Endocytosis is essential for the pathogenic development of Ustilago maydis. It has been shown that the initiation of pathogenicity relies upon the ability of the cell to recognize pheromone (a1 or a2) released from its mating partner and subsequently to form conjugated hyphae. The actin and microtubule cytoskeleton plays an essential role in all aspects of cell growth. A component of the actin cytoskeleton, the filamentous actin is required for cell-cell fusion, whereas the molecular motors, kinesin and dynein, move along microtubules and provide the long distance transport of many proteins and they are important in cell growth and pathogenicity. In this thesis, we investigated the role of the cytoskeleton in endocytosis and a1 pheromone transport, using a fluorescently labelled derivative of the a1 pheromone. We confirmed that uptake of the a1 pheromone is also receptormediated. In addition, we have shown that pheromone transport towards the cellular vacuole requires the actin and microtubule cytoskeletons. Furthermore, we revealed that the microtubule-dependent motors kinesin-1 and kinesin-3 and dynein were shown to be essential in the delivery of the pheromone to vacuoles. Moreover, a mutation in the early endosomal protein Yup1 gene causes a stop in delivery of the synthetic pheromone to the vacuole. This suggests that it travels with early endosomes. Within the actin cytoskeleton, we analysed the dynamics of actin patches in the presence of the synthetic pheromone and found that the dynamics of the patches increased significantly. Additionally, in the presence of an over-expression of the tail domain of the molecular motor myosin-5, the dynamics of the patches were significantly reduced and their intensity diminished.

Student Number : 9906751K - MSc dissertation - School of Molecular and Cell Biology - Faculty of Science / In addition to its crucial role in cell adhesion, β-catenin is also known to augment gene expression by forming a complex with lymphoid enhancer factor/T-cell factor in the nucleus. Unregulated β-catenin expression and/or its increased nuclear presence can lead to abnormal cell proliferation, tumour invasion and metastasis. Pertinent is the fact that the actin cytoskeleton is central to the translocation of several nuclear proteins. This study investigated whether the actin cytoskeleton influences the nuclear translocation of β-catenin in human oesophageal squamous cell carcinoma (HOSCC), a metastatic disease of common occurrence in South Africa. Disruption of the actin cytoskeleton of five moderately differentiated HOSCC cell lines, with cytochalasin D (cytoD), showed that the nuclear β-catenin level was unaltered in SNO, WHCO1 and WHCO5, but decreased in WHCO3 and WHCO6. CytoD treatment did not affect the cytoplasmic/membrane β-catenin level in these cell lines. Further examination of the possible association between the actin cytoskeleton and nuclear β-catenin translocation, required the design and stable transfection, of a vector containing full-length human β-catenin cDNA into one of the HOSCC lines. Stimulation of exogenous β-catenin expression in transfected WHCO1 cells did not increase cellular β-catenin level, nor did the stimulation of endogenous β-catenin expression with DMSO. In most cases (SNO, WHCO1 and WHCO5) the nuclear distribution of β-catenin in HOSCC is independent of a functional actin cytoskeleton, nonetheless there are some exceptions (WHCO3 and WHCO6). The observed variation within the HOSCC lines is possibly due to specific underlying event/s particular to the cell line. The stable level of β-catenin expression could be a consequence of regulatory pathways in WHCO1 compensating for the induced imbalance of β-catenin expression.

