Examining the contribution of histone modification to sex differences in learning and memory

X-linked ubiquitin-specific peptidase 11 increases tauopathy vulnerability in women

Although women experience significantly higher tau burden and increased risk for Alzheimer's disease (AD) than men, the underlying mechanism for this vulnerability has not been explained. Here, we demonstrate through in vitro and in vivo models, as well as human AD brain tissue, that X-linked ubiquitin specific peptidase 11 (USP11) augments pathological tau aggregation via tau deubiquitination initiated at lysine-281. Removal of ubiquitin provides access for enzymatic tau acetylation at lysines 281 and 274. USP11 escapes complete X-inactivation, and female mice and people both exhibit higher USP11 levels than males. Genetic elimination of usp11 in a tauopathy mouse model preferentially protects females from acetylated tau accumulation, tau pathology, and cognitive impairment. USP11 levels also strongly associate positively with tau pathology in females but not males. Thus, inhibiting USP11-mediated tau deubiquitination may provide an effective therapeutic opportunity to protect women from increased vulnerability to AD and other tauopathies.

Temporal endurance of exercise-induced benefits on hippocampus-dependent memory and synaptic plasticity in female mice

Exercise facilitates hippocampal neurogenesis and neuroplasticity that in turn, promotes cognitive function. Our previous studies have demonstrated that in male mice, voluntary exercise enables hippocampus-dependent learning in conditions that are normally subthreshold for long-term memory formation in sedentary animals. Such cognitive enhancement can be maintained long after exercise has ceased and can be re-engaged by a subsequent subthreshold exercise session, suggesting exercise-induced benefits are temporally dynamic. In females, the extent to which the benefits of exercise can be maintained and the mechanisms underlying this maintenance have yet to be defined. Here, we examined the exercise parameters required to initiate and maintain the benefits of exercise in female C57BL/6J mice. Using a subthreshold version of the hippocampus-dependent task called object-location memory (OLM) task, we show that 14d of voluntary exercise enables learning under subthreshold acquisition conditions in female mice. Following the initial exercise, a 7d sedentary delay results in diminished performance, which can be re-facilitated when animals receive 2d of reactivating exercise following the sedentary delay. Assessment of estrous cycle reveals enhanced wheel running activity during the estrus phase relative to the diestrus phase, whereas estrous phase on training or test had no effect on OLM performance. Utilizing the same exercise parameters, we demonstrate that 14d of exercise enhances long-term potentiation (LTP) in the CA1 region of the hippocampus, an effect that persists throughout the sedentary delay and following the reactivating exercise session. Previous studies have proposed exercise-induced BDNF upregulation as the mechanism underlying exercise-mediated benefits on synaptic plasticity and cognition. However, our assessment of hippocampal Bdnf mRNA expression following memory retrieval reveals no difference between exercise conditions and control, suggesting that persistent Bdnf upregulation may not be required for maintenance of exercise-induced benefits. Together, our data indicate that 14d of voluntary exercise can initiate long-lasting benefits on neuroplasticity and cognitive function in female mice, establishing the first evidence on the temporal endurance of exercise-induced benefits in females.

The epigenetic role of proteasome subunit RPT6 during memory formation in female rats

Reports of sex differences in the neurobiology of memory formation are becoming more prevalent. Despite this, much remains unknown about the role of sex in this process. We previously reported the first evidence of a novel epigenetic role for proteasome subunit RPT6 during memory formation in the hippocampus of male rodents whereby it associated with monoubiquitinated histone H2B (H2Bubi). Here, we used molecular, biochemical, and behavioral approaches to investigate whether RPT6 has a similar epigenetic role during memory formation in female rats. Following contextual fear conditioning, we found that RPT6 levels and DNA binding at regions coding for c-fos, the previously identified target of RPT6 in males, were unchanged in the hippocampus of females and that loss of RPT6 did not alter learning-induced increases in c-fos However, RPT6 was in complex with H2Bubi in the female hippocampus and this association increased with fear conditioning, suggesting that it could still retain an epigenetic function. Consistent with this, hippocampal siRNA-mediated knockdown of the RPT6-coding gene, Psmc5, impaired memory in females. These results suggest that while RPT6 does associate with epigenetic H2Bubi during memory formation in both males and females, it has sex-specific gene targets during the memory consolidation process.

