Mutations in String/CDC25 inhibit cell cycle re-entry and neurodegeneration in a Drosophila model of Ataxia telangiectasia

Ataxia Telangiectasia patient-derived neuronal and brain organoid models reveal mitochondrial dysfunction and oxidative stress

Ataxia Telangiectasia (AT) is a rare disorder caused by mutations in the ATM gene and results in progressive neurodegeneration for reasons that remain poorly understood. In addition to its central role in nuclear DNA repair, ATM operates outside the nucleus to regulate metabolism, redox homeostasis and mitochondrial function. However, a systematic investigation into how and when loss of ATM affects these parameters in relevant human neuronal models of AT was lacking. We therefore used cortical neurons and brain organoids from AT-patient iPSC and gene corrected isogenic controls to reveal levels of mitochondrial dysfunction, oxidative stress, and senescence that vary with developmental maturity. Transcriptome analyses identified disruptions in regulatory networks related to mitochondrial function and maintenance, including alterations in the PARP/SIRT signalling axis and dysregulation of key mitophagy and mitochondrial fission-fusion processes. We further show that antioxidants reduce ROS and restore neurite branching in AT neuronal cultures, and ameliorate impaired neuronal activity in AT brain organoids. We conclude that progressive mitochondrial dysfunction and aberrant ROS production are important contributors to neurodegeneration in AT and are strongly linked to ATM's role in mitochondrial homeostasis regulation.

ATM Kinase Small Molecule Inhibitors Prevent Radiation-Induced Apoptosis of Mouse Neurons In Vivo

ATM kinase is becoming an important therapeutic target for tumor radiosensitization. Radiation is known to cause neuro-inflammation and neurodegeneration; however, the effects of small molecule ATM inhibitors (ATMi's) and radiation on normal tissue, including healthy brain, are largely unexplored. Therefore, we examined the mouse CNS after ATMi radiosensitization with a focus on the fate of neurons. We used several approaches to assess the effects on the DNA damage response (DDR) and apoptosis of neurons using immunostaining. In vivo, a significant decrease in viable neurons and increase in degenerating neurons and apoptosis was observed in mice treated with radiation alone. On the other hand, an ATMi alone had little to no effect on neuron viability and did not induce apoptosis. Importantly, the ATMi's did not further increase radiation toxicity. In fact, multiplex immunostaining showed that a clinical candidate ATMi (AZD1390) protected mouse neurons from apoptosis by 90% at 4 h after radiation. We speculate that the lack of toxicity to neurons is due to a normal ATM-p53 response that, if blocked transiently with an ATMi, is protective. Altogether, in line with previous work using ATM knockout mice, we provide evidence that ATM kinase inhibition using small molecules does not add to neuronal radiation toxicity, and might, in fact, protect them from radiation-induced apoptosis at least in the short term.

Cell cycle re-entry in the aging Drosophila brain

The brain is an organ comprised mostly of long-lived, quiescent cells that perform vital functions throughout an animal's life. Due to the brain's limited regenerative ability, these long-lived cells must engage unique mechanisms to cope with accumulated damage over time. We have shown that a subset of differentiated neuronal and glial cells in the fruit fly brain become polyploid during adulthood. Cell cycle re-entry in the brain has previously been associated with neurodegeneration, but there may be a more complex relationship between polyploidy and cell fitness in the brain. Here, we examine how known lifespan modifiers influence the accumulation of polyploidy in the aging fly brain. Flies aged at a low temperature, or with a low protein diet, accumulate polyploid cells in the brain more slowly than expected if this phenotype were solely regulated by lifespan mechanisms. Despite the slower accumulation of polyploid cells, animals under conditions that extend lifespan eventually reach similar levels of polyploidy in the brain as controls. Our work suggests known lifespan modifiers can influence the timing of cell cycle re-entry in the adult brain, indicating there is a flexible window of cell cycle plasticity in the aging brain.

