Abstract
Author SummaryAxons and dendrites of synaptic partners must be targeted to a common region of the developing neural network so that appropriate connections can be formed. The mechanisms underlying this targeting are incompletely understood. We showed previously that a positional cue (Slit) acting in the medio-lateral axis of the Drosophila nerve cord controls the position of sensory terminals independently of their synaptic partners. This work revealed that there might be additional cues operating in a similar fashion in the dorso-ventral axis of the nerve cord. Here we report the discovery of a dorso-ventral system of positional cues, in the form of a gradient of secreted Semaphorin 2a acting at right angles to the Slit gradient, and membrane bound Semaphorin 1a differentially distributed across the neuropile. The two Semaphorins dictate the termination positions of sensory axons in the dorso-ventral axis. Together with a third signal acting in the antero-posterior axis, Semaphorins and Slit deliver axons to appropriate volumes of the neural network. These studies support a model in which axons branch and terminate, independently of synaptic partners, in response to pervasive systems of volumetric positional cues.
Highlights
During the development of neural circuitry, neurons of different kinds must establish specific synaptic connections by selecting appropriate targets from large numbers of different alternatives
We showed previously that a positional cue (Slit) acting in the medio-lateral axis of the Drosophila nerve cord controls the position of sensory terminals independently of their synaptic partners
We report the discovery of a dorso-ventral system of positional cues, in the form of a gradient of secreted Semaphorin 2a acting at right angles to the Slit gradient, and membrane bound Semaphorin 1a differentially distributed across the neuropile
Summary
During the development of neural circuitry, neurons of different kinds must establish specific synaptic connections by selecting appropriate targets from large numbers of different alternatives The range of these alternative targets is reduced by well organised patterns of growth, termination, and branching that deliver the terminals of appropriate pre- and postsynaptic partners to restricted regions of the developing nervous system. We have used the axons of embryonic Drosophila sensory neurons as a model system in which to study the way in which growing neurons are guided to terminate in a specific region of the developing nervous system These neurons have their cell bodies in the periphery of the embryo, either close to or embedded in the body wall. Because the sensory neurons provide us with an accessible set of cells whose terminals grow to different parts of the forming neuropile, we can readily use these neurons to investigate the guidance mechanisms
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