Editors' Choice

Science  24 Dec 2004:
Vol. 306, Issue 5705, pp. 2164

    Limits to Growth

    1. Stella M. Hurtley

    Plasticity in neurons is regulated, in part, by the degradation of specific proteins at synapses. Actively dividing cells rely on ubiquitin-dependent degradation to regulate the transitions through the phases of the cell cycle. Van Roessel et al. find that a key enzyme involved in the latter process, the anaphase-promoting complex (APC), plays a role in controlling synaptic size and plasticity. [APC has also been linked to axonal growth and patterning (Konishi et al. Reports, 13 February 2004, p. 1026).] In Drosophila, APC subunits are found at neuromuscular synapses, and when APC levels were reduced, the synaptic boutons of motor neurons increased in size because of the action of the protein Liprin-α, which is a substrate for APC-stimulated ubiquitinylation and degradation. Furthermore, muscles lacking APC displayed altered synaptic transmission, and the postsynaptic levels of glutamate receptor were increased. These pre- and postsynaptic functions of APC may explain why a cell cycle regulator is expressed in differentiated postmitotic cells. — SMH

    Cell 119, 707 (2004).


    Of Mice . . .

    1. Stephen J. Simpson

    Murid rodents are only one of the approximately 146 families of mammals, yet comprise nearly one-third of all mammalian species. A robust phylogeny would provide the framework for understanding their evolutionary success as well as their roles as model organisms in biomedical research and as hosts and vectors of human pathogens. Steppan et al. present analyses based on sequences from 53 genera of four nuclear genes (GHR, BRCA1, RAG1, and c-myc), which yield nearly identical phylogenies. Taken together, these resolve most relationships among the 16 subfamilies and identify four distinct explosive radiations. One occurred when the ancestor of most Sigmodontinae colonized South America; another as the Murinae (Old World mice and rats) expanded their range from Southeast Asia across Asia and Africa. The results also suggest that—through the attribution of fossil calibrations to the wrong nodes and the neglect of rate heterogeneity—nearly all past applications of a molecular clock calibrated using the mouse/rat divergence have overestimated dates (that is, placed them too far back in time) by 20 to 50%. — SJS

    Syst. Biol. 53, 533 (2004).


    Ingesting Nanotubes

    1. Phillip D. Szuromi

    One concern in nanotechnology is that the uptake and fate of nanomaterials in cells may differ from those of larger micrometer-scale particles. Two groups have imaged the uptake of carbon nano-tubes into mammalian cells. Cherukuri et al. incubated single-walled carbon nanotubes (SWNTs, about 1 nm in diameter and 1 μm in length) solubilized in Pluronic surfactant with cultured mouse peritoneal macrophage-like cells. Using near-infrared fluorescence imaging, they found that the macrophages ingested the SWNTs and apparently localized them in phagocytic compartments, without signs of acute toxicity. Monteiro-Riviere et al. looked at the uptake of multiwalled carbon nanotubes (MWNTs) by cultured human epidermal keratinocytes. Although most of the MWNTs, which were not modified after growth on silicon wafers, did not interact with the cells, enough did that 84% of the cells took up MWNTs after 48 hours of exposure at 0.4 mg/ml. After 24 hours at this concentration, the percentage of viable cells decreased by 30%, and transmission electron microscopy revealed MWNTs (some almost 4 mm in length) within cytoplasmic vacuoles in 60% of the cells. Although these cultured keratinocytes lack the protective stratum corneum of human skin, these results indicate that further studies of carbon nanotube exposure risks are in order. — PDS

    J. Am. Chem. Soc. 126, 15638 (2004); Toxicol. Lett.10.1016/ j. toxlet.2004.11.004


    Individual Differences

    1. Gilbert J. Chin

    Neuroimaging has begun to map specific patterns of brain activity associated with cognitive functions. The usual statistical analysis of these rather large data sets relies on having about a dozen subjects and looking for consistent neuronal activations, but an increasing interest in how personality traits and mood states might influence responses has led to looking at activations across subjects.

