Suppression of PKR promotes network excitability and enhanced cognition by interferon-γ-mediated disinhibition

PERK as a hub of multiple pathogenic pathways leading to memory deficits and neurodegeneration in Alzheimer's disease

Cell signaling in response to an array of diverse stress stimuli converges on the phosphorylation of eukaryotic initiation factor-2α (eIF2α). In the brain, eIF2α is a hub for controlling learning and memory function and for maintaining neuronal integrity in health and disease. Among four eIF2α kinases, PERK is emerging as a key regulator for memory impairments and neurodegeneration in Alzheimer's disease (AD). Genetic and pharmacological manipulations of PERK-eIF2α signaling have revealed that the overactivation of this pathway is not a mere consequence of the neurodegenerative process but play critical roles in AD pathogenesis and the occurrence of memory deficits. This review provides an overview of recent progress in animal model studies, which demonstrate that dysregulated PERK accounts for memory deficits and neurodegeneration not only as a detrimental mediator downstream of β-amyloidosis and tauopathy but also as an important determinant upstream of both pathogenic mechanisms in AD. A therapeutic perspective is also discussed, in which interventions targeting the PERK-eIF2α pathway are expected to provide multiple beneficial outcomes in AD, including enhanced mnemonic function, neuroprotection and disease modification.

Inhibition of the integrated stress response reverses cognitive deficits after traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of long-term neurological disability, yet the mechanisms underlying the chronic cognitive deficits associated with TBI remain unknown. Consequently, there are no effective treatments for patients suffering from the long-lasting symptoms of TBI. Here, we show that TBI persistently activates the integrated stress response (ISR), a universal intracellular signaling pathway that responds to a variety of cellular conditions and regulates protein translation via phosphorylation of the translation initiation factor eIF2α. Treatment with ISRIB, a potent drug-like small-molecule inhibitor of the ISR, reversed the hippocampal-dependent cognitive deficits induced by TBI in two different injury mouse models-focal contusion and diffuse concussive injury. Surprisingly, ISRIB corrected TBI-induced memory deficits when administered weeks after the initial injury and maintained cognitive improvement after treatment was terminated. At the physiological level, TBI suppressed long-term potentiation in the hippocampus, which was fully restored with ISRIB treatment. Our results indicate that ISR inhibition at time points late after injury can reverse memory deficits associated with TBI. As such, pharmacological inhibition of the ISR emerges as a promising avenue to combat head trauma-induced chronic cognitive deficits.

Try to Remember: Interplay between Memory and Substance Use Disorder

Memories associated with substance use disorders, or substance-associated cues increase the likelihood of craving and relapse during abstinence. There is a growing consensus that manipulation of synaptic plasticity may reduce the strength of substance abuse-related memories. On the biological front, there are new insights that suggest memories associated with substance use disorder may follow unique neurobiological pathways that render them more accessible to pharmacological intervention. In parallel to this, research in neurochemistry has identified several potential candidate molecules that could influence the formation and maintenance of long-term memory. Drugs that target these molecules (blebbistatin, isradipine and zeta inhibitory peptide) have shown promise at the preclinical stage. In this review, we shall provide an overview of the evolving understanding on the biochemical mechanisms involved in memory formation and expound on the premise that substance use disorder is a learning disorder.

How and why do T cells and their derived cytokines affect the injured and healthy brain?

The evolution of adaptive immunity provides enhanced defence against specific pathogens, as well as homeostatic immune surveillance of all tissues. Despite being 'immune privileged', the CNS uses the assistance of the immune system in physiological and pathological states. In this Opinion article, we discuss the influence of adaptive immunity on recovery after CNS injury and on cognitive and social brain function. We further extend a hypothesis that the pro-social effects of interferon-regulated genes were initially exploited by pathogens to increase host-host transmission, and that these genes were later recycled by the host to form part of an immune defence programme. In this way, the evolution of adaptive immunity may reflect a host-pathogen 'arms race'.

Coronavirus nonstructural protein 15 mediates evasion of dsRNA sensors and limits apoptosis in macrophages

Coronaviruses are positive-sense RNA viruses that generate double-stranded RNA (dsRNA) intermediates during replication, yet evade detection by host innate immune sensors. Here we report that coronavirus nonstructural protein 15 (nsp15), an endoribonuclease, is required for evasion of dsRNA sensors. We evaluated two independent nsp15 mutant mouse coronaviruses, designated N15m1 and N15m3, and found that these viruses replicated poorly and induced rapid cell death in mouse bone marrow-derived macrophages. Infection of macrophages with N15m1, which expresses an unstable nsp15, or N15m3, which expresses a catalysis-deficient nsp15, activated MDA5, PKR, and the OAS/RNase L system, resulting in an early, robust induction of type I IFN, PKR-mediated apoptosis, and RNA degradation. Immunofluorescence imaging of nsp15 mutant virus-infected macrophages revealed significant dispersal of dsRNA early during infection, whereas in WT virus-infected cells, the majority of the dsRNA was associated with replication complexes. The loss of nsp15 activity also resulted in greatly attenuated disease in mice and stimulated a protective immune response. Taken together, our findings demonstrate that coronavirus nsp15 is critical for evasion of host dsRNA sensors in macrophages and reveal that modulating nsp15 stability and activity is a strategy for generating live-attenuated vaccines.

Cytokines and cytokine networks target neurons to modulate long-term potentiation

Cytokines play crucial roles in the communication between brain cells including neurons and glia, as well as in the brain-periphery interactions. In the brain, cytokines modulate long-term potentiation (LTP), a cellular correlate of memory. Whether cytokines regulate LTP by direct effects on neurons or by indirect mechanisms mediated by non-neuronal cells is poorly understood. Elucidating neuron-specific effects of cytokines has been challenging because most brain cells express cytokine receptors. Moreover, cytokines commonly increase the expression of multiple cytokines in their target cells, thus increasing the complexity of brain cytokine networks even after single-cytokine challenges. Here, we review evidence on both direct and indirect-mediated modulation of LTP by cytokines. We also describe novel approaches based on neuron- and synaptosome-enriched systems to identify cytokines able to directly modulate LTP, by targeting neurons and synapses. These approaches can test multiple samples in parallel, thus allowing the study of multiple cytokines simultaneously. Hence, a cytokine networks perspective coupled with neuron-specific analysis may contribute to delineation of maps of the modulation of LTP by cytokines.

eEF2K/eEF2 Pathway Controls the Excitation/Inhibition Balance and Susceptibility to Epileptic Seizures

Alterations in the balance of inhibitory and excitatory synaptic transmission have been implicated in the pathogenesis of neurological disorders such as epilepsy. Eukaryotic elongation factor 2 kinase (eEF2K) is a highly regulated, ubiquitous kinase involved in the control of protein translation. Here, we show that eEF2K activity negatively regulates GABAergic synaptic transmission. Indeed, loss of eEF2K increases GABAergic synaptic transmission by upregulating the presynaptic protein Synapsin 2b and α5-containing GABAA receptors and thus interferes with the excitation/inhibition balance. This cellular phenotype is accompanied by an increased resistance to epilepsy and an impairment of only a specific hippocampal-dependent fear conditioning. From a clinical perspective, our results identify eEF2K as a potential novel target for antiepileptic drugs, since pharmacological and genetic inhibition of eEF2K can revert the epileptic phenotype in a mouse model of human epilepsy.

