Recovery of learning and memory is associated with chromatin remodelling

Specific N-terminal mutations in the human androgen receptor induce cytotoxicity

Polyglutamine (polyQ) stretch amplification in different proteins causes neurodegenerative disease. These proteins form intracellular aggregates thought to be cytotoxic but differ in pathology and tissue specificity. Here, we demonstrate that specific sequences outside the polyQ stretch of the human androgen receptor contribute to polyQ pathology. An exchange of two N-terminal serine phosphorylation residues to alanine in the wild type androgen receptor (ARQ22dm) resulted in cytoplasmic accumulation and increased early hormone-dependent aggregation of the receptor. In a Drosophila model, the ARQ22dm was cytotoxic, and developing larvae expressing this receptor showed behavioral abnormalities and severely impaired locomotion. In contrast, the same double mutation in an androgen receptor with an extended polyQ stretch was less toxic. The response of the receptors to inhibitors of polyglutamine toxicity is altered by the amino acid exchanges suggesting that careful consideration is needed in the choice of potential therapies of disorders involving toxic polyQ species.

Epigenetic regulation of BMP7 in the regenerative response to ischemia

Kidneys damaged by ischemia have the potential to regenerate through a mechanism involving intrarenal induction of protective factors, including bone morphogenetic protein-7 (BMP7). Epigenetic changes, such as alterations in histone modifications, have also been shown to play a role in various pathologic conditions, but their involvement in ischemic injury and regeneration remains unknown. This study investigated whether changes in histone acetylation, regulated by histone acetyltransferase and histone deacetylase (HDAC), are induced by renal ischemia and involved in the regenerative response. Ischemia/reperfusion of the mouse kidney induced a transient decrease in histone acetylation in proximal tubular cells, likely as a result of a decrease in histone acetyltransferase activity as suggested by experiments with energy-depleted renal epithelial cells in culture. During recovery after transient energy depletion in epithelial cells, the HDAC isozyme HDAC5 was selectively downregulated in parallel with the return of acetylated histone. Knockdown of HDAC5 by RNAi significantly increased histone acetylation and BMP7 expression. BMP7 induction and HDAC5 downregulation in the recovery phase were also observed in proximal tubular cells in vivo after transient ischemia. These data indicate that ischemia induces dynamic epigenetic changes involving HDAC5 downregulation, which contributes to histone re-acetylation and BMP7 induction in the recovery phase. This highlights HDAC5 as a modulator of the regenerative response after ischemia and suggests HDAC5 inhibition may be a therapeutic strategy to enhance BMP7 expression.

Conditional transgenic mouse models: from the basics to genome-wide sets of knockouts and current studies of tissue regeneration

Many mouse models are currently available, providing avenues to elucidate gene function and to recapitulate specific pathological conditions. To a large extent, successful translation of clinical evidence or analytical data into appropriate mouse models is possible through progress in transgenic or gene-targeting technology. Beginning with a review of standard mouse transgenics and conventional gene targeting, this article will move on to discussing the basics of conditional gene expression: the tetracycline (tet)-off and tet-on systems based on the transactivators tet-controlled transactivator (Tta) and reverse tet-on transactivator (rtTA) that allow downregulation or induction of gene expression; Cre or Flp recombinase-mediated modifications, including excision, inversion, insertion and interchromosomal translocation; combination of the tet and Cre systems, permitting inducible knockout, reporter gene activation or activation of point mutations; the avian retroviral system based on delivery of rtTA specifically into cells expressing the avian retroviral receptor, which enables cell type-specific, inducible gene expression; the tamoxifen system, one of the most frequently applied steroid receptor-based systems, allows rapid activation of a fusion protein between the gene of interest and a mutant domain of the estrogen receptor, whereby activation does not depend on transcription; and techniques for cell type-specific ablation. The diphtheria toxin receptor system offers the advantage that it can be combined with the ‘zoo’ of Cre recombinase driver mice. Having described the basics we move on to the cutting edge: generation of genome-wide sets of conditional knockout mice. To this end, large ongoing projects apply two strategies: gene trapping based on random integration of trapping vectors into introns leading to truncation of the transcript, and gene targeting, representing the directed approach using homologous recombination. It can be expected that in the near future genome-wide sets of such mice will be available. Finally, the possibilities of conditional expression systems for investigating gene function in tissue regeneration will be illustrated by examples for neurodegenerative disease, liver regeneration and wound healing of the skin.

