Massachusetts Institute of Technology

A theoretical physicist examines exotic particles lurking in new materials.

Axions are very light, very weakly interacting particles, whose existence was posited more than 30 years ago [1,2] in order to clean up our ‘standard model’ of particle physics ³. They close an annoying loophole in Kobayashi and Maskawa’s Nobel-prize-winning explanation of why the microscopic laws of physics look so nearly the same when running backwards as forwards in time (time reversal symmetry).

Despite heroic efforts — and several false alarms — axions have not yet been detected, but they have become increasingly important. They have been warmly embraced in unified field theories and in string theory. And when we run the equations through Big-Bang cosmology, we find that axions should contribute much of the dark matter that astronomers have inferred to explain the Universe ⁵.

Now Shou-Cheng Zhang and his colleagues (X.-L. Qi et al. Phys. Rev. B78, 195424; 2008) inform us that, all along, axions have been lurking unrecognized on surfaces of bismuth-tin alloys and other materials. To be more precise: the equations that arise in axion physics [6,7] are the same as those that describe the electromagnetic behaviour of a recently discovered class of materials known, collectively, as topological insulators [8,9].

The axion field inside topological insulators is an emergent — and subtle — property of collections of electrons that is connected to their spin–orbit coupling.

These ‘quasi-axions’ don’t improve our standard model, but they do have the charming advantage of being accessible, possibly even useful. There are ideas to exploit their behaviour to make anyons ¹⁰, potential building blocks for quantum computation.

No short summary can do justice to the wealth of ideas synthesized in this paper. Powerful, beautiful mathematics is at play in reality.

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3. Peccei, R. & Quinn, H. Phys. Rev. Lett. 38, 1440 (1977)

4. Svrcek, P. & Witten, E. J. High Energy Phys. 0606 (2006).

5. Hertzberg, M., Tegmark, M. & Wilczek. F. Phys.Rev. D78, 083507 (2008).

6. Huang, M. & Sikivie, P. Phys. Rev. D32, 1560 (1985).

7. Wilczek, F. Phys. Rev. Lett. 58, 1799 (1987).

8. Kane, C. & Mele,E. Phys. Rev. Lett. 95, 226801 (2005).

9. Fu, L., Kane, C. & Mele, E. Phys. Rev. 98, 106803 (2007).

10. Fu, L. & Kane, C. Phys. Rev. Lett. 100, 096407 (2008).

MRC Laboratory of Molecular Biology, Cambridge

A molecular biologist gets excited about making designer proteins in cells.

The genetic code describes the relationship between the heritable information in the genome and the amino acids that are strung together to make proteins. This code, like any that contains redundancy, is open to hacking, and I have long been fascinated by how the process of translation, by which cells string amino acids together, might be reprogrammed to make new polymers. Several labs have already manipulated cells to incorporate designer amino acids into their proteins.

But Peter Schultz and his colleagues at the Scripps Research Institute in La Jolla, California, have achieved something remarkable. Proteins are made from a set of 20 amino acids, each of which contains an amine and a carboxylic acid group flanking a central carbon atom. Schultz’s team engineered a bacterial cell to work with amino-acid-like molecules called -hydroxy acids that have an alcohol group where the amine would normally be. During translation, instead of forming an amide bond to link polymer subunits, this -hydroxy acid forms an ester bond (J. Guo et al. Angew. Chem. 120, 734–737; 2008).

Replacing a nitrogen and a hydrogen atom in a polymer chain with an oxygen atom might seem like a slight change, but it means that a protein can now be specifically cut at the ester bond in basic solution. Making esters from -hydroxy acids may first have been achieved with ribosomes in a test tube in the 1970s, but turning the process into a heritable, genetic property is a major advance: it takes synthetic biologists closer to creating organisms with designer codes to make new polymers.

One day soon, the creativity and skill with which chemists can make molecules will be coupled to the selective power of organismal evolution. And we will watch new life forms boot up.

Imperial College, London.

A crystallographer takes a jaunt into immunology.

Although I spend most of my time exploring a landscape formed by atoms and bonds, I know it is healthy to make occasional journeys into less familiar territories, and I was intrigued to spot a paper on the curious interplay between infection and immunity in cattle with foot-and-mouth disease virus (FMDV).

FMDV, a highly contagious pathogen that can cause lameness, low weight and decreased milk production, is a scourge of agricultural livestock around the world. Although the acute phase of infection is rarely fatal, infection may persist in animals that have apparently recovered, creating a viral reservoir that some fear could contribute to the spread of disease. Nicholas Juleff and colleagues, from the United Kingdom’s Institute for Animal Health, report a fascinating discovery that may have unlocked the secret of FMDV persistence.

They used an array of molecular techniques to search for traces of virus in tissues from the mouths and throats of infected cattle (N. Juleff et al. PLoS ONE 3, e3434; 2008). In a carefully controlled study, they found evidence of intact, non-replicating virus particles trapped by immune cells called follicular dendritic cells within the germinal centres of lymph nodes. Strikingly, virus was present for at least 38 days post infection, even though it was undetectable in surrounding tissues.

The retention of intact virus within germinal centres is likely to have a role in stimulating the long-lasting immune response of white blood cells that is characteristic of viral infections (but not current vaccine preparations) and echoes a pattern previously seen for HIV infection. The authors suggest that this capture may inadvertently also be responsible for preserving intact viruses capable of infecting susceptible cells as they come into contact with germinal centres. A causal relationship has yet to be firmly established but the paper illuminates a clear pathway by which to check this out.

Harvard Medical School, Boston, Massachusetts

An immunologist muses about inflammation through cell interactions.

I spend my lab hours trying to understand what prompts T cells — a type of white blood cell — to specialize. Some T cells produce soluble molecules that rattle the immune system into an inflamed state; other cells generate molecules that calm the system back down.

Upon infection, cells such as macrophages — another type of white blood cell — produce soluble molecules called interleukins that direct the fate of the responding T cells. An emerging curiosity in the field is which interleukins make certain T cells become pro-inflammatory, and which cause other T cells to become anti-inflammatory. This decision is crucial for determining whether an immune response induces or suppresses inflammation.

Recently, investigators have turned their attention towards an interleukin known as IL-27. This is produced by activated macrophages and was initially thought to induce IFN, a signalling molecule that activates macrophages even more.

But work by Nico Giraldi and his colleagues at Genentech in South San Francisco, and other groups, has recast IL-27 as a molecule that primarily directs T cells to suppress inflammation. In a paper published in March, Giraldi’s team confirmed that IL-27 acts in this way because it causes CD4+ and CD8+ T cells to make the anti-inflammatory IL-10, and does not work through an alternative pathway (M. Batten et al. J. Immunol. 180, 2752–2756; 2008). Mice with Listeria infections or autoimmune tissue inflammation in their brains and spinal cords generated fewer IL-10-producing T cells when they lacked an IL-27 receptor. Whether an analogous interaction occurs in humans is not known, but, if it does occur, this research could become medically useful.