Shaping and interpretation of Dpp morphogen gradient by endocytic trafficking

Dpp/BMP is a morphogen that controls patterning and growth in the Drosophila wing disc. Endocytic trafficking has been proposed to control extracellular Dpp spreading or removal, and Dpp signaling. However, how Dpp morphogen gradient is shaped and interpreted by endocytic trafficking remains unclear due to the lack of tools to visualize Dpp distribution at the physiological level. Here we generate functional fluorescent protein tagged dpp alleles to visualize both extracellular and intracellular Dpp distribution. Using these alleles, we found that, while Dynamin-mediated internalization of Dpp is required for Dpp signal activation, Rab5-mediated trafficking is not required for Dpp spreading or signaling as proposed before but for shutting off Dpp signaling through downregulating activated receptors. We provide evidence that Dpp signaling is terminated at Multivesicular body (MVB) likely through sorting activated receptors into intraluminal vesicles (ILVs) rather than Rab7-mediated lysosomal degradation. We further found that blocking MVB formation expanded Dpp signaling gradient without affecting extracellular Dpp gradient, thus compromising extracellular Dpp gradient interpretation. These results indicate that extracellular Dpp gradient is shaped by Dynamin-mediated internalization of Dpp and interpreted by duration of Dpp signaling.