Frizzled-dependent Planar Cell Polarity without Wnt Ligands

Abstract

Planar cell polarity (PCP) organizes the orientation of cellular protrusions and migratory activity within the tissue plane. PCP establishment involves the subcellular polarization of core PCP components. It has been suggested Wnt gradients could provide a global cue that coordinates local PCP with tissue axes. Here we dissect the role of Wnt ligands in the orientation of hairs of Drosophila wings, an established system for study of PCP. We found that PCP was normal in quintuple mutant wings that rely solely on membrane-tethered Wingless for Wnt signaling, suggesting that a Wnt gradient is not required. We then used a nanobody-based approach to trap Wntless in the endoplasmic reticulum, and hence prevent all Wnt secretion, specifically during the period of PCP establishment. PCP was still established. We conclude that, even though Wnt ligands could contribute to PCP, they are not essential, and another global cue must exist for tissue-wide polarization.

No Evidence that Wnt Ligands Are Required for Planar Cell Polarity in Drosophila

“…In contrast to previous reports, our results do not support the hypothesis that Wnt ligands impinge on PCP signaling. We note that our data are consistent with a study recently posted on bioRxiv ( Yu et al, 2020 ), which we discuss below.…”

“…Unexpectedly, although we did observe wnt4 expression in the margin of the pupal wing ( Figures 1 D and S2 ), we did not detect wnt4 expression in the dorsoventral (DV) boundary at the L3 larval stage, as has been demonstrated via RNA in situ hybridization ( Chen et al, 2008 ; Gieseler et al, 2001 ; Harris et al, 2016 ) and a GFP knockin ( Yu et al, 2020 ). Instead, our wnt4 reporter drove UAS:GFP expression in a small cluster of cells at the anterior ventral boundary of the wing pouch and was not detected in the wing margin until early pupal stages ( Figures 1B and S2 ).…”

“…During the preparation of this manuscript, a preprint by Yu et al (2020) reported related data and similar conclusions using several independent approaches. Specifically, Yu et al (2020) found that normal PCP patterning occurs in the wings of double-mutant flies expressing only membrane-tethered Wg and homozygous for mutations for wnt4 , as well as in quintuple mutant lacking wnt2, wnt4, wnt6 , and wnt10 and expressing solely membrane-tethered Wg. They also report that evi/wntless loss of function does not interfere with PCP, nor does driving apoptosis in all wg -expressing cells in the developing disc by overexpressing hid and rpr .…”

“…To address this issue, we performed double RNAi experiments against wg, wnt4 , and wnt6 in the developing wing using nub -Gal4, using transgenic double-stranded RNA (dsRNA) lines recently shown to produce strong loss-of-function phenotypes and co-expressed with UAS: dcr2 to strengthen RNAi-mediated knockdown ( Barrio and Milán, 2020 ). Under these conditions, wg RNAi led to both a complete loss of the wing margin and a profound growth defect, similar in appearance to the effects of inducing apoptosis in wg -expressing cells by overexpressing the pro-apoptotic genes hid and rpr ( Yu et al, 2020 ). However, even in these severely mispatterned wings, PCP appeared wild-type.…”

Widespread retention of ohnologs in key developmental gene families following whole genome duplication in arachnopulmonates

“…The expanded repertoire of frizzled genes in amblypygids is intriguing since they have also retained most Wnts and indeed several Wnt subfamilies are duplicated, and therefore it is possible that some specialised ligand-receptor interactions have evolved compared to other arthropods (Wu and Nusse 2002). However, although frizzled genes encode key receptors for Wnt ligands, they have other Wnt-independent functions, so the expansion of the frizzled gene repertoire could be related to the evolution of alternative signalling roles (Janssen et al 2015;Yu et al 2020).…”