Role of the histone variant H2A.Z.1 in memory, transcription, and alternative splicing is mediated by lysine modification

Creating long-lasting memories requires learning-induced changes in gene expression, which are impacted by epigenetic modifications of DNA and associated histone proteins. Post-translational modifications (PTMs) of histones are key regulators of transcription, with different PTMs producing unique effects on gene activity and behavior. Although recent studies implicate histone variants as novel regulators of memory, effects of PTMs on the function of histone variants are rarely considered. We previously showed that the histone variant H2A.Z suppresses memory, but it is unclear if this role is impacted by H2A.Z acetylation, a PTM that is typically associated with positive effects on transcription and memory. To answer this question, we used a mutation approach to manipulate acetylation on H2A.Z without impacting acetylation of other histone types. Specifically, we used adeno-associated virus (AAV) constructs to overexpress mutated H2A.Z.1 isoforms that either mimic acetylation (acetyl-mimic) by replacing lysines 4, 7 and 11 with glutamine (KQ), or H2A.Z.1 with impaired acetylation (acetyl-defective) by replacing the same lysines with alanine (KA). Expressing the H2A.Z.1 acetyl-mimic (H2A.Z.1KQ) improved memory under weak learning conditions, whereas expressing the acetyl-defective H2A.Z.1KA generally impaired memory, indicating that the effect of H2A.Z.1 on memory depends on its acetylation status. RNA sequencing showed that H2A.Z.1KQ and H2A.Z.1KA uniquely impact the expression of different classes of genes in both females and males. Specifically, H2A.Z.1KA preferentially impacts genes involved in synaptic function, suggesting that acetyl-defective H2A.Z.1 impairs memory by altering synaptic regulation. Finally, we describe, for the first time, that H2A.Z is also involved in alternative splicing of neuronal genes, whereby H2A.Z depletion, as well as expression of H2A.Z.1 lysine mutants influence transcription and splicing of different gene targets, suggesting that H2A.Z.1 can impact behavior through effects on both splicing and gene expression. This is the first study to demonstrate that direct manipulation of H2A.Z post-translational modifications regulates memory, whereby acetylation adds another regulatory layer by which histone variants can fine tune higher brain functions through effects on gene expression and splicing.

A rodent obstacle course procedure controls delivery of enrichment and enhances complex cognitive functions

Enrichment in rodents affects brain structure, improves behavioral performance, and is neuroprotective. Similarly, in humans, according to the cognitive reserve concept, enriched experience is functionally protective against neuropathology. Despite this parallel, the ability to translate rodent studies to human clinical situations is limited. This limitation is likely due to the simple cognitive processes probed in rodent studies and the inability to control, with existing methods, the degree of rodent engagement with enrichment material. We overcome these two difficulties with behavioral tasks that probe, in a fine-grained manner, aspects of higher-order cognition associated with deterioration with aging and dementia, and a new enrichment protocol, the 'Obstacle Course' (OC), which enables controlled enrichment delivery, respectively. Together, these two advancements will enable better specification (and comparisons) of the nature of impairments in animal models of complex mental disorders and the potential for remediation from various types of intervention (e.g., enrichment, drugs). We found that two months of OC enrichment produced substantial and sustained enhancements in categorization memory, perceptual object invariance, and cross-modal sensory integration in mice. We also tested mice on behavioral tasks previously shown to benefit from traditional enrichment: spontaneous object recognition, object location memory, and pairwise visual discrimination. OC enrichment improved performance relative to standard housing on all six tasks and was in most cases superior to conventional home-cage enrichment and exercise track groups.

Age-dependent attenuation of spatial memory deficits by the histone acetyltransferase p300/CBP-associated factor (PCAF) in 3xTG Alzheimer's disease mice

Histone acetylation, catalyzed by histone acetyltransferases, has emerged as a promising therapeutic strategy in Alzheimer's disease (AD). By longitudinally characterizing spatial memory at 3, 6, and 9 mo of age, we show that acute activation and inhibition of the histone acetyltransferase PCAF remediated memory impairments in 3xTG-AD mice in an age-related bidirectional manner. At 3 and 6 mo of age, PCAF activation ameliorated memory deficits. At 9 mo of age, PCAF activation had no effect on spatial memory, whereas PCAF inhibition improved memory deficits in females. This work reveals a complex potential therapeutic role for PCAF in AD, initially benefitting memory but becoming detrimental as the disease progresses.

