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The physical manifestation of learning and memory formation in the brain can be expressed by strengthening or weakening of synaptic connections through morphological changes. Local actin remodeling underlies some forms of plasticity and may be facilitated by local β-actin synthesis, but dynamic information is lacking. In this work, we use single-molecule in situ hybridization to demonstrate that dendritic β-actin messenger RNA (mRNA) and ribosomes are in a masked, neuron-specific form. Chemically induced long-term potentiation prompts transient mRNA unmasking, which depends on factors active during synaptic activity. Ribosomes and single β-actin mRNA motility increase after stimulation, indicative of release from complexes. Hence, the single-molecule assays we developed allow for the quantification of activity-induced unmasking and availability for active translation. Further, our work demonstrates that β-actin mRNA and ribosomes are in a masked state that is alleviated by stimulation.
Observing the Messenger
In order to elucidate the dynamics of individual components in the cell, single-molecule technologies are being developed (see the Perspective by Akbalik and Schuman). Park et al. (p. 422) used a mouse expressing fluorescent β-actin messenger RNAs (mRNAs) to visualize mRNA movements in living cells and tissues. Buxbaum et al. (p. 419) showed that neurons contain β-actin mRNAs and ribosomes packaged in a dense structure, impenetrable by oligonucleotide probes. This effectively masks the mRNAs until neuronal stimulation exposes the mRNA and ribosomes briefly, presumably reflecting the local stimulation and translation involved, for example, in the generation of memories.