Sex bias in systemic lupus erythematosus: a molecular insight

Acknowledging sex differences in immune response is particularly important when we consider the differences between men and women in the incidence of disease. For example, over 80% of autoimmune disease occurs in women, whereas men have a higher incidence of solid tumors compared to women. In general women have stronger innate and adaptive immune responses than men, explaining their ability to clear viral and bacterial infections faster, but also contributing to their increased susceptibility to autoimmune disease. The autoimmune disease systemic lupus erythematosus (SLE) is the archetypical sexually dimorphic disease, with 90% of patients being women. Various mechanisms have been suggested to account for the female prevalence of SLE, including sex hormones, X-linked genes, and epigenetic regulation of gene expression. Here, we will discuss how these mechanisms contribute to pathobiology of SLE and how type I interferons work with them to augment sex specific disease pathogenesis in SLE.

SEX-DEPENDENT IMPACT OF MICROBIOTA STATUS ON CEREBRAL μ -OPIOID RECEPTOR DENSITY IN FISCHER RATS

μ‐opioid receptors (MOPr) play a critical role in social play, reward and pain, in a sex‐ and age‐dependent manner. There is evidence to suggest that sex and age differences in brain MOPr density may be responsible for this variability; however, little is known about the factors driving these differences in cerebral MOPr density. Emerging evidence highlights gut microbiota's critical influence and its bidirectional interaction with the brain on neurodevelopment. Therefore, we aimed to determine the impact of gut microbiota on MOPr density in male and female brains at different developmental stages. Quantitative [³H]DAMGO autoradiographic binding was carried out in the forebrain of male and female conventional (CON) and germ‐free (GF) rats at postnatal days (PND) 8, 22 and 116–150. Significant ‘microbiota status X sex’, ‘age X brain region’ interactions and microbiota status‐ and age‐dependent effects on MOPr binding were uncovered. Microbiota status influenced MOPr levels in males but not females, with higher MOPr levels observed in GF versus CON rats overall regions and age groups. In contrast, no overall sex differences were observed in GF or CON rats. Interestingly, within‐age planned comparison analysis conducted in frontal cortical and brain regions associated with reward revealed that this microbiota effect was restricted only to PND22 rats. Thus, this pilot study uncovers the critical sex‐dependent role of gut microbiota in regulating cerebral MOPr density, which is restricted to the sensitive developmental period of weaning. This may have implications in understanding the importance of microbiota during early development on opioid signalling and associated behaviours.

The Histone H3 K4me3, K27me3, and K27ac Genome-Wide Distributions Are Differently Influenced by Sex in Brain Cortexes and Gastrocnemius of the Alzheimer’s Disease PSAPP Mouse Model

Background: Women represent the majority of Alzheimer’s disease patients and show typical symptoms. Genetic, hormonal, and behavioral mechanisms have been proposed to explain sex differences in dementia prevalence. However, whether sex differences exist in the epigenetic landscape of neuronal tissue during the progression of the disease is still unknown. Methods: To investigate the differences of histone H3 modifications involved in transcription, we determined the genome-wide profiles of H3K4me3, H3K27ac, and H3K27me3 in brain cortexes of an Alzheimer mouse model (PSAPP). Gastrocnemius muscles were also tested since they are known to be different in the two sexes and are affected during the disease progression. Results: Correlation analysis distinguished the samples based on sex for H3K4me3 and H3K27me3 but not for H3K27ac. The analysis of transcription starting sites (TSS) signal distribution, and analysis of bounding sites revealed that gastrocnemius is more influenced than brain by sex for the three histone modifications considered, exception made for H3K27me3 distribution on the X chromosome which showed sex-related differences in promoters belonging to behavior and cellular or neuronal spheres in mice cortexes. Conclusions: H3K4me3, H3K27ac, and H3K27me3 signals are slightly affected by sex in brain, with the exception of H3K27me3, while a higher number of differences can be found in gastrocnemius.

