Control of Drosophila perineurial glial growth by interacting neurotransmitter-mediated signaling pathways.

Drosophila peripheral nerves, similar structurally to the peripheral nerves of mammals, comprise a layer of axons and inner glia, surrounded by an outer perineurial glial layer. Although it is well established that intercellular communication occurs among cells within peripheral nerves, the signaling pathways used and the effects of this signaling on nerve structure and function remain incompletely understood. Here we demonstrate with genetic methods that the Drosophila peripheral nerve is a favorable system for the study of intercellular signaling. We show that growth of the perineurial glia is controlled by interactions among five genes: ine, which encodes a putative neurotransmitter transporter; eag, which encodes a potassium channel; push, which encodes a large, Zn(2+)-finger-containing protein; amn, which encodes a putative neuropeptide related to the pituitary adenylate cyclase activator peptide; and NF1, the Drosophila ortholog of the human gene responsible for type 1 neurofibromatosis. In other Drosophila systems, push and NF1 are required for signaling pathways mediated by Amn or the pituitary adenylate cyclase activator peptide. Our results support a model in which the Amn neuropeptide, acting through Push and NF1, inhibits perineurial glial growth, whereas the substrate neurotransmitter of Ine promotes perineurial glial growth. Defective intercellular signaling within peripheral nerves might underlie the formation of neurofibromas, the hallmark of neurofibromatosis.

Expression mosaic of odorant-binding proteins in Drosophila olfactory organs.

Deciphering the genome of the fruitfly, Drosophila melanogaster, has revealed 39 genes coding for putative odorant-binding proteins (OBPs), more than are known at present for any other insect species. Using specific antibodies, the expression mosaic of five such OBPs (OS-E, OS-F, LUSH, PBPRP2, PBPRP5) on the antenna and maxillary palp has been mapped in the electron microscope. It was found that (1) OBP expression does correlate with morphological sensillum types and subtypes, (2) several OBPs may be co-localized in the same sensillum, and (3) OBP localization is not restricted to olfactory sensilla. The expression of PBPRP2 in antennal epidermis sheds some light on the possible evolution of OBPs.

Molecular evolution of odorant-binding protein genes OS-E and OS-F in Drosophila.

The Drosophila olfactory genes OS-E and OS-F are members of a family of genes that encode insect odorant-binding proteins (OBPs). OBPs are believed to transport hydrophobic odorants through the aqueous fluid within olfactory sensilla to the underlying receptor proteins. The recent discovery of a large family of olfactory receptor genes in Drosophila raises new questions about the function, diversity, regulation, and evolution of the OBP family. We have investigated the OS-E and OS-F genes in a variety of Drosophila species. These studies highlight potential regions of functional significance in the OS-E and OS-F proteins, which may include a region required for interaction with receptor proteins. Our results suggest that the two genes arose by an ancient gene duplication, and that in some lineages, one or the other gene has been lost. In D. virilis, the OS-F gene shows a different spatial pattern of expression than in D. melanogaster. One of the OS-F introns shows a striking degree of conservation between the two species, and we identify a putative regulatory sequence within this intron. Finally, a phylogenetic analysis places both OS-E and OS-F within a large family of insect OBPs and OBP-like proteins.

Olfactory coding in a compound nose. Coexpression of odorant-binding proteins in Drosophila.

Odorant-binding proteins (OBPs) are small, soluble proteins present in the aqueous medium surrounding olfactory receptor neurons. Their function in olfaction is unknown: they have been proposed to facilitate the transit of hydrophobic molecules to olfactory receptors, to deactivate the odorant stimulus, and/or to play a role in chemosensory coding. We have examined the genomic organization and expression patterns of two olfactory-specific genes (OS-E and OS-F) of Drosophila melanogaster, the products of which are members of a protein family in Drosophila sharing sequence similarity with moth OBPs. We found that the OS-E and OS-F transcription units are located < 1 kb apart. They are oriented in the same direction and display a similar intron-exon organization. Expression of both OS-E and OS-F proteins is spatially restricted to the ventrolateral region of the Drosophila antenna. Within this region, both OS-E and OS-F proteins are expressed within two different types of sensory hairs: in most, if not all, sensilla trichodea and in approximately 40% of the interspersed small sensilla basiconica. We consistently observe that OS-E and OS-F are coexpressed, indicating that an individual sensillum can contain more than one odorant-binding protein. This finding has potential implications for the roles of odorant-binding proteins in olfactory coding.

