CHEMISTRY
Searching for a better mosquito repellent
N,N-diethyl-m-toluamide (DEET) has been the gold standard for mosquito repellents for >50 years. But because mosquitoes still transmit diseases such as malaria in many areas, the search for more effective repellents continues. Alan Katritzky et al. conducted a rigorous search of a library of N-acylpiperidines, using an artificial neural network to identify strong candidates, and then tested them in the laboratory on human volunteers. The authors based their neural network on a set of 200 previously characterized repellents, including such factors as molecular weight and surface area. Once the network had been validated, the authors set it to analyze >2,000 N-acylpiperidines. They then selected 23 compounds for synthesis by a novel method using benzotriazole activation to acylate piperidines. Volunteers wore arm patches impregnated with standard doses of repellents and were exposed to caged mosquitoes. The authors measured persistence (the time until repellence wore off, signaled by the onset of biting) and found several compounds significantly more potent than DEET, some lasting more than three times as long. In this assay, DEET provided repellence for 17.5 days; one of the new compounds lasted 85 days. — K.M.
Hands treated with DEET (Left) and untreated (Right).
“Synthesis and bioassay of improved mosquito repellents predicted from chemical structure” by Alan R. Katritzky, Zuoquan Wang, Svetoslav Slavov, Maia Tsikolia, Dimitar Dobchev, Novruz G. Akhmedov, C. Dennis Hall, Ulrich R. Bernier, Gary G. Clark, and Kenneth J. Linthicum (see pages 7359–7364)
BIOCHEMISTRY
Toxin attack in view
The bacteria that cause antibiotic-associated diarrhea, as well as botulism and other forms of food poisoning, produce toxins that attack the cytoskeleton of cells, ultimately killing them. The prominent role of these toxins, called actin-specific ADP-ribosylating toxins (ADPRTs), in bacterial virulence suggests that they may be attractive targets for antibacterial drug design. Although the structures for many of these ADPRTs have been determined, just how these toxins recognize actin, and how they selectively alter a single residue of the protein, remains unclear. Hideaki Tsuge et al. report the crystal structure of actin complexed with Ia, the enzymatic component of the Clostridium perfringens ι-toxin, showing the key sites involved in actin recognition. The authors confirm the importance of these sites for the toxin's activity and propose a mechanism by which the toxin selectively transfers an ADP-ribose to the Arg177 residue of actin: an event that reduces the ability of actin to polymerize, leading to disorganized cytoskeletal architecture and cell death. These insights, the authors say, may aid in the development of drugs that inhibit the interaction between the toxins and actin, providing new therapeutic strategies against serious infectious diseases. — M.M.
Structure of Ia (blue and yellow) and actin (green).
“Structural basis of actin recognition and arginine ADP-ribosylation by Clostridium perfringens ι-toxin” by Hideaki Tsuge, Masahiro Nagahama, Masataka Oda, Shinobu Iwamoto, Hiroko Utsunomiya, Victor E. Marquez, Nobuhiko Katunuma, Mugio Nishizawa, and Jun Sakurai (see pages 7399–7404)
BIOPHYSICS
And yet it moves
Rhodopsin-like receptors form a large class of G protein-coupled receptors (GPCRs). Exactly which conformational changes take place on activation (in the case of rhodopsin, by illumination) has been an open question, and a recent study concluded that the conformation of rhodopsin does not change substantially. Christian Altenbach et al. provide evidence for a conformational shift, based on the application of the double electron–electron resonance (DEER) EPR technique. Researchers previously have used site-directed spin labeling to investigate dynamic receptor structure. DEER, however, is a spin echo method that enables measurement of interelectron distances at much greater separation (20–60 Å) than previously possible. Altenbach et al. studied the motion of nitroxide moieties affixed to the transmembrane helices on the outer protein surface, rather than deep inside the protein where their presence might alter the overall structure. The authors used DEER data with a model conformation derived from structural data to obtain a maximally probable spatial distribution of the nitroxide chains for both inactive and light-activated rhodopsin. The major event upon activation is an outward 5-Å radial shift by helix 6, which could prove common to all rhodopsin-like GPCRs, the authors say. — K.M.
A spin label attached to transmembrane helix in activated rhodopsin.
“High-resolution distance mapping in rhodopsin reveals the pattern of helix movement due to activation” by Christian Altenbach, Ana Karin Kusnetzow, Oliver P. Ernst, Klaus Peter Hofmann, and Wayne L. Hubbell (see pages 7439–7444)
IMMUNOLOGY
Brain medicine from celery
Dietary flavonoids, natural compounds that have antiinflammatory properties, have been linked to beneficial effects in cancer and diseases such as arthritis. In one study, flavonoid intake was inversely related to dementia. Saebyeol Jang et al. investigated how luteolin—a flavonoid found in celery and green pepper—acts on microglia in mice to reduce inflammation. The brain is immune-privileged, however, and microglia provide its main immunological defense. Using levels of the inflammatory cytokine IL-6 as a generalized measure of inflammation, the authors studied primary mouse microglia and the immortalized microglia line BV-2 in cell cultures. Treatment with bacterial lipopolysaccharide, which induces inflammation, stimulated IL-6 production that was reduced similarly in both cell types and eliminated at high doses. Their analysis of transcription factor DNA binding revealed that luteolin exerts its effect primarily by reducing binding of the promoter AP-1, likely by preventing upstream phosphorylation of JNK. In mice, lipopolysaccharide injection triggered inflammation (and high IL-6) in both the periphery and the brain. In an in vivo study, the authors found that doses of luteolin in drinking water reduced IL-6 in plasma and in the hippocampus, which has a high density of microglia, but not in other brain regions. The authors suggest luteolin as a potential treatment for inflammatory brain conditions. — K.M.
“Luteolin reduces IL-6 production in microglia by inhibiting JNK phosphorylation and activation of AP-1” by Saebyeol Jang, Keith W. Kelley, and Rodney W. Johnson (see pages 7534–7539)
MICROBIOLOGY
Flu gets better at binding
Researchers have found that some North American strains of influenza have increased their affinity for human cell surface molecules. These findings suggest that the viruses are coming closer to attaining the characteristics needed to cause a human pandemic and that surveillance of avian flu viruses, and preparations for a possible pandemic, should continue. Jessica Belser et al. analyzed the binding strength between several recent avian flu viruses and the sialic acid sugar molecules found on the surface of human and ferret cells. The authors found that a few of the H7 strains that caused minor, untransmissible viral infections in individuals in North America between 2002 and 2004 have increased their affinity for the sialic acids found on human tracheal cells. They show that one strain of the H7N2 virus, isolated from a man in New York in 2003, replicated in the ferret respiratory tract and was capable of transmission through direct contact with other ferrets. Belser et al. suggest that these viruses could be evolving toward the same strong, sugar-binding properties that characterized the three pandemic viruses of the 20th century. If this evolution continues, avian flu viruses could potentially travel easily between animals and humans, according to the authors. — P.D.
“Contemporary North American influenza H7 viruses possess human receptor specificity: Implications for virus transmissibility” by Jessica A. Belser, Ola Blixt, Li-Mei Chen, Claudia Pappas, Taronna R. Maines, Neal Van Hoeven, Ruben Donis, Julia Busch, Ryan McBride, James C. Paulson, Jacqueline M. Katz, and Terrence M. Tumpey (see pages 7558–7563)