DNA repair protein DNA-PK protects PC12 cells from oxidative stress-induced apoptosis involving AKT phosphorylation.

BACKGROUND: Emerging evidence suggest that DNA-PK complex plays a role in the cellular response to oxidative stress, in addition to its function of double strand break (DSB) repair. In this study we evaluated whether DNA-PK participates in oxidative stress response and whether this role is independent of its function in DNA repair. METHODS AND RESULTS: We used a model of H2O2-induced DNA damage in PC12 cells (rat pheochromocytoma), a well-known neuronal tumor cell line. We found that H2O2 treatment of PC12 cells induces an increase in DNA-PK protein complex levels, along with an elevation of DNA damage, measured both by the formation of γΗ2ΑX foci, detected by immunofluorescence, and γH2AX levels detected by western blot analysis. After 24 h of cell recovery, γΗ2ΑX foci are repaired both in the absence and presence of DNA-PK kinase inhibitor NU7026, while an increase of apoptotic cells is observed when DNA-PK activity is inhibited, as revealed by counting pycnotic nuclei and confirmed by FACS analysis. Our results suggest a role of DNA-PK as an anti-apoptotic factor in proliferating PC12 cells under oxidative stress conditions. The anti-apoptotic role of DNA-PK is associated with AKT phosphorylation in Ser473. On the contrary, in differentiated PC12 cells, were the main pathway to repair DSBs is DNA-PK-mediated, the inhibition of DNA-PK activity causes an accumulation of DNA damage. CONCLUSIONS: Taken together, our results show that DNA-PK can protect cells from oxidative stress induced-apoptosis independently from its function of DSB repair enzyme.

Herpes Simplex Virus-Type1 (HSV-1) Impairs DNA Repair in Cortical Neurons.

Several findings suggest that Herpes simplex virus-1 (HSV-1) infection plays a role in the neurodegenerative processes that characterize Alzheimer's disease (AD), but the underlying mechanisms have yet to be fully elucidated. Here we show that HSV-1 productive infection in cortical neurons causes the accumulation of DNA lesions that include both single (SSBs) and double strand breaks (DSBs), which are reported to be implicated in the neuronal loss observed in neurodegenerative diseases. We demonstrate that HSV-1 downregulates the expression level of Ku80, one of the main components of non-homologous end joining (NHEJ), a major pathway for the repair of DSBs. We also provide data suggesting that HSV-1 drives Ku80 for proteasomal degradation and impairs NHEJ activity, leading to DSB accumulation. Since HSV-1 usually causes life-long recurrent infections, it is possible to speculate that cumulating damages, including those occurring on DNA, may contribute to virus induced neurotoxicity and neurodegeneration, further suggesting HSV-1 as a risk factor for neurodegenerative conditions.

DNA Double Strand Breaks: A Common Theme in Neurodegenerative Diseases.

Accumulation of DNA damage and impairment of DNA repair systems are involved in the pathogenesis of different neurodegenerative diseases. Whenever DNA damage is too extensive, the DNA damage response pathway provides for triggering cellular senescence and/or apoptosis. However, whether the increased level of DNA damage in neurodegenerative disorders is a cause rather than the consequence of neurodegenerative events remains to be established. Among possible DNA lesions, DNA double strand breaks (DSBs) are rare events, nevertheless they are the most lethal form of DNA damage. In neurons, DSBs are particularly deleterious because of their reduced DNA repair capability as compared to proliferating cells. Here, we provide a description of DSB repair systems and describe human studies showing the presence of several types of DNA lesions in three major neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD). Then, we analyze the role of DSB accumulation and deficiency of DSB repair systems in neurodegeneration by examining studies on animal models of neurodegenerative diseases.

The Response to Oxidative DNA Damage in Neurons: Mechanisms and Disease.

