Endosomal trafficking can influence the composition of the plasma membrane and the ability of cells to polarize their membranes. Here, we examined whether trafficking through clathrin-independent endocytosis (CIE) affects the ability of T cells to form a cell-cell conjugate with antigen-presenting cells (APCs). We show that CIE occurs in both the Jurkat T cell line and primary human T cells. In Jurkat cells, the activities of two guanine nucleotide binding proteins, Arf6 and Rab22 (also known as Rab22a), influence CIE and conjugate formation. Expression of the constitutively active form of Arf6, Arf6Q67L, inhibits CIE and conjugate formation, and results in the accumulation of vacuoles containing lymphocyte function-associated antigen 1 (LFA-1) and CD4, molecules important for T cell interaction with the APC. Moreover, expression of the GTP-binding defective mutant of Rab22, Rab22S19N, inhibits CIE and conjugate formation, suggesting that Rab22 function is required for these activities. Furthermore, Jurkat cells expressing Rab22S19N were impaired in spreading onto coverslips coated with T cell receptor-activating antibodies. These observations support a role for CIE, Arf6 and Rab22 in conjugate formation between T cells and APCs.

Endocytosis effectuates a critical interface between the eukaryotic cell and its apposing environment. It is, subsequently, paramount for many physiologically important processes and encompasses a diverse array of mechanisms and pathways. The classical endocytic routes mediated by clathrin and caveolin are the best understood and the molecular roles of their major regulators, such as dynamin, adaptor proteins and various lipid species, are the most comprehensively described. Recent identification of an assortment of constitutive, noncaveolar, clathrin-independent endocytic (CIE) pathways has expanded the endocytic system. Unlike the classical endocytic pathways, little is known about the guiding parameters of CIE routes. Consequently, it is not possible to understand the important cellular roles these pathways may be fulfilling. This study has begun to characterise the very basic parameters governing the morphologically striking Clathrin-Independent Carrier (CLIC) pathway. Development of a diverse molecular toolkit has now allowed the quantitation of endocytic capacity provided by CLICs, the visualisation of subtle sorting components of the CLIC pathway, the isolation of novel CLIC cargo and regulators, and has linked this mechanism to the critical cellular processes of cellular migration and membrane repair. Calculation of the individual capacity of endocytic routes provides important information about the contribution of each pathway to total plasma membrane (PM) uptake and turnover. Quantitation of the volume, surface area and number of structures forming per minute in this study shows that CLICs provide the vast majority of constitutive endocytosis, up to four times the capacity of the clathrin mediated endocytic (CME) pathway. As the equivalent of the entire PM area could pass through the CLIC pathway within 12 minutes it is evident that CLICs are fundamental housekeepers of bulk membrane internalisation. Thus, they are likely to be central regulators of PM homeostasis and turnover. High-resolution tomography, in conjunction with analysis of CLIC cargo trafficking, identifies these carriers as complex, pleiomorphic structures that sort the bulk of membrane to early endosomes and recycle cargo back to the cell surface. Such vast internalisation combined with an ability to rapidly recycle components quickly attributes the CLIC pathway as a complex sorting station. Isolation of novel cargo and regulators has identified a striking array of proteins now associated with the CLIC pathway for the first time. A significant proportion of identified targets localise to lipid-rafts and recycle from the PM, facets consistent with association to the CLIC pathway. Numerous targets have also been directly implicated in clathrin-independent endocytosis by independent groups. Verification of selected cargo, such as CD44, Thy-1 and myoferlin, showing specific internalisation through the CLIC pathway, has provided insight into the sorting ability of the CLIC pathway and links to adhesion turnover and membrane recycling. Consistent with a role in cellular adhesion turnover, it was found that CLICs become polarised within migrating cells. This has shown the first instance of spatial separation between three major endocytic routes, CLICs, caveolae and CME and highlights the important and coordinated roles of multiple endocytic pathways during physiologically significant processes. The specific internalisation of paxillin, Thy-1 and CD44 through CLICs at the leading edge of migrating cells suggests that CLICs rapidly turnover adhesion components for dynamic extracellular sensation during directional cell migration. Indeed, specific ablation of the CLIC pathway significantly impedes cellular migration, implying coordination with CME at the leading edge. This study has defined numerous parameters of the CLIC pathway, developing the current understanding of this poorly defined route and places the CLIC pathway as a unique player during critical cellular processes.