Inherited C-terminal TREX1 variants disrupt homology-directed repair to cause senescence and DNA damage phenotypes in Drosophila, mice, and humans

Age-related microangiopathy, also known as small vessel disease (SVD), causes damage to the brain, retina, liver, and kidney. Based on the DNA damage theory of aging, we reasoned that genomic instability may underlie an SVD caused by dominant C-terminal variants in TREX1, the most abundant 3'-5' DNA exonuclease in mammals. C-terminal TREX1 variants cause an adult-onset SVD known as retinal vasculopathy with cerebral leukoencephalopathy (RVCL or RVCL-S). In RVCL, an aberrant, C-terminally truncated TREX1 mislocalizes to the nucleus due to deletion of its ER-anchoring domain. Since RVCL pathology mimics that of radiation injury, we reasoned that nuclear TREX1 would cause DNA damage. Here, we show that RVCL-associated TREX1 variants trigger DNA damage in humans, mice, and Drosophila, and that cells expressing RVCL mutant TREX1 are more vulnerable to DNA damage induced by chemotherapy and cytokines that up-regulate TREX1, leading to depletion of TREX1-high cells in RVCL mice. RVCL-associated TREX1 mutants inhibit homology-directed repair (HDR), causing DNA deletions and vulnerablility to PARP inhibitors. In women with RVCL, we observe early-onset breast cancer, similar to patients with BRCA1/2 variants. Our results provide a mechanistic basis linking aberrant TREX1 activity to the DNA damage theory of aging, premature senescence, and microvascular disease.

Antagonistic effect of cyclin-dependent kinases and a calcium-dependent phosphatase on polyglutamine-expanded androgen receptor toxic gain of function

Spinal and bulbar muscular atrophy is caused by polyglutamine (polyQ) expansions in androgen receptor (AR), generating gain-of-function toxicity that may involve phosphorylation. Using cellular and animal models, we investigated what kinases and phosphatases target polyQ-expanded AR, whether polyQ expansions modify AR phosphorylation, and how this contributes to neurodegeneration. Mass spectrometry showed that polyQ expansions preserve native phosphorylation and increase phosphorylation at conserved sites controlling AR stability and transactivation. In small-molecule screening, we identified that CDC25/CDK2 signaling could enhance AR phosphorylation, and the calcium-sensitive phosphatase calcineurin had opposite effects. Pharmacologic and genetic manipulation of these kinases and phosphatases modified polyQ-expanded AR function and toxicity in cells, flies, and mice. Ablation of CDK2 reduced AR phosphorylation in the brainstem and restored expression of Myc and other genes involved in DNA damage, senescence, and apoptosis, indicating that the cell cycle-regulated kinase plays more than a bystander role in SBMA-vulnerable postmitotic cells.

Cell Cycle Re-entry in the Nervous System: From Polyploidy to Neurodegeneration

Terminally differentiated cells of the nervous system have long been considered to be in a stable non-cycling state and are often considered to be permanently in G0. Exit from the cell cycle during development is often coincident with the differentiation of neurons, and is critical for neuronal function. But what happens in long lived postmitotic tissues that accumulate cell damage or suffer cell loss during aging? In other contexts, cells that are normally non-dividing or postmitotic can or re-enter the cell cycle and begin replicating their DNA to facilitate cellular growth in response to cell loss. This leads to a state called polyploidy, where cells contain multiple copies of the genome. A growing body of literature from several vertebrate and invertebrate model organisms has shown that polyploidy in the nervous system may be more common than previously appreciated and occurs under normal physiological conditions. Moreover, it has been found that neuronal polyploidization can play a protective role when cells are challenged with DNA damage or oxidative stress. By contrast, work over the last two and a half decades has discovered a link between cell-cycle reentry in neurons and several neurodegenerative conditions. In this context, neuronal cell cycle re-entry is widely considered to be aberrant and deleterious to neuronal health. In this review, we highlight historical and emerging reports of polyploidy in the nervous systems of various vertebrate and invertebrate organisms. We discuss the potential functions of polyploidization in the nervous system, particularly in the context of long-lived cells and age-associated polyploidization. Finally, we attempt to reconcile the seemingly disparate associations of neuronal polyploidy with both neurodegeneration and neuroprotection.

Regulation of neuronal bioenergetics as a therapeutic strategy in neurodegenerative diseases

Neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis are a heterogeneous group of debilitating disorders with multifactorial etiologies and pathogeneses that manifest distinct molecular mechanisms and clinical manifestations with abnormal protein dynamics and impaired bioenergetics. Mitochondrial dysfunction is emerging as an important feature in the etiopathogenesis of these age-related neurodegenerative diseases. The prevalence and incidence of these diseases is on the rise with the increasing global population and average lifespan. Although many therapeutic approaches have been tested, there are currently no effective treatment routes for the prevention or cure of these diseases. We present the current status of our knowledge and understanding of the involvement of mitochondrial dysfunction in these diseases and highlight recent advances in novel therapeutic strategies targeting neuronal bioenergetics as potential approach for treating these diseases.