There is a growing body of evidence indicating that the mechanisms that control genome stability are of key importance in the development and function of the nervous system. The major threat for neurons is oxidative DNA damage, which is repaired by the base excision repair (BER) pathway. Functional mutations of enzymes that are involved in the processing of single-strand breaks (SSB) that are generated during BER have been causally associated with syndromes that present important neurological alterations and cognitive decline. In this review, the plasticity of BER during neurogenesis and the importance of an efficient BER for correct brain function will be specifically addressed paying particular attention to the brain region and neuron-selectivity in SSB repair-associated neurological syndromes and age-related neurodegenerative diseases.

Intrabody-mediated diverting of HP1ß to the cytoplasm induces co-aggregation of H3-H4 histones and lamin-B receptor.

Diverting a protein from its intracellular location is a unique property of intrabodies. To interfere with the intracellular traffic of heterochromatin protein 1ß (HP1ß) in living cells, we have generated a cytoplasmic targeted anti-HP1ß intrabody, specifically directed against the C-terminal portion of the molecule. HP1ß is a conserved component of mouse and human constitutive heterochromatin involved in diverse nuclear functions including gene silencing, DNA repair and nuclear membrane assembly. We found that the anti-HP1ß intrabody sequesters HP1ß into cytoplasmic aggregates, inhibiting its traffic to the nucleus. Lamin B receptor (LBR) and a subset of core histones (H3/H4) are also specifically co-sequestered in the cytoplasm of anti-HP1ß intrabody-expressing cells. Methylated histone H3 at K9 (Me9H3), a marker of constitutive heterochromatin, is not affected by the anti-HP1ß intrabody expression. Hyper-acetylating conditions completely dislodge H3 from HP1ß:LBR containing aggregates. The expression of anti-HP1ß scFv fragments induces apoptosis, associated with an alteration of nuclear morphology. Both these phenotypes are specifically rescued either by overexpression of recombinant full length HP1ß or by HP1ß mutant containing the chromoshadow domain, but not by recombinant LBR protein. The HP1ß-chromodomain mutant, on the other hand, does not rescue the phenotypes, but does compete with LBR for binding to HP1ß. These findings provide new insights into the mode of action of cytoplasmic-targeted intrabodies and the interaction between HP1ß and its binding partners involved in peripheral heterochromatin organisation.

Biochemical characterization of sirtuin 6 in the brain and its involvement in oxidative stress response.

Sirtuin 6 (SIRT6) is a member of nicotinamide adenine dinucleotide-dependent deacetylase protein family and has been implicated in the control of glucose and lipid metabolism, cancer, genomic stability and DNA repair. Moreover, SIRT6 regulates the expression of a large number of genes involved in stress response and aging. The role of SIRT6 in brain function and neuronal survival is largely unknown. Here, we biochemically characterized SIRT6 in brain tissues and primary neuronal cultures and found that it is highly expressed in cortical and hippocampal regions and enriched in the synaptosomal membrane fraction. Immunoblotting analysis on cortical and hippocampal neurons showed that SIRT6 is downregulated during maturation in vitro, reaching the lowest expression at 11 days in vitro. In addition, SIRT6 overexpression in terminally differentiated cortical and hippocampal neurons, mediated by a neuron-specific recombinant adeno-associated virus, downregulated cell viability under oxidative stress condition. By contrast, under control condition, SIRT6 overexpression had no detrimental effect. Overall these results suggest that SIRT6 may play a role in synaptic function and neuronal maturation and it may be implicated in the regulation of neuronal survival.

Therapeutic application of intrabodies against age-related neurodegenerative disorders.

Many neurodegenerative diseases, referred to as misfolding diseases, are characterized by the formation and accumulation of pathological extracellular and intracellular misfolded aggregates. Ageing is considered the major risk factor for neurodegenerative disorders and, due to increase of mean lifespan, the clinical relevance is growing dramatically with a urgent need to find new effective therapeutic approaches. The intracellular antibody technology is a gene-based strategy which exploits the specificity of recombinant antibodies to neutralize or modify the function of intracellular and extracellular target antigens. Intrabodies can potentially recognize all the pathological conformers of a misfolding-prone protein, and therefore they are emerging as therapeutic agents for the treatment of misfolding diseases as well as molecular tools for the understanding of their pathogenesis. Here we focus on the application of intrabodies against two major age-related neurodegenerative disorders, Alzheimer's disease (AD) and Parkinson's disease (PD) and the description of in vivo gene delivery systems available for their potential entering in the clinical setting.

