This Week in Science

Science  18 Dec 1998:
Vol. 282, Issue 5397, pp. 2149
  1. G Whiz

    Although it has been about 200 years since Cavendish first determined the gravitational constant G, most experiments have used a similar technique: A torsion balance measurement of the restoring force in a fiber between two large masses. In recent experiments by various groups, the systematic errors inherent in these measurements have resulted in scatter in the value for G varying by up to 40 times their individual errors. Schwarz et al. (p. 2230; see the news story by Kastenbaum) have been able to reduce or remove these systematic errors by using a different experimental setup: Laser interferometry is used to track a free-falling body under the influence of a nearby mass.

  2. Warm at the Top

    The climate during the Late Cretaceous was anomalously warm at times; dinosaurs could be found in Antarctica, and the Arctic region contained abundant plants (see the Perspective by Huber). Tarduno et al. (p. 2241) describe a vertebrate fossil assemblage from Axel Heiberg Island in the high Canadian Arctic. This assemblage includes fish, turtles, and champsosaurs, a large, nonmigratory crocodile-like predator. The assemblage implies that polar temperatures were high (perhaps exceeding 14° Celsius on average) about 92 to 86 million years ago, a period that followed the occurrence of extensive global basaltic volcanism.

  3. By Land as Well as by Sea

    A series of abrupt climate events punctuated the transition from the ice age to the Holocene climate in the North Atlantic region, including cooling in the Younger Dryas and Preboreal Oscillation. It has been difficult to recognize this sequence elsewhere, particularly in terrestrial records, and thus the global extent of these events has been uncertain. Yu and Eicher (p. 2235) now report evidence for this sequence in lake sediments in Ontario, Canada, further within the interior of North America.

  4. Round and Round

    Chiral molecules are characterized by an optical rotation angle, the angle by which plane-polarized light is rotated when it passes through a solution of chiral molecules. Exactly how this angle arises from the molecular conformation and chemical bonding in the molecule has been difficult to identify. Kondru et al. (p. 2247) have developed a quantitative method for computing individual atomic contributions to the optical rotation angle. The method provides insights into how molecular geometry, substituents, and chemical bonding can affect the optical rotation angle.

  5. Inside Scoop on Iron

    As an essential constituent of many enzymes, iron is avidly sought and sequestered by microorganisms. Siderophores are small secreted molecules that chelate iron in a bioavailable form. Ferguson et al. (p. 2215; see the Perspective by Braun) present the crystal structure at 2.5 angstroms of the integral membrane protein FhuA from Escherichia coli. This protein mediates the energy-dependent transport of the iron-containing siderophore across the outer membrane. FhuA exhibits a remarkable cork-in-a-bottleneck conformation, and the structure of the protein in complex with its substrate reveals how subtle deformations of the cork translocate the iron chelate from the extracellular medium into the periplasmic space.

  6. Stretch and Change the Channel

    The energy for regulating the downhill flow of inorganic ions across the cellular membrane is usually obtained chemically (opening an ion channel in response to binding of a neurotransmitter) or electrically (opening in response to a change in the transmembrane potential). Chang et al. (p. 2220) present the crystal structure of an ion channel that is regulated mechanically by lateral stretching of the cellular membrane. In the closed state, the channel consists of 10 helices, contributed as pairs from five identical subunits. A funnel-shaped pore is closed off near the cytoplasmic surface, and the suggested mechanism of energy transduction involves an outward displacement of the inner helix of each pair.

  7. One Way Trip

    Auxins, hormones that regulate many developmental processes in plants, are transported from the tip of the plant to other tissues. Continuing on hypotheses first proposed by Darwin, researchers have long been in search of the molecules responsible for the directed transport. Gälweiler et al. (p. 2226; see the Perspective by Jones) have now cloned the AtPIN1 gene from Arabidopsis. Mutants defective in PIN1 are deficient in auxin transport. The protein product, whose predicted sequence shows similarity to bacterial carrier proteins, is located at the basal ends of cells involved in auxin transport, thus providing insight into the molecular basis of unidirectional auxin transport.

  8. The Worm Gets the Bird

    Population cycles are a major theme in ecology, but definitive identification of their causes remains problematic. Now Hudson et al. (p. 2256) describe the interaction behind one celebrated cycle, namely, the regular crashes in numbers of red grouse in the north of England. Experimental reduction in the burden of a nematode worm in the birds can repeatedly prevent the decline in numbers, demonstrating that a single trophic interaction can induce regular fluctuations.

  9. Sleeping to Remember

    Can't quite name that catchy tune you heard? A good night's sleep may help you remember. Dave et al. (p. 2250; see the news story by Barinaga) recorded neuronal activity in the nucleus robustus archistriatalis (RA) of zebra finches during waking and sleeping periods. The RA area lies at the beginning of the motor pathway that supports singing. The RA neurons responded more strongly to the broadcast of a bird song when the bird was asleep or anesthetized than awake. This enhanced responsiveness was reduced by infusion of the neuromodulator norepinephrine into the brain area that provide input to the RA and suggests a mechanism by which auditory information might be consolidated as a function of wake-sleep state.

  10. Nutrient Micropatches in the Sea

    Aquatic microbial ecosystems are generally assumed to function within a homogeneous environment in which nutrient dispersal is virtually instantaneous. Blackburn et al. (p. 2254) show that this is not so: Patches of nutrients, related to the release of fresh dissolved organic matter, form through the clustering of chemotactic bacteria. Simulations reveal that chemotaxis confers a significant growth advantage to bacteria, thus explaining why the phenotype is present in this environment.

