- Email: [email protected]
Olfactory Coding: Connecting Odorant Receptor Expression and Behavior in the Drosophila Larva
Current Biology Vol 16 No 1 R16
in sporulating cells of Bacillus subtilis strain 3610. Virology 39, 265–275. 5. Osburne, M.S., and Sonenshein, A.L. (1976). Behavior of a temperate bacteriophage in differentiating cells of Bacillus subtilis. J. Virol. 19, 26–35. 6. Kawamura, F., and Ito, J. (1974). Bacteriophage gene expression in sporulating cells of Bacillus subtilis 168. Virology 62, 414–425. 7. Buu, A., and Sonenshein, A.L. (1975). Nucleic acid synthesis and ribonucleic acid polymerase specificity in germinating and outgrowing spores of Bacillus subtilis. J. Bacteriol. 124, 190–200. 8. Burgess, R.R., Travers, A.A., Dunn, J.J., and Bautz, E.K. (1969). Factor stimulating transcription by RNA polymerase. Nature 221, 43–46. 9. Losick, R., and Sonenshein, A.L. (1969). Change in the template specificity of RNA polymerase during sporulation of Bacillus subtilis. Nature 224, 35–37. 10. Haldenwang, W.G., Lang, N., and Losick, R. (1981). A sporulation-induced sigmalike regulatory protein from B. subtilis.
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Cell 23, 615–624. Stragier, P., and Losick, R. (1990). Cascades of sigma factors revisited. Mol. Microbiol. 4, 1801–1806. Fox, T.D., Losick, R., and Pero, J. (1976). Regulatory gene 28 of bacteriophage SPO1 codes for a phage-induced subunit of RNA polymerase. J. Mol. Biol. 101, 427–433. Meijer, W.J., Castilla-Llorente, V., Villar, L., Murray, H., Errington, J., and Salas, M. (2005). Molecular basis for the exploitation of spore formation as survival mechanism by virulent phage φ29. EMBO J. 24, 3647–3657. Strauch, M.A., and Hoch, J.A. (1993). Signal transduction in Bacillus subtilis sporulation. Curr. Opin. Genet. Dev. 3, 203-212. Levin, P.A., and Losick, R. (1996). Transcription factor Spo0A switches the localization of the cell division protein FtsZ from a medial to a bipolar pattern in Bacillus subtilis. Genes Dev. 10, 478–488. Moreno, F. (1979). On the trapping of phage genomes in spores of Bacillus
Olfactory Coding: Connecting Odorant Receptor Expression and Behavior in the Drosophila Larva The discovery of odorant receptors has significantly changed our understanding of how animals identify thousands of odorants. A recent study has shed new light on the central issue of how odor information is translated into meaningful behavior. Reinhard F. Stocker For many animal species, smell is the most crucial of all the senses. Odors are vital for locating food and egg-laying substrates, for avoiding dangers and for communicating with conspecific individuals. Understanding olfaction in the biological context requires analysis at multiple levels: reception of odors by sensory neurons, integration of odor information in the brain, and translation of odor perception into meaningful behavioral output. Recent years have seen tremendous progress in odorant receptor and olfactory circuit analysis, largely aided by the use of simple model systems such as Drosophila. In contrast, the issue of how odor perception drives behavior has remained largely unexplored. As they reported in a recent issue of Current Biology, Fishilevich et al. [1] have now used an even simpler model, the Drosophila larva, to investigate
how odorant receptor expression patterns might be linked to chemotaxis. Surprisingly, they found that olfactory sensory neurons — each of which expresses its proper odorant receptor — are not equivalent in driving chemotaxis. At the extreme, larvae with just a single functional sensory neuron, expressing the receptor OR42a, are attracted to a high fraction of odors. The identification of odorant receptors [2–4] and analysis of their expression patterns in the sensory neurons have had a great impact on our understanding of the principles of olfactory coding. In both mammals and insects, olfactory sensory neurons essentially express a single type of odorant receptor. Neurons expressing the same receptor type are scattered in the epithelium, but their axons converge onto one or two glomeruli in the olfactory brain centers [5–8]. The chemical
17.
18.
