A critical affinity window for IgSF proteins DIP-alpha and Dpr10 is required for proper motor neuron arborization

For flies to walk properly, motor neurons (MNs) from the ventral nerve cord (VNC) need to reach the correct muscle, and arborize appropriately during development. The canonical view of how this is achieved is that cell surface proteins are expressed pre- and post-synaptically that bind to each other like molecular "lock-and-keys" that guide neurons to their targets. The binding affinities of these molecules can vary by more than 100-fold. In the fly leg neuromuscular system, three MNs express DIP- and their target muscles express its cognate partner, dpr10, both of which encode members of the Immunoglobulin superfamily (IgSF). Although, both of these molecules are necessary for the maintenance of MN-muscle contacts, the role that specific affinities play in this process has not been examined. Here we use knock-in mutations into DIP- and dpr10 that either decrease or increase the affinity between these two proteins. Compared to control animals, decreasing the affinity results in phenotypes similar to DIP- or dpr10 null animals, where MN axons fail to maintain contacts with their muscle targets and retract their filopodia, resulting in stunted and/or branchless axons. We also find that the three DIP--expressing motor neurons behave differently to the loss of affinity. Surprisingly, if the affinity increases past a certain threshold, a similar branchless phenotype is observed in adult legs. Live imaging during pupal development shows that MN filopodia are unable to productively engage their muscle targets and behavioral assays suggest that these defects lead to locomotor deficits. These data suggest that CAM affinities are tuned to a specific range to achieve proper neuronal morphology.