was not fully appreciated until the work of Krebs and Fischer defined the first of many clear functions for phosphate modification on a protein8, O-GlcNAc cannot be classified as a key regulatory modification until we dis-cover the ‘smoking gun’—evidence that O-GlcNAc modification at a specific site on a given protein alters its biological properties.

The QUIC-Tag method gives the community a valuable tool for conducting this hunt.

COMPETING INTERESTS STATEMENTThe author declares no competing financial interests.

1. Khidekel, N. et al. Nat. Chem. Biol. 3, 339–348 (2007).

2. Khidekel, N., Ficarro, S.B., Peters, E.C. & Hsieh-Wilson, L.C. Proc. Natl. Acad. Sci. USA 101, 13132–13137

(2004).3. Bishop, A.C. et al. Nature 407, 395–401 (2000).4. Hsu, J.L., Huang, S.Y., Chow, N.H. & Chen, S.H. Anal.

Chem. 75, 6843–6852 (2003).5. Mikesh, L.M. et al. Biochim. Biophys. Acta 1764,

1811–1822 (2006).6. Vosseller, K. et al. Proteomics 5, 388–398 (2005).7. Zachara, N.E. & Hart, G.W. Biochim. Biophys. Acta

1761, 599–617 (2006).8. Fischer, E.H. & Krebs, E.G.J. Biol. Chem. 216, 121–

132 (1955).

Figure 1 The QUIC-Tag strategy for quantitative analysis of O-GlcNAc. O-GlcNAc–modified proteins (blue squares) from two samples are specifically labeled with a ketone-containing galactose (yellow circles), which is then further reacted with a biotin moiety (green hexagon). After tryptic digestion, light (purple) or heavy (red) methyl groups are added to the amino groups of the peptides, and the O-GlcNAc–modified peptides are enriched via avidin. Both identification (most robustly performed by ETD fragmentation) and relative quantification can then be achieved.

Stimulating the cell’s appetite for itselfAnne Simonsen & Harald Stenmark

New inducers of autophagy—the process by which cells use lysosomes to degrade portions of their cytoplasm—are lead compounds for new drugs targeting neurodegenerative protein aggregation diseases.

Induction of a process that augments the cell’s capacity to degrade intracellular pro-tein aggregates is a goal in therapy of neu-rodegenerative diseases such as Parkinson’s, Alzheimer’s and Huntington’s disease. These diseases are characterized by accumulation of intracellular protein aggregates in nerve cells,

which ultimately cause cell death and ensu-ing loss of brain functions1,2. On p. 331 of this issue, Sarkar et al.3 describe a set of new neuro-protective compounds that stimulate the cell’s digestion of protein aggregates.

One of the major cellular pathways for scavenging intracellular protein aggregates is autophagy (literally, “self-eating”)2. Autophagy is a bulk degradation process that involves the sequestration of portions of cytoplasm by a double-membrane autophagosome, followed by digestion of the sequestered material when the autophagosome fuses with a lysosome full of hydrolytic enzymes (Fig. 1)4. Recently,

researchers found that loss of autophagy causes neurodegeneration even in the absence of any disease-associated mutant proteins5,6, which suggests that the continuous clearance of cellular proteins through basal autophagy prevents their accumulation, and in turn pre-vents neurodegeneration. Experiments in fly and mouse models have provided proof of principle that stimulation of autophagy can prevent and even reverse neurodegenerative disease7.

The compound that has been used for such studies, the immunosuppressant rapamycin, stimulates autophagy and aggregate digestion

Anne Simonsen and Harald Stenmark are in the Centre for Cancer Biomedicine, University of Oslo, and the Department of Biochemistry, the Norwegian Radium Hospital, Montebello, N-0310 Oslo, Norway.e-mail: stenmark@ulrik.uio.no

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by inhibiting the evolutionarily conserved protein kinase TOR (target of rapamycin)2. Via a mechanism that remains to be clarified, TOR acts as a brake on autophagy; thus, when it is inhibited with rapamycin, autophagy is turned on. Unfortunately, TOR does not con-trol autophagy alone—it is also an activator of ribosome biogenesis and other pathways involved in cell growth (Fig. 1)8. This causes undesired side effects (immunosuppression may be considered a side effect in this con-text) during long-term administration of rapamycin, and therefore alternative inducers of autophagy are desired.

