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The Hippo Pathway is a highly conserved regulator of tissue growth and size determination, limiting the activity of the transcriptional co-activator Yorkie (Yki), which promotes proliferation and inhibits apoptosis. Hippo signalling integrates and transduces cell polarity and cell-cell adhesion inputs thereby responding to the state of tissue architecture. The transmembrane apical polarity protein Crumbs (Crb) controls the activity of Yki by regulating Expanded (Ex), a protein that promotes Hippo signalling through kinase-dependent and -independent mechanisms to robustly inhibit Yki activity. Crb plays a dual role in the regulation of Ex by controlling its apical localisation, facilitating Yki inhibition, and by promoting Ex degradation, thus activating Yki. Crb regulates the stability of Ex by stimulating a phosphorylation-dependent ubiquitylation and proteasomal degradation. Characterisation of the precise mechanisms by which Crb regulates Ex has been the focus of this thesis. Based on candidates identified by mass spectrometry and from literature, the Casein Kinase 1 (CK1) family of kinases, and the deubiquitylating enzyme (DUB) Usp2 have both been identified as novel regulators of Ex stability. CK1s promote Ex phosphorylation and degradation, acting as Ex inhibitors, while Usp2 promotes Ex function by promoting its stabilisation. Furthermore, in a screen to identify DUBs that regulate Drosophila adult wing size, CG10889 has been established as a novel regulator of growth that interacts with members of the Hippo pathway.

Thesis (Ph. D.)--Princeton University, 1923. / Bibliography: p. 66-69.

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No description available.

Au cours du développement, les organismes croissent de façon harmonieuse suivant un programme génétique intrinsèque et en adaptation avec les conditions environnementales. Chaque tissu atteint une taille cible qui est proportionnelle à la taille finale des autres organes et à celle de l’organisme. Des expériences de régénération effectuées sur différents modèles animaux ont révélé que chaque organe possède un programme autonome de croissance. Ainsi, des mécanismes de coordination entre la croissance tissulaire et le programme de développement sont nécessaires afin d’assurer une régulation fine de l’allométrie avant le passage du stade juvénile au stade adulte fixant la taille finale du corps. Dilp8 (Drosophila Insulin Like Peptide 8) est une hormone analogue aux peptides de la famille des insulines/relaxines. Elle est produite par les tissus dont la croissance est lésée, en régénération ou néoplasique. dilp8 inhibe la production d’hormone stéroïde et retarde ainsi le passage à la forme adulte. Les mutants dilp8 présentent une augmentation du bruit développemental qui se traduit par une perte de la symétrie bilatérale des organes (asymétrie fluctuante FA). Ceci suggère un rôle de dilp8 dans la coordination de la croissance entre les organes. Au cours de ma thèse, j’ai pu montrer que la voie de signalisation Hippo, son activateur transcriptionnel Yorkie et son co-partenaire Scalloped régulent directement les niveaux transcriptionnels de dilp8 via un Hippo Responsive Element (HRE) présent dans le promoteur de dilp8. La voie Hippo joue un rôle clé dans le contrôle de la taille des organes en couplant les paramètres biomécaniques des tissus avec la prolifération cellulaire. / Growth of different body parts needs to be coordinated and scaled with the overall body size to give rise to adults of correct proportions. Since different organs follow autonomous growth programs, mechanisms must operate to ensure that each organ has reached an appropriate size before proceeding through developmental transitions. We recently identified Dilp8 (Drosophila insulin-like peptide 8) as a key hormone in coupling organ growth with animal maturation. Dilp8 is secreted from abnormally growing tissues and acts on the brain complex to delay pupariation. In addition, dilp8 mutant flies exhibit elevated fluctuating asymmetry (FA) demonstrating a function for Dilp8 in coordinating organ growth and ensuring developmental stability. Identifying signals that control dilp8 expression is therefore likely to provide a better understanding of organ size assessment mechanisms. The Hippo tumour suppressor pathway plays a major function in restricting organ growth by promoting cell cycle exit and apoptosis. Hippo signalling is responsive to the mechanical forces operating in growing organs making it an ideal candidate for assessing organ size. Activation of the Hippo pathway restricts nuclear translocation of the transcriptional co-activator Yorkie (Yki), which together with its DNA-binding partner Scalloped (Sd), regulates downstream growth-promoting target genes. Using a molecular biology approach, we show that Yki/Sd directly regulate dilp8 expression through a Hpo Responsive Element (HRE) in the dilp8 promoter.

