Research Projects
Molecular and Cellular Biology of Short- and Long- Term Olfactory Memory in Drosophila
Schematic diagram of the
signaling systems used for memory formation
view larger image
image courtesy of Ron Davis Laboratory
A long-term project in the laboratory is to understand the molecular and cellular biology of olfactory memory in Drosophila. We have previously identified and studied several mutant strains that produce deficits in odor memories, including dunce, rutabaga, leonardo, fasciclinII, Volado, and DCO (protein kinase A). We are currently addressing specific issues regarding some of these mutants as well as others. For example, we are using systems described below to direct gene expression in time and space to determine where transgenes must be expressed to rescue mutant phenotypes. In addition, we are studying the roles in memory formation for GABA receptors, the neurofibromatosis 1 gene, and rhoGAPs. Each of these may allow us to place additional information on our model for how olfactory memories are formed and stored in Drosophila. We are also probing long-term memory formation in Drosophila with studies on the Drosophila CREB gene, combined with microarray technology to help identify genes that exhibit altered expression in specific areas of the fly brain as long-term memories are formed and consolidated.
Mushroom Body Organization and Memory Processing
Schematic
diagram of the Drosophila olfactory system
image courtesy of Ron Davis Laboratory
Although it is quite certain that memories are formed and stored in the mushroom bodies, the role of specific types of mushroom body neurons is less understood. We have previously obtained evidence that odor memories are formed and stored for at least three hours after training in the α/β mushroom body neurons. This was accomplished using transgenic reagents that can acutely silence specific neurons during the process of acquiring, consolidating, or retrieving memories. We are continuing to use these and other reagents to better define the role of the three principle types of mushroom body neurons in olfactory memory formation.
Ethanol Tolerance in Drosophila
We have developed procedures to induce both acute and chronic tolerance of adult Drosophila to the sedative effects of ethanol. We are interested in how these states of tolerance change the normal behavior of Drosophila and how changes in gene expression and neuronal morphology contribute to these behavioral changes. In addition, we are very interested in understanding the molecular biological relationship between rapid and chronic ethanol tolerance, and short- and long-term memory formation.
Drosophila Brain Mapping
Image of
the cellular cortex of the Drosophila brain
courtesy of the Ron Davis Laboratory
A much-needed resource for the continued understanding of Drosophila memory formation is a high-resolution map of the brain. We are using two-photon microscopy to visualize neurons in the Drosophila brain and developing the computational and software tools necessary for constructing a digital map of the brain and for analyzing brain organization within and between individual animals.
Technology for Controlling Gene Expression in Both Time and Space
Principle
behind the TARGET system
image courtesy of Ron Davis Laboratory
In order to understand in detail the function of any gene in memory formation, it is necessary to be able to turn any particular gene on or off during the development of the organism, or during the process of acquiring and retrieving memories. We have developed two technologies to do this and have also used these technologies to answer specific questions about certain genes involved in memory formation. One of the new technologies utilizes a hybrid transcription factor composed of an activation domain, the GAL4 DNA binding domain, and the ligand-binding domain from the human progesterone receptor. We have shown that when introduced into the fly by transgenesis, this transcription factor, known as Gene-Switch, can activate a second transgene with GAL4 binding sites in response to the anti-progestin, RU486. We have used specific enhancers to drive Gene-Switch only in the mushroom bodies and have rescued memory mutants upon activation of Gene-Switch with RU486 administered in the food. A second technology utilizes a temperature-sensitive repressor of GAL4, known as GAL80ts, which represses GAL4 activity at low temperature but becomes inactive at higher temperatures. This temperature-dependent switch can be used to repress or derepress any existing GAL4 element with simple temperature shifts. This technology, known as TARGET (temporal and regional gene expression targeting), has been used to solve several different biological problems, including the rescue of memory mutants in both time and space.
Imaging Neural Activity in Living Drosophila
Synaptic Recruitment
image courtesy of Ron Davis Laboratory
It is important to understand the activity of neurons in living animals during the process of sensing and learning about the environment. Since it is difficult to probe neuronal activity in flies using standard electrophysiological techniques, we have turned to optical imaging of neuronal activity. We have introduced transgenes into the fly that report changes in intracellular calcium concentration as well as synaptic transmission via GFP flourescence. We have used these to monitor calcium influx during neuronal depolarization and to visualize synaptic activity as the animals are exposed to odorants. The latter experiments have shown that specific antennal lobe glomeruli respond to different classes of odorants. We have used synaptic transmission reporters to visualize the changes in synaptic transmission that occur with conditioning and have discovered that new populations of synapses, defined by antennal lobe glomeruli, become activated after the flies are trained. In other words, we have visualized a olfactory memory trace for the first time. We are now examining other areas of the Drosophila brain to uncover other memory traces that may be formed upon conditioning.
Mouse Integrins and Memory Formation
Deficient
LTP in α3 integrin mouse mutants
image courtesy of Ron Davis Laboratory
The major focus of our work in the mouse is on the role of integrins in synaptic and behavioral plasticity. This project was an outgrowth of our discovery of the Drosophila Volado locus, which encodes an integrin involved in memory formation. We would like to understand the roles of integrins in the mouse brain using combined genetic, molecular, and cellular approaches. We have examined synaptic and behavioral plasticity in several integrin mutants of the mouse, looking for dominant effects on behavior and have indeed been able to demonstrate that integrins have a conserved role in behavior. We have also constructed several different floxed alleles for different integrins and are using these along with CRE lines to further delimit how integrins serve mammalian synaptic and behavioral plasticity.
Genetic Risk Factors for Mood Disorders
We have initiated a study to identify changes in certain genes that may confer susceptibility to bipolar disorder and other brain disorders.