Pharmacological Selectivity Within Class I Histone Deacetylases Predicts Effects on Synaptic Function and Memory Rescue

Distinct roles for the deacetylase domain of HDAC3 in the hippocampus and medial prefrontal cortex in the formation and extinction of memory

Histone deacetylases (HDACs) are chromatin modifying enzymes that have been implicated as powerful negative regulators of memory processes. HDAC3has been shown to play a pivotal role in long-term memory for object location as well as the extinction of cocaine-associated memory, but it is unclear whether this function depends on the deacetylase domain of HDAC3. Here, we tested whether the deacetylase domain of HDAC3has a role in object location memory formation as well as the formation and extinction of cocaine-associated memories. Using a deacetylase-dead point mutant of HDAC3, we found that selectively blocking HDAC3 deacetylase activity in the dorsal hippocampus enhanced long-term memory for object location, but had no effect on the formation of cocaine-associated memory. When this same point mutant virus of HDAC3 was infused into the prelimbic cortex, it failed to affect cocaine-associated memory formation. With regards to extinction, impairing the HDAC3 deacetylase domain in the infralimbic cortex had no effect on extinction, but a facilitated extinction effect was observed when the point mutant virus was delivered to the dorsal hippocampus. These results suggest that the deacetylase domain of HDAC3 plays a selective role in specific brain regions underlying long-term memory formation of object location as well as cocaine-associated memory formation and extinction.

Inhibition of HDAC3 prevents diabetic cardiomyopathy in OVE26 mice via epigenetic regulation of DUSP5-ERK1/2 pathway

Inhibition of total histone deacetylases (HDACs) was phenomenally associated with the prevention of diabetic cardiomyopathy (DCM). However, which specific HDAC plays the key role in DCM remains unclear. The present study was designed to determine whether DCM can be prevented by specific inhibition of HDAC3 and to elucidate the mechanisms by which inhibition of HDAC3 prevents DCM. Type 1 diabetes OVE26 and age-matched wild-type (WT) mice were given the selective HDAC3 inhibitor RGFP966 or vehicle for 3 months. These mice were then killed immediately or 3 months later for cardiac function and pathological examination. HDAC3 activity was significantly increased in the heart of diabetic mice. Administration of RGFP966 significantly prevented DCM, as evidenced by improved diabetes-induced cardiac dysfunction, hypertrophy, and fibrosis, along with diminished cardiac oxidative stress, inflammation, and insulin resistance, not only in the mice killed immediately or 3 months later following the 3-month treatment. Furthermore, phosphorylated extracellular signal-regulated kinases (ERK) 1/2, a well-known initiator of cardiac hypertrophy, was significantly increased, while dual specificity phosphatase 5 (DUSP5), an ERK1/2 nuclear phosphatase, was substantially decreased in diabetic hearts. Both of these changes were prevented by RGFP966. Chromatin immunoprecipitation (ChIP) assay showed that HDAC3 inhibition elevated histone H3 acetylation on the DUSP5 gene promoter at both two time points. These findings suggest that diabetes-activated HDAC3 inhibits DUSP5 expression through deacetylating histone H3 on the primer region of DUSP5 gene, leading to the derepression of ERK1/2 and the initiation of DCM. The present study indicates the potential application of HDAC3 inhibitor for the prevention of DCM.

The development prospection of HDAC inhibitors as a potential therapeutic direction in Alzheimer's disease

Alzheimer’s disease (AD) is a chronic neurodegenerative disease, which is associated with learning and memory impairment in the elderly. Recent studies have found that treating AD in the way of chromatin remodeling via histone acetylation is a promising therapeutic regimen. In a number of recent studies, inhibitors of histone deacetylase (HDACs) have been found to be a novel promising therapeutic agents for neurological disorders, particularly for AD and other neurodegenerative diseases. Although HDAC inhibitors have the ability to ameliorate cognitive impairment, successful treatments in the classic AD animal model are rarely translated into clinical trials. As for the reduction of unwanted side effects, the development of HDAC inhibitors with increased isoform selectivity or seeking other directions is a key issue that needs to be addressed. The review focused on literatures on epigenetic mechanisms in recent years, especially on histone acetylation in terms of the enhancement of specificity, efficacy and avoiding side effects for treating AD.

