Nat. Biotechnol. doi:10.1038/nbt.4016 (2017)

Credit: NAT. BIOTECHNOL

Neurons undergo rapid alterations in gene and protein expression in response to changing environmental conditions. However, detecting these changes in an in vivo environment can be technically challenging. Alvarez-Castelao et al. developed a bioorthogonal amino acid tagging approach utilizing methionyl-tRNA synthetase (MetRS) with an expanded amino acid binding site (L274G) that loads the noncanonical amino acid azidonorleucine (ANL) and directs ANL site-selectively into nascent proteins. The authors generated a transgenic mouse line expressing MetRS L274G that was responsive to neuronal Cre recombinases such as CaMK2a-Cre or GAD2-Cre, which are expressed in excitatory and inhibitory neurons, respectively. The system enabled proteomic analysis of nascent protein production in these distinct neuronal populations, and any other Cre-addressable cell population, by treating the metabolically labeled proteins with a cleavable biotin alkyne tag and performing affinity purification combined with LC–MS/MS. Alvarez-Castelao et al. compared the hippocampal proteomes in response to changes in sensory cues by housing mice in a standard cage or an enriched environment.They identified 225 proteins in excitatory hippocampal neurons that were differentially regulated and that were enriched for neuronal and synaptic function. Altogether, the use of the MetRS unnatural amino acid tagging approach offers exciting opportunities to reveal proteome changes in a cell-type-specific manner.