EGL-3 and EGL-21 are required to trigger nocifensive response of Caenorhabditis elegans to noxious heat

Caenorhabditis elegans (C. elegans) is a widely used model organism to examine nocifensive response to noxious stimuli, including heat avoidance. Recently, comprehensive analysis of the genome sequence revealed several pro-neuropeptide genes, encoding a series of bioactive neuropeptides. C. elegans neuropeptides are involved in the modulation of essentially all behaviors including locomotion, mechanosensation, thermosensation and chemosensation. The maturation of pro-neuropeptide to neuropeptide is performed by ortholog pro-protein convertases and carboxypeptidase E (e.g. EGL-3 and EGL-21). We hypothesized that C. elegans egl-3 or egl-21 mutants will have a significant decrease in mature neuropeptides and they will display an impaired heat avoidance behavior. Our data has shown that thermal avoidance behavior of egl-3 and egl-21 mutants was significantly hampered compared to WT(N2) C. elegans. Moreover, flp-18, flp-21 and npr-1 mutant C. elegans displayed a similar phenotype. EGL-3 pro-protein convertase and EGL-21 carboxypeptidase E are essential enzymes for the maturation of pro-neuropeptides to active neuropeptides in C. elegans. Quantitative mass spectrometry analyses with egl-3 and egl-21 mutant C. elegans homogenates demonstrated that proteolysis of ProFLP-18 and ProFLP-21 are severely impeded, leading to a lack of mature bioactive neuropeptides. Not only FLP-21 but also FLP-18 related mature neuropeptides, both are ligands of NPR-1 and are needed to trigger nocifensive response of C. elegans to noxious heat.