La voie Hippo est une voie conservée impliquée dans la croissance des tissus et la suppression de tumeurs. Des études ont démontré son implication dans le développement des cancers chez l'homme. Cette cascade contrôle l'activité du co-activateur transcriptionnel Yorkie (Yki) chez la drosophile et de la protéine YAP (Yes Associated Protein) chez les mammifères. En raison de l'épissage alternatif de leur transcrits, les protéines Yki et YAP existent sous deux isoformes contenant un domaine WW (Yki1/YAP1) ou deux (Yki2/YAP2). Puisque les domaines WW sont essentiels pour l’interaction avec des partenaires spécifiques, l’inclusion alternative de ce domaine dans la protéine Yki/YAP peut remodeler leur réseau d’interaction et donc leur activité. La régulation et les conséquences fonctionnelles de l’épissage alternatif de yki / YAP in vivo sont inconnues.Dans le cadre de ce doctorat, nous avons constaté que la déplétion du facteur d’épissage B52 chez la drosophile réduit l’inclusion de l’exon alternatif dans l’ARNm de yki et favorise l’expression de l’isoforme Yki1 aux dépens de l’isoforme Yki2. La déplétion en B52 dans l'aile réduit la croissance et l'activité de Yki. Nous montrons que l'isoforme Yki1 est une version atténuée de la protéine Yki qui peut entrer en concurrence avec l'isoforme Yki2 dans le noyau. Pour déterminer le rôle de l’épissage alternatif de yki in vivo et l'importance de l'isoforme courte Yki1, nous avons abrogé cet épissage en utilisant la technologie CRISPR/Cas9 et avons créé des mouches capables d'exprimer uniquement l'isoforme Yki2. Ces mouches yki2only sont viables mais présentent un phénotype aléatoire d’ailes asymétriques. Cette augmentation de l'«asymétrie fluctuante», qui traduit une déviation par rapport au développement normal, suggère que l’épissage alternatif de yki est crucial pour la stabilité développementale. Ces résultats mettent en évidence un nouveau niveau de modulation de la voie Hippo via l’épissage alternatif de yki.L'inclusion alternative du deuxième domaine WW est une caractéristique conservée entre Yki et YAP. Cela conforte l'idée que les isoformes Yki1 et YAP1 ont une fonction importante in vivo et que l'épissage alternatif de yki/YAP est un mécanisme conservé de contrôle de la voie Hippo. Cette étude ouvre de nouvelles perspectives pour la modulation de la voie Hippo dans les cellules cancéreuses en modifiant l’épissage alternatif de YAP. / The Hippo pathway is a conserved pathway involved in tissue growth and tumor suppression. Studies have demonstrated its significance in the development of human cancers. This cascade controls the activity of the transcription co-activator Yorkie (Yki) in flies and Yes-associated protein (YAP) in mammals. Due to Alternative Splicing (AS), both Yki and YAP proteins exist as two isoforms containing one (Yki1/YAP1) or two (Yki2/YAP2) WW domains. Since WW domains are essential for interaction with specific partners, the alternative inclusion of this domain in Yki/YAP protein may remodel their interaction network and therefore their activity. The regulation and functional consequences of AS of yki/YAP in vivo are unknown.In this Ph.D. project, we identified that depletion of splicing factor B52 in Drosophila lowers inclusion of the alternative exon in yki mRNAs and favors the expression of Yki1 isoform at the expense of the Yki2 isoform. B52 depletion in the wing reduces growth and Yki activity. We demonstrate that Yki1 isoform is an attenuated version of Yki protein that can compete with Yki2 isoform in the nucleus. To ascertain the role of yki AS in vivo and the importance of short isoform Yki1, we abrogated this splicing by using CRISPR/Cas9 technology and created flies that can express Yki2 isoform only. yki2only flies are viable but display a random phenotype of asymmetric wing size. This rise in “fluctuating asymmetry” that is the consequence of subtle deviation from normal development, suggests that AS of yki is crucial for the development robustness. Taking together, these results highlight a new layer of modulation of Hippo pathway via AS of yki.Alternative inclusion of the second WW domain is a conserved feature between Yki and YAP. This further supports the idea that Yki1 and YAP1 isoforms have an important function in vivo and that AS of yki/YAP is a conserved mechanism of control of the Hippo pathway. This study opens up new perspectives for modulation of the Hippo pathway in cancer cells by altering YAP AS.