Gene-based antibody strategies for prion diseases.

Prion diseases or transmissible spongiform encephalopathies (TSE) are a group of neurodegenerative and infectious disorders characterized by the conversion of a normal cellular protein PrP(C) into a pathological abnormally folded form, termed PrP(Sc). There are neither available therapies nor diagnostic tools for an early identification of individuals affected by these diseases. New gene-based antibody strategies are emerging as valuable therapeutic tools. Among these, intrabodies are chimeric molecules composed by recombinant antibody fragments fused to intracellular trafficking sequences, aimed at inhibiting, in vivo, the function of specific therapeutic targets. The advantage of intrabodies is that they can be selected against a precise epitope of target proteins, including protein-protein interaction sites and cytotoxic conformers (i.e., oligomeric and fibrillar assemblies). Herein, we address and discuss in vitro and in vivo applications of intrabodies in prion diseases, focussing on their therapeutic potential.

Nicotinic receptor and tobacco-related cancer.

Currently, tobacco smoking causes approximately 5-6 million deaths per year including more than 35% of all cancer deaths. Nicotine, the addictive constituent of tobacco, and its derived carcinogenic nitrosamines, contribute to cancer promotion and progression through the activation of nicotinic acetylcholine receptors (nAChR). Although the role of nicotine in cancerogenesis is still discussed controversially, it has been recently shown that nicotine induces DNA damages, via induction of oxidative stress, in bronchial epithelial cells. Moreover, nicotine is able to induce muscle sarcomas in A/J mice. In this mini-review we highlight the role of nAChR and nicotine in all cancer phases (induction, promotion and progression). Relevant new findings quoted in literature and some new experiments of our laboratory were reported and discussed.

A new "era" for the α7-nAChR.

The α7-nAChR plays critical roles in numerous organs and cells by regulating highly organ and cell typespecific functions. In this special issue different Authors have contributed to clarify the different roles played by the α7- nAChR. Post-translational processes such as receptor "underactivation" or "overactivation" are associated in the central nervous system with brain disorders including neurodegeneration, while also contributing to the regulation of nonneuronal cells and cancers derived from them. Current advances in the knowledge of α7-nAChR biology encourage the exploitation of this receptor as a therapeutic target for a variety of diseases, including Alzheimer's, disease, Parkinson's disease, cognitive decline, pain and cancer.

Sublethal doses of ß-amyloid peptide abrogate DNA-dependent protein kinase activity.

Accumulation of DNA damage and deficiency in DNA repair potentially contribute to the progressive neuronal loss in neurodegenerative disorders, including Alzheimer disease (AD). In multicellular eukaryotes, double strand breaks (DSBs), the most lethal form of DNA damage, are mainly repaired by the nonhomologous end joining pathway, which relies on DNA-PK complex activity. Both the presence of DSBs and a decreased end joining activity have been reported in AD brains, but the molecular player causing DNA repair dysfunction is still undetermined. ß-Amyloid (Aß), a potential proximate effector of neurotoxicity in AD, might exert cytotoxic effects by reactive oxygen species generation and oxidative stress induction, which may then cause DNA damage. Here, we show that in PC12 cells sublethal concentrations of aggregated Aß(25-35) inhibit DNA-PK kinase activity, compromising DSB repair and sensitizing cells to nonlethal oxidative injury. The inhibition of DNA-PK activity is associated with down-regulation of the catalytic subunit DNA-PK (DNA-PKcs) protein levels, caused by oxidative stress and reversed by antioxidant treatment. Moreover, we show that sublethal doses of Aß(1-42) oligomers enter the nucleus of PC12 cells, accumulate as insoluble oligomeric species, and reduce DNA-PK kinase activity, although in the absence of oxidative stress. Overall, these findings suggest that Aß mediates inhibition of the DNA-PK-dependent nonhomologous end joining pathway contributing to the accumulation of DSBs that, if not efficiently repaired, may lead to the neuronal loss observed in AD.

