Homophilic Binding of Dscam Isoforms

Abstract

Nervous system function depends on the specificity of individual neuronal connections, which in turn involves recognition of cell surface molecules during neuronal development. Dscam , which encodes an immunoglobulin (Ig) superfamily cell adhesion molecule (Down syndrome cell adhesion molecule) implicated in Drosophila axonal pathfinding, can give rise to 38,016 protein isoforms that share a common domain structure but differ in three extracellular Ig domains (Ig2, Ig3, Ig7). Individual neurons express multiple isoforms, and different neurons express distinct sets of isoforms. Wojtowicz et al. overexpressed a single Dscam isoform in Drosophila midline cells and a set of interneurons whose axons cross the midline and found a marked reduction in the number of axons crossing the midline at the posterior commissure. Dscam isoforms showed homophilic binding in assays involving aggregation of fluorescent beads bound to Dscam extracellular domain-Fc chimeras, decorated beads binding to COS cells expressing Dscam, and pull-downs from extracts of Drosophila S2 cells expressing FLAG-tagged Dscam. Further, the aggregation assay indicated that different isoforms differed in the rate and extent of aggregation. However, different isoforms did not bind to each other. Binding specificity depended on all three variable Ig domains, and even isoforms that differed in only 7 to 12 amino acids of Ig7 showed little if any heterophilic binding. The authors suggest that this specificity of Dscam interaction could promote repulsive interactions between sister branches of the same axon that share their entire Dscam complement, whereas low levels of signaling from neurites that share only some isoforms could lead to adhesion. W. M. Wojtowicz, J. J. Flanagan, S. S. Millard, S. L. Zipursky, J. C. Clemens, Alternative splicing of Drosophila Dscam generates axon guidance receptors that exhibit isoform-specific homophilic binding. Cell 118 , 619-633. [Online Journal]