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premaxilla in the front of the upper jaw, andnasal bones fused into a single unit) withprimitive characters (including relativelyblade-like teeth along the sides of the jaws anda powerful three-fingered hand). Indeed, someaspects of the pelvis are unexpectedly primi-tive — more typical of Triassic-to-MiddleJurassic theropods than of the more advancedforms (although such features are present in atleast one other coelurosaur12). The evolution-ary analysis by Xu et al. incorporates the newinformation from Guanlong, and confirmsthat the Tyrannosauroidea arose towards thebase of the Coelurosauria.

The most spectacular feature of Guanlong isits crest. All tyrannosauroids have some orna-mentation along their nasal bones, be it a row ofsmall hornlets as in Appalachiosaurus and Alio-ramus, a low ridge as in Dilong, or a roughenedtexture like all the others. But this newly dis-covered form has an especially impressiveexample — a tall, narrow projection withnumerous hollow excavations. Narrow mid-line crests seem to have been the fashion forMiddle and Late Jurassic theropods, as they are also present in Ceratosaurus, the primitivecoelurosaur Proceratosaurus11, and Guanlong’spossible predator Monolophosaurus (a previousrecord holder in terms of elaborate cranialcrests among carnivorous dinosaurs). The frag-ile nature of these structures suggests that theyserved for visual signalling, and so for speciesrecognition and mating displays, rather than asweapons. Indeed the fact that the smaller of thetwo Guanlong skeletons has a proportionatelymuch smaller crest is consistent with the dis-play being associated with sexual maturity.

Guanlong’s presence at the beginning of theLate Jurassic, and the record of compsog-nathids and maniraptorans11,12 somewhat later

in the same epoch, indicate that the majordivergences among the coelurosaurian dino-saurs had already occurred by 160 millionyears ago. Indeed, Eshanosaurus, a dinosaur

known only from an Early Jurassic lower jawfound in Yunnan, may represent a relativelyadvanced maniraptoran13. If so, the origin of allthe primary lines of the coelurosaurs —includ-ing the tyrannosauroids — lies even furtherback. The new ‘crowned dragon’ of Xinjiang issimply the latest discovery on the trail leadingback to the origin of the tyrant kings. ■

Thomas R. Holtz Jr is in the Department ofGeology, University of Maryland, College Park,Maryland 20742, USA. e-mail: tholtz@geol.umd.edu

1. Zhou, Z., Barrett, P. M. & Hilton, J. Nature 421, 807–814(2003).

2. Xu, X. et al. Nature 439, 715–718 (2006).3. Holtz, T. R. in The Dinosauria 2nd edn (eds Weishampel, D.

B., Dodson, P. & Osmolska, H.) 111–136 (Univ. CaliforniaPress, 2004).

4. Carr, T. D. J. Vert. Paleontol. 19, 497–520 (1999).5. Currie, P. J. Can. J. Earth Sci. 40, 651–665 (2003).6. Snively, E., Russell, A. P. & Powell, G. L. Zool. J. Linn. Soc.

142, 525–553 (2004).7. Osborn, H. F. Bull. Am. Mus. Nat. Hist. 22, 281–296

(1906).8. Matthew, W. D. & Brown, B. Bull. Am. Mus. Nat. Hist. 46,

367–385 (1922).9. Holtz, T. R. J. Paleontol. 68, 1100–1117 (1994).10. Carr, T. D., Williamson, T. E. & Schwimmer, D. R. J. Vert.

Paleontol. 25, 119–143 (2005).11. Xu, X. et al. Nature 431, 680–684 (2004). 12. Holtz, T. R., Molnar, R. E. & Currie, P. J. in The Dinosauria

2nd edn (eds Weishampel, D. B., Dodson, P. & Osmolska, H.) 71–110 (Univ. California Press, 2004).

13. Hartman, S., Lovelace, D. & Wahl, D. J. Vert. Paleontol. 25(Suppl. to No. 3), 67A–68A (2005).

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Figure 2 | Guanlong wucaii in evolutionary context. The bipedal and carnivorous theropod dinosaurs firstappeared around 235 million years (Myr) ago in the Triassic period, later diverging into several differentlines. In particular, during the Jurassic, one lineage (the Avetheropoda) divided into the Carnosauria and Coelurosauria, the latter including the Tyrannosauroidea3. Certain primitive features of the twoGuanlong fossils2 show that Guanlong lies at the base of the lineage of the tyrannosauroids, and close to the divergence between the major lineages of coelurosaurs. Numbers in parentheses indicate adult length.

CHEMICAL BIOLOGY

Aptamers in nanolandMichael Famulok and Günter Mayer

Chameleon-like nanoparticles of gold can be used to indicate the presenceof various biomolecules. Adding aptamers — DNA strands that bind only tospecific molecules — to the mix opens up further possibilities.