As an approach to obtaining new com-pounds that modulate autophagy, Sarkar et al. performed a small-molecule screen in yeast for compounds that either enhance or inhibit the growth-inhibitory effect of rapamycin in this organism. Out of more than 50,000 compounds screened, the authors identified 12 small-molecule enhancers of rapamycin

(SMERs)3. When these were tested in mam-malian-cell systems for their ability to mediate clearance of mutant huntingtin and α-synu-clein proteins associated with Huntington’s disease and familial Parkinson’s disease, respectively, three of these (SMERs 10, 18 and 28) were found to increase clearance of these known autophagy substrates. Studies with mouse embryonic fibroblasts lacking Atg5, a crucial component of the autophagic machin-ery, confirmed that autophagy is required for this activity of the SMERs. The authors then went on to demonstrate that SMERs 10, 18 and 28 induce autophagy in mammalian cells, and finally they showed that these compounds reverse the toxic effects of mutant huntingtin on photoreceptors in the fly eye, thus provid-ing proof of principle that the SMERs can reverse aggregation toxicity in vivo.

So how do the SMERs work? Sarkar et al.3 established that they act synergistically with rapamycin in autophagic protein aggregate

clearance in mammalian cells, which is con-sistent with the way they were identified in the yeast screen. Moreover, none of the three selected SMERs affected the phosphorylation of two canonical TOR substrates involved in protein synthesis. Although we cannot exclude the possibility that the SMERs influence some uncharacterized pathway downstream of TOR, it is more likely that they stimulate autophagy by a novel TOR-independent mechanism (Fig. 1). Insulin receptor sub-strate 2, a scaffold protein involved in insulin and insulin-like growth factor 1 signaling, has recently been shown to cause autophagic clearance of accumulated mutant huntingtin despite activation of TOR9, and it is possible that SMERs act in this same pathway.

There is still some way to go before we know whether the SMERs and their derivatives can serve as valuable drug leads. The compounds will have to be tested for efficacy and toxicity in suitable animal models for neurodegenerative

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Figure 1 SMERs as novel inducers of autophagy. Certain proteins have the propensity to form protein aggregates, and excessive amounts of intracellular protein aggregates, as seen in neurodegenerative diseases, are cytotoxic. Under normal conditions, cells dispose of protein aggregates by wrapping them into a double-membraned phagophore (also called an isolation membrane), forming an autophagosome. When the autophagosome fuses with a lysosome, the hydrolytic enzymes of the latter degrade the protein aggregate and thus detoxify it. Autophagy is negatively regulated by the protein kinase TOR, which is also a positive regulator of cell growth. Inhibition of TOR with rapamycin therefore stimulates autophagy but also inhibits growth. The new compounds SMER10, SMER18 and SMER28 stimulate autophagy by a pathway that seems independent of TOR. Thus, these drugs could potentially be used to boost autophagy of protein aggregates in the nerve cells of people with neurodegenerative diseases, without the side effects associated with rapamycin.

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diseases, and because the SMERs enhanced rapamycin’s growth-inhibitory effect in yeast, possible growth-inhibitory effects in mam-mals need to be monitored carefully. SMERs will almost certainly have to be derivatized before further development, and Sarkar et al. have already proven the feasibility of a struc-ture-activity relationship–based protocol for improvement of the SMERs3. However, in order to further investigate their clinical poten-tial, it will be crucial to know more about the precise mechanism of action of the SMERs, as this not only will hint about routes to optimize the activity of the compounds while minimiz-

ing possible side effects, but also may shed new light on the way autophagy is controlled.

The identification of the SMERs as autoph-agy-inducing drugs is exciting because it may bring us closer to a new therapy for neurode-generative diseases, for which current phar-macotherapy is insufficient. Moreover, the growth-inhibitory effects that rapamycin and its synthetic analogs have on certain cancers may in part be attributed to their autophagy-inducing effects10. The identification of novel autophagy inducers could therefore also pro-vide us with new possibilities for treating can-cer in the future.

COMPETING INTERESTS STATEMENTThe authors declare no competing financial interests.

