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Oratie uitgesproken door prof.dr. B.E. Snaar-Jagalska bij de aanvaarding van het ambt van hoogleraar op het gebied van Cellulaire Tumorbiologie aan de Universiteit Leiden op maandag 11 december 2017

Advances in scientific techniques have provided researchers with exceptional new opportunities to identify and monitor changes between different cancer types, during different stages of progression, between individual tumor cells and in the surrounding stroma. The wealth of information that can be obtained from new scientific techniques places additional requirements on the conventional cancer models. New models that could be used to rapidly access the (potential) functional importance of newly identified (epi)genetic and proteomic changes and test the efficacy on emerging (combinatorial) therapies are desperately required. The distinctive characteristics of zebrafish are progressively being applied to create more relevant models of human diseases. Zebrafish embryos provide a powerful tool to develop functional cancer models. This is a tool that can be used from drug discovery and development to assessment of drug toxicity. This review will summarise the use of zebrafish xenograft models to study human cancers, and discuss the benefits and limitations of these models.

Zebrafish embryos can be obtained for research purposes in large numbers at low cost and embryos develop externally in limited space, making them highly suitable for high-throughput cancer studies and drug screens. Non-invasive live imaging of various processes within the larvae is possible due to their transparency during development, and a multitude of available fluorescent transgenic reporter lines. To perform high-throughput studies, handling large amounts of embryos and larvae is required. With such high number of individuals, even minute tasks may become time-consuming and arduous. In this chapter, an overview is given of the developments in the automation of various steps of large scale zebrafish cancer research for discovering important cancer pathways and drugs for the treatment of human disease. The focus lies on various tools developed for cancer cell implantation, embryo handling and sorting, microfluidic systems for imaging and drug treatment, and image acquisition and analysis. Examples will be given of employment of these technologies within the fields of toxicology research and cancer research.

Improved targeted therapies are needed to combat metastatic prostate cancer. Here, we report the identification of the spleen kinase SYK as a mediator of metastatic dissemination in zebrafish and mouse xenograft models of human prostate cancer. Although SYK has not been implicated previously in this disease, we found that its expression is upregulated in human prostate cancers and associated with malignant progression. RNAi-mediated silencing prevented invasive outgrowth in vitro and bone colonization in vivo, effects that were reversed by wild-type but not kinase-dead SYK expression. In the absence of SYK expression, cell surface levels of the progression-associated adhesion receptors integrin α2β1 and CD44 were diminished. RNAi-mediated silencing of α2β1 phenocopied SYK depletion in vitro and in vivo, suggesting an effector role for α2β1 in this setting. Notably, pharmacologic inhibitors of SYK kinase currently in phase I–II trials for other indications interfered similarly with the invasive growth and dissemination of prostate cancer cells. Our findings offer a mechanistic rationale to reposition SYK kinase inhibitors for evaluation in patients with metastatic prostate cancer. Cancer Res; 75(1); 230–40. ©2014 AACR.

Dictyostelium cells use extracellular cyclic AMP both as a chemoattractant and as a morphogen inducing cell-type-specific gene expression. Cyclic AMP binds to surface receptors, activates one or more G-proteins, and stimulates adenylate cyclase, guanylate cyclase and phosphoinositidase C. Mutant fgdC showed aberrant chemotaxis, and was devoid of cyclic AMP-induced gene expression and differentiation. Both the receptor- and G-protein-mediated stimulation of adenylate cyclase and guanylate cyclase were unaltered in mutant fgdC as compared to wild-type cells. In wild-type cells phosphoinositidase C was activated about twofold by the cyclic AMP receptor. In mutant fgdC cells, however, the enzyme was inhibited by about 60%. These results suggest that phosphoinositidase C is regulated by a receptor-operated activation/inhibition switch that is defective in mutant fgdC. We conclude that activation of phosphoinositidase C is essential for Dictyostelium development.

