly of septin7 filaments, which may constrain the size and mobility of AP3 coats on the surface. Subsequent cell biology studies in HeLa cells linked septins to actin fibers on which they may control mobility of AP3-coated endosomes and thus their maturation. An actin nucleation complex, based on CYFIP1 was identified together with AP1 on liposomes before. Here we show on GUVs, that CYFIP1 is recruited on the surface surrounding clathrin coats. Upon supply of ATP, sustained actin polymerization generated a thick shell of actin on the GUV surface. The force generated by actin assembly lead to formation of long tubular protrusions, which projected from the GUV surface and were decorated with clathrin coats. Thereby the GUV model illustrated a possible mechanism for tubular carriers formation. The importance of CYFIP1-reliant actin polymerization for the generation of MPR-positive tubules at the trans-Golgi network (TGN) of HeLa cells was subsequently demonstrated in our lab. The notion that tubulation of artificial membranes could be triggered by actin polymerization allowed us to perform a comparative mass spectrometry screen. By comparing the abundance of proteins on liposomes under conditions promoting or inhibiting actin polymerization, candidates possibly involved in stabilization, elongation or fission of membrane tubules could be identified. Among the proteins enriched under conditions promoting tubulation, we identified type I phosphatidylinositol-4-phosphate 5-kinases. Their presence suggested an involvement of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) in tubule formation. By cell biology studies in HeLa we show, that down regulation of these enzymes altered the dynamics of fluorescently-tagged MPRs, illustrating the importance of locally confined PI(4,5)P2 synthesis during formation of coated carriers at the TGN. Bin–Amphiphysin–Rvs (BAR) domains are known to sense membrane curvature and induce membrane tubulation. Among various BAR domain proteins, Arfaptin2 was enriched under conditions allowing tubulation of liposomes. By microscopy studies on HeLa cells we show, that Arfaptin2 as well as its close paralog Arfaptin1 were present on AP1-coated MPR tubules emerging from the TGN. We further show, that tubule fission occurred at regions were Arfaptin1 is concentrated and that simultaneous down regulation of both Arfaptins lead to increased number and length of MPR tubules. Since fission of coated transport intermediates at the TGN is poorly understood, our findings contribute a valuable component towards a model describing the entire biogenesis of coated post-Golgi carriers. In conclusion, combining artificial membrane systems and cell biology studies allowed us to propose new models for formation as wall as for fission of AP1-coated transport intermediates at the TGN. Further we gained new insights on AP3 coats and the possible involvement of septin filaments in AP3-dependent endosomal maturation.

Gemini surfactants are versatile gene delivery agents because of their ability to bind and compact DNA and their low cellular toxicity. The aim of my dissertation work was to develop non-invasive mucosal formulations of novel amino acid-substituted gemini surfactants with the general chemical formula C12H25(CH3)2N+-(CH2)3-N(AA)-(CH2)3-N+(CH3)2-C12H25 (AA= glycine, lysine, glycyl-lysine, lysyl-lysine). These compounds were formulated with a model plasmid DNA, encoding for interferon-γ and green fluorescent protein, in the presence of helper lipid, 1,2 dioleyl-sn-glycero-phosphatidyl-ethanolamine. Formulations were assessed in Sf 1 Ep epithelial cells. Among the novel compounds, plasmid/gemini/lipid (P/G/L) nanoparticles formulated using glycine- and glycyl-lysine substituted gemini surfactants achieved significantly higher gene expression than the parent unsubstituted compound. The key physicochemical properties, e.g. size, surface charge, DNA binding, and toxicity of P/G/L complexes were correlated with transfection efficiency. The presence of amino-acid substitution did not