A instabilidade cromossômica e a aneuploidia são características associadas às células malignas. Sabe-se que essas alterações podem ser resultantes de erros em eventos durante a mitose. Com o objetivo de gerar uma população com ganho de ploidia, utilizamos dois inibidores de fases distintas da mitose. Resultando no aumento da frequência celular em G2/M, ou seja, células com 4 vezes o número de cromossomos (4C) e de células com DNA acima de 4, hipertetraploide. Quantificamos a quantidade de núcleos e demonstramos que o tratamento, especificamente, levou a uma NCI, e não a multinucleação. Ambas linhagens celulares submetidas ao tratamento apresentaram alterações morfológicas, como protrusões de membrana, indicando alterações no citoesquelto. Nas análises de mRNA e da expressão proteica, observamos alterações na regulação da actina, coincidindo com a elevação dos mRNAs de YAP/TAZ, efetores co-transcricionais da via Hippo, regulada por alterações no citoesqueleto de actina. Desde modo, propomos, que o tratamento utilizado é um método eficiente para o estudo de células aneuploides e da NCI, que o citoesqueleto de actina é modulado por esse fenótipo e requer YAP/TAZ, provavelmente para manter a sobrevivência e favorecer a proliferação celular observada após o tratamento. / Chromosomal instability and aneuploidy are characteristics associated with malignant cells. It is known that these changes may be due to errors in events during mitosis. In order to generate a population with gain of ploidy, we used two inhibitors of distinct phases of mitosis. Resulting in increasing cell frequency in G2/M, cells with 4 times the number of chromosomes (4C) and cells with DNA above 4, hypertetraploid. We quantified the number of nuclei and demonstrated that the treatment specifically led to NCI, not multinucleation. Both cell lines submitted to treatment presented morphological alterations, such as membrane protrusions, indicating changes in the cytoskeleton. In the analysis of mRNA and protein expression, we observed alterations in actin regulation, coinciding with the elevation of YAP / TAZ mRNAs, co-transcriptional effectors of the Hippo signaling pathway, regulated by changes in the actin cytoskeleton. We propose, that the treatment used is an efficient method for the study of aneuploid cells as well NCI. Also, the actin cytoskeleton is modulated by that phenotype which requires high YAP / TAZ, probably to maintain cell survival and promote cell proliferation observed.

Le bon fonctionnement des réseaux neuronaux dépend des interactions entre les neurones et les cellules gliales. Alors que de nombreux efforts ont été faits pour comprendre les interactions entre les neurones, moins est connu sur la nature des interactions entre les cellules gliales ; ceci est due à la complexité du système nerveux des vertébrés, qui comprend plus de cellules gliales que de neurones. Cependant, le système nerveux de la drosophile à un rapport neurones-cellules gliales faible, ce qui fait de cet animal simple un modèle idéal pour évaluer ce concept. J’ai utilisé des approches génétiques à résolution cellulaire pour disséquer les mécanismes cellulaires et moléculaires de la migration collective des cellules gliales in vivo. En résumé, mes données révèlent les bases du mécanisme contrôlant la migration cellulaire collective : 1) les cellules du front de migration interagissent entre elles en amont et en aval et 2) N-cad est nécessaire pour une migration optimal de la glie. / The functionality of the complex neural network depends on the interactions between neurons and glia. While many efforts have been made to understand the neuron-neuron interactions, less is known about those amongst glial cells. Due to the complexity of the vertebrate nervous system, which comprises manifold more glia than neurons, it is hard to tackle the role of glia-glia interactions. The nervous system of Drosophila, however, has a lower glia-neuron ratio, which makes this simple animal an ideal model. I use genetic approaches at cellular resolution to dissect the cellular and molecular mechanisms of glial collective migration in vivo. In Sum, I have shown some basic mechanism controlling collective cell migration: 1) cells at the front of the collective interact with each other through anterograde and retrograde bidirectional interaction. 2) N-cad appears necessary for timely movement of glial community.

