Induction of two DNA mismatch repair proteins, MSH2 and MSH6, in differentiated human neuroblastoma SH-SY5Y cells exposed to doxorubicin

CDK-independent role of D-type cyclins in regulating DNA mismatch repair

Although mismatch repair (MMR) is essential for correcting DNA replication errors, it can also recognize other lesions, such as oxidized bases. In G0 and G1, MMR is kept in check through unknown mechanisms as it is error-prone during these cell cycle phases. We show that in mammalian cells, D-type cyclins are recruited to sites of oxidative DNA damage in a PCNA- and p21-dependent manner. D-type cyclins inhibit the proteasomal degradation of p21, which competes with MMR proteins for binding to PCNA, thereby inhibiting MMR. The ability of D-type cyclins to limit MMR is CDK4- and CDK6-independent and is conserved in G0 and G1. At the G1/S transition, the timely, cullin-RING ubiquitin ligase (CRL)-dependent degradation of D-type cyclins and p21 enables MMR activity to efficiently repair DNA replication errors. Persistent expression of D-type cyclins during S-phase inhibits the binding of MMR proteins to PCNA, increases the mutational burden, and promotes microsatellite instability.

Quantitative proteomics and in-cell cross-linking reveal cellular reorganisation during early neuronal differentiation of SH-SY5Y cells

The neuroblastoma cell line SH-SY5Y is commonly employed to study neuronal function and disease. This includes cells grown under standard conditions or differentiated to neuron-like cells by administration of chemical reagents such as retinoic acid (RA) or phorbol-12-myristate-13-acetate (PMA). Even though SH-SY5Y cells are widely explored, a complete description of the resulting proteomes and cellular reorganisation during differentiation is still missing. Here, we relatively quantify the proteomes of cells grown under standard conditions and obtained from two differentiation protocols employing RA or a combination of RA and PMA. Relative quantification and KEGG pathway analysis of the proteins reveals the presence of early differentiating cells and provides a list of marker proteins for undifferentiated and differentiated cells. For characterisation of neuronal sub-types, we analyse expression of marker genes and find that RA-differentiated cells are acetylcholinergic and cholinergic, while RA/PMA-differentiated cells show high expression of acetylcholinergic and dopaminergic marker genes. In-cell cross-linking further allows capturing protein interactions in different cellular organelles. Specifically, we observe structural reorganisation upon differentiation involving regulating protein factors of the actin cytoskeleton.

Quantitative proteomic analyses are employed to explore the changes in the proteome that occur upon neuronal differentiation in the SH-SY5Y cell line.

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.

Somatic deletions of genes regulating MSH2 protein stability cause DNA mismatch repair deficiency and drug resistance in human leukemia cells

DNA mismatch repair enzymes (e.g., MSH2) maintain genomic integrity, and their deficiency predisposes to several human cancers and to drug resistance. We found that leukemia cells from a substantial proportion of patients (~11%) with newly diagnosed acute lymphoblastic leukemia (ALL) have low or undetectable MSH2 protein levels (MSH2-L), despite abundant wild-type MSH2 mRNA. MSH2-L leukemia cells contained partial or complete somatic deletions of 1–4 genes that regulate MSH2 degradation (FRAP1, HERC1, PRKCZ, PIK3C2B); these deletions were also found in adult ALL (16%) and sporadic colorectal cancer (13.5%). Knockdown of these genes in human leukemia cells recapitulated the MSH2 protein deficiency by enhancing MSH2-degradation, leading to significant reduction in DNA mismatch repair (MMR) and increased resistance to thiopurines. These findings reveal a previously unrecognized mechanism whereby somatic deletions of genes regulating MSH2 degradation result in undetectable levels of MSH2 protein in leukemia cells, MMR deficiency and drug resistance.

DNA repair in neurons: so if they don't divide what's to repair?

