A critical role for DNA end-joining proteins in both lymphogenesis and neurogenesis

XRCC4 was identified via a complementation cloning method that employed an ionizing radiation (IR)-sensitive hamster cell line. By gene-targeted mutation, we show that XRCC4 deficiency in primary murine cells causes growth defects, premature senescence, IR sensitivity, and inability to support V(D)J recombination. In mice, XRCC4 deficiency causes late embryonic lethality accompanied by defective lymphogenesis and defective neurogenesis manifested by extensive apoptotic death of newly generated postmitotic neuronal cells. We find similar neuronal developmental defects in embryos that lack DNA ligase IV, an XRCC4-associated protein. Our findings demonstrate that differentiating lymphocytes and neurons strictly require the XRCC4 and DNA ligase IV end-joining proteins and point to the general stage of neuronal development in which these proteins are necessary.

DNA ligase IV deficiency in mice leads to defective neurogenesis and embryonic lethality via the p53 pathway

DNA ligase IV (LIG4) is a nonhomologous end-joining (NHEJ) protein used for V(D)J recombination and DNA repair. In mice, Lig4 deficiency causes embryonic lethality, massive neuronal apoptosis, arrested lymphogenesis, and various cellular defects. Herein, we assess potential roles in this phenotype for INK4a/ARF and p53, two proteins implicated in apoptosis and senescence. INK4a/ARF deficiency rescued proliferation/senescence defects of Lig4-deficient fibroblasts but not other phenotypic aspects. In contrast, p53 deficiency rescued embryonic lethality, neuronal apoptosis, and fibroblast proliferation/senescence defects but not lymphocyte development or radiosensitivity. Young Lig4/p53 double null mice routinely died from pro-B lymphomas. Thus, in the context of Lig4 deficiency, embryonic lethality and neuronal apoptosis likely result from a p53-dependent response to unrepaired DNA damage, and neuronal apoptosis and lymphocyte developmental defects can be mechanistically dissociated.

Identification of redox/repair protein Ref-1 as a potent activator of p53

p53 can be isolated from cells in a form that is inert for binding to DNA but that can be stimulated dramatically by phosphorylation, antibody binding, or short single strands of DNA. This suggests that upon genotoxic stress, cells can convert latent p53 to one that is active for DNA binding. Surprisingly, we observed that latent p53 is as effective in activating transcription in vitro as is active p53. We found that HeLa nuclear extracts can stimulate DNA binding by latent p53 and have purified from them a p53-stimulating protein that we have determined to be the product of the Ref-1 gene. Interestingly, Ref-1 is a dual function protein that can both regulate the redox state of a number of proteins and function as a DNA repair (A/P) endonuclease. We observed that oxidized forms of full-length and carboxy-terminally truncated p53 (p53 delta30), which are inactive for DNA binding, are both stimulated by the Ref-1 protein. However, in the presence of reducing agent, Ref-1 is an extremely potent stimulator of full-length p53 but not p53 delta30. These and additional data indicate that Ref-1 protein stimulates p53 by both redox-dependent and -independent means and imply a key role for it in p53 regulation. Importantly, we have also determined that Ref-1 can stimulate p53 transactivation in vivo. This is the first example of a noncovalent protein modifier of p53 function identified in cells.

Birth weight and subsequent risk of obesity: a systematic review and meta-analysis

