In This Issue
ENGINEERING, CHEMISTRY
Competing mechanisms in carbon nanotube breakage
Traian Dumitrica et al. discuss the competition between brittle fracture and plastic deformation in carbon nanotubes and present results indicating that both breaking mechanisms coexist, with either dominating in failure during a given test. Although the extreme resistance to mechanical tension in carbon nanotubes has been well studied, it has been difficult to establish quantitative levels of breaking strain or to identify details of dominant atomic-scale rearrangements. Dumitrica et al. combined molecular dynamics simulations, quantum mechanical evaluation of energy characteristics for key configurations, and reaction rate theory for the probability of a yield event in a simultaneous study of bond rotation and brittle bond-breaking mechanisms. Breaking strain values were calculated for nanotubes of different symmetries and diameters at varying temperatures and load rates. The authors were able to determine domains of dominance for the competing routes of brittle breaking and plastic relaxation, and they mapped the carbon nanotube strength as a function of tensile test duration, temperature, and chiral symmetry of the sample. This approach can be applied to other nanotubes, the authors say, such as boron-nitride, metal disulfides, or perhaps cellular microtubules where an ideal cylindrical lattice can respond to tension in a similar manner. — R.N.
Nanotube failure varies with chirality, temperature, and test duration.
“Symmetry-, time-, and temperature-dependent strength of carbon nanotubes” by Traian Dumitrica, Ming Hua, and Boris I. Yakobson (see pages 6105–6109)
BIOPHYSICS
Nanomechanical measurements of nucleic acid hairpin folds
Michael Woodside et al. report the development of high-precision measurements of nucleic acid hairpin folding and unfolding in an optical trap. DNA and RNA hairpins are involved in important processes such as transcription regulation, recombination, and protein recruitment. Hairpin folding also supplies a model system for studying helix formation, which underlies all nucleic acid structure. Woodside et al. provide a direct measurement of the transition state for this elementary process, finding that it involves the formation of 1–2 bp next to the loop. Studying 20 different DNA hairpin sequences with varying stem length (6–30 bp), loop length (3–30 nt), and stem GC content (0–100%), the authors measured hairpin opening distances with subnanometer precision. Unfolding energies spanning one order of magnitude and transition rates covering six orders of magnitude were observed. The results agreed quantitatively with a hybrid energy landscape model that combines well-established thermodynamic nearest-neighbor free energies with the known nanomechanical properties of DNA. By testing and supporting both the nucleation model for helix formation and a quantitative landscape model for folding, the authors say this work should help guide future investigations of the fundamental kinetic and thermodynamic behavior of nucleic acids. — P.D.
DNA hairpin placed under tension in optical trap.
“Nanomechanical measurements of the sequence-dependent folding landscapes of single nucleic acid hairpins” by Michael T. Woodside, William M. Behnke-Parks, Kevan Larizadeh, Kevin Travers, Daniel Herschlag, and Steven M. Block (see pages 6190–6195)
DEVELOPMENTAL BIOLOGY
Tracking epithelial cell motility in transgenic Hydra
Jörg Wittlieb et al. have generated stable transgenic Hydra containing fluorescent epithelial cells, which, combined with the transparency of Hydra tissue, should greatly aid in tracking in vivo cell behavior during morphogenesis. The authors created GFP-expressing Hydra vulgaris and established several Hydra lines that expressed GFP in their endodermal epithelial cells. Under fluorescence, endodermal cell motility, rather than passive tissue displacement, was observed to play a major role in Hydra bud formation and tissue regeneration. When tissue from a regenerating polyp was grafted onto the gastric region of transgenic Hydra, endodermal epithelial cells rapidly mobilized to the graft site and formed a secondary body axis, demonstrating high motility. This recruitment could be induced by distantly related Hydra species, indicating that the Hydra signaling system has been highly conserved evolutionarily. Because Hydra were the first metazoans to develop complex body structures, these transgenic lines may serve as important tools to study the evolution of development. — N.Z.
