Patterning and growth control in vivo by an engineered GFP gradient

Morphogen gradients provide positional information during development.To uncover the minimal requirements for morphogen gradient formation, wehave engineered a synthetic morphogen inDrosophila wing primordia. We show that aninert protein, green fluorescent protein (GFP), can form a detectablediffusion-based gradient in the presence of surface-associated anti-GFPnanobodies, which modulate the gradient by trapping the ligand and limitingleakage from the tissue. We next fused anti-GFP nanobodies to the receptorsof Dpp, a natural morphogen, to render them responsive to extracellular GFP.In the presence of these engineered receptors, GFP could replace Dpp toorganize patterning and growth in vivo. Concomitant expression ofglycosylphosphatidylinositol (GPI)–anchored nonsignaling receptors furtherimproved patterning, to near–wild-type quality. Theoretical argumentssuggest that GPI anchorage could be important for these receptors to expandthe gradient length scale while at the same time reducing leakage.