Editors' Choice

Science  20 Feb 2004:
Vol. 303, Issue 5661, pp. 1106
  1. MICROBIOLOGY

    Sinusoidal Sporozoite SPECTacle

    1. Caroline Ash

    During malaria infection, sporozoites injected from the salivary gland of the biting mosquito must migrate to the hepatocytes of the victim's liver to multiply and develop. To access the hepatocytes, the sporozoites must pass from the circulation across a layer of liver sinusoidal cells. This stage of malaria transmission is a promising target for vaccines and drugs, but is not as well characterized as other stages of the parasite life cycle.

    Ishino et al. have explored the route by which sporozoites exit the bloodstream and cross the sinusoidal cell layer of the liver. They discovered a novel protein important for parasite motility and invasion called SPECT, which was specifically produced by the parasite at this stage, and was localized to and secreted from a specialized compartment of the parasite termed the microneme. Parasites were observed to attach to and migrate over the surface of the epithelial cells lining the sinusoidal veins until they reached a Kupffer cell (a liver macrophage-like cell that forms up to 30% of the sinusoidal layer), through which they migrated to reach the hepatocytes. SPECT-deleted parasites could not cross the sinusoidal layer even though they could still invade hepatocytes in vitro. — CA

    PloS Biol. 2, 77 (2004).

  2. ENVIRONMENTAL SCIENCE

    Outlook Smoggy

    1. H. Jesse Smith

    Automobiles emit approximately one-third of the CO2 produced by fossil fuel burning, along with huge quantities of carbon monoxide and hydrocarbons. Because diesel engines can be made to emit less than similar gasoline models, it would seem that replacing gasoline with diesel vehicles could help reduce air pollution and slow global warming. This is, in fact, a strategy being pursued or discussed in much of Europe and in California.

    Jacobson et al. present a study showing that nitrogen emissions from diesel engines may still enhance photochemical smog. The culprits in this inconvenient and previously neglected complication are NOx (NO and NO2) and the ratio of NO2 to NO in the emissions. Diesel vehicles emit 4 to 30 times more NOx than gasoline vehicles and have NO2/NO as much as 16 times higher. Because these two factors are critical in the production of smog, unless ways can be found to reduce them in the exhaust, large-scale conversion to diesel could worsen air pollution. Moreover, increased production of ozone caused by high NOx and higher particulate emissions make the choice of diesel more problematic than may be suggested if considering only CO, CO2, and hydrocarbons. — HJS

    Geophys. Res. Lett. 31, L02116 (2004).

  3. CHEMISTRY

    Microfluidics for Organics

    1. Marc S. Lavine

    Microfluidics devices have traditionally been fabricated from silicon and glass, using photolithography and etching techniques, or from poly(dimethylsiloxane) (PDMS), using soft lithography techniques. Both of these methods have their limitations: The former requires clean room facilities and is labor-intensive, and it is difficult to engineer components that require flexibility; PDMS is flexible, but can swell in various organic solvents.

    Rolland et al. show that the PDMS can be replaced with perfluoropolyethers (PFPEs), which exhibit low surface energy, low modulus, high gas permeability, and low toxicity, but also possess chemical resistivity similar to that of Teflon. Fabrication of devices was accomplished by partially curing layers to give them some rigidity, and fully curing after repositioning so that they properly fuse together. Fully cured PDMS and PFPE materials had similar elastic behavior at room temperature and also similar tensile moduli. The PFPE material was resistant to toluene and dichloromethane, which caused the PDMS channels to swell and plug shut. — MSL

    J. Am. Chem. Soc. 10.1021/ja031657y (2004).

  4. ECOLOGY/EVOLUTION

    Symmetry and Speciation

    1. Andrew M. Sugden

    Evolutionary theory predicts that there will be higher potential for diversification in groups of organisms that have traits that promote reproductive isolation between populations. For plants, one such feature is floral symmetry. Bilaterally symmetrical (zygomorphic) flowers such as orchids allow for more precise collection and placement of pollen by visiting animal pollinators than do radially symmetrical (actinomorphic) flowers such as buttercups. Mutations that cause changes in the shape and dimensions of zygomorphic flowers have the potential to generate reproductive isolation between populations by decreasing pollinator-mediated gene flow.

