[Show abstract][Hide abstract] ABSTRACT: Memory decline is one of the greatest health threats of the twenty-first century. Because of the widespread increase in life expectancy, 20 percent of the global population will be over 60 in 2050 and the problems caused by age-related memory loss will be dramatically aggravated. However, the molecular mechanisms underlying this inevitable process are not well understood. Here we show that the activity of the recently discovered mechanistic target of rapamycin (mTOR) complex 2 (mTORC2) declines with age in the brain of both fruit flies and rodents and that the loss of mTORC2-mediated actin polymerization contributes to age-associated memory loss. Intriguingly, treatment with a small molecule that activates mTORC2 (A-443654) reverses long-term memory (LTM) deficits in both aged mice and flies. In addition, we found that pharmacologically boosting either mTORC2 or actin polymerization enhances LTM. In contrast to the current approaches to enhance memory that have primarily targeted the regulation of gene expression (epigenetic, transcriptional, and translational), our data points to a novel, evolutionarily conserved mechanism for restoring memory that is dependent on structural plasticity. These insights into the molecular basis of age-related memory loss may hold promise for new treatments for cognitive disorders.
Full-text · Article · Oct 2015 · Scientific Reports
[Show abstract][Hide abstract] ABSTRACT: A study now provides proof of concept that restoration of Ras-Erk signaling during adulthood rescues cellular and cognitive phenotypes in mouse models of the genetic disorder Noonan syndrome.
No preview · Article · Nov 2014 · Nature Neuroscience
[Show abstract][Hide abstract] ABSTRACT: Activity-dependent changes in the strength of synaptic connections are fundamental to the formation and maintenance of memory. The mechanisms underlying persistent changes in synaptic strength in the hippocampus, specifically long-term potentiation and depression, depend on new protein synthesis. Such changes are thought to be orchestrated by engaging the signaling pathways that regulate mRNA translation in neurons. In this review, we discuss the key regulatory pathways that govern translational control in response to synaptic activity and the mRNA populations that are specifically targeted by these pathways. The critical contribution of regulatory control over new protein synthesis to proper cognitive function is underscored by human disorders associated with either silencing or mutation of genes encoding proteins that directly regulate translation. In light of these clinical implications, we also consider the therapeutic potential of targeting dysregulated translational control to treat cognitive disorders of synaptic dysfunction.
No preview · Article · Jul 2014 · Annual Review of Neuroscience
[Show abstract][Hide abstract] ABSTRACT: At hippocampal synapses, activation of group I metabotropic glutamate receptors (mGluRs) induces long-term depression (LTD), which requires new protein synthesis. However, the underlying mechanism remains elusive. Here we describe the translational program that underlies mGluR-LTD and identify the translation factor eIF2α as its master effector. Genetically reducing eIF2α phosphorylation, or specifically blocking the translation controlled by eIF2α phosphorylation, prevented mGluR-LTD and the internalization of surface AMPA receptors (AMPARs). Conversely, direct phosphorylation of eIF2α, bypassing mGluR activation, triggered a sustained LTD and removal of surface AMPARs. Combining polysome profiling and RNA sequencing, we identified the mRNAs translationally upregulated during mGluR-LTD. Translation of one of these mRNAs, oligophrenin-1, mediates the LTD induced by eIF2α phosphorylation. Mice deficient in phospho-eIF2α-mediated translation are impaired in object-place learning, a behavioral task that induces hippocampal mGluR-LTD in vivo. Our findings identify a new model of mGluR-LTD, which promises to be of value in the treatment of mGluR-LTD-linked cognitive disorders.
No preview · Article · Jun 2014 · Nature Neuroscience
[Show abstract][Hide abstract] ABSTRACT: Author Summary
Angelman syndrome (AS) is a devastating neurodevelopmental disorder diagnosed in young children, currently with no effective treatments. It is characterized by absence of speech, ataxia, intellectual disability, epilepsy, and a characteristic behavior of frequent laughter and smiling. The disease is caused by loss of the maternal allele of UBE3A, which is preferentially silenced on the paternal chromosome and expressed on the maternal chromosome in neurons due to genomic imprinting. It has been long proposed that by activating the originally silenced paternal allele of UBE3A, the disease may be cured. Here in our research, we demonstrated the feasibility of activating paternal Ube3a in mice by terminating the transcription of its antisense RNA Ube3a-ATS genetically. In the AS mouse model who additionally receives the terminated Ube3a-ATS allele from the paternal side, we observed restoration of Ube3a expression, amelioration of behavioral defects and reversal of the impaired long-term potentiation. We further studied the imprinting mechanisms of Ube3a and proposed a novel transcriptional collision model. These results provide solid in vivo evidence for a key regulatory role of Ube3a-ATS in the disease and open up an exciting possibility of a gene-specific treatment for Angelman syndrome.
