This Week in Science

Science  06 Oct 2000:
Vol. 290, Issue 5489, pp. 9
  1. Planets Without Orbits?

    About 47 extrasolar planets have been detected orbiting around stars, which indicates that planets may be relatively common in the universe. Zapatero Osorio et al. (p. 103; see the news story by Irion) have used optical and near-infrared imaging and spectroscopy to image and determine the temperature of several young, low-mass objects (5 to 15 Jupiter masses and 1 to 5 million years old) in the nearby stellar cluster around σ Orionis. Because these objects are very cool (1700 to 2200 kelvin), they cannot sustain nuclear burning, which suggests that they may be planets. If they are planets, however, they are not orbiting any star. Such isolated planets would add a new challenge to planet formation models and the time scales over which planets may form.

  2. Out of Order Comes Chaos

    Although the theory of nuclear magnetic resonance (NMR) spectroscopy makes a few critical assumptions, it is highly reliable and allows complex sequences of radio-frequency (RF) pulses to be used to produce predictable changes in magnetization. Lin et al. (p. 118) show that in aqueous solution, the milieu of many protein NMR experiments, two effects, radiation damping and the dipolar field, that are normally treated separately can actually combine even after very simple RF pulses to create chaotic spin dynamics. Magnetization that should be eliminated by a pulse sequence can reappear and can be amplified by slight temperature gradients. The authors discuss how these effects can be avoided and even put to use in imaging.

  3. Mesoporous Patterning

    Most patterning of inorganic thin films involves the selective formation of the film in certain areas with other regions left empty. Doshi et al. (p. 107) have used in situ photogeneration of acid and photomasking to pattern different silica mesoporous phases within a continuous film. The exposed areas can form a more densely packed hexagonal phase than the unexposed regions, and continuous “gray-scale” patterning was achieved by varying the incident flux. If greater amounts of surfactant were used, the exposed areas formed a less dense tetragonal phase. Because these regions have different densities and thus different refractive indices, such films can be used for optical waveguides and gratings.

  4. Vibrationally Excited Electron Transfer

    Most electron transfer reactions of interest occur in solution or condensed phases, but fundamental studies are difficult to unravel if there are competing solvent effects. Huang et al. (p. 111) looked at an intermediate case of vibrationally excited NO molecules scattering off of a single-crystal gold surface. The dipole of the NO molecule interacts with its image dipole in the metal, which mimics the effect solvent screening. What they find is a dramatic increase in electron transfer for high vibrational states that arise from more favorable crossings of the potential energy barrier. No such effect was seen for the same experiment performed with an insulating LiF surface.

  5. Thermally Enhanced Electron Transfer

    Electron transfer processes in proteins appear to require very precise structural arrangements, and thermal motions might be expected to decrease reaction rates. Balabin and Onuchic (p. 114; see the Perspective by Schulten) show that rather than being disturbed by them, proteins can use thermal motions to their advantage. Electrons can explore a web of pathways, some of which may occur in protein conformations far from equilibrium. Because of the quantum nature of electron motion, constructive and destructive interference effects arise between pathways that may dynamically amplify electron transfer.

  6. When Noble Gas Meets Noble Metal

    Noble gases have completely filled valence electron shells and are therefore quite unreactive. Nevertheless, a variety of compounds with covalent noble gas bonds have been made, although most of these compounds cannot be isolated. Seidel and Seppelt (p. 117; see the Perspective by Pyykkö) now show that even xenon can go for the gold—in this case, by combining with a square planar gold complex that could be crystallized and that was stable up to −40°C. In solution, the compound was even stable at room temperature under a xenon pressure of 10 bars.

  7. Neuropathic Pain Killer

    Peripheral nerve damage can lead to neuropathic pain, which is intense, persistent, and difficult to treat with presently available drugs. Boucher et al. (p. 124) present evidence that GDNF, but not other neurotrophic factors, can prevent the development of neuropathic pain and could also help revert already established neuropathic pain states. The effects of GDNF are caused by a reduction of spontaneous activity in myelinated sensory afferents. The underlying mechanism is a readjustment of voltage-gated sodium channel expression in dorsal root ganglion neurons.

  8. Having the Nerve to Go On

    In the nematode Caenorhabditis elegans, mutations that disrupt the signaling pathway of the insulin-like receptor daf-2 dramatically extend the animals' life-span and cause the accumulation of large amounts of fat. By selectively expressing normal versions of the mutated insulin-like receptors only in certain tissues, Wolkow et al. (p. 147) pinpoint the nervous system as responsible for this pathway's effect on life-span and the muscles as the site that controls the metabolic alterations. The authors suggest that defects in the daf-2 pathway allow overexpression of free-radical scavenging enzymes, which protects neurons from oxidative damage and allows them to secrete life-prolonging neuroendocrine signals.

  9. A Sense of Space

    T cells have acquired the ability to respond to the physical space in which they exist, as can be seen when they divide to fill lymphoid organs that are empty or contain only a few lymphocytes. Such homeostatic expansion is held in check when the space is fully occupied, and in this case, naïve T cells survive in a steady state. Seddon et al. (p. 127) examined the requirement of p56lck, a tyrosine kinase critically associated with T cell receptor signaling and T cell development. By switching off p56lck in mature naïve T cells, they made the surprising observation that although this signaling molecule was necessary for homeostatic expansion, it was not required for T cell survival.