Endocytosis effectuates a critical interface between the eukaryotic cell and its apposing environment. It is, subsequently, paramount for many physiologically important processes and encompasses a diverse array of mechanisms and pathways. The classical endocytic routes mediated by clathrin and caveolin are the best understood and the molecular roles of their major regulators, such as dynamin, adaptor proteins and various lipid species, are the most comprehensively described. Recent identification of an assortment of constitutive, noncaveolar, clathrin-independent endocytic (CIE) pathways has expanded the endocytic system. Unlike the classical endocytic pathways, little is known about the guiding parameters of CIE routes. Consequently, it is not possible to understand the important cellular roles these pathways may be fulfilling. This study has begun to characterise the very basic parameters governing the morphologically striking Clathrin-Independent Carrier (CLIC) pathway. Development of a diverse molecular toolkit has now allowed the quantitation of endocytic capacity provided by CLICs, the visualisation of subtle sorting components of the CLIC pathway, the isolation of novel CLIC cargo and regulators, and has linked this mechanism to the critical cellular processes of cellular migration and membrane repair. Calculation of the individual capacity of endocytic routes provides important information about the contribution of each pathway to total plasma membrane (PM) uptake and turnover. Quantitation of the volume, surface area and number of structures forming per minute in this study shows that CLICs provide the vast majority of constitutive endocytosis, up to four times the capacity of the clathrin mediated endocytic (CME) pathway. As the equivalent of the entire PM area could pass through the CLIC pathway within 12 minutes it is evident that CLICs are fundamental housekeepers of bulk membrane internalisation. Thus, they are likely to be central regulators of PM homeostasis and turnover. High-resolution tomography, in conjunction with analysis of CLIC cargo trafficking, identifies these carriers as complex, pleiomorphic structures that sort the bulk of membrane to early endosomes and recycle cargo back to the cell surface. Such vast internalisation combined with an ability to rapidly recycle components quickly attributes the CLIC pathway as a complex sorting station. Isolation of novel cargo and regulators has identified a striking array of proteins now associated with the CLIC pathway for the first time. A significant proportion of identified targets localise to lipid-rafts and recycle from the PM, facets consistent with association to the CLIC pathway. Numerous targets have also been directly implicated in clathrin-independent endocytosis by independent groups. Verification of selected cargo, such as CD44, Thy-1 and myoferlin, showing specific internalisation through the CLIC pathway, has provided insight into the sorting ability of the CLIC pathway and links to adhesion turnover and membrane recycling. Consistent with a role in cellular adhesion turnover, it was found that CLICs become polarised within migrating cells. This has shown the first instance of spatial separation between three major endocytic routes, CLICs, caveolae and CME and highlights the important and coordinated roles of multiple endocytic pathways during physiologically significant processes. The specific internalisation of paxillin, Thy-1 and CD44 through CLICs at the leading edge of migrating cells suggests that CLICs rapidly turnover adhesion components for dynamic extracellular sensation during directional cell migration. Indeed, specific ablation of the CLIC pathway significantly impedes cellular migration, implying coordination with CME at the leading edge. This study has defined numerous parameters of the CLIC pathway, developing the current understanding of this poorly defined route and places the CLIC pathway as a unique player during critical cellular processes.

