# This Week in Science

Science  01 Jan 2010:
Vol. 327, Issue 5961, pp. 10
1. # Be-Deviled Cancer

Recently, a deadly transmissible cancer has emerged in Tasmanian devils, the largest existing marsupial carnivore. This disease, devil facial tumor disease (DFTD), leads to the growth of large facial tumors that frequently metastasize to internal organs. DFTD is thought to be transmitted by biting, and leads to death of affected animals within months, usually by obstructing the animals' ability to feed. Consequently, in the last 10 years Tasmanian devil numbers have dropped by about 60%. There are no genetic tests, vaccines, or treatments available for this disease, and without intervention, models predict that DFTD could cause extinction of Tasmanian devils in the wild within 50 years. Several lines of evidence suggest that DFTD is transmitted as a clonal allograft, whereby the cancer cells themselves are the agents of tumor transmission. Murchison et al. (p. 84) examined this hypothesis in detail by genotyping 25 tumor-host pairs from around Tasmania at 14 microsatellite loci and at a variable mitochondrial polymorphism. DFTD tumors were indeed found to be genetically distinct from their hosts and almost completely genetically identical to one another, supporting the idea of transmission by allograft.

2. # Lipid Rafts Come of Age

Living cells are surrounded by cellular membranes composed of lipids and proteins. Much attention has been paid to the biogenesis and sorting of membrane proteins. The dynamics and sorting of lipids have been much more difficult to study. Lingwood and Simons (p. 46) review the evidence for, and the role played by, so-called lipid rafts—laterally segregated regions within membranes enriched for particular lipids and proteins.

3. # Dendrites Shape Interneuron Firing

Basket cells, a group of fast-spiking inhibitory interneurons, play an important part in the function of neuronal networks. The mechanisms underlying the high temporal precision and short latency of basket cell activity are unclear. Hu et al. (p. 52, published online 3 December) investigated dendrite functions in fast-spiking hippocampal basket cells and found that action potentials are initiated in the axon and propagate back into the dendrites without activity dependence but with strongly reduced amplitude. This is very different from what has been observed previously in widely investigated pyramidal cell dendrites, probably due to the high potassium to sodium conductance ratios in the dendrites of the interneurons. These dendritic mechanisms can explain the high-frequency firing and precise timing of basket cells seen in network activity in vivo.

4. # Activating Stubborn Dopants

Many applications of semiconductor light-emitting diodes and lasers, such as reading optical disks, benefit from shorter wavelengths, but this requires materials with larger energy gaps between their valance and conduction bands. The electronic conductivity of these materials often has to be increased by doping with impurity atoms. However, in nitride materials, such as GaN and AlGaN, hole doping with acceptor atoms such as Mg is ineffective at room temperature. Simon et al. (p. 60) grew a gradient of AlGaN on the surface of GaN and found that the polarization of the layer could field-ionize the acceptor dopants efficiently at room temperature. The heterostructure was used successfully in a light-emitting diode that emits in the ultraviolet.

5. # Carbon Nanotube Bridge for DNA Transport

The nanoporosity of carbon nanotubes has been exploited in the control of molecular transport—for example, in creating membranes. Liu et al. (p. 64) fabricated devices in which one single-walled carbon nanotube connects two fluid reservoirs. In some of these devices, apparently those in which the nanotube is metallic, the ionic conductivity is anomalously higher than that expected from the bulk resistivity of the electrolyte. This high conductivity was exploited for the transport of single-stranded DNA, which was accompanied by large but transient increases in the ion current.

6. # Metal in the Middle

Biphasic reaction mixtures allow the isolation of sensitive products, which can form in one solvent and then shift rapidly into another, protected from side reactions and interfering by-products. However, ensuring that catalysts remain in the proper phase can be challenging, and surfactants added to induce efficient mixing often prove difficult to separate from the product stream. Crossley et al. (p. 68; see the Perspective by Cole-Hamilton) tackle these issues by preparing easily recoverable amphiphilic nanoparticles that simultaneously stabilize aqueous-organic emulsions and catalyze organic reactions. The particles combine hydrophobic nanotubes with hydrophilic oxides, causing them to accumulate at water-hydrocarbon interfaces. Depositing palladium on specific portions of the particles' surfaces thus localizes the metal in one or both phases, facilitating hydrogenation of several compounds of interest in biofuel refining.