Neuronal DNA repair remains one of the most exciting areas for investigation, particularly as a means to compare the DNA repair response in mitotic (cancer) vs. post-mitotic (neuronal) cells. In addition, the role of DNA repair in neuronal cell survival and response to aging and environmental insults is of particular interest. DNA damage caused by reactive oxygen species (ROS) such as generated by mitochondrial respiration includes altered bases, abasic sites, and single- and double-strand breaks which can be prevented by the DNA base excision repair (BER) pathway. Oxidative stress accumulates in the DNA of the human brain over time especially in the mitochondrial DNA (mtDNA) and is proposed to play a critical role in aging and in the pathogenesis of several neurological disorders including Parkinson's disease, ALS, and Alzheimer's diseases. Because DNA damage accumulates in the mtDNA more than nuclear DNA, there is increased interest in DNA repair pathways and the consequence of DNA damage in the mitochondria of neurons. The type of damage that is most likely to occur in neuronal cells is oxidative DNA damage which is primarily removed by the BER pathway. Following the notion that the bulk of neuronal DNA damage is acquired by oxidative DNA damage and ROS, the BER pathway is a likely area of focus for neuronal studies of DNA repair. BER variations in brain aging and pathology in various brain regions and tissues are presented. Therefore, the BER pathway is discussed in greater detail in this review than other repair pathways. Other repair pathways including direct reversal, nucleotide excision repair (NER), mismatch repair (MMR), homologous recombination and non-homologous end joining are also discussed. Finally, there is a growing interest in the role that DNA repair pathways play in the clinical arena as they relate to the neurotoxicity and neuropathy associated with cancer treatments. Among the numerous side effects of cancer treatments, major clinical effects include neurocognitive dysfunction and peripheral neuropathy. These symptoms occur frequently and have not been effectively studied at the cellular or molecular level. Studies of DNA repair may help our understanding of how those cells that are not dividing could succumb to neurotoxicity with the clinical manifestations discussed in the following article.

Mitochondrial dysfunction and increased sensitivity to excitotoxicity in mice deficient in DNA mismatch repair

The expression profile in the hippocampus of mice lacking one allele of the MutS homologue (Msh2), gene, which is one of the most representative components of the DNA mismatch repair system, was analysed to understand whether defects in the repair or in response to DNA damage could impact significantly on brain function. The overall results suggested a reduction in mitochondrial function as indicated by gene expression analysis, biochemical and behavioural studies. In the hippocampus of Msh2+/- mice, array data, validated by RT-PCR and western blot analysis, showed reduced expression levels of genes for cytochrome c oxidase subunit 2 (CoxII), ATP synthase subunit beta and superoxide dismutase 1. Biochemically, mitochondria from the hippocampus and cortex of these mice show reduced CoxII and increased aconitase activity. Behaviourally, these alterations resulted in mice with increased vulnerability to kainic acid-induced epileptic seizures and hippocampal neuronal loss. These data suggest that lack of an efficient system involved in recognizing and repairing DNA damage may generate a brain mitochondriopathy.

Preservation of DNA integrity and neuronal degeneration

The mismatch repair system (MMR) is an important member of the DNA checkpoint, that includes a number of protein deputed to control genomic stability through cell cycle arrest, DNA repair, and apoptosis. Here we summarize some recent data from our and other groups underlining the contribution to neurodegeneration of MSH2, perhaps the most relevant component of the MMR system. These data suggest that this protein participates not only in the cancer prevention machinery for the body but also in neurodegenerative processes.

Degadration of mismatch repair hMutSalpha heterodimer by the ubiquitin-proteasome pathway

Mismatch repair plays a critical role in genome stability. This process requires several proteins including hMSH2/hMSH6 (hMutSalpha) heterodimer involved in the first stage of the process, the mispair recognition. We previously reported that in U937 and HL-60 cell lines, hMSH2 and hMSH6 protein expression was much lower than that in HeLa and KG1a cells. Here, we showed that the decreased expression of hMutSalpha results from differences in the degradation rate of both proteins by the ubiquitin-proteasome pathway. Our data suggest that in human cell lines, ubiquitin-proteasome could play an important role in the regulation of hMutSalpha protein expression, thereby regulating mismatch repair activity.

Striatal neurones show sustained recovery from severe hypoglycaemic insult

Glucose deprivation provides a reliable model to investigate cellular responses to metabolic dysfunction, and is reportedly associated with permanent cell death in many paradigms. Consistent with previous studies, primary cultures of rat striatal neurones exposed to 24-h hypoglycaemia showed dramatically decreased sodium 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) metabolism (used as a marker of cell viability) and increased TUNEL staining, suggesting widespread DNA damage typical of apoptotic cell death. Remarkably, restoration of normal glucose levels initiated a sustained recovery in XTT staining, along with a concomitant decrease in TUNEL staining, even after 24 h of hypoglycaemia, suggesting recovery of damaged neurones and repair of nicked DNA. No alterations in the levels of four DNA repair proteins could be detected during hypoglycaemia or recovery. A reduction in intracellular calcium concentration was seen in recovered cells. These data suggest that striatal cells do not die after extended periods of glucose deprivation, but survive in a form of suspended animation, with sufficient energy to maintain membrane potential.

