TRF2 dysfunction elicits DNA damage responses associated with senescence in proliferating neural cells and differentiation of neurons

Effects of nanomaterial exposure on telomere dysfunction, hallmarks of mammalian and zebrafish cell senescence, and zebrafish mortality

Environmental and occupational exposure to hazardous substances accelerates biological aging. However, the toxic effects of nanomaterials on telomere and cellular senescence (major hallmarks of the biological aging) remained controversial. This study was to synthesize all published evidence to explore the effects of nanomaterial exposure on the telomere change, cellular senescence and mortality of model animals. Thirty-five studies were included by searching electronic databases (PubMed, Embase and Web of Science). The pooled analysis by Stata 15.0 software showed that compared with the control, nanomaterial exposure could significantly shorten the telomere length [measured as kbp: standardized mean difference (SMD) = -1.88; 95% confidence interval (CI) = -3.13 - - 0.64; % of control: SMD = -1.26; 95%CI = -2.11- - 0.42; < 3 kbp %: SMD = 5.76; 95%CI = 2.92 - 8.60), increase the telomerase activity (SMD = -1.00; 95%CI = -1.74 to -0.26), senescence-associated β-galactosidase levels in cells (SMD = 8.20; 95%CI = 6.05 - 10.34) and zebrafish embryos (SMD = 7.32; 95%CI = 4.70 - 9.94) as well as the mortality of zebrafish (SMD = 3.83; 95%CI = 2.94 - 4.72)]. The expression levels of telomerase TERT, shelterin components (TRF1, TRF2 and POT1) and senescence biomarkers (p21, p16) were respectively identified to be decreased or increased in subgroup analyses. In conclusion, this meta-analysis demonstrates that nanomaterial exposure is associated with telomere attrition, cell senescence and organismal death.

Connecting telomere maintenance and regulation to the developmental origin and differentiation states of neuroblastoma tumor cells

A cardinal feature that distinguishes clinically high-risk neuroblastoma from low-risk tumors is telomere maintenance. Specifically, neuroblastoma tumors with either active telomerase or alternative lengthening of telomeres exhibit aggressive growth characteristics that lead to poor outcomes, whereas tumors without telomere maintenance can be managed with observation or minimal treatment. Even though the need for cancer cells to maintain telomere DNA-in order to sustain cell proliferation-is well established, recent studies suggest that the neural crest origin of neuroblastoma may enforce unique relationships between telomeres and tumor malignancy. Specifically in neuroblastoma, telomere structure and telomerase activity are correlated with the adrenergic/mesenchymal differentiation states, and manipulating telomerase activity can trigger tumor cell differentiation. Both findings may reflect features of normal neural crest development. This review summarizes recent advances in the characterization of telomere structure and telomere maintenance mechanisms in neuroblastoma and discusses the findings in the context of relevant literature on telomeres during embryonic and neural development. Understanding the canonical and non-canonical roles of telomere maintenance in neuroblastoma could reveal vulnerabilities for telomere-directed therapies with potential applications to other pediatric malignancies.

Ionizing Radiation-Induced Brain Cell Aging and the Potential Underlying Molecular Mechanisms

Population aging is occurring rapidly worldwide, challenging the global economy and healthcare services. Brain aging is a significant contributor to various age-related neurological and neuropsychological disorders, including Alzheimer's disease and Parkinson's disease. Several extrinsic factors, such as exposure to ionizing radiation, can accelerate senescence. Multiple human and animal studies have reported that exposure to ionizing radiation can have varied effects on organ aging and lead to the prolongation or shortening of life span depending on the radiation dose or dose rate. This paper reviews the effects of radiation on the aging of different types of brain cells, including neurons, microglia, astrocytes, and cerebral endothelial cells. Further, the relevant molecular mechanisms are discussed. Overall, this review highlights how radiation-induced senescence in different cell types may lead to brain aging, which could result in the development of various neurological and neuropsychological disorders. Therefore, treatment targeting radiation-induced oxidative stress and neuroinflammation may prevent radiation-induced brain aging and the neurological and neuropsychological disorders it may cause.

