C. elegans orthologs MUT-7/CeWRN-1 of Werner syndrome protein regulate neuronal plasticity

Abstract

Caenorhabditis elegans expresses human Werner syndrome protein (WRN) orthologs as two distinct proteins: MUT-7, with a 3'-5' exonuclease domain, and CeWRN-1, with helicase domains. How these domains cooperate remains unclear. Here, we demonstrate the different contributions of MUT-7 and CeWRN-1 to 22G small interfering RNA (siRNA) synthesis and the plasticity of neuronal signaling. MUT-7 acts specifically in the cytoplasm to promote siRNA biogenesis and in the nucleus to associate with CeWRN-1. The import of siRNA by the nuclear Argonaute NRDE-3 promotes the loading of the heterochromatin-binding protein HP1 homolog HPL-2 onto specific loci. This heterochromatin complex represses the gene expression of the guanylyl cyclase ODR-1 to direct olfactory plasticity in C. elegans. Our findings suggest that the exonuclease and helicase domains of human WRN may act in concert to promote RNA-dependent loading into a heterochromatin complex, and the failure of this entire process reduces plasticity in postmitotic neurons.

MUT-7 Provides Molecular Insight into the Werner Syndrome Exonuclease

“…Olfactory learning requires downregulation of the expression of a guanylyl cyclase, ODR-1, by endogenous odr-1 siRNA. The production of this siRNA is modulated by MUT-7 in the cytoplasm, and this siRNA is required for the formation of the complex with chromatin-associated heterochromatin protein 1 (HP1) homolog HPL-2 in the nucleus; HPL-2 binds to odr-1 mRNA to silence ODR-1 expression in a negative feedback loop [ 54 , 55 ]. These findings suggest that the WRN exonuclease is critical for the installation of epigenetic modifications that regulate gene expression.…”

“…This behavioral state is termed long-term olfactory learning and requires the accumulation of EGL-4 in the nucleus of AWC neurons [ 68 , 69 ]. When EGL-4 accumulates in the nucleus, MUT-7 acts specifically in the cytoplasm to promote the synthesis of 22G siRNA [ 54 , 55 ]. Endogenous 22G siRNAs are 22 nucleotides long and have a guanosine at the 5′ end [ 63 ].…”

“…The nuclear argonaute protein NRDE-3 acts as a shuttle to carry these 22G siRNAs from the cytoplasm to the nucleus [ 70 ]. The entrance of the 22G siRNAs into the nucleus induces nuclear EGL-4 to phosphorylate nuclear MUT-7 [ 54 ]. Activated MUT-7 is required for the association between HPL-2 and CeWRN-1 and their loading on the siRNA-targeted locus odr-1 [ 54 ].…”

“…The entrance of the 22G siRNAs into the nucleus induces nuclear EGL-4 to phosphorylate nuclear MUT-7 [ 54 ]. Activated MUT-7 is required for the association between HPL-2 and CeWRN-1 and their loading on the siRNA-targeted locus odr-1 [ 54 ]. This results in a reduction in the mRNA level of odr-1 , which encodes the guanylyl cyclase ODR-1 that generates the second messenger cGMP; it is this reduction that is highly correlated with the adaptation of the odor-seeking response [ 54 , 55 ].…”

“…Activated MUT-7 is required for the association between HPL-2 and CeWRN-1 and their loading on the siRNA-targeted locus odr-1 [ 54 ]. This results in a reduction in the mRNA level of odr-1 , which encodes the guanylyl cyclase ODR-1 that generates the second messenger cGMP; it is this reduction that is highly correlated with the adaptation of the odor-seeking response [ 54 , 55 ]. These findings support the idea that the 3′-5′ exonuclease of MUT-7 is required for siRNA synthesis and suggest that the import of siRNA into the AWC neuron nucleus is required for CeWRN-1-dependent olfactory learning ( Figure 3 ).…”