Transmembrane signal transduction was investigated in four Dictyostelium discoideum mutants that belong to the fgd A complementation group. The results show the following. (a) Cell surface cAMP receptors are present in fgd A mutants, but cAMP does not induce any of the intracellular responses, including the activation of adenylate or guanylate cyclase and chemotaxis. (b) cAMP induces down-regulation and the covalent modification (presumably phosphorylation) of the cAMP receptor. (c) The inhibitory effects of GTP gamma S and GDP beta S on cAMP binding are reduced; the stimulatory effect of cAMP on GTP gamma S binding is lost in fgd A mutants. (d) Basal high-affinity GTPase activity is reduced 40% and the stimulatory effect of cAMP is decreased from 40% in wild type to 30% in fgd A. (e) GTP-mediated stimulation and inhibition of adenylate cyclase is normal in mutant membranes. The results suggest a defective interaction between cell surface cAMP receptors and a specific G-protein in fgd A mutants. This interaction appears to be essential for nearly all signal transduction pathways in Dictyostelium discoideum.

Glucocorticoids are potent anti-inflammatory drugs widely used clinically to treat various inflammatory and immune conditions. However, two main clinical problems limit their use. GCs trigger severe side effects and they induce acquired glucocorticoid resistance, especially during chronic systemic treatment [6-11]. Therefore, developing novel strategies to improve the treatment inflammatory and immune conditions becomes urgent. Here, we present evidence that ginsenosides act as selective GR agonists, antagonists, and prodrugs. Moreover, our data illustrate that the number and positions of glucose groups bound to the steroid backbone of ginsenosides and different chemical structures are essential factors underlying the differential mechanistic effects of ginsenosides. Understanding the molecular mechanisms and the effects of natural compound (ginsenosides) opens a novel road towards developing improved anti-inflammatory GCs. Attaching one glucose group to classical GCs produced selective Gr agonists, such as GDex and GPdn, which exert anti-inflammatory effects without triggering side effects due to the absence of GR transactivation activities. Attaching two glucose groups to classical GCs resulted in the creation of GC prodrugs, such as GbPdn, which mediates its action locally at the site of inflammation, dependent on GBA2 activity, without triggering side effects.

The way organisms develop from the initial single-cellular state to a complex final assembly like the human body, and how the final body is maintained throughout life, is one of the greatest mysteries and it’s understanding one of the biggest scientific challenges. Lately, it came as a surprise that the initial assembly and the later maintenance of integrity is not only determined by intricate biochemical communication networks, but in part by physical forces that cells, their neighbors, and their environment apply in a bidirectional manner. The resulting collectivity of cell behavior determines the development of organisms, and are crucial to the health and disease state of the organism.In this thesis, we developed and utilized concepts from physics to quantitatively understand forces that develop between cells and their environment, and towards neighboring cells, and how the interplay between these forces regulates the arrangement, shape, and topology of tissue. The topics range from the development of novel experimental methods to the combination of experimental observations with theoretical descriptions. Our results contribute to a better understanding of cell and tissue integrity.

Death in all types of melanomas is generally caused by metastasis. Uveal melanoma (UM) is the most common intraocular melanoma, there are currently no (patient-derived) animal models that faithfully recapitulate metastatic dissemination of UM. Here we generate embryonic zebrafish models for both the primary and disseminated stage of ocular melanoma. In doing so we can recapitulate the etiology of cancer in its totality. Subsequently, we developed a patient-derived zebrafish xenograft (zf-PDX) model, using spheroid cultures generated from metastatic and primary UM tissues. Harnessing this versatile model, we reveal high sensitivity of circulating UM cells to ferroptosis induction in vivo by Erastin and RSL3, implicating ferroptosis as a new potential therapy in metastatic UM.Increased melanin levels in cutaneous melanoma are associated with decreased patient survival. Melanin levels in primary uveal melanoma patient cells positively correlate with their metastatic potential in zebrafish. Modulation of melanin levels of pan-melanoma cells results in enhanced/reduced metastatic potential upon increased or decreased melanin levels, respectively. Melanin depletion sensitizes melanoma cells to ferroptosis inducers in zebrafish leading to a decreased metastatic burden. Collectively, our data identify melanin biosynthetic enzymes as potential future target to treat melanoma and show that melanin protects metastasizing melanoma cells from ferroptosis.

