This Week in Science

Science  08 Aug 2003:
Vol. 301, Issue 5634, pp. 729
  1. Supersize Supershear

    Laboratory experiments have shown that centimeter-sized cracks can propagate at velocities higher than the shear velocity of the material. However, evidence for supershear rupture on the larger, kilometer-scale of earthquakes has been inconclusive. Bouchon and Vallée (p. 824) determined that the 400-kilometer-long surface rupture on the 2001 Kunlunshan earthquake underwent supershear and traveled at about 5 kilometers per second after about the first 100 kilometers of the fault had ruptured. The reality of supershear rupture heightens the potential hazards of these more energetic ruptures.

  2. Quantum Dots for Quantum Logic

    Although several technologies are being pursued for coherent quantum information processing, solid-state systems may prove the simplest to implement on a wide scale. Li et al. (p. 809) demonstrate a conditional quantum logic gate in which two qubits, each consisting of an exciton (an electron-hole pair), are coupled together in a single quantum dot. These qubits can be manipulated coherently with polarized light and can perform logical operations with a fidelity of 0.7.

  3. Balancing Birefringence

    Polymer processing usually causes the chains to stretch. Although this sort of molecular orientation can be exploited in the making of polarizing films, it is a shortcoming if one wants the film to be optically isotropic. Tagaya et al. (p. 812) have come up with a way to compensate for the anisotropy that is induced by adding small strontium carbonate crystals, which orient in the same way as the stretched polymer chains but have the opposite birefringence. This technique can be applied to polymers with either positive or negative birefringence through the selection of the correct crystalline material and particle sizes.

  4. From Methane to Acetic Acid in One Step

    Methane in natural gas is a clean starting source for chemical synthesis, but its strong C-H bonds make direct activation difficult. For example, the synthesis of acetic acid from methane requires three steps, the first of which being the high-temperature conversion (∼800°C) of methane to CO and H2. Periana et al. (p. 814) report a direct process at lower temperatures (180°C) using a simple if daunting catalyst—a low loading of PdSO4 in concentrated sulfuric acid. The authors suggest a mechanism in which a Pd-CH3 species is generated and then carbonylated by methanol that is produced in situ. Although less efficient than the three-step process, this approach could take advantage of previously reported Pt-catalyzed routes in this solvent for methanol synthesis.

  5. Retrograde Transport and ALS Therapy

    Amyotrophic lateral sclerosis (ALS) is a fatal disease characterized by degeneration of the motor neurons for which there is no effective therapy. Kaspar et al. (p. 839) now show in a mouse model of ALS that injecting an adeno-associated viral vector carrying the neurotrophic factor, insulin growth factor 1 (IGF-1), into mouse limb muscles delays disease progression and prolongs survival of the animals. Travel of the viral vector by retrograde transport along the motor neurons that innervate the muscles delivered IGF-1 to the neuronal cell bodies.

  6. Packing Zeolites with Polyurethane Films

    Microporous zeolites have found numerous uses in separations, ion exchange, and petrochemical cracking. However, they form small crystallites, and an ongoing challenge has been the formation of ordered crystalline arrays that can fully exploit their pore network. Lee et al. (p. 818) show that a uniform polyurethane film can serve as a template that can control the growth along all three principal crystal axes.

  7. Acoustic Profiles of Ocean Mixing

    The layers with contrasting temperature and salinity of the oceans are commonly mapped by depth profiling. Such mapping of thermohaline structure has limited horizontal resolution and often does not reveal the full complexity of major water masses. Holbrook et al. (p. 821; see the Perspective by Ruddick) present acoustic images of thermohaline structure in the ocean created from marine multichannel seismic reflection profiles that show the internal thermohaline structure across a major oceanographic front between the Labrador and the North Atlantic Currents. These detailed images reveal the lateral extent of thermohaline intrusions caused by double-diffusion, the depth extent of subthermocline eddies, and the possible location of the boundary between deep water and bottom water in this part of the North Atlantic.

  8. Antidepressants and Hippocampal Neurogenesis

    Antidepressants can cause rapid increases in serotonin and noradrenaline levels in patients, but usually the clinical benefits are not seen until 3 or 4 weeks have passed. One possible explanation for this delay is that antidepressant drugs change the mood of patients by stimulating neurogenesis. Santarelli et al. (p. 805; see the news story by Vogel) studied genetically engineered mice lacking the serotonin 1A receptor and mice whose ability to undergo hippocampal neurogenesis had been blocked by focal irradiation. In both cases, antidepressant-induced neurogenesis was inhibited and the behavioral actions of the antidepressants were abolished.

