MicroRNAs (miRNAs) regulate cellular fate by controlling the stability or translation of mRNA transcripts. Although the spatial and temporal patterning of miRNA expression is tightly controlled, little is known about signals that induce their expression nor mechanisms of their transcriptional regulation. Furthermore, few miRNA targets have been validated experimentally. The miRNA, miR132, was identified through a genome-wide screen as a target of the transcription factor, cAMP-response element binding protein (CREB). miR132 is enriched in neurons and, like many neuronal CREB targets, is highly induced by neurotrophins. Expression of miR132 in cortical neurons induced neurite outgrowth. Conversely, inhibition of miR132 function attenuated neuronal outgrowth. We provide evidence that miR132 regulates neuronal morphogenesis by decreasing levels of the GTPase-activating protein, p250GAP. These data reveal that a CREB-regulated miRNA regulates neuronal morphogenesis by responding to extrinsic trophic cues.

Both increases and decreases in methyl CpG-binding protein 2 (MeCP2) levels cause neurodevelopmental defects. We found that MeCP2 translation is regulated by microRNA 132 (miR132). Block of miR132-mediated repression increased MeCP2 and brain-derived neurotrophic factor (BDNF) levels in cultured rat neurons and the loss of MeCP2 reduced BDNF and miR132 levels in vivo. This feedback loop may provide a mechanism for homeostatic control of MeCP2 expression.

The activity-dependent regulation of ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors and the stabilization of synapses are critical to synaptic development and plasticity. One candidate molecule implicated in maturation, synaptic strengthening, and plasticity is PSD-95. Here we find that acute knockdown of PSD-95 in brain slice cultures by RNAi arrests the normal development of synaptic structure and function that is driven by spontaneous activity. Surprisingly, PSD-95 is not necessary for the induction and early expression of long-term potentiation (LTP). However, knockdown of PSD-95 leads to smaller increases in spine size after chemically induced LTP. Furthermore, although at this age spine turnover is normally low and LTP produces a transient increase, in cells with reduced PSD-95 spine turnover is high and remains increased after LTP. Taken together, our data support a model in which appropriate levels of PSD-95 are required for activity-dependent synapse stabilization after initial phases of synaptic potentiation.AMPA receptor ͉ long-term depression ͉ long-term potentiation ͉ postsynaptic density M odifications in synaptic strength and stabilization of synapses underlie developmental and activity-dependent plasticity in the brain. Understanding their molecular mechanisms provides insight into behavioral plasticity and brain dysfunction during disease. One postsynaptic modification regulating synaptic strength is the delivery and removal of ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (1-4). Recently, PSD-95, a member of the MAGUK (membraneassociated guanylate kinase) family, has been suggested to play important roles in synaptic plasticity in hippocampus and cortex by regulating AMPA-R trafficking. These studies used overexpression of wild-type and mutant forms of PSD-95 and suggested that PSD-95 drives synaptic maturation, controls AMPA-R number at synapses, and participates in receptor delivery or stabilization during synaptic plasticity in vitro and in vivo (5-9). Moreover, stronger synapses with increased PSD-95 levels show enhanced synaptic depression [long-term depression (LTD)] (7, 8). Because MAGUKs are highly homologous and several of them are present at hippocampal synapses (10, 11), issues of specificity weaken the conclusions of such studies. Predictions from overexpression experiments are that loss of PSD-95 should affect basal synaptic properties and strength as well as bidirectional synaptic plasticity. However, a genetic approach using mice modified at the PSD-95 locus showed enhanced long-term potentiation (LTP) and impaired learning (12) but no change in basal synaptic function, which might be due to undetected compensatory mechanisms. Here we examined the function of endogenous PSD-95 by a temporally and spatially restricted knockdown approach using RNAi.We find that knockdown of PSD-95 arrests the functional and morphological development of glutamatergic synapses. Although induction and early LTP are largely un...