Context and Auditory Fear are Differentially Regulated by HDAC3 Activity in the Lateral and Basal Subnuclei of the Amygdala

Histone acetylation is a fundamental epigenetic mechanism that is dynamically regulated during memory formation. Histone acetyltransferases (HATs) and histone deacetylases (HDACs) compete to modulate histone acetylation, allowing for rapid changes in acetylation in response to a learning event. HDACs are known to be powerful negative regulators of memory formation, but it is not clear whether this function depends on HDAC enzymatic activity per se. Here, we tested whether the enzymatic activity of an individual Class I HDAC, HDAC3, has a role in fear memory formation in subregions of the hippocampus and amygdala. We found that fear conditioning drove expression of the immediate early genes cFos and Nr4a2 in the hippocampus, which coincided with reduced HDAC3 occupancy at these promoters. Using a dominant-negative, deacetylase-dead point mutant virus (AAV-HDAC3(Y298H)-v5), we found that selectively blocking HDAC3 deacetylase activity in either the dorsal hippocampus or basal nucleus of the amygdala enhanced context fear without affecting tone fear. Blocking HDAC3 activity in the lateral nucleus of the amygdala, on the other hand, enhanced tone, but not context fear memory. These results show for the first time that the enzymatic activity of HDAC3 functions to negatively regulate fear memory formation. Further, HDAC3 activity regulates different aspects of fear memory in the basal and lateral subregions of the amygdala. Thus, the deacetylase activity of HDAC3 is a powerful negative regulator of fear memory formation in multiple subregions of the fear circuit.

Targeting Class I Histone Deacetylases in a "Complex" Environment

Histone deacetylase (HDAC) inhibitors are proven anticancer therapeutics and have potential in the treatment of many other diseases including HIV infection, Alzheimer's disease, and Friedreich's ataxia. A problem with the currently available HDAC inhibitors is that they have limited specificity and target multiple deacetylases. Designing isoform-selective inhibitors has proven challenging due to similarities in the structure and chemistry of HDAC active sites. However, the fact that HDACs 1, 2, and 3 are recruited to several large multi-subunit complexes, each with particular biological functions, raises the possibility of specifically inhibiting individual complexes. This may be assisted by recent structural and functional information about the assembly of these complexes. Here, we review the available structural information and discuss potential targeting strategies.

A First-in-Class Small-Molecule that Acts as a Dual Inhibitor of HDAC and PDE5 and that Rescues Hippocampal Synaptic Impairment in Alzheimer's Disease Mice

The targeting of two independent but synergistic enzymatic activities, histone deacetylases (HDACs, class I and HDAC6) and phosphodiesterase 5 (PDE5), has recently been validated as a potentially novel therapeutic approach for Alzheimer's disease (AD). Here we report the discovery of a new first-in-class small-molecule (CM-414) that acts as a dual inhibitor of PDE5 and HDACs. We have used this compound as a chemical probe to validate this systems therapeutics strategy, where an increase in the activation of cAMP/cGMP-responsive element-binding protein (CREB) induced by PDE5 inhibition, combined with moderate HDAC class I inhibition, leads to efficient histone acetylation. This molecule rescued the impaired long-term potentiation evident in hippocampal slices from APP/PS1 mice. Chronic treatment of Tg2576 mice with CM-414 diminished brain Aβ and tau phosphorylation (pTau) levels, increased the inactive form of GSK3β, reverted the decrease in dendritic spine density on hippocampal neurons, and reversed their cognitive deficits, at least in part by inducing the expression of genes related to synaptic transmission. Thus, CM-414 may serve as the starting point to discover balanced dual inhibitors with an optimal efficacy and safety profile for clinical testing on AD patients.

Inhibition of Histone Deacetylase 3 Restores Amyloid-β Oligomer-Induced Plasticity Deficit in Hippocampal CA1 Pyramidal Neurons

Neurodegenerative diseases such as Alzheimer's disease (AD) are associated with alterations in epigenetic factors leading to cognitive decline. Histone deacetylase 3 (HDAC3) is a known critical epigenetic negative regulator of learning and memory. In this study, attenuation of long-term potentiation by amyloid-β oligomer, and its reversal by specific HDAC3 inhibitor RGFP966, was performed in rat CA1 pyramidal neurons using whole cell voltage-clamp and field recording techniques. Our findings provide the first evidence that amyloid-β oligomer-induced synaptic plasticity impairment can be prevented by inhibition of HDAC3 enzyme both at the single neuron as well as in a population of neurons, thus identifying HDAC3 as a potential target for ameliorating AD related plasticity impairments.