Polyploidy in the adult Drosophila brain

Long-lived cells such as terminally differentiated postmitotic neurons and glia must cope with the accumulation of damage over the course of an animal's lifespan. How long-lived cells deal with ageing-related damage is poorly understood. Here we show that polyploid cells accumulate in the adult fly brain and that polyploidy protects against DNA damage-induced cell death. Multiple types of neurons and glia that are diploid at eclosion, become polyploid in the adult Drosophila brain. The optic lobes exhibit the highest levels of polyploidy, associated with an elevated DNA damage response in this brain region. Inducing oxidative stress or exogenous DNA damage leads to an earlier onset of polyploidy, and polyploid cells in the adult brain are more resistant to DNA damage-induced cell death than diploid cells. Our results suggest polyploidy may serve a protective role for neurons and glia in adult Drosophila melanogaster brains.

Drosophila Glia: Models for Human Neurodevelopmental and Neurodegenerative Disorders

Glial cells are key players in the proper formation and maintenance of the nervous system, thus contributing to neuronal health and disease in humans. However, little is known about the molecular pathways that govern glia-neuron communications in the diseased brain. Drosophila provides a useful in vivo model to explore the conserved molecular details of glial cell biology and their contributions to brain function and disease susceptibility. Herein, we review recent studies that explore glial functions in normal neuronal development, along with Drosophila models that seek to identify the pathological implications of glial defects in the context of various central nervous system disorders.

Role of Serotonin Transporter in Eye Development of Drosophila melanogaster

Serotonin transporter (SerT) in the brain is an important neurotransmitter transporter involved in mental health. However, its role in peripheral organs is poorly understood. In this study, we investigated the function of SerT in the development of the compound eye in Drosophila melanogaster. We found that SerT knockdown led to excessive cell death and an increased number of cells in S-phase in the posterior eye imaginal disc. Furthermore, the knockdown of SerT in the eye disc suppressed the activation of Akt, and the introduction of PI3K effectively rescued this phenotype. These results suggested that SerT plays a role in the healthy eye development of D.melanogaster by controlling cell death through the regulation of the PI3K/Akt pathway.

Modeling Neurodegenerative Disorders in Drosophila melanogaster

Drosophila melanogaster provides a powerful genetic model system in which to investigate the molecular mechanisms underlying neurodegenerative diseases. In this review, we discuss recent progress in Drosophila modeling Alzheimer's Disease, Parkinson's Disease, Amyotrophic Lateral Sclerosis (ALS), Huntington's Disease, Ataxia Telangiectasia, and neurodegeneration related to mitochondrial dysfunction or traumatic brain injury. We close by discussing recent progress using Drosophila models of neural regeneration and how these are likely to provide critical insights into future treatments for neurodegenerative disorders.

Evidence for premature aging in a Drosophila model of Werner syndrome

Werner syndrome (WS) is an autosomal recessive progeroid disease characterized by patients' early onset of aging, increased risk of cancer and other age-related pathologies. WS is caused by mutations in WRN, a RecQ helicase that has essential roles responding to DNA damage and preventing genomic instability. While human WRN has both an exonuclease and helicase domain, Drosophila WRNexo has high genetic and functional homology to only the exonuclease domain of WRN. Like WRN-deficient human cells, Drosophila WRNexo null mutants (WRNexoΔ) are sensitive to replication stress, demonstrating mechanistic similarities between these two models. Compared to age-matched wild-type controls, WRNexoΔ flies exhibit increased physiological signs of aging, such as shorter lifespans, higher tumor incidence, muscle degeneration, reduced climbing ability, altered behavior, and reduced locomotor activity. Interestingly, these effects are more pronounced in females suggesting sex-specific differences in the role of WRNexo in aging. This and future mechanistic studies will contribute to our knowledge in linking faulty DNA repair mechanisms with the process of aging.

A pharmacological screen for compounds that rescue the developmental lethality of a Drosophila ATM mutant

