This Week in Science

Science  13 Feb 2004:
Vol. 303, Issue 5660, pp. 921
  1. Intercellular Communication Route?

    Mammalian cells interact with one another in a variety of ways, for example, by secreting and binding diffusible messengers like hormones and growth factors, or between attached cells via gap junctions. Rustom et al. (p. 1007) now describe what may represent an independent form of cell-cell communication that they term tunneling nanotubes. In cell cultures from a variety of cell types, they observed the formation of thin tubules connecting cells. These fragile, actin-rich structures were observed to transport membrane components from one cell to another in a unidirectional fashion. The tubules allowed the passage of vesicles of endocytic origin but excluded other organelles like mitochondria and also did not appear to allow significant transfer of cytosolic proteins.

  2. Thin Thermal Barriers

    Thermal barrier coatings help protect materials and delicate machinery from the damaging effects of high temperatures and are also being pursued for applications in thermoelectric technology, where maintaining large temperature gradients is vital. Costescu et al. (p. 989) show that laminated structures made from several-nanometers-thick layers of tungsten and alumina form effective thermal barriers. The high density of interfaces, thought to give rise to the suppression of heat transfer, suggests another route to form thin and stable thermal coatings.

  3. Conflict and Control

    How do the brain processes involved in implementing cognitive control become engaged? In other words: what controls control? The conflict hypothesis states that monitoring of response conflict acts as a signal that engages control processes that are needed to overcome conflict and to perform effectively. In a combination of psychological tests and functional magnetic resonance imaging studies, Kerns et al. (p. 1023; see the Perspective by Matsumoto and Tanaka) found that activity in the anterior cingulate cortex predicts changes in performance and changes in activity in the prefrontal cortex. This result suggests that the anterior cingulate cortex is not responsible for the allocation of control, but rather for the monitoring of conflict that ensues when insufficient control has been allocated.

  4. Leader of the Packing

    The packing of particles depends on both particle shapes and sizes as well as on whether the packing is well ordered or random. For example, spherical particles can at best fill 74% of space when packed in a crystalline form but only 64% of space if they are not allowed to order. The crystalline packing of ellipsoids is about as efficient as that for spheres, but Donev et al. (p. 990; see the Perspective by Weitz) show that for random jammed states, ellipsoids pack more densely than spheres and approach the crystalline density. The observations are obtained from simulations, and from experiments using M&M's Candies.

  5. Hydrogen from Ethanol

    Ethanol is a renewable source of energy, but the use of ethanol from biomass sources as a combustion fuel requires the energetically expensive removal of excess water. Deluga et al. (p. 993; see the news story by Cho) describe a method for generating hydrogen from wet ethanol by its fast partial oxidization over rhodium or rhodium-cerium catalysts. The reactions run at about 700°C, but because the process is exothermic, the starting mixture only needs to be heated to about 140°C for the reaction to run. The only by-product is CO2, and some of the H2 actually comes from the water. The use of such “on-board” reforming of liquid fuel into H2 provides an alternative to storing H2 for fuel cell vehicles.

  6. Not So Different After All

    Botanists and zoologists have been asking similar fundamental questions about the evolution of reproductive systems, and many of the events that occur after insemination and pollination raise similar evolutionary issues. At this stage in the life cycle, players with different evolutionary interests (competing males, the female parent, gametophytes, gametes, and zygotes) come into contact and promote opportunities for cooperation and conflict. Bernasconi et al. (p. 971) identify potential analogies and differences between animals and flowering plants, summarize recent major advances, and outline the major challenges facing both fields.

  7. Winds Below

    Detailed observations of ocean surface winds should help improve weather forecasting and our understanding of upper ocean processes. However, direct surface wind measurements made over the ocean are relatively sparse. An alternative to direct measurements is radar observation of the ocean surface by satellite, from which the speed and direction of the overlying winds can be calculated. This method offers good spatial and temporal resolution—25-kilometer measurements covering 90% of the world every 24 hours—and provides estimates of wind speed and direction as good as those made by buoys. Chelton et al. (p. 978; see the Perspective by Kelly) present an analysis of 3 years of data collected by the QuikSCAT satellite radar scatterometer. They see fine-scale features that arise from orographic influence near landmasses and from differences in sea surface temperatures in the open ocean.

  8. Transcription in Plain View

    Initiation of transcription in eukaryotes involves the assembly of a large complex, composed of RNA polymerase II (pol II) and five general transcription factors, at the gene promoter. The transcription factor TFIIB plays a role in bridging between promoter DNA and pol II. Bushnell et al. (p. 983) have determined the structure of a complex of the 10-subunit yeast RNA pol II with TFIIB at 4.5 angstrom resolution. Using structural information available on the other four transcription factors, TFIID, TFIIE, TFIIF, and TFIIH, they propose a nearly complete model of the transcription initiation complex. Westover et al. (p. 1014) provide new insights into transcription itself, through the 3.6 angstrom structure of pol II in a complex with synthetic DNA and RNA oligonucleotides. The structure reveals how the RNA transcript is separated from the DNA template during transcription.

