This Week in PNAS
ENGINEERING
Micromovements with magnets
Benjamin Yellen et al. report the use of a fluid containing small magnetic particles to move cells, large molecules, and colloidal particles across a surface. The technique allows precise motion control without some of the problems associated with optical and electrical field traps. Yellen et al. used a chip with patterned magnetic patches and magnetic particles in fluid suspension to manipulate nonmagnetic objects ranging in size from <100 nm to several microns. The team manipulated the suspended magnetic particles above the substrate by controlling both the magnetic moments of the patches on the chip and the globally applied magnetic bias field. The particles pushed the nonmagnetic objects in controllable directions on the chip's surface, and the researchers could reprogram the chip's magnetization on the fly and redirect objects to new positions. The technique avoids heating associated with lasers and optical traps, as well as the potentially destructive effect that electric fields can have on biological materials. Although Yellen et al. were not able to freeze the objects in place or remove the suspended magnetic particles, the technique could lead to a useful tool for microfabrication and micromanipulation. — P.D.
Polystyrene beads assembling onto a magnetically patterned substrate inside a fluid.
“Arranging matter by magnetic nanoparticle assemblers” by Benjamin B. Yellen, Ondrej Hovorka, and Gary Friedman (see pages 8860–8864)
EVOLUTION
Ancient roots of South American plant–insect ecodiversity
The same types of diverse insect–plant relationships that typify modern South American forests appear to have grounding in the ancient past. Peter Wilf et al. report a wider variety of insect damage to South American plants as compared with North American contemporaries in the Eocene epoch. Currently, South America has a high diversity of plants that display specialized associations with diverse insects in tropical and subtropical forests. Wilf et al. investigated quarries exposing 52-million-year-old fossiliferous rocks to assess plant–insect associational diversity in the distant past. An examination of >3,500 fossil leaves from a site in the Patagonian region of Argentina revealed 52 different types of insect damage. Compared with the same type of examination of fossil leaves from three comparable sites in North America, the South American plants showed many more types of damage, indicating a higher richness of associated insect species. In addition, the total number of plant species found in the South American site was higher than that of the North American samples. This positive linkage between plant and insect diversity in early Eocene Patagonia supports an ancient history for high South American biodiversity. — T.D.
Diverse leaf feeding damage points to high insect richness in South America.
“Richness of plant–insect associations in Eocene Patagonia: A legacy for South American biodiversity” by Peter Wilf, Conrad C. Labandeira, Kirk R. Johnson, and N. Rubén Cúneo (see pages 8944–8948)
MEDICAL SCIENCES
Assessing double-strand DNA breaks caused by CT scans
Markus Löbrich et al. describe a rapid in vivo technique for assessing low-dose ionizing radiation damage in lymphocytes caused by computed tomography (CT) scans. Previous research in human radiation exposure has focused on studying victims of high radiation doses such as atomic bomb survivors, but little work has been seen in evaluating the effects of everyday radiation exposures. Löbrich et al. enumerated γ-H2AX foci by immunofluorescence in lymphocytes from individuals undergoing CT of the thorax and/or abdomen. γ-H2AX foci, phosphorylated histones, form as a result of radiation-induced double-strand breaks in DNA. Up to 1 h after CT, the authors observed a linear relationship between the number of foci formed and the dose length product, which is the product of the radiation dose deposited within the exposure field and the length of the body examined. Within 24 h, nearly all the individuals reached their preexposure foci level with a double-strand break repair rate similar to the rate of repair of fibroblasts irradiated in vitro. — F.A.
Measuring CT scan radiation damage.
6′-GNTI (red-orange) binding KOP-R (green)/DOP-R (blue) dimer, with activated Gα protein (yellow).
“In vivo formation and repair of DNA double-strand breaks after computed tomography examinations” by Markus Löbrich, Nicole Rief, Martin Kühne, Martina Heckmann, Jochen Fleckenstein, Christian Rübe, and Michael Uder (see pages 8984–8989)
PHARMACOLOGY
Relevance of G protein-coupled receptor
Maria Waldhoer et al. activated delta-(DOP-R) and kappa-opioid peptide receptor (KOP-R) heterodimers with the agonist ligand 6′-guanidinonaltrindole (6′-GNTI), demonstrating the functional relevance of G protein-coupled receptor (GPCR) heterodimers. Although many GPCRs form tissue-specific heterooligomers, the in vivo relevance of this oligomerization has not been well understood. Waldhoer et al. generated cell lines stably expressing DOP-R, KOP-R, or both. The authors observed that 6′-GNTI bound to the receptors individually and as heterodimers, and found that 6′-GNTI activated the coreceptor-expressing cells most potently. The cells expressing KOP-R alone had a lower potency, and 6′-GNTI had no agonist effect on DOP-R cells. In the coreceptor-expressing cells, selective antagonists of DOP-R and KOP-R were able to abolish the 6′-GNTI-mediated signaling. Because previous research has suggested that KOP/DOP-R heterodimers exist in the mouse spinal cord, the researchers administered 6′-GNTI intrathecally into the animals. The agonist ligand induced analgesia, with the effect blocked by DOP-R- and KOP-R-selective antagonists. When administered intracerebroventricularly, 6′-GNTI had no analgesic effect, which the authors attribute to the tissue-specific activity of KOP/DOP-R heterodimers in vivo. Based on these results, Waldhoer et al. suggest that selectively targeting the spinal cord may present a feasible approach for analgesia. — F.A.
“A heterodimer-selective agonist shows in vivo relevance of G protein-coupled receptor dimers” by Maria Waldhoer, Jamie Fong, Robert M. Jones, Mary M. Lunzer, Shiv K. Sharma, Evi Kostenis, Philip S. Portoghese, and Jennifer L. Whistler (see pages 9050–9055)
PLANT BIOLOGY
Hopping helitrons help haplotype variability
Jinsheng Lai et al. report that the genetic disparities between two lines of inbred maize are a result of a class of transposable elements, helitrons. Plant evolution has led to the existence of shared genetic sequences across species, a feature robustly illustrated by the similarity among the genomes of Arabidopsis, rice, and maize. However, different strains within a species can show significant genomic differences. The McC line of maize, for example, contains sequences from four genes that are absent from the equivalent location in the B73 line but are found elsewhere in the genome. To uncover the basis for these genomic differences, Lai et al. compared sequences in the bz genomic region of the two maize lines. The authors demonstrate that the movement of genes by two helitrons, HelA and HelB, accounts for the differences between the two bz locus haplotypes. These helitrons, which are large and carry fragments of several unrelated genes to new locations of the genome, may play an important role in gene evolution. — M.M.
“Gene movement by Helitron transposons contributes to the haplotype variability of maize” by Jinsheng Lai, Yubin Li, Joachim Messing, and Hugo K. Dooner (see pages 9068–9073)
