Local Dendritic Translation Research Articles

Dendritic distribution of CDK5 mRNA and p35 mRNA, and a glutamate-responsive increase of CDK5/p25 complex contribute to tau hyperphosphorylation

BackgroundIn Alzheimer's disease (AD), abnormally phosphorylated tau in the somatodendrite compartment of brain neurons causes synaptic loss, resulting in neuron death. Although the mechanism by which hyperphosphorylated tau appears in dendrites remains unclear, we have previously reported that local translation of tau mRNA and GSK3β mRNA in response to glutamatergic stimulation triggers an increase of tau protein and initiation of a cycle for amplification of reactivated preexisting GSK3β, respectively. In this study, we investigated the mechanism responsible for neural excitation-dependent activation of another major tau kinase, CDK5, within dendrites. MethodsPrimary hippocampal neurons were treated with glutamate and examined by in situ hybridization, immunocytochemistry and Western blotting. ResultsThe mRNAs for both CDK5 and its neural-specific activator, p35, were found to be constitutively distributed in dendrites. Glutamate treatment induced immediate local dendritic translation of these proteins as well as conversion of p35 to p25, which forms the hyper-activated CDK5/p25 complex. This neural excitation-dependent tau phosphorylation by CDK5 was suppressed in the presence of a calpain inhibitor or a NMDA receptor antagonist. ConclusionOur results indicate that in addition to an increase of dendritic tau and reactivation of preexisting GSK3β, increase and hyper-activation of CDK5 are evoked by translation of dendrite-distributed mRNAs upon NMDA receptor-mediated neural excitation. General significanceHyperphosphorylated tau with AD epitopes is locally produced in dendrites via translational activation of dendrite-distributed mRNAs in response to glutamatergic stimulation. Therefore, tau hyperphosphorylation may play a crucial role in synaptic transduction.

Semaphorin-3A Promotes Degradation of Fragile X Mental Retardation Protein in Growth Cones via the Ubiquitin-Proteasome Pathway.

Fragile X mental retardation protein (FMRP) is an RNA-binding protein that regulates local translation in dendrites and spines for synaptic plasticity. In axons, FMRP is implicated in axonal extension and axon guidance. We previously demonstrated the involvement of FMRP in growth cone collapse via a translation-dependent response to Semaphorin-3A (Sema3A), a repulsive axon guidance factor. In the case of attractive axon guidance factors, RNA-binding proteins such as zipcode binding protein 1 (ZBP1) accumulate towards the stimulated side of growth cones for local translation. However, it remains unclear how Sema3A effects FMRP localization in growth cones. Here, we show that levels of FMRP in growth cones of hippocampal neurons decreased after Sema3A stimulation. This decrease in FMRP was suppressed by the ubiquitin-activating enzyme E1 enzyme inhibitor PYR-41 and proteasome inhibitor MG132, suggesting that the ubiquitin-proteasome pathway is involved in Sema3A-induced FMRP degradation in growth cones. Moreover, the E1 enzyme or proteasome inhibitor suppressed Sema3A-induced increases in microtubule-associated protein 1B (MAP1B) in growth cones, suggesting that the ubiquitin-proteasome pathway promotes local translation of MAP1B, whose translation is mediated by FMRP. These inhibitors also blocked the Sema3A-induced growth cone collapse. Collectively, our results suggest that Sema3A promotes degradation of FMRP in growth cones through the ubiquitin-proteasome pathway, leading to growth cone collapse via local translation of MAP1B. These findings reveal a new mechanism of axon guidance regulation: degradation of the translational suppressor FMRP via the ubiquitin-proteasome pathway.

Cataloguing and Selection of mRNAs Localized to Dendrites in Neurons and Regulated by RNA-Binding Proteins in RNA Granules.