DNA repair in neural cells: basic science and clinical implications

As one part of a distinguished scientific career, Dr. Bryn Bridges focused his attention on the issue of DNA damage and repair in stationary phase bacteria. His work in this area led to his interest in DNA repair and mutagenesis in another non-dividing cell population, the neurons in the mammalian nervous system. He has specifically taken an interest in the magnocellular neurons of the central nervous system, and the possibility that somatic mutations may be occurring in these neurons. As part of this special issue dedicated to Bryn Bridges upon his retirement, I will discuss the various DNA repair pathways known to be active in the nervous system. The importance of DNA repair to the nervous system is most graphically illustrated by the neurological abnormalities observed in patients with hereditary diseases associated with defects in DNA repair. I will consider the mechanisms underlying the neurological abnormalities observed in patients with four of these diseases: xeroderma pigmentosum (XP), Cockayne's syndrome (CS), ataxia telangectasia (AT) and AT-like disorder (ATLD). I will also propose a mechanism for one of the observations indicating that somatic mutation can occur in the magnocellular neurons of the aging rat brain. Finally, as a parallel to Bridges inquiry into how much DNA synthesis is going on in stationary phase bacteria, I will address the question of how much DNA synthesis in going on in neurons, and the implications of the answer to this question for recent studies of neurogenesis in adult mammals.

Phosphorylation of mismatch repair proteins MSH2 and MSH6 affecting MutSalpha mismatch-binding activity

Mismatch repair (MMR) is involved in the removal of mispaired bases from DNA and thus plays an important role in the maintenance of genomic stability and the prevention of mutations and cancer. Moreover, MMR triggers genotoxicity and apoptosis upon processing of DNA lesions such as O6-methylguanine. Whereas the enzymology of MMR has been elucidated in great detail, only limited data are available concerning its regulation. Here we show that the major mismatch-binding proteins MSH2 and MSH6, forming the MutSalpha complex, are phosphorylated in vitro by protein kinase C and casein kinase II, but not by protein kinase A. Phosphorylation of MSH2 and MSH6 was also found within the cell, with MSH6 being more extensively phosphorylated than MSH2. Lack of MSH2 and MSH6 phosphorylation in vivo due to phosphate depletion, kinase inhibition (by H7 and quercetin) and treatment with phosphatases (CIP, SAP and lambda-PPase) significantly reduced mismatch-binding activity of MutSalpha. It also prevented methylation-induced nuclear translocation of the repair complex, indicating that nuclear translocation of MutSalpha upon mutagen treatment is dependent on protein phosphorylation. The finding that MSH2 and MSH6 are subject to phosphorylation resulting in increased mismatch binding by MutSalpha indicates a novel type of post-translational regulation of MMR which might be involved in the response of cells to genotoxic stress.

Identification and functional characterization of the promoter region of the human MSH6 gene

Postreplicative mismatch repair (MMR) corrects polymerase errors arising during DNA replication. Consistent with this role, the Saccharomyces cerevisiae MMR genes MSH2, MSH6, and PMS1 were reported to be transcriptionally upregulated during late G(1) phase of the cell cycle. Surprisingly, despite the high degree of conservation of the MMR system in evolution, the human MMR genes studied to date, MSH2, MLH1, and PMS2, appear to be transcribed from classical housekeeping promoters, and the amounts of the polypeptides encoded by them fluctuate little during the cell cycle. Only the amounts of the 160-kDa MSH6 protein were reported to vary, both during development and following stimulation of cell growth. Moreover, transcription of this gene was found to be downregulated by CpG methylation of the promoter region in a subset of clones treated with alkylating agents. In an attempt to understand the molecular basis underlying these phenomena, we isolated the 5' region of the MSH6 gene and subjected it to functional analysis. We now show that the MSH6 gene is also transcribed from a classical housekeeping gene promoter. Despite housing putative binding sites for the transcription factors AP1, NF-kappaB, and MTF-1, the MSH6 promoter failed to respond to ionizing radiation or heavy metals. Interestingly, MSH6 transcription was upregulated during late G(1) phase, even though the levels of the protein remained essentially constant during the cell cycle.

Chasing fate and function of new neurons in adult brains

Neuron production, migration and differentiation are major developmental events that continue, on a smaller scale, into adult life in a wide range of species from insects to mammals. Recent reports of adult neurogenesis in primates, including humans, have led to explosive scientific and public attention. During the last two years, significant discoveries have revealed that the generation, recruitment and survival of new neurons in adult brains are governed by principles similar to those that shape the developing brain, such as neuronal death, sensory experience, activity levels, and learning. Similarly, many factors implicated in embryonic neurogenesis are increasingly found to regulate adult neurogenesis and survival as well. These findings now allow the first manipulations of the numbers of adult-generated neurons to address their potential behavioral function.

