This Week in Science

Science  07 May 2004:
Vol. 304, Issue 5672, pp. 791
  1. Metals March to RNA's Orders


    The growth morphology of many inorganic and semiconducting materials can be controlled with the use of small organic compounds or peptides that alter nucleation or affect the relative growth rate of different crystal faces. Examples of nucleic acids controlling crystal growth, or control of metal particle growth, are fewer. Gugliotti et al. (p. 850) created a large library of modified RNA molecules and selected for RNAs that could change the growth habitat of palladium particles, which are usually cubic, to thin hexagonal crystallites on the micrometer scale. The RNAs, which could be grouped into five families, exerted this effect on aqueous solutions of dilute precursors at room temperature.

  2. Doing the Capillary Wave

    Although the interface between two fluids, such as that between a liquid and its vapor, appears to be smooth, thermal fluctuations inevitably lead to a dynamic roughening of the interface. For a liquid-vapor system, these capillary waves cannot be studied in real space because they are caused by local motions over a large area on short time scales; they have been observed instead through scattering techniques. Aarts et al. (p. 847; see the Perspective by Poon) used scanning laser confocal microscopy to follow interface fluctuations in real space in a colloid-polymer system that serves as a model for a molecular fluid. Fluctuations were observed down to the single-particle level.

  3. Threshold for Death

    Control of initiator caspases, the proteases that activate signaling pathways that lead to cell death or apoptosis, literally determines whether a cell dies. The regulation of caspase-2 is implicated in a particularly important death response, the response to anticancer drugs that induce genotoxic stress. Tinel and Tschopp (p. 843) report that caspase-2 becomes activated as part of a protein complex they call the PIDDosome. PIDD is a protein expressed in response to the tumor suppressor p53 and interacts with an adaptor protein called RAIDD, which in turn binds to caspase-2. Small increases in the amount of PIDD expressed in cells causes activation of caspase-2 and sensitizes cells to the genotoxic agent doxorubicin.

  4. The Low Down on Old Diamonds

    Impurities trapped in diamonds can provide information about the origin of the rock in which they are found. Cartigny et al. (p. 853) have measured high concentrations of unaggregated nitrogen and the nitrogen isotopic abundances in microdiamonds from the Akluilâk minette dikes of Nunavut, Canada. These findings suggest a formation process in which crustal metamorphic rocks were subducted to great depths and subjected to high pressures and temperatures. The metamorphic rocks with the encapsulated microdiamonds were then brought back to the near surface by entrainment in the minette magma about 1.8 billion years ago. These results push back evidence for active subduction zone processes by about 1.2 billion years.

  5. Malleable Memories of Fear

    The recall of a memory by a reminder stimulus places this memory back into an active and labile state, from which it is reconsolidated into an inactive and stable state. Is this cellular reconsolidation of memory simply a recapitulation of the events engaged at consolidation, or is there a more complicated process at work? (See the Perspective by Izquierdo and Cammarota.) Lee et al. (p. 839) show that brain-derived neurotrophic factor (BDNF), but not transcription factor Zif268, is necessary for the consolidation of contextual fear conditioning within the hippocampus. However, Zif268, but not BDNF, is required for reconsolidation of the contextual fear memory. Frankland et al. (p. 881) show that processing fear memories involves the activation of multiple cortical regions. Cortical activation was greater after remote, rather than recent, memory tests. This result is consistent with an increasingly important role for the cortex over time. The anterior cingulate cortex, an area involved in the processing contextual or emotional memories, played a critical role in remote memory for contextual fear conditioning.

  6. Structure of Copper-Containing Enzymes


    Copper enzymes play a role in many biological processes. Two examples are peptidylglycine-α-hydroxylating monoxygenase (PHM), which catalyzes the reduction of O2 as a step in the essential amidation of peptide hormones and neuropeptides, and nitrite reductase, which is involved in the geochemical nitrogen cycle. Understanding the mechanism of such enzymes has been hampered by the difficulty in obtaining atomic structures of enzyme intermediates with diatomic molecules bound to the copper, but two studies provide new insights (see the Perspective by Aboelella et al.). Prigge et al. (p. 864) report a 1.85 angstrom (Å) structure of a copper-O2 complex in PHM, and Tocheva et al. (p. 867) report a 1.3 Å structure with nitric oxide bound to copper in nitrite reductase.

  7. Deep Phase Transformation

    Perovskite [(Mg, Fe)SiO3] is thought to be a dominant mineral phase of the lower mantle. Murakami et al. (p. 855; see the cover and the Perspective by Garnero) conducted diamond anvil experiments on magnesium perovskite and found a structural transformation to sixfold silicon and eightfold magnesium in which the silica octrahedra share edges to create chains that are stacked into edge- and apex-sharing sheets. The transformation occurs above 125 gigapascals and 2500 Kelvin, which would correspond to a depth of about 2700 kilometers. This depth would be near the base of the lower mantle, where seismic data has deciphered a heterogeneous D'' layer at the core-mantle interface. The elastic and density properties of the new post-perovskite phase are consistent with the seismic properties of the D'' layer.

