<p>x</p> / <p>Platelet-derived growth factors (PDGF), a family of potent mitogens and chemoattractants for cells of mesenchymal origin, elicit their biological effects through the binding of two related receptor tyrosine kinases, denoted α- and β-receptors. The binding of PDGF to the receptors causes receptor dimerization and autophosphorylation on tyrosine residues. Src homology 2 (SH2) domain-containing proteins then bind the phosphorylated receptors, mediating further propagation of the signal. This thesis describes how the interaction between the PDGF receptors and some of their downstream targets can modify the cellular response to PDGF.</p><p>The tyrosine phosphatase SHP-2 has been implicated in activation of the Ras/MAPK pathway downstream of several receptor tyrosine kinases. We found that SHP-2 binds to phosphorylated Y763 in the PDGF β-receptor, in addition to the already reported binding to Y1009. Cells expressing PDGF β-receptors with Y763 and Y1009 mutated to phenylalanine exhibited decreased Ras-GTP loading and reduced activation of Erk2 in response to PDGF. Whereas these cells did not show any change in the mitogenic response to PDGF, the PDGF-induced chemotaxis was significantly reduced in cells expressing mutant compared to wild-type receptor.</p><p>The phosphorylation of Y771 of the PDGF β-receptor had been shown to be significantly lower in the αβ-heterodimeric receptor compared to in the ββ-homodimer, causing reduced binding of RasGAP to the heterodimer and increased Ras/MAPK activation. We could demonstrate that the reduced phosphorylation of Y771 is due to dephosphorylation by tyrosine phosphatases, including SHP-2.</p><p>SHP-2 had been shown to associate with the docking protein Gab1 after growth factor stimulation. We showed that the adaptor protein Grb2 was required for PDGF mediated phosphorylation of Gab1, and that phosphorylated Gab1, Grb2 and SHP-2 create a complex upon PDGF stimulation. Using a cell system with an inducible Gab1 expression, we further demonstrated that Gab1 increased SHP-2 activity in response to PDGF, without affecting the interaction between SHP-2 and the b-receptor. Induction of Gab1 correlated with an increase in both PDGF-induced Erk and p38 MAPK activation, whereas Akt activation was unaffected. The latter finding was in line with our observation that PDGF had no effect on the interaction between Gab1 and p85 of PI3’-kinase. The increase in MAPK activity after Gab1 induction and PDGF treatment did not correlate with an increase in PDGF-induced mitogenicity; instead these cells displayed more pronounced actin reorganization in response to PDGF.</p><p>In conclusion, our data indicate that SHP-2 regulates the PDGF response both through direct dephosphorylation of the receptor and through its interaction with Gab1. PDGF stimulated activation of SHP-2 seems to be correlated not only with mitogenesis, but also with reorganization of the actin cytoskeleton and cell migration.</p>

x / Platelet-derived growth factors (PDGF), a family of potent mitogens and chemoattractants for cells of mesenchymal origin, elicit their biological effects through the binding of two related receptor tyrosine kinases, denoted α- and β-receptors. The binding of PDGF to the receptors causes receptor dimerization and autophosphorylation on tyrosine residues. Src homology 2 (SH2) domain-containing proteins then bind the phosphorylated receptors, mediating further propagation of the signal. This thesis describes how the interaction between the PDGF receptors and some of their downstream targets can modify the cellular response to PDGF. The tyrosine phosphatase SHP-2 has been implicated in activation of the Ras/MAPK pathway downstream of several receptor tyrosine kinases. We found that SHP-2 binds to phosphorylated Y763 in the PDGF β-receptor, in addition to the already reported binding to Y1009. Cells expressing PDGF β-receptors with Y763 and Y1009 mutated to phenylalanine exhibited decreased Ras-GTP loading and reduced activation of Erk2 in response to PDGF. Whereas these cells did not show any change in the mitogenic response to PDGF, the PDGF-induced chemotaxis was significantly reduced in cells expressing mutant compared to wild-type receptor. The phosphorylation of Y771 of the PDGF β-receptor had been shown to be significantly lower in the αβ-heterodimeric receptor compared to in the ββ-homodimer, causing reduced binding of RasGAP to the heterodimer and increased Ras/MAPK activation. We could demonstrate that the reduced phosphorylation of Y771 is due to dephosphorylation by tyrosine phosphatases, including SHP-2. SHP-2 had been shown to associate with the docking protein Gab1 after growth factor stimulation. We showed that the adaptor protein Grb2 was required for PDGF mediated phosphorylation of Gab1, and that phosphorylated Gab1, Grb2 and SHP-2 create a complex upon PDGF stimulation. Using a cell system with an inducible Gab1 expression, we further demonstrated that Gab1 increased SHP-2 activity in response to PDGF, without affecting the interaction between SHP-2 and the b-receptor. Induction of Gab1 correlated with an increase in both PDGF-induced Erk and p38 MAPK activation, whereas Akt activation was unaffected. The latter finding was in line with our observation that PDGF had no effect on the interaction between Gab1 and p85 of PI3’-kinase. The increase in MAPK activity after Gab1 induction and PDGF treatment did not correlate with an increase in PDGF-induced mitogenicity; instead these cells displayed more pronounced actin reorganization in response to PDGF. In conclusion, our data indicate that SHP-2 regulates the PDGF response both through direct dephosphorylation of the receptor and through its interaction with Gab1. PDGF stimulated activation of SHP-2 seems to be correlated not only with mitogenesis, but also with reorganization of the actin cytoskeleton and cell migration.