The Phosphorylated Form of the Histone H2AX (γH2AX) in the Brain from Embryonic Life to Old Age

The γ phosphorylated form of the histone H2AX (γH2AX) was described more than 40 years ago and it was demonstrated that phosphorylation of H2AX was one of the first cellular responses to DNA damage. Since then, γH2AX has been implicated in diverse cellular functions in normal and pathological cells. In the first part of this review, we will briefly describe the intervention of H2AX in the DNA damage response (DDR) and its role in some pivotal cellular events, such as regulation of cell cycle checkpoints, genomic instability, cell growth, mitosis, embryogenesis, and apoptosis. Then, in the main part of this contribution, we will discuss the involvement of γH2AX in the normal and pathological central nervous system, with particular attention to the differences in the DDR between immature and mature neurons, and to the significance of H2AX phosphorylation in neurogenesis and neuronal cell death. The emerging picture is that H2AX is a pleiotropic molecule with an array of yet not fully understood functions in the brain, from embryonic life to old age.

Human iPSC-Derived Neurons as A Platform for Deciphering the Mechanisms behind Brain Aging

With an increased life expectancy among humans, aging has recently emerged as a major focus in biomedical research. The lack of in vitro aging models-especially for neurological disorders, where access to human brain tissues is limited-has hampered the progress in studies on human brain aging and various age-associated neurodegenerative diseases at the cellular and molecular level. In this review, we provide an overview of age-related changes in the transcriptome, in signaling pathways, and in relation to epigenetic factors that occur in senescent neurons. Moreover, we explore the current cell models used to study neuronal aging in vitro, including immortalized cell lines, primary neuronal culture, neurons directly converted from fibroblasts (Fib-iNs), and iPSC-derived neurons (iPSC-iNs); we also discuss the advantages and limitations of these models. In addition, the key phenotypes associated with cellular senescence that have been observed by these models are compared. Finally, we focus on the potential of combining human iPSC-iNs with genome editing technology in order to further our understanding of brain aging and neurodegenerative diseases, and discuss the future directions and challenges in the field.

The Intra- and Extra-Telomeric Role of TRF2 in the DNA Damage Response

Telomere repeat binding factor 2 (TRF2) has a well-known function at the telomeres, which acts to protect the telomere end from being recognized as a DNA break or from unwanted recombination. This protection mechanism prevents DNA instability from mutation and subsequent severe diseases caused by the changes in DNA, such as cancer. Since TRF2 actively inhibits the DNA damage response factors from recognizing the telomere end as a DNA break, many more studies have also shown its interactions outside of the telomeres. However, very little has been discovered on the mechanisms involved in these interactions. This review aims to discuss the known function of TRF2 and its interaction with the DNA damage response (DDR) factors at both telomeric and non-telomeric regions. In this review, we will summarize recent progress and findings on the interactions between TRF2 and DDR factors at telomeres and outside of telomeres.

Microcephalin 1/BRIT1-TRF2 interaction promotes telomere replication and repair, linking telomere dysfunction to primary microcephaly

Telomeres protect chromosome ends from inappropriately activating the DNA damage and repair responses. Primary microcephaly is a key clinical feature of several human telomere disorder syndromes, but how microcephaly is linked to dysfunctional telomeres is not known. Here, we show that the microcephalin 1/BRCT-repeats inhibitor of hTERT (MCPH1/BRIT1) protein, mutated in primary microcephaly, specifically interacts with the TRFH domain of the telomere binding protein TRF2. The crystal structure of the MCPH1-TRF2 complex reveals that this interaction is mediated by the MCPH1 ₃₃₀YRLSP₃₃₄ motif. TRF2-dependent recruitment of MCPH1 promotes localization of DNA damage factors and homology directed repair of dysfunctional telomeres lacking POT1-TPP1. Additionally, MCPH1 is involved in the replication stress response, promoting telomere replication fork progression and restart of stalled telomere replication forks. Our work uncovers a previously unrecognized role for MCPH1 in promoting telomere replication, providing evidence that telomere replication defects may contribute to the onset of microcephaly.