”Zum Golde drängt, am Golde hängt doch alles”

— “Towards gold throng all, to gold cling all”.This sentiment, from Goethe’s masterpieceFaust, reflects mankind’s fascination with thismetal. But the lustre that makes gold so attrac-tive in its bulk state changes entirely at thenanoscale. Gold particles of 10–100 nano-metres possess optical properties that changeaccording to their configuration: separatedparticles appear red in colour, and aggregatedparticles appear blue.

Juewen Liu and Yi Lu1 exploit thischameleon-like nature of gold nanoparticles tocreate a colorimetric indicator for the detectionof specific biomolecules. They publish theirresults in the journal Angewandte ChemieInternational Edition. The principle of the tech-nique is not new: nanoparticles were first usedas colour indicators to detect certain DNAsequences a decade ago2,3, and have sincebecome an important tool in biodiagnostics4.But Liu and Lu take the established principle

one step further by adding in the unique fea-tures of aptamers — short, single-strandedDNA sequences that recognize specific mol-ecules and bind to them — to create a fast, analyte-specific indicator.

The more conventional part of the newapproach1 is loosely based on the concept of nanoparticle aggregation through DNAhybridization, a process in which two comple-mentary single DNA strands anneal to form adouble strand. First, each separated — andtherefore red — gold nanoparticle has severalsingle-stranded DNA sequences attached to itby covalent bonds (Fig. 1a). Two strandsattached to neighbouring nanoparticles arethen bound to one another through a parallelDNA linker consisting of a series of nucleotidescomplementary to both strands. As severalidentical DNA strands are attached to eachnanoparticle, many nanoparticles can be gluedtogether in this way. An aggregate forms, witha characteristic blue-purple colour (Fig. 1b).

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50 YEARS AGO“The ‘Queen-Substance’ ofHoneybees and the Ovary-inhibiting Hormone of Crustaceans”— In the course of work on thesocial organization of honeybeecommunities, it has been foundthat worker honeybees obtain asubstance (‘queen-substance’)from their queens which, ifobtained in sufficient quantity,inhibits development of theirovaries and the production offurther queens. It will also, underexperimental conditions, inhibitovary development in worker ants(Formica fusca)... A similar hormonehas been shown to inhibit ovarydevelopment in decapodcrustaceans. These substancesappear to be similar in severalrespects. Thus both are stable toheat and to acids but less to alkalis,soluble in acetone and alcohol,both are active, at least undercertain conditions, when takenorally, and both serve to inhibitdevelopment of the ovary andrelated phenomena.From Nature 11 February 1956.

100 YEARS AGO“Result of War affected by Soldier’sStature” — The Japanese had anunquestionable advantage in therecent war as being smaller thanthe Russians; they were smallertargets for fire-arms. I wish to pointout that it is possible to expressthis advantage quantitatively onthe assumption...that bullets are,on average, uniformly distributedover the target presented by aman’s body, also that a manpresents a target proportional inarea to the square of his height. TheAnthropological Institute has kindlygiven me figures for the purpose; theaverageheight of 2500 Japanese...was 1585 millimetres as comparedwith an average of 1642 millimetresfor the average of177,948 EuropeanRussian conscripts. The averageRussian height thus exceeds thatof the Japanese by about 3.47 per cent. The squares of the twoaverage heights, representing, as I have said, the average targetsoffered by each to an enemy, differ therefore approximately by 7 per cent, so that the Russianfire was relatively ineffective tothat extent. John H. TwiggFrom Nature 8 February 1906. 50

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The innovative part of Liu and Lu’s strategyis that the DNA linker is itself an extension ofa DNA aptamer that binds specifically toadenosine5, a representative small molecule. In the absence of adenosine, the aptamerbehaves essentially like any short, linear DNAsequence, and does not adopt a well-definedthree-dimensional structure. So any part of theaptamer is equally accessible for hybridizationto a complementary sequence, and thereforeto function as a linker.

In the presence of adenosine, however, theaptamer markedly changes its conformation(Fig. 1c). It folds to form well-defined bindingpockets for adenosine molecules, exhibiting athree-dimensional structure stable enough tobe elucidated by NMR spectroscopy6. Thebinding process also requires the participationof endmost nucleotides of the DNA linker toform the compact structure of the aptamer–adenosine complex. This can only happen ifthe gold nanoparticle right at the end of theaptamer is released, and so the whole networkof aggregated gold nanoparticles disassembles.The colour of the ensemble therefore turnsback to an intense red, and so acts as a visualindicator of the presence of adenosine.

