In This Issue

CHEMISTRY, CELL BIOLOGY

Dynamic imaging of cellular biomolecules using a nongenetic approach

In vivo biological imaging systems typically tag proteins with genetically encoded peptide sequences that direct the chemical or enzymatic attachment of small molecule probes such as biotin or spectroscopicprobes like GFP. Although these methods are effective for visualizing proteins in living cells, they are insufficient for other biomolecules, including cell surface glycans and lipids. Jeremy Baskin et al. devised a nongenetic in vivo imaging system using a bioorthogonal chemical reporter strategy. Critical to the success of this system is a difluorinated cyclooctyne reagent that rapidly and selectively attaches to azides in a copper-free click chemistry reaction. The authors used this method to study the dynamics of cellular glycosylation in Chinese hamster ovary cells. In this approach, the cell's metabolic machinery installed an azide-containing analog of sialic acid into the cell surface glycans. These cell surface glycans were then covalently labeled with fluorophores using a cyclooctyne probe that was designed to incorporate two rate-enhancing features: ring strain and electron-withdrawing groups. This copper-free [3+2] azide-alkyne cycloaddition recapitulates the speed of traditional copper-catalyzed click chemistry but avoids the cytotoxicity of the copper metal, which is crucial in dynamic cellular imaging. — F.A.

“Copper-free click chemistry for dynamic in vivo imaging” by Jeremy M. Baskin, Jennifer A. Prescher, Scott T. Laughlin, Nicholas J. Agard, Pamela V. Chang, Isaac A. Miller, Anderson Lo, Julian A. Codelli, and Carolyn R. Bertozzi (see pages 16793–16797)

ECOLOGY

Driving food-web diversity

Food webs are an essential and complex part of biology that can contain many species interactions. Eldon Eveleigh et al. report that food-web structures vary in a predictable manner according to the abundance of dominant players in the web. The authors modeled changes in a food web by merging foraging theory with food-web ecology and data on the long-studied spruce budworm, a destructive herbivore that undergoes outbreaks approximately every 35 years. They found that budworm density provoked large shifts throughout food webs and that higher-order mobile generalists will converge rapidly in areas with abundant prey. Increasing budworm abundance provoked a cascade of increasing diversity and food-web complexity in the forests. The research results confirmed several recent predictions of food-web theory, which has predicted that this flexibility should stabilize webs in heterogeneous forests relative to homogeneous forest stands. Heterogeneous forests accordingly had greater food-web flexibility than did homogeneous ones. These results, the authors suggest, have implications for forest and pest management practices. — P.D.

“Fluctuations in density of an outbreak species drive diversity cascades in food webs” by Eldon S. Eveleigh, Kevin S. McCann, Peter C. McCarthy, Steven J. Pollock, Christopher J. Lucarotti, Benoit Morin, George A. McDougall, Douglas B. Strongman, John T. Huber, James Umbanhowar, and Lucas D. B. Faria (see pages 16976–16981)

MEDICAL SCIENCES

Crucial factor declines in banked blood

Millions of patients are treated each year by blood transfusion. However, transfusions carry a risk of ischemia, a dangerous drop in blood flow, for some groups. The body controls oxygen delivery largely by constricting or expanding the diameter of blood vessels; a nitric oxide (NO) signal transduced by hemoglobin induces vascular relaxation. That many anemic patients fare worse with transfusion suggests that stored blood has undergone biochemical changes. In two separate studies, Bennett-Guerrero et al. and Reynolds et al. report that stored blood undergoes a rapid decline in S-nitroso (SNO)-hemoglobin levels. This decline appears to be responsible for the risk of ischemia. Bennett-Guerrero et al. undertook a survey of factors relevant in the functional changes of stored red blood cells (RBCs). Only 3 h after donation, they found that SNO-hemoglobin had already declined to the long-term minimum. The ability of RBCs to relax blood vessels also dropped quickly. The authors found that the RBCs slowly became less deformable, which would impede their flow through capillaries. Reynolds et al. observed that SNO-hemoglobin levels dropped after day 1 and remained low thereafter. Vasodilation was correspondingly impaired. The authors found that RBCs replenished with nitric oxide relaxed blood vessels to the same degree as those from fresh blood. In transfusion experiments with live dogs, replenished RBCs increased coronary blood flow to a much greater degree than SNO-depleted RBCs. The authors conclude that it should be possible to replenish SNO-hemoglobin in banked blood and reduce the transfusion risk. — K.M.

