MDC1 is a mediator of the mammalian DNA damage checkpoint

To counteract the continuous exposure of cells to agents that damage DNA, cells have evolved complex regulatory networks called checkpoints to sense DNA damage and coordinate DNA replication, cell-cycle arrest and DNA repair. It has recently been shown that the histone H2A variant H2AX specifically controls the recruitment of DNA repair proteins to the sites of DNA damage. Here we identify a novel BRCA1 carboxy-terminal (BRCT) and forkhead-associated (FHA) domain-containing protein, MDC1 (mediator of DNA damage checkpoint protein 1), which works with H2AX to promote recruitment of repair proteins to the sites of DNA breaks and which, in addition, controls damage-induced cell-cycle arrest checkpoints. MDC1 forms foci that co-localize extensively with gamma-H2AX foci within minutes after exposure to ionizing radiation. H2AX is required for MDC1 foci formation, and MDC1 forms complexes with phosphorylated H2AX. Furthermore, this interaction is phosphorylation dependent as peptides containing the phosphorylated site on H2AX bind MDC1 in a phosphorylation-dependent manner. We have shown by using small interfering RNA (siRNA) that cells lacking MDC1 are sensitive to ionizing radiation, and that MDC1 controls the formation of damage-induced 53BP1, BRCA1 and MRN foci, in part by promoting efficient H2AX phosphorylation. In addition, cells lacking MDC1 also fail to activate the intra-S phase and G2/M phase cell-cycle checkpoints properly after exposure to ionizing radiation, which was associated with an inability to regulate Chk1 properly. These results highlight a crucial role for MDC1 in mediating transduction of the DNA damage signal.     

A 20-km-diameter multi-ringed impact structure in the North Sea

Most craters found on Earth are highly eroded, poorly preserved and only exposed on land. Here we describe a multi-ringed impact structure discovered in the North Sea from the analysis of three-dimensional seismic reflection data. The structure is 20 km in diameter, and has at least ten distinctive concentric rings located between 2 and 10 km from the crater centre. The structure affects Cretaceous chalk and Jurassic shales, and is well preserved below several hundred metres of post-impact Tertiary strata, which constrains its age to be 60-65 Myr old. The formation of concentric ringed impact structures at this relatively small scale had not previously been thought possible, especially on the terrestrial planets. We have mapped the ring structures at a resolution of tens of metres both laterally and in depth, and show that the rings are fault-bounded graben structures, similar to fault arrays formed in low-strain-rate detachment tectonic settings. Strata deeper than 500 m palaeodepth appear unfaulted, and we infer that the concentric ring structures may have accommodated post-impact extension towards the excavated crater, through detachment on weak layers within the chalk.     

Fibrosing alveolitis (pulmonary interstitial fibrosis) evoked by experimental inhalation of Gasoline vapours

Summary Rats exposed to an atmosphere contaminated with gasoline vapour for 6–12 weeks exhibit progressive focal interstitial fibrosis of the lung, which is associated with irregular alveolar collapse. It is suggested that this experimental model of diffuse fibrosing lung disease constitutes a useful tool for the study of the dynamics of pulmonary reactivity to atmospheric pollutants and which may facilitate an understanding of the pathogenesis of fibrosing alveolitis in man.This work was supported by a grant from the National Health and Medical Research Council of Australia.     

Studies on the hydrolysis of bradykinin by angiotensin-converting enzyme (kininase II)

Summary Arg-Pro-Pro-Gly-Phe, the N-terminal pentapeptide of bradykinin, is not an inhibitor of angiotensin-converting enzyme and is not hydrolyzed by the enzyme. Arg-Pro-Pro, the N-terminal tripeptide is a relatively potent (IC₅₀=2.3×10⁶ M) inhibitor but its higher homolog, Gly-Arg-Met-Lys-Arg-Pro-Pro is not an inhibitor of angiotensin-converting enzyme.This work was supported in part by grants from the US Public Health Service (HL 18415, HL 15691, HL 19764) and the John A. Hartford Foundation, Inc., and by the Medical Research Service of the Veterans Administration.     

IGF and FGF cooperatively establish the regulatory stem cell niche of pluripotent human cells in vitro

Distinctive properties of stem cells are not autonomously achieved, and recent evidence points to a level of external control from the microenvironment. Here, we demonstrate that self-renewal and pluripotent properties of human embryonic stem (ES) cells depend on a dynamic interplay between human ES cells and autologously derived human ES cell fibroblast-like cells (hdFs). Human ES cells and hdFs are uniquely defined by insulin-like growth factor (IGF)- and fibroblast growth factor (FGF)-dependence. IGF 1 receptor (IGF1R) expression was exclusive to the human ES cells, whereas FGF receptor 1 (FGFR1) expression was restricted to surrounding hdFs. Blocking the IGF-II/IGF1R pathway reduced survival and clonogenicity of human ES cells, whereas inhibition of the FGF pathway indirectly caused differentiation. IGF-II is expressed by hdFs in response to FGF, and alone was sufficient in maintaining human ES cell cultures. Our study demonstrates a direct role of the IGF-II/IGF1R axis on human ES cell physiology and establishes that hdFs produced by human ES cells themselves define the stem cell niche of pluripotent human stem cells.