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Nobel Prize Winner Says MicroRNAs and piRNAs Influence the Coordinated Regulation of Transcription in the Nucleus, and Translation at the Synapse—Neuroscience 2013

Classic behavioral studies of memory storage in people and animals have defined two temporally distinct phases for memory storage: a short-term memory lasting minutes that can be elicited by one training trial, and a long-term memory lasting days or more that typically requires repeated training trials. In earlier work, Eric Kandel, M.D., Director of the Kavil Institute for Brain Science at Columbia University, Howard Hughes Medical Institute senior investigator, and a co-recipient of the 2000 Nobel Prize in Physiology or Medicine for discoveries concerning signal transduction in the nervous system, together with colleagues, delineated these two behavioral memory phases in studies of learned fear, an implicit form of memory, using the simple gill-withdrawal reflex of Aplysia, which is a marine snail. This work revealed that there is a cellular representation of the learning process. The substrate of learning is the synapse, and learning leads to changes in the strength of synaptic connections. These studies found that short-term memory is mediated by a transient synaptic facilitation of pre-existing connections due to covalent modification of pre-existing proteins, whereas long-term memory results from a persistent facilitation mediated by transcription and synaptic growth. The critical transcriptional switch that converts short-term to long-term facilitation and long-term memory in Aplysia is mediated by the removal of the repressive step of CREB-2 and the activation of CREB-1. Because small RNAs are important in transcriptional control and post-transcriptional regulation of gene expression, Dr. Kandel and his group wondered whether they might also regulate this key transcriptional switch from short-term to long-term memory. Together with collaborators, Dr. Kandel’s group profiled the small RNAs of Aplysia and identified 170 distinct miRNAs, nine of which were enriched in the CNS, and several were rapidly down-regulated by transient exposure to serotonin. The most abundant and well-conserved brain-specific miRNA, miR-124, was exclusively present presynaptically in the sensory neuron, where it inhibits CREB-1 mRNA. Serotonin, the modulatory transmitter released during learning, inhibits miR-24 and leads to the translation of CREB-1s, the activator of long-term memory transcription. In the course of profiling these small RNAs, Dr. Kandel and collaborators discovered a relatively new class of small RNAs, piRNAs (piwi-interacting RNAs; piwi proteins are a class of regulatory proteins), which had previously been thought to be germ-cell specific. We found that these neuronal piRNAs have predominant nuclear localization and sensitivity to serotonin. In response to serotonin, an increase in the level of a particular piRNA, piRNA-F, led to methylation and silencing of the CREB-2 promoter. Because CREB-2 is the major inhibitory constraint of memory that represses CREB-1, methylation of CREB-2 presumably leads to prolonged activation of CREB-1 and of long-term memory. The finding that serotonin bi-directionally regulates piRNAs and miRNAs — such that a rise in piRNA-F levels silences CREB-2, while a fall in miR-124 levels activates CREB-1 — provides a coordinated small RNA-mediated gene regulatory mechanism that acts on both the nucleus and on cytoplasmic mRNAs to allow CREB-1 to be active for over 24 hours. This thereby establishes stable long-term changes in the sensory neurons for the consolidation of long-term memory storage. Dr. Kandel indicated that the discovery of piRNAs in the brain was surprising because piRNAs were thought to be restricted to germ cells. The abundance of these neuronal piRNAs, and their responsiveness to neuromodulators, suggests a much broader role for piRNA than had previously been appreciated. In a larger sense, this data indicates a new mechanism for epigenetic regulation of gene expression underlying long-term memory storage. Although epigenetic regulation was widely known to occur in the context of development and differentiation, the discovery of the relevance of epigenetics in learning-related adult brain function is relatively recent. Furthermore, the mechanisms that may recruit epigenetic factors in a target-specific manner have remained elusive, and the first study to suggest a role for small-RNAs in guiding chromatin modification and transcriptional control was in fission yeast. Placed in the larger context of these and other earlier studies, our study provides an activity-dependent, piRNA-mediated, mechanism for DNA methylation of specific loci that can translate transient signals into long-term synaptic changes underlying memory storage. The scientific presentation of Dr. Kandel’s work will be given today, Monday, November 11, in the period of 2:10–2:45 p.m PST. Neuroscience 2013 continues through Wednesday, November 13. 30,000 scientists are attending this meeting. [Neuroscience 2013 Program] [Neuroscience 2013 Meeting]