Lentivirus-mediated chronic expression of dominant-negative CREB in the dorsal hippocampus impairs memory for place learning and contextual fear conditioning

Memory strength gates the involvement of a CREB-dependent cortical fear engram in remote memory

Encoding and retrieval of contextual memories is initially mediated by sparsely activated neurons, so-called engram cells, in the hippocampus. Subsequent memory persistence is thought to depend on network-wide changes involving progressive contribution of cortical regions, a process referred to as systems consolidation. Using a viral-based TRAP (targeted recombination in activated populations) approach, we studied whether consolidation of contextual fear memory by neurons in the medial prefrontal cortex (mPFC) is modulated by memory strength and CREB function. We demonstrate that activity of a small subset of mPFC neurons is sufficient and necessary for remote memory expression, but their involvement depends on the strength of conditioning. Furthermore, selective disruption of CREB function in mPFC engram cells after mild conditioning impairs remote memory expression. Together, our data demonstrate that memory consolidation by mPFC engram cells requires CREB-mediated transcription, with the functionality of this network hub being gated by memory strength.

Little is known about mechanisms that regulate the involvement of cortical engram cells in remote memory. Here, authors demonstrate that memory consolidation by mPFC engram cells requires CREB-mediated transcription, with the functionality of this network hub being gated by memory strength.

Role of hippocampal activity-induced transcription in memory consolidation

Experience-dependent changes in the strength of connections between neurons in the hippocampus (HPC) are critical for normal learning and memory consolidation, and disruption of this process drives a variety of neurological and psychiatric diseases. Proper HPC function relies upon discrete changes in gene expression driven by transcription factors (TFs) induced by neuronal activity. Here, we describe the induction and function of many of the most well-studied HPC TFs, including cyclic-AMP response element binding protein, serum-response factor, AP-1, and others, and describe their role in the learning process. We also discuss the known target genes of many of these TFs and the purported mechanisms by which they regulate long-term changes in HPC synaptic strength. Moreover, we propose that future research in this field will depend upon unbiased identification of additional gene targets for these activity-dependent TFs and subsequent meta-analyses that identify common genes or pathways regulated by multiple TFs in the HPC during learning or disease.

HIPP neurons in the dentate gyrus mediate the cholinergic modulation of background context memory salience

Cholinergic neuromodulation in the hippocampus controls the salience of background context memory acquired in the presence of elemental stimuli predicting an aversive reinforcement. With pharmacogenetic inhibition we here demonstrate that hilar perforant path-associated (HIPP) cells of the dentate gyrus mediate the devaluation of background context memory during Pavlovian fear conditioning. The salience adjustment is sensitive to reduction of hilar neuropeptide Y (NPY) expression via dominant negative CREB expression in HIPP cells and to acute blockage of NPY-Y1 receptors in the dentate gyrus during conditioning. We show that NPY transmission and HIPP cell activity contribute to inhibitory effects of acetylcholine in the dentate gyrus and that M1 muscarinic receptors mediate the cholinergic activation of HIPP cells as well as their control of background context salience. Our data provide evidence for a peptidergic local circuit in the dentate gyrus that mediates the cholinergic encoding of background context salience during fear memory acquisition.

Intra-hippocampal circuits are essential for associating a background context with behaviorally salient stimuli and involve cholinergic modulation at SST⁺ interneurons. Here the authors show that the salience of the background context memory is modulated through muscarinic activation of NPY⁺ hilar perforant path associated interneurons and NPY signaling in the dentate gyrus.

Predator odor evokes sex-independent stress responses in male and female Wistar rats and reduces phosphorylation of cyclic-adenosine monophosphate response element binding protein in the male, but not the female hippocampus

