Responding to DNA double strand breaks in the nervous system

Mitochondrial, cell cycle control and neuritogenesis alterations in an iPSC-based neurodevelopmental model for schizophrenia

Schizophrenia is a severe psychiatric disorder of neurodevelopmental origin that affects around 1% of the world's population. Proteomic studies and other approaches have provided evidence of compromised cellular processes in the disorder, including mitochondrial function. Most of the studies so far have been conducted on postmortem brain tissue from patients, and therefore, do not allow the evaluation of the neurodevelopmental aspect of the disorder. To circumvent that, we studied the mitochondrial and nuclear proteomes of neural stem cells (NSCs) and neurons derived from induced pluripotent stem cells (iPSCs) from schizophrenia patients versus healthy controls to assess possible alterations related to energy metabolism and mitochondrial function during neurodevelopment in the disorder. Our results revealed differentially expressed proteins in pathways related to mitochondrial function, cell cycle control, DNA repair and neuritogenesis and their possible implication in key process of neurodevelopment, such as neuronal differentiation and axonal guidance signaling. Moreover, functional analysis of NSCs revealed alterations in mitochondrial oxygen consumption in schizophrenia-derived cells and a tendency of higher levels of intracellular reactive oxygen species (ROS). Hence, this study shows evidence that alterations in important cellular processes are present during neurodevelopment and could be involved with the establishment of schizophrenia, as well as the phenotypic traits observed in adult patients. Neural stem cells (NSCs) and neurons were derived from induced pluripotent stem cells (iPSCs) from schizophrenia patients and controls. Proteomic analyses were performed on the enriched mitochondrial and nuclear fractions of NSCs and neurons. Whole-cell proteomic analysis was also performed in neurons. Our results revealed alteration in proteins related to mitochondrial function, cell cycle control, among others. We also performed energy pathway analysis and reactive oxygen species (ROS) analysis of NSCs, which revealed alterations in mitochondrial oxygen consumption and a tendency of higher levels of intracellular ROS in schizophrenia-derived cells.

Prevalence and mechanisms of somatic deletions in single human neurons during normal aging and in DNA repair disorders

Replication errors and various genotoxins cause DNA double-strand breaks (DSBs) where error-prone repair creates genomic mutations, most frequently focal deletions, and defective repair may lead to neurodegeneration. Despite its pathophysiological importance, the extent to which faulty DSB repair alters the genome, and the mechanisms by which mutations arise, have not been systematically examined reflecting ineffective methods. Here, we develop PhaseDel, a computational method to detect focal deletions and characterize underlying mechanisms in single-cell whole genome sequences (scWGS). We analyzed high-coverage scWGS of 107 single neurons from 18 neurotypical individuals of various ages, and found that somatic deletions increased with age and in highly expressed genes in human brain. Our analysis of 50 single neurons from DNA repair-deficient diseases with progressive neurodegeneration (Cockayne syndrome, Xeroderma pigmentosum, and Ataxia telangiectasia) reveals elevated somatic deletions compared to age-matched controls. Distinctive mechanistic signatures and transcriptional associations suggest roles for somatic deletions in neurodegeneration.

Exploring the Origin and Physiological Significance of DNA Double Strand Breaks in the Developing Neuroretina

Genetic mosaicism is an intriguing physiological feature of the mammalian brain that generates altered genetic information and provides cellular, and prospectively functional, diversity in a manner similar to that of the immune system. However, both its origin and its physiological significance remain poorly characterized. Most, if not all, cases of somatic mosaicism require prior generation and repair of DNA double strand breaks (DSBs). The relationship between DSB generation, neurogenesis, and early neuronal cell death revealed by our studies in the developing retina provides new perspectives on the different mechanisms that contribute to DNA rearrangements in the developing brain. Here, we speculate on the physiological significance of these findings.

Disease-associated c-MYC downregulation in human disorders of transcriptional regulation

Cornelia de Lange syndrome (CdLS) is a rare multiorgan developmental disorder caused by pathogenic variants in cohesin genes. It is a genetically and clinically heterogeneous dominant (both autosomal and X-linked) rare disease. Increasing experimental evidence indicates that CdLS is caused by a combination of factors, such as gene expression dysregulation, accumulation of cellular damage and cellular aging, which collectively contribute to the CdLS phenotype. The CdLS phenotype overlaps with a number of related diagnoses such as KBG syndrome and Rubinstein-Taybi syndrome both caused by variants in chromatin-associated factors other than cohesin. The molecular basis underlying these overlapping phenotypes is not clearly defined. Here, we found that cells from individuals with CdLS and CdLS-related diagnoses are characterized by global transcription disturbance and share common dysregulated pathways. Intriguingly, c-MYC (subsequently referred to as MYC) is downregulated in all cell lines and represents a convergent hub lying at the center of dysregulated pathways. Subsequent treatment with estradiol restores MYC expression by modulating cohesin occupancy at its promoter region. In addition, MYC activation leads to modification in expression in hundreds of genes, which in turn reduce the oxidative stress level and genome instability. Together, these results show that MYC plays a pivotal role in the etiopathogenesis of CdLS and CdLS-related diagnoses and represents a potential therapeutic target for these conditions.

Senescence and impaired DNA damage responses in alpha-synucleinopathy models

α-Synuclein is a crucial element in the pathogenesis of Parkinson’s disease (PD) and related neurological diseases. Although numerous studies have presented potential mechanisms underlying its pathogenesis, the understanding of α-synuclein-mediated neurodegeneration remains far from complete. Here, we show that overexpression of α-synuclein leads to impaired DNA repair and cellular senescence. Transcriptome analysis showed that α-synuclein overexpression led to cellular senescence with activation of the p53 pathway and DNA damage responses (DDRs). Chromatin immunoprecipitation analyses using p53 and γH2AX, chromosomal markers of DNA damage, revealed that these proteins bind to promoters and regulate the expression of DDR and cellular senescence genes. Cellular marker analyses confirmed cellular senescence and the accumulation of DNA double-strand breaks. The non-homologous end joining (NHEJ) DNA repair pathway was activated in α-synuclein-overexpressing cells. However, the expression of MRE11, a key component of the DSB repair system, was reduced, suggesting that the repair pathway induction was incomplete. Neuropathological examination of α-synuclein transgenic mice showed increased levels of phospho-α-synuclein and DNA double-strand breaks, as well as markers of cellular senescence, at an early, presymptomatic stage. These results suggest that the accumulation of DNA double-strand breaks (DSBs) and cellular senescence are intermediaries of α-synuclein-induced pathogenesis in PD.

Excess levels of a protein involved in Parkinson’s disease can impair the brain’s capacity to repair DNA damage, leading to a state of cellular aging that accelerates neuronal death. When aggregated, the α-synuclein protein plays a major role in Parkinson’s disease and other neurodegenerative disorders. A team from South Korea, led by He-Jin Lee of Konkuk University, Seoul, and Seung-Jae Lee of Seoul National University College of Medicine, showed that human neuronal cells and mouse models with elevated expression of α-synuclein develop double-stranded breaks in their genomes as a consequence of deficient quality control mechanisms. The accumulated DNA damage spurs the cells to enter a state in which they show canonical signs of cellular aging but remain metabolically active in ways that fuel neurodegeneration. Therapies that target these processes could help prevent or treat α-synuclein–linked diseases.