[Show abstract][Hide abstract] ABSTRACT: The mechanistic target of rapamycin (mTOR) acts as a highly conserved signaling "hub" that integrates neuronal activity and a variety of synaptic inputs. mTOR is found in two functionally distinct complexes, mTORC1 and mTORC2, that crucially control long-term synaptic efficacy and memory storage. Dysregulation of mTOR signaling is associated with neurodevelopmental and neuropsychiatric disorders. In this Review, we describe the most recent advances in studies of mTOR signaling in the brain and the possible mechanisms underlying the many different functions of the mTOR complexes in neurological diseases. In addition, we discuss the medical relevance of these findings.
Full-text · Article · Nov 2013 · Nature Neuroscience
[Show abstract][Hide abstract] ABSTRACT: A major goal of biomedical research is the identification of molecular and cellular mechanisms that underlie memory storage. Here we report a previously unknown signaling pathway that is necessary for the conversion from short- to long-term memory. The mammalian target of rapamycin (mTOR) complex 2 (mTORC2), which contains the regulatory protein Rictor (rapamycin-insensitive companion of mTOR), was discovered only recently and little is known about its function. We found that conditional deletion of Rictor in the postnatal murine forebrain greatly reduced mTORC2 activity and selectively impaired both long-term memory (LTM) and the late phase of hippocampal long-term potentiation (L-LTP). We also found a comparable impairment of LTM in dTORC2-deficient flies, highlighting the evolutionary conservation of this pathway. Actin polymerization was reduced in the hippocampus of mTORC2-deficient mice and its restoration rescued both L-LTP and LTM. Moreover, a compound that promoted mTORC2 activity converted early LTP into late LTP and enhanced LTM. Thus, mTORC2 could be a therapeutic target for the treatment of cognitive dysfunction.
Full-text · Article · Mar 2013 · Nature Neuroscience
[Show abstract][Hide abstract] ABSTRACT: In Drosophila melanogaster few methods exist to perform rapid cell-type or tissue-specific expression profiling. A translating ribosome affinity purification (TRAP) method to profile actively translated mRNAs has been developed for use in a number of multicellular organisms although it has only been implemented to examine limited sets of cell- or tissue-types in these organisms. We have adapted the TRAP method for use in the versatile GAL4/UAS system of Drosophila allowing profiling of almost any tissue/cell-type with a single genetic cross. We created transgenic strains expressing a GFP-tagged ribosomal protein, RpL10A, under the control of the UAS promoter to perform cell-type specific translatome profiling. The GFP::RpL10A fusion protein incorporates efficiently into ribosomes and polysomes. Polysome affinity purification strongly enriches mRNAs from expected genes in the targeted tissues with sufficient sensitivity to analyze expression in small cell populations. This method can be used to determine the unique translatome profiles in different cell-types under varied physiological, pharmacological and pathological conditions.
[Show abstract][Hide abstract] ABSTRACT: Type I interferon is an integral component of the antiviral response, and its production is tightly controlled at the levels of transcription and translation. The eukaryotic translation-initiation factor eIF4E is a rate-limiting factor whose activity is regulated by phosphorylation of Ser209. Here we found that mice and fibroblasts in which eIF4E cannot be phosphorylated were less susceptible to virus infection. More production of type I interferon, resulting from less translation of Nfkbia mRNA (which encodes the inhibitor IκBα), largely explained this phenotype. The lower abundance of IκBα resulted in enhanced activity of the transcription factor NF-κB, which promoted the production of interferon-β (IFN-β). Thus, regulated phosphorylation of eIF4E has a key role in antiviral host defense by selectively controlling the translation of an mRNA that encodes a critical suppressor of the innate antiviral response.
No preview · Article · Apr 2012 · Nature Immunology
[Show abstract][Hide abstract] ABSTRACT: The double-stranded RNA-activated protein kinase (PKR) was originally identified as a sensor of virus infection, but its function in the brain remains unknown. Here, we report that the lack of PKR enhances learning and memory in several behavioral tasks while increasing network excitability. In addition, loss of PKR increases the late phase of long-lasting synaptic potentiation (L-LTP) in hippocampal slices. These effects are caused by an interferon-γ (IFN-γ)-mediated selective reduction in GABAergic synaptic action. Together, our results reveal that PKR finely tunes the network activity that must be maintained while storing a given episode during learning. Because PKR activity is altered in several neurological disorders, this kinase presents a promising new target for the treatment of cognitive dysfunction. As a first step in this direction, we show that a selective PKR inhibitor replicates the Pkr(-/-) phenotype in WT mice, enhancing long-term memory storage and L-LTP.
