Olfactory Neuron Precursor Diversity
In the adult mouse, the brain provides a steady supply of newly generated olfactory neurons. These cells are generated in the subventricular zone and migrate to the olfactory bulb. Merkle et al. (p. 381, published online 5 July) now show that different regions of the subventricular zone give rise to different types of olfactory neurons. Thus, the stem cells of the subventricular zone are not so much individually versatile and are better characterized as a starting point of an already diverse population.
Borrowing Power from Nature
Mechanical tasks are accomplished in the cell through an array of molecular machines and there has been interest in exploiting this machinery in artificial nanoscale structures. Van den Heuvel and Dekker (p. 333) review the recent progress on the use of rotary and linear motor proteins for tasks such as facilitating transport or powering a device. Although some clever applications have evolved, the authors note that many uses are still only at the proof-of-principle stage.
Eucrites are meteorites that trace igneous activity on small bodies, similar to the asteroid Vesta, early in the solar system's history. Dating them can tell us about geophysical processes at work when these bodies were differentiating to form a metallic core and silicate mantle. However, such attempts have been difficult because eucrites tend to be changed by later heating and fracturing, and also, the isotopic systems available for dating are hard to calibrate. By analyzing zircons within eucrites, Srinivasan et al. (p. 345) have dated their crystallization to within 6.8 million years of metal-silicate differentiation on their parent body. They were able to anchor the short-lived Hf-W isotope system with the slower U-Pb system to tie down the timing accurately. Later metamorphosis of the eucrites took place after another 9 million years and was likely caused by heating from impacts.
In a Spin
Imaging the surfaces of stars other than the Sun would allow astronomers to map the physical processes at work on them. With advanced optical interferometric techniques. Monnier et al. (p. 342, published online 31 May; see the Perspective by Quirrenbach) have resolved the surface of the main sequence star Altair, one of the brightest stars in the night sky, to a resolution of <1 milliarcsecond. Altair is unusual as it spins very rapidly, fast enough that it appears elongated through centrifugal forces. The amount of distortion and the attendant changes in surface temperatures, characterizing angular momentum transport within the star, diverge from the predictions of standard models, especially around the equator. Thus, extra processes, such as differential rotation and alternative gravity darkening laws, are needed to explain the appearance of rotating stars.
When films are grown on surfaces through vapor-phase deposition, complex heterostructures can form because of strains that arise through lattice mismatches. Robinson et al. (p. 355) show in a solution environment that the complex superlattices can form spontaneously in cadmium sulfide nanorods through the controlled introduction of silver cations. Alternating layers of cadmium sulfide and silver sulfide form along the axis of the rod because the lattice-mismatch strain that builds up during silver infiltration limits the growth of the silver sulfide domains. The control over growth achieved by changing the solution parameters and nanowire dimensions was used to tune the near-infrared emission from these nanorods.
Halogens in Antarctica
Tropospheric halogens affect the concentration of ozone, the oxidizing capacity of the atmosphere, and aerosol formation, all of which are linked to climate. The halogen chemistry of the frozen high latitudes has proven to be particularly interesting, not least because of the role of these regions as harbingers of global climate change, but a better understanding of that chemistry has been hampered by lack of data. Saiz-Lopez et al. (p. 348) present measurements of BrO and IO in the Antarctic boundary layer from January 2004 to February 2005. They observed high concentrations and persistence of these halogens throughout the sunlit period, contrary to expectations and unlike the situation in the Arctic, where IO has not been detected. The springtime IO levels they found are the highest reported anywhere in the atmosphere, and an apparent synergy between IO and BrO suggests an unknown halogen-activation mechanism. These levels of halogens also cause the rapid oxidation of dimethyl sulfide and mercury in the Antarctic boundary layer.
Gradually Becoming Dominant
Dinosaurs became the dominant land animals by the Jurassic. Whether their early ascension began by way of an extinction that preferentially affected their precursors, including the archeosaurs and amniotes, or through a more gradual replacement of these other groups, is unclear, but the earlier Triassic fossils needed to evaluate these questions have been relatively scarce. Irmis et al. (p. 358, see the cover) now describe a rich fossil assemblage from New Mexico dating to the Late Triassic that includes both dinosaurs and their reptilian precursors. Thus, some of the precursors persisted much longer than had been thought and existed along with dinosaurs for millions of years. These fossils support a model of a gradual rise of dinosaurs in the Late Triassic that preceded their dominance by the beginning of the Jurassic.
