Simulating Foam Formation
Foams are easily made whether in the kitchen sink in the form of soap bubbles or a frothy head on the top of a quickly poured beer. Saye and Sethian (p. 720; see the Perspective by Weaire) describe the mathematical simulation of foam dynamics by decomposing the process into rearrangement, drainage, and rupture phases that are then linked by coupling the flux boundary conditions.
Defense and Counter-Defense
Provided a pathogen can enter the body and survive coughing and spluttering, peristalsis, and mucus, the first active responses the host evokes to an invading organism will be at the level of the first cell encountered, well before classical cellular immunity and antibody responses are initiated. Randow et al. (p. 701) review the range of intracellular defenses against incoming pathogens and describe how compartmental boundaries within the cell provide multiple levels at which pathogens can be thwarted in their attempts to subjugate the cell to do their bidding. Baxt et al. (p. 697) review the range of evasion tactics that bacterial pathogens can summon to counter host repulsion and establish a niche in which to replicate and ensure onward transmission.
Building Better Vaccines
In the past few years, several highly potent, broadly neutralizing antibodies (bNAbs) specific for the gp120 envelope protein of HIV-1 have been discovered. The goal of this work is to use this information to inform the design of vaccines that are able to induce such antibodies (see the Perspective by Crowe). However, because of extensive somatic hypermutation, the epitope bound by these antibodies often does not bind to the germline sequence. Jardine et al. (p. 711, published online 28 March; see the cover) used computational analysis and in vitro screening to design an immunogen that could bind to VRC01-class bNAbs and to their germline precursors. Georgiev et al. (p. 751) took advantage of the fact that only four sites on the HIV viral envelope protein seem to bind bNAbs, and sera that contain particular bNAbs show characteristic patterns of neutralization. An algorithm was developed that could successfully delineate the neutralization specificity of antibodies present in polyclonal sera from HIV-infected patients.
A Phase for Fano
In spectroscopy, samples placed between a steady light source and a detector are characterized based on the relative intensities of light absorbed at different frequencies. Temporal behavior—the relaxation of a photoexcited state—can be indirectly inferred from the absorption band shapes. The advent of ultrafast laser technology has enabled increasingly sophisticated measurements directly in the time domain. Ott et al. (p. 716; see the Perspective by Lin and Chu) present an analytical framework to account for asymmetric band shapes, termed Fano profiles, on the basis of a phase shift in the temporal dipole response.
Controlling the propagation of electromagnetic waves is a key requirement in communication technologies. The components tend to be bulky, however, which can make it difficult to integrate with microelectronics circuits. Using arrays of metallic nanoantennae patterned on a substrate surface, Shitrit et al. (p. 724) fabricated a novel class of metamaterials: anisotropic materials without inversion symmetry. The materials may pave the way to polarization-dependent nanophotonics.
Dust in the Clouds
Sulfate aerosols have the greatest radiative impact on climate systems. Harris et al. (p. 727) report that the oxidation of sulfur dioxide gas, catalyzed by natural transition metal ions mostly on the surface of coarse mineral dust, is the dominant pathway for sulfate production in clouds. In view of the growing sulfur dioxide emissions from large, industrializing countries, including this process in climate models should improve the agreement between models and observations.
Setting the Pace
The heart beats rhythmically throughout life. Highly specialized cardiac pacemaker cells control the timing of this beating. Bressan et al. (p. 744, published online 21 March) identified the embryonic location of the pacemaker precursors in early avian development and traced the cells throughout their incorporation into the heart. The events that establish the pacemaker lineage occur prior to the initiation of heart formation, and are governed, at least in part, by a class of Wnt signaling molecules.
Identical and Still Different
Even in monozygotic twins reared together, there are always observable differences reflecting the influence of individual responses. Freund et al. (p. 756; see the Perspective by Bergmann and Frisén) developed an inbred mouse model for studying the environmental influences on genetically identical animals and examined their effects on behavioral and neural development.