Nicotine: specific role in angiogenesis, proliferation and apoptosis.

Nowadays, tobacco smoking is the cause of ~5-6 million deaths per year, counting 31% and 6% of all cancer deaths (affecting 18 different organs) in middle-aged men and women, respectively. Nicotine is the addictive component of tobacco acting on neuronal nicotinic receptors (nAChR). Functional nAChR, are also present on endothelial, haematological and epithelial cells. Although nicotine itself is regularly not referred to as a carcinogen, there is an ongoing debate whether nicotine functions as a 'tumour promoter'. Nicotine, with its specific binding to nAChR, deregulates essential biological processes like regulation of cell proliferation, apoptosis, migration, invasion, angiogenesis, inflammation and cell-mediated immunity in a wide variety of cells including foetal (regulation of development), embryonic and adult stem cells, adult tissues as well as cancer cells. Nicotine seems involved in fundamental aspects of the biology of malignant diseases, as well as of neurodegeneration. Investigating the biological effects of nicotine may provide new tools for therapeutic interventions and for the understanding of neurodegenerative diseases and tumour biology.

Phosphorylation changes of CaMKII, ERK1/2, PKB/Akt kinases and CREB activation during early long-term potentiation at Schaffer collateral-CA1 mouse hippocampal synapses.

Protein phosphorylation is the main signaling system known to trigger synaptic changes underlying long-term potentiation (LTP). The timing of these phosphorylations plays an essential role to maintain the potentiated state of synapses. However, in mice a simultaneous analysis of phosphorylated proteins during early-LTP (E-LTP) has not been thoroughly carried out. Here we described phosphorylation changes of alphaCaMKII, ERK1/2, PKB/Akt and CREB at different times after E-LTP induced at Schaffer collateral/commissural fiber-CA1 synapses by 1 s 100 Hz tetanic stimulation in mouse hippocampal slices. We found that phosphorylation levels of all the molecules examined rapidly increased after tetanisation and remained above the basal level up to 30 min. Notably, we observed a sustained increment in the phosphorylation level of Akt at Ser473, whereas the phosphorylation level of Akt at Thr308 was unchanged. Unexpectedly, we also detected a marked increase of CREB target genes expression levels, c-fos, Egr-1 and exon-III containing BDNF transcripts. Our findings, besides providing a detailed timing of phosphorylation of the major kinases involved in E-LTP in mice, revealed that a modest LTP induction paradigm specifically triggers CREB-mediated gene expression.

Mutation of SHOC2 promotes aberrant protein N-myristoylation and causes Noonan-like syndrome with loose anagen hair.

N-myristoylation is a common form of co-translational protein fatty acylation resulting from the attachment of myristate to a required N-terminal glycine residue. We show that aberrantly acquired N-myristoylation of SHOC2, a leucine-rich repeat-containing protein that positively modulates RAS-MAPK signal flow, underlies a clinically distinctive condition of the neuro-cardio-facial-cutaneous disorders family. Twenty-five subjects with a relatively consistent phenotype previously termed Noonan-like syndrome with loose anagen hair (MIM607721) shared the 4A>G missense change in SHOC2 (producing an S2G amino acid substitution) that introduces an N-myristoylation site, resulting in aberrant targeting of SHOC2 to the plasma membrane and impaired translocation to the nucleus upon growth factor stimulation. Expression of SHOC2(S2G) in vitro enhanced MAPK activation in a cell type-specific fashion. Induction of SHOC2(S2G) in Caenorhabditis elegans engendered protruding vulva, a neomorphic phenotype previously associated with aberrant signaling. These results document the first example of an acquired N-terminal lipid modification of a protein causing human disease.

