This Week in Science

Science  18 Sep 1998:
Vol. 281, Issue 5384, pp. 1765
  1. Getting to the Source of Black Holes

    A space interferometry program consisting of an 8-meter-diameter radio telescope in space and up to 40 ground-based telescopes from the Very Long Baseline Interferometry (VLBI) network provides a virtual telescope 25,000 kilometers in diameter with three times better resolution than previous radio interferometers. This international program requires five tracking stations and three data-correlating facilities to record simultaneously the radio signals from an extragalactic radio source at all of the telescopes and create an integrated, coherent radio image of the structure of the source near its core. Hirabayashi et al. (p. 1825; see the Perspective by Reid) have already seen exceptional detail in four images of such sources that show jetlike structures twisting and bending away from their cores. These jets are related to instabilities or precessing motions or both that are associated with a massive, spinning black hole at the core of these sources.

  2. Early Fishing Sites

    The earliest Paleo-Indian sites in the New World seem to indicate that most sustenance was based on hunting game, but many coastal sites that would provide a more detailed picture are now submerged. Sandweiss et al. (p. 1830) and Keefer et al. (p. 1833) describe two Paleo-Indian sites from coastal Peru that show evidence of a strong maritime-based sustenance economy (see the news story by Pringle). The sites are among the oldest in the New World and date to perhaps 12,000 years ago, considerably older than other evidence of maritime-based sustenance. One site contains possible evidence of early nets in younger levels, and both sites contain lithic artifacts derived from inland sources

  3. Changing the Bulk of the Planets

    The abundances of nonvolatile elements in the bulk of C1 carbonaceous chondrite meteorites have been assumed to represent the bulk composition from which all of the terrestrial planets formed in the solar nebula and subsequently evolved. Bertka and Fei (p. 1838) used the refined moment of inertia for Mars measured by Mars Pathfinder and recent high-pressure experiments on probable martian core phases to determine its bulk composition. They could not fit the data using a C1 chondrite accretion model. Instead, they suggest that the iron-silicon ratio in the solar nebula decreased with distance from the sun and thus each planet accreted a different proportion of iron to silicon. Given that this previously proposed hypothesis fits the new martian data better than the C1 chondrite hypothesis, models of planetary differentiation and core formation in our inner solar system will have to be revised to account for variation in iron-silicon ratio between the different planets versus a single iron-silicon ratio that has been assumed in the currently favored C1 chondrite model.

  4. Fire Power

    Several phenomena, such as earthquakes and impact cratering, show a distinct power-law relation between size and frequency. This relation allows the occurrence of small events to be used to predict the occurrence of larger events. Malamud et al. (p. 1840) examined several large databases and found that natural forest fires also show such a relation, a finding that should improve the prediction and management of forest fires. The behavior is also captured in a simple cellular automata computer model of forest fires.

  5. Clues to Controlling Plasma Turbulence

    Stars are formed from a hot vigorously convecting plasma confined in a strong magnetic field undergoing fusion reactions. This model also forms the basis for many attempts at developing controlled fusion reactors. One major problem has been the control of turbulence in the confined plasma. Turbulence hinders confinement and suppresses reactions. Lin et al. (p. 1835; see the Perspective by Burell) have explored the nature of turbulence and means to control it in such flows using a massively parallel computer simulation. The results show how small-scale zonal flows that regulate and turbulent transport develop and thus suggest ways to minimize turbulence.

  6. One Out of Many

    Complexation of two identical molecules (hosts) driven by the addition of a different molecule (a guest) is now well known. Using more host molecules becomes more difficult, however, because of the entropic penalties in organizing more components. Martin et al. (p. 1842) show that four identical molecules can be used to encapsulate and solvate adamantane suspended in dichloromethane. Weak interactions such as hydrogen bonding between the hosts and the guests and molecular curvature of the host had to be carefully tailored to direct such assembly.

  7. Overcoming Humoral Transplant Rejection

    Because of the shortage of human organs available for transplant, the pig, whose physiology closely resembles humans, is being seriously considered as an organ donor. A major barrier to successful transplantation, however, is the humoral rejection of the donor organs that is initiated by natural production of antibodies in humans to certain carbohydrate moieties (called αGal epitopes) on the pig cells. These antibodies are thought to be produced because of exposure to similar epitopes in normal bacterial flora. In a mouse model, Bracy et al. (p. 1845) have surmounted this barrier by transducing some of the recipient's bone marrow cells to produce the enzyme, GT, that generates the carbohydrate. The recipient mice stop producing the natural antibodies, even in the face of constant bacterial challenge from the normal flora. If similar results could be obtained in a human system, this approach would remove a major obstacle to xenotransplantation.

  8. Reexamining SOD's Role in ALS

    Genes linked to the production of amyotrophic lateral sclerosis (ALS) encode a mutant form of an enzyme known as superoxide dismutase (SOD). This finding has led to the hypothesis that the neurodegeneration of motor neurons characteristic of the disease is linked to anomalous enzyme activity and the production of reactive oxygen species. Bruijn et al. (p. 1851; see the cover) now provide evidence that challenges this hypothesis. Mice that completely lack endogenous SOD, and mice that possess six times the normal concentrations of SOD, all exhibit identical pathology in the presence of the mutant enzyme. These results suggest that therapies based on the replacement of SOD activity will be ineffective in treating ALS.

  9. Recovering Velocity

    Detecting moving visual stimuli presents the challenge of distinguishing a change in contrast due to motion of the stimulus from a change due to purely local patterns. Single and Borst (p. 1848) measured the intracellular concentration of calcium, as an indicator of electrical activity, in the arbor of dendrites of single sensory neurons in the blowfly. They find that calcium fluctuates within individual dendrites in response to both types of contrast changes but that integration between the dendrites and the cell body effectively cancels out the responses due to local patterns while retaining the overall direction of motion. Furthermore, to maximize integrative efficacy, the fanlike dendritic arbor is oriented so that neurons sensitive to vertical motion have their fans aligned with the dorsal-ventral axis, whereas horizontally sensitive neurons have their arbors arrayed orthogonally.

  10. Entering the Meiotic Cell Cycle

    The mechanisms that control DNA replication during meiosis (the formation of gametes, which reduces chromosome number) are less well understood than those that control entry into mitotic S phase. In the yeast Saccharomyces cerevisiae, Dirick et al. (p. 1854) found that entry into meiotic S phase required the B-type cyclins Clb5 and Clb6 as does the mitotic regulatory mechanism. However, in meiosis, the protein kinase Ime2 was required rather than the related cyclin-dependent kinase Cdc28. Ime2 was required for degradation of the cyclin-dependent kinase inhibitor Sic1. The results indicate that meiotic cell cycles might have evolved from mitotic cycles with conserved roles for the cyclin B-cyclin-dependent kinase complexes. However, the Ime2 protein kinase might have functionally replaced the complex of Cdc28 with G1-specific cyclins, which controls degradation of Sic1 at the transition from G1 to S phase in mitotic cycles.

  11. Fertilin and Sperm Function

    A multifunctional protein in the mammalian fertilization process, fertilin, was previously implicated in binding acrosome-reacted sperm to eggs. Using mice lacking fertilin β, Cho et al. (p. 1857) show that fertilin's functions are more diverse. Fertilin also affects binding to the zona pellucida, an outer coat of the egg. In addition, sperm lacking fertilin are deficient in making their way to the oviducts. Temporary adhesive interactions may be involved in the migratory process.

  12. Activating ASK1 in Cell Death

    The cell death receptor Fas activates two independent apoptosis pathways. The more well-established pathway activates caspase 8 and a protease cascade, and the other uses the Daxx adaptor protein to somehow activate the protein kinase JNK, whose phosphorylation activity appears to be critical for apoptosis in certain cells. Chang et al. (p. 1860) found that Daxx removes the self-inhibition of apoptosis signal-regulating kinase 1 (ASK1), the first member of a kinase cascade. They hypothesize that Daxx brings ASK1 into the Fas receptor complex by binding to the amino-terminal domain of ASK1. This step removes the block on the kinase activity, and phosphorylation by ASK1 ultimately leads to the activation of JNK and cell death.

  13. Promoting Dendrites

    Neurons respond to synaptic activity not only by forwarding the electrical signal but also by rearranging their growth to adapt to developing or changing signaling regimes. CPG15, a protein identified in the rat and Xenopus laevis, is induced in response to synaptic activity. Nedivi et al. (p. 1863) show by overexpressing the protein that it promotes growth of dendrites in projection neurons but has no effect on interneurons. The CPG15 protein appears to function in an intercellular signaling mechanism.

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