Spatiotemporal translational regulation plays a key role in determining cell fate and function. Specifically, in neurons, local translation in dendrites is essential for synaptic plasticity and long-term memory formation. To achieve local translation, RNA-binding proteins in RNA granules regulate target mRNA stability, localization, and translation. To date, mRNAs localized to dendrites have been identified by comprehensive analyses. In addition, mRNAs associated with and regulated by RNA-binding proteins have been identified using various methods in many studies. However, the results obtained from these numerous studies have not been compiled together. In this review, we have catalogued mRNAs that are localized to dendrites and are associated with and regulated by the RNA-binding proteins fragile X mental retardation protein (FMRP), RNA granule protein 105 (RNG105, also known as Caprin1), Ras-GAP SH3 domain binding protein (G3BP), cytoplasmic polyadenylation element binding protein 1 (CPEB1), and staufen double-stranded RNA binding proteins 1 and 2 (Stau1 and Stau2) in RNA granules. This review provides comprehensive information on dendritic mRNAs, the neuronal functions of mRNA-encoded proteins, the association of dendritic mRNAs with RNA-binding proteins in RNA granules, and the effects of RNA-binding proteins on mRNA regulation. These findings provide insights into the mechanistic basis of protein-synthesis-dependent synaptic plasticity and memory formation and contribute to future efforts to understand the physiological implications of local regulation of dendritic mRNAs in neurons.

THE ROLE OF CAPRIN-1 PROTEIN DYSREGULATION IN SYNAPSE DECLINE LEADING TO PROGRESSION OF TAUOPATHIES

Many neurodegenerative diseases are characterized by accumulation of proteins such as tau, a microtubule stabilization protein. Toxic tau forms tangles and affects neuronal synapse function, an early step of neurodegeneration. To focus on synapse function, we highlighted Caprin-1 protein. Through gene ontology, Caprin-1 is related to RNA granule proteins, important for transport and local translation in dendrites. Caprin-1 is of interest for neurodegeneration because it is a memory related protein, transports mRNA in RNA granules, and knock out mice demonstrate memory deficits. As we age, the performance of local translation in the dendrites is compromised and leads to synapse dysfunction. We hypothesize Caprin-1 binds to Tau and becomes disrupted. This leads to the dissolution of RNA granules, inhibition of mRNA transport in dendrites, suppression of translation, and failure of synapse. Early western blot data showed reduced Caprin-1 in PS19 Tau+, supporting our model that Caprin-1 is disrupted in disease models. Through immunohistochemistry, we investigated the localization of Caprin-1 in the mouse hippocampus. We observed Caprin-1 localization to dendrites of CA1 neurons in the hippocampus. Furthermore, Caprin-1 exhibited colocalization with Rps6, an RNA granule marker. This suggests Caprin-1 associates with RNA granules in mouse hippocampus. Finally, we investigated the localization of Caprin-1 in human iPSC-derived neurons. Similar to the mouse hippocampus, we observed localization of Caprin-1 to dendrites of human neurons. In future directions, we will examine whether pathogenic tau alters the association of Caprin-1 with RNA granules and the mechanisms by which pathogenic tau negatively effects synapse function.

Individual Copy Number of Ribosomal Genes as a Factor of Mental Retardation and Autism Risk and Severity.

Autism is a complex multifactorial developmental disorder characterized by deficits in communication and restricted interests, often followed by mental retardation. Autism spectrum disorders (ASD) are caused by defects in miscellaneous molecular mechanisms, many of which remain unclear. But a considerable part of the known pathways converges on protein synthesis or degradation processes at different stages in the dendrites, laying the foundation for a concept of disturbed “translational homeostasis” or “proteostasis” in autism. The protein synthesis is conducted on ribosomes, cellular organelles consisting from a complex of riboproteins and a ribosomal RNA (rRNA) framework. The rRNA is encoded by ribosomal genes (RG) existing in multiple copies in the genome. The more copies of RG that are contained in the genome, the higher is the peak (maximum possible) ribosome abundance in the cell. A hypothesis is proposed that the RG copy number, through determining the quantity of ribosomes available in the dendrites, modulates the level of local dendritic translation and thus is a factor of risk and severity of a series of neuropsychiatric disorders caused by aberrant dendritic translation. A carrier of very low copy number of ribosomal genes is expected to have a milder form of ASD than a subject with the same epigenetic and genetic background, but a higher ribosomal gene dosage. Various ways of evaluation and testing the hypothesis on clinical material and animal models are suggested.