A putative role for lncRNAs in epigenetic regulation of memory

The central dogma of molecular genetics is defined as encoded genetic information within DNA, transcribed into messenger RNA, which contain the instructions for protein synthesis, thus imparting cellular functionality and ultimately life. This molecular genetic theory has given birth to the field of neuroepigenetics, and it is now well established that epigenetic regulation of gene transcription is critical to the learning and memory process. In this review, we address a potential role for a relatively new player in the field of epigenetic crosstalk - long non-coding RNAs (lncRNAs). First, we briefly summarize epigenetic mechanisms in memory formation and examine what little is known about the emerging role of lncRNAs during this process. We then focus discussions on how lncRNAs interact with epigenetic mechanisms to control transcriptional programs under various conditions in the brain, and how this may be applied to regulation of gene expression necessary for memory formation. Next, we explore how epigenetic crosstalk in turn serves to regulate expression of various individual lncRNAs themselves. To highlight the importance of further exploring the role of lncRNA in epigenetic regulation of gene expression, we consider the significant relationship between lncRNA dysregulation and declining memory reserve with aging, Alzheimer's disease, and epilepsy, as well as the promise of novel therapeutic interventions. Finally, we conclude with a discussion of the critical questions that remain to be answered regarding a role for lncRNA in memory.

Ubiquitination of Histone H2B by Proteasome Subunit RPT6 Controls Histone Methylation Chromatin Dynamics During Memory Formation

BACKGROUND

Posttranslational histone modifications play a critical role in the regulation of gene transcription underlying synaptic plasticity and memory formation. One such epigenetic change is histone ubiquitination, a process that is mediated by the ubiquitin-proteasome system in a manner similar to that by which proteins are normally targeted for degradation. However, histone ubiquitination mechanisms are poorly understood in the brain and in learning. In this article, we describe a new role for the ubiquitin-proteasome system in histone crosstalk, showing that learning-induced monoubiquitination of histone H2B (H2Bubi) is required for increases in the transcriptionally active H3 lysine 4 trimethylation (H3K4me3) mark at learning-related genes in the hippocampus.

METHODS

Using a series of molecular, biochemical, electrophysiological, and behavioral experiments, we interrogated the effects of short interfering RNA-mediated knockdown and CRISPR (clustered regularly interspaced short palindromic repeats)-mediated upregulation of ubiquitin ligases, deubiquitinating enzymes and histone methyltransferases in the rat dorsal hippocampus during memory consolidation.

RESULTS

We show that H2Bubi recruits H3K4me3 through a process that is dependent on the 19S proteasome subunit RPT6 and that a loss of H2Bubi in the hippocampus prevents learning-induced increases in H3K4me3, gene transcription, synaptic plasticity, and memory formation. Furthermore, we show that CRISPR-dCas9-mediated increases in H2Bubi promote H3K4me3 and memory formation under weak training conditions and that promoting histone methylation does not rescue memory impairments resulting from loss of H2Bubi.

CONCLUSIONS

These results suggest that H2B ubiquitination regulates histone crosstalk in learning by way of nonproteolytic proteasome function, demonstrating a novel mechanism by which histone modifications are coordinated in response to learning.

The histone H3 lysine 9 methyltransferase G9a/GLP complex activity is required for long-term consolidation of spatial memory in mice

The G9a/G9a-like protein (GLP) histone lysine dimethyltransferase complex and downstream histone H3 lysine 9 dimethylation (H3K9me2) repressive mark have recently emerged as key transcriptional regulators of gene expression programs necessary for long-term memory (LTM) formation in the dorsal hippocampus. However, the role for hippocampal G9a/GLP complex in mediating the consolidation of spatial LTM remains largely unknown. Using a water maze competition task in which both dorsal hippocampus-dependent spatial and striatum-dependent cue navigation strategies are effective to solve the maze, we found that pharmacological inhibition of G9a/GLP activity immediately after learning disrupts long-term consolidation of previously learned spatial information in male mice, hence producing cue bias on the competition test performed 24 h later. Importantly, the inhibition of hippocampal G9a/GLP did not disrupt short-term memory retention. Immunohistochemical analyses revealed increases in global levels of permissive histone H3K9 acetylation in the dorsal hippocampus and dorsal striatum at 1 h post-training, which persisted up to 24 h in the hippocampus. Conversely, H3K9me2 levels were either unchanged in the dorsal hippocampus or transiently decreased at 15 min post-training in the dorsal striatum. Finally, the inhibition of G9a/GLP activity further increased global levels of H3K9 acetylation while decreasing H3K9me2 in the hippocampus at 1 h post-training. However, both marks returned to vehicle control levels at 24 h. Together, these findings support the possibility that G9a/GLP in the dorsal hippocampus is required for the transcriptional switch from short-term to long-term spatial memory formation.

