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Doctor of Philosophy / Department of Biology / Alexander E. Beeser / Multiple signaling pathways regulate development of the posterior zebrafish body, which is derived from a population of progenitor cells called the tailbud, a structure formed at the end of gastrulation. Fate specification and differentiation are closely linked with cell migration to ensure that, as some cells exit the tailbud and differentiate, other cells are retained in the tailbud as undifferentiated precursors to support later growth. The role of BMP signaling in specifying cell fate in the tailbud has been well-characterized. Among the lost ventral tissues like ventral tailfin and cloaca, embryos with compromised BMP signaling produce a curious phenotype-a ventrally located secondary tail containing both somitic muscle and notochord. This phenotype is proposed to be a fate-patterning defect when the BMP gradient lowered to a precise level. However, this morphogen mode is insufficient to explain secondary tail formation without considering BMP also regulates morphogenetic movements during gastrulation, promoting the convergence of lateral mesodermal cells towards the dorsal axis. In this study, we provide evidence that BMP signaling continues to mediate cell movements during tail development. Our data indicate that BMP signaling is activated in the ventroposterior tailbud to promote cell migration during tailbud protrusion, and that it is the defective migration of these cells which ultimately leads to bifurcation of the CNH domain, a presumptive stem cell pool in the tailbud, and formation of a secondary tail in BMP mutants. In parallel, the morphogenesis of tailbud cells is known to be under the control of noncanonical Wnt signaling, although the exact nature of the defect remains unclear. We find that inhibition of noncanonical Wnt signaling also leads to secondary tail formation. Additionally, we show that noncanonical Wnt signaling interacts with BMP signaling to maintain CNH integrity by affecting cadherin localization in CNH cells, possibly disrupting cell cohesion. We propose a model that BMP and a noncanonical Wnt pathway regulate tail morphogenesis by controlling cell migration and cell adhesion within the tailbud.

Die Wnt/ β-Catenin-Signaltransduktion nimmt eine exponierte Stellung in der kolorektalen Karzinogenere ein. Niclosamid ist ein Derivat der Salicylsäure, das bei Bandwurm- infektionen eingesetzt wird. Es konnte gezeigt werden, dass Niclosamid den Wnt/ β-Catenin-Signalweg moduliert. Ziel der vorliegenden Arbeit war es, den therapeutischen Einsatz des Niclosamids beim kolorektalen Karzinom zu evaluieren. Die Zellproliferation von kolorektalen Karzinomzelllinien (humane SW480 und SW620 Zellen sowie CC531 Zellen einer Ratte) und von Rattenfibroblasten wurde nach 12 und 24 Stunden Inkubation mit Niclosamid durch lichtmikroskopische Zellzahlbestimmungen beurteilt. Die Apoptoseraten wurden mit einem Zelltod-Assay ermittelt. Eine Immunfluoreszenzfärbung gab Aufschluss über das Expressionsmuster von aktivem β-Catenin. Die Promotoraktivität des LEF/ TCF-Transkriptionsfaktors wurde nach Transfektion mit TOPflash mithilfe eines Luciferase Assays analysiert. Die Genexpression von Wnt-modulierenden Faktoren (Bcl-9 und Wif1), von Komponenten des ß-Catenin- Degradationskomplexes (Axin2 und GSK 3β), von kanonischen Zielgenen (Met, MMP7 und Cyclin D1) und von c-jun als Schlüsselprotein des nicht-kanonischen Wnt/ JNK-Signalweges wurde in der RT-PCR untersucht. Auf Proteinebene wurden zur Bestätigung zusätzlich Western Blots mit Antikörpern gegen aktives β-Catenin und c-jun durchgeführt. Die Zellproliferation kolorektaler Karzinomzelllinien wurde dosisabhängig inhibiert, und Niclosamid führte zu Apoptose. Nach Inkubation mit Niclosamid kam es nicht zur Umverteilung von aktivem β-Catenin von der nukleären in die zytosolische Fraktion. Die Wnt-Promotor-Aktivität von LEF/ TCF wurde nach 12 Stunden Inkubation mit 10 und 50 μM Niclosamid jedoch signifikant gesenkt. Kanonische Wnt-Zielgene (Met, MMP7 und Cyclin D1) sowie der Koaktivator Bcl-9 wurden auf Transkriptionsebene gehemmt, während das nicht-kanonische Schlüsselprotein c-jun aktiviert wurde. Fasst man zusammen, so führt die Inkubation mit Niclosamid zu inhibitorischen Effekten auf kolorektale Karzinomzelllinien und zu einer reduzierten kanonischen Wnt-Aktivität. Diese Effekte können durch eine gestörte Formation des Triple-Komplexes aus Bcl-9, β- Catenin und LEF/ TCF und einer Aktivierung von c-jun und damit des nicht-kanonischen Wnt/ JNK-Signalweges bedingt sein. In in vivo-Untersuchungen beabsichtigen wir, in einem Tiermodell die Daten zu verifizieren und so den Einsatz des Niclosamids als Option für Patienten mit metastasiertem kolorektalem Karzinom weiterführend zu beurteilen.

