This Week in Science

Science  18 Jun 1999:
Vol. 284, Issue 5422, pp. 1889
  1. Balancing Blood Vessels

    Solid tumors need a blood supply in order to grow. Conventional models suggest that most tumors initially grow as avascular masses that induce growth of new blood vessels once they reach about 1 millimeter in size. Holash et al. (p. 1994) show that, at least for some tumors, early events in tumor angiogenesis may be more complex. Experimentally induced tumors in rodents (C6 gliomas) initially grew by “coopting” preexisting blood vessels from the surrounding tissue. Surprisingly, these coopted vessels then regressed (which resulted in transient tumor regression, only to be replaced by newly made vessels that facilitated tumor growth. The expression patterns of anti-angiogenic angiopoietin-2 and pro-angiogenic vascular endothelial growth factor suggest that these proteins are involved in a critical balancing act that helps determine whether a tumor successfully establishes itself in the host.

  2. Maintaining Their Bounce

    A rubber ball game has been played in Mesoamerica since at least 1600 B.C. and ancient figurines as well as bands used to hold together tools, such as axes, were also made of rubber. Historical records noted the superior elastic properties of the Mesoamerican rubber, which gave the balls their bounce and other artifacts their formability. Hosler et al. (p. 1988; see the news story by Stokstad) studied the modern latex processing methods of workers in Escuintla, Mexico, and compared the mechanical and chemical properties of modern unprocessed and processed latex with ancient rubber (processed latex). After the sticky latex is extracted from the Castilla elastica tree, it is mixed with the juice from morning glory vine (Ipomoea alba). The addition of the morning glory vine gives the latex its enhanced elasticity and formability.

  3. Floating Electrons for Quantum Computers

    Quantum computers offer the possibility of solving highly complex problems that cannot be solved with classical computers. The development of these computers is hindered, however, by the lack of physical systems suitable for their implementation. Ideally, a system should consist of a large number (>106) of interacting particles (or qubits), whose states can be readily manipulated. Platzman and Dykman (p. 1967) propose a system that consists of two-dimensional arrays of electrons (>108 per square centimeter) floating on superfluid liquid helium and discuss how the interacting qubits could be manipulated and read out.

  4. Old Leaves in the CO2 Record

    Ice core records have shown that atmospheric carbon dioxide (CO2) concentrations increased abruptly during major deglaciations and decreased during glaciations; however, it has been difficult to obtain comparable records at lower latitudes. Wagner et al. (p. 1971) now present a detailed CO2 record from the beginning of the Holocene based on stomatal frequency signatures in fossil birch leaves preserved in a bog in the Netherlands. The data reveal that CO2 dropped even during a short-lived cooling event in the beginning of the Holocene known as the Preboreal Oscillation (∼11,000 years ago), and may indicate that early Holocene CO2 concentrations may have exceeded “preindustrial” values.

  5. Do Not Disturb

    Researchers have observed not only increasing annual average atmospheric carbon dioxide (CO2) concentrations but also an increased seasonal amplitude of CO2 concentrations at high latitudes. Explanations for this trend have been hard to verify. Zimov et al. (p. 1973) have monitored the seasonal amplitude of carbon exchange over disturbed and undisturbed sites in forest tundra in northeast Siberia and found that disturbed sites show a much higher CO2 amplitude than undisturbed ones. Increased disturbance, for example, through fire and grazing, which results in species replacements, may account for the observed seasonal amplitude effects.

  6. This Won't Hurt a Bit...

    It is normal to experience pain as well as the uneasiness associated with the expectation of painful sensations. Ploghaus et al. (p. 1979) have applied functional magnetic resonance imaging to study the parts of the brain that are involved in these feelings. They describe a clear dissociation between the anticipation of pain and the experience of pain itself. Discrete areas in the brain, such as in the prefrontal cortex, the insula, and the cerebellum, are activated by anticipation of pain, whereas discrete but nearby locations mediate the actual pain experience. These findings may help us to better understand and treat a number of clinical pain syndromes.

  7. The Buzz on Fly Flight

    How do flies fly? Although steady states have proven amenable to theoretic and analytic approaches both in aerodynamics and enzyme kinetics, the regime of non-steady state behavior is miserly with its secrets. Dickinson et al. (p. 1954; see the cover and the Perspective by Dudley) have built a large-scale model of the wings of Drosophila and analyzed the forces generated in flight. They confirm that “delayed stall” is responsible for producing lift during the translational portion of the wing stroke. At the end of each half-stroke, when the direction of the wing reverses, they find that two rotational mechanisms, “Magnus effect” and “wake capture,” operate synergistically to provide large lift transients.

  8. Anoxic Energy Pathways

    Oxidative phosphorylation uses the energy derived from the sequential transfer of electrons to pump protons across the mitochondrial inner membrane; this proton gradient is then used to synthesize adenosine triphosphate. Five integral membrane enzyme complexes participate in this process; the crystal structures of two of these (complexes III and IV) and much of a third (complex V) have been determined in recent years. Iverson et al. (p. 1961; see the Perspective by Hederstedt) present the 3.3 angstrom structure of fumarate reductase, an enzyme that is homologous in primary structure and in mechanism to succinate dehydrogenase (complex II) and that catalyzes the terminal electron-transfer step in anaerobic respiration. The four subunits of fumarate reductase bind six redox centers in an almost linear arrangement from the electron donor menaquinone to the flavin moiety that reduces the substrate. The two quinone binding sites resemble those found in the bacterial photosynthetic reaction centers but are instead located on opposite sides of the membrane in a fashion similar to those found in cytochrome bc1 (complex III).

