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SummaryWe report the results of a genetic screen to identify molecules important for synapse formation and/or maintenance. siRNAs were used to decrease the expression of candidate genes in neurons, and synapse development was assessed. We surveyed 22 cadherin family members and demonstrated distinct roles for cadherin-11 and cadherin-13 in synapse development. Our screen also revealed roles for the class 4 Semaphorins Sema4B and Sema4D in the development of glutamatergic and/or GABAergic synapses. We found that Sema4D affects the formation of GABAergic, but not glutamatergic, synapses. Our screen also identified the activity-regulated small GTPase Rem2 as a regulator of synapse development. A known calcium channel modulator, Rem2 may function as part of a homeostatic mechanism that controls synapse number. These experiments establish the feasibility of RNAi screens to characterize the mechanisms that control mammalian neuronal development and to identify components of the genetic program that regulate synapse formation and/or maintenance.

The transcription factor CREB mediates diverse responses in the nervous system. It is not known how CREB induces specific patterns of gene expression in response to different extracellular stimuli. We find that Ca2+ influx into neurons induces CREB phosphorylation at Ser133 and two additional sites, Ser142 and Ser143. While CREB Ser133 phosphorylation is induced by many stimuli, phosphorylation at Ser142 and Ser143 is selectively activated by Ca2+ influx. The triple phosphorylation of CREB is required for effective Ca2+ stimulation of CREB-dependent transcription, but the phosphorylation of Ser142 and Ser143, in addition to Ser133, disrupts the interaction of CREB with its cofactor CBP. These results suggest that Ca2+ influx triggers a specific program of gene expression in neurons by selectively regulating CREB phosphorylation.

The RE1 binding silencer protein REST represses neuronal-specific gene expression in nonneuronal cell types. In this issue of Neuron, Ballas et al. show that REST inhibits gene expression via the recruitment of multiple histone deacetylase complexes.

SummaryMutations or duplications in MECP2 cause Rett and Rett-like syndromes, neurodevelopmental disorders characterized by mental retardation, motor dysfunction, and autistic behaviors. MeCP2 is expressed in many mammalian tissues and functions as a global repressor of transcription; however, the molecular mechanisms by which MeCP2 dysfunction leads to the neural-specific phenotypes of RTT remain poorly understood. Here, we show that neuronal activity and subsequent calcium influx trigger the de novo phosphorylation of MeCP2 at serine 421 (S421) by a CaMKII-dependent mechanism. MeCP2 S421 phosphorylation is induced selectively in the brain in response to physiological stimuli. Significantly, we find that S421 phosphorylation controls the ability of MeCP2 to regulate dendritic patterning, spine morphogenesis, and the activity-dependent induction of Bdnf transcription. These findings suggest that, by triggering MeCP2 phosphorylation, neuronal activity regulates a program of gene expression that mediates nervous system maturation and that disruption of this process in individuals with mutations in MeCP2 may underlie the neural-specific pathology of RTT.