An activity-dependent switch to cap-independent translation triggered by eIF4E dephosphorylation

Phosphorylation of eIF4E in the stroma drives the production and spatial organisation of collagen type I in the mammary gland

The extracellular matrix (ECM) plays critical roles in breast cancer development. Whether ECM composition is regulated by the phosphorylation of eIF4E on serine 209, an event required for tumorigenesis, has not been explored. Herein, we used proteomics and mouse modelling to investigate the impact of mutating serine 209 to alanine on eIF4E (i.e., S209A) on mammary gland (MG) ECM. The proteomic data have been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the dataset identifier PXD028953. We discovered that S209A knock-in mice, expressing a non-phosphorylatable form of eIF4E, have less collagen-I deposition in native and tumor-bearing MGs, leading to altered tumor cell invasion. Additionally, phospho-eIF4E-deficiency impacts collagen topology; fibers at the tumor-stroma boundary in phospho-eIF4E-deficient mice run parallel to the tumor edge but radiate outwards in wild-type mice. Finally, a phospho-eIF4E-deficient tumor microenvironment resists anti-PD-1 therapy-induced collagen deposition, correlating with an increased anti-tumor response to immunotherapy. Clinically, we showed that collagen-I and phospho-eIF4E are positively correlated in human breast cancer samples, and that stromal phospho-eIF4E expression is influenced by tumor proximity. Together, our work defines the importance of phosphorylation of eIF4E on S209 as a regulator of MG collagen architecture in the tumor microenvironment, thereby positioning phospho-eIF4E as a therapeutic target to augment response to therapy.

Molecular targets for modulating the protein translation vital to proteostasis and neuron degeneration in Parkinson's disease

Parkinson's disease (PD) is the most common neurodegenerative movement disorder, which is characterized by the progressive loss of dopaminergic neurons in the Substantia Nigra pars compacta concomitant with Lewy body formation in affected brain areas. The detailed pathogenic mechanisms underlying the selective loss of dopaminergic neurons in PD are unclear, and no drugs or treatments have been developed to alleviate progressive dopaminergic neuron degeneration in PD. However, the formation of α-synuclein-positive protein aggregates in Lewy body has been identified as a common pathological feature of PD, possibly stemming from the consequence of protein misfolding and dysfunctional proteostasis. Proteostasis is the mechanism for maintaining protein homeostasis via modulation of protein translation, enhancement of chaperone capacity and the prompt clearance of misfolded protein by the ubiquitin proteasome system and autophagy. Deregulated protein translation and impaired capacities of chaperone or protein degradation can disturb proteostasis processes, leading to pathological protein aggregation and neurodegeneration in PD. In recent years, multiple molecular targets in the modulation of protein translation vital to proteostasis and dopaminergic neuron degeneration have been identified. The potential pathophysiological and therapeutic significance of these molecular targets to neurodegeneration in PD is highlighted.

Constitutive translation of human α-synuclein is mediated by the 5'-untranslated region

Genetic and biochemical studies have established a central role for α-synuclein (SNCA) accumulation in the pathogenesis of Parkinson's disease. Uncovering and subsequently interfering with physiological mechanisms that control SNCA expression is one approach to limit disease progression. To this end, the long and GC-rich 5'-untranslated region (UTR) of SNCA, which is predicted to fold into stable hairpin and G-quadruplex RNA motifs, was investigated for its role in mRNA translation. Inclusion of SNCA 5'-UTR significantly induced expression of both SNCA and luciferase ORF constructs. This effect was not associated with a change in mRNA levels or differential nucleocytoplasmic shuttling. Further, the presence of the 5'-UTR enhanced SNCA synthesis when cap-dependent translation was attenuated with rapamycin treatment. Analysis using multiple methodologies revealed that the 5'-UTR harbours an internal ribosome entry site (IRES) element that spans most of its nucleotide sequence. Signals such as plasma-membrane depolarization, serum starvation and oxidative stress stimulated SNCA protein translation via its 5'-UTR as well as enhanced its IRES activity. Taken together, these data support the idea that the 5'-UTR is an important positive regulator of SNCA synthesis under diverse physiological and pathological conditions, explaining in part the abundance of SNCA in healthy neurons and its accumulation in degenerative cells.

Translational control in synaptic plasticity and cognitive dysfunction

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.

RNA binding proteins: a common denominator of neuronal function and dysfunction

In eukaryotic cells, gene activity is not directly reflected by protein levels because mRNA processing, transport, stability, and translation are co- and post-transcriptionally regulated. These processes, collectively known as the ribonome, are tightly controlled and carried out by a plethora of trans-acting RNA-binding proteins (RBPs) that bind to specific cis elements throughout the RNA sequence. Within the nervous system, the role of RBPs in brain function turns out to be essential due to the architectural complexity of neurons exemplified by a relatively small somal size and an extensive network of projections and connections. Thus far, RBPs have been shown to be indispensable for several aspects of neurogenesis, neurite outgrowth, synapse formation, and plasticity. Consequently, perturbation of their function is central in the etiology of an ever-growing spectrum of neurological diseases, including fragile X syndrome and the neurodegenerative disorders frontotemporal lobar degeneration and amyotrophic lateral sclerosis.

Characterization of the role of eIF4G in stimulating cap- and IRES-dependent translation in aplysia neurons

The rate-limiting step(s) of translation in the nervous system have not been clearly identified. We have been examining this question in the cell body of the Aplysia sensory neuron, where translational regulation is important for the regulation of synaptic strength. In the present study, we examined the role of the adaptor protein eIF4G. We cloned Aplysia eIF4G (Ap4G) and Ap4G contains all the standard metazoan eIF4G protein–protein interaction domains. Overexpressing Ap4G in Aplysia sensory neurons caused an increase in both cap-dependent and internal ribosome entry site (IRES)-dependent translation using a previously characterized bicistronic fluorescent reporter. Unexpectedly, measurement of overall translation using the methionine analog, L-azidohomoalanine, revealed that overexpression of Ap4G did not lead to an increase in overall translation rates. Indeed, the effect of Ap4G on the bicistronic reporter depended on the presence of an upstream open reading frame (uORF) in the 5’ UTR encoded by the vector. We have previously shown that Mnk strongly decreased cap-dependent translation and this depended on a putative 4G binding domain. Here we extend these results showing that even in the absence of the uORF, overexpression of Mnk strongly decreases cap-dependent translation and this depends on the Mnk binding site in eIF4G. Similarly, an increase in cap-dependent translation seen with overexpression of elongation factor 2 kinase did not depend on the uORF. Overall, we show that eIF4G is rate limiting for translation of an mRNA encoding an uORF, but is not generally a rate-limiting step for translation.