Late neurodevelopmental effects of early exposures to chemical contaminants: reducing uncertainty in epidemiological studies

Early exposures to environmental chemicals are reliably associated with late neurotoxicities in children. However, substantial scatter of observations exists around the estimated dose-effect relationships. This variability has many potential sources, one of which is interindividual differences in susceptibility. Such differences imply that the long-term impacts of exposure will not the same for all individuals, but will vary depending on a variety of factors that might either aggravate or mitigate contaminant effects. These include co-exposures, genetic polymorphisms and characteristics of the social environment. The context dependence of contaminant effects has implications both for study designs and analytical approaches. In addition, a systems approach to understanding the associations among contaminant exposures, covariates and health outcomes is necessary.

Histone deacetylase inhibitors: possible implications for neurodegenerative disorders

During the past six years numerous studies identified histone deacetylase (HDAC) inhibitors as candidate drugs for the treatment of neurodegenerative disorders. Two major neuroprotective mechanisms of HDAC inhibitors have been identified, namely the transcriptional activation of disease-modifying genes and the correction of perturbations in histone acetylation homeostasis, which have been shown to be intimately involved in the neurodegenerative pathomechanisms of Huntington's, Parkinson's and Kennedy disease, amyotropic lateral sclerosis, Rubinstein-Taybi syndrome as well as stroke. Based on the promising in vitro and in vivo analyses, clinical trials have been initiated to evaluate the safety and efficacy of HDAC inhibitors for the treatment of devastating diseases such as Huntington's disease, amyotropic lateral sclerosis and spinal muscular atrophy. Here, the authors summarize and discuss the findings on the emerging field of epigenetic therapy strategies in neurodegenerative disorders.

Neurodegeneration and neuroinflammation in cdk5/p25-inducible mice: a model for hippocampal sclerosis and neocortical degeneration

The cyclin-dependent kinase cdk5 is atypically active in postmitotic neurons and enigmatic among the kinases proposed as molecular actors in neurodegeneration. We generated transgenic mice to express p25, the N-terminally truncated p35 activator of cdk5, in forebrain under tetracycline control (TET-off). Neuronal expression of p25 (p25(ON)) caused high mortality postnatally and early in life. Mortality was completely prevented by administration of doxycycline in the drinking water of pregnant dams and litters until P42, allowing us to study the action of p25 in adult mouse forebrain. Neuronal p25 triggered neurodegeneration and also microgliosis, rapidly and intensely in hippocampus and cortex. Progressive neurodegeneration was severe with marked neuron loss, causing brain atrophy (40% loss at age 5 months) with nearly complete elimination of the hippocampus. Neurodegeneration did not involve phosphorylation of protein tau or generation of amyloid peptide. Degenerating neurons did not stain for terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling or activated caspase-3 but were marked by FluoroJadeB in early stages. Diseased neurons were always closely associated with activated microglia already very early in the disease process. Primary neurons derived from p25 embryos were more prone to apoptosis than wild-type neurons, and they activated microglial cells in co-culture. The inducible p25 mice present as a model for neurodegeneration in hippocampal sclerosis and neocortical degeneration, with important contributions of activated microglia.

Non-coding RNAs, epigenetics and complexity

Several aspects of epigenetics are strongly linked to non-coding RNAs, especially small RNAs that can direct the cytosine methylation and histone modifications that are implicated in gene expression regulation in complex organisms. A fundamental characteristic of epigenetics is that the same genome can show alternative phenotypes, which are based in different epigenetic states. Some of the most studied complex epigenetic phenomena including transposon activity and silencing recently exemplified by piRNAs (piwi-interacting RNAs), position effect variegation, X-chromosome inactivation, parental imprinting, and paramutation have direct or indirect participation of an RNA component. Conceivably, most of the non-coding RNAs with no described function yet, are players in epigenetic mechanisms that are still not completely understood. In that regard, RNAs were recently implicated in new mechanisms of genetic information transfer in yeast, plants and mice. In this review article, the hypothesis that non-coding RNAs might be the main component of complex organisms acquired during evolution will be explored. The question of how evolutionary theories have been challenged by these molecules in association with epigenetic mechanisms will also be discussed here.

