Expression of Quinone Reductase-2 in the Cortex Is a Muscarinic Acetylcholine Receptor-Dependent Memory Consolidation Constraint.

Learning of novel information, including novel taste, requires activation of neuromodulatory transmission mediated, for example, by the muscarinic acetylcholine receptors (mAChRs) in relevant brain structures. In addition, drugs enhancing the function of mAChRs are used to treat memory impairment and decline. However, the mechanisms underlying these effects are poorly understood. Here, using quantitative RT-PCR in Wistar Hola rats, we found quinone reductase 2 (QR2) to be expressed in the cortex in an mAChR-dependent manner. QR2 mRNA expression in the insular cortex is inversely correlated with mAChR activation both endogenously, after novel taste learning, and exogenously, after pharmacological manipulation of the muscarinic transmission. Moreover, reducing QR2 expression levels through lentiviral shRNA vectors or activity via inhibitors is sufficient to enhance long-term memories. We also show here that, in patients with Alzheimer's disease, QR2 is overexpressed in the cortex. It is suggested that QR2 expression in the cortex is a removable limiting factor of memory formation and thus serves as a new target to enhance cognitive function and delay the onset of neurodegenerative diseases. SIGNIFICANCE STATEMENT: We found that: (1) quinone reductase 2 (QR2) expression is a muscarinic-receptor-dependent removable constraint on memory formation in the cortex, (2) reducing QR2 expression or activity in the cortex enhances memory formation, and (3) Alzheimer's disease patients overexpressed QR2. We believe that these results propose a new mechanism by which muscarinic acetylcholine receptors affect cognition and suggest that inhibition of QR2 is a way to enhance cognition in normal and pathological conditions.

mAChR-dependent decrease in proteasome activity in the gustatory cortex is necessary for novel taste learning.

Regulation of protein degradation via the ubiquitin proteasome system is crucial for normal learning and synaptic plasticity processes. While some studies reveal that increased proteasome degradation is necessary for different types of learning, others suggest the proteasome to be a negative regulator of plasticity. We aim to understand the molecular and cellular processes taking place in the gustatory cortex (GC), which underlie appetitive and aversive forms of taste learning. Previously, we have shown that N-methyl d-aspartic acid receptor (NMDAR)-dependent upregulation of proteasome activity 4h after novel taste learning is necessary for the association of novel taste with malaise and formation of conditioned taste aversion (CTA). Here, we first identify a correlative increase in proteasome activity in the GC immediately after novel taste learning and study the upstream and downstream effectors of this modulated proteasome activity. Interestingly, proteasome-mediated degradation was reduced in the GC, 20min after novel taste consumption in a muscarinic acetylcholine receptor (mAChR)-dependent and NMDAR-independent manner. This reduction in protein degradation led to an increased amount of p70 S6 kinase (p70S6k), which was abolished in the presence of mAChR antagonist scopolamine. Infusion of lactacystin, a proteasome inhibitor, to the GC precluded the amnestic effect of scopolamine. This study shows for the first time that following novel taste learning there is a cortical, mAChR-dependent reduced proteasome activity that enables the memory of taste familiarity. Moreover, inhibition of degradation in the GC attenuates novel taste learning and of p70 S6 kinase correlative increased expression. These results shed light on the complex regulation of protein synthesis and degradation machineries in the cortex following novel taste experience.

NMDAR-dependent proteasome activity in the gustatory cortex is necessary for conditioned taste aversion.

Taste information is processed in different brain structures in the mammalian brain, including the gustatory cortex (GC), which resides within the insular cortex. N-methyl-d-aspartate receptor (NMDAR) activity in the GC is necessary for the acquisition of conditioned taste aversion (CTA) but not positive novel taste learning. Previous studies have shown that taste memory consolidation requires intact protein synthesis in the GC. In addition, the direct involvement of translation initiation and elongation factors was documented in the GC during taste learning. However, protein expression is defined by protein synthesis, degradation, and localization. Protein degradation is critical for the consolidation and reconsolidation of other forms of learning, such as fear learning and addiction behavior, but its role in cortical-dependent learning is not clear. Here, we show for the first time that proteasome activity is specifically increased in the GC 4h following experiencing of a novel taste. This increase in proteasome activity was abolished by local administration to the GC of the NMDA antagonist, APV, as well as a CaMKII inhibitor, at the time of acquisition. In addition, local application of lactacystin, a proteasome inhibitor, resulted in impaired CTA, but not novel taste learning. These results suggest that NMDAR-dependent proteasome activity in the GC participates in the association process between novel taste experience and negative visceral sensation.

Taste familiarity is inversely correlated with Arc/Arg3.1 hemispheric lateralization.