Emotional modulation of the synapse

Acute stress and emotional arousal can enhance the consolidation of long-term memories in a manner that is dependent on β -adrenoceptor activation in the basolateral complex of the amygdala (BLA). The BLA interacts with multiple memory systems in the brain to modulate a variety of classes of memory. However, the synaptic mechanisms of this interaction remain unresolved. This review describes the evidence of modulation of memory and synaptic plasticity produced by emotional arousal,stress hormones, and pharmacological or electrophysiological stimulation of the amygdala. The amygdala modulation of local translation and/or degradation of the synaptic plasticity-related proteins, activity-regulated cytoskeletal-associated protein and calcium/calmodulin dependent protein kinase II α , is offered as a potential mechanism for the rapid memory consolidation that is associated with emotionally arousing events. This model shares features with synaptic tagging and the emotional tagging hypotheses.

Pseudomonas HopU1 modulates plant immune receptor levels by blocking the interaction of their mRNAs with GRP7

Pathogens target important components of host immunity to cause disease. The Pseudomonas syringae type III-secreted effector HopU1 is a mono-ADP-ribosyltransferase required for full virulence on Arabidopsis thaliana. HopU1 targets several RNA-binding proteins including GRP7, whose role in immunity is still unclear. Here, we show that GRP7 associates with translational components, as well as with the pattern recognition receptors FLS2 and EFR. Moreover, GRP7 binds specifically FLS2 and EFR transcripts in vivo through its RNA recognition motif. HopU1 does not affect the protein-protein associations between GRP7, FLS2 and translational components. Instead, HopU1 blocks the interaction between GRP7 and FLS2 and EFR transcripts in vivo. This inhibition correlates with reduced FLS2 protein levels upon Pseudomonas infection in a HopU1-dependent manner. Our results reveal a novel virulence strategy used by a microbial effector to interfere with host immunity.

Memory-enhancing intra-basolateral amygdala infusions of clenbuterol increase Arc and CaMKIIα protein expression in the rostral anterior cingulate cortex

Activation of β-adrenoceptors in the basolateral complex of the amygdala (BLA) modulates memory through interactions with multiple memory systems. The cellular mechanisms for this interaction remain unresolved. Memory-modulating BLA manipulations influence expression of the protein product of the immediate early gene activity-regulated cytoskeletal-associated protein (Arc) in the dorsal hippocampus, and hippocampal expression of Arc protein is critically involved in memory consolidation and long-term potentiation. The present studies examined whether this influence of the BLA is specific to the hippocampus and to Arc protein. Like the hippocampus, the rostral portion of the anterior cingulate cortex (rACC) is involved in the consolidation of inhibitory avoidance (IA) memory, and IA training increases Arc protein in the rACC. Because the BLA interacts with the rACC in the consolidation of IA memory, the rACC is a potential candidate for further studies of BLA modulation of synaptic plasticity. The alpha isoform of the Calcium/Calmodulin-dependent protein kinase II (CaMKIIα) and the immediate early gene c-Fos are involved in long-term potentiation and memory. Both Arc and CaMKIIα proteins can be translated in isolated synapses, where the mRNA is localized, but c-Fos protein remains in the soma. To examine the influence of memory-modulating manipulations of the BLA on expression of these memory and plasticity-associated proteins in the rACC, male Sprague-Dawley rats were trained on an IA task and given intra-BLA infusions of either clenbuterol or lidocaine immediately after training. Findings suggest that noradrenergic stimulation of the BLA may modulate memory consolidation through effects on both synaptic proteins Arc and CaMKIIα, but not the somatic protein c-Fos. Furthermore, protein changes observed in the rACC following BLA manipulations suggest that the influence of the BLA on synaptic proteins is not limited to those in the dorsal hippocampus.

Compartment-specific, differential regulation of eukaryotic elongation factor 2 and its kinase within Aplysia sensory neurons

Long-term facilitation (LTF) in Aplysia is a leading model for elucidating the biochemical mechanisms of synaptic plasticity underlying learning. LTF requires translational control downstream of target of rapamycin complex 1. Our lab has previously shown that treatment with the facilitating neurotransmitter, 5-hydroxytryptamine (5-HT), causes a target of rapamycin complex 1-mediated decrease in phosphorylation of eukaryotic elongation factor 2 (eEF2) within the neurites of sensory neurons involved in LTF. Here, we characterize the Aplysia orthologue of eEF2 kinase (eEF2K). We show that the Aplysia eEF2K orthologue contains an S6 kinase phosphorylation site and that a serine-to-alanine mutation at this site blocks the ability of 5-HT to decrease eEF2 phosphorylation in neurites. We also show that within the soma, 5-HT has the opposite effect, decreasing eEF2K phosphorylation at the S6 kinase site and, concomitantly, increasing eEF2 phosphorylation. Surprisingly, while eEF2K over-expression inhibits translation of a marker for internal ribosome entry site-dependent translation, it stimulates the translation of a marker for cap-dependent translation. This study demonstrates that eEF2 is differentially regulated in separate compartments and contributes to a growing body of evidence that inhibition of elongation can stimulate the translation of certain transcripts.

Metaplasticity governs compartmentalization of synaptic tagging and capture through brain-derived neurotrophic factor (BDNF) and protein kinase Mzeta (PKMzeta)

Activity-dependent synaptic plasticity is widely accepted to be the cellular correlate of learning and memory. It is believed that associativity between different synaptic inputs can transform short-lasting forms of synaptic plasticity (<3 h) to long-lasting ones. Synaptic tagging and capture (STC) might be able to explain this heterosynaptic support, because it distinguishes between local mechanisms of synaptic tags and cell-wide mechanisms responsible for the synthesis of plasticity-related proteins (PRPs). STC initiate storage processes only when the strength of the synaptic tag and the local concentration of essential proteins are above a certain plasticity threshold. We present evidence that priming stimulation through the activation of metabotropic glutamate receptors substantially increases the "range of threshold" for functional plasticity by producing protein kinase Mζ (PKMζ) as a PRP through local protein synthesis. In addition, our results implicate BDNF as a PRP which is mandatory for establishing cross-capture between synaptic strengthening and weakening, whereas the newly generated PKMζ specifically establishes synaptic tagging of long-term potentiation. Most intriguingly, we show here that STC are confined to specific dendritic compartments and that these compartments contain "synaptic clusters" with different plasticity thresholds. Our results suggest that within a dendritic compartment itself a homeostatic process exists to adjust plasticity thresholds. The range in which these clusters operate can be altered by processes of metaplasticity, which will operate on the cluster independently of other clusters at the same dendrite. These clusters will then prepare the synaptic network to form long-term memories.