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subtilis 168: reciprocal exclusion of phages φ29 and φe during outgrowth of spores. Virology 93, 357–368. Wu, L.J., and Errington, J. (1994). Bacillus subtilis SpoIIIE protein required for DNA segregation during asymmetric cell division. Science 264, 572–575. Lee, P.S., Lin, D.C., Moriya, S., and Grossman, A.D. (2003). Effects of the chromosome partitioning protein Spo0J (ParB) on oriC positioning and replication initiation in Bacillus subtilis. J. Bacteriol. 185, 1326–1337. Sonenshein, A.L. (1970). Trapping of unreplicated phage DNA into spores of Bacillus subtilis and its stabilization against damage by 32P decay. Virology 42, 488–495.
Department of Molecular Biology and Microbiology, Tufts University School of Medicine, 136 Harrison Avenue, Boston, Massachusetts 02111, USA. DOI: 10.1016/j.cub.2005.12.007
information conveyed by the sensory neurons is thus translated into a pattern of glomerular activation [9–11]. The insect olfactory system therefore shares the design of the mammalian system [12] but comprises only a fraction of the receptors and neurons of the latter, providing an attractive system for analyzing odor coding. Insects undergoing metamorphosis exhibit an even simpler olfactory circuit during the larval stage. How is this larval pathway organized? Could it offer a yet simpler approach for understanding the sense of smell from odor reception to behavioral output? The odorant receptor family in Drosophila consists of 62 members [3,4,13] compared to more than 1000 in rodents. At least 25 of the 62 receptors are expressed in the fly larva [1,14,15]. Of these 25, 14 are larval-specific, while the rest are expressed in both adult and larval olfactory systems. As in the adult, the large majority of the 21 larval olfactory sensory neurons express one conventional odorant receptor, along with an atypical receptor, OR83b [1,15,16]. At least two neurons express two additional receptors apart from OR83b [1]. Because the number of receptors exceeds the number of olfactory sensory neurons, a few more cases of triple receptor expression are to be expected. In
Dispatch R17
summary, the rules of odorant receptor expression in the larval olfactory system appear to be similar to those in adult flies and mammals but differ from another simple system, the nematode Caenorhabditis elegans, in which sensory neurons express multiple odor receptors. Kreher et al. [15] investigated the electrophysiological responses of 11 larval odorant receptors to a panel of 29 compounds by expressing single odorant receptor genes in sensory neurons of anosmic adult mutant flies [17,18]. Their results showed that the responses of the different receptors are very diverse, ranging from activation by just one odorant to activation by many of the tested odorants. Some receptors were found to respond most strongly to aliphatic compounds whereas others seemed to be tuned to aromatic compounds. Single-cell labeling experiments in the Or83b-Gal4 driver line, as well as experiments in which transgenes were expressed under the control of promoters of odorant receptor genes, showed that the larval olfactory pathway has a surprisingly similar, but much simpler, design to its adult counterpart [1,15,19]. As in the adult, in the larva each olfactory sensory neuron projects to a single glomerulus in the antennal lobe. But compared to the 1,300 olfactory sensory neurons in the adult, there are no more than 21 of these in the larva. Each of these is unique and projects to one of 21 glomeruli. Hence, any given glomerulus is the target of a single sensory neuron and is therefore associated with a specific odorant receptor (except for the few neurons that express two conventional odorant receptors). The Drosophila larva thus provides a ‘minimal’ olfactory model system that still allows meaningful comparison with mammals. In their new work, Fishilevich et al. [1] went a step ahead and addressed directly the correlation between odorant receptor expression and behavioral output — chemotaxis. For this purpose, they used ‘subtractive’ and