Biochemical, electrophysiological, and imaging studies suggest that the anterior part of the insular cortex (IC) serves as primary taste cortex, whereas fMRI studies in human propose that the anterior IC is also involved in processing of general novelty or saliency information. Here, we compared activity regulated cytoskeleton associated protein (Arc)/Arg3.1 protein levels in the rat IC following administration of familiar versus novel tastes. Surprisingly, there was no correlation between novel taste and Arc/Arg3.1 levels when measured as the sum of both left and right insular cortices. However, when left and right IC were examined separately, Arc/Arg3.1 level was lateralized following novel taste learning. Moreover, Arc/Arg3.1 lateralization was inversely correlated with taste familiarity, whereas the high lateralization of Arc/Arg3.1 expression observed following novel taste learning is reduced proportionally to the increment in taste familiarity. In addition, unilateral inhibition of protein synthesis in the IC had asymmetrical effect on memory, inducing strong memory impairment similarly to bilateral inhibition or memory preservation, indicating that hemispheric lateralization is central for processing taste saliency information. These results provide indications, at the gene level of expression, for the role of IC lateralization in processing novel taste information and for the asymmetrical contribution of protein synthesis in each hemisphere during memory consolidation.

Specific quinone reductase 2 inhibitors reduce metabolic burden and reverse Alzheimer's disease phenotype in mice.

Biological aging can be described as accumulative, prolonged metabolic stress and is the major risk factor for cognitive decline and Alzheimer's disease (AD). Recently, we identified and described a quinone reductase 2 (QR2) pathway in the brain, in which QR2 acts as a removable memory constraint and metabolic buffer within neurons. QR2 becomes overexpressed with age, and it is possibly a novel contributing factor to age-related metabolic stress and cognitive deficit. We found that, in human cells, genetic removal of QR2 produced a shift in the proteome opposing that found in AD brains while simultaneously reducing oxidative stress. We therefore created highly specific QR2 inhibitors (QR2is) to enable evaluation of chronic QR2 inhibition as a means to reduce biological age-related metabolic stress and cognitive decline. QR2is replicated results obtained by genetic removal of QR2, while local QR2i microinjection improved hippocampal and cortical-dependent learning in rats and mice. Continuous consumption of QR2is in drinking water improved cognition and reduced pathology in the brains of AD-model mice (5xFAD), with a noticeable between-sex effect on treatment duration. These results demonstrate the importance of QR2 activity and pathway function in the healthy and neurodegenerative brain and what we believe to be the great therapeutic potential of QR2is as first-in-class drugs.

Biphasic activation of the mTOR pathway in the gustatory cortex is correlated with and necessary for taste learning.

Different forms of memories and synaptic plasticity require synthesis of new proteins at the time of acquisition or immediately after. We are interested in the role of translation regulation in the cortex, the brain structure assumed to store long-term memories. The mammalian target of rapamycin, mTOR (also known as FRAP and RAFT-1), is part of a key signal transduction mechanism known to regulate translation of specific subset of mRNAs and to affect learning and synaptic plasticity. We report here that novel taste learning induces two waves of mTOR activation in the gustatory cortex. Interestingly, the first wave can be identified both in synaptoneurosomal and cellular fractions, whereas the second wave is detected in the cellular fraction but not in the synaptic one. Inhibition of mTOR, specifically in the gustatory cortex, has two effects. First, biochemically, it modulates several known downstream proteins that control translation and reduces the expression of postsynaptic density-95 in vivo. Second, behaviorally, it attenuates long-term taste memory. The results suggest that the mTOR pathway in the cortex modulates both translation factor activity and protein expression, to enable normal taste memory consolidation.

Tyrosine phosphorylation of the 2B subunit of the NMDA receptor is necessary for taste memory formation.

We aimed to test whether tyrosine phosphorylation of the NMDA receptor (NMDAR) in the insular cortex is necessary for novel taste learning. We found that in rats, novel taste learning leads to elevated phosphorylation of tyrosine 1472 of the NR2B subunit of the NMDAR and increases the interaction of phosphorylated NR2B with the major postsynaptic scaffold protein PSD-95. Injection of the tyrosine kinase inhibitor genistein directly into the insular cortex of rats before novel taste exposure prevented the increase in NR2B tyrosine phosphorylation and behaviorally attenuated taste-memory formation. Functionally, tyrosine phosphorylation of NR2B after learning was found to determine the synaptic distribution of the NMDAR, since microinjection of genistein to the insular cortex altered the distribution pattern of NMDAR caused by novel taste learning.

Muscarinic-Dependent miR-182 and QR2 Expression Regulation in the Anterior Insula Enables Novel Taste Learning.