This principle is not limited to adenosinedetection, as Liu and Lu show with a neat sec-ond example1. In this case, the colour change istriggered by a small molecule that might needto be detected in situations when elaborateanalytical equipment is not readily available:cocaine. The authors found that, with slightadjustments of the appropriate aptamer, thedissociation of the DNA linker from one set ofgold nanoparticles could be brought aboutupon cocaine binding to the aptamer. Usingthis system, the amount of cocaine presentcould be quantified in the range 50 to 500micromoles per litre.

Both systems proved to be highly specific,exactly reflecting the specificity of the parentaptamers. The rapidity with which these sen-sors can detect analytes (a response is givenwithin seconds), as well as the sensors’ simpledesign and the fact that mere inspection is, in

principle, sufficient for semi-quantitativedetection, are outstanding advantages of thetechnique, and ones that set it apart from pre-viously developed aptamer-based detectionapproaches7.

The detection limit of the nanoparticle-based assay is correlated with the appropriatedissociation constant, a number that measuresthe strength of the small molecule’s binding tothe aptamer. As used by Liu and Lu1, eachnanoparticle has an average of 17 linkages, allof which need to be broken to disassemble theaggregate. So Liu and Lu’s method is moreenergy-intensive than other detection princi-ples, and so less sensitive. Improving the sen-sitivity of the technique could, however, easilybe accomplished simply by reducing the num-ber of linkages on each nanoparticle.

An alternative strategy for increasing detec-tion sensitivity might be to apply the principlesof Liu and Lu’s approach to the distance-con-trolled assembly of another class of nanoscalematerials — quantum dots. Quantum dots arenanocrystals (for example, zinc sulphide-capped cadmium selenide) that can be used asfluorescent probes in biodetection assays,including those for the detection of nucleicacids. Microbeads labelled with quantum dotsof different sizes in controlled ratios can bemade to exhibit a unique fluorescent signal forevery DNA sequence8. By controlling the dis-tance between two differently labelled quan-tum dots using an aptamer linker-and-targetcomplex, one might be able to take advantageof a phenomenon known as fluorescence resonance energy transfer (FRET) to alter theemission wavelength of a quantum-dot probe,and so its colour. Writing in a recent edition ofChemBioChem, Andrew Ellington and col-leagues9 have taken a first step in this direc-tion, using an aptamer-labelled quantum dot,the fluorescence of which is reduced by anannealed fluorescent sequence. This sequenceis then released in the presence of the analytemolecule to which the aptamer links9.

These aptamer-based tools1,9 could becomea very useful addition to the nanostructure

Figure 1 | Liu and Lu’s aptamer-based detection strategy4. a, Gold nanoparticles, red whenseparated, are prepared with strands of DNA (radiating spokes) attached. b, A DNA aptamer (green) is modified so that one end is formed of consecutive sequences complementary to the DNA strands of two nanoparticles. The nanoparticles are thus linked by DNA hybridization, causing their colour to change to a blue-purple. c, The addition of adenosine (orange dots), the target molecule of theaptamer, causes dramatic conformational changes of the aptamer structure, which leads to thedissociation of the nanoparticle network. The colour of the solution changes back to red, a visualindication of the presence of adenosine.

Goldnanoparticles

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biodiagnostics workbox. Imagine that simplelitmus tests for every analyte for whichaptamers exist were available! To be able tolook at a ‘paper strip’ to gain an instant idea ofwhether a compound is present and in whatquantities would be worth more than itsweight in gold. ■

Michael Famulok and Günter Mayer are at the Kekulé Institut für Organische Chemie und Biochemie, University of Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany.e-mail: m.famulok@uni-bonn.de

IMMUNOLOGY

Exhausted T cells perk upMatthew A. Williams and Michael J. Bevan

During persistent infections, the immune cells responsible for killinginfected cells and maintaining inflammation gradually stop functioning,allowing the pathogen to thrive. But can this process be reversed?

There are many checks and balances in placeto control the immune response to a patho-genic onslaught: too little of a reaction and theinvader can kill the host or take up residence;too much and the immune system can itselfstart to damage tissues. On page 682 of thisissue, Barber et al.1 demonstrate that a recep-tor–ligand pair involved in controlling theimmune response can be blocked by antibod-ies to revive dysfunctional T cells and cleara chronic viral infection.

Following many infections, immune cellsthat recognize antigens from the invad-ing pathogen (CD8+ T cells) undergo a dramatic expansion in number and developfunctions to deal with the intruders. These‘effector’ T cells can kill infected cells andproduce cytokine molecules, such as interferon-� and tumour-necrosis factor-�,to promote local inflammation. The patho-gen-specific T cells go through 15 or morecell divisions in five days, producing mil-lions of effector cells that go on a search-and-destroy mission to clear the infection.These enormous numbers of effector cellscannot be maintained once the infection isgone, so most of them die. But they leavebehind a pool of stable, long-lived memoryT cells that can deal promptly, it is hoped,with a second exposure to the same patho-gen (Fig. 1a). Some infections do not followthis textbook ‘acute’ course, however. The pathogen may find ways to avoid clear-ance, and the infection becomes persistent,or ‘chronic’.

