Ever since the idea of Wnt molecules as a morphogen gradient emerged, the visualization of that gradient has remained elusive. Antibodies against Wnt molecules are notoriously bad, and epitope tags had a tendency to to disrupt the normal function of the proteins. This lack of visualization has led to much uncertainty about the nature of Wnt gradients and the distance over which they can function. Now, for the first time Farin et al have succeeded in generating an HA-tagged version of the endogenous Wnt3 gene in mice giving homozygous viable and healthy animals by placing the epitope not at either end of the protein but in a lowly conserved region opposite from the interaction site with the Frizzled receptors.
The detection of this epitope-tagged Wnt3 in the gut is still problematic, but with a multistep detection protocol (rat anti-HA / rabbit anti-rat / goat anti-rabbit poly HRP / tyramide signal amplification) they succeed in visualizing the protein. Thus, Farin et al found that Wnt3 produced in and secreted by a Paneth cell binds to Frz receptors on the neighboring cell, the Lgr5+ intestinal stem cell where Frz keeps it tethered to the membrane. Next, cell division of these cells dilutes the surface-bound Wnt3 over daughter cells, thus creating a short-range (mainly one, occasionally two cell) gradient.
Of course, it is always dangerous to translate findings from one biological system to the next. This paper only looks ant Wnt3 and only looks at the Paneth cell / Lgr5+ stem cell system. This system is characterized by direct contact between the niche (Paneth cell) and stem cell (Lgr5+ cell). If we assume the same short-range mechanism holds true for Wnt9b secreted from the ureteric bud and its role in controlling the nephron progenitor cells, what does this mean for the control of the nephron progenitors / cap mesenchyme? If the majority of Wnt9b secreted from the ureteric bud would only reach the neighboring cells in the cap as suggested for Wnt3, what is the exact role of the stromal Fat4 signal that controls the Yap/Taz and directs the β-catenin signal towards differentiation targets, if there would hardly be any Wnt9b that reaches these cells in the first place? Is there a role for proliferation within the cap in further distributing the Wnt9b signal? Or is the Wnt3/Paneth/Lgr5 cell system not a good model for the mechanism via which Wnt9b controls the NPCs, and does this require longer-range activity (as discussed by Farin et al)? And how about the other Wnt family members having a role in the developing kidney? It is clear there is still much to be described about the role of Wnt signalling and this paper provides an elegant way to do this.
Peter Hohenstein