Characterization of Small RNAs in Aplysia Reveals a Role for miR-124 in Constraining Synaptic Plasticity through CREB

Abstract

Summary Memory storage and memory-related synaptic plasticity relies on precise spatiotemporal regulation of gene expression. To explore the role of small regulatory RNAs in learning-related synaptic plasticity we carried out massive parallel sequencing to profile the small RNAs of Aplysia californica. We identified 170 distinct miRNAs, 13 of which were novel and specific to Aplysia. Nine miRNAs were brain-enriched, and several of these were rapidly down-regulated by transient exposure to serotonin, a modulatory neurotransmitter released during learning. Further characterization of the brain-enriched miRNAs revealed that miR-124, the most abundant and well-conserved brain-specific miRNA, was exclusively present pre-synaptically in a sensory-motor synapse where it constrains serotonin-induced synaptic facilitation through regulation of the transcriptional factor CREB. We therefore present direct evidence that a modulatory neurotransmitter important for learning can regulate the levels of small RNAs and present a novel role for miR-124 in long-term plasticity of synapses in the mature nervous system.

MicroRNAs and synaptic plasticity—a mutual relationship

“…The effect most probably is mediated through inadequate EPAC-Rap1 regulation on miR-124 levels which in turn represses translation of Zif268 [66]. The function of miR-124 in memory formation appears to be highly conserved, because miR-124 also regulates synaptic facilitation and memory formation in Aplysia californica [67].…”

The brain-specific microRNA miR-128b regulates the formation of fear-extinction memory

“…Although there is evidence to suggest that the acquisition of conditioned fear and the encoding of fear extinction share some common molecular substrates 1 , the function of genes expressed at the time of retrieval of fear memory and how they are regulated to allow fear extinction to proceed has not been thoroughly investigated. MicroRNAs are a family of small non-coding RNAs that regulate gene function by inhibiting the expression of their target mRNAs [2][3] , and recent findings implicate microRNA activity in learning associated with cocaine and nicotine addiction 4-5 , as well as suggesting a more general role for miR-124, -132 and -134 in hippocampaldependent learning and memory [6][7][8] . In the following series of experiments we demonstrate that the brain-specific microRNA, miR-128b, is required for fear extinction memory and that its influence on fear extinction occurs via negative regulation of genes associated with neural plasticity and retrieval of fear memory.…”

Neuroligin-associated microRNA-932 targets actin and regulates memory in the honeybee

“…Although miRNA regulation of neuronal plasticity is thought to be a conserved mechanism among higher eukaryotes, there are few vertebrate and even fewer invertebrate studies that have attempted to characterize the specific roles of miRNAs in learning and memory 7,8,10 . The use of tractable insect models such as the honeybee that offers a sophisticated behavioural repertoire, a complete genome sequence, emerging RNA interference technologies and demonstrated adult brain plasticity provides an excellent opportunity to study molecular mechanisms underlying learning and memory 11 .…”