The ATM (ataxia-telangiectasia mutated) protein kinase mediates early cellular responses to DNA double-strand breaks (DSBs) generated during metabolic processes or by DNA-damaging agents. ATM deficiency leads to ataxia-telangiectasia, a disease marked by lymphopenia, genomic instability and an increased predisposition to lymphoid malignancies with chromosomal translocations involving lymphocyte antigen receptor loci. ATM activates cell-cycle checkpoints and can induce apoptosis in response to DNA DSBs. However, defects in these pathways of the DNA damage response cannot fully account for the phenotypes of ATM deficiency. Here, we show that ATM also functions directly in the repair of chromosomal DNA DSBs by maintaining DNA ends in repair complexes generated during lymphocyte antigen receptor gene assembly. When coupled with the cell-cycle checkpoint and pro-apoptotic activities of ATM, these findings provide a molecular explanation for the increase in lymphoid tumours with translocations involving antigen receptor loci associated with ataxia-telangiectasia.

V(D)J recombination and class switch recombination employ overlapping but distinct non-homologous end-joining (NHEJ) pathways to repair DNA double strand break (DSB) intermediates. 53BP1 is a DNA damage response protein that is rapidly recruited to sites of chromosomal DSBs, where it appears to function in a subset of ataxia-telangiectasia mutated (ATM) kinase, H2AX- and MDC1- dependent events1,2. A 53BP1 dependent end joining pathway has been described that is dispensable for V(D)J recombination but essential for class-switch recombination CSR3, 4. Here, we report a previously unrecognized defect in the joining phase of V(D)J recombination in 53BP1 deficient lymphocytes distinct from that found in classical NHEJ-, H2AX-, MDC1- and Atm-deficient mice. Absence of 53BP1 leads to impairment of distal V-DJ joining with extensive degradation of un-repaired coding ends and episomal signal joint reintegration at V(D)J junctions. This results in apoptosis, loss of T-cell receptor alpha locus integrity and lymphopenia. Further impairment of the apoptotic checkpoint causes propagation of lymphocytes bearing antigen receptor breaks. These data suggest a more general role for 53BP1 in maintaining genomic stability during long range joining of DNA breaks.

Phosphatases are important regulators of intracellular signaling events, and their functions have been implicated in many biological processes. Dual-specificity phosphatases (DUSPs), whose family currently contains 25 members, are phosphatases that can dephosphorylate both tyrosine and serine/threonine residues of their substrates. The archetypical DUSP, DUSP1/MKP1, was initially discovered to regulate the activities of MAP kinases by dephosphorylating the TXY motif in the kinase domain. However, although DUSPs were discovered more than a decade ago, only in the past few years have their various functions begun to be described. DUSPs can be categorized based on the presence or absence of a MAP kinase-interacting domain into typical DUSPs and atypical DUSPs, respectively. In this review, we discuss the current understanding of how the activities of typical DUSPs are regulated and how typical DUSPs can regulate the functions of their targets. We also summarize recent findings from several in vivo DUSP-deficient mouse models that studied the involvement of DUSPs during the development and functioning of T cells. Finally, we discuss briefly the potential roles of DUSPs in the regulation of non-MAP kinase targets, as well as in the modulation of tumorigenesis.

Abstractc‐Myc induces widely varying cellular effects, including cell proliferation and cell death. These different cellular effects are determined, in part, by c‐Myc protein expression levels, which are regulated through several transcriptional and post‐transcriptional pathways. c‐Myc transcripts can be detected in cells at all stages of B and T lymphocyte development. However, little is known about c‐Myc protein expression, and how it varies, in developing lymphocytes. Here mice have been generated in which the endogenous c‐Myc locus has been modified (c‐MycG) so that it encodes a GFP‐c‐Myc fusion protein. c‐MycG/G mice are viable, appear normal and exhibit grossly normal lymphocyte development. Flow cytometric analyses revealed significant heterogeneity in c‐Myc protein expression levels in developing c‐MycG/G B and T lymphocytes. GFP‐c‐Myc expression levels were highest in proliferating lymphocytes, suggesting that c‐Myc up‐regulation is important for promoting lymphocyte cell division, and demonstrating that GFP‐c‐Myc expression is a marker of proliferating lymphocytes in vivo.

Phosphoinositide3-kinase gamma (PI3K␥) in neutrophils plays a critical role in the directed migration of these cells into inflamed tissues. In this study, we demonstrate the importance of the endothelial component of PI3K␥ activity relative to its leukocyte counterpart in supporting neutrophil interactions with the inflamed vessel wall. Despite the reconstitution of class-Ib PI3K function in neutrophils of p110␥ ؊/؊ mice, we observed a 45% reduction in accumulation of these cells in an acute lung injury model. Mechanistically, this appears to result from a perturbation in selectin-mediated adhesion as manifested by a 70% reduction in wild-type (WT) neutrophil attachment to and 17-fold increase in rolling velocities on p110␥ ؊/؊ microvessels in vivo in response to tumor necrosis factor alpha (TNF␣). This alteration in adhesion was further augmented by a deficiency in p110␦, suggesting that the activity of both catalytic subunits is required for efficient capture of neutrophils by cytokinestimulated endothelium. Interestingly, Eselectin-mediated adhesion in p110␥ ؊/؊ mice was impaired by more than 95%, but no defect in nuclear factor kappa B (NF-B)-induced gene expression was observed. These findings suggest a previously unrecognized partnership between class-I PI3Ks expressed in leukocytes and endothelium, the combination of which is required for the efficient trafficking of immunocompetent cells to sites of inflammation. IntroductionClass-I phosphoinositide 3-kinases (PI3Ks) play a pivotal role in modulating innate and adaptive immune responses, as they are important transducers of external stimuli to cells such as granulocytes and lymphocytes. [1][2][3][4][5] Structurally, they exist as heterodimeric complexes in which a catalytic p110 subunit (designated as ␣, ␤, ␥, or ␦) is in association with a particular regulatory subunit (designated p50, p55, p85, or p101). 1,6,7 Functionally, all class-I PI3Ks catalyze the phosphorylation of phosphatidylinositol (4,5)-bisphosphate (PIP 2 ) to form phosphatidylinositol (3,4,5)-trisphosphate (PIP 3 ) in response to activation of either receptor tyrosine kinases (RTKs) or G-protein-coupled receptors (GPCRs), which ultimately regulates a diverse array of biologic functions. In regards to innate immunity, one major role of PI3Ks is to support chemoattractant-directed migration of neutrophils, macrophages, and specific populations of mast cells into sites of inflammation. [8][9][10][11] Mechanistically, activation of this intracellular signaling pathway is essential for reorganization of cytoskeleton and membrane structure in response to such agonists, events that result in cell polarity and pseudopodia extension. [12][13][14] In particular, it is believed that PI3K␥, a class-Ib PI3K, predominates in this process, as recruitment of neutrophils into inflamed tissues was reduced by more than 60% in p110␥-null mice as compared with wild-type (WT) controls. [9][10][11] These studies, however, did not take into account the contribution of other cell types required for neutrophil t...