Simulating the emergence of life’s building blocks
According to one theory, life on Earth originated around 4.00 to 3.85 billion years ago, when a barrage of extraterrestrial impacts during the Late Heavy Bombardment triggered the formamide molecule to spontaneously break down into the nucleobase building blocks of RNA and DNA. Martin Ferus et al. (pp. 657–662) reproduced the simultaneous, high-energy synthesis of four nucleobases from the formamide-containing plasma generated by an extraterrestrial body impact. The authors used the high-power Prague Asterix Laser System to induce the dielectric breakdown of plasma produced by an extraterrestrial body impact. The breakdown was accompanied by all anticipated hallmarks of a high-energy density event, including shock rises in temperature to 4,500 K, the formation of a shock wave, and the generation of secondary hard radiation in the form of UV, vacuum UV, extreme UV, and X-ray light. The findings reveal that stable but highly reactive CN and NH radicals attack the formamide parent molecule, thereby forming several intermediates and eventually leading to the creation of the nucleic bases adenine, guanine, cytosine, and uracil. According to the authors, the findings address a central problem in origin-of-life research and may aid in the search for biogenic molecules in the universe. — A.G.
How neurons detect edges in a visual scene
Neurons in a brain region called primary visual cortex (V1) are specialized for processing visual features important for edge detection, which is crucial for interacting with objects in the world. Each V1 cell responds selectively to particular features of a visual scene, and the ability to detect features in the environment depends on the activity of individual neurons as well as the number of V1 neurons attuned to those features. Using a theoretical approach, Charles Stevens (pp. 875–880Z) reports that edge detection could be enhanced by the existence of different numbers of V1 neurons attuned to different spatial frequencies—measures of the number of times a pattern repeats per unit distance. By analyzing previously collected data from cat visual cortex, the author calculated the relative number of neurons attuned to each spatial frequency. While few neurons responded selectively to low or high spatial frequencies, many neurons were attuned to intermediate spatial frequencies. The author’s calculations revealed that the varying abundances of neurons attuned to different spatial frequencies could allow the brain to filter the visual scene to rapidly and efficiently detect edges. According to the author, such a computation may contribute to our ability to recognize simple line drawings of objects that look different in reality. — J.W.
DNA origami machine elements with complex motion
DNA origami machine components.
DNA origami, an emerging technique for folding DNA into complex 3D shapes, has shown immense potential for nanodevice applications in industrial technology and medicine. Bottom-up self-assembly is a particularly promising area, but facilitating complex motion at the nanoscale level remains a challenge. Alexander Marras et al. (pp. 713–718) integrated concepts from engineering machine design and DNA origami to create nanoscale DNA origami machine elements with complex and reversible 1D, 2D, and 3D motions. The authors designed, fabricated, and characterized the mechanical behavior of flexible DNA origami rotational and linear joints that integrate stiff dsDNA components and flexible ssDNA components. In doing so, the authors constrained motion along a single degree of freedom and demonstrated the ability to tune the flexibility and range of motion. Multiple joints with simple 1D motion were then integrated into higher order mechanisms, including a crank-slider that couples rotational and linear motions, and a Bennett linkage that moves between a compacted bundle and an expanded frame configuration with a constrained 3D motion path. Finally, the authors demonstrated the structure’s ability to undergo reversible conformational changes on minute timescales. According to the authors, the findings demonstrate the use of a macroscopic machine design approach to construct DNA origami mechanisms with programmable 2D and 3D motion. — A.G.
Insecticide-treated bed nets and malaria mosquito adaptation
Anopheles gambiae mosquito. Image courtesy of the CDC.
Two common West African malaria mosquitoes, Anopheles gambiae and Anopheles coluzzii, are distinct species that periodically hybridize. However, hybrids typically die without producing offspring. In 2006, populations of A. coluzzii in Mali began developing tolerance to insecticides used to control the spread of malaria due to the appearance of an island of mutations in an insecticide resistance gene, known as the kdr gene, which had previously only existed in A. gambiae. To understand how the mutations appeared, Laura Norris et al. (pp. 815–820) collected mosquitoes from homes in Mali between 2001 and 2012 and analyzed their genomes for the presence of insecticide resistance mutations. The authors found that the mutations appeared in A. coluzzii populations around the same time that the two species experienced an increase in interbreeding, suggesting that hybrid mosquitoes introduced the insecticide resistance mutations into A. coluzzii populations by mating with the parent species. The emergence of the mutations also coincided with a campaign in Mali to distribute insecticide-treated bed nets (ITNs). According to the authors, selective pressure from insecticides in the ITNs may have favored survival of hybrid mosquitoes that facilitated evolution of insecticide resistance by A. coluzzii and blurred the boundaries between the species. — J.P.J.
Temperature and susceptibility to rhinoviruses
Rhinovirus infection of airway epithelial cells results in accumulation of double-stranded RNA (blue) during viral replication. Image courtesy of Ulysses Isidro and Jeffrey Grotzke (Yale University, New Haven, CT).
Most rhinoviruses, which cause common cold and trigger asthma, are thought to multiply less efficiently in the lungs, where temperatures hover around 37 °C, than in the cooler environment of the nasal cavity, where temperatures of 33 to 35 °C are common due to inhalation of ambient air. To determine whether temperature-dependent differences in the antiviral immune response mounted by hosts might enable rhinoviruses to establish infections more easily in the nasal cavity than in the lungs, Ellen Foxman et al. (pp. 827–832) examined viral replication and antiviral responses in airway epithelial cells, the virus’s main target. When mouse airway epithelial cells were exposed to a mouse-adapted rhinovirus strain at either 33 °C or 37 °C, the virus replicated less efficiently and produced lower levels of infectious viruses at the higher temperature. During replication, the virus induced more robust type I interferon-mediated immune responses—known to curb viral replication—in the cells at 37 °C than at 33 °C. Further, the authors report, two related immune signaling mechanisms—RIG-I–like receptor and interferon-αβR pathways—were more active at 37 °C than at 33 °C. According to the authors, the findings furnish a link between temperature-dependent rhinovirus replication and host immune defense, raising the possibility that exposure to cool air might lower resistance against rhinoviruses. — P.N.
Regulation of daily rhythms in plants
In plants, many biological processes follow daily rhythms maintained by the internal circadian clock acting in concert with external signals, such as temperature and light. However, the extent to which these rhythms vary within a species and how such variation is regulated remain unknown. To determine the source of intraspecies variation in response to changes in the day/night cycles, Amaury de Montaigu et al. (pp. 905–910) studied an evening-expressed gene, GIGANTEA (GI), in Arabidopsis. The authors transformed 77 Arabidopsis isolates with DNA encoding the GI promoter fused to a reporter gene. In all strains, longer day length led to later peak expression of GI, but the timing and level of expression varied. To find genetic loci responsible for these differences, the authors crossed two strains with different expression patterns, analyzed genetic markers and phenotypes among five generations of progeny, and identified four loci that precisely regulated the timing of GI expression during long days. One locus contained an in-frame deletion in the PHYTOCHROME B PHOTORECEPTOR (PHYB) gene that correlated with a late peak in GI expression, similar to that observed with a reduced-function phyB mutant. According to the authors, the results demonstrate that natural rhythms may be regulated by altering sensitivity to input signals without disrupting the circadian system. — C.B.