Olfaction in Drosophila - using Drosophila odorant receptor genes and the DSCAM gene to analyze the specific connections of olfactory neurons in order to unravel the logic of olfactory information coding.

We asked whether epigenetic control of genes in this class on the same chromosome was exerted coordinately or whether each such gene was epigenetically regulated independently of other genes in this class on the chromosome. Using fluorescence in situ hybridization (FISH) to assess asynchronous replication, a marker for allele-specific epigenetic differences, we found that epigenetic regulation was coordinated on each chromosome: i.e. paternally derived alleles were regulated in the same way on the same chromosome. However, within a given cell clone, chromosomes were regulated independently, so that maternally derived genes were activated on some chromosomes in a specific clone while paternally derived genes were activated on other chromosomes. These findings support a chromosome wide mechanism for choosing alleles of autosomal genes which are monoallelically expressed on a given chromosome. Excluded is a mechanism by which all monoallelically expressed genes in a single cell clone of one parental origin or the other are regulated in a coordinated manner. Initial observations were on mice, and recent experiments have extended the observations to humans and also shown that when three copies of a given autosome are present, still only one is early replicating. Thus, a process with similarities to X-inactivation appears to be occurring for all autosomal pairs. We are currently exploring the mechanistic basis for the distinction between the maternal and paternal copies of each autosome.

We are pursuing experiments to further define mechanisms controlling odorant receptor gene expression using transgenic mice and retroviral vectors to manipulate and analyze gene expression in olfactory neurons.

The Drosophila olfactory system. We and others have identified a family of novel seven transmembrane domain receptors that are likely Drosophila olfactory receptors. As in the vertebrate olfactory systems studied, members of the Drosophila olfactory receptor family are expressed in distinct subsets of olfactory neurons distributed amongst neurons expressing different receptors. Using transgenic approaches, we find that the neurons expressing a given receptor, while they are distributed amongst neurons expressing other receptors, project to distinct glomeruli in the antennal lobe (the Drosophila equivalent to the mouse olfactory bulb.)

Thus, like the mouse, the fly uses spatial segregation of the projections to the brain to encode the nature of the olfactory stimulus. We are pursuing experiments to further define the specific connections of neurons in the Drosophila olfactory system and will explore the mechanisms by which these neurons form appropriate connections and function to accomplish the discrimination of odorants. One intriguing gene we are studying is the DSCAM gene which has been shown to have over 36,000 different possible splice forms. We are using a combination of microarray analysis and single cell RT-PCR to look at the regulation of splicing of DSCAM in olfactory neurons and other neuronal and non-neuronal cells.

immunoglobulin gene allelic exclusion. Allelic exclusion in light-chain synthesis is thought to result from a feedback mechanism by which the expression of a functional light-chain on the surface of the B cell leads to an intracellular signal that downregulates the V(D)J recombinase, thus precluding rearrangement of the other allele. While such a feedback mechanism clearly plays a role in the maintenance of allelic exclusion, we have shown that the initial establishment of allelic exclusion involves differential availability of the two alleles for rearrangement. We have shown that asynchronous replication plays a role in determining which allele will be rearranged first in a given developing B cell. Allelic exclusion is critical for the proper functioning of the immune system including the prevention of autoimmunity.

Selected Publications
Singh, N., Ebrahimi, F.A.W., Qi, P. and Chess, A. Coordination of the Random Asynchronous Replication of Autosomal Loci. Nature Genetics. 33(3):339-341. (2003).

Singh, N., Bergman, Y., Cedar, H. and Chess, A. Biallelic germline transcription at the immunoglobulin locus. Journal of Experimental Medicine. 197(6): 743-750. (2003).

Mostoslavsky, R*., Singh, N*., Tenzen, T., Goldmit, M., Gabay, C., Elizur, S., Qi, P., Chess, A., Cedar, H., and Bergman, Y. Asynchronous replication and allelic exclusion in the immune system. Nature. 414:221-225. (2001). (*These authors contributed equally.)

Gao, Q., Yuan, B., and Chess, A. Convergent projections of Drosophila olfactory neurons to specific glomeruli suggest an odotopic map in the antennal lobe. Nature Neurosci. 3(8): 780-785, (2000).