Tie1 é um receptor tirosina quinase expresso em células endoteliais importante em angiogênese, formação de vasos sanguíneos a partir de vasos pré-existentes. Este receptor pertence a uma família pequena composta por apenas dois membros (Tie1 e Tie2) para os quais angiopoietinas foram identificadas como ligantes de Tie2. No entanto, Tie1 continua a ser um receptor órfão, sem ligantes identificados até o momento. Sendo assim, é difícil compreender completamente as propriedades biológicas de Tie1 e seus mecanismos moleculares em angiogênese sem um ligante identificado. Entretanto, como sugerido através de estudos de deleção gênica, este receptor é uma molécula essencial na angiogênese, apresentando um papel importante no desenvolvimento da vascularização da retina e desenvolvimento de tumores. O nosso objetivo foi estudar a participação do domínio extracelular de Tie1 na neovascularização e, no processo, identificar possíveis ligantes para este receptor. Através da tecnologia de phage display, identificamos um peptídeo específico e seletivo para Tie1, sugerindo a existência de um sítio de ligação único neste receptor. Mostramos que este peptídeo é capaz de inibir a proliferação de células endoteliais induzida por Ang1, um ligante bem caracterizado de Tie2 que também modula a atividade de Tie1. Além disso, também mostramos que este peptídeo inibe a angiogênese in vivo num modelo animal bastante relevante para estudo de doenças humanas, o modelo da retinopatia induzida por oxigênio. Uma vez que este peptídeo liga-se a um sítio único e seletivo para Tie1, hipotetizamos que o mesmo poderia mimetizar possíveis ligantes naturais deste receptor. Para identificá-los, proteínas com mimetopo cruzado com este peptídeo foram identificadas em extrato proteico de diferentes linhagens celulares. Tais proteínas são possíveis candidatos a interação com Tie1. Em resumo, demonstramos que o domínio extracelular de Tie1 é importante para a angiogênese patológica e identificamos proteínas como possíveis ligantes deste receptor, o que poderá contribuir para um melhor entendimento da participação de Tie1 na formação de vasos. O peptídeo aqui identificado poderá ser ainda uma ferramenta útil para o desenvolvimento de novas terapias anti-angiogênicas com importantes aplicações à saúde humana. / Tie1 is a tyrosine kinase receptor expressed by endothelial cells and important in angiogenesis, the formation of new blood vessels from pre-existing ones. This receptor belongs to a small family of receptors composed of two members only (Tie1 and Tie2) to which angiopoietins have been identified as ligands for Tie2. On the other hand, Tie1 is still an orphan receptor with no ligand identified to date. Thus, it is difficult to assess Tie1 mechanism of action in neovascularization without a known ligand. Nevertheless, gene deletion studies have shown that Tie1 is essential in angiogenesis, and plays an important role in retinal and tumoral vascularization. The aim of our study was to evaluate the participation of Tie1 extracellular domain in angiogenesis, and in the process, to identify putative ligands for this receptor. Utilizing phage display, we have identified and characterized a Tie1 specific and selective ligand peptide, which suggests the existence of a binding site unique to this receptor and not shared by other family members. We show that this peptide prevents endothelial cells proliferation, induced by angiopoetin-1, a ligand for Tie2 but which also modulates Tie1 activity. Using a well-accepted mouse model for human diseases, the oxygen induced retinopathy model, we show that this peptide inhibits angiogenesis in vivo. Since this peptide maps to a unique binding site in Tie1, we hypothesized that it might mimic a natural ligand for this receptor. To identify them, proteins with cross reactive epitopes with an anti-peptide sera were identified by proteomic approaches. These proteins are thus possible ligands for Tie1. In summary, we have shown that Tie1 extracellular domain is important in angiogenesis and we have identified putative ligand for this receptor, which might contribute to a better understanding of the molecular mechanisms associated with Tie1 in blood vessel formation. The peptide here characterized may also be an important tool for the development of novel anti-angiogenesis therapeutic approaches for disesase with an angiogenic component.