Endocytosis effectuates a critical interface between the eukaryotic cell and its apposing environment. It is, subsequently, paramount for many physiologically important processes and encompasses a diverse array of mechanisms and pathways. The classical endocytic routes mediated by clathrin and caveolin are the best understood and the molecular roles of their major regulators, such as dynamin, adaptor proteins and various lipid species, are the most comprehensively described. Recent identification of an assortment of constitutive, noncaveolar, clathrin-independent endocytic (CIE) pathways has expanded the endocytic system. Unlike the classical endocytic pathways, little is known about the guiding parameters of CIE routes. Consequently, it is not possible to understand the important cellular roles these pathways may be fulfilling. This study has begun to characterise the very basic parameters governing the morphologically striking Clathrin-Independent Carrier (CLIC) pathway. Development of a diverse molecular toolkit has now allowed the quantitation of endocytic capacity provided by CLICs, the visualisation of subtle sorting components of the CLIC pathway, the isolation of novel CLIC cargo and regulators, and has linked this mechanism to the critical cellular processes of cellular migration and membrane repair. Calculation of the individual capacity of endocytic routes provides important information about the contribution of each pathway to total plasma membrane (PM) uptake and turnover. Quantitation of the volume, surface area and number of structures forming per minute in this study shows that CLICs provide the vast majority of constitutive endocytosis, up to four times the capacity of the clathrin mediated endocytic (CME) pathway. As the equivalent of the entire PM area could pass through the CLIC pathway within 12 minutes it is evident that CLICs are fundamental housekeepers of bulk membrane internalisation. Thus, they are likely to be central regulators of PM homeostasis and turnover. High-resolution tomography, in conjunction with analysis of CLIC cargo trafficking, identifies these carriers as complex, pleiomorphic structures that sort the bulk of membrane to early endosomes and recycle cargo back to the cell surface. Such vast internalisation combined with an ability to rapidly recycle components quickly attributes the CLIC pathway as a complex sorting station. Isolation of novel cargo and regulators has identified a striking array of proteins now associated with the CLIC pathway for the first time. A significant proportion of identified targets localise to lipid-rafts and recycle from the PM, facets consistent with association to the CLIC pathway. Numerous targets have also been directly implicated in clathrin-independent endocytosis by independent groups. Verification of selected cargo, such as CD44, Thy-1 and myoferlin, showing specific internalisation through the CLIC pathway, has provided insight into the sorting ability of the CLIC pathway and links to adhesion turnover and membrane recycling. Consistent with a role in cellular adhesion turnover, it was found that CLICs become polarised within migrating cells. This has shown the first instance of spatial separation between three major endocytic routes, CLICs, caveolae and CME and highlights the important and coordinated roles of multiple endocytic pathways during physiologically significant processes. The specific internalisation of paxillin, Thy-1 and CD44 through CLICs at the leading edge of migrating cells suggests that CLICs rapidly turnover adhesion components for dynamic extracellular sensation during directional cell migration. Indeed, specific ablation of the CLIC pathway significantly impedes cellular migration, implying coordination with CME at the leading edge. This study has defined numerous parameters of the CLIC pathway, developing the current understanding of this poorly defined route and places the CLIC pathway as a unique player during critical cellular processes.

yes / The hERG potassium channel is critical for repolarisation of the cardiac action potential. Reduced expression of hERG at the plasma membrane, whether caused by hereditary mutations or drugs, results in long QT syndrome and increases the risk of ventricular arrhythmias. Thus, it is of fundamental importance to understand how the density of this channel at the plasma membrane is regulated. We used antibodies to an extracellular native or engineered epitope, in conjunction with immunofluorescence and ELISA, to investigate the mechanism of hERG endocytosis in recombinant cells and validated the findings in rat neonatal cardiac myocytes. The data reveal that this channel undergoes rapid internalisation, which is inhibited by neither dynasore, an inhibitor of dynamin, nor a dominant negative construct of Rab5a, into endosomes that are largely devoid of the transferrin receptor. These results support a clathrin-independent mechanism of endocytosis and exclude involvement of dynamin-dependent caveolin and RhoA mechanisms. In agreement, internalised hERG displayed marked overlap with glycosylphosphatidylinositol-anchored GFP, a clathrin-independent cargo. Endocytosis was significantly affected by cholesterol extraction with methyl-β-cyclodextrin and inhibition of Arf6 function with dominant negative Arf6-T27N-eGFP. Taken together, we conclude that hERG undergoes clathrin-independent endocytosis via a mechanism involving Arf6. / British Heart Foundation (grant number PG/10/68/28528; http://www.bhf.org.uk)