Post-traumatic stress disorder (PTSD) is characterized by memory disturbances following trauma. Acute predator threat has emerged as an ethological model of PTSD, yet the effects of predator odor on signaling cascades associated with long-term memory remain poorly understood. In this study, we exposed male and female Wistar rats to the synthetic predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) to assess behavioral and physiological responses as well as rapid modulation of signal transduction cascades associated with learning and memory in the male and female hippocampus. During exposure to TMT in the homecage, both male and female animals displayed robust immobility, avoidance, and altered activity as a function of time. Physiologically, TMT exposure increased circulating corticosterone and blood glucose in both male and female rodents, suggesting that TMT evokes sex-independent behavioral and physiological responses. With respect to signal transduction, TMT exposure rapidly reduced phosphorylation of cyclic-adenosine monophosphate response element binding protein (CREB) in the male, but not the female hippocampus. Furthermore, TMT exposure reduced phosphorylation of extracellular signal-regulated kinase 1/2 and increased nuclear expression of the synapto-nuclear messenger protein Jacob in the male hippocampus, consistent with activation of the CREB shut-off pathway. In a follow-up behavioral experiment, post-training exposure to TMT did not affect spatial water maze performance of male rats. However, male rats re-introduced to the context in which TMT had previously been presented displayed avoidance and hyperactivity, but not freezing behavior or elevated corticosterone responses, suggesting that TMT exposure supports a form of contextual conditioning which is not characterized by immobility. Taken together, our findings suggest that TMT evokes similar behavioral and physiological responses in male and female Wistar rats, but affects distinct signaling cascades in the male and female hippocampus which may contribute to behavioral disruptions associated with predator exposure.

Intrahippocampal administration of a domain antibody that binds aggregated amyloid-β reverses cognitive deficits produced by diet-induced obesity

BACKGROUND

The prevalence of high fat diets (HFD), diet-induced obesity (DIO) and Type 2 diabetes continues to increase, associated with cognitive impairment in both humans and rodent models. Mechanisms transducing these impairments remain largely unknown: one possibility is that a common mechanism may be involved in the cognitive impairment seen in obese and/or diabetic states and in dementia, specifically Alzheimer's disease (AD). DIO is well established as a risk factor for development of AD. Oligomeric amyloid-β (Aβ) is neurotoxic, and we showed that intrahippocampal oligomeric Aβ produces cognitive and metabolic dysfunction similar to that seen in DIO or diabetes. Moreover, animal models of DIO show elevated brain Aβ, a hallmark of AD, suggesting that this may be one source of cognitive impairment in both conditions.

METHODS

Intrahippocampal administration of a novel anti-Aβ domain antibody for aggregated Aβ, or a control domain antibody, to control or HFD-induced DIO rats. Spatial learning measured in a conditioned contextual fear (CCF) task after domain antibody treatment; postmortem, hippocampal NMDAR and AMPAR were measured.

RESULTS

DIO caused impairment in CCF, and this impairment was eliminated by intrahippocampal administration of the active domain antibody. Measurement of hippocampal proteins suggests that DIO causes dysregulation of hippocampal AMPA receptors, which is also reversed by acute domain antibody administration.

CONCLUSIONS

Our findings support the concept that oligomeric Aβ within the hippocampus of DIO animals may not only be a risk factor for development of AD but may also cause cognitive impairment before the development of dementia.

GENERAL SIGNIFICANCE AND INTEREST

Our work integrates the engineering of domain antibodies with conformational- and sequence-specificity for oligomeric amyloid beta with a clinically relevant model of diet-induced obesity in order to demonstrate not only the pervasive effects of obesity on several aspects of brain biochemistry and behavior, but also the bioengineering of a successful treatment against the long-term detrimental effects of a pre-diabetic state on the brain. We show for the first time that cognitive impairment linked to obesity and/or insulin resistance may be due to early accumulation of oligomeric beta-amyloid in the brain, and hence may represent a pre-Alzheimer's state.

Protein Phosphatase-1 Inhibitor-2 Is a Novel Memory Suppressor

Reversible phosphorylation, a fundamental regulatory mechanism required for many biological processes including memory formation, is coordinated by the opposing actions of protein kinases and phosphatases. Type I protein phosphatase (PP1), in particular, has been shown to constrain learning and memory formation. However, how PP1 might be regulated in memory is still not clear. Our previous work has elucidated that PP1 inhibitor-2 (I-2) is an endogenous regulator of PP1 in hippocampal and cortical neurons (Hou et al., 2013). Contrary to expectation, our studies of contextual fear conditioning and novel object recognition in I-2 heterozygous mice suggest that I-2 is a memory suppressor. In addition, lentiviral knock-down of I-2 in the rat dorsal hippocampus facilitated memory for tasks dependent on the hippocampus. Our data indicate that I-2 suppresses memory formation, probably via negatively regulating the phosphorylation of cAMP/calcium response element-binding protein (CREB) at serine 133 and CREB-mediated gene expression in dorsal hippocampus. Surprisingly, the data from both biochemical and behavioral studies suggest that I-2, despite its assumed action as a PP1 inhibitor, is a positive regulator of PP1 function in memory formation.

