Susanne Burkhardt

Altered Histone Acetylation Is Associated with Age-Dependent Memory Impairment in Mice

Age-Old Problem With the increase in human life span, there is an associated increase in incidence of age-associated cognitive decline, which causes a huge emotional and economic burden. However, the mechanisms underlying age-associated memory impairment are poorly understood. Now, Peleg et al. (p. 753; see the Perspective by Sweatt) have found that the memory disturbances in the aging mouse brain are associated with specific changes in learning-induced histone acetylation, which interferes with the hippocampal gene-expression program. Restoration of dynamic histone acetylation reinstated cognitive function in the aging mouse. Deregulated histone acetylation may represent an early biomarker of age-dependent cognitive decline. As the human life span increases, the number of people suffering from cognitive decline is rising dramatically. The mechanisms underlying age-associated memory impairment are, however, not understood. Here we show that memory disturbances in the aging brain of the mouse are associated with altered hippocampal chromatin plasticity. During learning, aged mice display a specific deregulation of histone H4 lysine 12 (H4K12) acetylation and fail to initiate a hippocampal gene expression program associated with memory consolidation. Restoration of physiological H4K12 acetylation reinstates the expression of learning-induced genes and leads to the recovery of cognitive abilities. Our data suggest that deregulated H4K12 acetylation may represent an early biomarker of an impaired genome-environment interaction in the aging mouse brain.

Free radical-mediated molecular damage. Mechanisms for the protective actions of melatonin in the central nervous system.

Abstract: This review briefly summarizes the multiple actions by which melatonin reduces the damaging effects of free radicals and reactive oxygen and nitrogen species. It is well documented that melatonin protects macromolecules from oxidative damage in all subcellular compartments. This is consistent with the protection by melatonin of lipids and proteins, as well as both nuclear and mitochondrial DNA. Melatonin achieves this widespread protection by means of its ubiquitous actions as a direct free radical scavenger and an indirect antioxidant. Thus, melatonin directly scavenges a variety of free radicals and reactive species including the hydroxyl radical, hydrogen peroxide, singlet oxygen, nitric oxide, peroxynitrite anion, and peroxynitrous acid. Furthermore, melatonin stimulates a number of antioxidative enzymes including superoxide dismutase, glutathione peroxidase, glutathione reductase, and catalase. Additionally, melatonin experimentally enhances intracellular glutathione (another important antioxidant) levels by stimulating the rate‐limiting enzyme in its synthesis, γ‐glutamylcysteine synthase. Melatonin also inhibits the proxidative enzymes nitric oxide synthase and lipoxygenase. Finally, there is evidence that melatonin stabilizes cellular membranes, thereby probably helping them resist oxidative damage. Most recently, melatonin has been shown to increase the efficiency of the electron transport chain and, as a consequence, to reduce election leakage and the generation of free radicals. These multiple actions make melatonin a potentially useful agent in the treatment of neurological disorders that have oxidative damage as part of their etiological basis.

N1-acetyl-N2-formyl-5-methoxykynuramine, a biogenic amine and melatonin metabolite, functions as a potent antioxidant.

The biogenic amine N1‐acetyl‐N2‐formyl‐5‐methoxykynuramine (AFMK) was investigated for its potential antioxidative capacity. AFMK is a metabolite generated through either an enzymatic or a chemical reaction pathway from melatonin. The physiological function of AFMK remains unknown. To our knowledge, this report is the first to document the potent antioxidant action of this biogenic amine. Cyclic voltammetry (CV) shows that AFMK donates two electrons at potentials of 456 mV and 668 mV, and therefore it functions as a reductive force. This function contrasts with all other physiological antioxidants that donate a single electron only when they neutralize free radicals. AFMK reduced 8‐hydroxydeoxyguanosine formation induced by the incubation of DNA with oxidants significantly. Lipid peroxidation resulting from free radical damage to rat liver homogenates was also prevented by the addition of AFMK. The inhibitory effects of AFMK on both DNA and lipid damage appear to be dose‐response related. In cell culture, AFMK efficiently reduced hippocampal neuronal death induced by either hydrogen peroxide, glutamate, or amyloid β25–35 peptide. AFMK is a naturally occurring molecule with potent free radical scavenging capacity (donating two electrons/molecule) and thus may be a valuable new antioxidant for preventing and treating free radical‐related disorders.

microRNA-34c is a novel target to treat dementias

MicroRNAs are key regulators of transcriptome plasticity and have been implicated with the pathogenesis of brain diseases. Here, we employed massive parallel sequencing and provide, at an unprecedented depth, the complete and quantitative miRNAome of the mouse hippocampus, the prime target of neurodegenerative diseases such as Alzheimers disease (AD). Using integrative genetics, we identify miR‐34c as a negative constraint of memory consolidation and show that miR‐34c levels are elevated in the hippocampus of AD patients and corresponding mouse models. In line with this, targeting miR‐34 seed rescues learning ability in these mouse models. Our data suggest that miR‐34c could be a marker for the onset of cognitive disturbances linked to AD and indicate that targeting miR‐34c could be a suitable therapy.