The Same Difference
Recent advances in sequencing technology have increased our power to study variation within a single organism. Clark et al. (p. 338) resequenced 20 strains of Arabidopsis thaliana with high-density nucleotide oligonucleotide arrays and found extensive variation. The comprehensive inventory of genome-wide DNA polymorphisms in Arabidopsis illustrates the extent of natural genetic variation, with many genes disabled in different wild strains, as well as high levels of polymorphism among gene family members, including those involved in disease resistance.
What's the Buzz?
The residents of bee hives are well known to be closely related, but hives can often exhibit more genetic diversity than might be anticipated from theories on the benefits of cooperation among closely related individuals. Mattila and Seeley (p. 362) show one reason for this is that more genetically diverse hives (those originating from a female mating with multiple males) perform better in the rate of comb building, foraging rates, and honey production than those originating from a single female and male. To advertise her presence in the colony and to exert influence over its members, a honeybee queen produces a complex blend of substances known as queen mandibular pheromone. Vergoz et al. (p. 384 see the Perspective by Galizia) found that exposure to queen pheromone leads to a reduction in aversive learning but not to a reduction in appetitive learning in young honeybees. The queen substance modulates the dopaminergic system of bees, which reduces the capacity of young workers to form aversive memories.
Location, Location, Location
Despite substantial effort, it has remained relatively mysterious how the protein known as Hedgehog (Hh) activates signaling pathways that regulate various biological processes, including stem cell function, development, and cancer. Rohatgi et al. (p. 372; see the Perspective by Christensen and Ott) show that mammalian cells use their primary cilium as an antenna that samples the surrounding environment for the presence of Hh. When Hh bound to its receptor Patched 1 (Ptc1), the receptor left the cilia, where (in the absence of stimulation) it acts to restrain Hh signaling by preventing accumulation of the signaling protein Smoothened (Smo). Accumulation of Smo in the cilia of stimulated cells corresponded to activation of Hh signaling. Further understanding the molecular mechanisms that influence cellular localization of Ptc1 and Smo will improve understanding of the signaling pathway and may lead to new therapeutic targets.
Longevity on the Brain
Several studies show that loss-of-function mutations in the insulin-like signaling cascade extends the life span of worms and flies; however, equivalent mutations are associated with metabolic disease and fatal diabetes in mice. In contrast, calorie restriction or genetic strategies in mice that enhance insulin sensitivity lower the risk of age-related disease and extend life span. Taguchi et al. (p. 369) resolve these conflicting results by pointing to the brain as the site where reduced insulin-like signaling can extend mouse life span.
The recent discovery that certain viruses express microRNAs (miRNAs) raises the question as to whether these pathogens might use miRNA to evade their hosts. Stern-Ginossar et al. (p. 376; see the Perspective by Cullen) find that for human cytomegalovirus this appears to indeed be the case. One of the virus' miRNAs was predicted to target the 3′ untranslated regions of two immune-related genes, which become activated in response to viral infections. Expression of one of these proteins was indeed dampened by the viral miRNA, which reduced recognition by antiviral natural killer cells. It remains to be seen if miRNA will turn out to be a widespread method exploited by viruses to evade host immunity.
Bands of Ordered Organic Molecules
The electronic structure of many inorganic materials can be determined with techniques such as angle-resolved photoelectron spectroscopy (ARPES), in part because these materials form well-ordered crystals or films. For organic materials used in electronics applications, it is often difficult to grow crystals of sufficient size, or films that are well ordered and oriented, that would allow spectra to be obtained that could be compared to theoretical band structure models. Koller et al. (p. 351) have grown well-ordered films of sexiphene, which packed flat onto an oxygen-covered copper surface. Using density function theory to interpret the ARPES results, they show that a quasi-one-dimensional band structure develops out of individual molecular orbitals in the direction parallel to the long molecular axis. However, in the perpendicular direction, the electronic structure reflects the periodicity of the lattice, and continuous bands are seen.
To Cluster or Not to Cluster
The PDZ domain is a frequently occurring interaction domain in eukaryotic proteins that binds to the C-termini of target proteins. PDZ domains have been thought to cluster into functional classes, each with distinct sequence binding preferences. Using a combination of experimental data and modeling, Stiffler et al. (p. 364) predict PDZ domain-peptide interactions across the mouse proteome. Instead of observing clustering, they find that PDZ domains are evenly distributed throughout selectivity space likely to minimize cross-reactivity. The study highlights focusing on families of interaction domains as a productive approach to gain insight into protein function.