Conditional knockout of tumor overexpressed gene in mouse neurons affects RNA granule assembly, granule translation, LTP and short term habituation

Development of single-molecule ubiquitination mediated fluorescence complementation to visualize protein ubiquitination dynamics in dendrites

The ubiquitination/proteasome system is important for the spatiotemporal control of protein synthesis and degradation at synapses, while dysregulation may underlie autism spectrum disorders (ASDs). However, methods allowing direct visualization of the subcellular localization and temporal dynamics of protein ubiquitination are lacking. Here we report the development of Single-Molecule Ubiquitin Mediated Fluorescence Complementation (SM-UbFC) as a method to visualize and quantify the dynamics of protein ubiquitination in dendrites of live neurons in culture. Using SM-UbFC, we demonstrate that the rate of PSD-95 ubiquitination is elevated in dendrites of FMR1 KO neurons compared with wild-type controls. We further demonstrate the rapid ubiquitination of the fragile X messenger ribonucleoprotein, FMRP, and the AMPA receptor subunit, GluA1, which are known to be key events in the regulation of synaptic protein synthesis and plasticity. SM-UbFC will be useful for future studies on the regulation of synaptic protein homeostasis.

Genetics, molecular control and clinical relevance of habituation learning

Habituation is the most fundamental form of learning. As a firewall that protects our brain from sensory overload, it is indispensable for cognitive processes. Studies in humans and animal models provide increasing evidence that habituation is affected in autism and related monogenic neurodevelopmental disorders (NDDs). An integrated application of habituation assessment in NDDs and their animal models has unexploited potential for neuroscience and medical care. With the aim to gain mechanistic insights, we systematically retrieved genes that have been demonstrated in the literature to underlie habituation. We identified 258 evolutionarily conserved genes across species, describe the biological processes they converge on, and highlight regulatory pathways and drugs that may alleviate habituation deficits. We also summarize current habituation paradigms and extract the most decisive arguments that support the crucial role of habituation for cognition in health and disease. We conclude that habituation is a conserved, quantitative, cognition- and disease-relevant process that can connect preclinical and clinical work, and hence is a powerful tool to advance research, diagnostics, and treatment of NDDs.

A native function for RAN translation and CGG repeats in regulating fragile X protein synthesis

Repeat-associated non-AUG translation of expanded CGG repeats (CGG RAN) from the FMR1 5’ UTR produces toxic proteins that contribute to neurodegeneration in Fragile X-associated Tremor/Ataxia Syndrome (FXTAS). Here we describe how unexpanded CGG repeats and their translation play conserved roles in regulating FMRP synthesis. In neurons, CGG RAN acts as an inhibitory upstream open reading frame to suppress basal FMRP production. Activation of mGluR5 receptors enhances FMRP synthesis. This enhancement requires both the CGG repeat and CGG RAN initiation sites. Using non-cleaving antisense oligonucleotides (ASOs), we selectively blocked RAN translation. This ASO blockade enhanced endogenous human neuronal FMRP expression. In human and rodent neurons, RAN blocking ASOs suppressed repeat toxicity and prolonged survival. These findings delineate a native function for CGG repeats and RAN translation in regulating basal and activity-dependent FMRP synthesis and demonstrate the therapeutic potential of modulating CGG RAN translation in fragile X-associated disorders.

Multivalent Molecules as Modulators of RNA Granule Size and Composition

RNA granules are ensembles of specific RNA and protein molecules that mediate localized translation in eukaryotic cells. The mechanisms for formation and selectivity of RNA granules are unknown. Here we present a model for assembly of one type of RNA granule based on experimentally measured binding interactions among three core multivalent molecular components necessary for such assembly: specific RNA molecules that contain a cis-acting sequence called the A2 response element (A2RE), hnRNP A2 proteins that bind specifically (with high affinity) to A2RE sequences or nonspecifically (with lower affinity) to other RNA sequences, and heptavalent protein cytoskeleton-associated protein 5 (CKAP5, an alternative name for TOG protein) that binds both hnRNP A2 molecules and RNA. Non-A2RE RNA molecules (RNA without the A2RE sequence) that may be recruited to the granules through nonspecific interactions are also considered in the model. Modeling multivalent molecular interactions in granules is challenging because of combinatorial complexity in the number of potential molecular complexes among these core components and dynamic changes in granule composition and structure in response to changes in local intracellular environment. We use a hybrid modeling approach (deterministic-stochastic-statistical) that is appropriate when the overall compositions of multimolecular ensembles are of greater importance than the specific interactions among individual molecular components. Modeling studies titrating the concentrations of various granule components and varying effective site pair affinities and RNA valency demonstrate that interactions between multivalent components (TOG and RNA) are modulated by a bivalent adaptor molecule (hnRNP A2). Formation and disruption of granules, as well as RNA selectivity in granule composition are regulated by distinct concentration regimes of A2. Our results suggest that granule assembly is tightly controlled by multivalent molecular interactions among RNA molecules, adaptor proteins, and scaffold proteins.