Reactive oxygen and nitrogen species and cellular and organismal decline: amelioration with melatonin.

Cellular and organismal decline is, in part, believed to be a consequence of oxygen and nitrogen-based reactants which persistently damage macromolecules throughout a lifetime. The resulting accumulation of damaged molecules eventually seriously compromises essential functions of cells leading to their death. Excessive cellular loss causes deterioration of organ function and inevitably to the demise of the organism. The sequence of events, known as the free radical theory of aging, is widely espoused by biological gerontologists. Antioxidants are commonly employed to combat molecular damage mediated by oxygen and nitrogen-based reactants. One of these protective agents is melatonin. Melatonin has several distinct advantages as a preserver of organelle structure and function. It is widely distributed in organisms and within cells. It works via a number of mechanisms to reduce oxidative damage. Thus, melatonin scavenges a number of reactants including the hydroxyl radical (*OH), hydrogen peroxide (H(2)O(2)), nitric acid (NO*), peroxynitrite (ONOO(-)) and peroxynitrous acid (ONOOH). One of the products of melatonins interaction with H(2)O(2), i.e., N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK), is also a highly efficient radical scavenger. The cascade of reactions where the secondary metabolites are also effective scavenges is believed to contribute to melatonins high efficacy in reducing oxidative damage. Besides its direct scavenging actions, melatonin stimulates several antioxidative enzymes including superoxide dismutase, glutathione peroxidase and glutathione reductase in addition to inhibiting a proxidative enzyme, nitric oxide synthase. This combination of actions assists melatonin in protecting cells from the degenerative changes normally associated with aging and age-related diseases.

Reducing HDAC6 ameliorates cognitive deficits in a mouse model for Alzheimer's disease

Histone deacetylases (HDACs) are currently being discussed as promising therapeutic targets to treat neurodegenerative diseases. However, the role of specific HDACs in cognition and neurodegeneration remains poorly understood. Here, we investigate the function of HDAC6, a class II member of the HDAC superfamily, in the adult mouse brain. We report that mice lacking HDAC6 are cognitively normal but reducing endogenous HDAC6 levels restores learning and memory and α‐tubulin acetylation in a mouse model for Alzheimers disease (AD). Our data suggest that this therapeutic effect is, at least in part, linked to the observation that loss of HDAC6 renders neurons resistant to amyloid‐β‐mediated impairment of mitochondrial trafficking. Thus, our study suggests that targeting HDAC6 could be a suitable strategy to ameliorate cognitive decline observed in AD.

Melatonin's unique radical scavenging properties - roles of its functional substituents as revealed by a comparison with its structural analogs

Melatonins O‐methyl and N‐acetyl residues are not only the basis of its amphilicity enabling the molecule to enter all organs and all subcellular compartments, but are also decisive for its antioxidant properties. We have compared melatonins redox chemistry with that of several structural analogs: tryptamine, N‐acetyltryptamine, serotonin, N‐acetylserotonin, 5‐methoxytryptamine, 6‐chloromelatonin and 2‐iodomelatonin. Scavenging of hydroxyl radicals (·OH) was measured in a scavenger competition assay based on ABTS cation radical (ABTS·+) formation. The capability of undergoing single‐electron transfer reactions was studied using an ABTS·+ reduction assay, reflecting the more general property of scavenging organic cation radicals. Direct scavenging of superoxide anions (O2·−), under non‐catalyzed conditions, was investigated in a hematoxylin autoxidation assay. Measurements of chemiluminescence were used for studying scavenging of O2·− under catalyzed conditions, either by hemin‐mediated interaction or by combination with the respective indolyl cation radicals. Light emission was determined in the absence or presence of the ·OH scavenger dimethylsulfoxide and the O2·− scavenger Tiron. Products formed by oxidation of the respective indoles in a moderately alkaline, hemin‐catalyzed H2O2 system were analyzed by thin‐layer chromatography and fluorometry. Absence of either the O‐methyl or the N‐acetyl residue causes marked diminutions in the capacities of scavenging ·OH and ABTS·+ as well as in chemiluminescence emitted during oxidation. The importance of the N‐acetyl group is insofar remarkable as it seems, at first glance, to be isolated from the indolic moiety; interactions between side chain and indolic moiety are therefore decisive for melatonins redox properties. The 5‐hydroxylated compounds are not generally more efficient scavengers, but particularly better reducers of ABTS·+; in the alkaline H2O2 system generating ·OH and O2·−, melatonin was much more rapidly oxidized than the 5‐hydroxylated and non‐substituted analogs. Oxidative products formed from any of the compounds studied contained much less of substituted kynuramines as in the case of melatonin, indicating that radical chain termination by O2·− is considerably more efficient with melatonin. These findings are supported by measurements of chemiluminescence, which largely reflects pyrrole ring cleavage as a result of combination with superoxide anions. In this regard, only 6‐chloromelatonin equalled melatonin, whereas the efficiency of 2‐iodomelatonin was much lower, another indication for the importance of 2,3‐dioxygenation.