CPEB1 is overexpressed in neurons derived from Down syndrome IPSCs and in the hippocampus of the mouse model Ts1Cje

Trisomy 21, also known as Down syndrome (DS), is the most frequent genetic cause of intellectual impairment. In mouse models of DS, deficits in hippocampal synaptic plasticity have been observed, in conjunction with alterations to local dendritic translation that are likely to influence plasticity, learning and memory. Here we show that expression of a local translational regulator, the Cytoplasmic Polyadenylation Element Binding Protein 1 (CPEB1), is enhanced in hippocampal neurons from the Ts1Cje DS mouse model. Interestingly, this protein, which is also involved in dendritic mRNA transport, is overexpressed in dendrites of neurons derived from DS human induced pluripotent stem cells (hIPSCs). Moreover, there is an increase in the mRNA levels of α-Calmodulin Kinase II (α-CaMKII) and Microtubule-associated protein 1B (MAP1B), two dendritic mRNAs, in Ts1Cje synaptoneurosomes. Taking into account the fundamental role of CPEB1 protein and its target mRNAs in synaptic plasticity, these data could be relevant to the intellectual impairment in the context of DS.

TAR DNA-Binding Protein 43 and Disrupted in Schizophrenia 1 Coaggregation Disrupts Dendritic Local Translation and Mental Function in Frontotemporal Lobar Degeneration

BackgroundNeurodegenerative diseases involving protein aggregation often accompany psychiatric symptoms. Frontotemporal lobar degeneration (FTLD) associated with TAR DNA-binding protein 43 (TDP-43) aggregation is characterized by progressive neuronal atrophy in frontal and temporal lobes of cerebral cortex. Furthermore, patients with FTLD display mental dysfunction in multiple behavioral dimensions. Nevertheless, their molecular origin for psychiatric symptoms remains unclear. MethodsIn FTLD neurons and mouse models with TDP-43 aggregates, we examined coaggregation between TDP-43 and disrupted in schizophrenia 1 (DISC1), a key player in the pathology of mental conditions and its effects on local translation in dendrites and psychiatric behaviors. The protein coaggregation and the expression level of synaptic proteins were also investigated with postmortem brains from patients with FTLD (n = 6). ResultsWe found cytosolic TDP-43/DISC1 coaggregates in brains of both FTLD mouse model and patients with FTLD. At the mechanistic levels, the TDP-43/DISC1 coaggregates disrupted the activity-dependent dendritic local translation through impairment of translation initiation and, in turn, reduced synaptic protein expression. Behavioral deficits detected in FTLD model mice were ameliorated by exogenous DISC1 expression. ConclusionsOur findings reveal a novel role of the aggregate-prone TDP-43/DISC1 protein complex in regulating local translation, which affects aberrant behaviors relevant to multiple psychiatric dimensions.

Molecular mechanisms of autism as a form of synaptic dysfunction

Autism spectrum disorders (ASDs) are a separate group of developmental disorders with a very large genetic component. Genetic screening has identified hundreds of mutations and other genetic variations associated with autism, and bioinformatic analysis of signaling pathways and gene networks has led to the understanding that many of these mutational changes are involved in the functioning of synapses. A synapse is a site of electrochemical communication between neurons and is a required subunit for learning and memory. Interneuronal communicative connections are plastic. The most prominent forms of synaptic plasticity are accompanied by changes in protein biosynthesis, both in the neuron body and in dendrites. Protein biosynthesis, or translation, is a finely regulated process, with the central role played by mTOR kinase (mammalian or mechanistic target of rapamycin). A mutational aberration in at least one of the links of the mTOR signaling pathway impairs the synaptic plasticity and behavior. The deregulation of local translation in dendrites is connected with the following monogenic ASDs: neurofibromatosis type 1, Noonan syndrome, Costello syndrome, Cowden syndrome, tuberous sclerosis, fragile X syndrome, and Rett syndrome (RS). The review considers the most important mutations leading to monogenic autism. The possibility of a mechanism-based treatment of certain ASDs is discussed.