Des données récentes montrent que la transition endothélio-mésenchymateuse (ou EndMT) joue un rôle important dans le développement et dans un certain nombre de maladies. L’objectif de ma thèse a été de déterminer la ou les fonctions du récepteur Tie1. Le récepteur Tie1 est un récepteur à activité tyrosine kinase exprimé sur les cellules endothéliales et impliqué dans la maturation des vaisseaux sanguins mais très peu de choses sont connues concernant Tie1. Nous avons montré que l’absence du récepteur Tie1 par ARN interférence dans les cellules endothéliales microvasculaires humaines HMVEC induit une EndMT dans ces cellules. Ces cellules acquièrent un phénotype de cellules fibroblastiques, perdent leurs marqueurs endothéliaux au profit des marqueurs des cellules mésenchymateuses et acquièrent un potentiel migratoire très important. Nous avons déterminé que les voies de transduction Erk1/2, Erk5 et d’Akt étaient activées lors de cette EndMT et montré que l’inhibition de Tie1 induit l’activation du promoteur du gène Slug. Le second objectif était de savoir si l’EndMT était présente dans les tumeurs humaines. L’étude de coupes d’adénocarcinomes pancréatiques montre qu’il y a une co-expression dans certaines cellules des marqueurs à la fois endothéliaux et mésenchymateux signe d’une EndMT. Les résultats obtenus au cours de ma thèse montrent que l’absence de Tie1 dans les cellules endothéliales induit l’EndMT qui peut être à l’origine des CAFs. Ces derniers représentent une nouvelle cible thérapeutique. En effet leur interaction avec les cellules cancéreuses est essentielle pour la croissance de la tumeur et l’apparition de métastases. / Recent data show that Endothelial–mesenchymal transition (EndMT) has a significant role in development and in various pathologies. The goal of my thesis was to determine the functions of the Tie1 receptor (Tyrosine kinase with immunoglobulin-like and EGF-like domains 1). Tie1 is a receptor with a tyrosine kinase activity expressed on endothelial cells and implicated in blood vessels maturation but few things are known for Tie1. We have showed that suppression of Tie1 with interfering RNA in human endothelial microvascular endothelial cells induces EndMT. These cells acquire a fibroblastic phenotype, lose their endothelial markers in favor of mesenchymal markers and acquire an important migratory potential. We find that Erk1/2, Erk5 and Akt cascades were activated in EndMT and showed that Tie1 inhibition induces Slug promoter activation. The second goal was to know if EndMT occurs in human tumors. Immunochemistry studies with confocal microscopy of pancreatic adenocarcinomas slides showed that there was a co-expression of endothelial and mesenchymal markers in some cells suggesting an EndMT.The results of my thesis show that Tie1 deficiency in endothelial cells induces endothelial-mesenchymal transition which can be at the origin of CAF. These CAF represent a new therapeutic target. Their interactions with cancerous cells are essential for tumor growth and metastasis appearance.