Neural tube defects (NTDs) are a world health issue, affecting approximately 1 in every 1000 live births. These congenital defects arise from the improper closure of the neural tube during development, resulting in significant, life-threatening malformations of the central nervous system. Although it has been observed that supplementing women of child-bearing age with folates greatly decreases the chances of having an NTD affected baby, unfortunately these defects still occur. It is accepted that these complex disorders arise from a combination of genetic, environmental, and dietary influences. One such dietary influence is the one-carbon metabolism metabolite, homocysteine. Homocysteine is a byproduct of methylation reactions in the cell that exists in an inverse homeostasis with folate. Homocysteine can also undergo a transformation that allows it to then react with exposed lysine or cysteine residues on proteins, in a process known as N-homocysteinylation or S-homocysteinylation respectively. High levels of homocysteine have been long correlated with many disease states, including NTDs. One potential mechanism by which homocysteine confers its negative effects is through protein N-homocysteinylation. Here, a novel and high-throughput assay for N-homocysteinylation determination is described. This assay is shown to be accurate with mass spectrometry then shown to be biologically relevant using known hyperhomocysteinemia mouse models. This assay was then applied to a cohort of neural tube closure staged mouse embryos with two different genetic mutations that have previously been shown to predispose mice to NTDs. The genotypes explored here are mutations to the LRP6 gene and the Folr1 gene, both of which have been described as folate-responsive NTD mouse models. It was seen that maternal diet and embryonic genotype had the largest influence on the developmental outcome of these embryos; however, the inverse relationship between folate and homocysteine seemed to be established at this early time point, emphasizing the importance of the balance in one-carbon metabolism. One of these genes, LRP6, was then explored in a human cohort of spina bifida cases. Four novel mutations to the LRP6 gene were found and compared to the mouse model used in the previous study. One of the mutations found in the human population was seen to mimic that of the LRP6 mouse model, therefore expanding the potential of this NTD model. / text