“S-nitrosohemoglobin deficiency: A mechanism for loss of physiological activity in banked blood” by James D. Reynolds, Gregory S. Ahearn, Michael Angelo, Jian Zhang, Fred Cobb, and Jonathan S. Stamler (see pages 17058–17062)

and

“Evolution of adverse changes in stored RBCs” by Elliott Bennett-Guerrero, Tim H. Veldman, Allan Doctor, Marilyn J. Telen, Thomas L. Ortel, T. Scott Reid, Melissa A. Mulherin, Hongmei Zhu, Raymond D. Buck, Robert M. Califf, and Timothy J. McMahon (see pages 17063–17068)

MICROBIOLOGY

West Nile virus traverses axons to infect the central nervous system

Since its emergence in 1999, West Nile virus (WNV) has become the most common cause of epidemic viral encephalitis in the United States.A neurotropic RNA virus, WNV is spread by mosquitoes, and the disease process entails a short asymptomatic period, followed by fever and encephalitis. The virus must travel from the skin and peripheral tissues to enter the central nervous system (CNS), but the mechanism of transport has not been well understood. Melanie Samuel et al. studied the role of axonal transport in viral progression. Their experiments on compartmentalized rat neurons demonstrated that WNV is transported in both the anterograde and retrograde directions to infect cells. Direct infection of hamster sciatic nerves resulted in viral transport to the spinal cord and development of acute flaccid paralysis, a signature symptom of WNV infection. However, treatment with a neutralizing IgG antibody prevented the virus from spreading and blocked disease symptoms. The authors' findings suggest that similar antibody-based therapeutics could disrupt transneuronal spread and limit WNV replication in the human CNS. — F.A.

“Axonal transport mediates West Nile virus entry into the central nervous system and induces acute flaccid paralysis” by Melanie A. Samuel, Hong Wang, Venkatraman Siddharthan, John D. Morrey, and Michael S. Diamond (see pages 17140–17145)

NEUROSCIENCE

Nicotine addiction pathway identified

For chronic smokers trying to quit, it is not the nicotine reward that is the main problem, it is the brain's adapations to that reward. Olivier George et al. report on recruitment of a brain pathway that is responsible for the strong desire to resume taking more nicotine after a withdrawal period, revealing a mechanism that could be manipulated to help people quit smoking. The authors tested the effects of nicotine withdrawal on the rat corticotropin releasing factor system (CRF), which is activated in the brain during periods of stress and has been implicated in withdrawal from other drugs. They found that the CRF system is activated when the nicotinic receptors of addicted rats are blocked. The activation prompts a negative emotional state in the rats that causes them to seek out high amounts of nicotine, and is also responsible for signs of anxiety during the withdrawal or abstinence period. The drives to avoid negative emotions and anxiety are thought to be principal causes of nicotine addiction, making smoking a difficult habit to break. The authors found that they could prevent increased nicotine intake by blocking the CRF1 receptor. They say that it may be a suitable target for drug interventions to control nicotine addiction. — P.D.

“CRF–CRF₁ system activation mediates withdrawal-induced increases in nicotine self-administration in nicotine-dependent rats” by Olivier George, Sandy Ghozland, Marc R. Azar, Pietro Cottone, Eric P. Zorrilla, Loren H. Parsons, Laura E. O'Dell, Heather N. Richardson, and George F. Koob (see pages 17198–17203)