Role of Drosophila Rap/Fzr (DCdh1) in Retinal Axon Targeting and its Interactions with Loco and Liprin-alpha

The development of the wild type Drosophila compound eye involves stereotypical targeting of photoreceptor axons to the specific layers of the optic ganglion, medulla and lamina, in the third instar larvae. To test the hypothesis that ubiquitin ligases play an important role during retinal axon targeting we have examined the patterns of axon targeting in the developing eye of the retina aberrant in pattern (rap/fzr) mutants. Rap/Fzr is a homolog of mammalian Cdh1, an activator of anaphase promoting complex (APC), a multi-subunit E3 ubiquitin ligase, regulating the cell cycle progression. Previous work has shown that Rap/Fzr is required during eye development for proper cell cycle regulation, glia differentiation and pattern formation. It was also necessary for proper neuromuscular junction development and circadian rhythms. Our results show that Rap/Fzr is required for proper retinal axon targeting in the developing eye. Using ro-tau-lacZ, we show that the R2-R5 axons fail to stop in the lamina and mis-target to the medulla levels. Also, mosaic analyses experiments using FLP-FRT and GAL4-UAS techniques show that Rap/Fzr functions in a cell autonomous manner. To test for possible role of other signalling molecules and interactions with Rap/Fzr, we have examined rap/fzr axon projection phenotypes in double mutant combinations with the RGS protein, locomotion defective (loco) mutants and a scaffolding protein, Liprin-. Our studies suggest that Rap/Fzr is required for proper axon targeting during Drosophila visual system development, and the phenotype is enhanced in double mutants with either loco or Liprin-. These results are consistent with other mammalian studies reporting a role of Cdh1 in axon growth and targeting and provides further insights into neuronal functions of the ubiquitin ligase APC/CCdh1. HighlightsO_LILoss of rap/fzr in the third instar Drosophila eye disc leads to photoreceptor axon overgrowth C_LIO_LIOverexpression of rap/fzr leads to photoreceptor axon leads to axon shortening and clumping C_LIO_LILoss of LocoP452 leads to photoreceptor overgrowth C_LIO_LIDouble mutants of rap and loco or rap and Liprin- show axon enhancement of the axon targeting defects in the Drosophila third instar larvae eye imaginal discs. C_LI