Mutant huntingtin impairs Ku70-mediated DNA repair

Broken strands, broken minds: Exploring the nexus of DNA damage and neurodegeneration

Neurodegenerative disorders are primarily characterized by neuron loss progressively leading to cognitive decline and the manifestation of incurable and debilitating conditions, such as Alzheimer's, Parkinson's, and Huntington's diseases. Loss of genome maintenance causally contributes to age-related neurodegeneration, as exemplified by the premature appearance of neurodegenerative features in a growing family of human syndromes and mice harbouring inborn defects in DNA repair. Here, we discuss the relevance of persistent DNA damage, key DNA repair mechanisms and compromised genome integrity in age-related neurodegeneration highlighting the significance of investigating these connections to pave the way for the development of rationalized intervention strategies aimed at delaying the onset of neurodegenerative disorders and promoting healthy aging.

Mutant huntingtin protein induces MLH1 degradation, DNA hyperexcision, and cGAS-STING-dependent apoptosis

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin (HTT) gene. The repeat-expanded HTT encodes a mutated HTT (mHTT), which is known to induce DNA double-strand breaks (DSBs), activation of the cGAS-STING pathway, and apoptosis in HD. However, the mechanism by which mHTT triggers these events is unknown. Here, we show that HTT interacts with both exonuclease 1 (Exo1) and MutLα (MLH1-PMS2), a negative regulator of Exo1. While the HTT-Exo1 interaction suppresses the Exo1-catalyzed DNA end resection during DSB repair, the HTT-MutLα interaction functions to stabilize MLH1. However, mHTT displays a significantly reduced interaction with Exo1 or MutLα, thereby losing the ability to regulate Exo1. Thus, cells expressing mHTT exhibit rapid MLH1 degradation and hyperactive DNA excision, which causes severe DNA damage and cytosolic DNA accumulation. This activates the cGAS-STING pathway to mediate apoptosis. Therefore, we have identified unique functions for both HTT and mHTT in modulating DNA repair and the cGAS-STING pathway-mediated apoptosis by interacting with MLH1. Our work elucidates the mechanism by which mHTT causes HD.

Necrosis Links Neurodegeneration and Neuroinflammation in Neurodegenerative Disease

The mechanisms of neuronal cell death in neurodegenerative disease remain incompletely understood, although recent studies have made significant advances. Apoptosis was previously considered to be the only mechanism of neuronal cell death in neurodegenerative diseases. However, recent findings have challenged this dogma, identifying new subtypes of necrotic neuronal cell death. The present review provides an updated summary of necrosis subtypes and discusses their potential roles in neurodegenerative cell death. Among numerous necrosis subtypes, including necroptosis, paraptosis, ferroptosis, and pyroptosis, transcriptional repression-induced atypical cell death (TRIAD) has been identified as a potential mechanism of neuronal cell death. TRIAD is induced by functional deficiency of TEAD-YAP and self-amplifies via the release of HMGB1. TRIAD is a feasible potential mechanism of neuronal cell death in Alzheimer's disease and other neurodegenerative diseases. In addition to induction of cell death, HMGB1 released during TRIAD activates brain inflammatory responses, which is a potential link between neurodegeneration and neuroinflammation.

Mono- and Biallelic Inactivation of Huntingtin Gene in Patient-Specific Induced Pluripotent Stem Cells Reveal HTT Roles in Striatal Development and Neuronal Functions

Background

Mutations in the Huntingtin (HTT) gene cause Huntington's disease (HD), a neurodegenerative disorder. As a scaffold protein, HTT is involved in numerous cellular functions, but its normal and pathogenic functions during human forebrain development are poorly understood.

Objective

To investigate the developmental component of HD, with a specific emphasis on understanding the functions of wild-type and mutant HTT alleles during forebrain neuron development in individuals carrying HD mutations.

Methods

We used CRISPR/Cas9 gene-editing technology to disrupt the ATG region of the HTT gene via non-homologous end joining to produce mono- or biallelic HTT knock-out human induced pluripotent stem cell (iPSC) clones.

Results

We showed that the loss of wild-type, mutant, or both HTT isoforms does not affect the pluripotency of iPSCs or their transition into neural cells. However, we observed that HTT loss causes division impairments in forebrain neuro-epithelial cells and alters maturation of striatal projection neurons (SPNs) particularly in the acquisition of DARPP32 expression, a key functional marker of SPNs. Finally, young post-mitotic neurons derived from HTT-/- human iPSCs display cellular dysfunctions observed in adult HD neurons.

Conclusions

We described a novel collection of isogenic clones with mono- and biallelic HTT inactivation that complement existing HD-hiPSC isogenic series to explore HTT functions and test therapeutic strategies in particular HTT-lowering drugs. Characterizing neural and neuronal derivatives from human iPSCs of this collection, we show evidence that HTT loss or mutation has impacts on neuro-epithelial and striatal neurons maturation, and on basal DNA damage and BDNF axonal transport in post-mitotic neurons.

R-LOOPs on Short Tandem Repeat Expansion Disorders in Neurodegenerative Diseases

Expansions of short tandem repeats (STRs) have been found to be present in more than 50 diseases and have a close connection with neurodegenerative diseases. Transcriptional silencing and R-LOOP formation, RNA-mediated sequestration of RNA-binding proteins (RBPs), gain-of-function (GOF) proteins containing expanded repeats, and repeat-associated non-AUG (RAN) translation of toxic repeat peptides are some potential molecular mechanisms underlying STR expansion disorders. R-LOOP, a byproduct of transcription, is a three-stranded nucleic acid structure with abnormal accumulation that participates in the pathogenesis of STR expansion disorders by inducing DNA damage and genome instability. R-LOOPs can engender a series of DNA damage, such as DNA double-strand breaks (DSBs), single-strand breaks (SSBs), DNA recombination, or mutations in the DNA replication, transcription, or repair processes. In this review, we provide an in-depth discussion of recent advancements in R-LOOP and systematically elaborate on its genetic destabilizing effects in several neurodegenerative diseases. These molecular mechanisms will provide novel targets for drug design and therapeutic upgrading of these devastating diseases.

Heat shock protein DNAJA2 regulates transcription-coupled repair by triggering CSB degradation via chaperone-mediated autophagy

Transcription-coupled nucleotide excision repair (TC-NER) is an important genome maintenance system that preferentially removes DNA lesions on the transcribed strand of actively transcribed genes, including non-coding genes. TC-NER involves lesion recognition by the initiation complex consisting of RNA polymerase II (Pol II) and Cockayne syndrome group B (CSB), followed by NER-catalyzed lesion removal. However, the efficient lesion removal requires the initiation complex to yield the right of way to the excision machinery, and how this occurs in a timely manner is unknown. Here we show that heat shock protein DNAJA2 facilitates the HSC70 chaperone-mediated autophagy (CMA) to degrade CSB during TC-NER. DNAJA2 interacts with and enables HSC70 to recognize sumoylated CSB. This triggers the removal of both CSB and Pol II from the lesion site in a manner dependent on lysosome receptor LAMP2A. Defects in DNAJA2, HSC70 or LAMP2A abolish CSB degradation and block TC-NER. Our findings discover DNAJA2-mediated CMA as a critical regulator of TC-NER, implicating the DNAJA2-HSC70-CMA axis factors in genome maintenance.