Forebrain-specific, conditional silencing of Staufen2 alters synaptic plasticity, learning, and memory in rats

BackgroundDendritic messenger RNA (mRNA) localization and subsequent local translation in dendrites critically contributes to synaptic plasticity and learning and memory. Little is known, however, about the contribution of RNA-binding proteins (RBPs) to these processes in vivo.ResultsTo delineate the role of the double-stranded RBP Staufen2 (Stau2), we generate a transgenic rat model, in which Stau2 expression is conditionally silenced by Cre-inducible expression of a microRNA (miRNA) targeting Stau2 mRNA in adult forebrain neurons. Known physiological mRNA targets for Stau2, such as RhoA, Complexin 1, and Rgs4 mRNAs, are found to be dysregulated in brains of Stau2-deficient rats. In vivo electrophysiological recordings reveal synaptic strengthening upon stimulation, showing a shift in the frequency-response function of hippocampal synaptic plasticity to favor long-term potentiation and impair long-term depression in Stau2-deficient rats. These observations are accompanied by deficits in hippocampal spatial working memory, spatial novelty detection, and in tasks investigating associative learning and memory.ConclusionsTogether, these experiments reveal a critical contribution of Stau2 to various forms of synaptic plasticity including spatial working memory and cognitive management of new environmental information. These findings might contribute to the development of treatments for conditions associated with learning and memory deficits.

Prosapip1-Dependent Synaptic Adaptations in the Nucleus Accumbens Drive Alcohol Intake, Seeking, and Reward

The mammalian target of rapamycin complex 1 (mTORC1), a transducer of local dendritic translation, participates in learning and memory processes as well as in mechanisms underlying alcohol-drinking behaviors. Using an unbiased RNA-seq approach, we identified Prosapip1 as a novel downstream target of mTORC1 whose translation and consequent synaptic protein expression are increased in the nucleus accumbens (NAc) of mice excessively consuming alcohol. We demonstrate that alcohol-dependent increases in Prosapip1 levels promote the formation of actin filaments, leading to changes in dendritic spine morphology of NAc medium spiny neurons (MSNs). We further demonstrate that Prosapip1 is required for alcohol-dependent synaptic localization of GluA2 lacking AMPA receptors in NAc shell MSNs. Finally, we present data implicating Prosapip1 in mechanisms underlying alcohol self-administration and reward. Together, these data suggest that Prosapip1 in the NAc is amolecular transducer of structural and synaptic alterations that drive and/or maintain excessive alcohol use.

Accumulation of Polyribosomes in Dendritic Spine Heads, But Not Bases and Necks, during Memory Consolidation Depends on Cap-Dependent Translation Initiation.

Translation in dendrites is believed to support synaptic changes during memory consolidation. Although translational control mechanisms are fundamental mediators of memory, little is known about their role in local translation. We previously found that polyribosomes accumulate in dendritic spines of the adult rat lateral amygdala (LA) during consolidation of aversive pavlovian conditioning and that this memory requires cap-dependent initiation, a primary point of translational control in eukaryotic cells. Here we used serial electron microscopy reconstructions to quantify polyribosomes in LA dendrites when consolidation was blocked by the cap-dependent initiation inhibitor 4EGI-1. We found that 4EGI-1 depleted polyribosomes in dendritic shafts and selectively prevented their upregulation in spine heads, but not bases and necks, during consolidation. Cap-independent upregulation was specific to spines with small, astrocyte-associated synapses. Our results reveal that cap-dependent initiation is involved in local translation during learning and that local translational control varies with synapse type.SIGNIFICANCE STATEMENT Translation initiation is a central regulator of long-term memory formation. Local translation in dendrites supports memory by providing necessary proteins at synaptic sites, but it is unknown whether this requires initiation or bypasses it. We used serial electron microscopy reconstructions to examine polyribosomes in dendrites when memory formation was blocked by an inhibitor of translation initiation. This revealed two major pools of polyribosomes that were upregulated during memory formation: one pool in dendritic spine heads that was initiation dependent and another pool in the bases and necks of small spines that was initiation independent. Thus, translation regulation differs between spine types and locations, and translation that occurs closest to individual synapses during memory formation is initiation dependent.

