Detection of histone acetylation levels in the dorsal hippocampus reveals early tagging on specific residues of H2B and H4 histones in response to learning.

The recent literature provides evidence that epigenetic mechanisms such as DNA methylation and histone modification are crucial to gene transcription linked to synaptic plasticity in the mammalian brain--notably in the hippocampus--and memory formation. We measured global histone acetylation levels in the rat hippocampus at an early stage of spatial or fear memory formation. We found that H3, H4 and H2B underwent differential acetylation at specific sites depending on whether rats had been exposed to the context of a task without having to learn or had to learn about a place or fear therein: H3K9K14 acetylation was mostly responsive to any experimental conditions compared to naive animals, whereas H2B N-terminus and H4K12 acetylations were mostly associated with memory for either spatial or fear learning. Altogether, these data suggest that behavior/experience-dependent changes differently regulate specific acetylation modifications of histones in the hippocampus, depending on whether a memory trace is established or not: tagging of H3K9K14 could be associated with perception/processing of testing-related manipulations and context, thereby enhancing chromatin accessibility, while tagging of H2B N-terminus tail and H4K12 could be more closely associated with the formation of memories requiring an engagement of the hippocampus.

Introduction

As a result of dynamic interactions between environmental constraints and an organism’s genome, synaptic plasticity and formation of enduring memories require modulations of gene transcription (expression, repression) at critical periods following learning [1], [2], [3]. Such changes implicate in part chromatin structure modifications catalyzed by epigenetic mechanisms, among which histone acetylation appears to be one of the critical processes [4]. Among the 5 core histones, studies investigating global changes in histone acetylation levels in the hippocampus of rodents after learning have mainly focused on H3 and H4. A few examples are rodents subjected to either fear conditioning [5], [6], [7], subsequent extinction [8], [9], object recognition [10], [11], [12], or place learning in the Morris water maze [13] (for reviews [14], [15]). However, a series of indirect evidence suggests that H2B histone could be an additional target for regulations involved in memory formation and consolidation processes. Indeed, HDAC2 knock-out mice have recently been shown to display improved memory functions, and increased acetylation levels of H2B (among others) were measured in their hippocampus [16]. Genetic inhibition of protein phosphatase 1 (PP1) in the mouse brain, previously shown to produce animals with prolonged vividness of a spatial memory [17], also presented increased H2B acetylation in the hippocampus [11]. A recent paper described that depolarization of hippocampal slices maintained in vitro induced H2BK5K12K15K20 acetylation within minutes [18], suggesting that the tetra acetylation of H2B could mediate activity-dependent signaling in the hippocampus. Finally, our recent work showed that acetylations of H2B histones on its N-terminus were dynamically regulated during the consolidation of a spatial memory: tetra acetylated H2B was increased in the dorsal hippocampus of rats having learned the location of an escape platform hidden in a water maze for 3 days [13]. Acetylated H2B was enriched on gene promoters involved in memory and plasticity, such as the BDNF promoter IV, cFos, FosB and Zif268. Moreover, spatial training-induced H2B acetylation was strongly diminished in a rat model invalidating spatial memory consolidation by selective damage to cholinergic and glutamatergic hippocampal inputs [13]. Together, these data strongly suggest a particular involvement of H2B acetylation in the molecular processes involved in spatial memory formation. However, it is yet unknown whether these acetylation changes measured on H2B histone N-terminus specifically concern place learning or more generally hippocampus-dependent learning. Therefore, in this paper, we compared the acetylation status of H2B in different hippocampal-dependent learning tasks; one taxed spatial memory formation, the other contextual fear conditioning. Moreover, we compared acetylation levels in the dorsal hippocampus of learning animals (location of a hidden platform, fear to context association) to a series of control situations that did not require the formation of a memory for spatial cues (rats had to swim to a visible platform) or for context signification (rats were exposed to context or shock-only conditions, or taken from their home). Together with H2B tetra-ac, we also assessed H2B acetylation on lysine 5, which, according to Valor and colleagues [19], seems to be dynamically regulated in CBP deficient mice. Lastly, we measured the acetylation levels of two other histones: H3 and H4. To this end, we chose specific acetylation modifications: H3K9K14 and H4K12, previously reported to be associated with learning and memory. Histone H3 acetylation on lysine 14 was one of the first modifications described to be modulated by experience-dependent behavior: H3K14 was found hyperacetylated in the CA1 region of the hippocampus of rats after a contextual fear conditioning vs. naive rats [6]. H3K9 acetylation, together with that of K14, was recently shown over promoters of actively transcribed genes in mouse cells [20] and we previously observed H3K9K14 hyperacetylation in the hippocampus of rats undergoing a spatial memory task compared to naïve rats [13]. H4K12 acetylation modification was selected for its described role in fear conditioning in mice [7]. Moreover, authors showed that aged mice displayed a specific deregulation of histone H4K12 acetylation during learning and failed to initiate a hippocampal gene expression program associated with memory consolidation [7]. Restoration of physiological H4K12 acetylation with HDAC inhibitors reinstated the expression of learning-induced genes and led to the recovery of cognitive abilities [7]. We previously showed an increase of H4K12 acetylation in rats undergoing spatial training, associated with the cFos and Zif268 gene promoters [13].