Oxidative Stress and Antioxidants in Neurodegenerative Disorders

Neurodegenerative disorders constitute a substantial proportion of neurological diseases with significant public health importance. The pathophysiology of neurodegenerative diseases is characterized by a complex interplay of various general and disease-specific factors that lead to the end point of neuronal degeneration and loss, and the eventual clinical manifestations. Oxidative stress is the result of an imbalance between pro-oxidant species and antioxidant systems, characterized by an elevation in the levels of reactive oxygen and reactive nitrogen species, and a reduction in the levels of endogenous antioxidants. Recent studies have increasingly highlighted oxidative stress and associated mitochondrial dysfunction to be important players in the pathophysiologic processes involved in neurodegenerative conditions. In this article, we review the current knowledge of the general effects of oxidative stress on the central nervous system, the different specific routes by which oxidative stress influences the pathophysiologic processes involved in Alzheimer's disease, Parkinson's disease, Amyotrophic Lateral Sclerosis and Huntington's disease, and how oxidative stress may be therapeutically reversed/mitigated in order to stall the pathological progression of these neurodegenerative disorders to bring about clinical benefits.

Modulation of Synaptic Plasticity Genes Associated to DNA Damage in a Model of Huntington's Disease

Huntington's disease (HD) is a disease characterized by the progressive degeneration of nerve cells in the brain. DNA damage has been implicated in many neurological disorders; however, the association between this damage and the impaired signaling related to neurodegeneration is still unclear. The transcription factor c-AMP-responsive element binding protein (CREB) has a relevant role in the neuronal plasticity process regulating the expression of several genes, including brain-derived neurotrophic factor (BDNF). Here we analyzed the direct link between DNA damage and the expression of genes involved in neuronal plasticity. The study was performed in model cell lines STHdhQ7 (wild type) and STHdhQ111 (HD model). Treatment with Etoposide (Eto) was used to induce double-strand breaks (DSBs) to evaluate the DNA damage response (DDR) and the expression of synaptic plasticity genes. Eto treatment induced phosphorylation of ATM (p-ATM) and H2AX (γH2AX), markers of DDR, in both cell lines. Interestingly, upon DNA damage, STHdhQ7 cells showed increased expression of activity-regulated cytoskeleton associated protein (Arc) and BDNF when compared to the HD cell line model. Additionally, Eto induced CREB activation with a differential localization of its co-activators in the cell types analyzed. These results suggest that DSBs impact differentially the gene expression patterns of plasticity genes in the normal cell line versus the HD model. This effect is mediated by the impaired localization of CREB-binding protein (CBP) and histone acetylation in the HD model. Our results highlight the role of epigenetics and DNA repair on HD and therefore we suggest that future studies should explore in depth the epigenetic landscape on neuronal pathologies with the goal to further understand molecular mechanisms and pinpoint therapeutic targets.

PQBP5/NOL10 maintains and anchors the nucleolus under physiological and osmotic stress conditions

Polyglutamine binding protein 5 (PQBP5), also called nucleolar protein 10 (NOL10), binds to polyglutamine tract sequences and is expressed in the nucleolus. Using dynamic imaging of high-speed atomic force microscopy, we show that PQBP5/NOL10 is an intrinsically disordered protein. Super-resolution microscopy and correlative light and electron microscopy method show that PQBP5/NOL10 makes up the skeletal structure of the nucleolus, constituting the granule meshwork in the granular component area, which is distinct from other nucleolar substructures, such as the fibrillar center and dense fibrillar component. In contrast to other nucleolar proteins, which disperse to the nucleoplasm under osmotic stress conditions, PQBP5/NOL10 remains in the nucleolus and functions as an anchor for reassembly of other nucleolar proteins. Droplet and thermal shift assays show that the biophysical features of PQBP5/NOL10 remain stable under stress conditions, explaining the spatial role of this protein. PQBP5/NOL10 can be functionally depleted by sequestration with polyglutamine disease proteins in vitro and in vivo, leading to the pathological deformity or disappearance of the nucleolus. Taken together, these findings indicate that PQBP5/NOL10 is an essential protein needed to maintain the structure of the nucleolus.

The Emerging Landscape of Natural Small-molecule Therapeutics for Huntington's Disease

Huntington's disease (HD) is a rare and fatal neurodegenerative disorder with no diseasemodifying therapeutics. HD is characterized by extensive neuronal loss and is caused by the inherited expansion of the huntingtin (HTT) gene that encodes a toxic mutant HTT (mHTT) protein having expanded polyglutamine (polyQ) residues. Current HD therapeutics only offer symptomatic relief. In fact, Food and Drug Administration (FDA) approved two synthetic small-molecule VMAT2 inhibitors, tetrabenazine (1) and deutetrabenazine (2), for managing HD chorea and various other diseases in clinical trials. Therefore, the landscape of drug discovery programs for HD is evolving to discover disease- modifying HD therapeutics. Likewise, numerous natural products are being evaluated at different stages of clinical development and have shown the potential to ameliorate HD pathology. The inherent anti-inflammatory and antioxidant properties of natural products mitigate the mHTT-induced oxidative stress and neuroinflammation, improve mitochondrial functions, and augment the anti-apoptotic and pro-autophagic mechanisms for increased survival of neurons in HD. In this review, we have discussed HD pathogenesis and summarized the anti-HD clinical and pre-clinical natural products, focusing on their therapeutic effects and neuroprotective mechanism/s.

Reduction in PA28αβ activation in HD mouse brain correlates to increased mHTT aggregation in cell models

Huntington's disease is an autosomal dominant heritable disorder caused by an expanded CAG trinucleotide repeat at the N-terminus of the Huntingtin (HTT) gene. Lowering the levels of soluble mutant HTT protein prior to aggregation through increased degradation by the proteasome would be a therapeutic strategy to prevent or delay the onset of disease. Native PAGE experiments in HdhQ150 mice and R6/2 mice showed that PA28αβ disassembles from the 20S proteasome during disease progression in the affected cortex, striatum and hippocampus but not in cerebellum and brainstem. Modulating PA28αβ activated proteasomes in various in vitro models showed that PA28αβ improved polyQ degradation, but decreased the turnover of mutant HTT. Silencing of PA28αβ in cells lead to an increase in mutant HTT aggregates, suggesting that PA28αβ is critical for overall proteostasis, but only indirectly affects mutant HTT aggregation.