  8. A Signaling Threesome

    The neurotrophins are secreted growth factors that play a range of roles in the development and maintenance of the vertebrate nervous system through binding to two classes of receptors, the shared p75 neurotrophin receptor and the Trk receptor tyrosine kinase. Neurotrophins exist as dimers, and a crystal structure of the neurotrophin, nerve growth factor (NGF), bound to a ligand binding domain of Trk possesses a 2:2 complex, suggesting that receptor dimerization may be involved in the activation mechanism. Now He and Garcia (p. 870; see the Perspective by Zampieri and Chao) have determined the structure of NGF bound to the extracellular domain of p75 at 2.4 Å resolution. In the structure, an NGF dimer is bound to a single p75 monomer, and this allosteric binding prevents binding of a second p75. Signaling may thus occur through the disassembly of p75 dimers, which would potentially allow NGF-p75 complexes to engage the Trk receptor and modulate its signaling.

  9. Processing Inflammatory Pain


    Hyperalgesia, an excessive sensitivity to pain, has been attributed exclusively to a sensitization of peripheral nociceptors. However, changes in the central processing of painful stimuli, particularly in the dorsal horn of the spinal cord, also play an important role. Harvey et al. (p. 884; see the news story by Marx) show that glycine receptors containing the α3 receptor subtype are primary molecular targets of prostaglandin-induced inflammatory reactions in the spinal cord. Furthermore, inhibition of glycinergic neurotransmission by prostaglandins underlies later stages of inflammatory pain sensitization.

  10. Old Old-World Hummingbirds

    Hummingbirds are limited to the New World and are thought to have originated in South America. Their sparse fossil record has hampered our understanding of their evolutionary history. Mayr (p. 861; see the news story by Stokstad) reports the discovery of a series of well-preserved hummingbird skeletons from the Oligocene period in Germany. This new species, Eurotrochilus inexpectus, has a long, narrow beak and characteristic hummingbird wing bones, and was likely able to hover and feed on nectar. Thus, the coevolution of birds and flowers could date back to this period. These results may also explain the presence of flowers in the Old World that seem to be adapted to hovering avian pollinators.

  11. Serotonin in the Prefrontal Cortex

    The forebrain serotonin (5-HT) system has been implicated in the etiology or treatment of several neuropsychiatric disorders, including obsessive-compulsive disorder, schizophrenia, and the cognitive sequelae of certain drugs of abuse. However, the effects of regionally selective serotonin depletion have never been investigated. Clarke et al. (p. 878) examined the behavioral effects of depleting 5-HT selectively within the prefrontal cortex. Prefrontal 5-HT depletion in the marmoset monkey induced a relatively specific deficit in behavioral flexibility on a serial visual discrimination reversal task comparable to that seen in patients with schizophrenia and obsessive-compulsive disorder.

  12. Consuming Interest

    The fate of organic carbon delivered to the oceans by rivers has been unclear. Studies have shown that terrigenous dissolved organic carbon (tDOC) mixes conservatively along continental margins (it is not degraded), but it does not accumulate in the deep sea, so somewhere in between it is being remineralized into inorganic carbon. This gap in understanding has been sustained by the difficulty of making measurements in coastal currents, which transport materials rapidly away from their origins and make their evolution hard to observe directly. Hansell et al. (p. 858) present measurements of the slow change of tDOC made in the Beaufort Gyre of the western Arctic Ocean, an ocean whose drainage system contains more than half of the global inventory of soil organic carbon. Taking advantage of the long residence time of surface water there (roughly 20 years), they calculated that the half-life of the tDOC was approximately 7 years. Less than one-third of the tDOC supplied to the Arctic Ocean by rivers survived until it reached the North Atlantic, and most of this material was mineralized in the Arctic.

  13. Generating Biological Oscillations

    Biochemical oscillations exist in nature; however, relatively little is known about the molecular circuits underlying these oscillations. An example of such an oscillation is found in the periodic pulses of cyclic adenosine monophosphate (cAMP) in Dictyostelium during development. Components of this oscillation include the cAMP receptor, adenylyl cyclase, extracellular phosphodiesterase, intracellular phosphodiesterase, cAMP-dependent protein kinase, and mitogen-activated protein (MAP) kinase 4. Maeda et al. (p. 875) provide experimental data to further elucidate the circuitry and show that phosphodiesterase RegA is directly regulated by the phosphorylating activity of the MAP kinase ERK2. Simulations show that the circuit would be effective in producing oscillations over a sustained period.

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