Les cellules épithéliales sont des cellules polarisées qui forment l’un des types cellulaires le plus abondant dans le corps humain. Leur polarité apico-basale (A/B) est établie et maintenue par la ségrégation asymétrique de protéines adaptatrices hautement conservées. Cette polarité est essentielle pour de nombreuses fonctions cellulaires clés comme l’adhésion (jonctions intercellulaires) ou la signalisation et la prolifération par la localisation et la concentration des complexes de signalisation. Durant la cancérogénèse, un grand nombre de ces processus est dérégulé aboutissant à la sur-prolifération, la migration et/ou l’invasion des cellules cancéreuses. Une meilleure compréhension des mécanismes à l’origine de ces processus est indispensable pour trouver de nouvelles cibles thérapeutiques pour le traitement du cancer. Dans l’équipe, nous sommes particulièrement intéressés par les protéines adaptatrices à domaines PDZ (domaine de liaison protéine-protéine). De par leur structure modulaire et la diversité de leurs partenaires, ces protéines adaptatrices sont impliquées dans la régulation de très nombreuses fonctions et fournissent des plateformes où différents processus peuvent être intégrés. Durant mon doctorat, j’ai étudié deux protéines adaptatrices dans le système animal modèle Drosophila melanogaster, Bbg et Magi, impliquées dans deux processus cellulaires essentiels : la dynamique des jonctions et la prolifération. Grâce aux molécules d’adhésion, les cellules non seulement restent cohésives dans un tissu, mais c’est aussi à ce niveau qu’elles peuvent obtenir une information concernant la densité cellulaire d'un tissu. Cette information est alors relayée au cytosquelette d’actine via des protéines adaptatrices spécialisées dans le but de réguler la prolifération et la voie Hippo. Cependant, le contrôle de la voie de signalisation Hippo par certaines protéines adaptatrices et par le cytosquelette d’actine n'est que partiellement compris à ce jour. Dans le laboratoire, nous étudions notamment le rôle d'une nouvelle protéine adaptatrice apicale nommée Big Bang (Bbg) dans le disque d’aile de la drosophile. Nous nous sommes intéressés à Bbg car c’est une cible de la voie Notch chez la drosophile et son homologue humain PDZD2 (pour PDZ domain-containing 2 protein) est sur-exprimé dans plusieurs cancers (sein et prostate).Mes résultats montrent que Bbg est un nouveau régulateur du cytosquelette d’actine et de la voie Hippo. Une étude détaillée de la fonction de Bbg et de ses partenaires permet de mieux comprendre les relations existantes entre dynamique de l’actine et prolifération. Bbg induit une accumulation d’actine filamenteuse en augmentant l’activité d’Enabled et la phosphorylation de Myosin Light Chain (MLC). Cette régulation résulte en une augmentation de l’activité de Yorkie, effecteur final de la voie Hippo, pour soutenir la prolifération cellulaire.La régulation des jonctions adhérentes est une étape cruciale lors de l’évolution d'une tumeur solide. Malgré les récentes avancées dans le domaine, de nombreux aspects clés de la dynamique des jonctions restent peu caractérisés.Dans le laboratoire, nous recherchons de nouveaux régulateurs de jonctions et grâce au modèle de remodelage des AJs lors du développement de l’œil de pupe de drosophile. Nous avons identifié Magi en tant que protéine adaptatrice recrutant le complexe formé de RASSF8 et ASPP. Magi régule le recrutement de Bazooka à la membrane, le dépôt d’E-Cadhérine et des Caténines et finalement le remodelage des jonctions pendant la morphogénèse. J’ai identifié Echinoid, une protéine de type immunoglobuline impliquée dans l’adhésion cellulaire et la régulation de la voie Hippo, comme un nouveau partenaire responsable du recrutement de Magi aux futures zones de jonctions. / Epithelial cells are polarised cells that form one of the most abundant cell types in the human body. Their apico-basal (A/B) polarity is established and maintained by the asymmetric segregation of highly conserved scaffold proteins. Proper A/B polarity is critical for many key cellular functions such as intercellular junctions and therefore adhesion, or signalling and proliferation by localising and concentrating signalling complexes. During carcinogenesis, many of these processes are mis-regulated leading to the over-proliferation, migration and/or invasion of cancer cells. A better understanding of the mechanisms underlying these processes is really needed to find new therapeutic targets in cancer treatment.In the team, we are particularly interested in scaffold proteins with PDZ domains (protein-protein interaction domains). Due to their modular structure, the high number of interactions they engage in, and the variety of their binding partners, these scaffold proteins are implicated in the regulation of many key cellular functions and processes. During my PhD, I have studied two scaffold proteins in the Drosophila melanogaster animal model, Bbg and