  9. Delivering Liver

    In mammals, the liver differentiates from foregut endoderm after receiving signals from the adjacent cardiac mesoderm. Jung et al. (p. 1998) used cultured tissues from mouse embryos to evaluate the role of fibroblast growth factors (FGFs) in the initial specification of cells to become liver and their subsequent morphogenetic outgrowth. They found that FGF1 and FGF2 could induce expression of liver-specific genes, whereas FGF8 (but not FGF1 and FGF2) promoted outgrowth of the liver bud. Modified inactive FGF receptors were used to block signaling from the cardiac mesoderm. Thus, organ precursor cells can be cultured and their differentiation controlled by adding factors normally secreted by adjacent tissue in the embryo.

  10. Unsticking the bubblegum Mutant

    Accumulation of very long chain fatty acids (VLCFAs) in the brain results in a neurodegenerative disorder in humans called adrenoleukodystrophy (ALD). Min and Benzer (p. 1985; see the news story by Barinaga) describe a new Drosophila mutant that is characterized by dramatic dilation of photoreceptor axons (hence, the name bubblegum) caused by excessive accumulation of VLCFAs. Supplementing the diet of bubblegum flies with glyceryl trioleate oil (one of the components of “Lorenzo's oil,” which was originally used to treat ALD patients) reduced VLCFA levels and, when given at the larval stage, prevented VLCFA accumulation and neurodegeneration. Although the mutant proteins in ALD and bubblegum are involved in different steps of the VLCFA-oxidation pathway, bubblegum flies may prove a valuable tool for screening new drugs to treat this incurable disease.

  11. Tailoring Carbohydrate Synthesis

    Complex carbohydrates serve a great variety of functions in biological systems, from forming the structural foundations for plant cell walls to specifying blood group identity. One type, the hemicelluloses, interweaves with cellulose microfibrils to form the plant cell wall. The identity of the terminal sugar residue affects the biochemical properties of the hemicellulose and thus the physical characteristics of the cell wall. Perrin et al. (p. 1976) cloned from Arabidopsis a xyloglucan fucosyltransferase, the enzyme responsible for adding a terminal fucosyl residue on the principal hemicellulose of dicotyledonous plants. Identification of this enzyme may allow many of the other highly specific enzymes that direct carbohydrate synthesis in plants to be isolated and may eventually lead to directed manipulation of complex carbohydrate structures.

  12. Dealing with Natural Disasters

    The cost of natural disasters globally has continued to increase, and the United Nations declared the 1990s as an International Decade for Natural Disaster Reduction. The Decade witnessed several major disasters, including large earthquakes, volcanic eruptions, and floods. The Board on Natural Disasters (p. 1943) reviews some of the major efforts during the past decade, emphasizes the shift in response to mitigation, and highlights some of the efforts that remain. [See also the editorial by Press and Hamilton.]

  13. Ever Decreasing Circles in Magnetism

    The ever-increasing memory density requirements in the computer industry need advances in magnetic materials and a decrease in memory size. One key technology in this area is the formation of magnetic bubble domains. These are small magnetically recordable regions on the surface of a magnetic material that usually form only in the presence of an external magnetic field. Using a double-layered manganite structure, Fukumura et al. (p. 1969) show that magnetic bubble domains can form spontaneously without an external magnetic field. These results may provide a route for designing even smaller magnetic recording media.

  14. Flexing the Muscle

    Neurotrophic factors are known to potentiate synaptic neurotransmitter release. Boulanger and Poo (p. 1982) shed new light on the underlying intracellular mechanisms. In co-cultures of Xenopus motoneurons and muscle cells, the potentiating effect of brain-derived neurotrophic factor (BDNF) is gated by adenosine 3′,5′-monophosphate (cAMP); cAMP also potentiates the effects of low doses of BDNF on neurotransmitter release. These results provide a link between neurotrophins and cAMP gating that is thought to be involved in Hebbian learning in a large range of organisms.

  15. Distinguishing Dead Cells

    When cells die by apoptosis, the corpses must be cleared away. Do organisms use the same mechanisms to do this as they use to remove intruders such as bacteria? In vertebrates and in the fruit fly Drosophila, specialized cells called macrophages or hemocytes, respectively, engulf (phagocytose) debris. Franc et al. (p. 1991) found that Drosophila need to use the Croquemort receptor for clearing apoptotic cells, but not for clearance of bacteria. The amount of Croquemort was increased under conditions that stimulated apoptosis. Thus, the recognition of different types of debris needing clearance involves different receptor systems that are responsive to the relevant stimuli.

  16. Protein Clues to Spliceosome History

    Introns are removed from pre-messenger RNA by complexes called spliceosomes. The RNA components of the two spliceosomes found in humans, the minor and major spliceosomes, have been identified, but their protein constituents have been unknown. Will et al. (p. 2003) now show that several proteins are found in both the major and minor spliceosomes, but each contains several proteins not contained in the other. Through the similarities and differences noted in the two complexes, functional and evolutionary mechanisms are suggested.

  17. Wind and Climate on Mars

    M. P. Golombek (Perspectives, 5 March 1999, p. 1470) found that the data from several missions to Mars support the idea that Mars once had a “warmer and wetter environment with vastly different erosion rates” and that much of the landscape has “changed little from when it was created by catastrophic floods some 1.8 to 3.5 billion years ago.”

    C. B. Leovy comments that “Golombek's estimate of erosion rate, based on the assumption of constancy of the wind regime over several billion years, should be scaled upward by a factor of at least 105” and that “wind modification of the surface has probably been much more important, and modification by flowing water may have been correspondingly less important, than Golombek infers.”

    In response, Golombek agrees that there are “uncertainties in deriving quantitative erosion rates from landed surface views of a planet,” but maintains that significant wind-related erosion on Mars is “basically incompatible with the observed suite of erosional forms and the estimated rate of erosion for these ancient terrains.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/284/5422/1891a

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