Conditional knockout of TOG results in CNS hypomyelination

The tumor overexpressed gene (TOG) protein is present in RNA granules that transport myelin basic protein (MBP) mRNA in oligodendrocyte processes to the myelin compartment. Its role was investigated by conditionally knocking it out (KO) in myelinating glia in vivo. TOG KO mice have severe motor deficits that are already apparent at the time of weaning. This phenotype correlates with a paucity of myelin in several CNS regions, the most severe being in the spinal cord. In the TOG KO optic nerve <30% of axons are myelinated. The number of oligodendrocytes in the corpus callosum, cerebellum, and cervical spinal cord is normal. In the absence of TOG, the most patent biochemical change is a large reduction in MBP content, yet normal amounts of MBP transcripts are found in the brain of affected animals. MBP transcripts are largely confined to the cell body of the oligodendrocytes in the TOG KO in contrast to the situation in wild type mice where they are found in the processes of the oligodendrocytes and in the myelin compartment. These findings indicate that MBP gene expression involves a post-transcriptional TOG-dependent step. TOG may be necessary for MBP mRNA assembly into translation permissive granules, and/or for transport to preferred sites of translation. GLIA 2017;65:489-501.

Conditional Knockout of Cav2.1 Disrupts the Accuracy of Spatial Recognition of CA1 Place Cells and Spatial/Contextual Recognition Behavior

Hippocampal pyramidal neurons play an essential role in processing spatial information as implicated with its place-dependent firing. Although, previous slice physiology studies have reported that voltage gated calcium channels contribute to spike shapes and corresponding firing rate in the hippocampus, the roles of P/Q type calcium channels (Cav2.1) underlying neural activity in behaving mice have not been well-investigated. To determine physiological and behavioral roles of Cav2.1, we conducted place cell recordings in CA1 and hippocampus dependent learning/memory tasks using mice lacking Cav2.1 in hippocampal pyramidal neurons under CamK2α-Cre recombinase expression. Results suggested that impairments shown in behavioral tasks requiring spatial and contextual information processing were statistically significant while general neurological behaviors did not differ between groups. In particular, deficits were more profound in recognition than in acquisition. Furthermore, place cell recordings also revealed that the ability to recollect spatial representation on re-visit in the conditional knockout was also altered in terms of the cue recognition while the capability of a place cell to encode a place was intact compared to the control group. Interestingly, CA1 pyramidal neurons of conditional knockout mice showed reduced burst frequency as well as abnormal temporal patterns of burst spiking. These results provide potential evidence that Cav2.1 in hippocampal pyramidal cells modulates temporal integration of bursts, which, in turn, might influence the recognition of place field and consequently disrupt spatial recognition ability.

Principles and Properties of Stress Granules

Stress granules are assemblies of untranslating messenger ribonucleoproteins (mRNPs) that form from mRNAs stalled in translation initiation. Stress granules form through interactions between mRNA-binding proteins that link together populations of mRNPs. Interactions promoting stress granule formation include conventional protein-protein interactions as well as interactions involving intrinsically disordered regions (IDRs) of proteins. Assembly and disassembly of stress granules are modulated by various post-translational modifications as well as numerous ATP-dependent RNP or protein remodeling complexes, illustrating that stress granules represent an active liquid wherein energy input maintains their dynamic state. Stress granule formation modulates the stress response, viral infection, and signaling pathways. Persistent or aberrant stress granule formation contributes to neurodegenerative disease and some cancers.

Is toxicant-induced Sertoli cell injury in vitro a useful model to study molecular mechanisms in spermatogenesis?

Sertoli cells isolated from rodents or humans and cultured in vitro are known to establish a functional tight junction (TJ)-permeability barrier that mimics the blood-testis barrier (BTB) in vivo. This model has been widely used by investigators to study the biology of the TJ and the BTB. Studies have shown that environmental toxicants (e.g., perfluorooctanesulfonate (PFOS), bisphenol A (BPA) and cadmium) that exert their disruptive effects to induce Sertoli cell injury using this in vitro model are reproducible in studies in vivo. Thus, this in vitro system provides a convenient approach to probe the molecular mechanism(s) underlying toxicant-induced testis injury but also to provide new insights in understanding spermatogenesis, such as the biology of cell adhesion, BTB restructuring that supports preleptotene spermatocyte transport, and others. Herein, we provide a brief and critical review based on studies using this in vitro model of Sertoli cell cultures using primary cells isolated from rodent testes vs. humans to monitor environmental toxicant-mediated Sertoli cell injury. In short, recent findings have shown that environmental toxicants exert their effects on Sertoli cells to induce testis injury through their action on Sertoli cell actin- and/or microtubule-based cytoskeleton. These effects are mediated via their disruptive effects on actin- and/or microtubule-binding proteins. Sertoli cells also utilize differential spatiotemporal expression of these actin binding proteins to confer plasticity to the BTB to regulate germ cell transport across the BTB.

