Repression of an activity-dependent autocrine insulin signal is required for sensory neuron development in C. elegans

The nervous system comprises diverse and highly specialized neuron-types, each expressing a unique set of genes that defines its functional properties. What molecular mechanisms generate diverse neuron types remains a central question in neuroscience. Our study of C. elegans chemosensory BAG neurons showed that a p38 MAP kinase (MAPK), PMK-3, is required for their proper differentiation and function (Brandt, J. and Ringstad, N. 2015). How p38 MAPKs function in neurodifferentiation is poorly understood. To better understand how pmk-3 promotes the BAG cell fate, we purified wild-type and pmk-3 mutant chemosensory BAG neurons and determined their transcriptomes using RNA-Seq. Expression of a number of genes that encode neuropeptides, including insulin-like peptides (ILPs), were up-regulated in pmk-3 mutant BAG neurons. Through analysis of mutations that restore expression of a BAG-fate marker to pmk-3 mutants, we additionally found that genes required for the release of neuropeptides, including UNC-31/CAPS, suppress pmk-3 mutant gene expression defects. These two observations suggested that the differentiation defects of pmk-3 mutants are associated with dysregulated release of peptide hormones. Indeed, we find that increased synthesis and release of ILPs from BAG during development causes a significant fraction of the gene expression and functional defects of pmk-3 mutant neurons. Together our data delineate a mechanism through which p38 MAPKs promote proper sensory neuron differentiation by inhibiting an autocrine insulin signal that represses expression of a BAG neuron fate. These findings reveal an unexpected role for insulin signaling in nervous system development and suggest that insulin-like factors are at the nexus of intrinsic genetic programs and extrinsic signaling mechanisms that regulate cell fate and neuronal differentiation. Overall design: Two biological replicates were analyzed for each of the 3 cell populations