The QF/QUAS System in Anopheles gambiae

Nathaniel Grubbs, Ph.D.

Nathaniel Grubbs, Ph.D., Department of Entomology, North Carolina State University MORE ABOUT THE AUTHOR

Riabinina et al. (2016) recently published research on the olfactory centers of the malaria mosquito, Anopheles gambiae, which may be of interest to the broader insect genomic research community, not only for what it reveals, but because of the technical aspects of the.  The authors use the Q System to regulate the expression of various transgenes.   The Q System could also be used in other insects.

Because olfaction is important for mosquitoes to find human hosts, understanding how this system works could prove helpful in protecting people from the diseases these insects vector. To accomplish this, Riabinina and company turned to the genetic model organism, Drosophila melanogaster, for inspiration. Olfaction is well studied in Drosophila, providing a useful framework for understanding other species. Also critical is that some of the genetic tools used in Drosophila are becoming available in mosquitoes.

Diagram of basic binary Q system with QF activator and QS inhibitor

The Q system showing the 3 major components. The QF transcription factor, the enhancer binding sites for QF known as QUAS and an inhibitor of QF call QS. QS can be inhibited by a small molecule – quinic acid, providing an additional level of control of this system. This image is from Chris Potter’s lab at Johns Hopkins. Dr. Potter was the original developer of this system.

For this study, Riabinina and colleagues turned to a transcription regulatory system analogous to the GAL4/UAS but from Neurospora and referred to as the Q System.  The senior author of the manuscript was the original developer of the Q System in D. melanogaster.   This system, the QF/QUAS, or Q-system, is composed of a transcription factor (QF) that binds to specific upstream activating sequences (QUAS).

Riabinina et al. modified the Q-system by cloning 15 copies of the QUAS upstream of the gene they wished to express, in this case a membrane-targeted GFP (mCD8:GFP) useful for visualizing neuronal axons (or any cell). To drive the expression of their QUAS-mCD8:GFP, this group created a QF “driver” construct using 9kb of sequence upstream of the olfactory receptor neuron-specific gene, Orco from Anopheles gambiae. The additional copies of the QUAS sequence in front of GFP ensured high levels of fluorescent expression in spite of the limited nature of the driver promoter, permitting reliable visualization of Orco-specific olfactory neurons when the two constructs were combined in the same individuals.

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Neurospora has a cluster of genes involved in quinic acid metabolism, allowing it to be used as a carbon source in glucose-limited conditions. The gene cluster is regulated by a transcription factor QF and genes regulated by QF have upstream activating sequences where QF binds (QUAS). In addition, the protein QS prevents QF-mediated expression by binding to QF. So QF/QS/QUAS this is very much analogous to Gal4/Gal80/UAS.

Much of what Riabinina et al. found was not surprising, given previous research in Drosophila and in Anopheles. Rather, they add distinct observations with a novel method which were able to confirm several important characteristics of mosquito olfaction, including the sexual dimorphism of adult mosquito olfactory anatomy. The details are worth a read for any interested in this specific field, especially to see the direct visualization of the olfactory system neural anatomy.

Use of the Q-system did lead to a novel and interesting finding: olfactory receptors previously discovered in the labellum, a predominantly gustatory organ, project to the subesophageal zone along with the gustatory receptors from the same organ, rather than to the antennal lobe with other olfactory receptors, raising interesting questions about the integration of gustatory and olfactory inputs in mosquito host finding and feeding.

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Quinic acid

The authors of this paper are already working to apply their modified tool to other chemosensory receptors which may be important for mosquitoes to identify human hosts, but their methods could easily be adapted to understanding other aspects of mosquito neural development. More broadly, these modified tools may prove useful in other under-studied insect species in need of novel ways to visualize cellular architecture and development.

The QF/QUAS system functions very much like the Gal4/UAS system and just as Gal4 can be inhibited by the Gal80 protein, QF can be inhibited by the QS protein.  Unique to the Q System, however, is the option of inhibiting QS with a small molecule – quinic acid.  So there is an added level of control that one can achieve under some conditions with the Q System.

 

Riabinina, O, Task, D, Marr, E, Lin, C-C, Alford, R, O’Brochta, DA, Potter, CJ. Organization of olfactory centres in the malaria mosquito Anopheles gambiae. Nat. Commun. 2016; 7:13010.    doi:10.1038/ncomms13010

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