Antagonist actions of CMK-1/CaMKI and TAX-6/Calcineurin along the C. elegans thermal avoidance circuit orchestrate nociceptive habituation

Habituation is a conserved physiological phenomenon, during which responses decrease following repeated exposure to innocuous or noxious stimuli. Impaired nociceptive habituation is associated with several pain conditions in human, but the underpinning molecular mechanisms are only partially understood. In the nematode Caenorhabditis elegans, thermo-nociceptive habituation was previously shown to be regulated by the Ca2+/Calmodulin-dependent protein kinase 1 (named CMK-1), but its downstream effectors were unknown. Here, using in vitro kinase assays coupled with mass-spectrometry-based phosphoproteomics, we empirically identified hundreds of CMK-1 phospho-substrates. Among them, we found that CMK-1 can phosphorylate the calcineurin A (CnA) protein TAX-6 in a highly conserved regulatory domain. Combined genetic and pharmacological manipulations revealed a network of antagonistic actions between CMK-1 and calcineurin pathways in the regulation of the responsiveness of naive worms and their habituation to repeated noxious heat stimuli. We further highlighted multiple places of action of the two signaling pathways in a subset of thermosensory neurons and downstream interneurons mediating avoidance behaviors. As a whole, our study has identified (i) CMK-1 substrate candidates, which will fuel further research on the intracellular actuation of CMK-1-dependent signaling, and (ii) a complex set of antagonistic interactions between CMK-1 and calcineurin signaling operating at distributed loci within a sensory-behavior circuit, acting to adjust baseline thermo-nociception and regulate thermo-nociceptive habituation.