Atypical Ataxia Presentation in Variant Ataxia Telangiectasia: Iranian Case-Series and Review of the Literature

Ataxia-telangiectasia (AT) is a rare autosomal recessive neurodegenerative multisystem disorder. A minority of AT patients can present late-onset atypical presentations due to unknown mechanisms. The demographic, clinical, immunological and genetic data were collected by direct interview and examining the Iranian AT patients with late-onset manifestations. We also conducted a systematic literature review for reported atypical AT patients. We identified three Iranian AT patients (3/249, 1.2% of total registry) with later age at ataxia onset and slower neurologic progression despite elevated alpha-fetoprotein levels, history of respiratory infections, and immunological features of the syndrome. Of note, all patients developed autoimmunity in which a decrease of naïve T cells and regulatory T cells were observed. The literature searches also summarized data from 73 variant AT patients with atypical presentation indicating biallelic mild mutations mainly lead to an atypical phenotype with an increased risk of cancer. Variant AT patients present with milder phenotype or atypical form of classical symptoms causing under- or mis- diagnosis. Although missense mutations are more frequent, an atypical presentation can be associated with deleterious mutations due to unknown modifying factors.

The Role of RAD6B and PEDF in Retinal Degeneration

RAD6B is an E2 ubiquitin-conjugating enzyme, playing an important role in DNA damage repair, gene expression, senescence, apoptosis and protein degradation. However, the specific mechanism between ubiquitin and retinal degeneration requires more investigation. Pigment epithelium-derived factor (PEDF) has a potent neurotrophic effect on the retina, protecting retinal neurons and photoreceptors from cell death caused by pathological damage. In this study, we found that loss of RAD6B leads to retinal degeneration in mice, especially in old age. Affymetrix microarray analysis showed that the PEDF signal was changed in RAD6B deficient groups. The expression of γ-H2AX, β-Gal, P53, Caspase-3, P21 and P16 was increased significantly in retinas of RAD6B knockout (KO) mice. Our studies suggest that RAD6B and PEDF play an important role in the health of retina, whereas the absence of RAD6B accelerates the degeneration.

The Cancer-Associated ATM R3008H Mutation Reveals the Link between ATM Activation and Its Exchange

ATM kinase is a tumor suppressor and a master regulator of the DNA damage response. Most cancer-associated alterations to ATM are missense mutations at the PI3-kinase regulatory domain (PRD) or the kinase domain. Expression of kinase-dead (KD) ATM protein solely accelerates lymphomagenesis beyond ATM loss. To understand how PRD suppresses lymphomagenesis, we introduced the cancer-associated PRD mutation R3008H (R3016 in mouse) into mice. R3008H abrogated DNA damage- and oxidative stress-induced activation of ATM without consistently affecting ATM protein stability and recruitment. In contrast to the early embryonic lethality of AtmKD/KD mice, AtmR3016H (AtmR/R ) mice were viable, immunodeficient, and displayed spontaneous craniofacial abnormalities and delayed lymphomagenesis compared with Atm-/- controls. Mechanistically, R3008H rescued the tardy exchange of ATM-KD at DNA damage foci, indicating that PRD coordinates ATM activation with its exchange at DNA-breaks. Taken together, our results reveal a unique tumorigenesis profile for PRD mutations that is distinct from null or KD mutations. SIGNIFICANT: This study functionally characterizes the most common ATM missense mutation R3008H in cancer and identifies a unique role of PI3-kinase regulatory domain in ATM activation.

Ubiquitin signalling in neurodegeneration: mechanisms and therapeutic opportunities

Neurodegenerative diseases are characterised by progressive damage to the nervous system including the selective loss of vulnerable populations of neurons leading to motor symptoms and cognitive decline. Despite millions of people being affected worldwide, there are still no drugs that block the neurodegenerative process to stop or slow disease progression. Neuronal death in these diseases is often linked to the misfolded proteins that aggregate within the brain (proteinopathies) as a result of disease-related gene mutations or abnormal protein homoeostasis. There are two major degradation pathways to rid a cell of unwanted or misfolded proteins to prevent their accumulation and to maintain the health of a cell: the ubiquitin–proteasome system and the autophagy–lysosomal pathway. Both of these degradative pathways depend on the modification of targets with ubiquitin. Aging is the primary risk factor of most neurodegenerative diseases including Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis. With aging there is a general reduction in proteasomal degradation and autophagy, and a consequent increase of potentially neurotoxic protein aggregates of β-amyloid, tau, α-synuclein, SOD1 and TDP-43. An often over-looked yet major component of these aggregates is ubiquitin, implicating these protein aggregates as either an adaptive response to toxic misfolded proteins or as evidence of dysregulated ubiquitin-mediated degradation driving toxic aggregation. In addition, non-degradative ubiquitin signalling is critical for homoeostatic mechanisms fundamental for neuronal function and survival, including mitochondrial homoeostasis, receptor trafficking and DNA damage responses, whilst also playing a role in inflammatory processes. This review will discuss the current understanding of the role of ubiquitin-dependent processes in the progressive loss of neurons and the emergence of ubiquitin signalling as a target for the development of much needed new drugs to treat neurodegenerative disease.

Ataxia-telangiectasia: epidemiology, pathogenesis, clinical phenotype, diagnosis, prognosis and management

INTRODUCTION

Ataxia-telangiectasia (A-T) is a rare autosomal recessive syndrome characterized by progressive cerebellar ataxia, oculocutaneous telangiectasia, variable immunodeficiency, radiosensitivity, and cancer predisposition. Mutations cause A-T in the ataxia telangiectasia mutated (ATM) gene encoding a serine/threonine-protein kinase.

AREAS COVERED

The authors reviewed the literature on PubMed, Web of Science, and Scopus databases to collect comprehensive data related to A-T. This review aims to discuss various update aspects of A-T, including epidemiology, pathogenesis, clinical manifestations, diagnosis, prognosis, and management.

EXPERT OPINION

A-T as a congenital disorder has phenotypic heterogeneity, and the severity of symptoms in different patients depends on the severity of mutations. This review provides a comprehensive overview of A-T, although some relevant questions about pathogenesis remain unanswered, probably owing to the phenotypic heterogeneity of this monogenic disorder. The presence of various clinical and immunologic manifestations in A-T indicates that the identification of the role of defective ATM in phenotype can be helpful in the better management and treatment of patients in the future.

Loss of RAD6B induces degeneration of the cochlea in mice

Presbycusis is a form of age-related hearing loss (AHL). Many studies have shown that the degeneration of various structures in the cochlea of the inner ear is related to AHL, and DNA damage is an important factor leading to the above process. As an E2 ubiquitin-conjugated enzyme, RAD6B plays an important role in DNA damage repair (DDR) through histone ubiquitination. However, the molecular mechanism is still unclear. In this study, we investigated the role of RAD6B in the morphological changes and DDR mechanisms in aging-related degeneration of the cochlea of mice. We observed that the hair cells, stria vascularis and spiral ganglion in the cochlea of the RAD6B knockout mice showed significant degenerative changes and abnormal expression of proteins associated with DDR mechanisms compared with those of the littermate wild-type mice. In conclusion, our results suggest that the deletion of RAD6B may lead to abnormalities in DDR, thereby accelerating the degeneration of various structures in the cochlea and senescence and apoptosis of cochlea cells.

