Regulation of Host-Seeking and Blood-Feeding Behavior in the Mosquito
Olfactory cues guide mosquitoes toward humans, from which the mosquitoes derive the blood they need to complete ovarian development. We are interested in understanding the specific chemical cues that mosquitoes use to detect humans, and the extent to which mosquitoes discriminate among humans when they choose a host. We are applying our work in Drosophila olfaction to this important vector insect and are launching an ambitious mosquito genetics project to begin probing which genes and pathways matter for mosquito host-seeking behavior.
Not all mosquitoes are attracted to humans, and we are interested in how certain mosquito species evolved a strong attraction to us. To probe the question of how human host attraction evolved, we are examining the molecular and genetic differences of two variants of Aedes aegypti, Aedes aegypti aegypti and Aedes aegypti formosus. These represent an interesting case of divergent host adaptation within the same species. These two strains live in close proximity in coastal Africa, but only one (Aedes aegypti aegypti) shows interest in human hosts; the other actively avoids humans. We are collecting both strains from the field to rear in the laboratory and will carry out behavioral and molecular genetic experiments to study how this mosquito evolved to specialize on humans. The results will have important implications for our understanding of how mosquitoes evolved to feed on humans.
Mosquito host-seeking behavior is suppressed or inhibited for about 72 hours after the mosquito takes a blood meal. The molecular basis for the regulation of this behavior is unknown, but it may be explained by a humoral control mechanism in which the sensitivity of the olfactory system is altered after blood feeding. We are examining the hypothesis that regulation of specific olfactory genes modifies the host-seeking behavior of female Aedes aegypti after blood feeding. We are interested in identifying the profiles of genes that are regulated in different tissue types before and after blood feeding and then probing the genetic requirement for these genes in the behavior.
Molecular Biology of the Insect Odorant Receptors
Insects have exquisitely sensitive olfactory systems that are tuned to food odors and pheromonal cues emitted by members of the same species. We have been studying the molecular mechanisms by which insect olfactory neurons respond to and discriminate among the numerous possible odors in the environment. Several years ago, we and others identified a divergent family of seven transmembrane domain receptors now known to be the insect odorant receptors. One member of the odorant receptor gene family, Or83b, it is expressed in nearly all olfactory neurons. Therefore, each olfactory neuron in the fly is likely to express a conventional odorant receptor along with the coreceptor Or83b.
Our recent work has shown that the insect odorant receptor is a heteromeric complex of the Or83b coreceptor with a conventional ligand-binding odorant receptor. Or83b is necessary and sufficient to target this OR/Or83b complex to the ciliated dendrite of the olfactory sensory neuron. In collaboration with Kazushige Touhara and colleagues (University of Tokyo), we investigated whether Or83b has signaling functions beyond its role in ciliary trafficking. Our recent work provides strong evidence that the OR/Or83b complex forms an odor-gated nonselective cation channel that does not depend on G protein signaling. We are carrying out a large-scale in vivo structure-function analysis of Or83b and the conventional odorant receptors to probe the biology of these unusual membrane receptors. Our goal is to map those domains that are necessary for the heteromeric association of the OR/Or83b complex, domains necessary for trafficking, and residues that are necessary for odor signal transduction. We are particularly interested in discovering which residues may contribute to forming the ion-conducting pore. Going beyond conventional genetics, we are using chemical biology to probe for small molecules that interfere with heterodimerization, trafficking, or signaling of OR/Or83b complexes. Some of these compounds may be useful elements in a chemical strategy to block olfactory host-seeking behaviors in mosquitoes and other pest insects. These compounds may act as insect repellents that could be useful to control insect vectors that transmit human infectious diseases.
Genetic Basis of Interindividual Variation in Human Smell Perception
Humans show large differences in olfactory perception, but the basis of this phenotypic variation is unknown. Some people find the odor of cilantro quite pleasant, while others find it vile. Still other people with an otherwise normal sense of smell are unable to detect certain odors. We are carrying out a large-scale human clinical study to test the hypothesis that variation in human odor perception is due in part to genetic variation in odorant receptors. To date, our study has collected psychophysical data on 391 subjects asked to smell 66 odors, and we have identified large cohorts of individuals with interesting olfactory deficits. In collaboration with Hiroaki Matsunami's lab (Duke University Medical Center), we have identified variation in a single human odorant receptor, OR7D4, as a major determinant of the ability to detect the odorous by-products of sex steroids, androstenone and androstadienone. In ongoing work, we plan to carry out a large-scale genotype-phenotype association study to link variation in odorant receptor genes to variability in olfactory perception. Following up on our findings of a human androstadienone receptor, we are measuring physiological responses in women who smell this male-derived odor. Our goal is to tease apart conscious and unconscious perception of these male-derived odors and to study the influence of the genetic makeup of odorant receptor genes in our subjects. Our long-term goal is to understand the genetic basis of odor coding in humans.
Olfactory Adaptation to Feeding State in Drosophila melanogaster
The ability of animals to adapt their feeding behavior in response to hunger and satiety cues is important for survival in the natural world where food resources fluctuate. Prior studies in vertebrates and in worms suggest that this adaptation may involve modulation of the chemosensory system in response to food intake, though little is known about the mechanism for this proposed sensory adaptation.
We have been examining the behavioral response of adult D. melanogaster flies to odor cues and food in starved and in fed states. Flies that have been starved for 24 or 48 hours show significantly increased olfactory behavior preference for a food odor when tested in a two-choice odor preference assay. There is a corresponding increase in food intake in such animals. Using whole-genome microarrays, we have identified a set of genes in Drosophila that are regulated by starvation and that are candidates for molecular regulators of this behavioral phenotype. Forty genes show at least a 10-fold increase or decrease in fly heads after 24 hours of starvation. These may represent transcripts that modulate behavior in response to feeding state. In antennae, 40 genes are likewise either up- or down-regulated in response to 24 hours of starvation, suggesting that feeding state modulates gene expression in peripheral olfactory organs. Our results demonstrate that insect olfactory preference can depend on feeding state, and we are investigating candidate genes that may regulate this behavior.
Identification of Novel Genes and Circuits in an Animal Model of Binge Eating Disorder
The etiology of compulsive feeding behaviors, including bulimia nervosa and binge eating disorder in humans, is poorly understood. We propose that studying these clinical conditions in a simpler genetic model system, the Drosophila larva, may shed new light on this important health problem. Fruit flies go through four distinct life stages: embryo, larva, pupa, and adult. While adult flies, like normal humans, regulate their feeding according to hunger status and the circadian clock, the larva resembles a binge eater: it feeds continuously for nearly 72 hours, eating 3–5 times its own weight in food. About 24 hours before puparation, the larva abruptly leaves the food medium and stops eating. This highly stereotyped behavior provides an attractive experimental model to explore the neuronal mechanisms that drive and sustain continuous (compulsive) feeding. The hypothesis to be evaluated is that continuous feeding in the Drosophila larva is a behavior accessible to genetic and pharmacological modulation.
We will carry out microarray analysis to identify candidate genes subject to regulation during continuous feeding. Using a genome-wide RNA interference (RNAi) screen, we hope to identify genes that modulate food intake. We will complement the RNAi screen with a small-molecule screen that will look for compounds that reduce food intake. Finally, we will study the neuronal circuits modulating continuous feeding. Our long-term goal is to identify genes and neuronal circuits mediating the continuous feeding behavior of larvae and to prove that this compulsive-like behavior can be decreased by specific pharmacological interventions. We hope to illuminate common principles underlying the regulation of feeding behavior that will be applicable to processes occurring in human patients suffering from compulsive eating disorders.