This Week in Science

Science  25 Apr 2003:
Vol. 300, Issue 5619, pp. 545
  1. A Molecular Record of Cannibalism

    A common human prion protein polymorphism at residue 129 offers a high degree of protection against induced and sporadic Creutzfeldt-Jakob disease. Kuru, a similar prion disease that devastated particular New Guinean tribes, was transmitted when relatives consumed the brains of dead relatives during death rituals. The kuru epidemic had a profound selection effect on the residue 129 polymorphism in the affected area of Papua New Guinea. Mead et al. (p. 640; see the 11 April news story by Pennisi) present an analysis of worldwide prion protein gene haplotype diversity and allele frequency of coding and noncoding polymorphisms and found that balancing selection at this locus is much older and geographically widespread. It is clear from classical and molecular anthropological evidence that cannibalism, a practice now viewed as taboo across all cultures, was widespread in human prehistory. Cannibalistic recycling of naturally occurring prions is thought to have led to the epidemics of BSE (bovine spongiform encephalopathy) in animals and to kuru itself.

  2. A Date with Prehistory

    Several major hominid fossils have been recovered from caves, notably in South Africa. Although caves have proved to be the main source of high-quality fossils, these locations pose problems for obtaining accurate dates on the fossils. Most of the deposits lack volcanic layers or have a clear contiguous stratigraphy. Partridge et al. (p. 607; see the news story by Gibbons) have used cosmogenic nuclides to date the burial of several recently discovered hominid fossils from caves in South Africa. The fossils, including the skeleton StW 573, date to about 4 million years ago, and an initial analysis implies that they are Australopithecus species. Along with similar hominids from East Africa, these fossils would be some of the earliest found.

  3. X-ray Snapshots of a Ferroelectric Transition

    Ultrafast x-ray pulses can be used to follow rapid structural transitions in single crystals. Collet et al. (p. 612; see the Perspective by Siders and Cavalleri) have now used pulsed x-ray diffraction to follow a paraelectric-to-ferroelectric in an organic crystal that undergoes internal charge transfer and subsequent structural changes after electronic excitation. They followed the ferroelectric transition, which occurs on the 100-picosecond time scale, initiated by an ultrashort laser pulse at a wavelength of 800 nanometers. The metastable ferroelectric phase forms through the cooperative action of a series of donor-acceptor pairs.

  4. Target Practices

    One system under consideration for drug delivery is micelles made from diblock copolymers. Using triple-labeling confocal microscopy, Savic et al. (p. 615; see the Perspective by Hubbell) found that dye-labeled poly(caprolactone)-poly(propylene oxide) targeted cellular organelles such as the Golgi and mitochondria but not the nucleus. They propose a model of how the uptake of the micelles occurs.

  5. Glasses to Glasses

    Glass formation can proceed through different mechanisms. In the regime of high temperatures and concentrations, the typical “repulsive glass” can form, whereas at low temperatures it is possible to form an “attractive glass” where cluster formation interferes with the motion of the constituent particles. When the particles have both repulsive and attractive interactions, two glassy states can form that have different dynamics. Chen et al.(p. 619) used small-angle neutron scattering and photon correlation methods to explore the transition in a dense copolymer micelle system where one glassy state converts to the other. They map out the phase diagram for this system and identify the critical point where the local structures of the two different glasses are identical.

  6. Dipeptide Nanotube Templates

    A number of peptides have been shown to self-assemble into nanotubes in solution. In a study of the ability of very short aromatic peptides to form amyloid fibrils, Reches and Gazit (p. 625) observed the formation of nanotubes from a simple diphenylalanine unit. The nanotubes have very high aspect ratios and were also very stiff. The nanotubes could be filled with silver to produce silver nanowires after enzymatic degradation of the peptide backbone.

  7. Icy Titan Surface

    Saturn's largest moon, Titan, is shrouded in a thick atmosphere of nitrogen, methane, and carbon monoxide that obscures its surface features. The surface has been inferred to be icy and to perhaps be covered with layers of organic sediments raining out from its enigmatic atmosphere. Griffith et al. (p. 628) combined multiple spectroscopic observations to derive a spectrum of Titan's surface. The spectrum indicates large exposures of water ice on the surface, which will help to refine models of atmospheric processes and climate change on this unusual satellite.

  8. Hummingbird Floral Arrangements

    In the island of St. Lucia, a bizarre sexual dimorphism occurs in the purple-throated carib hummingbird. The larger male has a short, straight bill and the female has a long curved bill. These characteristics are matched by a conditional floral polymorphism in the host plant Heliconia bihai, whereby one morph is preferred by each gender. A related species, Heliconia caribaea, supports only female hummingbirds. Temeles and Kress (p. 630; see the cover and the Perspective by Altshuler and Clark) investigated the relations between these plants and pollinators in Dominica, two islands north along the chain. There, the same species are present, but the pattern is exactly reversed: Dimorphism occurs in H. caribaea, supporting both hummingbird genders at lower elevations, whereas H. bihai supports only male birds at higher elevations.