A role for Drosophila dFoxO and dFoxO 5'UTR internal ribosomal entry sites during fasting

One way animals may cope with nutrient deprivation is to broadly repress translation by inhibiting 5'-cap initiation. However, under these conditions specific proteins remain essential to survival during fasting. Such peptides may be translated through initiation at 5'UTR Internal Ribosome Entry Sites (IRES). Here we show that the Drosophila melanogaster Forkhead box type O (dFoxO) transcription factor is required for adult survival during fasting, and that the 5'UTR of dfoxO has the ability to initiate IRES-mediated translation in cell culture. Previous work has shown that insulin negatively regulates dFoxO through AKT-mediated phosphorylation while dFoxO itself induces transcription of the insulin receptor dInR, which also harbors IRES. Here we report that IRES-mediated translation of both dFoxO and dInR is activated in fasted Drosophila S2 cells at a time when cap-dependent translation is reduced. IRES mediated translation of dFoxO and dInR may be essential to ensure function and sensitivity of the insulin signaling pathway during fasting.

Viagra for your synapses: Enhancement of hippocampal long-term potentiation by activation of beta-adrenergic receptors

Beta-adrenergic receptors (beta-ARs) critically modulate long-lasting synaptic plasticity and long-term memory storage in the mammalian brain. Synaptic plasticity is widely believed to mediate memory storage at the cellular level. Long-term potentiation (LTP) is one type of synaptic plasticity that has been linked to memory storage. Activation of beta-ARs can enhance LTP and facilitate long-term memory storage. Interestingly, many of the molecular signaling pathways that are critical for beta-adrenergic modulation of LTP mirror those required for the persistence of memory. In this article, we review the roles of signaling cascades and translation regulation in enabling beta-ARs to control expression of long-lasting LTP in the rodent hippocampus. These include the cyclic-AMP/protein kinase-A (cAMP-PKA) and extracellular signal-regulated protein kinase cascades, two key pathways known to link transmitter receptors with translation regulation. Future research directions are discussed, with emphasis on defining the roles of signaling complexes (e.g. PSD-95) and glutamatergic receptors in controlling the efficacy of beta-AR modulation of LTP.

Spatially restricting gene expression by local translation at synapses

mRNA localization and regulated translation provide a means of spatially restricting gene expression within each of the thousands of subcellular compartments made by a neuron, thereby vastly increasing the computational capacity of the brain. Recent studies reveal that local translation is regulated by stimuli that trigger neurite outgrowth and/or collapse, axon guidance, synapse formation, pruning, activity-dependent synaptic plasticity, and injury-induced axonal regeneration. Impairments in the local regulation of translation result in aberrant signaling, physiology and morphology of neurons, and are linked to neurological disorders. This review highlights current advances in understanding how mRNA translation is repressed during transport and how local translation is activated by stimuli. We address the function of local translation in the context of fragile X mental retardation.

Ribosomal protein S6 kinase is a critical downstream effector of the target of rapamycin complex 1 for long-term facilitation in Aplysia

Long-term facilitation (LTF) in Aplysia is a leading cellular model for elucidating the biochemical mechanisms of synaptic plasticity underlying learning. In Aplysia, LTF requires translational control downstream of the target of rapamycin (TOR) complex 1 (TORC1). The major known downstream targets of TORC1 are 4E binding protein (4E-BP) and S6 kinase (S6K). By removing the site within these regulators required for their interaction with TORC1, we have generated dominant negative proteins that disrupt specific pathways downstream of TORC1. Expression of dominant negative S6K, but not dominant negative 4E-BP, in Aplysia sensory neurons (SNs) blocked 24-h LTF. TORC1 is directly activated by the small GTP-binding protein, Ras homologue enriched in brain (Rheb). To determine the effects of TORC1 activation on translation in Aplysia neurons, we have examined the effects of expressing a constitutively active form of the Aplysia orthologue of Rheb, ApRheb (ApRheb(Q63L)). Expression of ApRheb(Q63L) increased 4E-BP phosphorylation and the level of general, cap-dependent translation within the SN cell soma in a rapamycin-sensitive manner. This increase in cap-dependent translation was blocked neither by dominant negative 4E-BP nor dominant negative S6K. Thus, we demonstrate that S6K is an important downstream target of TORC1 in Aplysia and that it is necessary for 24-h LTF, but not for TORC1-mediated increases in somatic cap-dependent translation.

Activation of host translational control pathways by a viral developmental switch

In response to numerous signals, latent herpesvirus genomes abruptly switch their developmental program, aborting stable host–cell colonization in favor of productive viral replication that ultimately destroys the cell. To achieve a rapid gene expression transition, newly minted capped, polyadenylated viral mRNAs must engage and reprogram the cellular translational apparatus. While transcriptional responses of viral genomes undergoing lytic reactivation have been amply documented, roles for cellular translational control pathways in enabling the latent-lytic switch have not been described. Using PEL-derived B-cells naturally infected with KSHV as a model, we define efficient reactivation conditions and demonstrate that reactivation substantially changes the protein synthesis profile. New polypeptide synthesis correlates with 4E-BP1 translational repressor inactivation, nuclear PABP accumulation, eIF4F assembly, and phosphorylation of the cap-binding protein eIF4E by Mnk1. Significantly, inhibiting Mnk1 reduces accumulation of the critical viral transactivator RTA through a post-transcriptional mechanism, limiting downstream lytic protein production, and impairs reactivation efficiency. Thus, herpesvirus reactivation from latency activates the host cap-dependent translation machinery, illustrating the importance of translational regulation in implementing new developmental instructions that drastically alter cell fate.

Kaposi's sarcoma-associated herpesvirus (KSHV) is an important human pathogen and, like all herpesviruses, establishes a state of permanent residency in the infected host called latency. Major sites of KSHV latency are cells of the immune system and cells lining blood vessels. In individuals with weakened immunity, inappropriate growth of these cells driven by the resident virus can give rise to primary effusion lymphoma and Kaposi's sarcoma, respectively. These life-threatening cancers are most common in patients with HIV/AIDS and have become a major source of mortality in parts of sub-Saharan Africa. Under appropriate stimuli, herpesviruses change their relationship with the host cell and begin to manufacture proteins required to assemble new infectious virus particles that can be released and spread. To achieve this, the virus hijacks key processes within the cell and conscripts them into producing viral proteins. In this study, we describe for the first time how KSHV carefully manipulates the host protein synthesis machinery during the switch from latency to this specialized infectious virus production mode. Our results show that although overall protein synthesis is diminished, key components of the host's protein manufacturing machinery are actually stimulated, presumably to accelerate viral protein production.

