Editors' Choice

Science  24 Feb 2006:
Vol. 311, Issue 5764, pp. 1073
  1. ECOLOGY/EVOLUTION

    Long-Term Loss of Landbirds

    1. Andrew M. Sugden

    Recent studies have documented the effects of climate variation on the distribution and local survival of a variety of animal species. However, the effects of contemporary climate change on population density cross the entire range of a species, and hence on its potential population decline, have remained mostly unexplored.

    Birds are the only group of organisms for which reliable data exist over ecologically significant stretches of time. Anders and Post quantified the relationships over four decades between climatic oscillations, local temperatures, and population biology of the yellow-billed cuckoo, a North American migrant landbird, using data from the U.S. Geological Survey's Breeding Bird Survey. The cuckoo population densities across their breeding range showed a lagged effect, declining after years when the local temperatures were high. The strength of this effect was predictive of longer-term population decline, which may be caused by a relative scarcity of invertebrate prey after warmer winters. — AMS

    J. Anim. Ecol. 75, 221 (2006).

  2. ASTRONOMY

    Stellar Construction Sites

    1. Joanne Baker

    How and when did galaxies assemble all their stars? Two studies report a census of galaxies across cosmic time and the evolution of star formation rates over the universe's history. Using near-infrared and optical emission data, Kong et al. found that in 80% of distant large galaxies, stars formed at a prodigious rate, much more rapidly than in galaxies of similar mass today. These ancient galaxies appear to have formed all of their stars in a vigorous burst, lasting only a hundred million years.

    Caputi et al. observed a similar pattern of exceptionally rapid star formation in old galaxies. By analyzing mid-infrared emission detected with the Spitzer space telescope, they also found evidence for the presence of complex molecules (polyaromatic hydrocarbons) in the interstellar region of these galaxies at early times. Both teams suggest that their findings favor a “cosmic downsizing” phenomenon, with galaxy formation being more active and rapid in the young universe than at present. — JB

    Astrophys. J. 638, 72; 637, 727 (2006).

  3. EARTH SCIENCE

    Weighing Ice Sheets

    1. H. Jesse Smith

    Melting of the Greenland and Antarctic ice sheets is the largest potential contributor to sea level rise, but calculating the mass balance over time of such large and topographically diverse areas is difficult. Many individual measurements must be combined to create a composite picture of the whole, and techniques that track surface elevation accurately in the larger, more uniform interior sections are not as accurate when applied to the relatively narrow, high-relief coastal margins. Moreover, mass change estimates based only on elevation data do not take into account the height variations caused by compaction of the snow that covers the ice. Recent studies have documented mass loss along the margins and concurrent mass gain in the interiors, but the net effect of these compensatory processes is unclear.

    Zwally et al. used satellite-based radar altimeters to track elevation changes for nearly all of Greenland and Antarctica over a decade. In addition to applying improved methods of data analysis, the authors incorporated estimates of density variation due to firn compaction. Their integrated assessments suggest that although Greenland is gaining mass, Antarctica is melting at a comparatively faster rate, resulting in a net rise in sea level. These conclusions differ both in sign and in magnitude from those of several other studies (for instance, see Rignot and Kanagaratnam, Reports, 17 February 2006, p. 986), leaving open the question of how to reconcile the findings. — HJS

    J. Glaciol. 51, 509 (2005).

  4. CHEMISTRY

    Two Rings to Bind Them All

    1. Jake S. Yeston

    Metallocene polymerization catalysts—two cyclic aromatic rings flanking a central metal (generally Ti, Zr, or Hf) center—have recently been optimized for the commercial production of plastics. Although heterogeneous catalysts are more widely used, the well-defined structure and ligand tunability of the metallocenes offer more rational control over the characteristics of the polymer product, particularly its stereochemistry. However, these molecular catalysts have generally been ineffective in making ultrahigh-molecular-weight polyethylene, an especially tough, resilient plastic.

    Starzewski et al. have designed a zirconocene that overcomes this deficiency and yields polyethylene with chain molecular weights exceeding a million g/mol. They tuned the catalyst's electronic properties to favor continual insertion of ethylene monomers into the growing polymer chain and achieved the necessary >10,000:1 selectivity for chain growth over termination by linking the cyclic ligands around Zr through a dative bond between a phosphine on one ring and a borane on the other. — JSY

    Angew. Chem. Int. Ed. 45, 10.1002/anie.200504173 (2006).