Development of Allosteric Hydrazide-Containing Class I Histone Deacetylase Inhibitors for Use in Acute Myeloid Leukemia

One of the biggest hurdles yet to be overcome for the continued improvement of histone deacetylase (HDAC) inhibitors is finding alternative motifs equipotent to the classic and ubiquitously used hydroxamic acid. The N-hydroxyl group of this motif is highly subject to sulfation/glucoronidation-based inactivation in humans; compounds containing this motif require much higher dosing in clinic to achieve therapeutic concentrations. With the goal of developing a second generation of HDAC inhibitors lacking this hydroxamate, we designed a series of potent and selective class I HDAC inhibitors using a hydrazide motif. These inhibitors are impervious to glucuronidation and demonstrate allosteric inhibition. In vitro and ex vivo characterization of our lead analogues' efficacy, selectivity, and toxicity profiles demonstrate that they possess low nanomolar activity against models of acute myeloid leukemia (AML) and are at least 100-fold more selective for AML than solid immortalized cells such as HEK293 or human peripheral blood mononuclear cells.

Histone Deacetylase (HDAC) Inhibitors - emerging roles in neuronal memory, learning, synaptic plasticity and neural regeneration

Epigenetic regulation of neuronal signalling through histone acetylation dictates transcription programs that govern neuronal memory, plasticity and learning paradigms. Histone Acetyl Transferases (HATs) and Histone Deacetylases (HDACs) are antagonistic enzymes that regulate gene expression through acetylation and deacetylation of histone proteins around which DNA is wrapped inside a eukaryotic cell nucleus. The epigenetic control of HDACs and the cellular imbalance between HATs and HDACs dictate disease states and have been implicated in muscular dystrophy, loss of memory, neurodegeneration and autistic disorders. Altering gene expression profiles through inhibition of HDACs is now emerging as a powerful technique in therapy. This review presents evolving applications of HDAC inhibitors as potential drugs in neurological research and therapy. Mechanisms that govern their expression profiles in neuronal signalling, plasticity and learning will be covered. Promising and exciting possibilities of HDAC inhibitors in memory formation, fear conditioning, ischemic stroke and neural regeneration have been detailed.

Histone deacetylase inhibition abolishes stress-induced spatial memory impairment

Acute stress induced before spatial training impairs memory consolidation. Although non-epigenetic underpinning of such effect has been described, the epigenetic mechanisms involved have not yet been studied. Since spatial training and intense stress have opposite effects on histone acetylation balance, it is conceivable that disruption of such balance may underlie acute stress-induced spatial memory consolidation impairment and that inhibiting histone deacetylases prevents such effect. Trichostatin-A (TSA, a histone deacetylase inhibitor) was used to test its effectiveness in preventing stress' deleterious effect on memory. Male Wistar rats were trained in a spatial task in the Barnes maze; 1-h movement restraint was applied to half of them before training. Immediately after training, stressed and non-stressed animals were randomly assigned to receive either TSA (1mg/kg) or vehicle intraperitoneal injection. Twenty-four hours after training, long-term spatial memory was tested; plasma and brain tissue were collected immediately after the memory test to evaluate corticosterone levels and histone H3 acetylation in several brain areas. Stressed animals receiving vehicle displayed memory impairment, increased plasma corticosterone levels and markedly reduced histone H3 acetylation in prelimbic cortex and hippocampus. Such effects did not occur in stressed animals treated with TSA. The aforementioned results support the hypothesis that acute stress induced-memory impairment is related to histone deacetylation.

Challenges in developing drugs for pediatric CNS disorders: A focus on psychopharmacology

Many psychiatric and behavioral disorders manifest in childhood (attention deficit hyperactivity disorder, obsessive compulsive disorder, anxiety, depression, schizophrenia, autism spectrum disorder, etc.) and the opportunity for intervening early may attenuate full development of the disorder and lessen long term disability. Yet, pediatric drug approvals for CNS indications are limited, and pediatric testing generally occurs only after establishing adult efficacy, more as an afterthought rather than with the initial goal of developing the medication for a pediatric CNS indication. With pharmaceutical companies decreasing funding of their neuroscience research divisions overall, the prospects for moving promising investigational drugs forward into pediatrics will only decline. The goal of this review is to highlight important challenges around pediatric drug development for psychiatric disorders, specifically during clinical development, and to present opportunities for filling these gaps, using new strategies for de-risking investigational drugs in new clinical trial designs/models. We will first present the current trends in pediatric drug efficacy testing in academic research and in industry trials, we will then discuss the regulatory landscape of pediatric drug testing, including policies intended to support and encourage more testing. Obstacles that remain will then be presented, followed by new designs, funding opportunities and considerations for testing investigational drugs safely.