Evolution, immunity and the emergence of brain superautoantigens

While some autoimmune disorders remain extremely rare, others largely predominate the epidemiology of human autoimmunity. Notably, these include psoriasis, diabetes, vitiligo, thyroiditis, rheumatoid arthritis and multiple sclerosis. Thus, despite the quasi-infinite number of "self" antigens that could theoretically trigger autoimmune responses, only a limited set of antigens, referred here as superautoantigens, induce pathogenic adaptive responses. Several lines of evidence reviewed in this paper indicate that, irrespective of the targeted organ (e.g. thyroid, pancreas, joints, brain or skin), a significant proportion of superautoantigens are highly expressed in the synaptic compartment of the central nervous system (CNS). Such an observation applies notably for GAD65, AchR, ribonucleoproteins, heat shock proteins, collagen IV, laminin, tyrosine hydroxylase and the acetylcholinesterase domain of thyroglobulin. It is also argued that cognitive alterations have been described in a number of autoimmune disorders, including psoriasis, rheumatoid arthritis, lupus, Crohn's disease and autoimmune thyroiditis. Finally, the present paper points out that a great majority of the "incidental" autoimmune conditions notably triggered by neoplasms, vaccinations or microbial infections are targeting the synaptic or myelin compartments. On this basis, the concept of an immunological homunculus, proposed by Irun Cohen more than 25 years ago, is extended here in a model where physiological autoimmunity against brain superautoantigens confers both: i) a crucial evolutionary-determined advantage via cognition-promoting autoimmunity; and ii) a major evolutionary-determined vulnerability, leading to the emergence of autoimmune disorders in Homo sapiens. Moreover, in this theoretical framework, the so called co-development/co-evolution model, both the development (at the scale of an individual) and evolution (at the scale of species) of the antibody and T-cell repertoires are coupled to those of the neural repertoires (i.e. the distinct neuronal populations and synaptic circuits supporting cognitive and sensorimotor functions). Clinical implications and future experimental insights are also presented and discussed.

Translational control of auditory imprinting and structural plasticity by eIF2α

The formation of imprinted memories during a critical period is crucial for vital behaviors, including filial attachment. Yet, little is known about the underlying molecular mechanisms. Using a combination of behavior, pharmacology, in vivo surface sensing of translation (SUnSET) and DiOlistic labeling we found that, translational control by the eukaryotic translation initiation factor 2 alpha (eIF2α) bidirectionally regulates auditory but not visual imprinting and related changes in structural plasticity in chickens. Increasing phosphorylation of eIF2α (p-eIF2α) reduces translation rates and spine plasticity, and selectively impairs auditory imprinting. By contrast, inhibition of an eIF2α kinase or blocking the translational program controlled by p-eIF2α enhances auditory imprinting. Importantly, these manipulations are able to reopen the critical period. Thus, we have identified a translational control mechanism that selectively underlies auditory imprinting. Restoring translational control of eIF2α holds the promise to rejuvenate adult brain plasticity and restore learning and memory in a variety of cognitive disorders.

DOI:http://dx.doi.org/10.7554/eLife.17197.001

Shortly after hatching, a chick recognizes the sight and sound of its mother and follows her around. This requires a type of learning called imprinting, which only occurs during a short period of time in young life known as the “critical period”. This process has been reported in a variety of birds and other animals where long-term memory formed during a critical period guides vital behaviors. In order to form imprinted memories, neurons must produce new proteins. However, it is not clear how new experiences trigger the production of these proteins during imprinting. Unraveling such mechanisms may help us to develop drugs that can recover plasticity in the adult brain, which could help individuals with brain injuries relearn skills after critical periods are closed.

It is possible to imprint newly hatched chicks to arbitrary sounds and visual stimuli by placing the chicks in running wheels and exposing them to repeated noises and videos. Later on, the chicks respond to these stimuli by running towards the screen, mimicking how they would naturally follow their mother. This system allows researchers to measure imprinting in a carefully controlled laboratory setting.

A protein called elF2α plays a major role in regulating the production of new proteins and has been shown to be required for the formation of long-term memories in adult rodents. Batista et al. found that elF2α is required to imprint newly hatched chicks to sound. During the critical period, this factor mediates an increase in “memory-spines”, which are small bumps on neurons that are thought to be involved in memory storage. On the other hand, elF2α was not required to imprint newly hatched chicks to visual stimuli, suggesting that there are different pathways involved in regulating imprinting to different senses. Batista et al. also demonstrate that using drugs to increase the activity of eIF2α in older chicks could allow these chicks to be imprinted to new sounds.

The next steps following on from this work are to identify proteins that eIF2α regulates to form memories, and to find out why eIF2α is only required to imprint sounds. Future research will investigate the mechanisms that control visual imprinting and how it differs from imprinting to sounds.

DOI:http://dx.doi.org/10.7554/eLife.17197.002

Evaluation of human microtubule affinity-regulating kinase 4 inhibitors: fluorescence binding studies, enzyme, and cell assays

Human microtubule affinity-regulating kinase 4 (MARK4) is considered as an encouraging drug target for the design and development of inhibitors to cure several life-threatening diseases such as Alzheimer disease, cancer, obesity, and type-II diabetes. Recently, we have reported four ligands namely, BX-912, BX-795, PKR-inhibitor, and OTSSP167 (hydrochloride) which bind preferentially to the two different constructs of human MARK4 containing kinase domain. To ensure the role of ubiquitin-associated (UBA) domain in the ligand binding, we made a newer construct of MARK4 which contains both kinase and UBA domains, named as MARK4-F3. We observed that OTSSP167 (hydrochloride) binds to the MARK4-F3 with a binding constant (K) of 3.16 × 10⁶, M^-1 (±.21). However, UBA-domain of MARK4-F3 doesn't show any interaction with ligands directly as predicted by the molecular docking. To validate further, ATPase inhibition assays of all three constructs of MARK4 in the presence of mentioned ligands were carried out. An appreciable correlation between the binding experiments and ATPase inhibition assays of MARK4 was observed. In addition, cell-proliferation inhibition activity for all four ligands on the Human embryonic kidney (HEK-293) and breast cancer cell lines (MCF-7) was performed using MTT assay. IC₅₀ values of OTSSP167 for HEK-293 and MCF-7 were found to be 58.88 (±1.5), and 48.2 (±1.6), respectively. OTSSP167 among all four inhibitors, showed very good enzyme inhibition activity against three constructs of MARK4. Moreover, all four inhibitors showed anti-neuroblastoma activity and anticancer properties. In conclusion, OTSSP167 may be considered as a promising scaffold to discover novel inhibitors of MARK4.

