The Drosophila retina is patterned by a morphogenetic wave driven by the Hedgehog signaling protein. Hedgehog, secreted by the first neurons, induces neuronal differentiation and hedgehog expression in nearby uncommitted cells, thereby propagating the wave. Evidence is presented here that the zebrafish Hedgehog homolog, Sonic Hedgehog, is also expressed in the first retinal neurons, and that Sonic Hedgehog drives a wave of neurogenesis across the retina, strikingly similar to the wave in Drosophila. The conservation of this patterning mechanism is unexpected, given the highly divergent structures of vertebrate and invertebrate eyes, and supports a common evolutionary origin of the animal visual system.

Epithelial cells are equipped with junctional complexes that are involved in maintaining tissue architecture, providing mechanical integrity and suppressing tumour formation as well as invasiveness. A strict spatial segregation of these junctional complexes leads to the polarisation of epithelial cells. In vertebrate epithelia, basally localised hemidesmosomes mediate stable adhesion between epithelial cells and the underlying basement membrane. Although components of hemidesmosomes are relatively well known, the molecular machinery involved in governing the formation of these robust junctions, remains elusive. Here, we have identified the first component of this machinery using a forward genetic approach in zebrafish as we show that the function of penner (pen)/lethal giant larvae 2(lgl2) is necessary for hemidesmosome formation and maintenance of the tissue integrity in the developing basal epidermis. Moreover, in pen/lgl2 mutant, basal epidermal cells hyper-proliferate and migrate to ectopic positions. Of the two vertebrate orthologues of the Drosophila tumour suppressor gene lethal giant larvae, the function of lgl2 in vertebrate development and organogenesis remained unclear so far. Here, we have unravelled an essential function of lgl2 during development of the epidermis in vertebrates.

The epidermis is a stratified epithelium, which forms a barrier to maintain the internal milieu in metazoans. Being the outermost tissue, growth of the epidermis has to be strictly coordinated with the growth of the embryo. The key parameters that determine tissue growth are cell number and cell size. So far, it has remained unclear how the size of epidermal cells is maintained and whether it contributes towards epidermal homeostasis. We have used genetic analysis in combination with cellular imaging to show that zebrafish goosepimples/myosin Vb regulates plasma membrane homeostasis and is involved in maintenance of cell size in the periderm, the outermost epidermal layer. The decrease in peridermal cell size in Myosin Vb deficient embryos is compensated by an increase in cell number whereas decrease in cell number results in the expansion of peridermal cells, which requires myosin Vb (myoVb) function. Inhibition of cell proliferation as well as cell size expansion results in increased lethality in larval stages suggesting that this two-way compensatory mechanism is essential for growing larvae. Our analyses unravel the importance of Myosin Vb dependent cell size regulation in epidermal homeostasis and demonstrate that the epidermis has the ability to maintain a dynamic balance between cell size and cell number.

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1461 Poster Board I-484 The ecotropic viral integration site-1 (Evi-1) locus was originally identified as a common site of retroviral integration in murine myeloid tumors and was later shown to be one of the most potent oncogenenes associated with murine and human myeloid leukemia. More recent data suggest involvement of Evi-1 in embryonic hematopoiesis (Goyama et al, Cell Stem Cell 2008; Yuasa et al, EMBO J, 2005), yet the precise role and molecular regulation of Evi-1 during blood development remains poorly understood. The zebrafish model offers powerful tools for genetic and embryonic studies. Here, we study zebrafish embryonic development and human pluripotent stem cells to understand how evi-1 modulates early hematopoietic development. Loss-of-function studies were performed in vivo by injecting Morpholino oligonucleotides in zebrafish zygotes to inhibit evi-1 pre-mRNA splicing. To control for off-target effects, two separate morpholinos were designed and injected. N=100 zebrafish were analysed pro experiment in each group. Inhibition of evi-1 was confirmed by quantitative PCR comparison in morpholino-injected and control embryos. Hematopoietic development was followed in both morphants and wild-type embryos by simple microscopy and in situ hybridizations using known hematopoeitic markers in order to investigate the developmental time-point in which evi-1 regulates blood development. evi-1 morpholino injected zebrafisch embryo showed severely reduced numbers of circulating blood cells, consistent with the phenotype observed in Evi-1−/− mice. Additionally, hemorrhages were observed, suggesting concomittant defects of the endothelial lineage in evi-1 deficient fish. In situ hybridization analysis on 11-12 somite stage embryos revealed strong reduction of myeloid embryonic hematopoiesis (measured by pu.1 expression in the anterior lateral plate mesoderm), while no change was observed in primitive erythroid progenitor cells (monitored by gata1 expression) or overall in blood and endothelial precursors in the posterior lateral plate mesoderm (as monitored by scl expression). Taken together, our studies demonstrate a strong impact of evi-1 on zebrafish blood development, confirming the results from Evi-1−/− mice. As gata1 expression and therefore erythroid precursor cells in the posterior blood islands are unaffected in evi-1 morphants, our results support the hypothesis that the reduction of primitive yolk-sac erythrocytes in mutant mice was caused from hemorrhages from pericardial effusions. Since erythroid and myeloid cells derive from a common precursor, but gata1 expression was unaffected in knock-down embryos, we anticipate that evi-1 plays a specific role in the myeloid lineage, as shown by abolished pu.1 expression in the anterior LPM. evi-1 therefore probably affects differentiation, survival or proliferation of myeloid cells. Previous reports in adult hematopoietic cells show that evi-1 can interact with both gata1 and pu.1. However, our data suggest that this is not the case during embryonic development, since gata1 expression remained unaltered in morpholino-injected embryos. Furthermore, data in mice suggest that Evi-1 may modulate embryonic hematopoiesis by affecting hematopoietic stem cell proliferation through regulation of Gata2. Currently ongoing experiments in our laboratories focus on characterization of genetic interactions between evi-1, gata2 and pu.1 during zebrafish blood development. Amongst other, gata2 and respectively pu.1 mRNA are co-injected in evi-1 morphants to analyse whether they can rescue the blood phenotype. Moreover, selected findings in zebrafish embryonic development will be verified in the human using using in vitro differentiating human induced pluripotent stem (iPS) cells. First expression data generated by real-time PCR analysis showed differential expression of EVI-1 in embryoid bodies generated from human iPS cells, confirming our hypothesis that EVI-1 has specific effects during human blood development. Disclosures No relevant conflicts of interest to declare.