Peripheral blood mononuclear cells from mild cognitive impairment patients show deregulation of Bax and Sod1 mRNAs.

Elevated oxidative stress-induced apoptosis has been found in peripheral cells from patients with Alzheimer's disease (AD). Furthermore, treatment of lymphocytes from AD patients, with Abeta(1-42) and H(2)O(2) results in enhanced apoptosis. Mild cognitive impairment (MCI), a clinical condition between normal aging and AD, shares with AD a similar pattern of peripheral markers of oxidative stress. In this study we investigated spontaneous and H(2)O(2)-induced oxidative stress and apoptosis levels in peripheral blood mononuclear cells (PBMCs) from MCI and AD patients, as well as from Parkinson's disease (PD) patients without cognitive impairment or age-matched healthy control. Sod1 mRNA levels were studied to analyse the anti-oxidative pathway, while Bax and Bcl-2 mRNAs levels and PARP protein cleavage were monitored to study apoptosis. We found that the expression of Sod1 and Bax mRNAs was statistically higher in both MCI and AD patients compared to controls or PD subjects. Since Bcl-2 mRNA level was not different among groups, the Bax/Bcl-2 ratio was statistically higher in AD and MCI patients. PARP cleavage was also enhanced in PBMCs from MCI and AD individuals and this finding was associated with a higher level of spontaneous apoptosis. Interestingly, exposure to H(2)O(2) induced a significant decrease of Bcl-2 mRNA transcript, while Sod1 and Bax mRNAs levels were unchanged in PBMCs derived from MCI and AD patients. In conclusion, our results show that Bax and Sod1 mRNA levels are altered in PBMCs from both MCI and AD patients and indicate these changes as potential biomarkers in the early diagnosis of AD.

The potential of intracellular antibodies for therapeutic targeting of protein-misfolding diseases.

Misfolding diseases are a wide group of devastating disorders characterized by the accumulation of pathological protein aggregates. Although these disorders still lack an effective treatment, new antibody-based strategies are emerging and entering clinical trials. The intrabody approach is a gene-based technology developed to neutralize or modify the function of intracellular and extracellular target antigens. Because intrabodies can potentially target all the different isoforms of a misfolding-prone protein, including pathological conformations, they are emerging as therapeutic molecules for the treatment of misfolding diseases, including Alzheimer's, Parkinson's, Huntington's and prion diseases. This review will provide a description of the intrabody approach, an update of preclinical studies on misfolding diseases and an outlook on the intrabody delivery issue for therapeutic purposes.

Combating protein misfolding and aggregation by intracellular antibodies.

Conformational or misfolding diseases are a large class of devastating human disorders associated with protein misfolding and aggregation. Most conformational diseases are caused by a combination of genetic and environmental factors, suggesting that spontaneous events can destabilize the protein involved in the pathology or impair the clearance mechanisms of misfolded aggregates. Aging is one of the risk factors associa-ted to these events, and the clinical relevance of conformational disorders is growing dramatically, as they begin to reach epidemic proportions due to increases in mean lifespan. Currently, there are no effective strategies to slow or prevent these diseases. Intrabodies are promising therapeutic agents for the treatment of misfolding diseases, because of their virtually infinite ability to specifically recognize the different conformations of a protein, including pathological isoforms, and because they can be targeted to the potential sites of aggregation (both intra- and extracellular sites). These molecules can work as neutralizing agents against amylo-idogenic proteins by preventing their aggregation, and/or as molecular shunters of intracellular traffic by re-routing the protein from its potential aggregation site. The fast-developing field of recombinant antibody technology provides intrabodies with enhanced binding specificity and stability, together with lower immunogenicity, for use in a clinical setting. This review provides an update on the applications of intrabodies in misfolding diseases, with particular emphasis on an evaluation of their multiple and feasible modes of action.