Mechanisms of enhanced quiescence in neural stem cell aging

The maintenance of neural stem cell function is vital to ensure neurogenesis throughout adulthood. During aging, there is a significant reduction in adult neurogenesis that correlates with a decline in cognitive function. Although recent studies have revealed novel extrinsic and intrinsic mechanisms that regulate the adult neural stem cell (NSC) pool and lineage progression, the precise molecular mechanisms that drive dysregulation of adult neurogenesis in the context of aging are only beginning to emerge. Recent studies have shed light on mechanisms that regulate the earliest step of adult neurogenesis, the activation of quiescent NSCs. Interestingly, the ability of NSCs to enter the cell cycle in the aged brain significantly declines suggesting a deepend state of quiescence. Given the likely contribution of adult neurogenesis to supporting cognitive function in humans, enhancing neurogenesis may be a strategy to combat age-related cognitive decline. This review highlights the mechanisms that regulate the NSC pool throughout adulthood and discusses how dysregulation of these processes may contribute to the decline in neurogenesis and cognitive function throughout aging.

Long-lived post-mitotic cell aging: is a telomere clock at play?

Senescence is a cellular response to stress for both dividing and post-mitotic cells. Noteworthy, long-lived post-mitotic cells (collectively named LLPMCs), which can live for decades in the organism, can exhibit a distinct type of cellular aging characterized by a progressive functional decline not associated to an overt senescence phenotype. The age-related drivers of senescence and aging in LLPMCs remain largely unknown. There is evidence that an increased production of reactive oxygen species (ROS) due to dysfunctional mitochondria, coupled with an inherent inability of cellular-degradation mechanisms to remove damaged molecules, is responsible for senescence and aging in LLPMC. Although telomeric DNA shortening, by nature linked to cell division, is generally not considered as a driver of LLPMC aging and senescence, we discuss recent reports revealing the existence of age-related telomere changes in LLPMC. These findings reveal unexpected roles for telomeres in LLPMC function and invite us to consider the hypothesis of a complex telomere clock involved in both dividing and non-dividing cell aging.

Downregulation of TMPRSS4 Enhances Triple-Negative Breast Cancer Cell Radiosensitivity Through Cell Cycle and Cell Apoptosis Process Impairment

Background:

Radioresistance remains a challenge for cancer radiotherapy. The present study aims to investigate the role of TMPRSS4 in triple negative breast cancer (TNBC) cell radiosensitivity.

Materials and Methods:

After transfection of MDA-MD-468 triple negative breast cancer cells line by using the lentivirus vector, the effect of TMPRSS4 down-regulation on TNBC radiosensitivity was evaluated by using cloning assay and CCK-8 assay. The CCK-8 assay was also used for performing cell proliferation analysis. Western blot was carried out to detect the expression of certain proteins related to cell cycle pathways (cyclin D1), cell apoptosis pathways (Bax, Bcl2, and Caspase3), DNA damage and DNA damage repair (TRF2, Ku80 , ˠH2AX) . The cell cycle and cell apoptosis were also investigated using flow cytometer analysis.

Results:

TMPRSS4 expression was down-regulated in MDA-MB-468 cells which enhanced MDA-MB-468 cells radiosensitivity. TMPRSS4 silencing also improved IR induced cell proliferation ability reduction and promoted cell arrested at G2/M phase mediated by 6 Gy IR associated with cyclin D1 expression inhibition. Moreover, TMPRSS4 inhibition enhanced TNBC apoptosis induced by 6 Gy IR following by over-expression of (Bax, Caspase3) and down-regulation of Bcl2 as the pro-apoptotic and anti-apoptotic proteins, respectively. Otherwise, TMPRSS4 down-regulation increases DNA damage induced by 6 Gy IR and delays DNA damage repair respectively illustrated by downregulation of TRF2 and permanent increase of Ku80 and ˠH2AX expression at 1 h and 10 h post-IR.

Conclusion:

Down-regulation of TMPRSS4 increases triple negative breast cancer cell radiosensitivity and the use of TMPRSS4 inhibitor can be encouraged for improving radiotherapy effectiveness in TNBC radioresistant patients.

