Dicer and eIF2c are enriched at postsynaptic densities in adult mouse brain and are modified by neuronal activity in a calpain‐dependent manner

Abstract

We have hypothesized that small RNAs may participate in learning and memory mechanisms. Because dendritic spines are important in synaptic plasticity and learning, we asked whether dicer, the rate-limiting enzyme in the formation of small RNAs, is enriched within dendritic spines. In adult mouse brain, dicer and the RNA-induced silencing complex (RISC) component eIF2c were expressed in the somatodendritic compartment of principal neurons and some interneurons in many regions, and dicer was enriched in dendritic spines and postsynaptic densities (PSDs). A portion of dicer and eIF2c were associated with each other and with fragile X mental retardation protein (FMRP), as assessed by coimmunoprecipitation. Calpain I treatment of recombinant dicer or immunopurified brain dicer caused a marked increase in RNAse III activity. Purified PSDs did not exhibit RNAse III activity, but calpain caused release of dicer from PSDs in an enzymatically active form, together with eIF2c. NMDA stimulation of hippocampal slices, or calcium treatment of synaptoneurosomes, caused a 75 kDa dicer fragment to appear in a calpain-dependent manner. The findings support a model whereby acute neuronal stimulation at excitatory synapses increases intracellular calcium, which activates calpain, which liberates dicer and eIF2c bound to PSDs. This supports the hypothesis that dicer could be involved in synaptic plasticity.

MicroRNA regulation of homeostatic synaptic plasticity

“…The experiments blocking these homeostatic changes with the miR-I support this conclusion. Mature miR-485 levels were elevated by increased excitatory activity in hippocampal neurons, and other studies show that the miRNA processing enzyme Dicer is regulated in a Ca 2+ -dependent manner (35), suggesting that pre-miRNAs targeted to synapses may undergo processing into mature miRNAs in an activity-dependent manner.…”

Human Dicer C-terminus functions as a 5-lipoxygenase binding domain

“…Although it remains to be determined whether they interact in primary leukocytes, the relatively weak coimmunoprecipitation between 5LO and Dicer proteins suggests that this Dicer 5LObd-mediated interaction may be transient, conditional and/or require structural changes in Dicer conformation. Whether Dicer 5LObd becomes exposed to the surface (i) of an alternatively spliced variant form of Dicer, (ii) following protease-directed cleavage, as observed in the case of calpain-mediated release of Dicer fragments [38], (iii) in association with an enhanced enzyme activity induced by limited proteolysis [3], or (iv) upon covalent modification, such as phosphorylation, also remains to be determined.…”

MicroRNAs and synaptic plasticity—a mutual relationship

“…Initially, it was found that Dicer is activated in response to activity by limited proteolysis due to cleavage by the calcium-dependent protease calpain. This resulted in increased RNAse III activity of Dicer on dsRNA substrates [29]. In a more recent study, BDNF-mediated stimulation of the ERK pathway was shown to stabilize Dicer protein levels via phosphorylation of TRBP [30].…”