This report describes the association between birth weight (BW) and obesity. Screening of 478 citations from five electronic databases resulted in the inclusion of 33 studies, most of medium quality. The meta-analysis included 20 of these published studies. The 13 remaining articles did not provide sufficient dichotomous data and were systematically reviewed, revealing results consistent with the meta-analysis. Our results revealed that high BW (>4000 g) was associated with increased risk of obesity (odds ratio [OR], 2.07; 95% confidence interval [CI], 1.91-2.24) compared with subjects with BW ≤ 4000 g. Low BW (<2500 g) was associated with decreased risk of obesity (OR, 0.61; 95% CI, 0.46-0.80) compared with subjects with BW ≥ 2500 g. However, when two studies exhibited selection bias were removed, the results indicated no significant association between low BW and obesity (OR, 0.77; 95% CI, 0.58-1.04). Sensitivity analyses showed that differences in the study design, sample size and quality grade of the study had an effect on the low BW/obesity association, which low BW was not associated with the risk of obesity in cohort studies, studies with large sample sizes and studies with high quality grades. Pooled results were similar when normal birth weight (2500-4000 g) was used as the reference category. Subgroup analyses based on different growth and developmental stages (pre-school children, school children and adolescents) also revealed that high BW was associated with increased risk of obesity from childhood to early adulthood. No significant evidence of publication bias was present. These results suggest that high BW is associated with increased risk of obesity and may serve as a mediator between prenatal influences and later disease risk.

Synergistic activation of caspase-3 by m-calpain after neonatal hypoxia-ischemia: a mechanism of "pathological apoptosis"?

The relative contributions of apoptosis and necrosis in brain injury have been a matter of much debate. Caspase-3 has been identified as a key protease in the execution of apoptosis, whereas calpains have mainly been implicated in excitotoxic neuronal injury. In a model of unilateral hypoxia-ischemia in 7-day-old rats, caspase-3-like activity increased 16-fold 24 h postinsult, coinciding with cleavage of the caspase-3 proenzyme and endogenous caspase-3 substrates. This activation was significantly decreased by pharmacological calpain inhibition, using CX295, a calpain inhibitor that did not inhibit purified caspase-3 in vitro. Activation of caspase-3 by m-calpain, but not mu-calpain, was facilitated in a dose-dependent manner in vitro by incubating cytosolic fractions, containing caspase-3 proform, with calpains. This facilitation required the presence of some active caspase-3 and could be abolished by including the specific calpain inhibitor calpastatin. This indicates that initial cleavage of caspase-3 by m-calpain, producing a 29-kDa fragment, facilitates the subsequent cleavage into active forms. This is the first report to our knowledge suggesting a direct link between the early, excitotoxic, calcium-mediated activation of calpain after cerebral hypoxia-ischemia and the subsequent activation of caspase-3, thus representing a tentative pathway of "pathological apoptosis."

Ku86-deficient mice exhibit severe combined immunodeficiency and defective processing of V(D)J recombination intermediates

Ku is a heterodimeric DNA end binding complex composed of 70 and 86 kDa subunits. Here, we show that Ku86 is essential for normal V(D)J recombination in vivo, as Ku86-deficient mice are severely defective for formation of coding joints. Unlike severe combined immunodeficient (scid) mice, Ku86-deficient mice are also defective for signal joint formation. Both hairpin coding ends and blunt full-length signal ends accumulate. Contrary to expectation, Ku86 is evidently not required for protection of either type of V(D)J recombination intermediate. Instead, V(D)J recombination appears to be arrested after the cleavage step in Ku86-deficient mice. We suggest that Ku86 may be required to remodel or disassemble DNA-protein complexes containing broken ends, making them available for further processing and joining.

Growth retardation and leaky SCID phenotype of Ku70-deficient mice

Ku70, Ku80, and DNA-PKcs are subunits of the DNA-dependent protein kinase (DNA-PK), an enzyme implicated in DNA double-stranded break repair and V(D)J recombination. Our Ku70-deficient mice were about 50% the size of control littermates, and their fibroblasts were ionizing radiation sensitive and displayed premature senescence associated with the accumulation of nondividing cells. Ku70-deficient mice lacked mature B cells or serum immunoglobulin but, unexpectedly, reproducibly developed small populations of thymic and peripheral alpha/beta T lineage cells and had a significant incidence of thymic lymphomas. In association with B and T cell developmental defects, Ku70-deficient cells were severely impaired for joining of V(D)J coding and recombination signal sequences. These unanticipated features of the Ku70-deficient phenotype with respect to lymphocyte development and V(D)J recombination may reflect differential functions of the three DNA-PK components.