Molecular Mechanisms of Hematopoietic Stem Cell Development- The Role of Retinoic Acid Signaling Bhaskar Chanda For the Doctor of Philosophy Medical Biophysics University of Toronto 2013 Abstract During mouse embryonic development, the formation of blood or hematopoiesis occurs in multiple phases. The first phase or primitive hematopoiesis generates a restricted subset of blood cell lineages but is devoid of lymphoid and hematopoietic stem cell (HSC) potential. The next phase of hematopoiesis, also known as definitive hematopoiesis, is characterized by its ability to generate multilineage hematopoietic progenitors and HSCs from a specialized population of endothelial cells known as hemogenic endothelium (HE). Such endothelial to hematopoietic transitions (EHT) have been recently observed at a clonal level, however, molecular mechanisms that underlie EHT leading to the specification of HSCs have remained poorly understood. Here we show that retinoic acid (RA) signaling plays a pivotal role in embryonic hematopoiesis and HSC development. RA signaling inhibits primitive hematopoiesis, and promotes definitive hematopoiesis. This inductive effect of RA signaling extends to the specification of HSCs. Activation of the RA signaling pathway ex vivo in AGM-derived HE dramatically enhanced the repopulating potential, whereas its conditional inhibition in vivo abrogated HSC development. These repressive and inductive effects of RA signaling were mediated primarily via retinoic acid receptor (RAR)- α. We further analyzed the mechanistic basis of RA signaling with a combined use of cellular, molecular and biochemical assays, and show that β-catenin dependent Wnt signaling is the downstream mediator of RA signaling. Collectively, this thesis provides new insight into molecular mechanisms that control embryonic hematopoiesis and identify the RA pathway as a key regulator of definitive hematopoiesis and HSC specification.

Medulloblastomas are tumours of cerebellar origin and are thought to arise from the malignant transformation of progenitor cells in the developing cerebellum. A number of developmental signalling pathways are required for the precise cell specification, proliferation, migration and differentiation involved in forming the mature cerebellum and it is the dysregulation of these processes that can lead to the eventual formation of a tumour. Genes encoding components of the canonical Wnt/β-catenin signaling pathway are mutated in around 15% of medulloblastomas and germline mutations that activate this pathway are known to predispose to medulloblastoma. Despite this, the contribution of Wnt/β-catenin signaling to normal cerebellum development is not yet well understood and the developmental origins of medulloblastoma arising from activation of this pathway are only beginning to be revealed. Therefore, the aims of this thesis were to characterise the spatio-temporal nature of Wnt/β-catenin signalling during cerebellum development and to investigate its function, with the broad goal of informing our understanding of how medulloblastoma arises from oncogenic activation of Wnt/β-catenin signalling. To address the first aim I utilised a LacZ expressing Wnt/β-catenin signalling reporter mouse to characterize the spatio-temporal pattern of Wnt/β-catenin pathway activation during cerebellum development. Analysis of LacZ reporter expression revealed a pattern of transient Wnt/β-catenin activity in discrete cell populations throughout cerebellum development. I found that Wnt/β-catenin activity is present during the early specification of granule cells at the cerebellar rhombic lip but not during the expansion of this cell population at later stages. During perinatal development Wnt/β-catenin activity shifts to the cerebellar ventricular zone, a known germinal centre for GABAergic interneurons and glia, and was observed in cells radiating out from this region. By early postnatal development the expression of the Wnt/β-catenin reporter became progressively restricted to the developing Bergmann glia population. To investigate the function of Wnt/β-catenin in these cell lineages and how its dysregulation could contribute to medulloblastoma, I used a combination of ex vivo organotypic culture, in utero electroporation and tissue-specific gene targeting to manipulate components of the pathway. Culturing slices of E18.5 cerebellum in the presence of small molecule activators of the Wnt/β-catenin pathway revealed a reduction in the expression of glial markers Sox9 and GFAP. In addition, interneuron lineage marker Pax2 was also reduced, supporting the conclusion that dysregulation of Wnt/β-catenin signalling affects the generation of cell lineages from the ventricular zone. To investigate this hypothesis further, I constitutively activated the Wnt/β-catenin signalling pathway in the developing cerebellum using Cre-Lox gene targeting to knock out Apc, a negative regulator of the pathway, in ventricular zone derived lineages. Cre-induced recombination of Apc resulted in nuclear accumulation of β-catenin, a sign that the pathway had become ectopically activated. Furthermore, a reduction in the expression of Sox9 and Pax2 was also observed in these mutant cells. From these data, I conclude a potential role for Wnt/β-catenin signaling in the regulation of glial/interneuron progenitors. Combined, these data support a model where Wnt/β-catenin signalling could perform multiple functions in specification of the granule lineage, regulation of glial/interneuron progenitors and in glial differentiation/maturation. Importantly, dysregulation of progenitor self-renewal and differentiation is widely acknowledged to promote tumourigenesis. Thus, the data in this thesis support a potential mechanism for the development of medulloblastoma from the dysregulation of ventricular zone progenitors.

