Fig. 4.
NMJ assembly in Drosophila and in C. elegans. (A) The Drosophila NMJ is a glutamatergic synapse. The Wnt ligands Wingless (Wg) and Wnt5 are released from presynaptic boutons. Binding to the Wnt receptor Frizzled on the postsynaptic membrane, Wg is thought to stimulate a Frizzled nuclear import (FNI) pathway for gene expression. This pathway might be crucial for both pre- and postsynaptic differentiation. Wg also activates Frizzled on the presynaptic membrane to promote presynaptic assembly via a pathway dependent on Gsk3β and MAP1B. Conversely, Wnt5 directly binds to and activates the receptor tyrosine kinase Derailed (Drl) on the postsynaptic membrane for postsynaptic assembly. (B) The C. elegans DA9 neuron, which is positioned in the ventral region of the worm, sends a dendrite anteriorly (green) and an axon (blue) that first migrates posteriorly; this soon turns toward the dorsal region and travels anteriorly in the dorsal region. It does not form synapses until it reaches a more anterior region, leaving an asynaptic domain in the axon. LIN-44/Wnt is produced by hypodermal cells and activates LIN-17/Frizzled in the axon to suppress presynaptic assembly.