Upstream Open Reading Frames Located in the Leader of Protein Kinase Mζ mRNA Regulate Its Translation

For protein synthesis that occurs locally in dendrites, the translational control mechanisms are much more important for neuronal functioning than the transcription levels. Here, we show that uORFs (upstream open reading frames) in the 5′ untranslated region (5′UTR) play a critical role in regulation of the translation of protein kinase Mζ (PKMζ). Elimination of these uORFs activates translation of the reporter protein in vitro and in primary cultures of rat hippocampal neurons. Using cell-free translation systems, we demonstrate that translational initiation complexes are formed only on uORFs. Further, we address the mechanism of translational repression of PKMζ translation, by uORFs. We observed an increase in translation of the reporter protein under the control of PKMζ leader in neuronal culture during non-specific activation by picrotoxin. We also show that such a mechanism is similar to the mechanism seen in cell stress, as application of sodium arsenite to neuron cultures induced translation of mRNA carrying PKMζ 5′UTR similarly to picrotoxin activation. Therefore, we suppose that phosphorylation of eIF2a, like in cell stress, is a main regulator of PKMζ translation. Altogether, our findings considerably extend our understanding of the role of uORF in regulation of PKMζ translation in activated neurons, important at early stages of LTP.

PERK regulates Gq protein-coupled intracellular Ca2+ dynamics in primary cortical neurons

PERK (EIF2AK3) is an ER-resident eIF2α kinase required for behavioral flexibility and metabotropic glutamate receptor-dependent long-term depression via its translational control. Motivated by the recent discoveries that PERK regulates Ca²⁺ dynamics in insulin-secreting β-cells underlying glucose-stimulated insulin secretion, and modulates Ca²⁺ signals-dependent working memory, we explored the role of PERK in regulating G_q protein-coupled Ca²⁺ dynamics in pyramidal neurons. We found that acute PERK inhibition by the use of a highly specific PERK inhibitor reduced the intracellular Ca²⁺ rise stimulated by the activation of acetylcholine, metabotropic glutamate and bradykinin-2 receptors in primary cortical neurons. More specifically, acute PERK inhibition increased IP₃ receptor mediated ER Ca²⁺ release, but decreased receptor-operated extracellular Ca²⁺ influx. Impaired G_q protein-coupled intracellular Ca²⁺ rise was also observed in genetic Perk knockout neurons. Taken together, our findings reveal a novel role of PERK in neurons, which is eIF2α-independent, and suggest that the impaired working memory in forebrain-specific Perk knockout mice may stem from altered G_q protein-coupled intracellular Ca²⁺ dynamics in cortical pyramidal neurons.

PERK Regulates Working Memory and Protein Synthesis-Dependent Memory Flexibility

PERK (EIF2AK3) is an ER-resident eIF2α kinase required for memory flexibility and metabotropic glutamate receptor-dependent long-term depression, processes known to be dependent on new protein synthesis. Here we investigated PERK’s role in working memory, a cognitive ability that is independent of new protein synthesis, but instead is dependent on cellular Ca²⁺ dynamics. We found that working memory is impaired in forebrain-specific Perk knockout and pharmacologically PERK-inhibited mice. Moreover, inhibition of PERK in wild-type mice mimics the fear extinction impairment observed in forebrain-specific Perk knockout mice. Our findings reveal a novel role of PERK in cognitive functions and suggest that PERK regulates both Ca²⁺ -dependent working memory and protein synthesis-dependent memory flexibility.

Biomarkers in Sporadic and Familial Alzheimer's Disease

Most forms of Alzheimer's disease (AD) are sporadic (sAD) or inherited in a non-Mendelian fashion, and less than 1% of cases are autosomal-dominant. Forms of sAD do not exhibit familial aggregation and are characterized by complex genetic and environmental interactions. Recently, the expansion of genomic methodologies, in association with substantially larger combined cohorts, has resulted in various genome-wide association studies that have identified several novel genetic associations of AD. Currently, the most effective methods for establishing the diagnosis of AD are defined by multi-modal pathways, starting with clinical and neuropsychological assessment, cerebrospinal fluid (CSF) analysis, and brain-imaging procedures, all of which have significant cost- and access-to-care barriers. Consequently, research efforts have focused on the development and validation of non-invasive and generalizable blood-based biomarkers. Among the modalities conceptualized by the systems biology paradigm and utilized in the "exploratory biomarker discovery arena", proteome analysis has received the most attention. However, metabolomics, lipidomics, transcriptomics, and epigenomics have recently become key modalities in the search for AD biomarkers. Interestingly, biomarker changes for familial AD (fAD), in many but not all cases, seem similar to those for sAD. The integration of neurogenetics with systems biology/physiology-based strategies and high-throughput technologies for molecular profiling is expected to help identify the causes, mechanisms, and biomarkers associated with the various forms of AD. Moreover, in order to hypothesize the dynamic trajectories of biomarkers through disease stages and elucidate the mechanisms of biomarker alterations, updated and more sophisticated theoretical models have been proposed for both sAD and fAD.

The Epilepsy Spectrum: Targeting Future Research Challenges

There have been tremendous recent advances in our understanding of the biological underpinnings of epilepsy and associated comorbidities that justify its representation as a spectrum disorder. Advances in genetics, electrophysiology, and neuroimaging have greatly improved our ability to differentiate, diagnose, and treat individuals with epilepsy. However, we have made little overall progress in preventing epilepsy, and the number of patients who are cured remains small. Likewise, the comorbidities of epilepsy are often underdiagnosed or not adequately treated. In this article, we suggest a few areas in which additional research will likely pay big dividends for patients and their families.

Translational control by eIF2α in neurons: Beyond the stress response

The translation of mRNAs is a tightly controlled process that responds to multiple signaling pathways. In neurons, this control is also exerted locally due to the differential necessity of proteins in axons and dendrites. The phosphorylation of the alpha subunit of the translation initiation factor 2 (eIF2α) is one of the mechanisms of translational control. The phosphorylation of eIF2α has classically been viewed as a stress response, halting translation initiation. However, in the nervous system this type of regulation has been related to other mechanisms besides stress response, such as behavior, memory consolidation and nervous system development. Additionally, neurodegenerative diseases have a major stress component, thus eIF2α phosphorylation plays a preeminent role and its modulation is currently viewed as a new opportunity for therapeutic interventions. This review consolidates current information regarding eIF2α phosphorylation in neurons and its impact in neurodegenerative diseases. © 2016 Wiley Periodicals, Inc.