The liver is the largest solid organ in the body and is frequently the site of injury. During disease, liver injury is usually compensated for by exceptionally efficient regeneration which occurs both from differentiated epithelia and also from an undifferentiated cell population with stem cell like qualities known as hepatic progenitor cells (HPCs). HPCs are particularly active during massive or chronic liver injury and therefore are an attractive target for much needed novel therapies to enhance regeneration in patients for whom the only current effective therapy is liver transplantation. Stem cells in other organs systems are believed to reside in a specialised microenvironment or niche which supports their maintenance and function. To investigate the hypothesis that HPCs are supported by a functional niche and are capable of regenerating hepatocytes, we commenced by establishing a number of murine in vivo models. Having shown a stereotypical niche, consisting of macrophages, myofibroblasts and laminin exists in both animal models and human disease, we investigated the active recruitment of extrahepatic cells into this niche and showed that macrophages are actively recruited from the bone marrow during liver injury. Macrophages were shown to influence HPC behaviour during injury. Furthermore using macrophages as a cellular therapy, induced HPC activation with corresponding changes to liver structure and function. Investigation of signalling pathways revealed and confirmed a TWEAK dependent activation of HPCs following macrophage transfer. Having demonstrated the potential for macrophage therapy via HPC activation, we aimed to study the ability of HPCs to regenerate the hepatic parenchyma. To do so we developed and characterised a novel model of hepatocellular injury and HPC activation. Using the genetic labeling of hepatocytes in this model we were able to show rapid and large scale repopulation of hepatocytes from a precursor source with HPCs being the critical precursor source of hepatocellular regeneration. In addition this process is again dependent on TWEAK signalling, without which HPC mediated regeneration fails resulting in mortality. Therefore HPCs are an attractive biological target for regenerative medicine, and both TWEAK signalling and autologous macrophage infusion offer genuine potential to manipulate these cells as future therapies.

Induction of vertebrate heart development requires inhibition of canonical/<i>β</i>-catenin dependent Wnt signalling, activation of non-canonical/<i>β</i>-catenin independent Wnt signalling and transcription factor activation. Wnt/<i>β</i>-catenin signalling may also have a later regulatory role in cardiogenesis. The recent discovery of Wnt6 expression next to and within the developing heart during the relevant stages of cardiomyogenesis, combined with knockdown and over-expression data suggests that Wnt6 may have a role in the regulation of this process. Inhibition of canonical signalling leads to increased expression of cardiac associated transcription factors such as members of the Nkx2 and GATA family. These families are expressed in overlapping regions which specify the early heart field prior to the expression of the later cardiomyocyte-specific genes. This study demonstrates the ability of <i>β</i>-catenin to inhibit cardiogenesis during later developmental stages, before the cardiac mesoderm begins to differentiate into myocardium (heart muscle) and that the newly discovered Wnt6 exerts inhibition of cardiogenesis in a <i>β-</i>catenin<i> </i>dependent manner. This inhibition of cardiogenesis by <i>β-</i>catenin can occur in a cell-autonomous manner and is a result of direct inhibition of cardiac transcription factors of the GATA family. Over-expression of these pro-cardiogenic transcription factors GATA4 and GATA6 can restore the cardiomyogenic differentiation programme in embryos where it has previously been inhibited by <i>β</i>-catenin. In conclusion GATA factors are the relevant targets of Wnt/<i>β-</i>catenin signalling in the inhibition of normal cardiac development. The subsequent loss of cardiac gene expression observed is therefore a result of insufficient GATA expression and function.

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