RNA Binding Proteins and Genome Integrity

Genome integrity can be threatened by various endogenous or exogenous events. To counteract these stressors, the DNA damage response network contributes to the prevention and/or repair of genomic DNA damage and serves an essential function in cellular survival. DNA binding proteins are involved in this network. Recently, several RNA-binding proteins (RBPs) that are recruited to DNA damage sites have been shown to be direct players in the prevention or repair of DNA damage. In addition, non-coding RNAs, themselves, are involved in the RNA-mediated DNA repair system. Furthermore, RNA modification such as m6A methylation might also contribute to the ultraviolet-responsive DNA damage response. Accumulating evidence suggests that RNA metabolism is more deeply involved in diverse cellular functions than previously expected, and is also intricately associated with the maintenance of genome integrity. In this review, we highlight the roles of RBPs in the maintenance of genome integrity.

Involvement of Atm and Trp53 in neural cell loss due to Terf2 inactivation during mouse brain development

Maintenance of genomic integrity is one of the critical features for proper neurodevelopment and inhibition of neurological diseases. The signals from both ATM and ATR to TP53 are well-known mechanisms to remove neural cells with DNA damage during neurogenesis. Here we examined the involvement of Atm and Atr in genomic instability due to Terf2 inactivation during mouse brain development. Selective inactivation of Terf2 in neural progenitors induced apoptosis, resulting in a complete loss of the brain structure. This neural loss was rescued partially in both Atm and Trp53 deficiency, but not in an Atr-deficient background in the mouse. Atm inactivation resulted in incomplete brain structures, whereas p53 deficiency led to the formation of multinucleated giant neural cells and the disruption of the brain structure. These giant neural cells disappeared in Lig4 deficiency. These data demonstrate ATM and TP53 are important for the maintenance of telomere homeostasis and the surveillance of telomere dysfunction during neurogenesis.

Different requirements of functional telomeres in neural stem cells and terminally differentiated neurons

Here, Lobanova et al. examine the roles of telomeres at distinct stages of murine brain development by using lineage-specific genetic ablation of TRF2, an essential component of the shelterin complex that protects chromosome ends from the DNA damage response machinery. These results suggest that telomeres are dispensable in terminally differentiated neurons and provide mechanistic insight into cognitive abnormalities associated with aberrant telomere length in humans.

Telomeres have been studied extensively in peripheral tissues, but their relevance in the nervous system remains poorly understood. Here, we examine the roles of telomeres at distinct stages of murine brain development by using lineage-specific genetic ablation of TRF2, an essential component of the shelterin complex that protects chromosome ends from the DNA damage response machinery. We found that functional telomeres are required for embryonic and adult neurogenesis, but their uncapping has surprisingly no detectable consequences on terminally differentiated neurons. Conditional knockout of TRF2 in post-mitotic immature neurons had virtually no detectable effect on circuit assembly, neuronal gene expression, and the behavior of adult animals despite triggering massive end-to-end chromosome fusions across the brain. These results suggest that telomeres are dispensable in terminally differentiated neurons and provide mechanistic insight into cognitive abnormalities associated with aberrant telomere length in humans.

The long and the short of TRF2 in neurogenesis

Gene expression patterns change dramatically during neuronal development. Proliferating cells, including neural stem cells (NSCs), express telomere repeat-binding factor 2 (TRF2), a nuclear protein that associates with telomeric proteins, DNA, and RNA telomeres. In NSCs TRF2 also binds to the transcription regulator REST to facilitate repression of numerous neuron-specific genes, thereby keeping the NSCs in a self-renewing state. Upon neuronal differentiation, TRF2 levels decline, REST-regulated neuronal genes are derepressed, and a short isoform of TRF2 arises (TRF2-S) which localizes in the cytoplasm, associates with different subsets of proteins and transcripts, and mobilizes axonal G-rich mRNAs. We recently identified two RNA-binding proteins, HNRNPH1 and H2 (referred to jointly as HNRNPH due to their high homology), which mediate the alternative splicing of an exon required for the expression of full-length TRF2. As HNRNPH levels decline during neurogenesis, TRF2 abundance decreases and TRF2-S accumulates. Here, we discuss the shared and unique functions of TRF2 and TRF2-S, the distinct subcellular compartment in which each isoform resides, the subsets of proteins and nucleic acids with which each interacts, and the functional consequences of these ribonucleoprotein interactions. This paradigm illustrates the dynamic mechanisms through which splicing regulation by factors like HNRNPH enable distinct protein functions as cells adapt to developmental programs such as neurogenesis.