Magi, which are involved in two important cell processes: adherens junctions (AJs) dynamic and cell proliferation.Through adhesion molecules, epithelial cells not only remain cohesive, but can also sense cellular density in a tissue and relay this information through dedicated scaffolds to the actin cytoskeleton to ultimately regulate the Hippo pathway and proliferation. However, many aspects of the control of Hippo signalling by apical scaffolds and the actin cytoskeleton are still poorly understood. In the laboratory, we are interested in the study of a new conserved apical scaffold, Big Bang (Bbg). Bbg is a new and quite unknown protein expressed in a variety of Drosophila epithelia, and appears as a potential Notch target in Drosophila. Its’ human homolog called PDZD2 (PDZ domain-containing 2 protein) has been shown to be over-expressed in several types of cancers (breast and prostate cancers).My results show that Bbg is a new regulator of the actin cytoskeleton and of the Hippo pathway in Drosophila. A detailed study of Bbg function and of its associated partners have helped to better understand the intricate relationships between actin dynamics and proliferation. My results suggest that Bbg promotes accumulation of filamentous actin (F-Actin) through the increase of the activity of Enabled (Ena) and the phosphorylation of the molecular motor Myosin Light Chain (MLC). This regulation leads to the increase of Yorkie activity, the final effector of the Hippo pathway, to promote cell proliferation.The regulation of adhesion, and in particular of Adherens Junctions (AJs), is a critical step during the evolution of solid tumours. A better understanding of how these structures are regulated will provide valuable insights into different phases of the disease. Despite the recent advances, many key aspects of AJ dynamics remain poorly understood. In the laboratory, we are interested in the identification of new AJs regulators. Using the remodelling of AJs during the development of the Drosophila pupal eye as a model, we have identified Magi as a scaffold recruiting a complex formed by RASSF8 and ASPP, regulating Bazooka membrane recruitment, E-Cadherin and catenins deposition, and ultimately AJs remodelling during morphogenesis. I uncovered Echinoid, an immunoglobulin-like protein involved in cell adhesion and in Hippo pathway regulation, as a new binding partner responsible for the recruitment of Magi at future AJ sites.

Glucagon-like peptide-1 (GLP-1) is secreted from the enteroendocrine L cell following nutrient ingestion. Although GLP-1 regulates several aspects of nutrient homeostasis, one important function is to enhance glucose-dependent insulin secretion. In type 2 diabetes, post-prandial GLP-1 secretion is impaired. Insulin resistance, which is required for the pathogenesis of type 2 diabetes, is also associated with impaired GLP-1 secretion. I, therefore, hypothesized that insulin modulates GLP-1 secretion from the intestinal L cell and, furthermore, insulin resistance directly impairs the function of the endocrine L cell. In well-characterized L cell models, I established that insulin stimulates GLP-1 secretion through the MEK1/2-ERK1/2 pathway, and induction of insulin resistance in vitro attenuated insulin- and heterologous secretagogue-induced GLP-1 release. Furthermore, glucose-stimulated GLP-1 secretion was decreased in hyperinsulinemic-insulin resistant MKR mice, demonstrating that insulin resistance is associated with impaired L cell function. I next examined the role of the actin cytoskeleton in insulin-stimulated GLP-1 secretion. Insulin treatment transiently induced actin depolymerization, and depolymerization of the actin cytoskeleton potentiated insulin-stimulated GLP-1 release from the L cell, demonstrating that the cytoskeleton functions as a permissive barrier. Central to insulin’s effects on actin dynamics is the Rho GTPase, Cdc42, as siRNA-mediated knockdown and over-expression of a dominant-negative mutant, prevented insulin-stimulated actin remodeling and GLP-1 release. Insulin also promoted activation of PAK1, the downstream kinase of Cdc42, and over-expression of a kinase-dead PAK1 mutant attenuated insulin-stimulated GLP-1 release. In cells that expressed dominant-negative Cdc42 or kinase-dead PAK1, activation of ERK1/2 following insulin treatment was attenuated, demonstrating that the Cdc42-PAK1 axis regulates the activity of the canonical ERK1/2 pathway. In summary, this thesis demonstrates, for the first time, that insulin is a GLP-1 secretagogue, and this effect of insulin is mediated through the canonical ERK1/2 pathway and the Cdc42-PAK1 axis. Insulin resistance in the L cell impairs the responsiveness of the L cell to heterologous secretagogues. Collectively, these findings suggest that an alternative approach to treat type 2 diabetes and/or insulin resistance may be to directly improve the function of the L cell, thereby enhancing endogenous GLP-1 release.

