MicroRNAs are small, non-coding RNAs that control the translation of target messenger RNAs, thereby regulating critical aspects of plant and animal development. In the mammalian nervous system, the spatiotemporal control of mRNA translation has an important role in synaptic development and plasticity. Although a number of microRNAs have been isolated from the mammalian brain, neither the specific microRNAs that regulate synapse function nor their target mRNAs have been identified. Here we show that a brain-specific microRNA, miR-134, is localized to the synapto-dendritic compartment of rat hippocampal neurons and negatively regulates the size of dendritic spines--postsynaptic sites of excitatory synaptic transmission. This effect is mediated by miR-134 inhibition of the translation of an mRNA encoding a protein kinase, Limk1, that controls spine development. Exposure of neurons to extracellular stimuli such as brain-derived neurotrophic factor relieves miR-134 inhibition of Limk1 translation and in this way may contribute to synaptic development, maturation and/or plasticity.

Summary The microRNA pathway has been implicated in the regulation of synaptic protein synthesis and ultimately dendritic spine morphogenesis, a phenomenon associated with long-lasting forms of memory. However, the particular microRNAs (miRNAs) involved are largely unknown. We performed a functional screen to identify specific miRNAs that function at synapses to control dendritic spine structure. One of the identified miRNAs, miR-138, is highly enriched in the brain, localized within dendrites and negatively regulates the size of dendritic spines in rat hippocampal neurons. miR-138 controls the expression of Acyl protein thioesterase 1 (APT1), an enzyme regulating the palmitoylation status of proteins that are known to function at the synapse, including G protein alpha subunits (Gα). RNAi-mediated knockdown of APT1 and expression of membrane-localized Gα both suppress spine enlargement caused by miR-138 inhibition, suggesting that APT1-regulated depalmitoylation of Gα might be an important downstream event of miR-138 function. Our results uncover a novel miRNA-dependent mechanism in neurons and demonstrate a previously unrecognized complexity of miRNA-dependent control of dendritic spine morphogenesis.

Dendritic spines are small, actin-rich protrusions on the surface of dendrites that receive the majority of excitatory synaptic inputs in the brain. The formation and remodeling of spines, processes that underlie synaptic development and plasticity, are regulated in part by Eph receptor tyrosine kinases. However, the mechanism by which Ephs regulate actin cytoskeletal remodeling necessary for spine development is not fully understood. Here, we report that the Rac1 guanine nucleotide exchange factor Tiam1 interacts with the EphB2 receptor in a kinase-dependent manner. Activation of EphBs by their ephrinB ligands induces the tyrosine phosphorylation and recruitment of Tiam1 to EphB complexes containing NMDA-type glutamate receptors. Either knockdown of Tiam1 protein by RNAi or inhibition of Tiam1 function with a dominant-negative Tiam1 mutant blocks dendritic spine formation induced by ephrinB1 stimulation. Taken together, these findings suggest that EphBs regulate spine development in part by recruiting, phosphorylating, and activating Tiam1. Tiam1 can then promote Rac1-dependent actin cytoskeletal remodeling required for dendritic spine morphogenesis.synapses ͉ Rho GTPase ͉ actin cytoskeleton ͉ NMDA receptor

The protocol for penetratin-coupling of siRNA-like molecules used in this Article incorporated suggestions by Carol M. Troy that were based on the method described in ref. It has been drawn to our attention that the wording used in the first sentence after equation (1) is ambiguous. For the example used to illustrate 3 4 power scaling, we implied that if metabolic rate had a body mass scaling exponent of 0.75, a 10-fold increase in size would increase metabolic rate 7.5-fold; however, we were referring to the power-law relationship on a log scale, which should have been clear from the context. A 10-fold increase in mass would therefore increase metabolic rate as a function of 10 0.75