interfere with DNA compaction and contributed to an overall low toxicity of all P/G/L complexes, comparable to the parent gemini surfactant. A cellular uptake mechanistic study revealed that both clathrin- and caveolae-mediated uptake were major uptake routes for P/G/L nanoparticles. However, amino acid substitution in the gemini surfactant imparted high buffering capacity, pH-dependent increase in particle size, and balanced DNA binding properties. These properties may enhance endosomal escape of P/12-7NGK-12/L resulting in higher gene expression. Finally, the P/G/L complexes were incorporated into an in-situ gelling dispersion containing a thermosensitive polymer, poloxamer 407, and a permeation enhancer, diethylene glycol monoethyl ether (DEGEE). A 16% w/v poloxamer concentration produced a dispersion that gelled at body temperature and exhibited sufficient yield value to prevent formulation leakage from the vaginal cavity. The formulations were prepared with a model plasmid, encoding for red fluorescent protein, and administered topically to rabbit vagina. In agreement with our in vitro results, confocal microscopy revealed that glycyl-lysine substituted gemini surfactant exhibited higher gene expression compared to the parent unsubstituted gemini surfactant. This provided proof-of-concept for use of amino acid-substituted gemini surfactant in non-invasive mucosal (vaginal) gene delivery systems with potential therapeutic applications. These formulations will be developed with therapeutically relevant genes to assess their potential as genetic vaccines. In addition, new gemini surfactants will be developed by grafting other amino acids via glycine linkage to retain conformation flexibility and enhance endosomal escape of DNA complexes for higher transfection efficiency.

Les maladies cardiovasculaires (MCV) sont les principales causes de mortalité et de morbidité à travers le monde. En Amérique du Nord, on estime à 90 millions le nombre d’individus ayant une ou plusieurs MCV, à près de 1 million le nombre de décès reliés par année et à 525 milliards de dollars les coûts directs et indirects en 2010. En collaboration avec l’équipe du Dre. Boileau, notre laboratoire a récemment identifié, le troisième locus impliqué dans l’hypercholestérolémie familiale. Une étude publiée dans le New Engl J Med a révélé que l’absence de la convertase PCSK9 réduit de 88% le risque de MCV, corrélé à une forte réduction du taux de cholestérol plasmatique (LDL-C). Il fut démontré que PCSK9 lie directement le récepteur aux lipoprotéines de faible densité (LDLR) et, par un mécanisme méconnu, favorise sa dégradation dans les endosomes/lysosomes provoquant ainsi une accumulation des particules LDL-C dans le plasma. Dans cet ouvrage, nous nous sommes intéressés à trois aspects bien distincts : [1] Quels sont les cibles de PCSK9 ? [2] Quelle voie du trafic cellulaire est impliquée dans la dégradation du LDLR par PCSK9 ? [3] Comment peut-on inhiber la fonction de PCSK9 ? [1] Nous avons démontré que PCSK9 induit la dégradation du LDLR de même que les récepteurs ApoER2 et VLDLR. Ces deux membres de la famille du LDLR (fortes homologies) sont impliqués notamment dans le métabolisme des lipides et de la mise en place de structures neuronales. De plus, nous avons remarqué que la présence de ces récepteurs favorise l’attachement cellulaire de PCSK9 et ce, indépendamment de la présence du LDLR. Cette étude a ouvert pour la première fois le spectre d’action de PCSK9 sur d’autres protéines membranaires. [2] PCSK9 étant une protéine de la voie sécrétoire, nous avons ensuite évalué l’apport des différentes voies du trafic cellulaire, soit extra- ou intracellulaire, impliquées dans la dégradation du LDLR. À l’aide de milieux conditionnées dérivés d’hépatocytes primaires, nous avons d’abord démontré que le niveau extracellulaire de PCSK9 endogène n’a pas une grande influence sur la dégradation intracellulaire du LDLR, lorsqu’incubés sur des hépatocytes provenant de souris déficientes en PCSK9 (Pcsk9-/-). Par analyses de tri