Les voies d’endocytose qui régissent l’internalisation d’éléments extracellulaires peuvent être classées selon que la protéine de manteau, la clathrine, est impliquée ou non dans le processus. Les voies indépendantes de la clathrine sont utilisées par de nombreuses toxines, des virus et des protéines endogènes. Les mécanismes permettant d’induire le recrutement des protéines cargoes et la déformation de la membrane plasmique dans le contexte de l'endocytose clathrine-indépendant restent encore mal compris. Cette étude montre que la galectine 3, une protéine humaine qui se lie aux glucides, induit la formation d’invaginations de la membrane plasmique de manière indépendante de la clathrine. Les glycosphingolipides (molécules jouant un rôle majeur dans la physiologie de la cellule) sont essentielles pour permettre à la galectine 3 d’induire ces invaginations et d’être internalisée dans la cellule. Les structures tubulaires induites par la galectine 3 présentent une morphologie étonnamment similaire à celle de compartiments intermédiaires de transport décrits dans la littérature pour l’endocytose indépendante de la clathrine. Des cargos utilisant la voie indépendante de la clathrine, tels que CD44 et les intégrines α5 et β1, sont retrouvés dans les tubules induits par la galectine 3. De plus, cette dernière est nécessaire à l’internalisation de CD44. Cela indique donc que la galectine 3 pourrait relier des protéines cargos glycosylés à des glycosphyngolipides de la membrane plasmique et ainsi induire une déformation de la membrane et leur internalisation dans les cellules. Ce mécanisme diffère de celui utilisé par la toxine pentamérique de Shiga et par la toxine cholérique, qui sont leur protéines cargos propores et interagissant directement avec le glycosphyngolipide leur servant de récepteur. Les tubules induits par la galectine 3 sont distincts de ceux induit par les autres lectines. Celles-ci présentent des spécificités différentes de liaison aux glucides, montrant ainsi la l'importance des interactions entre les lectines et les sucres dans ce processus. De plus, nous avons constaté que la galectine 3 module l’équilibre à l’état basal de l’intégrine β1 à la surface de la cellule. Cette protéine étant capitale pour les phénomènes d’adhésion et de migration cellulaires, nous avons donc exploré le rôle conjoint de la galectine 3 et des glycosphingolipides dans la migration cellulaire. La galectine 3 inhibe la migration des cellules humaines de carcinomes mammaires alors qu’elle stimule au contraire celle de cellules de tumeurs mammaires murines. Or, nous avons montré que la régulation par la galactine 3 de la migration de différentes lignées cellulaires est dépendante des glycosphingolipides. Il ressort donc de cette étude que la galectine 3 et les glycosphingolipides contribuent de manière synergique au processus d’induction de déformation de la membrane, à l’endocytose de protéines cargos et à la migration cellulaire. / Endocytic processes which govern the uptake of extracellular material into the cell can be classified based on their dependence on the coat protein, clathrin. Clathrin-independent mechanisms are used by many toxins, viruses and endogenous proteins. How cargo is recruited and membranes are bent is not well understood in these cases. Here, we discovered that galectin 3, a human carbohydrate binding protein induced the clathrin-independent formation of endocytic plasma membrane invaginations. Glycosphingolipids, which have established functions in key physiological processes, were found to be essential for the formation of galectin 3-induced invaginations and for the efficient uptake of the protein into the cell. Galectin 3-induced tubular structures were found to have a strikingly similar morphology to that of the clathrin-independent carriers described in literature. Clathrin-independent endocytic cargoes such as CD44, α5 and β1 integrin were present in galectin 3-induced tubules, and galectin activity and glycosphingolipids were required for the uptake of CD44. This indicated that galectin 3 could link glycosylated cargoes with glycosphingolipids for cargo recruitment and membrane bending. In contrast, the pentameric Shiga and cholera toxins are their own cargoes and drive membrane deformations by directly binding to their respective glycosphingolipid receptors. Galectin 3-induced tubules were distinct from those induced by lectins with different carbohydrate binding specificities, which revealed the importance of lectin-glycan interaction in this process. Further, we observed that galectin 3 modulated the steady state surface dynamics of β1 integrin, a protein which like CD44 is critical for cell adhesion and migration. Subsequently, we explored the interplay of galectins and glycosphingolipids in cell migration. Galectin 3 inhibited cell migration in human breast carcinoma cells, and stimulated migration in a mouse mammary tumor cell line. However, the regulation of migration by galectin 3 was in both cases found to be dependent on glycosphingolipids. In conclusion, galectin 3 and glycosphingolipids synergistically contribute to the clathrin-independent curvature generation process, cargo endocytosis and cell migration.