Cornelia de Lange syndrome: from molecular diagnosis to therapeutic approach

Cornelia de Lange syndrome (CdLS) is a severe genetic disorder characterised by multisystemic malformations. CdLS is due to pathogenetic variants in NIPBL, SMC1A, SMC3, RAD21 and HDAC8 genes which belong to the cohesin pathway. Cohesin plays a pivotal role in chromatid cohesion, gene expression, and DNA repair. In this review, we will discuss how perturbations in those biological processes contribute to CdLS phenotype and will emphasise the state-of-art of CdLS therapeutic approaches.

A Dual Role of ATM in Ischemic Preconditioning and Ischemic Injury

The ataxia-telangiectasia mutated (ATM) protein is regarded as the linchpin of cellular defenses to stress. Deletion of ATM results in strong oxidative stress and degenerative diseases in the nervous system. However, the role of ATM in neuronal ischemic preconditioning and lethal ischemic injury is still largely unknown. In this study, mice cortical neurons preconditioned with sublethal exposure to oxygen glucose deprivation (OGD) exhibited ATM/glucose-6-phosphate dehydrogenase pathway activation. Additionally, pharmacological inhibition of ATM prior to the preconditioning reversed neuroprotection provided by preconditioning in vitro and in vivo. Meanwhile, we found that ATM/P53 pro-apoptosis pathway was driven by lethal OGD injury, and pharmacological inhibition of ATM during fatal oxygen-glucose deprivation/reperfusion injury promoted neuronal survival. More importantly, inhibition of ATM activity after cerebral ischemia protected against cerebral ischemic-reperfusion damage in mice. In conclusion, our data show the dual role of ATM in neuronal ischemic preconditioning and lethal ischemic injury, involving in the protection of ischemic preconditioning, but promoting neuronal death in lethal ischemic injury. Thus, the present study provides new opportunity for the treatment of ischemic stroke.

RAD6B Plays a Critical Role in Neuronal DNA Damage Response to Resist Neurodegeneration

RAD6 participates in DNA double-strand breaks (DSBs) repair by ubiquitinating histone H2B in mitotic cells. In terminally differentiated cells, however, the mechanisms of DNA damage repair are less well known. In this study, we investigate whether RAD6B is involved in DSBs repair in neurons and effects of RAD6B deficiency on neuronal survival. We compared neurons of RAD6B-deficient mice with those of littermate wild type (WT) mice and induced DNA damage by X-ray irradiation. We provide evidence that RAD6B is essential for neural DDR and RAD6B deficiency results in increased genomic instability and neurodegeneration. Moreover, higher levels of p53 and p21 are present in the brains of RAD6B-deficient mice, which may be responsible for neuronal senescence, and degeneration. In addition, behavioral experiments show that RAD6B-deficient mice exhibit marked learning and memory deficits. In conclusion, these findings suggest that RAD6B is critical for neural integrity and that the absence of RAD6B accelerates neurodegeneration in mice.

Emerging Perspectives on DNA Double-strand Breaks in Neurodegenerative Diseases

DNA double-strand breaks (DSBs) are common events that were recognized as one of the most toxic lesions in eu-karyotic cells. DSBs are widely involved in many physiological processes such as V(D)J recombination, meiotic recombina-tion, DNA replication and transcription. Deregulation of DSBs has been reported in multiple diseases in human beings, such as the neurodegenerative diseases, with which the underlying mechanisms are needed to be illustrated. Here, we reviewed the recent insights into the dysfunction of DSB formation and repair, contributing to the pathogenesis of neurodegenerative dis-orders including Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), Huntington’s disease (HD) and ataxia tel-angiectasia (A-T).

Ataxia-telangiectasia: A review of clinical features and molecular pathology

Ataxia-telangiectasia (A-T) is an autosomal recessive primary immunodeficiency (PID) disease that is caused by mutations in ataxia-telangiectasia mutated (ATM) gene encoding a serine/threonine protein kinase. A-T patients represent a broad range of clinical manifestations including progressive cerebellar ataxia, oculocutaneous telangiectasia, variable immunodeficiency, radiosensitivity, susceptibility to malignancies, and increased metabolic diseases. This congenital disorder has phenotypic heterogeneity, and the severity of symptoms varies in different patients based on severity of mutations and disease progression. The principal role of nuclear ATM is the coordination of cellular signaling pathways in response to DNA double-strand breaks, oxidative stress, and cell cycle checkpoint. The pathogenesis of A-T is not limited to the role of ATM in the DNA damage response (DDR) pathway, and it has other functions mainly in the hematopoietic cells and neurons. ATM adjusts the functions of organelles such as mitochondria and peroxisomes and also regulates angiogenesis and glucose metabolisms. However, ATM has other functions in the cells (especially cell viability) that need further investigations. In this review, we described functions of ATM in the nucleus and cytoplasm, and also its association with some disorder formation such as neurologic, immunologic, vascular, pulmonary, metabolic, and dermatologic complications.

Antioxidant treatment ameliorates phenotypic features of SMC1A-mutated Cornelia de Lange syndrome in vitro and in vivo

Cornelia de Lange syndrome (CdLS) is a rare disease characterized by cognitive impairment, multisystemic alterations and premature aging. Furthermore, CdLS cells display gene expression dysregulation and genomic instability. Here, we demonstrated that treatment with antioxidant drugs, such as ascorbic acid and riboceine, reduced the level of genomic instability and extended the in vitro lifespan of CdLS cell lines. We also found that antioxidant treatment partially rescued the phenotype of a zebrafish model of CdLS. Gene expression profiling showed that antioxidant drugs caused dysregulation of gene transcription; notably, a number of genes coding for the zinc finger (ZNF)-containing Krueppel-associated box (KRAB) protein domain (KRAB-ZNF) were found to be downregulated. Taken together, these data suggest that antioxidant drugs have the potential to ameliorate the developmental phenotype of CdLS.

Genome Damage Sensing Leads to Tissue Homeostasis in Drosophila

DNA repair is a critical cellular process required for the maintenance of genomic integrity. It is now well appreciated that cells employ several DNA repair pathways to take care of distinct types of DNA damage. It is also well known that a cascade of signals namely DNA damage response or DDR is activated in response to DNA damage which comprise cellular responses, such as cell cycle arrest, DNA repair and cell death, if the damage is irreparable. There is also emerging literature suggesting a cross-talk between DNA damage signaling and several signaling networks within a cell. Moreover, cell death players themselves are also well known to engage in processes outside their canonical function of apoptosis. This chapter attempts to build a link between DNA damage, DDR and signaling from the studies mainly conducted in mammals and Drosophila model systems, with a special emphasis on their relevance in overall tissue homeostasis and development.

DNA-PK and P38 MAPK: A Kinase Collusion in Alzheimer's Disease?