New drugs for use as tuberculosis (TB) treatment are needed due to the constrains of classical antibiotics against TB and the rise of antibiotic-resistant strains, making TB a harder and harder disease to treat. This thesis is focused on using the in vivo whole animalzebrafish embryo model for TB to evaluate potential anti-TB host-directed therapeutics (HDTs) arising from in vitro screens. Although in vitro screens for HDTs using cellular models can be performed at high throughput, a limiting step is the validation in whole animal models and translation of results to clinical applications. Due to the complex infection dynamics of mycobacteria, the use of whole animal models is indispensable in research into TB and the zebrafish model has contributed key findings about host-pathogen dynamics during mycobacterial infection. One of the most promising host targets of HDTs is autophagy, which is recognized as an important host-protective pathway. Boosting autophagy levels using HDTs could be a way to overcome the pathogen’s autophagy evasion strategies and could therefore be a promising therapeutic route. For this thesis we took advantage of the possibilities of the zebrafish embryo model for TB and the zebrafish toolkit to study several autophagy-modulating HDTs as potential anti-TB drugs.

The function of TLRs in innate immunity has aroused worldwide attention soon after its discovery. Because of the broad functions of TLR2 in innate immunity, the drive for the development of TLR2-targeted vaccines or therapeutic treatments has accelerated in the last decades. However, its dual role in both activation and suppression of innate immune responses makes it very difficult to use the available results from basic research for the development of clinical trials. In addition, it is still not clear what is the function of TLR2 in regulating phagocytic cell migration. Therefore, we aimed to determine the function of TLR2 in mycobacterial infection and explore its role in regulating phagocytic cell migration in inflammatory tissue by using a zebrafish larval model in this thesis. We showed that infection of a tlr2 mutant in zebrafish larvae leads to a higher mycobacterial burden, accompanied by a lower number of granulomas and increased extracellular bacterial growth. Through a tail fin wounding and tail fin infection zebrafish model, we demonstrated that tlr2 is involved in modulating leukocyte migration. This thesis provides a better understanding of the functions of TLR2 in innate immune responses to infection and tissue wounding.

The work included in this thesis is aimed at developing novel tools to advance our understanding of prostate cancer. The clinical problem of prostate cancer is presented and discussed in the wider context of the current clinical knowledge, highlighting the genetic mechanisms at its base. A dedicated chapter focuses on bone metastases, highly morbid feature of advanced prostate cancer, discussing the known mechanisms and the available models to study it in translational research. Then, moving from the molecular analysis of clinical specimens of bone metastasis, a biochemical pathway is identified and further studied in vitro, ex vivo and in vivo, validating the initial findings. A novel, early-stage prostate cancer patient-derived xenograft is presented and extensively characterized and implemented in a drug screening. This allowed to screen the effect of over 70 known drugs on prostate cancer models, using three-dimensional cultures and a semi-automated platform. As all research builds on previously established findings, a bibliometric analysis tool is presented, to assist in the generation of a knowledge network arranged by topic and impact of research. All these aspects and findings are then discussed in the context of the current direction of prostate cancer research, its emerging tools and its long-known challenges.