A targeted DNA-PKcs-null mutation reveals DNA-PK-independent functions for KU in V(D)J recombination

The DNA-dependent protein kinase (DNA-PK) consists of Ku70, Ku80, and a large catalytic subunit, DNA-PKcs. Targeted inactivation of the Ku70 or Ku80 genes results in elevated ionizing radiation (IR) sensitivity and inability to perform both V(D)J coding-end and signal (RS)-end joining in cells, with severe growth retardation plus immunodeficiency in mice. In contrast, we now demonstrate that DNA-PKcs-null mice generated by gene-targeted mutation, while also severely immunodeficient, exhibit no growth retardation. Furthermore, DNA-PKcs-null cells are blocked for V(D)J coding-end joining, but retain normal RS-end joining. Finally, while DNA-PK-null fibroblasts exhibited increased IR sensitivity, DNA-PKcs-deficient ES cells did not. We conclude that Ku70 and Ku80 may have functions in V(D)J recombination and DNA repair that are independent of DNA-PKcs.

The influence of age on apoptotic and other mechanisms of cell death after cerebral hypoxia-ischemia

Unilateral hypoxia-ischemia (HI) was induced in C57/BL6 male mice on postnatal day (P) 5, 9, 21 and 60, corresponding developmentally to premature, term, juvenile and adult human brains, respectively. HI duration was adjusted to obtain a similar extent of brain injury at all ages. Apoptotic mechanisms (nuclear translocation of apoptosis-inducing factor, cytochrome c release and caspase-3 activation) were several-fold more pronounced in immature than in juvenile and adult brains. Necrosis-related calpain activation was similar at all ages. The CA1 subfield shifted from apoptosis-related neuronal death at P5 and P9 to necrosis-related calpain activation at P21 and P60. Oxidative stress (nitrotyrosine formation) was also similar at all ages. Autophagy, as judged by the autophagosome-related marker LC-3 II, was more pronounced in adult brains. To our knowledge, this is the first report demonstrating developmental regulation of AIF-mediated cell death as well as involvement of autophagy in a model of brain injury.

Measuring two-dimensional receptor-ligand binding kinetics by micropipette

We report a novel method for measuring forward and reverse kinetic rate constants, kf0 and kr0, for the binding of individual receptors and ligands anchored to apposing surfaces in cell adhesion. Not only does the method examine adhesion between a single pair of cells; it also probes predominantly a single receptor-ligand bond. The idea is to quantify the dependence of adhesion probability on contact duration and densities of the receptors and ligands. The experiment was an extension of existing micropipette protocols. The analysis was based on analytical solutions to the probabilistic formulation of kinetics for small systems. This method was applied to examine the interaction between Fc gamma receptor IIIA (CD16A) expressed on Chinese hamster ovary cell transfectants and immunoglobulin G (IgG) of either human or rabbit origin coated on human erythrocytes, which were found to follow a monovalent biomolecular binding mechanism. The measured rate constants are Ackf0 = (2.6 +/- 0.32) x 10(-7) micron 4 s-1 and kr0 = (0.37 +/- 0.055) s-1 for the CD16A-hIgG interaction and Ackf0 = (5.7 +/- 0.31) X 10(-7) micron 4 s-1 and kr0 = (0.20 +/- 0.042) s-1 for the CD16A-rIgG interaction, respectively, where Ac is the contact area, estimated to be a few percent of 3 micron 2.

