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Genetic Analysis of Neural Circuit Organization and Assembly
Summary: Liqun Luo seeks to understand how neural circuits assemble during development and contribute to sensory perception. He has developed genetic tools to track neurons in the brains of fruit flies and mice to investigate these problems.
We study how neural circuits are organized to process information and how they are assembled during development. To address these questions, we use fruit flies and mice as model organisms and combine advanced molecular genetics with anatomical, physiological, and behavioral approaches.
Organization of the Olfactory System
The olfactory systems from flies to mammals use a similar organizational principle. In the fly, olfactory receptor neurons (ORNs) expressing the same odorant receptor project their axons to the same glomerulus in the antennal lobe. Projection neurons (PNs) send dendrites to individual glomeruli and relay olfactory information via their axons to the mushroom body and lateral horn (Figure 1). Using the MARCM (mosaic analysis with a repressible cell marker) method to label individual neurons, we previously found that PN axon terminals exhibit striking stereotypy at the lateral horn according to the glomeruli to which they send dendrites. Axon terminals of PNs representing food odors are spatially segregated from those that represent mating pheromones. By contrast, PN axon terminal arborizations in the mushroom body, the olfactory learning and memory center, exhibit much less stereotypy. Recently, we have also characterized arborization patterns of local interneurons in the antennal lobe and found considerable diversity and variability in their wiring patterns. We are currently using two-photon calcium imaging, optogenetics, and quantitative behavioral assays to identify principles of information processing at the antennal lobe and in higher olfactory centers suggested by previous anatomical studies.
Analogous to flies, mouse ORNs expressing the same odorant receptor project their axons to the same glomerulus in the olfactory bulb, creating a spatial map for odor processing. Little is known about how the olfactory bulb glomerular map is represented in the olfactory cortex. We are using virus-mediated trans-synaptic tracing to investigate this problem.
Development of Wiring Specificity in the Fly Olfactory System
The assembly of the fly olfactory system requires precise glomerular targeting of axons from each of the 50 ORN classes, as well as dendrites of each of the 50 PN classes. We are using this neural circuit as a model to investigate the general principles by which precise wiring specificity arises during development. Our previous studies have shown that PN dendrite patterning precedes ORN axon targeting. PN dendrite targeting relies on global cues in the form of gradients, as well as local cues distributed in a "salt-and-pepper" fashion on dendrites projecting to different glomeruli. Targeting of ORN axons may use the same molecules as PN dendrite targeting, but via distinct mechanisms including axon-axon interactions and axon-target interactions. To identify the cell-surface code ORNs and PNs use to form specific connections at stereotypically organized glomeruli, we are performing systematic genetic screens coupled with targeted candidate gene approaches.
Developmental Neurobiology
In addition to our focus on the olfactory system, we are studying several other developmental neurobiological problems. For example, we have shown that fly mushroom body neuron axons undergo stereotypical developmental pruning. This pruning process utilizes a local axon degeneration mechanism and requires cell-autonomous action of a steroid hormone ecdysone receptor and the ubiquitin-proteasome system, as well as glial engulfment receptors that mediate removal of degenerated axons. We are studying cell-cell communications that regulate axon pruning.
Recently, using a genetic mosaic system we developed in the mouse, we have uncovered novel roles for genes important for neuronal migration. We have also identified a cell-autonomous role for the NMDA receptor in patterning dendrites according to input organization. We continue to investigate genes that regulate neuronal activity for their function in the maturation of individual neurons and their integration into functional circuits.
Creating Genetic Tools
In the process of dissecting the adult organization and developmental assembly of complex neural circuits, we have created several useful genetic tools. The MARCM method enables the visualization and genetic manipulation of small populations of cells or single neurons in a mosaic fly (Figure 1). It has been instrumental in many of our projects and has been widely used in the Drosophila field. Recently we have developed a new repressible binary expression system, the Q system. The Q system has many applications, and we are applying it to several problems described above.
We have also developed a mosaic method in the mouse. This method, called MADM (mosaic analysis with double markers), is conceptually similar to fly MARCM. MADM allows sparse labeling and genetic manipulation of individual cells or cells that share the same lineage with distinct colors in mosaic animals (Figure 2). Using MADM as a lineage analysis tool, we have shown that cerebellar granule cells that share a common lineage project their axons to a specific sublayer of the molecular layer. We are expanding the MADM technique to other mouse chromosomes, and will use MADM to explore neural developmental processes. We have also developed other useful mice (such as a double-fluorescent Cre reporter) and synapse-labeling tools in vivo.
These studies have also been supported by grants from the National Institute of Neurological Disorders and Stroke, the National Institute on Deafness and Other Communication Disorders, and the Human Frontier Science Program.
As of September 23, 2010
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