Ataxia-telangiectasia (A-T) is a neurodegenerative disease caused by mutation of the A-T mutated (ATM) gene. ATM encodes a protein kinase that is activated by DNA damage and phosphorylates many proteins, including those involved in DNA repair, cell cycle control, and apoptosis. Characteristic biological and molecular functions of ATM observed in mammals are conserved in Drosophila melanogaster. As an example, conditional loss-of-function ATM alleles in flies cause progressive neurodegeneration through activation of the innate immune response. However, unlike in mammals, null alleles of ATM in flies cause lethality during development. With the goals of understanding biological and molecular roles of ATM in a whole animal and identifying candidate therapeutics for A-T, we performed a screen of 2400 compounds, including FDA-approved drugs, natural products, and bioactive compounds, for modifiers of the developmental lethality caused by a temperature-sensitive ATM allele (ATM8) that has reduced kinase activity at non-permissive temperatures. Ten compounds reproducibly suppressed the developmental lethality of ATM8 flies, including Ronnel, which is an organophosphate. Ronnel and other suppressor compounds are known to cause mitochondrial dysfunction or to inhibit the enzyme acetylcholinesterase, which controls the levels of the neurotransmitter acetylcholine, suggesting that detrimental consequences of reduced ATM kinase activity can be rescued by inhibiting the function of mitochondria or increasing acetylcholine levels. We carried out further studies of Ronnel because, unlike the other compounds that suppressed the developmental lethality of homozygous ATM8 flies, Ronnel was toxic to the development of heterozygous ATM8 flies. Ronnel did not affect the innate immune response of ATM8 flies, and it further increased the already high levels of DNA damage in brains of ATM8 flies, but its effects were not harmful to the lifespan of rescued ATM8 flies. These results provide new leads for understanding the biological and molecular roles of ATM and for the treatment of A-T.

Targeting New Candidate Genes by Small Molecules Approaching Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are among the most feared of the disorders that afflict humankind for the lack of specific diagnostic tests and effective treatments. Understanding the molecular, cellular, biochemical changes of NDs may hold therapeutic promise against debilitating central nerve system (CNS) disorders. In the present review, we summarized the clinical presentations and biology backgrounds of NDs, including Parkinson's disease (PD), Huntington's disease (HD), and Alzheimer's disease (AD) and explored the role of molecular mechanisms, including dys-regulation of epigenetic control mechanisms, Ataxia-telangiectasia-mutated protein kinase (ATM), and neuroinflammation in the pathogenesis of NDs. Targeting these mechanisms may hold therapeutic promise against these devastating diseases.

A novel porcine model of ataxia telangiectasia reproduces neurological features and motor deficits of human disease

Ataxia telangiectasia (AT) is a progressive multisystem disorder caused by mutations in the AT-mutated (ATM) gene. AT is a neurodegenerative disease primarily characterized by cerebellar degeneration in children leading to motor impairment. The disease progresses with other clinical manifestations including oculocutaneous telangiectasia, immune disorders, increased susceptibly to cancer and respiratory infections. Although genetic investigations and physiological models have established the linkage of ATM with AT onset, the mechanisms linking ATM to neurodegeneration remain undetermined, hindering therapeutic development. Several murine models of AT have been successfully generated showing some of the clinical manifestations of the disease, however they do not fully recapitulate the hallmark neurological phenotype, thus highlighting the need for a more suitable animal model. We engineered a novel porcine model of AT to better phenocopy the disease and bridge the gap between human and current animal models. The initial characterization of AT pigs revealed early cerebellar lesions including loss of Purkinje cells (PCs) and altered cytoarchitecture suggesting a developmental etiology for AT and could advocate for early therapies for AT patients. In addition, similar to patients, AT pigs show growth retardation and develop motor deficit phenotypes. By using the porcine system to model human AT, we established the first animal model showing PC loss and motor features of the human disease. The novel AT pig provides new opportunities to unmask functions and roles of ATM in AT disease and in physiological conditions.

Physiological and pathophysiological functions of cell cycle proteins in post-mitotic neurons: implications for Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder for which no effective treatment is available. Increased insight into the disease mechanism in early stages of pathology is required for the development of a successful therapy. Over the years, numerous studies have shown that cell cycle proteins are expressed in neurons of AD patients. Traditionally, neurons are considered to be post-mitotic, which means that they permanently retract from the cell cycle. The expression of cell cycle proteins in adult neurons of AD patients has therefore been suggested to promote or even instigate pathomechanisms underlying AD. Interestingly, expression of cell cycle proteins is detected in post-mitotic neurons of healthy controls as well, albeit to a lesser extent than in AD patients. This indicates that cell cycle proteins may serve important physiological functions in differentiated neurons. Here, we provide an overview of studies that support a role of cell cycle proteins in DNA repair and neuroplasticity in post-mitotic neurons. Aberrant control of these processes could, in turn, contribute to cell cycle-mediated neurodegeneration. The balance between regenerative and degenerative effects of cell cycle proteins in post-mitotic neurons might change throughout the different stages of AD. In the early stages of AD pathology, cell cycle protein expression may primarily occur to aid in the repair of sublethal double-strand breaks in DNA. With the accumulation of pathology, cell cycle-mediated neuroplasticity and neurodegeneration may become more predominant. Understanding the physiological and pathophysiological role of cell cycle proteins in AD could give us more insight into the neurodegenerative process in AD.

