Tumor necrosis factor (TNF) signaling is required for inflammatory nociceptive sensitization in both Drosophila and vertebrates. In Drosophila larval model of nociceptive sensitization, UV irradiation in results in epidermal apoptosis and thermal allodynia. TNF/Eiger is produced from dying epidermal cells and acts its receptor in nociceptive sensory neurons to induce thermal allodynia. Inhibition of TNF signaling results in attenuation of nociceptive sensitization whereas epidermal apoptosis still occurs in the absence of TNF. Major gaps in this model are the precise relationship between apoptotic cell death and production of TNF/Eiger, downstream signaling mediators for TNFR/Wengen, and target genes that alter nociceptive behaviors. Here we show that apoptotic cell death and thermal allodynia are genetically and procedurally independent of each other while initiator caspase Dronc is required for both. An apoptotic function of Dronc activates downstream effector caspase leading to execution of epidermal cell death whereas a non-apoptotic function of Dronc induces activation of TNF signaling. Behavioral analyses with overexpression of full-length or processed soluble TNF/Eiger suggest that Dronc-mediated processing/secretion of TNF/Eiger is important for nociceptive sensitization. It is also supported by the fact that Dronc is required for thermal allodynia when TNF/Eiger is ectopically expressed in nociceptive sensory neurons that normally do not produce TNF/Eiger for nociceptive sensitization. We found that Traf3, Traf6, a p38 kinase, and the transcription factor nuclear factor kappa B mediates TNF signaling for nociceptive sensitization. Finally, downstream target genes of TNF signaling are revealed by sensory neuron specific microarray analysis and behavioral validation. A conserved epigenetic factor, Enhaner of zeste (E(z)) is required for thermal allodynia as a downstream target gene of TNF/Eiger signal transduction. Our findings suggest that an initator caspase is involved in TNF processing/secretion during nociceptive sensitization and that TNF pathway activation leads to transcription of genes required for sensory neurons to sensitize. These findings have implications for both the evolution of inflammatory caspase function following tissue damage signals and the action of TNF during sensitization in vertebrates