Herein we report that H2B acetylation is increased in both learning situations (spatial or fear memory) as compared to the respective controls, suggesting that this modification is not specific to spatial learning but seems to be part of the molecular mechanisms involved in hippocampus-dependent memory formation. Our results also point to distinct regulations on specific histone sites, that seem to depend on which component of a behavioural test rats have to deal with (overall environmental context vs. specific goal therein) and which are detectable in whole dorsal hippocampus homogenates: while histone modifications detected on H2B N-terminus and H4K12 are induced in learning conditions, H3K9K14 seems more responsive to contextual and environmental changes. In addition, our results show that such changes are very precocious during the timing of learning, as they are detected early in the course of task acquisition.

Materials and Methods

Animals and Ethics Statement

Seventy nine 3-4 month-old Long-Evans male rats (Centre d’Elevage René Janvier, France) were used. They were individually housed in standard cages with food and water provided ad libitum, in a temperature- and humidity-controlled room (22±1°C and 55±5%, respectively) under a 12 h–12 h light-dark cycle (lights on at 8:00 a.m.). Experimental protocols and animal care were in compliance with the institutional guidelines (council directive 87/848, October 19, 1987, Ministère de l’agriculture et de la Forêt, Service Vétérinaire de la Santé et de la Protection Animale ) and international (directive 86–609, 24 November 1986, European Community) laws and policies (personal authorizations N° 67–167 for A.B., N° 67–289 for M.M N° 67–215 for JCC). All efforts were made to minimize suffering.

Morris Water Maze

The specifications of the water maze and the testing procedures have been described previously [13]. Briefly, after a four-trial session using a visible platform (VPf), two groups of rats which had to learn the location of a hidden platform (HPf) were given four successive acquisition trials per day for 1 day or 3 consecutive days. Control rats had to swim to a visible platform (VPf) emerging 1 cm above the water surface, and of which the location was changed from trial to trial on each day. One hour after the last acquisition trial, rats trained with the HPf for 1 or 3 days were tested for retention in a probe trial (for the control group, rats had to swim to a VPf ). Rats from the day1 group were immediately euthanized for biochemical studies. For biochemical studies (see below), a group of control rats taken from their home cage (HC) was also used.

Contextual Fear Conditioning

Rats were handled for 6 consecutive days (1 min/day/rat) before conditioning. Fear conditioning was performed in two identical Plexiglas chambers (25×27×18 cm) placed in ventilated light- and sound-attenuated boxes (57×38×38 cm, Campden Instruments LTD). The grid floor of each chamber consisted of parallel 0.3 cm diameter stainless-steel bars, 0.8 cm apart, connected to a shock generator (0.6 mA, 0.8 s, scrambled) controlled by a computerized interface (Med-PC, Med Associates, Inc., St Albans, VT, USA). Four conditions were used. Contextual fear conditioned rats received 3 foot shocks 180 s, 241 s and 362 s after the placement in the chamber (context-shock, CS). A first control group received 3 foot shocks delivered 1 s, 3s and 5s after their placement in the chamber (immediate-shock, IS). Another control group was left in the context, receiving no foot shock during the session (context group, CX). A last control group consisted of rats taken from their home cage without any exposure to shock or context (HC). Each training condition lasted 8 min. After training, all rats were returned to their home cage and left undisturbed until either euthanasia for biochemical studies (1 h delay) or behavioural testing for retention (24 h delay). To this end, automatic freezing measurements were carried out during an 8-min session, as described in detail by Marchand et al. [21].

