Cellular interpretation of the long-range gradient of Four-jointed activity in the Drosophila wing

Abstract

To understand how long-range patterning gradients are interpreted at the cellular level, we investigate how a gradient of expression of the Four-jointed kinase specifies planar polarised distributions of the cadherins Fat and Dachsous in the Drosophila wing. We use computational modelling to test different scenarios for how Four-jointed might act and test the model predictions by employing fluorescence recovery after photobleaching as an in vivo assay to measure the influence of Four-jointed on Fat-Dachsous binding. We demonstrate that in vivo, Four-jointed acts both on Fat to promote its binding to Dachsous and on Dachsous to inhibit its binding to Fat, with a bias towards a stronger effect on Fat. Overall, we show that opposing gradients of Fat and Dachsous phosphorylation are sufficient to explain the observed pattern of Fat–Dachsous binding and planar polarisation across the wing, and thus demonstrate the mechanism by which a long-range gradient is interpreted.DOI: http://dx.doi.org/10.7554/eLife.05789.001

Planar cell polarity in development and disease

“…Despite the fact that expression of ft appears uniform, the kinase activity of Fj (that forms a gradient), which may phosphorylate cadherin repeats of both Ft and Ds, can seemingly promote Ft binding to Ds 143–146 and inhibit Ds binding to Ft 146 . Controlling Ds binding to Ft and Ft binding to Ds is thought to lead to a graded distribution of binding affinity between these two proteins along the axis of differential ds and fj expression, whereby a cell with more Fj activity than its neighbor is going to have “stronger” Ft and “weaker” Ds in the sense of binding capacity, promoting Ft in the cell with higher Fj to bind with Ds in the cell with lower Fj (and thus “stronger” Ds).…”

Planar Polarity: Converting a Morphogen Gradient into Cellular Polarity

“…In a recent article published in eLife , Hale et al . [2] set out to demonstrate how the graded phosphorylation of Ft and Ds by the Golgi-resident kinase Four-jointed (Fj) regulates the Ft–Ds interaction and the planar-polarized accumulation of Ft–Ds complexes across the entire wing.…”

“…In their recent work, Hale et al . [2] used a combination of in vivo imaging and computational methods to demonstrate that the Ft–Ds binding gradient can be explained by the graded activity of Fj and that this is sufficient to propagate the polarization of complexes across the whole tissue.…”

“…As a measure of stability, they monitored fluorescence recovery after photobleaching (FRAP), the basic idea being that the proteins in stable complexes are less mobile and so when they are bleached there is less interchange with unbleached proteins from elsewhere in the cell, and so the fluorescence signal does not return to its pre-bleach level: the greater the mobility and therefore the greater the level of protein exchange, the higher the level of recovery of the fluorescence signal. The authors elegantly use endogenously tagged Ft and Ds proteins to be sure to investigate how Fj functions in vivo, rather than interpreting an artificial expression system with all its caveats [2]. Analysis of endogenous protein dynamics revealed a stable pool of Ft and Ds present in puncta at cell junctions, as well as a more dynamic pool that is also found at junctions.…”

“…For instance, Fj activity resulted in only a two-fold increase in Ds asymmetry across an individual cell [2], and previous reports suggested that the changes in polarity that result from manipulating Ds expression can only be propagated over the distance of a few cells [8,9]. Coupled with this is the fact that, although fj clones can reorient the polarity of a tissue near clone borders, the fj null phenotype shows basically wild-type planar polarity [10], and uniform (i.e.…”