Generation of epithelial planar cell polarity
Epithelial cells - in the epidermis as well as in neural epithelia - are polarized with respect to the body axis. This is not only the case in Drosophila and other insects but is a widespread feature of epithelia in both invertebrates and vertebrates (e.g. the ear epithelium of vertebrates, Fig. 1). The apparent difference between whole epithelial tissue (such as the disc-epithelia in Drosophila) and cells in tissue culture is that, in addition to the apical-basolateral polarity, epithelial tissues develop an obvious polarity with respect to the body axis (epithelial planar cell polarity, PCP). The Drosophila tissue polarity genes are required for correct PCP generation and mutations in them affect all epithelial tissues (both neuronal and non-neuronal). Efforts to understand the mechanisms of PCP formation have focused on the wing and the compound eye. In the wing, PCP is reflected in the choice of the site at which hair out-growth initiates in each cell and the direction the hair points (Fig. 1). In the eye, PCP is reflected in the mirror-symmetric arrangement of ommatidia relative to the dorso-ventral midline, the equator (see Fig. 1). This pattern is generated early in eye development when immature ommatidia rotate 90 degrees towards the equator and adopt opposite chiral forms depending upon whether they lie dorsally or ventrally (Fig. 2). PCP mutations result in the loss of hair/bristle polarity in the wing and loss of mirror-image symmetry in the eye, with ommatidia being misrotated and adopting the chiral forms randomly. Interestingly, recent findings indicate that the underlying signaling pathway(s) are conserved throughout evolution and regulate also related developmental aspects of coordinated cellular polarization in vertebrates. In particular the molecular mechanism regulating the process of convergent extension in vertebrate gastrulation has emerged as a highly related pathway.
Our lab focuses on the molecular understanding of the signaling pathways that regulate these processes, namely Frizzled (Fz) and Notch signaling for the R3/R4 decision in the eye (Fig. 2) and the differences between the canonical Wnt/Fz pathway and Fz/PCP signaling (Fig. 3). In addition, we are studying how the other PCP genes interact with and regulate the Fz/PCP signal transduction cascade (Fig. 4).
Figure 1. Examples of PCP in Drosophila and vertebrates. Regions of the wing (A) and eye (B) are shown as Drosophila examples. Photoreceptor cells in the enlarged ommatidium in (B) are numbered 1-7 according to their identity (see Fig. 2 for specific aspects of PCP generation in the eye). The axis of polarity is different in these tissue, and each tissue displays a different aspect of tissue polarization. Whereas single cells are polarized in the wing (A; as evident by the ordered appearance of the hairs, groups of cells are reflecting polarization in the eye (B). Vertebrate examples: the organ of Corti in the inner ear (C) with the stereo-cilial bundles (V-shaped in this view) that are aligned within the ear epithelium. (D) Cartoon of the morphogenetic cell movements and organization during convergent extension in mesoderm gastrulation. The body axis is established by cells that lengthen, narrow and intercalate. Modified from a drawing by Ray Keller.
Figure 2. Establishment of planar polarity a nd R3/R4 specification in the eye. (A) Left side: Cartoon of the logic of polarity generation during eye development. Initially ommatidial preclusters are organized in the antro-posterior axis and are symmetrical. Subsequently they rotate 90 degrees towards the equator (the D/V-midline) and at the end of this process chirality is established by the positions of the R3/R4 cells. Right side: Schematic presentation of chiral organization of the respective ommatidia (compare to the numbered ommatidium in Fig. 1B). In addition to the two chiral forms, symmetrical clusters with R3/R3 or R4/R4 pairs can be found in PCP mutants. R3 cells are highlighted in green and R4 cells in blue. (B) The two-tiered Fz/Notch signaling interplay regulating R3 and R4 fate determination, with Notch upregulation in R4 mediated through Delta expression in R3.
Figure 3. The Frizzled/PCP pathway A simplified version of the canonical Wnt/Beta-catenin pathway is shown for comparison on the left. The Fz/PCP cascades are indicated for both fly PCP establishment (center) and vertebrate convergent extension (right). The nuclear signaling leading to the transcriptional activation of
Delta is specific to eye PCP in flies. The members of the MAPK kinase cascade (gray shaded box) acting downstream of the Msn/STE20 kinase appear largely redundant. The effectors of RhoA in nuclear signaling and Msn/STE20 in cytoskeletal regulation are unknown. Molecular interactions that are documented are shown by bold arrows (2 point width), whereas genetic links are shown by regular arrows (1 point width).
Figure 4. Asymmetric signaling and interplay of the primary polarity genes in wing cells. As an end result of these signaling events, Fz/Dsh become specifically localized to the distal side of each cell, whereas Pk is found at the proximal side. Fmi localizes to both proximal and distal membranes. The generation of the actin hair follows this asymmetric localizations and forms distally (where Fz/Dsh accumulate).
Early eye developmentA second focus of the lab is to understand how the head is subdivided into the eye and antennal portions. Although the selector genes for either organ are known (e.g. PAX-6/Eyeless for eye development), their specific interaction with the signaling pathways that regulate the subdivision of the head are still unclear. Expression of the selector genes (and ultimately size and position of the respective organs) depends on EGF-receptor, Notch, TGF-beta/Dpp, Wnt and Hedgehog signaling. We are studying these pathways in the context of eye and antennal development, their interaction with the selector genes and the resulting gene expression profiles using a combination of in vivo studies, genetics and genomics.
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