[Show abstract][Hide abstract] ABSTRACT: The protozoan parasite Leishmania alters the activity of its host cell, the macrophage. However, little is known about the effect of Leishmania infection on host protein synthesis. Here, we show that the Leishmania protease GP63 cleaves the mammalian/mechanistic target of rapamycin (mTOR), a serine/threonine kinase that regulates the translational repressor 4E-BP1. mTOR cleavage results in the inhibition of mTOR complex 1 (mTORC1) and concomitant activation of 4E-BP1 to promote Leishmania proliferation. Consistent with these results, pharmacological activation of 4E-BPs with rapamycin, results in a dramatic increase in parasite replication. In contrast, genetic deletion of 4E-BP1/2 reduces parasite load in macrophages ex vivo and decreases susceptibility to cutaneous leishmaniasis in vivo. The parasite resistant phenotype of 4E-BP1/2 double-knockout mice involves an enhanced type I IFN response. This study demonstrates that Leishmania evolved a survival mechanism by activating 4E-BPs, which serve as major targets for host translational control.
[Show abstract][Hide abstract] ABSTRACT: Both the formation of long-term memory (LTM) and late-long-term potentiation (L-LTP), which is thought to represent the cellular model of learning and memory, require de novo protein synthesis. The mammalian target of Rapamycin (mTOR) complex I (mTORC1) integrates information from various synaptic inputs and its best characterized function is the regulation of translation. Although initial studies have shown that rapamycin reduces L-LTP and partially blocks LTM, recent genetic and pharmacological evidence indicating that mTORC1 promotes L-LTP and LTM is controversial. Thus, the role of mTORC1 in L-LTP and LTM is unclear. To selectively inhibit mTORC1 activity in the adult brain, we used a "pharmacogenetic" approach that relies on the synergistic action of a drug (rapamycin) and a genetic manipulation (mTOR heterozygotes, mTOR(+/-) mice) on the same target (mTORC1). Although L-LTP and LTM are normal in mTOR(+/-) mice, application of a low concentration of rapamycin-one that is subthreshold for WT mice-prevented L-LTP and LTM only in mTOR(+/-) mice. Furthermore, we found that mTORC1-mediated translational control is required for memory reconsolidation. We provide here direct genetic evidence supporting the role of mTORC1 in L-LTP and behavioral memory.
Preview · Article · Feb 2011 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: The eIF4E-binding proteins (4E-BPs) repress translation initiation by preventing eIF4F complex formation. Of the three mammalian 4E-BPs, only 4E-BP2 is enriched in the mammalian brain and plays an important role in synaptic plasticity and learning and memory formation. Here we describe asparagine deamidation as a brain-specific posttranslational modification of 4E-BP2. Deamidation is the spontaneous conversion of asparagines to aspartates. Two deamidation sites were mapped to an asparagine-rich sequence unique to 4E-BP2. Deamidated 4E-BP2 exhibits increased binding to the mammalian target of rapamycin (mTOR)-binding protein raptor, which effects its reduced association with eIF4E. 4E-BP2 deamidation occurs during postnatal development, concomitant with the attenuation of the activity of the PI3K-Akt-mTOR signaling pathway. Expression of deamidated 4E-BP2 in 4E-BP2(-/-) neurons yielded mEPSCs exhibiting increased charge transfer with slower rise and decay kinetics relative to the wild-type form. 4E-BP2 deamidation may represent a compensatory mechanism for the developmental reduction of PI3K-Akt-mTOR signaling.
[Show abstract][Hide abstract] ABSTRACT: Oncolytic viruses constitute a promising therapy against malignant gliomas (MGs). However, virus-induced type I IFN greatly limits its clinical application. The kinase mammalian target of rapamycin (mTOR) stimulates type I IFN production via phosphorylation of its effector proteins, 4E-BPs and S6Ks. Here we show that mouse embryonic fibroblasts and mice lacking S6K1 and S6K2 are more susceptible to vesicular stomatitis virus (VSV) infection than their WT counterparts as a result of an impaired type I IFN response. We used this knowledge to employ a pharmacoviral approach to treat MGs. The highly specific inhibitor of mTOR rapamycin, in combination with an IFN-sensitive VSV-mutant strain (VSV(DeltaM51)), dramatically increased the survival of immunocompetent rats bearing MGs. More importantly, VSV(DeltaM51) selectively killed tumor, but not normal cells, in MG-bearing rats treated with rapamycin. These results demonstrate that reducing type I IFNs through inhibition of mTORC1 is an effective strategy to augment the therapeutic activity of VSV(DeltaM51).