Histone deacetylase inhibitors: overview and perspectives

Histone deacetylase inhibitors (HDACi) comprise structurally diverse compounds that are a group of targeted anticancer agents. The first of these new HDACi, vorinostat (suberoylanilide hydroxamic acid), has received Food and Drug Administration approval for treating patients with cutaneous T-cell lymphoma. This review focuses on the activities of the 11 zinc-containing HDACs, their histone and nonhistone protein substrates, and the different pathways by which HDACi induce transformed cell death. A hypothesis is presented to explain the relative resistance of normal cells to HDACi-induced cell death.

A role for epigenetics in hearing: Establishment and maintenance of auditory specific gene expression patterns

Epigenetics is a large and diverse field encompassing a number of different mechanisms essential to development, DNA stability and gene expression. DNA methylation and histone modifications work individually and in conjunction with each other leading to phenotypic changes. An overwhelming amount of evidence exists demonstrating the essential nature of epigenetics to human biology and pathology. This field has spawned a vast array of knowledge, techniques and pharmaceuticals designed to investigate and manipulate epigenetic phenomena. Despite its centricity to molecular biology, little work has been conducted examining how epigenetics affects hearing. In this review, we discuss both the basic tenets of epigenetics and highlight the most recent advances in this field. We discuss its importance to human development, genomic stability, gene expression, epigenetic modifying agents as well as briefly introduce the expansive field of cancer epigenetics. We then examine the evidence of a role for epigenetics in hearing related processes and hearing loss. The article concludes with a discussion of areas of epigenetic research that could be applied to hearing research.

Molecular and cellular mechanisms of memory allocation in neuronetworks

Determining how neuronal networks encode memories is a key goal of neuroscience. Although neuronal circuit processes involved in encoding, storing and retrieving memory have attracted a great deal of attention, the processes that allocate individual memories to specific neurons within a network have remained elusive. Recent findings unraveled the first insights into the processes that modulate memory allocation in neuronetworks. They showed that neurons in the lateral amygdala compete to take part in auditory fear conditioned memory traces and that the levels of the transcription factor CREB (cAMP-response element binding protein) can affect the probability of a neuron to be recruited into a given memory representation. CREB-mediated transcriptional regulation involves several signaling pathways, known to mediate nuclear responses to diverse behavioral stimuli, along with coordinated interactions with multiple other transcription activators, coactivators and repressors. Moreover, activation of CREB triggers an autoinhibitory feedback loop, a metaplastic process that could be used to allocate memories away from cells that have been recently involved in memory. Beyond CREB, there may be a host of other processes that dynamically modulate memory allocation in neuronetworks by shaping cooperation and competition among neurons.

DNA methylation and histone acetylation work in concert to regulate memory formation and synaptic plasticity

A clear understanding is developing concerning the importance of epigenetic-related molecular mechanisms in transcription-dependent long-term memory formation. Chromatin modification, in particular histone acetylation, is associated with transcriptional activation, and acetylation of histone 3 (H3) occurs in Area CA1 of the hippocampus following contextual fear conditioning training. Conversely, DNA methylation is associated with transcriptional repression, but is also dynamically regulated in Area CA1 following training. We recently reported that inhibition of the enzyme responsible for DNA methylation, DNA methyltransferase (DNMT), in the adult rat hippocampus blocks behavioral memory formation. Here, we report that DNMT inhibition also blocks the concomitant memory-associated H3 acetylation, without affecting phosphorylation of its upstream regulator, extracellular signal-regulated kinase (ERK). Interestingly, the DNMT inhibitor-induced deficit in memory consolidation, along with deficits in long-term potentiation, can be rescued by pharmacologically increasing levels of histone acetylation prior to DNMT inhibition. These observations suggest that DNMT activity is not only necessary for memory and plasticity, but that DNA methylation may work in concert with histone modifications to regulate plasticity and memory formation in the adult rat hippocampus.

