Eric C. Griffith

Brain-Specific Phosphorylation of MeCP2 Regulates Activity-Dependent Bdnf Transcription, Dendritic Growth, and Spine Maturation

Mutations 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.

Regulation of transcription factors by neuronal activity

Synaptic activity regulates the expression of a set of neuronal gene products that are important for neuronal survival and differentiation, synaptogenesis and, ultimately, complex behaviour. Activity-dependent signalling pathways induce neuronal gene transcription by modulating the function of both transcriptional activators and repressors, and recent studies have revealed significant diversity in the mechanisms that control the activity of these transcriptional regulators. Investigators have begun to elucidate the distinct functions of individual activity-regulated transcription factors, and to explore how these factors cooperate to provide stimulus specificity in the initiation of neuronal transcriptional programmes.

Activity-Dependent Regulation of MEF2 Transcription Factors Suppresses Excitatory Synapse Number

In the mammalian nervous system, neuronal activity regulates the strength and number of synapses formed. The genetic program that coordinates this process is poorly understood. We show that myocyte enhancer factor 2 (MEF2) transcription factors suppressed excitatory synapse number in a neuronal activity- and calcineurin-dependent manner as hippocampal neurons formed synapses. In response to increased neuronal activity, calcium influx into neurons induced the activation of the calcium/calmodulin-regulated phosphatase calcineurin, which dephosphorylated and activated MEF2. When activated, MEF2 promoted the transcription of a set of genes, including arc and synGAP, that restrict synapse number. These findings define an activity-dependent transcriptional program that may control synapse number during development.

The Angelman Syndrome Protein Ube3A Regulates Synapse Development by Ubiquitinating Arc

Angelman Syndrome is a debilitating neurological disorder caused by mutation of the E3 ubiquitin ligase Ube3A, a gene whose mutation has also recently been associated with autism spectrum disorders (ASDs). The function of Ube3A during nervous system development and how Ube3A mutations give rise to cognitive impairment in individuals with Angleman Syndrome and ASDs are not clear. We report here that experience-driven neuronal activity induces Ube3A transcription and that Ube3A then regulates excitatory synapse development by controlling the degradation of Arc, a synaptic protein that promotes the internalization of the AMPA subtype of glutamate receptors. We find that disruption of Ube3A function in neurons leads to an increase in Arc expression and a concomitant decrease in the number of AMPA receptors at excitatory synapses. We propose that this deregulation of AMPA receptor expression at synapses may contribute to the cognitive dysfunction that occurs in Angelman Syndrome and possibly other ASDs.

The Many Forks in FOXO's Road

The FOXO family of transcription factors constitute an evolutionarily conserved subgroup within the larger family known as winged helix or Forkhead transcriptional regulators. Building upon work in the nematode, researchers have uncovered a role for these proteins in a diverse set of cellular responses that include glucose metabolism, stress response, cell cycle regulation, and apoptosis. At the organismal level, FOXO transcription factors are believed to function in various pathological processes ranging from cancer and diabetes to organismal aging. A number of studies have also shed light on the signaling pathways that regulate FOXO activity in response to external stimuli and have identified multiple FOXO target genes that mediate this varied set of biological responses. Many biological processes require the expression of new genes. A cells ability to express new genes is controlled by proteins called transcription factors. One such family of transcription factors is the winged helix or Forkhead family of transcriptional regulators, of which the FOXO transcription factors are a subgroup. The FOXO subfamily of transcription factors can control various biological processes, including glucose metabolism, stress response, cell cycle regulation, and cell death, by regulating gene expression. At the organismal level, the regulation of gene expression by FOXO transcription factors is believed to play a role in pathological processes ranging from cancer and diabetes to organismal aging. A number of studies have shed light on how the FOXO transcription factors themselves are regulated and have also identified the gene targets controlled by FOXO transcription factors.

CREB Transcriptional Activity in Neurons Is Regulated by Multiple, Calcium-Specific Phosphorylation Events

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 Ca(2+) 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 Ca(2+) influx. The triple phosphorylation of CREB is required for effective Ca(2+) 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 Ca(2+) influx triggers a specific program of gene expression in neurons by selectively regulating CREB phosphorylation.

Vav Family GEFs Link Activated Ephs to Endocytosis and Axon Guidance

Ephrin signaling through Eph receptor tyrosine kinases can promote attraction or repulsion of axonal growth cones during development. However, the mechanisms that determine whether Eph signaling promotes attraction or repulsion are not known. We show here that the Rho family GEF Vav2 plays a key role in this process. We find that, during axon guidance, ephrin binding to Ephs triggers Vav-dependent endocytosis of the ligand-receptor complex, thus converting an initially adhesive interaction into a repulsive event. In the absence of Vav proteins, ephrin-Eph endocytosis is blocked, leading to defects in growth cone collapse in vitro and significant defects in the ipsilateral retinogeniculate projections in vivo. These findings suggest an important role for Vav family GEFs as regulators of ligand-receptor endocytosis and determinants of repulsive signaling during axon guidance.

An RNAi-Based Approach Identifies Molecules Required for Glutamatergic and GABAergic Synapse Development

We 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.

Reading the unique DNA methylation landscape of the brain: Non-CpG methylation, hydroxymethylation, and MeCP2

DNA methylation at CpG dinucleotides is an important epigenetic regulator common to virtually all mammalian cell types, but recent evidence indicates that during early postnatal development neuronal genomes also accumulate uniquely high levels of two alternative forms of methylation, non-CpG methylation and hydroxymethylation. Here we discuss the distinct landscape of DNA methylation in neurons, how it is established, and how it might affect the binding and function of protein readers of DNA methylation. We review studies of one critical reader of DNA methylation in the brain, the Rett syndrome protein methyl CpG-binding protein 2 (MeCP2), and discuss how differential binding affinity of MeCP2 for non-CpG and hydroxymethylation may affect the function of this methyl-binding protein in the nervous system.

REST Acts through Multiple Deacetylase Complexes

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.