A Wnt-Frz/Ror-Dsh Pathway Regulates Neurite Outgrowth in Caenorhabditis elegans

Abstract

One of the challenges to understand the organization of the nervous system has been to determine how axon guidance molecules govern axon outgrowth. Through an unbiased genetic screen, we identified a conserved Wnt pathway which is crucial for anterior-posterior (A/P) outgrowth of neurites from RME head motor neurons in Caenorhabditis elegans. The pathway is composed of the Wnt ligand CWN-2, the Frizzled receptors CFZ-2 and MIG-1, the co-receptor CAM-1/Ror, and the downstream component Dishevelled/DSH-1. Among these, CWN-2 acts as a local attractive cue for neurite outgrowth, and its activity can be partially substituted with other Wnts, suggesting that spatial distribution plays a role in the functional specificity of Wnts. As a co-receptor, CAM-1 functions cell-autonomously in neurons and, together with CFZ-2 and MIG-1, transmits the Wnt signal to downstream effectors. Yeast two-hybrid screening identified DSH-1 as a binding partner for CAM-1, indicating that CAM-1 could facilitate CWN-2/Wnt signaling by its physical association with DSH-1. Our study reveals an important role of a Wnt-Frz/Ror-Dsh pathway in regulating neurite A/P outgrowth.

Author Summary

How do individual nerve fibers find their way along specific paths in a complex environment such as the developing central nervous system? A principal mechanism in axon guidance is binding of a receptor protein on the axon surface to a guidance molecule. However, it remains a mystery exactly how a limited number of guidance molecules can pilot the growth of billions of neurons. In this study, we used the nematode worm Caenorhabditis elegans to establish a system to follow the outgrowth of a single neurite. We then searched for genes affecting neurite development and uncovered a guidance molecule, CWN-2 (a member of the well-known Wnt family of signaling proteins), which attracts neurite growth. We also identified two Wnt receptors (CFZ-2 and MIG-1, from the Frizzled family), a co-receptor (CAM-1, from the Ror family), and an effector (DSH-1, from the Dsh family). Together these proteins convey the Wnt guidance signal into the neurite and influence its growth. In addition, our study reveals that the restricted spatial localization of the Wnt signal, together with the specific combination of Wnt receptors and effectors expressed in the neurite, are important for the complex function of guidance molecules.