Tie1 é um receptor tirosina quinase expresso em células endoteliais importante em angiogênese, formação de vasos sanguíneos a partir de vasos pré-existentes. Este receptor pertence a uma família pequena composta por apenas dois membros (Tie1 e Tie2) para os quais angiopoietinas foram identificadas como ligantes de Tie2. No entanto, Tie1 continua a ser um receptor órfão, sem ligantes identificados até o momento. Sendo assim, é difícil compreender completamente as propriedades biológicas de Tie1 e seus mecanismos moleculares em angiogênese sem um ligante identificado. Entretanto, como sugerido através de estudos de deleção gênica, este receptor é uma molécula essencial na angiogênese, apresentando um papel importante no desenvolvimento da vascularização da retina e desenvolvimento de tumores. O nosso objetivo foi estudar a participação do domínio extracelular de Tie1 na neovascularização e, no processo, identificar possíveis ligantes para este receptor. Através da tecnologia de phage display, identificamos um peptídeo específico e seletivo para Tie1, sugerindo a existência de um sítio de ligação único neste receptor. Mostramos que este peptídeo é capaz de inibir a proliferação de células endoteliais induzida por Ang1, um ligante bem caracterizado de Tie2 que também modula a atividade de Tie1. Além disso, também mostramos que este peptídeo inibe a angiogênese in vivo num modelo animal bastante relevante para estudo de doenças humanas, o modelo da retinopatia induzida por oxigênio. Uma vez que este peptídeo liga-se a um sítio único e seletivo para Tie1, hipotetizamos que o mesmo poderia mimetizar possíveis ligantes naturais deste receptor. Para identificá-los, proteínas com mimetopo cruzado com este peptídeo foram identificadas em extrato proteico de diferentes linhagens celulares. Tais proteínas são possíveis candidatos a interação com Tie1. Em resumo, demonstramos que o domínio extracelular de Tie1 é importante para a angiogênese patológica e identificamos proteínas como possíveis ligantes deste receptor, o que poderá contribuir para um melhor entendimento da participação de Tie1 na formação de vasos. O peptídeo aqui identificado poderá ser ainda uma ferramenta útil para o desenvolvimento de novas terapias anti-angiogênicas com importantes aplicações à saúde humana. / Tie1 is a tyrosine kinase receptor expressed by endothelial cells and important in angiogenesis, the formation of new blood vessels from pre-existing ones. This receptor belongs to a small family of receptors composed of two members only (Tie1 and Tie2) to which angiopoietins have been identified as ligands for Tie2. On the other hand, Tie1 is still an orphan receptor with no ligand identified to date. Thus, it is difficult to assess Tie1 mechanism of action in neovascularization without a known ligand. Nevertheless, gene deletion studies have shown that Tie1 is essential in angiogenesis, and plays an important role in retinal and tumoral vascularization. The aim of our study was to evaluate the participation of Tie1 extracellular domain in angiogenesis, and in the process, to identify putative ligands for this receptor. Utilizing phage display, we have identified and characterized a Tie1 specific and selective ligand peptide, which suggests the existence of a binding site unique to this receptor and not shared by other family members. We show that this peptide prevents endothelial cells proliferation, induced by angiopoetin-1, a ligand for Tie2 but which also modulates Tie1 activity. Using a well-accepted mouse model for human diseases, the oxygen induced retinopathy model, we show that this peptide inhibits angiogenesis in vivo. Since this peptide maps to a unique binding site in Tie1, we hypothesized that it might mimic a natural ligand for this receptor. To identify them, proteins with cross reactive epitopes with an anti-peptide sera were identified by proteomic approaches. These proteins are thus possible ligands for Tie1. In summary, we have shown that Tie1 extracellular domain is important in angiogenesis and we have identified putative ligand for this receptor, which might contribute to a better understanding of the molecular mechanisms associated with Tie1 in blood vessel formation. The peptide here characterized may also be an important tool for the development of novel anti-angiogenesis therapeutic approaches for disesase with an angiogenic component.