SIGNIFICANCE STATEMENT

We found that inhibitor-2 acts as a memory suppressor through its positive functional influence on type I protein phosphatase (PP1), likely resulting in negative regulation of cAMP/calcium response element-binding protein (CREB) and CREB-activated gene expression. Our studies thus provide an interesting example of a molecule with an in vivo function that is opposite to its in vitro function. PP1 plays critical roles in many essential physiological functions such as cell mitosis and glucose metabolism in addition to its known role in memory formation. PP1 pharmacological inhibitors would thus not be able to serve as good therapeutic reagents because of its many targets. However, identification of PP1 inhibitor-2 as a critical contributor to suppression of memory formation by PP1 may provide a novel therapeutic target for memory-related diseases.

Towards a better understanding of cognitive behaviors regulated by gene expression downstream of activity-dependent transcription factors

In the field of molecular and cellular neuroscience, it is not a trivial task to see the forest for the trees, where numerous, and seemingly independent, molecules often work in concert to control critical steps of synaptic plasticity and signalling. Here, we will first summarize our current knowledge on essential activity-dependent transcription factors (TFs) such as CREB, MEF2, Npas4 and SRF, then examine how various transcription cofactors (TcoFs) also contribute to defining the transcriptional outputs during learning and memory. This review finally attempts a provisory synthesis that sheds new light on some of the emerging principles of neuronal circuit dynamics driven by activity-regulated gene transcription to help better understand the intricate relationship between activity-dependent gene expression and cognitive behavior.

Standardised extract of Bacopa monniera (CDRI-08) improves contextual fear memory by differentially regulating the activity of histone acetylation and protein phosphatases (PP1α, PP2A) in hippocampus

Contextual fear conditioning is a paradigm for investigating cellular mechanisms involved in hippocampus-dependent memory. Earlier, we showed that standardised extract of Bacopa monniera (CDRI-08) improves hippocampus-dependent learning in postnatal rats by elevating the level of serotonin (5-hydroxytryptamine, 5-HT), activate 5-HT3A receptors, and cyclic adenosine monophosphate (cAMP) response element binding (CREB) protein. In this study, we have further examined the molecular mechanism of CDRI-08 in hippocampus-dependent memory and compared to the histone deacetylase (HDACs) inhibitor sodium butyrate (NaB). To assess the hippocampus-dependent memory, wistar rat pups were subjected to contextual fear conditioning (CFC) following daily (postnatal days 15-29) administration of vehicle solution (0.5 % gum acacia + 0.9 % saline)/CDRI-08 (80 mg/kg, p.o.)/NaB (1.2 g/kg in PBS, i.p.). CDRI-08/NaB treated group showed enhanced freezing behavior compared to control group when re-exposed to the same context. Administration of CDRI-08/NaB resulted in activation of extracellular signal-regulated kinase ERK/CREB signaling cascade and up-regulation of p300, Ac-H3 and Ac-H4 levels, and down-regulation of HDACs (1, 2) and protein phosphatases (PP1α, PP2A) in hippocampus following CFC. This would subsequently result in an increased brain-derived neurotrophic factor (Bdnf) (exon IV) mRNA in hippocampus. Altogether, our results indicate that CDRI-08 enhances hippocampus-dependent contextual memory by differentially regulating histone acetylation and protein phosphatases in hippocampus.

Factors affecting graded and ungraded memory loss following hippocampal lesions

This review evaluates three current theories--Standard Consolidation (Squire & Wixted, 2011), Overshadowing (Sutherland, Sparks, & Lehmann, 2010), and Multiple Trace-Transformation (Winocur, Moscovitch, & Bontempi, 2010)--in terms of their ability to account for the role of the hippocampus in recent and remote memory in animals. Evidence, based on consistent findings from tests of spatial memory and memory for acquired food preferences, favours the transformation account, but this conclusion is undermined by inconsistent results from studies that measured contextual fear memory, probably the most commonly used test of hippocampal involvement in anterograde and retrograde memory. Resolution of this issue may depend on exercising greater control over critical factors (e.g., contextual environment, amount of pre-exposure to the conditioning chamber, the number and distribution of foot-shocks) that can affect the representation of the memory shortly after learning and over the long-term. Research strategies aimed at characterizing the neural basis of long-term consolidation/transformation, as well as other outstanding issues are discussed.