Global Analysis of Regulated Small RNA

The cellular and biochemical signature of memory is transformed as a memory transits a nascent labile state into a consolidated and stable form. In prior work, we found that the translation of mRNA at synapses, regulated by the RISC pathway, is associated with the induction of stable memory in Drosophila. We next considered whether the regulation of small RNA biogenesis is an additional control point in this pathway. In the odor, shock associative conditioning paradigm, repeated electric foot-shocks serve as the aversive stimulus, acting via the release of dopamine into the adult brain memory center known as the Mushroom Body. Using high-throughput RNA sequence analysis and RT-PCR analysis, we found this release controls the biogenesis of a number of small RNAs, including piRNAs, snoRNAs, tRNA fragments, and miRNAs. A number of the regulated small RNAs were derived from the patterned degradation of mRNA.

      Among the induced miRNA loci was let-7C, one of the original miRNA’s identified as heterochronic genes in C. elegans. Let-7 was induced specifically in the Mushroom Body, where its expression suppressed the synthesis of a nuclear BTB protein, Abrupt, in a specific subset of Mushroom Body neurons. Abrupt is known as a developmental regulator of dendritic arbor branching, and controls branch number in a dosage dependent fashion. We found that transgenic manipulation of Abrupt level in Mushroom Body neurons dramatically regulated the number of pre-synaptic sites at axonal termini. This regulation was likewise observed after odor, foot-shock conditioning and was required for memory consolidation. Our observations point to a role for widespread transient synapse assembly as a prologue to consolidation, a phenomenon that might be referred to as synaptic ‘priming’. We suspect that such priming events reflect the transfer of a memory ‘engram’ between neuronal populations, from neurons that hold short-term memory to those that encode it in a stable, consolidated state. Some current work in progress aims to extend these results to the zebrafish.