The Effects of Pharmacological Inhibition of Histone Deacetylase 3 (HDAC3) in Huntington's Disease Mice

An important epigenetic modification in Huntington’s disease (HD) research is histone acetylation, which is regulated by histone acetyltransferase and histone deacetylase (HDAC) enzymes. HDAC inhibitors have proven effective in HD model systems, and recent work is now focused on functional dissection of the individual HDAC enzymes in these effects. Histone deacetylase 3 (HDAC3), a member of the class I subfamily of HDACs, has previously been implicated in neuronal toxicity and huntingtin-induced cell death. Hence, we tested the effects of RGFP966 ((E)-N-(2-amino-4-fluorophenyl)-3-(1-cinnamyl-1H-pyrazol-4-yl)acrylamide), a benzamide-type HDAC inhibitor that selectively targets HDAC3, in the N171-82Q transgenic mouse model of HD. We found that RGFP966 at doses of 10 and 25 mg/kg improves motor deficits on rotarod and in open field exploration, accompanied by neuroprotective effects on striatal volume. In light of previous studies implicating HDAC3 in immune function, we measured gene expression changes for 84 immune-related genes elicited by RGFP966 using quantitative PCR arrays. RGFP966 treatment did not cause widespread changes in cytokine/chemokine gene expression patterns, but did significantly alter the striatal expression of macrophage migration inhibitory factor (Mif), a hormone immune modulator associated with glial cell activation, in N171-82Q transgenic mice, but not WT mice. Accordingly, RGFP966-treated mice showed decreased glial fibrillary acidic protein (GFAP) immunoreactivity, a marker of astrocyte activation, in the striatum of N171-82Q transgenic mice compared to vehicle-treated mice. These findings suggest that the beneficial actions of HDAC3 inhibition could be related, in part, with lowered Mif levels and its associated downstream effects.

Sensory Cortical Plasticity Participates in the Epigenetic Regulation of Robust Memory Formation

Neuroplasticity remodels sensory cortex across the lifespan. A function of adult sensory cortical plasticity may be capturing available information during perception for memory formation. The degree of experience-dependent remodeling in sensory cortex appears to determine memory strength and specificity for important sensory signals. A key open question is how plasticity is engaged to induce different degrees of sensory cortical remodeling. Neural plasticity for long-term memory requires the expression of genes underlying stable changes in neuronal function, structure, connectivity, and, ultimately, behavior. Lasting changes in transcriptional activity may depend on epigenetic mechanisms; some of the best studied in behavioral neuroscience are DNA methylation and histone acetylation and deacetylation, which, respectively, promote and repress gene expression. One purpose of this review is to propose epigenetic regulation of sensory cortical remodeling as a mechanism enabling the transformation of significant information from experiences into content-rich memories of those experiences. Recent evidence suggests how epigenetic mechanisms regulate highly specific reorganization of sensory cortical representations that establish a widespread network for memory. Thus, epigenetic mechanisms could initiate events to establish exceptionally persistent and robust memories at a systems-wide level by engaging sensory cortical plasticity for gating what and how much information becomes encoded.

Histone Deacetylase Inhibition via RGFP966 Releases the Brakes on Sensory Cortical Plasticity and the Specificity of Memory Formation

Research over the past decade indicates a novel role for epigenetic mechanisms in memory formation. Of particular interest is chromatin modification by histone deacetylases (HDACs), which, in general, negatively regulate transcription. HDAC deletion or inhibition facilitates transcription during memory consolidation and enhances long-lasting forms of synaptic plasticity and long-term memory. A key open question remains: How does blocking HDAC activity lead to memory enhancements? To address this question, we tested whether a normal function of HDACs is to gate information processing during memory formation. We used a class I HDAC inhibitor, RGFP966 (C21H19FN4O), to test the role of HDAC inhibition for information processing in an auditory memory model of learning-induced cortical plasticity. HDAC inhibition may act beyond memory enhancement per se to instead regulate information in ways that lead to encoding more vivid sensory details into memory. Indeed, we found that RGFP966 controls memory induction for acoustic details of sound-to-reward learning. Rats treated with RGFP966 while learning to associate sound with reward had stronger memory and additional information encoded into memory for highly specific features of sounds associated with reward. Moreover, behavioral effects occurred with unusually specific plasticity in primary auditory cortex (A1). Class I HDAC inhibition appears to engage A1 plasticity that enables additional acoustic features to become encoded in memory. Thus, epigenetic mechanisms act to regulate sensory cortical plasticity, which offers an information processing mechanism for gating what and how much is encoded to produce exceptionally persistent and vivid memories. Significance statement: Here we provide evidence of an epigenetic mechanism for information processing. The study reveals that a class I HDAC inhibitor (Malvaez et al., 2013; Rumbaugh et al., 2015; RGFP966, chemical formula C21H19FN4O) alters the formation of auditory memory by enabling more acoustic information to become encoded into memory. Moreover, RGFP966 appears to affect cortical plasticity: the primary auditory cortex reorganized in a manner that was unusually "tuned-in" to the specific sound cues and acoustic features that were related to reward and subsequently remembered. We propose that HDACs control "informational capture" at a systems level for what and how much information is encoded by gating sensory cortical plasticity that underlies the sensory richness of newly formed memories.