The pathogenesis of Alzheimer's disease (AD), characterized by prevalent neuronal death and extracellular deposit of amyloid plaques, is poorly understood. DNA lesions downstream of reduced DNA repair ability have been reported in AD brains. Neurons predominantly use a mechanism to repair double-strand DNA breaks (DSB), which is non-homologous end joining (NHEJ). NHEJ requires DNA-dependent protein kinase (DNA-PK) activity. DNA-PK is a holoenzyme comprising the p460 kD catalytic subunit (DNA-PKcs) and its activator Ku, a heterodimer of p86 and p70 subunits. Ku first binds and then recruits DNA-PKcs to double-stranded DNA ends before NHEJ process begins. Studies have shown reduced NHEJ activity as well as DNA-PKcs and Ku protein levels in AD brains suggesting possible contribution of unrepaired DSB to AD development. However, normal aging brains also show reduced DNA-PKcs and Ku levels thus challenging the notion of any direct link between NHEJ and AD. Another kinase, p38 MAPK is induced by various DNA damaging agents and DSB itself. Increased DNA damage with aging could induce p38 MAPK and its induction may be sustained when DNA repair is compromised in the brain with reduced DNA-PK activity. Combined, these two events may potentially set the stage for an awry nervous system approaching AD.

Cdk12 Regulates Neurogenesis and Late-Arising Neuronal Migration in the Developing Cerebral Cortex

DNA damage response (DDR) pathways are critical for ensuring that replication stress and various types of DNA lesion do not perturb production of neural cells during development. Cdk12 maintains genomic stability by regulating expression of DDR genes. Mutant mice in which Cdk12 is conditionally deleted in neural progenitor cells (NPCs) die after birth and exhibit microcephaly with a thinner cortical plate and an aberrant corpus callosum. We show that NPCs of mutant mice accumulate at G2 and M phase, and have lower expression of DDR genes, more DNA double-strand breaks and increased apoptosis. In addition to there being fewer neurons, there is misalignment of layers IV-II neurons and the presence of abnormal axonal tracts of these neurons, suggesting that Cdk12 is also required for the migration of late-arising cortical neurons. Using in utero electroporation, we demonstrate that the migrating mutant cells remain within the intermediate zone and fail to adopt a bipolar morphology. Overexpression of Cdk5 brings about a partially restoration of the neurons reaching layers IV-II in the mutant mice. Thus, Cdk12 is crucial to the repair of DNA damage during the proliferation of NPCs and is also central to the proper migration of late-arising neurons.

The ATM kinase inhibitor KU-55933 provides neuroprotection against hydrogen peroxide-induced cell damage via a γH2AX/p-p53/caspase-3-independent mechanism: Inhibition of calpain and cathepsin D

The role of the kinase ataxia-telangiectasia mutated (ATM), a well-known protein engaged in DNA damage repair, in the regulation of neuronal responses to oxidative stress remains unexplored. Thus, the neuroprotective efficacy of KU-55933, a potent inhibitor of ATM, against cell damage evoked by oxidative stress (hydrogen peroxide, H₂O₂) has been studied in human neuroblastoma SH-SY5Y cells and compared with the efficacy of this agent in models of doxorubicin (Dox)- and staurosporine (St)-evoked cell death. KU-55933 inhibited the cell death induced by H₂O₂ or Dox but not by St in undifferentiated (UN-) and retinoic acid-differentiated (RA)-SH-SY5Y cells, with a more pronounced effect in the latter cell phenotype. Furthermore, this ATM inhibitor attenuated the Dox- but not H₂O₂-induced caspase-3 activity in both UN- and RA-SH-SY5Y cells. Although KU-55933 inhibited the H₂O₂- and Dox-induced activation of ATM, it attenuated the toxin-induced phosphorylation of the proteins H2AX and p53 only in the latter model of cell damage. Moreover, the ATM inhibitor prevented the H₂O₂-evoked increases in calpain and cathepsin D activity and attenuated cell damage to a similar degree as inhibitors of calpain (MDL28170) and cathepsin D (pepstatin A). Finally, we confirmed the neuroprotective potential of KU-55933 against the H₂O₂- and Dox-evoked cell damage in primary mouse cerebellar granule cells and in the mouse hippocampal HT-22 cell line. Altogether, our results extend the neuroprotective portfolio of KU-55933 to a model of oxidative stress, with this effect not involving inhibition of the γH2AX/p-p53/caspase-3 pathway and instead associated with the attenuation of calpain and cathepsin D activity.

ATR maintains chromosomal integrity during postnatal cerebellar neurogenesis and is required for medulloblastoma formation

Microcephaly and medulloblastoma may both result from mutations that compromise genomic stability. We report that ATR, which is mutated in the microcephalic disorder Seckel syndrome, sustains cerebellar growth by maintaining chromosomal integrity during postnatal neurogenesis. Atr deletion in cerebellar granule neuron progenitors (CGNPs) induced proliferation-associated DNA damage, p53 activation, apoptosis and cerebellar hypoplasia in mice. Co-deletions of either p53 or Bax and Bak prevented apoptosis in Atr-deleted CGNPs, but failed to fully rescue cerebellar growth. ATR-deficient CGNPs had impaired cell cycle checkpoint function and continued to proliferate, accumulating chromosomal abnormalities. RNA-Seq demonstrated that the transcriptional response to ATR-deficient proliferation was highly p53 dependent and markedly attenuated by p53 co-deletion. Acute ATR inhibition in vivo by nanoparticle-formulated VE-822 reproduced the developmental disruptions seen with Atr deletion. Genetic deletion of Atr blocked tumorigenesis in medulloblastoma-prone SmoM2 mice. Our data show that p53-driven apoptosis and cell cycle arrest - and, in the absence of p53, non-apoptotic cell death - redundantly limit growth in ATR-deficient progenitors. These mechanisms may be exploited for treatment of CGNP-derived medulloblastoma using ATR inhibition.

Tyrosyl-DNA Phosphodiesterase I a critical survival factor for neuronal development and homeostasis

Tyrosyl-DNA phosphodiesterase I (TDP1), like most DNA repair associated proteins, is not essential for cell viability. However, dysfunctioning TDP1 or ATM (ataxia telangiectasia mutated) results in autosomal recessive neuropathology with similar phenotypes, including cerebellar atrophy. Dual inactivation of TDP1 and ATM causes synthetic lethality. A TDP1H⁴⁹³R catalytic mutant is associated with spinocerebellar ataxia with axonal neuropathy (SCAN1), and stabilizes the TDP1 catalytic obligatory enzyme-DNA covalent complex. The ATM kinase activates proteins early on in response to DNA damage. Tdp1-/- and Atm-/- mice exhibit accumulation of DNA topoisomerase I-DNA covalent complexes (TOPO1-cc) explicitly in neuronal tissue during development. TDP1 resolves 3'- and 5'-DNA adducts including trapped TOPO1-cc and TOPO1 protease resistant peptide-DNA complex. ATM appears to regulate the response to TOPO1-cc via a noncanonical function by regulating SUMO/ubiquitin-mediated TOPO1 degradation. In conclusion, TDP1 and ATM are critical factors for neuronal cell viability via two independent but cooperative pathways.

