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RESEARCH

The primary goal of the lab is to understand how the insect brain receives, interprets, and responds to odors.

Summary

The focus of my laboratory is to understand how the insect brain receives, interprets, and responds to odors. Insects rely on their sense of smell for all major life choices, from foraging to mating, from choosing where to lay eggs to avoiding predators and dangers. We are interested in understanding at the neuronal level how odors regulate these behaviors.

Our long-term aim is to apply this knowledge to better control insects that pose a threat to human health. Our general approach towards achieving this goal is to link neuron function with olfactory-guided behaviors by developing and employing new genetic methods that enable unprecedented control over neural circuits in both the model organism Drosophila melanogaster and human vector Anopheles gambiae

Olfactory System

Figure 1Figure 1 (click to enlarge)Figure 2Figure 2 (click to enlarge)The olfactory system is an excellent model for characterizing how sensory information can be translated into biological representations. The primary neurons of the olfactory system are the olfactory receptor neurons (ORNs) which are located in the antenna (1200 ORNs) and maxillary palp (120 ORNs) (Figure 1 ).

 

These structures are analogous to the nose in mammals. Each ORN that expresses the same odorant receptor targets its axons to a defined locus in the antennal lobe (olfactory bulb) called a glomerulus. In the fly, there are about 54 glomeruli. The antennal lobe is the first processing center for olfactory information, and in ways that are still being elucidated, functions to transform and organize olfactory information. Such transformation is influenced by local interneurons (LNs) that innervate much of the antennal lobe. Olfactory information is then relayed by the projection neurons (PNs), which send their axons to synapse with the mushroom bodies (the center of learning/memory) and also the lateral horn (Figure 2).

The function of the lateral horn in organizing, interpreting, and processing olfactory information is less understood, but likely mediates innate olfactory responses.

Food and Pheromones

Many behaviors occur at food sites, such as aggregation, courtship, and egg-laying site selection.  However, the odorants that influence these behaviors at food sites is not clear. 

We discovered a new olfactory pathway in Drosophila that links food-odor perception to pheromone signaling.  By using a large arena 4-field olfactory assay, in which dozens of flies are simultaneously stimulated with an odor, we found that male flies, when stimulated with food-odors such as apple cider vinegar, banana, and yeast paste, deposit a pheromone onto their surroundings.  This pheromone acts like a tag to link food-odor perception to social communications.  

We identified the pheromone as 9-tricosene, a low-volatility odorant.  By single sensillum recordings, we identified Or7a receptor neurons in ab4 sensilla as those most robustly activated by this pheromone.  This is likely the first basiconic sensilla identified to be activated by a Drosophila pheromone. 

By behavioral analyses, we determined that 9-tricosene influences multiple behaviors, including aggregation and egg-laying site selection.  This is likely the first male-derived cue identified that influences egg-laying site selection in females. 

For more details:  Lin, et al, eLife 2015.

Development of Genetic Technologies

The characterization and manipulation of complex biological systems often requires sophisticated genetic tools. The Potter lab develops such new genetic technologies.

We developed the Q repressible binary expression to allow for sophisticated in vivo manipulations.  The Q-system functions independently of the GAL4 system. Using the Q and GAL4 systems together allows for a number of powerful possibilities in the marking and manipulation of tissues of interest.  The Q-system was first described in Potter et al, Cell, 2010.

The transcription factor for the Q-system, QF, appeared toxic if expressed pan-neuronaly or in all tissues.  We identified that a large region of the middle part of QF was responsible for this toxicity.  Removing this region, and creating a transcription factor that directly combined the QF DNA binding domain with the QF activation domain, generated a fully functional protein with greatly reduced toxicity.  This new QF transcription factor is called QF2. We generated a number of pan-neuronal and pan-tissue transgenic lines.  We also generated chimeric transcription factors that use GAL4 or LexA modular domains with QF.  More details can be found in Riabinina et al, Nature Methods, 2015

Fluorescence Guided Single Sensillum Recording

Olfactory neurons are housed in sensory hairs called sensilla.  The single sensillum recording technique records the electrical activity of the neurons within a sensillum, and allows for high resolution mapping of odorant to olfactory neuron activities. However, the technique requires semi-random sampling of the sensory hairs followed by stimulation with a panel of test odorants.

We improved upon this technique by validating a set of OrX-GAL4 and UAS-GFP reagents that allow systematic labeling of any basiconic, intermediate, or trichoid sensilla for targeted recordings.  This also enables easy identification of sensilla whose odor response profiles are ambiguous.  More details can be found in Lin and Potter, PLoS One, 2015.

Encoding of Biological Olfactory Values in the Insect Brain

We hypothesize that the antennal lobe functions as an organizing center to convert olfactory inputs into outputs biologically relevant to the animal.  In our studies of how antennal lobe output neurons (projection neurons) organize higher olfactory centers, we found that some olfactory information may be organized in broad categories of food and pheromones.  Details of this initial study can be found in Jefferis, Potter et al, Cell 2007.  

The food-odor induced 9-tricosene pheromone activates Or7a neurons to guide oviposition site-selection.  The projection neurons likely activated by 9-tricosene, DL5 projection neurons, share striking similarities to other projection neurons likely activated by other odorants that guide oviposition site-selection. This suggests that the female brain may contain a brain region dedicated to odor-guided egg-site selection. (Lin et al, eLife, 2015).

We are also currently investigating how odor valences are encoded in the brain. 

Mosquito olfaction

The mosquito is one of the greatest insect threats to human health, with over 1 million deaths every year due to mosquito-borne diseases such as malaria and dengue fever. Currently half the world’s population lives in regions threatened by mosquitoes and new strategies are required to combat mosquito host-seeking. Mosquitoes find humans by detecting olfactory cues (body odor and CO2). Targeting the olfactory system of mosquitoes is therefore a promising approach for interfering with the transmission of infectious diseases. Yet we know little about how olfactory signals are received and interpreted by the mosquito brain. 

We have recently generated transgenic Anopheles gambiae that utilize the Q-system to enable, for the first time, selective genetic labeling and manipulation of most olfactory neurons in living mosquitoes. We have generated transgenic mosquito lines that: 1) provide genetic access to the olfactory system components controlling a mosquito’s ability to distinguish humans over other animals; 2) allow us to perform neural anatomical analyses; 3) can be used to assay neuronal activity in response to human body odors, repellants, and mosquito pheromones.  Our first publication describing these reagents is in at Nature Communications

Anopheles gambiae female mosquito

Larval Olfaction

We have recently begun investigating the larval olfactory system, and how it guides larval responses to external stimuli.