2000 West Coast Worm Meeting abstract 70
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Biology Dept., University of Utah, Salt Lake City, UT 84112
We previously characterized the behavior of migrating GABA motorneuron growth cones in C. elegans larvae using time-lapse confocal microscopy. VD growth cones exhibit specific behaviors that result from the interaction between growth cones and different cellular substrates encountered during migration . The most dramatic behavior exhibited by migrating VD motorneuron growth cones in vivo is their collapse at, and subsequent extension beyond the dorsal body wall muscle. To identify molecules required for specific growth cone behaviors such as collapse we characterized the motorneuron axon outgrowth phenotype of existing uncoordinated mutants. One mutant, unc-119(ed3), possessed abnormally branched axons, most of which were located at the body wall muscle. This suggested that unc-119(ed3) growth cones extend supernumerary branches between the muscle and epidermis. Analysis of the UNC-119 expression pattern indicated that UNC-119 is expressed in neurons. Expression of UNC-119 under heterologous promoters demonstrated that it functions cell-autonomously to suppress axon branching. However, time-lapse analysis of motor neuron development and axon outgrowth indicated that UNC-119 does not function during growth cone migration. Comparison of axon outgrowth patterns 1 hour and 48 hours after the completion of growth cone migration indicated that axon branching occurs after axon outgrowth is completed in unc-119(ed3) mutants. Time-lapse analysis of established axons demonstrated that secondary growth cones sprout from motorneuron axons and cell bodies. These secondary growth cones extend extra branches to the dorsal nerve cord. Concurrently axonal extensions along the dorsal nerve cord are retracted. As a result, unc-119(ed3) adults have many commissural branches as well as large gaps in the dorsal nerve cord. Synapse development occurs after outgrowth is completed. To determine if synaptogenesis is affected we characterized synapses in unc-119(ed3) mutants. Synapses form and are morphologically normal as determined by electron microscopy. However, synapses are localized to inappropriate locations of the axon, including branches and dendritic regions. These data suggest that UNC-119 is a member of a new class of cell intrinsic factors that preserve the architecture of the nervous system and regulate synapse localization.