Somatodendritic microRNAs identified by laser capture and multiplex RT-PCR

The catalog of RNAs present in dendrites represents the complete repertoire of local translation that contributes to synaptic plasticity. Most views hold that a pool of dendritic mRNAs is selectively transported to a dendritic destination. This view requires that some mRNAs in the dendrite are locally enriched relative to the cell body; however, quantitative comparisons that would support this assumption do not currently exist. These issues related to somatodendritic distribution of mRNAs also apply to the microRNAs, approximately 21 nucleotide noncoding transcripts that bind to target mRNAs and either inhibit their translation or destabilize them. We combined laser capture with multiplex real-time RT (reverse transcription) PCR to quantify microRNAs in the neuritic and somatic compartments separately. The samples were standardized by RT-PCR measurements of a set of mRNAs, including known dendritic mRNAs, in these two compartments. Most neuronal miRNAs were detected in dendrites. With a few notable exceptions, most miRNAs were distributed through the somatodendritic compartment across a nearly constant gradient. Thus for lower-abundance miRNAs, the total neuronal concentration of the miRNA can remain readily detectable in the cell body but vanish from the dendrite. A very small number of miRNAs deviate from the distribution gradient across the miRNA population as relatively enriched or depleted in the dendrite.