The Emerging Landscape of Small-Molecule Therapeutics for the Treatment of Huntington's Disease

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene (HTT). The new insights into HD's cellular and molecular pathways have led to the identification of numerous potent small-molecule therapeutics for HD therapy. The field of HD-targeting small-molecule therapeutics is accelerating, and the approval of these therapeutics to combat HD may be expected in the near future. For instance, preclinical candidates such as naphthyridine-azaquinolone, AN1, AN2, CHDI-00484077, PRE084, EVP4593, and LOC14 have shown promise for further optimization to enter into HD clinical trials. This perspective aims to summarize the advent of small-molecule therapeutics at various stages of clinical development for HD therapy, emphasizing their structure and design, therapeutic effects, and specific mechanisms of action. Further, we have highlighted the key drivers involved in HD pathogenesis to provide insights into the basic principle for designing promising anti-HD therapeutic leads.

Longevity-Associated Variant of BPIFB4 Confers Neuroprotection in the STHdh Cell Model of Huntington Disease

Huntington's disease (HD) is caused by the production of mutant Huntingtin (mHTT), characterized by long polyglutamine repeats with toxic effects. There are currently no clinically validated therapeutic agents that slow or halt HD progression, resulting in a significant clinical unmet need. The striatum-derived STHdh cell line, generated from mHTT knock-in mouse embryos (STHdh^Q111/Q111), represents a useful model to study mechanisms behind pathogenesis of HD and to investigate potential new therapeutic targets. Indeed, these cells show susceptibility to nucleolar stress, activated DNA damage response and apoptotic signals, and elevated levels of H3K9me3 that all together concur in the progressive HD pathogenesis. We have previously shown that the adeno-associated viral vector-mediated delivery of the longevity-associated variant (LAV) of BPIFB4 prevents HD progression in a mouse model of HD. Here, we show that LAV-BPIFB4 stably infected in STHdh^Q111/Q111 cells reduces (i) nucleolar stress and DNA damage through the improvement of DNA repair machinery, (ii) apoptosis, through the inhibition of the caspase 3 death signaling, and (iii) the levels of H3K9me3, by accelerating the histone clearance, via the ubiquitin-proteasome pathway. These findings pave the way to propose LAV-BPIFB4 as a promising target for innovative therapeutic strategies in HD.

Sequestration of cellular native factors by biomolecular assemblies: Physiological or pathological?

In addition to native-state structures, biomolecules often form condensed supramolecular assemblies or cellular membraneless organelles that are critical for cell life. These biomolecular assemblies, generally including liquid-like droplets (condensates) and amyloid-like aggregates, can sequester or recruit their interacting partners, so as to either modulate various cellular behaviors or even cause disorders. This review article summarizes recent advances in the sequestration of native factors by biomolecular assemblies and discusses their potential consequences on cellular function, homeostasis, and disease pathology.

Therapeutic Strategies in Huntington’s Disease: From Genetic Defect to Gene Therapy

Despite the identification of an expanded CAG repeat on exon 1 of the huntingtin gene located on chromosome 1 as the genetic defect causing Huntington’s disease almost 30 years ago, currently approved therapies provide only limited symptomatic relief and do not influence the age of onset or disease progression rate. Research has identified various intricate pathogenic cascades which lead to neuronal degeneration, but therapies interfering with these mechanisms have been marked by many failures and remain to be validated. Exciting new opportunities are opened by the emerging techniques which target the mutant protein DNA and RNA, allowing for “gene editing”. Although some issues relating to “off-target” effects or immune-mediated side effects need to be solved, these strategies, combined with stem cell therapies and more traditional approaches targeting specific pathogenic cascades, such as excitotoxicity and bioavailability of neurotrophic factors, could lead to significant improvement of the outcomes of treated Huntington’s disease patients.

Reparative properties of human glioblastoma cells after single exposure to a wide range of X-ray doses

Radiation therapy induces double-stranded DNA breaks in tumor cells, which leads to their death. A fraction of glioblastoma cells repair such breaks and reinitiate tumor growth. It was necessary to identify the relationship between high radiation doses and the proliferative activity of glioblastoma cells, and to evaluate the contribution of DNA repair pathways, homologous recombination (HR), and nonhomologous end joining (NHEJ) to tumor-cell recovery. We demonstrated that the GO1 culture derived from glioblastoma cells from Patient G, who had previously been irradiated, proved to be less sensitive to radiation than the Sus\fP2 glioblastoma culture was from Patient S, who had not been exposed to radiation before. GO1 cell proliferation decreased with radiation dose, and MTT decreased to 35% after a single exposure to 125 Gγ. The proliferative potential of glioblastoma culture Sus\fP2 decreased to 35% after exposure to 5 Gγ. At low radiation doses, cell proliferation and the expression of RAD51 were decreased; at high doses, cell proliferation was correlated with Ku70 protein expression. Therefore, HR and NHEJ are involved in DNA break repair after exposure to different radiation doses. Low doses induce HR, while higher doses induce the faster but less accurate NHEJ pathway of double-stranded DNA break repair.

Polymerases and DNA Repair in Neurons: Implications in Neuronal Survival and Neurodegenerative Diseases

Most of the neurodegenerative diseases and aging are associated with reactive oxygen species (ROS) or other intracellular damaging agents that challenge the genome integrity of the neurons. As most of the mature neurons stay in G0/G1 phase, replication-uncoupled DNA repair pathways including BER, NER, SSBR, and NHEJ, are pivotal, efficient, and economic mechanisms to maintain genomic stability without reactivating cell cycle. In these progresses, polymerases are prominent, not only because they are responsible for both sensing and repairing damages, but also for their more diversified roles depending on the cell cycle phase and damage types. In this review, we summarized recent knowledge on the structural and biochemical properties of distinct polymerases, including DNA and RNA polymerases, which are known to be expressed and active in nervous system; the biological relevance of these polymerases and their interactors with neuronal degeneration would be most graphically illustrated by the neurological abnormalities observed in patients with hereditary diseases associated with defects in DNA repair; furthermore, the vicious cycle of the trinucleotide repeat (TNR) and impaired DNA repair pathway is also discussed. Unraveling the mechanisms and contextual basis of the role of the polymerases in DNA damage response and repair will promote our understanding about how long-lived postmitotic cells cope with DNA lesions, and why disrupted DNA repair contributes to disease origin, despite the diversity of mutations in genes. This knowledge may lead to new insight into the development of targeted intervention for neurodegenerative diseases.