A family of mammalian F-box proteins

Ubiquitin-mediated destruction of regulatory proteins is a frequent means of controlling progression through signaling pathways [1]. F-box proteins [2] are components of modular E3 ubiquitin protein ligases called SCFs, which function in phosphorylation-dependent ubiquitination ([3] [4] [5], reviewed in [6] [7]). F-box proteins contain a carboxy-terminal domain that interacts with substrates and a 42-48 amino-acid F-box motif which binds to the protein Skp1 [2] [3] [4]. Skp1 binding links the F-box protein with a core ubiquitin ligase composed of the proteins Cdc53/Cul1, Rbx1 (also called Hrt1 and Roc1) and the E2 ubiquitin-conjugating enzyme Cdc34 [8] [9] [10] [11]. The genomes of the budding yeast Saccharomyces cerevisiae and the nematode worm Caenorhabditis elegans contain, respectively, 16 and more than 60 F-box proteins [2] [7]; in S. cerevisiae, the F-box proteins Cdc4, Grr1 and Met30 target cyclin-dependent kinase inhibitors, G1 cyclins and transcriptional regulators for ubiquitination ([3] [4] [5] [8] [10], reviewed in [6] [7]). Only four mammalian F-box proteins (Cyclin F, Skp1, beta-TRCP and NFB42) have been identified so far [2] [12]. Here, we report the identification of a family of 33 novel mammalian F-box proteins. The large number of these proteins in mammals suggests that the SCF system controls a correspondingly large number of regulatory pathways in vertebrates. Four of these proteins contain a novel conserved motif, the F-box-associated (FBA) domain, which may represent a new protein-protein interaction motif. The identification of these genes will help uncover pathways controlled by ubiquitin-mediated proteolysis in mammals.

Specific association of the gene product of PKD2 with the TRPC1 channel

The function(s) of the genes (PKD1 and PKD2) responsible for the majority of cases of autosomal dominant polycystic kidney disease is unknown. While PKD1 encodes a large integral membrane protein containing several structural motifs found in known proteins involved in cell-cell or cell-matrix interactions, PKD2 has homology to PKD1 and the major subunit of the voltage-activated Ca2+ channels. We now describe sequence homology between PKD2 and various members of the mammalian transient receptor potential channel (TRPC) proteins, thought to be activated by G protein-coupled receptor activation and/or depletion of internal Ca2+ stores. We show that PKD2 can directly associate with TRPC1 but not TRPC3 in transfected cells and in vitro. This association is mediated by two distinct domains in PKD2. One domain involves a minimal region of 73 amino acids in the C-terminal cytoplasmic tail of PKD2 shown previously to constitute an interacting domain with PKD1. However, distinct residues within this region mediate specific interactions with TRPC1 or PKD1. The C-terminal domain is sufficient but not necessary for the PKD2-TRPC1 association. A more N-terminal domain located within transmembrane segments S2 and S5, including a putative pore helical region between S5 and S6, is also responsible for the association. Given the ability of the TRPC to form functional homo- and heteromultimeric complexes, these data provide evidence that PKD2 may be functionally related to TRPC proteins and suggest a possible role of PKD2 in modulating Ca2+ entry in response to G protein-coupled receptor activation and/or store depletion.

Telomere dysfunction impairs DNA repair and enhances sensitivity to ionizing radiation

Telomeres are specialized nucleoprotein complexes that serve as protective caps of linear eukaryotic chromosomes. Loss of telomere function is associated with rampant genetic instability and loss of cellular viability and renewal potential. The telomere also participates in processes of chromosomal repair, as evidenced by the 'capture' or de novo synthesis of telomere repeats at double-stranded breaks and by the capacity of yeast telomeres to serve as repositories of essential components of the DNA repair machinery, particularly those involved in non-homologous end-joining (NHEJ). Here we used the telomerase-deficient mouse, null for the essential telomerase RNA gene (Terc), to assess the role of telomerase and telomere function on the cellular and organismal response to ionizing radiation. Although the loss of telomerase activity per se had no discernable impact on the response to ionizing radiation, the emergence of telomere dysfunction in late-generation Terc-/- mice imparted a radiosensitivity syndrome associated with accelerated mortality. On the cellular level, the gastrointestinal crypt stem cells and primary thymocytes showed increased rates of apoptosis, and mouse embryonic fibroblasts (MEFs) showed diminished dose-dependent clonogenic survival. The radiosensitivity of telomere dysfunctional cells correlated with delayed DNA break repair kinetics, persistent chromosomal breaks and cytogenetic profiles characterized by complex chromosomal aberrations and massive fragmentation. Our findings establish a intimate relationship between functionally intact telomeres and the genomic, cellular and organismal response to ionizing radiation.

