Role of cellular trafficking on Dpp morphogen gradient formation and signaling

Dpp/BMP is a well-studied morphogen that controls patterning and growth in the Drosophila wing disc. However, how the Dpp morphogen gradient is established and is interpreted by endocytic trafficking remains largely unknown. By utilizing the endogenously tagged dpp alleles with the monomeric proteins mGreenLantern and mScarlet, I investigated the role of different trafficking factors in shaping the intra- and the extracellular Dpp gradient. Using these alleles, I showed that dynamin is a major regulator of the Dpp gradient and blocking dynamin-dependent endocytosis expanded the extracellular Dpp gradient and impaired Dpp signaling. I also found that blocking the early endosomal trafficking by knocking down Rab5 not only expanded the extracellular Dpp gradient, but also increased the range of Dpp signaling possibly due to an impaired termination of its receptor Tkv. I also demonstrated that blocking multivesicular body (MVB) formation, but not the endo-lysosomal fusion, expanded the internalized Dpp distribution and signaling range without affecting the extracellular Dpp gradient. By investigating the role of recycling endosomes, I also showed that while the slow recycling endosomes slightly affected the intracellular Dpp distribution, the fast recycling endosomes minimally affected the extracellular Dpp gradient and neither of these factors influenced the Dpp signaling activity. My findings indicated that the early endocytic factors act as a sink for the extracellular Dpp gradient and are required to activate Dpp signaling, while the late endocytic factors terminate Dpp signaling activity by sorting the activated receptors into the intraluminal vesicles (ILVs). Taken together, our results suggest that extracellular Dpp morphogen gradient is shaped and interpreted by distinct endocytic trafficking pathways.