Translational control of long-lasting synaptic plasticity and memory

Long-lasting forms of synaptic plasticity and memory are dependent on new protein synthesis. Recent advances obtained from genetic, physiological, pharmacological, and biochemical studies provide strong evidence that translational control plays a key role in regulating long-term changes in neural circuits and thus long-term modifications in behavior. Translational control is important for regulating both general protein synthesis and synthesis of specific proteins in response to neuronal activity. In this review, we summarize and discuss recent progress in the field and highlight the prospects for better understanding of long-lasting changes in synaptic strength, learning, and memory and implications for neurological diseases.

Potent activation of FGF-2 IRES-dependent mechanism of translation during brain development

Fibroblast growth factor-2 (FGF-2) plays a fundamental role in brain functions. This role may be partly achieved through the control of its expression at the translational level via an internal ribosome entry site (IRES)-dependent mechanism. Transgenic mice expressing a bicistronic mRNA allowed us to study in vivo and ex vivo where this translational mechanism operates. Along brain development, we identified a stringent spatiotemporal regulation of FGF-2 IRES activity showing a peak at post-natal day 7 in most brain regions, which is concomitant with neuronal maturation. At adult age, this activity remained relatively high in forebrain regions. By the enrichment of this activity in forebrain synaptoneurosomes and by the use of primary cultures of cortical neurons or cocultures with astrocytes, we showed that this activity is indeed localized in neurons, is dependent on their maturation, and correlates with endogenous FGF-2 protein expression. In addition, this activity was regulated by astrocyte factors, including FGF-2, and spontaneous electrical activity. Thus, neuronal IRES-driven translation of the FGF-2 mRNA may be involved in synapse formation and maturation.

Dendritic mRNA: transport, translation and function

Many cellular functions require the synthesis of a specific protein or functional cohort of proteins at a specific time and place in the cell. Local protein synthesis in neuronal dendrites is essential for understanding how neural activity patterns are transduced into persistent changes in synaptic connectivity during cortical development, memory storage and other long-term adaptive brain responses. Regional and temporal changes in protein levels are commonly coordinated by an asymmetric distribution of mRNAs. This Review attempts to integrate current knowledge of dendritic mRNA transport, storage and translation, placing particular emphasis on the coordination of regulation and function during activity-dependent synaptic plasticity in the adult mammalian brain.

The 5' leader of the mRNA encoding the mouse neurotrophin receptor TrkB contains two internal ribosomal entry sites that are differentially regulated

A single internal ribosomal entry site (IRES) in conjunction with IRES transactivating factors (ITAFs) is sufficient to recruit the translational machinery to a eukaryotic mRNA independent of the cap structure. However, we demonstrate that the mouse TrkB mRNA contains two independent IRESes. The mouse TrkB mRNA consists of one of two 5′ leaders (1428 nt and 448 nt), both of which include the common 3′ exon (Ex2, 344 nt). Dicistronic RNA transfections and in vitro translation of monocistronic RNA demonstrated that both full-length 5′ leaders, as well as Ex2, exhibit IRES activity indicating the IRES is located within Ex2. Additional analysis of the upstream sequences demonstrated that the first 260 nt of exon 1 (Ex1a) also contains an IRES. Dicistronic RNA transfections into SH-SY5Y cells showed the Ex1a IRES is constitutively active. However, the Ex2 IRES is only active in response to retinoic acid induced neural differentiation, a state which correlates with the synthesis of the ITAF polypyrimidine tract binding protein (PTB1). Correspondingly, addition or knock-down of PTB1 altered Ex2, but not Ex1a IRES activity in vitro and ex vivo, respectively. These results demonstrate that the two functionally independent IRESes within the mouse TrkB 5′ leader are differentially regulated, in part by PTB1.

Ras Transformation of RIE-1 Cells Activates Cap-Independent Translation of Ornithine Decarboxylase: Regulation by the Raf/MEK/ERK and Phosphatidylinositol 3-Kinase Pathways

Ornithine decarboxylase (ODC) is the first and generally rate-limiting enzyme in polyamine biosynthesis. Deregulation of ODC is critical for oncogenic growth, and ODC is a target of Ras. These experiments examine translational regulation of ODC in RIE-1 cells, comparing untransformed cells with those transformed by an activated Ras12V mutant. Analysis of the ODC 5' untranslated region (5'UTR) revealed four splice variants with the presence or absence of two intronic sequences. All four 5'UTR species were found in both cell lines; however, variants containing intronic sequences were more abundant in Ras-transformed cells. All splice variants support internal ribosome entry site (IRES)-mediated translation, and IRES activity is markedly elevated in cells transformed by Ras. Inhibition of Ras effector targets indicated that the ODC IRES element is regulated by the phosphorylation status of the translation factor eIF4E. Dephosphorylation of eIF4E by inhibition of mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase (MEK) or the eIF4E kinase Mnk1/2 increases ODC IRES activity in both cell lines. When both the Raf/MEK/ERK and phosphatidylinositol 3-kinase/mammalian target of rapamycin pathways are inhibited in normal cells, ODC IRES activity is very low and cells arrest in G(1). When these pathways are inhibited in Ras-transformed cells, cell cycle arrest does not occur and ODC IRES activity increases, helping to maintain high ODC activity.

Viral alteration of cellular translational machinery increases defective ribosomal products

Here we show that cells expressing genes inserted into Semliki Forest virus (SFV) vectors generate a large fraction of defective ribosomal products (DRiPs) due to frequent initiation on downstream Met residues. In monopolizing the host cell translational machinery, SFV reduces levels of translation eukaryotic initiation factor 4E (eIF4E), diminishes phosphorylation of ribosome subunit S6, and phosphorylates translation initiation factor eIF2alpha. We show that the last event is required for SFV mistranslation of inserted genes. Downstream initiation is suppressed by fusing inserted genes with the open reading frame encoding the SFV capsid, demonstrating that one function of the capsid element is to enable ribosomes to initiate translation in the proper location. These results show that in modifying translation, viral vectors can unpredictably increase the generation of truncated polypeptides and thereby the DRiP fraction of inserted gene products, which can potentially affect their yield, therapeutic efficacy, and immunogenicity.

Signalling to translation: how signal transduction pathways control the protein synthetic machinery

Recent advances in our understanding of both the regulation of components of the translational machinery and the upstream signalling pathways that modulate them have provided important new insights into the mechanisms by which hormones, growth factors, nutrients and cellular energy status control protein synthesis in mammalian cells. The importance of proper control of mRNA translation is strikingly illustrated by the fact that defects in this process or its control are implicated in a number of disease states, such as cancer, tissue hypertrophy and neurodegeneration. Signalling pathways such as those involving mTOR (mammalian target of rapamycin) and mitogen-activated protein kinases modulate the phosphorylation of translation factors, the activities of the protein kinases that act upon them and the association of RNA-binding proteins with specific mRNAs. These effects contribute both to the overall control of protein synthesis (which is linked to cell growth) and to the modulation of the translation or stability of specific mRNAs. However, important questions remain about both the contributions of individual regulatory events to the control of general protein synthesis and the mechanisms by which the translation of specific mRNAs is controlled.