Preparation of Tissues for Western Blot Analyses

All animals were killed by decapitation, their brains rapidly removed from the skull and transferred on an ice-cold glass plate. Freshly dissected dorsal hippocampi were immediately frozen in liquid nitrogen and kept at −80°C. Western blots were performed as described previously [13] with polyclonal antibodies against acetylated-H2B histone (H2B tetra-Ac, H2BK5) and acetylated-H3 histone (Upstate Biotechnology, New York, NY, USA), acetylated-H4 histone (Active motif Carlsbad, CA, USA), H3 and H4 histones (Abcam, Cambridge, UK), H2B histone (Euromedex, France). Secondary HRP-conjugated antibodies were from Jackson Immunoresearch (Suffolk, UK). Blots were revealed with BioFX® HRP chemiluminescent substrates SERI (SurModics, Eden Prairie, MN, USA) and exposed with Kodak BioMax light film (Sigma-Aldrich). Results were quantified using the ImageJ software. For each histone (either total or modified), we performed western blot analyses on increasing amounts of a total protein extract mix and determined the adequate amount within the linear range of detection to be assessed for quantitative western blots analyses.

Statistical Analysis

Behavioural studies

The analysis of spatial learning performance recorded during acquisition used a two-way ANOVA with repeated measures considering days (1–3) and platform condition (HPf vs. VPf). Probe trial performance was analyzed using a one-way ANOVA. An additional one sample t-test was performed to compare the time spent in the each quadrant to chance level (i.e., 15 s). When appropriate, post hoc comparisons used the Newman–Keuls multiple range statistic. Freezing was computed as the percentage of time spent at freezing over the 8-min test session. It was analyzed using an ANOVA with “Training condition” as the between-subject factor. The ANOVA was complemented by post hoc Newmann-Keuls tests when appropriate. In all cases, the threshold for rejecting the null hypothesis was set at α<0.05. Biochemical studies. Statistical analyses were performed using one-way ANOVA followed by Newman-Keuls multiple comparison tests. Data are expressed as the mean ± SEM. Differences at p<0.05 were considered significant.

Results

Histone Acetylation Profiles during Spatial Reference Memory Formation

We investigated whether histone acetylation was modulated at the beginning of a spatial memory training (1-day training) in rats having to search for a hidden platform (HPf) in the Morris Water Maze. Acetylation levels were compared to those measured in naive rats (HC) or rats that had swum to a visible platform (VPf). At this time point, rats had experienced the learning task, but did not present any behavioural evidence for a consolidated memory trace during a probe trial (figure 1A). In order to verify that our test conditions permitted learning with prolonged training, another group of rats was trained for 3 days. Acquisition (distance to the platform, either hidden or visible) and retention (time spent in the target quadrant, no platform, HPf group only) performances are shown in figure 1A. As expected, the retention results now clearly showed that after three acquisition days, the probe trial performance was significantly above chance in the target quadrant (quadrant effect 2 way-Anova F(3,12) = 11.84, p<0.001; time in target quadrant versus 15 sec: t(3) = 3.18, p<0.05), indicating efficient memory formation. Histone acetylation profiles of 3 major histones (H2B, H3 and H4) at specific lysine residues in the 1-day experimental group were established by western blot analyses in dorsal hippocampi of the 3 animal groups (HPf, VPf and HC; figure 1B). Representative western blots are shown on the left (duplicates) and quantification is shown on the right (n = 5). Global H2B histone acetylation (tetra Ac) was significantly increased in the HPf group as compared to VPf and HC control groups (1.50-fold, when compared to VPf, p<0.01; 1.56-fold, when compared to HC, p<0.05). Tetra-acetylated-H2B histone levels were not significantly different between VPf and HC groups. As tetra-acetylated-H2B, H2B acetylation on the single lysine 5 (H2BK5) was also significantly up-regulated in the HPf group compared to VPf and HC groups (1.38-fold, when compared to VPf, p<0.001; 1.3-fold, when compared to HC, p<0.05). H4K12 acetylation was also significantly increased in the HPf group compared to controls (1.41-fold, when compared to HC, p<0.05; 1.29-fold, when compared to VPf, p<0.05). Here again, no significant difference was found between HC and VPf groups, a result similar to that found for H2B acetylation. Finally, H3K9K14 histone acetylation levels were increased in HPf rats as compared to HC rats (1.74-fold, p<0.001). However, and this is a major difference with the other histone marks measured on H4 and H2B, there was no significant difference between HPf and VPf rats, the latter also showing H3 K9K14 histone acetylation levels that significantly exceeded those found in HC rats (1.79-fold, p<0.001).

