Title: A genetic and biochemical analysis of Dscam signaling in dendrite morphogenesis
Description: Dscam is a single-pass trans-membrane protein that is alternately spliced to generate over 38,000 different isoforms. Each Dscam protein is capable of binding itself in trans in an isoform-specific manner. Although previous work has demonstrated that Dscam has an essential role in axon development, few studies have addressed whether Dscam signaling is also important for dendrite morphogenesis. We performed a genetic analysis of Dscam's role in the development of 'da' larval sensory neurons. We found that Dscam loss of function severely disrupts the patterning of dendrites. In the absence of Dscam, individual dendrites fail to avoid the territory of their sister branches resulting in tangled dendritic arborizations. Dscam-null neurons were capable of elaborating dendrite arborizations with the correct size, orientation and complexity, suggesting that Dscam does not influence cell fate determination, but instead mediates repulsive interactions between sister branches of the same cell.

Mis-expression of a single isoform of Dscam in neighboring cells induces inappropriate repulsion between neighboring 'da' neurons. Additionally, the over-expression of Dscam significantly reduces the length and number of higher-order dendrite branches, an observation consistent with increased repulsive signaling between sister branches. Taken as a whole, we conclude that differential expression of Dscam isoforms in 'da' neurons permits developing dendrites to recognize and avoid dendrites from the same cell. In axon guidance, Dscam signals through Pak, a kinase with an important role in regulating cytoskeletal dynamics. However, we found that Pak loss of function has no effect on dendrite morphogenesis, suggesting that Dscam's signaling machinery is different in dendrites than axons. To identify proteins that physically interact with Dscam, we used immuno-affinity purification to isolate Dscam receptor complexes and performed mass spectroscopy to identify co-purifying proteins. We found a number of attractive candidates for further study, including Ncd, IF2, and an uncharacterized Rab-GAP, CG7324. The most interesting candidate identified, however, was α-Spectrin which co-purifies with Dscam specifically from neural tissue. Western blot analysis confirmed that Dscam binds to α-Spectrin but not a closely related family member, β-Spectrin. α-Spectrin loss of function phenocopies Dscam over-expression, suggesting that α-Spectrin may negatively regulate Dscam signaling in dendrites.