Human Pluripotent Stem Cells and Derived Neuroprogenitors Display Differential Degrees of Susceptibility to BH3 Mimetics ABT-263, WEHI-539 and ABT-199

Human embryonic stem cells (hESCs) are hypersensitive to genotoxic stress and display lower survival ability relative to their differentiated progeny. Herein, we attempted to investigate the source of this difference by comparing the DNA damage responses triggered by the topoisomerase I inhibitor camptothecin, in hESCs, human induced pluripotent stem cells (hiPSCs) and hESCs-derived neuroprogenitors (NP). We observed that upon camptothecin exposure pluripotent stem cells underwent apoptosis more swiftly and at a higher rate than differentiated cells. However, the cellular response encompassing ataxia-telangiectasia mutated kinase activation and p53 phosphorylation both on serine 15 as well as on serine 46 resulted very similar among the aforementioned cell types. Importantly, we observed that hESCs and hiPSCs express lower levels of the anti-apoptotic protein Bcl-2 than NP. To assess whether Bcl-2 abundance could account for this differential response we treated cells with ABT-263, WEHI-539 and ABT-199, small molecules that preferentially target the BH3-binding pocket of Bcl-xL and/or Bcl-2 and reduce their ability to sequester pro-apoptotic factors. We found that in the absence of stress stimuli, NP exhibited a higher sensitivity to ABT- 263 and WEHI-539 than hESCs and hiPSCs. Conversely, all tested cell types appeared to be highly resistant to the Bcl-2 specific inhibitor, ABT-199. However, in all cases we determined that ABT-263 or WEHI-539 treatment exacerbated camptothecin-induced apoptosis. Importantly, similar responses were observed after siRNA-mediated down-regulation of Bcl-xL or Bcl-2. Taken together, our results suggest that Bcl-xL contrary to Bcl-2 contributes to ensure cell survival and also functions as a primary suppressor of DNA double-strand brake induced apoptosis both in pluripotent and derived NP cells. The emerging knowledge of the relative dependence of pluripotent and progenitor cells on Bcl-2 and Bcl-xL activities may help to predict cellular responses and potentially manipulate these cells for therapeutic purposes in the near future.

The new insight on the regulatory role of the vitamin D3 in metabolic pathways characteristic for cancerogenesis and neurodegenerative diseases

Apart from the classical function of regulating intestinal, bone and kidney calcium and phosphorus absorption as well as bone mineralization, there is growing evidence for the neuroprotective function of vitamin D3 through neuronal calcium regulation, the antioxidative pathway, immunomodulation and detoxification. Vitamin D3 and its derivates influence directly or indirectly almost all metabolic processes such as proliferation, differentiation, apoptosis, inflammatory processes and mutagenesis. Such multifactorial effects of vitamin D3 can be a profitable source of new therapeutic solutions for two radically divergent diseases, cancer and neurodegeneration. Interestingly, an unusual association seems to exist between the occurrence of these two pathological states, called "inverse comorbidity". Patients with cognitive dysfunctions or dementia have considerably lower risk of cancer, whereas survivors of cancer have lower prevalence of central nervous system (CNS) disorders. To our knowledge, there are few publications analyzing the role of vitamin D3 in biological pathways existing in carcinogenic and neuropathological disorders.

The associations between Parkinson's disease and cancer: the plot thickens

Epidemiological studies support a general inverse association between the risk of cancer development and Parkinson’s disease (PD). In recent years however, increasing amount of eclectic evidence points to a positive association between PD and cancers through different temporal analyses and ethnic groups. This positive association has been supported by several common genetic mutations in SNCA, PARK2, PARK8, ATM, p53, PTEN, and MC1R resulting in cellular changes such as mitochondrial dysfunction, aberrant protein aggregation, and cell cycle dysregulation. Here, we review the epidemiological and biological advances of the past decade in the association between PD and cancers to offer insight on the recent and sometimes contradictory findings.

Aspm sustains postnatal cerebellar neurogenesis and medulloblastoma growth in mice

Alterations in genes that regulate brain size may contribute to both microcephaly and brain tumor formation. Here, we report that Aspm, a gene that is mutated in familial microcephaly, regulates postnatal neurogenesis in the cerebellum and supports the growth of medulloblastoma, the most common malignant pediatric brain tumor. Cerebellar granule neuron progenitors (CGNPs) express Aspm when maintained in a proliferative state by sonic hedgehog (Shh) signaling, and Aspm is expressed in Shh-driven medulloblastoma in mice. Genetic deletion of Aspm reduces cerebellar growth, while paradoxically increasing the mitotic rate of CGNPs. Aspm-deficient CGNPs show impaired mitotic progression, altered patterns of division orientation and differentiation, and increased DNA damage, which causes progenitor attrition through apoptosis. Deletion of Aspm in mice with Smo-induced medulloblastoma reduces tumor growth and increases DNA damage. Co-deletion of Aspm and either of the apoptosis regulators Bax or Trp53 (also known as p53) rescues the survival of neural progenitors and reduces the growth restriction imposed by Aspm deletion. Our data show that Aspm functions to regulate mitosis and to mitigate DNA damage during CGNP cell division, causes microcephaly through progenitor apoptosis when mutated, and sustains tumor growth in medulloblastoma.

RNF8 deficiency results in neurodegeneration in mice

The progressive loss of neurons causes neurodegenerative diseases. Because the accumulation of DNA breaks results in neuronal apoptosis, the lack of a variety of DNA damage repair-related proteins contributes to neurodegeneration. The ubiquitin ligase RNF8 plays an important role in DNA double-strand break repair via histone ubiquitination. However, the function of RNF8 in terminally differentiated neurons remains unknown. This study aimed to determine whether RNF8 is involved in the DNA damage response in neurons and contributes to neurodegeneration. Here, we present evidence suggesting that RNF8 deficiency results in DNA damage accumulation and neuronal apoptosis. RNF8(-/-) mice exhibit neuronal degeneration and reactive astrocytosis. Neurons from RNF8(-/-) mice appear to be more susceptible to X-ray-induced DNA damage. These changes were consistent with the behavioral performances of the RNF8-deficient mice, which included impaired performances in the open-field test and step-down avoidance task. Overall, these findings show that RNF8 is required for DNA damage repair in neurons. RNF8 deficiency is sufficient to cause neuronal pathology and cognitive decline, and the loss of RNF8 results in neuron degeneration.

