A comparison of the connectome of the hermaphrodite and male nervous systems reveals the existence of sexually dimorphic synaptic connections between sex-shared neurons (Jarrell et al., 2012). We recently discovered that many neurons initially form synapses in a non-discriminatory manner in both the male and hermaphrodite pattern before sexual maturation, but sex-specific pruning events result in the sex-specific maintenance of subsets of the connections (Oren-Suissa et al., 2016). Synapse pruning is a widely used mechanism to shape neuronal circuits during development, but it has not been previously implicated in the context of generating sex-specific neuronal circuitry. Proteasome-mediated protein degradation is a potent biochemical pathway used to control protein stability in various cellular processes. Since the proteasome has been shown to be involved in synapse remodeling, we decided to test whether there is active degradation of synapses in a sex specific manner. We found that inhibiting protein degradation by growing animals on Bortezomib-containing plates prevented synapse pruning and resulted in non-dimorphic adult synaptic connections. The E1 ubiquitin-activating enzyme lies at the heart of the biochemical degradation reaction and is necessary for all subsequent steps. As in other eukaryotes, C. elegans has a single E1 protein, UBA-1 (Kulkarni et al., 2008). Using a temperature sensitive allele of
uba-1,
it129, we show that in
uba-1 mutants elimination of synapses doesn't occur and the adult neuronal state is non-dimorphic.
uba-1-mediated protein degradation is temporally restricted, as a temperature shift in late larval stages is not sufficient to prevent synapse elimination. At the genetic level, we have linked the globally-acting sex determination system and synaptic pruning, and showed that
dmd-5, a phylogenetically conserved transcription factor of the Doublesex/MAB-3 family is dimorphically expressed in the male AVG shared neuron. Using transsynaptic synapse labeling of male and hermaphrodite specific AVG connections, we dissected the interplay between
dmd-5 and the protein degradation machinery. We have previously shown that
dmd-5 is required for maintenance of the male specific PHB to AVG synapses. PHA to AVG synapses are hermaphrodite specific: male PHA to AVG synapses are pruned during development. Blocking the protein degradation machinery resulted in maintenance of PHA-AVG male synapses. Similarly, in
dmd-5 mutant males PHA-AVG synapses were not pruned. Blocking protein degradation in
dmd-5 mutant males resulted in maintenance of PHA-AVG synapses. Taken together, our results suggest that synapse elimination by protein degradation is balanced by active maintenance of synapses, and both processes are required for generating the adult dimorphic circuitry.