The Role of DNA Damage in Neural Plasticity in Physiology and Neurodegeneration

Damage to DNA is generally considered to be a harmful process associated with aging and aging-related disorders such as neurodegenerative diseases that involve the selective death of specific groups of neurons. However, recent studies have provided evidence that DNA damage and its subsequent repair are important processes in the physiology and normal function of neurons. Neurons are unique cells that form new neural connections throughout life by growth and re-organisation in response to various stimuli. This “plasticity” is essential for cognitive processes such as learning and memory as well as brain development, sensorial training, and recovery from brain lesions. Interestingly, recent evidence has suggested that the formation of double strand breaks (DSBs) in DNA, the most toxic form of damage, is a physiological process that modifies gene expression during normal brain activity. Together with subsequent DNA repair, this is thought to underlie neural plasticity and thus control neuronal function. Interestingly, neurodegenerative diseases such as Alzheimer’s disease, amyotrophic lateral sclerosis, frontotemporal dementia, and Huntington’s disease, manifest by a decline in cognitive functions, which are governed by plasticity. This suggests that DNA damage and DNA repair processes that normally function in neural plasticity may contribute to neurodegeneration. In this review, we summarize current understanding about the relationship between DNA damage and neural plasticity in physiological conditions, as well as in the pathophysiology of neurodegenerative diseases.

PQBP1: The Key to Intellectual Disability, Neurodegenerative Diseases, and Innate Immunity

The idea that a common pathology underlies various neurodegenerative diseases and dementias has attracted considerable attention in the basic and medical sciences. Polyglutamine binding protein-1 (PQBP1) was identified in 1998 after a molecule was predicted to bind to polyglutamine tract amino acid sequences, which are associated with a family of neurodegenerative disorders called polyglutamine diseases. Hereditary gene mutations of PQBP1 cause intellectual disability, whereas acquired loss of function of PQBP1 contributes to dementia pathology. PQBP1 functions in innate immune cells as an intracellular receptor that recognizes pathogens and neurodegenerative proteins. It is an intrinsically disordered protein that generates intracellular foci, similar to other neurodegenerative disease proteins such as TDP43, FUS, and hnRNPs. The knowledge accumulated over more than 20 years has given rise to a new concept that shifts in the equilibrium between physiological and pathological processes have their basis in the dysregulation of common protein structure-linked molecular mechanisms.

Analysis of ionizing radiation induced DNA damage response in human adult stem cells and differentiated neurons

Findings of neurodegenerative features associated with human radiosensitive syndromes such as Ataxia telangiectasia suggest that DNA repair efficiency is crucial for maintaining the functional integrity of central nervous system. To gain a better understanding of ionizing radiation (IR) induced DNA damage response in undifferentiated and differentiated neural cell types and to evaluate the role of ATM in DNA double strand break (DSB) repair, an in vitro human neural cell differentiation model system was utilized in this study. As compared to adult stem cells, differentiated neurons displayed an attenuated DSB repair response (as judged by the persistence of 53BP1 foci) after IR exposure and the attenuation was even more pronounced in stem cells and neurons after suppression of ATM (Ataxia Telangiectasia Mutated) gene product suggesting the importance of ATM for an optimal DSB repair efficiency in human neural cell types. In corroboration with an attenuated DNA damage response, a sharp decline in the expression levels of several DSB repair genes was observed in neurons. Our results suggest that cellular differentiation modulates the expression of several genes thereby compromising the DSB repair fidelity in post mitotic neurons. Further studies are required to verify whether or not ATM mediated exacerbation of DNA repair deficiency in differentiated neurons leads to neurodegeneration.

Autoantibodies against NCAM1 from patients with schizophrenia cause schizophrenia-related behavior and changes in synapses in mice

From genetic and etiological studies, autoimmune mechanisms underlying schizophrenia are suspected; however, the details remain unclear. In this study, we describe autoantibodies against neural cell adhesion molecule (NCAM1) in patients with schizophrenia (5.4%, cell-based assay; 6.7%, ELISA) in a Japanese cohort (n = 223). Anti-NCAM1 autoantibody disrupts both NCAM1-NCAM1 and NCAM1-glial cell line-derived neurotrophic factor (GDNF) interactions. Furthermore, the anti-NCAM1 antibody purified from patients with schizophrenia interrupts NCAM1-Fyn interaction and inhibits phosphorylation of FAK, MEK1, and ERK1 when introduced into the cerebrospinal fluid of mice and also reduces the number of spines and synapses in frontal cortex. In addition, it induces schizophrenia-related behavior in mice, including deficient pre-pulse inhibition and cognitive impairment. In conclusion, anti-NCAM1 autoantibodies in patients with schizophrenia cause schizophrenia-related behavior and changes in synapses in mice. These antibodies may be a potential therapeutic target and serve as a biomarker to distinguish a small but treatable subgroup in heterogeneous patients with schizophrenia.

•

Some patients with schizophrenia are positive for anti-NCAM1 autoantibodies

•

Anti-NCAM1 antibody from schizophrenia patients inhibits NCAM1-NCAM1 interactions

•

Anti-NCAM1 antibody from schizophrenia patients reduces spines and synapses in mice

•

Anti-NCAM1 antibody from patients induces schizophrenia-related behavior in mice

Polyglutamine Expansion in Huntingtin and Mechanism of DNA Damage Repair Defects in Huntington’s Disease

Emerging evidence suggests that DNA repair deficiency and genome instability may be the impending signs of many neurological diseases. Genome-wide association (GWAS) studies have established a strong correlation between genes that play a role in DNA damage repair and many neurodegenerative diseases, including Huntington’s disease (HD), and several other trinucleotides repeat expansion-related hereditary ataxias. Recently, many reports have documented a significant role played by the DNA repair processes in aging and in modifying many neurodegenerative diseases, early during their progression. Studies from our lab and others have now begun to understand the mechanisms that cause defective DNA repair in HD and surprisingly, many proteins that have a strong link to known neurodegenerative diseases seem to be important players in these cellular pathways. Mutations in huntingtin (HTT) gene that lead to polyglutamine repeat expansion at the N-terminal of HTT protein has been shown to disrupt transcription-coupled DNA repair process, a specialized DNA repair process associated with transcription. Due to the recent progress made in understanding the mechanisms of DNA repair in relation to HD, in this review, we will mainly focus on the mechanisms by which the wild-type huntingtin (HTT) protein helps in DNA repair during transcription, and the how polyglutamine expansions in HTT impedes this process in HD. Further studies that identify new players in DNA repair will help in our understanding of this process in neurons. Furthermore, it should help us understand how various DNA repair mechanism(s) coordinate to maintain the normal physiology of neurons, and provide insights for the development of novel drugs at prodromal stages of these neurodegenerative diseases.