<p>The endothelial receptor tyrosine kinase (RTK) Tie1 was discovered over 20 years ago, yet its precise function and mode of action remain enigmatic. To shed light on Tie1’s role in endothelial cell biology, we investigated a potential threonine phosphorylation site within the juxtamembrane domain of Tie1. Expression of a non-phosphorylatable mutant of this site (T794A) in zebrafish (Danio rerio) significantly disrupted vascular development, resulting in fish with stunted and poorly branched intersomitic vessels. Similarly, T794A-expressing human umbilical vein endothelial cells formed significantly shorter tubes with fewer branches in three-dimensional Matrigel cultures. However, mutation of T794 did not alter Tie1 or Tie2 tyrosine phosphorylation or downstream signaling in any detectable way, suggesting that T794 phosphorylation may regulate a Tie1 function independent of its activity as a kinase. Although T794 is within a consensus Akt phosphorylation site, we were unable to identify a physiological activator of Akt that could induce T794 phosphorylation, suggesting that Akt is not the physiological Tie1-T794 kinase. However, the small GTPase Ras-related C3 botulinum toxin substrate 1 (Rac1), which is required for angiogenesis and capillary morphogenesis, was found to associate with phospho-T794 but not the non-phosphorylatable T794A mutant. Pharmacological activation of Rac1 induced downstream activation of p21-activated kinase (PAK1) and T794 phosphorylation in vitro, and inhibition of PAK1 abrogated T794 phosphorylation. Our results provide the first demonstration of a signaling pathway mediated by Tie1 in endothelial cells, and they suggest that a novel feedback loop involving Rac1/PAK1-mediated phosphorylation of Tie1 on T794 is required for proper angiogenesis.</p> / Dissertation

Pathological angiogenesis is an essential component of tumor growth, development, and metastasis for which few effective therapeutic options exist. Though many cancer therapies target the function of cell surface receptors, mechanisms regulating membrane receptor crosstalk remain unclear. Two important families of receptors in angiogenesis, the Ties and Integrins, respond to the extracellular environment via outside-in and, in the case of Integrins, also inside- out signaling. Recent reports showed that the endothelial specific tyrosine kinase receptor, Tie2, forms complexes with two of the endothelial Integrin heterodimers, α5β1 and αVβ3, providing a convenient mechanism for the integration of extracellular stimuli. Our data confirm the interaction between Integrins and Tie2 and additionally indicate an interaction with the orphan co-receptor Tie1. To elucidate the biological role of these macromolecular complexes, biochemical and biophysical methods including co-immunoprecipitation, FRET microscopy, and cellular based assays were used to follow receptor/Integrin association in response to the Tie2 ligands Angiopoietin-1 and -2 as well as the Integrin ligand fibronectin. Furthermore, structural analysis by small angle x-ray scattering of Tie2-ligand complexes and specific Integrin and Tie complexes are being used to identify the basis for growth factor receptor and Integrin signal transduction.

<p>A functioning vasculature is critical for the supply of nutrients to other systems as well as a host of physiologic and pathologic processes. Vascular development and maintenance are tightly regulated by a number of signaling processes, among which the Tie proteins are two functioning receptors. Although they have been shown to exhibit essential roles in endothelial cell sprouting and quiescence, the mechanistic details of Tie interactions and the effects of their associations with the Angiopoietins have not been elucidated. Studies in this thesis investigated the effects of Ang2 on Tie1 activation, signaling, and cellular responses within the context of both native and immortalized endothelial cells. Additionally, we investigated the role of Ang2 in the cellular reorganization and subsequent downregulation of Tie1. We observed that Ang2, but not Ang1, induces phosphorylation of Tie1 in endothelial cells and that the extracellular domain of Tie2 is required for Ang2-mediated activation of Tie1. Furthermore, we demonstrated that Tie1 activation leads to signaling through the Akt axis, and the consequent stimulation of anti-apoptotic and pro-proliferative cellular effects. Additionally, we demonstrated that Ang2 induces a concentration and time dependent downregulation of Tie1, and that Tie2's role in the process appears to be recruitment of the ligand to the multimeric Tie complexes. Interestingly, although Ang2 stimulation is necessary, we demonstrated that Ang2 activation of Tie1 receptor complexes is not required for ligand induced downregulation of the receptor. Finally, we characterized the modulatory role of Tie1 with regards to Angiopoietin signaling through Tie2, and observed that removal of Tie1 from the surface of endothelial cells induces Ang2 activation of Tie2 leading to increases in cell survival signaling cascades. Taken together, these data shed new light on Angiopoietin signaling through the Tie receptors, further characterize the interactions between Tie1 and Tie2, suggest novel forms of endothelial cell regulation within developing and mature vasculature, and may have implications in signaling within a host of physiologic and pathologic states.</p> / Dissertation

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