This Week in Science

Science  24 Dec 1999:
Vol. 286, Issue 5449, pp. 2424
  1. Breaking Up Is Easier To Do

    Low-energy electron impact can ionize atoms and molecules. However, even for the simplest case, the ionization of a hydrogen atom, a complete solution to the problem has been lacking, and actual solutions to date have relied on final states in which one electron and the proton are treated as highly excited hydrogen atoms. Rescigno et al. (p. 2474; see the cover and the Perspective by Whelan) have transformed the Schrödinger equation with a scaling function so that numerical solutions to high accuracy can be calculated for the breakup of the system into three charge particles. Such an approach could also be used in more complex electron-impact systems.

  2. Rich Mud, Poor Mud

    Layers of organic-rich sediments, called sapropels, have been formed in the Mediterranean Sea at irregular intervals seven times during the last 200,000 years. These layers must have formed under dramatically different conditions than those that occur in the well-ventilated, nutrient-depleted environment that exists there today. Sachs and Repeta (p. 2485) measured nitrogen isotopic ratios in molecules derived from chlorophyll in eastern Mediterranean sapropels in order to infer what these conditions were. Their findings also demonstrate that some fundamental assumptions typically made about the preservation of nitrogen isotopic ratios in sediments are likely incorrect.

  3. Magnified Galaxy Gas

    Quasars are quasistellar radio sources that are believed to be powered by supermassive black holes residing within extremely distant galaxies. Planesas et al. (p. 2493) used the additional magnification of the gas around a galaxy pair in the quasar 0957+561 created by a gravitational lens to determine the spatial distribution and kinematics of the molecular gas. The main galaxy has an extensive molecular gas cloud that actually limits their ability to see the companion galaxy. The determination of the gas distribution around distant galaxies is useful in refining the value of Hubble's constant.

  4. Mantle Origins

    Biogenic processing of nitrates enriches Earth's crust in the heavier nitrogen isotope, 15N. Earth's upper mantle is enriched in lighter nitrogen, which is thought to be a signature of the planetesimals that accreted to form Earth. Dauphas and Marty (p. 2488) measured the nitrogen and argon isotopic composition of rocks from the Kola Peninsula, Russia, which are considered to be remnants of a 370-million-year old plume and representative of the lower mantle. The rocks had a nitrogen isotopic signature lighter than that of the crust but heavier than that of the upper mantle. These measurements, combined with noble gas isotopic data, suggest that nitrogen in the upper mantle is not primordial. Instead, Earth's early, more reduced atmosphere did not allow the formation of abundant nitrate, and this lighter nitrogen crust was subducted and mixed into the upper mantle and left a lower mantle enriched in heavy nitrogen.

  5. Molecular Plumbers' Units

    Figure 1

    Angiopoietin-1 (Ang1) and vascular endothelial growth factor (VEGF) each stimulate growth of blood vessels. Thurston et al. (p. 2511) compared the phenotypes of transgenic mice overexpressing one or both of these growth factors in the skin. The vessels induced by VEGF were leaky, whereas those induced by Ang1 were resistant to plasma leakage, even when the mice were exposed to an inflammatory agent. Mice overexpressing both factors had the greatest number of blood vessels, and these were also resistant to leakage. These results suggest that Ang1 may be useful for reducing microvascular leakage in diseases such as diabetic retinopathy, and that a combination of Ang1 and VEGF may be the optimal therapy in other disease settings, such as limb ischemia, that would benefit from new blood vessel growth.

  6. Feeling Less Pain

    Morphine is widely used clinically for the treatment of severe pain. Bohn et al.(p. 2495) show that this analgesic effect of morphine is enhanced in mice that lack the protein β-arrestin 2. Morphine produces its effects by binding to the μ opioid receptor, a heterotrimeric guanine nucleotide binding protein (G protein)-coupled receptor. However, the receptor has a desensitization mechanism through which it becomes phosphorylated and then interacts with the signaling inhibitor β-arrestin 2. There are actually four related arrestins, but animals lacking just β-arrestin 2 showed increased and prolonged pain-relieving effects of morphine. Thus, β-arrestin 2 appears to show specificity for the μ opioid receptor and to be required for its normal physiological response.

  7. Centromere Surprises

    Centromeres are the components of chromosomes responsible for their equal partitioning to daughter cells during mitosis and meiosis. Copenhaver et al. (p. 2468) exploited the advantages of a particular mutant in the plant Arabidopsis as well as complete sequence information for chromosomes II and IV to define DNA sequences responsible for centromere function. The centromeres consisted of a central, repetitive core, surrounded by moderately repetitive DNA that had a very low rate of recombination. Flanking the moderately repeated DNA were stretches of DNA with normal recombination rates and high proportions of mobile elements. Surprisingly, the centromere contained not only repetitive DNA but also sequences that expressed genes. Some of the repetitive elements previously thought to be “centromeric repeats” were more abundant in the DNA flanking the centromere than within the genetically defined centromere, suggesting that the repeats cannot be sufficient for function.

