- Kristen L. Mueller
Our guts are colonized by trillions of commensal microbiota, whose influence on our immune systems is just beginning to be appreciated. Altered colonization has been associated with diseases such as allergy and inflammatory bowel disease, suggesting that commensals may play an important role in regulating immune system responses; to what extent, however, is not yet understood.
Gaboriau-Routhiau et al. have addressed this issue by comparing germ-free and normally colonized mice. They found that commensal microbiota were critical for maintaining T cell homeostasis in the gut. Germ-free mice exhibited altered gene expression profiles of cytokines and transcription factors that were associated with T helper cell–mediated immune responses. Recolonization with microbiota derived from mouse fecal matter restored normal expression patterns. Surprisingly, this effect was largely restricted to one strain of bacteria: the segmented filamentous bacteria (SFB). Similar findings were obtained by Ivanov et al., who demonstrated the effects of SFB on interleukin 17–producing T helper cell responses. Thus, these results indicate that T cell immunity is regulated by both host- and microbiota-derived factors and that microbes may actively shape T cell populations in the gut.
Immunity 31, 677 (2009); Cell 139, 485 (2009).
How Antioxidants Might Help
- Paula A. Kiberstis
Most studies of mammalian aging eventually lead to the mitochondrion, the energy-producing organelle. A prevailing hypothesis has been that damaging byproducts of mitochondrial respiration, called reactive oxygen species (ROS), accumulate as we age and cause mitochondrial dysfunction, which manifests at the organismal level as age-related disorders. Yet this hypothesis is at odds with growing evidence that an increase in the biogenesis of mitochondria can have beneficial anti-aging effects. Illustrating this dilemma are two studies of mice in which genetic manipulations altered mitochondrial function. For mice deficient in the proapoptotic protein Bak, Someya et al. found that mitochondria actively contribute to age-related hearing loss by mediating the death of cochlear cells in the ear. The death of these cells was triggered by oxidative stress and could be suppressed by antioxidants. Independently, Wenz et al. found that increased muscle expression of peroxisome proliferator–activated receptor-γ coactivator α (PGC-1α), a protein that up-regulates mitochondrial biogenesis, not only prevented age-associated loss of muscle mass, but also had beneficial effects on whole-body metabolism. Why the salutary effects of increased mitochondrial biogenesis are not counteracted by a parallel increase in damaging ROS remains unclear, but could involve increased turnover of the “older” ROS-damaged organelles.
Proc. Natl. Acad. Sci. U.S.A. 106, 19432; 10.1073/pnas.0911570106 (2009).
- Climate Science
Holding the Line
- H. Jesse Smith
Roughly one-third of all the CO2 emitted by human activity is ultimately absorbed by the ocean, a process that has helped slow down the rate of increase of atmospheric CO2 concentrations. As the ocean continues to absorb CO2, however, the rate at which it does so is expected to decrease because of the changes in carbonate chemistry that CO2 uptake causes. Global warming should then accelerate, a frightening prospect considering how quickly temperatures are rising already. Several studies have shown that the uptake of atmospheric CO2 by some regions of the ocean has slowed already, but does that mean that the integrated world ocean has become a less effective CO2 sink? Knorr combines data from ice cores, direct atmospheric measurements, and emission inventories to show that the global fraction of emitted CO2 that remains in the atmosphere has stayed constant over the past 160 years, at least within the limits of uncertainty of the measurements. Khatiwala et al. also fail to detect a significant recent change in the fraction of CO2 that the ocean is absorbing, in an examination of both ocean and terrestrial CO2 sinks for the longer period of the past two and a half centuries. That is welcome news, but not reason to be complacent about the future, as sooner or later the capacity of the ocean to absorb CO2 will be reduced. The real question is why we have not seen evidence of that reduction already.
Geophys. Res. Lett. 36, L21710 (2009); Nature 462, 346 (2009).