Inactivation of PNKP by mutant ATXN3 triggers apoptosis by activating the DNA damage-response pathway in SCA3

Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease (MJD), is an untreatable autosomal dominant neurodegenerative disease, and the most common such inherited ataxia worldwide. The mutation in SCA3 is the expansion of a polymorphic CAG tri-nucleotide repeat sequence in the C-terminal coding region of the ATXN3 gene at chromosomal locus 14q32.1. The mutant ATXN3 protein encoding expanded glutamine (polyQ) sequences interacts with multiple proteins in vivo, and is deposited as aggregates in the SCA3 brain. A large body of literature suggests that the loss of function of the native ATNX3-interacting proteins that are deposited in the polyQ aggregates contributes to cellular toxicity, systemic neurodegeneration and the pathogenic mechanism in SCA3. Nonetheless, a significant understanding of the disease etiology of SCA3, the molecular mechanism by which the polyQ expansions in the mutant ATXN3 induce neurodegeneration in SCA3 has remained elusive. In the present study, we show that the essential DNA strand break repair enzyme PNKP (polynucleotide kinase 3’-phosphatase) interacts with, and is inactivated by, the mutant ATXN3, resulting in inefficient DNA repair, persistent accumulation of DNA damage/strand breaks, and subsequent chronic activation of the DNA damage-response ataxia telangiectasia-mutated (ATM) signaling pathway in SCA3. We report that persistent accumulation of DNA damage/strand breaks and chronic activation of the serine/threonine kinase ATM and the downstream p53 and protein kinase C-δ pro-apoptotic pathways trigger neuronal dysfunction and eventually neuronal death in SCA3. Either PNKP overexpression or pharmacological inhibition of ATM dramatically blocked mutant ATXN3-mediated cell death. Discovery of the mechanism by which mutant ATXN3 induces DNA damage and amplifies the pro-death signaling pathways provides a molecular basis for neurodegeneration due to PNKP inactivation in SCA3, and for the first time offers a possible approach to treatment.

Spinocerebellar ataxia type 3 (SCA3) is an untreatable neurodegenerative disease, and the most common dominantly inherited ataxia worldwide. SCA3 is caused by expansion of a CAG tri-nucleotide repeat sequence in the ATXN3 gene’s coding region. The expanded CAG sequences encode a run of the amino acid glutamine; the mutant ATXN3 interacts with multiple proteins in vivo to create insoluble aggregates in SCA3 brains. It is thought that the loss of function of the aggregated proteins contributes to cellular toxicity and neurodegeneration in SCA3. Despite significant progress in understanding SCA3’s etiology, the molecular mechanism by which the mutant protein triggers the death of neurons in SCA3 brains remains unknown. We now report that the mutant ATXN3 protein interacts with and inactivates PNKP (polynucleotide kinase 3’-phosphatase), an essential DNA strand break repair enzyme. This inactivation results in persistent accumulation of DNA damage, and chronic activation of the DNA damage-response ATM signaling pathway in SCA3. Our work suggests that persistent DNA damage/strand breaks and chronic activation of ATM trigger neuronal death in SCA3. Discovery of the mechanism by which mutant ATXN3 induces DNA damage and amplifies the pro-death pathways provides a molecular basis for neurodegeneration in SCA3, and perhaps ultimately for its treatment.

Methylation of the nonhomologous end joining repair pathway genes does not explain the increase of translocations with aging

Chromosome translocations are especially frequent in human lymphomas and leukemias but are insufficient to drive carcinogenesis. Indeed, several of the so-called tumor specific translocations have been detected in peripheral blood of healthy individuals, finding a higher frequency of some of them with aging. The inappropriate repair of DNA double strand breaks by the nonhomologous end joining (NHEJ) pathway is one of the reasons for a translocation to occur. Moreover, fidelity of this pathway has been shown to decline with age. Although the mechanism underlying this inefficacy is unknown, other repair pathways are inactivated by methylation with aging. In this study, we analyzed the implication of NHEJ genes methylation in the increase of translocations with the age. To this aim, we determined the relationship between translocations and aging in 565 Spanish healthy individuals and correlated these data with the methylation status of 11 NHEJ genes. We found higher frequency of BCL2-JH and BCR-ABL (major) translocations with aging. In addition, we detected that two NHEJ genes (LIG4 and XRCC6) presented age-dependent promoter methylation changes. However, we did not observe a correlation between the increase of translocations and methylation, indicating that other molecular mechanisms are involved in the loss of NHEJ fidelity with aging.

Stem cells: the pursuit of genomic stability

Stem cells harbor significant potential for regenerative medicine as well as basic and clinical translational research. Prior to harnessing their reparative nature for degenerative diseases, concerns regarding their genetic integrity and mutation acquisition need to be addressed. Here we review pluripotent and multipotent stem cell response to DNA damage including differences in DNA repair kinetics, specific repair pathways (homologous recombination vs. non-homologous end joining), and apoptotic sensitivity. We also describe DNA damage and repair strategies during reprogramming and discuss potential genotoxic agents that can reduce the inherent risk for teratoma formation and mutation accumulation. Ensuring genomic stability in stem cell lines is required to achieve the quality control standards for safe clinical application.

Chromatin regulation of DNA damage repair and genome integrity in the central nervous system

With the continued extension of lifespan, aging and age-related diseases have become a major medical challenge to our society. Aging is accompanied by changes in multiple systems. Among these, the aging process in the central nervous system is critically important but very poorly understood. Neurons, as post-mitotic cells, are devoid of replicative associated aging processes, such as senescence and telomere shortening. However, because of the inability to self-replenish, neurons have to withstand challenge from numerous stressors over their lifetime. Many of these stressors can lead to damage of the neurons' DNA. When the accumulation of DNA damage exceeds a neuron's capacity for repair, or when there are deficiencies in DNA repair machinery, genome instability can manifest. The increased mutation load associated with genome instability can lead to neuronal dysfunction and ultimately to neuron degeneration. In this review, we first briefly introduce the sources and types of DNA damage and the relevant repair pathways in the nervous system (summarized in Fig. 1). We then discuss the chromatin regulation of these processes and summarize our understanding of the contribution of genomic instability to neurodegenerative diseases.

Co-opting the Fanconi anemia genomic stability pathway enables herpesvirus DNA synthesis and productive growth

DNA damage associated with viral DNA synthesis can result in double-strand breaks that threaten genome integrity and must be repaired. Here, we establish that the cellular Fanconi anemia (FA) genomic stability pathway is exploited by herpes simplex virus 1 (HSV-1) to promote viral DNA synthesis and enable its productive growth. Potent FA pathway activation in HSV-1-infected cells resulted in monoubiquitination of FA effector proteins FANCI and FANCD2 (FANCI-D2) and required the viral DNA polymerase. FANCD2 relocalized to viral replication compartments, and FANCI-D2 interacted with a multisubunit complex containing the virus-encoded single-stranded DNA-binding protein ICP8. Significantly, whereas HSV-1 productive growth was impaired in monoubiquitination-defective FA cells, this restriction was partially surmounted by antagonizing the DNA-dependent protein kinase (DNA-PK), a critical enzyme required for nonhomologous end-joining (NHEJ). This identifies the FA-pathway as a cellular factor required for herpesvirus productive growth and suggests that FA-mediated suppression of NHEJ is a fundamental step in the viral life cycle.

Aberrant topoisomerase-1 DNA lesions are pathogenic in neurodegenerative genome instability syndromes

DNA damage is considered to be a prime factor in several spinocerebellar neurodegenerative diseases; however, the DNA lesions underpinning disease etiology are unknown. We observed the endogenous accumulation of pathogenic topoisomerase-1 (Top1)-DNA cleavage complexes (Top1ccs) in murine models of ataxia telangiectasia and spinocerebellar ataxia with axonal neuropathy 1. We found that the defective DNA damage response factors in these two diseases cooperatively modulated Top1cc turnover in a non-epistatic and ATM kinase-independent manner. Furthermore, coincident neural inactivation of ATM and DNA single-strand break repair factors, including tyrosyl-DNA phosphodiesterase-1 or XRCC1, resulted in increased Top1cc formation and excessive DNA damage and neurodevelopmental defects. Notably, direct Top1 poisoning to elevate Top1cc levels phenocopied the neuropathology of the mouse models described above. Our results identify a critical endogenous pathogenic lesion associated with neurodegenerative syndromes arising from DNA repair deficiency, indicating that genome integrity is important for preventing disease in the nervous system.