Control of synaptic consolidation in the dentate gyrus: mechanisms, functions, and therapeutic implications

Synaptic consolidation refers to the development and stabilization of protein synthesis-dependent modifications of synaptic strength as observed during long-term potentiation (LTP) and long-term depression (LTD). Activity-dependent changes in synaptic strength are thought to underlie memory storage and other adaptive responses of the nervous systems of importance in mood stability, reward behavior, and pain control. This chapter focuses on the mechanisms and functions of synaptic consolidation in the dentate gyrus, a critical structure not only in hippocampal memory function, but also in regulation of stress responses and cognitive aspects of depression. Recent evidence suggests that synaptic consolidation at excitatory medial perforant path-granule cell synapses requires brain-derived neurotrophic factor (BDNF) signaling and induction of the immediate early gene activity-regulated cytoskeleton-associated protein (Arc). Arc mRNA is strongly induced and transported to dendritic processes following high-frequency stimulation (HFS) that induces LTP in the rat dentate gyrus in vivo. Sustained synthesis of Arc during a surprisingly protracted time-window is required for hyperphosphorylation of actin depolymerizing factor/cofilin and local expansion of the actin cytoskeleton in vivo. Furthermore, this process of Arc-dependent synaptic consolidation is activated in response to brief infusion of BDNF. Microarray expression profiling has revealed a panel of BDNF-regulated genes that may cooperate with Arc during synaptic consolidation. In addition to regulating gene expression, BDNF signaling modulates the fine localization and biochemical activation of the translation machinery. By modulating the spatial and temporal translation of newly induced (Arc) and constitutively-expressed mRNA in dendrites, BDNF may effectively control the window of synaptic consolidation. Dysregulation of BDNF synthesis and Arc function, specifically within the dentate gyrus, is linked to behavioral symptoms and cognitive deficits in animal models of depression and Alzheimer's disease. Therapeutics strategies targeting synaptic consolidation hold promise for the future.

MNK1 and MNK2 regulation in HER2-overexpressing breast cancer lines

MAPK-interacting protein kinases 1 and 2 (MNK1 and MNK2) function downstream of p38 and ERK MAPK, but there are large gaps in our knowledge of how MNKs are regulated and function. As proteins activated in the HER2/Ras/Raf/ERK pathway, the MNKs are of potential interest in HER2-overexpressing cancers. We utilized a panel of breast cell lines (HCC1419, AU565, SKBR3, MCF7, and MCF10A), three of which overexpress HER2, to characterize the amounts and activation status of MNKs and other pathway enzymes (ERKs and RSKs) in these cells. We generated a phosphospecific antibody to Thr(P)-214 in the T-loop of MNKs and found that phosphorylations of both Thr-209 and Thr-214 in human MNK1 are required for activation. Increased phosphorylation and activity of the MNKs correlate with HER2 overexpression, and inhibition of the MNKs reduces colony formation in soft agar. Our work identifies the MNKs as potential therapeutic targets for breast cancer treatments.

Serotonin increases phosphorylation of synaptic 4EBP through TOR, but eukaryotic initiation factor 4E levels do not limit somatic cap-dependent translation in aplysia neurons

The target of rapamycin (TOR) plays an important role in memory formation in Aplysia californica. Here, we characterize one of the downstream targets of TOR, the eukaryotic initiation factor 4E (eIF4E) binding protein (4EBP) from Aplysia. Aplysia 4EBP contains the four critical phosphorylation sites regulated by TOR as well as an N-terminal RAIP motif and a C-terminal TOS site. Aplysia 4EBP was hypophosphorylated in synaptosomes, and serotonin addition caused a rapamycin-sensitive increase in 4EBP phosphorylation both in synaptosomes and in isolated neurites. Aplysia 4EBP was regulated in a fashion similar to that of mammalian 4EBPs, binding to eIF4E when dephosphorylated and releasing eIF4E after phosphorylation. Overexpression of 4EBP in the soma of Aplysia neurons caused a specific decrease in cap-dependent translation that was rescued by concomitant overexpression of eIF4E. However, eIF4E overexpression by itself did not increase cap-dependent translation, suggesting that increasing levels of free eIF4E by phosphorylating 4EBP is not important in regulating cap-dependent translation in the cell soma. Total levels of eIF4E were also regulated by 4EBP, suggesting that 4EBP can also act as an eIF4E chaperone. These studies demonstrate the conserved nature of 4EBP regulation and its role in cap-dependent translation and suggest differential roles of 4EBP phosphorylation in the soma and synapse.

Chronic fluoxetine induces region-specific changes in translation factor eIF4E and eEF2 activity in the rat brain

The delayed therapeutic onset observed in response to chronic antidepressant drug treatment is little understood. While current theories emphasize effects on gene transcription, possible effects of antidepressant drugs on translation control pathways have not been explored. We examined the effect of the selective serotonergic reuptake inhibitor fluoxetine on regulation of two major determinants of mRNA translation, eukaryotic initiation factor 4E (eIF4E) and eukaryotic elongation factor 2 (eEF2). Chronic fluoxetine treatment induced hyperphosphorylation of eEF2 (Thr56) in prefrontal cortex, hippocampus and dentate gyrus of rats. By contrast, phosphorylation of eIF4E (Ser209) was observed specifically in the dentate gyrus. Acute fluoxetine treatment had no effect on translational factor activity. These findings suggest that region-specific regulation of translation contributes to the delayed action of antidepressant drugs such as fluoxetine.

Mnk is a negative regulator of cap-dependent translation in Aplysia neurons

To investigate the mechanisms underlying regulation of eukaryotic initiation factor 4E (eIF4E) phosphorylation in Aplysia neurons, we have cloned the Aplysia homolog of the vertebrate eIF4E kinases, Mnk1 and -2. Aplysia Mnk shares many conserved regions with vertebrate Mnk, including putative eukaryotic initiation factor 4G binding regions, activation loop phosphorylation sites, and a carboxy-terminal anchoring site for MAP kinases. As expected, purified Aplysia Mnk phosphorylated Aplysia eIF4E at a conserved carboxy-terminal serine and over-expression of Aplysia Mnk in sensory neurons led to increased phosphorylation of endogenous eIF4E. Over-expression of Aplysia Mnk led to strong decreases in cap-dependent translation, while generally sparing internal ribosomal entry site (IRES)-dependent translation. However, decreases in cap-dependent translation seen after expression of Aplysia Mnk could only be partly explained by increases in eIF4E phosphorylation. In Aplysia sensory neurons, phosphorylation of eIF4E is reduced during intermediate memory formation. However, we found that this physiological regulation of eIF4E phosphorylation was independent of changes in Aplysia Mnk phosphorylation. We propose that changes in eIF4E phosphorylation in Aplysia neurons are a consequence of changes in cap-dependent translation that are independent of regulation of Aplysia Mnk.

