Planar cell polarity in the larval epidermis ofDrosophilaand the role of microtubules

Abstract

10We investigate the mechanisms of planar cell polarity (PCP) in the larval epidermis of 11 Drosophila. Measurements of the amount of Dachsous across the segment find a peak 12 located near the rear of the anterior compartment. Localisation of Dachs and 13 orientation of ectopic denticles reveal the polarity of every cell in the segment. We 14 discuss how well these findings evidence a zigzag gradient model of Dachsous activity. 15 Several groups have proposed that Dachsous and Fat fix the direction of PCP via 16 oriented microtubules that transport PCP proteins to one side of the cell. We test this 17 proposition in the larval cells and find that most microtubules grow perpendicularly 18 to the axis of PCP. We find no significant bias in the polarity of those microtubules 19 aligned close to that axis. We also reexamine published data from the pupal abdomen 20 and fail to find evidence supporting the hypothesis that microtubular orientation 21 draws the arrow of PCP. 22As cells construct embryos and organs they need access to vectorial information that 24 informs them, for example, which way to migrate, divide, extend axons or how to 25 orient protrusions. In Drosophila there are (at least) two conserved genetic systems 26 Page 2 of 32 that generate vectorial information, and here we are concerned with only one of those, 27 the Dachsous/Fat system. Dachsous (Ds) and Fat (Ft) are large atypical cadherin 28 molecules that form heterodimeric bridges from cell to cell that help build planar cell 29 polarity (PCP) in one cell and also convey polarity between cells (Ma et al., 2003; 30 Matakatsu and Blair, 2004; Casal et al., 2006; Lawrence and Casal, 2018). The 31 activity of Ft is increased and Ds is reduced when they are phosphorylated by a third 32 molecule, Four-jointed (Fj), a Golgi-based kinase (Ishikawa et al., 2008; Brittle et al., 33 2010; Simon et al., 2010). The distribution of Ds, Ft and Fj and the interaction 34 between these molecules together determine what we describe as "Ds activity", by 35 which we mean the propensity of Ds on one cell to bind to Ft in the neighbouring cell. 36 Experiments suggest that, using the disposition and orientation of Ds-Ft bridges, each 37 cell compares the Ds activity of its two neighbours and points its denticles towards the 38 neighbour with the higher Ds activity. In this way, the local slopes in a landscape of Ds 39 activity determine polarities of all the cells (Casal et al., 2002; see Lawrence and 40 Casal, 2018 for more explanation). 41 A model: the ventral epidermis of the Drosophila larva 42Each segment of the larva is divided by cell lineage into an anterior (A) and a 43 posterior (P) compartment. In the ventral epidermis, a limited region of the segment 44 makes rows of thorny denticles that are polarised in an almost invariant pattern, while 45 the larger part of each segment makes no denticles and therefore its polarity is not 46 known (Figure 1-figure supplement 1). The Ds/Ft system is alone effective in 47 48 the "core" or Stan/Fz system (Casal et ...

Deciphering the roles of cell shape and Fat and Dachsous planar polarity in arranging the Drosophila apical microtubule network through quantitative image analysis

“…Namely, we demonstrated that the overall direction of microtubules follows the cell's long axis whereas the alignment of microtubules with each other is consistent with the cell eccentricity in agreement with the robustness of this alignment previously reported (Płochocka et al, 2021). A similar role for cell shape in organising microtubule polarity independently of Ft-Ds polarity has also been suggested in the Drosophila larval epidermis (Pietra et al, 2020).…”

Planar cell polarity: intracellular asymmetry and supracellular gradients of Dachsous

“…Within the A compartment, about two rows of the most anterior cells (figure 5 c ) and about two rows of the most posterior cells (figure 5 d ) show D located posteriorly; thus, their Ds/Ft polarity is that normally characteristic of the P cells. The third instar larva, having fewer but larger polyploid cells told a similar story: a set of cells in the A compartment, those confined to the extreme posterior row, had variable polarity with some showing the same polarity as in the P compartment (D accumulating posteriorly, [40]).

Figure 5Asymmetric localization of Dachs vis-à-vis A/P boundary.

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How do the Fat–Dachsous and core planar polarity pathways act together and independently to coordinate polarized cell behaviours?

“…Ft–Ds may also regulate microtubule orientation and core protein polarity in the anterior compartment of the abdomen. Here, microtubule plus-end growth is weakly biased towards the posterior ends of cells [54], although predominantly growth is along the mediolateral axis of the abdomen, orthogonal to the axis of asymmetry [129]. Consistent with this slight plus-end bias, Dsh particles show a posterior bias in their transport, as expected from the axis of the polarity of the core proteins [54].…”

“…The core proteins localize asymmetrically to anterior–posterior cell edges, but loss of core protein activity has relatively little effect on denticle orientation [107,134–136]. However, in the embryo and larva, denticle polarity correlates with asymmetric localization of Ds and D [129,137,138], and loss or overexpression of Ft–Ds severely disrupts denticle belt polarity [107,136–139]. Fj is highly expressed in the tendon cells which are posterior to rows 1 and 4 in the larval epidermis, which may explain the pattern of denticle polarity [140].…”

“…One way in which Ft and Ds might influence the polarization of cuticular structures could be by regulating cell shape and alignment. In wild-type embryos, cells are elongated on the dorsoventral axis and microtubules are aligned on this long axis [129,138]. In the absence of Ft, cells fail to elongate on the dorsoventral axis, and microtubules are disorganized [138].…”