TCTP directly regulates ATM activity to control genome stability and organ development in Drosophila melanogaster

Translationally controlled tumour protein (TCTP) is implicated in growth regulation and cancer. Recently, human TCTP has been suggested to play a role in the DNA damage response by forming a complex with ataxia telangiectasia-mutated (ATM) kinase . However, the exact nature of this interaction and its roles in vivo remained unclear. Here, we utilize Drosophila as an animal model to study the nuclear function of Drosophila TCTP (dTCTP). dTCTP mutants show increased radiation sensitivity during development as well as strong genetic interaction with dATM mutations, resulting in severe defects in developmental timing, organ size and chromosome stability. We identify Drosophila ATM (dATM) as a direct binding partner of dTCTP and describe a mechanistic basis for dATM activation by dTCTP. Altogether, this study provides the first in vivo evidence for direct modulation of dATM activity by dTCTP in the control of genome stability and organ development.

EZH2-mediated H3K27 trimethylation mediates neurodegeneration in ataxia-telangiectasia

The symptoms of ataxia-telangiectasia (A-T) include a progressive neurodegeneration caused by ATM protein deficiency. We previously found that nuclear accumulation of histone deacetylase-4, HDAC4, contributes to this degeneration; we now report that increased histone H3K27 trimethylation (H3K27me3) mediated by polycomb repressive complex 2 (PRC2) also plays an important role in the A-T phenotype. Enhancer of zeste homolog 2 (EZH2), a core catalytic component of PRC2, is a new ATM kinase target, and ATM-mediated S734 phosphorylation of EZH2 reduces protein stability. Thus, PRC2 formation is elevated along with H3K27me3in ATM deficiency. ChIP-sequencing shows a significant increase in H3K27me3 ‘marks’ and a dramatic shift in their location. The change of H3K27me3 chromatin-binding pattern is directly related to cell cycle re-entry and cell death of ATM-deficient neurons. Lentiviral knockdown of EZH2 rescues Purkinje cell degeneration and behavioral abnormalities in Atm^−/− mice, demonstrating that EZH2 hyperactivity is another key factor in A-T neurodegeneration.

ATM and the epigenetics of the neuronal genome

Ataxia-telangiectasia (A-T) is a neurodegenerative syndrome caused by the mutation of the ATM gene. The ATM protein is a PI3kinase family member best known for its role in the DNA damage response. While repair of DNA damage is a critical function that every CNS neuron must perform, a growing body of evidence indicates that the full range of ATM functions includes some that are unrelated to DNA damage yet are essential to neuronal survival and normal function. For example, ATM participates in the regulation of synaptic vesicle trafficking and is essential for the maintenance of normal LTP. In addition ATM helps to ensure the cytoplasmic localization of HDAC4 and thus maintains the histone 'code' of the neuronal genome by suppressing genome-wide histone deacetylation, which alters the message and protein levels of many genes that are important for neuronal survival and function. The growing list of ATM functions that go beyond its role in the DNA damage response offers a new perspective on why individuals with A-T express such a wide range of neurological symptoms, and suggests that not all A-T symptoms need to be understood in the context of the DNA repair process.

The innate immune response transcription factor relish is necessary for neurodegeneration in a Drosophila model of ataxia-telangiectasia

Neurodegeneration is a hallmark of the human disease ataxia-telangiectasia (A-T) that is caused by mutation of the A-T mutated (ATM) gene. We have analyzed Drosophila melanogaster ATM mutants to determine the molecular mechanisms underlying neurodegeneration in A-T. Previously, we found that ATM mutants upregulate the expression of innate immune response (IIR) genes and undergo neurodegeneration in the central nervous system. Here, we present evidence that activation of the IIR is a cause of neurodegeneration in ATM mutants. Three lines of evidence indicate that ATM mutations cause neurodegeneration by activating the Nuclear Factor-κB (NF-κB) transcription factor Relish, a key regulator of the Immune deficiency (Imd) IIR signaling pathway. First, the level of upregulation of IIR genes, including Relish target genes, was directly correlated with the level of neurodegeneration in ATM mutants. Second, Relish mutations inhibited upregulation of IIR genes and neurodegeneration in ATM mutants. Third, overexpression of constitutively active Relish in glial cells activated the IIR and caused neurodegeneration. In contrast, we found that Imd and Dif mutations did not affect neurodegeneration in ATM mutants. Imd encodes an activator of Relish in the response to gram-negative bacteria, and Dif encodes an immune responsive NF-κB transcription factor in the Toll signaling pathway. These data indicate that the signal that causes neurodegeneration in ATM mutants activates a specific NF-κB protein and does so through an unknown activator. In summary, these findings suggest that neurodegeneration in human A-T is caused by activation of a specific NF-κB protein in glial cells.

