Drosophila caspase activity is required independently of apoptosis to produce active TNF/Eiger during nociceptive sensitization

Juyeon Jo,Srividya C Iyer,Michael J Galko,Felona Gunawan,Seol Hee Im,Daniel T Babcock,Daniel N Cox

doi:10.1038/cddis.2016.474

Abstract

Tumor necrosis factor (TNF) signaling is required for inflammatory nociceptive (pain) sensitization in Drosophila and vertebrates. Nociceptive sensitization in Drosophila larvae following UV-induced tissue damage is accompanied by epidermal apoptosis and requires epidermal-derived TNF/Eiger and the initiator caspase, Dronc. Major gaps remain regarding TNF function in sensitization, including the relationship between apoptosis/tissue damage and TNF production, the downstream signaling in this context, and the target genes that modulate nociceptive behaviors. Here, apoptotic cell death and thermal nociceptive sensitization are genetically and procedurally separable in a Drosophila model of UV-induced nociceptive sensitization. Activation of epidermal Dronc induces TNF-dependent but effector caspase-independent nociceptive sensitization in the absence of UV. In addition, knockdown of Dronc attenuated nociceptive sensitization induced by full-length TNF/Eiger but not by a constitutively soluble form. UV irradiation induced TNF production in both in vitro and in vivo, but TNF secretion into hemolymph was not sufficient to induce thermal nociceptive sensitization. Downstream mediators of TNF-induced sensitization included two TNF receptor-associated factors, a p38 kinase, and the transcription factor nuclear factor kappa B. Finally, sensory neuron-specific microarray analysis revealed downstream TNF target genes induced during thermal nociceptive sensitization. One of these, enhancer of zeste (E(z)), functions downstream of TNF during thermal nociceptive sensitization. Our findings suggest that an initiator caspase is involved in TNF processing/secretion during nociceptive sensitization, and that TNF activation leads to a specific downstream signaling cascade and gene transcription required for sensitization. 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.

Highlights

Nociceptive sensory neurons often sensitize following inflammation or tissue damage.[1]
TNF/Eiger is dispensable for UV-induced epidermal apoptosis, the initiator caspase Dronc is required within the epidermis for both epidermal apoptosis and thermal allodynia development.[13]
Three different transheterozygous combinations of Dark mutant alleles failed to undergo epidermal apoptosis (Figure 1b; Supplementary Figure 1) and to induce thermal allodynia (Figure 1c), suggesting that apoptosome formation is required for both processes

Summary

Introduction

Nociceptive sensory neurons often sensitize following inflammation or tissue damage.[1]. During Drosophila UV-induced nociceptive sensitization, the cytokine TNF/Eiger[8] is produced by apoptotic epidermal cells It acts through its receptor, TNFR/Wengen, in nociceptive sensory neurons to mediate thermal allodynia – aversive withdrawal to previously innocuous temperatures. APAF-1-related killer (Dark) to form an apoptosome that activates effector caspases such as Drice and Dcp-1.19 These cleave the cellular substrates that lead to apoptosis.[20,21] In addition to their roles in apoptosis, Drosophila caspases function non-apoptotically in sperm individualization,[22] compensatory proliferation,[23] and innate immunity.[24]

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Results

Conclusion