Understanding the Mechanism of Dscam Mediated Contact Dependent Repulsion.

摘要

The precise neuronal connection is essential for normal behavior in animal. During development, such extraordinary neuronal specificity is achieved through the communication of the neurites on the response to repulsive or adhesive guidance cues. Among them, cell surface molecules mediate contact dependent adhesion or contact dependent repulsion. While contact dependent adhesion is involved in establishment of correct synaptic partnership or fasciculation of developing neurites, contact dependent repulsion is involved in self-avoidance (intraneuronal avoidance) or tiling (interneuronal avoidance). One of cell surface molecules mediating contact dependent repulsion is Dscam with two interesting features: 1) Extracellular domain of Dscam that can possibly generate 19,008 isoforms via alternative splicing presents unique homophilic binding specificity. 2) Genetic analysis in mushroom body and dendritic arborization (da) sensory neurons demonstrates that Dscam is essential for segregation of sister branches and self-avoidance, respectively. Until now how cytoplasmic domain of Dscam mediates contact via homophilic contact into repulsion has been remained unknown. My thesis research identified that 10 conserved signaling motifs in the cytoplasmic domain of Dscam is necessary for contact dependent repulsive signaling. In da sensory neurons, misexpression of mutant Dscam in all 10 motifs induced interneuronal contact without repulsion, whereas that of wildtype Dscam induced interneuronal repulsion. Further phenotypic analysis of misexpressed Dscam with mutations in different group of signaling motifs argued that four SH3 binding motifs and three SH2 binding motifs are essential for Dscam signaling and function redundantly. Next, I presented that Dock, a known Dscam interacting protein, is not required for self-avoidance in da neurons by analyzing zygotic and maternal null of dock . Finally, in order to search Dscam downstream proteins, data from genetic interaction screening and proteomic screening were described and discussed. These studies provide the first clue to understand the mechanism of Dscam mediated contact dependent repulsion in vivo.