Defective repair of topoisomerase I induced chromosomal damage in Huntington's disease

Topoisomerase1 (TOP1)-mediated chromosomal breaks are endogenous sources of DNA damage that affect neuronal genome stability. Whether TOP1 DNA breaks are sources of genomic instability in Huntington's disease (HD) is unknown. Here, we report defective 53BP1 recruitment in multiple HD cell models, including striatal neurons derived from HD patients. Defective 53BP1 recruitment is due to reduced H2A ubiquitination caused by the limited RNF168 activity. The reduced availability of RNF168 is caused by an increased interaction with p62, a protein involved in selective autophagy. Depletion of p62 or disruption of the interaction between RNAF168 and p62 was sufficient to restore 53BP1 enrichment and subsequent DNA repair in HD models, providing new opportunities for therapeutic interventions. These findings are reminiscent to what was described for p62 accumulation caused by C9orf72 expansion in ALS/FTD and suggest a common mechanism by which protein aggregation perturb DNA repair signaling.

Activated or Impaired: An Overview of DNA Repair in Neurodegenerative Diseases

As the population ages, age-related neurodegenerative diseases have become a major challenge in health science. Currently, the pathology of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease, is still not fully understood. Remarkably, emerging evidence indicates a role of genomic DNA damage and repair in various neurodegenerative disorders. Here, we summarized the current understanding of the function of DNA damage repair, especially base excision repair and double strand break repair pathways, in a variety of neurodegenerative diseases. We concluded that exacerbation of DNA lesions is found in almost all types of neurodegenerative diseases, whereas the activities of different DNA repair pathways demonstrate distinct trends, depending on disease type and even brain region. Specifically, key enzymes involved in base excision repair are likely impaired in Alzheimer's disease and amyotrophic lateral sclerosis but activated in Parkinson's disease, while nonhomologous end joining is likely downregulated in most types of neurodegenerative diseases. Hence, impairment of nonhomologous end joining is likely a common etiology for most neurodegenerative diseases, while defects in base excision repair are likely involved in the pathology of Alzheimer's disease and amyotrophic lateral sclerosis but are Parkinson's disease, based on current findings. Although there are still discrepancies and further studies are required to completely elucidate the exact roles of DNA repair in neurodegeneration, the current studies summarized here provide crucial insights into the pathology of neurodegenerative diseases and may reveal novel drug targets for corresponding neurodegenerative diseases.

HMGB1 signaling phosphorylates Ku70 and impairs DNA damage repair in Alzheimer's disease pathology

DNA damage is increased in Alzheimer's disease (AD), while the underlying mechanisms are unknown. Here, we employ comprehensive phosphoproteome analysis, and identify abnormal phosphorylation of 70 kDa subunit of Ku antigen (Ku70) at Ser77/78, which prevents Ku70-DNA interaction, in human AD postmortem brains. The abnormal phosphorylation inhibits accumulation of Ku70 to the foci of DNA double strand break (DSB), impairs DNA damage repair and eventually causes transcriptional repression-induced atypical cell death (TRIAD). Cells under TRIAD necrosis reveal senescence phenotypes. Extracellular high mobility group box 1 (HMGB1) protein, which is released from necrotic or hyper-activated neurons in AD, binds to toll-like receptor 4 (TLR4) of neighboring neurons, and activates protein kinase C alpha (PKCα) that executes Ku70 phosphorylation at Ser77/78. Administration of human monoclonal anti-HMGB1 antibody to post-symptomatic AD model mice decreases neuronal DSBs, suppresses secondary TRIAD necrosis of neurons, prevents escalation of neurodegeneration, and ameliorates cognitive symptoms. TRIAD shares multiple features with senescence. These results discover the HMGB1-Ku70 axis that accounts for the increase of neuronal DNA damage and secondary enhancement of TRIAD, the cell death phenotype of senescence, in AD.

Mitochondrial Abnormalities and Synaptic Damage in Huntington's Disease: a Focus on Defective Mitophagy and Mitochondria-Targeted Therapeutics

Huntington's disease (HD) is a fatal and pure genetic disease with a progressive loss of medium spiny neurons (MSN). HD is caused by expanded polyglutamine repeats in the exon 1 of HD gene. Clinically, HD is characterized by chorea, seizures, involuntary movements, dystonia, cognitive decline, intellectual impairment, and emotional disturbances. Several years of intense research revealed that multiple cellular changes, including defective axonal transport, protein-protein interactions, defective bioenergetics, calcium dyshomeostasis, NMDAR activation, synaptic damage, mitochondrial abnormalities, and selective loss of medium spiny neurons are implicated in HD. Recent research on mutant huntingtin (mHtt) and mitochondria has found that mHtt interacts with the mitochondrial division protein, dynamin-related protein 1 (DRP1), enhances GTPase DRP1 enzymatic activity, and causes excessive mitochondrial fragmentation and abnormal distribution, leading to defective axonal transport of mitochondria and selective synaptic degeneration. Recent research also revealed that failure to remove dead and/or dying mitochondria is an early event in the disease progression. Currently, efforts are being made to reduce abnormal protein interactions and enhance synaptic mitophagy as therapeutic strategies for HD. The purpose of this article is to discuss recent research in HD progression. This article also discusses recent developments of cell and mouse models, cellular changes, mitochondrial abnormalities, DNA damage, bioenergetics, oxidative stress, mitophagy, and therapeutics strategies in HD.

The Childhood-Onset Neurodegeneration with Cerebellar Atrophy (CONDCA) Disease Caused by AGTPBP1 Gene Mutations: The Purkinje Cell Degeneration Mouse as an Animal Model for the Study of this Human Disease

Recent reports have identified rare, biallelic damaging variants of the AGTPBP1 gene that cause a novel and documented human disease known as childhood-onset neurodegeneration with cerebellar atrophy (CONDCA), linking loss of function of the AGTPBP1 protein to human neurodegenerative diseases. CONDCA patients exhibit progressive cognitive decline, ataxia, hypotonia or muscle weakness among other clinical features that may be fatal. Loss of AGTPBP1 in humans recapitulates the neurodegenerative course reported in a well-characterised murine animal model harbouring loss-of-function mutations in the AGTPBP1 gene. In particular, in the Purkinje cell degeneration (pcd) mouse model, mutations in AGTPBP1 lead to early cerebellar ataxia, which correlates with the massive loss of cerebellar Purkinje cells. In addition, neurodegeneration in the olfactory bulb, retina, thalamus and spinal cord were also reported. In addition to neurodegeneration, pcd mice show behavioural deficits such as cognitive decline. Here, we provide an overview of what is currently known about the structure and functional role of AGTPBP1 and discuss the various alterations in AGTPBP1 that cause neurodegeneration in the pcd mutant mouse and humans with CONDCA. The sequence of neuropathological events that occur in pcd mice and the mechanisms governing these neurodegenerative processes are also reported. Finally, we describe the therapeutic strategies that were applied in pcd mice and focus on the potential usefulness of pcd mice as a promising model for the development of new therapeutic strategies for clinical trials in humans, which may offer potential beneficial options for patients with AGTPBP1 mutation-related CONDCA.