La toxine bactérienne de Shiga se lie au glycosphingolipide (GSL) globotriaosylcéramide (Gb3) afin d’entrer par endocytose dans les cellules en utilisant une voie dépendante et indépendante de la clathrine. Dans la voie indépendante de la clathrine, la toxine de Shiga réorganise les lipides de la membrane de façon à imposer une contrainte mécanique sur la bicouche, conduisant ainsi à la formation de pic d’invagination d'endocytose profonds et étroits. Mécaniquement ce phénomène n’est pas encore compris, notamment il reste énigmatique, comment se traduisent les propriétés géométriques de l’agrégation des glycosphingolipides GSLS et de la toxine. Dans mon travail de thèse, via l’utilisation de la sous-unité B de la toxine de Shiga (STxB) comme un modèle, différentes espèces moléculaires de son récepteur Gb3 ont été synthétisés avec des structures délibérément choisis. Les études réalisées par imagerie de haute résolution et par la modélisation informatique ont permis d’élucider les contraintes mécano-chimique sous-jacente conduisant à une réorganisation efficace qui a pour résultat l’agrégation de la toxine et la réorganisation des lipides. En combinant des expériences de simulation sur ordinateur de dynamique des particules dissipatives (DPD) et des expériences sur des modèles de membranes cellulaires, nous avons fourni la preuve de l’induction d’une force de fluctuation-membrane, de type « force de Casimir », conduisant à l'agrégation des molécules de toxines associées à la membrane à des échelles de longueur mésoscoiques. Nous avons observé et mesuré, en outre la condensation lipidique induite par la toxine, quantitativement sur des monocouches de Langmuir en utilisant la réflectivité des rayons X (XR) et par la mesure de la diffraction des rayons X par incidence rasante (GIXD), fournissant ainsi une preuve directe de l'hypothèse que la toxine a le potentiel de réduire de façon asymétrique la surface moléculaire sur la partie membranaire exoplasmique, ce qui conduit à une déformation locale de la membrane. Durant ma thèse, nos efforts ont été consacrés à la réalisation de nouveaux glycosphinolipides (GSL) comme outils chimiques à visée biologique. Par ailleurs, une nouvelle stratégie de reconstitution de GSL fonctionnels sur la membrane cellulaire, basée sur une réaction de ligation de type « click » entre un glycosyl-cyclooctyne et un azido-sphingosine a été étudiée. Les résultats obtenus sur les cellules se sont avérés beaucoup moins efficace que ceux in vitro. Une poursuite de l'optimisation de cette méthodologie est actuellement en cours. Une sonde fluorescente du glycosphinolipide Gb3, marquée à l’Alexa Fluor 568 lui-même lié par l'intermédiaire d'un bras PEG-α à la position de la chaîne acyle, a été synthétisée. Cette sonde se lie à la STxB sur couche mince de TLC, mais pas sur des membranes modèles. D'autres améliorations sont discutées. / Bacterial Shiga toxins bind to the glycosphingolipid (GSL) globotriaosylceramide (Gb3) to enter cells by clathrin-dependent and independent endocytosis. In the clathrin-independent pathway, Shiga toxin reorganizes membrane lipids in a way such as to impose mechanical strain onto the bilayer, thus leading to the formation of deep and narrow endocytic pits. Mechanistically how this occurs is not yet understood, and notably how the geometric properties of toxin-GSLs complexes translate into function has remained enigmatic. In my thesis work, using the B-subunit of Shiga toxin (STxB) as a model, different molecular species of its receptor Gb3 have been synthesized with deliberately chosen structures, coupled with high resolution imaging and computational modeling, to understand the underlying mechano-chemical constraints leading to efficient toxin clustering and lipids reorganization. By combining dissipative particle dynamics (DPD) computer simulation and experiments on cell and model membranes, we provided evidence that a membrane fluctuation-induced force, termed Casimir-like force, drives the aggregation of tightly membrane-associated toxin molecules at mesoscopic length scales. Furthermore, toxin-induced lipid condensation was observed and measured quantitatively on Langmuir monolayers using X-ray reflectivity (XR) and grazing incidence x-ray diffraction (GIXD), thereby providing direct evidence for the hypothesis that the toxin has the potential to asymmetrically reduce the molecular area of the exoplasmic membrane leaflet, leading to local membrane deformation. During my PhD, effort was also invested to develop new GSL tools applied to the biological setting. A novel strategy based on the Cu-free click reaction between glycosyl-cyclooctyne and azido-sphingosine was designed with the goal to functionally incorporate GSLs into cellular membranes. Following the synthesis work, click reactions have been performed in solution and on cells. Compared to the former, results on cells were far less efficient. Further optimization is currently ongoing. A fluorescently labeled Gb3 probe with Alexa Fluor 568 coupled via a PEG linker to the α-position of the acyl chain, was synthesized, to which STxB bound on TLCs, but not on model membranes. Further improvements are discussed.