Testosterone regulates FGF-2 expression during testis maturation by an IRES-dependent translational mechanism

Spermatogenesis is a complex process involving cell proliferation, differentiation, and apoptosis. Fibroblast growth factor 2 (FGF-2) is involved in testicular function, but its role in spermatogenesis has not been fully documented. The control of FGF-2 expression particularly occurs at the translational level, by an internal ribosome entry site (IRES)-dependent mechanism driving the use of alternative initiation codons. To study IRES activity regulation in vivo, we have developed transgenic mice expressing a bicistronic construct coding for two luciferase genes. Here, we show that the FGF-2 IRES is age-dependently activated in mouse testis, whereas EMCV and c-myc IRESs are not. Real-time PCR confirms that this regulation is translational. By using immunohistological techniques, we demonstrate that FGF-2 IRES stimulation occurs in adult, but not in immature, type-A spermatogonias. This is correlated with activation of endogenous FGF-2 expression in spermatogonia; whereas FGF-2 mRNA transcription is known to decrease in adult testis. Interestingly, the FGF-2 IRES activation is triggered by testosterone and is partially inhibited by siRNA directed against the androgen receptor. Two-dimensional analysis of proteins bound to the FGF-2 mRNA 5'UTR after UV cross-linking reveals that testosterone treatment correlates with the binding of several proteins. These data suggest a paracrine loop where IRES-dependent FGF-2 expression, stimulated by Sertoli cells in response to testosterone produced by Leydig cells, would in turn activate Leydig function and testosterone production. In addition, nuclear FGF-2 isoforms could be involved in an intracrine function of FGF-2 in the start of spermatogenesis, mitosis, or meiosis initiation. This report demonstrates that mRNA translation regulation by an IRES-dependent mechanism participates in a physiological process.

RNA transport and local control of translation

In eukaryotes, the entwined pathways of RNA transport and local translational regulation are key determinants in the spatio-temporal articulation of gene expression. One of the main advantages of this mechanism over transcriptional control in the nucleus lies in the fact that it endows local sites with independent decision-making authority, a consideration that is of particular relevance in cells with complex cellular architecture such as neurons. Localized RNAs typically contain codes, expressed within cis-acting elements, that specify subcellular targeting. Such codes are recognized by trans-acting factors, adaptors that mediate translocation along cytoskeletal elements by molecular motors. Most transported mRNAs are assumed translationally dormant while en route. In some cell types, especially in neurons, it is considered crucial that translation remains repressed after arrival at the destination site (e.g., a postsynaptic microdomain) until an appropriate activation signal is received. Several candidate mechanisms have been suggested to participate in the local implementation of translational repression and activation, and such mechanisms may target translation at the level of initiation and/or elongation. Recent data indicate that untranslated RNAs may play important roles in the local control of translation.

Significance of Molecular Signaling for Protein Translation Control in Neurodegenerative Diseases

It has long been known that protein synthesis is inhibited in neurological disorders. Protein synthesis includes protein transcription and translation. While many studies about protein transcription have been done in the last decade, we are just starting to understand more about the impact of protein translation. Protein translation control can be accomplished at the initiation or elongation steps. In this review, we will focus on translation control at initiation. Neurons have long neurites in which proteins have to be transported from the cell body to the end of the neurite. Since supply of proteins cannot meet the need of neuronal activity at the spine, protein locally translated at the spine will be a good solution to replace the turnover of proteins. Therefore, local protein translation is an important mechanism to maintain normal neuronal functions. In this notion, we have to separate the concept of global and local protein translation control. Both global and local protein translation control modulate normal neuronal functions from development to cognitive functions. Increasing lines of evidence show that they also play significant roles in neurodegenerative diseases, e.g. neuronal apoptosis, synaptic degeneration and autophagy. We summarize all the evidence in this review and focus on the control at initiation. The new live-cell imaging technology together with photoconvertible fluorescent probes allows us to investigate newly translated proteins in situ. Protein translation control is another line to modulate neuronal function in neuron-neuron communication as well as in response to stress in neurodegenerative diseases.

Eukaryotic translation initiation factor 4E availability controls the switch between cap-dependent and internal ribosomal entry site-mediated translation

Translation of m7G-capped cellular mRNAs is initiated by recruitment of ribosomes to the 5' end of mRNAs via eukaryotic translation initiation factor 4F (eIF4F), a heterotrimeric complex comprised of a cap-binding subunit (eIF4E) and an RNA helicase (eIF4A) bridged by a scaffolding molecule (eIF4G). Internal translation initiation bypasses the requirement for the cap and eIF4E and occurs on viral and cellular mRNAs containing internal ribosomal entry sites (IRESs). Here we demonstrate that eIF4E availability plays a critical role in the switch from cap-dependent to IRES-mediated translation in picornavirus-infected cells. When both capped and IRES-containing mRNAs are present (as in intact cells or in vitro translation extracts), a decrease in the amount of eIF4E associated with the eIF4F complex elicits a striking increase in IRES-mediated viral mRNA translation. This effect is not observed in translation extracts depleted of capped mRNAs, indicating that capped mRNAs compete with IRES-containing mRNAs for translation. These data explain numerous reported observations where viral mRNAs are preferentially translated during infection.

Differential translation and fragile X syndrome

Fragile X syndrome (FXS) is caused by the transcriptional silencing of the Fmr1 gene, which encodes a protein (FMRP) that can act as a translational suppressor in dendrites, and is characterized by a preponderance of abnormally long, thin and tortuous dendritic spines. According to a current theory of FXS, the loss of FMRP expression leads to an exaggeration of translation responses linked to group I metabotropic glutamate receptors. Such responses are involved in the consolidation of a form of long-term depression that is enhanced in Fmr1 knockout mice and in the elongation of dendritic spines, resembling synaptic phenotypes over-represented in fragile X brain. These observations place fragile X research at the heart of a long-standing issue in neuroscience. The consolidation of memory, and several distinct forms of synaptic plasticity considered to be substrates of memory, requires mRNA translation and is associated with changes in spine morphology. A recent convergence of research on FXS and on the involvement of translation in various forms of synaptic plasticity has been very informative on this issue and on mechanisms underlying FXS. Evidence suggests a general relationship in which the receptors that induce distinct forms of efficacy change differentially regulate translation to produce unique spine shapes involved in their consolidation. We discuss several potential mechanisms for differential translation and the notion that FXS represents an exaggeration of one 'channel' in a set of translation-dependent consolidation responses.