Infections Against Infection
In the same way that infection with the bacteria Wolbachia spp. can make Aedes mosquitoes resistant to dengue virus, there have been hints that these bacteria can interfere with the reproduction of malaria parasites. Bian et al. (p. 748) established a heritable Wolbachia infection in anopheline mosquitoes, which simultaneously suppressed the reproduction of malaria parasites within the adult female mosquitoes. The results hold promise for developing the model into a biocontrol agent to assist malaria control.
Of Mice and Markets
Some goods, such as widgets, are freely bought and sold in markets without protest, whereas others, such as indulgences, are not. Some mice that have been bred for use in laboratory experiments turn out to be surplus to requirements and are subsequently sacrificed. Falk and Szech (p. 707) studied the effect that marketplace negotiation has had on experimental subjects' willingness to pay for the upkeep of these surplus mice. Individuals were willing to pay much more to save the mice, but market-like exchanges lowered these prices.
Creating Unstable Atomic Orbitals
A hallmark of atomic Bohr orbitals is that they are stable; that is, time independent. However, for a very highly charged nucleus, the electrons must be described with the relativistic Dirac equation; the motion becomes time dependent, with electrons spiraling into the nucleus and coupling to positrons at large distances from the nucleus. In graphene, charge carriers are mass-less and described by the relativistic Dirac equation, and could also exhibit “atomic collapse” states. Wang et al. (p. 734, published online 7 March) created highly charged clusters of calcium dimers by atomic manipulation with a scanning tunneling microscope. The emergence of atomic-collapse resonances with increasing cluster size and charge was observed with scanning tunneling microscopy.
bZIPping Through Evolution
The basic region-leucine zipper (bZIP) transcription factors are found in many species and can form complexes that bind to DNA and affect transcription. Reinke et al. (p. 730) analyzed interactions for over 3000 bZIPs within and among five metazoan and two unicellular species. The results reveal differences within bZIP interactive networks that have accumulated over time and identify plasticity among interactions and changes in binding specificity that relate to specific amino acid residue changes.
Keeping in Synch
Although it differs from mammalian clocks, the circadian clock of cyanobacteria is a valuable model for understanding how such clocks function. At the heart of the cyanobacterial clock is a posttranslational regulation (PTR) circuit in which the phosphorylation of the clock protein KaiC oscillates. This circuit is apparently sufficient for generating rhythms, but it is connected to a transcriptional-translational (TTR) feedback loop more similar to the one that functions in mammals. This TTR loop is, at least in some conditions, dispensable. To understand the role of the TTR circuit, Teng et al. (p. 737) engineered cyanobacteria so that the circadian behavior of individual cells in a population of growing cells could be monitored. Cells engineered to lack the TTR mechanism had rhythmic clocks but fell out of synch with the other cells in a population over time. The experimental results together with mathematical modeling indicate that the TTR mechanism is important to allow cells to robustly stay in rhythm with one another in the absence of synchronizing external cues.
Getting It Wright?
In 2004, a paper by Wright et al. comparing six leaf traits of over 2000 plant species showed that between-species variation among the traits was confined primarily to a single multidimensional axis, but only if traits were normalized by leaf mass. This “leaf economic spectrum” has been influential in guiding understanding of the roles of plants in global carbon cycling. Osnas et al. (p. 741, published online 28 March) now show that the principal finding of Wright et al. is primarily a mathematical consequence of the way that the data were normalized. Analysis of the same data suggests that traits are primarily proportional to leaf area, not leaf mass. Using a method to analyze relationships among traits without normalization-induced correlations revealed a multidimensional correlation between leaf traits. These relationships imply weaker effects of leaf nitrogen on rates of photosynthesis and respiration, with important implications for current models of global change.
Dendritic Precision Strikes
The effects of excitatory synaptic inputs are considered to be highly compartmentalized because of the biophysical properties of dendritic spines. Individual inhibitory synapses, however, are thought to affect dendritic integration in a more extended spatial region. Combining optogenetic stimulation of dendrite-targeting γ-aminobutyric acid—mediated interneurons with two-photon calcium imaging in postsynaptic pyramidal cell dendrites, Chiu et al. (p. 759) challenge this latter view. The findings suggest that the effect of an inhibitory synapse can be as compartmentalized as that of an excitatory synapse, provided that the synapses are localized on spine heads.