Fear extinction is regulated by the activity of long noncoding RNAs at the synapse.

Long noncoding RNAs (lncRNAs) represent a multidimensional class of regulatory molecules that are involved in many aspects of brain function. Emerging evidence indicates that lncRNAs are localized to the synapse; however, a direct role for their activity in this subcellular compartment in memory formation has yet to be demonstrated. Using lncRNA capture-seq, we identified a specific set of lncRNAs that accumulate in the synaptic compartment within the infralimbic prefrontal cortex of adult male C57/Bl6 mice. Among these was a splice variant related to the stress-associated lncRNA, Gas5. RNA immunoprecipitation followed by mass spectrometry and single-molecule imaging revealed that this Gas5 isoform, in association with the RNA binding proteins G3BP2 and CAPRIN1, regulates the activity-dependent trafficking and clustering of RNA granules. In addition, we found that cell-type-specific, activity-dependent, and synapse-specific knockdown of the Gas5 variant led to impaired fear extinction memory. These findings identify a new mechanism of fear extinction that involves the dynamic interaction between local lncRNA activity and RNA condensates in the synaptic compartment.

Homeostatic scaling is driven by a translation-dependent degradation axis that recruits miRISC remodeling.

Homeostatic scaling in neurons has been attributed to the individual contribution of either translation or degradation; however, there remains limited insight toward understanding how the interplay between the two processes effectuates synaptic homeostasis. Here, we report that a codependence between protein synthesis and degradation mechanisms drives synaptic homeostasis, whereas abrogation of either prevents it. Coordination between the two processes is achieved through the formation of a tripartite complex between translation regulators, the 26S proteasome, and the miRNA-induced silencing complex (miRISC) components such as Argonaute, MOV10, and Trim32 on actively translating transcripts or polysomes. The components of this ternary complex directly interact with each other in an RNA-dependent manner. Disruption of polysomes abolishes this ternary interaction, suggesting that translating RNAs facilitate the combinatorial action of the proteasome and the translational apparatus. We identify that synaptic downscaling involves miRISC remodeling, which entails the mTORC1-dependent translation of Trim32, an E3 ligase, and the subsequent degradation of its target, MOV10 via the phosphorylation of p70 S6 kinase. We find that the E3 ligase Trim32 specifically polyubiquitinates MOV10 for its degradation during synaptic downscaling. MOV10 degradation alone is sufficient to invoke downscaling by enhancing Arc translation through its 3' UTR and causing the subsequent removal of postsynaptic AMPA receptors. Synaptic scaling was occluded when we depleted Trim32 and overexpressed MOV10 in neurons, suggesting that the Trim32-MOV10 axis is necessary for synaptic downscaling. We propose a mechanism that exploits a translation-driven protein degradation paradigm to invoke miRISC remodeling and induce homeostatic scaling during chronic network activity.

Modulation of hippocampal synapse maturation by activity-regulated E3 ligase via non-canonical pathway.

Development of functional synapses is crucial for the transmission and storage of information in the brain. Post establishment of the initial synaptic contact, synapses are stabilized through neuronal activity-induced signals. Emerging studies have implicated ubiquitination; a reversible posttranslational modification, as a key regulatory switch that modulates synapse development through proteasomal degradation. Ubiquitination of proteins is precisely regulated by E3 ligases, a set of enzymes that bind to specific substrates to facilitate the conjugation of monomeric or polymeric ubiquitin. However, the identity of specific E3 ubiquitin ligases that influence activity-dependent maturation of synapses and the mechanism by which ubiquitination of proteins regulate functional synapse development remain elusive. Here, we have identified a RING domain containing E3 ligase, Rnf2, asan activity-regulated factor that modulates glutamatergic synapse development in the hippocampus. Rnf2 is a synapse associated E3 ligase that is stabilized by neuronal activity through self-polyubiquitination. We have shown that neuronal activity shifts the balance toward stabilization of Rnf2 through self-polyubiquitination rather than triggering its degradation through polyubiquitination by Ube3A, an E3 ligase implicated in Angelman Syndrome. Our synapse density measurements and whole-cell patch-clamp recordings have revealed that the loss of Rnf2 function in cultured hippocampal neurons result in the development of 'silent' synapses that lack GluA1 containing functional AMPA receptors. These results provide a plausible mechanistic approach toward understanding how synapse maturation is regulated via the activity-dependent stabilization of Rnf2 through a non-canonical function of polyubiquitination.