  10. Cancer: The Proximity Effect

    Chromosomal rearrangements between the RET gene and other distant loci are common in radiation-induced thyroid tumors and has been seen in children exposed to environmental radiation after the Chernobyl incident. What mediates these reciprocal and illegitimate rearrangements between sequences that are often very far apart in the linear DNA sequence? Nikiforova et al. (p. 138; see the Perspective by Savage) show that the RET gene and the H4 gene, with which it often recombines in these tumors, are in close physical association in the nuclei of human thyroid cells. The formation of this chimeric gene product is known to cause cancer in mice.

  11. Determining Fat Cell Fate

    Adipose tissue serves as the body's site for energy storage and expenditure. A fair amount is known about the gene functioning in adipocyte differentiation; however, we are lacking knowledge of the players involved in early adipogenesis. Tong et al. (p. 134) used a fruit fly model system to identify two vertebrate GATA factors that, like their homologs in Drosophila, function in adipose tissue. The proteins GATA-2 and GATA-3 hold differentiating cells in the preadipocyte stage and hence regulate the preadipocyte to adipocyte transition. Several mouse models of obesity display a reduction in adipose expression of GATA-2 and GATA-3. Because GATA factors function in adipose tissue in both the fly and mouse, these proteins may serve as appropriate targets for obesity studies and therapy.

  12. Making an Unkind Cut

    In Alzheimer's disease, the accumulation of b-amyloid peptide in the brain results from the cleavage of its precursor protein by the membrane-associated aspartic protease memapsin 2 (b-secretase). Hong et al. (p. 150) have determined the crystal structure of the protease domain of memapsin 2 complexed with an inhibitor at a resolution of 1.9 angstroms. Although the hydrogen bonds involving the inhibitor backbone resemble those of other aspartic proteases, contacts with inhibitor side chains are different, and the inhibitor backbone has an unusual bent structure. These features may facilitate rational design of drugs that specifically inhibit memapsin 2.

  13. Targeting Anxiety

    Benzodiazepines are widely used drugs that enhance inhibitory GABAergic neurotransmission in the central nervous system. This pharmacological profile causes both anxiolytic and sedative effects. In an attempt to better understand the enormous heterogeneity of GABAA receptors Löw et al. (p. 131; see the news story by Helmuth) selectively tried to silence specific receptor subunits. They discovered that the a2 subunit of the GABAA receptor mediates the anxiolytic action of benzodiazepines. This finding could lead to the development of new drugs for the treatment of anxiety without the side effects of sedation or motor impairment.

  14. Peering Inside a Quantum Dot

    The ability to tune the optical and electronic properties of quantum dots by controlling their size offers the potential for various optoelectronic applications on the nanoscale, but such applications will require precise knowledge of the dot's electronic structure. Vdovin et al. (p. 122) describe how magnetotunneling spectroscopy can be used to produce spatial maps of the wave functions of the ground and excited electron states confined within the self-assembled quantum dot.

  15. Attack and Counterattack

    Post-transcriptional gene silencing, or RNA “interference,” may have evolved in plants as a defense against viral attack. Not surprisingly, the viruses may have evolved counter-moves. Anandalakshmi et al. (p. 142) investigated the protein interactions that regulate gene silencing. HC-Pro, encoded by plant potyviruses, interacts with a calmodulin-related plant protein that seems to repress gene silencing.

  16. Growth Factor Receptors and STATs

    The STAT (signal transducers and activators of transcription) transcription factors are activated in response to cytokine receptors and associated kinases. The STATs then migrate to the nucleus where they regulate transcription of specific genes. Stimulation of growth factor receptors also activates STATs, but the mechanism by which this occurs is not well understood. Simon et al. (p. 144) show that STAT3 is activated in response to the small guanine nucleotide-binding protein Rac1. In fact, direct binding interaction of activated (but not inactive) Rac1 with STAT3 appears to contribute to STAT activation. The studies indicate that Rac functions in growth factor-induced activation of STAT3 in two ways. It apparently helps localize STAT3 to kinase complexes at the cell surface and also promotes activation of kinases, like Jak2, that phosphorylate STAT3. The results help clarify control of STAT3, which along with Rac (through cooperation with Ras) can contribute to cell transformation and cancer.

  17. Assessing the Mechanisms That Give Rise to Autoimmunity

    Kouskoff et al. (Reports, 31 March, p. 2501), using a transgenic (Tg) mouse model, showed that challenge with a self-mimicking foreign antigen could break B cell self-tolerance in a manner independent of T cell help, and thereby identified “a potentially important mechanism” for autoimmune reactions. Zinkernagel, in a comment, draws attention to previous work from his lab, not cited by Kouskoff et al., that concluded that against “highly repetitive, identical polymeric determinants, B cells are not tolerant and react in a TI-1 [T-independent type 1] fashion without an obvious polyclonal activator,” whereas “against monomeric antigens, B cells strictly and exclusively react in a T cell-dependent and linked fashion.” Zinkernagel concludes that “instead of the still unproven mimicry hypothesis,” these results suggest that “antigen patterns plus absence or presence of T cell help play the principal roles in regulating B cell responsiveness.”

    Nemazee et al. express regret at their failure to cite this previous work, but nonetheless argue that Zinkernagel's contention that immune tolerance does not occur at the B cell level “cannot be generalized without denying a large body of direct evidence to the contrary.” And, while recognizing the limitations of antibody Tg mice, Nemazee et al. stress that such experiments also allow determinations that “are difficult or impossible to make in a polyclonal model.” Thus, they conclude, “our results provide novel mechanistic insight that complements other types of studies.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/290/5489/11a

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