MADISON - Feathers are the essence of birds. Without them, birds couldnot fly or attract mates. But how exactly do feathers form molecularly?Experimentally testing one current hypothesis, developmental biologistsat University of Wisconsin Medical School believe they now have theanswer.
In a previous study, UW anatomy professor John F. Fallon and his teamshowed that Sonic hedgehog (Shh) and bone morphogenetic protein 2(Bmp2) must be expressed in order to produce barb ridges, which areamong the first structures to form in the tufted branches of the simpledowny chick feather. The two proteins, which tend to play off eachother in organ development, also are involved in the embryonicdevelopment of limbs, lungs, teeth and the gut.
In the current study, appearing in the Aug. 16 Proceedings of theNational Academy of Sciences (PNAS Online, Aug. 8), Fallon's team andcollaborators showed that during the development of barbs-filamentousstructures that form the feather-the function of these two proteinsinteract. SHH up-regulates its own expression and that of Bmp2, andBmp2 then signals the down-regulation of Shh expression. This dynamicsignaling interaction fits a longstanding mathematical model known asan activator-inhibitor mechanism, says lead author Matthew P. Harris,Fallon's graduate student now doing a postdoctoral fellowship withNobel Laureate Christiane Nusslein-Volhard at Max Plank Institute inTubingen, Germany.
"In this model, the inhibitor down-regulates activator function, theactivator up-regulates its own expression and the activator alsoincreases the activity of the inhibitor," Harris says. "The model is asimple way of explaining how feather patterning is achieved."
Theoretical biologist Hans Meinhardt, also at the Max PlankInstitute and a collaborator on the PNAS paper, posited the role of theactivator-inhibitor model in developmental patterning in animals yearsago. Through the combined efforts of Meinhardt, Richard Prum of YaleUniversity and Scott Williamson of Cornell University, the model wasplaced in the context of simulations of growing feathers.
The results suggested that a simple interaction between Shh and Bmp2 issufficient to model the creation and patterning of barbs in featherdevelopment. The team then tested whether such interactions truly existin the developing feather. In the first steps of feather development,cells exposed to essentially the same levels of Bmp2 and Shh grow froma small bud to form a uniform ring. Shh then is expressed in specificspots along the ring, giving rise to bumps, seen microscopically aslongitudinal stripes demarcating the edges of ridges in the developingbarb.
"Each barb ridge grows in length by recruiting new cells, whichproliferate at the growing base of the feather germ, to join the baseof that barb ridge," Harris says. "The variations in the initial numberof barb ridges will directly affect the shape, and consequent function,of the feather."
To test the activator-inhibitor model, Harris injected retroviruses toforce the expression of either Shh or Bmp2 into the skin of six-day-oldchick embryos. The virus infected only small patches of cells andallowed Harris to locally examine the effects of the treatment on barbpatterning during feather development.
To assess the specific role of Bmp2 in regulating Shh expression,Harris tricked the cells into believing that Bmp2 was signaling themcontinuously by altering receptors in the cells. The over-expression ofBmp2 signaling via the altered receptors led to ongoing down-regulationof normal Shh expression needed to form the barbs.
Harris and colleagues used a similar experiment to test whether Shhcould up-regulate its own expression during barb formation, and foundthat it could. Similarly, they found that regional expression of Shhled to detectable up-regulation of Bmp2 in feather buds as they firstgrew.
The underlying assumptions of the model were found to be true indeveloping feathers. These findings suggest that simple relationshipsbetween developmental genes can provide the basis for the formation ofcomplex forms.
The researchers predict that a more complicated version of the modelcan be applied to the formation of more complex feathers. Termedpennaceous, these feathers occur in the duck and other birds, includingadult chickens, and are not characterized as downy. The more primitiveyoung chicken feathers, which are downy, are called plumalaceous.
"We don't have empirical evidence for this yet, but mathematicalanalyses lead us to believe that the addition of a third signalingfactor leads to the development of the more complex pennaceousfeather," Fallon says. " Our model supports paleontologic evidence thatpennaceous feathers are more advanced than plumalaceous feathers."
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