Lymphocytic choriomeningitis virus(LCMV) is a natural pathogen of mice, andmost LCMV infections of healthy mice follow the acute course. However, there is alab-derived variant of LCMV that over-whelms the immune response (either by

replicating faster or by infecting different hostcells), resulting in a persistent infection withhigh levels of virus in the blood and manyother tissues. Initially, mice infected with thelab-derived variant develop robust T-cellresponses that are comparable to thoseinduced by the parent strain. Then things startto deteriorate. Instead of clearing the viruswithin a week, the viral levels remain high, and

the effector cells gradually dwindle in numberand lose the ability to kill cells and make cyto-kines. In immunology jargon, they becomeexhausted (Fig. 1b).

Barber et al.1 noted that although the early,functional T cells in acute or chronic infec-tions make a lot of a surface receptor proteinknown as PD-1, after clearance of the acuteinfection the resting memory cells lose expres-sion of this marker. In chronically infected ani-mals, however, the T cells retain high levels ofPD-1 expression. In addition, one of the lig-ands for this receptor, PD-L1, is expressed athigh levels on the surface of chronicallyinfected cells. This receptor–ligand system hasbeen postulated2 to inhibit signalling throughthe T-cell antigen receptor — the receptor thattriggers the response to a pathogen.

Spectacularly, the authors found that thedecline in number and function of the effectorT cells in chronically infected mice could bereversed by injecting an antibody that blockedthe interaction between PD-1 and PD-L1 (Fig. 1c). They show that exhausted T cells thatcould not divide in response to antigen recov-ered this ability in the presence of the anti-body. In addition, the T cells regained killeractivity, secreted increased levels of interferon-�and tumour-necrosis factor-� and — mostimportantly — rapidly reduced the viral load.

The question remains as to whether a short course of such blocking antibodies couldhave beneficial effects on persistent infections in people; for example, those caused by hepati-tis virus, cytomegalovirus or HIV. Barber et

al.1 show that even in mice that lack CD4+

helper T cells, and therefore experience amuch more pronounced course of LCMVinfection and T-cell exhaustion, PD-L1blockade can still revitalize the exhausted T cells and result in diminished virus levels.This is of particular significance for HIVinfection, which is characterized by declin-ing numbers of CD4+ T cells.

The first step in finding out whether thispathway can be exploited therapeuticallywill be to study the expression pattern of thisreceptor and its ligand in humans during a variety of chronic infections. But persist-ent antigen exposure also occurs in cancer,where continual presentation of antigensderived from abnormal ‘self ’ proteins can lead to dysfunctional, tumour-specific T cells3. Notably, many tumours express PD-L1, so blocking this pathway may be beneficial in encouraging the immune systemto recognize and kill off the abnormal cells.

Regardless of whether the PD-1–PD-L1pathway is relevant in other models ofchronic antigen presentation, Barber andcolleagues’ results demonstrate that theexhaustion of T cells during chronic infec-tion can be reversed, raising the possibilityof therapies designed to revive T-cell func-tion. But first, the balance between regula-tory and activation pathways that govern theT-cell responses during chronic infections

Figure 1 | Reviving exhausted T cells. a, In an acuteinfection, pathogen-specific T cells respond and developinto effector cells that help to eliminate the invader bykilling infected cells and secreting cytokines. Once theinfection is cleared, the remaining effector cells give riseto long-lived, protective memory cells. b, In a chronicinfection, with a high antigen load, the effectorsgradually lose function and the ability to proliferate. c, Barber et al.1 show that in mice these ‘exhausted’ T cells can be revived by treatment with an antibody that blocks the interaction between the PD-1 receptorand its ligand PD-L1.

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1. Liu, J. & Lu, Y. Angew. Chem. Int. Edn 45, 90–94 (2006).2. Alivisatos, A. P. et al. Nature 382, 609–611 (1996).3. Mirkin, C. A., Letsinger, R. L., Mucic, R. C. & Storhoff, J. J.

Nature 382, 607–609 (1996).4. Rosi, N. L. & Mirkin, C. A. Chem. Rev. 105, 1547–1562

(2005).5. Huizenga, D. E. & Szostak, J. W. Biochemistry 34, 656–665

(1995).6. Lin, C. H. & Patel, D. J. Chem. Biol. 4, 817–832 (1997).7. Stojanovic, M. N. & Landry, D. W. J. Am. Chem. Soc. 124,

9678–9679 (2002).8. Han, M., Gao, X., Su, J. Z. & Nie, S. Nature Biotechnol. 19,

631–635 (2001).9. Levy, M., Cater, S. F. & Ellington, A. D. ChemBioChem 6,

2163–2166 (2005).

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