Local translational control in dendrites and its role in long-term synaptic plasticity

Local protein synthesis in dendrites has emerged as a key mechanism contributing to enduring forms of synaptic plasticity. Although the translational capability of dendrites has been appreciated for over 20 years, it is only recently that significant progress has been made in elucidating mechanisms that contribute to its regulation. It is clear from work over the last few years that the control of translation in dendrites is complex, involving a host of unique (and often surprising) mechanisms that can operate together or in parallel to tightly control gene expression in time and space. Here, we discuss the strategies used by neurons to regulate translation in dendrites and how these are implemented in the service of long-term information storage.

Translational regulation during oogenesis and early development: the cap-poly(A) tail relationship

Metazoans rely on the regulated translation of select maternal mRNAs to control oocyte maturation and the initial stages of embryogenesis. These transcripts usually remain silent until their translation is temporally and spatially required during early development. Different translational regulatory mechanisms, varying from cytoplasmic polyadenylation to localization of maternal mRNAs, have evolved to assure coordinated initiation of development. A common feature of these mechanisms is that they share a few key trans-acting factors. Increasing evidence suggest that ubiquitous conserved mRNA-binding factors, including the eukaryotic translation initiation factor 4E (eIF4E) and the cytoplasmic polyadenylation element binding protein (CPEB), interact with cell-specific molecules to accomplish the correct level of translational activity necessary for normal development. Here we review how capping and polyadenylation of mRNAs modulate interaction with multiple regulatory factors, thus controlling translation during oogenesis and early development.

BDNF function in adult synaptic plasticity: the synaptic consolidation hypothesis

Interest in BDNF as an activity-dependent modulator of neuronal structure and function in the adult brain has intensified in recent years. Localization of BDNF-TrkB to glutamate synapses makes this system attractive as a dynamic, activity-dependent regulator of excitatory transmission and plasticity. Despite individual breakthroughs, an integrated understanding of BDNF function in synaptic plasticity is lacking. Here, we attempt to distill current knowledge of the molecular mechanisms and function of BDNF in LTP. BDNF activates distinct mechanisms to regulate the induction, early maintenance, and late maintenance phases of LTP. Evidence from genetic and pharmacological approaches is reviewed and tabulated. The specific contribution of BDNF depends on the stimulus pattern used to induce LTP, which impacts the duration and perhaps the subcellular site of BDNF release. Particular attention is given to the role of BDNF as a trigger for protein synthesis-dependent late phase LTP--a process referred to as synaptic consolidation. Recent experiments suggest that BDNF activates synaptic consolidation through transcription and rapid dendritic trafficking of mRNA encoded by the immediate early gene, Arc. A model is proposed in which BDNF signaling at glutamate synapses drives the translation of newly transported (Arc) and locally stored (i.e., alphaCaMKII) mRNA in dendrites. In this model BDNF tags synapses for mRNA capture, while Arc translation defines a critical window for synaptic consolidation. The biochemical mechanisms by which BDNF regulates local translation are also discussed. Elucidation of these mechanisms should shed light on a range of adaptive brain responses including memory and mood resilience.

Biochemical mechanisms for translational regulation in synaptic plasticity

Changes in gene expression are required for long-lasting synaptic plasticity and long-term memory in both invertebrates and vertebrates. Regulation of local protein synthesis allows synapses to control synaptic strength independently of messenger RNA synthesis in the cell body. Recent reports indicate that several biochemical signalling cascades couple neurotransmitter and neurotrophin receptors to translational regulatory factors in protein synthesis-dependent forms of synaptic plasticity and memory. In this review, we highlight these translational regulatory mechanisms and the signalling pathways that govern the expression of synaptic plasticity in response to specific types of neuronal stimulation.

Local protein synthesis during axon guidance and synaptic plasticity

mRNA localization and regulated translation take central roles in axon guidance and synaptic plasticity. By spatially restricting gene expression within neurons, local protein synthesis provides growth cones and synapses with the capacity to autonomously regulate their structure and function. Studies in a variety of systems have provided insight into the specific roles of local protein synthesis during axonal navigation and during synaptic plasticity, and have begun to delineate the mechanisms underlying mRNA localization and regulated translation. Several powerful new tools have recently been developed to visualize each of these processes.

Synaptic plasticity and translation initiation

It is widely accepted that protein synthesis, including local protein synthesis at synapses, is required for several forms of synaptic plasticity. Local protein synthesis enables synapses to control synaptic strength independent of the cell body via rapid protein production from pre-existing mRNA. Therefore, regulation of translation initiation is likely to be intimately involved in modulating synaptic strength. Our understanding of the translation-initiation process has expanded greatly in recent years. In this review, we discuss various aspects of translation initiation, as well as signaling pathways that might be involved in coupling neurotransmitter and neurotrophin receptors to the translation machinery during various forms of synaptic plasticity.

Regulation of dendritic protein synthesis by miniature synaptic events

We examined dendritic protein synthesis after a prolonged blockade of action potentials alone and after a blockade of both action potentials and miniature excitatory synaptic events (minis). Relative to controls, dendrites exposed to a prolonged blockade of action potentials showed diminished protein synthesis. Dendrites in which both action potentials and minis were blocked showed enhanced protein synthesis, suggesting that minis inhibit dendritic translation. When minis were acutely blocked or stimulated, an immediate increase or decrease, respectively, in dendritic translation was observed. Taken together, these results reveal a role for miniature synaptic events in the acute regulation of dendritic protein synthesis in neurons.

Phosphorylation of eIF4E by Mnk-1 enhances HSV-1 translation and replication in quiescent cells

Although the activity of the translation initiation factor eIF4F is regulated in part by translational repressors (4E-BPs) that prevent incorporation of eIF4E, the cap-binding protein, into the initiation complex, the contribution of eIF4E phosphorylation to translational control remains controversial. Here, we demonstrate that the herpes simplex virus-1 (HSV-1) ICP0 gene product, a multifunctional transactivator of viral gene expression with ubiquitin E3 ligase activity that is important for vegetative replication and reactivation of latent infections, is required to stimulate phosphorylation of eIF4E as well as 4E-BP1, and promote assembly of eIF4F complexes in infected cells. Furthermore, 4E-BP1 is degraded by the proteasome in an ICP0-dependent manner, establishing that the proteasome can control 4E-BP1 steady-state levels. Preventing eIF4E phosphorylation by inhibiting the eIF4E kinase mnk-1 dramatically reduced viral replication and the translation of viral polypeptides in quiescent cells, providing the first evidence that phosphorylation of eIF4E by mnk-1 is critical for viral protein synthesis and replication. Thus, in marked contrast to many viruses that inactivate eIF4F, HSV-1 stimulates eIF4F complex assembly in quiescent, differentiated cells; moreover, this is important for viral replication, and may be crucial for HSV-1 to initiate its productive growth cycle in resting cells, such as latently infected neurons.

