Stereotyped Terminal Axon Branching of Leg Motor Neurons Mediated by IgSF Proteins DIP-α and Dpr10
The ability of animals to perform coordinated movements depends on the precise organization of neural circuits controlling motor function. Motor neurons (MNs), which are key components of these circuits, must project their axons out of the central nervous system and form precise terminal branching patterns at specific muscles in the periphery. By focusing on the Drosophila adult leg neuromuscular system we show that the stereotyped terminal branching of a subset of leg MNs is mediated by interacting transmembrane Ig superfamily (IgSF) proteins DIP- and Dpr10, present in MNs and target muscles, respectively. Importantly, the DIP-/Dpr10 interaction is needed only after MN axons reach the vicinity of their muscle targets. Live imaging of this process suggests that precise terminal branching patterns are gradually established by DIP-/Dpr10-dependent interactions between fine axon filopodia and developing muscles. Further, different leg MNs depend on the DIP- and Dpr10 interaction to varying degrees that correlate with the morphological complexity of the MNs and their muscle targets, suggesting that some MNs depend upon multiple sets of interacting proteins to establish terminal axon branching.