The histone chaperone Anp32e regulates memory formation, transcription, and dendritic morphology by regulating steady-state H2A.Z binding in neurons

Rapid removal of histone H2A.Z from neuronal chromatin is a key step in learning-induced gene expression and memory formation, but mechanisms underlying learning-induced H2A.Z removal are unclear. Anp32e was recently identified as an H2A.Z-specific histone chaperone that removes H2A.Z from nucleosomes in dividing cells, but its role in non-dividing neurons is unclear. Moreover, prior studies investigated Anp32e function under steady-state rather than stimulus-induced conditions. Here, we show that Anp32e regulates H2A.Z binding in neurons under steady-state conditions, with lesser impact on stimulus-induced H2A.Z removal. Functionally, Anp32e depletion leads to H2A.Z-dependent impairment in transcription and dendritic arborization in cultured hippocampal neurons, as well as impaired recall of contextual fear memory and transcriptional regulation. Together, these data indicate that Anp32e regulates behavioral and morphological outcomes by preventing H2A.Z accumulation in chromatin rather than by regulating activity-mediated H2A.Z dynamics.

Epigenetic Mechanisms of Learning and Memory: Implications for Aging

The neuronal epigenome is highly sensitive to external events and its function is vital for producing stable behavioral outcomes, such as the formation of long-lasting memories. The importance of epigenetic regulation in memory is now well established and growing evidence points to altered epigenome function in the aging brain as a contributing factor to age-related memory decline. In this review, we first summarize the typical role of epigenetic factors in memory processing in a healthy young brain, then discuss the aspects of this system that are altered with aging. There is general agreement that many epigenetic marks are modified with aging, but there are still substantial inconsistencies in the precise nature of these changes and their link with memory decline. Here, we discuss the potential source of age-related changes in the epigenome and their implications for therapeutic intervention in age-related cognitive decline.

Inhibition of 5α Reductase Impairs Cognitive Performance, Alters Dendritic Morphology and Increases Tau Phosphorylation in the Hippocampus of Male 3xTg-AD Mice

Recent work has suggested that 5α-reduced metabolites of testosterone may contribute to the neuroprotection conferred by their parent androgen, as well as to sex differences in the incidence and progression of Alzheimer's disease (AD). This study investigated the effects of inhibiting 5α-reductase on object recognition memory (ORM), hippocampal dendritic morphology and proteins involved in AD pathology, in male 3xTg-AD mice. Male 6-month old wild-type or 3xTg-AD mice received daily injections of finasteride (50 mg/kg i.p.) or vehicle (18% β-cyclodextrin, 1% v/b.w.) for 20 days. Female wild-type and 3xTg-AD mice received only the vehicle. Finasteride treatment differentially impaired ORM in males after short-term (3xTg-AD only) or long-term (3xTg-AD and wild-type) retention delays. Dendritic spine density and dendritic branching of pyramidal neurons in the CA3 hippocampal subfield were significantly lower in 3xTg-AD females than in males. Finasteride reduced CA3 dendritic branching and spine density in 3xTg-AD males, to within the range observed in vehicle-treated females. In the CA1 hippocampal subfield, dendritic branching and spine density were reduced in both male and female 3xTg-AD mice, compared to wild type controls. Hippocampal amyloid β levels were substantially higher in 3xTg-AD females compared to both vehicle and finasteride-treated 3xTg-AD males. Site-specific Tau phosphorylation was higher in 3xTg-AD mice compared to sex-matched wild-type controls, increasing slightly after finasteride treatment. These results suggest that 5α-reduced neurosteroids may play a role in testosterone-mediated neuroprotection and may contribute to sex differences in the development and severity of AD.

MouseBytes, an open-access high-throughput pipeline and database for rodent touchscreen-based cognitive assessment

Open Science has changed research by making data accessible and shareable, contributing to replicability to accelerate and disseminate knowledge. However, for rodent cognitive studies the availability of tools to share and disseminate data is scarce. Automated touchscreen-based tests enable systematic cognitive assessment with easily standardised outputs that can facilitate data dissemination. Here we present an integration of touchscreen cognitive testing with an open-access database public repository (mousebytes.ca), as well as a Web platform for knowledge dissemination (https://touchscreencognition.org). We complement these resources with the largest dataset of age-dependent high-level cognitive assessment of mouse models of Alzheimer’s disease, expanding knowledge of affected cognitive domains from male and female mice of three strains. We envision that these new platforms will enhance sharing of protocols, data availability and transparency, allowing meta-analysis and reuse of mouse cognitive data to increase the replicability/reproducibility of datasets.

