Dynamic modulation of the motor neuron translatome during developmental synapse elimination.

The developmental sculpting of neuromuscular circuitry in early postnatal life occurs through the process of synapse elimination: Supernumerary axon inputs are gradually eliminated from the neuromuscular junction (NMJ), resulting in each muscle fiber being innervated by a single axon terminal. Here, we investigated the molecular pathways underlying this process using a ChAT-RiboTag mouse model in which we isolated ribosome-bound mRNAs in motor neurons during synapse elimination in vivo. Analysis of these mRNAs using translating ribosome affinity purification followed by RNA sequencing (TRAP-seq) revealed dynamic changes in the motor neuron translatome over the first 2 weeks of life, which were largely independent of parallel transcriptional changes and correlated with the progressive elimination of supernumerary inputs. Bioinformatic analysis identified distinct clusters of transcripts that were translated at specific time points during synapse elimination. Treating mice with two small molecules that were predicted to independently target the proteins or pathways encoded by the transcript cluster associated with neural metabolism increased the rate of synapse elimination in vivo. Together, these data provide a cell type-specific overview of temporal modifications occurring in the motor neuron translatome during synapse elimination, revealing rapid and dynamic responses to postnatal developmental cues.