Hepta-Histidine Inhibits Tau Aggregation

Tau aggregation is a central hallmark of tauopathies such as frontotemporal lobar degeneration and progressive supranuclear palsy as well as of Alzheimer's disease, and it has been a target for therapeutic development. Herein, we unexpectedly found that hepta-histidine (7H), an inhibitor of the interaction between Ku70 and Huntingtin proteins, suppresses aggregation of Tau-R3 peptides in vitro. Addition of the trans-activator of transcription (TAT) sequence (YGRKKRRQRRR) derived from the TAT protein to 7H increased its permeability into cells, and TAT-7H treatment of iPS cell-derived neurons carrying Tau or APP mutations suppressed Tau phosphorylation. These results indicate that 7H is a promising lead compound for developing anti-aggregation drugs against Tau-related neurodegenerative diseases including Alzheimer's disease (AD).

DNAzyme Cleavage of CAG Repeat RNA in Polyglutamine Diseases

CAG repeat expansion is the genetic cause of nine incurable polyglutamine (polyQ) diseases with neurodegenerative features. Silencing repeat RNA holds great therapeutic value. Here, we developed a repeat-based RNA-cleaving DNAzyme that catalyzes the destruction of expanded CAG repeat RNA of six polyQ diseases with high potency. DNAzyme preferentially cleaved the expanded allele in spinocerebellar ataxia type 1 (SCA1) cells. While cleavage was non-allele-specific for spinocerebellar ataxia type 3 (SCA3) cells, treatment of DNAzyme leads to improved cell viability without affecting mitochondrial metabolism or p62-dependent aggresome formation. DNAzyme appears to be stable in mouse brain for at least 1 month, and an intermediate dosage of DNAzyme in a SCA3 mouse model leads to a significant reduction of high molecular weight ATXN3 proteins. Our data suggest that DNAzyme is an effective RNA silencing molecule for potential treatment of multiple polyQ diseases.

The interplay between mitochondrial functionality and genome integrity in the prevention of human neurologic diseases

As mitochondria are vulnerable to oxidative damage and represent the main source of reactive oxygen species (ROS), they are considered key tuners of ROS metabolism and buffering, whose dysfunction can progressively impact neuronal networks and disease. Defects in DNA repair and DNA damage response (DDR) may also affect neuronal health and lead to neuropathology. A number of congenital DNA repair and DDR defective syndromes, indeed, show neurological phenotypes, and a growing body of evidence indicate that defects in the mechanisms that control genome stability in neurons acts as aging-related modifiers of common neurodegenerative diseases such as Alzheimer, Parkinson's, Huntington diseases and Amyotrophic Lateral Sclerosis. In this review we elaborate on the established principles and recent concepts supporting the hypothesis that deficiencies in either DNA repair or DDR might contribute to neurodegeneration via mechanisms involving mitochondrial dysfunction/deranged metabolism.

DNA damage in embryonic neural stem cell determines FTLDs' fate via early-stage neuronal necrosis

The early-stage pathologies of frontotemporal lobal degeneration (FTLD) remain largely unknown. In VCP^T262A-KI mice carrying VCP gene mutation linked to FTLD, insufficient DNA damage repair in neural stem/progenitor cells (NSCs) activated DNA-PK and CDK1 that disabled MCM3 essential for the G1/S cell cycle transition. Abnormal neural exit produced neurons carrying over unrepaired DNA damage and induced early-stage transcriptional repression-induced atypical cell death (TRIAD) necrosis accompanied by the specific markers pSer46-MARCKS and YAP. In utero gene therapy expressing normal VCP or non-phosphorylated mutant MCM3 rescued DNA damage, neuronal necrosis, cognitive function, and TDP43 aggregation in adult neurons of VCP^T262A-KI mice, whereas similar therapy in adulthood was less effective. The similar early-stage neuronal necrosis was detected in PGRN^R504X-KI, CHMP2B^Q165X-KI, and TDP^N267S-KI mice, and blocked by embryonic treatment with AAV-non-phospho-MCM3. Moreover, YAP-dependent necrosis occurred in neurons of human FTLD patients, and consistently pSer46-MARCKS was increased in cerebrospinal fluid (CSF) and serum of these patients. Collectively, developmental stress followed by early-stage neuronal necrosis is a potential target for therapeutics and one of the earliest general biomarkers for FTLD.

DNA damage and regulation of protein homeostasis

The accumulation of unrepaired DNA lesions is associated with many pathological outcomes in humans, particularly in neurodegenerative diseases and in normal aging. Evidence supporting a causal role for DNA damage in the onset and progression of neurodegenerative disease has come from rare human patients with mutations in DNA damage response genes as well as from model organisms; however, the generality of this relationship in the normal population is unclear. In addition, the relevance of DNA damage in the context of proteotoxic stress-the widely accepted paradigm for pathology during neurodegeneration-is not well understood. Here, observations supporting intertwined roles of DNA damage and proteotoxicity in aging-related neurological outcomes are reviewed, with particular emphasis on recent insights into the relationships between DNA repair and autophagy, the ubiquitin proteasome system, formation of protein aggregates, poly-ADP-ribose polymerization, and transcription-driven DNA lesions.

Breast cancer type 1 and neurodegeneration: consequences of deficient DNA repair

Numerous cellular processes, including toxic protein aggregation and oxidative stress, have been studied extensively as potential mechanisms underlying neurodegeneration. However, limited therapeutic efficacy targeting these processes has prompted other mechanisms to be explored. Previous research has emphasized a link between cellular senescence and neurodegeneration, where senescence induced by excess DNA damage and deficient DNA repair results in structural and functional changes that ultimately contribute to brain dysfunction and increased vulnerability for neurodegeneration. Specific DNA repair proteins, such as breast cancer type 1, have been associated with both stress-induced senescence and neurodegenerative diseases, however, specific mechanisms remain unclear. Therefore, this review explores DNA damage-induced senescence in the brain as a driver of neurodegeneration, with particular focus on breast cancer type 1, and its potential contribution to sex-specific differences associated with neurodegenerative disease.

