Summary MicroRNAs (miRNAs) are non-coding RNAs that suppress translation of specific mRNAs. The miRNA machinery interacts with Fragile X Mental Retardation Protein (FMRP), which functions as translational repressor. We show that miR-125b and miR-132, as well as several other miRNAs, are associated with FMRP in mouse brain. miR-125b and miR-132 had largely opposing effects on dendritic spine morphology and synaptic physiology in hippocampal neurons. FMRP knockdown ameliorates the effect of miRNA overexpression on spine morphology. We identified NMDA receptor subunit NR2A as novel target of miR-125b and show that NR2A mRNA is specifically associated with FMRP in brain. In hippocampal neurons, NR2A expression is negatively regulated through its 3’UTR by FMRP, miR-125b and Argonaute 1. Regulation of NR2A 3’UTR by FMRP depends in part on miR-125b. Because NMDA receptor subunit composition profoundly affects synaptic plasticity, these observations have implications for the pathophysiology of Fragile X Syndrome, in which plasticity is altered.

articlesThe AMPA class of ionotropic glutamate receptors mediates most of the fast excitatory synaptic transmission in mammalian brain 1 . Changing the responsiveness of postsynaptic AMPA receptors offers a powerful way to regulate excitatory synaptic transmission. Among the mechanisms proposed for modification of AMPA receptor activity, one of the simplest is a change in the number of AMPA receptors in the postsynaptic membrane. Studies suggest that AMPA receptors can move in and out of the postsynaptic membrane on a rapid time scale (for review, see ref.2). Moreover, changes in postsynaptic membrane trafficking or in synaptic targeting of AMPA receptors have been correlated with alterations in synaptic efficacy [2][3][4][5][6][7][8][9] .In developing neurons in culture, synaptic expression of AMPA receptors is modulated over a period of days by relative levels of AMPA and NMDA receptor activity [10][11][12] . Mature neurons in culture also exhibit slow changes in synaptic AMPA receptor expression in response to chronic changes in endogenous activity 9,13,14 . On a faster time scale, induction of long-term depression (LTD) or acute application of AMPA or insulin can induce a loss of AMPA receptors from the cell surface of cultured hippocampal neurons within minutes 3,6,9,15 . Conversely, induction of long-term potentiation (LTP) and activation of CaMKII are correlated with a rapid recruitment of AMPA receptors to the postsynaptic membrane 5,8 . Thus, the synaptic content of AMPA receptors can change bidirectionally on a fast time scale and result in altered synaptic transmission.Little is known about the cell biological pathways or molecular mechanisms of postsynaptic AMPA receptor trafficking. To date, studies of AMPA receptor trafficking have focused on the presence or absence of AMPA receptors at synaptic sites, or on internal versus surface expression [3][4][5][6][7][8][9][10][15][16][17][18] . However, the multiple subunits of AMPA receptors interact differentially with diverse cytoplasmic proteins including GRIP/ABP, PICK1, NSF and SAP97, all of which have been implicated in synaptic targeting of AMPA receptors 3,5,17,[19][20][21][22][23][24][25][26][27] . The diversity of AMPA receptor protein interactions, and their regulation by phosphorylation 28 , suggest that AMPA receptor trafficking is likely to be regulated by varied mechanisms.Here we report that multiple distinct pathways exist for AMPA receptor endocytosis in cultured hippocampal neurons. We have quantified a basal rate of AMPA receptor internalization, which can be enhanced by a variety of stimuli including synaptic activity, ligand binding to AMPA receptors, insulin and calcium influx. Different internalizing stimuli use distinct intracellular signaling mechanisms, different endosomal sorting pathways and distinct sequence determinants within AMPA receptor subunits to effect AMPA receptor endocytosis. RESULTS Ligand-dependent endocytosis of AMPA receptorsTranslocation of AMPA receptors from cell surface to intracellular compartments was visu...