Gene therapy to target ER stress in brain diseases

Gene therapy based on the use of Adeno-associated viruses (AAVs) is emerging as a safe and stable strategy to target molecular pathways involved in a variety of brain diseases. Endoplasmic reticulum (ER) stress is proposed as a transversal feature of most animal models and clinical samples from patients affected with neurodegenerative diseases. Manipulation of the unfolded protein response (UPR), a major homeostatic reaction under ER stress conditions, had proved beneficial in diverse models of neurodegeneration. Although increasing number of drugs are available to target ER stress, the use of small molecules to treat chronic brain diseases is challenging because of poor blood brain barrier permeability and undesirable side effects due to the role of the UPR in the physiology of peripheral organs. Gene therapy is currently considered a possible future alternative to circumvent these problems by the delivery of therapeutic agents to selective regions and cell types of the nervous system. Here we discuss current efforts to design gene therapy strategies to alleviate ER stress on a disease context. This article is part of a Special Issue entitled SI:ER stress.

Review: Modulating the unfolded protein response to prevent neurodegeneration and enhance memory

Recent evidence has placed the unfolded protein response (UPR) at the centre of pathological processes leading to neurodegenerative disease. The translational repression caused by UPR activation starves neurons of the essential proteins they need to function and survive. Restoration of protein synthesis, via genetic or pharmacological means, is neuroprotective in animal models, prolonging survival. This is of great interest due to the observation of UPR activation in the post mortem brains of patients with Alzheimer's, Parkinson's, tauopathies and prion diseases. Protein synthesis is also an essential step in the formation of new memories. Restoring translation in disease or increasing protein synthesis from basal levels has been shown to improve memory in numerous models. As neurodegenerative diseases often present with memory impairments, targeting the UPR to both provide neuroprotection and enhance memory provides an extremely exciting novel therapeutic target.

The Long and Winding Road to Gamma-Amino-Butyric Acid as Neurotransmitter

This review centers on the discoveries made during more than six decades of neuroscience research on the role of gamma-amino-butyric acid (GABA) as neurotransmitter. In doing so, special emphasis is directed to the significant involvement of Canadian scientists in these advances. Starting with the early studies that established GABA as an inhibitory neurotransmitter at central synapses, we summarize the results pointing at the GABA receptor as a drug target as well as more recent evidence showing that GABAA receptor signaling plays a surprisingly active role in neuronal network synchronization, both during development and in the adult brain. Finally, we briefly address the involvement of GABA in neurological conditions that encompass epileptic disorders and mental retardation.

Absence of IFNγ promotes hippocampal plasticity and enhances cognitive performance

Cognitive functioning can be differentially modulated by components of the immune system. Interferon-γ (IFNγ) is a pro-inflammatory cytokine whose production is altered in many conditions displaying some degree of cognitive deficits, although its role in cognitive functioning is still unclear. Here we show that the absence of IFNγ selectively enhances cognitive behaviours in tasks in which the hippocampus is implicated. Moreover, the absence of IFNγ leads to volumetric and cell density changes that are restricted to the dorsal part of the hippocampus. In the dorsal hippocampus, the absence of this pro-inflammatory cytokine leads to an increase in the numbers of newly born neurons in the subgranular zone of the dentate gyrus (DG), an adult neurogenic niche known to support learning and memory, and to an enlargement of the dendritic arborization of DG granule and cornu ammonis (CA)1 pyramidal neurons. Moreover, it also modestly impacts synaptic plasticity, by decreasing the paired-pulse facilitation in the Schaffer collateral to CA1 pyramidal cell synapses. Taken together, our results provide evidence that IFNγ is a negative regulator of hippocampal functioning, as its absence positively impacts on dorsal hippocampus structure, cell density, neuronal morphology and synaptic plasticity. Importantly, these neuroplastic changes are associated with improved performance in learning and memory tasks. Therefore, blockage of the IFNγ signalling may present as promising therapeutic targets for the treatment of inflammation-associated cognitive dysfunction.

PKR Inhibition Rescues Memory Deficit and ATF4 Overexpression in ApoE ε4 Human Replacement Mice

Sporadic Alzheimer's disease (AD) is an incurable neurodegenerative disease with clear pathological hallmarks, brain dysfunction, and unknown etiology. Here, we tested the hypothesis that there is a link between genetic risk factors for AD, cellular metabolic stress, and transcription/translation regulation. In addition, we aimed at reversing the memory impairment observed in a mouse model of sporadic AD. We have previously demonstrated that the most prevalent genetic risk factor for AD, the ApoE4 allele, is correlated with increased phosphorylation of the translation factor eIF2α. In the present study, we tested the possible involvement of additional members of the eIF2α pathway and identified increased mRNA expression of negative transcription factor ATF4 (aka CREB2) both in human and a mouse model expressing the human ApoE4 allele. Furthermore, injection of a PKR inhibitor rescued memory impairment and attenuated ATF4 mRNA increased expression in the ApoE4 mice. The results propose a new mechanism by which ApoE4 affects brain function and further suggest that inhibition of PKR is a way to restore ATF4 overexpression and memory impairment in early stages of sporadic AD. Significance statement: ATF4 mRNA relative quantities are elevated in ApoE4 allele carriers compared with noncarrier controls. This is true also for the ApoE ε4 human replacement mice. ApoE4 mice injected with PKR inhibitor (PKRi) demonstrate a significant reduction in ATF4 expression levels 3 h after one injection of PKRi. Treatment of ApoE4 human replacement mice with the PKRi before learning rescues the memory impairment of the ApoE4 AD model mice. We think that these results propose a new mechanism by which ApoE4 affects brain function and suggest that inhibition of PKR is a way to restore memory impairment in early stages of sporadic AD.

PKR deficiency alters E. coli-induced sickness behaviors but does not exacerbate neuroimmune responses or bacterial load

Background

Systemic inflammation induces neuroimmune activation, ultimately leading to sickness (e.g., fever, anorexia, motor impairments, exploratory deficits, and social withdrawal). In this study, we evaluated the role of protein kinase R (PKR), a serine-threonine kinase that can control systemic inflammation, on neuroimmune responses and sickness.

Methods

Wild-type (WT) PKR+/+ mice and PKR−/− mice were subcutaneously injected with live Escherichia coli (E. coli) or vehicle. Food consumption, rotarod test performance, burrowing, open field activity, object investigation, and social interaction were monitored. Plasma TNF-α and corticosterone were measured by ELISA. The percentage of neutrophils in blood was deduced from blood smears. Inflammatory gene expression (IL-1β, TNF-α, IL-6, cyclooxygenase (COX)-2, iNOS) in the liver and the brain (hypothalamus and hippocampus) were quantified by real-time PCR. Blood and lavage fluid (injection site) were collected for microbiological plate count and for real-time PCR of bacterial 16S ribosomal DNA (rDNA). Corticotrophin-releasing hormone (CRH) expression in the hypothalamus was also determined by real-time PCR.

Results

Deficiency of PKR diminished peripheral inflammatory responses following E. coli challenge. However, while the core components of sickness (anorexia and motor impairments) were similar between both strains of mice, the behavioral components of sickness (reduced burrowing, exploratory activity deficits, and social withdrawal) were only observable in PKR−/− mice but not in WT mice. Such alteration of behavioral components was unlikely to be caused by exaggerated neuroimmune activation, by an impaired host defense to the infection, or due to a dysregulated corticosterone response, because both strains of mice displayed similar neuroimmune responses, bacterial titers, and plasma corticosterone profiles throughout the course of infection. Nevertheless, the induction of hypothalamic corticotrophin-releasing hormone (CRH) by E. coli was delayed in PKR−/− mice relative to WT mice, suggesting that PKR deficiency may postpone the CRH response during systemic inflammation.