This thesis describes the possible applicability of mesenchymal stromal cells (MSCs) in the treatment of liver fibrosis. In CCL4 induced animal models for liver fibrosis, we showed that local administration of MSCs after partial hepatectomy, results in a dose‐dependent on‐site amelioration of fibrosis. Furthermore, we compared the pro-regenerative and anti-fibrotic effects of four different subpopulations of MSCs, categorized on Endoglin (CD105) and VCAM (CD106) membrane expression. Our results showed that VCAM-positive subpopulations of MSCs are superior compared to VCAM-negative subpopulations in relation to their anti-fibrotic and pro-regenerative properties. In another study we showed that TAA induce liver fibrogenesis in zebrafish embryos through mechanisms similar to man and mice. In addition, we found that MSCs ameliorate fibrogenesis in this model.CRIPTO-1 is an (onco)foetal protein and is correlated to poor prognosis in HCC. The observations of our HCC study are suggestive for the existence of a more aggressive subgroup of HCCs recognized by their high CRIPTO-1 expression which also seems to be resistant to Sorafenib treatment. Cell survival and cell proliferation are some of the processes stimulated by CRIPTO-1, which are also known to be important during liver regeneration and fibrogenesis. We identified that multiple species show enhanced CRIPTO-1 during fibrogenesis and that elevated CRIPTO-1 plasma levels in humans with cirrhosis normalize after liver transplantation.

Streptomyces are filamentous bacteria that produce more than two-thirds of known antibiotics. Due to their multicellular lifestyle as well as their prolific production of secondary metabolites, Streptomyces are of unique fundamental and applied importance. However, Streptomyces have unstable genomes, an attribute that can cause genomic rearrangements and dramatically alter their phenotype. Previous studies have failed to explain this phenomenon. In this dissertation, we investigate the evolutionary functions and mechanisms of genomic instability in Streptomyces coelicolor. We first find that a subpopulation of cells generated through large genomic rearrangements becomes specialized to produce antibiotics. This results in a division of labor which benefits the entire colony, while the yield and diversity of antibiotics are maximized, despite significant fitness costs to this altruistic subpopulation. Next, we show that these altruistic mutants continue to lose fitness due to the irreversible accumulation of large deletions and deleterious mutations, coupled to an increased mutation rate. Finally, we explore the molecular consequences of large genomic rearrangements for development and antibiotic production using detailed proteomics and metabolomics analyses, which highlight key pathways that are impacted by these genomic events. Overall, this dissertation provides new insights of genomic instability in Streptomyces.

Prostate cancer (PCa) is one of the most prevalent cancer in males. Although the majority of the patients can benefit from the present clinical treatments, 20%-30% of the patients who originally respond to the therapy still develop incurable, castration-resistance bone metastases, which is a main cause of death in PCa . In this thesis, I combined an advanced zebrafish xenograft model with in vitro cellular approaches and mice xenografts to study the early stage of PCa metastasis. Using this comprehensive esearch platform, I identified multiple key signaling pathways that play essential roles in promoting the onset of PCa metastatis. The pathways I discovered include Cripto-associated EMT plasticity, CDC-42-N-Wasp-Cortactin associated mechanosensing and mechanotransduction, microenvironment dependent NF-ĸB-Activin A signaling pathway, and AMPK-Autophagy dependent metabolic stress coping pathway.

In this thesis, we will utilize embryonic zebrafish tumour models to understand the interaction between engrafted human cancer cells and macrophages from the host, test drug administration modalities and anti-cancer efficacies of newly-developed PDT and PACT compounds, and test a light-triggered liposomal system for targeted drug delivery specifically to cancer cells in vivo. In chapter 2, we investigate the role of macrophages in tumour-induced angiogenesis. We show that macrophage-dependent angiogenesis is driven by macrophage recruitment to lactic acid secreted by glycolytic B16 melanoma cells. Chemical inhibition of macrophages and glycolysis blocks the initiation of angiogenesis in these models, suggesting that macrophages attracted to glycolytic melanoma cells contribute to the tumour-induced angiogenesis process.In chapters 3 and 4, we explore novel PDT and PACT compounds, respectively, for treatment of conjunctival melanoma in zebrafish. We inject conjunctival melanoma cells into the retro-orbital site to establish an orthotopic model and into the Duct of Cuvier to generate an ectopic model. Our results prove that zebrafish provides a fast vertebrate cancer model to test the optimal administration regimen of drugs, conditions of light irradiation, host toxicity and anti-cancer efficacy of PDT and PACT drugs against conjunctival melanoma.In chapter 5, we focus on modifying liposomes to be light triggered in order to deliver drugs specifically to cancer cells. We inject MDA231 breast cancer cells into the Duct of Cuvier at 2 days post fertilization (dpf) to initiate metastasis to the CHT. We successfully demonstrate that light-triggered, cell-specific delivery of liposome-encapsulated doxorubicin reduces the xenograft cancer cell burden without enhanced cytotoxicity of the zebrafish embryos. In chapter 6, we summarize the novel anti-cancer strategies, which we have developed using zebrafish xenograft models. In the same chapter, we frame our findings in the current scientific landscape and discuss future perspectives.

