The Conserved SKN-1/Nrf2 Stress Response Pathway Regulates Synaptic Function in Caenorhabditis elegans

Abstract

The Nrf family of transcription factors plays a critical role in mediating adaptive responses to cellular stress and defends against neurodegeneration, aging, and cancer. Here, we report a novel role for the Caenorhabditis elegans Nrf homolog SKN-1 in regulating synaptic transmission at neuromuscular junctions (NMJs). Activation of SKN-1, either by acute pharmacological treatment with the mitochondrial toxin sodium arsenite or by mutations that cause constitutive SKN-1 activation, results in defects in neuromuscular function. Additionally, elimination of the conserved WD40 repeat protein WDR-23, a principal negative regulator of SKN-1, results in impaired locomotion and synaptic vesicle and neuropeptide release from cholinergic motor axons. Mutations that abolish skn-1 activity restore normal neuromuscular function to wdr-23 mutants and animals treated with toxin. We show that negative regulation of SKN-1 by WDR-23 in the intestine, but not at neuromuscular junctions, is necessary and sufficient for proper neuromuscular function. WDR-23 isoforms differentially localize to the outer membranes of mitochondria and to nuclei, and the effects of WDR-23 on neuromuscular function are dependent on its interaction with cullin E3 ubiquitin ligase. Finally, whole-transcriptome RNA sequencing of wdr-23 mutants reveals an increase in the expression of known SKN-1/Nrf2-regulated stress-response genes, as well as neurotransmission genes not previously implicated in SKN-1/Nrf2 responses. Together, our results indicate that SKN-1/Nrf2 activation may be a mechanism through which cellular stress, detected in one tissue, affects cellular function of a distal tissue through endocrine signaling. These results provide insight into how SKN-1/Nrf2 might protect the nervous system from damage in response to oxidative stress.

Author Summary

Transcriptional programs control cellular responses in the face of environmental stress, such as dietary restriction, hypoxia, or oxidative stress. Furthermore, in order to promote survival of the organism in response to insult, communication between tissues must be established. Using the model system C. elegans, we investigate functional changes in the nervous system mediated by the transcription factor SKN-1. We establish that activation of SKN-1, either genetically or through exposure to the mitochondrial toxin arsenite, results in locomotion changes that take place at the neuromuscular junction. Furthermore, these changes in the nervous system are brought about through signaling from the intestine. Lastly, we use whole-transcriptome RNA sequencing to identify new transcriptional targets of SKN-1 that might be affecting locomotory behavior. Our results indicate that neuronal function can be regulated at the level of the synapse in response to environmental stress.