‘additive’ strategies: in the former they genetically ablated selected olfactory sensory neurons via toxin expression, and in the latter they created animals with only one or two functional sensory neurons. In wild-type larvae, 36 of 53 tested compounds elicited robust chemotactic responses. In the subtractive approach, they obtained two types of results. Animals in which either the Or1aexpressing neuron or the Or49aexpressing neuron was ablated showed reduced chemotaxis to only one of 20 odors tested. This relatively mild effect is consistent with the broad and overlapping ligand tuning of many olfactory sensory neurons in adults [17] and larvae [15]. In contrast, loss of the neuron expressing Or42a resulted in anosmy to four of the 20 odors. In the additive approach, larvae with one or two functional olfactory sensory neurons were generated using Or1a, Or42a or Or49a driver lines [1]. Consistent with the stronger Or42a-ablated phenotype, Or42a-functional larvae responded to 22 of 53 odors tested (compared to 36 odors in the wild-type), including three of four odors to which Or42a-ablated animals are anosmic. The broad response profile for Or42a-functional larvae is in agreement with the broad ligand tuning of this receptor [15,18]. In contrast, Or1a- and Or49a-functional larvae did not exhibit significant chemotaxis to any of the 53 odors, consistent with the weak phenotype of the corresponding ablated larvae and with electrophysiological responses [15]. Animals with two functional neurons responded to a somewhat different subset of odors than larvae having either single functional neuron alone [1]. Statistical analysis suggested six cases of potential cooperativity between Or1a and Or42a chemotaxis. For example, 1-pentanol elicited significantly stronger chemotaxis in Or1a/Or42a-functional larvae than in either single Or-functional animal. Three major conclusions can be drawn from these data [1]. First, the minimal effects on chemotaxis after ablating the Or1a or Or49a
neurons suggest a certain level of redundancy in the system. This sounds surprising, given the small number of olfactory sensory neurons in the larval system. Yet, subtle effects exerted by seemingly ‘unimportant’ neurons could be crucial for cooperative processes (see below). Second, the Or42a neuron plays a particularly important role in odor detection and is sufficient to initiate chemotaxis to a high fraction of odors. Moreover, its loss leads to severe behavioral defects. Why there is functional heterogeneity between the Or42a neuron and the Or1a or Or49a neurons remains a puzzle for the moment. And third, cooperative action is suggested by the enhanced chemotactic responses of Or1a/Or42a-functional animals compared to the single Or1a- or Or42a-functional animals. Olfactory coding thus does not simply rely on additive activation of 21 parallel pathways, but involves horizontal interactions as well. Cross-talk may occur at many levels of the circuit, from the sensory neurons themselves to olfactory target neurons in the brain. A primary candidate are local interneurons in the antennal lobe that provide lateral connections among the glomeruli [19]. Significant transformation of olfactory signals is indeed known from the adult antennal lobe [20]. Integration of olfactory information may sharpen both quantitative and qualitative aspects of the signals, such as detection threshold and odor discrimination, respectively. Although chemotaxis assays do not answer how odors are distinguished from each other, it is reasonable to postulate that integrative processes may be particularly crucial if very few channels have to cope with many odors. The findings by Fishilevich et al. [1], together with the other recent data on the olfactory system of the Drosophila larva [14,15,19], provide links between odorant receptors, their ligands, odordriven behavior and the underlying neuronal substrate, and put forward the Drosophila larva as a highly attractive
Current Biology Vol 16 No 1 R18
‘minimal’ model in olfactory research. References 1. Fishilevich, E., Domingos, A.I., Asahina, K., Naef, F., Vosshall, L.B., and Louis, M. (2005). Chemotaxis behavior mediated by single larval olfactory neurons in Drosophila. Curr. Biol. 15, 2086-2096. 2. Buck, L., and Axel, R. (1991). A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 65, 175–187. 3. Clyne, P.J., Warr, C.G., Freeman, M.R., Lessing, D., Kim, J., and Carlson, J.R. (1999). A novel family of divergent seventransmembrane proteins: candidate odorant receptors in Drosophila. Neuron 22, 327–338. 4. Vosshall, L.B., Amrein, H., Morozov, P.S., Rzhetsky, A., and Axel, R. (1999). A spatial map of olfactory receptor expression in the Drosophila antenna. Cell 96, 725–736. 5. Ressler, K.J., Sullivan, S.L., and Buck, L.B. (1994). Information coding in the olfactory system: Evidence for a stereotyped and highly organized epitope map in the olfactory bulb. Cell 79, 1245–1255. 6. Vassar, R., Chao, S.K., Sitcheran, R., Nunez, J.M., Vosshall, L.B., and Axel, R. (1994). Topographic organization of sensory projections to the olfactory bulb. Cell 79, 981–991. 7. Gao, Q., Yuan, B., and Chess, A. (2000).