Molecular mechanisms of autism as a form of synaptic dysfunction

Autism spectrum disorders are a separate group of defects with a very high genetic component. Genetic screening has identified hundreds of mutations and other genetic variations associated with autism, and bioinformatic analysis of signaling pathways and gene networks has led to understanding that many of these mutational changes are involved in the functioning of synapses. A synapse is a site of electrochemical communication between neurons and an essential subunit for learning and memory. Interneuronal communicative relationships are plastic. The most prominent forms of synaptic plasticity are accompanied by changes in protein biosynthesis, both in neuron body and in dendrites. Protein biosynthesis or translation is a carefully regulated process, with a central role played by mTOR (mammalian or mechanistic target of rapamycin). Normally mTOR-regulated translation is slightly inhibited, and in most cases mutational damage to at least one of the links of the mTOR signaling pathway, increases translation and leads to impaired synaptic plasticity and behavior. Deregulation of the local translation in dendrites is connected with the following monogenic autism spectrum disorders: neurofibromatosis type 1, Noonan syndrome, Costello syndrome, Cowden syndrome, tuberous sclerosis, fragile X chromosome, syndrome, and Rett syndrome. The review considers the most important mutations leading to monogenic autism, as well as the possibility of a mechanism-based treatment of certain disorders of the autism spectrum.

Novel RNA- and FMRP-binding protein TRF2-S regulates axonal mRNA transport and presynaptic plasticity.

Despite considerable evidence that RNA-binding proteins (RBPs) regulate mRNA transport and local translation in dendrites, roles for axonal RBPs are poorly understood. Here we demonstrate that a non-telomeric isoform of telomere repeat-binding factor 2 (TRF2-S) is a novel RBP that regulates axonal plasticity. TRF2-S interacts directly with target mRNAs to facilitate their axonal delivery. The process is antagonized by fragile X mental retardation protein (FMRP). Distinct from the current RNA-binding model of FMRP, we show that FMRP occupies the GAR domain of TRF2-S protein to block the assembly of TRF2-S–mRNA complexes. Overexpressing TRF2-S and silencing FMRP promotes mRNA entry to axons and enhances axonal outgrowth and neurotransmitter release from presynaptic terminals. Our findings suggest a pivotal role for TRF2-S in an axonal mRNA localization pathway that enhances axon outgrowth and neurotransmitter release.

Meet the players: local translation at the synapse.

It is widely believed that activity-dependent synaptic plasticity is the basis for learning and memory. Both processes are dependent on new protein synthesis at the synapse. Here, we describe a mechanism how dendritic mRNAs are transported and subsequently translated at activated synapses. Furthermore, we present the players involved in the regulation of local dendritic translation upon neuronal stimulation and their molecular interplay that maintain local proteome homeostasis. Any dysregulation causes several types of neurological disorders including muscular atrophies, cancers, neuropathies, neurodegenerative, and cognitive disorders.

The Akt-mTOR Pathway in Down’s Syndrome: The Potential Use of Rapamycin/Rapalogs for Treating Cognitive Deficits

An increasing amount of evidence suggests that the dysregulation of the Akt-mTOR (Akt-mammalian Target Of Rapamycin) signaling network is associated with intellectual disabilities, such as fragile X, tuberous sclerosis and Rett's syndrome. The Akt-mTOR pathway is involved in dendrite morphogenesis and synaptic plasticity, and it has been shown to modulate both glutamatergic and GABAergic synaptic transmission. We have recently shown that the AktmTOR pathway is hyperactive in the hippocampus of Ts1Cje mice, a model of Down's syndrome, leading to increased local dendritic translation that could interfere with synaptic plasticity. Rapamycin and rapalogs are specific inhibitors of mTOR, and some of these inhibitors are Food and Drug Administration-approved drugs. In this review, we discuss the molecular basis and consequences of Akt-mTOR hyperactivation in Down's syndrome, paying close attention to alterations in the molecular mechanisms underlying synaptic plasticity. We also analyze the pros and cons of using rapamycin/rapalogs for the treatment of the cognitive impairments associated with this condition.