A cell that senses signals from its environment uses proteins for signal transduction via post translational modifications (PTMs) and protein- protein interactions (PPIs) from cell membrane into the nucleus where genes controlling cell proliferation, differentiation and apoptosis can be turned on or off, i.e. changing the phenotype or fate of the cell. Aberrations within such proteins are prone to cause diseases, such as cancer. Therefore, it is important so study aberrant signaling to be able to understand and treat diseases. In this thesis, signaling aberrations of PTMs and PPIs were analyzed with the use of the in situ proximity ligation assay (in situ PLA), and the thesis also contain method development of rolling circle amplification (RCA), which is the method used for signal amplification of in situ PLA reaction products. Paper I considers the integrity of RCA products. Here, the aim was to generate a smaller and more compact RCA product, for more accurate either visual or automated analysis. This was achieved with the use of an additional so called compaction oligonucleotide that during RCA was able to bind and pull segments of RCA products closer together. The compaction oligonucleotide served to increase the signal to noise ratio and decrease the number of false positive signals. The crosstalk between the Hippo and TGFβ signaling pathways were studied in paper II. Activity of the Hippo signaling pathway is regulated by cell density sensing and tissue control. We found differences in amounts and localization of interactions between the effector proteins of the two pathways depending on cell density and TGFβ stimulation. In paper III the NF-кB signaling pathway constitutively activated in chronic lymphocytic leukemia (CLL) was studied. A 4 base-pair frameshift deletion within the NFKBIE gene, which encodes the negative regulator IкBε, was found among 13 of a total 315 cases by the use of targeted deep sequencing. We found reduced levels of IкBε protein, decreased p65 inhibition, and increased phosphorylation, along with increased nuclear localization of p65 in NFKBIE deleted cases compared to healthy cases. Crosstalk between the Hippo and Wnt signaling pathway are studied within paper IV. Here, we found differences in cellular localization of TAZ/β-catenin interactions depending on colon cancer tumor stage and by further investigate Hippo/WNT crosstalk in cell line model systems we found an increase of complex formations involved in the crosstalk in sparse growing HEK293 cells compared to dense growing cells. Also, active WNT3a signaling was affected by cell density. Since cell density showed to have a big effect on Hippo/WNT crosstalk we continued to investigated the effect of E-cadherin, which has a function in cell junctions and maintenance of epithelial integrity on Hippo/WNT crosstalk. Interestingly, we found that E-cadherin is likely to regulate Hippo/WNT crosstalk.

The Hippo signaling pathway offers an intrinsic mechanism to control organ sizes, and dysfunction of this pathway can often lead to cancer. Great advancement has been made in recent years into understanding this pathway. Despite all this invaluable knowledge, much remains to be explored. Mass spectrometry offers an unbiased approach to characterize the interactome of any protein of interest and is particularly powerful for identifying potential novel regulators of signalling pathways. I therefore set out to characterize the interactome of all the Hippo pathway main components using mass spectrometry, with the goal of uncovering novel regulatory mechanism(s) of the Hippo pathway. In the end, I was able to identify over 250 novel interactors of the Hippo pathway in total. This study demonstrates the utility of mass spectrometry to identify novel regulators of the Hippo pathway and characterization of one such interactor.

The Hippo signaling pathway offers an intrinsic mechanism to control organ sizes, and dysfunction of this pathway can often lead to cancer. Great advancement has been made in recent years into understanding this pathway. Despite all this invaluable knowledge, much remains to be explored. Mass spectrometry offers an unbiased approach to characterize the interactome of any protein of interest and is particularly powerful for identifying potential novel regulators of signalling pathways. I therefore set out to characterize the interactome of all the Hippo pathway main components using mass spectrometry, with the goal of uncovering novel regulatory mechanism(s) of the Hippo pathway. In the end, I was able to identify over 250 novel interactors of the Hippo pathway in total. This study demonstrates the utility of mass spectrometry to identify novel regulators of the Hippo pathway and characterization of one such interactor.