Identification of genetic modifiers of TDP-43 neurotoxicity in Drosophila

Cytosolic aggregation of the nuclear RNA-binding protein TDP-43 is a histopathologic signature of degenerating neurons in amyotrophic lateral sclerosis (ALS), and mutations in the TARDBP gene encoding TDP-43 cause dominantly inherited forms of this condition. To understand the relationship between TDP-43 misregulation and neurotoxicity, we and others have used Drosophila as a model system, in which overexpression of either wild-type TDP-43 or its ALS-associated mutants in neurons is sufficient to induce neurotoxicity, paralysis, and early death. Using microarrays, we have examined gene expression patterns that accompany TDP-43-induced neurotoxicity in the fly system. Constitutive expression of TDP-43 in the Drosophila compound eye elicited widespread gene expression changes, with strong upregulation of cell cycle regulatory genes and genes functioning in the Notch intercellular communication pathway. Inducible expression of TDP-43 specifically in neurons elicited significant expression differences in a more restricted set of genes. Genes that were upregulated in both paradigms included SpindleB and the Notch target Hey, which appeared to be a direct TDP-43 target. Mutations that diminished activity of Notch or disrupted the function of downstream Notch target genes extended the lifespan of TDP-43 transgenic flies, suggesting that Notch activation was deleterious in this model. Finally, we showed that mutation of the nucleoporin Nup50 increased the lifespan of TDP-43 transgenic flies, suggesting that nuclear events contribute to TDP-43-dependent neurotoxicity. The combined findings identified pathways whose deregulation might contribute to TDP-43-induced neurotoxicity in Drosophila.

ATM kinase inhibition in glial cells activates the innate immune response and causes neurodegeneration in Drosophila

To investigate the mechanistic basis for central nervous system (CNS) neurodegeneration in the disease ataxia-telangiectasia (A-T), we analyzed flies mutant for the causative gene A-T mutated (ATM). ATM encodes a protein kinase that functions to monitor the genomic integrity of cells and control cell cycle, DNA repair, and apoptosis programs. Mutation of the C-terminal amino acid in Drosophila ATM inhibited the kinase activity and caused neuron and glial cell death in the adult brain and a reduction in mobility and longevity. These data indicate that reduced ATM kinase activity is sufficient to cause neurodegeneration in A-T. ATM kinase mutant flies also had elevated expression of innate immune response genes in glial cells. ATM knockdown in glial cells, but not neurons, was sufficient to cause neuron and glial cell death, a reduction in mobility and longevity, and elevated expression of innate immune response genes in glial cells, indicating that a non-cell-autonomous mechanism contributes to neurodegeneration in A-T. Taken together, these data suggest that early-onset CNS neurodegeneration in A-T is similar to late-onset CNS neurodegeneration in diseases such as Alzheimer's in which uncontrolled inflammatory response mediated by glial cells drives neurodegeneration.

Histone Deacetylase Inhibitors and Mithramycin A Impact a Similar Neuroprotective Pathway at a Crossroad between Cancer and Neurodegeneration

Mithramycin A (MTM) and histone deacetylase inhibitors (HDACi) are effective therapeutic agents for cancer and neurodegenerative diseases. MTM is a FDA approved aureolic acid-type antibiotic that binds to GC-rich DNA sequences and interferes with Sp1 transcription factor binding to its target sites (GC box). HDACi, on the other hand, modulate the activity of class I and II histone deacetylases. They mediate their protective function, in part, by regulating the acetylation status of histones or transcription factors, including Sp1, and in turn chromatin accessibility to the transcriptional machinery. Because these two classes of structurally and functionally diverse compounds mediate similar therapeutic functions, we investigated whether they act on redundant or synergistic pathways to protect neurons from oxidative death. Non-protective doses of each of the drugs do not synergize to create resistance to oxidative death suggesting that these distinct agents act via a similar pathway. Accordingly, we found that protection by MTM and HDACi is associated with diminished expression of the oncogene, Myc and enhanced expression of a tumor suppressor, p21^waf1/cip1. We also find that neuroprotection by MTM or Myc knockdown is associated with downregulation of class I HDAC levels. Our results support a model in which the established antitumor drug MTM or canonical HDACi act via distinct mechanisms to converge on the downregulation of HDAC levels or activity respectively. These findings support the conclusion that an imbalance in histone acetylase and HDAC activity in favor of HDACs is key not only for oncogenic transformation, but also neurodegeneration.

