| Nociception is the process of detecting and encoding noxious stimuli in the nervous system. Normal or abnormal activation of nociceptive pathways results in pain sensation, an unpleasant sensory experience linked to actual or potential tissue damage. Pain symptoms in human significantly decrease well-being and are still often not satisfactorily alleviated. Understanding the molecular and cellular processes modulating nociception, and in particular the nociceptive plasticity mechanisms occurring in normal and pathological conditions, may ultimately help to develop better pain management solutions. Progresses on this topic are hindered in mammals by ethical concerns, by the size and the complexity of their nervous system, and by the relative slowness of genetic approaches. Because they circumvent all these limitations, and because their nociceptive molecular pathways are remarkably conserved, invertebrates (such as Caenorhabditis elegans) have recently emerged as efficient complementary research models.
The general goal of this project is to shed new light on the mechanisms underlying nociceptive plasticity by using the C. elegans model. The project leverages on robust thermal nociception plasticity paradigms in which innate heat-evoked noxious heat avoidance responses are modulated by repeated heat stimulations or by long-term starvation. We will make an extensive use of automated computer-assisted behavioral analysis platforms, which we have established in our laboratory and that allows high-throughput thermal avoidance analyses. The project has five specific aims:
Aim 1 will seek to establish worm models for human pain genes. We will focus on genetic variants recently identified in genome-wide association studies (GWAS) and for which the mechanisms of action in pain pathways are largely unknown. In a previous screen with mutants in worm genes that are orthologous to these human pain genes, we identified 26 mutations that affected thermal avoidance and/or its plasticity. Using a series of heterologous expression and genetic rescue experiments, we plan to determine for which genes the modelling in worms is the most promising. Furthermore, we will characterize these genes and variants in more details, to understand how they control thermal nociception in worms.
Aim 2 and 3 will address how Protein Kinase A and Calcineurin signaling each control thermal nociception plasticity. Mutations affecting these pathways in worms cause striking plasticity phenotypes and will be used here as entry points. In particular, we will determine the nature of the enzyme isoforms involved, their place and timing of action in the nervous system, their potential crosstalk with other pathways, and we will identify their downstream targets. To that end, we will engage a combination of approaches including genome editing to ablate specific gene isoforms or regulatory residues, cell-specific rescues and overactivation mutations, biosensors for in situ kinase activity monitoring, and phosphoproteomics.
Aims 4 and 5 will address how thermal avoidance behaviors are regulated by neuropeptide communication to and from thermosensory neurons. Aim 4 will focus on how the FLP tonic thermonociceptor neurons can regulate long-term locomotory changes. Aim 5 will focus on how the acute heat-avoidance response triggered by AWC thermosensory neurons is modulated by food availability. For both aims, preliminary screens have identified candidate neuropeptides and receptors. In the present project, we will dissect the neural circuit involved by using tissue-specific gene knockout, functional manipulations with opto- and chemo-genetics, and in vivo calcium imaging.
The significance of the project is twofold. First, it will advance our fundamental knowledge of the mechanisms controlling thermal nociception plasticity at the molecular, neuronal, and circuit levels, which will be essential for the development of research using the C. elegans model. Second, since the nociception mechanisms studied here are well conserved in higher organisms, the project will provide determinant insights on potential therapeutic targets in pain management, including by establishing tractable models for recently identified human pain genes, about which much remains to be discovered.
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