Gradient Formation of the TGF-β Homolog Dpp

Abstract

Secreted morphogens such as the Drosophila TGF-beta homolog Decapentaplegic (Dpp) are thought to spread through target tissues and form long-range concentration gradients providing positional information. Using a GFP-Dpp fusion, we monitored a TGF-beta family member trafficking in situ throughout the target tissue and forming a long-range concentration gradient. Evidence is presented that long-range Dpp movement involves Dpp receptor and Dynamin functions. We also show that the rates of endocytic trafficking and degradation determine Dpp signaling range. We propose a model where the gradient is formed via intracellular trafficking initiated by receptor-mediated endocytosis of the ligand in receiving cells with the gradient slope controlled by endocytic sorting of Dpp toward recycling versus degradation.

Xenopus Lefty requires proprotein cleavage but not N‐linked glycosylation to inhibit nodal signaling

“…It remains possible that in our range of movement experiments, we are only detecting a fraction of the Xlefty gradient in regions where enough protein accumulates to generate a detectable immunostaining signal, while lower levels are in fact functional beyond these domains. A similar problem exists with the published studies on EGFP-tagged TGF␤-related molecules (Entchev et al, 2000;Teleman and Cohen, 2000;Sakuma et al, 2002;Belenkaya et al, 2004;Williams et al, 2004;Nakamura et al, 2006), which must also rise above a detection threshold that is currently unknown with respect to the level that induces downstream gene pathways. This issue is further complicated because the tag location in principle cannot distinguish between the active or inactive ligands or indeed the proprotein forms.…”

Two‐dimensional morphogen gradient in Xenopus: Boundary formation and real‐time transduction response

“…There is, therefore, general agreement that a morphogen gradient is formed in early Xenopus and fish embryos by diffusion in intercellular space. In contrast, it is likely that gradient spreading in the Drosophila wing disc, which occurs on a much slower timescale, uses other mechanisms, including transcytosis, argosomes, and cytonemes (Ramirez-Weber and Kornberg, 1999;Entchev et al, 2000;Dubois et al, 2001;Greco et al, 2001;Torroja et al, 2004).…”

Morphogen gradients, positional information, and Xenopus: Interplay of theory and experiment

“…However, growth factor-soaked beads have been used to visualize artificial gradients of activin and TGF-␤1 at physiologically functional levels that produce the appropriate dose-dependent effects (McDowell et al, 2001). In Drosophila, use of green fluorescent protein (GFP) -tagged Decapentaplegic (Dpp) has allowed the visualization of a gradient of that morphogen (Entchev et al, 2000;Teleman and Cohen, 2000).…”

“…The transcytosis model has arisen from several observations of Dpp and Wingless gradient formation in Drosophila. Direct visualization of morphogen gradients (using either antibodies or biologically active GFP-conjugated morphogens) has shown a significant fraction of the molecules to be localized to intracellular vesicles (Entchev et al, 2000). In the case of Dpp, its spread and consequent gradient formation is dependent on dynamin, a GTPase that mediates clathrin-coated endocytosis that is mutated in shibire mutant embryos (see Entchev and Gonzalez-Gaitan, 2002).…”

“…It also could be a mechanism for preventing a morphogen from escaping from a tissue. Such escape could be problematic in cases such as the Drosophila imaginal disc where the epithelial sheet being patterned is highly folded and such escape could allow inappropriate hopping of the morphogen from one fold to another (Entchev et al, 2000). However, the mathematical modeling described above shows that endocytosis is just as important for diffusion-based gradient formation as it is for transcytosis.…”