Figure 1

Short spatial memory training differentially modulates histone acetylation in the rat hippocampus.

(A) Performance of rats trained in the Morris water maze task during one or three consecutive days in the Morris Water Maze (left panel) and probe trial performance after 1 or 3 days of training (right panel). During training, rats had to search for the location of a platform hidden at a constant location (HPf); their controls swam to a visible platform (VPf) whose location was changed from trial to trial. Probe trial performances of the HPf groups are presented after 1- or 3 days of training (right panel) as the mean time (+ SEM) spent in the target quadrant. After 3 days of training, the rats trained with the hidden platform performed significantly above chance (i.e., 15 s), *p<0.05, an effect not observed after only 1 day of training. (B) Comparison of acetylated and total histone levels between home cage rats (HC, n = 5), rats trained to swim to a visible platform (VPf, n = 5) and rats trained to learn the location of a hidden platform (HPf, n = 5) in a single daily session (4 trials). Acetylation levels were measured by western blot performed on total extracts from dorsal hippocampus with specific antibodies (Tetra Ac: H2BK5K12K15K20, K5Ac: H2BK5, H4K12 and H3K9K14). Typical western blots are presented in duplicates on the left. Corresponding quantifications are shown on the right. Ratios of acetylated/total histone corresponding to the home cage rats (HC) were arbitrarily set at 100% and other values normalized accordingly. Newman-Keuls multiple comparisons test: ***p<0.001, **p<0.01, *p<0.05, for comparisons with the HC group or as indicated. Both H2B and H4 histones showed hyperacetylation in the group trained to find the hidden platform (HPf) compared to either control (VPf or HC), while H3 was hyperacetylated in the VPf and HPf groups, thus more reflecting task-related context processing.

In summary, these observations show that some acetylation modifications on H2B (K5 and tetra-acetylation) and H4 (K12) histones are consistently associated with early stages of spatial learning. Similarly, acetylation of H3K9K14 histones are also rapidly increased, but conversely to tetra-acetylated-H2B and H4K12 histone marks, it is also the case under all control situations when compared to HC, thus suggesting a role of this histone mark in task/context processing (swimming, stress, exploration…). Are these changes specific to a spatial learning situation? To address this question, we used a similar approach in rats that were subjected to a task that, being non spatial by nature, is also hippocampus-dependent, namely contextual fear conditioning (CFC).

Histone Acetylation Profiles during Contextual Fear Conditioning

CFC is one of the most widely used tests to study memory processes, and a few studies have reported histone modifications during the consolidation of conditioned fear. Indeed, H3 histone acetylation was consistently found up-regulated in the rat hippocampus after contextual fear conditioning [5], [6], [7]. H4 histone acetylation was reported unchanged in early studies [6], but was found to be increased in more recent ones [7], [22]. To the best of our knowledge, H2B has never been investigated in relation with this type of memory.

We thus analyzed histone acetylation of H2B, H4 and H3 in rats trained for contextual fear conditioning using 3 shocks at random time points within an 8-min training period (CS). As illustrated in Figure 2A, histone acetylation was compared to that found in context-only rats (CX) and in immediate-shock rats (IS). An additional group consisted of rats taken from their home cage (HC). As shown in figure 2B, only rats of the Context-Shock group exhibited conditioned freezing to the context after this delay. Freezing levels were very low in the two other groups. The ANOVA showed a significant effect of “Training condition” [F (1,27) = 112,28 P<0.0001] and the post hoc comparisons indicated that freezing levels in the CS group significantly differed from those measured in the CX and IS groups (p<0.001 in each case), which did not differ significantly from each other.

Figure 2

Impact of contextual fear conditioning on histone acetylation in the rat hippocampus.