Alternative Splicing of Neuronal Differentiation Factor TRF2 Regulated by HNRNPH1/H2

During neuronal differentiation, use of an alternative splice site on the rat telomere repeat-binding factor 2 (TRF2) mRNA generates a short TRF2 protein isoform (TRF2-S) capable of derepressing neuronal genes. However, the RNA-binding proteins (RBPs) controlling this splicing event are unknown. Here, using affinity pull-down analysis, we identified heterogeneous nuclear ribonucleoproteins H1 and H2(HNRNPH) as RBPs specifically capable of interacting with the spliced RNA segment (exon 7) of Trf2 pre-mRNA. HNRNPH proteins prevent the production of the short isoform of Trf2 mRNA, as HNRNPH silencing selectively elevates TRF2-S levels. Accordingly, HNRNPH levels decline while TRF2-S levels increase during neuronal differentiation. In addition, CRISPR/Cas9-mediated deletion of hnRNPH2 selectively accelerates the NGF-triggered differentiation of rat pheochromocytoma cells into neurons. In sum, HNRNPH is a splicing regulator of Trf2 pre-mRNA that prevents the expression of TRF2-S, a factor implicated in neuronal differentiation.

Transcriptional outcome of telomere signalling

Telomeres protect chromosome ends from degradation and inappropriate DNA damage response activation through their association with specific factors. Interestingly, these telomeric factors are able to localize outside telomeric regions, where they can regulate the transcription of genes involved in metabolism, immunity and differentiation. These findings delineate a signalling pathway by which telomeric changes control the ability of their associated factors to regulate transcription. This mechanism is expected to enable a greater diversity of cellular responses that are adapted to specific cell types and telomeric changes, and may therefore represent a pivotal aspect of development, ageing and telomere-mediated diseases.

Telomeres-structure, function, and regulation

In mammals, maintenance of the linear chromosome ends (or telomeres) involves faithful replication of genetic materials and protection against DNA damage signals, to ensure genome stability and integrity. These tasks are carried out by the telomerase holoenzyme and a unique nucleoprotein structure in which an array of telomere-associated proteins bind to telomeric DNA to form special protein/DNA complexes. The telomerase complex, which is comprised of telomeric reverse transcriptase (TERT), telomeric RNA component (TERC), and other assistant factors, is responsible for adding telomeric repeats to the ends of chromosomes. Without proper telomere maintenance, telomere length will shorten with successive round of DNA replication due to the so-called end replication problem. Aberrant regulation of telomeric proteins and/or telomerase may lead to abnormalities that can result in diseases such as dyskeratosis congenita (DC) and cancers. Understanding the mechanisms that regulate telomere homeostasis and the factors that contribute to telomere dysfunction should aid us in developing diagnostic and therapeutic tools for these diseases.

Recruitment of TRF2 to laser-induced DNA damage sites

Several lines of evidence suggest that the telomere-associated protein TRF2 plays critical roles in the DNA damage response. TRF2 is rapidly and transiently phosphorylated by an ATM-dependent pathway in response to DNA damage and this DNA damage-induced phosphoryation is essential for the DNA-PK-dependent pathway of DNA double-strand break repair (DSB). However, the type of DNA damage that induces TRF2 localization to the damage sites, the requirement for DNA damage-induced phosphorylation of TRF2 for its recruitment, as well as the detailed kinetics of TRF2 accumulation at DNA damage sites have not been fully investigated. In order to address these questions, we used an ultrafast femtosecond multiphoton laser and a continuous wave 405-nm single photon laser to induce DNA damage at defined nuclear locations. Our results showed that DNA damage produced by a femtosecond multiphoton laser was sufficient for localization of TRF2 to these DNA damage sites. We also demonstrate that ectopically expressed TRF2 was recruited to DNA lesions created by a 405-nm laser. Our data suggest that ATM and DNA-PKcs kinases are not required for TRF2 localization to DNA damage sites. Furthermore, we found that phosphorylation of TRF2 at residue T188 was not essential for its recruitment to laser-induced DNA damage sites. Thus, we provide further evidence that a protein known to function in telomere maintenance, TRF2, is recruited to sites of DNA damage and plays critical roles in the DNA damage response.