Systemic HDAC3 inhibition ameliorates impairments in synaptic plasticity caused by simulated galactic cosmic radiation exposure in male mice

Deep space travel presents a number of measurable risks including exposure to a spectrum of radiations of varying qualities, termed galactic cosmic radiation (GCR) that are capable of penetrating the spacecraft, traversing through the body and impacting brain function. Using rodents, studies have reported that exposure to simulated GCR leads to cognitive impairments associated with changes in hippocampus function that can persist as long as one-year post exposure with no sign of recovery. Whether memory can be updated to incorporate new information in mice exposed to GCR is unknown. Further, mechanisms underlying long lasting impairments in cognitive function as a result of GCR exposure have yet to be defined. Here, we examined whether whole body exposure to simulated GCR using 6 ions and doses of 5 or 30 cGy interfered with the ability to update an existing memory or impact hippocampal synaptic plasticity, a cellular mechanism believed to underlie memory processes, by examining long term potentiation (LTP) in acute hippocampal slices from middle aged male mice 3.5-5 months after radiation exposure. Using a modified version of the hippocampus-dependent object location memory task developed by our lab termed "Objects in Updated Locations" (OUL) task we find that GCR exposure impaired hippocampus-dependent memory updating and hippocampal LTP 3.5-5 months after exposure. Further, we find that impairments in LTP are reversed through one-time systemic subcutaneous injection of the histone deacetylase 3 inhibitor RGFP 966 (10 mg/kg), suggesting that long lasting impairments in cognitive function may be mediated at least in part, through epigenetic mechanisms.

Approaching Sex Differences in Cardiovascular Non-Coding RNA Research

Cardiovascular disease (CVD) is the biggest cause of sickness and mortality worldwide in both males and females. Clinical statistics demonstrate clear sex differences in risk, prevalence, mortality rates, and response to treatment for different entities of CVD. The reason for this remains poorly understood. Non-coding RNAs (ncRNAs) are emerging as key mediators and biomarkers of CVD. Similarly, current knowledge on differential regulation, expression, and pathology-associated function of ncRNAs between sexes is minimal. Here, we provide a state-of-the-art overview of what is known on sex differences in ncRNA research in CVD as well as discussing the contributing biological factors to this sex dimorphism including genetic and epigenetic factors and sex hormone regulation of transcription. We then focus on the experimental models of CVD and their use in translational ncRNA research in the cardiovascular field. In particular, we want to highlight the importance of considering sex of the cellular and pre-clinical models in clinical studies in ncRNA research and to carefully consider the appropriate experimental models most applicable to human patient populations. Moreover, we aim to identify sex-specific targets for treatment and diagnosis for the biggest socioeconomic health problem globally.

A Unique Mouse Model of Early Life Exercise Enables Hippocampal Memory and Synaptic Plasticity

Physical exercise is a powerful modulator of learning and memory. Mechanisms underlying the cognitive benefits of exercise are well documented in adult rodents. Exercise studies targeting postnatal periods of hippocampal maturation (specifically targeting periods of synaptic reorganization and plasticity) are lacking. We characterize a model of early-life exercise (ELE) in male and female mice designed with the goal of identifying critical periods by which exercise may have a lasting impact on hippocampal memory and synaptic plasticity. Mice freely accessed a running wheel during three postnatal periods: the 4^th postnatal week (juvenile ELE, P21–27), 6^th postnatal week (adolescent ELE, P35–41), or 4^th-6^th postnatal weeks (juvenile-adolescent ELE, P21–41). All exercise groups increased their running distances during ELE. When exposed to a subthreshold learning stimulus, juv ELE and juv-adol ELE formed lasting long-term memory for an object location memory task, whereas sedentary and adol ELE mice did not. Electrophysiological experiments revealed enhanced long-term potentiation in hippocampal CA1 in the juvenile-adolescent ELE group. I/O curves were also significantly modulated in all mice that underwent ELE. Our results suggest that early-life exercise, specifically during the 4^th postnatal week, can enable hippocampal memory, synaptic plasticity, and alter hippocampal excitability when occurring during postnatal periods of hippocampal maturation.