Conclusions

Taken together, our findings show that (1) loss of PKR could alter E. coli-induced sickness behaviors and (2) this was unlikely to be due to exacerbated neuroimmune activation, (3) elevated bacterial load, or (4) dysregulation in the corticosterone response. Further studies can address the role of PKR in the CRH response together with its consequence on sickness.

Single-Gene Determinants of Epilepsy Comorbidity

Common somatic conditions are bound to occur by chance in individuals with neurological disorders as prevalent as epilepsy, but when biological links underlying the comorbidity can be uncovered, the relationship may provide clues into the origin and mechanisms of both. The expanding list of monogenic epilepsies and their associated clinical features offer a remarkable opportunity to mine the epilepsy genome for coordinate neurodevelopmental phenotypes and examine their pathogenic mechanisms. Defined single-gene-linked epilepsy syndromes identified to date include all of the most frequently cited comorbidities, such as cognitive disorders, autism, migraine, mood disorders, late-onset dementia, and even premature lethality. Gene-linked comorbidities may be aggravated by, or independent of, seizure history. Mutations in these genes establish clear biological links between abnormal neuronal synchronization and a variety of neurobehavioral disorders, and critically substantiate the definition of epilepsy as a complex spectrum disorder. Mapping the neural circuitry of epilepsy comorbidities and understanding their single-gene risk should substantially clarify this challenging aspect of clinical epilepsy management.

TORC2: a novel target for treating age-associated memory impairment

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.

Phasic Treatment with Interferon Gamma Stimulates Release of Exosomes that Protect Against Spreading Depression

The detrimental effects of T-cell-secreted interferon gamma (IFNγ) on oxidative stress (OS) and demyelination in multiple sclerosis (MS) are well recognized. Recently, we demonstrated that IFNγ-mediated damage to myelin also increases susceptibility to spreading depression (SD; the likely basis of migraine with aura). However, before onset of MS, induction of physiological levels of IFNγ, like that produced by environmental enrichment (EE), protects against demyelination and OS. Accordingly, we focused on the potential for physiological levels of IFNγ to protect against SD. EE, which occurs with a moderate and phasic increase in proinflammatory cytokines, reduces migraine frequency. Thus, we applied phasic or pulsed IFNγ to brain slice cultures to emulate EE. This treatment reduced OS, increased myelin basic protein, a marker for myelin, and reduced susceptibility to SD. Building on our research on exosomes in EE-based neuroprotection, we found that IFNγ stimulation of slice cultures induced release of exosomes, likely from the microglia that produce the same protective effects as IFNγ treatment when applied to naive cultures. Finally, nasal administration of IFNγ to rats recapitulated in vitro effects, reducing OS, increasing myelin, and reducing SD. These results support phasic IFNγ signaling as a therapeutic target for prevention of SD and, by extension, migraine.

Neonatal vaccination with bacillus Calmette-Guérin and hepatitis B vaccines modulates hippocampal synaptic plasticity in rats

Immune activation can exert multiple effects on synaptic transmission. Our study demonstrates the influence of neonatal vaccination on hippocampal synaptic plasticity in rats under normal physiological conditions. The results revealed that neonatal BCG vaccination enhanced synaptic plasticity. In contrast, HBV hampered it. Furthermore, we found that the cytokine balance shifted in favour of the T helper type 1/T helper type 2 immune response in BCG/HBV-vaccinated rats in the periphery. The peripheral IFN-γ:IL-4 ratio was positively correlated with BDNF and IGF-1 in the hippocampus. BCG raised IFN-γ, IL-4, BDNF and IGF-1 and reduced IL-1β, IL-6, and TNF-α in the hippocampus, whereas, HBV triggered the opposite effects.

Pro-Apoptotic Kinase Levels in Cerebrospinal Fluid as Potential Future Biomarkers in Alzheimer's Disease

Alzheimer’s disease (AD) is characterized by the accumulation of Aβ peptides, hyperphosphorylated tau proteins, and neuronal loss in the brain of affected patients. The causes of neurodegeneration in AD are not clear, but apoptosis could be one of the cell death mechanisms. According to the amyloid hypothesis, abnormal aggregation of Aβ leads to altered kinase activities inducing tau phosphorylation and neuronal degeneration. Several studies have shown that pro-apoptotic kinases could be a link between Aβ and tau anomalies. Here, we present recent evidences from AD experimental models and human studies that three pro-apoptotic kinases (double-stranded RNA kinase (PKR), glycogen synthase kinase-3β, and C-Jun terminal kinase (JNK) could be implicated in AD physiopathology. These kinases are detectable in human fluids and the analysis of their levels could be used as potential surrogate markers to evaluate cell death and clinical prognosis. In addition to current biomarkers (Aβ_1–42, tau, and phosphorylated tau), these new evaluations could bring about valuable information on potential innovative therapeutic targets to alter the clinical evolution.

Soluble amyloid-β oligomers as synaptotoxins leading to cognitive impairment in Alzheimer's disease

Alzheimer’s disease (AD) is the most common form of dementia in the elderly, and affects millions of people worldwide. As the number of AD cases continues to increase in both developed and developing countries, finding therapies that effectively halt or reverse disease progression constitutes a major research and public health challenge. Since the identification of the amyloid-β peptide (Aβ) as the major component of the amyloid plaques that are characteristically found in AD brains, a major effort has aimed to determine whether and how Aβ leads to memory loss and cognitive impairment. A large body of evidence accumulated in the past 15 years supports a pivotal role of soluble Aβ oligomers (AβOs) in synapse failure and neuronal dysfunction in AD. Nonetheless, a number of basic questions, including the exact molecular composition of the synaptotoxic oligomers, the identity of the receptor(s) to which they bind, and the signaling pathways that ultimately lead to synapse failure, remain to be definitively answered. Here, we discuss recent advances that have illuminated our understanding of the chemical nature of the toxic species and the deleterious impact they have on synapses, and have culminated in the proposal of an Aβ oligomer hypothesis for Alzheimer’s pathogenesis. We also highlight outstanding questions and challenges in AD research that should be addressed to allow translation of research findings into effective AD therapies.

Brain metabolic stress and neuroinflammation at the basis of cognitive impairment in Alzheimer's disease

Brain metabolic dysfunction is known to influence brain activity in several neurological disorders, including Alzheimer’s disease (AD). In fact, deregulation of neuronal metabolism has been postulated to play a key role leading to the clinical outcomes observed in AD. Besides deficits in glucose utilization in AD patients, recent evidence has implicated neuroinflammation and endoplasmic reticulum (ER) stress as components of a novel form of brain metabolic stress that develop in AD and other neurological disorders. Here we review findings supporting this novel paradigm and further discuss how these mechanisms seem to participate in synapse and cognitive impairments that are germane to AD. These deleterious processes resemble pathways that act in peripheral tissues leading to insulin resistance and glucose intolerance, in an intriguing molecular connection linking AD to diabetes. The discovery of detailed mechanisms leading to neuronal metabolic stress may be a key step that will allow the understanding how cognitive impairment develops in AD, thereby offering new avenues for effective disease prevention and therapeutic targeting.