cellulaire (FACS), nous avons ensuite remarqué que la surexpression de PCSK9 diminue localement les niveaux de LDLR avec peu d’effet sur les cellules voisines. Lorsque nous avons bloqué l’endocytose du LDLR dans les cellules HepG2 (lignée de cellules hépatiques pour l’étude endogène de PCSK9), nous n’avons dénoté aucun changement des niveaux protéiques du récepteur. Par contre, nous avons pu démontrer que PCSK9 favorise la dégradation du LDLR par l’intermédiaire d’une voie intracellulaire. En effet l’interruption du trafic vésiculaire entre le réseau trans-Golgien (RTG) et les endosomes (interférence à l’ARN contre les chaînes légères de clathrine ; siCLCs) prévient la dégradation du LDLR de manière PCSK9-dépendante. [3] Par immunobuvardage d’affinité, nous avons identifié que la protéine Annexine A2 (AnxA2) interagit spécifiquement avec le domaine C-terminal de PCSK9, important pour son action sur le LDLR. Plus spécifiquement, nous avons cartographié le domaine R1 (acides aminés 34 à 108) comme étant responsable de l’interaction PCSK9AnxA2 qui, jusqu’à présent, n’avait aucune fonction propre. Finalement, nous avons démontré que l’ajout d’AnxA2 prévient la dégradation du LDLR induite par PCSK9. En somme, nos travaux ont pu identifier que d’autres membres de la famille du LDLR, soit ApoER2 et VLDLR, sont sensibles à la présence de PCSK9. De plus, nous avons mis en évidence que l’intégrité du trafic intracellulaire est critique à l’action de PCSK9 sur le LDLR et ce, de manière endogène. Finalement, nous avons identifié l’Annexine A2 comme unique inhibiteur naturel pouvant interférer avec la dégradation du LDLR par PCSK9. Il est indéniable que PCSK9 soit une cible de premier choix pour contrer l’hypercholestérolémie afin de prévenir le développement de MCV. Cet ouvrage apporte donc des apports considérables dans notre compréhension des voies cellulaires impliquées, des cibles affectées et ouvre directement la porte à une approche thérapeutique à fort potentiel. / Cardiovascular disease (CVD) is the primary cause of death and morbidity worldwide, claiming about 900 000 lives yearly in North America alone. A high level of circulating LDL-cholesterol is a major risk factor positively correlated with premature development of complex CVD mainly due to a rapid buildup of lipid deposition in the arteries. In collaboration with Dre Boileau, we recently discovered that the convertase PCSK9 is the third locus of familial hypercholesterolemia. A study published in the New Eng J Med revealed that the absence of PCSK9 reduces the risk of CVD by ~88%, resulting from a strong reduction of cholesterol in the bloodstream (LDL-C). It has been shown that PCSK9 directly binds the low-density lipoprotein receptor (LDLR) and by an unknown mechanism, reroutes it towards degradation in late endosomes/lysosomes, resulting in the accumulation of LDL-C particles in plasma. In this thesis, we addressed three different aspects of PCSK9 biology: [1] What are the targets of PCSK9? [2] Which cellular trafficking components are involved in PCSK9-induced LDLR degradation? [3] How can we inhibit the function of PCSK9? [1] We first demonstrated that PCSK9 induces the degradation of the LDLR and two of its closest family members. These include the very-low-density-lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2) implicated in neuronal development and lipid metabolism. In addition, we demonstrated that these receptors enhance the cellular association of PCSK9 independently of the presence of the LDLR. This study represents the first evidence that PCSK9 could target other proteins for degradation, reinforcing its role as a key regulator of some members of the LDLR family. [2] Since PCSK9 is a secreted protein, we decided to investigate the contributions of both the intra- and extracellular trafficking pathways in LDLR degradation. Using conditioned media derived from mice primary hepatocytes, we showed that endogenously secreted PCSK9 was not able to influence LDLR levels of PCSK9-deficient primary hepatocytes (Pcsk9-/-). By