The interplay between TNF signaling, apoptosis, and tissue damage-induced pain sensitization in Drosophila larvae. Juyeon Jo¹, Felona Gunawan², Daniel Babcock¹, Michael Galko¹. 1) UT M.D. Anderson cancer center, Housotn, TX; 2) Rice University.

   Nociception is the detection of painful stimuli and is a fundamental protective mechanism to prevent potential tissue damage. Recently we established a novel nociceptive sensitization model using Drosophila larvae where tissue damage induced by UV radiation results in both apoptotic epidermal cell death and thermal allodynia, or aversive withdrawal to previously innocuous temperatures. Although TNF signaling and apoptotic cell death were previously correlated in the development of allodynia it is not yet known whether TNF-mediated induction of allodynia functionally requires apoptosis or other canonical downstream members of the TNF signaling pathway. To clarify the functional relation between UV-induced apoptosis and allodynia, components of the canonical cell death pathway both upstream and downstream of the initiator caspase Dronc were knocked down in the epidermis and both cell death and UV-induced thermal allodynia were measured. Surprisingly, we found that only Dronc knockdown was capable of blocking allodynia (whereas all knockdowns blocked epidermal apoptosis). Therefore, we suggest that Dronc has a non-apoptotic function in the induction UV-induced allodynia. This conclusion is supported by the fact that Dronc is required for the ectopic allodynia caused by TNF misexpression in nociceptive sensory neurons, a context where no apoptotic cell death accompanies sensitization. When we tested possible downstream mediators of TNF signaling by nociceptive sensory neuron-specific RNAi knockdown we found that the kinase p38, the adaptors TRAF3 and TRAF6, and the rel-like transcription factor Dorsal are all required downstream of the TNF receptor, Wengen, for induction of UV-induced thermal allodynia. Our results reveal a surprising independence between TNF signaling and apoptosis in tissue damage-induced pain sensitization, suggest an apoptosis-independent role for Dronc in TNF production, and identify the conserved molecular architecture of downstream TNF signaling in a pain sensitization context.