molecular and genetic analysis of olfactory system in Drosophila
	John Carlson, Ph.D. Eugene Higgins Professor of Molecular, Cellular, and Developmental Biology Email: john.carlson@yale.edu Room: KBT 1132 Phone: 432-3541/ 432-3542 Fax: 432-5631 Carlson Lab Web site: http://pantheon.yale.edu/~jcarlso/ A.B. Harvard College 1977; Ph.D. Stanford University Medical School 1982

Olfaction offers a wealth of biological problems awaiting understanding in molecular terms. We are interested in the exquisite sensitivity of the olfactory system, its ability to distinguish among odors, and the means by which a system of such rich capabilities arises during development.

Antenna of Drosophila. Sensilla of one functional type, ab3, are labeled. ab3 sensilla contain two olfactory receptor neurons, ab3A and ab3B, that express different odor receptors and that differ in their odor-specificity.

Antennal lobes of brain. ab3A neurons send axons that converge in a symmetric pair of spherical glomeruli called DM2, labeled in yellow. Functional map of antennal surface. Seven types of sensilla are shown, including ab3 ("3"). Type 1 contains 4 distinct neurons and the other 6 types contain 2, combined according to a strict pairing rule, to yield 16 neuronal classes. Individual odor receptors have been mapped to most of these neuronal classes.

The fruit fly Drosophila, which is highly sensitive to a wide variety of odors, offers several advantages as an organism in which to study olfaction. Its olfactory system is relatively simple, containing ~10³ receptor cells. Powerful genetic and molecular techniques are available in Drosophila to identify components of the olfactory system, and its genome is small and sequenced. Most important, the function of the system can conveniently be analyzed in vivo, either physiologically or behaviorally.

We have discovered a large family of seven-transmembrane-domain odorant receptors in the fly. We identified them by using a novel computer algorithm to search the fly genome database for proteins with a particular structure, as opposed to proteins with a particular sequence. Different family members are expressed in different subsets of olfactory receptor cells.

We are exploring the organization of the olfactory system with a functional genomics approach. Through physiological recordings we have provided a functional map of olfactory receptor neurons, showing that neurons with a particular odor specificity are restricted to a particular spatial domain of the antenna. We are now integrating the functional map with the molecular map, by determining which receptors are expressed in neurons of a particular odor specificity. In this manner we are creating a receptor-to-neuron map and identifying ligands for individual receptors.

The large size of this receptor family presents an intriguing problem: how do individual olfactory receptor cells select, from among a large repertoire of receptor genes, which genes to express? One clue comes from a behavioral mutant we isolated, acj6 (abnormal chemosensory jump 6), in which the response of the olfactory organs is severely reduced to some, but not all, odors. Some cells in acj6 acquire a novel odor-specificity that is different from any we have observed in wild-type. Molecular analysis has shown that acj6 encodes a POU domain transcription factor that regulates a subset of receptor genes. Thus the Acj6 transcription factor plays a role in the process by which olfactory receptor cells choose which receptor genes to express. We are now testing the possibility that several transcription factors act combinatorially to help govern the odor-specificity of all olfactory receptor cells in the fly.

As a reciprocal approach to the problem of receptor gene choice, we are using bioinformatics to identify cis-acting regulatory sequences that dictate in which olfactory organ, in which sensillum type of an organ, and in which neuron of a sensillum an individual receptor is expressed. We are interested in determining whether the expression of particular receptor genes is governed by a combinatorial code of cis-acting regulatory elements.

We recently identified another large family of seven-transmembrane-domain genes, the Gr genes, whose tissue-specificity of expression and structure suggested that they encoded the first insect taste receptors. Genetic analysis and heterologous expression data have shown that one of these, Gr5a, in fact encodes a taste receptor for the sugar trehalose.

Finally, we have initiated a functional analysis of odorant receptors in the mosquito Anopheles gambiae, which transmits malaria. Female Anopheles detects and locates humans largely through olfactory cues. We have expressed Anopheles odorant receptors in a "decoder": a mutant olfactory receptor neuron of Drosophila that has lost odorant response due to a deletion of its endogenous receptor genes. Expression of a female-specific Anopheles odorant receptor in this neuron conferred response to the odorant 4-methyl phenol, a component of human sweat.

Selected Publications

Ray A., van der Goes van Naters W., Shiraiwa T., and Carlson J.R. (2007) Mechanisms of odor receptor gene choice in Drosophila, Neuron 53, 353-69.

Hallem, E., and Carlson, J.R. (2006) Odor coding by a receptor repertoire. Cell 125, 143-160.

Kreher, S.A., Kwon, J.Y., and Carlson, J.R. (2005) The molecular basis of odor coding in the Drosophila larva. Neuron 46, 445-456.

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.

Hallem, E.A., Ho, M.G., and Carlson, J. (2004) The Molecular Basis of Odor Coding in the Drosophila Antenna, Cell 117, 965-979.

Hallem, E.A., Fox, N., Zwiebel, L., and Carlson, J.R. (2004) Olfaction: Mosquito Receptor for Human-Sweat Odorant, Nature 427, 212-213.

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