This Week in Science

Science  05 Nov 2010:
Vol. 330, Issue 6005, pp. 727
  1. Dissecting Self-Incompatibility


      Although the pollen may be available for a flower to fertilize itself, molecular determinants on the pollen and the pistil prevent inbreeding in a process termed self-incompatibility. In the Petunia self-incompatibility, if male determinants (F-box proteins) on pollen are recognized by a female ribonuclease determinant on the pistil, the pollen tube is killed when its ribosomal RNA is digested. Outcrossed fertilizations can occur because of allelic diversity in the female that fails to recognize its male counterparts; however, the genetic diversity of the ribonuclease gene is greater than that of the known F-box gene. Kubo et al. (p. 796; see the Perspective by Indriolo and Goring) have discovered that there are several related F-box genes in Petunia, each of which brings its own allelic diversity to bear—thus, increasing the variety of potential mating partners.

    1. Tonic Inhibition

        Neuronal inhibition has recently drawn much attention; however, the mechanisms involved in tonic release of and the cellular source of the neurotransmitter involved, γ-aminobutyric acid (GABA), have been difficult to pin down. Lee et al. (p. 790, published online 23 September) showed that tonic release of GABA in the cerebellum occurs through the Bestrophin 1 anion channel of cerebellar astrocytes and Bergmann glial cells. These results confirm that glia can serve as a source of GABA for tonic inhibition of neurons and provide more evidence for interactions between neurons and glia cells that have implications for our understanding of brain-signaling mechanisms.

      1. The Dependable Warmer

          During the middle of the Eocene, about 40 million years ago, a transient warming event interrupted the long-term cooling trend that had been in progress for the previous 10 million years. Bijl et al. (p. 819; see the Perspective by Pearson) constructed records of sea surface temperature and atmospheric CO2 concentrations across the warming period. It appears that vast amounts of CO2 were injected into the atmosphere, and a sea surface temperature increase of as much a 6°C accompanied the atmospheric CO2 rise.

        1. Through a Lens Brightly

            Astronomical sources detected in the submillimeter range are generally thought to be distant, dusty galaxies undergoing a vigorous burst of star formation. They can be detected because the dust absorbs the light from stars and reemits it at longer wavelengths. Their properties are still difficult to ascertain, however, because the combination of interference from dust and the low spatial resolution of submillimeter telescopes prevents further study at other wavelengths. Using data from the Herschel Space Telescope, Negrello et al. (p. 800) showed that by searching for the brightest sources in a wide enough area in the sky it was possible to detect gravitationally lensed submillimeter galaxies with nearly full efficiency. Gravitational lensing occurs when the light of an astronomical object is deflected by a foreground mass. This phenomenon increases the apparent brightness and angular size of the lensed objects, making it easier to study sources that would be otherwise too faint to probe.

          1. Volatiles Versus Vegetation

              Plants act as both global sources and sinks of highly reactive volatile organic compounds (VOCs). Models typically treat the uptake and degradation of these compounds as if they are mostly unreactive, like other more commonly studied biogenic gases such as ozone. A study by Karl et al. (p. 816, published online 21 October) suggests that VOCs may be more reactive than expected. By monitoring six field sites representing a range of deciduous ecosystems, several oxidized VOCs were found to have high deposition fluxes. Fumigation experiments in the laboratory confirmed that leaves are capable of oxidizing these compounds, and do so through an enzymatic detoxification or stress-response mechanism. Budgets for VOC flux in the atmosphere suggests that, on a global scale, plants may take up significant levels of VOCs in polluted regions, especially in the tropics.

            1. Bend It, Stretch It

                CREDIT: HOSONO ET AL.

                Materials (such as those found in some artificial muscle systems) that are used for stimulus-controlled bending or stretching, require long-range ordering so that local chemical triggers will result in long-range bending or flexing motions. Hosono et al. (p. 808) observed that a sandwich of a polymethacrylate between uniaxially stretched Teflon sheets develops three-dimensional ordering. Upon hot pressing, the main chains of the polymer brushes aligned homeotropically to the film plane, whereas the side chains, containing azobenzene groups, oriented horizontally along the drawing direction of the Teflon sheets. With some azobenzene groups, alternate irradiation with ultraviolet and visible light made the composites bend and stretch reversibly.

              1. Gut Stem Cell Replacement

                  Gut cell turnover is characteristically rapid and relies on stem cells in the crypts that lie between the intestinal villi. The prevailing view is that stem cell division is asymmetric with one daughter retaining a stem cell character; however, this pattern of stem cell turnover does not always apply. Using long-term lineage tracing, Lopez-Garcia et al. (p. 822, published online 23 September) showed that the loss of a stem cell was compensated for by the multiplication of a neighboring cell. The rate of stem-cell loss was found to be equivalent to the rate of cell division, indicating that symmetric cell division was the rule for gut stem cells and implying stochastic expansion, contraction, and extinction of clones occurs.

                1. Keeping Fit

                    Mutations may be deleterious, neutral, or advantageous. Understanding the relative effect of a new mutation on an organism's fitness is important for many systems from complex diseases to conservation biology. Lind et al. (p. 825) used the bacterium Salmonella typhimurium to study the effects of random mutations in two ribosomal proteins on fitness. Most mutations, whether synonymous or nonsynonymous, had significant fitness costs, thus overturning the prevailing dogma that most point mutations are either neutral or lethal and indicating that the mutations influenced messenger RNA structure and/or stability.