Convergent projections of Drosophila olfactory neurons to specific glomeruli in the antennal lobe. Nat. Neurosci. 3, 780–785. 8. Vosshall, L.B., Wong, A.M., and Axel, R. (2000). An olfactory sensory map in the fly brain. Cell 102, 147–159. 9. Joerges, J., Küttner, A., Galizia, C.G., and Menzel, R. (1997). Representations of odours and odour mixtures visualized in the honeybee brain. Nature 387, 285–287. 10. Malnic, B., Hirono, J., Sato, T., and Buck, L.B. (1999). Combinatorial receptor codes for odors. Cell 96, 713–726. 11. Wang, J.W., Wong, A.M., Flores, J., Vosshall, L.B., and Axel, R. (2003). Twophoton calcium imaging reveals an odorevoked map of activity in the fly brain. Cell 112, 271–282. 12. Hildebrand, J.G., and Shepherd, G. (1997). Mechanisms of olfactory discrimination: converging evidence for common principles across phyla. Annu. Rev. Neurosci. 20, 595–631. 13. Robertson, H.M., Warr, C.G., and Carlson, J.R. (2003). Molecular evolution of the insect chemoreceptor gene superfamily in Drosophila melanogaster. Proc. Natl. Acad. Sci. USA 100, 14537–14542. 14. Couto, A., Alenius, M., and Dickson, B.J. (2005). Molecular, anatomical, and functional organization of the Drosophila olfactory system. Curr. Biol. 15, 1535–1547. 15. Kreher, S.A., Kwon, A.Y., and Carlson,
Nebulin: Does It Measure up as a Ruler? A recent study has shown that the giant protein nebulin maintains the lengths of actin filaments in striated muscle cells. Although on the surface, nebulin looks like a molecular ruler, it may be playing a more complex role in regulating dynamics at the pointed end of actin filaments in striated muscle. Velia M. Fowler, Caroline R. McKeown and Robert S. Fischer In a striated muscle cell, the lengths of the sarcomeric actin filaments — the thin filaments — are precisely regulated, both in terms of their particular lengths, and in the variation of their length distributions [1,2]. Actin monomers assemble in vitro into filaments with an exponential length distribution, and they can organize into a wide variety of lengths in non-muscle cells, indicating that length controls are not intrinsic to actin filament polymers. Historically, a ‘molecular ruler’ mechanism has been the most attractive model for how thin filament lengths are determined in striated muscle. By definition, a
molecular ruler must meet a number of criteria: the ruler must be the precise length of the target filament, with the length of the ruler dictating the length of the filaments; the end of the ruler should bind to a terminator protein to prevent actin subunit addition or loss once the filament has polymerized to the length of the ruler protein; and the ruler must associate in a stoichiometric ratio with its target filament. In this mechanism, filament lengths are fixed precisely to the ruler length once they have polymerized. Thus, only filaments associated with rulers will be the length of the ruler, and filaments without a ruler will assume random and variable lengths (Figure 1A). The giant molecule nebulin has been postulated to be a molecular
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J.R. (2005). The molecular basis of odor coding in the Drosophila larva. Neuron 46, 445–456. Larsson, M.C., Domingos, A.I., Jones, W.D., Chiappe, M.E., Amrein, H., and Vosshall, L.B. (2004). Or83b encodes a broadly expressed odorant receptor essential for Drosophila olfaction. Neuron 43, 703–714. Hallem, E.A., Ho, M.G., and Carlson, J.R. (2004). The molecular basis of odor coding in the Drosophila antenna. Cell 117, 965–979. Goldman, A.L., van der Goes van Naters, W., Lessing, D., Warr, C.G., and Carlson, J.R. (2005). Coexpression of two functional odor receptors in one neuron. Neuron 45, 661–666. Ramaekers, A., Magnenat, E., Marin, E.C., Gendre, N., Jefferis, G.S.X.E., Luo, L., and Stocker, R.F. (2005). Glomerular maps without cellular redundancy at successive levels of the Drosophila larval olfactory circuit. Curr. Biol. 15, 982–992. Wilson, R.I., Turner, G.C., and Laurent, G. (2004). Transformation of olfactory representations in the Drosophila antennal lobe. Science 303, 366–370.