Molecular genetic characterization of Drosophila ATM conserved functional domains

ATM-related kinases promote repair of DNA double-strand breaks and maintenance of chromosome telomeres, functions that are essential for chromosome structural integrity in all eukaryotic organisms. In humans, loss of ATM function is associated with ataxia telangiectasia, a neurodegenerative disease characterized by extreme sensitivity to DNA damage. Drosophila melanogaster has recently emerged as a useful animal model for analyzing the molecular functions of specific domains of this large, multifunctional kinase. The gene encoding Drosophila ATM kinase (dATM) was originally designated tefu because of the telomere fusion defects observed in atm mutants. In this report, molecular characterization of eight atm (tefu) alleles identified nonsense mutations predicted to truncate conserved C-terminal domains of the dATM protein, as well as two interesting missense mutations. One of these missense mutations localized within a putative HEAT repeat in the poorly characterized N-terminal domain of dATM (atm4), whereas another associated with a temperature-sensitive allele (atm8) changed the last amino acid of the conserved FATC domain. Leveraging this molecular information with the powerful genetic tools available in Drosophila should facilitate future analysis of conserved ATM-mediated molecular mechanisms that are important for telomere maintenance, DNA repair, and neurodegeneration.

Human HDAC1 and HDAC2 function in the DNA-damage response to promote DNA nonhomologous end-joining

DNA double-strand break (DSB) repair occurs within chromatin and can be modulated by chromatin-modifying enzymes. Here we identify the related human histone deacetylases HDAC1 and HDAC2 as two participants in the DNA-damage response. We show that acetylation of histone H3 Lys56 (H3K56) was regulated by HDAC1 and HDAC2 and that HDAC1 and HDAC2 were rapidly recruited to DNA-damage sites to promote hypoacetylation of H3K56. Furthermore, HDAC1- and 2-depleted cells were hypersensitive to DNA-damaging agents and showed sustained DNA-damage signaling, phenotypes that reflect defective DSB repair, particularly by nonhomologous end-joining (NHEJ). Collectively, these results show that HDAC1 and HDAC2 function in the DNA-damage response by promoting DSB repair and thus provide important insights into the radio-sensitizing effects of HDAC inhibitors that are being developed as cancer therapies.

Differential histone deacetylase mRNA expression patterns in amyotrophic lateral sclerosis

Histone deacetylases (HDACs) are important regulators of gene expression and cell differentiation. The HDAC inhibitors have recently been considered as potential novel neuroprotective drugs for the treatment of neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). A major limitation, however, lies in the broad spectrum of action of currently available HDAC inhibitors that may cause a variety of toxic side effects. The mRNA expression levels of the HDAC isoforms HDACs 1 to 11 have previously been characterized in rat brain but have not been studied in human tissue. Using in situ hybridization histochemistry and immunohistochemistry we assessed the distribution and expression levels of HDACs 1to 11 in postmortem ALS and control brain and spinal cord specimens (n = 6 cases each) to determine alterations in the mRNA expression pattern that could provide a basis for disease-specific therapies. We found a reduction of HDAC 11 mRNA and increased HDAC 2 levels in ALS brain and spinal cord compared with controls. A more precise knowledge of the disease-related expression pattern could lead to the development of more specific pharmacotherapeutic approaches.

Ubiquilin modifies TDP-43 toxicity in a Drosophila model of amyotrophic lateral sclerosis (ALS)

TDP-43 (43-kDa TAR DNA-binding protein) is a major constituent of ubiquitin-positive cytosolic aggregates present in neurons of patients with amyotrophic lateral sclerosis (ALS) and ubiquitin-positive fronto-temporal lobar degeneration (FTLD-U). Inherited mutations in TDP-43 have been linked to familial forms of ALS, indicating a key role for TDP-43 in disease pathogenesis. Here, we describe a Drosophila melanogaster model of TDP-43 proteinopathy. Expression of wild-type human TDP-43 protein in Drosophila motor neurons led to motor dysfunction and dramatic reduction of life span. Interestingly, coexpression of ubiquilin 1, a previously identified TDP-43-interacting protein with suspected functions in autophagy and proteasome targeting, reduced steady-state TDP-43 expression but enhanced the severity of TDP-43 phenotypes. Finally, ectopically expressed TDP-43 was largely localized to motor neuron nuclei, suggesting that expression of wild-type TDP-43 alone is detrimental even in the absence of cytosolic aggregation. Our findings demonstrate that TDP-43 exerts cell-autonomous neurotoxicity in Drosophila and further imply that dose-dependent alterations of TDP-43 nuclear function may underlie motor neuron death in ALS.

