Control of signaling molecule range during developmental patterning

Abstract

Tissue patterning, through the concerted activity of a small number of signaling pathways, is critical to embryonic development. While patterning can involve signaling between neighbouring cells, in other contexts signals act over greater distances by traversing complex cellular landscapes to instruct the fate of distant cells. In this review, we explore different strategies adopted by cells to modulate signaling molecule range to allow correct patterning. We describe mechanisms for restricting signaling range and highlight how such short-range signaling can be exploited to not only control the fate of adjacent cells, but also to generate graded signaling within a field of cells. Other strategies include modulation of signaling molecule action by tissue architectural properties and the use of cellular membranous structures, such as signaling filopodia and exosomes, to actively deliver signaling ligands to target cells. Signaling filopodia can also be deployed to reach out and collect particular signals, thereby precisely controlling their site of action.

Keywords: BMP; Cytoneme; Dpp; Embryo; FGF; HSPG; Hh; Nodal ECM; Signaling; Tissue architecture; Wg; Wnt; stem cell.

Fig. 1

Short-range signaling in tissue patterning. a Drosophila germline stem cell (GSC) identity is maintained by Dpp secreted by adjacent niche cap cells. Dally, secreted by and presented on cap cells, promotes short-range signaling possibly as a ligand co-receptor. Hemocytes also deposit collagen IV in between cap cells and GSCs, which binds Dpp and restricts its diffusion. Expression of the Tkv receptor on escort cells (EC) acts as a ligand sink. b Intestinal stem cell identity (yellow) is maintained by neighbouring Paneth cells (green) that secrete Wnt3. Wnt3 is bound by Fzd receptors on adjacent cells. Membrane clearance of the ubiquitin ligases Rnf43 and Znf3 is driven by the stem cell factors Lgr4/5 and R-spondin to maintain Fzd levels. c Left panel maternally loaded ndr1 in dorsal margin cells and yolk syncytial layer (YSL) ndr1 expression drives early Nodal signaling (pink) in the dorsal-most cells of the presumptive mesoendoderm. Middle subsequently a positive feedback loop potentiates the Nodal signal which begins to signal to adjacent cells. The ubiquitously expressed miR430 negatively regulates ndr1 and lft1/2 expression. Right by the 50% epiboly stage, miR430 expression is lost, enabling lft1/2 expression which inhibits further activation of Nodal signaling, thereby restricting the domain of responding cells to 5–6 cell tiers. The Nodal signal is maintained over several hours, potentially by signaling from internalised receptors. The cells that receive the signal for the longest (purple) have higher levels of pSmad2 resulting in graded signaling

Fig. 2

Influence of tissue architecture on signaling range. a Dorsally secreted Dpp–Scw heterodimers bind to collagen IV along with Sog and Tld. Inhibitory complex formation on Collagen IV (ColIV) acts to restrict dorsolateral signaling but also enables release of a freely diffusing shuttling complex by Tsg. Proteolytic cleavage of Sog by Tld releases the Dpp–Scw heterodimer which, if dorsally localized, can promote Dpp signaling. However, if cleavage occurs dorsolaterally, then the inhibitory complex is reformed. b Left panel the zebrafish lateral line epithelial primordium migrates along the flanks of the fish and secretes FGF ligands. Middle apical constriction of the epithelial cells drives the formation of a ‘rosette’ shaped organ with a central microlumen into which FGF is secreted. Right after FGF levels reach a threshold, the cells stop migrating and continue mechanosensory organ development. c Left panel the early chick gut epithelium (green) retains a stem cell-like identity (Lgr⁺, Sox9⁺) in response to Wnt signaling and secretes Shh to drive low-level Bmp4 expression in the underlying mesenchyme. Middle mechanical stress later results in the buckling of the epithelium. Right the concentration of Shh at the tip of the folds results in increased Bmp4 expression in the fold mesenchyme. BMP4 signals back to the gut epithelium to repress Wnt signaling to promote the differentiation of the fold epithelium, or villus, and restrict the stem cell pool to those cells at the base of the fold. For simplicity, the cytoplasmic colour is used to depict the stem cell or differentiated fates, depending on the presence or absence of Wnt signaling, respectively, whereas the nuclear colour depicts expression of Shh or Bmp4

Fig. 3

Cytonemes deliver and reach out for ligands in Air Sac Primordium development. a Schematic of the wing disc in third instar Drosophila larvae showing the tracheal branch, bound to the wing disc, with the ASP budding from the transverse connective (grey box) in response to the morphogens Dpp (purple, A/P border), Wg (green), and FGF (orange). b Enlarged view of the box in a showing that cells of the medial ASP (purple) extend short Tkv-loaded cytonemes and long FGFR-loaded cytonemes from the tip to capture distant Dpp and FGF secreted by cells of the wing disc epithelium. c Magnified, 90 °C rotated view in b showing that myoblasts underlying the ASP extend Fz-loaded cytonemes toward Wg-expressing cells, activating signaling to inhibit Dl expression. In turn, myoblasts extend cytonemes carrying Dl to the ASP, activating Notch signaling to promote correct ASP development

Fig. 4

Hh hitches a ride on exosomes that travel along cytonemes in the Drosophila wing disc. Left panel Hh producing cells (pink) of the wing disc epithelium produce cytonemes that extend over the domain of Hh signaling. Right Hh and its co-receptor Ihog are initially secreted apically before being endocytosed in a Disp and Rab5-dependent manner. Re-internalised Hh, Ihog, and Disp undergo transcytosis and are secreted basally; some are found within MVBs on intraluminal vesicles (grey). MVBs fuse with the basal membrane and release exosome-bound Hh, Ihog, and Disp that travel along cytonemes