Increased levels of cerebrospinal fluid JNK3 associated with amyloid pathology: links to cognitive decline

BACKGROUND

Alzheimer disease is characterized by cognitive decline, senile plaques of β-amyloid (Aβ) peptides, neurofibrillary tangles composed of hyperphosphorylated τ proteins and neuronal loss. Aβ and τ are useful markers in the cerebrospinal fluid (CSF). C-Jun N-terminal kinases (JNKs) are serine-threonine protein kinases activated by phosphorylation and involved in neuronal death.

METHODS

In this study, Western blots, enzyme-linked immunosorbent assay and histological approaches were used to assess the concentrations of Aβ, τ and JNK isoforms in postmortem brain tissue samples (10 Alzheimer disease and 10 control) and in CSF samples from 30 living patients with Alzheimer disease and 27 controls with neurologic disease excluding Alzheimer disease. Patients with Alzheimer disease were followed for 1-3 years and assessed using Mini-Mental State Examination scores.

RESULTS

The biochemical and morphological results showed a significant increase of JNK3 and phosphorylated JNK levels in patients with Alzheimer disease, and JNK3 levels correlated with Aβ42 levels. Confocal microscopy revealed that JNK3 was associated with Aβ in senile plaques. The JNK3 levels in the CSF were significantly elevated in patients with Alzheimer disease and correlated statistically with the rate of cognitive decline in a mixed linear model.

LIMITATIONS

The study involved different samples grouped into 3 small cohorts. Evaluation of JNK3 in CSF was possible only with immunoblot analysis.

CONCLUSION

We found that JNK3 levels are increased in brain tissue and CSF from patients with Alzheimer disease. The finding that increased JNK3 levels in CSF could reflect the rate of cognitive decline is new and merits further investigation.

Neuronal stress signaling and eIF2α phosphorylation as molecular links between Alzheimer's disease and diabetes

Mounting evidence from clinical, epidemiological, neuropathology and preclinical studies indicates that mechanisms similar to those leading to peripheral metabolic deregulation in metabolic disorders, such as diabetes and obesity, take place in the brains of Alzheimer's disease (AD) patients. These include pro-inflammatory mechanisms, brain metabolic stress and neuronal insulin resistance. From a molecular and cellular perspective, recent progress has been made in unveiling novel pathways that act in an orchestrated way to cause neuronal damage and cognitive decline in AD. These pathways converge to the activation of neuronal stress-related protein kinases and excessive phosphorylation of eukaryotic translation initiation factor 2α (eIF2α-P), which plays a key role in control of protein translation, culminating in synapse dysfunction and memory loss. eIF2α-P signaling thus links multiple neuronal stress pathways to impaired neuronal function and neurodegeneration. Here, we present a critical analysis of recently discovered molecular mechanisms underlying impaired brain insulin signaling and metabolic stress, with emphasis on the role of stress kinase/eIF2α-P signaling as a hub that promotes brain and behavioral impairments in AD. Because very similar mechanisms appear to operate in peripheral metabolic deregulation in T2D and in brain defects in AD, we discuss the concept that targeting defective brain insulin signaling and neuronal stress mechanisms with anti-diabetes agents may be an attractive approach to fight memory decline in AD. We conclude by raising core questions that remain to be addressed toward the development of much needed therapeutic approaches for AD.

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.

Somatostatin, neuronal vulnerability and behavioral emotionality

Somatostatin (SST) deficits are common pathological features in depression and other neurological disorders with mood disturbances, but little is known about the contribution of SST deficits to mood symptoms or causes of these deficits. Here we show that mice lacking SST (Sst(KO)) exhibit elevated behavioral emotionality, high basal plasma corticosterone and reduced gene expression of Bdnf, Cortistatin and Gad67, together recapitulating behavioral, neuroendocrine and molecular features of human depression. Studies in Sst(KO) and heterozygous (Sst(HZ)) mice show that elevated corticosterone is not sufficient to reproduce the behavioral phenotype, suggesting a putative role for Sst cell-specific molecular changes. Using laser capture microdissection, we show that cortical SST-positive interneurons display significantly greater transcriptome deregulations after chronic stress compared with pyramidal neurons. Protein translation through eukaryotic initiation factor 2 (EIF2) signaling, a pathway previously implicated in neurodegenerative diseases, was most affected and suppressed in stress-exposed SST neurons. We then show that activating EIF2 signaling through EIF2 kinase inhibition mitigated stress-induced behavioral emotionality in mice. Taken together, our data suggest that (1) low SST has a causal role in mood-related phenotypes, (2) deregulated EIF2-mediated protein translation may represent a mechanism for vulnerability of SST neurons and (3) that global EIF2 signaling has antidepressant/anxiolytic potential.

The small molecule ISRIB reverses the effects of eIF2α phosphorylation on translation and stress granule assembly

Previously, we identified ISRIB as a potent inhibitor of the integrated stress response (ISR) and showed that ISRIB makes cells resistant to the effects of eIF2α phosphorylation and enhances long-term memory in rodents (Sidrauski et al., 2013). Here, we show by genome-wide in vivo ribosome profiling that translation of a restricted subset of mRNAs is induced upon ISR activation. ISRIB substantially reversed the translational effects elicited by phosphorylation of eIF2α and induced no major changes in translation or mRNA levels in unstressed cells. eIF2α phosphorylation-induced stress granule (SG) formation was blocked by ISRIB. Strikingly, ISRIB addition to stressed cells with pre-formed SGs induced their rapid disassembly, liberating mRNAs into the actively translating pool. Restoration of mRNA translation and modulation of SG dynamics may be an effective treatment of neurodegenerative diseases characterized by eIF2α phosphorylation, SG formation, and cognitive loss.

Pathway-driven discovery of epilepsy genes

Epilepsy genes deliver critical insights into the molecular control of brain synchronization and are revolutionizing our understanding and treatment of the disease. The epilepsy-associated genome is rapidly expanding, and two powerful complementary approaches, isolation of de novo exome variants in patients and targeted mutagenesis in model systems, account for the steep increase. In sheer number, the tally of genes linked to seizures will likely match that of cancer and exceed it in biological diversity. The proteins act within most intracellular compartments and span the molecular determinants of firing and wiring in the developing brain. Every facet of neurotransmission, from dendritic spine to exocytotic machinery, is in play, and defects of synaptic inhibition are over-represented. The contributions of somatic mutations and noncoding microRNAs are also being explored. The functional spectrum of established epilepsy genes and the arrival of rapid, precise technologies for genome editing now provide a robust scaffold to prioritize hypothesis-driven discovery and further populate this genetic proto-map. Although each gene identified offers translational potential to stratify patient care, the complexity of individual variation and covert actions of genetic modifiers may confound single-gene solutions for the clinical disorder. In vivo genetic deconstruction of epileptic networks, ex vivo validation of variant profiles in patient-derived induced pluripotent stem cells, in silico variant modeling and modifier gene discovery, now in their earliest stages, will help clarify individual patterns. Because seizures stand at the crossroads of all neuronal synchronization disorders in the developing and aging brain, the neurobiological analysis of epilepsy-associated genes provides an extraordinary gateway to new insights into higher cortical function.