Glucagon-like peptide-1 (GLP-1) is secreted from the enteroendocrine L cell following nutrient ingestion. Although GLP-1 regulates several aspects of nutrient homeostasis, one important function is to enhance glucose-dependent insulin secretion. In type 2 diabetes, post-prandial GLP-1 secretion is impaired. Insulin resistance, which is required for the pathogenesis of type 2 diabetes, is also associated with impaired GLP-1 secretion. I, therefore, hypothesized that insulin modulates GLP-1 secretion from the intestinal L cell and, furthermore, insulin resistance directly impairs the function of the endocrine L cell. In well-characterized L cell models, I established that insulin stimulates GLP-1 secretion through the MEK1/2-ERK1/2 pathway, and induction of insulin resistance in vitro attenuated insulin- and heterologous secretagogue-induced GLP-1 release. Furthermore, glucose-stimulated GLP-1 secretion was decreased in hyperinsulinemic-insulin resistant MKR mice, demonstrating that insulin resistance is associated with impaired L cell function. I next examined the role of the actin cytoskeleton in insulin-stimulated GLP-1 secretion. Insulin treatment transiently induced actin depolymerization, and depolymerization of the actin cytoskeleton potentiated insulin-stimulated GLP-1 release from the L cell, demonstrating that the cytoskeleton functions as a permissive barrier. Central to insulin’s effects on actin dynamics is the Rho GTPase, Cdc42, as siRNA-mediated knockdown and over-expression of a dominant-negative mutant, prevented insulin-stimulated actin remodeling and GLP-1 release. Insulin also promoted activation of PAK1, the downstream kinase of Cdc42, and over-expression of a kinase-dead PAK1 mutant attenuated insulin-stimulated GLP-1 release. In cells that expressed dominant-negative Cdc42 or kinase-dead PAK1, activation of ERK1/2 following insulin treatment was attenuated, demonstrating that the Cdc42-PAK1 axis regulates the activity of the canonical ERK1/2 pathway. In summary, this thesis demonstrates, for the first time, that insulin is a GLP-1 secretagogue, and this effect of insulin is mediated through the canonical ERK1/2 pathway and the Cdc42-PAK1 axis. Insulin resistance in the L cell impairs the responsiveness of the L cell to heterologous secretagogues. Collectively, these findings suggest that an alternative approach to treat type 2 diabetes and/or insulin resistance may be to directly improve the function of the L cell, thereby enhancing endogenous GLP-1 release.

Various genes have evolved to protect the cell against stressor-induced damage or death including the heat shock proteins (HSPs). Stressor-induced HSP gene expression involves the activation of heat shock factor (HSF), which binds to the heat shock element (HSE) found in the promoter region of <em>hsp</em> genes. Previously, our laboratory has examined the expression and function of <em>hsp</em> genes in the South African clawed frog, <em>Xenopus laevis</em>. Amphibians are particularly susceptible to adverse environmental conditions, including high temperatures and toxicants. In contrast to the many known inducers of HSF activation in poikilothermic vertebrates, few inhibitors have been either discovered or described in the literature. The present study has compared for the first time the effect of two heat shock response (HSR) inhibitors, quercetin and KNK437, on <em>hsp</em> gene expression in <em>Xenopus</em> A6 cells, demonstrating their efficacy in poikilotherms. Northern blot and densitometric analysis showed that cells treated with either quercetin or KNK437 decreased the heat shock-induced accumulation of <em>hsp70</em>, <em>hsp47</em>, and <em>hsp30</em> mRNAs. Additionally, constitutive levels of <em>hsp47</em> and <em>hsc70</em> mRNAs were reduced. In comparison, neither quercetin nor KNK437 affected the levels of constitutively expressed <em>ef1&alpha;</em> mRNAs under control or heat shock conditions. Western blot and densitometric analysis in this study showed that under heat shock conditions, exposure to quercetin or KNK437 significantly decreased the accumulation of HSP30, and that KNK437 was more effective in doing so than quercetin. In comparison, levels of actin were not significantly affected by either heat shock or exposure to DMSO, quercetin, or KNK437. These findings suggest that one mechanism by which quercetin and KNK437 inhibits the HSR in <em>Xenopus</em> is through the inhibition of HSF activity. <br /><br /> Results of this study also suggest that KNK437 inhibits the acquisition of thermotolerance in poikilotherms, similar to observations in mammalian systems. In the presence of KNK437, cells given a 2 h heat pretreatment at 33ºC followed by a thermal challenge for 1 h at 37ºC, showed numerous ruffled membrane edges and some aggregates of disrupted stress fibers. In comparison, cells directly challenged for 1 h at 37ºC, showed a marked decrease in HSP30, which was located predominantly at the cellular periphery in conjunction with actin aggregates. These cells showed virtually no intact stress fibers spanning cells and no coherent cell-cell connections. A 3-D analysis of cells given a 1 h thermal challenge at 37ºC (after a prior 2 h heat shock at 33ºC) in the absence of KNK437, showed numerous linear actin bundles transversing the entire cell, even extending into areas of cell-cell contact, and abundant HSP30 concentrated in the perinuclear region surrounding an intact nucleus. However, in the presence of KNK437, there was a significant emergence of membrane ruffles indicating global instability of cellular adhesion. This study has demonstrated that KNK437, which is the more specific and efficient HSR inhibitor, will be an important inhibitor to compare with the well-documented quercetin for future investigations.