CAG RNAs induce DNA damage and apoptosis by silencing NUDT16 expression in polyglutamine degeneration

DNA damage plays a central role in the cellular pathogenesis of polyglutamine (polyQ) diseases, including Huntington's disease (HD). In this study, we showed that the expression of untranslatable expanded CAG RNA per se induced the cellular DNA damage response pathway. By means of RNA sequencing (RNA-seq), we found that expression of the Nudix hydrolase 16 (NUDT16) gene was down-regulated in mutant CAG RNA-expressing cells. The loss of NUDT16 function results in a misincorporation of damaging nucleotides into DNAs and leads to DNA damage. We showed that small CAG (sCAG) RNAs, species generated from expanded CAG transcripts, hybridize with CUG-containing NUDT16 mRNA and form a CAG-CUG RNA heteroduplex, resulting in gene silencing of NUDT16 and leading to the DNA damage and cellular apoptosis. These results were further validated using expanded CAG RNA-expressing mouse primary neurons and in vivo R6/2 HD transgenic mice. Moreover, we identified a bisamidinium compound, DB213, that interacts specifically with the major groove of the CAG RNA homoduplex and disfavors the CAG-CUG heteroduplex formation. This action subsequently mitigated RNA-induced silencing complex (RISC)-dependent NUDT16 silencing in both in vitro cell and in vivo mouse disease models. After DB213 treatment, DNA damage, apoptosis, and locomotor defects were rescued in HD mice. This work establishes NUDT16 deficiency by CAG repeat RNAs as a pathogenic mechanism of polyQ diseases and as a potential therapeutic direction for HD and other polyQ diseases.

Genome instability and loss of protein homeostasis: converging paths to neurodegeneration?

Genome instability and loss of protein homeostasis are hallmark events of age-related diseases that include neurodegeneration. Several neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis are characterized by protein aggregation, while an impaired DNA damage response (DDR) as in many genetic DNA repair disorders leads to pronounced neuropathological features. It remains unclear to what degree these cellular events interconnect with each other in the development of neurological diseases. This review highlights how the loss of protein homeostasis and genome instability influence one other. We will discuss studies that illustrate this connection. DNA damage contributes to many neurodegenerative diseases, as shown by an increased level of DNA damage in patients, possibly due to the effects of protein aggregates on chromatin, the sequestration of DNA repair proteins and novel putative DNA repair functions. Conversely, genome stability is also important for protein homeostasis. For example, gene copy number variations and the loss of key DDR components can lead to marked proteotoxic stress. An improved understanding of how protein homeostasis and genome stability are mechanistically connected is needed and promises to lead to the development of novel therapeutic interventions.

Determining the Fate of Neurons in SCA3: ATX3, a Rising Decision Maker in Response to DNA Stresses and Beyond

DNA damage response (DDR) and apoptosis are reported to be involved in the pathogenesis of many neurodegenerative diseases including polyglutamine (polyQ) disorders, such as Spinocerebellar ataxia type 3 (SCA3) and Huntington's disease (HD). Consistently, an increasing body of studies provide compelling evidence for the crucial roles of ATX3, whose polyQ expansion is defined as the cause of SCA3, in the maintenance of genome integrity and regulation of apoptosis. The polyQ expansion in ATX3 seems to affect its physiological functions in these distinct pathways. These advances have expanded our understanding of the relationship between ATX3's cellular functions and the underlying molecular mechanism of SCA3. Interestingly, dysregulated DDR pathways also contribute to the pathogenesis of other neurodegenerative disorder such as HD, which presents a common molecular mechanism yet distinct in detail among different diseases. In this review, we provide a comprehensive overview of the current studies about the physiological roles of ATX3 in DDR and related apoptosis, highlighting the crosslinks between these impaired pathways and the pathogenesis of SCA3. Moreover, whether these mechanisms are shared in other neurodegenerative diseases are analyzed. Finally, the preclinical studies targeting DDR and related apoptosis for treatment of polyQ disorders including SCA3 and HD are also summarized and discussed.

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.

Activation of proprotein convertase in the mouse habenula causes depressive-like behaviors through remodeling of extracellular matrix

The lateral habenula (LHb) attracts a growing interest as a regulator of monoaminergic activity which were frequently reported to be defective in depression. Here we found that chronic social defeat stress (CSDS) increased production of pro-inflammatory cytokines in LHb associated with mobilization of monocytes and remodeling of extracellular matrix by increased matrix metalloproteinase (MMP) activity. RNA-seq analysis identified proprotein convertase Pcsk5 as an upstream regulator of MMP activation, with upregulation in LHb neurons of mice with susceptibility to CSDS. PCSK5 facilitated motility of microglia in vitro by converting inactive pro-MMP14 and pro-MMP2 to their active forms, highlighting its role in mobilization of microglia and monocytes in neuroinflammation. Suppression of Pcsk5 expression via small interfering RNA (siRNA) ameliorated depressive-like behaviors and pathological mobilization of monocytes in mice with susceptibility to CSDS. PCSK5-MMPs signaling pathway could be a target for development of the antidepressants targeting the inflammatory response in specific brain regions implicated in depression.

DNA Damage Triggers a New Phase in Neurodegeneration

Subcellular compartmentalization contributes to the organization of a plethora of molecular events occurring within cells. This can be achieved in membraneless organelles generated through liquid-liquid phase separation (LLPS), a demixing process that separates and concentrates cellular reactions. RNA is often a critical factor in mediating LLPS. Recent evidence indicates that DNA damage response foci are membraneless structures formed via LLPS and modulated by noncoding transcripts synthesized at DNA damage sites. Neurodegeneration is often associated with DNA damage, and dysfunctional LLPS events can lead to the formation of toxic aggregates. In this review, we discuss those gene products involved in neurodegeneration that undergo LLPS and their involvement in the DNA damage response.

Role of the C9ORF72 Gene in the Pathogenesis of Amyotrophic Lateral Sclerosis and Frontotemporal Dementia

Since the discovery of the C9ORF72 gene in 2011, great advances have been achieved in its genetics and in identifying its role in disease models and pathological mechanisms; it is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). ALS patients with C9ORF72 expansion show heterogeneous symptoms. Those who are C9ORF72 expansion carriers have shorter survival after disease onset than non-C9ORF72 expansion patients. Pathological and clinical features of C9ORF72 patients have been well mimicked via several models, including induced pluripotent stem cell-derived neurons and transgenic mice that were embedded with bacterial artificial chromosome construct and that overexpressing dipeptide repeat proteins. The mechanisms implicated in C9ORF72 pathology include DNA damage, changes of RNA metabolism, alteration of phase separation, and impairment of nucleocytoplasmic transport, which may underlie C9ORF72 expansion-related ALS/FTD and provide insight into non-C9ORF72 expansion-related ALS, FTD, and other neurodegenerative diseases.

