This Week in Science

Science  11 Oct 2013:
Vol. 342, Issue 6155, pp. 159
  1. Mapping Mantle Mixing

    CREDIT: SCOTT W. FRENCH

    Mantle convection is the primary driving force for plate tectonics, but mantle convection also mixes material in the interior of Earth and controls heat flow from the core. The patterns of convection are often difficult to image directly with seismic waves—particularly on a global scale. French et al. (p. 227, published online 5 September) constructed a global tomographic model of the upper mantle and transition zone that is sensitive to changes in seismic velocity and anisotropy. The approach identifies elongated, horizontal structures in the upper mantle that are parallel to overlying plate motions. At greater depths, however, vertical plume-like structures extend from the lower mantle and disappear near the base of low velocity zones like those observed beneath Hawaii.

  2. Intestinal Villus Formation

    The intestinal villi are essential elaborations of the lining of the gut that increase the epithelial surface area for nutrient absorption. Shyer et al. (p. 212, published online 29 August; see the Perspective by Simons) show that in both the developing human and chick gut, the villi are formed in a step-wise progression, involving the sequential folding of the endoderm into longitudinal ridges, via a zigzag pattern, to finally form individual villi. These changes are established through the differentiation of the smooth muscle layers of the gut, restricting the expansion of the adjacent proliferating and growing endoderm and mesenchyme, generating compressive stresses that lead to the buckling and folding of the tissue.

  3. The Origins of Europeans

    To investigate the genetic origins of modern Europeans, Brandt et al. (p. 257) examined ancient mitochondrial DNA (mtDNA) and were able to identify genetic differences in 364 Central Europeans spanning the early Neolithic to the Early Bronze Age. Observed changes in mitochondrial haplotypes corresponded with hypothesized human migration across Eurasia and revealed the complexity of the demographic changes and evidence of a Late Neolithic origin for the European mtDNA gene pool. This transect through time reveals four key population events associated with well-known archaeological cultures, which involved genetic influx into Central Europe from various directions at various times.

  4. Elastic to Plastic

    When a crystal is mechanically compressed, it first reacts elastically (reversibly), and then enters the plastic regime, in which the structure of the material is irreversibly changed. This process can be studied with molecular dynamics (MD) simulations on very fine temporal and spatial scales, but experimental analysis has lagged behind. Milathianaki et al. (p. 220) shocked polycrystalline copper with a laser beam, and then took successive snapshots of the crystal structure at 10-picosecond intervals. The results were compared directly with atomistic simulations and revealed that the yield stress—the point of transition from plastic to elastic response—agreed well with MD predictions.

  5. Glassy Eyed

    In crystalline materials, the collective motion of atoms in one- and two-dimensional defects—like dislocations and stacking faults—controls the response to an applied strain, but how glassy materials change their structure in response to strain is much less clear. Huang et al. (p. 224; see the Perspective by Heyde) used advanced-transmission electron microscopy to investigate the structural rearrangements in a two-dimensional glass, including the basis for shear deformations and the atomic behavior at the glass/liquid interface.

  6. Remnants of a Water-Bearing World

    Stars like the Sun end their lives as white dwarfs. Farihi et al. (p. 218) used detailed spectroscopic analysis of a debris-accreting white dwarf, along with knowledge that such systems accrete this debris from remnants of rocky planetary bodies, to derive the water content in a disrupted extrasolar body. The findings suggest that the white dwarf contains the signature of a rocky minor planet composed of 26% water by mass.

  7. Viral Defenses

    CREDIT: IMAGE COURTESY OF OLIVIER VOINNET AND SHOU WEI DING

    In plants and invertebrates, RNA interference (RNAi) functions as an innate antiviral defense mechanism. Viruses that infect plants and invertebrates have evolved viral suppressors of RNAi (VSRs) that disable the RNAi pathway. Whether mammals use RNAi as a defense against viruses has been less clear (see the Perspective by Sagan and Sarnow). Li et al. (231) and Maillard et al. (235) studied mammalian cell lines and baby mice productively infected with RNA viruses and observed the production of virus-derived small RNAs (vsRNAs). When the putative VSR proteins of the infecting viruses were disabled, host RNAi-derived vsRNAs were much increased and the viruses were rapidly cleared and unable to mount a full-blown infection. Thus, RNAi also has an innate antiviral function in mammals.