DNA oxidatively damaged by chromium(III) and H2O2 is protected by the antioxidants melatonin, N1-acetyl-N2-formyl-5-methoxykynuramine, resveratrol and uric acid

Chromium (Cr) compounds are widely used industrial chemicals and well known carcinogens. Cr(III) was earlier found to induce oxidative damage as documented by examining the levels of 8-hydroxydeoxyguanosine (8-OH-dG), an index for DNA damage, in isolated calf thymus DNA incubated with CrCl(3) and H(2)O(2). In the present in vitro study, we compared the ability of the free radical scavengers melatonin, N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK), resveratrol and uric acid to reduce DNA damage induced by Cr(III). Each of these scavengers markedly reduced the DNA damage in a concentration-dependent manner. The concentrations that reduced 8-OH-dG formation by 50% (IC(50)) were 0.10 microM for both resveratrol and melatonin, and 0.27 microM for AFMK. However, the efficacy of the fourth endogenous antioxidant, i.e. uric acid, in terms of its inhibition of DNA damage in the same in vitro system was about 60--150 times less effective than the other scavengers; the IC(50) for uric acid was 15.24 microM. These findings suggest that three of the four antioxidants tested in these studies may have utility in protecting against the environmental pollutant Cr and that the protective effects of these free radical scavengers against Cr(III)-induced carcinogenesis may relate to their direct hydroxyl radical scavenging ability. In the present study, the formation of 8-OH-dG was likely due to a Cr(III)-mediated Fenton-type reaction that generates hydroxyl radicals, which in turn damage DNA. Once formed, 8-OH-dG can mutate eventually leading to cancer; thus the implication is that these antioxidants may reduce the incidence of Cr-related cancers.

HDAC1 Regulates Fear Extinction in Mice

Histone acetylation has been implicated with the pathogenesis of neuropsychiatric disorders and targeting histone deacetylases (HDACs) using HDAC inhibitors was shown to be neuroprotective and to initiate neuroregenerative processes. However, little is known about the role of individual HDAC proteins during the pathogenesis of brain diseases. HDAC1 was found to be upregulated in patients suffering from neuropsychiatric diseases. Here, we show that virus-mediated overexpression of neuronal HDAC1 in the adult mouse hippocampus specifically affects the extinction of contextual fear memories, while other cognitive abilities were unaffected. In subsequent experiments we show that under physiological conditions, hippocampal HDAC1 is required for extinction learning via a mechanism that involves H3K9 deacetylation and subsequent trimethylation of target genes. In conclusion, our data show that hippocampal HDAC1 has a specific role in memory function.

A hippocampal insulin‐growth factor 2 pathway regulates the extinction of fear memories

Extinction learning refers to the phenomenon that a previously learned response to an environmental stimulus, for example, the expression of an aversive behaviour upon exposure to a specific context, is reduced when the stimulus is repeatedly presented in the absence of a previously paired aversive event. Extinction of fear memories has been implicated with the treatment of anxiety disease but the molecular processes that underlie fear extinction are only beginning to emerge. Here, we show that fear extinction initiates upregulation of hippocampal insulin‐growth factor 2 (Igf2) and downregulation of insulin‐growth factor binding protein 7 (Igfbp7). In line with this observation, we demonstrate that IGF2 facilitates fear extinction, while IGFBP7 impairs fear extinction in an IGF2‐dependent manner. Furthermore, we identify one cellular substrate of altered IGF2 signalling during fear extinction. To this end, we show that fear extinction‐induced IGF2/IGFBP7 signalling promotes the survival of 17–19‐day‐old newborn hippocampal neurons. In conclusion, our data suggest that therapeutic strategies that enhance IGF2 signalling and adult neurogenesis might be suitable to treat disease linked to excessive fear memory.