Branch-specific axon pruning induced by Dpr4/DIP-{Theta} transneuronal interactions
Neuronal remodeling is a conserved, late developmental mechanism to refine neural circuits. Although remodeling typically occurs with remarkable spatiotemporal precision, its underlying molecular mechanisms remain poorly understood. In the Drosophila mushroom body (MB) circuit, {gamma}-Kenyon cells ({gamma}-KCs) undergo stereotyped remodeling during metamorphosis, in which they prune their larval vertical and medial axonal branches and subsequently regrow a medial, adult-specific branch. Our previous transcriptional profiling of developing {gamma}-KCs revealed dynamic expression of Defective proboscis extension response (Dpr) proteins and their binding partners, Dpr-interacting proteins (DIPs), members of the Immunoglobulin (Ig) superfamily. Despite their established roles in neurodevelopment, how Dpr/DIPs function - given their lack of intracellular domains - remains unclear. Here, we show that overexpression of Dpr4 in developing {gamma}-KCs cell-autonomously inhibits axon pruning. Strikingly, this effect is branch-specific: the vertical axonal branch fails to prune, while the medial branch prunes normally. To our knowledge, this represents the first demonstration of branch-specific control of pruning in this system. Moreover, the adult medial branch regrows normally, indicating that pruning and regrowth are independently regulated at the level of individual branches. We demonstrate that this unique branch-specificity arises from trans-neuronal interactions between Dpr4 in {gamma}-KCs and DIP-{theta} in dopaminergic neurons that selectively innervate the vertical larval MB lobe. Furthermore, our findings suggest that this phenotype relies on an Ig2 domain of a Dpr family member, implying the involvement of a third binding partner. Leveraging this robust overexpression phenotype to probe downstream mechanisms, we find that loss of the transmembrane adhesion protein N-Cadherin suppresses the Dpr4-induced pruning defect. Together, our findings highlight the local impact of Dpr/DIP-mediated trans-neuronal interactions on the spatial regulation of remodeling, and provide genetic evidence implicating N-Cadherin as a potential downstream mediator of Dpr/DIP function within a developing neural circuit.