Nuclear Export of NBN Is Required for Normal Cellular Responses to Radiation▿

Nijmegen breakage syndrome arises from hypomorphic mutations in the NBN gene encoding nibrin, a component of the MRE11/RAD50/nibrin (MRN) complex. In mammalian cells, the MRN complex localizes to the nucleus, where it plays multiple roles in the cellular response to DNA double-strand breaks. In the current study, sequences in mouse nibrin required to direct the nuclear localization of the MRN complex were identified by site-specific mutagenesis. Unexpectedly, nibrin was found to contain both nuclear localizing signal (NLS) sequences and a nuclear export signal (NES) sequence whose functions were confirmed by mutagenesis. Both nuclear import and export sequences were active in vivo. Disruption of either the NLS or NES sequences of nibrin significantly altered the cellular distribution of nibrin and Mre11 and impaired survival after exposure to ionizing radiation. Mutation of the NES sequence in nibrin slowed the turnover of phosphorylated nibrin after irradiation, indicating that nuclear export of nibrin may function, in part, to downregulate posttranslationally modified MRN complex components after DNA damage responses are complete

The Postcolonial Citizen: The Intellectual Migrant

The twentieth century has witnessed the rise of a large population of postcolonial intellectual migrants willingly arriving from formerly colonized countries into the United Kingdom, the United States, and Canada to pursue intellectual goals. Embedded in this movement from the formerly colonized spaces into the West is the vexed question of dislocation and displacement for these intellectual subjects. The Postcolonial Citizen traces how such modes of (un)belonging are represented within literary and cultural space and how migrancy, and in particular the postcolonial intellectual migrant, is symbolically and philosophically understood as a cultural icon of displacement in the West. Using literary texts, autobiographical narrative of displacement, and cultural criticism, this book treats the cultural reception of intellectual migrancy (particularly within America) as both an uneasy and ambiguous condition. What is timely about this book\u27s treatment of migrancy is the current threat imposed on postcolonial writers and scholars in the United States post-9/11. The book examines and exposes the consequences of intellectually intervening into democratic ideals after the rise of the national security state - giving the migrant sensibility of dislocation a socio-political dimension. Thus, in dealing with the cultural reception of migrancy, The Postcolonial Citizenclearly marks the shift between pre- and post-9/11 migrant subjectivity and particularly addresses how the third world intellectual migrant has become synonymous with the voice of dissent and threat to the established democratic order in the United States.https://digitalcommons.linfield.edu/linfauth/1003/thumbnail.jp

Nibrin, a novel DNA double-strand break repair protein, is mutated in Nijmegen breakage syndrome

Nijmegen breakage syndrome (NBS) is an autosomal recessive chromosomal instability syndrome characterized by microcephaly, growth retardation, immunodeficiency, and cancer predisposition. Cells from NBS patients are hypersensitive to ionizing radiation with cytogenetic features indistinguishable from ataxia telangiectasia. We describe the positional cloning of a gene encoding a novel protein, nibrin. It contains two modules found in cell cycle checkpoint proteins, a forkhead-associated domain adjacent to a breast cancer carboxy-terminal domain. A truncating 5 bp deletion was identified in the majority of NBS patients, carrying a conserved marker haplotype. Five further truncating mutations were identified in patients with other distinct haplotypes. The domains found in nibrin and the NBS phenotype suggest that this disorder is caused by defective responses to DNA double-strand breaks

Distinct Functional Domains of Nibrin Mediate Mre11 Binding, Focus Formation, and Nuclear Localization

The inherited chromosomal instability disorder Nijmegen breakage syndrome (NBS) results from truncating mutations in the NBS1 gene, which encodes the protein nibrin. Nibrin is part of a nuclear multiprotein complex that also contains the DNA repair proteins Mre11 and Rad50. Upon irradiation, this complex redistributes within the nucleus, forming distinct foci that have been implicated as sites of DNA repair. In NBS cells, nibrin is absent and Mre11 and Rad50 are cytoplasmic. In this study, the interacting domains on nibrin and Mre11 were mapped using the yeast two-hybrid system and expression of epitope-tagged constructs in NBS fibroblasts. Deletion of the carboxy-terminal 101 amino acids of nibrin eliminated its ability to interact with Mre11 and to complement the radiation sensitivity of NBS cells. However, this truncated form of nibrin could localize to the nucleus and form radiation-inducible foci. Expression of a carboxy-terminal 354-amino-acid fragment of nibrin was sufficient to direct the nuclear localization of nibrin, as well as that of Mre11 and Rad50. Despite providing some partial complementation of the radiation-sensitive phenotype, the nibrin-Mre11-Rad50 complexes in these cells were unable to form foci. These results indicate that nibrin directs not only the nuclear localization of the nibrin-Mre11-Rad50 complexes but also radiation-induced focus formation. However, direct interaction between nibrin and Mre11 is required for normal cellular survival postirradiation. Distinct domains of nibrin are required for each of these functions, focus formation, nuclear localization, and Mre11 interaction

Discovery, Optimization, and in Vivo Evaluation of Benzimidazole Derivatives AM-8508 and AM-9635 as Potent and Selective PI3Kδ Inhibitors

Lead optimization efforts resulted in the discovery of two potent, selective, and orally bioavailable PI3Kδ inhibitors, <b>1</b> (AM-8508) and <b>2</b> (AM-9635), with good pharmacokinetic properties. The compounds inhibit B cell receptor (BCR)-mediated AKT phosphorylation (pAKT) in PI3Kδ-dependent in vitro cell based assays. These compounds which share a benzimidazole bicycle are effective when administered in vivo at unbound concentrations consistent with their in vitro cell potency as a consequence of improved unbound drug concentration with lower unbound clearance. Furthermore, the compounds demonstrated efficacy in a Keyhole Limpet Hemocyanin (KLH) study in rats, where the blockade of PI3Kδ activity by inbibitors <b>1</b> and <b>2</b> led to effective inhibition of antigen-specific IgG and IgM formation after immunization with KLH