  9. Getting Connected

    According to the “small world” hypothesis, human social networks are structured in such a way that a short chain of around six social ties separates any two individuals from each other. The advent of e-mail connectivity offered Dodds et al. (p. 827; see the Perspective by Granovetter) the opportunity to examine such searches experimentally for social contacts. More than 53,000 volunteers in 163 countries tried to pass a message to a randomly assigned target. They needed to pass the message to a person whom they thought would be closer in the social network to the target and supplied information on their reasoning as they did so. Relatively weak ties were important within the search, and highly connected individuals did not appear to be essential to a successful search. Instead, motivation appeared to be an important factor in completion of the search chains.

  10. Shuffling Genes?

    Lateral gene transfer among bacteria has been considered to be so widespread that it renders the concept of species among bacteria meaningless and makes it impossible to resolve an evolutionary tree. Daubin et al. (p. 829; see the news story by Pennisi) directly address this controversial hypothesis by testing whether a core set of genes can track prokaryote phylogeny. Although they show that there is robust evidence for very few bona fide cases of lateral gene transfer among prokaryotes, they do not dispute that some widely distributed genes are laterally transferred nor that there have been some ancient transfers.

  11. Sorting Out Pre-Messenger RNA Processing

    Two methods have been developed to screen modifications that expand the number of protein products that arise from pre-mRNA molecules. The select group of pre-mRNAs that undergo adenosine to inosine (A-to-I) editing, an enzyme-mediated modification that can expand the repertoire of proteins encoded by a given gene, has been discovered largely by chance. Using a comparative genomics strategy, Hoopengardner et al. (p. 832) identified a signature sequence for RNA editing and used it to identify 16 new edited genes in fruit flies and 1 new edited gene in mammals. The nature of these genes suggests that a primary role of A-to-I RNA editing is the modification of fast signaling components within the nervous system. One way in which an organism as complex as human being results from relatively small complement of genes is through processes such as alternative pre-mRNA splicing, which can produce thousands of distinct protein isoforms from a single gene. To facilitate the study of the alternatively spliced transcriptome, Zhu et al. (p. 836) developed a single molecule-based technology, “digital polony exon-profiling,” that allowed them to identify and quantify individual splicing variants. This technology will help future studies on the role of specific splicing events in normal development and in disease.

  12. Fear and Synchronization

    Behavioral data suggest that two structures in the brain, the hippocampus and the amygdala, play a role in fear conditioning. However, physiological evidence of their interaction remains scant and their relation has been unclear. Seidenbecher et al. (p. 846) recorded electrical activity within both structures in mice undergoing fear conditioning. In the amygdala, synchronized activity at electroencephalogram theta frequencies increased in amplitude and became entrained to hippocampal theta frequencies after conditioning, particularly when exposed to the threat stimulus. This finding suggests that a functional interaction is established during memory formation between the two structures.

  13. Matters of Taste

    Mammalian receptors of the T1r family have been implicated in taste detection. The heterodimers T1r2+T1r3 and T1r1+T1r3 are thought to sense sweet and umami (glutamate), respectively. However, analysis of mice lacking T1r3 by Damak et al. (p. 850) indicates that the molecular mechanisms underlying taste responsiveness are more complicated. T1r3 null mice could still detect sugars and glutamate, but the mice no longer responded to artificial sweeteners. This observation points to the existence of other receptors in taste cells that are required to sample sweet and umami compounds.

  14. Life and Death in the Developing Spinal Cord

    The developing spinal cord follows morphogenic gradients that emanate from the floor plate and notochord. One of the signals is Sonic hedgehog (Shh). Shh does more than direct morphogenesis because it is also required for survival of the neuroepithelium. Thibert et al. (p. 843; see the Perspective by Guerrero and Ruiz i Altaba) show that the receptor for Shh, known as Patched, stimulates apoptotic cell death in the absence of Shh. Thus, refinement of spinal cord architecture results from a balance of morphogenic and apoptotic signals negotiated through signaling by Shh and its receptor Patched.