Neuroinflammation and Aβ accumulation linked to systemic inflammation are decreased by genetic PKR down-regulation

Alzheimer's disease (AD) is a neurodegenerative disorder, marked by senile plaques composed of amyloid-β (Aβ) peptide, neurofibrillary tangles, neuronal loss and neuroinflammation. Previous works have suggested that systemic inflammation could contribute to neuroinflammation and enhanced Aβ cerebral concentrations. The molecular pathways leading to these events are not fully understood. PKR is a pro-apoptotic kinase that can trigger inflammation and accumulates in the brain and cerebrospinal fluid of AD patients. The goal of the present study was to assess if LPS-induced neuroinflammation and Aβ production could be altered by genetic PKR down regulation. The results show that, in the hippocampus of LPS-injected wild type mice, neuroinflammation, cytokine release and Aβ production are significantly increased and not in LPS-treated PKR knock-out mice. In addition BACE1 and activated STAT3 levels, a putative transcriptional regulator of BACE1, were not found increased in the brain of PKR knock-out mice as observed in wild type mice. Using PET imaging, the decrease of hippocampal metabolism induced by systemic LPS was not observed in LPS-treated PKR knock-out mice. Altogether, these findings demonstrate that PKR plays a major role in brain changes induced by LPS and could be a valid target to modulate neuroinflammation and Aβ production.

PKR downregulation prevents neurodegeneration and β-amyloid production in a thiamine-deficient model

Brain thiamine homeostasis has an important role in energy metabolism and displays reduced activity in Alzheimer's disease (AD). Thiamine deficiency (TD) induces regionally specific neuronal death in the animal and human brains associated with a mild chronic impairment of oxidative metabolism. These features make the TD model amenable to investigate the cellular mechanisms of neurodegeneration. Once activated by various cellular stresses, including oxidative stress, PKR acts as a pro-apoptotic kinase and negatively controls the protein translation leading to an increase of BACE1 translation. In this study, we used a mouse TD model to assess the involvement of PKR in neuronal death and the molecular mechanisms of AD. Our results showed that the TD model activates the PKR-eIF2α pathway, increases the BACE1 expression levels of Aβ in specific thalamus nuclei and induces motor deficits and neurodegeneration. These effects are reversed by PKR downregulation (using a specific inhibitor or in PKR knockout mice).

Genetic or pharmacological reduction of PERK enhances cortical-dependent taste learning

Protein translation initiation is controlled by levels of eIF2α phosphorylation (p-eIF2α) on Ser51. In addition, increased p-eIF2α levels impair long-term synaptic plasticity and memory consolidation, whereas decreased levels enhance them. Levels of p-eIF2α are determined by four kinases, of which protein kinase RNA-activated (PKR), PKR-like endoplastic reticulum kinase (PERK), and general control nonderepressible 2 are extensively expressed in the mammalian mature brain. Following identification of PERK as the major kinase to determine basal levels of p-eIF2α in primary neuronal cultures, we tested its function as a physiological constraint of memory consolidation in the cortex, the brain structure suggested to store, at least in part, long-term memories in the mammalian brain. To that aim, insular cortex (IC)-dependent positive and negative forms of taste learning were used. Genetic reduction of PERK expression was accomplished by local microinfusion of a lentivirus harboring PERK Short hairpin RNA, and pharmacological inhibition was achieved by local microinfusion of a PERK-specific inhibitor (GSK2606414) to the rat IC. Both genetic reduction of PERK expression and pharmacological inhibition of its activity reduced p-eIF2α levels and enhanced novel taste learning and conditioned taste aversion, but not memory retrieval. Moreover, enhanced extinction was observed together with enhanced associative memory, suggesting increased cortical-dependent behavioral plasticity. The results suggest that, by phosphorylating eIF2α, PERK functions in the cortex as a physiological constraint of memory consolidation, and its downregulation serves as cognitive enhancement.

Type-I interferon signaling through ISGF3 complex is required for sustained Rip3 activation and necroptosis in macrophages

Myeloid cells play a critical role in perpetuating inflammation during various chronic diseases. Recently the death of macrophages through programmed necrosis (necroptosis) has emerged as an important mechanism in inflammation and pathology. We evaluated the mechanisms that lead to the induction of necrotic cell death in macrophages. Our results indicate that type I IFN (IFN-I) signaling is a predominant mechanism of necroptosis, because macrophages deficient in IFN-α receptor type I (IFNAR1) are highly resistant to necroptosis after stimulation with LPS, polyinosinic-polycytidylic acid, TNF-α, or IFN-β in the presence of caspase inhibitors. IFN-I-induced necroptosis occurred through both mechanisms dependent on and independent of Toll/IL-1 receptor domain-containing adaptor inducing IFN-β (TRIF) and led to persistent phosphorylation of receptor-interacting protein 3 (Rip3) kinase, which resulted in potent necroptosis. Although various IFN-regulatory factors (IRFs) facilitated the induction of necroptosis in response to IFN-β, IRF-9-STAT1- or -STAT2-deficient macrophages were highly resistant to necroptosis. Our results indicate that IFN-β-induced necroptosis of macrophages proceeds through tonic IFN-stimulated gene factor 3 (ISGF3) signaling, which leads to persistent expression of STAT1, STAT2, and IRF9. Induction of IFNAR1/Rip3-dependent necroptosis also resulted in potent inflammatory pathology in vivo. These results reveal how IFN-I mediates acute inflammation through macrophage necroptosis.