FACS-array-based cell purification yields a specific transcriptome of striatal medium spiny neurons in a murine Huntington disease model

Huntington disease (HD) is a neurodegenerative disorder caused by expanded CAG repeats in the Huntingtin gene. Results from previous studies have suggested that transcriptional dysregulation is one of the key mechanisms underlying striatal medium spiny neuron (MSN) degeneration in HD. However, some of the critical genes involved in HD etiology or pathology could be masked in a common expression profiling assay because of contamination with non-MSN cells. To gain insight into the MSN-specific gene expression changes in presymptomatic R6/2 mice, a common HD mouse model, here we used a transgenic fluorescent protein marker of MSNs for purification via FACS before profiling gene expression with gene microarrays and compared the results of this "FACS-array" with those obtained with homogenized striatal samples (STR-array). We identified hundreds of differentially expressed genes (DEGs) and enhanced detection of MSN-specific DEGs by comparing the results of the FACS-array with those of the STR-array. The gene sets obtained included genes ubiquitously expressed in both MSNs and non-MSN cells of the brain and associated with transcriptional regulation and DNA damage responses. We proposed that the comparative gene expression approach using the FACS-array may be useful for uncovering the gene cascades affected in MSNs during HD pathogenesis.

Exploring the role of high-mobility group box 1 (HMGB1) protein in the pathogenesis of Huntington's disease

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder caused by an increased and unstable CAG DNA expansion in the Huntingtin (HTT) gene, resulting in an elongated polyglutamine tract in huntingtin protein. Despite its monogenic cause, HD pathogenesis remains elusive and without any approved disease-modifying therapy as yet. A growing body of evidence highlights the emerging role of high-mobility group box 1 (HMGB1) protein in HD pathology. HMGB1, being a nuclear protein, is primarily implicated in DNA repair, but it can also translocate to the cytoplasm and participate into numerous cellular functions. Cytoplasmic HMGB1 was shown to directly interact with huntingtin under oxidative stress conditions and induce its nuclear translocation, a key process in the HD pathogenic cascade. Nuclear HMGB1 acting as a co-factor of ataxia telangiectasia mutated and base excision repair (BER) complexes can exert dual roles in CAG repeat instability and affect the final DNA repair outcome. HMGB1 can inhibit mutant huntingtin aggregation, protecting against polyglutamine-induced neurotoxicity and acting as a chaperon-like molecule, possibly via autophagy regulation. In addition, HMGB1 being a RAGE and TLR-2, TLR-3, and TLR-4 ligand may further contribute to HD pathogenesis by triggering neuroinflammation and apoptosis. Furthermore, HMGB1 participates at the unfolded protein response (UPR) system and can induce protein degradation and apoptosis associated with HD. In this review, we discuss the multiple role of HMGB1 in HD pathology, providing mechanistic insights that could direct future studies towards the development of targeted therapeutic approaches.

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).

DNA repair and neurological disease: From molecular understanding to the development of diagnostics and model organisms

In both replicating and non-replicating cells, the maintenance of genomic stability is of utmost importance. Dividing cells can repair DNA damage during cell division, tolerate the damage by employing potentially mutagenic DNA polymerases or die via apoptosis. However, the options for accurate DNA repair are more limited in non-replicating neuronal cells. If DNA damage is left unresolved, neuronal cells die causing neurodegenerative disorders. A number of pathogenic variants of DNA repair proteins have been linked to multiple neurological diseases. The current challenge is to harness our knowledge of fundamental properties of DNA repair to improve diagnosis, prognosis and treatment of such debilitating disorders. In this perspective, we will focus on recent efforts in identifying novel DNA repair biomarkers for the diagnosis of neurological disorders and their use in monitoring the patient response to therapy. These efforts are greatly facilitated by the development of model organisms such as zebrafish, which will also be summarised.

Age-Related Oxidative Changes in Primary Porcine Fibroblasts Expressing Mutated Huntingtin

BACKGROUND

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by CAG triplet expansions in the huntingtin gene. Oxidative stress is linked to HD pathology, although it is not clear whether this is an effect or a mediator of disease. The transgenic (TgHD) minipig expresses the N-terminal part of human-mutated huntingtin and represents a unique model to investigate therapeutic strategies towards HD. A more detailed characterization of this model is needed to fully utilize its potential.

METHODS

In this study, we focused on the molecular and cellular features of fibroblasts isolated from TgHD minipigs and the wild-type (WT) siblings at different ages, pre-symptomatic at the age of 24-36 months and with the onset of behavioural symptoms at the age of 48 months. We measured oxidative stress, the expression of oxidative stress-related genes, proliferation capacity along with the expression of cyclin B1 and D1 proteins, cellular permeability, and the integrity of the nuclear DNA (nDNA) and mitochondrial DNA in these cells.

RESULTS

TgHD fibroblasts isolated from 48-month-old animals showed increased oxidative stress, which correlated with the overexpression of SOD2 encoding mitochondrial superoxide dismutase 2, and the NEIL3 gene encoding DNA glycosylase involved in replication-associated repair of oxidized DNA. TgHD cells displayed an abnormal proliferation capacity and permeability. We further demonstrated increased nDNA damage in pre-symptomatic TgHD fibroblasts (isolated from animals aged 24-36 months).

CONCLUSIONS

Our results unravel phenotypic alterations in primary fibroblasts isolated from the TgHD minipig model at the age of 48 months. Importantly, nDNA damage appears to precede these phenotypic alterations. Our results highlight the impact of fibroblasts from TgHD minipigs in studying the molecular mechanisms of HD pathophysiology that gradually occur with age.

Repair kinetics of DNA double-strand breaks and incidence of apoptosis in mouse neural stem/progenitor cells and their differentiated neurons exposed to ionizing radiation

Neuronal loss leads to neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease and Huntington's disease. Because of their long lifespans, neurons are assumed to possess highly efficient DNA repair ability and to be able to protect themselves from deleterious DNA damage such as DNA double-strand breaks (DSBs) produced by intrinsic and extrinsic sources. However, it remains largely unknown whether the DSB repair ability of neurons is more efficient compared with that of other cells. Here, we investigated the repair kinetics of X-ray-induced DSBs in mouse neural cells by scoring the number of phosphorylated 53BP1 foci post irradiation. We found that p53-independent apoptosis was induced time dependently during differentiation from neural stem/progenitor cells (NSPCs) into neurons in culture for 48 h. DSB repair in neurons differentiated from NSPCs in culture was faster than that in mouse embryonic fibroblasts (MEFs), possibly due to the higher DNA-dependent protein kinase activity, but it was similar to that in NSPCs. Further, the incidence of p53-dependent apoptosis induced by X-irradiation in neurons was significantly higher than that in NSPCs. This difference in response of X-ray-induced apoptosis between neurons and NSPCs may reflect a difference in the fidelity of non-homologous end joining or a differential sensitivity to DNA damage other than DSBs.