(A) Experimental design. Three groups of rats (n = 16/group) were used. In one group, rats were kept in the context but received no shock (CX). Others received three immediate and consecutive shocks and were subsequently left in the context for 8 min (IS). In the last group, rats received three randomly-distributed shocks while being kept in the context as noted (CS). Animals (n = 10/group) were then either tested for freezing behavior after 24 h (probe) (B; n = 10/group) or euthanized after 1 h for tissue collection (dorsal hippocampus) and western blot analyses of acetylated histones (C; n = 6/group). (B) Freezing levels at 24 h. Notice that marked freezing was observed only in the Context-shock group (CS), demonstrating that rats of this group were the only ones to have associated the shock with the context and memorized this association. (C) Comparison of acetylated and total histone levels in the three groups relative to their counterparts taken from the home cage (HC, n = 6). Lysine acetylations measured were H2BK5 (K5Ac, plain histograms), H2BK5K12K15K20 (Tetra Ac, stripped histograms), H4K12 (K12Ac) and H3K9K14 (K9K14Ac). Typical western blots are shown in duplicates. Quantified results are represented as % induction of the Acetylated/total ratio for each histone. The ratio obtained in the HC condition was arbitrarily set at 100%. Newman-Keuls multiple comparisons test: ***p<0.001**p<0.01, *p<0.05, as compared to HC group. Global H2B and H4 histone acetylation levels were clearly increased in the group exhibiting fear towards the context (CS) as compared to the other situations, while H3 acetylation levels were increased in CS and both controls (CX and IS ) as compared to rats completely naive to the test situation (HC).

Histone acetylation levels were measured by immunoblotting in the dorsal hippocampus of rats trained in parallel and euthanized one hour after the training (figure 2C). When fear conditioned rats (CS) were compared to home-cage rats (HC), all histone marks measured on H2B, H3 and H4 displayed a significant increase in acetylation (H2BK5, 2.35 fold, p<0,001; tetra-Ac, 1.42-fold, p<0.05; H3K9K14, 1.52 fold, p<0.05; H4K12, 1.74 fold, p<0.01). Nevertheless, these marks were differentially responsive to the control situations. In is noteworthy that H3K9K14 histone acetylation was significantly increased in both the CX (1.3 fold, p<0.05) and the IS (1.42 fold, p<0.05) control groups as compared to the HC group. H2B N-terminus and H2BK5 acetylations showed a non significant trend to increase in response to IS (H2B tetra-Ac, 1.22 fold, p = 0.163; H2BK5, 1.34 fold, p = 0.052), while H4K12 acetylation remained unchanged in the CX or IS condition. Altogether, and as was also the case in the water maze test, these observations suggest that acetylation on H2B N-terminus and H4K12 are increased when shocks are paired with the context (i.e. when training subsequently results in established fear), whereas the increased H3K9K14 acetylation appears less specific to the establishment of such a context-shock association.

Discussion

We recently identified H2B tetra-acetylation as a major chromatin mark associated with plasticity/memory gene promoters in the hippocampus of rats which had learnt a spatial reference memory task over three consecutive days [13]. In the current report, we describe that this chromatin mark is consistently activated in response to learning engaging the hippocampus (spatial memory or contextual fear conditioning). We also report that the H4K12 acetylation pattern follows that of H2B N-terminus in the two behavioral tasks. Finally, we confirm that H3K9K14 acetylation seems more sensitive to manipulations of the rats’ environmental context in the Morris water maze and we extend this observation to contextual fear memory formation. Our results emphasize that the integration of memory-associated behaviors at the level of histone acetylation occurs on specific lysine residues, that can be detected at a global level in the dorsal hippocampus. In addition, our results suggest that such changes may reflect the type of information to be stored.

Acetylation of H2B and H4 Histones at Specific Sites is Induced in Tasks Requiring Memory Formation

A remarkable result presented herein is that the tetra-acetylated-H2B and H4K12ac histones were consistently found to be hyperacetylated in the hippocampus of rats subjected to a training resulting in memory formation, be it for the location of a platform hidden in the water maze or for the context-associated shocks in the fear conditioning paradigm. The acetylation status of these histone marks (H2BK5, H2BK5K12K15K20 and H4K12) could represent a molecular step towards memory formation.