Department of Biology, University of Fribourg, 10 rue du Musée, CH-1700 Fribourg, Switzerland. DOI: 10.1016/j.cub.2005.12.008
ruler that determines thin filament length in striated muscle [3–7]. Nebulin extends along the thin filament, with its amino terminus oriented near the pointed (free) end and its carboxyl terminus near the barbed end in the Z disc. A number of properties of nebulin appear to fulfill many of the a priori requirements for a molecular ruler. First, the molecular sizes of nebulin isoforms correlate with the lengths of thin filaments in the striated muscles in which the isoform is found [6,8]. Second, nebulin molecules are composed of a modular series of repeats corresponding to the repeats of the actin subunits of the thin filament, thus ‘measuring’ polymer length [6,9]. Third, a region in the unique amino-terminal domain of nebulin (M1M2M3) is located near the thin filament pointed end [9], and interacts with the actin pointed end capping protein, tropomodulin [10,11], thus potentially providing a mechanism to arrest filament elongation at precisely the length of the nebulin template [7] (Figure 1A). In a recent study, Gregorio and colleagues [12] have shown for the first time that nebulin regulates thin filament length. Using RNA
in sporulating cells of Bacillus subtilis strain 3610. Virology 39, 265–275. 5. Osburne, M.S., and Sonenshein, A.L. (1976). Behavior of a temperate bacteriophage in differentiating cells of Bacillus subtilis. J. Virol. 19, 26–35. 6. Kawamura, F., and Ito, J. (1974). Bacteriophage gene expression in sporulating cells of Bacillus subtilis 168. Virology 62, 414–425. 7. Buu, A., and Sonenshein, A.L. (1975). Nucleic acid synthesis and ribonucleic acid polymerase specificity in germinating and outgrowing spores of Bacillus subtilis. J. Bacteriol. 124, 190–200. 8. Burgess, R.R., Travers, A.A., Dunn, J.J., and Bautz, E.K. (1969). Factor stimulating transcription by RNA polymerase. Nature 221, 43–46. 9. Losick, R., and Sonenshein, A.L. (1969). Change in the template specificity of RNA polymerase during sporulation of Bacillus subtilis. Nature 224, 35–37. 10. Haldenwang, W.G., Lang, N., and Losick, R. (1981). A sporulation-induced sigmalike regulatory protein from B. subtilis.
11.
12.
13.
14.
15.
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Cell 23, 615–624. Stragier, P., and Losick, R. (1990). Cascades of sigma factors revisited. Mol. Microbiol. 4, 1801–1806. Fox, T.D., Losick, R., and Pero, J. (1976). Regulatory gene 28 of bacteriophage SPO1 codes for a phage-induced subunit of RNA polymerase. J. Mol. Biol. 101, 427–433. Meijer, W.J., Castilla-Llorente, V., Villar, L., Murray, H., Errington, J., and Salas, M. (2005). Molecular basis for the exploitation of spore formation as survival mechanism by virulent phage φ29. EMBO J. 24, 3647–3657. Strauch, M.A., and Hoch, J.A. (1993). Signal transduction in Bacillus subtilis sporulation. Curr. Opin. Genet. Dev. 3, 203-212. Levin, P.A., and Losick, R. (1996). Transcription factor Spo0A switches the localization of the cell division protein FtsZ from a medial to a bipolar pattern in Bacillus subtilis. Genes Dev. 10, 478–488. Moreno, F. (1979). On the trapping of phage genomes in spores of Bacillus
Olfactory Coding: Connecting Odorant Receptor Expression and Behavior in the Drosophila Larva The discovery of odorant receptors has significantly changed our understanding of how animals identify thousands of odorants. A recent study has shed new light on the central issue of how odor information is translated into meaningful behavior. Reinhard F. Stocker For many animal species, smell is the most crucial of all the senses. Odors are vital for locating food and egg-laying substrates, for avoiding dangers and for communicating with conspecific individuals. Understanding olfaction in the biological context requires analysis at multiple levels: reception of odors by sensory neurons, integration of odor information in the brain, and translation of odor perception into meaningful behavioral output. Recent years have seen tremendous progress in odorant receptor and olfactory circuit analysis, largely aided by the use of simple model systems such as Drosophila. In contrast, the issue of how odor perception drives behavior has remained largely unexplored. As they reported in a recent issue of Current Biology, Fishilevich et al. [1] have now used an even simpler model, the Drosophila larva, to investigate
how odorant receptor expression patterns might be linked to chemotaxis. Surprisingly, they found that olfactory sensory neurons — each of which expresses its proper odorant receptor — are not equivalent in driving chemotaxis. At the extreme, larvae with just a single functional sensory neuron, expressing the receptor OR42a, are attracted to a high fraction of odors. The identification of odorant receptors [2–4] and analysis of their expression patterns in the sensory neurons have had a great impact on our understanding of the principles of olfactory coding. In both mammals and insects, olfactory sensory neurons essentially express a single type of odorant receptor. Neurons expressing the same receptor type are scattered in the epithelium, but their axons converge onto one or two glomeruli in the olfactory brain centers [5–8]. The chemical
17.
18.
19.
subtilis 168: reciprocal exclusion of phages φ29 and φe during outgrowth of spores. Virology 93, 357–368. Wu, L.J., and Errington, J. (1994). Bacillus subtilis SpoIIIE protein required for DNA segregation during asymmetric cell division. Science 264, 572–575. Lee, P.S., Lin, D.C., Moriya, S., and Grossman, A.D. (2003). Effects of the chromosome partitioning protein Spo0J (ParB) on oriC positioning and replication initiation in Bacillus subtilis. J. Bacteriol. 185, 1326–1337. Sonenshein, A.L. (1970). Trapping of unreplicated phage DNA into spores of Bacillus subtilis and its stabilization against damage by 32P decay. Virology 42, 488–495.
