Neurons are polarized cells possessing axonal and dendritic processes with distinct morphologies, cytoskeletal architectures and signaling properties. Polarity is established during neuronal differentiation and is essential for the assembly and function of the nervous system. The trajectories of axons and dendrites are shaped by growth cones at their tips responding to guidance cues in their environment. We are interested in understanding the cellular and molecular mechanisms that polarize neurons and pattern their axons and dendrites. The nematode Caenorhabditis elegans has a simple, well-described nervous system that can be visualized at a single cell resolution in living animals using fluorescent protein reporters. Neurons in C. elegans extend processes with characteristic trajectories along the dorsoventral and/or anteroposterior (AP) body axis and form reproducible synaptic connections. Using genetic and molecular approaches, we identified several molecules that play key roles in controlling neuronal polarity as well as molecules that influence axon formation, extension and maintenance.

Wnts are secreted proteins that control a wide range of essential developmental processes. Wnt binding to the serpentine receptor Frizzled activates signaling cascades that cause changes in gene transcription and organization of the cytoskeleton. We uncovered a new role for Wnts and Frizzleds in establishing the AP polarity of the mechanosensory neurons ALM and PLM, which extend anterior and posterior processes with different attributes. Disruption of Wnt signaling leads to a complete inversion of ALM and PLM polarity: the anterior process adopts the length, branching pattern and synaptic properties of the wild-type posterior process and vice versa. Wnts act directly on PLM via a Frizzled and different but overlapping sets of Wnts function redundantly to polarize neurons in different body regions. Together with other studies, these results reveal that Wnts are global organizers of the nervous system along the AP body axis in C. elegans: they regulate AP neuronal polarity, axon guidance and cell migration.

Our analysis of neuronal polarity and Wnt signaling has also led to new insight into the mechanism of Wnt secretion. We found that the retromer, which is a conserved protein complex that mediates transcytosis and endosome-to-Golgi protein trafficking, is needed in Wnt producing cells to generate an effective Wnt signal. Deletion mutations of retromer subunits cause ALM and PLM polarity and other Wnt-related defects.

Many Wnts are expressed during development, yet it is unclear how the cellular response to different Wnts is coordinated. We identified a conserved transmembrane RING finger protein, PLR-1, that governs the response to Wnts by reducing the cell surface levels of the Wnt receptor Frizzled. Loss of PLR-1 activates Wnt signaling, while ectopic expression of PLR-1 blocks Wnt signaling. When PLR-1 and Frizzled are coexpressed, Frizzled is not detected on the cell surface but instead is colocalized with PLR-1 in endosomes. Our results show for the first time that regulation of Frizzled trafficking can control the spatial and temporal response to Wnts.