Molecular mechanisms mediating asymmetric subcellular localisation of the core planar polarity pathway proteins

Abstract

Planar polarity refers to cellular polarity in an orthogonal plane to apicobasal polarity, and is seen across scales from molecular distributions of proteins to tissue patterning. In many contexts it is regulated by the evolutionarily conserved 'core' planar polarity pathway that is essential for normal organismal development. Core planar polarity pathway components form asymmetric intercellular complexes that communicate polarity between neighbouring cells and direct polarised cell behaviours and the formation of polarised structures. The core planar polarity pathway consists of six structurally different proteins. In the fruitfly Drosophila melanogaster, where the pathway is best characterised, an intercellular homodimer of the seven-pass transmembrane protein Flamingo interacts on one side of the cell junction with the seven-pass transmembrane protein Frizzled, and on the other side with the four-pass transmembrane protein Strabismus. The cytoplasmic proteins Diego and Dishevelled are co-localised with Frizzled, and Prickle co-localises with Strabismus. Between these six components there are myriad possible molecular interactions, which could stabilise or destabilise the intercellular complexes and lead to their sorting into polarised distributions within cells. Post-translational modifications are key regulators of molecular interactions between proteins. Several post-translational modifications of core proteins have been reported to be of functional significance, in particular phosphorylation and ubiquitination. In this review, we discuss the molecular control of planar polarity and the molecular ecology of the core planar polarity intercellular complexes. Furthermore, we highlight the importance of understanding the spatial control of post-translational modifications in the establishment of planar polarity.

Keywords: PCP; phosphorylation; planar cell polarity; planar polarity; post-translational modification; ubiquitination.

Figure 1.. Principles of planar cell polarity.

(A) Schematic of localisation of core planar polarity pathway protein complexes in cells of the Drosophila pupal wing. Proximal localisation of Stbm (orange) and distal localisation of Fz (green) at apicolateral cell junctions leads to trichome (grey) emergence from distal cell edges. (B and C) Evolution of symmetry breaking cluster formation. (B) Fmi homodimers are stabilised by Fz localisation on one side of the complex at junctions between neighbouring cells. (C) Stbm localises to the apposing cell edge and the cytoplasmic proteins Pk, Dsh and Dgo localise proximally and distally in the complex as shown. (D) Image of dorsal surface of a wild-type Drosophila wing, showing uniform distal orientation of trichomes. Proximal is left, and anterior is up. (E) Image of dorsal surface of wing from a pk^pk-sple13 mutant fly, showing a swirled trichome pattern. (F) Confocal microscope image of Drosophila pupal wing epithelium, immunolabelled for Fmi (green, localised preferentially to proximodistal cell boundaries) and showing actin-rich trichomes (red, emerging from distal cell edges). (G and H) Confocal microscope image of a Drosophila pupal wing epithelium genetically mosaic for dsh^V26 mutant tissue, marked by loss of blue β-gal immunolabelling and outlined in white (G). Fmi (green) and Stbm (red, or white in H) are asymmetrically localised at proximodistal cell boundaries at the apicolateral cell junctions (left and right cell edges) in wild-type tissue, but lose this asymmetric localisation in mutant tissue. This shows that Dsh activity is required for planar polarisation of core pathway components such as Fmi and Stbm. (I–K) Molecularly asymmetric complexes (I) are sorted by feedback interactions (J) so that they all align in the same orientation (K). Complexes of opposite orientation are destabilised (red inhibitory symbols) and complexes of the same orientation are stabilised (black arrows).

Figure 2.. Mechanisms of planar cell polarity establishment.

(A–D) Putative stabilising and destabilising interactions between the core proteins on two neighbouring cell membranes (grey solid lines). Black arrows indicate stabilising interactions and red inhibitory symbols indicate destabilising interactions. Dotted grey arrows indicate removal of proteins from the intercellular complex. (A) Pk and Dgo compete for Dsh binding. Pk binding to Dsh leads to destabilisation of Fz. (B) Pk acts through Stbm to stabilise Fz in the adjacent cell. (C) Pk promotes Fmi and Stbm endocytosis in the presence of Fz on the same membrane. (D) Dsh stabilises Pk in the adjacent cell. (E,F) Post-translational modifications of the core proteins. Purple stars indicate ubiquitination of a core protein (E) and yellow stars indicate phosphorylation (F). Modifying enzymes acting in Drosophila are in grey and those only known to act in vertebrates are in blue. (G) Stoichiometry of core pathway complexes. A homodimer of Fmi associates with one Fz, two Dsh and one Dgo molecule on one side of the complex and with six Stbm and one Pk molecule on the other. (H) Clustering of intercellular complexes in parallel arrays results in multiple stabilising interactions, and complexes undergo a phase transition to form stable puncta. (I) Outside of puncta, core protein complexes are in both orientations, and destabilising feedback interactions prevent clustering of complexes, leading to complexes that are less densely packed.