7. # The Depths of the Changes

Over the course of the past glacial cycle, there have been two major types of rapid, large climate warming events: shorter-lived warm intervals lasting on the order of 1000 years and the last glacial-interglacial transition. Although both involved dramatic changes in large-scale ocean circulation, the extent to which those changes were similar is unclear. Roberts et al. (p. 75) analyzed the neodymium isotopic composition of the Fe-Mn oxide coatings of planktonic foraminifera and reconstructed patterns of Atlantic Ocean circulation during Heinrich event 1, a rapid global climate fluctuation about 14,000 years ago involving the destruction of Northern Hemisphere ice shelves and the last deglaciation. While both the source of deep water and the whole-ocean overturning rate shifted rapidly and synchronously during the last deglacial transition, only upper ocean circulation strength was affected during Heinrich event 1.

8. # Evolution in Action

Rates of evolution in gene and genome sequences have been estimated, but these estimates are subject to error because many of the steps of evolution over the ages are not directly measurable or are hidden under subsequent changes. Ossowski et al. (p. 92) now provide a more accurate measurement of how often spontaneous mutations arise in a nuclear genome. Mutations arising over 30 generations were compared by sequencing DNA from individual Arabidopsis thaliana plants. UV- and deamination-induced mutagenesis appeared to bias the type of mutations found.

9. # We Are Stardust

Supernovae form as the result of stellar explosions and are classified according to the properties of their spectra. Poznanski et al. (p. 58, published online 5 November) present a peculiar supernova that is characterized by extremely fast temporal evolution and unusual spectroscopic features, such that it defies classification. SN2002bj appears to be a member of a new class of supernovae, possibly formed by a helium detonation on a white dwarf ejecting a small envelope of material.

10. # Modifying Protein Modification

Alpha-dystroglycan (α-DG) is a cell-surface receptor that anchors the basal lamina to the sarcolemma by binding proteins containing laminin-G domains. This binding is essential for protecting muscle from contraction-induced injury, and defective binding is thought to cause a subclass of congenital muscular dystrophy (CMD) in humans. Mutations in six (putative) glycosyltransferase genes have been identified in patients with CMD, suggesting that glycosylation of α-DG may confer the ability to bind laminin. Despite extensive efforts for over 20 years, the actual laminin-binding moiety has remained unclear. Now, Yoshida-Moriguchi et al. (p. 88) have identified a phosphorylated O-mannosyl glycan on α-DG. This modification occurred in the Golgi via an unidentified kinase and was required for the maturation of α-DG into its laminin-binding form.

11. # Flowery Regulator

Control of gene transcription is multilayered, depending on transcription factors, epigenetic mechanisms, and interactions with small RNA molecules. Liu et al. (p. 94, published online 3 December) have now found that for the FLOWERING LOCUS C (FLC) gene of the plant Arabidopsis, a backwards transcript of the gene conspires with 3′ RNA-processing tools and histone demethylation to regulate the transcription of the protein-coding gene. The 3′-processing events require the antisense, not the sense, RNA transcript. It is then the sense transcript that, in the end, regulates onset of flowering.

12. # Bacterial Compartmentalization

In diverse bacteria, reactions that involve toxic or volatile metabolites are carried out by enzymes inside proteinaceous microcompartments. Tanaka et al. (p. 81; see the Perspective by Kang and Douglas) now report high-resolution crystal structures for four homologous proteins that are constituents of the shell that sequesters the metabolism of ethanolamine in bacteria. While the structures have similar overall folds, they have distinctive structural features that provide insight into how they build the shell and participate in microcompartment function.

13. # A Hidden Template

The entropic challenge inherent in forming a ring-shaped molecule generally increases considerably with the size of the ring. Assuming that a linear precursor must bind its ends together, extending its length diminishes the likelihood of the opposite ends approaching one another. In the absence of an external force, how then can a family of molybdenum oxide rings, several nanometers in diameter (quite large at the molecular scale), self-assemble? Miras et al. (p. 72, see the cover; see the Perspective by Whitmire) have now uncovered an internal template guiding the process. By carefully controlling conditions in a flow reactor, they were able to halt the assembly process partway through and characterize a smaller molybdenum oxide core cluster, around which the larger ring was forming. Ejection of this template then yielded the hollow finished product.

15. # Consciousness and Reproducibility

What exactly does it mean for neural information to reach subjective awareness? Previous work has suggested that the intensity and the duration of activation in brain areas that encode the contents of perception determine whether or not the information contributes directly to subjective experience. Now Schurger et al. (p. 97, published online 12 November; see the Perspective by Schwarzkopf and Rees) describe a third property that distinguishes conscious from nonconscious neural encoding reproducibility. Spatiotemporal patterns of brain activity were recorded while subjects performed a simple visual category-discrimination task. Activation patterns corresponding to conscious information were more reproducible—i.e., more reliable, across different presentations of the same stimulus category, compared to nonconscious activation patterns coding for the same perception.