In this thesis, we aimed to better understand the underlying mechanisms involved in TNBC progression and metastasis formation and discover new targets to reduce breast cancer related deaths. We performed an imaging-based RNAi phenotypic cell migration screen in two highly motile TNBC cancer cell lines to provide a repository of signaling determinants that functionally drive TNBC cell motility. Interestingly, two modulators essential for cancer cell migration were known to be involved in RNA splicing, making us decide to focus on the role of RNA splicing in breast cancer progression. We next summarized the current knowledge about splicing factors in breast cancer development and progression and identified co-regulated splicing factors that were associated with aggressive breast cancer phenotypes and metastasis formation that was not only restricted to breast cancer, increasing the global understanding of the role of the spliceosome in cancer development and progression. Moreover, the role of splicing factors in two major processes in cancer progression, cell migration and proliferation, was examined. Finally, using RNA sequencing, we systematically compared the transcriptomes of 14 breast cancer cell lines cultured both in 2D and 3D conditions to unravel the reprogramming that is associated with the invasive phenotype of basal B TNBC.

The effective treatment of tuberculosis (TB) remains a major challenge to global health. Drug-resistant Mycobacterium tuberculosis (Mtb) strains and co-infection with HIV further increase the difficulty of controlling TB. Thus, under the current situation, it is essential to develop effective treatment strategies for Mtb infections. Autophagy is a lysosomal degradation process and substantial experimental evidence has demonstrated that autophagy is an important host immune defense mechanism against mycobacterial infection. However, the development of effective therapies requires a better understanding of the interaction between the host and invading pathogens to identify host processes that can be targeted. A useful tool for such studies is the zebrafish model for TB. Zebrafish can be infected with Mycobacterium marinum (Mm), which is closely related to Mtb and causes similar disease characteristics. Taking advantage of the zebrafish TB model, this thesis presents new in vivo evidence for the important function of autophagy to inhibit mycobacterial proliferation inside macrophages. Furthermore, this study supports that stimulating the innate host defense processes that are dependent on the autophagy modulator, Dram1, and the selective autophagy receptors, p62 and Optineurin, could be a useful strategy to explore for adjunctive treatment of antibiotic-resistant TB infections.

The focus of this thesis is uveal melanoma (UM) which, once metastasized, is lethal due to lack of effective treatment options. To repress p53 activity approximately 65% of UM tumors express high levels of the p53 inhibitory proteins MDMX or MDM2. The aim of this thesis is to unravel the oncogenic function of MDMX and provide new treatment options for patients with metastasized UM. Chapter 2 describes the regulation of the transcriptome by MDMX in UM and proposes novel p53-independent effects of MDMX, i.e. FOXO inhibition. In chapter 3 the opportunities of a combined targeting of two common signaling pathways as therapeutic intervention for metastasized UM patients is investigated. Genetic interference with either MDMX or PKC δ expression or activity showed that beneficial effects can already be achieved by a more specific targeting, which is presumable less toxic to the patient. In chapter 4 it is described, opposed to what has been reported before, that enhancer of zeste homolog 2 (EZH2) inhibition poses a valuable novel therapeutic invention for UM. In chapter 5 it is shown that combining two clinically approved drugs, Quisinostat and Flavopiridol, could serve as an effective therapeutic intervention for UM patients.