The functions of H2B histone modifications are poorly documented. Nevertheless, the few available data suggest interesting features in relation with transcription and memory. At the level of gene transcription, it is noteworthy that H2BK5 was recently reported to be consistently found within the 5′ proximal region of high CpG content promoters (HCP) [23]. Hence, H2BK5Ac binding seems predictive for expression of HCP genes [23], which represent about 70% of the regulated genes expressed in most tissues [24]. These include memory/plasticity-related immediate-early genes (e.g., zif268,…), kinases (e.g. catalytic subunit of cAMP-dependent protein kinase,…), and neurotrophic factors (e.g. BDNF,…) [24]. In line with this, we previously showed that tetra-acetylated-H2B histones were enriched at specific plasticity/memory-related promoters (bdnf exon IV, cFos and zif268) in the hippocampus during consolidation of spatial memory, an event associated with higher gene expression levels [13]. At the global level, increased acetylated-H2B levels have been measured in hippocampi of transgenic mice models displaying enhanced long term potentiation (LTP) and improved memory functions (HDAC2 knock-out mice [16] and NIPP1 mice [11]). H2B tetra-acetylation at K5K12K15K20 can also be rapidly triggered by depolarization in hippocampal slices [18]. Altogether, these data suggest that H2B tetra-acetylation could represent an early subcellular step of memory formation, triggering the transcription of specific genes likely related to memory consolidation. Of note, H2B is itself the preferred histone target of CBP in the hippocampus [19], [25], [26], an acetyltransferase playing an important role in memory formation and consolidation [10], [19], [25], [26], [27]. We showed that CBP is up-regulated during spatial learning, while its proximal promoter was enriched in acetylated-H2B histone [13]. Thus, CBP-induced acetylation of H2B might be a means to activate specific plasticity/memory-related gene transcription programs. CBP-dependent transcription has also been described as an important mediator of environmental enrichment-induced adult neurogenesis, acetylated-H2B histone being associated with neurogenesis-related gene promoters [28]. Future studies using ChIP-sequencing will certainly help to identify and characterize acetylated H2B-regulated genetic programs in the hippocampus during memory formation. Remarkably, our previous immunohistochemistry studies performed on VPf and HPf after 3 days of training showed that acetylated H2B N-terminus levels were increased in all nuclei of hippocampal neurons (data not shown) - as was already the case for CBP [13] - rather than in a subset of the neuronal population [29]. This is in line with the fact that these changes are detectable by western blot analyses performed on total dorsal hippocampi extracts and further suggests that a global response to behavior takes place in the dorsal hippocampus. If, and also how this general modification will be subsequently integrated into only a subpopulation of neurons to sustain the memory trace remains to be established.

Acetylation of H4K12 has been more widely studied and its association with memory formation is documented, particularly after fear conditioning [7], [22], latent inhibition training [6] and spatial memory formation [13]. Acetylated H4K12 enrichment has been shown on different bdnf promoters in response to fear conditioning in the hippocampus [7], [30], [31] or in the frontal cortex [9], and our recent data show an enhancement of acetylated H4K12 on cFos and zif268 promoters in the hippocampus after spatial memory training [13]. A recent study remarkably showed that H4K12 acetylation was altered by aging in mice subjected to fear conditioning [7]. Histone H4 acetylation, including other lysine residues than K12, might also be involved in Alzheimer’s disease (AD) pathology as it is reduced in transgenic models of this disease [22], [32], [33], [34]. Furthermore, acetylated H4K12 associated genetic programs were recently identified by ChIP-sequencing in the hippocampus during fear learning [7]. Thus, the study presented here further emphasizes that this epigenetic mark is specific to memory formation as it is consistently induced in hippocampus-dependent learning paradigms, and not in the different control situations used herein and elsewhere.