Various genes have evolved to protect the cell against stressor-induced damage or death including the heat shock proteins (HSPs). Stressor-induced HSP gene expression involves the activation of heat shock factor (HSF), which binds to the heat shock element (HSE) found in the promoter region of <em>hsp</em> genes. Previously, our laboratory has examined the expression and function of <em>hsp</em> genes in the South African clawed frog, <em>Xenopus laevis</em>. Amphibians are particularly susceptible to adverse environmental conditions, including high temperatures and toxicants. In contrast to the many known inducers of HSF activation in poikilothermic vertebrates, few inhibitors have been either discovered or described in the literature. The present study has compared for the first time the effect of two heat shock response (HSR) inhibitors, quercetin and KNK437, on <em>hsp</em> gene expression in <em>Xenopus</em> A6 cells, demonstrating their efficacy in poikilotherms. Northern blot and densitometric analysis showed that cells treated with either quercetin or KNK437 decreased the heat shock-induced accumulation of <em>hsp70</em>, <em>hsp47</em>, and <em>hsp30</em> mRNAs. Additionally, constitutive levels of <em>hsp47</em> and <em>hsc70</em> mRNAs were reduced. In comparison, neither quercetin nor KNK437 affected the levels of constitutively expressed <em>ef1&alpha;</em> mRNAs under control or heat shock conditions. Western blot and densitometric analysis in this study showed that under heat shock conditions, exposure to quercetin or KNK437 significantly decreased the accumulation of HSP30, and that KNK437 was more effective in doing so than quercetin. In comparison, levels of actin were not significantly affected by either heat shock or exposure to DMSO, quercetin, or KNK437. These findings suggest that one mechanism by which quercetin and KNK437 inhibits the HSR in <em>Xenopus</em> is through the inhibition of HSF activity. <br /><br /> Results of this study also suggest that KNK437 inhibits the acquisition of thermotolerance in poikilotherms, similar to observations in mammalian systems. In the presence of KNK437, cells given a 2 h heat pretreatment at 33ºC followed by a thermal challenge for 1 h at 37ºC, showed numerous ruffled membrane edges and some aggregates of disrupted stress fibers. In comparison, cells directly challenged for 1 h at 37ºC, showed a marked decrease in HSP30, which was located predominantly at the cellular periphery in conjunction with actin aggregates. These cells showed virtually no intact stress fibers spanning cells and no coherent cell-cell connections. A 3-D analysis of cells given a 1 h thermal challenge at 37ºC (after a prior 2 h heat shock at 33ºC) in the absence of KNK437, showed numerous linear actin bundles transversing the entire cell, even extending into areas of cell-cell contact, and abundant HSP30 concentrated in the perinuclear region surrounding an intact nucleus. However, in the presence of KNK437, there was a significant emergence of membrane ruffles indicating global instability of cellular adhesion. This study has demonstrated that KNK437, which is the more specific and efficient HSR inhibitor, will be an important inhibitor to compare with the well-documented quercetin for future investigations.

No description available.