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Overview We use molecular genetics to study the logic of neural circuit organization and assembly. Much of our current knowledge of neuronal structure, connectivity and development derives from Ramon y Cajal’s classic studies a century ago. He used the Golgi staining method, which randomly labels a small number of individual neurons in their entirety unobscured by the mass of other brain connections. We developed a modern genetic analog of Golgi staining, Mosaic Analysis with a Repressible Cell Marker (MARCM), that has allowed us to label small groups or isolated single neurons in the Drosophila brain. With MARCM we can also genetically manipulate only these labeled neurons, such as deleting a gene of interest to assess its function in the assembly of neural circuits. We have used MARCM to study the morphological development of individual neurons and the formation of specific connections between neurons. Neuronal Morphogenesis and Axon Pruning The first steps in the assembly of functional neural circuits are for individual neurons to send axons and dendrites, which eventually form specific connections with their targets. Our previous studies have shown that Rho GTPases, acting as intracellular molecular switches that transduce extracellular signals to regulate the actin cytoskeleton, play important roles in many aspects of morphological development of neurons and in structural plasticity in adults. We now focus on studying axon and dendrite pruning during development. Using candidate gene and forward genetic screening, we are studying the cell biological machinery that executes the pruning process and the regulation of such machinery. We have also provided evidence that similar machinery might be used in degeneration of axons in response to injury and disease. Organization and Assembly of Olfactory Circuits Another focus in the lab is to study the logic that governs the assembly of individual neurons into functional circuits. We use the Drosophila olfactory system as a model. As in mammals, Drosophila olfactory sensory neurons that express a given receptor converge their axons onto a specific glomerulus in the antennal lobe, creating an odor map. This information is received and relayed by antennal lobe projection neurons (PNs), whose dendrites innervate single glomeruli and whose axons project to higher brain centers. Using MARCM-based lineage analysis, we found that PNs are prespecified by lineage and birth order to send dendrites to specific glomeruli and therefore carry specific olfactory information. Furthermore, PNs relaying information from specific glomeruli have stereotyped axon branching patterns and terminal fields in higher centers. Thus this circuit could allow specific odorants to activate stereotyped sets of third order neurons, which could then release an appropriate innate olfactory-driven behavior. Using MARCM to test candidate genes and to identify new genes through mosaic mutant screens, we are exploring the molecular mechanisms that govern the assembly of this remarkable circuit. Mouse MADM We have recently developed a genetic method in mice termed MADM (Mosaic Analysis with Double Markers), which is analogous to MARCM in Drosophila for labeling and genetic manipulation of individual neurons. We are starting to use MADM to explore neural circuits and the rules of their assembly in the mammalian brain. |
| Updated 05/2005 |