Maintaining the brain: insight into human neurodegeneration from Drosophila melanogaster mutants

The fruitfly Drosophila melanogaster has enabled significant advances in neurodegenerative disease research, notably in the identification of genes that are required to maintain the structural integrity of the brain, defined by recessive mutations that cause adult onset neurodegeneration. Here, we survey these genes in the fly and classify them according to five key cell biological processes. Over half of these genes have counterparts in mice or humans that are also associated with neurodegeneration. Fly genetics continues to be instrumental in the analysis of degenerative disease, with notable recent advances in our understanding of several inherited disorders, Parkinson's disease, and the central role of mitochondria in neuronal maintenance.

ATM localization and gene expression in the adult mouse eye

PURPOSE

High levels of metabolism and oxygen consumption in most adult murine ocular compartments, combined with exposure to light and ultraviolet (UV) radiation, are major sources of oxidative stress, causing DNA damage in ocular cells. Of all mammalian body cells, photoreceptor cells consume the largest amount of oxygen and generate the highest levels of oxidative damage. The accumulation of such damage throughout life is a major factor of aging tissues. Several multiprotein complexes have recently been identified as the major sensors and mediators involved in the maintenance of DNA integrity. The activity of these complexes initially seemed to be restricted to dividing cells, given their ultimate role in major cell cycle checkpoints. However, it was later established that they are also active in post-mitotic cells. Recent findings demonstrate that the DNA damage response (DDR) is essential for the development, maintenance, and normal functioning of the adult central nervous system. One major molecular factor in the DDR is the protein, ataxia telangiectasia mutated (ATM). It is required for the rapid induction of cellular responses to DNA double-strand breaks. These cytotoxic DNA lesions may be caused by oxidative damage. To understand how ATM prevents oxidative stress and participates in the maintenance of genomic integrity and cell viability of the adult retina, we determined the ATM expression patterns and studied its localization in the adult mouse eye.

METHODS

Atm gene expression was analyzed by RT-PCR experiments and its localization by in situ hybridization on adult mouse ocular and cerebellar tissue sections. ATM protein expression was determined by western blot analysis of proteins homogenates extracted from several mouse tissues and its localization by immunohistochemistry experiments performed on adult mouse ocular and cerebellar tissue sections. In addition, subcellular localization was realized by confocal microscopy imaging of ocular tissue sections, with a special focus on retinal cells.

RESULTS

Using RT-PCR, we detected a band of the expected size, with its sequence matching the amplified Atm cDNA sequence. Atm mRNA was detected in most cell bodies of the adult mouse eye by in situ hybridization of ocular tissue sections with specific digoxigenin-labeled PCR-amplified cDNA probes. Western blotting with different specific antibodies revealed bands corresponding to the expected sizes of ATM and its active forms (ATMp). These bands were not observed in the analysis of protein homogenates from Atm-deficient mouse tissues. ATM immunoreactivity was detected in the nucleus of all adult mice retinal cells and in most non-neuronal ocular cell types. The active phosphorylated form of ATM was also present in the retina as well as in non-neuronal cells of the adult mouse eye. However, its subcellular localization differed as a function of the cell type examined. A major finding of this study was that ATMp immunostaining in photoreceptor cells was exclusively in the cytoplasm, whereas ATM immunostaining was only in the nucleus of these cells. Furthermore, the specific and distinct ATM and ATMp immunolabeling patterns in photoreceptor cells were identical to those observed in the adult mouse cerebellar granule cells.

CONCLUSIONS

We report the expression profile of Atm gene and protein in the adult mouse eye. In particular, we observed a difference between the localization patterns of the active and inactive forms of ATM in photoreceptor cells. These localization patterns suggest that ATM and its phosphorylated activated form may be involved in both the protection of cells from oxidative damage and the maintenance of ocular cell structure and function. The protection mechanisms mediated by the two forms of ATM appear to be particularly important in maintaining photoreceptor integrity.