Reprint of "Functional overlaps between XLF and the ATM-dependent DNA double strand break response"

Developing B and T lymphocytes generate programmed DNA double strand breaks (DSBs) during the V(D)J recombination process that assembles exons that encode the antigen-binding variable regions of antibodies. In addition, mature B lymphocytes generate programmed DSBs during the immunoglobulin heavy chain (IgH) class switch recombination (CSR) process that allows expression of different antibody heavy chain constant regions that provide different effector functions. During both V(D)J recombination and CSR, DSB intermediates are sensed by the ATM-dependent DSB response (DSBR) pathway, which also contributes to their joining via classical non-homologous end-joining (C-NHEJ). The precise nature of the interplay between the DSBR and C-NHEJ pathways in the context of DSB repair via C-NHEJ remains under investigation. Recent studies have shown that the XLF C-NHEJ factor has functional redundancy with several members of the ATM-dependent DSBR pathway in C-NHEJ, highlighting unappreciated major roles for both XLF as well as the DSBR in V(D)J recombination, CSR and C-NHEJ in general. In this review, we discuss current knowledge of the mechanisms that contribute to the repair of DSBs generated during B lymphocyte development and activation with a focus on potential functionally redundant roles of XLF and ATM-dependent DSBR factors.

Functional overlaps between XLF and the ATM-dependent DNA double strand break response

Developing B and T lymphocytes generate programmed DNA double strand breaks (DSBs) during the V(D)J recombination process that assembles exons that encode the antigen-binding variable regions of antibodies. In addition, mature B lymphocytes generate programmed DSBs during the immunoglobulin heavy chain (IgH) class switch recombination (CSR) process that allows expression of different antibody heavy chain constant regions that provide different effector functions. During both V(D)J recombination and CSR, DSB intermediates are sensed by the ATM-dependent DSB response (DSBR) pathway, which also contributes to their joining via classical non-homologous end-joining (C-NHEJ). The precise nature of the interplay between the DSBR and C-NHEJ pathways in the context of DSB repair via C-NHEJ remains under investigation. Recent studies have shown that the XLF C-NHEJ factor has functional redundancy with several members of the ATM-dependent DSBR pathway in C-NHEJ, highlighting unappreciated major roles for both XLF as well as the DSBR in V(D)J recombination, CSR and C-NHEJ in general. In this review, we discuss current knowledge of the mechanisms that contribute to the repair of DSBs generated during B lymphocyte development and activation with a focus on potential functionally redundant roles of XLF and ATM-dependent DSBR factors.

ATM-deficient human neural stem cells as an in vitro model system to study neurodegeneration

Loss of ATM kinase, a transducer of the DNA damage response and redox sensor, causes the neurodegenerative disorder ataxia-telangiectasia (A-T). While a great deal of progress has been made in elucidating the ATM-dependent DNA damage response (DDR) network, a key challenge remains in understanding the selective susceptibility of the nervous system to faulty DDR. Several factors appear implicated in the neurodegenerative phenotype in A-T, but which of them plays a crucial role remains unclear, especially since mouse models of A-T do not fully mirror the respective human syndrome. Therefore, a number of human neural stem cell (hNSC) systems have been developed to get an insight into the molecular mechanisms of neurodegeneration as consequence of ATM inactivation. Here we review the hNSC systems developed by us an others to model A-T.

DNA repair mechanisms in dividing and non-dividing cells

DNA damage created by endogenous or exogenous genotoxic agents can exist in multiple forms, and if allowed to persist, can promote genome instability and directly lead to various human diseases, particularly cancer, neurological abnormalities, immunodeficiency and premature aging. To avoid such deleterious outcomes, cells have evolved an array of DNA repair pathways, which carry out what is typically a multiple-step process to resolve specific DNA lesions and maintain genome integrity. To fully appreciate the biological contributions of the different DNA repair systems, one must keep in mind the cellular context within which they operate. For example, the human body is composed of non-dividing and dividing cell types, including, in the brain, neurons and glial cells. We describe herein the molecular mechanisms of the different DNA repair pathways, and review their roles in non-dividing and dividing cells, with an eye toward how these pathways may regulate the development of neurological disease.

DNA damage response in peripheral nervous system: coping with cancer therapy-induced DNA lesions

In the absence of blood brain barrier (BBB) the DNA of peripheral nervous system (PNS) neurons is exposed to a broader spectrum of endogenous and exogenous threats compared to that of the central nervous system (CNS). Hence, while CNS and PNS neurons cope with many similar challenges inherent to their high oxygen consumption and vigorous metabolism, PNS neurons are also exposed to circulating toxins and inflammatory mediators due to relative permeability of PNS blood nerve barrier (BNB). Consequently, genomes of PNS neurons incur greater damage and the question awaiting investigation is whether specialized repair mechanisms for maintenance of DNA integrity have evolved to meet the additional needs of PNS neurons. Here, I review data showing how PNS neurons manage collateral DNA damage incurred in the course of different anti-cancer treatments designed to block DNA replication in proliferating tumor cells. Importantly, while PNS neurotoxicity and concomitant chemotherapy-induced peripheral neuropathy (CIPN) are among major dose limiting barriers in achieving therapy goals, CIPN is partially reversible during post-treatment nerve recovery. Clearly, cell recovery necessitates mobilization of the DNA damage response and underscores the need for systematic investigation of the scope of DNA repair capacities in the PNS to help predict post-treatment risks to recovering neurons.

DNA-dependent protein kinase and DNA repair: relevance to Alzheimer's disease

The pathological hallmark of Alzheimer's disease (AD), the leading cause of senile dementia, involves region-specific neuronal death and an accumulation of neuronal and extracellular lesions termed neurofibrillary tangles and senile plaques, respectively. One of the biochemical abnormalities observed in AD is reduced DNA end-joining activity. The reduced capacity of post-mitotic neurons for some types of DNA repair is further compromised by aging. The predominant mechanism to repair double-strand DNA (dsDNA) breaks (DSB) is non-homologous end joining (NHEJ), which requires DNA-dependent protein kinase (DNA-PK) activity. DNA-PK is a holoenzyme comprising the p460 kDa DNA-PK catalytic subunit (DNA-PKcs) and the Ku heterodimer consisting of p86 (Ku 80) and p70 (Ku 70) subunits. Ku binds to DNA ends first and then recruits DNA-PKcs during NHEJ. However, in AD brains, reduced NHEJ activity has been reported along with reduced levels of DNA-PKcs and the Ku proteins, indicating a potential link between AD and dsDNA damage. Since age-matched control brains also show a reduction in these protein levels, whether there is a direct link between NHEJ ability and AD remains unknown. Possible mechanisms involving the role of DNA-PK in neurodegeneration, a benchmark of AD, are the focus of this review.

