Maintenance of genome stability depends on the appropriate response to DNA damage. This response is based on complex networks of signaling pathways that activate numerous processes and lead ...ultimately to damage repair and cellular survival — or apoptosis. The protein kinases ATM and ATR are master controllers of some of these networks, acting either in concert or separately to orchestrate the responses to specific types of DNA damage or stalled replication. Understanding their mode of action is essential to our understanding of how cells cope with genotoxic stress.
Gene mutations provide valuable clues to cellular metabolism. In humans such
insights come mainly from genetic disorders. Ataxia-telangiectasia (A-T) and
Nijmegen breakage syndrome (NBS) are two ...distinct, but closely related, single
gene disorders that highlight a complex junction of several signal transduction
pathways. These pathways appear to control defense mechanisms against specific
types of damage to cellular macromolecules, and probably regulate the
processing of certain types of DNA damage or normal intermediates of DNA
metabolism. A-T is characterized primarily by cerebellar degeneration,
immunodeficiency, genome instability, clinical radiosensitivity, and cancer
predisposition. NBS shares all these features except cerebellar deterioration.
The cellular phenotypes of A-T and NBS are almost indistinguishable, however,
and include chromosomal instability, radiosensitivity, and defects in cell
cycle checkpoints normally induced by ionizing radiation. The recent
identification of the gene responsible for A-T,
ATM
, has revealed its
product to be a large, constitutively expressed phosphoprotein with a
carboxy-terminal region similar to the catalytic domain of phosphatidylinositol
3-kinases (PI 3-kinases). ATM is a member of a family of proteins identified in
various organisms, which share the PI 3-kinase domain and are involved in
regulation of cell cycle progression and response to genotoxic agents. Some of
these proteins, most notably the DNA-dependent protein kinase, have an
associated protein kinase activity, and preliminary data indicate this activity
in ATM as well. Mutations in A-T patients are null alleles that truncate or
destabilize the ATM protein. Atm-deficient mice recapitulate the human
phenotype with slower nervous-system degeneration. Two ATM interactors, c-Abl
and p53, underscore its role in cellular responses to genotoxic stress. The
complexity of ATM's structure and mode of action make it a paradigm of
multifaceted signal transduction proteins involved in many physiological
pathways via multiple protein-protein interactions. The as yet unknown NBS
protein may be a component in an ATM-based complex, with a key role in sensing
and processing specific DNA damage or intermediates and signaling their
presence to the cell cycle machinery.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Cellular responses to DNA damage are crucial for maintaining homeostasis and preventing the development of cancer. Our understanding of the DNA-damage response has evolved: whereas previously the ...focus was on DNA repair, we now appreciate that the response to DNA lesions involves a complex, highly branched signaling network. Defects in this response lead to severely debilitating, cancer-predisposing ‘genomic instability syndromes’. Double strand breaks (DSBs) in DNA are potent triggers of the DNA-damage response, which is why they are used to study this pathway. The chief transducer of the DSB signal is the nuclear protein kinase ataxia-telangiectasia mutated (ATM). Genetic, biochemical and structural studies have recently provided insights into the ATM-mediated DSB response, reshaping our view of this signaling pathway while raising new questions.
The ATM protein, encoded by the gene responsible for the human genetic disorder ataxia telangiectasia (A-T), regulates several cellular responses to DNA breaks. ATM shares a phosphoinositide ...3-kinase-related domain with several proteins, some of them protein kinases. A wortmannin-sensitive protein kinase activity was associated with endogenous or recombinant ATM and was abolished by structural ATM mutations. In vitro substrates included the translation repressor PHAS-I and the p53 protein. ATM phosphorylated p53 in vitro on a single residue, serine-15, which is phosphorylated in vivo in response to DNA damage. This activity was markedly enhanced within minutes after treatment of cells with a radiomimetic drug; the total amount of ATM remained unchanged. Various damage-induced responses may be activated by enhancement of the protein kinase activity of ATM.
Maintenance of genome stability is essential for avoiding the passage to neoplasia. The DNA-damage response--a cornerstone of genome stability--occurs by a swift transduction of the DNA-damage signal ...to many cellular pathways. A prime example is the cellular response to DNA double-strand breaks, which activate the ATM protein kinase that, in turn, modulates numerous signalling pathways. ATM mutations lead to the cancer-predisposing genetic disorder ataxia-telangiectasia (A-T). Understanding ATM's mode of action provides new insights into the association between defective responses to DNA damage and cancer, and brings us closer to resolving the issue of cancer predisposition in some A-T carriers.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
DNA damage is one of the most acute threats to cellular homeostasis and life. The cell responds to such damage by activating a vast array of responses, ranging from DNA repair to numerous signalling ...pathways, which temporarily slow down the cellular life cycle while the damage is being repaired. Sophisticated relays convey the DNA damage alarm to all these systems immediately after damage infliction. Such relays must be capable of sensing the damage and rapidly creating functional contact with many signalling networks. The ataxia telangiectasia mutated (ATM) protein is a prominent example of such a relay. It responds swiftly to a critical DNA damage - the double strand break (DSB) - by phosphorylating key proteins in numerous signalling pathways. Evidence is emerging, however, that the ATM protein might also be involved in other processes related to cellular homeostasis, which are not directly associated with the damage response. ATM is the protein product of the gene mutated in the multisystem disorder ataxia-telangiectasia (AT), which is characterized by neuronal degeneration, immunodeficiency, chromosomal instability and cancer predisposition. The AT phenotype and the functions of the ATM protein revealed to date demonstrate the exceptionally multifaceted nature of this protein.
Most patients with anti-NMDA receptor (NMDAR) encephalitis have intrathecal synthesis of antibodies, which cause a decrease of cell surface and synaptic NMDAR. Antibodies are immunoglobulin G (IgG)1 ...and IgG3 subtypes and can potentially activate complement. We examined whether complement immunoreactivity and antibody-secreting cells (plasma cells/plasmablasts) are present in the brain of these patients.
Cultured rat hippocampal neurons were used in an immunocytochemical assay to test whether patients' antibodies can fix complement. Using the same reagents (antibodies to C9neo, C(5b-9), C3), complement immunoreactivity was determined in the brain of 5 patients, the teratoma of 21 patients, and appropriate control tissues. A set of markers for B (CD20), T (CD3, CD4, CD8) and antibody-secreting cells (plasma cells/plasmablasts, CD138) were used to examine the brain inflammatory infiltrates.
Patients' antibodies were able to bind complement in vitro, but deposits of complement were not detected in patients' brain. Parallel experiments with teratomas showed that in contrast to the brain, the neural tissue of the tumors contained complement. Analysis of the inflammatory infiltrates in brain samples from autopsy or biopsy performed 3-4 weeks after symptom presentation demonstrated numerous antibody-secreting cells (CD138+) in perivascular, interstitial, and Virchow-Robin spaces, and B and T cells predominantly located in perivascular regions.
Complement-mediated mechanisms do not appear to play a substantial pathogenic role in anti-NMDAR encephalitis. In contrast, there are copious infiltrates of antibody-secreting cells (plasma cells/plasmablasts) in the CNS of these patients. The demonstration of these cells provides an explanation for the intrathecal synthesis of antibodies and has implications for treatment.
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