Nociceptive neurons innervate the skin with complex dendritic arbors that respond to pain-evoking stimuli such as harsh mechanical force or extreme temperatures. Here we describe the structure and development of a model nociceptor, the PVD neuron of C. elegans, and identify transcription factors that control morphogenesis of the PVD dendritic arbor. The two PVD neuron cell bodies occupy positions on either the right (PVDR) or left (PVDL) sides of the animal in posterior-lateral locations. Imaging with a GFP reporter revealed a single axon projecting from the PVD soma to the ventral cord and an elaborate, highly branched arbor of dendritic processes that envelop the animal with a web-like array directly beneath the skin. Dendritic branches emerge in a step-wise fashion during larval development and may use an existing network of peripheral nerve cords as guideposts for key branching decisions. Time-lapse imaging revealed that branching is highly dynamic with active extension and withdrawal and that PVD branch overlap is prevented by a contact-dependent self-avoidance, a mechanism that is also employed by sensory neurons in other organisms. With the goal of identifying genes that regulate dendritic morphogenesis, we used the mRNA-tagging method to produce a gene expression profile of PVD during late larval development. This microarray experiment identified > 2,000 genes that are 1.5X elevated relative to all larval cells. The enriched transcripts encode a wide range of proteins with potential roles in PVD function (e.g., DEG/ENaC and Trp channels) or development (e.g., UNC-5 and LIN-17/frizzled receptors). We used RNAi and genetic tests to screen 86 transcription factors from this list and identified eleven genes that specify PVD dendritic structure. These transcription factors appear to control discrete steps in PVD morphogenesis and may either promote or limit PVD branching at specific developmental stages. For example, time-lapse imaging revealed that MEC-3 (LIM homeodomain) is required for branch initiation in early larval development whereas EGL-44 (TEAD domain) prevents ectopic PVD branching in the adult. A comparison of PVD-enriched transcripts to a microarray profile of mammalian nociceptors revealed homologous genes with potentially shared nociceptive functions. We conclude that PVD neurons display striking structural, functional and molecular similarities to nociceptive neurons from more complex organisms and can thus provide a useful model system in which to identify evolutionarily conserved determinants of nociceptor fate.
Bibliographical noteFunding Information:
GFP reporter strains for transcripts enriched in the PVD/OLL data set were obtained from the British Columbia C. elegans Gene Expression Consortium and are listed in Supplemental Table 2 . Some of the nematode strains were provided by the Caenorhabditis Genetics Center, which is funded by the NIH National Center for Research Resources (NCRR). All studies in this work used C. elegans hermaphrodites.
We thank Oliver Hobert for helpful advice on the manuscript; Sylvia Lee for the mec-7::RFP transgenic line; Chieh Chang and Sarah Kucenas for help with time-lapse imaging; Braden Boone, John Mote and Shawn Levy of the Vanderbilt Functional Genomics Shared Resource (FGSR) for help with microarray experiments; Shenglong Wang, Nurith Kurn and Joe Don Heath of NuGEN Technologies for help with RNA amplification; members of the Miller lab for technical advice and for comments on the manuscript. Some of the strains used in this work were provided by the C. elegans Genetic Center which is supported by NIH NCRR . This work was supported by U. S.-Israel Binational Science Foundation Grant 2005036 (M.T. and D.M.M.)), NIH R01 NS26115 (D.M.M.), R21 NS06882 (D.M.M.), NIH F31 NS49743 (J.D.W.) and by NIH grants to Vanderbilt University: P30 CA68485 , P60 DK20593 , P30 DK58404 , HD15052 , P30 EY08126 and PO1 HL6744 .
- Dendritic morphogenesis
- Nociceptor development
- Transcription factors