Alternative roles for Cdk5 in learning and synaptic plasticity

Protein kinases mediate the intracellular signal transduction pathways controlling synaptic plasticity in the central nervous system. While the majority of protein kinases achieve this function via the phosphorylation of synaptic substrates, some kinases may contribute through alternative mechanisms in addition to enzymatic activity. There is growing evidence that protein kinases may often play structural roles in plasticity as well. Cyclin-dependent kinase 5 (Cdk5) has been implicated in learning and synaptic plasticity. Initial scrutiny focused on its enzymatic activity using pharmacological inhibitors and genetic modifications of Cdk5 cofactors. Quite recently Cdk5 has been shown to govern learning and plasticity via regulation of glutamate receptor degradation, a function that may not dependent on phosphorylation of downstream effectors. From these new studies, two roles emerge for Cdk5 in plasticity: one in which it controls structural plasticity via phosphorylation of synaptic substrates, and a second where it regulates functional plasticity via protein-protein interactions.

Histone deacetylase mediated transcriptional activation reduces proviral loads in HTLV-1 associated myelopathy/tropical spastic paraparesis patients

Epigenetic modifications of chromatin may play a role in maintaining viral latency and thus persistence of the human T-lymphotropic virus type 1 (HTLV-1), which is responsible for HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP). A major determinant of disease progression is increased peripheral blood proviral load (PVL), possibly via the accumulation of infected cells in the central nervous system (CNS) creating a damaging inflammatory response. Current therapeutic approaches that focus on reducing either cell proliferation, viral replication, or tissue invasion are still unsatisfactory. Contrasting with these inhibitory strategies, we evaluated the efficacy of a novel approach aimed, paradoxically, at activating viral gene expression to expose virus-positive cells to the host immune response. We used valproate (VPA), a histone deacetylase inhibitor that has been used for decades as a chronic, safe treatment for epileptic disorders. Based on in vitro and in vivo data, we provide evidence that transient activation of the latent viral reservoir causes its collapse, a process that may alleviate the condition of HAM/TSP. This represents the first such approach to treating HAM/TSP, using gene activation therapy to tilt the host-pathogen balance in favor of an existing antiviral response. This trial is registered at http://clinicaltrials.gov/as no. NCT00519181.

Environment enrichment rescues the neurodegenerative phenotypes in presenilins-deficient mice

Presenilin (PS) 1 and 2 conditional double knockout (cDKO) mice show progressive memory dysfunction and forebrain degeneration. Gene expression profiling results revealed a strong activation of immunity and inflammation responses in the brains of 10-month-old cDKO mice. As environmental enrichment (EE) has been shown to be able to improve memory and induce neurogenesis of the brain, we assessed the effects of EE on the memory performance and the neurodegeneration in cDKO mice. We found that EE effectively enhanced memory and partially rescued the forebrain atrophy of the cDKO mice. Our results suggest that immunity and inflammation could play important roles in the neurodegeneration of cDKO mice. Furthermore, the beneficial effects of EE may be associated with the inhibition of the expression of immunity and inflammation-related genes in the brain.

Turning back the clock on neurodegeneration

Does neuronal loss associated with dementia necessarily impair the ability to learn new information and recall old memories? In a recent report in Nature, Fischer et al. (2007) show that the ability to learn and remember can be reestablished in a mouse model of dementia through either environmental enrichment or chronic treatment with an inhibitor of histone deacetylase.

A Richness that Cures

A study in Nature by Fischer et al. shows that environmental enrichment or increasing histone acetylation rescue the ability to form new memories and re-establish access to remote memories even in the presence of brain degeneration. Chromatin remodeling may be the final gate environmental enrichment opens to enhance plasticity and represents a promising target for therapeutical intervention in neurodegenerative diseases.