Department of Molecular Biology and Microbiology, Tufts University School of Medicine, 136 Harrison Avenue, Boston, Massachusetts 02111, USA. DOI: 10.1016/j.cub.2005.12.007
information conveyed by the sensory neurons is thus translated into a pattern of glomerular activation [9–11]. The insect olfactory system therefore shares the design of the mammalian system [12] but comprises only a fraction of the receptors and neurons of the latter, providing an attractive system for analyzing odor coding. Insects undergoing metamorphosis exhibit an even simpler olfactory circuit during the larval stage. How is this larval pathway organized? Could it offer a yet simpler approach for understanding the sense of smell from odor reception to behavioral output? The odorant receptor family in Drosophila consists of 62 members [3,4,13] compared to more than 1000 in rodents. At least 25 of the 62 receptors are expressed in the fly larva [1,14,15]. Of these 25, 14 are larval-specific, while the rest are expressed in both adult and larval olfactory systems. As in the adult, the large majority of the 21 larval olfactory sensory neurons express one conventional odorant receptor, along with an atypical receptor, OR83b [1,15,16]. At least two neurons express two additional receptors apart from OR83b [1]. Because the number of receptors exceeds the number of olfactory sensory neurons, a few more cases of triple receptor expression are to be expected. In
Dispatch R17
summary, the rules of odorant receptor expression in the larval olfactory system appear to be similar to those in adult flies and mammals but differ from another simple system, the nematode Caenorhabditis elegans, in which sensory neurons express multiple odor receptors. Kreher et al. [15] investigated the electrophysiological responses of 11 larval odorant receptors to a panel of 29 compounds by expressing single odorant receptor genes in sensory neurons of anosmic adult mutant flies [17,18]. Their results showed that the responses of the different receptors are very diverse, ranging from activation by just one odorant to activation by many of the tested odorants. Some receptors were found to respond most strongly to aliphatic compounds whereas others seemed to be tuned to aromatic compounds. Single-cell labeling experiments in the Or83b-Gal4 driver line, as well as experiments in which transgenes were expressed under the control of promoters of odorant receptor genes, showed that the larval olfactory pathway has a surprisingly similar, but much simpler, design to its adult counterpart [1,15,19]. As in the adult, in the larva each olfactory sensory neuron projects to a single glomerulus in the antennal lobe. But compared to the 1,300 olfactory sensory neurons in the adult, there are no more than 21 of these in the larva. Each of these is unique and projects to one of 21 glomeruli. Hence, any given glomerulus is the target of a single sensory neuron and is therefore associated with a specific odorant receptor (except for the few neurons that express two conventional odorant receptors). The Drosophila larva thus provides a ‘minimal’ olfactory model system that still allows meaningful comparison with mammals. In their new work, Fishilevich et al. [1] went a step ahead and addressed directly the correlation between odorant receptor expression and behavioral output — chemotaxis. For this purpose, they used ‘subtractive’ and
‘additive’ strategies: in the former they genetically ablated selected olfactory sensory neurons via toxin expression, and in the latter they created animals with only one or two functional sensory neurons. In wild-type larvae, 36 of 53 tested compounds elicited robust chemotactic responses. In the subtractive approach, they obtained two types of results. Animals in which either the Or1aexpressing neuron or the Or49aexpressing neuron was ablated showed reduced chemotaxis to only one of 20 odors tested. This relatively mild effect is consistent with the broad and overlapping ligand tuning of many olfactory sensory neurons in adults [17] and larvae [15]. In contrast, loss of the neuron expressing Or42a resulted in anosmy to four of the 20 odors. In the additive approach, larvae with one or two functional olfactory sensory neurons were generated using Or1a, Or42a or Or49a