Nuclear translocation of mismatch repair proteins MSH2 and MSH6 as a response of cells to alkylating agents

Mammalian mismatch repair has been implicated in mismatch correction, the prevention of mutagenesis and cancer, and the induction of genotoxicity and apoptosis. Here, we show that treatment of cells specifically with agents inducing O(6)-methylguanine in DNA, such as N-methyl-N'-nitro-N-nitrosoguanidine and N-methyl-N-nitrosourea, elevates the level of MSH2 and MSH6 and increases GT mismatch binding activity in the nucleus. This inducible response occurs immediately after alkylation, is long-lasting and dose-dependent, and results from translocation of the preformed MutSalpha complex (composed of MSH2 and MSH6) from the cytoplasm into the nucleus. It is not caused by an increase in MSH2 gene activity. Cells expressing the DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT), thus having the ability to repair O(6)-methylguanine, showed no translocation of MutSalpha, whereas inhibition of MGMT by O(6)-benzylguanine provoked the translocation. The results demonstrate that O(6)-methylguanine lesions are involved in triggering nuclear accumulation of MSH2 and MSH6. The finding that treatment of cells with O(6)-methylguanine-generating mutagens results in an increase of MutSalpha and GT binding activity in the nucleus indicates a novel type of genotoxic stress response.

Contribution of NF-kappaB and p53 in the glutamate-induced apoptosis

Exposure of primary cultures of cerebellar granule cells for 15 min to micromolar concentrations of glutamate results in cell death of both necrotic and apoptotic types. Among the intracellular events triggered by glutamate, we identified two transcriptional factors: the p50 member of the NF-kappaB family and the tumor suppressor phosphoprotein p53. Pretreatment of the cultures with aspirin, which inhibits NF-kappaB activation, or with specific p53 antisense oligonucleotide, which inhibits p53 transcription, resulted in a complete prevention of glutamate-induced p53 induction and apoptosis. These findings suggest the existence of a transcriptional program activated by glutamate receptor stimulation in which p50 and p53 play a relevant role. Then, we studied the expression of two p53 downstream genes that could participate in the glutamate-induced pro-apoptotic pathway: p21, which codes for an inhibitor of different cyclin dependent kinases, and MSH2, which codes for a protein involved in the recognition and repair of DNA mismatches. We found that primary cerebellar neurons expressed p21 and MSH2 at very low levels in basal conditions. However, very soon after a brief exposure of the cells to glutamate, the expression of both proteins was dramatically enhanced.On these bases, we propose NF-kappaB, p53, p21 and MSH2 as relevant contributors of the glutamate-induced pro-apoptotic pathway. Understanding this cascade of nuclear events may unravel specific targets for pharmacological intervention for those neurological diseases in which excitatory amino acid-induced apoptosis plays a relevant role.

Distribution and kainate-mediated induction of the DNA mismatch repair protein MSH2 in rat brain

DNA repair is one of the most essential systems for maintaining the inherited nucleotide sequence of genomic DNA over time. Repair of DNA damage would be particularly important in neurons, because these cells are among the longest-living cells in the body. MSH2 is one of the proteins which are involved in the recognition and repair of a specific type of DNA damage that is characterized by pair mismatches. We studied the distribution of MSH2 in rat brain by immunohistochemical analysis. We found the level of MSH2 expression in rat brain to be clearly heterogeneous. The highest intensity of staining was found in the pyramidal neurons of the hippocampus and in the entorhinal and frontoparietal cortices. Positive cells were observed in the substantia nigra pars compacta, in cerebellar granular and Purkinje cells, and in the motor neurons of the spinal cord. We investigated the possible modulation of MSH2 expression after injection of kainate. Systemic administration of kainate induces various behavioural alterations and a typical pattern of neuropathology, with cell death in the hippocampal pyramidal neurons of the CA3/CA4 fields. Kainate injection also resulted in a marked, dose-dependent increase of MSH2 immunoreactivity in the hippocampal neurons of the CA3/CA4 fields. The effect was specific, since no changes in immunoreactivity were detected in the dentate gyrus nor in other brain areas. In summary, our data suggest that a mismatch DNA repair system, of which MSH2 protein is a representative component, is heterogeneously expressed in the rat brain and specifically induced by an experimental paradigm of excitotoxicity.