  11. Asthma via Interleukin-13

    Allergic asthma is becoming more prevalent—it is estimated that 15 million people in the United States alone have the condition. Animal models have provided insights into the immunologic elements that set up the condition, such as a type 2 T helper cell response and the production of interleukin-4 (IL-4) and IL-5. However, it is not clear what causes the acute reaction—airway hyperresponsiveness and severe mucus secretion, the symptoms that can lead to asphyxiation. Grünig et al. (p. 2261) and Wills-Karp et al. (p. 2258) have found that, in the mouse model of asthma, cytokine IL-13, working through activation of the α subunit of the IL-4 receptor, induces those symptoms; blocking IL-13 also blocked the symptoms. This work suggests that reagents that inhibit IL-13 action have potential therapeutic benefits (see the news story by Vogel).

  12. T Cells On

    T cells become activated after they meet their antigen, but the most efficient and persistent signaling requires costimulation of a T cell needs through another receptor, such as CD28. Wülfing and Davis (p. 2266) determined that only when the T cell antigen receptor (TCR) of T cells binds antigen while simultaneously receiving a costimulatory signal, do other proteins that are linked to the actin cytoskeleton move to the TCR complex. This costimulation-dependent cellular reorganization relies upon myosin motors and may be a key component of the costimulatory effect.

  13. T Cells Off

    T cells not only can be activated, but also must be turned off or the organism runs the risk of rampant autoimmunity, as is the case in mice genetically deficient in the protein CTLA-4. Lee et al. (p. 2263) determined the stage of the activation process that is affected by CTLA-4. CTLA-4 complexes with a phosphatase, SHP-2, and was found to bind to and dephosphorylate the ξ chain of the T cell antigen receptor. This direct action at the “source” of the activation signal implies that the TCR signal may be thwarted before the cell is committed to proliferate.

  14. Cocaine and CREB

    Psychoactive drug use can cause permanent molecular adaptations of neurons in the brain. Carlezon et al. (p. 2272) investigated a sequence of events in cells of the nucleus accumbens after exposure to cocaine. Chronic cocaine is known to increase adenosine 3′,5′-monophosphate (cAMP) formation and subsequently increase activity of cAMP-dependent protein kinase A (PKA). These events in turn increase phosphorylation of the transcription factor CREB (cAMP responsive binding element protein). The authors used virus-induced transient overexpression of normal CREB and a defective mutant to show that CREB effects the expression of dynorphin. Dynorphin is a well-known endogenous ligand for κ-opioid receptors that are involved in the valence (reward versus aversion) of cocaine action.

  15. Carbon on a Cosmic Scale

    The carbon atoms that make up organic matter on Earth was originally synthesized in interiors of stars. Henning and Salama (p. 2204) review the current understanding of the distribution of carbon in interstellar space, which encompasses species ranging from atomic carbon and molecular forms to larger solid particles.

  16. Sea Ice Sieves

    The properties of sea ice change dramatically with temperature. It abruptly becomes much more porous at temperatures above about −5° Celsius in contact with a typical brine; this change allows colonization by bacteria and transfer of fluid that carries heat and nutrients through the ice. Golden et al. (p. 2238) used percolation theory to explain this transition and resolve results from several experimental studies.

  17. Made on Many Scales

    Silicon, niobium, and titanium oxides with nanometer-scale pores have been patterned on a micrometer scale to create hierarchically ordered materials. Yang et al. (p. 2244) used rubber micromolds to create surface patterns the sol-gel precursors, which could contain triblock copolymers or polystyrene spheres that directed pore formation.

  18. Source of Resistance

    A subset of human tumors are resistant to the growth inhibitory effects of transforming growth factor-β (TGF-β). An understanding of the molecular mechanisms underlying this resistance may provide new insights into tumorigenesis. In a study of cultured cells, Sun et al. (p. 2270) found that TGF-β resistance can be conferred by overexpression of MDM2, an oncoprotein shown previously to interact with the p53 tumor suppressor. MDM2 appears to mediate resistance by interfering with signaling by the Rb/E2F transcription factor complex.

  19. Connecting cAMP and Ras Superfamily Signaling

    Activation of many receptors on the cell surface results in generation of the “second messenger” molecule cAMP (adenosine 3′-5′ monophosphate). Most of the biological effects of cAMP result from activation of the cAMP-dependent protein kinase. Kawasaki et al. (p. 2275) found evidence for another mechanism by which signals may be generated in response to cAMP. They identified a family of guanine nucleotide exchange proteins (cAMP-GEFs) that are particularly abundant in brain and that bind cAMP. Binding of cAMP activated the exchange activity of the cAMP-GEFs and resulted in selective activation of Rap1A, a member of the Ras superfamily of guanine nucleotide binding proteins. The cAMP-GEFS appear to connect signaling pathways that generate cAMP to those mediated by members of the Ras superfamily.

  20. The Hippocampus and Human Navigation

    E. A. Maguire et al. (Reports, 8 May, p. 921) investigated “the neural basis of navigation by humans … with functional neuroimaging of brain activity during navigation in a familiar, yet complex virtual reality town.” One finding was that accurate navigation was associated with activation of the right hippocampus.

    I. Fried comments that such activation is “primarily outside the hippocampus” according to the figures and stereotactic coordinates provided in the report. He notes that other recent studies have found such activity to be associated with the para hippocampus.

    In response, Maguire discusses the brain coordinate system and template used in the report and provides a figure showing “the location of the peaks of activity for the two relevant activations,” which falls within “the subicular regions of the hippocampal formation.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/282/5397/2151a

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