The sensation of pain triggers self-protective responses to noxious insults. Pain is perceived by specialized sensory neurons called nociceptors that utilize elaborate webs of dendritic processes to detect harmful stimuli. These features of nociceptive neurons are universally observed in animals and are thus likely to depend on conserved genetic programs that control nociceptor morphogenesis throughout phylogeny. The mechanisms that govern the creation of these complex structures are poorly understood but recent studies in Drosophila have established that sensory neuron morphology is defined by surprisingly diverse arrays of transcription factors. The identities of downstream genes and their orchestration by these transcriptional networks are largely unknown, however. Here, we propose to solve this problem by exploiting a powerful gene expression profiling method to identify targets of transcription factors that control morphogenesis of a model sensory neuron. In the nematode, C. elegans, a single pair of bilaterally symmetric neurons, PVDL and PVDR envelop the body with a regular array of highly branched dendrites that can be readily visualized with a fluorescent (GFP) marker in living animals. The PVDs respond to strong mechanical force (""""""""harsh touch"""""""") to trigger a withdrawal reflex, a behavior indicative of nociceptive function. We have used a cell specific microarray profiling strategy to identify transcripts that are highly expressed in PVD neurons. Specifically, we will: (1) Use available mutants and RNAi clones to test transcription factor genes in this data set for roles in PVD dendritic morphogenesis;(2) Use the mRNA tagging method to reveal genes downstream of the conserved LIM- homeodomain protein, MEC-3, an established determinant of PVD dendritic branching and;(3) Extend this mRNA tagging strategy to at least five additional key transcription factor genes identified in Aim 1. These data sets of transcription factor target genes will provide a foundation for future studies that link transcriptional control to the cell biology of dendritic arborization. We believe that our approach represents a useful alternative to direct studies of mammalian tissues and moreover, that it offers a uniquely powerful strategy for identifying molecules with fundamental, conserved roles in nociceptor morphogenesis.
Specialized nerve cells that mediate the sensation of pain utilize an elaborate array of spaghetti-like sensory processes directly beneath the skin to detect noxious stimuli. To facilitate the identification of genes that govern the creation of these sensory networks, we propose a genetic approach in the nematode C. elegans, a simple model organism with a single pair of PVD sensory neurons that display a stereotypical and readily visible array of nerve processes. The results of this work should reveal similar human genes with crucial roles in the creation of sensory neuron arrays and therefore potentially lead to treatments for injuries or diseases that disrupt human sensory neuron development.
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