Oxidative and cellular stresses can contribute to neurodegeneration, but it is unclear how synaptic transmission is altered in response to stress. We show that a conserved stress-response pathway activated by the transcription factor SKN-1/Nrf2 regulates neurotransmission at the C. elegans neuromuscular junction. We found that the WD40 repeat protein WDR-23, which is reported to target SKN-1 for degradation, is required for normal neurotransmitter secretion.
wdr-23 null mutants are resistant to the paralytic effects of aldicarb, have defects in synaptic vesicle exocytosis and have reduced neuropeptide secretion from motor neurons. A WDR-23-GFP fusion protein localizes to presynaptic terminals in motor neurons. We found the primary function of WDR-23 in neurons is to negatively regulate SKN-1 activity-
skn-1 mutants are hypersensitive to aldicarb, whereas worms over-expressing functional SKN-1-GFP specifically in neurons are resistant to aldicarb. Furthermore, the aldicarb resistance of
wdr-23 mutants is completely dependent on the presence of SKN-1-animals lacking both
wdr-23 and
skn-1 are identical in their response to aldicarb as
skn-1 mutants. A transcriptional
skn-1 reporter reveals that SKN-1 is expressed in cholinergic motor neurons. Furthermore, a SKN-1-GFP fusion protein can be seen in both the cytoplasm and nuclei of motor neurons, where its distribution is regulated by WDR-23. To identify transcriptional targets of SKN-1 that specifically regulate synaptic transmission, we profiled the transcriptomes of
wdr-23 mutants compared to wild type controls using whole genome RNAseq. As expected, most genes that are up-regulated in
wdr-23 mutants are involved in detoxification, including the known SKN-1 target
gcs-1. RNAi knockdown of the top 30 up-regulated genes has no impact on the aldicarb responsiveness of
wdr-23 mutants, suggesting that, independently, these genes do not regulate neurotransmission. Several genes with known roles in neuronal function are also up-regulated in
wdr-23 mutants. One of these is ACR-2, a subunit of a presynaptic acetylcholine receptor, which is specifically expressed in motor neurons and increases three fold by RNAseq. The
acr-2 promoter contains one predicted SKN-1 binding site that is conserved among nematodes. Using qRT-PCR and an
acr-2 reporter construct, we confirm that ACR-2 expression increases significantly in
wdr-23 mutants, and this increase is abolished in
skn-1 mutants. Together, our results suggest a model in which the abundance of SKN-1 in neurons is regulated by the WD40 adaptor protein WDR-23. Furthermore, neurotransmission is mediated by SKN-1-dependent transcriptional programs that may include the regulation of the acetylcholine receptor, ACR-2.