  8. Unlucky Lakes

    The negative consequences of increased loading of nitrogen and phosphorus into aquatic ecosystems are well known. Management strategies aimed at reducing the sources of these excess nutrients, such as fertilizer runoff or sewage outflows, can largely mitigate the increases in nitrogen and phosphorus levels; however, it is unclear if these strategies are influencing other spects of these ecosystems. Using a global lake data set, Finlay et al. (p. 247; see the Perspective by Bernhardt) found that reducing phosphorus inputs reduced a lake's ability to export reactive nitrogen, exacerbating nitrate pollution.

  9. Lymphocyte Metabolism

    Lymphocytes are highly dynamic cells, undergoing extensive proliferation upon infection and then reducing in number upon pathogen clearance. Lymphocytes also circulate through many different tissue environments that vary in their nutrient and oxygen availability. Recent studies have revealed changes in metabolic programming that facilitate this dynamic behavior. How these changes occur and the specific effects that they have on lymphocyte function and on the ultimate outcome of an infection are not well understood. Pearce et al. (1242454) review recent progress in this area, suggest how parallels might be found in studying the metabolic changes seen in tumor cells, and propose challenges for the future.

  10. Complexity and Diversity

    Complex organisms must produce and maintain an extraordinary diversity of cell and tissue types with a limited number of genes and molecular pathways. Cells accomplish this by reusing the same signaling networks at different times, in different tissues, and for different purposes, yet how this context-specificity is achieved is poorly understood. Jukam et al. (1238016, published online 29 August) show how a set of genes that function in cell and tissue growth can be used again in nondividing fly photoreceptor neurons to ensure that flies develop appropriate sensitivity to both blue and green light. The Hippo pathway undergoes a regulatory change—from negative to positive feedback—that requires a tissue-specific transcription factor network. This network uses evolutionarily conserved regulatory factors whose mutations in humans result in degenerative retinal diseases. The context-appropriate positive feedback in flies ensures an all-or-nothing fate decision necessary to establish a functional visual system.

  11. Two Viruses to Bind

    CREDIT: SHI ET AL.

    Structural studies of two different H7N9 influenza viruses isolated from humans—A/Shanghai/1/2013 and A/Anhui/1/2013—which have different amino acid sequences in the receptor binding site, provide data indicating that the virus is in transition with respect to host adaptation. The Shanghai virus was one of the first isolated in humans that binds avian receptor glycans with high affinity, but binds poorly to human receptors. However, the later Anhui isolates can bind both avian and human receptors at high affinity. Shi et al. (p. 243, published online 5 September) show that four hydrophobic mutations contribute to acquisition of affinity for the human receptor by the virus hemagglutinin (HA) and confirm this effect in binding studies with virus particles. Further comparison of a mutant H7N9 A/Anhui/1/2013 HA with the bird flu H5N1 virus revealed the significance of some of the naturally occurring changes observed in circulating H7N9 viruses, which helps to explain how these viruses have been able to cause many severe human infections in a short time.

  12. RTEL1 in DNA Replication

    Genome stability requires the coordinate action of a variety of DNA maintenance systems. The DNA helicase, RTEL1 (regulator of telomere length 1), disassembles recombination intermediates to avoid dangerous by-products. RTEL1 also limits excessive meiotic crossing over and disassembles telomere T loops. Vannier et al. (p. 239) now show that mammalian RTEL1 is part of the DNA replication machinery. RTEL1 binds to proliferating cell nuclear antigen (PCNA), an interaction that was important for normal DNA replication, replication fork stability, and telomere stability. The RTEL1-PCNA interaction was also critical for protecting cells against tumorigenesis but was not required for telomere T-loop disassembly.

  13. Bacterial Détente?

    Type VI secretion systems (T6SS) correspond to dynamic intracellular organelles that are functionally analogous to contractile bacteriophage tails. The T6SS of several bacteria species have been found to be responsible for antagonistic behavior that likely reflects the translocation of toxic proteins (effectors) between cells. Pseudomonas aeruginosa is able to sense exogenous T6SS attack and assemble its own T6SS apparatus to launch a retaliatory attack aimed directly at the attacker. Now, Ho et al. (p. 250) describe how exogenous attack is sensed in a process that involves membrane disruption and suggest that the T6SS provides a general cellular defense mechanism against not only T6SS but also conjugative DNA elements delivered via the type IV secretion system involved in mating pair formation.

  14. BCL11A Variants

    Recent chromatin mapping data have suggested that trait-associated variants often mark regulatory DNA. However, there has been little rigorous experimental investigation of regulatory variation. Bauer et al. (p. 253; see the Perspective by Hardison and Blobel) performed an in-depth study of the BCL11A fetal hemoglobin-associated locus. The trait-associated variants revealed a chromatin signature that enhanced erythroid development. The enhancer was required for erythroid expression of BCL11A and thus for globin gene expression.

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