Visualizing Wnt secretion from Endoplasmic Reticulum to Filopodia

Abstract

Wnts are a family of secreted palmitoleated glycoproteins that play a key role in cell to cell communications during development and regulate stem cell compartments in adults. Wnt 20 receptors, downstream signaling cascades and target pathways have been extensively studied while less is known about how Wnts are secreted and move from producing cells to receiving cells. We used the synchronization system called Retention Using Selective Hook (RUSH) to study Wnt trafficking from endoplasmic reticulum to Golgi and then to plasma membrane and filopodia in real time. Consistent with prior studies, inhibition of porcupine (PORCN) or 25 knockout of Wntless (WLS) blocked Wnt exit from the ER. Indeed, WLS was rate-limiting for Wnt ER exit. Wnt-containing vesicles paused at sub-cortical regions of the plasma membrane before exiting the cell. Wnt-containing vesicles were transported to adjacent cells associated with filopodia. Increasing the number of filopodia by expression of LGR5 in the producing cell increased the ability of a cell to send a Wnt signal. The RUSH system is a 30 powerful tool to provide new insights into the Wnt secretory pathway.

Cytonemes in development

“…Furthermore, the downstream regulators of the PCP pathway, Cdc42 and N-Wasp control length and branching behavior of Wnt cytonemes [23 •• ]. In the mouse intestinal crypt, filopodia formation can be stimulated through activation of the stem cell markers Lgr4/5, therefore, it is tempting to speculate that these filopodia carry signalling factors [32,33]. In conclusion, it seems that signalling pathway-specific components such as co-receptors are required to regulate the filopodia nucleation machinery.…”