driver lines [1]. Consistent with the stronger Or42a-ablated phenotype, Or42a-functional larvae responded to 22 of 53 odors tested (compared to 36 odors in the wild-type), including three of four odors to which Or42a-ablated animals are anosmic. The broad response profile for Or42a-functional larvae is in agreement with the broad ligand tuning of this receptor [15,18]. In contrast, Or1a- and Or49a-functional larvae did not exhibit significant chemotaxis to any of the 53 odors, consistent with the weak phenotype of the corresponding ablated larvae and with electrophysiological responses [15]. Animals with two functional neurons responded to a somewhat different subset of odors than larvae having either single functional neuron alone [1]. Statistical analysis suggested six cases of potential cooperativity between Or1a and Or42a chemotaxis. For example, 1-pentanol elicited significantly stronger chemotaxis in Or1a/Or42a-functional larvae than in either single Or-functional animal. Three major conclusions can be drawn from these data [1]. First, the minimal effects on chemotaxis after ablating the Or1a or Or49a
neurons suggest a certain level of redundancy in the system. This sounds surprising, given the small number of olfactory sensory neurons in the larval system. Yet, subtle effects exerted by seemingly ‘unimportant’ neurons could be crucial for cooperative processes (see below). Second, the Or42a neuron plays a particularly important role in odor detection and is sufficient to initiate chemotaxis to a high fraction of odors. Moreover, its loss leads to severe behavioral defects. Why there is functional heterogeneity between the Or42a neuron and the Or1a or Or49a neurons remains a puzzle for the moment. And third, cooperative action is suggested by the enhanced chemotactic responses of Or1a/Or42a-functional animals compared to the single Or1a- or Or42a-functional animals. Olfactory coding thus does not simply rely on additive activation of 21 parallel pathways, but involves horizontal interactions as well. Cross-talk may occur at many levels of the circuit, from the sensory neurons themselves to olfactory target neurons in the brain. A primary candidate are local interneurons in the antennal lobe that provide lateral connections among the glomeruli [19]. Significant transformation of olfactory signals is indeed known from the adult antennal lobe [20]. Integration of olfactory information may sharpen both quantitative and qualitative aspects of the signals, such as detection threshold and odor discrimination, respectively. Although chemotaxis assays do not answer how odors are distinguished from each other, it is reasonable to postulate that integrative processes may be particularly crucial if very few channels have to cope with many odors. The findings by Fishilevich et al. [1], together with the other recent data on the olfactory system of the Drosophila larva [14,15,19], provide links between odorant receptors, their ligands, odordriven behavior and the underlying neuronal substrate, and put forward the Drosophila larva as a highly attractive
Current Biology Vol 16 No 1 R18
‘minimal’ model in olfactory research. References 1. Fishilevich, E., Domingos, A.I., Asahina, K., Naef, F., Vosshall, L.B., and Louis, M. (2005). Chemotaxis behavior mediated by single larval olfactory neurons in Drosophila. Curr. Biol. 15, 2086-2096. 2. Buck, L., and Axel, R. (1991). A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 65, 175–187. 3. Clyne, P.J., Warr, C.G., Freeman, M.R., Lessing, D., Kim, J., and Carlson, J.R. (1999). A novel family of divergent seventransmembrane proteins: candidate odorant receptors in Drosophila. Neuron 22, 327–338. 4. Vosshall, L.B., Amrein, H., Morozov, P.S., Rzhetsky, A., and Axel, R. (1999). A spatial map of olfactory receptor expression in the Drosophila antenna. Cell 96, 725–736. 5. Ressler, K.J., Sullivan, S.L., and Buck, L.B. (1994). Information coding in the olfactory system: Evidence for a stereotyped and highly organized epitope map in the olfactory bulb. Cell 79, 1245–1255. 6. Vassar, R., Chao, S.K., Sitcheran, R., Nunez, J.M., Vosshall, L.B., and Axel, R. (1994). Topographic organization of sensory projections to the olfactory bulb. Cell 79, 981–991. 7. Gao, Q., Yuan, B., and Chess, A. (2000).
