Matthew E. Klein

Homeostatic regulation of MeCP2 expression by a CREB-induced microRNA.

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.

PSD-95 is required for activity-driven synapse stabilization

The activity-dependent regulation of α-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.

Roles of stargazin and phosphorylation in the control of AMPA receptor subcellular distribution

Understanding how the subcellular fate of newly synthesized AMPA receptors (AMPARs) is controlled is important for elucidating the mechanisms of neuronal function. We examined the effect of increased synthesis of AMPAR subunits on their subcellular distribution in rat hippocampal neurons. Virally expressed AMPAR subunits (GluR1 or GluR2) accumulated in cell bodies and replaced endogenous dendritic AMPAR with little effect on total dendritic amounts and caused no change in synaptic transmission. Coexpressing stargazin (STG) or mimicking GluR1 phosphorylation enhanced dendritic GluR1 levels by protecting GluR1 from lysosomal degradation. However, STG interaction or GluR1 phosphorylation did not increase surface or synaptic GluR1 levels. Unlike GluR1, STG did not protect GluR2 from lysosomal degradation or increase dendritic GluR2 levels. In general, AMPAR surface levels, and not intracellular amounts, correlated strongly with synaptic levels. Our results suggest that AMPAR surface expression, but not its intracellular production or accumulation, is critical for regulating synaptic transmission.

Role reversal : the regulation of neuronal gene expression by micrornas

In a similar fashion to transcription factors, non-coding RNAs can be essential regulators of gene expression. The largest class of non-coding RNAs is the microRNAs. These approximately 22 nt double-stranded RNA molecules can repress translation or target mRNA degradation. There has been a surge of research in the past year stimulated by the recent availability of specialized techniques, both in vitro and in silico, for predicting and characterizing microRNAs. The accumulating evidence suggests that microRNAs are ubiquitous regulators of gene expression during development. The combined actions of microRNAs and transcription factors are able to tune the expression of proteins on a global level in a manner that cannot be achieved by transcription factors alone.

RNA-binding protein Sam68 controls synapse number and local β-actin mRNA metabolism in dendrites

Proper synaptic function requires the spatial and temporal compartmentalization of RNA metabolism via transacting RNA-binding proteins (RBPs). Loss of RBP activity leads to abnormal posttranscriptional regulation and results in diverse neurological disorders with underlying deficits in synaptic morphology and transmission. Functional loss of the 68-kDa RBP Src associated in mitosis (Sam68) is associated with the pathogenesis of the neurological disorder fragile X tremor/ataxia syndrome. Sam68 binds to the mRNA of β-actin (actb), an integral cytoskeletal component of dendritic spines. We show that Sam68 knockdown or disruption of the binding between Sam68 and its actb mRNA cargo in primary hippocampal cultures decreases the amount of actb mRNA in the synaptodendritic compartment and results in fewer dendritic spines. Consistent with these observations, we find that Sam68-KO mice have reduced levels of actb mRNA associated with synaptic polysomes and diminished levels of synaptic actb protein, suggesting that Sam68 promotes the translation of actb mRNA at synapses in vivo. Moreover, genetic knockout of Sam68 or acute knockdown in vivo results in fewer excitatory synapses in the hippocampal formation as assessed morphologically and functionally. Therefore, we propose that Sam68 regulates synapse number in a cell-autonomous manner through control of postsynaptic actb mRNA metabolism. Our research identifies a role for Sam68 in synaptodendritic posttranscriptional regulation of actb and may provide insight into the pathophysiology of fragile X tremor/ataxia syndrome.

Coordination between Translation and Degradation Regulates Inducibility of mGluR-LTD

SUMMARY Dendritic protein homeostasis is crucial for most forms of long-term synaptic plasticity, and its dysregulation is linked to a wide range of brain disorders. Current models of metabotropic glutamate receptor mediated long-term depression (mGluR-LTD) suggest that rapid, local synthesis of key proteins is necessary for the induction and expression of LTD. Here, we find that mGluR-LTD can be induced in the absence of translation if the proteasome is concurrently inhibited. We report that enhanced pro-teasomal degradation during the expression of mGluR-LTD depletes dendritic proteins and inhibits subsequent inductions of LTD. Moreover, proteasome inhibition can rescue mGluR-LTD in mice null for the RNA binding protein Sam68, which we show here lack mGluR-dependent translation and LTD. Our study provides mechanistic insights for how changes in dendritic protein abundance regulate mGluR-LTD induction. We propose that Sam68-mediated translation helps to counterbalance degradation, ensuring that protein levels briefly remain above a permissive threshold during LTD induction.

Oxytocin improves behavioral and electrophysiological deficits in a novel Shank3-deficient rat

Mutations in the synaptic gene SHANK3 lead to a neurodevelopmental disorder known as Phelan-McDermid syndrome (PMS). PMS is a relatively common monogenic and highly penetrant cause of autism spectrum disorder (ASD) and intellectual disability (ID), and frequently presents with attention deficits. The underlying neurobiology of PMS is not fully known and pharmacological treatments for core symptoms do not exist. Here, we report the production and characterization of a Shank3-deficient rat model of PMS, with a genetic alteration similar to a human SHANK3 mutation. We show that Shank3-deficient rats exhibit impaired long-term social recognition memory and attention, and reduced synaptic plasticity in the hippocampal-medial prefrontal cortex pathway. These deficits were attenuated with oxytocin treatment. The effect of oxytocin on reversing non-social attention deficits is a particularly novel finding, and the results implicate an oxytocinergic contribution in this genetically defined subtype of ASD and ID, suggesting an individualized therapeutic approach for PMS. DOI: http://dx.doi.org/10.7554/eLife.18904.001

Proteostasis and RNA Binding Proteins in Synaptic Plasticity and in the Pathogenesis of Neuropsychiatric Disorders

Decades of research have demonstrated that rapid alterations in protein abundance are required for synaptic plasticity, a cellular correlate for learning and memory. Control of protein abundance, known as proteostasis, is achieved across a complex neuronal morphology that includes a tortuous axon as well as an extensive dendritic arbor supporting thousands of individual synaptic compartments. To regulate the spatiotemporal synthesis of proteins, neurons must efficiently coordinate the transport and metabolism of mRNAs. Among multiple levels of regulation, transacting RNA binding proteins (RBPs) control proteostasis by binding to mRNAs and mediating their transport and translation in response to synaptic activity. In addition to synthesis, protein degradation must be carefully balanced for optimal proteostasis, as deviations resulting in excess or insufficient abundance of key synaptic factors produce pathologies. As such, mutations in components of the proteasomal or translational machinery, including RBPs, have been linked to the pathogenesis of neurological disorders such as Fragile X Syndrome (FXS), Fragile X Tremor Ataxia Syndrome (FXTAS), and Autism Spectrum Disorders (ASD). In this review, we summarize recent scientific findings, highlight ongoing questions, and link basic molecular mechanisms to the pathogenesis of common neuropsychiatric disorders.