Sex and the Circuitry: The ubiquitin and netrin pathways interact to pattern sex-specific dimorphic circuits

Sexual dimorphism in brain structure and function is evident across phyla, but little is known about the mechanisms that establish dimorphic circuits. We have recently shown that synaptic elimination plays a central role in generating sexually dimorphic circuits in the nematode C. elegans - synapses between sex-shared neurons are removed dimorphically during development to accommodate for sex-specific behaviors.

Here we show that sex-specific synapse elimination is mediated by the ubiquitin-proteasome system (UPS). Dimorphic synapse pruning in sensory neurons fails to occur under genetic or pharmacological UPS inhibition. Cell-specific rescue experiments place UPS activity in the presynaptic cell. Among the many E3 ligases encoded in C. elegans, we identified the conserved F-box protein SEL-10/FBW7 as required for dimorphic synapse pruning. In a search for potential synaptic substrates we found that the UNC-6/netrin receptor UNC-40/DCC contains a SEL-10 phosphodegron binding site (CPD) and is indeed degraded when co-expressed with SEL-10 in culture. Genetic studies revealed that sel-10 acts downstream of unc-6/netrin but upstream of unc-40/DCC, indicating a role in regulating this ligand-receptor interaction. We generated an undegradable unc-40/DCC gain-of-function allele by CRISPR-mutating the UNC-40 CPD. In such unc-40/DCC(gof) animals, dimorphic synapses were retained in both sexes, implying that SEL-10- UNC-40 binding is necessary for sex-specific synapse removal. Surprisingly, dimorphic synapses were retained in unc-40/DCC(gof) animals even in unc-6/netrin- null background, suggesting that unc-6/netrin functions here to protect unc-40/DCC from degradation rather than strictly activate it.

Thus we show how sexual identity intersects with signaling pathways and the UPS to specify synapse elimination.