Cell mechanics: mechanical response, cell adhesion, and molecular deformation

As the basic unit of life, the cell is a biologically complex system, the understanding of which requires a combination of various approaches including biomechanics. With recent progress in cell and molecular biology, the field of cell mechanics has grown rapidly over the last few years. This review synthesizes some of these recent developments to foster new concepts and approaches, and it emphasizes molecular-level understanding. The focuses are on the common themes and interconnections in three related areas: (a) the responses of cells to mechanical forces, (b) the mechanics and kinetics of cell adhesion, and (c) the deformation of biomolecules. Specific examples are also given to illustrate the quantitative modeling used in analyzing biological processes and physiological functions.

Fundamentals of biomechanics in tissue engineering of bone

The objective of this review is to provide basic information pertaining to biomechanical aspects of bone as they relate to tissue engineering. The review is written for the general tissue engineering reader, who may not have a biomechanical engineering background. To this end, biomechanical characteristics and properties of normal and repair cortical and cancellous bone are presented. Also, this chapter intends to describe basic structure-function relationships of these two types of bone. Special emphasis is placed on salient classical and modern testing methods, with both material and structural properties described.

Brain-specific expression of an exogenous gene after i.v. administration

The treatment of brain diseases with gene therapy requires the gene to be expressed throughout the central nervous system, and this is possible by using gene targeting technology that delivers the gene across the blood-brain barrier after i.v. administration of a nonviral formulation of the gene. The plasmid DNA is targeted to brain with pegylated immunoliposomes (PILs) using a targeting ligand such as a peptidomimetic mAb, which binds to a transporting receptor on the blood-brain barrier. The present studies adapt the PIL gene targeting technology to the mouse by using the rat 8D3 mAb to the mouse transferrin receptor. Tissue-specific expression in brain and peripheral organs of different exogenous genes (beta-galactosidase, luciferase) is examined at 1-3 days after i.v. injection in adult mice of the exogenous gene packaged in the interior of 8D3-PIL. The expression plasmid is driven either by a broadly expressed promoter, simian virus 40, or by a brain-specific promoter taken from the 5' flanking sequence of the human glial fibrillary acidic protein (GFAP) gene. The transgene is expressed in both brain and peripheral tissues when the simian virus 40 promoter is used, but the expression of the exogenous gene is confined to the brain when the transgene is under the influence of the brain-specific GFAP promoter. Confocal microscopy colocalizes immunoreactive bacterial beta-galactosidase with immunoreactive GFAP in brain astrocytes. These studies indicate that tissue-specific gene expression in brain is possible after the i.v. administration of a nonviral vector with the combined use of gene targeting technology and tissue-specific gene promoters.

Effect of long-term mild hypothermia therapy in patients with severe traumatic brain injury: 1-year follow-up review of 87 cases

OBJECT

The goal of this study was to investigate the protective effects of long-term (3-14 days) mild hypothermia therapy (33-35 degrees C) on outcome in 87 patients with severe traumatic brain injury (TBI) (Glasgow Coma Scale score < or = 8).

METHODS

In 43 patients assigned to a mild hypothermia group, body temperatures were cooled to 33 to 35 degrees C a mean of 15 hours after injury and kept at 33 to 35 degrees C for 3 to 14 days. Rewarming commenced when the individual patient's intracranial pressure (ICP) returned to the normal level. Body temperatures in 44 patients assigned to a normothermia group were maintained at 37 to 38 degrees C. Each patient's outcome was evaluated 1 year later by using the Glasgow Outcome Scale. One year after TBI, the mortality rate was 25.58% (11 of 43 patients) and the rate of favorable outcome (good recovery or moderate disability) was 46.51% (20 of 43 patients) in the mild hypothermia group. In the normothermia group, the mortality rate was 45.45% (20 of 44 patients) and the rate of favorable outcome was 27.27% (12 of 44 patients) (p < 0.05). Induced mild hypothermia also markedly reduced ICP (p < 0.01) and inhibited hyperglycemia (p < 0.05). The rates of complication were not significantly different between the two groups.

