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

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Abstract
α- and β-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 β-Spectrin is required in neurons for proper midline axon guidance in the Drosophila embryonic CNS. In β-spectrin mutants many axons inappropriately cross the CNS midline, suggesting a role forβ -Spectrin in midline repulsion. Surprisingly, neither the Ankyrin-binding nor the pleckstrin homology (PH) domains of β-Spectrin are required for accurate guidance decisions. α-Spectrin is dependent upon β-Spectrin for its normal subcellular localization and/or maintenance, whereas α-spectrin mutants exhibit a redistribution of β-Spectrin to the axon scaffold.β -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 β-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.

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