In a similar manner to other learning paradigms, intact muscarinic acetylcholine receptor (mAChR) neurotransmission or protein synthesis regulation in the anterior insular cortex (aIC) is necessary for appetitive taste learning. Here we describe a parallel local molecular pathway, where GABAA receptor control of mAChR activation causes upregulation of miRNA-182 and quinone reductase 2 (QR2) mRNA destabilization in the rodent aIC. Damage to long-term memory by prevention of this process, with the use of mAChR antagonist scopolamine before novel taste learning, can be rescued by local QR2 inhibition, demonstrating that QR2 acts downstream of local muscarinic activation. Furthermore, we prove for the first time the presence of endogenous QR2 cofactors in the brain, establishing QR2 as a functional reductase there. In turn, we show that QR2 activity causes the generation of reactive oxygen species, leading to modulation in Kv2.1 redox state. QR2 expression reduction therefore is a previously unaccounted mode of mAChR-mediated inflammation reduction, and thus adds QR2 to the cadre of redox modulators in the brain. The concomitant reduction in QR2 activity during memory consolidation suggests a complementary mechanism to the well established molecular processes of this phase, by which the cortex gleans important information from general sensory stimuli. This places QR2 as a promising new target to tackle neurodegenerative inflammation and the associated impediment of novel memory formation in diseases such as Alzheimer's disease.

Fluid consumption and taste novelty determines transcription temporal dynamics in the gustatory cortex.

BACKGROUND: Novel taste memories, critical for animal survival, are consolidated to form long term memories which are dependent on translation regulation in the gustatory cortex (GC) hours following acquisition. However, the role of transcription regulation in the process is unknown. RESULTS: Here, we report that transcription in the GC is necessary for taste learning in rats, and that drinking and its consequences, as well as the novel taste experience, affect transcription in the GC during taste memory consolidation. We show differential effects of learning on temporal dynamics in set of genes in the GC, including Arc/Arg3.1, known to regulate the homeostasis of excitatory synapses. CONCLUSIONS: We demonstrate that in taste learning, transcription programs were activated following the physiological responses (i.e., fluid consumption following a water restriction regime, reward, arousal of the animal, etc.) and the specific information about a given taste (i.e., taste novelty). Moreover, the cortical differential prolonged kinetics of mRNA following novel versus familiar taste learning may represent additional novelty related molecular response, where not only the total amount, but also the temporal dynamics of transcription is modulated by sensory experience of novel information.

The differential role of cortical protein synthesis in taste memory formation and persistence.

The current dogma suggests that the formation of long-term memory (LTM) is dependent on protein synthesis but persistence of the memory trace is not. However, many of the studies examining the effect of protein synthesis inhibitors (PSIs) on LTM persistence were performed in the hippocampus, which is known to have a time-dependent role in memory storage, rather than the cortex, which is considered to be the main structure to store long-term memories. Here we studied the effect of PSIs on LTM formation and persistence in male Wistar Hola (n ≥ 5) rats by infusing the protein synthesis inhibitor, anisomycin (100 µg, 1 µl), into the gustatory cortex (GC) during LTM formation and persistence in conditioned taste aversion (CTA). We found that local anisomycin infusion to the GC before memory acquisition impaired LTM formation (P = 8.9E - 5), but had no effect on LTM persistence when infused 3 days post acquisition (P = 0.94). However, when we extended the time interval between treatment with anisomycin and testing from 3 days to 14 days, LTM persistence was enhanced (P = 0.01). The enhancement was on the background of stable and non-declining memory, and was not recapitulated by another amnesic agent, APV (10 µg, 1 µl), an N-methyl-d-aspartate receptor antagonist (P = 0.54). In conclusion, CTA LTM remains sensitive to the action of PSIs in the GC even 3 days following memory acquisition. This sensitivity is differentially expressed between the formation and persistence of LTM, suggesting that increased cortical protein synthesis promotes LTM formation, whereas decreased protein synthesis promotes LTM persistence.

ERK-dependent PSD-95 induction in the gustatory cortex is necessary for taste learning, but not retrieval.

The processes underlying long-term memory formation in the neocortex are poorly understood. Using taste learning, we found learning-related induction of PSD-95 in the gustatory cortex, which was temporally restricted, coupled to the learning of a novel, but not familiar, taste and controlled by ERK. Using temporally and spatially restricted RNA interference knockdown of PSD-95 in vivo, we found that PSD-95 induction is necessary for learning novel tastes, but not for the recollection of familiar ones.

Juvenile stress-induced alteration of maturation of the GABAA receptor alpha subunit in the rat.