  9. Protecting the Rain Forest?

    Applying a combination of field, aerial, and remote sensing surveys, Curran et al. (p. 1000) quantify the spatial extent and distribution of lowland tropical forest remaining in protected areas across Indonesian Borneo (Kalimantna). Millions of hectares of lowland forest conservation areas have been degraded since 1985, mainly by logging stimulated by state policies. Because Kalimantan's protected areas are now being intensively logged, this biodiversity hotspot is suffering irreversible ecological degradation.

  10. Controlling Flowering Time

    How plants measure day length and use it to synchronize flowering to the changing seasons has been of wide interest since photoperiodism was first described in the 1920s. Recently, major advances have been made in understanding the underlying mechanisms by using Arabidopsis and rice as models. In Arabidopsis, the CONSTANS transcription factor promotes flowering specifically in response to long days. Valverde et al. (p. 1003; see the Perspective by Klejnot and Lin) show how convergence between light signaling pathways and circadian control combine to generate the response to day length. They identified a new layer of control mechanisms by studying regulation of CONSTANS at the posttranscriptional level.

  11. Primordial Condensates

    The first solid particles to form in the solar nebula probably condensed directly from a gas phase at high temperatures (greater than 1500 kelvin). Pack et al. (p. 997) have found anomalous abundances of rare earth elements in two ordinary chondrites, and these anomalous abundances were probably produced by heterogeneous fractionation of the elements during condensation. The abundances indicate that the grains formed in a more reduced gas that was enriched in carbon relative to oxygen. These early condensates hint at a solar nebula that was heterogeneous and at one time contained carbon-rich gas and dust, perhaps derived from the interstellar medium.

  12. Not Only How Many, But What Type

    In progenitor cells of the central nervous system, Wnt signaling has been reported to affect cell proliferation. Lee et al. (p. 1020; see the Perspective by Bronner-Fraser) now examine the conditional expression of a constitutively active β-catenin gene in neural crest cells in mice. In contrast to the cell proliferation effect seen previously, analysis of the mutant mice indicates that neural crest cells migrate properly but differentiate into sensory neurons at the expense of other neural crest cell types. This work suggests an unexpected role for Wnt signaling in promoting a sensory neuron fate.

  13. Growing Axons Need APC

    The anaphase-promoting complex (APC) plays a crucial role in the exit of cells from mitosis because it acts as a ubiquitin ligase that promotes the degradation of mitotic cyclins. Why, then, would APC be highly expressed in neurons—cells that will never again enter the cell cycle? Konishi et al. (p. 1026) examined the role of APC in neurons and found that it promoted axonal but not dendritic growth. Reducing the levels of APC in developing rat cerebellum affected axonal growth and parallel fiber patterning. Lack of APC allowed primary neurons to grow axons on myelin—a substrate that normally inhibits axonal growth. Thus, APC in neurons is important in development and axonal growth, and may also be important in restraining the growth of injured axons in adult brains.

  14. Cytosolic Role of p53

    The tumor suppressor protein p53 promotes apoptosis of tumor cells. Although p53 functions as a transcription factor, it appears to have important cytosolic effects that are independent of its effects in the nucleus. Chipuk et al. (p. 1010) find that p53 directly enhances permeabilization of mitochondria by the pro-apoptotic protein Bax. In vitro or in transfected cells, p53 also caused release of pro-apoptotic proteins that are normally sequestered by binding to the anti-apoptotic protein Bcl-xL. The cytosolic effects of p53 appear to be functionally important because p53 still promoted apoptosis in cells in which nuclear transport of p53 and consequent transcriptional regulation were blocked.

  15. Forkhead and Autoimmunity

    The forkhead family of transcription factors are involved in a variety of systems. The forkhead family member, Foxj1, is thought to play a role in the immune system, but genetic deficiency of this factor proves lethal, making direct in vivo analysis difficult. To overcome this, Lin et al. (p. 1017) devised a chimeric system in mice, in which effects of Foxj1 deficiency could be explored. In these chimeras, Foxj1−/− T cells displayed an exaggerated inflammatory cytokine expression, eventually generating systemic autoimmunity. Normal repression of cytokine production by Foxj1 resulted from inhibition of the transcriptional activator NF-κB, possibly via the up-regulation of the inhibitory NF-κB cofactor, IκB.