flow cytometry (FACS), we observed that overexpression of the gain-of-function PCSK9-D374Y, but not wild type PCSK9, decreases cell surface LDLR on adjacent cells suggesting that its spectrum of action is local. We also noticed that blockade of endocytosis in HepG2 cells (commonly used to study endogenous LDLR degradation by PCSK9) does not affect total LDLR protein levels. In contrast, disruption of the intracellular trafficking between the trans-Golgi network (TGN) and endosomes (siRNAs against clathrin light chains; CLCs) prevented LDLR degradation in a PCSK9-specific manner. [3] By Far Western blotting, we identified that Annexin A2 (AnxA2) specifically interacts with the C-terminal domain of PCSK9, which is crucial for its function in LDLR degradation. Moreover, we determined that the R1 domain (amino acids 34 to 108) is responsible for the PCSK9AnxA2 interaction, which confers a new function for this protein. Finally, we showed that addition of AnxA2 prevents PCSK9-induced LDLR degradation. In summary, this work allowed us to identify that PCSK9 induces the degradation of the LDLR and its closest family members, ApoER2 and VLDLR. We also highlighted that the integrity of the intracellular trafficking pathway is crucial for endogenous PCSK9-induced LDLR degradation. Furthermore, we discovered that AnxA2 is a unique, natural inhibitor capable of interfering with the action of PCSK9 in LDLR degradation. It is undeniable that PCSK9 is a genetically validated target to reduce circulating LDL-cholesterol and prevent CVD. This thesis brings forth important contributions in our understanding of the cellular pathways involved and opens the door for novel therapeutic approaches.

Thesis (Ph. D.) -- University of Texas Southwestern Medical Center at Dallas, 2005. / Vita. Bibliography: 106-116.

Thesis (Ph.D. in Immunology) -- University of Colorado at Denver and Health Sciences Center, 2005. / Typescript. Includes bibliographical references (leaves 144-178). Free to UCDHSC affiliates. Online version available via ProQuest Digital Dissertations;

Orientador: Edgard Graner / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Odontologia de Piracicaba / Made available in DSpace on 2018-08-09T14:11:16Z (GMT). No. of bitstreams: 1 Agostini_Michelle_D.pdf: 6485719 bytes, checksum: f82ba343c6049746e2b63420116af03c (MD5) Previous issue date: 2007 / Resumo: O sistema ubiquitina-proteossomo degrada proteínas marcadas com etiquetas de Ub. A ubiquitinação é um processo reversível e moléculas de Ub podem ser desconjugadas pelas enzimas desubiquitinantes (DUBs), que evitam a degradação e aumentam a meia vida de seus substratos. A DUB USP2a foi identificada na próstata humana, é regulada por andrógenos e tem sua expressão aumentada em adenocarcinomas. USP2a protege a enzima ácido graxo sintase (FAS) da degradação, a qual é superexpressa em vários tipos de tumores, inclusive nos carcinomas espinocelulares (CECs) bucais. O objetivo deste trabalho foi estudar a expressão desta DUB e seu papel biológico em células derivadas de CECs bucais humanos. Foram detectados RNAs mensageiros para USP2a nas quatro linhagens celulares estudadas, principalmente nas linhagens SCC-4 e -15. Os níveis protéicos de USP2a foram semelhantes nas quatro linhagens, sendo ligeiramente maiores na SCC-9 e -25. Portanto, não foi encontrada uma correlação entre a quantidade de RNAs mensageiros e dos produtos protéicos de USP2a. Através de experimentos de imunofluorescência, demonstramos USP2a no citoplasma das células SCC-9, havendo uma concentração na região perinuclear em algumas células. A expressão forçada de USP2a nas células SCC- 9 não conferiu vantagem proliferativa, no entanto, a superexpressão de um duplomutante parece ter diminuído a proliferação. Ao contrário do que ocorre nas células LNCaP, a inibição da expressão de USP2a através de RNAi nas células SCC-9 causou discreta indução de apoptose. O tratamento das células SCC-9 com diferentes concentrações do fator de crescimento epidérmico (EGF) foi capaz de modular a expressão de