Single-Molecule Imaging of PSD-95 mRNA Translation in Dendrites and Its Dysregulation in a Mouse Model of Fragile X Syndrome

Fragile X syndrome (FXS) is caused by the loss of the fragile X mental retardation protein (FMRP), an RNA binding protein that regulates translation of numerous target mRNAs, some of which are dendritically localized. Our previous biochemical studies using synaptoneurosomes demonstrate a role for FMRP and miR-125a in regulating the translation of PSD-95 mRNA. However, the local translation of PSD-95 mRNA within dendrites and spines, as well as the roles of FMRP or miR-125a, have not been directly studied. Herein, local synthesis of a Venus-PSD-95 fusion protein was directly visualized in dendrites and spines using single-molecule imaging of a diffusion-restricted Venus-PSD-95 reporter under control of the PSD-95 3'UTR. The basal translation rates of Venus-PSD-95 mRNA was increased in cultured hippocampal neurons from Fmr1 KO mice compared with WT neurons, which correlated with a transient elevation of endogenous PSD-95 within dendrites. Following mGluR stimulation with (S)-3,5-dihydroxyphenylglycine, the rate of Venus-PSD-95 mRNA translation increased rapidly in dendrites of WT hippocampal neurons, but not in those of Fmr1 KO neurons or when the binding site of miR125a, previously shown to bind PSD-95 3'UTR, was mutated. This study provides direct support for the hypothesis that local translation within dendrites and spines is dysregulated in FXS. Impairments in the regulated local synthesis of PSD-95, a critical regulator of synaptic structure and function, may affect the spatiotemporal control of PSD-95 levels and affect dendritic spine development and synaptic plasticity in FXS.

Nervous translation, do you get the message? A review of mRNPs, mRNA-protein interactions and translational control within cells of the nervous system

In neurons, translation of a message RNA can occur metres away from its transcriptional origin and in normal cells this is orchestrated with perfection. The life of an mRNA will see it pass through multiple steps of processing in the nucleus and the cytoplasm before it reaches its final destination. Processing of mRNA is determined by a myriad of RNA-binding proteins in multi-protein complexes called messenger ribonucleoproteins; however, incorrect processing and delivery of mRNA can cause several human neurological disorders. This review takes us through the life of mRNA from the nucleus to its point of translation in the cytoplasm. The review looks at the various cis and trans factors that act on the mRNA and discusses their roles in different cells of the nervous system and human disorders.

Synaptic control of local translation: the plot thickens with new characters

The production of proteins from mRNAs localized at the synapse ultimately controls the strength of synaptic transmission, thereby affecting behavior and cognitive functions. The regulated transcription, processing, and transport of mRNAs provide dynamic control of the dendritic transcriptome, which includes thousands of messengers encoding multiple cellular functions. Translation is locally modulated by synaptic activity through a complex network of RNA-binding proteins (RBPs) and various types of non-coding RNAs (ncRNAs) including BC-RNAs, microRNAs, piwi-interacting RNAs, and small interference RNAs. The RBPs FMRP and CPEB play a well-established role in synaptic translation, and additional regulatory factors are emerging. The mRNA repressors Smaug, Nanos, and Pumilio define a novel pathway for local translational control that affects dendritic branching and spines in both flies and mammals. Recent findings support a role for processing bodies and related synaptic mRNA-silencing foci (SyAS-foci) in the modulation of synaptic plasticity and memory formation. The SyAS-foci respond to different stimuli with changes in their integrity thus enabling regulated mRNA release followed by translation. CPEB, Pumilio, TDP-43, and FUS/TLS form multimers through low-complexity regions related to prion domains or polyQ expansions. The oligomerization of these repressor RBPs is mechanistically linked to the aggregation of abnormal proteins commonly associated with neurodegeneration. Here, we summarize the current knowledge on how specificity in mRNA translation is achieved through the concerted action of multiple pathways that involve regulatory ncRNAs and RBPs, the modification of translation factors, and mRNA-silencing foci dynamics.