H3 Histone Acetylation Might be a Marker of Contextual Changes Processing

Another striking result is that H3 was found to be significantly acetylated at K9 and K14 in the hippocampus of rats subjected to learning, but to almost the same extent than in rats exposed to other control situations as compared to naive home cage rats. H3K14 acetylation is known to be induced in the hippocampus of animals undergoing unpleasant shocks paired to a context [5], [6], [35]. However, in all these studies, tissue collected in learners was compared to tissue from naive controls or from animals exposed to unpaired shocks. Our results indicating that a « new » situation, even when not associated with fear learning, is able to modify this epigenetic mark, further suggest that certain acetyltransferases could be rapidly activated in the hippocampus of animals placed in a novel situation to acetylate K9 and/or K14 of H3 histone in the nucleus. This would result in the opening of the chromatin and favor some gene transcription. Contextual fear learning was actually reported to induce bdnf mRNA in the CA1 area of the hippocampus, bdnf exon IV being more specifically activated when the context was paired to shocks and bdnf exon I being activated in the context-only situation [36]. It is noteworthy that bdnf exon I transcripts in the hippocampus are very responsive to a HDAC inhibitor directly modulating histone acetylation levels [37]. Thus, it is likely that acute changes in usual situations, either mild, such as having to wander in a novel environment when having been taken out from the home cage, or strong, such as having to experience unpaired shocks or swimming towards a visible escape platform, impact H3K9K14 histone acetylation, whereby the chromatin structure can be modified and specific gene profiles regulated. Of note, K9 and K14 acetylation has been recently shown to co-occur at active enhancers, and it was found to trigger transcriptional activation in mouse cells [20]. Which genetic programs are indeed activated in the behavioral conditions remains to be established, but they should definitely depend on how stressful and/or novel environmental changes may be. An interesting study demonstrated that rats either trained in associative or in non associative fear learning displayed similar gene expression profiles in the hippocampus, whereas greater levels of gene regulation were seen in the amygdala in response to associative fear conditioning compared to the non associative control [38]. This study was performed 30 min after training, a time point chosen to optimally detect immediate-early gene induction. In light of our observation that the acetylated-H3K9K14 histone is increased in all conditions compared to home cage controls in the hippocampus, these results suggest that there is a step of hippocampal activation in response to conditioning, whether more specific associative learning-dependent responses have to be formed or not. It would be of prime interest to compare the dependency of these genes [38] to acetylated-H3K9K14 histone versus acetylated H2B N-terminus or H4K12.

Early Engagement of Histone Acetylation in Memory Processing

Little is known about biochemical studies of memory formation in the Morris Water Maze (MWM). Indeed, MWM is a complex protocol requiring several days of training and daily repetitions of several learning trials. Thus, acquisition/consolidation/recall signals are mixed all along the learning days. In our previous studies [13], we measured increased H2BK5K12K15K20 and H4K12 acetylation levels after the 3rd day of acquisition, a moment at which performance can still be improved and thus memory undergo further consolidation, suggesting a role of this modification in memory consolidation. However, the study presented here in the Morris Water Maze shows that specific acetylation modifications occurring on H2BK5K12K15K20, H2BK5 and H4K12 are already elevated in the hippocampus after a single day of training, when no evidence for consolidation can be measured yet in a probe trial and learning experience has just started, suggesting that these modifications accompany or might even be a substrate of the earliest stages of task integration/memory formation. This does not necessarily mean that the processes brought to light in the current study are associated with short term memory processes, as early molecular events could serve to implement the transcriptional response for long-term memory processes over repetition of the task. A hypothesis could be that iterative training allows a gradual increase of acetylation marks over days. Repetition of the training could also impact persistence of the acetylation marks over time, thereby maintaining specific memory−/plasticity- gene transcription throughout the memorisation/consolidation process. It is noteworthy that levels of acetylation on H2B measured in this study at day 1 seem comparable to those measured in the study by Bousiges et al. [13] at day 3, suggesting that repetitive training would in fact not support accumulation of molecular events over the three days, but rather reflect behavior-induced molecular events after a given training session. However, measurements of acetylation levels by western blot are technically limited to assess subtle changes at the global level. Therefore, this kind of study should be conducted at the promoter level by chromatin immunoprecipitation on specific loci. In addition, whether or not acetylated chromatin is present on the same genes at early and later time points (day 1 and day 3) is not known. It must be considered that other epigenetic changes, such as histone phosphorylation [35] or histone methylation [39] could take place at later time points (between day 1 and days 3) and act in concert with acetylation modifications. Lastly, our global approach might have missed more discrete changes occurring in different hippocampal sub-structures (e.g. CA1, dentate gyrus….).

Taken together, our water maze and fear conditioning data support the idea that specific acetylation modifications might be engaged in the hippocampus at early stages of task training (water maze and fear conditioning) and maintained during further training over the process of memory formation in tasks based on cumulative learning (water maze). In addition, our findings indicate that H3K9K14 might be the more sensitive to changes in the environmental context than to the mnemonic dimension of the task itself, whereas H2BK5K12K15K20/H4K12 seem more sensitive to the formation of a memory for the platform location or for the meaning of the context. These outcomes support the hypothesis of a language within the chromatin [40] in response to behavior/environment and might therefore contribute to identify co-activator recruitment (e.g. CBP-dependent acetylation of H2B in the hippocampus, [19], [26]) to specific plasticity/memory-related promoters. Such knowledge will help to better define therapeutic options, especially in the perspective of treating cognitive alterations by a pharmacological action on acetylation or deacetylation of specific lysine residues on histones in order to directly stimulate appropriate transcriptional programs [41], [42], [43].