Oxidative stress, DNA damage, and the telomeric complex as therapeutic targets in acute neurodegeneration

Oxidative stress has been identified as an important contributor to neurodegeneration associated with acute CNS injuries and diseases such as spinal cord injury (SCI), traumatic brain injury (TBI), and ischemic stroke. In this review, we briefly detail the damaging effects of oxidative stress (lipid peroxidation, protein oxidation, etc.) with a particular emphasis on DNA damage. Evidence for DNA damage in acute CNS injuries is presented along with its downstream effects on neuronal viability. In particular, unchecked oxidative DNA damage initiates a series of signaling events (e.g. activation of p53 and PARP-1, cell cycle re-activation) which have been shown to promote neuronal loss following CNS injury. These findings suggest that preventing DNA damage might be an effective way to promote neuronal survival and enhance neurological recovery in these conditions. Finally, we identify the telomere and telomere-associated proteins (e.g. telomerase) as novel therapeutic targets in the treatment of neurodegeneration due to their ability to modulate the neuronal response to both oxidative stress and DNA damage.

Increased learning and brain long-term potentiation in aged mice lacking DNA polymerase μ

A definitive consequence of the aging process is the progressive deterioration of higher cognitive functions. Defects in DNA repair mechanisms mostly result in accelerated aging and reduced brain function. DNA polymerase µ is a novel accessory partner for the non-homologous end-joining DNA repair pathway for double-strand breaks, and its deficiency causes reduced DNA repair. Using associative learning and long-term potentiation experiments, we demonstrate that Polµ^−/− mice, however, maintain the ability to learn at ages when wild-type mice do not. Expression and biochemical analyses suggest that brain aging is delayed in Polµ^−/− mice, being associated with a reduced error-prone DNA oxidative repair activity and a more efficient mitochondrial function. This is the first example in which the genetic ablation of a DNA-repair function results in a substantially better maintenance of learning abilities, together with fewer signs of brain aging, in old mice.

DNA polymerases in nonhomologous end joining: are there any benefits to standing out from the crowd?

Chromosome breaks, often with damaged or missing DNA flanking the break site, are an important threat to genome stability. They are repaired in vertebrates primarily by nonhomologous end joining (NHEJ). NHEJ is unique among the major DNA repair pathways in that a continuous template cannot be used by DNA polymerases to instruct replacement of damaged or lost DNA. Nevertheless, at least 3 out of the 17 mammalian DNA polymerases are specifically employed by NHEJ. Biochemical and structural studies are further revealing how each of the polymerases employed by NHEJ possesses distinct and sophisticated means to overcome the barriers this pathway presents to polymerase activity. Still unclear, though, is how the resulting network of overlapping and nonoverlapping polymerase activities contributes to repair in cells.

Betamethasone therapy in ataxia telangiectasia: unraveling the rationale of this serendipitous observation on the basis of the pathogenesis

Ataxia telangiectasia (A-T) is a rare autosomal recessive disorder characterized by progressive neurological dysfunction. To date, only supportive care aimed to halt the progressive neurodegeneration is available for the treatment. Recently, an improvement of neurological signs during short-term treatment with betamethasone has been reported. To date, the molecular and biochemical mechanisms by which the steroid produces such effects have not yet been elucidated. Therefore, a review of the literature was carried out to define the potential molecular and functional targets of the steroid effects in A-T. Glucocorticoids (GCs) are capable of diffusing into the CNS by crossing the blood-brain barrier (BBB) where they exert effects on the suppression of inflammation or as antioxidant. GCs have been shown to protect post-mitotic neurons from apoptosis. Eventually, GCs may also modulate synaptic plasticity. A better understanding of the mechanisms of action of GCs in the brain is needed, because in A-T during the initial phase of cell loss the neurological impairment may be rescued by interfering in the biochemical pathways. This would open a new window of intervention in this so far incurable disease.

Neuronal responses to physiological stress

Physiological stress can be defined as any external or internal condition that challenges the homeostasis of a cell or an organism. It can be divided into three different aspects: environmental stress, intrinsic developmental stress, and aging. Throughout life all living organisms are challenged by changes in the environment. Fluctuations in oxygen levels, temperature, and redox state for example, trigger molecular events that enable an organism to adapt, survive, and reproduce. In addition to external stressors, organisms experience stress associated with morphogenesis and changes in inner chemistry during normal development. For example, conditions such as intrinsic hypoxia and oxidative stress, due to an increase in tissue mass, have to be confronted by developing embryos in order to complete their development. Finally, organisms face the challenge of stochastic accumulation of molecular damage during aging that results in decline and eventual death. Studies have shown that the nervous system plays a pivotal role in responding to stress. Neurons not only receive and process information from the environment but also actively respond to various stresses to promote survival. These responses include changes in the expression of molecules such as transcription factors and microRNAs that regulate stress resistance and adaptation. Moreover, both intrinsic and extrinsic stresses have a tremendous impact on neuronal development and maintenance with implications in many diseases. Here, we review the responses of neurons to various physiological stressors at the molecular and cellular level.

Oxidative DNA damage in neurons: implication of ku in neuronal homeostasis and survival

Oxidative DNA damage is produced by reactive oxygen species (ROS) which are generated by exogenous and endogenous sources and continuously challenge the cell. One of the most severe DNA lesions is the double-strand break (DSB), which is mainly repaired by nonhomologous end joining (NHEJ) pathway in mammals. NHEJ directly joins the broken ends, without using the homologous template. Ku70/86 heterodimer, also known as Ku, is the first component of NHEJ as it directly binds DNA and recruits other NHEJ factors to promote the repair of the broken ends. Neurons are particularly metabolically active, displaying high rates of transcription and translation, which are associated with high metabolic and mitochondrial activity as well as oxygen consumption. In such a way, excessive oxygen radicals can be generated and constantly attack DNA, thereby producing several lesions. This condition, together with defective DNA repair systems, can lead to a high accumulation of DNA damage resulting in neurodegenerative processes and defects in neurodevelopment. In light of recent findings, in this paper, we will discuss the possible implication of Ku in neurodevelopment and in mediating the DNA repair dysfunction observed in certain neurodegenerations.

In vivo importance of homologous recombination DNA repair for mouse neural stem and progenitor cells

We characterized the in vivo importance of the homologous recombination factor RAD54 for the developing mouse brain cortex in normal conditions or after ionizing radiation exposure. Contrary to numerous homologous recombination genes, Rad54 disruption did not impact the cortical development without exogenous stress, but it dramatically enhanced the radiation sensitivity of neural stem and progenitor cells. This resulted in the death of all cells irradiated during S or G2, whereas the viability of cells irradiated in G1 or G0 was not affected by Rad54 disruption. Apoptosis occurred after long arrests at intra-S and G2/M checkpoints. This concerned every type of neural stem and progenitor cells, showing that the importance of Rad54 for radiation response was linked to the cell cycle phase at the time of irradiation and not to the differentiation state. In the developing brain, RAD54-dependent homologous recombination appeared absolutely required for the repair of damages induced by ionizing radiation during S and G2 phases, but not for the repair of endogenous damages in normal conditions. Altogether our data support the existence of RAD54-dependent and -independent homologous recombination pathways.