Münster Drosophila Mini-Symposium

Neuronal information conductance depends on transmission of action potentials. The
conductance of action potentials is based on three physical parameters: The axial resistance of
the axon, the axonal insulation by glial membranes, and the positioning of voltage-gated ion
channels. In vertebrates, myelin and channel clustering allow fast saltatory conductance. Here
we show that in Drosophila melanogaster voltage-gated sodium and potassium channels, Para
and Shal, co-localize and cluster in an area of motor axons resembling the axon initial segment.
Para but not Shal localization depends on peripheral glia. In larvae, relatively low levels of Para
channels are needed to allow proper signal transduction and nerves are simply wrapped by glial
cells. In adults, the concentration of Para at the axon initial segment increases. Concomitantly,
these axon domains are covered by a mesh of glial processes forming a lacunar structure that
serves as an ion reservoir. Directly flanking the voltage-gated ion channel rich axon segment,
the lacunar structures collapse forming a myelin-like insulation. Thus, Drosophila development
may reflect the evolution of myelin which forms in response to increased levels of clustered
voltage-gated ion channels.