Dissociable involvement of estrogen receptors in perirhinal cortex-mediated object-place memory in male rats

Estrogens and the estrogen receptors (ER) - ERα, ERβ, and the G-protein coupled estrogen receptor (GPER) - are implicated in various forms of hippocampus (HPC)-dependent memory. However, the involvement of ER-related mechanisms in perirhinal cortex (PRh), which is necessary for object memory, remains much less clear. Moreover, there is a paucity of data assessing ER contributions to cognition in males,despite documented sex differences at the cellular level.We hypothesized that estrogens in PRh are important for object memory in males, assessingthe role of 17-βestradiol (E2), ERα, ERβ, GPER, and their downstream signaling pathways, in PRh-mediated object-in-place (OiP) memory in gonadally-intact male rats. Intra-PRh administration of E2 enhanced both long-term memory (LTM; 24 h) and short-term memory (STM; 20 min). Conversely, aromatase inhibition with letrozole impaired LTM and STM. The semi-selective ER inhibitor ICI 182780 impaired LTM, but not STM. This effect may be due to inhibition of ERβ, as the ERβagonist DPN, but not ERαagonist PPT, enhanced LTM. GPER was also found to be necessary in PRh, as the antagonist G15 impaired both LTM and STM. Western blot analyses demonstrated that phosphorylation levels of the extracellular signal-related kinase (ERK2 isoform), awell-establisheddownstream signaling pathway activated by estrogens through ERα/ERβ, was elevated in PRh 5 min following OiP learning.We also reportincreased levels of c-Jun N-terminal kinase (JNK; p46 and p54 isoforms) phosphorylation in PRh 5 min following learning,consistent with recent research linking GPER activation and JNK signaling in the HPC. This effect was abolished by intra-PRh administration of G15, but not letrozole, suggesting that JNK signaling is triggered via GPER activation during OiP learning, and is possibly E2-independent, similar to findings in the HPC. These results, therefore, reveal interesting dissociations between the roles of various ERs, possibly involving both estrogen-dependent and independent mechanisms, in PRh-mediated object-place learning in male rats.

The Clock Mechanism Influences Neurobiology and Adaptations to Heart Failure in Clock∆19/∆19 Mice With Implications for Circadian Medicine

In this study we investigated the role of the circadian mechanism on cognition-relevant brain regions and neurobiological impairments associated with heart failure (HF), using murine models. We found that the circadian mechanism is an important regulator of healthy cognitive system neurobiology. Normal Clock∆19/∆19 mice had neurons with smaller apical dendrite trees in the medial prefrontal cortex (mPFC), and hippocampus, showed impaired visual-spatial memory, and exhibited lower cerebrovascular myogenic tone, versus wild types (WT). We then used the left anterior descending coronary artery ligation model to investigate adaptations in response to HF. Intriguingly, adaptations to neuron morphology, memory, and cerebrovascular tone occurred in differing magnitude and direction between Clock∆19/∆19 and WT mice, ultimately converging in HF. To investigate this dichotomous response, we performed microarrays and found genes crucial for growth and stress pathways that were altered in Clock∆19/∆19 mPFC and hippocampus. Thus these data demonstrate for the first time that (i) the circadian mechanism plays a role in neuron morphology and function; (ii) there are changes in neuron morphology and function in HF; (iii) CLOCK influences neurobiological gene adaptations to HF at a cellular level. These findings have clinical relevance as patients with HF often present with concurrent neurocognitive impairments. There is no cure for HF, and new understanding is needed to reduce morbidity and improve the quality of life for HF patients.

Dissociable roles for histone acetyltransferases p300 and PCAF in hippocampus and perirhinal cortex-mediated object memory

The importance of histone acetylation for certain types of memory is now well established. However, the specific contributions of the various histone acetyltransferases to distinct memory functions remain to be determined; therefore, we employed selective histone acetyltransferase protein inhibitors and short-interference RNAs to evaluate the roles of CREB-binding protein (CBP), E1A-binding protein (p300) and p300/CBP-associated factor (PCAF) in hippocampus and perirhinal cortex (PRh)-mediated object memory. Rats were tested for short- (STM) and long-term memory (LTM) in the object-in-place task, which relies on the hippocampus and PRh for spatial memory and object identity processing, respectively. Selective inhibition of these histone acetyltransferases by small-interfering RNA and pharmacological inhibitors targeting the HAT domain produced dissociable effects. In the hippocampus, CBP or p300 inhibition impaired long-term but not short-term object memory, while inhibition of PCAF impaired memory at both delays. In PRh, HAT inhibition did not impair STM, and only CBP and PCAF inhibition disrupted LTM; p300 inhibition had no effects. Messenger RNA analyses revealed findings consistent with the pattern of behavioral effects, as all three enzymes were upregulated in the hippocampus (dentate gyrus) following learning, whereas only CBP and PCAF were upregulated in PRh. These results demonstrate, for the first time, the necessity of histone acetyltransferase activity for PRh-mediated object memory and indicate that the specific mnemonic roles of distinctive histone acetyltransferases can be dissociated according to specific brain regions and memory timeframe.