Planar cell polarity: a conserved mechanism of cellular polarization
Epithelial cells - in the epidermis as well as in neural epithelia - are polarized with respect to the body axis. This is a widespread feature of epithelia in both invertebrates and vertebrates (e.g. the orientation of wing cells or ommatidia in Drosophila, or the ear epithelium and fur of mammals, 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 apical-basolateral polarity, cells in tissues/organs develop a polarity with respect to the body axes (planar cell polarity, PCP). In Drosophila the core PCP genes are required for correct PCP generation in all adult tissues (neuronal and non-neuronal). Efforts to understand the mechanisms of PCP formation have focused in Drosophila on the wing and the compound eye. In the wing, PCP is reflected in the choice of the site at which an actin-based 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 D/V-midline, the equator (Fig. 1). This pattern is generated during eye development, when ommatidial preclusters rotate 90 degrees towards the equator, adopting 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 (Fig. 1), with ommatidia being misrotated and adopting the chiral forms randomly. Recent findings indicate that the underlying signaling pathway(s) are evolutionarily conserved. In addition to the polarization of epithelial tissues, the PCP genes also regulate the process of convergent extension during vertebrate gastrulation.
Our lab focuses on the molecular understanding of the signaling pathways that regulate PCP, namely Frizzled (Fz)/PCP signaling and associated regulatory factors, and in the eye in addition Notch signaling, which is also required for the PCP specification in the eye (Fig. 2). In parallel, we study the signaling specificity regulation and differences between the canonical Wnt/Fz pathway and Fz/PCP pathway (Fig. 3).
Figure 1. Examples of PCP in Drosophila and vertebrates.
(A-D) Distal cell orientation of appendages in Drosophila and mouse. Mutations in PCP genes disrupt this orientation and instead, cellular hairs create swirls and waves (B and D). (E-H) PCP aspects of sensory cell orientation: In the Drosophila eye (E,F) the ommatidia, or facets, are composed of photoreceptors, which are arranged in precisely oriented trapezoids. In PCP mutants, the arrangement of photoreceptors within each ommatidium and the arrangement of ommatidia with respect to the eye become disorganized. (G,H) Individual sensory hair cells of the mouse inner ear generate polarized bundles of actin-based stereocilia (labeled green with phalloidin). In Fz/PCP mutants these bundles form but their orientation becomes randomized. (I-L) PCP effects on convergent extension in vertebrate gastrulation and neurulation (examples shown are from zebrafish): PCP mutant vertebrate embryos fail to extend their A-P axis, as cells do not migrate and intercalate medially in a coordinated manner, leading to a shortened and broadened phenotype; lateral view (I,J) and dorsal view (K,L). The pictures shown in panels C-D, and G-H were kindly provided by Jeremy Nathans, and Matthew Kelley, respectively.
Figure 2. PCP establishment and R3/R4 specification in the Drosophila eye.
(A) Cartoon of polarity generation during eye development. Initially ommatidial preclusters are organized in the anterior-posterior axis and are symmetrical. Subsequently they rotate 90 degrees towards the equator (the D/V-midline) and chirality is established by the distinct specification and positioning of the R3/R4 photoreceptor cells. Right side: Schematic presentation of chiral organization of the ommatidia (compare to pictures shown in Fig. 1E and 1F). 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/PCP-Notch signaling interplay regulating R3 and R4 fate determination, with Delta upregulation in R3 mediating Notch activation in R4. Some known interactions among the PCP factors are indicated.
Figure 3. The Frizzled/PCP and canonical Wnt/beta-catenin pathways.
(A) A simplified version of the canonical Wnt/beta-catenin pathway, and (B) the Wnt-Fz/PCP are shown for comparison. The Fz/PCP cascades are indicated for fly PCP establishment (it is largely the same in vertebrates). 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 not yet known.
Early eye development
A 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 selector genes and the resulting gene expression profiles using a combination of genetics and genomics.