Epigenetic Alterations in Alzheimer's Disease

Alzheimer’s disease (AD) is the major cause of dementia in Western societies. It progresses asymptomatically during decades before being belatedly diagnosed when therapeutic strategies have become unviable. Although several genetic alterations have been associated with AD, the vast majority of AD cases do not show strong genetic underpinnings and are thus considered a consequence of non-genetic factors. Epigenetic mechanisms allow for the integration of long-lasting non-genetic inputs on specific genetic backgrounds, and recently, a growing number of epigenetic alterations in AD have been described. For instance, an accumulation of dysregulated epigenetic mechanisms in aging, the predominant risk factor of AD, might facilitate the onset of the disease. Likewise, mutations in several enzymes of the epigenetic machinery have been associated with neurodegenerative processes that are altered in AD such as impaired learning and memory formation. Genome-wide and locus-specific epigenetic alterations have also been reported, and several epigenetically dysregulated genes validated by independent groups. From these studies, a picture emerges of AD as being associated with DNA hypermethylation and histone deacetylation, suggesting a general repressed chromatin state and epigenetically reduced plasticity in AD. Here we review these recent findings and discuss several technical and methodological considerations that are imperative for their correct interpretation. We also pay particular focus on potential implementations and theoretical frameworks that we expect will help to better direct future studies aimed to unravel the epigenetic participation in AD.

Why Hydroxamates May Not Be the Best Histone Deacetylase Inhibitors--What Some May Have Forgotten or Would Rather Forget?

Hydroxamate-based histone deacetylase inhibitors (HDACIs) have been approved as therapeutic agents by the US Food and Drug Administration for use in oncology applications. While the potential utility of such HDACIs in other areas of medicinal chemistry is tremendous, there are significant concerns that "pan-HDAC inhibitors" may be too broadly acting and/or toxic for clinical use beyond oncology. In addition to the isozyme selectivity challenge, the potential mutagenicity of hydroxamate-containing HDAC inhibitors represents a major hindrance in their application to other therapeutic areas. Herein we report on the mutagenicity of known hydroxamates, discuss the mechanisms responsible for their genotoxicity, and review some of the current alternatives to hydroxamates. We conclude that the hydroxamate group, while providing high-potency HDACIs, is not necessarily the best zinc-binding group for HDACI drug discovery.

Concomitant histone deacetylase and phosphodiesterase 5 inhibition synergistically prevents the disruption in synaptic plasticity and it reverses cognitive impairment in a mouse model of Alzheimer's disease

BACKGROUND

Given the implication of histone acetylation in memory processes, histone deacetylase inhibitors (HDACIs) have been postulated as potential modulators of cognitive impairment in Alzheimer's disease (AD). However, dose-dependent side effects have been described in patients with the currently available broad-spectrum HDACIs, explaining why their therapeutic potential has not been realized for chronic diseases. Here, by simultaneously targeting two independent enzyme activities, histone deacetylase (HDAC) and phosphodiesterase-5 (PDE5), we propose a novel mode of inhibitory action that might increase the therapeutic specificity of HDACIs.

RESULTS

The combination of vorinostat, a pan-HDACI, and tadalafil, a PDE5 inhibitor, rescued the long-term potentiation impaired in slices from APP/PS1 mice. When administered in vivo, the combination of these drugs alleviated the cognitive deficits in AD mice, as well as the amyloid and tau pathology, and it reversed the reduced dendritic spine density on hippocampal neurons. Significantly, the combination of vorinostat and tadalafil was more effective than each drug alone, both against the symptoms and in terms of disease modification, and importantly, these effects persisted after a 4-week washout period.

CONCLUSIONS

The results highlight the pharmacological potential of a combination of molecules that inhibit HDAC and PDE5 as a therapeutic approach for AD treatment.