My PhD project studies how the temporally sequential Hox gene expression is regulated during head-tail patterning of the frog embryo. This question has not been solved and remains one of the greatest unsolved mysteries in developmental biology. Currently, a very attractive explanation attributes sequential Hox gene activation to progressive opening of the chromosome, because it parallels the linear arrangement of Hox genes on the chromosome. While promising, this explanation is surrounded by extra complexities that ensure Hox expression is synchronized between different cells. By using gain-of-function and loss-of-function approaches, I discovered that Hox-Hox interactions play a crucial role in the regulation of Hox expression, and for the first time, I dissected different roles for these interactions in axis formation. The findings are likely to resolve the above complexities, and reveal a new facet of the mechanisms underlying Hox gene regulation. Furthermore, by timed modulation of the actions of BMP signaling, I showed that there is a BMP-dependent timing mechanism in the head that could be continued by the Hox temporal sequence in the trunk. The two constitute an integrative timer which can be translated into spatial patterns of gene expression along the whole head-tail axis via a BMP/anti-BMP dependent mechanism.

Metastatic cancer is aggressive and rapidly developing, making it difficult to treat, often leading to mortality. Cancer cells are not isolated, but rather survive and proliferate in complex tumor microenvironments. Importantly, tumor cells “educate” immune cells to play a supportive role during cancer progression. Therefore, understanding how cancer cells and immune cells communicate is of pivotal importance to limit tumor progression. In this project, we identified the chemokine receptor CXCR4 and its correspondent ligand, CXCL12, as a key couple that regulate the interaction between tumor cells and immune cells. The research has been performed using the zebrafish xenograft model. This model offers the advantage of looking at tumor-immune cell interaction on a single cell level and in a living whole organism. The transparency of the embryo and the current genetic tools available, in combination with the ease in pharmacological approaches, make this model an excellent tool to determine and impair cancer-microenvironment inter-communication. We showed that inhibiting the CXCL12-CXCR4 axis either on the tumor side or on the immune cell side leads to impaired tumor progression, during the early phases of metastasis formation. We propose that blocking cancer-stroma communication represents a promising therapeutic strategy to fight cancer.

Ewing sarcoma (ES) is a special type of bone cancer, first described by Dr. James Ewing in his paper __Diffusive endothelioma of bone__. Today Ewing sarcoma represents the second most common bone cancer among adolescents and young adults. Contrary to the positive achievement in treatment of localized tumors, the long-term (5-years) survival for Ewing sarcoma patients with metastasis, however, remain below the 30% mark. In this thesis a report on experimental work aiming for a better understanding of the mechanisms underlying Ewing sarcoma metastasis is presented. Two distinct mechanisms are investigated: (1) a biochemical approach in which the initial steps in the CXCR4 signaling cascade are followed, and (2) a biophysical approach in which the guidance of Ewing sarcoma metastasis by the stiffness of their microenvironment is demonstrated. The results presented in this thesis provide deeper insights into the mechanisms controlling signaling of the chemokine receptor CXCR4 and into the role of the micro-environment in Ewing sarcoma cells behavior.Through various experimental approaches it was shown that both biochemical and biophysical guidance control how Ewing sarcoma develops into its distinct metastatic phenotype.

Ewing sarcoma (EWS) is a disease with a high need for novel therapeutic strategies. To aid in investigating such compounds in an in vivo setting, we have developed several zebrafish model systems for EWS, which are presented in this thesis. The first is a manual xenograft model in 2-day-old zebrafish embryos, in which cell behaviour on the level of proliferation and migration has been described, as well as their interaction with the host innate immunesystem. With this model, we found that simultaneous induction of the p53 pathway and disruption of EWS-FLI1 transcriptional activity had an additive effect on the reduction of EWS malignancy in vivo. The same model was used to explore the mechanism of NOTCH-induced tumour suppression, via Sirtuin1 (SIRT1) inhibition. In addition to this manual xenograft model, we also established a high-throughput automated implantation model in blastula stage embryos. Furthermore, a flexible transgenic model has been developed, where human EWSR1-ERG expression has been placed under an UAS promoter, allowing the gene to be expressed in different tissues at different times. When this EWS-driving fusiongene was expressed neuronally, overlap in gene and protein expression was found with other EWS models, as well as a histologic similarity to EWS tumours.

