A major project in the laboratory is to determine the mechanisms involved in controlling alternative splicing of the Drosophila Down Syndrome Cell Adhesion Molecule (Dscam) gene. The Dscam gene, which was discovered in Larry Zipursky’s lab, encodes an axon guidance receptor that is most similar to one of the human genes implicated in causing Down Syndrome. Perhaps the most interesting aspect of this gene is that it is the most extensively alternatively spliced gene that we know of in any organism. This single gene can generate over 38,000 different isoforms by virtue of extensive alternative splicing. In fact, the number of proteins generated by this gene is two to three times the number of genes in the entire Drosophila genome! It is thought that the diversity of Dscam isoforms contributes to the specificity of neuronal wiring. We have found that the alternative splicing of Dscam transcripts is regulated throughout development and in a tissue-specific manner. Moreover, this regulated alternative splicing is evolutionarily conserved. We are now using RNAi, genetics, evolutionary, and biochemical approaches to identify trans-acting factors and cis-acting RNA sequences that participate in controlling this extraordinarily complex alternative splicing event.

By performing an RNAi screen in which we depleted hundreds of RNA binding proteins in the Drosophila genome, we have identified about 40 different proteins that regulate the splicing of various Dscam exons. We have also identified several splicing regulatory elements that are required for the inclusion of any alternative exons and we are currently working to identify the mechanisms by which these elements act. In addition, we are using a variety of techniques to determine the role of the different Dscam isoforms in specifying axon guidance in the fly. Although there are differences in the properties of the human and Drosophila Dscam proteins, these studies may lead to insights regarding the role of this gene in the development of Down syndrome in humans.