Profound evidence indicates that GABAA receptors are important in the control of physiological response to stress and anxiety. The alpha subunit type composition contributes significantly to the functional characterization of the GABAA receptors. The alpha2, alpha3, alpha5 subunits are predominately expressed in the brain during embryonic and early postnatal periods of normal rats, whilst alpha1 are most prominent during later developmental stages. In the present study, we examined the long-term effects of juvenile stress on GABA alpha subunit expression in adulthood in the amygdala and hippocampus. We applied the elevated platform stress paradigm at juvenility and used the open-field and startle response tests to assess anxiety level in adulthood. Juvenile stress effects without behavioural tests in adulthood were also examined since previous studies indicated that the mere exposure to these tests might be stressful for rats, enhancing the effects of the juvenile exposure to stress. In adulthood, we quantitatively determined the level of expression of alpha1, alpha2 and alpha3 in the hippocampus and amygdala. Our results indicate that subjecting juvenile stressed rats to additional challenges in adulthood results in an immature-like expression profile of these subunits. To test for potential functional implications of these alterations we examined the effects of the anxiolytic (diazepam) and the sedative (brotizolam) benzodiazepines on juvenile stressed and control rats following additional challenges in adulthood. Juvenile stressed rats were more sensitive to diazepam and less sensitive to brotizolam, suggesting that the alterations in GABA alpha subunit expression in these animals have functional consequences.

A role for eukaryotic translation initiation factor 2B (eIF2B) in taste memory consolidation and in thermal control establishment during the critical period for sensory development.

All species exhibit critical periods for sensory development, yet very little is known about the molecules involved in the changes in the network wiring that underlies this process. Here the role of transcription regulation of the translation machinery was determined by evaluating the expression of eIF2Bepsilon, an essential component of translation initiation, in both taste-preference development and thermal control establishment in chicks. Analysis of the expression pattern of this gene after passive-avoidance training revealed clear induction of eIF2Bepsilon in both the mesopallium intermediomediale (IMM) and in the striatum mediale (StM). In addition, a correlation was found between the concentration of methylanthranilate (MeA), which was the malaise substrate in the passive-avoidance training procedure, the duration of memory, and the expression level of eIF2Bepsilon. Training chicks on a low concentration of MeA induced short-term memory and low expression level of eIF2Bepsilon, whereas a high concentration of MeA induced long-term memory and a high expression level of eIF2Bepsilon in both the IMM and StM. Furthermore, eIF2Bepsilon-antisense "knock-down" not only reduced the amount of eIF2Bepsilon but also attenuated taste memory formation. In order to determine whether induction of eIF2Bepsilon is a general feature of neuronal plasticity, we checked whether it was induced in other forms of neuronal plasticity, with particular attention to its role in temperature control establishment, which represents hypothalamic-related plasticity. It was established that eIF2Bepsilon-mRNA was induced in the preopotic anterior hypothalamus during heat conditioning. Taken together, these results correlate eIF2Bepsilon with sensory development.

Behavioral interference and C/EBPbeta expression in the insular-cortex reveal a prolonged time period for taste memory consolidation.

Memory consolidation is defined as the time window during which the memory trace is susceptible to behavioral, electrical, or pharmacological interventions. Here, we presented rats with two novel tastes at consecutive time intervals. Clear interference was evident when a novel taste formed the second taste input whereby, surprisingly, the time window for interference was found to last more than 10 h. In addition, we detected an increase of C/EBPbeta protein expression in the gustatory cortex 18 h after novel taste learning. This modulation was attenuated by a subsequent novel taste. Our findings reveal temporal constraints and a lingering nature of taste memory consolidation.

Different signal transduction cascades are activated simultaneously in the rat insular cortex and hippocampus following novel taste learning.

Novel taste learning is a robust one-trial incidental learning process, dependent on functional activity of the insular (taste) cortex. In contrast to that of the cortex, the role of the hippocampus in taste learning is controversial. We set out to identify the time courses of the activation of mitogen-associated protein kinase (MAPK), transcription factor cAMP-response element-binding protein (CREB) and Akt/PKB (protein kinase B) in the insular cortex and hippocampus of rats subsequent to novel taste learning. Following taste learning, an early response (20 min) occurred at the same time in the insular cortex and the hippocampus. However, whereas MAPK was activated specifically in the insular cortex, CREB and Akt were phosphorylated in the hippocampus but not in the cortex. In addition, the immediate early gene, CCAAT/enhancer binding protein (C/EBPbeta) was induced in both the hippocampus and the insular cortex 18 h following taste learning. The results demonstrate, for the first time, correlative activation and gene expression in the hippocampus following novel taste learning. Moreover, the results suggest that different signal transduction cascades necessary for taste learning are activated in concert in different brain structures, to enable taste learning and consolidation.