Group Leader: Felix Loosli
tel.: +49 721 608 28743
Cells within epithelial sheets are highly polarized with distinct apical and basal domains. This cellular organization is critical to both epithelial form and function, such as the uptake of nutrients in the gut, the secretion in epithelial glands or light reception in the neuro-retina of visual systems. Our research aims to identify genes that are essential for epithelial cell polarity in the vertebrate nervous system and to analyse their specific function. The genetic model systems that we use are zebrafish (Danio rerio) and medaka (Oryzias latipes; Figure 1).
Figure 1: A: zebrafish (Danio rerio); B: medaka (Oryzias latipes).
Mutations that affect apico-basal polarity often severely disturb the epithelial arrangement of cells and by that also the morphology of the affected tissue (Figure 2). Prior to organogenesis, the progenitor cells of these organs require information concerning apico-basal polarity for their proper development.
Figure 2: Wildtype (A) and mutant (B) retina of medaka fish hatchlings. The mutation that affects a-b polarity results in an abnormal shape and cellular architecture of the neural retina.
Furthermore, epithelial polarity is also tightly linked to proliferation. Mutations in epithelial polarity genes often result in ectopic proliferation, leading to an increased size of the affected tissue (Figure 3). Thus, epithelial polarity controls two aspects that play a critical role in tumor formation, namely epithelial-mesenchymal transition and cell proliferation. In fact, many genes that function in apico-basal polarity have initially been isolated as tumor supressor genes.
Figure 3: Proliferation in wild type (A) and mutant (B) retina. The anti-phospho Histone 3 antibody labels M-phase nuclei (red). A high number of ectopic M-phase nuclei are visible in the mutant retina.
Genes involved in epithelial polarity can be grouped into three classes: transmembrane proteins that are clustered in adhesion complexes (such as JAMs, cadherins, integrins) and mediate cell-cell and cell-surface interactions, scaffold proteins that link the receptors to the cytoskeleton and are required for protein complex formation (for example FERM and PDZ domain proteins) and signal transduction proteins that relay extracellular signals into the cell (b-catenin, small GTPases) (Figure 4).
Figure 4: Adhesion complexes (red) contain transmembrane receptors. Scaffold proteins (blue) connect these to the cytoskeleton and to signal transduction proteins (green) that relay signals to the nucleus. AJ adherens junctions; D desmosomes; FA focal adhesions; TJ tight junctions.
The development of the completely transparent fish embryos can be examined by non-invasive microscopy. Thus, their development is not affected by the analysis. To visualize apico-basal polarity in vivo we use GFP-tagged proteins that are specifically localized along the apico-basal axis. The simple architecture of neuroepithelia during early fish development allows to visualize the subcellular localization in the living embryo using a fluorescence dissection microscope (Figure 5).
Figure 5: A: Transversal section where the apical side of progenitor cells in the developing CNS is labeled with an anti-aPKC antibody (green). B: dorsal view of a live embryo that expresses apically localized Par3-GFP. The subcellular localization of Par3-GFP is visible due to the alignment of the apical side of the progenitor cells.
- Functional analysis of apico-basal polarity genes that have been identified in a mutagenesis screen using medaka fish. Interaction of these genes with known components of epithelial polarity.
- Genetic screens to identify novel genes required for epithelial polarity. Transgenic lines that express GFP-fusion proteins with a specific localization along the apico-basal axis will be used in chemical mutagenesis screens of zebrafish and medaka to isolate mutations that affect epithelial polarity.
Updated: January 17, 2011