Planar cell polarity (PCP) in the Drosophila eye is established by the distinct fate specifications of photoreceptors R3 and R4, and is regulated by the Frizzled (Fz)/PCP signaling pathway. Before the PCP proteins become asymmetrically localized to opposite poles of the cell in response to Fz/PCP signaling, they are uniformly apically colocalized. Little is known about how the apical localization is maintained. Evidence is provided that the PCP protein Diego (Dgo) promotes the maintenance of apical localization of Flamingo (Fmi), an atypical Cadherin-family member, which itself is required for the apical localization of the other PCP factors. This function of Dgo is redundant with Prickle (Pk) and Strabismus (Stbm), and only appreciable in double mutant tissue. The initial membrane association of Dgo depends on Fz, and Dgo physically interacts with Stbm and Pk through its Ankyrin repeats, providing evidence for a PCP multiprotein complex. These interactions suggest a positive feedback loop initiated by Fz that results in the apical maintenance of other PCP factors through Fmi (Das, 2004).
A crucial region for PCP signaling in the eye is in rows 2-5 in the 3rd instar larval disc behind the morphogenetic furrow (MF). Four lines of evidence support this assumption: (1) cells that take part in PCP signaling (R3/R4) are specified as photoreceptor subtypes in this region; (2) Frizzled-Notch signaling-dependent transcription in the R4 cell is initiated in this region, as detected by the mdelta0.5 reporter for the E(spl)mdelta gene; (3) the sev-enhancer, which is active in R3/R4 cells in this region, can drive a PCP gene in order to fully rescue the respective mutant phenotype; and (4) in the region ahead of the MF to the first row behind it, the PCP proteins are uniformly apically localized in all cells, before they begin at row 2 to display the characteristic PCP protein localization pattern (Das, 2004).
Following their initial symmetric apical localization, the PCP factors become asymmetrically enriched across the respective cell boundaries in the proximodistal axis in the wing or the dorsoventral axis in the eye. Although several models have been proposed as to how these complexes might be formed and maintained, the mechanism behind the early aspect of PCP establishment remains largely unclear. The data suggest a complex mechanism that involves redundancy among several PCP genes (Das, 2004).
Based on the analysis of single mutant clones in the eye, only Fz and Fmi affect PCP gene localization in a general non-redundant manner (and Stbm affects Pk localization). The single and double mutant clone data indicate the following (Das, 2004).
In addition to these initial requirements for apical localization and maintenance, the subsequent asymmetric resolution of the respective PCP proteins to the R4 cell is affected and often delayed in mutant backgrounds (Das, 2004).
How is the initial apical localization of all these factors maintained? As outlined above, none of the single mutant PCP genes, except fz and fmi, has a significant effect on the whole complex. However, in double mutant clones for either dgo and pk, or dgo and stbm, localization of the PCP proteins is severely affected. Most strikingly, the apical localization of Fmi and Fz is affected in these double mutant combinations. In addition, the localization of Stbm and Dsh are also affected. This could be either a direct effect of Dgo and Pk or could be mediated through their effect on Fmi [as in fmi- tissue, Stbm and Dsh as well as Fz are reduced apically]. These data suggest that the cytoplasmic PCP proteins, which are initially recruited to the membrane by Fz (i.e. Dgo and Dsh) and Stbm (i.e., Pk), form a protein complex that is required to maintain Fmi apically. This interpretation is supported by the observation that Dgo physically interacts with Stbm and Pk, and thus possibly stabilizes the initial complex. Thus, these studies reveal that Dgo, Stbm and Pk are required to maintain apical Fmi localization, possibly through the physical interactions among themselves and possibly other PCP factors, during the early stages preceding PCP signaling (i.e., anterior to MF in eye). In turn, apical Fmi promotes the maintenance of an initial PCP complex at adjacent cell membranes to facilitate their signaling specific interactions (Das, 2004).
It is possible to speculate on further implications of these data. During later stages of PCP signaling, the localization of the PCP factors is resolved into two types of complexes on adjacent cell membranes. The differential localization of either Fz/Dgo or the Stbm/Pk complex in the neighboring cells (R3 versus R4) suggests that asymmetric localization of PCP factors is maintained across the border of the R3 and R4 cells in the eye and across proximodistal cell borders in the wing. In the eye, the PCP proteins analyzed in this manner indeed localize to specific sides of the R3/R4 cell border. Similarly, proximodistal localization in the wing correlates with the respective R3/R4-specific localization. For example, the localization of Fz and Diego in the distal side of a wing cell correlates with the localization on the R3 side of the R3/R4 border; conversely, Stbm localization to the proximal side of a wing cell correlates with its localization on the R4 side of the R3/R4 border. The localization to either the R3 or R4 side also corresponds to the genetic requirements in either cell, as established in mosaic analyses. Thus, since Dgo, which is initially recruited by Fz, localizes to R3 and the pk/stbm complex localizes to R4, it is likely that at later stages during PCP signaling (posterior to MF) Fmi localization is maintained and stabilized through feedback loops on both sides of the R3/R4 boundary (Das, 2004).
A prediction from such a scenario is that Fz/Dgo are performing this function in R3 and the Stbm/Pk complex in R4. Since Fmi is known to function as a homophilic cell-adhesion molecule, the removal of the feedback loop on one side could be overcome through the homophilic recruitment of Fmi from the other side. Only when both feedback loops are weakened on either side, can Fmi localization become affected. This is supported by the different effects of the respective double mutants posterior to the MF; those that affect both sides of the R3/R4 boundary, e.g., dgo and stbm (R3side/R4side) or dgo and pk (R3side/R4side) can cause Fmi delocalization, whereas double mutants affecting only one cell, e.g., pk and stbm (both R4side), have no significant effect (Das, 2004).
Epithelial planar cell polarity (PCP) is evident in the cellular organization of many tissues in vertebrates and invertebrates. In mammals, PCP signalling governs convergent extension during gastrulation and the organization of a wide variety of structures, including the orientation of body hair and sensory hair cells of the inner ear. In Drosophila melanogaster, PCP is manifest in adult tissues, including ommatidial arrangement in the compound eye and hair orientation in wing cells. PCP establishment requires the conserved Frizzled/Dishevelled PCP pathway. Mutations in PCP-pathway-associated genes cause aberrant orientation of body hair or inner-ear sensory cells in mice, or misorientation of ommatidia and wing hair in Drosophila. This study provides mechanistic insight into Frizzled/Dishevelled signalling regulation. The ankyrin-repeat protein Diego binds directly to Dishevelled and promotes Frizzled signalling. Dishevelled can also be bound by the Frizzled PCP antagonist Prickle. Strikingly, Diego and Prickle compete with one another for Dishevelled binding, thereby modulating Frizzled/Dishevelled activity and ensuring tight control over Frizzled PCP signalling (Jenny, 2005).
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