Photocharged by Graphene
- Phil Szuromi
Although the basal plane of graphite is relatively unreactive, the unusually high conductivity and zero band-gap of graphene (an isolated single layer of graphite) portend greater reactivity. Liu et al. examined the photoreactivity of benzoyl peroxide with a graphene sheet suspended in toluene. Laser excitation at several visible wavelengths led to the addition of phenyl groups to the graphene surface, and the rate of reaction increased with decreasing irradiation wavelength at the same total power. The reaction did not proceed thermally but could occur at very low laser power. The presence of air only led to unrelated photo-oxidation reactions. The authors argue that the reaction proceeds through a hot-electron mechanism, in which a surface-adsorbed benzoyl peroxide molecule accepts a photoelectron ejected from graphene into its lowest unoccupied orbital, and then decomposes into phenyl radicals.
J. Am. Chem. Soc. 131, 10.1021/ja9043906 (2009).
- Laura M. Zahn
Fig trees are totally dependent on species-specific fig wasps that carry pollen between trees. The wasps are weak nocturnal fliers and live for only 2 days at most, and neighboring trees flower asynchronously, so species survival (of both tree and wasp) depends entirely on successful transits.
By physical mapping and genetic finger-printing, Ahmed et al. measured the gene flow in 79 fig trees along 250 km of the Ugab River in Namibia. Fruit was collected from individual trees and paternity was assigned to enable tracking of the insect pollinators; fig wasps were shown to move over distances as large as 160 km. Within this population, gene flow was predominantly unidirectional, suggesting that the insects were borne by the easterly winds. The authors conclude that the long-distance pollination of isolated individuals helps to maintain both the plant and insect populations and enables them to overcome the barrier effects of habitat fragmentation.
Proc. Natl. Acad. Sci. U.S.A. 106, 10.1073/pnas.0902213106 (2009).
- Cell Biology
Stress Testing the ER
- Guy Riddihough
The antibody-producing cells of our immune system are able to secrete their own weight in antibodies each day. This prodigious feat relies on the endoplasmic reticulum (ER), which is a network of internal membranes in eukaryotic cells and which ensures that secreted proteins are folded and packaged correctly. Misfolded proteins are horrifically sticky and potentially harmful; luckily, the ER detects, via the unfolded protein response (UPR), when it is full up and boosts its handling capacity by, for example, increasing the expression of ER-resident chaperone proteins that untangle protein aggregates.
Schuck et al. find that a dramatic expansion of the area and volume of the ER during the UPR helps to alleviate the refolding bottleneck. The UPR drives ER expansion through increased synthesis of membrane lipids, which are incorporated into the peripheral ER. The shape of the ER enlargements, be they sheets or tubules, does not seem critical; rather, it is the increase in volume, which facilitates protein refolding and reduces the chance of aggregation, and the increase in surface area, which promotes membrane-associated degradation processes.
J. Cell Biol. 187, 525 (2009).
- Materials Science
- Julia Fahrenkamp-Uppenbrink
There are many approaches available to pattern soft materials such as colloidal films, but they often require multiple processing steps and allow deposition of only a few particle layers. A method called evaporative lithography overcomes these limitations, and Harris et al. now show that the method also enables the creation of patterns from binary mixtures of particles. Evaporative lithography exploits the fact that when aqueous and organic droplets dry, particles within the droplets are deposited in different patterns. Aqueous droplet evaporation leads to the familiar coffee-ring effect, whereas when nonaqueous droplets evaporate, most particles are deposited at the center. Use of a mask modulates the evaporative landscape of a drying droplet or film, resulting in pattern formation. The authors demonstrate generation of diverse patterns by tuning the mask design, mixture composition, and particle size ratio for an aqueous mixture of silica microspheres and sulphonated polystyrene nanoparticles. The method is simple and versatile and may be used to pattern colloidal, polymeric, and biomolecular species.
Philos. Trans. R. Soc. London Ser. A 367, 5157 (2009).