MicroRNAs (miRs) are small, conserved, noncoding RNAs that act in association with the RNA-induced silencing complex (RISC) to regulate gene expression post-transcriptionally (Bartel, 2004
). miR mediated translation repression may occur at pre- and/or post-initiation steps (Abu-Elneel et al., 2008
; Filipowicz et al., 2008
; Lee et al., 1993
; Nottrott et al., 2006
; Richter, 2008
). Numerous miRNAs are expressed at high levels in the nervous system and regulate development (Kosik, 2006
), spine morphology and synapse function (Schratt, 2009b
). miRNAs are implicated in neurological and neuropsychiatric disease (Kocerha et al., 2009
; Kosik, 2006
; Lee et al., 2008
). A subset of miRNAs are localized to dendrites (Kye et al., 2007
; Schratt et al., 2006
; Siegel et al., 2009
), where local translation may affect dendritic spine morphology (Schratt, 2009b
; Schratt et al., 2006
). A critical gap is understanding how the repression of target mRNA translation by RISC components and miRNAs may be dynamically regulated by receptor signaling pathways. The ability of a cell to dynamically regulate RISC interactions with target mRNAs would allow for more precise temporal and spatial control of miRNA function than achieved solely by regulation of miRNA biogenesis. The physiological signals and molecular mechanisms that promote or inhibit the association of RISC complexes onto specific mRNAs are unknown.
A major advance in our understanding of the regulation of miRNA-mediated translation has been the evidence of reversibility in cultured dividing cells deprived of serum. The interaction of mRNA binding proteins with cis-acting elements can affect miRNA activity (Bhattacharyya et al., 2006
; Vasudevan et al., 2007
). These studies suggest the hypothesis that mRNA binding proteins may act as molecular intermediates downstream of receptor signaling pathways to modulate the interactions of RISC-associated miRNAs to target sequences. Since specific mRNA binding proteins can be localized non-uniformly within the cytoplasm to influence mRNA localization, they are uniquely positioned to co-opt RISC as a means to regulate local mRNA translation in polarized cells.
Dynamic regulation of synaptic protein synthesis in response to activation of neurotransmitter receptors, such as metabotropic glutamate receptors (mGluRs), plays a key role in long-term synaptic plasticity underlying learning and memory (Bramham and Wells, 2007
; Martin and Ephrussi, 2009
). microRNAs appears to be ideally suited to reversibly inhibit mRNA translation at synapses in response to receptor signaling (Chang et al., 2009
; Kosik, 2006
). This, in principle, could provide selective, bidirectional and spatial control for the regulation of mRNA translation. We sought to identify a molecular mechanism whereby a specific mRNA can be selectively silenced by a miRNA through RISC components, and investigate whether a selective mRNA binding protein in response to physiological signals may reversibly regulate these interactions.
Here we elucidate the molecular mechanism of reversible and selective regulation of postsynaptic density protein (PSD-95) mRNA translation in response to stimulation of gp1 mGluRs. miR-125a and fragile X mental retardation protein (FMRP) cooperate on the 3′UTR of PSD-95 mRNA to enable inhibition and confer flexibility allowing for translational activation at synapses in response to receptor activation. Loss of FMRP causes fragile X syndrome (FXS), the most common monogenetic form of inherited intellectual disability and autism. FMRP is localized to dendrites and synapses, where it regulates mRNA transport and local protein synthesis necessary for neuronal development and synaptic plasticity (Bassell and Warren, 2008
). FMRP interacts with mammalian eIF2C2 (AGO2) and associates with miRNAs (Edbauer et al.; Jin et al., 2004b
). We show that FMRP phosphorylation promotes the formation of an AGO2-miR125a inhibitory complex on PSD-95 mRNA translation. Conversely, mGluR stimulation leads to dephosphorylation of FMRP, release of AGO2 from the mRNA, and activation of translation. The control of translation by mGluR signaling to FMRP and AGO2 is on the time scale of minutes. This demonstrates that the phosphorylation status of an mRNA binding protein can affect the rapid and reversible control of miRNA-mediated translational regulation. This study provides mechanistic insight into the selective and cooperative interactions that may function as reversible switches to dynamically regulate miRNA function and has important implications for understanding neuropsychiatric disorders resulting from altered regulation of miRNA pathways.