Full-text · Article · Jan 2010 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Synaptic activity-dependent protein synthesis is required to convert a labile short-term memory (STM) into a persistent long-term memory (LTM). Indeed, genetic or pharmacological inhibition of translation impairs LTM, but not STM. Long-lasting biochemical and morphological changes of synapses, which underlie learning and memory, also require new protein synthesis. In recent years, a large number of experiments have yielded much new information about the processes that govern translational control of synaptic plasticity during learning and memory processes. Signaling pathways that modulate mRNA translation play critical roles in these processes. In this chapter, we review the mechanisms by which certain translational regulators including eIF2alpha, 4E-BP, S6K, and CPEB control long-term synaptic plasticity and memory consolidation and their involvement in neurologic disease.
No preview · Article · Dec 2009 · Progress in molecular biology and translational science
[Show abstract][Hide abstract] ABSTRACT: eIF4E, the mRNA 5' cap-binding translation initiation factor, is overexpressed in numerous cancers and is implicated in mechanisms underlying oncogenesis and senescence. 4E-BPs (eIF4E-binding proteins) inhibit eIF4E activity, and thereby act as suppressors of eIF4E-dependent pathways. Here, we show that tumorigenesis is increased in p53 knockout mice that lack 4E-BP1 and 4E-BP2. However, primary fibroblasts lacking 4E-BPs, but expressing p53, undergo premature senescence and resist oncogene-driven transformation. Thus, the p53 status governs 4E-BP-dependent senescence and transformation. Intriguingly, the 4E-BPs engage in senescence via translational control of the p53-stabilizing protein, Gas2. Our data demonstrate a role for 4E-BPs in senescence and tumorigenesis and highlight a p53-mediated mechanism of senescence through a 4E-BP-dependent pathway.
[Show abstract][Hide abstract] ABSTRACT: Hypusination is a unique posttranslational modification by which lysine is transformed into the atypical amino acid hypusine. eIF5A (eukaryotic initiation factor 5A) is the only known protein to contain hypusine. In this study, we describe the identification and characterization of nero, the Drosophila melanogaster deoxyhypusine hydroxylase (DOHH) homologue. nero mutations affect cell and organ size, bromodeoxyuridine incorporation, and autophagy. Knockdown of the hypusination target eIF5A via RNA interference causes phenotypes similar to nero mutations. However, loss of nero appears to cause milder phenotypes than loss of eIF5A. This is partially explained through a potential compensatory mechanism by which nero mutant cells up-regulate eIF5A levels. The failure of eIF5A up-regulation to rescue nero mutant phenotypes suggests that hypusination is required for eIF5A function. Furthermore, expression of enzymatically impaired forms of DOHH fails to rescue nero clones, indicating that hypusination activity is important for nero function. Our data also indicate that nero and eIF5A are required for cell growth and affect autophagy and protein synthesis.
Full-text · Article · Jul 2009 · The Journal of Cell Biology
[Show abstract][Hide abstract] ABSTRACT: Hippocampal interneurons synchronize the activity of large neuronal ensembles during memory consolidation. Although the latter process is manifested as increases in synaptic efficacy which require new protein synthesis in pyramidal neurons, it is unknown whether such enduring plasticity occurs in interneurons. Here, we uncover a long-term potentiation (LTP) of transmission at individual interneuron excitatory synapses which persists for at least 24 h, after repetitive activation of type-1 metabotropic glutamate receptors [mGluR1-mediated chemical late LTP (cL-LTP(mGluR1))]. cL-LTP(mGluR1) involves presynaptic and postsynaptic expression mechanisms and requires both transcription and translation via phosphoinositide 3-kinase/mammalian target of rapamycin and MAP kinase kinase-extracellular signal-regulated protein kinase signaling pathways. Moreover, cL-LTP(mGluR1) involves translational control at the level of initiation as it is prevented by hippuristanol, an inhibitor of eIF4A, and facilitated in mice lacking the cap-dependent translational repressor, 4E-BP. Our results reveal novel mechanisms of long-term synaptic plasticity that are transcription and translation-dependent in inhibitory interneurons, indicating that persistent synaptic modifications in interneuron circuits may contribute to hippocampal-dependent cognitive processes.
Full-text · Article · May 2009 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience