This Week in Science

Science  15 Jul 2005:
Vol. 309, Issue 5733, pp. 351
  1. The Virtual Synapse


    In a multiparameter, multidimensional system, building a quantitative and detailed model can be a helpful adjunct to experimental studies in exploring parameter space. Coggan et al. (p. 446, see the Perspective by Lučić and Baumeister) have taken a step toward describing dynamic events at a neuronal synapse by reconstructing the architecture of the pre- and postsynaptic membranes and underlying cytoplasmic vesicles, and incorporating kinetic measurements of neurotransmitter receptor properties as well as other physical and chemical parameters of neurotransmitters. At this simulated synapse, the known electrophysiology of transmitter release could not be explained by vesicle fusion only at the active zone, the classical region of membrane apposition between the pre- and postsynaptic neurons. Instead, the modeled electrophysiology fits better to observations if ectopic release by vesicle fusion outside of active zones is included.

  2. A Field Day for Fast Computing

    The Earth's magnetic field is produced by vigorous convection in its liquid iron outer core, and it is thought that many details of the field and its behavior over time, including reversals, are produced by the dynamics of these convections. However, it has been difficult to obtain the fluid state of the core in numerical simulations; in particular, the ratio of viscous to rotational forces is very small, so the core is in a Taylor state in which the axial magnetic torque vanishes. Takahashi et al. (p. 459; see the news story by Kerr) have now obtained these conditions in numerical experiments using a very fast supercomputer, the Earth Simulator. Their model reproduces many aspects of the current and past magnetic field and reveals how the field may vary during a reversal.

  3. Some Wiggle Room

    Below the glass transition temperature, Tg, of an amorphous material, large-scale molecular motions are no longer possible, and the material is thought of as being frozen. However, even below Tg, local atomic motions are still possible. Priestly et al. (p. 456) look at the roles of free, fixed, and internal surfaces on the relaxations in a polymer glass, by tagging the chains with fluorescent dyes so that their motion can be tracked. Relaxations are strongly influenced by the surfaces, and this effect extends considerably into the bulk of the material into regions where surface effects do not affect Tg.

  4. A Threat to Tokyo from Below


    The Philippine Sea Plate subducts to the north underneath Japan just south of Tokyo. Seismic hazards related to a subduction zone earthquake depend greatly on the depth to the fault. The plate boundary was thought to be about 20 to 40 kilometers (km) or more beneath the city, which is home to about 33 million people. Using seismic imaging, Sato et al. (p. 462) show that the boundary fault flattens and is no deeper than 25 km beneath Tokyo.

  5. Not the First Stars We See Tonight?

    The first stars born in the universe formed from primordial gases that contain no “metals”—astrophysical shorthand for carbon and heavier elements. Once these first-generation stars died in supernova explosions, second- and later-generation stars formed as the metal-enriched debris gravitationally coalesced. The recent discovery of “hyper metal poor” stars led to hopes that the earliest generation of stars, the so-called Population III, had been found. Iwamoto et al. (p. 451, published online 2 June 2005; see the Perspective by Beers) describe computer modeling that indicates these prime candidate stars are in fact second-generation objects that formed from the supernovae of an earlier population of stars. The results, which accurately reproduce the abundance of chemical elements in the hyper metal poor stars, will have important implications for identifying the true “first” stars.

  6. Controlled Single-Photon Emission

    The ability to deliver single photons on demand is an important requirement for quantum information processing and secure quantum communication. Reproducibility, in terms of the photon states from one to the other, as well as ease of implementation must also be considered for practical uses. Existing single photon sources generally meet one of these requirements, but not both. Darquié et al. (p. 454) present an approach, based on exciting a single, optically trapped rubidium atom with short laser pulses, that can meet both requirements. Each pulse stimulates the atom to emit a single photon.

  7. Trypanosomes Beware

    Recently, human serum apolipoprotein L-I (apoL-I) was found to lyse African trypanosomes, the parasite responsible for sleeping sickness. Pérez-Morga et al. (p. 469) have now elucidated the mechanism by which apoL-I kills trypanosomes. ApoL-I contains a membrane pore-forming domain that targets the lysosomal membrane of incoming trypanosomes. An ionic pore forms that triggers uncontrolled osmotic swelling of the lysosome and leads to trypanosome lysis. This function of apoL-I helps provide humans with an innate form of immunity against this pathogen. The parasite Trypanosoma cruzi goes through four life-cycle stages during its development in insects and humans; in humans, it causes Chagas disease. Complementing the sequencing of 3 kinetoplastid genomes reported in this issue, Atwood et al. (p. 473) present a proteomic analysis of the life-cycle stages of T. cruzi. The parasite appears to use histidine as an energy source during its development in insect vectors, but uses fatty acids when it resides in mammalian cells. Knowledge of stage-specific pathways may aid in selection of targets for drug intervention.

  8. Aging and Death

    Mutations in mitochondrial DNA (mtDNA) are thought to play a central role in mammalian aging, but the underlying cellular mechanisms have remained elusive. Kujoth et al. (p. 481) examined mice genetically manipulated to accumulate high levels of mtDNA mutations. The mutant mice had a significantly reduced life span compared with wild-type littermates and showed features of premature aging such as hearing loss, a decline in muscle mass, and dysfunction of tissues that normally undergo rapid cellular turnover. Surprisingly, the aging phenotype did not appear to arise from increased oxidative stress, as predicted by current working hypotheses, but rather from an increase in cell death (apoptosis). Mutations in mtDNA may drive the aging of certain tissues by triggering the loss of irreplaceable cells.

  9. Reversing Neurodegenerative Change


    Neurofibrillary tangles are the most common intraneuronal inclusion in the brains of patients with neurodegenerative diseases and are composed, at least in part, of deposits of the protein tau. SantaCruz et al. (p. 476) describe the remarkable effects of suppressing transgenic tau overexpression in mice: the recovery of memory loss after significant neuron loss and brain atrophy despite continued accumulation of neurofibrillary tangles. This dissociation of cognitive deficits from tangle pathology suggests that recovery of cognitive function is possible even after considerable neurodegeneration has occurred during the development of tauopathies such as Alzheimer's disease.

  10. Distorted Body Awareness

    A strange and disturbing neurological condition, anosognosia, can cause obviously intelligent, awake, and talking individuals to be unaware of paralysis on one side of their body. Berti et al. (p. 488) investigated patients with spatial neglect and found that about half of them were also anosognosic for their left hemiplegia because of lesions in the right brain hemisphere. Comparison of the two groups with and without anosognosia revealed that damage to frontal areas (particularly brain areas 6 and 44, motor cortex BA 4, and the somatosensory cortex) underpins the loss of awareness of motor impairment in these patients.

  11. Three Toxins Are Better Than One

    Understanding molecular mechanisms of neurotransmitter release and short-term synaptic plasticity is one of the central questions in neuroscience. Sakaba et al. (p. 491) studied the roles of SNARE proteins in neurotransmitter release using clostridial neurotoxins. A detailed kinetic analysis of the action of several toxins revealed that the kinetics of transmitter release differs, depending on which SNARE proteins were cleaved. Toxins cleaving synaptobrevin and syntaxin reduced the number of fusion-competent vesicles without changing Ca2+-sensitivity of the release apparatus of remaining vesicles. In contrast, toxins cleaving the C terminal of SNAP-25 reduced intracellular Ca2+-sensitivity of vesicle fusion, suggesting that the C terminal is important for driving rapid fusion. Furthermore, toxins cleaving synaptobrevin led to a modification of the coupling between Ca2+-channels and release-competent vesicles.

  12. Heat Flux Beneath Antarctic Ice

    Geothermal heat fluxes at the base of large ice sheets can lead to melting that makes the sheets much more prone to sliding than if they were frozen to the ground below. Determining geothermal heat flow beneath ice sheets is difficult, and rarely done, because measurements have required the drilling of boreholes down to bedrock, or close to it. Fox Maule et al. (p. 464, published online 9 June 2005) describe a way to do large-scale mapping much more easily by making magnetic measurements of the crust with an instrument on a satellite and then taking advantage of the relation between the magnetic properties of rock and its temperature. They have generated a heat-flux map for the entire continent of Antarctica that reveals large variations at scales of several hundred kilometers, and verify that areas of known volcanism or streaming ice also have high heat flows.

  13. RNAi and Heterochromatin

    In fission yeast, RNA interference (RNAi) machinery converts pericentromeric transcripts into small interfering RNAs (siRNAs), which are required for the pericentromeric heterochromatin formation. Kato et al. (p. 467, published online 9 June 2005) describe a mutation in the second largest subunit of RNA polymerase II (RNAPII), which specifically disrupts RNAi-dependent pericentromeric heterochromatin in fission yeast. The mutant could transcribe the pericentromeric region as well as other coding regions, but could not generate siRNA from the transcripts. It appears that RNAPII couples pericentromeric transcription with siRNA production needed for the heterochromatin assembly.

  14. High and Low Lights for Photosynthesis

    For photosynthesis to remain efficient, the composition of the photosynthetic apparatus must change in response to light conditions. Scheuring and Sturgis (p. 484) used atomic force microscopy to examine the native membranes of a photosynthetic bacterium. Under high-light conditions, the membrane has an amorphous structure with homogeneously distributed reaction centers with light-harvesting antennae between them. Under low-light conditions, the amorphous structure is maintained, but extra light harvesting antennae separate into paracrystalline domains. This structural adaptation prevents photodamage under high-light conditions but allows efficient photon capture under low-light conditions.