Nontelomeric splice variant of telomere repeat-binding factor 2 maintains neuronal traits by sequestering repressor element 1-silencing transcription factor

Telomere repeat-binding factor 2 (TRF2) is critical for telomere integrity in dividing stem and somatic cells, but its role in postmitotic neurons is unknown. Apart from protecting telomeres, nuclear TRF2 interacts with the master neuronal gene-silencer repressor element 1-silencing transcription factor (REST), and disruption of this interaction induces neuronal differentiation. Here we report a developmental switch from the expression of TRF2 in proliferating neural progenitor cells to expression of a unique short nontelomeric isoform of TRF2 (TRF2-S) as neurons establish a fully differentiated state. Unlike nuclear TRF2, which enhances REST-mediated gene repression, TRF2-S is located in the cytoplasm where it sequesters REST, thereby maintaining the expression of neuronal genes, including those encoding glutamate receptors, cell adhesion, and neurofilament proteins. In neurons, TRF2-S-mediated antagonism of REST nuclear activity is greatly attenuated by either overexpression of TRF2 or administration of the excitatory amino acid kainic acid. Overexpression of TRF2-S rescues kainic acid-induced REST nuclear accumulation and its gene-silencing effects. Thus, TRF2-S acts as part of a unique developmentally regulated molecular switch that plays critical roles in the maintenance and plasticity of neurons.

Dynamics of telomeric chromatin at the crossroads of aging and cancer

Telomeres are nucleoprotein structures that protect the ends of human chromosomes through the formation of a 'cap', thus preventing exonucleolytic degradation, inter- and intra-chromosomal fusion, and subsequent chromosomal instability. During aging, telomere shortening correlates with tissue dysfunction and loss of renewal capacity. In human cancer, telomere dysfunction is involved in early chromosome instability, long-term cellular proliferation, and possibly other processes related to cell survival and microenvironment. Telomeres constitute an attractive target for the development of novel small-molecule anti-cancer drugs. In particular, individual protein components of the core telomere higher-order chromatin structure (known as the telosome or 'shelterin' complex) are promising candidate targets for cancer therapy.

Squelching glioblastoma stem cells by targeting REST for proteasomal degradation

Glioblastoma brain tumors harbor a small population of cancer stem cells that are resistant to conventional chemotherapeutic and radiation treatments, and are believed responsible for tumor recurrence and mortality. The identification of the epigenetic molecular mechanisms that control self-renewal of glioblastoma stem cells will foster development of targeted therapeutic approaches. The transcriptional repressor REST, best known for its role in controlling cell fate decisions in neural progenitor cells, may also be crucial for cancer stem cell self-renewal. Two novel mechanisms for regulating the stability of REST have recently been revealed: these involve the telomere-binding protein TRF2 and the ubiquitin E3 ligase SCFbeta-TrCP. Reduced TRF2 binding to REST, and increased SCFbeta-TrCP activity, target REST for proteasomal degradation and thereby inhibit cancer stem cell proliferation. Neurological side effects of treatments that target REST and TRF2 may be less severe than conventional brain tumor treatments because postmitotic neurons do not express REST and have relatively stable telomeres.

Nontelomeric TRF2-REST interaction modulates neuronal gene silencing and fate of tumor and stem cells

Removal of TRF2, a telomere shelterin protein, recapitulates key aspects of telomere attrition including the DNA-damage response and cell-cycle arrest [1]. Distinct from the response of proliferating cells to loss of TRF2 [2, 3], in rodent noncycling cells, TRF2 inhibition promotes differentiation and growth [4, 5]. However, the mechanism that couples telomere gene-silencing features [6-8] to differentiation programs has yet to be elucidated. Here we describe an extratelomeric function of TRF2 in the regulation of neuronal genes mediated by the interaction of TRF2 with repressor element 1-silencing transcription factor (REST), a master repressor of gene networks devoted to neuronal functions [9-12]. TRF2-REST complexes are readily detected by coimmunoprecipitation assays and are localized to aggregated PML-nuclear bodies in undifferentiated pluripotent human NTera2 stem cells. Inhibition of TRF2, either by a dominant-negative mutant or by RNA interference, dissociates TRF2-REST complexes resulting in ubiquitin-proteasomal degradation of REST. Consequentially, REST-targeted neural genes (L1CAM, beta3-tubulin, synaptophysin, and others) are derepressed, resulting in acquisition of neuronal phenotypes. Notably, selective damage to telomeres without affecting TRF2 levels causes neither REST degradation nor cell differentiation. Thus, in addition to protecting telomeres, TRF2 possesses a novel role in stabilization of REST thereby controlling neural tumor and stem cell fate.