Cells communicate in multicellular organisms; by secreting and sensing signals, in order to adjust their behavior to the environment. Extracellular signals such as cytokines and growth factors bind to cell surface receptors and trigger the activation of multiple protein signal transduction cascades that mediate cellular responses such as proliferation, differentiation, apoptosis and motility. The Mitogen-activated protein kinase (MAPK) family is a group of homologous proteins forming several linear signal transduction pathways. The MAPK family is conserved among eukaryotes and most vertebrates contain at least 14 MAPKs. We are interested in the molecular mechanisms of MAPK signalling that facilitate proper development of the zebrafish embryo. The zebrafish is an excellent model to delineate MAPK associated embryonic processes. The bodyplan is completed within 24hours and within a week most organs are formed. With the current available zebrafish tools molecular mechanisms could be identified and linked to cellular processes and morphological observations. We constructed constitutive active zebrafish Erk2 mutants for identification of new signalling events. We completed expression analysis of all P38 isoforms by expression analysis of the P38_ and P38_ isoforms. We also performed a comparative study between P38_ and Erk2 MAPK in gastrulation. Finally, We addressed the role of P38_ in zebrafish angiogenesis.

Chemokines are signaling proteins that direct the migration and function of many cell types during development and responses of the immune system. The zebrafish embryo model is highly suited to investigate cell migration because of its optical transparency and availability of transgenic lines with fluorescently labeled cell types of the innate immune system. In this thesis, we analyzed the phylogenetic relationships between zebrafish and human chemokines and used Salmonella and Mycobacterium infection models to study the function of a zebrafish chemokine receptor gene, cxcr3.2, homologous to human CXCR3. Our data have demonstrated that cxcr3.2 is predominantly expressed in macrophages of the zebrafish embryo and plays an essential role in bacterial-induced macrophage migration and control of local infections. Furthermore, we used in vivo cell migration assays in wild type and cxcr3.2 mutant embryos to discover the putative ligand of the cxcr3.2 receptor. Injection of Cxcl11, an infection-inducible chemokine, resulted in the directional migration of macrophages in a cxcr3.2-dependent manner. As the first ligand-receptor pair with a proposed function in migration of zebrafish macrophages in response to bacterial infection, the identification of Cxcl11-Cxcr3.2 interaction is an important step towards understanding the chemokine signaling network underlying innate immune responses in the zebrafish model.

Multiple processes are influenced by a protein family named the function of Mitogen Activated Protein Kinases (MAPK), where among tissue morphogenesis, cell-proliferation, cell-differentiation, cell-survival and immune reactions. A consequence of this broad role of these proteins is that disturbed MAPK signaling possibly leads to severe affects, and could even lead to the development of a disease or disturb embryogenesis. The human genome encodes for 14 mapk genes. One of the best characterized MAPK signaling transductions pathways is the Extracellular signal Regulated Kinase 1 and 2 (ERK1/ERK2). The zebrafish is a relatively new, genetic accessible model organism, which is because of their transparency of the embryos, very suitable to study the role of the MAPKs in embryonic development. The study described in this thesis shows that the zebrafish genome encodes for the same mapk genes, which makes it possible to study the functions of these proteins, using the zebrafish. Further investigations to the roles of ERK1 and ERK2 during embryonic development led to the new insight that ERK1 and ERK2 have distinct roles during crucial developmental cell migration processes and the differentiation of tissues and cell types. In addition, it was found that ERK1 and ERK2 affect the expression of common as well as distinct target genes.