# Shaping and interpretation of Dpp morphogen gradient by endocytic trafficking

## TL;DR

This study shows how cells interpret the Dpp morphogen gradient by controlling how long the signal lasts inside the cell, not just how much Dpp is present.

## Contribution

The study reveals that intracellular signal duration, not extracellular Dpp levels, determines how cells interpret morphogen gradients.

## Key findings

- Dynamin-mediated internalization is required for Dpp signaling activation.
- Signal termination occurs at the multivesicular body via ESCRT-dependent sorting, not Rab7-mediated lysosomal degradation.
- Blocking MVB formation expands the Dpp signaling gradient without changing extracellular Dpp distribution.

## Abstract

Dpp/BMP is a morphogen that controls the patterning and growth of the Drosophila wing disc. Although endocytic trafficking has been proposed to influence both extracellular Dpp distribution and signaling, how this process shapes and interprets the Dpp gradient under physiological conditions remains unclear due to limitations in visualizing endogenous Dpp. Here, we generated fluorescent protein-tagged functional dpp alleles that allow simultaneous visualization of extracellular and intracellular Dpp distributions. Using these tools, we found that, while Dynamin-mediated internalization is required for Dpp signaling activation, Rab5-mediated early endosomal trafficking is dispensable for Dpp spreading and signaling initiation but is required for signal termination by promoting the downregulation of activated receptors. We show that Dpp signaling is terminated at the multivesicular body (MVB), likely through ESCRT-dependent sorting of activated receptors into intraluminal vesicles (ILVs), rather than via Rab7-mediated lysosomal degradation. Notably, blocking MVB formation expanded the Dpp signaling gradient without altering the extracellular Dpp gradient, thus compromising extracellular Dpp gradient interpretation. Together, our findings reveal that the extracellular Dpp gradient is shaped by Dynamin-dependent internalization and interpreted through the duration of intracellular signaling.

During development, cells need instructions to form organs and tissues with the right size and shape. These instructions often come from signaling molecules called morphogens, which travel through tissues in gradients. One such morphogen, called Dpp, helps shape the fruit fly wing by telling cells where they are and what to become. To better understand how cells respond to Dpp, we created fruit flies that produce a fluorescent version of the molecule, allowing us to track this molecule both outside and inside cells. We found that cells must uptake Dpp from their surface to start the signaling. The signal is terminated when Dpp reaches a specific compartment inside the cell. When this termination step is blocked, cells continue responding to Dpp longer than they should—even when the amount of Dpp outside the cells remains unchanged. This shows that cells respond not only to how much Dpp they see on the cell surface, but also to how long they are exposed to it inside cells. Our findings help explain how cells use these signals to determine where they are and what they should become during development.