    Canli et al. used the emotional Stroop interference task to show that negative words elicited greater activation of the anterior cingulate region, which is known to be involved in processing cognitive/emotional stimuli, with greater negative mood of the subject; whereas activation due to positive words correlated with higher scores for the trait of extraversion. This dissociation might plausibly be interpreted as reflecting a greater susceptibility to being distracted by negative interfering stimuli while in a negative frame of mind and, conversely, being more receptive to positive stimuli if one is inherently an outgoing sort. Kumari et al. have used the n-back task to show that with increasing cognitive demands, activation in the anterior cingulate increased in all subjects, but much more so for the ones who scored as extroverts, consistent with them being less aroused or anxious at rest and hence having to mobilize more cognitive resources to perform at the same level. — GJC

    Behav. Neurosci. 118, 897 (2004); J. Neurosci. 24, 10636 (2004).


    Salting in Nanotubes

    1. Marc S. Lavine

    Single-walled carbon nanotubes (SWNTs) are of interest because of their outstanding mechanical and electrical properties, and the tendency of SWNTs to aggregate into bundles has been overcome by modifying them chemically, dissolving them in superacids, or by sonicating them with the addition of surfactants or polymers. Unfortunately, all of these methods are based on an intercalating mediator that prevents the strong sidewall van der Waals forces from reaggregating the tubes, and many of these methods cut or damage the tubes.

    Pénicaud et al. show that SWNTs can be reduced using alkali metals to form polyelectrolyte salts that dissolve in aprotic polar organic solvents such as dimethyl sulfoxide. Elemental analysis indicated that the metals removed one negative charge for every 10 carbon atoms; however, only one of five charges was dissociated, whereas the others were balanced by the condensation of alkali cations. The nanotube polyelectrolyte solutions appear to be stable indefinitely, although they need to be kept under an inert atmosphere because the reduced SWNTs are sensitive to air. — MSL

    J. Am. Chem. Soc. 10.1021/ja0443373 (2004).


    Making a Copy of a Copy

    1. Guy Riddihough

    MicroRNAs (miRNAs) are small noncoding RNAs that are complementary to their targets and are encoded in the genomes of most plants and animals as self- complementary fold-back precursors, which undergo processing into ∼21-nucleotide (nt) effector species. The fold-back structure of miRNA precursors suggests that miRNA genes may have evolved from inverted duplications of their target genes, and Allen et al. explore this possibility in Arabidopsis. If miRNAs arose in this manner, they should have regions of homology extending beyond the ∼21-nt complementary core. Of the 91 miRNA loci used to search the Arabidopsis genome, only miR161 and miR163 showed extended sequence similarity to their target genes and to closely related family members. Unlike other miRNA multigene families, miR161 and miR163 are represented by single genes and are not found outside Arabidopsis, supporting the idea that they might be evolutionarily recent additions. Potential evolutionary intermediates of miRNAs were also identified; one of these loci is located close to its putative targets, as are miR161 and miR163, and phlyogenetic analysis indicates that all three are related to their targets. — GR

    Nature Genet. 36, 1282 (2004).


    . . . and Men

    1. Andrew M. Sugden

    Fossil and molecular evidence have hitherto suggested that the cercopithecoids (Old-World monkeys) and hominoid (ape and human) lineages diverged around the Oligocene/ Miocene boundary, 23 to 25 million years ago (Ma). In a challenge to the recentness of this estimate, Steiper et al. adopt a molecular approach called quartet analysis, which uses sequence data from two pairs of species from two clades to assess divergence dates with greater precision. For the hominoid branch, chimpanzee and human were chosen, and for the cercopithecoids, baboon and macaque; the divergence dates between the members of each pair were calibrated from fossil data. The resulting model suggests that the hominoid/cercopithecoid divergence took place in the Early Oligocene, 29 to 34.5 Ma. The implication of this result is that several million years of early hominoid history have yet to be sampled paleontologically and that Proconsul—hitherto considered the earliest of all hominoids—may have had earlier hominoid ancestors. — AMS

    Proc. Natl. Acad. Sci. U.S.A. 101, 17021 (2004).