Postgenomic global analysis of translational control induced by oncogenic signaling

It is commonly assumed that developmental and oncogenic signaling achieve their phenotypic effects primarily by directly regulating the transcriptional profile of cells. However, there is growing evidence that the direct effect on transcription may be overshadowed by differential effects on the translational efficiency of specific existing mRNA species. Global analysis of this effect using microarrays indicates that this mechanism of controlling protein production provides a highly specific, robust, and rapid response to oncogenic and developmental stimuli. The mRNAs so affected encode proteins involved in cell-cell interaction, signal transduction, and growth control. Furthermore, a large number of transcription factors capable of secondarily rearranging the transcriptional profile of the cell are controlled at this level as well. To what degree this translational control is either necessary or sufficient for tumor formation or maintenance remains to be determined.

Activity-Dependent Regulation of Neurohormone Synthesis and Its Impact on Reproductive Behavior in Aplysia1

The bag cell neurons (BCNs) of the mollusk Aplysia californica provide a simple model system for investigating cellular and molecular events regulating synthesis and secretion of a reproductive neuropeptide and their impact on physiology and behavior. The BCNs secrete a large amount of egg-laying hormone (ELH) in response to an electrical afterdischarge. The afterdischarge also triggers cellular and molecular events leading to upregulation of ELH biosynthesis to replenish the supply of releasable hormone that was lost because of secretion. In the present review, we discuss signal-transduction events that link membrane excitability to ELH biosynthesis. We present evidence that the afterdischarge stimulates ELH synthesis by upregulating translation of ELH mRNA rather than by activating ELH gene transcription. This increase in ELH synthesis is accompanied by a decrease in total protein synthesis, suggesting that the synthetic machinery is being funneled selectively toward ELH. We also discuss work showing that afterdischarge-induced ELH synthesis uses a novel mechanism of translation initiation, one involving a switch from cap-dependent to cap-independent translation initiation that activates an internal ribosome entry site (IRES) located in the 5'-untranslated region of ELH mRNA. The IRES-regulated translation provides a unique cellular mechanism to selectively upregulate synthesis of a critical reproductive hormone at the expense of nonessential proteins.

Compartmentalized synthesis and degradation of proteins in neurons

An important aspect of gene expression in neurons involves the delivery of mRNAs to particular subcellular domains, where translation of the mRNAs is locally controlled. Local synthesis of protein within dendrites plays a key role in activity-dependent synaptic modifications. In growing axons, local synthesis in the growth cone is important for extension and guidance. Recent evidence also documents the existence of mechanisms permitting local protein degradation, providing bidirectional control of protein composition in local domains. Here, we summarize what is known about local synthesis and degradation of protein in dendrites and axons, highlighting key unresolved questions.

Protein kinase M zeta synthesis from a brain mRNA encoding an independent protein kinase C zeta catalytic domain. Implications for the molecular mechanism of memory

Protein kinase M zeta (PKM zeta) is a newly described form of PKC that is necessary and sufficient for the maintenance of hippocampal long term potentiation (LTP) and the persistence of memory in Drosophila. PKM zeta is the independent catalytic domain of the atypical PKC zeta isoform and produces long term effects at synapses because it is persistently active, lacking autoinhibition from the regulatory domain of PKC zeta. PKM has been thought of as a proteolytic fragment of PKC. Here we report that brain PKM zeta is a new PKC isoform, synthesized from a PKM zeta mRNA encoding a PKC zeta catalytic domain without a regulatory domain. Multiple zeta-specific antisera show that PKM zeta is expressed in rat forebrain as the major form of zeta in the near absence of full-length PKC zeta. A PKC zeta knockout mouse, in which the regulatory domain was disrupted and catalytic domain spared, still expresses brain PKM zeta, indicating that this form of PKM is not a PKC zeta proteolytic fragment. Furthermore, the distribution of brain PKM zeta does not correlate with PKC zeta mRNA but instead with an alternate zeta RNA transcript thought incapable of producing protein. In vitro translation of this RNA, however, generates PKM zeta of the same molecular weight as that in brain. Metabolic labeling of hippocampal slices shows increased de novo synthesis of PKM zeta in LTP. Because PKM zeta is a kinase synthesized in an autonomously active form and is necessary and sufficient for maintaining LTP, it serves as an example of a link coupling gene expression directly to synaptic plasticity.

Isoform-specific knockout ofFE65 leads to impaired learning and memory

FE65 is a multimodular adapter protein that is expressed predominantly in brain. Its C-terminal phosphotyrosine interaction domain (PID) binds to the intracellular tail of the beta-amyloid precursor protein (betaPP), a protein of central importance to the pathogenesis of dementias of the Alzheimer type. To study the physiological functions of FE65, we generated a line of FE65 knockout mice via gene targeting. By Western analysis with a panel of FE65-specific antibodies, we demonstrate that the 97-kDa full-length FE65 (p97) was ablated in the mutant mice, and that a previously undescribed FE65 isoform with apparent molecular mass of 60 kDa (p60) was expressed in both wild-type and mutant mice. p60 had a truncated N-terminus and was likely to be generated through alternative translation. Expressions of the two isoforms appeared to be brain region distinct and age dependent. The p97FE65(-/-) mice were viable and showed no obvious physical impairments or histopathological abnormalities. However, p97FE65(-/-) and p97FE65(+/-) mice exhibited poorer performances than wild-type mice on a passive avoidance task when tested at 14 months (P <.05). p97FE65(-/-) mice at 14 months also exhibited impaired hidden-platform acquisition (P <.05) and a severe reversal-learning deficit (P <.002) but normal visual-platform acquisition in the Morris water maze tests. Probe trials confirmed impairments in p97FE65(-/-) mice in relearning of new spatial information, suggesting a hippocampus-dependent memory-extinction deficit. Reduced secretion of Abeta peptides was observed in primary neuronal cultures of hybrids of p97FE65(-/-)/betaPP transgenic (Tg2576) mice. These studies suggest an important and novel function of FE65 in learning and memory.