Local protein synthesis in cortical layer 1

Synaptic plasticity is a key process for brain function and memory formation. This depends on the ability of neurons to remodel their synaptic proteomes with high temporal and spatial resolution, promptly delivering and degrading the right proteins at the correct locations. Decades of work have shown that local protein synthesis at synapses is one of the mechanisms that enables remodeling of the local proteome. In vitro and ex vivo studies, mainly focusing on the hippocampus, have shown that neurons locally translate thousands of mRNAs, and that this process is required for several forms of long-term synaptic plasticity. Here we focus on cortical layer 1, a structure critical for learning that is devoid of neuronal cell bodies but enriched in their dendrites and axons. Combining laser-capture and RNAseq, we characterized the layer 1 transcriptome and found over 900 excitatory transcripts enriched in layer 1, including more than 100 excitatory synaptic transcripts. We validated the presence of candidate synaptic mRNAs with fluorescence in situ hybridization and digital droplet PCR, confirming their localization to the layer 1 neuropil. In order to assess whether these mRNAs are translated, we metabolically labelled nascent proteins in acute slices and observed abundant signal in layer 1, also when the label was specific to excitatory neurons, revealing that the signal arises from processes. These data unveil the potential of local protein synthesis in cortical layer 1; future work can exploit its optical accessibility to study this process dynamically.