  8. Stabilizing Influence

    Many human tumors contain mutant forms of the tumor suppressor protein p53, and much research effort has been directed toward the development of compounds that restore p53 function. In a screen of a chemical library, Foster et al.(p. 2507; see the news story by Pennisi) identified small molecules that restore p53 activity by stabilizing the active conformation of its DNA binding domain. The compounds activated p53 target genes in cultured cells and inhibited tumor growth in mice. These results establish the feasibility of rescuing p53 function with a pharmacological agent.

  9. Faulty Checkpoints in Cancer

    The hCHK2 gene encodes the human homolog of a yeast kinase required for activation of the G2 checkpoint, a pathway that prevents cells containing damaged DNA from entering mitosis. Bell et al. (p. 2528; see the news story by Hagmann) identified germ line mutations in hCHK2 in a rare subset of families with Li-Fraumeni syndrome, a cancer predisposition syndrome typically associated with germ line mutations in the gene encoding the tumor suppressor p53. This unanticipated link between p53 and the well-defined G2 checkpoint in yeast highlights the importance of cell cycle checkpoints in tumorigenesis and may lead to the development of more selective anticancer drugs.

  10. Sex, Aging, and Death

    Reproductive activity is known to decrease lifespan in animals and plants. In an experimental study with fruit flies, Sgrò and Partridge (p. 2521; see the Perspective by Reznick and Ghalambor) show that these “costs of reproduction” consist of a wave of mortality that is delayed until the onset of aging, and that these delays underlie the evolution of aging. The results support the idea that aging evolves through pleiotropic genes that have beneficial effects at young ages and detrimental effects later in life, rather than by an accumulation of deleterious mutations.

  11. Life with Father

    Solely maternal inheritance of mitochondrial DNA (mtDNA) is one of the major dogmas of human evolutionary genetics and has been a key element in the dating of certain events such as the spread of humans into Asia and Europe. Awadalla et al. (p. 2524; see the news story by Strauss) now provide convincing evidence that mtDNA is also inherited paternally. Results from their statistical analyses of mtDNA data from humans and chimpanzees are consistent with the occurrence of recombination. Inferences about the pattern and tempo of human evolution may now have to be reconsidered.

  12. Driven to Distraction

    Does the brain process words when we are looking at them even if we are not paying attention? Rees et al.(p. 2504) resolve this age-old issue by functionally imaging brain activity while asking human subjects to focus on either words or pictures that had been superimposed. By ratcheting up the difficulty of the picture task, they find that words and non-words (consonant strings) produce exactly the same brain activations. Words were not recognized when all of the attentional resources were diverted to a different task.

  13. Holding onto Zinc

    Amyotrophic lateral sclerosis (ALS), also called Lou Gehrig disease, is an incurable neurodegenerative condition. Its cause in most cases is not known, but 2% of ALS patients carry mutations in Cu,Zn superoxide dismutase (SOD), an enzyme that scavenges the superoxide free radical. Estévez et al. (p. 2498) now report that mutant SOD, which is unable to bind zinc (but still binds copper), induces cultured motor neurons to undergo apoptosis. If the wild-type SOD was forced to give up its zinc, it also caused motor neurons to die. When both wild-type and mutant SOD were replete with zinc, then both SODs protected motor neurons from apoptosis upon removal of nurturing growth factors. The authors propose that loss of zinc from SOD induces motor neuron apoptosis through an oxidative mechanism that produces nitric oxide.

  14. Quantum Critical Points in Magnetic Systems

    Understanding phase transitions in highly correlated electron systems is an important problem in condensed matter physics. A particularly interesting region of the phase diagram to study is the quantum-critical point, where characteristic quantum-mechanical energies and thermal energies contribute interchangably to the evolution of the system. Sachdev et al. (p. 2479) studied the effect of introducing a single nonmagnetic impurity into a nearly quantum-critical two-dimensional antiferromagnet and looked at the effect the impurity would have on experimentally measurable quantities—in this case, the magnetic susceptibility, a measure of the ease of ordering the magnetic spins. They show how the magnetic properties of the system evolve from the paramagnetic (no order) ground state to antiferromagnetic (opposite signs) near the quantum-critical point and how the Néel state can be characterized by a universal effective spin.