                  1. Tumor Vaccination Success

                      Vaccination with tumor-specific antigens is one of several attempted therapies seeking to harness the immune system, but—unfortunately—this strategy has been unsuccessful, possibly because of the immunosuppressive properties of the tumor microenvironment. Kraman et al. (p. 827; see the Perspective by Schreiber and Rowley) have identified immunosuppressive cells of mesenchymal origin in mice comprising 2% of the tumor stromal cell population. They were identified by expression of the fibroblast activation protein–α. Deletion of these cells in lung or pancreatic cancers in mice allowed successful therapeutic vaccination against the tumors, which was dependent on the adaptive immune system and the cytokines interferon-γ and tumor necrosis factor–α. These findings reveal that multiple cell types contribute to the immunosuppressive tumor microenvironment and will inform therapeutic cancer vaccine design.

                    1. Blocking Interfering Microbes

                        CREDIT: WALLACE ET AL.

                        Irinotecan is a widely used anticancer pro-drug that is converted in the liver into the active form, but when it gets into the gut, the normally benign microbial flora can convert it into the toxic form, which kills the rapidly multiplying gut epithelium as it would kill rapidly dividing tumor cells, and thus causes diarrhea. Wallace et al. (p. 831; see the Perspective by Patel and Kaufmann) used high-throughput screening to identify inhibitors that target the offending bacterial enzyme, β-glucuronidase, without killing the bacteria or affecting orthologous mammalian enzymes. Crystal structures revealed the molecular basis of selectivity, and in vivo studies showed that an inhibitor protected mice from irinotecan-induced toxicity.

                      1. Primitive Origins for Microglia

                          Microglia are the resident macrophages of the central nervous system and are associated with neurodegeneration and brain inflammatory diseases. Although the developmental origins of other tissue macrophage populations are well established, the origins of microglia remain controversial. Ginhoux et al. (p. 841, published online 21 October) used in vivo lineage tracing studies to show that microglia arise early in mouse development and derive from primitive macrophages in the yolk sac. This is in contrast to other cells of the mononuclear phagocyte system, which arise later in development from a distinct progenitor population.

                        1. Tuned for Faces

                            The temporal lobe of macaques' brains contains six patches of face-selective cortex. This observation has prompted systems neuroscientists to ask, why so many and what do they do? Freiwald and Tsao (p. 845; see the Perspective by Connor) targeted four of these regions for single-unit recordings and found that the different face-selective patches in macaques have independent functions. The areas where earliest processing occurred were most sharply tuned for individual views and least sharply tuned for identity. The mid-level area was more sharply tuned for identity, and the highest processing stage was strongly tuned for identity in a strikingly view-invariant way. These results yield fundamental insights into the computational process of object recognition, the functional organization of the brain, and how representations are transformed through processing hierarchies.

                          1. Weakly Interacting Graphene

                              Many unusual properties of graphene are a consequence of the Dirac dispersion of its electrons—a linear relationship between an electron's momentum and energy. Naïvely, this dispersion leads to the conclusion that electrons in graphene are strongly affected by mutual electrostatic interactions; however, there is little experimental evidence for strong interaction. Reed et al. (p. 805) resolved this discrepancy by using inelastic x-ray scattering spectra of graphite (which consists of loosely bound layers of graphene) to estimate how much the electric field was damped by the presence of mobile charge carriers. In fact, damping was strong at distances in excess of 1 nanometer, suggesting that graphene is more weakly interacting than was assumed.

                            1. Broken Symmetries

                                Bilayer graphene samples are expected exhibit quantum Hall states that are ferromagnetic with different types of spin ordering. Weitz et al. (p. 812, published online 14 October) studied the conductance of high-quality suspended bilayer graphene samples. They used an applied perpendicular electric field to induce transitions between the different broken-symmetry states that appear at low carrier densities and deduced their order parameters. These states appeared in both the absence of a magnetic field, as well as in the presence of a symmetry-breaking magnetic field. They also showed that, even in absence of both an applied magnetic or electric field, the bilayer exhibits an energy gap, which indicates that electron-electron interactions contribute to the band structure.

                              1. A Likely Conformation

                                  At several stages of protein synthesis, guanosine triphosphate hydrolyzing enzymes (GTPases) interact with the ribosome, and GTP hydrolysis is coupled to progression of synthesis. Voorhees et al. (p. 835) have determined a 3.2-resolution structure of the GTPase, elongation factor Tu, which delivers amino-acyl transfer RNA (tRNA) to the ribosome. The GTPase and tRNA were bound to the ribosome and were stalled in an active conformation by a GTP analog. The structure revealed that activation of the enzyme only required small changes in conformation to move a catalytic histidine into the correct position for hydrolysis. A similar mechanism likely applies to the activation of other translational GTPases.

                                1. Kicking Out Introns

                                    Many genes in eukaryotes contain introns that must be removed from the messenger RNA for proper gene function. Humans have on average eight introns per gene, whereas more than 90% of the genes in the yeast species Saccharomyces cerevisiae and Candida albicans have none at all. To understand how introns can be lost from genes, Mitrovich et al. (p. 838) compared non–protein-coding genes among the yeasts and found that genes for small nucleolar RNAs (snoRNAs) in C. albicans are intronic. By contrast in S. cerevisiae, snoRNAs are processed from unmodified RNA, suggesting a massive loss of snoRNAs—associated introns in the common ancestor of the Saccharomyces species. The introns seem to have been lost through splice-site degeneration, and associated compaction of linked exons resulted in nested splicing of some snoRNAs.