Utilizing Caenorhabditis elegans as a novel platform to model human RET receptor function
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In this study, we report a novel use of Caenorhabditis elegans as a platform to model the function of human RET isoforms, RET9 and RET51. C. elegans does not encompass an orthologue of RET. Thus, we ectopically expressed the intracellular domain of RET9 or RET51 under the control of a C. elegans neuronal specific reporter, mec-4. We showed that the resulting transgenic animals expressed RET isoforms that were phosphorylated in the six touch neurons of the animal. RET-expressing animals exhibited a premature axon termination phenotype, with the phenotype of RET51- expressing animals being significantly more penetrant than RET9 animals. The premise of the study was to use the RET model as a platform to study individual RET isoforms using the premature termination phenotype as a functional readout of isoformic activity. We developed and expressed mutant RET constructs of RET51 and RET9 harboring an activating mutation, M918T, and tyrosine substitutions to further dissect signalling events and differences between the RET isoforms. We identified Y1062 as a key signalling hub on RET51 that when abrogated, significantly suppressed the premature termination phenotype. Further, we showed that human RET interacted with worm orthologues of Cbl, Grb2, and Nck, key adapter proteins known to bind RET. We hypothesize that RET may be sequestering signalling proteins that normally associate and signal through C. elegans pro-growth receptors, SAX-3 and UNC-5, on the growing axon. Alternatively, or in addition, RET may potentially be actively signalling through MAPK pathways to promote growth cone collapse and premature termination of axons. We investigated the application of our model for drug testing, and identified the kinase inhibitor sorafenib as a potent suppressor of the RET induced phenotype. Together, our results demonstrate a novel application for C. elegans where individual RET isoforms and various derived mutants can be studied in isolation to further our understanding of signalling events contributing to RET-mediated disease.