USP2a, interferindo na quantidade de formas ubiquitinadas de FAS. Também foi investigada neste trabalho a possível interação entre USP2a e a proteína clatrina. De acordo com resultados prévios de experimentos realizados no laboratório do Dr. Massimo Loda, no Dana-Farber Cancer Institute, a cadeia pesada de clatrina é também substrato de USP2a. Clatrina é uma proteína que participa do processo de internalização e endocitose de proteínas localizadas na membrana plasmática. Demonstramos que USP2a e a cadeia pesada de clatrina estão co-localizadas no citoplasma de células AR-iPrEC e SCC-9 e que a produção de clatrina é regulada por andrógenos em células LNCaP. Houve uma maior produção de clatrina em células que superexpressam de forma estável USP2a. Um achado interessante foi que USP2a, além de presente no citoplasma, foi também encontrada na membrana plasmática de células LNCaP e o tratamento com EGF interferiu na localização sub-celular desta DUB, como ocorre com clatrina durante a endocitose. Estes resultados sugerem que USP2a participe do processo de endocitose mediada por clatrina / Abstract: The ubiquitin (Ub)-proteasome pathway controls cellular protein turnover by degrading targeted intracellular proteins tagged with poly-Ub chains. Ubiquitination is a reversible process and the deubiquitinating enzymes (DUBs) are proteases that specifically cleave off Ub from Ub-protein conjugates. They can act in a preproteasomal level removing the poly-Ub tag from specific substrates and preventing and modulating their degradation. The DUBs USP2a and USP2b were recently identified in the prostate of men and rats. USP2a is androgen-regulated, overexpressed in prostate cancer, and interacts with and stabilizes fatty acid synthase (FAS) and the protein murine double minute (Mdm2). FAS is overexpressed in several human malignancies, including oral squamous cell carcinoma, and is correlated with a poor prognosis for some tumors. Mdm2 is an Ub-protein ligase responsible for its own ubiquitination and ubiquitination of p53, that is degraded by the proteasome. When overexpressed in nontransformed cells USP2a exhibits oncogenic behavior both in vitro and in vivo and prevents apoptosis induced by chemotherapeutic agents. Considering that USP2a stabilizes FAS and Mdm2 and then protects tumoral cells from apoptosis, the purpose of the present study was to investigate the USP2a expression and its biological role in human oral squamous carcinoma cells. mRNAs for USP2a were detected in the four studied cell lines, mainly in SCC-4 and -15. The USP2a protein levels were similar in all cell lines, being slightly higher in SCC-9 and -25. By using immunofluorescence we showed that USP2a is located in the cytoplasm of SCC-9 cells and eventually concentrated around the nuclei. No significant differences were found in the proliferative rates of USP2a overexpressing SCC-9 cells, however, cells overexpressing mutant USP2a had lower proliferative potential. In contrast with LNCaP cells, USP2a silencing by siRNA slightly induced apoptosis. The treatment with different concentrations of EGF was able to modulate the USP2a expression in SCC-9 cells and change the amount of ubiquitinated forms of FAS. We also show in the present study experiments performed in the laboratory of Dr. Massimo Loda at the Dana-Farber Cancer Institute, in which the possible interaction between USP2a and clathrin was analyzed. Clathrin is involved in the internalization and endocytosis of proteins located in at the plasma membrane. Here we show that USP2a and clathrin heavy chain colocalize in the cytoplasm of AR-iPrEC and SCC-9 cells and that clathrin protein expression is regulated by androgens in LNCaP cells. We found higher amounts of clathrin in cells that stably express USP2a than in the controls. USP2a was found at the plasma membrane in LNCaP cells and after EGF stimulation a granular positivity for USP2a was observed in the cytoplasm. These results suggest that USP2a may have a role in the clathrin mediated endocytosis / Doutorado / Patologia