Convergent projections of Drosophila olfactory neurons to specific glomeruli in the antennal lobe. Nat. Neurosci. 3, 780–785. 8. Vosshall, L.B., Wong, A.M., and Axel, R. (2000). An olfactory sensory map in the fly brain. Cell 102, 147–159. 9. Joerges, J., Küttner, A., Galizia, C.G., and Menzel, R. (1997). Representations of odours and odour mixtures visualized in the honeybee brain. Nature 387, 285–287. 10. Malnic, B., Hirono, J., Sato, T., and Buck, L.B. (1999). Combinatorial receptor codes for odors. Cell 96, 713–726. 11. Wang, J.W., Wong, A.M., Flores, J., Vosshall, L.B., and Axel, R. (2003). Twophoton calcium imaging reveals an odorevoked map of activity in the fly brain. Cell 112, 271–282. 12. Hildebrand, J.G., and Shepherd, G. (1997). Mechanisms of olfactory discrimination: converging evidence for common principles across phyla. Annu. Rev. Neurosci. 20, 595–631. 13. Robertson, H.M., Warr, C.G., and Carlson, J.R. (2003). Molecular evolution of the insect chemoreceptor gene superfamily in Drosophila melanogaster. Proc. Natl. Acad. Sci. USA 100, 14537–14542. 14. Couto, A., Alenius, M., and Dickson, B.J. (2005). Molecular, anatomical, and functional organization of the Drosophila olfactory system. Curr. Biol. 15, 1535–1547. 15. Kreher, S.A., Kwon, A.Y., and Carlson,
Nebulin: Does It Measure up as a Ruler? A recent study has shown that the giant protein nebulin maintains the lengths of actin filaments in striated muscle cells. Although on the surface, nebulin looks like a molecular ruler, it may be playing a more complex role in regulating dynamics at the pointed end of actin filaments in striated muscle. Velia M. Fowler, Caroline R. McKeown and Robert S. Fischer In a striated muscle cell, the lengths of the sarcomeric actin filaments — the thin filaments — are precisely regulated, both in terms of their particular lengths, and in the variation of their length distributions [1,2]. Actin monomers assemble in vitro into filaments with an exponential length distribution, and they can organize into a wide variety of lengths in non-muscle cells, indicating that length controls are not intrinsic to actin filament polymers. Historically, a ‘molecular ruler’ mechanism has been the most attractive model for how thin filament lengths are determined in striated muscle. By definition, a
molecular ruler must meet a number of criteria: the ruler must be the precise length of the target filament, with the length of the ruler dictating the length of the filaments; the end of the ruler should bind to a terminator protein to prevent actin subunit addition or loss once the filament has polymerized to the length of the ruler protein; and the ruler must associate in a stoichiometric ratio with its target filament. In this mechanism, filament lengths are fixed precisely to the ruler length once they have polymerized. Thus, only filaments associated with rulers will be the length of the ruler, and filaments without a ruler will assume random and variable lengths (Figure 1A). The giant molecule nebulin has been postulated to be a molecular
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J.R. (2005). The molecular basis of odor coding in the Drosophila larva. Neuron 46, 445–456. Larsson, M.C., Domingos, A.I., Jones, W.D., Chiappe, M.E., Amrein, H., and Vosshall, L.B. (2004). Or83b encodes a broadly expressed odorant receptor essential for Drosophila olfaction. Neuron 43, 703–714. Hallem, E.A., Ho, M.G., and Carlson, J.R. (2004). The molecular basis of odor coding in the Drosophila antenna. Cell 117, 965–979. Goldman, A.L., van der Goes van Naters, W., Lessing, D., Warr, C.G., and Carlson, J.R. (2005). Coexpression of two functional odor receptors in one neuron. Neuron 45, 661–666. Ramaekers, A., Magnenat, E., Marin, E.C., Gendre, N., Jefferis, G.S.X.E., Luo, L., and Stocker, R.F. (2005). Glomerular maps without cellular redundancy at successive levels of the Drosophila larval olfactory circuit. Curr. Biol. 15, 982–992. Wilson, R.I., Turner, G.C., and Laurent, G. (2004). Transformation of olfactory representations in the Drosophila antennal lobe. Science 303, 366–370.
Department of Biology, University of Fribourg, 10 rue du Musée, CH-1700 Fribourg, Switzerland. DOI: 10.1016/j.cub.2005.12.008
ruler that determines thin filament length in striated muscle [3–7]. Nebulin extends along the thin filament, with its amino terminus oriented near the pointed (free) end and its carboxyl terminus near the barbed end in the Z disc. A number of properties of nebulin appear to fulfill many of the a priori requirements for a molecular ruler. First, the molecular sizes of nebulin isoforms correlate with the lengths of thin filaments in the striated muscles in which the isoform is found [6,8]. Second, nebulin molecules are composed of a modular series of repeats corresponding to the repeats of the actin subunits of the thin filament, thus ‘measuring’ polymer length [6,9]. Third, a region in the unique amino-terminal domain of nebulin (M1M2M3) is located near the thin filament pointed end [9], and interacts with the actin pointed end capping protein, tropomodulin [10,11], thus potentially providing a mechanism to arrest filament elongation at precisely the length of the nebulin template [7] (Figure 1A). In a recent study, Gregorio and colleagues [12] have shown for the first time that nebulin regulates thin filament length. Using RNA