Transgenic Hydra expressing GFP in endodermal epithelial cells.
“Transgenic Hydra allow in vivo tracking of individual stem cells during morphogenesis” by Jörg Wittlieb, Konstantin Khalturin, Jan U. Lohmann, Friederike Anton-Erxleben, and Thomas C. G. Bosch(see pages 6208–6211)
GENETICS
Neurogenetic examination of impulsivity and aggression
Andreas Meyer-Lindenberg et al. present neuroimaging data that suggest neural mechanisms for the association of a monoamine oxidase A (MAOA) gene variation with impulsive aggression. The authors studied the effects of MAOA-L, a common polymorphism of the X-linked MAOA gene, on brain structure and function as assessed by MRI in a sample of 142 healthy volunteers. The MAOA-L variant, for which MAO-A enzyme expression is relatively low, has been associated with increased risk for impulsive aggression in interaction with early life adversity. Meyer-Lindenberg et al. found that, compared with volunteers with the MAOA-H variant in which MAO-A expression is relatively high, volunteers with the MAOA-L genotype showed reduced volume in limbic brain regions, including the cingulate gyrus and bilateral amygdalae, increased amygdala activity during viewing of fearful stimuli, and decreased reactivity in orbitofrontal and cingulate cortex. Sex–genotype interactions were also observed, with MAOA-L men having increased orbitofrontal volume, increased amygdala and hippocampal reactivity during retrieval of emotionally negative material, and impaired cingulate activation during cognitive inhibition. The observed variations did not imply increased risk for violence in this mentally healthy sample, the authors say, but instead point to neural mechanisms mediating a gene–environment interaction. — R.N.
Limbic region volume reductions in MAOA-L individuals.
“Neural mechanisms of genetic risk for impulsivity and violence in humans” by Andreas Meyer-Lindenberg, Joshua W. Buckholtz, Bhaskar Kolachana, Ahmad R. Hariri, Lukas Pezawas, Giuseppe Blasi, Ashley Wabnitz, Robyn Honea, Beth Verchinski, Joseph H. Callicott, Michael Egan, Venkata Mattay, and Daniel R. Weinberger (see pages 6269–6274)
MEDICAL SCIENCES
Bone marrow-derived cells fuse with intestinal stem cells
Adnan Rizvi et al. show that adult bone marrow-derived cells (BMDCs) can repopulate an injured gut with all epithelial intestinal cell lineages, including enterocytes, goblet cells, Paneth cells, and enteroendocrine cells, possibly by fusing with long-lived intestinal progenitors. Cell fusion has arisen as one explanation of how transplanted BMDCs might adopt the role and appearance of a nonhematopoietic cell lineage. To track the fate of BMDCs in intestinal epithelium, Rizvi et al. transplanted GFP-tagged hematopoietic stem cells into mice that had received lethal doses of radiation to obliterate native bone marrow and to damage intestinal cells. Beginning at 2 weeks and continuing for 14 months after transplantation, cells derived from the stem cell donors were detected in both crypt and villus epithelium and the lamina propria of the small intestine. Also, the fusion of BMDCs with intestinal tumor cells did not initiate tumor formation, but may contribute to tumor progression; this cell fusion may be the root of phenotypic and genotypic diversity in tumors. Additionally, the authors suggest that fusion may enable BMDCs to regenerate intestinal cells, as has been shown for diseased liver cells, Purkinje neurons, and skeletal and cardiac muscle cells. — B.T.
Fusion between BMDCs and intestinal epithelium.
“Bone marrow-derived cells fuse with normal and transformed intestinal stem cells” by Adnan Z. Rizvi, John R. Swain, Paige S. Davies, Alexis S. Bailey, Adria D. Decker, Holger Willenbring, Markus Grompe, William H. Fleming, and Melissa H. Wong (see pages 6321–6325)