    Sargent compared species richness between zygomorphic and actinomorphic lineages in related animal-pollinated groups of plants. The zygomorphic lineages, in 15 out of 19 cases, were more species-rich than the actinomorphic lineages. Assuming that extinction rates are not higher in the actinomorphic lineages, this suggests that animal pollination indeed promotes the likelihood of reproductive isolation and speciation in plants with bilaterally symmetrical flowers. — AMS

    Proc. R. Soc. London Ser. B 10.1098/rspb.2003.2644 (2004).

  5. PALEONTOLOGY

    The World at Their Feet

    1. Linda Rowan

    Although mammals and dinosaurs evolved at about the same time about 200 million years ago, the fossil record of mammals is scant. Because mammals tended to be small in numbers and stature, their delicate bones are rarely preserved and the record consists mostly of teeth.

    One way to sidestep this gap in the fossil record is to look for tracks made by mammals. Lockley and Foster have discovered several sets of mammal tracks from the latest Cretaceous (about 75 to 65 million years ago) in Colorado. The three- to five-toed footprints resemble those of modern rodents; however, rodents did not evolve until later. The footprints probably belong to a multituberculate or a marsupial. From the tracks it is clear that these mammals were small and agile and lived among birds, dinosaurs, and other reptiles on a fertile coastal plain. More work will be needed to connect the partial record from head to toe to determine more about how mammals evolved in the Mesozoic. — LR

    Ichnos 10, 269 (2003).

  6. BIOMEDICINE

    Shock Therapy

    1. Paula A. Kiberstis

    Streptococcus pyogenes is a versatile bacterial pathogen that causes a wide range of illnesses in humans, from mild throat infections to toxic shock syndrome (STSS). STSS is an acute infection characterized by vascular injury and multi-organ failure. Even with aggressive treatment, STSS is fatal in 30 to 70% of cases, so new therapies are desperately needed.

    A mouse model of S. pyogenes infection has helped researchers delineate the pathophysiological chain of events leading to STSS. Herwald et al. find that a complex consisting of a protein shed by the bacteria (M1) and a blood clotting factor (fibrinogen) binds to and activates polymorphonuclear neutrophils (PMNs) of the immune system through interaction with β2-integrins. Rather than ingesting and destroying the bacteria, as they are designed to do, the activated PMNs undergo a process called “frustrated phagocytosis.” As a result, they release proteins that induce vascular leakage and massive inflammation, which in turn causes severe tissue damage. Importantly, lung damage in this mouse model was diminished by administration of a peptide that disrupts the interaction between fibrinogen and β2-integrins. — PAK

    Cell 116, 367 (2004).

  7. STKE

    Crosstalk to Death

    1. Lisa D. Chong

    Excessive nitric oxide (NO) and the release of zinc from intracellular stores have been linked to neuronal death associated with stroke and some neurodegenerative diseases. The mechanisms that mediate the neurotoxic effects have not been clear.

    Bossy-Wetzel et al. found that zinc is an important component of the NO neurotoxicity pathway. Increased intracellular zinc within mitochondria was observed in cultured cerebrocortical neurons exposed to NO. A free-radical scavenger blocked this effect. Zinc blocked respiration in isolated mitochondria, suggesting that this organelle is a key target of zinc-mediated toxicity. A zinc chelator blocked NO-induced activation of p38 mitogen-activated protein kinase (MAPK), a critical signaling molecule in the NO response pathway. NO exposure was also associated with potassium efflux and cell shrinkage, a morphological change characteristic of neuronal apoptosis. Treatment of neurons with a potassium channel antagonist, a zinc chelator, a reactive oxygen species (ROS) scavenger, or an inhibitor of p38 MAPK decreased NO-mediated neuronal cell death. Thus it appears that NO triggers zinc-mediated inhibition of mitochondrial function via an increase in ROS production. ROS then reacts with NO to activate p38 MAPK, which could then alter potassium channel function, causing progressive cell death. — LDC

    Neuron 41, 351 (2004).

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