Fly wing cells carry a built-in compass to organise tissue polarity
Quantitative fruit fly wing experiments show cells polarise via a robust internal signal, not a depleted protein pool or microtubule transport.
Summary
Cells in many tissues, such as skin hair follicles and airway cilia, line up in a shared direction through planar cell polarity. In the fruit fly wing, six 'core' proteins sort onto opposite cell edges to build this pattern. Models disagree on whether cells need an internal, cell-wide signal to do this sorting. Carayon, Strutt and Strutt switched core pathway activity on late in pupal development, tracked a fluorescent-timer-tagged Frizzled to measure complex stability, and quantified polarity angle and strength. Their results support a robust cell-scale signal. They argue against two proposed mechanisms: depletion of a limited core protein pool and polarised transport along microtubules. Spreading polarity from a boundary of Frizzled expression across the tissue was also difficult, which suggests individual cells have a strong intrinsic ability to establish and maintain polarity.
Detailed Summary
Planar cell polarity coordinates cells within the plane of a tissue. It orients hair follicles, airway cilia and the cell movements of gastrulation and neural tube closure. In the Drosophila wing, six core proteins (Frizzled, Dishevelled, Diego, Strabismus/Vang, Prickle and Flamingo) form intercellular complexes on proximal and distal cell edges. Disrupting any one of them scrambles polarity. Understanding how cells robustly sort these complexes matters for the many developmental defects linked to this pathway.
The prevailing view combines tissue-level global cues, such as mechanical tension from wing hinge contraction, with local feedback between neighbouring complexes. Computational models predict that this is not enough to polarise cells without a global cue. Yet earlier experiments switching on core pathway activity late produce swirling polarity patterns unrelated to tissue axes. This implies a non-local, cell-scale signal that promotes segregation of complexes to opposite sides of each cell. No systematic investigation of such a signal had been done before this study.
The authors used conditional and restrictive expression tools to turn core protein activity on de novo at defined times in the pupal wing. They measured polarity angle and strength with principal component analysis in QuantifyPolarity. Complex stability was read out with Frizzled tagged with a fluorescent timer (fast-maturing sfGFP for total protein, slower mKate2 for older, presumed stable protein). They tested a depletion model, in which a limiting core protein acts as a long-range inhibitor, by varying core gene dosage during de novo polarisation. They also tested whether polarised microtubule transport of core proteins could act as the signal. Finally, they induced a boundary of Frizzled expression to see how far polarity propagates across the tissue.
The results provide evidence for a robust cell-scale signal. They argue against both the limited-pool depletion mechanism and polarised microtubule transport as its basis. Polarity propagation across tissue boundaries was hard to induce even over hours. This suggests individual cells strongly establish and maintain their own polarity rather than depending on neighbours to impose it.
These findings narrow the field of candidate mechanisms and favour alternatives such as a diffusible biochemical inhibitor or other intracellular signalling. Caveats: the source text available for this summary was truncated after the early results, so detailed quantitative outcomes could not be reviewed and this summary relies mainly on the abstract and introduction. The work is in fly pupal wings, and the molecular identity of the cell-scale signal remains unresolved.
Key Findings
- Experiments support a robust cell-intrinsic, cell-scale signal that drives core planar polarity protein segregation in the Drosophila pupal wing.
- Depletion of a limited pool of a core protein does not appear to explain cell-scale polarity signalling.
- Polarised transport of core proteins on microtubules is argued against as the cell-scale signal.
- Propagating polarity across a tissue from a Frizzled expression boundary is hard to induce, even over several hours.
- A Frizzled mKate2/sfGFP fluorescent timer quantified stable versus total Frizzled alongside polarity angle and strength.
Methodology
The authors used conditional, heat-shock-induced de novo expression of core pathway components in the Drosophila pupal wing, with varied gene dosage. They measured polarity angle and strength with PCA-based QuantifyPolarity, and complex stability with a Frizzled::mKate2-sfGFP fluorescent timer validated by western blot. They also tested microtubule involvement and polarity propagation from induced Frizzled expression boundaries.
Study Limitations
The full text provided was truncated after the early results, so this summary relies mainly on the abstract and introduction and cannot report detailed quantitative findings. The work is confined to Drosophila pupal wings, and the molecular nature of the cell-scale signal is not identified. The study argues against two mechanisms rather than demonstrating the true one.
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