CONCLUSIONS

The data produced by this study demonstrate that long-term mild hypothermia therapy significantly improves outcomes in patients with severe TBI.

A five-microRNA panel in plasma was identified as potential biomarker for early detection of gastric cancer

BACKGROUND

Circulating microRNAs (miRNAs) have been implicated as novel biomarkers for gastric cancer (GC) diagnosis. However, the mixture of GC subtypes may have led to the inconsistent circulating miRNA profiles, and the clinical performance of circulating miRNAs has not yet been evaluated independently on early detection of GC.

METHODS

A four-phase study was designed with a total of 160 cancer-free controls, 124 patients with gastric non-cardia adenocarcinoma (GNCA) and 36 patients diagnosed gastric cardia adenocarcinoma (GCA). In the discovery phase, we screened the miRNA expression profile in plasma of 40 GNCA patients (stage I) and 40 matched controls by TaqMan low density array (TLDA) chips with pooled samples. Differentially expressed miRNAs were further validated in individual sample using quantitative reverse-transcriptase PCR (qRT-PCR) in the training phase. Subsequently, in an independent validation phase, the identified miRNAs were evaluated in 48 GNCA patients (stage I) and 102 matched controls. Finally, the identified miRNAs were further assessed in an external validation phase including advanced GNCA and GCA patients. Additionally, the expression levels of identified miRNAs were measured in the media of BGC823 and MGC803 cell lines.

RESULTS

Five miRNAs (miR-16, miR-25, miR-92a, miR-451 and miR-486-5p) showed consistently elevated levels in plasma of the GC patients as compared with controls, and were identified to be potential markers for GNCA with area under the receiver operating characteristic (ROC) curves (AUCs) ranging from 0.850 to 0.925 and 0.694 to 0.790 in the training and validation phases, respectively. The five-miRNA panel presented a high diagnostic accuracy for the early-stage GNCA (AUCs=0.989 and 0.812 for the training and validation phases, respectively). Three miRNAs (miR-16, miR-25 and miR-92a) were excreted into the culture media of GC cell lines.

CONCLUSIONS

The five-miRNA panel in plasma may serve as a potential non-invasive biomarker in detecting the early-stage GC.

Apoptosis-inducing factor is a major contributor to neuronal loss induced by neonatal cerebral hypoxia-ischemia

Nine-day-old harlequin (Hq) mice carrying the hypomorphic apoptosis-inducing factor (AIF)(Hq) mutation expressed 60% less AIF, 18% less respiratory chain complex I and 30% less catalase than their wild-type (Wt) littermates. Compared with Wt, the infarct volume after hypoxia-ischemia (HI) was reduced by 53 and 43% in male (YX(Hq)) and female (X(Hq)X(Hq)) mice, respectively (P<0.001). The Hq mutation did not inhibit HI-induced mitochondrial release of cytochrome c or activation of calpain and caspase-3. The broad-spectrum caspase inhibitor quinoline-Val-Asp(OMe)-CH(2)-PH (Q-VD-OPh) decreased the activation of all detectable caspases after HI, both in Wt and Hq mice. Q-VD-OPh reduced the infarct volume equally in Hq and in Wt mice, and the combination of Hq mutation and Q-VD-OPh treatment showed an additive neuroprotective effect. Oxidative stress leading to nitrosylation and lipid peroxidation was more pronounced in ischemic brain areas from Hq than Wt mice. The antioxidant edaravone decreased oxidative stress in damaged brains, more pronounced in the Hq mice, and further reduced brain injury in Hq but not in Wt mice. Thus, two distinct strategies can enhance the neuroprotection conferred by the Hq mutation, antioxidants, presumably compensating for a defect in AIF-dependent redox detoxification, and caspase inhibitors, presumably interrupting a parallel pathway leading to cellular demise.

