Binding specificities of the Dscam family of axon guidance receptors

Proper wiring of a nervous system is an intricate process necessitating that developing axons are guided in a precise manner to their final targets. Guidance occurs through the integration of environmental cues by a structure at the leading edge of the axon known as the growth cone. These cues are interpreted by receptors on the surface of the growth cone and translated into local movements. Here I describe the characterization of a new family of axon guidance receptors, the Down syndrome cell adhesion (Dscam) family. I provide data demonstrating that each transmembrane domain regulates Dscam1 localization in the Drosophila mushroom bodies. Transmembrane alternative 17.1 leads to localization in the cell bodies and dendrites, whereas alternative 17.2 localized protein to the axons.; Dscam1 was also tested for its ability to bind in a homophilic manner. I provide data showing that Dscam1 exhibits homophilic binding and that this binding is isoform-specific. I also show that all three of the alternatively spliced immunoglobulin domains in the ectodomain are required for this binding specificity. Moreover, these unique binding properties continue to be exhibited when present on the cell surface as shown through cell aggregation assays.; We have also isolated new members of the Dscam family: Dscam2, Dscam3, and Dscam4. I also demonstrate that these Dscam1 paralogues exhibit homophilic binding. Moreover, Dscam1, Dscam2, and Dscam3 are shown to bind specifically to themselves in trans and not to other family members. Dscam2 has two isoforms encoding different sequences in Ig7 and these two proteins show the same isoform-specific binding characteristics as Dscam1. All paralogues were also tested for their ability to bind in cis on the same cell surface. This binding was paralogue-specific but not isoform-specific. The non-isoform specific cis binding is demonstrated to inhibit the ability of cognate isoforms to bind in trans.; We propose a mechanism by which stochastic expression of Dscam1 isoforms gives neurons the ability to distinguish self from non-self, recognition of non-self allows for proper fasciculation of axons, while recognition of self results in axonal repulsion. This model is substantiated by data presented here by others demonstrating Dscam1 loss of function phenotypes in the Drosophila mushroom bodies and multidendritic neurons.