  5. CELL BIOLOGY

    Recruitment to an Organization

    1. Stella M. Hurtley

    Eukaryotic cells contain a dynamic array of microtubules (tubulin polymers), which play diverse roles in interphase but are dramatically rearranged into a spindle during mitosis to promote chromosome segregation. The centrosome, which is composed of a pair of centrioles and associated material, is a key organizer of microtubules and contains the γ isoform of tubulin. In mammalian cells, γ-tubulin is found in a ring-like complex together with other proteins, and Haren et al. characterize NEDD1, a protein of the centrosome that is associated with γ-tubulin ring structures. NEDD1 is not required for γ-tubulin ring complex assembly; in its absence, the complex is not correctly recruited to the centrosomes although NEDD1 is targeted to the centrosome even in the absence of γ-tubulin. Depletion of NEDD1 causes centrosomal defects and compromises the quality of the mitotic spindle and microtubule organization in interphase cells. Interfering with the NEDD1-γ-tubulin interaction blocks centriole duplication. Thus, it appears that NEDD1 mediates the interaction between γ-tubulin and the centrosome, which is necessary for centriole duplication and the fidelity of mitosis. — SMH

    J. Cell Biol. 172, 505 (2006).

  6. BIOMEDICINE

    Innate Immunity and Tumor Growth

    1. Stephen J. Simpson

    The Nod-like family of receptors of the intraepithelial cell are considered important sensors of pathogenic bacteria. Nod1 is activated by bacterial peptidoglycan and is associated with apoptotic pathways in the cell.

    Using retroviral mutagenesis in a human breast cancer epithelial cell line (MCF-7), da Silva Correia et al. tested the possibility that the proapoptotic character of Nod1 might be involved in another context where the regulation of cell death is critical. In a Nod1-deficient MCF-7 clone, the sensitivity to tumor necrosis factor α-induced cell death and the apoptotic response to the specific Nod1 activator diaminopimelic acid were both greatly reduced. The disruption of Nod1 also resulted in an increased ability of the MCF-7 clone to generate tumors in immunodeficient mice and an enhanced sensitivity to estrogen-induced tumor growth. It will be interesting to explore how a bacterial cell wall detector is involved in regulating tumor growth and whether this might afford a therapeutic opportunity. — SJS

    Proc. Natl. Acad. Sci. U.S.A. 103, 1840 (2006).

  7. STKE

    Targeting Downstream Repercussions

    1. Nancy R. Gough

    The tumor suppressor p53, which enforces cell-cycle arrest or cell death, is mutated in roughly half of malignant tumors. Nutlins are imidazoline compounds that disrupt the interaction between p53 and the E3 ubiquitin ligase MDM2 that targets p53 for degradation. Tovar et al. show that in 10 cell lines (representing a range of solid tumors) that express wild-type p53, adding nutlin-3a (and hence freeing p53) resulted in cell-cycle arrest although the extent of apoptosis varied. The osteosarcoma cell line SJSA-1, which has a highly amplified mdm2 gene, was the most sensitive to nutlin-3a-induced apoptosis. To verify that mdm2 amplification was responsible, two other osteosarcoma cell lines—MHM, which has a moderately amplified mdm2, and U2OS, which has a single copy of mdm2—were also analyzed. All three exhibited cell-cycle arrest when exposed to nutlin-3a; however, the induction of apoptosis varied with mdm2 copy number. Microarray analysis showed that proapoptotic genes, such as puma, noxa, and bax, were more strongly stimulated in cells with amplified mdm2 in response to nutlin-3a. Finally, nutlin-3a caused tumor regression in nude mice with MHM and SJSA-1 tumors and halted the growth of tumors that had normal MDM2 and p53. These results suggest that (i) nutlins may be effective clinically, especially for tumors with mdm2 amplification and (ii) cancer cells with normal p53 may have defects in the p53 apoptotic pathway. — NRG

    Proc. Natl. Acad. Sci. U.S.A. 103, 1888 (2006).