Dysfunctional metacognition across psychopathologies: A meta-analytic review

BACKGROUND

Dysfunctions in metacognition have been reported in individuals with anxiety disorders. Although recent studies have examined metacognition in other disorders, how dysfunctional metacognition compares across disorders is not clear. This review aimed to ascertain the importance of dysfunctional metacognition in various psychopathologies, and to identify similarities and differences in metacognitive profiles across disorders.

METHODS

Forty-seven studies were selected from 586 articles published between 1990 and August 2015, including a total sample of 3772 patients and 3376 healthy individuals. Studies that measured metacognition using the Meta-Cognitions Questionnaire (MCQ) and its variants were included. We conducted five meta-analyses including 49 to 55 effect sizes, comparing psychiatric patients to healthy individuals on respective metacognitive dimensions of the MCQ.

RESULTS

We found elevated metacognitive dysfunctions in patients, as a group, on all MCQ dimensions. Group effects were large and robust for the two negative beliefs (i.e., beliefs about the uncontrollability and danger of thoughts, and beliefs about the need to control thoughts), and moderate and unstable for the positive beliefs. Patients showed decreased cognitive confidence and heightened cognitive self-consciousness on moderate to large levels. Moderator analyses revealed that negative beliefs about uncontrollability and danger of thoughts were most prevalent in generalized anxiety disorder, whereas heightened cognitive self-consciousness was more characteristic in obsessive-compulsive disorder. Generalized anxiety disorder, obsessive-compulsive disorder and eating disorders manifested more similar metacognitive profiles than other disorders.

CONCLUSIONS

Our findings supported dysfunctional metacognition as common processes across psychopathologies, with certain dimensions being more prevalent in particular disorders.

Interaction of ATF6 and serum response factor

Serum response factor (SRF) is a transcription factor which binds to the serum response element (SRE) in the c-fos promoter. It is required for regulated expression of the c-fos gene as well as other immediate-early genes and some tissue-specific genes. To better understand the regulation of SRF, we used a yeast interaction assay to screen a human HeLa cell cDNA library for SRF-interacting proteins. ATF6, a basic-leucine zipper protein, was isolated by binding to SRF and in particular to its transcriptional activation domain. The binding of ATF6 to SRF was also detected in vitro. An ATF6-VP16 chimera activated expression of an SRE reporter gene in HeLa cells, suggesting that ATF6 can interact with endogenous SRF. More strikingly, an antisense ATF6 construct reduced serum induction of a c-fos reporter gene, suggesting that ATF6 is involved in activation of transcription by SRF. ATF6 was previously partially cloned as a member of the ATF family. The complete cDNA of ATF6 was isolated, and its expression pattern was described.

Characterization of broken DNA molecules associated with V(D)J recombination

We previously demonstrated that DNA molecules with double-strand breaks at variable-(diversity)-joining [V(D)J] recombination signal sequences are relatively abundant in mouse thymocytes. This abundance strongly suggests that the mechanism of V(D)J recombination involves double-strand cleavage at recombination signals. As a first step toward understanding the mechanism of cleavage, we used a sensitive PCR assay to characterize the structure of one class of cleavage products, the signal ends, in detail. Here we demonstrate that most of these ends are blunt and terminate in 5' phosphoryl groups. Virtually all of the flush signal ends are full length. A minor subpopulation of broken ends terminates in short single-strand extensions. We have found no evidence for covalent DNA-protein linkages involving the signal ends. These data allow further refinement of the double-strand cleavage model for V(D)J recombination.

Ecology. Giant pandas in a changing landscape

The giant panda has been restricted to several disjunct montane forest populations, and habitat loss and fragmentation are the primary threats to its survival. For pandas to survive, conservation efforts must focus on larger landscapes rather than individual nature reserves. China recently initiated several policies, including the Natural Forest Conservation Program and Grain-to-Green Policy, which provide a historic opportunity to integrate panda conservation into national policies. Simultaneously, China is promoting the Western China Development Program, which calls for substantial infrastructure and hydropower development and economic investments. Integrating panda conservation into these development policies will be a critical challenge.