PKR is activated by cellular dsRNAs during mitosis and acts as a mitotic regulator

dsRNA-dependent protein kinase R (PKR) plays a key role in innate immunity. PKR binds viral dsRNA and undergoes autophosphorylation, which leads to translational repression and signaling pathway modulation in infected cells. Kim et al. now show that PKR is activated during mitosis in uninfected cells. PKR interacts with dsRNAs formed by inverted Alu repeats, which become accessible to PKR during mitosis. Phosphorylated PKR then suppresses translation and coordinates mitosis. This study unveils a novel function of PKR and endogenous dsRNA mitosis in uninfected cells.

dsRNA-dependent protein kinase R (PKR) is a ubiquitously expressed enzyme well known for its roles in immune response. Upon binding to viral dsRNA, PKR undergoes autophosphorylation, and the phosphorylated PKR (pPKR) regulates translation and multiple signaling pathways in infected cells. Here, we found that PKR is activated in uninfected cells, specifically during mitosis, by binding to dsRNAs formed by inverted Alu repeats (IRAlus). While PKR and IRAlu-containing RNAs are segregated in the cytosol and nucleus of interphase cells, respectively, they interact during mitosis when nuclear structure is disrupted. Once phosphorylated, PKR suppresses global translation by phosphorylating the α subunit of eukaryotic initiation factor 2 (eIF2α). In addition, pPKR acts as an upstream kinase for c-Jun N-terminal kinase and regulates the levels of multiple mitotic factors such as CYCLINS A and B and POLO-LIKE KINASE 1 and phosphorylation of HISTONE H3. Disruption of PKR activation via RNAi or expression of a transdominant-negative mutant leads to misregulation of the mitotic factors, delay in mitotic progression, and defects in cytokinesis. Our study unveils a novel function of PKR and endogenous dsRNAs as signaling molecules during the mitosis of uninfected cells.

Translational control of mGluR-dependent long-term depression and object-place learning by eIF2α

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.

mTOR complex 1: a key player in neuroadaptations induced by drugs of abuse

The mammalian (or mechanistic) target of rapamycin (mTOR) complex 1 (mTORC1) is a serine and threonine kinase that regulates cell growth, survival, and proliferation. mTORC1 is a master controller of the translation of a subset of mRNAs. In the central nervous system mTORC1 plays a crucial role in mechanisms underlying learning and memory by controlling synaptic protein synthesis. Here, we review recent evidence suggesting that the mTORC1 signaling pathway promotes neuroadaptations following exposure to a diverse group of drugs of abuse including stimulants, cannabinoids, opiates, and alcohol. We further describe potential molecular mechanisms by which drug-induced mTORC1 activation may alter brain functions. Finally, we propose that mTORC1 is a focal point shared by drugs of abuse to mediate drug-related behaviors such as reward seeking and excessive drug intake, and offer future directions to decipher the contribution of the kinase to mechanisms underlying addiction. Recent studies suggesting that exposure to diverse classes of drugs of abuse as well as exposure to drug-associated memories lead to mTORC1 kinase activation in the limbic system. In turn, mTORC1 controls the onset and the maintenance of pathological neuroadaptions that underlie several features of drug addiction such as drug seeking and relapse. Therefore, we propose that targeting mTORC1 and its effectors is a promising strategy to treat drug disorders.

Genes and signaling pathways involved in memory enhancement in mutant mice

Mutant mice have been used successfully as a tool for investigating the mechanisms of memory at multiple levels, from genes to behavior. In most cases, manipulating a gene expressed in the brain impairs cognitive functions such as memory and their underlying cellular mechanisms, including synaptic plasticity. However, a remarkable number of mutations have been shown to enhance memory in mice. Understanding how to improve a system provides valuable insights into how the system works under normal conditions, because this involves understanding what the crucial components are. Therefore, more can be learned about the basic mechanisms of memory by studying mutant mice with enhanced memory. This review will summarize the genes and signaling pathways that are altered in the mutants with enhanced memory, as well as their roles in synaptic plasticity. Finally, I will discuss how knowledge of memory-enhancing mechanisms could be used to develop treatments for cognitive disorders associated with impaired plasticity.

Oxidative stress-associated protein tyrosine kinases and phosphatases in Fanconi anemia

SIGNIFICANCE

Fanconi anemia (FA) is a genetic disorder featuring chromosomal instability, developmental defects, progressive bone marrow failure, and predisposition to cancer. Besides the predominant role in DNA damage response and/or repair, many studies have linked FA proteins to oxidative stress. Oxidative stress, defined as imbalance in pro-oxidant and antioxidant homeostasis, has been considered to contribute to disease development, including FA.

RECENT ADVANCES

A variety of signaling pathways may be influenced by oxidative stress, particularly the equilibrium between protein kinases and phosphatases, consequently leading to an aberrant phosphorylation state of cellular proteins. Dysfunction of kinases/phosphatases has been implicated in the pathophysiology of human diseases. In FA, evidence is emerging that links abnormal phosphorylation/de-phosphorylation of signaling molecules to clinical complications and malformations.

CRITICAL ISSUES

In this study, we review the recent findings on the oxidative stress-related kinases and phosphatases, particularly tyrosine phosphatases in FA.

FUTURE DIRECTIONS

Understanding the role of oxidative stress-related kinases and phosphatases in FA may provide unique and generic possibilities for the future development of therapeutic strategies by targeting the dysregulated protein kinases and phosphatases in a clinical setting.

Small molecule kinase inhibitors alleviate different molecular features of myotonic dystrophy type 1

Expandable (CTG)n repeats in the 3′ UTR of the DMPK gene are a cause of myotonic dystrophy type 1 (DM1), which leads to a toxic RNA gain-of-function disease. Mutant RNAs with expanded CUG repeats are retained in the nucleus and aggregate in discrete inclusions. These foci sequester splicing factors of the MBNL family and trigger upregulation of the CUGBP family of proteins resulting in the mis-splicing of their target transcripts. To date, many efforts to develop novel therapeutic strategies have been focused on disrupting the toxic nuclear foci and correcting aberrant alternative splicing via targeting mutant CUG repeats RNA; however, no effective treatment for DM1 is currently available. Herein, we present results of culturing of human DM1 myoblasts and fibroblasts with two small-molecule ATP-binding site-specific kinase inhibitors, C16 and C51, which resulted in the alleviation of the dominant-negative effects of CUG repeat expansion. Reversal of the DM1 molecular phenotype includes a reduction of the size and number of foci containing expanded CUG repeat transcripts, decreased steady-state levels of CUGBP1 protein, and consequent improvement of the aberrant alternative splicing of several pre-mRNAs misregulated in DM1.

The eIF2α kinase PERK limits the expression of hippocampal metabotropic glutamate receptor-dependent long-term depression

The proper regulation of translation is required for the expression of long-lasting synaptic plasticity. A major site of translational control involves the phosphorylation of eukaryotic initiation factor 2 α (eIF2α) by PKR-like endoplasmic reticulum (ER) kinase (PERK). To determine the role of PERK in hippocampal synaptic plasticity, we used the Cre-lox expression system to selectively disrupt PERK expression in the adult mouse forebrain. Here, we demonstrate that in hippocampal area CA1, metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) is associated with increased eIF2α phosphorylation, whereas stimulation of early- and late-phase long-term potentiation (E-LTP and L-LTP, respectively) is associated with decreased eIF2α phosphorylation. Interesting, although PERK-deficient mice exhibit exaggerated mGluR-LTD, both E-LTP and L-LTP remained intact. We also found that mGluR-LTD is associated with a PERK-dependent increase in eIF2α phosphorylation. Our findings are consistent with the notion that eIF2α phosphorylation is a key site for the bidirectional control of persistent forms of synaptic LTP and LTD and suggest a distinct role for PERK in mGluR-LTD.