-Spectrin functions independently of Ankyrin to regulate the establishment and maintenance of axon connections in the Drosophila embryonic CNS

- and -Spectrin are major components of a submembrane cytoskeletal network connecting actin filaments to integral plasma membrane proteins. Besides its structural role in red blood cells, the Spectrin network is thought to function in non-erythroid cells during protein targeting and membrane domain formation. Here, we demonstrate that -Spectrin is required in neurons for proper midline axon guidance in the Drosophila embryonic CNS. In -spectrin mutants many axons inappropriately cross the CNS midline, suggesting a role for -Spectrin in midline repulsion. Surprisingly, neither the Ankyrin-binding nor the pleckstrin homology (PH) domains of -Spectrin are required for accurate guidance decisions. -Spectrin is dependent upon -Spectrin for its normal sub-cellular localization and/or maintenance, whereas -spectrin mutants exhibit a redistribution of -Spectrin to the axon scaffold. -spectrin mutants show specific dose-dependent genetic interactions with the midline repellent slit and its neuronal receptor roundabout (robo), but not with other guidance molecules. The results suggest that -Spectrin contributes to midline repulsion through the regulation of Slit-Robo pathway components. We propose that the Spectrin network is playing a role independently of Ankyrin in the establishment and/or maintenance of specialized membrane domains containing guidance molecules that ensure the fidelity of axon repulsion at the midline.