The identification of zebrafish mutants showing alterations in senescence-associated biomarkers

There is an interesting overlap of function in a wide range of organisms between genes that modulate the stress responses and those that regulate aging phenotypes and, in some cases, lifespan. We have therefore screened mutagenized zebrafish embryos for the altered expression of a stress biomarker, senescence-associated β-galactosidase (SA-β-gal) in our current study. We validated the use of embryonic SA-β-gal production as a screening tool by analyzing a collection of retrovirus-insertional mutants. From a pool of 306 such mutants, we identified 11 candidates that showed higher embryonic SA-β-gal activity, two of which were selected for further study. One of these mutants is null for a homologue of Drosophila spinster, a gene known to regulate lifespan in flies, whereas the other harbors a mutation in a homologue of the human telomeric repeat binding factor 2 (terf2) gene, which plays roles in telomere protection and telomere-length regulation. Although the homozygous spinster and terf2 mutants are embryonic lethal, heterozygous adult fish are viable and show an accelerated appearance of aging symptoms including lipofuscin accumulation, which is another biomarker, and shorter lifespan. We next used the same SA-β-gal assay to screen chemically mutagenized zebrafish, each of which was heterozygous for lesions in multiple genes, under the sensitizing conditions of oxidative stress. We obtained eight additional mutants from this screen that, when bred to homozygosity, showed enhanced SA-β-gal activity even in the absence of stress, and further displayed embryonic neural and muscular degenerative phenotypes. Adult fish that are heterozygous for these mutations also showed the premature expression of aging biomarkers and the accelerated onset of aging phenotypes. Our current strategy of mutant screening for a senescence-associated biomarker in zebrafish embryos may thus prove to be a useful new tool for the genetic dissection of vertebrate stress response and senescence mechanisms.

By performing genetic mutant screens using senescence-associated biomarkers, we show that the zebrafish is a tractable model system for the study of aging. In vertebrate organisms, it has not previously been possible to carry out systematic screens for genes that are important for stress responses and aging in an unbiased way. However, such vertebrate models are of considerable importance, given the provocative evidence of common biochemical and functional pathways modulating stress responses and lifespan as well as aging in a wide range of organisms. Our present study has successfully employed a colorimetric high-throughput method using a senescence-associated β-galactosidase-based assay to screen for mutations that alter the stress responses in zebrafish embryos, in the hope that these might represent potential aging mutants. Subsequently, the mutations identified by embryonic senescence have indeed displayed adult aging-related phenotypes in zebrafish. Hence, our method for the identification of mutant zebrafish has the immediate potential to accelerate the discovery of novel genes and new functions relevant for our understanding of aging processes in vertebrates. Such knowledge will be essential for the ultimate development of pharmacological, nutritional, and behavioral interventions for the amelioration of oxidative stress- and age-associated diseases and disabilities in humans.

Gene expression atlas of the mouse central nervous system: impact and interactions of age, energy intake and gender

The transcriptional profiles of five regions of the central nervous system (CNS) of mice varying in age, gender and dietary intake were measured by microarray. The resulting data provide insights into the mechanisms of age-, diet- and gender-related CNS plasticity and vulnerability in mammals.

Background

The structural and functional complexity of the mammalian central nervous system (CNS) is organized and modified by complicated molecular signaling processes that are poorly understood.

Results

We measured transcripts of 16,896 genes in 5 CNS regions from cohorts of young, middle-aged and old male and female mice that had been maintained on either a control diet or a low energy diet known to retard aging. Each CNS region (cerebral cortex, hippocampus, striatum, cerebellum and spinal cord) possessed its own unique transcriptome fingerprint that was independent of age, gender and energy intake. Less than 10% of genes were significantly affected by age, diet or gender, with most of these changes occurring between middle and old age. The transcriptome of the spinal cord was the most responsive to age, diet and gender, while the striatal transcriptome was the least responsive. Gender and energy restriction had particularly robust influences on the hippocampal transcriptome of middle-aged mice. Prominent functional groups of age- and energy-sensitive genes were those encoding proteins involved in DNA damage responses (Werner and telomere-associated proteins), mitochondrial and proteasome functions, cell fate determination (Wnt and Notch signaling) and synaptic vesicle trafficking.

