Highlights d The ALA axon instructs the trajectory of PVD primary dendrites in C. elegans d SAX-7/L1CAM and SAX-3/Robo form a ligand-receptor pair in trans for fasciculation d L1CAM-Robo signaling aligns F-actin dynamics in the dendrite growth cone d PVD dendrite fasciculation is important for somatosensory functions Authors

Self-avoidance is a conserved mechanism that prevents crossover between sister dendrites from the same neuron, ensuring proper functioning of the neuronal circuits. Several adhesion molecules are known to be important for dendrite self-avoidance, but the underlying molecular mechanisms are incompletely defined. Here, we show that FMI-1/Flamingo, an atypical cadherin, is required autonomously for self-avoidance in the multidendritic PVD neuron of Caenorhabditis elegans. The fmi-1 mutant shows increased crossover between sister PVD dendrites. Our genetic analysis suggests that FMI-1 promotes transient F-actin assembly at the tips of contacting sister dendrites to facilitate their efficient retraction during self-avoidance events, probably by interacting with WSP-1/N-WASP. Mutations of vang-1, which encodes the planar cell polarity protein Vangl2 previously shown to inhibit F-actin assembly, suppress self-avoidance defects of the fmi-1 mutant. FMI-1 downregulates VANG-1 levels probably through forming protein complexes. Our study identifies molecular links between Flamingo and the F-actin cytoskeleton that facilitate efficient dendrite self-avoidance.

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