Roles of DSCAM in axonal decussation and fasciculation of chick spinal interneurons.

The ventral midline of the embryonic neural tube, the floor plate, has a profound role in guiding axons during embryonic development. Floor plate-derived guidance cues attract or repel axons, depending on the neuronal subtype and developmental stage. Netrin-1 and its receptor, Deleted in Colon Carcinoma (DCC), are the key constituents of commissurral axons guidance cues toward the floor plate. Recent studies have implicated Down Syndrome Cell Adhesion Molecule (Dscam) as an additional Netrin-1 receptor. In this study, we examined the role of Dscam in guiding defined spinal dorsal interneuron populations. In vivo knockdown and ectopic expression of Dscam were performed in the dorsal dI1, dI2 and dI3 interneurons of chick embryos, by separately increasing or decreasing Dscam expression in each of these three specific interneuronal populations. Neuron-specific gain and loss of function of Dscam had no effect on the axonal trajectories of dI1-3 neurons. The commissural neurons, dI1c and dI2, crossed the midline, and the ipsilaterally projecting neurons, dI1i and dI3, projected ipsilaterally. However, the fasciculation of dI1 axons was diminished when Dscam expression was attenuated. Dscam is not required for either attraction to or repulsion from the floor plate. In contrast, Dscam is required for the fasciculation of axons, probably via homophilic interaction.