Coexpression of two odorant-binding protein homologs in Drosophila: implications for olfactory coding.

Odorant-binding proteins (OBPs) are small soluble proteins present in the aqueous medium surrounding olfactory receptor neurons. Their function in olfaction is still unknown: they have been proposed to facilitate the transit of hydrophobic molecules to olfactory receptors, to deactivate the odorant stimulus, and/or to play a role in chemosensory coding. In this study we examine the genomic organization and expression patterns of two olfactory-specific genes (OS-E and OS-F) of Drosophila melanogaster, the products of which are members of a protein family in Drosophila sharing sequence similarity with moth OBPs. We show that the OS-E and OS-F transcription units are located <1 kb apart. They are oriented in the same direction and display a similar intron-exon organization. Expression of both OS-E and OS-F proteins is restricted spatially to the ventrolateral region of the Drosophila antenna. Within this region both OS-E and OS-F proteins are expressed within two different types of sensory hairs: in most, if not all, sensilla trichodea and in approximately 40% of the interspersed small sensilla basiconica. We consistently observe that OS-E and OS-F are coexpressed, indicating that an individual sensillum can contain more than one odorant-binding protein. The functional significance of the observed expression pattern and its implications for olfactory coding are discussed.

Genetic and molecular studies of olfaction in Drosophila.

Drosophila melanogaster, an insect amenable to convenient molecular and genetic manipulation, has a highly sensitive olfactory system. A number of Drosophila olfactory mutants have been isolated and characterized. The smellblind mutant has a defect affecting a voltage-gated Na+ channel. The norpA mutant, defective in a phospholipase C, has a reduced response to odorants in one type of olfactory organ, providing genetic evidence for use of the inositol-1,4,5-trisphosphate signal transduction pathway in olfaction. Since the norpA gene is also required for phototransduction, this work demonstrates overlap in the molecular genetic basis of vision and olfaction. Interestingly, genetic analysis indicates that some olfactory information flows through a pathway which does not depend on norpA. Some mutants, such as ptg, acj6 and Sco, show odorant specificity, in the sense that some odorant responses are greatly reduced, whereas others are little affected, if at all. Some, but not all, mutations affect both larval and adult olfactory responses. Two tightly-linked Drosophila genes encode homologues of moth pheromone-binding proteins (PBPs). Genetic analysis may help determine whether PBPs facilitate transit of pheromones to or from olfactory receptor neurons. Information from Drosophila could be useful in designing means of controlling mosquitoes. It may also be possible to study olfactory genes, such as those encoding PBPs, from other insects by mutating them, introducing them into Drosophila and analysing their function in vivo.

Putative Drosophila pheromone-binding proteins expressed in a subregion of the olfactory system.

Four genes expressed in the olfactory system of Drosophila melanogaster have been identified by subtractive hybridization. Two of these genes, OS-E and OS-F, are related to genes encoding moth pheromone-binding proteins. The OS-E and OS-F genes are tightly linked and are expressed in a subregion of the antenna (the primary olfactory organ). A protein sequence analysis suggests the possibility that pheromone-binding proteins are members of a larger class of proteins, extending beyond the olfactory system. The predicted product of a third gene, OS-D, shares features common to vertebrate odorant-binding proteins, but has a primary structure unlike odorant-binding proteins. The fourth gene, OS-C, encodes a novel 13-kDa protein that contains a putative nuclear import sequence and an acid-rich region. The expression patterns of these genes differ within the antenna; their transcript distributions support the notion of specialized roles for different olfactory sensilla. The functions of the OS gene products have not been demonstrated; however, the potential identification of pheromone-binding proteins in Drosophila, a species with well characterized genetics, may offer a means of analyzing the function of these molecules that is not available in other systems.