Conclusion

Mouse CNS transcriptomes responded to age, energy intake and gender in a regionally distinctive manner. The systematic transcriptome dataset also provides a window into mechanisms of age-, diet- and sex-related CNS plasticity and vulnerability.

Telomere length and risk of Parkinson's disease

We investigated whether telomere length was associated with the risk of Parkinson's disease (PD) in a case-control study (96 cases and 172 age-matched controls) nested within the Health Professionals Follow-up Study. Relative ratio of telomere repeat copy number to single-gene copy number in peripheral blood leukocytes was determined by quantitative real time PCR. Men with shorter telomeres had a lower PD risk (multivariate adjusted relative risk for the lowest vs. the highest quartile 0.33; 95% confidence interval: 0.12-0.90). Our results suggest that, contrary to telomere attrition observed in several aging-related diseases, shorter telomeres are not associated with an increased risk of PD.

Telomere neurobiology

The ends of chromosomes consist of a hexanucleotide DNA repeat sequence and specialized DNA-binding and telomere-associated proteins. An enzyme activity called telomerase maintains telomere length by using an RNA template (TR) and a reverse transcriptase (TERT) to add the hexanucleotide sequence to the free chromosome end. The structure of telomeres is maintained and modified by telomere repeat-binding factors (TRF1 and TRF2) and proteins known for their role in DNA damage responses, including poly(ADP-ribose) polymerase-1, Werner, and ATM. Telomerase activity can be quantified using a telomere repeat amplification protocol (TRAP) assay, and levels of TERT and telomere-associated proteins are evaluated by immunoblot and immunocytochemical methods. Levels of TERT and telomere-associated proteins can be overexpressed or knocked down using viral vector-based methods. Using the kinds of approaches described here, evidence has been obtained suggesting that telomeres play important roles in regulating neural stem cell proliferation, neuronal differentiation, senescence of glial cells, and apoptosis and DNA damage responses of neural cells.

DNA damage responses in neural cells: Focus on the telomere

Postmitotic neurons must survive for the entire life of the organism and be able to respond adaptively to adverse conditions of oxidative and genotoxic stress. Unrepaired DNA damage can trigger apoptosis of neurons which is typically mediated by the ataxia telangiectasia mutated (ATM)-p53 pathway. As in all mammalian cells, telomeres in neurons consist of TTAGGG DNA repeats and several associated proteins that form a nucleoprotein complex that prevents chromosome ends from being recognized as double strand breaks. Proteins that stabilize telomeres include TRF1 and TRF2, and proteins known to play important roles in DNA damage responses and DNA repair including ATM, Werner and the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs). We have been performing studies of developing and adult neurons aimed at understanding the effects of global and telomere-directed DNA damage responses in neuronal plasticity and survival in the contexts of aging and neurodegenerative disorders. Deficits in specific DNA repair proteins, including DNA-PKcs and uracil DNA glycosylase (UDG), render neurons vulnerable to adverse conditions of relevance to the pathogenesis of neurodegenerative disorders such as Alzheimer's disease and stroke. Similarly, early postmitotic neurons with reduced telomerase activity exhibit accentuated responses to DNA damage and are prone to apoptosis demonstrating a pivotal role for telomere maintenance in both mitotic cells and postmitotic neurons. Our recent findings suggest key roles for TRF2 in regulating the differentiation and survival of neurons. TRF2 affects cell survival and differentiation by modulating DNA damage pathways, and gene expression. A better understanding of the molecular mechanisms by which neurons respond to global and telomere-specific DNA damage may reveal novel strategies for prevention and treatment of neurodegenerative disorders. Indeed, work in this and other laboratories has shown that dietary folic acid can protect neurons against Alzheimer's disease by keeping homocysteine levels low and thereby minimizing the misincorporation of uracil into DNA in neurons.