FIGURE 3.

Modes of extracellular Wnt transport. The table shows a diagrammatic representation of different modes employed to facilitate the extracellular travel of hydrophobic Wnt proteins and the tissues these mechanisms are observed. (A) Swim, an extracellular carrier protein that binds secreted Wg in Drosophila wing epithelium and enables their transport to the receiving cells. (B) Lipoprotein particle, a complex of proteins, phospholipids and fats. Wnts are transferred to lipoproteins from Evi, which increases the solubility of Wnts in the extracellular space and allows them to reach the receiving cells. (C) HSPGs, these membrane-bound glypicans bind to lipid-modified Wnts and enable their transport by moving along the membrane and relaying Wnts to other HSPGs and finally to Fz receptors present on receiving cells. (D) Exosomes, small vesicular structures loaded with Wnts both free and Evi bound form in the MVBs and released outside cells, further carrying them to receiving cells. (E) Cytonemes, cell membrane extensions directed by the cytoskeletal reorganization that extend up to several cell distances and carry Wnts along with them to the receiving cells. In some cases, the cytonemes extending from the receiving cells carrying Fz receptors bring receptors to the Wnt-producing cells.