  9. Missteps in the First Meiotic Metaphase

    Insights into human fertility and infertility can be gained by better understanding the events and factors involved in cell cycle division of model systems. Spruck et al. (p. 647) examine the role of the murine Cks2, a factor that associates with the mitotic cell cycle regulatory complex containing cyclin B and Cdk1. Targeted disruption of Cks2 yields mice that are viable but sterile in both sexes because of a block in gamete formation at meiosis I. Cks2 functions at the metaphase/anaphase transition of meiosis I, where homologous chromosomes segregate and where most cases occur of human aneuploidy (extra or missing chromosomes).

  10. Interrupting Early Oocyte Development

    The genetic material in vertebrate germ cells is highly condensed, so mechanisms must be in place to remodel and decondense the DNA of male and female gametes so that they can form the diploid genome. Burns et al. (p. 633) now show that the phenotype of the nucleoplasmin 2 (Npm2) null mutant mouse is not what would be expected from prior in vitro studies conducted with Xenopus NPM2. Whereas NPM2 decondenses sperm DNA in frog extract, when Npm2 is eliminated in mice, the resultant males appear normal and show normal fertility, but females are subfertile or infertile. This reduced fertility is caused by embryo loss as a result of defects in the nuclear and nucleolar organization of oocytes and mitotic arrest in early embryos. NPM2 represents one of only a few oocyte-derived genes that functions in postfertilization development.

  11. Cooperative Fibril Formation

    Many neurodegenerative diseases are characterized by the formation of pathological intraneuronal inclusions containing fibrils of polymerized proteins. For example, tau fibrils constitute the neurofibrillary tangles characteristic of Alzheimer's disease, and alpha-synuclein fibrils are the principal constituent of Lewy bodies, the pathological hallmark of Parkinson's disease. Giasson et al. (p. 636) now show that alpha-synuclein can induce the tau protein to form fibrils and that, when coincubated, these two proteins can induce fibrillization of each other.

  12. Regulating Hydrogen Peroxide Signaling

    Hydrogen peroxide (H2O2) is both a source of oxidative stress and a second messenger in signal transduction. Two reports provide insight into how 2-cys peroxiredoxins (Prxs) can both reduce H2O2 and regulate H2O2 signaling (see the Perspective by Georgiou and Masip). Wood et al. (p. 650) show that eukaryotic 2-Cys Prxs have structural features not present in bacterial Prxs that make them sensitive to inactivation by overoxidation of the peroxidatic cysteine to the sulfinic acid form. They suggest that this capacity for inactivation has evolved so that 2-Cys Prxs function as a floodgate—signaling is blocked until there is a burst in the H2O2 concentration that is sufficient to inactivate 2-Cys Prx and open the floodgate. Oxidation of cysteine to sulfinic acid was thought to be irreversible in cells. However, Woo et al. (p. 653) show that in mammalian cells the sulfinic form of peroxiredoxin I is rapidly reduced back to the catalytically active thiol form. The reversibility of this reaction is consistent with its possible involvement in the regulation of H2O2 signaling.

  13. Breaking a Square Deal

    Aromatic molecules gain extra stabilization from the interaction of 4n + 2 delocalized electrons in a cyclic, usually π-bonded system. Originally understood in organic molecules, many unusual aromatic systems, even all-metal ones, have been reported. However, experimental studies of the corresponding destabilization for antiaromatic 4n electron systems have been largely restricted to organic molecules. Kuznetsov et al. (p. 622) now report the synthesis of an all-metal aromatic species that is antiaromatic, the Al44− tetraanion stabilized with three Li+ cations. Calculations indicate that the addition of two electrons to Al42− changes the square arrangement of the Al atoms to a rectangular shape.

  14. Targets for Longevity

    Decreased functioning of the signaling pathway triggered by peptide hormones of the insulin family leads to more efficient use of food, increased resistance to stress, and longer lives. To identify genes that are the ultimate targets of this pathway responsible for these effects, Lee et al. (p. 644) looked for binding sites for the transcription factor DAF-16/FOXO that are conserved in the Drosophila and Caenorhabditis genomes. They then identified genes regulated by the insulin family that influenced metabolism and longevity, including those for pantothenate kinase, 4-hydroxyphenylpyruvate dioxygenase, an ABC transporter, and a protein similar to retinoblastoma binding protein 2.