The Ron Davis Laboratory at the Scripps Research Institute Florida and the Baylor College of Medicine

Research Projects

Molecular and cellular biology of short- and long- term olfactory memory in Drosophila

A long-term project in the laboratory is to dissect 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 and newly identified mutants. For example, we are using transgenic systems to direct gene expression in time and space to determine where transgenes must be expressed to rescue mutant phenotypes. The two “time-and-space” expression systems that we invented for use in Drosophila include “TARGET” and “Gene-Switch.” More information about these can be found in our publications. We are also using these to express inhibitory RNA molecules in specific neurons only in adult animals, as a reverse genetic approach to memory formation in Drosophila. In addition, we are studying the roles in memory formation for GABA receptors, the neurofibromatosis 1 gene, CREB, and new protein kinases that we have identified to function in memory formation. Discoveries about each of these allow us to update our molecular/cellular model for how olfactory memories are formed and stored in Drosophila.

Mushroom body circuitry and memory processing

Some of the olfactory memories that are formed and stored by the fly occur in the mushroom bodies. However, it remains less certain how each type of mushroom body neuron contributes to the overall storage of memory. In addition, several different types of neurons that are thought to synapse on the mushroom body neurons also contribute to olfactory memory and we are intensively investigating their roles. Many of these studies utilize a “systems neuroscience” approach, to inhibit or activate specific sets of neurons in a living and behaving fly.

Imaging neural activity in living Drosophila

It is important to understand the types of neuronal activity that occur 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 GFP-based transgenes into the fly that report changes in intracellular calcium concentration, synaptic transmission, and cAMP. 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. In addition to this antennal lobe memory trace, we have now identified three other memory traces that form after olfactory conditioning in the fly. These occur in different sets of neurons and form and exist across different intervals after training. The dorsal paired medial neurons form a delayed memory trace that forms at 30 min after training and persists for about 2 hours. It seems to correlate with medium-term memory. The alpha/beta neurons of the mushroom bodies form a memory trace that forms by nine hours after training and persists for at least 24 hours. This memory trace depends upon the activity of CREB and upon normal protein synthesis at the time of training. Finally, we have identified a memory trace that forms in GABAergic neurons that innervate the mushroom bodies. This memory trace is registered as a decreased calcium influx (decreased activity of the GABAergic, inhibitory neurons) immediately after learning. We are searching for other memory traces that may account for behavior at different times after training and are using reporter molecules to monitor other physiological events.

 

Mouse integrins, memory formation and Alzheimer's disease

The major focus of our work in the mouse is on the role of integrins in synaptic and behavioral plasticity, and in the potential role for this family of proteins in human neurological and psychiatric diseases. This project was an outgrowth of our discovery of the Drosophila Volado locus, which encodes an integrin involved in memory formation. We are probing 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 effects on behavior and have indeed been able to demonstrate that integrins have a conserved role in behavior. Most recently, we have demonstrated that beta1- and alpha3-integrin molecules are critical for one type of short-term memory named “working memory.”
The connection between integrin function and working memory is exceptionally important, in part, because of the many human diseases that affect working memory. Although we have several different projects that are probing this translational aspect of science, one project focuses specifically on the relationship between integrin function and the peptide Abeta that is involved in Alzheimer’s disease.

 

Genetic risk factors for bipolar disorder

We have recently completed a very large study of more than 75 candidate genes for bipolar disorder. This study included a case: control association study utilizing nearly two thousand individuals with bipolar I or bipolar II and the same number of control individuals. We have identified certain human genes that are associated with these diseases and are continuing to study them at deeper levels.