1. Bredesen, D.E., Rao, R.V. & Mehlen, P. Nature 443, 796–802 (2006).

2. Rubinsztein, D.C. Nature 443, 780–786 (2006).3. Sarkar, S. et al. Nat. Chem. Biol. 3, 331–338 (2007).4. Seglen, P.O. & Bohley, P. Experientia 48, 158–172

(1992).5. Hara, T. et al. Nature 441, 885–889 (2006).6. Komatsu, M. et al. Nature 441, 880–884 (2006).7. Ravikumar, B. et al. Nat. Genet. 36, 585–595

(2004).8. Wullschleger, S., Loewith, R. & Hall, M.N. Cell 124,

471–484 (2006).9. Yamamoto, A., Cremona, M.L. & Rothman, J.E. J. Cell

Biol. 172, 719–731 (2006).10. Easton, J.B. & Houghton, P.J. Oncogene 25, 6436–

6446 (2006).

Smells like breadDavid R Walt

Mammalian olfactory receptor genes have been engineered into a yeast expression system. The resulting cells provide a high-throughput screening system for studying receptor specificity and may find use as biosensors.

Dogs are known to have exquisitely sensitive noses. Most of us have encountered dogs at airports sniffing for contraband in the cus-toms area after international flights. Dogs are also employed to search for people that are lost in remote areas, and they are used exten-sively to seek explosives, such as those found in land mines. Most common explosives are comprised of trinitrotoluene (TNT). During manufacture, trace amounts of dinitrotoluene (DNT) impurity end up in TNT. DNT is sig-nificantly more volatile than TNT and is sus-pected to be the compound that enables dogs to detect the presence of explosives, including those in buried land mines. In this issue of Nature Chemical Biology, Radhika et al.1 report that they have engineered yeast to contain a library of olfactory receptors. By screening the library, they have identified a putative DNT receptor. The resulting yeast cells can serve as biosensors for DNT.

The operating principle of the mammalian olfactory system is unique. Rather than rely on the exquisite ligand-receptor interactions that pervade biology, the olfactory system relies on a combinatorial code arising from the inter-action of an odor with many semiselective olfactory receptors2,3. The resulting response pattern is processed and recognized by higher-level cortical functions. This strategy is much

more adaptive and flexible than other biologi-cal ligand-receptor–based systems in that odors can be recognized even if the organism has not encountered them before. To cover most of the ‘odor space’ in its environment, the typical mammal’s olfactory system has nearly 1,000 different receptors—coded for by 3 to 5% of the genome4! One of the most vexing issues in the study of olfaction has been the lack of a system for expressing functional olfactory receptors outside of mammalian cells.

To this end, Radhika et al.1 chose to express olfactory receptors in the yeast Saccharomyces cerevisiae. The olfactory receptor and its sig-naling pathway involve a complex cascade of various effector molecules. The interaction of an odor with an olfactory receptor activates a G protein, which causes adenylate cyclase to produce cyclic AMP (cAMP). Elevated levels of cAMP cause an ion channel to open, leading to an influx of calcium and sodium ions and pro-ducing an action potential. In order to express a functional receptor in a non-neuronal cell, it is essential to (i) pare down the complex-ity of the pathway and (ii) introduce a scheme whereby the receptor produces a signal when it binds an odorant molecule. To accomplish this task, the researchers cloned the receptor, the G protein, and the adenylate cyclase effector into S. cerevisiae (Fig. 1). In addition, human cAMP response element–binding protein and a cAMP-responsive gene coding for green fluorescent protein (GFP) were inserted into the yeast. The resulting yeast strain had three olfactory system genes that generated cAMP

upon activation by an odorant. The cAMP led to expression of GFP, thereby resulting in a fluorescent signal indicating the presence of the odorant.

After demonstrating that the yeast strain responded to a known odorant, the authors developed a generic system for inserting many different odorant receptors. While keeping the effector genes from their previous construct intact, they inserted known odorant receptors in the yeast construct and observed expression of GFP upon exposure to the cognate odor-ants. Next the researchers inserted a library of ‘orphan’ receptors into the yeast. Such recep-tors are known to be involved in olfaction, but the particular odorants that stimulate them are unknown. With this library of receptors inserted into different yeast cells, it was a sim-ple matter to identify the putative odorants—the authors simply had to expose the yeast to a particular odorant and observe which ones expressed GFP. By exposing the yeast library to DNT and observing the appearance of green fluorescence, the authors identified a yeast cell, and therefore an orphan odorant receptor, that responded to DNT.

Although this system and the results are very promising, some questions remain and future work must be performed. The authors have clearly identified a functional DNT receptor, but it still requires physiological validation. For example, with the sequence of the putative receptor known, antibody or mRNA labeling of intact olfactory epithelium would confirm that the receptor is present and expressed.

David R. Walt is in the Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155, USA.e-mail: david.walt@tufts.edu

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