Le cœur humain est un organe composé de différents types cellulaires tels que les cardiomyocytes, les fibroblastes, les muscles lisses et les cellules endothéliales. Ces cellules se diversifient grâce à des mécanismes moléculaires spécifiques en acquérant leurs propriétés fonctionnelles spécifiques. L’embryon de Drosophile est un modèle simple et adapté pour étudier la diversification des cellules cardiaques et leurs propriétés spécifiques. Le but du projet est d’améliorer notre connaissance sur les acteurs moléculaires qui contrôlent la diversification des cellules cardiaques. Pour atteindre cet objectif nous avons appliqué la méthode TRAP-rc ("rare cell Translation Ribosome Affinity Purification") suivie du séquençage ARN pour identifier les ARN messagers en cours de traduction spécifiques des cellules cardiaques Tin et Lb (Tin CBs et Lb CBs) à deux stades de développement corrélés avec la morphogenèse du cœur embryonnaire. Dans une première analyse focalisée sur l'analyse des données issues des TRAP-Seq sur cellules Tin nous avons mis en évidence que CAP et MSP-300 sont impliqués dans la migration des cardioblasts pendant la fermeture du cœur. En parallèle, nous avons également identifié deux autres gènes impliqués dans la morphogenèse, kon-tiki et dGrip qui semblent contrôler la cohésion des CBs au cours de la migration. En outre, nous avons trouvé qu'au stade 16, environ 60% des gènes enrichis sont communs entre les populations Tin et Lb. Parmi ces gènes, Src42, sqa et flr participent à la régulation du cytosquelette d'actine et nos analyses ont permis de démontrer qu'ils avaient également des fonctions dans la morphogenèse cardiaque. Nous avons également identifié des groupes de gènes plus spécifiques à chacune des populations ciblées. Une catégorie fonctionnelle fortement associée à la population Lb, comprend les gènes qui régulent l'épissage des ARN messagers et certains de ces gènes semblent être requis au cours de la morphogenèse cardiaque. Enfin, nous avons comparé nos données de TRAP-seq cardiaque avec des données de TRAP-Seq issues du muscle somatique (de l'équipe), et ainsi identifié près de 90 gènes qui présentent des isoformes protéiques spécifiques à chaque tissu notamment impliquées dans la formation de l'unité contractile sarcomérique. Ceci suggère que des mécanismes d'épissage spécifiques sont mis en place dans différents types cellulaires pour moduler les fonctions de certaines protéines musculaires. A travers ce projet, nous avons identifié de nouveaux acteurs généraux de la migration collective des cardioblastes au cours de la fermeture du cœur mais également de nouveaux gènes potentiellement impliqués dans l’acquisition des propriétés spécifiques dessous populations cardiaques Tin et Lb et de tissus musculaires distincts. Nous espérons que les données générées permettront dans le futur de mieux comprendre les mécanismes de la cardiogenèse des vertébrés ainsi que l’étiologie de maladies cardiaques. / Cardiac cells diversification is required for the formation of a functional heart. Human heart is a multi-lineage organ that develops through progressive diversification of progenitors derived from different heart fields. This process is underlined by numerous changes in the expression of a repertory of genes that allow cells to acquire their own identity and functions. The Drosophila embryo is a relatively simple model to study the diversification of cardiac cells and their properties. The goal of this project is to identify the repertories of genes that control the formation of different types of cardiac cells. To reach this objective we applied Translation Ribosome Affinity Purification (TRAP) method followed by RNA sequencing in order to identify mRNA engaged in translation specific to two cardiac cell types (Tinman (Tin) and Labybird (Lb) expressing cells), at two different time windows. We obtained a list of enriched genes for the different types of cardiac cells and time points. In a first part, we focused our attention on the Tindatasets and found that two genes, CAP and Msp300, are involved in cardioblasts migration during the heart closure. Then we identified two other candidate genes kontiki and dGrip that seem to contribute to maintain cohesion between CBs during heartmorphogenesis. Moreover by comparing our spatial datasets, we found that for the same time point, around 60% of Tin CBs enriched genes are common with Lb CBs enriched population and within this group we identified evolutionary conserved genes such as Src42, flr and sqa known to be involved in the cytoskeleton organization and in the actinpolymerization and depolymerisation. Our premiminary analyses show that they seem to be required for correct cardiac morphogenesis. We also identified sets of genes more specific for each cardiac cell population. Indeed, Lb CBs datasets show that in early stage there is the enrichment of genes mostly involved in transcriptional regulation and RNA splicing and some of these genes (prp8 and prp38) are involved in cardiac development. In parallel, we compared our TRAP-Seq dataset in the cardiac system with the TRAP-seqon muscle cells, and identified close to 90 genes that present cardiac or muscular specific isoforms. It is known that the alternative splicing, by increasing proteins diversity, contributes to the acquisition of specific cell properties. Furthermore, some cardiomyopathies are associated to defects in the alternative splicing of genes encoding sarcomeric proteins that we found in our dataset such as Tropomyosin and Zasp52. With this project, we have identified new actors of collective cardioblast migration and a set of genes with potential role in the acquisition of individual properties of Tin and Lbcardiac cells or of specific type of muscle tissue. We hope that our data could provide new insights into the genetic control of vertebrate cardiogenesis and into etiology of cardiac diseases.