Bidirectional Control of mRNA Translation and Synaptic Plasticity by the Cytoplasmic Polyadenylation Complex

Translational control of mRNAs in dendrites is essential for certain forms of synaptic plasticity and learning and memory. CPEB is an RNA-binding protein that regulates local translation in dendrites. Here, we identify poly(A) polymerase Gld2, deadenylase PARN, and translation inhibitory factor neuroguidin (Ngd) as components of a dendritic CPEB-associated polyadenylation apparatus. Synaptic stimulation induces phosphorylation of CPEB, PARN expulsion from the ribonucleoprotein complex, and polyadenylation in dendrites. A screen for mRNAs whose polyadenylation is altered by Gld2 depletion identified >100 transcripts including one encoding NR2A, an NMDA receptor subunit. shRNA depletion studies demonstrate that Gld2 promotes and Ngd inhibits dendritic NR2A expression. Finally, shRNA-mediated depletion of Gld2 in vivo attenuates protein synthesis-dependent long-term potentiation (LTP) at hippocampal dentate gyrus synapses; conversely, Ngd depletion enhances LTP. These results identify a pivotal role for polyadenylation and the opposing effects of Gld2 and Ngd in hippocampal synaptic plasticity.

Hypothalamic BDNF and obesity: found in translation

A recent study provides new insights into the central control of energy balance and obesity, showing that feeding behavior in mice can be modulated by local dendritic translation of a key protein in neuronal plasticity brain-derived neurotrophic factor (pages 564–571).

Patent Highlights

Patent Highlights

An Increase in Basal BDNF Provokes Hyperactivation of the Akt-Mammalian Target of Rapamycin Pathway and Deregulation of Local Dendritic Translation in a Mouse Model of Down's Syndrome

As in other diseases associated with mental retardation, dendrite morphology and synaptic plasticity are impaired in Down's syndrome (DS). Both these features of neurons are critically influenced by BDNF, which regulates local dendritic translation through phosphatidylinositol 3-kinase-Akt-mammalian target of rapamycin (mTOR) and Ras-ERK signaling cascades. Here we show that the levels of BDNF and phosphorylated Akt-mTOR (but not Ras-ERK) pathway proteins are augmented in hippocampal dendrites of Ts1Cje mice, a DS model. Consequently, the rate of local dendritic translation is abnormally high and the modulatory effect of exogenous BDNF is lost. Interestingly, rapamycin (a Food and Drug Administration-approved drug) restores normal levels of phosphorylated Akt-mTOR proteins and normal rates of local translation in Ts1Cje neurons, opening new therapeutic perspectives for DS. The NMDAR inhibitors APV, MK-801, and memantine also restore the normal levels of phospho-mTOR in dendrites of Ts1Cje hippocampal neurons. We propose a model to explain how BDNF-mediated regulation of local translation is lost in the Ts1Cje hippocampus through the establishment of a glutamatergic positive-feedback loop. Together, these findings help elucidate the mechanisms underlying altered synaptic plasticity in DS.

P111. RNG105 deficiency impairs the dendritic transport of mRNA and leads to the degeneration of neuronal networks

mRNA transport and subsequent local translation are key mechanisms for several cellular processes during development. In neurons, local translation in dendrites plays important roles in use-dependent synaptic modification and in higher-order brain functions. RNA granules that consist of clusters of ribosomes and mRNAs play central roles in the mRNA transport and local translational control in neuronal dendrites. We identified RNG105 (RNA granule protein 105) as a novel RNA-binding protein that localizes to RNA granules in neurons. We further identified RNG105-associated mRNAs, which included mRNAs encoding Na+/K+ ATPase (NKA) subunit isoforms, i.e., α3, FXYD1, FXYD6, and FXYD7. We generated RNG105-knockout mice and found that the mice died soon after birth because of respiratory failure, suggesting that their brainstem function was impaired. In Rng105–/– neurons, dendritic localization of the NKA mRNAs was reduced, which was accompanied by the loss of function of NKA in dendrites without affecting the NKA function in the soma. Furthermore, Knockout of RNG105 impaired the formation of synapses and neuronal networks, which was mimicked by inhibition and knockdown of NKA in neurons. We propose that RNG105 participates in the dendritic transport of the mRNAs to regulate the development and maintenance of functional networks.