  15. Slowed Down by Solvents

    Aqueous ions must often traverse organic interfaces, such as biological membranes or in “phase transfer” catalysis where reactions occur at water-organic interfaces. Studies of the motion of ions across real aqueous-organic interfaces have been hampered by difficulties in probing such complex systems. Wu et al. (p. 2482) have created a model interface by landing hydronium ions (D3O+) on an solid organic glass formed on a metal surface. Varying amounts of water were then coadsorbed with the ions; upon warming, a voltage change was seen as the ions migrated through the organic layer toward the metal. The presence of a water solvent shell lowered ion mobility across the interface compared to the bare ions, and these changes in mobility could be accounted for theoretically.

  16. Looking Out Below

    The Tonga-Kermadec island arc in the southwest Pacific Ocean was formed by subduction of Pacific oceanic crust beneath the Australian plate. The subduction produces melts by either releasing fluids that lower the melting point of the rock above the subducting plate or by decompression melting. Bourdon et al. (p. 2491) measured the protactinium and uranium isotopic concentrations in basalts and determined that the Tonga segment of the arc was formed by fluid-enhanced melting, whereas the Kermadec segment of the arc was formed by decompression melting. These results improve our understanding of the subduction process and provide a useful chemical tracer for melting processes.

  17. Channeling For Behaviors

    The causal relation between the mutation in a gene and its effect on the behavior of an organism is often difficult to establish. Davis et al. (p. 2501) describe a potassium channel encoded by the Caenorhabditis elegans exp-2 gene that controls the action potentials in the nematode's pharnyx muscles, which connect the mouth to the gut. Analysis of wild-type and mutant channels demonstrates that the changes in the electrophysiological characteristics of the channels determine the observed changes in animals' feeding behavior.

  18. Tied into Protein Degradation

    The list of key signaling events that are regulated by controlled ubiquitin-dependent degradation of proteins continues to grow. Brondello et al. (p. 2514) report that MAP kinase phosphatase-1 (MKP-1), which dephosphorylates and inactivates the p42 and p44 mitogen-activated protein (MAP) kinases, is degraded by the proteasome. The phosphatase itself is a target of the p42MAPK and p44MAPK enzymes that it inactivates. This phosphorylation of MKP-1 appears not to regulate its enzymatic activity, but rather to decrease its ubiquitin-dependent degradation. The MAP kinases also increase transcription of the gene encoding MKP-1, thus providing two signals that increase the abundance of MKP-1. Such regulation would appear to limit prolonged activation of the p42 and p44 MAP kinases.

  19. Light Motif (I)

    Phytochromes trigger light photoreceptors to regulate growth and development of photosynthetic organisms such as plants, algae, and cyanobacteria. However, phytochromes are not restricted to photosynthetic organisms—Davis et al.(p. 2517) searched genomic databases and found homologs to phytochrome receptors in the bacteria Deinococcus radiodurans and Pseudomonas aeruginosa. As in the photosynthetic systems, the Deinococcus phytoreceptor-like protein covalently attaches chromophores, albeit by a different linkage group. Because photosynthesis is not performed in Deinococcus, the bacterial phytochrome-like proteins may represent light-regulated histidine kinases that serve to protect the microbe from damaging intense visible light.

  20. Humans See, Humans Do

    An important ability in learning is imitation. Neurophysiological studies of monkeys have identified neurons that are active during arm movements whether self-executed or viewed, with greater activity during viewing of that movement. These neurons have been proposed to provide the neural basis for acquisition through imitation. Iacoboni et al. (p. 2526) now conduct functional imaging studies of humans to locate analogous brain areas with similar characteristics in a task in which the subject looks at a finger movement and then replicates the same movement.

  21. Light Motif (II)

    The circadian clock keeps organisms functioning in tune with their environment, but it is a flexible relationship. In insects, light can reset the clock by acting directly on a light-sensitive cog in the mechanism, the cryptochrome protein. Cryptochromes isolated from vertebrates do not seem to react this way, however, and now Okamura et al. (p. 2531) have confirmed in knockout mice that lack both types of cryptochromes that these proteins are not necessary for proper light-induced resetting. Previous work has excluded rhodopsin as the light detector for resetting, which leaves the identity of the vertebrate circadian light receptor open to future investigation. (See the Perspective by Hardin and Glossop.)

  22. Population Cycles and Parasitism

    Hudson et al. (Reports, 18 Dec. 1998, p. 2256) presented experiments showing that interactions between red grouse and a parasite were responsible for periodic crashes in the grouse population and thus implicated wildlife diseases in explaining population fluctuations. Lambin et al. question whether the control experiment used by Hudson et al. in which a population was treated to inhibit the parasite was sufficient and unbiased and argue that the inferred “change in fluctuation pattern is equivocal.” Hudson et al. respond that their approach does not confound the interpretation of the experiments and clarify that the “decrease in vicariance only occurred when grouse densities were relatively high….” The full text of this comment can be seen at www.sciencemag.org/cgi/content/full/286/5449/2425a

Navigate This Article